Hydrogen-rich gas production by catalytic steam gasification of rice husk using CeO2-modified Ni-CaO sorption bifunctional catalysts

Abstract

Hydrogen-rich gas production from rice husk via steam gasification and catalytic reforming using CeO2-modified Ni-CaO sorption bifunctional catalysts synthesized by sol–gel method in a two-stage system was investigated. The results show that the Ce0.7Ni1Ca5 catalyst achieves maximum H2 concentration (85.81(±0.39) vol.%) and H2 yield (35.82(±0.28) mmol g-1biomass) under a condition of 500 °C, S/C (steam/carbon in biomass) molar ratio of 5, catalyst/biomass mass ratio of 2.5, producing the lowest content of CO2 (3.62(±0.16) vol.%), CO (4.27(±0.11) vol.%), CH4 (4.49(±0.18) vol.%), and C2-C3 (1.81(±0.09) vol.%), correspondingly. Ce0.7Ni1Ca5 also exhibits excellent cyclic stability in H2 production, CO2 sorption, and inhibition of carbon deposition. The H2 concentration and H2 yield remain above 81.88 vol% and 32.11 mmol g-1biomass, respectively, and CO2 emission keeps below 4.85 vol% during 10 cyclic tests. The carbon deposition of Ce0.7Ni1Ca5 is only about 30% of that of Ni1Ca5 after 10 cycles and hardly increased after 5 cycles. It is found that well-dispersed CeO2 can effectively prevent the sintering of NiO and delay agglomeration of CaO species, stabilizing CaO carbonation and CO2 sorption, and the strong Ni-O-Ce interaction induces the creation of oxygen vacancies that facilitate the fracture of O–H bonds of water for the formation of H2. Furthermore, the high oxygen transport capacity of CeO2 not only forms abundant mesopores structure to promote WGS and SMR reactions for enhancing H2 production, but also contributes to reforming the carbon deposited on the catalyst surface by lowering the oxidation temperature of amorphous carbon containing low-molecular aromatic or aliphatic compounds with lower degree of graphitization.