The devolatilisation of particles of a complex fuel (dried sewage sludge) in a fluidised bed

A complex fuel is understood here to be a solid whose pyrolysis or devolatilisation (i.e., thermal decomposition) involves many reactions. Sewage sludge is such a fuel and when dry can contain more than 80 wt% of volatile matter. Consequently, the combustion of a sewage sludge is mainly a question of burning the volatile matter. Sewage sludge particles in a fluidised bed of size 0.6-8 mm pyrolyse (in the absence of O-2) or devolatilise (in the presence of O-2, when the volatile material bums) at a rate determined by (i) chemical kinetics, or (ii) heat transfer, either by conduction within the pellet or from the fluidised bed to the pellet's exterior. A scaling analysis, in conjunction with kinetic information obtained from thermogravimetric experiments, showed that most of the volatile matter in our sludge was lost in a manner controlled by heat transfer, after the sludge was added to the fluidised bed. This observation is consistent with the times taken for the evolution of 60% or 80% of the volatile matter, when the fuel was pyrolysed in argon, varying approximately as: constant(1) x d(s)(2) + constant(2) x d(s), d(s) being the diameter of the pellet. Devolatilisation time, t(v), can be correlated with d(s) by t(v) = constant(2) x d(s)(n), where n = 1.45 +/- 0.15. Previous workers obtained lower values of n. It is suggested that some differences in the value of n are related to the definition used for the devolatilisation time. The variation with d(s), for the evolution of 95% of the volatile material, for the smaller particles used, was much less clear than for the release of 60-80% of the volatiles. Given the scatter, a value of n = 0 could not be discounted. Such a value of n is consistent with the most tightly bound volatile material being released at a rate determined by chemical kinetics. (c) 2006 Elsevier Ltd. All rights reserved