Friday, 19 October 2012

Petrol from Carbon Dioxide

A UK company called Air Fuel Synthesis have announced that their demonstration unit on Teeside has successfully produced petrol from carbon dioxide taken from the air. Okay, so the current return of 5 litres of petrol for a cool £1m is hardly forecourt pricing (at the moment), but this is a chemistry blog, so how have they done it?

Well actually, the chemistry, according to their technical review, isn't all that difficult.

Step 1: Mix air with a mist of Sodium Hydroxide solution

Right, as I said, a relatively straightforward start to proceedings. Large quantities of air is blown into a huge tower contain a mist of sodium hydroxide solution (I say a huge tower, they're currently running this business out of a shipping container, but think long-term!). The CO2 in the air reacts with the sodium hydroxide solution:

CO2 + 2NaOH --> NaCO3 + H2O

Anything else in the air, for example Nitrogen, Oxygen and Argon, will pass straight through the tower, leaving a nice puddle of sodium carbonate solution at the bottom of it. As an optional extra, they've suggested condensing water vapour out of the air before blowing it up the tower, as the process needs pure water from somewhere (see step 3).

Step 2: Electrolysis of Sodium Carbonate

Basically, all step 1 does is get the carbon dioxide out of the air, now we need to regenerate it. The simplest way here is to electrolyse the sodium carbonate solution, regenerating the CO2 gas, which can be easily collected and stored.

Step 3: More electrolysis

I mentioned water in step 1, and this is where it's needed. Water is electrolysed to give hydrogen gas. Not a lot else to say about this, if you're not familiar with electrolysis, there's a nice video of it here.

Step 4: Making Syngas

Now you've got CO2 and H2 gas,the reverse-water-gas-shift reaction can be used to produce syn gas:

H2 + CO2 --> CO + H2O

Syn gas can then be used in two ways:

A: A fuel can be made by the Fischer-Tropsch Synthesis, which is already really common industrially (take a look at Sasol). I'm not going to give the equation here, but the important thing is that CO, H2 and a catalyst (probably Cobalt or Iron based in this case), combine at around 150-300oC to give a mixture of linear and branched alkanes and alkenes, probably with the odd oxygen containing functional group (alcohols, aldehydes and ketones) thrown in for good measure. This is the basic fuel, which can then be adjusted to suit needs.

B: Rather than a Fischer-Tropsch route, the Syngas can alternatively be combined to give methanol (CH3OH), itself an important chemical feedstock. The Mobil process then converts methanol to a more recognisable fuel. It basically works by dehydrating (removing water) from the methanol, to leave first a dimethyl ether, and then finally a hydrocarbon fuel.

Where's this technology heading?

Well the company admit themselves that it's early days when it comes to this technology, and they also say they can't see this being a long-term solution to running out of oil. Simply, this process is hugely energy-intensive as it is, so is going to need a really cheap source of electricity to be a viable process. At the moment, they're targeting the motorsport industry as being interested in their fuel. Now beyond F1, my motorsport knowledge isn't great, but the Mclaren and Sauber F1 teams have recently been declared carbon neutral, so may have an interest in this technology in helping to achieve those goals. Also, Lotus Cars have been active with their Exige Tri-Fuel sports car, something Air Fuel Synthesis highlight on their website.

 If Air Fuel Synthesis could find a source of cheap energy (such as off-peak electricity produced by renewables), then that's a start, and then it's all down to how efficient the process is. As you can see, two electrolysis steps, and two energy intensive reactions mean energy costs will be huge, and there's a definite opportunity for new chemistry to innovate a way around this. So good luck to them! It's a nice idea, and great to see a proof of concept, so I'll be keeping an eye out for this in the future.

If you've got anything to say, please leave a comment. Feel free to ask any questions about what I've written, and let me know if you find any faults.

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