Co-Combustion of Sewage Sludge and High Ash Coal Thermal Behavior, Ash Formation Behavior, Interaction Mechanisms and Economic Analysis

Abstract

Co-combustion of sewage sludge (SS) with high-ash coal (HC) offers a promising method for SS disposal and resource utilization. However, the impact of SS-HC co-combustion on boiler combustion efficiency, slagging, and economic performance is poorly understood. This study examined the combustion and ash characteristics, focusing on how minerals in SS affect combustion behavior and ash migration behavior. A multi-dimensional evaluation system, including combustion characteristics, operational safety, and techno-economic analysis, was developed to assess the feasibility of co-combustion. The comprehensive combustion characteristic indices for 5SS5HC and 8SS2HC were 4.80 × 10-8 and 1.02 × 10-7, representing 100.58% and 200.01% of HC alone, respectively.  The most effective ratio was 6SS4HC, where the actual fixed carbon loss rate (2.49%/min) surpassed the theoretical rate (1.97%/min) by 25.12%.The minerals CaO, Fe2O3, and AlPO4 in SS had both promoting and inhibiting effects on co-combustion, with CaO > AlPO4 > Fe2O3 in effectiveness. As SS blending increased, alkaline oxides formed low ionic potential modifiers, disrupting silicate polymerization and lowering ash fusion temperature (AFT). Blending 50% SS resulted in a dynamic payback period (DPP) of 2.91 years, while 60% SS increased it to 7.67 years. Balancing these effects, a 5:5 ratio of HC to SS was recommended.

Keywords: Sewage sludge, High ash coal, Combustion characteristics, Ash formation characteristics, Interaction