Ca-enhanced hematite oxygen carriers for chemical looping reforming of biomass pyrolyzed gas coupled with CO_2 splitting

摘要

Chemical looping reforming (CLR) of biomass pyrolyzed gases coupled with CO2 splitting was proposed to improve the H-2/CO ratio of gaseous product by using Ca-enhanced hematite oxygen carriers. The oxygen carriers mainly consist of Fe2O3, Ca2Fe2O5, CaO and SiO2. The impurity gas in biomass pyrolyzed gases is consumed in CLR stage, attributing to the dry reforming reaction over a catalytic effect of oxygen carriers. The highest CH4 conversion 98.26% and CO2 conversion 71.92% are realized in the CLR process with the crystal transformation of Fe2O3 and Ca2Fe2O5 to Fe and CaO. The improved reactivity of hematite oxygen carriers by CaO is mainly ascribed to the intermetallic synergistic effect in Ca2Fe2O5, which influences the lattice oxygen release and selectivity of gaseous products. The maximum H-2/CO ratio of gaseous product in CLR process is nearly 3 times as high as the initial H-2/CO ratio of biomass pyrolyzed gas. Moreover, up to 80.95% CO2 conversion efficiency and 2.32 mmol/min/g CO yield are achieved in CO2 splitting step with the generation of Fe3O4 and CaFe3O5 in oxygen carriers. The CO2 conversion shows positive correlation with reaction temperature but negative correlation with weight hourly space velocity. The metallic synergistic effect between Ca/Fe are conductive to improve CO2 conversion though shifting the reaction equilibrium and enhancing ion diffusion. Modest reactivity and stability of Ca-modified oxygen carriers are confirmed in 15 cycles reaction, which indicates that potentially application prospect in CLR of biomass pyrolyzed gases coupled with CO2 splitting process.