Experimental study of the supercritical water reforming of glycerol without the addition of a catalyst

• Glycerol reforming using supercritical water without adding a heterogenous catalyst is feasible.
• The use of a glycerol concentration as high as 30 wt.% produces a gas with high hydrogen yield.
• The effect of operating variables on the process performance and approach to chemical equilibrium was discussed.
• Two probable reaction pathways were proposed that make it possible to explain the experimental results.
• The reactor material (Inconel 625) showed a high catalytic activity, which was decreasing to reach a steady state.

Hydrogen production from the supercritical water reforming of glycerol was studied in a tubular reactor without adding a catalyst. Experiments were carried out at a pressure of 240 bar, temperatures of 750–850 °C, and glycerol feed concentrations of 5–30 wt.%. Likewise, the residence time was changed from 12 to 160 s, by handling the feed flow-rate. The dry gas is mainly consisted of H2, CO2, CO, CH4. In addition, small concentrations of glycerol were measured in the liquid phase analysis, but barely traces of others like glycolaldehyde, glyceraldehyde, dihydroxyacetone and lactic acid were detected. Thus, two probable reaction pathways are discussed, which makes it possible to explain the experimental results by using a method applicable to other similar processes. The results showed that the glycerol conversion was almost complete, except at the highest glycerol feed concentration, in which the conversion was of 88%. Hydrogen yields from 2 to 4 mol H2/mol glycerol were obtained at high and low glycerol feed concentrations, respectively, when operating at high temperature and residence time. Besides, it was verified the catalytic effect of the reactor material (Inconel 625) from the trend of the gas product yields with time and the structured carbon nanotubes encountered. The catalytic activity of the reactor material was decreasing to reach a steady state after a few tens of operating hours. This study illustrates that the reforming of glycerol using supercritical water without added catalyst is feasible to achieve a high-yield hydrogen production, and it encourages to continue the research line, to obtain a process economically interesting.