RU2297272C2 - Method of selective catalytic cleaning of exhaust gases from nitrogen oxides in industrial plants - Google Patents

Abstract

FIELD: cleaning exhaust gases from nitrogen oxides in industrial plants by means of selective catalytic cleaning with the use of ammonia.

SUBSTANCE: proposed method includes reduction of nitrogen oxides by ammonia on catalyst in presence of hydrogen. Before delivery to catalyst, exhaust gases are mixed with purge gases from the ammonia synthesis cycle. Content of hydrogen in mixture is maintained below low limit of ignition. Purge gases are enriched with ammonia by mixing them with ammonia synthesis gases.

EFFECT: reduced consumption of ammonia; enhanced mixing of exhaust gases with ammonia; reduced emissions of ammonia into atmosphere; reduced power requirements.

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Description

The invention relates to a technology for the selective purification of exhaust gases from nitrogen oxides before discharge into the atmosphere using ammonia and is primarily intended for purification of exhaust gases in industrial plants for the production of nitric acid, but can also be used in other industrial plants and energy facilities.

In modern conditions, the catalytic purification of the exhaust gases of nitric acid production plants is becoming an integral element of the technological process, providing fine purification to a residual content of NO _{x of the} order of 0.005 vol.%.

In the countries of the former USSR, all numerous nitric acid plants were equipped with such a treatment.

In large-capacity plants (UKL-7, AK-72), catalytic systems with high-temperature (700-760 ° C) purification using natural gas prevailed, in installations with low-power aggregates (according to the 1 / 3.5 ata scheme), a selective cleaning system prevailed using ammonia in the temperature range of 250-280 ° C.

The source of information is the Nitrogen Handbook, Volume 2, Moscow, Chemistry, 1987, pp. 59-62, 65-92.

In recent years, selective catalytic cleaning has replaced high-temperature cleaning in UKL-7 units; this process has also begun for the reconstruction of AK-72 units. This is due to a number of reasons, but the main thing is that this eliminates or saves a catalyst containing a precious metal (palladium) and a sharp decrease in the content of another harmful substance in the exhaust gases - carbon monoxide (CO).

Like the high-temperature natural gas purification method, selective purification using ammonia provides a residual NO _x content of 0.005 vol.%.

The widespread use of the selective purification method using pure gaseous ammonia, which is taken as a prototype of the invention, draws attention to some of its disadvantages:

- additional consumption of ammonia (3-4 kg per 1 ton of HNO ₃ );

- the residual content of NH ₃ in the exhaust gases (0.005-0.01 vol.%), which in the atmosphere can transform into NO _x , and therefore as a harmful substance is equal to NO _x .

The latter is associated with the need to maintain a certain excess of NH ₃ against stoichiometry with respect to NO _x to ensure complete purification of exhaust gases from them. The amount of NH ₃ excess is affected by the imperfection of mixing gaseous ammonia with exhaust gases in front of the reactor, the volume ratio of which is ~ 1: 1000.

The increase in ammonia consumption, although insignificant (1.05-1.4%), increases the cost of nitric acid.

In the existing method for the selective catalytic purification of exhaust gases, i.e. according to the prototype, pure ammonia gas is supplied to the mixer in front of the reactor at a pressure slightly higher than the pressure of the exhaust gases.

Therefore, in large-capacity units of the AK-72 type, gaseous ammonia under the required pressure (11-12 atm) is forced to receive liquid ammonia vapor in a special installation, which requires additional investment and complicates operation.

The technical problems to which the present invention is directed are:

- saving ammonia;

- improving the mixing of exhaust gases with ammonia (achieving homogeneity of the mixture) and, as a result, reducing the content of unreacted ammonia in the exhaust gases;

- saving of energy resources (electricity, natural gas).

According to the invention, these technical problems are solved together by mixing exhaust gases with a mixture of ammonia with hydrogen-containing gases. Such a mixture can either be composed of gaseous ammonia, hydrogen or hydrogen-containing gases, or be a finished mixture without further preparation.

At enterprises where nitric acid production plants are operated, ammonia is also usually produced.

Continuous ammonia production wastes are purge gases from the ammonia synthesis cycle and tank gases released in the tanks when liquid ammonia is synthesized from the synthesis system with a pressure relief of up to 20-40 atm.

Typical composition of purge gases at existing ammonia plants (AM-70, AM-76), vol.%:

NH ₃ 1.3-2.0 H ₂ 58.6-59.7 N ₂ 19.6-9.9 CH ₄ 13.5-15.5 Ar 5.0-5.2

In the ammonia unit with a capacity of 450-500 thousand tons / year, the amount of purge gases ranges from 7500-8300 nm ³ / hour.

Tank gases have the composition: Before freezing ammonia, vol.%: After freezing, vol.%: NH ₃ - 20-30 4-6.5 H ₂ - 33.0-39.0 43-44.5 N ₂ - 12.0-18.0 14.3-14.8 CH ₄ - 13.0-20.0 23-25 Ar - 3.9-4.3 4.8-5.1

In the ammonia aggregate, 1,500-2,000 nm ³ / hr of tank gases are averaged.

Purge and tank gases after freezing ammonia from them are sent to combustion in a mixture with natural gas in the burners of reforming furnaces of ammonia production, i.e. used as fuel. In this case, the ammonia contained in them is lost.

In some installations, ammonia from tank gases is washed with water to produce 15-20% ammonia water.

According to the present invention, tank and purge gases containing ammonia are used for the selective purification of exhaust gases from the production of nitric acid as ready-made ammonia-hydrogen-containing mixtures.

In the production of nitric acid according to the scheme 1 / 3.5 ATA and UKL-7, it is advisable to use tank gases, and before freezing or partial freezing of ammonia, depending on the production capacity of nitric acid.

The essence and example of the use of the present invention should be illustrated with reference to the production of nitric acid in units according to the scheme 7.3 ata (UKL-7), the most common at enterprises in Russia and the CIS countries.

As a typical case, for an ammonia unit with a capacity of 450-500 thousand tons / year, the enterprise has 5 UKL-7 units with a total capacity of 600 thousand tons / year HNO ₃ or hourly - 5 × 14.8 = 74 tons / hour.

For 1 ton of HNO _3, 3150-3200 nm ^{3 of} exhaust gases with a residual NO _x content after absorption of 0.1 vol.% Are subject to selective purification. When the degree of oxidation of NO _x equal to 50%, the exhaust gases contain 3175 × 0.001 × 0.5≈1.59 nm ³ NO and NO ₂ .

The stoichiometric consumption of ammonia for the reduction of NO and NO ₂ is determined by the reactions:

6NO + 4NH ₃ = 5N ₂ + 6H ₂ O

6NO ₂ + 8NH ₃ = 7N ₂ + 12H ₂ O

It will be 1.06 + 2.12 = 3.18 nm ³ per 1 ton of HNO ₃ , and with an excess of 5% for completeness of purification from NO _x against stoichiometry - 3.34 nm ³ .

Hourly demand for ammonia for 5 units 3.34 × 74 = 247.2 nm ³ .

Tank gases — a waste from one ammonia unit — contain, on average, ~ 1750 × 0.2 = 350 nm ³ NH ₃ before freezing, i.e. the hourly need for ammonia to clean the exhaust gases is provided.

The amount and composition of tank gases to the exhaust gas purification system of 5 UKL-7 units (according to the average composition):

nm ³ / hour about.% NH ₃ - 250,0 24.0 H ₂ - 406.5 39.0 N ₂ - 135.0 13.0 CH ₄ - 208.5 20,0 Ar - 41.5 4.0 1041.5 100.0

The ratio of the volumes of the ammonia-hydrogen mixture and exhaust gases is 1041.5: 3175 × 74.0 = 1: 226, i.e. ~ 5 times higher than in the prototype the ratio of "gaseous ammonia - exhaust gases", which is a prerequisite for a uniform distribution of NH ₃ and NO _x in gases.

The composition of the mixture of exhaust gases with ammonia in front of the catalytic treatment reactor (per 1 t of HNO ₃ ):

nm ³ about.% NH ₃ - 3.34 0.105 About ₂ - 82.6 2.59 H ₂ - 5,4 0.17 N ₂ - 3050.0 95.64 CH ₄ - 2,8 0.08 Ar - 0.6 0.02 H ₂ O - 44.3 1.43 NO _x - 3.0 0.1 3191.0 100.0

The content of H ₂ and CH _{4 is} significantly lower than the lower ignition limit of these combustible substances in mixtures even with air (4% for H ₂ and 5.5% for CH ₄ ), especially in a mixture with an inert content> 95%.

The effectiveness of the proposed method in UKL-7 units when they are converted to selective exhaust gas treatment is increased if instead of the commonly used vanadium catalyst AVK-10, aluminum-copper-zinc catalyst AMC-10 is used, which is designed to prevent vanadium corrosion of stainless steels at temperatures> 600 ° C.

Unlike the AVK-10 catalyst at AMC-10, hydrogen interacts with oxygen at 250-280 ° С. In accordance with the heat balance at the above hydrogen content in the mixture (0.17 vol.%), A temperature increase of up to 20 ° C can be obtained. If tank gas with a higher freezing point of ammonia and, accordingly, with a higher hydrogen content is used, this will allow either to reduce the preliminary heating of the exhaust gases in front of the selective purification reactor by 30-50 ° C, and to simplify this unit with an increase in the steam output, or to increase the temperature of the exhaust gases in front of the turbine combustion chamber to 320 ° C (by 50 ° C), and slightly reduce the specific consumption of natural gas per 1 ton of HNO ₃ , and also additionally release air to produce HNO ₃ .

In the above example, the positive effect consists of the following components:

- ammonia saving - not less than 280 tons / year for 5 units UKL-7 (which is burned);

- improves the homogeneity of the mixture of exhaust gases with ammonia and reduces the emission of ammonia into the atmosphere;

- artificial cold (electricity) is saved, except for the freezing of ammonia ~ 80 thousand kWh.

The most effective use of the proposed method in the reconstruction of nitric acid aggregates AK-72 with their transfer to low-temperature selective cleaning while maintaining a high-temperature reactor for heating exhaust gases in front of a gas turbine to 740-760 ° C.

In addition to the positive effect described above for the reconstruction of UKL-7, a significant improvement in the operating conditions of the PVG-1200 exhaust heater and the ability to not increase the surface of the convective part of the heater to achieve an exhaust gas temperature of 300 ° C are added.

This is due to the ability to increase the temperature of the exhaust gases in the selective purification reactor due to the combustion of hydrogen on the catalyst AMC-10, which allows the temperature to rise to 320 ° C. Therefore, in the convective part of PVG-1200, it is possible to limit oneself to heating the exhaust gases to 270-280 ° C, to get heating to 320 ° C in the reactor. As a result, the thermal load of the radiation part of PVG-1200 is reduced, the consumption of natural gas due to heat loss with exhaust gases after PVG-1200, emitted into the atmosphere.

In the AK-72 aggregate, the NO _X content after absorption is 0.07 vol.%.

The consumption of ammonia with a 5% excess against stoichiometry is 2.34 nm ³ .

In order to obtain a temperature increase of 40 ° С (from 280 to 320 ° С) in the selective purification reactor, the hydrogen content should be ~ 0.5%.

With the above compositions of the tank, purge and exhaust gases, it is necessary, as calculations show, for 1 ton of HNO ₃ to supply 7.91 nm ^{3 of} tank gases and 22.1 nm ^{3 of} purge gases.

The amount and composition of the mixture of tank and purge gases per 1 ton of HNO _{3 is} as follows:

nm ³ about.% NH ₃ - 234 - 7.8 H ₂ - 16.06 - 53.63 CH ₄ - 4.78 - 15.93 N ₂ - 5.39 - 17.97 Ar - 1.42 - 4.73 30.00 100.0

The amount and composition of the mixture of exhaust gases on the power of the unit AK-72 50 tons of HNO ₃ per hour: nm ³ about.% NH ₃ - 117.0 - 0,073 H ₂ - 803.0 - 0.50 CH ₄ - 239.0 - 0.149 N ₂ - 152669.0 - 95.18 Ar - 236.5 - 0.147 O ₂ - 4,130.0 - 2,575 H ₂ O- 2215.0 - 1.38 NO _x - 111.0 - 0,07 160,520.5 100.0

The hourly consumption of tank gases is 7.91 × 50≈400 nm ³ . Thus, one unit of ammonia type AM-70.76 provides the need for 2 units of AK-72.

The hourly flow rate of purge gases is 22.1 × 50≈1100 nm ³ , incl. hydrogen ≈590 nm ³ .

Thus, 2 AK-72 units are provided.

With membrane separation of purge gases and the return of hydrogen to the synthesis cycle, the amount of hydrogen in the waste from one ammonia unit is ~ 400 nm ³ / h. Therefore, for 2 AK-72 units, purge gas should be used at this stage before separation.

The above examples do not exhaust the possibility of the proposed method. These examples are limited by the properties used at the enterprises of domestic selective purification catalysts such as AVK-10 and AMC-10.

So, on the AVK-10 vanadium catalyst, hydrogen does not interact with oxygen in the operating temperature range for this catalyst of 250-280 ° C. If the entrainment of the catalyst dust were not threatened with vanadium corrosion of stainless steel equipment at temperatures above 600 ° C, then its use would allow the AK-72 unit to fully use the positive aspects of the proposed method. Namely, it would allow significantly more purge gases to be fed into the exhaust gas before selective purification of the ammonia synthesis shop, to increase the hydrogen content in the mixture to 2.5 vol.%

This would make it possible to reduce the ignition temperature of the catalyst in a high-temperature reactor to 320-350 ° C and eliminate the need to heat the gas after the selective purification reactor in the radiation part of PVG-1200, which would simplify the circuit of this unit and provide significant additional savings in natural gas.

The AMC-10 catalyst is devoid of this drawback of AVK-10, but the process described above cannot be carried out with its use due to the temperature limit (no more than 320 ° C), and hydrogen on this catalyst reacts with oxygen with increasing temperature. But since one can expect the creation of other selective purification catalysts, allowing them to increase the temperature of the selective purification process to ~ 500 ° C at an initial ignition temperature of 250-300 ° C, the possibilities of the proposed method will be used in a wider scope than in the above examples .

It should be noted that hydrogen, as a reducing agent, can partially participate in the process of selective purification along with ammonia. Therefore, in addition to the above positive effect (saving ammonia, natural gas, electricity), the technical solution of the present invention allows us to expect a further reduction in the emission of harmful substances into the atmosphere by reducing the residual ammonia content in the exhaust gases, not only due to improved mixing quality, but also due to that hydrogen removes the need to maintain an excess of ammonia in the mixture against a stoichiometric ratio with NO _x .

Literature

1. The Nitrogen Handbook, Volume 2. Moscow, Chemistry, 1987.

Claims (2)

1. The method of selective catalytic purification of exhaust gases from nitrogen oxides, including the reduction of nitrogen oxides with ammonia on a catalyst in the presence of hydrogen, characterized in that the exhaust gases are mixed before being fed to the catalyst with purge gases from the ammonia synthesis cycle, the hydrogen content in the mixture being kept below the lower ignition limit. 2. The method according to claim 1, characterized in that the purge gases are enriched with ammonia, mixing them with tank gases for the synthesis of ammonia.