CO2 assisted steam gasification for hydrogen-rich syngas from rice straw: Evolution of carbon structure and its effect

Abstract

CO₂-assisted steam gasification of rice straw (RS) offers a promising approach for both the sustainable utilization of rice straw and the reduction of CO₂ emissions. This study investigates the transformation of RS during pyrolysis and gasification in a fixed-bed reactor, with structural changes observed through solid-state13C nuclear magnetic resonance and Fourier-transform infrared (FT-IR) spectroscopy. The gas composition is analyzed by gas GC, and covalent bonds are quantified through carbon structure and elemental analysis. The results demonstrate that steam acts as a key reactant in the gasification process, significantly enhancing the pyrolysis of RS and leading to higher hydrogen yields. CO₂ serves as an oxidant above 500 ◦C, oxygenating aromatic rings and initiating ring-opening reactions to form active C(O) intermediates that are crucial for hydrogen production. The hydrogen yield increases similarly to H₂O gasification, while methane and carbon monoxide decrease significantly after gasification over 700 ◦C. The H₂/CO ratio improves from 0.64 in N₂ to 1.46 in H₂O, and further to 1.71 in CO₂-H₂O at 700 ◦C. CO₂-assisted steam gasification optimizes the reactive interface activity, promoting the selective formation of hydrogen and improving the efficiency of the gasification process. Furthermore, CO₂ plays a pivotal role in enhancing the formation of active C(O) intermediates, which further facilitates the production of high-purity hydrogen. The process also induces the formation of a regular micro-pore structure, improving the overall efficiency and selectivity of directional hydrogen generation.

Keywords

Carbon structure / Steam gasification / Hydrogen-rich syngas / CO2 reduction / Biochar

CRediT authorship contribution statement

Peng Liu: Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis, Conceptualization. Meiling Xie: Writing – review & editing, Writing – original draft, Data curation. Ying Chen: Data curation. Yanling Li: Conceptualization. Tanglei Sun: Funding acquisition. Tingzhou Lei: Investigation. Shijie Liu: Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was funded by the Natural Science Foundation of China (52436007, 52306224 and 52306223).

References

[1] Bhurse, R.R., Gope, P.P., Yadav, H., Prasad, K.R., Chaurasia, A.S., 2025. Syngas and hydrogen production from co-gasification of rice husk biomass and plastic waste mixture using Aspen plus. Environ. Dev. Sustain. 27 (7), 16033-16052. https://doi.org/10.1007/s10668-024-04571-z.

[2] Chianese, S., Fail, S., Binder, M., Rauch, R., Hofbauer, H., Molino, A., Blasi, A., Musmarra, D., 2016. Experimental investigations of hydrogen production from CO catalytic conversion of tar rich syngas by biomass gasification. Catal. Today 277, 182-191. https://doi.org/10.1016/j.cattod.2016.04.005.

[3] Dhanavath, K.N., Shah, K., Bhargava, S.K., Bankupalli, S., Parthasarathy, R., 2018. Oxygen-steam gasification of karanja press seed cake: fixed bed experiments, Aspen plus process model development and benchmarking with saw dust, rice husk and sunflower husk. J. Environ. Chem. Eng. 6 (2), 3061-3069. https://doi.org/10.1016/j.jece.2018.04.046.

[4] Feng, D., Zhang, Y., Zhao, Y., Sun, S., Gao, J., 2018. Improvement and maintenance of biochar catalytic activity for in-situ biomass tar reforming during pyrolysis and H2O/CO2 gasification. Fuel Process. Technol. 172, 106-114. https://doi.org/10.1016/j.fuproc.2017.12.011.

[5] Galih Pangestu, M.R.,Razzak, S.A.,Uddin, S., 2025. Microbial biomass conversion for hydrogen production: a review. Green Energy Resourc. 3 (3), 100131. https://doi.org/10.1016/j.gerr.2025.100131.

[6] Gao, N., Sliz, M., Quan, C., Bieniek, A., Magdziarz, A., 2021. Biomass CO2 gasification with CaO looping for syngas production in a fixed-bed reactor. Renew. Energy 167, 652-661. https://doi.org/10.1016/j.renene.2020.11.134.

[7] Gao, S., Zhao, J.,Wang, Z.,Wang, J., Fang, Y., Huang, J., 2013. Effect of CO2 on pyrolysis behaviors of lignite. J. Fuel Chem. Technol. 41 (3), 257-264. https://doi.org/10.1016/S1872-5813(13)60017-1.

[8] Guizani, C., Jeguirim, M., Gadiou, R., Escudero Sanz, F.J., Salvador, S., 2016. Biomass char gasification by H2O, CO2 and their mixture: evolution of chemical, textural and structural properties of the chars. Energy 112, 133-145. https://doi.org/10.1016/j.energy.2016.06.065.

[9] He, X., Liu, Z., Niu, W., Yang, L., Zhou, T., Di, Qin, Niu, Z., Yuan, Q., 2018. Effects of pyrolysis temperature on the physicochemical properties of gas and biochar obtained from pyrolysis of crop residues. Energy 143, 746-756. https://doi.org/10.1016/j.energy.2017.11.062.

[10] Hecht, E.S., Shaddix, C.R., Geier, M., Molina, A., Haynes, B.S., 2012. Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char. Combust. Flame 159 (11), 3437-3447. https://doi.org/10.1016/j.combustflame.2012.06.009.

[11] Hu, M., Deng, W., Su, Y., Wang, L., Chen, G., 2024. Production of hydrogen-rich syngas through microwave-assisted gasification of sewage sludge in steam-CO2 atmosphere. Fuel 357, 129855. https://doi.org/10.1016/j.fuel.2023.129855.

[12] Islam, M.W., 2020. Effect of different gasifying agents (steam, H2O2, oxygen, CO2, and air) on gasification parameters. Int. J. Hydrogen Energy 45 (56), 31760-31774. https://doi.org/10.1016/j.ijhydene.2020.09.002.

[13] Jin, Y., Hu, S., Zhang, Z., Zhu, B., Bai, D., 2022. The path to carbon neutrality in China: a paradigm shift in fossil resource utilization. Resour. Chem. Mater. 1 (1), 129-135. https://doi.org/10.1016/j.recm.2022.01.003.

[14] Jung, J., Kim, S., Lee, J., Oh, J.I., Choi, Y., Kwon, E.E., 2019. Tailoring pyrogenic products from pyrolysis of defatted Euglena gracilis using CO2 as reactive gas medium. Energy 174, 184-190. https://doi.org/10.1016/j.energy.2019.02.153.

[15] Kong, G., Liu, Q., Ji, G., Jia, H., Cao, T., Zhang, X., Han, L., 2023. Improving hydrogen-rich gas production from biomass catalytic steam gasification over metal-doping porous biochar. Bioresour. Technol. 387, 129662. https://doi.org/10.1016/j.biortech.2023.129662.

[16] Lina, Q., Zhenzhen, W., 2025. Chain reaction mechanism study of active groups in coal. Case Stud. Therm. Eng. 73, 106466. https://doi.org/10.1016/j.csite.2025.106466.

[17] Liu, M., Li, Y., Wang, X., Gai, Z., Rao, Q., Yang, T., Zhang, J., Tang, S., Pan, Y., Jin, H., 2024. Synergistic promotions between high purity H2 production and CO2 capture via sorption enhanced chemical looping reforming. Fuel Process. Technol. 254, 108042. https://doi.org/10.1016/j.fuproc.2024.108042.

[18] Liu, P., Chen, Y., Su, S., Li, Y., Li, X., Lei, T., 2025a. Insight into carbon structural variation from steam gasification of rice straw on enhancing hydrogen generation. J. Environ. Manag. 379, 124815. https://doi.org/10.1016/j.jenvman.2025.124815.

[19] Liu, P.,Lang, P., Chen, Z., Li, Y., Sun, T., Yang, Y., Huhe, T., Lei, T., 2023. Relevance of chemical structure in different wood wastes to pyrolysis behavior: Kinetics and hydrogen release. J. Energy Inst. 111, 101416. https://doi.org/10.1016/j.joei.2023.101416.

[20] Liu, P., Song, Y., Li, Y., Sun, T., Wang, Z., Lei, T., 2025b. Accurate carbon structure for prediction of reaction pathway during bio-coal production using torrefaction. Fuel 381 (PA). https://doi.org/10.1016/j.fuel.2024.133247, 133247- 133247.

[21] Liu, P., Wang, Y., Zhou, Z., Yuan, H., Zheng, T., Chen, Y., 2020. Effect of carbon structure on hydrogen release derived from different biomass pyrolysis. Fuel 271, 117638. https://doi.org/10.1016/j.fuel.2020.117638.

[22] Liu, P., Zhang, D., Wang, L., Zhou, Y., Pan, T., Lu, X., 2016. The structure and pyrolysis product distribution of lignite from different sedimentary environment. Appl. Energy 163, 254-262. https://doi.org/10.1016/j.apenergy.2015.10.166.

[23] Liu, P., Zhang, Y., Pan, T., Zhang, D., Chen, L., Sun, C., 2022. Accurately quantifying carbon structural types and predicting pyrolysis behavior of coal using solid 13C NMR Cp/MAS spectra. Fuel 327, 125092. https://doi.org/10.1016/j.fuel.2022.125092.

[24] Luo, S., Xiao, B., Hu, Z., Liu, S., Guo, X., He, M., 2009. Hydrogen-rich gas from catalytic steam gasification of biomass in a fixed bed reactor: influence of temperature and steam on gasification performance. Int. J. Hydrogen Energy 34 (5), 2191-2194. https://doi.org/10.1016/j.ijhydene.2008.12.075.

[25] Mavukwana, A., Burra, K.G., Sempuga, C., Castaldi, M., Gupta, A.K., 2024. Effect of spent fluid catalytic cracking (FCC) catalyst on syngas production from pyrolysis and CO2-assisted gasification of waste tires. Fuel 355, 129446. https://doi.org/10.1016/j.fuel.2023.129446.

[26] McBeath, A.V., Smernik, R.J., 2009. Variation in the degree of aromatic condensation of chars. Org. Geochem. 40 (12), 1161-1168. https://doi.org/10.1016/j.orggeochem.2009.09.006.

[27] Mojaver, P., Khalilarya, S., 2024. Sustainable waste-to-hydrogen energy conversion through face mask waste gasification integrated with steam methane reformer and water-gas shift reactor. Int. J. Hydrogen Energy 85, 947-961. https://doi.org/10.1016/j.ijhydene.2024.08.369.

[28] Pandey, B., Prajapati, Y.K., Sheth, P.N., 2019. Recent progress in thermochemical techniques to produce hydrogen gas from biomass: a state of the art review. Int. J. Hydrogen Energy 44 (47), 25384-25415. https://doi.org/10.1016/j.ijhydene.2019.08.031.

[29] Pang, Y., Zhu, X., Li, N., Wang, Z., 2023. Investigation on reaction mechanism for CO2 gasification of softwood lignin by ReaxFF MD method. Energy 267, 126533. https://doi.org/10.1016/j.energy.2022.126533.

[30] Panhwar, M.A., Geng, B., Lougou, B.G., Rafique, M., Ali, I., Khursheed, R., Garg, A., Shuai, Y., 2025. Recent advances in biochar-based materials for CO2 capture: from preparation to application in value-added chemicals and fuels. J. Environ. Chem. Eng. 13 (6), 120244. https://doi.org/10.1016/j.jece.2025.120244.

[31] Parthasarathy, P., Zuhara, S., Al-Ansari, T., McKay, G., 2023. A review on catalytic CO2 pyrolysis of organic wastes to high-value products. Fuel 335, 127073. https://doi.org/10.1016/j.fuel.2022.127073.

[32] Peng, L., Panpan, L., Zhuo, C., Yanling, L., Tanglei, S., Yantao, Y., Taoli, H., Tingzhou, L., 2023. Relevance of chemical structure in different wood wastes to pyrolysis behavior: Kinetics and hydrogen release. J. Energy Inst. 111. https://doi.org/10.1016/J.JOEI.2023.101416.

[33] Qi, P., Su, Y., Jiang, M., Wang, J., Xiong, Y., 2026. Synthesis of CH4 by catalytic reforming of rice husk pyrolysis gas over NiO/CuO-CeO2 catalysts. Renew. Energy 256, 124209. https://doi.org/10.1016/j.renene.2025.124209.

[34] Qing, M., Su, S., Chi, H., Xu, J., Sun, Z., Gao, J., Xu, K., Hu, S., Wang, Y., Hu, X., Xiang, J., 2019. Relationships between structural features and reactivities of coal-chars prepared in CO2 and H2O atmospheres. Fuel 258, 116087. https://doi.org/10.1016/j.fuel.2019.116087.

[35] Rahimi, M., Salaudeen, S.A., 2024. Optimizing hydrogen-rich gas production by steam gasification with integrated CaO-based adsorbent materials for CO2 capture: machine learning approach. Int. J. Hydrogen Energy 95, 695-709. https://doi.org/10.1016/j.ijhydene.2024.11.179.

[36] Rodrigues, J.P., Ghesti, G.F., Silveira, E.A., Lamas, G.C., Ferreira, R., Costa, M., 2022. Waste-to-hydrogen via CO2/steam-enhanced gasification of spent coffee ground. Clean. Chem. Eng. 4, 100082. https://doi.org/10.1016/j.clce.2022.100082.

[37] Ruoppolo, G., Ammendola, P., Chirone, R., Miccio, F., 2012. H2-rich syngas production by fluidized bed gasification of biomass and plastic fuel. Waste Manage 32 (4), 724-732. https://doi.org/10.1016/j.wasman.2011.12.004.

[38] Sakheta, A., Raj, T., Nayak, R., O'Hara, I., Ramirez, J., 2024. Improved prediction of biomass gasification models through machine learning. Comput. Chem. Eng. 191, 108834. https://doi.org/10.1016/j.compchemeng.2024.108834.

[39] Shadle, L.J., Indrawan, N., Breault, R.W., Bennett, J., 2025. Gasification technology. In: Lackner, M., Sajjadi, B., Chen, W. (Eds.), Handbook of Climate Change Mitigation and Adaptation. Springer Nature, Switzerland, Cham, pp. 793-882.

[40] Tang, F., Chi, Y., Jin, Y., Zhu, Z., Ma, J., 2022. Gasification characteristics of a simulated waste under separate and mixed atmospheres of steam and CO2. Fuel 317, 123527. https://doi.org/10.1016/j.fuel.2022.123527.

[41] Tang, F., Zhu, Z., Xu, C., Chi, Y., Jin, Y., 2023. Effects of steam and CO2 on gasification tar composition and evolution of aromatic compounds. Waste Manage 157, 219-228. https://doi.org/10.1016/j.wasman.2022.12.019.

[42] Van Hung, N., Maguyon-Detras, M.C., Migo, M.V., Quilloy, R., Balingbing, C., Chivenge, P., Gummert, M., 2020. Rice straw overview: availability, properties, and management practices. In: Gummert, M., Hung, N.V., Chivenge, P., Douthwaite, B. (Eds.), Sustainable Rice Straw Management. Springer International Publishing, Cham, pp. 1-13. https://doi.org/10.1007/978-3-030-32373-8.

[43] Wang, J., Jiang, M., Yao, Y., Zhang, Y., Cao, J., 2009. Steam gasification of coal char catalyzed by K2CO3 for enhanced production of hydrogen without formation of methane. Fuel 88 (9), 1572-1579. https://doi.org/10.1016/j.fuel.2008.12.017.

[44] Wang, Y., Bell, D.A., 2017. Competition between H2O and CO2 during the gasification of Powder River Basin coal. Fuel 187, 94-102. https://doi.org/10.1016/j.fuel.2016.08.109.

[45] Wilk, V., Hofbauer, H., 2013. Co-gasification of plastics and biomass in a dual fluidized-bed steam gasifier: possible interactions of fuels. Energy Fuels 27 (6), 3261-3273. https://doi.org/10.1021/ef400349k.

[46] Zhang, C., Jun, K., Gao, R., Kwak, G., Kang, S.C., 2016. Efficient utilization of associated natural gas in a modular gas-to-liquids process: technical and economic analysis. Fuel 176, 32-39. https://doi.org/10.1016/j.fuel.2016.02.060.

[47] Zhang, K., Kim, W., Park, A.A., 2020. Alkaline thermal treatment of seaweed for high-purity hydrogen production with carbon capture and storage potential. Nat. Commun. 11 (1), 3783. https://doi.org/10.1038/s41467-020-17627-1.

[48] Zhang, S., Wu, M., Qian, Z., Li, Q., Zhang, Y., Zhou, H., 2024a. CO rich syngas production from catalytic CO2 gasification-reforming of biomass components on Ni/CeO2. Fuel 357, 130087. https://doi.org/10.1016/j.fuel.2023.130087.

[49] Zhang, X., Cao, T., Zhang, G., Liu, Q., Kong, G., Wang, K., Jiang, Y., Zhang, X., Han, L., 2024b. Sustainable hydrothermal carbon for advanced electrochemical energy storage. J. Mater. Chem. A 12 (9), 4996-5039. https://doi.org/10.1039/D3TA07372F.

[50] Zhang, X., Kong, G., Zhang, X., Wang, K., Liu, Q., Shi, S., Han, L., 2023. Gasification integrated with steam co-reforming of agricultural waste biomass over its derived CO2/O2/steam-mediated porous biochar for boosting H2-rich syngas production. J. Environ. Chem. Eng. 11 (2), 109556. https://doi.org/10.1016/j.jece.2023.109556.

[51] Zhang, X., Qin, C., Lu, Y., Liu, Q., Liu, H., Wang, C., Zhang, G., Kong, G., Wang, K., Li, J., Shang, J., Zhang, X., Han, L., 2025. Green hydrogen and porous carbon materials co-harvested through CO2-steam co-mediated gasification. Bioresour. Technol. 415, 131726. https://doi.org/10.1016/j.biortech.2024.131726.

[52] Zhao, D., Wang, Y., Li, Q., Han, W., Wang, G., Shu, S., Wang, H., Liu, H., Liu, H., Shuai, G., Kou, J., Shen, L., 2025. Investigating the interaction between H2O-Char and CO2-Char in co-gasification through isotope tracer and ReaxFF method. Int. J. Hydrogen Energy 101, 863-874. https://doi.org/10.1016/j.ijhydene.2024.12.425.

[53] Zhou, Y., Zhu, S., Yan, L., Li, F., Bai, Y., 2019. Interaction between CO2 and H2O on char structure evolution during coal char gasification. Appl. Therm. Eng. 149, 298-305. https://doi.org/10.1016/j.applthermaleng.2018.12.037.

[54] Zou, X., Zhai, M., Yang, D., Liu, G., Wang, T., Guo, L., Zhang, Y., 2023. New insights into the mechanism of biomass char steam gasification process by oxygen-containing functional group as aromatic carbon boundaries: experimental and DFT study. Chem. Eng. J. 465, 142947. https://doi.org/10.1016/j.cej.2023.142947.