The method of cleaning flue gases from nitrogen oxides

 

(57) Abstract:

The invention relates to the field of purification of industrial gases from nitrogen oxides and can be used to reduce the content of NOxin the combustion products. Flue gases are treated at a temperature of 700-1200oC products of thermal decomposition of urea, obtained by heating solid urea outside the zone of reduction of nitrogen oxides while blowing a layer of solid urea and the subsequent introduction of the gas mixture formed in the zone of reduction of nitrogen oxides. The invention improves the purification of flue gases and to simplify the process. 3 C.p. f-crystals, 2 ill., table 2.

The invention relates to the field of purification of industrial gases from nitrogen oxides and can be used to reduce the content of NOx in the combustion products of thermal power plants and gas emissions technological units.

Known methods non-catalytic high-temperature flue gases from nitrogen oxides using as reducing agent urea (U.S. Patent N 4719092, class C 01 B 21/00 from 12.01.88; Patent Austria N 390208, class B 01 D 53/34 from 10.04.90 and others). All of these methods basics of the om gas, at a temperature of 700-1200oC with maximum cleaning efficiency in the temperature range 900-1100oC.

The reaction products, the recovery of nitric oxide are molecular nitrogen, carbon dioxide and water vapor:

2CO(NH2)2+ 4NO + O2--> 2CO2+ 4N2+ 4H2O (1)

The disadvantages of these methods include the reduction of the degree of purification with decreasing temperature below 900oC, and that the urea is used in the form of an aqueous solution of a certain concentration, which is obtained preliminary dissolution of solid urea. The preparation of an aqueous urea solution requires the use of special technological equipment and significant production areas for placement. Equipment must be stored in heated premises in connection with the fact that the aqueous urea solution freezes at negative ambient temperatures. This complicates the technological scheme of purification in General, and significantly increases the cost of implementing the cleanup process.

Closest to the present invention is a method, the essence of which is in the preliminary heat treatment and evaporation of an aqueous solution of urea F. the flow of cleaned gases, the temperature is in the range from 700 to 1200oC (RF Patent N 2040737, CL F 23 J 15/00 from 10.08.92).

The above disadvantages associated with the use for the reduction of nitrogen oxides aqueous solution of urea, typical for this cleaning method.

The present invention is to increase the degree of purification of flue gases, as well as simplification of the process by eliminating technological operations preparation of a solution of urea and improving, thus, the technical and economic indicators of the process.

The problem is solved by the proposed method of cleaning flue gases from nitrogen oxides, including processing of flue gases at a temperature of 700-1200oC products of thermal decomposition of urea, which according to the invention are used for the products of thermal decomposition of urea, obtained by heating solid urea outside the zone of reduction of nitrogen oxides while blowing a layer of solid urea carrier gas and the subsequent introduction of the gas mixture formed in the zone of reduction of nitrogen oxides.

In preferred variants:

use the products of thermal time is Gogo urea is performed by the carrier gas, having a temperature above the temperature of decomposition of urea by 10-50oC;

as the carrier gas used is air or nitrogen, or flue gas, or water vapor, or a mixture.

The method is as follows.

Thermal processing of solid urea is conducted at a temperature of 50-200oC, without prior dissolution in water, and the products of thermal decomposition of urea is fed to the zone of reduction of nitrogen oxides using a carrier gas, which may be air or nitrogen, or flue gas, or water vapor, or a mixture. Thermal treatment of solid urea can be hot carrier gas having a temperature above the temperature of decomposition of urea by 10-50oC, or by electric heating.

The end products of thermal decomposition of solid urea are ammonia and solanova acid:

CO(NH2)2= NH3+ HNCO,

to selectively restore the oxides of nitrogen at a temperature of 700-1200oC.

The proposed principle, the process can simplify the process flow of flue gases, so as to eliminate site preparation R is e equipment. In addition, experimentally set a new effect, namely, that the process of reduction of nitrogen oxides products of thermal decomposition of solid urea in the region of low temperatures (700-900oC) occurs with higher efficiency than the recovery process using the products of thermal decomposition of an aqueous solution of urea (Fig. 1). When using the products of thermal decomposition of an aqueous solution of urea purification efficiency gas in this temperature interval is not more than 55%, when using solid urea cleaning efficiency increases to 90%. Presumably this is caused by changing the ratio of components of the decomposition products (ammonia and isocyanates acid).

Since the temperature of the flue gases depends on their thermal load during operation of units with variable load, the temperature of the cleaned gas is often less than optimal, the corresponding maximum efficiency reduction of nitrogen oxides using as a reducing agent of the products of thermal decomposition of the urea solution. In these cases, the efficiency of gas cleaning using products Tripolitania for reducing oxides of nitrogen of urea in solid form, excluding the stage of dissolution, it is possible to expand the temperature range effective process that enables you to use the proposed method for the purification of gases of thermal units, working with a variable load.

In Fig. 2 shows a schematic diagram of the installation to implement the proposed method of cleaning flue gases from nitrogen oxides. Solid urea is fed to the device 1 for thermal decomposition, in which a temperature is maintained that ensures the selection of the reducing agent in an amount necessary for the reduction of nitrogen oxides. This device is supplied with gaseous media, providing the necessary gas-dynamic parameters of the expiration of the reducing agent mixture in the flow of cleaned gases. The mixture of products of thermal decomposition of solid urea from the carrier gas through the distribution device 2 is fed to tableaggregate 3, where the flue gas temperature is in the range of 700-1200oC.

Below are examples of gas purification from nitrogen oxides of the proposed method.

Example 1. Solid urea in the amount of 1.5 g is placed in an electric heater, where the support temperature is remotely gas mixture flows in a temperature-controlled quartz reactor, the temperature at which the change in the range of 700-1100oC. the mixture Flow rate of 120 l/h NOx at the inlet of the reactor to 110 ppm. The oxygen content of 7.8 about. %. The rest is nitrogen. The residence time of the gas mixture in the reaction zone of 0.5 C. the Content of nitrogen oxides in the analyzed gas mixture is measured at the entrance and at the exit from the installation. The research results are summarized in table. 1.

Example 2. The experiment is carried out under conditions identical to those given in example 1. The heating temperature of the solid urea is 100oC. comparison of the effectiveness of the reduction of nitrogen oxides using the products of thermal decomposition of solid urea and effectiveness of the process in the same conditions with use of the products of thermal decomposition of an aqueous solution of urea parallel conduct experiments with feeding to a reactor an aqueous solution of urea in a quantity sufficient for the reduction of NOx. The research results are summarized in table. 2.

As can be seen from the presented data, the proposed method of cleaning flue gases from nitrogen oxides using as reducing agent solid urea, preheated to a temperature of 50-200oC, and input products heat treatment which is effective and provides a high degree of purification in a wide range of changes in gas temperature and, therefore, under different thermal loads. The technology of the proposed method is considerably simplified in comparison with the prototype, as it eliminates the need for building site preparation of a solution of urea.

1. The method of cleaning flue gases from nitrogen oxides, including processing of flue gases at 700 - 1200oC products of thermal decomposition of urea, characterized in that the processing using the products of thermal decomposition of urea, obtained by heating solid urea outside the zone of reduction of nitrogen oxides while blowing a layer of solid urea carrier gas and the subsequent introduction of the gas mixture formed in the zone of reduction of nitrogen oxides.

2. The method according to p. 1, characterized in that use the products of thermal decomposition of urea, obtained by heating solid urea to 50 - 200oC.

3. The method according to PP. 1 and 2, characterized in that the heating of the solid urea is performed by the carrier gas having a temperature above the temperature of decomposition of urea by 10 - 50oC.

4. The method according to p. 1, characterized in that as the carrier gas using air or nitrogen, or flue gases, the

 

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