Absolutely dry wood is almost absent. Wet wood must be dried before its decomposition starts. Until its surface layers are moist, the wood temperature cannot exceed 100 °С, i. e. water boiling point. After the outer layers are dried completely, the wood temperature starts increasing. The wood is composed of several components with different chwmical compositions. First, when the lump surface temperature is approximately 180–200 °С, hemicelluloses start decomposing, which is followed by the cellulose and then lignite decomposition. When the surface temperature is up to 280–290 °С, only small chains forming gases and light liquid products are separated. When the temperature approaches 300 °С, a more intensive process of decomposition starts and it is accompanied by heat emission (exotherm). In this case, the wood temperature increases spontaneously until all heat of exotherm is emitted. The next stage requires external heat supply again. This is coa incineration. Let’s consider the basic stages in detail.
METHODS OF HEATING MATERIAL
Heating is the heat supply to a mateial. There are many ways to heat the material (induction, heat radiation, etc.); however, either direct heating with gas flow passing through the layer or bypassing the surface of individual lumps or indirect heating through the device wall is used for charring. In the latter case, the heat transfer is less efficient. Direct heating. This heat supply method itself is more efficient than others, but it is associated with some process problems to be discussed later. The gas flow through the layer is not uniform across the section. In case of vertical flow upwards, the flow near the walls is always larger than that through the layer. There are always more voids near walls, thereby the flow resistance is less. Heat emitted by hot gas – coolant – is transferred to cold wood through the lump surface. Finer the material, larger the specific surface. But the channels, namely voids between individual lumps are also less in size. If we load a layer of cuttings into the device, the flow filtration through them will be negligent. The flow will pass along the walls and will make channels in the layer. Most particles will not be bypassed. Available closed voids between particles make cuttings a bad heat carrier (it is no coincidence that cuttings are often added to lower crowns of rural houses for warmth-keeping). This complicates the heat transfer through the layer. Drying and pyrolysis of cuttings and other fine materials in the layer is inefficient in this case.
There are some specifics in the flow distribution through the layer, depending on the device orientation, locations of coolant input and output, and device height to length ratio. It should be taken into account that if the flow rate is low, dead zones visible from other areas are formed in the device space.
Below are the process problems related to direct heating. It is difficult to prepare a coolant that does not contain oxygen. Even when natural gas is burnt, the air excess is required to ensure complete incineration. Other types of fuel require higher air excess than that required for natural gas to ensure normal burning. Therefore, the coolant always contains oxygen. An additional system can be installed to remove oxygen from the coolant, but it makes the process difficult and creates an additional facility to be monitored and maintained.
When the oxygen-containing coolant enters a drier, more strict control is required because the dried wood may ignite in the drier. If the oxygen-containing coolant enters inside devices where the pyrolysis is performed, the oxidation of some coal is inevitable. In this case, the coal yield is reduced. Liquid and gaseous products of the wood decomposition aer mixed with incombustible flue gases; resultant gases have a low calorific caacity and they are difficult to incinerate. If it is possible to incinerate them with flashing of fuel with higher calorific capacity, the thermal efficiency will be low because the gases are diluted. To return the combustion products to the pyrolysis chamber, a hot smoke exhauster should be used. The smoke exhauster must be made of an acid-resistant material because acids from the decomposition products may enter it. Below is one more complexity: evil-smelling substances remain in the gas coolant and coal also has bad smell. To remove this smell, the coal cooling zone should be a separate cycle and cooling should be performed with a separate gas flow, which does not contain fragrant components.
Heating through the wall. External heating of vessels built into heaters (kilns, hearths) where drying and pyrolysis are performed is widely used in charring. Heat is supplied to walls of these vessels mostly by the gas flow of the coolant, sometimes by means of radiation from the burning fuel and hot walls of the hearth.
This is primarily vessel walls that are heated. The heated gases start lifting upwards along the walls and lower downwards while cooling down due to the heat transfer to the material. Cyclic flows shown in Fig. 3 are formed. Such flows are less critical in a horizontal device.
WHAT IS DRYING?
The wood should be dried before pyrolysis. The lumpy material drying process is composed of several periods (Fig.4).
When the moisture content is high, the process rate is defined by a possibility for moisture removal from the surface and the device. This period of constant drying speed (I). The rate depends on the gas medium condition. The higher the temperature, the quicker the moisture removal from the surface of the material and the higher the drying rate. While the moisture content is reduced, the moisture flow inside a lump is decreased.
There comes a time when the moisture supply to the surface becomes less than the medium can remove – the period of decreasing drying rate starts (II). Increasingly less moisture enters the surface each next step. When the moisture front goes back inside the lump, the material surface starts heating to a temperature exceeding the water boiling point.
Meanwhile, the temperature inside the lump is still above 100 °С. That is why, the pyrolysis often occurs on the lump surface and drying occurs inside the lump. After most free moisture removal, the drying rate starts decreasing sharply. This is a period of capillary moisture removal (III). The moisture is distributed inside the wood in large and small pores, in cell cavities. Some small pores can be overlapped in case of moisture reduction. Such mechanism protects a live plant from complete drying, but complicates the residual moisture removal from the wood. The inflexion point between I and II periods is called the first critical point. Both the drying rate in I period and the time of II period occurrence from the beginning of drying depend on temperature and regiome of the gas coolant flow bypassing the wood. If drying is performed by means of the coolant, which temperature does not exceed 100 °С (as it is performed for sawn timber drying), moisture content of the coolant is also critical.
However, higher temperatures are usually used for wood drying. The second critical point is the passage from II period to III period. In fact, drying in III period is performed extremely slowly. For practical purposes, it may be considered to be terminated. The exact position of critical points cannot be indicated. Their coordinates depend on external factors and structural features of the wood. Most researchers define the position of the first critical point in different positions as correspondent to the moisture content of 25–35 % rel., and that of the second critical point to 8–12 % rel. It follows that increase in the drying duration in excess of the time required for approaching the first critical point is most often economically unprofitable because the removal of each next per cent of moisture requires more time than the removal of the previous one.
The lump size is critical in the drying process. The less the lump, the more open surfaces fall within the unit mass and the quicker the drying. However, this is true if the material layer is blown uniformly by the flow.
Cuttings create the resistance to the flow, form closed voids between particles and thus dry out slower than larger lumps. The structural features of the wood cause irregular conditions of the moisture flow from inside towards the surface along and across the trunk: a round trunk even of small diameter dries out much worse than a thicker split log. The method providing for the storage of split wood in woodpiles in summer months for natural drying was used before. Stockpiling was performed manual, in accordance with definite rules, using gaskets, which ensured good ventilation. A stockpile was covered by bark residue. In dry weather, the wood reached the air-dry condition (25–28 % of moisture) for the first weeks. Such method in modern conditions is unprofitable because funds for felling and storage are taken out of the cycle and large areas are required. This practice was declined everywhere. Manual stacking is not paid off. The experience in drying in heaps on grating supports was successful. In any case, the wood stacking for drying and their subsequent delivery for processing is an additional cost chain. In addition, the wood storage in modern financial situations means the reduction in rates of using the floating assets. Therefore, artificial drying is optimal.
As it was mentioned before, the drying rate depends on the coolant temperature. Drying is the moisture removal process. The wood structure changes during the drying. If the drying is intensive (which occurs at a high coolant temperature), much steam is generated at the same time. Pressure in the lump increases with its size and heating temperature increase. This pressure causes the wood burst.
Coal becomes fine and fractured. On the contrary, slow heating (a process lasting several days with gradual temperature increase) allows receiving perfectly burnt coal without cracks. That is the reason for receiving coal of good quality by some primitive methods, starting from heat charring to kilns, are used in South-East Asia. These conditions may be satisfied in any kilns, not only in conventional Asian ones. The literature often contains references to some peculiarities of Asian kiln, which are the only possible for receiving coals of particular quality, but such idea propagators should not be trusted. They have either limited knowledge of the process or disinform consciously readers to promote “own” technology.
These “environmentally dirty” and unprofitable kilns allow receiving good coal due to the highly prolonged process. Nevertheless, such conditions may be used in other devices as well. Only economic issues (low specific capacity) do not allow spreading this mode everywhere.
Author of this article: Yuriy Yudkevich, PhD in Technical Sciences This material was published upon authorization of the author’s daughter.