Pyrolysis, heating wood with little or no air, splits it into three products: solid charcoal, a combustible gas, and a liquid of tar, water and organic acids. By mass the charcoal is only a minority of the wood, so the gas and liquid are the larger part and are worth capturing. Charcoal further treated with steam at high temperature becomes activated carbon, a filter material whose surface area per gram is many times that of plain charcoal.
Part of Organic Chemistry
The products of wood pyrolysis and how charcoal, tar, wood gas and activated carbon each come from one process.
One Process, Three Product Streams
When wood is heated away from oxygen it decomposes. The solid residue is charcoal. The vapours either burn off as wood gas (the gas the gasifier articles use, see Wood Gas) or condense as a liquid. That liquid separates into watery acid (pyroligneous acid, which contains acetic acid and, as the carrier notes in its biodiesel section, methanol from wood distillation) and tar. Further distillation of tars yields phenolic compounds such as the creosote described in Creosote and the antiseptic phenol of Antiseptic Compounds. Tar and pitch are treated in Wood Tar Production and Pitch Making.
Making charcoal as fuel is covered in Charcoal Production; the biochar articles are about soil and are not the place for the production method. Safe handling is in Safety Practices.
Activated Carbon
Activated carbon is charcoal treated with steam at high temperature so its internal pores open. Making Activated Carbon gives the steam activation range as 600 to 900 °C and the resulting area as 1,000 to 2,000 square metres per gram, against a few hundred for plain charcoal. Uses are water and air filtration (Charcoal Filtration); a filter adsorbs many dissolved organic compounds and taste, but it does not make unsafe water safe and it does not remove all poisons.
What You Need
- A retort or sealed pyrolysis vessel with a vent and condenser if capturing liquids.
- Dry hardwood in uniform pieces.
- A way to burn off or flare the gas, at a distance, outdoors.
- A steam source for activation.
Method: Reading A Pyrolysis Run
Step 1: Heat the sealed retort gradually. White steam comes first (water), then yellow smoke (acids and tars) and then flammable gas.
Step 2: Flare the vent gas at a distance.
Step 3: Stop when the vent smoke thins and let the retort cool sealed. Opening it hot lets air in and the charcoal burns.
Step 4: Collect the liquid and let it settle into tar and watery acid. Treat both as hazardous.
Worked Example: Where The Wood’s Mass Goes
Assume an illustrative charcoal yield of 25% of dry wood mass (a general figure, not a corpus value) for a 1,000 kg charge.
- Charcoal: 1,000 × 0.25 = 250 kg.
- Everything else leaving as gas and liquid: 1,000 − 250 = 750 kg.
- Ratio: 750 ÷ 250 = 3 times as much mass leaves as is kept.
A burner that vents all that to the air wastes three quarters of the wood’s mass and its heat value; a retort that burns the gas to heat the next charge uses it. The corpus’s tar-yield pages give the liquid fractions: Wood Tar Production gives pine heartwood 15-25% of dry weight as tar.
Common Mistakes
- Opening a hot retort. Air reaches hot charcoal and it ignites.
- Using the watery acid as a medicine or food additive. It contains toxic compounds.
- Expecting activated carbon to purify any water. See Safety.
- Confusing charcoal with biochar. Biochar is a soil article.
Safety
The vent gas is carbon monoxide, and the liquids are toxic
The pyrolysis gas contains carbon monoxide and burns; flare it outdoors at a distance and never test it with a flame. Smouldering charcoal in an enclosed space also gives off carbon monoxide: never sleep with a charcoal fire in a closed room. The tar and acid liquid is toxic and corrosive; do not drink, do not pour it in a water source, and wear gloves. See Gas Hazards.
What’s Next
- Learn next: Materials Synthesis — making materials from the products
- Combine with: Gasifier Design — the same chemistry run for the gas