Nitrogen oxide

  • 5.1 Books
  • 5.2 Papers

Differentiation between nitrogen oxides, nitrous gases or nitrogen oxides

Nitrogen oxides, Nitrogen oxides or nitrogen oxides are collective terms for the gaseous oxides of nitrogen. They will be abbreviated as NOx, since there are many due to the oxidation of the nitrogen more nitrogen -oxygen compounds. Sometimes the abbreviation NOx is also used for the so-called nitrous fumes (see below).

The nitrogen oxides are formed from the elements without exception by endothermic reactions, that is, they are formed from the elements only under external compulsion ( energy intake). This requires on the other hand their technical usefulness as an oxidizing agent (for example, dinitrogen tetroxide in rocketry, or laughing gas for hot flames). With the exception of the nitrous oxide they behave with respect to water (eg, in the atmosphere) as an acid. Partly because of this acid formation ( on the mucous membranes ) they are irritating and toxic. So they went ( with the exception of the laughing gas ) at an early stage an environmental attention. Nitrous oxide has on the one hand medical and technical applications, on the other hand it is discharged accidentally into the atmosphere in technical and agricultural processes. There it acts as a greenhouse gas and ozone killers (see nitrous oxide ).

Nitric oxide occurs in the human body as a messenger and takes place in the treatment of angina pectoris, among other applications.

Dinitrogen trioxide is in condensed form (-21 ° C) and deep blue solid ( -102 ° C) stained pale blue. At temperatures above 0 ° C, the compound decomposes into nitrogen monoxide and nitrogen dioxide.

Also formal Trinitramid (N ( NO2) 3, or N4O6 ), which Nitrylazid ( O2N -N3 or N4O2 ), and the Nitrosylazid ( ON or N4O N3 ) can be assigned to the nitrogen oxides. The compounds are extremely unstable. Trinitramid and Nitrylazid have so far been produced and detected only in solution. Nitrosylazid exists below -50 ° C as a pale yellow solid. Further still exist only below -142 ° C resistant, nitrogen trioxide radical (NO3, white), which is also present in an isomeric form of peroxide, as well as the dimer Distickstoffhexoxid ( N2O6, O2N -OO- NO2), which is also the peroxides counts.

Nitrogen oxides

Nitrous gases is the common name for the mixture of nitric oxide ( NO) and nitrogen dioxide ( NO2). Nitrous gases, inter alia, by the reaction of nitric acid (HNO3) with the organic materials or metals. ( In the reaction of nitric acid with silver and copper occurs very much NOx). Another source of nitrogen oxides are gases which result from the combustion of fossil fuels, such as coal or oil.

The typical red-brown color of the nitrous gases is caused mainly by the nitrogen dioxide ( NO2). Nitrogen oxides have a characteristic pungent odor and may cause a delay of more than 24 hours ( latency ) after inhaling yet to pulmonary edema. In men, impotence can also occur as a late effect in more frequent inhalation.

Effects of nitrogen oxides

  • Irritation and damage to respiratory organs (especially nitrogen )
  • The origin of acid rain: formation of nitric acid (HNO3 ) by reaction of (2 NO2 H2O → HNO3 HNO2 ) or by uptake of N2O5 in aerosol particles and subsequent formation of NO3- in the liquid phase.
  • Smog formation
  • Ozone formation under the influence of UV radiation
  • Nitrogen oxides are effective climate and amplify global warming. In particular, nitrous oxide ( N2O) is a greenhouse gas with a global warming effectiveness in a time horizon of 100 years, 298 times as large as that of CO2.
  • Nitrous oxide contributes to an extent like no other fabric indirectly to the depletion of ozone in the stratosphere.

NOx in the furnace

Nitrogen oxides are usually divided according to their sources and their formation mechanism in three ways:

  • Thermal NOx
  • Fuel or fuel- NOx
  • Prompt NOx

Mentioned in this context, "NOx " break down in the furnace to about 95 % NO and 5% NO2. With the help of the reaction kinetics can be the change in the concentrations of NOx describe. Here are ultimately the concentrations of N2 and O as well as the temperature relevant factors:

The exponential term is the approach of the Arrhenius equation, CN2 and cO the concentrations at the start of the reaction.

Thermal NOx

The term " thermal" refers to the relatively high temperatures required to initiate the formation of thermal NOx reaction over N2. The nitrogen source of the thermal NOx is present in the combustion air, nitrogen, which also originates from the oxidation of the N2 necessary oxygen in the combustion air. Zeldovich describes the origin in two or three steps, the scheme is known as a simple or extended " Zeldovich mechanism ".

Initiation reaction is the conversion of atmospheric nitrogen with atomic oxygen, produced in the nitrogen radicals. Oxidize in the second reaction further:

The third step takes into account that the evolved during combustion hydroxyl radicals (OH ) can react with nitrogen at oxygen deficiency as well ( reaction in the flame zone ):

With the formation of thermal NOx in combustion temperatures of about 1000 ° C to be expected, the formation rate increases exponentially with temperature. Dominated by nitrogen containing fuels below the so-called " fuel NOx " or "fuel NOx ". Likewise, the oxygen and the residence time offered the reactants in the combustion zone have an influence on the NOx - formation rate. Studies on the formation of nitrogen oxides at the arc furnace show that in addition to technical combustion processes of fossil fuels such as oil or natural gas have also O2/N2-Plasmen good educational conditions for nitrogen oxides.

Fuel NOx

Source of NOx species are bound in the fuel shares in nitrogen, which will be implemented during combustion NOx. The amount of entrained nitrogen is strongly depending on fuel, accordingly, the shares resulting from the combustion of thermal and fuel NOx vary in the flue gas.

Some examples are ( in% ):

There are two types of nitrogen release in solid fuels. The homogeneous release describes the outgassing of the bound nitrogen in the fuel with the volatile constituents during the heterogeneous combustion of the residual coke describes.

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

Fuel NOx is formed at temperatures around 800 ° C mainly in the flame fronts of the boilers. Here, the entrained fuel goes through several steps, starting on hydrogen cyanide ( HCN) and hydrazine ( NHN ) to NO and N2 lead. N2 and NO can with hydrocarbon radicals ( CHn ) is a back reaction to HCN through ( " reburning " ) and this again to NO or convert to molecular nitrogen (N2). Thus in total, the amount increases to molecular nitrogen. This effect is in the so -called " fuel - staging ", a primary pollutant mitigation measure advantage.

Prompt NOx

Instead of conversion to N2, the reaction of the fuel radicals ( CHn ) can lead to the formation of NOx N2 again. This amount of generated NOx is referred to as the " prompt " NOx and is also known as " Fenimore mechanism".

The key factor here is the resulting hydrocarbon radicals which are present as intermediates in the combustion of carbon fossil fuels. Their formation mechanisms are extremely complex and not yet fully recognized and understood. Prompt NOx formed in a very rapid formation reactions in comparatively small amounts, and is compared to the thermal NOx hardly temperature-dependent, while the contribution increases with increasing temperature.

NOx reduction

At the power plants there are different ways of NOx reduction.

The primary measures relating to the firing process and prevent the emergence of:

  • Air staging
  • Fuel staging
  • Internal EGR
  • External exhaust gas recirculation
  • Primäradditivierung
  • Quenching ( injection of water for temperature reduction)

The secondary measures reduce the NOx in the exhaust gas ( flue gas denitrification ):

  • SCR process
  • SNCR

Also for the exhaust gases of motor vehicles come measures to reduce the use.

Main products of reduction measures shown are elemental nitrogen, as it occurs to about 78 vol. % In air, and water. As a side reactions can occur at low levels of nitrous oxide. In the catalytic secondary measures can also handle small amounts escape of ammonia ( NH3 slip ).

606078
de