Supercritical CO2 coupled with water washing to enhance the removal of chlorine in MSWI fly ash together with its carbon sequestration function

Abstract

The high chlorine content in municipal solid waste incineration (MSWI) fly ash is a key factor restricting its treatment and disposal. In this study, a new treatment method was proposed to enhance the deep dechlorination of fly ash by coupling supercritical CO₂ (SC) treatment with water washing. Simultaneously the alkaline compounds in fly ash can fix CO₂ and achieve CO₂ credits. The results showed that supercritical CO₂ significantly improved the removal of chlorine. The treatment of “double water washing + SC + washing again” reduced the chlorine content in fly ash to 0.83 %, while “SC + double water washing + SC + washing again” reduced the chlorine content to 0.88 %, and the removal rate of chlorine was as high as 96.8 % and 96.5 %, respectively. In addition, supercritical CO₂ promoted the decomposition of insoluble chloride (Friedel’s salt) and the removal rate of insoluble chloride salts can reach 89.1 %. On the other hand, after supercritical CO₂ treatment, fly ash can quickly and efficiently absorb CO₂, and the amount of CO₂ absorbed after two supercritical CO₂ treatments was 64 g/kg. Lastly, supercritical CO₂ could significantly stabilize heavy metals in fly ash and reduce the concentrations of Pb and Cr in the washing solution.