Steam reforming of dimethyl ether for generating hydrogen-rich fuel-cell feeds.

摘要

This research evaluates both the thermodynamic and experimental aspects of dimethyl ether as a hydrogen carrier for fuel-cell feeds. The thermodynamics of generating hydrogen from dimethyl ether via partial oxidation, by steam reforming, or by the combined processes of partial oxidation and steam reforming were investigated as functions of temperature (100°C--600°C), steam-to-carbon ratio (0.00--4.00), oxygen-to-carbon ratio (0.00--2.80), pressure (1--5 atm) and product species. Thermodynamically, the resulting optimal processing conditions for dimethyl ether steam reforming occurred at a steam-to-carbon ratio of 1.50, a pressure of 1 atm and a temperature of 200°C, resulting in a hydrogen production efficiency of 97%. These results also showed that dimethyl ether hydrolysis to methanol, the first step in the steam reforming process to form hydrogen, was equilibrium limited.; Modeling the start-up energies and efficiencies of an automotive scale on-board fuel processor was also carried out as a function of fuel source (methane, methanol, ethanol, dimethyl ether and gasoline). Results demonstrated that a fuel processor utilizing dimethyl ether yielded the highest overall efficiency.; Dimethyl ether hydrolysis to form methanol (the desired intermediate) was studied experimentally over a series of as-received catalysts of varying acidity (zeolites Y and ZSM-5, ZrO2, gamma-Al2O 3, SiO2 and BASF K3-110) and also a series of in-house prepared composite catalysts containing copper and zinc, using a packed-bed reactor. All the acid catalysts, with the exception of ZrO2, attained the previously calculated equilibrium-limited conversions to methanol in the temperature range of interest (125°C--400°C).; Several homogeneous physical mixtures containing both an as-received acid catalyst and BASF K3-110 (a methanol-to-hydrogen catalyst) were used to examine the process of converting dimethyl ether to hydrogen, (i.e., dimethyl ether steam reforming). Hydrogen production efficiencies exceeding 95% and hydrogen production rates exceeding 85 mmoles gcat -1 h-1 were observed. In-house prepared composite catalysts, consisting of copper and zinc loaded onto a solid-acid substrate via incipient wetness or co-ion exchange, were also evaluated for dimethyl ether steam reforming activity, but were generally found less active than the physical mixture catalysts. Variations in catalyst activity and selectivity are discussed in terms of zeolite characteristics.; In situ DRIFTS experiments were also performed.