CN103100294A - Method for removing oxynitride from flue gas through ozone oxidation method - Google Patents

Abstract

The invention discloses a method for removing oxynitride from flue gas through an ozone oxidation method. The method comprises the following steps of: cooling the flue gas to 30-110 DEG C; introducing ozone into the cooled flue gas to react for 0 to 10 seconds; introducing the gas after the reaction into an absorbing tower; and spraying alkali liquor into the absorbing tower to absorb the gas after the reaction, wherein the gas after absorption can be discharged into air or put into other devices, and the alkali liquor after the absorption is reused. By adopting the method, the oxynitride in the flue gas can be efficiently and highly-selectively removed, so that the method can be used on industrial devices generating a lot of flue gas, such as the industrial devices of coal-fired power plants, cement plants, oil refineries and steel works.

Description

Ozone Oxidation Method for Removing Nitrogen Oxides in Flue Gas

technical field

The invention belongs to the technical field of environmental protection, and in particular relates to a method for removing nitrogen oxides in flue gas by an ozone oxidation method.

Background technique

As we all know, environmental protection has become a hot issue in the world today, and the level of environmental quality control has become a measure of a country's development. As a kind of polluting gas, the content of NOx in the air is increasing year by year, which is mainly due to the exhaust gas emitted by stationary sources such as coal-fired power plants and catalytic cracking (FCC) units in refineries and chemical plants, as well as mobile sources such as automobiles and trains. . Among them, NO is easy to form acid rain and photochemical smog under the action of sunlight, which seriously harms the human respiratory system; while N ₂ O can strongly absorb infrared radiation and contribute to the strengthening of the greenhouse effect. Therefore, eliminating NOx is of great significance to environmental protection. Many countries have established strict NOx emission standards.

At present, scholars at home and abroad have done a lot of research work in the field of flue gas denitrification technology. At present, the measures to control NOx emissions are roughly divided into two categories, one is flue gas purification technology, which removes NOx in flue gas; the other is Low NOx combustion technology, through various technical means, suppresses or reduces NOx in the combustion process to achieve the purpose of reducing NOx emissions. Wet denitrification technologies include selective catalytic reduction (Selective Catalytic Reduction, referred to as SCR) method, selective non-catalytic reduction (Selective Non-Catalytic Reduction, referred to as SNCR) method, wet complexation absorption method, etc. SNCR uses NH ₃ , urea and other reducing agents to spray into the furnace for selective reaction with NO _x without catalyst, so reducing agents must be added in the high temperature zone. The reducing agent is sprayed into the furnace at a temperature between 900°C and 1200°C. The reducing agent is rapidly thermally decomposed into NH ₃ and undergoes SNCR reaction with NO _x in the flue gas to generate N ₂ . The characteristic of this method is that no catalyst is needed, the amount of old equipment modification is small, and the investment is smaller than that of the SCR method, but the consumption of ammonia liquid is more than that of the SCR method. The SCR method refers to the selective catalytic reduction process of nitrogen oxides (NOx), that is, under the action of a catalyst, NOx can be converted into nitrogen (N ₂ ) and water by adding ammonia (NH ₃ ), because NH ₃ can "select The reaction is said to be "selective" by reacting with NOx rather than being oxidized by oxygen (O ₂ ). The reaction can be carried out between 280-450°C. The advantages of this method are: low reaction temperature, high purification rate, compact process equipment, reliable operation, and the reduced nitrogen is vented without secondary pollution. But there are also some obvious disadvantages: the composition of the flue gas is complex, and some pollutants can poison the catalyst; highly dispersed dust particles can cover the surface of the catalyst, reducing its activity; the investment and operating costs are high, and NH ₃ is easy to leak. Handling and storage are difficult, and easy to form (NH ₄ ) ₂ SO ₄ .

Chinese patent CN1923341A discloses a coal-fired boiler flue gas ozone oxidation simultaneous desulfurization and denitrification device and its method. The method comprises the following steps: 1) spraying ozone O ₃ into the low temperature section of 110-150°C before or after the electrostatic precipitator of the boiler flue, and the molar ratio of the injected ozone to the nitrogen oxide in the boiler flue gas is 0.5-1.5 , to oxidize the water-insoluble low-valence nitrogen oxides in the boiler flue gas into water-soluble high-valence nitrogen oxides, and oxidize sulfur dioxide to form sulfur trioxide. The reaction time is at least 0.5 seconds; 2) After the previous step The treated boiler flue gas is sent to the lye scrubber, where the flue gas is washed and the high-valence nitrogen oxides and sulfur oxides in the flue gas are absorbed. However, ozone is completely decomposed into oxygen in about 3 minutes during the day at normal temperature and pressure, and the decomposition is very violent when the temperature reaches 100 ° C. However, the technology points out that the reaction temperature is between 110 and 150 ° C. At this temperature, ozone instantly turns into oxygen. It will no longer have strong oxidizing properties before reacting with nitrogen oxide compounds, so it is difficult for this technology to achieve good results in practical applications.

Chinese patent CN101485957A discloses a coal-fired boiler flue gas treatment technology, aiming to provide a device and method for simultaneous desulfurization and denitrification of coal-fired boiler flue gas by ozone oxidation combined with double-tower washing. The device includes a flue, and the flue is connected with the heat exchanger, the lye washing tower A, and the lye washing tower B in turn; the lye washing tower A and the lye washing tower B are respectively provided with a circulating spray device A and a circulating spraying device. The shower device B and the ozone generator are respectively connected to the liquid phase area at the lower part of the lye washing tower A and the pipeline before entering the lye washing tower B. Using this technology can achieve a denitrification efficiency of more than 80% and a desulfurization efficiency of more than 95%. However, this technology uses the sequence of desulfurization first and then denitrification. In fact, nitrogen oxides are more likely to react with ozone than sulfur oxides, so the process route of this patent is just reversed; at the same time, the sulfur content in the flue gas is generally greater than the nitrogen oxide content. Ozone desulfurization requires a large amount of ozone. The cost of ozone generation lies not only in the source of oxygen, but also in its high power consumption. Therefore, for desulfurization, ozone oxidation is rarely used because its cost is too high.

Chinese patent CN101259369A discloses an integrated flue gas desulfurization, denitrification and dust removal process, which is to desulfurize and denitrify the flue gas through a reactor for oxidation reaction and acid-base neutralization reaction, and the treated flue gas is subjected to dust removal to obtain pure flue gas. The whole set of process equipment, including the ozone generator, supplies the water produced by the gas-dissolving mechanism to obtain the oxygen-rich absorption liquid, which is sent to the reactor to mix with the flue gas for oxidation reaction. The reactor includes venturi tubes, elbows, slaked lime distribution plates, straight tubes and right-angle baffles. The invention is a brand-new process with high efficiency, less investment and low operation cost. This technology first dissolves ozone in water and then reacts with flue gas, because few components in flue gas can dissolve in water except water vapor. Even if ozone can dissolve in water, ozone needs to break the gas-liquid interface before proceeding. The reaction increases the resistance of the reaction and reduces the contact chance of the reactant.

Contents of the invention

The object of the present invention is to provide a method for removing nitrogen oxides in flue gas by ozone oxidation, so as to overcome the disadvantages of high reaction temperature and high cost in the prior art. Nitrogen oxides in flue gas can be efficiently removed by using the method of the invention, and the method of the invention has low cost and good selectivity. The method of the invention can be used in coal-fired power plants, cement plants, oil refineries, steel plants and other industrial devices that produce a large amount of flue gas.

The invention provides a method for removing nitrogen oxides in flue gas by ozone oxidation, which is characterized in that it comprises the following steps:

1) The flue gas is cooled to 30-110°C, preferably 40-90°C, and then ozone is passed into the cooled flue gas for reaction. In this reaction, the heat released by the reaction is very small, so the reaction temperature Basically remain unchanged, the reaction time is between 0-10 seconds, preferably between 2-8 seconds; the molar ratio of the amount of ozone introduced to the nitrogen oxide compound is between 0.5-2:1, preferably 1.2-1.8: 1.

2) Introduce the reacted gas in step 1) into the absorption tower, spray lye into the upper part of the absorption tower, absorb the reacted gas with lye, the gas after the absorption can be discharged into the air or enter other devices, after the reaction The lye is recycled, and the pH value of the lye is kept between 6.5 and 8, preferably between 6.8 and 7.5; the molar ratio of the amount of lye sprayed into the absorption tower to the gas that has reacted in the absorption tower is 20～ 1:1, preferably 5-1.5:1.

3) According to the change of the pH value in the absorption tower in step 2), fresh lye is introduced, and a part of the crystalline material at the bottom of the absorption tower is discharged.

The further technical feature of the present invention is that: the reaction between the ozone and the flue gas is carried out in a reactor, the ozone enters the reactor from the middle and upper part of the reactor, and the flue gas enters the reactor from the bottom of the reactor.

The further technical feature of the present invention is that: the gas reacted in the step 1) is introduced into the middle and lower part of the absorption tower.

The ozone required by the method of the present invention is mainly prepared by an oxygen source or an air source.

Ozone reacts with nitrogen in the flue gas in the inventive method, and its main reaction is as follows:

NO+O ₃ →NO ₂ +O ₂

2NO ₂ +O ₃ →N ₂ O ₅ +O ₂

N ₂ O ₅ +H ₂ O→2HNO ₃

HNO ₃ +NaOH→NaNO ₃ +H ₂ O

The lye in the absorption tower in the method of the present invention can be more than one of sodium hydroxide, calcium hydroxide, ammonia and calcium carbonate.

Compared with the prior art, the present invention has excellent selectivity and reactivity because it adopts the ozone oxidation method, can react rapidly with nitrogen oxides, and the generated N ₂ O ₅ is extremely soluble in water, so the reaction It is very fast and fast, and is suitable for industrial devices with large flue gas volume; and the method of the invention has high denitrification efficiency. In addition, the invention has lower investment than the conventional SCR method, and there is no phenomenon of ammonia escape, which is beneficial to the post-treatment process. In addition, O ₃ is self-decomposing, and the decomposition product is non-toxic O ₂ without secondary pollution. The method of the invention has high denitrification efficiency and low cost.

The present invention will be described in further detail below in conjunction with the specific embodiments of the accompanying drawings, but the scope of the present invention is not limited.

Description of drawings

The figure is a simple flow chart of the present invention.

In the figure: 1-reactor, 2-ozone, 3-flue gas, 5-pump,

6-lye, 7-absorption tower, 8-fresh lye,

9- Flue gas after removal of nitrogen oxides.

As shown in the figure, the flue gas 3 with a temperature of 30-110°C, preferably 40-90°C, is introduced from the bottom of the reactor 1, and the ozone 2 is introduced from the middle and upper part of the reactor 1. The flue gas 3 and the ozone 2 Two streams of gases react inside the reactor 1, and the reaction time is 0～10 seconds, preferably 2～8 seconds; the mol ratio of the amount of ozone that passes into and nitrogen oxide compound is 0.5～2: 1, preferably 1.2～1.8: 1. Since the heat released by the reaction is very small, the reaction temperature remains constant.

The reacted gas in the reactor 1 enters the middle and lower part of the absorption tower 7, and the upper part of the absorption tower 7 is sprayed with lye 6 to absorb the reacted gas, and the absorbed gas is flue gas 9 after removing nitrogen oxides The flue gas 9 after removing nitrogen oxides can be discharged into the air or enter other devices, and the lye after the reaction is pumped into the absorption tower 7 for recycling through the pump 5, and the pH value of the lye is kept at 6.5 to 8, preferably Between 6.8 and 7.5; the molar ratio of the amount of lye sprayed into the absorption tower to the reacted gas in the absorption tower is between 20 and 1:1, preferably between 5 and 1.5:1. According to the change of pH value in the absorption tower 7, fresh lye 8 is introduced from the upper part of the absorption tower 7; as the absorbed lye increases, some salt crystals will appear at the bottom of the absorption tower, these crystals can be taken out, and the upper layer is clear The lye can still be recycled. These crystals mainly contain sodium nitrate, sodium sulfate, calcium nitrate, calcium sulfate, sodium sulfite and other substances, which can be used as industrial raw materials after purification.

Example

Example 1

Reduce the temperature of 500ppmvNOx flue gas to below 90°C, pass through the ozone concentration of 500ppmv, and the reaction time is 5 seconds, then send the flue gas into the absorption tower, pass into 4% sodium hydroxide solution for absorption, the liquid-gas ratio The ratio is 3:1, the absorbed flue gas can be emptied directly, the concentration of NOx is 10ppmv, the denitrification efficiency is 98%, and the lye can continue to be recycled. The entire flue gas flow remains unchanged, and NOx emissions are reduced. Experiments have shown that most of the nitrogen oxides are converted into nitrate ions, achieving the purpose of emission reduction.

Example 2

Reduce the temperature of 300ppmvNOx flue gas to below 80°C, pass through the ozone concentration of 500ppmv, and the reaction time is 3 seconds, then send the flue gas into the absorption tower, pass through 4% sodium hydroxide solution for absorption, the liquid-gas ratio 4:1, the absorbed flue gas can be directly emptied, the concentration of NOx is 10ppmv, the denitrification efficiency is 97%, and the lye can continue to be recycled. The entire flue gas flow remains unchanged, and NOx emissions are reduced. Experiments have shown that most of the nitrogen oxides are converted into nitrate ions, achieving the purpose of emission reduction.

Example 3

Lower the temperature of 200ppmvNOx flue gas to 60°C, pass through the ozone concentration of 300ppmv, and the reaction time is 5 seconds, then send the flue gas into the absorption tower, and pass into 4% calcium hydroxide solution for absorption. The liquid-gas ratio is 3.5:1, the absorbed flue gas can be emptied directly, the concentration of NOx is 0ppmv, the denitrification efficiency is 100%, and the lye can continue to be recycled. The entire flue gas flow remains unchanged, and NOx emissions are reduced. Experiments have shown that most of the nitrogen oxides are converted into nitrate ions, achieving the purpose of emission reduction.

Example 4

Lower the temperature of 420ppmvNOx flue gas to 40°C, pass through the ozone concentration of 600ppmv, and the reaction time is 5 seconds, then send the flue gas into the absorption tower, and pass into 4% calcium hydroxide solution for absorption. The liquid-gas ratio is 5:1, the absorbed flue gas can be directly emptied, the concentration of NOx is 0ppmv, the denitrification efficiency is 100%, and the lye can continue to be recycled. The entire flue gas flow remains unchanged, and NOx emissions are reduced. Experiments have shown that most of the nitrogen oxides are converted into nitrate ions, achieving the purpose of emission reduction.

Example 5

Lower the temperature of 180ppmvNOx flue gas to 40°C, pass through the ozone concentration of 300ppmv, and the reaction time is 4 seconds, then send the flue gas into the absorption tower, pass through 4% calcium carbonate solution for absorption, and the liquid-gas ratio is 2.5 : 1, the absorbed flue gas can be directly emptied, wherein the concentration of NOx is 0ppmv, the denitrification efficiency is 100%, and the lye can continue to be recycled. The entire flue gas flow remains unchanged, and NOx emissions are reduced. Experiments have shown that most of the nitrogen oxides are converted into nitrate ions, achieving the purpose of emission reduction.

Claims (10)

1. an Ozonation removes the method for oxynitrides in flue gas, it is characterized in that comprising the steps:

1) between flue gas cool-down to 30～110 ℃, then ozone is passed in the flue gas after cooling and react, the reaction time is 0～10 second, and the ozone amount that passes into and the mol ratio of oxynitrides are 0.5～2: 1;

2) with step 1) middle gas introducing absorption tower of having reacted, top, absorption tower sprays into alkali lye, with alkali lye, the gas that has reacted is absorbed, gas discharging after absorption is in air or enter other device, alkali lye after absorption recycles, in the absorption tower, the pH value of alkali lye is 6.5～8, and the mol ratio that sprays into the complete gas of alkali lye amount in the absorption tower and absorption tower internal reaction is 20～1: 1;

3) according to step 2) in the absorption tower situation of change of pH value introduce fresh alkali lye, and discharge a part of crystallized stock bottom the absorption tower.

2. method according to claim 1, it is characterized in that: the reaction of described ozone and flue gas is carried out in reactor, and ozone enters reactor from the reactor middle and upper part, and flue gas enters reactor from reactor bottom.

3. method according to claim 1 is characterized in that: the middle and lower part on the gas introducing absorption tower of react described step 1).

4. method according to claim 1, it is characterized in that: the alkali lye in described absorption tower is more than one in NaOH, calcium hydroxide, ammoniacal liquor and calcium carbonate.

5. method according to claim 1, it is characterized in that: described flue-gas temperature is 40～90 ℃.

6. method according to claim 1, is characterized in that: described ozone employing oxygen source or the preparation of air source of the gas.

7. method according to claim 1, it is characterized in that: the described reaction time is 2～8 seconds.

8. method according to claim 1, it is characterized in that: the mol ratio of described ozone amount and oxynitrides is 1.2～1.8: 1.

9. method according to claim 1, it is characterized in that: described step 2) in the absorption tower, pH value is 6.8～7.5.

10. method according to claim 1 is characterized in that: the mol ratio that described step 2) sprays into the complete gas of alkali lye amount in the absorption tower and absorption tower internal reaction in the absorption tower is 5～1.5: 1.

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