SOFC is an electrochemical device that converts the chemical energy of a fuel into electricity at high temperature (700 – 900°C). The choice of fuel stock to be utilized with a SOFC can vary greatly, from hydrocarbon fuels such as diesel, natural gas, propane, ethanol, methanol and bio-derived fuels to pure hydrogen. Although SOFC is most efficient when using pure hydrogen as a fuel stock, the infrastructure for hydrogen production and storage are still in development stages and difficult to maintain for safe keeping; therefore, hydrocarbon-based fuels serve as the fuels of choice and will likely be the choice of fuel for SOFC for the foreseeable future. The high operating temperatures of SOFC promotes faster kinetics with nonprecious materials (such as Ni), internal reforming of hydrocarbon fuels and the production of high quality waste heat. SOFC can be configured to produce power at both kilowatt and mega-watt levels. The application areas of SOFC include distributed power generation, electric utility and backup power. A single SOFC is comprised of a dense electrolyte and two porous electrodes.
The most conventional electrolyte is yttria-stabilized-zirconia (YSZ), which is an electronic insulator and conducts only O2- ions through the oxygen ion vacancy in its crystal lattice. The ionic conductivity of YSZ is on the order of 10-2 S cm-1 at 800°C. YSZ is also chemically stable over a wide range of O2 partial pressure experienced at SOFC electrodes (10-1 to 10-23 atm).
The anode is the electrode where the fuel is oxidized electrochemically. The most established anode is a composite of ceramics, yttria-stabilized-zirconia (YSZ), and metal (Ni) – otherwise known as Ni/YSZ anode. The required conductivity of anode is 1 to 102 S cm-1, while the required porosity is 30-35%. Among other properties, the anode needs to be tolerant to ‘coking’ and ‘sulfur poisoning’, particularly when operating in hydrocarbon fuels.
The cathode is the electrode at which the O2 molecule is electrochemically reduced to O2- ions. The most common cathode is a composite of La-based-oxide and YSZ. The required porosity of cathode is 30-35%. The oxygen reduction kinetics at the cathode is often the rate-limiting step among the SOFC reactions. The high operating temperature of SOFC allows the reduction of high polarization resistance of the cathode.
The RUBICON™ SOFC produces direct-current (DC) that is inverted to three-phase alternating current (AC) by means of an inverter to adapt the RUBICON™ SOFC output to suit the electrical requirements at the point of power delivery (power distribution system). The “Inverter” works as an interface between the RUBICON™ SOFC and the electrical network. The “Inverter” also acts as the voltage and frequency adjuster to the final load.
The power output from a single SOFC is relatively low wattage, and hence, SOFC cells are stacked to obtain meaningful power output (kW, MW). Between each SOFC cell, there is an “Interconnect” that helps to combine the cells in series. The “Interconnect” is an electrical conductor and within the RUBICON™ is chemically stable, both at reducing (anode, 10-23 atm) and oxidizing (cathode,10-1 atm) conditions. “Interconnects” are made of either metal or metal-ceramic composite.
The RUBICON™ SOFC is a fuel-flexible power solution that can generate electricity from almost any of the hydrocarbon fuels. The hydrocarbon fuel is reformed upstream of the SOFC stack in order to avoid potential ‘coking’ and ‘sulfur poisoning’ issues at the anode. Several components including a desulfurizer, a reformer and a number of heat exchangers are integrated in the RUBICON™ SOFC. The desulfurizer, the reformer and the associated heat exchangers are collectively known as the Fuel Processor.
The operational parameters such as steam to carbon ratio, current density and temperature are tuned in the RUBICON™ SOFC, so as to minimize the potential issues of sulfur poisoning and ‘coking’ of the stack and the corresponding performance loss.