Drying-free sludge gasification via moisture-tolerant co-processing

Abstract Sewage sludge management is challenged by its high moisture content, which necessitates energy-intensive drying before thermochemical conversion. This study proposes a novel approach by investigating the co-gasification of wet sewage sludge (WSS) with diverse biomass feedstocks to eliminate the pre-drying requirement. Using Aspen Plus simulations, the process was systematically evaluated. First, the gasification of dried sewage sludge (DSS) was optimized, achieving a cold gas efficiency (CGE) of 75%. In contrast, gasification of WSS alone yielded a significantly lower CGE of 40%. Co-gasification with biomass markedly improved efficiency by mitigating moisture content. The extent of improvement was highly dependent on biomass type and blending ratio. Rice straw (RS), with its inherently low moisture content, proved to be the most effective co-feed, achieving a CGE of 65% in a 30% WSS–70% RS blend—a performance comparable to DSS gasification. Economically, this co-processing strategy reduced the payback period from 12 years (for WSS) and 4 years (for DSS) to just 3 years, primarily by eliminating the capital and operational costs of the dryer. This study provides a validated simulation framework and demonstrates that moisture-tolerant co-processing, particularly with rice straw, presents a technically viable and economically advantageous alternative to conventional sludge drying, enhancing the sustainability of sludge-to-energy conversion.