Methane, electrochemical combustion

Carbon dioxide, 0=C=6 , mp —57°C (5.2 atm), bp —79 °C (sublimes), is obtained from the combustion of carbon and hydrocarbons in excess air or oxygen or by the pyrolysis ( calcination ) of CaCOs (limestone). The photosynthesis in plants reduces CO2 to organic matter, but the similar reduction of CO2 in a nonliving system ( in vitro ) appears to be very difficult. However, CO2 can be reduced electrochemically to methanol, formate, oxalate, methane, and/or CO depending upon the conditions. Numerous transition metal complexes of CO2 are known,which exhibit the modes of metal-C02 bonding depicted in Figure 2. 

Figure 2.13 shows T ideai versus temperature for various fuels electrochemically converted in the cells [411], For methane, T ideai is close to 100% and independent of temperature (reflecting that the entropy of the combustion reaction is almost zero). T ideai for hydrogen fuel is less than 100% and decreases strongly with temperature (AS <0). 

The hot clean fuel gas and the compressed ambient air are electrochemically combined within the high-pressure fuel cell with fuel and oxidant utilizations of 90 percent and 24.5 percent, respectively. The SOFC module is set (sized) to operate at 0.69 volts per cell. The spent fuel and air effluents of the SOFC are combusted within the module to supply heat for oxidant preheating. Unlike the natural gas case, the fuel does not require a pre-reformer with only 0.3 percent methane along with 36 percent hydrogen and 43 percent carbon monoxide. The carbon monoxide will be either water gas shifted to hydrogen or utilized directly within the fuel cell. A... 

Various t)q5es of modern power plants can operate on natural gas highly diluted with nitrogen, up to methane content below 50% in electrochemical generators (fuel cells), 40% in spark ignition engines, 30% in conventional gas turbines, 5% in diesel engines, and 1% in gas turbines with catalytic combustion [300]. 

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