I will provide links to other installments in this series for help with the nomenclature. Those will come after the jump.
Hydrocarbon fuels are the most commonly used ones, after coal. The "light" hydrocarbons, by my definition, include methane, ethane, propane, and the two butanes, with from one to four carbons per molecule. I classify these as "light" because they are all gases at normal temperatures and pressures.
These are the cleanest of the hydrocarbons as far as carbon dioxide production is concerned, because they have the highest molar ratio of hydrogen to carbon of all of the hydrocarbons. For methane that is 4:1, whilst for the butanes it is 2.5:1. As the number of carbon atoms per molecule increases, the relative proportion of hydrogen decreases.
For the introduction to this series, go here. Several key terms that we will use over and over are defined there. For the previous diary about the fallacy of the hydrogen economy, go here.
Natural gas is mostly methane, and I will use the properties of methane to describe it. Depending on the source, it may contain up to 6% or so of ethane, propane, and the butanes (mainly ethane), so pure methane is a pretty good approximation. Almost all natural gas is produced from wells. Gas often but not always is associated with significant amounts of petroleum ("gusher" oil wells are often pressurized with methane). Consequently, environmental impacts are similar to those associated with oil production, except not as severe in most cases.
After gas is extracted, it has to be dried and heavy hydrocarbons, if any, have to be removed. Sour gas also undergoes a sulfur (mostly hydrogen sulfide) removal step. Since methane has almost no odor, an odorant (usually ethanethiol) is added at a few parts per billion so that leaks can be easily detected my scent.
Methane is conveniently transported by pipeline under high pressure, and that is by far how the largest volume is moved. In recent years tankers of liquefied natural gas (LNG) have been moving methane from some producing regions overseas. Methane is hard to liquefy, because it has a low critical temperature of minus 82.25 degrees C (colder than dry ice) (see the introductory installment). That means that it has to be transported as a refrigerated liquid, a decided disadvantage.
As an automotive fuel, natural gas has be used as a highly compressed gas, requiring very strong (and large) tanks. For the equivalent of a 20 gallon tank of gasoline, roughly 100 gallons of high pressure storage capacity is required for natural gas. Thin sheet metal will not do, like for a gasoline or even a propane tank, because we are talking internal pressures of 3000 psi or greater. The biggest drawback at present, at least in the US, is a relative lack of fueling stations. Cars can be fueled from household gas supplies, but that requires a compressor and electricity and is slow compared to stopping at a high pressure fueling station. A downside to high pressure refueling is adiabatic heating of the fuel tanks. (Those of you who are scuba divers know about putting your air tanks in a pool of water to keep them from getting too hot whilst being filled).
For fixed installations, such as factories and power plants, natural gas probably provides the least polluting source of heat. For things that move, our infrastructure has not caught up yet. One real advantage to natural gas is that it is only about half as dense as air, making it tend to escape from buildings where there are leaks. It is the only fuel in common use with that property. The lower and upper explosive limits (see the introductory installment) for methane in air are about 5% to 15% volume/volume, fairly wide but nothing like that for hydrogen (4% to 75%). Methane is also a very potent greenhouse gas in comparison to carbon dioxide and, in my opinion, needs to be looked at more closely than it is, since all the attention seems to be directed towards carbon dioxide.
UPDATE: it may be that there are huge, untapped supplies of methane at the bottom of cold, deep ocean waters in the form of methane hydrate. This ice-like material is essentially pure methane and water, and, if the technology can be developed, may make methane the fuel of choice for the foreseeable future. An added benefit is that this methane would be prevented from eventually entering the atmosphere and the subsequent greenhouse heating.
Liquefied petroleum (aka LP gas) is mainly propane with greater or lesser quantities of the butanes. Propane is clean burning and, since it can be liquefied at room temperature (its critical temperature is 96.8 degrees C, almost the boiling point of water. This greatly facilitates transportation since it can be shipped either as a gas under pressure of as a liquid in pipelines, and as a liquid in tankers. This also makes it very useful for automobile applications.
Propane liquefies at minus 42 degrees C at atmospheric pressure, making it a better fuel in cold areas than the butane that was used decades ago. (Butane liquefies at minus 0.5 degrees C, so will not move from the tank to the house in cold weather. Those of you that are old timers may remember that butane tanks were buried and the incoming line was too to avoid "freezing up).
The biggest disadvantage with propane is that it is mainly a byproduct from the petroleum industry, so it is essentially made from oil. Thus, it has the same limitations insofar as finite supply as other hydrocarbon fuels except for natural gas has. In addition, when demand for other petroleum products becomes greater, the supply of propane becomes lesser because it can be reformed into larger molecules. Thus, there can be relatively large price swings in propane compared to natural gas.
Engines run on propane (and natural gas, for that matter) run more cleanly, both from a carbon standpoint and from other pollutants than engines run on gasoline or Diesel fuel. Propane is often used for forklifts in warehouses because little unburnt hydrocarbons and carbon monoxide are produced. In addition, engine wear is much less with these fuels. Propane fueled car engines can, in theory, run many hundreds of thousands of miles until overhaul is required.
Propane and the butanes have, like methane, little odor, so the same odorants used for natural gas are added to propane for fuel uses. (To get the idea of their natural odors, sniff a butane lighter (DON'T LIGHT IT!). The smell is very light and not at all like a gas leak (some of the smell comes from trace amounts of butene, an unsaturated hydrocarbon with a heavier smell). I do not know the reason that an odorant is not required for lighters. If anyone knows, please let us know in the comments.
A decided disadvantage to propane is that it is about 1.5 times more dense than air, thus allowing it to "pool" in low areas in case of a leak. For that reason, propane is more apt to cause an explosion than in methane, since the methane tends to escape, as mentioned above. Propane has a narrower explosive range than methane, 2.4% to 9.5%, so it is harder to make an explosive mixture than for methane, but the pooling phenomenon more than offsets that. Propane is not a greenhouse gas, and is becoming an important refrigerant to replace the chlorofluorocarbons.
Next week we will return to food additives, I think with thickeners. There are a bunch of them. Please respond with comments, questions, corrections, and any other science and technology related topics. Even though I realize that you blow a gasket every time I say this, I really mean that I learn much more than I teach by writing this series.
Warmest regards,
Doc