Nitrogen oxide

From Freepedia

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

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 3 listed above are unstable.

See the articles for these oxides for details on their properties, as well as NOx.

Contents

NOx

NOx 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". With this burns, combustion temperatures reach a high enough level to actually 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, CA in the summer in the form of brown clouds of smog.

The term Nox is also sometimes used to refer to Nitrous Oxide in the context of its use as a booster for internal combustion engines.

Definition of NOx in atmospheric science

In atmospheric science 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.

Sources of NOx

Three primary sources of NOx formation in combustion processes are documented: prompt NO, fuel NO and thermal NO. Thermal NOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas.

Thermal NOx

Thermal NOx refers to NOx formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primary function of temperature and the residence time of nitrogen at temperature. At high temperatires, usually above 2200F, 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:

  • N2 + O → NO + N
  • N+ O2 → NO + O
  • N + OH → NO + H

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 a s 50% of total emissions when combusting oil and as much as 80% when combusting coal.

Although the complete mechanism is 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. The second path involves the release of nitrogen radicals during the combustion of the char portion of the fuels. This reaction occurs much more slowly than the volatile phase.

Emission control technologies

Technologies such as flameless oxidation (FLOX®) and staged combustion significantly reduce thermal NOx in industrial processes. 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 Environmental Protection Agency (EPA) regulates and enforces NOx 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.
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