METHOD AND POWER PLANT FOR THE USE OF A SULPHUR ABSORBENT IN A FLUIDIZED BED

Abstract: Method for improved utilization of sulphur-absorbent when burning fuel in a fluidized bed and power plant for imple­ menting said method. Partially consumed bed material is withdrawn from the ash chamber (24) of the bed vessel (12) and returned to the combustion chamber (22) of the bed (18) by means of pneumatic transportation using steam as propellant. The steam constitutes a reactant which effects disintegration of the bed material particles. A power plant for implementing said method comprises a bed vessel (12) having an air distributor (16) with nozzles (42) for supplying air to a combustion chamber (22) to effect flu­ idization of the bed (18) and combustion of a fuel. The power plant further comprises an ash chamber (24) below the air distributor (16) for removal of ash and bed material, and transport means (68,70,72) for withdrawing partially consumed bed material from the ash chamber (24), treating this bed material with steam and returning the steam-treated bed material to the combustion chamber (22). The transport pipe (72) is suitably provided with a chamber (86) to prolong the dwelling time of the bed material in said transport means.

Description: The invention relates to a method for improved utilization of sulphur-absorbent when burning fuel in a fluidized bed according to the precharacterising part of Claim 1. The invention also relates to a power plant for implementing said method. Such a method and plant is known from the US-A-4 312 280. Improved utilization of sulphur-absorbent is of great importance in order to reduce consumption thereof when burning sulphur-containing coal in a fluidized bed of particulate material. The bed material contains substances which, upon combustion, combine with sulphur and prevent sulphur oxides from being released into the atmosphere along with the combustion gases. The sulphur-absorbent normally consists of a calcium compound, usually calcium carbonate (CaCO₃) or dolomite (CaCO₃ MgCO₃) or a mixture thereof. Hydrated calcium sulphate is a stable compound which can be deposited into the environment without any risk of environmental damage. The invention is particularly intended for power plants in which combustion occurs at a pressure considerably above atmospheric pressure, and in which the combustion gases are used to drive a gas turbine, and steam is generated to drive a steam turbine. A number of chemical reactions are obtained at the temperature prevailing in the bed at combustion (800-950° C). The absorbent particles are converted partially to calcium oxide (CaO) and are sulphated, and a surface layer of hydrated calcium sulphate is formed. Gradually, an absorbent particle will develop a surface layer of calcium sulphate (CaSO₄) while its core consists of CaCO₃ and/or CaO. The outer layer of calcium sulphate (CaSO₄) impedes access of sulphur to the unused absorbent thus limiting utilization of the absorbent material. In order to improve the coefficient of utilization it is known to remove bed material from the bed and crush the particles so that the calcium sulphate layer is disintegrated and unconsumed bed material inside the particles becomes exposed. The crushed bed material is returned to the bed so that unused absorbent material in the bed particles is accessible for absorption of sulphur. The US-A-4 312 280 discloses a method for improved utilization of sulphur-absorbent when burning coal in a fluidized bed, and a power plant for implementing said method. Sulfated sulphur-absorbent is extracted from the combustor accommodating the fluidized bed and is transported to a second bed vessel in which the sulfated absorbent is fluidized by air. Water or steam is injected into the fluidized bed in order to disintegrate the sulfated absorbent particles thus exposing unused sulphur-absorbent. After termination of the hydration of the absorbent in the second fluidized bed, the absorbent is re-supplied to the fluidized bed combustor. This method and installation is costly and space-demanding because it requires a second bed vessel with means for fluidization in order to treat the sulfated absorbent with steam. Another known method for more economically utilize sulphur-absorbing additives is mentioned in the EP-A-0 119 661. According to this method the unused sulphur-absorbent particles in the form of calcium oxide, which particles are coated with calcium sulphate, are withdrawn from the waste material from the fluidized bed and are then slaked with water which converts the calcium oxide into hydroxide whereas the calcium sulphate binds water. The absorption of the water causes a swelling of the particles. Upon reheating of these particles after having been recirculated into the fluidized bed a new pore structure of the particles is created that enables the particles to absorb additional amounts of sulphur dioxide. The EP-A-0 119 661 discloses yet another method to more effectively use sulphur-absorbing additives which consists in bringing the particles of sulphur-absorbing additives, comprised in the waste material from the fluidized bed, into contact with the fuel to be burnt in the fluidized bed. In order to blend the sulphur-absorbent particles with the fuel particles this method uses the present system for conveying the fuel from the storage to the combustion chamber. This recirculation of the sulphur-absorbent is supposed to be made several times. No additional water is added under this method. The EP-A-0 132 061 discloses a device for classifying the particles of a fluidized bed into fractions in order to eliminate oversized ash particles which are unacceptable for an efficient fluidization of the bed. For this purpose bed material is extracted from the bottom of the bed vessel through a tube and then blown into a vertical pipe the lower end of which is connected to a classifier while the upper end is led back to the freeboard of the bed vessel. Via the classifier a propellant gas is administered to the extracted material at such a pressure that the lighter particles of the extracted bed material are entrained in the propellant gas and transported back to the bed vessel while the bigger and heavier particles fall down by force of gravity into a discharge container. The propellant gas regularly consists of air. However, in cases where a reaction of the particles could take place with the oxygen in the air, for example in the case of a gasifier, nitrogen or steam may also serve as propellant gas. The invention aims at developing a method for improved utilization of sulphur-absorbent when burning fuel in a fluidized bed of the above-mentioned kind, which is much simpler, less space-demanding and less costly than the known method and which is particularly suitable for a plant with a pressurized fluidized bed. The invention also aims at designing a power plant for carrying out said method. To achieve this aim the invention suggests a method according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the method are characterized by the features of the additional Claims 2 to 5. A power plant for implementation of the method according to the invention is characterized by the features of Claim 6. Further developments of this power plant are characterized by the features of the additional Claims 8 and 9. According to the invention the bed material is withdrawn from the fluidized bed and mixed with steam. Calcium oxide (CaO) and calcium carbonate (CaCO₃) in the hot bed material react with the steam as follows CaO + H₂O→Ca(OH)₂ + 66 kJ/g-mol CaCO₃ + H₂O→Ca(OH)₂ + CO₂ - 112 kJ/g-mol These reactions are sufficiently violent to shatter the bed material particles. Unconsumed absorbent material inside the particles is exposed and becomes accessible for absorption. The calcium hydroxide (Ca(OH)₂) thus formed is an extremely fine-grained powder. Ca(OH)₂ is a good sulphur absorbent and has high reactivity in powder form thanks to the large reaction surface available. The following reaction is obtained: Ca(OH)₂ + SO₂ + 1/2 O₂→CaSO₄ + H₂O + 435 kJ/g-mol At temperatures below 640° C calcium hydroxide (Ca(OH)₂) is always in solid state. At temperatures above about 640° C liquid phases may be formed. This may cause particles to adhere to each other, forming lumps which clog the transport pipe. It is therefore advisable to cool the bed material withdrawn to a temperature of below 640° C before allowing it to react with the steam. The reaction of the bed material withdrawn may occur in a pneumatic transport pipe between an ash chamber below the combustion chamber and a re-supply point in the combustion chamber. Steam is supplied through an ejector nozzle in the transport pipe and acts as transport gas. The transport pipe may be provided with means to extend the transport time, and thus the time during which steam can react with the particles of the bed material. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 the invention utilized in a PFBC power plant, Figure 2 schematically what happens with a hot bed particle upon contact with steam. In the drawings, 10 designates a pressure vessel, housing a bed vessel 12 and a gas-cleaning plant symbolized by a cyclone 14. In reality the gas cleaning plant comprises parallel-connected groups of series-connected cyclones. In the lower part of the bed vessel 12 an air distributor 16 is arranged to disperse air for fluidization of a bed 18 of particle material and for combustion of fuel supplied to the bed 18 through a fuel-supply pipe 20. Fresh bed material is added to the bed vessel 12 through a pipe 21. The air distributor divides the bed vessel 12 into an upper combustion chamber 22 and a lower ash chamber 24. The upper part of the combustion chamber 22 forms a free-board 22a where combustion gases from the bed 18 collect. These gases are conducted from the free-board 22a via conduit 26 to the cyclone 14. Dust separated in the cyclone 14 is removed through pipe 50 and a pressure-reducing dust discharge device 52 for collection in a container outside the pressure vessel. The cleaned gas is conducted through the pipe 28 to a gas turbine 30 which drives a generator 32 and a compressor 34. The compressor compresses combustion air which is supplied through the pipe 36 to the space 38 between pressure vessel 10 and bed vessel 12. The air distributor 16 is constructed of elongate distribution chambers 40 with nozzles 42. Air for fluidization and combustion is supplied to these distribution chambers 40 from the space 38 via valve members or dampers, not shown, which determine the amount of the air flow. The distribution chambers 40 form slits or apertures 44 through which bed material can flow from the bed 18 in the combustion chamber 22 to the ash chamber 24. The bed material, sulphur absorbent and residual products from the combustion are removed from the ash chamber through the cell-feeder 46 in discharge pipe 48. Tubes 54 are provided in the bed 18 in the combustion chamber 22, to generate steam and to cool the bed, and tubes 56 are provided in the ash chamber 24 for superheating this steam. The steam drives a steam turbine 58 and a generator 60 connected to said tubes. The steam leaving the turbine 58 is condensed in a condenser 62, and the condensate is returned by the feed-water pump 64 to the tubes 54 in the bed 18. The plant is provided with a means for disintegrating the bed material particles in order to expose the unused absorbent present in their interior. The disintegrated bed material particles are returned to the bed where the absorbent, previously difficult to access, has now become easily accessible for absorption reaction. The bed material is treated with steam in order to disintegrate it. As shown in Figure 1, the treating means consists of a pneumatic transport means 66 for returning bed material from the ash chamber 24. Steam is used as transport gas. This transport means 66 includes a suction nozzle 68 connected to the ash chamber 24, an ejector 70, a pipe 72 and at least one supply nozzle 74. The orifice of the nozzle/nozzles in the combustion chamber 22 may be located in the bed 18, or/and in the free board 22a above the bed 18. The ejector nozzle 76 is connected to a steam conduit 78 via pipe 80, valve 82 and conduit 84. The transport pipe 72 may have an enlarged portion 72a forming a chamber 86 which extends the time the particles spend in the transport pipe 72, thus extending the time the bed material is exposed to the steam atmosphere. The quantity of material circulated through the transport means, treated with steam and returned from the ash chamber 24 to the combustion chamber 22 is regulated by the flow of steam to the ejector nozzle 76. The steam flow is regulated by valve 82, said valve having a control device 90 connected to an actuating device 92. The requisite steam pressure is determined by the pressure in the combustion chamber 18 and by the quantity of material to be circulated. The pressure must be slightly in excess of the pressure in the combustion chamber 18. The steam conduit 84 may also include a reducing valve to reduce the pressure of the steam withdrawn from the conduit 78. The temperature of the steam should be above 200° C but should not exceed the temperature T B of the bed material withdrawn. As mentioned, the temperature of the bed material treated with steam should be less than about 640° C. The material in the bed 18 in combustion chamber 22 has a temperature T A of 800-950° C. When the falling bed material has passed the tubes 56 in the ash chamber 24, it will have been cooled to a temperature considerably below 640° C and may lie within the interval of 300-500° C. The reaction caused by the treatment of the bed material is illustrated in Figure 2. When a particle comes into contact with steam, CaO and CaCO₃ will react with water to form Ca(OH)₂ in the form of a fine powder, while CO₂ is released. The surface layer of CaSO₄ is disintegrated, thus exposing unconsumed absorbent present in the interior of the particles. Ca(OH)₂ is obtained, as mentioned, in the form of a fine powder and a large effective surface is obtained making the powder an extremely efficient absorbent. It will therefore be well utilized even if the time of remaining in the bed 18 is short.

Claims: Method for improved utilization of sulphur-absorbent when burning sulphur-containing fuel, preferably coal, in a fluidized bed (18) of particulate material consisting at least partially of a sulphur absorbent containing calcium (Ca) or a suitable calcium compound, by means of extracting bed material from the fluidized bed, treating the bed material with steam such as to disintegrate the bed material to exposed unused absorbent inside the particles and then re-supplying the bed material to the bed, characterized in that the extracted bed material is returned to the fluidized bed by way of a pneumatic transport means driven by steam, the steam acting both as transport gas and as a reactant to effect the disintegration of the bed material particles. Method according to Claim 1, characterized in that the bed material is cooled to a temperature below 640° C before steam is supplied. Method according to claim 1 or 2, characterized in that in a combustion installation with a bed vessel (12) comprising a combustion chamber (22) above an air distributor (16) for air to fluidize the bed (18), and an ash chamber (24) for the removal and cooling of ash and consumed bed material, partially consumed bed material is removed from said ash chamber (24), treated with steam and returned to the bed (18) in the combustion chamber (22). Method according to Claim 3, characterized in that bed material is transported pneumatically from the ash chamber to the bed in the combustion chamber with steam as propellant. Method according to Claim 4, characterized in that steam is supplied to an ejector (70) in a transport pipe (72) between the ash chamber (24) and the combustion chamber (22). Power plant for burning a sulphur-containing fuel, primarily coal, in a fluidized bed (18) of particulate material comprising a bed vessel (12), an air distributor (16) with nozzles (42) for supplying air to the bed vessel (12) to effect fluidization of the bed material and combustion of a fuel supplied to the bed (18), said air distributor (16) dividing the bed vessel into a combustion chamber (22) and an ash chamber (24), openings (44) in the air distributor (16), permitting bed material to flow from the combustion chamber (22) to the ash chamber (24), tubes arranged in the bed (18) in the combustion chamber (22) for generating steam and for cooling the bed, and a means for cooling bed material in said ash chamber (24), characterized in that it also includes a pneumatic transport means (68) driven by steam for withdrawing cooled bed material containing sulphur absorbent from the ash chamber (24), a means (70) for mixing the moving bed material withdrawn with steam, and a pneumatic transport means (72) for returning the bed material to the bed in the combustion chamber (22), said transport means (72) being adapted to be driven by steam, which also acts as reactant with the bed material. Power plant according to Claim 6, characterized in that a pneumatic transport pipe (72) is arranged between the ash chamber (24) and the combustion chamber (22) said transport pipe having an ejector (70) with a nozzle for propellant gas which is connected via a control valve (82) to a steam source (78), the ejector being arranged to effect the mixing of bed material with steam and the reaction between steam and bed material in the ejector (70) and in the transport pipe (72) downstream of the ejector (70). Power plant according to Claim 9, characterized in that the transport pipe (72) includes an enlarged portion or chamber (86) adapted to extend the time spent by the bed material in the steam atmosphere.

Inventors: PILLAI, KRISHNA K
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1988
Main classification: F23C11
IPC codes: F23C10, F22B31