A hectare of corn made into ethanol gives 3,800-4,500 litres only at mechanised yields of 9.5-12.2 tonnes; at the 1.7-3.4 tonnes per hectare of hand-worked ground the same corn gives 622-1,345 litres, and every litre uses about 3 person-days of food energy while returning roughly half of it as fuel. Counting the heat for mashing and distilling, a plain wood-fired still on the stated assumptions burns more energy as wood than the alcohol it makes, so grain ethanol is a convenience fuel, not an energy gain.
Part of Ethanol Fuel Production
The arithmetic behind ethanol yields: what each crop gives per hectare, what the same land would feed, and what the process costs in heat.
What The Parent’s Table Implies About The Field
The parent gives litres of ethanol per hectare and per tonne. Dividing one by the other recovers the crop weight per hectare the table assumes.
| Crop | L/ha | L/tonne | Implied crop (t/ha) |
|---|---|---|---|
| Sugarcane | 6,000-8,000 | 70-80 | 75-114 |
| Sugar beet | 5,000-6,500 | 80-100 | 50-81 |
| Sweet sorghum | 3,500-5,000 | 50-60 | 58-100 |
| Corn | 3,800-4,500 | 370-400 | 9.5-12.2 |
| Wheat | 2,500-3,500 | 340-380 | 6.6-10.3 |
| Potatoes | 2,500-4,000 | 95-110 | 23-42 |
| Apples | 1,500-2,500 | 60-70 | 21-42 |
(Lowest L/ha ÷ highest L/t, highest ÷ lowest. Corn: 3,800 ÷ 400 = 9.5 and 4,500 ÷ 370 = 12.2.) These are mechanised, fertilised, well-watered yields.
The Same Land At The Corpus’s Hand-Worked Yields
The caloric guide’s non-industrial yields (Crop Calorie Yields Per Acre) are in pounds per acre; 1 lb/acre is 1.1209 kg/ha. Applying the parent’s own litres per tonne:
| Crop | Hand-worked yield | kg/ha | Ethanol (L/ha) | Same land as food (person-years at 2,800 kcal/day) |
|---|---|---|---|---|
| Corn | 1,500-3,000 lb/acre | 1,681-3,363 | 622-1,345 | 6.0-12.0 |
| Wheat | 800-1,500 lb/acre | 897-1,681 | 305-639 | 2.9-5.5 |
| Potatoes | 8,000-15,000 lb/acre | 8,967-16,813 | 852-1,849 | 6.8-12.7 |
| Beets | 15-25 short tons/acre | 33,600-56,000 | 2,690-5,600 | not computed |
Corn: 1,681 kg/ha (1.7 tonnes) × 370 ÷ 1,000 = 622 and 3,363 kg/ha (3.4 tonnes) × 400 ÷ 1,000 = 1,345. Person-years: acres converted to hectares (÷ 0.4047), times kcal per acre, ÷ (2,800 × 365 = 1,022,000). So hand-worked corn gives only 16-30% of the parent’s litres per hectare, wheat 12-18% and potatoes 34-46%. The beet row uses the sugar guide’s 15-25 tons per acre, which that guide itself shows does not fit its own plant spacing (Growing Sugar Beets at Scale); treat it as unverified.
Two Of The Parent’s Sentences Cannot Both Be True
The parent says a hectare of corn gives “roughly 4,000 liters of ethanol OR feeds 8-10 people for a year”. At the 9.5-12.2 t/ha that 4,000 L needs, the corn holds 9,500-12,200 kg × 3,638 kcal/kg = 34.6-44.4 million kcal, which feeds 34-43 people at 2,800 kcal a day (47-61 at 2,000). The 8-10 people figure belongs to hand-worked yields (6.0-12.0 person-years above). The two halves of the sentence use different fields. Its other number holds up: 4,000 L × 21.1 MJ ÷ 32 MJ per litre of gasoline is about 2,640 litres of gasoline-equivalent, against the parent’s “about 2,500”.
Food-versus-fuel, litre for litre: 2.5-2.7 kg of corn per litre, 9,100-9,800 kcal in, 5,040 kcal out (Feedstocks).
Heat Balance Per Litre Of Ethanol
Inputs (assumptions marked A are for you to replace with your own):
- Wash at 10% alcohol by volume: 100 L contains 10 L of ethanol and weighs about 98.5 kg, so about 9.9 kg of wash per litre of ethanol (A).
- Heat the wash from 25 to about 92°C, specific heat 4.1 kJ/kg·K: 9.9 × 4.1 × 67 = 2,720 kJ.
- Boil off the product: 1 L of ethanol at 95% by volume is about 1.05 L, about 0.85 kg. Latent heat of a 93-94% ethanol vapour is about 930 kJ/kg (846 kJ/kg for ethanol and 2,257 for water, weighted by mass; recalled constants). With a reflux ratio R, the boiler produces (R + 1) times the product as vapour: R = 2 gives 3 × 0.85 × 930 = 2,380 kJ; R = 4 gives 5 × 0.85 × 930 = 3,950 kJ.
- Mashing grain: about 10 kg of mash heated from 15°C to 70°C (4.0 kJ/kg·K) is 2,200 kJ; to 100°C for cooking corn, 3,400 kJ.
Total at 100% heat transfer: 2,720 + 2,380 + 2,200 = 7.3 MJ up to 2,720 + 3,950 + 3,400 = 10.1 MJ per litre, which is 35-48% of the 21.1 MJ in the litre. A plain firebox under a pot delivers perhaps 25% of the wood’s heat to the liquid (A). Then the wood burned carries 29-40 MJ per litre, and at about 15 MJ/kg for air-dry wood (A) that is 1.9-2.7 kg of wood per litre. The energy in the wood exceeds the energy in the fuel.
Heat recovery changes this, which is why large plants do better, and why sugarcane stills in the tropics burn the pressed stalks to run their own heat. Published full-life-cycle studies of grain ethanol disagree about whether it sits just above or just below break-even; this page does not choose between them, because the answer depends on exactly the assumptions marked A.
Verdict, With The Inputs Open
Grain ethanol turns food, wood heat and labour into a liquid that stores and pours. It is a good deal when the feedstock is waste and the heat is waste heat, and a poor one when it is food grown for the purpose. The parent’s last conclusion, that growing food is almost always the better use of farmland, follows from the arithmetic above.
Sources And Assumptions
- Molar masses from atomic masses: glucose 180.16, ethanol 46.07, starch unit 162.14, sucrose 342.30 g/mol.
- Ethanol 0.789 kg/L and LHV 26.8 MJ/kg (the corpus’s cooking-fuel table); gasoline about 32 MJ/L (recalled).
- Pasteur’s 48.4 g ethanol per 100 g glucose; specific heats and latent heats are standard tables (recalled, not opened here).
- Wash strength, reflux ratio, firebox efficiency and wood heating value (marked A) are assumptions.
What’s Next
- Learn next: Distillation, where most of the heat is spent
- Combine with: Caloric Budget for a Community, which sets what the land must feed first