Oxygen carriers for chemical-looping combustion of solid fuels

The main economic advantage of CLC is likely realized for combustion of solid fuels such as coal, petroleum coke and biomass. In a process where the fuel and air reactor are designed as interconnected fluidized beds and where the solid fuel is well-mixed with the oxygen carrier particles there are two main mechanisms for char and volatile conversion: i) Normal CLC where the volatiles and gasification intermediates react directly with the oxygen carrier particles through a gas-solid reaction and ii) through CLOU or chemical-looping with oxygen uncoupling, where the oxygen carrier particles release gaseous oxygen which reacts with the solid fuel components through normal combustion. In addition, several oxygen carriers have been identified to have some CLOU properties, and for these so-called semi-CLOU materials, it could be speculated that the solids and volatiles react through both CLC and CLOU processes in parallel. The role of the oxygen carrier is thus different and depends upon the process which is used above. For route i) the oxygen carrier needs to convert the intermediate syngas components, mainly H2 and CO, as well as volatiles at a sufficient rate to achieve high gaseous yield to carbon dioxide. Here the oxygen carrier may have a limited effect on the char conversion. For route ii), or CLOU, the oxygen carrier needs to supply oxygen to the fuel at a sufficient rate to achieve a high rate of char and volatile conversion. For semi-CLOU materials, the role of the oxygen carrier material could depend upon a variety of factors, including the rate of oxygen release, and where it is released. The paper discusses the three processes described above and proposes a simple generic analytical model to evaluate the three types of processes above. From the model, quantitative reactivity criteria for oxygen carrier particles can be obtained. It is suggested that it is likely a substantial advantage to utilize oxygen carriers with CLOU properties to process solid fuels, both with respect to char and volatile components. The advantage of semi-CLOU materials is less clear