Combustion of any fuel (solid or liquid) creates emissions, potentially released to the air, and by-products, including ash, for reuse or disposal. Emissions to the air contain several pollutants of increasing concern: trace amounts of metals (of particular concern mercury, Hg), sulfur compounds (SO2 and SO3, collectively called SOX), nitrogen compounds (NO and NO2, collectively called NOX), hydrogen–halogen compounds (HCl and HF) and fine particles (particulate).
With the Clean Air Act of 1990, the U.S. Environmental Protection Agency set various regulations that specifically addressed most of the major air pollutants (NOX, SO3 and particulates) from coal-fired power plants. The result of these regulations has been a dramatic improvement in air quality in the United States, despite a growing population and expanded economy. Recently, the EPA promulgated the Mercury and Air Toxics Standard (MATS), which for the first time in this country requires utilities that operate oil and coal plants to
There are three basic forms of mercury that can be emitted from the stack of coal-fired power plants – elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp).
These three forms make up the total mercury (Hg) that must be reduced for utilities to comply with MATS. Of these forms, the most difficult to remove is elemental mercury, since it is not easily removed by standard sorbents and, since it is rather insoluble, is not removed in scrubbers. The form of mercury is highly dependent on the type of coal that is combusted, the makeup of the ash and the type and arrangement of installed emission control equipment (selected catalytic reduction columns [SCRs], baghouses, electrostatic precipitator [ESPs], scrubbers, etc.) and the operations of the plant. Depending on these factors, the amount of Hg0 that is generated and potentially emitted can vary from 10–90 percent of the total mercury, with a typical range of 20–80 percent.
Consequently, for utilities to meet the 90 percent reduction goal as targeted by MATS, they will have to utilize technologies that are effective at removing elemental mercury.
To achieve compliance with MATS, utilities are looking at low-cost options such as sorbent injection.
However, removing elemental mercury using injection of sorbents such as activated carbon (AC) has proven to be challenging. Early tests showed that activated carbon injection (ACI) was somewhat effective at capturing mercury, but was, in most cases, limited to about 60 percent removal, regardless of how much AC was injected. Several plants with high levels of Hg0 showed less than 50 percent removal using standard ACs, even at injection rates exceeding 10 lb/Macf (pounds/million actual cubic feet) of flue gas, a level considered uneconomical and unacceptable by most utilities. To overcome this, and in anticipation of stringent mercury reductions as called for under EPA’s MATS, Midwest Energy Emissions Corp. (ME2C), along with the Energy & Environmental Research Center (EERC) of the University of North Dakota, has, during the past several years, been developing, testing and successfully demonstrating a new technology that will provide a low-cost option for utilities to meet MATS.
A new approach to control mercury
Through extensive research, the EERC developed a complex sorbent–flue gas interaction model that led to the development of a revolutionary technology for controlling mercury known as the Sorbent Enhancement Additive (SEA™) technology. As shown in Figure 1, the capture of mercury using sorbents is complex, involving many forms of mercury, oxidation and the role and impact that various flue gas components have on chemisorption.