RU2481890C1 - Method of selective catalytic cleaning of exhaust and flue gas of nitrogen oxide - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to selective catalytic cleaning of exhaust and flue gases of nitrogen oxides. Proposed method comprises catalytic removal of nitrogen oxide from gas using ammonia as reducing agent and removal of unreacted ammonia from gas phase. Note here that removal of said unreacted ammonia from gas phase is performed with the help of cation exchanger charged with metal/metals forming strong complexes with ammonia.

EFFECT: higher efficiency of cleaning, reduction of ammonia concentration to, in fact, whatever values.

3 cl, 1 dwg, 3 tbl

Description

The invention relates to the field of selective catalytic purification of nitrogen oxides of exhaust, flue and exhaust gases released during the operation of steam generating and heating systems, thermal power plants, internal combustion engines, as well as industries associated with the production of nitric acid and nitration.

In known methods for the catalytic purification of exhaust gases from nitrogen oxides, ammonia is usually used as a reducing agent. Ammonia, necessary for the reaction of reduction of nitrogen oxides to nitrogen, proceeds to the reaction zone either from an external source, or by catalytic hydrolysis of urea, or by decomposition of an ammonium salt (US 3885019, B01D 53/00, 05.20.1975; US 5869419, B01J 23/00 , 02/09/1999; RU 2405946, F01N 3/00, 08/15/2007; US 7749938, B01J 23/00, 06/06/2010), or is formed as a result of the reaction of exhaust gas components between themselves, initiated catalytically (US 6872365, F01N 3 / 10, May 29, 2005), or by passing a poor combustible mixture through an electric discharge plasma (US 6334986, B01D 53/58, 01/01/2002).

Known methods for the selective catalytic purification of gases from nitrogen oxides using ammonia as a reducing agent can effectively reduce the concentration of nitrogen oxides in gas streams from their initial sufficiently high values. However, a common drawback of these methods is the fact that after the cleaning procedure a rather large amount of ammonia remains in the composition of the gas to be cleaned, which is associated with the need to maintain a certain excess of NH ₃ against stoichiometry with respect to NO _x . Ammonia in the atmosphere can be transformed into NO _x , and therefore, as a harmful substance is equated to NO _x . Before emitting into the atmosphere, it is advisable to reduce the ammonia concentration to at least the maximum single maximum permissible concentration _mr = 0.2 mg / m ³ , and better to the maximum daily maximum permissible concentration _ss = 0.04 mg / m ³ according to the hygienic standards GN 2.1.6.1338-03.

Some methods of selective catalytic reduction of NO _x provide for the post-treatment of exhaust gases from ammonia after removal of nitrogen oxides from them (see, for example, US 6334986, B01D 53/58, 01/01/2002), which allows to reduce the ammonia concentration to a certain extent. However, these methods have an irreparable disadvantage: they are based on the catalytic oxidation of ammonia, as a result of which nitrogen oxides are formed again, which were previously removed at the stage of basic purification.

Closest to the proposed method for the selective catalytic purification of exhaust and flue gases from nitrogen oxides is an exhaust gas treatment method in which nitrogen oxides are catalytically removed from the exhaust gas using ammonia as a reducing agent, with decomposition and removal of unreacted ammonia - moreover, as the reduction of nitrogen oxides and the decomposition (oxidation) of unreacted ammonia is carried out by the same catalyst containing titanium oxide and, at least the least measure, one compound selected from the group consisting of vanadium (V) oxides, tungsten (W) oxides and molybdenum (Mo) oxides (RU 2406567, class B01J 23/652, 12.20.2010 - prototype).

The disadvantage of the prototype method (as well as the aforementioned method according to US Pat. No. 6,334,986) is that when using bifunctional (redox) catalysts, a decrease in the concentration of ammonia in the gas stream is possible only due to the re-formation of nitrogen oxides, that is, a "vicious circle ": a decrease in the concentration of nitrogen oxides in the reaction of their selective catalytic reduction is accompanied by the accumulation of ammonia, and a decrease in the concentration of ammonia due to the reaction of its catalytic oxidation of dreams It leads to the formation of nitrogen oxides.

The test results of the bifunctional catalysts given in the description of the prototype method show that when the concentration of nitrogen oxides in the exhaust gas stream is reduced by a factor of 20, the ammonia concentration remains quite high (2-8 ppm = 1.5-6 mg / m ³ ), then there are 7.5–40 and 37.5–200 times higher than MPC _MP and MPC _ss , respectively. If we achieve an even greater decrease in the concentration of nitrogen oxides, then the concentration of ammonia increases even more.

The objective of the invention is the development of such a method for the selective catalytic purification of exhaust and flue gases from nitrogen oxides when using ammonia as a reducing agent, in which the purification of the stream of purified gas from unreacted ammonia will not be accompanied by the repeated formation of nitrogen oxides, which will reduce the concentration of ammonia to almost any values, in particular, to values below the MPC _ss , and will improve the efficiency of catalytic reduction of oxides of az ota.

The solution to this problem is achieved by the proposed method for the selective catalytic purification of exhaust and flue gases from nitrogen oxides, including the catalytic removal of nitrogen oxides from the gas to be purified using ammonia as a reducing agent with the removal of unreacted ammonia from the gas phase, in which the removal of unreacted ammonia is not accompanied by the re-formation of oxides nitrogen, for which the removal of unreacted ammonia from the gas phase is carried out using cation exchange resin, yazhennogo metal / metals forming stable complexes with ammonia.

As the cation exchanger charged with the metal / metals, which form strong complexes with ammonia, sulfonation cationite of the following formula is used: [(P-SO ₃ ^- ) ₂ Me ²⁺ ), where P is a copolymer of styrene and divinylbenzene, Me ²⁺ are copper ions or / and calcium.

Before applying the gas stream to sulfocationite to remove ammonia, the gas stream is cooled to the optimum operating temperature of the sorbent.

Catalysts for the selective catalytic reduction of nitrogen oxides using ammonia as a reducing agent are commercially available and widely used; they are mixtures of metal oxides containing V ₂ O ₅ (in Russia, the AVK-10 vanadium catalyst is usually used).

The choice of the sorbent in the development of the proposed method was determined by the task - the method should provide a decrease in ammonia concentration to values lower than the MPC _ss (MPC _SS = 0.04 mg / m), that is, the sorbent used in the method should effectively absorb ammonia from the gas phase and hold it firmly avoiding the slip.

Our experimental studies of several cation exchangers with different structure of ionic groups showed that, during the sorption of ammonia from the gas phase on cation exchangers in the H-form, which are usually used for the sorption of ammonia from aqueous solutions, dehydration of the cation exchange phase is observed, leading to a decrease in the equilibrium and kinetic characteristics of the sorbent .

For further experiments, cation exchangers in the H form were modified by us - charged with metals. When using for the sorption of ammonia from a gas stream Me-forms of sulfocationionites: [(П-SO ₃ ^- ) ₂ Ме ²⁺ ), where Me ²⁺ are copper or / and calcium ions, it was found that the sorption capacity for ammonia increases by 3 -3.5 times. So, the Me form binds up to 4 moles of ammonia per mole of sulfo groups: [(P-SO ₃ ^- ) ₂ · Me (NH ₃ ) _n ²⁺ ], while the unmodified metal form of the H form of sulfocationite can bind no more than 1 mole of ammonia per mole sulfo groups: [P-SO ₃ NH ₄ ]. No less important was the fact that the strength and thermal stability of the outer-sphere complex of ammonia with metal: [(P-SO ₃ ^- ) ₂ · Me (NH ₃ ) _n ²⁺ ] is significantly higher than the strength of a purely ionic form: [P-SO ₃ NH ₄ ].

In the study of the influence of the chemical structure of the polymer in sulfocationite on the process of ammonia sorption, it was found that sulfocationites based on styrene-divinylbenzene copolymers exhibit the best sorption properties. Such cation exchangers most strongly retain water, which makes it possible to work efficiently at low humidity and, therefore, at elevated temperature (≥120 ° C) of the gas mixture being cleaned. In addition, it was found that in the studied gas environments sulfystyrene cation exchangers exhibit high thermal stability, especially their salt forms (Ca, Cu), this made it possible to raise the working temperature of the gas mixture being cleaned to 120-140 ° С, avoiding the need for its preliminary cooling and thereby to improve the technical and economic characteristics of the proposed method.

Further studies of the sorption process on sulfocathionites revealed a number of additional factors that significantly affect the efficiency of ammonia sorption. Thus, the Me forms of sulfocationionites, in comparison with the H form, have a higher affinity for ammonia (have large exchange constants), which leads to an increase in the distribution coefficients for low ammonia concentrations, and also have a higher selectivity for ammonia, which makes it possible to remove very low initial ammonia concentrations in the gas to be cleaned. Regulation of the ratio of copper and calcium during charging of cation exchanger allows to increase the affinity and heat resistance due to preferential charging with copper or to increase the capacity for ammonia when predominantly charging with calcium.

The proposed method for cleaning exhaust and flue gases is as follows (see drawing). Ammonia is added to the stream of gas to be purified before it enters the catalytic cell (1), either from an external source or from catalytic hydrolysis of urea or from the decomposition of an ammonium salt. In the catalytic cell, nitrogen oxides are reduced to molecular nitrogen. Next, the gas stream is sent to the sorption cell (2) for its purification from unreacted ammonia. If necessary, the gas stream is precooled before being fed to the sorbent to the optimum operating temperature of the sorbent in the cooling device (3).

The sorbents used in the proposed method, which are the Me-form of sulfonated styrene-divinylbenzene cation exchangers, were tested by us to confirm their ability to effectively absorb ammonia from the gas phase and hold it firmly.

In tables 1-3, as an example, data are given on the dependence of the time before the breakthrough (t) (the time when the concentration of NH ₃ at the outlet of the sorbent layer begins to exceed the maximum permissible value, in this case, the MPC _ss = 0.04 mg / m ³ ) on the moisture content of cation exchange resin (φ), on the concentration of NH ₃ in the gas-air mixture (C) and on the gas flow rate (W) for styrene-divinylbenzene sulfocationionite charged with copper: [(P-SO ₃ ^- ) ₂ Cu ²⁺ ].

By measuring the time before the breakthrough and knowing the velocity of the gas stream and the concentration of ammonia in the gas-air mixture, we can calculate the dynamic capacity of the sorbent to the breakthrough δ (mg / cm ³ ) (slip capacity), which is the amount of substance (NH ₃ ) sorbed and held by the sorbent layer an area of 1 cm ² and a length l 1 cm up to the moment when the concentration of NH ₃ at the outlet of the sorbent layer exceeds the maximum permissible value, in this case, the MPC _ss = 0.04 mg / m ³ :

δ = W × C × t / l,

where W l · min ^-1 cm ^-2 is the gas flow rate, C mg / l is the concentration of ammonia in the gas-air mixture, t min is the time before the breakthrough, l cm is the length of the sorbent layer. Tables 1-3 show data for the dynamic capacitance δ under various conditions.

Table 1 Dependence of the time before the breakthrough (t) and the dynamic capacity (δ) of styrene-divinylbenzene sulfocationionite: [(P-SO ₃ ^- ) ₂ Cu ²⁺ ] on its moisture content (φ) at a gas flow rate of W = 0.47 l · min ^-1 · cm ^-2 , the concentration of NH ₃ C = 2.5 mg / l and the length of the sorbent layer l = 1.3 cm. Parameter Parameter value φ,% fifty 75 90 t min 17 22 28 δ, mg / cm ³ 15.4 19.9 25.3

table 2 The dependence of the time before the breakthrough (t) and the dynamic capacity (δ) of styrene-divinylbenzene sulfocationite: [(P-SO ₃ ^- ) ₂ Cu ²⁺ ] on the concentration of NH ₃ at a gas flow rate of W = 0.47 l · min ^-1 · cm ^-2 , the length of the sorbent layer l = 1.3 cm and the moisture content of the sorbent φ = 50%. Parameter Parameter value C, mg / l 0.2 1,0 2.0 t min 567 45 22 δ, mg / cm ³ 41 16.3 15.9

Table 3 Dependence of time before slip (t) and dynamic capacity (δ) of styrene-divinylbenzene sulfocationionite: [(P-SO ₃ ^- ) ₂ Cu ²⁺ ] on the gas flow rate at a concentration of NH ₃ C = 2.0 mg / l, sorbent layer length l = 1.3 cm and humidity sorbent φ = 50%. Parameter Parameter value The gas flow rate W, l · min ^-1 · cm ^-2 0.236 0.47 0.71 t min 45 23 fifteen δ, mg / cm ³ 16.3 16.6 16,4

From the above data, it can be seen that at a cation exchanger humidity of 50%, which was present in all experiments, the dynamic capacity of the sorbent before breakthrough fluctuated around 16 mg / cm. An exception was observed only for small concentrations of ammonia in the gas-air flow: δ = 41 mg / cm ³ for C = 0.2 mg / l (table 2). However, with an increase in cation exchange moisture up to 90%, the dynamic capacity of the sorbent before breakthrough increases to δ = 25.3 mg / cm ³ (table 1).

The value of the dynamic capacity of the sorbent to the breakthrough δ allows us to evaluate its effectiveness in the absorption of ammonia from the gas phase.

Example. Suppose that as a result of selective catalytic purification of exhaust or flue gases from nitrogen oxides when using ammonia as a reducing agent, unreacted ammonia in a concentration of 2 ppm (1.5 mg / m ³ ) remains in the gas to be cleaned, for example, as was observed during testing of the method prototype (RU 2406567). If the proposed method uses a sorbent with a dynamic capacity of δ = 16 mg / cm ³ and a volume of 1 l, then the inventive method will allow, without replacing the sorbent, to clear the total volume of exhaust (or furnace) gases from nitrogen oxides while simultaneously removing unreacted ammonia to a concentration below MPC _ss = 0.04 mg / m ³ equal to 10 ⁴ m ³ :

Note that according to a rough estimate, such an amount of exhaust (furnace) gases is emitted by an average passenger car with a mileage of 10 ⁵ km or a heating system using a 10 kW diesel fuel for 400 days.

Thus, in the proposed method for the selective catalytic purification of exhaust and flue gases from nitrogen oxides when using ammonia as a reducing agent, the post-treatment of the stream of purified gas from unreacted ammonia is not accompanied by the repeated formation of nitrogen oxides, which allows reducing the ammonia concentration to almost any value, in particular, to values below the MPC _ss , and increase the efficiency of catalytic reduction of nitrogen oxides.

Claims (3)

1. A method for the selective catalytic purification of exhaust and flue gases from nitrogen oxides, comprising the catalytic removal of nitrogen oxides from the gas to be purified using ammonia as a reducing agent with the removal of unreacted ammonia from the gas phase, characterized in that the removal of unreacted ammonia is not accompanied by the re-formation of nitrogen oxides why removal of unreacted ammonia from the gas phase is carried out using cation exchange resin charged with metal / metals forming ammonia durable complexes. 2. The method according to claim 1, characterized in that as the cation exchange resin charged by the metal / metals forming strong complexes with ammonia, sulfocation ionite of the following formula is used: [(P-SO ₃ ^- ) ₂ Me ²⁺ ], where P is a copolymer styrene and divinylbenzene, Me ²⁺ - copper and / or calcium ions. 3. The method according to claim 1, characterized in that before applying the gas stream to the sulfocationite to remove ammonia, the gas stream is cooled to the optimum operating temperature of the sulfationite.

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