SE2050526A1

SE2050526A1 - Method and apparatus for defibrating wood chips, and a return steam pipe for such an apparatus

Info

Publication number: SE2050526A1
Application number: SE2050526A
Authority: SE
Inventors: Thommy Lindblom
Original assignee: Valmet Oy
Priority date: 2020-05-07
Filing date: 2020-05-07
Publication date: 2021-08-17
Also published as: EP4146860A1; EP4146860A4; WO2021225490A1; SE543857C2

Abstract

The invention relates to a method and an apparatus for defibrating wood chips comprising a preheater and a defibrator, said preheater comprising a wood chips inlet and a wood chips outlet, the defibrator comprising a wood chips inlet and a wood fiber outlet, the wood chips outlet of the preheater is connected to said wood chips inlet of said defibrator via a feeder pipe comprising at least one wood chips feeding arrangement for feeding wood chips to said defibrator, a return steam pipe is arranged between said feeder pipe and said preheater, characterized in that said return steam pipe is provided with a cleaning pipe for providing a gas into said return steam pipe.

Description

METHOD AND APPARATUS FOR DEFIBRATING WOOD CHIPS Technical field of the Invention The present invention relates to a method and apparatus for defibrating wood chips and a return steam pipe for such an apparatus.

Background of the Invention When def1brating wood chips in a defibrator, steam is generated from the moisture in thewood chips. Some of that moisture may attempt to travel backwards against the wood flow.To facilitate evacuation of steam and to avoid conﬂict with wood flow going in the oppositedirection, the wood chips are fed into the def1brator with a ribbon feed screw that allowssteam to go backwards in the center while the heavier wood chips are fed forward in theperiphery (ribbon). To evacuate the steam travelling backwards in the center of the ribbonscrew a retum steam pipe at a feeder pipe in which the ribbon screw is provided is connectedto a top portion of a preheater in which the wood chips are heated to a temperature at or above 150 °C.

However cleaver this construction is by evacuating the steam generated in said defibratorback to the preheater to allow more stable feeding of wood chips into the defibrator, it onlyworks if the pressure in the ribbon feed screw is higher than that of the top of the preheaterplus the pressure drop in the retum steam pipe. During certain conditions this may not be the case and during such conditions said retum steam pipe may be clogged.

The problem with a defibrating process in which the retum steam pipe is clogged is that thedefibration process needs to be shut down in order to clean said retum steam pipe. Such ashut-down will result in a low yield from the def1brating process as well as manhours in service which will reduce the profitability from the process.

Object of the Invention The present invention aims at obviating the afore-mentioned problem. A primary object ofthe present invention is to provide an improved apparatus for defibrating wood chips which has a higher yield than prior art apparatuses. Another object of the present invention is to provide an improved apparatus for defibrating Wood chips Which require less process-mode dependent service than prior art apparatuses.

Summarv of the Invention According to the invention at least the primary object is attained by means of the apparatushaving the features defined in the independent claims. Preferred embodiments of the present invention are further defined in the dependent claims.

According to a first aspect of the present invention it is provided an apparatus for defibratingWood chips comprising a preheater and a defibrator, said preheater comprising a Wood chipsinlet and a Wood chips outlet, the defibrator comprising a Wood chips inlet and a Wood fiberoutlet, the Wood chips outlet of the preheater is connected to said Wood chips inlet of saiddefibrator via a feeder pipe comprising at least one Wood chips feeding arrangement forfeeding Wood chips to said defibrator, a return steam pipe is arranged between said feederpipe and said preheater, Wherein said return steam pipe is provided With a cleaning pipe forproviding a gas into said return steam pipe. An advantage of this embodiment is that cloggingof the return steam pipe may be prohibited regardless of the process settings and/or process conditions of the defibration apparatus.

In various example embodiments of the present invention said cleaning pipe is provided insaid retum steam pipe closer to the feeder pipe than said preheater. The advantage of theseembodiments is that the cleaning pipe is provided in the close to an inlet of said retum steam pipe for alloWing cleaning of the full length of the same.

In various example embodiments of the present invention said Wood chip inlet of saidpreheater is arranged in a top portion of said preheater and said Wood chip outlet is arrangedin a bottom portion of said preheater. The advantage of these embodiments is that thetemperature, moisture and/or added chemical substances may be more evenly provided tothe ﬁall contents of Wood chips since all Wood chips are, at least during the movement fromthe top to the bottom of said preheater, exposed to the same atmosphere. This Will enhance the process speed of sufficiently pretreated Wood chips for the later defibrating step.

In various example embodiments of the present invention said cleaning pipe is at least partlyprovided inside said return steam pipe. The advantage of these embodiments is that suchinsertion of the cleaning pipe into said return steam pipe may cause so-called venturi effects in the retum steam pipe which may enhance the cleaning properties.

In various example embodiments according to the present invention said gas provided tosaid cleaning pipe is steam. And in embodiments of the invention, the steam is fresh steam.The advantage of these embodiments is that the added cleaning component is the same as the base component supposed to be running in said retum steam pipe.

In various example embodiments according to the present invention the cleaning pipe isprovided with a controllable valve arrangement. The advantage of these embodiments is thatsaid valve may be used to adjust the amount of cleaning component, the duty time of cleaningand introduce cleaning component pressure waves in order to further increase the cleaning properties.

In various example embodiments of the present invention said retum steam pipe is connectedto a top portion of said preheater. The advantage of these embodiments is that the top portionof the preheater is supposed to be free from wood chips so that the steam easily can be recirculated.

In various example embodiments according to the present invention said wood chips feedingarrangement in said feeder pipe a ribbon screw feeder. The advantage of these embodimentsis that a feeding of wood chips into the defibrator is not disturbed by the backwardly travelling ﬂow of steam.

In various example embodiments according to the present invention a screw feeder isarranged in said preheater for feeding wood chips from said preheater to said ribbon screwfeeder. The advantage of these embodiments is that the ribbon screw feed may be fed with a constant flow of wood chips as long as wood chips is present in the preheater.

In various example embodiments according to the present invention said retum steam pipe is arranged between said preheater and said ribbon screw on said feeder pipe. The advantage of these embodiments is that the return steam pipe is arranged at the rear of the ribbon feed screw in order to recirculate the steam generated in the defibrating unit.

In various example embodiments according to the present invention said portion of saidcleaning pipe provided inside of said return steam pipe is essentially in parallel With thereturn steam pipe. In embodiments of the invention, the cleaning pipe is provided in thecenter of the return steam pipe. The advantage of these embodiments is that the gas provided by said cleaning pipe is assisting the ﬂoW in the return steam pipe.

In various example embodiments according to the present invention at least 3 cm of saidcleaning pipe is provided inside said retum steam pipe. The advantage of these embodimentsis that a suff1cient amount of said cleaning pipe inserted into the retum steam pipe may createventuri effects Which may yet ﬁ1rther improve the cleaning properties. HoWever, one Wantsto keep the insertion as little as possible since the volume of the retum steam pipe not affected by the gas stream emanating from be cleaning pipe may still be clogged.

In various example embodiments of the present invention a cross-section area of saidcleaning pipe is less than half of the cross-section area of the retum steam pipe. Theadvantage of these embodiments is that the amount of gas emanating from the cleaning pipemay be suff1ciently high but not limiting the retum ﬂoW of steam in the retum steam pipe.For this reason, said cleaning pipe must have a sufficiently smaller cross-sectional area thanthe retum steam pipe hoWever not so small that the ﬂoW from said cleaning pipe is not enough from cleaning purpose.

In another aspect of the present invention it is provided a method for def1brating Wood chipscomprising the step of: a. preheating the Wood chips is a preheater, b. feeding the Wood chips from said preheater to a defibrator in a feeder pipe, retuming steam from the feeder pipe to the preheater in a retum steam pipe, and P- providing extemal gas to said retum steam pipe via a cleaning pipe.

In yet another aspect of the present invention it is provided a retum steam pipe configuredfor being connectable in a first end to a Wood chips feeding arrangement provided between a preheater and a def1brator in an apparatus for def1brating Wood chips and in a second end to a preheater of the apparatus for defibrating wood chips, wherein said return steam pipe is provided with a Cleaning pipe for providing a gas into said return steam pipe.

Further advantages with and features of the invention will be apparent from the following detailed description of preferred embodiments.

Brief description of the drawings A more complete understanding of the abovementioned and other features and advantagesof the present invention will be apparent from the following detailed description of preferredembodiments in conjunction with the appended drawings, wherein: Fig. 1 is a schematic side view of an example embodiment of a defibrating apparatusaccording to the present invention, Fig. 2 is an enlarged view of a portion of a first example embodiment of a retum steam pipeaccording to the present invention, and Fig. 3 is an enlarged view of a portion of a second example embodiment of a retum steam pipe according to the present invention.

Detailed description of preferred embodiments of the invention Fig. 1 depicts a schematic side view of an example embodiment of a defibrating apparatus100 according to the present invention. The def1brating apparatus comprising a preheater130 and a defibrator 150. Wood chips 140 to be defibrated by the defibrating apparatus 100may first enter a steaming bin 142. Wood chips 140 from the steaming bin 142 may beinserted into the preheater by a screw feeder 132, which in the depicted embodiment isprovided at the bottom of the steaming bin 142. Wood chips 140 from the steaming bin 142may enter a wood chips inlet 131 at a top portion of the preheater 130.

The steaming bin 142 essentially provides for driving out air trapped in the interior of thewood chips and increasing the water contents of the wood chips. This may be performed byexposing the wood chips to an increased humidified atmosphere. The steaming bin 142 mayregulate the humidity by inserting steam from a steam pipe 127. A valve 126 may open end close depending on the level of humidity in the steaming bin.

In the preheater 130 wood chips may be heated to about 175 °C. It is well known in the artthat a significant drop in the power required to fiberize wood chips is required when thetemperature of the wood chips exceeds 150 °C. The preheating may be performed in anenvironment of constant temperature and pressure. The preheating time is deterrninate bythe heating time of the wood chips needed for lignin in the middle lamella to reach a glasstransition point. A therrnal softening of lignin may allow for a relatively easy mechanical separation of f1bers in the def1brator 150.

As depicted in figure 1, the preheater 130 may be provided with steam from a steam pipe125. Steam pipe 125 may be provided with a regulated valve 124 for deterrnining the amountand time said steam may enter the preheater. Steam may also be provided at a top portion ofthe preheater 130 via steam pipe 129 which is provided with a controllable valve 128. Aconveyor screw 170 at the bottom of the preheater 130 feeds the wood chips from thepreheater to a ribbon screw 152 via a feeder pipe 139. The conveyor screw 170 in thepreheater feed the wood chips in the form of a dense or semi dense plug through the feederpipe 139. This plug comprising soft wood chips and water. The plug of wood chips will prohibit escape and insert of steam from and into the preheater via the feeder pipe 139.

When defihrating or ieiiriing wood chips in a defibrator or reffiiier, steam is geiierateii. fromthe mo isture in the chip. Some of that rnoisture niust travel backtvards against the wood flotv.To facilitate evfacuation. and to avoid conflict wfith wood ﬂow going forward, the wood chipsare fed into the deﬁliratoi' with a ltollown/'riblion feed screw' 152 that allows steam to gobaclivvards in the center while the heafviei' chips a fed forvvfard in the periphery' (ribbon). Toevacuate the steam traveling hackwards in the center of the iilihtin feed screw l52 a returnsteam pipe l 10 at the hack portion ofthe rihhon feed screw feeder pipe 139 is coniiected. tothe top of the preheater l30. Steam flotving in the reverse direction with respect to the tvoodchips in the ribbon feed screw 152 is forced to travel via the return steam pipe l 10, arrangedat the liack portion of the ribbon. feed screw 152, to the preheater instead. of entering thepreheater l30 via the screw feed pipe since the screw feeder pipe is blocked wfith a rnoving iilug of vvood chips.

Hovvever clever this construction is evactiating steam to allow more stable teediiig of wfood.citips, it only vvork if the pressure in the rilihon feed screw l52 is higher than that of the top of the prelieater l30 plus the pressure drop of the return steam-/balance pipe ll0. During certain conditions this may not be the case and sometimes the differential pressure over thereturn steam or balance pipe is not high enough to ensure enoiigh flow' and/or speed to makethis steam evaciiatioii stable or smooth. In such cases there may be a need for eleaniiig thereturn steam or balance pipe llll. By inserting fresh steam into the return steam (balance)pipe lit) in an inserted smaller Cleaning pipe 120, l20* a so-Called Venturi effect can becreated facilitating flow in it regardless of the amount of steam generated between the discsin the defibrator 150. Consequently, the flow Velocity in the return steam (balance) pipe l 10can be controlled (by the ainount of gas, such as steain or fresh steam iiiserted) avoiding the loyv Velocity situations that cause the clogging and/or build-ups.

With the Cleaning pipe l20 provided inside the return steam pipe lll), iinder-pressiire niaybe created at the inlet of said return steam pipe. This iinileir-pressure at the inlet of said retumsteam pipe allows for forciiig (veiituri effect) steam generated at the defibrator lSO into therettim steam pipe although the over-pressure in the return steam pipe is small compared tothe top of the preheater tvhere the return steam pipe is ending. The venturi effect is increasedwith an increased flow of steam in the Cleaning pipe. With the Venturi effect the Cloggiiig of the return steam pipe may be pifohilvited for the ﬁill length of the return steam iiipe.

As an example, as gas or steam is ejected from the Cleaning pipe, it mixes with the steam inthe return steam pipe. Imagine that the gas or steam from the Cleaning pipe mixes with fourtimes as much steam in the return steam pipe. As momentum is conserved, this cloud ofsteani/gas, now five times as much as came out of the Cleaning pipe, is now moving at onefifth the speed of the ﬂow from the Cleaning pipe. All that gas/steam moving in an upwarddirection of the return steam pipe requires replacement steam to be pulled in from the inletof the return steam pipe. In this way a suction at the inlet of the return steam pipe is created by inserting a flow of gas inside the return steam pipe by said Cleaning pipe.

Moving five times as much gas/steam may seem efficient, but the kinetic energy of themoving gas or steam is a function of Velocity squared. Thus, we may have five times as muchgas or steam moving, but the kinetic energy per volume of air is now 1/25th of what it wasin the ﬂow of gas or steam from the Cleaning pipe, and the total kinetic energy is one fifthof what we started with. The Venturi effect in this case may be useful to suck up steam fromthe feeder pipe 139 that otherwise shouldn't go through the return steam pipe. At least a portion of the Cleaning pipe l20” provided inside said return steam pipe may be concentricaily arranged in the return steam pipe. In various example ernbodiinent the returnsteam pipe has a cross sectional area which is several times larger than the cross-sectionalarea of the portion of the cieaning pipe 120” provided inside said return steam pipe 110. Asa further deveiopinent, adding measuring devices on the return steam or balance pipe maygive a better understanding of the eriviroiiment around. it and, eventuaiiy, controlliiig theVelocity of the ﬂow' rnay be a vvay' of controliing the pressure of the ribbon (feed) screvviVVith this possibility and iiicreased understanding of the pressure balance, a new more stablecontrol of the same, might lead to a more smooth/staiile feediiig of chips into the defibiator or refiner.

In ifarious examph: embodinieiits the cleaning pipe 120 is provided at said return steam pipe110 closer to the feeder pipe 139 (at the back portion of the ribbon feed screw 152) than saidpreheater 130. The cleaning pipe 120 may be in arranged in close vicinity to a back side ofsaid ribbon feed screw 152. The cleaning pipe 120 may be inserted into the return steam pipe110. The inserted length of said cleaning pip 120 in said return steam pipe is denoted by120”. The inserted length may vary in between 1 cm to 10 cm. In various exampleembodiments said cleaning pipe 120 is having a cross sectional area being less than half thecross-sectional area of the return steam pipe 110. In various example embodiments theinserted length 120” of said cleaning pipe 120 into said return steam pipe 110 may be 1-5 %of the total length of the return steam pipe 110. The cleaning pipe 120 can have a constantcross section throughout the portion 120” being inside said return steam pipe 110.Alternatively said cleaning pipe 120 may have a smaller cross section at the outlet compared to the cross section in the beginning of the portion being inside said return steam pipe 110.

The pressure of the gas in said cleaning pipe 120 is higher than the pressure in the returnsteam pipe 110. The pressure of the gas in said cleaning pipe may be constant or vary. Incertain circumstances high pressure pulses may be inserted by the cleaning pipe into thereturn steam pipe for breaking loose clogged material inside said return steam pipe. Thewood chip inlet 131 of said preheater 130 may be arranged in a top portion of said preheater130 and said wood chip outlet 134 may be arranged in a bottom portion of said preheater 130.

The gas provided to said cleaning pipe may be steam and can preferably be fresh steam.

However other gases may be used which will not affect the process. One may for instance add acid to the steam. The acid may increase the cleaning properties which in addition may be a process component in the preheater 130.

The retum steam pipe 110 is with its first end 114 attached to the feeder pipe 139 comprisingthe ribbon feed screw and with a second end 112 attached to a top portion of the preheater130. A desired ﬂow of direction of steam in said retum steam pipe 110 is from said first end114 to said second end 112. A ﬂow of gas in said cleaning pipe 120 is therefore in the samedirection for assisting the ﬂow of steam in said retum steam pipe 110, i.e., the cleaning pipe120 is attached to said retum steam pipe 110 in the vicinity of where said retum steam pipe110 attaches to said feeder pipe 139 such as their ﬂow will assist each other in a direction to the top of the preheater 130.

The ribbon feed screw 152 is providing wood chips to the def1brator 150 Via a wood chipsinlet 151 of said defibrator 150. In said deﬁbrator 150 the wood chips are tumed into fiberby grinding the wood chips in between to grinding discs with Variable distance to each other.From an outlet 154 of said defibrator 150 comes wood fiber in a suitable form for inter alia making paper.

The cleaning pipe 120 may be provided with a controllable valve arrangement 122. Thecontrollable valve arrangement may be used for causing the above-mentioned pressurepulses and/or for controlling the pressure and time said gas is ﬂowing in said cleaning pipe120. The portion 120°of said cleaning pipe 120 provided inside of said retum steam pipe 110can essentially be in parallel with the retum steam pipe 110. Preferably the portion 120” of said cleaning pipe 120 is provided in the center of the retum steam pipe 110.

In figure 1 it is depicted a common gas inlet 175 for providing gas or steam to the cleaningpipe 120, steam pipe 127 to the steaming bin 142 and steam pipe 125 to the preheater 130.However, in various example embodiments said cleaning pipe, steam pipe 125 and steampipe 127 may be connected to separate steam generating units, preferably for generating fresh steam.

A method for defibrating wood chips may comprising the steps of: preheating the wood chipsis a preheater 130, feeding the wood chips from said preheater to a defibrator in a feeder pipe, retuming steam from the feeder pipe to the preheater in a retum steam pipe and providing external gas to said return steam pipe via a cleaning pipe for allowing free passagein said return steam pipe. The provision of gas may be constant with a constant pressure andﬂow. The gas may be pure steam or steam blended with a suitable component for assisting the defibrating process.

In figure 2 it is depicted an enlarged view of a portion of an inventive retum steam pipe 110according to the present invention. The bottom portion is to be connected to the feeder pipe139 and the upper portion is to be connected to the preheater 130. At the bottom portion thecleaning pipe 120 is inserted into the retum steam pipe 110. The portion of the cleaning pipe120” inside said retum steam pipe is depicted to be essentially directed in the same manneras the retum steam pipe in order to create a ﬂow of steam from the cleaning pipe whichessentially coincides with the ﬂow of direction of steam in the retum steam pipe 110. Theﬂow of steam in figure 2 is from the bottom to the top in the cleaning pipe 110 and in the retum steam pipe 120.

Figure 3 depicts an enlarged view of a portion of a second example embodiment of a retumsteam pipe according to the present invention. The difference compared to figure 2 is thatthe cleaning pipe 120 is provided at the retum steam pipe further away from the inlet of saidretum steam pipe. The inserted portion 120' of the cleaning pipe 120 is in parallel with thesurrounding retum steam pipe. Preferably the cleaning pipe 120 is arranged in the center ofthe steam pipe 110. Although the cleaning pipe 120 is further away from the inlet of theretum steam pipe 110, the suction force when introducing a stream of gas/steam from thecleaning pipe 120 into the retum steam pipe may be enough to direct steam from the backportion of the ribbon feed screw 152 into the retum stem pipe 110 and forward said steam to the top portion of the preheater 130.The retum steam pipe 110 according to the present invention may be sold as a spare part.Said retum steam pipe 110 may be retrof1tted to old types of defibrating apparatuses for improving the reliability.

Feasible modif1cations of the Invention The invention is not limited only to the embodiments described above and shown in the drawings, which primarily have an illustrative and exemplifying purpose. This patent ll application is intended to cover all adjustnients and Variants of the preferred en1bodin1entsdescribed herein, thus the present invention is defined by the Wording of the appended clainisand the equivalents thereof. Thus, the equipment may be modified in all kinds of Ways Within the scope of the appended clainis.

Throughout this specification and the clain1s Which follows, unless the context requiresotherwise, the Word “con1prise”, and Variations such as “con1prises” or “con1prising”, Willbe understood to iniply the inclusion of a stated integer or steps or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

12 C1aims

1. An apparatus (100) for defibrating Wood chips (140) comprising a preheater (130)and a def1brator (150), said preheater (130) comprising a Wood chips in1et (131) and a Woodchips out1et (134), the defibrator (150) comprising a Wood chips in1et (151) and a Wood fiberout1et (154), the Wood chips out1et (134) of the preheater(130) is connected to said Woodchips in1et (151) of said def1brator(150) via a feeder pipe (139) comprising at 1east one Woodchips feeding arrangement (152) for feeding Wood chips (140) to said def1brator (150), areturn steam pipe (110) is arranged between said feeder pipe (139) and said preheater (130),characterized in that said return steam pipe (110) is provided With a c1eaning pipe (120, 120°) for providing a gas into said return steam pipe (110).

2. The apparatus (100) according to c1aim 1, Wherein said c1eaning pipe (120, 120') isprovided in said return steam pipe (110) c1oser to the feeder pipe (139) than said preheater(130).

3. The apparatus according to c1aim 1 or 2, Wherein said Wood chip in1et (131) of saidpreheater (130) is arranged in a top portion of said preheater (130) and said Wood chip out1et(134) is arranged in a bottom portion of said preheater (130).

4. The apparatus (100) according to any one of the preceding c1aims, Wherein said c1eaning pipe (120, 120°) is at 1east part1y provided inside said retum steam pipe (110).

5. The apparatus (100) according to any one of the preceding c1aims, Wherein said gas provided to said c1eaning pipe (120) is steam.

6. The apparatus (100) according to any one of the preceding c1aims, Wherein the c1eaning pipe (120, 120') is provided With a contro11ab1e va1ve arrangement (122).

7. The apparatus (100) according to any one of the preceding c1aims, Wherein said retum steam pipe (110) is connected to a top portion of said preheater (130).

8. The apparatus (100) according to any one of the preceding c1aims, Wherein said Wood chips feeding arrangement (152) in said feeder pipe (139) a ribbon screw feeder. 13

9. The apparatus (100) according to claim 8, wherein a screw feeder (170) is arrangedin said preheater (130) for feeding wood chips (140) from said preheater (130) to said ribbonscrew feeder (152).

10. The apparatus (100) according to claim 8, wherein said return steam pipe (110) isarranged between said preheater (130) and said ribbon screw (152) on said feeder pipe (139).

11. The apparatus (100) according to claim 4, wherein said portion of said cleaning pipe(120, 120') provided inside of said return steam pipe (110) is essentially in parallel with the return steam pipe (1 10).

12. The apparatus (100) according to claim 4, wherein said portion of said cleaning pipe(120, 120') provided inside of said return steam pipe (110) is provided essentially in the center of the retum steam pipe (1 10).

13. The apparatus (100) according to claim 11, wherein at least 3 cm of said cleaning pipe (120°) is provided inside said retum steam pipe (110).

14. The apparatus (100) according to any one of the preceding claims, wherein a cross-section area of said cleaning pipe (120”) is less than half of the cross-section area of the retum steam pipe (1 10).

15. The apparatus (100) according to any one of the preceding claims, wherein a ﬂow ofgas from said cleaning pipe (120, 120') is configured for assisting the ﬂow of steam in the retum steam pipe (110) from the feeder pipe (139) to the preheater (130).

16. The apparatus (100) according to any one of claim 1-14, wherein said preheater (130)further comprising at least one steam inlet (125, 129) for providing steam generated extemal to said apparatus into said preheater.

17. A method for defibrating wood chips comprising the steps of:a. preheating the wood chips is a preheater, b. feeding the wood chips from said preheater to a defibrator in a feeder pipe, 14 c. retuming steam from the feeder pipe to the preheater in a return steam pipe, and d. providing external gas to said return steam pipe via a cleaning pipe.

18. The method according to claim 16, further comprising the steps of: e. feeding wood chips from said feeder pipe to said defibrator by means of a ribbonfeed screw, and f increasing a pressure of steam in said feeder pipe by allowing steam generated insaid defibrator to retum in a backward direction compared to the wood chips in said ribbon feed screw.

19. The method according to claim 16 or 17, wherein said extemal gas provided to said cleaning pipe is steam.

20. The method according to any one of claim 16 or 18, wherein a flow of gas from saidcleaning pipe is assisting the ﬂow of steam in the retum steam pipe from the feeder pipe to the preheater.

21. A retum steam pipe (110) configured for being connectable in a first end to a woodchips feeding arrangement provided between a preheater and a defibrator in an apparatus fordefibrating wood chips and in a second end to a preheater of the apparatus for defibratingwood chips, wherein said retum steam pipe is provided with a cleaning pipe for providing a gas into said retum steam pipe.

22. The retum steam pipe according to claim 20, wherein said cleaning pipe is provided in said retum steam pipe closer to the wood chips feeding arrangement than said preheater.

23. The retum steam pipe according to claim 20 or 21, wherein said cleaning pipe is at least partly provided inside said retum steam pipe.

24. The retum steam pipe according to any one of claim 20-22, wherein the cleaning pipe is provided with a controllable valve arrangement.

25. The return steam pipe according to any one of claim 22-23, Wherein said portion ofsaid cleaning pipe provided inside of said return steam pipe is essentially in parallel With the return steam pipe.

26. The return steam pipe according to claim 24, Wherein said portion of the cleaning pipe is provided at least 3 cm into said return steam pipe.

27. The retum steam pipe according to any one of claim 20-25, Wherein an outlet of said cleaning pipe is directed in said retum steam pipe in a direction towards said preheater.