Synergistic Characteristics of Co-Hydrothermal Carbonization of Sewage Sludge and Waste Biomass

Abstract

This study investigated the synergistic characteristics of co-hydrothermal carbonization (co-HTC) of sewage sludge (SS) and wheat straw (WS). Co-HTC of SS and WS synergistically promoted the conversion of organic matter into combustible hydrochar components. The synergistic coefficients for hydrochar yield (YHC), energy recovery efficiency (REN), carbon recovery efficiency (RC) and hydrogen recovery efficiency (RH) first increased and then decreased as the WS blending ratio or hydrothermal carbonization (HTC) temperature increased. Additionally, blending WS or increasing the HTC temperature shifted the main combustion process of hydrochar to the low-temperature region. The average activation energy of hydrochar combustion first decreased and then increased as the WS blending ratio increased. However, the average combustion activation energies of hydrochars prepared via HTC at 200, 230 and 260 °C were essentially identical (160.61, 161.88, and 161.04 kJ mol^-1, respectively), with a relative difference of < 1%. Maintaining an HTC temperature of 200 °C avoids the higher energy input required for elevated temperatures (230 or 260 °C), which is crucial for reducing the industrial application costs of HTC while preserving hydrochar quality. Consequently, from the perspective of energy consumption, 200 °C is more suitable for the HTC. The minimum average activation energy for hydrochar combustion (160.61 kJ mol^-1) was achieved at a SS:WS mass ratio of 2:1 and an HTC temperature of 200 °C. These research findings provide a theoretical basis for the energy-oriented utilization of SS and waste biomass.

Keywords:
sewage sludge; waste biomass; hydrochar; synergistic effect; combustion; kinetic analysis