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Causes and preventive measures of wood microwave drying defects

2018-10-22 11:51:25

Abstract: The main causes of wood microwave drying defects were analyzed from the aspects of wood dielectric properties, microwave penetration depth, microwave equipment and drying technology, and the prevention measures were discussed from the aspects of optimizing dryer design, improving microwave field distribution uniformity, controlling microwave radiation power and time, changing microwave radiation mode, etc. The main measures to produce drying defects are expected to provide a reference for the research and application of wood microwave drying technology.
Key words: wood; microwave drying; drying defects; preventive measures

Conventional steam drying is widely used in wood drying because of its mature technology, strong practicability and large loading capacity. Its drying volume accounts for more than 80% of the total wood drying volume in China. However, in conventional steam drying, the direction of heat transfer inside wood is generally opposite to that of water transfer, and the speed of heat exchange and heat transfer on the surface and inside wood is slow, which directly leads to the problems of long drying time, high energy consumption, Difficult-to-dry wood and poor drying quality of thick square wood.

Microwave drying of wood is to put wet wood as a dielectric in microwave alternating electromagnetic field. Under the action of frequent alternating electromagnetic field, the polarized molecules in wood rotate rapidly, friction with each other and generate heat, so as to heat and dry wood. Due to the characteristics of instantaneity, integrity, selectivity and high efficiency of microwave heating, wood microwave drying not only has good drying quality, but also has speed advantages that other drying methods can not compare. Therefore, considering the drying quality and drying speed, microwave drying is an ideal drying method. However, research and production practice show that some drying quality defects, such as cracking and carbonization, may still occur during microwave drying of wood. In this paper, the causes of wood drying defects in the process of microwave drying were analyzed theoretically, and the main measures to prevent wood drying defects were put forward.

1 theoretical analysis of the causes of defects in wood microwave drying
Theoretically, the causes of wood drying defects during microwave drying are mainly reflected in the following three aspects: First, wet wood is a typical anisotropic heterogeneous material. In microwave field, the microwave energy absorbed by different parts of wet wood can be expressed in the following way:
P =k fE 2 epsilon r TG Delta (W/m 3)
In the formula: K is a constant; f is a microwave frequency (Hz); E is an electric field strength (V/m); epsilon R is a dielectric constant of wet wood; TG delta is a tangent of loss angle (or loss factor) of wet wood.
It can be seen from equation (1) that the ability of wet wood to absorb microwave (or convert microwave energy into heat energy) depends on the dielectric constant, loss factor, microwave frequency and field strength of wet wood. The absorption ability of wet wood to microwave depends entirely on the dielectric constant (dielectric constant and loss factor) of wet wood when the microwave field intensity and frequency are constant.
Wet wood is a typical anisotropic heterogeneous biomass material. Within the wood, there are differences in wood properties between early wood and late wood, heartwood and sapwood, and significant differences in moisture content in different parts of the wood, resulting in differences in dielectric properties between different parts of the wood, making different parts of the wood. The temperature difference is caused by the different ability of absorbing microwave.

When the temperature difference exceeds the ultimate strength of wood, the thermal stress may lead to wood cracking. Antti A.L. and Perrie P. used infrared imaging technology to study the temperature distribution on the surface of birch and pine wood during microwave drying. The results showed that the temperature distribution on the surface of birch and pine wood was not uniform during microwave drying. The higher the moisture content of wood, the longer the heating time, the more uniform the temperature distribution on the surface of the wood. The worse the sex. For example, from the first minute to the seventh minute of heating, the maximum difference of surface temperature of dry plate rises from 6 C to 25 C, and the maximum difference of surface temperature of wet plate rises from 13 C to 30 C in corresponding time.

Secondly, as a kind of high-energy electromagnetic wave, microwave has good penetration. For not too thick wood, it can directly penetrate into the wood interior, and heat the whole wood interior and exterior simultaneously, thus greatly shortening the heating time and speeding up the drying rate. When microwave enters wood, the energy density of wood surface is the highest. With the penetration of microwave into wood, the energy decays exponentially, and the energy of microwave field is released to wood. Because the energy of microwave field decreases gradually with the depth of wood entering, the thickness of wood is limited when wood is dried by microwave. Therefore, in microwave drying, thickness is also an important factor affecting wood temperature distribution and drying quality. Penetration depth (DE) refers to the distance when microwave power decays from the material surface to 1/e of the surface value (36.8). The penetration depth of microwave in wood can be approximated by the following formula:

Lambda 0
DE =
PI e * TG Delta
Type 0 is the electromagnetic wave length in vacuum;
The electrical constant; TG delta is the loss tangent (or loss factor) of wet wood. From the formula (2), we can see that under certain frequency, microwave
The penetration depth decreases with the increase of wet wood loss factor and dielectric constant. Because water is a material with very strong molecular polarity, its ability to absorb microwave is much higher than that of wood, so the state and quantity of water in wood is the decisive factor affecting the dielectric coefficient of wood. Without considering the influence of temperature and other factors, the penetration depth of microwave will follow.
With the increase of wood moisture content, it decreases. The author calculates by theory.