Mineral carbonation in a nutshell
• Mineral carbonation involves reactions of magnesium or calcium oxides (typically contained in mineral silicates and industrial wastes) with CO2 (dilute in the atmosphere or exhaust gases, or as captured pure CO2) to give inert carbonates. Due to the lower energy state of carbonates compared to CO2, these reactions release significant amounts of energy and, in nature, occur spontaneously (but slowly). Vast amounts of suitable and readily accessible mineral silicates exist – many times more than is needed to sequester all anthropogenic CO2 emissions.
• Slow speed of reactions of natural silicate rocks (requiring mechanical, chemical or biological treatments) and the large amounts of minerals or wastes that must be handled (2-3 tonnes rock per tonne of CO2) are the primary challenges for commercially viable industrial mineral carbonation applications. Processes that accelerate kinetics and maximise materials values with minimal additional costs and environmental impacts are the focus of R&D by Cambridge Carbon Capture and others around the world.
• Mineral Carbonation offers a commercial route to large-scale CCS starting with bottom-up, market-driven and highly profitable distributed niche applications where cost-reduction learning curves apply. Valuable and useful by-products such as silica, metals, chemicals, cements and construction materials, as well as remediation of waste feedstocks, enable a business case to be made. It is already being implemented across different industries and scales in profitable business operations and demonstration projects around the world.
• Mineral carbonation enables CO2 sequestration without capital and energy-intensive large-scale solvent capture and avoids the cost and public acceptability challenges of supercritical CO2 pipeline transport and storage infrastructure. It is primarily a disruptive, not an additive technology to conventional geo-CCS and UK policy makers risk missing a major opportunity by ignoring it.
In 1989, as I was finishing up four years as a British Gas research scholar in solid oxide fuel cells at the Wolfson Unit for Solid State Ionics at Imperial College, I accepted a job with Cookson Group in Oxford to lead their SOFC materials development programme.
The company was gearing up to supply the advanced electronic ceramic powders and components needed for Westinghouse’s and Dornier’s SOFC generator systems, but my first task alongside colleagues from Johnson Matthey, British Gas and Alstom, was to organise a conference celebrating the 150th Anniversary of the invention of the fuel cell by Sir William Grove. We were apparently just five years away from mass-market fuel cells delivering 70% electrical efficiency, high-grade waste heat, zero pollutants and (unremarked at the time) an exhaust output of pure CO2.
I mention this for several reasons (and fuel cells are relevant to the subject of this article, mineral carbonation) – cleantech innovation is a dangerous area for any business to engage; it seems especially risky if it is policy led and technology pushed. Fuel cell technology is a prime example where policy makers and funding agencies have pushed and led most technology developers down one R,D&D roadmap while a few business-minded companies have quietly got on with addressing what customers actually want and delivering it to them.
Today there is a global market for commercial fuel cell systems approaching $1bn/year and most do not use hydrogen. Although still tiny and niche, the fuel cell market is growing exponentially and, as units costs reduce by ~20-25% for every doubling of sales, applications will inevitably progress from niche to mainstream.
For reasons which I don’t need to rehearse here, commercial implementation of the chosen technical solution to CCS – geological sequestration – faces hugely greater challenges than fuel cells. We all want to stop CO2 molecules going into the atmosphere in the cheapest and least economically and environmentally disruptive way; we want CO2 sequestration to be commercially driven and cost reduced and we know that policy makers shouldn’t try to pick winners. So why in the UK are we wasting time and money and risking our climate by allowing the same divergence of policy and reality again?
Leading global chemicals companies like Solvay and SABIC don’t see CO2 as a waste to be stored underground, but are making strategic investments in the processes to use it as future industrial feedstock that will inevitably replace syngas. As Parliamentary Office of Science & Technology Note 403 points out, there are unsubsidised market driven technology solutions & applications for CO2 sequestration that are out there and already being implemented commercially.
They may be small and niche today – e.g. the storage of stranded excess renewable electricity via conversion of CO2 and water into fuels; the use of CO2 to convert alkaline dust wastes from paper burning into aggregates; and carbonation to stabilise aluminium industry wastes to avoid the risk of toxic red-mud spills – but with the right policy approaches, applications can be stimulated massively and the 20% cost-learning curve will do the rest.
Mineral carbonation – this is one of those rare technologies where the business models exist today to make CO2 sequestration commercially attractive in niche applications. Economic feasibility is underpinned not by the price of CO2 but by the release of substantial energy when gaseous CO2 reacts to form a lower energy state carbonate and by the value of the various by-products.
If we can understand all that makes these first commercial applications stack-up, and identify what scope there is for process improvements and changes (and dare I say policy measures) that can widen the commercially-driven applications, then we stand a chance of stimulating and accelerating a disruptive emergence of a market-led sequestration industry.
At the largest scales, carbonation of minerals can strip CO2 directly from the air – a geo-engineering approach featuring strongly in Virgin Earth Challenge’s £25m competition – or reactions of exhaust gas or captured CO2 with minerals to form construction materials can be used to substitute for pipelines and underground storage infrastructure in conventional power station CCS applications. Making the business case and developing the appropriate carbonation technologies across these scales of implementation will be challenging, but with energetics, economics & scaleability going with it rather than against it, mineral carbonation would seem a more feasible solution than geo-sequestration.
These are some of the themes that around forty industrial and academic members of the Mineralisation Research Cluster of the UK’s CO2 Chemistry Network will be discussing at its first workshop on November 28th (http://co2chem.co.uk/calendar).
Mineral carbonation explained
So what exactly is mineral carbonation? It encompasses a diverse range of processes and applications developed and researched globally over the last couple of decades, but most typically, mineral carbonation involves the use of exhaust gas CO2 to transform industrial wastes and minerals into valuable materials products, and because it consumes CO2 it can avoid any need to separate, transport and store CO2 gas.
Process variants can range in scale and complexity from simple sprinkling of suitable rock dusts onto large areas of land or sea to soak up atmospheric CO2 (e.g. www.smartstones.nl), to sophisticated multi-step chemical digestion, sequential precipitation and carbonation processes designed to produce high purity metal, silica, zero-carbon calcium & magnesium oxides and carbonate mineral powders (e.g. www.cacaca.co.uk).
Mineral carbonation brings opportunities particularly to the energy and materials intensive industries such as iron & steel, cement, glass, waste and minerals & mining, and also can provide low-cost CO2 capture materials as a scalable solution for the wider industrial and power generation CCS markets.
We are talking here about a fast and potentially profitable industrial version of the Earth’s natural silicate-carbonate cycle that strips around a billion tons of CO2 out of the atmosphere every year. This natural CO2 capture and sequestration is driven by the energy released when magnesium and calcium-containing silicate rocks are slowly weathered by wind and rain and react chemically with CO2 and water to form carbonates and silica. Over millennia, this process has locked up the vast majority of the Earth’s carbon into limestone rock.
The natural process is slow because, in rocks, ions of magnesium and calcium are protected from reaction with CO2 by the silicon dioxide structure that surrounds them. However, the carbonation reaction speeds up massively when mining companies dig up, crush and process these types of rocks to get at useful metals they contain or to provide aggregates and fillers for construction materials.
Crushing rock to powder is fairly energy intensive and the mixed product of a simple direct carbonation reaction may have little if any commercial value at large scales. An alternative approach is to chemically extract the magnesium or calcium ions from the rock or from appropriate industrial wastes before carbonation. In this approach, with the appropriate chemical processes, it may be possible to achieve faster, less energy intensive and more complete carbonation and also to enable metals, silica and carbonates to be recovered as separate and valuable products.
For example, Cambridge Carbon Capture is developing a fast low-cost chemical digestion process to extract amorphous silica and magnesium and calcium as pure oxides from olivines and steel slags; these oxides can then be used as zero or at least lower-carbon substitutes for conventional CO2-intensive lime in industry and also used directly as a medium for CO2 capture from exhaust emissions (one of our customers, Polarcus, plans to use CCC’s process for on-board sequestration in their fleet of marine survey vessels).
Product carbonates, if they have sufficient purity, may then find high value and volume application as white pigments or fillers for e.g. paper making. Silica powders, if they are produced with appropriate purity and particle size, are worth thousands of pounds per tonne as chemical inputs to the glass, electronics, construction and plastics industries.
It’s not just rocks that can be used as sources of magnesium and calcium for mineral carbonation processes. Industrial processes such as metals refining and combustion generate huge volumes of silicate slags and ashes. In their initial state, many of these wastes are unstable and risk leaching heavy metals into the environment and therefore often have a financial penalty associated with their storage as hazardous wastes. Mineral carbonation processes can offer an economic route to remediate these problematic wastes while also sequestering CO2. As well as these opportunities, my company, Cambridge Carbon Capture Ltd, is also investigating the by-product recovery of valuable trace metals such as platinum, rare-earths, and stainless steel alloys from a variety of industrial slags and mining wastes.
Power from mineral carbonation
So back to fuel cells – which is also where Cambridge Carbon Capture started. Fuel cells convert chemical energy directly and extremely efficiently into electricity; chemical energy is available in the oxidation of carbon-based fuels and there is another 15% or so available from the further reaction of CO2 with mineral silicates to form carbonates.
In 2010, with a team from Cambridge University, we put the two of these together with an alkaline fuel cell to create a high-efficiency zero-carbon electrochemical fossil power generation system: fossil fuel and mineral silicates IN and zero-carbon electricity, carbonates and silica OUT. This breakthrough won us a 2011 Shell Springboard Award and the fact that our electrochemical mineral carbonation system stripped net CO2 out of the air at the same time took us through several rounds of the Virgin Earth Challenge.
So what would we prefer, a CCS infrastructure that uses a quarter of a power station’s electricity to sequester its CO2 emissions under the North Sea or one that generates additional electricity and useful materials products?