Boiler Modifications Cut Mercury
Emissions 70%, Researchers Find
Source: GreenBiz.com
BETHLEHEM, Pa., Oct. 6, 2005 - 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% 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% by 2010
and 69% 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% 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.
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