Over the past six months, I have read dozens of articles that address how natural gas may play a role as a transportation fuel.
The simplest way to use natural gas in a vehicle is to compress the gas to a high pressure (CNG) and use it as a direct substitute for gasoline in a spark ignited internal combustion engine. This is probably the best alternate as only 3 to 5% of the energy in the gas is wasted in compression of the gas and 95 to 97% of the fuel’s heating value remains for use as an alternate to gasoline. Of course the vehicle will have a heavier fuel tank resembling a welding cylinder and will lose some of its trunk space, but even with this disadvantage at least 95% of the fuel value remains intact for usage as a fuel aboard the vehicle. Also as methane has a ratio of four atoms of hydrogen to one atom of carbon it is a lower carbon fuel than gasoline and when burned to yield the same amount of energy methane will emit at least 20% less CO2 emissions per mile traveled than will gasoline.
The second simplest method to use natural gas as a transportation fuel is to liquefy the gas (LNG) and store the gas as a cryogenic liquid aboard the vehicle at a very low temperature. Approximately 10% of the energy content of the natural gas is used in the liquefaction process. The advantage of LNG over CNG is that the fuel tank takes up less volume or more gas can be stored in the same volume on board the vehicle to increase the vehicle’s range. The cost to make LNG is also much higher than to make CNG and the LNG alternate in my opinion is less preferable than CNG but both alternates still are what I call “green” options as they lower carbon emissions.
The third alternate is to convert the natural gas into gasoline in a gas to liquids (GTL) process. Approximately half of the energy content of the natural gas is lost in the GTL process and also the resulting gasoline like fuel is higher in carbon content than methane. Hence the desired goal of lowered carbon emissions simply does not result in this option. The only driver for this option is that the liquid fuel in GTL can be directly substituted for oil derived gasoline and there is no storage disadvantage. But this scenario basically defeats any purpose of emissions reduction and simply wastes half the energy in the natural gas. While there may be economic merit in GTL projects given the arbitrage in the value of liquid fuels over natural gas, there is no thermodynamic merit at all in GTL. In fact there is major thermodynamic and hence ecologic disadvantage.
The fourth alternate is to convert the natural gas to hydrogen and then compress or liquefy hydrogen as a transportation fuel. This is kind of dumb as hydrogen gas is 1/8 as dense as natural gas and only has 2.5 times the heating value per pound. Hence compressing hydrogen means only 1/3 the fuel can be carried aboard the vehicle compared with natural gas. In addition one quarter of the energy in the natural gas is lost in its conversion to hydrogen. Liquid hydrogen is also approximately 1/8 as dense as LNG so it too makes no sense. Also liquid hydrogen needs about ¼ the energy in the gas for the compression energy to liquefy this extremely low boiling point gas.
The fifth alternate being proposed is to convert the natural gas to hydrogen and then combine hydrogen with nitrogen to produce anhydrous ammonia as a liquid fuel. This has all the losses of producing hydrogen plus the energy to compress hydrogen and nitrogen to synthesize ammonia. On top of that the resulting fuel is only 3/17 by mass hydrogen and the remainder is nitrogen that adds no fuel value to the equation. Besides ammonia is a toxic and dangerous gas and the idea of using ammonia as a transportation fuel is almost a Rube Goldberg idea.
The sixth idea is to reform (react) natural gas with steam and produce hydrogen and carbon monoxide. These gases are then fed to living organisms in bio reactors that convert the gases into liquid fuels. Of course this is a very expensive method of producing liquid fuels and probably the companies that are trying this will die of carbon monoxide poisoning. I actually prefer the ammonia idea to this almost idiotic idea.
The final alternate is my own invention of the dual fuel vehicle with natural gas converted to methanol and then using a small quantity of methanol to augment a gasoline fired engine. My dual fuel idea has the most thermodynamic merit as the dual fuel vehicle will attain diesel type MPGs while using gasoline and disassociated methanol as the combined fuel in a high compression, lean burning, and non-throttled engine. When the added fuel efficiency of the dual fuel engine is credited against the energy lost to synthesize the methanol, the overall energy balance actually results in more energy saved than energy used in the natural gas to produce the methanol. This is basically because the engine should enjoy at least a 10% improvement in MPG while using methanol for perhaps only 5% of its fuel needs.
Of course I wish the dual fuel approach will be widely adopted but failing that, I suggest we simply compress the natural gas and have many more CNG vehicles on the road. Presently, the US and Bolivia have approximately the same number of CNG vehicles in their respective fleets. The US must increase its natural gas use in vehicles by a hundred fold (two orders of magnitude) in the next ten to fifteen years. Natural gas is a great fuel and we are blessed with significant quantities. Let’s just use it wisely.