Straw Pellets Burning in a Fixed Bed with Low Ash Agglomeration

Melting of the straw pellets’ ash leads to bed agglomeration with incomplete burning out and even to burning extinguishing. The aims of the work are to study the ash agglomeration during the combustion of straw pellets in a fixed bed at different primary air velocities, which differs by being carried out at various thicknesses of the pellets bed, and to substantiate the mode of burning with minimized ash agglomeration. The aims are achieved by: batch burning of straw pellets with recording bed temperatures; determining the mass fraction of agglomerated ash; determining the ash fusibility temperatures; analyzing the chemical composition of ash and calculating the agglomeration indices; determining energy loss because of incomplete carbon burnout. The most important results are: experimental evidence that decreasing the primary air velocity, combined with lowering the pellets bed thickness, resulted in lower maximal temperatures in the bed and decreased bottom ash agglomeration; when maximal bed temperature decreased the chemical composition of bottom ash changed with ash indices shifting to lower slagging and agglomeration. The practical significance of the obtained results is that at maximal bed temperatures ranging from 1061 to 915°C, the content of agglomerated bottom ash, having initial deformation temperatures of 1065–1045°C, was 0.03 of ash mass. This is a so-called low-temperature mode of burning where the maximal temperature in the burning bed is lower than the initial deformation temperature of the formed bottom ash, thus preventing its melting and agglomeration, and is recommended for straw pellets combustion in a continuously operated burner with acceptable energy loss because of incomplete carbon burnout.