Mercury Emissions Control Technologies
Pilot Plant Study of Technologies for Reducing Hg, SO3, & NOx Project Summary
CONSOL, Inc., Library, PA, will construct a pilot-plant facility producing flue gas from a coal-fired utility to test technologies that remove not only mercury, but will reduce nitrogen, sulfur and carbon dioxide emissions as well. The facility will be composed of an air preheater, an electrostatic precipitator (ESP) to collect fine particulates, and an alkaline-sorbent injection system to control sulfur condensation. An alkaline additive is injected into the air heater, which will operate at 200-250o F, to neutralize the sulfur. Mercury will be collected with the fly ash in the ESP. The work addresses several utility issues: mercury removal at lower-than-normal temperatures, using spray cooling to lower temperatures, and the additive's effects on specific plant components performance.
Project Abstract: The U.S. Environmental Protection Agency is evaluating the need to regulate mercury (mercury) releases to the environment. EPA estimates that coal-fired utilities are the largest source category of anthropogenic mercury emissions, with annual air releases of ~52 tons. EPA currently is evaluating the mercury emission data from 84 utility systems and has given strong indications that they will promulgate mercury regulations for the coal-fired utility industry.
There is no proven mercury control technology for coal-fired utility systems. Activated carbon injection, in conjunction with flue gas cooling, has been suggested as a control option. This suggestion is based on mercury control from municipal waste incinerators. There are specific problems in adapting this control technology to coal-fired power plants. DOE estimates that control costs for this technology (at 90% mercury removal) are between $25,000 and $70,000 per pound of mercury removed. This technology could cost the coal-fired utility industry $2.6 billion to $7.3 billion dollars per year. mercury is not the only pollutant under scrutiny. EPA continues to investigate the need for additional reductions in NOx, fine particulate and fine particulate precursors, and CO2.
From 1993 to 1999, CONSOL R&D conducted a number of utility mercury measurement programs that show that ESP/wet FGD remove 60-70% of the total mercury in the coal. This technology may provide a cost-effective mercury control strategy, depending on the level of control imposed by EPA. However, only one-third of the coal-fired utilities are equipped with wet FGD systems. Most utilities are equipped with electrostatic precipitators (ESPs) as their primary emission control device. These facilities would require an add-on control unit specific for mercury control.
In 1999, CONSOL R&D conducted a program showing that fly ash alone can remove a substantial portion of the flue gas mercury when the gas is cooled below typical exhaust temperatures (from 300 degrees F to 220 degrees F). At reduced flue gas temperatures, the mercury is absorbed directly on the fly ash particles and is removed with the particulate in the ESP. At a flue gas temperature of ~200 degrees F, 80-90% of the flue gas mercury was removed. However, operating at reduced flue gas temperatures carries a risk of increased "back-end" corrosion as a result of SO3 condensation. In a separate CONSOL R&D research program, flue gas SO3 concentrations were reduced by ~80% with the injection of an alkaline sorbent. The reduction of SO3 in the flue gas will allow for safe operation at lower flue gas temperatures conducive to mercury absorption.
In this proposal, CONSOL and the contract team propose to construct a pilot plant facility operating on a flue gas slip stream from a coal-fired utility boiler to investigate mercury control technologies. The pilot plant will consist of an air preheater, ESP, and an alkaline sorbent injection system for SO3 control. The technology is twofold. Alkaline sorbent is injected prior to the air preheater to neutralize the SO3. The air preheater is operated in a manner to reduce flue gas temperature to the 220-250 F range. Flue gas mercury is collected with the fly ash in the ESP.
The proposed work addresses a variety of utility issues associated with mercury removal. The contract team will determine mercury removal with the fly ash at a variety of temperature ranging from ~320 degrees F to ~220 degrees F at the outlet of the pilot scale ESP. The team will evaluate the option of spray cooling as a technique to lower the temperature. The team will complete a series of long-term tests to evaluate the impact of this technology on the performance of specific utility components.
The control technology proposed by CONSOL has several economic advantages over carbon injection. The first is reagent cost. A second advantage is the potential for improving station heat rate. A 90 degree F reduction in flue gas temperature is equivalent to a 2% efficiency improvement. This equates to a 2% decrease in fuel usage and pollution production. A typical 600 MW coal-fired utility boiler fires 1.3 million tpy of coal. A 2% fuel savings equates to 26,000 tons of coal (>$600,000 in fuel costs). A 2% decrease in fuel usage with no generating penalty results in a 2% reduction in SOx, NOx, and CO2 emissions.
The reduction of SO3 has other economic benefits. Flue gas SO3 is associated with visible plumes and plays a role in the formation of undesirable ammonium salts in NOx control technologies. Reduction in flue gas SO3 would minimize these problems and lead to additional utilization of SCR/SNCR NOx control technologies.
Technical Reports and Presentations:
Contacts:
- For further information on this project, contact NETL Project Manager, Lynn A. Brickett.
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