WO1988004196A1

WO1988004196A1 - Flue gas purifying procedure

Info

Publication number: WO1988004196A1
Application number: PCT/FI1987/000168
Authority: WO
Inventors: Ari Asikainen; Pekka Riala; Eija Moilanen
Original assignee: Imatran Voima Oy
Priority date: 1986-12-12
Filing date: 1987-12-10
Publication date: 1988-06-16
Also published as: AU1049488A; FI865084A; FI865084A0; FI76931B

Abstract

A procedure for purifying flue gases, wherein into the firebox of a boiler (4) is conveyed an absorbent substance and said substance is allowed to react with the sulphur in the flue gases, the flue gases are further conveyed from the firebox of the boiler by an exhaust passage (a) to a pre-separator (8), which separates from the flue gas the absorbent that has not reacted with sulphur and conveys said non-reacted product to a separate hydrating unit. The solid matter separated from the pre-separator (8) is hydrated and converted into a form which is reactive with sulphur and transported concurrently with the flow to a reactor (16) which is located after the flue gas pre-separator (8) as referred to the direction of travel of the flue gases.

Description

Flue gas purifying procedure

The present invention concerns a procedure for purifying flue gases, wherein into the firebox of the boiler is conveyed an absorbent substance and said agent is allowed to react with the sulphur in the flue gases, the flue gases are further conveyed from the firebox of the boiler by an exhaust passage to a pre- separator, which separates from the flue gas that absorbent which has not reacted with sulphur and conveys said non-reacted product to a separate hydration unit.

Several methods are known in the art which serve the separation from flue gases of sulphur compounds produced in connection with combustion, especially of sulphur dioxide. Such procedures are known in which the sulphur is bound to a reagent containing calci¬ um. Also known are for instance the so-called wet limestone method and a semi-dry purifying procedure based on use of lime. So-called dry desulphurizing methods are also known, which are based on applying firebox injection. In some of the procedures the desulph¬ urizing taking place in the firebox has been supplemented with a reactor or with a hose filter. In these, attempts have been made to elevate the degree of absorbent utilization by means of re- circulation.

The object of the invention is specifically a flue gas purifying procedure wherein it is possible in connection with combustion to purify the flue gases from sulphur compounds and, specifically, the sulphur dioxide can be removed from the flue gas.

The invention has also for its object a procedure in which one is enabled to utilize the heat liberated in the firebox of the boiler in decomposing carbonate stone to more readily reactive oxide.

further object of the invention is a procedure which can be applied in conjunction with combustion installations intended for solid, liquid as well as gaseous fuels.

The procedure of the invention is mainly characterized in that the solid matter separated and drawn from a pre-separator is hydrated and converted into a form which is reactive with sulphur, and is then conveyed concurrent with the flow into a reactor located after the flue gas pre-separator as seen in the direction in which the flue gases travel.

The concurrent flow conduction principle of the invention affords numerous advantages. The solid matter quantities that have to be handled in the hydration step will be small, whereby the apparatus is simpler and therefore less expensive than any apparatus of prior art. The dust separating apparatus proper is not subjected to load because no recirculation is applied. Therefore the proce¬ dure of the invention can easily be applied also in existing in¬ stallations, without making any changes of the filters. With the concurrent flow conduction of the invention, the dust content in the gas going to the reactor is markedly reduced, owing to pre- separation, whereby the mechanical wear of those parts of the reactor which the gas touches will be minimal. Moreover, with pre- separation the soiling of the reactor is less than in the case in which no pre-separation is practiced.

In the procedure of the invention, binding of sulphur is accom¬ plished in two steps.

As taught by the invention, an absorbent substance is conveyed into the firebox of the boiler construction, into contiguity with the flue gases; this absorbent may in the general embodiment of the invention consist of powdery carbonate stone containing alkali and/or earth alkali metal. The carbonate stone will under influ¬ ence of the heat in the firebox decompose to become the corres¬ ponding oxide. In the most advantageous embodiment of the inven- tion calcium carbonate is used, for which the equations following below are also presented. In the first step, finely ground limestone is blown into the fire¬ box of the boiler, into 900-1200°C, whereby the calcium carbonate decomposes according to the formula (1) :

CaC0₃ > CaO + C0₂ (1)

Part of the calcium oxide thus formed reacts further with sulphur dioxide, (2):

CaO + S0 + 1/20₂ > CaS0₄ (2)

Calcium sulphate is then produced. The contribution of sulphur reacting in the firebox may amount to 10 to 70% of the total sulphur quantity. When a quantity of reagent is used which is economically sensible in view of the sulphur contained in coal, the sulphur separation taking place in the firebox is about 30 to 50% of the total SO2 quantity. In order to be able to keep within the emission standards that have been set up, it is in most instances necessary to supplement the sulphur binding at treatment after the boiler.

In the second step of the procedure the CaO which has not reacted and the reaction products are carried out from the boiler together with the flue gas and end up in a pre-separator. Any type of dry dust separating means can be used for pre-separator. Selection and dimensioning of the pre-separator depend essentially on the ash content of the fuel that is used, and on the mode of combustion. If the fuel contains little or no ash, no special requirements are imposed on the pre-separator. The degree of separation must be adequate so that a large enough quantity of lime can be separated from the flue gas for further treatment.

If the fuel contains ash in abundance and the mode of combustion is such that the greater part thereof goes along with the flue gases (dust burning), it is advantageous to make the pre-separator selective. Advantageously a cyclone separator is then dimensioned so that only those particles are separated which are over a suit- able particle size. It is hereby possible to achieve substantial reduction of the mass flow going to further treatment.

When the particle size of the limestone that is blown into the firebox has been suitablye chosen in relation to the quick ash, merely the fraction containing mainly CaO will go to further treatment. The ash and CaS0 particles, which are unnecessary in view of the further reactions, will instead go along with the flue gas.

The fraction separated in the further treatment is hydrated, in dry or wet condition, according to the following equation:

CaO + H₂0 > Ca(0H)₂ (3)

Hydration is indispensable in order that the S0₂ still present in the flue gases can be made to react and can be eliminated from the gas.

The fraction separated in wet hydration is conducted into a tank and mixed in water, which is kept in continuous circulation with the aid of a stirrer. The reaction of equation (3) takes place in this tank. Part of the calcium is solved in water, while yet the greater part remains in suspension.

From the tank, the suspension is pumped into a reactor in which reacting of S0₂ and drying of the solid matter take place. Spray¬ ing of the suspension into the reactor may be accomplished by using rotation or pressure-atomizing nozzles known in the art. The liquid quantity entering the reactor is regulated so that the temperature after the reactor is higher than the water dewpoint temperature but at the same time close enough to it to ensure efficient S0₂ absorption. The S0₂ separation proceeds mainly according to equation (4) :

Ca(0H)₂ + S0₂ > CaS0₃ + H₂0 (4) Into the fraction separated in the dry hydration alternative steam is conducted, which causes the reaction (3) . The dust has to be intensely agitated during this steam blowing, e.g. with a screw mixer of the kind shown in the figure. Grinding before the screw improves the hydration properties of the dust. It is also possible to combine the hydration and grinding steps by applying advantage¬ ously a jet grinding process operating with steam. The steam that is set free is conducted directly to the reactor from the hydra¬ tion apparatus.

The hydrated dust is blown into the reactor with the aid of carrier air. It is advisable to cool the flue gas by water spraying before dust admixture, in order to create favourable conditions of reac¬ tion.

After the reactor, the quick ash and the reaction products are separated from the flue gases. The separator may for instance be an electrical or hose filter. When a hose filter is used, sulphur precipitation reaction also occurs in the dust layer which forms on the surface of the hose.

An advantage of the procedure of the invention e.g. over the wet method is its simple, and therefore advantageously priced, equip¬ ment, absence of waste water, and a dry end product. Compared with the semi-dry method, the decisive advantage is the less expensive absorbent as well as equipment which is structurally simpler and more favourable in price. One advantageous embodiment of the pro¬ cedure of the invention is expressively based on using limestone for absorbent, by reason of which the costs will be considerably less.

Compared with those desulphurizing methods which are based on use of firebox injection, and with those methods in which desulphuriz¬ ing in the firebox is supplemented with a reactor or with a hose filter, the advantages and essential differences marking the pro¬ cedure of the present invention include controlled activation of the injection product in a separate process, implying that with the procedure of the invention good sulphur separating power is ensured, and concurrent flow of solid matter in the process.

The invention is described in the following, referring to certain advantageous embodiments of the invention, presented in the fig¬ ures of the attached drawing, yet to which the invention is not meant to be exclusively confined.

The procedure has been schematically depicted in Figs 1 and 2.

In the procedure of the invention, in an advantageous embodiment thereof, limestone powder is fed so that It Is calcinated to calcium oxide (CaO) . Part of the sulphur will then immediately react with CaO. Calcium sulphate CaS04 is then formed. As taught by the in¬ vention, the reaction products and the quick ash have been conveyed in the procedure of the invention along with the flue gases to a desulphurizing unit following after the boiler, where at first selective separation of non-reacted CaO takes place. The pre-separ- ated calcium oxide (CaO) is hydrated as taught by the invention applying the concurrent flow principle and effecting hydration in a separate process, so that reactive calcium hydroxide (Ca(0H)₂) is produced, which is conveyed in concurrent flow into the reactor. Only the particles containing most calcium oxide are conducted to the hydration process of the invention, while the quick ask and the particles containing CaSU4 that have been produced in the firebox go along with the flue gas by the passage a₂ directly to the reactor 16.

The Ca(0H) produced in the concurrent flow process of the Inven¬ tion is blown advantageously by water spraying into the cooled flue gas, whereby the residual S0 reacts, becoming mainly calcium sulphite (CaSO ) . Finally, the solids are removed from the flue gas, advantageously in an electric and/or hose filter.

In Fig. 1, the silo for absorbent, preferably limestone, has been indicated with reference numeral 1. An absorbent dispensing means 2 is located in the lower part of the silo. A carrier air blower 3 blows the carrier air, with admixed absorbent, into, the firebox of the boiler 4. Combustion air and coal are introduced by the pas- sages 5. In the most advantageous embodiment, the boiler is a dust-burning boiler. But such an embodiment is equally feasible in which the boiler is a so-called grate boiler, or a combined grate/dust-burning boiler, in which case the grate has been de¬ noted with reference numeral 6, as in the figure. The slag is taken out from the boiler construction by the passage 7. The flue gases are conducted from the boiler construction by the flue gas passage a to the flue gas pre-separator 8. In the most advan-. tageous embodiment the pre-separator 8 consists of a cyclone. The sorted particles are separated into an intermediate silo 9. The isolating means 10, advantageously a so-called rotary compartment seal, distributes the fraction that has arrived there from the pre-separator 8, into a mixing tank 11. Water supply to the pro¬ cess is by the line 15, and the water is supplied by this line into the mixing tank 11. From the mixing tank 11, the hydrated substance Ca(0H)₂ is conveyed, transported by a pump means 12, through a control valve 14 into the reactor 16. In the reactor 16 may advantageously be introduced any auxiliary substance that may be required in the spraying operation, for instance air or steam, by the passage 17. From the reactor 16, the flue gas is trans- ported by the passage 21 to a dust separating means 18. This dust separating means may comprise an electric or hose separator means for separating the dust. A flue gas blower 19 draws the flue gas from the dust separating means 18 and blows it into the smokestack 20, and thence further into ambient air. The flue gas impurities which have been separated are removed from the units 16 and 18 by the dumping line 28.

In Fig. 2 is presented another advantageous embodiment of the invention. In this embodiment a so-called hydration screw 22 is employed. The fraction separated in the separator means is carried from the intermediate silo 9 through the rotary seal 10 to the hydration screw 22, also steam being supplied by the passage 23 to this same screw. The hydrated matter moves, conveyed by the screw, to the feed passage E, and said hydrated matter is fed together with carrier air, with the aid of a carrier air blower 24, into the reactor 16. The material is fed into said reactor 16 through a distribution pipe.system 25. The flue gas wetting water is sup¬ plied by the passage 26, int® the reactor 16. As taught by the invention, the hydration design has been implemented in accordance with the concurrent flow principle. At the pre-separator point in the flue gas passage system a given fraction is recovered, and this fraction is treated so that it becomes reactive with the sulphur in the flue gas, and it is thereafter conveyed into the reactor 16, specifically in concurrent flow as referred to the direction in which the flue gas is conveyed.

Claims

1. A procedure for purifying flue gases, wherein into the firebox of a boiler (4) is conveyed an absorbent substance and said sub- stance is allowed to react with the sulphur in the flue gases, the flue gases are further conveyed from the firebox of the boiler by an exhaust passage (a) to a pre-separator (8) , which separates from the flue gas the absorbent that has not reacted with sulphur and conveys said non-reacted product to a separate hydrating unit, characterized in that the solid matter separated from the pre- separator (8) is hydrated and converted to a form which is reac¬ tive with sulphur and transported concurrently with the flow to a reactor (16) which is located after the flue gas pre-separator (8) as referred to the direction of travel of the flue gases.

2. Procedure according to claim 1, characterized in that from the pre-separator (8) a separated fraction is conducted to an inter¬ mediate silo (9) and thence further to a mixing tank (11) .

3. Procedure according to claim 1 or 2, characterized in that in the hydration process a mixing tank (11) is used into which the fraction separated in the pre-separator is conducted and into which along a line (15) is supplied water, and that with the line between the main flue gas passage and the mixing tank (11) con- nects a feedback branch (13) going to the mixing tank (11) .

4. Procedure according to claim 1, characterized in that in the hydration process the fraction conducted from the pre-separator (8) is conveyed to a hydration screw (22) , to said screw means advantageously also being brought steam by a line (23) and from said screw means the separated fraction being transported advan¬ tageously along with carrier air blown by a carrier air blower (24) to the reactor (16).

5. Procedure according to any one of the preceding claims, charac¬ terized in that such a reactor (16) following after the pre- ' separator is used into which the hydrated calcium oxide is con¬ ducted according to the concurrent flow principle and into which is further conducted a spraying auxiliary substance by a line (17) , advantageously air or steam.

6. Procedure according to any one of the preceding claims, characterized in that in the procedure the flue gas is conducted after the pre-separator (8) to a separate dust separating means (18) , which comprises advantageously an electric and/or hose separator.

7. Procedure according to any one of the preceding claims, characterized in that for absorbent substance is used carbonate stone, which advantageously contains alkali and/or earth alkali metals, said carbonate stone being in the procedure of the inven¬ tion under effect of the heat in the firebox decomposed to become the corresponding oxide.

8. Procedure according to claim 7, characterized In that calcium 0 carbonate is used for absorbent substance.

9. Procedure according to the preceding claim, characterized in that in the pre-separator are separated calcium oxide particles of a certain size that have not reacted with the flue gas, and that 5 said particles are carried to a hydration process, the calcium oxide being allowed to react with water, whereby calcium hydroxide is formed, which is further conducted in concurrent flow into contiguity with the flue gas and to react with the sulphur in the flue gas, the reaction products of calcium hydroxide and sulphur 0 dioxide being removed from the flue gas.

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