Analysis and optimisation of H2 production from crude glycerol by steam reforming using a novel two step process

•New two-step concept developed and optimised for H2 production from crude glycerol•pH influences glycerol purification with CH3COOH; an optimum occurs at a pH = 6.•Thermodynamic gas composition is experimentally achieved at T > 550 °C.•High H2 selectivity obtained from concentrated crude glycerol solutions (10–50 wt.%)•Optimum H2 production found at 680 °C, 37 wt.% glycerol and 3 g cat min/g glycerol

This work studies the valorisation of biodiesel-derived glycerol to produce a hydrogen rich gas by means of a two-step sequential process. Firstly, the crude glycerol was purified with acetic acid to reduce problematical impurities. The effect of the final pH (5–7) on the neutralisation process was addressed and it was found that a pH of 6 provided the best phase separation and the greatest glycerol purity. Secondly, the refined glycerol was upgraded by catalytic steam reforming and this step was theoretically and experimentally studied. The theoretical study analyses the effect of the temperature (400–700 °C), glycerol concentration (10–50 wt.%) and N2 (225–1347 cm3 STP/min) and liquid flow (0.5–1 mL/min) rates on the thermodynamic composition of the gas. The results show that the temperature and glycerol concentration exerted the greatest influence on the thermodynamics. The experimental study considers the effect of the temperature (400–700 °C), glycerol concentration (10–50 wt.%) and spatial time (3–17 g catalyst min/g glycerol) on the product distribution in carbon basis (gas, liquid and solid) and on the composition of the gas and liquid phases. The experiments were planned according to a 2 level 3 factor Box–Wilson Central Composite Face Centred (CCF, α: ± 1) design, which is suitable for studying the influence of each variable as well as all the possible interactions between variables. The results were analysed with an analysis of variance (ANOVA) with 95% confidence, enabling the optimisation of the process. The gas phase was made up of a mixture of H2 (65–95 vol.%), CO2 (2–29 vol.%), CO (0–18 vol.%) and CH4 (0–5 vol.%). Temperatures of 550 °C and above enabled thermodynamic compositions for the gas to be achieved and helped diminish carbon formation. A possible optimum for H2 production was found at a temperature of around 680 °C, feeding a glycerol solution of 37 wt.% and using a spatial time of 3 g catalyst min/g glycerol. These conditions provide a 95% carbon conversion to gas, having the following composition: 67 vol.% H2, 22 vol.% CO2, 11 vol.% CO and 1 vol.% CH4.