CN105437342B - A kind of timber heat modification method based on normal pressure intensification negative pressure heat preservation technique - Google Patents

Abstract

The invention provides a wood thermal modification method based on normal pressure heating and negative pressure heat preservation process without heating or protection medium, capable of rapid heating, low energy consumption, simple implementation equipment, and no safety hazard. It comprises the following steps: The sawn timber is placed in the processing kiln and heated to 115-125°C in an atmospheric air environment, and kept for 50-70min to preheat the wood and reduce the moisture content of the wood; then the wood is heated at a heating rate of 8-12°C/h The temperature rises to 150°C-190°C; then vacuumize the treatment kiln, reduce the pressure in the kiln to 0.2-0.3atm, and keep the temperature constant, and the holding time is 1-3h; finally stop heating, and gradually reduce the pressure of the treatment kiln. temperature, and air is introduced into the treatment kiln to gradually restore the pressure to normal pressure.

Description

A thermal modification method of wood based on normal pressure heating and negative pressure heat preservation process

technical field

The invention relates to a novel wood modification method for adjusting the appearance color and dimensional stability of wood, and belongs to the field of wood modification.

Background technique

Wood thermal modification, also known as wood heat treatment, is a method of heat-treating wood in an inert environment around 200°C. At this temperature level, the hemicellulose in the wood is partially degraded, the crystallinity of the cellulose increases, and the chemical components of the lignin also reorganize, reducing the moisture and hydrophilic groups in the wood, and the chromophoric groups Changes to adjust its material color and increase its dimensional stability.

The laboratory exploration of thermal modification of wood originated in the 1930s, and the industrialization began in the 1990s. According to the pressure level of the processing medium, there are currently four main processes: normal pressure type, negative pressure type, pressurized type and mixed type.

The atmospheric pressure process uses water vapor, nitrogen or hot oil as the heating medium to remove or reduce the oxygen content in the treatment environment while heating the wood, so as to prevent excessive oxidation of the wood at high temperature and reduce the fire during the treatment process. risk. The Thermowood process in Finland is a typical atmospheric pressure heat treatment process (see Figure 1). The process mainly treats Nordic pine, spruce, birch and poplar, and consists of 3 stages. The wood is first dried at a maximum temperature of 130°C (stage 1), and the drying time is determined according to the initial moisture content of the wood. Adjustment. In the next heat treatment stage (stage 2), the wood is heated at a temperature of 185-215°C, depending on the wood type and product use. At this stage, the chemical components of the wood cell wall are degraded and reorganized, resulting in changes in the properties of the wood. The heat treatment stage lasts for 2-3 hours, and then enters the cooling and humidity-adjusting stage (stage 3). The temperature in the kiln is reduced by spraying water, and the wood also absorbs a certain amount of water during this process, and finally the moisture content is controlled at 4-7. %. The main disadvantage of the atmospheric pressure process is that the preparation of the heating medium increases the cost of treatment, and at the same time, corresponding waste water and waste oil recovery treatment is required.

The pressurized process uses pressurized steam or water as the treatment medium, and its closed high-humidity environment is conducive to promoting the hydrolysis of hemicellulose in wood, and can achieve a better modification effect at a lower temperature level; however, due to the wood hydrolyzate The acidic, high-humidity environment also intensifies the erosion of the treatment equipment, and the increased pressure level also means that more safety issues need to be considered during operation. A typical pressure heat treatment process is shown in Figure 2.

The negative pressure treatment process reduces the oxygen content in the treatment device by vacuuming. The pressure in the treatment kiln is generally about 0.15 to 0.35 atm, so there is no need to prepare inert media such as steam, nitrogen or hot oil. The discharge during the treatment process The level is also lower, but the reduction in the amount of air also reduces the amount of heat transfer medium in the process environment, making convective heat transfer difficult. In order to improve the heat transfer efficiency, generally, the speed of the fan is increased. The speed of the fan needs to reach 1930rpm under the pressure of 0.2atm, while only 635rpm can achieve the same heat transfer effect under normal pressure. At the same time, the negative pressure environment also has a certain inhibitory effect on the degradation of hemicellulose, so the temperature level of this process generally reaches 225-230 ° C, which is higher than other processes (see Figure 3). Some negative pressure treatment technology schemes install a heating plate in the kiln to heat the wood through conduction. However, since the heating plate itself needs to occupy a certain space in the kiln, the effective volume of the treatment kiln has been significantly lost.

The hybrid treatment process completes the heat treatment step by step in two different devices. The first step is to place the wood in a high-pressure water or steam environment for treatment. After the treatment, the moisture content of the wood is relatively high, so before the second step of treatment Firstly, the wood should be dried in a drying kiln to reduce the moisture content to about 10%, and then the wood should be put into an environment with atmospheric pressure steam or nitrogen as the protective gas for treatment. The mixed treatment process decomposes the heat treatment process into several relatively independent steps. Through the optimization of the equipment ratio and process, a smaller device can be used to achieve higher output, but overall it is much more complicated than other processes in terms of equipment and process. , so it is rarely used at present.

Contents of the invention

The purpose of the present invention is to provide a wood thermal modification method based on normal pressure heating and negative pressure heat preservation process without heating or protection medium, fast heating, low energy consumption, simple implementation equipment, and no safety hazard.

The wood thermal modification method based on normal pressure temperature rise and negative pressure heat preservation process of the present invention, it comprises the following steps:

a. Wood sawn timber is heated in an atmospheric air environment: place the timber in a treatment kiln and heat it to 115-125°C, keep it for 50-70min, preheat the timber and reduce the moisture content of the timber; then use 8 -12°C/h heating rate to raise the wood temperature to 150°C-190°C;

b. Heat preservation and pressure reduction: vacuumize the treatment kiln, reduce the pressure in the kiln to 0.2-0.3 atm, and keep the temperature constant, and the heat preservation time is 1-3 hours;

c. Cooling and repressurization: Stop heating, gradually lower the temperature of the treatment kiln, and introduce air into the treatment kiln to gradually restore the pressure to the normal pressure level.

Beneficial effects of the present invention: the feature of the present invention is mainly in normal pressure heating and negative pressure heat preservation.

Atmospheric pressure heating means that the wood is heated in the normal pressure air environment. This step can be divided into two stages. The first stage is heated to 115-125°C and kept for 50-70min to preheat the wood and reduce the moisture content of the wood. Rate. In the second stage, the temperature of the wood is raised to 150°C-190°C (the temperature is generally raised to about 160°C only to adjust the color of the material, and the temperature is generally raised to about 180°C to adjust the color of the material and improve the dimensional stability), and the heating rate is about 8 -12°C/h. Due to the presence of oxygen in the treatment environment, the level of degradation and recombination of chemical components in wood can reach the effect of traditional negative pressure treatment at higher treatment temperatures. The treatment temperature of this method is not more than 200 ° C, because the degradation rate of wood is significantly increased after the temperature is higher than that, the mechanical strength loss of wood is too large, and at this time, the generation of combustible organic volatiles in wood is significantly increased, giving The handling process poses a safety hazard.

Negative pressure insulation is to vacuumize the treatment kiln after the wood temperature reaches the target temperature, reduce the pressure in the kiln to 0.2-0.3atm, and keep the treatment temperature unchanged. The negative pressure environment in this step can play two roles: (1) reduce the oxygen content in the treatment environment, avoid excessive degradation of wood in a continuous high temperature environment, and at the same time ensure the safety of the treatment environment; (2) in this method , When the kiln starts vacuuming, the wood has reached the treatment temperature. At this time, increasing the vacuum degree of the treatment environment can increase the heat preservation of the treatment device, reduce the heat consumption of wood, and reduce the energy consumption in the heat preservation stage. The present invention is completely different from the existing negative pressure treatment process. The existing negative pressure treatment process is to heat up the wood under negative pressure. The improvement of the vacuum degree makes it difficult to convectively heat the wood, which becomes a barrier that restricts the wood from reaching the target treatment temperature. major factor. The invention adopts the negative pressure method in the heat preservation stage, which effectively prevents the heat dissipation of the treatment kiln from being too fast.

The technical scheme of the invention is mainly applicable to the broad-leaved wood species of indoor products, which can adjust the material color of the wood and significantly improve the dimensional stability of the wood. The main advantages of this method:

1) This method does not need to prepare any heating or protective medium (steam, hot oil, nitrogen, etc.), which simplifies the processing system, reduces the cost, and greatly reduces the corrosion of the device.

2) This method is carried out under normal pressure and negative pressure conditions, and there is no potential safety hazard caused by high pressure.

3) The two pressure conditions of this method can be realized in the same device without preparing different devices separately, which is an "integrated" treatment process.

4) This method overcomes the defects of heat conduction difficulties caused by insufficient heat transfer medium or the need for additional heating plates in the traditional negative pressure process, and can achieve rapid heating.

5) This method can realize the modification level of the traditional negative pressure process at a higher temperature at a lower temperature, shorten the heating time, and reduce energy consumption.

The above-mentioned thermal modification method of wood based on normal pressure heating and negative pressure heat preservation process, in step c, when the temperature is lower than 100 ° C, air is introduced into the treatment kiln, the pressure is gradually restored to the normal pressure level, and at the same time, the air is injected into the treatment kiln. Spray water inside. This adjusts the moisture content of the wood.

In the above-mentioned wood thermal modification method based on the normal pressure heating and negative pressure heat preservation process, in step a, spray water into the treatment kiln during the heating process at a heating rate of 8-12°C/h. During the heating process, water can be properly sprayed into the treatment kiln. The water is rapidly vaporized at high temperature, which can adjust the humidity of the treatment kiln, prevent wood from cracking and cause quality problems, and at the same time ensure the safety of the treatment process.

The above-mentioned wood heat modification method based on normal pressure heating and negative pressure heat preservation process has steps before step a: a0. Wood pretreatment: firstly dry the sawn timber at high temperature so that the moisture content of the wood is 2-4%. In this way, quality problems caused by drying of the wood during high-temperature treatment can be prevented.

In the above-mentioned wood thermal modification method based on the normal pressure temperature rise and negative pressure heat preservation process, the temperature of the wood is raised to 155°C-165°C at a heating rate of 8-12°C/h in step a.

The above-mentioned wood thermal modification method based on the normal pressure temperature rise and negative pressure heat preservation process, in step a, the temperature of the wood is raised to 175°C-185°C at a heating rate of 8-12°C/h.

In the above-mentioned wood thermal modification method based on the normal pressure heating and negative pressure heat preservation process, in step b, for wood with a thickness of 2cm, the heat preservation time is 1h; for wood with a thickness of 5cm, the heat preservation time is 3h.

Description of drawings

Fig. 1 is the temperature curve of Thermowood heat treatment process;

Fig. 2 is press heat treatment process temperature, pressure curve figure;

Fig. 3 is negative pressure heat treatment process temperature, pressure graph;

Fig. 4 is a curve diagram of temperature and pressure in normal pressure heating and negative pressure heat preservation heat treatment process.

detailed description

The wood thermal modification method based on the normal pressure heating and negative pressure heat preservation process, the specific process steps are as follows:

1) Wood pretreatment: This method is suitable for wood sawn timber. The wood to be treated should be dried at high temperature first, and the moisture content should be at a low level, generally around 3%, to prevent the wood from drying out during high temperature treatment. quality problems.

2) Wood heating: The treated wood is heated in an atmospheric air environment. Referring to Figure 4, this step can be divided into two stages, the first stage is heated to about 120°C to preheat the wood and further reduce the moisture content of the wood. Due to the low moisture content of wood, the heating rate at this stage can be set at a relatively high level without causing cracking or deformation of the wood. Generally, the initial temperature in the kiln can be set at 70°C. Rapidly raise the temperature to 120°C after internal temperature, and keep it for about 1h. In the second stage, the temperature of the wood is raised to about 160°C (only to adjust the color of the material) or 180°C (to adjust the color of the material and improve the dimensional stability), and the heating rate is about 10°C/h. Due to the presence of oxygen in the treatment environment, the level of degradation and recombination of chemical components in wood can reach the effect of traditional negative pressure treatment at higher treatment temperatures. The treatment temperature of this method is not more than 200 ° C, because the degradation rate of wood is significantly increased after the temperature is higher than that, the mechanical strength loss of wood is too large, and at this time, the generation of combustible organic volatiles in wood is significantly increased, giving The handling process poses a safety hazard. During the treatment process at this stage, water can be properly sprayed into the kiln, and the water is rapidly vaporized at high temperature, which can adjust the humidity of the treatment kiln, prevent wood from cracking, and ensure the safety of the treatment process.

3) Heat preservation and pressure reduction: After the wood temperature reaches the target temperature, vacuumize the treatment kiln to reduce the pressure in the kiln to 0.2-0.3 atm, and keep the treatment temperature unchanged. See Figure 4, the holding time is adjusted according to the processing requirements and size of the wood, and it is about 1 to 3 hours. For floor material blanks with a thickness of about 2cm, the processing time can meet the processing requirements in 1 hour, while for furniture materials with a thickness of about 5cm, the processing time can be extended to 2-3 hours. The negative pressure environment at this stage can play two roles: (1) reduce the oxygen content in the treatment environment, avoid excessive degradation of wood in a continuous high temperature environment, and at the same time ensure the safety of the treatment environment; In the pressure treatment process, the improvement of the vacuum degree makes the convective heating of the wood difficult, which becomes the main factor restricting the wood to reach the target treatment temperature. In this method, the wood has reached the treatment temperature when the vacuum is started in the kiln. The vacuum of the environment can instead increase the thermal insulation of the treatment device, reduce the heat consumption of wood, and reduce the energy consumption in the thermal insulation stage.

4) Cooling and humidity adjustment: Stop heating, gradually lower the temperature of the kiln, and when the temperature is lower than 100°C, air is introduced into the kiln to gradually restore the pressure to normal pressure, and at the same time, spray water into the kiln to adjust moisture content of wood.

The color of the wood modified by the method can be significantly deepened, and the dimensional stability can be greatly improved. According to existing studies, when the wood temperature reaches 160°C, the chemical components in the wood, especially the hemicellulose, begin to degrade significantly, and the color of the wood begins to deepen significantly. Therefore, this temperature is also regarded as the starting point of heat treatment modification. starting temperature. When the temperature level was further increased to about 180°C, the degradation level of hemicellulose was increased, the hydrophilic groups in the wood were reduced, and the dimensional stability of the wood was significantly improved. This is the main theoretical basis for the Thermowood process in Finland to set the treatment temperature of Thermo-S (dimensionally stable) grade hardwood at 185°C. The heating stage of the process of this method is carried out under normal pressure air conditions. Due to the existence of oxygen, the degradation level of wood chemical components in this stage is higher than that of the existing treatment process. High-temperature pyrolysis has a certain inhibitory effect, so the pyrolysis level of wood at this stage is lower than that of conventional processes. Therefore, in general, its modification effect is equivalent to that of conventional treatments at the same temperature level.

Claims (7)

1. a kind of timber heat modification method based on normal pressure intensification negative pressure heat preservation technique, is characterized in that：It comprises the steps：

A, timber sawn timber is heated in atmospheric air environment：Timber is placed in process kiln and is heated to 115-125 °C, 50-70min is kept, and the moisture content of timber is preheated and is reduced to timber；Then with 8-12 °C/h heating rates by timber temperature Degree rises to 150 °C -190 °C；

B, insulation step-down：Vacuumize to processing kiln, reduce kiln inner pressure to 0.2～0.3atm, and keeping temperature is constant, protect The warm time is in 1～3h；

C, the multiple pressure of cooling：Stop heating, be gradually lowered the temperature for processing kiln, and air be passed through to processing in kiln, by pressure gradually Return to normal pressure level.

2. the timber heat modification method based on normal pressure intensification negative pressure heat preservation technique as claimed in claim 1, is characterized in that：Step In c, air is passed through when temperature is less than 100 °C into process kiln, pressure is gradually restored to normal pressure level, while to process kiln Internal spraying water.

3. the timber heat modification method based on normal pressure intensification negative pressure heat preservation technique as claimed in claim 1, is characterized in that：Step In a with 8-12 °C/h heating rate temperature-rise periods in, to process kiln internal spraying water.

4. the timber heat modification method based on normal pressure intensification negative pressure heat preservation technique as claimed in claim 1, is characterized in that：In step There is step before rapid a：A0, timber pretreatment：High temperature drying is carried out to timber sawn timber first so that moisture content is in 2-4%.

5. the timber heat modification method based on normal pressure intensification negative pressure heat preservation technique as claimed in claim 1, is characterized in that：Step Wood temperature is risen to 155 °C -165 °C with 8-12 °C/h heating rates in a.

6. the timber heat modification method based on normal pressure intensification negative pressure heat preservation technique as claimed in claim 1, is characterized in that：Step Wood temperature is risen to 175 °C -185 °C with 8-12 °C/h heating rates in a.

7. the timber heat modification method based on normal pressure intensification negative pressure heat preservation technique as claimed in claim 1, is characterized in that：Step In b, for the timber of thickness 2cm, temperature retention time 1h；For the timber of thickness 5cm, temperature retention time 3h.