U.S. Navy looking at obtaining fuel from seawater

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USS Fife, a Spruance-class destroyer powered by gas turbines

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Tell someone that you’ve invented a car that runs on water and they're liable to report you for fraud. That hasn’t stopped scientists and engineers at the U.S. Naval Research Laboratory (NRL) who want to run warships on seawater – or at least, to turn seawater into jet fuel. This may sound like they’ve been standing too close to the ether again, but the idea is to extract carbon dioxide and hydrogen from seawater and then convert these into jet fuel by a gas-to-liquids process. If this proves practical, American naval vessels could refuel themselves at sea.

At first, it seems odd that the NRL wants to make jet fuel, but many modern warships now run on gas turbines, a type of jet engine. Every year the U.S. Navy’s fleet of 15 oilers carries 600 million gallons (2.27 billion liters) of fuel to ships at sea. This is a major logistical problem made worse by dependence on hostile or unstable nations who may cut off or interfere with fuel supplies in times of crisis. Needless to say, a ship that can make its own fuel while underway would be an advantage.

A U.S. Navy ship being refueled at sea

Seawater contains about three percent carbon dioxide in the form of dissolved carbonic acid, carbonate and bicarbonate. That’s 140 percent more than air. Along with the hydrogen bound in the water molecules, there’s all that’s needed to make hydrocarbon fuels at sea. The tricky bit is how to do it.

According to research chemist Dr. Heather Willauer, the NRL’s approach is based on established technology. "The reduction and hydrogenation of CO2 to form hydrocarbons is accomplished using a catalyst that is similar to those used for Fischer-Tropsch reduction and hydrogenation of carbon monoxide,” she said. “By modifying the surface composition of iron catalysts in fixed-bed reactors, NRL has successfully improved CO2 conversion efficiencies up to 60 percent."

The Fischer-Tropsch reduction was invented by Franz Fischer and Hans Tropsch in Germany in the 1920s. It converts coal, natural gas or biomass into fuel by means of iron or some other catalyst and is used commercially in countries with abundant coal, but little oil. Despite being very inefficient and costly, the U.S. Defense Department has long been interested in it.

The NRL process begins by extracting carbon dioxide and hydrogen from seawater. To do this, it uses a three-chambered electrochemical acidification cell. As seawater passes through this, it’s subjected to a small electric current. This causes the seawater to exchange hydrogen ions produced at the anode with sodium ions. As a result, the seawater is acidified.

An Electrochemical Acidification Carbon Capture Skid, used for the process

Meanwhile, at the cathode, the water is reduced to hydrogen gas and sodium hydroxide is formed. The cells recover dissolved and bound carbon dioxide by re-equilibrating carbonate and bicarbonate to carbon dioxide gas from the acidified seawater. The end product is hydrogen and carbon dioxide gas. As a bonus, the sodium hydroxide is added to the leftover seawater to neutralize its acidity.

In the next step, the hydrogen and carbon dioxide are passed into a heated reaction chamber with an iron catalyst. The gases combine and form long-chained unsaturated hydrocarbons with methane as a by-product. The unsaturated hydrocarbons are then oligomerized – that is, they are made to form longer hydrocarbon molecules containing six to nine carbon atoms. Using a nickel-supported catalyst, these are then converted into jet fuel.

The process has been tested under open ocean conditions in the Gulf of Mexico, and the NRL is now working to improve the process and scale it up to practical levels. The estimated cost of the fuel is projected to be between US$3.00 and $6.00 per gallon (US$0.79 - $1.58 per liter) and that may be something of a problem because the current price of jet fuel is about $3.30 per gallon ($0.87 per liter), which makes the NRL product potentially almost twice as expensive.

Another problem is that processes based on the Fischer-Tropsch reduction are very energy intensive and inefficient, which adds to the cost. Also, the end product is very pure and this can cause lubrication and sealing problems in engines.

However, the big question is, where does the energy come from to make the fuel while at sea? Most Fischer-Tropsch reduction processes work because the raw material is itself a fuel. To make fuel from coal, you burn coal to run the process. The same goes for natural gas, biomass and other examples. With the NRL process, the raw material is seawater, so what is running the machinery? The jet fuel produced is only an energy storage medium, not an energy source. To use that is like trying to lift yourself off the ground by yanking on your belt. Until that question is answered, a vital piece of the puzzle is still missing.

Source: NRL

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