NOx is a generic term for mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide). They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures. In areas of high motor vehicle traffic, such as in large cities, the amount of nitrogen oxides emitted into the atmosphere as air pollution can be significant. NOx gases are formed whenever combustion occurs in the presence of nitrogen – as in an air-breathing engine; they also are produced naturally by lightning. In atmospheric chemistry, the term means the total concentration of NO and NO2. NOx gases react to form smog and acid rain as well as being central to the formation of tropospheric ozone.
Formation and reactions
Oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash.
In atmospheric chemistry, the term NOx means the total concentration of NO and NO2. During daylight, these concentrations are in equilibrium; the ratio NO/NO2 is determined by the intensity of sunshine (which converts NO2 to NO) and the concentration of ozone (which reacts with NO to again form NO2).
|NO concentration in air
|Time required for half NO
to be oxidized to NO2 (min)
When NOx 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 particularly susceptible to adverse effects of smog such as damage to lung tissue and reduction in lung function.
Formation of nitric acid and acid rain
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:
- 2 NO2 + H2O → HNO2 + HNO3
Nitrous acid then decomposes as follows:
- 3 HNO2 → HNO3 + 2 NO + H2O
- 4 NO + 3 O2 + 2 H2O → 4 HNO3
This nitric acid may end up in the soil, where it makes nitrate, where it is of use to growing plants.
Nitric oxide is produced during thunderstorms due to the extreme heat of lightning, and is caused by the splitting of nitrogen molecules. This can result in the production of acid rain, if nitrous oxide forms compounds with the water molecules in precipitation.
The three primary sources of NOx 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 is also a minor contributor.
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 (the extended Zeldovich mechanism) producing thermal NOx are:
- 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 the hydroxyl radical, •HO, was added by Lavoie, Heywood and Keck to the mechanism and makes a significant contribution to the formation of thermal 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 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 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.
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 NOx is especially minimal and it is generally only of interest for the most exacting emission targets.
NO from N2O
At high pressures NO formation via N2O becomes important:
- N2 + O + M → N2O + M
- N2O + O → 2 NO (activation energy 97 kJ/mol)
- N2O + O → N2 + O2
Competing Reactions :
- N2O + O → NO + N (thermal NO)
- N2O + O + M → N2O2 + M
NOx reacts with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases, such as emphysema or bronchitis, or may also aggravate existing heart disease.
NOx reacts with volatile organic compounds in the presence of sunlight to form ozone. Ozone can cause adverse effects such as damage to lung tissue and reduction in lung function mostly in susceptible populations (children, elderly, asthmatics). Ozone can be transported by wind currents and cause health impacts far from the original sources. The American Lung Association estimates that nearly 50 percent of United States inhabitants live in counties that are not in ozone compliance.
NOx destroys ozone in the stratosphere. Ozone in the stratosphere absorbs ultraviolet light, which is potentially damaging to life on earth. NOx from combustion sources does not reach the stratosphere; instead, NOx is formed in the stratosphere from photolysis of nitrous oxide.
NOx also readily reacts with common organic chemicals, and even ozone, to form a wide variety of toxic products: nitroarenes, nitrosamines and also the nitrate radical some of which may cause biological mutations. Recently another pathway, via NOx, to ozone has been found that predominantly occurs in coastal areas via formation of nitryl chloride when NOx comes into contact with salt mist.
Regulation and emission control technologies
As discussed above, atmospheric NOx eventually forms nitric acid, which contributes to acid rain. NOx emissions are regulated in the United States by the 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 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, whereby 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 (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). Inline emulsified fuel/water mixtures show NOx reductions between 4 and 83%. Other technologies, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion NOx.
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