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;
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.
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.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Chemical energy to enthalpy.
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.
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.