What is the formula for nitric oxide

Nitrogen oxides or Nitrogen oxides is a collective name for the gaseous oxides of nitrogen. You will also get NOx Abbreviated because there are several nitrogen-oxygen compounds due to the many oxidation states of nitrogen. Sometimes the abbreviation NOx also used for the so-called nitrous gases (see below).

The nitrogen oxides are without exception endothermic compounds, i.e. they are only formed from the elements under external pressure (energy supply). On the other hand, this means that they can be used technically as an oxidizing agent (e.g. nitrous oxide in rocket technology or nitrous oxide for hot flames). With the exception of laughing gas, they behave as acid generators in relation to water (e.g. in the atmosphere). Because of this acid formation (on the mucous membranes), among other things, they are irritating and toxic. With the exception of laughing gas, they attracted environmental attention at an early stage. On the one hand, nitrous oxide has medical and technical uses, on the other hand it is unintentionally released into the atmosphere during technical and agricultural processes. There it acts as a greenhouse gas and ozone killer. More under laughing gas.

Nitric oxide occurs in the human body as a messenger substance and is used in the treatment of angina pectoris, among other things.

Dinitrogen trioxide: is deep blue in condensed form (-21 ° C) and pale blue in solid form (-102 ° C). At temperatures above 0 ° C, the compound breaks down into nitrogen monoxide and nitrogen dioxide. Because of this property, the boiling point cannot be determined.

Nitrous gases

Nitrous gases is the trivial name for the mixture of nitrogen monoxide, NO, and nitrogen dioxide, NO2. Nitrous gases are produced, among other things, by the reaction of nitric acid, ENT3, with organic substances or metals. (The reaction of nitric acid with silver and copper produces a lot of NOx). Another source of nitrogen oxides are the exhaust gases that are produced when burning fossil fuels such as coal or oil.

The typical red-brown vapors of the nitrous gases are mainly caused by nitrogen dioxide, NO2caused. Nitrous gases have a characteristic pungent odor and can lead to pulmonary edema after a delay of more than 24 hours after inhalation. In men, impotence can also occur as a long-term consequence when inhaling more frequently.

Effects of nitrogen oxides

  • Irritation and damage to the respiratory system (especially nitrogen dioxide)
  • Formation of acid rain: Formation of nitric acid (ENT3) by reaction of (2NO2 + H2O → ENT3 + ENT2) or by adding N2O5 in aerosol particles and subsequent formation of NO3- in the liquid phase.
  • Smog formation
  • Ozone formation under the influence of UV radiation

NOx in the furnace

As a rule, nitrogen oxides are divided into 3 types according to their sources and their mechanism of formation:

  • thermal NOx
  • Fuel or fuel-NOx
  • prompt NOx

The “NOx“About 95% NO and 5% NO settle in the furnace2 together. With the help of the reaction kinetics, the change in the concentrations of NOx describe. Here, ultimately, the concentrations of N2 and O as well as the temperature are decisive influencing factors:

The exponential term is the approach via the Arrhenius equation, cN2 and cO the concentrations at the beginning of the reaction.

Thermal NOx

The term “thermal” refers to the relatively high temperatures that initiate the thermal NO formation reactionx about N2 are needed. The nitrogen source of thermal NOx is the nitrogen present in the combustion air that is used to oxidize the N2 The oxygen required also comes from the combustion air. Zeldovich describes the creation in two or three steps, the scheme is known as a simple or extended "Zeldovich mechanism".

The start reaction is the conversion of atmospheric nitrogen with atomic oxygen, in which nitrogen radicals are formed. These oxidize further in the second reaction:


The third step takes into account that the hydroxide radicals (OH) generated during combustion can also react with nitrogen:


With the formation of thermal NOx is to be expected at combustion temperatures from about 1250 ° C, the formation rate increases exponentially with the temperature. Below, the so-called “fuel NO” dominates in the case of fuels containing nitrogenx"Or" fuel-NOx". The oxygen available and the residence time of the reactants in the combustion zone also have an influence on the NOx-Creation rate. Studies of nitrogen oxide formation in the electric arc furnace show that in addition to the technical combustion processes of fossil fuels such as crude oil or natural gas, O2/ N2-Plasmas have good nitrogen oxide formation conditions.

Fuel NOx

Source of this NOx-Type are the proportions of nitrogen bound in the fuel that are converted into NO during combustionx implemented. The amount of nitrogen carried along is heavily dependent on the fuel, and the proportions of thermal and fuel NO generated by the combustion also vary accordinglyx in the flue gas.

Some examples are (proportions in%):

fuel thermal NOx BS-NOx out
volatile constituents
from coke
Prompt NOx
Diesel fuel
Inside the engine
90-95 - - 5-10
gas 100 - - -
Heavy fuel oil 40-60 60-40 - -
Hard coal
Dry firing
10-30 50-70 20-30 -
Hard coal
Melt firing
40-60 30-40 10-20 -
Brown coal

A distinction is made between two types of nitrogen release with solid fuels. The homogeneous release describes the outgassing of the nitrogen bound in the fuel with the volatile components, while the heterogeneous describes the burn-up of the residual coke.

Essential source of fuel NOx are the volatile components of the fuel.

Fuel NOx arises from temperatures around 800 ° C mainly in the flame fronts of the furnace. The fuel carried through goes through several reaction steps, which are carried out via hydrocyanic acid (HCN) and hydrazine (NHn) to NO or N2 to lead. N2 and NO can be linked to hydrocarbon radicals (CHn) undergo a reverse reaction to HCN ("reburning") and in doing so again to NO or to molecular nitrogen (N2) implement. This increases the total amount of molecular nitrogen. This effect is used in what is known as “fuel grading”, a primary pollutant reduction measure.

Prompt NOx

Instead of converting to N2 the reaction of the fuel radicals (CHn) with N2 again to the formation of NOx to lead. This proportion of NOx is called "prompt" NOx and is also known as the "Fenimore Mechanism".

The main influencing factor are the resulting hydrocarbon radicals, which are present as intermediate products of the combustion of carbonaceous fossil fuels. Their educational mechanisms are extremely complex and have not yet been fully recorded and understood. Prompt NOx arises in very rapid formation reactions in comparatively small amounts and is in comparison to thermal NOx hardly temperature-dependent, although the proportion increases with increasing temperature.


There are various options for NO in the power plantsx-Reduction.

The primary measures concern the combustion process and prevent the occurrence,

  • Air staging
  • Fuel grading
  • internal exhaust gas recirculation
  • external exhaust gas recirculation
  • Primary additives
  • Quenching (injection of water to reduce temperature)

while the secondary measures the NOx in the exhaust gas (flue gas denitrification).

  • SCR process
  • SNCR procedure

Mitigation measures are also used for vehicle exhaust gases.

The common feature of the reduction measures is that they do not mean that the problem is completely eliminated. This would be the case if nitrogen (as a natural component of the atmosphere) were to appear as the product of the detoxification reaction. Instead, the available methods often lead to nitrous oxide. See there for details.



  • Helmut Effenberger: Steam generation. Springer-Verlag, ISBN 3-540-64175-0
  • J. Warnatz, U. Maas, R. W. Dibble: combustion. 3rd edition, Springer-Verlag, ISBN 3-540-42128-9
  • G.P. Merker, G. Stiesch: Technical combustion Engine combustion. B.G. Teubner Verlag Stuttgart,ISBN 3-519-06381-6


  • Erich Fitzer, Dieter Siegel: Nitrogen oxide emissions from industrial combustion systems depending on the operating conditions. Chemie Ingenieur Technik 47 (13), p. 571 (1975), ISSN 0009-286X
  • Rainer Römer, Wolfgang Leckel, Alfred Stöckel, Gerd Hemmer: Influencing the formation of nitrogen oxide from fuel-bound nitrogen by means of combustion measures. Chemie Ingenieur Technik 53 (2), pp. 128-129 (1981), ISSN 0009-286X
  • Heinrich Wilhelm Gudenau, Klaus E. Herforth: Formation of nitrogen oxide when solid fuels are converted into various gas media. Chemie Ingenieur Technik 53 (9), pp. 742-743 (1981), ISSN 0009-286X
  • Manfred Schrod, Joachim Semel, Rudolf Steiner: Process for reducing NOx emissions in flue gases. Chemie Ingenieur Technik 57 (9), pp. 717-727 (1985), ISSN 0009-286X
  • Hans-Georg Schäfer, Fred N. Riedel: About the formation of nitrogen oxides in large combustion plants, their influence on the environment, their reduction and their removal from the exhaust gases of the power plants. Chemiker-Zeitung 113 (2), pp. 65-72 (1989), ISSN 0009-2894
  • Ulrich Förstermann: Nitric oxide (NO): environmental toxin and endogenous messenger substance. Biology in our time 24 (2), pp. 62-69 (1994), ISSN 0045-205X

See also:

Categories: Fabric Group | gas