Thesis (M.S. in Aeronautical Engineering)--Naval Postgraduate School, June 2003. / Thesis advisor(s): Oscar Biblarz, Garth Hobson. Includes bibliographical references (p. 45-47). Also available online.

Thesis (Ph. D.)--University of Sydney, 2008. / Includes graphs and tables. Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Aerospace, Mechanical and Mechatronic Engineering. Bibliography: pp. 209-222. Also available in print form.

Hydrogen is considered a possible alternative to fossil fuels. Hydrogen can be produced through dark fermentation with 1 mol hexose yielding a maximum of 4 mol hydrogen in association with acetate production, and 2 mol hydrogen in association with butyrate production. However, an economically and technically feasible process is yet to be established. So far research into fermentative hydrogen production has focused on pure and soluble carbohydrates, particularly glucose. To reduce substrate costs, use of more complex low-cost co- and waste products of the food industry or biomass crops which have undergone minimum pre-treatment would be desirable. Also, whilst much research to date has focused on use of pure bacterial strains, an easily obtainable mixed microflora would be preferable to avoid costs of substrate sterilisation. Therefore this research project focused on fermentative hydrogen production from three abundant (in the UK) low cost substrates, namely a wheat starch co-product, sugarbeet and perennial ryegrass. Anaerobic digester sludge obtained from the local sewage works was used as inoculum in a continuously stirred tank reactor. Production of hydrogen and other fermentation products was measured to gain information about the main metabolic pathways used. To lower hydrogen partial pressure the reactor was sparged with nitrogen and the effect on hydrogen production observed. It was demonstrated that stable fermentative hydrogen production from the wheat starch co-product and sugarbeet water extract was possible in continuous operation. Hydrogen production from grass extract was demonstrated in batch mode. Sparging with nitrogen significantly increased hydrogen yields, by 46% for the wheat starch co-product, by 67% for sugarbeet water extract, and by 184% for ryegrass extract. Maximum yields achieved were 1.9 mol hydrogen per mol hexose converted for 16 days on starch, 1.7 mol per mol hexose converted for 5 days on sugarbeet water extract and 0.8 mol hydrogen per mol hexose converted in batch from grass extract. Therefore up to 48% of the maximum theoretical hydrogen yield was produced. Various factors were identified as preventing higher hydrogen yields. Hydrogen production was more closely related to butyrate than acetate concentration. Also, lactate, ethanol and propionate, which are products of carbohydrate fermenting metabolic pathways that do not produce hydrogen, were detected, as were signs of hydrogen consuming homoacetogenesis in continuous operation.

Approved for public release; distribution is unlimited / The conversion to hydrogen as a naval aviation fuel would allow for independence on fuel cost and supply, as hydrogen is globally accessible. The biggest obstacle to using hydrogen is its very low density, a property that even combined with hydrogen's high heat of combustion still results in very large fuel tanks. Liquid hydrogen (LH2) with its higher density would still require a larger volume than kerosene for the aircraft to achieve the same mission. Another problem with using LH2 is its cryogenic nature, a property that requires complicated fuel tanks and more careful fueling. A design study has been conducted for this report to determine the feasibility of using LH2. A Lockheed-Martin P-3 Orion configuration was modified to accommodate LH2 as its fuel, its mission parameters kept unchanged. It is concluded from this design study that using LH2 would significantly limit the amount of usable cabin space, as the fuel tank takes up 65% of the aircraft's internal volume. Despite the large LH2 tank weight of about 14,865lb, due to the low fuel weight the aircraft's takeoff gross weight is only 113,646lb, about 80% of the current petroleum-fueled P-3. The total cost of LH2 as fuel is currently undetermined. / Ensign, United States Navy

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Andrei Fedorov; Committee Member: Srinivas Garimella; Committee Member: Timothy Lieuwen; Committee Member: William Koros; Committee Member: William Wepfer. Part of the SMARTech Electronic Thesis and Dissertation Collection.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (M.S.)--University of Akron, Dept. of Chemical and Biomolecular Engineering, 2006. / "May, 2006." Title from electronic thesis title page (viewed 01/15/2008) Advisor, Lu-Kwang Ju; Committee members, Bi-min Zhang Newby, Donald Ott; Department Chair, Lu-Kwang Ju; Dean of the College, George K. Haritos; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Dionisios G. Vlachos, Dept.of Chemical Engineering. Includes bibliographical references.

Hydrogen has gained interest as fuel recently as the harmful effects of fossil fuels on the environment can no longer be ignored. Hydrogen, which produces no pollutants, forms the feed for cleaner fuel cells systems currently in use. Fuel cells, although not as economically viable as fossil fuels, have found a foothold in the energy market in various markets like power backup and use in remote locations. Production of hydrogen is still largely done via fossil fuel reforming and this technology has received renewed interest for use with fuel cells in the form of micro- reformers or fuel processors. This study entailed the performance benchmarking of a so called Best-in-Class commercial micro reformer (as available in 2010), the 1 kW WS FLOX Reformer, and was undertaken under the auspices of the national HySA programme. The study’s focus was primarily on reformate output quality (carbon monoxide concentration), and start up time, thermal efficiency and hydrogen output (15 SCLM). The reformer consisted of a combustion section encased in an outer reforming section consisting of three reactors in series, steam reforming, water gas shift and selective methanation. As-provided temperature control is simplified though the use of only one temperature setpoint in the combustion chamber and temperature control in the CO clean up stages obtained through means of heat transfer with incoming water being evaporated. Combustion takes place through flame combustion or by means of the supplier’s patented FLOX (flameless oxidation) combustion. The purchased FLOX Reformer assembly was integrated into a fully automated unit with all balance of plant components as well as microGC and flue gas analysis for measurement of outlet conditions. The FLOX Reformer was tested at multiple combustion temperatures, combustion flowrates, reforming loads and steam-to-carbon ratios to obtain a wide set of benchmark data. From the testing it was found that the reformer was able to produce the necessary 15 SCLM hydrogen with a carbon monoxide purity of less than 10 ppm as required in fuel cells for all testing if the reaction temperatures were within the recommended limits. Intermediary water gas shift analysis showed methane and carbon monoxide conversion in the reforming and water gas shift stages to be identical to thermodynamic equilibrium conversion – 95% and higher for all temperatures. iii Selective methanation conversion obtained was 99%, but not always at equilibrium conversion due to increased selective methanation temperatures, where carbon dioxide methanation was also observed at the higher temperatures. Temperature control through heat exchange with incoming water in the CO removal stages was found to be less than ideal as the temperature inside these stages fluctuated dramatically due to inaccuracies in the water pump and a lagged response to flowrate changes. Startup times of less than an hour was observed for multiple combustion flowrates and the reformer boasts a standby function to reduce this to less than half an hour. The thermal efficiency was independently confirmed and tested and found to be higher than 70 % for flame combustion and on par with other commercially available fuel processors. The suppliers trademark FLOX combustion only reaching 65% due to decreased combustion efficiency.

Increasing carbon emissions and insecurities in oil supply have led to heightened interest in hydrogen powered fuel cells. Preferably, the cell runs on hydrogen gas, though due to the sensitivity of the catalytic components in the fuel cell to carbon monoxide, the hydrogen must be extremely pure (typically <50 ppm CO). Due to a lack of hydrogen infrastructure, it is envisaged that a medium term solution will be the reforming of more conventional fuels such as gasoline. The gas mixture produced however, contains impurities such as CO, CO₂ and CH₄. Purification may be achieved using palladium membranes, which allow selective permeation of hydrogen. This thesis describes the research carried out in conjunction with Johnson Matthey on thin (typically 7.5 μm) palladium/silver alloy membranes supported on both ceramic and stainless steel porous tubular substrates. Extensive experimental flow testing has been performed to assess the effect of temperature, feed composition, including wet feeds, and membrane thickness on the hydrogen purification properties. An existing Fortran based model was validated and revised to accurately account for the effects of operating conditions such as temperature and carbon monoxide concentration. This work provided excellent correlation between experimental and simulated results. The validated and improved model was incorporated in the design of a hydrogen refuelling station in Aspen Plus and the palladium membrane requirements assessed to supply 650 fuel cell vehicles per day. The system incorporated a steam reformer, membrane clean-up module, water trap and high pressure compressor for hydrogen storage at 1000 bara. Operating conditions such as system pressure, fuel feed and steam to carbon ratio were investigated and adjusted to optimise the overall system efficiency. An efficiency of 52% was achieved with a steam to carbon ratio of SCR = 2.5. A membrane requirement of 6000 standard tubes was found to provide a 90% hydrogen recovery efficiency.