An undesirable byproduct of the burning of coal is mercury emissions. On average, elemental mercury constitutes approximately 0.17 ppm of US coal. When coal is burned, mercury is vaporized and enters the atmosphere. These mercury emissions eventually condense and enter the food chain as methylmercury. Developing fetuses exposed to mercury emissions are incredibly vulnerable and may be born with learning disabilities.
The Mercury and Air Toxics Standards (MATS) rule is a United States Environmental Protection Agency (EPA) rule. It was developed to reduce the emissions of toxic air pollutants from power plants. One of the pollutants of concern is mercury emission from coal-fired electric generating units (EGUs). Since the rule’s implementation, several technologies have been implemented to reduce EGU mercury emissions.
Some plants initially installed systems such as fabric filters (FF) or electrostatic precipitators (ESPs) to control particulate emissions. They also captured a small percentage of mercury emissions as a side effect. Plants later installed flue gas desulphurization (FGD) systems to remove sulfur oxides. Again, these also captured a small portion of mercury emissions as a side effect. After Implementing MATS, many plants installed another technology: activated carbon injection (ACI) to capture mercury. This simple and inexpensive modification sprayed powdered activated carbon into the cold flue gas stream to adsorb mercury. However, this additional particulate injection also increased the unplanned load on the FF and ESP systems.
To avoid a costly requirement to reconfigure or enlarge the FF or ESP, a less common technology is being added to supplement the mercury removal capability of these systems: calcium bromide (CaBr2) systems. This system sprays a CaBr2 solution onto the coal, usually as it enters the bunkers. The solution may also be sprayed directly into the boiler or the flue gas stream. The installation of these systems resulted in significant increases in mercury capture. As an added benefit, the amount of carbon injected was reduced, resulting in savings. The table below summarizes the results of each capture technology as a percentage of flue gas mercury removed.
|Technology||FF Alone||ESP Alone||ESP +FGD|
|With CaBr2 and ACI||>90%||>88%||>80%|
A GAO report on DOE data showed coal-fired power plants using technologies such as calcium bromide addition resulted in about a 90% reduction of mercury emissions across all coal types.
The relatively low capital cost of equipment and the high availability of the chemicals domestically make the technology a potentially cost-effective solution to meeting the MATS requirements. Those EGUs subject to MATS guidelines should weigh the cost of their existing mercury abatement systems to explore whether adding a new MATS system could act as a cost-saving measure. As shown in the discussion above, the system’s simplicity combined with the relatively low cost of the chemical could pay for itself almost immediately.