DK201670531A1

DK201670531A1 - Method and system for thermal modification of wood, as well as a control program for carrying out the improved method

Info

Publication number: DK201670531A1
Application number: DKPA201670531A
Authority: DK
Inventors: Peter Klaas
Original assignee: Wtt Holding Aps
Priority date: 2016-07-15
Filing date: 2016-07-15
Publication date: 2017-10-23
Also published as: DK179094B1; WO2018010748A2; WO2018010748A3

Abstract

Method for thermal modification of wood, comprising the foliowing steps: a. placing a temperature probe in a central/core part of the wood to be treated or in a sample of tbe wood to be treated; b. placing the wood and optionally the sample to be treated in a treatment chamber, said treatment chamber, during the wood modification being closed from tbe ambient environment; c. providing an inert gas atmosphere inside the closed treatment chamber; d. providing temperature measurement inside the treatment chamber; e. heating the atmosphere in the treatment chamber to between 160 to 195C; f. shutting off the heating once the temperature in the wood reaches the woods exothermic peak; g. opening the treatment chamber and taking out the treated wood when the temperature in the atmosphere is below 50C.

Description

[Titel]

Field of the Invention

The present invention relates to an improved method for thermal modification of wood, as well as a control program for carrying out the improved method.

Background of the Invention

It is well known that thermo treatment of wood can add some advantageous characteristics to the treated wood as compared to untreated wood. In particular the desired characteristics are increased durability, dimensional stability, increased insulation, improved acoustic properties and color of the wood.

Particularly the Finnish Thermowood association has published a “Thermowood® Handbook”, 2003, describing how to treat various types of wood in order to obtain the desired results.

The thermodynamics of the individual components of wood are highly specific to each component. At the same time, the thermal modification of wood in order to improve its dimensional stability and durability is also highly specific to its components. In particular, the improvements are a specific function of the modification of Hemicellu-loses.

Wood mainly consists of three components: Hemicelluloses, Celluloses and Lignin. For thermal modification, the goal is to maximize Hemicelluloses modification while at the same time minimizing the modification of Celluloses and Lignin. The modification of Hemicelluloses achieves the desired purpose of the modification process which is to increase durability, dimensional stability, increased insulation, improved acoustic properties and color of the wood. However undesired negative side effect from thermal modification on the physical properties of the wood, such as reduction in Modulus of Elasticity (MOE) and increased brittleness, is related primarily to the modification and degradation of Celluloses and Lignin. In this way, traditional heat treatment processes is a trade-off between the desired and the undesired effects of modification, which is subject to optimization. The most effective control program will be a pro gram which can reliably determine the optimal trade-off for a given load of wood to be modified.

Cellulose is crystalline and strong. Furthermore cellulose is also resistant to hydrolysis. Hemicellulose on the other hand is comparably weaker, i.e. has little strength, and is rather easily modified when exposed to hydrolysis. The chemical composition of hemicellulose varies depending on the species of wood, but the general differences between cellulose and hemicellulose remains.

Hemicellulose contains varying concentrations of xylan - depending on species (hardwood most, while softwood has smaller concentrations). The particular characteristic with xylan is that when degrading during heat treatment, xylose reactions are exothermic (an exothermic reaction is a chemical reaction that releases energy. It is the opposite of an endothermic reaction).

Existing control programs do not control the process parameters (predominantly temperature and time) with reference to the modification process itself (e.g. the degree to which Hemicelluloses is actually modified) - see examples in the Thermowood® Handbook, but is based on a recipe typically developed by experience and lab tests, for a specific species. Because the modification process is significantly influenced by parameters which are subject to significant variability even within the same species, such as density and moisture content, a standard recipe will often not attain the optimum degree of modification so that the wood is either not modified enough or too much.

An example of a control regime is suggested in “Thermowood® Handbook” published by the Finnish Thermowood association, dated 08.04.2003, see page 2-1 and chapter 3. The process according to the Thermowood Handbook is divided in three phases: 1 (drying - time consuming 0° - 125°), 2 (heat treatment 185° to 215°C), and 3 (condi-tioning/cooling).

Object of the Invention

The idea of this invention is to utilize the specific thermodynamics of Hemicelluloses as a control parameter in order to achieve an optimal degree of thermal modification of wood and to improve the method for this treatment such that a minimum of energy is used and at the same time an improved result is achieved.

Description of the Invention

In the following, a novel control program is presented which program control the process parameters (temperature and time) with reference to the modification process of Hemicelluloses. Because the modification of Hemicellulosis is highly specific and different from Celluloses and Lignin, the exotherm of Hemicelluloses modification can be used as a direct, real time proxy for the modification process. This enables that every charge of wood is modified optimally - irrespectively of variations in density and moisture content in raw materials.

The invention has at least two main objectives:

The first object is to reduce cycle time. Theoretically it takes 1:35 hours to heat a 100 x 53 mm board of Spruce (12 % moisture content (MC)), density 527 kg/m3) from 25 oC to 170 oC , and another 1:35 hours to cool it back down to 30 oC. If it is assumed that the modification process at 170 oC takes 2:00 hours for Spruce, then the total theoretical cycle time from start to finish is 5:10 hours. However, the existing process takes 24:00 hours or more. A 4 to 5 fold reduction of process time will lead to a significant reduction of the modification costs and, as a result, obtain a decisive competitive advantage compared to the alternatives.

The second objective is to improve the quality and usefulness of thermally modified wood. A poor control in existing processes of dysfunctional partial pressure dynamics inside and outside of the wood during heating and cooling may lead to internal cracks and a brittle surface, especially for susceptible species such as many hardwoods. But it can also cause problems for other products of high commercial value such as glue laminated window frames used for PVC coating in the windows and door industry or the acoustic properties of modified wood for music instruments. If dysfunctional par tial pressure dynamics are eliminated, the quality and usefulness of thermally modified wood increases significantly.

The invention consequently provides a method for thermal modification of wood which method addresses these issues, where said novel and inventive method comprises the following steps: a. placing a temperature probe in a central/core part of the wood to be treated or in a sample of the wood to be treated; b. placing the wood and optionally the sample to be treated in a treatment chamber, said treatment chamber, during the wood modification being closed from the ambient environment; c. providing an inert gas atmosphere inside the closed treatment chamber; d. providing temperature measurement inside the treatment chamber;

e. heating the atmosphere in the treatment chamber to between 165 to 190 °C

f. shutting off the heating once the temperature in the wood or wood sample reaches between 165 to 195 °C

g. opening the treatment chamber and taking out the treated wood when the temperature in the atmosphere is below 50 °C

Once the atmosphere inside the closed treatment chamber is being heated the internal pressure in the treatment chamber will also increase. In fig. 2 is illustrated the heat transfer characteristic differences between steam (water vapour) traditionally used, and present in the wood, and Nitrogen - which is the main constituent of the atmosphere used in the treatment chamber with the present invention. Once the wood is placed in the treatment chamber there will be a certain natural moisture content in the wood. Typically 10% to 24 % moisture - mainly water. Due to the increased temperature and pressure the water will turn to vapour/steam. This could cause internal damage inside the wood but due to the overall vapour pressure inside the treatment chamber, the water vapour can unhindered leave the wood structure as there is no steam pressure inside the treatment chamber to hinder the escape.

Consequently the present invention presents a method for thermal modification of wood. By realizing that the three main components of wood are highly specific and varies with type of process, temperature and other process parameters, the inventive method provides a very specific, fast and improved treatment method.

Hemi Cellulose Cellulose Lignin

Hydrolysis/low temp Yes No No

Pyrolysis/high temp Yes Yes Yes

Exothermic peak in

Nitrogen atmosphere 1) (oC) 290 (Xylan) 360 320

Table 1.

Note 1) temperatures are cited at ambient pressure. When the pressure in the nitrogen atmosphere increases the corresponding exothermic peak temperature decreases.

As summarized in table 1 above, the thermodynamic properties of Hemicelluloses, Celluloses and Lignin are different and highly specific. In particular, Hemicelluloses is much more readily modified by the hydrolysis process than is Celluloses and Lignin. Further, the modification of Hemicelluloses becomes exothermic at lower temperatures than does Lignin and Celluloses, so that these exotherms become a reliable proxy of the type of component (Hemicelluloses, Celluloses and Lignin) which is being modified. This causal relationship between exotherm and type of component forms the basis of the control system according to the present invention. By measuring the exotherm, as the difference in temperature in the center of the wood or a representative sample and in the Nitrogen atmosphere surrounding the wood, a real-time proxy measure of the modification of Hemicelluloses can be obtained, which allows a precise determination of when the modification of the Hemicelluloses starts, when it is at its highest and when it has ended. In other words, once the exothermic reaction is started with respect to hemicellulose, the heating of the treatment chamber is shut off. The precise determination of the specific modification of Hemicelluloses, as opposed to overall modification of the wood, has significant impact on quality control. A classical measure of degree of modification is the mass loss of the wood. Both the positive effects of mass loss on increased durability and the negative effects on decreased physical strength properties increase with mass loss. There is a trade-off between increasing durability and decreasing physical strength. But this tradeoff is non linear. Initially, for smaller mass losses the increase in durability (by the decrease in equilibrium moisture content, EMC) is relative higher than the loss of strength. At a certain characteristic point, this relation reverses so that beyond this point, further increases in mass loss will cause relatively lower increases in EMC but higher losses of strength. We hypothesize here that this characteristic point marks the termination of the modification of Hemicelluloses, and the beginning of the modification of Celluloses and Lignin. We base this on three observations:

First, the change of rate in decrease in EMC from high to low may be caused by the affinity of free water bonding being much higher with Hemicelluloses than with celluloses and lignin. This is due to the many hydroxyl groups in Hemicelluloses but not in Celluloses and Lignin. As a result, for the same degree of mass loss, modification of Hemicelluloses will cause a larger reduction in EMC than will Celluloses and Lignin. Second, the density of Hemicelluloses is lower than that of Celluloses and Lignin. At the same time, Hemicelluloses has lower strength than Celluloses and Lignin due to differences in their chemical and physical composition; Together, this implies that mass loss related to the modification of Hemicelluloses has relatively lower negative impact on the overall physical properties of the wood, than does the mass loss related to Celluloses and Lignin.

The third observation is that the changes in the rate of change in EMC and mass loss happens at the same point in time, indicating that they are both related to the same physical component, which we suggest is Hemicelluloses.

The resulting control strategy is as follows:

In the first phase, where the modification of Hemicelluloses is endothermic, heat is continuously transferred to the wood until the exotherm is reached. Once the exotherm is reached, no more heat is transferred and cooling is initiated. During the exotherm phase, the purpose of the cooling is to remove as much energy from the wood as possible, in order to minimize the temperature increase in the core of the wood. The purpose of this is to minimize the risk of modifiying and degrading Celluloses and Ligning by keeping the temperature below the thresholds where this happens. Once the exotherm is completed and all Hemicelluloses is modified, continued cooling will terminate any farther modification of the wood and bring it down to the desired end temperature.

Once the exotherm is reached, the heat generated by the exotherm process is greater than the heat supplied. Consequently the temperature increases faster in the wood (as measured by the sensors) than in the treatment chamber. This is a clear indication, that the exotherm temperature has been reached, and that the supply of heat can be shutoff, and optionally active cooling implemented.

The invention therefore also provides a control program for controlling the method discussed above, wherein: a. Tn is the temperature measured in the atmosphere inside a treatment chamber; b. Tw is the core temperature of wood placed in the treatment chamber; c. Tmax is a desired temperature in the core of the wood; d. Tmin is the temperature of the wood before and after treatment; e. wherein the control program in response to input (Tw) from a first temperature sensor placed in the core of the wood to be treated and input (Tn)from a second temperature sensor positioned inside the treatment chamber, regulates the supply of heat to the treatment chamber, if i. Tn < Tmax increases Tn up to Tmax; ii. Tn = Tmax and Tw < Tmax keeps Tn constant; iii. Tn = Tmax and Tw = Tmax keeps Tn constant iv. Tn < Tw decrease Tn TN: Actual temperature of the atmosphere TW: Actual temperature of the wood (measured at the center of the board)

Tmax: The desired temperature for the thermal modification, depending on the species

Tmin: The temperature of the wood before and after modification (room temperature)

Phase no. Condition Desired temperature Temperature ofT;N control strategy 1 If TN < Tmax TMax Increase TN (Heat up) 2 If TN = Tmax AND TW < TNmax TMax Hold TN (Keep constant) 2a If TN = Tmax AND TW = Tmax TMax Hold TN (Keep constant) 3 IfTN<TW Tmin Decrease TN (Cool down)

Table 2: overview of control program

In a further embodiment Tn, Tw, Tmax and Tmin are process parameters pre-selected according to the species of wood, the treatment gas and the moisture content of the wood.

The inventive method is graphically illustrated in fig 1. Curve 1 illustrates the temperature in the atmosphere inside the treatment chamber, as controlled by the means for heating the gas. Curve 2 illustrates the registered temperature inside the wood to be treated or alternatively in a representative sample. The registration in the wood is accomplished by embedding a temperature probe inside the wood at a predetermined depth, in order that the measured temperature resembles a situation where the entire wood batch to be treated has attained the desired temperature. Often three or more temperature probes/sensors are used in order to assure that all wood, regardless of position in the treatment chamber has reached the desired temperature. The desired temperature is the temperature where the exotherm reaction of hemicellulose is active. This is illustrated by curve 2 having an increased temperature even though the heating of the atmosphere in the treatment chamber has ceased.

Description of the drawing

Fig. 1 illustrates the temperature regime as function of time for the process Fig. 2 illustrates steam resp. nitrogen’s heat transfer coefficient as function of temperature

As the desired temperature will vary from wood species to wood species, and also within species depending on moisture content, wood density etc., it is not possible for all wood species to pre-determine desired temperatures. This is also not necessary as the addition of heat to the atmosphere is only continued after reaching the desired treatment temperature, for example 180 °C as illustrated in fig.l. Once the temperature is reached, the temperature is maintained. Once the registered wood temperature (registered by one or more of the embedded temperature probes/sensors) is increasing above the desired temperature, the heating is shut off, or a controlled cooling is activated.

Reduced cycle time

The reduction in cycle time is achieved in the following way: • Decoupling of pressure and temperature in the heating and cooling phase. In existing HPD process, pressure is created by producing steam by means of heating up water. This process is very time consuming because the increase in steam pressure is lagging behind temperature increase (see fig 2) and because the relative humidity must be kept above 85% to avoid damage to the wood. The lagging effect is aggravated by the slow increase in pressure as a function of temperature, at low temperatures. Fig 2 illustrates how pressure builds very slowly in temperatures below 140 oC, which is the temperature range where most of the heating and cooling takes place. The decoupling is achieved by using Nitrogen as a means of building pressure. In this context it is important to realize that any gas, and in particular any inert gas may be used with the present invention in order to achieve the described advantages. • First, pressure is built with Nitrogen. Next, the Nitrogen atmosphere is heated up in order to heat up the wood. Because of the decoupling and the fact that relative humidity does not need to be controlled, heating and cooling can be done at the systems maximum capacity. • Increase in energy transfer to and from the wood. In existing HPD process, steam at low temperatures has a very low capacity to transfer energy to and from the wood. This is clearly illustrated in fig. 2. The substitution of steam with Nitrogen (or any inert gas) enables a very large increase in the rate of energy transfer to and from the wood in the heating and cooling phase • Increase in nominal heating and cooling capacity. With the increase in energy transfer capacity achieved by exchanging Nitrogen for steam, the limiting factor now becomes the system’s capacity to heat and cool the atmosphere. In order to minimize the time of the heating phase, the nominal heating capacity pr. m3 wood must increase from approximately 45 Kwh in the existing HPT process to approximately 220 Kwh in the process according to the present invention. In a similar manner, cooling capacity must be increased from approximately xx kWh to yy kWh

In practice the method is carried out by selecting one or more samples of the wood to be treated. In each sample is positioned a temperature sensor, approximately in the core of the wood. This may be accomplished by drilling into the wood, inserting the temperature probe, and resealing the hole with a suitable compound.

The one or more samples are thereafter positioned in the batch of wood to be treated, such that a representative temperature development may be registered by the embedded temperature sensors. The sensors are connected to a control system - typically a computer, where the input from the sensors are used to control the heating means inside the treatment chamber, such that once the exothermic process (of the hemicellu-loses) commences, the heating is turned off, or drastically reduced inside the treatment chamber.

Once the treatment is over, the temperature sensors (and samples) are retrieved. Normally samples for each batch to be treated shall be prepared as disclosed above, however for very large batches, where multiple treatment cycles are necessary the samples may be reused. However the modification process also alters the samples, and it is therefore preferred to use new samples before each treatment cycle.

Claims

1. Method for thermal modification of wood, comprising the following steps: a. placing a temperature probe in a central/core part of the wood to be treated or in a sample of the wood to be treated; b. placing the wood and optionally the sample to be treated in a treatment chamber, said treatment chamber, during the wood modification being closed from the ambient environment; c. providing an inert gas atmosphere inside the closed treatment chamber; d. providing temperature measurement inside the treatment chamber; e. heating the atmosphere in the treatment chamber to between 160 to 195 9C; f. shutting off the heating once the temperature in the wood reaches the woods exothermic peak; g. opening the treatment chamber and taking out the treated wood when the temperature in the atmosphere is below 50 5C

2. Method according to claim 1 wherein three or more temperature sensors or probes are embedded in the wood to be treated, where the sensors/probes are in communication with a control unit outside the treatment chamber, and where the input from the sensors/probes is used to determine the heat shut off point.

3. Method according to claim 1 wherein three or more temperature sensors or probes are embedded in representative samples of the wood to be treated, where the sensors/probes are in communication with a control unit outside the treatment chamber, and where the input from the sensors/probes is used to determine the heat shut off point.

4. Method according to claim 1, wherein the woods exothermic peak temperature is either determined by measuring on a sample of the wood to be treated before commencing treatment, or by estimating the woods exothermic peak temperature based on pre-selected temperatures.

5. Method according to claim 1 wherein the inert gas is Nitrogen, and optionally the gas is recycled through a cleaning process and reused.

6. Method according to claim 1 wherein after step f. an active cooling procedure is implemented, before removing the wood from the treatment chamber.

7. A control program for controlling the method according to claim 1, wherein: a. Tn is the temperature measured in the atmosphere inside a treatment chamber; b. Tw is the core temperature of wood placed in the treatment chamber; c. Tmax is a desired temperature in the core of the wood; d. Tmj„ is the temperature of the wood before and after treatment; e. wherein the control program in response to input (Tw) from a first temperature sensor placed in the core of the wood to be treated and input (Tn)from a second temperature sensor positioned inside the treatment chamber, regulates the supply of heat to the treatment chamber, if i. Tn < Tmax increases Tn up to Tmax; ii. Tn = Tmax and Tw < Tmax keeps Tn constant; iii. Tn = Tmax and Tw = Tmax keeps Tn constant iv. Tn < Tw decrease Tn v.

8. A control program according to claim 7 where Tn, Tw, Tmax and Tmin are process parameters pre-selected according to the species of wood, the treatment gas and the moisture content of the wood.