energy density of CNG at 3600 PSi

Methane 100 moles/hour

TEMPERATURE DEG F 60.00000

PRESSURE PSIA 3614.000

ENTHALPY BTU/HR -129743.7

ENTROPY BTU/R/HR -1258.874

VAPOR FRACTION 1.000000

PROPERTIES TOTAL VAPOR

----------------------- ---------- ----------

FLOWRATE LBMOL/HR 100.0000 100.0000

MOLECULAR WT. 16.0430 16.0430

ENTHALPY BTU/LBMOLE -1297.4370 -1297.4370

ENTROPY BTU/LBMO/R -12.5887 -12.5887

CP BTU/LBMOL/R 13.5698

CV BTU/LBMOL/R 7.6169

CP/CV 1.7816

DENSITY LB/FT3 12.0997

Z-FACTOR 0.859352

FLOWRATE FT3/SEC T-P 0.036831

MMSCFD STP 0.910626

VISCOSITY CP 0.023171

TH.COND BTU/FT/HR/F 0.039636

STREAM CRITICAL TEMPERATURE (CUBIC EOS) F -116.4

STREAM CRITICAL PRESSURE (CUBIC EOS) PSIA 673.1

TS IS STANDARD TEMPERATURE AT 60.00 DEG F

You seem to be going about this the wrong way round. The calorific value per unit mass is the more basic figure.

Doing a couple of checks on expected density at 3600psi and expected CV, it looks like your gas is mainly methane. Then net CV ~ 50MJ/kg = 13889watt-hr/kg.

Also if you know the Btu/ft³ quoted by gas supplier, at supply pressure to appliances just above atmospheric, and you know or can estimate density at that pressure, you can work out watt-hour/kg.

You're a factor about 1000 out (depending on just how you do the calcs) in your 207514 kg/m³. Dividing 266000 Btu/ft³ by 50MJ/kg gives 198kg/m³, roughly as calculated for methane at 3600psi.

Advantage of mass CV is it is pretty constant for various fuels. For all the usual hydrocarbons above about butane it's 44MJ/kg (net CV). This is figure used for petrol, diesel etc. It rises a bit thru propane and ethane to 50 for methane.

Incidentally, couldn't assume your natural gas is methane. I have data on nat gas in USA which has CV per ft³ 2.5 x higher than methane so has higher molecular weight. Also nat gas (in UK anyway) has some higher MW gas in it, so has higher volume CV than pure methane, but somewhat lower mass CV.

Cheers....Codey

Damn youre right. I was wondering what is it that i was doing wrong. its not the KW h/kg that changes but the BTU/Ft^3 because the mass remains the same, its the density thats changing. cant believe i actually made the mistake when i used the calorific value to calculate that of diesel and gasoline.

a kilogram of nat gas is equal to 1 1/2 litres of gasoline.

a 77 litre tank held 11 kilos of cng at 3300 psi.[former truck]

go to "http://encyclopedia.airliquide.com/Encyclopedia.asp?gasID=41"

55% of air density. good luck