Nitrogen oxide
286685
224146951
2008-07-07T15:08:37Z
J.delanoy
2372780
Reverted edits by [[Special:Contributions/88.105.167.11|88.105.167.11]] to last version by NJGW (using [[WP:HG|Huggle]])
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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:
* [[Nitric oxide]] (NO), nitrogen(II) oxide
* [[Nitrogen dioxide]] (NO<sub>2</sub>), nitrogen(IV) oxide
* [[Nitrous oxide]] (N<sub>2</sub>O), nitrogen (I) oxide
* [[Dinitrogen trioxide]] (N<sub>2</sub>O<sub>3</sub>), nitrogen(II, IV) oxide
* [[Dinitrogen tetroxide]] (N<sub>2</sub>O<sub>4</sub>), nitrogen(IV) oxide
* [[Dinitrogen pentoxide]] (N<sub>2</sub>O<sub>5</sub>), nitrogen(V) oxide
(Note that the last three are unstable.)
[[Chemical reaction]]s that produce nitrogen oxides often produce several different compounds, the proportions of which depend on the specific reaction and conditions. For this reason, secondary{{huh}} production of N<sub>2</sub>O is undesirable, as NO and NO<sub>2</sub> — which are extremely toxic — are liable to be produced as well.
<gallery>
Image:Nitric-oxide-3D-vdW.png|<center>'''[[Nitric oxide]]''', NO</center>
Image:Nitrogen-dioxide-3D-vdW.png|<center>'''[[Nitrogen dioxide]]''', NO<sub>2</sub></center>
Image:Nitrous-oxide-3D-vdW.png|<center>'''[[Nitrous oxide]]''', N<sub>2</sub>O</center>
Image:Dinitrogen-trioxide-3D-vdW.png|<center>'''[[Dinitrogen trioxide]]''', N<sub>2</sub>O<sub>3</sub></center>
Image:Dinitrogen-tetroxide-3D-vdW.png|<center>'''[[Dinitrogen tetroxide]]''', N<sub>2</sub>O<sub>4</sub></center>
Image:Dinitrogen-pentoxide-3D-vdW.png|<center>'''[[Dinitrogen pentoxide]]''', N<sub>2</sub>O<sub>5</sub></center>
</gallery>
==NO<sub>''x''</sub>==
{{dablink|This section refers to the chemical term for nitrogen oxides produced during combustion. For other definitions see [[Nox]]}}
'''NO<sub>''x''</sub>''' is a generic term for mono-nitrogen oxides (NO and NO<sub>2</sub>). 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 [[oxide]]s 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 (O<sub>2</sub>), [[nitric oxide]] (NO) will be converted to [[nitrogen dioxide]] (NO<sub>2</sub>), with the time required dependent on the concentration in air as shown below:<ref> {{cite web|url=http://www.branchenv.com/nox/nox_info.asp |title=NOx Removal |accessdate=2007-12-26 |publisher=Branch Environmental Corp }}</ref>
{| class=”wikitable”
|-
! NO concentration in air
(ppm)
! Time required for half NO
to be oxidized to NO<sub>2</sub>
(min)
|-
|20,000
|0.175
|-
|10,000
|0.35
|-
| 1,000
| 3.5
|-
|100
|35
|-
|10
|350
|-
|1
|3500
|}
When NO<sub>''x''</sub> and [[volatile organic compound]]s (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.<ref>{{cite web|url=http://www.epa.gov/airprogm/oar/urbanair/nox/hlth.html |title=Health and Environmental Impacts of NOx |accessdate=2007-12-26 |publisher=[[United States Environmental Protection Agency]] }}</ref>
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:
:2NO<sub>2</sub> + H<sub>2</sub>O → HNO<sub>2</sub> + HNO<sub>3</sub>
(nitrogen dioxide + water → nitrous acid + nitric acid).
Nitrous acid then decomposes as follows:
:3HNO<sub>2</sub> → HNO<sub>3</sub> + 2NO + H<sub>2</sub>O
(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 + 3O<sub>2</sub> + 2H<sub>2</sub>O → 4HNO<sub>3</sub> (nitric oxide + oxygen + water → nitric acid).
Mono-nitrogen oxides are also involved in [[tropospheric]] production of [[ozone]].<ref>
{{cite journal
|author=D. Fowler, ''et al.''
|year=1998
|title=The atmospheric budget of oxidized nitrogen and its role in ozone formation and deposition
|journal=New Phytologist
|volume=139
|pages=11–23
|doi=10.1046/j.1469-8137.1998.00167.x
}}</ref>
NO<sub>''x''</sub> should not be confused with NOS, a term used to refer to [[Nitrous|nitrous oxide]] (N<sub>2</sub>O) in the context of its use as a power booster for internal combustion engines.
==Definition of NO<sub>''x''</sub> and NO<sub>''y''</sub> in atmospheric chemistry==
In [[atmospheric chemistry]] the term NO<sub>''x''</sub> is used to mean the total concentration of [[Nitric oxide|NO]] plus [[Nitrogen dioxide|NO<sub>2</sub>]]. During daylight NO and NO<sub>2</sub> are in equilibrium with the ratio NO/NO<sub>2</sub> determined by the intensity of sunshine (which converts NO<sub>2</sub> to NO) and the concentration of [[ozone]] (which reacts with NO to give back NO<sub>2</sub>). NO and NO<sub>2</sub> are also central to the formation of [[tropospheric ozone]]. This definition excludes other oxides of nitrogen such as nitrous oxide (N<sub>2</sub>O). NO<sub>''y''</sub> (reactive odd nitrogen) is defined as the sum of NO<sub>''x''</sub> plus the compounds produced from the oxidation of NO<sub>''x''</sub> which include [[nitric acid]] and [[peroxyacetyl nitrate]]. In this context nitrous oxide and [[ammonia]] are not considered as reactive nitrogen compounds.
== Industrial sources of NO<sub>''x''</sub> ==
The three primary sources of NO<sub>''x''</sub> in [[combustion]] processes:
* thermal NO<sub>''x''</sub>
* fuel NO<sub>''x''</sub>
* prompt NO<sub>''x''</sub>
Thermal NO<sub>''x''</sub> formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel NO<sub>''x''</sub> 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<sub>''x''</sub>. The contribution of prompt NO<sub>''x''</sub> is normally considered negligible. A fourth source, called ''feed NO<sub>''x''</sub>'' 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<sub>''x''</sub> ===
Thermal NO<sub>''x''</sub> refers to NO<sub>''x''</sub> 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 (N<sub>2</sub>) and oxygen (O<sub>2</sub>) in the combustion air disassociate into their atomic states and participate in a series of reactions.
The three principal reactions producing thermal NO<sub>''x''</sub> are:
(Extended [[Zeldovich]] Mechanism)
*'''N<sub>2</sub> + O → NO + N'''
*'''N + O<sub>2</sub> → 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 Lavoie, Heywood and Keck to the mechanism and makes a significiant contribution to the formation of thermal NO<sub>''x''</sub>.
===Fuel NO<sub>''x''</sub>===
The major source of NO<sub>''x''</sub> production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NO<sub>''x''</sub> during combustion. During combustion, the nitrogen bound in the fuel is released as a [[Radical (chemistry)|free radical]] and ultimately forms free N<sub>2</sub>, or NO. Fuel NO<sub>''x''</sub> 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<sub>''x''</sub>. The second path involves the combustion of nitrogen contained in the char matrix during the combustion of the [[Charring|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<sub>''x''</sub>, since much of the NO<sub>''x''</sub> that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.
It is possible revert NO<sub>''x''</sub> emitted from a diesel engine back into Nitrogen and Water by using an SCR [[Selective Catalytic Reduction]] Unit. Ths requires the addition of a urea based liquid re-agent namely [[AdBlue]] to initialise the chemical reaction.
===Prompt NO<sub>''x''</sub>===
This third source is attributed to the reaction of atmospheric nitrogen, N<sub>2</sub>, with radicals such as C, CH, and CH<sub>2</sub> 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]]), H<sub>2</sub>CN ([[dihydrogen cyanide]]) and CN- ([[cyano]] radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt NO<sub>''x''</sub> is especially minimal and it is generally only of interest for the most exacting emission targets.
=== Regulation and emission control technologies ===
The [[Kyoto Protocol]], ratified by 54 nations in 1997, classifies N<sub>2</sub>O as a [[greenhouse gas]], and calls for substantial worldwide reductions in its emission.<ref>{{cite book | last = Gerrard | first = Michael B. | authorlink = | coauthors = | title = Global Climate Change and U.S. Law | publisher = American Bar Association | date = 2007-09-25 | location = | pages = 38 | url = | doi = | id = | isbn = 1590318161 }}</ref>
As discussed above, atmospheric NO<sub>''x''</sub> eventually forms nitric acid, which contributes to acid rain.<ref> {{cite web|url=http://www.bayjournal.com/article.cfm?article=2179 |title=NOx in the Air: Multiple Effects |accessdate=2008-06-04 |last=Blankenship |first=Karl |date=1997-10|publisher=Chesapeake Bay Journal }}</ref> NO<sub>''x''</sub> emissions are regulated in the United States by the [[United States Environmental Protection Agency|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<sub>''x''</sub> in industrial processes. [[LO-NOx burner|Bowin low NO<sub>''x''</sub> 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.<ref>Bob Joynt & Stephen Wu, ''Nitrogen oxides emissions standards for domestic gas appliances background study'' Combustion Engineering Consultant; February 2000</ref> [[Water injection (engines)|Water Injection]] technology, wherby water is introduced into the combustion chamber, is also becoming an important means of NO<sub>''x''</sub> 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<sub>''x''</sub>.
The use of [[Exhaust gas recirculation]] and [[catalytic converter]]s in motor vehicle engines have significantly [[automobile emissions control|reduced emissions]].
== Biogenic sources ==
[[Agricultural]] [[fertilization (soil)|fertilization]] and the use of [[nitrogen fixing]] [[plant]]s also contribute to atmospheric NO<sub>''x''</sub>, by promoting [[nitrogen fixation]] by microorganisms.<ref>
{{cite journal
|author=J.N. Galloway, ''et al.''
|month=Sep
|year=2004
|title=Nitrogen cycles: past, present, and future
|journal=Biogeochemistry
|volume=70
|issue=2
|pages=153–226
|doi=10.1007/s10533-004-0370-0
}}</ref><ref>
{{cite journal
|author=E.A. Davidson & W. Kingerlee
|year=1997
|title=A global inventory of nitric oxide emissions from soils
|journal=Nutrient Cycling in Agroecosystems
|volume=48
|pages=37–50
|doi=10.1023/A:1009738715891
}}</ref>
==References==
{{reflist}}
==External Links==
[http://www.epa.gov/air/urbanair/nox/ How Nitrogen Oxides Affect the Way We Live and Breathe :: US EPA Information]
[[Category:Oxides]]
[[Category:Pollutants]]
[[Category:Smog]]
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