Semi-Batch Gasification of Refuse-Derived Fuel (RDF)

processes

Article Semi-Batch Gasiﬁcation of Refuse-Derived Fuel (RDF)

Juma Haydary 1,* , Patrik Šuhaj 1 and Michal Šoral 2

1 Institute of Chemical and Environmental Engineering, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovakia; [email protected] 2 Analytical Department, Institute of Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, SK-845 38 Bratislava, Slovakia; [email protected] * Correspondence: [email protected]

Abstract: Gasiﬁcation is a promising technology for the conversion of mixed solid waste like refuse-

derived fuel (RDF) and municipal solid waste (MSW) into a valuable gas consisting of H2, CO, CH4 and CO2. This work aims to identify the basic challenges of a single-stage batch gasification system related to tar and wax content in the producer gas. RDF was first gasified in a simple semi-batch laboratory-scale gasification reactor. A significant yield of tars and waxes was received in the produced gas. Waxes were analyzed using gas chromatography-mass spectrometry (GC- MS) and nuclear magnetic resonance (NMR) spectrometry. These analyses indicated the presence

of polyethylene and polypropylene chains. The maximum content of H2 and CO was measured 500 seconds after the start of the process. In a second series of experiments, a secondary catalytic stage with an Ni-doped clay catalyst was installed. In the two-stage catalytic process, no waxes were captured in isopropanol and the total tar content decreased by approximately 90 %. A single one-stage semi-batch gasification system is not suitable for RDF gasification; a large fraction of tar and waxes can be generated which can cause fouling in downstream processes. A secondary catalytic stage can significantly reduce the tar content in gas. Keywords: gasification; RDF; MSW; tar; wax Citation: Haydary, J.; Šuhaj, P.; Šoral, M. Semi-Batch Gasification of Refuse-Derived Fuel (RDF). Processes 2021, 9, 343. https://doi.org/ 1. Introduction 10.3390/pr9020343 Based on the EU directive 2018/850 on the landfill of waste [1], member states are obliged to reduce the yield of municipal waste landfilled to less than 10% by 2035. Currently, Academic Editor: Albert Ratner many countries, including Slovakia, send more than 50% of MSW to landfills. Reaching EU Received: 26 January 2021 targets in MSW landfilling requires the intensive implementation of alternative recycling Accepted: 9 February 2021 Published: 13 February 2021 and recovery technologies. Thermochemical conversion is a promising method of solid waste conversion, including heterogeneous mixed waste like MSW, into valuable materials

Publisher’s Note: MDPI stays neutral and energy [2]. with regard to jurisdictional claims in Refuse-Derived Fuel (RDF) is produced from municipal solid waste (MSW) by the published maps and institutional affil- separation of incombustibles and often also biologically degradable fractions. The main iations. components of RDF are plastic fraction (low density polyethylene–LDPE, high density polyethylene–HDPE, polypropylene–PP, polyethylene terephthalate–PET, polystyrene and other synthetic polymers), paper fraction (white paper, colored papers, cardboards, soft paper, etc.), textile fraction (natural and synthetic fibers), and other organic materials such as wood, rubber etc. [3]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Gasification of this waste produces gases consisting mainly of H2, CO, CH4 and CO2. This article is an open access article However, this gas can contain many impurities such as solid particles, tars, compounds distributed under the terms and of sulfur, chlorine, etc. [4]. Although much attention has been devoted to gasification in conditions of the Creative Commons recent years, there are still a number of challenges to the full commercialization of waste Attribution (CC BY) license (https:// gasification [5]. To be used in internal combustion engines, turbines or in methanol or creativecommons.org/licenses/by/ ammonia processes, the gas should meet relatively strict requirements for gas composition, 4.0/). heating value, tar content and sulfur and chlorine content [6,7].

Processes 2021, 9, 343. https://doi.org/10.3390/pr9020343 https://www.mdpi.com/journal/processes Processes 2021, 9, 343 2 of 12

The quality of the gas is significantly affected by the gasification technology used and the conditions in the gasifier. Moving bed gasifiers [8], fluidized bed gasifiers [9] and entrained flow gasifiers [10] commercialized and used in coal gasification have been employed also in the gasification of biomass and waste [11,12]. Although large industrial- scale gasification is usually carried out in continuous mode, many small or pilot scale gasifiers were designed for batch or semi-batch modes. In a semi-batch gasification system, the gas phase is continuously removed from the reactor and usually a carrier gas is flowing into the reactor. The conditions of fixed-bed, or better called, moving-bed gasifiers are very similar to batch gasifiers and many challenges in batch gasification systems can be expected also in fixed-bed gasifiers. The high tar content of gas is one of the most important disadvantages of fixed-bed and semi-batch gasifiers. In updraft gasifiers, the yield of tar typically reaches values up to 100 g/Nm3 [13] (101.325 kPa, 0 ◦C). The yield and characteristics of tars depend also on the feed composition. Yu et al. [14] reported characteristics of tar formation during cellulose, hemicellulose and lignin gasification. They found different compositions of tars received from these materials; however, polycyclic aromatic hydrocarbons (PAHs) were the dominant components in all cases. Benzene, toluene, ethylbenzene, xylene (BTEX), phenols and oxygenated hydrocarbons were other types of components found. However, as reported by Li and Suzuki [15], the characteristics of tars from biomass gasification depend also on the gasification temperature. At 400 ◦C, mixed oxygenates are the dominant type of tar components; at 500 ◦C, phenolic ethers; at 600 ◦C, alkyl phenolics; at 700 ◦C, heterocyclic ethers; at 800 ◦C, PAHs; and at 900 ◦C, larger PAHs. In the case of plastic wastes, primary tars are alkanes and alkenes or aromatics if the gasified polymers contain aromatic rings. Linear hydrocarbons are not thermally stable; they are cracked at higher gasification temperatures and secondary (aromatics, alkyl aromatics, phenols) and tertiary (PAHs) tars are created [16,17]. RDF is a mixture of natural polymers (cellulose, hemicellulose, lignin ... ) and synthetic polymers such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyamide (PA), etc. Based on this composition, mixed characteristics of tars from lignocellulosic biomass and from plastic waste can be expected. However, reactions between primary tars from different resources can result in a synergic effect. For example, C2–C4 olefins can act as tar precursors and promote tar formation reactions [16]. Reports on the presence of waxes in producer gas from MSW or RDF gasification were not found in the open literature. However, many pilot- and laboratory-scale units with updraft fixed bed or semi-batch reactors where MSW or RDF was used as feedstock reported pipes plugging because of the high tar and wax content in the gas. However, waxes were reported as the pyrolysis product of MSW, RDF and plastics. In some cases, up to 93 % of the product yields were created by light and heavy waxes [18]. Buah et al. [19] pyrolyzed RDF at 400 to 700 ◦C and measured an oil/wax yield from 30 to 50 wt.%. Selectivity to waxes depends on the pyrolysis technology used. Waxes as segments of polyolefin chains are the primary pyrolysis product of polyolefins; the short reaction time and rapid cooling of pyrolysis vapors increase their yield. Arabiourrutia et al. [20] reported the characteristics of waxes from the pyrolysis of three different polyolefin plastics (HDPE, LDPE and PP) in a conical-spouted bed reactor, the yield of waxes varied between 50 and 92% with the highest yield for PP; the authors found that 90% of waxes were collected as white powder in the filter downstream of the reactor, the melting point of waxes was between 60 and 95 ◦C. Taking into account the wax formation during the pyrolysis of plastics and consider- able amounts of plastics in MSW and RDF, it can be assumed that high tar and wax yields in pilot-scale gasification units can be related to no appropriate gasification technology and process conditions used. In contrast to fast pyrolysis of plastics when tar and waxes are usually desired products, they are undesired and have to be removed from the producer gas in the gasification process. Tar and wax yields can be influenced by the gasification technology used and process conditions like temperature, residence time or oxygen flow [21]. Processes 2021, 9, 343 3 of 12

Further reduction of tar can be achieved by physical or chemical ex-situ methods such as scrubbing or secondary catalytic cracking. A detailed discussion of the secondary catalytic cracking of tars is presented in previous works [22–24]. This work aims to identify the basic challenges of a single-stage semi-batch gasiﬁcation system related to tar and wax content in the producer gas. A more detailed characterization of the tar and wax yield is provided. The effect of process parameters such as temperature and residence time on product yields and gas composition is observed. A secondary catalytic stage using cheap catalysts was tested and results were compared with those of the one-stage batch gasiﬁcation system.

2. Materials and Methods 2.1. RDF Characterization The RDF used in this work was obtained from a waste recycling company in Slovakia. The company produces RDF from municipal solid waste by the separation of inorganics and biodegradables. A plastic rich fraction coming from industrial waste was used to control the RDF heating value within a required range. The company produces RDF for use in cement kilns. The obtained RDF was characterized by thermogravimetric analysis, elemental analysis, calorimetric analysis and differential scanning calorimetry. Samples for RDF characterization were homogenized by grounding of around 2 kg of the waste to particles of sizes lower than 1 mm. Then, 200 g of RDF were ground to powder in a cryogenic mill. However, some waste components remained in the form of small-sized fibers or particles. Therefore, all measurements of RDF characterization were carried out at least in triplicate to obtain average values with a relative deviation lower than 10%. For thermogravimetric analysis, a simultaneous TG/DSC thermal analyzer (STA 409 PC Luxx® from NETZSCH, Selb, Germany) was used. In this work, TG measurements were performed with a dual purpose: to determine the approximate analysis (moisture, volatile matter, fixed carbon and ash content), and to monitor the behavior of RDF thermal decomposition. The samples were heated from room temperature up to 800 ◦C in an inert atmosphere (nitrogen) at a heating rate of 10 K min−1. Measurements continued in an inert atmosphere at 800 ◦C for 30 min. After this period, oxygen was introduced to the system and the sample was combusted at 800 ◦C. The inert gas flow rate was 120 ml min−1. The used oxygen flow rate for sample combustion was 60 ml min−1. Elemental analysis was performed using an elemental analyzer (Vario Macro Cube ELEMENTAR, Elementar Analysensystem, GmbH, Langenselbold, Germany). The content of carbon, hydrogen, nitrogen and sulfur was obtained using a CHNS module with the combustion tube temperature at 1150 ◦C and the reduction tube temperature at 850 ◦C. The content of chlorine was obtained using a Cl 5000 ppm detector (Elementar Analysensystem, GmbH, Langenselbold, Germany). The content of oxygen was estimated by calculation to 100%. The higher heating value (HHV) of RDF was measured using an FTT isoperibolic calorimetric bomb (Fire Testing Technology Ltd., East Grinstead, UK). Combustion of the sample took place in a calorimetric bomb under an oxygen atmosphere at 30 bar. Benzoic acid was used as a standard material.

2.2. RDF Gasification Experiments Initially, a single-stage laboratory-scale batch gasification system (Figure1a) was used to observe the effect of temperature and residence time on tar and wax content in the producer gas. A 20 gram sample of RDF with a particle size lower than 1 mm was placed into a 400 mm stainless steel tubular reactor with a 25 mm inner diameter. The reactor, connected to air flow, was introduced into a preheated tube furnace. Experiments were realized at reactor temperatures of 700, 800 and 900 ◦C. The term ‘’reactor temperature” in this work indicates the temperature measured by a thermocouple placed near the ceramic wall of the tube surface. Of course, the sample temperature does not immediately reach the value of the furnace temperature. At the beginning of the experiment, the direction of heat flux is from the furnace wall to the sample; however, in later stages, after Processes 2021, 9, x FOR PEER REVIEW 4 of 12

in this work indicates the temperature measured by a thermocouple placed near the ceramic wall of the tube surface. Of course, the sample temperature does not immediately reach the value of the furnace temperature. At the beginning of the experiment, the direction of heat flux is from the furnace wall to the sample; however, in later stages, after exothermic reactions start, the sample temperature rapidly increases and the heat flux direction can reverse. Compressed air from a bottle was used as the gasification medium. The air flow was continuously recorded and maintained at 15.5 ± 1.0 l.min−1. This value of air flow corresponded to the optimum oxygen demand for RDF gasification. The gas leaving the reactor was led to a system of scrubbers filled with isopropanol and in some cases also glass beads, one of which was non-thermostated at room temperature; four scrubbers were thermostated at 30 °C, and two thermostated at −20 °C in a cryostat. All scrubbers were weighed before and after each experiment to determine the tar and condensed water yield. The gas leaving the condenser system was fed to a microchromatograph (GCM® Micro Box III microchromatograph, SLS MicroTechnology GmbH, Hamburg, Germany) where the content of the main gaseous components (H2, CO, CO2, CH4, N2) was determined every 3 min. The duration of the experiment was 1.5 hours. During this time, complete conversion of RDF was reached in all cases. In terms of the gasification reaction, the first 15 minutes of each experiment represented the most important period. At each temperature, the experiments were performed three times. For all measured values, the arithmetic mean of all three measurements was considered as the final value. In a second series of experiments, a secondary catalytic reactor with natural red clay containing mainly SiO2 and some amount of Al2O3, Fe2O3, CaCO3, MgCO3, calcined at 800 °C and impregnated with Ni as the catalyst was installed after the gasification reactor to reduce the tar and wax Processes 2021, 9, 343 fraction. This catalyst was characterized in the framework of a previous work [22] by 4pore of 12 structure and specific surface measurements at different stages of preparation, thermogravimetric analysis, elemental analysis, and X-ray diffraction analysis, X-ray fluorescence (XRF) analysis, and scanning electron microscopy (SEM). In addition, in the previ- ousexothermic work [23], reactions catalytic start, activity the of sample the used temperature catalyst was rapidly confirmed increases by cracking and the of heat model ﬂux tardirection components. can reverse.

FigureFigure 1. 1. LaboratoryLaboratory scale scale gasification gasiﬁcation unit: ( (aa)) without without secondary secondary catalysis catalysis ( (bb)) with with secondary secondary catalysis. catalysis.

2.3. TarCompressed and Wax Yield air Characterization from a bottle was used as the gasification medium. The air flow was continuously recorded and maintained at 15.5 ± 1.0 l.min−1. This value of air flow The tar and wax yield was determined using a simple gravimetric method. Isopro- corresponded to the optimum oxygen demand for RDF gasification. The gas leaving the panol with tar, water and wax collected in scrubbers was distilled in a vacuum rotary reactor was led to a system of scrubbers filled with isopropanol and in some cases also evaporator (Hei-VAP Advantage from Heidolph Instruments GmbH, Schwabach, Ger- glass beads, one of which was non-thermostated at room temperature; four scrubbers were many)thermostated at 55 °C at and 30 ◦10C, kPa. and When two thermostated the time betw ateen−20 two◦C drops in a cryostat. of the distillate All scrubbers was longer were thanweighed 4 s, the before distillation and after was each prolonged experiment for to another determine 45 themin. tar The and distillation condensed residue water yield.was weightedThe gas leaving and used the to condenser calculate system the tar was and fed wax to yield. a microchromatograph (GCM® Micro Box III microchromatograph,As shown in Figure 2, SLS the MicroTechnology tar and waxes, besides GmbH, a dissolved Hamburg, phase Germany) in isopropanol, where the created an undissolved solid phase. For the characterization of the tar and wax fraction content of the main gaseous components (H2, CO, CO2, CH4,N2) was determined every 3 min. The duration of the experiment was 1.5 h. During this time, complete conversion of RDF was reached in all cases. In terms of the gasification reaction, the first 15 min of each experiment represented the most important period. At each temperature, the experiments were performed three times. For all measured values, the arithmetic mean of all three measurements was considered as the final value. In a second series of experiments, a secondary catalytic reactor with natural red clay containing mainly SiO2 and some amount ◦ of Al2O3, Fe2O3, CaCO3, MgCO3, calcined at 800 C and impregnated with Ni as the catalyst was installed after the gasification reactor to reduce the tar and wax fraction. This catalyst was characterized in the framework of a previous work [22] by pore structure and specific surface measurements at different stages of preparation, thermogravimetric analysis, elemental analysis, and X-ray diffraction analysis, X-ray fluorescence (XRF) analysis, and scanning electron microscopy (SEM). In addition, in the previous work [23], catalytic activity of the used catalyst was confirmed by cracking of model tar components.

2.3. Tar and Wax Yield Characterization The tar and wax yield was determined using a simple gravimetric method. Iso- propanol with tar, water and wax collected in scrubbers was distilled in a vacuum rotary evaporator (Hei-VAP Advantage from Heidolph Instruments GmbH, Schwabach, Ger- many) at 55 ◦C and 10 kPa. When the time between two drops of the distillate was longer than 4 s, the distillation was prolonged for another 45 min. The distillation residue was weighted and used to calculate the tar and wax yield. As shown in Figure2, the tar and waxes, besides a dissolved phase in isopropanol, created an undissolved solid phase. For the characterization of the tar and wax fraction by gas chromatography-mass spectrometry (GC-MS), this undissolved phase was separated from isopropanol by ﬁltration and analyzed separately. GC-MS analysis was carried out using a GC 7890A gas chromatograph (Agilent Technologies, Wilmington, NC, USA) and a Processes 2021, 9, x FOR PEER REVIEW 5 of 12

by gas chromatography-mass spectrometry (GC-MS), this undissolved phase was sepa- Processes 2021, 9, 343 rated from isopropanol by filtration and analyzed separately.5 of 12 GC-MS analysis was carried out using a GC 7890A gas chromatograph (Agilent Technologies, Wilmington, USA) and a mass spectroscope 5975C (Agilent Technologies, Wilmington, USA). Conditions of GC- mass spectroscope 5975C (Agilent Technologies, Wilmington, USA). Conditions of GC-MS measurementsMS measurements were the same aswere in a previous the same work [as25]. in a previous work [25].

FigureFigure 2. Tar 2. and Tar wax and captured wax in isopropanol. captured in isopropanol. Nuclear Magnetic Resonance (NMR) spectroscopy was applied for a more detailed characterizationNuclear of the Magnetic undissolved Resonance fraction. All NMR (NMR) spectra spectroscopy were acquired using was a applied for a more detailed 600 VNMRS NMR spectrometer (Varian Inc., Palo Alto, Santa Clara, CA, USA) operating at thecharacterization frequencies of 599.76 MHz of forthe1H undissolved and 150.82 MHz for fraction.13C nuclei. The All spectrometer NMR spectra was were acquired using a 600 equippedVNMRS with NMR a standard spectrometer tunable X/H double-resonance (Varian Inc., probe, Palo an inverse Alto, HCN California, triple- USA) operating at the fre- resonance probe and a 3.2 mm magic-angle spinning (MAS) probe. All liquid state NMR spectraquencies (1H, 13C, of COSY 599.76 spectrum MHz with gradient for 1H coherence and selection,150.82 HSQC MHz (Heteronuclear for 13C nuclei. The spectrometer was Singleequipped Quantum with Coherence) a standard and band-selective tunable HMBC X/H (Heteronuclear double-resonance Multiple Bond probe, an inverse HCN triple- Correlation) spectrum with gradient coherence selection) were measured in d6-benzene atresonance 25 ◦C. The chemical probe shift and axis for a solid3.2 statemm was magic-angle referenced using spinning hexamethylbenzene (MAS) probe. All liquid state NMR (132.3spectra and 17.3 (1 ppmH, 13 [26C,,27 ]).COSY The solid spectrum state sample waswith spun gradient at 7 kHz at roomcoherence temperature. selection, HSQC (Heteronuclear First, a simple one-pulse experiment was run with a relaxation time of 30 s. After that, a 1H-Single13C cross-polarization Quantum experiment Coherence) with spinning and sidebandband-selective suppression wasHMBC run with (Heteronuclear Multiple Bond linear ramping during a 7 ms cross-polarization period. For 1H decoupling, the two-pulse phaseCorrelation) modulation (TPPM) spectrum decoupling with scheme gradient was used. coherence All NMR spectra selection) are shown in were measured in d6-benzene Figuresat 25 S1–S9 °C. inThe the Supplementary chemical shift Materials axis to this for work solid (NMR state images). was referenced using hexamethylbenzene Elemental composition of both dissolved and undissolved fractions of tar and wax was(132.3 measured and by the17.3 same ppm method [26,27]). described The in Chapter solid 2.1. state sample was spun at 7 kHz at room temperature. First, a simple one-pulse experiment was run with a relaxation time of 30 s. After 3. Results and discussion 1 13 3.1.that, Results a ofH- RDFC Characterization cross-polarization experiment with spinning sideband suppression was run withThe courselinear of thermalramping decomposition during of a RDF 7 ms is demonstrated cross-polarizati by TG andon DSC period. curves For 1H decoupling, the two- in Figure3. In an inert environment, four basic thermal decomposition steps were observed. Inpulse the temperature phase range modulation of up to 105 ◦ C,(TPPM) water was evaporated.decoupling The second scheme step in was the used. All NMR spectra are temperatureshown in range Figures of 230 to 400NMR1–NMR9◦C is due to the decomposition in the supplemen of the cellulosictary fraction. material to this work (NMR im- Plastic fraction represented mainly by polyethylene and polypropylene is decomposed in theages). temperature range of 400 to 500 ◦C. A separate thermogravimetric analysis of cellulose and low densityElemental polyethylene composition [28] confirmed of these both findings. dissolved The final decomposition and undissolved step fractions of tar and wax under an inert atmosphere in the temperature range of 700 to 750 ◦C corresponds with the decompositionwas measured of inorganic by saltsthe suchsame as CaCO method3 which described are present in thein paperChapter fraction. 2.1. The study on the thermal decomposition of CaCO3 by Singh and Singh [29] shows its 3. Results and discussion 3.1. Results of RDF Characterization The course of thermal decomposition of RDF is demonstrated by TG and DSC curves in Figure 3. In an inert environment, four basic thermal decomposition steps were observed. In the temperature range of up to 105 °C, water was evaporated. The second step in the temperature range of 230 to 400 °C is due to the decomposition of the cellulosic fraction. Plastic fraction represented mainly by polyethylene and polypropylene is decomposed in the temperature range of 400 to 500 °C. A separate thermogravimetric analysis of cellulose and low density polyethylene [28] confirmed these findings. The final decomposition step under an inert atmosphere in the temperature range of 700 to 750 °C corresponds with the decomposition of inorganic salts such as CaCO3 which are present in the paper fraction. The study on the thermal decomposition of CaCO3 by Singh and

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Singhdecomposition [29] shows at its temperatures decomposition from at temper 700 to 800atures◦C. from The 700 last to decomposition 800 °C. The last step decom- is the positionresult of step char is combustion the result of underchar combustion oxygen. Except underfor oxygen. the last Except combustion for the last steps, combustion all other steps,decomposition all other decomposition steps are endothermic steps are as endo it resultsthermic from as theit results DSC curve.from the DSC curve.

Figure 3. ThermogravimetricFigure 3. Thermogravimetric record of RDF. record of RDF. The measured proximate and elemental composition of RDF shown in Table1 is in The measured proximate and elemental composition of RDF shown in Table 1 is in relatively good coherence with data from the literature [28,29]. However, the composition relatively good coherence with data from the literature [28,29]. However, the composition of RDF depends on many factors including location and method of production. Sulfur of RDF depends on many factors including location and method of production. Sulfur and and chlorine content can significantly affect the downstream processes required for gas chlorine content can significantly affect the downstream processes required for gas clean- cleaning. Sulfur content in the studied RDF is lower than in coal. Chlorine content of ing. Sulfur content in the studied RDF is lower than in coal. Chlorine content of 0.73 mass 0.73 mass % includes the studied RDF in Class 3 of a scale with three classes defined by % includes the studied RDF in Class 3 of a scale with three classes defined by European European standard EN 15359: 2011. The RDF content of chlorine reported in the literature standardvaries between EN 15359: 0.3 and 2011. 0.8 massThe RDF %. The content studied of RDFchlorine had relativelyreported highin the calorific literature value. varies The betweenmeasured 0.3 higher and 0.8 heating mass value%. The (HHV) studied of 20.81RDF MJhad·kg relatively−1 is comparable high calorific with thevalue. calorific The −1 measuredvalue of sub-bituminous higher heating coal.value (HHV) of 20.81 MJ.kg is comparable with the calorific value of sub-bituminous coal. Table 1. Proximate and elemental composition and calorific value of RDF. Table 1. Proximate and elemental composition and calorific value of RDF. Proximate composition (wt. %): Proximate composition (wt. %): Moisture 10.00 Volatile matterMoisture 66.25 10.00 Fixed carbonVolatile matter 9.29 66.25 AshFixed carbon 14.46 9.29 Elemental composition (wt.Ash %): 14.46 ElementalCarbon composition (wt. %): 46.47 HydrogenCarbon 6.43 46.47 NitrogenHydrogen 0.84 6.43 Sulfur 0.35 Nitrogen 0.84 Chlorine 0.73 Oxygen *Sulfur 30.72 0.35 HHV (MJ.kg−1) Chlorine 20.81 0.73 * calculated by difference. Oxygen* 30.72 HHV (MJ.kg−1) 20.81 * calculated by difference.

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3.2. Single Stage RDF Gasification

3.2.In Single a single-stage Stage RDF batch Gasification gasification reactor, the amount of generated gas and its composition are changing. During the reaction of the material under the constant flow of an In a single-stage batch gasification reactor, the amount of generated gas and its com- oxidizing agent, the equivalent ratio (oxygen flow to stoichiometric oxygen requirement) position are changing. During the reaction of the material under the constant flow of an increases. This change in condition results in a change of gas composition. The contents oxidizing agent, the equivalent ratio (oxygen flow to stoichiometric oxygen requirement) of increases.important This combustible change in components—H condition results2 in, CO, a change and CH of gas4—change composition. during The the contents process ofand shows a maximum at a specific point of the process. Figure 4 shows the gas composition important combustible components—H2, CO, and CH4—change during the process and changeshows up a maximumto 90 minutes at a specificfrom the point beginning of the process. of the process Figure 4when shows the the reactor gas composition temperature waschange 900 °C. up Both to 90 minH2 and from CO the contents beginning show of the a maximum process when of around the reactor 500 temperature seconds after was the ◦ start900 ofC. the Both gasification. H2 and CO Lower contents concentrations show a maximum in the of initial around stage 500 secof the after experiment the start of thewere causedgasification. by gradual Lower heating concentrations of the sample in the to initial the required stage of thetemperature experiment and were by causedstarting by the gasificationgradual heating reactions. of the After sample reaching to the requiredthis maxi temperaturemum (24 mol.% and by H starting2 and 21 the mol.% gasification CO), the contentreactions. of H After2 rapidly reaching decreased this maximum to values (24 below mol.% 1% H 2afterand 35 21 min mol.% from CO), the the beginning content of of H 2the process.rapidly It decreased was assumed to values that belowduring 1% this after period, 35 min mainly from the beginningvolatile fraction of the process.of the waste It wouldwas assumedbe gasified. that The during decrease this period, in the CO mainly conten thet volatile is not so fraction rapid; ofit shows the waste relatively would behigh valuesgasified. up to The 50 decreaseminutes inafter the the CO start content of the is notprocess. so rapid; After it showsthe gasification relatively of high the values volatile fraction,up to 50 gasification min afterthe of fixed start ofcarbon the process. continues; After this the process gasification is much of the slower volatile and fraction, a longer gasification of fixed carbon continues; this process is much slower and a longer time is time is required for total gasification of the char. As the char is consumed in the reactor, required for total gasification of the char. As the char is consumed in the reactor, the content the content of CO decreases and the content of CO2 starts to increase again. The maximum of CO decreases and the content of CO2 starts to increase again. The maximum content content of CH4 and CO2 was recorded at around 350 seconds from the process beginning; of CH4 and CO2 was recorded at around 350 sec from the process beginning; then the then the content of CH4 decreased below 1% and that of CO2 to around 3% in the following content of CH4 decreased below 1% and that of CO2 to around 3% in the following minutes. 2 minutes.However, However, by increasing by increa the equivalentsing the equivalent ratio above ratio a critical above value, a critical the CO value,2 content the CO started con- tentto started increase to again increase while again the CO while content the CO decreases. content Thedecreases. course ofThe gas course composition of gas composi- can be tioninfluenced can be influenced by the composition by the composition of feed as well of feed as by as the well flow as by of oxidizingthe flow of agent oxidizing (air in thisagent (aircase). in this In thiscase). work, In this the work, air flow the rate air was flow maintained rate was maintained at 15.5 ± 1.0l at min 15.5− 1±. 1.0l.min−1.

90.00 H2 N2 CO CH4 CO2 80.00

70.00

60.00

50.00 Vol. % Vol. 40.00

30.00

20.00

10.00

0.00 0 1000 2000 3000 4000 5000 6000

Time (s)

Figure 4. Time dependence of gas composition at 900 ◦C. Figure 4. Time dependence of gas composition at 900 °C. Reactor temperature showed signiﬁcant inﬂuence on product yields and gas composition.Reactor Table2 temperature shows product showed yields significant at all three in reactorfluence temperatures on product selectedyields and in this gas work.composition. Table 2 shows product yields at all three reactor temperatures selected in this work.

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Gas released from the reactor originated from sample gasiﬁcation and from air introduced to the reactor. The gas product yields in Table2 were calculated by differences between the mass of the sample and the mass of other products. Thus, the mass of air introduced to the system is not included but only the portion of the sample transformed to gas product. By increasing the temperature, both tar and wax and aqueous yields decreased. However, waxes captured in isopropanol were visible as a solid phase at all temperatures (Figure2 ). The mass yield of RDF transformed to syngas increased from 0.46 at 700 ◦C to 0.67 at 900 ◦C. At 700 ◦C, a slightly higher ash yield was recorded, which indicates lower carbon conversion at this temperature.

Table 2. Product mass yields of single-stage batch gasiﬁcation of RDF [kg/kg RDF].

Temperature (◦C) 700 800 900 Gas a 0.46 0.63 0.67 Tar and Wax 0.1 0.08 0.06 Aqueous 0.25 0.13 0.10 Ash 0.19 0.16 0.17 a calculated by difference between mass of the sample and mass of other products. (mass of added air is excluded).

For the used RDF and air ﬂow rate, the maximum content of H2 and CO at all temperatures was measured around 500 sec after the process start. Table3 shows the gas composition at this point for all reactor temperatures. The maximum content of H2, CO, ◦ and CH4 increased from 10.5, 8.5 and 6.5 mol.% at 700 C to 20, 16.5 and 9 mol.%. at 900 ◦ ◦ C, respectively. The CO2 content decreased from 21.5 mol.% at 700 C to 14.5 mol% at 900 ◦ C. The decrease in the mole fraction of nitrogen is a result of the increasing amount of H2, CO, and CH4 since the amount of nitrogen was practically constant.

Table 3. Gas composition (mol.%) at 500 s.

◦ Temperature ( C) H2 N2 CO CH4 CO2 700 10.5 51.5 8.5 6.5 21.5 800 16 45 7.5 9.5 18 900 20 36.5 16.5 9 14.5

3.3. Tar and Wax Yield Characterization To determine the nature of undissolved solid phase, GC-MS analysis was performed separately for every phase. As shown in Table4, the Pyro-GC-MS analysis of undissolved phase identified mainly C5–C12 alkenes, which indicates that the undissolved solid phase captured in isopropanol is formed by segments of polyolefin chains. Formation of this phase can be considered as the main disadvantage of a simple one-stage non-catalytic fixed- bed or semi-batch gasification system. Such heavy tar and wax fraction can cause major fouling problems in downstream devices installed next to the reactor. GC-MS analysis of the dissolved phase identified ethanol, aliphatic and aromatic hydrocarbons and traces of many oxygenated compounds. Nuclear magnetic resonance confirmed the nature of the solid undissolved phase as segments of polymer chains. All NMR spectra are shown in the Supplementary Materials to this work (NMR images), particularly the 1H spectrum (Figure S1), 13C (Figure S2), 2D COSY (Figures S3 and S4), 2D HSQC (Figures S5 and S6), 2D HMBC (Figure S7), directly measured 13C MAS spectrum (Figure S8) and the 13C MAS spectrum with the utilization of 1H-13C cross-polarization (Figure S9). Both liquid- and solid-state NMR revealed dominant content of linear and branched alkyl chains. A small amount of molecules with terminal double-bond moieties were also observed in the liquid state. 2D correlation NMR spectra (especially multiple bond correlations in the HMBC spectrum) indicated the presence of polyethylene and polypropylene chains. A very good correlation with the literature was found for the relative chemical shifts of polyethylene signals in [30] measured at 110 ◦C. Processes 2021, 9, x FOR PEER REVIEW 9 of 12

Cyclodecane 5.64 Hexadecane 0.09 Toluene 4.71 Tetradecane 0.09 Hexadiene 4.15 Tetradecene 0.08 Dodecene 2.91 Benzene 0.08 Nonene 2.59 Pentdecene 0.08 Nuclear magnetic resonance confirmed the nature of the solid undissolved phase as segments of polymer chains. All NMR spectra are shown in the supplementary material to this work (NMR images), particularly the 1H spectrum (Figure NMR1), 13C (Figure NMR2), 2D COSY (Figure NMR3, Figure NMR4), 2D HSQC (Figure NMR5, Figure Processes 2021, 9, 343 NMR6), 2D HMBC (Figure NMR7), directly measured 13C MAS9 of 12 spectrum (Figure NMR8) and the 13C MAS spectrum with the utilization of 1H-13C cross-polarization (Figure NMR9). Both liquid- and solid-state NMR revealed dominant content of linear and Table 4. GC-MS analysis. branched alkyl chains. A small amount of molecules with terminal double-bond moieties were alsoUndissolved observed Phase in the liquid state. 2D Dissolved correlation Phase NMR spectra (especially multiple Component Peak Area (%) Component Peak Area (%) bond correlations in the HMBC spectrum) indicated the presence of polyethylene and pol- Pentene 15.08 Isopropanol 93.87 ypropyleneHexene chains. A 14.8very good correlation Ethanol with the literature 1.15 was found for the relative Heptene 11 Dimethylbenzene 0.37 chemicalOctene shifts of polyethylene 7.56 signals Nonadecene in [30] measured 0.18 at 110 °C. Dimethylbenzene 6.42 Octadecene 0.11 Cyclodecane 5.64 Hexadecane 0.09 3.4. EffectToluene of Secondary 4.71Catalysis on the Tetradecane Tar and Wax Yield 0.09 Hexadiene 4.15 Tetradecene 0.08 DodeceneA natural clay catalyst 2.91 doped with Benzene Ni was employed 0.08 in a secondary reactor to reduce Nonene 2.59 Pentdecene 0.08 the yield of tar and wax in the produced gas. The clay containing mainly SiO2 and some

3.4.amount Effect of Secondaryof Al2O Catalysis3, Fe2O on3 the, CaCO Tar and3 Wax, MgCO Yield 3, etc. [31] had the initial specific surface area of 2 −1 2 −1 approximatelyA natural clay catalyst 100 dopedm g with; however, Ni was employed its surface in a secondary area reactor decreased to reduce by calcination to 35 m g . theDetails yield of of tar the and two-stage wax in the produced RDF gasification gas. The clay containing with a mainly clay catalyst SiO2 and some in the secondary reactor were amount of Al2O3, Fe2O3, CaCO3, MgCO3, etc. [31] had the initial specific surface area of approximatelypublished 100in ma2 previousg−1; however, work its surface [22]. area Here, decreased we by focus calcination on tothe35 meffect2 g−1. of the secondary catalyst Detailson the of thetar two-stageand wax RDF yield gasification only. with Figure a clay 5 catalyst show ins thetar secondary fraction reactor captured in isopropanol after werethe publishedsecondary in a previouscatalytic work stage. [22]. Here,A comparison we focus on the of effect this of picture the secondary with Figure 2 shows a signifi- catalyst on the tar and wax yield only. Figure5 shows tar fraction captured in isopropanol aftercant the visible secondary effect catalytic of stage.secondary A comparison catalysis of this on picture the withtar Figureand wax2 shows yield. a After installing the cat- significantalytic stage, visible no effect solid of secondary fraction catalysis was on captur the tar anded in wax isopropanol yield. After installing and the used isopropanol had the catalytic stage, no solid fraction was captured in isopropanol and the used isopropanol hada clearer a clearer color.

FigureFigure 5. Tar 5. andTar wax and captured wax incaptured isopropanol in after isopropanol installing a secondary after installing catalytic reactor. a secondary catalytic reactor.

A comparison of tar yields of a two stage catalytic system with a system without a catalystA (data comparison from Table2) shown of tar in Figureyields6a provedof a two that Ni-impregnatedstage catalytic clay reducedsystem with a system without a thecatalyst tar and wax(data yield from by up Table to 91.8%. 2) The shown effect ofin the Figure Ni-based 6a catalyst proved on the that removal Ni-impregnated clay reduced of tar from waste and biomass gasification was confirmed also by other authors [32,33]. Furthermore,the tar and the wax influence yield of the by temperature up to 91.8%. in the catalyticThe effect process of is the significant, Ni-based as can catalyst on the removal of ◦ ◦ betar seen from in Figure waste6b (Y and axis zoom biomass in); an increasegasification in the temperature was confirmed from 700 C also to 900 byC other authors [32,33]. Fur- decreased the tar yield by around 45%. Compared to the simple one-stage semi-batch gasification,thermore, no waxthe fractioninfluence in the formof the of a temperature solid phase in isopropanol in the wascatalytic observed process in the is significant, as can be two-stageseen in catalytic Figure system. 6b (Y High axis molecular zoom weight in); polymeran incr segmentsease in in the the gasifiertemperature prod- from 700 °C to 900 °C uct can have an extremely negative effect on the downstream processing of the generated gas;decreased therefore, simple the tar one-stage yield semi-batch by around gasification 45%. ofCompared RDF is not recommended. to the simple one-stage semi-batch gasification, no wax fraction in the form of a solid phase in isopropanol was observed in the two-stage catalytic system. High molecular weight polymer segments in the gasifier product can have an extremely negative effect on the downstream processing of the generated gas; therefore, simple one-stage semi-batch gasification of RDF is not recommended.

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Figure 6. Tar yield from RDFFigure gasiﬁcation 6. Tar with yield a secondaryfrom RDF catalyticgasification stage with with a Ni-dopedsecondary clay catalytic catalyst. stage (a )with Comparison Ni-doped with clay cat- non catalytic process, (b) Effectalyst. of temperature.

4.4. Conclusions Conclusions AA single-stage single-stage semi-batch semi-batch gasification gasification of of RDF RDF can can provide provide a a relatively relatively high high yield yield of of heavyheavy tars tars and and waxes. waxes. The The tar tar and and wax wax fraction fraction can can amount amount to to up up to to 10% 10% of of the the feed feed mass. mass. SinceSince high high tar tar content content gas gas has has extremely extremely negative negative effect effect on on downstream downstream processing, processing, simple sim- one-stageple one-stage semi-batch semi-batch gasification gasification of RDF of RDF is not is recommended. not recommended. GC-MS GC-MS and NMRand NMR analysis anal- haveysis shownhave shown that wax that fraction wax fraction captured captured in the form in the of aform solid of powder a solid inpowder isopropanol in isopropanol consists mainlyconsists of mainly polyethylene of polyethylene and polypropylene and poly chains.propylene The chains. nature ofThe the nature waxes of proves the waxes that theyproves come that from they the come plastic from fraction the plastic of RDF frac andtion furtherof RDF thermal and further and thermal catalytic and processing catalytic canprocessing lead to theircan lead total to decomposition. their total decomposition. The content The of content most important of most important gas components gas com- changesponents duringchanges the during process; the forprocess; the used for RDFthe used and RDF air flow and rate, air flow the maximumrate, the maximum content ofcontent H2 and of COH2 and atall CO temperatures at all temperatures was measured was measured around around 500 sec500 afterseconds the after start the of thestart ◦ process.of the process. The maximum The maximum content content of H2 andof H CO2 and varied CO varied from from 10.5 and10.5 8.5and vol.% 8.5 vol.% at 700 at 700C to°C 20 to and 20 and 16.5 16.5 vol.% vol.% at 900 at 900◦C, °C, respectively. respectively. Calcined Calcined red red clay clay can can serve serve as as a a low-cost low-cost catalystcatalyst in in thethe removal of tar tar and and waxes waxes from from the the gas. gas. Impregnation Impregnation of these of these catalysts catalysts with withNi leads Ni leads to better to better tar removal tar removal and anda higher a higher content content of H of2 in H 2thein thegas; gas; however, however, it can it can also alsorepresent represent additional additional costs costs related related to to cataly catalystst preparation preparation and and after useuse treatment.treatment. Full Full commercializationcommercialization of of MSW MSW gasification gasification requires requires further further investigation investigation of of thermo-catalytic thermo-catalytic thermalthermal decomposition decomposition and and downstream downstream gas gas purification purification processes. processes.

SupplementarySupplementary Materials: Materials:The The following following are are available available online online at at https://www.mdpi.com/2227-971 www.mdpi.com/xxx/s1: 7/9/2/343/s1: Figure S1. Aliphatic part of the liquid state 1H NMR spectrum. Figure S2. Aliphatic Figure NMR1. Aliphatic part of the liquid state 1H NMR spectrum part of the liquid state 13C NMR spectrum. Figure S3. 2D COSY NMR spectrum of the liquid state Figure NMR2. Aliphatic part of the liquid state 13C NMR spectrum sample.Figure Figure NMR3. S4. Homonuclear 2D COSY NMR correlations spectrum of aliphaticof the liquid protons state in sample a zoom of the 2D COSY NMR spectrumFigure of the NMR4. liquid state Homonuclear sample. Figure correlations S5. 2D HSQC of aliphatic NMRspectrum protons in from a zoom the aliphatic of the chemical2D COSY 1 13 shiftNMR regions spectrum of Hof andthe liquidC. Figure state sample S6. 2D HSQC NMR spectrum from the double-bond chemical 1 13 shift regionsFigure of NMR5.H and 2D C.HSQC Figure NMR S7. spectrum Band-selective from the gHMBC aliphatic NMR chemical spectrum shift from regions the aliphaticof 1H and 1 13 chemical13C shift regions of H and C. Figure S8. Directly measured solid-state magic-angle spinning 13 (7 kHz spinFigure rate) NMR6.C NMR 2D HSQC spectrum NMR of spectrum the sample. from Figure the do S9.uble-bond Solid-state chemical magic-angle shift regions spinning of 1H (7and kHz 13C spin rate) 13C NMR spectrum with 1H-13C cross-polarization Figure NMR7. Band-selective gHMBC NMR spectrum from the aliphatic chemical shift re- Author Contributions: Conceptualization, J.H.; Formal analysis, J.H.; Funding acquisition, J.H.; gions of 1H and 13C. Investigation, J.H., P.Š. and M.Š.; Methodology, J.H., P.Š. and M.Š.; Project administration, J.H.; Figure NMR8. Directly measured solid-state magic-angle spinning (7 kHz spin rate) 13C NMR Supervision, J.H.; Writing—original draft, J.H.; Writing—review & editing, P.Š. and M.Š. All authors spectrum of the sample have read and agreed to the published version of the manuscript. Figure NMR9. Solid-state magic-angle spinning (7 kHz spin rate) 13C NMR spectrum with 1H-13C cross-polarization

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Funding: This research was funded by Slovak Research and Development Agency, Grant APVV-15- 0148 and Grant APVV-19-0170. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article or Supplementary Materials. Acknowledgments: In this work we used equipment obtained under the OP Research and Devel- opment project ITMS 26240220084, co-financed by the Fund of European Regional Development to whom we express our thanks. Conflicts of Interest: The authors declare no conflict of interest.

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