Natural Gas as an Alternative Fuel

Natural gas is a medium-tern solution to the demise of petrol ... but not the greenhouse problem

How would you like it if, perhaps in the future, your energy company delivered fuel direct to your door? You could refuel in your garage, or at the office. If we can home-deliver everything from groceries to roses – why not fuel? Think of the real estate it would free up via suddenly redundant servos; the time it would save. It’s not as if the service station experience is sentimental … being asked, ad nauseum, if you have a shopper docket, or Fly Buys, or if the notion of two-for-one Kit Kats appeals.

As a sweetener, how about if the home-delivery option meant – kilojoule for kilojoule – you’d get the equivalent of a tankfull of petrol for less than half today’s going petrol price? Everyone would go for it.

In fact, even today, many Australians should be asking their local members why you can buy 58 litres of petrol for, say, $90. And yet, companies like AGL can sell the same amount of energy – home delivered, remember – for something like $40. It really makes you think.

That’s right: such a fuel is probably delivered to your street now. Australia has vast reserves – at least a century’s worth on current consumption, which (although admittedly there’s a major greenhouse issue) could at least help solve our Persian Gulf/crude oil dependency problem. (Two-thirds of the world’s known reserves of crude oil come from the Middle East.)

The fuel is of course natural gas. Delivery infrastructure is in place. A licensed gasfitter could install the fittings in your garage this afternoon. A purpose-built compressor and sundry plumbing (and, of course, the right tank in the car) is all it would take to get the job done.

AVAILABLE NOW

If you’ll pardon the pun it’s not a pipe dream: NG home refuelling stations can be bought in North America and Europe now. They’re especially popular in New York and California. All you do is plug your car into the compressor in the garage wall, which refills a pressurised tank in the car while you watch re-runs of The Simpsons and burn the dinner.

Fiat, Volkswagen, General Motors and Peugeot sell dual-fuel (NG and petrol) cars now. Plenty more, including Honda, have experimental NG-only cars on trial. All petrol cars could run NG, which enjoys a higher octane rating than petrol. Engines running on NG alone can exploit higher compression ratios and develop greater efficiency. There are currently an estimated 5.7 million NG vehicles worldwide (about 0.6 per cent of the vehicles on earth). These include trucks – diesel engines, with appropriate modifications, also run happily on NG.

UNDERSTANDING NATURAL GAS

NG is clear and odourless (but, for safety a chemical called Ethyl Mercaptan is added so you can smell any leak). It’s lighter than air, unlike LPG, so it won’t form combustible ‘pools’ in depressions if it does leak. It spontaneously ignites at just under 600 degrees C, and only burns if it’s mixed with air in the range of five to 15 per cent. It’s non-toxic, but doesn’t support life (so it won’t poison you the way cyanide does, but you can certainly suffocate in it).

NG is mainly methane. It’s the simplest hydrocarbon, one atom of carbon joined with four hydrogens. Methane is the first in a family of hydrocarbons called ‘alkanes’, in which methane is followed by ethane, propane (LPG), butane (lighter fluid), pentane, hexane, heptane and octane (petrol), etc. Each in this list has one more carbon atom than its predecessor, respectively, and there are plenty more after octane.

The alkanes methane through to butane, inclusive, exist as gasses. From pentane up to the one with 17 carbons, they’re liquids (like petrol and diesel), and from 18 carbons up they’re solids (like paraffin and bitumen).

THE MAIN HURDLE

Liquid fuels are practical because they pack a lot of energy into a small volume (this is often called a ‘high energy density’), and they can easily be pumped into an engine. Gasses are somewhat less practical as fuels because, uncompressed, you don’t get much energy in a given volume.

The reason LPG – a.k.a. propane – is a popular fuel is simply that it liquefies easily when moderately compressed, such as in the boot of practically every taxi in the country. Liquefied, it packs almost the same punch, per tankfull, as petrol.

This is not the case with methane. You’d need 56 cubic metres of it, uncompressed (a box four metres by seven metres by two metres) to hold the same energy as a 60-odd litre petrol tank – rather impractical for your average car. There are two options: compression or liquefaction. Compression leads to compressed natural gas (CNG), which stores the gas in a pressure vessel at up to 250 atmospheres, which is about a third of the pressure in the best compressed hydrogen fuel systems. That shrinks its volume to less than one per cent of the uncompressed size – which is quite manageable. CNG has about a quarter the energy density of petrol, so the range for any particular tank volume is about a quarter that of petrol.

Liquefied natural gas (LNG) isn’t practical for cars. (Hydrogen has similar problems.) It needs expensive, aerospace-spec super-insulating pressure vessels and must be maintained at around minus-150 degrees C. Too hard, except for bulk (supertanker) transport.

AUSTRALIA’S VAST RESERVES

In 2003, ExxonMobil made the largest discovery of natural gas in Australia’s history – bigger, even, than the acclaimed Northwest Shelf – when it located the Jansz gas field 200km off the WA coast. It totals the equivalent of 20 years’ worth of our NG production, and added one-fifth to the country’s known reserves. It’s located entirely within Australia’s economic zone, and appears to be unaffected by native title claims.

The CSIRO recently dropped the bombshell, in its thought-provoking Fuel for Thought report, that natural gas will be one of only two alternative fuels with any real clout over the next decade (the other is LPG). Why is it, then, that the darling of the alternative fuel set appears to be the problematic alcohol, ethanol, while natural gas seems swept well and truly under the rug?

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