Alkali sulphation in flames

Övrigt konferensbidrag, 2015

The use of low quality fuels in power generation is typically motivated by a potential reduction in fuel costs or CO2 emissions, the latter in case the fuel is based on biomass. These features make low quality fuels attractive at the same time as such fuels are usually problematic to use in power generation due to their fuel composition. One of the main issues is high temperature corrosion (HTC), caused by alkali containing chlorides and sulphates. The alkali chlorides are particularly problematic and these are formed from the alkali metals and chlorine when released during the combustion process. These chlorides form salts which are deposited on surfaces at relatively low temperatures, typically in the range 500-600°C. The HTC problem is often related to combustion of biomass and it has a significant effect on the achievable thermal efficiency and/or maintenance cost of the power plant. However, when sulphur is present, the alkali metals may form sulphates instead of chlorides. Sulphate salts are typically less corrosive and are deposited at higher temperatures and the negative impact on the thermal efficiency is therefore less pronounced. The sulphur content in biomass is typically low compared to the content of alkali metals and chlorine. Therefore, sulphur has to be added if alkali sulphation should be significant. One option is to blend the biomass with another fuel with higher sulphur content, typically coal. In this way, co-combustion can increase the thermal efficiency of power plants using biomass and at the same time lower the use of fossil fuels. However, the current understanding of the chemistry related to the formation of alkali chlorides and sulphates is limited and needs to be improved for a better control of the HTC process when using low quality fuels for power generation, both in todays’ air-fired boilers as well as for future boiler technologies such as oxy-combustion.