BE1021577B1 - GLOBAL METHOD FOR WET CLEANING SMOKE - Google Patents

Abstract

In accordance with the process according to the invention: - fumes to be purified (1) are sent successively: to an acid scrubber (101), the pH of which is kept below 3 and in which the nitrogen oxides contained in the fumes are oxidized , by an oxidizing agent (4) introduced into the acid scrubber, in N02 and / or nitrogen compounds having a higher degree of oxidation, then in an alkaline scrubber (102), the pH of which is maintained above 5 , 5 and in which the oxidized nitrogen compounds contained in the fumes (2) from the acid scrubber, as well as said sulfur oxides contained in the fumes to be purified (1) are captured in the presence of an oxidation inhibitor (5) introduced into the alkaline washer, - a deconcentration purge (7), which is withdrawn from the acid scrubber (101), and a deconcentration purge (8), which is withdrawn from the alkaline washer (102) separately from that of the washer acid, are mixed and sent to a stripping column (201) which also receives a flow of stripping gas (10), and - the flow of gaseous effluents (11) leaving the stripping column (201) is recycled and mixed with the fumes upstream of the alkaline scrubber (102).

Description

GLOBAL METHOD FOR WET CLEANING SMOKE

The present invention relates to a method and a plant for the wet scrubbing of fumes containing nitrogen oxides and sulfur oxides.

In these processes, a washing liquid is brought into contact with fumes to be purified in a scrubber so that the dusts suspended in the flue gases are transferred into the washing liquid together with the gaseous pollutants present in the flue gases. especially hydrochloric acid HCl and sulfur dioxide S02. The flow rate of liquid necessary to ensure good purification is generally high, for example several tens to a few hundred m3 / h, so that the liquid is recycled and only a small fraction is withdrawn for deconcentration. This purge withdrawn is generally treated with a water treatment before being discharged to the surrounding environment.

Furthermore, in addition to the aforementioned acidic pollutants such as hydrochloric acid HCl and sulfur dioxide S02, the fumes to be purified generally contain oxides of nitrogen NOx in concentration too high to be rejected so that a treatment complementary is usually necessary. The reduction of these nitrogen oxides NOx can be carried out mainly in three ways. A first possibility is a reduction at the source, by a so-called SNCR process, in which a reducing agent, such as urea or ammonia, is introduced. This solution is inexpensive but suffers from a limitation: beyond a certain reduction rate of nitrogen oxides, side effects, such as the generation of other pollutants, such as nitrous oxide, appear. Also this solution is limited to cases where it is desired to obtain a limited reduction of nitrogen oxides, for example less than 60%. A second solution is to use catalysts in the context of a so-called SCR solution: this option can make it possible to obtain significant reductions of nitrogen oxides, especially more than 80%, but it is expensive in investment and requires a location important soil, in addition to that used for the reduction of the aforementioned acid pollutants. A third way is to use a wet denox treatment, as proposed in FR 2 643 286. In this solution, an oxidation reagent, such as sodium chlorite, is used. However, this solution suffers from limitations. In particular, when a high percentage of reduction of nitrogen oxides NOx is required, it is relatively common to obtain an undesired leakage of by-products, such as chlorine dioxide, or even chlorine. In addition, it is known that the oxygen of the fumes to be treated, in the presence of the nitrogen dioxide generated, will oxidize very rapidly the reducing compounds present as sulfites and hydrogen sulfites, so that the ability of the scrubber to check the NOx nitrogen oxides and the oxidant compound leak.

Finally, all wet solutions are likely to generate soluble nitrogen compounds, such as nitrates, which are undesirable and must therefore be limited.

For its part, US Pat. No. 4,400,362 discloses a process for the wet scrubbing of fumes containing nitrogen oxides and sulfur oxides, in which the fumes are successively sent to an acid scrubber and then to a scrubber. alkaline scrubber. In the acidic scrubber, the NOx of the fumes to be treated react with a reducing solution of H2SO3 introduced into the scrubber, forming compounds N203 and H2SO4. In the alkaline scrubber, the fumes from the acid scrubber are only washed with an alkaline solution, without further input. The liquid purge of the acid scrubber is sent to a stripper from which an outgoing gas stream, containing in particular SO2, is recycled only in the acid scrubber in order to maintain in the latter a high concentration of sulfites or assimilated species. In a completely disjointed manner, the liquid purge of the alkaline scrubber is sent to an oxidation tank by addition of air, before being evacuated without any recycling.

The object of the present invention is to provide a global process for the wet cleaning of fumes, which is capable of capturing, inter alia, the nitrogen oxides and the sulfur dioxide present in the fumes, by limiting or even avoiding the disadvantages listed above. For this purpose, the subject of the invention is a process for the purification of fumes by the wet process, in which: fumes to be purified are sent successively: in an acidic scrubber, whose pH is kept below 3 and in which the The nitrogen oxides contained in the flue gases are oxidized by an oxidizing agent introduced into the acid scrubber, into NO 2 and / or into nitrogenous compounds having a higher oxidation state, then into an alkaline scrubber, the pH of which is maintained greater than 5.5 and in which the oxidized nitrogen compounds contained in the fumes from the acid scrubber, as well as the said sulfur oxides contained in the fumes to be purified, are captured in the presence of an oxidation inhibitor introduced into the alkaline scrubber - a purge of deconcentration, which is withdrawn from the acid scrubber, and a purge of deconcentration, which is withdrawn from the alkaline scrubber separately from that of the acid scrubber, are mixed and sent to a scrubber. a stripping column which also receives a stream of stripping gas, and the flow of gaseous effluents leaving the stripping column is recycled and mixed with the fumes upstream of the alkaline scrubber. The invention also relates to a wet scrubber plant containing nitrogen oxides and sulfur oxides, comprising: - an acid scrubber, which is fed with fumes to be purified and in which the oxides of nitrogen contained in the flue gases are oxidized by an oxidation agent (4) introduced into the acid scrubber, with NO 2 and / or nitrogenous compounds having a higher oxidation state, - an alkaline scrubber, which is fed with the fumes leaving the acid scrubber and in which the oxidized nitrogen compounds contained in the fumes from the acid scrubber, as well as the said sulfur oxides contained in the fumes to be purified, are collected in the presence of an oxidation inhibitor introduced into the alkaline scrubber a stripping column, which is fed, at the same time, by the mixture of a purge of deconcentration, withdrawn from the acid scrubber, and a purge of deconcentration, withdrawn from the alkaline scrubber of mani it is separated from that of the acid scrubber, and by a stream of stripping gas, and - means for recycling the flow of gaseous effluents leaving the stripping column, which are suitable for mixing this flow of gaseous effluents with the fumes at least upstream of the alkaline scrubber.

Thus, the invention makes it possible, at the same time, to carry out a wet reduction of the nitrogen oxides, by using an oxidation reagent in an acidic scrubber, to then wetly capture, on the one hand, the compounds resulting therefrom. that are the oxidized nitrogen compounds and either an excess of the aforementioned oxidation reagent or a by-product formed by the latter and secondly the sulfur dioxide of the fumes, using an oxidation inhibitor in an alkaline scrubber, to submit the purges, which are extracted separately from the acidic scrubber and the alkaline scrubber, to a stripping by a gaseous flow, and reintroducing the gaseous effluent, result of stripping, into the flue gas at least upstream of the alkaline scrubber, the case upstream of the acid scrubber.

According to additional advantageous features of the method and the installation according to the invention, taken individually or in any technically possible combination: the flow of gaseous effluents leaving the stripping column is recycled and mixed with the fumes downstream of the acidic scrubber; the flow of gaseous effluents leaving the stripping column is recycled and mixed with the fumes upstream of the acidic scrubber; from a measurement of the redox potential of the mixture of the two purges, the quantity of oxidation inhibitor introduced in the alkaline scrubber is controlled so as to maintain the redox potential of the alkaline scrubber in a range of values of -80 mV to +300 mV; from a measurement of the redox potential of the purging of the alkaline scrubber, the quantity of oxidation inhibitor introduced into the alkaline scrubber is controlled so as to maintain the redox potential of the alkaline scrubber in a range of values; 150 mV to +200 mV; the oxidizing agent is sodium chlorite or chlorine dioxide; the oxidation inhibitor is sodium thiosulfate; the stripping gas is air; the stripping column is a packed column. 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 FIGS. 2 and 3 are views similar to FIG. 1, respectively illustrating two variants of the installation and of the method according to the invention.

In the installation shown in FIG. 1, the fumes to be purified 1 are first introduced into a first washer 101 called an acid scrubber in the sense that, in this scrubber, the pH is kept at a frankly acidic value, namely lower at least 3, preferably less than 2. Two functions are performed in the washer 101. On the one hand, very soluble pollutants, such as hydrochloric acid HCl, are collected by washing and on the other hand, part of the nitrogen oxides NOx is oxidized so that it can be solubilized and thus captured. In particular, nitric oxide NO, which is very sparingly soluble in water, is oxidized to nitrogen dioxide NO 2 and / or nitrogen compounds at a higher oxidation state.

It should be noted that, since the fumes 1 often contain hydrochloric acid which will solubilize, the pH of the washer 101 is normally low. However, it is possible, optionally, to feed the washer 101 into an acid flow, not shown in Figure 1, for example hydrochloric acid.

To carry out the oxidation of a portion of the NOx nitrogen oxides, an oxidizing agent, which may also be described as an oxidizing compound, is introduced into the washer 101 in the form of the flow referenced 4. Preferably, this oxidizing agent 4 is a solution of sodium chlorite NaClO 2, but other oxidants may be used, for example chlorine dioxide CIO 2.

A purge of deconcentration 7 is withdrawn from the scrubber 101. Insofar as very oxidizing conditions are to be maintained in the scrubber 101, otherwise the essential oxidation function of the NOx nitrogen oxides would not be satisfactorily ensured, it is advisable to limit, as much as possible, the admission into this washer 101 of reducing compounds, and thus to avoid reintroducing into the washer 101 liquid and reducing streams, such as the purges of other washers that the washer 101. In practice, however, it will be noted that it is not possible to completely avoid the arrival of reducing compounds in the acid scrubber 101, if only because sulfur dioxide S02 is present in the fumes 1. Sulfur, even if it is only slightly soluble in acidic medium and will be effectively captured only with the alkaline scrubber which follows the acidic scrubber, nevertheless contributes to bringing a few reducing agents. This undesired effect is limited by the low pH of the acidic scrubber 101.

The partially purified fumes 2 leaving the acid scrubber 101 are introduced into a second scrubber 102 called an alkaline scrubber, whose pH is maintained at a value greater than 5.5, preferably greater than 7.5. According to the invention, the alkaline scrubber 102 is introduced with an oxidation inhibitor 5, which is preferably sodium thiosulfate Na 2 S 2 O 3, but which may also be, for example, native S sulfur. This oxidation inhibitor is essential. Indeed, in order to be able, in the alkaline scrubber 102, to both satisfactorily capture the oxidized nitrogen compounds, including the mixture of nitrogen monoxide and nitrogen dioxide, from the acid scrubber 101, and absorb all excess of the oxidizing agent 4, introduced into the acid scrubber 101, and / or its by-products, it is essential to maintain frankly reducing conditions to the alkaline scrubber 102. In this perspective, naturally, the S02 sulfur present in the fumes 2 is captured in the scrubber 102 in the form of sulfites and / or hydrogen sulfites which are strong reducing agents. However, it is also known, in particular the work of Mrs de Vincentis and Rochelle of the University of Texas, that, in the presence of oxygen fumes and nitrogen dioxide N02, is established a radical chain d It is noted that up to 30 moles of sulfites and hydrogen sulfites can be oxidized by one mole of nitrogen oxide captured. Also, in most cases, this oxidation is enough to deplete the alkaline scrubber 102 too much in reducing compounds. The interest of counteracting this oxidation is thus understood by introducing into the scrubber 102 the oxidation inhibitor 5.

Optionally, an additional amount of sulphites may be added to the scrubber 102 by a stream 6, for example in the form of a solution of an alkaline sulphite or of an alkaline hydrogen sulphite. Other reagents, such as soda, can also be introduced at the level of the scrubber 102, as well as water.

According to the invention, a purge 8 is withdrawn from the alkaline scrubber 102 separately from the purge 7 of the acid scrubber 101. As the conditions of the scrubber 102 are reducing, the separation of the purges 7 and 8 avoids the harmful effect which would occur in introducing the purge 8 in the acid washer 101, as is commonly done in the form of a hydraulic cascade, because this reductive purge would neutralize the effect of the oxidizing agent 4 introduced into the acidic scrubber.

In addition, according to the invention, the purges 7 and 8 are introduced into a Stripping column 201, which is of a known technology, and which may be a packed column, being noted that the purges 7 and 8 are thus introduced by being mixed with each other just at the inlet of the stripping column 201 or, as in FIG. 1, upstream of this stripping column by forming a mixture of the purges, referenced 9. The column stripping 201 also receives a stripping gas stream 10, advantageously constituted by air. The gaseous flow 10 is admitted to the base of the stripping column, from where it goes up in the latter, by crossing countercurrently the mixture of the purges 7 and 8, introduced at the top of the column. This stripping operation has several beneficial effects. First, by mixing the purges 7 and 8, it recreates in this mixture conditions more favorable to oxidation, since the purge 7 is both acidic and oxidizing. In addition, in the stripping column 201, a portion of the sulfites and hydrogen sulfite is converted to sulphates so that the resulting final purge 12 exiting the stripping column 201 is detoxified. On the other hand, in the stripping column 201, the stripping gas stream 10 is charged with sulfur dioxide SO2 so that the off-gas stream 11 leaving the column contains SO 2 sulfur dioxide.

According to the invention, the flow of gaseous effluents 11 is mixed with the fumes supplying the installation: it is sufficient for this flow 11 to be mixed with the fumes 2, that is to say upstream of the alkaline scrubber 102 but downstream. of the acid scrubber 101, in the sense that it is in the alkaline scrubber 102 that the reducing potential of the sulphites resulting from the uptake of the sulfur dioxide S02 by this scrubber 102 is used. In practice, as represented in FIG. the flow of gaseous effluents 11 can be introduced immediately upstream of the scrubber 101, that is to say mixed with the fumes 1, it being understood that the sulfur dioxide S02 contained in the effluent stream 11 then passes through the acid scrubber 101 to reach the alkaline scrubber 102 in which it is used. In any case, it is understood that the recycling of the gaseous effluent stream 11 creates a turn-round of sulfur dioxide S02, which increases the concentration actually seen in sulfur dioxide by the alkaline scrubber 102 and which helps to maintain a high concentration of sulfites in the scrubber wash liquid 102. Preferably, the concentration of sulfites and hydrogen sulfites in the alkaline scrubber 102, and therefore in its bleed 8, is greater than 10 g / l.

Thus, in accordance with the invention, the stripping column 201 has a triple role: it oxidizes part of the sulphites to sulphates, which the alkaline scrubber 102 does only in a very limited way because of the inhibitor 5 which is introduced, thereby providing an outlet, via the purge 12, to the sulfur contained in the fumes 1; it eliminates part of the sulphites and hydrogen sulphites contained in the mixture of purges 7 and 8 by transferring them in the form of sulfur dioxide SO 2 into the flue gas stream 11, the latter being reintroduced into the flue gases in a manner that is profitable for the washer 101 and especially, for the washer 102; and it detoxifies the final purge 12 by oxidation, notably reducing its chemical oxygen demand.

Furthermore, the stripping column 201 has the additional potential effect of reducing the nitrate content, which has been found experimentally. The chemical mechanism causing this decrease in nitrate content is poorly understood. It is probable that sulpho-nitrated species are created and that, in fine, a significant part of the nitrates and nitrites and nitrogen oxides captured is transformed into nitrogen by a global reaction of the type 4 Na2SO3 +2 N02 -> 4 Na2SO4 + N2, although this approach is simplistic and much more complex mechanisms are actually implemented.

As mentioned above, it is preferable to maintain a sufficient reducing potential for the alkaline scrubber 102. According to an advantageous optional feature of the invention, which is implemented in the plant of FIG. 2, the quantity of inhibitor of 5, introduced into the alkaline scrubber 102, is slaved to the redox potential of the mixture 9 of the purges 7 and 8, the value of this redox potential being measured by an ad hoc apparatus 301, which is of a known technology and which controls an information flow 3000 for the dosing means of the oxidation inhibitor 5: the apparatus 301 thus drives the injection of the oxidation inhibitor 5, that is to say controls the quantity this inhibitor introduced into the alkaline scrubber 102, so as to maintain in the range -80 mV to +300 mV the redox potential that this device.

Alternatively, which is illustrated in FIG. 3, the injection of the oxidation inhibitor 5 is controlled by the information flow 3000 provided by the apparatus 301 measuring the redox potential of the purge 8 of the alkaline scrubber 102. , so as to maintain this redox potential measured in the range -150 mV to +200 mV.

It will be noted that, above, the redox potential values are given with respect to the reference potential of a saturated calomel electrode.

Claims (10)

1.- Process for the wet scrubbing of fumes containing nitrogen oxides and sulfur oxides, in which: fumes to be purified (1) are sent successively: in an acidic scrubber (101), the pH of which is kept below 3 and in which the nitrogen oxides contained in the flue gases are oxidized, with an oxidation agent (4) introduced into the acid scrubber, with NO 2 and / or with nitrogen compounds having a degree of oxidation higher, and then in an alkaline scrubber (102), whose pH is maintained greater than 5.5 and wherein the oxidized nitrogen compounds contained in the fumes (2) from the acid scrubber, and said sulfur oxides contained in the fumes to be purified (1) are collected in the presence of an oxidation inhibitor (5) introduced into the alkaline scrubber, - a purge of deconcentration (7), which is withdrawn from the acid scrubber (101), and a purge of deconcentration ( 8), which is withdrawn from the alkaline scrubber (102) separately e of that of the acid scrubber, are mixed and sent to a stripping column (201) which also receives a stream of stripping gas (10), and - the flow of gaseous effluents (11) leaving the stripping column ( 201) is recycled and mixed with the fumes upstream of the alkaline scrubber (102). 2. - Process according to claim 1, wherein the gaseous effluent stream (11) leaving the stripping column (201) is recycled and mixed with the fumes (2) downstream of the acid scrubber (101). 3. The method of claim 1, wherein the gaseous effluent stream (11) leaving the stripping column (201) is recycled and mixed with the fumes (1) upstream of the acid scrubber (101). 4, - Process according to any one of the preceding claims, wherein, from a redox potential measurement of the mixture of the two purges (7, 8), the quantity of oxidation inhibitor (5) introduced is controlled in the alkaline scrubber (102) so as to maintain the redox potential of the alkaline scrubber in a range of values -80 mV to +300 mV. 5, - Process according to any one of claims 1 to 3, wherein, from a measurement of the redox potential of the purge (8) of the alkaline scrubber (102), the quantity of oxidation inhibitor is controlled (5) introduced into the alkaline scrubber so as to maintain the redox potential of the alkaline scrubber in a range of -150 mV to +200 mV. 6. The process of any one of the preceding claims, wherein the oxidizing agent (4) is sodium chlorite or chlorine dioxide. 7. The process of any one of the preceding claims, wherein the oxidation inhibitor (5) is sodium thiosulfate. The method of any one of the preceding claims, wherein the stripping gas (10) is air. 9, - Installation for wet scrubbing of fumes containing nitrogen oxides and sulfur oxides, comprising: - an acid scrubber (101), which is fed with fumes to be purified (1) and in which the oxides nitrogen contained in the flue gases are oxidized by an oxidizing agent (4) introduced into the acid scrubber, with NO 2 and / or nitrogenous compounds having a higher oxidation state, - an alkaline scrubber (102), which is fed by the fumes (2) leaving the acid scrubber (101) and in which the oxidized nitrogen compounds contained in the fumes (2) originating from the acid scrubber, as well as the said sulfur oxides contained in the fumes to be purified (1) are taken in the presence of an oxidation inhibitor (5) introduced into the alkaline scrubber, - a stripping column (201), which is fed, at the same time, by mixing a stripping purge (7), withdrawn from the acid washer (101), and from a purge of deconcentration (8), withdrawn from the lav alkali (102) separately from that of the acid scrubber, and by stripping gas flow (10), and - means for recycling the flue gas stream (11) exiting the stripping column (201) , which are adapted to mix this flow of gaseous effluents with the fumes at least upstream of the alkaline scrubber (102). 10, - Installation according to claim 9, wherein the stripping column (201) is a packed column.