Thermogravimetric measurements of the kinetics of pyrolysis of dried sewage sludge

In a previous paper [Scott SA, Dennis JS, Davidson JF, Hayhurst AN. Chem Eng Sci; 2005; in press [1]] an algorithm was described, which provided a detailed description of the kinetics of devolatilisation of quite complex fuels from conventional thermogravimetric analyses. Such experiments are described here. They measure the loss of mass of a solid whilst it undergoes thermal decomposition, when subjected to a linear rise in temperature. The algorithm uses information from such experiments, at several different rates of heating, to identify how many parallel, first-order reactions participate in devolatilisation. Also, values of the activation energy and pre-exponential factor for each individual step can be determined. Here, this algorithm is used to investigate the devolatilisation of two dried sewage sludges. An ‘undigested’ sludge turns out to lose its volatile matter in eight parallel reactions; a ‘digested’ sludge devolatilises in a more complex way. In both cases the activation energies ranged from 100 to ∼300 kJ mol−1 and the pre-exponential factors varied widely from 1010 to 1018 s−1. Interestingly, the steps with the lowest activation energy have the smaller pre-exponential factors.