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12 October 2005

Boiler modifications cut mercury emissions 70%

A new technique can reduce mercury emissions from coal-fired power plants.

In full-scale tests at three power plants, the new system reduced flue-gas emissions of mercury by as much as 70% or more with modest impact on plant performance and fuel cost, said Carlos E. Romero, lead researcher at Lehigh University's Energy Research Center (ERC).

The reductions came by modifying the physical conditions of power-plant boilers, including flue gas temperature, the size of the coal particles burned, the size and unburned carbon level of the fly ash, and the fly ash residence time, Romero said. These modifications promote the in-flight capture of mercury, he said.

Mercury enters the atmosphere as a gas and can remain airborne several years before it precipitates with rain and falls into bodies of water, where fish ingest it. Because mercury is a neurotoxin, people who consume large quantities of fish can develop brain and nervous ailments. Mercury advisories exist in 44 states.

Coal-fired power plants are the largest single-known source of mercury emissions in the U.S. Estimates of total mercury emissions from coal-fired plants range from 40 to 52 tons.

The U.S. Environmental Protection Agency last March issued first-ever regulations restricting the emission of mercury from coal-fired power plants. The order mandates reductions of 23% by 2010 and 69% by 2018. Four states, Massachusetts, New Jersey, Connecticut, and Wisconsin, issued their own restrictions before the 15 March action by the EPA.

The changes in boiler operating conditions prevent mercury emission at the stack and promote its oxidation in the flue gas and adsorption into the fly ash instead, Romero said. Scrubbers, filters, and other boiler pollution-control equipment can easily capture oxidized mercury.

The ERC team used computer software to model boiler operating conditions and alterations and then collaborated with Western Kentucky University on the field tests. Analysis of stack emissions showed the new technology achieved a 50% to 75% reduction of total mercury in the flue gas with minimal to modest impact on unit thermal performance and fuel cost. This came from units burning bituminous coals.

Officials capture only about one-third of mercury from coal-burning power plant boilers not equipped with special mercury-control devices, Romero said.

Romero estimated the new ERC technology could save a 250-megawatt power unit as much as $2 million a year in mercury-control costs. The savings could occur by applying the ERC method solely or in combination with a more expensive technology called activated carbon injection, which coal-fired power plants would use to reduce mercury emissions, Romero said. The resulting hybrid method would reduce the approximately 250 lbs per hour of activated carbon that a 250-MW boiler needs to inject to curb mercury emissions, he said.

The breakthrough follows years of work by ERC researchers in optimizing boiler operations to control emissions of NOx, CO, particulates, and other pollutants.

It is expensive to check for levels of mercury emissions because officials measure mercury levels in parts per billion, while they measure NOx levels in parts per million.

The ERC ran tests at a power plant in Alexandria, Va., and at two units of a power plant in Massachusetts. The ERC and Western Kentucky University will conduct tests next year at an additional unit firing Powder River Basin sub-bituminous coals.

For related information, go to www.isa.org/environment.