Heating Value of Natural Gas
Posted on | April 21, 2008 | 4 Comments
In determining the heating value of natural gas, whether the composition hydrocarbon (such as ethane or propane) also affects?
Heating value has nothing to do with something hot to heat the substance. Heating value is the value of the heat generated from complete combustion of a substance at a particular temperature.
Excellent hydrocarbon combustion reactions like this:
CxHy + (x + y / 4) O2 -> x CO 2 + y / 2 H2O
By definition heat of combustion is calculated as if the reactants and reaction products have the same temperature. Normally the standard conditions used for the calculation of heating value is 25 ° C and 1 atm. As we all know at 25 ° C and 1 atm H2O has a liquid phase, the calculations assume H2O HHV-burning results condensed into liquid phase, so in addition to the heat obtained from combustion, also obtained by condensation heat energy from H2O. If the calculation is assuming that LHV H2O remain in the gas phase at 25 ° C. So the difference between the HHV and LHV is the heat condensation H2O at standard temperature and pressure.
It should be noted that the HHV and LHV is the theoretical notation, only used for indication and do not show the actual conditions in practice. The reason for the fuel and gas combustion results have never been at the same temperature as the assumptions used to calculate HHV and LHV. In practice, the energy we can get from burning fuel will always be smaller than the HHV or LHV, because there is energy in the form of heat carried away by the gases of combustion. That is why the efficiency of all energy conversion engines (steam power plant, internal combustion engines, gas turbine) can never be 100%.
So HHV and LHV absolutely nothing to do with the phase of the fuel, either solid fuel and liquid, both had HHV and LHV. If it’s easy or difficult to burn, also has nothing to do with HHV & LVH. Remember! Exothermal combustion process, so do not take the heat (energy) from the environment it provides heat to the environment. Actually that can be burned it is the gas phase, if there is liquid fuel, it must be sufficient vapor formed above the surface in order to start combustion. If we start from the ambient temperature, for certain liquid fuels such as diesel oil, the temperature must be sufficient so that the vapor pressure high enough to form a vapor phase that can be burned (hence the term flash point). But it was burned, the heat from the combustion will always provide enough energy to produce steam phase that is ready to be burned.
The higher the carbon number, heating value in kJ / kmol (but not dlm kJ / kg!) Was also higher. For gas, heating value is usually expressed in Btu / MMSCF, and we know that for the gas mole was proportional to the volume, so for natural gas more severe the higher the fraction value in Volumetric heating base.
Please note, the unit which states the value of LHV / HHV too, that the value of LHV / HHV enlarge as an increase in the amount of carbon, of course, for the unit Btu / lbmol (kJ / kmol). Because if the units used are mass-based, LHV / HHV of methane greater than the longer chain (because smaller MW).
Determining gross heating value is not passed through the GC but the numerical computation (which is generally already in his soft machine which took GC composition data peak GC). Why? Because we have to enter natural gas properties in 60 oF and 14.7 psia.
Essentially, the GC is not limited to the measurement of C9 alone, could be more depending on the settings /instruments and standards are implemented method.
GHV not a direct measure of the GC like measure the pressure from a pressure transmitter, but no computation of the GC after the gas composition obtained.
It can be calculated until the heavy carbon chain, but usually is not accurate, after all, in fact very small fraction compared with the fraction of C1 (which is usually used as a standard specification of the gas to sales), so if there is a small change of composition in the carbon chain Weighing just give a small impact on the value of GHV.
GC can perform calculations GHV. Conducted by the GC is to see the composition of the gas based on chromatogram peak. Then based on the standards used, whether it’s GPA-2172, or ISO 6976, GC will be calculated based on input data GHV mol% of the measured gas itself, base pressure and base pressure measurement used. In some places this is not usually done because the GC does not measure the value of the composition of H2O and H2S and some other components that are not detected by GC. So the common practice is to give all this data to information flow computer (GC data and H2O and other components required), and the flow computer will calculate gross heating value.
For the ISO6976 standard, we can not choose the standard pressure used, it is not specified by default. The unit output of the ISO6976 standard for calculating the volume MJ/Sm3. ISO6976 provides the option to count in some combustion GHV / metering temp. Which is commonly used (15.15) and (20.20). All inputs and outputs of ISO6976 is in the form of metric.
For the 2172 GPA standards, we can choose GHV would be calculated on how much pressure is up to us. Commonly used is 14.73 psia. Standard calculations GPA2172 use a pressure 14,696. However, GPA 2172 did not provide the option for the calculation of the temperature input is always considered to temperatures of 60 oF standards. Another input required is the value of the gas compressibility at standard pressure (14.73 in this case) is usually derived from AGA 8 calculations.
To measure the gas itself, which is the fraction of weight on fiscal gas measurement value is small enough, but if there is any calculation will give a significant difference due to the heavy fraction has a heating value of the greatest value. If such a gas values should be 0.01% read 0.1%, that’s enough to give errors of about 0.4% error which, when cashed would provide a number of around several hundred thousand dollars per year depending on the amount of gas flowing.
C1 – C4 has a contribution greater HHV, because among other C heating value of C1 – C4 bigger. As a reference in Perry’s Chemichal Engineers Handbook 6th edition, table 3-207. Heating value listed there for each component:
C1 == 21,502 (BTU / lb)
C2 == 20,416 (BTU / lb)
C3 == 19,929 (BTU / lb)
IC4 == 19,614 (BTU / lb)
Nc4 == 19,665 (BTU / lb)
iC5 == 19,451 (BTU / lb)
Nc5 == 19,499 (BTU / lb)
e6 == 19,391 (BTU / lb)
There ’size’ other than the heating value of the volume, with units of Btu / SCF. Usually when we talk metering gas chromatograph with on-line measurement of the heating value is based on this volume, Btu / SCF, so the total energy passing yards (Btu per hour or per day) is the product of volume and value mmscfd this heating value (Btu / SCF). If the heating value is calculated based on the logical volume heating value of C2 will be higher than C1 and C3 is higher than the C2 and so on because the C2 molecular weight of more than C1, etc., is based on the principle that the volume of 1 mole of C1 will be equal to the volume of 1 mol C2 (about 379 SCF / mol).
GHV can be measured by Mass and volume, if based on the Mass (BTU / kg), higher C will contribute to the Heating Value is the lower, while if based on volume (BTU / SCF) is the opposite C higher contribution of Heating Value higher.
2145 Year of the GPA 2003, Physical Constants for Hydrocarbon. If the LNG component in the molar fraction, the GHV at 60 F as an ideal gas is:
C1 == 52.673 BTU / Kg == 1010.0 BTU / SCF
C2 == 49.238 BTU / Kg == 1769.7 BTU / SCF
C3 == 47.739 BTU / Kg == 2516.2 BTU / SCF
i-C4 == 46.808 BTU / Kg == 3252.0 BTU / SCF
n-C4 == 46.958 BTU / Kg == 3262.4 BTU / SCF
i-C5 == 46.394 BTU / Kg == 4000.9 BTU / SCF
n-C5 == 46.484 BTU / Kg == 4008.7 BTU / SCF
n-C6 == 46.174 BTU / Kg == 4756.0 BTU / SCF
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4 Responses to “Heating Value of Natural Gas”
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November 18th, 2009 @ 20:55
Please tell me about
How to convert Btu/scf to Btu/kg.
Thank you.
November 26th, 2009 @ 07:03
Natural gas has a standard density value (in mass/volume units), so the volume conversion to mass unit can be done.
February 18th, 2010 @ 08:58
Please tell me about
The composition of H2O and H2S have to normalise with GC composition before calculate GHV ?
What the standard document is the confirmation in the combination H2O and H2S with GC data for calculation GHV ?
Thank you for your information.
April 20th, 2010 @ 23:25
@ JJ:
All compositions are always normalized before calculating GHV
Involvement of the composition of H2O and H2S in the calculation can be seen on standard GHV GPA 2172 or ISO 6076
warm regards