ASH DEPOSITION DURING BIOMASS AND COAL COMBUSTION - A MECHANISTIC APPROACH

The variability in both inorganic and organic properties of biomass fuels is large. This paper discusses combustion-driven transformations and deposition of inorganic material found in solid fuels, with a focus on the formation of deposits and their properties. A small number of mechanisms is used to describe both the transformations and deposition. The discussion below outlines this mechanistic approach to describing the fate of inorganic material in solid fuels with a particular focus on the mechanisms of ash deposition. This mechanistic approach has the potential of embracing a large range of fuel variations, combustor types, and operating conditions without the need of developing extensive databases or testing procedures for each new situation. The approach has been successfully demonstrated for coal combustion, and examples from coal experiments will be used as illustrations. The same methodology and logic can be applied to biomass combustion. A comparison of coal and biomass is briefly presented, including the chemical structures and the modes of occurrence of inorganic material in the fuels. The major mechanisms of ash deposition during combustion of coal and biomass are related to the types of inorganic material in the fuel and the combustion conditions. The effects of fuel (biomass or coal) characteristics and combustor operating conditions on ash deposit properties such as tenacity, emissivity, thermal conductivity, morphology, strength, chemical composition, viscosity, and rate of growth are discussed. A mechanistic model describing ash deposition in solid-fuel combustors is presented and used to postulate characteristics of ash deposits formed in biomass combustors.