EVC3: Sugar and starch fermentation to ethanol
Sugar crops are predominantly sugar cane, but also sugar beets and sweet sorghum are used. These plants produce sucrose, a dimer of C6 sugars such as mainly glucose and fructose. By milling and leaching at slightly elevated temperatures the sucrose is extracted into approximately 20 wt% sugar juice, which is then pre-treated by clarification and a heat treatment. A sugar mill may produce only sugar, sugar and ethanol in parallel on a more or less equal scale or mainly sugar and some ethanol from molasses (concentrated syrup residue after sugar crystallisation) only. Yeast and nutrients are injected into the clarified juice and routed to fermenters where the sucrose is enzymatically split into the C6 sugars and then fermented to alcohol (“wine”).
In the case of starch crops, mainly wheat, barley, corn (maize) but also e.g. cassava, the initial step is dry milling the crop grains, separation of the starch “meal” and addition of water and enzymes to obtain the starch as a thick gel slurry. There is also a wet milling process where the grains are soaked in a dilute sulphuric acid solution prior to milling and recovery of the starch, but also with possibilities for a range of valuable by-products. Starch is a polymer of C6 sugars - mainly glucose -, and enzymes are added to the slurry (“mash”) to depolymerise the starch to release the sugars. The slurry is then heat treated and sent to fermenters where yeast and nutrients are added, and the sugars converted to alcohol.
The glucose-to-ethanol reaction is represented by the equation below:
C6H12O6 + 2 ADP + 2 Pi → 2 C2H5OH + 2 CO2 + 2 H2O + 2 ATP
The fermentation process takes 1-2 days to complete. In both cases (sugar crops, starch crops), ethanol is recovered from the “wine” by a typically two-stage distillation to produce approx. 94 wt% ethanol (hydrous ethanol, used as E100 in Brazil) followed by mol sieve dehydration to reach above 99 wt% minimum ethanol content (dehydrated or anhydrous ethanol) for blending into gasoline.
In addition to ethanol (and sugar), by-products from cane ethanol are CO2 from the fermentation, bagasse fibre used as fuels and vinasse recycled as fertiliser. By-products from the starch crop-based ethanol is in addition to CO2, dry distillers grain solids (DDGS) used as cattle fodder and depending on the feedstock and milling technology, also technical corn oil, starch, syrup, gluten and bran. Due to increased requirements for GHG reduction and also because it is a revenue-generating by-product from a waste stream, anaerobic digestion technologies for residue streams are being more and more integrated into ethanol production.
In the EU, ethanol from these pathways is a biofuel and is subject to a cap.
The global production of bio-ethanol was 108 000 billion liters[1] (86 million tonnes, 638 TWh or 55 Mtoe, (tonnes of oil equivalents), of which over half was in the USA (some 200 plants) and one quarter in Brazil (close to 400 plants[2]), and 5 % in the EU (some 50 plants[3]), with no other country producing above 5 %. The feedstock used is 46 % corn, 38 % sugar cane, 5 % wheat and then followed by molasses and other crops.
Fact Sheet: Ethanol
Acknowledgement: Large parts of the texts were taken from Lars Waldheim´s contribution to the report “The Contribution of Advanced Renewable Transport Fuels to Transport Decarbonisation in 2030 and beyond”
[2] USDA Gains Report Brazil 2018
[3] USDA Gains Report EU 2018