Electricity and hydrogen from solid fuels [electronic resource] : a study of carbon fuel cell systems
Keywords: Mechanical Engineering

The astounding advances in technology and standards of living over the past three centuries have been supported by the ready availability of cheap and reliable energy sources throughout the world. From coal to oil and natural gas, new fuels have continuously been discovered and developed in order to enable further increases in consumption. These fuels that power our 21st century lives, however, are finite and increasingly constrained, signifying that the modern energy supply system is unsustainable. Further, mounting evidence has made clear the global environmental implications of continued fossil fuel use. With the worldwide appetite for energy continuing to grow at a rapid pace, the development of new technologies that can produce electricity and fuels in a sustainable and environmentally friendly manner is of paramount importance. One such technology that holds promise as a clean and efficient energy generation source is the solid oxide based carbon fuel cell. These cells, which operate directly on pulverized coals or biomasses, have the ability to convert solid carbonaceous fuels into electricity and hydrogen spontaneously and at high overall efficiencies. Further, the cell itself operates as an air-seperation device, resulting in a product stream of near-capture ready carbon dioxide that can be sequestered, enabling zero or even negative emissions power production to be realized. In order to explore the potential of carbon fuel cell devices and understand the operational potential of systems built around carbon fuel cells, an integrated and coupled model of carbon fuel cell behavior was developed. The model includes a comprehensive dry gasification mechanism used to model the conversion process of the solid fuel, as well as detailed electrochemistry kinetics based upon reaction mechanisms at each electrode surface. Mass and heat transport phenomena are included in the model in order to accurately predict carbon fuel cell behavior. Experimental measurements of carbon fuel cell devices were performed in order to both validate and inform the model by providing necessary kinetic parameters. Finally, the model was exercised for a variety of carbon fuel cell designs, fuels, materials, and geometries, in order to reveal and explain the operational characteristics of solid oxide based carbon fuel cells. The results of this modeling study reveal that carbon fuel cells operating with air as an oxidizer have a fundamental tradeoff between system efficiency and cell power density. An optimal operational point was identified for a tubular system geometry that allowed for power densities of 1 W/cm2 with an overall efficiency approaching 65% for a device with a bed height of 50 mm. For cells operating with steam as the oxidizer, the co-production of hydrogen and electricity allowed for higher overall efficiencies, albeit with a required heat input. For the geometry studied, a maximum efficiency point near 90% was found, corresponding to a hydrogen production rate of 8 kg/m2 day at the short circuit condition. Finally, a coupled device made up two carbon fuel cells, known as a steam- carbon-air fuel cell, was studied, which allowed for the coproduction of hydrogen and electricity with no external heat or work input. For the modeled geometry, a maximum efficiency of over 78% was demonstrated near a solution point with a hydrogen production rate of 1.3 kg/(m2 day) and an electric power density of 55 mW/cm2. Further, a maximum hydrogen production point was also identified, which represented a six-fold increase in hydrogen production rate density when compared to a single carbon fuel cell operating with steam as the oxidizer.