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Nitrogen oxide formation is an inherent consequence of fossil fuels combustion. Over the next several decades, such combustion will continue to be our major source of energy for electricity generation and motor vehicle propulsion. Unlike many other atmospheric pollutants, more nitrogen oxides are formed at higher combustion efficiencies. Fixation of atmospheric nitrogen and oxygen and oxidation of nitrogen compounds in fuel are the two processes by which fuel combustion results in nitrogen oxide formation.
The term nitrogen oxide typically refers to any binary compound of oxygen and nitrogen, or to a mixture of such compounds: Nitric oxide (NO), nitrogen(II) oxide Nitrogen dioxide (NO2), nitrogen(IV) oxide Nitrous oxide (N2O), nitrogen (I) oxide Dinitrogen trioxide (N2O3), nitrogen(II, IV) oxide Dinitrogen tetroxide (N2O4), nitrogen(IV) oxide Dinitrogen pentoxide (N2O5), nitrogen(V) oxide (Note that the last three are unstable.)
NOx is a generic term for mono-nitrogen oxides (NO and NO2). These oxides are produced during combustion, especially combustion at high temperatures. At ambient temperatures, the oxygen and nitrogen gases in air will not react with each other. In an internal combustion engine, combustion of a mixture of air and fuel produces combustion temperatures high enough to drive endothermic reactions between atmospheric nitrogen and oxygen in the flame, yielding various oxides of nitrogen. In areas of high motor vehicle traffic, such as in large cities, the amount of nitrogen oxides emitted into the atmosphere can be quite significant. When NOx and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog, a significant form of air pollution, especially in the summer. Children, people with lung diseases such as asthma, and people who work or exercise outside are susceptible to adverse effects of smog such as damage to lung tissue and reduction in lung function. Mono-nitrogen oxides are also involved in tropospheric production of ozone. NOx should not be confused with NOS, a term used to refer to nitrous oxide (N2O) in the context of its use as a power booster for internal combustion engines.
NOx react with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases such as emphysema, bronchitis it may also aggravate existing heart disease. NOx react with volatile organic compounds in the presence of heat and sunlight to form Ozone. Ozone can cause adverse effects such as damage to lung tissue and reduction in lung function mostly in susceptible populations (children, elderly, asthmatics). Ozone can be transported by wind currents and cause health impacts far from the original sources. Millions of Americans live in areas that do not meet the health standards for ozone.
NOx also readily react with common organic chemicals, and even ozone, to form a wide variety of toxic products: nitroarenes, nitrosamines and also the nitrate radical some of which may cause biological mutations.
The three primary sources of NOx in combustion processes: thermal NOx fuel NOx prompt NOx Thermal NOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel NOx tends to dominate during the combustion of fuels, such as coal, which have a significant nitrogen content, particularly when burned in combustors designed to minimise thermal NOx. The contribution of prompt NOx is normally considered negligible. A fourth source, called feed NOx is associated with the combustion of nitrogen present in the feed material of cement rotary kilns, at between 300° and 800°C, where it is also a minor contributor.
Agriculturalfertilization and the use of nitrogen fixingplants also contribute to atmospheric NOx, by promoting nitrogen fixation by microorganisms.
Nitrogen and oxygen combine to form several stable oxides of different molecular composition. The two oxides identified as important pollutants in the lower atmosphere are nitric oxide and nitrogen dioxide. Nitrous oxide occurs in significant concentrations even in the natural unpolluted atmosphere. It arises from natural biologic processes that occur in the soil and is not classified as an air pollutant.
Among the various oxides of nitrogen presenting the sunlight-irradiated, polluted atmospheres, nitric oxide and nitrogen dioxide, designated by the composite formula, play the most important role in chemical and photochemical changes. A major source of these oxides is the combustion of fossil fuels, which releases a predominance of nitric oxide. The rapid cooling of the gases in the combustion chamber prevents the return of nitric oxide is converted to nitrogen dioxide by reaction with oxygen during the exhaust dilution process; however, the major pathway leading to formation of nitrogen dioxide from nitric oxide is the photochemical interaction between hydrocarbons and various other compounds and intermediate free radicals generated in the sunlight-irradiation polluted atmosphere.
The extent to which nitrogen dioxide reduces visibility and colors the horizon sky depends on the concentration of the pollutant, the viewing distance, and the accompanying aerosol concentration. The presence of photochemical aerosol or other particulates matter suppresses the coloration effect and increases the visibility-reduction effect.
Significant concentrations of nitric oxide and smaller concentrations of nitrogen dioxide are formed at the high temperatures accompanying the burning of fossil fuels in mixtures of air. These pollutants are emitted to the atmosphere from auto exhausts, power plant and furnace stacks, incinerators, and vents from certain chemical processes. Natural biologic reactions also generate large quantities of nitric oxide and nitrogen dioxide. Nitrous is not considered an air contaminant since there is no evidence involving it in the series of complex chemical reactions producing photochemical smog.
Nitrous oxide is a product is a product of natural chemical reactions that occurs in the soil. The nitrogen oxides, symmetrical nitrogen trioxide, dinitrogen trioxide, dinitrogen tetroxide, and dinitrogen pentoxide have not been identified as trace levels is inferred from the thermodynamic and kinetics of their reactions as studied in the laboratory. The concentrations of the oxides and acids of nitrogen are controlled largely by the reaction rates and usually much greater than those expected at equilibrium. Nitric oxide formation is favored at high temperatures. The concentrations of nitric oxide and nitrogen dioxide are limited by the thermodynamics and kinetic properties of nitrogen, oxygen, nitric oxide, and nitrogen dioxide molecules, and nitrogen and oxygen atoms.
