High-purity hydrogen production from real biomass pyrolysis vapors via a chemical looping process

Abstract

H₂ production from renewable bio-oil is a promising way to supply green hydrogen; however, this technology suffers from the high viscosity, high corrosiveness, and complex compositions of the bio-oil. Thus, in the present study, we propose a novel method that converts real biomass pyrolysis vapors into H₂via a chemical looping process. Fe–Al–Ni composite oxide pellets were prepared in a simple and industrial way and then used as the oxygen carrier (OC), and their redox activity and cycle stability were assessed in a fixed-bed reactor system under different conditions with pine sawdust volatiles as the fuel. The results indicated that the Fe–Al–Ni composites exhibited nearly 99% CO₂ selectivity in the reduction stage and high H₂ purity (>98%) in the H₂ production stage when the redox temperature was higher than 850 °C. X-ray diffraction (XRD) analysis of the Fe–Al–Ni composite oxide pellets at different bed layers indicated that Fe₂O₃ can be reduced to FeO by sawdust pyrolysis vapors, and an inert spinel phase of Fe–Al–O was formed concurrently. Coke deposited on the OC would hinder reduction, thereby decreasing the fuel conversion and H₂ energy efficiency, but it can be improved by increasing the pyrolysis temperature. Long cycling tests showed that a relatively stable H₂ energy efficiency of 20% and a H₂ purity of 98% could be obtained in whole tests; nevertheless, the conversion of CH₄ declined rapidly after 25 cycles, which could be attributed to the interior sintering, iron migration to the outer surface and garnet phase (Al₃Fe₅O₁₂) formation of the OC according to the scanning electron micrograph, micro texture, and XRD analysis of the OC with different cycling tests.