Diesel Fuel From Air: Neat Idea, Very Inefficient Use Of Energy

It sounds like an intriguing idea: Make diesel fuel from air rather than fossil-fuel petroleum.

Better yet, make it from carbon dioxide--the greenhouse gas whose concentration in the atmosphere has risen precipitously since the dawn of the Industrial Age.

Diesel fuel, after all, is a hydrocarbon chain. Take hydrogen and carbon dioxide, add some energy, and voila: Carbon-neutral liquid fuel.

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Indeed, according to a BBC report, two small companies--Sunfire in Germany and Carbon Engineering in Canada--are doing just that in pilot programs.

Sunfire produced usable e-diesel at its Dresden plant in April, and Johanna Wanka, a German minister, ran the first 5 liters in her Audi A8 TDI--an event the automaker proudly promoted.

Carbon Engineering has just completed a pilot plant that will produce 500 liters of carbon-neutral diesel fuel a day from 1 to 2 tons of atmospheric carbon dioxide.

Synthesizing e-diesel starts with boiling water into steam, and then separating it into hydrogen and oxygen through electrolysis at roughly 800 degrees Celsius (1500 degrees Fahrenheit).

The hydrogen then reacts with CO2 under pressure, and again at high temperature, yielding a liquid called "blue crude," composed of long-chain hydrocarbons.

The blue crude is similar to the crude oil extracted from oil wells; the final e-diesel is created by conventional refining processes, and can be mixed with conventional diesel fuel.

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As the BBC notes, "The process requires electricity, but if the start-ups use renewable electricity they can produce diesel that is carbon neutral."

And that's the challenge: It takes truly large amounts of electricity to produce relatively small volumes of diesel fuel.

The energy content of the resulting diesel fuel is only 13 percent of the energy required to manufacture it.

When that is run through a diesel engine, at 30 to 35 percent efficiency, only 4 percent of that energy is actually used to turn the wheels of the car--the rest is wasted in heat and noise.