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Boiler modifications cut mercury emissions 70 percent or more, research team finds

October 04, 2005

Inexpensive technique verified in full-scale tests at three coal-fired power plants

Researchers at Lehigh University's Energy Research Center (ERC) have developed and successfully tested a cost-effective technique for reducing mercury emissions from coal-fired power plants.

In full-scale tests at three power plants, says lead investigator Carlos E. Romero, the Lehigh system reduced flue-gas emissions of mercury by as much as 70 percent or more with modest impact on plant performance and fuel cost.

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

The ERC researchers reported their findings in an article titled "Modification of boiler operating conditions for mercury emissions reductions in coal-fired utility boilers," which will be published in a forthcoming issue of the journal Fuel.

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 it is ingested by fish. Because mercury is a neurotoxin, people who consume large quantities of fish can develop brain and nervous ailments. Forty-four states have mercury advisories.

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

The changes in boiler operating conditions, said Romero, prevent mercury from being emitted at the stack and promote its oxidation in the flue gas and adsorption into the fly ash instead. Oxidized mercury is easily captured by scrubbers, filters and other boiler pollution-control equipment.

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 that the new technology achieved a 50- to 75-percent reduction of total mercury in the flue gas with minimal to modest impact on unit thermal performance and fuel cost. This was achieved at units burning bituminous coals.

Only about one-third of mercury is captured by coal-burning power plant boilers that are not equipped with special mercury-control devices, Romero said.

Romero estimated that 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 be achieved, he said, by applying the ERC method solely or in combination with a more expensive technology called activated carbon injection, which would be used by coal-fired power plants to reduce mercury emissions. The resulting hybrid method, says Romero, would greatly reduce the approximately 250 pounds per hour of activated carbon that a 250-MW boiler needs to inject to curb mercury emissions.

The new ERC technology was developed by Romero, ERC director Edward Levy, ERC associate director Nenad Sarunac, ERC research scientist Harun Bilirgen, and Ying Li, who recently received an M.S. in mechanical engineering from Lehigh.

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

For their mercury-emission research, the ERC group received a total of $1.2 million in funding from a consortium of utility companies, the Pennsylvania Infrastructure Technology Alliance and the U.S. Department of Energy.

It is expensive to check for levels of mercury emissions, says Romero, because mercury levels are measured in parts per billion, while NOx levels are measured in parts per million.

The ERC tests were performed at a power plant in Alexandria, Virginia, 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.

Romero discussed his group's findings at the 2004 Pittsburgh Coal Conference in Osaka, Japan, where he gave a paper titled "Impact of Boiler Operating Conditions on Mercury Emission in Coal-Fired Utility Boilers."

He has given half a dozen presentations on his group's findings so far this year, including an address at the ICAC (Institute of Clean Air Companies) Clean Air Technologies and Strategies Conference in Baltimore in March.

Lehigh University