The role of alkali in agglomeration during combustion in fluidised beds

Biofuels originating from process industries as by-products can be utilised to produce onsite power which can reduce their dependence on the National Grid. Beet Sugar industry byproducts vinasse, raffinate and pressured sugar beet pulp are cofired with Thoresby coal in a 25kW fluidised bed combustor. Agglomeration indices were used and muffle furnace tests were carried out before firing the materials in order to pre-assess the suitability of the materials for the firing tests as well as for finding out validity of indices and muffle furnace tests in relation to actual firing tests. The effect of the presence of alkali and its concentration in the fuel on the onset of agglomeration is investigated. Presence of Calcium in coal ash extends defluidization time which indicates that lime can be used as a bed material to increase operational times while firing these troublesome fuels. However, no signs of agglomeration were observed during prolonged tests with blends of coal and pressed pulp. Therefore, pulp can be used as fuel in fluidised bed without the use of alkali getters as long as operational parameters are properly controlled. But, vinasse and raffinate can't be used without adapting measures such as addition of alkali getters to reduce agglomeration. It is found that accumulation of alkali as well as its feed rate in to fluidised bed are very important parameters in determining agglomeration behaviour of the bed.