High-Efficiency High Power Density Direct Carbon Fuel Cell

Faria, Christian

Etd

High-Efficiency High Power Density Direct Carbon Fuel Cell

Public

For over 80 years, combustion turbines have been the preferred method of generating power in excess of 10MW. In addition to producing heat and power, the combustion of fossil fuels releases pollutants in the form volatile organic compounds, oxides of carbon, nitrogen, and sulfur, and fine particulates. Simple cycle (SC) combustion turbines are limited by Carnot efficiency, resulting in thermal efficiencies of 34-39% with the most modern ultra-supercritical plants approaching 50%. By 2040, global electricity demand is projected to increase 57% from its current level, with renewables accounting for only 37% of electricity generation. Increasing electricity demand coupled with stringent emissions regulations drives the need for a more efficient way of generating dispatchable power from carbon-based fuels. Fuel cells are electrochemical devices that continuously convert the energy from a chemical reaction into electricity. The use of solid carbon fuel allows for separation of reactant and product streams allowing nearly 100% fuel utilization, while low reaction entropy permits high theoretical efficiency. Pure CO2 gas produced can be captured for sequestration or use in downstream applications. Practical direct carbon fuel cell (DCFC) efficiency is projected to be more than 70%, leading to nearly a 50% reduction in emissions from power generation relative to SC turbines. Prior DCFC designs have suffered from slow transport of reactants to the electrolyte and formation of corrosive compounds resulting in material degradation, short cell lifetime, and power densities less than 1 W/cm2. A high-efficiency high power density DCFC is proposed which uses a cathode-supported solid oxide electrolyte such as yttria-stabilized zirconia and an iron and/or manganese based liquid anode alloy with high carbon solubility. Solid fuel containing carbon is submerged and dissolved in the anode, with impurities accumulating in a slag layer at the top of the feed reservoir. Anode reaction kinetics are rapid due to carbon solubility and gas lift stirring. Calculation of Phase Diagrams (CALPHAD) methodology is used to select an alloy composition that has the best combination of low liquidus temperature and high carbon solubility. Mass and energy balance models forecasting operation at 10 bar show this DCFC achieving 40% efficiency at 1000°C with a power density of 5 W/cm2, and 66% efficiency at 800°C with a power density of 1.37 W/cm2. Model open-circuit voltage estimates are validated by experiments at various temperatures and with several anode alloy compositions, with empirical results compared to previous works. This thesis contains an in-depth investigation of the anode alloy development, selection, characterization, and experimentation in a proof-of-concept DCFC apparatus. Models for cost, energy, and net emissions for various types of coal and biomass fuel are compared to conventional methods of power generation. Measurements and data from this research will be contributed to open literature, advancing the future of DCFC technology.

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