Chemical-looping combustion in a 100-kW unit using a mixture of ilmenite and manganese ore as oxygen carrier

Abstract

Chemical-looping combustion (CLC) is a novel carbon-capture technology with potential to drastically reduce the cost of CO2 capture. Relying on interconnected fluidized bed technology, CLC systems can achieve CO2 capture by using oxygen carrying bed material. This so-called oxygen carrier transports oxygen from combustion air to fuel, thus making carbon capture inherent to the CLC process. In this study, we present findings from a 100kW chemical-looping combustor for solid fuels. The 100kW unit uses the dual-CFB concept, where both air reactor and fuel reactor are designed as circulating fluidized beds.The oxygen carrier material used in this study consisted of a mixture of ilmenite – which has been used in several studies in CLC – and a manganese ore. Previous studies have shown that gas conversion can be significantly increased by using manganese ore particles as oxygen carrier. However, previous testing has also shown that the production of fines, i.e. particle attrition, may be high when using manganese ore. The reason for mixing the two materials is thus to obtain an oxygen carrying material that has high reactivity, and yet does not produce too much fines during fuel operation.The 100kW unit was operated in total for 18h with fuel. Three fuels were used in the experiments: two bituminous coals and wood char. Gas conversion was high, and increased with increasing fraction of manganese ore in the oxygen-carrier mixture. At the end of the experiments, the fraction of manganese ore in the bed material was approximately 8%, which also was the highest fraction during all tests. The mixture of ilmenite and manganese ore gave significant improvements in gas conversion in comparison to only ilmenite. The highest gas conversion observed during testing with bituminous coal was 91.5%, as compared to 84% with only ilmenite as oxygen carrier during similar conditions in the 100kW unit.These test results indicate that the addition of manganese ore could almost halve the fraction of unconverted gas. Thus, mixing mechanically stable ilmenite with more reactive manganese ore can give reductions in costs as compared to using manganese ore only, and still give significantly reduced oxygen demand as compared to ilmenite. In the present case – mixing of ilmenite and manganese ore – the high reactivity was also possible to combine with improved operability of the material, primarily manifested as lower production rate of fines.