Charcoal has paramagnetic properties due to the presence of stabilizing macro radicals (paramagnetic centers – PMC). The presence of macro radicals determines the high reactivity of charcoal in relation to the oxygen.
Hot charcoal unloaded from the charcoal kiln absorbs oxygen from the air, at the same time it heats up even more, which can lead to spontaneous combustion. The charcoal burnt at low temperatures and containing up to 30% of volatile substances have the greatest ability to spontaneous combustion, the temperature of spontaneous combustion of such coals is below 150 °C. Charcoal with a small amount of volatile substances can ignite spontaneously at temperatures above 250 °C. Unloaded charcoal at 30-90°C absorbs from the air 0.5-2% of oxygen (from charcoal weight) per 1 hour; at the same time low molecular products, in particular water, are exuded from charcoal.
Spontaneous ignition of charcoal — the result of its auto oxidation, developing at a rapid pace, with fast temperature rise under the influence of existing paramagnetic centers in charcoal. It is a branched chain process having certain critical parameters. If during the contact of charcoal with air these parameters (concentration of PMC, temperature, O2 concentration and geometrical dimensions of the mass of charcoal) are not exceeded, the charcoal will not ignite.
Based on this, one can conclude that the stabilization of the hot charcoal can be performed by controlled cooling of the air on the conveyor. The optimal conditions of this process: the temperature of the charcoal at the time of its unloading from the vertical retort is 170 °C, the height of the layer of charcoal on the conveyor is from 60 to 100 mm, cooling time is 7 min., the temperature of the charcoal coming down from conveyor is 70-80 °C. Under these conditions, the charcoal absorbs oxygen from the air, not heating up, stabilizes and loses its ability to ignite spontaneously. At low ambient temperature charcoal is cooled too quickly and doesn’t have enough time to stabilize, so in the winter the casing of the conveyor must be insulated.
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