A Novel Method for CO2 Sequestration via Indirect Carbonation of Coal Fly Ash

Coal fly ash is a potential candidate for CO2 mineral sequestration. If calcium is extracted selectively from coal fly ash prior to carbonation (namely indirect carbonation), a high-purity and marketable precipitated calcium carbonate (PCC) can be obtained. In the extraction process, recyclable ammonium salt (i.e., NH4Cl/NH4NO3/CH3COONH4) solution was used as a calcium extraction agent in this study. The influence of time, temperature, agent concentration, and solid-to-liquid ratio on calcium extraction efficiency was explored. NH4Cl/NH4NO3/CH3COONH4 are confirmed to be effective calcium extraction agents for the high-calcium coal fly ash investigated, and about 35–40% of the calcium is extracted into the solution within an hour. The calcium extraction performance is best for CH4COONH4, followed by NH4NO3 and NH4Cl. Increasing temperature from 25 to 90 °C and agent concentration from 0.5 to 3 mol/L only subtly increases calcium extraction efficiency for NH4Cl and NH4NO3, while the positive effect of increasing temperature and agent concentration is more obvious for CH3COONH4. In the carbonation process, carbonation efficiency, namely conversion of Ca2+ into precipitated calcium carbonate(PCC), is only 41–47% when the leachate is carbonated by CO2. A newly proposed method of substituting CO2 with NH4HCO3 as the source of CO3 2– yields much higher carbonation efficiency (90–93%). Furthermore, the carbonation reaction rate is also largely improved when carbonating the leachate by NH4HCO3. In addition to these benefits, CO2 capture and storage can be simultaneously realized on-site if integrating the leachate carbonation process with an ammonia–water CO2 capture process using NH4HCO3 as a connector. In this way, the costs associated with CO2 compression and transportation can be eliminated. PCC with a purity up to 97–98% is obtained, which meets the purity requirement (≥97%) of industrially used PCC. It is estimated based on the experimental results that 0.17 tons of PCC can be produced from 1 ton of coal fly ash by this method, bounding 0.075 tons of CO2 at the same time, and 0.036 tons more CO2 can be avoided if the obtained PCC is substituted for the PCC manufactured by the conventional energy-intensive method.