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Doctoral Thesis in Materials Science and Engineering Enhancing the circular economy: Resource recovery through thermochemical conversion processes of land ll waste and biomass
ILMAN NURAN ZAINI
ISBN TRITA ITM AVL : KTH KTH www.kth.se Stockholm, Sweden Enhancing the circular economy: Resource recovery through thermochemical conversion processes of landfill waste and biomass
ILMAN NURAN ZAINI
Academic Dissertation which, with due permission of the KTH Royal Institute of Technology, is submitted for public defence for the Degree of Doctor of Technology on Friday the 11th June 2021, at 02:00 p.m. digital.
Doctoral Thesis in Materials Science and Engineering KTH Royal Institute of Technology Stockholm, Sweden 2021 © Ilman Nuran Zaini
ISBN 978-91-7873-913-4 TRITA-ITM-AVL 2021:29
Printed by: Universitetsservice US-AB, Sweden 2021 Abstract Currently, the global economy looses a considerable amount of potential secondary raw materials from the disposed waste streams. Furthermore, the existing landfill sites that often do not have proper environmental protection technologies pose a long-lasting risk for the environment, which urge immediate actions for landfill remediations. At the same time, the energy recovery from waste through conventional incinerators has been criticized for its CO2 emissions. Alternatively, pyrolysis and gasification offer the potential to recover secondary resources from waste and biomass streams, which can increase the circularity of the material resources and limit the CO2 emissions. This thesis aims to realize feasible thermochemical processes to enhance the material resources' circularity by treating landfill waste and biomass. Correspondingly, fundamental studies involving experimental works and process developments through lab-scale experiments and process simulations are carried out. The thesis is written based on the results from five different studies that cover the investigation regarding the effect of waste/biomass fuel properties on the performance of the pyrolysis and gasification processes, as well as the process development and improvement of thermochemical conversion processes of waste and biomass. The first study investigates the primary fragmentation behaviour of waste fuel pellets during the pyrolysis stage of thermochemical conversion processes. This study shows that the fragmentation degree of waste pellets correlates well with their volatile matter contents. Meanwhile, there is no clear relation between the fragmentation degree and the pellets’ mechanical strength. Generally, due to the high volatile matter content from plastic, fuel pellets from waste tend to fragment into a high number of smaller particles than typical biomass or coal pellets during thermochemical processes. Hence, for some processes, improving the thermal stability of waste pellets is more relevant than improving their mechanical strength. Subsequently, the second study examines the reactivity and kinetics behaviour of waste-derived char during gasification. In general, it is found that the char reactivity is a function of the ash amount and the ratio of inorganic catalytic elements (K, Ca, Na, Mg, and Fe) to the inhibitor elements (Si, Al, and Cl). More importantly, the char gasification test results demonstrated the significance of the waste sorting processes' operating conditions on the thermal behaviour of the waste fuel, especially during the gasification process. Meanwhile, the third study investigates the syngas and tar formations resulting from different interactions between plastic and paper fractions of solid waste. The results show that the interaction between plastic and paper significantly depends on the hydrocarbon chain structures of the plastic polymer. Specifically, the interactions of iii aliphatic-structured plastic polymers (represented by PE) and paper cause synergistic effects that reduce the tar and increase the syngas yields. Meanwhile, the synergistic effects tend to be less evident in the case of co-gasification between paper and an aromatic hydrocarbon polymer, represented by PS. Based on the results of the previous studies, a co-gasification process of waste with biomass or biochar is proposed in the fourth study. It is found that adding biochar during the gasification of waste could significantly increase the syngas and H2 production to become higher than that of when adding biomass. Synergistic effects are observed in the form of an extensive syngas yield increment and a tar yield reduction, due to the tar reforming reactions over biochar particles. In general, both biochar and biomass additions result in a higher energy yield ratio, suggesting that it could improve the efficiency of the waste gasification. Finally, the fifth study focuses on process simulations and operational cost assessments of co-production of H2, biochar, and bio-oil from biomass. The process simulation study is carried out to evaluate different scenarios for producing biochar, bio-oil, and H2 based on a biomass pyrolysis process coupled with a steam reforming and a WGS process. Based on the calculations of the total operating cost and the potential revenue, it is found that the production of bio-oil is more economically beneficial than the production of H2. The estimated minimum selling price for biochar and bio-oil based on the operating cost alone is within the price ranges of related commodities in Sweden (i.e., charcoal, coal, coke and oil crude). Nevertheless, capital and operating costs for post-processing of bio-oil should also be considered in the future to obtain a more complete economic judgement.
Keywords: gasification; pyrolysis; waste-to-energy; landfill waste; hydrogen production; circular economy; municipal solid waste
iv Sammanfattning
Förnärvarande så försvinner en stor mängd sekundära råmaterial som avfall istället för att bidra till en global ekonomi. Dessutom så är de existerande avfallsdeponierna inte utrustade med teknologier för att förhindra långsiktiga risker för miljöförstöringar, vilket gör att det är av yttersta vikt att vidta åtgärder för att hantera dessa avfallsdeponier. Samtidigt så har energiåtervinning baserat på förbränning av avfall kritiserats för den leder till ökade CO2 utsläpp. Här utgör pyrolys och förgasningsprocesser bra alternativ för att möjliggöra en återvinning av sekundära råmaterial från avfall och biomassa, vilket kan leda till en ökad cirkularitet av materialresurser och minskade CO2 emissioner. I denna avhandling diskuteras möjligheten att använda termokemiska processer för att öka cirkulationen av material baserat på utnyttjande av avfallsdeponier och biomassa. Fundamentala studier inriktade på utveckling av experimentella tekniker och processer har utförts baserat på försök I laboratorieskala och simuleringar. Avhandlingen innefattar resultat från fem olika undersökningar som behandlar studier av påverkan av bränsleegenskaperna hos avfall/biomassa på effektiviteten av pyrolys- och förgasningsprocesser samt på processutvecklingar och förbättringar av termokemiska konverteringsprocesser för avfall och biomassa. Den första undersökningen fokuserar på den primära sönderdelningen av pellets tillverkade av avfall vid pyrolyssteget av den termokemiska konverteringsprocesen. Resultaten visar att graden av sönderdelning av pellets från avfall är starkt kopplad till innehållet av lättflyktiga ämnen. Däremot så finns ingen klar koppling mellan graden av sönderdelning och den mekaniska styrkan hos pellets. Generellt så innebär den höga andelen av lättflyktiga ämnen i plaster att bränsle pellets tillverkade av avfall tenderar i att sönderdelas i ett stort antal små partiklar jämfört med typiska pellets tillverkade av biomassa eller kol under den termokemiska konverteringsprocessen. Därför så är det mer relevant att förbättra den termiska stabiliteten av pellets tillverkade av avfall i jämförelse med den mekaniska stabiliteten för några processer. Den andra undersökningen fokuserar på att studera reaktiviteten och kinetiken av kol tillverkat av avfall vid förgasning. Resultaten visar att reaktiviteten hos kol är beroende av askinnehållet och kvoten mellan oorganiska katalytiska ämnen (K, Ca, Na, Mg, och Fe) och hämmande ämnen (Si, Al, och Cl). Fast av större betydelse, att förgasningen av kol visar på betydelsen av sorteringsmetodernas effektivitet på den termiska egenskapen hos bränsle tillverkat av avfall, speciellt under förgasningsprocessen. Den tredje undersökningen fokuserar på att studera bildningen av tjära och syngas som ett resultat av interaktioner mellan plast- och pappersfraktioner som finns i det fasta avfallet. Resultaten visar att interaktionen mellan plast och papper i hög grad beror av kedjestrukturen hos kolväten i polymerer. Specifikt, så orsakar interaktionen v mellan alifatiskt strukturerade polymerer (representerade av PE) och papper synergiska effekter som leder till minskade tjär- och syngasutbyten. Samtidigt så är den synergiska effekten mindre uppenbar när papper och aromatiska kolvätespolymerer, representerade av PS, förgasas tillsammans. Den fjärde undersökningen presenterar resultat från en kombinerad förgasningsprocess som använder avfall I combination med biomassa eller biotjära, baserat på resultaten i de tidigare tre studierna. Resultaten visar att om biotjära tillsätts vid förgasning av avfall så kan utbytet av syngas och H2 ökas mer än om biomassa tillsätts vid förgasning av avfall. Dessutom framkom synergiska bidrag i form av väsentlig ökning av syngasutbytet och en minskning av tjärutbytet, orsakat av tjärreaktioner som sker i biotjärpartiklarna. Generellt så visar resultaten att både tillsatser av biotjära och biomassa leder till ett högre energiutbyte, vilket indikerar att dessa tillsatser kan öka effektiviteten vid förgasning av avfall. I den femte undersökningen användes processsimuleringar och kostnadsberäkningar för bedöma processen innefattande att samproducera H2, biokol, och bio-olja från biomassa. Simuleringarna av processen utfördes för att bestämma olika scenarios vid tillverkning av biokol, bioolja och H2 vid pyrolys av biomassa kopplat med en ångreformering och en WSG process. Baserat på beräkningar av den totala driftskostnaden och den potentiella förtjänsten så framkom att produktion av bioolja är mer fördelaktig i jämförelse med produktion av H2. Den uppskattade lägsta försäljningspriset för biokol och bioolja baserat på enbart driftskostnaden ligger inom prisintervallet för relaterade existerande produkter ( t ex träkol, kol, koks och råolja). Icke desto mindre så måste kostnader för kapital och driftskostnader relaterade till förbehandling av bioolja tas i beaktande i framtida utvärderingar för att erhålla en mer realistisk ekonomisk bedömning.
Nyckelord: förgasning, pyrolys, avfall-till-energi, avfallsdeponi, vätgasproduktion, circular ekonomi, kommunalt solitt avfall.
vi
Acknowledgements
I would like to express my gratitude to my supervisors, Weihong Yang and Pär Göran Jönsson, to give me the opportunity to pursue a PhD education and provide support and guidance during my study. I am grateful to be part of the NEW-MINE project. The supports from the NEW- MINE’s ESRs and supervisors have been crucial for my research works, which are greatly acknowledged. I also thank the European Commission for providing the funding of my study through the NEW-MINE project. I would also like to thank the Swedish Energy Agency for the financial support to conduct research during my study. I would like to thank the co-authors of the supplements presented in this thesis for their help in assisting the experimental works, engaging in insightful discussions, and preparing the manuscripts: Peter Nagy, Nanta Sophonrat, Katarzyna Jagodzińska, Yamid Gomez-Rueda, Cristina García López, Yuming Wen, Elsayed Mousa, Devy Kartika Ratnasari, Lieve Helsen, Kurt Sjöblom, and Thomas Pretz. I would like to thanks all of my friends from the Energy and Furnace Technology group, KTH, for all of their supports during my study: Nanta, Kasia, Devy, Rikard, Shule, Tong, Panos, Yuming, Hanmin, and Henry. I will certainly miss all of the fun things we did together. I would also like to thank all friends I have met in Stockholm, especially my Indonesian friends. Thanks for all of the fun we have shared. Lastly, my deepest gratitude goes to my beloved family for their endless love, prayer, and care. Thanks to my parents, Bapak Ahmad Zaini Bisri and Umi Mustabsyirotul Ummah, for always believing in me and encouraging me to aim high. Thanks to my son, Taqi, for always being the source of joy and happiness. Finally, I would like to thank my wife, Anissa Nurdiawati, for always being by my side through the good times and the bad. These past four years have not been easy, and I am deeply grateful for her unconditional love and patience. I love you.
Ilman Nuran Zaini Stockholm, May 2021
vii List of scientific supplements in the thesis
I. I. N. Zaini, Y. Wen, E. Mousa, P. G. Jönsson, W. Yang. Primary fragmentation behavior of Refuse Derived Fuel pellets during rapid pyrolysis. Fuel Processing Technology, 216, 2021.
II. I. N. Zaini, C. García López, T. Pretz, W. Yang, P. G. Jönsson. Characterization of pyrolysis products of high-ash excavated-waste and its char gasification reactivity and kinetics under a steam atmosphere. Waste Management, 97, 149-163, 2019.
III. I. N. Zaini, P. Nagy, K. Jagodzińska, P. G. Jönsson, W. Yang. Steam co- gasification of plastic and paper waste fractions: Synergistic effects on the tar and syngas formations. To be submitted to Chemical Engineering Journal.
IV. I. N. Zaini, Y. Gomez-Rueda, C. García López, D. K. Ratnasari, L. Helsen, T. Pretz, P. G. Jönsson, W. Yang. Production of H2-rich syngas from excavated landfill waste through steam co-gasification with biochar. Energy, 207, 2020.
V. I. N. Zaini, N. Sophonrat, K. Sjöblom, W. Yang. Creating values from biomass pyrolysis in Sweden: Co-production of H2, Biocarbon, and Bio-oil. Processes, 9, 2021.
Contribution statement: Supplement I: Performed experiments (excl. the compressive strength measurements of the pellets), analysed data, prepared, and reviewed the manuscript. Supplement II, III, IV: Performed experiments, analysed data, prepared, and reviewed the manuscript. Supplement V: Performed process simulation (excl. pyrolysis yield calculations), analysed data related to the simulation results, prepared, and reviewed the manuscript.
viii List of scientific contributions not included in the thesis
1. I. N. Zaini, W. Yang, P. G. Jönsson. Steam gasification of solid recovered fuel char derived from landfill waste: A kinetic study. Energy Procedia, 142, 723-729, 2017.
2. I. N. Zaini, W. Yang, P. G. Jönsson. Pyrolysis of solid recovered fuel from landfill waste: Gas and oil product composition. 4th International Symposium on Enhanced Landfill Mining, Mechelen (Belgium), 5-6 February 2018.
3. N. Sophonrat, L. Sandström, I. N. Zaini, W. Yang. Stepwise pyrolysis of mixed plastics and paper for separation of oxygenated and hydrocarbon condensates. Applied Energy, 229, 314-325, 2018
4. A. Nurdiawati, I. N. Zaini, M. Aziz. Efficient hydrogen production from algae and its conversion to methylcyclohexane. Chemical Engineering Transactions, 70, 1507-1512, 2018.
5. A. Nurdiawati, I. N. Zaini, M. Aziz. Dual-stage chemical looping of microalgae for methanol production with negative-carbon emission. Energy Procedia, 158, 842-847, 2019.
6. A. Nurdiawati, I. N. Zaini, M. Amin, D. Sasongko, M. Aziz. Microalgae-based coproduction of ammonia and power employing chemical looping process. Chemical Engineering Research and Design, 146, 311-323, 2019.
7. A. Nurdiawati, I. N. Zaini, A. R. Irhamna, D. Sasongko, M. Aziz. Novel configuration of supercritical water gasification and chemical looping for highly- efficient hydrogen production from microalgae. Renewable and Sustainable Energy Reviews, 112, 369-381, 2019.
8. A. M. Salem, I. N. Zaini, M. C. Paul, W. Yang. The evolution and formation of tar species in a downdraft gasifier: numerical modelling and experimental validation. Biomass and Bioenergy, 130, 2019.
9. I. N. Zaini, C. García López, T. Pretz, P. G. Jönsson, W. Yang. Gasification of refuse derived fuel obtained from a ballistic separation process of landfill waste. 17th International Waste Management and Landfill Symposium, Cagliari (Sardinia, Italy), 30 Sept - 4 Oct 2019.
ix 10. Y. Gomez-Rueda, I. N. Zaini, W. Yang, L. Helsen. Landfill solid waste-based syngas purification by a hybrid pulsed corona plasma unit. 27th European Biomass Conference and Exhibition, Lisbon (Portugal), 27 – 30 May 2019.
11. Y. Gomez-Rueda, I. N. Zaini, W. Yang, L. Helsen. Thermal tar cracking enhanced by cold plasma - a study of naphthalene as tar surrogate. Energy Conversion and Management, 208, 2020.
12. K. Jagodzinska, I. N. Zaini, R. Svanberg, W. Yang, P. G. Jönsson. Pyrolysis of excavated waste from landfill mining: Characterisation of the process products. Journal of Cleaner Production, 279, 2020.
13. Y. Gomez-Rueda, I. N. Zaini, W. Yang, L. Helsen. Seashell waste-derived materials for secondary catalytic tar reduction in municipal solid waste gasification. Biomass and Bioenergy, 143, 2020.
14. Y. Wen, I. N. Zaini, S. Wang, H. Yang, W. Mu, W. Yang, P. G. Jönsson. Synergistic effect of the co-pyrolysis of cardboard and polyethylene: a kinetic and thermodynamic study. Energy, 229, 2021.
x
Table of Contents 1. Introduction ...... 1 Introduction ...... 1 Objectives ...... 5 Structure of the dissertation ...... 7 Sustainability aspects of the thesis ...... 8 2. Background ...... 10 Overview of the thermochemical conversion technologies ...... 10 Pyrolysis ...... 11 Gasification ...... 12 2.3.1 Fixed bed gasifiers ...... 13 2.3.2 Tar definition and mitigation ...... 13 Thermochemical conversion processes of landfill waste ...... 15 3. Methodology ...... 17 Feedstock materials ...... 17 3.1.1 Excavated landfill waste ...... 17 3.1.2 Commercial RDF pellets ...... 18 3.1.3 Virgin materials for simulated waste ...... 20 3.1.4 Biomass and biochar ...... 20 Experimental facilities ...... 20 3.2.1 Pelletization ...... 20 3.2.2 Thermogravimetric analysis (TGA) instrument ...... 21 3.2.3 Bench-scale fixed bed reactors ...... 22 Feedstock and product characterization ...... 23 3.3.1 Syngas analysis ...... 23 3.3.2 Liquid/tar analysis ...... 24 3.3.3 Solid analysis...... 24 Process simulation ...... 24 3.4.1 Overview of the investigated processes ...... 24 3.4.2 Calculation of biomass pyrolysis yields ...... 25 xi 3.4.3 Process simulation of condenser, steam reformer, and WGS processes ...... 25 4. Influence of feedstock characteristics on the physical and thermochemical conversion phenomenon during pyrolysis/gasification ...... 28 Effect of different plastic-paper mixtures on the primary fragmentation behavior of RDF pellets (Supplement I) ...... 29 4.1.1 Background and aims ...... 29 4.1.2 Results and discussion ...... 30 4.1.3 Summary ...... 35 Influence of waste composition on the char reactivity during gasification (Supplement II) ...... 36 4.2.1 Background and aims ...... 36 4.2.2 Results and discussion ...... 36 4.2.3 Summary ...... 40 Investigation on the syngas and tar formation during gasification of plastic- paper waste mixtures (Supplement III) ...... 41 4.3.1 Background and aims ...... 41 4.3.2 Results and discussion ...... 42 4.3.3 Summary ...... 51 5. Process improvement and development of thermochemical processes of waste and biomass ...... 52 Co-gasification of waste with biomass or biochar for enhancing the gasification performance (Supplement IV) ...... 52 5.1.1 Background and aims ...... 52 5.1.2 Results and discussion ...... 53 5.1.3 Summary ...... 60
Process simulation and operational cost assessment of co-production of H2, biochar, and bio-oil (Supplement V) ...... 61 5.2.1 Background and aims ...... 61 5.2.2 Results and discussion ...... 62 5.2.3 Summary ...... 70 6. Conclusions ...... 71 Influence of feedstock characteristics on the physical and thermochemical conversion phenomenon during pyrolysis/gasification ...... 71 xii Process improvement and development of thermochemical processes of waste and biomass ...... 72 7. Recommendations of future works ...... 74 8. References ...... 75
xiii Table of Figures
Figure 1. The role of thermochemical conversion processes of waste in the circular economy...... 3 Figure 2. The illustration of the conversion process of waste fuels during pyrolysis or gasification for H2 production...... 4 Figure 3. The overview of the thesis work...... 5 Figure 4. The thermochemical conversion routes of waste/biomass and their respective products...... 10 Figure 5. Conversion stages during gasification process of waste or biomass (modified from [40])...... 12 Figure 6. The flowchart of the landfill waste sample preparation procedures in Supplement II...... 17 Figure 7. The illustration of the single pellet production equipment used for making RDF pellets...... 21 Figure 8. The schematic diagram of the vertically-oriented bench-scale fixed bed reactor...... 23 Figure 9. The schematic diagram of the horizontally-oriented bench-scale fixed bed reactor...... 23 Figure 10. The schematic diagram of (a) Scenario 1 and (b) Scenario 2 of biochar/bio-oil/H2 co-production...... 25 Figure 11. The developed flowsheet diagram of the steam reforming process in Scenario 1 (upper) and Scenario 2 (lower)...... 26 Figure 12. The flowsheet diagram of the WGS and PSA processes...... 26 Figure 13. The mass degradation (TG) and the first derivative of the mass degradation (DTG) plot of cardboard and PE decompositions during the non-isothermal pyrolysis test using a TGA instrument...... 30 Figure 14. Volume flowrate of generated syngas during pyrolysis of RDF pellet samples at different temperatures...... 31 Figure 15. Fragmentation ratio (FR) values of different RDF pellet samples after pyrolysis treatment at different temperatures and residence times...... 33 Figure 16. Relation between FR values of the fragmented RDF char and the RDF pellets’ (a) compressive strength and (b) volatile matter content obtained after pyrolysis tests at 700 °C...... 34 Figure 17. The amount of inorganic species in the char produced from pyrolysis of different waste samples...... 37 Figure 18. Average of reactivity of chars during steam gasification at 900 ᵒC as a function of the value of inorganic indexes proposed in this study...... 39
xiv Figure 19. Syngas and tar yields produced from co-gasification of PE-cardboard mixtures (upper) and PS-cardboard mixtures (lower) at different blending ratios and gasification temperature of 900 °C...... 42 Figure 20. The yields of H2, CXHY, CO, and CO2 produced from co-gasification of PE and cardboard at different blending ratios and a gasification temperature of 900 °C...... 44 Figure 21. The yields of H2, CXHY, CO, and CO2 produced from co-gasification of PS and cardboard at different blending ratios and a gasification temperature of 900 °C...... 45 Figure 22. The composition of tar (according to the number of aromatic rings) produced from the co-gasification of PE and cardboard at 900 °C presented as area% of the GC-MS spectrums...... 48 Figure 23. The composition of tar (according to the number of aromatic rings) produced from the co-gasification of PS and cardboard at 900 °C...... 49 Figure 24. The measured and predicted values of tar yields obtained from co- gasification of 50 wt.% PE and 50 wt.% cardboard at different operating temperatures...... 50 Figure 25. The conversion rate, syngas yield, and tar yield obtained from steam co- gasification of RDF-landfill with (a) biomass and (b) biochar at a gasification temperature of 800 ᵒC...... 53 Figure 26. The measured and predicted values of H2 yields produced from the steam co-gasification of RDF-landfill with biomass and biochar at gasification temperature of 800 °C...... 54 Figure 27. Illustration of tar reforming mechanisms over biochar during the co- gasification process...... 57 Figure 28. BET surface area of gasified biochar obtained at different operating conditions...... 58 Figure 29. The amount of AAEM species in the biochar before and after gasification in respect to the initial mass of the biochar...... 58 Figure 30. The sankey diagrams of the mass flow (upper) and energy flow (lower) of the co-production Scenario 1 (biochar and H2 productions)...... 63 Figure 31. The sankey diagrams of the mass flow (upper) and energy flow (lower) of the co-production Scenario 2 (biochar, bio-oil, and H2 productions). . 64 Figure 32. The relation between the temperature of bio-oil condenser and the efficiency values of Scenario 2...... 66
xv List of Tables
Table 1. Overview of the objectives of each Supplement included in the thesis work ...... 6 Table 2. Roles of the doctoral thesis in regards to the UN’s Sustainable Development Goals...... 8 Table 3. Classification of some pyrolysis processes [26,27]...... 11 Table 4. Definition and description of tar classes according to TNO.ECN [46]. ... 14 Table 5. Material composition of excavated landfill waste in some countries...... 16 Table 6. The material composition of excavated landfill waste samples used in Supplements II and IV...... 18 Table 7. Values from the proximate, ultimate, inorganic content, and heating value analyses of the raw materials used in different supplements...... 19 Table 8. Summary of blocks used in the Aspen Plus model...... 27 Table 9. The properties of char produced from pyrolysis of different waste samples at maximum temperature of 900 ᵒC...... 37 Table 10. Time required for 50% conversion level (tX=50%), average char reactivity (Ravg), and the calculated kinetic parameters of chars during steam gasification at different temperatures...... 38 Table 11. Monomer structure of PE and PS...... 41 Table 12. The main composition of tar produced from co-gasification of PE and cardboard at 900 °C...... 46 Table 13. The results from the steam gasification experiments with different fuel mixtures at different gasification temperatures...... 56 Table 14. Efficiencies and yield of products obtained from different co-production scenarios...... 65 Table 15. Summary of the main process parameters for OPEX calculation...... 67 Table 16. Annual operation costs for the co-production process of biochar, H2, and bio-oil...... 68
xvi List of Abbreviations
AAEM Alkali and alkaline earth metal BET Brunauer-Emmett-Teller BTX Benzene, toluene, and xylene CB Cardboard DTA Differential thermal analysis DTG Derivative thermogravimetric ELFM Enhanced landfill mining EN European standard EU European union FF-LW Fine fractions of the excavated landfill waste FR Fragmentation ratio GC Gas chromatography HDPE High density polyethylene HHV Higher heating value ISO International organization for standardization LAGA The Germany’s federal states working party on waste LHV Lower heating value MS Mass spectrometry MSW Municipal solid waste NGOs Non-governmental organizations OPEX Operating expenses PAHs Polycyclic aromatic hydrocarbons PE Polyethylene PS Polystyrene PSA Pressure swing adsorption RDF Refuse derived fuel xvii RDF-LW Uncleaned RDF fractions of the excavated landfill waste RDF-LWcln Cleaned RDF fractions of the excavated landfill waste RPM Random pore model SDG Sustainable development goal SS Swedish standard S/C Steam to carbon ratio TG Thermogravimetric TGA Thermogravimetric analysis UN United nations VCN Vanadium carbonitride WGS Water-gas shift WtE Waste to energy
List of symbols 퐴 Pre-exponential factor (min-1)
-1 퐸푎 Activation energy (kJ mol ) 푑푎푓 Dry ash free
-1 푘푅푃푀 The rate constant for the random pore model (min ) mash The mass of the ash residue mo The initial mass of the char mt The mass at conversion time of t
Rt The reactivity of char during gasification time of t
-1 푅푎푣푔 Average char reactivity (min ) R2 Coefficient of determination
푡푥=50% Time required for 50% char conversion level (min) 푋 Char conversion ratio
푋푚푎푥 The char conversion ratio at which the maximum reactivity occurs xviii 휂푏𝑖표−표𝑖푙 Efficiency of the bio-oil production
휂퐻2 Efficiency of the hydrogen production
휂푡ℎ Total thermal efficiency 휓 The structural parameter
xix
1. Introduction Introduction
The current existing economy model with its linear extract-produce-use-dispose of material and energy flow is considered unsustainable; thus, circular economy has been proposed as a replacement [1]. In this new proposed economy system, an alternative circular flow model is introduced to develop an environmentally and economically sustainable growth [2]. It is estimated that the current European economy is only 12% circular (slightly higher than the total global number of 9%), which shows that the linear model is still systemically an integral part of the current economic model [3]. In 2019, 225 million tonnes of municipal solid waste (MSW) were generated in the European Union (EU), with half of them either being incinerated (26%) or landfilled (24%), and the rest being recycled (30%) and composted (17%) [4]. The complexity of the materials used in today’s modern society has been one factor that hinders waste recycling. For example, adding additives or combining plastics with other materials (paper, metal, fibres) for packaging materials causes a complicated separation process, which leads to the materials being rejected for recycling [5]. According to an Ellen MacArthur Foundation’s report, it is estimated that globally, the amount of plastic packaging being recycled is only 14%, with 72% of them are either landfilled or leaking to the environment [6]. They also concluded that 30% of the plastic packaging would never be reused or recycled without fundamental innovations [6]. As there is still less than half of the waste being recycled, the EU’s economy loses a considerable amount of potential secondary raw materials from the disposed waste streams. Correspondingly, resource recovery from waste is the crucial key to achieving the circular economy to ensure that the value of products, materials, and resources in the market could be prolonged; thus, minimising the waste and resource use [7]. Since 2015, the European Commission has launched an ambitious Circular Economic Package to close the product life cycle loop [8,9]. The package includes various proposals on modifying waste policies with one of the targets to push the member states in limiting the amount of landfilled waste to less than 10 wt.% of the total waste amount by 2035 [10]. Moreover, the released communications within the Circular Economic Package also highlight the significant role of energy recovery from waste in assisting the EU in reaching its circular economy goal [9]. Explicitly, these documents suggest that some waste-to-energy (WtE) processes (e.g., gasification) should be implemented to different stages of the waste hierarchy [9]. Implementation of WtE could link and increase the resource and energy efficiencies
1 that can assist EU’s member states in achieving their targets related to the waste management, energy union, and environmental policies [7]. As a result of landfill disposal practices in the past decades, it is estimated that at least 500,000 landfill sites exist in the EU-28 countries, of which 90% are non-sanitary landfill sites with insufficient environmental protection technologies [11]. Thus, those landfills pose a long-lasting risk for humans and the environment, which urge immediate actions for landfill remediation. Enhanced Landfill Mining (ELFM) has recently been proposed as an alternative strategy to address unwanted implications of landfills while simultaneously reclaiming deposited materials, energy carriers, and land resources [12]. In the ELFM, advanced thermochemical conversion processes of waste are proposed to treat non-recyclable fractions of the landfill waste [13]. Biomass is another essential key in enhancing the circularity of the product lifecycle. Biomass and waste are the largest renewable energy sources in the EU, accounting for more than 60% of the total share of renewable energy sources [14,15]. Therefore, biomass as a renewable source would enable the Member States to meet the constantly increasing EU energy target from renewables. More importantly, biomass provides a source for renewable carbonaceous raw materials, which can allow a close loop material flow, unlike their fossil-based production system counterpart. The current biomass used in the industry mainly comes from the residues of forestry industries. At the same time, there is currently an emerging market of biomass energy from postconsumer wood waste. Wood waste as a fuel has grown interest in the EU as it is cheaper than conventional biomass fuels such as clean wood chips [16]. Following the EU waste classification code, the potential wood waste streams come from the construction and demolition waste; municipal and household waste; and materials from mechanical treatment of waste [16]. It is estimated that the potential amount of wood waste from construction and demolition waste can reach 51 million tonnes annually; while, there are potentially 22.6 million tonnes of wood waste from the MSW generated in the EU [17].
2
Figure 1. The role of thermochemical conversion processes of waste in the circular economy.
As an advanced thermochemical conversion technology, pyrolysis/gasification offers the potential to recover secondary resources from the waste or biomass stream, which conventional incineration could not achieve. These technologies provide higher flexibilities with respect to waste valorization, allowing waste conversion into various fuels and chemicals. Figure 1 illustrates the potential roles of the thermochemical conversion processes in the circular economy. Also, pyrolysis and gasification processes have a less environmental impact than the incineration process [18]. In order to improve the waste recycling rate, Sweden has introduced back the tax on waste incineration since 2020, which its price will reach 175 SEK per tonne in 2022 [19]. This tax is also considered essential to reach Sweden’s ambition of zero greenhouse gas emissions by 2045 [20]. Meanwhile, a new stricter measure on waste incineration has also been implemented in the Netherlands, through the introduction of a €31 per tonne tax on the import of waste for incineration starting from 2020 [21]. As a result, the application of pyrolysis or gasification has recently gaining more interest as an alternative to the incinerator. One of the promising pathways is the application of pyrolysis or gasification for hydrogen (H2) production. H2 is considered as a potential energy carrier of the future. Its application has been considered for various decarbonization proposals in the energy-intensive industries (e.g., steel industries) and transportation sectors [22]. In addition, the production of H2 through the upgrading of pyrolysis or gasification products would typically
3 produce a separated CO2 stream that can be further compressed and stored; thus, allowing for the development of a carbon-negative process. Despite the potentials, it has been indicated that the complexity and heterogeneity of waste, along with the purification process of the produced syngas, are the most relevant obstacles for an implementation of the current pyrolysis/gasification technologies [23,24]. Figure 2 shows the illustration of the waste fuels conversion process during pyrolysis or gasification followed by the upgrading of their vapor products in the case of H2 production. As illustrated in the figure, the fuel particles undergo a series of physical phenomena (e.g., particle fragmentation) and thermochemical reactions, which significantly depend on the fuel properties. Therefore, careful process developments concerning the thermochemical conversion phenomenon during the pyrolysis/gasification of waste are required to achieve an efficient management of residues.
Figure 2. The illustration of the conversion process of waste fuels during pyrolysis or gasification for H2 production.
This thesis summarizes the works on the development of thermochemical processes of landfill waste and biomass. The first part of the works presented in the thesis studies the influence of feedstock characteristics on the physical and thermochemical conversion phenomenon during pyrolysis and gasification. Meanwhile, the second part of the works investigates the process improvements and developments of pyrolysis and gasification processes of waste and biomass. The layout of the thesis and the details of the scope of works are described in the following sections.
4 Objectives
This thesis's general objective is to realize feasible thermochemical processes to enhance the material resources' circularity by treating unrecyclable fractions of waste and biomass. To achieve that goal, multiscale research works are carried out in this thesis, from lab-scale fundamental studies to a process modeling. The specific objectives of those studies are listed as follows. To investigate the physical and thermochemical characteristics of waste during pyrolysis/gasification processes. o To investigate the primary fragmentation behavior of Refuse Derived Fuel (RDF) pellets during rapid pyrolysis (Supplement I). o To study the kinetic behavior of steam gasification of char produced from landfill waste (Supplement II). o To examine the syngas and tar formation during gasification of waste (Supplement III). To propose a process to recover fuels and materials from waste/biomass. o To investigate the possibility of biomass-based auxiliary fuel addition to improving the quality of syngas produced from waste (Supplement IV). o To evaluate the process and economic aspect of H2, bio-oil, and biochar production from biomass (Supplement V). Based on their objectives, the overview of the study in each supplement is shown in Figure 3. The objectives are further specified in Table 1.
Figure 3. The overview of the thesis work.
5 Table 1. Overview of the objectives of each Supplement included in the thesis work
Supplement Supplement title Objectives
I Primary fragmentation behavior of Study the effect of the paper-to-plastic Refuse Derived Fuel pellets during ratio on the fragmentation degree of RDF rapid pyrolysis pellets. Determine the relationship between the pellet properties and the fragmentation ratio.
II Characterization of pyrolysis Investigate the characteristics of gas, products of high-ash excavated- liquid, and char obtained from a slow waste and its char gasification pyrolysis of landfill waste. reactivity and kinetics under a steam Investigate the kinetics behavior of atmosphere different waste char during gasification. Determine the effect of inorganic contents of waste on the kinetic parameters.
III Steam co-gasification of plastic and Investigate the influence of different paper waste fractions: Synergistic plastic-paper fraction mixtures on the effects on the tar and syngas syngas and tar formations produced from formations waste gasification. Investigate the effect of the blending ratio and gasification temperature on the synergistic effects between plastic and paper waste fractions.
IV Production of H2-rich syngas from Compare the effect of adding biomass or excavated landfill waste through biochar on the performance of gasification steam co-gasification with biochar of landfill waste. Determine the synergistic effects that occur during the steam co-gasification of different fuel blends. Investigate the syngas and tar composition produced from the co-gasification.
V Creating values from biomass Evaluate the process and economic pyrolysis in Sweden: Co-production aspects of H2, biocarbon (biochar), and of H2, biocarbon, and bio-oil bio-oil production from biomass residue.
6 Structure of the dissertation
This thesis is a compendium of five supplements formulated during the doctoral thesis study. The supplements can be classified into two main works based on their objectives as follows. Fundamental investigations: Supplement I, II, III Process development by lab-scale facilities and process simulations: Supplement IV, V The thesis is organized into seven chapters. Chapter 2 contains a brief literature review on the pyrolysis and gasification processes of waste and biomass. The composition of the waste/biomass, fundamentals of the processes, the characteristics of the pyrolysis/gasification products, and related purification technologies are presented. Chapter 3 provides the methodology used in the Supplements. The feedstock materials, the analysis methods for product characterization, the lab-scale facilities, and the process simulation are further specified. Chapter 4 is written based on Supplement I, II, and III, where the feedstock's physical and fuel characteristics are related to the kinetic and product distribution of the processes. Supplement I investigates the relation between waste fuel pellets' properties and their fragmentation behavior during rapid pyrolysis. Supplement II studies the product distribution of waste pyrolysis and its subsequent char gasification, focusing on the kinetic behavior. Supplement III examines the interactions between plastic and paper waste fractions and the mechanism of their syngas and tar formation during gasification. Chapter 5 presents the process development works conducted in Supplement IV and V. Supplement IV investigates the possibility of enhancing the syngas quality and the performance of waste gasification by adding biomass or biochar. Supplement V studies the evaluation of process and economic aspects of the pyrolysis-based co- production of H2, biochar, and bio-oil for steel industries. Chapter 6 summarizes the general conclusions based on the results presented in the Supplements. Chapter 7 provides suggestions and recommendations for future studies or applications based on the findings obtained in this thesis.
7 Sustainability aspects of the thesis
The works conducted in this doctoral thesis cover the utilization of landfill waste and biomass to improve the circularity of resources, which is in accordance with our efforts to achieve a more sustainable future world. In detail, the sustainability aspects of the works in this thesis in regards to the UN’s Sustainable Development Goals (SDG) are presented in Table 2.
Table 2. Roles of the doctoral thesis in regards to the UN’s Sustainable Development Goals. Sustainable Development Goals [25] Roles of the doctoral thesis No. Scopes 7 Affordable and clean energy This thesis aims to use thermochemical conversion processes to recover fuels, chemicals, and carbonaceous solids from waste and biomass residue. Thus, the use of fuels from the proposed process as a replacement for fossil fuels could be considered a part of SDG-7’s target of increasing renewable energy share. Also, the process could be developed in a decentralized way, in which locally generated waste/biomass could be used as a feedstock. This aspect is following the SDG-7’s target in terms of ensuring universal access to affordable energy service. 9 Industry, innovation, and The recovered fuel or chemical from waste could be infrastructure supplied back to the industries as secondary raw materials; thus, it can prolong the lifecycle of resources while minimizing environmental impacts due to the waste exposure. This aspect is in accordance with SDG-9’s target to improve the industries' sustainability by increasing resource-use efficiency and adopting more clean and environmentally sound technologies. 11 Sustainable cities and One of the targets of SDG-11 is to reduce the adverse communities environmental impact of cities with particular attention to municipal solid waste and air quality. Thus, the proposed waste treatment in this thesis could help achieve this target as it can minimize the amount of the final discharged solid waste. 12 Responsible consumption and The possibility of using the chemicals or production carbonaceous materials derived from the waste and biomass as raw materials for the industries could substantially increase the recycling rate and reduce waste generation. This outcome is following the targets within SDG-12. 13 Climate action The thermochemical conversion of waste/biomass can accelerate the SDG-13 targets in combating
8 climate change. The conversion of carbon-neutral biomass into fuel and chemical for industrial and transportation sectors could be crucial in mitigating the climate change.
9 2. Background
In this chapter, the background on the characteristics of waste and biomass and the pyrolysis and gasification technologies are presented. Factors and challenges that affect the operability of the pyrolysis and gasification of waste/biomass are also briefly discussed. Meanwhile, specific background for each supplement is stated in the background of Chapter 4 and 5.
Overview of the thermochemical conversion technologies
The primary methods for thermochemical conversion of waste or biomass are pyrolysis, gasification, and combustion, as shown in Figure 4. Among those methods, combustion or incineration is currently the most established technology for energy recovery of MSW or biomass. Compared to the biochemical treatment of MSW (e.g., anaerobic digestion), incineration offers a higher mass and volume reduction of waste and significant conservation of land. In an incinerator, heat is generated by burning the waste, which is used either as a direct heating source or to produce electricity. An incinerator plant consists typically of a combustor, a recovery boiler, a flue gas cleaning system, and a steam cycle.
Figure 4. The thermochemical conversion routes of waste/biomass and their respective products.
Despite the established application, the use of the incineration process as an option of waste management disposal has been debated due to their bad reputation of released toxins and greenhouse gases. Moreover, some people argue that incinerators can discourage recycling. Some NGOs declare that incinerators should be abandoned in the circular economy as the material loops are closed when ‘there is nothing left to burn’ [7]. Consequently, the development of alternative new processes for treating
10 waste is a significant concern, in which special attention could be given to pyrolysis and gasification routes.
Pyrolysis
Pyrolysis is a thermal decomposition process of a carbonaceous material that occurs in the absence of oxygen to produce oil, syngas, and solid char residue. Pyrolysis processes typically are operated at a temperature between 300–600 °C. The distribution of the pyrolysis products can be varied depends on the operation conditions. For instance, a high yield of oil can be achieved at moderated pyrolysis temperatures and short vapor residence times. While operating the pyrolysis process at higher temperatures and longer vapor residence times would optimize the syngas yield. On the other hand, optimum solid char production is achieved at low temperatures and long residence times. Those parameters, along with the pyrolysis process's heating rate, can be used to classify the pyrolysis process as shown in Table 3. Table 3. Classification of some pyrolysis processes [26,27]. Heating rate Vapor Pyrolysis process Main products (K/min) residence time Slow pyrolysis / carbonization < 10 Hours to days Charcoal Intermediate pyrolysis ~10–100 10 s–30 min High yield of oil products Fast pyrolysis ~100–1000 < 2 s Moderate yields of oil and syngas
Having the ability to co-produce oil, syngas, and char has been a main advantage for pyrolysis over other thermochemical conversion processes. It offers higher flexibility for waste/biomass valorization. Furthermore, intensive research activities have been going in the past decades to study the upgrading of pyrolysis products into renewable materials with high economic values, which include production of carbon nano materials [28], activated adsorbent for wastewater treatment [29], transportation fuels [30], or valuable aromatic hydrocarbons and olefins [31]. The upgrading of pyrolysis vapor has been intensively studied in the past decade. This include the H2-rich syngas production through the catalytic upgrading of pyrolysis vapor [32,33] and catalytic steam reforming of pyrolysis vapor [34–37]. In the literature, the main pyrolysis process is typically operated at temperature of 500- 750 °C, while the catalytic steam reformer temperature is in the range of 600-900 °C. In most cases, Ni-based catalyst is employed together with steam to biomass ratio (S/B) of 4. The highest H2 yield is around 10-11 wt% which is close to the maximum
11 theoretical yield based on steam reforming of pyrolysis vapor of around 11 wt.% of dry biomass. This result is achieved when the liquid/bio-oil is fully converted into gases [34,37]. In most cases, the catalytic upgrading processes produce a high concentration of CO in the syngas. This CO content can be further converted to H2 via the WGS reaction by adding more steam [38]. It should be noted that catalyst deactivation could also be a problem during an operation [34].
Gasification
Gasification represents a thermochemical conversion of carbonaceous feedstock into synthetic gas (syngas) involving partial oxidation process in the presence of gasifying agents [39]. Common solid feedstock typically used in gasification includes coal, biomass, and municipal solid waste. Meanwhile, air, steam, oxygen, CO2, or a mixture of these gases usually are used as gasifying agents. Other than the syngas as the main product, the gasification by-products also consist of unreacted carbonaceous particles (char), ash, and tar/oil. Depending on the reactor type, gasification can be operated at various temperatures in the range of 500–1400 °C and operating pressure between an atmospheric pressure to 3.3 MPa [39].
Figure 5. Conversion stages during gasification process of waste or biomass (modified from [40]).
During the gasification process, the feedstock particle undergoes a sequence of conversion stages that consists of drying, pyrolysis/devolatilization, char gasification/reduction, and char combustion, as illustrated in Figure 5 [40]. In the drying stage, the feedstock's inherent moisture is initially released at temperatures between 100–200 °C. After that, the dried particles go through the pyrolysis or devolatilization stage at higher temperatures. Thereby, they are thermally decomposed into smaller condensable compounds (i.e., liquid compounds) and non- condensable gasses, leaving char as a solid residue. At this stage, the conversion occurs in the absence of any major chemical reactions with the external gasifying
12 agent. Subsequently, the pyrolysis step is then followed by further gas-phase reactions involving the gas and liquid products and the char reaction with the gasifying agents. The gasification reactions are mainly endothermic; thus, a certain amount of exothermic char combustion reaction is necessary to provide a sufficient supply of heat in the reactor. At the end of those stages, the gasification process typically produces syngas, tar, and ash (or unreacted char).
2.3.1 Fixed bed gasifiers
Current technologies of the gasifiers can be classified into three main types, which are fixed bed gasifiers, fluidized bed gasifiers, and entrained flow gasifiers. Among those gasifiers, fixed-bed gasifiers are the most commonly used technologies to treat waste or biomass. Fixed-bed gasifiers are favored normally due to their easy construction process, simple operation, and high thermal efficiency [41]. Moreover, the use of fixed-bed gasifiers is particularly preferred for a decentralized power generation or for other thermal applications in a small to medium scale plant (<10 MW) [41]. Depending on the direction of flow of the feedstock and the syngas, fixed bed gasifiers can be classified as updraft and downdraft gasifiers. Having the similar principal to the conventional updraft gasifiers, the slagging gasification technology is a proven technology for waste treatment at higher temperatures. This technology operates using a similar principle as blast furnaces used in the steel industry, where a high-temperature gasifying agent is injected to the bottom part of a shaft furnace. Due to the high operating temperature, the gasifier could convert waste into syngas and at the same time vitrify its ash into slag [42]. Typically, during the operation of the slagging gasifier, coke is added to provide a sufficient heat to reach the high-temperature gasification and ash melting zones between 1000 to 1800 °C [42]. Currently, there are more than 40 commercial WtE plants around the globe operated based on this gasification technology [43]. Nevertheless, despite the promising ability for converting MSW into syngas and slag, these gasifiers often suffer from the high amount of tar generation. Zhang et al. [44] performed gasification of MSW using a plasma-heated shaft gasifier and found a high amount of tar generation, which is ranged between 20 – 40 wt.% of the treated MSW.
2.3.2 Tar definition and mitigation
Various definitions of tar have been proposed in the literature. Those definitions generally differ in terms of the scope of compounds that should be classified as tar. Specifically, the available literature has defined tar as follows [45],
13 1. any organic compounds produced from a partial-oxidation thermal process (gasification) of organic materials, which generally are large aromatics compounds, 2. a mixture of condensable hydrocarbons, which consists of single to multiple ring aromatic compounds, oxygenated hydrocarbons, and complex polycyclic aromatic hydrocarbons (PAHs), or 3. hydrocarbons with a molecular weight larger than that of benzene. Tar compounds can be classified into five classes according to the physical tar properties of the tar solubility in water and tar condensation. This classification is developed by TNO.ECN (Netherland) [46]. The details of the classification are presented in Table 4.
Table 4. Definition and description of tar classes according to TNO.ECN [46].
Class Definition Description Tar components Class 1 GC-undetectable This class includes the heaviest gravimetric tars tars that condense at high temperatures, even at very low concentrations. Class 2 Heterocyclic These are components that pyridine, phenol, cresol, aromatics generally exhibit high water quinoline solubility due to their polarity. Class 3 1-ring aromatics Light hydrocarbons that are xylene, styrene, toluene not important in condensation and water solubility issues. Class 4 2 to 3-ring aromatics These components condense Naphthalene, methyl- (light PAH) at relatively high naphtalene, biphenyl, concentrations and ethenylnaphtalene, intermediate temperatures. acenaphtylene, acenaphtene, etc. Class 5 >3-ring aromatics These components condense Fluoranthene, pyrene, (heavy PAH) at relatively high temperatures benzo-anthracene, at low concentrations. chrysene, benzo- fluoranthene, benzo- pyrene, etc.
An excessive tar amount in the syngas is problematic for the subsequent downstream processes such as the syngas utilization in fuel cells, methanation reactors, and Fischer-Tropsch processes [47]. Tars also cause blockage of gas downstream, fouling, and erosion for equipment [48]. Hence, the mitigation or removal of tar from syngas
14 is an intensively investigated topic. The method for tar removal can be categorized into two main approaches based on the tar removal location, which are in-situ and ex- situ methods. In the in-situ methods, the amount of tar is reduced inside the gasifier by adjusting the process conditions such as maximizing the operation conditions (e.g., pressure, temperature, etc.), adding bed additives, adding auxiliary fuel, and improving the reactor design [49]. Meanwhile, the ex-situ methods aim to remove tar after the main gasification process by adding mechanical/physical processes; reforming processes via partial oxidation, thermal, or plasma; catalytic tar elimination; or a combination of those processes [50]. The tar removal efficiency is significantly affected by the composition of the tar compounds, as they may differ from each other in terms of the physical and chemical properties. For example, a partial oxidation at temperature higher than 1100 °C is able to eliminate most tar compounds, except for stable aromatic hydrocarbons such as toluene, phenol and naphthalene [51]. It has also been suggested that the reforming of aromatic ring compounds containing side chains (e.g., toluene and phenol) needs a higher amount of energy compared to the reforming of simple aromatic ring compounds (e.g., naphthalene) [52].
Thermochemical conversion processes of landfill waste
According to the literature, the fuel characteristics of the excavated waste from aged landfills are different compared to the original MSW [53,54]. One of the main obstacles in using excavated landfill waste as a fuel in thermochemical conversion processes is the high amount of ash due to the presence of impurities such as dirt/soil. It is reported that the excavated waste contain 34-60 wt.% of impurities depending on the age of the landfill site and the composition of waste [55]. Together with the high content of volatile matter in waste, the high amount of ash could cause a low performance of the thermochemical conversion process such as a low thermal output, a high ash clinker formation, and a high tar production [56]. In addition, the landfilled waste materials suffer from a degradation which may affect its characteristics during the thermochemical conversion following the alteration of their physicochemical properties. Zhou et al. [54] indicate that the plastic fractions obtained from landfill sites typically have a less fixed carbon content and a higher ash content than a normal plastic waste, in which the differences are linearly correlated with the age of the landfilled plastics. Canopoli et al. [57] also reported that landfilled plastics contain higher amounts of heavy metals compared to the fresh plastic waste. There are many studies that have reported the characterization of landfill waste compositions and fuel properties [57–61]. Table 5 shows the material composition of excavated landfill waste obtained from landfill mining projects in some countries.
15 Nevertheless, the studies on the utilization of landfill waste through thermochemical conversion processes are still limited. Agon et al. [62] conducted single-stage plasma gasification tests of RDF samples obtained from the excavated landfill waste. The tests showed promising results as the produced syngas had a medium calorific value and a high concentration of CO2 and H2. However, the gasification performance was lower than that of biomass due to the influence of the ash and moisture contents as well as the particle sizes of the landfill RDF. Rotheut and Quicker [63] investigated the use of a non-pretreated RDF derived from the excavated landfill waste in a commercial-scale incinerator. The results show that the untreated landfill RDF cannot be used in the plant without any additions of fresh municipal solid wastes, due to the problem of an uncontrolled fire propagation in the furnace.
Table 5. Material composition of excavated landfill waste in some countries. Kuopio (Finland) Kudjape (Estonia) Lower Austria (Austria) Materials (%) [59] [60] [61] Moisture content - - 29.0 – 55.0 Plastics 23.0 22.4 18.0 Paper 4.0 – 8.0 5.1 3.0 Textiles 7.0 - 6.0 Wood 6.0 – 7.0 4.7 9.0a Glass and ceramics 4.6 1.0 Metals 3.0 – 4.0 3.1 5.0 Stones and inerts 17.5 6.0 Fine fraction 50.0 – 54.0 28.7 47.0 Rest 2.0 13.4 1.0 awood, leather, and rubber
16 3. Methodology Feedstock materials
3.1.1 Excavated landfill waste
Real excavated landfill waste samples were used in Supplements II and IV. The samples were obtained from two excavation projects of 40-50 years old landfill sites located in Halbenrain (Austria) and Mont-Saint-Guibert (Belgium). RWTH Aachen carried out the excavation projects and the subsequent sorting processes of the waste [64,65]. Thereafter, RWTH Aachen supplied the samples in the form of powder-like materials with particle sizes being less than 3 mm.
Figure 6. The flowchart of the landfill waste sample preparation procedures in Supplement II.
After being excavated, the waste materials obtained from the Halbenrain site were processed and sorted by treatment in a Biological and Mechanical Treatment Plant. A series of waste sampling and particle size reduction was done by following the LAGA procedures/standards [64]. The potential RDF fraction with particle size of 20-40 mm and the fine fraction from the waste sorting process were selected for the study in Supplement II. A further separation process was also carried out to reduce the inert materials from the RDF fraction. As a result, three samples with different amounts of inorganic impurities were obtained namely RDF-LW (the original RDF),
17 RDF-LWcln (the cleaned RDF), and FF-LW (the fine fractions) as explained in Figure 6.
Table 6. The material composition of excavated landfill waste samples used in Supplements II and IV. Excavated landfill waste samples Composition Halbenrain site Mont-Saint-Guibert site Commercial
(wt.%, db) (Supplement II) (Supplement IV) RDF pellets 20-40 mm RDF fraction 90-200 mm RDF fraction Plastics 25 43 35 Textile 1 8 9 Paper/cardboard 4 4 40 Wood 13 1 16 Other combustible 9 10 - Metals 5 2 - Inert/fine particles 23 33 -
The excavated landfill waste from the Mont-Saint-Guibert site was processed using a two-stage commercial-scale ballistic separation process [65]. The separation process produced three main fractions: the light/2D, the heavy/3D, and the fine fraction. For the study in Supplement IV, the light waste fraction sorted by mesh opening size between 90-200 mm was selected to represent the potential RDF fraction of excavated landfill waste. This fraction accounted for the highest calorific value than other fractions. Table 6 presents the material compositions of the excavated waste samples used in Supplements II and IV. Furthermore, the fuel properties of the samples are listed in Table 7.
3.1.2 Commercial RDF pellets
Other than the excavated waste sample, the experiments conducted in Supplement II also investigated a commercial RDF pellet supplied by Renewi Icova B.V. (Belgium). The pellets consisted of selectively collected commercial waste and pre- sorted industrial waste. Hence, the behavior of the landfill waste samples could be compared to that of relatively fresh MSW. The RDF pellets were made of approximately 9 wt.% textiles, 16 wt.% wood, 40 wt.% paper/cardboard, and 35 wt.% plastics.
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Table 7. Values from the proximate, ultimate, inorganic content, and heating value analyses of the raw materials used in different supplements. Beech Analyses Polyethylene Polystyrene Landfill Beech Cardboard wood (dry basis) (PE) (PS) RDFa wood char No. of supplement I, III III I, III IV IV IV Proximate (wt.%) Volatile matter 100 100 77.3 55.3 84.2 26.9 Fixed carbonb 0 0 11.7 1.8 15.0 67.3 Ash <0.1c <0.1c 11.0 46.5 0.8 5.8 Ultimate (wt.%) C 84.2 86.3 43.0 40.5 49.1 79.9 H 11.9 7.2 5.5 6.1 6.1 2.6 N <0.11c <0.11c 0.58 1.10 0.12 0.29 Oa 7.1 6.3 39.7 4.3 43.8 11.4 S 0.132 0.068 0.091 0.234 0.026 0.051 Cl 0.031 0.033 0.150 1.203 <0.01 0.014 Inorganic (mg/kg) Al 17.4 <5 7900 17700 31 172 Ca 118 <50 32900 29800 1880 8480 Fe <10 <2 950 21600 93 405 K 881 <100 364 4950 1010 4510 Mg <7 <5 1120 2470 401 1710 Na 93 <50 701 2910 63 543 Si <700 <200 4780 160000 140 802 Heating value (MJ/kg) HHV 44.2 38.2 15.7 24.1 19.6 30.1 LHV 41.7 36.7 14.4 22.9 18.3 29.6 aObtained from the Mont-Saint-Guibert landfill site. bCalculated from 100% difference. cLower limit of detection.
19 3.1.3 Virgin materials for simulated waste
The studies in Supplements I and III utilized virgin materials to simulate the real MSW-derived RDF. Polyethylene (PE) and polystyrene (PS) plastic materials were purchased from Goodfellow Cambridge Ltd. (United Kingdom) in the form of granules with particle sizes of 2–4 mm. In addition, PE materials in the form of powder with particle sizes less than 0.3 mm were also purchased from the same company for thermogravimetric analysis (TGA) tests. Meanwhile, to represent the paper fraction of waste, corrugated cardboard material from used packaging boxes was cut into particles size of 2.5 – 5 mm and used in the pyrolysis/gasification tests.
3.1.4 Biomass and biochar
In Supplement IV, beech wood sawdust provided by J. Rettenmaier & Söhne Gmbh, Rosenberg (Germany) was used as the raw biomass sample. The biochar sample was also produced from the same biomass sample using a vertical lab-scale fixed bed reactor at pyrolysis temperature of 500 ᵒC and solid residence time of 30 min. The details of the reactor can be found in the next section.
Experimental facilities
3.2.1 Pelletization
The simulated RDF pellets used in Supplement I were produced using a lab-scale single pellet production equipment, as illustrated in Figure 7. The equipment consisted of pellet dies made from a VCN alloy steel, a heating element, and a hydraulic press. The hydraulic press has a maximum operating load of 10 kN. For each process, approximately 2-3 g of feedstock was placed into the pellet dies heated at a specific temperature. The produced RDF pellets' mechanical strength was determined through compression tests in the radial and longitudinal directions. The tests were conducted using a lab-scale compressive tester machine based on ISO 4700:2007 standard [66]. The machine was equipped with an automatic data logger that record the compression force values (N) as an increasing load was applied to the pellet. The force value recorded when the pellets started to deform was used to define the pellet compressive strength.
20
Figure 7. The illustration of the single pellet production equipment used for making RDF pellets.
3.2.2 Thermogravimetric analysis (TGA) instrument
The feedstock's thermal degradation was characterized by using a TGA instrument (STA 449 F1 Jupiter, NETZSCH). The TGA instrument was equipped with a steam generator connected to the main chamber. Under a steam atmosphere, the TGA can be operated at a maximum temperature of 1250 °C. In Supplement II, the data obtained from the TGA tests are converted and normalized into the char conversion ratio (X) according to the following formula,
푚 −푚 푋 = 표 푡 (Eq. 1) 푚표−푚푎푠ℎ where mo is the initial mass of the char, mt is the mass at conversion time of t, and mash is the mass of the ash residue after a complete conversion of char. The reactivity of char during gasification time of t (Rt), was quantified by the ratio (푑푋⁄푑푡) that is formulated as follows,
푑푋 푑푚 1 푅푡 = = − (Eq. 2) 푑푡 푑푡 푚표−푚푎푠ℎ where 푑푚⁄푑푡 is the mass degradation rate of the char during gasification time of t. The random pore model (RPM) is used to fit the experimental char conversion data from the TGA. In RPM, the char reactivity can be described by the following equation [67]:
21 푑푋 = 푘 (1 − 푋)√1 − 휓ln(1 − 푋) (Eq. 3) 푑푡 푅푃푀
-1 where 푘푅푃푀 is the rate constant for the random pore model (min ) and 휓 is the structural constant/parameter. The structural parameter 휓 is determined by using the following equation [68]:
2 휓 = (Eq. 4) [2푙푛(1−푋푚푎푥+1)] where 푋푚푎푥 can be defined as the char conversion ratio at which the maximum reactivity occurs. Finally, the char gasification rate is dependent on the temperature according to the Arrhenius relationship which may be described as follows:
퐸 푘 = 퐴 푒푥푝 (− 푎 ) (Eq. 5) 표 푅푇 where 푘 is represented by 푘푅푃푀 in the case of RPM; 퐴표 is the Arrhenius equation -1 pre-exponential factor; 퐸푎 is the activation energy; 푅 is the gas constant (8.314 J K mol-1); and 푇 is the reaction temperature.
3.2.3 Bench-scale fixed bed reactors
Two bench-scale fixed bed reactors, a vertically- and a horizontally-oriented one, were used to perform the pyrolysis and gasification processes. Approximately a maximum sample amount of ±15 g can be used for each test. In general, these reactors consist of a stainless-steel (316L) tube, an electric heater with an insulator, and a gas cleaning system, as seen in Figure 8 and Figure 9. Depends on the process, the reactor's temperature was controlled by placing a thermocouple either inside the stainless tube or on the outer surface of the tube. The reactor can be operated up to a maximum temperature of 1000 °C. The reactor was connected to a N2 gas cylinder and a steam generator system to supply the required pyrolysis carrier gas and gasifying agent. The flow rate of water going into the steam generator was controlled by using a peristaltic pump. Furthermore, the gas cleaning system consisted of a series of washing bottles placed inside a cooling bath to condense the oil/tar produced from the thermochemical processes. Additional cleaning by solid adsorbent was used to ensure the produced gas is free from oil/tar and water vapor.
22
Figure 8. The schematic diagram of the vertically-oriented bench-scale fixed bed reactor.
Figure 9. The schematic diagram of the horizontally-oriented bench-scale fixed bed reactor.
Feedstock and product characterization
3.3.1 Syngas analysis
The syngas produced from the pyrolysis/gasification test was analyzed by using a Micro-GC instrument (Agilent 490 micro GC quad), which was equipped with four columns, namely MS5A, PPU, Al2O3/KCl, and CP-Sil 5CB columns. The Micro-GC instrument is calibrated to detect and measure the concentration of H2, CO, CO2, and CxHy gases (CH4, C2H2, C2H4, C2H6, C3H6, and C3H8). The syngas composition and flowrate were measured every 3 min.
23 3.3.2 Liquid/tar analysis
The liquid/tar products obtained from Supplements II and III study were analyzed using a GC/MS analysis system consisting of an Agilent 7890A gas chromatographer (GC) and an Agilent 5975C MSD mass spectrometer (MS). The GC can be equipped with various columns, such as HP-5ms columns and DB-1701 columns. The analysis of the tar samples collected from the gasification experiments in Supplement IV was performed by an external laboratory, Verdant Chemical Technologies (Sweden), using a GC/FID analysis system.
3.3.3 Solid analysis
The proximate, ultimate, and heating value analyses of raw feedstock and their corresponding char were performed by an external laboratory Eurofins (Sweden), based on the SS-EN 15400:2011, SS-EN 15402:2011, SS-EN 15403:2011, SS-EN 15407:2011, and SS-EN 15408:2011 standards. Furthermore, the inorganic content analysis was performed by another external laboratory ALS Scandinavia AB (Sweden).
Process simulation
3.4.1 Overview of the investigated processes
A process simulation work was carried out in Supplement V as follows to evaluate two co-production scenarios:
Scenario 1: co-production of biochar and H2 and,
Scenario 2: co-production biochar, bio-oil, and H2. In the co-production system, the biochar is produced from a biomass pyrolysis process. Simultaneously, the bio-oil is collected by condensing some parts of the pyrolysis vapor in the bio-oil condenser. The vapor from the pyrolyzer (Scenario 1) or the bio-oil condenser (Scenario 2) is subsequently converted into H2 through a combination of steam reformer and water-gas-shift processes. The illustrations of those two co-production scenarios are presented in Figure 10.
24 (b) Scenario 1: Co-production of biochar and H2
(a) Scenario 1: Co-production of biochar, bio-oil, and H2
Figure 10. The schematic diagram of (a) Scenario 1 and (b) Scenario 2 of biochar/bio-oil/H2 co- production.
3.4.2 Calculation of biomass pyrolysis yields
In this study, the biomass pyrolysis products' yields were determined based on the operation of the Envigas biochar pilot plant in Bureå, Sweden. An intermediate pyrolysis process of 100 kg/h biomass is operated in an electrically heated screw reactor at an operating temperature between 550-650 °C. The biochar yield used in the study was obtained directly from the process at an operating temperature of 550 °C. The value of the biochar yield is 23 wt.% of the biomass input. After that, further calculations to determine the composition of the pyrolysis vapor are carried out based on the elemental balance.
3.4.3 Process simulation of condenser, steam reformer, and WGS processes
The results from the calculation of biomass pyrolysis yields were used as an input value for the process simulation in Aspen Plus. The process simulation scope includes the bio-oil condenser, steam reformer, WGS, and PSA processes. The process simulations are performed using the Aspen Plus version 9.0 (Aspen Technology, Inc.) process simulation software. The flowsheet diagram of the steam reformer section's developed model in the Aspen Plus is presented in
25 Figure 11. Meanwhile, the flowsheet diagram of the WGS and the PSA modules is shown in Figure 12.
Figure 11. The developed flowsheet diagram of the steam reforming process in Scenario 1 (upper) and Scenario 2 (lower).
Figure 12. The flowsheet diagram of the WGS and PSA processes.
26 Table 8. Summary of blocks used in the Aspen Plus model. Abbreviations Type of block Function COOL1 Heater Cooling of syngas input to the WGS reactor.
COOL2 Heater Cooling of H2-rich gas products. COOL3 Heater Cooling within the PSA process.
COMP1 Compressor Compression of the H2-rich gases to 1 MPa. COND Flash2 Bio-oil condenser. DECOMP RYield Breakdown of the high molecular weight lignin fraction into C, H2, and O2. Only for Scenario 1. HX1 HeatX Heat exchanger between reformer's syngas and pyrolysis vapor stream. HX2 HeatX Heat exchanger between reformer's syngas and steam input to the reformer. HX3 HeatX Preheating of WGS steam input. MIX1 Mixer Addition of the make-up water. REFORM RGibbs Steam reforming at 850 °C and ambient pressure. SEP1 Sep Separation of pyrolysis vapor stream into a light fraction (conventional) and a high molecular weight lignin fraction (non-conventional). SEP2 Sep Water condenser. PSA Sep PSA module operated at 1 MPa and 50 °C. PUMP1 Pump Water supply to the steam reformer. PUMP2 Pump Water supply to the WGS reactor. WGS RStoic WGS process at 425 °C and ambient pressure.
The details of the blocks’ specifications used in the simulation are shown in Table 8. The simulations were then carried out as follows based on conditions and assumptions: The process is operated under steady-state conditions. The biomass input is 1000 kg/h, which includes its moisture content. Gases are assumed as ideal gases. The compressor and pump efficiencies are 90 and 75%, respectively [69]. The minimum temperature approach of heat exchangers is 10 °C [69].
27 4. Influence of feedstock characteristics on the physical and thermochemical conversion phenomenon during pyrolysis/gasification
This chapter is a compilation of the results from works conducted in Supplements I, II, and III. These works study the influence of the waste/biomass feedstock characteristics on their physical and thermochemical conversion phenomenon during pyrolysis/gasification processes. Specifically, this chapter focuses on the investigation of the primary fragmentation behavior of waste pellets (Supplement I), the kinetics of the char decomposition (Supplement II), and the mechanisms of the syngas and tar formations (Supplement III). The operability of fixed-bed gasifiers is significantly influenced by the fuel pellets/particles' comminution as the fuel is converted in the gasifier chamber [70,71]. This particle comminution subsequently occurs during different stages, namely primary fragmentation, secondary fragmentation, percolative fragmentation, and attrition [72]. Among those stages, the primary fragmentation is considered as being the crucial reduction mechanism. It decides the particle size distribution of the charred fuel particles; hence, it significantly influences the char conversion rates in the gasifier [73]. This stage occurs during the pyrolysis stage of the gasification process due to the release of volatiles and the collapsing of internal bridges of char particles [74]. In the fixed-bed gasifiers, the fuel particles undergo the pyrolysis stage at 500–700 °C before the subsequent char gasification stage [19]. Considering its significance, the work conducted in Supplement I is focused on studying the primary fragmentation behavior of RDF fuels during the rapid pyrolysis stage of gasification. Other critical parameters that influences the gasification process are the char gasification reactivity and kinetics. Comprehensive knowledge of the waste-derived char gasification reactivity and kinetics could help to design and simulate reliable and efficient gasifiers [75]. It has been well known that the reactivity of char during gasification is significantly affected by the inorganic content in the chars, especially for gasification at temperatures lower than 1000 °C [76]. Hence, the work conducted in Supplement II focuses on the investigation of the kinetic behavior of waste- derived char gasification under a steam atmosphere, which emphasizes the relation between the char’s inorganic contents and its reactivity. Another study conducted within Supplement III focuses on another aspect concerning the heterogeneity of the waste material compositions. Specifically, it investigates the synergistic effects of the interactions between plastic and paper fractions of the waste feedstock during gasification. These interactions were observed by evaluating the amounts and compositions of the produced syngas and tar. The knowledge of those interactions is important in optimizing the gasification
28 performance of waste and developing co-gasification of plastic with biomass that recently gained more interest.
Effect of different plastic-paper mixtures on the primary fragmentation behavior of RDF pellets (Supplement I)
4.1.1 Background and aims
The fragmentation behavior of various solid fuels has been reported by previous studies [71,77–79]. According to the literature, the fragmentation of fuel particles during thermochemical processes is mainly determined by their composition, mechanical properties, and textural properties. For instance, fuel particles with a higher fixed carbon content tend to have a higher fragmentation degree [79]. It is also found that fuel particles with a lower mechanical strength most likely would be fragmented into a higher number of particles [78,79]. Furthermore, it has been suggested that the influence of the mechanical strength of the fuel particles is more significant than the volatile matter content [78]. Nevertheless, those explanations from the literature were mainly based on biomass and coal particles' fragmentation in fluidized-bed gasifiers. Meanwhile, investigations on pelletized RDF particles' fragmentation, which notably have contrast properties than biomass or coal particles, are relatively rare. The work conducted in Supplement I aims to study the primary fragmentation behavior of RDF pellets during the rapid pyrolysis stage of the gasification process. The pyrolysis experiments were carried out at 500–700 °C using simulated RDF pellet samples. These pellets consisted of different cardboard-to-plastic mass ratios ranging from 25:75 to 75:25 with a total weight of approximately 2 g. The pellet's dimension had varying lengths between 15–20 mm in length and a diameter of about 12 mm. The fragmentation degree was evaluated by analyzing the number and particle size distribution of the pellets obtained from different pyrolysis temperatures and solid residence times. The relations between the fragmentation degree and the pellets’ properties (i.e., mechanical strength and composition) are further examined.
29 4.1.2 Results and discussion
Figure 13. The mass degradation (TG) and the first derivative of the mass degradation (DTG) plot of cardboard and PE decompositions during the non-isothermal pyrolysis test using a TGA instrument.
Figure 13 presents mass degradation (TG) and the first derivative of the mass degradation (DTG) plots of cardboard and PE decompositions during the non- isothermal pyrolysis test by using a TGA instrument. As indicated in the figure, cardboard starts to thermally decompose at approximately 230 °C, mainly due to a hemicellulose decomposition. Hemicellulose is well known to have the lowest thermal stability compared to other lignocellulose components of biomass as it decomposes between 150–300 °C [80]. As indicated by the DTG plot in the figure, the highest mass decomposition rate was recorded at a temperature around 360 °C, at which cellulose, as the main component of cardboard, plays a significant role in the devolatilization. After that, the decomposition of lignin accounts for a further gradual mass degradation at temperatures higher than 400 °C [81]. In contrast to cardboard, the mass degradation of PE starts at a higher temperature of approximately 420 °C. Besides, instead of a multi-stages decomposition, PE only has a single-stage process, as it is fully devolatilized at approximately 507 °C. The temperature range of the PE decomposition measured in this study is in accordance with previous studies that report a range temperature between 390–495 °C [82].
30
Figure 14. Volume flowrate of generated syngas during pyrolysis of RDF pellet samples at different temperatures.
31 Figure 14 shows the volume flowrate of the generated syngas during pyrolysis of RDF pellet samples with different cardboard-to-plastic mass ratios at different temperatures. As seen in the figure, the CO generation occurs earlier than the other gases, especially at low pyrolysis temperatures. The earlier production of CO is mainly due to the decomposition of lignocellulose components of cardboard at lower temperatures than for the decomposition of PE, as explained previously. In general, pyrolysis of pellets with higher cardboard amounts resulting in a production of higher amount of CO and CO2. In contrast, pyrolysis of plastic contributes to the higher yields of hydrocarbon gases. In the case of PE, the pyrolysis process generates more C2H4, C2H6, and C4H10 [83]. This is also confirmed by the finding of this study shown in Figure 14. Operating the pyrolysis test at a higher temperature causes a higher volume rate of syngas, especially in H2, CH4, and CXHY. The higher yield of these gases is a result of the more intensive oil/wax cracking reactions at higher pyrolysis temperatures. The degree of fragmentation was quantified and evaluated by using the FR values. Figure 15 presents the FR values of different RDF pellet samples after pyrolysis treatment at different temperatures and residence times. In most cases, it can be seen that the FR value raises with the longer residence times until residence time of 15 min as there are more number of fragmented char particles. Thereafter, the values gradually decrease as the smaller char particles undergo further comminution due to either fragmentation or shrinkage. Thus, the number of counted particles decrease due to the loss of these small particles. In addition, pellets with high plastic concentrations tend to be sticky at the beginning of the pyrolysis process, due to the presence of fused plastic particles. As seen in Figure 15, pellets with higher concentrations of cardboard tend to be fragmented into smaller number of particles at the end of the pyrolysis test. This is mainly due to the lignin solid bridge binding force between cardboard particles that are formed during the densification of RDF. As explained from the TGA results, lignin tends to have a higher thermal stability than PE. Furthermore, higher pyrolysis temperatures typically increase the FR values for all types of pellet. Nevertheless, the effect of pyrolysis temperature in the range of 500–700 °C on the fragmentation of RDF pellets is less significant than that of the RDF pellet compositions.
32
Figure 15. Fragmentation ratio (FR) values of different RDF pellet samples after pyrolysis treatment at different temperatures and residence times.
Figure 16a shows the relation between the compressive strength of the pellets (both radial and longitudinal) and the FR values. As depicted in the figure, it can be suggested that no clear linear relationship exist between the mechanical strength the fragmentation degree of the RDF pellets. These results are in contradictive with
33 other studies suggesting that fuel particles with a lower mechanical strength tends to have a higher fragmentation degree [79]. This trend is mainly due to the complex nature of mechanical strength of such multi-component pellets as a result of the different binding mechanisms between different materials particles [85–87]. Consequently, it is rather difficult to predict the degree of fragmentation by only considering the mechanical strength of RDF pellets. In contrast, the volatile matter content of the samples correlates well with the FR values, as shown by Figure 16b, as a higher volatile matter content consistently results in a higher FR value. This may illustrates that the thermal stability of RDF pellets significantly depends more on the volatile matter contents rather than their mechanical strength. Thus, it can be concluded that in the case of RDF pellets, their volatile matter content may provide a better results in predicting the degree of RDF pellets’ fragmentation.
(a) 10.0 Radial Longitudinal FR 7.5 25C-75P 25C-75P 5.0 50C-50P 50C-50P
2.5 Fragmentation ratio, ratio, Fragmentation 75C-25P 75C-25P 0.0 0 10 20 30 40 50 60 Compressive strength (MPa)
(b) 10.0
FR 7.5 25C-75P
5.0 50C-50P R² = 0.9703
2.5 Fragmentation ratio, ratio, Fragmentation 75C-25P 0.0 80 85 90 95 100 Volatile matter (wt.%) Figure 16. Relation between FR values of the fragmented RDF char and the RDF pellets’ (a) compressive strength and (b) volatile matter content obtained after pyrolysis tests at 700 °C.
34 4.1.3 Summary
The densification of different material compositions of RDF results in varied particle binding forces that has their own unique characteristics of mechanical strength and thermal stability during pyrolysis. Lignin solid bridges are formed between cardboard particles and they have a binding force with a higher thermal stability than that of fused plastics. Thus, pellets with a higher concentration of cardboard tends to be less fragmented. Within the investigated temperature range of 500–700 °C, the effect of the pyrolysis temperature on the fragmentation degree of RDF pellets is less pronounced than the effect of the plastic and cardboard compositions of RDF pellets. The fragmentation degree of RDF pellets correlates well with the pellet’s volatile matter content. Whereas, there is not a clear relation between the mechanical strength of pellets and the pellet’s fragmentation degree. Generally, due to the high volatile matter content from plastic, RDF pellets tend to fragment into a high number of smaller particles compared to typical biomass or coal pellets during thermochemical processes. This characteristics could be a crucial concern when the high-contained plastic RDF pellets are used in a some specific reactors. Hence, in this case, improving the thermal stability of RDF pellets is more relevant rather than their mechanical strength. Mixing the RDF with additional auxiliary fuel that potentially have a higher thermal stability could be useful.
Further results and discussion of the experimental study are available in Supplement I.
35 Influence of waste composition on the char reactivity during gasification (Supplement II)
4.2.1 Background and aims
It is well known that the inorganic contents in the char is one of the main factors that determine char gasification reactivity. Specifically, the presents of alkali and alkaline earth metal (AAEM) species such as K, Na, Mg, and Ca can simultaneously act as catalysts that accelerate the char conversion rate [76,88]. In contrast, the present of Si, has been shown to act as an inhibitor that cause a lower reactivity of the char gasification [89,90]. Due to the heterogeneity of the waste material compositions, their inorganic content is more varied than that of the biomass feedstock. Moreover, in the case of landfill waste, there are typically tremendous amounts of ash due to the presence of impurities that comes from mainly soil/dirt. As a result, a thorough investigation of the influence of the inorganic content on the results is of high concern. In the work in Supplement II, steam gasification experiments were conducted to investigate the char reactivity behavior by using a TGA instrument. The char samples were produced from pyrolysis of high-ash RDF and fine fractions obtained from excavated landfill waste. These samples represent different amounts and compositions of inorganic contents. In addition, a char sample produced from pyrolysis of commercially available RDF pellets was used to represent a low inorganic contained waste sample.
4.2.2 Results and discussion
Table 9 shows the ultimate analysis results of the char samples used for the gasification tests. Moreover, the amount of inorganic contents of the char is shown in Figure 17. As shown in Table 9, the char produced from excavated landfill waste generally dominated by ash content due to the high dirt/soil impurities. The ash amount in the landfill waste char is at least 76.5 wt.%. The high amount of dirt in the landfill waste samples also results in the domination of Si over other inorganic species, as can be seen in Figure 17. The highest Si concentration is found in the case of a RDF-LW char sample which contains 20.8 wt.% of Si. This value is significantly higher than that of the RDF-MSW sample which only has 9.9 wt.% of Si. In contrast to the RDF fraction of landfill waste (RDF-LW and RDF-LW samples), the fine fraction (FF-LW) char is dominated by Ca (15.4 wt.%) instead of Si. This result is in accordance with previous studies related to the inorganic content of landfill waste fine fraction, which reported that Ca and Fe are the most abundant metal species in the fine fractions [91].
36
Table 9. The properties of char produced from pyrolysis of different waste samples at maximum temperature of 900 ᵒC.
Char composition (wt.%, db) Char structural properties Sample BET surface Pore volume C H N Cl S Ash area (m2/g) (cm3/g) FF-LW 11.8 0.2 <0.10 0.64 1.54 90.6 49.6 0.072 RDF-LW 15.8 0.5 0.18 1.46 0.45 83.0 37.8 0.028 RDF-LWcln 20.3 <0.10 0.3 4.13 0.45 76.5 75.0 0.043 RDF-MSW 44.6 0.4 0.79 1.49 054 53.5 7.8 0.035
25
FF-LW RDF-LW RDF-LWcln RDF-MSW
20
15
10
5 Mineral compositions Mineral compositions (wt.% of chars)
0 Si Al Ca Fe K Mg Na P
Figure 17. The amount of inorganic species in the char produced from pyrolysis of different waste samples.
Table 10 presents the reactivity indexes of the char gasification as well as its calculated kinetic parameter values. Overall, char from the fine fraction of landfill waste is the most reactive sample compared to other landfill waste fractions and the normal RDF samples. This result is specifically indicated by the shortest tX=50% and the highest Ravg values compared to the other samples. The highest reactivity of the fine fraction char is mainly caused by the its high AAEM contents especially Ca, and metal transition Fe, which are well known to act as catalytic elements that contribute to the higher reactivity [92].
37 Despite its role as an inhibitor during gasification, the presence of a smaller Si content does not guarantee a higher reactivity of the char. This trend can be seen in the case of RDF-LWcln that has a lower Ravg value than that of RDF-LW, even though it contains a lower amount of Si. One possible reason that cause this trend is the higher Cl concentration than the RDF-LW char sample following the impurities reduction process. It has been investigated that the presence of inorganic chlorine has an inhibiting effect on the reactivity of MSW char [93]. Generally, all studied char samples have relatively low porosity as shown by the low range values of BET surface area. Thus, no notable trend is found related to the influence of the char porosity on the char reactivity. This result is in accordance with results from previous studies which state that the role of inorganic AAEM is more significant than the porosity of the char at a gasification temperature below 1000 ᵒC [76].
Table 10. Time required for 50% conversion level (tX=50%), average char reactivity (Ravg), and the calculated kinetic parameters of chars during steam gasification at different temperatures. Temp. Reactivity indexes Kinetic parameters Sample -3 -1 -1 -1 2 (ᵒC) tX=50% Ravg (x10 min ) Ea (kJ mol ) A (min ) R 800 10.5 13.9 FF-LW 850 6.0 25.1 127.7 9.25 x 104 0.9912 900 3.5 48.9 800 15.0 8.7 RDF-LW 850 7.5 18.4 104.5 6.23 x 103 0.9486 900 6.0 26.7 800 32.0 6.1 RDF-LWcln 850 23.0 8.2 117.9 1.12 x 104 0.9975 900 12.0 16.4 800 13.5 11.8 RDF-MSW 850 10.0 20.3 58.3 4.70 x 101 0.9998 900 7.0 35.1
38 0.08 FF-LW RDF-LW RDF-LWcln 0.06 RDF-MSW
) R² = 0.8695 1 - 0.04 (min avg R
0.02
0.00 0 0.5 1 1.5 [(K+Ca+Mg+Na+Fe) / (Si+Al+Cl)] x ash%
Figure 18. Average of reactivity of chars during steam gasification at 900 ᵒC as a function of the value of inorganic indexes proposed in this study.
In this study, an inorganic index is proposed as a tool to summarize the relation between the inorganic element amounts and compositions on the char reactivity. This index is developed upon the previously proposed indexes by other studies [94,95], which are normally used as a way to illustrate the role of the inorganic elements on the char reactivity. The index is defined as the ratio of the amount of the inorganic elements having catalytic to the inhibitor ones, which is multiplied by the amount of ash in the char. The formula of the inorganic index is written as follows,
퐾+퐶푎+푀푔+푁푎+퐹푒 퐼푛표푟푔푎푛𝑖푐𝑖푛푑푒푥 = 푥푎푠ℎ푤푡. % 푆𝑖+퐴푙+퐶푙 (Eq. 6) where ash wt.% is the amount of ash in dry basis.
Figure 18 shows the relation plot between the values of inorganic index and the Ravg values. As depicted in the figure, the plot exhibits a well-defined linear fit with an R2 value of 0.8695. Furthermore, both the reactivity and inorganic index values of the char samples follow the order of RDF-LWcln < RDF-LW < RDF-MSW < FF-LW. Thus, the results suggest that the proposed index could well relate the amounts and compositions of inorganic species to the char reactivity in the case of a widely varied high-ash waste feedstock.
39 4.2.3 Summary
The excavated landfill waste contain significantly higher amounts of ash than the fresh MSW. Depending on the composition of the inorganic species in the ash, the higher amount of ash in landfill waste could cause a higher or lower char reactivity compared to that of fresh MSW during gasification. The char produced from the pyrolysis of fine fraction contains the highest amount of ash with a higher concentration of catalytic inorganic species (Ca and Fe); thus, it has the highest char reactivity. On the other hand, the RDF fractions of landfill waste exhibit the lowest char reactivities due to the significant domination of Si. In general, it is found that the char reactivity depends on the the ratio of the catalytic inorganic elements (K, Ca, Na, Mg, and Fe) over the inhibitor elements (Si, Al, and Cl). The results from the gasification tests of the landfill waste char demonstrated the significance of the process parameters during waste sorting processes on the thermal behavior of the waste fuel, especially during the gasification. The proposed inorganic index could be useful for providing information with respect to the expected reactivity of RDF from landfill waste in which its inorganic elements composition is significantly varied depending on the landfill age, type of waste, and pre-processing method.
Further results and discussion of the experimental study are available in Supplement II.
40 Investigation on the syngas and tar formation during gasification of plastic-paper waste mixtures (Supplement III)
4.3.1 Background and aims
According to the literature, synergistic effects due to the interaction between plastic and biomass particles mainly exist in form of alteration of either the plastic or biomass’s decompositions due to the release of radicals from their counterpart feedstock [96–98]. This alteration occurs in a series of overlapping reactions, which includes the promotion of the cracking of plastic’s hydrocarbon due to the release of free radicals from biomass [98]; and the stabilization of those biomass radicals by the donation of H atoms released from the devolatilization of plastics [97,99]. Another possible synergistic mechanism exist in a form of interactions between volatiles released by one component with the char derived from other components [100]. Furthermore, those synergistic mechanisms are significantly influenced by the gasification operation parameters, especially the gasifying agent [98,101] and the gasification temperature [102]. Despite that the interactions between plastic and biomass have been widely investigated so far, there is still lack information on the tar formation mechanisms resulting from the plastic-biomass mixtures. In addition, the gasification of paper and plastic mixtures has not been investigated as intensely as plastic and woody biomass mixtures [96–104]. The work conducted in Supplement III, aims to examine the synergistic effects on the syngas and tar formation due to the interaction of paper with aromatic or aliphatic hydrocarbon polymers during steam gasification. PE and PS plastics were used to represent the aliphatic and aromatic-structured polymers (see Table 11), respectively.
Table 11. Monomer structure of PE and PS.
Materials Monomer structure
PE
PS
41 4.3.2 Results and discussion
1.5 0.50 Measured syngas Predicted syngas Measured tar Predicted tar 1.2 0.40 daf) - daf) - fuel
- 0.9 0.30 fuel -
0.6 0.20 Tar yield yield Tar(g/g
Syngas Syngas yield (g/g 0.3 0.10
0.0 0.00 0 25 50 75 100 PE content (wt.%)
1.5 0.50 Measured syngas Predicted syngas
1.2 Measured tar Predicted tar 0.40 daf) - daf) - fuel - 0.9 0.30 fuel -
0.6 0.20 Tar yield yield Tar(g/g
Syngas Syngas yield (g/g 0.3 0.10
0.0 0.00 0 25 50 75 100 PS content (wt.%) Figure 19. Syngas and tar yields produced from co-gasification of PE-cardboard mixtures (upper) and PS-cardboard mixtures (lower) at different blending ratios and gasification temperature of 900 °C.
Figure 19 shows the measured yields of syngas and tar obtained from co-gasification tests of PE-cardboard mixtures at 900 °C. In addition, the predicted values of product yields were also determined based on the weighted average values from mono-gasifications of each feedstock. The results are plotted in form of lines to evaluate the synergetic effects that occurred during the co-gasification tests. As seen in the figure, a mono-gasification of PE produces 63% less syngas and 3.8 times more tar yield than the gasification of pure cardboard. Furthermore, it is found that
42 additions of cardboard during gasification of PE causes synergistic effects that enhance the generation of syngas while lowering the tar yield. This trend can especially be seen in the case of cardboard proportions of 25 wt.%, at which the tar yield is drastically reduced by 80%, from 0.081 to only 0.016 g/g-fuel-daf. This value is lower than that of pure cardboard gasification. The measured value is also 76% lower than the predicted value of tar yield of 25 wt.% of cardboard proportion, which indicate an occurring of extensive synergistic effects on the tar reduction. At cardboard amounts of higher than 25 wt.%, the degree of synergistic effects on the tar reduction decreases as the gap between the predicted and measured values is getting closer. Following the reduction of the tar yield, the co-gasification test consisting of 25 wt.% of cardboard and 75 wt.% PE mixture produces the highest value of syngas yield among the studied fuel mixtures. Specifically, it produces 1.00 g/g-fuel-daf of syngas, which is almost three times of the measured value from the 100 wt.% PE gasification. The measured value is also twice as high as the predicted syngas yield value for the same fuel mixture proportions. This result again confirms that the highest degree of synergistic effect is observed for cardboard additions of 25 wt.%, with respect to both tar yield reduction and syngas yield increment. The results from the co-gasification of PS and cardboard at 900 °C is also presented in Figure 19. As shown the figure, the steam gasification of PS generally results in a higher yield of tar than the gasification of PE. Its measured value reaches 0.261 g/g- fuel-daf, which is more than three times of the tar yield produced from gasification of PE. Moreover, in contrast to that of PE-cardboard co-gasification, it can be seen that no clear synergistic effects are observed in the case of PS-cardboard mixtures. The measured values of both the syngas and tar yields are proportionally shifted with increased of cardboard proportions. Also, the different between measured and predicted values are found to be insignificant.
43 0.15 0.50 Measured H₂ Measured CₓHᵧ Predicted H₂ Predicted CₓHᵧ 0.12 0.40 daf) - daf) - fuel
0.09 - 0.30 fuel -
0.06 0.20 yield yield (g/g yield yield (g/g y 2 H x H 0.03 0.10 C
0.00 0.00 0 25 50 75 100 0 25 50 75 100 PE content (wt.%) PE content (wt.%)
0.50 1.00 Measured CO Measured CO₂ Predicted CO Predicted CO₂ 0.40 0.80 daf) daf) - - fuel fuel
0.30 - 0.60 -
0.20 0.40 yield yield (g/g 2 CO CO yield (g/g
0.10 CO 0.20
0.00 0.00 0 25 50 75 100 0 25 50 75 100 PE content (wt.%) PE content (wt.%)
Figure 20. The yields of H2, CXHY, CO, and CO2 produced from co-gasification of PE and cardboard at different blending ratios and a gasification temperature of 900 °C.
44 0.15 0.50 Measured H₂ Measured CₓHᵧ Predicted H₂ Predicted CₓHᵧ 0.12 0.40 daf) - daf) - fuel
0.09 - 0.30 fuel -
0.06 0.20 yield yield (g/g yield yield (g/g y 2 H x H 0.03 0.10 C
0.00 0.00 0 25 50 75 100 0 25 50 75 100 PS content (wt.%) PS content (wt.%) 0.50 1.00 Measured CO Measured CO₂ Predicted CO Predicted CO₂ 0.40 0.80 daf) daf) - - fuel fuel
0.30 - 0.60 -
0.20 0.40 yield yield (g/g 2 CO CO yield (g/g
0.10 CO 0.20
0.00 0.00 0 25 50 75 100 0 25 50 75 100 PS content (wt.%) PS content (wt.%)
Figure 21. The yields of H2, CXHY, CO, and CO2 produced from co-gasification of PS and cardboard at different blending ratios and a gasification temperature of 900 °C.
45
Table 12. The main composition of tar produced from co-gasification of PE and cardboard at 900 °C.
PE content (wt.%) No. 0 25 50 75 100 Tar compounds Area% Tar compounds Area% Tar compounds Area% Tar compounds Area% Tar compounds Area% 1 Benzene 36.16 Naphthalene 31.2 Naphthalene 26.22 Naphthalene 27.53 Cyclododecane 4.25 2 Toluene 22.99 Indene 9.21 Indene 9.03 Indene 7.62 1-Tridecene 4.03 3 Naphthalene 14.70 Biphenylene 7.90 Biphenylene 6.74 Biphenylene 5.32 1-Octadecene 3.98 4 Styrene 11.04 Anthracene 7.16 Anthracene 6.13 Anthracene 4.85 9-Octadecenamide 3.96 5 Indene 6.77 Pyrene 3.91 Pyrene 5.81 9-Octadecenamide 4.00 3-Heptadecene 3.69 6 Biphenylene 3.36 Naphthalene, 2- 3.65 Naphthalene, 2- 4.21 Naphthalene, 2- 3.36 n-Nonadecanol-1 3.63 methyl- methyl- methyl- 7 Phenanthrene 2.78 Naphthalene, 1- 3.11 Naphthalene, 1- 3.41 Styrene 3.44 3-Eicosene 3.51 methyl- methyl- 8 Octane, 4- 2.21 Styrene 2.77 Naphthalene, 2- 2.83 Naphthalene, 1- 3.12 3-Hexadecene 3.19 methyl- ethenyl- methyl- 9 Phenanthrene 2.50 Biphenyl 2.58 Fluorene 2.39 1-Nonadecene 3.11 10 Naphthalene, 2- 2.43 Fluorene 2.10 Pyrene 2.27 2-Tetradecene 2.95
ethenyl-
46 Figure 20 presents the yield of H2, CO, CO2, and CXHY gases obtained from the co- gasification of PE and cardboard at 900 °C. As shown in the figure, the syngas produced from a mono-gasification of PE is dominated by the presence of CXHY gases (0.246 g/g-fuel-daf), which contribute to 70% of the total syngas yield. In contrast, the gasification of cardboard produce more CO and CO2 as the sum of their yield values reaches 80% of the syngas yield. Similar to the trend with respect to the syngas and tar yields, synergistic effects are observed for all syngas components. Specifically, at cardboard proportions lower than 50 wt.%, the measured values of all gas yields are higher than the predicted values. In the case of H2, CO, and CXHY gases, their measured yield values peak at cardboard contents of 25 wt.%, at which the value also shows the largest deviation from their corresponding predicted values. Figure 21 shows the yield of syngas components based on the results from the co- gasification of PS and cardboard mixtures. As depicted in the figure, the syngas obtained from PS gasification differs to that of PE gasification in term of the CO and CXHY gas yield values. In the case of PS, the gasification process produces at least a three times higher yield of CO, which is 0.111 g/g-fuel-daf, compared to PE’s value of 0.034 g/g-fuel-daf. In contrast, the yield of CXHY gases from gasification of PS is significantly lower than that of PE, as its value is only 0.046 g/g-fuel-daf. As explained previously, no clear synergistic effects could be found in the case of the total syngas production from a co-gasification of PS and cardboard. Despite that result, deviations of the measured yield values from their predicted values are found in the case of CO and CO2, which might indicate synergistic effects occurs on the formation of those gases. Specifically, the highest deviations are found when the fuel consists of 25% PE and 75% cardboard. At this fuel blending ratio, the measured value of CO yield is 20% higher than that of the predicted value; whereas, the measured yield of CO2 is 30% lower than its predicted value. The composition of tar produced from the co-gasification of PE and cardboard (CB) at 900 °C is presented in Figure 22. The tar compounds are categorized according to their aromatic ring number. Furthermore, the results are presented as area percentage value (area%) of the detected spectrum by the GC-MS instrument, which do not actually represents the actual weight percentage of the compounds. As shown in the figure, the tar produced for a 100% PE gasification consists of aliphatic hydrocarbon compounds. This result is in accordance with the original structure of PE, which is constructed from aliphatic alkene hydrocarbons. As listed in Table 12, the compositions are mainly dominated by aliphatic hydrocarbon such as decane, pentadecane, octadecane, etc. A very small percentage of PAH is detected as shown by the presence of naphthalene which has area percentage of only 1%.
47 100
80 0 ring (aliphatic) 1 ring (BTX & others) 60 2 ring (PAH) 3 ring (PAH)
40 4 ring (PAH)
Peak percentagearea (area%) 20
0 100PE 75PE-25CB 50PE-50CB 25PE-75CB 100CB
Figure 22. The composition of tar (according to the number of aromatic rings) produced from the co-gasification of PE and cardboard at 900 °C presented as area% of the GC-MS spectrums.
In contrast, a drastic change in the tar composition is found when PE is mixed with 25 wt.% of cardboard. As presented in the figure, the concentration of aliphatic compounds is significantly reduced from 98 to only 19 area%, when 25 wt.% cardboard is added. Also, the following reduction of aliphatic compounds, the concentration of aromatic compounds steeply increases with PAHs dominates the tar composition. The concentration of PAHs and single aromatic compounds are 70 and 10 area%, respectively. Also, naphthalene is found to be the main compounds of the tar sample that has a concentration of 28 area%, followed by indene (8 area%) and biphenylene (5 area%). As explained previously, the tar yield of PE gasification significantly decreases with the presence of cardboard materials, especially at mass proportions of 25 wt.%. From the tar composition for this fuel blending ratio, it can be suggested that the tar compounds derived from the PE gasification undergo cracking reactions due to the interactions with cardboard. Specifically, its long chain hydrocarbons are cracked into lighter aromatic compounds. The concentration of PAHs increases at the higher cardboard’s mass proportion, which is followed by the decrease of single aromatic compounds. For instance, there is a decrease of single aromatic compounds at cardboard proportions of 75 wt.%, despite the higher tar yield, compared to the addition of 25 wt.% cardboard. This result suggests that the lighter aromatic compounds tend to transform into larger aromatic compounds at higher cardboard proportions. This phenomenon consequently increases the PAHs concentration, as explained above. The main component of the PAHs is naphthalene which has a concentration in the range of 26.2–31.2 area%. The result also explains the lower syngas yields obtained from the
48 co-gasification of higher cardboard contents due to the more single aromatic compounds being transformed into larger PAHs instead of cracked into syngas.
100
80 0 ring (aliphatic)
1 ring (BTX & others) 60 2 ring (PAH)
3 ring (PAH) 40 4 ring (PAH)
Peak percentagearea (area%) 20
0 100PS 75PS-25CB 50PS-50CB 25PS-75CB 100CB
Figure 23. The composition of tar (according to the number of aromatic rings) produced from the co-gasification of PS and cardboard at 900 °C.
The compositions of the SPA tar samples from the co-gasification of PS and cardboard at 900 °C are presented in Figure 23. In contrast to the gasification of PE, the tar obtained from the PS gasification mainly consists of aromatic compounds with smaller carbon atom numbers. The GCMS result of the SPA tar sample obtained from 100% PS gasification shows the domination of styrene, which accounts for 13.1 area%. In total, the amount of single aromatic compounds in the tar sample could reach 23.7 area%. In addition, the tar sample contains a considerable amount of heavy PAHs as there are 31.4 and 17.1 area% of tar species that have more than two aromatic rings. A notable tar species of this fraction is anthracene (three aromatic rings) that accounts for almost 10 area%. At a higher cardboard proportion, the SPA tar samples from the co-gasification of PS show a tendency to contain a higher concentration of lighter aromatics. As shown in Figure 23, the total concentration of PAHs decreases gradually from 76.1 to 67.4 area%, with the increase of cardboard content from 25 to 75 wt.%. This trend is followed by the increase of single aromatics concentration from 23.6 to 31.5, respectively. As described in Table 3, styrene and toluene are mainly responsible for this result as they account for almost 3 area% rise in the concentration.
49 0.10 50PE-50CB Predicted tar Measured tar 0.08 daf)
- 0.064
fuel 0.06 - 0.051
0.039 0.04 0.037
Tar yield yield Tar(g/g 0.023 0.019 0.02
0.00 800 900 1000 Gasification temperature (°C) Figure 24. The measured and predicted values of tar yields obtained from co-gasification of 50 wt.% PE and 50 wt.% cardboard at different operating temperatures.
Figure 24 presents the measured and predicted values of the tar yields obtained from co-gasification of 50 wt.% PE and 50 wt.% of cardboard at different gasification temperatures in the range of 800–1000 °C. The results presented in the figure suggests that the degree of the synergistic effects on the tar productions are different for each gasification temperature. At 800 °C, the measured value of tar yield is 0.037 g/g-fuel-daf, which is 0.027 g/g-fuel-daf or 42% lower than that of predicted value. The degree of the tar reduction is more intense when the gasification temperature is raised to 900 °C, as the measured tar yield value is 0.032 g/g-fuel-daf or 63% lower than that of predicted value. Nevertheless, the degree of the synergistic effect decreases when the gasification temperature is raised further to a temperature of 1000 °C. At this temperature, the difference in the values of measured and predicted yields is only 41%. Moreover, the measured tar yield at 1000 °C (0.023 g/g-fuel-daf) is slightly higher than that of 900 °C (0.019 g/g-fuel-daf). The results suggest that the interaction between PE and cardboard is less intense at 1000 °C.
50 4.3.3 Summary
The interaction between aliphatic-structured PE and cardboard causes synergistic effects that lead to the reduction of tar and the increase of syngas yields. The presence of cardboard volatiles enhance the reforming of the aliphatic tar derived from the decomposition of PE to form lighter aromatic compounds. As a result of these synergistic effects, the tar produced from the co-gasification contains a significant amount of 2-ring aromatic compounds, especially naphthalene. There are not any notable synergistic effects found in the case of co- gasification between aromatic-structured PS and cardboard, as their syngas and tar yields shift proportionally with the change of the fuel blending ratio. In contrast to the co-gasification of PE, the tar obtained from the PS- cardboard co-gasification contains considerable amounts of single aromatic compounds. The gasification temperature affects the degree of the synergistic effects, in which the highest degree of synergistic effects during co-gasification of PE and cardboard is found at 900 °C.
Further results and discussion of the experimental study are available in Supplement III.
51 5. Process improvement and development of thermochemical processes of waste and biomass
This chapter is a compilation of the works conducted in Supplement IV and V. These works study the process improvement and development of thermochemical processes of waste and biomass. Specifically, this chapter focuses on the co- gasification of waste with biomass or biochar for enhancing the gasification performance (Supplement IV), as well as the process simulation and operational cost assessment of co-production of H2, biochar, and bio-oil (Supplement V)
Co-gasification of waste with biomass or biochar for enhancing the gasification performance (Supplement IV)
5.1.1 Background and aims
As shown in Chapter 4, gasification of plastic-containing waste is quite problematic due to the tendency to produce a high amount of tar which can cause damages in the subsequent downstream processes such as blockage of gas downstream, fouling, and erosion for equipment [48]. It is also harmful for syngas utilization in fuel cells, methanation reactors, and Fischer-Tropsch processes [47]. Moreover, the presence of a high ash amount such as in the case of excavated landfill waste, could decreases the gasification performance by reducing the thermal output, increasing the ash clinker formation, and emitting higher amount of CO2 [56]. Therefore, improvements are required to prevent severe problems during gasification of such low quality waste fuels. The work conducted in Supplement IV investigates the co-gasification of waste with other feedstock to improve the gasification performance. In this study, potential RDF fraction obtained from excavated landfill waste was used as the main feedstock for a steam gasification process. Biochar was added as an auxiliary fuel at mass concentrations between 15–35 wt.% and its influences were observed especially in terms of an increased H2 concentration and a lower tar content in the syngas. In addition, the performance of the waste-biochar co-gasification was also compared to that of waste-biomass co-gasification as the same operating conditions.
52 5.1.2 Results and discussion
(a) 1.5 0.15
1.2 0.12 daf) daf) - 0.9 0.09 - fuel fuel - -
0.6 0.06 Conversion Conversion rate Tar yield(g/g
Syngas yield(g/g 0.3 0.03
0.0 0.00 0 25 50 75 100 Biomass content (wt.%) Measured syngas yield Predicted syngas yield Conversion rate Measured tar yield Predicted tar yield
(b) 1.5 0.15
1.2 0.12 daf) daf) - - 0.9 0.09 fuel fuel - -
0.6 0.06 Conversion Conversion rate Tar yield(g/g 0.3 0.03 Syngas yield(g/g
0.0 0.00 0 25 50 75 100 Biochar content (wt.%)
Measured syngas yield Predicted syngas yield Conversion rate Measured tar yield
Figure 25. The conversion rate, syngas yield, and tar yield obtained from steam co-gasification of RDF-landfill with (a) biomass and (b) biochar at a gasification temperature of 800 ᵒC.
53 Figure 25 shows the measured conversion rate, syngas yield, and tar yield obtained from steam co-gasification of RDF-landfill materials with biomass and biochar at a gasification temperature of 800 ᵒC. In addition, predicted values of product yields were also determined based on the weighted average values from mono-gasifications of each feedstock. The results are plotted in form of lines to evaluate the synergetic effects that occurred during the co-gasification tests. As shown in the figure, tar yield of the RDF-landfill gasification reduces when biochar is added as an auxiliary fuel. At a biochar concentration ≥ 25 wt.%, the measured value of tar yield is at least 52% lower than that of the predicted value, which suggests that a notable synergetic effect occurs for the tar formation. Furthermore, the measured value of syngas yield raises following the reduction of tar production. At a biochar concentration of 35 wt.%, the measured syngas increase from 0.73 to 1.2 g/g-fuel-daf. However, biochar reacts relatively slow during gasification; hence, the gasification suffers from a low conversion rate when biochar is added as a fuel. In contrast to that of biochar co- gasification, no notable synergistic effects are found in term of syngas yield during co-gasification of RDF-landfill and biomass. As shown in the figure, the measured values of the syngas yield are approximately similar to those of predicted predicted values. Nevertheless, an addition of more biomass can still contribute positively to the increase of the syngas yield owing to the higher volatile and fixed carbon contents of the biomass.
0.10 0.10 (a) (b) daf) daf) - 0.08 - 0.08 fuel fuel - - 0.05 0.05
0.03 0.03 ₂ Measured H₂ Measured H Gas yield(g/g Gas
Gas Gas yield(g/g ₂ Predicted H₂ Predicted H 0.00 0.00 0 25 50 75 100 0 25 50 75 100 Biomass content (wt.%) Biochar content (wt.%)
Figure 26. The measured and predicted values of H2 yields produced from the steam co- gasification of RDF-landfill with biomass and biochar at gasification temperature of 800 °C.
54 The measured and predicted values of the H2 yields produced from the steam co- gasification of RDF-landfill with biomass and biochar at different blending proportions are presented in Figure 26. Those values were obtained at gasification temperature of 800 °C. As shown in the figure, the H2 production significantly increases as biochar is added to the gasification process. The highest H2 yield is achieved at a biochar mass proportion of 35 wt.% as the value reaches 0.075 g/g- fuel-daf. This value is almost three times than that of mono-gasification of RDF- landfill and double than that of biomass co-gasification with the same blending proportion. In addition, this measured value is more than 50% higher than that of the predicted value, which suggest there is a significant synergistic effect on the H2 production. Compared to the results from the biomass co-gasification, the syngas produced from co-gasification of waste with biochar generally has a higher concentration of H2, as can be seen in Table 13. The H2 concentration in the syngas increases with an increased biochar proportion. In addition, the CO2 concentration in the syngas also increases with an increased biochar amount. In contrast, the CO concentration significantly decreases, especially at low biochar proportions. Specifically, it can be 45% lower than that of mono-gasification of RDF-landfill. As same trend also found in the case of CxHy-gases as its concentration decreases with an increased in biochar content.
55
Table 13. The results from the steam gasification experiments with different fuel mixtures at different gasification temperatures.
Sample Fuel composition (wt.%) Temp. Gas concentration (vol.%) Yield (g/g-fuel-daf) EY ratio
no. RDF-landfill Biomass Biochar (ᵒC) H2 CO CO2 CxHy Total syngas H2 Tar 1 100 - - 800 38.0 17.9 20.1 25.6 0.73 0.029 0.050 0.44 2 - 100 - 800 48.2 21.6 22.5 7.7 0.83 0.043 0.031 1.03 3 - - 100 800 56.6 14.1 26.2 3.1 1.27 0.084 0.002 0.73 4 85 15 - 800 44.1 14.0 20.1 21.8 0.76 0.034 0.051 0.53 5 75 25 - 800 45.2 14.6 23.4 16.8 0.79 0.036 0.051 0.55 6 65 35 - 800 42.6 20.6 20.7 15.9 0.83 0.035 0.063 0.69 7 85 - 15 800 51.2 9.9 23.7 15.2 0.92 0.051 0.038 0.50 8 75 - 25 800 52.6 10.7 22.0 15.6 1.03 0.061 0.019 0.59 9 65 - 35 800 53.5 10.4 25.0 11.1 1.20 0.075 0.014 0.56 10 100 - - 900 41.1 11.2 15.5 32.2 0.81 0.035 0.04 0.51 11 100 - - 1000 47.5 10.3 11.7 30.5 0.78 0.046 0.05 0.54 12 65 35 - 900 45.5 18.0 18.7 17.8 0.94 0.046 0.03 0.80 13 65 35 - 1000 47.9 20.0 15.8 16.3 0.96 0.054 0.02 0.84 14 65 - 35 900 54.8 13.8 20.6 10.9 1.46 0.096 0.02 0.88 15 65 - 35 1000 55.8 20.1 15.1 9.1 1.53 0.111 0.023 1.11
56
Figure 27. Illustration of tar reforming mechanisms over biochar during the co-gasification process.
The synergetic effects on the syngas and tar yields during biochar co-gasification are mainly due to the heterogeneous reactions of the tar reforming process over biochar particles. These reactions typically have a higher conversion rate than that of homogeneous tar reforming reactions such as dry and steam reforming reactions [105]. In general, this heterogeneous tar reforming process begins with the tar adsorption, which is followed by the dehydrogenation to form soot, and the subsequent soot gasification [106]. As shown in Figure 27, the tar generated from the waste particles initially are adsorbed into the biochar structure through the active sites on the surface of the biochar particles. Subsequently, polymerization reactions occur as the H2 is released, which leaves soot deposition on the biochar surface. Thereafter, the produced soot particles react with H2O or CO2 from the surrounding and transform into non-condensable gases. During this process, the presence of AAEM particles in the biochar is essentially important in promoting the tar reforming process, especially with respect to Ca and K [107]. At higher temperatures, the co-gasification of biochar and waste still produces higher H2 yield values compared to when using other fuel mixtures, as can be seen in Table 13. In fact, the H2 yield elevates by 48% to 0.111 g/g-fuel-daf, as the temperature increases from 800 to 1000 ᵒC at biochar proportion of 35 wt.%. This yield value correspond to the H2 concentration of 55.8 vol.% at 1000 ᵒC. Meanwhile, the CO
57 yield also increases with the rise of the gasification temperature, whereas CO2 yield decreases when the gasification temperature is raised from 800 to 1000 ᵒC. Despite the higher amount of H2 production, the tar yield generated from biochar co- gasification increases at higher temperatures. Specifically, the tar yield increases from 0.014 to 0.023 g/g-fuel-daf, when the temperature is raised from 800 to 1000 ᵒC, respectively.
1000 /g) 2 800 746.3
622.1 600 611.7 394.4 400
200
BET BET surfacearea of biochar (m Gasification at 800 °C
10.1 Gasification at 1000 °C 0 0 5 10 15 20 25 30 Gasification time (min)
Figure 28. BET surface area of gasified biochar obtained at different operating conditions.
10000 8480 Ca K Mg Na 8000 biochar) - 6891 raw
- 6118 6000
4510 4351 4000 2694
2000 1710 1577 1466 543 Amount Amount of species (mg/kg 318 211 0 Raw biochar Gasified biochar at 800 °C Gasified biochar at 1000 °C Figure 29. The amount of AAEM species in the biochar before and after gasification in respect to the initial mass of the biochar.
58 The increase of the tar yield during biochar co-gasification at higher temperatures could be explained by considering the structural changes of biochar and the release of AAEM species. Figure 28 presents the BET surface areas of biochar measured at different steam gasification temperatures and residence times. As shown in the figure, the surface area of the biochar particle extensively increases after 5 min of the gasification process. It can also be seen that after 5 min, the surface area of the gasified biochar at 1000 °C is almost double than that at 800 °C. The higher rate of pore enlargement at higher gasification temperatures consequently causes the release of more AAEM species, as can be seen in Figure 29. Consequently, the number of active sites on the biochar particles decreases following the higher release of AAEM, which cause the tar reforming to be less extensive at higher gasification temperatures. In contrast to that of biochar co-gasification, the tar yield produced from the co- gasification of biomass and waste notably decreases at higher gasification temperatures. Specifically, as the tar yield at 1000 ᵒC is 67% lower than that of 800 ᵒC. Correspondingly, the syngas yield increases due to the reforming of tar into smaller hydrocarbon molecules and non-condensable gases. At 1000 ᵒC, addition of 35 wt.% biomass produces syngas with H2 concentration of 47.9 vol.%. In addition, the performance of the gasification tests is evaluated by using the energy yield ratio (EY), which can be expressed as follows:
퐿퐻푉 푥푉 퐸푛푒푟푔푦푦𝑖푒푙푑푟푎푡𝑖표(퐸푌) = 푠푦푛푔푎푠 푠푦푛푔푎푠 (Eq. 7) 퐿퐻푉푓푢푒푙푥푚𝑖
3 where, 퐿퐻푉푠푦푛푔푎푠 represents the calorific value of the produced syngas (MJ/Nm ), 3 푉푠푦푛푔푎푠 is the total volume of produced syngas (Nm ), and 퐿퐻푉푓푢푒푙 is the calorific value of the fuel (MJ/kg).
The gasification of RDF-landfill samples has low EY values due to the low syngas yield and eminent chemical energy losses in the form of tar. The EY value is 0.44 at a gasification temperature of 800 ᵒC, which rises to 0.54 at 1000 ᵒC. The use of biomass or biochar as an auxiliary fuel could enhance the gasification performance as indicated by higher EY values. Specifically, at 800 ᵒC, one can see that the value of the EY ratio increases proportionally with the amount of biomass content in the fuel, as its maximum value (0.69) is obtained at a biomass content of 35 wt.%. In contrast, the optimum EY value in the case of biochar co-gasification is obtained at a 25 wt.% biochar content. Greater amounts of biochar reduce the EY value, due to the low conversion rate of biochar at a low gasification temperatures. Nevertheless, co-gasification of 35 wt.% biochar exhibits better gasification performance at higher temperatures than that of other fuel mixtures. At 1000 ᵒC, this gasification has an EY value of 1.11, whereas gasification of RDF-landfill without any fuel addition and with 35 wt.% biomass addition only show EY values of 0.54 and 0.84, respectively.
59 5.1.3 Summary
Adding biochar during gasification of waste could significantly increase the syngas and H2 production to higher values compared to when biomass is added. Synergistic effects are observed in form of extensive syngas yield increment and tar yield reduction due to the tar reforming reactions over biochar particles. In contrast, no notable synergetic effects on the syngas and H2 yield during co- gasification of biomass and waste within the investigated range of operating conditions. Overall, the H2 yield linearly increases at higher biomass amounts. At low gasification temperatures, the addition of biochar can reduce a larger amount of tar than that of biomass addition. Nevertheless, the tar conversion rate reduces at higher gasification temperatures, due to the structural change of the biochar and the loss of more AAEM. Both biochar and biomass addition results in a higher energy yield ratio which suggest that it could improve the efficiency of waste gasification. Further results and discussion of the experimental study are available in Supplement IV.
60 Process simulation and operational cost assessment of co-
production of H2, biochar, and bio-oil (Supplement V)
5.2.1 Background and aims
In Sweden, iron and steel industries are the largest industry that relies most on the use of fossil fuels and these industries emit a large amount of CO2. In 2019, the Swedish iron and steel industries at least emits 1.97 ton of CO2 per ton steel produced, which can be compared to to the total amount of 57 million tons CO2 [108,109]. The main CO2 emission source is the blast furnaces operation, due to the high consumption of coke. It is estimated that the operation of a blast furnace approximately consumes 250 – 300 kg of coke per ton of hot metal being produced [110]. Thus, new processes have been proposed to reduce CO2 emission from the iron and steel making process, especially to mitigate the CO2 emission from the conventional blast furnaces. These processes includes the use of H2 as a reduction agent in a direct reduction iron (DRI) process instead of a coke-based blast furnace [111] and the replacement of coal and coke with biochar from biomass [112]. Consequently, the possibility to produce biomass-based H2 and biochar for steel industries has gained an increasing interest recently. The main thermochemical process for producing biochar is biomass pyrolysis, in which pyrolysis vapor consisting of bio-oil and gas is also produced as by-products. The reforming of this vapor fraction to produce H2 presents a very attractive option as H2 is co-produced together with biochar. In addition, the reforming process can be operated and installed independently without altering the main biochar production process. Another possibility to valorize the pyrolysis by-product is from the production of crude bio-oil for refinery industries. This can be achieved by condensing the heavy hydrocarbon compounds of the pyrolysis vapor, where the remaining fraction is reformed into H2 as previously explained. The application of bio-oil in existing refinery is gaining a significant interest with an initiative to refine bio-oil on an industrial scale is on-going in Sweden [113]. Correspondingly, further investigation of the co-production process is important to obtain an optimum products yields with a high economic value. The work conducted in Supplement V aims to evaluate different co-production scenarios of biochar, H2, and bio-oil by means of process simulation. In addition, preliminary operation expenses (OPEX) comparison between scenarios were carried out to determine the scenario that offers the most economic benefit in the scope of Swedish industries and market.
61 5.2.2 Results and discussion
In this study, process simulations were conducted to evaluate two main co- production scenarios, which are
Scenario 1: Co-production of biochar and H2 through combination of pyrolysis, steam reforming, and WGS processes, and
Scenario 2: Co-production of biochar, bio-oil, and H2 through combination of pyrolysis, bio-oil condenser, steam reforming, and WGS processes. Figure 30 presents the Sankey diagram of the mass and energy flow obtained from the process simulation of Scenario 1. In this scenario, 217 kg/h of biochar and 783 kg/h of vapor are produced from pyrolysis of 1000 kg/h biomass. In the steam reformer, 393 kg/h of steam is required to maintain the process at the required S/C ratio. Subsequently, another 263 kg/h of steam is also needed for the WGS reactor to convert the syngas output from the steam reformer into H2. From the WGS reactor, 93.5 kg/h of H2 can be generated which correspond to a 10.0 wt.% dry biomass input, as can be seen in Table 14. Correspondingly, about 943 kg/h of CO2 is produced in the final stream which equals to 101.1 wt.% of the dry biomass input. As shown in Figure 30, the pyrolysis of 1000 kg/h biomass at 550 °C requires 420 kW of electrical power. This value is equal to 1.6 MJ/kg of dry biomass input, which is similar to that of reported values from other studies [114,115]. Meanwhile, a greater electrical power (699.7 kW) is needed to maintain the endothermic steam reforming process at 850 °C. On the other hand, no external energy is required to operate the
WGS process, as it is an exothermic process. In summary, the values of 휂퐻2 and 휂푡ℎ of Scenario 1 are 47.1 and 79.0, respectively.
62
Figure 30. The sankey diagrams of the mass flow (upper) and energy flow (lower) of the co-production Scenario 1 (biochar and H2 productions).
63
Figure 31. The sankey diagrams of the mass flow (upper) and energy flow (lower) of the co-production Scenario 2 (biochar, bio-oil, and H2 productions).
64 Figure 31 presents the mass and energy flow diagrams based on the process simulation of Scenario 2, with the temperature of the bio-oil condenser is set at 50 °C. As shown in the figure, approximately 430 kg/h of crude bio-oil can be collected at the condenser. This value equals to at least 46 wt.% of biomass input in dry basis. However, the crude bio-oil contains a significant amount of water (almost 42 wt.%); thus, its calorific value is relatively low (nearly 6.0 MJ/kg). Correspondingly, as the water content is mostly condensed together with the heavy hydrocarbon fractions, the vapor input to the steam reformer contains a negligible amount of water vapor. Hence, more steam input is required in the steam reforming process than that when using Scenario 1. In addition, due to the lower amount of vapor input (55% lower), the steam reformer require a 21% lower amount of electrical power than that of Scenario 1. The syngas output from the steam reformer contains a sufficient amount of steam; thus, no additional steam is needed for the WGS process. During the final process, the amount of H2 yield in Scenario 1 is equal to 4.7 wt.% of biomass input.
The values of 휂퐻2 and 휂푏𝑖표−표𝑖푙 in Scenario 2 are 22.9 and 23.1, respectively, whereas the value of 휂푡ℎ is similar to that of Scenario 1 (79.0%). In addition, setting the condenser at different temperatures only slightly shift the value of 휂푡ℎ, as increasing the condenser temperature from 50 to 130 °C results only change the value by 1% higher as can be seen in Figure 32.
Table 14. Efficiencies and yield of products obtained from different co-production scenarios. Parameters Scenario 1 Scenario 2a
H2 yield (wt.% dry biomass) 10.0 4.7
CO2 yield (wt.% dry biomass) 101.1 53.6 Bio-oil yield (wt.% dry biomass) - 46.0
휂퐻2 (%) 47.1 22.9
휂푏𝑖표−표𝑖푙 (%) - 23.1
휂푡ℎ (%) 79.0 79.0 a temperature of the bio-oil condenser at 50 °C.
65 40 100
30 90
20 80 oil oil efficiency (%) -
10 ηH2 70 and bio Thermal efficiency (%)
2 ηbio-oil
H ηth 0 60 50 60 70 80 90 100 110 120 130 Condenser temperature (°C)
Figure 32. The relation between the temperature of bio-oil condenser and the efficiency values of Scenario 2.
Based on the results obtained from the process simulation, an OPEX calculation was performed to compare the cost for operating the co-production plant at different operating conditions as follows, Case 1: Production of biochar and H2 through Scenario 1 Case 2: Production of biochar, bio-oil, and H2 through Scenario 2, with the temperature of the bio-oil condenser is set at 50 °C. Case 3: Production of biochar, bio-oil, and H2 through Scenario 2, with the temperature of the bio-oil condenser is set at 130 °C. The main process parameters of the OPEX analysis and their corresponding values for each cases are listed in Table 15. For Case 2 and 3, the bio-oil prices are adjusted based on the water content of the bio-oil.
66 Table 15. Summary of the main process parameters for OPEX calculation. Parameter Case 1 Case 2 Case 3 Pyrolysis Biomass pellet (kg/h) 1000 1000 1000 Reactor heating (kW) 420.3 420.3 420.3 Bio-oil condenser Recirculation pump (kW) - 7.5 7.5 Reformer Reactor heating (kW) 699.70 553.80 464.30 Water supply (kg/h) 287.60 350.57 202.88 WGS reactor Make-up catalyst (kg/h) 0.71 0.37 0.47 Water supply (kg/h) 0.00 0.00 0.00 Solid disposal (kg/h) 0.71 0.37 0.47 PSA Compressor and utilities (kW) 178.50 86.40 111.10 Products yields Biochar (kg/h) 217.20 217.20 217.20
H2 (kg/h) 93.55 43.83 57.00 Bio-oil (kg/h) 0.00 429.69 178.15
CO2 (kg/h) 944.39 500.32 628.18 Bio-oil price Water content in bio-oil (wt.%) - 41.70 0.64 Bio-oil price based on water content - 5.25 8.94
67 Table 16. Annual operation costs for the co-production process of biochar, H2, and bio-oil. Parameter Case 1 Case 2 Case 3 Variable operating cost (MSEK/year) Pyrolysis Biomass pellet 24.00 24.00 24.00 Reactor heating 2.35 2.35 2.35 Bio-oil condenser Recirculation pump - 0.04 0.04 Reformer Reactor heating 3.92 3.10 2.60 Water supply 0.02 0.03 0.02 WGS reactor Make-up catalyst 0.001 0.001 0.001 Solid disposal 0.022 0.011 0.015 PSA Compressor and utilities 1.00 0.48 0.62 Fixed operating cost (MSEK/year) Salary 5.76 5.76 5.76 Overhead 5.184 5.184 5.184 Products values (MSEK/year) Biochar 13.90 13.90 13.90
H2 24.70 11.57 15.05 Bio-oil 0.00 17.19 12.83
CO2 83.86 44.43 55.78 Total OPEX (MSEK/year) 42.26 40.96 40.59 Total revenue (MSEK/year) 38.60 42.66 41.78
Total revenue including CO2 (MSEK/year) 122.46 87.09 97.56 Gross production cost1 (SEK/kg-biomass) 5.28 5.12 5.07 Loss/Gain based on OPEX2 (SEK/kg- -0.46 0.32 0.14 biomass) 2 Loss/Gain based on OPEX with CO2 23.85 13.97 16.92 (SEK/kg-biomass) Minimum biochar selling price3 (SEK/kg) 10.11 Minimum bio-oil selling price4 (SEK/kg) 3.44 5.59 1 Gross production cost = Total OPEX/kg biomass fed. 2 Loss/Gain based on OPEX = (Total revenue - Total OPEX)/kg biomass fed. 3 Not including capital investment. 4 Not including capital investment. And using the same biochar price as Case 1.
68 The results of the annual OPEX calculation is presented in Table 16. The cost of biomass and personnel costs, which are fixed for all cases, accounts for the largest expense. Specifically, it corresponds to at least 59 and 27% of the total operating cost, respectively. Meanwhile, the operating cost of the steam reformer and WGS reactor varies for each case. The highest operating cost of the steam reformer is found in Case 1, as it has to process a higher amount of pyrolysis vapour. As a result, Case 1 has the highest total operating cost of 42.3 MSEK/year, which is actually only 3% higher than the other cases. Despite the little difference in the total operating cost, Case 1 has the lowest revenue. This can be up to 11% lower than that of other cases. Hence, the analysis shows that Case 1 is the least economical, based on the operating cost consideration. Nevertheless, Case 1 could have a lower capital cost than the other cases as it does not requires additional costs for the bio-oil condenser. The result also suggests that production of bio-oil in Scenario 2 is more economically beneficial based on the current situation in Sweden. Despite the lower price of crude bio-oil per kg than that of H2, a significant higher amount of bio-oil can be produced than that of H2. Hence, in total, it can provide more revenue. Overall, without considering the economic value of CO2 by-products, the most economical process is found in Case 2 with a gain of 0.31 SEK/kg biomass input. However, it should be noted that this value is obtained by assuming that the bio-oil could be sold to the refineries without further processing. Hence, there is still some future consideration that might hinder this process's economics concerning the investment on the post- processing of the high-water containing bio-oil. In contrast to the aforementioned OPEX results, a different trend is found when CO2 is considered as a commodity in the revenue calculations. By assuming that CO2 can be sold at 11.1 SEK/kg, the total revenue can be increased by four times for Case 1, and by twice higher for both Case 2 and 3. As results, all cases can achieve a considerable economic gain more than 10 SEK/kg of biomass. Nevertheless, instead of commodity, CO2 is normally considered an emission that could arguably be carbon neutral in this process. Based on the total operating cost, adjustments on the minimum price of biochar and bio-oil are suggested as can be seen in Table 16. To cover at least the total operating cost, it is suggested that the biochar should be at least sold at a minimum price of 10.11 SEK/kg, which is 2 SEK higher than the current price provided by Envigas. This price is actually within the range of the retail prices for grill charcoal in Sweden (12-35 SEK/kg [116]), as well as coking coal (10-20 SEK/kg), and coal (5-10 SEK/kg) [117]. Meanwhile, the calculated minimum price for bio-oil should be at least 3.44 and 5.59 SEK/kg for Case 2 and 3, respectively. These prices are comparable to that of crude oil.
69 5.2.3 Summary
The results indicate that both Scenario 1 (co-production of biochar and H2) and Scenario 2 (co-production of biochar, bio-oil, and H2) have a similar value with respect to the thermal efficiency despite the different in the H2 and bio-oil yields. Moreover, no notable effects on the thermal efficiency of the system is found at different bio-oil condenser temperatures. Based on the calculation of the total operating cost and the potential revenue, it is found that the production of bio-oil is more economically benefit than that of H2. Nevertheless, capital and operating costs for post-processing of bio-oil should also be considered in the future. The estimated minimum selling price for biochar and bio-oil based on only OPEX is within the price ranges of related commodities in Sweden (i.e., charcoal, coal, coke and oil crude). Further results and discussion of the experimental study are available in Supplement V.
70 6. Conclusions
This doctoral thesis consists of two major sections based on the works conducted in Supplement I – V. The conclusion for each supplement is presented according to their section as follows.
Influence of feedstock characteristics on the physical and thermochemical conversion phenomenon during pyrolysis/gasification Effect of different plastic-paper mixtures on the primary fragmentation behavior of RDF pellets The densification of different material compositions of RDF results in varied particle binding forces that have their unique characteristics with respect to the mechanical strength and thermal stability during the pyrolysis process. Thus, pellets with a higher concentration of paper fractions tend to be less fragmented. Within the range of the investigated pyrolysis temperatures, the effect of the pyrolysis temperature on the fragmentation degree of RDF pellets is less pronounced than the effect of the plastic and cardboard compositions of RDF pellets. The fragmentation degree of RDF pellets correlates well with the pellet’s volatile matter contents. At the same time, there is no clear relation between the mechanical strength of pellets and the pellet’s fragmentation degree. Generally, due to the high volatile matter content from plastics, RDF pellets tend to fragment into a high number of smaller particles than what is found for typical biomass or coal pellets during thermochemical processes. Hence, for some gasifiers, an improvement of the thermal stability of RDF pellets is more relevant rather than their mechanical strength. Mixing the RDF with additional auxiliary fuel that potentially has a higher thermal stability could be useful.
Influence of waste composition on the char reactivity during gasification Depending on the composition of the inorganic species in the ash, the amount of ash in the waste could cause a higher or lower char reactivity during gasification. The study shows that high-ash waste fractions with a higher concentration of catalytic inorganic species (Ca and Fe) have the highest char reactivity, as shown in fine fractions from excavated landfill waste. On the other hand, the significant domination of Si could lower the char reactivity. Thus, the results from the gasification test of the landfill waste char demonstrated the significance of the process parameters during waste sorting processes on the waste fuel's thermal
71 behavior, especially during the gasification process. In general, it is found that the char reactivity is a function of the ash amount and the ratio of the amount of inorganic catalytic elements (K, Ca, Na, Mg, and Fe) to the inhibitor elements (Si, Al, and Cl). An inorganic index based on that relationship is proposed. It could be useful for providing information regarding the expected reactivity of RDF from landfill waste in which its inorganic elements composition is significantly varied depending on the landfill age, type of waste, and pre-processing method.
Investigation on the syngas and tar formation during gasification of plastic-paper waste mixtures Steam co-gasification experiments of paper and paper fractions of MSW have been carried out using PE, PS, and cardboard materials. It is found from the results that the interaction between PE and cardboard causes synergistic effects that lead to a reduction of tar and an increase of the syngas yields. The presence of cardboard volatiles enhance the reforming of the aliphatic tar compound derived from the decomposition of PE to form lighter aromatic compounds. As a result of these synergistic effects, the tar produced from the co-gasification contains a significant amount of 2-ring aromatic compounds, especially naphthalene. Meanwhile, there are no apparent synergistic effects found in the case of co-gasification between PS and cardboard, as their syngas and tar yields shift proportionally with the change of the fuel blending ratio. Furthermore, the result also suggests that the gasification temperature affects the degree of the synergistic effects. The highest degree of synergistic effects during co-gasification of PE and cardboard is found at 900 °C.
Process improvement and development of thermochemical processes of waste and biomass Co-gasification of waste with biomass or biochar for enhancing the gasification performance Adding biochar during the gasification of waste could significantly increase the syngas and H2 production to higher values than those found when adding biomass. Synergistic effects are observed in the form of extensive syngas yield increments and tar yield reductions due to the tar reforming reactions over biochar particles. In contrast, no notable synergetic effects on the syngas and H2 yield during co- gasification of biomass and waste within the investigated range of operating conditions. Overall, the H2 yield increases linearly at higher biomass amounts. At low gasification temperatures, the addition of biochar can reduce a more significant
72 amount of tar compared to when adding biomass. Nevertheless, the tar conversion rate reduces at higher gasification temperatures, due to the biochar's structural change and the loss of more AAEM. Both biochar and biomass addition results in a higher energy yield ratio, suggesting that it could improve the efficiency of the waste gasification.
Process simulation and operational cost assessment of co-production of H2, biochar, and bio-oil A process simulation study is carried out to evaluate a different scenario for producing biochar, bio-oil, and H2 based on a biomass pyrolysis process coupled with a steam reforming and a WGS process. In general, no significant difference in total thermal efficiency is found when the process is shifted from the “biochar and H2 co-production” to “biochar, bio-oil, and H2 co-production”. Moreover, no notable effects on the system's thermal efficiency are found at different bio-oil condenser temperatures. Based on the calculation of the total operating cost and the potential revenue, it is found that the production of bio-oil is more economically beneficial than that of H2. Nevertheless, capital and operating costs for post- processing of bio-oil should also be considered in the future. The estimated minimum selling price for biochar and bio-oil based on only OPEX is within the price ranges of related commodities in Sweden (i.e., charcoal, coal, coke, and oil crude).
73 7. Recommendations of future works
Further research on the implementation of thermochemical processes of waste/biomass within the circular economy context is recommended to improve their technical and economic feasibilities. Based on this thesis's conducted works, the following aspects can be considered for further investigations: An implementation of the findings obtained from the lab-scale studies on the pilot-scale experiments. Development of computation fluid dynamics model to further evaluate the developed process models in terms of the reactors' operability. Investigate and evaluate the performance of developed thermochemical processes by scaling up further into a continuous process. Deep techno-economic and life cycle analyses comparisons between different thermochemical routes to evaluate their feasibility in enhancing the material circularity on a commercial scale.
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