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Coal plants turn to mercury controls to meet compliance deadlines

Published by , Editorial Assistant
World Coal,

Based on data recently published in EIA’s preliminary annual electric generator survey, several coal-fired electricity generators in the US installed mercury control equipment using activated carbon injection systems just prior to compliance deadlines.

The nature and timing of control additions indicate a strategy to maintain the availability of affected coal-fired generators by requesting extensions to compliance deadlines and investing in flexible, low-cost environmental control technology.

At the end of 2011, the US Environmental Protection Agency (EPA) announced standards to limit mercury, acid gases, and other toxic pollution from power plants. EPA’s final ruling, called the Mercury and Air Toxics Standards(MATS), was released on 16 February, 2012. MATS required all coal- and oil-fired generators that sell power and have a capacity greater than 25 MW to comply with emissions limits for toxic air pollutants associated with fuel combustion such as mercury, arsenic, and heavy metals. At the time, the rule applied to 76% of all operating coal units, which represented 99% of generating capacity. The initial compliance deadline was 16 April, 2015.

Between January 2015 and April 2016, about 87 GW of coal-fired plants installed pollution-control equipment, and nearly 20 GW of coal capacity retired. About 26% of those retirements occurred in April 2015, meeting the MATS rule's initial compliance date. According to analysis by MJ Bradley & Associates, 142 GW of coal plants had applied for and received one-year extensions that allowed them to operate until April 2016 while finalising compliance strategies.

An additional one-year extension to April 2017 was granted to a few units critical to ensuring electric reliability. Five coal plants with a combined capacity 2.3 GW received this extension. Since then, two of the five plants have retired, one converted to natural gas, and one installed MATS-compliant controls. The remaining plant, Oklahoma’s Grand River Energy Center, was given another emergency extension for reliability issues in April.

Of the coal capacity installing pollution control equipment to comply with MATS, activated carbon injection (ACI) was the dominant compliance strategy, with close to 78 GWs of coal capacity adding ACI. Activated carbon injection systems work by injecting powdered activated carbon into the flue stack (exhaust) of a coal-fired power plant. This powered activated carbon then absorbs the vaporised mercury from the flue gas and is collected from the plant's particulate collection device. Activated carbon is a carbonaceous, highly porous adsorptive medium that has a complex structure composed primarily of carbon atoms.

ACI technologies have the shortest construction lead time of the compliance control technologies — between 12 and 18 months — and the lowest installation cost — about US$11 per kilowatt (kW). Other technologies, such as electrostatic precipitators (ESP) and baghouses have longer lead times and higher costs. Flue gas desulfurisation (FGD) has the highest average lead time, at 50 months, and the highest installation cost, at US$228/kW.

Environmental control technologies vary in terms of what air pollutants they remove. For example, FGD technologies can control mercury, sulfur dioxide, and acid gases, whereas ESPs can control mercury, non-mercury metals, and acid gases.

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