Waste glycerol gasified to make hydrogen

CANADIAN chemical engineers have found that subjecting glycerol and methanol biodiesel waste mixtures to a supercritical water gasification process makes useful hydrogen gas.

As the global production of biodiesel increases, so too does the production of glycerol. While it does have some uses, in food and pharmaceuticals for example, the costs of purification are prohibitive for biorefineries. The team from York University, Ontario, led by Janusz Kozinski, investigated the potential to produce clean-burning hydrogen from it instead. Canada alone produces almost 44,000 t/y of crude glycerol, with the potential to make enough hydrogen to fuel 26,800 cars.

Crude glycerol is a mixture of glycerol and unreacted methanol. Kozinski and the team tested mixtures of a variety of ratios of glycerol to methanol to simulate crude glycerol, to help understand the impact of the concentrations of the two components. They used four alkali catalysts – K₂CO₃, Na₂CO₃, KOH and NaOH. Supercritical water gasification uses supercritical water – at a temperature and pressure at which it is neither a liquid nor a gas – to gasify and reform an aqueous substance into other products, in this case glycerol into hydrogen. The water acts as a reactant, rather than just a solvent. The researchers carried out the supercritical water gasification process in a continuous flow tubular reactor, with a residence time of 45 seconds to prevent secondary reactions which would make methane. They used temperatures ranging from 450–600?C at two different pressures, 23 MPa and 25 MPa. The products of the reaction were separated and the gas portion passed through a moisture trap before being analysed by gas spectrometry.

The best yield of hydrogen, 25.3 mmol/g came from crude glycerol to methanol in a 1:1 ratio, at 600?C with the pressure at 25 MPa. The most effective catalyst was K₂CO₃ at 0.5 wt%. Lower temperatures were found to favour the formation of methane instead of hydrogen, while increased temperatures promote the breakdown of intermediates into smaller molecules. The carbon gasification efficiency was found to be above 85%, producing the hydrogen-rich syngas that the researchers say proves the process is viable for untreated crude glycerol waste at biorefineries.

“The results suggest that effluents from biodiesel refineries can be used as potential feedstock for waste-to-energy conversion,” say the researchers.

Chemical Engineering Research and Design DOI: 10.1016/j.cherd.2016.07.005