DOI: 10.3303/CET24111113 Paper Received: 15 March 2024; Revised: 06 June 2024; Accepted: 15 May 2024 Please cite this article as: Mavukwana A.-E., Aktas F., Burra K.G., Sempuga C., Gupta A.K., 2024, Effect of Spent Fluid Catalytic Cracking Catalysts on Syngas Yield during CO2-Assisted Gasification of Solid Waste, Chemical Engineering Transactions, 111, 673-678 DOI:10.3303/CET24111113 CHEMICAL ENGINEERING TRANSACTIONS VOL. 111, 2024 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Valerio Cozzani, Bruno Fabiano, Genserik Reniers Copyright © 2024, AIDIC Servizi S.r.l. ISBN 979-12-81206-11-3; ISSN 2283-9216 Effect of Spent Fluid Catalytic Cracking Catalysts on Syngas Yield during CO2-Assisted Gasification of Solid Waste Athi-enkosi Mavukwanaa,b, Fatih Aktasa,c, Kiran G Burraa, Celestin Sempugab, Ashwani K Guptaa aThe Combustion Laboratory, University of Maryland, Department of Mechanical Engineering, College Park, MD, USA. bDepartment of Chemical Engineering, College of Science, Engineering, and Technology, University of South Africa (UNISA), c/o Christiaan de Wet & Pioneer Avenue, Florida Campus 1710, Johannesburg, South Africa. cDepartment of Mechanical Engineering, Faculty of Engineering, Gazi University, 06570 Maltepe, Ankara, Türkiye. mavukae@unisa.ac.za This research examines the synergistic effects of using CO2, spent fluid cracking catalyst (sFCC), and municipal solid wastes (MSW) for energy and chemical production. The effect of utilizing sFCC catalysts on syngas yield and its quality and energy yield is examined during CO2-assisted gasification of MSW components, pinewood, polyethylene terephthalate (PET), polystyrene, and waste tyres. The influence of catalyst position (in-situ and quasi-in-situ) and temperature were also investigated. In all cases, the sFCC catalyst resulted in doubling the yield of hydrogen and tripling methane which reveals that the spent FCC catalyst promoted the hydrocracking and CO2 reforming of hydrocarbons in the gas phase. As such the presence of sFCC in-situ catalytic CO2- assisted gasification increased the overall syngas yield by 62.5%, for PET, by 55% for waste tyres, 3.5 % for polystyrene and no change for wood when compared to their respective non-catalytic gasification cases. A comparison of quasi-in-situ gasification to in-situ catalytic gasification showed that quasi-in-situ gasification increased syngas yield and energy yields of pinewood by 21% and 22%, respectively. For PET, the quasi-in- situ catalytic gasification increased the syngas yield and energy by 27.5% and 23.8%, respectively. In the case of waste tyres, the syngas yield and energy yields increased by 24% and 23%, respectively. For polystyrene, the quasi-in-situ catalytic gasification increased the syngas yield and energy by 103.4% and 62.5% as compared to the in-situ catalytic gasification case. This implies that quasi-in-situ catalytic gasification is more efficient and effective at increasing the syngas yield. When comparing the carbonaceous sources, materials with higher volatile matter such as polystyrene had the highest overall syngas yield compared to other materials. Materials that have a higher tendency to form char had the least increase in syngas yield and energy due to soot and coke formation over the catalyst. 1. Introduction In the USA, over 60% of the 292.8 million tons of municipal solid waste generated annually is biomass. Currently, only 34.6 million tons of MSW are processed through energy recovery in 75 facilities across the United States, which accounts for only about 12 % of available waste (EPA, 2020). The problem is that existing facilities are faced with severe pollutants emission challenges requiring energy-intensive flue gas cleaning, offering low energy recovery, and high investment costs which results in marginal to subpar economic viability, especially in competition with low-cost landfilling (Abubakar et al., 2022). Gasification is a good technology that can transform organic waste into useful energy and other chemical products (Peres et al., 2013). Synthetic gas which is a mixture of carbon monoxide and hydrogen is the most used raw material for the production of energy and many useful products. Converting biomass to synthetic gas seems a good practical method of making use of the large energy content available in the organic portion of waste in MSW (Figueroa et al., 2013). The predominant challenge with gasification is that there is a significant generation of tar and char, which significantly affects the gas yield and composition thus reducing the efficiency of the process (Ruoppolo et al., 2013). This work explores the effects of using the spent fluid cracking catalyst (sFCC) during the gasification of MSW, to increase the yield 673 mailto:mavukae@unisa.ac.za of syngas through the thermal decomposition of tars. In the petroleum sector, the sFCC catalyst is utilized as a hydrocracking, hydro refining, and catalytic reforming catalyst to improve the output of gasoline and other hydrocarbons derived from crude oil (Wang et al., 2019, 2021). The sFCC catalyst is discarded when it degrades so that it remains the largest source of solid waste in the petroleum industry. Owing to its substantial metal impurity level, sFCC is considered hazardous waste. This sFCC catalyst is still active (Bertero et al., 2019) and the metallic oxides can function as reaction catalysts for pyrolysis and gasification (Mastry et al., 2023). Pinewood, polyethylene terephthalate (PET), polystyrene, and waste tyres are used as representatives of the organic waste of MSW whereas CO2 is used as a gasification agent. The MSW components are gasified under non-catalytic, in-situ, and quasi-in-situ catalytic configurations using a fixed-bed reactor in a carbon dioxide atmosphere at temperatures of 800 ℃ and 900 ℃ to study the effect of catalyst positioning on syngas yield. The distribution of gaseous products as well as the amount of energy produced by the various gasification configurations were analyzed and compared. There are many studies where sFCC is used to improve the yield of bio-oil during pyrolysis of different materials in pyrolysis (Heo et al., 2023; Johansson et al., 2023; Mendoza- Martinez et al., 2023). However, to the best of the authors' knowledge, no study has been found that investigates the effect of sFCC catalyst on syngas yield during the CO2-assist gasification of four different solid waste materials. This study aims to unravel the potential of using CO2 greenhouse gas in gasification in an optimized catalytic route to enhance energy yield while also providing waste management. 2. Methodology 2.1 Materials Pinewood pellets, waste tyres, recycled polyethylene terephthalate (PET), and polystyrene pellets were used as a representation of MSW. All the samples were ground down to a maximum size of 1000 µm with an industrial blender. The proximate and ultimate analyses of materials used in this test are shown in Table 1. The spent fluid cracking catalyst (sFCC) was a commercially available catalyst sourced from the largest chemicals manufacturer in the USA. The catalyst composition included zeolites, silicon dioxide, aluminium oxide, kaolin, calcined, magnesium oxide, aluminium calcium oxide, as well as small amounts of metal oxides and rare earth oxides. The study of catalytic and non-catalytic CO2-assisted gasification of MSW was conducted at 800 ℃ and 900 ℃. For all catalytic tests the organic MSW sample was kept constant at 10 g and the catalyst at 2 g. For the in-situ catalytic studies, 2 g of catalyst was uniformly distributed in 10 g of the MSW sample, while for the quasi- in-situ catalytic experiment, the catalyst bed was placed about 8 mm downstream of the feedstock. 2.2 Reactor Facility Figure 1. A schematic diagram of the reactor test facility A lab-scale reactor facility was used for the gasification of all four types of samples, see Figure 1. The preheater and the fixed-bed reactor were both electrically heated. Before starting the experiment, Argon gas was used to flush the system from any residual gases present from the previous test run. Nitrogen (N2) was used as the tracer gas due to its inert nature which assisted in analyzing the various gases produced during the reaction quantitatively. For gasification, a mixture of 75% CO2 and 25% N2 was used as the gasifying agent. The temperature was set at 800 or 900 ℃ for both the preheater and main reactor. The feedstock, mentioned in the 674 previous section, was only introduced into the reactor once the set temperature reached the desired value. The gases produced from the reactor were first passed through the condensers placed in an ice bath to remove tar, water, and other condensable vapours. The dry and tar-free gases were then passed through a filter to remove any particulates, such as soot or some other particulates. Finally, the gases were examined using a micro-GC to analyze the mole fractions of H2, CO, CO2, CH4, C2H4, C2H6, C2H2, C3H8, and N2. The gases were analyzed at 0.5, 1, 2, 3, and 4-minute time intervals from the start of the experiment, and then at 2.7-minute intervals from 5 minutes onwards up to 50 minutes. All the experiments were conducted three times to ensure the consistency of the results. The mean values from the 3 tests are reported here. Table 1. Ultimate and Proximate analysis of carbon feedstocks (Liu et al., 2020). Sample Ultimate analysis (wt.%) Proximate analysis (wt.%) C H O N Cl S Moisture VM FC ash LHV (kJ/kg) Pinewood 48.72 6.52 44.41 0.23 0 0.12 0 85.6 14.2 0.2 17.65 Waste tyre 81.85 6.66 8.42 1.7 0 1.37 0 62.5 27.9 8.9 33.33 PET 62.57 4.4 33.03 0 0 0 0 94.4 5.6 0 22.04 Polystyrene 92.8 7.2 0 0 0 0 0 99.9 0.1 0 11.022 3. Results and discussion 3.1 Temporal evolution of major syngas components (a) (b) (c) (d) Figure 2. Temporal evolution of (a) H2, (b) CO, (c) CH4, and (d) C2Hx flowrates during non-catalytic and catalytic- in-situ CO2-gasification of pinewood, waste tyres, PET, and polystyrene. Figure 2 shows the transient behaviour of syngas during non-catalytic and in-situ catalytic gasification of pinewood, PET, waste tyres, and polystyrene. The characteristics of the thermal decomposition behaviour of the materials are compared at 900 ℃ temperature. It is evident from the results that direct in-situ gasification using the sFCC catalyst promoted the thermal cracking and dehydrogenation reactions of the larger hydrocarbons leading to increased H2 yield for the materials studied. Polystyrene with nearly 100% volatile matter and the largest composition of H2, as shown in Table 1 produced the highest peak of H2 followed by PET 675 and wood, and lastly the waste tyres. Figure 2(a) shows that the peak height of H2 evolution doubled from 0.04 g/min to 0.08 g/min with sFCC addition for polystyrene. For PET, tyres, and wood, the H2 peak height increased from 0.02 g/min to about 0.034 g/min, and the temporal evolution of H2 terminated at about 15 minutes instead of 11 minutes observed during the non-catalytic case. The CO evolution peak is shown Figure 2(b). During PET gasification, adding sFCC increases the peak height CO evolution by nearly 56%, signifying an increase in CO yield. However, adding sFCC to polystyrene-CO2-gasification there is a sharper decrease in CO peak compared to that of CO2 gasification only. A similar behaviour is observed for wood, while sFCC didn’t affect waste tyres by much. This means that sFCC inhibits the influence of the Boudouard reactions that produce CO. Figure 2(c) shows that the presence of sFCC catalyst increased the CH4 production since larger hydrocarbons are reformed/cracked into smaller (light) hydrocarbons, including methane. CH4 peak height tripled for PET and quadrupled for polystyrene when sFCC was added, whereas for tyres the CH4 peak increased by 33.33.%. For both wood and tyres in the absence of a catalyst, the CH4 production terminated at 10 minutes but in the presence of sFCC the CH4 production terminated after 15 minutes, meaning that the volatiles that would leave the reactor are cracked in the presence of sFCC. Hence CO2 and sFCC catalysts aided the production of CH4. However, the opposite was observed for C2Hx (Figure 2(d)) wherein the sFCC catalyst significantly reduced the peak height of C2Hx by nearly 50%. Thus, reveals that sFCC facilitated the reforming of C2 and above hydrocarbons but not CH4. Figure 3 shows that the addition to sFCC improved the cumulative syngas yield and syngas energy for all materials with polystyrene and wood outperforming PET and waste tyres. This is because, under thermal treatment, PET and tyres will decompose into a series of heavy hydrocarbon compounds and some of those compounds form radicals that eventually form soot. These products are a result of complex reactive mechanisms of the volatile matter in PET and tyres, which include random chain scission, end-chain scission, chain-stripping, cross-linking, and coke formation. Therefore, tyres and PET are less reactive in the CO2 atmosphere compared to wood and polystyrene. Figure 3. Syngas yield (bar plot) and energy yield (line plot) from CO2-gasification of pinewood, PET, waste tyres, and polystyrene without and with catalyst. 3.2 Influence of catalytic position in CO2-assisted gasification In these tests, we placed the catalyst about 8 mm downstream of the test sample to facilitate thermal cracking without inhibition of CO2 + char reactions as observed. This configuration is called quasi-in-situ. Figure 4 shows the accumulative yield of syngas major components for the in-situ, quasi-in-situ CO2 gasification configurations. For all materials H2, CO, and CH4 yield increased when the catalyst was placed downstream of the feedstock revealing the sFCC catalyst to promote hydrocracking and reforming of hydrocarbons. For pinewood, the overall syngas yield increased by some 21.27% and the syngas energy increased by 22%, respectively, as compared to the in-situ case. For PET gasification, the overall syngas and energy yields increased by 27.5% and 23.8%, respectively in quasi-in-situ cases when compared to in-situ. Polystyrene provided the highest increase, with the quasi-in-situ catalytic gasification increasing the syngas yield and energy by 103.4% and 62.5% as compared to the in-situ catalytic gasification case, respectively. This, therefore, means that the quasi-in-situ catalytic gasification method is suitable for using sFCC catalyst in gasification. The spent FCC catalyst is a hydrocarbon cracking catalyst and does not participate in the solid reaction or the solid thermal decomposition of pinewood. It only participates during the thermal cracking of the volatiles that are evolved in the reactor. Therefore, this catalyst should be placed downstream of the reactor for secondary cracking. 676 Figure 4. Syngas yield (bar plot) and energy yield (line plot) during in-situ, quasi-in-situ CO2-assisted gasification of pinewood, waste tyres, PET, and polystyrene. 3.3 Influence of temperature (a) (b) Figure 5. Effect of temperature on Syngas yield (bar plot) and energy yield (line plot) from catalytic CO2-assisted gasification at 800 ℃ and 900 ℃ of Pinewood (a) and Polystyrene (b). The effect of temperature was examined for pinewood and polystyrene. Figure 5 shows the influence of temperature during CO2-assisted gasification. The results show that operating at a lower temperature of 800 ℃ has a negative impact on the CO yield because the Boudouard reaction responsible for carbon conversion to CO is favoured only at high temperatures. The other observation is that at lower temperatures there is a higher yield of C2Hx hydrocarbons which suggests that CO2-reforming of hydrocarbons is favoured at high temperatures which affects the overall yield of CO. For pinewood (Figure 5a) the yield of syngas energy decreased by 38.24% when the temperature was decreased from 900 to 800 ℃. The in-situ catalytic case at 900 ℃ provided 42.42% higher energy yield compared to the in-situ case at 800 ℃. Also, the quasi-in-situ case at 900 ℃ provided a 58.33% higher energy yield compared to the quasi-in-situ case at 800 ℃. A similar behavior was observed for polystyrene, see Figure 5(b). At a higher temperature of 900 ℃ polystyrene provided a higher 677 yield of syngas compared to pinewood, PET, and waste tyres. This is because polystyrene is 100% volatile matter (Table 1) and has no tendency to form char. 4. Conclusions The effect of adding spent FCC (sFCC) catalyst during CO2-assisted gasification of MSW is explored to better understand the capabilities and potential of sFCC catalysts in assisting the thermal cracking of organic solid waste into syngas. Pinewood, waste tyres, PET, and polystyrene were evaluated as representative components in MSW. The results showed that the presence of sFCC increased the yields of total syngas and syngas energy compared to the non-catalytic cases. In-situ catalytic CO2-assisted gasification increased the overall syngas yield by 81.25% for waste tyres, 62.5% for PET, 50% for pinewood, and 3.5 % for polystyrene when compared to respective non-catalytic gasification cases. For polystyrene, the quasi-in-situ catalytic gasification increased the syngas yield and energy by 103.4% and 62.5% as compared to the in-situ catalytic gasification case, respectively. However, for PET the syngas yield and energy increased by 27.5% and 23.8%. In the case of waste tyres, the syngas yield and energy yields increased by 24% and 23%, respectively. This suggests that quasi-in-situ catalytic gasification is more efficient and effective than in-situ catalytic gasification for increasing the syngas yield. High-temperature quasi-in-situ catalytic CO2-assisted gasification is better suited to achieve high syngas energy yield. The findings presented here provide conclusive evidence of the synergistic potential of combining waste materials from FCC plants with those from MSW recovery facilities to recover valuable products while simultaneously reducing carbon footprint via catalytic CO2-assisted gasification. Acknowledgements This research was supported by ARPAE and is gratefully acknowledged. The CUNY City College team's support is also greatly appreciated. The financial support provided to Athi-enkosi Mavukwana by the Fulbright and the University of South Africa is gratefully acknowledged. Fatih Aktas gratefully acknowledges TUBITAK (The Scientific and Technological Research Council of Turkey) and Gazi University for their financial support to him while at the Combustion Laboratory, University of Maryland. References Abubakar, I.R., Maniruzzaman, K.M., Dano, U.L., AlShihri, F.S., AlShammari, M.S., Ahmed, S.M.S., Al-Gehlani, W.A.G., Alrawaf, T.I., 2022. Environmental Sustainability Impacts of Solid Waste Management Practices in the Global South. International Journal of Environmental Research and Public Health 19, 12717. Bertero, M., García, J.R., Falco, M., Sedran, U., 2019. Equilibrium FCC catalysts to improve liquid products from biomass pyrolysis. Renewable Energy 132, 11–18. EPA, 2020. Advancing Sustainable Materials Management. United States Environmental Protection Agency. Office of Resource Conservation and Recovery 184. Figueroa, J.E.J., Ardila, Y.C., Hoss Lunelli, B., Filho, R.M., Wolf Maciel, M.R., 2013. Evaluation of pyrolysis and steam gasification processes of sugarcane bagasse in a fixed bed reactor. Chemical Engineering Transactions 32, 925–930. Liu, X., Burra, K.G., Wang, Z., Li, J., Che, D., Gupta, A.K., 2020. On deconvolution for understanding synergistic effects in co-pyrolysis of pinewood and polypropylene. Applied Energy 279, 115811. Mastry, M.C., Dorazio, L., Fu, J.C., Gómez, J.P., Sedano, S., Ail, S.S., Castaldi, M.J., Yilmaz, B., 2023. Processing renewable and waste-based feedstocks with fluid catalytic cracking: Impact on catalytic performance and considerations for improved catalyst design. Frontiers in Chemistry 11, 1–10. Peres, A.P.G., Lunelli, B.H., Filho, R.M., 2013. Application of biomass to hydrogen and syngas production. Chemical Engineering Transactions 32, 589–594. https://doi.org/10.3303/CET1332099 Ruoppolo, G., Miccio, F., Brachi, P., Picarelli, A., Chirone, R., 2013. Fluidized bed gasification of biomass and biomass/coal pellets in oxygen and steam atmosphere. Chemical Engineering Transactions 32, 595–600. Wang, C., Tian, X., Zhao, B., Zhu, L., Li, S., 2019. Experimental Study on Spent FCC Catalysts for the Catalytic Cracking Process of Waste Tyres. Processes 7, 335. https://doi.org/10.3390/pr7060335 Wang, Q., Li, Yi, Benally, C., Li, Yiming, Chen, C., An, Z., Gamal El-Din, M., 2021. Spent fluid catalytic cracking (FCC) catalyst enhances pyrolysis of refinery waste activated sludge. Journal of Cleaner Production 295, 126382. 678 337Mavukwana-PAGATO.pdf Effect of Spent Fluid Catalytic Cracking Catalysts on Syngas Yield during CO2-Assisted Gasification of Solid Waste