Direct Conversion of Coal and Coal-Derived Carbon in Fuel Cells

Abstract

A direct carbon fuel cell (DCFC) using a carbon-rich derivative of coal would maximize the conversion efficiency of this vast energy resource by avoiding the efficiency limitations of heat engines. A total conversion efficiency of 80% (based on heat of combustion of carbon) has been achieved at 30–120 mA/cm2 using carbon materials extracted from coal and other fossil resources. High experimental efficiency is grounded in two favorable aspects of the reaction thermodynamics. The net fuel cell reaction (C + O2 = CO2) has a nearly zero entropy change and therefore a theoretical efficiency of 100%. The fixed chemical potentials of carbon reactant and CO2 product make possible the full utilization of fuel in a single pass through the cell. The pure CO2 product can be used directly in enhanced oil and gas recovery, or sequestered. Historically, the development of carbon fuel cells have been limited by low anode rates, accumulation of impurities in the electrolyte, logistics of refueling, and lack of suitable cathodes. These problems are being addressed by recent developments of highly reactive carbon materials, low-cost techniques for separation of coal from ash, the possibility of pneumatic distribution of solid particulate fuel to the cells, and availability of cathodes from the molten carbonate fuel cell technology. Rate depends on atomic scale disorder and accessibility of reactive sites, but not on purity. Sources of suitable anode fuel include thermally decomposed products of (1) mechanical and chemical coal/ash separation or (2) solvent extraction. With current understanding of the cell basics, the next steps are demonstration of an engineering scale fuel cell stack (∼1 kW), supported by development of coal-to-carbon processes and techniques of electrolyte management. Successful development of a direct conversion fuel cell for coal (or coal-derived carbon) has extraordinary implications in extending the energy reserves of coal-producing nations, easing the control of regulated emissions at the plant, and expanding the use the earth’s greatest fossil resource while decreasing emissions of greenhouse gas.