CO2 sequestration by indirect carbonation of high-calcium coal fly ash

The use of coal fly ash for CO2 sequestration has been proposed as a promising option for its utilization. However, research is quite necessary for advancing this technology. Indirect carbonation of high-calcium coal fly ash for CO2 sequestration was investigated in this study. In these processes, calcium was first extracted from a high-calcium coal fly ash sample with NH4Cl solution. The obtained leachate was subsequently carbonated by bubbling CO2. It was shown that NH4Cl could extract about 35% of the total calcium into the solution under the conditions investigated. The dissolution of calcium is nearly accomplished in half an hour. Further increasing the extraction solution temperature (10-90 degrees C) and the concentration of NH4Cl(0.5-3mol/L) only has subtle positive effects on the calcium extraction efficiency. The carbonation efficiency of the extraction leachate, namely the percentage of the calcium in the solution transformed into calcium carbonates is about 47%. The sharp drop in pH after bubbling CO2 due to weak acid buffering capacity of the solution hinders the further precipitation of Ca2+. Calcium carbonate samples with a purity of up to 97% are obtained, meeting the purity requirements for industrial use.