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The term 'nitrogen oxide' is used to refer to any of these oxygen compounds of nitrogen, or to a mixture of them:

★ Nitric oxide (NO), nitrogen(II) oxide

★ Nitrogen dioxide (NO₂), nitrogen(IV) oxide

★ Nitrous oxide (N₂O)

★ 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, the proportions depending on the specific reaction and conditions. This is one reason why domestic production of N₂O is undesirable; the other two stable oxides — which are extremely toxic — are liable to be produced.

Contents

NOx

Definition of NOx and NOy in atmospheric chemistry

Industrial sources of NOx

Thermal NOx

Fuel NOx

Prompt NOx

Regulation and emission control technologies

Biogenic sources

References

See also

External links

NO_x

'NO_x' is a generic term for mono-nitrogen oxides (NO and NO₂). These oxides are produced during combustion, and are of interest as air pollution. They are believed to aggravate asthmatic conditions, and are involved in tropospheric production of ozone.^[1] They eventually form nitric acid when dissolved in atmospheric moisture, forming a component of acid rain.
If nitric oxide (NO) is in the presence of excess oxygen (O₂) then it will be converted to nitrogen dioxide (NO₂) depending on the concentration of air as shown below:

NO concentration in air(ppm)	Time required for half NOto be oxidized to NO2 (min)
10,000	0.175
20,000	0.35
1,000	3.5
100	35
10	350
1	3500

^[2]
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:
HNO₂ → HNO₃ + 2NO + H₂
Nitrous acid → nitric acid + nitrogen oxide + water
Where nitric oxide will oxidize to form nitrogen dioxide again reacting with water ultimately forming nitric acid
4NO + 3O₂ + 2H₂O → 4HNO₃, nitric oxide + oxygen + water → nitric acid ^[3]
In an internal combustion engine, a mixture of air and fuel is burned. When the mixture is tuned so as to consume every molecule of reactant (in this case fuel and oxygen) it is said to be "running at stoichiometry". When this burns, combustion temperatures reach a high enough level to drive endothermic reactions between nitrogen and oxygen in the flame, yielding various oxides of nitrogen, the results of which can be seen over major cities such as Los Angeles, California in the summer in the form of brown clouds of smog.
Not to be confused with the term NOS which is used to refer to nitrous oxide in the context of its use as a booster for internal combustion engines.

Definition of NO_x and NO_y 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 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. 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, peroxyacetyl nitrate. In this context nitrous oxide and ammonia are not considered as reactive nitrogen compounds.

Industrial sources of NO_x

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 NO_x

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 producing thermal NO_x 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 NO_x.

Fuel NO_x

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 NO_x

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.

Regulation and emission control technologies

The United States Environmental Protection Agency (EPA) regulates and enforces NO_x 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 NO_x 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 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 NOx. Of particular importance is the introduction of catalytic converters which have significantly reduced emissions from motor vehicles.

Biogenic sources

Agricultural fertilization and the use of nitrogen fixing plants have also contributed to increases in atmospheric NO_''x'' through promoting nitrogen fixation by microorganisms.^[5][6]

References

1.
The atmospheric budget of oxidized nitrogen and its role in ozone formation and deposition, D. Fowler, ''et al.'', , , New Phytologist, 1998
2. Nox
3. Process Design Technical Note
4. Bob Joynt & Stephen Wu, ''Nitrogen oxides emissions standards for domestic gas appliances background study'' Combustion Engineering Consultant; February 2000
5.
Nitrogen cycles: past, present, and future, J.N. Galloway, ''et al.'', , , Biogeochemistry, 2004
6.
A global inventory of nitric oxide emissions from soils, E.A. Davidson & W. Kingerlee, , , Nutrient Cycling in Agroecosystems, 1997


★ Fire and ICE: Revving Up for H₂, Adrian Cho, , , Science, 2004

★ Seinfeld, John H.; Pandis, Spyros N (1998). Atmospheric Chemistry and Physics - From Air Pollution to Climate Change. John Wiley and Sons, Inc. ISBN 0-471-17816-0

★ NO_x Formation Literature Review and Research Project, Jack Ponton; Department of Chemical Engineering; School of Engineering and Electronics; Edinburgh, UK

★ FACT SHEET #2; NOx Formation and Control in Cement Rotary Kilns (html) (MS Word doc), Southern Illinois University at Edwardsville

★ Prompt NO_x definition, boiler glossary, McKenzie Corporation

See also

★ Criteria air contaminants

External links

★ Australia's Nitrogen oxides emissions standards for domestic gas appliances - background study

★ Australia's National Pollutant Inventory - Oxides of nitrogen Fact Sheet

★ in-line emulsification of water with oil for NOx reduction

★ NASA report on non combustion NOx in atmosphere