The main causal factor of climate change is the release of carbon dioxide (CO2) and other greenhouse gasses into the atmosphere. As natural processes will be insufficient to absorb future anthropogenic CO2 emissions, it is generally agreed that carbon capture, use and storage technologies are the optimal way to tackle this problem, by capturing CO2 and converting it for reuse or storage, thereby preventing its release into the atmosphere.
To date, carbon capture followed by transportation to a storage site with subsequent structural storage, where CO2 is injected under pressure into a geological formation and kept in place by an impermeable layer of cap rock, has been the most common option for the mitigation of CO2 emissions. However, alternatives exist to the storage of CO2 gas. Mineral carbonation (MC) is a process whereby CO2 is chemically reacted with metal oxide bearing-minerals to form stable carbonates, offering an attractive solution for the permanent and safe storage of CO2. This reaction can take place either below (in situ) or above ground (ex situ). In situ mineral carbonation involves the injection of CO2 into underground reservoirs to promote the reaction between CO2 and alkaline-minerals to form carbonates. Ex situ mineral carbonation relates to above-ground processes, which require rock mining and material comminution as pre-requisites for MC.1
The CO2SolStock project, funded under the EU’s Seventh Framework Programme (FP7), investigated a biomimetic approach to CO2 carbonation and aimed to investigate microbial carbonation as an alternative way to store carbon. The project aimed to map the various microbiological pathways of capturing CO2 through carbonation and establish a methodology and a testing toolkit, to enable future research teams to investigate and evaluate scientifically similar pathways. Finally, the project aimed to validate its technological strategy with at least two novel recipes that were potentially competitive and ready for a proof of concept test.
The project investigated four main CO2 storage pathways. In the first of these, subterranean pathways using bacteria in deep saline aquifers were shown to be potentially complex and energy intensive for low results in terms of carbon storage. However, this option might still prove to be of interest for sealing saline aquifers used to store supercritical CO2 in some carbon capture and storage (CCS) schemes. Another approach sought to combine two sources of industrial by-products: desalination brines as a calcium source and domestic wastewater as a carbon source. For this pathway, the potential for precipitation of calcium carbonate in terms of bacterial strains was demonstrated in the lab, but the correct recipe has yet to be worked out and needs further experimentation. Dual wastewater anaerobic treatment and silicate rocks weathering was the third pathway, in a first stage, a bacterial acid attack on silicate minerals frees the necessary calcium, while in a second stage, other bacteria produce the alkalinity needed to precipitate limestone and generate high-quality biogas. Finally, in an oxalate-carbonate pathway, an ecosystem management approach was developed based on the discovery of a triple symbiosis between some special trees, fungi and bacteria, leading to the precipitation of limestone in acidic soils around and below the tree roots.
The project found that bio-carbonation pathways represent a real paradigm shift, as they deal with CO2 that could be beyond the reach of classical CCS. Bio-carbonation pathways also mimic the natural-geological CO2 storage mechanism and fix CO2 as a stable solid, which can be either stored or could potentially be used as a building material. Hence storage sites do not necessarily need to be big or subterranean with a sealing cap rock. Bio-pathways also have the significant advantage that they can address past emissions by fixing atmospheric CO2 through photosynthesis, unlike CSS.
While CO2SolStock dealt primarily with in situ carbonation, ex situ processes were the focus of the CO2NOR2 project, funded under Horizon 2020. The two-year project, launched in September 2015, will investigate an innovative and sustainable method for mineral carbonation to ensure the safe storage of CO2. This method includes the creation of novel nanomaterials via a ball milling process, based on low-cost ultramafic and mafic rocks from the Troodos ophiolite (Cyprus). Ophiolitic rocks are considered to be one of the most promising lithotypes for CO2 storage due to their high reactivity.
A systematic study of the applicability of these rocks for CCS will be carried for the first time as part of this project. It is anticipated that ball milling will accelerate the kinetics of rock-fluid reactions during the carbonation procedure. Hence, carbonate minerals, which are stable over geological timescales, will provide a safe long term CCS solution. Additives will also be tested in the nanomaterials in an attempt to increase their CO2-storage capacity. The proposal also involves applied research into the use of the end-product carbonates in the building industry.
Mineral carbonation is not the only option available. It is also possible to capture CO2 released by large-scale industrial sources and feed it immediately into a conversion unit that will convert it into a marketable carbon derivative.3 As many of the feedstocks for the most widely used commodity chemicals are currently derived from non-sustainable carbon sources such as petroleum, the replacement of these sources with recycled CO2 becomes an even more attractive proposition. Furthermore, the technologies exist to reuse the CO2 captured in this way as a carbon source for the manufacture of commodity chemicals, particularly liquid and gaseous synthetic fuels.
The ESBCO2 project, which was funded under FP7 and ran from 2012 to 2015, looked at the production of biofuels through microbial electrosynthesis (MES). MES is a process that exploits the ability of microbes to make electrical contacts with electrodes and other cells and the production of biofuels through MES is of great interest. Specifically, the project aimed to examine mechanisms by which microorganisms conserve energy when directly accepting electrons for MES from electrodes, and to further explore carbon and electron flow during CO2 reduction to biofuels at a cathode. The project will contribute to the development of a cost effective alternative to current fuel production, using greenhouse gas CO2 as a feedstock. It will use new concepts based on electron (e-) transfer/exchange, conductive biofilms and other novel materials to deliver an environmentally sustainable solution for biofuel production.
The European Union’s Bioeconomy Strategy supports the development of production systems with reduced greenhouse gas emissions, including increased carbon sequestration in agricultural soils, sea beds and the appropriate enhancement of forest resources.4 The research conducted in the above projects and other projects funded under Horizon 2020 will feed into this support. The fragmentation of know-how and activities across Europe is one barrier to the fast development and uptake of CO2 conversion technologies. With respect to bio-conversion, this is something that is being addressed by the European Commission’s Bio-observatory, which is managed by the Joint Research Centre. The task ahead is enormous. For the technologies outlined above to influence CO2 levels on a scale that would impact on climate they will need to span the chasm from R&D to large-scale market uptake, requiring billions of euros in investment. That said, given that the stakes are so high, it is clear that carbon dioxide conversion technologies will have a key role to play, along with emission reduction and storage solutions, in future strategies to restore balance to the global carbon cycle.
2 Project full title: "Carbon dioxide storage in nanomaterials based on ophiolitic rocks and utilization of the end-product carbonates in the building industry”.