energies

Article Process Modelling and Simulation of Waste Gasification-Based Flexible Polygeneration Facilities for Power, Heat and Biofuels Production

Chaudhary Awais Salman 1,* and Ch Bilal Omer 2

1 School of Business, Society and Engineering, Mälardalen University, SE 721 23 Västerås, Sweden 2 Kot Addu Power Company Limited (KAPCO), Kot Addu 34050, Pakistan; [email protected] * Correspondence: [email protected]



Received: 19 July 2020; Accepted: 13 August 2020; Published: 18 August 2020


Abstract: There is increasing interest in the harnessing of energy from waste owing to the increase in global waste generation and inadequate currently implemented waste disposal practices, such as composting, landfilling or dumping. The purpose of this study is to provide a modelling and simulation framework to analyze the technical potential of treating municipal solid waste (MSW) and refuse-derived fuel (RDF) for the polygeneration of biofuels along with district heating (DH) and power. A flexible waste gasification polygeneration facility is proposed in this study. Two types of waste—MSW and RDF—are used as feedstock for the polygeneration process. Three different gasifiers—the entrained flow gasifier (EFG), circulating fluidized bed gasifier (CFBG) and dual fluidized bed gasifier (DFBG)—are compared. The polygeneration process is designed to produce DH, power and biofuels (methane, methanol/dimethyl ether, gasoline or diesel and ammonia). Aspen Plus is used for the modelling and simulation of the polygeneration processes. Four cases with different combinations of DH, power and biofuels are assessed. The EFG shows higher energy efficiency when the polygeneration process provides DH alongside power and biofuels, whereas the DFBG and CFBG show higher efficiency when only power and biofuels are produced. RDF waste shows higher efficiency as feedstock than MSW in polygeneration process.

Keywords: process simulations; cogeneration; trigeneration; multigeneration; waste-to-energy

1. Introduction Approximately 2.0 billion tonnes of municipal solid waste (MSW) is generated globally every year—this figure is forecast to reach 3.4 billion tonnes annually in 2050 [1]. MSW contains significant amounts of non-recyclable organic and inorganic content. There is a growing interest in recovering energy and nutrients from MSW owing to the increase in waste generation and inadequate waste disposal practices currently implemented, such as composting, landfilling and dumping [2]. MSW is a heterogeneous mixture of waste fractions, thus, the conversion of MSW to energy requires the source separation of the non-recyclable organic fraction from the recyclable organic fraction (such as metal, glass, etc.). MSW also has high moisture and ash content and low heating value. The heterogenous nature of MSW makes it difficult to convert it into energy [3]. The properties of MSW can be improved by processing it into refuse-derived fuel (RDF) through physiochemical treatments, such as shredding, magnetic sorting, grinding, screening and bag ripping [4], resulting in low moisture and ash content and higher heating value than MSW. Some developed countries (Sweden, Denmark and Japan) have state-of-the-art waste management systems with well-established waste collection and source-separation practices [1]. Consequently, the production of RDF through MSW processing is easier in these countries than in low-income

Energies 2020, 13, 4264; doi:10.3390/en13164264 www.mdpi.com/journal/energies Energies 2020, 13, 4264 2 of 22 countries. In addition, some developed countries, such as the United States, Australia, and Canada, still dispose of 50% of their waste to landfill [1]. Thus, not every country has the ability and existing structure to prepare RDF from MSW. This study therefore considers both MSW and RDF as potential feedstocks for polygeneration facilities. Incineration of waste for heat or power production is a commercially mature technology; however, there is a growing interest among researchers and industrial stakeholders in designing waste-to-biofuels processes [5]. The efficiency of waste combustion to produce electrical power is ~30%. The overall efficiency increases with the co-production of district heating (DH) with power, but not all regions require DH, and its production depends on the local demand. Different thermochemical processes can convert MSW and RDF into biofuels, for instance, gasification, pyrolysis, hydrothermal liquefaction, hydrothermal carbonization and torrefaction [6]. This study uses the gasification process for the treatment of MSW and RDF. Gasification converts waste to syngas, which provides flexibility to produce a variety of biofuels along with DH and power. Syngas from waste gasification can be converted to power at a higher efficiency than waste combustion [7,8]. However, waste gasification remains in the research and development phase, with only a few demonstration plants currently in operation. The overall energy efficiency of waste gasification can also be increased by the simultaneous conversion of syngas to multiple products (biofuels, power and DH) in polygeneration plants [9]. Polygeneration is the integration of various processes to produce multiple products simultaneously [10]. The overall aim of polygeneration process design is to increase the conversion efficiency by the production of value-added products and to enhance the economic outcomes. The secondary aim of polygeneration processes is to increase the system flexibility in terms of products, operation and feedstock. According to Ciuta et al. [11], the best path forward for waste gasification is to combine gas turbine and biofuel synthesis. The net system efficiency of the gasification process may also increase through polygeneration of DH, power and other bio-based products [9]. Several authors conducted polygeneration-based studies with coal as a feedstock [12–21]. Gootz et al. [12] studied the polygeneration of coal to liquid biofuels based on entrained flow gasification. Buttler at al. [13] evaluated the coal-based polygeneration plant by integrating a gasification-combined cycle with electrolysis and natural gas synthesis. Yu et al. [14] analyzed the polygeneration of Fischer–Tropsch (FT) fuels with power from coal gasification. Narvez et al. [15] and Li et al. [16] studied the polygeneration plant with methanol and power production. A coal pyrolysis-based polygeneration plant was analyzed by Guo et al. [17]. Li et al. [18] designed a coal to natural gas and methanol-based polygeneration plant and established the techno-economic feasibility. Huang et al. [19] designed coal-based polygeneration processes for chemicals and power. Heinz et al. [20] simulated a coal-based polygeneration plant with fluidized bed gasification. Kim et al. [21] evaluated the polygeneration of methanol, power and heat from coke oven gas. Biomass-based polygeneration plants have also been studied by numerous authors [22–26]. Fan et al. [22] studied the biomass- and coal-based polygeneration plant for power and natural gas. Meerman et al. [23] extensively studied the polygeneration of various biofuels along with power from both biomass and coal. Sahoo et al. [24] developed the hybrid polygeneration plant for power, cooling and desalination by integrating solar with a biomass power plant. Jana et al. [25] studied the feasibility of an agricultural-based polygeneration plant. In a different study, Jana et al. [26] reported the sustainability of polygeneration design. Gladysz et al. [27] studied polygeneration systems based on thermochemical conversion of biomass. There are few studies considering waste as feedstock for a polygeneration plant. Villarroel-Schneider et al. [28] designed a biogas-based polygeneration plant using dairy farm waste. Jana et al. [29] performed the life-cycle assessment of an agricultural waste-based polygeneration plant. Fuente et al. [30] designed a concentrated solar and waste-based polygeneration plant, mainly for power production. To our knowledge, there are no reports in the literature that consider MSW and RDF conversion to multiple products through flexible polygeneration facilities. Energies 2020, 13, 4264 3 of 22

There are various types of gasifiers that can treat waste: fixed bed, fluidized bed, and entrained flow gasifiers (EFGs) [31]. Waste feedstock differs from biomass and coal in that the size and properties of the waste are heterogeneous. Waste also has higher inorganic content, and its heating value is generally lower due to the high amount of ash and moisture. It is generally perceived that circulating fluidized bed gasifiers (CFBG) or bubbling fluidized bed gasifiers (BFBG) produce high-quality syngas for biofuel synthesis. However, CFBG and BFBG require an air separation unit (ASU) to produce the nitrogen-free syngas. The dual fluidized bed gasifier (DFBG) is another type of gasifier that contains twoEnergies separate 2020, 13, x FOR reactors PEER REVIEW for gasification and combustion and can therefore produce3 of 22 syngas without nitrogen.value is generally Fluidized lower bed due gasification to the high amount also producesof ash and moisture. tar, which It is must generally be removedperceived that from syngas in downstreamcirculating processes fluidized priorbed gasifier to synthesiss (CFBG) or of bubbling biofuels. fluidized The EFGbed gasifiers is another (BFBG) type produce of gasifier,high- which operates atquality high pressuresyngas for andbiofuel temperature synthesis. However, and can CFBG produce and BFBG tar-free require syngas. an air separation However, unit the (ASU) EFG requires feed pretreatment,to produce the e.g., nitrogen-free size reduction syngas. The through dual fluidize grinding,d bed gasifier torrefaction, (DFBG) is andanother drying. type of gasifier Performance of that contains two separate reactors for gasification and combustion and can therefore produce syngas different gasifierswithout nitrogen. has previously Fluidized bed been gasification compared also forproduces the production tar, which must of methanebe removed through from syngas syngas from biomass orin coal-based downstream feedstock processes prior [32– 34to ].synthesis There of has biofuels. been littleThe EFG or no is another consideration type of gasifier, of evaluating which suitable gasifiers foroperates RDF toat biofuelshigh pressure or MSWand temperature to biofuels an processes.d can produce tar-free syngas. However, the EFG The purposerequires feed of pretreatment, this study e.g., is to size design reduction and through simulate grinding, RDF torrefaction, and MSW-based and drying. gasification Performance of different gasifiers has previously been compared for the production of methane polygenerationthrough plants syngas for from DH, biomass power or and coal-based biofuels feedstock production. [32–34]. Methane,There has methanolbeen little /dimethylor no ether (DME), FTconsideration fuels (diesel of orevaluating gasoline) suitable and gasifiers ammonia for RDF from to thebiofuels waste or MSW polygeneration to biofuels processes. plant are considered as potential biofuels.The purpose Waste of polygenerationthis study is to design processes and aresimulate designed RDF and MSW-based compared gasification with three different gasifiers—thepolygeneration EFG, CFBG, plants andfor DH, DFBG. power Theand biofuels following production. section Me describesthane, methanol/dimethyl the methodology ether for the (DME), FT fuels (diesel or gasoline) and ammonia from the waste polygeneration plant are process design,considered simulation as potential approach biofuels. Waste and polygeneration the studied cases,processes and are isdesigned followed and compared by results, with discussion and conclusions.three different gasifiers—the EFG, CFBG, and DFBG. The following section describes the methodology for the process design, simulation approach and the studied cases, and is followed by 2. Materialsresults, and discussion Methods and conclusions.

2.1. Polygeneration2. Materials Concepts and Methods This section2.1. Polygeneration provides Concepts a technical description of the waste-based polygeneration concepts simulated in this study. FigureThis section1 presents provides the a simplified technical description process flowof the diagram waste-based of the polygeneration waste-based concepts polygeneration concept. Twosimulated types in of this feed—RDF study. Figure and 1 presents MSW—are the simplified considered process and flow three diagram diff oferent the waste-based gasifiers—the EFG, polygeneration concept. Two types of feed—RDF and MSW—are considered and three different CFBG, andgasifiers—the DFBG—are EFG, evaluated. CFBG, and DFBG—are evaluated.

Figure 1. FigureSimplified 1. Simplified process process flow flow diagram diagram of a awaste-based waste-based polygeneration polygeneration facility. Two facility. types of Two types feed—RDF and MSW—are considered and three different gasifiers—EFG, CFBG, and DFBG—are of feed—RDF and MSW—are considered and three different gasifiers—EFG, CFBG, and evaluated. DFBG—are evaluated. Energies 2020, 13, 4264 4 of 22

2.2. Ultimate and Proximate Analyses of MSW and RDF As described above, two different types of waste were selected as the raw material: RDF and MSW. The ultimate and proximate analyses of RDF and MSW used for simulations are displayed in Table1.

Table 1. Ultimate and proximate analyses of RDF and MSW (wt%) [7,35].

RDF MSW Carbon 52.11 58.45 Hydrogen 7.40 6.87 Nitrogen 0.85 1.36 Sulfur 0.46 2.49 Chlorine 0.07 0.30 Oxygen 39.11 30.53 Ash 17.47 30.81 Moisture 13.12 51.87 Note: C, H, N, S, Cl, O are presented on a dry and ash-free basis; ash is on a dry basis.

Higher heating value (HHV) and lower heating value (LHV) of RDF and MSW are estimated using the following relations [36,37]: ! MJ    O  HHV = 0.3289 + 0.0117(100 C )0.25 C + 1.129 H + 0.105S (1) kg − daf ab − 10 ! MJ 18.015 H%  5.85 LHV = HHV × + Moisture(%) (2) kg − 2 × 100 where C, H and O are carbon, hydrogen and oxygen, respectively; daf refers to the dry and ash-free basis, and ab refers to the ash-free basis.

2.3. Modelling and Simulation This section describes the modelling and simulation approach used for the waste gasification-based polygeneration process. Aspen Plus was used for modelling and simulation [38].

2.3.1. Air Separation Unit Modelling The ASU was only considered for the EFG and CFBG; it was not modelled in detail in this study. Instead, the separator block of Aspen Plus was used to separate oxygen and nitrogen. The power consumed by the ASU was estimated as 0.4 kWhe/m3 oxygen [33].

2.3.2. Pretreatment (Grinding and Drying) Modelling Size reduction of waste (up to 2 mm) was carried out as waste pretreatment through grinding in order to make it suitable for gasification. The grinding was modelled in three stages by using the three crushers and the screen unit of Aspen Plus. The power required for grinding was estimated by the crusher blocks. Both RDF and MSW contain moisture, so drying is required to achieve the optimum conditions in the gasifier. RDF and MSW were dried to 10% final moisture. The RSTOICH reactor and separator block of Aspen Plus was used to model the dryer [39]. The heat required for drying was calculated using Equation (3) [40].

heatdrying = F0CpwastedT + MCpwaterdT + LhvMv (3) where, F0 is mass of waste feed in the dryer, Cpwaste is heat capacity of dry waste, Cpwater is heat capacity of water, M is mass of water present in the waste, dT is difference between drying and ambient temperature, Lhv is the latent heat of vaporization of water and Mv is the amount of water evaporated. Energies 2020, 13, 4264 5 of 22

The CALCULATOR block of Aspen Plus determined the heat required for drying using Equation (3). The required heat for drying was exported from the heat recovery steam generation (HRSG) section in the form of low pressure (LP) steam.

2.3.3. Modelling of Gasifiers, Syngas Cooling, Acid Gas Removal and Water–Gas Shift Reactor Zero-dimensional models of gasifiers—EFG, CFBG and DFBG—were developed in this study with Aspen Plus. Figures2–4 describe the process flow diagram of EFG, CFBG and DFBG, respectively. EFG and CFBG were modelled using two blocks of Aspen Plus. The RYIELD reactor decomposes the waste into its constituent elements and compounds (C, H, N, S, Cl, O, Ash and moisture). Gasification of waste was modelled by the RGIBBS reactor by assuming equilibrium conditions through minimization of Gibbs free energy. The pressure and temperature considered for EFG were 24 bar and 1300 ◦C[41]. For the CFBG, a temperature of 850 ◦C and a pressure of 1 bar were considered. Pure oxygen from ASU and steam from the HRSG section were also introduced into the gasifier. The DFBG was modelled by using three Aspen Plus blocks: RYIELD for the decomposition of waste into its constituent elements and RGIBBS reactor for gasification and combustion. Unreacted char from the gasification reactor was transferred to the combustor. Char combustion was modelled by the RSTOICH reactor with combustion reactions at adiabatic conditions. DFBG operated at a temperature of 850 ◦C and a pressure of 1 bar. Air entered into the combustor reactor of DFBG, while steam was used as the oxidizing agent in the gasifier. All the sulfur, chlorine and nitrogen present in waste was hydrogenated to hydrogen sulfide, hydrogen chloride and ammonia, respectively. Ash was removed from the syngas using the separator block. Syngas from the gasifier was cooled in three stages—in the first stage, syngas was cooled to 350 ◦C and waste heat was recovered in the form of steam. Syngas was then scrubbed with water in the second stage and finally, syngas was further cooled prior to the water–gas shift (WGS) reactor. The flash drum was used to remove water from syngas. Ammonia and hydrogen chloride in syngas was removed with water in the flash drum. Sulfur was removed by acid gas removal (AGR). For simplicity, the separator block was used to model AGR. Syngas from the gasifier was split and sent to the gas turbine and HRSG section without upgradation in the WGS reaction. As CFBG and DFBG operated at atmospheric conditions, cooled syngas was compressed to 24 bar before sending it to the gas turbine section and WGS reactor. The compressor block with 85% isentropic efficiency and 95% mechanical efficiency was used to model the syngas compression. The WGS reactor was used to maintain the H2/CO ratio of syngas according to R1: CO + H O CO + H ∆G◦ = 41 kJ/mol (R1) 2 → 2 2 − The syngas was required to produce methane, methanol/DME, FT diesel and FT gasoline fuels, which require a H2/CO of 2–3 for their synthesis. Thus, for conservative assessment a H2/CO ratio of 3 was selected in this study. The REQUIL reactor of Aspen Plus was used with adiabatic and equilibrium conditions. Steam was added to the reactor. The design specification block of Aspen Plus adjusted the flow of steam to achieve the desired H2/CO ratio of syngas leaving the WGS reactor. The temperature of WGS varied between 300 and 450 ◦C for different cases and gasifiers. Upgraded syngas leaving the WGS reactor was cooled to 150 ◦C and waste heat was recovered as steam. Pure hydrogen is required for ammonia synthesis, so cooled syngas (without WGS) was sent to the ammonia synthesis island, where carbon monoxide was converted to hydrogen in the presence of steam in two shift reactors—modelling details of which are presented in Section 2.3.7. CO2 was removed from upgrade syngas. CO2 removal can be carried out through absorption with N-methyl diethanolamine (MDEA). For simplicity, modelling of CO2 removal was omitted from this study. Instead, a separator block of Aspen Plus was used to remove 98% of CO2. The heat required for CO2 removal was considered as 4 MJ/kg CO2 removed and required heat was exported from the HRSG section [42]. EnergiesEnergies 20202020,, 1313,, 4264x FOR PEER REVIEW 6 of 22 Energies 2020, 13, x FOR PEER REVIEW 6 of 22

Figure 2. Process flow diagram of entrained flow gasification of MSW and RDF. Figure 2. ProcessProcess flow flow diagram of entrained flow flow gasification gasification of MSW and RDF.

Figure 3. Process flow flow diagram of gasificationgasification of MSW and RDF in the directly heatedheated CFBG.CFBG. Figure 3. Process flow diagram of gasification of MSW and RDF in the directly heated CFBG.

Figure 4. Process flow flow diagram of gasificationgasification of MSWMSW and RDF in the indirectlyindirectly heatedheated DFBG.DFBG. Figure 4. Process flow diagram of gasification of MSW and RDF in the indirectly heated DFBG. Energies 2020, 13, 4264 7 of 22

2.3.4. Methane Synthesis Modelling Energies 2020, 13, x FOR PEER REVIEW 7 of 22 Figure5 depicts the methane production process diagram from syngas modelled in Aspen Plus. Syngas2.3.4. was Methane converted Synthesis to methane Modelling in a methanation process in the presence of a catalyst. Reactions R2 and R3 occurFigure in 5 the depicts reactor. the methane Upgraded production syngas wasprocess preheated diagram to from 280 syngas◦C and modelled compressed in Aspen to 6 Plus. bar prior to methanationSyngas was [ 43converted]. The compressor to methane blockin a methanation of Aspen Plus process was in used the presence to compress of a catalyst. syngas. Reactions The design of the compressorR2 and R3 occur was in beyond the reactor. the scope Upgraded of this syngas study, was and preheated only the to compression280 °C and compressed power required to 6 bar was estimated.prior to Isentropic methanation effi [43].ciency The ofcompressor 85% and block mechanical of Aspen effi Plusciency was ofused 98% to werecompress used. syngas. The The RGIBBS design of the compressor was beyond the scope of this study, and only the compression power reactor was used to model the methanation by restricting the equilibrium at 280 ◦C and 6 bar. Reactions required was estimated. Isentropic efficiency of 85% and mechanical efficiency of 98% were used. The R2 and R3 are exothermic, so heat must be removed from the reactor to maximize the methane yield. RGIBBS reactor was used to model the methanation by restricting the equilibrium at 280 °C and 6 Vaporsbar. leaving Reactions the R2 methanation and R3 are exothermic, were cooled so heat and must waste be removed heat was from recovered the reactor in to the maximize form of the steam. Vaporsmethane were upgradedyield. Vapors to 98%leaving pure the methanemethanation by usingwere cooled the separator and waste blocks heat was of Aspenrecovered Plus. in the 90% of unconvertedform of steam. gases Vapors were recycled, were upgraded while theto 98% rest pure were methane sent for by combustion. using the separator blocks of Aspen Plus. 90% of unconverted gases were recycled, while the rest were sent for combustion. CO + 3H CH + H O ∆H = 206 kJ/mol (R2) 2 → 4 2 ◦ − CO + 3H2 → CH4 + H2O ΔH◦ = −206 kJ/mol (R2) CO2 + 4H2 CH4 + 2H2O ∆H = 165 kJ/mol (R3) CO2 + 4H→2 → CH4 + 2H2O ΔH◦◦ = −165− kJ/mol (R3)

FigureFigure 5. Process 5. Process flow flow diagram diagram of of methane methane synthesis synthesis from syngas syngas produced produced from from waste waste gasification. gasification.

2.3.5.2.3.5. Methanol Methanol/DME/DME Synthesis Synthesis Modelling Modelling FigureFigure6 shows 6 shows the process the process flow diagramflow diagram for methanol for methanol and DME and productionDME production from syngasfrom syngas modelled in Aspenmodelled Plus. in Upgraded Aspen Plus. syngas Upgraded from thesyngas WGS from reactor the WGS was compressedreactor was comp to 95ressed bar using to 95 the bar compressor using blockthe of compressor Aspen Plus. block The of design Aspen ofPlus. the The compressor design of the was compressor beyond thewas scopebeyond of the this scope study of this and study only the and only the compression power required was estimated. Isentropic efficiency of 85% and mechanical compression power required was estimated. Isentropic efficiency of 85% and mechanical efficiency of efficiency of 98% were used. Compressed syngas was preheated to 220 °C for methanol synthesis 98% were[44]. The used. RGIBBS Compressed reactor was syngas usedwas for modelling preheated of to methanol 220 ◦C forsynthesis methanol fromsynthesis syngas by [assuming44]. The RGIBBSthe reactorminimization was used forof Gibbs modelling free energy of methanol. The reaction synthesis R4 occurs from in syngas the methanol by assuming reactor. the Vapors minimization leaving of Gibbsthe free reactor energy. were The cooled reaction to 35 °C. R4 Unreacted occurs in gases the methanol were separated reactor. from Vapors methanol leaving by using the reactorthe flash were cooleddrum to 35 of ◦AspenC. Unreacted Plus. Part gases of the were unreacted separated vapors from was methanol recycled while by using the remainder the flash was drum sent of for Aspen Plus.combustion. Part of the unreactedLiquid methanol vapors from was the recycled flash drum while was the distilled remainder in two wasstages sent using for the combustion. separator and Liquid methanolDSTWU from block the of flash Aspen drum Plus was with distilled 30 stages in twoand stages99% recovery using theof methanol. separator The and off-gases DSTWU from block of Aspenmethanol Plus with distillation 30 stages were and recycled. 99% recovery of methanol. The off-gases from methanol distillation Half of the pure methanol was collected as a separated product, and the other half was converted were recycled. to DME. Methanol was dehydrated to DME via R5. Liquid methanol was pumped to 56 bar and Half of the pure methanol was collected as a separated product, and the other half was converted to preheated to 280 °C prior to the DME synthesis reactor. The REQUIL reactor was used to model DME DME.synthesis Methanol by was assuming dehydrated equilibrium to DME viaconditions. R5. Liquid Vapors methanol leaving was the pumped reactor to were 56 bar cooled and preheated and to 280condensed.◦C prior to DME the DMEwas recovered synthesis reactor.via distillati Theon. REQUIL The DSTWU reactor block was usedwas used to model to model DME DME synthesis by assumingpurification equilibrium with 30 stages conditions. and 99% Vapors recovery leaving of DME. the Unreacted reactor were methanol cooled was and pumped condensed. back to DME the was methanol purification section. Energies 2020, 13, 4264 8 of 22 recovered via distillation. The DSTWU block was used to model DME purification with 30 stages and 99% recovery of DME. Unreacted methanol was pumped back to the methanol purification section. Energies 2020, 13, x FOR PEER REVIEW 8 of 22 4H + 2CO 2CH OH ∆H = 181 kJ/mol (R4) 2 → 3 ◦ − 4H2 + 2CO → 2CH3OH ΔH◦ = −181 kJ/mol (R4) 2CH OH CH OCH + H O ∆H = 23.4 kJ/mol (R5) 2CH3 3OH→ → CH3 3OCH3 3 + H22O ΔH◦ ◦= −23.4− kJ/mol (R5)

FigureFigure 6. Process6. Process flow flow diagram of of methanol methanol and andDME DME synthesis synthesis from syngas from produced syngas producedfrom waste from wastegasification. gasification.

2.3.6.2.3.6. FT Fuels FT Fuels Synthesis Synthesis Modelling Modelling The processThe process flow flow diagram diagram of of FT FT fuels fuels synthesissynthesis from syngas syngas is isshown shown in inFigure Figure 7. Upgraded7. Upgraded syngas was compressed to 36 bar and preheated to 220 °C for FT fuels production [45]. The FT fuels syngas was compressed to 36 bar and preheated to 220 ◦C for FT fuels production [45]. The FT fuels product distribution was modelled and simulated by adopting the method from Pondini et al. [45] product distribution was modelled and simulated by adopting the method from Pondini et al. [45] (Figure 7). The product distribution follows the Anderson–Schultz–Flory (ASF) model with an α- (Figure7). The product distribution follows the Anderson–Schultz–Flory (ASF) model with an α-value value of 0.8 Equation (4) [45,46]. CO conversion is the limiting step in the FT reactor; 70% CO of 0.8conversion Equation was (4) [selected45,46]. CO[46]. conversion is the limiting step in the FT reactor; 70% CO conversion was selected [46]. W𝑊n = 𝑛(1n(1 α − ) 2α)2α(𝑛α(n 1 ) −1) (4) (4) − − The FTThe fuels FT fuels are are composed composed of of C1–C30 C1–C30++ hydrocarbonshydrocarbons containing containing paraffins paraffi andns and olefins, olefins, according according to reactionsto reactions R6 and R6 and R7. R7. The The CALCULATOR CALCULATOR blockblock of Aspen Aspen Plus Plus calculated calculated the the yield yield of FT of fuels FT fuels and and the resultsthe results were were transferred transferred to to the the RYIELD RYIELD reactor. reactor. ParaffiParaffinsns formation formation nCO + (2n + 1)H2 CnH2n+2 + nH2O (R6) nCO + (2n + 1)H2→ → CnH2n+2 + nH2O (R6) Olefins formation Olefins formation nCO + 2nH (—CH —)n + nH O (R7) 2 → 2 2 Products leaving the FT reactornCO were + 2n cooledH2 → (—CH by using2—)n the+ nH three-phase2O flash drum of Aspen(R7) Plus into the water, heavy ends and light ends. Light ends were sent to a separator block, which separated the off-gasesProducts (unreacted leaving syngasthe FT reactor and C1–C4) were cooled and C5–C11by using the contents. three-phase For simplicity,flash drum of heavy Aspen ends Plus were into the water, heavy ends and light ends. Light ends were sent to a separator block, which separated refined in a column modelled using the separator block. Off-gases, C5–12 and C12–C64 contents the off-gases (unreacted syngas and C1–C4) and C5–C11 contents. For simplicity, heavy ends were were separated in the column. C12–C64 contents of FT fuels were pumped to the hydrocracker at refined in a column modelled using the separator block. Off-gases, C5–12 and C12–C64 contents were 50 bar.separated Hydrogen in the from column. off-gases C12–C64 was contents separated of FT byfuels pressure were pumped swing to adsorption the hydrocracker (PSA) at and 50 bar. sent for hydrocrackingHydrogen offrom C12–C64 off-gases contents was separated of FT fuels. by pressure The remainder swing adsorption of the off-gases (PSA) wereand sentsent backfor for combustion.hydrocracking The RSTOICH of C12–C64 block contents was of used FT fuels. to model The remainder the hydrocracker of the off-gases at 50 barwere and sent 350 back◦C. for Diesel and gasolinecombustion. were The then RSTOICH separated block andwas collectedused to model as final the hydrocracker products by at using 50 bar the and separator 350 °C. Diesel block of Aspenand Plus. gasoline were then separated and collected as final products by using the separator block of Aspen Plus. Energies 2020, 13, x FOR PEER REVIEW 9 of 22

Energies 2020, 13, 4264 9 of 22 EnergiesFigure 2020, 7.13 ,Process x FOR PEER flow REVIEW diagram of FT diesel and FT gasoline synthesis from syngas produced from9 of 22 waste gasification.

2.3.7. Ammonia Synthesis Modelling Figure 8 shows the process flow diagram of ammonia synthesis modelled in Aspen Plus. Syngas from the CFBG and DFBG contains a significant amount of methane, which needs to be reformed to H2 in the presence of steam (R8). Thus, a reformer was also modelled for the CFBG and DFBG only using the RSTOICH reactor in Aspen Plus. Carbon monoxide and carbon dioxide in syngas from the reformer was converted to hydrogen via two WGS reactors: a high-temperature shift reactor (300–450 °C) and a low-temperature shift reactor (220–250 °C). The REQUIL reactor was used to model shift reactors at adiabatic conditions with a temperature approach of 20 °C. CO2 was removed via a separator and the required heat for CO2 removal was imported from the HRSG section. Hydrogen was separated via a PSA and off-gases were sent for combustion. Hydrogen was preheated to 300 °C from vapors leaving the ammonia synthesis reactor. Nitrogen from the ASU entered the ammonia synthesis reactor. The ammonia synthesisFigureFigure reactor 7.7. Process operates flowflow diagramdiagramat 300 °C of and FTFT dieseldiesel 100 bar andand [23]. FTFT gasolinegasoline synthesissynthesis fromfrom syngassyngas producedproduced fromfrom wasteThewaste REQUIL gasification.gasification. reactor was used to model ammonia production from nitrogen and hydrogen via R9. Vapors leaving the ammonia synthesis reactor were cooled and waste heat was recovered in the 2.3.7.form2.3.7. of Ammonia steam. Ammonia Synthesis was Modelling then liquefied by condensation and unreacted vapors were separated and recycled back to the reactor. FigureFigure8 8 shows shows the the process process flow flow diagram diagram of of ammonia ammonia synthesis synthesis modelled modelled in in Aspen Aspen Plus. Plus. Syngas Syngas from the CFBG and DFBG contains a significant amount of methane, which needs to be reformed to H from the CFBG and DFBG containsCH4 + aH significant2O → CO + amount 3H2 ΔH of◦ = methane,206 kJ/mol which needs to be reformed (R8) to 2 in the presence of steam (R8). Thus, a reformer was also modelled for the CFBG and DFBG only using H2 in the presence of steam (R8). Thus, a reformer was also modelled for the CFBG and DFBG only N2 + 3H2 → 2NH3 ΔH◦ = −92.4 kJ/mol (R9) theusing RSTOICH the RSTOICH reactor reactor in Aspen in Aspen Plus. Plus. Carbon monoxide and carbon dioxide in syngas from the reformer was converted to hydrogen via two WGS reactors: a high-temperature shift reactor (300–450 °C) and a low-temperature shift reactor (220–250 °C). The REQUIL reactor was used to model shift reactors at adiabatic conditions with a temperature approach of 20 °C. CO2 was removed via a separator and the required heat for CO2 removal was imported from the HRSG section. Hydrogen was separated via a PSA and off-gases were sent for combustion. Hydrogen was preheated to 300 °C from vapors leaving the ammonia synthesis reactor. Nitrogen from the ASU entered the ammonia synthesis reactor. The ammonia synthesis reactor operates at 300 °C and 100 bar [23]. The REQUIL reactor was used to model ammonia production from nitrogen and hydrogen via R9. Vapors leaving the ammonia synthesis reactor were cooled and waste heat was recovered in the form of steam. Ammonia was then liquefied by condensation and unreacted vapors were separated and recycled back to the reactor.

CH4 + H2O → CO + 3H2 ΔH◦ = 206 kJ/mol (R8) FigureFigure 8. 8.Process Process flowflow diagramdiagram of ammonia synthesis from syngas produced produced from from waste waste gasification. gasification. N2 + 3H2 → 2NH3 ΔH◦ = −92.4 kJ/mol (R9) Carbon monoxide and carbon dioxide in syngas from the reformer was converted to hydrogen via two WGS reactors: a high-temperature shift reactor (300–450 ◦C) and a low-temperature shift reactor (220–250 ◦C). The REQUIL reactor was used to model shift reactors at adiabatic conditions with a temperature approach of 20 ◦C. CO2 was removed via a separator and the required heat for CO2 removal was imported from the HRSG section. Hydrogen was separated via a PSA and off-gases were sent for combustion. Hydrogen was preheated to 300 ◦C from vapors leaving the ammonia synthesis reactor. Nitrogen from the ASU entered the ammonia synthesis reactor. The ammonia synthesis reactor operates at 300 ◦C and 100 bar [23]. The REQUIL reactor was used to model ammonia production from nitrogen and hydrogen via R9. Vapors leaving the ammonia synthesis reactor were cooled and waste heat was recovered in the form of steam. Ammonia was then liquefied by condensation and unreacted vapors were separated and recycled back to the reactor.

CH + H O CO + 3H ∆H = 206 kJ/mol (R8) 4 2 → 2 ◦ Figure 8. Process flow diagram of ammonia synthesis from syngas produced from waste gasification. Energies 2020, 13, x FOR PEER REVIEW 10 of 22

2.3.8. Power and DH Production Modelling A combination of the Brayton and Rankine cycles was used for power and DH generation (Figure 9). Syngas at high pressure was combusted and expanded in the gas turbine. The heat from hot flue gases leaving the gas turbine was recovered in the form of superheated steam. Superheated steam was expanded in a series of steam turbines. Saturated steam was condensed to provide the DH. Operating pressure selected for the gas turbine was 24 bar [42]. The EFG operates at 24 bar, so syngas from the EFG was already at the required pressure, but syngas produced in the CFBG and DFBG was at atmospheric pressure and was therefore compressed to 24 bar. Isentropic efficiency of Energies85% and2020 ,mechanical13, 4264 efficiency of 95% were considered for the modelling of the compressor.10 of 22 Compressed syngas was combusted by using the RSTOICH reactor with combustion reactions at adiabatic conditions. The air required for combustion was estimated using the CALCULATOR block. N + 3H 2NH ∆H = 92.4 kJ/mol (R9) Air was preheated by steam in2 the HRSG2 → section.3 ◦Combustion− gases were expanded in the gas turbine. A discharge pressure of 1.07 bar was considered. Isentropic efficiency of 85% and mechanical 2.3.8. Power and DH Production Modelling efficiency of 95% were selected for the gas turbine. AThe combination heat from combustion of the Brayton gases and leaving Rankine the gas cycles turbine was was used recovered for power in andthe HRSG DH generation section in (Figurethe form9). of Syngas superheated at high pressuresteam at was70 bar combusted and 470 and°C. expandedThree heat in exchangers—an the gas turbine. economizer, The heat from an hotevaporator flue gases and leaving a superheater—were the gas turbine used was recoveredto recover in heat the in form the ofform superheated of steam. steam.Water was Superheated pumped steaminto the was economizer, expanded where in a series it was of heated steam turbines.to 220 °C. Saturated The evaporator steam wasconverted condensed the vapor-liquid to provide the stream DH. Operatingto vapor. Steam pressure was selected heated forto 470 the °C gas in turbine the supe wasrheater. 24 bar Three [42]. The different EFG operates steam pressures at 24 bar, were so syngas used fromin this the study: EFG washigh-pressure already at thesteam required at 70 pressure,bar, medium-pressure but syngas produced steam at in the17 bar CFBG and and low-pressure DFBG was atsteam atmospheric at 4 bar. Superheated pressure and steam was was therefore expanded compressed in three turbines—high-pressure, to 24 bar. Isentropic efficiency medium-pressure of 85% and mechanicaland low-pressure efficiency turbines—with of 95% were considered isentropic forefficien the modellingcies of 90%, of the 85% compressor. and 80%, Compressedrespectively, syngasand a wasmechanical combusted efficiency by using of 95% the [32]. RSTOICH The steam reactor leaving with the combustion medium-pressure reactions and at low-pressure adiabatic conditions. turbines Thewere air condensed required forto produce combustion the wasDH. estimated Waste heat using was the also CALCULATOR recovered in the block. biofuel Air synthesis was preheated section, by steamwhich inwas the introduced HRSG section. in steam Combustion turbines gasesto produce were expandedextra power. in the Drying gas turbine. of waste, A gasifier, discharge WGS, pressure CO2 ofremoval, 1.07 bar distillation was considered. columns Isentropic and biofuel efficiency synthesis of 85% also and required mechanical steam, efficiency which of was 95% exported were selected from forthe theHRSG. gas turbine.

Figure 9.9. Process flowflow diagram of power and heat producedproduced fromfrom combustioncombustion of syngassyngas producedproduced from wastewaste gasification.gasification.

2.4. PerformanceThe heat from Indicators combustion and Case gases Studies leaving the gas turbine was recovered in the HRSG section in the form of superheated steam at 70 bar and 470 ◦C. Three heat exchangers—an economizer, an evaporator and aThe superheater—were effects of the steam-to-waste used to recover ratio Equation heat in the (1) form on syngas of steam. composition Water was and pumped biofuel synthesis into the for all three gasifiers—EFG, CFBG and DFBG—were estimated and reported. Similarly, the changes economizer, where it was heated to 220 ◦C. The evaporator converted the vapor-liquid stream to in syngas composition and biofuel production with the variation of equivalence ratio (ER) were also vapor. Steam was heated to 470 ◦C in the superheater. Three different steam pressures were used inreported this study: Equation high-pressure (2). steam at 70 bar, medium-pressure steam at 17 bar and low-pressure steam at 4 bar. Superheated steam was expanded in three turbines—high-pressure, medium-pressure and low-pressure turbines—with isentropic efficiencies of 90%, 85% and 80%, respectively, and a mechanical efficiency of 95% [32]. The steam leaving the medium-pressure and low-pressure turbines were condensed to produce the DH. Waste heat was also recovered in the biofuel synthesis section, which was introduced in steam turbines to produce extra power. Drying of waste, gasifier, WGS, CO2 removal, distillation columns and biofuel synthesis also required steam, which was exported from the HRSG. Energies 2020, 13, 4264 11 of 22

2.4. Performance Indicators and Case Studies The effects of the steam-to-waste ratio Equation (1) on syngas composition and biofuel synthesis for all three gasifiers—EFG, CFBG and DFBG—were estimated and reported. Similarly, the changes in syngas composition and biofuel production with the variation of equivalence ratio (ER) were also Energies 2020, 13, x FOR PEER REVIEW 11 of 22 reported Equation (2).  kg  Total steam supplied in gasifierkg Steam Total steam supplied in gasifier ( )s Steam= s (5) waste =  kg  (5) waste Waste entered in gasifer kg Waste entered in gasifer ( s) s  Oxygen  Oxygen fuel used ER = fuel (6) ER =  Oxygen  Oxygen (6) fuel Stoichiometric fuel Four cases were designed to determine how the output and polygeneration efficiencies differ for differentFour gasifiers cases were and designed waste (Figureto determine 10). Inhow case the 1,ou antput equal and polygeneration amount of syngas efficiencies was distributed differ for to producedifferent biofuels, gasifiers power and and waste DH. (Figure Case 10). 1 was In simulatedcase 1, an equal for all amount three gasifiersof syngas with was RDFdistributed and MSW to as feed.produce In case 2,biofuels, MSW power and RDF and wereDH. Case co-gasified, 1 was simula andted syngas for all was three equally gasifiers distributed with RDF and to produceMSW as DH, feed. In case 2, MSW and RDF were co-gasified, and syngas was equally distributed to produce DH, power and biofuels. In case 3, syngas is equally distributed in the polygeneration facility to produce power and biofuels. In case 3, syngas is equally distributed in the polygeneration facility to produce two or three biofuels at a time along with DH and power. The last case considers the trigeneration of two or three biofuels at a time along with DH and power. The last case considers the trigeneration of heat,heat, power power and and one one biofuel biofuel simultaneously simultaneously from from MSWMSW and and RDF. RDF.

FigureFigure 10. 10.Description Description of of waste waste to to energyenergy polygenerationpolygeneration cases cases simulated simulated in this in thisstudy. study.

ColdCold gas egasfficiency efficiency (CGE) (CGE) of of gasifiers gasifiers was was obtainedobtained from from the the following following equation: equation:

LHV CGE (%) = LHVsyngas × 100 (7) CGE (%) = LHV 100 (7) LHVwaste feed × Energy efficiencies of the polygeneration systems were estimated using the following relations: Energy efficiencies of the polygeneration systemsDH were + power estimated + biofuels using the following relations: Energy efficiency with DH (%) = × 100 (8) Waste DH + power + biofuels ( ) = power + biofuels EnergyEnerg effiy ciencyefficienc withy without DH DH% (%) = × 100 100(9) (8) Waste × where power is the net power generated from the gas turbinepower and+ steambiofuels turbines in MW. DH is the Energy efficiency without DH (%) = 100 (9) heat recovered via steam condensation in steam turbines in MW.Waste Biofuels produced× (as MW) in each case were entered in the above equations to determine the energy efficiency of different cases.

Energies 2020, 13, x FOR PEER REVIEW 12 of 22

3. Results Energies 2020, 13, 4264 12 of 22 Simulations were performed according to the cases described in Section 2.3 and the results are presented in the following sections. First, the effect of the steam/waste feed ratio and ER on syngas wherecomposition power is is described the net power for all generated three gasifiers— from thethe gas EFG, turbine CFBG and and steam DFBG—followed turbines in MW. by the DH results is the heatof simulations recovered for via cases steam 1 condensationto 4. in steam turbines in MW. Biofuels produced (as MW) in each case were entered in the above equations to determine the energy efficiency of different cases. 3.1. Influence of the Steam/Waste Ratio and ER on Syngas Composition and Energy Efficiency of the 3.Polygeneration Results Plant SimulationsFigure 11 presents were performed the effect according of the steam/wa to the casesste describedratio and inER Section on the 2.3 syngasand the volumetric results are presentedcomposition in and the followingenergy efficiency sections. of First,the polygenera the effecttion of the process steam from/waste the feed EFG. ratio The and steam/waste ER on syngas ratio compositionis varied within is described the range for of all0.1 three to 1.0. gasifiers—the The carbon monoxide EFG, CFBG volumetric and DFBG—followed percentage decreases, by the results while of simulationsthe methane for and cases carbon 1 to 4.dioxide percentages increase with steam/waste ratio. MSW gasification produces syngas with lower carbon monoxide and hydrogen percentage than RDF gasification in the 3.1.EFG. Influence Change ofin the the Steam energy/Waste efficiency Ratio and of the ER onpolygene Syngasration Composition facility and with Energy the steam/waste Efficiency of theratio is also Polygeneration Plant estimated. Efficiency for RDF gasification is higher than for MSW gasification with the EFG because RDF Figurehas less 11 presentsmoisture the and effect higher of the steamelemental/waste carbon ratio and and ER onhydrogen the syngas content volumetric than compositionMSW. The andpolygeneration energy effi ciencyprocess of with the DH polygeneration also shows higher process energy from efficiency the EFG. Thethan steamthe polygeneration/waste ratio is process varied withinwith no the DH range output. of 0.1 Energy to 1.0. Theefficiency carbon increases monoxide up volumetric to a steam/waste percentage ratio decreases, of 0.4, whilebut changes the methane only andmarginally carbon with dioxide the percentagesfurther increase increase in steam with in steam EFG./waste ratio. MSW gasification produces syngas withThe lower ER carbon in the EFG monoxide is varied and from hydrogen 0.1 to percentage0.5, and its effect than RDFon syngas gasification volumetric in the composition EFG. Change is inpresented the energy in Figure efficiency 11. ofIn thethe polygenerationEFG, the volumetr facilityic percentages with the steam of carbon/waste ratiomonoxide is also and estimated. carbon Edioxidefficiency in forsyngas RDF increase, gasification while is higher the percentages than for MSW of hydrogen gasification and with methane the EFG decrease because with RDF increasing has less moistureER. The polygeneration and higher elemental efficiency carbon increases and hydrogen up to an content ER of than 0.2, MSW.and then The polygenerationdecreases with processfurther withincreases DH alsoin ER. shows Increases higher in energyER in the effi EFGciency initially than increase the polygeneration the energy efficiency, process with but no then DH decrease output. Energythe efficiency efficiency when increases DH is not up considered to a steam as/waste a produc ratiot; the of 0.4,polygeneration but changes efficiency only marginally does not withincrease the furtherwhen ER increase is at or inabove steam 0.2 in for EFG. EFG with DH.

Figure 11. EffectEffect of the the steam-to-waste steam-to-waste ratio and ER on syngas syngas composition composition and biofuel synthesis from the EFG. Energies 2020, 13, 4264 13 of 22

The ER in the EFG is varied from 0.1 to 0.5, and its effect on syngas volumetric composition is presented in Figure 11. In the EFG, the volumetric percentages of carbon monoxide and carbon dioxide in syngasEnergies increase, 2020, 13, x FOR while PEERthe REVIEW percentages of hydrogen and methane decrease with increasing13 of 22 ER. The polygeneration efficiency increases up to an ER of 0.2, and then decreases with further increases in Figure 12 shows the effect of the steam/waste ratio and ER on the syngas volumetric composition ER. Increasesand polygeneration in ER in the energy EFG efficiency initially from increase the CFBG the energy. Similar effi tociency, EFG, syngas but then from decrease MSW gasification the efficiency whencontains DH is not a lower considered percentage as a of product; hydrogen the than polygeneration RDF gasification effi ciencyfor the doesCFBG. not However, increase the when carbon ER is at or abovemonoxide 0.2 for percentage EFG with in DH. syngas from MSW gasification is higher than that from RDF gasification. FigureThe volumetric 12 shows percentage the effect of carbon the steam monoxide/waste ratioand methane and ER onin syngas the syngas decrease volumetric with increasing composition and polygenerationsteam/waste ratio, energy whereas effi ciencythe percentages from the of CFBG. hydrogen Similar and tocarbon EFG, dioxide syngas fromincrease. MSW There gasification is a containsmarginal a lower increase percentage in polygeneration of hydrogen energy than efficien RDFcy gasification in the CFBG for with the a CFBG.steam/waste However, ratio of the up carbonto monoxide0.4, and percentage efficiency decreases in syngas with from further MSW increases gasification in steam is content. higher than that from RDF gasification. The carbon dioxide percentage in syngas increases with the increase in ER, whereas carbon The volumetric percentage of carbon monoxide and methane in syngas decrease with increasing monoxide and methane content of syngas decrease with ER in the CFBG. The increase in hydrogen steam/waste ratio, whereas the percentages of hydrogen and carbon dioxide increase. There is a percentage with ER in the CFBG is marginal. The energetic efficiency of the polygeneration process marginaldecreases increase with inER. polygeneration Similar to the EFG, energy the energy efficiency efficiency in the of CFBGRDF gasification with a steam is higher/waste than ratio that of of up to 0.4, andMSW effi gasificationciency decreases in CFBG. with further increases in steam content.

FigureFigure 12. E 12.ffect Effect of theof the steam-to-waste steam-to-waste ratio ratio andand ER on on syngas syngas composition composition and and biofuel biofuel synthesis synthesis from from the CFBG.the CFBG.

TheThe carbon effects dioxide of the steam/waste percentage ratio in syngas on the increasessyngas volumetric with the composition increase inand ER, polygeneration whereas carbon monoxideefficiency and in methane the DFBG content are shown of syngas in Figure decrease 13. The with volumetric ER in thepercentage CFBG. of The carbon increase monoxide in hydrogen in percentagesyngas withdecreases ER in with the steam/waste CFBG is marginal. ratio, wherea Thes energetic the percentages efficiency of carbon of the dioxide polygeneration and methane process increase with the steam/waste ratio in the DFBG. Carbon dioxide and methane percentages in syngas decreases with ER. Similar to the EFG, the energy efficiency of RDF gasification is higher than that of from MSW gasification are higher than those from RDF gasification in the DFBG. Hydrogen and MSW gasification in CFBG. carbon monoxide volumetric percentages are lower in syngas from MSW gasification than from RDF Thegasification effects in of the the case steam of DFBG./waste The ratio energy on the effici syngasency of volumetric the polygeneration composition process and with polygeneration the DFBG efficiencyincreases in the with DFBG the steam/waste are shown ratio inFigure for both 13 MSW. The and volumetric RDF gasification, percentage but the of efficiency carbon monoxidetends to in syngasdecrease decreases at very with high steam steam-to-waste/waste ratio, ratios. whereas the percentages of carbon dioxide and methane increase with the steam/waste ratio in the DFBG. Carbon dioxide and methane percentages in syngas from MSW gasification are higher than those from RDF gasification in the DFBG. Hydrogen and Energies 2020, 13, 4264 14 of 22 carbon monoxide volumetric percentages are lower in syngas from MSW gasification than from RDF gasification in the case of DFBG. The energy efficiency of the polygeneration process with the DFBG increases with the steam/waste ratio for both MSW and RDF gasification, but the efficiency tends to

Energiesdecrease 2020 at, 13 very, x FOR high PEER steam-to-waste REVIEW ratios. 14 of 22

Figure 13. 13. EffectEffect of the of thesteam-to-waste steam-to-waste ratio on ratio syngas on syngascomposition composition and biofuel and synthesis biofuel from synthesis the DFBG. from the DFBG. Figures 11–13 provide the optimum steam/waste ratio and ER for the polygeneration process. TableFigures 2 shows 11– 13the provide steam/waste the optimum ratio steamand ER/waste selected ratio andfor ERthe for further the polygeneration analysis of process.various polygenerationTable2 shows the cases steam explained/waste ratio in andSection ER selected2.4 and forsyngas the further composition analysis under of various these polygeneration conditions. A steam/wastecases explained ratio in of Section 0.3 was 2.4 selected and syngas for the composition EFG, whereas under for the these CFBG conditions. and the DFBG A steam a steam/waste/waste ratio ratioof 0.3 of was 0.4 selectedwas considered. for the EFG, An ER whereas of 0.2 was for the chos CFBGen for and the the EFG DFBG and 0.1 a steam was /selectedwaste ratio for the of 0.4 CFBG. was Theconsidered. syngas volumetric An ER of 0.2composition was chosen (dry) for theof these EFG gasifiers and 0.1 wasfor both selected RDF for and the MSW CFBG. gasification The syngas is shownvolumetric in Table composition 2. Syngas (dry) from of thesethe EFG gasifiers and DFBG for both has RDF higher and hydrogen MSW gasification percentage is shown than that in Table from2. theSyngas CFBG. from The the EFG EFG produces and DFBG syngas has higher with a hydrogenhigher volumetric percentage percentage than that of from carbon the CFBG.monoxide The than EFG theproduces CFBG syngasand DFBG. with aThe higher methane volumetric percentage percentage is negl ofigible carbon in monoxidesyngas from than the the EFG, CFBG but and higher DFBG. in syngasThe methane from percentagethe CFBG and is negligible DFBG. Syngas in syngas from from the the DFBG EFG, butand higher CFBG in has syngas higher from carbon the CFBG dioxide and percentagesDFBG. Syngas than from syngas the DFBG from andthe EFG. CFBG The has CGE higher of carbongasifiers dioxide is higher percentages with RDF than waste syngas as feedstock from the thanEFG. with The CGEMSW. of gasifiers is higher with RDF waste as feedstock than with MSW.

Table 2.2. Volumetric composition of syngas from gasifiersgasifiers (the EFG, CFBG and DFBG) for both MSW and RDF gasification gasification selected for heat, power and biofuelbiofuel synthesis in polygeneration casescases 11 toto 4.4.

EFGEFG CFBGCFBG DFBG DFBG FeedstockFeedstock RDFRDF MSW MS RDFW RDF MSW MSW RDF MSW MSW SteamSteam/waste/waste ratio ratio 0.3 0.3 0.3 0.3 0.4 0.4 0.40.4 0.4 0.4 0.4 ER 0.2 0.2 0.1 0.1 - - ER 0.2 0.2 0.1 0.1 - - H2, % 54.3 49.7 42.4 40.5 55.6 54 CO, %H2, % 40.0 54.3 41.4 49.7 33.8 42.4 35.040.5 55.6 24.7 54 14.7 CO2, %CO, % 5.7 40.0 8.9 41.4 13.5 33.8 16.235.0 24.7 14.4 14.7 21.2 CH4, % negligible negligible 10.3 8.3 5.3 10.1 LHV, MJ/COkg2, % 16.4 5.7 14.2 8.9 16.2 13.5 14.116.2 14.4 16.5 21.2 15.2 CGE, %CH4, % 84.4 negligible 80.2 negligible 83.3 10.3 79.48.3 84.9 5.3 10.1 78.4 LHV, MJ/kgNote: Syngas 16.4 composition is 14.2 on a volumetric16.2 and dry14.1 basis. 16.5 15.2 CGE, % 84.4 80.2 83.3 79.4 84.9 78.4 As described above,Note: two Syngas different composition types of is waste on a volumetric feedstock—RDF and dry andbasis. MSW—were considered in this study. RDF and MSW have different moisture contents, elemental compositions and heating values.As Adescribed 500 MW above, polygeneration two different plant types was consideredof waste feedstock—RDF as a basis in this and study. MSW—were Different wasteconsidered types inand this compositions study. RDF wouldand MSW therefore have changedifferent the moisture amount contents, of waste elemental fed into the compositions 500 MW polygeneration and heating values.plant. TableA 5003 MWshows polygenera the masstion flow plant rate was of waste considered required as a in basis the 500in this MW study. polygeneration Different waste plant types for andRDF, compositions MSW and mixture would of therefore both types change of feedstock. the amount RDF of contains waste fed less into moisture the 500 and MW higher polygeneration LHV than plant. Table 3 shows the mass flow rate of waste required in the 500 MW polygeneration plant for RDF, MSW and mixture of both types of feedstock. RDF contains less moisture and higher LHV than MSW, so less RDF is required than MSW. Similarly, when a mixture of RDF and MSW is fed into the polygeneration plant, the total amount is higher than for 100% RDF but lower than for 100% MSW.

Energies 2020, 13, 4264 15 of 22

MSW, so less RDF is required than MSW. Similarly, when a mixture of RDF and MSW is fed into the polygeneration plant, the total amount is higher than for 100% RDF but lower than for 100% MSW.

Energies 2020, 13, x FOR PEER REVIEW 15 of 22 Table 3. Mass flow rate of waste (kg/s) required for a 500 MW polygeneration plant for different types

and compositionTable 3. Mass offlow wastes. rate of waste (kg/s) required for a 500 MW polygeneration plant for different types and composition of wastes. RDF, % MSW, % Mass Flow of Waste Equivalent to 500 MW, kg/s RDF,100 % MSW, % 0 Mass Flow of Waste Equivalent 23.25 to 500 MW, kg/s 0100 0 100 23.25 40.88 750 100 25 40.88 26 50 50 29.6 2575 25 75 26 34.4 50 50 29.6 25 75 34.4 Cases 1 to 4 for polygeneration plants (as described in Section 2.4) were simulated under the conditionsCases presented 1 to 4 for in Tablepolygeneration2. The following plants (as sections described present in Section and discuss2.4) were the simulated simulation under results the for all polygenerationconditions presented cases. in Table 2. The following sections present and discuss the simulation results for all polygeneration cases. 3.2. Case 1 Results 3.2. Case 1 Results In case 1, a polygeneration facility of 500 MW was simulated. An equal amount of syngas In case 1, a polygeneration facility of 500 MW was simulated. An equal amount of syngas generated was transferred to produce power and various biofuels (methane, methanol/DME, FT fuels generated was transferred to produce power and various biofuels (methane, methanol/DME, FT fuels and ammonia). The results are shown in Figure 14. The EFG produces more power than the CFBG and and ammonia). The results are shown in Figure 14. The EFG produces more power than the CFBG DFBG.and Methane DFBG. Methane production production from the from CFBG the CFBG and DFBG and DFBG is higher is higher than than from from the EFG.the EFG. One One reason reason for the higherfor methane the higher production methane production in the CFBG in the and CFBG DFBG and DFBG is the is higher the higher methane methane content content in in the the syngas syngas from thesefrom gasifiers. these Thegasifiers. CFBG The and CFBG DFBG and produce DFBG produc similare similar amounts amounts of methane of methane biofuel biofuel from from RDF, RDF, but the DFBGbut produces the DFBG more produces methane more biofuel methane than biofuel the CFBGthan the from CFBG MSW. fromAmmonia MSW. Ammonia production production is higher is in the DFBGhigher than in the in DFBG the EFG than and in the CFBG. EFG and Methanol CFBG. /Methanol/DMEDME production production in the CFBGin the CFBG and DFBG and DFBG is higher thanis in higher the EFG, than whereas in the EFG, FT whereas diesel and FT diesel FT gasoline and FT productiongasoline production is similar is similar for all for gasifiers. all gasifiers. The EFG showsThe the EFG highest shows overall the highest polygeneration overall polygeneration efficiency efficiency when the when low-temperature the low-temperature excess excess heat is heat utilized is utilized for DH, followed by the DFBG and the CFBG. The DFBG shows higher polygeneration for DH, followed by the DFBG and the CFBG. The DFBG shows higher polygeneration efficiency when efficiency when DH is not considered as an output. Overall, polygeneration efficiency for RDF DH is not considered as an output. Overall, polygeneration efficiency for RDF gasification is higher gasification is higher than that for MSW gasification, mainly due to lower moisture and high carbon than thatand hydrogen for MSW content. gasification, mainly due to lower moisture and high carbon and hydrogen content.

FigureFigure 14. Power, 14. Power, biofuels biofuels and and heat heat distribution distribution from RDF RDF and and MSW MSW gasification gasification in three in three gasifiers gasifiers (EFG,(EFG, CFBG; CFBG; and and DFBG). DFBG). Feed Feed= 500= 500 MW; MW; an an equalequal amount of of syngas syngas was was transferred transferred for biofuels for biofuels and and powerpower production production in each in each case. case.

Energies 2020, 13, 4264 16 of 22

3.3. CaseEnergies 2 Results2020, 13, x FOR PEER REVIEW 16 of 22 In3.3. case Case 2, 2 polygenerationResults processes with feed comprising a mix of RDF and MSW were simulated. Three sub-casesIn case 2, with polygeneration 75% RDF/25% processes MSW, with 50% feed RDF comprising/50% MSW, a mix and of 25% RDF RDF and/75% MSW MSW were were considered.simulated. Syngas Three generatedsub-cases with from 75% the RDF/25% mixed-feed MSW, gasification 50% RDF/50% was MSW, transferred and 25% equallyRDF/75% to MSW produce powerwere and considered. various biofuels Syngas generated (methane, from methanol the mixe/DME,d-feed FT gasification fuels, and was ammonia). transferred Polygeneration equally to efficiencyproduce with power and without and various DH was biofuels also estimated. (methane, The methanol/DME, results are shown FT in fuels, Figure and 15. Aammonia). large amount of powerPolygeneration and DH is efficiency produced with with and mixed-feed without DH gasification was also estimated. with 75% The RDF results/25% are MSW shown fraction, in Figure whereas 25% RDF15. A/ 75%large MSW amount result of power in lower and powerDH is produced output. Similarly,with mixed-feed biofuels gasification production with also 75% decreases RDF/25% with MSW fraction, whereas 25% RDF/75% MSW result in lower power output. Similarly, biofuels the increase in MSW content in mixed feed. The DFBG shows greater efficiency when only biofuels production also decreases with the increase in MSW content in mixed feed. The DFBG shows greater and power are produced, whereas the EFG shows higher efficiency when DH is also considered along efficiency when only biofuels and power are produced, whereas the EFG shows higher efficiency with biofuelswhen DH and is also power. considered along with biofuels and power.

FigureFigure 15. Power,15. Power, biofuels biofuels and and heatheat distribution for for gasification gasification of mixed of mixed RDF and RDF MSW and feed MSW in the feed in the threethree gasifiersgasifiers (EFG,(EFG, CFBG CFBG and and DFBG). DFBG). Total Total feed feed = =500500 MW; MW; an anequal equal amount amount of syngas of syngas was was transferredtransferred for biofuelsfor biofuels and and power power production production in in eacheach case.

3.4. Case3.4. Case 3 Results 3 Results In caseIn case 3, the 3, the polygeneration polygeneration plantplant produced produced only only 2 or 2 3 or biofuels 3 biofuels along along with DH with and DH power. and Six power. Six didifferentfferent subcases were were simulated, simulated, as described as described in Figure in 10 Figure and Section 10 and 2.4. Section The results 2.4 .are The presented results are in Figure 16. The waste polygeneration plant producing methane, methanol/DME, power and DH presented in Figure 16. The waste polygeneration plant producing methane, methanol/DME, power shows the highest energetic return, followed by the plant producing methane, FT fuels, power and and DH shows the highest energetic return, followed by the plant producing methane, FT fuels, power DH, and the plant producing methane, ammonia, power and DH. Polygeneration plants with the and DH,CFBG and or theDFBG plant show producing higher energy methane, efficiency ammonia, than those power with and the DH.EFG Polygenerationwhen no DH is considered. plants with the CFBGPolygeneration or DFBG show efficiencies higher energyare similar effi forciency all gasi thanfiersthose when withlow-temperature the EFG when waste no heat DH is recovered is considered. Polygenerationas DH alongside efficiencies power and are biofuels similar forproduction. all gasifiers The polygeneration when low-temperature efficiency reaches waste heatup to is 80% recovered in as DHcases alongside with DH, power power, and methane biofuels and production. methanol/DME, The or polygeneration DH, power, methane, efficiency and reachesFT fuels upwith to the 80% in casesEFG with and DH, DFBG. power, methane and methanol/DME, or DH, power, methane, and FT fuels with the EFG and DFBG. Energies 2020, 13, 4264 17 of 22 Energies 2020,, 13,, xx FORFOR PEERPEER REVIEWREVIEW 17 of 22

Figure 16. Polygeneration of power, heat and two biofuels from RDF and MSW gasification in the three Figure 16. Polygeneration of power, heat and two biofuels from RDF and MSW gasification in the gasifiers (EFG, CFBG and DFBG). Total feed = 500 MW; an equal amount of syngas was transferred for three gasifiers (EFG, CFBG and DFBG). Total feed = 500 MW; an equal amount of syngas was the production of power and the two biofuels in each case. transferred for the production of power and the two biofuels in each case. 3.5. Case 4 Results 3.5. Case 4 Results In this last case, the trigeneration plant with waste gasification was simulated. Figure 17 shows In this last case, the trigeneration plant with waste gasification was simulated. Figure 17 shows the simulation results. Trigeneration of methane with power and DH achieve the highest energy the simulation results. Trigeneration of methane with power and DH achieve the highest energy efficiency, followed by methanol/DME, ammonia and FT fuels with DH and power. The EFG shows efficiency, followed by methanol/DME, ammonia and FT fuels with DH and power. The EFG shows efficient trigeneration of methane, DH and power, and ammonia, DH and power. The DFBG shows efficient trigeneration of methane, DH and power, and ammonia, DH and power. The DFBG shows higher energy efficiency for the trigeneration of methanol, DH and power, and FT fuels, DH and power. higher energy efficiency for the trigeneration of methanol, DH and power, and FT fuels, DH and When DH is not considered, the DFBG and CFBG show better performance than the EFG. Similar to power. When DH is not considered, the DFBG and CFBG show better performance than the EFG. cases 1–3, RDF shows better results than MSW when used for the gasification feed. Similar to cases 1–3, RDF shows better results than MSW when used for the gasification feed.

FigureFigure 17. 17. TrigenerationTrigeneration ofof power,power, heatheat and biofuel fr fromom RDF RDF and and MSW MSW gasification gasification in in the the three three gasifiersgasifiers (EFG, (EFG, CFBG CFBG and and DFBG). DFBG). Total Total feed feed= =500 500 MW; MW; an an equal equal amount amount of of syngas syngas was was transferred transferred for thefor production the production of power of power and and two two biofuels biofuels in each in each case. case. Energies 2020, 13, 4264 18 of 22

3.6. Carbon Dioxide Emissions Table4 shows the carbon dioxide emissions in tonne /MJ of waste feed for the gasifiers—EFG, CFBG and DFBG. RDF has higher cumulative carbon content than MSW; therefore, the polygeneration plant has higher specific carbon dioxide emissions with RDF as feedstock. Among the gasifiers, EFG and DFBG produce slightly higher carbon dioxide emissions than the CFBG.

Table 4. CO2 emissions as t/MJ of feed with different gasifiers for a 500 MW polygeneration plant.

CO2 Emissions, Tonne/MJ Feed Feed EFG CFBG DFBG RDF 37.38 35.52 37.06 MSW 31.6 31.2 31.2

4. Discussion This section discusses some of the limitations of this study. The polygeneration processes studied in this paper are simulated through the commercially available software Aspen Plus. The modelling of waste gasification requires a set of assumptions, such as for syngas composition, which produced uncertainties in the results. In addition, the models for waste gasification were developed by considering equilibrium conditions due to a lack of information in the literature regarding the reaction kinetics for MSW and RDF gasification. Thus, the reliability of simulation results from gasification requires checking and validation. CGE was selected to compare the results in this study with the published literature. According to Meerman et al. [23], the CGE of gasification obtained from the model should be in line with the literature value of 79–83%. The CGE of gasification in our models is in the range of 79–84%, which is consistent with this reported range. This study considers the properties of waste as constant for steady-state simulations, but waste is actually heterogeneous, and the properties of MSW and RDF vary in different regions and at different times of year, which alters the actual operation of the gasification process. Gasification of waste and biomass also produces tar in syngas, which needs to be removed before further processing of syngas to biofuels. For simplicity, this study excludes tar production. Similarly, the removal of sulfur and carbon dioxide was not modelled. Although the DFBG shows higher biofuel conversion efficiencies than the CFBG and EFG, it requires two reactors, which may increase the required capital and operating costs. However, the EFG and CFBG require an ASU to produce nitrogen-free syngas, which increases their capital expenses. The EFG requires liquid feedstock (such as black liquor) or very fine-sized feed. For biomass-based feedstock, Meijdan et al. [33] recommend torrefaction as a pretreatment before the EFG. This study considered only grinding and crushing of waste to less than 2 mm as a pretreatment. However, the use of torrefaction as a pretreatment may also increase the capital and operating expenses. The study used gasification of MSW and RDF as a base thermochemical process for the polygeneration of biofuels, DH and power; however, pyrolysis of waste is an alternative thermochemical process that can convert waste into biofuels, heat and power. Polygeneration systems comprise a complex set of multiple processes for the synthesis of biofuels alongside heat and power. The demand for bioenergy products is dependent on local conditions and demand, e.g., DH is not a potential product in countries with a warm climate. Similarly, the production of biofuels is dependent on the energy mix of the country. Thus, the primary aim of the study was to perform a system analysis of waste-based polygeneration plants and report optimum conditions and gasifier reactors for waste conversion to energy. Further optimization with specific country and regional case studies and economic analyses will be the next step towards waste gasification-based polygeneration processes. Energies 2020, 13, 4264 19 of 22

5. Conclusions This study provides the process-modelling framework for waste gasification-based flexible polygeneration facilities for the production of heat, power and various biofuels. The technical performance was analyzed for three different gasifiers—the EFG, the CFBG and the DFBG. Various combinations of products were assessed through different case studies. The main conclusions are as follows. The properties of waste are critical for their conversion into useful bioproducts. Waste with high moisture and ash content (such as MSW) require a significant amount of energy for preprocessing such as drying and grinding. The lower carbon and hydrogen content of MSW also decreases the overall efficiency of the polygeneration process. Processing of MSW into RDF significantly improves the system technical performance, as RDF has lower moisture and ash content as well as higher carbon and hydrogen content and high LHV. In terms of waste gasification, the steam/waste ratio and ER is decisive for the production of syngas with high heating value. Different gasifiers have different optimum conditions for producing high-quality syngas. The EFG shows the best energetic performance when DH is also produced in the polygeneration process alongside power and biofuels. The DFBG and CFBG show better polygeneration efficiency when only power and biofuels are produced. The polygeneration process with 2 or 3 biofuels alongside DH and power shows better overall performance than the polygeneration process with 5 biofuels, DH and power. Overall, modelling and simulation results from waste polygeneration processes show that this is a promising option for treating and reducing MSW. However, further analyses, such as the economic potential of the polygeneration process, uncertainty analysis and determination of optimum products, will provide a clearer picture of waste-based polygeneration systems. Furthermore, the financial implications of these polygeneration facilities to treat waste must be studied and compared for different regions and scenarios. The flexibility of these processes in terms of full- and part-load operation must also be assessed in future work.

Author Contributions: Both authors contributed equally to the manuscript. Conceptualization, C.A.S. and C.B.O.; Data curation, C.B.O.; Formal analysis, C.B.O.; Methodology, C.A.S.; Software, C.A.S.; Visualization, C.A.S. and C.B.O.; Writing—original draft, C.A.S and C.B.O. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Swedish Knowledge Foundation (20120276) (KKS) and the co-production partners within the framework Future Energy: ABB, Castellum and the VEMM group (VafabMiljö, Eskilstuna Energi och Miljö, and Mälarenergi). Bilal Omer also want to acknowledge support from KAPCO for data used in this study. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

AGR Acid gas removal ASF Anderson–Schultz–Flory ASU Air separation unit CFBG Circulating fluidized bed gasifier CGE Cold gas efficiency DFBG Dual fluidized bed gasifier DH District heating DME Dimethyl ether DSTWU Shortcut distillation column block in Aspen Plus EFG Entrained flow gasifier ER Equivalence ratio FT Fischer Tropsch Energies 2020, 13, 4264 20 of 22

HHV Higher heating value HP High pressure HRSG Heat recovery steam generation LHV Lower heating value LP Low pressure MDEA N-methyl diethanolamine MP Medium pressure MSW Municipal solid waste PSA Pressure swing adsorption RDF Refuse derived fuel REQUIL Aspen Plus reactor block based on equilibrium reactions RGIBBS Aspen Plus reactor block based on minimizing Gibbs energy RSTOICH Aspen Plus reactor block based on known stoichiometric reactions RYIELD Aspen Plus reactor block based on component yields WGS Water–gas shift

References

1. Bhaskar, T.; Pandey, A.; Rene, E.; Tsang, D. Waste Biorefinery Integrating Biorefineries for Waste Valorisation; Elsevier: Amsterdam, The Netherlands, 2020; Volume 53. [CrossRef] 2. Istrate, I.-R.; Iribarren, D.; Galvez-Martos, J.; Dufour, J. Review of life-cycle environmental consequences of waste-to-energy solutions on the municipal solid waste management system. Resour. Conserv. Recycl. 2020, 157, 104778. [CrossRef] 3. Moharir, R.V.; Gautam, P.; Kumar, S. Waste Treatment Processes/Technologies for Energy Recovery. In Current Developments in Biotechnology and Bioengineering; Elsevier: Amsterdam, The Netherlands, 2019; pp. 53–77. [CrossRef] 4. Giugliano, M.; Ranzi, E. Thermal Treatments of Waste. In Waste to Energy (WtE); Elsevier Inc.: Amsterdam, The Netherlands, 2016. [CrossRef] 5. Chen, H.; Jiang, W.; Yang, Y.; Yang, Y.; Man, X. Global trends of municipal solid waste research from 1997 to 2014 using bibliometric analysis. J. Air Waste Manag. Assoc. 2015, 65, 1161–1170. [CrossRef][PubMed] 6. Miandad, R.; Rehan, M.; Barakat, M.A.; Aburiazaiza, A.S.; Khan, H.; Ismail, I.M.I.; Dhavamani, J.; Gardy, J.; Hassanpour, A.; Nizami, A.-S. Catalytic Pyrolysis of Plastic Waste: Moving Toward Pyrolysis Based Biorefineries. Front. Energy Res. 2019, 7, 1–17. [CrossRef] 7. Bioenergy IEA. Gasification of Waste for Energy Carriers; IEA: Paris, France, 2018. 8. Whiting, K.; Wood, S.; Fanning, M. Waste Technologies: Waste to Energy Facilities—A Report for the Strategic Waste Infrastructure Planning (SWIP) Working Group. Available online: http://large.stanford.edu/courses/ 2017/ph240/kim-d2/docs/wsp-may13.pdf (accessed on 5 July 2020). 9. Segurado, R.; Pereira, S.; Correia, D.; Costa, M. Techno-economic analysis of a trigeneration system based on biomass gasification. Renew. Sustain. Energy Rev. 2019, 103, 501–514. [CrossRef] 10. Jana, K.; Ray, A.; Majoumerd, M.M.; Assadi, M.; De, S. Polygeneration as a future sustainable energy solution—A comprehensive review. Appl. Energy 2017, 202, 88–111. [CrossRef] 11. Ciuta, S.; Tsiamis, D.; Castaldi, M.J. Gasif Waste Mater Technol Gener Energy, Gas, Chem from Munic Solid Waste, Biomass, Nonrecycled Plast Sludges, Wet Solid Wastes. In Critical Development Needs; Elsevier: Amsterdam, The Netherlands, 2017; pp. 121–141. [CrossRef] 12. Gootz, M.; Forman, C.; Wolfersdorf, C.; Meyer, B. Polygeneration-Annex: Combining lignite-based power generation and syngas-based chemicals production. Fuel 2017, 203, 989–996. [CrossRef] 13. Buttler, A.; Kunze, C.; Spliethoff, H. IGCC–EPI: Decentralized concept of a highly load-flexible IGCC power plant for excess power integration. Appl. Energy 2013, 104, 869–879. [CrossRef] 14. Yu, G.-W.; Xu, Y.-Y.; Hao, X.; Li, Y.-W.; Liu, G.-Q. Process analysis for polygeneration of Fischer–Tropsch

liquids and power with CO2 capture based on coal gasification. Fuel 2010, 89, 1070–1076. [CrossRef] 15. Narvaez, A.; Chadwick, D.; Kershenbaum, L. Small-medium scale polygeneration systems: Methanol and power production. Appl. Energy 2014, 113, 1109–1117. [CrossRef] 16. Li, Y.; Zhang, G.; Yang, Y.; Zhai, D.; Zhang, K.; Xu, G. Thermodynamic analysis of a coal-based polygeneration system with partial gasification. Energy 2014, 72, 201–214. [CrossRef] Energies 2020, 13, 4264 21 of 22

17. Guo, Z.; Wang, Q.; Fang, M.; Luo, Z.; Cen, K. Thermodynamic and economic analysis of polygeneration system integrating atmospheric pressure coal pyrolysis technology with circulating fluidized bed power plant. Appl. Energy 2014, 113, 1301–1314. [CrossRef] 18. Li, M.; Zhuang, Y.; Zhang, L.; Liu, L.; Du, J.; Shen, S. Conceptual design and techno-economic analysis for a coal-to-SNG/methanol polygeneration process in series and parallel reactors with integration of waste heat recovery. Energy Convers. Manag. 2020, 214, 112890. [CrossRef] 19. Huang, H.; Yang, S.; Cui, P. Design concept for coal-based polygeneration processes of chemicals and power

with the lowest energy consumption for CO2 capture. Energy Convers. Manag. 2018, 157, 186–194. [CrossRef] 20. Heinze, C.; May, J.; Peters, J.; Ströhle, J.; Epple, B. Techno-economic assessment of polygeneration based on fluidized bed gasification. Fuel 2019, 250, 285–291. [CrossRef] 21. Kim, S.; Kim, M.; Kim, Y.T.; Kwak, G.; Kim, J. Techno-economic evaluation of the integrated polygeneration system of methanol, power and heat production from coke oven gas. Energy Convers. Manag. 2019, 182, 240–250. [CrossRef] 22. Fan, J.; Hong, H.; Jin, H. Biomass and coal co-feed power and SNG polygeneration with chemical looping combustion to reduce carbon footprint for sustainable energy development: Process simulation and thermodynamic assessment. Renew. Energy 2018, 125, 260–269. [CrossRef] 23. Meerman, H.; Ramírez, A.; Turkenburg, W.; Faaij, A. Performance of simulated flexible integrated gasification polygeneration facilities. Part A: A technical-energetic assessment. Renew. Sustain. Energy Rev. 2011, 15, 2563–2587. [CrossRef] 24. Sahoo, U.; Kumar, R.; Pant, P.; Chaudhary, R. Development of an innovative polygeneration process in hybrid solar-biomass system for combined power, cooling and desalination. Appl. Therm. Eng. 2017, 120, 560–567. [CrossRef] 25. Jana, K.; De, S. Polygeneration using agricultural waste: Thermodynamic and economic feasibility study. Renew. Energy 2015, 74, 648–660. [CrossRef] 26. Jana, K.; De, S. Sustainable polygeneration design and assessment through combined thermodynamic, economic and environmental analysis. Energy 2015, 91, 540–555. [CrossRef] 27. Gładysz, P.; Saari, J.; Czarnowska, L. Thermo-ecological cost analysis of cogeneration and polygeneration energy systems—Case study for thermal conversion of biomass. Renew. Energy 2020, 145, 1748–1760. [CrossRef] 28. Villarroel-Schneider, J.; Mainali, B.; Martí-Herrero, J.; Malmquist, A.; Martin, A.; Alejo, L. Biogas based polygeneration plant options utilizing dairy farms waste: A Bolivian case. Sustain. Energy Technol. Assess. 2020, 37, 100571. [CrossRef]

29. Jana, K.; De, S. Environmental impact of an agro-waste based polygeneration without and with CO2 storage: Life cycle assessment approach. Bioresour. Technol. 2016, 216, 931–940. [CrossRef][PubMed] 30. De La Fuente, E.; Martín, M. Optimal CSP-Waste Based Polygeneration Coupling for Constant Power Production; Elsevier BV: Amsterdam, The Netherlands, 2019; Volume 46, pp. 1645–1650. 31. McKendry, P. Energy production from biomass (Part 2): Conversion technologies. Bioresour. Technol. 2002, 83, 47–54. [CrossRef] 32. Heyne, S.; Thunman, H.; Harvey, S. Exergy-based comparison of indirect and direct biomass gasification technologies within the framework of bio-SNG production. Biomass Convers. Biorefin. 2013, 3, 337–352. [CrossRef] 33. Van Der Meijden, C.M.; Veringa, H.J.; Rabou, L.P. The production of synthetic natural gas (SNG): A comparison of three wood gasification systems for energy balance and overall efficiency. Biomass Bioenergy 2010, 34, 302–311. [CrossRef] 34. Gassner, M.; Wang, L. Thermo-economic process model for thermochemical production of Synthetic Natural Gas (SNG) from lignocellulosic biomass. Biomass Bioenergy 2009, 33, 1587–1604. [CrossRef] 35. Tungalag, A.; Lee, B.; Yadav, M.; Akande, O. Yield prediction of MSW gasification including minor species through ASPEN plus simulation. Energy 2020, 198, 117296. [CrossRef] 36. Buckley, T.J. Calculation of higher heating values of biomass materials and waste components from elemental analyses. Resour. Conserv. Recycl. 1991, 5, 329–341. [CrossRef] 37. Özyu˘guran,A.; Yaman, S.; Küçükbayrak, S. Prediction of calorific value of biomass based on elemental analysis. Int. Adv. Res. Eng. J. 2018, 2, 254–260. Energies 2020, 13, 4264 22 of 22

38. Aspen Technology, Inc. Aspen Plus. USA. 2020. Available online: https://www.aspentech.com/ (accessed on 25 March 2020). 39. Aspentech. Getting Started Modeling Processes with Solids; Aspen Technology: Burlington, VT, USA, 2013. 40. Salman, C.A. Techno Economic Analysis of Wood Pyrolysis in Sweden; KTH Stockholm: Stockholm, Sweden, 2014. 41. Harris, D.; Roberts, D. Coal Gasification and Conversion; Elsevier BV: Amsterdam, The Netherlands, 2013; Volume 2, pp. 427–454. 42. Li, K.; Cousins, A.; Yu, H.; Feron, P.H.; Tade, M.; Luo, W.; Chen, J. Systematic study of aqueous

monoethanolamine-based CO2 capture process: Model development and process improvement. Energy Sci. Eng. 2015, 4, 23–39. [CrossRef] 43. Heyne, S.; Thunman, H.; Harvey, S. Extending existing combined heat and power plants for synthetic natural gas production. Int. J. Energy Res. 2011, 36, 670–681. [CrossRef] 44. Clausen, L.R.; Elmegaard, B.; Ahrenfeldt, J.; Henriksen, U.B. Thermodynamic analysis of small-scale dimethyl ether (DME) and methanol plants based on the efficient two-stage gasifier. Energy 2011, 36, 5805–5814. [CrossRef] 45. Pondini, M.; Ebert, M. Process Synthesis and Design of Low Temperature Fischer-Tropsch Crude Production from Biomass Derived Syngas. Master’s Thesis, Chalmers University of Technology, Göteborg, Sweden, 2013. 46. Song, H.-S.; Ramkrishna, R.; Trinh, S.; Wright, H. Operating strategies for Fischer-Tropsch reactors: A model-directed study. Korean J. Chem. Eng. 2004, 21, 308–317. [CrossRef]

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