CET Volume 86 DOI: 10.3303/CET2186256 Paper Received: 7 October 2020; Revised: 29 January 2021; Accepted: 1 May 2021 Please cite this article as: Im-Orb K., Arpornwichanop A., 2021, Process Analysis of an Integrated Gasification and Methanol Synthesis Process for Bio-methanol Production from Untreated and Torrefied Biomass, Chemical Engineering Transactions, 86, 1531-1536 DOI:10.3303/CET2186256 CHEMICAL ENGINEERING TRANSACTIONS VOL. 86, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-84-6; ISSN 2283-9216 Process Analysis of an Integrated Gasification and Methanol Synthesis Process for Bio-Methanol Production from Untreated and Torrefied Biomass Karittha Im-orba,*, Amornchai Arpornwichanopb aProgram in Food Process Engineering, Faculty of Food-Industry, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand bCenter of Excellence in Process and Energy Systems Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand karittha.im@kmitl.ac.th The integrated biomass gasification and methanol synthesis process is investigated in this study. The different types of biomass i.e., the untreated and torrefied biomass at 250 oC (TB250) and torrefied biomass at 300 oC (TB300) are considered feedstock. The influence of torrefying temperature on the yield and composition of raw syngas derived gasifier is investigated. The biomass processed torrefaction leads to an increase in syngas and methanol yields. Moreover, the bio-methanol production process using torrefied biomass releases lower amount of CO2 than the raw one. An energy analysis is also performed using overall energy consumption and cold gas efficiency (CGE) of the integrated process as the indicators. The TB300 offers better performance in methanol production and CO2 emission. However, it requires high energy for methanol synthesis unit and offers low CGE. 1. Introduction Energy production from biomass has been attracting a considerable amount of attention due to concern in energy security and global climate change. Methanol is an important chemical that can be used as a clean burning fuel for replacing the liquid fossil fuel without changing the existing infrastructures. It can also be used as intermediate for the synthesis of numerous chemicals. Generally, methanol is produced from syngas derived fossil fuels, i.e., partial oxidation of methane, steam reforming of natural gas, or gasification of coal (Zhang et al., 2010), which causes high CO2 emission during the process. Therefore, the bio-methanol production from syngas through biomass gasification has attracted crescent interest, especially, in agricultural countries. However, the use of biomass for gasification requires large amount of biomass because it has low energy density which sequentially results in high transportation costs. The high moisture content of raw biomass is another a topic of concern because a high energy demand is required to evaporate the contained water (Siew Ng et al., 2011). To overcome these limitations, torrefaction, a thermal pretreatment process improving the energy density, gridability and hydrophobicity of biomass, is introduced. Torrefaction is carried out in absence of oxygen at a temperature range of 200-300 °C with a residence time from few minutes to several hours. The derived products are known as torgas, consisting of light gas (i.e., CO, CO2, CH4, and H2) and other organic condensable compounds, and torrefied biomass which is the residual solid (Basu, 2013). Previously, most studies focused on the gas production from the gasification of raw biomass and torrefied biomass. Jamin et al. (2020) studied and compared the gasification performance of raw and torrefied wood wasted. They reported that the syngas yield, higher heating value (HHV), CGE and carbon conversion (CC) increased for both feedstocks when gasifying temperature increased. Tapasvi et al. (2015) found that the torrefied biomass gave higher H2 and CO contents and higher cold gas energy and exergy efficiencies than untreated biomass. Kuo et al. (2014) compared the syngas production from raw and torrefied bamboo. They found that the syngas yield increased with torrefying temperature and the torrefied bamboo at 250 °C was the most feasible fuel. Muslim et al. (2017) studied the effect of gasifying condition on the syngas production from 1531 raw empty fruit branch (EFB) and torrefied EFB. They summarized that the gasification of torrefied biomass at high temperature could enhance the H2 yield. However, the study of bio-methanol production via an integrated gasification and methanol synthesis process using torrefied biomass is limited only on untreated biomass. Bio-methanol synthesis from syngas derived from pine biomass gasification was studied over different operating conditions. The offgas separated from methanol reactor was utilized as fuel for the combustion unit to reduce the requirement of a carbon source at the gasifier (Puig-Gamero et al., 2018). Im-orb et al. (2020) performed thermodynamic analysis to investigate the performance of bio-methanol production from untreated oil palm biomass residues (i.e., trunk, frond, and EFB). They found that the maximum yield of methanol was achieved at gasifying temperature of 750 °C and equivalent ratio (ER) of 0.25 when the trunk was a feedstock. Therefore, the potential of bio-methanol from torrefied biomass is examined in this study using a process model developed in Aspen Plus V.8.8. The untreated and torrefied bamboo at different torrefying temperatures of 250 °C (TB250) and 300 °C (TB300) are used as biomass model compound. The effect of torrefying temperature on the yield of raw syngas produced from gasifier and methanol as well as the CO2 emission is firstly investigated. The energy analysis using overall energy consumption and cold gas efficiency (CGE) are also investigated. 2. Process modelling description The integrated biomass gasification and methanol synthesis process for bio-methanol production consists of three main sections (i.e., gasification, syngas cleaning and conditioning, and methanol synthesis) as shown in Figure 1. Modelling of the integrated process was done in Aspen Plus V8.8 using the untreated biomass, TB250 and TB300 as biomass model compound which their ultimate and proximate analyses are presented in Table 1 (Kuo et al., 2014). The Aspen plus model flowsheet is shown in Figure 2. 2.1 Biomass gasification process The biomass gasification model is a thermodynamic equilibrium model simulated based on the main assumptions as: (1) the process is performed under isothermal and steady state conditions, (2) pyrolysis is instantaneous, (3) char consists of only carbon, (4) ash is a non-reactive compound, and (5) tar consists of toluene, naphthalene, phenol, and pyrene. In the simulation, biomass was defined as a non-conventional component and the HCOALGEN and DCOALGEN models were used to determine the enthalpy and density of the solid biomass. Biomass was firstly converted to conventional component in RYIELD reactor (DECOMP) by identifying the yield distribution in the calculator block according to its ultimate and proximate analyses. The tar yield which was assumed to contain 65 wt% toluene, 20 wt% naphthalene, 10 wt% phenol, and 5 wt% pyrene (Sharma et al., 2017) was specified in RYIELD reactor (R-TAR). Oxygen-rich air was used as gasifying agent because it provided high concentration of major components in methanol synthesis. The gasification reactions were simulated using RGIBBS reactor (GASIF), in which the syngas composition was estimated using Gibbs free energy minimization method. The results of the developed gasification model were validated with those of published experiment (Lan et al., 2019) at the same operating conditions and the model results were match well with experimental data with a root mean square error (RMSE) of approximately 2.55% (Im- orb et al., 2020). In this study, the operating conditions of gasification were controlled at gasifying temperature of 900 °C to prevent the operational problems from tar formation (Berry et al., 2017), the ER and biomass feed rate were set at 0.25 and 0.74 kg/h, respectively. Table 1: Ultimate and proximate analyses of untreated biomass, TB250 and TB300 Biomass Ultimate Analysis (%wt dry biomass) Proximate Analysis (%wt dry biomass) C N H O S FC VM Ash HHV Raw 48.64 0.52 5.64 44.09 0.03 15.28 83.57 1.15 18.94 TB 250 56.58 0.52 5.55 35.90 0.02 25.05 70.20 1.43 20.99 TB 300 69.56 0.12 4.77 23.6 0.00 48.47 49.52 2.01 27.23 Figure 1: Simplified diagram of bio-methanol production process Gasification (CPOX) Reformer PSA Methanol synthesis Untreated or torrefied biomass O2-rich air H2S, CO2Ash Raw methanol Offgas Syngas cleaning and conditioning process 1532 Figure 2: Aspen plus model flowsheet of the Integrated gasification and methanol synthesis process 2.2 Syngas cleaning and conditioning process 2.2.1 The catalytic partial oxidation (CPOX) process The CPOX process was used to reform methane and tars to syngas (H2 and CO) via partial oxidations (Berry et al., 2017). The RTOIC reactor (REFORM) was used to simulate the CPOX process, and its operating condition was set to be similar to gasifying condition. The oxygen enriched air was supplied to the reactor until methane and tars were completely reformed. 2.2.2 Pressure swing adsorption (PSA) process The PSA process was used to remove some amount of CO2 until the ratio of syngas was achieved the feed gas specification of methanol synthesis (Eqs(1)-(2)) (Hernandez et al., 2016). PSA process consisted of three sub- units which simulated using SEP blocks (PSA1, PSA2 and PSA3) based on the real operating conditions of 35 oC, 3.03 MPa. The syngas was adjusted to these operating conditions through the multistage compressors (COMP1, COMP2 and COMP3). The separation percentage of considered components was specified based on actual plant data (Puig-Gamero et al., 2018). The pressure of syngas satisfying methanol synthesis specification from PSA process was increased through compressor (COMP4) to achieve the operating pressure of 5.07 MPa for methanol synthesis. 2 2 2 2.1 H CO CO CO − ≈ + (1) 2 2.4 2.5 H CO = − (2) 2.3 Bio-methanol synthesis process The methanol reactor containing CuO-ZnO-AlO-based catalyst was simulated using REQUIL reactor (MEOHSYN). The derived product was calculated based on the chemical equilibriums of the reactions given in Eqs(3)-(5). It was noted that the accuracy of the model should be improved by using reaction kinetics for industrial scale up purpose. The operating condition was maintained at 220 °C and 50 atm to ensure that the catalysts were active, and the heat of reaction was effectively used (Rafael et al., 2018). The product from MEOHSYN reactor was cooled down to 25 °C via COOLER1 and the generated offgas was separated from raw bio-methanol at METSEP. The separated offgas was recycle to MEOHSYN to enhance the methanol yield. 2 32CO H CH OH+ ↔ (3) 2 2 2CO H CO H O+ ↔ + (4) Gasification section Syngas cleaning and conditioning section Methanol synthesis section DECOMP GASIF R-T AR SEP-1 MIX-1 REFORM SEP-2 COMP1 SEP-3 PSA1 V1 COMP2 PSA2 V2 PSA3 V3 SPLIT 2 MIXER1 COMP3 SPLIT 1 MIXER2 COMP4 MEOHSYN V4 COOLER1 MET SEP V5 MIXER3GUARDBED V6 SPLIT 3 HEAT ER1 V7 MIXER4 SPLIT 4 S101 S102 S201 S106 S103 S104 S105 S107 S108 S202 S109 S110-CHA S111 S112 S113-H2O S114-H2 S115 S116 S117 S118-CO S119 S120-CO S121-CO2 S123-CO2 S124-CO2 S125 S126 S127 S128 S129 S131 S134 S135 S136 S137 S138 S139 S140 S141 S142 S143 S144-VEN S138-1 S139-1 S144 AT M 1533 2 2 3 23CO H CH OH H O+ ↔ + (5) 3. Energy analysis The energy requirement of the overall methanol production process as well as individual unit using different types of biomass was investigated. The overall energy consumption was determined by sum of energy demanded in every single unit. The CGE of the system reveals the conversion efficiency of biomass to raw methanol was defined as a ratio of the LHV of the methanol product and biomass input (Eq(6)) Muslim et al. (2017). MEOH MEOH Biomass Biomass m LHV CGE m LHV = (6) 4. Results and discussions 4.1 Effect of torrefying temperature on raw syngas production Figure 3 shows the concentration of each component, yield and H2/CO ratio of the raw syngas produced from gasification of raw biomass, TB250 and TB300. It indicates that syngas yield increased as torrefying temperature increased. The concentration of H2 of raw syngas deriving from each biomass was not different, whereas that of CO from TB300 showed the highest value followed by TB250 and untreated biomass due to its high carbon content. As a result, the H2/CO ratio of raw TB300-syngas exhibited the highest value. Moreover, the concentration of CO2 was found to decrease as torrefying temperature increases. However, the yield of TB300-syngas after cleaning and conditioning to satisfy the specification of methanol synthesis decreased and showed the lowest value compared to others (Figure 4). The concentration of H2 and CO manifested the same trend as those of raw syngas. 4.2 Performance comparison of methanol production from untreated and torrefied biomass Methanol production, CO2 emission, overall energy consumption and CGE of the integrated biomass gasification and methanol synthesis process is summarized in Table 2. Although the TB300 offered the lowest amount of feed gas for methanol synthesis, it provided the highest yield of bio-methanol due to the high concentration of H2 and CO in feed gas. Moreover, the use of TB300 also had the least environmental impact based on CO2 emission. Regarding the energy analysis, the integrated biomass gasification and methanol synthesis process was an exothermic process. The use of TB250 led to the highest energy consumption because it produced largest amount of feed gas for bio-methanol synthesis, hence, the highest energy was required at compressor 4 and methanol reactor (Table 3). For the CGE, TB300 had a higher HHV than TB250 and untreated biomass, therefore, it had the lowest CGE of approximately 28 % followed by TB250 and untreated biomass, respectively. Figure 3: Composition, yield, and H2/CO ratio of syngas derived from gasifier 1534 Figure 4: Composition, yield, and H2, CO2 and CO ratio of methanol synthesis feed gas derived from gas cleaning and conditioning processes Table 2: Performance comparison of methanol production from untreated and torrefied biomass Performance indicator Raw TB250 TB300 Bio-methanol production (kmol/h) 0.00702 0.00781 0.00843 CO2 emission (kmol/h) 0.00686 0.00475 0.00057 Overall energy consumption (KW) 3.25863 3.66957 3.50999 CGE (%) 35.06 34.60 27.99 Table 3: Energy consumption of each operating unit in methanol production process from untreated and torrefied biomass Performance indicator Raw TB250 TB300 Gasifying temperature (°C) 900 900 900 Equivalent ratio (ER) 0.25 0.25 0.25 Biomass feed rate (kg/h) 0.74 0.74 0.74 Methanol reactor temperature (°C) 220 220 220 Methanol reactor pressure (MPa) 1.013 1.013 1.013 Energy consumption (KW) Gasifier 0.1574 0.2405 0.1771 CPOX -1.4168 -1.2103 -0.8331 Compressor 1 0.5816 0.6388 0.6813 PSA1 0.0014 0.0013 0.0010 Compressor 2 0.1106 0.1329 0.1577 PSA2 0.0005 0.0004 0.0000 PSA3 0.0002 0.0002 0.0001 Compressor 3 0.0865 0.1184 0.1504 Compressor 4 0.3811 0.4242 0.2890 MeOH reactor 0.4604 0.5126 0.4468 Cooler1 0.1804 0.2006 0.1534 Heater1 -0.0018 -0.0018 -0.0014 5. Conclusions The performance of the integrated biomass gasification and methanol synthesis process for bio-methanol production was investigated using the model developed in Aspen Plus V.8.8. The untreated biomass, TB250, and TB300 were used as feedstock. The yield of raw syngas leaving gasifier which controlled the gasifying 1535 condition at 900 °C, 1.013 MPa and ER at 2.50 increased as torrefying temperature increased. The TB300 offers the highest yield raw syngas but the opposite trend was observed when the raw syngas was cleaned and conditioned to meet the specification of methanol synthesis. However, the use of TB300 still offered the highest yield of methanol due to the high concentration of H2 and CO in the feed gas. The energy analysis indicated that the use of TB250 required the largest energy and the use of TB300 offered the lowest CGE. Regarding an environmental impact, the bio-methanol production from TB300 showed the best performance. 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