Optimization of mineral carbonation process for CO2 sequestration by lime-rich coal ashes

Mineral carbonation is one of the alternatives considered for CO2 sequestration. This consists of inducing the artificial weathering of alkaline minerals to produce stable carbonate solids. This work presents the wet carbonation of lime-rich ashes from the fluidized bed combustion of two different fuels: a Spanish sulfur-rich lignite, with calcium carbonate and quartz as main mineral constituents, and the waste produced during the extraction of the coal. The novelty of the work is its experimental approach, which explores all the variables (pH, temperature, time, liquid to solid ratio, NaCl ratio) from a statistical survey centered on finding optimized conditions that maximize the degree of carbonation. CO2 uptake is determined by direct thermogravimetric tests. The results indicate that high carbonation efficiency (78% efficiency) is achieved for both solids at basic pH and moderate treatment times. Those conditions cause anhydrite to dissolve, increasing the availability of Ca2+ for reprecipitation as carbonate and hydroxide. At neutral pH, 72% conversion efficiency is equivalent to a capture of 8% of the CO2 theoretically emitted in the combustion of coal wastes. The proposed conditions for fly ash carbonation enables rapid CO2 capture that is applicable for upscaling and industrial use.