ScienceWise - Summer 2010

Electricity – Carbon = Good

How Maths May be the Key to Clean Coal Power

Australia currently generates around 85% of its electricity by burning coal, resulting in 170 million tones of CO2 being pumped into the atmosphere each year. It’s a major factor in making Australians the highest polluters in the world, ahead even of the US. Although alternative energy such as wind and solar is slowly coming on line, most realistic analysts agree that coal must continue to provide at least some of our power for the foreseeable future. But that’s not to say that coal can’t clean up its act. There are technologies available to extract CO2 from the flues of power stations though many of these are far from economically viable.

However much the average person wants to save the environment most people and most industries are simply not in a position to pay vastly more for electricity than they currently do. So what we need are CO2 extraction technologies that are simple, efficient and highly cost effective. And the only way we can achieve this is with some advanced science.
Dr Rowena Ball is an applied mathematician and physical chemist, currently working on the complex thermo chemistry and kinetics involved in flue gas carbon dioxide capture.

Carbon capture involves two main processes. The first is the “scrubbing”, or selective removal of carbon dioxide from the mixture of flue gases in the emissions stream. The second is “sequestration” – the stable, long-term geologic storage of the compressed pure carbon dioxide. In collaboration with an Australian company, Calix Ltd, Dr Ball is developing a novel looping technique to scrub the flue gases of power stations.

The team is refining what’s known as an Endex reactor, based on some initial work Dr Ball did back in the 90’s. In the Endex reactor, carbon dioxide is adsorbed by calcium oxide to form synthetic limestone, a reaction that is exothermic, so generates large amounts of heat. The second stage is desorption of pure carbon dioxide from this limestone, a reaction which is endothermic so requires a lot of heat energy to drive it. The clever part about the Endex reactor is that the adsorption and desorption reactions are coupled thermally and matched kinetically, meaning that the energy generated by the exothermic first stage is harnessed directly to drive the endothermic second stage making the process highly energy efficient and therefore cheap.

“Calcium oxide is ubiquitous in the Earth. It very slowly adsorbs carbon dioxide from the air to form limestone. But at normal temperatures the process occurs only over millions of years, and is too slow to be of any use in blotting up carbon dioxide on time scales of interest to human responses to climate change.” Dr Ball explains.

The Endex process operates between 750 and 850 degrees Celsius, with the carbon dioxide adsorption/desorption cycle taking only seconds to minutes. Perhaps most importantly, the system can be retrofitted to existing power plants, avoiding the cost of entirely new power stations.

“Cost has been the big hurdle in flue gas carbon dioxide capture”, says Dr Ball, “All the methods considered so far have been prohibitively expensive. Unless the capture step can be done for less than $10 per tonne of carbon dioxide captured, it is not going to be economically viable. Developing countries in particular are not going to be able to afford it.”
The company has received support from both the Federal and Victorian Governments to build a large-scale demonstration Endex plant, which is currently under construction at a site in Victoria.

Although the development of the Endex process was facilitated by modern mathematical modelling methods and advanced computing, the underlying science has its roots in stability theory, developed during the late nineteenth century and early twentieth century. The original applications of this branch of mathematics were in celestial mechanics and control theory. Stability theory aims to answer the question: Given a dynamic system in a particular state, will a perturbation to that state grow or decay? In other words, is the system stable?

“The application to Endex carbon capture is an interesting example of how high quality fundamental mathematics research can turn out to have important applications to new problems,” Dr Ball says. “In the late 19th century and for most of the 20th century fossil fuels were cheap, climate change was not a problem, and carbon dioxide capture had barely been heard of. Times have changed dramatically. This is a very good argument for support for mathematics and for training young people in the mathematical sciences.”

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