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Two-step Gasification of Biochar for Hydrogen-Rich Gas Production: Effect of the Biochar Type and Catalyst

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journal contribution
posted on 18.07.2019, 16:15 by Hakan Cay, Gozde Duman, Jale Yanik
The two-stage steam gasification process, including gasification and volatile reforming steps, was proposed to produce hydrogen-rich gas from spent coffee residues. For comparison purposes, the one-stage steam gasification process was also performed. It was aimed to enhance the hydrogen-rich gas production by using coffee residue biochars. Special focus was given to the comparison of gasification properties of biochars from wet (subcritical water, 200 °C) and dry (nitrogen atmosphere, 300 and 500 °C) pyrolysis of spent coffee residues. The steam gasification of biochars was then performed at 850 °C. For all biochars, the two-step process produced more hydrogen gas than the one-step process, and higher hydrogen yields were obtained from pyrochars produced by dry pyrolysis. Besides the biochar type, effects of the temperature and catalyst (dolomite and red mud) in the volatile reforming step on tar yield and hydrogen production efficiency were comparatively investigated. The results indicated that temperature has a significant effect on the composition and yield of product gas for raw biomass and hydrochar produced by hydrothermal carbonization (HTC), while catalysts were effective in the case of P300. In catalytic experiments (ex situ), dolomite acted as the CO2 capture sorbent, while the reaction pathway of the red mud was a redox cycle. Depending on the feedstock, syngas contained around 59–64% of H2 and 27–30% of CO2, and the maximum hydrogen yields ranged between 51.5 and 116.5 mmol/g feedstock. The present study demonstrates that dry pyrolysis is a more effective pretreatment of biomass compared to HTC for production of hydrogen-rich gas via steam gasification. The pretreatment temperature of 500 °C and the temperature of 850 °C in the volatile reforming step were identified as the suitable conditions for hydrogen production.