Kinetic models for the pyrolysis and combustion of two types of sewage sludge

Abstract

Pyrolysis and combustion of two types of sewage sludge have been studied. The two sludges considered, considering their thermal decomposition, have different behaviors when comparing pyrolysis and combustion TG runs of each sludge. In one case, the combustion can be considered as an oxidative pyrolysis, where the decomposition process is accelerated by the presence of oxygen. In the other case, the TG combustion run has two parts, the first one coinciding with the pyrolysis run and the second one with combustion of the char formed previously. For the pyrolysis of each sludge, three thermogravimetric runs at different heating rates (5, 15 and 30 K/min) were carried out to obtain a kinetic model, considering three parallel reactions. Following the simplex method, and with the same set of parameters for the three runs, and for each one of the three fractions considered, the values of pre-exponential factors, the apparent activation energies (103.9, 169.7 and 209.4 kJ/mol), the reaction orders and the maximum amount of volatiles were obtained. A discussion of the different values, in comparison with others obtained from literature is presented. Following a similar methodology, for the combustion of each sludge, two series of three runs at different oxygen concentrations (10 and 20%) and heating rates were carried out. For the sludge with oxidative pyrolysis behaviour, the scheme of three parallel reactions (as in pyrolysis) was appropriate for correlating the data, using the same values of activation energy, reaction order and maximum amount of volatiles evolved, and optimizing only the value of the pre-exponential factor, that is correlated with oxygen pressure. On the other hand, for the other sludge, with the first stage coinciding with pyrolysis and the second stage concerning the char combustion, the kinetic parameters of the second stage are obtained, with an apparent activation energy of 144.1 kJ/mol.