Flammability limits of methane at high pressure eg 100 barg

[LordKelvin]

I am seeking information / guidance on the flammability of methane when it is mixed with nitrogen that contains a residual 5 mole % oxygen content at around 100 barg. Any advice very much appreciated, thanks

 

[monaco8774]

Methane at low pressurs has a LEL around 5% and a HEL around 15%, (that is methane in 100% air) also although not a lot of work has been done to my knowledge on high pressure operation, generally the envelope gets smaller not bigger at higher pressures, i.e. the LEL moves up and the HEL moves down.

So if you have 5% 02 in the nitrogen, you can compare this to the O2 in air and back calc a conservative value for your nitrogen mix based on low pressure. However you can see that your going to struggle here to get an explosive mixture at all.

 

[25362]

To monaco8774, my sources indicate that the LEL doesn't change much with higher pressures, while the HEL increases with pressure broadening the flammability range!

One formula from the literature:

HEL[sub]p[/sub] = HEL + 20.6(logP + 1)​

P is in MPa absolute
HEL is in vol% of fuel in air at 1 atm abs

Would you care to comment ?

 

[j2bprometheus]

Hi-
There a some references on the effect of pressure on the lean flammability limit.

Here's what [1] says:

For a methane-air mixture, the percent of methane limits are 6.0 and 17.1 at 10 atmospheres.

At 125 atmospheres, the limits are 5.7 and 45.5 percent methane.

There are some other references. Burgess and Hertzberg wrote a paper in 1975 on the effect of pressure on the lean flammbility limit (LFL). I cannot find my copy of the paper so I can't give you a title. But Burgess & Hertzberg indicate that for methane, the LFL remains constant up to 100 atmospheres. For natural gas, the LFL goes from 5 % at 100 atm to 2 % at 700 atm.


I have another reference regarding the effect of pressure on the lean limit for natural gas (Wierzba and Karim, 1990) if you are interested.

Also, there is a 1997 paper by Yamamoto that says pressure has little effect up to pressures of about 41 atmospheres.


1. Rose, J.W., and J.R. Cooper (editors). "Technical Data on Fuel", Seventh Edition, A Halsted Press Book printed by John Wiley & Sons, New York, New York, 1977. See Table 5.3, which is called "Effect of Pressure on Inflammability Limits". There are also a couple plots related to the effect of pressure.


Hope that this helps.

 

[butlen]

Its a long time since your post so I won't spend much time on reply.

As posted above increased pressure will dramatically increase HEL once you get above 1 atmosphere gauge pressure. LEL is not significantly affected.

Temperature also affects HEL at about 7% per 100 degrees C.

This is worth knowing as your pressure will have had to come from somewhere - ie a compressor which will develop temperature.

First you need to calculate the LEL and HEL of your mixture. Is the flammable gas just methane or is it a natural gas? If its just methane fine, but if its natural gas then you need to look at the individual flammable gas constituents because other alkanes like pentane and propane have different flammability limits to methane.

Your 5% nitrogen will suppress the flammability of the gas mix slightly. The way to work out specific limits is to use Le Chatalier's Theory (google it). Le Chataliers will give LEL and HEL for the specific mix you have.

Then use Zebatakis, or Coward and Jones (google it) to calculate the HEL at 100 bar based on the HEL at atmospheric we calculated from Le Chatliers.

For natural gas at 100 bar (without the N2) we get 55.4% HEL. British Coal Guidance OI 26 says that you are safe to compress methane down to 66.8% methane, or up to 8.48% oxygen at 100 bar, but note the safety factor in this.

If you want le chateliers or zebatakis references let me know.