The kinetics of the pyrolysis or devolatilisation of sewage sludge and other solid fuels

It has been previously shown that the pyrolysis of one, dried sewage sludge can be modelled using 25 first-order, parallel reactions. Here a “computer experiment” has been performed on the pyrolysis of particles of that sewage sludge, whose thermal decomposition has been simplified to involve nine parallel, first-order reactions, which “cover” the 25 steps measured previously. The activation energies of these nine reactions range from 110 to 310 kJ/mol in equal steps of 25 kJ/mol. The pre-exponential factors of these, equally weighted, reactions increase from 1.0 × 109 to 1.5 × 1018 s−1, in accord with the previous measurements. It is found that the Arrhenius plot for the effective, overall rate constant governing the fall in mass during the kinetically controlled pyrolysis of this surrogate sewage sludge is curved, but approximated well by the expression: 8.5 × 1010 exp {− (140 ± 30 kJ/mol)/RT} s−1. This simple result enables a time-scale to be evaluated for the kinetics of this fuel’s pyrolysis and suggests that the thermal decomposition of this solid fuel might sometimes be described by just one chemical reaction. It turns out that such a “one-step model” often fails as a description of pyrolysis, because there is a wide, intermediate range of temperature, where some steps are reaction-controlled, whilst others are governed by heat transfer. Thus devolatilisation of a solid fuel has to be considered in terms of many reactions being involved. The situation is rationalised by assigning to each single reaction involved in devolatilisation its own “decomposition temperature”, at which, depending on particle size, that reaction changes from kinetic to heat transfer control at higher temperatures. Otherwise, if the Biot number is larger than unity, reaction fronts pass through a pyrolysing particle, creating a shrinking core, where that reaction has not yet occurred. However, if the Biot number is much less than unity, each step in the chemical mechanism proceeds uniformly throughout a heated particle, but attains an almost constant rate, as soon as its decomposition temperature is reached. Decomposition temperatures depend on particle size and are evaluated for the sewage sludge under consideration. The importance of particle size and also the Nusselt and pyrolysis numbers is also investigated, as well as how best to measure the kinetics of pyrolysis. Such kinetic measurements are more difficult to make for biomasses and waste fuels than for coals.