Methane Number Calculation

[msonnet]

Hi everybody,

I previously posted this question in another forum. Can anybody help?

I am trying to figure out how to calculate the Methane Number (MN) for a given gas composition. I tried to find some information in the literature but was not successful.

I only found the following formulas:

MON = -406.14 + 508.04*(H/C) – 173.55*(H/C)^2 + 20.17*(H/C)^3
MN = 1.624*MON – 119.1

with H/C being the hydrogen/carbon ration. (MON = motor octane number).

Having tried it on several gas compositions, I am always off by some 10% of the real MN number. Even if I take 100% CH4,I get a MN=108 which is obviously wrong.

Would it be possible to calculate the MN of the gas composition based on the MN of the various constituents multiplied by the Mol %, such as done to calculate the Molecular Weight of the gas? If yes, where can I find the MN numbers for the various elements?

Thanks in advance.

 

[TD2K]

What's the source of this equation? I'm frankly suspicious of it, the octane number is strongly dependent on the structure of the molecule. octane and iso-octane have widely different octane numbers yet this equation has no way to differentiate between the two.

There's a excellent book called 'Guidebook to Petroleum Product blending' that has correlations for blending of lots of hydrocarbon properties and octane is one but you need to know the octane number of each blend stream, they make no attempt to predict it from basic physical properties.

This isn't my area of speciality but last stupid question is: what is methane number? I've not come across this term.

 

[msonnet]

I found the formulas in the following documents.

http://www.arb.ca.gov/regact/cng-lpg/appd.PDF

http://www.arb.ca.gov/regact/cng-lpg/appa.PDF

The MN number is used for gas engine fuel compatibility as you would use the WI (wobbe index) for the gas turbines. Why MN? I guess this has to do with the natural gas that is normally burned and composed of around 85 to 90 %mol in Methane.

Target is to have a MN above 80 to avoid knock problems on the engine. Certain engines can go lower.

The knock resistance of gaseous fuels is often evaluated by the methane number. Methane, which has high knock resistance, is given the index 100. Hydrogen, which has low knock resistance, is given the index 0. If a certain gas mixture has a methane number of 70, its knock resistance is equivalent to that of a gas mixture of 70 % methane and 30 % hydrogen.