Processing of glycerol under sub and supercritical water conditions

Converting glycerol, a by-product from biodiesel production into useful products and energy could contribute to a positive life cycle for the biodiesel process. One kg of glycerol is produced for every 10 kg of biodiesel and has the potential to be used as a source of H2, syngas or CH4 by an appropriate conversion process. Catalytic Supercritical Water Gasification (CSCWG) processing of crude glycerol solutions is one such viable option. Above its critical point [>221 barg, >374 °C], the properties of water, such as the low relative permittivity and high ionic product make it capable of dissolving non-polar organic compounds, allowing for high reactivity, and the ability to act as an acid/base catalyst. In this work, the degradation of glycerol by CSCWG at temperatures [400–550 °C] and pressures [170–270 barg] was investigated using a packed bed reactor (PBR) containing a Fe2O3 + Cr2O3 catalyst. Glycerol feed concentrations were between 2 and 30 wt% at flow rates from [10–65 ml/min], which gave weight hourly space velocities (WHSV) of [38–125 h−1]. The results indicated that high temperature and low feed concentration tended to increase the gas yield and selectivity toward H2 production with some char (<2.7 wt%). Syngas of up to 64 mole% was obtained with minimum 4:1 mole ratio of H2:CO. High yields of volatile hydrocarbons were also obtained: 14 and 69 mole % for methane and ethylene, respectively, which could be used for energy generation in SOFCs or turbines, reformed to syngas or converted to chemicals by an appropriate route. Pressure had little effect on the gas yields in the subcritical water region, but had a positive effect on H2 and CO2 in the supercritical region where char formation also was increased resulting in loss of catalyst activity. Complete conversion of glycerol was achieved at high temperature (550 °C). A maximum of 11 wt% liquid products were obtained at 400 °C (mainly allyl alcohol, methanol and formaldehyde). Catalyst stability was also evaluated, which was found to reach relative stability in the supercritical water environment for up to 9 h of operation.