How much does the methane content of natural gas vary, and what is the impact?

Question

According to Wikipedia, natural gas

is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide, or helium.

I'm curious about a few things that aren't explained in the entry:

• How much methane is typically in natural gas?
• How much does this vary?
• What influences the methane level?

Most importantly, I'm wondering: are there any sustainability factors influenced by the methane content of natural gas? For instance, I know that methane is a potent greenhouse gas, so perhaps natural gas with a lower methane content would be better for the environment. On the other hand, perhaps combustion efficiency improves with higher methane content, or air pollution is reduced.

Answer 1

Apparently Wiki is sort of wrong. True for gas at a well head . But most gas in transmission lines delivered to customers is methane and a trace of air. The ethane ( 2 carbons) and higher carbons are worth more money than methane. Separating these other components is what the many gas plants around the world do : They take in "wet" gas ( aka well head raw gas) into a cryogenic separation to take out everything that is not methane. As noted in the other answer there are liquid slugs in the wet gas , so the gas plants have "slug catchers" , usually a bunch of pipe ),Like everything "cold" costs money , so some plants do not cool enough to get all the ethane. depending on location , season, market ,etc , there will be some ethane. Ethane gives more BTU per volume and this might be a problem for some end users so a little air is bled in to adjust the heating value to 1000 BTU /cubic meter ( In US ),( I probably have the units wrong ). This is for sweet gas with no mercury ( very bad for the aluminum cryogenic units) . Sulfur ( sour gas) and mercury must be removed before separations.

Answer 2

Content is typically very high percentage methane. First stage processing removes propane, butane etc because they tend to liquefy at the pressures used in the pipelines. Then they collect in the low spots when the line isn't running at capacity. When the flow increases, you get a slug of liquid propane being pushed by the gas. A pipeline pump designed for a gas gets really unhappy when it gets a slug of liquid.

Detail analysis: https://www.uniongas.com/about-us/about-natural-gas/Chemical-Composition-of-Natural-Gas

Note that these are in moles, and hence will be volumetrically correct. Percentage by mass will skew away from methane. (Remember that a mole of any gas is essentially the same volume at reasonable temperature and pressure)

The higher alkanes are green house gasses, but have short lifetimes. See this good answer on quora: https://www.quora.com/If-methane-is-a-significant-greenhouse-gas-then-do-all-alkanes-also-have-potential-to-be-a-significant-greenhouse-gas

Turns out that right now the liquids are more valuable than the methane. Natural gas for heating would be a lot more expensive, but right now the drilling and fracking is being driven by the price of butane and propane. (Don't ask me for a source for that. Somewhere in the last year.)

The other significant variable component is hydrogen sulphide (H₂S ) Smells of rotten eggs at < 2 ppm. Anesthetizes the nose at slightly higher levels. Recommended safety levels no more than 10 ppm for 10 minutes. High levels are deadly. 1000 ppm is "1 breath unconscious, a few minutes dead" Since you can't smell it, and it's heavier than air this is a major threat in confined spaces. There are wells in Alberta that produce up to 30% H₂S (300,000 ppm)

Answer 3

If your transmission system is losing a lot of methane to the atmosphere, you've got more immediate concerns, and it should all be burnt at point of use, giving CO₂ and water vapour. Small leaks do occur, but the big leaks are the point of production, where methane is what's present, rather than in the distribution grid.

The higher alkanes also have significant global warming potential if they get into the air. The only reason you don't see them listed as often as methane is that methane is common, and produced by lots of processes (like ruminant digestive systems). They're no better for producing CO₂ than methane when burnt (from the composition they should be worse - I'll try to run the numbers later).

The one thing that can be done is to mix in some hydrogen, which is being trialled in the UK. This produces only water when burnt, and isn't a significant greenhouse gas if it leaks. This can only replace a fraction of the methane without everyone replacing all their gas-burning appliances.

Answer 4

Using the resource "Chemical composition of natural gas" linked in Sherwood Botsford's answer, I calculated the range of CO₂ emissions for various compositions of natural gas, using this process:

1. Using the molar mass and chemical formula from Wikipedia, calculate the % mass of carbon in each constituent hydrocarbon
2. Combine this with the heat of combustion from Engineering Toolbox, to determine a heat content to carbon content ratio -- a sort of "carbon efficiency"
3. Within the typical range of each gas, determine the value that makes the end product more or less carbon efficient overall
4. Sum up the total heat of combustion and carbon mass from the resulting mix to calculate the ratio of carbon dioxide to heat.

Here are the results:

• Typical analysis: 50.26 kg CO₂ / GJ
• Maximum emissions: 51.62 kg CO₂ / GJ
• Minimum emissions: 49.76 kg CO₂ / GJ

The total variation from the typical analysis is -1% to +3%. Because methane is the most "carbon efficient" constituent of natural gas, the emissions rate essentially correlates to how much methane it contains.