(583bw) Investigation of Zeolites for Mercury Oxidation in SCR Units

The Mercury and Air Toxic Standard (MATS) promulgated by the Environmental Protection Agency in 2011 seeks to limit mercury emissions from stationary power sources by up to 91% over the next several years.  Of those stationary sources covered by MATS, coal fired power plants currently represent the greatest anthropogenic source of mercury emissions in the US.  Mercury exists in the flue gas stream in the elemental (Hg­⁰), oxidized (Hg⁺²) and particulate (Hg^P) form.   Of these the latter two are easily captured in the flue gas desulfurization unit and electric static precipitator respectively, while the former is much harder due to its relative inertness.    Currently employed technologies for reducing Hg emissions consist of injecting oxidizers or sorbents, such as CaBr₂ or activated carbon respectively, into the flue gas steam to limit emissions.  However these additives come at a cost and increase power plants operating expenditures.  To minimize the need of these additives, research is being conducted to augment or replace the catalysts being used in selective catalytic reduction (SCR) units used to reduce nitrogen oxide (NO_X­) emissions.  The goal is to develop a novel catalyst that will perform the deNO_X­ and mercury oxidation chemistry simultaneously, which would limit or eliminate the need for additives.

Small pore zeolites SSZ-13 and SAPO-34, which have shown promise as SCR catalysts in other deNO_X applications, are studied for their mercury oxidation potential.  The catalysts are exposed to a simulated flue gas stream that is created by combusting methane, which is representative of the environment present in a coal-fired boiler.  Prior to combustion, the air stream is premixed with varying concentrations of NO_X, SO₂, and Cl₂ so that each component’s effect on mercury oxidation could be examined.  The catalysts are studied with and without impregnated Fe or Cu and compared to state-of-the-art V₂O₅/TiO₂ commercial SCR catalysts.