GECO - Geothermal Emission Gas Control (Horizon 2020)

Ultramafic rocks are a major component of the ocean lithosphere commonly exposed near and along slow and ultra-slow spreading ridges and in ophiolites environments. The serpentinization of mantle rocks is the reaction of water with the mineral phases olivine and pyroxene to predominantly form serpentine, brucite, and magnetite. The mineralogical assemblages and textures are complex and reflect multiple phases of alteration, deformation, and veining during emplacement, hydrothermal alteration, and weathering. Serpentinization is a widespread process that occurs along mid-ocean ridges where tectonic activity causes exposure of mantle rocks to seawater. It has been studied in modern and fossil oceanic lithosphere as along the Mid-Atlantic Ridge (Früh-Green et al., 2003; Boschi et al., 2013) and at the Ligurian ophiolites (Schwarzenbach et al., 2021), among many others. During serpentinization, various elements (e.g., Mg, Ca, Cl, F, C, S, B, Sr) are exchanged between the rock and the interacting fluid. Additionally, oxidation of Fe2+ in the primary mineral phases of olivine and pyroxene is oxidized to Fe3+ resulting in the formation of H2, while CO2 can be reduced to CH4; the release of H-rich fluids fueling microbial life is also relevant to the origin of life (Frueh-Green at al., 2003). As the oceanic lithosphere experiences significant geochemical transformation, serpentinization also controls the transport of elements (e.g., H2O, C, S, B, Cl, F) into the mantle, and influences its seismic and rheological properties at convergent plate margins. Finally, serpentinites are also highly reactive with CO2 such that they are prime targets for carbon sequestration (Boschi et al., 2009). Hence, serpentinization has wide-ranging effects on various tectonic and magmatic processes and numerous geochemical cycles, strongly controlling the chemical exchange between hydrosphere, lithosphere, and biosphere and is considered a key player in the global cycles in the ocean and continental settings.

La CO2 in forma gassosa o disciolta in acqua si combina con ossidi di magnesio, sodio e calcio e forma dei minerali, ovvero carbonato di magnesio, di sodio e di calcio. Questa reazione chimica permette un “sequestro mineralogico” della CO2 che può essere così tolta dall’atmosfera o “catturata” direttamente da sorgenti puntiformi: industrie, impianti di produzione di energia, cementifici, e acciaierie. https://www.rinnovabili.it/ambiente/cambiamenti-climatici/quanto-ci-piacciono-i-carbonati-il-sequestro-mineralogico-della-co2/

To meet Paris Agreement's goal of limit global warming to +1.5/2°C from pre-industrial level, Carbon Capture and Storage (CCS) technologies are required to sequester 100 to 1000 Gt of CO2 over the 21st century. Most of the existing large-scale CCS facilities are based on enhanced oil recovery, where CO2 is trapped, mainly physically, below an impermeable cap rock. An alternative is the permanent storage of CO2 through mineral carbonation. This approach is based on the reaction between CO2 and divalent cations (e.g., Mg2+) resulting in the bonding of CO2 into the structure of newly formed carbonate minerals (MgCO3), achieving "mineral trapping". This technology has virtually no risk of release, can be applied at the global scale either in-situ, directly injecting CO2 into ultramafic formations, or ex-situ in bespoke reactors, and promises a very large storage capacity (105-108 Gt). In addition, recovery of valuable metals from the feedstock material and commercialization of the carbonate produced as construction material could represent a significant source of revenue. Despite these advantages, to date the large-scale deployment of mineral carbonation is still hindered by slow reaction rates and elevated storage costs. Here we present an innovative approach for the development of alternative carbonation paths in the framework of the Horizon 2020 GECO project (https://geco-h2020.eu). This consists in the study of natural carbonation systems using field geology, petrology, geochemistry and combining the results with batch reactor experiments and geochemical modelling. The aim is to overcome current technological barriers by learning from natural systems which spontaneously sequestered large amount of CO2. The overall goal is to define new cost-effective solutions for the storage of CO2 in ultramafic rocks capable of speeding up the large-scale deployment of CCS so that efficient CO2 storage could be achieved within the timeframe considered by the Paris agreement.

The EU H2020 GECO (Geothermal Emission Gas Control) project is aimed to produce new technologies to limit emissions from geothermal power generation by either gas re-injection or its use to produce commercial material through serpentinite carbonation. In this framework, the realization of a closed loop geothermal power plant has been planned in the Lardello geothermal area (Italy), where gas will be re-injected in a reservoir constituted by phyllites and micaschists. A set of water-gas-rock interaction experiments was performed in order to: i) investigate the interaction between CO2-H2S gas mixture, representative of the geothermal fluids exploited at Larderello, and phyllites and micaschists of the reservoir ii) optimize the conditions for CO2 mineral sequestration by reacting CO2-H2S gas mixture with different serpentinised ultramafic rocks buried in the nearby area. During the experiments, rock powder suspended in ultrapure ion-free MilliQ® water were reacted with a gas phase (CO2-H2S=98-2% or CO2=100%) in a PARR 5500 HP stirred reactor at P-T conditions ranging from 20 to 60 bar and 90 to 250 °C, respectively. The liquid phase resulting from the experiments was analysed via ion chromatography and ICP-MS to determine ion contents, whilst rock and rock powder were examined with SEM-EDS and EPMA to identify mineral phases and determine mineral chemistry. Preliminary results highlighted that H2S plays a pivotal role in controlling the reaction pathways with phyllites and micaschists, allowing the formation of pyrite in a wide range of P-T conditions. This process induces a selective removal of Fe from the solution, while the exceeding SiO2 deriving from mica and chlorite alteration re-precipitate as quartz. In this experiment, carbonate precipitation is prevented by the low Ca and Mg content of the samples and by the high water to rock ratio constrained by the experimental set-up. Experiments with ultramafic rocks were performed using serpentinised harzburgite and brucite-rich dunite in order to identify the most reactive lithology for mineral carbonation. Preliminary results show that CO2 sequestration is strongly enhanced by the presence of brucite compared to serpentine but further experiments are required to establish the most efficient reaction conditions. This research is supported by European Horizon 2020 “GECO” project (Grant n° 818169).

Understanding low temperature carbon sequestration through serpentinite-H2O-CO2 interaction is becoming increasingly important as it is considered a potential approach for carbon storage required to offset anthropogenic CO2 emissions. In this study, we present new insights into spontaneous CO2 mineral sequestration through the formation of hydromagnesite + kerolite with minor aragonite incrustations on serpentinite walls of the Montecastelli copper mine located in Southern Tuscany, Italy. On the basis of field, petrological, and geochemical observations coupled with geochemical modeling, we show that precipitation of the wall coating paragenesis is driven by a sequential evaporation and condensation process starting from meteoric waters which emerge from fractures into the mine walls and ceiling. A direct precipitation of the coating paragenesis is not compatible with the chemical composition of the mine water. Instead, geochemical modeling shows that its formation can be explained through evaporation of mine water and its progressive condensation onto the mine walls, where a layer of serpentinite powder was accumulated during the excavation of the mine adits. Condensed water produces a homogeneous film on the mine walls where it can interact with the serpentinite powder and become enriched in Mg, Si, and minor Ca, which are necessary for the precipitation of the observed coating paragenesis. The evaporation and condensation processes are driven by changes in the air flow inside the mine, which in turns are driven by seasonal changes of the outside temperature. The presence of "kerolite", a Mg-silicate, is indicative of the dissolution of Si-rich minerals, such as serpentine, through the water-powder interaction on the mine walls at low temperature (~17.0 to 18.1 °C). The spontaneous carbonation of serpentine at low temperature is a peculiar feature of this occurrence, which has only rarely been observed in ultramafic outcrops exposed on the Earth's surface, where instead hydromagnesite predominantly forms through the dissolution of brucite. The high reactivity of serpentine observed, in this study, is most likely due to the presence of fine-grained serpentine fines in the mine walls. Further study of the peculiar conditions of underground environments hosted in Mg-rich lithologies, such as that of the Montecastelli Copper mine, can lead to a better understanding of the physical and chemical conditions necessary to enhance serpentine carbonation at ambient temperature.

The EU H2020 GECO project is primarily aimed to setup technologies to lower emissions from geothermal power generation by capturing them for either reuse or storage, to turn captured emissions in to commercial products and demonstrate the technical and economic feasibility of the injection method. To achieve this goal a site specific characterization and modelling of geology and geochemistry of the geothermal reservoir are in progress for optimization of the injection experiments at four distinct geothermal systems throughout Europe are in progress. The Italian GECO demonstration site is Castelnuovo, which is located in the northeaster side of the Larderello geothermal area and where Graziella Green Power and Storengy are planning to exploit a deep seated high temperature resource for power production with the scope of no non condensable gases (NCGs), release in atmosphere. The geothermal fluid will be extracted from 2 production wells and then it will be reinjected in the reservoir by mean of 1 reinjection well. A Zero emissions ORC power plant will produce 5MWe. For the geological assessment of Castelnuovo site an integrated methodology was designed and is applied to obtain the most reliable and accurate assessment of the site. The geological, reservoir and chemical reactive modelling are performed using field or labs data and observations (i.e., geology, geophysics, geochemistry surveys). Moreover, we adopted a double scale approach where regional, and larger, model was built to provide constrains to local, smaller and detailed, models. At regional scale a geological model including the main structures of the area is modelled to create the geometries needed to local models. At locale scale, firstly the thermal steady state of the rock volume and secondly the reinjection simulations on reservoir and fluids-rocks interaction during the reinjection are numerically computed.

The goal of the GECO (Geothermal emission gas control) project is to advance our ability to provide cleaner, and cost-effective non-carbon emitting geothermal energy. GECO is based on the application of an innovative technology that can limit the emissions from geothermal plants by condensing and re-injecting gases or utilising them to produce commercial products. This approach leads to long-term environmentally friendly storage of waste gases and lowers the cost of cleaning geothermal gas compared to standard industry solutions. We are primarily focused on implementing a new ex-situ CO2 mineral sequestration approach for the production of commercial materials. Because the efficiency of the carbonation process is intimately linked to the mineralogy of the starting material, the first step has been to perform a regional survey to identify the most reactive ultramafic lithology to accelerate the CO2 fixation process. We have found that in Southern Tuscany natural carbonation has been particularly efficient in the formation of magnesite and hydromagnesite when CO2-bearing fluids interacted respectively with serpentinised harzburgites and dunites [1, 2]. Therefore, these lithologies will be first characterised petrologically and geochemically and then used to perform carbonation experiments to constrain silicate dissolution and carbonation precipitation rates. Results will be used to model reaction paths for industrial applications in order to create a variety of potential commercial materials.

In the framework of Horizon 2020 program, the European Commission funded GECO project that has the overall aim to generate viable, safe and cost-effective technologies for cleaning geothermal power plant exhaust gases to be applied widely at European and global scale. The rationale of GECO project relays on a successful technology recently tested in Iceland at pilot scale, where the gas emissions (mostly steam and CO2) from geothermal power plant were condensed and re-injected in the geothermal reservoir, or turned into commercial products. The GECO partners are committed to characterise 4 different demonstration sites located in Iceland, Italy, Germany and Turkey providing a pre-feasibility assessment of the baseline reservoir conditions and to demonstrate the feasibility of the re-injection (including NGC's) of geothermal fluids. To this latter aim, studies on the re-injection conditions beside the development of efficient and environmentally safe and economic viable methods to purify the geothermal emissions are planned. The Castelnuovo demonstration site is located a few kilometres northeast from Larderello geothermal area where a deep, superheated steam reservoir with 8% of expected NGCs (98% CO2, 2% H2S) is hosted mainly in the metamorphic units and characterised by temperature ranging between 300° and 350°C and pressure up to 70 bar at depths ranging from 4 and 4 km. For the pre-feasibility assessment of the Castelnuovo (Italy) site a geological, geophysical and geochemical integrated approach is proposed. To this aim, different scales and typologies of modelling are ongoing. The area of modelling is about 25 Km x 37 Km in order to include information from the main geological regional structures, which are crucial to increase the accuracy of the reservoir scale models. A first level of modelling produced a geological regional model and a steady state thermal regional model, where existing data and information from structural geologic, geophysical and geochemical field works are embedded. The geometries from geological regional model and the natural thermal state of the underground are acting as input and boundary conditions for the reservoir modelling. The reservoir modelling is the second level of modelling, i.e. local model, and is carried out to assess the exploitation scenario accounting for injection and production. Moreover, at reservoir scale, geochemical modelling of the gas-water-rock interaction during the exploitation is ongoing to predict the fate of reinjected fluids at depth. Laboratory experiments will also provide some constrains on the fluid-rock geochemical reactions expected in the reservoir as consequence of re-injection.