CET vol 99 DOI: 10.3303/CET2399106 Paper Received: 10 December 2022; Revised: 17 March 2023; Accepted: 21 April 2023 Please cite this article as: Ramos N.M., Lesak G.V., Lima Luz Junior L.F., Corazza M.L., 2023, Analysis of Syngas Production from Catalytic Biogas Reforming and Upgrading, Chemical Engineering Transactions, 99, 631-636 DOI:10.3303/CET2399106 CHEMICAL ENGINEERING TRANSACTIONS VOL. 99, 2023 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Sauro Pierucci, Flavio Manenti Copyright © 2023, AIDIC Servizi S.r.l. ISBN 978-88-95608-98-3; ISSN 2283-9216 Analysis of Syngas Production from Catalytic Biogas Reforming and Upgrading Nadia M. V. Ramos*, Giuliana V. G. Lesak, Luiz Fernando L. L. Júnior, Marcos L. Corazza Department of Chemical Engineering, Federal University of Paraná, Av. Cel. Francisco H. dos Santos, 100 - Jardim das Américas, Curitiba, Paraná, Brazil. nadia.ramos@ufpr.br The development of alternative and sustainable technologies for the production of liquid biofuels has gained ground worldwide. Biogas is one of the most potential feedstocks to be used and the Fischer-Tropsch (FT) route is one of the most studied processes for the production of these liquid biofuels. In this work, different processes for the conversion of biogas to syngas by catalytic reforming were studied, in order to evaluate the better route to obtain the syngas and its usage as feedstock in FT synthesis. Two steps were analyzed using Aspen Plus V12.1®: 1) biogas upgrading; and 2) catalytic reforming process. Dry reforming (DR) and Bi reforming (BR) were selected for the reforming process assessment. CH4 and CO2 conversions were selected to evaluate the process performance. Results showed that water scrubbing is a better route to perform biogas upgrading, based on the H2S final content and the CH4 recovery. Besides, for the biogas reforming process, all scenarios presented an increase in conversions when the temperature was increased. For bi reforming, which uses biogas and water steam as reactants, a decrease in the molar fraction of H2O contributed to higher CO2 conversion. The BR process with a molar ratio of CH4, CO2, and H2O at 3:2:1, respectively, at elevated temperatures and pressure of 0.1 MPa, showed the most promising combination of results for syngas production, considering environmental and operational factors. The CH4 and CO2 conversions obtained in 900 °C were 96.45% and 98.06%, respectively. 1. Introduction Recently, there has been growing concern about global warming and the greenhouse effect. Because of that environmental and energy solutions have been developed to contribute for a sustainable future, reducing some negative impacts caused by industrial and technologic activities (Ashraf et al., 2015). Biofuels are a fundamental element of this context. Although substantial research has been devoted to their production, there is still great potential for computer modelling and simulation of these processes. To produce biofuels, one of the most potential feedstocks to be used is biogas, composed mostly of CH4, CO2 and minor amounts of other compounds (Angelidaki et al., 2018). Meanwhile, Fischer-Tropsch (FT) synthesis is one of the most prominent processes for the production of renewable fuels (Dieterich et al., 2020). It uses syngas (a mixture of CO and H2) as a reactant and produces different hydrocarbon chains, which can be refined to some specific fuels, lubricants and other compounds (Ramos et al., 2011). The first step of FT route is the production of syngas by catalytic reforming, in this case using biogas as feedstock, mitigating greenhouse gases emissions (Zhao et al., 2020). An additional stage for cleaning biogas is required to remove some impurities of the mixture, such as H2S (Angelidaki et al., 2018; Ashraf et al., 2015). It can be conducted by physical, chemical or biological processes, such as adsorption, absorption or membrane separation (Chen et al., 2015). In the present work, processes for the biogas conversion to syngas by catalytic reforming were studied, in order to assess the better route and process configuration for the syngas production from biogas, aiming its application in the bio-fuel production by FT route. The two upgrading/reforming stages were analyzed using software Aspen Plus V12.1®: 1) biogas upgrading by water scrubbing and membrane separation; and 2) catalytic reforming process by dry reforming and bi reforming. 631 2. Methodology 2.1 Biogas Upgrading To produce syngas by the catalytic reforming of biogas, it is necessary to remove some impurities, such as H2S and in some cases CO2. Water scrubbing is a physical absorption process, and it has the ability to dissolve H2S and CO2 in a single stage (Angelidaki et al., 2018). Considering the CO2 demand for dry reforming or bi reforming processes, a water scrubbing system to remove particularly H2S from biogas was proposed using Aspen Plus V12.1®. A flowsheet of the process is presented in Figure 1. The fed biogas contains 60 % CH4, 38 % CO2, 2 % N2, 2,000 ppm H2S and 100 ppm NH3 (Ashraf et al., 2015). The electrolytic NRTL thermodynamic model with the Redlich-Kwong equation of state was selected. The absorber and stripping columns were modeled using the RadFrac model. Since the focus of this research was on the efficiency of the absorber column, pressurization and heating equipment were not considered. On the other hand, membrane technology is a competitive and compact alternative to perform biogas desulphurization. The principle relies on the selective permeability properties of membranes, allowing the separation of biogas components (Angelidaki et al., 2018). Makaruk et. al. studied a system to upgrade the biogas using a rubbery polyimide membrane of poly(amide-6-b-ethylene oxide), capable of permeate H2S. It’s selectivity for H2S/CH4 is 54 and for CO2/CH4 is 12 (Makaruk et al., 2013; Miltner et al., 2016). This membrane was used for biogas upgrading and compared with the proposed water scrubbing system on this paper. Figure 1 - Simulation flowsheet representing Water Scrubbing. 2.2 Catalytic Reforming The dry reforming (DR) is a reaction that occurs using both CO2 and CH4 reforming agent, generating CO and H2. The main reaction (1), presented in Table 1, is endothermic occurring at temperatures above 850 °C and atmospheric pressure. The DR catalysis can be operated with noble metals or nickel, supported on alumina. The biggest challenge of this technology is the catalyst deactivation, caused by coke formation (Ashraf et al., 2015; Cao et al., 2018; Zhao et al., 2020). The combination of DR with steam reforming is called bi reforming (BR), a process that uses CO2 and H2O vapor reacting with CH4 to produce syngas. It has higher endothermicity than DR, occurring at temperatures above 800 °C and pressures from 1 to 7 bar. Due to the addition of steam into the reactor, coke formation is minimized when compared to DR. Moreover, the conversion of biogas fed as a reactant to syngas is lower, when compared to DR technology (Entesari et al., 2020; Zhao et al., 2020). Table 1 shows several side reactions that take place along with the reforming processes. CH4 decomposition, Boudouard reaction, reverse water-gas shift and carbon-water reaction occur in DR (Cao et al., 2018). Meanwhile, due the presence of steam and the operating temperature above 800 °C, coke formation in BR can be neglected (Entesari et al., 2020). Thus, reactions 1, 3, 4 and 6 were dismissed for BR simulations. Table 1: Main reactions of reforming processes. Nº Reaction Description ΔH 298 K (kJ/mol) Reforming process 1 𝐶𝐻4 + 𝐶𝑂2 ↔ 2 𝐶𝑂 + 2 𝐻2 Dry reforming + 247 DR 2 3 𝐶𝐻4 + 2 𝐻2𝑂 + 𝐶𝑂2 ↔ 4 𝐶𝑂 + 8 𝐻2 Bi reforming + 659 BR 3 𝐶𝐻4 → 𝐶(𝑠) + 2 𝐻2 Methane Decomposition + 75 DR 4 2 𝐶𝑂 ↔ 𝐶(𝑠) + 𝐶𝑂2 Boudouard reaction - 171 DR 5 𝐶𝑂2 + 𝐻2 ↔ 𝐶𝑂 + 𝐻2𝑂 Reverse Water Gas Shift + 41 DR and BR 6 𝐶 + 2 𝐻2𝑂 ↔ 𝐶𝑂2 + 2 𝐻2 Carbon-Water reaction + 90 DR 632 To simulate the two catalytic reforming processes (DR and BR) using the Aspen Plus V12.1® simulator, the Peng-Robinson equation of state was selected to represent the liquid-vapor equilibrium. Also, for both processes, RGibbs equilibrium reactor was used. Two outputs were established to evaluate the systems behavior: CH4 and CO2 conversions (equations 1 and 2, respectively). The impact caused on these responses was analyzed based on changes in temperature, pressure and feed composition. The proposed scenarios are described in Table 2 and 3 for both dry reforming and bi reforming processes, respectively. 𝐶𝐻4 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = 100% ∙ 𝐶𝐻 4 𝑖𝑛 − 𝐶𝐻4 𝑜𝑢𝑡 𝐶𝐻 4 𝑖𝑛 (1) 𝐶𝑂2 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = 100% ∙ 𝐶𝑂2 𝑖𝑛 − 𝐶𝑂2 𝑜𝑢𝑡 𝐶𝑂2 𝑖𝑛 (2) Table 2: Analyzed scenarios for the dry reforming process (DR). Scenarios Temperature (ºC) Pressure (MPa) Composition (CH4:CO2) 01 400 - 1100 0.05, 0.1, 1.0, 5.0 3:2 02 400 - 1100 0.1 2:1, 3:2, 1:1 Table 3: Analyzed scenarios for the bi-reforming process (BR). Scenarios Temperature (ºC) Pressure (MPa) Composition (CH4:CO2:H2O) 01 400 - 1100 0.05, 0.1, 1.0, 5.0 3:2:2 02 400 - 1100 0.1 6:3:4, 3:2:2, 3:3:2 03 400 - 1100 0.1 3:2:1, 3:2:2, 3:2:3 3. Results and Discussions 3.1 Biogas Upgrading Table 4 shows the results of both systems proposed for biogas upgrading. For water scrubbing process, the results imply that it is possible to perform almost 99.7 % of H2S removal, assuming an initial content of 2,000 ppm. Although CH4 recovery was higher than 98 %, CO2 losses were significant, considering its demand for DR or BR processes. This can be justified by the effectiveness of water scrubbing technology to also remove CO2 from biogas, since it is more soluble in water than CH4 (Ashraf et al., 2015). To ensure the complete withdrawn of CO2 from 1,000 Nm³/hr of biogas, it will be necessary around 200 m³/hr of water (Angelidaki et al., 2018). Also, this system guarantees the full regeneration of H2O, which minimizes the environmental and economic impacts. Meanwhile, the membrane system proposed by Makaruk et. al. was also able to remove almost entirely the content of H2S. Similarly, it is possible to ensure the complete removal of CO2 by using a second stage equipped with a glassy membrane, which has higher selectivity for CO2, achieving 70 % of CH4 recovery. Considering the H2S final content on clean biogas, both processes can achieve values within the recommended limit of less than 10 ppm (Chen et al., 2015). However, water scrubbing was superior to membrane separation analyzing the CH4 recovery, which makes this process better for biogas upgrading. Nevertheless, it is necessary to study the economic and energetic perspective and extending the criteria for choosing the best process. Besides, it is crucial to consider the CO2 losses, since it is a reforming agent on both dry and bi reforming. Table 4: Simulation results for water scrubbing and theoretical results for membrane separation. Technology CO2 losses CH4 recovery H2S final content Reference Water Scrubbing 41.52 % 98.24 % 8.9 ppm This study Membrane system - 70 % < 3.3 ppm (Makaruk et al., 2013) 3.2 Catalytic Reforming Primarily, the DR was simulated in Aspen Plus V12.1®. Figure 2(a-b) depicts the results considering the effect of the pressure in CH4 and CO2 conversions, respectively. Regardless of the chosen pressure, both conversions increased with the increase in temperature due to the endothermicity of the DR, which ends up being favored at higher temperatures. Moreover, at temperatures below 700 °C, the pressure showed a significant impact on both conversions by influencing the reactions 1 and 3, presented in Table 1. Also, higher pressures changed the chemical equilibrium of the main reaction of DR towards the reactants side. Thus, higher temperatures and lower pressures have a positive impact on both CH4 and CO2 conversions. As shown in Figure 1b, the CO2 633 conversion had a different trend at temperatures within 400 °C to 700 °C, represented by a slight decrease in the profiles. At temperatures below 643 °C, the DR (reaction 1) is influenced by equilibrium limitations, decreasing the CO2 conversion. Particularly, at 1 MPa and 5 MPa, the initial trend proposes that higher pressures would provide better conversions. Since carbon-water reaction (6) is favorable at temperatures above 630 °C, it is expected a change in the equilibrium in lower conditions, consuming CO2 (Cao et al., 2018). Besides, based on Le Chatelier's Principle, the increased pressure is favorable for the balance to move in the direction of the reactants, consuming CO2 and promoting better conversions for it. A second scenario was proposed for DR. Figure 3(a-b) shows the results of the effect of feedstock composition in CH4 and CO2 conversions for DR, respectively. For the same operational conditions, higher ratios of CH4 to CO2 (2:1) decreased the CH4 conversion and increased the CO2 conversion. This behavior indicates that CO2 is the limiting reactant for the DR (reaction 1), so its feed ratio controls the conversion of the mixture. Similarly to the first scenario, the CO2 conversion showed a slight decrease at lower temperatures, also given by the spontaneity of the DR reverse reaction under these conditions. Figure 2 - Pressure effect on DR process in terms of a) CH4 conversion; and b) CO2 conversion. Figure 3 - Feedstock composition effect on DR process in terms of a) CH4 conversion; and b) CO2 conversion. Figure 4(a-b) shows the results for the pressure effect in CH4 and CO2 conversions for the BR process. Notably, the conversions behavior in BR reactions is similar to that presented in DR. Both increased with increasing temperature due to the extremely endothermic character of BR (reaction 2). Figure 4b exhibits that CO2 conversion does not reach 100 % for any of the proposed pressures, as reported by other researchers (Entesari et al., 2020). This behavior indicates that the steam shifts the chemical equilibrium of the water gas shift reaction towards the products, increasing the amount of H2 and CO2. This concept also justifies the null and negative conversion of CO2 at temperatures below 550 °C. Under these conditions, specifically below 827 °C, the direct water gas shift reaction is favored, producing more H2 and CO2 for all pressure conditions. With increasing 634 temperatures, the system tends to follow the reverse order and ends up increasing CO2 consumption. The lasts scenarios proposed for BR show the impact of the input composition in the system. Figure 5 shows the results of the CO2 composition effect in CH4 conversion for BR reactions. They were similar to those previously presented for the DR process, where all of which can reach values close to 100 % at high temperatures. Also, it is clear that higher ratios of H2O enhance the CH4 conversion, due to the thermodynamics of reaction 2. Figure 4 - Pressure effect on BR process in terms of a) CH4 conversion; and b) CO2 conversion. Figure 5 - CO2 composition effect on BR reaction in terms of a) CH4 conversion; and b) CO2 conversion. Figure 6 - H2O composition effect on BR reaction in terms of a) CH4 conversion; and b) CO2 conversion. 635 Figure 5b depicts the lower conversion of CO2, one of the biggest challenges of BR, when compared to DR. However, in Figure 6b, where only the steam feed was changed, the results showed that smaller amounts of H2O guaranteed better CO2 conversions. This can be attributed to the water gas shift reaction (WGS), linked to the Le Chatelier’s Principle. Smaller compositions of a given compound shift the chemical balance of the reaction so as to produce that same compound, which in this case is water. Consequently, the reverse WGS is shifted to the product side, consuming CO2. In opposite situations (greater amount of water), the equilibrium tends to the reactants side, to produce CO2. 4. Conclusions In this study, the production of syngas by catalytic biogas reforming and upgrading was analyzed using software Aspen Plus V12.1®. Water scrubbing and membrane separation proved to be feasible processes for biogas upgrading, considering an H2S removal above 99 % for both approaches evaluated. However, for catalytic reforming that uses biogas as a feedstock (CH4 and CO2), improvements in the recovery of these gases are necessary to minimize environmental, economic and production impacts. In this work, the H2S final content and the CH4 recovery were used to choose the best process. Since water scrubbing offered a better combination of results for it, it was superior to membrane separation for biogas upgrading. It was also possible to compare DR and BR for catalytic biogas reforming process. At high pressures and low temperatures condition (5 MPa and 600 ºC), CH4 and CO2 conversions are lower in BR (7.08 % and 1.75 %, respectively) than DR (62.06 % and 56.00 %, respectively). This suggests that the presence of steam had a negative impact in both conversions. Nevertheless, at low pressures and high temperatures, in both processes, the conversions reached values near 100 %. Especially for CO2 conversion in BR, this happened when steam feed was in smaller ratios (Figure 6b). Considering the environmental impact, BR is a better process, since it can achieve similar results to those obtained in DR, but without coke formation. The best operational condition was on scenario 03 for the BR process, at temperatures above 900 °C and CH4:CO2:H2O composition of 3:2:1. 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