WO2021225490A1

WO2021225490A1 - Method and apparatus for defibrating wood chips, and a return steam pipe for such an apparatus.

Info

Publication number: WO2021225490A1
Application number: PCT/SE2021/050271
Authority: WO
Inventors: Thommy LINDBLOM
Original assignee: Valmet Ab
Priority date: 2020-05-07
Filing date: 2021-03-26
Publication date: 2021-11-11
Also published as: SE543857C2; SE2050526A1; EP4146860A1

Abstract

The invention relates to a method and an apparatus for defibrating wood chips comprising a preheater (130) and a defibrator (150), 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 (139) comprising at least one wood chips feeding arrangement (152) for feeding wood chips to said defibrator, a return steam pipe (110) is arranged between said feeder pipe and said preheater, characterized in that said return steam pipe (110) is provided with a cleaning pipe (120) for providing a gas into said return steam pipe.

Description

METHOD AND APPARATUS FOR DEFIBRATING WOOD CHIPS, AND A RETURN STEAM PIPE FOR SUCH AN APPARATUS

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 defibrating wood chips in a defibrator, steam is generated from the moisture in the wood 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 opposite direction, the wood chips are fed into the defibrator with a ribbon feed screw that allows steam to go backwards in the center while the heavier wood chips are fed forward in the periphery (ribbon). To evacuate the steam travelling backwards in the center of the ribbon screw a return steam pipe at a feeder pipe in which the ribbon screw is provided is connected to 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 defibrator back to the preheater to allow more stable feeding of wood chips into the defibrator, it only works if the pressure in the ribbon feed screw is higher than that of the top of the preheater plus the pressure drop in the return steam pipe. During certain conditions this may not be the case and during such conditions said return steam pipe may be clogged.

The problem with a defibrating process in which the return steam pipe is clogged is that the defibration process needs to be shut down in order to clean said return steam pipe. Such a shut-down will result in a low yield from the defibrating 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 of the 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.

Summary of the Invention

According to the invention at least the primary object is attained by means of the apparatus having 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 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, wherein said return steam pipe is provided with a cleaning pipe for providing a gas into said return steam pipe. An advantage of this embodiment is that clogging of 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 in said return steam pipe closer to the feeder pipe than said preheater. The advantage of these embodiments is that the cleaning pipe is provided in the close to an inlet of said return 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 said preheater is arranged in a top portion of said preheater and said wood chip outlet is arranged in a bottom portion of said preheater. The advantage of these embodiments is that the temperature, moisture and/or added chemical substances may be more evenly provided to the full contents of wood chips since all wood chips are, at least during the movement from the 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 partly provided inside said return steam pipe. The advantage of these embodiments is that such insertion of the cleaning pipe into said return steam pipe may cause so-called venturi effects in the return steam pipe which may enhance the cleaning properties.

In various example embodiments according to the present invention said gas provided to said 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 return steam pipe.

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

In various example embodiments of the present invention said return steam pipe is connected to a top portion of said preheater. The advantage of these embodiments is that the top portion of 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 feeding arrangement in said feeder pipe a ribbon screw feeder. The advantage of these embodiments is 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 is arranged in said preheater for feeding wood chips from said preheater to said ribbon screw feeder. 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 return 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 said cleaning pipe provided inside of said return steam pipe is essentially in parallel with the return steam pipe. In embodiments of the invention, the cleaning pipe is provided in the center 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 said cleaning pipe is provided inside said return steam pipe. The advantage of these embodiments is that a sufficient amount of said cleaning pipe inserted into the return steam pipe may create venturi effects which may yet further improve the cleaning properties. However, one wants to keep the insertion as little as possible since the volume of the return 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 said cleaning pipe is less than half of the cross-section area of the return steam pipe. The advantage of these embodiments is that the amount of gas emanating from the cleaning pipe may be sufficiently high but not limiting the return flow of steam in the return steam pipe. For this reason, said cleaning pipe must have a sufficiently smaller cross-sectional area than the return 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 defibrating wood chips comprising 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, c. returning 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.

In yet another aspect of the present invention it is provided a return steam pipe configured for being connectable in a first end to a wood chips feeding arrangement provided between a preheater and a defibrator in an apparatus for defibrating 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 advantages of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the appended drawings, wherein:

Fig. 1 is a schematic side view of an example embodiment of a defibrating apparatus according to the present invention,

Fig. 2 is an enlarged view of a portion of a first example embodiment of a return steam pipe according to the present invention, and

Fig. 3 is an enlarged view of a portion of a second example embodiment of a return 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 apparatus 100 according to the present invention. The defibrating apparatus comprising a preheater 130 and a defibrator 150. Wood chips 140 to be defibrated by the defibrating apparatus 100 may first enter a steaming bin 142. Wood chips 140 from the steaming bin 142 may be inserted into the preheater by a screw feeder 132, which in the depicted embodiment is provided at the bottom of the steaming bin 142. Wood chips 140 from the steaming bin 142 may 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 the wood chips and increasing the water contents of the wood chips. This may be performed by exposing the wood chips to an increased humidified atmosphere. The steaming bin 142 may regulate 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 art that a significant drop in the power required to fiberize wood chips is required when the temperature of the wood chips exceeds 150 °C. The preheating may be performed in an environment of constant temperature and pressure. The preheating time is determinate by the heating time of the wood chips needed for lignin in the middle lamella to reach a glass transition point. A thermal softening of lignin may allow for a relatively easy mechanical separation of fibers in the defibrator 150.

As depicted in figure 1, the preheater 130 may be provided with steam from a steam pipe 125. Steam pipe 125 may be provided with a regulated valve 124 for determining the amount and time said steam may enter the preheater. Steam may also be provided at a top portion of the preheater 130 via steam pipe 129 which is provided with a controllable valve 128. A conveyor screw 170 at the bottom of the preheater 130 feeds the wood chips from the preheater to a ribbon screw 152 via a feeder pipe 139. The conveyor screw 170 in the preheater feed the wood chips in the form of a dense or semi dense plug through the feeder pipe 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 deflbrating or refining wood chips in a defibrator or refiner, steam is generated from the moisture in the chip. Some of that moisture must travel backwards against the wood flow. To facilitate evacuation and to avoid conflict with wood flow going forward, the wood chips are fed into the defibrator with a hollow/ribbon feed screw 152 that allows steam to go backwards in the center while the heavier chips a fed forward in the periphery (ribbon). To evacuate the steam traveling backwards in the center of the ribbon feed screw 152 a return steam pipe 110 at the back portion of the ribbon feed screw feeder pipe 139 is connected to the top of the preheater 130 Steam flowing in the reverse direction with respect to the wood chips in the ribbon feed screw 152 is forced to travel via the return steam pipe 110, arranged at the back portion of the ribbon feed screw 152, to the preheater instead of entering the preheater 130 via the screw feed pipe since the screw feeder pipe is blocked with a moving plug of wood chips.

However clever this construction is evacuating steam to allow more stable feeding of wood chips, it only work if the pressure in the ribbon feed screw 152 is higher than that of the top of the preheater 130 plus the pressure drop of the return si earn -/balance pipe 110. During certain conditions this may not be the case and sometimes the differential pressure over the return steam or balance pipe is not high enough to ensure enough flow and/or speed to make this steam evacuation stable or smooth. In such cases there may be a need for cleaning the return steam or balance pipe 110. By inserting fresh steam into the return steam (balance) pipe 110 in an inserted smaller cleaning pipe 120, 120’ a so-called Venturi effect can be created facilitating flow in it regardless of the amount of steam generated between the discs in the defibrator 150. Consequently, the flow velocity in the return steam (balance) pipe 110 can be controlled (by the amount of gas, such as steam or fresh steam inserted) avoiding the low velocity situations that cause the clogging and/or build-ups.

With the cleaning pipe 120 provided inside the return steam pipe 110, under-pressure may be created at the inlet of said return steam pipe. This under-pressure at the inlet of said return steam pipe allows for forcing (venturi effect) steam generated at the defibrator 150 into the return steam pipe although the over-pressure in the return steam pipe is small compared to the top of the preheater where the return steam pipe is ending. The venturi effect is increased with an increased flow of steam in the cleaning pipe. With the venturi effect the clogging of the return steam pipe may be prohibited for the full length of the return steam pipe.

As an example, as gas or steam is ejected from the cleaning pipe, it mixes with the steam in the return steam pipe. Imagine that the gas or steam from the cleaning pipe mixes with four times as much steam in the return steam pipe. As momentum is conserved, this cloud of steam/gas, now five times as much as came out of the cleaning pipe, is now moving at one fifth the speed of the flow from the cleaning pipe. All that gas/steam moving in an upward direction of the return steam pipe requires replacement steam to be pulled in from the inlet of 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 the moving gas or steam is a function of velocity squared. Thus, we may have five times as much gas or steam moving, but the kinetic energy per volume of air is now l/25th of what it was in the flow of gas or steam from the cleaning pipe, and the total kinetic energy is one fifth of what we started with. The Venturi effect in this case may be useful to suck up steam from the feeder pipe 139 that otherwise shouldn't go through the return steam pipe. At least a portion of the cleaning pipe 120’ provided inside said return steam pipe may be concentrically arranged in the return steam pipe. In various example embodiment the return steam pipe has a cross sectional area which is several times larger than the cross-sectional area of the portion of the cleaning pipe 120’ provided inside said return steam pipe 110. As a further development, adding measuring devices on the return steam or balance pipe may give a better understanding of the environment around it and, eventually, controlling the velocity of the flow may be a way of controlling the pressure of the ribbon (feed) screw. With this possibility and increased understanding of the pressure balance, a new more stable control of the same, might lead to a more smooth/stable feeding of chips into the defibrator or refiner.

In various example embodiments the cleaning pipe 120 is provided at said return steam pipe 110 closer to the feeder pipe 139 (at the back portion of the ribbon feed screw 152) than said preheater 130. The cleaning pipe 120 may be in arranged in close vicinity to a back side of said ribbon feed screw 152. The cleaning pipe 120 may be inserted into the return steam pipe 110. The inserted length of said cleaning pip 120 in said return steam pipe is denoted by 120’. The inserted length may vary in between 1 cm to 10 cm. In various example embodiments said cleaning pipe 120 is having a cross sectional area being less than half the cross-sectional area of the return steam pipe 110. In various example embodiments the inserted 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 constant cross 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 return steam pipe 110. The pressure of the gas in said cleaning pipe may be constant or vary. In certain circumstances high pressure pulses may be inserted by the cleaning pipe into the return steam pipe for breaking loose clogged material inside said return steam pipe. The wood chip inlet 131 of said preheater 130 may be arranged in a top portion of said preheater 130 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 return steam pipe 110 is with its first end 114 attached to the feeder pipe 139 comprising the ribbon feed screw and with a second end 112 attached to a top portion of the preheater 130. A desired flow of direction of steam in said return steam pipe 110 is from said first end 114 to said second end 112. A flow of gas in said cleaning pipe 120 is therefore in the same direction for assisting the flow of steam in said return steam pipe 110, i.e., the cleaning pipe 120 is attached to said return steam pipe 110 in the vicinity of where said return steam pipe 110 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 defibrator 150 via a wood chips inlet 151 of said defibrator 150. In said defibrator 150 the wood chips are turned into fiber by 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. The controllable valve arrangement may be used for causing the above-mentioned pressure pulses and/or for controlling the pressure and time said gas is flowing in said cleaning pipe 120. The portion 120’of said cleaning pipe 120 provided inside of said return steam pipe 110 can essentially be in parallel with the return steam pipe 110. Preferably the portion 120’ of said cleaning pipe 120 is provided in the center of the return steam pipe 110.

In figure 1 it is depicted a common gas inlet 175 for providing gas or steam to the cleaning pipe 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 steam pipe 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 chips is a preheater 130, feeding the wood chips from said preheater to a defibrator in a feeder pipe, returning steam from the feeder pipe to the preheater in a return steam pipe and providing external gas to said return steam pipe via a cleaning pipe for allowing free passage in said return steam pipe. The provision of gas may be constant with a constant pressure and flow. 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 return steam pipe 110 according to the present invention. The bottom portion is to be connected to the feeder pipe 139 and the upper portion is to be connected to the preheater 130. At the bottom portion the cleaning pipe 120 is inserted into the return steam pipe 110. The portion of the cleaning pipe 120’ inside said return steam pipe is depicted to be essentially directed in the same manner as the return steam pipe in order to create a flow of steam from the cleaning pipe which essentially coincides with the flow of direction of steam in the return steam pipe 110. The flow of steam in figure 2 is from the bottom to the top in the cleaning pipe 110 and in the return steam pipe 120.

Figure 3 depicts an enlarged view of a portion of a second example embodiment of a return steam pipe according to the present invention. The difference compared to figure 2 is that the cleaning pipe 120 is provided at the return steam pipe further away from the inlet of said return steam pipe. The inserted portion 120' of the cleaning pipe 120 is in parallel with the surrounding return steam pipe. Preferably the cleaning pipe 120 is arranged in the center of the steam pipe 110. Although the cleaning pipe 120 is further away from the inlet of the return steam pipe 110, the suction force when introducing a stream of gas/steam from the cleaning pipe 120 into the return steam pipe may be enough to direct steam from the back portion of the ribbon feed screw 152 into the return stem pipe 110 and forward said steam to the top portion of the preheater 130.

The return steam pipe 110 according to the present invention may be sold as a spare part. Said return steam pipe 110 may be retrofitted to old types of defibrating apparatuses for improving the reliability.

Feasible modifications 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 application is intended to cover all adjustments and variants of the preferred embodiments described herein, thus the present invention is defined by the wording of the appended claims and the equivalents thereof. Thus, the equipment may be modified in all kinds of ways within the scope of the appended claims.

Throughout this specification and the claims which follows, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply 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

1. An apparatus (100) for defibrating wood chips (140) comprising a preheater (130) and a defibrator (150), said preheater (130) comprising a wood chips inlet (131) and a wood chips outlet (134), the defibrator (150) comprising a wood chips inlet (151) and a wood fiber outlet (154), the wood chips outlet (134) of the preheater(130) is connected to said wood chips inlet (151) of said defibrator (150) via a feeder pipe (139) comprising at least one wood chips feeding arrangement (152) for feeding wood chips (140) to said defibrator (150), a return 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 cleaning pipe (120, 120’) for providing a gas into said return steam pipe (110).

2. The apparatus (100) according to claim 1, wherein said cleaning pipe (120, 120') is provided in said return steam pipe (110) closer to the feeder pipe (139) than said preheater (130).

3. The apparatus according to claim 1 or 2, wherein said wood chip inlet (131) of said preheater (130) is arranged in a top portion of said preheater (130) and said wood chip outlet (134) is arranged in a bottom portion of said preheater (130).

4. The apparatus (100) according to any one of the preceding claims, wherein said cleaning pipe (120, 120’) is at least partly provided inside said return steam pipe (110).

5. The apparatus (100) according to any one of the preceding claims, wherein said gas provided to said cleaning pipe (120) is steam.

6. The apparatus (100) according to any one of the preceding claims, wherein the cleaning pipe (120, 120') is provided with a controllable valve arrangement (122).

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

8. The apparatus (100) according to any one of the preceding claims, wherein said wood chips feeding arrangement (152) in said feeder pipe (139) a ribbon screw feeder.

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

10. The apparatus (100) according to claim 8, wherein said return steam pipe (110) is arranged 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 (110).

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 return steam pipe (110).

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

15. The apparatus (100) according to any one of the preceding claims, wherein a flow of gas from said cleaning pipe (120, 120') is configured for assisting the flow of steam in the return 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 external 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, c. returning 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 ribbon feed screw, and f. increasing a pressure of steam in said feeder pipe by allowing steam generated in said defibrator to return 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 external 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 said cleaning pipe is assisting the flow of steam in the return steam pipe from the feeder pipe to the preheater.

21. A return steam pipe (110) configured for being connectable in a first end to a wood chips feeding arrangement provided between a preheater and a defibrator in an apparatus for defibrating 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.

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

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

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