NOx

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 NO_x 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).

In the presence of excess oxygen (O₂), nitric oxide (NO) reacts with the oxygen to form nitrogen dioxide (NO₂). The time required depends on the concentration in air as shown below:

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 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. This chemical reaction occurs when nitrogen dioxide reacts with water:

2 NO₂ + H₂O → HNO₂ + HNO₃

where nitric oxide will oxidize to form nitrogen dioxide that again reacts with water, ultimately forming nitric acid:

4 NO + 3 O₂ + 2 H₂O → 4 HNO₃

Combining these three equations gives a single equation that describes the aerobic conversion of nitrogen dioxide to nitric acid:

4 NO₂ + 2 H₂O + O₂ → 4 HNO₃

Mono-nitrogen oxides are also involved in tropospheric production of ozone.

This nitric acid may end up in the soil, where it makes nitrate, where it is of use to growing plants.

Environmental effects

NO_x 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.

NO_x reacts with volatile organic compounds in the presence of sunlight to form and to destroy 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. In South East England, ground level ozone pollution tends to be highest in the countryside and in suburbs, while in central London and on major roads NO emissions are able to "mop up" ozone to form NO₂ and oxygen.

NO_x 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 NO_x, to ozone has been found that predominantly occurs in coastal areas via formation of nitryl chloride when NO_x comes into contact with salt mist.

NO_x emissions also cause global cooling through the formation of ^•OH radicals that destroy methane molecules, countering the effect of greenhouse gases. The effect can be significant. For instance, according to the OECD "the large NO_x emissions from ship traffic lead to significant increases in hydroxyl (OH), which is the major oxidant in the lower atmosphere. Since reaction with OH is a major way of removing methane from the atmosphere, ship emissions decrease methane concentrations. (Reductions in methane lifetimes due to shipping-based NO_x emissions vary between 1.5% and 5% in different calculations)." "In summary, most studies so far indicate that ship emissions actually lead to a net global cooling. However, it should be stressed that the uncertainties with this conclusion are large, in particular for indirect effects, and global temperature is only a first measure of the extent of climate change in any event."

Biodiesel and NOx

Biodiesel and its blends in general are known to produce lower carbon monoxide, soot, hydrocarbon emissions, and higher NO_x emissions compared with regular diesel. Because of the lower heating value of biodiesel, more biodiesel should be burned to produce the equivalent energy of ULSD. Also, due to the presence of high oxygen content in biodiesel fuels, generally biodiesel fuels emit more NO_x than regular diesel for the same heat generation. The reduction of NO_x emissions is one of the most important technical challenges facing biodiesel, especially in light of the increasingly stringent exhaust emission regulations on diesel engines. NO_x formation during biodiesel combustion is associated with a number of factors such as the property of biodiesel and combustion conditions. Combustion temperature influences thermal NO_x emissions. Therefore, low-temperature may help thermal NO_x reduce during combustion, leading to low-temperature combustion or LTC technology.

Regulation and emission control technologies

Technologies such as flameless oxidation (FLOX) and staged combustion significantly reduce thermal NO_x in industrial processes. Bowin low NO_x 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 NO_x reduction through increased efficiency in the overall combustion process. Alternatively, the water (e.g. 10 to 50%) is emulsified into the fuel oil before 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 NO_x reductions between 4 and 83%.

Other technologies, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion NO_x by reacting the exhaust with urea or ammonia to produce nitrogen and water. SCR is now being used in ships, diesel trucks and in some diesel cars. The use of exhaust gas recirculation and catalytic converters in motor vehicle engines have significantly reduced vehicular emissions. NO_x was the main focus of the Volkswagen emissions violations.