Defects and Prevention of Wood Carbonization
When wood is carbonized, if the process or operation is improper, many defects will occur, mainly including cracking, deformation, uneven color, knot falling off, and even fire. However, if the causes of these defects are mastered, basically these defects can be prevented.
to crack
The cracks of carbonized wood are mainly surface cracks and end cracks, and there may also be internal cracks when thick plates or squares are carbonized. Surface cracking occurs during the drying stage before carbonization. Often, the surface cracks produced during the first drying are relatively fine and difficult to detect. When the second drying occurs, the moisture content of the wood will further decrease from 12%, and the wood will shrink further, making the surface cracks larger and more obvious. Inner cracks are mainly produced during secondary drying. When the temperature rises or the humidity drops too fast during the secondary drying, the inner layer with a certain amount of moisture shrinks quickly, and internal cracks occur due to greater tensile stress. Therefore, before the average moisture content of the wood drops to 4%, the temperature cannot be raised too quickly.
out of shape
There are several kinds of deformations, such as horizontal bending (tile bending), forward bending (longitudinal bending), side bending (side bending), twisting and warping.
uneven color
The uneven color of the wood after carbonization reflects the uneven degree of carbonization. This is mainly due to the uneven air circulation in the kiln, or there is no fan in the carbonization tank, which cannot produce forced circulation, resulting in a large difference in color between the surrounding wood and the internal wood. Therefore, there must be forced air circulation in the carbonization kiln (tank), and the air velocity through the material pile must be large to ensure uniform drying and carbonization. In addition, if there are many kinds of trees and woods with different thicknesses or different moisture contents in the same kiln, it will cause uneven drying and carbonization. Therefore, the species, thickness and moisture content of wood charred in the same kiln should be the same or similar.
Knot off
After the wood is carbonized, the knots sometimes fall off during the planing process, and the knots that fall off are usually dead knots. Therefore, the carbonized wood should contain as little or no dead knots as possible. After the knots fall off, the wood of the same species can be used to fill the knots.
on fire
At the end of the carbonization process, the temperature of the wood is very high (up to 180-212°C) and the moisture content is very low (less than 0.5%). When the hot wood comes into contact with the oxygen in the air, it is easy to catch fire. In addition, if the airtightness of the kiln shell is not good, more air will leak into the kiln, which will also cause the charred wood to burn. Therefore, the correct way to cool wood should be to spray atomized water and spray steam into the kiln at the same time, instead of spraying air. After the kiln temperature rises to 125°C, during the whole process of wood drying, carbonization, subsequent cooling, and humidity control, steam must be sprayed into the kiln without interruption. If there is a power failure in these stages, the steaming cannot be done, and the generator must be powered immediately to ensure the normal steaming. Otherwise, due to the lack of water vapor in the kiln and the leakage of air outside the kiln into the kiln, it is easy to cause dry and hot wood to burn.
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