Boiler Modifications Cut Mercury Emissions 70 Percent Or More, Research Team Finds

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

The reductions were achieved, says Romero, by modifying the physicalconditions of power-plant boilers, including flue gas temperature, thesize of the coal particles that are burned, the size and unburnedcarbon level of the fly ash, and the fly ash residence time. Thesemodifications 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 emissionsreductions in coal-fired utility boilers," which will be published in aforthcoming issue of the journal Fuel.

Mercury enters the atmosphere as a gas and can remain airborne severalyears before it precipitates with rain and falls into bodies of water,where it is ingested by fish. Because mercury is a neurotoxin, peoplewho consume large quantities of fish can develop brain and nervousailments. Forty-four states have mercury advisories.

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

The U.S. Environmental Protection Agency last March issued itsfirst-ever regulations restricting the emission of mercury fromcoal-fired power plants. The order mandates reductions of 23 percent by2010 and 69 percent by 2018. Four states - Massachusetts, New Jersey,Connecticut and Wisconsin - issued their own restrictions before theMarch 15 action by the EPA.

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

The ERC team used computer software to model boiler operatingconditions and alterations and then collaborated with Western KentuckyUniversity on the field tests. Analysis of stack emissions showed thatthe new technology achieved a 50- to 75-percent reduction of totalmercury in the flue gas with minimal to modest impact on unit thermalperformance and fuel cost. This was achieved at units burningbituminous coals.

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

Romero estimated that the new ERC technology could save a 250-megawattpower unit as much as $2 million a year in mercury-control costs. Thesavings could be achieved, he said, by applying the ERC method solelyor in combination with a more expensive technology called activatedcarbon injection, which would be used by coal-fired power plants toreduce mercury emissions. The resulting hybrid method, says Romero,would greatly reduce the approximately 250 pounds per hour of activatedcarbon that a 250-MW boiler needs to inject to curb mercury emissions.

The new ERC technology was developed by Romero, ERC director EdwardLevy, ERC associate director Nenad Sarunac, ERC research scientistHarun Bilirgen, and Ying Li, who recently received an M.S. inmechanical engineering from Lehigh.

The breakthrough follows years of work by ERC researchers in optimizingboiler operations to control emissions of NOx, CO, particulates andother pollutants.

For their mercury-emission research, the ERC group received atotal of $1.2 million in funding from a consortium of utilitycompanies, the Pennsylvania Infrastructure Technology Alliance and theU.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 NOxlevels 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 WesternKentucky University will conduct tests next year at an additional unitfiring Powder River Basin sub-bituminous coals.

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

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