CET Volume 86 DOI: 10.3303/CET2186032 Paper Received: 23 August 2020; Revised: 23 January 2021; Accepted: 29 April 2021 Please cite this article as: Mirzaei N., Toffolo A., Engvall K., Kantarelis E., 2021, Flexible Production of Liquid Biofuels via Thermochemical Treatment of Biomass and Olefins Oligomerization: a Process Study, Chemical Engineering Transactions, 86, 187-192 DOI:10.3303/CET2186032 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 Flexible Production of Liquid Biofuels via Thermochemical Treatment of Biomass and Olefins Oligomerization: a Process Study Nima Mirzaeia, Andrea Toffolob, Klas Engvalla, Efthymios Kantarelisa,* a KTH Royal Institute of Technology, Department of Chemical Engineering, Teknikringen 42, 10044, Stockholm, Sweden b Luleå University of Technology, Department of Engineering Sciences & Mathematics, University Campus, Porsön, 97187, Luleå, Sweden ekan@kth.se The use of residual biomass streams for production of liquid fuels will help in achieving the goal of renewable transportation fuels and sustainable society. Hence, efficient, and reliable processes offering a flexible product distribution proven at commercial scale are required. This study explores the technical feasibility of producing gasoline and diesel range hydrocarbons from thermochemical processing of biomass via the production of light olefins (C2-C4) and their subsequent oligomerization through mathematical modelling and simulation using MATLAB software. Different biomass processing scenarios were considered, including a standalone biomass gasification plant and integrated biomass pyrolysis- char gasification (O2- or air-blown) process. Process analysis indicated that the integrated plant offers 10-11% higher carbon efficiency. The processing step with the highest carbon penalty for all the cases is the syngas composition tailoring via water-gas shift reaction. 1. Introduction Transition to a bio-based economy entails the development of innovative and highly efficient processes based on renewable materials. In Sweden, this transformation urges the production of millions of cubic meters of bio- based transportation fuels (Eriksson 2017). At the same time, the fuel type (gasoline or diesel) demand is expected to fluctuate considerably, and thus different fuel mix (gasoline and diesel) will be required. Thermochemical conversion of biomass to transportation fuels via catalytic deoxygenation of pyrolysis liquids results in low overall carbon efficiencies (Venderbosch 2015); in addition, Fischer-Tropsch synthesis (FT) exhibits low carbon efficiency to specific fuel cuts due to the unfavourable Anderson-Schulz-Flory (ASF) product distribution (Abelló et al.,2011). The production of highly valuable intermediates such as light olefins (C2-C4) would allow the selective production of diesel and/or gasoline at desired proportions by adjusting the operating conditions, as demonstrated by Mobil’s Olefins to Gasoline and Distillate process (MOGD) (Keil, 1999). MOGD shows great selectivity to gasoline and diesel (>95%) and is versatile (Tabak et al. 1986). The product composition varies from <1% to 83% diesel with the rest being gasoline (Tabak et al. 1990). It is mainly iso- paraffinic (after hydrotreatment) and has a very high cetane number, when compared to the typical FT process which produces low octane normal paraffins (Wei et al., 2017). Recent developments indicate that direct synthesis of lower olefins from syngas is achieved with high selectivity without the limitations of the ASF product distribution (Jiao et al. 2016), offering higher carbon efficiencies. Separation of pyrolysis and gasification steps in an integrated plant can offer higher carbon efficiency (Rytter et al. 2013). Light olefins are of high demand in the chemical industry which adds extra product flexibility considering that in the future the transportation fleet will be electrified. All the above suggest that the direct conversion of pyrolysis vapors and syngas to olefins is a more carbon efficient route and thus, is a promising alternative for conversion of biomass to liquids fuels. 187 In this study, the technical feasibility of gasoline and diesel range hydrocarbons production via oligomerization of olefins synthesized from biomass-derived syngas, is investigated through mathematical modelling and simulation using MATLAB software. Different scenarios with biomass and/or bio-char feeding were regarded for meeting different proportions of product (gasoline or diesel) demand based on Swedish forecast for 2030. 2. Methods Two different process layouts were considered, based on biomass gasification, or combined biomass pyrolysis- char gasification process, coupled to syngas and/or biooil-to-olefins conversion followed by the MOGD process. The two simplified layouts are shown in Figure 1. Biomass/char gasification is followed by gas cleaning and conditioning (water-gas shift reaction -WGS), direct syngas-to-olefins synthesis, and subsequent olefins oligomerization (MOGD process). Figure 1: Simplified process flow diagram of biomass gasification and MOGD process (top) and combined pyrolysis-gasification and MOGD process (bottom). The fuel type (fossil-based Gasoline and Diesel) demand ratio (G/D) for the years 2020 and 2030 was determined based on statistics for transportation fuel consumption, assuming a linear projection for the year 2030 without accounting for any other market parameters (e.g., electrification etc.) (Figure 2). Three process scenarios were investigated with the thermochemical treatment of the biomass being the differing point, namely, O2-blown biomass gasification in entrained-flow gasifier; biomass pyrolysis-char air gasification in fluidized-bed reactor; biomass pyrolysis-char O2 gasification in fluidized-bed reactor (Table 1). 188 Figure 2: Fossil-based gasoline and diesel demand in Sweden with projection for the year 2030 Table 1: Different Scenarios investigated Biomass Process G/D Notes O2-blown gasification of biomass 2020 Entrained-flow gasification (T>1000 °C) O2-blown gasification of biomass 2030 Entrained-flow gasification (T>1000 °C) Biomass pyrolysis-air gasification of char 2020 Fluidized-bed gasification (T< 1000 °C) Biomass pyrolysis-air gasification of char 2030 Fluidized-bed gasification (T< 1000 °C) Biomass pyrolysis-O2 gasification of char 2020 Fluidized-bed gasification (T< 1000 °C) Biomass pyrolysis-O2 gasification of char 2030 Fluidized-bed gasification (T< 1000 °C) Gasification of biomass and/or char was modelled using a constrained non-stoichiometric Gibbs minimization model with temperature approach for methane reforming and tuned with the data from a pilot plant in Sweden (Cortus Energy). Contaminants such as H2S, COS, NH3 and HCN were included in the model while the tar was not considered. The latter holds as a valid assumption given the high operating temperature of the gasifier (>1000 °C) in the biomass gasification scenarios; in addition, the gasification of the volatile-stripped char in the integrated pyrolysis-gasification case should not produce any tars. Producer gas cleaning section included high- and low- temperature sulfur removal units using ZnO for H2S and COS removal and hydrolysis. H2/CO tailoring for olefins synthesis was achieved by sorption enhanced sour water-gas shift (WGS) reactor (Mobed et al. 2014). The combination of WGS catalyst with CO2 sorbents has shown itself to be an efficient method to improve conversion with simultaneous CO2 capture (Abbasi et al. 2014). A heterogeneous plug flow packed bed model was employed for the WGS reactor. Gas separation units (for plant capacities 5-50 MWth biomass) were based on pressure- and vacuum swing adsorption processes and were modelled using multi-component Langmuir adsorption models. The limitation of 50MWth biomass was considered due to biomass logistics constraints (Haarlemmer et al. 2014). Syngas-to-olefins process used a pseudo homogeneous plug flow model based on kinetic expressions from Do et al. (2020), while biomass pyrolysis model used experimental data from Garcia-Perez et al. (2008) for fast pyrolysis conditions (fast heating rates and short (<2s) residence time). The bio-oil to olefins process was based on data by Gong et al., (2011). Oligomerization of olefins considered compounds from C2 to C20.The oligomerization model was based on (Alberty, 1987) while isomer group properties were estimated based on Alberty’s model (Alberty,1987). C7-C11 fraction was considered as gasoline cut and C12 to C20 as diesel cut. The different scenarios and process layouts were evaluated with regards to carbon efficiency (ηC) and well as to energy efficiency based on biomass HHV (ηEHHV), defined as: ηΕHHV= mgasoline and dieselHHVgasoline and dieselmbiomassHHVbiomass (1) The overall energy efficiency includes the electricity requirements for gas compression and any additional heat for the process. ηΕoveral= mgasoline and dieselHHVgasoline and diesel mbiomassHHVbiomass+Electricity+Heat (2) 189 3. Results and Discussion It was found that gasification operation at 2.5 bar was enough to overcome pressure drop of the system until the syngas-to-olefins section which operated at 1.5 bar. It is assumed that the slightly increased pressure does not affect the pyrolysis reactor yields. The obtained carbon efficiencies for the different scenarios are shown in Figure 3. Figure 3: Carbon and energy efficiencies (based on biomass HHV) to gasoline and diesel for the different scenarios studied. As seen, the combined pyrolysis-gasification route offers a carbon effciency of ~40%, while for the biomass gasfication route this is approximately 29%. The efficiency achieved for the biomass gasfication route is typical for reported Fischer-Tropsch processes using entrained-flow gasfication for syngas generation with the exception that the olefins generation from the syngas here is performed at considerably lower pressure while the product compositon can be readiliy alterred. The combined pyrolysis-gasification route offers higher efficieny. Τhe difference of the two routes lies in the utilization of gases from conversion of bio-oil to olefins and also to the high carbon penalty one has to pay tailoring the H2/CO for olefins synthesis from the syngas (Figure 4). Indeed, more than 25% of biomass contained carbon is lost to CO2 during CO shifting(WGS Losses). Therefore, external renewable H2 supply can further improve overall carbon effciencies. It has to be noted that the final MOGD product is still subject to hydrotreatment and thus additional H2 will be required. The H2 needed for MOGD product for full hydrogenation and hydrotreatment of bio-oil are shown in Table 2. Table 2: H2 requirements for complete hydrogenation of products Biomass Process G/D H2 Requirements (kgH2/m 3 product) O2-blown gasification of biomass 2020 8.6 – 25.7 O2-blown gasification of biomass 2030 7.4 – 22.1 Biomass pyrolysis-air gasification of char 2020 8.3 – 24.9 Biomass pyrolysis-air gasification of char 2030 7.5 – 22.6 Biomass pyrolysis-O2 gasification of char 2020 8.4 – 25.1 Biomass pyrolysis-O2 gasification of char 2030 7.5– 22.4 Bio-oil hydrotreatment 62 – 80 190 The amount of H2 is determined assuming purely mono-olefinic or purely tri-olefinc composition of the product (Tabak et al. 1990). Carbon efficiencies of ~35% have been reported for pyrolysis-hydrotreatment processing route (Venderbosch 2015), however the H2 requirements for complete hydrogenation are high. Figure 4: Carbon flow for the O2-blown biomass gasification (left) and the combined biomass pyrolysis- O2 gasification (right) for the 2030 scenario. The energy efficiency based on biomass energy content, as expected, follows the carbon effieciency trends exceeding the 50% mark (for the MOGD product) for the integrated plant. In all cases process integration resulted in heat self-suffiecnt process by combustiing the off-gases. A simple steam Rankine cycle (one evaporation pressure level and one condensation pressure level) was chosen to generate electricity from the excess heat (Table 3). The electricity requirements for compression and separation are higher for the O2-blown gasification due to the air separation unit. Table 3: Electricity production and requirements and energy efficiency of the scenarios investigated. Process G/D El. Requirements for compression (kWe/ton biom. daf) El. production (kWe/ton biom daf) Overall energy efficiency (%) O2-blown gasification of biomass 2020 975.6 617.5 37.1 O2-blown gasification of biomass 2030 973.9 612.8 36.3 Biomass pyrolysis-air gasification of char 2020 423.1 321.5 53.4 Biomass pyrolysis-air gasification of char 2030 450.6 321.1 52.4 Biomass pyrolysis-O2 gasification of char 2020 502.0 457.9 52.0 Biomass pyrolysis-O2 gasification of char 2030 502.0 465.4 51.9 4. Conclusions In this study, the conversion of biomass-derived syngas and pyrolysis vapours to liquid fuels with an integrated stepwise process targeting for the production of gasoline and diesel via the selective MOGD has been studied. Separation of pyrolysis and gasification steps result in similar carbon losses to direct gasification, but the reduced need for syngas tailoring improves the overall carbon conversion. WGS as syngas tailoring step is the stage with the highest carbon penalty both for the standalone gasification as well as the integrated process with carbon losses greater than 25% of the total carbon. Thus, external renewable hydrogen supply would improve the overall efficiency considerably. The targeted production of light olefins in the standalone gasification route results in carbon efficiencies comparable (but lower) to direct FT synthesis without the use of high pressure. An increase of ~10-11% in carbon efficiency is achieved when biomass pyrolysis and char gasification are combined due to the utilisation of gases from bio-oil to olefins process. 191 Acknowledgments The authors would like to thank Cortus Energy for the pilot plant data and help with the gasifier modelling and Swedish Energy Agency and f3 Swedish Knowledge Centre for Renewable Transportation Fuels for funding this work. References Abbasi E., Hassanzadeh A., Zarghami S., Arastoopour H., Abbasian J.,2014, Regenerable MgO-based sorbent for high temperature CO2 removal from syngas: 3. CO2 capture and sorbent enhanced water gas shift reaction, Fuel, 137,260-268 Abelló S., Montané D., 2011, Exploring Iron-based Multifunctional Catalysts for Fischer-Tropsch Synthesis: A Review, ChemSusChem. 4, 1538–1556. Alberty R.A.,1987, Kinetics of the polymerization of alkenes on zeolites, Journal of Chemical Physics, 87, 3660-3667. Alberty R.A.,1987, Thermodynamics of the catalytic polymerization of alkenes in the gas phase, Chemical Engineering Science, 42, 2325-2330. Do T. N., Kim J., 2020, Green C2-C4 hydrocarbon production through direct CO2 hydrogenation with renewable hydrogen: Process development and techno-economic analysis, Energy conversion and Management, 214, 112866. Duan Y., Sorescu D. C., Luebke D., 2011, Efficient theoretical screening of solid sorbents for CO2 capture applications, Proceedings of 28th Ann. Int. Pittsburgh Coal Conference, Sept.12-15, 2011, Pittsburgh, PA, 1-17. Eriksson S., 2017, Preem < https://www.svebio.se/app/uploads/2017/05/Eriksson_- So%CC%88ren_ABC17.pdf > accessed 18.12.2020. Garcia-Perez M., Wang X. S., Shen J., Rhodes M. J., Tian F., Lee W.-J., Wu H., Li C.-Z., 2008, Fast Pyrolysis of Oil Mallee Woody Biomass: Effect of Temperature on the Yield and Quality of Pyrolysis Products, Industrial and Engineering Chemistry Research, 47, 1846-1854. Gong F., Yang Z., Hong, Wei C.Huang W., Ning S., Zhang Z., Xu Y., Li Q., 2011, Selective conversion of bio- oil to light olefins: Controlling catalytic cracking for maximum olefins Bioresource Technology, 102, 9247- 9254 Haarlemmer G., Boissonnet G., Peduzzi E., Setier P.-A., 2014, Investment and production costs of synthetic fuels – A literature survey, Energy, 66, 667,676. Jiao F., Li J., Pan X., Xiao J., Li H., Ma H., Wei M., Pan Y., Zhou Z., Li M., Miao S., Li J., Zhu Y., Xiao D., He T., Yang J., Qi F., Fu Q., Bao X. 2016, Selective conversion of syngas to light olefins, Science, 351, 1065- 1068. Keil F. J., 1999, Methanol-to-hydrocarbons: process technology, Microporous and Mesoporous Materials,29,49-66. Mobed P., Maddala J., Rengaswamy R., Bhattacharyya D., Turton R.,2014, Data Reconciliation and Dynamic Modeling of a Sour Water Gas Shift Reactor, Industrial and Engineering Chemistry Research, 53, 19855−19869. Rytter Ε., Ochoa-Fernández E., Fahmi A., 2013, Biomass-to-Liquids by the Fischer-Tropsch Process, Chapter Ιn: P. Imhof, Jan Cornelis van der Waal (Ed), Catalytic Process Development for Renewable Materials., Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 265–308. Tabak S. A, Avidan, A. A., Krambeck, F. J.,1986, Production of synthetic gasoline and diesel fuel from non- petroleum resources. Am. Chem. Soc., Div. Gas Fuel Chem., Prepr. American Chemical Society national meeting, New York, NY, USA, 13 Apr Tabak S.A., Yurchak S.,1990, Conversion of methanol over ZSM-5 to fuels and chemicals, Catalysis Today, 6, 307-327. Venderbosch R. H., 2015, A Critical View on Catalytic Pyrolysis of Biomass, ChemSumChem, 8,1306 –1316. Wei J., Ge Q., Yao R., Wen Z., Fang C., Guo L., Xu H., Sun J.,2017, Directly converting CO2 into a gasoline fuel, Nature Communications, 8:15174. 192