The invention relates to a method for gasifying fuel, such as wood chips, with a co- current gasifier, which has a combustion chamber and a liquid channel for circulating a cooling liquid, such as water. The invention also relates to an apparatus used in the method.

Combustible product gas can be manufactured from solid fuels, such as wood chips, by means of a co-current gasifier. The co-current gasifier has a fuel silo and beneath it a combustion chamber. The fuel is fed from above into the upper part of the fuel silo, from where it flows due to gravity downwards into the combustion chamber, where gasification of the fuel occurs. The combustion air required in the gasification is fed to the central part of the gasifier and the generated product gas exits from the lower part of the reactor.

In many co-current gasifiers the heat generated in the combustion chamber is used for heating up the combustion air led into the gasifier and for drying the fuel, such as the wood chips, in the fuel silo. In such gasifiers pyrolysis of the fuel occurs partly already in the fuel silo. A weakness with these gasifiers is the difficult control of the circumstances of the pyrolysis reaction, wherefore contaminants are easily generated in the product gas. Co-current gasifiers are also known, in which the aim is to make the pyrolysis reaction as short as possible with regards to time. In these gasifiers the heating and drying of the fuel in the silo is actively prevented, so that the pyrolysis would occur as close to the combustion chamber as possible or only in the combustion chamber.

Reference publication WO 2008/145814 A1 shows a co-current gasifier, which has a fuel silo and beneath it a combustion chamber. Between the fuel silo and the combustion chamber there is a ring-shaped air channel, through which the combustion air is led into the combustion chamber. The purpose of the air channel is to limit the transfer of heat from the combustion chamber to the fuel silo. In addition to air, or instead of the air, some other cooling medium, such as water, may also be circulated in the air channel. Reference publication US 1524466 shows a fuel feeding device, which may be used in gasifiers and furnaces. The feeding device comprises three ring-shaped parts, which are fitted on top of each other. In the lowest ring-shaped part there is a cooling channel, where cooling water may be circulated. In the solutions described in reference publications WO 2008/145814 A1 and US 1524466 the purpose of circulating cooling liquid is only to prevent the transfer of heat from the combustion chamber to the fuel silo.

There are many drawbacks related to prior art liquid-cooled co-current gasifiers. The fuel burns at a high temperature in the combustion chamber, wherefore the temperature of the outer surface of the combustion chamber of the gasifier is high. The high temperature of the combustion chamber easily causes a fire risk. A cooling liquid circulation between the fuel silo and the combustion chamber cannot solve the heating problem of the outer surface of the combustion chamber. In the combustion of fuel, ash is generated in the combustion chamber, which ash must periodically be removed. In order to remove the ash, the combustion chamber must be equipped with an ash removal hatch, which can be opened, which hatch further decreases the fire safety of the gasifier. The fire in the combustion chamber must be extinguished for the duration of the ash removal, whereby the gas production is interrupted.

It is an object of the invention to provide an improved method and apparatus for gasifying bio fuel, by which the drawbacks and disadvantages relating to the prior art can be removed.

The objects of the invention are obtained with a method and an apparatus, which are characterised in what is presented in the independent claims. Some advantageous embodiments of the invention are presented in the dependent claims.

The invention relates to a method for gasifying combustion fuel, advantageously solid combustion fuel, such as wood chips. The gasification occurs with a co- current gasifier, which has a combustion chamber and a liquid channel for circulating cooling liquid. The co-current gasifier may also comprise a fuel silo or the fuel silo may be a separate part in connection with the co-current gasifier. Fuel is dispensed into the fuel silo, from where it flows into the combustion chamber, where pyrolysis and gasification of the fuel occurs. Normal water can be used as the cooling liquid circulated in the liquid channel. The purpose of the circulated cooling liquid is to decrease the transfer of heat from the combustion chamber to the fuel silo and to thus prevent the heating and drying of the fuel in the fuel silo. In the method the cooling liquid is led from the liquid channel to the bottom of the combustion chamber and from the bottom of the combustion chamber onwards out of the combustion chamber. At the bottom of the combustion chamber there is during the gasification thus continuously a layer of cooling liquid, in which the ash and coal particles generated in the combustion are mixed. The cooling liquid is advantageously led from the combustion chamber back into the liquid channel, i.e. the same circulated cooling liquid is used in the cooling.

In one advantageous embodiment of the method according to the invention the product gas generated in the gasification is led out of the combustion chamber along the flow duct of the cooling liquid and contaminants are removed from the product gas in a product gas washer. In a second advantageous embodiment of the method according to the invention the ash and coal particles generated in the gasification are led out of the combustion chamber with the cooling liquid along the flow duct. Ash and coal is advantageously removed from the cooling liquid in a product gas washer.

In a third advantageous embodiment of the method according to the invention the temperature of the circulated cooling liquid is lowered with a condenser. After the condensing the liquid is directed into the liquid channel of the co-current gasifier.

In the apparatus according to the invention for the gasification of fuel there is a co- current gasifier, which comprises a combustion chamber and a liquid channel for circulating the cooling liquid, such as water. The co-current gasifier comprised in the apparatus further comprises a first flow duct leading from the liquid channel to the bottom of the combustion chamber and a second flow duct leading out of the combustion chamber.

In one advantageous embodiment of the apparatus according to the invention there is a fuel silo and the liquid channel is at least partly between the fuel silo and the combustion chamber, whereby the cooling liquid circulated in the liquid channel can be used to efficiently decrease the transfer of heat from the combustion chamber to the fuel silo. The co-current gasifier advantageously has a hole leading from the fuel silo to the combustion chamber, through which the fuel flows from the fuel silo to the combustion chamber, and the liquid channel is a ring-shaped channel part surrounding said hole.

A second advantageous embodiment of the apparatus according to the invention further comprises a pump and flow ducts for leading the cooling liquid from the combustion chamber back into the liquid channel. A third advantageous embodiment of the apparatus according to the invention further comprises a product gas washer, which washer is attached to the end of the second flow duct leading out of the combustion chamber. The product gas washer advantageously has a cleaning layer for cleaning the product gas and a product gas duct for leading out the cleaned product gas and the apparatus has means for leading the cooling liquid onto the cleaning layer.

A fourth advantageous embodiment of the apparatus according to the invention further comprises a coal separator for removing ash and coal particles in the cooling liquid and a condenser for lowering the temperature of the circulating cooling liquid.

It is an advantage of the invention that by means if it, the temperature of the outer surface of the combustion chamber of the co-current gasifier remains very low, which decreases the risk of a fire breaking out.

It is an advantage of an embodiment of the invention that in the cooling of the combustion chamber the cooling liquid is used also for removing the ash generated in the combustion chamber. The ash exits the combustion chamber with the cooling liquid in a fire safe way. The fire in the combustion chamber does not need to be extinguished for the duration of the ash removal, but the ash is removed from the combustion chamber continuously during the use of the gasifier. In the following, the invention will be described in detail. In the description, reference is made to the appended drawings, in which

Figure 1 a shows as an example a co-current gasifier according to the

invention as a vertical cross-section,

Figure 1 b shows the co-current gasifier of Figure 1a as a horizontal cross-section at the sectional plane A-A and

Figure 1 c shows the co-current gasifier of Figure 1a as a horizontal cross-section at the sectional plane B-B.

Figure 1a shows as an example a co-current gasifier according to the invention and a cooling liquid circulation apparatus in connection thereto as a vertical cross- section. Figure 1 b shows a horizontal cross-section of the co-current gasifier in Figure 1a at the sectional plane A-A and Figure 1c at the sectional plane B-B. The co-current gasifier is in a use situation in the vertical position according to Figure 1a. In the following, expressions used in the figure descriptions, which describe directions, such as upwards, downwards, above, beneath, on the top surface or on the lower surface, mean the directions when the gasifier is in the position shown in Figure 1a. If the gasifier is in some other position than the one according to the figure, the expressions describing directions change accordingly.

The gasifier has a cylindrical outer casing 10, the upwards pointing end of which has an airtight lid 12, which can be opened. Inside the outer casing there are two essentially parallel floors at a distance from each other, an upper floor 16a and a lower floor 16b, which limit a cooling space 80 between them. The outer casing of the gasifier forms the outer wall of the cooling space. The part below the cooling space forms the gasification part 20 of the co-current gasifier, where the actual gasification of the fuel occurs. Inside the outer casing, above the cooling space 80, there is a cylindrical fuel silo 14, which is open at its both ends, where the fuel to be gasified is dispensed via the lid 12, which can be opened.

Inside the gasification part there is a combustion chamber 32, which has a triple wall structure comprising an internal jacket 34, an intermediate jacket 36 and an external jacket 38. The internal jacket, which forms the innermost wall surface of the combustion chamber, is a cylindrical part having its upper edge at the level of the upper floor 16a and extending through concentric holes 30 leading through the upper floor, a baffle plate 16c and the lower floor 16b close to the bottom of the combustion chamber. The internal jacket is attached to the edges of the holes in the floors and baffle plate with a welding attachment, whereby it simultaneously forms one wall of the air channel 18 and the liquid channel 19. At the level of the lower edge of the internal jacket there is a circular grate 40, which is supported by its edge on the lower edge of the internal jacket. On the inner surface of the internal jacket, a little below the height level of the lower floor 16b, there is a horizontal fire ring 50, which forms in the upper part of the combustion chamber a narrowing, which reduces its cross-section. The fire ring is supported in its place in a way that enables thermal movement by attaching a lower support ring to the inner wall of the internal jacket and on top of it an upper support ring and by fitting the fire ring in the gap between the support rings.

Around the internal jacket there is an intermediate jacket 36, which forms a closed wall surface outside the cylindrical sidewall of the internal jacket and below the grate 40. The intermediate jacket turns inward at its upper edge and connects with the internal jacket a little above the fire ring. The intermediate jacket is advantageously made of steel and attached by its upper edge to the internal jacket with a welding attachment. Around the intermediate jacket there is an external jacket 38, which forms the outermost casing of the combustion chamber. A space surrounding the combustion chamber is formed between the external jacket and the intermediate jacket, which space functions as a preheating space for the gasification air. The upper surface of the preheating space is formed by the lower floor 16b.

The cooling space 80 between the fuel silo and the gasification part functions as a structural part, which reduces the transfer of heat from the gasification part to the fuel silo. The cooling space is divided into two parts with a baffle plate 16c in the direction of the upper and lower floor, so that an air channel 18 is formed above the baffle plate and a liquid channel 19 below the baffle plate. The air channel 18 is a ring-shaped channel part, through which the gasification air needed in the gasification of fuel is led into the gasifier. Inside the air channel there is a spacer plate 48 (Figure 1 b), which closes the channel entirely at one point. On the first side of the spacer plate in the outer wall of the channel there is an inlet opening 57, through which the gasification air can flow into the air channel, and on the second side of the spacer plate there is an outlet opening 58, from which air can exit from the air channel into an air duct 64. The gasification air thus circulates an substantially full round in the air channel. The second end of the air duct is led through a hole made in the external jacket to a space between the external jacket and the intermediate jacket. Through the outlet opening the gasification air can flow from the air channel into the air duct 64 and along the air duct further to the preheating space. In the part between the lower floor and the upper edge of the intermediate jacket, the preheating space is limited by the internal jacket 34. This ring-shaped wall portion of the preheating space, limited by the internal jacket, is equipped with air nozzles 60, through which the gasification air is led from the preheating space to above the fire ring.

Figure 1c shows a co-current gasifier according to the invention as a horizontal cross-section at the sectional plane B-B. The liquid channel 19 below the air channel is a ring-shaped channel part limited by the external jacket 38, the internal jacket 34, the baffle plate 16c and the lower floor 16b. The external jacket has an inlet 70 for the inflow of cooling liquid and an outlet 72 for the outflow of cooling liquid. Water is advantageously used as the cooling liquid. The inlet opens up through the external jacket 38 to outside the gasifier and the outlet opens up through the lower floor 16b into the space between the external jacket and the intermediate jacket. The inlet and the outlet are situated on opposite edges of the ring-shaped liquid channel. As a continuation of the outlet 72 there is a first flow duct 74a, the downwards pointing second end of which is led through a hole in the intermediate jacket 36 to below the grate 40. The first flow duct thus forms an inflow route for the cooling liquid from the liquid channel to inside the combustion chamber below the grate. The wall of the intermediate jacket further has a second hole, as a continuation of which there is a second flow duct 74b. The second flow duct leads through the external jacket 38 to outside the gasifier, i.e. it forms an outflow route for the cooling liquid from the combustion chamber to outside the gasifier. The holes in the intermediate jacket for attaching the first flow duct and the second flow duct are situated on opposite edges of the combustion chamber at a distance from the lowest point of the intermediate jacket. In a use situation of the gasifier there is thus always some cooling liquid, advantageously water, at the bottom of the combustion chamber.

The co-current gasifier according to the invention naturally includes an ignition mechanism, by which the fuel to be gasified is ignited (the ignition mechanism is not shown in the figures). Various prior art ignition mechanisms, which are not described in this context in further detail, can be used in the gasifier. The ignition mechanism is preferably an automatic, liquid gas operated or electrically operated mechanism.

The apparatus according to the invention further comprises a product gas washer 100, a coal separator 200, a condenser 300 and a pump 400. The product gas washer is a device, by means of which contaminants, such as soot, generated in the gasification process are removed from the product gas formed in the co- current gasifier. The washer comprises a vertical cylindrical container 108, on the inside of which a net 102 has been installed in a horizontal position, which net divides the inner part into an upper part 104 and a lower part 106. In the upper part there is a group of substantially round pieces 110, which are stacked into a cleaning layer 122 on top of the net 102. The material of the pieces may for example be metal or some organic material, such as wood or coal. The size and shape of the pieces is selected so that several labyrinthine flow routes are formed between them for the flowing through of product gas and cooling liquid. A duct 1 14 leads through the lid 1 12 of the container into the container, at the end of which duct there is a sprayer 1 16. The sprayer divides the cooling liquid moving along the duct into an even droplet shower on the surface of the cleaning layer. A product gas duct 124 leads through the lid of the container, along which duct the product gas, from which contaminants have been cleaned, is suctioned out of the container. In the other end of the product gas duct there may be an extractor or it may be attached to a motor, which as it runs forms a suitable suction effect for removing the product gas (the extractor and motor are not shown in the figure).

At the bottom of the lower part 106 of the container there is a liquid space 1 18, where the liquid coming from the sprayer, which flows through the cleaning layer, is accumulated. The second end of the second flow duct 74b coming from the co- current gasifier is led through the wall of the container into the lower part of the container. The inlet point of the second flow duct is at a distance from the bottom of the container. Near the bottom 120 of the container there is a hole leading through the wall of the container, as a continuation of which hole there is a third flow duct 74c leading out of the container. The product gas generated in the co- current gasifier flows into the container along the second flow duct 74b and rises inside the container due to the suction effect upwards through the cleaning layer toward the first end of the product gas duct. Simultaneously liquid flows down through the cleaning layer, which liquid cools the product gas and removes contaminants from it. The contaminants flow into the liquid space with the liquid.

The cooling liquid flows from inside the combustion chamber 32 along the second flow duct 74b into the washer 100 to the liquid space 118. The ash generated as the fuel is combusted is mixed in the combustion chamber with the cooling liquid, whereby the ash exits from the combustion chamber with the cooling liquid. The co-current gasifier does thus not need a separate ash removal system, but the ash is removed from the combustion chamber with the cooling liquid circulated in the apparatus. In the end of the third flow duct 74c departing from the washer there is a pump 400. The pump advantageously works with electricity. A fourth flow duct 74d has been attached to the pump, the second end of which is attached to the inlet 70 of the liquid channel 19 of the co-current gasifier. The flow ducts 74a, 74b, 74c and 74d and the pump 400 thus form a flow route for the cooling liquid leading from the liquid channel to the combustion chamber, from the combustion chamber to the product gas washer 100 and from the washer back to the liquid channel, in which flow route the circulation of the cooling liquid is maintained with the aid of the pump 400. The duct 114 leading to the upper part 104 of the washer is connected at its other end to the fourth flow duct 74d through a valve 126. With the aid of the valve a part of the cooling liquid to be circulated is directed through the duct 114 onto the cleaning layer. In the part of the flow duct 74d between the pump 400 and the valve 126 there is a coal separator 200 and a condenser 300. The coal separator is used to remove ash and fine-grained coal mixed in with the cooling liquid from the cooling liquid. The coal separator may be any separator, by means of which small solid particles may be removed from liquid. The condenser is used to remove thermal energy from the circulated cooling liquid, whereby the temperature of the cooling liquid can be kept at a desired level. The effect of the condenser is dimensioned so that the temperature of the cooling liquid after the condensation is about 30 Ό. The structure and function of the coal separator and condenser are generally known prior art, hence they are not described in more detail in this context.

The co-current gasifier according to the invention functions in the following manner. The lid 12 of the fuel silo 14 is opened and a suitable amount of fuel is dispensed into the fuel silo, whereby a part of the fuel flows into the combustion chamber 32. Almost any bio fuel can be used as fuel for the gasifier, such as pine, birch, fir, willow or other kinds of wood chipped with the bark. The wood chips may be air-dried, whereby their moisture content can be 30-40 percent by weight. After dispensing the fuel, the lid is shut in an airtight manner.

The extractor connected to the product gas duct 124 is started and the fuel in the combustion chamber 32 is ignited. Due to the negative pressure provided by the extractor, gasification air flows into the air channel 18 and along the air duct 64 further into the preheating space between the intermediate jacket 36 and the external jacket 38, from where it is led through the air nozzles 60 above the fire ring 50 in the combustion chamber. The fuel is gasified in the combustion chamber at a high temperature of 1100-1300 Since the te mperature of the fuel in the fuel silo is low, pyrolysis does in practice not occur at all in the fuel silo. Drying of the fuel does also not significantly occur in the fuel silo, but the fuel is nearly at its original moisture state when arriving in the combustion chamber. The pyrolysis thus occurs during a very short distance between the upper floor 16a and the fire ring 50.

In the pyrolysis reaction, product gas is generated in the combustion chamber 32, which product gas contains hydrogen, carbon monoxide, carbon dioxide, nitrogen and water vapour. The product gas exits from the combustion chamber due to suction along the second flow duct 74b to the lower part 106 of the product gas washer 100, from where it flows through the cleaning layer upwards to the upper part 104 and further along the product gas duct 124 out of the washer. The product gas is cleansed from contaminants therein, when it flows through the cleaning layer. During the use of the gasifier, cooling liquid, advantageously water, is circulated in the apparatus by means of the pump 400 comprised in the apparatus. The pump suctions cooling liquid from the liquid space 1 8 of the product gas washer and leads it through the coal separator 200 and condenser 300 to the liquid channel 19 of the co-current gasifier. The cooling liquid cooled in the condenser has a temperature of about 30 Ό as i t arrives in the liquid channel. The cooling liquid flowing in the liquid channel binds heat to itself and prevents the transfer of thermal energy generated in the combustion chamber to the fuel silo 14 and to the fuel in the silo. From the liquid channel the cooling liquid flows along the first flow duct 74a below the grate 40 to the bottom of the combustion chamber. Hot coals and brands generated in the combustion of the fuel fall through the grate straight into the cooling liquid, where they are quickly extinguished. It is believed that dropping glowing fuel brands into water may increase the production of hydrogen gas. The ash generated in the combustion is mixed in the combustion chamber into the cooling liquid. The cooling liquid flows from the combustion chamber 32 along the second flow duct 74b to the product gas washer, from where the pump suctions it to circulate again. Part of the circulating cooling liquid is directed through the valve 126 and duct 114 onto the cleaning layer.

The co-current gasifier comprised in the apparatus may be a one-combustion chamber gasifier as described above, or it may have several combustion chambers, such as is presented in the earlier application WO 2008/145814 A1 by the applicant. The individual devices further comprised in the apparatus, such as the coal separator, the product gas washer, the condenser, the pump and the co- current gasifier may along the flow route of the cooling liquid be in some other order than in the description presented above. The apparatus may also have a separate fifth flow duct 74e placed above the second flow duct 74b, for the flow of the product gas.

Some advantageous embodiments of the method and apparatus according to the invention have been described above. The invention is not limited to the solutions described above, but the inventive idea can be applied in numerous ways within the scope of the claims.