The following provides a brief description of 16 clean coal technologies. All but one are in use today at coal-fueled electric generating units (EGUs); carbon capture and storage (CCS) is still under development. Alone or in combination, clean coal technologies are capable of reducing sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), mercury, acid gases, and other emissions from coal-fueled EGUs by 90 percent to 99.9 percent. In addition, the high efficiencies of advanced coal-fueled electric generating technologies contribute to reductions in carbon dioxide (CO2) emissions.
Wet Scrubbers (Wet FGD)
Wet scrubbers (or flue gas desulfurization) combine a mixture of lime or limestone and water with power plant flue gases to remove SO2 and acid gases. The mixture is either injected into the scrubber with the flue gas, or the flue gas bubbles up through the mixture. According to EPA, wet scrubbers achieve SO2 and acid gas removal efficiencies of 90 percent to 98 percent. Scrubbers in combination with selective catalytic reduction (SCR) for NOx control can remove up to 80 percent of mercury emissions.1
Dry Scrubbers (Dry FGD)
Dry scrubbers spray very finely powdered lime or other absorbents into a vessel where they combine with power plant flue gases to remove SO2 and acid gases. The resulting sorbent is captured with a fabric filter (also called a baghouse). According to EPA, dry scrubbers achieve removal efficiencies of 90 percent to 93 percent.2
Dry Sorbent Injection (DSI)
DSI systems inject sorbents, such as Trona (a naturally-occurring mineral), into power plant flue gas ductwork to remove SO2 and acid gases. The sorbent is then captured in the PM removal system (either an electrostatic precipitator or a baghouse). DSI systems achieve 40 percent to 75 percent removal.3
Low-NOx Burners (LNB)
Burners inject coal and air into power plant boilers. Low-NOx burners are designed to reduce the level of NOx formed as the coal is burned. Depending on the burner design, DOE reports that low-NOx burners achieve from 40 percent to 50 percent NOx removal.4
Low-NOx Burners with Overfire Air (LNB with OFA)
The removal efficiency of low-NOx burners can be improved by injecting air into the area above the burner. With certain burner types, LNB with OFA can achieve nearly 70 percent NOx removal.5
Selective Non-catalytic Reduction (SNCR)
SNCR systems inject ammonia into power plant flue gases to remove NOx. According to EPA, SNCR achieves about a 35 percent reduction in NOx emissions.6
Selective Catalytic Reduction (SCR)
With SCR, ammonia is injected along with the power plant flue gas into a device that contains a catalyst to improve NOx removal efficiencies. According to EPA, SCR can achieve up to 90 percent NOx removal. SCR in combination with SO2 scrubbers can achieve up to 80 percent mercury removal.7
Activated Carbon Injection (ACI)
ACI systems inject finely-powdered activated carbon into power plant flue gases. The carbon is then removed by the PM control system (fabric filter or ESP). ACI can achieve up to 90 percent mercury removal.8
Halogenated ACI (HACI)
Halogenated ACI improves the mercury removal efficiency of ACI by adding a halogen (either bromide or chloride). Removal efficiencies of greater than 90 percent have been reported.9
Fabric Filter Systems/Baghouses
Fabric filters (also called baghouses) remove PM from power plant flue gases by capturing PM on fabric bags that are analogous to vacuum cleaner bags. The captured PM is removed by injecting high-pressure air into the bags or by shaking the bags. Fabric filters can achieve 99 percent to 99.9 percent PM removal.10
Electrostatic Precipitators (ESPs)
ESPs capture PM from power plant flue gases by passing the gases through a device in which charged metal plates are suspended. The PM is attracted to the plates by static electricity. ESPs are classified as either “hot-side” or “cold-side” depending on their location in the flue gas train. According to EPA, new ESPs capture 99 percent to 99.9 percent of PM.11
ESP with Baghouse
A baghouse is installed downstream of an ESP for improved PM control. Additionally, sorbents can be injected to capture mercury, and, in some cases, SO2 and NOx. These systems have been reported to achieve 90 percent mercury capture and, in certain cases, 90 percent SO2 removal and up to 30 percent NOx removal.12
Supercritical boilers operate at higher steam temperatures and pressures and, therefore, are more efficient than subcritical boilers. Supercritical boilers can achieve thermal efficiencies of up to 40 percent, compared with an average of 33 percent for the existing fleet of coal-fueled EGUs. Because of their efficiency, supercritical units can emit 10 percent to 20 percent less CO2 than subcritical EGUs.13
Ultra-Supercritical Boilers (USC)
USC boilers operate at even higher pressures and temperatures than supercritical boilers. American Electric Power’s John W. Turk plant in Arkansas uses USC technology. The plant recently began commercial operation. USC units can achieve up to 44 percent thermal efficiency, resulting in CO2 emissions that are as much as 30 percent lower than those from the existing fleet of coal- fueled EGUs.14
Integrated Gasification Combined Cycle (IGCC)
IGCC converts coal to a synthetic gas that is combusted in a combined-cycle system (a combustion turbine and a heat recovery steam generator). IGCC can achieve thermal efficiencies that exceed 40 percent, thus emitting as much as 30 percent less CO2 than the existing fleet of coal-fueled power plants. IGCC plants also have very low SO2, NOx, particulate matter and mercury emissions.15
Carbon Capture and Storage (CCS)
CCS is a technology system to capture CO2 from power plants and permanently store it underground safely in geologic formations. Currently, CCS is estimated to add 40 percent to 80 percent to the cost of electricity generated by a coal- fueled electric generating unit. CCS can capture and store up to 90 percent of the CO2 from a power plant. No full-scale CCS-equipped power plants are operating in the U.S., although several small-scale demonstrations have taken place.16
1 U.S. EPA, “Documentation: Updates to EPA Base Case v4.10_MATS.”
4 Watts, J.U., U.S. Department of Energy, “An Overview of NOx Control Technologies Demonstrated under the Department of Energy’s Clean Coal Technology Program.”
5 Ibid. 6 U.S. EPA, “Documentation: Updates to EPA Base Case v4.10_MATS.” 7 Ibid. 8 Ibid.
9 Ondrey, Gerald, “Chementator: Halogenated Activated Carbon Reduces Mercury Emissions from a Coal-Fired Plant,” Chemical Engineering, April 15, 2008.
10 U.S. EPA, “Air Pollution Control Technology Fact Sheet: Fabric Filter,” EPA-452/F-03-024.
11 U.S. EPA, “Air Pollution Control Technology Fact Sheet: Dry Electrostatic Precipitator,” EPA-452/F-03-028.
12 Bustard, C. Jeanne, Charles Lindsay, and Paul Brignac, “Field Test Program for Long-Term Operation of a COHPAC® System for Removing Mercury from Coal-Fired Flue Gas,” June 2006; Derenne, Steven, “TOXECONTM Clean Coal Demonstration for Mercury and Multi-pollutant Control,” presentation at EUEC 2008, Tucson, Arizona.
13 U.S. DOE, National Energy Technology Laboratory, “Coal-Fired Power Plants (CFPPs): Supercritical and Ultra Supercritical Boilers;” Massachusetts Institute of Technology, “The Future of Coal: Options for a Carbon-Constrained World,” 2007; U.S. EPA NEEDS database v4.10; Beer, Janos, “Higher Efficiency Power Generation Reduces Emissions,” National Coal Council Issue Paper 2009.
14 Ibid and Southwestern Electric Power Company, “Ultra-supercritical Generation: Increased Efficiency with Improved Environmental Performance.”
15 U.S. DOE National Energy Technology Laboratory, “Cost and Baseline Performance for Fossil Energy Plants: Volume 3a: Low-Rank Coal to Electricity: IGCC Cases,” May 2011.
16 “Report of the Interagency Task Force on Carbon Capture and Storage,” August, 2010.