Self-combustion of charcoal
Hot charcoal discharged from the retorts absorbs oxygen from the air, while it heats up even more, as a result of which spontaneous combustion of coal can occur. Coals burnt at low temperatures and containing up to 30% volatile substances have the greatest capacity for spontaneous combustion; the spontaneous combustion temperature of such coals is below 150 ° C. Coals with a low content of volatile substances can self-ignite at temperatures above 250 ° C.
Spontaneous combustion of charcoal is the result of its autoxidation, which develops like an avalanche, with a rapid rise in temperature under the influence of paramagnetic centers in the coal. It is a chain-branched process with certain critical parameters. If these parameters are not exceeded when coal comes into contact with air, the coal will not ignite.
Based on this, it was concluded that the stabilization of hot coal can be carried out by controlled cooling with air on a conveyor. Optimal conditions for this process: the temperature of coal at the time of unloading from a vertical continuously operating retort is 170 ° C, the height of the coal layer on the conveyor is 60–100 mm, the cooling time is 7 min, the temperature of coal coming off the conveyor is 70–80 ° C. Under these conditions, coal absorbs oxygen from the air without warming up, stabilizes and loses the ability to ignite spontaneously. At low ambient temperatures, coal cools too quickly and does not have time to stabilize, therefore, the conveyor casing must be thermally insulated in winter.
The maximum permissible concentration of charcoal aerosol in the air of the working area is 6 mg / m3. The minimum self-ignition temperature is 340 ° C, the lower concentration limit of ignition of charcoal dust in air is 128 g / m3.