Thermodynamic analysis of gasification-driven direct carbon fuel cells

The gasification-driven direct carbon fuel cell (GD-DCFC) system is compared with systems using separate gasification steps prior to work extraction, under autothermal or indirect constraints. Using simple system exergy analysis, 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. A more detailed calculation for the DCFC and indirect gasification plants, using common solid fuel compositions, gives conversion efficiencies in the range of 51-58% at an operating voltage of 0.7 V selected for both systems in this study. In contrast, the conversion efficiency of the autothermal gasification approach is estimated to be 33-35% at 0.7 V. DCFC efficiencies can be increased to over 60% by an increase in operating voltage and/or inclusion of a bottoming cycle. The thermodynamic model also indicates that steam gasification yields similar work output and thermal efficiency as for CO2 gasification. Open circuit potential measurements agree with equilibrium calculations both for the C-O and C-H-O gasification systems, confirming the governing mechanism and feasibility of the GD-DCFC. Current-voltage measurements on an un-optimized system demonstrate power densities of 220 mW cm(-2) at 0.68 V during operation at 1178 K. (C) 2009 Elsevier B.V. All rights reserved.