Chemical looping gasification of coal using calcium ferrites as oxygen carrier

Chemical-looping gasification (CLG) is a novel technology to convert solid fuels like coal and biomass into combustible gases. Without the requirement of high-cost pure oxygen production, CLG utilizes lattice oxygen from oxygen carrier (OC) for partial oxidation of fuels. In this work, the bimetallic CaO/Fe2O3 oxygen carriers (OCs) prepared by the wet-impregnation method were tested in a batch fluidized bed reactor using coal (char) as the fuel. The results indicate that increasing CaO addition tends to elevate the low heating value (LHV) of the gas products, and the maximum H-2/CO ratio of 4.6 was attained with coal when FeCa50 (the molar ratio of CaO to Fe2O3 is 0.5:0.5) was used as OC. It was found that the char gasification rate is closely dependent on both lattice oxygen donation ability and catalytic activity of OCs. Ca2Fe2O5 was determined as a catalytic substance, and the catalytic effect on char gasification was shown to be prominent at the later stage of char gasification when the lattice oxygen donation ability of OCs reduces. Moreover, the catalytic effect of partially reduced OCs on the water-gas shift (WGS) and steam-iron reactions (which may regulate the quality of gas products) was also explored. Ca2Fe2O5, with the presence of oxygen vacancies, exhibits better catalytic activity to WGS for attaining higher CO conversion and selectivity, as compared with CaFe3O5 (moderate catalysis) and Fe3O4 (weak catalysis). However, Ca2Fe2O5, with a poor reducibility, limits the hydrogen production during the iron-steam process, but it indeed improves the hydrogen production rate owing to its active sites.