CN101279185B - Gas phase oxidation-liquid phase reduction absorption method for removing nitrogen oxides in exhaust gas - Google Patents

Abstract

The invention provides a gas-phase oxidation/liquid-phase reduction method for absorbing and removing NOx. The method utilizes O2 in waste gases as an oxidizer; after be processed by non-catalyst oxidization or catalyzed oxidation by modified active carbon materials, NOx waste gas with low oxidization degree reacts with an alkaline reducing solution fully to remove NOx. After going through multi-grade oxidization, reduction and absorption, the NOx in waste gases can fully meet the emission standard. The method of the invention has the advantages of being able to realize up-to-standard emission of NOx waste gas with low oxidization degree, simple technique, small cost and high removing efficiency.

Description

Gas phase oxidation-liquid phase reduction absorption method for removing nitrogen oxides in exhaust gas

(1) Technical field

The invention relates to a gas-phase oxidation-liquid-phase reduction absorption method for removing nitrogen oxides (NOx) in waste gas.

(2) Background technology

There are nitrification and nitrosation reactions in the production process of fine chemical industry, such as pharmaceutical factory, and the NOx content in the exhaust gas is high, forming the "yellow dragon" phenomenon, which is one of the important pollutants causing air pollution at present. NOx not only forms photochemical smog, acid rain and acid mist in the atmosphere, but also is closely related to the destruction of the ozone layer, and seriously endangers human health. Therefore, the treatment of NOx is not only an important issue in the environmental field, but also an important task to achieve sustainable economic development in our country. inevitable requirement. The removal methods of nitrogen oxides are mainly divided into two types: dry method and wet method: in dry method, SCR method is widely used in Europe and the United States, but it requires the flue gas temperature to be above 300 ℃, which is suitable for the treatment of coal-fired flue gas ; while the wet method is more suitable for the treatment of industrial waste gas, and has a better effect on the treatment of NOx waste gas with a high degree of oxidation (NO ₂ /(NO ₂ +NO) concentration ratio).

In industrial production, NOx exhaust gas is almost discharged at normal temperature and pressure, and generally contains water vapor, oxygen, etc., and the oxygen content is relatively high, sometimes the same as air, so it is not suitable for SCR treatment. At the same time, the main component of NOx is NO, and the degree of oxidation is very low, ~10-20%. Due to the low solubility of NO in water, lye and other absorption liquids, the conventional wet absorption liquid has a low rate of removal of this type of NOx. Very low, difficult to meet emission requirements. At present, the liquid absorption research shows that the denitrification effect of the reduction absorption method is better. When the oxidation degree of the exhaust gas is greater than 50%, the denitrification efficiency of more than 90% can be obtained [Yang Yang Nitrogen Oxide Emission Reduction Technology and Flue Gas Denitrification Engineering 2007 Metallurgical Industry Publishing Society], so using an oxidant to oxidize NO in the exhaust gas to NO ₂ and increasing the oxidation degree of the exhaust gas is one of the effective ways to remove this kind of NOx.

专利200610012414.1、200610051721.0、200710052129.7、200610012525.2、200610051737.1、200510062185.X、200710067082.1、200710061515.2等利用含氯强氧化剂(亚氯酸钠、次氯酸钠等)、双氧水、臭氧氧化及光催化氧化方式对废气中的NO进行氧化The single or combined use of these oxidation methods greatly improves the NOx removal rate, but due to the use of additional oxidants, the investment and operating costs increase, and it is more difficult to apply in small and medium-sized enterprises, and when the NOx exhaust gas treatment volume is relatively large There are certain problems in the application of large-scale photocatalytic oxidation; patents 01105698.3 and 02110646.0 use oxygen in the air as the oxidant, and ethylenediamine cobalt solution oxidizes NO in liquid phase, which can achieve the standard emission of NOx, but the use of ethylenediamine cobalt will increase the exhaust gas Processing costs, and there may still be some problems in engineering control in industrial devices; patents 200510062296.0 and 200510062297.5 realize complex absorption of NOx by adding effective additives to Fe ^II EDTA solution. When the reaction time is 20-30s, it can make The NOx removal rate reaches 65-95%, but the cost input of the absorption liquid increases, and the composition of the waste liquid after absorption is complicated and difficult to handle.

Young Sun Mok[Absorption-reduction technique assisted by ozoneinjection and sodium sulfide for NOx removal from exhaust gas, Chemical Engineering Journal, 2006] After 300ppm NO is oxidized by ozone and absorbed by Na ₂ S alkaline solution, the removal rate can reach more than 95%; CN101036851A oxidizes the waste gas containing NOx through ozonation and absorbs polyethylene glycol, and almost reaches a removal rate of 100%. However, the ozone generator has a large investment and high cost, and it is difficult to apply it in small and medium-sized enterprises. Isao Mochida et al [Oxidation of NOinto NO ₂ over Active Carbon Fibers, Energy & Fuels 1994, 8: 1341-1344] used modified activated carbon fibers (ACF) to carry out NO (400ppm) oxidation experiments at room temperature. The conversion rate was 73% under dry conditions at 25°C, but the presence of water vapor in the exhaust gas severely inhibited the conversion of NO, and the conversion rate was only 21% under saturated water vapor. It can be seen that the presence of high content of water vapor in industrial waste gas seriously hindered the catalytic oxidation of NO by ACF. application.

Therefore, how to remove nitrogen oxides in exhaust gas at low cost and efficiently, there is still room for improvement in the prior art.

(3) Contents of the invention

The purpose of the present invention is to provide a gas-phase oxidation-liquid-phase reduction method for absorbing and removing nitrogen oxides in low-oxidation NOx waste gas containing saturated water vapor discharged from industry, which ensures reasonable emission of NOx-containing waste gas at low cost.

The technical scheme adopted in the present invention is:

A gas-phase oxidation-liquid-phase reduction method for absorbing and removing nitrogen oxides in exhaust gas, the nitrogen oxides being a mixture of NO and NO ₂ , the method is as follows: the exhaust gas containing nitrogen oxides is subjected to non-catalytic oxidation or catalytic Oxidation, the gas-phase oxidation of NO is converted into _NO2 , and then the liquid-phase reduction and absorption of the reducing absorption liquid removes nitrogen oxides in the exhaust gas; the oxidation degree of the exhaust gas containing nitrogen oxides is lower than 50%; The exhaust gas may or may not contain water vapor; the non-catalytic oxidation or catalytic oxidation process uses the oxygen contained in the exhaust gas as an oxidant; the reducing absorption liquid is a mixed aqueous solution of a reducing agent and an alkali, and the reducing agent is One of the following: ① urea, ② sodium sulfide, ③ sodium sulfite, ④ sodium thiosulfate, ⑤ ammonium sulfite; the alkali is sodium hydroxide or potassium hydroxide; the mass content of the reducing agent in the mixed aqueous solution is 0.02 ～20%, the mass content of alkali is 0.01～20%.

The non-catalytic oxidation step is as follows: at normal temperature, the exhaust gas containing nitrogen oxides is passed into the oxidation tower, and the NO is oxidized by oxygen in the exhaust gas.

The catalytic oxidation step is as follows: a fixed-bed oxidation reactor is filled with modified activated carbon or activated carbon fiber as a catalyst, and the exhaust gas is heated to 20-100° C. with water vapor, so that oxygen in the exhaust gas reacts with NO; the modified activated carbon It is prepared by the following method: Soak activated carbon in 0.5-2.0M alkali solution for 18-24 hours, wash it with deionized water until neutral, dry it and roast it at 400-1000°C for 1 hour under the protection of nitrogen or hydrogen (preferably 600-800°C ), heating rate 10K/min, to obtain modified activated carbon.

The liquid-phase reduction and absorption is carried out in a bubble column, and waste gas enters from the bottom of the column, reacts with the reducing absorption liquid through bubbling, and is discharged from the top of the column.

Alternatively, the liquid-phase reduction and absorption is carried out in a packed tower, and the waste gas enters from the bottom of the tower, and is discharged from the top of the tower after being reversely contacted and reacted with the reducing absorption liquid. The packing used in the packed tower usually refers to granular packing or structured packing. Granular packing mainly refers to Pall ring, Raschig ring, arc saddle, and moment saddle packing. The material can be plastic or ceramics; structured packing mainly refers to wire mesh Corrugated packing or grid packing, wire mesh corrugated packing material can be plastic or carbon fiber, grid packing can be plastic or wood.

The gas phase oxidation and liquid phase reduction absorption can be carried out alternately. In order to achieve the emission of NOx up to the standard, gas-phase oxidation and liquid-phase reduction and absorption can alternately carry out multi-stage reactions. The number of reaction stages is determined by the actual situation, and generally 1 to 4 stages are used. Under the same conditions, NOx can also be oxidized first and then absorbed, but at this time, the oxidation tower has a large volume and a large amount of catalyst, so it is generally not used. For ordinary industrial waste gas, generally 2-3 stages of treatment can meet the NOx emission requirements.

The present invention uses a low-cost oxidant to oxidize the low oxidation degree NOx containing saturated water vapor in the gas phase at normal temperature and pressure, so that the oxidation degree of NOx in the waste gas can reach more than 50%, and the NOx can reach the standard through liquid phase reduction and absorption. discharge, and the process is simple and easy to operate.

The features of the present invention are:

(1) Oxidation technology is used to oxidize the difficult-to-treat NO into easy-to-treat NO ₂ , and the reducing lye is used to treat NOx to ensure that the exhaust gas is discharged up to the standard;

(2) The oxygen in the exhaust gas is fully utilized, and no additional oxidant is used, which reduces the cost of exhaust gas treatment;

(3) The use of modified activated carbon materials as catalysts effectively avoids catalyst deactivation caused by water vapor in exhaust gas and improves the oxidation effect;

(4) The synergistic effect of lye and reducing agent accelerates the NOx absorption rate, reduces equipment size, and reduces equipment investment;

(5) The reducing agent reduces NOx to harmless nitrogen, which reduces the amount of alkali. On the one hand, it reduces the treatment cost of the absorption waste liquid, and on the other hand, it reduces the concentration of salt formed in the absorption liquid, effectively preventing the filling of the absorption tower. Absorber clogging that may be caused by salt precipitation;

(6) Exhaust gas treatment is carried out under normal temperature and pressure to reduce energy consumption;

(7) A salt recovery system can be installed before absorbing waste liquid for waste water treatment to recover useful salts such as sodium nitrate and sodium nitrite, reducing the cost of waste gas treatment and the cost of waste liquid treatment.

The beneficial effects of the present invention are mainly reflected in: low-oxidation degree NOx waste gas can be discharged up to the standard under low-cost conditions, and the process is simple; the oxygen in the waste gas is fully utilized without adding additional oxidant, and the difficult-to-treat NO is converted into easy Treat NO ₂ with high efficiency.

(4) Description of drawings

Fig. 1 is the flow chart of the method of embodiment 1 of the present invention; Wherein 1 is exhaust gas containing NOx, 2 is a sampling port, 3 is an oxidation tower, 4 is an absorption tower, 5 is a reduction absorption liquid, 6 is a tail gas discharge port, and 7 is a Waste drain.

Fig. 2 is the flow chart of the method for embodiment 4 of the present invention; Wherein 1 is the exhaust gas containing NOx, 2 is a sampling port, 8 is a catalyst bed, 9 is an oxidation reactor, 4 is an absorption tower, 5 is a reduction absorption liquid, 6 7 is a tail gas discharge port, and 7 is a waste liquid discharge port.

Fig. 3 is the flow chart of the method for embodiment 10 of the present invention; wherein 1 is NOx-containing waste gas, 2 is a sampling port, 3 is an oxidation tower, 10 is a packed tower, 11 is a liquid storage tank, and 12 is a fresh reducing absorption liquid inlet , 6 is the tail gas discharge port, and 7 is the waste liquid discharge port.

(5) Specific implementation methods

The present invention is further described below in conjunction with specific embodiment, but protection scope of the present invention is not limited thereto:

Example 1:

The NOx exhaust gas is passed into the oxidation tower 3 and is oxidized by _O2 , then enters the absorption tower from the bottom of the absorption tower 4 and undergoes reduction and absorption reaction with the absorption liquid 5 by bubbling. After the reaction, the gas is discharged from the top of the tower, and the exhaust gas undergoes three-stage oxidation-reduction absorption treatment , the tail gas is emptied through the tail gas discharge port 6. Oxidation tower 3 has a diameter of 40 mm and a height of 300 mm, with a total oxidation volume of 2.0 L. Bubble absorption tower 4 has a diameter of 40 mm and a height of 600 mm. Exhaust gas contains 1907ppmvNOx, saturated water vapor and about 20% O ₂ at normal temperature, the amount of reducing absorption liquid 5 added is 200ml, and its composition is 4% (W/W) NaOH and 8% (W/W) CO(NH ₂ ) ₂ . Gas flow rate: 0.08m ³ /h, reaction at normal temperature and pressure, NOx concentration at inlet and outlet is detected and analyzed at sampling port 2 by flue gas analyzer (testo350-XL).

Example 2:

The reactor and operating conditions are the same as in Example 1. The exhaust gas contains NOx1330ppmv, saturated water vapor and about 20% O2 at room temperature, and the addition of the reduction absorption solution ₅ is 200ml, and its composition is 4% (W/W) NaOH and 8% (W/W)CO(NH ₂ ) ₂ , gas flow rate: 0.08m ³ /h, reaction at normal temperature and pressure, NOx concentration at the inlet and outlet is detected and analyzed by a flue gas analyzer (testo 350-XL).

Example 3:

The exhaust gas is oxidized by _O2 in the oxidation tower 3, and then enters the absorption tower 4 for reaction, and undergoes primary oxidation, reduction and absorption treatment. Oxidation tower 3 has a diameter of 80 mm and a height of 600 mm (oxidation volume 3.0 L), and bubbling absorption tower 4 has a diameter of 40 mm and a height of 600 mm. Exhaust gas contains 1785ppmvNOx, saturated water vapor and about 20% _O2 at normal temperature, the amount of reducing absorption solution 5 added is 300ml, containing 1% (W/W) NaOH and 8% (W/W) CO(NH ₂ ) ₂ , Gas flow rate: 0.08m ³ /h, reaction at normal temperature and pressure, NOx concentration at inlet and outlet is detected and analyzed at sampling port 2 by flue gas analyzer (testo350-XL).

Example 4:

The fixed-bed oxidation reactor 9 is filled with modified activated carbon, and the waste gas 1 heated to a suitable temperature by water vapor undergoes an oxidation reaction with the _O2 in the waste gas through the fixed-bed oxidation reactor 9. After the reaction, the gas is reduced and absorbed by the absorption tower 4, The exhaust gas is evacuated through the exhaust outlet 6 after being catalyzed and oxidized by the second stage. The oxidation reactor 9 has an inner diameter of Φ9.0 mm and a height of 140 mm, and the bubbling absorption tower 4 has a diameter of 40 mm and a height of 600 mm. Preparation method of modified activated carbon: Commercial activated carbon is ground and sieved into 20-30 meshes, soaked in 1M KOH solution for 24 hours, washed with deionized water until neutral, dried in an oven, and placed in a resistance furnace under the protection of _H2 for 800 ℃ roasting for 1h, heating rate 10K/min.

The main components of exhaust gas are: 845ppmvNOx, saturated water vapor at 20°C, and about 20% O ₂ . Oxidation reactor 9 temperature: 50°C, absorbing solution 5 added to 300ml, containing 0.5% (W/W) NaOH and 2% (W/W) CO(NH ₂ ) ₂ , gas flow rate: 0.12m ³ /h, The NOx concentration at the inlet and outlet is detected and analyzed by the flue gas analyzer (testo 350-XL) at the sampling port 2, wherein the NOx oxidation degree after the exhaust gas is oxidized by the oxidation reactor 9 is 50%.

Example 5:

After the exhaust gas containing saturated water vapor (20°C) is heated to a certain temperature by water vapor, it passes through the integral fixed bed oxidation reactor 9 to undergo oxidation reaction with _O2 in the air by using modified activated carbon, and uses a flue gas analyzer (testo 350- XL) Continuously detect the change of NOx concentration at the oxidation inlet and outlet until the reaction reaches a steady state, and the outlet exhaust gas is emptied through the tail gas discharge port 6 . The oxidation reactor 9 has an inner diameter of Φ9.0 mm, a height of 140 mm, a mass of modified activated carbon of 4.5 g, a reaction temperature of 50° C., and a reaction time of 2 s. The preparation method of the modified activated carbon is the same as in Example 4.

Embodiment 6:

The reactor and operating conditions are the same as in Example 4, and the NOx exhaust gas 1 is subjected to primary catalytic oxidation-reduction absorption treatment. The exhaust gas contains NOx 741ppmv, saturated water vapor at 20°C and about 20% O ₂ . After the waste gas is oxidized by modified activated carbon and O ₂ , the oxidation degree of the gas is 52%, and the amount of reduction absorption liquid 5 is 300ml, containing 0.5% (W/W) NaOH and 2% (W/W) Na ₂ SO ₃ .

Embodiment 7:

The reactor and operating conditions are the same as in Example 4, and the NOx exhaust gas 1 is subjected to primary catalytic oxidation-reduction absorption treatment. The exhaust gas contains NOx 662ppmv, saturated water vapor at 20°C and about 20% O ₂ . After the exhaust gas is oxidized by modified activated carbon and _O2 , the oxidation degree of the gas is 52%, and the amount of reduction absorption solution 5 is 300ml, containing 0.5% (W/W) NaOH and 1% (W/W) Na ₂ S ₂ O ₃ .

Embodiment 8:

The reactor and operating conditions are the same as in Example 4, and the NOx exhaust gas 1 is subjected to primary catalytic oxidation-reduction absorption treatment. The exhaust gas contains NOx 771ppmv, saturated water vapor at 20°C and about 20% O ₂ . After the exhaust gas is oxidized by modified activated carbon and _O2 , the oxidation degree of _the gas is 51%.

Embodiment 9:

The reactor and operating conditions are the same as in Example 4, and the NOx exhaust gas 1 is subjected to two-stage catalytic oxidation-reduction absorption treatment. The exhaust gas contains NOx 783ppmv, saturated water vapor at 20°C and about 20% O ₂ . After the exhaust gas is oxidized by modified activated carbon and _O2 , the oxidation degree of the gas is 50%, the amount of reduction absorption liquid 5 is 300ml, and the composition is 0.5% (W/W) NaOH and 2% (W/W) (NH ₄ ) ₂ SO ₃ .

Example 10:

Under normal temperature and pressure, the exhaust gas is oxidized by the oxidation tower 3 (oxidation volume 3.05L) and then enters the packed tower 10 from the bottom of the tower. It is discharged into the liquid storage tank 11, and the absorbed gas is discharged from the top of the tower. The waste gas undergoes secondary oxidation, reduction and absorption reactions, and the tail gas is emptied through the tail gas discharge port 6. Oxidation tower 3 has a diameter of 900 mm and a height of 1100 mm. Packed tower 10 has a diameter of 400 mm and a height of 1500 mm, wherein the packing layer is 750 mm high, and the packing is Dg25 plastic Pall ring (Ningbo Haoyu Chemical Co., Ltd.). The exhaust gas contains 1540ppmvNOx, saturated water vapor and about 20% O ₂ at room temperature, and the absorption liquid contains 1% (W/W) NaOH and 6% (W/W) CO(NH ₂ ) ₂ . Gas flow rate: 0.5m ³ /h, liquid spray density 15m ³ /m ² ·h, inlet and outlet NOx concentrations are detected and analyzed by flue gas analyzer (testo 350-XL).

See Table 1 for the NOx treatment effects of each embodiment under different conditions.

Table 1: NOx treatment effect under different conditions

serial number   Test process import exit Removal rate or conversion rate/% NOx/ppmv Oxidation degree/% NOx/ppmv Oxidation degree/% 1 Three-stage oxidation tower oxidation, three-stage absorption 1907 20.1 429 6.85 77.5 2 Three-stage oxidation tower oxidation, three-stage absorption 1330 14.8 410 4.49 69.2 3 Primary absorption after oxidation in the primary oxidation tower 1785 18.2 372 20.2 79.0 4 Secondary catalytic oxidation - urea lye absorption 845 4.14 160 16.5 81.1 5 Catalytic oxidation of modified activated carbon 740 5.00 337 54.0 55.0 6 Sodium sulfite lye absorption after catalytic oxidation 741 6.48 263 25.5 64.5 7 Absorption of sodium thiosulfate lye after catalytic oxidation 662 4.83 279 6.10 58.0 8 Sodium sulfide lye absorption after catalytic oxidation 771 3.58 241 6.64 69.0 9   Secondary catalytic oxidation - ammonium sulfite lye absorption 783 4.47 201 5.47 74.3

10   Secondary oxidation tower oxidation-absorption 1540 22.5 210 37.8 86.4

Note: the numbers in the table correspond to the serial numbers of the examples, and the specific operation steps of each group of tests are shown in the relevant examples; in examples 1 to 9, except for example 4, the liquid absorption is carried out in the bubbling absorption tower, the examples 10 The absorption test was carried out in a packed tower.

Claims (4)

1. the method for a gas phase oxidation-liquid phase reduction absorbing and removing nitrogen oxides of exhaust gas, described nitrogen oxide is NO and NO ₂Mixture, described method is as follows: the waste gas of nitrogen-containing oxide through non-catalytic oxidation or catalytic oxidation, is made NO carry out gaseous oxidation and is converted into NO ₂After, the liquid-phase reduction through the reproducibility absorption liquid absorbs again, removes nitrogen oxides from exhaust gas; Described reproducibility absorption liquid is the mixed aqueous solution of reducing agent and alkali, and described reducing agent is one of following: 1. urea, 2. vulcanized sodium, 3. sodium sulfite, 4. sodium thiosulfate, 5. ammonium sulfite; Described alkali is NaOH or potassium hydroxide; In the described mixed aqueous solution, the mass content of reducing agent is 0.02～20%, the mass content of alkali is 0.01～20%; Described non-catalytic oxidation step is as follows: under the normal temperature, the waste gas of nitrogen-containing oxide is fed in the oxidizing tower, utilize oxygen in the waste gas with the NO oxidation; Described catalytic oxidation step is as follows: as catalyst, with steam waste gas is heated to 20～100 ℃ with the active carbon of fixed-bed oxidation reactor filling modification or NACF, the oxygen in the waste gas is reacted with NO; The active carbon of described modification is prepared by following method: with the aqueous slkali soaking 18～24h of active carbon with 0.5～2.0M, be washed till neutrality with deionized water, 400～1000 ℃ of roasting 1h under dry back nitrogen or the hydrogen shield obtain the active carbon of modification.

2. the method for claim 1, it is characterized in that: described liquid-phase reduction is absorbed in the bubble tower and carries out, and waste gas enters at the bottom of by tower, by bubbling and reproducibility absorption liquid reaction back, discharged by cat head.

3. the method for claim 1, it is characterized in that: described liquid-phase reduction is absorbed in the packed tower and carries out, waste gas enters at the bottom of by tower, with the reverse haptoreaction of reproducibility absorption liquid after, discharge by cat head.

4. the method for claim 1 is characterized in that: described gaseous oxidation and liquid-phase reduction absorb and hocket.

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