Thermochemical Equilibrium as an Indicator of Ash Deposition Problems in Biomass Boilers

Combustion of many biomass fuels, in particular herbaceous materials such as straws and grasses, often leads to unmanageable ash deposition problems. The inorganic material from such fuels deposits on heat transfer and structural surfaces and undergoes several chemical and physical changes. For many herbaceous fuels, the resulting deposits have proven to be unmanageable for long-term operation.The formation of fly ash and its deposition on surfaces are now reasonably well understood. The complex thermochemical reactions that can occur after deposition and, to a lesser extent, during flight of fly ash particles, are less well understood. This paper presents rational means of using non-ideal thermochemical equilibrium as an indicator of the potential difficulty of managing ash depositsin furnaces. The application of this theory to practical systems and the role of changing the chemical composition of deposits to influence their manageability are discussed. The equally important roles of chemical kinetics and transport processes are also acknowledged.