GB2169887A

GB2169887A - A process for removing gaseous sulphur compounds, such as sulphur dioxide, from the flue gases of a furnace

Info

Publication number: GB2169887A
Application number: GB08527454A
Authority: GB
Inventors: Sirpa Haemaelae
Original assignee: Tampella Oy AB
Current assignee: Tampella Oy AB
Priority date: 1984-11-09
Filing date: 1985-11-07
Publication date: 1986-07-23
Also published as: GB2169887B; IT8567942A0; NL8503082A; IT1237362B; NZ213857A; SE461957B; SE8505269D0; FI78845B; AU579901B2; BG60232B1; SE8505269L; BG60232B2; AU4907685A; FI851622L; GB8527454D0; CA1289337C; FI851622A0

Abstract

In addition to a sulphur-containing fuel (4) and an oxygen-containing gas (5), a pulverous alkali metal or alkaline earth metal oxide or carbonate (6) (particularly calcium or magnesium) is fed into the furnace (1) in an amount in excess in proportion to the sulphur dioxide gas formed in the combustion chamber of the furnace, and water and/or steam (9) is sprayed separately into the calcium oxide-containing and/or magnesium oxide-containing flue gases (8) either in the furnace or after leaving the furnace. Alternatively, or in addition, the pulverous oxide (6') can be fed directly into the flue gases (8) leaving the furnace (1). <IMAGE>

Description

SPECIFICATION A process for removing gaseous sulphur compounds such as sulphur dioxide, from the flue gases of a furnace The present invention relates to a process for removing gaseous sulphur compounds, such as sulphur dioxide, from the flue gases of a furnace which burns sulphur-containing fuel, e.g. coal or oil.

It is already known to decrease the sulphur 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 circulating bed, it is possible by means of a calcium addition to decrease the sulphur 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 sulphur 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 time of the additive is short due to the nature of the combustion, it is to be expected that the decrease in the sulphur dioxide content of the flue gases would stay 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 sulphur dioxide content of flue gases can be decreased by various absorption processes outside the furnace. One such process, known per se, is the so-called spray or semi-dry process, in which the flue gas leaving the furnace is led into a separate reactor, into which an aqueous 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 the upper part of the vessel. The temperature of the reactor is at this time about 50-80 C, 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 density of the aqueous slurry of calcium hydroxide should be maintained at such a level 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 sulphur dioxide. The disadvantages of the process include the tendency of the nozzles to become clogged, the need for an additional preparing and batching apparatus for the aqueous slurry of calcium hydroxide which raises the investment costs, and the problem of controlling the drop size during spraying.

It is a object of the present invention to provide a process for removing gaseous sulphur compounds, such as sulphur dioxide, from the flue gases of a furnace by means of which the gaseous sulphur compounds can be converted to solid sulphur 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 invention, there is provided a process for removing gaseous sulphur compounds, particularly sulphur dioxide, from the flue gases of a furnace, comprising the steps of (a) feeding to said furnace, in addition to sulphur-containing material to be burned and an oxygen-containing gas, a pulverous material comprising an alkali metal oxide and/or alkaline earth metal oxide, and/or a compound (other than the hydroxide) which will be converted in the furnace to such compound, and/or feeding such a pulverous oxide to the flue gases after leaving said furnace.

(b) separately spraying liquid water and/or steam into said furnace and/or into the flue gases after leaving said furnace, so as to convert oxide present to a corresponding hydroxide which reacts with sulphur dioxide or other sulphur compound present in the flue gases, and finally (c) separating from said flue gases a reaction product obtained in the form of a solid containing the corresponding alkali metal and/or alkaline earth metal sulphate, and possibly sulphite.

In the process according to the present invention, a material which reacts with gaseous sulphur compounds, particularly with sulphur dioxide, and water are fed separately into the process whereby the problems, of preparing, handling and feeding-in a slurry are avoided.

The basic idea of the invention is thus that the alkali metal or alkaline earth metal oxides, e.g.

calcium and magnesium oxides, which are inactive from the point of view of the removal of sulphur dioxide are activated only in situ in the flue gases by means of water and/or steam, whereby they are converted to the respective hydroxides and react with sulphur dioxide to form a solid sulphate/sulphite mixture which can thereafter be effectively removed from the flue gases by physical separation methods.

A pulverous oxide and/or carbonate is fed into the combustion chamber of the furnace in accordance with the sulphur content of the fuel in such a way that the amount of alkali and/or alkaline earth metals is at least equivalent to the amount of sulphur in the molar proportion according to the reaction formula, but preferably higher than the amount required for the reaction. By feeding the oxide and/or carbonate in the form of powder separately into the combustion chamber, or the oxide directly into the flue gas ducts, it does not have to be fed in the form of a slurry through nozzles, so that nozzle clogging and the need for the use of extra preparation and batching devices for the aqueous slurry are avoided. On the contrary, the feeding of water and steam through nozzles is uncomplicated and easy.

The feeding of water or steam into the flue gases is in practice carried out at a temperature of 50-800 C, preferably within the temperature range 90-200 C. If it is desired to recover the absorption product substantially in the form of dry powder, water is used only in such a amount that the thermal energy and the heat of reaction of the flue gases suffice to vaporize it; a small amount of energy introduced from outside the system may be used as a supplement to the heat of reaction.

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 present invention.

In the drawing, the furnace in general is indicated by reference numeral 1. A sulphur-containing fuel for combustion 4, usually preheated, an oxygen-containing gas 5, and calcium and/or magnesium oxide 6' and/or carbonate 6, preferably in an amount in excess in proportion to the amount of the sulphur dioxide gas forming in the combustion chamber, are fed into the combustion chamber of the furnace 1. By the expression "in excess" is meant in this context that the amount of calcium, magnesium, or calcium and magnesium, present in the calcium and/or magnesium oxide and/or carbonate is greater than would in theory, according to the reaction formula, be required to react with all of the sulphur fed into the combustion chamber.

The carbonate fed into the furnace breaks down in the furnace into the corresponding oxide and carbon dioxide. The oxide, for its part, can react with the sulphur dioxide, forming first sulphite and thereafter, upon oxidation, sulphate. Owing to the short retention time in the furnace, only part of the oxide has time to react with the sulphur dioxide at a temperature sufficiently high for the reaction, and for this reason the flue gases 8 leaving the combustion chamber of the furnace which contain combustion residues and also unabsorbed sulphur dioxide, also carry calcium oxide and/or magnesium oxide. These flue gases leave the combustion chamber of the furnace through a flue gas conduit 7. Additionally or alternatively, pulverous oxide 6' can be fed as shown directly into the flue gas conduit 7 or into a subsequent reactor 2.

In practice, the temperature of the flue gases 8 is so low that the reaction between the calcium and/or magnesium oxide and sulphur dioxide is relatively weak, and the oxides can under these conditions be regarded as relatively inactive in terms of sulphur removal.

However, the flue gases 8 can be used in a heat exchanger 12 to preheat the air 5 fed into the furnace 1.

The flue gases 8 containing calcium and/or magnesium oxide and sulphur dioxide, which emerge from the combustion chamber of the furnace 1 are thereafter directed into a reactor, which is generally indicated at 2. In order to activate the calcium and/or magnesium, water 9 or steam is sprayed into the flue gases in the reactor 2, and this water or steam reacts with the calcium and/or magnesium oxide, thereby forming the respective hydroxide. The hydroxide for its part reacts with the remaining sulphur dioxide in the flue gases 8, thereby forming the respective sulphite, which, in the presence of oxygen, at least in part, further oxidizes to the respective sulphate.

The amount of water 9 fed into the reactor 2 is adjusted to so low a value that the heat of the flue gases 8 suffices to evaporate the water fed into the reactor 2. Thereupon the dry, fly ash-like reaction product can be removed, in the same manner as the other fly ash, in a conventional fly ash separator 3, from which the cleaned flue gases 11 are directed further into a flue 13 and the separated fly ash 12 and the reaction product are possibly directed to a further treatment step.

The order in which the water or steam and the pulverous oxide are added is in no way critical. Thus, for example, the water or steam can be fed into the furnace and the pulverous oxide only at a point subsequent to the furnace, either into the flue gas conduit 7 or into the subsequent reactor 2. The additional advantages of the process according to the present invention include the fact 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 oxide in the combustion chamber, whereby the expensive circulatingbed alternative with its complicated recirculating devices and at the same time the excessive dust which is a disadvantage of the recirculating-bed alternative due to its principle of operation, as well as the separation of the dust, are avoided. Compared with the previously known spray process, the spraying of water or steam into the reactor 2 is, furthermore, considerably less complicated and easier to implement than when using slurry which clogs the nozzles and is difficult to mix. It is an additional advantage that carbonate can be economically burned in the combustion chamber of the furnace itself.

The invention is illustrated by the following examples:~ Example 1 Coal having a sulphur content of 1.4% by weight is fed at a rate of 70 t/h into a pulverizedcoal furnace having a thermal output of 600 MW, the furnace being operated at full capacity. An excess of combustion air is fed in, so that the oxygen content in the flue gases leaving the furnace is 4% by volume Calcium, which may be, for example, in the form of calcium carbonate, dolomite, or calcium oxide, is fed into the furnace. For example, crude calcium carbonate having a calcium carbonate content of 90% by weight is fed into the furnace at a predetermined varying proportion to the amount of sulphur entering the furnace in the fuel. The theoretical equivalent amount is about 3.4 t/h calcium carbonate.

The calcium carbonate decomposes in accordance with the equation:~ (1) CaCO3#CaO + CO2 in the furnace at a high temperature to form calcium oxide and carbon dioxide, which leave the furnace along with the flue gases through a flue gas duct. Part of the calcium oxide in the furnace reacts with the oxides of sulphur present in the flue gases, thereby forming calcium sulphate or calcium sulphite.

(2) CaO+S02+ 1/2 O2#CaSO4 or CaO + SO2 ,CaS03 CaSO2+ 1/2 O2#CaS04 Water and/or steam is sprayed into the flue gases in the furnace and/or in the flue gas conduit, and/or in a separate reactor downstream of the flue gas conduit.

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

The increased moisture content of the flue gases enables a highly reactive calcium hydroxide to be formed in the furnace from the unreacted calcium oxide:~ (3) CaO + H2O~Ca(OH)2 Ca(OH)2+ SO2-CaS0#/H2O, which rapidly reacts with the oxides of sulphur present in the flue gases. The higher the moisture content of the flue gases upon their leaving the furnace, the more effectively the sulphur dioxide is removed from the flue gases. In terms of energy economy it is, however advantageous to proceed in such a way that the heat released in the chemical reactions suffices to evaporate the water amount added. If it is desired to increase the final temperature of the flue gases, this may be done either by using external heat or by using external heat or by means of a warm flue gas flow.

It is essential that the compound arriving in the reaction zone, when derived from calcium carbonate or dolomite, is in the form of oxide.

The results obtained are given in Table 1 below, which shows as a percentage how much sulphur dioxide was removed from the flue gases when varying amounts of calcium carbonate were fed into the furnace in accordance with the present invention, the amounts of the calcium carbonate being reported as molar ratios of the calcium content of the pulverous calcium carbonate to the sulphur content of the fuel fed into the furnace. The temperatures of the flue gases were measured at a point immediately prior to the feeding point of the water or steam, except at 800 C, at which temperature value the water or steam was fed directly into the furnace.

Table I Ca/S Flue gas81 Flue gas SO, T in T out reduction 0.48 8000CA 1080C 42% 0.52 500C 650C 56% 1.52 2020C 740C 77% 1.56 900C 680C 82% 2.20 2000C 720C 87% 2.22 1200C 620C 96% 2.3 1100C 68 C 93% 2.5 900C 660C 97% 4.1 8000C 1100C 72% 4.0 120 C 680C 98% A) water or steam fed into the furnace B) at a point immediately before the feeding point of water Example 2 Dolomite which contained 45% by weight calcium carbonate (CaCO2), 45% by weight magnesium carbonate (MgCO2) and 10% by weight impurities was fed into a pulverized-coal furnace in accordance with the procedure of Example 1, using the same operating values. On an equivalent basis, the amount of dolomite required in proportion to the amount of sulphur fed in is about 6.8 tn/h.

The calcium and magnesium carbonates contained in the domomite break down in the furnace into calcium oxide, magnesium oxide and carbon -dioxide, which leave the furnace along with the flue gases through a flue gas conduit. Part of the oxides in the furnace reacts with the oxides of sulphur present in the flue gases, thereby forming sulphate or sulphite.

Water and/or steam is sprayed into the flue gases either in the furnace or in the flue gas conduit, or in a separate reactor situated at a point subsequent to the flue gas conduit, whereupon the oxides which have not reacted in the furnace, owing to the increased moisture, can form hydroxide. The hydroxide for its part reacts with the oxides of sulphur present in the flue gases, thereby forming a pulverous reaction product.

When dolomite is used, the highly reactive calcium hydroxide reacts before the reaction of the slower magnesium hydroxide, which, when the amount of calcium present is sufficient, passes through the reactor almost unreacted. By designing the process to be carried out on the basis of the use only of the calcium present in the dolomite, the equivalent amount given above is arrived at. When the molar ratio of calcium to sulphur is at least 1, the results of the process are substantially in agreement with the corresponding values in Table 1.

Example 3 Calcium dioxide which contains impurities in an amount of 10% by weight is fed into a furnace in accordance with the procedure of Example 1, using corresponding operating values. In terms of the reaction, the theoretical equivalent amount of calcium oxide in proportion to the amount of sulphur entering the furnace in the fuel is about 1.9 tn/h.

Part of the calcium oxide reacts in the furnace with the oxides of sulphur present in the flue gases, thereby forming calcium sulphate or sulphite.

Water and/or steam is sprayed into the flue gases either in the furnace or in the flue gas conduit, or in a separate reactor located at a point after the flue gas conduit.

Owing to the increase in moisture, the calcium oxide forms highly reactive calcium hydroxide which reacts rapidly with the oxides of sulphur still present in the flue gases. The higher the moisture content in the flue gases upon leaving the furnace, the more effectively the sulphur dioxide is 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 reaction will suffice to evaporate the amount of water added.

When the calcium fed in, as calcium oxide, is calculated in a molar ratio to the sulphur, the results are in accordance with those shown above in Table 1.

Claims

1. A process for removing gaseous sulphur compounds, particularly sulphur dioxide, from the flue gases of a furnace, comprising the steps of (a) feeding to said furnace, in addition to sulphur-containing material to be burned and an oxygen-containing gas, a pulverous material comprising an alkali metal oxide and/or alkaline earth metal oxide, and/or a compound (other than the hydroxide) which will be converted in the furnace to such compound, and/or feeding such a pulverous oxide to the flue gases after leaving said furnace.

2. A process as claimed in Claim 1, wherein said compound is a carbonate.

3. A process according to Claim 1 or Claim 2, wherein said pulverous material is fed in an amount in excess in proportion to the sulphur present in the flue gases.

4. A process according to Claim 1 or Claim 2, or Claim 3, wherein the spraying of the water and/or steam is carried out when the temperature of the flue gases is 50-800 C.

5. A process according to Claim 4, wherein the temperature of said flue gases is 90-200 C.

6. A process according to any one of the preceding Claims, wherein water is sprayed into the flue gases at most in an amount that can be evaporated by the thermal energy provided by the flue gases and the reactions taking place therein.

7. A process according to any one of Claims 1 to 5, wherein a small amount of additional energy is introduced into said flue gases from the outside such as to ensure evaporation of added water prior to the separation step.

8. A process according to any one of the preceding Claims, wherein the pulverous material fed in comprises calcium carbonate, calcium-magnesium carbonate, and/or the corresponding oxides.

9. A process as claimed in Claim 1 for removing sulphur compounds from the flue gases of a furnace, substantially as hereinbefore described with reference to the drawing.

10. A process as claimed in Claim 1 for removing sulphur compounds from the flue gases of a furnace, substantially as hereinbefore described with reference to any of the Examples.