KEY POINTS:
Capturing carbon dioxide is an exercise in separating the gas you want from the gases you don't want.
The problem is that we release the energy in coal by combining its carbon with the oxygen in air, and air is about four-fifths nitrogen. At some point in the
process, all that unwanted nitrogen has to be got out of the way.
Inevitably that comes at a cost in capital plant and energy losses.
One option is to extract the oxygen from air before combustion. This is a well-known industrial process, used in the steel industry.
The "oxyfuel" process for carbon capture involves burning coal not in air, but in a mixture of oxygen and recycled CO2.
The hot flue gas coming out of the boilerhouse, instead of going up a chimney, can then be compressed and sent on its way to geological sequestration - locking it up underground - with little further processing.
Another option is to gasify the coal by reacting it with steam and pure oxygen. This gives you a mixture of carbon monoxide, carbon dioxide and hydrogen.
You can then either burn them in a highly efficient power plant combining gas and steam turbines, or react them together to form chemicals like diesel, as Solid Energy wants to do in Southland.
Either way, the nitrogen has already been eliminated, so capturing the CO2 can be done with little additional cost.
Coal Research in New Zealand and HRL, the former research arm of the Victorian State Electricity Commission, are working on gasifiers specifically for lignites.
A third option is to separate CO2 from the unwanted nitrogen after combustion.
This might involve scrubbers in which some solvent compound selectively absorbs the CO2 from the flue gas, then releases it before being recycled.
Alternatively, it might involve using some type of solid material which mops up the CO2 and then releases it when the temperature is raised or the pressure is dropped.
Or it might involve membranes that allow CO2 to pass through, but not other gases.
Which technology is used would be a horses for courses exercise.
Dr Barry Hooper, chief technologist at Australian-based research group CO2CRC, says there is no clear winner yet.
"Which technology is selected may depend on whether it is for power generation or for synfuels," he says.
`We are looking at pre- and post-combustion. Oxyfuel is particularly relevant for new builds."
He rejects the view that retrofitting existing power stations with carbon capture technology is likely to be prohibitively expensive. Without minimising the challenges of carbon capture, they are within a business-as-usual range for chemical engineers. It is not like trying to crack nuclear fusion.
"Physically it's not a problem," Hooper says. "It's the economics that's the issue."
But he is confident costs can be reduced. "The issues are solvable and we will be able to drive those costs down."
When it comes to storing CO2 in underground formations, the first thing to understand is that at the pressures and depths involved, CO2 is not a gas but a "supercritical" fluid .
Supercritical fluids are like gases in that they can diffuse readily through the pores of solids, but like liquids they take up much less space than they would in a gaseous state - about 1/400th in CO2's case.
The first places to look for potential storage sites are likely to be depleted oil or natural gas fields. Their geology has been mapped already and the cap rock that trapped the hydrocarbons there for millions of years should do the same for CO2. Indeed, CO2 has been pumped into some fields for many years - not to dispose of the CO2, but to flush out more oil or gas.
Such disposal sites have a potential weakness, though. Any abandoned production wells need to be properly sealed to prevent CO2 leaking out.
A much more widespread resource is deep saline aquifers, layers of porous rock full of water too salty to be of any other use. The idea is that injected CO2 will dissolve in the saline water and may eventually combine chemically with the surrounding rocks, locking it up even more securely.
So you are looking for layers of rock which are porous enough, permeable enough and extensive enough to stash a lot of CO2 - all under a layer of rock that has very low permeability and porosity so that it will act as a seal.
Sites also need good "injectivity" - the rate at which CO2 can be injected into the storage reservoir.
"The trade-offs between the injectivity required, the reservoir storage capacity and the quality of the seal can be intricate and require careful evaluation by geologists and engineers," says CO2CRC.
Its Otway project in western Victoria has begun to store 100,000 tonnes of CO2 2km underground in a depleted gas field.
It will monitor soil, air and water in the area in order to refine the models for how CO2 should behave in such a site.
A later stage of the project is intended to inject CO2 into a saline aquifer not quite as far down.
"That's the big target," says CO2CRC director Dr Peter Cook. "Deep saline aquifers are much more widespread than depleted oil or gas fields."
An extensive report on carbon capture and storage by the UN Intergovernmental Panel on Climate Change concludes that it is likely to be technically possible to store at least two trillion tonnes of CO2 in geological formations worldwide.