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Nitrogen oxide (NOx) is a general term used to describe a group of gases that contain varying amounts of nitrogen and oxygen. NOx forms at high temperatures during fossil fuel combustion (see How NOx is Formed ). The primary sources of NOx emissions in the United States are motor vehicles, power plants, and other commercial, industrial, and residential sources that burn fossil fuels. Direct NOx emissions from these sources include nitrogen dioxide (NO₂), nitrous oxide (N₂O), and nitric oxide (NO). NOx emissions can also react with other compounds in the atmosphere to form secondary products that includes ozone (O₃), nitric acid (HNO₃), and nitrate particles. Because of the health and environmental impacts associated with NOx emissions, coal-fired power plants are subject to local, state, and Federal emission regulations (see NOx Regulatory Drivers ).

There are a number of NOx control technologies that have been developed for coal-fired power plants since enactment of the 1990 Clean Air Act Amendments (see NOx Reduction Technologies ). These NOx control technologies can be grouped into two broad categories: combustion modifications and post-combustion processes. Combustion modifications manage the mixing of fuel and air, thereby reducing temperature and initial turbulence, which minimizes NOx formation in the boiler. Post-combustion control relies on various chemical processes to convert the NOx formed in the boiler to inert nitrogen. In addition to NOx control processes, more sophisticated combustion control systems using neural networks are being developed to further reduce NOx emissions while optimizing overall boiler performance.

Commercially-available low-NOx burners (LNB) and selective catalytic reduction (SCR) NOx control technologies are enabling industry compliance with today's regulatory requirements. However, the NOx control technologies under development by DOE/NETL will provide more cost-effective options for coal-fired power plants to comply with the ever more stringent environmental regulatory and legislative requirements of tomorrow. NOx emission control costs are significant and can exceed 20% of the total cost for environmental controls on today's coal-fired power plants. The capital and operating costs of SCR controls are relatively high and may not be cost-effective for older, smaller coal-fired power plants, or even for some larger base load plants. In particular, the high capital cost for SCR can result in a levelized cost of control significantly greater than current and projected NOx allowance prices for units with a relatively low capacity factor. Potentially more stringent state, regional, and/or federal regulations in the near future will require the retrofit of NOx controls on a greater proportion of existing coal-fired power plants. As a result, a portion of these plants could be at-risk for early retirement if more cost-effective control technologies than today's SCR are not developed.

DOE/NETL's NOx R&D program focuses on the development of advanced in-furnace NOx controls that can approach the performance of SCR, but at significantly lower cost. Further, the technologies under development are: (1) to have negligible impact on balance-of-plant issues, (2) applicable to a wide range of boiler types and configurations, and (3) capable of maintaining performance over a wide range of feed coals and operating conditions. The DOE/NETL portfolio of NOx control technology R&D projects encompasses laboratory studies, modeling, and pre-commercial demonstration full-scale testing. The success of the projects is intimately tied to key collaborations and partnerships with industry, federal, state, and local agencies, and the academic and research communities.