WO2006091120A1 - Method for reducing a nitrogen oxide emitted by power plans using a natural or associated gas - Google Patents

Abstract

The inventive method for reducing a nitrogen oxide emitted by power plants using a natural or associated gas consists in reducing a combustion temperature by supplying an incombustible gas to a burning area. The supply of an incombustible gas consists in partially pre-oxidising a natural or associated gas by compressing said gas to a pressure equal to or higher than 3MPa, in heating the gas to a temperature equal to or greater than 300 °C, in supplying the air whose oxygen volume content ranges from 2 to 8 % with respect to the initial natural or associated gas content and in subsequently supplying said partly oxidised natural or associated gas, which contains the mixture of incombustible gas and nitrogen from air, to the burning area. In order to additionally reduce the nitrogen oxide emission, the inventive method also consists in extracting liquid oxidation products ( oxidate, substantially containing methanol or water) from the partly oxidised natural or associated gas, in introducing said products into exhaust combustion products at a temperature equal to or greater than 430 °C and, subsequently, in collecting the thus formed nitrogen oxide.

Description

 METHOD FOR REDUCING THE EMISSION OF NITROGEN OXIDES BY POWER PLANTS ON NATURAL OR ASSOCIATED GAS

Technical field

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 operating on natural or associated gas, and can be used in industrial energy.

BACKGROUND OF THE INVENTION Nitrogen oxides are among the major air pollutants in industrialized countries, having 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, energy accounts for up to 30% of their emissions. In the total volume of harmful emissions from large thermal power plants operating on natural gas, their share reaches 20% (Khodakov Yu.C., “Nitrogen oxides and heat power engineering: problems and solutions”, Moscow, ECT-M LLC, 2001, 432 s. ) 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 natural gas-fired power plants, 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. Increasing the temperature from 1500 ⁰ C to 2100 ⁰ C leads to a tenfold

SUBSTITUTE SHEET (RULE 26) an increase in the concentration of NO. Appearing in noticeable quantities only at temperatures above 1300 ⁰ C _{5, 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 combustion temperature (Druskin LI, "Effective Use of Natural Gas in Industrial Installations," Directory, Moscow, 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%. At the same time, 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 basin during fuel combustion", Leningrad, "Nedra", 1977, 295 s )

However, the known 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.

There is a method of reducing the formation of NO during combustion by diluting combustible gases with inert or non-reactive gases, for example, specially added argon (Samapiegro J.-M., Lagorer B., Eggolos FN, Daranto M., Sautet J.-C, Charop. "27th Sumrosium (Ipteroptiopl) on Combustiop", Combiop Ipstitute, 1998, p. 1385-1395). It was shown that the yield of NO decreases very much 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 introduced into the gas supplied for combustion, argon in a concentration of 40% vol.

SUBSTITUTE SHEET (RULE 26) NO concentration in the flue gas is reduced to 480 million ^'1 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 43O ⁰ C to 83O ⁰ C to oxidize nitric oxide to nitrogen dioxide, followed by trapping nitrogen dioxide in a scrubber (Zamapsku VM, But L ., MaIy R.M. et al, "Ohhidiopof NO to NO ₂ bu hudrope repohide apd its mihture with metapol ip patal gas apd coalustiop gasess", Combust. Ssi. Tim. .

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 the recirculation smoke exhauster 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 ⁰ C, and the formation of nitric oxide is reduced by two or three times (Druskin JI. And .. "Effective use of natural gas in industrial plants", Directory, Moscow, Energoatomizdat, 1992) or (Sigal I.Ya., "Protection of the air basin during fuel combustion", Leningrad, "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 known that production of methanol by oxidation of compressed and heated natural gas by air at an initial temperature of up to 500 ⁰ C _{5 with a} pressure of up to 10 MPa and an oxygen content of not more than 8% vol. followed by cooling the reaction mixture and the separation of methanol from the cooled gas-liquid mixture (RU 2162460, Cl, C07C31 / 04, 01.27.2000). The resulting crude methanol can be used as

SUBSTITUTE SHEET (RULE 26) 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 by partially oxidizing 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 ⁰ C, natural or associated gas is partially oxidized by air with an oxygen content of 2% to 8% vol. in relation to the source of 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.

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 ⁰ C, natural or associated gas is partially oxidized by air with an oxygen content of 2% to 8% vol. in relation to the source of 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 42O ⁰ C to 45O ⁰ 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 the content of nitrogen in it from 20% to 35% vol.

An additional difference also lies in the fact that liquid oxidation products (an 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 combustion exhaust products at a temperature of at least 43O ⁰ C followed by capture of the resulting nitrogen dioxide.

SUBSTITUTE SHEET (RULE 26) 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

In the future, the invention is illustrated by specific examples of its implementation and the attached drawing, in which: Fig. L shows the calculated dependence of the final temperature and the concentration of NO in adiabatic combustion products of a stoichiometric mixture with air of a preliminarily partially oxidized air of natural gas on the concentration of non-combustible gas - nitrogen, introduced by partial oxidation.

BEST MODE FOR CARRYING OUT THE INVENTION The results of studies conducted during the development of this method are shown in tables 1 and 2 and in FIG. 1. In FIG. 1. and table 1 presents 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 the 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%.

SUBSTITUTE SHEET (RULE 26) Table 1

Adiabatic calculation of the equilibrium temperature and NO concentration during combustion of a stoichiometric 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 volumetric

In real power plants, the combustion temperature is approximately 200 ⁰ C lower than the adiabatic combustion temperature. Calculation of the temperature and the 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. The calculated temperature of the products of combustion decreased by about 175 ⁰ C.

A partial oxidation of a mixture of air until the nitrogen concentration in the combustion gas supplied to 30% relative to the total concentration therein CH ₄ + N _2, NO concentration decreased with a real value 990 ^{million, "1} to about 550 ^{million," 1,} ie . almost 50%. When oxidized with air until the nitrogen concentration in the gas supplied for combustion reaches 56% relative to the total concentration of CH ₄ + N _{2 in it} , the NO concentration decreased to -190 ppm ^{, 1} ie by 80% or five times. Thus, dilution of a combustible gas with nitrogen by partially oxidizing it makes it possible to significantly reduce NO emissions.The data in Table 2 can be used to calculate the reduction of nitric oxide emissions by partially oxidizing natural or associated gas with air.

SUBSTITUTE SHEET (RULE 26) Table 2.

Adiabatic calculation of the dependence of the equilibrium temperature and the NO concentration during 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 .

Examples of various embodiments of the proposed method are presented in table 3. Example 1.

The source gas at a flow rate of 14,000 m ³ / h is compressed by a compressor to a pressure of 5.0 MPa, heated to a temperature of 42O ⁰ C and oxidized in a reactor by supplying 5,300 m ³ / h of air with an oxygen content of 8% vol. in relation to the source of associated gas, and the resulting gas containing 25% vol. nitrogen, without the allocation of raw methanol is fed into the burner of the combustion chamber. In this case, a decrease in the combustion temperature by 78 ⁰ C is achieved, which leads to a decrease in the concentration of NO in flue gases to 670 ppm ^, or to a decrease in the emission of nitric oxide by 32% by volume. 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 45O ⁰ C and oxidized in three reactors in series with a total supply of 11,862 m ³ / h of air with an oxygen content in each of the reactors 2 , 0% vol. in relation to the source of natural gas, and the resulting gas containing 20.6% vol. nitrogen, after the allocation of 2780 kg / h of liquid oxidation products, is fed into the burner of the combustion chamber. Wherein

SUBSTITUTE SHEET (RULE 26) a reduction of the combustion temperature by 62 ⁰ C is achieved, which leads to a decrease in the concentration of NO in flue gases to 728 ppm ^, or to a reduction in the emission of nitric oxide by 26% vol.

Example 3

The source natural gas with a flow rate of 7,330 m ³ / h is compressed by a compressor to a pressure of 7.0 MPa, heated to a temperature of 45O ⁰ C and oxidized in three reactors installed in series with a total supply of 4,048 m ³ / h of air with an oxygen content in each of the reactors 3 , 9% vol. in relation to the source of natural gas, and the resulting gas, after the separation of 496 kg / h of liquid oxidation products, is fed into the burner of the combustion chamber. In this case, a reduction in the combustion temperature by 114 ⁰ C is achieved, which leads to a decrease in the concentration of NO in flue gases to 552 ppm ^, or to a reduction in the emission of nitric oxide by 44% by volume. Example 4.

The source natural gas with a flow rate of 7,330 m ³ / h is compressed by a compressor to a pressure of 7.0 MPa, heated to a temperature of 45O ⁰ C and oxidized in three reactors installed in series with a total supply of 4,048 m ³ / h of air with an oxygen content in each of the reactors 3 , 9% vol. in relation to the source natural gas, and the resulting gas, after separation of 496 kg / h of liquid oxidation products (oxidate) is fed to the burner of the combustion chamber, and 125 kg / h of separated liquid oxidate, containing mainly methanol and water, to further reduce nitric oxide emissions are introduced into the exhaust products of combustion at a temperature of 49O ⁰ C, followed by capture of the resulting nitrogen dioxide. In this case, a general decrease in the concentration of NO in flue gases to UO ppm ^"1 or a decrease in the emission of nitric oxide by 89% vol., I.e., by almost 10 times, is achieved.

Table 3.

Состав попутного газа, % об.: CH₄ - 80,0; C₂H₆ - 14,0%; C₃H₈ - 3,0; N₂ - 3,0. *^*} Состав природного газа, % об.: CH₄ - 98,0; N₂, CO₂ - 2,0.SUBSTITUTE SHEET (RULE 26)

Associated gas composition,% 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 .: CH ₄ - 98.0; N ₂ , CO ₂ - 2.0.

^ After separation of liquid oxidation products (oxidate). * ^***) JJ _{00J16 introducing LI of} any oxidate into the exhaust products of combustion (flue gases).

SUBSTITUTE SHEET (RULE 26)

Claims

Claim 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 pre-compressed to a pressure of at least 3 MPa and partially heated to a temperature of at least 300 ⁰ C oxidize with air at an oxygen content of 2% to 8% vol. in relation to the source of natural or associated gas, and partially oxidized and containing non-combustible gas - air nitrogen, natural or associated gas is fed into the combustion zone. 2. The method according to p. 1, characterized in that the natural or associated gas is preferably compressed to a pressure of 5 MPa to 8 MPa, heated to a temperature of 42O ⁰ C to 45O ⁰ 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 a nitrogen content of air from 20% to 35% vol. 3. The method according to any one of claims 1 to 2, characterized in that before combustion from the partially oxidized natural or associated gas, liquid oxidation products (oxidate) are isolated and introduced into the exhaust products of combustion at a temperature of at least 43O ⁰ C, followed by capture of the resulting nitrogen dioxide. SUBSTITUTE SHEET (RULE 26)