I just ran across this
thought-provoking eye-popping post on Brave New Climate by Dr. John Morgan, a chemical engineer.
One thing the ocean has plenty of is hydrogen, which is bound up in water molecules. But did you also know that there is 140 times more carbon in a liter of seawater than in a liter of air? Hydrogen and carbon can be combined chemically to form hydrocarbons, otherwise known as liquid fuels.
So I guess it's no surprise that the US Navy is interested in manufacturing synthetic jet fuel from seawater. What's surprising is that it could be done at a cost comparable to fossil fuel. And that nobody's heard about it yet.
When we burn fossil fuels we release CO2 into the air, but not all of what we release stays there. A large fraction ends up in the sea, because the ocean and atmosphere exchange CO2 freely. In the air, CO2 causes climate change; in the ocean, CO2 forms carbonate and bicarbonate ions, which cause ocean acidification. Both of these are very bad things.
Because there is so much carbon in seawater, it's easier (and cheaper) to remove carbon from the ocean than from the air, and the effect on climate is about the same, since the air and ocean exchange CO2 so freely: lowering carbon in the ocean would also draw down CO2 in the air.
A paper from the Naval Research Lab (Willauer et. al. 2010) outlines the process. Seawater is broken into hydrogen and oxygen through electrolysis; the oxygen is vented off and the hydrogen is retained. At the same time, CO2 is captured from a much larger volume of seawater. The CO2 is converted to carbon monoxide at first, then to hydrocarbons through the Fischer-Tropsch process. Chemically it looks like this:
34 H2O → 34 H2 + 17 O2 (Electrolysis)
11 CO2 + 11 H2 → 11 CO + 11 H2O (Reverse water gas shift)
11 CO + 23 H2 → C11H24 + 11 H2O (Fischer-Tropsch)
Combined, the process sums to:
11 CO2 + 12 H2O → C11H24 + 17 O2
That C11H24 you get at the end is the jet fuel.
Extracting CO2 from seawater turns out to be surprisingly easy, using a new method called bipolar membrane electrodialysis. Eisman et. al. (2012) have described BPMED and shown it can be done efficiently and fairly cheaply.
The Navy is interested in manufacturing jet fuel at remote locations, either remote islands or at sea. Electricity (mostly for the electrolysis step) is the main energy input and therefore the key cost driver. The NRL report envisions two types of electricity production to drive this process. For remote islands, they are looking at OTEC (Ocean-Thermal Electric Conversion), which generates power from the difference in temperature between the surface of the sea and the depths. For at-sea synfuel manufacturing, they are looking at existing Navy nuclear power plants, such as those currently found on submarines and aircraft carriers.
OTEC is not a mature technology, as there are no existing powerplants of this type anywhere in the world. Accordingly cost estimates for OTEC vary widely, and Willauer uses estimates ranging from $900 million to $1.5 billion for a 200 MW OTEC plant. Under these scenarios, the final cost of jet fuel would range from $5.78 to $8.70 per gallon, according to Willauer.
Although these costs are higher than current jet fuel prices, Willauer points out that generating fuel on site eliminates all transportation and storage costs for the fuel, which are considerable; therefore, even at these prices, on-site manufacturing of jet fuel might be cost-effective. Further, the cost of jet fuel has risen about fivefold since the turn of the century, and if those increases were to continue, synthetic jetfuel could quickly become the cheaper option.
For at-sea applications, Willauer assumes current Navy nuclear power plant capital costs of $1200/kW (which would be $240 million for a 200 MW reactor) in a ship costing $650 million. Under that scenario, fuel cost would be $5.74 per gallon, according to Willauer.
But Morgan points out that by beaching the reactor, that $650 million for the ship could be saved. Zeroing out the cost of the ship would result in synthetic jet fuel from seawater costing $3.00 per gallon, according to Morgan's spreadsheet. (Note that Morgan is a bit more conservative about process energy usage than Willauer, so the total fuel output from the 200 MWe input is different in the spreadsheet than in Willauer's paper.) That's about what jet fuel from fossil has cost on the open market for the past 2 years.