There are two main methods for making bamboo charcoal in China, dry distillation and pyrolysis and direct firing in earth kilns.
(1) Dry distillation pyrolysis method: The equipment is mainly an externally heated vertical dry distillation kettle. When this dry distillation kettle is used to burn bamboo charcoal, it can use bamboo that has been pre-dried to a moisture content of 20% to 25%. You can use unpre-dried bamboo, but it is better to use pre-dried bamboo. Since there is basically no problem of bamboo charcoal oxidation during the firing process, the yield of bamboo charcoal is relatively high, generally about 25%, and the higher one can reach 35%. The firing cycle is generally 48 to 72 hours. However, the refining temperature is not high, which affects the density of bamboo charcoal; the volume of the retort is small and the output of bamboo charcoal is low.
(2) Earth kiln firing method: it uses fuel (wood) direct heating method, that is, the heat generated by the fuel combustion at the kiln mouth rises to the top of the kiln and diffuses into the kiln. Most of the hot air flows in the upper layer. A small part of the heat radiates to the surroundings, slowly drying from top to bottom, and reaches pre-carbonization; burning some bamboo in the kiln makes the temperature in the kiln continue to rise and remove volatile substances. At this time, the flue gas in the kiln circulates, and the heat and temperature of each point Basically uniform, complete the carbonization and refining stages to obtain bamboo charcoal with dense structure. The earth kiln firing method usually includes smoke pre-drying, drying, pre-carbonization, carbonization, calcination (refining), natural cooling and other stages. Each stage has a different temperature. The pre-drying stage of smoking is 60-100℃, the drying stage is 100-150℃, the pre-carbonization stage is 150-270℃, the carbonization stage is 270-450℃, and the calcination stage is 450℃-1000. ℃ around. From the perspective of the process of charcoal burning in earth kilns, the temperature and carbonization speed at each stage are determined by the operator’s “watching and sniffing”. One is to observe the changes in smoke from the chimney and kiln door; . The fresh bamboo used in the earth kiln is generally placed outdoors for about a week, and it is smoked and pre-dried in the kiln. It takes about 1 week, and it also takes a week when it is naturally cooled to the kiln mouth temperature of 50～60℃. It takes 2 days. Therefore, it usually takes 25-30 days from kiln loading to charcoal discharge, and the bamboo charcoal yield is generally about 20%. The process is reasonable. If the amount of later oxidation and combustion is small, the yield will exceed 20%, otherwise it will be less than 20%, or even lower (due to air leakage or excessive air intake). Earth kilns are cheap, high refining temperature, high density of bamboo charcoal, but poor quality stability and low yield.