Chemical energy to enthalpy.

[rgm12]

Hi,

I am new to this forum. I am Having an issue with regards calculating the enthalpy in flue gases leaving a furnace.

I know the fuel Net Calorific Value (NCV) of 38MJ/kg
and I need to determine Cp of the flue gases.

Now assuming an adiabatic process constant pressure, hence no heat transfer dQ=0 and considering adiabatic temperature to be 2102 degC, ignoring potential energy, H = U (internal energy) which gives us H= mdot*Cp*T

using energy balance, Qin=Qout;

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NVC=Cp*T

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(mdot is common on both sides)

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38MJ/2102=Cp= 18.07 KJ/kg degC.

Doesn't it seem to be quite high? I've never seen such high specific heat capacities. Am I doing something wrong?

Thanks for your attention.

 

[Latexman]

Flue gases are mainly CO2 and H2O. I'd expect a Cp in the range of 2-4 kJ/(kg.^oC). Look up the Cp for CO2 and H2O as pure components at that temperature. Nowhere near 18 kJ/(kg.^oC).

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[rgm12]

Thanks for your reply Latexman,

Well to be more precise flue gases are mainly N2, and Cp for N2 in that temperature range is around 1.3KJ/KgK.

Still my big question is what's wrong with the my equations?

The issue with using Cp as 1.3KJ/kgK is that the enthalpy will be far less then the energy input.

ex. mdot*NCV=0.0039kg/s*38MJ/Kg= 148.2KW

now using adiabatic flame temp and Cp for N2 at specified adiabatic flame temp (2300 Kelvin): h=mdot*Cp*T = (2.14*0.0039)*1300*2300
= 24.95KW


148.2-24.95 = 123.25KW ?? any idea why is this big discrepancy ?

 

[rgm12]

Hi there,

Sry for the hassle, I found the problem... I calculated the mass flow rate wrong at the side of the flue gases. That was the issue. stupid me...

Thanks anyway.