Assessing the bioenergy potential of high-ash anaerobic sewage sludge using pyrolysis kinetics and thermodynamics to design a sustainable integrated biorefinery

Abstract

A new opportunity for producing valuable biorefinery products can be found by integrating biochemical and thermochemical processing with municipal wastewater treatment. This study is the first to evaluate the kinetic triplet and thermodynamic parameters from the pyrolysis of typical Brazilian anaerobic sewage sludge performed in the framework of a multi-step solid-state process. The physicochemical characteristics of the anaerobic sewage sludge are comparable to those obtained from low-rank coals. The pyrolysis characteristics were analyzed by non-isothermal thermogravimetry under different heating rates (10, 25, 50, and 90 K min−1) in an inert atmosphere. Two devolatilization stages were distinguished from the active pyrolysis zone, with an average mass loss of 47.56 wt% (sum) in the range of 398–953 K. For each devolatilization stage, three isoconversional methods (Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Starink) were utilized to calculate the activation energy, and then the compensation effect method was applied to find the pre-exponential factor. The average activation energies calculated ranged from 113.7 to 117.3 kJ mol−1 for the first stage and from 115.7 to 121.9 kJ mol−1 for the second stage, with respective pre-exponential factors of 7.39 × 109 min−1 and 8.80 × 107 min−1. According to the master-plots method, it was found that the first stage followed the fourth-order (F4) model, while the second stage was described by the second-order (F2) model. Based on the statistical evaluation, the devolatilization behaviors reconstructed from overall kinetic expression agree reasonably well with the experimental data, proving its practical importance for designing a pyrolytic processing system using anaerobic sewage sludge as raw material. This study contributes by providing useful insights that can be applied to a large-scale biorefinery as a critical step towards producing biofuels coupled to municipal wastewater treatment in an environmentally sustainable manner.