Exploration of integrated fuel transformations for high efficiency energy systems

Increasing the efficiency of fuel-based energy systems will reduce greenhouse gas emissions and the consumption of natural resources. High efficiency energy system architectures are designed to maximize work extraction and minimize exergy destruction throughout the process. Energy conversion devices and processes are the building blocks of energy systems. This goal of this thesis is to illustrate how including fuel transformations as an additional building block in energy system design can lead to more efficient energy system designs where, ultimately, they can be deployed to benefit society. Two small-scale, high-efficiency systems were developed that illustrate the benefits of using integrated fuel transformations. These systems are discussed in detail in this thesis, including their theoretical foundations, the models used to refine the system design and understand system performance, and experimental investigations of the fuel transformations used in these systems. The first system discussed is the mixed combustion/electrochemical energy conversion engine. The main driving force behind this concept was to try and increase exergy efficiency by reducing exergy destruction due to combustion which destroys around 20-25% of the input exergy in most fuel-based energy systems. The modeling exploration indicated exergy efficiencies exceeding 60% were possible using this architecture by successfully reducing combustion and other reaction-related exergy losses. The model results of the fuel reformer in the system were experimentally validated using surrogate mixtures of rich-engine exhaust gas, methane, and steam at the modeled conditions (1000 degrees C and 5 bar). The other system explored in this thesis is the thermochemically recuperated Diesel engine. This system was developed based on recent low-heat-rejection Diesel engine research that showed it was possible to achieve around 30% more power than conventional Diesel engines while achieving exergy efficiencies in the 50-60% range. However, the main drawback of the low-heat-rejection Diesel is that it requires a fuel that will produce nearly zero soot at stoichiometric operation in a direct-injection, compression-ignition engine. The thermochemically recuperated Diesel strategy aims to eliminate this issue by integrating a fuel reformer into the engine, enabling the use of the high-performance low-heat-rejection concept with commercial fuels. Modeling results suggest it is possible to fully convert gasoline and steam into a methane and synthesis gas mixture that is expected to produce sufficiently low engine-out soot emissions. Additionally, the model indicates that the thermal energy recovery is effective, and the system can obtain exergy efficiencies of 55%. Finally, an initial experimental investigation concluded that gasoline steam reforming—at pressures up to 8 bar—is possible at the temperatures and water concentrations of interest. Gasoline steam reforming at Diesel injection pressures (> 250 bar) is proposed as the next step.

Keywords: Mechanical Engineering.
Advisor: Edwards Chris
Cargnello Matteo 
Mitchell Reginald