A small community of researchers increasingly see the potential for certain types of rocks to offer a cost-effective carbon capture and storage (CCS) approach that could one day help reverse climate change. Yes: plain, old rocks. Here is the story behind the potential CCS strategy hiding under our feet.
A back-to-the-future CCS approach
The phrase “carbon capture and storage” often evokes images of enormous coal-fired power plants, complex industrial systems for scrubbing CO2 from exhaust gas, and pipes boring thousands of feet underground to dispose of CO2 in geologic reservoirs.
However, geologists have long known that nature has an alternative method for CCS. When certain types of rocks are exposed to air, they undergo a chemical reaction that transforms CO2 into a stable carbon-based rock in a process called “CO2 mineralization.” The geochemistry of mineralization reactions is fairly well understood: metal-oxide minerals (such as those rich in magnesium like serpentine and olivine) that react with CO2 in the air are widely abundant deep in the Earth and play an important role in transforming CO2 into the carbonate rocks (e.g. limestone) that comprise a large portion of the Earth’s crust.
But while such CO2-reactive minerals are widely abundant deep below the Earth’s surface, most of these minerals are shielded from exposure to air, so natural CO2 mineralization processes only sequester a tiny fraction of the CO2 emitted from the burning fossil fuels each year. (Physicist Klaus Lackner of Arizona State University (ASU) likes to point out that this naturally-slow CO2 mineralization rate is a good thing in the long run: if it were any faster, rocks would slowly draw down all of the CO2 in the atmosphere, ending life on Earth). So when it comes to climate change, natural CO2 mineralization processes won’t be anywhere near enough to solve the problem alone.
With some clever-but-low-tech engineering, however, it could be possible to accelerate natural CO2 mineralization processes substantially. And this is exactly why some climate researchers have explored the potential of engineered CO2 mineralization processes to serve as a large-scale carbon sequestration climate solution (also called "enhanced/accelerated weathering" or "mineral carbonation").
When it comes to engineering effective CO2 mineralization climate strategies, "it’s all about speed!” says Roger Aines, a scientist at Lawrence Livermore National Lab (LLNL). To speed up natural CO2 mineralization processes, scientists are exploring ways to increase the surface area of CO2-reactive minerals that is exposed to air (or another concentrated CO2 stream such as a power plant exhaust), and/or changing the chemistry of the original minerals (by heat treatment, enzyme treatment, etc.). In practice, this looks like:
Grinding or crushing CO2-reactive minerals mined from deep within the Earth (what geologists call ex situ mineralization).
- Increasing mineralization deep within the Earth (called in situ mineralization) by drilling into the lithosphere and fracturing subterranean rocks with CO2-reactive minerals to increase exposure and store CO2 underground.
(Note: Scientists have also proposed using CO2 reactive minerals to enhance the ocean’s alkalinity -- e.g LLNL scientist Greg Rau and Cardiff University scientist Phil Renforth have proposed -- as a related cousin to ex-situ Earth-based CO2 mineralization approaches, which is itself deserving of a separate post.)
Compared to conventional CCS projects, CO2 mineralization approaches offer a number of benefits. For example, CO2 mineralization approaches face few concerns about leakage, induced earthquakes, and/or land-use concerns that can accompany conventional CCS projects. In addition, engineered CO2 mineralization processes could create small but potentially meaningful revenue streams beyond carbon credits for businesses in the mining, agriculture, and manufacturing sectors, opening new frontiers for these difficult-to-decarbonize industries to becomes leaders in the fight against climate change.
So if turning CO2 into stone to fight climate change has so many benefits, why has it gained so little attention in the climate conversation to date?
The discouraging academic history on CO2 mineralization
Aines credits Lackner with first proposing the idea of CO2 mineralization back in the 90s, which in turn catalyzed a flurry of academic study of the topic, focused primarily around the mining (i.e ex situ) CO2 mineralization approaches. However, the results from this initial investigation into CO2 mineralization approaches have not been particularly encouraging. These researchers found that dedicated mining and processing CO2-reactive minerals as a standalone CCS strategy is probably more expensive than traditional CCS approaches at power plants (which themselves cost upwards of $60/ton CO2 unsubsidized). As the IPCC’s 2005 Special Report on CCS chapter on CO2 mineralization (on which Lackner was a lead author) puts it, “the kinetics of natural mineral carbonation is slow; hence all currently implemented processes require energy intensive preparation of the solid reactants to achieve affordable conversion rates and/or additives that must be regenerated and recycled using external energy sources.”
But here’s where the clever engineering enters the picture. Researchers have also looked at the potential for existing industrial processes to produce “waste” CO2-reactive minerals that could be re-purposed for CCS in a cost-effective way. For example, Jennifer Wilcox, a researcher at the Colorado School of Mines, has assessed the potential for waste products from cement production, coal power, and steel manufacturing wastes to supply the feedstock for CO2 mineralization processes. While these waste piles could provide cost-effective CCS, Wilcox and her team found that the available supply of these low-cost inputs for CO2 mineralization pale in comparison to the scale of CCS needed to mitigate climate change: even if all of these industrial wastes were harnessed for CO2 mineralization purposes, they would only be able to capture around 1% of US emissions (depending on assumptions used).
CO2 mineralization, it seemed, just wasn’t that attractive a target for CCS, as there seemed to be little way around the fundamental constraint of finding cheap and voluminous mineral sources that are not energy intensive to break down.
Between a rock and a hard place: CO2 mineralization commercialization challenges
This expert consensus that CO2 mineralization was at best high-hanging fruit on the CCS tree has had a self-fulfilling effect on innovation and advances in the field. Whereas governments have invested billions of dollars in conventional CCS projects at power plants and industrial facilities, the US government has only spent small amounts to research CO2 mineralization, primarily based out of the Albany Research Center shortly after Lackner and team proposed this idea.
Lackner notes that all Federal funding for CO2 mineralization “stopped because everyone was convinced that other forms of geological storage would be a lot cheaper.” Furthermore, Roger Aines of the Lawrence Livermore National Lab notes that CO2 mineralization approaches have also struggled to gain political support because “the real challenge in CO2 mineralization is that it is not a method for controlling point sources like a power plant, simply because the lowest cost sources of minerals that are so good at absorbing CO2 are rarely co-located next to power plants or factories. The idea of cleaning up CO2 from the atmosphere is still new, and government investments to date have almost all been made in controlling individual emitters, not reducing existing atmospheric CO2.”
Industry and civil society have also remained almost entirely on the sidelines in exploring CO2 mineralization approaches. Just this year De Beers became the first major mining or energy company to announce any research or projects around CO2 mineralization. And research from the Center for Carbon Removal identified no grants for CO2 mineralization from philanthropists in the U.S. over the past decade.
While a handful of intrepid startups such as Green Minerals and Mineral Carbonation International are working to build CO2 mineralization businesses, they are finding it difficult to gain traction. With little government support and low awareness among industry and civil society, it is incredibly difficult for the companies to find the capital needed to develop and deploy effective CO2 mineralization solutions.
Challenging the conventional wisdom
Because of the growing importance of CCS in meeting our climate goals, more and more researchers believe that it is too early to cross CO2 mineralization off the list of potential CCS strategies.
For one, as Aines bluntly puts it, “the idea that new mines grinding up new rock is the only way to engage CO2 process as a large-scale, economically viable climate technology is wrong. We can take advantage of what the mining industry is doing, and has done in the past, to start the testing and evaluation of new processes.” What Aines is noting is that certain mining wastes (including asbestos, tailings from diamond, and nickel mines, for example), offer a potentially substantial, yet unexplored source of extracted and processed CO2-reactive minerals. The re-processing and/or engineering of certain mine waste piles could turn these supplies of CO2-reactive minerals into passive CCS projects (see Georges Beaudoin’s research, for example). And as Wilcox et al have concluded, “comparatively low-cost methods for the advancement of mineral carbonation technologies... may be extended to more abundant yet expensive natural alkalinity sources,” increasing the economically-viable supply of CO2 mineralization CCS strategies in the future.
Second, researchers at universities (such as Peter Kelemen at Columbia) and at national research labs (such as the CarbFix project in Iceland and the PNNL in the U.S.) are exploring the often-overlooked category of underground (i.e. in situ) strategies for potential opportunities to serve as large-scale CCS projects. In situ mineralization involves drilling “wells” into CO2-capturing rock formations to speed up natural mineralization rates in places where CO2-capturing rock formations are relatively close to the surface (such as in Oman). Drilling into these formations and/or injecting compressed CO2 into these formations in similar fashion to a geothermal energy project would allow water and air to have much greater exposure to these rocks, vastly enhancing the rate at which they capture and store CO2.
Exploration of these previously overlooked CO2 mineralization CCS strategies is seen by many in this community as a low-cost path forward with potentially enormous returns for fight against climate change. At best, these initial explorations could lead to unexpected discoveries and innovations around CO2 mineralization that enable costs for these approaches to fall more than thought possible with existing technology. “The value for that initial testing phase is very real,” notes Aines. According to Aines, existing mine tailings alone could sequester a few billions of tons of CO2 in total (the U.S. emits roughly 6B tons CO2 each year as a comparison), which he calls “a huge win for the first phases of implementation of a new technology.” And at worst, these projects will provide robust science to help inform any future action on CO2 mineralization.
Towards an action plan for CO2 mineralization
In December of 2016, Aines, Kelemen, and Greg Dipple from the University of British Columbia convened a workshop of the leading practitioners in the field of CO2 mineralization to discuss where the greatest opportunities for CO2 mineralization CCS strategies existed, and how this community could marshal the resources needed to overcome the immediate barriers facing these projects. Researchers discussed a wide range of new proposals for CO2 mineralization (including new ideas such as mining olivine as an agricultural fertilizer, building high-temperature, high-pressure reactors for CO2 mineralization, and even using CO2 capturing minerals as a concrete or aggregate replacement). The goal of the workshop was to figure out “what next?” A few big ideas emerged from the discussion:
1. Now is the time to invest in more research and pilot projects to test costs, performance, and environmental impacts of CO2 mineralization strategies. It will be difficult to get accurate numbers on costs and performance of CO2 mineralization strategies without pilot projects of meaningful scale. Cost estimates for various CO2 mineralization solutions range from very cheap (just a few $/ton CO2) to expensive (upwards of $100/ton CO2). Furthermore, CO2 mineralization projects come with many site-specific challenges, such as navigating mine safety protocols and environmental regulations (many CO2-reactive minerals are found in rock formations that also include heavy metals that can contaminate local water and air supplies) that could lead to significant costs for project developers. It is very hard to estimate all of these project costs and performance variables in theory -- actual projects are needed to hone in on the true scale of the opportunity around CO2 mineralization.
2. Developing accounting protocols is critical to enable CO2 mineralization projects to participate in carbon markets. Another challenge for would-be CO2 mineralization project developers is the lack of protocols for measuring and verifying lifecycle CO2 capture and storage that results from CO2 mineralization projects. Regulators and/or third-party certification groups will need to validate the efficacy and reliability of CO2 mineralization efforts to enable buyers of CO2 mineralization credits to trust that their projects sequester as much CO2 as needed. The long lead-time for developing and implementing accounting protocols makes it worthwhile to begin the process now. As Lackner puts it, “there is no motivation to do CO2 mineralization for carbon sequestration if you cannot get credit for it. You can't get credit for it if there are no good accounting protocols. So you need to figure the accounting out soon.”
3. Start dialogues with key stakeholders. How do you get community advocates to push for funding for CO2 mineralization projects? Inspire the next generation of entrepreneurs and scientists? Proactively engage regulators to make regulatory process as fair, robust, and transparent and possible? Get industry champions to build projects and incorporate CO2 mineralization in their supply chains? One thread that addressed all of these questions was the urgent need to start dialogues today in the communities that will build and deploy projects (often rural mining communities and tropic agricultural communities -- far removed from the university research on this topic). On the ground engagement and collaboration between research, industry, and government with the communities that will build and deploy these projects is critical today to ensure that first projects are of the highest value to getting to scale in the future.
Despite less than optimistic preliminary investigations into the economics and potential of CO2 mineralization as a CCS solution to climate change, researchers and entrepreneurs alike have worked diligently to show that this field is worth a second look. New approaches to an old idea show promising pathways forwards, but it will be up to governments and businesses to take the leap and begin funding new approaches to determine the potential of this frontier in climate action.