WO2001024910A1

WO2001024910A1 - Method and installation for purifying fumes containing organic pollutants

Info

Publication number: WO2001024910A1
Application number: PCT/FR2000/002787
Authority: WO
Inventors: Bernard Siret; Claire Arnaud
Original assignee: Lab S.A.
Priority date: 1999-10-07
Filing date: 2000-10-06
Publication date: 2001-04-12
Also published as: FR2799388B1; FR2799388A1

Abstract

The invention concerns a method which consists in contacting (through 4) the fumes to be purified, containing dust and organic pollutants, with a washing liquid, in a humid washer (2); introducing into the washing liquid the adsorbent particles through (14); and evacuating (through 16) the purified fumes outside the humid washer, which contain solid particles; conveying the evacuated fumes towards at least a filtering element (26) capable of retaining the solid particles, so as to form a cake (40) of said particles, through which continuously passes at least part of the fumes derived from the washer, thereby enabling an additional purification in said cake, such that the washer may even operate with relatively low efficacy, which is economically advantageous.

Description

PROCESS AND PLANT FOR PURIFYING FUMES CONTAINING ORGANIC POLLUTANTS

The present invention relates to a process and an installation for purifying smoke containing organic pollutants.

Industrial smoke, for example from the incineration of household waste, contains many organic pollutants, some of which, such as dioxins and furans, have a particularly high toxicity. It is therefore necessary to purify this smoke before discharging it into the atmosphere, to lower the rate of these pollutants to sufficiently low values.

EP-A-0 509 258 discloses a process for the purification of smoke by the wet route, which consists in bringing them into contact with a washing liquid, in an enclosure called a wet washer. This contacting causes the condensation of organic pollutants which are thus separated from the fumes.

Furthermore, this document teaches to introduce, into the washing liquid, adsorbent particles, for example activated carbon, which immobilize the pollutants. These adsorbent particles are then separated from the washing liquid, for example by means of an activated carbon filter, which allows the pollutants thus immobilized to be removed from the washing liquid.

However, this process has certain drawbacks. In fact, the fumes from the washer, although they are purified, still contain dust, in particular fly ash which must be eliminated before the smoke is discharged into the atmosphere, insofar as this dust still contains pollutants that it was not possible to condense during the wet wash. In addition, the implementation of this method involves a significant consumption of adsorbent particles, since the latter are periodically removed from the washing liquid.

Furthermore, wet electrostatic precipitators are known, or still electrofiltrant modules, such as for example those sold by the company LAB SA under the commercial reference EDV, making it possible to separate the solid particles contained in a smoke flow. However, such devices are bulky and involve high costs, both in terms of investment and operation.

In order to overcome these various drawbacks, the invention proposes to implement a process which, while ensuring a thorough purification of the fumes, is economical in terms of investment and operation, and induces a consumption of adsorbent particles reduced compared to to the prior art.

For this purpose, it relates to a process for the purification of smoke by the wet route, in which the smoke to be purified, containing dust and organic pollutants, is brought into contact with a washing liquid in a wet washer, in order to condensing a fraction of said pollutants, adsorbent particles are introduced into the washing liquid allowing the immobilization of part of said condensed pollutants, and partially purified fumes, which contain solid particles, are removed from the wet washer, characterized in that the fumes discharged from the washer are directed towards at least one filtering element capable of retaining the solid particles, so as to form a cake of said particles, through which flows continuously at least a portion of the fumes from the washer .

The invention provides, in a known manner, a prior purification of the fumes in the wet scrubber. Then, the fumes discharged from the washer are directed to a filter element capable of retaining the solid particles. The latter, which is commonly used in the field of gas filtration, is for example formed from a nylon fabric, acrylic material or even polytetrafluoroethylene (PTFE). This filtering element causes an accumulation of solid particles, which leads to the formation of a thin layer, or cake of the latter, on the upstream side of the flue gas flow. These solid particles include a part dust present in the fumes, as well as the entrained part of the adsorbent particles used to immobilize the pollutants condensed in the washing liquid.

A substantial part, advantageously all of the smoke flows through this cake, which induces an additional purification. Indeed, these fumes will circulate in particular in the vicinity of the adsorbent particles, capable of immobilizing the pollutants and therefore of removing them from the fumes. In service, this cake is separated from the filter, periodically or else continuously, for example by rinsing or else by vibrating this filter.

The invention achieves the previously mentioned objectives. Indeed, it implements filter elements which have a reduced bulk, a simple structure and imply low costs.

Furthermore, the use of these filter elements causes additional purification of the fumes, in two different ways. In fact, at the outlet of the washer, the fumes contain pollutants which are either immobilized in the solid particles, or present in the gaseous phase proper.

However, the filter elements allow the solid particles, and therefore the pollutants trapped by these particles, to be removed from the fumes. In addition, the cake formed on the filter elements by the solid particles, in particular due to the presence of the adsorbent particles, entrained by the fumes and still having an adsorption capacity for the pollutants, induces an immobilization of the pollutants initially present in the gas phase, and therefore their separation from the smoke admitted upstream of the filter elements.

The invention also ensures contact between the fumes and the adsorbent particles, which is much better in the vicinity of the filter elements than in the wet washer. This makes it possible to confer on the adsorbent particles, entrained by the fumes out of the washer, improved efficiency and thus contributes to reducing consumption thereof, in comparison with the prior art.

According to an advantageous characteristic of the invention, the the organic pollutants content of the smoke, immediately downstream of the wet scrubber, is substantially higher than the desired content downstream of the filter element.

One can for example aim for a content of fumes in organic pollutants, immediately downstream of the wet scrubber, which is up to 10 times, preferably 3 to 5 times, greater than the desired content before the fumes are released into the atmosphere. , namely downstream of the filter element. This makes it possible to give the wet washer a relatively low purification efficiency, which is advantageous in economic terms. Indeed, this washer is then likely to have limited dimensions and its installation and operating costs can be lowered.

Furthermore, carrying out only partial purification at the level of the washer is not detrimental to obtaining fumes having a concentration of pollutants compatible with their release into the atmosphere. In fact, when the washer provides only partial purification, the fumes which are evacuated therefrom have a concentration of solid particles, and therefore of adsorbent particles, which is relatively high. In this way, these particles will be able, once arranged in a cake on the filter element, to operate an additional immobilization of the pollutants present in the gas phase at the outlet of the washer. It will also be advisable for the skilled person to vary it voluntarily, in particular to increase the concentration of solid particles in the flue gases admitted downstream of the washer, by modulating the operating parameters of the latter.

According to another advantageous characteristic of the invention, the filter element is made of a material capable of retaining pollutants in the gas phase. In this case, this filter element is for example made of carbon fibers or polypropylene. This makes it possible to carry out an ultimate purification phase, through the filter element itself. In this way, for a given purification rate downstream of the filter element, the efficiency of the washer can be lowered, which ensures a reduction in costs, in particular of operation. According to another characteristic of the invention, at least a substantial part of the adsorbent particles belonging to the cake of solid particles is recycled to the washer. Such recycling makes it possible to take advantage, in an optimal manner, of the extremely high adsorption capacities which the adsorbent particles have, such as activated carbon or coke, and induces a reduction in their consumption.

According to another characteristic of the invention, a reaction of destruction of the pollutants immobilized by the adsorbent particles is induced. This destruction, which is described in EP-A-0 585 316, can in particular be implemented by the injection of an external reagent, such as hydrogen peroxide, or even by taking advantage of the presence of the oxygen in the fumes, which dissolves in the washing liquid. This destruction reaction contributes to the continuous regeneration of the adsorbent particles, by liberation of their active sites, which gives the latter a particularly long period of use and makes it possible to reduce their consumption.

According to another characteristic of the invention, the fumes discharged from the wet washer are subjected to a separation of the liquid droplets. This operation, which makes it possible to eliminate substantially any liquid phase of the fumes admitted upstream of the filter element, gives the latter an optimal efficiency.

The invention also relates to a smoke purification installation comprising means for supplying smoke to be purified, containing dust and organic pollutants, means for supplying a washing liquid, a wet washer. allowing said fumes to be brought into contact with said washing liquid, means for introducing adsorbent particles into the washing liquid and means for discharging partially purified fumes, which contain solid particles, outside the wet washer, characterized in that the discharge means are placed in communication with at least one filter element capable of retaining said solid particles, so as to form a cake of these particles, interposed at least partially on the flow path of the fumes from the washer.

According to other characteristics of the invention:

the or each filter element is made of a material capable of retaining the pollutants present in the gas phase of the fumes;

- the or each filter element is made of carbon fibers or polypropylene;

the installation includes means for recycling, towards the washer, adsorbent particles belonging to the cake;

- The installation also includes a separator, interposed between the washer and the filter elements, suitable for separating the water droplets present in the fumes. The invention will be described below, with reference to the single appended figure, given only by way of nonlimiting example, which is a schematic representation of a smoke purification installation according to the invention.

The installation illustrated in this figure comprises a wet washer 2, of known type, supplied by line 4 with fumes to be purified, which contain organic pollutants, in particular dioxins and furans. This washer 2 also receives a recycling conduit 6 conveying a washing liquid, which is distributed in the interior volume of the washer by means of sprayers 8. There is also provided an unrepresented makeup and purging of this washing liquid. washing .

The outlet of the washer is connected, by a line 10, with a tank 12 which can be integrated into the washer 2, a tank on which an inlet 14 is stuck allowing the supply of this tank with adsorbent particles, such as activated carbon or lignite coke. The outlet from this tank 12 communicates with line 6 conveying the recycled washing liquid in the direction of the washer 2. The inlet 14 can also be tapped on line 4. Two recycling lines, which will be explained below, open out also in this tray 12.

The tank 12 can also receive an inlet (not shown) of hydrogen peroxide, so as to generate a reaction of oxidation aimed at destroying in situ the pollutants immobilized by the adsorbent particles, in accordance with the teaching of EP-A-0 585 316.

The fumes purified within the washer 2 are removed therefrom through a duct 16 connected to a separator 18, for example with chevrons, allowing the separation of the droplets present in the fumes, as well as large solid particles.

A line 20 allows recycling, towards the tank 12, of what has been captured by the separator 18. The latter is also put in communication, by a conduit 22, with a filtration enclosure 24 containing filter elements 26, produced carbon fiber.

The filtration chamber 24 receives an inlet 28 for rinsing water from the cake formed in the vicinity of the filter elements 26, as will be explained below. A pipe 30, allowing the injection of water vapor, is placed in communication with the interior of the filtration enclosure 24. A line 32 ensures the evacuation of the purified fumes out of the filtration enclosure 24. This the latter is also linked to a pipe 34, opening into an intermediate tank 36, itself placed in communication with the tank 12 via a line 38.

The operation of the installation described above will be explained below.

Fumes to be purified, which contain organic pollutants and dust, are admitted into the washer 2 by line 4. They are brought into contact, in this washer 2, with the washing liquid supplied by the sprayers 8, which ensures the condensation, in a known manner, of a fraction of the pollutants initially present in these fumes. Furthermore, the adsorbent particles admitted into the washing liquid through the inlet 14, also immobilize by adsorption almost all of the thus condensed fraction of these pollutants.

The partially purified fumes, discharged through line 16, contain dust containing pollutants, adsorbent particles immobilizing pollutants, pollutants present in the gas phase as well as droplets of washing liquid. The concentration of pollutants in these fumes, immediately at the outlet of the scrubber, can be maintained substantially higher than the target concentration before the rejection of fumes into the atmosphere by line 32, since these fumes will be subjected to additional purification. , as will be explained in the following.

The fumes from the washer are admitted into the separator 18, which allows the elimination of the droplets of entrained washing liquid. The latter are returned, with part of the dust and adsorbent particles, towards the tank 12, by the line 20.

The conduit 22 directs the fumes separated from the water droplets to the filtration chamber 24. The latter is dimensioned in such a way that the fumes pass through the filter elements 26 with a speed of between 0.5 and 5 m / s , preferably between 1.5 and 3 m / s.

The fumes carried by line 22 still contain a significant amount of solid particles, namely dust and adsorbent particles, that is to say between

5 and 50 mg / Nm ³ of smoke and, preferably, between 5 and 20 mg / Nm ³ of smoke.

When the fumes arrive in contact with the filter elements 26, an accumulation of solid particles occurs on the upstream side of these elements 26. This contributes to the formation of a cake 40 of these solid particles, through which the fumes are brought to walk. This cake 40 causes an additional purification of the fumes, since the pollutants are immobilized by the adsorbent particles that this cake contains.

Furthermore, since the filter elements 26 are made of carbon fiber, the pollutants present in the gas phase in the fumes are retained by adsorption by these filter elements, which induces ultimate purification of the fumes, before their rejection into the atmosphere through line 32. Since the formation of this cake is rapid, it is not necessary to increase the efficiency of the washer during the initial phase of constitution of this cake. The latter is rinsed periodically by the line 28, which can be supplied at least partially by the intermediate tank 36. The duct 34 conveys dust, rinsing liquid and adsorbent particles, which are enriched in pollutants compared to the particles fed by line 22, namely upstream of filtration. All of the adsorbent particles are returned to the tank 12, at which a purge, not shown, of these particles is provided, in accordance with the teaching of EP-A-0 585 316.

When their content of adsorbed pollutants is too high, the filter elements 26 can be regenerated, in a known manner, by injection of water vapor through line 30. An example of implementation of the process of the invention will be given below. -after, without limitation.

Fumes to be purified, containing approximately 2.8 ng TEQ / Nm ³ of dioxins and furans are admitted via line 4, at a flow rate of approximately 1.4 m ³ / s. The washing liquid conveyed through line 6 contains 20 g / l of adsorbent particles, consisting of lignite coke. This liquid is sprayed at a rate of 9 liters per m ³ of smoke admitted. The temperature in the washer is 61 ° C. The fumes carried by the line 22, at the outlet of the separator 18, contain suspended solids, namely dust as well as adsorbent particles at a rate of approximately 30 mg / Nm ³ . Furthermore, the fumes present in this line 22 contain 0.14 ng TEQ / Nm ³ of dioxins and furans, ie a purification factor 20 compared to the inlet of the washer.

The fumes from line 22 are directed to filter elements made of carbon fibers, the filter surface of which is 2.9 m ² . The fumes ready to be discharged into the atmosphere, i.e. present in line 32, have a concentration of suspended solids of less than 10 mg / Nm ³ and a concentration of dioxins and furans of 0.04 ng TEQ / Nm ³ . The passage of smoke through filter elements 26 therefore makes it possible to obtain an additional factor of a value of 3.5 in terms of purification of the fumes.

Claims

1. Process for the purification of smoke by the wet route, in which the fumes to be purified, containing dust and organic pollutants, are brought into contact (by 4) with a washing liquid in a wet washer (2), in order to condensing a fraction of said pollutants, adsorbent particles are introduced into the washing liquid (par 14) allowing the immobilization of part of said condensed pollutants, and partially purified fumes (which contain particles) are removed (par 16) solids, outside the wet washer, characterized in that the fumes discharged from the washer (par 16) are directed towards at least one filter element (26) capable of retaining the solid particles, so as to form a cake (40) of said particles , through which flows at least part of the fumes from the washer continuously.

2. Purification process according to claim 1, characterized in that, immediately downstream of the wet scrubber (2), the content of fumes in organic pollutants is substantially higher than the desired content downstream of the filter element (26) .

3. Purification process according to claim 2, characterized in that the content of organic pollutants in the smoke, immediately downstream from the wet scrubber (2), is up to 10 times, preferably 3 to 5 times, greater than the desired content downstream of the filter element.

4. Purification process according to one of claims 1 to 3, characterized in that the filter element (26) consists of a material capable of retaining the pollutants present in the gaseous phase of the fumes.

5. Purification process according to one of claims 1 to 4, characterized in that it is recycled (by 34) to the washer

(2), at least a substantial part of the adsorbent particles belonging to the cake (40) of solid particles.

6. Purification process according to one of the preceding claims, characterized in that a reaction of destruction of the pollutants immobilized by the particles is induced. adsorbent.

7. Purification process according to any one of the preceding claims, characterized in that the exhaust fumes (by 16) of the wet washer (2) are subjected to a separation (in 18) of the liquid droplets.

8. Smoke purification installation comprising means (4) for supplying smoke to be purified, containing dust and organic pollutants, means of arrival

(6, 8) of a washing liquid, a wet washer (2) allowing said fumes to be brought into contact with said washing liquid, means for introducing (14) adsorbent particles into the washing liquid and means exhaust (16) partially purified fumes, which contain solid particles, outside the wet washer (2), characterized in that the exhaust means (16) are placed in communication with at least one filter element (26) suitable for retaining said solid particles, so as to form a cake (40) of these particles, interposed at least partially on the flow path of the fumes from the washer.

9. Purification installation according to claim 8, characterized in that the or each filter element (26) consists of a material capable of retaining the pollutants present in the gaseous phase of the fumes.

10. Purification installation according to claim 9, characterized in that the or each filter element (26) is made of carbon fibers or polypropylene.

11. Purification installation according to one of claims 8 to 10, characterized in that it comprises means (34, 38) for recycling, to the washer (2), adsorbent particles belonging to the cake (40).

12. Purification installation according to one of claims 8 to 11, characterized in that it further comprises a separator (18), interposed between the washer (2) and the filter elements (26), suitable for separating the water droplets present in the fumes.