Environmental aspects of slag derived from co-gasification of municipal solid waste, incineration bottom ash and sewage sludge

Abstract

As an option to improve existing waste management structure in Singapore — slagging gasification or also known as the ‘Direct Melting System’ has been considered as it is known to achieve high energy recovery, at the same time produce reusable vitrified slag. To further enhance a circular economy model and to divert more incineration bottom ash (IBA) away from Pulau Semakau in Singapore, the incorporation of either IBA or sludge mixed into municipal solid waste (MSW) feedstock was proposed for the pilot-scale 11.5 tonnes/day slagging gasifier in the Nanyang Technological University’s Waste to Energy Research Facility (WTERF). First, a baseline study of MSW slagging gasification products was carried out. Following that, the study of slags derived from the co-gasification processes was carried out. Microwave-assisted acid digestion (MAD) and Toxicity Characteristic Leaching Procedure (TCLP) were conducted for products of MSW gasification with coal coke or biochar, co-gasification with MSW and IBA, co-gasification with MSW and sludge, and IBA to determine toxic constituent and release respectively. After that, Ion chromatography (IC), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) were used to analyse sample compositions which would be used for various calculations. Results has shown that MSW co-gasification with IBA slag was within TCLP limits. However, leached Fe concentrations from MSW co-gasification with sludge slag have exceeded TCLP limits potentially due to presence of loose magnetic metals. Notably, Zn had been identified to be the most susceptible to leaching as it had the highest leaching ratio. In addition, to understand how the sludge and incineration bottom ash (IBA) are treated by Singapore and developed countries, a detailed literature review on common sludge and IBA treatment methods was done to evaluate environmental impacts alongside MSW gasification technology in accordance to two factors: (1) emission control and (2) reusability of final products. It was found that high temperature slagging gasification has a good standing as complementary waste treatment technique to commonly use in Singapore incineration.