Ethanol and Biodiesel By-products
Depending on the feedstock and process design, ethanol production results in several by-products which may include crop residues, stillage, evaporator condensate, condensed solubles, spent cake and/or distillers grains, all of which have a high potential for methane production. Stillage, a residual of the distillation of ethanol from fermentation liquor, contains a high level of biodegradable COD as well as nutrients and has a high pollution potential. Up to 20 L of stillage may be generated for each liter of ethanol produced. Conversion of stillage to biogas and application of effluent to croplands results in a more sustainable ethanol production system.
Many ethanol plants minimize effluent discharges by evaporation of the stillage to produce evaporator condensate (used partially for make-up water) and condensed solubles. The evaporator condensate contains volatile fermentation products that can inhibit ethanol fermentation. Anaerobic digestion can remove these fermentation products and provide a liquid more suitable for process recycling. The distillers grains and condensed solubles are normally blended for use in animal feed as dried distillers grains and solubles (DDGS). However, the current rapid expansion of ethanol production could lead to saturation of the feed market with DDGS, affecting the sale value of this by-product. Thus, there is an opportunity for biogas production from these by-products to offset facility energy requirements. In cellulosic ethanol production, non-fermentable hydrolysis products can also be converted to methane. Finally, crop residues may also be harnessed for biogas production, which can greatly improve the energy yield from ethanol production.
Biodiesel is normally produced from either virgin plant oils or waste vegetable oils through a catalytic transesterification process. The typical biodiesel production process uses an alkaline hydrolysis reaction to convert vegetable oil into biodiesel using methanol, potassium hydroxide, and heat. A transesterification reaction splits the glycerol group from the triglyceride oils, producing methyl esters (biodiesel) and glycerol by-product. To purify the biodiesel, a washing process is employed to remove soaps, free fatty acids and excess methanol, producing a washwater by-product. For every 100 L of oil, approximately 25 L of methanol and 0.8 kg of KOH/NaOH is consumed, yielding around 75 L of biodiesel and 25 L of crude glycerol. The washing process produces another 30 L of biodiesel washwater. Both the crude glycerol and the biodiesel washwater have significant methane production potential. When vegetable oil is pressed from seeds (or algae), there is also a press-cake by-product along with crop residues from harvesting that are both amenable to biogas production. Conversion of biodiesel by-products to methane offers a sustainable treatment solution, while also providing additional energy. Methane can also be converted to methanol, an ingredient used in biodiesel production. Also, digester effluent could be used to grow algae for biodiesel production.
Stillage characterization and anaerobic treatment of ethanol stillage from conventional and cellulosic feedstocks.
Biomass and Bioenergy, 19(2):63-102 (2000). [PDF]
Biogas and Biofuel Synergies.
2007 Farm to Fuel Summit, St. Petersburg, Florida. 19 July 2007. [PDF]
Glycerol and Its Uses.
Biodiesel Production Workshop, Orange County Public Schools, Westside Tech, Winter Park, Florida. 2 June 2007. [PDF]