Enhancing hydrogen-rich fuel gas production from dewatered sludge pyrolysis: The effect of hybrid conditioning with peroxyacetic acid, polyaluminium chloride, and KOH-modified activated carbon

Abstract

Pyrolysis is a promising technology for the disposal of sewage sludge, because it not only greatly reduces the volume of sludge but also converts it into high calorific and pollution-free clean energy, H2. Direct pyrolysis of raw sludge (with moisture content of about 98%) consumes a lot of energy, thus it is necessary to dewater the raw sludge before pyrolysis. However, sludge is a heterogeneous colloidal system, making it difficult to reduce water content with traditional pressurization. Conditioning agents often are needed to improve the dewatering efficiency. In this work, we applied peroxyacetic acid (PA) and polyaluminium chloride (PAC) combined with KOH-modified activated carbon (KAC) to condition the raw sludge (denoted as PA/PAC/KAC) before conducting dewatering and pyrolysis. We found that changes of organic matter composition of the dewatered sludge exerted by the conditioning significantly influenced pyrolysis gas production. The pyrolysis H2 yield from the conditioned sludge can be 4–5-fold higher than the unconditioned. The PA oxidation mediated by KAC led to more olefin substances appearing in the dewatered sludge, which further promoted the production of low aromatic hydrocarbons through the Diels-Alder reaction in pyrolysis and enhanced tar cracking rate. Potassium loaded on KAC catalyzed tar cracking. KAC remained in the dewatered sludge promoted water–gas-shift (WGS) and steam reforming reactions to produce more H2. This study provides a new path to enhance the resource utilization of dewatered sludge and facilitate the resource utilization of the dewatered sludge in potential applications.