RU2550864C2 - Method for high-temperature non-catalytic removal of nitrogen oxides from combustion products with multi-zone input of reducing agent - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to high-temperature purification of combustion products of all types of organic fuel from nitrogen oxides (NO_x) via selective non-catalytic reduction thereof. A method for high-temperature purification of combustion products in the exhaust duct of a thermal generating unit from nitrogen oxides via selective non-catalytic reduction thereof includes feeding an ammonia-containing reducing agent into selected zones of the exhaust duct that are bounded by the temperature range of 850 - 1100°C. The method is characterised by that the reducing agent is simultaneously fed into all said selected zones, where in each of said zones the reducing agent is fed with a flow coefficient of less than one relative to the stoichiometric flow rate. The number of the reducing agent input zones is preferably 2-5. The flow coefficient of the reducing agent fed into each purification zone is preferably in the range of 0.2-0.8. The reducing agent is preferably ammonia or carbamide.

EFFECT: providing a maximum degree of reduction of NO_x without exceeding the stoichiometric ratio of the flow of the reducing agent.

5 cl, 1 dwg, 4 tbl

Description

Area of use

The invention relates to high-temperature purification of combustion products of all types of fossil fuels from nitrogen oxides (NO _x ) by selective non-catalytic reduction (SNCR) and can be used to reduce the emission of NO _x into the atmosphere with flue gases of thermal units for various purposes, mainly steam boilers of thermal power plants .

State of the art

Flue gas purification by SNCR is usually carried out by introducing an ammonia-containing reducing agent into the gases. Known methods for high-temperature purification of combustion products from nitrogen oxides by their SNCR (US 4208386, B01D 53/56, 1980; US 4325924, B01D 53/56, 1982 [1]), using urea as a reducing agent. In this case, the temperature range of the gas stream into the zone of which the reducing agent is introduced is 1900 ... 2500 ° F (1000 ... 1350 ° C). The disadvantage of these methods is the low efficiency of the gas stream purification process due to the significant temperature dependence of the efficiency of the reduction of nitrogen oxides within the specified interval. In addition, a decrease or increase in temperature within the indicated range is relatively optimal and leads to an increase above the permissible limits of the content of the secondary pollutant in the purified gases - ammonia. The fact is that during the operation of thermal units there are significant fluctuations in their heat load, and therefore the temperature of the gas stream being cleaned. If the temperature of the gas stream in the input zone of the reducing agent decreases, the efficiency of the SNCR process decreases. To solve this problem, reducing agents are used to reduce the temperature of the NO _x reduction process, for example, oxygen-containing organic compounds: aldehydes, ketones, ethylene glycol (US 4719092, B01D 53/56 [2]) or hydrogen-containing inorganic compounds: guanidine, melamine and others ( US 4751065; US 4770863; US 4927612; US 4119702 (all B01D 53/56) [3]). However, the introduction of these additives, which reduce the temperature of the effective NO _x reduction process, nevertheless, does not provide sufficient efficiency of the cleaning process under load fluctuations of thermal units, and in some cases leads to the formation of secondary pollutants.

A known method of high-temperature purification of combustion products in a gas duct of a thermal unit from nitrogen oxides by selective non-catalytic reduction, which consists in the fact that ammonia-containing reducing agent is introduced into the selected gas duct between the heating surfaces located in it in a temperature range of 850 ... 1100 ° C, is adopted (Zellinger G., Tauschitz J. Betrieb-serfahrungen mit der nichtkatalytischen Stickstoffoxidreduktion in den Dampfkraftwerken der Osterreichischen Draukraftwerke AG // VGB Kraftwerkstechnik, 1989, Bd. 69, H. 12, S.1194-2000 [4]. According to [4], each of these zones is designed to operate at a specific heat load of the unit, at which the temperature of the flue gases is closest to optimal for SNCR. The disadvantages of this technical solution are the complexity of the regulatory process for selecting a working area with switching the operation of the corresponding equipment, as well as the possibility of flue gas pollution with ammonia due to the need to work with the highest possible ratio of reducing agent relative to its stoichiometric value.

Disclosure of invention

The objective of the invention is to increase the efficiency of purification of flue gases from nitrogen oxides while eliminating the emission of a secondary pollutant into the atmosphere in the form of an excess of a reducing agent, and the technical result is to ensure the maximum degree of reduction of NO _x without exceeding the stoichiometric ratio of the reducing agent consumption.

The solution of this problem and the achievement of the specified technical result are ensured by the fact that when implementing the method of high-temperature cleaning of the combustion products in the duct of the thermal unit from nitrogen oxides by their SNCR, which consists in the fact that, in the temperature range of 850 ... 1100 ° C, the selected zone of the duct between located in the heating surfaces are used to introduce an ammonia-containing reducing agent according to the invention, the reducing agent is introduced simultaneously to all of these selected zones, and to each of these zones it is supplied with a flow coefficient with respect to a stoichiometric flow rate of less than one. The number of input zones of the reducing agent is mainly 2 ... 5; the flow coefficient of the reducing agent supplied to each cleaning zone is set in the range of 0.2 ... 0.8; ammonia or urea is used as a reducing agent.

The causal relationship between the specified technical result and the distinguishing features of the invention is as follows. It is known that the optimum temperature corresponding to the maximum degree of reduction of nitrogen oxides using an ammonia-containing compound as a reducing agent is in the range of 900 ... 1000 ° C, while the practical temperature range of the combustion products in the possible processing area is wider and amounts to 850 ... 1100 ° C . This temperature range occurs in almost all thermal units when operating at various loads. But it is advisable to introduce the reducing agent not in one narrow temperature zone with optimal conditions for the recovery process, but in the entire possible temperature range in which the SNCR process can occur.

The efficiency of gas purification in each zone with a multi-zone input of the reducing agent may be less than the efficiency of a single-zone input scheme. This is because the temperature of the flue gases in each input zone of the reducing agent is different and may differ from the optimum process temperature. However, the total efficiency of the multi-zone cleaning scheme will be higher compared to single-zone. This is due to the fact that the length of the gas path on which the reduction process takes place in them increases, and the reaction time for a given volume of gases also increases. In addition, when implementing a multi-zone input of a reducing agent, the amount of emission of a secondary pollutant is also reduced by increasing the volume and time of the reaction.

The reducing agent is supplied to each cleaning zone with a flow coefficient (the ratio of the actual reducing agent consumption to stoichiometrically necessary) of less than one. In this case, the total amount of reducing agent supplied must be sufficient for the reduction of nitrogen oxides (not less than the stoichiometrically necessary amount) contained in the flue gases.

Brief Description of the Drawing

The drawing shows a schematic flow diagram of one of the possible examples of the implementation of the method of high-temperature purification of combustion products from nitrogen oxides according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the invention is illustrated by the example shown in the drawing, a flow chart. In the gas duct section 1 of the thermal unit, in the examples considered below - a pulverized coal-fired steam boiler, with an initial gas temperature in this section of approximately 1100 ° C, heating surfaces are installed, including the first screen superheater (BFS) 2, the second stage 3 of the second BFS and the first stage 4 of the second BPC, after which the products of combustion of fossil fuels are cooled to a temperature of 850 ° C. For high-temperature cleaning of the combustion products from the nitrogen oxides contained in them in the space of the duct 1 free from the heating surfaces 2-4, three temperature zones A, B, C are selected, for which the reducer is provided with fittings 5-8, respectively. The reducing agent in its pure form (ammonia, urea) or in a mixture with water vapor as a carrier is simultaneously introduced into the initial sections of all these zones using valves 5-8, while the flow coefficient of the reducing agent supplied to each cleaning zone is set in the range 0, 2 ... 0.8. When a reducing agent is mixed with combustion products, a reduction reaction of nitrogen oxides takes place. The results of an experimental verification of the indicated technological scheme are given below for four different examples. In the first two examples, only one zone is used, in the next two - three. The content of nitrogen and ammonia oxides after purification was measured in chilled products of combustion (flue gases) at the outlet of the boiler before being discharged into the chimney.

Example 1. The flow rate of the purified gases through the duct 1 was 1,100,000 nm ³ / h. When the load was 100% in the zone B of the flue 1, located in the cross-section between steps 3, 4 of the second BPC, the vapor-gas recovery mixture previously obtained using a 40% carbamide solution was fed. The temperature of the combustion products in this zone averaged 1006 ° C. The urea flow rate was 1. The test results are shown in table 1.

Table 1 Nn Urea consumption, l / h The content of NO _x in flue gases after purification, mg / m ³ The degree of purification,% The content of NH ₃ in the flue gas, mg / m ³ one. 0 1080 0 0 2. 1930 490 55 35 3. 1900 540 fifty 27 four. 1850 530 51 twenty

Example 2. The volume of purified gases 800000 nm ³ / h At a load of 70%, the vapor-gas recovery mixture previously obtained using a 40% carbamide solution was fed into the high-temperature zone of the boiler located in zone B. The temperature of the combustion products in this zone averaged 860 ° C. The urea flow rate was 1. The test results are shown in table 2.

table 2 Nn Urea consumption, l / h The content of NO _x in flue gases after purification, mg / m ³ The degree of purification,% The content of NH ₃ in the flue gas, mg / m ³ one. 0 850 0 0 2. 1100 620 27 38 3. 1120 630 26 45 four. 1080 645 24 33

Example 3. At one hundred percent load, the recovery mixture was supplied simultaneously to three zones A, B, C of treatment. The temperature of the combustion products in the first zone A was on average 1049 ° C, in the second zone B - on average 1006 ° C, in the third zone C - on average 963 ° C. The urea consumption coefficient in each treatment zone ranged from 0.2 to 0.5. The remaining test conditions are similar to those in example 1. The test results are shown in table 3.

Table 3 Nn Urea consumption, l / h The content of NO _x in flue gases after purification, mg / m ³ The degree of purification,% The content of NH ₃ in the flue gas, mg / m ³ Zone A Zone B Zone C one. 0 0 0 1080 0 0 2. 390 980 570 240 78 3 3. 390 770 770 260 76 5 four. 580 390 770 300 72 2

Example 4. At a load of 70%, the reducing mixture was supplied simultaneously to three zones A, B, C of treatment. The temperature of the combustion products in zone A averaged 1045 ° C, in zone B - an average of 933 ° C, in zone C - an average of 890 ° C. The urea consumption coefficient in each treatment zone ranged from 0.2 to 0.6. The remaining test conditions are similar to those in example 1. The test results are shown in table 4.

Table 4 Nn Urea consumption, l / h The content of NO _x in flue gases after purification, mg / m ³ The degree of purification,% The content of NH ₃ in the flue gas, mg / m ³ Zone A Zone B Zone C one. 0 0 0 850 0 0 2. 330 550 220 240 72 3 3. 220 650 770 220 74 2 four. 220 550 330 215 75 3

An analysis of the results of purification of combustion exhaust products from nitrogen oxides shown in Tables 1-4 showed a significant increase in the degree of gas purification during multi-stage introduction of a reducing agent compared with a single-stage one, a significant decrease in the ammonia content in the treated flue gases, and also a consistently high degree of gas purification when the boiler load changes .

Claims (5)

1. The method of high-temperature purification of combustion products in the duct of a thermal unit of nitrogen oxides by selective non-catalytic reduction, which consists in the fact that in the limited temperature range of 850-1100 ° C selected zone of the duct between the heating surfaces located in it, an ammonia-containing reducing agent is introduced, characterized in that that the reducing agent is introduced simultaneously into all of these selected zones, and in each of these zones it is supplied with a flow coefficient in relation to the stoichiometric flow dy smaller than unity. 2. The method according to claim 1, characterized in that the number of input zones of the reducing agent is 2-5. 3. The method according to claim 1 or 2, characterized in that the flow coefficient of the reducing agent supplied to each cleaning zone is set in the range of 0.2-0.8. 4. The method according to claim 1 or 2, characterized in that as the reducing agent use ammonia or urea. 5. The method according to claim 3, characterized in that ammonia or urea is used as a reducing agent.

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