Saturday, 27 March 2010

Vehicle Carbon Capture

We all hope battery technology will improve enough to make electric cars attractive, but if it does not I for one would pay a very hefty price, perhaps £5/litre of fuel ($3200/tonne of CO2 produced at today’s exchange rate) before I abandoned my car. Fortunately it is possible to capture carbon dioxide from the atmosphere or from vehicles for much less than that.

The options I favour revolve around quicklime (CaO). The average factory gate quicklime cost in the USA is published and was $101/ton in 2009. The material is made by decomposing limestone (CaCO3) in a kiln. When quicklime recombines with carbon dioxide it may form either the carbonate (CaCO3) or, if in solution, the bicarbonate (Ca(HCO3)2). Based on the carbonate, the cost of quicklime per ton of carbon dioxide absorbed would be $129. There are of course other essential costs including the cost of capturing and sequestering the carbon dioxide produced as the quicklime is made.

One way to use quicklime is simply to dump it in the ocean.

Another is to use quicklime to turn sodium or potassium carbonate into the hydroxide by precipitating the insoluble calcium carbonate as in the Kraft paper process. Solutions of sodium or potassium hydroxide can be used to capture carbon dioxide from the air by reforming the carbonate.

Alternatively the hydroxide could be used in a vehicle either in solution or perhaps, as weight is crucial, as the solid or a slurry. At the limit of solubility the range would be 100 miles for 150 kg of potassium carbonate solution in a vehicle doing 30 miles per imperial gallon. This compares favourably with current battery technology and recharging with potassium hydroxide solution at the gas station would be much quicker than recharging a battery. Moreover the vehicle could still be driven for hundreds of miles with the hydroxide exhausted, if necessary.

If quicklime could be used directly as solid particles, 150 kg would give a range of 228 miles but reaction rate might be too low, although the high temperatures available in the exhaust should help. Also carbonate might blanket the quicklime giving poor conversion and to make matters worse the particles might stick together with all the water vapour in the exhaust gases, making discharge very difficult. Quicklime is of course the key component of cement.

Solid lithium hydroxide is used to remove carbon dioxide at ambient temperature in spacecraft, where weight is crucial. In a vehicle 150 kg of lithium carbonate would correspond to a range of 308 miles. But unfortunately regeneration is currently difficult and expensive because of the low solubility of lithium carbonate when using the Kraft process and the relatively low melting point of lithium carbonate combined with the very high decomposition temperatures when emulating quicklime manufacture.

I have not yet worked out whether the higher concentration of carbon dioxide in exhaust gas is enough to tip the cost advantage against capture from the atmosphere. The logistics of the latter certainly look much easier and of course there are economies of scale and a free choice of location to suit sequestration and perhaps provide low cost energy/fuel.