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Natural Gas Composition Unit Conversions

Natural Gas Composition Unit Conversions


Firstly, I have been struggling with the units associated with natural gas composition and converting said units to other units. A common roadblock that I experience is a customer will send me a natural gas composition with "ppm" listed for certain contaminants such as H2S, CO2, N2 and H2O. I know I can, and probably should, always ask them to clarify what their units are, but the use of "ppm" without specifying volume, mass or molar seems to be all over the internet so I'm wondering if I'm missing something here? Is there a rule of thumb for when "ppm" is used? Can I assume that ppm is given as a molar basis when it concerns gas composition? From my understanding, ppm_molar, ppm_mass and ppm_volume will have completely different values (provided gas compressibility is taken into account). I should mention that I'm a recent Mechanical Engineering grad and Chemistry was never my strong subject.

Secondly, given a complete (or nearly complete) gas analysis on a molar basis (mole fractions), part of my job is to verify the gas suitability for use in our Thermoelectric Generators. For example, I was given a spec with 0.79%mol H2O. The customer converted this to mg/Sm3 where the standard conditons are taken at 1 atm and 15C. The value they gave was 17160 mg/Sm3 (for 0.79%mol H2O), but when I do the calculation myself, I end up with 6022.6 mg/Sm3. My calculation is as follows:

1) Calculate mass of H2O per mol of gas mixture using molar mass of H2O: 0.0079 [mol_H2O/mol_mixture] * 18015.28 [mg_H20/mol_H2O] = 142.32 [mg_H2O/mol_mixture]

2) Calculate molar volume at 15C and 1 atm: V = 8.314*288/101325 = 0.023631 [m3/mol_mixture] <---- Ideal gas law

3) Calculate mg/Sm3 of H2O: 142.32 [mg_H2O/mol_mixture] / 0.023631 [m3/mol_mixture] = 6022.6 [mg/Sm3]

Am I doing something wrong/not accounting for something in my calculation? What could be the reason the customer calculated such a high number?

Sorry for the multiple questions in one thread - I maybe should have split them up into separate ones. Thank you in advance for your feedback.

RE: Natural Gas Composition Unit Conversions

In the industrial hygiene field, ppm or better expressed as ppmV is a volume of a contaminant in vapor or gaseous form to 10^6 volume of air. In your case such relationship is probably not true and I would ask the customer for clarification. Normally in the field of combustion, relationship of constituents are expressed in percentages of volume for gaseous fuels, percentage by weight for liquid and solid fuels. Since none of my references indicate units in ppm for fuel constituents, not that it should not be done, but if it is, you should get clarification so that molecular weight of the fuel can be used the calculations. It would also be best to tell us the origin of the natural gas, so that, I can do a little research that may be of help.
For your second question, ask your customer if the 0.79% mole of H20 means .79 mole of H20 to 100 moles of combustible fuel, air or products of combustion, then, you can recalculate. I would also ask the customer at what temperature/pressure was the 0.79% value listed for, so that you could verify their adjusted value to standard condition.

RE: Natural Gas Composition Unit Conversions

In Oil & Gas "ppm" is taken to be "ppmV" unless some other measure is explicitly given. Like everything else in this goofy industry, it is not always adhered to, but mostly it is.

For a water content of 17,600 I get a mole percent of 2.2% (if C=2.3765E-5, then the equation is:

Mole% = W*C/(MWwater+W*C)

where the "W" is your 17,600 mg/SCM, and the definition P & T for SCM doesn't matter to the water side). You seem a lot closer than your client.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

RE: Natural Gas Composition Unit Conversions

I would add that there is a conversion formula from ppm V to ppm weight and vice versa given by ISO 20765 (appendix sections).

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