WO1996004202A1

WO1996004202A1 - Method and equipment for recovery and treatment of sulphur dioxide contained in exhaust gases

Info

Publication number: WO1996004202A1
Application number: PCT/FI1995/000410
Authority: WO
Inventors: Viljo JÄRVENPÄÄ
Original assignee: Wiser Oy
Current assignee: Wiser Oy
Priority date: 1994-08-02
Filing date: 1995-08-01
Publication date: 1996-02-15
Anticipated expiration: 1997-02-02
Also published as: EP0796221A1; AU3079395A; CA2196531A1; FI943589L; FI943589A0; FI943589A7

Abstract

The invention concerns a method and an equipment (10) for recovery and treatment of sulphur dioxide (SO2) contained in exhaust gases. In the method, the flue gases are passed as a flow (13) into the washer part (11), and the sulphur dioxide (SO2) contained in the flue gases is dissolved into the washing liquid passed into the washer part (11). The mixture that contains sulphur dioxide (SO2) and that has been dissolved into the washing liquid is passed along a flow line (17) into a liquid container (12). The mixture that is placed in the liquid container (12) and that contains sulphur dioxide (SO2) is oxidized at a pressure of free choice, by means of a gas, preferably air, that is placed in the liquid container (12) and that contains oxygen (O2), into sulphur trioxide (SO3), which forms sulphuric acid (H2SO4) together with the water present in the liquid container (12).

Description

Method and equipment for recovery and treatment of sulphur dioxide contained in exhaust gases

The invention concerns a method for recovery and treatment of sulphur dioxide contained in exhaust gases.

Further, the invention concerns an equipment for recovery and treatment of sulphur dioxide contained in exhaust gases.

Cleaning of exhaust gases, such as, for example, flue gases, has been considered to be highly important for a long time. Initially, attempts were made to recover the dust contained in flue gases only, but at present, it is stipulated by law that especially large quantities of exhaust gases must also be cleaned in respect of their gas content. In particular the sulphur dioxide contained in flue gases has been restricted to certain emission limits. The emission limits are staggered so that little amounts of exhaust gases are allowed to contain somewhat more sulphur than, for example, large power- plant gas emissions, in which hundreds of thousands of cubic metres of gas per hour can be concerned. In the case of smaller amounts of burning, i.e. lower emissions, the emissions of sulphur dioxide have been limited by means of fuels of lower contents of sulphur, in which case separate removal of sulphur is, as a rule, not at all necessary. The costs of investment of the devices used for cleaning of flue gases are high, and their operation also produces expenses.

The commonest device for cleaning of flue gases is an electric filter, which usually removes the dust from flue gases to a sufficient extent, but which allows free passage for the gaseous components of the flue gases. Thus, if the limits of sulphur emissions stipulated by law are to be reached by means of an electric filter, it is necessary to use a fuel substantially free from sulphur. In the removal of sulphur dioxide, primarily three different processes are employed, which are the dry, semi-dry and wet separation processes.

The neutralizing agent needed in the dry-separation process, as a rule caustic- lime/limestone powder, is fed either directly into the combustion or to among the hot flue gases. By adding a suitable proportion of moisture to the exhaust gases that are departing, the temperature of the flue gases is lowered, whereby the sulphur dioxide (SO₂) and the moisture and the caustic lime react with each other and form calcium sulphite powder (CaSO₃), which is then separated by means of dry filtering. In this way, besides the dust in the flue gases, also the sulphur dioxide can be eliminated. It is a drawback of this prior-art process that the particle size of the neutralizing agent used must be fine, and the material must also be sufficiently pure in order that the neutralization should be sufficiently efficient. It is a further drawback of the process that the consumption of the neutralizing agent is by orders higher than what would be required by the stoichiometric need (stoichiometry means the molar ratios of calcium to sulphur). This is why, in the future, the dry-separation process will not be a viable solution from the point of view of nature.

In the semi-dry separation process, most commonly, milk of lime is atomized into the flue gases as the neutralizing agent. The supply of milk of lime must be large enough so that the sulphur dioxide can be neutralized reliably and, at the same time, the temperature of the flue gases be lowered sufficiently for dry filtering. The result of this process is a dry powder which contains, besides the solids contained in the flue gases, also calcium sulphite that has been formed. True enough, part of the calcium sulphite may be oxidized to calcium sulphate, i.e. gypsum, but the main part of the product of neutralization consists of calcium sulphite powder.

The drawbacks of the semi-dry separation process are, in principle, the same as those of the dry separation process, i.e. disproportionately high consumption of pure neutralizing agent, but additionally also the fact that the calcium sulphite (CaSO₃) formed as a powder is a material that has no compression strength. This results from the "hairiness" of calcium sulphite, i.e. calcium sulphite consists of pin-covered granules which cannot be compressed into a compact mix. Thus, the use of the calcium sulphite formed, for example, as filling soil for foundations of buildings or roads is out of the question. In principle, the calcium sulphite formed is a material suitable for skiing pistes only. Nor do experiments carried out by means of stabiliz- ation with cement provide an adequate, chemically stable solution, because, for example, the oxygen content in ground water oxidizes the calcium sulphite to calcium sulphate. This is why the calcium sulphite ought to be oxidized by heating at about 1000°C to gypsum, i.e. calcium sulphate. This is, however, so expensive that the solution is not economically reasonable. On the basis of the above, the semi- dry process of separation is also a solution unacceptable from the point of view of nature for removal of sulphur.

From the point of view of nature, the best process known so far for removal of sulphur is the so-called wet- ashing process, in which the flue/combustion gases are washed with a neutralizing washing liquid. In such a case, the sulphur dioxide dissolves from the flue gases into the washing liquid and makes the washing liquid acid. The acid washing liquid is then neutralized by means of a suitable chemical, most commonly with limestone (CaCO₃) or milk of lime, i.e. calcium hydroxide (Ca(OH)₂). The details of carrying out the wet-washing process, however, involve a few difficulties. First, the calcium bisulphite (CaHSO₃) formed in the neutralizing stage must be oxidized to calcium sulphate. This reaction must take place within a relatively precise pH-range, which imposes high requirements, besides on the pH- meter, also on the metering of the neutralizing agent. These difficulties together make the wet-separation process rather difficult to carry out and to monitor. If these circumstances have been taken into account sufficiently well, the final product that is obtained is even good gypsum, for which there is use, for example, as raw- material for the gypsum industry and also as filling soil, because gypsum is slaky and is compacted well with a good compression strength. At the same time, the slaky gypsum is chemically stable. A drawback of the wet-separation process is the high investment cost, and the operation of the process also causes expenses. This is why the wet-separation process has been used primarily for large flue-gas quantities and at large plants, in which the washing of the flue gases must be sufficiently efficient, among other things, in respect of sulphur.

The object of the present invention is to provide a method and an equipment in which the numerous drawbacks present in the prior-art solutions are avoided.

The method in accordance with the invention is characterized in that

a) the flue gases are passed as a flow into the washer part, and the sulphur dioxide (SO₂) contained in the flue gases is dissolved into the washing liquid passed into the washer part,

b) the mixture that contains sulphur dioxide (SO₂) and that has been dissolved into the washing liquid is passed along a flow line into a liquid container, and

c) the mixture that is placed in the liquid container and that contains sulphur dioxide (SO₂) is oxidized at a pressure of free choice, by means of a gas, preferably air, that is placed in the liquid container and that contains oxygen (O₂), into sulphur trioxide (SO₃), which forms sulphuric acid (H₂SO ) together with the water present in the liquid container.

The equipment in accordance with the invention is characterized in that the equip¬ ment includes

a) a washer part, into which the flue gases are passed along a first flow line and the washing liquid along a second flow line and/or along a third flow line, in which connection the sulphur dioxide contained in the flue gases is dissolved into the washing liquid that has been passed into the washer part,

b) a fourth flow line for passing the mixture that has been dissolved into the washing liquid and that contains sulphur dioxide (SO₂) from the washer part into the liquid container, and c) a liquid container, in which the mixture that contains sulphur dioxide is oxidized at a pressure of free choice, by means of a gas, preferably air, that is placed in the liquid container and that contains oxygen (O₂), into sulphur trioxide (SO₃), which forms sulphuric acid (H₂SO₄) together with the water present in the liquid container.

In the present invention, it has been realized to oxidize the sulphur dioxide in connection with the washing directly into sulphuric acid. In the method and equip¬ ment in accordance with the present invention, the sulphur dioxide contained in the flue gases is dissolved into the washing liquid, which is oxygenated/aerated inten¬ sively, whereby the sulphur dioxide dissolved in the washing liquid is oxidized into sulphur trioxide (SO₂ + V2O1 - > SO₃), whi . . together with the water present in the v hing liquid, forms sulphuric acid (H ^), which again dissolves sulphur diox, (SO₂) into itself, for example 35 g ms of SO₂ per kilogram of H SO₄ 50 ^CA it a temperature of 50°C, i.e. an almost equal amount as compared with the amount dissolved into pure water: 40 grams of SO per kilogram of H₂O at 50°C, and, dissolved in oxidizing conditions, the sulphur dioxide is again oxidized into sulphur trioxide and from it further into sulphuric acid.

The invention is in no way critical in respect of the washing-liquid system that is used. Washing liquid can be passed into the washer part either from the liquid container in which sulphuric acid is formed or from a separate washing-liquid duct or from both.

By means of the method and equipment in accordance with the present invention, a number of significant advantages are obtained compared with the processes of separation known from the prior art. In the method and equipment of the present invention, it is unnecessary to use a number of control units, a measuring detector or containers of neutralizing agents by whose means the dissolved sulphur dioxide can be neutralized, a deposit, removal of deposit, filtering, conveyors, or various treatments. According to a preferred embodiment of the invention, it is sufficient to regulate a suitable supply of additional liquid so that the order of magnitude of the amount of the washing liquid that flows out, i.e. the sulphuric acid, corresponds to the desired concentration of sulphuric acid. The pumps and pipe systems with their washing-liquid containers remain clean and free from any blocking in the event of pH variations, which blocking tends to occur frequently in neutralization with lime if the pH is not within the desired limits out of some reason.

In the process of the present invention, the treatment of the acid mixture that flows out may be either concentration or neutralization with a suitable lime, which lime is now, however, of secondary importance and can be even impure lime. Thus, the result is a chemical product of reasonably low cost, concentrated sulphuric acid, or gypsum that is suitable for filling soil and raw-material. In any case, the neutraliz¬ ation taking place in the process of the present invention is directly stoichiometric without any extra consumption of neutralizing agent. These advantages of the invention correspond to neutralization and sulphur-removal acceptable to nature. At the same time, the invention proves that sulphuric acid can also be produced without lead chambers or V₂O₅ catalysts.

The invention will be described in detail with reference to a solution of principle of the invention illustrated in the figure in the accompanying drawing, the invention being, however, not supposed to be confined to said embodiment alone.

In the figure in the drawing, the equipment in accordance with the invention is denoted generally with the reference numeral 10. The flue gases that contain sulphur dioxide are passed into the washer part 11 as a flow 13. At the same time, into the washer part 11 , washing liquid is passed, for example, from a circulation water pump 15, which sucks washing liquid out of the washing-liquid container 12. If necessary, into the washer part 11 , additional washing liquid, such as additional water, can be passed along the flow line 16 as a washing liquid of its own, which replaces any amount of washing liquid that may evaporate and the acid solution that flows out along the discharge line 22. In the washing-liquid container 12, there is a stirrer shaft 18, at whose end there is a stirrer-disperser 19, in which case the air/oxygen mixture which is fed out of the feed line 20, preferably, for example, through the hollow stirrer shaft 18 is passed and dispersed by means of the stirrer/disperser 19 into the washing liquid present in the washing-liquid container 12, whereby the sulphur dioxide dissolved in the washing liquid is oxidized into sulphur trioxide. A motor 21 is fitted to rotate the stirrer shaft 18. From the washer. part 11, the washing liquid into which the sulphur dioxide dissolved from the flue gases has been dissolved flows along the flow line 17 into the washing-liquid container 12. The circulation water pump 15 is preferably such that it sucks the air that flows along the flow line 23. Owing to this, additional air is supplied into the oxygenous liquid in the washing-liquid/water container, which air is dissolved into the washing liquid in the flow duct 14 after the circulation water pump 15 under pressure, preferably of an order of about 1...10 bar, before the liquid enters into the washer part 11. Owing to this, the liquid that flows into the washer part 11 is highly oxygenous and, at the same time, takes care of the circumstance that the sulphur dioxide that has been dissolved into the washing liquid, the liquid that contains sulphuric acid and water, is oxidized under pressure ever more completely into sulphur trioxide.

Thus, in the preferred embodiment of the process and equipment of the present invention, oxidization of sulphur dioxide takes place both in the flow duct/pipe 14 placed after the circulation water pump 15 and passing into the washer part 11, in the washer part 11, and in the washing-liquid container 12. It should be noted that, in the washing-liquid container 12, oxidization takes place with the aid of the addition of air/oxygen supplied along the feed line 20 and with the aid of the stirrer 18 and the stirrer/disperser 19 just at a very low positive pressure. The bottom part of the washing-liquid container 12 is provided with a removal of deposit 24 for the removal of the deposit that arises as a result of solid matter that may be contained in the flue gases and as a result of precipitation resulting from probable oxidization.

The process of oxidization in accordance with the present invention can also be carried out in a number of different ways and by means of different solutions of equipment, but an essential feature is oxidization of the sulphur dioxide in the liquid/water, at a pressure which can be chosen freely in principle, in the washing- liquid container 12, and the ultimate oxidization under pressure in the pipe/duct 14 after the circulation water pump 15. The addition of air/ oxygen can also be carried out in a way differing from the embodiment shown in the figure in the drawing, but even then the final result is the desired one. The supply of air/oxygen into the circulation water pump 15 or into the pressurized flow duct or pipe 14 after the pump can be carried out in a number of different ways, but the major part or even all of the sulphur dioxide is oxidized in the pressurized state 14. The washing-liquid container 12 is kept rich in oxygen just otherwise. Thus, the process of the present invention is elastic and simple.

Regarding its construction and principle of operation, the washer part 11 may be a washer part known from the prior art. It is the essential idea that the sulphur dioxide is dissolved into the washing liquid as completely as possible, which liquid contains a substantial amount of sulphuric acid, as a result of concentration, according to the basic realization of the invention. A suitable concentration of acid is, for example, a 50 % sulphuric acid, which is taken out as the flow 22, this concentration being the capacity of dissolution of sulphur dioxide by sulphuric acid and being almost equal to the corresponding capacity of dissolution of sulphur dioxide by pure water.

As the oxidizing gas passed along the line 20, it is possible to use air or air enriched with oxygen, but also pure oxygen gas can be used. The latter mode is just an expensive mode of oxidizing.

Above, a solution of principle of the invention is described, and it is obvious to a person skilled in the art that numerous modifications can be made to said solution within the scope of the inventive idea stated in the accompanying patent claims.

Claims

1. A method for recovery and treatment of sulphur dioxide (SO₂) contained in exhaust gases, characterized in that

a) the flue gases are passed as a flow (13) into the washer part (11), and the sulphur dioxide (SO₂) contained in the flue gases is dissolved into the washing liquid passed into the washer part (11),

b) the mixture that contains sulphur dioxide (SO₂) and that has been dissolved into the washing liquid is passed along a flow line (17) into a liquid container (12), and

c) the mixture that is placed in the liquid container (12) and that contains sulphur dioxide (SO₂) is oxidized at a pressure of free choice, by means of a gas, preferably air, that is placed in the liquid container (12) and that contains oxygen (O₂), into sulphur trioxide (SO₃), which forms sulphuric acid (H₂SO ) together with the water present in the liquid container (12).

2. A method as claimed in claim 1, characterized in that the sulphuric acid (H₂SO ) is allowed to be concentrated in the liquid container (12) to the desired H₂SO₄-concentration, preferably to a concentration of 50 %, before the solution of sulphuric acid is taken out.

3. A method as claimed in claim 1 or 2, characterized in that additional washing liquid / additional water is fed along the line (16) into the washer part (11) to replace any amount of washing liquid that may evaporate and to replace the H₂SO₄- containing solution that is removed.

4. A method as claimed in any of the claims 1 to 3, characterized in that additional oxidization is carried out under pressure in the flow duct / flow line (14) passing into the washer part (11).

5. A method as claimed in claim 4, characterized in that the circulation water pump (15) is fitted to suck air / oxygen-rich gas into the flow duct / flow line (14) that passes into the washer part (11).

6. A method as claimed in any of the claims 1 to 5, characterized in that sulphuric acid is removed from the liquid container (12) along a discharge line (22).

7. A method as claimed in any of the claims 1 to 5, characterized in that sulphuric acid is removed from the flow duct / flow line (14) that passes into the washer part (11).

8. A method as claimed in any of the claims 1 to 7, characterized in that oxygen- containing gas is passed into the liquid container (12).

9. An equipment (10) for recovery and treatment of sulphur dioxide (SO₂) contained in exhaust gases, characterized in that the equipment (10) includes

a) a washer part (11), into which the flue gases are passed along a first flow line (13) and the washing liquid along a second flow line (14) and/or along a third flow line (16), in which connection the sulphur dioxide (SO₂) con¬ tained in the flue gases is dissolved into the washing liquid that has been passed into the washer part (11),

b) a fourth flow line (17) for passing the mixture that has been dissolved into the washing liquid and that contains sulphur dioxide (SO₂) from the washer part (11) into the liquid container (12), and

c) a liquid container (12), in which the mixture that contains sulphur dioxide (SO₂) is oxidized at a pressure of free choice, by means of a gas, preferably air, that is placed in the liquid container (12) and that contains oxygen (O₂), into sulphur trioxide (SO₃), which forms sulphuric acid (H₂SO₄) together with the water present in the liquid container (12). 11

10. An equipment as claimed in claim 9, characterized in that the liquid container (12) is provided with a duct (20) for the feed of gas that contains oxygen.

11. An equipment as claimed in claim 9 or 10, characterized in that the equipment (10) includes a circulation water pump (15), which is fitted to suck washing liquid out of the liquid container (12) and to pass the washing liquid along the second flow line (14) under pressure into the washer part (11).

12. An equipment as claimed in claim 11, characterized in that the circulation water pump (15) communicates with a flow line (23) that contains air / oxygen-rich gas, the circulation water pump (15) being fitted to suck the air / the oxygen-rich gas into the second flow line (14) passing into the washer part (11).

13. An equipment as claimed in any of the claims 9 to 12, characterized in that the liquid container (12) is provided with a duct (22) for the discharge of the sulphuric acid.

14. An equipment as claimed in any of the claims 9 to 13, characterized in that the liquid container (12) is provided with a stirrer shaft (18), at whose end there is a stirrer-disperser (19).

15. An equipment as claimed in claim 14, characterized in that the stirrer shaft (18) is hollow, the oxygen-containing gas being fitted to flow from a feed duct (20) through the hollow stirrer shaft (18) to the stirrer-disperser (19).

16. An equipment as claimed in any of the claims 9 to 15, characterized in that in the lower part of the liquid container (12), there is a duct (24) for the removal of deposit.