EP3700659A1

EP3700659A1 - Method for removal of mercury from gaseous effluents

Info

Publication number: EP3700659A1
Application number: EP18792938.5A
Authority: EP
Inventors: Bernard Siret; Frank Tabaries
Original assignee: LAB SAS
Current assignee: LAB SAS
Priority date: 2017-10-26
Filing date: 2018-10-25
Publication date: 2020-09-02
Also published as: WO2019081634A1; FR3072888A1; FR3072888B1

Abstract

In order to effectively, inexpensively and easily remove mercury from gaseous effluents, the invention proposes a mercury-removal method in which a reagent for capturing mercury (3), which consists of a sulfur-containing compound and an alkali metal compound, is injected in liquid form into a gaseous stream (2) having a temperature higher than 140°C, by being atomised therein in the form of droplets inside a cloud (5) of solid particles (4) having a concentration between 1 g/Nm3 and 10 g/Nm3, so that, by evaporation of the water contained in the droplets placed in contact with the gaseous stream, the droplets produce microparticles in which, at the same time, the sulfur-containing compound of the solution forms sulfur-containing nuclei, capable of fixing the mercury present in the gaseous stream, and the alkali metal compound of the solution forms alkali metal nuclei which are adhesive and/or which have a strong affinity for the sulfur-containing nuclei and the solid particles of the cloud.

Description

Process for the demercurization of gaseous effluents

The present invention relates to a process for the demercurization of gaseous effluents.

Many industries generate gaseous effluents containing mercury, which is a highly toxic compound and must be removed from these gaseous effluents. In particular, mercury is found in the gaseous effluents of power plants that burn coal, coal naturally containing a little mercury which, during combustion, will end up in combustion fumes in the form of mercury metal or oxidized mercury. Mercury is also found in flue gases from waste-to-energy plants and waste incineration plants because the waste contains some mercury.

Several processes are used to demercurise the gaseous effluents, especially the aforementioned fumes, that is to say to purify these gaseous effluents by retaining mercury.

The most common way to proceed is to bring the gaseous effluents into contact with powdery or granular adsorbents. The most used of these adsorbents is activated carbon because it is inexpensive and effective for other pollutants, such as volatile organic compounds, dioxins and furans. This activated carbon can be doped with compounds such as halogens, such as chlorine, bromine or iodine, sulfur or selenium. However, at high temperature and particularly when the temperature exceeds 200 ° C, the adsorption isotherms become very unfavorable: it is then necessary to use significant amounts of coal, which adversely affects the operating cost, as well as the quality of the products. solid residues generated. In addition, an increase in activated charcoal dosages may not be sufficient and peaks, beyond allowable limits, occur. Compared with the use of simple activated charcoal, the use of doped coals improves this situation a little, but the cost of these products is high and secondary problems, for example due to the corrosivity of the halogenated coals, occur. Likewise, the use of brominated coals can also, by association with ammonium chloride present in the gaseous effluents, generate corrosive mixtures for exchangers operating at low relative temperatures, such as economizers.

For example, US 2016/279568 discloses a gaseous effluent demineralization process, in which carbon particles, activated by hydrobromic acid, are injected into a hot gas stream to oxidize and adsorb mercury present therein. gas flow. D1 plans to inject into the hot gas stream, in addition to the above-mentioned carbon particles, a solution of sodium hydroxide and sodium carbonate. calcium, which may also contain sodium sulphide: this solution does not capture mercury present in the gas stream, but reacts with carbon dioxide and sulfur oxides, present in this gas stream. In particular, by reaction with carbon dioxide, sodium bicarbonate is produced.

For its part, US 2014/0050640 discloses a gaseous effluent demercurization process, wherein a solution containing an active mercury capture compound is injected into a gas stream to be treated. This active compound is based on silica and may in particular result from the reaction between a precursor containing silica and a sulphide, it being noted that this sulphide is no longer available as such in the injected solution since it has been consumed by reaction with the precursor containing silica. This solution can be mixed with an alkaline reagent that does not capture mercury, but that captures sulfur oxides, this alkaline reagent may be trona.

As for WO 2014/062438, it also discloses a process for the demercurisation of gaseous effluents, in which a solution containing alkali metal polysulfides is injected into a hot gaseous stream to be treated: the mercury is captured by being complexed by the metal of the polysulfides of alkali metals.

In addition, it has been proposed to use clays, or mixtures of clays and lime. But, as a rule, at equivalent dosage and at equivalent temperature, these clay products are less capacitive and less effective than activated carbons.

It has also been proposed to percolate the gaseous effluents in towers filled with granular activated carbon. However, this technique is unattractive in terms of bulk volume and presents potential safety risks in view of the large static tonnages of coal used, the fumes to be demercurized being generally hot. The phenomena of coal self-combustion at operating temperatures, that is to say between 140 and 200 ° C, are common and their consequences are important for the loss of availability of the installation.

Wet processes are also used, in which the solubility of the mercury salts is used, or in which an oxidation of the mercury metal to ionic mercury is carried out before or during its transfer in the liquid phase. These wet processes are effective, but not always usable: this is for example the case when the gaseous effluents to be treated are located in an area with no water or at which a significant wet discharge is problematic.

The object of the present invention is to propose a new method of demercurization, which is effective, economical and simple to implement.

For this purpose, the subject of the invention is a process for the demercurisation of gaseous effluents, as defined in claim 1. The idea underlying the invention is to form in situ, that is to say in a sheath which conveys a hot gas stream to demercurize, solid grains having a large active surface to capture and retain the mercury present in the gas stream. These grains result from the deposition, on solid particles of a sufficiently dense cloud which is present in the gas flow, of microparticles which come from droplets produced by atomization of a solution of a sulfur compound and a compound of a alkali metal, introduced directly into the gas stream, these microparticles being generated by evaporation of the water contained in the droplets, if appropriate after explosion of the latter: the sulfur compound of the solution forms, in the microparticles, nuclei even to fix the mercury; at the same time, the alkali metal compound forms, in the microparticles, nuclei which are tacky and / or which have a high affinity for both the nuclei resulting from the sulfur compound and the solid particles of the cloud present in the gas flow. Therefore, it is understood that by atomizing the aforementioned solution in the cloud of solid particles, the microparticles bind to the solid particles, lining the surface of the latter, while maintaining their microstructure and therefore their ability to capture and retain mercury . The active surface of the grains obtained by binding of the microparticles to the solid particles is thus reduced, in the sense that the ratio between the active surface and the volume of these grains is greatly increased, and this by using the solid particles as a support for these microparticles , being noticed that the solid particles without the microparticles would be inactive with respect to the mercury whereas, thanks to the microparticles which are fixed there, the solid particles become traps with mercury. The demercurization of the gas stream is thus very efficient, while being simple and economical to implement, as detailed below.

Additional advantageous features of the method according to the invention are specified in the dependent claims.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the drawings in which:

- Figure 1 is a diagram of an installation implementing a first embodiment of the method according to the invention; and

- Figure 2 is a view similar to Figure 1, illustrating a second embodiment of the method according to the invention.

In Figures 1 and 2 is shown a sheath 1 in which a gas stream 2 flows, and from left to right in the figures. The embodiment of the sheath 1 is not limiting.

The gas stream 2 circulates in the sheath 1 at a temperature above 140 ° C, typically between 140 and 350 ° C, preferably between 170 and 220 ° C. The gas stream 2 contains mercury. In order to demercurise the gas stream 2, a mercury capture reagent 3 is injected into the gas stream, at one or more injection points in the sheath 1, which, where appropriate, are distributed along the the latter.

This mercury capture reagent 3 is injected into the gas stream 2 in liquid form, consisting, before injection, of a solution of a sulfur compound and an alkali metal compound. The reagent 3 thus consists essentially of two compounds, namely the sulfur compound and the alkali metal compound, which will be detailed just below.

The sulfur compound is, for example and without limitation, a sulfide or a polysulfide. The sulfur compound preferably contains an alkali metal sulfide and / or alkali polysulfide. In all cases, the sulfur compound is intended, when the reagent solution 3 is injected into the gas stream 2, to form solid nuclei able to fix the mercury present in the gas stream 2.

The alkali metal compound is, for example and without limitation, sodium salts and / or potassium salts and / or sodium hydroxide. Thus, the alkali metal compound contains, for example, sodium carbonate and / or sodium bicarbonate and / or sodium hydroxide and / or potassium carbonate and / or potassium bicarbonate and / or sodium borate and / or borax. In all cases, the alkali metal compound is intended, when the solution of the reagent 3 is injected into the gas stream 2, to form nuclei having a high affinity with other solids or even solid nuclei.

Of course, the reagent 3 solution, which essentially comprises the sulfur compound and the alkali metal compound, may also include additives.

As clearly visible in FIGS. 1 and 2, the solution of the reagent 3 is injected into the gas stream 2 while being atomized therein in a cloud of solid particles: in FIG. 1, the solid particles are referenced 4 and form a referenced cloud 5; in FIG. 2, the solid particles are referenced 6 and form a cloud referenced 7.

In the preferred embodiment of FIG. 1, the solid particles 4 result from the injection of a dedicated stream 4 'into the gas stream 2 upstream of the injection of the reagent 3, the injection of this stream 4' being operated at one or more injection points in the sheath 1, which, where appropriate, are distributed along the latter and which are all located before the injection point (s) of the reagent 3. In practice, the flow 4 'is introduced inside the sheath 1 by any suitable material, for example rods, nozzles, straight or bevelled pipes. Once the stream 4 'is injected into the gas stream 2 inside the sheath 1, the solid particles 4 from the stream 4' are dispersed and gradually form the cloud 5 which, along the sheath 1 from of the points 4 'flow injection, extends downstream, to cover the entire section of the sheath 1, as shown schematically in Figure 1.

According to a particularly advantageous optional arrangement, the solid particles 4 are intended to capture acidic pollutants present in the gas stream 2, such as hydrogen chloride (HCl) and sulfur dioxide (S0 ₂ ): for this purpose, the 4 'stream then contains sodium bicarbonate or sodium sesquicarbonate as the trona, which is a natural mineral. Bicarbonate or sodium sesquicarbonate are injected into the gaseous stream 2 in pulverulent form, typically obtained by grinding, and, in contact with the gas stream 2, produce the solid particles 4 by partial decomposition. Of course, the stream 4 'may contain, in addition to or as an alternative to sodium bicarbonate and sesquicarbonate, other reactive products which, in particular after decomposition in contact with the gas stream 2, form in the latter solid particles 4 able to capture the acid pollutants present in the gas stream 2.

In the embodiment of FIG. 2, the solid particles 6 are not added to the gas stream 2 by being injected into the sheath 1 by a separate dedicated stream such as the stream 4 'of FIG. 1, but pre-exist in the stream gaseous 2 flowing in the sheath 1. These solid particles 6 include in particular fly ash which is carried in combustion fumes constituting the gas stream 2, these fly ash being present from the production of such combustion fumes.

Of course, in a variant, the embodiments of FIG. 1 and FIG. 2 can be combined, so that the reagent 3 is atomized within a cloud which contains, at the same time, solid particles resulting from the injection of a dedicated stream which is added to the gas stream 2, such as the solid particles 4 resulting from the injection of the stream 4 'in FIG. 1, and pre-existing solid particles in the gas stream 2, such as the solid particles 6 in Figure 2.

In all cases, the solid particles 4 and 6 have a characteristic size, especially an equivalent diameter, which is much greater than one micrometer.

In addition, also in all cases, the atomization of the reagent solution 3 in the cloud 5 or 7 is carried out in the form of droplets of small diameter, typically less than 200 microns, preferably less than 100 microns. In practice, the injection, in atomized form, of the reagent solution 3 is carried out by means known per se, for example bi-fluid air nozzles or by any suitable equipment.

Once the solution of the reagent 3 is atomized in the form of the aforementioned droplets in the hot gas stream 2, several physicochemical processes follow each other. At first, the contacting of the droplets of the reagent 3 with the hot gas stream 2 leads, if appropriate after the explosion of the droplets, to the evaporation of water from the latter. As the water evaporates, the reagent 3 of each of the droplets begins to oversaturate with sulfur compound and / or alkali metal compound. The return to the thermodynamic saturation equilibrium then produces microparticles of very small size, which, in particular, is clearly smaller than the size characteristic of the solid particles 4 or 6.

Specifically, on the surface of the evaporating droplets, alkali metal solid nuclei develop, for example, formed by sodium carbonate when the alkali metal compound of the reagent 3 contains carbonate or bicarbonate of the alkali metal. sodium or when the alkali metal compound of the reagent 3 contains soda which, by carbonation in the gas stream 2, has the same effect. These alkali metal nuclei have high affinity for other solids or are sticky. In other words, the nuclei of the alkali metal act as a glue binder.

At the same time, also on the surface of each of the droplets evaporating, sulfur-containing solid nuclei develop, for example nuclei or even micro-clusters of sodium polysulfide when the sulfur compound of the reagent 3 contains sodium polysulfide. These sulfur nuclei have the property of fixing the mercury, while being bound to the alkali metal nuclei thanks to the fixing capacity that they have.

In a second step, the microstructures described above, containing the alkali metal nuclei and the sulfur nuclei, bind, in particular by physicochemical bonding, to the solid particles 4 or 6 of the cloud 5 or 7, while retaining their microstructure. The microparticles can thus coat the surface of the solid particles 4 or 6. In all cases, chemical bonds between the solid particles 4 or 6 and the alkali metal nuclei of the microstructures are formed and ensure the stability of the grains thus obtained. In particular, both in the case of the embodiment of FIG. 1 and in the case of the embodiment of FIG. 2, the sticky, high affinity, and microstructural character of the alkali metal solid nuclei, which belong to the microparticles. resulting from the atomization and evaporation of the reagent 3 solution, these microparticles are fixed to the surface of the solid particles 4 and 6, adhering with a high affinity. Since these microparticles also contain the sulfur-containing nuclei, which can fix the mercury, the grains obtained by aggregation of the microparticles with each other and on the solid particles 4 or 6 have their active surface for the demercurization which is multiplied, in the sense that the surface ratio active on volume of these grains is greatly increased. Of course, the density of the cloud 5 or 7 is decisive for the processes described above to proceed satisfactorily: according to one characteristic of the invention, the concentration of the solid particles 4 or 6 within the gas stream 2 is included between 1 g / Nm ³ and 10 g / Nm ³ .

With regard to the quantity of reagent 3 that is injected into the gas stream 2, this can be either fixed, the reagent 3 then being injected at a predetermined constant flow rate, or slaved to the flow rate of the gas stream 2, especially for keep constant the concentration, in the gas stream 2, microparticles resulting from the atomization and evaporation of the reagent 3, is still driven by a mercury analyzer indicating the concentration of mercury in the gas stream 2, upstream or downstream downstream of the introduction of reagent 3 into this gas stream. In practice, the concentration, in the gas stream 2, of the microparticles resulting from the atomization and the evaporation of the solution of the reagent 3 is typically between 20 mg / Nm ³ and 500 mg / Nm ³ , this concentration being expressed in dry matter.

The advantages conferred by the invention are numerous:

the combined use of the sulfur compound and the alkali metal compound, one of which serves to capture and retain the mercury, the other serving as a glue binder and helping to ensure the affinity of the abovementioned microparticles with each other and with the solid particles 4 and 6, makes it possible to increase the effective surface / volume ratio by using solid particles 4 and 6 as support; in other words, the solid particles 4 and 6, which would normally be inactive with respect to mercury, become in a way mercury traps;

the installation for implementing the method is simple; and

the reagent for capturing mercury 3 is cheap, stable and easy to prepare on site.

Claims

1 .- A process for the demercurization of gaseous effluents, in which a mercury capture reagent (3), which consists of a solution of a sulfur compound and an alkali metal compound, is injected in the form of liquid in a gaseous stream (2) having a temperature above 140 ° C, being atomized therein as droplets within a cloud (5; 7) of solid particles (4; 6) having a concentration of between 1 g / Nm ³ and 10 g / Nm ³ , so that, by evaporation of the water contained in the droplets brought into contact with the gas stream, the droplets produce microparticles in which, at the same time, the sulfur compound of the solution forms sulfur-containing nuclei capable of binding mercury present in the gas stream, and the alkali metal compound of the solution forms alkali metal nuclei which are tacky and / or which have a high affinity for the sulfur nuclei and the solid particles of the cloud.

The process according to claim 1, wherein the alkali metal compound of the mercury capture reagent (3) contains sodium carbonate and / or sodium bicarbonate and / or sodium hydroxide.

3. - Method according to one of claims 1 or 2, wherein the alkali metal compound of the mercury capture reagent (3) contains potassium carbonate and / or potassium bicarbonate.

4. A process according to any one of the preceding claims, wherein the alkali metal compound of the mercury capture reagent (3) contains sodium borate and / or borax.

5. - Process according to any one of the preceding claims, wherein the sulfur compound of the mercury capture reagent (3) contains a sulfide and / or a polysulfide, preferably alkaline.

6. - Method according to any one of the preceding claims, wherein at least a portion of the solid particles (4) of the cloud (5) results from the injection of a dedicated stream (4 ') into the gas stream (2). upstream of the injection of the mercury capture reagent (3).

7. - Process according to claim 6, wherein said dedicated stream (4 ') contains a reagent for capturing acid pollutants contained in the gas stream (2).

The method of claim 7, wherein said acid pollutant uptake reagent comprises sodium bicarbonate and / or sodium sesquicarbonate.

9. - Process according to any one of the preceding claims, wherein the gaseous flow to demercurize (2) consists of combustion fumes, and wherein at least a portion of the solid particles (6) of the cloud (7) is constituted fly ash present in said combustion fumes.

10. - Process according to any one of the preceding claims, wherein the gas stream (2) wherein the mercury capture reagent (3) is injected has a temperature between 170 and 220 ° C.