Slipped, dissolved, and loosed: can weathering of rocks help tackle climate change?
Phil Renforth & Huw Pullin – School of Earth and Ocean Sciences, Cardiff University.
Some think that reacting rocks with CO2 may help prevent climate change. An idea that was initially thought to be expensive may now become a crucial component in our toolbox.
In his wide-ranging book, The planets, their origin and development (1951, Yale University Press) the physical chemist Harold Urey observed that the carbon dioxide (CO2) concentration in the atmosphere was substantially lower than what he thought it should be. His speculation on the cause of this difference has gone on to shape how we study the Earth and climate. He suggested that the CO2 had reacted with silicate minerals in rocks (like basalt) to produce new carbonate minerals (i.e. limestone), and that water and erosion were responsible. The idea that one mineral could transform into another was nothing new, but that CO2 was involved, was profound. It would take another 30 years before this relationship was fleshed out further.
Every year, the Earth’s rivers naturally add around 500 million tonnes of dissolved calcium to the oceans. That’s equivalent to the contents of several thousand large cargo ships emptying their holds. The enigmatic journey of this element begins as a rebirth deep in the Earth’s crust where it is bound in a cage of silicon and oxygen (‘silicate rock’). Through volcanism, these rocks eventually reach the surface, and slightly acidic rainwater dissolves the bonds to release the calcium. The liberated calcium seeks a new partner, it is very amenable to any available CO2, and both are swept towards the ocean in river water (as dissolved calcium bicarbonate). Eventually, the CO2 and calcium form a stronger long-term marriage as a calcium carbonate mineral, which settles onto the ocean floor. This journey is called ‘weathering’ and a similar story can be told for other elements in rocks (e.g., magnesium).
In the late 1970s, scientists realised that the relationship between weathering and atmospheric carbon dioxide may have an impact on climate over millions of years. This has been the subject of intense and increasingly sophisticated research ever since1. The hypothesis goes that as global temperatures increased, weathering increased, which removed CO2 from the atmosphere, reducing temperatures. Weathering, scientists postulated, is an inertia damper for the climate.
By the 1990s, some were speculating whether we could enhance these natural processes to counteract climate change. Prof. Klaus Lackner from Arizona State University, one of the early pioneers of this idea, knew that “CO2 needs to be taken care of, and societies must prohibit the dumping of CO2”. Him and his colleagues “were naturally drawn to forming carbonates”.
The high temperature and pressure reactor developed by Mineral Carbonation International. (Courtesy of Marcus Dawe, CEO).
Several proposals in this area have gained notoriety. One idea is to mine silicate rock, put it into a pressure cooker containing CO2 and water and let the carbonate mineral form (known as ‘mineral carbonation’). As an alternative to mining rocks, anthropogenic minerals, materials produced from human activity (e.g., waste cement, iron and steel slag) could be used in place of the natural silicates. Another approach is the direct CO2 injection into underground silicate rock formations2. Finally, and more recently, the idea of spreading crushed minerals onto the land surface (‘enhanced weathering’) has been put forward.
Mineral carbonation – Cooking up something
If weathering rates, and their effect on CO2 removal, increase naturally when it gets warmer, then, notionally, all we need to do is put the minerals into a heated reactor. The first experiments demonstrated that the concept worked, a large proportion of the magnesium and calcium in the silicates was converted to carbonate minerals in a matter of hours. The achievement in this early work is often understated, but here was a reaction that naturally influences atmospheric CO2 over 100,000 years, now working over the time it takes to watch an episode of The Great British Bake Off.
For the reaction to work, the temperature needs to be greater than 150°C and the pressure over 50 atmospheres of pure CO2. This is a considerable engineering challenge, especially if you’re trying to deal with the emissions from a power plant. A 2005 Special Report from the Intergovernmental Panel on Climate Change (IPCC) highlighted that the costs of mineral carbonation were greater than injecting CO2 into underground storage reservoirs3. There has been promising work since, and Mineral Carbonation International, an Australian based start-up, have commissioned several pilot plants. Their Chief Scientist, Dr Geoff Brent suggests that “perhaps the most important feature is that the carbonates and silicates arising from the mineral carbonation process can actually be used in valuable products, making this a carbon utilisation solution as opposed to a waste disposal approach”. As the construction industry consumes tens of billions of tonnes of aggregate and cement every year, replacing some of these with mineralised CO2 is a tantalising prospect.
Microscope image of thin section through a grain of slag taken under cathodoluminescence. The image shows a range of minerals and small air pockets typical of the material.
The Blaenavon Iron and Steel works is nestled at the head of the Afon Lwyd valley. It was one of several works on the edge of the South Wales coal field that operated during the Industrial Revolution. Although production at Blaenavon ceased in 1904, it is now listed by UNESCO as a World Heritage Site due to it being an outstanding example of 19th century industrial landscape. At the base of one of the decommissioned blast furnaces, a loud speaker in booming Welsh tones tells you of how workers would tap the furnaces for molten iron and cast them into metal ingots that bared a resemblance to suckling piglets (or ‘pig iron’). It was important, so we are told, before tapping the iron to remove the slag that floated over its surface. This slag is a wonderful material. Its chemistry is somewhere between cement and volcanic glass, created in its own artificial volcano. It is also rich in silicate minerals that react with CO2 much faster than their natural counterparts. If you look closely, you can find deposits of calcium carbonate at the base of heaps of slag, but it was only relatively recently that we were able to show that the CO2 trapped in the carbonate had originated from the atmosphere. So, leaving slag heaps to weather will slowly remove CO2 from the atmosphere. However, scientists are now wondering if this material, along with others (e.g., cement), could be harnessed to promote the capture of CO2.
Our team at Cardiff University, funded as part of the UKRI programme on Greenhouse Gas Removal, are investigating the potential of slag to be used for CO2 capture. Globally, about half a billion tonnes of slag are produced per year (much of which is recycled in cement). This equates to a total CO2 capture potential of 250 million tonnes, which is trifling when compared to global CO2 emissions of 40 billion tonnes. However, this potential may be more important than it first appears as it could mop up the residual emissions that are hard to deal with once other reduction technologies have been deployed (e.g., low carbon electricity and carbon capture and storage). When you stack this potential with materials from other industries, the CO2 capture may be as much as any other proposal to counteract climate change.
Are such proposals a long way from being deployed? The answer might be surprising: they are not. Launched in 2010, Carbon8 Aggregate is a UK based company that specialise in reacting waste with CO2 to produce construction materials. They have expanded production capacity to 50,000 tonnes per year with factories in Suffolk and Avonmouth. Prof. Colin Hills of the University of Greenwich, and founder of Carbon8 considers the CO2 mineral reaction to be relatively cheap, and that “key costs are associated with obtaining the CO2, constructing the 'reactor', any additives/reagents needed, and transporting the product market”.
Enhanced weathering –Spreading the load
In 2015 at a small conference in Oxford, the retired Professor of Geochemistry at Utrecht University, Olaf Schuiling held up a bag of olivine, a silicate mineral rich in magnesium, and proclaimed that he held in his hands the solution to climate change. Such a tongue-in-cheek boast was made with a specific process in mind, and not without reason. In 2006, Schuiling proposed that spreading crushed silicate rocks onto the land surface may be enough to accelerate weathering. The idea proved controversial, and gaps in our knowledge mean that it is difficult to accurately assess the feasibility of such an idea. On the one hand, it could be an exceptionally cheap method of capturing atmospheric CO2 if the rocks dissolved quickly, doing away with the expensive high temperature, high pressure, reactor-based processes. On the other hand, the speed of these reactions in the environment are poorly understood (meaning that they could be more expensive than theory would suggest, although ongoing experiments will hopefully shed light on this5), and the impact on the environment is equally hard to assess. However, we have been spreading rocks onto the land surface for millennia (lime, potash, phosphate), and some silicate rocks may be able to contribute nutrients for plant growth, lower the acidy of soils, lower the emission of nitrous oxide (another potent greenhouse gas), while the resulting slightly alkaline drainage waters may make a positive contribution to ocean acidification.
Enhanced weathering proposes to spread crushed minerals onto the land, similar to current practices of agricultural liming.
Breaking the bonds
The challenge of preventing climate change remains daunting. The Carbon Clock produced by the MCC in Berlin predicts that we have 9 years before the CO2 levels in the atmosphere are enough to warm the planet by 1.5 °C6. Cutting emissions is essential, but it’s unlikely to be scaled quickly enough to meet our targets. So, what do we do with the CO2 we cannot avoid creating? It is likely that a lot of the CO2 will end up being injected underground into unreactive rock formations. However, the potential for locking some CO2 up in minerals should not be ignored. As Dr Brent suggests “the mineral feedstocks are abundant and widespread and more than sufficient to deal with global CO2 emissions”. Indeed, the UK has enough silicate rock to capture over 400 billion tonnes of CO2, more than will ever be needed. There are some exciting proposals on the table that need to be explored in much more detail, some that are already helping to reduce emissions, and others that may not make it further than the lab door. Ten years ago, our first investigations into storing CO2 using minerals unearthed what we thought were expensive, radical ideas. It is time to take a second look.
Phil Renforth and Huw Pullin are funded by the UK’s Greenhouse Gas Removal Programme, supported by the Natural Environment Research Council, the Engineering and Physical Sciences Research Council, the Economic & Social Research Council, and the Department for Business, Energy & Industrial Strategy under grant nos. NE/P019943/1 and NE/P019730/1. All views expressed here are their own.
1. For a discussion on weathering and climate interaction, see Berner R.A., and Kothavala, Z. (2001) Geocarb III: A Revised Model of Atmospheric CO2 over Phanerozoic Time. American Journal of Science. 201. 182-204.
2. Injecting CO2 into underground silicate rock formations was considered in detail in a 2018 report by the National Academies of Sciences, Engineering, Medicine, US, Negative Emissions Technologies and Reliable Sequestration https://www.nap.edu/catalog/25259/negative-emissions-technologies-and-reliable-sequestration-a-research-agenda see also the CarbFix project.
3. The IPCC 2005 Special Report on Carbon Dioxide Capture and Storage can be downloaded from here https://www.ipcc.ch/report/srccs/
4. Options for carbonising the steel industry are considered by the IPCC Annual Report 5 (2014) Working Group 3 http://www.ipcc.ch/report/ar5/wg3/ and a 2015 technical report by WSP for UK Government ‘Industrial Decarbonisation & Energy Efficiency Roadmaps to 2050’
5. Enhanced weathering experiments are currently underway by the Leverhulme Centre for Climate Change Mitigation led by Sheffield University http://lc3m.or
6. See https://www.mcc-berlin.net/en/research/co2-budget.html for the MCC Carbon Clock.
Copyright Phil Renforth 2018