Different effects of mesophilic and thermophilic anaerobic digestion on subsequent pyrolysis: Characteristics, kinetics and thermodynamic analysis

Anaerobic digestion (AD) combined with pyrolysis has been used as an innovative sludge-to-energy process that enhances energy recovery. This study investigated the different impacts of mesophilic and thermophilic AD on subsequent pyrolysis. The pyrolysis characteristics, kinetics, and thermodynamics of raw sludge (RS) and digested sludge (DS) after both mesophilic and thermophilic AD were investigated using thermogravimetric analysis (TGA) and three kinetic models (Starink, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose). TGA results indicated that thermophilic AD reduced the weight loss from 71.2-75.6 % for RS to 55.0–59.1 % for DS, more considerably than mesophilic AD (reducing from 70.6-75.2 % to 61.3–69.5 %), indicating higher organic matter removal. Kinetic analysis showed that the activation energy (E) of RS and DS pyrolysis ranged from 128.9 to 399.9 kJ/mol and 117.3 to 470.4 kJ/mol, respectively. Mesophilic AD generally increased the E, while thermophilic AD did not markedly alter E at low conversion rates (0.2–0.5) but increased it at high conversion rates (0.6–0.8), changing from 203.7–399.9 kJ/mol for RS to 248.9–470.4 kJ/mol for DS. Thermodynamic analysis showed that mesophilic AD generally enhanced pyrolysis favorability and reactivity but impeded activated complex formation. Thermophilic AD enhanced favorability and favored complex formation at low conversion rates but hindered it at high conversion rates. Reactivity varied with thermophilic AD extent, with excessive AD reducing reactivity. Pyrolysis–gas chromatography/mass spectrometry analysis confirmed that AD, on the one hand, decomposed biodegradable organics into low-molecular-weight organics, retained them in the DS, and facilitated subsequent pyrolysis. On the other hand, AD could accumulate large bio-refractory molecules, rendering DS less prone to pyrolytic decomposition. This effect of thermophilic AD was more significant than that of mesophilic AD. Given more favorable change was observed in kinetics and thermodynamics at lower conversion rates, low-temperature pyrolysis to produce biochar might be a promising strategy for DS treatment. The insights gained in this study may guide the optimization of AD conditions to maximize the efficiency of subsequent pyrolysis, providing more insight into the thermochemical conversion of DS and pyrolysis design.