When fossil fuels burn at sufficiently high temperatures, nitrogen oxides (NOx) are formed. Although there are many sources of NOx - for example, gasoline-powered automobiles are major sources of NOx - coal-fired power plants account for approximately 25% of the emissions of NOx in the U.S. annually. NOx can contribute to smog and acid rain. The Department of Energy's Fossil Energy program, therefore, is focusing considerable attention on new technologies to reduce NOx emissions from existing and future coal-fired power plants.

When coal burns, NOx is formed in two ways: (1) when nitrogen bound in the coal is released and combines with oxygen to form "fuel NOx," and (2) when high combustion temperatures break apart stable nitrogen molecules in the air which then recombine with oxygen to form "thermal NOx."

Because NOx emissions do not form in significant amounts until flame temperatures reach 2800 degrees F, technology development historically has focused largely on ways to burn fuel in stages. Staged combustion limits the amount of air in the initial stages of combustion when fuel bound nitrogen is released, and also reduces flame temperature during some phases of combustion to reduce thermal NOx.

	IF DOE IS SUCCESSFUL...
	The estimated market penetration for advanced NOx control technologies is 170,000 megawatts under the Clear Skies Initiative caps. If DOE can help produce lower cost NOx control technologies, U.S. ratepayers could save nearly $300 million a year by 2010.

Burners using staged combustion are called "low-NOx burners." Demonstrated in the Energy Department's Clean Coal Technology Program, low-NOx burners are now installed on approximately 75 percent of the suitable coal-fired generating capacity in the United States. They are one of the Energy Department's major clean coal "success stories."

But in recent years, environmental laws have tightened. The burners demonstrated in the Clean Coal Technology Program could cut NOx by approximately 40 to 70 percent - reducing emissions to roughly 0.65 to 0.25 pounds per million Btus (depending on coal type and combustion conditions). New Clean Air Act regulations, however, restrict NOx emissions to as low as 0.15 pounds per million Btus (for 22 eastern states and the District of Columbia.) President Bush's Clear Skies Initiative together with the Clean Air Interstate Rule lowers the nationwide cap on NOx emissions and will likely require major NOx reduction measures at virtually all of the Nation's coal-fired power plants. Moreover, new limits on microscopic particulate matter (PM2.5) will also lead to more stringent NOx limits, since NOx can be a precursor of these airborne particles.  Finally, many states have adopted NOx limits that are considerably lower than federal requirements.

To meet these new challenges, many utilities have turned to other types of NOx controls. The most effective is called "selective catalytic reduction" or SCR. SCR uses ammonia or urea along with catalysts in a post-combustion vessel to transform NOx into nitrogen and water. It can achieve the 0.15-pound-per-million Btu standard, however it requires additional equipment that is costly and complex to operate, and can create its own environmental problems (if ammonia leaks into the air). A concept called "selective non-catalytic reduction," or SNCR, does not use catalysts and typically injects the ammonia/urea directly into the boiler.

A New Generation of Low-NOx Burners. The Energy Department is focusing much of its research on a new generation of low-NOx burners. The goal is to develop advanced low-NOx burners that can also achieve the 0.15-pound-per-million Btu standard but at a cost 25 to 50 percent below that of SCR. Eliminating the post-combustion chemical plant (e.g., selective catalytic reactor) would reduce a power plant's complexity and lower the costs of electricity produced for consumers.

Advanced "Reburning" Concepts. Another possible way to reduce NOx emissions is a concept called "reburning." In the reburning process, part of the fuel is injected into a separate reburn zone, typically after the main combustion zone. Oxygen levels are kept low in the reburn zone, and the resulting fuel-rich conditions break apart NOx formed in the main combustion zone. Although early versions of reburning were demonstrated in the Clean Coal Technology Program, the Energy Department is studying more advanced reburn concepts, including some approaches in which reburning is combined with selective non-catalytic reduction.

Oxygen-Enhanced Combustion. Another advanced approach for reducing NOx may be to add oxygen to the combustion process. Although this seems counter-intuitive, because oxygen promotes higher flame temperatures, research has shown that by carefully controlling the way in which oxygen is added, the efficiency of the combustion process can be improved (meaning less fuel is consumed) and nitrogen content can be reduced. NOx emissions reductions of as much as 80-90% have been demonstrated at commercial glass melting furnaces that have been converted to oxygen-enhanced fuel firing. Key to this approach will be an economical way to produce the required oxygen. Ceramic membranes, which allow only the oxygen in air to pass through, could be one way to lower the cost of oxygen production.