Nitrogen oxide

Nitrogen oxide is typically any binary compound of oxygen and nitrogen, or 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.)

Chemical reactions that produce nitrogen oxides often produce several different compounds, the proportions of which depend on the specific reaction and conditions. For this reason, secondary^{[clarification needed]} production of N₂O is undesirable, as NO and NO₂ — which are extremely toxic — are liable to be produced as well.

Nitric oxide, NO	Nitrogen dioxide, NO2	Nitrous oxide, N2O	Dinitrogen trioxide, N2O3
Dinitrogen tetroxide, N2O4	Dinitrogen pentoxide, N2O5

NO_x

It has been suggested that some sections of this article be split into a new article entitled NOx. (Discuss) (June 2009)

NO_x 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.

In the presence of excess oxygen (O₂), nitric oxide (NO) will be converted to nitrogen dioxide (NO₂), with the time required dependent on the concentration in air as shown below:^[1]

When NO_x 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.^[2]

Mono-nitrogen oxides eventually form nitric acid when dissolved in atmospheric moisture, forming a component of acid rain. The following chemical reaction occurs when nitrogen dioxide reacts with water:

2 NO₂ + H₂O → HNO₂ + HNO₃

Nitrous acid then decomposes as follows:

3 HNO₂ → HNO₃ + 2 NO + H₂O

where nitric oxide will oxidize to form nitrogen dioxide that again reacts with water, ultimately forming nitric acid:

4 NO + 3 O₂ + 2 H₂O → 4 HNO₃

Mono-nitrogen oxides are also involved in tropospheric production of ozone.^[3]

NO_x should not be confused with nitrous oxide (N₂O) which has many uses as an oxidizer, an anesthesia, and a food additive.

In atmospheric chemistry

In atmospheric chemistry the term NO_x is used to mean the total concentration of NO plus NO₂. During daylight NO and NO₂ are in equilibrium with the ratio NO/NO₂ determined by the intensity of sunshine (which converts NO₂ to NO) and the concentration of ozone (which reacts with NO to give back NO₂). NO and NO₂ are also central to the formation of tropospheric ozone. This definition excludes other oxides of nitrogen such as nitrous oxide (N₂O). NO_y (reactive odd nitrogen) is defined as the sum of NO_x plus the compounds produced from the oxidation of NO_x which include nitric acid.

Natural sources

Nitrous oxide is produced during thunderstorms due to the extreme heat of lightning,^[4] and is caused by the splitting of nitrogen molecules. This can result in the production of acid rain, if nitrous oxide forms compounds with the water molecules in precipitation, thus creating acid rain.

Industrial sources

The three primary sources of NO_x in combustion processes:

• thermal NO_x
• fuel NO_x
• prompt NO_x

Thermal NO_x formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel NO_x 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 NO_x. The contribution of prompt NO_x is normally considered negligible. A fourth source, called feed NO_x 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.

Thermal

Thermal NO_x refers to NO_x formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1600°C (2900°F), molecular nitrogen (N₂) and oxygen (O₂) in the combustion air disassociate into their atomic states and participate in a series of reactions.

The three principal reactions (the extended Zeldovich mechanism) producing thermal NO_x are:

N₂ + O → NO + N

N + O₂ → NO + O

N + OH → NO + H

All 3 reactions are reversible. Zeldovich was the first to suggest the importance of the first two reactions. The last reaction of atomic nitrogen with the hydroxyl radical, OH, was added by Lavoie, Heywood and Keck to the mechanism and makes a significiant contribution to the formation of thermal NO_x.

Fuel

The major source of NO_x production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NO_x during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N₂, or NO. Fuel NO_x can contribute as much as 50% of total emissions when combusting oil and as much as 80% when combusting coal.

Although the complete mechanism is not fully understood, there are two primary paths of formation. The first involves the oxidation of volatile nitrogen species during the initial stages of combustion. During the release and prior to the oxidation of the volatiles, nitrogen reacts to form several intermediaries which are then oxidized into NO. If the volatiles evolve into a reducing atmosphere, the nitrogen evolved can readily be made to form nitrogen gas, rather than NO_x. The second path involves the combustion of nitrogen contained in the char matrix during the combustion of the char portion of the fuels. This reaction occurs much more slowly than the volatile phase. Only around 20% of the char nitrogen is ultimately emitted as NO_x, since much of the NO_x that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.

Prompt

This third source is attributed to the reaction of atmospheric nitrogen, N₂, with radicals such as C, CH, and CH₂ fragments derived from fuel, where this cannot be explained by either the aforementioned thermal or fuel processes. Occurring in the earliest stage of combustion, this results in the formation of fixed species of nitrogen such as NH (nitrogen monohydride), HCN (hydrogen cyanide), H₂CN (dihydrogen cyanide) and CN- (cyano radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt NO_x is especially minimal and it is generally only of interest for the most exacting emission targets.

Health effects

NO_x 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.^[5]

NO_x 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. The American Lung Association estimates that nearly 50 percent of United States inhabitants live in counties that are not in ozone compliance.^[6]

NO_x (especially N₂O) destroys ozone layer.^[7] This layer absorbs ultraviolet light, which is potentially damaging to life on earth. ^[8]

NO_x 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.

Regulation and emission control technologies

The Kyoto Protocol, ratified by 54 nations in 1997, classifies N₂O as a greenhouse gas, and calls for substantial worldwide reductions in its emission.^[9]

As discussed above, atmospheric NO_x eventually forms nitric acid, which contributes to acid rain.^[10] NO_x emissions are regulated in the United States by the Environmental Protection Agency, and in the UK by the Department for Environment, Food and Rural Affairs.

Technologies such as flameless oxidation (FLOX) and staged combustion significantly reduce thermal NO_x in industrial processes. Bowin low NO_x technology is a hybrid of staged-premixed-radiant combustion technology with a major surface combustion preceded by a minor radiant combustion. In the Bowin burner, air and fuel gas are premixed at a ratio greater than or equal to the stoichiometric combustion requirement.^[11] Water Injection technology, wherby water is introduced into the combustion chamber, is also becoming an important means of NO_x reduction through increased efficiency in the overall combustion process. Alternatively, the water (e.g. 10 to 50%) is emulsified into the fuel oil prior to the injection and combustion. This emulsification can either be made in-line (unstabilized) just before the injection or as a drop-in fuel with chemical additives for long term emulsion stability (stabilized). Other technologies, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion NO_x.

The use of exhaust gas recirculation and catalytic converters in motor vehicle engines have significantly reduced emissions.

Biogenic sources

Agricultural fertilization and the use of nitrogen fixing plants also contribute to atmospheric NO_x, by promoting nitrogen fixation by microorganisms.^[12][13]

Derivatives

Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO−2), nitrate (NO−3), nitronium (NO+2), and nitrosonium (NO⁺). NO₂ is intermediate between nitrite and nitronium:

NO₂⁺ + e⁻ → NO₂

NO₂ + e⁻ → NO₂⁻

See also

• Nitrogen oxide sensor

References

External links