The oceans are already the world’s largest carbon sink, soaking up a staggering 2bn tonnes of carbon every year. Adding alkalis would make them even better, by increasing the seas’ ability to take up CO2 from the atmosphere and reducing their ability to desorb it back (Scheme 1).
Scheme 1. Adding alkalinity to seawater
CO2+H2O ←→ H2CO3 ←→ H+ + HCO3- ←→ 2H+ + CO32-
Haroon Kheshgi at Exxon Mobil suggested adding lime (CaO), which is soluble, to seawater in 1995 (Energy, 1995, 20, 915). However, the idea was quickly dismissed as too energy intensive and so costly. Producing lime involves heating or calcining limestone [Equation 1] at a temperature of 9000C and consumes 2.67GJ/tonne of limestone calcined. It also generates carbon dioxide in the process.
CaCO3 → CaO + CO2 Equation 1 (produces 1 mol CO2)
But Tim Kruger, a management consultant with London firm Corven, and a Cambridge natural sciences graduate, believes he has a way of making the idea workable. To carry out the process more cost-effectively, Kruger proposes mining limestone in regions where there is access to plenty of stranded energy, too remote from the market to make it economically viable.
Possible energy sources include natural gas, that is currently flared, solar or nuclear power.
Australia’s Nullarbor Plain is a prime location as it not only has 10 000km3 of limestone, but soaks up roughly 20MJ m-2 of solar irradiation every day.
The idea has already captured the attention of Shell, which has agreed to fund a study into its economic feasibility. And one of the top researchers in the field, Klaus Lackner, at Columbia University, US, commented that it ‘is certainly worth thinking through carefully’.
Calcium oxide added to the oceans – probably as the pre-reacted hydroxide – reacts with carbon dioxide dissolved in seawater to produce calcium bicarbonate (Equation 2). The carbon dioxide from Equation 1 can also be sequestered (see carbon conundrum, right).
CaO + H2O + 2CO2 → Ca(HCO3)2 Equation 2 (absorbs 2 mol CO2)
Crucially, for every mol of carbon dioxide produced by calcination [Equation 1], two mols of carbon dioxide are sequestered by the oceans.
‘This “carbon negative” process has the potential to reverse the accumulation of carbon dioxide in the atmosphere,’ Kruger says. ‘It would be possible to reduce carbon dioxide levels to pre-industrial levels, without the need to restrict production.’
In addition, the process should counter the continued rise in ocean acidification, which threatens marine organisms, such as molluscs, corals and crustaceans.
Kruger estimates, for example, that ‘it would require the consumption of approximately 5% of the limestone in the Nullarbor Plain to return the concentration of carbon dioxide in the atmosphere back to pre-industrial levels’.
But while the ‘theoretical CO2 balance is roughly right’, Lackner cautions that getting an exact figure for the amounts of CO2 generated and absorbed by all of these various inputs and processes is critical.
The actual product of adding calcium oxide to seawater is not bicarbonate but a mixture of carbonate and bicarbonate, he notes. That makes the multiplicative factor 1.8 mol of CO2 for every mol of CO2 produced by calcination. Assuming stranded methane is used as the energy to drive the initial calcination reaction, this would generate a further 0.3 to 0.4 mol of CO2, so Lackner estimates the actual ratio would be 1.3–1.4 mol CO2 consumed for every 1.8 mol absorbed. (This gives a net 0.4–0.5 mol of CO2 sequestered, against Kruger’s own estimate of 0.7 mol.)
Sequestering the CO2 from Equation 1 would make the process ‘worth thinking about’, Lackner adds (see carbon conundrum, right).
More worrying is the possibility that the process of bicarbonate formation could reverse to reform the original carbonate. According to Harvard professor Daniel Schrag, Lackner says, ‘the ocean will probably shed excess limestone and thus release the CO2 in what he thinks is a few hundred years. If this were the case, then the CO2 balance becomes a bit dubious’.
Adding Ca(OH)2 to seawater could potentially accelerate that process, ‘partially undoing the carbon uptake achieved’ by adding it to seawater in the first place, says Gideon Henderson, professor of earth sciences at Oxford University.
Henderson and colleagues have recently begun investigating the environmental impacts of adding Ca(OH)2 to seawater, including on biological organisms. As well as determining what volumes to use, the aim is to identify the best locations to add alkali to seawater.
‘Uptake of CO2 could happen pretty quickly. But it depends on how the Ca(OH)2 is added, where and in what volumes,’ he says.
‘The basic science makes sense but the challenge is the scale of what needs to be done: the amount of calcite that needs digging up and how to put that in the oceans.’
Even assuming limestone does crash out of seawater, Lackner says that the process could buy researchers more time to develop other technologies. ‘If nothing else, one could make the argument that there will be some peak saving right when we need it.’
And as Shell’s Gilles Bertherin makes clear, ‘It’s a promising idea – we are providing some early stage funding to find out whether it can work… We have to be sure, however, that there are no countervailing environmental impacts of the process before we consider taking it further.’
Kruger is developing the process in an ‘open source’ way, and invites people to participate via a new website: www.cquestrate.com.
What to do with the carbon dioxide from the calcination reaction (Equation 1) is another problem yet to be ironed out. Kruger suggests one option would be to use this CO2 to grow biomass in deserts. Sealing the gas in an algae-filled water tank would promote its conversion to sugars and oxygen via photosynthesis.
‘Were such a system to yield 10 tonnes of glucose/hectare/year, comparable to a conventional sugar cane plantation, then 6 tonnes of water would be consumed,’ he says. ‘As a single mm of rain falling on a hectare amounts to 10 tonnes of water, such a system could allow the production of crops in all but the most arid environments.’
As well as crops, the process could be harnessed to grow biofuels.
Ultimately, however, any carbon taken up by the algae would eventually have to be returned to the atmosphere, making the ‘net carbon balance quite tenuous,’ Lackner points out.
A more viable option may be to sequester the CO2 by standard techniques to entrap the gas in rocks, in aquifers and disused oil wells. This process would be much cheaper than for sequestering CO2 from power plants, Kruger points out, as the main costs typically involve separating the CO2 from the other flue gases. The CO2 from the calcination route is pure.
Electrolysis provides the acid test
Last year, Daniel Schrag and coworkers at Harvard University reported an alternative approach to boosting seawater alkalinity by removing hydrogen chloride (Environ. Sci. Technol., 2007, 41, 8464). The acid is neutralised by reaction with silicate rocks – in a process that mimics the natural weathering process, mediated by carbonic acid in rainwater. (A similar idea is the subject of a recent patent application by Lackner.) One drawback, however, is that the HCl needs to be produced by electrolysis, which is potentially very costly.