(423d) Effect of Acidic Sites and Calcium Hydroxide on Adsorption of Mercuric Chloride in Activated Carbon: a Molecular Simulation Study

Authors: 
Kotdawala, R., Worcester Polytechnic Institute
Kazantzis, N., Worcester Polytechnic Institute
Thompson, R. W., Worcester Polytechnic Institute


The clean air act amendments of 1990 identify a number of hazardous air pollutants (HAPs) of particular concern to human health and the environment. Data suggest that coal-fired power plants and municipal solid waste (MSW) incinerators are a significant source of some of these compounds particularly elemental mercury and mercuric chloride. In the combustion zone, all mercury in coal is vaporized yielding vapor concentrations of mercury in the range of 1-20 μg/m3 (0.1-2 ppbv). At flame temperatures (1700 K), all of the mercury is expected to be as elemental mercury in the gas phase. In the post flame region (1700-400 K), equilibrium predicts the oxidation of Hg0 to HgCl2 in the gas phase. Measurements in pilot and full-scale systems show that 10-80 % of the vapor phase mercury is likely to be HgCl2 which is easier to remove from flue gas streams than the elemental mercury [1,2,3].

These emissions of mercuric chloride from MSW incinerators and coal burning power plants can be reduced by source separation, product substitution, and flue gas cleanup. Cleanup technologies, required to meet the air quality standards, include wet scrubbing and adsorption on dry sorbents, which can be carried out either by injecting the sorbents into the exhaust gases, or by using multistage fixed beds for selective adsorption of acid gases, mercuric chloride and dioxins. Processes which use adsorption on dry sorbents do not pose the problem of the treatment and stabilization of waste liquid streams, and therefore seem very attractive for both small and large combustors such as those used for incineration of hospital wastes, respectively. The need to develop technologies capable of achieving high removal efficiencies for mercury chloride emission control led many researchers to focus their attention on the evaluation of the adsorption capacity and selectivity shown by different solids. Selections of appropriate adsorbent compel researchers to understand the adsorption behavior of mercuric chloride at the molecular level [1, 2, and 3].

In the present research study, we attempt to understand the physical adsorption of mercuric chloride and mixture of mercuric chloride and nitrogen in activated carbon through detailed Monte?Carlo simulations and computational quantum chemistry techniques. The activated carbon is modeled with slit carbon pores with hydroxyl, carboxyl and carbonyl sites as well as with presence of calcium hydroxide. Layers of calcium hydroxide are introduced in the carbon slit pore at different sites. The capacity and selectivity of activated carbon is compared for different acid sites and calcium hydroxide concentrations, as well as different pore sizes, by simulating single component mercuric chloride isotherms. The adsorption of binary mixture (N2-HgCl2) is simulated in both types of micro pores adsorbents at different HgCl2 concentrations (in the limits of infinite dilution) in the temperature range of 100 to 180 °C.

References:

1. B. Hall, O.Lindquvist, and E.Ljungstrom, Env. Sci. Techn., 24, 108 (1990) 2 S.V.Krishnan, B.K. Gullett and W. Jozevicz, Env. Sci. Techn., 28, 1506 (1994) 3. D. Karatza, A. Lancia, D.Musmarra, F.Pepe and G.Volpicelli, Comb. Sci. Tech., 112, 163 (1996)