RU2271856C2 - Method of purification of flue gases from nitrogen oxide - Google Patents

Abstract

FIELD: methods of purification of flue gases.

SUBSTANCE: the invention is pertaining to the method of purification of flue gases and may be used to decrease the outbursts of nitrogen oxide with the help of the method of the high-temperature selective non-catalytic reduction. The method provides for feeding of the previously prepared steam-gaseous reduction mixture into the gas flue of the flue gas burning aggregate with the temperature of the flow of the purification gases of 700-1200°C. At that the steam-gaseous reduction admixture is prepared in the connected to the gas flue high-speed reactor during 0.5-5 seconds by introduction of a water solution of carbamide with a superheated steam at the pressure of 3-10 atm. Concentration of the water solution of carbamide makes 20-40 mass %. The time of the contact of the water solution of carbamide with the superheated steam predominantly makes 0,5-2 seconds. The pressure in the reactor predominantly makes 3-6 atm. The temperature of the superheated steam makes 200-400°C. The invention ensures simplification of the production process of purification of the effluent gases from nitrogen oxides, to increase the level of purification of such a gas in the broad range of the temperatures of the purified combustion products, and also to decrease significantly the share of the secondary contaminant - ammonia, in the purified gases.

EFFECT: the invention ensures simplification of the production process of the effluent gases purification from nitrogen oxides, to increase the level of purification of the gases in the broad range of the temperatures of the purified combustion products, to decrease significantly the share of the secondary contaminant - ammonia in the purified gases.

5 cl, 2 ex, 1 dwg, 2 tbl

Description

The invention relates to the field of flue gas purification and can be used to reduce emissions of nitrogen oxides from the products of combustion of fuel-burning units by the method of high-temperature selective non-catalytic reduction of nitrogen oxides using urea (urea).

Known methods for selective non-catalytic high-temperature cleaning of flue gases (SNCR) from nitrogen oxides US No. 4208386, C1 423/235, 1980 [1]; US No. 4325924, C1 423/235, 1982 [2], using urea for this purpose. Urea can be injected into the gas stream being cleaned with a temperature exceeding 1300 ° F (moreover, the temperature of the gas being purified should not exceed 1900 [1] - 2500 ° F [2]), in the form of a dry powder, melt or its solution in one solvent or mixture solvents.

The disadvantages of these methods are the low efficiency of the process of purification of exhaust gases from nitrogen oxides, the high content in the purified gases of a secondary pollutant - ammonia, as well as the limited temperature range within which the process of more efficient gas purification from nitrogen oxides proceeds.

In a number of patents, including the aforementioned, to improve the efficiency of the process of purification of gases from nitrogen oxides using urea, it is proposed to use a number of other reducing chemicals together with urea (US No. 4751065, C1 423/235 1988; US No. 4844878, C1 423 / 235, 1989; US No. 4888165, C1 423/235, 1989).

The disadvantages of these methods are also the low efficiency of the process of purification of exhaust gases from nitrogen oxides, the high content in the purified gases of a secondary pollutant - ammonia, as well as the limited temperature range within which the process of the most effective gas purification from nitrogen oxides proceeds.

Improving the process of gas purification from nitrogen oxides using urea in the direction of increasing the efficiency of the process and reducing the emission of secondary pollutants, the main of which is ammonia, is described in patents: US No. 4777024, US C1 423/235, 1988; US No. 4780289, US C1 423/235, 1988. By adding various chemicals, it was possible to achieve an ammonia content of 30-90 ppm in the purified gases.

The disadvantage of these methods is the need to enter additional chemical reagents, as well as the insufficiently high efficiency of the process with respect to the purification of gases from nitrogen oxides (up to 50-60%).

A known method of purification of flue gases from nitrogen oxides US Patent No. 5240688, US C1 423/235, 1993, which consists in the fact that urea hydrolysis products are introduced into the stream of purified gases, and this patent states that its aqueous solution is used for hydrolysis of urea, the hydrolysis process itself is carried out at a temperature of 300-500 ° F and for at least three minutes (for the urea hydrolysis reaction to proceed completely, the time should be at least six minutes) with the formation of at least one of the following product s, or mixtures thereof: ammonium carbamate, ammonium carbonate, ammonium bicarbonate and ammonia. The hydrolysis rate can be increased by carrying out the reaction under increased pressure (1200-1500 psi), or by introducing hydrolyzing agents into the neutral urea aqueous solution - substances that change the pH of the aqueous urea solution. It is also possible to use compositions for purification of exhaust gases from nitrogen oxides that include, in addition to the products of urea hydrolysis, a number of organic compounds, which makes it possible to satisfactorily clean exhaust gases with a lower temperature from nitrogen oxides.

The disadvantages of this method are also the low degree of purification of exhaust gases from nitrogen oxides, as well as the use of additional reagents that would increase the efficiency of this purification.

Closest to the invention in technical essence and the achieved result is a method of purification of flue gases from nitrogen oxides according to the patent of the Russian Federation No. 2040737, class. F 23 J 15/00, 1992, according to which the cleaning of the products of combustion of fuel-burning units from nitrogen oxides is carried out by injecting into the zone of exhaust gases with a temperature of 700-1200 ° C a vapor-gas reducing mixture previously obtained by thermohydrolysis of urea in its aqueous solution at a temperature of 60 -150 ° C followed by evaporation of the reaction mixture at a temperature of 100-350 ° C using superheated steam. The processes of thermohydrolysis and evaporation of the resulting mixture of thermohydrolysis are carried out in successive reactors.

The disadvantages of this method are the insufficient efficiency of purification of exhaust gases from nitrogen oxides in a wide temperature range of the purified gases, pollution of the exhaust gases with a secondary pollutant - ammonia, as well as the complexity of the process for producing a reducing mixture.

The objective of the invention is to simplify the technological scheme of purification of exhaust gases from nitrogen oxides, increasing the degree of gas purification in a wide temperature range of the purified combustion products, as well as a significant reduction in the amount of secondary pollutant - ammonia, in the purified gases.

The problem according to the invention is solved in that in a method for the selective non-catalytic purification of flue gases from nitrogen oxides, comprising supplying a pre-obtained steam-gas recovery mixture to a gas duct of a fuel-burning unit in a stream of cleaned gases with a temperature of 700-1200 ° C, according to the invention, a gas-vapor reduction mixture is obtained in gas duct of a high-speed reactor by introducing into contact for 0.5-5 s an aqueous urea solution with superheated steam at a pressure of 3-10 atm.

The problem is also solved by the fact that the concentration of the aqueous urea solution is 20-40 wt.%, The pressure is mainly 3-6 atm, and the temperature of the superheated steam is 200-400 ° C.

The problem is also solved by the fact that air, water vapor, flue gases are used as a carrier gas uniformly distributing the reducing mixture in the stream of purified gases.

The essence of the claimed invention is as follows.

Schematic diagram of the process of selective non-catalytic high-temperature cleaning of flue gases from nitrogen oxides is shown in the drawing.

An aqueous urea solution with a concentration of 20-40 wt.% Is supplied to the high-speed reactor 1 through line 2, and superheated steam with a temperature of 200-400 ° C is fed through line 3. The flow rate of the urea solution is determined by the volume of purified gases and the concentration of nitrogen oxides in them. The consumption of superheated steam depends on the consumption of the urea solution and can be 15-30 kg per 1 kg of solution. In a high-speed reactor under a pressure of 3-10 atm, an aqueous solution of urea and superheated steam contact for 0.5-5.0 s, resulting in thermal decomposition of urea and evaporation of decomposition products with the formation of a vapor-gas recovery mixture, which through nozzle devices 4 connected to the output of the high-speed reactor, using the supply line 5 of the carrier gas is introduced into the duct 6, the temperature of the gas stream in which is at a level of 700-1200 ° C.

The following examples only illustrate the invention, but do not limit it.

Example 1 (prototype). A steam boiler with a steam capacity of 100 t / h was operating at 50% load. The flue gas purification process was carried out with a gas-vapor recovery mixture obtained by preliminary thermohydrolysis of a 20% aqueous urea solution and subsequent evaporation of the resulting substances. Evaporation was ensured by adding superheated steam with a temperature of 250 ° C and a pressure of 5 atm. The test results are shown in table 1.

Table 1
Mode Gas temperature in the urea feed zone, ° С The content of NO _x , mg / m ³ The degree of purification,% The content of NH ₃ after purification, mg / m ³ Before entering urea After entering urea one 1006 181 54 70 26 2 1006 131 32 76 21 3 1006 121 38 69 19

Example 2. The process of cleaning flue gases was carried out on an incinerator. The volume of purified gases is 40,000 nm ³ / h. An aqueous urea solution with a concentration of 40 wt.% Was used as a reducing agent. The flow rate of the urea solution was 10-20 kg / h. The flue gas cleaning process was carried out with a gas-vapor recovery mixture previously obtained in a high-speed reactor by contacting for 1 sec a 40% aqueous urea solution and superheated water vapor with a temperature of 250 ° C and a pressure of 5 atm. The consumption of superheated steam was 300-600 kg / h. The test results are shown in table 2.

table 2
Mode Gas temperature in the urea feed zone, ° С The content of NO _x , mg / m ³ The degree of purification,% The content of NH ₃ after purification, mg / m ³ Before entering urea After entering urea one 990 289 35 88 2 2 950 314 fifty 84 5 3 860 295 89 70 eleven four 1010 288 32 89 2 5 870 291 81 72 10 6 960 300 41 86 four 7 840 280 92 67 13 8 890 266 67 75 9

A comparative analysis of the results of purification of flue gases from nitrogen oxides according to the prototype and according to the invention showed a significant increase in the degree of gas purification, as well as the expansion of the temperature range of the effective process using the method according to the invention. In addition, the concentration of the secondary pollutant, ammonia, is significantly reduced in the purified gases.

Claims (5)

1. The method of selective non-catalytic purification of flue gases from nitrogen oxides, comprising supplying a pre-prepared vapor-gas recovery mixture to the gas duct of a fuel-burning unit with a flow temperature of treated gases of 700-1200 ° C, characterized in that the gas-vapor recovery mixture is obtained in a high-speed reactor connected to the gas duct during 0.5-5 s by contacting an aqueous urea solution with superheated steam at a pressure of 3-10 atm. 2. The method according to claim 1, characterized in that the concentration of the aqueous urea solution is 20-40 wt.%. 3. The method according to claim 1, characterized in that the contact time of the aqueous urea solution and superheated steam is preferably 0.5-2 s. 4. The method according to claim 1, characterized in that the pressure in the reactor is preferably 3-6 atm. 5. The method according to claim 1, characterized in that the temperature of the superheated steam is 200-400 ° C.