Hydrogen Production by Partial Catalytic Dehydrogenation of Kerosene

Abstract

Due to decreasing supplies of fossil fuels and increasing environmental pollution, the introduction of a more fuel efficient electrical power system in aircraft applications is necessary. One possibility to improve the efficiency is to run the auxiliary power unit (APU), which provides electric energy on airplanes, with an efficient proton exchange membrane fuel cell system (PEMFC). The hydrogen for this concept can be provided by partial catalytic dehydrogenation (PCD) of Jet fuel stored onboard. The difference of this alternative thermochemical catalytic process to the more common reforming process is that no water is needed as a reaction partner. Therefore, no CO is generated, which would poison the catalyst in PEMFC. Other than gaseous hydrocarbons, no gaseous side products are expected. Beyond that, a high hydrogen purity of 98 vol.-% can be achieved. The partial conversion of jet fuel of about 10 to 15 % allows further use of the converted fuel in combustion processes on board. Since the composition of kerosene is very diverse, suitable reaction conditions for a process concept of the PCD of kerosene Jet A-1 have to be defined and the efficiency of the process has to be evaluated. In this thesis, two different process concepts for PCD of jet fuel are developed and their efficiency is evaluated by process simulation. One process concept is designed to run with regular kerosene Jet A-1, which involves a desulfurization step of the jet fuel before the PCD to reduce catalyst deactivation by sulfur poisoning. Since the sulfur containing components in Jet A-1 are found in the higher boiling range of kerosene, the desulfurization is accomplished by thermal distillation of desulfurized Jet A-1 fractions by rectification. The second concept is designed to run with desulfurized kerosene which differs in its chemical composition from regular Jet A-1. The first part of this thesis deals with the experimental characterization of the fuels. As the hydrogen yield, conversion of the fuel and product compositions highly depend on the composition of the hydrocarbon groups in kerosene, the detailed chemical composition of kerosene Jet A-1 was investigated and model components have been defined. These model components represent the hydrocarbon groups in the Jet fuel and they can be used for the design of model mixtures to experimentally investigate hydrogen yield, product composition, conversion rates, stability of the catalytic reaction and the reaction conditions. The catalyst used for the experimental investigation is platinum with tin on an aluminum oxide carrier. The experimental results using the model components show, that the hydrocarbon group of cycloalkanes leads to high hydrogen yield and stable reaction conditions. On the other hand, n-alkanes lead to catalyst deactivation by carbon formation on the catalyst surface and side reactions, thus causing a decline of hydrogen purity of the product gas by evolution of gaseous hydrocarbons. In a next step, the previously defined reaction conditions from the model mixture tests are applied to real kerosene. Due to the content of long chain hydrocarbons of up to 22 carbon atoms causing catalyst deactivation by carbon formation, the stability of this reaction is strongly reduced in comparison to the model mixtures. So far, a more suitable catalyst for more stable process conditions does not yet exist. In the second part of the thesis, the experimental results of the model components and model mixture are used for modelling the two process concepts for PCD in the process simulation. To achieve the highest possible system efficiency, a heat and material integration of the two process concepts is accomplished within the process simulation. For the definition of the system efficiency, the hydrogen yield is a key figure since it is the output of the process. The electric efficiency of both process concepts includes system losses of the fuel cell and product gas conditioning. With the experimentally investigated hydrogen yields of the model mixtures, a system efficiency for the process concept, including the desulfurization of the Jet fuel, of 17% is achieved. The process concept working with desulfurized Jet fuel has no additional energy demand for the desulfurization and achieves for system efficiency a value of 20.7%. To compete with a regular gas turbine APU, with average efficiency of 15 to 18%, the fuel cell APU system provided with hydrogen from PCD of kerosene has to be advanced to higher hydrogen yield. This could be accomplished by the development of design fuels for aircraft applications which suit PCD conditions and catalyst development. The results in this work can provide the boundary conditions for these investigations.