Ranking low cost sorbents for mercury capture from simulated flue gases

Coal fired utility boilers are the largest anthropogenic source of mercury release to the atmosphere, and mercury abatement legislation is already in place in the USA. The present study aimed to rank low cost mercury sorbents (char and activated carbon from the pyrolysis of scrap tire rubber and two coal fly ashes from UK power plants) against Norit Darco Hg™ for mercury retention by using a novel bench-scale reactor. In this scheme, a fixed sorbent bed was tested for mercury capture efficiency from a simulated flue gas stream. Experiments with a gas stream of only mercury and nitrogen showed that while the coal ashes were the most effective in mercury capture, char from the pyrolysis of scrap tire rubber was as effective as the commercial sorbent Norit Darco Hg™. Tests conducted at 150 °C, with a simulated flue gas mix that included N₂, NO, NO'2, CO₂, O₂ SO₂ and HCl, showed that all the sorbents captured approximately 100% of the mercury in the gas stream. The introduction of NO and NO₂ was found to significantly improve the mercury capture, possibly by reactions between NO_x and the mercury. Since the sorbents' efficiency decreased with increasing test temperature, physical sorption could be the initial step in the mercury capture process. As the sorbents were only exposed to 64 ng of mercury in the gas stream, the mercury loadings on the samples were significantly less than their equilibrium capacities. The larger capacities of the activated carbons due to their more microporous structure were therefore not utilized. Although the sorbents have been characterized by BET surface area analysis and XRD analysis, further analysis is needed in order to obtain a more conclusive correlation of how the characteristics of the different sorbents correlate with the observed variations in mercury capture ability.