Mercury Removal from Flue Gas by Aqueous Precipitation

Abstract

Mercury has drawn the public’s concern due to its bio-accumulative nature and its adverse impact on human health. At the same time, concern for greenhouse gas release has also grown, leading to a range of carbon capture strategies. Previous mercury removal studies, however, focused on either dry techniques or acidic solution conditions, and a major uncertainty is how high alkalinity (which is used in carbon mineralization capture methods) may result in a different mercury removal mechanism. To examine this question in detail, in this study, a lab-scale liquid-gas scrubbing system is manufactured and installed, based on the carbon mineralization by aqueous precipitation (CMAP) process for CO₂ sequestration. Sodium hydroxide solution is selected as a typical candidate to remove mercury. Based on the test results, the CMAP process, like most other capture methods, has relatively low elemental mercury removal efficiency, i.e. less than 20%, but it can remove oxidized mercury with more than 90% efficiency. This performance is very similar to that of the traditional flue gas desulfurization processes which are operated under acidic conditions, at pH from 4 to 6. Equilibrium calculations by Visual MINTEQ predict the speciation of mercury equilibrium in the liquid phase, and the result is in agreement with the experimental findings.