Thermal analysis of chemical-looping combustion
Artikel i vetenskaplig tidskrift, 2006
In chemical-looping combustion, a gaseous fuel is burnt with inherent separation of the greenhouse gas CO2. Oxygen is transferred from the combustion air to the fuel by an oxygen carrier, which is usually a metal oxide, and therefore direct contact between the fuel and the combustion air is avoided. Thus, the products of combustion, i.e., CO2 and H2O, are not mixed with the rest of the flue gases and after condensation almost pure CO2 is obtained, without any energy lost for the separation. A thermal analysis of the process using a large number of possible oxygen carriers was performed by simulating reactions using the HSC Chemistry 5.0 software. Three fuels were used in the investigation, CH4, CO and H2. Based on the ability of the oxygen carriers to convert the fuel to the combustion products CO2 and H2O, stability in air and the melting temperatures of the solid material some metal oxides based on Ni, Cu, Fe, Mn, Co, W and sulphates of Ba and Sr showed good ther-modynamic properties and could be feasible oxygen carriers. Only a few of these possible oxygen carrier systems, based on Cu, Fe and Mn, showed complete conversion of the fuel gas, but still the other systems had limited equilibrium restrictions, with only small and acceptable amounts of unreacted CO and H2 released from the fuel reactor. The promising systems were investigated further with respect to temperature changes in the fuel reactor as well as possible carbon, sulphide and sulphate formation in the fuel reactor. For some systems the reactions in the fuel reactor were endothermic, resulting in a temperature drop in the fuel reactor. However, this drop can be limited by applying a sufficient circulation of particles from the air reactor to the fuel reactor. When Ni or Co is used as oxygen carrier the fuel may need to be desulphurized prior to combustion to avoid formation of solid or liquid sulphides or sulphates. On the other hand, to prevent decomposition of the sulphates BaSO4 and SrSO4, in the fuel reactor, to sulphur-containing gases and metal oxides, it is necessary that some sulphur is present in the fuel and that high temperatures are avoided. Formation of carbon should not be a problem as long as the process is run under conditions of high fuel conversion.