Laboratory measurements of alkali metal containing vapors released during biomass combustion
Alkali metals, in particular potassium. have been implicated as key ingredients for enhancing fouling and slagging of heat transfer surfaces in power generating facilities that convert biomass to electricity. When biomass is used as a fuel in boilers, the deposits formed reduce efficiency, and in the worst case lead to unscheduled plant downtime. Blending biomass with other fuels is often used as a strategy to control fouling and slagging problems. Depending on the combustor, sorbents can be added to the fuel mixture to sequester alkali metals. Another possibility is to develop methods of hot gas cleanup that reduce the amount of alkali vapor to acceptable levels. These solutions to fouling and slagging, however, would greatly benefit from a detailed understanding of the mechanisms of alkali release during biomass combustion. Identifying these alkali vapor species and understanding how these vapors enhance deposit formation would also be beneficial.
Our approach is to directly sample the hot gases liberated from the combustion of small biomass samples in a variable-temperature quartz-tube reactor employing a molecular beam mass spectrometer (MBMS) system. We have successfully used this experimental technique to identify alkali species released during the combustion of selected biomass feedstocks used in larger scale combustion facilities. Multiple combustion conditions have been investigated to target those conditions that minimize alkali metal release. The results of these laboratory studies indicate that initial feedstock composition has the most pronounced effect on alkali metal released during combustion. Four mechanisms of alkali metal release have been identified depending on the feedstock composition. Primary alkali metal release in the combustion of relatively low alkali metal containing woody feedstocks is through the vaporization or decomposition of alkali sulfates. Alkali metal chlorides are the primary alkali metal species released during combustion of herbaceous feedstocks, grasses, and straws with high alkali metal and chlorine contents. For feedstocks with high alkali metal and low chlorine content, alkali metal hydroxides are the most abundant alkali vapor released. If a high alkali content is coupled with high levels of fuel-bound nitrogen, the dominant form of alkali metal vapor is the alkali cyanate. In general, the chlorine content of biomass has been identified as an important parameter that facilitates alkali release.