CH706528B1

CH706528B1 - Method and installation for capturing highly oxidizing pollutants in a wet gas stream

Info

Publication number: CH706528B1
Application number: CH00959/13A
Authority: CH
Inventors: Tabaries Frank; Siret Bernard
Original assignee: Lab Sa
Priority date: 2012-05-15
Filing date: 2013-05-15
Publication date: 2017-03-31
Also published as: BE1021986B1; CH706528A2; FR2990630B1; FR2990630A1

Abstract

According to the process according to the invention, the gaseous stream (1) is introduced into a wet scrubber (101), and a first stream containing a reducing reagent (3) and a second stream containing an oxidation inhibitor (4) , Starting from at least the redox potential measurement of a purge (5) leaving the scrubber (101), controlling the quantity of the oxidation inhibitor (4) introduced by the second stream into the scrubber In maintaining the redox potential of this purge below a given level and from at least the direct or indirect measurement of the reducing reagent concentration (3) in the purge (5) leaving the scrubber, The quantity of reducing reagent introduced by the first stream into the scrubber is controlled.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an installation for capturing, in a gas stream, highly oxidizing pollutants of which at least one belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine. More generally, the invention refers to the purification of gaseous streams by a wet process.

In such processes a washing liquid is brought into contact with the gases to be purified in a scrubber so that the pollutants to be collected are transferred into the washing liquid. Wet scrubbers are commonly used to kill acid pollutants such as hydrochloric acid HCl and sulfur dioxide SO2 but are not 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. Furthermore, in addition to the above-mentioned main pollutants, the gases to be purified may also contain heavy metals such as mercury.

[0003] Wet scrubbers offer great flexibility and are therefore currently used extensively.

[0004] In practice, various reagents are used in these scrubbers depending on the pollutant to be captured. Thus, for example, to capture sulfur dioxide SO2 or hydrochloric acid HCl, an alkaline neutralizer, such as lime, limestone or sodium hydroxide, is used.

In order to capture nitrogen oxides, several methods are possible: they generally use strong oxidants to oxidize a portion of the nitrogen monoxide NO to nitrogen dioxide NCk and then exploit the reaction NO + NO 2 - N203, the product of which is a soluble compound which is captured by being transferred to the wash liquid. Other methods use various reagents, such as sodium chlorite, as proposed in FR 2,643,286, or such as ozone. Overall, these various processes perform well, but to the extent that the leakage of the oxidizing compound is to be avoided, guard washers are necessary.

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

[0007] In still other cases, it is compounds such as chlorine or chlorine dioxide that have to be removed. These situations are encountered in the paper industry. Reduction washing solutions, often composed of sodium sulfite at a pH above 8, are then used.

[0008] This being recalled, a problematic case arises 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 of chemical kinetics and gas-liquid transfer kinetics. This problematic situation is particularly acute when oxidants are used for the wet scrubbing of nitrogen oxides: in fact, some of the nitrogen monoxide NO is oxidized to nitrogen dioxide, this oxidation being desired, and then The mixture of nitrogen monoxide and nitrogen dioxide is collected in a scrubber containing sulfites. However, the oxygen of the fumes gives a chain oxidation of the sulphites, So that well over one mole of sulphite is converted to sulphate per mole of the captured nitrogen monoxide / nitrogen dioxide mixture, which can rapidly lead to depletion such as sulfites that the nitrogen oxides are no longer Well captured. Furthermore, this loss of reducing power in the scrubber is detrimental to the good uptake of the excess oxidizing agent, such as the chlorine dioxide CI02 used upstream of the scrubber to reduce 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. Which can lead rapidly to a depletion such as sulphites that nitrogen oxides are no longer well captured. Furthermore, this loss of reducing power in the scrubber is detrimental to the good uptake of the excess oxidizing agent, such as chlorine dioxide CI02 used upstream of the scrubber to reduce 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. Which can lead rapidly to a depletion such as sulphites that nitrogen oxides are no longer well captured. Furthermore, this loss of reducing power in the scrubber is detrimental to the good uptake of the excess oxidizing agent, such as chlorine dioxide CI02 used upstream of the scrubber to reduce 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 sulphites using oxidation inhibitors, such as sodium thiosulfate. However, sodium thiosulphate is much more expensive than sulfites, the latter being in many cases supplied by the sulfur dioxide SO2 present in the incoming gases, so that it is sought to minimize its consumption.

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

OBJECT AND SUMMARY OF THE INVENTION The object of the present invention is to improve the capture of strongly oxidizing gaseous pollutants by a wet process, taking place under economic and technically optimum conditions in order to achieve this capture.

To this end, the subject of the invention is a process for capturing, in a gas stream, highly oxidizing pollutants of which at least one belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine , In which: - the gaseous stream is introduced into a wet scrubber, as well as a first stream containing a reducing reagent and a second stream containing an oxidation inhibitor, - on the basis of at least the measurement of the redox potential, A purge exiting from the scrubber, the quantity of the oxidation inhibitor introduced by the second stream into the scrubber is controlled so as to maintain the redox potential of this purge below a given level and, At least the direct or indirect measurement of the reducing reagent concentration in the purge exiting the scrubber,The quantity of reducing reagent introduced by the first stream into the scrubber is controlled.

The subject of the invention is also an installation for capturing, in a gas stream, highly oxidizing pollutants, at least one of which belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine, Installation comprising: a wet scrubber fed by the gas stream, as well as by 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 leaving the wet scrubber so as to maintain the redox potential of this purge below a given level, and second means for Controlling the amount of reducing reagent introduced by the first stream into the scrubber from at least the measurement,Direct or indirect, of the concentration of reducing reagent in the purge leaving the scrubber.

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

According to advantageous additional characteristics of the process and the installation according to the invention, taken in isolation or in all technically possible combinations: the quantity of oxidation inhibitor introduced by the second stream is controlled in the scrubber Exclusively from the measurement of the redox potential of the purge coming out of the scrubber, and the quantity of reducing reagent introduced by the first stream into the scrubber is controlled exclusively from the measurement of the reducing reagent concentration in the purge of the scrubber; The respective amounts of oxidation inhibitor and reducing reagent introduced by the second stream and the first stream, In the scrubber from the respective measurements of the redox potential of the purge coming out of the scrubber and of the concentration of reducing reagent in this purge, as well as, if appropriate, the pH of this purge; The redox potential of the purge leaving the scrubber is maintained at a value of 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 nitrogen oxides and oxygen; The gas flow originates from a wet scrubber other than the washer into which this gas flow is introduced; The first and second means include a joint multivariable controller. The redox potential of the purge leaving the scrubber is maintained at a value of 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 nitrogen oxides and oxygen; The gas flow originates from a wet scrubber other than the washer into which this gas flow is introduced; The first and second means include a joint multivariable controller. The redox potential of the purge leaving the scrubber is maintained at a value of 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 nitrogen oxides and oxygen; The gas flow originates from a wet scrubber other than the washer into which this gas flow is introduced; The first and second means include a joint multivariable controller. - in addition to the highly oxidizing pollutant (s), the gas stream contains nitrogen oxides and oxygen; The gas flow originates from a wet scrubber other than the washer into which this gas flow is introduced; The first and second means include a joint multivariable controller. - in addition to the highly oxidizing pollutant (s), the gas stream contains nitrogen oxides and oxygen; The gas flow originates from a wet scrubber other than the washer into which this gas flow is introduced; The first and second means include a joint multivariable controller.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the drawings in which: FIG. 1 is a diagram of an installation according to the invention, implementing a method in accordance with the invention; And FIG. 2 is a view similar to FIG. 1, illustrating a variant of the installation and the method according to the invention.

[0017] In the installation shown in FIG. 1, a gas stream to be purified 1 is introduced into a wet scrubber 101. This gas stream 1 may in particular consist of a stream originating from an upstream industrial process, or else be the outlet of a scrubber other than scrubber 101.

The gas stream to be treated 1 is polluted by the presence of at least one highly oxidizing compound, from the group consisting of ozone 03, chlorine Cl 2, chlorine dioxide CI0 2 and bromine Br 2. In practice, the gas stream 1 may also contain nitrogen oxides, in particular nitrogen monoxide NO and nitrogen dioxide NO 2, as well as oxygen.

According to the invention, the wet scrubber 101 receives a liquid stream containing a reducing reagent 3, which, in a non-limiting manner, may be an alkali sulfite or an alkali hydrogen sulfite, in particular sodium sulfite.

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

[0021] From the scrubber 101, a treated gas stream 2 and a deconcentration liquid 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 all times a sufficient concentration of the corresponding main reactive species , Especially for reasons of chemical kinetics with the strongly oxidizing pollutant (s) to be captured. For example, in the case of wet denitrification using sodium chlorite NaClO 2, the gas stream 1 exits another scrubber, which is therefore upstream of scrubber 101 and in which the sodium chlorite introduced therein Carries out the oxidation of part of the nitrogen oxides present in the gas stream admitted into this other scrubber. In addition to water vapor, carbon dioxide, nitrogen and oxygen, The gas stream 1 then contains nitrogen monoxide NO and nitrogen dioxide NO 2, as well as chlorine dioxide CIO 2 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 mixture 2 can be released to the atmospheric medium. In this case, the reducing reagent 3 used is sodium sulfite or sodium hydrogen sulphite in aqueous solution. Especially the mixture of the carbon monoxide NO and the carbon dioxide NO 2, as well as the chlorine dioxide CI 2, so that the gas discharge 2 can be released to the atmospheric medium. In this case, the reducing reagent 3 used is sodium sulfite or sodium hydrogen sulphite in aqueous solution. Especially the mixture of the carbon monoxide NO and the carbon dioxide NO 2, as well as the chlorine dioxide CI 2, so that the gas discharge 2 can be released to the atmospheric medium. In this case, the reducing reagent 3 used is sodium sulfite or sodium hydrogen sulphite in aqueous solution.

In practice, in order to capture nitrogen oxides, it is essential to maintain a minimum concentration of sulphites, in particular SO 3 - and HSO 3 - ions, in the scrubber 101, in other words in its purge 5. The absence of an oxidation inhibitor 4, the oxygen contained in the gas stream 1 will consume, by oxidation, a significant portion of the reducing agents present, including the reducing reagent 3. This oxidation is, moreover, greatly accelerated by certain oxides of nitrogen Such as NO 2, and in the absence of an oxidation inhibitor 4, the reducing agents, including the sulfites, are capable of practically disappearing from the medium. We would then find ourselves with washer 101 too depleted in reducers to be able to perform its function. By way of example, in the case where sulfites are used as reducing agents in scrubber 101,

This being the case, this simple condition is not sufficient to ensure the capture of the highly oxidizing pollutants in the scrubber 101 and the redox potential must also be maintained at a rather low 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 in the scrubber 101 of the upstream leakage of chlorine dioxide CI02 will only be poorly performed and the concentration of chlorine dioxide in Outlet of the scrubber 101, into the gas discharge 2, will exceed the tolerable limit.

As an option (not shown), the scrubber 101 can receive other inlets, such as, for example, caustic soda to maintain a certain pH value and water.

The pH control of the scrubber, which is not the subject of the invention as such and which can be achieved by an ad hoc measuring apparatus, shown under the reference 203 in FIG. 1, directly influences the redox potential of the scrubber since this redox potential decreases as the pH increases. Typically, it is desired to maintain the pH in the range of values 6 to 10, maintain the redox potential in the range -150 mV to +200 mV, and maintain the concentration of reducing reagent 3 in the range Range of 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 scrubber 101 is of a technology known per se. Preferably, this scrubber consists of a packed tower, comprising at least two meters of a packing with a specific surface area of at least 100 m 2 / m 3.

According to the invention, the redox potential is measured in the purge 5 of the scrubber 101 using an ad hoc apparatus 201 and the 2001 information is used on the redox potential measured so that, for example, Using a PID regulator, otherwise known as a derived integrated proportional control unit, known per se, to control the oxidation inhibitor flow 4, that is to say to control the quantity of this oxidation inhibitor Introduced into the scrubber 101.

Similarly, according to another characteristic of the invention, the concentration of reducing reagent 3 in the purge 5 is measured and the 2002 information is used on this measured concentration in order to control the replenishment of reducing reagent 3. [ Information 2002 also makes it possible to ensure that there is sufficient deconcentration. In practice, the concentration of the reducing reagent 3 in the purge 5 is measured by an ad hoc device 202 known per se, making it possible to measure this concentration of reducing reagent either directly or indirectly, Ie by measuring another concentration, or even another characteristic, making it possible, especially by calculation, to go back to the reducing reagent concentration. This device 202 can for example be a conductivity meter,

In the light of the foregoing, it will be understood that the essence of the invention is to maintain in the scrubber 101 both a sufficient concentration of reducing reagent 3 and a relatively low redox potential. These two conditions are necessary in order to obtain a gaseous discharge 2 in accordance with the regulatory requirements when a strong oxidizing compound is present in the incoming gas stream 1. This is particularly the case for wet denitrification with sodium chlorite or Chlorine dioxide, as mentioned above: for such denitrification, on the one hand, the low redox potential of the scrubber 101 is indispensable in order to 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,

[0031] According to an alternative shown in FIG. 2, a multivariable controller 301 is used to jointly drive the flow of the reducing reagent 3 and the flux of the oxidation inhibitor 4, this multivariable controller 301 having for inputs both

Claims

The 2001 information on the measured redox potential of the purge 5, the 2002 information on the concentration of reducing reagent 3 measured in the purge 5, and optionally a 2003 information on a measured pH value of the purge 5 The necessity and the pertinence of controlling not only the concentration of reducing reagent 3 but also the redox potential in the scrubber 101 are confirmed by tests detailed below, carried out with a pilot installation. 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. In the context of the pilot plant, a gas stream of 1.7 liters / minute, Containing 440 mg / m3 of nitrogen oxide NO in air is admitted into a bubbler containing 400 ml of a 1 M hydrochloric acid solution. In this bubbler, 30 ml / hour Of a 24 g / l sodium chlorite solution. In contact with hydrochloric acid, chlorine dioxide is evolved according to the reaction of Na 2 CO 2 + 4 HCl -> 4 CIO 2 + 5 NaCl + 2 H 2 O and converts NO 2 to nitrogen dioxide NO 2. At the same time, a second bubbler, separate 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 washer downstream of the first bubbler described above, Which collects oxides of nitrogen NOx as well as the leakage of active oxidizing agent originating from the first bubbler: the second bubbler thus simulates the scrubber 101 of the installation of FIG. 1 or Fig. 2. The concentration of the compounds in the effluent gas exiting the second bubbler is determined by analyzers and / or by chemical assays in order to evaluate both the denitrification yield and the leakage of active oxidizing agent. In a first case, hereafter referred to as test A, the second bubbler contains 30 g / l sodium sulphite and enough oxidation inhibitor to maintain a low redox potential. In a second case, hereinafter referred to as Test B, sodium thiosulfate is added in insufficient quantity, in this case divided by two compared to Test A.

Thus, as can be seen in the table above, the redox potential of the second bubbler is higher for test B, while for test A the redox potential is lower. Moreover, while in the two tests the denitrification yield is similar, the leakage of CI02 active oxidizing agent 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 incapable of effectively collecting the strong oxidizing compound 01 (¾ and the concentration of the latter is too high for the exiting gaseous effluent to be released to the atmosphere. , The redox potential values mentioned herein are given by reference to a saturated calomel electrode.

1. A process for the capture, in a gas stream, of strongly oxidizing pollutants of which at least one belongs to the group consisting of chlorine, chlorine dioxide, ozone and bromine, in which: (1) and a first stream containing a reducing reagent (3), and a second stream which is distinct from the first stream and which contains an oxidation inhibitor (4), the amount of oxidation inhibitor (4) is controlled from at least the redox potential measurement of a purge (5) leaving the wet scrubber for collecting highly oxidizing pollutants (101) Introduced by the second stream into the wet scrubber for capturing highly oxidizing pollutants so as to maintain the redox potential of this purge below a given level,And - from at least the direct or indirect measurement of the reducing reagent concentration (3) in the purge (5) leaving the wet scrubber for collecting the highly oxidizing pollutants (101), the quantity of reagent Reducing agent introduced by the first stream into the wet scrubber for capturing highly oxidizing pollutants.
2. The process as claimed in claim 1, wherein the quantity of oxidation inhibitor (4) introduced by the second stream is controlled in the wet scrubber for collecting highly oxidizing pollutants (101) exclusively from the measurement of the redox potential Of the purge (5) issuing from the wet scrubber for collecting strongly oxidizing pollutants, and in which the quantity of reducing reagent (3) introduced by the first stream is controlled in the wet scrubber for collecting highly polluting pollutants From the measurement of the reducing reagent concentration in the purge of the wet scrubber of capture of the highly oxidizing pollutants.
3. The process as claimed in claim 1, wherein the respective amounts of oxidation inhibitor (4) and reducing reagent (3) introduced respectively by the second stream and the first stream are controlled in the wet scrubber of (101) from the respective measurements of the redox potential of the purge (5) leaving the wet scrubber for collecting the highly oxidizing pollutants and the concentration of reducing reagent in this purge, and optionally the pH of This purge.
4. The method according to claim 1, wherein the redox potential of the purge leaving the wet scrubber for collecting highly oxidizing pollutants is maintained at a value of between -100 mV and + 150 mV And the pH of the wet scrubber for collecting highly oxidizing pollutants is maintained between 6 and 10.
5. The process according to claim 1, wherein the oxidation inhibitor (4) is sodium thiosulfate.
6. The process according to claim 1, wherein the reducing reagent (3) is sodium sulfite.
7. The process as claimed in claim 1, wherein, in addition to the highly oxidizing pollutant (s), the gas stream (1) contains nitrogen oxides and oxygen.
8. A method according to claim 1, in which the gas stream (1), which is introduced into the wet scrubber for collecting highly oxidizing pollutants (101), originates from an upstream wet scrubber which is other than The wet scrubber for capturing strongly oxidizing pollutants.
9. An installation for collecting strongly oxidizing pollutants in a gaseous stream, at least one of which consists of chlorine, chlorine dioxide, ozone and bromine, this installation comprising: a wet scrubber (101) (1), and 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 quantity of d (4) introduced by the second stream into the scrubber (101) from at least the redox potential measurement of a purge (5) issuing from the wet scrubber so as to maintain the redox potential of this scrubber Purges below a given level, and second means (202, 202,301) for controlling the quantity of reducing reagent (3) introduced by the first stream into the scrubber (101) from at least the direct or indirect measurement of the reducing reagent concentration in the purge (5) washer.
10. An installation according to claim 9, wherein said first and second means include a joint multivariable controller (301).