Coal Powered Aircraft
Published on May 16, 2006 at 11:57 am.
3 Comments.
Filed under aeronautics.
Now for an aviation related entry! I was reading in the local paper recently about coal powered aircraft. Naturally, one things of steam engines, with a boilerman stoking a fire with a shovel. Yeah, right. Like that sort of thing would fly. Granted, I recall some fanciful stories about similar coal/steam driven flying machines written in the early 20th Century. I guess, you’d call them early science fiction. Needless to say, it was more fiction than science.
However, this is not the intent of the research. Rather, the researchers, including leaders in the field working at Penn State, are looking at an alternative to jet fuel that is derived from coal. So, they are looking at some liquid or gas fuel that is derived from coal. This, in fact, is not unreasonable. In fact, there already exist methods to gasify or liquify coal to produce fluids similar to natural gas or to diesel or fuel oil. None of these products are useful as aviation fuel, but the researchers are developing something that might do the trick.
So, how does this work? I have not read much on the specifics of the procedure, but there are several things that we can guess from the properties that aviation fuel needs. Basically, there are two types of aviation fuel: Avgas (basically a high octane gasoline) and various grades of jet fuel (basically high grade kerosine). The liquification process will produce a thicker, less explosive fuel, more like kerosine than like gasoline. So, the new fuel will be a jet fuel. Jet fuel has some specific needs that keep gasoline from being used as a suitable fuel. For one, gasoline is too volatile and explosive. It would tend to ignite prematurely at the injector rather than dispersing in the combustion chamber of the engine. Also, gasoline breaks down much too easily in high temperatures typical of jet engines. High performance jet engines also tend to have even more specific needs. The compression of air during intake into the engine makes the engine itself very hot, not to mention heat from combustion. Getting rid of the excess heat is an issue. This is where the high grade jet fuel comes in. Kerosine can tolerate quite high temperatures without breaking down or spontaneously igniting — much higher temperatures than gasoline. Kerosine also has a much higher boiling point, also a plus in high temperature environments. And, kerosine has a fairly high specific heat, so a given volume of kerosine can absorb more heat to raise it to a high temperature than an equal amount of gasoline. These properties mean that the jet fuel itself can be used to cool the engine. The fuel circulates and picks up heat before being injected into the engine to burn. This has an added advantage of putting the surplus heat back into the engine’s exhaust. The way that a jet engine works is that air enters the engine at a much lower speed than it exits the engine. Momentum is the product of mass times velocity. So, a higher exhaust velocity has higher momentum. The difference in momentum between the air into the engine and the exhaust gasses is called impulse. Newton’s third law says that for every force, there is an equal and opposite force. Accelerating the gasses to exit faster than they come into the engine requires a force. So, there is an equal and opposite force on the engine, driving the aircraft forward. (Note that this description is strictly for a ramjet, a turbojet, or a scramjet. Turbofans, increasingly popular with commercial airlines, use the jet engine to drive a large fan that provides as much or more thrust than the jet exhaust. Turboprops are similar, in that a jet drives the propeller.) The force on the gasses comes from an increase in pressure of the gas in the combustion chamber. This increase in pressure results from heating the gas through burning the fuel. In addition to the heat, burning fuel converts liquid fuel into gas. Converting a liquid to a gas typically results in an increase of volume of nearly 100,000% (assuming constant pressure), so this is the major source of pressure in the engine. Additional heat can’t hurt, though it would represent such a tiny percent increase in thrust that I’d imagine it to be virtually undetectable. Thust, jet fuel must be stable under very high temperatures. By the way, the same rationale is why one of the most commonly used rocket fuels is RP-1, basically very highly refined high grade kerosine. Rocket engines work the same way as jet engines, only they don’t take in air.Â
So, how does all this lead to coal power? Well, the Air Force has been working on alternative jet fuels for many years.  One problem with very high performance engines is that they typically heat their kerosine fuel about as high as can be safely done, meaning that extra cooling is needed besides just the fuel. Naturally greater efficiency and performance would result from a fuel that could do the job by itself. Also, the United States imports most of its oil. This imported oil, of course, is the source of the kerosine jet fuel. From simple national defense arguments, it is clear that it would be a very good idea for the military to depend upon some fuel source that does not come from other countries. OK, this is why the government stockpiles oil, but still … . The United States, however, has vast supplies of coal. The problem with coal is that it is harder to transport than the liquids derived from petroleum. Coal is also much heavier than kerosine and burns slower, so it is not useful in aircraft engines. Hmm.
But, wait. I said that you can liquify or gasify coal. This produces a fluid more useful for our purposes. The Germans did something like this during World War II. However, the process for converting coal to a liquid or gas is expensive. It is cheaper to buy and refine crude oil. Well, it was cheaper, I should say. With oil becoming more expensive, coal is looking better. The newer forms of the procedure produce a liquid with properties not too dissimilar to kerosine. In fact, one advantage is that the new fluid is stable to even higher temperature than most kerosine products. This makes it an ideal fuel for high performance jet engines. The Penn State group has produced what seems to be a perfect ballance, called JP900. The name means that it is stable to 900 degrees Fahrenheit (over 480C). Even better, it seems to burn cleaner, and with the same energy content. The only catch is that it costs a lot to make. However, if oil prices remain high, the cost factor is not such a problem. Furthermore, those jet fighters drink jet fuel at an amazing rate. The Air Force alone accounts for over half of the entire government’s fuel bill! Well, I suppose that if the Navy didn’t use so many nuclear reactors, they’ be up there, too. Still, a fuel source that doesn’t depend upon foreign oil would be a significant advantage. Now, if it works for military jets, perhaps it can be adapted for civilian jets, too. This too, could become important if the cost of fuel stays so high. After all, airlines can’t keep spending more than they make forever, can they? And if it works in airplanes, could truck engines eventually be adapted? The more of this fuel that is produced, the cheaper it will be to make it.Â
What also interests me, given my interest in space, is that JP900 will have properties very similar to RP-1, only a bit cheaper. Hmm. Also, it burns cleaner, meaning that as a rocket fuel it would cause less damage to the stratosphere as it shoots through. Again, hmm. I hope that the fuel gets perfected soon. It would be interesting to see if RP-1 gets a run for its money as a dominent rocket fuel.
-Astroprof
Ben Bowman on May 7, 2007 at 10:24 pm: 1
Almost a year has passed since this was published. Has anything happened in the coal liquification process?
David C. on March 10, 2008 at 12:04 pm: 2
Now two years. I wondered what happened.
Astroprof on March 10, 2008 at 6:15 pm: 3
I have not heard anything. There has apparently been little in the news, and if the researchers are publishing, they are doing so in journals that I do not read.