Thursday, March 15, 2007

Methane: a dossier

Methane is the simplest hydrocarbon, a single carbon atom surrounded by four hydrogen atoms, each at the vertex of a regular tetrahedron.

Natural gas is over 99% methane, the remainder being some heavier hydrocarbons such as ethane, propane, and butane.

Methane lasts for about 10 years in the atmosphere. Its main sink is photochemical oxidation. Additional methane interferes with the photochemical sink so that any additional emitted methane is expected to last 14 years rather than only 10.

The pre-industrial concentration of methane in the atmosphere was 0.7 parts per million. The atmosphere contains 4,850 teragrams of methane, 4.85 billion metric tons.

Methane is the third most abundant greenhouse gas in our atmosphere, after water vapor and carbon dioxide.

Anaerobic bacteria emit methane as a waste product. Anarobic bacteria live in swamps and other wetlands, sanitary landfills, animal waste, domestic sewage, and in the intestines of various animals and insects.

The combustion of organic matter emits methane. Biomass burning (for forest clearing among other things) emits an estimated 40 million tons of methane per year.

Termites emit 20 million tons of methane per year.

Natural gas leaks emit 40 million tons of methane per year.

Swamps and other wetlands emit 115 million tons of methane per year.

Total worldwide emissions of methane from all sources, natural and anthropogenic, are an estimated 535 (plus or minus 125) million tons per year.

Methane is the eighth most abundant gas in the atmosphere, behind nitrogen, oxygen, argon, water vapor, carbon dioxide, neon, and helium.

The melting point of methane is -182.48 C, and the boiling point is -161.49 C, at standard pressure.

Replacing one of methane's hydrogen atoms with something else gives a "methyl" compound. The simplest replacements are for the halogen elements, fluorine, chlorine, bromine, iodine. CH3F is methyl fluoride, CH3Cl is methyl cloride, and so forth.

Global methane concentrations vary annually by about 3% from the highest value (during summer) to the lowest value (during winter).

Methane concentrations vary with latitude and hemisphere. The highest concentrations of methane are in the Arctic during summer months, the lowest concentrations in the Antarctic during winter.

The global average concentration of methane in 1982 was about 1.6 parts per million.

The global average concentration of methane in 1972 was about 1.4 parts per million.

The global average concentration of methane in 1992 was about 1.7 parts per million.

The global average concentration of methane leveled off in 2000 at about 1.75 parts per million.

The growth rate of methane concentrations in the northern hemisphere in the 1990s was much smaller than in the preceding two decades. No one knows exactly why.

Rice paddies emit 60 million tons of methane per year.

Coal mines emit 30 million tons of methane per year.

The danger of methane in coal mines is from asphyxiation and explosion. Miners carry canaries, which pass out from a lack of oxygen more quickly than do humans, to warn them of asphyxiation dangers. The "miner's lamp" was invented by Sir Humphrey Davies to reduce the dangers of explosion.

The methane produced in wetlands sometimes bubbles to the surface in substantial quantities, and occasionally this methane catches fire, causing mysterious "lights" in the middle of a dark swamp at night. This "swamp gas" explanation was once used to explain some UFO sightings.

On a "pound for pound" (unit mass) basis, methane is 58 times more potent a greenhouse gas as carbon dioxide. However, CO2 concentrations in the atmosphere are 200 times greater than those of methane.

Methane is used for motor vehicles in the form of "CNG," which stands for "compressed natural gas." It is one of the cleanest forms of internal combustion.

The hydrogen for space launch rockets is made from methane, via a catalytic process.

Methanol is made from methane, and methanol in turn is used to make MTBE, methyl tertiary butyl ether.

Methane is a colorless, odorless, tasteless gas. When used in commercial natural gas, an odorant is mixed with it to alert users of gas leaks.


Anonymous said...

Great report on methane!

Please continue to report more as you see fit.

Know much about the infamous Reagan comment about more pollution being emitted by trees than factories? I believe he was referring to terpenes.

James Killus said...

Sorry to be so tardy in reply.

In a clarification issued after Reagan's statement, the White House said that he was referring to nitrogen oxides, which further compounded the confusion. If nitrous oxide (N2O) is taken as a nitrogen oxide (which it technically is, it just doesn't participate in smog formation; it does have a stratospheric impact however), then the statement is technically true.

It is true that there are more terpenes emitted by trees (and isoprene as well, by hardwoods) than by factories, but, of course, the biogenic source is much more distributed, and terpenes decay very rapidly on contact with--ozone. Biogenic hydrocarbons do contribute to smog in urban areas, though, which caused a big fight in the mid-1980s, with a guy by the name of Chamedies being the big winner (he used my chemical kinetics mechanmisms though, heh, heh, heh).