WO2011101540A1

WO2011101540A1 - A fluidized bed gasification system and a method of gasifying fuel in a fluidized bed gasification system

Info

Publication number: WO2011101540A1
Application number: PCT/FI2011/050138
Authority: WO
Inventors: Arto Ahvenainen; Eero Berg
Original assignee: Foster Wheeler Energia Oy
Current assignee: Amec Foster Wheeler Energia Oy
Priority date: 2010-02-17
Filing date: 2011-02-15
Publication date: 2011-08-25
Anticipated expiration: 2012-08-17
Also published as: FI20105154A7; FI20105154L; FI20105154A0

Abstract

The invention relates to a fluidized bed gasification system and a method of gasifying fuel in a fluidized bed gasification system, said gasification system comprising a fluidized bed gasification reactor and a fluidized bed combustion reactor, and in which a first fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor is a circulating fluidized bed reactor, a first portion of there circulating means of which is arranged gas- tight inside the reaction chamber of a second fluidized bed reactor for gasifying fuel to be gasified by transferring heat from the particles of the fluidized bed combustion reactor through the outer wall of the first portion to the particles of the fluidized bed gasification reactor.

Description

A FLUIDIZED BED GASIFICATION SYSTEM AND A METHOD OF GASIFYING FUEL IN A FLUIDIZED BED GASIFICATION SYSTEM

[0001 ] The present invention relates to a fluidized bed gasification sys- tern and a method of gasifying fuel in a fluidized bed gasification system according to the preambles of the independent patent claims.

[0002] Thus, the invention especially relates to a fluidized bed gasification system comprising a fluidized bed gasification reactor, provided with a reaction chamber, means for supplying fuel to be gasified to the reaction chamber, means for fluidizing a particle bed being formed in the reaction chamber, means for gasifying the fuel to be gasified and a gas channel connected to the upper portion of the reaction chamber for removing gas produced in the gasification and particles entrained therewith from the reaction chamber; and a fluidized bed combustion reactor, provided with a reaction chamber, means for supplying fuel to be combusted to the reaction chamber, means for fluidizing a particle bed being formed in the reaction chamber, means for combusting the fuel to be combusted, and a gas channel connected to the upper portion of the reaction chamber for removing gas gener- ated in the combustion and particles entrained therewith from the reaction chamber; and in which a first fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor is a circulating fluidized bed reactor, comprising recirculating means for particles, which have a particle separator arranged in the gas channel connected to the upper portion of the reaction chamber of the first fluidized bed reactor for separating particles entrained with the gas from the gas and a return channel for recirculating the separated particles from the particle separator back to the lower portion of the reaction chamber of the first fluidized bed reactor. [0003] On the other hand, the invention especially relates to a method of gasifying fuel in a fluidized bed gasification system, comprising a fluidized bed gasification reactor provided with a reaction chamber and a gas channel connected to the upper portion of the reaction chamber for removing gas produced in the gasification and particles entrained therewith from the reaction chamber, and a fluidized bed combustion reactor provided with a reaction chamber and a gas channel connected to the upper portion of the reaction chamber for removing gas generated in the combustion and particles entrained therewith from the reaction chamber, in which a first fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor is a circulating fluidized bed reactor, comprising recirculating means for particles, which have a particle separator arranged in the gas channel connected to the upper portion of the reaction chamber of the first fluidized bed reactor for separating particles entrained with the gas from the gas, and a return channel for recirculating separated particles from the particle separator to the lower portion of the reaction chamber of the first fluidized bed reactor, in which method fuel to be gasified is supplied to the reaction chamber of the fluidized bed gasification reactor, the fuel to be gasified is fluidized and gasified in a particle bed formed in the reaction chamber and gas produced in the gasification and particles entrained with the gas are removed from the reaction chamber to the gas channel connected to the upper portion of the reaction chamber; fuel to be combusted is supplied to the reaction chamber of the fluidized bed combustion reactor, fuel to be combusted is fluidized and combusted in a particle bed formed in the reaction chamber and gas generated in the combustion and particles entrained with the gas are removed to the gas channel connected to the upper portion of the reaction chamber. [0004] The gasification of fuel is an endothermic process, which requires external thermal energy. A conventional method to produce the required thermal energy is to combust a portion of the fuel to be gasified in a gasification reactor. A disadvantage of this method is that the amount of the product gas gained from a certain amount of fuel decreases and carbon dio- xide generating in the combustion is mixed with the product gas, which degrades the quality of the product gas and diminishes the heat value thereof. Another conventional solution is to combust suitable fuel in a separate combustion reactor and transfer hot ash from the combustion reactor to the gasifi- cation reactor. The transfer of hot ash from the combustion reactor to the reaction chamber of the gasification reactor requires, however, rather complicated equipment, especially if the reactors operate at different pressures. The manufacture of the apparatus and the maintenance thereof can, thus, be ex- pensive.

[0005] US Patent No. 4,459,201 discloses a pyrolyzer, in which residual carbon generated in a pyrolyzer is combusted in a separate combustion reactor and in a combustion space situated in the reaction chamber of the py- rolyzer, said space being formed either of a multipart combustion zone or of multiple parallel combustion tubes. Thereby, thermal energy is transferred through the walls of the multipart combustion space situated in the reaction chamber of the pyrolyzer to the pyrolyzer. This apparatus is rather complicated, difficult to control and liable to disturbances. Moreover, the mainten- ance of the multipart combustion space arranged in the reaction chamber of the pyrolyzer, and possible repairing operations thereof, are difficult to perform.

[0006] EP patent no. 1 377 650 B1 discloses a solution, in which the reaction chamber of a pyrolyzer is arranged in the reaction chamber of a flui- dized bed reactor combusting residual carbon of the pyrolyzer. In this arrangement, the heat exchange surface between the reaction chamber of the pyrolyzer and the reaction chamber of the combustion reactor is relatively small, whereby the heat exchange efficiency may in some cases be inade- quate. On the other hand, in solutions, in which the heat exchange surface is directly between the combustion reactor and the gasification reactor, too hot or poorly fluidized areas may be generated close to the heat exchange surfaces. There the temperature of the gasification reactor may locally rise too high, which can decrease the product gas yield. Maintenance of the reaction chambers situated one within the other and possible repairing operations thereof may also be difficult to perform. [0007] An object of the present invention is to provide an efficient fluidized bed gasification system and a method of gasifying fuel in a fluidized bed gasification system, in which problems of the prior art have been minimized.

[0008] In order to diminish above-mentioned problems of the prior art, a fluidized bed gasification system in accordance with the characterizing part of the independent apparatus claim is provided. A fluidized bed gasification system is thereby provided, in which the means for gasifying the fuel to be gasified comprise a first portion of the recirculating means of the first fluidized bed reactor, which first portion is arranged gas-tight inside the reaction chamber of a second fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor, which second fluidized bed reactor is not the above-mentioned first fluidized bed reactor, for gasifying the fuel to be gasified by transferring heat from the particles of the fluidized bed combustion reactor through an outer wall of the first portion to the particles of the fluidized bed gasification reactor.

[0009] According to another aspect of the present invention, a method in accordance with the characterizing part of the independent method claim is provided in order to diminish above-mentioned problems of the prior art. Thus, a method of gasifying fuel is provided, in which the fuel to be gasified is gasified by transferring heat from the particles of the fluidized bed combustion reactor to the particles of the fluidized bed gasification reactor through the outer wall of a first portion of the recirculating means of the first fluidized bed reactor, which first portion is arranged gas-tight inside the reaction chamber of a second fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor, which second fluidized bed reactor is not the above-mentioned first fluidized bed reactor.

[0010] In the arrangement in accordance with the present invention, heat is transferred thus from the hot particulate bed material of the fluidized bed combustion reactor, which is hereafter also referred to as combustion reactor, by conduction to the particulate bed material of the fluidized bed gasi- fication reactor, which is hereafter also referred to as gasification reactor, through the outer wall limiting the first portion of the recirculating means of the first fluidized bed reactor arranged inside the reaction chamber of the second fluidized bed reactor. In practice, there is only a relatively thin metal wall in said place between the bed materials of the reactors, so the heat transfer is efficient. Since the temperatures of the reaction chamber and the recirculating means of the fluidized bed reactor can be well controlled, the temperatures in different sides of the metal heat exchange surface can advantageously be continuously adjusted to be stable and such that the desired heat exchange efficiency can be obtained, but the temperature of the heat exchange surface will not rise too high in view of the gasification of the fuel.

[001 1 ] The gas-tight arrangement of the first portion of the recirculating means of the first fluidized bed reactor inside the reaction chamber of the second fluidized bed reactor refers to that no direct transfer of gas or any oth- er substance takes place between the first portion of the recirculating means of the first fluidized bed reactor and the reaction chamber of the second fluidized bed reactor. Merely thermal energy is transferred by conduction through the wall limiting the first portion of the recirculating means, in other words through the outer wall thereof. Thus, the processes of the first fluidized bed reactor and the second fluidized bed reactor do not affect each other otherwise, and the processes can be, for example, at different pressures.

[0012] Especially the feature that the heat exchange surface is the outer wall limiting the first portion of the recirculating means of the first fluidized bed reactor results in that the heat exchange efficiency can be advantageous- ly controlled by adjusting the amount of the material recirculating in the recirculating means of the first fluidized bed reactor, in other words the amount of the particles separating in the particle separator of the fluidized bed reactor. This may be advantageously carried out, for example, by varying the total amount of the bed material in the first fluidized bed reactor. [0013] According to a first preferred embodiment of the present invention, the first fluidized bed reactor is a combustion reactor and the second fluidized bed reactor is a gasification reactor. Thus, the combustion reactor is a circulating fluidized bed reactor. Thereby, the gasification reactor may be either a circulating fluidized bed reactor or a fluidized bed reactor of another kind , for example, a bubbling fluidized bed reactor. According to this embodiment, a portion of the recirculating means of the fluidized bed combustion reactor, a so called first portion of the recirculating means, is arranged gas- tight inside the reaction chamber of the gasification reactor. Thus, external thermal energy is introduced to the gasification process indirectly from the relatively hot particles flowing in the recirculating means of the combustion reactor through the wall between the first portion of the recirculating means and the reaction chamber of the gasification reactor.

[0014] According to a second preferred embodiment of the present in- vention, the first fluidized bed reactor is a gasification reactor and the second fluidized bed reactor is a combustion reactor. Thus, the gasification reactor is a circulating fluidized bed reactor. Thereby, the combustion reactor can be either a circulating fluidized bed reactor or a fluidized bed reactor of another kind, for example, a bubbling fluidized bed reactor. According to this embodi- ment, the first portion of the recirculating means of the fluidized bed gasification reactor is arranged gas-tight inside the reaction chamber of the combustion reactor. Thus, external thermal energy required for the process is introduced to the gasification process indirectly from the hot particles of the reaction chamber of the combustion reactor to the particles recirculating in the re- circulating means of the gasification reactor through the wall between the first portion of the recirculating means and the reaction chamber of the combustion reactor.

[0015] Said first portion of the recirculating means of the first fluidized bed reactor preferably comprises at least a portion of the return channel of the recirculating means. Said portion of the return channel can be either a conventional, simple tube channel, but according to a preferred embodiment of the invention, said portion of the return channel is at least partially divided into two or more parallel channel branches. Such a return channel, which is divided into channel branches, provides a larger heat exchange surface than a simple return channel and, thus, intensifies the heat exchange between the particles flowing in the return channel and the particles in the reaction chamber of the second fluidized bed reactor.

[0016] According to a preferred embodiment of the present invention, it is possible to alternatively or additionally connect to the portion of the return channel comprising the first portion of the recirculating means also other means, with which heat exchange between the particles of the first fluidized bed reactor flowing in the return channel and the particles in reaction chamber of the second fluidized bed reactor is improved. Such means can be, for example, guiding means or chambers arranged in the return channel or on the walls thereof, by means of which heat exchange between the particles flowing in the return channel and the walls of the return channel, is improved.

[0017] The temperature of the fluidized bed combustion reactor is preferably adjusted such that a required amount of heat for maintaining the gasification process is transferred from the hot particles of the combustion reactor to the gasification reactor, but the temperature of the bed of the gasification reactor does not rise too high at any point. Since in the arrangements in accordance with the present invention, the heat exchange always takes place between the particles of the fluidized bed and the particles flowing in the recirculating means, the heat exchange is efficient and accurately adjustable. [0018] The particle separator of the recirculating means of the first fluidized bed reactor is preferably a cyclone, but it can in some cases also be of other type, such as a multicyclone or an impact separator. According to a preferred embodiment of the invention, the first portion of the recirculating means of the first fluidized bed reactor, in other words, the portion of the recirculating means of the fluidized bed reactor remaining inside the reaction chamber of the second fluidized bed reactor, comprises a particle separator or at least a portion of the particle separator. The particle separator remaining either completely or partially inside the reaction chamber of the second fluidized bed reactor is advantageously a cyclone, the geometry of which can be conven- tional. The particle separator is, however, preferably a cyclone, the height of which is greater than that of a conventional separator in order to improve the heat exchange, more preferably at least 30%, and most preferably at least 50% of the height of the reaction chamber of the second fluidized bed reactor.

[0019] The shape of the reaction chamber of the second fluidized bed reactor comprising a particle separator of the recirculating means of the first fluidized bed reactor or a portion thereof may be conventional, in other words the cross-section of the reaction chamber may be constant in the majority of the reaction chamber. According to a preferred embodiment of the invention, the cross-sectional area of the upper portion of the reaction chamber of the second fluidized bed reactor is, however, greater than the cross-sectional area of the rest of the reaction chamber. Thereby, the particle separator of the recirculating means of the first fluidized bed reactor is preferably arranged in the upper portion of the above-mentioned reaction chamber of the second fluidized bed reactor, in which upper portion the horizontal cross-sectional area of the reaction chamber is greater than that in the portion of the reaction chamber below the particle separator.

[0020] According to a preferred embodiment of the present invention also the second fluidized bed reactor is a circulating fluidized bed reactor, comprising recirculating means for particles provided with a particle separator arranged in the gas channel connecting to the upper portion of the reaction chamber of the second fluidized bed reactor for separating particles entrained with the gas and a return channel for returning separated particles to the lower portion of the reaction chamber of the second fluidized bed reactor, and a first portion of the recirculating means of the second fluidized bed reactor is arranged gas-tight inside the reaction chamber of the first fluidized bed reac- tor. A fluidized bed gasification system in accordance with this embodiment thus comprises two circulating fluidized bed reactors, of which the first portion of the recirculating means of the first fluidized bed reactor is arranged inside the reaction chamber of the second fluidized bed reactor and the first portion of the recirculating means of the second fluidized bed reactor is arranged inside the reaction chamber of the first fluidized bed reactor. Thus, heat transfers from the hot particles of the combustion reactor to the particles of the gasification reactor especially efficiently, in two different stages.

[0021 ] In this embodiment, the first portion of the recirculating means of both fluidized bed reactors can comprise a portion of the return channel and/or at least a portion of the particle separator. Alternatively, the first portions of the recirculating means of the fluidized bed reactors can vary, for example, in such a way that the first portion of the recirculating means of the first fluidized bed reactor comprises a portion of the return channel and the first portion of the recirculating means of the second fluidized bed reactor comprises at least a portion of the particle separator.

[0022] The portion of the return channel of either of the reactors being inside the reaction chamber of the other reactor can be a simple tube structure or it may be formed of parallel channel branches in the above-described manner. The particle separator of either of the reactors being at least partially inside the reaction chamber of the other reactor can be of conventional shape, or it can be clearly higher than the conventional in the above- described manner. The cross-sectional area of the reaction chamber of either of the fluidized bed reactors containing at least a portion of the particle sepa- rator of the other fluidized bed reactor can be mainly constant as in the conventional cases, or it can be greater than the cross-sectional area of the rest of the reaction chamber in the above-described manner.

[0023] The fuel used in the fluidized bed combustion reactor can be any suitable solid fuel. The fuel used in a combustion reactor in accordance with a preferred embodiment is ash containing unburned carbon removed from the gasification reactor. Especially when the gasification reactor is a py- rolysis reactor, it is possible to advantageously use residual carbon produced by the pyrolysis reactor as fuel for the combustion reactor. According to a preferred embodiment, the gasification reactor relates to a Fischer-Tropsch (FT) process, whereby methane and other tail gases obtained as a side product from the FT process can preferably be used as fuels of the combustion reactor. The operational temperature of the fluidized bed combustion reactor is preferably about 700 - 800 °C.

[0024] The fuel to be gasified in the fluidized bed gasification reactor is preferably solid carbonaceous fuel, for example, biofuel. The operational temperature of the fluidized bed gasification reactor is preferably 400-500 °C, whereby the fuel to be gasified pyrolyzes due to the heat and is ground to fine residual carbon (coke dust). The gasification process can be intensified, if necessary, for example, by means of oxygen or oxygen/steam mixture used as fluidizing gas. Especially when the gasification reactor is a pyrolysis reactor, it is also possible to use purified product gas at an appropriate pressure or, for example, tail gas removed from the FT process as a fluidizing gas for the reactor.

[0025] Since the heat exchange in the process in accordance with the present invention takes place by means of fluidizing sand, an efficient heat exchange from the particles of the combustion reactor to the gasification reactor is provided by means of the process. Since the internal temperature differences of the circulating fluidized bed reactors are small and accurately adjustable, the temperatures of the heat exchange surfaces positioned in the recirculating means according to the invention can be well controlled. Thus, by means of the invention, it is possible to advantageously obtain a relatively high gasification temperature, which is optimal in view of the gasification process.

[0026] For example, in applications, in which the further processing or use of the product gas of the gasification reactor takes place at a pressure which is higher than the atmospheric pressure, it is advantageous that also the gasification process is pressurized. When using an arrangement in accordance with the present invention, the combustion process and the gasification process can be separated as for material flows, and therefore the gasification process can preferably be carried out pressurized.

[0027] The above described gasification process, in which heat is transferred to the particles of the fluidized bed gasification reactor heat from the hot particles of the fluidized bed combustion reactor, can be completed by supplying heat to the gasification reactor in some other known manner, for example, by means of superheated steam generated by the combustion reactor or by some other separately generated hot medium.

[0028] The invention is described below by way of example, with reference to the accompanying drawings, in which schematically illustrates a fluidized bed gasification system in accordance with a preferred embodiment of the present invention;

Fig. 2 schematically illustrates a fluidized bed gasification system in accordance with a second preferred embodiment of the present invention;

Fig. 3 schematically illustrates a fluidized bed gasification system in accordance with a third preferred embodiment of the present invention;

Fig. 4 schematically illustrates a fluidized bed gasification system in accordance with a fourth preferred embodiment of the present invention;

Fig. 5 schematically illustrates a fluidized bed gasification system in accordance with a fifth preferred embodiment of the present invention; [0029] Fig. 1 shows a fluidized bed gasification system in accordance with a preferred embodiment of the present invention, which comprises a fluidized bed gasification reactor 10 and a fluidized bed combustion reactor 30, which both are circulating fluidized bed reactors. Thus, a gas channel 14 con- nected to the upper portion of a reaction chamber 12 of the gasification reactor 10, through which gas channel the product gas and particles entrained therewith are removed from the reaction chamber 12, is provided with a particle separator 16 separating particles from the product gas, which particles are then recirculated along a return channel 18 back to the lower portion of the reaction chamber 12. The lower portion of the reaction chamber comprises means for fluidizing the particle bed forming to the reaction chamber 12, in other words a grid 20, an inlet means 22 for fluidizing gas and a wind box 24. The reaction chamber also comprises means 26 for introducing the fuel to be gasified, for example, a feed channel or feed screw for fuel, and means 28 for removing the bottom ash generated in the gasification, for example, a bottom ash screw.

[0030] Respectively, a gas channel 34 connected to the upper portion of a reaction chamber 32 of the fluidized bed combustion reactor 30, through which gas channel the flue gas generated in the combustion and particles en- trained therewith are removed from the reaction chamber 32, is provided with a particle separator 36 separating particles from the flue gas, which particles are then recirculated along a return channel 38 back to the lower portion of the reaction chamber 32. The lower portion of the reaction chamber comprises means for fluidizing the particle bed forming to the reaction chamber 32, in other words a grid 40, an inlet means 42 for fluidizing gas and a wind box 44. The reaction chamber comprises also means 46 for introducing the fuel to be gasified, for example, a feed channel or feed screw for fuel, and means 48 for removing the bottom ash generated in the combustion, for example, a bottom ash screw. [0031 ] According to the present invention, a portion of the return channel 18 of the gasification reactor 10, a so called first portion 50 is arranged inside the reaction chamber of the combustion reactor 30. Thus, when the particles separated by the particle separator 16 of the gasification reactor are recirculated along the return channel 18 to the reaction chamber 12, thermal energy is transferred to the separated particles through a wall 52 of the first portion of the return channel from the hot particles in the reaction chamber 32 of the combustion reactor 30.

[0032] In order to obtain a sufficiently efficient heat transfer from the particles of the combustion reactor to the gasification reactor, in the arrangement in accordance with Fig. 1 , the height of the first portion 50 of the return channel 18 of the gasification reactor 10 is preferably a considerable portion of the height of the reaction chamber 32, preferably at least 50%, more preferably at least 70% of the height of the reaction chamber 32. It is possible to control the heat exchange efficiency in the first portion 50 of the return channel, and, thus, the temperature of the gasification reactor by adjusting the amount of the particles to be separated in the particle separator 16 of the gasification reactor. This can be performed, for example, by changing the volume or particle size of the particle bed formed in the gasification reactor. Another possibility to control the temperature of the gasification reactor is to vary the temperature in the combustion reactor or the volume or particle size or fluidization velocity of the particle bed in the combustion reactor.

[0033] In practice, the fluidized bed gasification systems in accordance with the present invention also comprise many other portions and details known per se, which, however, are not important in view of the present invention, and have therefore not been shown in Figs. 1 -6. In Figs. 2-6, same reference numbers as in Fig. 1 have been used with elements that are similar to the corresponding elements in Fig. 1 .

[0034] The combustion reactor 30 in the arrangement shown in Fig. 1 is a circulating fluidized bed reactor, but it could also be other type of fluidized bed reactor, for example, a bubbling bed reactor. In the arrangement of Fig. 1 , a portion of the return channel 18 of the gasification reactor 10 is arranged inside the reaction chamber 32 of the combustions reactor 30. A second alternative arrangement, which comes into question when the combustion reac- tor is a circulating fluidized bed reactor, is that a portion of the return channel of the combustion reactor is arranged inside the reaction chamber of the gasification reactor. Naturally, the gasification reactor may then be also of other fluidized bed reactor type, such as a bubbling bed reactor.

[0035] A fluidized bed gasification system in accordance with a second preferred embodiment of the present invention shown in Fig. 2 differs from the gasification system shown in Fig. 1 especially in that the portion of the return channel 18 of the gasification reactor 10 in the reaction chamber 32 of the combustion reactor 30 is divided into a number of parallel channel branches 54. In order to divide the particles separated in the particle separator evenly in the channel branches 54, the particle separator of Fig. 2 is preferably a multicyclone 56. The channel branches 52 preferably continue through the wind box 44 of the combustion reactor to a bottom funnel 58, from which they are returned along a common final portion of the return channel 18 to the gasification reactor 10. [0036] When the return channel of the gasification reactor 10 is divided into channel branches 54 the total surface area of the wall separating the return channel from the reaction chamber 32 of the combustion reactor 30 is greater than in case of Fig. 1 . Thus, division of the return channel into parallel channel branches 54 intensifies the heat transfer from the hot particles of the bed of the combustion reactor 30 to the gasification reactor 10.

[0037] In the arrangement illustrated in Fig. 2, the combustion reactor 30 is a circulating fluidized bed reactor, but, as in case of Fig. 1 , it could also be of other fluidized bed reactor type, such as a bubbling bed reactor. The channel branches 54 of the gasification reactor 10 in the embodiment illu- strated in Fig. 2 are arranged inside the reaction chamber of the combustion reactor 30. An alternative that comes into question when the combustion reactor is a circulating fluidized bed reactor is that the return channel of the combustion reactor is divided into parallel channel branches, which are arranged inside the reaction chamber of the gasification reactor. Naturally, the gasification reactor can then be also of other fluidized bed reactor type, such as bubbling bed reactor.

[0038] A fluidized bed gasification system in accordance with a third preferred embodiment of the present invention illustrated in Fig. 3 differs from the gasification system illustrated in Fig .1 in that the particle separator 36 of the combustion reactor 30 and a portion of a return channel 38 are arranged inside the reaction chamber 12 of the gasification reactor 10. Thus, in the arrangement disclosed in Fig. 3, thermal energy is transferred from the particles of the combustion reactor to the particles of the gasification reactor both in a portion of the return channel 18 of the gasification reactor 10 and in the par- tide separator 36 of the combustion reactor 30 and in a portion of the return channel 38 of the combustion reactor.

[0039] The particle separator of a conventional fluidized bed reactor is relatively low compared, for example, with the height of the reaction chamber of the same reactor, typically approximately 20% of the height of the reaction chamber. The circumference of the outer wall of the particle separator is, however, clearly larger than the circumference of the return channel, and therefore the use of the outer surface of the particle separator 36 of the combustion reactor 30 as a heat exchange surface significantly increases the heat exchange surface area between the combustion reactor and the gasification reactor. Thus, the heat exchange efficiency of the arrangement disclosed in Fig 3 is clearly greater than, for example, the heat exchange efficiency of the embodiment disclosed in Fig.1 .

[0040] In the arrangement of Fig. 3, the efficiency of the heat exchange from the particles of the combustion reactor 30 to the particles of the gasifica- tion reactor 10, and, thus, the temperature of the gasification reactor may be controlled by adjusting the amount of the particles separated in the particle separator of either of the fluidized bed reactors or by adjusting the temperature of the combustion reactor, in the same manner as shown in connection with Fig. 1 . [0041 ] The cross-sectional area of the return channel arranged inside the reaction chamber of the fluidized bed reactor is generally clearly smaller than the cross-sectional area of the reaction chamber, and therefore the return channel arranged inside the reaction chamber does not considerably disturb the process taking place in the reaction chamber. The cross-sectional area of the particle separator 36 of the combustion reactor 30 arranged in the reaction chamber of the gasification reactor 10 instead can be a considerable portion of the cross-sectional area of the reaction chamber 12. In the following, the portion of the reaction chamber 12, in which the particle separator is positioned, is called an upper portion 60 of the reaction chamber. To avoid a considerable disturbance of the reaction chamber 1 2 to the gasification process, the cross-sectional area of the upper portion 60 of the reaction chamber is preferably greater than the cross-sectional area of the portion of the reaction chamber below the upper portion.

[0042] In Fig. 3, the portion of the return channel 18 of the gasification reactor 10 inside the reaction chamber 32 of the combustion reactor 30 comprises guiding plates 62, by means of which the heat transfer is intensified from the particles in the reaction chamber 32 of the combustion reactor 30 to the particles flowing in the return channel 18. Instead of guiding plates it is also possible to use guiding means of other types, or chambers or other suit- able constructions for intensifying the heat transfer. Such constructions may be arranged either inside the return channel 18 or outside thereof, in the reaction chamber 32 of the combustion reactor 30.

[0043] A fluidized bed gasification system in accordance with a fourth preferred embodiment of the present invention disclosed in Fig. 4 differs from the gasification system of Fig. 3 in that the height of the particle separator 36 of the combustion reactor 30 arranged inside the reaction chamber 12 of the gasification reactor 10 is clearly greater than that of a conventional separator. The height of the particle separator 36 is preferably at least 30%, most preferably at least 50%, of the height of the reaction chamber 12. In the embodi- ment of Fig. 4, the surface area of the wall between reaction chamber 12 of the gasification reactor 10 and the particle separator 36 of the combustion reactor 30 arranged inside the reaction chamber 12 of the gasification reactor 10 is thus greater than that in the arrangement in accordance with Fig.3. This results in that the heat transfer efficiency from the particles of the combustion reactor 32 to the particles of the gasification reactor 10 is greater than in the arrangement in accordance with Fig. 3.

[0044] As the particle separator 36 of the combustion reactor 30 in the embodiment in accordance with Fig. 4 is higher than that in the arrangement in accordance with Fig. 3, the upper portion 60 of the reaction chamber 12, the cross-sectional area of which is greater than the cross-sectional area of the rest of the reaction chamber, is preferably higher than that in the arrangement in accordance with Fig. 3. Naturally, in the arrangements in accordance with Figs. 3 and 4, it is possible to intensify the heat exchange further in the manner shown in Fig. 2 by dividing the portions of the return channel inside the reaction chamber either entirely or partially into parallel channel branches.

[0045] In the embodiments disclosed in Figs. 3 and 4, the particle separator 16 of the gasification reactor 10 is arranged outside the reaction chamber 32 of the combustion reactor 30, but the return channel 18 of the gasification reactor is mainly inside the reaction chamber 32 of the combustion reactor 30 and the particle separator 36 of the combustion reactor 30 is entirely inside the reaction chamber 12 of the gasification reactor 10. Naturally, corresponding arrangements can also be reversely realized, in other words in such a way that the particle separator of the combustion reactor is ar- ranged outside the reaction chamber of the gasification reactor, but the return channel 38 of the combustion reactor 30 is mainly arranged inside the reaction chamber 12 of the gasification reactor and the particle separator 16 of the gasification reactor 10 is arranged entirely inside the combustion chamber 32 of the combustion reactor 30. [0046] The fluidized bed gasification system in accordance with a fifth preferred embodiment of the present invention disclosed in Fig. 5 differs from the gasification systems of Figs. 3 and 4 in that the particle separator 16 of the gasification reactor 10 is partially arranged inside the reaction chamber 32 of the combustion reactor 30. Moreover, the height of the particle separator 16 of the gasification reactor 10 arranged inside the reaction chamber 32 of the combustion reactor 30 is clearly greater than that of a conventional separator. The height of the particle separator 16 is preferably at least 30%, more preferably at least 50%, of the height of the reaction chamber 32. Thus, the heat exchange efficiency from the particles of the combustion reactor 30 to the gasification reactor 10 is still higher than that in the arrangements in accordance with Figs. 3 and 4.

[0047] In the embodiment shown in Fig. 5, the particle separator 16 of the gasification reactor 10 is mainly arranged inside the reaction chamber 32 of the combustion reactor 30 and the particle separator 36 of the combustion reactor 30 is arranged entirely inside the combustion chamber 12 of the gasification reactor 10. Naturally, the corresponding arrangement can also be realized reversely, in other words in such a way that the particle separator 36 of the combustion reactor 30 is arranged mainly inside the reaction chamber 12 of the gasification reactor 10 and the particle separator 16 of the gasification reactor 10 is arranged entirely inside the combustion chamber 32 of the combustion reactor 30.

[0048] An arrangement closely related to the present invention, which, however, to a certain extent deviates from the above described arrangements, is to arrange the first portion of the recirculating means of the gasifica- tion reactor, which is a circulating fluidized bed reactor, inside the flue gas channel of the combustion reactor, especially to the first portion of a flue gas channel downstream of the particle separator, a so called back pass. Thus, in this arrangement, heat is transferred to the recirculating particulate material of the gasification reactor from the flue gases of the combustion reactor through a wall of the first portion of the recirculating means. Since the temperature of the flue gases in the back pass of the combustion reactor can be controlled relatively accurately, it is possible, by means of this arrangement, to adjust the amount of heat energy to be transferred to the gasification reactor.

[0049] While the invention has been described herein by way of exam- pies 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..

Claims

1 . A fluidized bed gasification system, comprising

a fluidized bed gasification reactor, provided with a reaction chamber; means for supplying fuel to be gasified to the reaction chamber; means for fluidizing a particle bed being formed in the reaction chamber; means for gasifying the fuel to be gasified; and a gas channel connected to the upper portion of the reaction chamber for removing gas produced in the gasification and particles entrained therewith from the reaction chamber; and

a fluidized bed combustion reactor, provided with a reaction chamber; means for supplying fuel to be combusted to the reaction chamber; means for fluidizing a particle bed being formed in the reaction chamber; means for combusting the fuel to be combusted and a gas channel connected to the upper portion of the reaction chamber for removing gas generated in the com- bustion and particles entrained therewith from the reaction chamber;

and in which a first fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor is a circulating fluidized bed reactor, comprising recirculating means for particles, which have a particle separator arranged in the gas channel connected to the upper portion of the reaction chamber of the first fluidized bed reactor for separating particles entrained with the gas from the gas and a return channel for returning separated particles from the particle separator to the lower portion of the reaction chamber of the first fluidized bed reactor;

characterized in that the means for gasifying the fuel to be gasified comprise a first portion of the recirculating means of the first fluidized bed reactor, which first portion is arranged gas-tight inside the reaction chamber of a second fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor, which second fluidized bed reactor is not the above-mentioned first fluidized bed reactor, for gasifying the fuel to be ga- sified by transferring heat from the particles of the fluidized bed combustion reactor through an outer wall of the first portion to the particles of the fluidized bed gasification reactor.

2. Fluidized bed gasification system in accordance with claim 1 , characterized in that the first fluidized bed reactor is the fluidized bed combustion reactor and the second fluidized bed reactor is the fluidized bed gasification reactor.

3. Fluidized bed gasification system in accordance with claim 1 , characterized in that the first fluidized bed reactor is the fluidized bed gasification reactor and the second fluidized bed reactor is the fluidized bed combustion reactor.

4. Fluidized bed gasification system in accordance with claim 2 or 3, characterized in that the first portion of the recirculating means of the first fluidized bed reactor comprises at least a portion of the return channel.

5. Fluidized bed gasification system in accordance with claim 4, characterized in that said portion of the return channel comprises at least two parallel channel branches.

6. Fluidized bed gasification system in accordance with claim 4, characterized in that guiding means or chambers are connected to said portion of the return channel, for improving heat transfer through the outer wall of the first portion.

7. Fluidized bed gasification system in accordance with any of the preceding claims, characterized in that the first portion of the recirculating means of the first fluidized bed reactor comprises at least a portion of the particle separator.

8. Fluidized bed gasification system in accordance with claim 7, characterized in that the particle separator of the recirculating means of the first fluidized bed reactor is a cyclone, the height of which is greater than that of a conventional particle separator, preferably at least 30%, most preferably at least 50%, of the height of the reaction chamber of the second fluidized bed reactor.

9. Fluidized bed gasification system in accordance with claim 7 or 8, cha- racterized in that the particle separator of the recirculating means of the first fluidized bed reactor is arranged in the upper portion of the reaction chamber of the second fluidized bed reactor, in which the horizontal cross section of the reaction chamber is larger than that in the portion of the reaction chamber below the particle separator.

10. Fluidized bed gasification system in accordance with claim 1 , characterized in that the second fluidized bed reactor is a circulating fluidized bed reactor, comprising recirculating means for particles, which comprise a particle separator arranged in the gas channel connected to the upper portion of the reaction chamber of the second fluidized bed reactor for separating particles entrained with the gas from the gas and a return channel for returning the separated particles to the lower portion of the reaction chamber of the second fluidized bed reactor; and a first portion of the recirculating means of the second fluidized bed reactor is arranged gas-tight inside the reaction chamber of the first fluidized bed reactor.

1 1 . Fluidized bed gasification system in accordance with claim 10, characterized in that the first portion of the recirculating means of at least one fluidized bed reactor of the first fluidized bed reactor and the second fluidized bed reactor comprises at least a portion of the return channel of said one fluidized bed reactor.

12. Fluidized bed gasification system in accordance with claim 1 1 , characterized in that said portion of the return channel comprises at least two parallel channel branches.

13. Fluidized bed gasification system in accordance with claim 10, 1 1 or 12, characterized in that the first portion of the recirculating means of the at least one fluidized bed reactor of the first fluidized bed reactor and the second fluidized bed reactor comprises at least a portion of the particle separator of said one fluidized bed reactor.

14. A method of gasifying fuel in a fluidized bed gasification system, comprising a fluidized bed gasification reactor provided with a reaction chamber and a gas channel connected to the upper portion of the reaction chamber for removing gas produced in the gasification and particles entrained therewith from the reaction chamber, and a fluidized bed combustion reactor provided with a reaction chamber and a gas channel connected to the upper portion of the reaction chamber for removing gas generated in the combustion and particles entrained therewith from the reaction chamber, in which

a first fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor is a circulating fluidized bed reactor, comprising recirculating means for particles, which have a particle separator arranged in the gas channel connected to the upper portion of the reaction chamber of the first fluidized bed reactor for separating particles entrained with the gas from the gas, and a return channel for recirculating separated particles from the particle separator to the lower portion of the reaction chamber of the first fluidized bed reactor, in which method:

fuel to be gasified is supplied to the reaction chamber of the fluidized bed gasification reactor, the fuel to be gasified is fluidized and gasified in a particle bed formed in the reaction chamber and gas generated during gasification and particles entrained therewith are removed from the reaction chamber to the gas channel connected to the upper portion of the reaction chamber;

fuel to be combusted is supplied to the reaction chamber of the flui- dized bed combustion reactor, the fuel to be combusted is fluidized and combusted in a particle bed formed in the reaction chamber and gas generated in the combustion and particles entrained therewith are removed to the gas channel connected to the upper portion of the reaction chamber;

characterized in that the fuel to be gasified is gasified by transferring heat from the particles of the fluidized bed combustion reactor to the particles of the fluidized bed gasification reactor through the outer wall of a first portion of the recirculating means of the first fluidized bed reactor, which first portion is arranged gas-tight inside the reaction chamber of a second fluidized bed reactor of said fluidized bed gasification reactor and fluidized bed combustion reactor, which second fluidized bed reactor is not the above-mentioned first fluidized bed reactor.

15. Method of gasifying fuel in a fluidized bed gasification system in accordance with claim 14, characterized in that the efficiency of the heat transfer is adjusted by adjusting the amount of particles separating in the particle se- parator of the first fluidized bed reactor.

16. Method of gasifying fuel in a fluidized bed gasification system in accordance with claim 14, characterized in that the second fluidized bed reactor is a circulating fluidized bed reactor, comprising recirculating means for par- tides, which have a particle separator arranged in the gas channel connected to the reaction chamber of the second fluidized bed reactor for separating particles entrained with the gas from the gas and a return channel for returning separated particles from the particle separator to the lower portion of the reaction chamber of the second fluidized bed reactor; and the gasification of the fuel to be gasified is further improved by transferring heat from the particles of the fluidized bed combustion reactor to the particles of the fluidized bed gasification reactor through the outer wall of a first portion of the recirculating means of the second fluidized bed reactor, which first portion is arranged gas-tight inside the reaction chamber of the first fluidized bed reactor.