Views: 0 Author: Site Editor Publish Time: 2023-04-01 Origin: Site
Responsibility and Status of Wood Drying
The waste of wood resources in China is mostly due to the non drying or improper treatment of wet wood, resulting in the degradation or even loss of use value of wood. The purpose of wood drying treatment is to:
(1) prevent deformation and cracking of wood;
(2) Improve the mechanical strength and physical properties of wood;
(3) Prevent wood decay and insect damage;
(4) Reduce the weight of wood.
From the above, it can be seen that the responsibility of the wood drying industry lies in improving the performance and utilization of wood. At the same time, as wood drying is the process with the largest energy consumption, it is one of the crucial links in the production of wood products, whether from the perspective of product quality or economic benefits.
Current Situation of Wood Drying Technology
1) Huge development potential of wood drying industry
According to estimates by relevant experts, the existing equipment in China has an annual drying capacity of about 4 million cubic meters of wood, accounting for only about 17% of the sawn wood needed to be dried in 2000. However, the drying equipment capacity in moderately developed countries can reach 30% of the required drying capacity, while in the United States, it can reach as high as 60%. This indicates that the capacity of drying equipment in China is far from that of developed and moderately developed countries, and also indicates that the development potential of China's wood drying industry is great.
2) High drying energy consumption and environmental pollution cannot be ignored
Steam drying accounts for over 80% of the existing drying equipment in China. However, furnace gas and direct fire flue gas drying are more common in small enterprises. Steam drying uses boilers as heating equipment, and the average thermal efficiency of industrial boilers and their pipe networks in China is only 60%. At the same time, steam drying has a large heat loss in the intake and exhaust, accounting for about 40% of the steam thermal energy. In addition, there is heat dissipation loss from walls, gates, etc., making the primary energy utilization rate of steam drying generally lower than 30%. High drying energy consumption not only increases product costs, but also increases the amount of smoke and exhaust gas emitted from boilers and drying chambers, thereby increasing the pollution of these harmful substances to the atmosphere. Taking a steam drying workshop with an annual drying capacity of 10000 cubic meters of wood as an example, a boiler with a capacity of about 4 tons/h is required. The harmful substances it emits per hour are: about 40 kg of smoke and dust, 1900 cubic meters of CO2, 45 cubic meters of SO2 and a small amount of NO2. These substances are the main factors causing greenhouse effect, acid rain and ozone destruction. According to relevant data, Shenyang, Xi'an, Beijing, Shanghai and Guangzhou ranked among the top 10 cities with the most severe particle pollution on the global monitoring network. As the contribution rate of energy to the process can reach 70% to 80%, energy conservation in wood drying must be paid attention to.
3) There are many old and crude equipment, resulting in severe degradation losses in drying
Currently, most small and medium-sized (especially small) wood processing plants in China have outdated and crude drying equipment, low quality operators, and most of them have not received professional training. As a result, the loss of wood grade due to improper drying is quite serious, generally greater than 20%, and about 5% of the wood completely loses its use value. According to incomplete statistics, the total annual drying amount of this type of equipment nationwide is about 1 million cubic meters, with a reduction of 20%. If the reduction loss per cubic meter of material is calculated as 50 yuan, the direct economic loss is more than 10 million yuan. If 5% of the wood loses its use value, the wasted forest resources are more than 50000 cubic meters. However, microwave drying equipment has a beautiful appearance and simple operation, greatly reducing equipment and human losses, thereby improving the quality of wood products.
Principles of Microwave Drying of Wood
Microwave drying of wood is a new technology, which has a series of advantages such as uniform heating, fast drying speed, no deformation, good quality, and is conducive to continuous and automated production.
Microwave wood drying is different from traditional wood drying equipment in that it is an electromagnetic wave with a frequency ranging from 300MHZ to 300GMHZ and a corresponding wavelength ranging from MM to DM (10 to 10M), which is longer than the wavelength of light and infrared waves and belongs to the high-frequency band. There are two types of fixed dedicated frequencies commonly used for industrial microwave heating equipment in China: 915MHZ and 2450MHZ. Microwave has wave characteristics such as reflection, transmission, interference, diffraction, polarization and energy transmission accompanying electromagnetic waves; Active molecules such as water molecules of materials are repeatedly arranged, transformed and released at frequencies of 915MHZ and 2450 in the electromagnetic field for heating and drying purposes. Microwave heating of materials inside and outside simultaneously has the characteristics of instant heating, without thermal inertia, so it is easy to control during the drying process; And there is no noise and harmful gas emissions, which belongs to environmental protection drying technology. Simply put, microwave is directly heated by the moisture contained in the material and the wood itself through the internal absorption. Due to the evaporation of water on the surface of the material, the internal temperature of the material is higher than the surface. This creates an outward pressure difference inside the material, accelerating the surface migration of water, so the speed of microwave drying of wood is much faster than the speed of convective drying by 20 to 30 times. Compared to far infrared and steam drying, the energy consumption is reduced by more than 25%.
Microwave drying of materials can make the temperature inside the wood higher than 60 ℃, and the parasitic insect eggs in the materials can be quickly killed, achieving the effect of mothproof and mildew prevention. For various wood conditions with different properties, the best benefits can be obtained.