Direct Carbon Fuel Cell System Utilizing Solid Carbonaceous Fuels

Abstract

This 1-year project has achieved most of its objective and successfully demonstrated the viability of the fluidized bed direct carbon fuel cell (FB-DCFC) approach under development by Direct Carbon technologies, LLC, that utilizes solid carbonaceous fuels for power generation. This unique electrochemical technology offers high conversion efficiencies, produces proportionately less CO 2 in capture-ready form, and does not consume or require water for gasification.FB-DCFC employs a specialized solid oxide fuel cell (SOFC) arrangement coupled to a Boudouard gasifier where the solid fuel particles are fluidized and reacted by the anode recycle gas CO 2 . The resulting CO is electrochemically oxidized at the anode. Anode supported SOFC structures employed a porous Ni cermet anode layer, a dense yttria stabilized zirconia membrane, and a mixed conducting porous perovskite cathode film. Several kinds of untreated solid fuels (carbon and coal) were tested in bench scale FB-DCFC prototypes for electrochemical performance and stability testing. Single cells of tubular geometry with active areas up to 24 cm 2 were fabricated. The cells achieved high power densities up to 450 mW/cm 2 at 850 o C using a low sulfur Alaska coal char. This represents the highest power density reported in the open literature for coal based DCFC. Similarly, power densities up to 175 mW/cm 2 at 850 o C were demonstrated with carbon. Electrical conversion efficiencies for coal char were experimentally determined to be 48%. Long-term stability of cell performance was measured under galvanostatic conditions for 375 hours in CO with no degradation whatsoever, indicating that carbon deposition (or coking) does not pose any problems. Similar cell stability results were obtained in coal char tested for 24 hours under galvanostatic conditions with no sign of sulfur poisoning. Moreover, a 50-cell planar stack targeted for 1 kW output was fabricated and tested in 95% CO (balance CO 2 ) that simulates the composition of the coal syngas. At 800 o C, the stack achieved a power density of 1176 W, which represents the largest power level demonstrated for CO in the literature.Although the FB-DCFC performance results obtained in this project were definitely encouraging and promising for practical applications, DCFC approaches pose significant technical challenges that are specific to the particular DCFC scheme employed. Long term impact of coal contaminants, particularly sulfur, on the stability of cell components and cell performance is a critically important issue. Effective current collection in large area cells is another challenge. Lack of kinetic information on the Boudouard reactivity of wide ranging solid fuels, including various coals and biomass, necessitates empirical determination of such reaction parameters that will slow down development efforts. Scale up issues will also pose challenges during development of practical FB-DCFC prototypes for testing and validation.To overcome some of the more fundamental problems, initiation of federal suppor...

Solid Fuel Utilization in High Temperature Fuel Cells

“…A major portion of the presentation will be devoted to describing and discussing a new approach in solid fuel utilization in a novel solid carbon fuel cell (SCFC) [2][3][4]. SCFCs hold the potential to double the system level conversion efficiency.…”