Nitrogen oxide

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The term nitrogen oxide typically refers to any binary compound of oxygen and nitrogen, or to a mixture of such compounds:

(Note that the last three are unstable.)

Chemical reactions that produce nitrogen oxides often produce several, the proportions depending on the specific reaction and conditions. This is one reason why secondary production of N2O is undesirable; the other two stable oxides — which are extremely toxic — are liable to be produced.


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.

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]

NO concentration in air


Time required for half NO

to be oxidized to NO2 (min)

20,000 0.175
10,000 0.35
1,000 3.5
100 35
10 350
1 3500

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.[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:
2NO2 + H2O → HNO2 + HNO3 (nitrogen dioxide + water → nitrous acid + nitric acid).
Nitrous acid then decomposes as follows:
3HNO2 → HNO3 + 2NO + H2O (nitrous acid → nitric acid + nitric oxide + water),
where nitric oxide will oxidize to form nitrogen dioxide that again reacts with water, ultimately forming nitric acid:
4NO + 3O2 + 2H2O → 4HNO3 (nitric oxide + oxygen + water → nitric acid).

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

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.

Definition of NOx and NOy in atmospheric chemistry

In atmospheric chemistry the term NOx 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 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. NOy (reactive odd nitrogen) is defined as the sum of NOx plus the compounds produced from the oxidation of NOx which include nitric acid, peroxyacetyl nitrate. In this context nitrous oxide and ammonia are not considered as reactive nitrogen compounds.

Industrial sources of NOx

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.

Thermal NOx

Thermal NOx refers to NOx 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 producing thermal NOx are:

(Extended Zeldovich Mechanism)

  • 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 Hydroxyl radical, OH, was added by Lavovie, Heywood and Keck to the mechanism and makes a significiant contribution to the formation of thermal NOxx.

Fuel NOx

The major source of NOx production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO. Fuel NOx 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 NOx. 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 NOx, since much of the NOx that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.

Prompt NOx

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 NOx is especially minimal and it is generally only of interest for the most exacting emission targets.

Regulation and emission control technologies

The United States Environmental Protection Agency (EPA) regulates and enforces NOx emission limits in the U.S. in accordance to legislation passed by the United States Congress. The Kyoto Protocol, ratified by 54 nations in 1997, calls for a substantial world wide reduction of greenhouse gases including nitrous oxide.

Technologies such as flameless oxidation (FLOX®) and staged combustion significantly reduce thermal NOx in industrial processes. Bowin low NOx 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.[4] Water Injection technology, wherby water is introduced into the combustion chamber, is also becoming an important means of NOx 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 NOx.

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 NOx, by promoting nitrogen fixation by microorganisms.[5][6]


  1. "NOx Removal". Branch Environmental Corp. Retrieved 2007-12-26.
  2. "Health and Environmental Impacts of NOx". United States Environmental Protection Agency. Retrieved 2007-12-26.
  3. D. Fowler; et al. (1998). "The atmospheric budget of oxidized nitrogen and its role in ozone formation and deposition". New Phytologist. 139: 11–23.
  4. Bob Joynt & Stephen Wu, Nitrogen oxides emissions standards for domestic gas appliances background study Combustion Engineering Consultant; February 2000
  5. J.N. Galloway; et al. (2004). "Nitrogen cycles: past, present, and future". Biogeochemistry. 70 (2): 153–226. doi:10.1007/s10533-004-0370-0. Unknown parameter |month= ignored (help)
  6. E.A. Davidson & W. Kingerlee (1997). "A global inventory of nitric oxide emissions from soils". Nutrient Cycling in Agroecosystems. 48: 37–50.

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