Sunday, May 30, 2010

The Big Chill, and Fire!

After making biochar in our kiln, we allow the retort drum to cool to ambient temperature, for ease of handling and to avoid the risk of the hot char self-igniting. Curiously, when we take the lid of the drum, the char cools abruptly. What's up with that?! We kicked it around and concluded that it's probably due to rapid evaporation of volatile compounds. On completion of pyrolysis the atmosphere in the retort is saturated with residual volatiles. When the lid is removed, these diffuse into the air, triggering evaporation from the manifold char surfaces as the system equilibrates to the new atmosphere. The coolness we're experiencing is the latent energy of evaporation (in the same way that rubbing alcohol feels cool on the skin, and then is gone).

Far more consequential is spontaneous combustion! We had just bagged some biochar (day-old and auto-chilled as above) and chucked it into the back of a station wagon for transport. Then we resumed work re-loading the kiln. Fortunately, one of our group noticed the smoke and we quickly removed the burning bags and quenched them with water.  If you google "spontaneous combustion" (bypass the apocryphal tales of "Human Spontaneous Combustion"), you find sites insisting that spontaneous combustion of a bag of charcoal is the stuff of urban myths (I can assure you, it is NOT). The best explanation I found was here: Crushing the char before stuffing it in the bag exposes heaps of surface area to fresh air; stir in the heat of friction from crushing, add a lick of water from humidity condensing on the char surfaces to catalyze oxidation reactions, and Voilá! Instant fire. Take care with handling fresh biochar! 

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