How to use natural carbon mineralization?
Many scientific discoveries have been made through nature observation. From Newton who presented gravity’s concepts by receiving an apple on his head, to the Velcro tape invention (a commonly used scratch), inspired by Geckos who climb on walls, nature offers a multitude of solutions to many problems. Today, bio-inspiration or bio-mimic, which respectively consist of inspiration or mimic nature, are widely used to find innovative solutions.
It seems natural that in order to find solutions to fight greenhouse gases accumulation in the atmosphere, researchers looked up to nature. Carbon cycle study allowed us to identify the reactions involved in natural carbon content regulation on Earth. Today, many technologies aiming at reducing the amount of CO2 in the atmosphere are based on the observation of these natural reactions.
Carbon mineralization enables atmospheric CO2 sequestration within carbonate minerals. The idea of using this reaction was presented in the 90s (Seifritz 1990 Lackner et al., 1995) and this way of carbon sequestration offers several advantages. First as carbonates are stable minerals on the geological time scale, carbon mineralization offers an excellent guarantee for durability of CO2 storage (Sipilä 2008; Olajire 2013; Sanna et al., 2014; NASEM 2019; Kelemen et al., 2020; Sandalow et al., 2021). Furthermore, multiple feedstocks can be used for this reaction and these materials are accessible almost everywhere around the globe. Three main types of materials can be used as reactants :
a. Natural rocks, more precisely ultramafic rocks (peridotite, dunite, serpentinite) or mafic rocks (oceanic or terrestrial basalts)
b. Industrial residues, (e.g. iron and steel metallurgical slags and others).
c. Mining residues, issued for example from diamond or nickel deposits exploitation.
Carbon mineralization also allows to revalue the factory or mining residues. By extension companies and communities can save money on waste storage. This solution fits perfectly into the principles of sustainable growth and circular economy.
Despite these numerous advantages, natural carbon mineralization is too slow at atmospheric conditions. Research efforts have therefore focused on the three key points of the reaction to optimise this process and maximise CO2 sequestration:
1. Dissolution of CO2
2. Dissolution of reactive elements
3. Precipitation of carbonate minerals
Many parameters (physical and biological) influence these key points. The pathways of carbon mineralization, aiming at controlling these parameters, are therefore multiple (Figure 1). The following paragraphs present these different solutions in a general perspective. First the different carbon mineralization protocols could be divided into three main families (Sandalow et al., 2021; J Wilcox, B Kolosz, & J Freeman (2021); NASEM 2019; Kelemen et al., 2020)
I. In situ, the reaction proceeds directly in the natural rock;
II. Surface, the reaction takes place above ground under natural pressure and temperature conditions;
III. Ex-situ, the reaction takes place above ground within reactors at high pressure and/or high temperature.
If we wish to have an impact on the amount of CO2 within the atmosphere, regardless of the methodology used, the source of CO2 is the key.
- If the CO2 comes from human emissions (industrial stacks, industrial brine, etc.) carbon sequestration will "only" decrease the amount of anthropogenic CO2 emitted.
- If the CO2 comes from the atmosphere, carbon sequestration allows to remove the excess of anthropogenic carbon accumulated in the atmosphere since the industrial era (1750). This is what we refer to as a negative emission technology.
Several sequestration routes are now considering the use of CO2 sources qualified as hybrids, by coupling the two processes mentioned above (ICEF 2021, J Wilcox, B Kolosz, & J Freeman (2021)).
IN SITU
In this type of carbon mineralization, CO2 dissolved in water or industrial brines is injected into the hardrock at high pressures (Kelemen et al., 2020; NASEM 2019; Sandalow et al.,2021). These injections can be performed in two major rock types: (a) peridotites and (b) basalts. The main objective is to stimulate the natural mineralization reaction in these rocks. The CarbFix project, located in Iceland, catched the eye of the press. It is a large-scale application of in situ mineralization. Monitoring a reaction which takes place several hundred meters below the ground surface is one of the main difficulties encountered in In situ projects. Numerous research efforts are being made to develop indirect methods to monitor the reaction (without taking samples). For example, numerical modelling allows the use of real data and the extrapolation of results.
SURFICIAL
Surficial mineralization processes use solid waste (mining or industrial) or natural minerals milled to sequester CO2. The carbon mineralization reaction is performed under natural pressure and temperature conditions and materials are grounded beforehand. The surface area of the mineral is therefore increased which allows it to accelerate and maximise dissolution of the reactive elements (Mg2+ and Ca2+). Surficial processes can be divided in two categories depending on the location of the reaction: (i) close to the source of the feedstocks and (ii) far from the source of the materials also called Enhanced weathering. The projects close to the source of the materials mainly concern mine tailings issued from metal or diamond deposits exploitation. Project Vesta aims to spread olivine on beaches to sequester CO2 (i.e Enhanced weathering & Olivine articles). In this pilot project, waves increase physical alteration of olivine minerals and allow to achieve a better mineral dissolution. Project Vesta therefore belongs to the Coastal enhanced weathering branch of carbon mineralization pathways.
EX SITU
Natural minerals, mining or industrial waste are also used to sequester CO2 in this carbon mineralization pathway. These feedstocks are placed in reactors designed to boost the carbon sequestration. One benefit of these technologies is to control all the external parameters at each key point of the carbon mineralization process. For example, in order to promote carbon mineralization reaction, materials could be chemically, thermally or physically pre-treated (grinded). Ex-situ mineralization protocols can be divided into two subcategories: (a) direct and (b) indirect. In the first one, the reaction takes place in a single step. Mineral dissolution and carbonate precipitation are influenced by different even antagonist physical and biological parameters. Indirect mineralization is appealing because mineral dissolution and carbonate precipitation could be performed in at least two separated steps. The external parameters could be adjusted in order to increase each part of the reaction and therefore increase carbon sequestration. As an example, processes called "pH swing" propose to put the reactive elements in an acidic solution in order to enhance the import of Mg or Ca in water. Then the pH will be increased (by addition of chemicals) to promote carbonate precipitation (Olajire 2013, Sanna et al., 2014). In most cases, ex-situ mineralization techniques are still under development. The Australian company MCi developed a platform, on the one hand sequester CO2 using different types of reactors, and on the other hand recover valuable materials. For example, the products of the mineralization reaction could be used as building materials.
Conclusion
All these carbon mineralization pathways aim at storing CO2 in a solid carbonate. In addition to the sustainability of the carbon sequestration, these solutions allow to reduce the post-sequestration monitoring, particularly compared CO2 storage in geological reservoirs. Carbon mineralization is therefore a good solution to sequester CO2 and fight against the greenhouse gases accumulation in the atmosphere. However, several challenges must be met to use these solutions at large scale. Increasing the number of pilot projects to test the different mineralization pathways is now necessary.
