Liquefied Natural Gas is natural gas, the same stuff that’s piped to your house for heating and cooking, except that it’s been chilled down to -260°F (-160°C) so that it’s a liquid at normal atmospheric pressure.

Natural gas, in general, is a near-ideal fuel. We will never have the “hydrogen economy” because the molecule of hydrogen, which contains two atoms, is so small that it is almost impossible to store. Hydrogen is smaller than the spaces between molecules (or crystals) of anything you might make a tank out of, so keeping it is like storing marbles in a net-bag — inevitably some seeps out. If the tank is metal, on the way out the hydrogen bonds to the metal atoms to form hydrides, which are soft and brittle, not at all what you’d want to make a fuel tank out of[1]. Natural gas is mostly methane, which is one carbon atom and four hydrogens. Most of the energy from burning natural gas comes from the hydrogen. The carbon atom serves as a ball and chain to keep the hydrogen safely confined.

The disadvantages of natural gas as a fuel primarily come down to density. At normal temperature and pressure it’s a gas, slightly heavier than air (no: methane is lighter than air. Thanks, Cajun), with an energy content so close to 1,000 BTU per cubic foot that the units are interchangeable until you get to fine details. Gasoline, by contrast, has over 800,000 BTU per cubic foot, which is why it is, so far, preferred as a vehicle fuel. Replacing a 16-gallon tank of gasoline with the same energy content of natural gas would require over 13,000 gallons of volume, not particularly practical — a big tanker truck might carry 5,500 to 9,000 gallons, and towing two of them with your car would be clumsy and inefficient. There are two ways of overcoming this problem: Compression and liquification.

Compression is simpler. Natural gas at 200 bar has 200 times the energy density, so storing it in a vehicle becomes less problematic. You still need four times the volume to replace gasoline entirely, which isn’t practical in a small car but may be doable in a van or a truck. A bigger problem is the pressure. 200 bar is 29 thousand pounds per square inch, or roughly a thousand times the pressure in your tires. The tanks have to be strong, which makes them heavy. Again, that’s a problem in a small car but may not be in a truck. It might also be a problem in accidents, but a tank strong enough to hold 200 bar is also strong enough that it won’t be easily punctured. CNG is becoming available, and there are vehicles manufactured to use it, but it seems to me to be only a step — a step in the right direction, but only a step nevertheless.

[Correction from Fred Abernathy of Harvard: 200 bar is 2,900 PSI, or a hundred times normal tire pressure. Thanks, Fred. Mumble grumble. I used to be fairly good at arithmetic]

The more modern choice is liquification. This has two advantages. It results in even higher energy density — liquified natural gas has over 600,000 BTU per cubic foot, only a 1/3 penalty vis-a-vis gasoline. It also gets rid of the other stuff in it. “Natural” natural gas, straight from the ground, has all sorts of other compounds mixed in with it, and during the liquifying process they get separated out, so what’s left is almost pure methane. That’s important from a pollution point of view, since the other “stuff” has more carbon in it than methane, so it produces more CO2[2].

A tank for LNG needs to be strong, but not as strong as one for CNG. It also has to be insulated, and as a vehicle fuel this comes out to possibly the biggest disadvantage of LNG: if it isn’t used right away it warms up and escapes. One of the target markets for LNG is over-the-road trucks, which fill up and go, so the fuel doesn’t stay in the tank long. I don’t drive much, so a tank of gas lasts me over a week, sometimes two. If my car ran on LNG it would need a chiller to keep the fuel cold when I’m not driving, which would have to be run off the electricity in the house or use up some of the fuel to keep it going. For a city dweller with natural gas piped to the house, that would be an advantage — the same gadget could liquify the piped-in gas and store it, much more efficiently than battery charging. As a side benefit, in an LNG-fueled vehicle an air conditioner would be an existential definition of “redundant”. If you have a store of liquid at -260°F, compressing and expanding Freon® to get cool would be totally unnecessary.

One huge advantage of natural gas as a fuel is its octane rating. That’s a complicated subject, made even more complex by past advertising campaigns that label high-octane fuel as “premium”. An engine with high compression is more efficient, and for high compression you need a high octane rating. Natural gasoline has a low octane rating, so it needs additives to make it usable in an efficient engine[3]. When high-compression engines first started being produced that problem was quickly noted, and fuel to be used in the newer, better, more expensive engines was made available; the primary additive was tetraethyl lead, produced by the Ethyl Corporation and advertised to the skies as Newer Better Fancier — “premium”. High-octane gasoline actually has less energy than the cheap stuff! Nowadays, of course, pollution fighters have serious hots for lead in any form, and the additives that replaced it, mostly various forms of alcohol, are either more expensive, reduce the energy content even more, or both, and in any case don’t increase the octane rating as much as lead did. Natural gas has an equivalent octane rating of about 130, higher than any form of gasoline including “purple racing gas”, and as a bonus its burning process doesn’t form as much oxides of nitrogen as other fuels, so efficient engines are practical. They don’t even need catalytic reactors[4].

Another advantage of natural gas is that it’s here. The United States imports some natural gas (in LNG form, because that’s the most efficient way of transporting it) but it doesn’t need to, except where it’s more economic to buy it from an easily-exploited reservoir than to drill for it, or where population density plus safety concerns make pipelines less attractive[5]. There’s natural gas almost everywhere down deep[6], and we keep finding reservoirs of it where it’s seeped upwards and been trapped in rock that’s less porous. There are also clathrates, blobs of natural gas mixed with other things found in the deep, high-pressure, cold oceans — it’s difficult and dangerous to retrieve them, but if we run short the technology could be developed, and clathrates are abundant, possibly even containing more methane than rock does. There is lots of natural gas, and exploiting it would reduce both pollution and dependence on exports.

So why aren’t we using natural gas more?

Well, we’re starting to — now that it’s become apparent that it’s abundant, and therefore cheap and likely to remain so, new fixed installations tend to go that way. Among other things, it’s the ideal fuel for “peaking” and “backing” plants designed to generate the electricity supposedly produced by windmills and solar panels. As a vehicle fuel it’s a bit more problematic because of the storage problem, which boils down to infrastructure and thence to history. Gasoline was originally an unwanted byproduct of drilling for oil; the original market was for lubricants, to replace whale-killing and other animal fats, and for kerosene for oil lamps. The early invention of the Kettering spark system made gasoline-fueled, spark-ignition engines easier to build than Diesels, and the gasoline was there in quantity, despiséd by most and available for use. It’s relatively easy to handle and has high energy density, so the familiar setup of storage tanks and gas stations grew as the use of automobiles did.

The technology of the early Twentieth Century wasn’t up to achieving and maintaining the low temperatures and high pressures necessary for liquifying natural gas and storing the result, which is another reason they based the system on gasoline instead. Now, though, we have relatively efficient and inexpensive ways of doing that, many of them derived directly or indirectly from the space program — liquid nitrogen, at a temperature of -320°F, is considerably colder than LNG, and nowadays costs about as much as beer or gasoline. It’s time to look more closely at LNG as a vehicle fuel. The fact that the EPA isn’t clamoring for its use and offering subsidies to build up the infrastructure for it is conclusive evidence that they don’t know what they’re doing and/or have some agenda other than pollution and efficiency.


[1] This is one of the factors limiting the life of a nuclear fission reactor. Hydrogen is just a proton with an electron for company. A nuclear reactor produces free protons as a byproduct of its operation, and the protons latch on to electrons to form hydrogen — which promptly begins seeping through the piping and the reactor vessel, which are usually steel. The resulting hydrides make the steel brittle and likely to crack, so if it goes on very long the vessel and piping start breaking and it’s time to shut the thing down because it isn’t safe any more.

[2] The debate over whether carbon dioxide qualifies as a “pollutant” is not addressed here. The Law says it is, and that’s what we have to work with. There are also components of natural gas that don’t burn as efficiently, producing unburned hydrocarbons and nitrous oxides, which really are pollutants.

[3] It’s also possible to change the mix of petroleum products in gasoline to make a fuel with higher octane rating. This is being done in a small way — in most parts of the US it’s possible to buy “pure gasoline”, with no -ols in it — but it requires changing the refining process and produces less fuel per barrel of oil input, so it’s more expensive.

[4] The thing under the floorboards beneath your feet is a strong vessel in which a chemical reaction occurs, promoted by a catalyst. It is therefore a catalytic reactor, by definition. The term “catalytic converter” is a mealymouthed euphemism designed to keep Greenies and other ignoramuses from riffing on “well dayum, reactors is noocular, git that thang away from me!” Confuse the bastards. Use the right terminology — it’s even Yuropeen (and therefore sophisticated) to do that.

[5] Which is silly. LNG is delivered to places like Boston because of the perception that pipelines are dangerous, which isn’t a foolish concern — if they break it’s a problem (to put it mildly) — but a tanker full of liquified natural gas contains a lot of energy in a small space, and if one should ever catch fire and explode in a harbor next to a city full of people, it’s going to make the Texas City disaster look like a squib.

[6] This seems to me strong support for “abiogenic petroleum”, a.k.a. “the Gold hypothesis” after Horace Gold, the scientist who proposed it. To a very close approximation the Universe is made of hydrogen, and carbon is one of the main byproducts of the fusion process that makes stars hot and bright. Methane, natural gas, is the simplest combination of the two, and is even found in interstellar clouds. The Gold hypothesis is that when the Earth formed from a cloud of particles around the Sun, hydrogen and carbon were incorporated into it and formed methane; oil then occurs because heat and pressure forced methane molecules to combine into heavier fractions. This theory is pooh-poohed because we find microbes in oil, and it’s assumed that the microbes ate the carbon and produced petroleum; to me it seems equally reasonable to assume that the microbes found the oil and went “o yum, food!” (thus adding to the effects of heat and pressure to form heavier molecules). If the Gold hypothesis is true there is methane, and therefore oil, literally everywhere; there is no possibility of running out, because we’ll run out of oxygen to burn it with first.

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