Ash-Related Issues in Fluidized-Bed Combustion of Biomasses: Recent Research Highlights

Finland and Sweden are leaders in the use of biomass fuels in large-scale boilers. In these countries, the dominating large-scale combustion technology for biomass fuels is fluidized-bed combustion (FBC). Biomass fuels differ in many ways from the standard fossil fuels used in FBC, such as coal. They often have high moisture contents, lower heating values, and a variety of impurities, such as chlorine, sulfur, phosphorus, nitrogen, and a variety of ash-forming metals. FBC of biomass fuels is often connected with operational challenges, which are related to the fuel chemistry and fuel properties. Bed sintering, superheater fouling, and high-temperature corrosion are crucial factors to take into account when fuels are selected for FBC. It is of vital interest to find ways of predicting the degree of these kinds of ash-related problems for various fuels or fuel mixtures. This paper reviews some of the recent progress in our understanding of the fate and behavior of ash-forming matter in FBC. The following topic areas are discussed: fuel characterization, release of the ash-forming matter during combustion, interaction of the ash and bed material, fly ash formation, fly ash properties, ash deposits, and fouling and corrosion.