Chemical Looping Combustion of Biomass/Coal with Natural Iron Ore as Oxygen Carrier in a Continuous Reactor

Chemical looping combustion (CLC) is a new innovative technology with inherent separation of CO2 without energy penalty. Experiments on chemical looping combustion of biomass/coal were conducted in a 1 kW(th) continuous reactor, and an Australia iron ore was selected as oxygen carrier. Both biomass/coal mixture and biomass were used as fuels. The effect of temperature on gas composition of both the fuel reactor and the air reactor, conversion efficiency of carbonaceous gases, carbon capture efficiency, and oxide oxygen fraction was investigated. An increase in the fuel reactor temperature produced a higher CO2 concentration in the fuel reactor for biomass/coal mixture, whereas it produced a lower one for pure biomass. CO concentration in the fuel reactor increased in both fuel conditions. Due to the poor oxygen transport capacity and the thermodynamic constraint of the iron ore conversion from Fe2O3 to Fe3O4, a higher temperature would contribute to decreasing the conversion efficiency of carbonaceous gases for both biomass and biomass/coal mixture. Both carbon capture efficiency and oxide oxygen fraction were enhanced with increasing the fuel reactor temperature, and the deviation between them was caused by the combustible carbonaceous gases in the fuel reactor. Both the fresh and the used oxygen carrier particles were characterized. X-ray diffraction (XRD) results indicated that the iron ore as oxygen carrier possesses a good regenerable ability in the CLC process. This is attributed to the existence of quartz in the iron ore particles and its sintering inhibition. Reactions between SiO2 and Fe3O4 may occur at a high temperature under a reducing condition. Scanning electron microscope (SEM) analysis showed that as a consequence of accumulative effect of redox reaction and thermal stress, the used oxygen carrier particles obtained a porous structure facilitating the gas solid reactions. Energy dispersive X-ray (EDX) results demonstrated the deposition of alkali metals on the particle surface of oxygen carrier during the CLC process of biomass. Blending biomass with coal and adding some additives might be effective measures to reduce the potential negative influence of biomass ash on oxygen carrier.