Journal of Power Sources 194 (2009) 774–785 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Thermodynamic analysis of gasification-driven direct carbon fuel cells Andrew C. Lee a,∗ , Reginald E. Mitchell a , Turgut M. Gür b,c a Department of Mechanical Engineering, Stanford University, 452 Escondido Mall, Building 520, Stanford, CA 94305, USA b Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Durand Building, Stanford, CA 94305, USA c Direct Carbon Technologies, LLC, Palo Alto, CA 94301 USA article info Article history: Received 22 April 2009 Received in revised form 21 May 2009 Accepted 22 May 2009 Available online 30 May 2009 Keywords: Direct carbon fuel cell Coal Electricity generation Gasification Exergy abstract The gasification-driven direct carbon fuel cell (GD-DCFC) system is compared with systems using sepa- rate 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 gasifi- cation plants, using common solid fuel compositions, gives conversion efficiencies in the range of 51–58% at an operating voltage of 0.7V selected for both systems in this study. In contrast, the conversion effi- ciency 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 effi- ciency as for CO 2 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Currently, half of the electricity produced in the United States comes from coal fired power plants. Coal’s share of electricity pro- duction in many developing countries exceed this amount reaching over 70% in China and India. Given coal’s abundance and low price, it is expected that this naturally occurring solid fuel will continue to be relevant to the world’s energy portfolio for decades to come. Historically, pulverized coal has been burned in air to produce sensible enthalpy that is transferred as heat to a Rankine steam cycle equipped with a generator to produce electrical work. This approach has several important drawbacks. Primarily, the conver- sion efficiencies are in the low 30% range, which are limited by the relatively low temperature steam cycle, and the exhaust stream is highly diluted by N 2 originating from the combustion air and hence, is not ready for CO 2 capture. Driven by global warming concerns, particularly in regards to CO 2 emissions, emerging coal utilization strategies prevent addi- tion of N 2 into the fuel stream by using pure oxygen from an air separation unit (ASU), thus producing a concentrated CO 2 stream ∗ Corresponding author. Tel.: +1 510 304 9211; fax: +1 650 723 0335. E-mail addresses: aclee@stanford.edu (A.C. Lee), remitche@stanford.edu (R.E. Mitchell), turgut@stanford.edu (T.M. Gür). for geological sequestration [1]. Modern efforts to address these concerns have led to more advanced processes like Integrated Gasi- fication Combined Cycle (IGCC) plants, where coal is gasified with steam and oxygen to a syngas that is subsequently cleaned of sul- fur and other impurities before utilization in a cascaded, combined gas-steam cycle. This approach does not introduce nitrogen gas into the process stream, and thus the exhaust is more readily suitable for sequestration of CO 2 . With IGCC, the higher temperature oper- ation regime of the gas turbine allows higher thermal efficiencies than that of the standalone Rankine cycle. The syngas can also be converted electrochemically via solid oxide fuel cell, similar to the FutureGen program [2,3]. Although long and rich in history [4], direct carbon fuel cells (DCFCs) have recently gained renewed interest as an area of active research and development, encompassing a variety of fuel cell con- figurations and approaches [5]. All of these approaches share an end goal of eventually converting practical and economically feasi- ble carbonaceous solids (coal, biomass, municipal solid waste, char, etc.) electrochemically to electric power with efficiencies higher than contemporary arrangements, producing an effluent stream of concentrated CO 2 . High temperature, solid oxide fuel cells (SOFCs) based on oxide- ion conducting membranes have garnered widespread attention in the last several decades due to inherent high system efficien- cies. Inspired by their compatibility and fuel flexibility, SOFC based 0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2009.05.039