Small-Scale Production of Fuel-Cell Hydrogen

Sammanfattning: The work presented here aims at producing hydrogen for proton-exchange fuel-cell applications from hydrocarbons. This thesis describes an integrated catalytic converter comprising a steam-reforming reactor (heat of reaction supplied by catalytic combustion), a water-gas shift reactor and a preferential oxidation of carbon monoxide reactor, and the work performed on the fuel processing catalysts therein. The first section describes the fuel processing unit itself and its performance. A number of hydrocarbons were used as feedstock, both gases (methane, natural gas, propane) and liquids (methanol, ethanol, kerosene). The reactor system was optimised concerning the steam-to-carbon ratio in the reformer, steam to carbon monoxide in the water-gas shift, oxygen to carbon monoxide in the preferential oxidation etc. The reactor system performed according to specifications, achieving 106 % of the 10 Nm3/h of hydrogen that it was designed for (natural gas as feedstock), with good emission control (carbon monoxide below 10 ppm and methane acceptable). The reactor performed satisfactorily with the other feedstocks (methane, propane, methanol, ethanol and kerosene) as well. The reactor start-up behaviour was investigated and the start-up time from fuel-feed to reformate with less than 20 ppm carbon monoxide was lowered from 30 to 20 min by using partial oxidation/auto-thermal reforming during start-up. A long-term experiment with a 3 Nm3/h of hydrogen unit showed acceptable performance for the ~500 h it was run. The second section deals with the fuel-processing catalysts used, starting with the steam-reforming catalyst. There has been work performed both on nickel- and noble-metal-based catalysts. By doping the nickel-based catalysts with rare earth metals, the sulphur resistance was improved as well as the regenerability of the catalysts. The regenerability of the nickel-based catalysts was also improved by substitution of nickel into a hexa-aluminate structure. The improvements in the sulphur tolerance of the nickel-based catalysts were, however, not great enough and this, in association with lower activity, made noble-metal-based catalyst the choice for the steam reformer. Various noble-metal-based catalysts were manufactured and evaluated, and one of them showed both good initial activity and was stabile under the reaction conditions, even compared to a commercial catalyst formulation. In the water-gas shift, a Pt-based CeO2 catalyst was investigated, a catalyst with high initial activity and susceptible to a high initial rate of deactivation. The catalysts were characterized by H2 and CO temperature programmed reduction, and the gas species with the highest impact on the rate of deactivation was concluded to be carbon monoxide. It was also concluded that the rate of deactivation was counteracted by the presence of water. The catalyst was stabilised in various ways both by adding dopants to the active phase and to the CeO2-based carrier. The stabilisations resulted in a more stabile catalyst with equal or higher initial activity. Doping with tungsten (W) had a positive effect on the Pt/ CeO2 catalyst whilst doping the CeO2 with zirconia had the greatest impact. The regeneration behaviour of the catalyst was investigated and it was shown that steam and soft oxidation in air had a regenerative effect. The sulphur tolerance of the catalyst system was also investigated and the results show that the catalysts are affected by sulphur; even in as low concentrations as 3 ppm, but that the effect is reversible. The discussion takes up the various deactivation mechanisms proposed for this system in the literature and conclusions are drawn concerning proposed deactivation mechanisms etc. Several noble-metal-based catalysts were investigated for their activity, selectivity and susceptibility to side reactions in the selective oxidation of carbon monoxide in hydrogen-rich streams. A total of 15 catalysts were tested, 5 different noble metals mounted both on NiO and CoO and on γ-alumina. The catalyst showing the best activity/selectivity was Pt mounted on CoO and this was further investigated and characterised. It was shown that as the activity increased with increased Pt cluster size, the selectivity reached a maximum and this information was used to make predictions about a sample run on-stream for 1000 h. This sample showed high activity but poor selectivity, leading to the conclusion that a growth of Pt crystallite size is a possible reason for catalyst aging. By using high-resolution transmission electron microscopy with X-ray energy-dispersive spectroscopy and a high-angle annular dark-field detector it was concluded that there were Pt particles on the larger CoO particles. These Pt particles were concluded to be the ones causing increased activity and selectivity. The major conclusion of this study is that it is possible to produce fuel-cell-quality hydrogen by using an integrated catalytic approach to reactor design. By using this design and the developed catalysts described, it is also possible to achieve good power density of the system (1 kWe/dm3 of reactor volume, including water gas shift and preferential oxidation reactor).