emissions will have to be supplemented by post-combustion controls.

Advanced cost-effective NOx control technologies that can be retrofitted to existing coal-fired electric utility boilers and are capable of meeting a target NOx emission limit of 0.15 lb NOx/million Btu will have to be developed. Although, in general, currently available selective catalytic reduction control technology can achieve this emission limit, high capital costs and the lack of long-term availability of catalysts are major concerns.

In most cases, existing control technologies for the removal of sulfur dioxide will be adequate to meet future mandated reduction goals. However, retrofitting existing equipment could pose problems for many utilities because of space limitations, and innovative retrofitting concepts will be required for these sites.

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. At present, average primary particulate emissions from all coal-fired utility boilers are about 0.043 lb/million Btu (FETC, 1999c).

To comply with current PM emission limits, the majority of coal-fired electric utility boilers control primary particulates with electrostatic: precipitators, which use electric fields to remove particulates from boiler flue gas. A smaller but growing number of boilers employ fabric filter collectors (bag houses), which control PM by passing the flue gas through a tightly woven fabric that collects the particles in the form of dust cake. When operating properly, state-of-the-art electrostatic precipitators and bag houses can achieve overall collection efficiencies of 99.9 percent of total primary particulate mass to below the 1978 NSPS of 0.03 lb/million Btu.

However, even with high-performance particulate control systems, collection is less efficient for particle sizes of less than 1 μm. Particles in the 0.1 to 0.5 μm size range are an especially challenging problem for stationary sources that employ wet scrubbers and electrostatic precipitators. Moreover, more than half of the existing electrostatic precipitators installed on electric-utility boilers in the United States have been in operation for more than 30 years, and almost 17 percent for 40 years. These older models of electrostatic precipitators are less efficient than more recent models.

Potential national strategies for reducing PM may include limiting coal-fired units to a more stringent emissions rate than the current 1978 NSPS. In the near term, it may be possible to improve the operation and maintenance of existing particulate control technologies and achieve higher overall collection efficiencies. However, strategic research, development, and testing of efficient, cost-effective, innovative PM control technologies, processes, and concepts may be necessary. If so, new fine particulate control technologies are likely to require long development times.

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