Hydrochar prepared from municipal sewage sludge as renewable fuels: Evaluation of its devolatilization performance, reaction mechanism, and thermodynamic property

Abstract

Hydrothermal treatment (HT) of sewage sludge (SS) is a promising method to improve its dewaterability. Effect of HT on devolatilization performance, kinetic parameters, and thermodynamic property during the pyrolysis of SS was investigated by thermogravimetric analysis (TGA), isoconversional method, and master plots method. Derivative thermogravimetry (DTG) curves showed two distinct devolatilization peaks #1 at 283.21 °C and #2 at 327. 43 °C along with a shoulder between 400 and 600 °C. HT temperature influenced peak #2 more significantly as its peak devolatilization rate decreased consecutively from 4.25 % to 1.70 %∙min−1. Meanwhile it moved towards higher temperature and overlapped with the shoulder ultimately. The value of comprehensive pyrolysis index (CPI) decreased greatly from 3.37 × 10−5 to 1.24 × 10−6 %3∙°C−3∙min−2 with HT temperature. Activation energy was determined by Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink, and Friedman methods. It generally increased against conversion with diverse variation patterns. Its average value reduced obviously after HT. Despite HT, all samples followed identical Avrami-Erofeev model with decreased order of reaction from 7.60 to 6.56. The variation trends of enthalpy, Gibbs free energy, and entropy verified that HT improved the thermal stability of SS, which coincided with CPI. Pyrolysis process of SS evolved from ordered state to disordered state, indicating increased reactivity due to the probable catalytic effect of inorganics in the ash. The work provides guidance for the design, optimization, and upscale of pyrolysis reactors and operation parameters when SS is utilized as renewable fuels.