Experimental Study Of CO₂Sequestration And Simultaneously Stabilizing Heavy Metals Through MSWI Fly Ash Carbonation Process Under Low-pressure Condition

Nowadays, solid waste incineration technology has been widely used in the processingof municipal solid waste in our country. However, large amounts of toxic fly ash could beproduced during the solid waste incineration process, which definitely need stabilizingtreatment before its final landfill disposal since it contains high concentration of dioxin,soluble salts and heavy metals. In the meantime, certain amount of CO₂also could bereleased into the atmosphere to aggravate the green house effect. In this regard, fly ashcarbonation treatment on the one hand can be able to absorb a part of the CO₂to formcarbonate minerals, and on the other hand, the heavy metals in the fly ash can be trapped inthe newly formed carbonate mineral, hence reducing its leaching toxicity. It has beenrecognized as a potential way of better application prospect.In this paper, fly ash samples of three different municipal solid waste incinerationpower plants were collected and respectively treated through carbonation process with pureCO₂under different water-solid-ratio, temperature, and reaction time conditions. The X-raydiffraction （XRD）, Scanning Electron Microscopy, thermogravimetry-differential scanningcalorimetry （TG-DSC） and Horizontal Vibration Method （HJ557-2009） were used toanalyze the crystalline phase and microstructure of fly ash sample, the amount of CO₂absorbed by the carbonation process and leaching characteristic of heavy metals （Pb、Cu、Zn、Cr、Cd、Co、Ni、As、Hg） before and after the carbonation process. In addition, finishthe preliminary design of cold test bench of bubbling fluidized bed for further dynamiccarbonation experimental study.The XRD analysis results showed that the peak of Ca（OH）2and CaClOH of Shanghaifly ash samples disappeared while peak of CaCO3was increased and enhanced evidentlyafter the carbonation process. While the peak of CaCO3in Wuhan fly ash samples wasenhanced for a little bit.It is found that the largest absorption and fixed amount of CO₂by carbonation process can be achieved when the liquid-solid ratio is0.2ml/g for Shanghai and WH2samples,while0.3ml/g for WH1samples, which is54.95L/kg,9.89L/kg and9.63L/kg, respectively.Besides, the following repeated experiment of SH samples also draws a consistentconclusion. Compared the absorption and fixed amount of CO₂of the three samples, it isobvious that the amount of Ca（OH）₂and CaClOH which is much easier to react with CO₂to form CaCO₃contained in fly ash has a substantial influence on the amount of CO₂absorbed by fly ash during the carbonation process. Under the experimental condition inthis study, CO₂absorption amount during the carbonation process of SH ash samplesincreased with the growth of reaction time, and ultimately reach a stable value. At the sametime, carbonation process of Shanghai ash samples under100℃condition has a larger CO₂absorption amount than other three temperature condition.The leaching toxicity of heavy metals in fly ash such as Pb, Cr, Co, Ni, Cu, As and Hgwere effectively reduced after the carbonation process. However, the leaching toxicity ofCd was increased after the carbonation treatment. For the Zn in Wuhan ash samples, itsleaching toxicity was increased, while the Zn in Shanghai ash samples was increased insome experiment times but decreased in other times. Moreover, the lowest leaching toxicityof Pb, Cr, Co, Ni, Cu, As and Hg usually can be achieved when carbonation process wasperformed under the optimum Liquid-solid ratio condition.