Control of Nitrogen Oxide Emissions

At very high combustion temperatures, the following reaction occurs  

High temperatures favor both a high equilibrium concentration and a rapid rate of formation of NO. Especially in internal combustion engines, rapid cooling of the exhaust gas from combustion freezes NO at a relatively high concentration because equilibrium is not maintained. Thus, by its nature, the combustion process both in the internal combustion engine and in furnaces produces high levels of NO in the combustion products. 

The level of NOx emitted from stationary sources such as power plant furnaces generally falls within the range of 50-1000 ppm. Generation of NO is favored both kinetically and thermodynamically by high temperatures and by high excess oxygen concentrations, and these factors must 

Removal of NOx from stack gas presents some formidable problems. Use of liquid scrubbers such as those employed to remove SOj from stack gas is not very effective for NOx removal because of the low solubilities of nitrogen oxides. Sorption onto solids followed by destruction of the sorbed gases has been tried. Catalytic reduction and decomposition of nitrogen oxides are employed in automobile catalytic converters (see Section 8.8) and may be applicable to stack gas, although sulfur gases and particles in stack gas may interfere and poison the catalysts. Another possibility is the use of biofilters in which microorganisms held on support media metabolize NOx Section 8.9). 

---

U.S. Department of Energy, Clean Coal Technology Topical Reports. (1999). Reburmng Technologies for the Control of Nitrogen Oxides Emissions from Coal-Fired Boilers. Report No. 14 (May). Washington, DC U.S. 

Kemal, A., J. Dorn, and C. T. Bowman. 1997. Control of nitrogen oxide emissions from air-breathing combustors using partial premixing of fuel and air. Western States Section of the Combustion Institute. Paper No. 97F-136. 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

Epperly, W. R., Broderick, R. G., and Peter-Hoblyn, J. D., 1988A, Control of Nitrogen Oxides Emissions From Stationary Sources, Proceedings of the American Power Conference, Chicago, IL, April 20, pp. 911-915. 

Ammonia Plants - New ammonia plants should set as a target the achievement of nitrogen oxide emissions of not more than 0.5 kg/t of product (expressed as NOj at 3%). Ammonia releases in liquid effluents can be controlled to 0.1 kg/t of product. Condensates from ammonia produetion should be reused. 

Less, but still significant, information is available on the surface chemistry of other nitrogen oxides. In terms of N20, that molecule has been shown to be quite reactive on most metals on Rh(110), for instance, it decomposes between 60 and 190 K, and results in N2 desorption [18]. N02 is also fairly reactive, but tends to convert into a mixed layer of adsorbed NO and atomic oxygen [19] on Pd(lll), this happens at 180 K, and is partially inhibited at high coverages. Ultimately, though the chemistry of the catalytic reduction of nitrogen oxide emissions is in most cases controlled by the conversion of NO. 

Early emission control methods were based on the use of a thermal reactor for hydrocarbon and carbon monoxide oxidation, combined with exhaust gas recirculation (EGR) for reduction of nitrogen oxide emissions (Fig. 3.2a). Hydrocarbons and carbon monoxide in the hot exhaust fed to the reactor, once heated, were rapidly oxidized to carbon dioxide and water by the additional pumped air which was fed to the reactor (e.g., Eqs. 3.3 and 3.4). 

The first commercial appHcation of precious metals for the reduction of nitrogen oxides in power plant emission control was in 1989. W. R. Grace s... 

Selective Catalytic Reduction of Nitrogen Oxides The traditional approach to reducing ambient ozone concentrations has been to reduce VOC emissions, an ozone precurssor. In many areas, it has now been recognized that ehmination of persistent exceedances of the National Ambient Air Qnality Standard for ozone may reqnire more attention to reductions in the other ingredients in ozone formation, nitrogen oxides (NOJ. In such areas, ozone concentrations are controlled by NO rather than VOC emissions. 

Title rV Acid Deposition Control - As we all know, acid rain occurs when sulfur dioxide and nitrogen oxide emissions are transformed in the atmosphere and return to the earth in rain, fog, or snow. Approximately 20 million tons of SOj are emitted annually in the United States, mostly from the burning of fossil fuels by electric... 

New low-NO, burners are effective in reducing emissions from both new power plants and existing plants that are being retrofitted. Low NO, burners limit the formation of nitrogen oxides by controlling the mixing of fuel and air, in effect... 

While natural emissions of sulphur and nitrogen exist, over 95% of the sulphur emissions in eastern North America are of man-made origin. Natural sources of nitrogen are less well estabUshed but are estimated to be small when compared to the man-made emissions 21). The distribution of North American sources of sulphur dioxide and nitrogen oxides are shown in Figure 2. In 1980, which has served as the base period for the assessment of emissions,it was estimated that sulphur dioxide emissions were Canada - 4.8 million tonnes (metric) and the United States - 24 million tonnes nitrogen oxides emissions were Canada - 1.8 million tonnes and the United States - 20 million tonnes. The more recent trends for sulphur dioxide emissions in Canada and the emission control limits are shown in Figure 3 14). 

---