Elucidation of synergistic effects in straw/sludge co-pyrolysis through gaseous product monitoring and biochar analysis

Abstract

The environmental problems posed by the increasing global production of sludge highlight the need for efficient sludge treatment methods ideally affording value-added products, as exemplified by the co-pyrolysis of sludge and biomass. In this study, we aimed to investigate the effects of temperature (500, 600, 700, and 800 °C) and straw content (0, 25, 50, 75, and 100 wt%) on the properties of gaseous products and biochar during pyrolysis of sludge–straw mixtures in a fixed-bed reactor. Gas (e.g., CO2, CO, CH4, and H2) release increased with increasing temperature (up to 700 °C) and straw content, which was ascribed to the high content of volatiles in straw and its ability to act as a hydrogen source for sludge. The specific surface area and pore volume of biochar first decreased and then increased with increasing temperature, whereas the reverse was true for pore size. The synergy between sludge and straw, quantified as the difference between experimental and predicted biochar yields and specific surface areas, was maximized at a straw content of 75 wt% and was analyzed to design a pyrolysis product framework based on the circular economy concept. Thus, this study paves way for the effective utilization of sludge co-pyrolysis products and the preparation of mixed biochar, providing a reference for solid-waste treatment and efficient energy utilization.