Glycerol reforming in supercritical water; a short review

Due to the rise in global biodiesel production, the amount of crude glycerol, the main byproduct, has increased steadily. Identification of high value added outlets for crude glycerol has been explored in detail to increase the overall economics of the biodiesel process. Examples are the use of glycerol for the synthesis of green biobased chemicals and the conversion to hydrogen and/or syngas. Supercritical water has received considerable attention as reaction medium for glycerol reforming because it allows the processing of streams with high moisture content and the production of clean gas at high pressure, being attractive from an environmental point of view. The recent interest in supercritical water reforming of biomass, especially glycerol, has created the need for the available data in this specific field to be collected and compared. This review, the first to deal exclusively with glycerol processing, focuses on the production of gas and of organic components in supercritical water. Both catalytic and non-catalytic reforming of glycerol is considered and thermodynamics are addressed. The results suggest that the process conditions during supercritical water reforming of glycerol should depend on the aimed products. Low temperatures, high pressures, concentrated feed solutions and acidic catalyst will generate liquid products, with acrolein as the main compound. For gasification, high temperatures, lower pressures, dilute feed solutions and alkali or metal catalysts should be used.