| Description |
Mercury contamination in the planet's oceans and lakes presents a growing threat to human health and safety. The bioaccumulation of methyl mercury in fish and other aquatic life is rendering them increasingly unfit for human consumption. Accumulation of mercury in groundwater is also a growing hazard. Coal fired power plants are currently the largest anthropogenic source of mercury, but luckily, their contribution to global mercury levels can be abated with the installation of proper controls. Activated carbon injection (ACI) is among the most developed and promising of the available control technologies. To optimize ACI processes, models are necessary to properly gauge the effectiveness of various sorbents. This study developed an entrained flow model to build on previous work and further investigate factors that may influence mercury adsorption. For qualitative analysis, the model requires an understanding of reactor conditions, sorbent parameters, and packed-bed isotherms. Quantitative prediction of mercury adsorption is possible if enough information is provided about the sorbent and reactor. Namely, apparent density, porosity, average pore diameter, BET surface area, maximum uptake, equilibrium sorbent parameters, residence time, temperature, sorbent particle size and distribution, inlet concentration, and sorbent feed rate composition must be known. External mass transfer effects exclusively limit mercury adsorption when sorbent particle diameter is less than roughly 1 micron, and intraparticle diffusions exclusively limit mercury adsorption at diameters greater than one 100 microns. As expected, large BET surface areas and residence times favor adsorption. It is shown that the isotherm chosen in a model is not nearly as important as the adsorption parameters used in it. The Langmuir and Freunlich adsorption models are both good choices for modeling of intraparticle diffusion. Increased mercury adsorption can be shown with decreasing average particle size, increasing feed rate, and increasing average pore diameter. Full scale data are not entirely consistent with the trend of decreasing particle size and increasing adsorption; the cause is not yet understood. |
