RU2081685C1 - Method for purifying flue gases from nitrogen oxides - Google Patents

Abstract

FIELD: environmental control. SUBSTANCE: proposed method resides in following: flue gases to be purified are treated with carbamide at its decomposition temperature, followed by catalytic after-purification of gases from nitrogen oxides using products resulting from thermal decomposition of carbamide. Inventive novelty of proposed method resides in fact that catalytic after-purification of gases is conducted at temperature 90-170 C, preferably at 90-150 C, while in first process step of treating gases with carbamide it is advisable to use temperature 300-1200 C. EFFECT: proposed method can be advantageously used to purify flue gases from thermal power generating plants and off-gases discharged by various items of process equipment. 3 cl, 2 dwg

Description

The invention relates to the field of purification of industrial gas emissions from nitrogen oxides (NO _x and can be used to reduce the content of NO _h in the combustion products of thermal power plants, as well as in gas emissions of technological units for various purposes to any given level (below sanitary standards).

 Known methods for cleaning gases from nitrogen oxides by selective catalytic and non-catalytic reduction methods.

A known method of selective catalytic purification, based on the reduction of NO _h with ammonia at a temperature of 250 6000 ^o C on catalysts, which include transition metal oxides: V ₂ O ₅ , Fe ₂ O ₃ , Cr ₂ O ₃ , TiO ₂ , MnO ₂ and etc. Flue gases are preheated in a heat exchanger to 320 ^o C. The method provides a cleaning efficiency of 80 to 90% (US Pat. Germany N P3627086, B 01 D 53/34, 1988).

 The disadvantage of this method is the need to use significant amounts of catalyst, which increases the aerodynamic resistance of the unit, leads to additional energy consumption for the drive of draft blowers and increases operating costs. In addition, the cost of the catalyst and reactor for its placement is the main share of the specific costs of gas purification.

 Certain technological and structural difficulties are also associated with the increased operating temperature of the catalysts. In order to ensure the necessary efficiency of the process, either a heat exchanger for heating gases to the required temperature is included in the technological scheme (Nitrogen Handbook, vol. 2, M. Khimiya, 1987, p. 63 66, fig. 1 40), or the catalytic reactor is stirred in the area of the unit, where the gases to be purified have this temperature in accordance with the technological conditions. This requires a significant reconstruction of the unit, which complicates and increases the cost of the cleaning system.

There is also a method of selective non-catalytic purification based on high-temperature reduction of NO _x in the absence of a catalyst with amine-containing reductants: gas-exchange ammonia, aqueous ammonia, urea, cyanuric acid, etc. The process temperature is 850 - 1150 ^o C, the cleaning efficiency is up to 90% (US Pat. USA N 4325924, CL 423-235, 1982).

 The non-catalytic purification process has significant advantages over the catalytic purification due to the absence of problems associated with the use of the catalyst, but the efficiency of this process, as well as the catalytic one, depends on the temperature of the gases and the degree of purification decreases with a decrease or increase in temperature relative to the optimal value.

When the load of the heat generator changes, the temperature of the flue gases in the input zone of the reducing agent can vary over a wide range: from 1150 to 800 ^o C, which is adequate to reduce the efficiency of non-catalytic treatment from maximum to zero.

Close to the proposed method in technical essence and the achieved result is a combined method for catalytically-non-catalytic reduction of nitrogen oxides with a nitrogen-containing processing agent, which is introduced into the flue gas stream under conditions that ensure the presence of ammonia in the gases after the first processing stage (at a temperature of 648 - 1150 ^o C) then the gas flows at a temperature of 204 538 ^o C through a catalyst bed, which is a variable valency metal oxides, such as chromium, vanadium, titanium, w lezo et al. for later recovery of remaining nitrogen oxides. In the examples cited in the patent, a temperature of 427 ^° C. is maintained in the second catalytic stage, and vanadium pentoxide on a ceramic honeycomb carrier is used as a catalyst. Moreover, the total degree of purification is 80 90% (US Pat. USA N 4978514, CL 423 235, 1990).

 The disadvantage of the prototype is also due to the increased temperature of the catalytic stage, which requires, as was shown above, either the inclusion of heat exchangers in the technological scheme, or the placement of the catalytic reactor in a certain zone of the heat generator, which complicates the technological scheme of treatment as a whole.

 The objective of the present invention is to develop a method of purification of flue gases from nitrogen oxides, which simplifies the process technology while maintaining a stable, maximum degree of purification under various loads of the heat generator.

The problem is solved by the proposed method of purification of flue gases from nitrogen oxides, including the treatment of flue gases with urea at a temperature of its decomposition followed by catalytic purification of gases from nitrogen oxides by products of thermal decomposition of urea, a distinctive feature of which is that the catalytic purification of gases is carried out at a temperature of 90,170 ^o C.

It is advisable to treat the carbamide in the first stage at a temperature of 300 1200 ^o C, and the post-treatment of flue gases from nitrogen oxides in the second stage at a temperature of 90 150 ^o C.

This process implementation scheme allows to significantly simplify the flue gas purification technology, since the catalytic stage is proposed to be carried out at a temperature corresponding to the temperature of the exhaust gases before they enter the chimney (90 170 ^o C), which makes it possible to place the catalytic unit in the area between the heat generator and the flue pipe, without embedding it in the heat generator and without heating the gases in heat exchangers before feeding it to the catalyst. As it was unexpectedly established and experimentally confirmed with the use of catalysts of various compositions, the temperature of 90 170 ^o C provides an efficient mode of operation of the catalyst provided that the thermal decomposition products of urea containing ammonia and isocyanic acid are used as a reducing agent (see graph in Fig. 1). At the same time, a stable and high level of purification of flue gases from nitrogen oxides (99 100%) is achieved, regardless of the mode of the first non-catalytic stage of the process, and, consequently, of changing eniya mode TEPLOAGREGAT.

 Fig. 2 shows the installation diagram, which allows to implement the proposed method for purifying gases from nitrogen oxides, where 1 is a heat generator, 2 is a distribution device, 3 is a catalytic unit, 4 is a smoke exhauster, 5 is a chimney.

A urea solution in an amount sufficient to reduce nitrogen oxides is fed into the flue of the thermal unit 1 with a temperature of 300 to 1200 ^o C using a distributor 2. In the injection zone, the urea is decomposed into ammonia and isocyanic acid: CO (NH ₂ ) ₂ NH ₃ + HNCO , which are in the zone of input of urea is 800 1150 ^o C, then there is a partial reduction of nitrogen oxides. Then, flue gases containing urea decomposition products and nitrogen oxides remaining after the first purification step pass through catalyst bed 3 at a temperature of 90-170 ^° C. (preferably 90,150 ^° C.

 As a catalyst can be used any known catalysts for the reduction of nitrogen oxides with ammonia, for example, oxides of metals of variable valency, such as iron, chromium, vanadium, etc.

 After the second stage of purification, the purified gases, practically free of nitrogen oxides, are discharged into the chimney 5 using a smoke exhauster 4.

In the event that the temperature of the gases in the urea injection zone is less than 800 ^° C., only the urea decomposition process occurs in the first stage, and the reduction of nitrogen oxides proceeds in the second catalytic stage.

Below are examples of gas purification from nitrogen oxides of the proposed method
Example 1 (laboratory studies).

A mixture of gases containing nitrogen, oxygen and nitrogen oxides is fed into the quartz reactor of the 1st stage. The consumption of the gas mixture is 180 l / h. The content of NO _x at the outlet in the reactor is 150 mg / m ³ , the content of O _{2 is} 0.5% vol. the rest is nitrogen. The temperature in the reactor 1050 ^o C.

The mixture of gases from the reactor of the 1st stage is fed into the catalytic reactor of the 2nd stage, filled with a catalyst consisting of 90% Fe ₂ O ₃ and 10% Cr ₂ O ₃ . The volume of catalyst is 0.02 l. The temperature in the catalyst bed is 150 ^° C. The content of NO _x at the outlet from the first stage reactor and from the second stage catalytic reactor is the same and is 150 mg / m ³ .

A urea solution with a concentration of 0.5% wt. in an amount of 0.03 l / h, after which the content of nitrogen oxides at the outlet of the first stage reactor is reduced to 52 mg / m ³ , and at the outlet of the second stage reactor to 1 mg / m ³ .

 Example 2 (pilot tests).

In a steam generator with a capacity of 13.5 tons of steam per hour, natural gas is burned in an amount of 800 m ³ / hour. A catalyst is placed in the flue in front of the chimney. The composition of the catalyst is shown in example 1. The volume of catalyst is 1 m ³ . The temperature in the zone of catalyst placement is 100 ^o C. The content of nitrogen oxides in the flue gas is 360 mg / m ³ .

In the zone of the steam generator with a flue gas temperature of 800 ^o C is fed a 20% aqueous urea solution in an amount of 24 kg / h.

The content of NO _h after stage I of purification (before the catalyst) is 325 mg / m ³ , after the catalyst 2 mg / m ³ .

 Example 3 (laboratory studies).

The temperature in the stage I reactor is 300 ^° C, in the stage II catalytic reactor is 170 ^° C, the remaining conditions are identical to those in Example 1. After feeding the urea solution, the content of NO _h at the outlet of the stage I reactor was 150 mg / m ³ , after stage II mg / m ³ .

 Example 4 (laboratory studies).

As the catalyst was used industrial catalyst AVK-10, containing 10% wt. V ₂ O ₅ on a γ-Al ₂ O ₃ support.

At a temperature in the reactor of stage I, 700 ^° C, and in the reactor of stage II, 120 ^° C (other conditions are identical to those in Example 1), the NO _x content at the outlet in the reactor of stage I is 345 mg / m ³ , after stage I, 345 mg / m ³ , after stage II 0 mg / m ³ .

 Example 5 (laboratory studies).

During the experiment, the temperature conditions specified in the prototype were used. In the catalytic reactor of the second stage was loaded industrial catalyst ABK-10, the composition is also close to that specified in the prototype. The remaining conditions were identical to those specified in example 1. At a temperature in the first stage of 760 ^o C, and in the second stage of 430 ^o C, the content of NO _h at the outlet of the stage reactor was 136 mg / m ³ , after the first stage of purification 136 mg / m ³ , after stage II 0 mg / m ³ .

 The results are shown in table 1.

As can be seen from the data presented, the proposed two-stage method of purification of flue gases from nitrogen oxides using urea as a reducing agent and the second catalytic stage at a temperature of 90
180 ^o C is effective and provides a high degree of purification under various loads of heat units (the temperature of the process in the first stage can vary within wide limits). The technology of the proposed method is simple, because it eliminates the need to incorporate a catalytic purification unit into a heat generator or preheat gases in heat exchangers, which makes it possible to apply the proposed method to existing thermal units and process units using the temperature of the exhaust gases determined by the technological mode of their operation.

Claims (3)

1. The method of purification of flue gases from nitrogen oxides, including the treatment of flue gases with urea at a temperature of its decomposition followed by catalytic purification of gases from nitrogen oxides by products of thermal decomposition of urea, characterized in that the catalytic purification of gases from nitrogen oxides is carried out at 90 170 ^o C. 2. The method according to claim 1, characterized in that the processing of urea is carried out at 300 to 1200 ^o C. 3. The method according to claim 1, characterized in that the post-treatment of flue gases from nitrogen oxides is carried out at 90 150 ^o C.

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