Nitrogen oxide

The term nitrogen oxide is a general term and can be used to refer to any of these oxides (oxygen compounds) of nitrogen, or to a mixture of them:

• Nitric oxide (NO), nitrogen(II) oxide
• Nitrogen dioxide (NO₂)
• Dinitrogen monoxide (N₂O), nitrous oxide
• Dinitrogen trioxide (N₂O₃)
• Dinitrogen tetroxide (N₂O₄)
• Dinitrogen pentoxide (N₂O₅)

A mixture is often formed in chemical reactions that produce nitrogen oxides, with the proportions depending on the specific reaction and the conditions it is performed in. This is one reason why home production of N₂O is undesirable; the other two stable oxides - which are extremely toxic - are liable to be produced.

Note that the last three listed above are unstable.

See the articles for these oxides for details on their properties, as well as the section NO_x below.

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

NO_x

This section refers to the chemical term for nitrogen oxide produced during combustion. For other definitions see Nox

NO_x is a generic term for the various nitrogen oxides produced during combustion. They are believed to aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, which is also an irritant and eventually form nitric acid when dissolved in water. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems.

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 burn some of the nitrogen in the air, 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 confsed 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 NO_{2). 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 and other compounds. In this context nitrous oxide and ammonia are not considered as reactive nitrogen compounds.}

Sources of NOx

Generally, three primary sources of NO_x formation in combustion processes are documented:

• 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 three 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 which has a significiant contribution to the formation of thermal NOx. --Tanin Su 19:35, 5 November 2006 (UTC)Tanin Su

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 imto 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 (chloracetophenone or CN gas) 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.

Emission control technologies

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. 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. 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.

Regulation

The USA Environmental Protection Agency (EPA) regulates and enforces NO_x emission limits in the U.S. in accordance to legislation passed by Congress.

The Kyoto Treaty, ratified by 54 nations in 1997, calls for a substantial world wide reduction of greenhouse gases including Nitrous Oxide.

References

• Adrian Cho (2004). "Fire and ICE: Revving Up for H₂". Science. 305 (5686): 964–965.
• 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

External links

• Australia's Nitrogen oxides emissions standards for domestic gas appliances - background study
• Australia's National Pollutant Inventory - Oxides of nitrogen Fact Sheet