Ever held a stethoscope to a tree?
Try it sometime. Find a stethoscope at a medical supplier. After that, find a large tree — something broad, something with thin bark. You can hear a heartbeat through a chest even without a stethoscope, but skin is full of water, and water moves sound. Bark is, well, bark. It's dry. The rivers inside trees flow quietly.
You'll find that it sounds like you're holding a seashell up to your ear — except this sound is not sound waves circulating around a well-shaped piece of calcium carbonate: it is treeblood. We call it sap. You can hear it flowing in the grain.
The point is that trees are like us.
Actually a better illustration would be Carbon.
Carbon: Carbon-based. Carbon-based organism means earthly organism. Atomic number six. An orb of buzzing particles and empty space: an atom. Fatten up the core, you've got Gold, or Lead. Carbon catches fire: Gold does not. The mathematics of chemistry — this why we barbecue with coal rather than Cadmium. The mathematics of chemistry make you and me and our hearts and our fingertips and diamonds and stars the same as trees.
Pyrolysis is the chemical decomposition of organic materials in the application of heat without oxygen. Usually the end product is Carbon.
Industrially, the process of pyrolysis is catalyzed under controlled circumstances: under controlled pressures, and under controlled temperatures. We have invented devices similar to furnaces that operate at lower temperatures (800°F rather than 1500°F (~427°C & ~816°C repectively) and with less air. Crude oil is pressurized and very aged dead matter. When heated accordingly, it releases some tars, some Hydrogen, some CH4; what you're left with Carbon. Specifically, charcoal.
But pyrolysis is good for more than making charcoal. Tires wear out; they get old. Instead of burning their rubber in fire pits and filling the sky with smoke, or packing them in landfills for our grandchildren to make themselves sick over, we burn them without air. Rubber is bad: carbon is not. Again, chemistry at work.
Do the same thing to soil contaminated with pesticides and fuels.
As of 2002, we were testing the application of pyrolysis to treat mixed radioactive wastes.
Pyrolysis at home
Think we've got no immediate use for pyrolysis? Think again. Think about Kansas.
Think about burning things for heat. Moreover, think about carrying and storing things to burn for heat. On planet Earth the average guerilla farmer burns trees for warmth, rather than buying charcoal at the grocery store. Logs are heavy and take up space: charcoal is light and compact. But your average guerilla farmer has too much pride to walk into a Giant Eagle for a bag of coal. Rightly so.
No, no thank you — we'll be making our own charcoal.
Separating Carbon from wood is easy.
Dig a pit and stack logs inside, packed tightly together. Cover the mound with earth and ignite it in a few places. Maintain a slow burn by allowing very limited amounts of air — the fire's got to keep going. The temperature of the wood will reach 500°F; the heat will consume oxygen and break the chemical bonds holding the Carbon with the oil, the hydrogen, and the carbon monoxide. (That's right, carbon monoxide — don't try this one in your kitchen).
Making lots of charcoal? Give it a few days. Let it cool completely before you uncover it, lest the breath of oxygen ignite the whole thing afresh and decompose the precious Carbon.
Use the free space in your rammed earth dwelling for playing cards, etc.
Sources
Tawrell, Paul. "Camping & Wilderness Survival: The Ultimate Outdoors Book." Tawrell: Ontario. 1996.
Wikipedia
http://en.wikipedia.org/wiki/pyrolysis
http://en.wikipedia.org/wiki/woodgas (reproduced at Answers.com)
CPEO
http://www.cpeo.org/techtree/ttdescript/pyrols.htm
Mrs. Keech's third grade class, 1993.