Chemical Looping Combustion of a Biomass Char in Fe2O3-, CuO-, and SrFeO3−δ-Based Oxygen Carriers

This research focuses on the combustion of biomass char in fluidized beds of various particulate solids, which, under the conditions of the reaction, were either inert or capable of supplying oxygen to reactions. The latter were termed oxygen carriers. The solids used were SiO2, as an inert material, and three oxygen carriers: (1) Fe2O3 prepared from a natural pyrite ore, (2) CuO supported on mayenite, and (3) SrFeO3-delta strontium ferrite perovskite. Combustion experiments were undertaken by introducing a sample of partially devolatilized biomass (commercial "biochar") to a hot bubbling bed (inner diameter of 30 mm), fluidized by a mixture of oxygen and nitrogen, then analyzing the composition of the off-gas and the burnout time of the char sample. In the temperature range investigated in this work (1023- 1168 K), CuO and SrFeO3-delta but not Fe2O3 thermally decomposed, releasing gaseous O-2 [so-called "chemical looping oxygen uncoupling" (CLOD)]. Hence, to make the combustion conditions comparable to various oxygen carriers, all experiments were performed using a fluidizing gas with a fixed partial pressure of O-2 (pO(2)) of similar to 0.015 bar. Despite the same nominal pO(2) , the occurrence of the oxygen uncoupling reaction increased the total net amount of O-2(g) available in the process, affecting external mass transfer of O-2 to the char particle and accelerating its rate of combustion. The time needed to totally combust 0.1 g of biochar particles in different beds at 1168 K followed the trend CuO < SrFeO3-delta < Fe2O3 approximate to silica sand. The difference in the performance of CuO and SrFeO3-delta was ascribed to the lower oxygen availability via CLOU in perovskite compared to copper oxide. Interestingly, combustion in the bed of Fe2O3 particles took a similar amount of time as combustion in the inert bed of SiO2, despite iron oxide playing an active role in the process. The finding is explained by Fe(2)O(3 )reacting with CO produced from incomplete char combustion, which results in the reduced oxide competing with char for O-2(g) and effectively decreasing the local pO(2).