As biodiesel manufacturing continues to skyrocket, glycerol, previously a sought-after by-product has become an increasingly threatening waste crisis (Sabourin-Provost and Hallenbeck, 2009). The most commercially used process in biodiesel production is the base-catalyzed trans-esterification of oil, which produces about 10 kg of glycerol per 100 kg of biodiesel. As demands for biodiesel multiply in order to reduce petroleum dependence and greenhouse gas emissions, the development of a feasible method to convert glycerol into a usable resource becomes increasingly critical. Fuel is one of the only viable resources that fulfills this long-term demand. Therefore, conversion of glycerol into either ethanol or hydrogen has been deemed the most apt solution. Ethanol production from crude glycerol has been met with many setbacks such as low yields and impracticality. Thus, current research looks to hydrogen, which is currently being investigated as a future fuel, to resolve the glycerol crisis.—Alec Faggen
Sabourin-Provost, G., Hallenbeck, P., 2009. High yield conversion of a crude glycerol fraction from biodiesel production to hydrogen by photofermentation. Bioresource Technology 100, 3513–3517.
Sabourin-Provost and Hallenbeck working in the Department of Microbiology and Immunology at the University of Montreal found that the purple non-sulfur photosynthetic bacterium, Rhodopseudomonas palustris, photoferments glycerol to hydrogen via an active nitrogenase. The authors of the paper were able to collect 6 moles of hydrogen gas/mole of glycerol, (almost 75% of theoretical calculations). Although not required for synthesis, the addition of a nitrogen source maximized hydrogen formation. Crude glycerol and pure glycerol had about the same yields of hydrogen production.
A number of benefits to hydrogen manufacturing exist. Hydrogen is a water insoluble product; thus, it is relatively easy to collect in comparison to water-soluble products such as ethanol. Furthermore, necessary dilutions, in order to counter possible contaminant complications, are not as consequential if producing a gaseous product like hydrogen.
Nonetheless, many obstacles subsist before hydrogen synthesis from glycerol can be efficient. An economic photobioreactor where the reactions can take place is yet to be constructed. The photobioreactor must be transparent so light can catalyze the reactions and also hydrogen-impermeable so the hydrogen gas can ultimately be collected. In addition, R. palustris is inefficient at utilizing light, and future research should investigate optimal light intensity for these organisms.