Basic Thermodynamic Studies and Exergetic Analysis of Fuel Cell Steam Reforming for Natural Gas in Waste Landfill and Anaerobic Digestor Gas

Abstract

One of the great opportunities for waste utilization and reduction of global warming (carbon footprint) is the utilization of the methane in the gas to produce electric power with a fuel cell. These waste gases are primarily methane which must be reformed to hydrogen before use in a fuel cell. There are three basic types of methane reforming—steam, autothermal and partial oxidation. Temperature, pressure, and steam-to-carbon ratio play an important role in the performance of steam reforming. These variables also influence the reversible work and reaction enthalpy of the reforming products. A fuel cell stack or cell integrated steam reformer allows a recirculation of reversible heat as long as the operational temperature of the fuel cell is appropriate to the temperature of the reformer. The maximum total reversible work (highest exergetic efficiency) of the hydrogen and the carbon monoxide W_rev,H2,CO,ox is achieved at 762 kJ per 1 mol methane which is converted in the fuel processor. This corresponds to a maximum intrinsic thermal efficiency η_Th,Max, of 95% at an optimum temperature of 1023 K. This optimum temperature is in the operating temperature range of the high-temperature fuel cells like the molten carbonate and solid oxide fuel cells. Higher efficiencies can be expected with these fuel cells with steam reforming when utilizing natural gas, anaerobic digestor gas, or landfill gas.