Bioethanol: Efficiency
Why cane ethanol returns far more energy than maize
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.For ethanol the useful measure is the energy ratio: energy in the fuel divided by the fossil energy used to make it. US corn ethanol returns about 1.5 without counting its animal-feed co-product and 2.1-2.3 with it; Brazilian sugarcane ethanol returns about 9.
Efficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| US corn ethanol, dry mill, no co-product credit | 1.5 energy ratio1 | In service | Energy in ethanol divided by energy used to grow corn and run the plant |
| US corn ethanol, dry mill, with co-product credit | 2.1 to 2.3 energy ratio1 | In service | As above, crediting the energy embodied in distillers grains sold as animal feed |
| Corn ethanol selling wet distillers grains (Iowa) | 4 energy ratio1 | Record | With co-product credit; the plant skips drying the grains (2.3 without credit) |
| Brazilian sugarcane ethanol | 9.3 energy ratio2 | In service | Renewable energy out per unit of fossil energy in, at the factory gate (7.6 after shipping to New Zealand) |
| Cellulosic ethanol from switchgrass | 5.4 energy ratio3 | Record | Renewable energy out per unit of non-renewable energy in, 10 US farms over 5 years (reported as 540% more renewable than non-renewable energy) |
| Process energy at the ethanol plant | 31 to 50% of ethanol energy1 | In service | Plant energy (23,424-38,141 Btu per gallon) divided by ethanol heat content (76,300 Btu per gallon) (calculated) |
Where the energy is lost
| Step | Loss |
|---|---|
| Cooking, distillation and drying of co-products at the plant | 31-50% of the ethanol's energy content (calculated)1 |
| Energy carried away in the distillers grains co-product | Not given as a share; it is why the ratio rises from 1.5 to 2.1-2.3 when credited1 |
Yeast ferments sugar into ethanol and CO2, so the chemistry is simple; the losses come from everything around it. Corn starch must be cooked and broken into sugar with enzymes, and after fermentation the ethanol must be distilled from the broth and the leftover grains dried, which is why a conventional US dry mill burns about 38,141 Btu for every gallon holding 76,300 Btu. A typical plant yields 2.76 gallons per bushel of corn.
Sugarcane does far better because the cane's fibrous residue (bagasse) is burned on site in combined heat and power plants that supply all the mill's steam and electricity and often export surplus power, so little fossil energy is needed. Cellulosic ethanol, made from straw, grasses or wood, could in principle combine high ratios with non-food feedstocks, but breaking down cellulose remains costly.
Sources
- 2015 Energy Balance for the Corn-Ethanol Industry (published February 2016) US Department of Agriculture, 2016
- The Sustainability of Brazilian Sugarcane Bioethanol: A Literature Review (Barber, Pellow and de Aragao Pereira, for EECA) AgriLINK NZ / Bioenergy Association of New Zealand, 2008
- Net energy of cellulosic ethanol from switchgrass (Schmer, Vogel, Mitchell and Perrin, PNAS 105(2):464-469) University of Nebraska-Lincoln Digital Commons (author record), 2008