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NOx Reductions Under Phase II of the Acid Rain Program

Group 1 and Group 2 Boilers

The final rule implements the second stage of the Nitrogen Oxides (NOx) Reduction Program under title IV of the Clean Air Act Amendments (CAAA) of 1990 by establishing NOx emission limitations for certain coal-fired utility units and by revising NOx emission limitations for others.

See also: full text of the final rule

Benefits of Reducing NOx

Emissions of nitrogen oxides discharged into the atmosphere from the burning of fossil fuels have significant adverse effects on human health and the environment, contributing substantially to the formation of ozone, acid deposition, eutrophication of water bodies, inhaleable fine particles, and visibility degradation. Substantial, additional regional NOx reductions from current levels are likely to be necessary to address these problems. Electric utilities are a major contributor to NOx emissions nationwide, and approximately 90 percent of electric utility NOx comes from coal-fired power plants. The emission limitations established by this rule are some of the most cost-effective means of achieving NOx reductions. The level of needed reductions will likely be greater than those achievable under the title IV NOx emission limitations established under today's final rule. The additional reductions from this final rule represent a reasonable step toward achieving necessary NOx reductions.

First Stage of the NOx Reduction Program

Title IV specifies a two-part strategy to reduce emissions from coal-fired electric power plants. The first stage of the program, promulgated April 13, 1995 will reduce annual NOx emissions in the United States by over 400,000 tons per year between 1996 and 1999 (Phase I), and by approximately 1.17 million tons per year beginning in the year 2000 (Phase II). These reductions are achieved by coal-fired dry bottom wall-fired boilers and tangentially fired boilers (Group 1). The total annual cost of this regulation to the electric utility industry is estimated at $267 million, resulting in an overall cost-effectiveness of $227 per ton of NOx removed.

Second Stage of the NOx Reduction Program

In the second stage of the title IV Program EPA has: (1) determined that more effective low NOx burner (LNB) technology is available to establish more stringent standards for Phase II, Group 1 boilers than those established for Phase I; and (2) established limitations for other boilers known as Group 2 (wet bottom boilers, cyclones, cell burner boilers, and vertically fired boilers), based on NOx control technologies that are comparable in cost to LNBs. These new determinations increase the reductions from the Title IV NOx Reduction Program to 2.1 million tons per year, beginning in 2000.

The final rule sets lower Group 1 emission limits and establishes emission limits for several other types of coal-fired boilers (Group 2) in Phase II. The annual cost of these additional reductions will be approximately $200 million, at an average cost-effectiveness of $229 per ton of NOx removed. By the year 2000, the Phase II NOx rule will achieve an additional reduction of 890,000 tons of NOx annually, increasing the overall annual reductions to 2.1 million tons. Overall, this rule achieves significant NOx reductions very cost-effectively.
Emission Limits for Phase II

The following table presents the boiler types affected by this rule, their population, and the NOx emission limitations:

Boiler Types

Number of Boilers

Phase II Emission Limits

Group 1 Boilers
dry bottom wall-fired


dry bottom wall-fired: 0.46 lb/mmBtu
tangential: 0.40 lb/mmBtu

Group 2 Boilers
cell burners
cyclones > 155 MW
wet bottoms > 65 MW
vertically fired


cell burners: 0.68 lb/mmBtu
cyclones: 0.86 lb/mmBtu
wet bottoms: 0.84 lb/mmBtu
verticals: 0.80 lb/mmBtu

Compliance and Deadlines

A utility can choose to comply with the rule in one of three ways: (1) meet the standard annual emission limitations, (2) average the emissions rates of two or more boilers, which allows utilities to over-control at units where it is technically easier and less expensive to control emissions, or (3) if a utility cannot meet the standard emission limit, it can apply for a less stringent alternative emission limit (AEL) if it uses the appropriate NOx emission control technology on which the applicable emission limit is based.
EPA's determination of an AEL will be based on evidence that control equipment was properly designed, installed, and operated during a demonstration period.

Phase I affected units are required to meet the applicable limits by 1996, Phase II affected units are required to meet the applicable limits by 2000. The final rule relies upon target performance standards, but also allows emissions averaging and the use of alternative, higher emissions limits where meeting the applicable limits is infeasible. Utilities choose the method of compliance that best suits their needs. This approach provides flexibility, promotes technology development and competition, and provides opportunities to reduce the cost of control.


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