Analysis of solid state, solid oxide electrolyte based direct carbon fuel cells [electronic resource]
Keywords: Mechanical Engineering

Efficient use of solid fuels, including coal and biomass, is of paramount importance in light of limited fossil fuel reserves and environmental concerns over pollution and atmospheric carbon dioxide. The solid state, direct carbon fuel cell (DCFC), using an oxide ion conducting electrolyte, is investigated here as a means to efficiently extract work from carbonaceous solids, while producing a capture ready stream of CO2. The fuel is supplied as a free particle bed in direct contact with an SOFC anode. Thermodynamic performance and behavior, carbon reactivity, and experimental DCFC performance are used as metrics to explore feasibility. Thermodynamic system analyses are performed in order to estimate DCFC system performance, as compared to systems using separate autothermal or indirect gasification steps upstream of the fuel cell. The maximum work output of the indirect gasification scheme is 4--7% lower than the unconstrained direct approach, while the work output of the autothermal gasification approach is 12--13% lower than the unconstrained case. Detailed calculations for the DCFC and indirect gasification plants give nominal system efficiencies between 51 and 58%, compared to the autothermal gasification approach which was found to be 33--35% efficient. DCFC efficiencies can be increased to over 60% by an increase in operating voltage and/or inclusion of a bottoming cycle. Open circuit potential measurements agree with equilibrium calculations both for the C-O and C-H-O chemical systems, confirming the governing mechanism and feasibility of the DCFC. The Boudouard reaction governs the cell potential in the presence of solid carbon. Reactivities (to CO2) of various pulverized carbons and chars were measured and compared using thermogravimetric analysis. Coal and biomass chars created under high heating rates demonstrate the highest reactivities, as compared to slowly heat coals or highly processed fuels. A five-step heterogeneous chemical reaction mechanism characterizing the Boudouard reaction was employed to model activated carbon gasification at a pressure of 1 atm in the temperature range of 973 to 1273 K. These experimentally verified kinetic parameters were then included in a transport model to calculate scalar concentration fields established in the carbon bed as a consequence of convection, diffusion, and heterogeneous reaction. The model, validated using fixed bed experiments, is used to simulate the effect of an imbedded SOFC, in contact with the carbon bed. Current densities in the practical range of 100--1000 mA/cm2 can be supported by the considered geometry. Current--voltage measurements performed on a lab-scale system demonstrate power densities of 220mW/cm2 at 0.68V during operation at 1178 K using activated carbon. Several additional experiments, including DC measurements and gas chromatographic measurements, were conducted to determine the effects of carrier gas on performance and to verify Faradaic operation via an oxygen balance. In addition to work production from the DCFC, hydrogen production using the SOFC architecture is also considered. The carbon assisted steam electrolysis (CASE) system is analyzed as a means to reduce the losses from conventional electrolysis. Thermodynamic analysis suggests efficiencies of nominally 60% are possible using a coupled CASE-DCFC plant. Measured open circuit voltages agree with theoretical calculations for the carbon-steam cell, and gas analysis verifies hydrogen production. Current-voltage measurements were taken of several un-optimized configurations of CASE cells.