Gas rocks! Mineralising carbon dioxide into rock

Late in 2016, an international consortium of scientists behind a project called CarbFix announced that it had successfully begun the industrial-scale capture and mineralisation of carbon dioxide (CO2) from the emissions of Hellisheiði, Iceland’s largest geothermal power station. They had turned an unwanted and harmful greenhouse gas into a benign and solid mineral contained within bedrock, and they had done it on a timescale previously thought impossible. CarbFix had turned gas into rock in just two years.

Carbon capture and storage (CCS) is not a new idea. The first large-scale CCS facility started operations in the USA in 1972. Today there are 17 CCS projects operating in various countries around the world, from Brazil to Saudi Arabia, with a combined CO2 capture capacity of around 30 million tonnes per annum. According to the Global Carbon Project, more than 36 gigatonnes of CO2 is emitted into the global atmosphere every year as a consequence of fossil fuel use and industry alone.

In order to limit global warming caused by this high rate of emissions to well below 2°C of warming – which experts from across the scientific community agree would signal irreversible environmental catastrophe – total global emissions of all greenhouse gases (of which CO2 makes up 65 per cent) will need to be at least halved by 2050. The Global CCS Institute thinks CCS is key to reaching this target, claiming that “it will be impossible to deliver the ‘well below’ 2°C climate goal if CCS is not adopted as a key mitigation option within five to seven years”.

During conventional CCS, CO2 is captured from the emissions produced by fossil fuel power plants, heavy industry or refineries using a chemical process. The CO2 is liquefied under pressure so that it can be transported and injected into underground rock formations, often an old oil or gas field, at depths of up to 5km. Successful storage can only be achieved if there is an overlying layer of impermeable rock to stop the still-buoyant CO2 escaping to the surface and eventually returning to the atmosphere. Given the right chemical composition of rock in the storage formation, it is thought that the CO2 will eventually transform into limestone over a timescale of hundreds to thousands of years.

Such long timescales, prohibitive costs and the sheer quantity of energy required (for every three coal power plants undertaking CCS, a fourth is needed to provide the power) are thought to have held back the expansion of CCS. “The process does need improving,” admits John Scowcroft, executive adviser to the Europe region at the Global CCS Institute. “The thing people worry about is whether the CO2 is going to come back – and the subsidiary worry, whether it is dangerous.”

CarbFix has the potential to address these worries. “The importance of CarbFix, in my opinion, is that it is showing definitively, in fast-forward, that all CO2 sequestered will turn out like this, that the CO2 will be rock and it will stay there,” Scowcroft says.

Currently ranked second in the Environmental Performance Index, Iceland regularly tops global ‘green lists’. This is largely due to its use of alternative energy, particularly geothermal. Generally considered a ‘clean’ technology, geothermal energy does produce waste emissions of both CO2 and hydrogen sulphide. An investigation into whether CCS techniques could provide a solution to unwanted emissions evolved into what has become the $10m CarbFix project.

“We wanted to mimic the natural process that is happening in nature here in Iceland,” explains project manager Edda Aradóttir at Orkuveita Reykjavíkur (Reykjavik Energy), one of the principal CarbFix partners. “We expected that the process would be fairly rapid and we knew it would be quicker than conventional CCS, but this was the fastest outcome we could have expected.” 

The apparent alchemy of permanently mineralising CO2 in just two years was achieved through subtle deviations from standard CCS techniques. Rather than injecting liquefied CO2, CarbFix dissolves the gas in water and injects the resulting carbonated solution to depths below 1,000m. The solution has a CO2 concentration of 0.823 mol/kg and a slightly acidic pH of 3.85 at 20°C. Not only do interactions between the carbonated solution and the host rock begin immediately, the speed of the reaction is aided by the acidity, which helps to break down the host rock to release elements that combine with the carbon and oxygen of the CO2.

Another important benefit is that CO2-saturated water is denser than liquefied gas, eradicating the problem of buoyant CO2 escaping back to the surface. An impermeable cap rock over the sequestration site is no longer needed.

However, the real magic comes from the selection of the host rock: basalt. Basalt is highly reactive because it contains relatively large amounts (up to 25 per cent by weight) of the more readily interactive elements such as calcium, magnesium and iron. These are the elements that react with the injected fluid to form carbonate minerals, particularly the solid crystalline mineral calcite.

“We are scientists, so we are always sceptical,” says Aradóttir. “But we got indications that the process was working very quickly. We labelled the injected gas with different tracers and tracked it from nearby monitoring wells. By taking samples of residual water, we could see the CO2 was not coming through. Later we drilled cores and we could detect our tracers in the calcite we found.”

‘We have to find a way of building the needed infrastructure and incentivising the use of CCS.’

The deposits appear as white-ish veins in the test cores. Since pilot injections began in 2010, CarbFix has achieved over 95 per cent permanent mineral CO2 sequestration in under two years, compared to the centuries required by other methods.

Approximately 10 per cent of the planet’s continental surface and most of the ocean floor is basalt. So in theory, CarbFix could be widely replicated. It has been calculated that while mineral storage capacity in Iceland could be as much as 400 gigatonnes of CO2, the ocean ridges could exceed 250,000 gigatonnes – more than enough to account for the safe sequestration of all CO2 emissions resulting from the use of all fossil fuel resources on Earth.

While this tantalising potential is hard to ignore, critics are quick to point out that CarbFix has yet to prove itself viable on a huge scale. CarbFix is injecting 10,000 tonnes of CO2 annually at the Hellisheiði site. This is alongside its sister project, SulFix, which injects 7,000 tonnes of hydrogen sulphide per year into basalt to form iron pyrite. Larger-scale testing has been impossible in Iceland as there are simply not enough emissions. Geothermal Hellisheiði produces only 5 per cent of the emissions generated elsewhere by fossil-fuel plants.

Other criticisms include concerns that mineralisation may not be permanent if deposited rock is subjected to substantial heating, but Aradóttir rejects this. “You can reverse the process by dissolving the carbonite in acid, but we don’t intend to do that.” Then there is the amount of water required to dissolve the unwanted gases before injection. CarbFix uses 22 tonnes of water for every tonne of CO2 dissolved. Seawater can be used instead of freshwater, but because salinity influences the process, around 30 tonnes of seawater is required to dissolve a tonne of CO2. Aradóttir is quick to point out that, along with CO2, all heavy metals precipitate out of the injected solution into the mineral deposits, resulting in purified groundwater that is immediately potable and could be re-used. “Any water can be purified,” she says, “even waste water.”

The price of CarbFix at the Hellisheiði site is $30 per tonne of CO2. This compares favourably with conventional CCS methods, which cost between $60 and $130 per tonne, but Aradóttir believes cost is still the biggest drawback. “Within Europe, industry can buy its CO2 quota cheaply, at approximately €6 per tonne,” she says. “The politicians have to be involved.” Scowcroft at the Global CCS Institute agrees: “The CCS community has a lot of work to do in getting the message across and the legislation in place. We have to find a way of building the infrastructure and incentivising the use of CCS.”

Nevertheless, he sees great value in the CarbFix project beyond the surprising speed of mineralisation. “They’ve found a more efficient storage method that provides greater reassurance to the public. We talk a lot about technology, but public engagement is as important. A method can be as efficient as you like but if the public don’t like it, it won’t succeed.”

A new phase of the project, CarbFix2, was launched late last year, aiming to move the technology from the demonstration phase to and economically viable complete CCS chain that can be used in Europe and the world.

So, is CarbFix the climate solution miracle we’ve all been waiting for? “There is no one climate miracle,” says Aradóttir. “It’s very risky if everyone thinks they don’t have to worry about their emissions now. We have to apply every available solution and everyone has to play a part.”