CHARACTERISTICS

There are two classes of environmental pollution control used to comply with local or national criteria air pollutant emission requirements (i.e., SO₂, NO_x): precombustion/combustion and post-combustion. Flue gas desulfurization (FGD) equipment—"scrubbers"—are the most widely-used post-combustion technology for removal of sulfur dioxide. The current generation of scrubbers is able to remove up to 90% of SO₂ in stack gases. Advanced scrubbers use finely ground lime or limestone slurries to convert more than 95% of the SO₂ in flue gases to gypsum and other reusable byproducts. Sorbent injection, which is capable of reducing up to 70% of the SO₂ emissions, can be used in the convection pass or duct work; it is also referred to as dry scrubbing.

For NO_x, post-combustion technologies convert NO_x to nitrogen and water vapor by combining the (flue gas) waste stream with ammonia, urea or other compounds. The most effective currently available technology is Selective Catalytic Reduction (SCR). SCR injects ammonia in the presence of a catalyst; this reduces NO_x by 80-90% at a retrofit capital cost of $100-200/kW. Selective Non-Catalytic Reduction (SNCR), used for smaller boilers, injects a reagent into the flue gas at temperatures of 900-1100º Celsius without using a catalyst; this achieves 30-50% NO_x reductions at a cost of from $10-20/kW. Pre-combustion techniques for NO_x control include reburning (i.e., the controlled injection of coal or natural gas in the boiler reducing NO_x emissions by 50-70%), low-NO_x burners (LNB) (creation of a fuel-rich flame for efficient combustion and NO_x reduction of 40-50%), and advanced overfire air (the injection of air in the combustor to reduce NO_x emissions by 10-25%. Other more novel approaches (and combinations) are also being proposed, but have not been implemented extensively.

Neither pre- nor post-combustion control techniques significantly affect CO₂ emissions. However, modern post-combustion SO₂ technologies can consume about 1% of the energy produced in coal-fired power plants. Pre-combustion NO_x control techniques can improve the efficiency of the combustion process, thereby slightly reducing CO₂ emissions. However, the effect that these pollution control devices have on CO₂ emissions is small.

SIZE:
FGD: units up to 100-1000 MW
SNCR: small units 100-500 MW
SCR: units from 100-1000 MW
LNB: units from 100-500 MW

FEATURES:
Addition of advanced scrubbers reduces plant electric output, but by less than 1%. The latest pre-combustion technologies can increase efficiency while reducing NO_x².

COST:
FGD: $75-200/kW
SCR: retrofit cost of $100-200/kW
SNCR: $10-20/kW (used for small boilers)

CURRENT USAGE:
In U.S., scrubbers are installed at over 400 coal-fired facilities; LNB are used extensively; SNCR and SCR are being installed on U.S. utility boilers for compliance with new requirements under the 1990 Amendments to the Clean Air Act.

POTENTIAL USAGE:
Cost may limit widespread deployment of SO₂ scrubbing technologies. Where low-sulfur fuels are available, it may be more cost-effective to switch fuels. As interest in the control of NO_x control grows, LNBs and, to a lesser extent, SCR, are growing in popularity.

CLIMATE CHANGE IMPACT

EMISSION EFFECT:
    

CONDITIONS FOR EMISSIONS MITIGATION:

• CO₂ will only be mitigated where the use of environmental control technologies increase the efficiency of the combustion process.

EMISSION ESTIMATE:
CO₂ emissions may be reduced by 1-2% or increase by 1-2% depending upon the control technique used.

COST-EFFECTIVENESS:
High. These techniques are not designed to control CO₂ emissions.

SECONDARY EFFECTS:
These technologies are designed to remove high amounts of NO_x and SO₂of unnecessary emissions of other pollutants.

ISSUES ASSOCIATED WITH IMPLEMENTING ACTION

• Byproducts have to be disposed of.
• Capital costs of environmental pollution control technologies can be prohibitive.

RESOURCES

• The World Bank hosts information on a variety of environmental pollution control technologies on at its Environmental Management (EM) Power website, along with guidelines as to conducting environmental assessments. http://www.worldbank.org/html/fpd/em/.
• The U.S. DOE Federal Energy Technology Center (FETC) sponsors the Emissions Control Byproducts Consortium (ECBC), dedicated to develop and demonstrate technologies for solving problems related to the utilization of by-products from coal combustion processes. It is hoped that these technologies, by the year 2005, will lead to a doubling of the current rate of FGD by-product use, a 10% increase in the overall national rate of byproduct use, and a 25% increase in the number of uses considered "allowable" under state regulations.
• Tavoulareas, E. Stratos and Jean-Pierre Charpentier, 1995, Clean Coal Technologies for Developing Countries, World Bank Technical Paper No. 286, Energy Series, (July).
• United States Department of Energy, 1998, Clean Coal Technology Demonstration Program Project Fact Sheets 1997, DOE/FE-0369. 150p
• South, D.W., et al., Technologies and Other Measures for Controlling Emissions: Performance, Cost and Applicability, NAPAP SOS/T-25, National Acid Precipitation Assessment Program, Acidic Deposition: State of Science and Technology, Volume IV., (Washington D.C.: December 1990).

CONTACTS

ABB Environmental Systems
KNO_xville, TN
Tel: (800) 346-8944
ABBMarket@usapc.mail.abb.com
http://www.abb.com

Electric Power Research Institute
Palo Alto, CA
http://www.epri.com

Institute for Clean Air Companies
Jeffrey C. Smith
Executive Director
Washington, DC
Tel: (202) 457-0911
Jsmith@icac.com
http://www.icac.com

U.S. Department of Energy
Randolph Pennington
Program Manager
Environmental Systems
Germantown, MD
Tel: (301) 903-3485
Randy.Pennington@hq.doe.gov
http://www.doe.gov

World Bank
Stratos Tavoulearas
Washington, DC
Tel: (202) 458-9118
http://www.worldbank.org