The fuel which is being burned has a higher specific energy than pure carbon, (compare the specific energy of natural gas with coal [1]). The chemical process is methane -> pure carbon with CO2 as an intermediate state.
Of course you're not generating as much power as just burning the methane completely.
Thanks. So disregarding any heat losses, you get (12-7.5)/12 = 37.5% of the stored energy. Gas power plants appear to have an efficiency of 54% [1]. I honestly can't imagine how you could reasonably get electrical energy out of that system.
Of course you're not generating as much power as just burning the methane completely.
[1] http://www.engineeringtoolbox.com/co2-emission-fuels-d_1085....