A method of converting an existing fluidised bed boiler into a fluidised bed gasifier, and a fluidised bed gasifier retrofitted thereby

Abstract

Invention relates to a method of converting an existing fluidised bed boiler into a fluidised bed gasifier. The existing fluidised bed boiler (1) comprises a furnace chamber (10), a windbox (20), a particle separator (30). Said method comprises step of providing a first gas-tight partition wall structure (40) inside the furnace chamber (10) so as to divide the furnace chamber (10) into a first gas-tight compartment (101, 121) and a second gas-tight compartment (102, 122), wherein a gasification reactor is provided in the first gas-tight compartment which (101, 121) is connected to inlet (32) of the particle separator (30). Said method further comprises step of providing a second gas-tight partition wall structure (50) inside the windbox (20), such that a first windbox chamber thus formed is configured to deliver fluidising gas to the gasification reactor. Invention relates also to a fluidised bed gasifier (1') retrofitted by said method.

Description

A method of converting an existing fluidised bed boiler into a fluidised bed gasifier, and a fluidised bed gasifier retrofitted therebyTechnical field

[0001] The present invention relates to a method of converting an existing fluidised bed boiler into a fluidised bed gasifier according to the preamble of claim 1 .

[0002] The present invention relates to a fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to the preamble of the second independent claim.Background art

[0003] Bubbling fluidised bed (BFB) and circulating fluidised bed (CFB) technologies are two well-known basic technologies of fluidised bed combustion. As compared to BFB in which the bed of fluidised solids has a relatively high density, CFB is distinguished by a greater velocity of fluidisation gas and relatively high solids recycling, which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations and stabilizing emissions at a low level. In this regard, the CFB technology was considered particularly suitable for coal incineration and has been applied to a large number of fossil fuel power plants.

[0004] As conventional CFB boiler for coal combustion, EP0365723A1 discloses a fluidized bed reactor which comprises a furnace section 4 at least partially consisting of water tubes, a separating section 6 including one or more cyclone separators, and a heat recovery area 8 including an enclosure housing a plurality of heat exchange surfaces. In the furnace section 4, particulate material, which can include coal and relatively fine particles of an adsorbent material such as limestone, is fluidized by fluidization gas such as air in a fluidized bed. Flue gas and entrained particulate material from the fluidized bed are introduced into the separating section 6, by which the particulate material is disengaged from the flue gas. Separate particulate material is then recycled to the furnace section 4,whereas clean flue gas passes through the heat recovery area 8 before exiting to external equipment.

[0005] Nowadays the transition to a net zero world is reshaping industries and even entire economies. Companies, including fossil fuel power plants utilising CFB boilers for coal combustion, are expected to change the way they operate so as to address the challenges of decarbonization and net zero. In the face of such a serious environmental situation, quite a few fossil fuel CFB boilers in fairly good condition have to be shut down, which would adversely lead to unnecessary waste of boiler components in good working order.

[0006] An object of the invention is to provide method of converting an existing fluidised bed boiler into a fluidised bed gasifier in which components of a CFB boiler can be reutilised to the maximum level for fluidised bed gasification for biomass and various types of waste. It is also an object of the invention to provided fluidised bed gasifier retrofitted from existing fluidised bed boiler.Disclosure of the Invention

[0007] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.

[0008] According to an embodiment of the invention a method of converting an existing fluidised bed boiler into a fluidised bed gasifier, the existing fluidised bed boiler comprising:- a furnace chamber, which has a top wall, a grid, and walls,- a windbox arranged in connection with the grid,- a particle separator arranged adjacent to the furnace chamber, wherein the particle separator comprises an inlet connected to one of the walls, a gas outlet, and a particle outlet, the method of converting the existing fluidised bed boiler comprises steps of- providing a first gas-tight partition wall structure inside the furnace chamber, which extends from the grid to the top wall of the furnace chamber so as to divide the furnace chamber into a first gas-tight compartment and a second gas-tightcompartment, wherein a gasification reactor is provided in the first gas-tight compartment which is connected to the inlet of the particle separator,- providing a second gas-tight partition wall structure inside the windbox, such that a first windbox chamber thus formed is configured to deliver fluidising gas to the gasification reactor.

[0009] This enables a new way to set the fluidised bed boiler for conventional steam generation using fossil fuels back on track as fluidised bed gasifier for converting biomass and various types of waste into sustainable fuels. The furnace chamber as well as the windbox of the existing fluidised bed boiler are divided into two or more gas-tight compartments by means of gas-tight partition wall structures, which allows for a smooth shift from fluidised bed boiler to fluidised bed gasifier. In addition, since infrastructure for the fluidised bed gasifier is straight from the existing fluidised bed boiler, the fluidised bed gasifier can be put into use with shortened delivery time and relatively low infrastructure investment cost.

[0010] According to an aspect of the invention the method of converting the fluidised bed boiler further comprises step of forming the first gas-tight partition wall structure by two partition walls that are horizontally spaced apart from each other. In other words, the partition walls are spaced in a direction from the first gas-tight compartment toward the second gas-tight compartment. As such, the space sandwiched between the two partition walls can act as a seal layer, by which the gas and thermal insulating abilities of the first gas-tight partition wall structure are enhanced.

[0011] According to an aspect of the invention the method of converting the fluidised bed boiler further comprises a step of arranging a system for leading inert gas into space between the two partition walls.

[0012] According to an aspect of the invention the method of converting the fluidised bed boiler further comprises step of filling a space defined between the two partition walls with a pressurised inert gas. The pressurised inert gas, such as N2and CO2, filled between the two partition walls may further prevent the diffusion of gas from one gas-tight compartment to the other gas-tight compartment, thereby ensuring that no gas diffusion or gas transfer between the first andsecond gas-tight compartments takes place through the first gas-tight partition wall structure.

[0013] According to an aspect of the invention the method of converting the fluidised bed boiler further comprises step of forming a heat-resistant refractory layer on the top wall, the walls and the first gas-tight partition wall structure of the first gas-tight compartment. This provides the fluidised bed gasifier with increased heat resistance particularly suitable for gasification processes which are usually subjected to higher operating temperatures.

[0014] According to an aspect of the invention the method of converting the fluidised bed boiler further comprises steps of- providing a gas conduit extending between the gas outlet of the particle separator and top of the second gas-tight compartment,- providing a burner at upper part of the second gas-tight compartment defining a partial oxidation reactor in the second gas-tight compartment, and- providing a product gas outlet in the vicinity of bottom of the second gas-tight compartment, for discharging reformed product gas.

[0015] As such, a second gas-tight compartment other than the first gas-tight compartment in which a gasification process takes place can be reutilised for tar removal process for the product gas derived from the gasification process in the first gas-tight compartment. By newly installing a gas conduit allowing fluid communication between the first gas-tight compartment and the second gas-tight compartment as well as modifying the second gas-tight compartment with new burner and product gas outlet, rate of utilisation of the old parts from the existing fluidised bed boiler can be further increased and the product gas can be reformed by partial oxidation in same reactor where the gasification process takes place.

[0016] According to an aspect of the invention the method of converting the fluidised bed boiler further comprises step of providing the burner at an outlet of the gas conduit in the second gas-tight compartment.

[0017] According to an aspect of the invention the existing fluidised bed boiler comprises at least two particle separators arranged adjacent to the furnace chamber, each of the particle separators comprising an inlet connected to one ofthe walls, a gas outlet, and a particle outlet, the method of converting the fluidised bed boiler further comprises steps of- providing the first gas-tight partition wall structure inside the furnace chamber such that both the first gas-tight compartment and the second gas-tight compartment are provided with at least one particle separator, wherein the gasification reactor is provided in the first gas-tight compartment which is connected to inlet of a first particle separator, and wherein a combustion reactor is provided in the second gas-tight compartment which is connected to inlet of a second particle separator,- providing the second gas-tight partition wall structure inside the windbox, such that the first wind box chamber is arranged to deliver fluidising gas to the gasification reactor and a second windbox chamber is arranged to deliver fluidising gas to the combustion reactor,- providing a first particle transfer line extending between particle outlet of the first particle separator and the combustion reactor, and- providing a second particle transfer line extending between particle outlet of the second particle separator and the gasification reactor.

[0018] In doing so, a second gas-tight compartment other than the first gas-tight compartment in which a gasification process takes place can function as a combustor that provides thermal energy to the first gas-tight compartment for the gasification process. Such thermal energy can be transferred by means of heattransfer solids moving between the first and second gas-tight compartments through the newly installed first and second particle transfer lines. Hence, rate of utilisation of the old parts from the existing fluidised bed boiler can be further increased and the retrofitted fluidised bed boiler is provided with more flexible operation.

[0019] According to one aspect of the invention the existing fluidised bed boiler further comprises a back pass in which one or more heat exchangers are located, and a flue gas duct system arranged between gas outlets of the particle separators and the back pass for conducting cleaned flue gas to the back pass, the method of converting the fluidised bed boiler further comprises step of- modifying the flue gas duct system into a first duct sub-system connected to gas outlet of the first particle separator for drawing product gas to furtherprocessing and a second duct sub-system connected to gas outlet of the second particle separator for drawing exhaust gas to further processing.

[0020] This way the flue gas duct system is divided into two sub-systems separated from each other. In doing so, the flue gas duct system of the existing fluidised bed boiler can be reutilised to draw the product gas and the exhaust gas in an independent manner.

[0021] A fluidised bed gasifier retrofitted from an existing fluidised bed boiler, the existing fluidised bed boiler comprising:- a furnace chamber which has a top wall, a grid, and walls,- a windbox arranged in connection with the grid,- a particle separator arranged adjacent to the furnace chamber, wherein the particle separator comprises an inlet connected to one of the walls, a gas outlet, and a particle outlet, the retrofitted fluidised bed gasifier comprises:- a first gas-tight partition wall structure inside the furnace chamber, which extends from the grid to the top wall of the furnace chamber, such that the furnace chamber is divided into a first gas-tight compartment and a second gas-tight compartment,- a gasification reactor arranged to the first gas-tight compartment which is connected to the inlet of the particle separator,- a second gas-tight partition wall structure located inside the windbox, such that a first windbox chamber is formed by at least part of the windbox and arranged to deliver fluidising gas to the gasification reactor.

[0022] According to an aspect of the invention the first gas-tight partition wall structure is comprised of two partition walls spaced apart in a direction from the first gas-tight compartment toward the second gas-tight compartment.

[0023] According to as aspect of the invention a space defined between the two partition walls is filled with a pressurised inert gas.

[0024] According to an aspect of the invention a heat-resistant refractory layer is formed on the top wall, the walls and the first gas-tight partition wall structure of the first gas-tight compartment.

[0025] According to an aspect of the invention a gas conduit is arranged to extend between the gas outlet of the particle separator and top of the second gastight compartment, a burner is arranged to upper part of the second gas-tight compartment, and a product gas outlet is arranged in the vicinity of bottom of the second gas-tight compartment, for discharging reformed product gas.

[0026] According to an aspect of the invention, the burner is arranged at an outlet of the gas conduit in the second gas-tight compartment.

[0027] According to an aspect of the invention the existing fluidised bed boiler comprises at least two particle separators arranged adjacent to the furnace chamber, each of the particle separators comprising an inlet connected to one of walls, a gas outlet, and a particle outlet, the first gas-tight partition wall structure is arranged inside the furnace chamber such that both the first gas-tight compartment and the second gas-tight compartment are provided with at least one particle separator, the gasification reactor is arranged in the first gas-tight compartment, which is connected to inlet of a first particle separator, a combustion reactor is arranged in the second gas-tight compartment, which is connected to inlet of a second particle separator, the second gas-tight partition wall structure is arranged inside the windbox such that the first windbox chamber is arranged to deliver fluidising gas to the gasification reactor and a second windbox chamber is arranged to deliver fluidising gas to the combustion reactor, a first particle transfer line is arranged to extend between particle outlet of the first particle separator and the combustion reactor, and a second particle transfer line is arranged to extend between particle outlet of the second particle separator and the gasification reactor.

[0028] According to an aspect of the invention the existing fluidized bed boiler further comprises a back pass in which one or more heat exchangers are located, and a flue gas duct system arranged between gas outlets of the particle separators and the back pass for conducting cleaned flue gas to the back pass, and the retrofitted fluidised bed gasifier comprises:first duct sub-system arranged to a first part of the flue gas duct system and connected to gas outlet of the first particle separator, for drawing product gas to further processing, and second duct sub-system arranged to a second part the flue gas duct system and connected to gas outlet of the second particle separator, for drawing exhaust gas to further processing.

[0029] The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.Brief Description of Drawings

[0030] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in whichFig. 1A illustrates a cut out view A-A of Fig. 1C of an existing fluidised bed boiler before retrofit for the present invention,Fig. 1 B illustrates a top view of the existing fluidised bed boiler of Fig. 1 A,Fig. 1C illustrates a side of the existing fluidised bed boiler of Fig. 1A,Fig. 2A illustrates a cut out view A-A of Fig. 2C of a fluidised bed gasifier obtained by converting the existing fluidised bed boiler according to an embodiment of the invention,Fig. 2B illustrates a top view of the fluidised bed gasifier of Fig. 2A,Fig. 2C illustrates a side of the fluidised bed gasifier of Fig. 2A,Fig. 3A illustrates a cut out view of a fluidised bed gasifier obtained by converting the existing fluidised bed boiler according to another embodiment of the invention, which is observed in the same manner as in Fig. 2A,Fig. 3B illustrates a top view of the fluidised bed gasifier of Fig. 3A,Fig. 4A illustrates a cut out view of a fluidised bed gasifier obtained by converting the existing fluidised bed boiler according to still another embodiment of the invention, which is observed in the same manner as in Fig. 2A,Fig. 4B illustrates a top view of the fluidised bed gasifier of Fig. 4A,Fig. 4C illustrates a back view of the fluidised bed gasifier of Fig. 4A,Fig. 5A illustrates a cut out view of a fluidised bed gasifier obtained by converting the existing fluidised bed boiler according to still another embodiment of the invention, which is observed in the same manner as in Fig. 2A,Fig. 5B illustrates a top view of the fluidised bed gasifier of Fig. 5A,Fig. 6 illustrates a cut out view of a fluidised bed gasifier obtained by converting the existing fluidised bed boiler according to still another embodiment of the invention, which is observed in the same manner as in Fig. 2A, andFig. 7 illustrates a cut out view of a fluidised bed gasifier obtained by converting the existing fluidised bed boiler according to still another embodiment of the invention, which is observed in the same manner as in Fig. 2A.Detailed Description of Drawings

[0031] Fig. 1A, 1 B and 1C depict schematically an existing fluidised bed boiler 1 being specifically a circulating fluidised bed boiler 1 , which is configured to combust fuel material and produce superheated steam in a manner known per se. The circulating fluidised bed boiler will be referred to as CFB boiler for sake of conciseness. The CFB boiler 1 comprises a furnace chamber 10, a windbox 20 located at a bottom of the furnace chamber 10, and at least one particle separator 30,31 arranged adjacent to the furnace chamber 10.

[0032] The furnace chamber 10 of the CFB boiler 1 is configured to combust fuel with combustion gas, such as air, therein and to generate a stream of flue gas in a bed of fluidised particles. The furnace chamber 10 comprises, as shown in Fig. 1A and 1 B, side walls 11 which horizontally enclose the furnace chamber 10. The furnace chamber 10 further comprises a top wall 12 arranged preferably perpendicularly to the side walls 11 , and a grid 13 located in lower part of the furnace chamber 10. Fig. 1 B discloses that the furnace chamber 10 has a rectangular shape consisting of four side walls 11 , namely a first wall 11.1 , a second wall 11 .2, a third wall 11 .3 and a fourth wall 11 .4. As it can be seen in the Fig. 1 B, the first wall 11.1 and the second wall 11 .2 are substantially twice as wide (horizontal length) as the third wall 11.3 and the fourth wall 11.4, which provides advantageous, but not essential, starting point for the conversion according to the invention.

[0033] However, the shape of cross section of the furnace chamber 10 may vary and the CFB boiler may still be applicable for conversion according to the invention. For example, the furnace chamber 10 may comprise five or more side walls forming a cross section of polygonal shape. The side walls 11 can be tube walls i.e. so-called finned tube walls or membrane walls wherein fins are welded between tubes forming a gas tight wall structure. The wall tubes are connected to a water-steam circuit of boiler system (not shown).

[0034] In Fig. 1A, 1B and 1C the configuration in which two particle separators 30, 31 are arranged in the vicinity of the second wall 11 .2 of the furnace chamber 10 is exemplified. However, the particle separators 30, 31 may be arranged on different walls of the furnace chamber 10, such as on two opposite walls. Additionally, or alternatively, the CFB boiler 1 may comprise two or more particle separators on different walls of the furnace chamber 10. Depending on size, or nominal power of the CFB boiler, the CFB boiler 1 may comprise only one particle separator.

[0035] The grid 13 at the bottom of the furnace chamber 10 is provided with several gas nozzles 14 which provide fluid connection between the windbox 20 and the furnace chamber 10. The windbox 20 is configured to feed fluidisation gas, which is typically air in a fluidised bed boiler, through nozzles 14 of the grid 13 at the bottom of the furnace chamber 10. The air introduced through the grid13 acts as fluidisation gas and is the primary combustion air. The fluidising / combustion gas is usually air, but they can also comprise other components such as circulated flue gas and / or oxygen or a mixture therefor.

[0036] An existing fluidised bed boiler is provided with means for introducing pressurized air into the windbox 20 as a source of air, as well as a suitable ductwork and a blower (not shown). The windbox 20, and also other air inlets, are in connection with the source of air. This is an example of an air operated CFB boiler. There is at least one inlet 15 for fuel in connection with the furnace chamber 10. Operation of the CFB boiler involves a process of circulation of solid material particles, which in this connection may also be referred to as bed material, as well. The bed material may comprise sand, limestone, and / or clay, that in particular may comprise kaolin, and also unburned fuel. Due to the fluidised bed of material inside the boiler, CFB boilers have high heat transfer coefficients and substantially uniform temperature distribution and have a considerably low stable combustion temperature. Combustion of fuel in the circulating fluidised bed results in heating, evaporating the water in the water-steam circuit and superheating the steam, which can be used in a manner known as such, for example, production of electric power in a steam turbine generator. The steam cycle is not described here in more detailed manner.

[0037] When used as CFB boiler the particle separators 30, 31 each has an inlet 32 connected to the second wall 11 .2 for receiving the stream of flue gas and particles from the furnace chamber 10, a gas passage 34 for discharging cleaned flue gas from the particle separator. The separator is connected to lower part of the furnace chamber 10 via a dip leg 35 and a loop seal 36. A particle outlet (not shown) is arranged in connection with a loop seal 36 for returning separated particles to the furnace chamber 10.

[0038] The CFB boiler 1 may further comprise a back pass 38, where one or more heat exchange surfaces for transferring heat from flue gas to a heat transfer medium are located. In practice there are usually several heat exchange surfaces, such as superheaters, reheaters, economisers and air heaters. The back pass is usually connected to the gas outlets of the particle separators via a flue gas duct system, for conducting the cleaned flue gas to the back pass.(First embodiment)

[0039] Fig. 2A, 2B and 2C depict schematically a fluidised bed gasifier 1’ obtained by converting the CFB boiler 1 shown in the Fig. 1A, 1 B and 1C according to a first embodiment of the present invention. In the fluidised bed gasifier retrofitted from the CFB boiler 1 according to the present embodiment, a substantially vertical, first gas-tight partition wall structure 40 is provided inside the furnace chamber 10. More specifically, the first gas-tight partition wall structure 40 extends from the grid 13 to the top wall 12 in an extension direction and preferably between two opposite side walls 11 , in this case between the first wall 11.1 and the second wall 11.2, of the furnace chamber 10 so as to divide the furnace chamber 10 into a first gas-tight compartment 101 and a second gas-tight compartment 102. The second wall 11 .2 is the wall in which the gas passages 34, 34 to the separators 30,31 from the furnace chamber 10 are located. Since the ratio of widths of the first wall 11.1 and the second wall 11 .2 to the third wall 11.3 and the fourth wall 11.4 in the horizontal cross section is 1 :N where N >1 , here substantially 1 :2, and the partition wall structure 40 is provided in the middle of the original cross section, the converted cross section of the first gas-tight compartment 101 and the second gas-tight compartment 102 is substantially square. One of the gas passages 34 is now in the first compartment 101 and the other one in the second compartment 102. As such, both the first and the second compartment has a gas-tight structure and thus can serve as process chamber in a fluidised bed gasifier 1 ’. Heat transfer surfaces such as tube walls can be modified suitably, if needed, to recover heat from the gasification process and producing steam, thereby fully utilising the existing power plant infrastructure, which may otherwise to be even decommissioned.

[0040] In the first embodiment of the invention, a gasification reactor is provided in the first gas-tight compartment 101 which is connected to the inlet of the particle separator 30. Therefore, the gasification reactor may also be referred to with the numeral 101. Since in this example the original CFB boiler included two separators 30, 31 , one of the separators may become unused and the second outlet 32 for the second separator is preferably blocked which is described by a cross on the outlet. Instead of deactivating the second separator, it may be conceivable in some practical applications to utilize both of the separators by arranging aparallel or series coupling of the separators for removing solid particles more efficiently.

[0041] As it becomes clear for the description above, since one of the separators may become unused after the conversion, the method according to the invention is well applicable for converting a fluidised bed boiler provided with only one particle separator into a fluidised bed gasifier.

[0042] In the fluidised bed gasifier according to the present embodiment, gas pressure inside the gasification reactor is usually higher than normal boiler furnace pressure in a CFB boiler. To protect the gasification reactor against the increased gas pressure, the first gas-tight partition wall structure 40 is designed to have a relatively high pressure resistance. As shown in Fig. 2A, for this specific embodiment, the first gas-tight partition wall structure 40 is formed by a single gas-tight partition wall, however, it can also be comprised of two or more gastight partition walls. Preferably, an additional support structure can be installed outside the gasification reactor 101 against any desired side walls bordering the first compartments 101 to endure the increased gas pressure. Additionally, at least one gas-tight partition wall is preferably provided with a fluid flow circuit through which a cooling fluid flows. This is optionally applicable to all embodiments of the invention. Said fluid flow circuit can be formed as part of the watersteam circuitto which wall tubes of the side walls 11 are connected. In such case, the at least one gas-tight partition wall can be same type of tube wall, i.e. so- called finned tube wall or membrane wall as the side walls 11 and the cooling fluid is water. Alternatively, said fluid flow circuit in the partition wall can be connected to a closed circuit separated from the water-steam circuit of boiler system and accordingly, the cooling fluid can be water, air, inert gas or the like.

[0043] Moreover, the part of the grid 14 which is not horizontally in the region of the gasification reactor 101 is deactivated in this embodiment. Inside the windbox 20 is provided a second gas-tight partition wall structure 50 so as to create a separate windbox chamber for delivering fluidising gas independently to the gasification reactor. As shown in Fig. 2A, the second gas-tight partition wall structure 50 is installed along the extension direction of the first gas-tight partition wall structure 40. This way the second gas-tight partition wall structure divides the windbox 20 into a first windbox part and a second windbox part. In other words,preferably the first and the second partition wall structures 40, 50 are arranged to same horizontal location, the first partition wall structure on top of the second partition wall structure. However, the arrangement of the second gas-tight partition wall structure is not limited to this. For example, the second gas-tight partition wall structure may be formed on a line extending from a lower end of the first gas-tight partition wall structure in any downward direction inclined from vertical direction, or on a line extending in parallel to the first gas-tight partition wall structure. In addition, the second gas-tight partition wall structure may be arranged as substantially horizontal wall under the grid 13 in the second gas-tight compartment 102. In such case, substantially the entire windbox 20 can function as the chamber for delivering fluidising gas to the gasification reactor and the grid in connection with the second gas-tight compartment 102 is blocked.

[0044] Further, it is preferable to form a refractory layer 43 on the top wall 12, side walls 11 and the first gas-tight partition wall structure 40 inside of the first gas-tight compartment 101 , for providing heat resistance to the gasification reactor. Preferably, the first gas-tight compartment 101 is completely surrounded by refractory material. It may often be a case that a CFB boiler has its membrane walls provided with a light refractory for erosion protection, at least at its lower part, and when converted into a gasifier according to the invention the additional refractory layer may be formed on an existing refractory lining, or the existing refractory lining is renewed or replaced with a new, optionally thicker refractory lining. In the areas of the walls 11 and the top 12 where no refractory is present heat-resistant refractory layer provided on the top wall, the side walls and the first gas-tight partition wall structure of the first gas-tight compartment.

[0045] In the retrofitted gasification reactor, carbonaceous feedstock such as municipal solid waste (MSW) or biomass is supplied to the fuel inlet 15 via the existing fuel handling and feeding systems (not shown) of the CFB boiler 1. In the CFB gasification process, which is practised in the gasification reactor 101 , the heat required for gasification reactions is produced through the partial combustion of the fuel, while the rest of the fuel is converted into product gas. Product gas and particles entrained with the gas is led to the cyclone separator 30 via the gas passage 34. The supplied carbonaceous feedstock is converted in thegasification reactor into product gas, which mainly consists of carbon monoxide, carbon dioxide, hydrogen, and hydrocarbon components.

[0046] According to the present embodiment a partial oxidation reactor is formed into the second gas-tight compartment 102. To conduct the product gas from the particle separator 30 which is still after the conversion connected to the gasification reactor 101 for further processing, a gas conduit 103 is newly installed at the gas outlet of the particle separator 30, which is in connection with the gasification reactor 101. In the present embodiment, the gas conduit 103 is extending between the gas outlet of the particle separator 30 and top of the second gas-tight compartment 102 such that the second gas-tight compartment 102 can receive the product gas from the cyclone separator 30 of the first gas-tight compartment 101. The gas conduit 103 is preferably coupled to middle of the top wall of the second gas-tight compartment 102, and is configured to direct the gas flow substantially downwards in the second compartment 102.

[0047] At upper part of the second gas-tight compartment 102 is provided a burner 60, the flame provided by the burner, when in use, being illustrated by dotted line in the figure. Preferably, the burner 60 is arranged at an outlet of the gas conduit in the second gas-tight compartment 102. The burner 60 is provided with an inlet for oxygen 61 and / or steam 62, which may be fed through the burner 60 into the second gas-tight compartment 102. Oxygen and steam react with portion of the product gas to rise the temperature to enable melt of ash and crack of higher hydrocarbon components in the product gas. Depending on the product gas composition, the second gas-tight compartment 102 is configured to operate at temperature in a range of 1100 to 1400 °C.

[0048] As such, the second gas-tight compartment 102 serves as a partial oxidation reactor for reforming the received product gas. In the conversion according to the embodiment of the invention the second compartment is provided with a product gas outlet channel 70 in the vicinity of bottom of the second gas-tight compartment 102, at which the grid has been disabled in the conversion. The reformed product gas is discharged via the newly installed product gas outlet channel 70. The product gas discharged from the second gas-tight compartment 102 may be further conducted to the existing back pass and / or filter for gas cooling and / or cleaning, if so desired. In other words, the conversion may compriseproviding the fluidised bed gasifier with connection channel between the product gas outlet channel 70 and the back pass 38.

[0049] As described above in the first embodiment, a fluidised bed gasifier together with a partial oxidation (POx) reactor is retrofitted into an existing fluidised bed boiler by making use of most of the existing main infrastructures belonging for example to a coal-fired power plant, with few modifications. Thus, in-service components such as boiler furnace, particle separators, and back pass can remain operational despite cease of the power plant operation, and the new gasification plant can be delivered quickly with reduced investment costs. It should be understood that some additional auxiliary equipment may need to be added, replaced or reconfigured for the new use as gasifier-POx reactor.(Second embodiment)

[0050] Fig. 3A and 3B depict schematically a fluidised bed gasifier obtained by converting the CFB boiler 1 according to a second embodiment of the present invention. The second embodiment is another example of the present invention in which the existing fluidised bed boiler is converted into a fluidised bed gasifier combined with a POx reactor. More precisely in the method the conversion results in two gasification reactors and one POx reactor common to the gasification reactors. In Fig. 3A and 3B, the same elements as shown in Fig. 2A and 2B are numbered with the same reference signs, and the description thereof is referred to for applicable parts. The arrangement of Fig. 3A and 3B differs from that of Fig. 2A and 2B mainly in that two gas-tight partition wall structures 41 and 42 are provided inside the furnace chamber 10.

[0051] Both of the gas-tight partition wall structures 41 and 42 extends from the grid 13 to the top wall 12, in a first extension direction and a second extension direction, respectively, so as to divide the furnace chamber 10 into a first gastight compartment 111 , a second gas-tight compartment 112, and a third gastight compartment 113. The first gas-tight compartment 111 is located between the fourth wall 11 .4 and a first partition wall structure 41 , and is connected to inlet of a first particle separator 30 at its upper part, thereby converted into and serving as a first gasification reactor. The second gas-tight compartment 112 is located between the third side wall 11.3 and a second partition wall structure 42 and isconnected to inlet of a second particle separator 31 , thereby converted into and serving as a second gasification reactor. The third gas-tight compartment 113 is located between the partition wall structures 41 , 42 and it has been converted into a POx reactor. It has a similar structure to that of the second gas-tight compartment 102 of Fig. 2A and 2B. A first gasification reactor is provided in the first gas-tight compartment 111 which is connected to the inlet of the particle separator, and a second gasification reactor is provided in the second gas-tight compartment 112 which is connected to the inlet of the particle separator. A partial oxidation reactor is provided in the third gas-tight compartment 113, which is formed horizontally between the first gas-tight compartment 111 and the second gas-tight compartment 112.

[0052] Moreover, inside the windbox 20 are provided one or more gas-tight partition wall structures so as to create one or more separate windbox chambers for delivering fluidising gas to the gasification reactors, in independently controllable manner. As shown in Fig. 3A, it is preferable that the windbox 20 is divided into three separate chambers by two gas-tight partition wall structures 51 , 52. The gas-tight partition wall structure 51 is formed on a vertical line extending along the first extension direction of the partition wall structure 41 , and the gas-tight partition wall structure 52 is formed on a vertical line extending along the second extension direction of the partition wall structure 42. However, arrangement of the partition wall structures inside the windbox is not limited to this. For example, there may be only one partition wall structure arranged as substantially horizontal wall under the grid 13 in the third gas-tight compartment 113, such that fluidising gas is delivered through the whole windbox 20 to both the first and second gastight compartments 111 , 112.

[0053] Further, a first gas conduit is extending between the gas outlet of the particle separator 30 and top of the third gas-tight compartment 113, and a second gas conduit is extending between the gas outlet of the particle separator 31 and top of the third gas-tight compartment 113. As such, the third gas-tight compartment 113 can receive the product gas from the first gas-tight compartment 111 and / or second gas-tight compartment 112 for reforming the received product gas by a POx reaction, which is practised in a similar manner as that of the first embodiment. Thus, in this embodiment, in the conversion method, twoindependently operable gasification reactors 111 ,112 are formed with the partition wall structures 42 and a POx reactor common to the gasification reactors is formed between the gasification chambers.

[0054] As described above in the second embodiment, two separate gasification reactors are formed in the existing furnace chamber. Therefore, the retrofitted fluidised bed gasifier can provide versatile operational modes in relation to gasification.(Third embodiment)

[0055] Fig. 4A and 4B depict schematically a fluidised bed gasifier obtained by converting the CFB boiler 1 according to a third embodiment of the present invention. In Fig. 4A and 4B, the same elements as shown in Fig. 2A and 2B are numbered with the same reference signs, and the description thereof is referred to for applicable parts. The arrangement of Fig. 4A and 4B differs from that of Fig. 2A and 2B mainly in that a first gas-tight compartment 121 and a second gas-tight compartment 122 serve as a gasification reactor and a combustion reactor, respectively.

[0056] Similarly to the first embodiment, the furnace chamber 10 is divided by the first gas-tight partition wall structure 40 into the first gas-tight compartment 121 and the second gas-tight compartment 122. The windbox 20 is also divided by the second gas-tight partition wall structure 50 into a first chamber for delivering a first fluidisation gas independently to the first gas-tight compartment 121 , and a second chamber for delivering a second fluidisation gas independently, the gas being either identical with or different from the first fluidisation gas, to the second gas-tight compartment 122. However, there is no limitation thereto, so long as the second gas-tight partition wall structure can divide the windbox 20 into two separate chambers.

[0057] In the third embodiment, the first gas-tight compartment 121 is provided with a first particle separator 30, wherein a gasification reactor is provided in the first gas-tight compartment 121 which is connected to inlet of the first particle separator 30. The second gas-tight compartment 122 is provided with a second particle separator 31 , wherein a combustion reactor is provided in the secondgas-tight compartment 122 which is connected to inlet of the second particle separator 31.

[0058] Moreover, a first particle transfer line 80 is arranged to extend between particle outlet of the particle separator 30 and the combustion reactor 122, and a second particle transfer line 82 is arranged to extend between particle outlet of the particle separator 31 and the gasification reactor 121 . Preferably a refractory layer 43 is arranged to the side walls and top wall of the gasification reactor. Thereby, particles can be efficiently recirculated between the gasification reactor 122 and combustion reactor 121 by utilising the existing particle separators, thus enabling heat and material transfer between the gasification reactor and the combustion reactor. Even if not shown the first and the second particle transfer lines 80, 82 are preferably provided with gas seals to prevent undesired back flow of gas.

[0059] This way the converted plant comprises a gasification reactor 121 and a combustion reactor 122 which have solid material return system combined at their dip legs. Thermal energy required for the gasification reactions can be partially introduced to the gasification reactor 121 by transferring hot solid material from the combustion reactor 122. Respectively, char from gasification reactor 121 can be transferred to the combustion reactor 122 via the first particle transfer line for completing combustion of remaining combustible material. Such solids transfer can be achieved by utilising, for example, an arrangement for controlling a flow of solid particles as described in the European patent application EP3840871A1.

[0060] Furthermore, the back pass 38 (as shown in Fig. 1C) or more generally, exhaust gas duct system of the CFB boiler 1 is modified into a first duct subsystem 38’ and a second duct sub-system 38”. The fist duct sub-system 38’ is connected to gas outlet of the particle separator 30 for drawing product gas from the gasification reactor to further processing. The second duct sub-system 38” is connected to gas outlet of the particle separator 31 for conducing exhaust gas from the combustion reactor to further processing. According to an aspect of the invention the modification of the back pass 38 is performed such that heat exchangers are divided to cool both the product gas and the exhaust gas. According to another aspect of the invention the modification is of the back pass 38 isperformed such that heat exchangers are used for cooling either one of the exhaust gas and the product gas, and a new gas cooler or even a new back pass, inside which one or more heat exchanging tubes are located, is provided for cooling the other gas not cooled by the existing heat exchangers.

[0061] As described above in the third embodiment, an existing fluidised bed boiler can be converted into a fluidised bed gasifier capable of indirect gasification. Furthermore, heat can be transferred between the gasification reactor and the combustion reactor efficiently by utilising the existing furnace chamber and particle separators.(Fourth embodiment)

[0062] Fig. 5A and 5B depict schematically a fluidised bed gasifier obtained by converting the CFB boiler 1 according to a fourth embodiment of the present invention. The fourth embodiment is still another example of the present invention in which the existing fluidised bed boiler is converted into a fluidised bed gasifier combined with a POx reactor. In Fig. 5A and 5B, the same elements as shown in Fig. 2A and 2B are numbered with the same reference signs, and the description thereof is referred to for applicable parts. The arrangement of Fig. 5A and 5B differs from that of Fig. 2A and 2B mainly in that an inner wall assembly 54, configured on suspended support, is provided inside the second gas-tight compartment 102.

[0063] The inner wall assembly 54 is arranged in a coaxial configuration with respect to the burner 60 and / or the chamber of the second gas-tight compartment 102 horizontally enclosed by the walls 11.1 , 11.2, 11.3 and the partition wall structure 40. The inner wall assembly 54 is preferably suspended from the top wall 12 and it extends downwards from the region of the burner 60 to the vicinity of the product gas outlet 70. The inner wall assembly 54 is preferably formed by a number of water tube walls, each of which is made of water / steam-cooled tubes connected by welded fins, for example. The water tube walls are preferably planar panel walls. Since these water tube panel walls can be easily formed as spare parts, their maintenance and replacement can be done in a fast and easy manner. Moreover, each water tube panel wall is hanging on its own suspended support and thus forming an array of inner panel walls. The suspended support enablesthe inner panel wall to be cleaned easily by a rapping device (not shown), because the top suspended structure can vibrate or swing when rapped.

[0064] Further, the inner wall assembly functions as a protective device which captures slag / dust formed from carbon compounds and sticky components contained in the product gas received from the first gas-tight compartment 101 . More specifically, when reacted with steam, for example, sticky components of the product gas are decomposed, and residuals shall accumulate as a protective slag / dust layer on the inner wall assembly. Further, each panel section of the inner wall assembly will be separately cleanable by a rapping device such as spring hammer rapping, thereby the protective slag / dust layer accumulated on the inner wall assembly can be maintained within a desired range of thickness. In doing so, the product gas received from the first gas-tight compartment 101 is purified while plugging of the second gas-tight compartment 102 is prevented.

[0065] As described above in the fourth embodiment, a protective slag / dust layer is formed on the inner wall assembly in a controllable manner. Therefore, the retrofitted fluidised bed gasifier may be operated at a regular lower temperature to avoid issues with alkali and other corrosive components.

[0066] In above first to fourth embodiments, each of the gas-tight partition wall structures 40, 41 , 42 that divide the furnace chamber 10 is formed by a single gas-tight partition wall. Next, other configuration of the gas-tight partition wall structure will be described.(Fifth embodiment)

[0067] Fig. 6 depicts schematically a fluidised bed gasifier obtained by converting the CFB boiler 1 according to a fifth embodiment of the present invention. The fifth embodiment is yet another example of the present invention in which the existing fluidised bed boiler is converted into a fluidised bed gasifier combined with a POx reactor. In Fig. 6, the same elements as shown in Fig. 2A are numbered with the same reference signs, and the description thereof is referred to for applicable parts. The arrangement of Fig. 6 differs from that of Fig. 2A mainly in that the first gas-tight partition wall structure 40 is comprised of two partition walls 40.1 , 40.2.

[0068] The partition walls 40.1 , 40.2 extend from the grid 13 to the top wall 12, each of which is substantially impervious to gas. They together form the first gastight partition wall structure 40, which divides the furnace chamber 10 into a first gas-tight compartment 101 and a second gas-tight compartment 102. The partition wall 40.1 and the partition wall 40.2 are spaced apart in a direction from one of the two gas-tight compartments 101 , 102 toward the other. Thereby, between the two partition walls 40.1 , 40.2 is defined a space 40.3, which acts a seal or cushion layer that increases the gas and thermal insulating abilities of the first gas-tight partition wall structure 40.

[0069] Furthermore, in the conversion, a system 91 ,90 for leading inert gas into the space 40.3 is provided to the fluidized bed gasifier. This way the space 40.3 defined between the partition walls 40.1 , 40.2 is preferably filled with a pressurised inert gas such as N2and CO2. As shown in Fig. 6, such inert gas can be pressurised and introduced into said space from an external source of gas 90 through line 91 , for example. During operation of the retrofitted fluidised bed gasifier according to the present embodiment, product gas is produced in the first gas-tight compartment 101 and then sent to the second gas-tight compartment 102 to be reformed. The pressurised inert gas between the partition walls 40.1 ,40.2 can help prevent the product gas from passing through the partition wall40.1 , or the reformed product gas from passing through the partition wall 40.2, thus further enhancing the gas insulating ability of the first gas-tight partition wall structure 40. Preferably, the system 91 ,90 for leading inert gas into the space40.3 or partition walls 40.1 , 40.2 are further provided with a pressure regulation system, configured to adjust pressure differential between the pressurised inert gas filled inside the space 40.3 and product gas produced I reformed in the first I second gas-tight compartment 101 1 102. In doing so, the pressure of the pressurised inert gas can be kept higherthan process pressures, i.e. pressures of the product gas inside the first gas-tight compartment 101 and / or of the reformed product gas inside the second gas-tight compartment 102, within a predetermined range for forming a sufficient pressure lock between the first and second gas-tight compartments 101 and 102 without damaging the partition walls 40.1 ,40.2. Distance between the partition walls needs only be enough to make it possible to arrange the system for leading inert gas to the gasifier and pressurize the space. However, if desired the space 40.3 may be dimensioned so that the wallstructure 40 can be inspected or serviced via the space 40.3 and the space may also be used for arranging auxiliary equipment of the gasifier and / or the POX reactor 102.(Sixth embodiment)

[0070] Fig. 7 depicts schematically a fluidised bed gasifier obtained by converting the CFB boiler 1 according to a sixth embodiment of the present invention. The sixth embodiment is another embodiment of the present invention in which the existing fluidised bed boiler is converted into a gasification reactor and a combustion reactor. In Fig. 7, the same elements as shown in Fig. 4A are numbered with the same reference signs, and the description thereof is referred to for applicable parts. The arrangement of Fig. 7 differs from that of Fig. 4A mainly in that the first gas-tight partition wall structure 40 is comprised of two partition walls 40.1 , 40.2.

[0071] Like the fifth embodiment as described above, the partition walls 40.1 and 40.2 are substantially impervious to gas, and together form the first gas-tight partition wall structure 40 so as to divide the furnace chamber 10 into a first gas-tight compartment 121 and a second gas-tight compartment 122. The partition wall 40.1 and the partition wall 40.2 are spaced apart in a direction from one of the two gas-tight compartments 121 , 122 toward the other. Thereby, between the two partition walls 40.1 , 40.2 is defined a space, which acts as a seal or cushion layer that increases the gas and thermal insulating abilities of the first gas-tight partition wall structure 40.

[0072] Furthermore, also for this embodiment, in the conversion, a system 91 ,90 for leading inert gas into the space 40.3 is provided to the fluidized bed gasifier. This way the space defined between the partition walls 40.1 , 40.2 is preferably filled with a pressurised inert gas such as N2and CO2, possibly introduced from the source of gas 90 through the line 91 . Such pressurised inert gas between the partition walls 40.1 , 40.2 can further prevent or reduce gas transfer between the first gas-tight compartment 121 and the second gas-tight compartment 122, such that substantially no product gas will pass through the partition wall 40.1 from the first gas-tight compartment 121 , and that substantially no flue gas will pass through the partition wall 40.2 from the second gas-tight compartment 122.

[0073] Also, in this embodiment distance between the partition walls needs only be enough to make it possible to arrange the system for leading inert gas to the gasifier and pressurize the space. And similarly to the fifth embodiment, if desired the space 40.3 may be dimensioned so that the wall structure 40 can be in- spected or serviced via the space 40.3 and the space may also be used for arranging auxiliary equipment.

[0074] 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 obvious to the skilled person that, along with the technical pro- gress, the basic idea of the invention can be implemented in many ways. The invention and its embodiments are thus not limited to the examples and samples described above but they may vary within the contents of patent claims and their legal equivalents. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combi- nation is technically feasible.

Claims

25Claims1 . A method of converting an existing fluidised bed boiler (1) into a fluidised bed gasifier (1 ’), the existing fluidised bed boiler (1) comprising:- a furnace chamber (10), which has a top wall (12), a grid (13), and walls (11.1 , 11.2, 11.3, 11.4),- a windbox (20) arranged in connection with the grid (13),- a particle separator (30) arranged adjacent to the furnace chamber (10), wherein the particle separator (30) comprises an inlet (32) connected to one of the walls (11.1 , 11 .2, 11 .3, 11 .4), a gas outlet, and a particle outlet, characterised in that the method of converting the existing fluidised bed boiler (1) comprises steps of- providing a first gas-tight partition wall structure (40) inside the furnace chamber (10), which extends from the grid (13) to the top wall (12) of the furnace chamber (10) so as to divide the furnace chamber (10) into a first gas-tight compartment (101 , 121) and a second gas-tight compartment (102, 122), wherein a gasification reactor is provided in the first gas-tight compartment (101 , 121) which is connected to the inlet (32) of the particle separator (30),- providing a second gas-tight partition wall structure (50) inside the windbox (20), such that a first windbox chamber thus formed is configured to deliver fluidising gas to the gasification reactor.

2. The method of converting a fluidised bed boiler according to claim 1 , wherein the method of converting the fluidised bed boiler (1) further comprises step of forming the first gas-tight partition wall structure (40) by two partition walls (40.1 , 40.2) that are spaced apart in a direction from the first gas-tight compartment (101 , 121) toward the second gas-tight compartment (102, 122).

3. The method of converting a fluidised bed boiler according to claim 2, wherein the method of converting the fluidised bed boiler (1) further comprises step of filling a space (40.3) defined between the two partition walls (40.1 , 40.2) with a pressurised inert gas (90).

4. The method of converting a fluidised bed boiler according to any one of claims 1 to 3, wherein the method of converting the fluidised bed boiler (1) further comprises step of forming a heat-resistant refractory layer (43) on the top wall (12), the walls (11.1 , 11 .2, 11 .4) and the first gas-tight partition wall structure (40) of the first gas-tight compartment (101 , 121).

5. The method of converting a fluidised bed boiler according to any one of claims 1 to 4, wherein the method of converting the fluidised bed boiler (1) further comprises steps of- providing a gas conduit (103) extending between the gas outlet of the particle separator (30) and top of the second gas-tight compartment (102),- providing a burner (60) at upper part of the second gas-tight compartment (102) defining a partial oxidation reactor in the second gas-tight compartment (102), and- providing a product gas outlet (70) in the vicinity of bottom of the second gastight compartment (102), for discharging reformed product gas.

6. The method of converting a fluidised bed boiler according to claim 5, wherein the method of converting the fluidised bed boiler (1) further comprises step of providing the burner (60) at an outlet of the gas conduit (103) in the second gas-tight compartment (102).

7. The method of converting a fluidised bed boiler according to any one of claims 1 to 4, wherein the existing fluidised bed boiler (1) comprises at least two particle separators (30, 31) arranged adjacent to the furnace chamber (10), each of the particle separators (30, 31) comprising an inlet connected to one of the walls (11.1 , 11.2, 11.3, 11.4), a gas outlet, and a particle outlet, the method of converting the fluidised bed boiler (1) further comprises steps of- providing the first gas-tight partition wall structure (40) inside the furnace chamber (10) such that both the first gas-tight compartment (121) and the second gastight compartment (122) are provided with at least one particle separator (30, 31), wherein the gasification reactor is provided in the first gas-tight compartment (121) which is connected to inlet of a first particle separator (30), and wherein a combustion reactor is provided in the second gas-tight compartment (122) which is connected to inlet of a second particle separator (31),- providing the second gas-tight partition wall structure (50) inside the windbox (20), such that the first wind box chamber is arranged to deliver fluidising gas to the gasification reactor and a second windbox chamber is arranged to deliver fluidising gas to the combustion reactor,- providing a first particle transfer line (80) extending between particle outlet of the first particle separator (30) and the combustion reactor, and- providing a second particle transfer line (82) extending between particle outlet of the second particle separator (31) and the gasification reactor.

8. The method of converting a fluidised bed boiler according to claim 7, wherein the existing fluidised bed boiler (1) further comprises a back pass (38) in which one or more heat exchangers are located, and a flue gas duct system arranged between gas outlets of the particle separators (30, 31) and the back pass (38) for conducting cleaned flue gas to the back pass, the method of converting the fluidised bed boiler further comprises step of- modifying the flue gas duct system into a first duct sub-system connected to gas outlet of the first particle separator (30) for drawing product gas to further processing and a second duct sub-system connected to gas outlet of the second particle separator (31) for drawing exhaust gas to further processing.

9. A fluidised bed gasifier (1 ’) retrofitted from an existing fluidised bed boiler (1), the existing fluidised bed boiler (1) comprising:- a furnace chamber (10) which has a top wall (12), a grid (13), and walls (11.1 , 11.2, 11.3, 11.4),- a windbox (20) arranged in connection with the grid (13),- a particle separator (30) arranged adjacent to the furnace chamber (10), wherein the particle separator (30) comprises an inlet (32) connected to one of the walls (11.1 , 11 .2, 11 .3, 11 .4), a gas outlet, and a particle outlet, characterised in that the retrofitted fluidised bed gasifier (1 ’) comprises:- a first gas-tight partition wall structure (40) inside the furnace chamber (10), which extends from the grid (13) to the top wall (12) of the furnace chamber (10), such that the furnace chamber (10) is divided into a first gas-tight compartment (101 , 121) and a second gas-tight compartment (102, 122),28- a gasification reactor arranged to the first gas-tight compartment (101 , 121) which is connected to the inlet (32) of the particle separator (30),- a second gas-tight partition wall structure (50) located inside the windbox (20), such that a first windbox chamber is formed by at least part of the windbox (20) and arranged to deliver fluidising gas to the gasification reactor.

10. The fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to claim 9, wherein the first gas-tight partition wall structure (40) is comprised of two partition walls (40.1 , 40.2) spaced apart in a direction from the first gas-tight compartment (101 , 121) toward the second gas-tight compartment (102, 122).11 . The fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to claim 10, wherein a space (40.3) defined between the two partition walls (40.1 , 40.2) is filled with a pressurised inert gas (90).

12. The fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to any one of claims 9 to 11 , wherein a heat-resistant refractory layer (43) is formed on the top wall (12), the walls (11.1 , 11.2, 11.4) and the first gastight partition wall structure (40) of the first gas-tight compartment (101 , 121).

13. The fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to any one of claims 9 to 12, wherein a gas conduit (103) is arranged to extend between the gas outlet of the particle separator (30) and top of the second gas-tight compartment (102), a burner (60) is arranged to upper part of the second gas-tight compartment (102), and a product gas outlet (70) is arranged in the vicinity of bottom of the second gas-tight compartment (102), for discharging reformed product gas.

14. The fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to claim 13, wherein the burner (60) is arranged at an outlet of the gas conduit (103) in the second gas-tight compartment (102).

15. The fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to any one of claims 9 to 12, wherein the existing fluidised bed boiler29(1) comprises at least two particle separators (30, 31) arranged adjacent to the furnace chamber (10), each of the particle separators (30, 31) comprising an inlet connected to one of walls (11.1 , 11.2, 11.3, 11.4), a gas outlet, and a particle outlet, the first gas-tight partition wall structure (40) is arranged inside the furnace chamber (10) such that both the first gas-tight compartment (121) and the second gastight compartment (122) are provided with at least one particle separator (30, 31), the gasification reactor is arranged in the first gas-tight compartment (121), which is connected to inlet of a first particle separator (30), a combustion reactor is arranged in the second gas-tight compartment (122), which is connected to inlet of a second particle separator (31), the second gas-tight partition wall structure (50) is arranged inside the windbox (20) such that the first windbox chamber is arranged to deliver fluidising gas to the gasification reactor and a second windbox chamber is arranged to deliver fluidising gas to the combustion reactor, a first particle transfer line (80) is arranged to extend between particle outlet of the first particle separator (30) and the combustion reactor, and a second particle transfer line (82) is arranged to extend between particle outlet of the second particle separator (31) and the gasification reactor.

16. The fluidised bed gasifier retrofitted from an existing fluidised bed boiler according to claim 15, wherein the existing fluidized bed boiler (1) further comprises a back pass (38) in which one or more heat exchangers are located, and a flue gas duct system arranged between gas outlets of the particle separators (30, 31) and the back pass (38) for conducting cleaned flue gas to the back pass (38), and the retrofitted fluidised bed gasifier (1 ’) comprises: first duct sub-system arranged to a first part of the flue gas duct system and connected to gas outlet of the first particle separator (30), for drawing product gas to further processing, and second duct sub-system arranged to a second part the flue gas duct system and connected to gas outlet of the second particle separator (31), for drawing exhaust gas to further processing.

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