Method of Reducing the Sulfur Oxide Content of Combustion Gases Resulting From Combustion of Sulfur-Containing Fossil Fuels

The invention relates to a method of reducing the sulfur oxide content of combustion gases resulting from combustion of sulfur-containing fossil fuels by means of calcium carbonate (e.g. limestone) or calcium magnesium carbonate (e.g. dolomite) conveyed to the combustion zone during the combustion process, the sulfur oxides thereby resulting, particularly sulfur trioxide which forms by oxidation of sulfur dioxide, combining during the combustion process with the calcium oxide or calcium magnesium oxide resulting from de-acidification of the carbonate during the combustion process, whereby the sulfur oxide content of the combustion gases is reduced.

Combustion of fossil fuels such as oil, hard coal, shale, are used, for example, in industrial furnaces, for instance for current generation.

A method of the type specified above is known from the publication "Fluidised Bed Combustion Process Evaluation" Phase II, Pressurized Fluidized Bed Coal Combustion Development, Westinghouse Research Laboratories, Prepared For National Research Center, September 1975, pages 52 to 59. According to this method, flue gases can be desulfurized at temperatures of about 800.degree. C. by the addition of limestone or dolomite, the freshly de-acidified (and at this point highly reactive) limestone or dolomite reacting to form calcium sulfate (gypsum) or calcium magnesium sulfate. The method is applicable to combustion of fuel in a flame as well as to combustion of fuel in a fluidized bed.

In this known method, the degree of utilization of the added limestone or dolomite is very low. Conversion of above 30 to 35% are barely obtained. The reason for this is that the reactions of the sulfur oxides with the limestone or dolomite are surface reactions which virtually come to a halt when the surface of the limestone or dolomite particles are covered with the reaction product, i.e. the sulfate.

The degrees of utilization when ground limestone is blasted into a free burning flame are not much higher than when fuels in lump form, for instance, are burned in a fluidized bed of granular limestone. In any case, conversions or more than about 35% of the limestone used are not obtained.

In practice, therefore, for effective desulfurization, the quantities of limestone or dolomite used must be about 4 to 10 times the stoichiometric quantity. Since heating and de-acidifying of the limestone or dolomite requires a heat consumption of about 600 kcal/kg of limestone or dolomite, the economy of the process, as well as being encumbered with the costs of the excess limestone or dolomite, is also encumbered with the loss of heat from the de-acidification of too great an amount of limestone or dolomite. Furthermore, considerable expenses have to be borne for the harmless deposition of the non-reacted quantities of limestone or dolomite.

The object of the invention is to devise a method of the type specified above in which it is possible to reduce the considerable amounts of calcium carbonate (e.g. limestone) or calcium magnesium carbonate (e.g. dolomite) hitherto required for desulfurization, so as to improve the rationality during the combustion of fossil fuels. The invention is based on the fact that the reaction of the sulfur oxides with calcium carbonate or calcium magnesium carbonate is a surface reaction and that the utilization of the oxide is improved with an increasing specific surface area.

In order to achieve the object, it is suggested according to the invention that the calcium oxide or calcium magnesium oxide be removed and slaked with water, whereupon the calcium hydroxide or calcium magnesium hydroxide thereby produced is recycled to the combustion zone or to the gases emanating from the combustion zone. The recycling of the hydroxide which is obtained by slaking and which is of fine granulation and therefore has a large specific surface area, is undertaken together with the fossil fuel or separately therefrom or at the same time along with the fossil fuel. This re-supplying may be effected by, for example, blasting it into the combustion zone.

When putting the method into practice, it has surprisingly emerged that the considerable quantities of the carbonate, e.g. limestone or dolomite, hitherto required can be reduced to essentially stoichiometric quantities, while ensuring adequate desulfurization. This economy achieved by the invention can be improved still further if the conversion speed of the sulfur oxides with the limestone or dolomite is raised. This arises from the known fact that sulfur dioxide, for instance, reacts relatively slowly with calcium carbonate or calcium oxide, whilst sulfur trioxide for example, has a high reaction rate.

The retardation of the reactions between sulfur oxides and calcium oxide or calcium magnesium oxide is thus also contingent on the fact that the reaction of sulphur dioxide with oxygen to form sulfur trioxide in the firing or gas chamber takes place very slowly. It is suggested, therefore, that to establish the equilibrium SO.sub.2 +1/2 O.sub.2 .revreaction.SO.sub.3 more rapidly, a catalyst be supplied to the combustion zone together with the fossil fuel. The catalyst can be, for example, an oxide of a metal having more than one valency state such as vanadium pentoxide. The catalyst can be added together with the fuel both in the pure form and with carrier material which contains such catalysts in significant amounts. Particularly advantageously, ground oil shale or oil shale combustion waste can be used since oil shale or oil shale combustion waste contain quantities of vanadium pentoxide which can amount to a few hundred grams per tonne.

The addition of the catalyst with a carrier material, e.g. oil shale or oil shale combustion waste, has the advantage that, with this carrier material, an extremely fine distribution of the catalyst is obtained.

In an embodiment of the method of the invention, combustion of the fossil fuel takes place in a fluidized bed, namely a fluidized bed formed by particles of the carbonate used or a fluidized bed of inert eddy bodies.

Expediently, during the combustion process, the ashes, together with the resulting calcium oxide or calcium magnesium oxide, are removed from the fluidized bed. The very fine grained ashes usually obtained in the fluidized bed method are separated from the oxide, e.g. by sifting. Then the partially reacted calcium oxide or calcium magnesium oxide which has been separated and which is now de-acidified, is dry slaked in a slaking apparatus, so that an extremely fine-grained calcium hydroxide or calcium magnesium hydroxide powder is produced. Before this, the calcium oxide or calcium magnesium oxide which has been separated from the ashes is separated from the resulting adhering calcium sulfate (gypsum) or calcium magnesium sulfate in a drum or some other suitable apparatus in which there is enough movement.

The fine grained calcium hydroxide or calcium magnesium hydroxide produced by slaking is then blasted by a suitable device either into the fluidized bed or into the gas chamber where an extremely intensive reaction takes place with the sulfur oxides contained in the combustion gases.

The calcium hydroxide or calcium magnesium hydroxide, which has fully reacted, is obtained in part in such a fine granular form that it can be separated in electric filters. A coarser component left behind can be separated by suitable cyclone dust extractors or other devices preceding the electric filters.

In another embodiment of the method of the invention, combustion of the fossil fuel is conducted in a free-burning flame, e.g. in coal dust furnaces or oil furnaces, and the calcium carbonate or calcium magnesium carbonate used to react with the sulfur oxides is first finely ground and then supplied to the combustion zone together with or along with the fossil fuel. By way of example, finely ground limestone or dolomite can be added to the coal dust furnace or be blasted into the furnace together with the oil. As a result of the high flame temperature, de-acidification of the limestone or dolomite takes place in a short period, while reaction of the sulfur oxides therewith takes place until the surface is blocked with sulfate.

After the particles have been subsequently separated by cyclone separators or by electric filters from the flue gases, and after they have been slaked at suitable temperatures to form an extremely fine powder, they are again blasted into the flame chamber and, because of their extremely fine granulation, they react very quickly with the sulfur oxides contained in the flame gases.

Even with free-burning flames, in order to prevent any over-burning of the hydroxide in the flame zone, the hydroxide can be blasted into a zone the temperature of which is below the de-acidification temperature of the carbonate, the very high reactivity of the hydroxide and its fine granularity being retained.

The problem arises, when extracting the dust from the hot waste gases from the boiler, of separating in the same filter both flue ashes and also the oxide and the sulfate produced by the reaction. This may take place in an electric filter, expediently by making use of the differences of the dust resistances of the various substances. In the electric filter, substances with a high dust resistance require a stronger electrostatic field for separation than substances with a low dust resistance. This effect is made use of in the separation of the various substances by using an electric filter which has various sections through which the gases flow successively. In the sections, electrostatic fields of varying strength are maintained such that the lowest field prevails in the first section and the highest field in the last section. When the gases pass through the electric filter, the substances with the lowest dust resistance are then separated in the first section and the substances with the highest dust resistance are separated in the last section. With a mode of operation such as this, extremely good degrees of desulfurization can be achieved.

In the same way as in the reaction in the fluidized bed, the reaction time for obtaining the equilibrium SO.sub.2 +1/20.sub.2 .revreaction.SO.sub.3 with free-burning flame can also be determinative for the reaction rate of the sulfur oxides with the carbonate. In such a case, as in the method with the fluidized bed, catalysts are blasted in with the carbonate powder or with the finely ground coal or with the oil mists in pure form or with other substances containing such catalysts, with the result that establishment of the equilibrium SO.sub.2 +1/20.sub.2 .revreaction.SO.sub.3 can take place with sufficient speed.

The fine calcium oxide or calcium magnesium oxide can be separated after the combustion zone by a cyclone or by electric filters, or can be separated from the ash formed during combustion by sifting.

In the drawings, examples of apparatus for implementing the method according to the invention are represented. In the drawings:

FIG. 1 is a diagrammatic representation of an apparatus for fluidized bed firing; and

FIG. 2 is a diagrammatic representation of an apparatus for coal dust firing.

The reference numerals used in the Figures have the following significance:

1. is a fluidized bed furnace;

2. is a fluidized bed contained in the furnace;

3. is a blowing device for injecting hydroxide into the furnace;

4. is a device for the feeding of fuel to the furnace;

5. is a device for the feeding of limestone or dolomite to the furnace;

6. is a device for the feeding of catalyst to the furnace;

7. is a discharge line for discharge of calcium oxide or calcium magnesium oxide and ashes from the furnace;

8. is a cooling apparatus;

9. is a sifter;

10. is the ashes separated in sifter 9;

11. is the calcium oxide or calcium magnesium oxide separated in sifter 9;

12. is a drum for separating calcium sulfate;

13. is a slaking apparatus for slaking the calcium oxide or calcium magnesium oxide;

14. is a device for receiving calcium hydroxide (lime hydrate or slaked lime) or calcium magnesium hydroxide formed in apparatus 13;

15, 16, and 17. are metering devices for recycling hydroxide from device 14;

18. is an inlet for feeding hydroxide supplied from device 14 via metering device 15 to the blasting device 3;

19. is a further inlet for feeding hydroxide supplied from device 14 via metering device 15 to the fluidized bed;

20. is an inlet for feeding hydroxide supplied from device 14 via metering device 16 to the gas chamber;

21. is a boiler;

22. is a mechanical dust extracting apparatus;

23. is an electric filter;

24. is a blower;

25. is a store device for limestone or dolomite;

26. is a store device for the catalyst;

27. is a store for fossil fuel;

28. is a store for coal dust;

29. is a burner;

30. is a blower;

31. is a metering device for the supply of coal dust;

32. is a metering device for the supply of limestone or dolomite;

33. is a metering device for the supply of catalyst;

34. is the gypsum removed in filter 23;

35. is the carbonate removed in filter 23;

36. is the calcium oxide and calcium magnesium oxide;

37. is a metering device for calcium hydroxide or calcium magnesium hydroxide; and

38. is a zone below de-acidification temperature.

The invention will now be illustrated by the following Examples.

In a fluidized bed combustion plant constructed in accordance with FIG. 1, 15 tonne of coal were burned per hour. The coal contained 3% of sulfur (0.45 t S/h).

To react with this quantity of sulfur according to the overall equation S+1.5O.sub.2 +CaO.dbd.CaSO.sub.4, 787.5 kg of CaO or 1227.5 kg of CaCO.sub.3 are required per hour, i.e. 8.18% of the coal quantity used. CaO is calcium oxide, CaCO.sub.3 is calcium carbonate and CaSO.sub.4 is anhydrite (gypsum).

The calculated quantity of limestone was added to the fluidized bed furnace 1 together with the coal. This has the advantage that distribution of the limestone is effected at the same time as distribution of the coal thereby avoiding the presence at individual points in the fluidized bed furnace 1 of quantities of lime which are above or below the required quantity thereof, both of which would result in multiple consumption thereof.

The limestone was broken down to the same grain size as the coal. In the combustion process, about one quater of the sulfur oxides reacted withe the lime formed from the limestone during the combustion process. However, there were still about 590 kg/h of lime which did not react with sulfur, but were de-acidified.

This quantity of lime was removed continuously by an overflow from the furnace, together with anhydrit (CaSO.sub.4) and the coal ashes produced by the reaction. After the coal ashes had been sieved, the lime was dry slaked with 190 l of water.

The fine slaked lime powder thus obtained was then either added in measured amounts to the fuel or was blasted into fluidized bed 2 or into the gas chamber above the fluidized bed.

The lime powder which has not fully reacted was finally separated in the dust extracting apparatus 22 and 23 downstream of the boiler 21.

The addition of catalyst took place either with the fuel or the lime, or by the device 6.

Since the effectiveness of the catalyst varies subject to its composition and nature, the correct amount must be determined by practical tests.

In a coal dust firing plant as represented in FIG. 2, 100t of coal dust were burned per hour. The coal dust contained 1.3% of sulfur. The proportion of sulfur was therefore 1.3 t S/h. To react with the sulfur, therefore, 2.28 t/h of CaO or 4.06 t/h of CaCO.sub.3 were required.

The limestone was either finely ground separately or, more expediently, it was added to the coal before the grinding process and was ground with it.

During the combustion of the coal dust, a particularly homgeneous distribution of lime in the combustion gases is thereby ensured. During combustion, about a third of the sulfur oxides formed react in the gas chamber with the lime which was formed. The reacted lime was conveyed, together with the combustion gases, through the boiler system to the dust extracting apparatus.

A large portion of the ashes formed during combustion was separated in the boilers, whilst the finest component was separated in the dust extracting apparatus connected downstream of the boiler.

When the ash particles and the lime particles differed substantially in their grain size, separation of the ash particles from the lime particles was effected upstream of the electric filter 23 by a sifter 9. Where this was impossible, separation was effected as described earlier, by the use of electrostatic fields of varying strength in electric filter 23. The lime was then dry slaked with about 490 l/h of water.

The slaked lime thus produced was added to the coal to be injected into the furnace or was injected into the gas chamber at point 38 in boiler 21 where the temperature is so low that the Ca(OH).sub.2 was not converted into CaO.