Method and a Device for Removing Gas from Wood Chips

Abstract

A method and a device for removing gas from wood chips, wherein the wood chips flowing downwards are heated with a steam flow that is cross-directional in relation to the wood chip flow.

Description

FIELD OF THE INVENTION

The invention relates to a method for removing gas from wood chips according to the preamble of the appended claim 1. The invention also relates to a device for implementing the aforementioned method in accordance with the preamble of the appended claim 12.

BACKGROUND OF THE INVENTION

When manufacturing cellulose and paper pulp, the ligno-cellulose-containing wood chips used as raw material are cooked in an alkaline solution in order to separate the fibres and lignin contained in them from each other. The manufacture comprises several stages, both before and after the cooking. Before cooking, the wood chips are directed to the gas removal phase, where gases, primarily air, are removed from both inside the wood chips and the spaces between them by directing hot steam to the wood chips. Removing gas from the spaces between the wood chips decreases the amount of gases disturbing the operation of the digester. In addition, when the gases are removed from inside the wood chips, the chips absorb cooking chemicals better, which improves the yield of the cooking and the quality of the pulp it provides.

Vertical bins or vessels or horizontal screws are generally used for removing gas from wood chips. The screws being used are generally wood chip conveyor screws, which convey wood chips, for example, from chipping to the digester. Generally, this type of screws are installed horizontally or only in a small angle with the horizontal level. The steam needed for removing gas is generally fed from nozzles installed on the lower surface of the shell forming the bottom of the screw. However, the loading of the screws varies in relation to time, i.e. in relation to the length of the screw, the amount of wood chips moving inside varies, resulting in slow warming of individual wood chips to the maximum temperature and varying their delay in the screw. Consequently, the processing of the wood chips is not uniform. In addition, the delay of the wood chips in the screw is not long enough in order for the gas removal to be complete.

When using vertical bins or vessels in removing gas, the wood chips are generally fed to the bin from its upper end and the steamed wood chips are removed from the lower end of the bin. The steam is generally fed to the lower part of the bin and it flows upwards against the direction of motion of the wood chip material. This kind of a solution for steaming wood chips is disclosed, for example, in the U.S. Pat. No. 4,867,845, where wood chips are fed to a bin with an downward-increasing diameter from the upper end of the bin and the steamed wood chips are removed from the lower end of the bin, by means of a rotating discharger installed at the bottom of the bin. Steam is fed vertically to the centre of the bin, via a steam pipe installed parallel to the vertical central axis of the bin. The end of the steam pipe extending close to the bottom of the bin, from where the steam is discharged to the bin, is formed as an expanding taper. Steam flows upwards from the steam pipe, through the wood chip column packed in the bin.

The publication U.S. Pat. No. 6,199,299 also discloses a chip bin, where steam is fed to wood chips flowing downwards in the bin from the middle of the height of the bin, via steam nozzles installed in the shell of the bin.

US-publication 5,628,873 discloses a downwards-tapered steam bin, where steam is fed to the wood chip flow flowing downward in the chip bin from above the conical narrowing part. Steam feeding takes place both via the shell of the bin and the steam feeding pipes installed within the bin.

The problem with the above-mentioned bins is that the gas removal in them is not even throughout the entire wood chip flow. The steam fed via the bin shell cannot penetrate to the middle of the wood chip flow in such a manner that all the wood chips would receive an identical steam processing. The steam fed to the middle of the bin, to its lower part, does not penetrate into the wood chips next to the walls of the bin or into the chips on the wood chip surface of the bin efficiently enough to attain an even gas removal result. In addition, in the solutions presented above, the warming of the wood chips is slow in the bin, in which case a part of the wood chips do not receive a proper gas removal processing.

BRIEF DESCRIPTION OF THE INVENTION

The purpose of the present invention is therefore to provide a method for removing gas from wood chips, which avoids the above-mentioned problems and in which the gas removal from wood chips takes place in such a manner that all the wood chips receive an identical steam processing. Thus, the pulp resulting from the cooking following the gas removal is homogenous.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The device according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 12.

The other, dependent claims present some preferred embodiments of the invention.

The invention is based on the idea that gas removal from wood chips is preformed in a vertical, elongated gas removal vessel, to which the wood chips are fed from the upper part of the vessel and the wood chips are removed from its lower part. Steam is fed to the wood chips to the wood chip flow travelling downwards in the vessel from the middle and edges of the flow, in cross-direction in relation to the wood chip flow. The feed rate of wood chips to the vessel and the discharge rate from it are maintained on a level that the wood chip flow is moving as a so-called plug flow, in which case the wood chips move evenly in relation to the entire cross-section area of the vessel. Therefore, steam is fed into these wood chips in the plug flow. The level of the surface of wood chips in the vessel is controlled by means of the feed rate and discharge rate of the wood chips. The surface of the wood chips is maintained on such a level that the steam fed to the wood chips does not escape directly to the steam space in the upper part of the vessel. The steam feeding means are thus arranged to feed steam to a distance from the surface of the wood chips, inside the wood chip flow. The height of the gas removal vessel and the placement of the steam feeding means in relation to the height of the vessel is arranged in such a manner that the delay time of wood chips in the vessel is long enough for the gas removal to be as efficient as possible.

The majority of the steam required in the gas removal from wood chips is fed via the steam feeding pipe arranged in the middle of the wood chip flow and the rest of the steam is fed from around the wood chip flow, via steam feeding means arranged in the shell of the vessel. In order for the gas removal to be efficient enough, the temperature of the wood chips in the vessel is raised quickly to approx. 100° C. and maintained there substantially over the entire time the wood chips remain in the vessel. The amount of the steam being fed is controlled by means of the temperature and pressure measurements of the steam gathering in the upper part of the vessel. The steam forming in the gas removal is removed from the upper part of the vessel, from above the surface of the wood chips and the condensate is removed from the lower part of the vessel.

An advantage of the gas removal method according to the invention is that as a result of both the design of the gas removal vessel and the transverse steam flow, which does not prevent the flow of the wood chips, an even, downward plug flow of wood chips is created, resulting in each wood chip receiving an equally long and identical steam processing. Consequently, the gas removal within individual wood chips is efficient and fast. The transverse steam flow does not prevent the downward flow of the wood chips either. Further, the quick raising of the temperature of the wood chips to the processing temperature with a transverse steam flow ensures enough delay time for the wood chips in the processing temperature, which improves gas removal significantly. Also, the dimensions of the gas removal vessel and the placement of the steam feeding means in relation to the height of the vessel improve gas removal. The gas removal device used in implementing the method, i.e. the gas removal vessel is simple to manufacture and easy to install in its place and it takes little room in the cramped device environments of pulp and paper mills.

The invention can also be applied in renewing equipment in factories, which manufacture pulp. Thus, acquiring a completely new gas removal device can be avoided and economical savings can be created.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail with reference to the appended drawings, in which

FIG. 1 shows a schematic side view of a device according to the invention,

FIG. 2 shows a schematic side view of another device according to the invention and

FIG. 3 shows a schematic side view of a steam feeding device used in a device according to an embodiment of the invention.

In FIGS. 1, 2 and 3, the same numerals refer to corresponding parts and they will not be explained separately later on, unless required by the illustration of the subject matter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a device 1 for removing gas from wood chips. The device is placed in a pulp and/or paper factory before the digester where the cooking of pulp takes place. The device comprises is an elongated, by its diameter downwards tapered and advantageously conical vessel 2 installed in the vertical direction. The vessel can be any vessel suitable for the purpose, such as, for example, a bin 2. The wood chips are fed to the bin 2 by means of a feeding device 3, from the upper end of the bin. The feeding device 3 is, for example, a rotary vane feeder or a screw. The wood chips can be fed directly to the bin 2 by means of the feeding device or, such as in the advantageous embodiment shown in the figure, the wood chips are fed with the feeding device to a chip packer 4 arranged between the roof 19 closing the upper part of the bin and the feeding device. The chip packer 4 spreads the wood chips as an even layer to the gas removal vessel 2 by means of the steam fed to the chip packer. The steam fed to the packer also increases the temperature of the wood chips before they fall to the bin 2. In the winter, if the wood chips are frozen, the steam fed to the chip packer also melts the wood chips. The amount of steam fed to the chip packer 4 is controlled by the temperature of the wood chips. The operation of the chip packer is obvious to a man skilled in the art, which is why it is not described more in detail here.

The wood chips that have gone through the gas removal process are removed from the bin 2 by the discharge means 5 that are connected to the bottom of the bin. The discharge means comprises several pushing means 6, which move the wood chips to a discharge opening 7 that is centralized with the midpoint of the bottom of the bin 2. The pushing means 6 are placed substantially evenly over the periphery of the entire shell of the bin 2 defining the bottom of the bin. The power transmission of the discharge means is implemented, for example, hydraulically. The discharge opening is connected to a discharge means arranged under it, i.e. to a screw 8, by means of which the gas-free wood chips are transferred forward.

The wood chips fed to the bin fill the bin 2 substantially evenly to the level of the surface 9 of the wood chips. The surface of the wood chips settles within a distance from the roof 19 of the bin. A free space, i.e. a gas space 13 remains between the surface 9 of the wood chips and the roof of the bin. The level of the surface 9 of the wood chips in the bin is controlled by controlling the feed rate of the wood chip feeding device 3 and/or the discharge rate of the wood chip discharge means 5 and the screw 8 discharging the wood chips from the bin. The discharge rate is controlled according to the production of the digester. For the purpose of controlling, one or more sensors 10 measuring the level of the surface of the wood chips are arranged in the bin 2. On the basis of the data provided by the surface level sensor 10, the controller 21 controls the rate of the feeding device 3 and/or the discharge means 5 and the screw 8 and at the same time the delay time of the wood chips in the delay zone 25 between the steam feeding means 11 and 12 as well as the discharge means 5. The surface level of the wood chips is to be the wood chips must be controlled onto such level that the wood chip surface extends all the time above the steam feeding openings of the steam feeding means 11 and 12 feeding steam to the bin. Thus, the steam is prevented from directly escaping to the steam space 13 above the surface of the wood chips.

As can be understood from what is described above, the wood chips move downward in the bin 2 by forming a downward moving wood chip flow, which is described with the arrow A in the figure. The downward movement and the effect of the steam fed to the wood chip flow causes the wood chip flow to become dense, thus forming a so-called plug flow. Thus, the downward expanding design of the bin promotes the even movement of the flow in such a manner that no blockages or other obstacles blocking the even flow form in it. The wood chip flow is guided by means of the flow controller 14 arranged in the lower part of the bin, which flow controller evens the chip flow in the bin. In addition, the flow controller 14 prevents the wood chips from directly discharging to the discharge opening 7 and thus the blockage of the discharge opening 7.

Steam is fed to the wood chip flow in cross direction in relation to the wood chip flow with first steam feeding means 11 and second steam feeding means 12. The first steam feeding means 11, i.e. the steam feeding pipe 11 is arranged in the middle of the bin 2. The steam feeding pipe 11 thus directs steam to the middle of the bin 2 in such a manner that at least a part of its length combines with the vertical mid-axis of the bin. The end of the steam feeding pipe 11 that extends inside the bin is closed, preventing the steam from flowing into the chip flow directly, in the same direction with it. In the lower end of the steam feeding pipe 11 there is an around the steam feeding pipe extending steam distribution means 15, which can be a perforation extending around the feeding pipe, or some suitable means attached to the feeding pipe, such as, for example, a screen, which distributes steam from the steam feeding pipe 11 evenly to the wood chips horizontally on every side of the pipe. In the embodiment of FIG. 1, a screen 15 for dividing steam is arranged in the steam feeding pipe. The steam feeding pipe 11 thus feeds steam to the middle of the bin in a cross-direction in relation to the wood chip flow, which steam is marked with arrows in the figure. In the shell of the bin, substantially on the same level with the screen 15, there are the second steam feeding means 12 extending around the entire shell of the bin. The second steam feeding means comprising a distribution chamber 12 and a steam distribution means 20 direct steam to the bin in a cross-direction in relation to the wood chip flow. The distribution means 12 is arranged on the outer surface of the bin and it extends around the entire bin shell. The steam distribution means 20 is, for example, a screen and it is arranged on the inner surface of the bin shell, in connection with the distribution chamber and also extends around the bin shell. The cross-directional steam flow enables the quick raising of the temperature of the wood chips to a temperature that is advantageous for gas removal. It has been experimentally established that gas removal from wood chips takes place in approx. 100° C. The wood chips are quickly heated by means of the steam directed to the bin to this temperature and are maintained in this temperature by means of continuous steam feeding. The majority of the steam required in gas removal is fed from the steam feeding pipe 11. The rest of the steam is fed from the steam feeding means 12 and/or the steam feeding units 17 arranged on the bottom of the discharge means 5. It is also possible to feed steam to the bin only from the steam feeding pipe 11, if the steam amount coming through it is enough to maintain a high enough temperature required in the gas removal. Controlling the gas removal temperature is done by controlling the amount of steam fed to the bin. The amount of steam is controlled by measuring the temperature prevailing in the delay zone 25 below the second steam feeding means 12 by means of a controller 23, which sends a control message to the steam feeding means (not shown in the figure). The control message is in the figure referred to with the letter B and dashed lines. It has been experimentally established, that the delay time of an individual wood chip piece in the delay zone 25 should be approximately 10 to 30 minutes, advantageously approximately 15 to 25 minutes in order for the gas removal from it to be as complete as possible. The steam feeding means 11 and 12 are placed in relation to the height of the bin 2 in such a manner that below the second steam feeding means 12, between the steam feeding means 11 and 12 and the wood chip discharge means is formed a so-called delay zone 25, where an adequate delay time, which is required for gas removal, is guaranteed for the downward flowing wood chips. The delay zone is approximately ⅔ of the height of the vessel 2.

The gases that the steam has displaced from the wood chips and the spaces between them, gathers in the gas space 13 between the surface 9 of the wood chips and the roof 19 of the bin, from where it is advantageously directed out via a unit 16. It is also possible to remove gases from the bin 2 by sucking them via the steam feeding pipe 11 or the distribution chamber 12. Thus, the steam flow can be effectively directed in a cross-direction through the wood chip flow. In addition, this way the channelling of the steam in the wood chip flow can be prevented. The steam gases can be sucked from only one of the steam feeding means at a time, because feeding enough steam to the wood chip flow and maintaining a high enough temperature in it must be guaranteed throughout the entire gas removal process. In other words, if the steam gases are sucked via the distribution chamber 12, enough steam must be fed from the steam feeding pipe 11. Further, if the sucking of steam gases takes place via the steam feeding pipe 11, enough steam must be fed via the distribution chamber 12 and the steam feeding units 17.

By means of the temperature sensor 22 arranged in the gas space 13 above the wood chip surface 9, the temperature of the gas space 13 above the wood chip surface is monitored. If the temperature rises too high, it is a sign of a malfunction of the bin, in which case an alarm is given. The temperature prevailing in the gas space 13 above the wood chip surface is lower than the processing temperature of the wood chips. This is because a part of the steam fed to the wood chips flows upstream in the upper part of the wood chip column towards the surface 9 of the wood chips and displaces the colder gas in the wood chips, which flows upwards to the gas space 13.

The condensate formed in connection with the wood chip gas removal flows to the bottom of the discharge means 5 connected to the lower part of the bin and are removed from there via the condensate removal units 18. The condensate can also be removed from the screw 8 (not shown in the figure).

FIG. 2 shows another embodiment of the gas removal device 1 according to the invention. This embodiment differs from the above-described embodiment of FIG. 1 for the part of the wood chip discharge means. Gas removal from the wood chips takes place in a corresponding manner as is described in connection with FIG. 1. In FIG. 2 the downward flowing wood chip flow formed by wood chips is removed from the bin 2, such as in the embodiment of FIG. 1, by means of the wood chip discharge means 5 in connection with the bottom of the bin 2. There are several hydraulically moving pushing means 6 in the discharge means, which move the wood chips from one side of the bin to the discharge opening 7 placed on the other edge of the bin 2. The screw 8 moves the gas-free wood chips forward. In this embodiment no separate flow controller is required for directing the wood chip flow to the discharge means 5.

The flow of the wood chips in the bin 2 can, if desired, be directed by means of the design of the steam feeding means 11. FIG. 3 shows an in the bin 2 arranged steam feeding pipe 11 having a closed, downward convergent tapered flow guide 24 attached to the lower surface of a screen provided in the steam feeding pipe. The effect of the flow guide is based on that it prevents the sudden expansion of the free cross-sectional area of the bin after the screen 15 and thus evens the wood chip flow downwards.

The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims. The gas removal vessel can thus also be of the same diameter in relation to its entire height, a vessel that is round in its cross section. Further, as the steam used in gas removal can be used either fresh steam or expansion steam received from another process of the pulp or paper mill. Also, the steam gas received from the gas removal device can be compressed and recycled back as heating steam to the device.

Claims

1-19. (canceled)

20. A method for removing gas from wood chips, in which method the downwards flowing wood chips are heated with a steam flow that is cross directional in relation to the wood chip flow and is directed to the wood chip flow both from the middle of the wood chip flow and from the edges of the wood chip flow, wherein the wood chips are heated to a gas removal temperature, after which heating the wood chips are maintained in the gas removal temperature for a certain delay time.

21. The method according to claim 1, wherein the wood chips are heated in a vertically installed, elongated vessel, where steam is directed to the wood chip flow by means of first steam feeding means arranged in the middle of the wood chip flow and second steam feeding means arranged in the shell of the vessel.

22. The method according to claim 21, wherein the wood chips are fed to the vessel from the upper end of the vessel and removed from the bottom of the vessel, by means of wood chip discharge means in connection with the bottom of the vessel.

23. The method according to claim 22, wherein the level of the surface of the wood chips fed to the vessel is controlled by controlling the feed rate of the wood chips and the discharge rate of the wood chips.

24. The method according to claim 20, wherein the delay time of the wood chips after the heating is approximately of approximately 10 to 30 minutes, advantageously 15 to 25 minutes.

25. The method according to claim 20, wherein the delay time of the wood chips fed to the vessel in the delay zone is controlled by controlling the feed rate of the wood chips and the discharge rate of the wood chips.

26. The method according to claim 21, wherein the steam feeding means are arranged in relation to the height of the vessel in such a manner that a delay zone is formed below the steam feeding means in the flow direction of the wood chips.

27. The method according to claim 20, wherein the temperature of the wood chips in the vessel is controlled by controlling the amount of steam directed from the steam feeding means to the vessel.

28. The method according to claim 20, wherein gas is removed from the wood chips before the wood chips are directed to cooking.

29. A device for removing gas from wood chips, which device comprises a vessel, means for feeding wood chips to the vessel from its upper end, wherein the wood chips are arranged to flow downward in the vessel, and steam feeding means for feeding steam in a cross-direction in relation to the wood chip flow in order to heat the wood chips and to remove gas from it, wherein the steam feeding means are arranged to direct steam to the wood chip flow from the middle of it and from its edges, wherein the wood chips are arranged to be heated to a gas removal temperature, after which heating the wood chips are arranged to be maintained in the gas removal temperature for a certain delay time.

30. The device according to claim 29, wherein the steam feeding means contain first steam feeding means, which are arranged in the middle of the wood chip flow flowing in the vessel, and second steam feeding means, which are arranged in the shell of the vessel.

31. The device according to claim 29, wherein the steam feeding means comprise steam distribution means for directing steam in a cross-direction in relation to the wood chip flow.

32. The device according to claim 29, wherein the device contains wood chip discharge means for discharging wood chips from the vessel, from the lower end of the vessel.

33. The device according to claim 29, wherein the steam feeding means are arranged in relation to the height of the vessel in such a manner that a delay zone is formed between the steam feeding means and the wood chip discharge means.

34. The device according to claim 33, wherein the delay time of the wood chips in the delay zone is of approximately 10 to 30 minutes, advantageously 15 to 25 minutes.

35. The device according to claim 29, wherein the vessel is a vertically installed elongated vessel.

36. The device according to claim 29, wherein the device comprises a surface level sensor, which is arranged to measure the level of the surface of the wood chips in the vessel, and a controller, which is arranged to control the level of the surface of the wood chips by controlling the discharge rate of the wood chip discharge means and the feed rate of the wood chip feeding means on the basis of the data provided by the surface level sensor.

37. The device according to claim 29, wherein the controller is arranged to control the delay time of the wood chips in the delay zone by controlling the discharge rate of the wood chip discharge means and the feed rate of the wood chip feeding means on the basis of the data provided by the surface level sensor.

38. The device according to claim 29, wherein the device comprises a controller, which is arranged to measure the temperature of the wood chips in the vessel and to control the amount of steam fed to the steam feeding means.