WO1996030465A1

WO1996030465A1 - Method for feeding regeneration offgas into a gasifier

Info

Publication number: WO1996030465A1
Application number: PCT/FI1996/000171
Authority: WO
Inventors: Tero Mikael VÄÄRÄNEN
Original assignee: Enviropower Oy
Current assignee: Enviropower Oy
Priority date: 1995-03-30
Filing date: 1996-03-27
Publication date: 1996-10-03
Anticipated expiration: 1997-09-30
Also published as: AU5006796A

Abstract

The invention relates to a method for feeding regeneration offgas into a gasifier (1) in the production of product gas by gasifying sulfurous fuel. The obtained product gas is passed into a sulfur removal reactor (5) containing metal oxide sorbent, the metal oxide sorbent binding the sulfur and being converted into metal sulfide; the product gas is discharged; the metal sulfide is passed into a regeneration reactor (9) into which air and steam are introduced for regenerating the metal sulfide into metal oxide; the metal oxide is recycled into the sulfur removal reactor (5); and sulfur-oxides-bearing regeneration offgas is passed from the regeneration reactor (9) into an ejector (19), where the pressure of the offgas is increased to enable the offgas to be introduced into the gasifier (1).

Description

Method for feeding regeneration offgas into a gasifier

The invention relates to a method for feeding regeneration offgas into a gasifier in the production of product gas by gasifying sulfurous fuel at a pressure exceeding atmospheric pressure.

In power plants based on the gasification of fuel, it is necessary to separate sulphur from the product gas formed in the gasificatio- , after which the gas is passed into a gas turbine for producing electricity. In a technique used widely, sulfur is removed by cooling the formed product gas to a suit¬ able temperature, whereafter the sulfur compounds are washed off the gas in a conventional way with a water solution, and the gas is re-heated to the required high operating temperature. This process is very expensive, as additional energy is required both for cooling and heating the gas.

Another technique for realizing this process is to first remove part of the sulfur contained in the fuel in gasification typically by introducing lime into the fluidized bed of the gasifier, the lime removing 60 to 70% of the sulfur. Since more than 95% of the sulfur has to be removed, preferably substan- tially all of it, any sulfur remaining in the gas is purified with a metal oxide sorbent, through which the product gas is passed and which is circulated through the regeneration apparatus so that sulfur compounds will be separated from the sorbent, and the sorbent is recycled into the purification of the product gas. During regeneration the sorbent is oxidized back to metal oxide, thus also obtaining so-called generation offgas containing S0₂. The regeneration reaction is a highly exothermic reaction, and therefore the tem- perature of the regeneration reactor has to be adjusted in order that the sorbent would not be destroyed. The temperature is adjusted by adding so- called dilution gas, i.e. steam or nitrogen, into the regeneration reactor. The proportion of steam or nitrogen in the formed regeneration offgas is thus from 70 to 80% by volume. As used in this patent application and claims, the term regeneration offgas refers to the sulfurous gas leaving the regeneration reactor of the metal oxide sorbent. This gas is formed when sorbent that has bound sulfur and is in a metal sulfide form as well as air and steam or nitrogen are introduced into the regeneration reactor and they react with the metal sulfide so that it is converted to a metal oxide while the sulfide forms a sulfurous gas, mainly sulphur dioxide. This regeneration offgas is utilized as such by passing it through a pressure reduction step into other useful applications, such as the production of elementary sulfur or sulfuric acid. Alternatively, it can be pressurized and recycled into the gasifier. In such a case, the sulfur contained in the regeneration offgas is bound to the sulfur removal sorbent introduced into the gasifier, such as lime, e.g. dolomite, whereby the resultant calcium sulfate can be removed from the gasifier along with the bottom ash. Metal oxides, such as iron oxide Fe₂0₃, zinc ferrite ZnFe₂0₄, or zinc titanate Zn₂Ti0₄, or cerium oxide, are used widely as sorbent in the sulfur removal of product gas. A common practice is also to use fluidized bed reactors both as sulfur removal and regeneration reactors.

U.S. Patent 4,769,045 describes a method similar to that described above, where two parallel sulfur removal reactors based on the use of zinc ferrite oxide are operated in turn. The patent teaches that one of the reactors is used for sulfur removal while the other is used for sorbent regeneration by using air and steam. The patent does not disclose in any way how air and steam are pressurized. On the other hand, it discloses a solution where a circulating fluidized bed reactor-regeneration reactor combination of the same kind as described above is used for purifying product gas and regenerating metal oxide. However, the patent does not disclose in any way how the pressures and temperatures could be adjusted in practice. An essential feature of the gasification process is that all gas streams to be introduced into the gasifier should be at a pressure at least equal to that of the gasifier. Air, steam, regeneration offgas, and other gases to be introduced into the gasifier therefore have to be pressurized e.g. by means of a compressor to a pressure at least equal to the pressure of the gasifier. Gases can be introduced into the gasifier most easily when they are at a pressure exceeding that of the gasifier. in practice, problems have occurred in the pressurization of regeneration offgas in that the gas has to be cooled to a very low temperature, about 60 to 80°C, to obtain the required pressure increase of about 4 bar by means of a compressor. If the dilution gas is steam, the temperature drop is problematic as the water forming upon the condensation of steam forms sulfuric acid with gaseous sulfur dioxide. Sulfuric acid in turn corrodes the compressor and other equip¬ ment. As a result, nitrogen has to be used as dilution gas in the regeneration reactor. One drawback of the use of nitrogen is that it is expensive. Another draw¬ back is that the use of nitrogen results in a significant increase in the needless inert gas stream in the gasifier process. The object of the present invention is to provide a method which allows regeneration offgas to be fed simply and advantageously into the gasifier for removing sulfur in power plant use. In the method according to the invention, the regeneration offgas is passed into an ejector to increase its pressure at least to a level equal to the pressure of the gasi¬ fier. Air at a pressure higher than that of the air introduced into the gasifier is thus injected into the ejector as motive gas to increase the pressure of the regeneration offgas, the air inducting regeneration offgas while flowing through the ejector, thus produc¬ ing a gas mixture being at a pressure at least equal to that of the gasifier. The gas mixture formed in the ejector is then passed into the gasifier. One essential feature of the invention is that the dilution gas used in the regeneration reactor is preferably steam, which is advantageously easily available at the power plant. Another essential feature of the invention is that the regeneration offgas is cooled to a temperature exceeding the steam condensing temperature, which is about 300°C, typical¬ ly to a temperature between 350 and 400°C, whereby the steam will not condense and thus no sulfuric acid will be formed. Still another essential feature of the invention is that the regeneration offgas is pressur¬ ized to a pressure at least equal to that of the gasifier by means of an ejector, where preheated air pressurized to an appropriate pressure is used as motive gas in pressurization. In this way this air can be taken from the air stream also otherwise introduced into the gasifier, wherefore the gas flow volume of the gasifier will not increase, as both the steam and the air amounts are merely passed partly along a dif¬ ferent path to the gasifier. An advantage of the invention is that the dimensions and total mass flows of the gasifier remain unchanged while the costs are reduced significantly as compared with the use of nitrogen as dilution gas. Moreover, contrary to the compressor, the ejector has no moving parts nor seals that require maintenance and wear out in use. Another advantage of the invention is that it is simple and easy to realize, and it can utilize gaseous mixtures or substances that are anyway available at the power plant. The invention will be described more fully with reference to the attached drawing, which schematically shows an equipment needed for applying the invention.

The figure schematically shows a gasifier 1, into which coal and lime, such as dolomite, are intro- duced through feed lines 2 and 3. The gasifier 1 typically comprises a fluidized bed la, where the coal is gasified, thus forming product gas, and where lime reacts with sulfurous compounds contained in the product gas, such as H₂S, forming calcium sulfate. The following reaction formulas 1 and 2 represent sulfur- removal reactions occurring in the gasifier. It appears from the formulas that calcium oxide reacts with hydrogen sulfide, resulting in calcium sulfate remaining in the ash.

1) CaO_(β) + H₂S_(g) → CaS_(β) + H₂0_(g)

2) CaS,., + 2 0_a(g) → CaS0_4(β) - TO ASH

From the gasifier 1 the product gas is passed through a conduit 4 into a sulfur removal reactor 5. The product gas is cooled by means of a cooler 6 provided in connection with the conduit 4 in order that its temperature would be as appropriate as possible for the operating conditions of the sulfur removal reactor and the sorbent used. In the sulfur removal reactor 5 the product gas is passed into a fluidized bed, where the fluidized bed material is metal oxide which will react with the sulfur. This is illustrated schematically in reaction formulas 3 and 4. Reaction formula 3 represents the reaction of metal oxide and hydrogen sulfide in general, and formula 4 represents the reaction occurring when zinc oxide is used as a sorbent.

3) Me0₍„ + H₂S_(g) → MeS₍„ + H₂0₍₂₎ 4) ZnO + H₂S -. ZnS + H₂0

Product gas purified from sulfurous compounds is then passed through a dust separation filter 7 and further through a conduit 8 into a gas turbine not shown in the figure. From the sulfur removal reactor 5 the spent sorbent is passed into a regeneration reactor 9, where air and steam mixed with it are passed through the sorbent. In the regeneration reactor 9 the sorbent, now in a metal sulfide form, reacts with the oxygen of air, thus forming sulfur dioxide. At the same time the sorbent is again con¬ verted to metal oxide, which is recycled into the sulfur removal reactor 5. Reaction formulas 5 and 6 represent the regeneration process, reaction formula 5 representing the reaction of metal sulfide in general, and reaction formula 6 representing the reaction when zinc oxide is used as a sorbent.

5) MeS₍.₎ + 1.5 0_2(g) - Me0 ₎ + S0_2(g) 6) ZnS + 1.5 0₂ → ZnO + S0₂

The regeneration reactor may also be a fluidized bed reactor. Sulfur-dioxide-containing regeneration offgas leaving the reactor is passed into a cooler 10, where it is cooled to a required temperature, typical¬ ly about 350 to 400^βC. Solids are then removed from the regeneration offgas by means of a dust separation filter 11, and the gas is passed through an ejector 19 into the gasifier.

In order that air could be introduced into the gasifier, it normally has to be compressed to a press¬ ure at least equal to the pressure of the gasifier, usually to 15 to 25 bar, depending on the gasifier. Pressurized air from the gas turbine is cooled in an air cooler 12, whereafter its pressure is elevated in a compressor 13 to a level adequate for introducing the air into the gasifier 1. Air from the compressor 13 is further passed through another air cooler 14, where it is cooled. The air is thereafter fed mainly into the fluidized bed la of the gasifier 1 to bring about fluidization and required combustion. However, part of the air leaving the compressor 13 is passed through a conduit 15 into the regeneration reactor 9, as mentioned above. Part of the stream of air from the compressor 13 is passed through a separate conduit 16 into an ejector compressor 17, which increases the pressure of the air to the level required. After the ejector compressor 17, the air is cooled in an ejector cooler 18, whereafter it is passed into the ejector 19 to increase the pressure of the regeneration offgas. In the ejector 19 the high-pressure air acts as so- called motive gas, the flow of which effects the ejector phenomenon, whereby the pressure of the regeneration offgas rises to a higher level than the pressure of the air otherwise fed into the gasifier. As a result of this, the mixture of air and regener¬ ation offgas can be passed through a conduit 20 and mixed with the stream of air from the compressor 13; if desired, it can alternatively be passed directly into the gasifier 1, as shown by the broken line.

The solution of the invention does not increase the amount of air to be introduced into the gasifier nor does it increase the stream of gas or the amount of inert gas flowing through the gasifier 1, as most of the gas stream required in sulfur removal is taken from the normal gas stream fed into the gasifier and returned into this gas stream after the regeneration step of the sulfur removal process. In this way, sul- fur can be removed by using dolomite or some other lime.

Calcium sulfate is discharged through the bottom of the gasifier with the ashes, and transported to a dumping area. The invention has been described above only by way of example, and the equipment used for applying the method is not or need not be connected as shown in the figure. It is essential that the final sulfur removal of product gas is performed by a metal oxide sorbent, which is again regenerated for sulfur removal by introducing steam and air from the stream of air passed into the gasifier into the regeneration reactor. It is also essential that the regeneration offgas is cooled only to such an extent as is necess- ary to pressurize it in the ejector by means of air into a pressure suitable for introduction into the gasifier. Besides zinc oxide mentioned above, the sorbent may also be zinc titanate or any metal oxide and their combinations which are thermodynamically stable at the pressure and temperature of the sulfur removal process of product gas and capable of binding sulfur and being regenerated. Instead of steam, it is also possible to use some other dilution gas than e.g. nitrogen.

Claims

Claims :

1. Method for feeding regeneration offgas into a gasifier 1 in the production of product gas by gasify- ing sulfurous fuel at a pressure exceeding atmospheric pressure by introducing air, steam, fuel to be gas¬ ified, and lime into the gasifier 1, the lime binding part of sulfurous compounds formed in the gasifier 1; the method comprising passing the obtained product gas containing various sulfur compounds into a sulfur removal reactor 5 containing metal oxide sorbent for removing the remaining sulfur compounds, the metal oxide sorbent reacting with the sulfur and being converted into metal sulfide; discharging purified product gas from the sulfur removal reactor 5; passing the metal sulfide from the sulfur re¬ moval reactor 5 into a regeneration reactor 9, into which air and steam or nitrogen are introduced for regenerating the metal sulfide again into metal oxide; recycling the metal oxide obtained by regener¬ ating into the sulfur removal reactor 5; c h a r a c t e r i z e d in that the sulfur- oxides-bearing regeneration offgas formed in the regeneration reactor 9 is passed into an ejector 19, where the pressure of the offgas is increased to a level at least equal to the pressure prevailing in the gasifier 1 to enable the offgas to be introduced into the gasifier 1.

2. Method according to claim 1, c h a r a c¬ t e r i z e d in that air at a pressure higher than that of the air introduced into the gasifier 1 is injected into the ejector 19 as motive gas to increase the pressure of the regeneration offgas, said air inducting regeneration offgas while flowing through the ejector 19, thus producing a gas mixture being at a pressure at least equal to that of the gasifier 1, said gas mixture formed in the ejector 19 being passed into the gasifier.

3. Method according to claim 1 or 2, c h a r a c t e r i z e d in that part of the total amount of air to be introduced into the gasifier 1 is used as the air to be introduced into the ejector 19.

4. Method according to any one of claims 1 to 3, c h a r a c t e r i z e d in that part of the total amount of air to be introduced into the gasifier 1 is introduced into the regeneration reactor 9, the air introduced into the regeneration reactor 9 being recycled into the gasifier 1 through the regeneration reactor 9 and the ejector 19.

5. Method according to claim 3 or 4, c h a r a c t e r i z e d in that air to be introduced into the ejector 19 is taken from the stream of air to be introduced into the gasifier 1 after the pressure of the air has been increased.

6. Method according to any one of claims 1 to 5, c h a r a c t e r i z e d in that the sulfurous fuel to be gasified in the gasifier 1 is coal or coke.

7. Method according to claim 6, c h a r a c- t e r i z e d in that the sulfurous fuel to be gasified in the gasifier 1 is coke.

8. Method according to any one of claims 1 to 7, c h a r a c t e r i z e d in that the regeneration offgas is pressurized to a pressure higher than that of the gasifier 1.

9. Method according to any one of claims 1 to 8, c h a r a c t e r i z e d in that the metal oxide sorbent is zinc titanate.

10. Method according to any one of claims 1 to 9, c h a r a c t e r i z e d in that the pressure of the gas mixture is maintained higher than the pressure of the gasifier 1.