EP1670622B1 - Method and system for heating frozen logs - Google Patents

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

Field of the invention

The present invention relates to a method and a system for heating frozen balls. More particularly, the present invention relates to a method and system for heating frozen logs to facilitate their debarking.

Description of the prior art:

Sawmills in forest-rich countries produce lumber from trees taken from the forest, which must be stripped of their bark before cutting into planks and planks. Debarking is often performed by a ring debarker with a motorized rotary ring to which bent tools are attached which remove the bark by friction as the ball moves longitudinally within the ring due 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 cut in order to remove the bark. The lower the temperature of the balls, the stronger the adhesion and the greater the pressure must be.

Increasing the pressure on the cutting tools, however, has several disadvantages. On the one hand, the increase in debarking pressure produces accelerated wear of the cutting tools and possibly damage to the components of the debarker (cutting tool, arm, tensioning system, etc.). On the other hand, the debarkers then tear too much wood fiber with the bark, which is a significant loss. In the summer, about 10 to 20% of the bark mass consists of wood fibers, which are burned or buried with the bark. In winter, at some Canadian sawmills, up to 50% of the bark mass is wood fiber. Typically, about 25% of the revenue from a conventional sawmill come from the ligneous fiber that it sells in the form of chips to a paper mill. These chips come from sections of the log that are not found in the form of lumber.

Another disadvantage arises from the fact that if the pressure on the cutting tools is decreased in order to minimize the loss of wood, bark is then found in the chips. However, the presence of bark particles in the chips is at the root of various problems in the paper mill: decreased yield of the digester, dirt in the dough (pitch), and especially presence of sclerite which increases, ultimately, the rate incidence of breakage of the sheet during its manufacture and printing defects ('fish eye', grain of rice) at the printer. For so long, the bark rate in the chips of suppliers is one of the most important quality criteria for pulp manufacturers, and paper mills are becoming more demanding.

There is therefore a strong advantage in increasing the temperature of the fillet before debarking. Typically, winter log thaw can reduce wood losses by about 2% while increasing the quality of chips delivered to pulp and paper mills. For a mill that consumes about 500 000 m ³ of wood annually, this improvement represents a saving of over half a million dollars (the amounts mentioned herein are expressed in Canadian dollars).

To achieve this temperature increase, sawmills are currently using heating methods including immersion and spraying of the logs in hot water ponds or by means of water jets and / or steam devices. These methods work well in terms of preheating, but pose a number of problems, the most important of which is the environment. Indeed, emptying and cleaning are frequently required to rid the basins that accumulate dirt and bark. Since contaminated water can no longer be released to the environment because of government regulations, sawmills must have settling ponds that must be frequently emptied and cleaned, resulting in high costs.

1. a) empiler, éléver et culbuter les billes de bois gelées afin de favoriser le frottement et les chocs entre billes produisant l'écorçage, et
2. b) soumettre les billes de bois à de l'eau chaude ou de la vapeur d'eau.

US-A-5,263,522 describes a method for heating frozen logs to facilitate their debarking, each log having a longitudinal axis, a peripheral surface and a given diameter, the method comprising the steps of:

1. a) stacking, elevating and tumbling the frozen logs to promote friction and shocks between logs producing debarking, and
2. (b) subject the logs to hot water or steam.

An object of the present invention is therefore to provide a method and / or a heating system that can solve some of the problems mentioned above of the prior art. In particular, an object is to propose a new method for heating frozen wood logs in winter to facilitate their debarking. Another object is to propose a system for implementing this method.

Summary of the invention

1. a) entraîner les billes de bois gelées le long d'un parcours déterminé; et
2. b) soumettre les billes de bois à un rayonnement infrarouge le long du parcours pour chauffer lesdites billes de bois par rayonnement infrarouge.

The present invention provides a method for heating frozen logs to facilitate their debarking, each log having a longitudinal axis, a peripheral surface and a given diameter, the method comprising the steps of:

1. a) driving the frozen logs along a specified course; and
2. b) subjecting the logs to infrared radiation along the path to heat said logs with infrared radiation.

Preferably, step a) further comprises the step of pivoting the logs around their respective longitudinal axes along at least a portion of the path to allow the peripheral surface of each log be subjected to infrared radiation.

1. a) un convoyeur pour entraîner les billes de bois gelées le long d'un parcours déterminé; et
2. b) une surface de rayonnement infrarouge positionnée par rapport au convoyeur pour soumettre les billes de bois à un rayonnement infrarouge le long du parcours et chauffer lesdites billes de bois par rayonnement infrarouge.

The present invention provides a system for heating frozen logs to facilitate their debarking, each log having a longitudinal axis, a peripheral surface and a given diameter, the system comprising:

1. a) a conveyor for driving the frozen logs along a determined path; and
2. b) an infrared radiation surface positioned relative to the conveyor to subject the logs to infrared radiation along the path and heat said wood logs by infrared radiation.

Preferably, the system comprises any means, apparatus and / or device for implementing the various steps and / or sub-steps of the method according to the present invention.

The objects, advantages, and other features of the present invention will become more apparent upon reading the following non-limiting 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 displacement 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 wood pellets according to another preferred embodiment of the present invention, the wood logs being moved by the conveyor and being subjected to infrared radiation.
• Figure 3 is a side view of a frozen ball heating system driver according to the present invention.
• Figure 4 is a front view of what is illustrated in the Figure 3 .
• Figure 5 is a top view of what is illustrated at the Figure 3 .

Detailed description of the drawings:

In the following description, the same numerical references 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 "heat" can also be used to melt or sublimate the ice and the word "infrared" includes any electromagnetic radiation of a thermal nature (visible type radiation, infrared radiation). short, medium infrared, long infrared, etc.).

In addition, although the present invention is primarily designed to heat frozen wood pellets to facilitate their debarking, the invention may be used in other fields for other applications, as obvious to a person. versed in art. For these reasons, expressions such as "logs", "wood", "jellies" and / or "debarking" and any other reference and / or any other equivalent or similar expression shall not be construed 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 means for moving and rotating the logs 3 include certain components, such as chains, reflectors, etc., all 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 so as to limit the scope of the present invention. It should be understood, as is also obvious to one skilled in the art, that other suitable components and geometries and other cooperations 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 "set", as well as any other equivalent expression and / or compound words thereof, may be used interchangeably in context of the present description. This also applies to other expressions that are mutually equivalent, such as "infrared radiation", "infrared heating" and "thermal radiation" for example, as is obvious to a person skilled in the art.

With reference to the attached figures, and in general, the present invention relates to a method of heating frozen balls. More particularly, the present invention relates to a method of heating frozen wood pellets to facilitate their debarking, and also relates to a system 1 for carrying out this method. As known in the art, each log 3 normally has a longitudinal axis 5 (typically a central axis 5), a peripheral surface 7 (outer surface 7 of the ball), and a given diameter (average).

According to the present invention, the method comprises two main steps, in particular step a) of driving the frozen logs 3 along a determined path; and step b) subjecting the wood pellets 3 to infrared radiation 11 along the path for heating said wood pellets 3 by infrared radiation 11.

As will become more apparent from the following description, each of the above steps may comprise several substeps depending on the applications for which this method is used and the final results that are desired.

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

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

In addition, steps a) and b) can be intimately linked according to the applications for which this method is intended, as is also obvious to a person skilled in the art. For example, step a) could include the substep of sorting the logs 3 by their diameter, the logs 3 whose diameter falls within a first range of diameters being driven along a first determined course, and the logs 3 whose diameter falls within a second range of diameters being driven along a determined second course. By therefore, step b) could further comprise the step of subjecting the sorted logs 3 to infrared radiation 11 along each of said paths, i.e., there could be a first system of infrared heating 11 for heating the logs 3 of the first diameter range and another infrared heating system 11 separated by infrared radiation 11 (or a re-routing to the first heating system after treating the logs 3 of the first diameter range) for heating the logs 3 of the second range of diameters.

As previously explained, the present invention also relates to a system 1 for heating frozen wood logs 3 to facilitate their debarking. The system 1 comprises a conveyor 13 for driving the frozen wood logs along a determined path and an infrared radiation surface 11 positioned relative to the conveyor 13 for subjecting the wood pellets 3 to infrared radiation 11 along the route and heat said logs 3 by infrared radiation 11.

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

For example, and preferably, the system 1 according to the present invention comprises: means for pivoting the logs 3 about their respective longitudinal axes along at least a portion of the path to allow the peripheral surface 7 of each wooden ball 3 to be subjected to infrared radiation 11 (these means preferably comprising toothed chains or rack chains 19 cooperating with the conveyor 13, or any other suitable device); means for placing the logs 3 parallel to each other; means for maintaining a distance between the adjacent logs 3; means for joining the logs 3 to each other; a conveyor 13 adapted to drive the logs 3 along a horizontal plane; an infrared radiation surface 11 positioned above the horizontal plane; means to train the logs 3 in a direction transverse to the longitudinal axes 5 of the logs 3; means for sorting the logs 3 according to their diameter, the logs 3 whose diameter falls within a first range of diameters being driven along a first determined course, and the logs 3 whose diameter falls within a second range of diameters being driven along a determined second course; an infrared radiation surface for each path; at least one chain of stops 23 cooperating with the conveyor 13 for driving the logs 3; an evaluation device for evaluating the characteristics of the logs 3 before subjecting them to infrared radiation 11; means for controlling, depending on the characteristics (eg the initial temperature, etc.) of the logs 3, the period during which the logs 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 logs 3 to logs 3 (either the same logs 3 or other logs 3 to be heated), the reflector 21 being preferably positioned below conveyor 13 if infrared radiation surface 11 is positioned above it; etc.

In addition, the infrared radiation 11 preferably has a wavelength of between about 0.7 and 10 microns and a power density determined according to the characteristics of the wood pellets 3 and limitations relating to flammability. Also preferably, the infrared radiation surface 11 may take the form of electric emitters, gas radiants, catalytic type gas radiants, or any other emitter suitable for emitting infrared radiation 11 or any other radiation. suitable heat for heating the logs 3 according to the present invention and having the preferred features mentioned in the present description.

Various other aspects, features and advantages of the present invention will become more apparent upon reading the following description of general operation and basic principles, experimental work performed, test results, description of a typical installation. and improvements and benefits achievable in the context of the present invention.

General operation and basic principles

The solution that is proposed here and which is the subject of the invention involves infrared technology. This preferred technology was chosen for its simplicity and ability to heat effectively at a reasonable cost. At the base of the technology, a simple heating element is brought to high temperature, which allows it to emit radiation called "infrared", 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 wavelength range between about 0.7 and 10 microns. There is often a distinction between "short" type infrared, "medium" infrared and "long" infrared. Short IR involves an emission temperature above about 2000 ° C and its spectral emission range mainly covers the range of about 0.7 to 3.0 microns. Medium infrared involves an emission temperature between about 700 and 1300 ° C and covers a range of about 1 to 7 microns. Long type infrared involves an emission temperature of about 400 to 600 ° C and mainly covers the wavelength range of about 2 to 10 microns.

Just like the technique of soaking with hot water, the infrared heats strictly on the surface and has little or no ability to heat deeply. Even the short-type infrared, which has the ability to penetrate certain materials, penetrates only marginally the outer surface of the bark. On the other hand, the zone to be heated, the bark, is directly under the outer surface of the ball. In fact, it is necessary to heat the so-called "wet" bark located under the so-called "dry" bark to the wood-bark interface where the cambium is located. This zone is thus heated by thermal conduction from the surface exposed to the infrared towards the inside of the ball. On the other hand, the entire periphery of the balls 3 must be exposed to the infrared radiation 11, which can be achieved by rolling the ball 3 on itself below a plane of emitters 25.

In other words, the heating by infrared radiation 11 does not consist in completely thawing the ball 3 but rather in preheating the surface 15. the facts, the outer bark, the visible part, contains practically no water. On the other hand, 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 force between the wood and the bark. It is therefore necessary to reheat this part if it is desired to have a good operation of the debarker, which maximizes the sawing yield and improves the quality of the chips (by reducing the amount of bark in the chips).

Ideally, an infrared preheating system should comprise a conveying system where all the balls 3 would be contiguous to each other, in order to minimize the radiation losses between the balls 3. In fact, the displacement by rotation of the balls involves a movement inverse tangential between two adjacent balls, as best illustrated in FIG. Figure 1 .

Naturally, it is physically very difficult to perform lateral displacement without providing a certain spacing between the balls 3. Moreover, the balls 3 do not all have the same diameter and keeping them side by side would imply hiding the balls. small between two large diameter balls.

The conveyor 13 envisaged involves chains with stops 23 to push each ball 3 and fixed or movable toothed chains or rack chains 19 to ensure their rotation. The distance between each abutment is uniform along the conveyor 13 and is greater than the maximum diameter of the balls 3 entering a plane of infrared emitters, as better illustrated in FIG. Figure 2 .

The use of infrared completely eliminates the use of water and associated environmental problems. On the other hand, the implementation of this type of system 1 requires the respect of certain power density limits, the choice of a particular type of infrared emitter, and the use of a conveyor 13 making it possible to expose the entire periphery of the balls. 3. A research project was therefore initiated to answer certain technical questions and to demonstrate the principle.

Experimental work done

At first, it has been demonstrated that it is not necessary to completely defrost the logs 3. Obtaining, at the interface between the wood and the bark, a temperature of a few degrees below the point freezing substantially reduces the shearing force to the debarker. This demonstration was made from samples of logs 3 of different species at various temperatures below the freezing point and using a device measuring the shear force to be provided to detach the bark from samples of balls 3.

In a second step, as better illustrated at Figures 3 to 5 a pilot comprising a conveyor 13 under infrared emitters was constructed to characterize the thermal behavior of the balls 3 under various infrared radiation power densities and exposure periods, on balls 3 of different diameters and thicknesses of bark and according to various initial temperatures. The Figures 3 to 5 show the pilot with his main components.

This pilot was used to establish operating parameters, such as the power and the exposure time of beads 3 to infrared radiation. As illustrated on the figure 3 (and similarly, at the Figure 2 ), this equipment was characterized by infrared emitters placed above a conveyor 13 allowing the displacement and rotation of the balls 3; the outer surface 7 of each of the balls 3 is thus completely exposed to the infrared radiation 11. Thermocouples were installed at different depths inside the ball 3 to measure the rise in temperature. Black spruce logs frozen at temperatures of about -15 ° C. at -30 ° 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 at depth of the ball 3 by thermocouple and at the surface by pyrometry. Ball 3 with thermocouples was placed on the conveyor 13 with two balls 3 neighbors on both sides. At first, the stop chains 23 moved the three balls 3 under the infrared exposure zone. Once the balls 3 under the emitters 25, the toothed chains rolled the balls 3 on themselves for a predetermined time ranging from about 1 to 8 minutes. In a third step, the balls 3 were removed from the heating zone and the rotational movement continued without infrared exposure for several minutes, the total time of exposure and non-exposure corresponding to a period of time typical between the entrance of a ball 3 in a sawmill and the debarking station.

Another type of test has also been to place pieces of wood such as bark, sawdust and chips on a metal plate under different levels of radiation power density to observe the tendency to ignite.

Test results Shear tests

The results of shear tests as a function of temperature showed that the shear stress decreases sharply with increasing temperature. This decrease is particularly strong below the freezing point, and less marked near and above the freezing point. Since the energy expenditure would be very great if the infrared heating system were to pass all the water from the bark from the solid state to the liquid state, it became clear that it is advantageous to limit temperature rise up to a few degrees below the freezing point. The shear forces are not much larger than above the freezing point (the freezing point of water in the wood is about 2 or 3 degrees lower than the normal temperature of pure water , because of the presence of minerals in the water).

The ball heating tests on the infrared driver

The infrared heating results on the pilot confirmed the validity of infrared technology to preheat ball 3 and obtain an adequate temperature at the interface between bark and wood (i.e. cambium). The infrared radiation 11 is strictly absorbed on 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).

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

The tests showed that the lower the initial temperature of ball 3, the more effective the infrared exposure. So much so that the energy expenditure required to rise to the debarking temperature is substantially the same, whether the ball is at an initial temperature of about -20 ° C or -30 ° C. An explanation for this is that the water in the bark changes to the liquid state later when ball 3 has a starting temperature of about -30 ° C. However, ice has a thermal conductivity greater than water (by a factor of four) and this allows the heat absorbed on the surface to propagate better inside the ball. Also, the passage 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 deeper.

Moreover, the tests carried out tend to indicate that the energy required to make a rise in temperature to the desired temperature level 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 larger balls 3 determines the energy consumption for all the balls. Which is to say that the smaller 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 possible. 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, the energy expenditure to raise the temperature at the wood / bark interface of a typical ball of about 3.8 meters in length from a very low temperature (about -20 ° C). C at -30 ° C) to about -6 ° C is estimated at about 3 kWh per meter in diameter. For a ball 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 a fixed spacing between the balls 3, and that the larger balls 3 are about 20 cm in diameter, this means that the energy expenditure is about 0.6 kWh per ball, regardless of diameter. At a cost of about 10 to 15 ¢ per kilowatt-hour (marginal cost of electricity in Quebec, taking into account a high penalty on the power demand), this corresponds to a cost of less than about 10 ¢ per ball .

All of the infra-red pilot tests focused exclusively on black spruce. Among all the test results, the most out of the general trend of experimental points were those associated with bark with partially detached and sometimes superimposed pieces. However, the presence of air gap between two scales of bark or between the bark and the wood is a barrier to the transfer of heat to the inside of the ball, 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. Infrared technology is therefore likely to apply to all species that are the raw material of Nordic sawmills.

Flammability tests

The use of infrared raises the question of flammability. Indeed, the infrared emitters heat everything that faces them with a relatively high intensity. However, the radiation power densities involved in the case of the preheating of balls 3 can not cause the balls 3 to be ignited directly. Conditions (radiation power density level, surrounding air temperature, exposed surface to volume, exposure time, etc.) are simply not likely to initiate and maintain a combustion of full ball. On the other hand, wood particles, such as sawdust and pieces of bark, which are exposed for a long time to the infrared can come into combustion, and these parameters can be easily and suitably controlled, as is clear for a person paid in art.

On the infrared driver, the central area of the conveyors 13 comprises metal plates (reflectors 21) partially reflecting the radiation that would otherwise be lost downwards. The reflected part is directed in part towards the balls 3, partially redirected towards the surface of the infrared emitters, which improves the energy efficiency. These plates are placed between the chains of displacement of the balls and face directly the emitters 25 which overhang them: they are therefore subjected to radiation and, without the presence of beads 3, their temperature can rise. The wood particles found on these plates are thus heated, on the one hand by direct contact with the metal plates, and on the other hand by radiation from the infrared emitters.

It is in these conditions that the flammability tests were carried out.

The flammability tests have shown that the power density at the infrared emitter must be lower than a given level to prevent the inflammation of the various wood particles found at the conveyor, as is obvious for a person paid in art.

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

Description of a typical installation

In most sawmills, the balls 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 stronger and smaller diameters are then routed to the debarkers. Between the moment of entry of the ball in the sawmill and the debarking thus flow several minutes. Often, a conveyor 13 with a width equivalent to the length of the balls 3 and of appreciable length (more than twenty meters) is already present.

An infrared installation would simply be to cover the infra-red transmitter conveyor 13 over the full width and over a length compatible with the required infrared exposure time. These emitters can be of different types. The emitters 25 envisaged so far are of the "infrared panels" type. These panels generally have a thickness of about ten centimeters. A certain distance between the level of the plane of the conveyor 13 and the emitting surface must be respected, and transverse bars can ensure the protection of the surface 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 building of entry of the balls 3 in the sawmill is ample. The implementation of an infrared heating system in a conventional sawmill does not therefore pose an insurmountable problem: only the length required for adequate heating time is likely to require reorganization in the sawmill.

The use of infrared emitters powered by electricity is self-evident in northern areas where this form of energy is available. However, the use of gas radiants, particularly of catalytic combustion type, is equally conceivable.

The biggest consideration is at the conveyor 13. Usually, the conveyors 13 used do not impose a continuous rotational movement of 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 the radiation coming from the plane of the emitters 25 overlying the balls 3. And it is advantageous to carry out the rotation while by moving the balls 3 laterally.

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 displacement.

In an actual 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 one is not hindered in its movement by the presence of the neighboring balls 3. Ideally, the distance between each ball 3 could be adjusted dynamically to minimize the space between two adjacent balls 3, thus minimizing radiation losses passing between two balls 3 side by side. Because even if there is a reflective plate under the balls 3, it can absorb a portion 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 previously mentioned, the energy expended for the logs entering the mill at about -20 ° C or -30 ° C is substantially the same. But the energy required is naturally less for beads 3 entering at about -10 ° C, since the temperature to be obtained in the cambium is around -6 ° C. In this case, it would be advantageous to reduce the power delivered to the infrared transmitters, either by lowering the power of all transmitters by electronic control systems, or (preferably) by turning off a number of transmitters. Infrared. In both cases, the control parameter could be the temperature of the balls 3 at the inlet, read by pyrometry or direct contact with a temperature probe, such as a thermocouple for example.

Improvements and achievable benefits

The application of infrared preheating logs before debarking provides a target temperature within minutes wood-bark that facilitates debarking. The invention therefore provides all the advantages of the hot water technique with respect to the loss of wood fibers, wear and damage to debarking cutters and the reduction of bark in the chips.

Compared to the traditional technique of soaking basins or hot water jets, it eliminates the problems of emptying and treating waste water. It also eliminates the space occupied and maintenance costs of the soaking and settling ponds, as well as the drip platform (the soaking tank residues). must be placed on a platform adjacent to the soaking basin so that excess water will return to the pond). The use of an infrared system above a conveyor makes it possible to use a parcel method, in a space that is already occupied by the conveyor ensuring the lateral displacement of the balls 3 in the sawmills.

In terms of energy, infrared technology is probably more effective than water heating, with hot water tanks for dipping logs often in the open air. The surface of hot water exposed to the environment involves large heat transfers by convection and radiation and significant losses by evaporation.

In addition, the bark detached from the ball 3 contains less water than in the case of the use of hot water tanks, which increases the calorific value of the bark. In the event of burning bark, this is an energetic asset and relieves typical problems associated with the burning of bark laden with water.

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

For example, with respect to the productivity criterion, the present invention allows: a decrease in wood tearing of about 30%; an increase in sawing volume; reduced wear and damage to cutting tools, and standardization of the pulp and paper manufacturing process (due to less seasonal variability in chip bark content).

Concerning the quality / value criterion of the product, the present invention allows: an improvement of the quality of the chips (less bark and fine particles) and a improvement of the paper quality (less inconvenience due to the presence of bark particles: dirt, sclerite, printing defects in the printer).

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

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

Although the present invention has been previously explained through preferential embodiments thereof, it should be made clear that any modification to these preferred embodiments within the scope of the appended claims is not considered to be altered or altered. the nature and scope of the present invention.

Claims (35)

1. A method for heating frozen logs of wood (3) in order to make it easier to strip off the bark, each wooden log (3) having a longitudinal axis (5), a peripheral surface (7) and a given diameter, the method comprising the following steps:

   a) driving the frozen wooden logs (3) along a predetermined path; and

   b) subjecting the wooden logs (3) to infrared radiation (11) along the path in order to heat the said wooden logs (3) by infrared radiation (11).
2. Method according to claim 1 characterised in that the step a) comprises furthermore the step of making the wooden logs pivot around their respective longitudinal axes (5) along at least one section of the path in order to allow the peripheral surface (7) of each wooden log (3) to be subjected to the infrared radiation (11).
3. Method according to claim 1 or 2 characterised in that the step a) comprises furthermore the step of placing the wooden logs (3) parallel to one another.
4. Method according to any one of claims 1 to 3 characterised in that the step a) comprises furthermore the step of maintaining a certain distance between the adjacent wooden logs (3).
5. Method according to any one of claims 1 to 3 characterised in that the step a) comprises furthermore the step of setting the wooden logs (3) adjacent to one another.
6. Method according to any one of claims 1 to 5 characterised in that the step a) comprises furthermore the step of driving the wooden logs (3) along a horizontal plane.
7. Method according to claim 6 characterised in that the step b) comprises furthermore the step of subjecting the wooden logs (3) to infrared radiation (11) positioned above the horizontal plane.
8. Method according to any of claims 1 to 7 characterised in that the step a) comprises furthermore the step of driving the wooden logs (3) in a direction transversely to the longitudinal axes (5) of the wooden logs (3).
9. Method according to any of claims 1 to 8 characterised in that the step a) comprises furthermore the step of sorting the wooden logs (3) according to their diameter, the wooden logs (3) whose diameter falls within a first band of diameters being driven along a predetermined first path, and the wooden logs (3) whose diameter falls within a second band of diameters being driven along a predetermined second path, and characterised in that the step b) comprises furthermore the step of subjecting the sorted wooden logs (3) to infrared radiation (11) along each of the said paths.
10. Method according to any of claims 1 to 9 characterised in that the step b) comprises furthermore the step of subjecting the wooden logs (3) to an infrared radiation (11) having a wavelength located between about 0.7 and 10 microns.
11. Method according to any of claims 1 to 10 characterised in that the step b) comprises furthermore the step of subjecting the wooden logs (3) to an infrared radiation (11) having a power density determined as a function of the characteristics of the wooden logs (3).
12. Method according to any of claims 1 to 11 characterised in that the step b) comprises furthermore the step of subjecting the wooden logs (3) to an infrared radiation (11) for a period of time predetermined as a function of the characteristics of the wooden logs (3).
13. Method according to any of claims 1 to 12 characterised in that the step b) comprises furthermore the step of reflecting a part of the infrared radiation (11) not absorbed by the wooden logs (3) once more towards the wooden logs (3).
14. Method according to any of claims 1 to 13 characterised in that the step a) furthermore comprises the step of evaluating the characteristics of the wooden logs (3) before subjecting them to the infrared radiation (11).
15. A system (1) for heating frozen wooden logs (3) with the aim of making it easier to strip the bark, each wooden log (3) having a longitudinal axis (5), a peripheral surface (7) and a given diameter, the system (1) comprising:

    a) a conveyor (13) for driving the frozen wooden logs (3) along a predetermined path; and

    b) an infrared radiation (11) surface (15) positioned in relation to the conveyor (13) in order to subject the wooden logs (3) to infrared radiation (11) along the path and to heat the said wooden logs (3) through infrared radiation (11).
16. System (1) according to claim 15 characterised in that it comprises means for making the wooden logs (3) pivot around their respective longitudinal axes (5) along at least one section of the path in order to enable the peripheral surface (7) of each wooden log (3) to be subjected to infrared radiation (11).
17. System (1) according to claim 16 characterised in that the means for making the wooden logs (3) pivot comprises at least one cogged chain interacting with the conveyor (13).
18. System (1) according to claim 16 characterised in that the means for making the wooden logs (3) pivot comprise at least one toothed rack (19) interacting with the conveyor (13).
19. System (1) according to any of claims 15 to 18 characterised in that it comprises the means for placing the wooden logs (3) parallel to one another.
20. System (1) according to any of claims 15 to 19 characterised in that it comprises means for maintaining a certain distance between adjacent wooden logs (3).
21. System (1) according to any of claims 15 to 20 characterised in that it comprises means for placing the wooden logs (3) side by side.
22. System (1) according to any of claims 15 to 21 characterised in that the conveyor (13) is adapted to drive the wooden logs (3) along a horizontal plane.
23. System (1) according to claim 22 characterised in that the infrared radiation (11) surface (15) is positioned above the horizontal plane.
24. System (1) according to any of claims 15 to 23 characterised in that it comprises means for driving the wooden logs (3) in a direction transversely to the longitudinal axes (5) of the wooden logs.
25. System (1) according to any of claims 15 to 24 characterised in that it comprises means for sorting the wooden logs (3) according to their diameter, the wooden logs (3) whose diameter falls within a first band of diameters being driven along a predetermined first path, and the wooden logs (3) whose diameter falls within a second band of diameters being driven along a predetermined second path, the system (1) being equally characterised in that it comprises an infrared radiation (11) heating surface (15) for each of the said paths.
26. System (1) according to any of claims 15 to 25 characterised in that the infrared radiation (11) has a wavelength located between about 0.7 and 10 microns.
27. System (1) according to any of claims 15 to 26 characterised in that the infrared radiation (11) has a power density determined as a function of the characteristics of the wooden logs (3).
28. System (1) according to any of claims 15 to 27 characterised 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) have to be subjected to the infrared radiation (11).
29. System (1) according to any of claims 15 to 28 characterised in that it comprises at least one reflector (21) in order to reflect a part of the infrared radiation (11) not absorbed by the wooden logs (3) back towards the wooden logs.
30. System (1) according to any of claims 15 to 29 characterised in that the said at least one reflector (21) is positioned below the conveyor (13).
31. System (1) according to any of claims 15 to 30 characterised in that it comprises an evaluation device for evaluating the characteristics of the wooden logs (3) before subjecting them to the infrared radiation (11).
32. System (1) according to any of claims 15 to 31 characterised in that it comprises at least one support chain (23) interacting with the conveyor (13) in order to drive the wooden logs (3).
33. System (1) according to any of claims 15 to 32 characterised in that the infrared radiation (11) surface (15) comprises electrical transmitters (25).
34. System (1) according to any of claims 15 to 32 characterised in that the infrared radiation (11) surface (15) comprises gas radiators.
35. System (1) according to any of claims 15 to 32 characterised in that the infrared radiation (11) surface (15) comprises gas radiators of the catalytic type.

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