RU2411065C1 - Method of removing nitrogen oxides from flue gases - Google Patents

Abstract

FIELD: process engineering. ^ SUBSTANCE: invention relates to uncatalysed purification of flue gases of nitrogen oxides and may be used to reduced their content in flue gases of fuel-firing arrangements of whatever power and application. Water solution of carbamide stays in contact with overheated steam at 150 - 500C and 3-10 atm for 0.5 - 5.0 s. Obtained has-steam reducing mix is mixed with ozone-air or ozone-oxygen mix, while mixed product is fed into flue gas flow with temperature of 150 - 1000C. Zone-to-carbamide weight ratio makes 0.01 - 1.0. Preferable, mixed product is fed in flow of flue gases with temperature of 250 - 450C, and contact between said product with flue gas flow lasts 0.2 - 1.0 s. Mixed product is fed by carrier gas selected from the group comprising air, steam, flue gases, inert gases or mix thereof. ^ EFFECT: higher degree of purification in wide temperature range. ^ 4 cl, 3 tbl, 3 ex

Description

The invention relates to processes for the non-catalytic purification of flue gases from nitrogen oxides (NO _x ) and can be used to reduce the content of NO _x in the flue gases of fuel-burning plants of any capacity and purpose.

It is known that carbamide, ammonia, ammonium salts, isocyanic acid and other amine-containing compounds (US No. 3900504, US No. 4208386, US No. 4726302) can serve as reducing agents for the process of selective non-catalytic reduction of nitrogen oxides.

The disadvantages of these methods are the low efficiency of the process of purification of exhaust gases from nitrogen oxides, the presence 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.

Of the large number of possible reducing agents for nitrogen oxides, ammonia and urea are of practical importance, with urea being more preferable from the point of view of environmental safety of the process (RU No. 2271856, 2006).

The optimal temperature of the process of non-catalytic gas purification using urea is in a fairly narrow range of 900-1000 ° C (US No. 4208386, US No. 4726302, RU No. 2271856).

However, there are a number of thermal units in which the combustion chamber zone, where the temperature of the flue gas is optimal for the non-catalytic cleaning process, is not available for introducing the reducing agent, and the temperature of the flue gas at the outlet of the heat generator is 300-500 ° С. These are internal combustion engines, diesel engines, gas turbine and gas engine plants, process furnaces. It is also known that during operation of thermal units, significant fluctuations in the heat load occur. If the temperature of the flue gases in the input zone of the reducing agent decreases, the efficiency of the process of non-catalytic cleaning of flue gases from nitrogen oxides decreases.

To make it possible to carry out the process of reducing nitrogen oxides at a lower temperature, additives to the reducing agent are used, for example, oxygen-containing organic compounds: aldehydes, ketones, ethylene glycol, etc. (US No. 4719092), guanidine, melamine, furfural, calcium cyanamide, methyl phenols, etc. (US No. 4751065, US No. 4770863, US No. 4927612), as well as ozone, nitric acid, hydrogen peroxide, chlorine dioxide, perchloric acid, sodium chloride, sodium hypochlorite, etc. (US No. 4119702, 1978, RU No. 2314861, US No. 4213944 , 1980).

The use of these additives to an aqueous urea solution allows the process of selective reduction of nitrogen oxides in the temperature range 200-800 ° C.

The disadvantages of the above methods of purification using additives are the possibility of the formation of hazardous secondary pollutants during gas purification from nitrogen oxides, as well as the insufficiently high degree of gas purification from nitrogen oxides.

Closest to the technical nature of the invention is a method of non-catalytic purification of flue gases described in US patent No. 4119702, 1978.

According to the specified method, the purification of flue gases from nitrogen oxides is carried out in the presence of an aqueous solution of urea and oxygen-containing additives, in particular ozone at a temperature of 200-800 ° C.

However, according to the examples of the specific implementation of the method described in the patent, the degree of gas purification from nitrogen oxides using ozone in the temperature range 400-700 ° C is 0-30%.

Thus, the disadvantage of this method is the low degree of purification of flue gases.

The objective of the present invention is to increase the efficiency of cleaning flue gases from nitrogen oxides and expanding the temperature range of effective gas cleaning.

The problem is solved by a method of purifying flue gases from nitrogen oxides, which consists in contacting an aqueous urea solution with superheated water vapor at a temperature of 150-500 ° C, a pressure of 3-10 atm, for 0.5-5.0 s, the resulting vapor-gas reduction mixture is mixed with an ozone-air or ozone-oxygen mixture and the mixing product is fed into the flue gas stream with a temperature of 150-1000 ° C, while the mass ratio of ozone: urea is 0.01-1.0.

Preferably, the mixing product is fed into a flue gas stream having a temperature of 250-450 ° C.

Preferably, the residence time of the mixing product in the flue gas stream is 0.2-1.0 s.

The mixing product may be supplied with a carrier gas selected from the group consisting of air, water vapor, flue gases, inert gases, or a mixture thereof.

The technical result achieved is to increase the degree of purification of flue gases from nitrogen oxides in a wide temperature range.

The essence of the claimed invention is as follows.

To prepare a gaseous reducing mixture, an aqueous urea solution is used in an amount sufficient to reduce the nitrogen oxides contained in the flue gas. The initial urea solution is transferred to the vapor-gas phase by thermal decomposition at a temperature of 150-500 ° C, a pressure of 3-10 atm, for 0.5-5.0 s. An ozone-containing mixture, namely, an ozone-air or ozone-oxygen mixture, is added to the resulting vapor-gas reduction mixture, and the mixing product is introduced using a carrier gas into a stream of cleaned flue gases with a temperature of 150-1000 ° C, preferably 250-450 ° C. The mass ratio of ozone: urea is 0.01-1.0. The residence time of the mixing product in the flue gas stream is 0.2-1.0 s. As the carrier gas, water vapor, flue gases, compressed air, nitrogen, inert gases or a mixture thereof can be used.

The process according to the invention provides an unexpected result, which consists in a sharp increase in the degree of purification of flue gases due to the introduction of the above ozone-containing mixture directly into the vapor-gas recovery mixture obtained by the above technology.

The following are examples illustrating but not limiting the invention described.

Example 1

An initial mixture of gases simulating flue gases and containing nitrogen, oxygen, nitrogen oxides (NO and NO ₂ ) is fed into a quartz reactor with a diameter of 20 mm, placed in an electric furnace, at a speed of 210 l / h. The oxygen content in the mixture is 10 vol.%, The content of nitrogen oxides is indicated in table 1, the rest is nitrogen. The vapor-gas recovery mixture obtained by thermal decomposition of a 0.5% aqueous urea solution is mixed with an ozone-air mixture. The urea consumption ratio with respect to nitrogen oxides is 1.5. The mass ratio of ozone: urea is 0.02. The residence time of the gas mixture in the stream of purified flue gas in the reaction zone is 1 s. The experimental results are shown in table 1.

Table 1 T, ° C Reactor inlet concentration, ppm Concentrations at the outlet of the reactor, ppm The degree of recovery of NO _x ,% NO ₂ NO NO _x NO ₂ NO NO _x 158 95 167 263 thirty 192 222 15,52 226 95 164 258 82 0 82 68.26 253 96 164 260 27 0 27 89.61 293 93 164 258 3 0 3 98.84 328 93 164 258 2 0 2 99.22 353 93 164 258 2 0 2 99.22 400 93 164 258 3 one four 98.45 426 93 164 258 2 four 6 97.67 515 93 164 258 one fourteen fifteen 94.18 625 93 164 258 0 21 21 91.85 720 93 164 258 one 25 26 89.91 820 93 164 258 one fourteen fifteen 94.18 915 93 164 258 one 8 9 96.51 1000 87 164 251 one 26 27 89.25

Example 2

The experimental conditions are similar to those described in example 1, except for the following:

- feed rate of the initial mixture of gases - 660 l / h;

- the oxygen content in the mixture is 15 vol.%;

- mass ratio of ozone: urea is 0.2;

- the residence time of the gas mixture in the reaction zone is 0.32 s.

The experimental results are shown in table.2.

table 2 T, ° С Reactor inlet concentration, ppm Concentrations at the outlet of the reactor, ppm The degree of recovery of NO _x ,% NO ₂ NO NO _x NO ₂ NO NO _x 180 166 eighteen 184 64 0 64 24.75 260 162 eighteen 179 35 0 35 80.50 308 176 17 193 four 0 four 97.93 363 163 17 180 6 0 6 96.66 417 166 16 183 2 34 36 95.46 517 172 17 189 10 eighteen 28 85.19 545 173 17 190 9 0 9 95.25 622 172 17 190 9 four 13 93.14 721 172 17 189 fourteen 29th 43 77.26 822 172 17 189 10 26 36 80.96 912 172 17 189 2 eleven 13 84.96 960 176 17 193 2 17 19 80,10

Example 3

The experimental conditions are similar to those described in example 1, except for the following:

- feed rate of the initial mixture of gases - 420 l / h;

- the oxygen content in the mixture is 20 vol.%;

- mass ratio of ozone: urea is 0.9;

- the residence time of the gas mixture in the reaction zone is 0.5 s.

The experimental results are shown in table.3.

Table 3 T, ° С Reactor inlet concentration, ppm Concentrations at the outlet of the reactor, ppm The degree of recovery of NO _x ,% NO ₂ NO NO _x NO ₂ NO NO _x 206 146 twenty 166 137 0 137 17.33 250 147 twenty 167 24 0 24 85.58 300 147 twenty 167 3 0 3 98.20 336 143 twenty 163 2 0 2 98.77 392 145 twenty 16 four one 5 96.96 451 140 twenty 160 5 6 eleven 93.14 526 140 21 161 5 9 fourteen 91.28 595 141 21 162 four 13 17 89.48 620 142 21 163 5 fourteen 19 88.34 717 146 22 168 6 19 25 85.09 829 145 17 162 four 22 26 83.94 929 141 17 158 one 13 fourteen 91.16 1010 141 19 160 one 32 33 89.38

Similar results are obtained using an ozone-oxygen mixture.

As can be seen from the above data, as a result of the method according to the invention, the degree of purification of flue gases from nitrogen oxides reaches 99%, while a high degree of purification is observed in a wide temperature range (250-1000 ° C and above).

Claims (4)

1. The method of purification of flue gases from nitrogen oxides, which consists in contacting an aqueous urea solution with superheated water vapor at a temperature of 150-500 ° C, a pressure of 3-10 atm, for 0.5-5.0 s, obtained the vapor-gas reduction mixture is mixed with the ozone-air or ozone-oxygen mixture and the mixing product is introduced into the flue gas stream with a temperature of 150-1000 ° C, while the mass ratio of ozone: urea is 0.01-1.0. 2. The method according to claim 1, characterized in that the mixing product is introduced into the flue gas stream having a temperature of 250-450 ° C. 3. The method according to claim 1, characterized in that the residence time of the mixing product in the flue gas stream is 0.2-1.0 s. 4. The method according to claim 1, characterized in that the mixing product is supplied with a carrier gas selected from the group consisting of air, water vapor, flue gases, inert gases or a mixture thereof.