Carbonation of flue gas desulfurization gypsum for CO2 sequestration

Abstract

The IPCC asserts that to prevent a 2°C global temperature increase by the year 2050, CO2 must be removed from the atmosphere by sequestration. The goal of this study was to use FGD gypsum for CO2 mineralization and experimentally explore to find the optimal conditions for the highest conversation rates at ambient temperature while eliminating ammonia usage. While maintaining an alkaline solution using NaOH, a stirred reactor was utilized to study the effects of PCO2 (0.69, 2.07, 4.14, 6.89, and 17.24 bar), solution pH (12, 13, 13.5 and 14), solid-to-solution ratio (1:100, 1:80, 1:40, 1:100), and reaction time (10, 15, 30, and 120+ min) variation on the rate of conversion. The CaCO3 produced was calculated by Rietveld refinement of XRD patterns to determine the impact of each experimental variable.Experimental results showed solution pH was a primary control on mineralization, with nearly 100% conversion of FGD gypsum to CaCO3 occurring at initial pH = 13.5 and 14, for PCO2 > 2 bar and S:L = 1:100. At initial pH of 12, no gypsum conversion occurred. Reaction time also affected the amount of gypsum conversion to CaCO3. At initial pH = 13, S:L = 1:100 and PCO2 = 2.07 bar, 15 min was the optimum reaction time, achieving 75% conversion. However, with the same conditions at 360 min, a 61% conversion occurred, due to final pH’s below 7. Increasing S:L ratio resulted in increased gypsum-to-carbonate conversion. The optimal conditions for conversion of gypsum into calcite occurred at short reaction times of 15 min, low pressures at around PCO2 = 2.07 bar, and low solution ratios of S:L = 1:100, achieving 75% conversion. In contract, a reaction time of 360 min produced a result of only 61% conversion at the same PCO2 and S:L ratio, due to the pH dropping below 7. The results of this study demonstrate that FGD gypsum is a viable feedstock for CO2 mineralization, potentially offering a cheap and rapid method for carbon sequestration.