EP1670622B8

EP1670622B8 - Method and system for heating frozen logs

Info

Publication number: EP1670622B8
Application number: EP04789713A
Authority: EP
Inventors: Normand Bedard; Benoît LAGANIERE
Original assignee: FPInnovations; Hydro Quebec
Current assignee: FPInnovations; Hydro Quebec
Priority date: 2003-10-03
Filing date: 2004-10-04
Publication date: 2009-08-19
Anticipated expiration: 2024-10-04
Also published as: WO2005032779A3; EP1670622B1; DE602004019341D1; ATE422177T1; CA2540477A1; EP1670622A4; WO2005032779A2; CA2443006A1; EP1670622A2; CA2540477C

Abstract

The invention relates to a method and system (1) for heating frozen timber logs (3) in order to ease the debarking thereof. The inventive method consists in carrying the frozen timber logs (3) through a determined distance and in exposing said timber logs (3) to infrared radiation (11) during said travel in order to heat the logs (3) by means of said infrared radiation (11). The inventive system (1) comprises a conveyor (13) for carrying and rotating the frozen logs (3) during a determined distance and a surface (15) for infrared radiation (11) which is arranged with respect to the conveyor (13) in such a way that the timber logs (3) are exposed to infrared radiation (11) along said distance and heated by said infrared radiation (11).

Description

METHOD AND SYSTEM FOR HEATING FROZEN BALLS

Field of the Invention: The present invention relates to a method and a system for heating frozen beads. More particularly, the present invention relates to a method and a system for heating frozen wooden logs in order to facilitate their debarking.

Description of the prior art:

Sawmills in forest-rich countries produce timber from trees taken from the forest, which must be stripped of their bark before cutting into planks and planks. Debarking is often done by a ring debarker consisting of a motorized rotating ring to which are bent tools which remove the bark by friction, as the ball moves longitudinally inside the ring thanks to drive rollers. Ideally, only the bark needs to be removed.

When the temperature of the logs is below the freezing point (which is the normal situation in winter in the Nordic countries), the adhesion between the bark and the wood is strong and greater pressure must be exerted on the tools. cut in order to remove the bark. The lower the temperature of the beads, the stronger the adhesion and the higher this pressure must be.

Increasing the pressure on the cutting tools has several drawbacks, however. On the one hand, the increase in debarking pressure produces accelerated wear of the cutting tools and possibly damage to the components of the debarking machine (cutting tool, arm, tensioning system, etc.). On the other hand, the debarkers then pluck excess wood fibers with the bark, which constitutes a significant loss. In summer, approximately 10 to 20% of the barked mass is made up of wood fibers, which are burnt or buried with the bark. In winter, in some Canadian sawmills, up to 50% of the debarked mass consists of wood fibers. However, typically, about 25% of the revenues of a conventional sawmill come from the wood fiber it sells in the form of chips to a paper mill. These shavings come from sections of the log which are not found in the form of lumber. Another disadvantage results from the fact that if the pressure on the cutting tools is reduced in order to minimize the loss of wood, the bark is then found in the chips. However, the presence of bark particles in the shavings is the cause of various problems in the paper mill: lower yield of the digester, dirt in the pulp (pitch), and especially the presence of sclerite which ultimately increases the rate incidence of breakage of the sheet during its production and printing defects ('fish eye', grain of rice) at the printer. As long as the bark content in supplier shavings is one of the most important quality criteria for pulp manufacturers, and paper mills are becoming more and more demanding. There is therefore a strong advantage in increasing the temperature of the ball before debarking. Typically, thawing logs in winter can reduce wood losses by about 2% while increasing the quality of the chips delivered to pulp and paper mills. For a sawmill that consumes approximately 500,000 m ³ of wood annually, this improvement represents a saving of more than half a million dollars (the amounts mentioned in this description are expressed in Canadian dollars).

To effect this temperature increase, the sawmills currently use heating methods comprising immersing and sprinkling the logs in hot water basins or by means of water and / or steam jets. These methods work well in terms of preheating, but pose various problems, the most important of which concerns the environment. Indeed, emptying and cleaning are frequently required to rid the basins which accumulate dirt and bark. Because contaminated water can no longer be released to the environment due to government regulations, sawmills have to set up settling ponds which must be frequently emptied and cleaned, resulting in high costs. An object of the present invention is therefore to propose a method and / or a heating system which can solve some of the problems reported above of the prior art. More particularly, an object is to propose a new method for heating frozen wooden logs in winter in order to facilitate their debarking. Another object is to propose a system for implementing this method.

Summary of the invention:

The present invention relates to a method for heating frozen wooden logs in order to facilitate their debarking, each wooden ball having a longitudinal axis, a peripheral surface and a given diameter, the method being characterized in that it comprises the following steps : a) entrain frozen wooden logs along a determined route; and b) subjecting the wooden balls to infrared radiation along the path to heat said wooden balls by infrared radiation.

Preferably, step a) further comprises the step of rotating the wooden balls around their respective longitudinal axes along at least a portion of the path in order to allow the peripheral surface of each wooden ball to 'be subjected to infrared radiation.

The present invention relates to a system for heating frozen wooden balls in order to facilitate debarking, each wooden ball having a longitudinal axis, a peripheral surface and a given diameter, the system being characterized in that it comprises: a) a conveyor for driving the frozen wooden logs along a determined route; and b) an infrared radiation surface positioned relative to the conveyor to subject the wooden balls to infrared radiation along the path and to heat said wooden balls by infrared radiation. Preferably, the system comprises any means, apparatus and / or device for implementing the various steps and / or substeps of the method according to the present invention. The objects, advantages, and other characteristics of the present invention will become more apparent on reading the following nonlimiting description of preferred embodiments shown in the accompanying drawings.

Brief description of the drawings:

Figure 1 is a schematic view of a lateral and rotational movement of adjacent wooden logs according to a preferred embodiment of the present invention.

Figure 2 is a partial schematic view of a system for heating frozen wooden balls according to another preferred embodiment of the present invention, the wooden balls being moved by the conveyor and being subjected to infrared radiation.

Figure 3 is a side view of a pilot of a frozen bead heating system according to the present invention.

Figure 4 is a front view of what is illustrated in Figure 3.

Figure 5 is a top view of what is illustrated in Figure 3.

Detailed description of the drawings:

In the following description, the same reference numerals designate similar elements. The embodiments shown in the figures are for illustrative purposes only. In the context of the present description, and in addition to its usual definition, the term "heating" can also mean melting or sublimating the ice and the word

"infrared" includes any electromagnetic radiation of a thermal nature

(visible type radiation, short infrared, medium infrared, long infrared, etc.).

In addition, although the present invention is mainly designed to heat frozen wooden logs 3 in order to facilitate debarking, the invention can be used in other fields for other applications, as is obvious to a person. versed in art. For these reasons, expressions such as "logs", "wood", "jellies" and / or "peeling" and any other reference and / or any other equivalent or similar expression should not be considered as limiting the scope of the present invention and include any other object and any other application with which the present invention may be used and may be useful. In addition, although the preferred embodiment of the conveyor 13 and the means for moving and rotating the wooden balls 3 have certain components, such as chains, reflectors, etc., all of these components are not necessarily essential to the invention and consequently should not be taken in their restrictive sense, that is to say should not be considered in a way that limits the scope of the present invention. It should be understood there, as also obvious to a person skilled in the art, that other appropriate components and geometries and other cooperation between them can be used for the conveyor 13 or any other component of the system 1 according to the present invention, in order to implement the different steps and sub-steps of the method, as easily inferred from the present description, without departing from the scope of the invention.

In addition, expressions such as "system", "apparatus", "machine" and / or "assembly", as well as any other equivalent expression and / or words composed of these, may be used interchangeably. in the context of this description. This also applies to other expressions which are mutually equivalent, such as "infrared radiation", "infrared heating" and "thermal radiation" for example, as evident to a person skilled in the art.

Referring to the attached figures, and in general, the present invention relates to a method of heating frozen beads. More particularly, the present invention relates to a method of heating wooden logs 3 frozen in order to facilitate their debarking, and also relates to a system 1 for the implementation of this method. As known in the art, each wooden ball 3 normally has a longitudinal axis 5 (typically, a central axis 5), a peripheral surface 7 (external surface 7 of the ball), and a given (average) diameter.

According to the present invention, the method comprises two main steps, in particular step a) of driving the frozen wooden balls 3 along a determined path; and step b) subjecting the wooden balls 3 to infrared radiation 11 along the path to heat said wooden balls 3 by infrared radiation

11.

As will become more apparent on reading the description which follows, each of the above-mentioned steps may comprise several sub-steps depending on the applications for which the present method is used and the final results which are desired.

Indeed, for example, and as will be better explained below, step a) could include the following sub-steps: i) rotate the wooden logs 3 around their respective longitudinal axes 5 along d 'at least a portion of the path in order to allow the peripheral surface 7 of each wooden ball 3 to be subjected to infrared radiation 11; ii) placing the wooden balls 3 parallel to each other; iii) alternatively, either maintain a certain distance between the adjacent wooden balls 3, so that wooden balls 3 of different diameters can also be subjected to infrared radiation 11, and thus prevent wooden balls 3 of small diameter from being hidden by wooden balls 3 of larger diameter, or adjoin wooden balls 3 adjacent to each other, when for example these wooden balls are of similar diameter, in order to maximize the use of infrared radiation 1 1, that is to say maximize the amount of infrared radiation 11 emitted being absorbed by the wooden balls 3; iv) driving the wooden balls 3 along a horizontal plane, such as for example along a conveyor 13; v) driving the wooden balls 3 in a direction transverse to the longitudinal axes 5 of the wooden balls 3, in particular for reasons of efficiency of arrangement space of the components of the corresponding system 1; vi) evaluate the characteristics of the wooden balls 3 (eg temperature, dimension, etc.) before subjecting them to infrared radiation 11; or vii) any other suitable and desired step, as evident to a person skilled in the art.

Similarly, the above-mentioned step b) can also comprise several sub-steps, such as for example, as better explained below: i) subjecting the wooden balls 3 to infrared radiation 11 positioned above the horizontal plane ; ii) subjecting the wooden balls 3 to infrared radiation 11 having a wavelength between about 0.7 and 10 microns; iii) subjecting the wooden balls 3 to infrared radiation 11 having a power density determined as a function of the characteristics of the wooden balls 3 and of the flammability limitations; iv) subjecting the wooden balls 3 to infrared radiation 11 for a period determined according to the characteristics of the wooden balls 3 (eg their initial temperature, etc.); v) reflect a portion of the infrared radiation 11 not absorbed by the wooden balls 3 again towards wooden balls 3 (either the same wooden balls 3 or other wooden balls 3 having to be subjected to infrared radiation 11 ); or vi) any other suitable and desired step, as evident to a person skilled in the art.

In addition, steps a) and b) can be intimately linked depending on the applications for which the present method is intended, as also obvious to a person skilled in the art. For example, step a) could include the sub-step of sorting the wooden balls 3 according to their diameter, the wooden balls 3 whose diameter falls within a first range of diameters being entrained along d 'a first determined course, and the wooden balls 3 whose diameter falls within a second range of diameters being driven along a second determined course. Through Consequently, step b) could also comprise the step of subjecting the sorted wooden balls 3 to infrared radiation 11 along each of said paths, that is to say, there could be a first system of heating by infrared radiation 11 to heat the wooden balls 3 of the first range of diameters and another infrared heating system 11 by infrared radiation 11 (or a re-routing to the first heating system after having treated the wooden balls 3 the first range of diameters) for heating the wooden balls 3 of the second range of diameters. As previously explained, the present invention also relates to a system 1 for heating frozen wooden balls 3 in order to facilitate debarking. The system 1 comprises a conveyor 13 for driving the frozen wooden balls along a determined path and a surface 15 of infrared radiation 11 positioned relative to the conveyor 13 for subjecting the wooden balls 3 to infrared radiation 11 along the run and heat said wooden balls 3 by infrared radiation I I.

Since the system 1 according to the present invention is intended to implement the aforementioned method, it preferably comprises several means, apparatuses and / or devices for carrying out each of the steps and substeps described above.

For example, and preferably, the system 1 according to the present invention comprises: means for pivoting the wooden balls 3 around their respective longitudinal axes 5 along at least a portion of the path in order to allow the peripheral surface 7 of each wooden ball 3 to be subjected to infrared radiation 11 (these means preferably comprising tooth chains or rack chains 19 cooperating with the conveyor 13, or any other suitable device); means for placing the wooden balls 3 parallel to each other; means for maintaining a certain distance between the adjacent wooden logs 3; means for attaching the wooden balls 3 to each other; a conveyor 13 capable of driving the wooden balls 3 along a horizontal plane; an infrared radiation surface 15 positioned above the horizontal plane; means to train wooden logs 3 in a direction transverse to the longitudinal axes 5 of wooden logs 3; means for sorting the wooden balls 3 according to their diameter, the wooden balls 3 whose diameter falls within a first range of diameters being driven along a first determined path, and the wooden balls 3 whose diameter falls within a second range of diameters being driven along a second determined path; an infrared radiation surface for each route; at least one chain of stops 23 cooperating with the conveyor 13 to drive the wooden balls 3; an evaluation device for evaluating the characteristics of the wooden balls 3 before subjecting them to infrared radiation 11; means for controlling, as a function of the characteristics (eg the initial temperature, etc.) of the wooden balls 3, the period during which the wooden balls 3 must be subjected to infrared radiation 11; at least one reflector 21 for reflecting a portion of the infrared radiation 11 not absorbed by the wooden balls 3 towards wooden balls 3 (either the same wooden balls 3 or other wooden balls 3 having to be heated), the reflector 21 being preferably positioned below the conveyor 13 if the surface 15 of infrared radiation 11 is positioned above it; etc.

In addition, the infrared radiation 1 i preferably has a wavelength between about 0.7 and 10 microns and a power density determined according to the characteristics of the wooden balls 3 and limitations relating to inflarnability. Preferably also, the infrared radiation surface 11 can take the form of electric emitters 25, gas radiant heaters, catalytic type gas radiant heaters, or any other emitter 25 suitable for emitting infrared radiation 11 or any other radiation. adequate thermal intended to heat the wooden balls 3 according to the present invention and having the preferential characteristics mentioned in the present description.

Various other aspects, characteristics and advantages of the present invention will become more apparent on reading the following description of the general operation and basic principles, of the experimental work carried out, of the test results, of a description of a typical installation. and improvements and advantages achievable in the context of the present invention. General operation and basic principles

The solution which is proposed here and which constitutes the object of the invention involves infrared technology. This preferred technology was chosen for its simplicity and its ability to heat efficiently at a reasonable cost. At the basis of the technology, a simple heating element is brought to high temperature, which allows it to emit so-called "infrared" radiation, not visible to the naked eye and of a thermal nature, heating the objects it " enlightened". Infrared equipment used in an industrial environment emits, in practice, radiation covering the range of wavelengths between about 0.7 and 10 microns. We often distinguish between "short" infrared, "medium" infrared and "long" infrared. Short infrared implies an emission temperature above about 2000 ° C and its spectral emission range mainly covers the range from about 0.7 to 3.0 microns. Medium infrared implies an emission temperature between about 700 and 1300 ° C and covers a range from about 1 to 7 microns. Long type infrared implies an emission temperature of about 400 to 600 ° C and mainly covers the wavelength range from about 2 to 10 microns. Like the hot water soaking technique, infrared heats strictly on the surface and has little or no capacity to heat deep down. Even short-type infrared, which has the capacity to penetrate certain materials, penetrates only very marginally the external surface of the bark. However, the area to be heated, the bark, is directly below the external surface of the log. In fact, it is necessary to heat the so-called "wet" bark located under the so-called "dry" bark until the wood-bark interface where the cambium is found. This area is therefore heated by thermal conduction from the surface exposed to infrared towards the interior of the ball. On the other hand, the entire periphery of the balls 3 must be exposed to infrared radiation 11, which can be done by rolling the ball 3 on itself below an emitter plane 25.

In other words, heating by infrared radiation 11 does not consist in carrying out a complete thaw of the ball 3 but rather in preheating the surface 15. In in fact, the outer bark, the visible part, contains practically no water. However, the underlying layer, the inner bark between the outer bark and the wood, contains a significant amount of water. When strongly frozen, this water increases the adhesion strength between the wood and the bark. It is therefore necessary to reheat this part if you want a good functioning of the debarker, which maximizes the sawing yield and improves the quality of the chips (by reducing the quantity of bark in the chips).

Ideally, an infrared preheating system should include a conveying system where all the balls 3 would be joined together, in order to minimize the losses of radiation between the balls 3. In reality, the displacement by rotation of the balls implies a movement inverse tangential between two adjacent balls, as best illustrated in Figure 1. Naturally, it is physically very difficult to perform a lateral displacement without providing a certain spacing between the balls 3. In addition, the balls 3 do not all have the same diameter and holding them side by side would involve hiding small balls between two large diameter balls. The conveyor 13 envisaged involves chains with stops 23 for pushing each ball 3 and chains with teeth or chains with fixed or movable racks 19 to ensure their rotation. The distance between each stop is uniform along the conveyor 13 and is greater than the maximum diameter of the balls 3 entering under a plane of infrared emitters 25, as best illustrated in FIG. 2.

The use of Pinfrarouge makes it possible to completely get rid of the use of water and associated environmental problems. On the other hand, the implementation of this type of system 1 requires respecting certain power density limits, choosing a particular type of infrared emitters, and using a conveyor 13 making it possible to expose the entire periphery of the balls. 3. A research project has therefore been initiated in order to answer certain technical questions and to demonstrate the principle. Experimental work carried out

Firstly, it has been demonstrated that it is not necessary to completely thaw the balls 3. The fact of obtaining, at the interface between the wood and the bark, a temperature of a few degrees below the point freezing substantially reduces the bark shearing effort. This demonstration was made from samples of 3 balls of different species at various temperatures below the freezing point and using a device measuring the shear force to be provided for detaching the bark from samples of balls 3.

Secondly, as best illustrated in FIGS. 3 to 5, a pilot comprising a conveyor 13 under infrared emitters 25 was constructed to characterize the thermal behavior of the balls 3 under various power densities of infrared ray and exposure periods , on balls 3 of different diameters and thicknesses of bark and at various initial temperatures. Figures 3 to 5 show the pilot with its main components.

This pilot was used to establish operating parameters, such as the power and the time of exposure of the balls 3 to infrared radiation. As illustrated in FIG. 3 (and in a similar manner, in FIG. 2), this equipment was characterized by infrared transmitters 25 placed above a conveyor 13 allowing the movement and rotation of the balls 3; the external surface 7 of each of the balls 3 is thus completely exposed to the infrared radiation 11. Thermocouples have been installed at different depths inside the ball 3 in order to measure the increase in temperature. Black spruce logs frozen at temperatures of about -15 ° C to -30 ^σ C, and of different diameters, were exposed to several levels of power density and several exposure times. These tests made it possible to know the optimal operating parameters.

More precisely, these tests made it possible to measure the temperature in depth of the ball 3 by thermocouple and at the surface by pyrometry. Ball 3 comprising thermocouples was placed on the conveyor 13 with two neighboring balls 3 on either side. Initially, the chains with stops 23 moved the three balls 3 under the infrared exposure zone. Once the balls 3 under the transmitters 25, the tooth chains made the balls 3 roll on themselves for a predetermined time ranging from approximately 1 to 8 minutes. Thirdly, the balls 3 were removed from the heating zone and the rotational movement continued without exposure to infrared for several minutes, the total time of exposure and non-exposure corresponding to a period of time typical between the entry of a log 3 in a sawmill and the debarking station.

Another type of test also involved placing pieces of wood such as bark, sawdust and shavings on a metal plate under different levels of radiation power density to observe the tendency to flammability.

Test results

Shear tests The results of the shear tests as a function of temperature have shown that the shear force decreases sharply with an increase in temperature. This decrease is particularly strong below the freezing point, and less marked near and above the freezing point. As the energy expenditure would be very great if the infrared heating system were asked to transfer all the water from the bark from the solid state to the liquid state, it therefore became obvious that it is advantageous to limit the 'temperature rise up to a few degrees below freezing. The shear forces are then not much greater than above the freezing point (the freezing point of water in wood is about 2 or 3 degrees lower than the normal temperature of pure water. , due to the presence of minerals in the water).

The ball heating tests on the infi'arouge pilot The infrared heating results on the pilot confirmed the validity of the infrared technology to preheat the ball 3 and obtain an adequate temperature at the interface between the bark and the wood (ie with cambium). The infrared radiation 11 is strictly absorbed at the surface of the bark: the heat is transmitted more deeply, by thermal conduction through the bark. During the period of exposure to infrared, the surface temperature rises rapidly and the temperature gradient in the thickness of the bark is high. During the subsequent "non-exposure" period, this gradient decreases sharply. At the end of this period, the temperature gradient in the thickness of the bark is lower: the energy contained in the surface layer (essentially "dry bark") has been largely transferred to the lower layers colder ("wet bark" and sapwood).

Tests have shown that it is indeed possible, even with very cold beads 3 (approximately -30 ° C.), to obtain, after a few minutes of exposure to infrared and a period of "non-exposure", a temperature (approximately -6 ° C) at the interface between the wood and the bark at which debarking is facilitated in a satisfactory manner. When this temperature is reached at the wood-bark interface, all the bark is then found at a temperature higher than the temperature of the wood-bark interface. The outer surface of the bark is then at an almost stable temperature between around 5 and 10 ° C. So some of the water in the bark is no longer frozen.

It emerged from the tests that the lower the initial temperature of the ball 3, the more effective the exposure to infrared. So much so that the energy expenditure required for a rise to the debarking temperature is substantially the same, whether the ball is at an initial temperature of about -20 ° C or -30 ° C. One explanation for this is that the water contained in the bark goes into the liquid state later when the ball 3 has a starting temperature of about -30 ° C. However, ice has a greater thermal conductivity than water (by a factor of four) and this allows the heat absorbed on the surface to better propagate inside the ball. Also, the transition from the solid state to the liquid state of the water contained in the bark requires an appreciable amount of energy, which is then no longer available to penetrate more deeply.

Furthermore, the tests carried out tend to indicate that the energy required to raise the temperature to the desired level of temperature is substantially proportional to the diameter of the ball 3. In the case where the space between two successive balls 3 on the conveyor 13 is always the same, this implies that the diameter of the largest balls 3 determines the energy consumption for all the balls. This amounts to saying that the smallest balls 3 will receive more energy than necessary. One criterion to consider in this situation is the energy bill. However, a conveyor 13 adjusting the distance between the balls 3 is conceivable. The small balls 3 would then receive less energy and the overall energy bill would be minimized. Tests have shown that for electric infrared emitters 23, the energy expenditure to raise the temperature at the wood / bark interface of a typical log about 3.8 meters in length from a very low temperature (around - 20 ° C at -30 ° C) up to approximately -6 ° C is estimated at approximately 3 kWh per meter in diameter. For a log about 20 cm in diameter, this means an energy expenditure of about 0.6 kWh. If we assume the use of a conveyor 13 with fixed spacing between the balls 3, and that the largest balls 3 are about 20 cm in diameter, this means that the energy expenditure is about 0.6 kWh per log, whatever the diameter. At a cost of around 10 to 15 days per electric kilowatt hour (marginal cost of electricity in Quebec, taking into account a strong penalty on the call for power), this corresponds to a cost less than around 10 per log.

All tests on the infrared pilot focused on black spruce only. Among all the test results, the cases most remote from the general tendency of the experimental points were those associable with a bark presenting pieces partially detached and sometimes supeiposed. However, the presence of an air gap between two bark scales or between the bark and the wood constitutes a barrier to the transfer of heat towards the interior of the log, which explains the greater consumption. energy. Black spruce, which often has this bark morphology, is therefore an unfavorable case compared to other northern species such as balsam fir. The iiifrarouge technology is therefore probably applicable to all the species constituting the raw material of Nordic sawmills.

Flammability tests y The use of infrared raises the question of flammability. In fact, the infrared emitters heat everything that faces them with a relatively high intensity. However, the radiation power densities involved in the preheating of balls 3 cannot cause the balls 3 to ignite directly. The conditions (level of radiation power density, temperature of the surrounding air, surface exposed in relation to the volume, exposure time, etc.) are simply not likely to initiate and maintain combustion of a full ball. On the other hand, particles of wood, such as sawdust and pieces of bark, which are found exposed for a long time to infrared can enter into combustion, and these parameters can be easily and suitably controlled, as evident for a skilled person. in art.

On the infrared pilot, the central zone of the conveyors 13 comprises metal plates (reflectors 21) partially reflecting the radiation which would otherwise be lost downwards. The reflected part is directed in part towards the balls 3, in part redirected towards the surface of the infrared emitters, which improves energy efficiency. These plates are placed between the chains for moving the balls and directly face the emitters 25 which overhang them: they are therefore subjected to radiation and, without the presence of balls 3, their temperature can rise. The wood particles found on these plates are therefore heated, on the one hand by direct contact with the metal plates, and on the other hand by radiation coming from the infrared emitters. It was under these conditions that the flammability tests were carried out. Flammability tests have shown that the power density at the infrared emitter must be below a certain given level to avoid the ignition of the various particles of wood found on the conveyor, as evident for a person versed in art.

On the other hand, an inflammation of these particles could not probably cause the ignition of whole beads. In addition, if the flammability constitutes an obstacle, fire prevention systems such as water or steam jet devices, and / or a particle sweeping / evacuation system, could also be provided according to the present invention, such as that also evident to a person skilled in the art.

Description of a typical installation In most sawmills, the logs 3 are cut and sorted according to their length, then transported parallel to each other on a conveyor 13 which routes them to other sorting systems. The balls 3 of larger and smaller diameters are then conveyed to the debarkers. Several minutes pass between the moment the log enters the sawmill and the debarking. Often, a conveyor 13 with a width equivalent to the length of the balls 3 and an appreciable length (more than twenty meters) is already present.

An infrared installation would simply consist in covering the conveyor 13 with infrared transmitters over the full width and over a length compatible with the required infrared exposure time. These transmitters 25 can be of different types. The transmitters 25 envisaged so far are of the "infrared panel" type. These panels are generally about ten centimeters thick. A certain distance between the level of the plane of the conveyor 13 and the emitting surface 15 must be respected, and transverse bars can ensure the protection of the surface 15 of the emitters 25 against the lifting of a ball 3. However, the height occupied by the infrared panels and by these bars would be relatively small: the vertical space available between the plane of the conveyor 13 and the ceiling of the log entry building 3 in the sawmill is more than enough. The installation of an infrared heating system in a conventional sawmill does not therefore pose an insurmountable problem: only the length necessary for an adequate heating time is likely to require reorganizations in the sawmill.

The use of electrically powered infrared emitters goes without saying in northern regions where this form of energy is available. However, the use of gas radiant heaters, particularly of the catalytic combustion type, is equally possible.

The biggest consideration is at the level of the conveyor 13. Usually, the conveyors 13 used do not impose a continuous rotational movement on the balls. However, in a possible infrared system, a rotational movement is preferable in order to expose the entire periphery of the ball 3 to radiation coming from the plane of the emitters 25 overhanging the balls 3. And it is advantageous to perform the rotation while by moving the balls laterally 3.

'The pilot used for the experimental tests included two chains with stops and two chains with teeth, the first to move the balls 3 laterally and the last to rotate the balls 3 on themselves without lateral movement.

In a real installation, the toothed chains could be replaced by fixed racks, and the rotation would be ensured by the lateral displacement of the balls 3 on the teeth of the racks 19. The type of conveyor 13 may however be different. The important thing is to make sure that the balls 3 rotate on themselves and that each is not hampered in its movement by the presence of neighboring balls 3. Ideally, the distance between each ball 3 could be dynamically adjusted in order to minimize the space between two neighboring balls 3, therefore minimize the losses of radiation passing between two balls 3 side by side. Because even if there is a reflective plate under the balls 3, it can absorb part of the infrared radiation 11 from the emitters 25 and this implies certain energy losses. To minimize the energy expended, it would be advantageous, in the case where the distance between two balls 3 is not adjustable, to discriminate the balls 3 according to the diameter. One can imagine an infrared heating system for the balls 3 of smaller diameter and another for the balls 3 of a larger diameter.

Power control

As mentioned previously, the energy expended for the logs 3 entering the sawmill at approximately -20 ° C or at -30 ° C is substantially the same. However, the energy required is naturally less for the balls 3 entering at approximately -10 ° C., since the temperature to be obtained in the cambium is around approximately -6 ° C. In this case, it would be advantageous to reduce the power delivered to the infrared transmitters 25, either by lowering the power of all the transmitters 25 by electronic control systems, or (preferably) by switching off a certain number of transmitters. 25 infrared. In both cases, the control parameter could be the temperature of the balls 3 at the input, read by pyrometry or direct contact, with a temperature probe, such as a thermocouple for example.

Achievements and Benefits

The application of infrared to preheating the balls before debarking allows a target temperature to be obtained in a few minutes at the wood-bark interface which facilitates debarking. The invention therefore provides all the advantages of the hot water technique relative to the loss of wood fibers, to wear and damage to the cutting tools of debarkers and to the reduction of the presence of bark. in the shavings.

Compared to the classic technique of soaking tanks or hot water jets, it eliminates the problems of draining and treating used water. It also eliminates the space occupied and the maintenance costs of the soaking and settling tanks, as well as the drip platform (the residue from the soaking tank must be placed on a platform adjacent to the soaking basin so that the excess water returns to the basin). The use of an infrared system above a conveyor allows a process to be used, in a space already currently occupied by the conveyor ensuring the lateral movement of the balls 3 in the sawmills.

In terms of energy, infrared technology is probably more efficient than water heating, the hot water basins for soaking the beads often being in the open air. The surface of the hot water exposed to the environment involves significant heat transfers by convection and radiation and significant losses by evaporation.

Furthermore, the bark detached from the ball 3 contains less water than when using hot water basins, which increases the calorific value of the bark. In the event of a recovery of the bark by combustion, this constitutes an energy advantage and eliminates the typical problems associated with the combustion of bark loaded with water.

In other words, as can now be appreciated, the present invention overcomes several problems and disadvantages of the prior art, and also offers several advantages over conventional methods and systems.

For example, with regard to the productivity criterion, the present invention allows: a reduction in the wood pulling of approximately 30%; an increase in sawing volume; a reduction in the wear and damage of the cutting tools, and a standardization of the pulp and paper manufacturing process (given the less seasonal variability of the bark content of the chips). Regarding the quality / value criterion of the product, the present invention allows: an improvement in the quality of the chips (less bark and fine particles) and a improved paper quality (fewer disadvantages linked to the presence of bark particles: dirt, sclerite, printing defects at the printer).

Regarding the energy efficiency criterion, the present invention allows: a) cost: on average, for a northern sawmill in Quebec and over a winter season, approximately $ 0.06 / log in electricity, or approximately $ 1 per cubic meter of volume of wood and b) return on investment: approximately 2 to 3 years if only investment costs, energy costs and gains in volume of fibers and wood are taken into account; less than a year, taking into account the increase in the value of the chips or the reduction of costs in chemical products to the paper mill.

Regarding the environmental criterion, the present invention allows: a reduction in the amount of debarking residue (less wood in the bark); elimination of the use of water in soaking tanks and spills of contaminated water; improving energy efficiency in bark-burning processes and reducing polluting emissions; and a reduction in the use of chemicals in pulp and paper mills to extract the bark particles from the pulp.

Although the present invention has been previously explained by way of preferred embodiments thereof, it should be clarified that any modification to these preferential embodiments, within the scope of the appended claims, is not considered to change or alter the nature and scope of the present invention.

Claims

Claims:

1. A method for heating frozen wooden balls (3) in order to facilitate debarking, each wooden ball (3) having a longitudinal axis (5), a peripheral surface (7) and a given diameter, the method being characterized in that it comprises the following stages: a) driving the frozen wooden balls (3) along a determined route; and b) subjecting the wooden balls (3) to infrared radiation (11) along the path to heat said wooden balls (3) by infrared radiation (11).

2. Method according to claim 1, characterized in that step a) further comprises the step of rotating the wooden balls (3) around their respective longitudinal axes (5) along at least a portion of the path in order to allow the peripheral surface (7) of each log (3) to be subjected to infrared radiation (11).

3. Method according to claim 1 or 2, characterized in that step a) further comprises the step of placing the wooden balls (3) parallel to each other.

4. Method according to any one of claims 1 to 3, characterized in that step a) further comprises the step of maintaining a certain distance between adjacent wooden logs (3).

5. Method according to any one of claims 1 to 3, characterized in that step a) further comprises the step of joining wooden logs (3) adjacent to each other.

6. Method according to any one of claims 1 to 5, characterized in that step a) comprises the step of driving the wooden balls (3) along a horizontal plane.

7. Method according to claim 6, characterized in that step b) further comprises the step of subjecting the wooden balls (3) to infrared radiation (11) positioned above the horizontal plane.

8. Method according to any one of claims 1 to 7, characterized in that step a) further comprises the step of driving the wooden balls (3) in a direction transverse to the longitudinal axes (5) of wooden logs (3).

9. Method according to any one of claims 1 to 8, characterized in that step a) further comprises the step of sorting the wooden balls (3) according to their diameter, the wooden balls (3) including the diameter falls within a first range of diameters being driven along a first determined path, and the wooden balls (3) whose diameter falls within a second range of diameters being driven along a second determined route, and characterized in that step b) further comprises the step of subjecting the sorted logs (3) to infrared radiation (11) along each of said routes.

10. Method according to any one of claims 1 to 9, characterized in that step b) further comprises the step of subjecting the wooden balls (3) to infrared radiation (11) having a length of wave located between about 0.7 and 10 microns.

11. Method according to any one of claims 1 to 10, characterized in that step b) further comprises the step of subjecting the wooden balls (3) to infrared radiation (11) having a power density determined according to the characteristics of the wooden logs (3).

12. Method according to any one of claims 1 to 11, characterized in that step b) further comprises the step of subjecting the wooden balls (3) to infrared radiation (11) for a period determined by according to the characteristics of the wooden logs (3).

13. Method according to any one of claims 1 to 12, characterized in that step b) further comprises the step of reflecting a portion of the infrared radiation (11) not absorbed by the wooden balls (3) again to wooden logs (3).

14. Method according to any one of claims 1 to 13, characterized in that step a) further comprises the step of evaluating the characteristics of the wooden balls (3) before subjecting them to infrared radiation (11 ).

15. A system (1) for heating frozen wooden balls (3) in order to facilitate debarking, each wooden ball (3) having a longitudinal axis (5), a peripheral surface (7) and a given diameter, the system (1) being characterized in that it comprises: a) a conveyor (13) for driving the frozen wooden balls (3) along a determined path; and b) an infrared radiation surface (15) positioned relative to the conveyor (13) to subject the wooden balls (3) to infrared radiation (11) along the course and to heat said wooden balls (3 ) by infrared radiation (11).

16. System (1) according to claim 15, characterized in that it comprises means for pivoting the wooden balls (3) around their respective longitudinal axes (5) along at least a portion of the course so to allow the peripheral surface (7) of each wooden ball (3) to be subjected to infrared radiation (11).

17. System (1) according to claim 16, characterized in that the means for rotating the wooden balls (3) comprise at least one tooth chain cooperating with the conveyor (13).

18. System (1) according to claim 16, characterized in that the means for rotating the wooden balls (3) comprise at least one rack chain (19) cooperating with the conveyor (13).

19. System (1) according to any one of claims 15 to 18, characterized in that it comprises means for placing the wooden balls (3) parallel to each other.

20. System (1) according to any one of claims 15 to 19, characterized in that it comprises means for maintaining a certain distance between adjacent wooden balls (3).

21. System (1) according to any one of claims 15 to 20, characterized in that it comprises means for joining the wooden balls (3) to each other.

22. System (1) according to any one of claims 15 to 21, characterized in that the conveyor (13) is capable of driving the wooden balls (3) along a horizontal plane.

23. System (1) according to claim 22, characterized in that the surface (15) of infrared radiation (11) is positioned above the horizontal plane.

24. System (1) according to any one of claims 15 to 23, characterized in that it comprises means for driving the wooden balls (3) in a direction transverse to the longitudinal axes (5) of the wooden balls.

25. System (1) according to any one of claims 1 5 to 24, characterized in that it comprises means for sorting the wooden balls (3) according to their diameter, the wooden balls (3) whose diameter falls within a first range of diameters being driven along a first determined path, and the wooden balls (3) whose diameter falls within a second range of diameters being driven along a second determined route, the system (1) being also characterized in that it comprises a surface (15) of heating by infrared radiation (11) for each of said routes.

26. System (1) according to any one of claims 15 to 25, characterized in that the infrared radiation (11) has a wavelength between approximately 0.7 and 10 microns.

27. System (1) according to any one of claims 15 to 26, characterized in that the infrared radiation (11) has a power density determined as a function of the characteristics of the wooden balls (3).

28. System (1) according to any one of claims 15 to 27, characterized in that it comprises means for controlling, as a function of the characteristics of the wooden logs (3), the period during which the wooden logs ( 3) must be subjected to infrared radiation (11).

29. System (1) according to any one of claims 15 to 28, characterized in that it comprises at least one reflector (21) for reflecting a portion of the infrared radiation (11) not absorbed by the wooden balls ( 3) to wooden logs.

30. System (1) according to any one of claims 15 to 29, characterized in that said at least reflector (21) is positioned below the conveyor (13).

31. System (1) according to any one of claims 15 to 30, characterized in that it comprises an evaluation device for evaluating the characteristics of the wooden balls (3) before subjecting them to infrared radiation (11) .

32. System (1) according to any one of claims 15 to 31, characterized in that it comprises at least one chain of stops (23) cooperating with the conveyor (13) to drive the wooden balls (3).

33. System (1) according to any one of claims 15 to 32, characterized in that the surface (15) of infrared radiation (11) comprises electric transmitters (25).

34. System (1) according to any one of claims 15 to 32, characterized in that the surface (15) of infrared radiation (11) comprises gas radiant heaters.

35. System (1) according to any one of claims 15 to 32, characterized in that the surface (15) of infrared radiation (11) comprises gas radiant catalytic type.