you'll need a gas analysis for the exact value. But to get close enough, methane has about 23,900 BTU/lb gross or upper heating value or 21,500 net or lower heating value/lb.
Why must everything be available on the internet? Perry's Chemical Engineers' Handbook (in any good library) has heats of combustion for a great many (over a hundred) hydrocarbons.
On the Internet, go to the NIST Chemistry WebBook at
(1) Go to that website and click on "name". Then enter methane and select "Gas Phase" and "Thermodynamic data". Then press "Search".
(2) You will then find the molecular weight of methane to be 16.04 and the [Δ]cHo of the methane gas to be 891 kJ/mol ... (where [Δ]cHo is the gross or higher heat of combustion and the mol is a gram-mol).
(3) Multiply kJ/mol by (430.12/molecular wt) to obtain Btu/lb. Thus, the gross heat of combution for methane is (891)(430.12/16.04) = 23,893 Btu/lb.
Follow the same procedure for any other gas for which you want the same data.
Milton Beychok
(Visit me at www.air-dispersion.com)
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If using natural gas I would use 1030 as the minimum btu per cubic foot as the basis to determine the flow rate. I might also calculate at a higher value when checking for rangeability.
Natural gas is not purely methane. Most natural gas includes ethane, propane, butane and condensates once called "drip gas" or gasoline. Some gas includes carbon dioxide, hydrogen sulfide, etc. Natural gas processing plants extract liquids to sell as Natural Gas Liquids or liquified petroleum gas such as propane and butane. Older plants may only extract the heavier molecules but the turbo-expander plants extract most of the ethane too. However the interstate pipelines have a minimum btu/cf value. I think that the contracts require over 1030 btu per cubic foot. The bigger molecules have a higher heating value per cubic foot. Sufficient ethane or such must be left in low btu gas to exceed the 1030 or so minimum.
As a further note, you are doing yourself a favor by working on a mass basis (BTU/lb rather than BTU/cuft) for hydrocarbon gases. Mass directly reflects on carbon atoms and the number of bonds to be broken. JLSeagull's last post "bigger molecules have a higher heating value per cuft" shows this. If hydrogen is not present (very high BTU/lb but very low BTU/cuft) then you can use methane's net HV of 21,500 BTU/lb with acceptable accuracy for many calculations even with a good amount of ethane or propane present. My experience anyway.
