Analysis of Fuel Options for SOFC-based Power Systems in Undersea Vehicles

Previously demonstrated studies using anode recycle and carbon dioxide sequestration showed promising results for solid oxide fuel cell (SOFC) systems in unmanned undersea vehicles (UUVs). However, multiple heaters were used to maintain system temperature, and recent tests of a Delphi Corporation SOFC stack and fuel processor has uncovered thermal management issues regarding steam reformer operation in an anode-recycle loop. An experiment was attempted in which the fuel reformer was placed downstream of the SOFC stack and was to be driven (heated) by the anode exhaust from the stack. However, the maximum temperature achievable was only 400 degrees C, much lower than the 600 degrees C minimum for steam reforming of liquid hydrocarbon fuels. In order to provide more heat to the steam reformer, the combustor channel of Delphi's reformer was employed, and suitable conversion of JP-10 and S-8 liquid fuels was demonstrated. However, in a UUV application, continual operation of a burner (or CPOX reformer) will consume the limited supply of oxygen that is available. This extra consumption of oxygen for combustion detracts from the intended benefits of using energy-dense logistics fuels and highly efficient SOFCs. In a departure from NUWCDIVNPT's original focus on diesel-type (logistics) fuels, methane is looking more attractive for UUV applications, at least for first generation SOFC-powered UUVs. Even though the energy density for methane is roughly half that of diesel-type fuel, the savings in oxygen and carbon dioxide sorbent storage result in system energy metrics similar to those for S-8-based systems. In addition, the fuel processor control and system start-up can be greatly facilitated by using methane instead of a liquid hydrocarbon fuel. Switching to a methane fuel feed has the following advantages: high hydrogen-to-carbon ratio (this lowers O-2 and CO2 sorbent storage requirements); facilitates steam reforming versus diesel-type fuels; greater ease of start-up and system power transient control; risk mitigation for potential SOFC coking; and low boiling point, thus avoiding logistics of vaporizing diesel-type fuels and producing carbon residue.