Ethanol can be fermented from three kinds of plant material: sugars such as cane, beet and fruit, which ferment as they are; starches such as corn, wheat and potatoes, which must first be converted to sugar; and cellulose such as wood and straw, which a village cannot break down. Almost every usable feedstock is also food, so each litre of fuel is made from something that could have been eaten.
Part of Ethanol Fuel Production
A feedstock is the plant material that supplies the sugar yeast turns into alcohol; the choice sets the work needed before fermentation and the food given up.
Sugar Ferments, Starch Needs One Extra Step, Cellulose Needs A Factory
Yeast eats only simple sugars (Sugar Sources). The three classes of plant material differ in how far they are from that.
| Class | Examples | Work before yeast can use it |
|---|---|---|
| Sugar | fruit, sugarcane, beets, sweet sorghum | crush or press, ferment the juice |
| Starch | corn, wheat, barley, potatoes, cassava | cook, then convert with the enzymes in malt |
| Cellulose | wood, straw, grass | acid or industrial enzymes; not realistic for a village |
Each class has its own page: Sugar Crops, Starch Crops and Cellulosic Sources. How much fuel a hectare gives is on Yield per Hectare Comparison.
Every Practical Feedstock Is Also Food
The parent guide’s table gives corn 370-400 litres of ethanol per tonne. Turn that around:
- Grain per litre: 1,000 kg ÷ 400 = 2.5 kg, and 1,000 kg ÷ 370 = 2.7 kg.
- Food energy in that grain: the caloric guide gives corn 1,650 kcal per lb, which is 1,650 ÷ 0.4536 = 3,638 kcal per kg (Crop Calorie Yields Per Acre). So 2.5 kg is about 9,100 kcal and 2.7 kg about 9,800 kcal.
- In person-days at the 2,800 kcal a day the community guides use: 9,100 ÷ 2,800 = 3.2 and 9,800 ÷ 2,800 = 3.5.
- Energy in the fuel: a litre of ethanol holds about 21.1 MJ (26.8 MJ/kg from the alcohol-fuel guide times 0.789 kg/L), which is 21,100 ÷ 4.184 = about 5,040 kcal.
So one litre of grain ethanol uses 3.2-3.5 person-days of food and returns about 51-55% of that energy as fuel (5,040 ÷ 9,800 and 5,040 ÷ 9,100), before a single joule is spent heating the mash or the still. The distillers’ grain left over returns some protein as feed (Distiller’s Grain).
Use Feedstocks In This Order
- Waste first: fruit pomace after juicing, windfalls too bruised to eat, spoiled grain, potato peelings.
- Surplus that would rot: a glut of apples or plums with no way to store it.
- Crops grown for fuel on poorer ground: sweet sorghum and beets, which still occupy land that could grow food.
- Food-grade grain last, and not while anyone in the community is short of calories (Winter Calorie Gap Planning).
What’s Next
- Learn next: Fermentation, the step that turns the feedstock’s sugar into alcohol
- Combine with: Yield per Hectare Comparison, which puts each crop’s fuel against its food