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Chemical energy to enthalpy.

Chemical energy to enthalpy.

Chemical energy to enthalpy.

(OP)
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;
NVC=Cp*T
(mdot is common on both sides)

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.

RE: Chemical energy to enthalpy.

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.

RE: Chemical energy to enthalpy.

(OP)
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 ?

RE: Chemical energy to enthalpy.

(OP)
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.

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