CN107570004A - A Method for Reducing NO Emission in Boiler Flue Gas - Google Patents

Abstract

A kind of method of NOx emission in reduction boiler smoke, by setting catalyst metals heating surface in 750 DEG C~950 DEG C cigarette warm areas, under catalyst action, NO is reduced directly using CO caused by fuel combustion.It is required that CO and NO molar concentration rate is more than 1.2 in flue gas, catalyst metals heating surface outside wall surface temperature is not less than 595 DEG C, and residence times of the reacting gas CO and NO on catalyst metals heating surface surface is not less than 0.17 second.If CO concentration deficiency caused by burning, gaseous state or liquid hydrocarbon are sprayed into flue to being coated with before the heating surface of nanocatalyst, its imperfect combustion produces CO so that total CO and NO molar concentration rate are not less than 1.2.By the present invention program, NO concentration in flue gas can be made to reduce by more than 75%, reduce environmental pollution.

Description

A Method for Reducing NO Emission in Boiler Flue Gas

technical field

The invention belongs to the technical field of boiler flue gas pollutant control, in particular to a low-cost method for reducing boiler flue gas NOx emissions.

Background technique

NOx is one of the important culprits causing environmental problems such as acid rain and smog. In 2015, China's nitrogen oxide emissions reached 18.51 million tons, the main source of which was the combustion process of various fuels, especially emissions from power stations and industrial boilers. According to the latest revised "Emission Standards of Air Pollutants for Thermal Power Plants" (GB13223-2011), the NOx emission requirement for new boilers is <100mg/m ³ , which is the most stringent environmental protection standard in the world. The ultra-low emission standard proposed in recent years requires NOx emission to be less than 50mg/m ³ .

At present, the methods for controlling NOx emissions in flue gas mainly include low-nitrogen combustion technology and flue gas denitrification. Common low-nitrogen combustion technologies include low excess air combustion (LEA, also known as low-oxygen combustion), concentration deviation, and air staging. These technologies generally require the redesign and modification of traditional boilers, which are costly and may have a certain impact on combustion efficiency. For example, the low excess air combustion technology may increase fly ash combustibles during actual operation, reduce combustion efficiency, and cause side effects such as coking in the furnace.

The more mature flue gas denitrification methods include selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). SNCR refers to the use of ammonia water, urea solution, etc. as reducing agents at high temperatures, without the use of catalysts to reduce NOx to nitrogen and water. This technology has low cost, but the denitrification efficiency is low, and the thermal efficiency of the boiler is reduced to a certain extent. Compared with SNCR, SCR uses catalysts to catalytically reduce NOx under medium and low temperature conditions. Its denitrification efficiency is higher, but the initial investment and operation costs are higher, the floor area is large, and the catalyst needs to be replaced regularly. In addition, both SNCR and SCR use ammonia as a reducing agent. In order to achieve high denitrification efficiency, excessive input of ammonia is necessary, so a small amount of NH ₃ escapes. The escaping NH ₃ combines with the SO ₃ produced by combustion to form NH ₄ HSO ₄ or (NH ₄ ) ₂ SO ₄ . When the temperature is lower than a certain value, it condenses on the surface of the air preheater, blocks the air preheater, and air preheater If it is not synthesized with SO ₃ , it will be directly discharged into the atmosphere, causing smog.

Contents of the invention

In order to overcome the deficiencies in the prior art, the present invention proposes a method for reducing NOx emission in boiler flue gas, which can not only reduce the NO content in the flue gas, reduce the environmental pollution caused by NO, but also have low cost.

To achieve the above object, the technical scheme adopted in the present invention is as follows:

1) Set the catalytic metal heating surface in the smoke temperature range of 750°C-950°C. The heating surface is composed of a metal tube and a nano-particle catalytic active coating solidified on the outer surface of the metal tube, so that the flue gas containing reaction gases CO and NO The residence time on the surface of the catalyst metal heating surface is at least 0.17 seconds, and the temperature of the outer wall surface of the catalyst metal heating surface is at least 595°C, and the CO produced by combustion is used to directly reduce NO;

2) Control CO concentration and NO concentration during the combustion process, so that the molar concentration ratio of CO and NO is at least 1.2.

Further, preferably, when the molar concentration ratio of CO and NO is lower than 1.2, use a spray gun to spray gaseous or liquid hydrocarbons into the flue in front of the heated catalyst metal, (the general chemical formula is C _m H _n , its incomplete combustion produce CO), so that the molar concentration ratio of the total CO to NO is not lower than 1.2.

Further, preferably, when the residence time of the flue gas on the surface of the catalyst metal heating surface is lower than 0.17 seconds, the distance between the tubes on the catalyst metal heating surface is adjusted, or a flue gas baffle is set in the flue in front of the catalyst metal heating surface, for The flue gas flow rate is adjusted so that the residence time of the flue gas containing reaction gases CO and NO on the surface of the metal heating surface of the catalyst is not less than 0.17 seconds.

Preferably, the nanoparticle catalytically active coating has a thickness of 0.3-1.2 mm.

The nanoparticle catalytically active coating of the present invention can adopt tungsten-nickel-iron composite oxide, and the tungsten, nickel and iron elements in the tungsten-nickel-iron composite oxide all exist in the form of oxide, and the molar ratio of tungsten, nickel and iron element is 1:20~32:67~79. The preparation method of the catalyst metal heating surface comprises the following steps:

1) The tungsten-nickel-iron composite oxide powder is solidified on the outer surface of the metal tube by spraying to form a layer of nanoparticle catalytic active coating, in which tungsten, nickel and iron elements all exist in the form of oxides; tungsten, The molar ratio of nickel and iron is 1:20-32:67-79;

2) The metal tube solidified with the catalytically active coating of nanoparticles is subjected to heat treatment in a dry flue gas atmosphere containing _O2 and _CO2 , and the heat treatment temperature is 660°C-688°C to obtain the catalyst metal heating surface.

Preferably, in the dry flue gas atmosphere in the above step 2), the O ₂ concentration is 6.4%-8.8%, the CO ₂ concentration is 10.2%-12.4%, and the heat treatment time is 32-45 minutes.

Preferably, the nanoparticle catalytically active coating can also use iron-based nickel-manganese composite oxides, and nickel, manganese and iron elements in the iron-based nickel-manganese composite oxides all exist in the form of oxides, wherein nickel and manganese The molar ratio is 1:2-2:1, and the total molar content of nickel and manganese is 5-12.7%. The preparation method of the catalyst metal heating surface comprises the following steps:

1) The iron-based nickel-manganese composite oxide powder is solidified on the outer surface of the metal tube by high-temperature spraying to form a layer of nanoparticle catalytic active coating, wherein nickel, manganese and iron elements all exist in the form of oxides; The molar ratio of nickel to manganese is 1:2-2:1, and the total molar content of nickel and manganese is 5-12.7%;

2) heat-treating the metal tube solidified with the nanoparticle catalytic active coating in an oxygen-containing atmosphere at a treatment temperature of 680° C. to 695° C. to obtain the catalyst metal heating surface.

Preferably, the oxygen content of the oxygen-containing atmosphere in the above step 2) is 6%-8%, and the heat treatment time is 20-24 minutes.

Compared with the prior art, the present invention has the following advantages and outstanding technical effects: ① can reduce the NO concentration in the flue gas by more than 75%, reducing the emission of NOx to the environment; ② has almost no influence on the combustion process in the furnace, It overcomes the reduction of boiler combustion efficiency caused by existing low-nitrogen combustion technologies such as low-oxygen combustion and air classification; ③ No reducing agents such as ammonia water and urea are used, and there is no "ammonia escape" problem compared with SNCR or SCR; ④ It is only necessary to spray catalyst on the outer surface of some heating surfaces, or add a reducing agent spray gun if necessary, and the initial investment and operating costs are low; The concentration of CO emitted in the environment is reduced.

Description of drawings

Fig. 1 is a structural schematic diagram of a boiler combustion equipment.

In the figure: 1- furnace; 2- catalytic metal heating surface; 3- reducing agent spray gun; 4- flue gas baffle.

Fig. 2 is an effect diagram of two embodiments of the present invention.

detailed description

The technical solutions in the present invention will be further described and explained below in conjunction with the accompanying drawings and specific embodiments.

The invention provides a method for reducing NO emissions in boiler flue gas, the method comprising the steps of:

1) Set the catalytic metal heating surface in the smoke temperature range of 750°C-950°C. The heating surface is composed of a metal tube and a nano-particle catalytic active coating solidified on the outer surface of the metal tube, so that the flue gas containing reaction gases CO and NO The residence time on the surface of the catalyst metal heating surface is at least 0.17 seconds, and the temperature of the outer wall surface of the catalyst metal heating surface is at least 595°C, and the CO produced by combustion is used to directly reduce NO;

2) Control CO concentration and NO concentration during the combustion process, so that the molar concentration ratio of CO and NO is at least 1.2.

The principle of the present invention to reduce NO emission in boiler flue gas is: under the catalytic action of a specific nano-catalyst, NO can be reduced by CO in the temperature range of 750°C to 950°C:

Fig. 1 is a structural schematic diagram of a boiler combustion equipment for reducing flue gas NO emission by applying the content of the present invention, which includes 1-furnace, catalyst metal heating surface 2, reducing agent spray gun 3 and flue gas baffle 4.

During the combustion process of fuel in the furnace, especially when the oxygen content is insufficient, part of the reducing CO gas will be generated by incomplete combustion. The catalytic metal heating surface 2 is arranged in an area where the smoke temperature is 750°C to 950°C and the temperature of the outer wall surface of the metal heating surface tube is not lower than 595°C. When the flue gas containing CO and NO flows over the metal heating surface of these catalysts, the catalytic reduction reaction shown in formula (1) will occur, thereby effectively reducing the emission of NO in the flue gas; specific technical measures include:

The molar concentration ratio of CO and NO in the flue gas on the heating surface of the catalyst metal is above 1.2. When the original CO concentration in the flue gas is insufficient, a reducing agent spray gun 3 is installed in front of the heating surface of the catalyst metal, and the chemical form of C _m can be sprayed into the flue. The gaseous or liquid hydrocarbons of _Hn , these external reducing agents undergo the following incomplete combustion reactions in the oxygen-deficient flue gas:

This reaction can supplement the CO content in the flue gas to meet the requirement that the molar concentration ratio of total CO and NO is not lower than 1.2.

When the residence time of the flue gas on the surface of the catalyst metal heating surface is less than 0.17 seconds, adjust the distance between the tubes on the catalyst metal heating surface, or set a flue gas baffle 4 in the flue in front of the catalyst metal heating surface to adjust the flow of flue gas To meet the above requirement that the residence time of flue gas containing reaction gases CO and NO on the surface of the catalytic metal heating surface is not less than 0.17 seconds.

On the heating surface of the catalyst metal, the thickness of the nanoparticle catalytically active coating is generally 0.3-1.2 mm. Two kinds of catalyst metal heating surfaces and their preparation methods are as follows:

The first nanoparticle catalytically active coating uses tungsten-nickel-iron composite oxides, in which tungsten (W), nickel (Ni) and iron (Fe) elements all exist in the form of oxides (WO ₃ , NiO, Fe ₂ O ₃ ), the molar ratio of tungsten, nickel and iron is 1:20-32:67-79, and the preparation method of the catalyst metal heating surface includes the following steps:

1) The tungsten-nickel-iron composite oxide powder is solidified on the outer surface of the metal tube by spraying to form a layer of nanoparticle catalytic active coating, in which tungsten, nickel and iron elements all exist in the form of oxides; tungsten, The molar ratio of nickel and iron is 1:20-32:67-79;

2) The metal tube solidified with the nanoparticle catalytic active coating is heat-treated in a dry flue gas atmosphere with an _O2 concentration of 6.4% to 8.8% and a _CO2 concentration of 10.2% to 12.4%. The heat treatment temperature is 660 ° C to 688 °C, the heat treatment time is preferably 32-45 minutes, and the catalyst metal heating surface can be prepared.

The second nanoparticle catalytic active coating adopts iron-based nickel-manganese composite oxides, and nickel (Ni), manganese (Mn) and iron (Fe) elements in the iron-based nickel-manganese composite oxides all exist in the form of oxides ( NiO, MnO, Fe ₂ O ₃ ), wherein the molar ratio of nickel to manganese is 1:2-2:1, the total molar content of nickel-manganese is 5-12.7%, the preparation method of the catalyst metal heating surface comprises the following steps:

1) The iron-based nickel-manganese composite oxide powder is solidified on the outer surface of the metal tube by high-temperature spraying to form a layer of nanoparticle catalytic active coating, wherein nickel, manganese and iron elements all exist in the form of oxides; The molar ratio of nickel to manganese is 1:2 to 2:1, and the total molar content of nickel and manganese is 5 to 12.7%; the high temperature spraying method can adopt supersonic flame spraying or high temperature and low pressure plasma spraying process, etc. A solid nanoparticle catalytically active coating is formed on the outer surface of the tube;

2) heat-treating the metal tube solidified with the catalytically active coating of nanoparticles in an oxygen-containing atmosphere at a temperature of 680° C. to 695° C. to obtain the catalyst metal heating surface. The oxygen content of the oxygen-containing atmosphere is 6%-8%, the heat treatment time is preferably 20-24 minutes, and the catalyst metal heating surface can be prepared.

Example: Utilize the catalytic metal heating surface to catalyze the reduction of NO by CO under the required environment, and reduce the emission of NO in the flue gas:

Embodiment 1: The tungsten-nickel-iron composite oxide nano-catalyst is sprayed on the outer surface of a metal heating surface pipe with a diameter of 38 mm and a length of 300 mm by plasma spraying to form a layer of nano-particle catalytic active material coating with a thickness of 0.8 to 1.1 mm; the molar ratio of tungsten, nickel and iron is 1:25:73, and they all exist in the form of oxides; the metal heating surface tube sprayed with nano-catalysts is placed in an _O2 concentration of 7.1%, and a _CO2 concentration of Heat treatment at 670°C for 36 minutes in an atmosphere of 11.2% dry flue gas.

Embodiment 2: The iron-based nickel-manganese composite oxide catalyst is sprayed on the outer surface of a metal heating surface pipe with a diameter of 38 mm and a length of 300 mm by a plasma spraying process to form a layer of nanoparticle catalytic active material coating with a thickness of 0.8 to 1.1 mm. mm; where the nickel-manganese molar ratio is 1:1.1, the total molar content of nickel-manganese is 8.4%, and nickel, manganese and iron all exist in the form of oxides; the metal heating surface tubes sprayed with nano-catalysts are placed in O ₂ Heat treatment at 690°C for 22 minutes in an atmosphere with a concentration of 7.1%.

The above two catalyst metal heating surface tubes, fly ash and quartz sand are respectively placed in a quartz glass tube reactor and heated by an electric heating furnace, wherein the fly ash and quartz sand exist in the form of a fixed bed. As shown in Figure 2, in the temperature range of 750°C to 950°C, when the CO-NO molar concentration ratio is 1.2 and the flue gas residence time is 0.17 seconds, compared with the common solid particles in the furnace such as fly ash and quartz sand , the catalytic activity of the metal heating surface tube sprayed with two nano-catalysts for the CO-NO reaction was significantly enhanced. For example, at 900°C, under the catalysis of iron-based nickel-manganese composite oxide nano-catalysts, the NO reduction conversion rate reaches 97%; under the catalysis of tungsten-nickel-iron composite oxide nano-catalysts, the NO reduction conversion rate is close to 75%.

Claims (10)

1. A method for reducing NOx emissions in boiler flue gas, characterized in that the method may further comprise the steps: 1) Set the catalytic metal heating surface in the smoke temperature range of 750°C-950°C. The heating surface is composed of a metal tube and a nano-particle catalytic active coating solidified on the outer surface of the metal tube, so that the flue gas containing reaction gases CO and NO The residence time on the surface of the catalyst metal heating surface is at least 0.17 seconds, and the temperature of the outer wall surface of the catalyst metal heating surface is at least 595°C, and the CO produced by combustion is used to directly reduce NO; 2) Control CO concentration and NO concentration during the combustion process, so that the molar concentration ratio of CO and NO is at least 1.2. 2. A method for reducing NOx emissions in boiler flue gas according to claim 1, characterized in that: when the molar concentration ratio of CO and NO is lower than 1.2, utilize a spray gun to inject gaseous state into the flue in front of the heated catalyst metal or liquid hydrocarbons such that the total molar concentration ratio of CO to NO is not less than 1.2. 3. A method for reducing NOx emissions in boiler flue gas according to claim 1, characterized in that: when the residence time of the flue gas on the surface of the catalyst metal heating surface is lower than 0.17 seconds, the distance between the tubes of the catalyst metal heating surface is adjusted. The distance between them, or the flue gas baffle is set in the flue in front of the catalyst metal heating surface, which is used to adjust the flue gas flow, so that the residence time of the flue gas containing reaction gases CO and NO on the surface of the catalyst metal heating surface is not less than 0.17 seconds. 4. A method for reducing NO emissions in boiler flue gas according to claim 1, characterized in that: the nanoparticle catalytically active coating has a thickness of 0.3-1.2mm. 5. A method for reducing NOx emissions in boiler flue gas according to any one of claims 1-4, characterized in that: the nanoparticle catalytically active coating adopts tungsten-nickel-iron composite oxide, tungsten-nickel-iron The tungsten, nickel and iron elements in the composite oxide all exist in the form of oxides, and the molar ratio of tungsten, nickel and iron elements is 1:20-32:67-79. 6. A method for reducing NO emissions in boiler flue gas according to claim 5, characterized in that: the preparation method of the catalyst metal heating surface comprises the steps: 1) The tungsten-nickel-iron composite oxide powder is solidified on the outer surface of the metal tube by spraying to form a layer of nanoparticle catalytic active coating, in which tungsten, nickel and iron elements all exist in the form of oxides; tungsten, The molar ratio of nickel and iron is 1:20-32:67-79; 2) The metal tube solidified with the catalytically active coating of nanoparticles is subjected to heat treatment in a dry flue gas atmosphere containing _O2 and _CO2 , and the heat treatment temperature is 660°C-688°C to obtain the catalyst metal heating surface. 7. A method for preparing a catalyst metal heating surface for reducing NOx content in flue gas according to claim 6, characterized in that: in the dry flue gas atmosphere in step 2), the _O2 concentration is 6.4% to 8.8% %, the _CO2 concentration is 10.2% to 12.4%, and the heat treatment time is 32 to 45 minutes. 8. A method for reducing NOx emissions in boiler flue gas according to any one of claims 1-4, characterized in that: the nanoparticle catalytically active coating adopts iron-based nickel-manganese composite oxide, iron-based The nickel, manganese and iron elements in the nickel-manganese composite oxide all exist in the form of oxides, wherein the molar ratio of nickel to manganese is 1:2-2:1, and the total molar content of nickel-manganese is 5-12.7%. 9. A method for reducing NOx emissions in boiler flue gas according to claim 8, characterized in that: the preparation method of the catalyst metal heating surface comprises the steps: 1) The iron-based nickel-manganese composite oxide powder is solidified on the outer surface of the metal tube by high-temperature spraying to form a layer of nanoparticle catalytic active coating, wherein nickel, manganese and iron elements all exist in the form of oxides; The molar ratio of nickel to manganese is 1:2-2:1, and the total molar content of nickel and manganese is 5-12.7%; 2) heat-treating the metal tube solidified with the nanoparticle catalytic active coating in an oxygen-containing atmosphere at a treatment temperature of 680° C. to 695° C. to obtain the catalyst metal heating surface. 10. A method for reducing NOx emissions in boiler flue gas according to claim 9, characterized in that: the oxygen content of the oxygen-containing atmosphere in step 2) is 6% to 8%, and the heat treatment time is 20 to 24 minute.