Evaluation of hematite oxygen carrier in chemical-looping combustion of coal

Abstract

Chemical-looping combustion (CLC) using a ShenHua bituminous coal and an HuaiBei anthracite as fuel was investigated in 1kWth interconnected fluidized beds under continuous operation. The evaluation of hematite oxygen carrier in coal CLC was performed. Meanwhile, the fate of fuel-N in the CLC process was investigated. According to a 20h continuous operation with the natural hematite at a fuel reactor temperature of 970°C, results showed that the CO2 capture efficiency for ShenHua bituminous coal was 82%, whereas the one for HuaiBei anthracite was 65% due to a low gasification rate for the anthracite. With the fuel reactor temperature range of 880–970°C, there were neither hydrocarbons heavier than CH4 nor tars in the exit of the fuel reactor. For both of the two coals used, the CO2 fraction in the flue carbonaceous gas of the fuel reactor reached 92% at a fuel reactor temperature of 970°C, indicating a high reactivity of the hematite as an oxygen carrier. Under the continuous operation, the inert material of SiO2 in the hematite was difficult to react with the active phases of Fe2O3 or Fe3O4, resulting in the alleviated sintering degree of the oxygen carrier. The mass loss rate of the hematite was about 0.12wt.%/h. There was no nitrogen oxides evolution in the exit gas of the fuel reactor with the fuel reactor temperature range of 880°C–970°C. N2 was the sole product of the nitrogen transfer of fuel-N in the fuel reactor. A high fuel reactor temperature produced more fuel-N to N2. Along with residual char circulating to the air reactor, there was some nitrogen oxides formation in the air reactor. The NO concentration was mostly influenced by the amount of char coming into the air reactor, which was less at a high fuel reactor temperature. Enough residence time for the fuel in the fuel reactor should be ensured with respect to eliminate NO formation in the air reactor, and a carbon stripper is better to be employed and developed in the future design and operation of CLC plant.