There are various outlets for disposal and utilization of the crude glycerol generated in biodiesel plants. For large scale biodiesel producers, crude glycerol can be refined into a pure form and then be used in food, pharmaceutical, or cosmetics industries. For small scale producers, however, purification is too expensive to be performed in their manufacturing sites. Their crude glycerol is usually sold to large refineries for upgrading. In recent years, however, with the rapid expansion of biodiesel industry, the market is flooded with excessive crude glycerol. As a result, biodiesel producers only receive 2.5-5 cents/lb for this glycerol. Therefore, producers must seek new, value-added uses for this glycerol.
There have been many investigations into alternative uses of crude glycerol. Combustion, composting, animal-feeding, thermo-chemical conversions, and biological conversion methods for glycerol usage and disposal have all been proposed. Johnson and Taconi (2007) reported that combustion of crude glycerol is a method that has been used for disposal. However, this method is not economical for large producers of biodiesel. It has also been suggested that glycerol can be composted or used to increase the biogas production of anaerobic digesters . DeFrain et al. (2004) attempted to feed biodiesel-derived glycerol to dairy cows in order to prevent ketosis, but found that it was not useful.
Also, Lammers et al. (2008) studied supplementing the diet of growing pigs with crude glycerol. This study found that the metabolizable-to-digestible energy ratio of glycerol is similar to corn or soybean oil when fed to pigs. Therefore, the study concludes that “crude glycerol can be used as an excellent source of energy for growing pigs,” but also cautions that little is known about the impacts of impurities in the glycerol. Furthermore, Cerrate et al. (2006) have had some success with feeding glycerol to broiler chickens. Birds fed 2.5 % of 5% glycerin diets had higher breast yield than the control group, but the authors caution that there is still concern about methanol impurities in the glycerol.
Converting crude glycerol into valued-added products through thermo-chemical methods or biological methods is an alternative for utilizing this waste stream. It has been reported that glycerol can be thermochemically converted into propylene glycol , acetol , or a variety of other products . Cortright et al. (2002) have developed an aqueous phase reforming process that transforms glycerol into hydrogen. Virent Energy Systems is currently trying to commercialize this technology and claim that sodium hydroxide, methanol, and high pH levels within crude glycerol help the process .
For biological conversions of crude glycerol, the glycerol serves as a feedstock in various fermentation processes. For example, Lee et al. (2001) have used glycerol in the fermentation of Anaerobiospirillum succiniciproducens for the production of succinic acid. The fermentation of E. coli on glycerol leads to the production of a mixture of ethanol, succinate, acetate, lactate, and hydrogen . Glycerol can also be converted to citric acid by the yeast Yarrowia lipolytica. It has been reported that this organism produces the same amount of citric acid when grown on glucose or on raw glycerol. Rymowicz et al. (2006) found that acetate mutant strains of Y. lipolytica can produce high levels of citric acid while producing very little isocitrate. Furthermore, it has been shown that Clostridium butyricum can utilize biodiesel-derived glycerol to produce 1,3-propanediol (an important chemical building block with many industrial uses) in both batch and continuous cultures. During the fermentation process, the organism also produces byproducts of acetic and butyric acid . The researchers at Virginia Tech also developing algal fermentation processes to convert crude glycerol into high value omega-3 polyunsaturated fatty acids