Describing sewage sludge pyrolysis kinetics by a combination of biomass fractions decomposition

The sewage sludge is considered to be highly heterogeneous and comprising a mixture of various compounds. The characterization was performed in order to follow their compositions according to biomass fractions. The analytical method used was a sequence of ethanol–toluene extraction and digestion by hot water and sulfuric acid, achieving the fractionation of extractives, water- and acid-insoluble materials and lignin contents, respectively. Pyrolysis behavior was studied by Thermogravimetric Analysis run on original sludge and its extracted/digested residues. Typical main decomposition occurred in the range of 250–600 °C. The decomposition mechanism was more obviously elucidated when extracted and digested residues were tested. According to DTG data, the kinetic modeling was performed using a combination of four reactions, each reaction contributed by an individual compound. The apparent activation energies were found at ca. 82, 121, 64 and 121 kJ mol−1. The first three values were comparable to that of biomass species decomposition, namely hemicellulose, cellulose and lignin, respectively. The first compound may be also explained by a complex protein presented in bacteria. Another reaction, however, was categorized into extractives such as lipids, waxes or oils. Instead of 1.0 as realized in biomass species, reaction order of sludge component decomposition was somewhat deviated from 1.0 (between 0.91 and 1.63). This indicated some properties of sludge constituents that deviate from natural biomass fractions. The hypothesis was also confirmed by FTIR run on original and extracted/digested sludges which indicate the closely analogy to those biomass species.