[arvindpj]

Hello.

In my non-premixed combustion problem, I would like to burn 30000 BTU/hr of methane sent from the gas inlet.

In order to impose the corresponding mass flow of methane, I need two parameters. Firstly, the inlet gas temperature, that would dictate the gas density and secondly, the heating value of methane.

Could anyone let me know, where this heating value is specified in Fluent. Is it computed from the standard enthalpy of methane? Could you explain how we can deduce the heating value from the standard enthalpy?

Thanks,
Jay

[vinerm]

Yes, it is calculated from standard state enthalpy values using stoichiometric coefficients. Essentially, it is just difference in the standard state enthalpy (enthalpy of formation) values for reactant and product. If product has a higher value, then the heating value is positive, implying endothermic reaction.

[arvindpj]

I wrote a small python code to compute the heating value of CH4 based on Fluent Std State values:

# Fuel
CH4_molarmass = 16.04304 #g/mol 
CH4_std_state_entalphy = -7.49e7/1000 #kJ/kgmol @ 25 C
CH4_Hf = CH4_std_state_entalphy/1000  # KJ/mol

# Oxidizer
O2_molarmass = 31.9988 #g/mol
O2_std_state_entalphy = 0/1000 #kJ/kgmol @ 25 C
O2_Hf = O2_std_state_entalphy/1000  # KJ/mol

# Combustion Products
CO2_molarmass = 44.00995 #g/mol
CO2_std_state_entalphy = -3.935e8/1000 #kJ/kgmol @ 25 C
CO2_Hf = CO2_std_state_entalphy/1000  # KJ/mol

CO_molarmass = 28.01055 #g/mol
CO_std_state_entalphy = -1.105e8/1000 #kJ/kgmol
CO_Hf = CO_std_state_entalphy/1000  # KJ/mol

H20_molarmass = 18.01534 #g/mol
H20_std_state_entalphy = -2.418e8/1000 #kJ/kgmol @ 25 C
H20_Hf = H20_std_state_entalphy/1000  # KJ/mol


#Heat of Combustion
Hc = ((CO2_Hf + 2*H20_Hf) - (CH4_Hf + O2_Hf))*-1.0
print ("Lower Heat of Combustion of CH4 = ", Hc, "KJ/mol")

Hc_mass = Hc*1000/CH4_molarmass
print ("Lower Heat of Combustion of CH4 = ", Hc_mass, "kJ/kg")
print ("Lower Heat of Combustion of CH4 = ", Hc_mass*0.429923, "BTU/lb")

CH4_density = 0.648 #kg/m3 @ 25 degC

CH4_Heating_Value = Hc_mass*CH4_density  #KJ/m3

print ("Lower Heating Value of CH4 = ", CH4_Heating_Value, "kJ/m3")
print ("Lower Heating Value of CH4 = ", CH4_Heating_Value*0.0268392, "BTU/ft3")

CH4_HV = CH4_Heating_Value*0.0268392 #BTU/ft3

And the heating value of Methane/Air combustion is:

Lower Heat of Combustion of CH4 = 802.2 KJ/mol
Lower Heat of Combustion of CH4 = 50002.99195165006 kJ/kg
Lower Heat of Combustion of CH4 = 21497.43630882925 BTU/lb
Lower Heating Value of CH4 = 32401.93878466924 kJ/m3
Lower Heating Value of CH4 = 869.6421154294947 BTU/ft3

[vinerm]

I am sorry, is there a question or you just wanted to share some information?

[arvindpj]

I was sharing the info.

But, I have another question:

My intention is to simulate in Fluent with 100 % CH4 to mimic the combustion of Natural Gas which is 85%-95% CH4.

The heating values of the two fuels are different and their air-fuel ratio are different as well. 10.9:1 vs. 9.5:1 (approx.)

If I burn equivalent mass with same energy content (say 30000 BTU/hr):

Will they require same oxidizer (air) amount?

or

Will the exhaust products mole fractions be the same?

I have data of air exhaust from experiments for NAT combustion. But, I would like to simulate the furnace with 100% methane.

[vinerm]

The requirement of air will be as per the stoichiometric equation. If you use only Methane then air requirement will be as per Methane. So, if you use higher or smaller amount of Methane, air requirement will change accordingly, consequently, altering the mole fractions at the outlet.