A Survey of Nitrogen-Oxides Control Technology and the Development of a Low NO_x Emissions Combustor

The problem of NO_x emissions reduction is gaining increased attention from those concerned with air pollution control activities. HEW proposes to issue criteria for the control of nitrogen oxides within the year. The Los Angeles Air Pollution Control District (LAAPCD) has already published regulations which limit emissions from combustion sources to a fixed rate of 140 lb/hr NO_x. This is equivalent to an allowable emission concentration of about 20 ppm for a 500 MW power station. Two of the major contributors to oxides of nitrogen air pollution in the United States are Industrial Processes and Stationary Combustion Sources, both being responsible for over 50 percent of the total NO_x emissions. Motor vehicles contribute the remainder for a total of 20 million tons per year. Although some advances have been made in the development of commercial control equipment for Industrial Process emissions, there is very little well-developed technology available for NO_x emissions control from Stationary Combustion Sources. Two of the most promising areas being investigated are: • Stoichiometric variations of the air-fuel feed and partial recycling of the combustion products. • Advanced design of combustion equipment. The former system of control, in a test program with 17 commercial boilers, has been responsible for the reduction of NO_x emissions from about 350–150 ppm. An advanced design combustor, operated at a heat release level of 10⁵ Btu/hr, has produced NO_x emission values of about 150 ppm. When coupled with simulated combustion gas recycle, the emissions were further reduced to 100 ppm. Although both the “stoichiometric” and “combustor” approach to the problem of NO_x emissions represent a good beginning, greater efforts must be expended on these and other promising control techniques in preparation for future regulations.