A steam generator produces 20 kg/s of high-pressure steam at T = 300o C and P = 1.2 MPa. The feed of steam generator is water at 25 o C and atmospheric pressure. Methane is used as fuel; oxygen percent in stack outlet and heat loss through the surface of the generator are 5% and 10%, respectively. Determine the molar flow rate of methane and the excess air percentage so that the maximum temperature in the combustion chamber is 2000 K.