Coal is a mined material. Because it naturally occurs, it has a lot of contaminants that are mined with the coal. Radioactive isotopes, silica, arsenic, chromium, etc., as well as mercury. Power plants are currently the dominant polluters of mercury in the United States (~50 percent). In 2011 the EPA finalized and issued the first standards required by the power industry to reduce mercury emissions. Unfortunately, 20 years after the 1990 Clean Air Act power plants are still the major emitters of mercury.
The effects of mercury exposure vary depending on the form and level of exposure. Acute exposure to mercury vapor can produce serious effects on the nervous system including psychotic reactions, hallucinations, suicidal tendencies and delirium. Continued exposure can produce violent muscular spasms and even death. When it enters the body, it is stored in the kidneys, blood, spleen, brain, liver, bones and fatty tissues. Although it’s a natural element found in the earth’s crust, often in regions with volcanic activity, its natural status doesn’t mean it’s safe. In fact, it was previously thought to be a good and healthful substance. The first emperor of China was said to have taken mercury pills to give him eternal life. Instead the mercury pills destroyed his vital bodily systems and eventually caused his death.
Obviously, coal emissions must remove the mercury from the flue gas it emits. The problem as noted above is that when coal is burned the mercury is vaporized. Currently, no method is available to remove vaporized mercury. Therefore, the mercury must be oxidized (made into a particulate) that can then be filtered out, usually via the Flue Gas Desulfurization (FGD) equipment already installed at a large number of power plants. The most common method of oxidation involves the injection of powdered activated carbon (PAC) into the flue gas stream upstream of the FGD so that the mercury can be captured and removed. When injected into the flue gas, PAC captures mercury in the pores of the carbon particles. The PAC/mercury particles can be removed in the filtering process of the FGD.
Another method of mercury removal that is gaining wider acceptance is the use of a special catalyst used in the Selective Catalytic Reduction (SCR) already installed in power plants. By replacing the catalyst with a special catalyst for the removal of mercury the SCR becomes not just a NOx reducer but also a means for mercury removal.
Regardless of the method used, the plant will use site-specific constraints to determine which method of mercury removal is most appropriate for them. The plant must look at the types of coal being combusted, the amount of naturally occurring mercury in the coal, the existing plant equipment and how efficient they are in the mercury removal process. New technologies that are being developed may make 99.99% of mercury removable. Right now the industry standard is about 90% mercury removal. New standards being implemented in the US and Europe are driving the development of the new mercury removal technologies. Power plant managers, owners, and operators must be aware of new standards being implemented and the technologies being developed to help existing power plants meet the new standards.