FI84885C - Procedure for the purification of flue gases - Google Patents

Description

1 84885

The present invention relates to a process for the purification of flue gases and in particular to a process for reacting a sulfur and nitrogen oxide in a hot flue gas suspension containing powdered alkali and / or alkaline earth metal oxide to solidify and thus easily react which occurs by spraying water into the hot flue gas suspension to activate said powdered oxide and separating the finally obtained solid reaction product from the flue gas suspension.

Numerous different methods are known for purifying flue gases from solid and gaseous contaminants, such as sulfur dioxide. These methods can be divided into two main groups, namely dry and wet methods and their various combinations. The advantage of the dry processes 20 is that the separated solid products are recovered dry and the disadvantage is the relatively low reduction of sulfur dioxide. Wet methods, on the other hand, are more efficient, but the sludge obtained from them is difficult to store without drying and this in turn consumes energy. Combinations of the above-mentioned methods are also known, in which the flue gas stream is divided into two sub-streams, one of which has been treated by the dry method and the other by the wet method, but even this method has not been able to eliminate the disadvantages of the wet or dry method.

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It is already known to feed finely divided limestone into a boiler to decompose it into calcium oxide, which reacts with sulfur oxides in the flue gases to form a solid dusty reaction product. It is also known to activate unreacted calcium oxide in the flue gases from said boilers in a special activation reactor with water or steam to convert the calcium oxide to highly reactive calcium hydroxide, which further binds the 2,84885 sulfur oxides in the flue gas suspension to solid dusty calcium sulphite. However, within the short delay time available, usually up to about 5 seconds, it is technically difficult 5 to react the calcium oxide quantitatively with the sulfur oxides and thus to apply a large excess of lime. This in turn causes dust problems, whether the dust is recovered dry.

It is therefore an object of the present invention to provide a combined wet and dry process for cleaning flue gases, which has made it possible to eliminate the disadvantages of wet and dry processes while retaining the advantages of both methods.

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The main features of the invention appear from the appended claims.

In the process of the present invention, the flue gas suspension is not divided into two partial streams, as in the previously known combined dry and wet processes, but the flue gas suspension is washed completely after activation with an alkaline aqueous solution. to evaporate the water and the reaction product is separated from the flue gas suspension substantially dry. Thanks to the washing, the flue gas suspension can be cleaned even more thoroughly and using less absorbent.

However, the reaction product is recovered substantially dry by recirculating the scrubbing solution to the hot flue gas suspension to activate the oxide with the water contained in the aqueous solution, while evaporating excess water and reacting any unreacted absorbent 35 with the sulfur oxides in the flue gas suspension.

In a preferred embodiment of the invention, the washed flue gas suspension is heated and then filtered, for example

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3,84885 with an electrostatic precipitator so that the reaction product is recovered dry. A portion of the reaction product separated by filtration may be returned to the flue gas suspension in the reaction product to be recovered to react any remaining unreacted absorbent, and a portion of the reaction product may be separated from the activated flue gas suspension prior to scrubbing.

Part of the washing solution can, of course, be recycled back to the washing, if desired.

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In an alternative embodiment of the invention, part of the scrubbing solution is returned to the scrubbing and the remainder is subjected to evaporation and the crystal slurry thus obtained is returned to the hot flue gas suspension to evaporate the water therein. At least a portion of the flue gas suspension can be filtered between activation and scrubbing so that less dust does not enter the scrubber. In this way, the amount of dust containing absorbent which enters the wash and has not yet reacted can be adjusted to suit the washing.

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The invention will now be described in more detail with reference to the accompanying drawings, which show schematic vertical views of three different apparatus for carrying out the method according to the invention.

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In Fig. 1, the boiler is generally indicated by reference number 2. Sulfur-containing fuel 3 is burned in the boiler furnace with air 4 and powdered calcium carbonate is fed to the top of boiler 2 from silo 1, whereby the carbonate decomposes into an oxide which reacts with sulphide oxides, especially sulfur dioxide. The flue gas suspension from the boilers is led in pipes 5 to the lower part of the vertical activation reactor 6, which is also fed with an aqueous solution 7, whereby unreacted calcium oxide in the flue gas suspension is converted to highly reactive calcium hydroxide . From the activation reactor 6, the flue gas suspension is led to a scrubber 8, where the flue gas suspension is washed with an alkaline aqueous solution 9 in a jet scrubber, where not a large amount of dust in the flue gas suspension causes clogging problems. If desired, the walls of the scrubber 8 can be rinsed with water to remove dust deposits. By adding suitable chemicals to the scrubbing solution 9, not only sulfur oxides but also other gaseous impurities, such as nitrogen oxides, can be removed from the flue gas suspension.

The spent scrubbing solution is recovered from the lower part of the scrubber 8 and passed in a tube 7 to the nozzles at the bottom of the activation reactor 6, from which the spent scrubbing solution is sprayed into the Akti lubrication reactor 6. the effect of the content. The washed flue gas suspension is heated indirectly by a heat coil 10 and further passed through a duct 11 to an electrostatic precipitator 12 where the reaction product is recovered as dry dust 14, after which the purified gases are passed to a barrel 13. If desired, a coarse separator 22

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In the embodiment according to Fig. 2, a downwardly directed after-activation channel 6 'is arranged between the activation reactor 6 and the scrubber 8, at the lower end of which a coarse separator 18 removes solid reaction product 18 and at the top of the post-activation channel 6' a dust 14 separated by an electric filter 12 is fed. This separated dust 14 is also added to the lower part of the activation reactor 6 via a pipe 16. The remainder of the dust 14 separated by the electrostatic precipitator 12 is removed from the process along the pipe 15.

In this case too, the spent washing solution 7 removed from the scrubber 8 is led to the lower part of the activation reactor 6, but a part is returned to the scrubber 8 in the return pipe 9. To the return pipe 35

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5 84885 9 water and possible additional chemical are added via the supply pipe 19. Some of the used washing solution 7 can also be removed from the process by means of a pipe 17.

By feeding a portion of the ash 14 separated by the electrostatic precipitator 12 along a pipe 16 to the upper part of the after-activation channel 6 'and along the pipe 16' to the lower part of the activation reactor 6, any unreacted absorbent in this solid reaction product can be recovered. to regulate within the desired limits. In this case, too, the reaction product is recovered dry, if desired.

In the embodiment according to Figure 3, the scrubber 8 is arranged only after the electrostatic precipitator 12. In this case too, the spent washing solution is led along a pipe 7 to the lower part of the activation reactor 6, but an evaporation unit 20 is arranged in the pipe 7, where the used washing solution is evaporated by an indirect heating device 10 to provide a crystal slurry. In addition, water or another alkaline solution can be introduced into the lower part of the activation reactor 6.

In order to control the amount of alkali entering the scrubber 8, a bypass line 21 can be arranged between the after-activation channel 6 'and the pipe 11 leading from the electrostatic precipitator 12 to the scrubber 8, by means of which the amount of dust entering the scrubber 8 and thus the washing solution concentration can be controlled.

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The dry reaction product obtained by the process of the invention can be disposed of as such or disposed of as it is substantially insoluble, containing mainly calcium sulphite and sulphate. The method 35 according to the invention is also very easy to adjust so that its operation is optimal by controlling the amounts of washing solution and solid dust circulating therein.

Claims (6)

A process for reacting sulfur and nitrogen oxides of a hot flue gas suspension containing powdery alkali and / or alkaline earth metal oxide to a solid and thus readily separable reaction product by injecting into the hot flue gas suspension to activate said powdery oxide the obtained solid reaction product from the flue gas suspension, characterized in that the flue gas suspension (5) is washed (8) after the activation (6) with an alkaline aqueous solution (9), the so-called washing solution (7) containing water and the reaction product are recirculated at least partially to the hot flue suspension (5) for activating (6) of said oxide and extending the excess water and the reaction product (14) before separating the flue gas suspension into substantially dry form. Process according to claim 1, characterized in that the washed flue gas suspension is heated (10) and filtered (12) for separating the reaction product (14) substantially. 20 dry form. Process according to claim 2, characterized in that the reaction product (14) separated by filtration is recirculated to the flue gas suspension and the reaction product (18) is separated from the activated flue gas suspension before washing (8). Method according to any of the preceding claims, characterized in that a portion (9) of the washing solution (7) is recirculated to the washing (8). 5. A process according to claim 1, characterized in that a portion (9) of the washing solution (7) is recirculated to the wash (8) and the remainder is subjected to displacement (20) and the thus obtained crystal slurry is recirculated to the hot flue gas suspension (5). . 9 84885 Process according to claim 5, characterized in that at least part of the flue gas suspension is filtered (12) after the activation (6) and before washing (8).