CHEMICAL ENGINEERING TRANSACTIONS VOL. 76, 2019 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar S. Varbanov, Timothy G. Walmsley, Jiří J. Klemeš, Panos Seferlis Copyright © 2019, AIDIC Servizi S.r.l. ISBN 978-88-95608-73-0; ISSN 2283-9216 Biomass Conversion to Methanol Integrating Solid Oxide Cells and Two-Stage Gasifier: Effects of Carbon Dioxide Recirculation and Pressurized Operation Giacomo Buteraa,*, Søren Højgaard Jensenb, Rasmus Østergaard Gadsbøllc, Jesper Ahrenfeldtc, Lasse Røngaard Clausena aDTU Mechanical Engineering, Technical University of Denmark, Nils Koppels Allé 403, Kgs.Lyngby, 2800, DK. bDTU Link, DynElectro, 4621, Gadstrup, DK. cDTU Chemical Engineering, Technical University of Denmark, Frederiksborgvej 399, Roskilde, 4000, DK. giabut@mek.dtu.dk Synthesis of biofuels is an important step in the phase out of fossil fuels in the transportation sector, especially in long-distance sea, air and road transport where direct electrification seems unfeasible. Integration of renewable electricity enables efficient electricity storage as well as an increased utilization of the biomass carbon, which lowers the biomass demand. This paper presents a flexible system for the conversion of biomass and electricity to methanol. The system is based on the deep integration of a Two-Stage gasifier and solid oxide cells (SOC). The integration enables efficient production of a nitrogen-free high-quality syngas, suitable for methanol production. This study focuses on the system in electrolysis mode, and analyzes the effects of recirculating CO2 from the gas conditioning and methanol synthesis process back to the SOC, as well as the effects of pressurized operation of the gasifier and increased H2O content in the gasifier. Thermodynamic modeling shows that CO2-recirculation allows an increase in conversion of the carbon in the biomass to methanol from 80 % up to 92 %, with an energy efficiency of 71 %. Only a slight pressurization seems feasible, as an increase in pressure beyond ~3 bar results in significant methane formation inside the SOC. 1. Introduction The phase out of fossil fuels from the transportation sector requires several solutions in order to be cost efficient and technically feasible. Battery electric vehicles are an important part of the solution, especially for cars, but biofuels or electro-fuels are needed for long-distance transports, for example by ships, airplanes and trucks (Mathiesen et al., 2015). Gasification represents a pathway to convert biomass to a syngas suitable for production of biofuels, such as methanol (MeOH), dimethyl ether (DME) or synthetic natural gas (SNG) - all suitable for internal combustion engines. A way to limit the consumption of biomass for biofuels is to mix the biomass syngas with electrolytic H2. This can approximately double the biofuel output per biomass input (Clausen, 2015). Among the different gasification technologies, the Two-Stage Gasifier developed at the Technical University of Denmark (DTU) has high cold gas efficiency and low tar content of the produced syngas, adopting only a bag-filter for gas cleaning (Gadsbøll et al., 2019). New concepts for the Two-Stage gasifier based on updraft fixed bed pyrolysis and fluid bed char gasification can be scaled to 50-100 MW (Gadsbøll et al., 2019), or even more if pressurized. The integration of Two-Stage gasification and solid oxide cells (SOC) has previously been studied, both for electricity production when operating as solid oxide fuel cells (SOFC) (Bang-Møller et al., 2011) or for biofuel production when operated as solid oxide electrolysis cells (SOEC) (Pozzo et al., 2015). One study has also considered both operating modes of the SOC in a polygeneration plant employing Two-Stage gasification and SOC, converting biomass to SNG (69 % efficiency) or electricity (46 % efficiency), depending on electricity prices (Sigurjonsson and Clausen, 2018). Instead of adding electrolytic hydrogen to the syngas, it is also possible to feed the SOEC directly with syngas, as suggested by (Pozzo et al., 2015). In this way, co-electrolysis is performed directly on the syngas, and it becomes much easier to change operation to SOFC mode, as the syngas flow does not need to be redirected. In this paper, the integration DOI: 10.3303/CET1976197 Paper Received: 28/02/2019; Revised: 29/04/2019; Accepted: 29/04/2019 Please cite this article as: Butera G., Jensen S.H., Gadsboll R.O., Ahrenfeldt J., Clausen L.R., 2019, Biomass Conversion to Methanol Integrating Solid Oxide Cells and Two-Stage Gasifier: Effects of Carbon Dioxide Recirculation and Pressurized Operation, Chemical Engineering Transactions, 76, 1177-1182 DOI:10.3303/CET1976197 1177 between gasifier and SOC is increased further by placing the SOC between the pyrolysis reactor and the char gasification reactor of the Two-Stage gasifier. The novel configuration features the operation of the SOC as either SOFC or SOEC, creating a system able to produce a high quality syngas for biofuels production no matter the electricity price. Compared to the paper by (Pozzo et al., 2015), this system has simple oxygen handling, as the gasifier does not need oxygen. The fluid bed char reactor is instead electrically heated with heating elements in the bed. Furthermore, when using electric heating the size of the SOC is reduced. Internal reforming in the SOC of pre-reformed tars also increases efficiency in SOFC mode. This novel concept has been named the Two-Stage Electro-Gasifier. This paper focuses on the operation in SOEC mode, as this is the main mode of operation. The syngas is used for methanol synthesis. The system, shown schematically in Figure 1, is designed and analyzed by thermodynamic modelling in energy and exergy terms. A parametric analysis has the objective of investigating the effect of varying pressure and steam content in the gasifier. An increase in pressure could possibly push towards smaller components, whereas the increase in steam content in the gasifier improves the kinetics of the gasification reactions, making the reactor smaller. A number of cases are evaluated by changing these two parameters. Furthermore, the cases will show the impact of recirculating CO2 from the acid gas removal (AGR) and the topping column in the methanol synthesis section back to the SOEC. Figure 1: Schematic view of the plant for biomass conversion to methanol, when the SOC operates as SOEC. CO2 recirculation, not adopted in the base case, is also shown. 2. Methods The thermodynamic modeling is carried out using the software Dynamic Network Analysis (DNA) (Elmegaard, 1999) for the gasification section and AspenPlus from AspenTech® for the methanol synthesis plant, where PR- BM equations of states are used for the syngas processing and SR-POLAR equations of states are used for the methanol synthesis and distillation and for the recirculation (Clausen, 2015). 2.1 Plant description and modelling Wet wood chips are dried using superheated steam in an updraft fixed bed. The generated steam is cleaned for particles and added to the gas downstream to provide the H2O for both electrolysis and the gasification reactions. The dry biomass from the dryer undergoes pyrolysis and is cleaned for particles and sulfur before mixing with steam. To avoid tar condensation in the tar-laden volatiles exiting the pyrolysis, temperature is kept above 250 °C. Sulfur removal is important to preserve the lifetime of the pre-reformer and the SOEC, as both are very sensitive to sulfur poisoning. To effectively remove sulfur, present as H2S, COS and sulfur-containing organic molecules, the combined effects of Zinc-oxide- and Copper-oxide-bed technologies can be applied (Haldor Topsøe, 2019). Furthermore, the pre-reformer will act as a secondary guard bed for the SOEC, lowering sulfur levels to sub ppm levels. Before sending the gas to the SOEC, the adiabatic pre-reformer converts tar compounds and higher hydrocarbons to methane, to prevent a fast decay of the SOEC lifetime. In the SOEC, the gas is reduced, and the methane in the gas is reformed to H2 and CO. The gas then enters the electrically heated fluid bed char gasifier, where char is gasified by the steam and CO2 content in the gas. After particle removal the syngas goes to acid gas removal (AGR) carried out by an amine wash. Dry CO2-lean syngas is compressed and sent to the methanol synthesis loop. The methanol reactor is cooled with boiling water at ~255 °C. The produced steam condenses at around the same temperature releasing heat primarily to the steam superheater in the steam dryer section. Methanol and water are condensed from the methanol reactor product gas. Most of the residual gas is recirculated to the methanol reactor (97 %). The purged gas is burned to provide high temperature heat to other sections of the plant. The liquid stream is sent to distillation, consisting of a topping column for removal of absorbed gasses, and a distillation column at atmospheric pressure for separation of methanol and water. Table 1 shows all the process parameters used to model the system. The pyrolysis process has been modeled as a balanced reaction (neither exothermic or endothermic), and an “equivalent gas Two-Stage Electro-Gasifier Syngas MeOH AshChar Electricity CO2 CO2 Dry Biomass Electricity Steam Electricity Purge Gas Burner Pyrolysis Particle removal Dryer Biomass Particle removal Sulfur removal SOEC Acid gas removal Pre reform er Char Gasifier Particle removal CO2 MeOH synthesis 1178 composition” has been calculated based on this and a normal mass (H,C,O etc.) and energy balance. The SOEC has been modeled as described in (Butera et al., 2019). 2.2 Parametric analysis Besides the modeling and analysis of a base case, a parametric analysis is performed to evaluate the effects on the system by 1) including CO2 recirculation from the AGR and the topping column, 2) increasing the steam content at the outlet of the char gasifier (10-20 mol. %) and 3) increasing the pressure. The change in pressure is done indirectly by changing the maximum allowable methane content in the syngas. The methane content will be determined by the SOEC, as the nickel containing fuel electrode catalyzes methane synthesis (and reforming). An increase in methane content will therefore correspond to an increase in pressure. Grand Composite Curves are built for each case to assess whether an external heat source is needed. Table 1: Process design parameters for the biomass to methanol conversion plant. Component Parameters Woody Biomass feed Beech wood chips (Gadsbøll et al., 2019) Dry composition [wt %]: C 48.1, O 44.8, H 6.4, N 0.081, ash 0.619. Biomass in [MWth,dry]=100 LHV [kJ/kg]=18280 Cp,dry biomass [kJ/kg∙K]=1.35 Tbiomass,in [°C]=25 Steam Dryer Tinlet,steam [°C]=250 Tout,steam,atm pressure [°C]=120 Tout,biomass [°C]=240 Tout,steam,high pressure [°C]=170 Moisture out [wt %]=2 ∆psteam dryer [mbar]=30 Pyrolysis reactor (updraft fixed bed) Char Composition [wt %]: C 90.7, O 4.5, H 2.1, N 0.2, ash 2.5. Cp,char [kJ/kg∙K]=1.276 mchar/mbiomass,dry [-]=0.25 heat loss [MW]=1 LHV [kJ/kg]=33130 Tin,recirculated volatiles [°C]=750 Tout,volatiles [°C]=250 Tout,char [°C]=740 ∆ppyrolysis [mbar]=30 Gas Cleaning ∆pparticle removal [mbar]=5 ∆psulfur removal [mbar]=20 Compressors, Blowers and Ejector ηis,compressor [-]=0.8 ηis,turbine [-]=0.85 ηis,compressor part-load [-]=0.4 ηis,blower [-]=0.4 ηel-mech [-]=0.95; ηejector [-]=0.20* Solid Oxide Cell ASRcell = 0.2** iSOEC [A/cm2]=-1 xO2,air, out [mol. %] = 50 Tfuel gas,in [°C]=750 Tfuel gas,out [°C]=750 Tair,in [°C]=700 Tair,out [°C]=750 no heat loss ∆pSOEC [mbar]=30 Chemical equilibrium at the fuel outlet. Char Gasifier (fluid bed) C conversion*** [-]=0.95 heat loss [MW]=1 Tsyngas,out [°C]=850 Tash,out [°C]=850 Tash,out [°C]=850 ∆pgasifier [mbar]=150 WGS at equilibrium at the outlet (CH4 inert). Methanol synthesis Based on (Clausen, 2015). Acid Gas Removal Heat [MJ/kgCO2,removed]=3.8 Theat for stripping column [°C]=120 xCO2,syngas out [mol. %] =1.2 xH2O,syngas out [mol. %] =0 ∆pAGR [mbar]=50 Heat exchangers ∆T/2evap-cond[°C]=2.5 ∆T/2gas[°C]=25 ∆T/2syngas-H2O-condensing[°C]=5 ∆T/2MeOH reactor[°C]=5 ∆T/2steam heater[°C]=5 ∆pHE,gasif. section [mbar]=10 ∆pHE,int. pressure[mbar]=100 ∆pHE,high pressure [mbar]=800 Burner xO2,exhausts out [mol. %] = 16 ∆pburner [mbar]=10 *Value from (Wendel et al., 2016). **Value from (Noponen et al., 2014). ASR = ASRcell + ASRinterconnects. ASRinterconnects is deliberately increased to provide heat for the electrolysis and reforming reactions. The value of ASRinterconnects is an output of the model. ***Considering the combined process of pyrolysis and gasification. 3. Results and discussion The full plant is shown in Figure 2 for the base case, where the gasification section operates at atmospheric pressure and the steam content at the outlet of the gasifier is set to 10 mol %. CO2 recirculation is not used but shown in light grey to clarify how CO2 recirculation is incorporated in the other cases. The exergy analysis performed on the base case, drawn in Figure 3, shows an exergy efficiency of 74.5 %. The analysis shows that the least efficient process from an exergetic point of view is heat transfer (10.2 %, ~11 MW). The temperature difference between fluids is the major responsible for this phenomenon and its reduction could improve the exergy efficiency. Nevertheless, larger-area and more expensive heat exchangers would be required. Table 2 includes the inputs and results from all the cases studied. At 1 and 2 mol. % CH4 content at the outlet of the SOEC, the pressure in the gasification section increases from atmospheric conditions to 2-6 bar. Furthermore, at constant CH4 molar fraction (cases B or C), it is seen that an increase in H2O content results 1179 in an increase in gasifier pressure, as higher steam content inhibits CH4 formation. An increase in pressure from atmospheric up to 6.3 bar (case C.3), results in approx. 6 times smaller components for the gasification section, which becomes attractive when pushing towards large-scale plants of 100 MWth or more of dry biomass input. The pressure increase also reduces compressor cost and power, from 14 MWel (case A.3) to 7 MWel (case C.3). The CO2-recirculation only slightly increases efficiency (70.0 % to 70.6 %) but instead increases overall carbon conversion to methanol from 79.7 % to 92.2 %. Efficiency is reduced when increasing the methane content above 1 %, as inert methane builds up in the methanol synthesis loop, leading to a higher purge gas loss. Figure 2: Full plant for the conversion of woody biomass to methanol. Data refers to the base case. (note: components in grey are not used in the base case). Note: the reactivity in the char gasifier is calculated by the method explained in (Gøbel, 1999). A high H2O content (20 %) also has a negative impact on efficiency, as there is not enough waste heat to cover the needed heat for steam drying, AGR and the distillation column reboiler. This can also be seen on the Grand Composite Curve for case B.3 (Figure 4). Here a maximum external heat source of 7.97 MW is needed to satisfy the thermal energy demand of the process. The need for an external heat source is detrimental for the efficiency of the system, as the heat is assumed to be provided by electric heating. A solution is to use a heat pump to cover the distillation column heat demand by using excess low temperature heat from the plant. This could considerably decrease the demand for external heat. When AGR heat is supplied through electric heating, the use of a high temperature heat pump to cover the AGR heat demand would improve the energy efficiency of the process even further. Table 2 also reports the char gasifier reactivity evaluated at the outlet of the gasifier. By changing the steam content from 10 mol. % to 20 mol. % the reactivity increases by ~140 %, indicating the potential size reduction of the fluid bed char gasifier. The specific size of the methanol reactor is indirectly shown by the ratio between the molar flow rate of gas at the inlet of the methanol reactor divided by the molar flow rate of product methanol. Adding CO2 recirculation does not change this ratio, as the increase in gas flow to the methanol reactor is compensated by an increase in methanol production. When increasing the CH4 content to 1-2 %, the reactor size is increased by 66 % (case C.3), as the methane builds up in the methanol synthesis loop. Burner Wet Wood (45% moisture) 100 MWth (dry) Syngas 138.3 MWth Part icles Removal 1.0 MWel Updraft fixed bed St eam Dryer 11.7 MWth Reactivi ty 1.95E-05 s -1 Ash 4.4 MWth Fluid Bed Char Gasifier Updraft fixed bed Pyrolysis Reactor Electricity Part icles Removal Acid Gas Removal Methanol Reactor Flash Tank Topping Column Distillation Column H2O H2O MeOH 111.4 MWth Air 0.0 MWel Air 9.99 1.01 3 25 6.04 85.8 20 9 12.02 85.0 23 0 6.17 79.9 40 12.02 79.9 402.62 1.06 3 30 6.04 0.80 3 30 5.58 - 24 0 1.37 - 74 0 0.17 - 85 0 5.13 1.04 3 31 5.32 1.02 3 80 0 5.13 1.01 3 25 5.32 1.01 3 30 0.0 MWel 2.55 1.05 3 75 0 6.76 1.01 8 25 0 4.41 1.06 8 25 0 4.41 1.06 3 25 0 6.76 1.06 3 26 8 11.18 1.05 3 75 0 7.02 0.85 3 30 6.94 1.01 3 90 0 2.62 1.01 3 25 4.30 1.01 3 30 4.30 1.02 3 75 0 2.62 1.05 3 70 0 SO C unit 33.5 MWel DC 8.63 1.05 3 75 0 6.94 1.02 3 75 0 m[kg/s] p[bar] T[°C] 51.18 1.06 8 25 0 55.59 1.03 8 12 0 0.2 MWel CO2 0.4 MWel 6.7 MWel 5.5 MWel 8.15 0.86 3 85 0 6.04 10 42 1 6.04 9.9 30 6.04 85.8 34 9 12.02 80.7 26 0 5.99 85.8 56 5.85 79.9 40 5.85 8.1 43 5.85 8.0 64 1.13 0.85 3 30 5.13 1.03 3 75 0 0.19 1.03 3 36 0.19 79.9 40 5.62 8.0 12 8 5.62 1.01 3 65 5.60 1.01 3 64 0.23 8.0 30 0.97 8.0 30 H2O 55.59 1.07 8 12 9 Sulfur Removal 11.18 1.06 3 26 0 6.76 1.02 3 25 0 6.76 1.08 3 26 8 Pre- reformer 1180 Figure 3: Exergy analysis for the base case. Reference temperature T0 and pressure p0 are set respectively to 25 °C and 1.013 bar. Reference environment for the chemical exergy is the Model II, Appendix C from (Bejan et al. 1999). ED and EL stand for exergy destruction and exergy loss in each process. Table 2: inputs and relevant outputs of the analyzed cases. Note that pressure is an input at atmospheric pressure when the methane content is not set, but pressure is an output when the methane content is set. Case Base A.1 A.2 A.3 B.1 B.2 B.3 C.1 C.2 C.3 Inputs xCH4,SOEC-outlet [mol. %] - - - - 1 1 1 2 2 2 xH2O,gasifier-outlet [mol. %] 10 10 15 20 10 15 20 10 15 20 CO2 recycle No Yes Yes Yes Yes Yes Yes Yes Yes Yes pSOEC-outlet [bar] 1.02 1.02 1.02 1.02 2.27 3.08 4.18 3.38 4.60 6.28 Outputs Syngas compressors [MWel] 12.1 14.0 14.0 14.0 10.4 9.3 8.4 8.9 8.0 7.1 Total power demand [MWel] 58.8 81.9 82.0 87.8 77.7 77.0 83.1 75.6 74.7 76.9 Methanol output [MWth] 111.4 128.4 128.7 128.9 125.1 124.7 124.1 120.9 119.9 118.8 AGR heat demand [MWth] 2.0 2.3 7.8 12.1 2.2 7.7 11.9 4.1 7.5 11.6 External heat source [MWth] 0.0 0.0 0.0 5.7 0.0 0.3 8.0 0.0 0.0 3.0 Efficiency [ %] 69.8 70.5 70.6 68.5 70.4 70.4 67.7 68.7 68.6 67.2 Carbon Conversion to methanol [ %] 79.7 91.8 92.1 92.2 89.5 89.2 88.7 86.4 85.8 85.0 Char gasifier reactivity (outlet) ∙105 [s-1] 1.95 1.95 3.17 4.62 1.98 3.24 4.76 2.01 3.29 4.82 Eq biomass moisture content [wt. %] 45.3 50.9 54.4 57.9 50.8 54.2 57.7 50.6 53.9 57.4 MeOH reactor size [kmolgas / kmolMeOH] 5.78 5.88 5.74 5.67 7.11 7.27 7.51 8.76 9.13 9.58 Figure 4: Grand Composite Curve for the case B.3. Air (0.1%) Wet Biomass (100%) Total Input (155%) ED (9.8%) Electricity (54.9%) EL (0.4%) H e a t T ra n s fe r ED (2.4%) T u rb o m a c h in e s ED (4.7%) S O E C ED (6.5%) P y ro ly s is a n d c le a n in g EL (4.1%) ED (4.2%) G a s if ic a ti o n ED (2.2%) S te a m D ry e r EL (0.4%) ED (0.9%) A G R ED (1.1%) M e O H r e a c to r ED (0.7%) EL (0.8%) D is ti lla ti o n B u rn e r ED (0.7%) ED (0.5%) M ix in g MeOH (115.5%) ED (0.3%) E x p a n s io n v a lv e s 7.97 MWth Methanol reactor Burner AGR Dist-Col 0 200 400 600 800 1000 1200 1400 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 S h if te d T e m p e ra tu re [ °C ] Net Heat flow [kW] 1181 4. Conclusion A novel system integrating a Two-Stage gasifier and solid oxide cells for the conversion of biomass and electricity to methanol has been proposed and analyzed by thermodynamic modelling. The effects of CO2 recirculation, pressurization and H2O content in the gasification have been evaluated. In general, the analysis showed that the system overall benefits from CO2 recirculation, as carbon conversion increases from 80 % up to 92 %, while plant efficiency is stable. The increase in pressure of the Two-Stage Electro-Gasifier decreases component sizes as well as compressor power and cost. Increasing the pressure beyond ~3 bar results in decreased carbon conversion and efficiency and increased size of the MeOH reactor, as methane is formed in the SOEC. Further analysis is needed to assess whether an increase in pressure is beneficial for the economy of the system. Increasing steam content in the char gasifier increases the reactivity, allowing it to be downsized, but when the steam content is increased beyond 15 mol. % the plant efficiency decreases. The decrease in efficiency can almost be offset by integrating a heat pump. Further experimental campaign will also investigate SOC running on pyrolysis gas, to test the SOC operation without use of the pre-reformer. 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