[enviro-vlc] Catching carbon - tricky, but it's not nuclear fusion
vern at coombs.anu.edu.au
Tue Apr 15 02:26:43 EST 2008
Catching carbon - tricky, but it's not nuclear fusion
4:00AM Monday April 14, 2008
By Brian Fallow
NZ green firm among world's best
The green test: Gary McCormick
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
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.
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