EP2668458B1 - Secure method for the heat treatment of wood - Google Patents

Description

The invention relates to a method of heat treatment of wood, the principle of which is based on the generation of hot gases from the combustion of biomass, said gases feeding a furnace designed to provide this treatment. Heat treatment of wood can improve its biological resistance to insects and xylophagous fungi, or increase its dimensional stability through a modification of the material and without the introduction of chemical.

Wood heat treatment processes using a gas generator using biomass are generally confronted with two major risks, which are fire and environmental pollution, both of which are related to the production of toxic vapors. . Indeed, wood heated to a temperature above 180 ° C undergoes chemical modifications of its carbon chains. Cellulose, hemicelluloses and wood lignin undergo a gradual degradation producing molecules such as aldehydes, such as furfural or formaldehyde, methanol formic and acetic acids as well as combustible gases such as hydrogen, monoxide of carbon and methane. These gases are likely to self-ignite and thus cause a fire in wood heat treatment furnaces. The most commonly used solutions to limit the risk of starting and spreading a fire consist in limiting the oxygen content of the enclosure constituted by the oven, and to eliminate the possible sources of ignition. In processes based on the generation of gases from biomass, the combustion gases loaded with 10% oxygen are reused for the supply of the furnace, without the presence of naked flames in the enclosure. However, it turns out that preliminary tests of heat treatment of wood have shown that a rate of oxygen of the order of 10% in the enclosure remained too high to guarantee, for sure, the safety of the installations. to a fire departure. In addition, certain safety standards, such as the EN1539 standard for ovens and ovens containing emissions of volatile organic compounds, recommend an oxygen content of less than 7% for high temperatures. above 150 ° C. Other means have already been implemented to reduce the risk of fire, among which, the setting overpressure of the chamber to limit oxygen inputs, forced evacuation of vapors by means of a pump to reduce the concentration of flammable gases, or the establishment in the chamber of temperature probes for detecting possible starting points of fire. However, the main disadvantages of all these techniques are that it remains particularly difficult to lower the oxygen level from 10% to 7%, and to control that this rate has been reached appropriately. In addition, all these techniques have a high cost, and are not without influence on the process of heat treatment of wood, and on the quality of this treatment. Finally, these techniques are not necessarily adaptable to fixed input data, which are, for example, the flammable nature of wood, the use as a starting point of a gas at 10% oxygen at a minimum. Such methods are defined in WO 02/16848 A1 . Regarding the recovery of vapors from heat treatment of wood, it is currently carried out by condensation at the outlet of the furnace, by means of a condenser type "tubes and calender" with a water cooling system, this solution having been validated following tests carried out on a small-scale station. If the results showed acceptable levels of recovery, it is clear that retention rates, including the heaviest compounds, could be improved. Other means have already been used for the recovery of vapors, such as forced extraction of these vapors by means of a pump, or storage in a set of two successive tanks. The main drawbacks of these techniques are that they do not systematically allow the uptake of products in small quantities and with high vapor pressures, that they can not, moreover, surely avoid the re-aspiration of fluids in the form of vapors by the pumping system, and finally, that they do not provide a self-cleaning of the condensation system. In addition, they do not guarantee condensation of vapors without lowering the gas flow, and do not guarantee to make the liquid state permanent, including in a low vacuum system.

The methods of heat treatment of wood according to the invention, implement means for limiting the risk of fire, and means for recovering the vapors from the treatment, these means overcoming all the aforementioned drawbacks. In order to ensure that the oxygen level does not exceed a threshold value in the oven throughout the duration of the process, said method includes an additional step of controlling this oxygen level, in particular to organize an intervention fast at the facility in the event that this control step would detect an abnormally high value of this rate.

The subject of the invention is a method for heat treatment of wood, comprising a step of heating the wood in an oven placed in an enclosure, by means of hot gases supplied by a generator, a step of recovering the vapors resulting from the treatment and a step of injecting water into the oven so as to create water vapor. The main characteristic of a thermal treatment process according to the invention is that it implements a step of controlling the oxygen level in the oven, by the presence of at least one oxygen probe implanted in said oven and connected to a reading means of the measurement. The enclosure, which represents the specific part of the installation in which is implemented the wood heat treatment process, corresponding to the space dedicated to the furnace, and the means for recovering the vapors from the heat treatment wood, will be filled with water vapor. The injection of water into the furnace will generate the production of water vapor in said furnace, these vapors constituting a large volume of gas mixing with the vapors of gases resulting from the heat treatment of the wood, this mixture having the consequence of significantly lower the oxygen level in the oven. In this way, the risk of fire is reduced or even canceled. In addition, the creation of an abundant quantity of water vapor in the enclosure will generate zones of condensation of these water vapors at the level of the cold parts of said enclosure, this condensation being particularly suitable for the trapping of vapors from the heat treatment of the wood, by solubilization in the water produced by said condensation. The process of treatment according to the invention implements a simple and perfectly well controlled means, to significantly reduce the risk of fire in the enclosure, and to recover completely or partially, the harmful vapors emanating from the heat treatment of wood. The injection of water into the furnace can be effected either by means of at least one directional jet or by diffusion in the form of droplets, via at least one injection nozzle. Similarly, the orientation of the water inlet in the furnace is flexible, and will be a function of geometric constraints and congestion encountered. The water injection step can be continuous throughout the heat treatment of the wood. According to another preferred embodiment of a method according to the invention, it can be carried out intermittently, at predefined time intervals depending on the constraints encountered. According to another preferred embodiment of a method according to the invention, it can be performed "à la carte" over time ranges random, dictated by the constraints for a real-time adjustment. The implantation of an oxygen probe in the furnace makes it possible to continuously inform an operator of the oxygen content present in said furnace during the heat treatment of the wood, and to evaluate the risk of fire. If the flow rate of injected water is constant, that is to say operating all or nothing, an unacceptable rise in the oxygen level will indicate to an operator that it is necessary to activate the injection of water into the furnace. If the flow of water is variable, the operator will adjust its flow punctually, to adapt to the critical situation encountered, and thus reduce the oxygen level. Advantageously, the oxygen sensor is connected to a buzzer to inform, in real time, an operator that the oxygen level has reached a value too high. This alarm may be visual, such as a light that comes on, or sound, such as an alarm or a siren.

Advantageously, the water injection rate is variable. This possibility increases the flexibility of use of the wood heat treatment system, by adjusting the quality of the wood to be treated, and the dimensions and geometry of the oven and the enclosure, the amount of water to be injected . Similarly, if the rate of oxygen were to grow during the same thermal treatment of wood, a temporary increase in the flow of injected water would bring it down to an acceptable level.

According to a preferred embodiment of a wood treatment method according to the invention, the gas generator uses the biomass. Although the interest of using biomass is no longer to be proven, on the other hand, it generates disadvantages related to the emission of vapors heavily loaded with water and various dusts and varied. For this gas generation configuration, vapor recovery is therefore a major issue, in particular to avoid environmental pollution.

Preferably, the water injected into the oven is preheated to accelerate the vaporization phenomenon. In other words, the water injected into the oven is hot, and is heated to a temperature of the order of 80 ° C. This preheating phenomenon will accelerate the process of formation of water vapor in the enclosure containing the furnace, and instantly make operational this water vapor, against the occurrence of a fire, and vis-à-vis the collection of vapors charged with dust, resulting from the heat treatment of wood.

Advantageously, the hot gases supplied by the gas generator are used to heat the water which is injected into the furnace. In this way, the energy used to heat the water pre-exists, and therefore does not have to be brought or created from the outside. Either the water to be injected is initially cold, or it is already hot, the gases supplied by the generator allowing, in these two cases, to increase the temperature of the water to accelerate the phenomenon of vaporization. In the case of a gas generator using biomass, the temperature of the gases emitted can reach 600 ° C., said gases then being able to heat the water rapidly and significantly. It is understood that in this configuration, the major objective of the gases emitted is the heat treatment of the wood, and that, with appropriate development, these gases may, in whole or in part, also serve to heat the water. In the invention, the oxygen sensor is connected to a system for self-regulation of the flow of water injected into the oven, so as to maintain the oxygen level in the oven below a threshold value, at each instant of the process. In the case of a constant water flow, that is to say operating all or nothing, the device triggering the injection of water is connected to the oxygen sensor, and is programmed to trigger automatically from that the measured value of the oxygen level is found above a threshold value. In the case of a variable-rate water injection, the flow-regulating device is connected to the oxygen probe, and is programmed to adapt the flow of water to be injected, as soon as the measured value of the oxygen level is above a threshold value. Such configurations constantly ensure safe conditions, to avoid the risk of fire. The threshold value below which the oxygen level must be maintained is 7%. Indeed, this rate guarantees conditions of non-occurrence of fire, and this, so almost safe.

• Le captage de la phase gazeuse des vapeurs nocives émises, par une solubilisation dans l'eau produite par cette condensation. Ce type de piégeage est particulièrement efficace pour les composés fortement solubles dans l'eau, comme les acides et les alcools. La solubilisation dans l'eau de corps légers comme les acides et les alcools améliore également leur rétention, en les empêchant d'être réaspirés par le système de pompage.
• Le nettoyage en continu de l'échangeur de refroidissement.

Advantageously, the step of recovering the vapors resulting from the heat treatment of the wood, involves a cooling exchanger, part of the water vapor present in the enclosure condensing at said exchanger. Indeed, a cooling exchanger may already be present in the chamber to recover the vapors from the heat treatment of wood. In the case of an injection of water into the chamber, this exchanger will concentrate, in condensed form, a portion of the emitted water vapor. This important condensation will allow:

• The capture of the gaseous phase of the harmful vapors emitted, by a solubilization in the water produced by this condensation. This type of entrapment is particularly effective for compounds that are highly soluble in water, such as acids and alcohols. The solubilization in water of light bodies such as acids and alcohols also improves their retention, preventing them from being re-aspired by the pumping system.
• Continuous cleaning of the cooling exchanger.

Preferably, the water injected into the furnace comes from a parallel and autonomous circuit. Indeed, it is desirable that the water supply circuit to be injected into the furnace, is dissociated from the rest of the plant specially designed for the heat treatment of wood, to avoid accidental interference, between said circuit of water and said installation, which could be detrimental to the smooth running of the treatment method according to the invention. Thus, by this separation, an incident that would occur at this water circuit, would not require the interruption of the treatment process.

The wood treatment methods according to the invention, have the advantage of using water, which is a simple, inexpensive and well controlled, allowing both to avoid the occurrence of a fire and to recover the harmful vapors resulting from the heat treatment of the wood. Moreover, this means, which is based on an injection of water into the furnace, can not in any way, including in the event of a malfunction, have any negative influence on the progress of the wood treatment process and on the quality of this treatment.

• La figure 1 est une vue schématisée d'un circuit d'eau monté dans une installation prévue pour la mise en oeuvre d'un procédé de traitement du bois selon l'invention.

The following is a detailed description of a preferred embodiment of a method of heat treatment of wood according to the invention, with reference to the figure 1 .

• The figure 1 is a schematic view of a water circuit mounted in an installation provided for the implementation of a wood treatment method according to the invention.

Referring to the figure 1 , the wood heat treatment processes according to the invention implement a closed circuit 1 of hot water, parallel to the main gas circuit, developed from biomass for heat treatment of wood. The heat treatment processes of the wood according to the invention in fact comprise a step of producing gas at about 600 ° C., a stage of heat treatment of the wood itself. speak, involving an oven 2 in which these particular hot gases arrive, and a terminal stage of recovery of the vapors emitted during said treatment, these vapors being heavily loaded with water and harmful dust. With this type of process, then arise two problems: one related to the occurrence of a fire, and the other, related to the recovery of harmful vapors from the oven. Indeed, beyond a temperature of 150 ° C in the enclosure of the installation containing the furnace 2, there is a real risk of occurrence of a fire, mainly because of the presence of a high oxygen level, usually greater than 10%. In addition, since the vapors from the heat treatment of wood are loaded with water and dust, it is necessary to recover them efficiently and sufficiently early, before they spread to the atmosphere and pollute the environment. Usually these two problems are treated separately, with specific equipment, bulky and therefore expensive. The treatment methods according to the invention propose to solve these two problems by implementing a single technique, which is that of injecting hot water into the furnace 2, in order firstly to create instantaneously water vapor, the main consequence of which is to contribute to lowering the oxygen content in said furnace 2 to values of less than 7% and, secondly, to promote condensation at the level of the cold elements of the chamber, for trapping the vapors from the furnace 2. This hot water injection into the furnace 2 is made possible by the circuit 1 of hot water, implanted parallel and autonomous, in the installation specially dedicated to the heat treatment of wood. The production of hot water is carried out by means of a tubular 3 gas / water heat exchanger, fed by hot gases 16 at 600 ° C. from a gas generator using the biomass, said exchanger 3 being placed in the circuit 1 closed hot water. The hot water rotates in the circuit 1 by means of a circulator 4 and transmits its heat to a hot water tank 5, the water of which is sent to the oven 2 by a pump 6 for increasing the pressure. The overheating of the circuit 1 is avoided by the introduction of a cooling loop 7 implanted on said circuit 1, said loop 7 comprising an exchanger 8 water / water plate cooled by a cold unit 14. This exchanger 8 ensures the sampling calories from the water instead of the hot water preparer 5, which has reached its equilibrium temperature, of the order of 85 ° C-90 ° C. A temperature probe 9 placed at the outlet of the hot water preparer 5 triggers the tilting of a valve 10 and the deviation of the flow of water towards the cooled circuit 7. The temperature of the water in the circuit is measured by a number of temperature probes 11, 12, 13, distributed along the latter, and in particular at the inlet 12 and at the outlet 13 of the gas / water exchanger 3. The gases 17 coming out of the 3 gas / water heat exchanger, have a temperature of the order of 300 ° C, and are routed to the furnace 2 via a system of valves 18. The introduction into the oven 2 of gas 17 at this temperature , allows a lower heat input, making it a regulating element for the establishment of bearings in the furnace 2 during processing. The gases 17 leaving the exchanger 3 gas / water, are used for an optimized supply of heat during the phases of temperature steps in the furnace 2. The control of the oxygen level in the chamber is achieved by means of an oxygen sensor. According to a first variant of use, this probe can be connected to a visual or audible alarm, which is triggered as soon as the oxygen level is higher than a threshold value. Preferably, this rate is 10%. According to another variant of use, the probe is connected to a regulator of the flow of hot water, which increases said flow rate, if the rate of oxygen tends to increase towards the threshold value.

Claims (7)

1. A method for the heat treatment of wood, comprising a step for heating the wood in an oven (2) placed in an enclosure, using hot gases (17) supplied by a generator, a step for recovering vapors from the processing, and a step for injecting water into the oven (2) so as to create water vapor, said method also implementing a step for monitoring the oxygen level in the oven (2), through the presence of at least one oxygen sensor installed in the oven (2) and connected to a means for reading the measurement, characterized in that said oxygen sensor is connected to a self-regulation system for the water flow rate injected into the oven (2), so as to keep the oxygen level in the oven (2) below a threshold value of 7%, at each moment of the method.
2. The method according to claim 1, characterized in that the water injection flow rate is variable.
3. The method according to any one of claims 1 or 2, characterized in that the gas generator uses biomass.
4. The method according to any one of claims 1 to 3, characterized in that the water injected into the oven (2) is heated beforehand, to accelerate the vaporization phenomenon.
5. The method according to any one of claims 1 to 4, characterized in that the hot gases (16) provided by the gas generator are used to heat the water that is injected into the oven (2).
6. The method according to any one of claims 1 to 5, characterized in that the step for recovering vapors resulting from the heat treatment of the wood involves a cooling exchanger, part of the water vapors present in the enclosure condensing at said exchanger.
7. The method according to any one of claims 1 to 6, characterized in that the water injected into the oven (2) comes from a parallel and autonomous circuit (1).

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