Critical perspective of using natural gas for vehicles
Current technology in context
Two factors have spurred the manufacturer of compressed natural gas (CNG) and liquefied natural gas (LNG) vehicles – fuel economy and environmental factors. A frequent confusion needs to be cleared up, the difference between the two forms of the gas. CNG is similar to LPG, in that it too is a fossil fuel and is methane. However, the central difference is that the former is a highly compressed gas – signified by the “C”, and the latter is an uncompressed very cold liquid, as the “L” means.
When not transported by pipeline, LNG (CH4, or methane), is liquefied as a result of extreme compression. (25 kPa/3.6 psi) to about 1/600 of its natural volume and is stored in thick-walled cylinders at about −162 °C (−260 °F). Contaminants, such as dirt, helium, heavy hydrocarbons, gases with acid, and water are removed in the processing. Odorants are added to this otherwise odorless, non-toxic, and colorless gas so as to make it detectable . In that form it is safe, but when the gas is let out or escapes, it expands and is subject to ignition at a 5-15 percent gas-to-air concentration. While the storage cylinders are very efficiently insulated, there is a tendency for the gas to warm. Temperatures are kept at storage levels by a small continuous boil-off called “cryogenic boiling”. This purposeful leaking subtracts heat from the remaining gas, in a similar manner that heat passes from water to create steam.
Natural gas, of course, is a fossil fuel, a hydrocarbon, subject to the same restraints as regular gasoline, but it does burn much cleaner, hence the original and primary motivation to switch to it several decades ago. However, it has been found that methane is a greenhouse gas, more threatening than carbon dioxide, but is there is less natural gas emitted into the atmosphere at present. On the other hand, natural gas has 20 times the ability to create radiative energy than carbon dioxide. A ton of methane can capture just as much radiation as carbon dioxide, though it stays in the atmosphere from eight to 40 times shorter . As the permafrost warms up huge quantities of methane are released, and there is concern that this could accelerate global warming. In the event of spillage, natural gas will disperse more quickly than gasoline or diesel fuel. LNG comes primarily from Trinidad, Tobago, Algeria, Indonesia, Malaysia, and Qatar. Iran and Russia are potentially large sources, as well. The cost of LNG plants is high due to lacked of skilled persons construct them, material costs, and general rising prices of petrochemicals.
CNG is less than 1% of its original volume in a free state at standard pressure and temperature and about 42% of the volume occupied by LPG. It is stored in thick walled cylinders or spheres at a pressure of 200–248 bar (2900–3600 psi. Because there is no cryogenic storage as with LPG, CNG costs less to keep on site. An alternative storage method is adsorbed natural gas (ANG) with pressures around 500 psi, the same as in a natural gas pipeline. The storage tanks contain spongy materials like activated carbon and matrices of substances composed of metals combined with organic materials – organic metal frameworks (MOF). Hence, vehicles can be refueled at sites that don’t need to compress the gas further. As a result of the lower storage pressures, the vehicle tank can be of lighter material, and the walls do not have to be as thick. The storage containers have thicker walls than those of LPG. It is CNG that is primarily used as a vehicle fuel, there being some 12.6 million worldwide in 2010 . Many of these vehicles are bi-fueled, i.e., having the capability of running either on gasoline or CNG. This is to say that the vehicle’s combustive apparatus – injectors, cylinders, emissions control, and so forth are used for either fuel.
An advantage of CNG is that it can be mixed with biofuels, such as methane produced from landfills and animal waste. Like LNG, CNG does not foul spark plugs as readily and contaminants don’t befoul crankcase oil. There is 1.6 times the amount of energy in standard gasoline . Like LPG, CNG is in closed spaces, thus doing away with spillage or evaporation as with conventional liquid petrochemicals. However, because of its storage requirements, CNG-powered vehicles need storage space – lots of it – for the tanks, which, at the same time, are much heavier than conventional tanks used for regular liquid fuels.
Even with a redesign of the tank, such as flattening it out and storing it on the roof or underneath the vehicle, the vertical profile of the vehicle, perforce, will be higher. While the gas is the same, methane, CNG is the method of choice for vehicle, as opposed to LNG because there are no cryogenic storage requirements for the former.
References (Subject is indicated by URL – accessed 18 September 2011)