Aug 24 2009

Jet fuel from Seawater

The New Scientist headling reads: “How to turn seawater into jet fuel.” The “water into fuel” meme is one that will just not die – like lead into gold, it is an iconic example of turning something of little or no value into something of great value, which apparently appeals to our ape brains.

This is an old story that crops up from time to time. I recently blogged about a Florida inventor, John Kanzius, who found a way to burn salt water, resulting in many breathless news reports imagining a day in the near future when we would simply fill our gas tanks with seawater. The New Scientist story has a couple of new wrinkles, but first let me review the reason we will not be running our cars on water – the laws of thermodynamics.

As I discussed last week, in any new process that claims to produce energy it is a useful exercise to consider where the energy is ultimately coming from. In the case of water, the claims invariably are that the hydrogen and oxygen is split through some sort of electrolysis process, and then the hydrogen is burned back with oxygen to produce energy. Of course, thermodynamics tells us that this process must produce less energy than it consumes – it costs energy, it does not produce energy.

Looked at another way – in order to get energy out of a chemical process the end products must contain less energy than the starting products, and the energy released will be a portion of the difference in this energy content. Fossil fuels, for example, are energy dense molecules, and they burn to more stable products, like carbon dioxide, that contain less energy.  The energy released comes from the bonds in the hydrocarbon that are being broken.

If the end products are the same as the starting products then the process cannot produce energy. It must, in fact, lose at least a little bit of energy, for no process can be 100% efficient. So if you start with water and end with water, you have not released any energy, you have only consumed energy going around in circles. John Kanzius, for example, failed to take into account the energy going into the radio waves he was using to help electrolyze the water.

So what about the New Scientist article? The new wrinkle (probably not really new, but I haven’t written about it before) is that the process included extracting carbon dioxide dissolved in the seawater. Rather than splitting hydrogen and oxygen and then burning them back together, they split off the hydrogen from the water and then combine it with carbon from carbon dioxide to make a hydrocarbon. The article reports:

It uses a variant of a chemical reaction called the Fischer-Tropsch process, which is used commercially to produce a gasoline-like hydrocarbon fuel from syngas, a mixture of carbon monoxide and hydrogen often derived from coal.

So they are developing a variant of an existing commercial process, which at least makes it plausible that the process will work. But note that the process uses carbon monoxide, not carbon dioxide – the latter is more stable, which means lower energy. Also, hydrogen as a free gas contains usable energy. The process derives its starting materials from coal, natural gas, or biomass – actual energy sources.

Getting hydrogen from water does not create energy – it uses it. And starting with carbon dioxide is problematic because carbon dioxide is the usual end product of burning hydrocarbons. So again we have the problem that we are ending with products that are similar to our starting products – so where is the energy coming from? Somewhere else.

Unless I am missing something, this process will not generate energy. The New Scientist article was not clear on that point. It may still turn out to be a useful process for the Navy, however. Imagine a nuclear aircraft carrier being able to create its own jet fuel out in the middle of the ocean, using the energy from its own nuclear reactor. I believe General Norman Schwarzkopf said during a press conference regarding the Gulf War, “Amateurs talk about tactics, professionals talk about logistics.” (Sorry I could not find a specific reference – if anyone can, please leave it in the comments.)

The process may therefore be of tremendous logistical value to aircraft carriers, making them less dependent on the supply chain to keep their birds in the air. But this is only an option if you are carrying around a nuclear reactor.

Of course, other energy sources could be used – solar, wind, etc. This or a similar process may become valuable in a future with diminishing fossil fuel for those vehicles that simply need high-energy fuel to operate. In other words, this could be a valuable energy storage system, if not an energy generation system.

Incidentally, back when the coming hydrogen fuel economy was all the rage, the same point could be made. We have little or no free hydrogen on the earth, it has to be created. So hydrogen as fuel is an energy storage system, not a source of energy. We would still need an energy generation infrastructure. This is identical to battery-operated cars. The batteries just store energy that needs to be created elsewhere.

It can also be mentioned that since this process takes CO2 out of the ocean and then ultimately puts it back into the environment (into the air, but the ocean and atmosphere CO2 are in equilibrium), this is a carbon-neutral process. It does not take previously sequestered carbon and put it into the environment.

This is therefore an interesting and potentially useful process – I just wish the media would get the emphasis correct rather than repeated the tired “gas from water” theme.

7 responses so far