A convenient truth: Minerals for CO2 sequestration

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Published: Monday, 01 October 2012

Pol Knops investigates solutions to rising global carbon dioxide (CO2) levels and shows how industrial minerals may play a role in the solving the problem

Global warming, the term used to describe the rise in the average temperature of Earth’s atmosphere, has been noted and assessed since the late 19th century, but it is only in the past 20 years that the phenomenon has been common knowledge.

The consequences of global warming are well known and well documented. And now, more than ever, action is being taken to try to combat it.

There are three main options in this drive: to use considerably less fossil energy, to switch to renewable energy, and to correct the balance between CO2 input and output.

The preferred option, especially from an environmentalist’s perspective, is to use less fossil fuel.

But as the global population continues to increase and economies and infrastructure develop, that appears impossible. Coal, oil and gas are increasingly used more, are more readily available and are cheaper to extract than any alternatives.

There is a fundamental need for energy - and the feedstock is available.

Although renewable energy may be an answer in the very long term, possibly hundreds of years, action to lower CO2 emissions must be taken in a considerably shorter timeframe, which would not allow for this technology’s storage, location and cost requirements to be met.





Sequester CO2:

To prevent the atmospheric CO2 concentrations from rising, one option is to put the CO2 back into an inert form.

There are three options to store the CO2:

- Geological Carbon Capture and Storage (CCS);

- Injection into oceans;

- Mineralisation.

Geological CCS (injection of CO2 into formations), preferably from power plants, is the most-researched option, but has many drawbacks associated with location, energy demand, public acceptance and costs. Most of the current CCS research and attention is given to this process.

The second theory is to simply inject the CO2 deep into Earth’s oceans.

But, as the oceans naturally circulate and release CO2, this would harm marine life and is therefore no longer considered a valid option.

Carbon storage occurs in the following form;

Carbon deposits

Amount (I0E15 kg)

Relative (%)

Limestone

35,000

47

Dolomite

25,000

33

Sedimentary carbon

15,000

20

Recoverable fossil fuels

4

0.005

Atmospheric CO2

0.72

0.004

Biomass

0.056

0.0007












Olivine


Olivine is the mineral which has seen the most amount of examination when it comes to sequestration.

It is more commonly found as peridotite, which is a combination of olivine and pyroxene.

Various naturally occurring and residual materials react with CO2 and sustainably store it.

The carbonation of carbon dioxide via magnesium and calcium-rich minerals such as olivine and wollastonite permanently disposes of CO2 in a geologically stable form.

The products formed by carbonation are thermodynamically stable, and this route is viable owing to the massive global reserves of suitable minerals.

Much of the research into mineral carbon sequestration has looked at methods of speeding up the carbonation process, which is inherently slow in natural conditions, to create an industrially viable system.

Some research suggests that in order to accelerate the process from years to hours, the reaction can be performed in a high-pressure/high-temperature autoclave.

Suitable processing of the mineral products, through size reduction and thermal or mechanical activation, has improved carbonation rates.

The resulting carbonated products provide a useful source of additional revenue, as they can be used in construction applications or mine reclamation.

Olivine reacts with CO2 to form magnesium bicarbonate solutions and dissolved silica.

In Earth’s oceans, this forms limestone and dolomite caverns, the ultimate storage facility for CO2.

In industrial installations, the olivine can be converted to magnesium carbonate and silica.

In formula form: Mg2SiO4 + 2 CO2 => MgCO3 + SiO2.

The end product now has a lower energy state than the gaseous CO2.

Most of the carbon is stored in this form.

The vast majority of CO2 is now sequestered in a safe and stable form.

Is there enough olivine to tackle global warming?

There are various options to increase CO2 sequestration capacity:

- Increase the natural weathering rate;

- Replace products;

- Increase the reaction rate by high temperatures and high pressures.

Due to the properties olivine holds, one solution would be to mine, mill and spread more of this material.

Global olivine capacity had been falling due to a combination of operation scale-backs and mine closures. World capacity in 2008 was assessed at about 9m tpa, but this has since decreased.

Sibelco, the world’s leading olivine producer, restarted production at its 400,000-tpa Raubergvik and 1.9m tpa Grubse olivine mines in Norway in September last year, after a two-year closure.

Production from the Raubergvik mine was scheduled to be shipped to the US, mainly for foundry use.

According to an address given in July 2009, entitled In Situ Mineral Carbonation in Peridotite and Basalt for CO2 Capture and Storage, by Peter Kelemen, JŸrg Matter and Dave Goldberg, there is enormous storage capacity in peridotite.

The Oman ophiolite is 70,000 km3 of which 30% is peridotite.

Similar size ophiolites are in Papua New Guinea, New Caledonia and along the east coast of the Adriatic Sea.

“All of these, except perhaps for the Balkan examples, extend offshore beneath marine sediments. This is particularly evident where peridotite outcrops along the shoreline. In general, near-surface mantle perodotite is present on all continents except perhaps in Antarctica, and for example in North America the cumulative volume of several smaller bodies taken together is comparable to the volume of peridotite in Oman,” the key note explains.

Research

Carbonation of olivine is a relatively new process and was first suggested in a letter to the Nature journal in 1990, so this research field is still new and developing.

The first companies to use these principles are starting to appear and interest has increased (see chart).

CCS Competition

In the UK, National Grid, the energy network transmittor, agreed to take part in a competition in 2009 to upgrade some of its transmission system for CCS.

The plans were to potentially use 300km of the National Transmission System (NTS) to pipe CO2 from the central belt to the St Fergus terminal in Scotland. From there, the CO2 would be potentially piped offshore for storage.

The move was part of a competition held by the Department of Energy and Climate Change (DECC) to demonstrate commercial-scale CCS in the UK. Following completion of the competition, a contract was promised to be awarded to a project to realise the proposal.

National Grid submitted its proposal in April 2009, and regulator Ofgem sought the views of the industry at this time.

The project wavered and was taken off the boiler several times, due to funding uncertainty. This year, the Energy and Climate Change Secretary Ed Davey launched a new competition.

This competition, named the CCS Commercialisation Programme, will support commercial scale CCS with £1bn in capital funding that the government made available.

Summary

While the CO2 is sequestered in a geological time scale, there is, as yet, no real discussion about its release, stability and any risks inherent in this process.

As an alternative to sequestering CO2, other techniques can be used to tackle the problems of global warming such as magnesium release and neutralising acidic soils.

Although this sector is still small, it is growing at considerable speed and new products are being developed.