Table of Contents

• Primary Particulates
• Secondary Particulates
• Impact of PM2.5 NAAQS Regulations
• Visibility (Regional Haze)
• Opacity
• Toxic Release Inventory (TRI)

Primary Particulates
Currently, coal-fired electric utility boilers built or modified after August 17, 1971, must comply with a New Source Performance Standard (NSPS) limit on primary particulate emissions of 0.10 lb/million Btu. Units built or modified after September 18, 1978, must comply with a more stringent NSPS of 0.03 lb/million Btu, or 1 percent of the potential combustion concentration (99 percent reduction). Average primary particulate emissions from all coal-fired utility boilers are about 0.043 lb/million Btu. Airborne particles are also regulated as "criteria pollutants" under EPA's National Ambient Air Quality Standards (NAAQS) program, so the emissions of primary PM10 (particles smaller than 10 micrometers) and PM2.5 (particles smaller than 2.5 micrometers) from coal power plants are also subject to limitations set forth under State Implementation Plans (SIPs) for achieving the ambient standards for these pollutants.

The utility industry has already made great strides in controlling the emissions of primary particulates. Emissions of PM10 in 1996 were less than 260,000 tons/year, compared with early-1970 emission levels that exceeded 1.6 million tons/year. These declines in emissions are made even more dramatic considering that during the period from 1970 to the present there has been a greater than 150 percent increase in coal consumed to produce electricity. However, despite these successes, primary particulate emissions from coal-fired power plants may continue to be targeted in reaction to environmental issues such as visibility, acidification, and air toxics.

Secondary Particulates
Gaseous emissions from coal-fired power plants, most notably SO₂ and NOx, can react in the atmosphere to form "secondary" fine particles (sulfate and nitrate salts). Emissions of SO₂ and NOx are regulated as gaseous pollutants under several provisions of the 1990 Clean Air Act Amendments. EPA may also consider additional restrictions on SO₂ and NOx as a result of their potential to form secondary fine particles. The current regulations applicable to air emissions of gaseous SO₂ and NOx are summarized below.

New Source Performance Standards (NSPS) - Title I, Part A, Section 111
The table below summarizes the SO₂ and NOx emission limits specified under NSPS for coal-fired electric utility steam generating units:

Acid Rain - Title IV
Reductions in SO₂ releases are being obtained under Title IV through a program of emission allowances. EPA issues allowances to power plants covered by the acid rain program; each allowance is worth one ton of SO₂ released. To obtain reductions in SO₂ pollution, allowances are set below the current level of SO₂ releases. Plants may only release as much SO₂ as they have allowances. Plants can exceed these allowances by buying them from another power plant that has reduced its SO₂ releases below its number of allowances and therefore has allowances to sell or trade. There are stiff penalties for plants that release more pollutants than their allowances cover.

Title IV also restricts the maximum allowable emission rates of NOx from coal-fired utility units. Phase I, which began in 1996, sets NOx emission limits at 0.45 lb/million Btu for tangentially fired boilers and 0.50 lb/million Btu for dry bottom wall-fired boilers (other than units applying cell burner technology). Phase II, which began in 2000, set additional limitations on boilers required to comply with Phase I reducing NOx emission limits to 0.40 lb/million Btu for tangentially fired boilers and 0.46 lb/million Btu for dry bottom wall-fired boilers. NOx emission limits also were applied to additional boiler sources, including cell burner boilers (0.68 lb/million Btu), cyclone boilers (0.86 lb/million Btu), vertically fired boilers (0.80 lb/million Btu), and wet bottom boilers (0.84 lb/million Btu). Thus, the Title IV restrictions on NOx emissions are more stringent than the NSPS limits listed above.

Full implementation of Title IV will result in an annual cap on power plant SO₂ emissions of 8.9 million tons, down from a 1970 level of nearly 16 million tons. In addition, NOx emissions during Phase I of Title IV will be reduced by 400,000 tons/yr between 1996 and 1999, while Phase II will reduce annual emissions by another 1.2 million tons.

National Ambient Air Quality Standards (NAAQS) -
Title I, Part A, Sections 109-110

SO₂ and NO₂ are identified as gaseous "criteria pollutants" under EPA's National Ambient Air Quality Standards (NAAQS) program, so the emissions of SO₂ and NOx from coal power plants are subject to restrictions set forth under State Implementation Plans (SIPs) for achieving the ambient standards for these pollutants. NOx is also a key precursor to ozone, another a NAAQS criteria pollutant; therefore, emissions of NOx from coal plants may also be regulated under SIPs for achieving the ambient ozone standard.

Impact of PM2.5 NAAQS Regulations
Although most of the primary PM produced by coal-fired power plants is captured by existing pollution control devices, the portion that does escape falls mostly into the PM2.5 (less than 2.5 microns) size category, as does almost all of the secondary PM formed from SO₂ and NOx. EPA promulgated new ambient standards for PM2.5 in July 1997. The schedule for implementing the PM2.5 standards requires the collection and analysis of data from a nationwide ambient monitoring network through 2003. Contingent upon the outcome of a five-year scientific review of the standards to be completed in 2002, EPA will designate non-attainment areas starting in 2002 and ending by 2005. States that have areas that are not in compliance with the standards will be required to submit SIPs by 2008; full compliance with the PM2.5 NAAQS will be required by 2017.

It is not certain at this time what implications, if any, PM2.5 non-compliance status will have on emissions of primary or secondary particulates from coal-based power systems. Quantitative correlations between electric-utility emissions of primary PM, secondary PM, and ambient PM2.5 have not yet been established. The NETL Air Quality Research program is gathering and interpreting data to improve the accuracy of such quantitative correlations. Although research has generally indicated a consistent positive correlation between elevated ambient PM2.5 mass concentrations and adverse human health effects, the specific components of PM2.5 that are most detrimental to human health have not yet been identified. It is uncertain whether the portion of PM2.5 associated with coal plant emissions is responsible for the observed adverse health effects. However, further restrictions on power plant emissions of primary fine particles and gaseous precursors in response to PM2.5 regulations remains a possibility. Of particular concern are the ultra fine (less than 0.1 micron) particles that may be associated with trace metals and semi-volatile and volatile organics.

Visibility (Regional Haze)
On April 22, 1999, EPA announced a major effort to improve air quality in national parks and wilderness areas. The Regional Haze Rule calls for state and federal agencies to work together to improve visibility in 156 national parks and wilderness areas such as the Grand Canyon, Yosemite, the Great Smokies and Shenandoah. The rule requires the states, in coordination with the Environmental Protection Agency, the National Park Service, U.S. Fish and Wildlife Service, the U.S. Forest Service, and other interested parties, to develop and implement air quality protection plans to reduce the pollution that causes visibility impairment. The first State plans for regional haze are due in the 2003-2008 time frame. Five multi-state regional planning organizations are working together now to develop the technical basis for these plans.

Both the Regional Haze Rule and the PM2.5 NAAQS (and their associated implementation processes) provide similar regulatory mechanisms for reducing power plant emissions of primary PM and gaseous precursors to secondary PM. However, the technical basis for implementation of the two standards is significantly different. For example, the effect on human health must be considered as a part of the standard-setting and implementation processes for the PM2.5 NAAQS; however, health effects are not relevant to implementing the Regional Haze Rule. Furthermore, EPA has proposed guidelines for implementation of the best available retrofit technology (BART) requirements under the Regional Haze Rule for certain large stationary sources put in place between 1962 and 1977, including power plants.The scientific evidence for a causal relationship between power plant emissions and regional haze is generally considered to be stronger than the link between such emissions and PM-related health effects. Also, the proposed BART guidelines specifically allow states to consider the possibility of restricting SO₂ emissions from power plants to levels that are below the existing NSPS. It is therefore possible that power sources that do not face additional emission restrictions under the PM2.5 NAAQS implementation process may still face additional emission restrictions under the Regional Haze Rule.

Opacity
Opacity is a measure of the degree to which a substance prevents the transmission of light. The percentage opacity of a substance is defined as:

%Opacity = (1-I/I_o) * 100%

where I_o and I are, respectively, the intensity of the light before and after it passes through the substance.Opacity of the combined gas/particle releases from the coal-fired electricity generating units is regulated under the same section of NSPS that restricts the emission of particulate matter (40 CFR Chapter I, Part 60, Subpart D, Section 60.42 and 60.42a). All such units built or modified after August 17, 1971, are prohibited from releasing any gases that exhibit greater than 20 percent opacity (6-minute average), except for one 6-minute period per hour of not more than 27 percent opacity.Opacity in coal-fired utility boiler exhaust streams can be caused by many factors, including primary PM escaping the particulate collection system, particles generated in flue gas desulfurization (FGD) systems downstream of the primary particulate control device, condensable aerosols such as sulfuric acid in the flue gas, secondary PM formation, and colored gases such as NO₂. For this reason, some units can violate the opacity standard while still meeting the particulate matter emission standard. Development of appropriate control technologies depends on the accurate identification of the actual source(s) of stack opacity.

Toxic Release Inventory (TRI)
Two rules, Section 313 of the Emergency Planning and Community Right-To-Know Act (EPCRA) and section 6607 of the Pollution Prevention Act (PPA), mandate that a publicly accessible toxic chemical database be developed and maintained by U.S. EPA. This database, known as the Toxics Release Inventory (TRI), contains information concerning waste management activities and the release of toxic chemicals by facilities that manufacture, process, or otherwise use said materials. The TRI regulation is merely a reporting requirement; it does not restrict the emissions of toxic materials. However, the ready accessibility of this information has resulted in increased public pressure on industries to reduce these emissions voluntarily, and has mobilized public opposition to the expansion of activities by traditionally "heavy polluting" industries.

Electric utilities began reporting under TRI in July 1999, including releases of certain trace metals and acid gases. Although the concentrations of toxic chemicals in utility flue gases are extremely low, the total amount released from a facility in a given year can be significant because of the vast quantities of flue gases produced. Using the EPRI PISCES model, Rubin et al. (November 1997) estimated annual TRI-related emissions for a 650 MW (net) electric-utility boiler burning bituminous coal in compliance with Phase I acid rain provisions and equipped with an ESP. The dominant chemical species released at the power plant stack were sulfuric acid (H₂SO₄), hydrogen chloride (HCl), and hydrogen fluoride (HF). While these emissions are only indirectly related to PM, the development of control technologies for PM may also have an impact on or be driven by public pressure to reduce TRI-related emissions.