Scaling down direct dimethyl ether synthesis for biogas conversion

Abstract

The feasibility of direct DME synthesis from biogas conversion as a small scale energy storage system was studied. Anaerobic digestion (AD) of grass silage was used as a feedstock for production of biogas and it was found that grass silage has high affinity for methane enriched biogas. A mathematical model was developed to investigate the effect of operation conditions in reforming of this biogas and it was found that flexible H2/CO ratios can be produced by controlling the O2/H2O ratio. The syngas produced from the above model was used to develop a mathematical model for direct DME synthesis. The water gas shift reaction was studied over CuO/ZnO/Al203 (CZA) and 1 % Au/CeZr04 catalysts and it was found that the gold based catalysts are superior to the copper based catalysts for low temperature WGS. The kinetics of the WGS reaction over fresh gold based catalyst and the deactivation kinetics model was then studied. It was found that the best fit of the experimental data was the surface redox kinetic model with surface oxidation as rate limiting step and water induced deactivation. Methanol synthesis was studied over a series of catalysts (CZA, Cu/Ce02, CuO/MnO/ZnO, 1% Au/CeZr04 and 5% Pd/AhC^) at different operating conditions: T=200-260 °C, P=20-50 bar, H2/CO molar ratio=2/l, GHSV=2640 mlg'V. It was found that CZA showed the highest activity and there was no effect of addition of gold. Different solid acid catalysts (NH4ZSM-5, HZSM- 5 or Y-AI2O3) as a dehydration function were physically mixed with CZA for direct DME synthesis from syngas. It was found that the CZA/ HZSM-5 bifunctional catalyst with 0.25 acid fraction showed high stability over a period of 212 hr.

Date of Award	Dec 2010
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	David Rooney (Supervisor)