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 O2) or devolatilise (in the presence of O2, when the volatile material burns) 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: constant1×ds2+constant2×ds, dsds being the diameter of the pellet. Devolatilisation time, tvtv, can be correlated with dsds by tv=constant2×dsn, where n=1.45±0.15n=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 dsds 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=0n=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.