FLUIDIZED BED REACTOR SYSTEM AND A METHOD OF OPERATING A FLUIDIZED BED REACTOR SYSTEM

Abstract

A reactor system and control method. The method includes feeding solid fuel and oxygen containing gas to a first fluidized bed reactor to form a fluidized bed of particles and combusting a first portion of the fuel in the bed with the oxygen containing gas to generate hot bed particles and a first stream of hot flue gas, conveying the first stream to the flue gas channel, transferring hot bed particles including a second portion of the solid fuel at a predetermined hot particles transfer rate from the first reactor to a second fluidized bed reactor, feeding fluidizing gas to the second reactor to form a second fluidized bed, and transferring bed particles from the second reactor to the first. The method includes first and second operation modes. In the first, the fluidizing gas is oxygen containing gas and, in the second, the gas includes steam, CO.sub.2, or inert gas.

Claims

1. A method of controlling a reactor system comprising first and second fluidized bed reactors, a flue gas channel in gas flow connection with the first fluidized bed reactor and a product gas channel in gas flow connection with the second fluidized bed reactor, the method comprising the steps of: feeding solid fuel and oxygen containing gas to the first fluidized bed reactor so as to form therein a fluidized bed of particles and combusting a first portion of the solid fuel in the fluidized bed of particles with the oxygen containing gas so as to generate hot bed particles and a first stream of hot flue gas; conveying the first stream of hot flue gas to the flue gas channel; transferring hot bed particles including a second portion of the solid fuel at a predetermined hot particles transfer rate from the first fluidized bed reactor to the second fluidized bed reactor; feeding fluidizing gas to the second fluidized bed reactor so as to form therein a second fluidized bed of particles; and transferring bed particles from the second fluidized bed reactor to the first fluidized bed reactor, wherein the method comprises first and second operation modes, in which first operation mode the fluidizing gas is oxygen containing gas and the method comprises the steps of: combusting a portion of the second portion of the solid fuel in the second fluidized bed of particles so as to generate a second stream of hot flue gas, and conveying the second stream of hot flue gas to the flue gas channel, and, in which second operation mode the fluidizing gas comprises steam, CO.sub.2, or non-oxygen containing inert gas and the method comprises the steps of: pyrolyzing or gasifying a portion of the second portion of the solid fuel in the second fluidized bed of particles so as to generate product gas, and conveying the product gas to the product gas channel.

2. The method of controlling a reactor system according to claim 1, wherein the first fluidized bed reactor is ramped down by switching the reactor system from the first operation mode to the second operation mode.

3. The method of controlling a reactor system according to claim 2, wherein the predetermined hot particles transfer rate is in the second operation mode at least 20% higher than in the first operation mode.

4. The method of controlling a reactor system according to claim 1, wherein, in the first operation mode, the predetermined hot particles transfer rate and the rate of feeding fluidizing gas are controlled so as to maintain the second fluidized bed reactor at least at a predefined temperature.

5. The method of controlling a reactor system according to claim 4, wherein the predefined temperature is at least 400° C.

6. The method of controlling a reactor system according to claim 1, wherein the predetermined hot particles transfer rate is in the second operation mode controlled so as to maintain the second fluidized bed reactor at least at a predefined pyrolyzing or a gasifying temperature.

7. The method of controlling a reactor system according to claim 6, wherein the predefined pyrolyzing or gasifying temperature is at least 400° C.

8. The method of controlling a reactor system according to claim 1 wherein the second operation mode comprises a step of feeding second solid fuel to the second fluidized bed reactor.

9. A fluidized bed reactor system comprising: first and second fluidized bed reactors; means for feeding solid fuel and means for feeding oxygen containing gas to the first fluidized bed reactor; a flue gas channel in gas flow connection with the first fluidized bed reactor; means for transferring bed particles from the first fluidized bed reactor to the second fluidized bed reactor; means for feeding fluidizing gas to the second fluidized bed reactor; and means for transferring bed particles from the second fluidized bed reactor to the first fluidized bed reactor, wherein the means for feeding fluidizing gas is in controllable gas flow connection with the means for feeding oxygen containing gas to the first fluidized bed reactor and the means for feeding fluidizing gas is in controllable gas flow connection with a supply of at least one of steam, CO.sub.2, and non-oxygen containing inert gas, and the second fluidized bed reactor is in controllable exhaust gas flow connection with the flue gas channel and with a product gas channel.

10. The fluidized bed reactor system according to claim 9, wherein the means for transferring bed particles from the first fluidized bed reactor to the second fluidized bed reactor comprises a particle flow controller.

11. The fluidized bed reactor system according to claim 10, wherein the particle flow controller is a mechanical or pneumatic flow controller or comprises a fluidized bed.

12. The fluidized bed reactor system according to claim 11, wherein the means for feeding fluidizing gas comprises a channel with a valve to a supply of oxygen containing gas and a channel with a valve to a supply of steam, CO.sub.2 or non-oxygen containing inert gas.

13. The fluidized bed reactor system according to claim 9, wherein the controllable exhaust gas flow connection comprises a channel with a valve to the flue gas channel and a channel with a valve to the product gas channel.

14. The fluidized bed reactor system according to claim 9, wherein the product gas channel is in gas flow connection with a product gas storage that is in gas flow connection with a burner arranged to produce and to convey hot gas to a superheater chamber arranged in the flue gas channel.

15. The fluidized bed reactor system according to claim 9, wherein the product gas channel is in gas flow connection with a product gas storage that is in gas flow connection with the first fluidized bed reactor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 schematically illustrates an arrangement according to a first preferred embodiment of the present invention.

[0027] FIG. 2 schematically illustrates an arrangement according to a second preferred embodiment of the present invention.

[0028] FIG. 3 schematically illustrates an arrangement according to a third preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The diagram of FIG. 1 schematically illustrates a fluidized bed reactor system 10 according to a preferred embodiment of the present invention. The reactor system comprises a first fluidized bed reactor 12 and a second fluidized bed reactor 14. In the embodiment shown in FIG. 1, both first and second fluidized bed reactors are circulating fluidized bed reactors, but alternatively either one or both of the reactors could be another kind of fluidized bed reactor, such as a bubbling bed reactor.

[0030] The first fluidized bed reactor 12 is a steam generator, i.e., a boiler, and comprises a furnace 16 with a bottom grid 18 connected by an air channel 20 to an air supply 22, i.e., to an air feeding fan, means for feeding solid fuel 24, a particle separator 26 connected to the upper portion of the furnace, a return duct 28 for returning particles separated in the particle separator 26 to the lower portion of the furnace, and a flue gas channel 30 for conveying cleaned flue gas from the particle separator 26 to a back pass. The return duct 28 also advantageously comprises a conventional gas-lock (not shown in FIG. 1).

[0031] Walls 32 enclosing the furnace 16 of the first fluidized bed reactor 12 are of conventional evaporative tube-wall construction and the back pass comprises conventional heat recovery devices, such as an air preheater, economizers and superheaters, and flue gas cleaning devices, such as a fly ash collector and a wet or dry flue gas desulfurizer. Because such devices are well-known to persons skilled in the art and not specifically relevant for the present invention, they are not shown in FIG. 1 or described here in more detail. The furnace 16 of the first fluidized bed reactor 12 advantageously comprises conventional means for discharging bottom ash (not shown in FIG. 1) from the bottom portion of the furnace.

[0032] To the lower portion of the furnace 16 of the first fluidized bed reactor 12 is connected a particle transfer channel 34 equipped with a gas-lock forming particle flow controller 36, such as a fluidized bed, mechanical or pneumatic flow controller, for conveying hot bed particles at a controlled rate from the furnace 16 of the first fluidized bed reactor 12 to a furnace 38 of the second fluidized bed reactor 14. The furnace 38 of the second fluidized bed reactor comprises a bottom grid 40, which is connected via a fluidizing gas channel 42 and a valve 44 to the air channel 20 of the first fluidized bed reactor 12, and also via a valve 46 to a steam supply (not shown in FIG. 1). The furnace 38 of the second fluidized bed reactor 14 is also equipped with means for feeding solid fuel 48, and a particle separator 50 connected to the upper portion of the furnace of the second fluidized bed reactor 14. Walls enclosing the furnace 38 of the second fluidized bed reactor 14 are preferably covered with a relatively thick layer of refractory.

[0033] The lower portion of the particle separator 50 of the second fluidized bed reactor 14 is connected via a second particle transfer channel 52 to the furnace 16 of the first fluidized bed reactor 12 for returning particles separated in the particle separator of the second fluidized bed reactor to the first fluidized bed reactor. The particle separator 50 of the second fluidized bed reactor 14 may also be equipped with a return duct (not shown in FIG. 1), similarly as the return duct 28 of the particle separator 26 of the first fluidized bed reactor 12, for returning a portion of the separated particles to the lower portion of the furnace 38 of the second fluidized bed furnace 14. The particle transfer channel 52 and/or the return duct (not shown in FIG. 1) of the particle separator 50 of the second fluidized bed reactor 14 is advantageously equipped with means for controlling (not shown in FIG. 1) the shares of particles that are returned to the furnace 38 of the second fluidized bed reactor 14 and particles that are transferred to the furnace 16 of the first fluidized bed reactor 12, respectively. The second particle transfer channel 52, and/or the return duct (not shown in FIG. 1) of the particle separator 50 of the second fluidized bed reactor 14, may thus be equipped with a particle flow controller (not shown in FIG. 1), such as a fluidized bed, mechanical or pneumatic flow controller, for conveying separated particles at a controlled rate into the furnace/s. The second particle transfer channel 52, as well as the return channel (not shown in FIG. 1) of the particle separator 50 of the second fluidized bed reactor 14, advantageously also comprises a gas-lock (not shown in FIG. 1), preferably, as a part of the above described particle flow controller, to prevent gas flow from the furnace/s to the particle separator 50.

[0034] The upper portion of the particle separator 50 is connected to an exhaust gas channel 54 that is, via a valve 56, connected to a product gas channel 58 and via another valve 60 to the flue gas channel 30. The product gas channel generally comprises product gas cooling surfaces, such as heat transfer surfaces 62 as shown in FIG. 1, in the exhaust gas channel 54, and leads to conventional means for utilizing the product gas, such as devices for synthesis process treatment (not shown in FIG. 1).

[0035] The fluidized bed reactor system is advantageously operated in one of first and second operating modes, wherein the furnace 16 of the first fluidized bed reactor 12 is always in operation operated as a circulating fluidized bed boiler that is in a relatively small flow solids circulation with the furnace 38 of the second fluidized bed reactor 14, which solids flow circulation is controlled by the particle flow controller 36 in the particle transfer channel 34.

[0036] In the first operation mode, the valve 44 between the air channel 20 and the fluidizing gas channel 42 is open and both first and second fluidized bed reactors, 12, 14, are fluidized with air. Thereby, both fluidized bed reactors operate as a combustion reactor. The valve 46 is closed so that steam is not supplied from the steam supply into the fluidizing gas channel 42. In the first operation mode, also, the valve 60 between the exhaust gas channel 54 and flue gas channel 30 is open, whereby flue gas from combustion processes in the both fluidized bed reactors 12, 14 are conveyed to the flue gas channel 30. Respectively, the valve 56 to the product gas channel 58 is closed. At least a portion of particles separated in the particle separator 50 of the second fluidized bed reactor 14 are returned via the second particle transfer channel 52 to the furnace 16 of the first fluidized bed reactor 12.

[0037] The purpose of the first operating mode is to generate steam with a good efficiency and to keep the furnace 38 of the second fluidized bed reactor 14 in a temperature that enables its rapid switching to a pyrolyzing or gasification mode. The desired temperature is achieved by controlling the rate of transferring hot solids from the furnace 16 of the first fluidized bed reactor by the particle flow controller 36 and/or by feeding auxiliary fuel to the furnace 38 of the second fluidized bed reactor 14 by the solid fuel feeder 48. When rapid ramp down of the combustion process in the reactor system 10 is needed, the rate of transferring hot solids from the first fluidized bed reactor 12 to the second fluidized bed reactor 14 is increased and the second fluidized bed reactor is shifted to a gasifier or pyrolyzing mode by closing the valve 44 and opening the valve 46 to fluidize the fluidized bed in the furnace 38 with steam. In the gasifying or pyrolyzing mode of the second fluidizing reactor 38, the valve 60 to the flue gas channel 30 is closed and the valve 56 to the product gas channel 58 is opened so as to convey the generated product gas to other use. Auxiliary fuel is generally in the gasifying or pyrolyzing mode fed via the fuel feeder 48 to the furnace 38 of the second fluidized bed reactor 14.

[0038] Above is described an embodiment in which the fluidizing gas of the first fluidized bed reactor is air, but, in other embodiments of the present invention, the fluidizing gas can alternatively be other oxygen containing gas, such as a mixture of oxygen and CO2 rich gas. In the above described embodiment, the first fluidized bed reactor is a boiler, but, in other embodiments of the present invention, the first fluidized bed reactor can alternatively be another fluidized bed reactor, such as an incinerator generating steam or hot water to another use. Above is described an embodiment in which the second fluidized bed reactor is in the second operating mode fluidized with steam, but, in other embodiments of the present invention, the second fluidized bed reactor can, in the second operating mode, be fluidized with another gasifying or pyrolyzing inducing gas, such as CO.sub.2 or non-oxygen containing inert gas, or with a mixture of such gases.

[0039] The diagram of FIG. 2 schematically illustrates a fluidized bed reactor system 10′ according to another preferred embodiment of the present invention. The embodiment shown in FIG. 2 differs from that shown in FIG. 1 mainly in that product gas produced in the second operation mode in the second fluidized bed reactor 14 is conveyed via the channel 58 to a product gas storage 64. The product gas storage is advantageously associated with means (not shown in FIG. 2) for compressing the product gas. The product gas storage 64 is according to the embodiment shown in FIG. 2 connected via a channel 66 to a burner 68 arranged to produce and to convey hot gas to a superheater chamber 70 arranged in the flue gas channel 30 of the first fluidized bed reactor 12. While the reactor system and the method of controlling a reactor system described in connection with FIG. 1 are directed to enable especially rapid ramp down of a combustion process, by switching from the first operation mode to the second operation mode, the arrangement shown in FIG. 2 is directed to also enable rapid ramp up of the combustion process.

[0040] Rapid ramp up of the combustion process is advantageously realized by switching from the second operation mode to the first operation mode and by simultaneously firing by the burner 68 product gas collected during the second operation mode in the product gas storage 64 so as to rapidly increase superheating of steam in the superheater chamber 70. Alternatively, product gas collected to the product gas chamber 64 can be fired at any time, during either a first or a second operation mode, when rapid increase of the heat output of the boiler is desired.

[0041] The diagram of FIG. 3 schematically illustrates a fluidized bed reactor system 10″ according to still another preferred embodiment of the present invention. The embodiment shown in FIG. 3 differs from that shown in FIG. 2 in that product gas collected in the product gas storage 64 is not conveyed to a superheater chamber 70, but via a channel 72 with a valve 74 to the furnace 16 of the first fluidized bed reactor 12. Thereby, product gas collected in the product gas storage 64 can be used, when desired, to rapidly increase temperature in the furnace 16 of the first fluidized bed actor 12 to increase the heat output of the first fluidized bed reactor.

[0042] Alternatively, in FIGS. 2 and 3, the product gas channel 58 can be divided in two branches, one directed into the product gas storage 64 and the other into the product gas upgrading such as leading to conventional means for utilizing the product gas, such as devices for synthesis process treatment (not shown in FIGS. 2 and 3). It is also possible to combine any of the embodiments described above into one, as technically feasible.

[0043] While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features and several other applications included within the scope of the invention as defined in the appended claims.