This the the first part of a multipart series (I have not decided whether there will be three or four parts yet) about fuels. This installment will consider different classes of fuels, and future installments will cover each class is some detail. I will link back to this installment in future ones for easy access.
A fuel can be defined as a material that can be combusted with oxygen (usually but not always from the atmosphere) to produce combustion products and to release energy, usually in the form of heat. Under this definition, nuclear materials are not strictly fuels, since they do not combust. We will, however, cover them as well since they are a source of heat.
Before we get started, I wanted to set the record straight about the ice storm. At latest count, there are 28 dead in Kentucky as a result of the storm. Last week a commentor, who shall remain nameless, bragged about how he toughed out the 1998 ice storm in Canada, and that ignorant Kentuckians just do not know how to deal with the cold like the smart Canadians do.
Here is a quote from Environment Canada, a federal agency that performs, amongst other functions, services similar to the US National Weather Service.
How did the storm affect Canada:
* at least 25 deaths, many from hypothermia.
* about 900,000 households without power in Quebec; 100,000 in Ontario.
Sounds sort of similar to the Kentucky situation. I guess that my points are that:
a) It is rude to deride others whom one perceives to be as backwards or ignorant. Both of those are treatable conditions, with education as the treatment.
b) It is foolish, some might say stupid, to make false claims that are easily checked. Unfortunately, stupidity is not as easily treated as ignorance is.
c) The commentors who took offense at the braggadocio of that particular commentor have my thanks.
Now, on to fuels.
The heat produced from fuels can be used in several ways: for heating living spaces, for internal combustion engines, for production of electricity, and for industrial processes, like smelting iron. Obviously there are other uses, but these account for by far the bulk of the fuels used today.
Fuels can be classified in several ways, and the method that I am going to use divides them into hydrocarbons and coal (I will also treat hydrogen in this category), oxygenates like alcohols, metals, "novel" materials, and nuclear materials. Obviously there can be some overlap, especially between the novel materials and other classes.
Hydrogen is quite different from other fuels, and an entire diary devoted to it is here. I will not have much to say about it tonight.
Hydrocarbons include natural gas, liquefied petroleum (LP gas, propane and butane), gasoline, kerosene, Diesel oils (along with jet fuel) and heavy heating oils, and coal. These all have one common element: the produce carbon dioxide when combusted. The order of carbon dioxide production per unit mass of these fuels is the order in which they are listed above, on account of increasing carbon to hydrogen ratios in the molecules that comprise them.
Natural gas in primarily methane, the lightest saturated hydrocarbon. Natural gas is normally provided in a compressed form, but can be liquefied at greatly reduced temperature. Refrigerated tankers carry huge amounts of liquefied natural gas (LNG) from gas producing countries worldwide. LP can be liquefied at ambient temperatures in relatively lightweight metal tanks, a decided advantage over LNG. From gasoline through the heavy heating oils the materials are liquids at ambient temperature and pressures, whilst coal is a solid. This makes these materials particularly easy to transport by tanker or pipeline, and easy to use both industrially and by the consumer.
A real advantage to hydrocarbons, especially the liquid ones, is they do not tend to pick up water since they are nonpolar and do not hydrogen bond. Fuel lines do sometimes ice up in very cold weather, but that is most often caused by condensation of water from the atmosphere in the fuel tank. Keeping a fuel tank full reduces this because there is less air and thus moisture for condensation. Hydrocarbons have another advantage in that they are not "aggressive" in that they are essentially inert to fuel system components.
Oxygenated fuels include methanol, ethanol, some of the ethers, and a few others. Their heat content on a molar basis is always smaller than that of hydrocarbons of similar molecular mass because they contain oxygen. In that sense, they are already partially combusted. This automatically requires more fuel for the same amount of heat.
Oxygenates have other problems as well. The alcohols are miscible with water and hydrogen bond, and the ethers dissolve some water because they hydrogen bond. This can be a really big problem in fuel systems for several reasons, including corrosion of metal components. They are also hygroscopic, meaning that they actually attract water from the atmosphere. The alcohols, and to a lesser extent, the ethers are also very aggressive towards fuel system components, so special materials have to be used if their content exceeds around 10%. This is the reason that 95% ethanol can be used only in vehicles designed for them.
Another problem with at least one of the oxygenates, methyl tert-butyl ether, (MTBE), is that they tend to migrate into the water table from leaking underground storage tanks. Unlike hydrocarbons, which can be skimmed off during remediation, the oxygenates dissolve in water and can not be separated easily. MTBE was required in gasoline to reduce air pollution (and boost the octane rating) until it was discovered that the water pollution that it caused was worse than the air pollution that is prevented.
We normally do not think of metals as fuels, but the are essential for some processes. In particular aluminum is used as a fuel in metallothermic reactions (the thermite reaction). This reaction is used for welding large steel pieces and for military incendiary devices. Metals have a huge fuel value for their mass in general and often become white hot. For example, the combustion of aluminum powder produces molten (and vapor) iron for the weld.
Novel fuels are sort of a catch all category that I intend to use for new fuels like biodiesel, dimethyl ether, ammonia, and several others. It is difficult to make categorical statements since this is a diverse group, so I will leave it to a future diary.
Nuclear "fuels" almost always refer to enriched uranium, but several other elements can be used, notably plutonium. Fuel grade uranium is enriched to about 5% uranium-235 (the natural abundance is about 0.72%). In contrast, bomb grade material is around 90% uranium-235. Plutonium could be diluted down with inert materials to a few per cent and used in fission reactors, but there is a problem.
It is technically challenging and exceedingly expensive to enrich uranium, as will be discussed in an upcoming part of this series. On the other hand, it is easy to separate plutonium from uranium since the two have quite different chemical properties. Agencies are very reluctant to release plutonium for the commercial electrical generating community for fear of diversion and enrichment of plutonium by nefarious forces.
The holy grail of nuclear fuel is a feasible fusion process that can be controlled and is inexpensive. This, in theory, could be a supply of essentially limitless energy, just like the sun works. Unfortunately, it is not possible to control the fusion reaction economically at present.
For future installments, a few definitions will be useful to know. They are:
Critical temperature: the temperature above which a gas can not be liquefied, regardless of how much pressure is applied.
Critical pressure: the pressure below which a gas can not be liquefied at its critical temperature.
Heat content: the amount of heat in Joules per unit contained in a fuel. We shall use kilojoules per kilogram (kJ/kg). For scale, one kilojoule would heat one kilogram of water by 0.24 degrees C.
Lower and upper explosive limits (LEL and UEL): the least and greatest amount of a fuel/air mixture that will ignite and burn explosively, expressed as %fuel/%air in a volume to volume basis.
Flash point (Fp): the temperature below which a fuel will not ignite even if exposed to an open flame. The lower the Fp, the more flammable a given fuel is. Gasoline has a low flash point, while cooking oil has a high one.
Autoignition temperature: the temperature at which a fuel will ignite without being exposed to an open flame.
Well, that will get the ball rolling for this series. As always, any question, comment, criticism, or other input (on any science or technical subject) is welcome. I know that you cringe when I say this, but I always learn much more than I teach by posting this series.
Epilogue: folks, the next few installments will be math intensive. I strive to be accurate in my derivations, but it is easy to make a mistake. Those of you who are hip to it, the relative energy densities of the different fuels, and just are good at catching arithmetical errors, please keep me honest. The last thing that I want to do is to represent a math error for a fact. I depend on everyone here for a crosscheck.
Footnote: on a completely unrelated subject, I found the Holy Grail in Lincoln Memorial cents today. I could not believe my eyes when I found a 1972 Philadelphia doubled die obverse cent this afternoon. This is by far the most valuable die error in the Lincoln Memorial series, the one since 1959.
Warmest regards,
Doc