Methane Number calculation

[msonnet]

Hi everybody,

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.

 

[buzz41]

Since side branching of a fuel hydrocarbon has a big effect on octane number, I am not sure one can be very predictive of fuel performance by using merely the carbon/hydrogen ratios.

For example, normal octane (C8H18) has very poor performance, but Tri Methyl Pentane, also (C8H18), and also known as iso-octane, is very good. In fact, it is the standard for the 100 octane number as you may know. In this case, the H/C ratios of these two fuels are the same, but the motor octane numbers differ by more than 100 points.

Maybe that H/C formula is restricted to linear molecules?

Your formula relating methane numbers to motor octane is interesting. The arithmetic gives:
MN 100 = MON 135 and
MN 0 = MON 73.5

This means that since the MN scale has methane = 100 and hydrogen = 0, that the motor octane of methane is 135 and hydrogen is 73.5.

I googled a goldmine of info on Compressed Natural Gas as vehicle fuel at

http://www.iangv.org/html/sources/sources/reports/IANGVREPpart2final.pdf

There are actually 7 parts. Just play with the URL to get the others.

The site has a lot of tech content.