Sulphation of potassium chloride in air and oxy-fuel combustion
Conference contribution, 2013
During combustion of biomass, alkali and chlorine species are released to the gas phase, species which can react to form corrosive salts that potentially may cause severe corrosion problems on heat transfer surfaces upon condensation. This corrosion process is referred to as high-temperature corrosion (HTC) and it limits steam data in biomass combustion, with corresponding limitation in thermal efficiency. The flue gas composition and also the HTC conditions can be altered by introducing a second fuel, e.g. coal for co-combustion. The flue gas composition can be altered even more drastically when applying oxy-fuel combustion where the concentration of combustion products are higher than in air firing due to the use of pure oxygen and flue gas recycling. In this work, the alkali sulphation process in different oxy-fuel and air-fuel atmospheres has been investigated by modelling the gas phase chemistry. The overall purpose is to examine the combined effects of oxy-combustion and co-combustion on the HTC process and to make a comparison with air-fired conditions. The flue gas composition in oxy-fuel combustion depends on the recirculation strategy used which in turn also influences the sulphation. According to the modelling results the degree of sulphation of gas-phase alkali metals is in general higher for oxy-fuel combustion compared to air-fuel combustion. It is concluded from the modelling results that an increased amount of sulphur and water contribute to a substantial effect on the degree of sulphation of alkali species. On the other hand, the modelling also shows that an increased content of HCl, which is present during wet recirculation, has a negative effect on the sulphation; the degree of sulphation is therefore lower in wet compared to dry recirculation. The effects of other important parameters such as sulphur-to-potassium and air-to-fuel ratios, temperature and residence time are also discussed further in the paper.