CS268668B2 - Method of nitrogen oxides removal from gas current - Google Patents

Abstract

1. A method for removing nitric oxides from a flue gas resulting from combustion in which the flue gas is cooled, undergoes a preliminary purification and its SO2 content is reduced, where the flue gas pre-treated in this way is heated in at least one regenerator, mixed with NH3 and passed through a catalyst bed suitable for the conversion of nitric oxides, whereupon the purified hot flue gas is passed through at least one more regenerator and cooled and discharged.

Description

KRISTO? POLPGAIIG dr., T? CST3ERG SCHLIEBEIER CLAUS dr., STRASSLACii LINDA GERHARD dr., GRCltaID (DE)

LINDE AKTIEKGESELLSCHAF7, V.IESBADEL (DÉ)

A method for removing nitrogen oxides from a gas stream (57) by removing nitrogen oxides from a gas stream comprising nitrogen oxides, sulfur oxides, hydrogen chloride, hydrogen fluoride, dust and soot, by reducing nitrogen oxide. ammonia at high temperatures ηξ of the catalyst in solid form, in which sulfur oxides, hydrogen chloride, hydrogen fluoride, dust and soot are first removed from the gas, and the gas is passed through a heat exchanger and heated, followed by reduction e heat exchanger. The principle of this solution is that the heat exchange is carried out by means of regenerators and the reduction takes place with the addition of a stream of ammonia-containing gas containing sulfur dioxide and sulfur dioxide of at least 200 mg / m 2.

CS 268668 92

The invention relates to a process for the purification of waste gases, in particular to a process for removing nitrogen oxides from a gas stream.

It is known from the prior art to remove nitrogen oxides from a gas stream in which a gas stream containing, in addition to the nitrogen oxides, other contaminants such as sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, dust and soot is subjected to ammonia reduction on the catalyst. solid form at a temperature ranging from 250 to 450 ° C and then and optionally removing any remaining impurities (see Ind. Ehg. Chem. Prod. Res. Dev. 1980, 19, 218-225).

The disadvantage of this known process is, in particular, that the efficiency of the process is not satisfactory because of the short life of the catalyst used.

In addition, after three weeks of operation, a decrease in the activity of the catalyser can be observed. Also from this point of view, the economy of the above known cleaning process is unsatisfactory.

A further disadvantage of the known process is that, at peak load, it is very difficult to regulate the amount of addition, since the variation in the amount of gas simultaneously causes the temperature in the catalyst bed to fluctuate, which negatively affects the selectivity of the catalyst.

The object of the present invention is not to find a more economical process for purifying gases from nitrogen oxides.

Principle of a method for removing nitrogen oxides from a stream of gas containing nitrogen oxides, sulfur oxides, hydrogen chloride, hydrogen fluoride, dust and soot, which is carried out by reducing nitrogen oxides with ammonia at high temperatures on a solid catalyst and first removing sulfur oxides from the gas , hydrogen chloride, hydrogen fluoride, dust and soot, the gas is passed through a heat exchanger and heated and then reduced and after the reduction the gas stream is passed through another heat exchanger. adding a stream of ammonia-containing gas with a sulfur dioxide and sulfur trioxide content of not more than 200 mg / m 2;

The advantage of this process according to the invention is first and foremost the cost-effective cleaning and prolongation of the catalyst life.

As already mentioned, the gas stream being fed to the reduction may contain a maximum of 200 mg / m @ 2 of sulfur oxides (i.e. sulfur dioxide and sulfur trioxide) and in addition a maximum of 50 mg / m @ 2 of dust and soot.

Separation of sulfur, hydrogen chloride and hydrogen fluoride oxides is carried out after prior separation of the dust by known processes, for example by washing with an absorbent at temperatures in the range of 0 to 60 ° C or below.

Since the separation of the remaining impurities generally takes place at temperatures substantially lower than those required to operate the catalyst, the gas stream must be heated. According to the invention, such heating is carried out in heat exchangers, preferably regenerators (as will be explained in more detail below), which are heated with hot nitrogen-free gas, and then a gas stream containing ΝΟ _χ passes through the heat exchanger. In this way, it is possible to ensure sufficient heating of the gas flow to the reduction treatment without significant energy consumption. After heating in a heat exchanger, the gas is reacted with ammonia. .

In the process according to the invention, it is advantageous to use heat exchangers with a heat storage mass. According to the invention, these exchangers are arranged in such a way that they can be switched over and rotated by one another. This means that the cleaned gas is cooled for cooling by a heat exchanger which has been adapted in the previous switching cycle to pass the gas to be reduced, during which the cleaned and heated gas is passed through the cooled heat exchanger again. In this process, heat exchangers of the conventional type can be used with a system of heat exchangers, but also of bulk-filled heat exchangers, for example regenerators and rotating gas preheaters. · After a period of time, the gas flow in the regenerator switches over. · From the cold end of the regenerator, gas is now blown into the interior of the regenerator, which removes accumulated heat from the regenerator charge. bundles of mutually parallel sheets with a smooth surface, in which the heat storage mass is placed

In this solution, it is particularly advantageous to use a ceramic mass as a heat storage material, since this mass ensures the most complete heat exchange. These ceramics can be used at least in part as their own catalysts.

in which the following rea catalyst occurs ____________ 4 N? ♦ 6 H ₂ 0 catalyst 3 + 6 H 0

The hot gas is then passed through a catalyst to:

NH ₃ + 4 N0 ♦ 0 ₂ eventually for reaction:

NH ₃ * 2 NCL · + 0 ₂

In particular, the first reaction occurs, since the gas stream contains mostly more than 90% NO and the remainder is NO ₂ · N ₂ ⁸ is formed during the catalytic reaction, or at higher temperatures water vapor is harmless and can be harmless Consequences released into the air ·

The hot scrubbed gas then transfers its heat to the heat exchanger used in the previous cycle to heat the NOx-containing gas stream, so that the heat exchanger can be reused in the next cycle to heat another portion of the NOx-containing gas stream. According to a preferred embodiment, the NOx-containing gas stream is transferred to a further heat exchanger after a period of time, for example from 1 to 20 minutes, in particular 3 to 5 minutes.

According to a particularly preferred embodiment of the process according to the invention, the purification process utilizes a catalyst divided into several separate layers. This treatment is particularly useful in those cases where the space available for the catalyst bed cannot be provided at the required space velocity and linear velocity of the gas stream due to the limited space available. Then, the division of the catalyst bed into several layers in the form of layers increases its leading cross-section, i.e. the surface area of the bed, since the sum of the surface areas of the individual layers is greater than the cross-sectional area of the flow path. In a series arrangement, it is preferred that the catalyst layers overlap at least partially. In this arrangement, the catalyst layers are spaced apart in different planes perpendicular to the direction of gas passage, with at least portions of the catalyst layers overlapping each other.

It is further preferred according to the invention if the gas stream passes through the catalyst layers successively. Furthermore, this solution provides the possibility of dividing the gas stream into several partial streams which are separately fed to the catalyst layers. This means that the gas can be divided into mutually parallel partial streams and in this form it can pass through the catalyst layers and can also be introduced into the layers gradually. .

In addition, it is also possible to change the direction of the gas flow in the heat exchangers in the region of the catalyst or between the heat exchanger and the catalyst. This allows the gas flow to pass through the heat exchanger or the catalyst both horizontally and vertically. '

An arrangement of the catalyst (i.e. catalyst layers) and heat exchangers in which the catalyst is located below the heat exchanger proves to be a particularly advantageous solution.

Known catalysts, for example zeolites or vanadium / titanium oxides on support materials or rare earth catalysts on support materials, can be used to reduce nitrogen oxides. Depending on the type of catalyst used, the gas stream is then heated to the temperature required to effect the reaction on the catalyst, typically in the range of 250 to 450 ° C. Heat losses resulting from imperfect heat exchange can be replaced by a heat source. The heat source may be any source, the flue gas being particularly preferred in this case. These flue gases may be produced, for example, by burning high calorific fuels in a combustion chamber and may be supplied from the outside or may be generated directly by burning fuels with a high thermal energy content in the reactor. Usually, only a small amount of fuel is sufficient to cover the heat loss. Naturally, heat losses can also be covered from other heat sources, for example from electric heaters or through steam condensing in the heat exchanger. Heat can be supplied from both sides of the catalyst bed. In addition, at least one odeb _χ- containing partial gas stream may be withdrawn from the reactor prior to passing through the catalyst and contacted with the catalyst after heating in the exhaust duct.

Examples of apparatus for carrying out the process according to the invention are shown in the accompanying drawings, in which Fig. 1 shows a longitudinal vertical section through a dual-flow reactor with two catalyst layers, Fig. 2 shows a vertical flow section with a single catalyst layer; 3 is a vertical section through an exemplary embodiment of a catalyst bed.

In the apparatus of FIG. 1, 210,000 Nm < 3 > / h of gas having a temperature of about 30 [deg.] C. are supplied via line J. This gas stream is compressed in the blower 2 to a pressure of about 1 bar. This gas stream, which is free of dust, soot, hydrogen fluoride, hydrogen chloride, sulfur dioxide and sulfur trioxide in the upstream plant, still contains a total of 50 mg / m @ 2 of dust and soot, 200 mg / m @ 2 of sulfur dioxide and sulfur dioxide. ppm Ν0 _χ

The compressed gas stream is then passed through the four-way flap 4 via line 2 ^{to the} reactor 6.

The reactor 6 is formed as a two-stream regenerator comprising a lower regenerative charge 2, over which a catalytic bed layer 8 is placed, above which a partition 9 and an upper catalytic bed layer 10 are placed, with a space filled with an upper regenerative charge 11 above. for example ceramic crushed material.

In the illustrated embodiment, the gas stream containing, _χ is fed to the bottom regeneration charge 2 and heated to 300 ° C therein. The heated gas is argued

B2 by feeding it into the catalyst bottom layer 8, it mixes in a stoichiometric ratio of about 1: 1 (based on NO) with ammonia »via line 2. The mixture thus formed is then passed to the bottom layer of the catalyst bed, where 90% is converted to nitrogen and water »so that there are still about 26 volume ppm of NO in the cleaned gas · This value is well below the legal limit for the maximum escape of nitrogen oxides into the atmosphere ·

The cleaned gas is then passed through a gas-permeable partition 2 into the second system through which the cold gas passed through the pre-charge switching cycle. The four-way flap 6 directs the gas into the line 13 through which it is evacuated. After a period of 4 minutes has elapsed in this example, the four-way flap 6 is switched to a position such that the gas flow »coming to the reduction. enters the reactor 6 via line 12 and is discharged via line 2 · This second option is indicated by additional arrows ·

To compensate for heat losses, the reactor 6 is further provided with a burner 14 to which are connected the conduits 15 and 16 for supplying, for example, natural gas and air. This burner produces flue gases which are fed through the conduit 17 to the regenerator charges to heat them. temperature ·

The second exemplary embodiment of the apparatus according to the invention shown in FIG. 2 has, in contrast to the apparatus in FIG. 1, only one catalyst layer which is the catalyst bed 18. In addition, hot gas can be supplied via line 19 to cover heat losses.

Also in this embodiment, a gas stream containing 0 _χ is fed to a bottom regeneration charge 6 formed, for example, by poured ceramic grits and heated to about 300 ° C. After mixing the ammonia supplied through line 2, the gas is passed through the catalyst bed 18, in which the reaction of Ν0 _χ occurs to nitrogen and water, so that only 28 volume ppm of NO remains in the cleaned gas. The cleaned gas is then passed through the upper regenerative charge 11 which transfers its heat capacity and then leaves the regenerator through a pipe 12 having a temperature of about 40 ° C. In the four-way flap 6, the gas is transferred to the line 13 through which it is discharged.

In this example too, the four-way flap 6 switches over a period of time, for example 4 minutes, so that the gas supplied to the reduction enters the reactor 6 via line 12 and is discharged via line 2. This second option is also indicated by additional arrows.

Fig. 3 shows the process gas flow path in the direction of the arrows 23 in a stepped catalyst with horizontal catalyst layers 21 ' partially overlapping each other in the flow direction. The process gas stream passes in the direction of the arrow 23. 1 and 2. The individual catalyst layers 21 may be formed by bed layers, the catalyst being in the form of particles, for example of annular or star shape. Honeycomb catalysts can also be used.

Between the catalyst layers 21 there are separating walls 22 which extend from the inner edge of one layer to the outer edge of the next catalyst layer. The catalyst layers 21 and the thin separating walls 22 are arranged symmetrically about the central axis of the flow path 20. greater than the cross-sectional area of the flow path 20.

OBJECT OF THE INVENTION

Claims (1)

Process for removing nitrogen oxides from a stream of gas containing nitrogen oxides, sulfur oxides, hydrogen chloride, hydrogen fluoride, dust and soot, by reducing nitrogen oxide with ammonia at high CS 268668 B2 5 at the catalyst in solid form at which sulfur oxides, hydrogen chloride, hydrogen fluoride, dust and soot are first removed from the gas, the gas is passed through a heat exchanger and heated and then reduced and after the reduction the gas stream is passed A heat exchanger characterized in that the heat exchange is carried out by means of regenerators and the reduction takes place by adding a stream of ammonia-containing gas, the sulfur dioxide and sulfur trioxide content of the gas being not more than 200 mg / m @ 2. . 3 drawings CS 268 668 B2 Giant. 1 CS 268 668 B2 Giant. 2