Converting sludge to slag through a high temperature slagging co-gasification process: an evaluation based on a demonstration trial and life cycle assessment

Abstract

A variety of sludge-based waste materials with distinctive physical and chemical characteristics are co-treated with municipal solid waste (MSW) in a demonstration-scale high-temperature slagging gasifier. The waste materials tested include dewatered sewage sludge (originated from industrial and municipal wastewater treatment processes, respectively), dried sewage sludge, sludge incineration ashes, and waterworks sludge (originated from freshwater treatment process). Up to 49.5% by mass of dried sewage sludge is successfully co-gasified with MSW and up to 89.5% of the solid residues from the gasification process is recovered as reusable slag and recyclable metals. Despite feeding of various waste mixtures, stable operation of the slagging gasifier is achieved with the proper control of the temperature profiles inside the gasifier, the melting of inorganic residues, smooth discharge of the molten slag, and effective flue gas emissions control. Various types of waste-derived slag are characterized to evaluate their potentials to be applied as sustainable construction materials. Toxicity characteristic leaching procedure (TCLP) and compliance batch leaching test results indicate that the sludge-derived slag is environmentally safe with very low pollution potential. In addition, life cycle assessment (LCA) is performed to comprehensively evaluate the treatment of sludge-based waste materials via slagging co-gasification technology. These findings reveal the promising potentials of converting sludge to slag through the high temperature slagging gasification process based on both engineering and environmental aspects, as well as the possible areas for further advancement in the future.