Reducing mercury emissions

The Electric Power Research Institute, Texas Genco and URS Corporation have completed initial testing of a novel, low-cost technology aimed at reducing mercury emissions from coal-fired power generators.

The Electric Power Research Institute (EPRI), Texas Genco and URS Corporation have completed initial testing of a novel, low-cost technology aimed at reducing mercury emissions from coal-fired power generators equipped with an electrostatic precipitator and wet scrubber for particulate and SO2 reduction.

The tests were conducted at Texas Genco’s Limestone Station in Jewett, Texas.

The concept involves oxidizing elemental mercury with small amounts of halogen injected into the boiler and then capturing the oxidized mercury by conventional SO2 control devices. The mercury in the flue gas from coal-fired power plants is a mixture of elemental and oxidized mercury, but only the oxidized mercury is soluble in water and, therefore, can be captured by SO2 controls.

Limestone’s existing electrostatic precipitator (ESP) and SO2 scrubber capture virtually all the soluble oxidized mercury – 50 to 65 per cent of the mercury found in the exhaust gas produced by the plant. There is no evidence to suggest the devices capture significant quantities of the remaining insoluble elemental mercury that is generated at Limestone by burning a blend of Texas lignite and Powder River Basin coal.

EPRI researcher Ramsay Chang said the approach tested at Limestone was rooted in experience with plants that burn other forms of coal. “We know that mercury in the flue gas at plants burning coals with higher chlorine content, such as Eastern bituminous coal, appears mostly in the oxidized form, which is removable by SO2 controls,” Chang said. “Therefore, we tried to create similar conditions at plants where the coal does not naturally contain the halogen believed necessary to promote the formation of oxidized mercury.”

Following some short-term tests that EPRI conducted over the last two years at other power plants burning North Dakota and PRB coal, the Texas Genco/EPRI/URS team decided to test this approach at Limestone.

Texas Genco injected a liquid stream of calcium chloride or bromide directly into the boiler over a three-day test period. “Our preliminary results show that, at the low concentrations injected into the boiler, the approach can increase mercury removal from the 50-65 per cent baseline to around 75 per cent,” Carl Richardson of URS Corporation said.

More work is necessary, however, because what is not known is whether higher mercury removals can be obtained with higher halogen injection rates, what corrosive effects these halogen additives might have, or what impact the additives might have on plant discharge streams.

“Our goal now is to validate the results and attempt to determine the impacts of halogen addition on the performance and operation of the plant. Nonetheless, these early results are intriguing and encouraging,” Chang said.

Going forward, the project team plans to solicit participation from other power generators in the region while continuing its investigation at the Limestone Station in 2005 and 2006.

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