The igniting and sealing after igniting

(3) Heating (dry and pre-carbonization stages)
It is a process raising the temperature inside the kiln by dismounting the arc-feeding intake
on the gate everyday and feeding firewood to keep burning. Once finished feeding, the
feeding intake should be sealed again. Usually the feeding keeps 2 to 3 times according to
combustion that can be adjusted by changing the opening of bottom air intakes on the gate.
To avoid feeding firewood at midnight, it is necessary that not only the feeding intake but
also the air intakes should be sealed after last feeding in the evening every day. Of cause, the
air intakes couldn’t be sealed completely and leave a small portions to remain slow
combustion. The bamboo in the kiln will crack if the temperature escalates quickly. The
temperature in the kiln should be controlled under the self-igniting point of bamboo in seven
to eight days after igniting. Firewood feeding should be decreased or stopped if the
temperature is enough. Blocking the intakes and flue rim with bricks can regulate the
temperature in the kiln. Figure 15 is a curve of heating, which delineates the change of the
temperature in the process of heating.

(4) Carbonization

When the temperature in the kiln reaches 260℃, bamboo will be decomposed rapidly and give out a lot of offspring and reaction heat. When it is over 450℃, bamboo pyrolysis enters into refining or calcining stage. In fact, it is a contracting process of high temperature
pyrolyzation for improving the quality of bamboo charcoal and enhancing its hardness as well. After the end of heating open the intakes on the gate and feed more firewood quickly to raise the temperature inside the kiln. In this process, the intakes of kiln gate shouldn’t be opened wholly in a short time, and they are to be opened gradually within twenty-four hours or so to make the bamboo charcoal contracted absolutely. At the end of refining stage, all the intakes should be opened again for one or two hours to raise the temperature of the charcoal in the kiln to 1000℃ or more. Fig. 16 depicted the curves of temperature during the refining stage. The beginning and ending of refining process will be controlled according to the temperature on the curve.

The kiln gate situation at the end of refining

In practice, workers, especially in small plant, determine the carbonization stages by
watching the color and smelling the smoke coming out from the flue. At the beginning of
drying, the smoke shows white color containing a lot of steam, and then slight acid smelled.
Beginning carbonization, the smoke shows slight yellow companying with tar smell. When
smoke color turns to slight blue, it indicates the end of carbonization and the start of
refining.

(5) Sealing for cooling and unloading
The kiln gate must be fully sealed with brick and mud pile at the end of refining, and let the
bamboo charcoal in kiln cools naturally. If the kiln is not sealed well, bamboo charcoal
inside will be easily oxidized. The cooling time depends on weather. Usually it takes five to
six days. When temperature in the kiln is lower than 50℃, it is the time of put out the
bamboo charcoal. At first the gate should be opened little to note if the bamboo charcoal
re-burns and then opening the gate completely.

bamboo charcoal cooling in kiln