BE1021986B1 - METHOD AND APPARATUS FOR CAPTURING HIGHLY OXIDIZING POLLUTANTS IN A WET GASEOUS FLOW - Google Patents

Abstract

In accordance with the method according to the invention: - the gas stream (1) is introduced into a wet scrubber (101), as well as a first stream containing a reducing reagent (3) and a second stream containing an oxidation inhibitor (4 ), - from at least the measurement of the redox potential of a purge (5) leaving the washer (101), the quantity of the oxidation inhibitor (4) introduced by the second flow into the washer is controlled so as to keep the redox potential of this purge below a given level, and - from at least the measurement, direct or indirect, of the concentration of reducing reagent (3) in the outgoing purge (5) of the washer, the amount of reducing reagent introduced by the first flow into the washer is controlled.

Description

METHOD AND APPARATUS FOR CAPTURING HIGHLY OXIDIZING POLLUTANTS IN A WET GASEOUS FLOW

The present invention relates to a method and a system for capturing, in a gas flow, strongly oxidizing pollutants, at least one of which belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine. More generally, the invention refers to the purification of gaseous flow by wet process.

In processes of this type, a washing liquid is contacted with the gases to be purified in a scrubber so that the pollutants to be trapped are transferred into the scrubbing liquid. Wet scrubbers are commonly used to kill acidic pollutants, such as hydrochloric acid HCl and sulfur dioxide S02, but are not, however, restricted to these pollutants and are used to kill a large number of potential gaseous pollutants such as chlorine. , bromine, chlorine dioxide, ozone and nitrogen oxides. Moreover, in addition to the main pollutants mentioned above, the gases to be purified may also contain heavy metals such as mercury.

Wet scrubbers offer great flexibility and are therefore currently widely used.

In practice, different reagents are used in these scrubbers according to the pollutant to be collected. Thus, to capture for example S02 sulfur dioxide or hydrochloric acid HCl, an alkaline neutralizer, such as lime, limestone or sodium hydroxide is used.

Several processes are possible for capturing nitrogen oxides: they generally use strong oxidants to oxidize part of the nitrogen monoxide NO to nitrogen dioxide NO 2, and then exploit the reaction NO + NO 2 → N 2 O 3 product is a soluble compound that is captured by being transferred to the washing liquid. Other processes use various reagents, such as sodium chlorite, as proposed in FR 2 643 286, or such as ozone. Overall, these various processes are efficient, but insofar as the leakage of the oxidizing compound is to be avoided, guard washers are necessary.

For the metals, it is also possible to use oxidants, in particular for solubilizing the mercury in the +0 oxidation state by passing it to the +2 oxidation state. Thus, patent FR 2 724 577 proposes such a method. Again, leakage of the oxidizing reactive compound can not be tolerated.

In still other cases, it is compounds such as chlorine or chlorine dioxide that must be removed. These situations are encountered in the paper industry. Reducing washing solutions, often composed of sodium sulphite at a pH greater than 8, are then used.

This being recalled, a problematic case occurs when it is necessary to maintain in the scrubber a reducing solution, that is to say having a low redox potential, and also a sufficient concentration of reducing agent, for example sulphites, for reasons chemical kinetics and kinetics of gas-liquid transfer. This problematic situation is particularly acute when oxidants are used for the purification of nitrogen oxides by the wet route: in fact, part of the nitric oxide NO is oxidized to nitrogen dioxide, this oxidation being desired, then the nitric oxide and nitrogen dioxide mixture is captured in a scrubber containing sulphites. However, the flue gas oxidizes the sulphites so that well over one mole of sulphite is converted to sulphate per mole of the nitric oxide-nitrogen dioxide mixture, which can lead rapidly to an impoverishment such as sulphites that nitrogen oxides are no longer well captured. Moreover, this loss of reducing power in the scrubber is detrimental to the good uptake of the excess of oxidation agent, such as the chlorine dioxide CI02 used upstream of the scrubber to reduce the nitrogen oxides. A double problem therefore arises: both a sufficient concentration of reducing reagents must be maintained in the scrubber and a sufficiently low redox potential must also be maintained in the scrubber, which is not exactly the same.

In this context, it has been proposed, in particular by Mrs Rochelle and Vincentis of the University of Texas, to limit the oxidation of sulfites by using oxidation inhibitors, such as sodium thiosulfate. However, sodium thiosulphate is much more expensive than sulphites, the latter being in many cases brought by the sulfur dioxide S02 present in the incoming gas, so we try to minimize the consumption.

For its part, EP-A-2 123 344 discloses a method for treating combustion fumes, into which a mixed solution of a reducing agent and sodium hydroxide is introduced into a wet scrubber. This document teaches that this mixed solution maintains the pH and redox potential of the scrubber purge within predetermined ranges. Furthermore, it is clearly stated in this document that the reducing agent is either sodium sulphite or sodium thiosulfate.

The object of the present invention is to improve the uptake of gaseous pollutants strongly oxidizing wet, placing themselves in economic and technically optimal conditions to achieve this capture. For this purpose, the subject of the invention is a process for the capture, in a gaseous flow, of strongly oxidizing pollutants of which at least one belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine, in which the gaseous flow is introduced into a wet scrubber, as well as a first stream containing a reducing reagent and a second stream containing an oxidation inhibitor, starting from at least the measurement of the redox potential of an outgoing purge; of the scrubber, the amount of the oxidation inhibitor introduced by the second flow into the scrubber is controlled so as to maintain the redox potential of this purge below a given level, and - from at least measuring, directly or indirectly, the reductive reagent concentration in the purge leaving the scrubber, controlling the amount of reducing reagent introduced by the first flow in the scrubber. The invention also relates to a system for capturing, in a gaseous flow, strongly oxidizing pollutants, at least one of which belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine, the installation comprising: a wet scrubber fed by the gas stream, as well as a first stream containing a reducing reagent and a second stream containing an oxidation inhibitor; a first means for controlling the quantity of oxidation inhibitor introduced by the second stream; in the scrubber from at least the measurement of the redox potential of a purge exiting the wet scrubber so as to maintain the redox potential of this purge below a given level, and - a second means for controlling the quantity of reducing reagent introduced by the first flow into the scrubber from at least the direct or indirect measurement of the concentration of reducing reagent in the purge leaving the scrubber.

Thus, the invention provides for maintaining highly oxidizing pollutants, both a concentration of sufficient reducing reagents and a relatively low redox potential, in the capture washer: these two conditions are necessary in order to obtain sufficiently purified gas discharges, in particular in accordance with regulatory requirements applicable to gas streams containing at least one strongly oxidizing compound.

According to advantageous additional features of the process and the installation according to the invention, taken separately or in any technically possible combination: the amount of oxidation inhibitor introduced by the second flow into the scrubber is controlled exclusively from measuring the redox potential of the purge leaving the scrubber, and controlling the amount of reducing reagent introduced by the first flow into the scrubber exclusively from the measurement of the reducing reagent concentration in the scrubber purge; the respective amounts of oxidation inhibitor and reducing reagent introduced, respectively by the second flow and by the first flow, respectively, are controlled in the scrubber from the respective measurements of the redox potential of the purge leaving the scrubber and the scrubber; concentration of reducing reagent in this purge, as well as possibly the pH of this purge; the redox potential of the purge leaving the scrubber is maintained at a value between -100 mV and +150 mV and the pH of the scrubber is maintained between 6 and 10; the oxidation inhibitor is sodium thiosulfate; the reducing reagent is sodium sulphite; in addition to the highly oxidizing pollutant (s), the gas stream contains oxides of nitrogen and oxygen; the gaseous flow comes from a wet scrubber other than the scrubber into which this gaseous flow is introduced; the first and second means include a joint multivariable controller. The invention will be better understood on reading the description which will follow, given solely by way of example and with reference to the drawings in which: FIG. 1 is a diagram of an installation according to the invention; implementing a method according to the invention; and - Figure 2 is a view similar to Figure 1, illustrating a variant of the installation and the method according to the invention.

In the installation shown in FIG. 1, a gaseous stream to be purified 1 is introduced into a wet scrubber 101. This gaseous flow 1 can in particular be constituted by a flow originating from an upstream industrial process, or else be the output of a washer other than the washer 101.

The gas stream to be treated 1 is polluted by the presence of at least one strongly oxidizing compound, from the group consisting of ozone O3, chlorine Cl2, chlorine dioxide CI02 and Br2 bromine. In practice, the gas stream 1 may also contain nitrogen oxides, in particular nitric oxide NO and nitrogen dioxide NO 2, and also oxygen.

According to the invention, the wet scrubber 101 receives a liquid stream containing a reducing reagent 3, which, without limitation, may be an alkaline sulfite or an alkaline hydrogen sulfite, in particular sodium sulfite.

According to the invention, the washer 101 also receives an oxidation inhibitor 4, which is distinct from the reducing reagent 3 and consists for example of a solution of sodium thiosulfate or a suspension of sulfur.

From the scrubber 101, a treated gas stream 2 and a liquid deconcentration purge 5 are discharged. The injection of the reducing reagent 3 into the scrubber 101, which is decoupled from that of the oxidation inhibitor 4, is intended to maintain in this scrubber at any time, a sufficient concentration of the corresponding main reactive species, in particular for reasons of chemical kinetics with the strongly oxidizing pollutant (s) to be collected. For example, in the case of wet denitrification using sodium chlorite NaClO 2, the gas stream 1 leaves another scrubber, which is therefore upstream of the scrubber 101 and in which the sodium chlorite introduced therein performs the oxidation of a part of the nitrogen oxides present in the gas stream admitted to this other scrubber. In addition to water vapor, carbon dioxide, nitrogen and oxygen, the gas stream 1 then contains nitric oxide NO and nitrogen dioxide NO2, as well as chlorine dioxide. CI02 as an active oxidizing agent not consumed in the upstream scrubber. The role of the scrubber 101 is then to capture a large part of the nitrogen oxides NOx, especially the mixture between the carbon monoxide NO and the carbon dioxide NO2, as well as the chlorine dioxide CI02, so that the rejection gas 2 can be released to the atmosphere. In this case, the reducing reagent 3 used is sodium sulphite or sodium hydrovesulfite, in aqueous solution.

In practice, in order to capture the oxides of nitrogen, it is essential to maintain a minimum concentration of sulphites, in particular ions S03-- and HS03-, in the washer 101, in other words in its purge 5. In the absence, however, in the absence of oxidation inhibitor 4, the oxygen contained in the gas stream 1 will consume by oxidation a significant part of the reducing agents present, including the reducing reagent 3. This oxidation is also greatly accelerated by certain nitrogen oxides such as N02 and, in the absence of oxidation inhibitor 4, the reducing agents, including sulphites, are likely to virtually disappear from the medium. It would end up with the washer 101 too depleted in reducers to perform its function. By way of example, in the case where sulphites are used as reducing agents in the scrubber 101, it is desirable to maintain a concentration of at least 10 g / l in sulphites, more specifically in SO 3 - and HSO 3 - ions.

This being so, this simple condition is not sufficient to ensure the capture of highly oxidizing pollutants in the washer 101 and it is also necessary that the redox potential is kept at a low enough level. Typically, this redox potential must be at most +200 mV and, preferably, must be less than +100 mV. In the case of the denitrification mentioned above, if the redox potential is not low enough, the uptake into the scrubber 101 of the upstream CI02 chlorine dioxide leak will only be bad, and the concentration of chlorine dioxide in Exit of the washer 101, in the gas discharge 2, will exceed the tolerable limit. As an option not shown, the washer 101 may receive other arrivals, such as caustic soda to maintain a certain pH value and water.

The control of the pH of the scrubber, which is not the subject of the invention as such and which can be carried out by an ad hoc measuring apparatus, shown under the reference 203 in FIG. 1, also influences directly on the redox potential of the washer since this redox potential decreases when the pH increases. Typically, it is desired to maintain the pH in the range of 6 to 10, to maintain the redox potential in the range of -150 mV to +200 mV, and to maintain the reducing reagent concentration in the range of values 5 to 50 g / l. It will be noted that the concentration of oxidation inhibitor 4 depends strongly on the nature of this inhibitor: in the case of sodium thiosulfate, a concentration of between 0.5 and 5 g / l is preferred.

In practice, the washer 101 is of a known technology. Preferably, this scrubber consists of a packed tower, comprising at least two meters of a lining with a specific surface area of at least 100 m 2 / m 3.

According to the invention, the redox potential is measured in the bleed 5 of the washer 101 by means of an ad hoc apparatus 201 and the 2001 information on the measured redox potential is used, for example with the help of a PID regulator, otherwise known as a derived integral proportional control unit, known per se, to control the flow of oxidation inhibitor 4, that is to say in order to control the amount of this oxidation inhibitor introduced into the washer 101.

Similarly, according to another characteristic of the invention, the concentration of reducing reagent 3 is measured in the purge 5 and the 2002 information on this measured concentration is used to control the addition of reducing reagent 3. The aforementioned information 2002 also ensures that there is sufficient deconcentration. In practice, the measurement of the concentration of the reducing reagent 3 in the purge 5 is carried out by an ad hoc device 202 known per se, making it possible to measure this concentration of reducing reagent either directly or indirectly, that is, that is to say, by measurement of another concentration, or even of another characteristic, allowing, in particular by calculation, to go back to the concentration of reducing reagent. This device 202 may for example be a conductivity meter, an ion chromatograph or a density measurement probe.

In view of the above, it is understood that, in essence, the invention aims to maintain in the washer 101 both a sufficient reducing reagent concentration 3 and a low enough redox potential. These two conditions are necessary to obtain a gaseous discharge 2 that meets the regulatory requirements when a strongly oxidizing compound is present in the incoming gas stream 1. This is particularly the case for wet denitrification with sodium chlorite or sodium chlorite. chlorine dioxide, as mentioned above: for such denitrification, on the one hand, the low redox potential of the washer 101 is essential to properly capture the upstream leak of chlorinated oxidant CI02, which is present in the gas stream 1 and, on the other hand, a sufficient concentration of sulphites is necessary, both for capturing the nitrogen oxides and the chlorine dioxide CI02 mentioned above.

According to an alternative shown in FIG. 2, a multivariable controller 301 is used to jointly control the flow of the reducing reagent 3 and the flow of the oxidation inhibitor 4, this multivariable controller 301 having as inputs both the 2001 information. on the measured redox potential of purge 5, information 2002 on the concentration of reducing reagent 3 measured in the purge 5, and, optionally, 2003 information on a measured value of the pH of the purge 5.

The need and the relevance of controlling not only the concentration of reducing reagent 3 but also the redox potential in the washer 101 are confirmed by the tests detailed below, carried out with a pilot plant. These tests simulate wet denitrification with sodium chlorite, such as that mentioned above: in this case, as also indicated above, the active oxidizing agent is chlorine dioxide CI02.

As part of the pilot plant, a gas stream of 1.7 liters / minute, containing 440 mg / m3 dry nitrogen oxide NO in air is admitted into a bubbler containing 400 ml of a solution 1M hydrochloric acid. In this bubbler, 30 ml / hour of a solution of sodium chlorite at 24 g / liter are also introduced. In contact with hydrochloric acid, chlorine dioxide is evolved according to the reaction NaClO 2 + 4 HCl → 4 CIO 2 + 5 NaCl + 2H 2 O and converts nitric oxide NO to nitrogen dioxide NO 2.

At the same time, a second bubbler, distinct from the first bubbler presented above and fed by the gases produced by the first bubbler, also contains 400 ml of solution, a reducing reagent and an oxidation inhibitor. This second bubbler simulates an alkaline scrubber downstream of the first bubbler described above, which captures oxides of nitrogen NOx and the leakage of active oxidizing agent from the first bubbler: the second bubbler thus simulates the washer 101 of the FIG. 1 or FIG. 2. The concentration of the compounds in the effluent gas leaving the second bubbler is determined by analyzers and / or by chemical assays, in order to evaluate both the denitrification efficiency and the leakage rate. active oxidizing agent.

In a first case, subsequently referred to as Test A, the second bubbler contains sodium sulphite at 30 g / l and enough oxidation inhibitor to maintain a low redox potential. In a second case, subsequently qualified as test B, sodium thiosulphate is added in insufficient quantity, in this case divided by two compared to test A. The table below groups the characteristics of tests A and B.

Thus, as can be seen in the table above, the redox potential of the second bubbler is higher for test B, whereas for test A, the redox potential is lower. Moreover, while in both tests, the denitrification efficiency is similar, the leakage of active oxidizing agent CI02 is much greater for test B in which the redox potential is not controlled and becomes too high: the solution liquid of the second bubbler is then unable to effectively capture the strongly oxidizing compound CI02 and the concentration in the latter is much too high for the outgoing gaseous effluent can be released into the atmosphere.

Of course, in practice, the redox potential values referred to herein are given by reference to a saturated calomel electrode.

Claims (10)

1. - Method for capturing, in a gaseous flow, strongly oxidizing pollutants, at least one of which belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine, in which: - it is introduced into a wet scrubber (101) the gas stream (1), as well as a first stream containing a reducing reagent (3) and a second stream containing an oxidation inhibitor (4), - from at least the measurement of the redox potential of a purge (5) leaving the scrubber (101) controls the amount of oxidation inhibitor (4) introduced by the second flow into the scrubber so as to maintain the redox potential of this purge below a given level, and - from at least the measurement, direct or indirect, of the concentration of reducing reagent (3) in the purge (5) leaving the washer, the amount of reducing reagent introduced by the first flow in the washer. 2. - Process according to claim 1, wherein the amount of oxidation inhibitor (4) introduced by the second stream into the scrubber (101) is controlled exclusively from the measurement of the redox potential of the purge (5) leaving the scrubber, and in which the amount of reducing reagent (3) introduced by the first flow into the scrubber is controlled exclusively from the measurement of the concentration of reducing reagent in the purge of the scrubber. 3. - Process according to claim 1, wherein the respective amounts of oxidation inhibitor (4) and reducing reagent (3) introduced, respectively by the second stream and by the first stream, into the scrubber ( 101) from the respective measurements of the redox potential of purge (5) leaving the scrubber and the concentration of reducing reagent in this purge, as well as possibly the pH of this purge. 4. - Method according to any one of the preceding claims, wherein the redox potential of the purge (5) leaving the scrubber (101) is maintained at a value between -100 mV and +150 mV and the pH of the scrubber is maintained between 6 and 10. 5. - Process according to any one of the preceding claims, wherein the oxidation inhibitor (4) is sodium thiosulfate. 6. - Process according to any one of the preceding claims, wherein the reducing reagent (3) is sodium sulfite. 7. - Process according to any preceding claim, wherein, in addition to the highly oxidizing pollutant or pollutants, the gas stream (1) contains oxides of nitrogen and oxygen. 8. - Process according to any one of the preceding claims, wherein the gas stream (1) is from a wet scrubber other than the scrubber (101) into which this gas stream is introduced. 9. - Installation for capturing, in a gaseous flow, highly oxidizing pollutants of which at least one belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine, this installation comprising: - a wet scrubber (101 ) fed by the gas stream (1), as well as by a first stream containing a reducing reagent (3) and a second stream containing an oxidation inhibitor (4), - first means (201; 201, 301) for controlling the amount of oxidation inhibitor (4) introduced by the second stream into the scrubber (101) from at least the measurement of the redox potential of a purge (5) exiting the wet scrubber so as to maintain the potential redox from this purge below a given level, and - second means (202; 202, 301) for controlling the amount of reducing reagent (3) introduced by the first flow into the scrubber (101) from at least the direct or indirect measurement of the concentration of reducing reagent in the rge (5) exiting the scrubber. The apparatus of claim 9, wherein said first and second means include a multivariable controller (301).