Principles and Models of Solid Fuel Combustion

Doctoral thesis, 2001

Combustion of solid fuels stands for a substantial part of the heat and power production in the world. In this thesis different aspects related to the thermochemical conversion of solid fuels in fluidized and fixed beds are treated, such as: the combustion temperature of a fuel particle related to the surrounding temperature in a fluidized bed combustor (FBC); the fuel loading in a FBC; a transient description of the conversion of non-spherical fuel particles; the thermochemical properties and composition of volatiles released from wood; the thermal conductivity of wood during different stages of conversion; the modelling of a grate furnace; the general combustion behaviour of a fixed bed on a reciprocating or a travelling grate; and finally, the design and construction of experimental units for investigation of combustion in a fixed bed. The main result is a number of sub models validated by comparison with measurements. The sub models are to be used to describe and understand various combustion behaviours. Besides the sub models derived, it is concluded that the generally accepted behaviour of combustion on a reciprocating or a travelling grate is not correct for wet biofuels. Measurements and supplementary modelling show that the ignition of the bed is not caused by reflection of radiation from specially designed arches in the furnace. Instead ignition takes place at the bottom of the bed, close to the surface of the grate. This is important for new designs of reciprocating and travelling grates.