Effective utilization of crude glycerol is very crucial to the commercialization and further development of biodiesel production. In the long term, utilization of biomass-derived glycerol will not only contribute to reducing society's dependence on nonrenewable resources but also will promote the development of integrated biorefineries.
This review addresses the value-added opportunities for crude glycerol from biodiesel production, mainly as a feedstuff for animal feed and as feedstocks for chemicals. Promising results have been achieved by researchers, especially for non-ruminant animals such as pigs, laying hens, and broilers. However, there still are some considerations that need to be taken into account for broad-scale use of this biomass-derived chemical in animal feeds. First of all, the chemical composition of crude glycerol varies significantly with the methods and feedstocks used to produce biodiesel. Since different researchers have used different qualities of crude glycerol for their studies, animal producers need to pay attention when they decide to include crude glycerol as a component of animal feed rations. Secondly, to some extent the impurities in crude glycerol affect feed performance. Finally, the amount of crude glycerol included in feed formulations needs to be considered. The establishment of a crude glycerol feed specification is recommended. The resulting "standard" crude glycerol would have a higher value simply because it would be consistent from all producers.
Conventional catalysis and biotransformation are the two main routes available for converting crude glycerol to a variety of value-added chemicals. In more recent years, there have been extensive studies, and some encouraging results, on processes for crude glycerol conversion. For example, the productions of 1,3-propanediol, citric acid, poly (hydroxyalkanoates), butanol, hydrogen, docosahexaenoic acid-rich algae, monoglycerides, lipids and syngas from crude glycerol are promising. However, many of the technologies offered still need further development to make them cost-effective and operationally feasible for incorporation into biorefineries.
Impurities in crude glycerol can greatly influence the conversion of glycerol into other products. In some biological conversion processes, pollutants in crude glycerol, inhibit cell and fungal growth and result in lower production rates and product yields (compared with pure or commercial glycerol under the same culture conditions). On the other hand, for conventional catalytic conversions, impurities poison the catalysts, increasing char production and influencing product yield. Many of the technologies need to be more fully understood and optimized, such as optimizing reaction parameters, production yields, and fermentation conditions, developing mutant strains and efficient bioreactors for stable cultures, and improving the activity and selectivity of catalysts. Additionally, cell growth has been shown to be inhibited during fermentation when the initial crude glycerol concentration was high. Conversely, higher crude glycerol concentrations are necessary for improving production efficiency.
In brief, it has been shown that biological conversions of crude glycerol can produce higher value chemicals. However, it is impossible for the current state of the technologies to convert glycerol at rates necessary to prevent a large accumulation of glycerol. Both improved biological and conventional techniques will be needed in future biodiesel plants. Other promising uses, starting with crude glycerol, also have been disclosed. Although hopeful results have been achieved, there are still aspects that need to be improved.
In summary, in recent years, scores of utilization opportunities of crude glycerol have been presented and promising results have been achieved. However, it is imperative to point out that there are still more technical hurdles to jump for developing practical processes to directly utilize crude glycerol from biodiesel production on a large scale. Quick overviews of what has been investigated, with respect to potential chemicals from biological and conventional catalytic conversions of crude glycerol, are summarized in Tables and , respectively.
Biological conversions of crude glycerol to chemicals
Conventional catalytic conversions of crude glycerol to chemicals