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China has initially formed a large-scale professional dry wood production capacity and dry sawn timber market, as well as strong wood drying technology research and development, innovation capabilities and equipment manufacturing capabilities, and has formed a relatively independent wood drying industrial system.
In 2008, the output of sawn timber in China was about 66 million m3, mainly used for solid wood furniture, solid wood flooring, joinery board, interior decoration, and construction.
Doors, windows, sports equipment and vehicle production. The total amount of artificially dried sawn timber is about 13 million m3/d[1], which accounts for about 20% of the total amount of dry sawn timber. In developed countries, artificial drying of sawn timber usually accounts for 60% to 70% of the total amount of sawn timber. The amount of artificially dried sawn timber in China is so low that it is inconsistent with China’s status as the world’s largest furniture, wood floor producing country, and producer of wood-based panels.
1.1 Sawn timber applications
1) Solid wood furniture In 2007, China's solid wood furniture consumed about 15 million to 25 million m3 of sawn wood, most of which were hard broad-leaved trees, mainly from northeast China and Russia, followed by North America, Europe, and the tropics. There are many tree species and specifications for furniture and artificial drying materials account for about 40%. The main problems are that some larger-sized parts are often difficult to dry; in addition, furniture products produced in the South are sold to the North due to the high final moisture content of the wood and the residual stress in the wood, especially to North America and Europe. In the process of use, furniture is prone to cracking and deformation.
2) Solid wood flooring In 2007, the output of solid wood flooring in China was approximately 50 million m2, and the consumption of sheet steel was approximately 1.25 million m3. Most of the species are tropical imported materials, but also the traditional Northeast wood and Russian imports. The cross-sectional dimensions (width x length) of the solid wood floor blank are mostly 22 mm×(110-130) mm, and all of them have been artificially dried.
3) Construction and interior decoration In 2007, the consumption of sawn timber for construction and decoration in China was about 35 million m3. The building materials were mainly coniferous materials such as Pinus sylvestris, Chinese fir, larch, Pinus massoniana, and wetland pine; the interior decoration was made of eucalyptus, oak, Hard maple trees such as maple and black walnut are the main products. Most building materials are not artificially dried or only atmospherically dried. Therefore, cracking and deformation are serious.
4) Blockboard core board The output of blockboard in 2007 was 13.24 million m3, and the consumption of sheet was about 10 million m3, mainly poplar and fir of plantation forest. Artificial drying materials account for about 40%.
1.2 Drying equipment
At present, the domestic wood drying equipment manufacturing industry has formed an annual production capacity of approximately 600 million yuan. It supplies nearly 2,000 units/sets of wood drying equipment to the market each year, which can basically meet the needs of domestic wood processing companies. In the current drying equipment, conventional steam drying equipment still dominates, accounting for about 80% of the drying equipment market. In addition to using steam as the heat medium, conventional drying equipment using hot water (including high-temperature hot water) and furnace gas as the heat medium also occupies a large market share. With the increase of poplar, fir, and easy-to-import materials such as radiata pine, the demand for high-temperature and rapid drying kiln in the market has also increased.
In regions with abundant hydropower resources or severe environmental pollution, dehumidifying and drying facilities also have a certain market. Vacuum drying as a good auxiliary drying method can make up for the lack of conventional drying, and is mostly used for drying thick planks or square materials of hard-leaved hardwood. In order to improve the dimensional stability and durability of wood, in recent years, equipment for integrating wood drying and high-temperature heat treatment (carbonized wood) has appeared on the market.
1.3 Technology Status
1.3.1 Major Scientific and Technological Achievements
Since the "Eighth Five-Year Plan", China's timber drying industry has experienced a period of relatively rapid development. It has built a number of wood drying technology innovation platforms, production, research and research bases based on the National Wood Science and Technology Development Center and provincial and ministerial-level key laboratories; A number of national scientific and technological research projects, provincial and ministerial-level key projects, and national natural fund projects have achieved a number of scientific research achievements.
1) In addition to conventional steam drying, cyclone drying with wood processing residues as energy source, indirect heating and drying of furnace gas, hot drying with hot water (high-temperature hot water), high-efficiency vacuum drying, dehumidifying drying, etc. A complete wood drying industrial system is commonly used in industrial production to meet the drying requirements of different wood products.
2) The kiln dry benchmark [2] for nearly one hundred wood species in China is formulated to ensure the quality of sawn timber.
3) Established a domestic wood drying equipment manufacturing industrial system. In recent years, some large-scale wood processing companies that have been newly put into operation have often used imported production lines or host equipment, while domestic dryers have been used for drying equipment. It shows that the general technical performance of China's conventional kiln drying equipment is close to the advanced level in foreign countries and has formed a series.
4) Balanced moisture content in wood; Moisture transfer mechanism in wood; Heat and mass transfer model; Capillary tension and shrinkage mechanism of wood cells; Stress, strain and regularity during wood drying process, etc. With certain results, it has effectively promoted the academic status of China's wood drying discipline in the industry.
For a long time, the research on wood drying technology in China has been closely linked with the actual production. More than 70% of the research results have been promoted and applied in enterprises, and the contribution rate to the industry is relatively high. According to incomplete statistics, in the past 10 years, the output value of wood drying technology has generated about 3 billion yuan, saving about 5 billion yuan for wood processing companies, and saving about 13 million tons of standard coal. The development of wood drying technology has also made significant contributions to the creation of brand names for furniture and flooring companies in China and to international markets.
1.3.2 Independent Intellectual Property
China has formulated 3 national standards for wood drying and 5 forestry industry standards. In terms of wood drying new technologies, new processes and new installations, 30 patents have been declared and 18 have been authorized, including 4 invention patents and 14 utility model patents.
1.3.3 Application of R&D Results
1) Clean energy drying, heat recovery technology. The heat pump dehumidification and drying technology and the solar wood drying technology using electricity as energy source have no pollution to the environment, and the waste heat of exhaustable recyclable wood is a kind of green drying technology with energy saving and consumption reduction. Through the cooperation of schools and enterprises, research has been carried out on wood heat pump dehumidification drying energy-saving technology, high-temperature double heat source dehumidification drying, high-temperature dual heat source dehumidification, and solar combined drying. It has been widely used and promoted in Russia.
2) Efficient and rapid drying technology. Including vacuum drying, high temperature drying, microwave, high-frequency drying and hot plate contact drying. For example, the vacuum vacuum drying technology with negative pressure superheated steam and heat recovery device; the mechanism and process of high-frequency vacuum drying; the vacuum drying of wood under vacuum; the hot-press drying of veneer and lumber.
For hard-leaved hardwoods with good permeability, vacuum, high-frequency vacuum and microwave drying should be adopted. The drying efficiency is high and the quality is good. For artificial fast-growing materials, high-temperature drying or hot-pressing plates should be used for contact drying, and the speed should be fast. Smooth and smooth, the surface density and intensity are slightly increased.
3) Improve the process and control the drying quality. Broad-leaved tree materials, such as beech wood and poplar wood, tend to shrink during the drying process. By studying the mechanism and timing of shrinkage formation, a technical method to reduce or eliminate wood shrinkage is proposed. Some of the light-white broad-leaved trees, such as maple, birch, paulownia, etc., have undergone studies on the mechanism, conditions, and prevention of discoloration in the process of drying, and have achieved initial success.
4) Drying, modification and integration. For the poplar, Chinese fir, and masson pine plantation wood, easy to warp and deform in the processing and use of defects, the use of drying, modified integration technology, not only high-quality drying of sawn timber, but also improve the wood properties, improve Density, strength and flatness. If the resin is impregnated first, or pre-pressed, then dried and set, the strength and density of the wood are improved.
1 Overview of China's Wood Drying Industry