Advanced SCR Catalysts Tune Oxidized Mercury Removal

By Scot Pritchard, Cormetech Inc.; Masashi Kiyosawa and Katsumi Nochi, Mitsubishi Heavy Industries, Hiroshima, Japan

Pages: 1 2 3 4

Catalysts used in selective catalytic reduction (SCR) systems in utility boilers provide high NO_x removal efficiencies that routinely exceed 90%. A major co-benefit of applying SCR to coal-fired power plants is that the SCR catalyst also oxidizes the vapor phase mercury from an elemental form to a soluble ionic form, which can be readily captured in a downstream flue gas desulfurization process. Mitsubishi Heavy Industries and Cormetech have developed an advanced SCR catalyst technology with high mercury oxidation activity capable of achieving 95% oxidized mercury over a wide range of operating conditions.

While U.S. mercury emission control rules are stalled at the federal level, many individual states have filled the gap with a patchwork of rules and requirements. It’s reasonable to expect that the Environmental Protection Agency (EPA) will eventually develop mercury rules that will pass judicial scrutiny and survive the inevitable series of lawsuits that follow (see sidebar). In the meantime, the important research and development work required to accurately quantify the co-benefits of air quality control system components continues.

A recent article in POWER (“Determining AQCS Mercury Removal Co-Benefits,” July 2010) discussed Southern Company’s extensive test program to determine the combined mercury removal co-benefits possible with the electrostatic precipitator (ESP), selective catalytic reduction (SCR), and wet flue gas desulfurization (FGD) technology combination. In this article, we explore another approach to maximizing mercury co-benefits: using an advanced SCR catalyst tuned to maximize mercury oxidation for a particular set of operating conditions.

Status of the Clean Air Mercury Rule

In 2005, the U.S. Environmental Protec¬tion Agency (EPA) issued the Clean Air Mercury Rule (CAMR) establishing perfor¬mance standards capping mercury emis¬sions from coal-fired power plants and creating a market-based cap-and-trade program. CAMR was set up to reduce power plant mercury emissions from the then-current rate of 48 tons per year to 15 tons per year by 2018, a reduction of nearly 70%.
Not content with the regulatory ap¬proach adopted by the EPA, petitioners from 15 states, other governmental enti¬ties, and environmental groups filed suit maintaining that the EPA did not have the authority to delist power plants from the federal Clean Air Act list of sources of hazardous air pollutants. The petitioners challenged that mercury and other haz¬ardous air pollutants should be subject to the National Emission Standards for Haz¬ardous Air Pollutants and thus be subject to maximum achievable control technol¬ogy rules. In 2008, the U.S. District Court of Appeals in Washington, D.C., vacated CAMR, leaving regulation to the 20 states that have implemented their own regula¬tions covering mercury emissions. Most of these states have requirements for an 80% to 90% reduction in mercury emissions.
To address a subsequent lawsuit by mul¬tiple plaintiffs, the EPA developed a draft consent decree in October 2009 calling for the EPA to develop air toxin emissions standards for coal- and oil-fired power plants under Section 112(d) of the Clean Air Act. The decree calls for a notice of proposed rulemaking to be published in the Federal Register no later than March 16, 2011, followed by enactment of a fi¬nal rule by November 16, 2011.

Pages: 1 2 3 4