Hydrothermal gasification of a biodiesel by-product crude glycerol in the presence of phosphate based catalysts

•The effects of phosphate based catalysts on SCWG of glycerol were investigated.•We studied with pure and crude glycerol for comparison.•Experiments were carried out at 300–600 °C and a pressure range of 8.5–33.0 MPa.•In the case of the production of hydrogen K2HPO4 and K3PO4 are more effective.•Acetaldehyde, propionaldehyde, and acetone identified in the aqeuous phase mainly.

Energy from biomass can be provided in various ways, such as burning the solid wastes, production of biogas (by anaerobic digesters), biofuels (i.e. methanol, ethanol, biodiesel, and derivatives), and methane via the utilization of landfills. Biodiesel is a widely used biofuel produced by the conversion of first-generation biomass feedstock via bio-chemical conversion platforms.Crude glycerol is the by-product of biodiesel production being 10 wt.% of the produced biodiesel. The objective of this study is to utilize this glycerol fraction by converting it to fuel gas or to chemical feedstock.In this study, the concentration of glycerol feedstock solution and the catalyst concentration were 50 g/L and 5 g/L, respectively. Crude glycerol was gasified in a sub and supercritical water medium by using a batch autoclave with an inner volume of 100 ml. A temperature range from 300 to 600 °C was studied. Experiments were performed with pure and crude glycerol samples in the absence and the presence of homogeneous acidic and alkali catalysts, namely H3PO4, KH2PO4, K2HPO4, and K3PO4. These were used to obtain higher gasification efficiencies and hydrogen and/or methane yields.Subsequent to each experiment liquid, solid, and gaseous products were collected and analyzed by GC, TCA (total carbon analyzer), HPLC, and GC/MS.The order of the effectiveness of the catalysts on gasification was found as: K3PO4 > K2HPO4 > H3PO4 > KH2PO4 for crude glycerol and K3PO4 > K2HPO4 > KH2PO4 > H3PO4 for pure glycerol. K2HPO4 and K3PO4 were found to be more effective in terms of hydrogen production while H3PO4 and KH2PO4 showed the best performance for the maximized methane production.