Enhanced Microwave Induced Thermochemical Conversion of Waste Glycerol for Syngas Production

Abstract

Glycerol waste from biodiesel production can be converted to syngas (CO+H2) via a thermochemical conversion process. In this study, microwave was used to initiate a glycerol conversion reaction in a specially fabricated quartz tube reactor with a silicon carbide bed as the microwave absorber. A nickel-based catalyst and steam were added to the reacting bed to enhance production of hydrogen. By adjusting the microwave power level from 110 to 880 watt (W), the reaction temperature of 500°C to more than 1400°C could be rapidly achieved within a few minutes, which is much faster than heating by conventional furnaces. The gasification reaction commenced by feeding raw material continuously through the hot silicon carbide bed at a rate of 1 g/min with the O2 to fuel ratio varying from 0-0.25. The overall time for each trial was 20-30 minutes including preheating of the bed material. In contrast to typical biomass gasification, char and tar yields were small in most runs. In general, glycerol waste yielded higher syngas when compared with pure glycerol conversion. Complete conversion to gas product may be achieved at a power level of 440W. The maximum syngas production from glycerol waste without a catalyst was more than 23.98 L over 20 min run at 660 W with 0.25 O2 to fuel ratio. Overall content of other hydrocarbon gases was around 3-28 vol.% depending on operating conditions and raw material. Lower heating values (LHV) of product gas for glycerol waste were much higher for runs at 1.0 L/min carrier gas flow, ranging from 3.75-17.64 MJ/m3 while relatively stable LHV of 1.96-5.88 were obtained from 2.0 L/min flow. The addition of a catalyst significantly increased gas production at lower wattage runs where overall conversions were comparable to those of higher wattage experiments without catalysts. The maximum total conversion and LHV were obtained from 1%Ni/SiC catalyst at a reaction temperature of 600°C (330W) and no external O2 with a gas product heating value of 9.18 MJ/m3 and 1.32 H2 to CO ratio. From these results, the novel microwave-induced heating technique can be considered as an efficient option for conversion of glycerol waste via the gasification process to acceptable quality syngas.