Nitrogen oxides or nitrogen oxides is a collective term for numerous gaseous oxides of nitrogen . Nitric oxide (NO) and nitrogen dioxide (NO 2 ) are summarized as NO x . The species N 2 O 3 and N 2 O 4 also occur in higher concentration NO x mixtures . These mixtures are known as nitrous gases , especially in the field of occupational safety . In air chemistry , NO x and other nitrogen oxides with an oxidation level of 2 or more, including acids and organic species, are grouped together with the symbol NO y .
Education and characteristics
The nitrogen oxides are formed from the elements without exception through endothermic reactions, that is, they are only formed from the elements under external pressure (energy supply). On the other hand, this means that they can be used industrially as an oxidizing agent, for example dinitrogen tetroxide in rocket technology, or laughing gas (dinitrogen monoxide, N 2 O) for hot flames . With the exception of laughing gas, they behave as an acid generator in relation to water (for example 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 attention from an environmental perspective at an early stage. On the one hand, nitrous oxide has medical and technical applications, on the other hand it is unintentionally released into the atmosphere during technical and agricultural processes.
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.
Formally, trinitramide (N (NO 2 ) 3 or N 4 O 6 ), nitryl azide (O 2 N – N 3 or N 4 O 2 ) and nitrosyl azide (ON – N 3 or N 4 O) can also be assigned to nitrogen oxides become. The connections are extremely unstable. Until now, trinitramide and nitryl azide could only be produced and detected in solution. Nitrosyl azide exists as a pale yellow solid below −50 ° C. Furthermore, there is the nitrate radical (NO 3 , colorless), which is only stable below −142 ° C , and which also occurs in an isomeric peroxide form, as well as the dimer nitrous oxide (N 2 O 6 , O 2 N-OO-NO 2 ), which is also one of the peroxides.
|Oxidation state of N||Molecular formula||designation|
|+0.5||N 4 O||Nitrosyl azide|
|+1||N 2 O||Nitrous oxide (laughing gas)|
|+1||N 4 O 2||Nitryl azide|
|+3||N 2 O 3||Nitrous oxide|
|+3||N 4 O 6||Trinitramide|
|+4||NO 2||Nitrogen dioxide|
|+4||N 2 O 4||Nitrous tetroxide|
|+5||N 2 O 5||Nitrous oxide|
Nitrous gases are formed, among other things, when nitric acid (HNO 3 ) reacts with organic substances or metals. The reaction of nitric acid with silver and copper produces large amounts of NO x . The typical red-brown color of nitrous gases is mainly caused by nitrogen dioxide (NO 2 ). Nitrous gases have a characteristic pungent odor and can lead to pulmonary edema with a delay of more than 24 hours ( latency period ) after inhalation .
Nitrogen oxides in the air
Nitrogen oxides enter the atmosphere from natural and anthropogenic sources.
One of the main sources of nitrogen oxides in the atmosphere is exhaust gases from burning fossil fuels such as coal or oil. In Europe in 2000, more than half were in some cities, such as London , up to three-quarters of the NO x - emissions caused by traffic. NO x concentrations directly at the exhaust of vehicles range between 1000 mg / m 3 and 5000 mg / m 3 . However, emissions have been falling continuously since the 1990s. According to the Federal Environment Agency , annual nitrogen oxide emissions in Germany fell by 59% between 1990 and 2015 from 2.887 to 1.186 million tons. In the European Union (EU28), NO x emissions fell by 56% between 1990 and 2015, from 17.664 to 7.751 million tons. During this period, the share of transport in NO x emissions in the EU28 fell to 46% (39% road transport and 7% other forms of transport). Up to 80% of this comes from diesel engines . The emissions from shipping contribute significantly to this. For example, almost a third of nitrogen oxide emissions in Hamburg are caused by the port operations of the Port of Hamburg .
Nitrogen oxides are also produced as a result of natural atmospheric processes. As early as 1997, a study by Colin Price and his colleagues came to the conclusion that lightning made a significant contribution to the generation of nitrogen oxides in the atmosphere. They assumed that between 60 and 120 lightning bolts occur worldwide every second, depending on the year or month. Lightning occurs mainly over land and its frequency increases with increasing temperatures. Since nitrates are formed from nitrogen oxides together with water , lightning is also a very important natural source for the “ nitrogen fertilization ” of wild plants or plankton , which cannot metabolize elemental nitrogen.
Contrary to previous assumptions, most nitrogen oxides are not created automatically because of the higher lightning frequency in the tropics - in addition to the frequency, the lightning length is also decisive for the amount of NO x generated . Calculations that were carried out on the basis of the determined frequency of lightning and effects came to the result that up to 20 million tons of nitrogen oxides are produced annually by this natural cause. According to a study published in 2003 (in the USA ) 90% of all nitrogen oxides in the middle and upper troposphere (between 5 and 15 km) will be due to this cause in summer . In the lower troposphere (<5 km), up to 20% of the total amount of NO x is generated by lightning in the summer months , while the amount is negligible in the winter months.
Effects of nitrogen oxides
Nitrogen oxides - especially nitrogen dioxide - irritate and damage the respiratory system. Increased concentrations in the air they breathe have a negative effect on the lung function of children and adults. They are largely responsible for the formation of acid rain , whereby nitric acid (HNO 3 ) is caused by the reaction of (2 NO 2 + H 2 O → HNO 3 + HNO 2 ) or by the absorption of N 2 O 5 in aerosol particles and the subsequent formation of NO 3 - arises in the liquid phase.
Nitrogen oxides close to the ground are responsible for the formation of ozone (O 3 ) under the influence of UV radiation in so-called summer smog . The formation of the ozone is triggered here by the UV light from the sun:
Since the process is reversible , the amount of ozone is highest on sunny days in the afternoon and falls again overnight.
When the pollutants are transported away from the emission centers, the NO is increasingly converted into NO 2 , which accelerates the formation reaction of ozone (1) and reduces the decomposition reaction (2). Accordingly, higher ozone levels can be measured in more rural areas than in cities.
To protect human health, the EU limit value was set as the annual mean value for nitrogen dioxide concentration (NO 2 ) in the outside air at 40 µg / m 3 . For certain workplaces in industrial branches and trade, where significantly higher nitrogen oxide emissions are to be expected, a separate workplace limit value applies, which allows up to 950 µg / m 3 . This value applies only to healthy workers and a maximum of eight hours a day and a maximum of 40 hours a week.
Nitrogen oxides from air traffic and, indirectly, nitrous oxide also contribute to the depletion of ozone in the stratosphere. Nitrous oxide is photolyzed by UV radiation and forms NO, which in turn breaks down ozone according to equation (2).
NO x in the furnace
As a rule, nitrogen oxides are divided into 3 types according to their sources and their mechanism of formation:
- thermal NO x
- Fuel-NO x (English fuel-NO x )
- prompt NO x
The “NO x ” mentioned in this context are composed of around 95% NO and 5% NO 2 in the furnace . The change in the concentrations of NO x can be described with the aid of the reaction kinetics . The concentrations of N 2 and O as well as the temperature are decisive influencing factors:
The exponential term is the approach using the Arrhenius equation , c N2 and c O are the concentrations of N 2 and O at the respective point in time.
Thermal NO x
The term “thermal” refers to the relatively high temperatures that are required to initiate the formation reaction of thermal NO x via N 2 . The nitrogen source of the thermal NO x is the nitrogen present in the combustion air; the oxygen required to oxidize the N 2 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 hydroxyl radicals (OH) produced during combustion can also react with nitrogen if there is a lack of oxygen (reaction in the flame zone):
The formation of thermal NO x is to be expected at combustion temperatures from around 1000 ° C, the rate of formation of NO increases exponentially from around 1200 ° C. If the gas cools down to below 600 ° C, it is oxidized to NO 2 . Below 1000 ° C, so-called fuel NO x dominates in nitrogen-containing fuels . The oxygen available and the residence time of the reactants in the combustion zone also have an influence on the rate of NO x formation. High pressures such as B. occur in internal combustion engines, also promote NO x formation. Studies of nitrogen oxide formation in electric arc furnaces show that, in addition to the technical combustion processes of fossil fuels such as crude oil or natural gas, O 2 / N 2 plasmas also have good nitrogen oxide formation conditions.
Fuel NO x
The source of this type of NO x is the amount of nitrogen bound in the fuel, which is converted into NO x during combustion . The amount of nitrogen carried is heavily dependent on the fuel; accordingly, the proportions of thermal and fuel NO x in the flue gas resulting from the combustion also vary .
Some examples are (proportions in%):
fuel thermal NO x BS-NO x from volatile
Prompt NO x Diesel / petrol
90-95 - - 5-10 gas 100 - - - Heavy fuel oil 40-60 40-60 - -
Dry coal firing
10-30 50-70 20-30 - Black coal
40-60 30-40 10-20 - Brown coal <10 > 80 <10 -
There are two types of nitrogen release for 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 , for example .
The main source of fuel NO x are the volatile components of the fuel.
From temperatures of around 800 ° C, fuel NO x is mainly produced in the flame fronts of the furnace. The fuel carried through goes through several reaction steps, which lead to NO and N 2 via hydrogen cyanide (HCN) and hydrazine (NH n ) . N 2 and NO can undergo a reverse reaction to HCN with hydrocarbon radicals (CH n ) (“reburning”) and convert them back to NO or to molecular nitrogen (N 2 ). 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 NO x
Instead of the conversion to N 2 , the reaction of the fuel radicals (CH n ) with N 2 can lead to the formation of NO x again . This proportion of NO x formed is called “prompt” NO x and is also known as the “Fenimore mechanism”.
The main influencing factor are the hydrocarbon radicals that are formed as intermediate products of the combustion of fossil fuels containing carbon . Their educational mechanisms are extremely complex and so far not fully recorded and understood. Prompt NO x arises in very rapid formation reactions in comparably small quantities and, in comparison to thermal NO x, is hardly temperature-dependent, although the proportion increases with increasing temperature.
NO x reduction
There are various options for reducing NO x in power plants.
The primary measures concern the combustion process and prevent the formation of NO x . These include air grading , fuel grading , internal exhaust gas recirculation , external exhaust gas recirculation, primary additives and quenching (injection of water to reduce the temperature).
The secondary measures reduce the NO x in the exhaust gas through catalytic (SCR process) or non-catalytic (SNCR process) reduction to elemental nitrogen. Catalytic exhaust gas cleaning is also used for the exhaust gases from motor vehicles . The main products of the reduction measures shown are elemental nitrogen , of which around 78% by volume occurs in the air, and water. Small amounts of laughing gas can arise as side reactions . In addition, small amounts of ammonia can escape (NH 3 slip) in both the catalytic and non-catalytic secondary measures.
Natural NO x degradation
The processes in the atmosphere by which nitrogen oxides are formed and broken down are extremely complicated. They consist of a large number of reactions that are influenced to varying degrees by the prevailing temperature, the strength of the sunlight, the pressure, pollution and the respective concentration ratios. When the molecules rise into the atmosphere, for example, on the one hand the reaction probability decreases due to decreasing pressure, on the other hand some of the collisions with unreactive molecules are eliminated and the radiation increases in intensity. This is why the half-life of the substances involved in the reactions changes with altitude, and some intermediate products, which are only very short-lived near the ground, are much more stable in the stratosphere. On the other hand, compounds like nitrous oxide, which are relatively inert on the ground, can slowly rise into the stratosphere, where they eventually react with other compounds. The typical (but highly variable) lifetime of NO x is a few hours for the lower troposphere and a few days for the upper troposphere. In the stratosphere and mesosphere, the typical lifespan is from days to a few weeks.
Nitrous acid could form in the atmosphere as a result of the likely heterogeneous reaction.
However, it is then subject to photolysis , which is why it is viewed as a source of OH radicals in many chemical models.
Dinitrogen pentoxide can occur as an intermediate in the atmosphere. It arises z. B. from nitrogen dioxide and nitrogen trioxide.
As the anhydride of nitric acid, it reacts with water (vapor) to form nitric acid, although the speed of the reaction in the gas phase is not precisely known.
It is assumed that the reaction takes place primarily heterogeneously on moist aerosols. The formation of nitric acid in the atmosphere according to the reaction is also of great importance:
The reaction proceeds about ten times faster at 25 ° C than the recombination of OH radicals with sulfur dioxide. However, nitric acid can also be permanently removed from the reaction cycle, e.g. B. by accumulation on aerosol particles or by new formation (condensation) of such particles. The gaseous nitric acid is chemically very stable in the troposphere near the ground and is removed from the atmosphere by dry and wet deposition due to its good water solubility.
Nitrogen trioxide, in turn, is an important part of the clean and polluted troposphere at night . During the day the compound is subject to photolysis:
In addition, nitrogen trioxide reacts quickly with nitrogen monoxide to form nitrogen dioxide:
The lifespan of nitrogen trioxide is therefore less than 10 s during the day. Influencing factors for these reactions are the ozone concentration, the light intensity (sun), the residence time of the compounds in the atmosphere, the air pollution (e.g. dust, sulfur dioxide) and the smog formation.
Metrological proof of nitrogen oxides
Chemiluminescence methods can be used to measure nitrogen oxides in exhaust gases from stationary sources . For this purpose, the property of nitrogen monoxide is used to emit light when it is converted into nitrogen dioxide (chemiluminescence). A representative partial flow is taken from the exhaust gas and, after passing through a converter, which converts any nitrogen dioxide that may be present into nitrogen monoxide, is brought into contact with ozone . The light emitted during the reaction is converted by a photomultiplier into an electrical signal that provides information about the nitrogen oxide concentration.
To determine nitrogen monoxide and nitrogen dioxide using ion chromatography , these gases are converted into nitric acid with the help of ozone or hydrogen peroxide and water. The nitrate concentration is then analyzed.
Other methods for measuring nitrogen oxides in exhaust gases from stationary sources are the sodium salicylate method and the dimethylphenol method. In the sodium salicylate process, nitric oxide and nitrogen dioxide are oxidized to nitric acid and then converted into nitrosalicylic acid with sodium salicylate . The concentration of the yellow anion of nitrosalicylic acid that is formed after a further treatment step can be determined photometrically and thus provide information about the nitrogen oxide concentration of the gas being sampled. For the dimethylphenol process, nitrogen monoxide and nitrogen dioxide are oxidized in the gas phase by means of ozone to form nitrous oxide, which after absorption in water is converted with 2,6-dimethylphenol in sulfuric and phosphoric acid solutions to 4-nitro-2,6-dimethylphenol , the anion of which is also measured photometrically can be.
One method for determining nitrous oxide in exhaust gases from stationary sources is the non-dispersive infrared method . For this purpose, the gas in a measuring gas cell and a reference cell are illuminated by an infrared radiator. Both beams are detected by a receiver and compared with one another. The beam, which is weakened in comparison to the reference cell, is a measure of the nitrous oxide concentration. In order to minimize cross-sensitivities of the process due to interfering carbon monoxide , it is converted into carbon dioxide by means of a converter made of metal oxides .
It is important that all components of the measuring device that come into contact with the gas to be sampled do not react with the nitrogen oxides.
The Saltzman method can be used to measure the immission of nitrogen dioxide , in which the sample air is passed through a reaction solution that reacts with the gas component to be detected to form a red azo dye . The color intensity of the reaction solution is determined photometrically and is a measure of the mass of nitrogen dioxide. By using an oxidizing agent, the Saltzman method can also be used to determine nitrogen monoxide.
As with emission measurement, chemiluminescence methods can also be used for immission measurement.
Another possibility of measuring the immission of nitrogen dioxide is the use of passive collectors . A wire mesh prepared with triethanolamine on which nitrogen dioxide is deposited is located in a glass tube that is open at the bottom . At the end of the collection time, the wire mesh is treated with a combination reagent in order to analyze the resulting discoloration photometrically.
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