NO x is the collective name for nitrogen oxides NO and NO 2 formed in chemical reactions in the atmosphere and during combustion . Together with volatile organic substances , near-surface ozone , lead , carbon monoxide , sulfur oxides and dust particles are among the harmful emissions for which the restrictions are established by the US Environmental Protection Agency [1] .
NO x in the atmosphere is formed both due to natural phenomena, such as lightning and forest fires, and as a result of human activities. Impurities of NO 2 color industrial fumes brown, so emissions from plants with a noticeable content of nitrogen oxides are called "fox tails." NO x emissions are considered one of the main causes of photochemical smog formation. Combining with water vapor in the atmosphere, they form nitric acid , and, together with sulfur oxides, cause acid rain . Elevated concentrations of NO x have a detrimental effect on human health, therefore, standards have been adopted in different countries to limit the maximum allowable concentrations of NO x in the exhausts of boilers in power plants, gas turbine plants, automobiles, airplanes and other devices. Improvement of combustion technologies is largely aimed at reducing NO x emissions while improving the energy efficiency of devices.
Nitrous oxide N 2 O is not included in NO x , but it affects the state of the ozone layer in the upper atmosphere.
Health effects of NO x
NO is odorless, but when inhaled, it can bind to hemoglobin , similar to carbon monoxide, converting it into a form that is not able to carry oxygen [2] . NO 2 irritates the lungs and can lead to serious health effects. NO 2 combines with water, dissolves well in fat and can penetrate into the capillaries of the lungs, where it causes inflammation and asthmatic processes. A concentration of NO 2 above 200 ppm is considered lethal, but even at a concentration above 60 ppm, discomfort and burning sensation in the lungs can occur. Long-term exposure to lower concentrations can cause headaches, digestive problems, coughing, and lung disease.
In the clinic of acute poisoning with nitrogen oxides, four periods are distinguished: latent , increase in pulmonary edema , stabilization and reverse development [3] . In the latent period of imaginary well-being, which can last 4-12 hours, the patient may be disturbed by conjunctivitis , rhinitis and pharyngitis due to irritation of the mucous membranes , manifested by coughing , lacrimation, general malaise , but his general condition is generally satisfactory [3] . Then the patient's condition worsens: as pulmonary edema develops, a wet cough with mucous or bloody sputum , shortness of breath , cyanosis , tachycardia , low-grade or febrile temperature increase appears. There is a feeling of fear , psychomotor agitation and convulsions . In the absence of qualified medical care, this can be fatal.
NO x formation mechanisms
Three main mechanisms of NO x formation are known [4] :
- thermal mechanism, or Zeldovich’s high-temperature mechanism
- "Fast" mechanism ( eng. Prompt NO ), also called chemical
- the mechanism associated with the formation of NO x from nitrogen-containing fuel components ("fuel NO x ", eng. fuel NO )
Additional ways of NO formation are associated with reactions of nitrous oxide N 2 O and the NNH radical [5] .
Zeldovich's Mechanism
The high-temperature mechanism of nitrogen oxidation in the combustion zone was proposed by Ya. B. Zeldovich in the mid-1940s [6] and is considered the main mechanism for the formation of nitrogen oxides during combustion. This mechanism includes the following elementary stages:
to which the reaction is added (Fenimore and Jones, 1957 [7] ):
The set of reactions (1-3) is called the extended Zeldovich mechanism. Due to the fact that the triple bond energy in the N 2 molecule is about 950 kJ / mol, reaction (1) has a large activation energy and can pass at a noticeable rate only at high temperatures. Therefore, this mechanism plays an important role in the case of high temperatures in the reaction zone, for example, during the combustion of near-stoichiometric mixtures or during diffusion combustion. It is believed that increasing the maximum temperature in the combustion zone above 1850 K leads to unacceptably high emissions of NO x , and one of the main ways to reduce emissions by the thermal mechanism is to prevent the formation of hot spots in the flame front.
Fast Mechanism
The Zeldovich mechanism well describes NO x emissions in the case of thermal factors (for example, during the combustion of hydrogen or carbon monoxide in air), but for hydrocarbon fuels it turned out that the experimentally measured concentrations of NO x significantly exceed those predicted by the thermal mechanism. Direct measurements performed by Fenimore in 1971 showed that NO is formed already at the beginning of the chemical reaction zone [8] . This mechanism has been called the "prompt" ( English prompt NO ) or the Fenimore mechanism.
The formation of NO x by a fast mechanism is associated with the reaction of the CH radical, which is present only in the initial decomposition zone of hydrocarbon fuels, with molecular nitrogen:
NO can be formed in a number of subsequent reactions involving various radicals, for example:
The general reaction scheme of the fast mechanism is shown in the figure.
For a long time, it was believed that instead of reaction (4), the CH radical reacts with N 2 along the path [9] :
however, this reaction is forbidden on the back [10] , and quantum chemical calculations and experimental studies carried out in recent years have shown that reaction (4) [11] [12] plays the main role.
Since reaction (4) proceeds with a low activation energy in the fast mechanism, it is possible at relatively low temperatures of the order of 1000 K, and in rich mixtures (with an excess of fuel) the formation of NO is promoted by an increased concentration of CH radicals.
The formation of NO x from nitrogen-containing fuel components
Almost all solid combustible materials contain organic substances, which contain nitrogen. Coal, hay, wood, and household waste can contain up to 1-3% nitrogen by weight. During the pyrolysis and combustion of such materials, NO can form as a result of decomposition of these components. Part of the nitrogen may be converted to N 2 or remain bound in ash, resins and other non-volatile residues. Since the combustion of coal and other solid combustible materials takes place at relatively low temperatures, when the Zel'dovich mechanism does not work, the contribution of this mechanism to emissions and NO x can be significant [13] .
Ways to Reduce NO x Emissions
Since the bulk of NO x emissions in real devices is attributed to the Zeldovich high-temperature mechanism, most of the methods developed so far to reduce these emissions are aimed at lowering the maximum temperature in the combustion zone and reducing the residence time of the reactants in this zone. Ways to reduce NO x emissions are divided into primary and secondary [14] . Primary measures include the optimization of the combustion process and the design of devices, as a result of which the concentration of NO x in the exhaust gases is reduced. Secondary methods (selective or non-selective catalytic reduction) involve the purification of exhaust gases before they are released into the atmosphere and entail significant additional costs. The actual methods used are, to a large extent, conditioned by the regulations for NOx emissions in various countries in industrial plants (gas turbine power plants, industrial furnaces), airplanes, automobiles and devices for the consumer sector, such as boilers or heaters. The more stringent standards are set for emissions, the more expensive the design of the combustion chamber is and the more difficult it is to control its working process.
Water Injection
In installations with diffusion combustion, the oxidizing agent and fuel are fed separately to the combustion chamber, where they are mixed and burned. This organization of component supply is relatively safe and allows you to control the workflow over a wide range, varying the consumption of components. However, in the diffusion flame in the combustion zone, the oxidizing agent and fuel automatically turn out to be in a stoichiometric ratio and inevitably foci of elevated temperature are formed, which leads to the formation of NO x . Emission reduction in such installations is achieved by diluting the mixture with a cooling component, most often by injecting water or water vapor into the combustion chamber. However, this method inevitably leads to a decrease in the power and efficiency of the installation, and its applicability is limited by the fact that, together with a reduction in NO x , emissions of CO and other products of incomplete combustion begin to increase [15] .
Multistage Combustion
To reduce the temperature in the chemical reaction zone, the components can be burned in several stages. For example, in the first stage, a rich mixture (with excess fuel) can be burned, followed by cooling of the products of incomplete combustion by means of heat-removing elements. At the second and subsequent stages, additional air is added to the mixture, which allows complete combustion of the mixture to the final products of combustion (CO 2 and H 2 O). At the same time, the temperature throughout the combustion zone does not reach anywhere the values at which thermal NO x is formed , however, this is achieved at the cost of reducing the plant's power and complicating its design.
One of the most common options for multi-stage combustion technology is the three-stage combustion ( English reburning process ). The essence of this method is that in the first stage a rich mixture is burned, in the second, main stage - a mixture with excess air, and in the third stage, hydrocarbon fuel is again added to the combustion products, which acts as a reducing agent for the oxides already formed in the main combustion zone nitrogen. As a result of this, NO x is reduced to N 2 [16] [17] .
Exhaust Gas Recirculation
The injection of water vapor or the supply of additional air to the reaction zone is associated with a complication of the design of the device and is mainly used in systems created before the invention of simpler and more economical methods. In more modern systems, the products of combustion themselves, taken from the flue or exhaust stream, are used as a diluent for the initial mixture. These products, consisting mainly of CO 2 and H 2 O, have a high heat capacity and effectively remove heat from the reaction zone, reducing its temperature. The energy efficiency of the installation is reduced to a lesser extent than when air or water vapor is supplied from outside, since the energy of the combustion products is partially returned to the system. The design of the installation itself is simplified, since it is not necessary to create separate units that supply external components. These methods have found application both in automobile engines in exhaust gas recirculation systems ( English EGR , exhaust gas recirculation), and in boilers and furnace devices ( flue gas recirculation , English FGR , flue gas recirculation) [18] .
Burning lean mixtures
This method of reducing emissions has been widely used in gas turbine plants. A pre-mixed mixture of fuel and air is fed into the combustion chamber in a non-stoichiometric ratio with excess air (lean mixture). This ensures almost complete combustion of the initial fuel, and the temperature of the combustion products decreases, which also leads to a reduction in NO x emissions by the high-temperature mechanism. If natural gas is used as fuel, then virtually all NO x emissions are attributable to the fast mechanism.
Gas turbine plants using the combustion of lean mixtures with low NOx emissions are sometimes referred to as “dry” ( dry dry NOx ) because there is no water vapor injection in them. However, the range of sustainable combustion in such plants is markedly reduced; they are subject to the dangers of the development of combustion instabilities, breakthroughs and flame outbreaks [19] . Therefore, their power range is limited, and sudden changes in the load in the network led to accidents due to spontaneous shutdown of gas turbine units in power generation networks. In addition, the range of sustainable combustion is sensitive to the type of fuel, and switching to fuel from another supplier can be very difficult, since the composition of natural gas from different fields varies greatly.
Selective Catalytic Reduction
The method (SCR, English selective catalytic reduction ) is used to purify flue and exhaust gases from NO x with an efficiency reaching 90%. A reagent, usually ammonia or urea, is injected into the exhaust stream, and the mixture enters the catalyst. The catalyst operates in the temperature range from 450 to 900 K and provides the following reactions in which nitric oxide is reduced to molecular nitrogen:
in the case of adding ammonia and
in case of use of urea (a more expensive reagent).
The catalysts use titanium oxide with the addition of vanadium , molybdenum or tungsten , zeolites , iron oxides with a thin film of iron phosphates or activated carbon in the form of agglomerated granules. The catalyst material is selected taking into account its price and durability under specified operating conditions [20] .
Selective Non-Catalytic Recovery
The method of SN , the English selective non-catalytic reduction ) is widespread in the global energy sector and is used in Russia at thermal power plants. In this method, ammonia or urea is added to the flue gas, which reduces NO to molecular nitrogen. Failure to use a catalyst can significantly reduce the cost of the process. The method was patented by Exxon Research Engineering in 1975 [21] .
The method is used in the temperature range from about 1100 to 1400 K and is described by the gross reaction
At lower temperatures, the reaction proceeds too slowly, and at higher temperatures, the reaction begins to compete with it.
The main difficulty in the application of this method is related to the need to ensure the mixing of the reagent with flue gases precisely in a given temperature window and stay in it for 200-500 ms [22] .
Notes
- ↑ What are the Six Common Air Pollutants? (eng.) . US Environmental Protection Agency . Date of treatment March 19, 2014.
- ↑ Stamler JS, Gow AJ Reactions between nitric oxide and haemoglobin under physiological conditions // Nature. - Macmillan Publishers Ltd, 1998. - Vol. 391, No. 6663 . - P. 169-173. - DOI : 10.1038 / 34402 .
- ↑ 1 2 Artamonova V.G., Mukhin N.A., 2004 , p. 351.
- ↑ Wünning and Wünning, Flameless oxidation, 1997 , p. 82.
- ↑ Lefebvre, Ballal, Gas Turbine Combustion, 2010 , p. 378.
- ↑ Zel'dovich et al., Oxidation of nitrogen during combustion, 1947 .
- ↑ Fenimore CP, Jones GW Nitric Oxide Decomposition at 2200–2400 ° K // The Journal of Physical Chemistry. - American Chemical Society, 1957. - Vol. 61, No. 5 . - P. 654-657. - DOI : 10.1021 / j150551a034 .
- ↑ Fenimore CP Formation of nitric oxide in premixed hydrocarbon flames // Symposium (International) on Combustion. - Elsevier, 1971. - Vol. 13, No. 1 . - P. 373-380. - DOI : 10.1016 / S0082-0784 (71) 80040-1 .
- ↑ Warnatz, Combustion, 2006 , p. 262.
- ↑ Cui Q., Morokuma K., Bowman JM, Klippenstein SJ The spin-forbidden reaction CH ( 2 Π) + N 2 → HCN + N ( 4 S) revisited. II. Nonadiabatic transition state theory and application // The Journal of chemical physics. - American Institute of Physics, 1999. - Vol. 110, No. 19 . - P. 9469-9482. - DOI : 10.1063 / 1.478949 .
- ↑ Moskaleva LV, Lin MC The spin-conserved reaction CH + N 2 → H + NCN: A major pathway to prompt NO studied by quantum / statistical theory calculations and kinetic modeling of rate constant // Proceedings of the Combustion Institute. - Elsevier, 2000. - Vol. 28, No. 2 . - P. 2393-2402. - DOI : 10.1016 / S0082-0784 (00) 80652-9 .
- ↑ Lamoureux N., Desgroux P., El Bakali A., Pauwels JF Experimental and numerical study of the role of NCN in prompt-NO formation in low-pressure CH 4 –O 2 –N 2 and C 2 H 2 –O 2 –N 2 flames // Combustion and Flame. - Elsevier, 2010 .-- Vol. 157, No. 10 . - P. 1929-1941. - DOI : 10.1016 / j.combustflame.2010.03.01.03 .
- ↑ Glarborg ea, Fuel nitrogen conversion, 2003 , p. 91.
- ↑ Warnatz, Combustion, 2006 , p. 267.
- ↑ Lefebvre, Ballal, Gas Turbine Combustion, 2010 , p. 387.
- ↑ Smoot, Hill, Xu, NOx control through reburning, 1998 , p. 386.
- ↑ Kotler V. R. Three-stage combustion . Date of treatment April 23, 2014.
- ↑ Kotler V.R. Flue gas . Date of treatment April 23, 2014.
- ↑ Wünning and Wünning, Flameless oxidation, 1997 , p. 84.
- ↑ Kotler V.R. Selective catalytic reduction - SLE (SNR) . Date of treatment April 23, 2014.
- ↑ US Patent No. 3,900,554 of August 19, 1975. Method for the reduction of the concentration of NO in combustion effluents using ammonia . Description of the patent on the website of the United States Patent and Trademark Office .
- ↑ Kotler V.R. Selective non-catalytic reduction - SNCR (SNCR) . Date of treatment April 8, 2014.
Literature
- Zeldovich Ya. B., Sadovnikov P. Ya., Frank-Kamenetsky D.A. Oxidation of nitrogen during combustion. - M.-L .: Publishing house of the Academy of Sciences of the USSR, 1947. - 148 p.
- Warnatz J., Maas U., Dibble RW Combustion: Physical and Chemical Fundamentals, Modeling and Simulation, Experiments, Pollutant Formation . - Springer, 2006 .-- 378 p. - ISBN 978-3-540-45363-5 . (eng.)
- Lefebvre AH, Ballal DR Gas Turbine Combustion: Alternative Fuels and Emissions. 3rd edition. - CRC Press, 2010 .-- 538 p. - ISBN 978-1420086041 . (eng.)
- Artamonova V.G., Mukhin N.A. Intoxication with irritating substances (chlorine, hydrogen chloride, sulfur dioxide, hydrogen sulfide, nitrogen oxides) // Occupational Diseases . - M .: Medicine, 2004 .-- 480 p. - 3000 copies. - ISBN 5-225-04789-0 .
Reviews
- Miller JA, Bowman CT Mechanism and modeling of nitrogen chemistry in combustion (English) // Progress in Energy and Combustion Science. - Elsevier, 1989 .-- Vol. 15, no. 4 . - P. 287–338. - DOI : 10.1016 / 0360-1285 (89) 90017-8 .
- Wünning JA, Wünning JG Flameless oxidation to reduce thermal NO-formation (English) // Progress in Energy and Combustion Science. - Elsevier, 1997 .-- Vol. 23, no. 1 . - P. 83-94. - DOI : 10.1016 / S0360-1285 (97) 00006-3 .
- Smoot LD, Hill SC, Xu H. NO x control through reburning // Progress in Energy and Combustion Science. - Elsevier, 1998 .-- Vol. 24, no. 5 . - P. 385–408. - DOI : 10.1016 / S0360-1285 (97) 00022-1 .
- Glarborg P., Jensen AD, Johnsson JE Fuel nitrogen conversion in solid fuel fired systems (English) // Progress in Energy and Combustion Science. - Elsevier, 2003 .-- Vol. 29, no. 2 . - P. 89-113. - DOI : 10.1016 / S0360-1285 (02) 00031-X .
- Konnov AA, Javed MT, Kassman H., Irfan N. NO x Formation, Control and Reduction Techniques (Eng.) // Handbook of Combustion. Volume 2: Combustion Diagnostics and Pollutants. - Wiley, 2010 .-- P. 439-464. - DOI : 10.1002 / 9783527628148.hoc037 .
- Glarborg P., Miller JA, Ruscic B., Klippenstein SJ Modeling nitrogen chemistry in combustion // Progress in Energy and Combustion Science. - 2018 .-- Vol. 67. - P. 31-68. - ISSN 0360-1285 . - DOI : 10.1016 / j.pecs.2018.01.002 .
Links
- NOx Chemistry - video of a lecture by Professor Michael Pilling at the 2013 Summer School on Combustion at Princeton University (in English, slides for the lecture ).