Biodiesel can be made from waste cooking oil, rendered animal fat or pressed oilseeds such as sunflower and rapeseed, and every one of them is also food: a litre of oil holds about 8,280 kcal, roughly three days of one person’s calories. A diesel generator burning 8 litres a day therefore consumes the food energy of about 24 people, so feedstock should come from true waste first and from dedicated crops only when the community is well fed.
Part of Biodiesel Production
A biodiesel feedstock is any triglyceride-rich oil or fat that can be reacted with methanol; the three practical sources differ in cost, quality and what they displace.
The Three Sources Compared
| Source | Strength | Weakness | Page |
|---|---|---|---|
| Waste cooking oil | already collected; dark colour and smell do not matter | variable quality, free fatty acid, water | Waste Cooking Oil |
| Animal fat (tallow, lard) | rendered from slaughter | solid at room temperature; fuel gels in the cold | Animal Fats |
| Oilseed crops (sunflower, rapeseed) | sustainable, scalable | needs land and a press | Oilseed Crops |
Every Litre Of Oil Is Also Food
- Oil holds 9 kcal per gram (Oil Extraction). A litre weighs about 0.92 kg (assumed), so 920 g × 9 = 8,280 kcal.
- At the 2,800 kcal a day the community guides use, 8,280 ÷ 2,800 = 3.0 person-days of food energy per litre.
- The parent’s generator example burns 2 litres an hour for 4 hours a day = 8 litres a day, so 8 × 8,280 = 66,240 kcal, which is 66,240 ÷ 2,800 = 23.7 person-days every day, the energy that feeds about 24 people.
Fat is the densest food a community has and is scarce in lean times. The alternative uses of the same oil are food, soap (Plant Oils), lamp fuel (Fuel Sources) and candles. Whether it should become fuel is a decision about how well fed the community is, not a chemistry question.
Waste cooking oil is only waste if it was surplus in the first place. In a food-short community, used frying oil can be strained and used for cooking again, or burned in a lamp, before it is sent to a reactor.
Methanol Is The Other Feedstock
Biodiesel also needs about 200 ml of methanol per litre of oil. Methanol is not made by fermentation; the parent’s equipment list says to salvage it from fuel and solvent stores, which is a finite stock. Ethanol can substitute in principle (Triglyceride + Methanol → FAME + Glycerin).
Methanol is part of the feedstock bill
Methanol is poisonous through skin and vapour and causes permanent blindness; the lye is caustic. Whichever oil you choose, the reaction needs both (Methanol Hazards).
What’s Next
- Learn next: Production Process, what to do with the feedstock
- Combine with: Oilseed Crops, for what a hectare gives