Biomass calcium looping gasification via cement-modified carbide slag in fluidized bed: an examination on enhanced multi-cycle CO2 capture and hydrogen production

Biomass calcium looping gasification (CLG) for CO₂ capture and H₂ production is rather attractive but still needs developing low-cost CaO-based sorbent with enhanced mechanical strength, cyclic CO₂ absorption reactivity, and multi-cycle gasification performance. In this work, cement-modified carbide slag was developed as a type of cheap CaO sorbent which was prepared by an economical method of wet mixing. Characterization of modified carbide slag revealed that mayenite (Ca₁₂Al₁₄O₃₃) was successfully synthesized which played important role in promoting sintering resistance and mechanical strength of carbide slag sorbent. Cyclic CO₂ capture tests with a thermogravimetric analyzer (TGA) verified that cement-modified carbide slag showed much higher CO₂ absorption capacity than natural carbide slag. Modified carbide slag with 15 wt% cement (CS15) presented the best CO₂ capture performance in this work, and the CaO conversion approached 58.3% after 15 cycles. Biomass calcium looping gasification tests for H₂ production were also conducted in a fluidized bed reactor under different conditions, with special attention to the cyclic gasification performance of CS15. Gasification results proved that adding carbide slag not only apparently increased H₂ concentration and H₂ yield but also reduce the yield of tar species. Cement-modified carbide slag showed better gasification performance than natural carbide slag. Although increasing gasification temperature was beneficial to promote biomass conversion and H₂ yield, too high temperature should be avoided which would inhibit water gas shift reaction, CO₂ capture, and result in the conversion of tar species to polycyclic aromatic hydrocarbons (PAHs). Moreover, results of cyclic gasification tests presented great potential to adopt modified carbide slag in fluidized bed calcium looping gasification. After 8 cycles of gasification/regeneration, H₂ concentration and H₂ yield with CS15 were stabilized at 49.8% and 32.7 mg/g_biomass, which were increased by 11.5% and 17.7% in comparison to those with CS.