[001] The present invention relates to an arrangement for gasifying solid fuel in accordance with the preamble of claim 1.

[002] The invention also relates to a method of gasifying solid fuel in accordance with the preamble of claim 9. [003] Hot gases treated in certain industrial processes contain components that have a tendency to stick on the heat surfaces. Sticky compounds may also be generated as a result of cooling. This complicates the recovery of heat from the gases or cooling the gas. [004] Problems also appear in the gasification processes because of the substances sticking on heat exchange surfaces. Gasification or combustion of solid carbonaceous material in a circulating fluidized bed reactor, in which such a high gas flow velocity is maintained that a considerable portion of the solid particles is entrained with gas from the reaction chamber and after particle separation mainly returned to the fluidized bed, has been noted to have many advantages compared to the conventional gasification or combustion methods.

[005] When gasifying carbonaceous fuels, such as biofuels or waste derived fuels, generally air and/or oxygen as well as steam are supplied to the gasification reactor, whereby an object is to generate product gas, the main components of which are carbon monoxide CO, hydrogen H ₂ , and hydrocarbons C _x H _y . Ash particles and residual carbon are usually entrained with the product gas exiting from the gasification reactor. Depending on the concept, they must possibly be separated by a particle separator, for example, by a filter, prior to further use of the product gas. Generally, the aim is to optimize the efficiency of the gasification system in such a way that the coal conversion level of the fuel is as high as possible, in other words the content of the residual carbon in the ash removed from the equipment is as low as possible. [006] Especially with gasification gases derived from biofuels, heat recovery and also possibly further use of the gas are substantially complicated by components contained in the biofuels that have a tendency to stick on the surfaces. Sticky compounds may also be generated as a result of cooling.

[007] The product gas exiting from the gasification reactor generally also contains ash particles, which need to be removed, for example, by means of a particle filter prior to further use of the product gas. Since the particle filters filtering gas at a high temperature are expensive and prone to get damaged, the product gas is generally cooled prior to filtering. Especially when gasifying waste materials and biomass, considerable amounts of tar compounds can be generated. Here, tar compounds refer to compounds or components which are gaseous at the gasification temperature, but are condensed at lower temperatures to droplets, which stick easily, and further even to solid particles, which can build, for example, on heat exchange surfaces of the gas cooler or filter deposits that are difficult to be removed. Thus, tar compounds, for example, reduce the heat exchange efficiency of the heat exchange surfaces weakening the operation of the equipment and clog filtering elements of the filter increasing the pressure loss. [008] The amount of tar compounds can be diminished by means of thermal cracking. The tar compounds are then decomposed by thermal cracking and the amount of tar compounds in the final product gas diminishes. The thermal cracking of the product gas is performed by raising the gas temperature after gasification high enough, whereby the generated tars are decomposed to simpler compounds. The simplest way to do this is to introduce to the product gas either oxygen or air. A portion of the combustible components of the gas thereby burns and the temperature rises. The temperature required for cracking of tar compounds is about 1000 - 1200 °C. The product gas consumed for combustion is compensated by compounds generated in thermal cracking.

[009] Publication JP 1 1043681 discloses gasification of biofuels in a fluidized bed reactor. The product gas from a fluidized bed reactor is guided to an oxidizing oven operating at a temperature higher than the fluidized bed reactor, in which oven secondary gasification takes place. The temperature in the oxidizing oven is 1200 - 1600 °C, whereby, for example, tar compounds decompose. The lower portion of the oxidizing oven is provided with a cooling portion, in which gas and the formed melt material are cooled by conducting them to water. The quick water cooling solidifies the melt material and the thus granulated material is removed from the cooler and the gas is guided to further treatment.

[0010] Publication US 2007/0175095 discloses a biomass gasification system, in which the product gas from the actual gasification stage is conducted to a down- stream reforming unit, in which the tar components of the product gas are decomposed by thermal cracking. Oxygen is supplied to the reforming unit, whereby the fuel oxidizes, which increases the temperature to a level required by the thermal cracking. This causes cracking of tar compounds. Gas is cooled after the reforming unit and conducted to be used. Here, the melt material from the gasification stage is led to act as fuel in a separate heater providing heat to the gasification. In the method disclosed in the publication, the question of the treatment of melt components generating in the actual thermal cracking remains completely open. Thus, the solution is especially prone to clogging of the heat surfaces downstream of the reforming unit.

[0011] It is an object of the invention to provide an arrangement for and method of gasifying solid fuel, by means of which it is possible to minimize the problems of the prior art. [0012] Objects of the invention are achieved by means of an arrangement for gasifying solid fuel, which arrangement comprises a gasification reactor for producing oxidizable product gas from solid fuel and a gas treatment reactor arranged in flow direction of the product gas in gas flow connection with the gasification reactor, said gas treatment reactor comprising means for supplying oxygenous gas to the gas treatment reactor for partial oxidization and thermal cracking of the product gas. The main characteristic feature of the invention is that a radiation heat exchange cooler of product gas is arranged in connection with the gas treatment reactor to solidify melt components in the product gas and that a discharge connection is arranged in the lower portion of the radiation heat exchange cooler for removing solid material separated from the product gas, especially solidified melt components, from the radiation heat exchange cooler.

[0013] By using such an arrangement, no catalysts are needed for decomposing the tar components of the gasification gas, whereby the operation of the arrangement is reliable. At the same time, it is possible to cool down gas containing sticky and/or melt components produced in thermal cracking in a reliable manner, and to remove said components in solid form. The arrangement is also energy efficient, as considerable amounts of heat is recovered from sticky and/or melt components before their removal in solid form.

[0014] According to a preferred embodiment of the invention, the radiation heat exchange cooler is formed of walls defining a gas space of the radiation heat exchange cooler. The walls of the radiation heat exchange cooler comprise heat exchange surfaces and the gas space remaining inside the walls is substantially free space.

[0015] This way, the risk of clogging the cooler is minimized and a reliable cooling and the change of sticky and/or melt components to non-sticky are obtained prior to their removal from the cooler.

[0016] The gas treatment reactor is preferably a vertical reactor, the upper portion thereof being provided with an inlet for supplying the product gas to the reactor, and the lower portion thereof being provided with said radiation heat exchange cooler.

[0017] The lower portion of the radiation heat exchange cooler is preferably arranged with a turn chamber for the gas flow, the lower portion of which chamber is provided with said discharge connection and which chamber is provided with a gas discharge opening in such a manner that the flow direction of the gas flowing through the turn chamber substantially changes in the turn chamber. The gas discharge opening opens to the turn chamber preferably in such a manner that the gas flow direction changes in the turn chamber by at least 90°. [0018] Preferably, the upper portion of the gas treatment reactor comprises a refractory coating, for example, masonry.

[0019] According to a preferred embodiment, the above mentioned gas discharge opening is connected to a convection boiler, comprising at least one heat exchanger. Preferably, the convection boiler comprises at least two heat exchangers, which are subsequently arranged in horizontal direction. The heat exchanger or the heat exchangers of the convection boiler are preferably arranged directly above the bottom portion of the convection boiler and a conveyor for solid material is arranged at the bottom portion of the convection boiler. The conveyor is preferably arranged to transfer solid material from the bottom portion of the convection boiler to the lower portion of the turn chamber of the gas flow arranged in the lower portion of the radiation heat exchange cooler. [0020] The gasification reactor is preferably a circulating fluidized bed reactor, which comprises a solids separator, the gas discharge connection of which is in gas flow connection with the gas treatment reactor.

[0021] The objectives of the invention are achieved also by means of a method of gasifying solid fuel in a gasification reactor, in which oxidizable product gas is produced from solid fuel, said product gas being led from the gasification reactor to a gas treatment reactor, to which gas treatment reactor oxygenous gas is introduced and product gas is partially oxidized and its temperature is raised, whereby thermal cracking of the components of the product gas is achieved. It is a charac- teristic feature of the method that in the method solid components of the product gas are melt and/or softened to become sticky forming melt components, whereafter the gas is directed to a radiation heat exchange cooler, in which the temperature of the product gas is decreased by means of radiation heat exchange in such a way that melt components in the product gas solidify and solidified components are discharged from the radiation heat exchange cooler in solid form through a discharge connection arranged in the lower portion thereof. [0022] The product gas is preferably guided to flow in the gas treatment reactor substantially vertically from the top downwards, and the direction of the product gas flow in the lower portion of the radiation heat exchange cooler is changed, whereafter the product gas flow is conducted to a convection boiler. The direction of the production gas flow is preferably changed by 90 - 180 degrees.

[0023] In the method, oxidizable product gas is produced from solid fuel in a flui- dized bed, whereby the material composition of the fluidized bed is controlled at least partially based on the melting or softening behavior of the gas components in the gas treatment reactor.

[0024] Other additional characteristic features typical of the invention become apparent in the accompanying claims and in the description of the embodiments in the figures.

[0025] The invention and the operation thereof is described below with reference to the enclosed schematic drawing, in which

Fig. 1 schematically illustrates an embodiment of an arrangement in ac- cordance with the invention; and

Fig. 2 schematically illustrates another embodiment of an arrangement in accordance with the invention.

[0026] Fig. 1 discloses an embodiment in accordance with the invention of an arrangement 10 for gasifying solid fuel. The embodiment of Fig. 1 comprises a gasification reactor 12', in which fuel is gasified in such a way that the product gas generated can be further oxidized. The arrangement also comprises a gas treatment reactor 20 arranged in flow direction of the product gas in gas flow connection with the gasification reactor 12' for thermal cracking of the product gas and a radiation heat exchange cooler 41 of gas arranged in connection with the gas treatment reactor. This entity provides an arrangement for generating oxidizable product gas from solid fuel, by means of which arrangement good quality product gas can be generated in a reliable manner by utilizing thermal cracking and at the same time taking care of the melt components generated in the thermal cracking in an operationally reliable manner by solidifying them to a non-sticky form and by treating them in a non-sticky form.

[0027] Product gas is thus generated in a gasification reactor and conducted substantially non-cooled to a gas treatment reactor following the gasification reactor 12' in flow direction of the product gas. A radiation heat exchange cooler 41 for gas is in connection with the gas treatment reactor 20, which in this embodiment is further connected, for example, to a convection boiler 40 for further cooling of the product gas.

[0028] The gas treatment reactor 20 is preferably a vertical reactor, in which gas is arranged to flow substantially from the top downwards. The upper portion thereof is provided with an inlet 26 for introducing product gas to the reactor 20. The gas treatment reactor preferably comprises means 22 for introducing oxygenous gas to the reactor, preferably arranged into connection with the inlet 26. Means 22 are in connection with a gas source 24, preferably containing either oxygen or a mixture of oxygen and steam. Means 22 for introducing oxygenous gas to the reactor can also comprise separate channels for oxygenous gas and steam, whereby means 22 are in connection both with a source of oxygenous gas and a source of steam (not shown). In order to efficiently treat the product gas, means 22 for introducing oxygenous gas have preferably been arranged to the centerline of the inlet 26 in such a way that oxygenous gas and steam can be led to the reactor in such a way that the flow thereof is directed substantially in parallel with the flow direction of the product gas.

[0029] The oxygen supplied through means 22 oxidizes a portion of the combustible components of the product gas and the temperature of the gas rises. Thus, when the apparatus is operated, an oxidizing zone 27 is formed in connection with the inlet 26. The inlet area in the upper portion of the gas treatment reactor is provided with refractory lining 34, such as masonry. The masonry lining has been used for coating substantially all surfaces in the upper portion of the gas treatment reactor. The lining continues to a distance from the inlet in such a manner that it extends at least until it covers the oxidation zone of the gas treatment reactor. The refractory lining acts as heat insulation and, thus, the structure allows the increase of the gas temperature high enough to bring about thermal cracking. The structure external of the refractory lining may as such be a cooled structure because of the endurance of the structure. A temperature of about 1100 - 1400°C is preferably maintained in the upper portion of the gas treatment reactor 20. Although herein is referred to an oxidation zone, it must be understood that the product gas is only partially oxidized at this stage and that also the final product gas is still oxidizable gas. At a high temperature, tar compounds of the product gas are cracked by thermal cracking, whereby the amount of tar compounds in the product gas diminishes, since the tar compounds formed in the product gas decompose to simpler compounds. At the same time, the product gas used for combustion is compensated by compounds generated by thermal cracking.

[0030] The high temperature maintained in the gas treatment reactor 20 softens or even melts the solids, which can also be called fly ash, arriving to the gas treatment reactor 20 through the separator 14. The softened fly ash particles stick on the surrounding surfaces, from which they can be removed by soot blowing. For this purpose, the arrangement preferably comprises soot blowers. High pressure water injection means have preferably been arranged in connection with the re- fractory lined surface of the treatment reactor, whereby it is possible to remove ash stuck on the refractory lined surface by high pressure water injection.

[0031] The radiation heat exchange cooler 41 begins from beneath the refractory lined portion, from the close proximity of thereof. In other words, the walls 21 of the lower portion of the gas treatment reactor 20 act as radiation heat exchangers, which cool the product gas. The radiation heat exchange cooler is formed of walls 21 , which define a gas volume in the radiation heat exchange cooler, which gas volume is substantially free space. In other words, no heat exchanger structures affecting the gas flow are arranged in the gas volume. Softened and/or melt fly ash thereby also sticks on the walls of the lower portion of the gas treatment reactor 20. Preferably, there are soot blowers 44 in connection with the walls of the lower portion of the gas treatment reactor, by means of which it is possible to remove the solidified material accumulated on the walls. The soot blowers 44 may be, for example, rapping hammer type soot blowers, which can provide impacts to a wall of the radiation heat exchanger from the outside thereof. Soot blowers are preferably positioned to cause effect on all surfaces of the radiation heat exchange cooler.

[0032] A turn chamber 28 for the gas flow is provided in the lower portion of the gas treatment reactor, from which chamber a gas outlet opening 30 opens to convection boiler 40. Also, the walls of the turn chamber 28 act at the same time as radiation heat exchangers. In the lower portion of the turn chamber, there is a discharge connection 46 for removing solid material separated in a solid form from the product gas. The solid material separated from the walls of the lower portion of the gas treatment reactor 20 is guided along the walls of the reactor and turn chamber 25 to the discharge connection 46 to be further treated. [0033] Especially biofuels contain ash, which have alkali components, such as potassium and sodium. The alkali components melt at the high temperatures of the thermal cracking. In the presence of chlorine and other ash components, the sodium and potassium salts form a very strongly corroding mixture in the melt phase, which is very harmful for many lining materials and pressure vessel steels. This can be, according an embodiment of the invention, considerably decreased by adding an appropriate amount of peat or other fuel containing acid components, such as silicon or sulphur. Thereby, the corroding effect of the melt ash generating in thermal cracking will substantially decrease. [0034] Fig. 2 illustrates another embodiment of an arrangement 10 in accordance with the invention for gasifying solid fuel. The embodiment of Fig. 2 comprises a circulating fluidized bed reactor 12, which acts as gasification reactor, and fuel is gasified in a fast fluidized bed formed said gasification reactor in such a way that oxidizable product gas is generated. The arrangement also comprises a treatment reactor 20 for gas generated in the reactor and connected in flow direction of the product gas in gas flow connection with the circulating fluidized bed reactor 12 and a radiation heat exchange cooler 41 arranged in connection therewith. [0035] The arrangement is especially advantageous, when the fuel used is bio- mass. The structure and basic operation of a circulating fluidized bed reactor 12 is known as such. The circulating fluidized bed reactor comprises, for example, inlet means 16 for fluidizing gas and inlet means 18 for fuel and/or bed material. The circulating fluidized bed reactor 12 also comprises a separation apparatus 14 for solid material, such as one or more cyclones, in which solid material, especially bed material, is separated from the product gas and returned as so called external circulation back to the reactor. The product gas is conducted from the separation apparatus 14 of the circulating fluidized bed reactor 12 to a gas treatment reactor 20 following it in gas flow direction, shown with arrow A, substantially non-cooled. There is a radiation heat exchange cooler 41 for gas in connection with the gas treatment reactor 20, which is in this embodiment further connected to a convection boiler 40, which is a so called horizontal boiler. All the main heat exchangers 42 in the horizontal boiler are horizontally subsequently supported. The gas cooler 41 is mainly formed of radiation heat exchanger surfaces 21.

[0036] The gas treatment reactor 20 is also in this case preferably a vertical reactor, in which gas is arranged to flow substantially from the top downwards. An inlet 26 is arranged in the upper portion thereof for introducing product gas to the reac- tor 20. The gas treatment reactor 20 preferably comprises means 22 for supplying oxygenous gas to the reactor arranged in connection with the inlet 26. The means 22 are in connection with a gas source 24 preferably containing either oxygen or mixture of oxygen and steam. Means 22 for supplying oxygenous gas to the reactor can also comprise separate channels for oxygenous gas and steam, whereby means 22 are in connection both with the source of oxygenous gas and the source of steam (not shown). In order to efficiently treat the product gas, the means 22 for feeding oxygenous gas are preferably arranged to the centerline of the inlet 26 and in such a manner that oxygenous gas and steam can be supplied to the reactor in such a way that the flow thereof is directed substantially in parallel with the flow direction of the product gas.

[0037] The oxygen supplied through means 22 oxidizes a portion of the combustible components of the product gas and the gas temperature rises. Thus, when the apparatus is in operation, an oxidation zone 27 is formed in connection with the inlet 26. The inlet area in the upper portion of the gas treatment reactor is supplied from the inside with a refractory lining 34, such as masonry. The refractory lining is used for substantially all surfaces in the upper portion of the gas treatment reactor. The refractory lining continues from the inlet to a distance therefrom in such a manner that it extends at least to such a distance that the oxidation zone of the gas treatment reactor is within the area of the refractory lining. The refractory lining acts as heat insulation and the structure thus allows the rise of the gas temperature high enough to bring about thermal cracking. The structure external of the refractory lining may as such be a cooled structure because of the endurance of the structure. Preferably, a temperature of about 1 100 - 1400°C is maintained in the upper portion of the gas treatment reactor 20. Although herein is referred to an oxidation zone, it must be understood that the product gas is only partially oxidized at this stage and also the final product gas is still oxidizable gas. At a high temper- ature, tar compounds of the product gas are decomposed by means of thermal cracking, whereby the amount of tar compounds in the product gas diminishes, because the tar compounds formed in the product gas decompose to simpler compounds. At the same time, the product gas consumed to combustion is compensated by compounds generated by thermal cracking.

[0038] When the circulating fluidized bed reactor 12 is operated in the embodiment of Fig. 2 according to an embodiment in such a way that the gasification temperature is decreased in the reactor, whereby the amount of solid carbon and/or hydrocarbons entrained from the gasifier reactor 12 to the gas treatment reactor 20 through a separator 14 increases. The partial oxidation of the gas treatment reactor thereby changes in such a way that the flame formed therewith is more advantageous as for the radiation heat exchange and, thus, the efficiency of the radiation heat exchange can be increased in the gas treatment reactor. [0039] The high temperature maintained in the gas treatment reactor 20 softens or even melts solid material arriving to the gas treatment reactor 20 through the separator 14, which may also be called fly ash. Softened fly ash particles stick on the surrounding surfaces, from which they can be removed by soot blowing. Therefore, the arrangement also preferably comprises soot blowers. High pressure water injection means are preferably arranged in connection with the refractory lined surface of the treatment reactor, whereby ash stuck on the refractory lined surface can be successfully removed, for example, by means of high pressure water injection.

[0040] The walls of the lower portion of the gas treatment reactor 20 below the refractory lined portion act as radiation heat exchangers, cooling down product gas. The radiation heat exchange cooler is formed of walls defining a gas volume in the radiation heat exchange cooler, the gas volume being substantially free space. In other words, no heat exchanger structures affecting the gas flow are arranged in the gas volume. When gas is cooled down, softened and/or melt fly ash sticks to a certain extent also to the walls of the lower portion of the gas treatment reactor 20 and solidifies to the surface thereof. For this purpose, soot blow- ers 44 are preferably provided in connection with the walls of the lower portion of the gas treatment reactor, by means of which material solidified and accumulated on the walls can be removed. Soot blowers 44 are rapping hammer type soot blowers, by means of which impacts can be generated on the wall of the radiation heat exchanger from the outside thereof.

[0041] As can be seen in the drawings, the radiation heat exchanger, in other words cooled wall, comprises heat exchange channels, such as tubes. The collecting headers of the tubes in the cooled wall are referred to with reference number 23 in the figures. The heat exchange channels of the radiation heat exchange cooler 41 extend in the drawings only below the refractory lined portion or to the lower end thereof. Thereby, the structure of the upper portion can be joined with the radiation heat exchange cooler in such a manner that the use of soot blowers 44 arranged in connection with the radiation heat exchange cooler does not cause any significant transmission of soot blowing impacts, which are adverse to the endurance of the refractory lining, to the refractory lining. It has also been disclosed in Fig. 2, how the refractory lining of the upper portion can be of separately cooled structure, the collecting headers of cooling tubes of which are shown with reference number 23'. [0042] A turn chamber 28 for the gas flow is arranged to the lower portion of the gas treatment reactor, from which a gas discharge opening 30 opens to a convection boiler 40, substantially upwards. The walls of the turn chamber 28 also oper- ate at the same time as radiation heat exchangers. The lower portion of the turn chamber is provided with a discharge connection 46 for the discharge of solid material separated from the product gas. The solid material separated from the walls of the lower portion of the gas treatment reactor 20 is conducted along the walls of the reactor and the turn chamber 28 to the discharge connection 46 to be further treated.

[0043] In the embodiment of Fig 2, the turn chamber 28 is formed in the gas treatment reactor in such a manner that it comprises with the convection chamber 40 a common wall 32, the gas being arranged to flow beneath said common wall. Thus, the direction of the product gas flow is changed in the lower portion of the gas treatment reactor by 90 to 180 degrees, whereafter the product gas flow is conducted to the convection boiler 40. The direction of the product gas flow is preferably changed by 135 to 180 degrees. [0044] Gas is conducted from the turn chamber 28 to the convection boiler 40. At least one heat exchanger, preferably two heat exchangers 42, which are horizontally subsequently supported, are arranged to the gas space thereof. Solid material from the product gas also sticks on the surfaces of the heat exchangers of the convection boiler and it needs to be removed from the surfaces. When the heat exchangers are arranged horizontally subsequently, in other words not one on top of the other, it is possible to prevent the solid material dislodged from the heat exchanger first in the gas flow direction from being drifted to the surfaces of the following heat exchanger. [0045] Collecting spaces 48 for solid material are arranged beneath the heat exchangers 42. The first heat exchanger is preferably, however, partially above the discharge opening 30 of the turn chamber 28. More solid material accumulates on the surface of the first heat exchanger than to other heat exchangers 42 of the convection boiler and it is thus advantageous that the solid material removed from the first heat exchanger may fall, due to gravity, directly to the lower portion of the turn chamber 28 to be removed. There is a conveyor 50, such as a screw conveyor, in connection with collecting space beneath the other heat exchangers subsequent to the first heat exchanger, by means of which solid material sepa- rated from these heat exchangers is conducted also to the lower portion of the turn chamber 28 through a channel 52 connecting them.

[0046] The cooled gases are conducted from the convection boiler 40 through a possible filtering apparatus 55 to be further used.

[0047] By mixing peat with biofuel, it is possible to have effect on the behavior of the ash at the same time in such a way that the stickiness of the ash to the refractory lining of the gas treatment reactor diminishes or the ash can be easily removed from the refractory lined surfaces.

[0048] According to an embodiment of the invention, the fuel to be gasified is biofuel, whereby a pre-determined amount of peat is dosed to the fuel and/or bed material. The method of gasifying solid fuel thereby comprises a step of determining the amount and/or quality of melt and/or sticky material generating in the gas treatment reactor and adjusting the amount of peat in the fuel in such a manner that the amount and/or quality of melt and/or sticky material generating in the gas treatment reactor is within pre-determined limits. Thus, the fouling of the convection boiler can also be diminished and the soot blowing of ash from the heat surfaces made easier by adding peat to the biofuels. The bed material or the bed material mixture used in a fluidized bed gasifier may also be used for influencing the stickiness or easiness of soot blowing of the ash.

[0049] It must be noted that the above description discloses only some of the most preferred embodiments of the invention. Thus it is obvious that the invention is not limited to the disclosed embodiments, but it can be applied in many ways. The arrangement can be realized also in such a way that a so called slow fluidized bed is used as the gasification reactor. Features described in connection with different embodiments can also be used in connection with other embodiments within the basic concept of the invention and/or disclosed features can be bined to different entities, if so desired and they are technically feasible.