Hydrogen Production in Supercritical Water
Type of Degreedissertation
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Hydrogen has been considered as an environmentally friendly energy carrier for the future. It is not found freely in nature but must be obtained by processing hydrogen containing compounds. Current industrial-scale hydrogen production relies on the fossil fuel feedstocks. The processing of fossil fuels for hydrogen production may represent a more efficient use, but ultimately still contributes greenhouse gases into the atmosphere. Fossil fuels are not the only possibility for a hydrogen containing feedstock, however. Biomass and biomass-derived materials also can be processed to generate hydrogen. Carbon dioxide generated during hydrogen production from biomass will later be fixed by plants during photosynthesis, thus creating a closed loop with no net increase in CO2. The properties of water above its critical point (T: 374°C, P: 221 bar) are markedly different than under ambient conditions, making it interesting both as solvent and reactant. In the supercritical phase the dielectric constant of water is greatly reduced and accordingly it behaves as an organic solvent, easily dissolving many organic species and gases while precipitating polar salts. As a homogenous phase with low viscosity and high diffusivity, transport limitations can be overcome in supercritical water. Physical properties of supercritical water such as density, heat capacity, and ion product can be tuned by small changes in temperature and pressure to enhance reaction rate and reduce volume requirement for reactors. A further benefit conferred by the supercritical water process is that water is compressed in the liquid state allowing the produced hydrogen to be obtained directly at high pressure without the need for energy intensive compression. Four biomass-derived compounds have been examined as feedstocks for hydrogen production in supercritical water by catalytic reforming in a continuous flow reactor. The flow-type reactor allowed the attainment of short residence times of seconds unavailable to previous researchers operating batch reactors. First, glucose was used as a model compound for biomass (Chapter 2). The presence of the ruthenium catalyst greatly increased the conversion and hydrogen yield from glucose while significantly reducing char and tar formation. Feed concentrations of up to 5 wt% glucose gave a hydrogen yield near the theoretical maximum at 700°C with a residence time of only two seconds. Ethanol (Chapter 3) was investigated as a feedstock for hydrogen production as it is already produced for use as an automotive fuel additive, however its conversion to hydrogen for use in a fuel cell would greatly increase its efficiency. Full conversion to gaseous products was seen above 700°C with no coke formation being observed below 10 wt% ethanol feed. Varying pressure from 221 to 276 bar had little effect on the gas yields. The third biomass-derived feedstock used was glycerol (Chapter 4), which is obtained as a byproduct from biodiesel manufacturing by transesterification of vegetable oils. Hydrogen yields near the theoretical limit were obtained for dilute solutions with a 1s residence time at 800°C, while hydrogen yields dropped with longer residence times due to methanation. Feed concentrations of up to 40 wt% glycerol were also gasified at 800°C and 1 s residence time with no coke formation and the yield of product gases closely following equilibrium values. Liquefied switchgrass biocrude was evaluated as the fourth feedstock for hydrogen production in Chapter 5. Nickel, cobalt, and ruthenium catalysts were prepared on titania, zirconia, and magnesium aluminum spinel supports to create a suite of nine catalysts. These were evaluated for hydrogen production by gasification of switchgrass biocrude in supercritical water at 600°C and 250 bar. Magnesium aluminum spinel was seen to be an inappropriate support as reactors quickly plugged. Ni/ZrO2 gave 0.98 mol H2/mol C, the highest hydrogen yield of all tested catalysts; however, over time, increase in pressure drop lead to reactor plugging with all zirconia supported catalysts. Titania supported catalysts gave lower conversions, however they did not plug during the course of the study. Charring of all catalysts was seen to occur at the entrance of the reactor as the biocrude was heated. All support materials suffered significant surface area loss due to sintering. The severity of water’s critical point can lead to sintering and phase transformations of catalyst support materials. Cerium-coated gamma-alumina (Chapter 6) and binary oxides of aluminum, titanium, and zirconium (Chapter 7) were synthesized as potential catalyst supports and evaluated for their stability in hot compressed water. Gamma-Al2O3 modified with 1-10 wt% Ce was examined, specifically in the temperature range of 500 – 700°C at 246 bar. Transformations of the gamma phase were slowed but not prevented. Based on X-ray analysis, the transformation of gamma-Al2O3 proceeded through the kappa phase toward the stable alpha phase. Reduced cerium species were seen to be oxidized in the supercritical water environment, and low Ce-loading supports maintained the highest BET surface areas. The stabilization was greatest at 700°C, where Ce-modified aluminas retained significantly higher specific surface areas than unmodified alumina. Binary oxides of aluminum, titanium, and zirconium with 1:1 mole ratios of the component metals were synthesized by a coprecipitation method. Their stability in sub- and supercritical water was evaluated at 25 MPa over a temperature range of 350 – 650 °C for a period of three hours by XRD and BET studies. The compound ZrTiO4 was crystallographically stable at all conditions. It maintained its surface area in subcritical water, although it sintered and lost much of its pore volume in supercritical water. ZrO2/Al2O3 maintained high surface area up to 450°C, but sintered above this temperature as a result of phase transformation of both ZrO2 and Al2O3. The TiO2/Al2O3 mixed oxide, while having the highest initial surface area, sintered extensively following all hydrothermal treatments. Alumina in the TiO2/Al2O3 system hydrolyzed in subcritical water and transformed to corundum in supercritical water, while anatase titania was transformed to rutile only at 650°C.