Everything about Nitrogen Oxide totally explained
The term
nitrogen oxide typically refers to any
binary compound of
oxygen and
nitrogen, or to a mixture of such compounds:
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 N
2O is undesirable; the other two stable oxides — which are extremely toxic — are liable to be produced.
Image:Nitric-oxide-3D-vdW.png|Nitric oxide, NO
Image:Nitrogen-dioxide-3D-vdW.png|Nitrogen dioxide, NO2
Image:Nitrous-oxide-3D-vdW.png|Nitrous oxide, N2O
Image:Dinitrogen-trioxide-3D-vdW.png|Dinitrogen trioxide, N2O3
Image:Dinitrogen-tetroxide-3D-vdW.png|Dinitrogen tetroxide, N2O4
Image:Dinitrogen-pentoxide-3D-vdW.png|Dinitrogen pentoxide, N2O5
NOx
NO
x is a generic term for mono-nitrogen oxides (NO and NO
2). These oxides are produced during
combustion, especially combustion at high temperatures.
At ambient temperatures, the oxygen and
nitrogen gases in air won't 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 (O
2),
nitric oxide (NO) will be converted to
nitrogen dioxide (NO
2), with the time required dependent on the concentration in air as shown below:
| NO concentration in air(ppm)
|
Time required for half NOto be oxidized to NO2
(min)
|
| 20,000 |
0.175 |
| 10,000 |
0.35 |
| 1,000 |
3.5 |
| 100 |
35 |
| 10 |
350 |
| 1 |
3500 |
When NO
x 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.
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
2 + H
2O → HNO
2 + HNO
3
(nitrogen dioxide + water → nitrous acid + nitric acid).
Nitrous acid then decomposes as follows:
» 3HNO
2 → HNO
3 + 2NO + H
2O
(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
2 + 2H
2O → 4HNO
3 (nitric oxide + oxygen + water → nitric acid).
Mono-nitrogen oxides are also involved in
tropospheric production of
ozone.
NO
x shouldn't be confused with NOS, a term used to refer to
nitrous oxide (N
2O) 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 NO
x is used to mean the total concentration of
NO plus
NO2. During daylight NO and NO
2 are in equilibrium with the ratio NO/NO
2 determined by the intensity of sunshine (which converts NO
2 to NO) and the conventration of
ozone (which reacts with NO to give back NO
2). NO and NO
2 are also central to the formation of
tropospheric ozone. This definition excludes other oxides of nitrogen such as nitrous oxide (N
2O). 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 and
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 NO
x 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's 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 Lavoie, 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 isn't 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.
It is possible revert NOx 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 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 can't 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's 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. 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 (for example 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.
Further Information
Get more info on 'Nitrogen Oxide'.
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