Thermochemical heat storage utilization of MSWI fly ash after carbonation enhancement with ammonia water regulation: Carbon sequestration efficiency, heavy metal immobilization, and heat storage characteristics analysis

Accelerated carbonation is an effective method of treating municipal solid waste incineration fly ash (FA). However, the efficiency of CO2 sequestration is limited and the usage of products is uncertain. This study proposed a method for enhancing carbonation by ammonia water (NH4OH) regulation and thermochemical heat storage of its products. The influences of NH4OH on CO2 sequestration and heavy metals immobilization were investigated. The results demonstrated that the carbon sequestration efficiency improved from 65.2% to 98.3%, with the NH4OH addition increasing (0–20%). NH4OH promoted the conversion of calcium chloride (CaCl2) and calcium sulfate (CaSO4), which were not originally involved in the carbonation reaction, to calcium carbonate (CaCO3). Excess NH4OH increased the release of heavy metals into the liquid phase due to the chelation of heavy metals by ammonia ions (NH4+) and the effect of high pH. The appropriate amount of NH4OH (8%) offered a high-efficient immobilization of heavy metals. The energy density of FA samples increased from 226.6 kJ/kg to 1207.8 kJ/kg with the NH4OH addition increasing, positively correlating with the content of CaCO3. After the thermal cycle treatment, the non-ammonia carbonated FA had a serious sintering phenomenon with chlorellestadite (Ca5(P,Si,S)3O12(Cl,OH,F)), which was produced from the reaction between CaSO4 and other compounds; while the carbonated FA with NH4OH addition (8%) showed good thermal cycle stability with fine particles and single component (calcium oxide). Therefore, the carbonation treatment in this study is a novel and highly-efficient method, with the final products providing an alternative material for thermochemical heat storage.