A fuel is an organic compound, or a gas mixture, that releases energy when it burns with oxygen, ideally into carbon dioxide and water. Burning ethanol takes three oxygen molecules per ethanol molecule and gives two carbon dioxide and three water. When air is short, combustion stops at carbon monoxide, an odourless poison, so every fuel is both a resource and a gas-hazard question.
Part of Organic Chemistry
Energy from organic compounds: what burning is, which fuels a community can make, and the shared hazards of all of them.
Burning Is A Reaction With A Balance Sheet
Fuel chemistry in the carrier covers three practical fuels: wood gas from gasification, biodiesel from vegetable oil, and bioethanol from fermentation. All three are carbon-hydrogen-oxygen compounds or mixtures. Combustion pairs the fuel with oxygen: carbon becomes carbon dioxide, hydrogen becomes water, and the heat released is the fuel’s value.
- Wood Gas and Gasifier Design: a gas, mostly nitrogen, carbon monoxide and hydrogen.
- Biodiesel: a liquid for diesel engines from oil.
- Bioethanol: an alcohol for spark-ignition engines and lamps.
- Charcoal: a solid fuel, and a by-product source.
What Decides How Good A Fuel Is
- Energy per unit mass or volume. A dense liquid stores more per litre than a gas. The producer-gas energy per cubic metre is on Energy Content of Wood Gas.
- Ease of ignition and storage. Ethanol vapour ignites at ordinary temperatures; see Fire Safety.
- Cleanliness. Tars and water in a fuel foul an engine.
- Competition with food. The carrier warns never to divert food crops to fuel without a substantial food surplus, and notes that in a rebuilding community food is almost always more valuable.
What You Need
- A clear sense of which engine or burner the fuel is for. Diesel engines, spark engines, stoves and lamps need different fuels; see Ethanol in Gasoline Engines and Lamp Fuel.
- A tested supply of feedstock (oil, sugar or starch, wood).
- Ventilated outdoor workspace.
Worked Example: The Balance For Burning Ethanol
Ethanol, C2H6O, burns as: C2H6O + 3 O2 → 2 CO2 + 3 H2O. With rounded atomic masses C = 12, H = 1, O = 16:
- Left side: ethanol 46 g + 3 × 32 g of oxygen (3 × 32 = 96) = 142 g.
- Right side: 2 × 44 g of CO2 (88) + 3 × 18 g of water (54) = 142 g. Mass balances.
- Air is about 21% oxygen by volume (a recalled value), so the air needed per mole of ethanol is 3 ÷ 0.21 ≈ 14.3 moles of air.
An engine or burner given less air than this cannot burn all the carbon to CO2 and makes carbon monoxide.
Common Mistakes
- Burning indoors with a poor air supply. It produces carbon monoxide.
- Confusing a fuel’s usefulness with its safety. All the fuels here are flammable and several are toxic.
- Ignoring the food cost.
Safety
Carbon monoxide is odourless and is what kills
Incomplete combustion makes carbon monoxide, which cannot be smelled and displaces oxygen from the blood. Burn fuels outdoors or with a proper flue, never run an engine or stove in a closed room, and never use a flame to test for a flammable gas. Fuel alcohol is poison: do not drink it, and see Methanol Hazards.