Methane gas at 400 K and 1 atm (100 kPa) enters a combustion chamber, where it i

Question

Methane gas at 400 K and 1 atm (100 kPa) enters a combustion chamber, where it is mixed with air entering at 500 K and 1 atm. The products of combustion exit at 1800 K and 1 atm with the product analysis on a dry basis given as 9.7% CO_2, 0.5% CO, 2.95% O_2, and 86.85% N_2. The average value for the specific heat C_p of methane between 298 and 400 K is 38 kJ/kmol.K. Determine the following: The complete balanced equation for the reaction. Rewrite the balanced equation in terms of per mole of fuel Air-fuel ratio on a mass basis The percent theoretical air The dew point temperature of the product The enthalpy of reaction.

Explanation / Answer

The balanced reaction is CH4+2O2--àCO2+2H2O and incomplete combustion CH4+1.5O2àCO+2H2O

Basis: Moles of product on dry basis : 100 moles

N2 in product = 86.85 moles. The soure of N2 is air. Air contains 21% O2 and 79% N2. Moles of air supplied = 86.85/0.79=110 moles

Mass of air = 110*29=3190 gm

Moles of O2 in air = 110*0.21= 23 moles.

The reactions per mole of feed are CH4+2O2--àCO2+2H2O and incomplete combustion CH4+1.5O2àCO+2H2O

CH4 supplied = That used for formation of CO2= 9.7 moles and that used for formation of CO= 0.5, moles of CH4 supplied = 9.7+0.5= 10.2 moles, mass of methane= 10.2*16= 163.2 gm

Theoretcial Oxygen to be supploed = 10.2*2 ( since each mole of CH4 requires 2 moles of O2)=20.4

Percent excess oxygen =100*(23.20.4)/20.4=12.74%

Air to fuel ratio ( Molar)= 110/23= 4.78

Air fuel ratio ( Mass)= 3190/163.2=19.54

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