Wood distillation

A special case of dry distillation where the solid material being heated is wood.

Use

This process takes in a readily available material (wood) and produces three separate streams of useful materials: charcoal, flammable gas, and a variety of liquid hydrocarbons, including aromatic compounds.

Chemistry

The base chemistry is simple: heat wood in an oxygen-free environment until all the free water is driven off. Then raise the temperature to 270°C at which point a self-sustaining exothermic reaction takes place, breaking down the cellulose and other materials in the wood into solid, liquid, and gaseous products. Source wood, temperature control and residence time in the reaction and distillation zones govern what reaction products are favored.

Consumables

• Wood (cellulose, really)
• Starting fuel. Could be more wood, charcoal, alcohol or something else

Products

• Solids: Charcoal and wood ash
• Liquids: Collectively referred to as Wood tar, and further partitioned into soluble tar and insoluble tar. These are the liquid products formed during the reaction that do not boil away. They are a highly viscous mixture of high melting point hydrocarbons. Wood tar can be purified and fractionated by distillation at a sufficiently temperature, or it can be used as-is for a variety of purposes, including wood treatment and waterproofing.
• Vapors: These products escape the reaction as gasses, but liquify at environmental temperatures, thus giving the term "wood distillation" its name. The primary vapors collected are water, methanol, acetic acid and BTX aromatics. Many other compounds are produced as well, including but not limited to:
  • pyrolignious amines: ammonia, methylamine, pyridine, methyl pyridine...
  • pyrolignious acids: acetic, formic, propionic, butyric, valeric, caproic, crotonic, angelic, pyromucic...
  • pyrolignious alcohols: methanol isoamyl, allyl, isobutyl ...
  • methyl/ethyl/propyl ketones, furfural, methyl furfural...
• Gases: Carbon dioxide, carbon monoxide and hydrogen escape the reaction zone as gasses, but do not liquify at environmental temperatures. Kept away from oxygen, they are collectively flammable and can be used as a fuel. This mixture is referred to as producer gas.

Yield

Source 1

• According to Pine Tar; History and Uses, the average yield for one cord (4,000 lb.) of "light wood" might be:
  • Charcoal 25 to 35 bushels or 403 to 564 lbs
  • Wood turpentine 8 to 15 gal.
  • Total oils; including tar 65 to 100 gal
    • Tar 40 to 60 gal.

Source 2

Chemical composition of wood distillation products

Material	Pine	Birch
Charcoal	31.80	37.83
Water	27.81	22.27
Carbon dioxide	9.96	10.13
Soluble tar	8.15	8.03
Tarry residue	7.93	11.79
Acetic acid	7.08	3.50
Carbon monoxide	3.32	3.74
Methane	0.54	0.59
Unsaturated hydrocarbons	0.19	0.23
Methanol	1.60	0.88
Acetone	0.19	0.18
Methyl acetate	0.02	0.01

Tools

• A kiln with an airtight heating compartment separate from the fire compartment.
• A condenser and receiver, or a fractionating column

See Also

• charcoalmaking

Publications

• Maybery, Charles F. (1882) "On the Products of the Dry Distillation of Wood at Low Temperatures"
  Proceedings of the American Academy of Arts and Sciences 18; pp47-54. American Academy of Arts and Sciences
  link courtesy jstor.org.
• Hawley, I. F. (1921) "The Distillation of Resinous Wood in the Southern States"
  Naval Stores; pp251-252. Review Publishing and Printing Company, Savannah, Georgia
  link courtesy Google books.
• Beglinger, Edward (1956) "Hardwood Distillation Industry #738"
  Forest Products Laboratory, Forest Service, U. S. Department of Agriculture in association with the University of Wisconsin
  link courtesy Oregon State University.