CA1289336C

CA1289336C - Process for removing gaseous sulfur compounds and sulfur dioxide fromthe flue gases of a furnace

Info

Publication number: CA1289336C
Application number: CA000494920A
Authority: CA
Inventors: Sirpa Hamala
Original assignee: Tampella Oy AB
Current assignee: Ivo Power Engineering Oy
Priority date: 1985-04-24
Filing date: 1985-11-08
Publication date: 1991-09-24
Anticipated expiration: 2008-09-24
Also published as: IT8567943A0; SE8505270L; DE3539348A1; FR2580950B1; FI851623L; AU4907785A; CN1005312B; PL148176B1; SE8505270D0; HU202422B; GB8527455D0; SE461958B; FI851623A0; GB2174082A; DD240839A5; ES8700307A1; CN85108066A; FI78846B; BG60231B1; NL8503081A

Abstract

Abstract The invention relates to a process for removing sulfur dioxide from the flue gases of a furnace. In a manner deviating from prior known methods, in addition to a sulfur-containing material (4) to be burned and an oxygen-bearing gas (5), a pulverous calcium or magnesium hydroxide (6) is fed, in excess proportion to the sulfur dioxide gas produced in the combustion chamber, into the combus-tion chamber of the furnace (1), and water (9) or steam is sprayed separately into the thus obtained calcium oxide bearing or magne-sium oxide bearing flue gases (8) in a stage (2) separate from the combustion chamber. Alternatively and preferably the pulverous hydroxide (6) can be fed directly into the flue gases which leave the furnace (1), either in the flue gas conduit (7) or in a reactor (2) subsequent to it, where the hydroxide is activated by means of water (9) or steam.

Description

8~.~36 A process for removing gaseous sulfur compounds and sulfur dioxide from the flue gases of a furnace The present invention relates to a process for removing gaseous sulfur compounds, and especially sulfur dioxide, from the flue gases of a furnace which burns sulfur-containing fuels, such as coal or oil.

It is previously known to decrease the sulfur dioxide content of the flue gases of a furnace by feeding calcium oxide, calcium carbonate or some other alkaline compound into the combustion chamber of the furnace. In a fluidized-bed furnace with a circul-ating bed it is possible by means of a calcium addition to de-crease the sulfur dioxide content of the flue gases by as much as 90 ~ when the furnace is operating within the temperature range which is optimal for the chemical reactions, i.e. 800-1000 C.
The sulfur dioxide thus absorbed leaves the furnace in the form of gypsum, along with the fly ash.

In other furnaces, in which it is necessary to use temperatures higher than those mentioned above and in which the retention of the additive is short due to the nature of the combustion, it is to be expected that the decrease in the sulfur dioxide content of the flue gases remain substantially lower, about 50 ~ or less, and therefore this process has not been applied on an industrial scale to such furnaces.

On the other hand, it is known that the sulfur dioxide content of flue gases can be decreased by various absorption processes outside the furnace. One such process, known _er se, is the so-called semidry process, in which the flue gas emerging from the furnace is led into a separate reactor, into which an aqueous a~

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slurry of calcium hydroxide is sprayed in the form of small droplets through specific nozzles. The reactor is typically a rather large vessel, in which the velocity of the flue gases is allowed to decrease and the aqueous slurry is sprayed downwards from above, from the upper part of the vessel. The temperature of the reactor is at this time about 50-80C, and the control of the spraying of the aqueous slurry of calcium hydroxide is very important, since drops which are too large will remain as liquid on the bottom of the reactor. The thickness of the aqueous slurry of calcium hydroxide should be maintained thick enough that the heat content in the flue gases would evaporate the water entering the reactor, so that the adsorption product can be recovered in the form of dry powder. By this process it is possible to remove as much as 90~ of the sulfur dioxide. The disadvantages of the process include the tendency of the nozzle to become clogged, an extra preparing and batching apparatus for the aqueous slurry of calcium hydroxide which raises the investment costs, and problems of controlling the drop size during the spraying.
The goal of the present invention is to provide a process for removing gaseous sulfur compounds, such as sulfur dioxide, from the flue gases of a furnace, a process by which the gaseous sulfur compounds can be converted to solid sulfur compounds which can easily be separated from the gases and thereby effectively removed from the flue gases of the furnace in a simple and economical manner.
According to the present invention there is provided a process for removing a gaseous sulfur compound from a flue gas of a furnace comprising:
a) feeding a pulverous alkali metal hydroxide or alkaline earth metal hydroxide, in addition to the flue gas to be burned and an oxygen-bearing :
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gas, into a furnace into the sulfur dioxide containing flue gases which leave the furnace, b) separately spraying water or steam into the furnace or into the flue gas, and finally c) separating the solicl obtained as a reaction product, which contains alkali metal or alkaline earth metal sulfate, from the gases.
The gaseous sulfur compound to be removed is preferably sulfur dioxide. The quantity of pulverous hydroxide used is preferably an excess in proportion to the amount of sulfur present in the flue gas. Preferably the water or steam is sprayed while the temperature of the flue gas is 50 to 800C, more preferably 90 to 200C, and the quantity of water or steam preferably does not exceed the amount which can be evaporated by the thermal energy of the flue gas and by the reactions in the furnace.
Preferably the hydroxide is calcium hydroxide or a calcium/magnesium hydroxide mixture.
in the process according to the present invention, a material which reacts with gaseous sulfur compounds, and particularly with sulfur dioxide, and water are fed into the process separately, whereby the problems of preparing, handling and feeding in a slurry are avoided, in the following manner:
a) a pulverous alkali metal hydroxide and/or alkaline earth metal hydroxide is fed, in addition to the sulfur-containing material to be burned and the oxygen-containing gas, into the furnace or into the sulfur dioxide containing flue gases emerging from the furnace, ~, , ' - ' ., ~I ~d~3~j b) water and/or steam is sprayed separately into the furnace and/or into the flue gases, and finally c) a solid which contains alkali metal and/or alkaline earth metal sulfate, and possibly sulfite, is separated from the gases.
The basic idea of the invention is thus that the hydroxide is fed into the flue gases in the form of powder and is activated only ~n situ in the flue gases by means oE water and/or steam, whereupon it reacts with sulfur dioxide and forms a sulfate/sulfite mixture which can thereafter be removed effectively from the flue gases by conventional physical ash separation methods.
A pulverous hydroxide is fed into the combustion chamber of the furnace, or into the flue gases emerging from the Eurnace, in accordance with the sulfur content of the fuel in such a way that the amount of alkali and/or alkaline earth metal in the molar ratio according to the reaction formula, is at least the amount equivalent to the sulfur, preferably, however, greater than the amount necessary for the reaction.
By feeding hydroxide in the form of powder separately into the combustion chamber, or into the flue gas conduit, it is possible to use simple feeding devices, e.g. pneumatic ones, whereby the clogging of the nozzles and the use of extra preparing and batching devices for an aqueous slurry are avoided. Respectively, the feeding of water and steam through nozzles is uncomplicated and easy.

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The feeding of water or steam into the flue gases is in practice carried out at a temperature range 50-800 C, preferably within the temperature range 90-200 C. If it is desired to recover the product of absorption substantially in the form of dry powder, water is used in the spraying only in such an amount that the thermal energy of the flue gases suffices to evaporate it.

The invention is described below in greater detail with reference to the accompanying drawing, which depicts diagrammatically an apparatus suitable for carrying out the process according to the invention.

In the drawing the furnace in general is indicated by reference numeral 1. The sulfur-containing material 4 to be burned, an oxygen-bearing gas 5, and a pulverous calcium and/or magnesium hydroxide 6, are fed into the combustion chamber of the furnace 1, preferably in excess in proportion to the sulfur dioxide gas pro-duced in the combustion chamber. By the expression "in excess" is meant in this context that the amount of the calcium, magnesium, or calcium and magnesium compound is greater than would in theory be needed, according to the reaction formula, to react with all of the sulfur dioxide fed into the combustion chamber.

The hydroxide fed into the furnace is first dehydrated in the furnace to oxide. The oxide, for its part, can react with the sulfur dioxide, first forming sulfite and, when oxidizing there-after, sulfate. Owing to the short retention time in the furnace only part of the oxide has time to react with the sulfur dioxide at a temperature sufficiently high in terms of the reaction, and for this reason, calcium oxide and/or magnesium oxide bearing flue gases 8 which contain combustion residue and steam, and also unabsorbed sulfur dioxide leave, the combustion chamber of the furnace through the flue gas conduit 7 In practice the temperature of the flue gases 8 is so low that the reaction between -the calcium and/or magnesium oxide and sulfur oxide is relatively weak, and under such conditions the oxides can be regarded as inactive in terms of sulfur removal. When the temperature of the flue gases decreases, the oxides may react with the steam present in the flue gases and reconver to hydroxi-de. Thus it is preferable to feed the pulverous hydroxide directly into the flue gas conduit 7 or into the reactor 2 subsequent to it. In addition, the flue gases 8 can be used in the heat ex-changer 12 to heat the air 5 to be fed into the furnace 1.

The sulfur dioxide containing flue gases emerging from the com-bustion chamber of the furnace 1, which contain calcium and/or magnesium oxide and possibly calcium and/or magnesium hydroxide, are thereafter directed into a reactor, which is generally indi-cated by reference numeral 2. In order to activate the oxide and/
or hydroxide, water or more steam is sprayed into the flue gases of the reactor 2, and this water or steam reacts with the calcium and/or magnesium oxide and forms the respective hydroxide, acti-vating it and also activating the hydroxide possibly already pre-sent in the flue gases. The hydroxide for its part reacts with the sulfur dioxide still present in the flue gases 8, thereby forming the respective sulfite, which, in the presence of oxygen, at least in part further oxidiæes to the respective sulfate. The amount of water 9 fed into the reactor 2 is adjusted to so low a level that the heat of the flue gases 8 will suffice to evaporate the water fed into the reactor 2. Thereupon the substantially dry, fly-ashlike reaction product can be removed, in the same manner as other dust, in a conventional dust separator 3, from which the flue gases 11 are further directed into the flue 13 and the sepa-rated dust 12 is directed to a possible further treatment.

The order in which the water and the hydroxide are added is in no way critical. Thus it is possible, for example, to feed the water or steam into the furnace and the pulverous hydroxide only at a point subsequent to the furnace, either into the flue gas conduit or into -the reactor subsequent to it.

One of the further advantages of the present invention is that the process can be applied to a furnace provided with any type of burner. The size of the furnace is not a restricting factor, and it is not necessary to circulate the calcium and/or magnesium hydroxide in the combustion chamber, whereby the expensive circu-lating-bed alternative with its complicated circulation devices, and at the same time the excessive dust due to its operating principle and also dust separation are avoided. Compared with the prior known spray method, the spraying of water or steam into the reactor 2 is, furthermore, considerably less complicated and easier to implement than when using a slurry which clogs the nozzles and is difficult to mix.

The invention is described below in greater detail with the aid of examples.

Example 1 Coal having a sulfur content of 1.4 % is fed at a rate of 70 tn/h into a 600 MW pulverized-coal furnace, while operating at full capacity. An excess of combustion air is fed in , so that the oxygen content in the flue gases is 4 %.

Calcium hydroxide having a calcium hydroxide content of 90 % is fed into the furnace at a certain varying proportion to the sulfur amount entering the furnace in the fuel. The theoretical equivalent amount is about 2.5 tn/h of the said calcium hydroxide.

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3 Ei Calcium hydroxide and water and/or steam are sprayed into the flue gases either in the flue gas duct or in a separate reactor located at a point subsequent to the flue gas duct.

In terms of energy economy it is most advantageous to increase the moisture content of the flue gases by spraying water into them in a separate reactor located at a point subsequent to all heat recovery surfaces.

The increased moisture content of the flue gases makes the calcium hydroxide highly reactive, whereupon it rapidly reacts with the oxides present in the flue gases. The higher the moisture content of the flue gases upon leaving, the more effectively the the sulfur dioxide becomes removed from the flue gases. In terms of energy economy it is, however, advantageous to operate in such a way that the heat released in the chemical reactions will suffice to evaporate the water amount added. If it is desired to raise the final temperature of the flue gases, this is done either by using external heat or by means of a warm flue gas flow.

The results are shown in the table below, which shows in percent how much sulfur dioxide was removed from the flue gases when varying amounts of calcium hydroxide were fed into the furnace in accordance with the present invention; the amounts of the calcium hydroxide are indicated as molar proportions of the calcium con-tent of the pulverous calcium hydroxide to the sulfur content of the fuel fed into the furnace. The temperatures of the flue gases were measured immediately prior to the feeding point of the water or steam, except at 800 C, at which the water or steam was fed directly into the furnace.

333~ j Table 1 B) Ca/S Flue gas Flue gas SO
temperature ternperature reduction o A) o 0.48 800 C 108 C 42 %
o o 0.52 500c 650c 56 %
1.52 202 C 74 C 77 %
o o 1.56 90 C 68 C 82 %
o ' o 2.20 200 C 72 C 87 %
o o 2.22 120 C 62 C 96 %
o o 2.3 llQ C 68 C 93 %
o o 2.5 90 C 66 C 97 %
o o 4.1 800 C 110 C 72 %
4.0 120 C 68C 98 %

A) water or steam fed into the furnace B) Immediately prior to the feeding point of water Example 2 A calcium-magnesium hydroxide which contains calcium hydroxide 45 ~, magnesium hydroxide 45 % and impurities 10 % is fed into the furnace according to Example 1, under corresponding conditions.
Calcium-magnesium hydroxide and water and/or steam are fed into the flue gases either in the furnace or in a separate reactor located at a point subsequent to the furnace.

The increase in the moisture content makes particularly calcium hydroxide highly reactive, whereupon it rapidly reacts with the oxides of sulfur present in the flue gases. If the molar ratio of the calcium contained in the calcium hydroxide to the sulfur is at least 1, the reaction occurs primarily between the calcium hydroxide and the oxides of sulfur, and the magnesium hydroxide, being slower, passes through the reaction zone substantially unchanged.

3;336 If the calcium-magnesium hydroxide is fed into hot flue gases, the consequence may be either that the magnesium hydroxide breaks down into magnesium hydroxide and water or that the entire calcium-magnesium hydroxide breaks down into calcium oxide and magnesium oxide and water. In this case each oxide can as such react witll the oxides of sulfur and, when the flue gases cool and the moisture content increases, re-form hydroxide, which further reacts with the oxides of sulfur. If the molar proportion of the calcium to the sulfur is at least 1, the reaction results and conditions are substantially in accordance with the corresponding values in Table 1, owing to the ability of calcium compound to react more rapidly.

Claims

1. A process for removing a gaseous sulfur compound from a flue gas of a furnace comprising:
a) feeding a pulverous alkali metal hydroxide or alkaline earth metal hydroxide, in addition to the flue gas to be burned and an oxygen-bearing gas, into a furnace into the sulfur dioxide containing flue gases which leave the furnace, b) separately spraying water or steam into the furnace or into the flue gas, and finally c) separating the solid obtained as a reaction product, which contains alkali metal or alkaline earth metal sulfate, from the gases.

2. a process according to claim 1 wherein the gaseous sulfur compound is sulfur dioxide.

3. A process according to Claim 1, wherein an excess of pulverous hydroxide is fed in proportion to the amount of sulfur present in the flue gas.

4. A process according to Claim 1, 2 or 3, wherein the water or steam is sprayed while the temperature of the flue gas is 50-800°C.

5. A process according to Claim 1, 2 or 3, wherein the water or steam is sprayed while the temperature of the flue gas is 90-200°C.

6. A process according to Claim 1, 2 or 3, wherein water is sprayed into the flue gas in a quantity which does not exceed the amount which can be evaporated by the thermal energy produced by the flue gas and by the reaction in the furnace.

7. A process according to Claim 1, 2 or 3 wherein the hydroxide is calcium hydroxide or a calcium-magnesium hydroxide mixture.