Fuel from Seawater

Very interesting article since it potentially indicates a relatively efficient way to convert nuclear or other clean sources of energy into fuel for existing hydrocarbon burning engines, which I imagine is the point of the process. Since the research is being performed by the US Navy it's worth noting that all current US aircraft carriers are nuclear powered and therefore are themselves sources of clean energy for this process.

Since the CO2 in seawater comes originally from the atmosphere (not all of it, but increases in atmospheric CO2 cause corresponding increases in seawater CO2 concentration), assuming you got your power from a carbon-neutral source, this would essentially provide you with a carbon-neutral way to burn diesel or other hydrocarbons.

It seem to me that, on an industrial scale and for general purpose use as a fuel and not just for carrier-based aircraft, it might be possible to construct a long (?miles?), shallow (?inches?) pond with a fresh water inlet at one end and a nuclear plant and large industrial version of the gas-to-liquid fuel system at the other. Fresh water would be added at one end; the high surface ratio would maximize it's ability to absorb CO2 on it's way to the other end, where it would be converted to fuel. The purpose of using an enclosed pool being to get maximum efficiency by using a high efficiency electrolyte such as nafion which is quite a bit more efficient than sodium chloride. (Also quite a bit more expensive, but it would be maintained in the system and not consumed.) I guess the practicality of such an arrangement would be determined by exactly what size pool would be necessary which I guess would depend on the exact rate that water absorbs carbon dioxide, which I do not know although I am sure there are many engineers who could readily perform the calculation and figure if it is possible from a practical standpoint.

Hydrocarbons are really a very nice and dense way of storing energy; provided they can be themselves produced and used efficiently and without damaging the environment. Absent small civilian shipboard nuclear power becoming practical (both from an engineering and political standpoint), this would provide a nice way to continue to use current diesel engines in an environmentally safe manner.

I know you know a lot more about engines that I do; am I right to think that since diesel produced this way would have (I assume) no impurities and would be pretty much custom tailored in regards to the fractions of the various hydrocarbons used, that it would be able to be burnt very cleanly even in current engines? Just in terms of designing fuel tanks diesel is certainly far more compact than LNG or hydrogen, and also requires much simpler construction than LNG or hydrogen tanks. Therefore, it would be nice to have a way to produce (and burn) diesel that was as easy on the environment as either of those sources. In point of fact, given a clean power source, production would be much cleaner than current LNG production methods, which is why I would be interested to know if burning it could be done as cleanly as LNG.

Not unlimited certainly, but possibly sufficient for the purposes of extending the time before the carrier's aviation fuel needs to be replenished. I'm not saying it is sufficient, just thinking that since the Navy is funding it they may think it may be. They could also possibly convert one of their tankers to be a dedicate processing ship if a full-scale version of the system is too large to install aboard an already tightly-packed carrier. Or they could simply be looking at it as an alternate source of aviation fuel that can be produced in quantities on land without resorting to reliance on foreign sources if the total power or space requirements are too great to allow any shipboard installation. From this article you linked it is apparent that are specifically using sodium ions as part of the process, so it would make sense to just use seawater rather than my idea of using a higher efficiency electrolyte; I guess then that any industrial installation would be located on the coast (which is in any case I believe where most of our current refining capacity is located?).

Right, but I assume they mean small relative to current means of extracting hydrogen from water, and recovering most of the dissolved CO2 as a nice bonus. You will in any case have to use at least a little more energy than could be recovered from burning the hydrogen; there is always going to be some inefficiency in any system otherwise you would essentially have a perpetual motion machine. Also of course there will be some energy lost in the chemical reactor that converts the CO2 and hydrogen into hydrocarbons. It looks like they are then using a normal catalytic process then to convert the hydrocarbons produced to the specific fractions they require. This last step is already well understood and could probably be performed in any of our current refineries if an industrial-sized extraction operation was co-located with an existing coastal refinery (or if it wasn't; but locating it on-site would help with maximize return since you wouldn't have to spend energy transporting the feedstock.)

While the Navy seams to be interested in this mainly as a source of jet fuel, I was thinking more long term of it's specific application to civilian boating (and other transportation uses) since it should be able to produce other hydrocarbon based fuels as well once implemented on a very large-scale basis.

The reason I specifically mentioned nuclear is because it is a portable, very high density power source that is already successfully operated on naval vessels. The number of solar panels that would fit on a ship would not produce enough power to produce the large amounts of fuel consumed in aviation operations, even if they can achieve the high efficiencies they seem to believe they can (and I assume if they think they can produce it for $3-$6 per gallon that they think they be very efficient indeed). Obviously a land based installation would not have this space restriction and could use any available non-hydrocarbon energy source; it could certainly use nuclear but any of the developed and developing renewable energy sources could then be used as well.

In any case; even if they never successfully produce fuel, being able to produce CO2 in industrial amounts efficiently from seawater would be a tremendous development. The current processes for producing industrial amounts of CO2 almost all involve extraction from fossil fuels; in addition to releasing large amounts of CO2 into the atmosphere while powering the process, the CO2 produced of course eventually makes it into the atmosphere as well. On the other hand, since the CO2 produced in the Navy's process is essentially extracted from the atmosphere (by being dissolved in the ocean), you would end up with less net CO2 released, especially if you used a power source that does not itself release CO2. And you get hydrogen as a bonus, which is useful as a fuel and industrial chemical itself quite apart from it's usefulness as a feedstock for hydrocarbon production.

-Harry