Marcosdias:
You are obviously all mixed up in your basic Thermodynamics (as well as fuel combustion). It is far wiser to stop trying to understand an accurate thermo database - such as NIST - and go back to basics, studying what Enthalpy, Entropy, and Heats of Combustion are and how they are calculated. You will find that the Thermodynamic states of Enthalpy and Entropy both rely on a relative reference temperature and pressure in order to be calculated. As defined, they are not absolute values, they are relative values and you must specify the base conditions under which they are calculated. Simply go to:
Now select “Propane” in the drop-down window as well as all the units that you prefer to use.
Select the Isobaric process and continue on. You will be asked for the range of temperatures (10 – 50
oC) and the base pressure. I selected 10 bar(A).
Press for Data and subsequently, on the next page, press “View data in HTML table”.
You will get a result that can be easily copied and pasted into an Excel spreadsheet, listing all the pertinent Thermo data you ever need – and the physical state the fluid finds itself in. You will see that NIST gives the specific heats (at both constant pressure and constant volume) for each specific temperature at that pressure (10 bara).
Most importantly here, look at the bottom of the table and you will see the immediate important information relative to the printed results, the Reference States:
Reference States
For Internal energy, U = 0 at 273.16 K for saturated liquid.
For Entropy, S = 0 at 273.16 K for saturated liquid.
The Lower Heating Value (LHV) of propane has absolutely nothing to do with Thermophysical data. It has everything to do with the chemical properties of the fuel and its combustion.
