RU2359170C2 - Method of reducing emission of nitrogen oxides by power plants on natural or associated gas - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: present invention relates to a method of reducing emission of nitrogen oxides by power plants on natural or associated gas, which includes reduction of combustion temperature by supplying incombustible gas to the combustion zone. Incombustible gas is supplied through preliminary partial oxidation of natural or associated gas by compressing to pressure not less than 3 MPa, heating to temperature not below 300°C and supply of air with oxygen content of 2-8% vol., relative the initial natural or associated gas, with subsequent supply into the combustion zone of partially oxidised and incombustible gas containing nitrogen air from natural or associated gas. For further reduction of emission of nitrogen oxides from partially oxidised natural or associated gas, liquid products of oxidation are separated (oxidate, mainly containing methanol and water), and added to products of combustion at temperature of not less than 430°C, with subsequent trapping of the formed nitrogen dioxide.

EFFECT: reduced emission of nitrogen oxides into the atmosphere.

3 cl, 1 dwg

Description

The invention relates to the field of protection of atmospheric air from industrial emissions, and in particular to methods for reducing the emission of nitrogen oxides in thermal power plants and power plants running on natural or associated gas, and can be used in industrial energy.

State of the art

Nitrogen oxides are among the main atmospheric pollutants in industrialized countries, with harmful effects on human health and the environment. Participating in photochemical reactions with oxygen and hydrocarbons, nitrogen oxides generate other highly toxic pollutants. In addition, nitrogen oxides, along with carbon dioxide, methane and freons, are among the main greenhouse gases and also participate in cycles leading to the destruction of the ozone layer. Although the main source of anthropogenic emissions of nitrogen oxides is transport, in developed countries the energy sector accounts for up to 30% of their emissions. Their share in the total amount of harmful emissions of large thermal power plants operating on natural gas reaches 20% (Y. Khodakov, “Nitrogen oxides and heat power engineering: problems and solutions.” - M .: EST-M LLC, 2001, 432 from).

At high temperatures developed in power plants, nitric oxide NO is formed mainly, since higher nitrogen oxides are thermodynamically unstable under these conditions. According to modern concepts, during the combustion of fuels, nitric oxide can be formed by three different mechanisms. Firstly, during the reaction of molecular nitrogen present in the combustion zone with oxygen (“thermal” NO). Secondly, due to the activation of molecular nitrogen in reactions with hydrocarbon radicals (“fast” NO). Thirdly, due to the organic nitrogen compounds present in the fuel (“fuel” NO). For power plants and power plants operating on natural gas, the main source of NO formation is the thermal mechanism. When the temperature of burning fuel in thermal power up to 2200 K and the residual oxygen concentration above 1% of the equilibrium concentration of NO can exceed 2000 million ^-1. In this case, the combustion temperature has the greatest influence on the level of nitrogen oxide emissions. An increase in temperature from 1500 ° C to 2100 ° C leads to a tenfold increase in the concentration of NO. Appearing in appreciable quantities only at temperatures above 1300 ° C, the concentration of thermal NO further increases exponentially.

Without the use of special measures to suppress the formation of nitrogen oxides, their level in the emissions of energy boiler units during the combustion of natural gas with a flow rate of more than 35 thousand m ³ / h reaches 1,500 mg / m ³ . Currently, the most common methods for reducing the content of nitrogen oxides in flue gases are high-temperature non-catalytic reduction with the addition of ammonia or its derivatives or selective catalytic reduction of NO with ammonia. A more economical way to reduce the NO content is to modify the combustion of fuel, and the goal of most of these modifications is to minimize the temperature of combustion (Druskin LI "Effective use of natural gas in industrial plants", Handbook. - M .: Energoatomizdat, 1992).

There are a number of ways to reduce the emission of NO in thermal power plants by lowering the combustion temperature of natural gas by recirculation into the combustion zone of the combustion chamber of non-combustible gas (flue gases). With a degree of recirculation of flue gases of 20%, it is possible to reduce the yield of nitrogen oxides by about half, and with a degree of recirculation of 30%, by three times. It is believed that under conditions of sustainable combustion, the recirculation of combustion products should be limited to between 20% and 30%. Moreover, the reduction in the efficiency of the installation is only from 0.02% to 0.03% per 1% of recirculating gases (Sigal I.Ya. “Protection of the air pool during fuel combustion.” - L .: Nedra, 1977, 295 pp. )

However, the known method requires large capital costs for the conversion of furnace units and a significant energy consumption for the recirculation of a large volume of flue gases.

A known method of reducing NO formation during combustion by diluting flammable gases with inert or non-reactive gases, for example, specially added argon (Samaniegro J.-M., Labegorre B., Egolfopoulos FN, Ditaranto M., Sautet J.-C, Charon O. " 27th Symposium (International) on Combustion ", Combustion Institute, 1998, p. 1385-1395). It was shown that the yield of NO decreases very strongly with the dilution of combustible gases with argon, and a particularly rapid decrease in the yield of NO is observed at argon concentrations in the gas entering the burner of less than 50 vol.%. When argon is introduced into the gas supplied for combustion at a concentration of 40 vol.%, The concentration of NO in the combustion products decreases from 480

^-1 million to about 100 million ^-1 or almost 80%.

However, the known method is economically unacceptable due to the high cost of inert gases and oxygen-free non-reactive gases such as nitrogen.

Another known way to reduce NO emissions in power plants is to introduce methanol and / or hydrogen peroxide into the waste products of combustion at a temperature of from 430 ° C to 830 ° C for the oxidation of nitric oxide to nitrogen dioxide, followed by trapping of nitrogen dioxide in a scrubber (Zamansky VM, But L ., Maly PM et al, "Oxidation of NO to NO ₂ by hydrogen peroxide and its mixture with methanol in natural gas and coal combustion gases", Combust. Sci. Technol. 1996. Vol.120. P.255). However, due to the need to supply large volumes of these expensive chemical products to power plants, this method is not widespread.

The closest is a way to reduce the emission of nitrogen oxides in thermal power plants and power plants operating on natural or associated gas by recirculation into the combustion zone of the combustion chamber of non-combustible combustion products - flue gases. Flue gases from the tail of the combustion chamber in an amount equal to 30% of the volume of air supplied to the combustion are cooled to a temperature of 300 ° C and mixed with blower exhaust air or fed into the combustion torch through slots in the furnace hearth or slots under the burners. In this case, the maximum flame temperature decreases by almost 200 ° C, and the formation of nitric oxide is reduced by two or three times (Druskin LI "Effective use of natural gas in industrial plants", Handbook. - M .: Energoatomizdat, 1992) or ( Seagal I.Ya. “Protection of the air basin during fuel combustion.” - L .: Nedra, 1977.295 s.).

However, the method requires large capital costs for the conversion of the furnace units and a significant energy consumption for the recirculation of a large volume of flue gases.

It is also known that methanol is obtained by oxidizing air with compressed and heated natural gas at an initial temperature of up to 500 ° C, pressure up to 10 MPa and an oxygen content of not more than 8 vol.%, Followed by cooling of the reaction mixture and the separation of methanol from a cooled gas-liquid mixture (RU 2162460, C1, С07С 31/04, 01/27/2000). The resulting crude methanol can be used as an environmentally friendly (not containing sulfur and other harmful impurities) liquid reserve fuel for thermal power plants and power plants, replacing oil reserve fuel, for example, fuel oil.

However, this does not solve the problem of reducing the emission of nitrogen oxides in thermal power plants and power plants.

SUMMARY OF THE INVENTION

The objective of the present invention is to develop a method for reducing the emission of nitrogen oxides by thermal power plants and power plants operating on natural or associated gas by lowering the combustion temperature due to the partial oxidation of natural or associated gas, in which it is pre-compressed to a pressure of at least 3 MPa and not heated to a temperature below 300 ° C, the natural or associated gas is partially oxidized with air at an oxygen content of 2% to 8% by volume relative to the source natural or associated gas and then fed Stična oxidized and comprising an incombustible gas - air, nitrogen or natural gas passing into the combustion zone.

The problem is solved in that pre-compressed to a pressure of at least 3 MPa and heated to a temperature of at least 300 ° C, the natural or associated gas is partially oxidized with air at an oxygen content of 2% to 8% by volume relative to the source natural or associated gas and then partially oxidized and containing non-combustible gas - air nitrogen, natural or associated gas is supplied to the combustion zone.

Additional differences of the proposed method are that natural or associated gas is preferably compressed to a pressure of 5 MPa to 8 MPa, heated to a temperature of 420 ° C to 450 ° C and oxidized with air in one or more successive reactors with an oxygen content of 2% up to 4 vol.% in relation to the source of natural or associated gas to an air nitrogen content of 20% to 35 vol.%.

An additional difference also lies in the fact that prior to combustion, liquid oxidation products (oxidate) containing mainly methanol and water, as well as a small amount of formaldehyde and ethanol are isolated from partially oxidized natural or associated gas and introduced into the exhaust products during a temperature of at least 430 ° C followed by trapping of the resulting nitrogen dioxide.

The proposed method is more efficient and economical in comparison with the prototype and other known methods and does not require the supply of additional chemical products to the power plant, which leads to lower costs for reducing the emission of nitrogen oxides. In addition, the need for the importation of reserve liquid fuel can be reduced due to the use of raw methanol extracted from liquid oxidate as a reserve fuel.

Brief List of Drawings

The invention is further illustrated by specific examples of its implementation and the accompanying drawing, which shows the calculated dependence of the final temperature and the concentration of NO in the adiabatic combustion products of a stoichiometric mixture with air of previously partially oxidized air of natural gas on the concentration in it of a non-combustible gas - nitrogen introduced by partial oxidation.

The best embodiment of the invention

The results of studies conducted during the development of this method are shown in tables 1 and 2 and in the drawing. The drawing and table 1 shows the thermodynamic calculation of the effect of nitrogen in the air introduced into natural gas during its partial oxidation by the proposed method on the combustion temperature and the equilibrium concentration of nitric oxide. However, such an adiabatic calculation of the temperature and composition of the products of combustion of a gas-air mixture does not take into account heat losses, therefore, it gives overestimated values of the final temperature, and accordingly, overestimated values of the concentration of NO in comparison with real burner devices. Nevertheless, the tendency of a decrease in the concentration of NO, which is the main nitric oxide in flue gases, as the combustible gas is diluted with nitrogen, is quite distinct. For a stoichiometric mixture, according to calculations, the introduction of 30% nitrogen during partial oxidation of natural gas leads to a decrease in the NO concentration by about 20%, and 60% of nitrogen leads to a decrease in the NO concentration by more than 50%.

Table 1 Adiabatic calculation of the equilibrium temperature and concentration of NO during combustion: a techniometric mixture of products of partial oxidation of natural gas and air at a pressure of 1 atm and an initial temperature of 298.15 K, depending on the volume concentration of nitrogen introduced by partial oxidation. The concentration of CH ₄ in combustible gas, vol.% The concentration of N ₂ in combustible gas, vol.% The equilibrium temperature of the combustion products, K The equilibrium concentration of NO in the combustion products, million ^-1 one hundred 0 2226.5 1972 92 8 2217.5 1896 84 16 2206.8 1810 76 24 2194.0 1709 68 32 2178.1 1591 60 40 2158.2 1451 52 48 2132.3 1284 44 56 2097.4 1083 36 64 2047.9 842

In real power plants, the combustion temperature is approximately 200 ° C lower than the adiabatic combustion temperature. Calculation of the temperature and products of combustion of a mixture of natural gas with a 50% addition of carbon dioxide, carried out for closer correspondence to real temperature, is given in Table 2. Moreover, the calculated temperature of the products of combustion decreased by about 175 ° C.

A partial oxidation with air, such mixture to achieve therein CH nitrogen concentration in the combustion gas supplied to 30% relative to the total concentration of ₄ + N ₂ concentration of NO decreased with a real value ^-1 990 million to about 550 million ^-1, i.e. almost 50%. In the oxidation with air to achieve a nitrogen concentration in the combustion gas supplied to 56% relative to the total concentration therein CH ₄ + N _2, NO concentration decreased to ~ 190 million ^-1, i.e. 80% or five times. Thus, dilution of a combustible gas with nitrogen by its partial oxidation makes it possible to significantly reduce the emission of NO. The data in table 2 can be used to calculate the reduction of nitric oxide emissions by partial oxidation of natural or associated gas by air.

table 2 Adiabatic calculation of the dependence of the equilibrium temperature and the NO concentration during the combustion of a stoichiometric mixture of partial oxidation products of a 50% mixture of CO ₂ with natural gas and air at a pressure of 1 atm and an initial temperature of 298.15 K, depending on the volume concentration of nitrogen introduced by partial oxidation . The concentration of CH ₄ relative to the concentration of CH ₄ + N ₂ in a combustible gas, vol.% The concentration of N ₂ relative to the concentration of CH ₄ + N ₂ in a combustible gas, vol.% The equilibrium temperature of the combustion products, K The equilibrium concentration of NO in the combustion products, million ^-1 one 2 3 four one hundred 0 2051.0 991 92 8 2029.2 891 84 16 2003.8 785 76 24 1973.8 671 68 32 1937.6 552 44 56 1767.8 194

Examples of various embodiments of the proposed method are presented in table 3.

Example 1

The source gas at a rate of 14,000 m ³ / h is compressed by compressor to a pressure of 5.0 MPa, heated to a temperature of 420 ° C and oxidized in the reactor by supplying 5300 m ³ / h of air with an oxygen content of 8 vol.% With respect to the source gas. and the resulting gas containing 25% vol. nitrogen, without the allocation of raw methanol is fed into the burner of the combustion chamber. This achieves reduction in combustion temperature to 78 ° C, resulting in reduced NO concentration in the flue gas to 670 million ^-1 or to reduce emissions of nitrogen oxide at 32 vol.%.

Example 2

The source natural gas with a flow rate of 40,234 m ³ / h is compressed by a compressor to a pressure of 8.0 MPa, heated to a temperature of 450 ° C and oxidized in three reactors installed in series with a total supply of 11862 m ³ / h of air with an oxygen content of 2.0 in each of the reactors vol.% with respect to the source of natural gas, and the resulting gas containing 20.6 vol.% nitrogen, after separation of 2780 kg / h of liquid oxidation products, is fed to the burner of the combustion chamber. This achieves reduction in combustion temperature to 62 ° C, resulting in reduced NO concentration in the flue gas to 728 million ^-1 or to reduce emissions of nitrogen oxide at 26 vol.%.

Example 3

The source natural gas with a flow rate of 7330 m ³ / h is compressed by a compressor to a pressure of 7.0 MPa, heated to a temperature of 450 ° C and oxidized in three reactors installed in series with a total supply of 4048 m ³ / h of air with an oxygen content of 3.9 in each of the reactors vol.% in relation to the source of natural gas, and the resulting gas, after separation of 496 kg / h of liquid oxidation products is fed into the burner of the combustion chamber. This achieves reduction in combustion temperature to 114 ° C, which leads to lower NO concentrations in flue gas to 552 million ^-1 or to reduce emissions of nitrogen oxide at 44 vol.%.

Example 4

The source natural gas with a flow rate of 7330 m ³ / h is compressed by a compressor to a pressure of 7.0 MPa, heated to a temperature of 450 ° C and oxidized in three reactors installed in series with a total supply of 4048 m ³ / h of air with an oxygen content of 3.9 in each of the reactors vol.% with respect to the source of natural gas, and the resulting gas after the separation of 496 kg / h of liquid oxidation products (oxidate) is fed into the burner of the combustion chamber, and 125 kg / h of the separated liquid oxidate, containing mainly methanol and water, for further reducing azo oxide emissions and introduced into the waste combustion gases at a temperature of 490 ° C followed by trapping of the resulting nitrogen dioxide. This achieves reduction in total NO concentrations in flue gas to 110 million ^-1 or reduce emissions of nitrogen oxide at 89 vol.%, I.e. almost 10 times.

Table 3 Parameter Example 1 ^*) Example 2 ^**) Example 3 ^**) Example 4 ^**) one 2 3 four 5 Consumption of natural or associated gas, m ³ / h 14000 40234 7330 7330 Natural or associated gas pressure, MPa 5,0 8.0 7.0 7.0 Natural or associated gas temperature, ° С 420 450 450 450 Supply air consumption, m ³ / h 5300 11862 4048 4048 The oxygen content in the reactors relative to natural or associated gas, vol.% 8.0 2.0 3.9 3.9

Table 3 (end) one 2 3 four 5 The proportion of oxidized natural or associated gas, vol.% 3.8 5.5 11.3 11.3 Partially oxidized natural or associated gas supplied to the burner of the combustion chamber, m ³ / h 18417 47707 ^***) 10110 ^***) 10110 ^***) Composition of partially oxidized natural or associated gas, vol.% Methane and other hydrocarbons 69.0 76,4 62.6 62.6 Nitrogen 25.0 20.6 32,3 32,3 With 3,1 2.0 3.2 3.2 CO ₂ 0.9 1,0 1,6 1,6 Other 2.0 - 0.3 0.3 The heat of combustion supplied to the combustion of natural or associated gas, kcal / m ³ - 6540 5420 5420 The emitted liquid oxidation products, including (kg / h): - 2780 496 496 Methanol - 1,140 196 196 Formaldehyde - 106 18.5 18.5 Ethanol - 22 5.5 5.5 Estimated combustion temperature, ° С 1700 1716 1664 1664 Achievable reduction in combustion temperature AT, ° C 78 62 114 114 Estimated concentration of N0 in flue gases, million ' ¹ 670 728 552 110 ^****) Achievable decrease in the concentration of NO, vol.% 32 26 44 89 ^*) Composition of associated gas, vol.%: CH ₄ - 80,0; C ₂ H ₆ - 14.0%; C ₃ H ₈ - 3.0; N ₂ - 3.0. ^**) Composition of natural gas, vol.%: СН ₄ - 98,0; N ₂ , CO ₂ - 2.0. ^***} After separation of the liquid oxidation products (oxidate). ^****) After the introduction of liquid oxidate into the exhaust products of combustion (flue gases).

Claims (3)

1. A method of reducing the emission of nitrogen oxides by power plants using natural or associated gas by lowering the combustion temperature by supplying a non-combustible gas to the combustion zone, characterized in that the natural or associated gas is partially compressed to a pressure of at least 3 MPa and partially heated at a temperature of at least 300 ° C. they are oxidized with air at an oxygen content of 2 to 8 vol.% with respect to the source natural or associated gas, and partially oxidized and containing non-combustible gas is supplied to the combustion zone — air nitrogen, natural or associated gas. 2. The method according to claim 1, characterized in that the natural or associated gas is preferably compressed to a pressure of from 5 to 8 MPa, heated to a temperature of from 420 to 450 ° C and oxidized with air in one or more successive reactors with an oxygen content in each reactor from 2 to 4 vol.% in relation to the source of natural or associated gas to an air nitrogen content of from 20 to 35 vol.%. 3. The method according to any one of claims 1 and 2, characterized in that prior to combustion, liquid oxidation products (oxidate) are isolated from the partially oxidized natural or associated gas and introduced into the exhaust products of combustion at a temperature of at least 430 ° C, followed by capture of the resulting nitrogen dioxide.

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