Carbon Capture and Storage . A case study of mineral storage of CO 2 in Greece

While the demand in reduction of CO2 increases, the need for CO2 sequestration processes is very high. One promising technology is the Carbon Capture and Storage (CCS). In this paper we refer to several papers which study the three main steps in CCS chain. CO2 capture technologies, CO2 transportation to the storage sites and the very critical step the CO2 storage. Recently a novel method (mineral carbonation) for CO2 sequestration has been proposed which is based in the reaction of CO2 with calcium or magnesium oxides or hydroxides to form stable carbonate materials. Greece is a country that emits CO2 mainly from the lignite fired power plant in Western Greece. After the study of the bibliographic references about the use of mineral carbonation process while injecting CO2 in the appropriate geological forms we concluded that there are also these forms in our country and mainly in the area near to the power plant such as in sites Vourinos and Pindos. In these sites exist minerals rich in oxides and hydroxides of Ca, Mg and Fe representing the perfect materials for mineral carbonation.


Introduction
Nowadays there is an increasing demand for energy resulting in the increase in the use of fuels, particularly the conventional fossil fuels (coal, oil and natural gas).
Despite the fact that the fossil fuels are the key energy source since the industrial revolution they are causing the same time, through their combustion, a serious threat for the Environment emitting to the atmosphere high amounts of CO2, a major anthropogenic greenhouse gas.It is clear that the human activities influence the climate system. 1 In 2016 the average concentration of CO2 (403 ppm) was 40% than in the mid-1800s.It was estimated that the CO2 concentration increased about 2 ppm/year in the last ten years. 2 In the light of global commitment achieved in Paris 2015, the rise of global temperature should be kept below 2 o C compared to preindustrial levels and also the temperature increase should be limited to no more than 1.5 o C (UN Paris Agreement 2015).According to IEA the goal of Paris Agreement requires the storage of at least 1 gigatonne of CO2 annually by 2030.A critical technology that could help in the fulfillment of the above goals is the Carbon Capture and Storage) CCS.The objective of CCS is to capture and store CO2 in several manners.The CCS uses existing processes and technologies available in the oil and gas industries to capture the CO2 and store it deep below the surface in appropriate geological formations for permanent storage.
The aim of this paper is to present the several capture, transportation and storage strategies according to bibliography.It also presents the CCS technologies around the Europe.Giving emphasis on the third part of the CCS chain (the storage) it concludes that the mineral carbonation could be a promising CO2 storage technique.Taking into account that lignite combustion is the main industrial way to produce electricity in Greece and emits high amounts of CO2, we tried to give some ideas in using CCS technology in Greece power plants.Based only on bibliographical references about the geology around the Greek Power Plant area, we concluded that mineral carbonation in sites Vourinos and Pindos under appropriate circumstances could be a potential way of sequestrating CO2 in a safe and permanent way.

CO 2 Capture Technology
There are three technological routes for CO2 capture from power plants: Precombustion capture where fuels are converted to H2 and CO2 and the CO2 produced is separated before combustion, Post-combustion capture where CO2 is separated from flue gas, which is produced by fuel combustion and Oxy-fuel, where pure oxygen is used instead of air during combustion, leading to a flue gas stream of nearly pure CO2.However, the application of this technology may reduce the efficiency of the plant by 14% and increase the cost of electricity by 30-70%). 3The post combustion capture is of particular interest because it is a possible near-term CO2 capture technology that can be used to existing power plant. 4As a result we are going to focus mainly on post combustion technologies in this paper.
Chemical absorption is one of CO2 Capture Technologies.According to [5][6][7] the classic CO2 absorbent is aqueous monoethanolamine (MEA) especially for CO2 separation in electricity generation.The first full-scale commercial post-combustion carbon capture and sequestration (CCS) project operated in coal fired power plant in Estevan, Saskatchewan, Canada used an amine based process reducing CO2 emissions.There are also new absorbents 5 that has been studied for this purpose as single amine absorbents, amine blends, multi face absorbents e.g. the formulation of aqueous piperazine (PZ) and 2-amino-2-methyl-1-propanol (AMP), econamine FG+, KS-1 and Cansolv.
Adsorption is another technology for CO2 capture.The use of adsorption process in electric power plants by [8][9] , indicates that this technique can be used to power plants.Some classical adsorbents are carbons, aluminas, zeolites, silicas, metal organic frameworks, hydrotalcites, poliymers etc.More details about the adsorption in CO2 capture technologies and their development are indicated in 5 .
There is another process, relatively new, proposed by Shimizu T. 1999 10 for CO2 removal from flue gas released from air-blown combustion systems.The calcium looping process separates CO2 using the reaction CaO + CO2 →CaCO3 and regeneration of CaO using O2 combustion.It is of interest the key advantages of this technique 11 : Large amount of high recoverable heat (600-900 o C), Possible increase in power plant energy penalty (40-60%), no flue gas cooling and pretreatment (SOx) and finally it has low emissions and affordable price.A review of the calcium looping technology and its progress is indicated by 5 .
Another technology, in an early stage of development, for capturing CO2 from coal fired power plants is chemical looping [12][13] having the potentially of very low efficiency penalty and low CO2 avoidance cost. 14Details about the progress of this technology are shown by 5 .Membrane-based processes can be used in precombustion, oxy-combustion and post -combustion and are suitable for coal fired power plants.The development of this technology is described by Mai Bui 2018. 5nic liquids (ILs) technology has attracted attention due to the energy and costefficient separation of CO2 from post-combustion flue gas. 15The progress of this technique is represented by 5 .
Finally, we should mention two methods of Carbon Dioxide Removal from the atmosphere.The first one is BECCS where oxidation or gasification of biomass extracts energy known as bioenergy, capturing at the same time the CO2 from these two processes.However this approach has serious problems such as the need of the most arable land to be used for the food demand and not for biomass (National Academy of Sciences).The increase in electricity cost and decrease in energy security is another serious problem. 16e second method is Direct Air Capture and Sequestration (DAC).The capture is taking place directly through the atmosphere via absorption or adsorption separation processes.There is a DAC plant in Hilwil, Switzerland which filters CO2 from the atmosphere and supply 900 tones of it annually to a nearby greenhouse helping the plants to grow (Grand opening of Climeworks commercial DAC plant, Gasword, 2017).DAC is a promising approach, however it can't replace the conventional CCS systems because the CO2 concentration in air is 100 to 300 times lower than in the flue gas of gas or coal fired power plants.This results in a high cost of capturing CO2 from air than from point sources and constrains the use of DAC. 17

CO 2 Transportation
In CCS process, after the CO2 capture and separation, the gas is transported to the storage site via pipeline when it is in dense phase or by trucks, rail and ships when is in the liquid phase.The efficacy of the methods depends on the distance of each point of storage.Ideally CO2 would be stored where captured.According to (Zero emissions platform) for large distances >1500 Km, transportations via ships is preferable because of lower cost.9] There are also some difficulties.The amount of CO2 that is transported should be in dense phase, otherwise the system will have operational problems.For this purpose the appropriate temperature and pressure must be chosen so that the phase will remain the same along the length of the pipeline. 5,18 urthermore the impurities in the CO2 stream are a vital subject and impact on the design and operation of the pipeline system. 18Generally, it is considered that the cost of transporting CO2 may be considerably reduced by using multiple diameter trunk lines which lower operating cost and ensure at the same time the right operating pressure throughout the whole pipeline. 5,18,20 Ohe other hand, the CO2 transportation via ships can be an effective cost solution for very long distances and for low quantities from small sources.21 Details about the technology of CO2 shipping can be found in (Dr Peter Brownshort 2015).22

CO 2 storage
CO2 storage is the last step in the CCS chain.The CCS process comprises from ocean storage, geological storage and mineral carbonation. 23Geological storage is considered to be the most viable option and includes depleted oil and gas reservoirs, coal formations, saline formations, basalt formation and Hydrate storage of CO2 within the subsurface environment.Another option is deep ocean storage, however there is a constrain in this option (ocean acidifiacation and eutrophication) which limits this technology and mineral carbonation.25][26][27][28]

Mineral Carbonation
Developing a method for secure sequestration of the CO2 in geological formation is one of the most serious difficulties that the scientists have to overcome.
The mineral carbonation is a method assembles many advantages.There are several features that make it unique among the other CO2 storage procedures.[31] As a result, in this study we are focused on this alternative storage option where the CO2 gas is injected underground under optimized conditions and converted to stable carbonate minerals.During this method CO2 is reacted chemically with calcium or magnesium oxides to form stable carbonate materials through the below reaction: MO + CO2→MCO3 + heat M is the divalent metal.The amount of heat depends on the metal and on the material containing the metal oxide.
The above reaction releases heat and this means that the mineralization thermodynamically is realized at low temperatures, otherwise the calcinations are taken place.The big challenge in this method is to accelerate the carbonation exploiting the appropriate amount of heat without causing problems to the environment. 32neral carbonation can be carried out in two ways.The first one is the in-situ method where the CO2 is injected into a geologic formation for the production of stable carbonates as calcite CaCO3, dolomite Ca0.5Mg0.5CO3,Magnesite MgCO3 and siderite FeCO3.The formed products are thermodynamically stable as a result the sequestration is permanent and safe. 33This method differs from the conventional geological storage because CO2 is injected underground in the appropriate conditions so as to accelerate the natural process of mineral carbonization.The second one is the ex-situ method where the process takes place above the ground in a processing plant. 23,34 he mineral carbonation process routes are described in details by Olajire 2013. 32e in situ mineralization is preferable because there is no need for additional facilities and mining, the CO2 is injected directly into porous rocks in the subsurface and reacts directly with the rocks.Moreover there is no need for transportation of the reactants which could be a difficult process.Finally, the amount of the minerals is larger compared to minerals from industrial wastes. 27,32 owever there are also challenges in this way of mineralization as the critical choice of the rocks which should contain the metals and have the appropriate physical and chemical properties to accelerate the carbonation.Another challenge that the scientists have to overcome is to achieve the carbonation acceleration and to achieve the utilization of the heat released from the reactions. 329][40] Generally, the in situ method may be preferable for high volumes of CO2. 41 the other hand, the main advantages of the ex-situ method are: the availability of minerals at low cost and also their high reactivity when compared to natural minerals. 32

Minerals for potential CO 2 storage
Oxides and hydroxides of Ca and Mg are proposed as the appropriate materials for the mineral carbonation because they provide alkalinity.Although magnesia (MgO) and lime (CaO) are the most naturally common earth metal oxides, they usually bonded up as silicate such as olivine and serpentine (typically containing 30-60 wt% MgO) . 33The carbonation of Ca is more effective however the MgO is more common in nature. 33Basalts and ophiolite rocks are enriched in magnesium, calcium and iron silicates. 42Among silicate rocks, mafic and ultramafic rocks contain high amounts of Mg, Ca and Fe and low content of sodium and potassium.Some of the main minerals in these rocks are olivines, serpentine, enstatite and wollastonite. 32cording to Coleman 1977  43 and Nicolas 1989 44 olivine, serpentine, peridotite and gabbro are mainly found in ophiolite belts geological zones.3] RCO2 is the mass ratio of rock to CO2 and Rc mass ratio of rock needed for CO2 fixation to carbon burned.It can be seen that Basalt consists of a relatively small amount of MgO compared to Dunite and Serpentine however it's capacity is higher probably due to CaO and also it is required >1.  1. Composition of minerals and their CO2 sequestration characteristics. 31,45ere are several studies and projects have been conducted in natural minerals for CO2 sequestration.6] There are several studies that have been conducted for CO2 storage in basalt rocks 39,[47][48][49][50][51][52][53][54]  ).1][62] Dunite can also be a possible mineral for CO2 storage. 30,63 ][68] Another deposit could be zeolite according to Vatalis 2012 69 and also the sandstone formations according to Koukouzas 2018. 70Finally, it was investigated the role of water carbonation of forsterite in scCO2 and found that the reactions could be more extensive under specific conditions. 71

CCS technologies in Europe
There are 78 commercial scale projects around the Europe and they are in various stages of development (  2).The UK hosts most of these plants (22) followed by Norway (12), Netherlands (10) and Germany (9).The highest amount of these plants (35%) do not use storage site for the CO2 but they follow the process of utilization, 23% store the CO2 in saline

The case of Carbfix (Iceland)
As we mentioned in previous part the mineral carbonization is a new, environmental safe and low cost technique.The Carbfix is a project in Iceland that is injecting solutions of mixed CO2 and H2S into basaltic rocks (basaltic lava flows and hyaloclastite) at 1000m.The field site is situated in SW Iceland close to a geothermal power plant which produces up to 30000 tones of CO2 per year and it is estimated to be increased.The source of CO2 is the geothermal gas which is a by product of the geothermal steam production. 51The project started in 2007 and it's operation is since 2012.It has been estimated that in 2017 it was injected about 10000t CO2.The percentage of CO2 that is mineralized as carbonate in the basalt rocks is found to be almost complete ( 95%) in 2 years (Carbon Capture and Storage Association).The existence of large available area of basaltic rocks associated with the rapid carbonation reactions may result in a safe and permanent solution.

CO 2 storage in Greece
The the global temperature rise below 2 o C compared to pre industrial levels and also limit the temperature increase to no more than 1.5 o C aiming to reduce the risks and impact in the climate change. 72The CCS technologies in Europe as mentioned above are far away from the Greek Power Plants and the CO2 transportation is a very difficult process.As a result, an appropriate CO2 storage site in Greece would be the perfect solution.
There are several studies conducted about the CO2 storage through the application of CCS technique in Greece.According to Tasianas 2016 73 one potential storage site in oil and gas fields lies in Prinos, Kavala in NE Greece.Furthermore, it was estimated through a model the potential storage capacity in the Pentalofos (Tsarnos and Kalloni members) and Eptahori reservoirs in NW Greece and found to be 728 Gt CO2 for both storage sites. 73In Prinos (Thassos-Kavala path) hydrocarbon field offshore in Northern Greece a monitoring system which simulated a potential CO2 leakage from Prinos field was investigated and found that CO2 reaches the seabed in approximately 13.7 years after the injection and it reaches it's peak after 32.9 years.The model results show that CO2 will flow towards the Natura protected areas only in 5 days after the leakage and between this period the authorities should take the appropriate measures in order to avoid environmental problems.As a result a possible leakage would affect the environment. 74However, according to Koukouzas 2016 74 the consequences of a CO2 are considered limited and the ecosystem is capable of recovering.Finally it was calculated the amount for operating this system 0.38$/ton of CO2 and 0.45$ /ton of CO2 is found to be cost for EOR.
Vatalis 2012 69 proposed the CO2 storage in the known deposite of Zeolite in Evros (Northern Greece).Koukouzas 2018 70 concluded that Pentalofos and Tsotyli sandstone formations could be a potential CO2 storage site under specific conditions.
However, this approach need more investigation.
Another promise technique for CO2 storage without such an environmental risk, as we mentioned in previous part of this study, is the mineral carbonation.A study was conducted for the storage of captured CO2 in magnesium silicates.For the experiment samples from ultramafic rocks from mountain Vourinos in Western Macedonia, Greece, were used.It was used the aqueous technique.The results indicated limited carbonation.However, this situation will probably change in different experimental conditions.For example, longer reaction time, the particle size and the discharge of impurities which poison the reaction, would probably improve the carbonation. 30nerally, mineral carbonation is a new CCS process which promises the permanent storage of CO2.The most important aspect is that under specific conditions ensures that carbonates formed are environmentally benign and geologically stable.
Taken into account the geological forms that are appropriate for CO2 storage through mineral carbonation we concluded that Greece could be a potential site for CO2 storage because throughout the continental part all of these geological forms can be found.
The most capable sites for CO2 injections are the basaltic rocks.Several sites with basalts in Greece could be potential CO2 storage sites for mineralization.
6][77][78][79] In Othris ophiolite complex (Figure 1) was found olivine phyric lavas from the Agrillia area (about six Km NW from Lamia) and high MgO basaltic dykes from Pournari area (about 31 KM NW from Lamia).The major (in wt%) elements determined for ultramafic lavas from Agrillia area show the highest values for SiO2, MgO, CaO and FeO in all sample cases and for high-Mg basalts from Pournari show the highest values for SiO2 , FeO, MgO and CaO in all sample cases. 75rthermore, the lower unit of the Pindos ophiolitic belt is composed mainly by basaltic rocks. 802] Finally, basalts can be found in ophiolitic rocks of the Attic-Cycladic crystalline belt.According to Stouraiti 2017 83 in Paros, western Samos (Kallithea), Naxos, central Samos, Skyros, Tinos and S. Evia are found basalts exhibiting high MgO concentrations.Moreover on Acrotiri Peninsula, Santorini, Greece are found basalts 84 and also in Kos-Nisyros. 85However, the major factor that eliminates the potential CO2 storage in the last areas is that they are islands with limited storage areas and the transportation of CO2 in this case is a very difficult and high cost process.
The ophiolites in Greece are widespread mostly exposed in central and northern Greece.Large ultramafic bodies are found in East Othris ophiolite belt (Figure 1).It was indicated [86][87] that in Vrinera ophiolitic unit the ultramafic rocks consist of serpentinized harzburgites and are below gabbros and diorites.The ophilithic units of Eretria, Aerino, Velestino consist mainly of serpentites the same case in the southern part of Aerino.Finally, serpentinites can be found in ophiolitic mélange of Ag.
Giorgios but it is rather small (2Km 2 ).The ophiolite units of two Greek islands Evia and Lesvos comprise from amphibolites and below them underlie ultramafic masses which consist of serpentinized harzburgites , patches of dunites and serpentinized depleted iherzolites and harzburgites, respectively. 88A study that has been conducted in the East part of Thessaly, Central Greece shows that the metaophiolites of this region consist mainly of serpentinites and metabasites. 89[95] There are several studies conducted in Vourinos and show that dunite surrounded by serpentinized harzburgites with some lenses of serpentinized dunite. 92Furthermore, the Koziakas mountain ophiolite in western Thessaly, also belongs to West Greek ophiolite belt and comprises mantle peridoites with harzburgites and secondary plagioscale bearing Iherzolites. 92,96 we can see there are several sites in Greece that could be potential CO2 storage sites since their underground hosts rocks rich in olivine, serpentine, harzburgites, dunites, peridotites and basaltic glass which include high amounts of Mg, Ca and Fe oxides and hydroxides.As we mentioned the islands could not be a part of these sites because the CO2 transportation cost will be high.There are a few studies about the establishment of CCS technologies in Greece.
Greece emits CO2 in the atmosphere with biggest source the lignite fired power plants in western Macedonia.As a result we tried to give some ideas in using CCS technology in Greece in order to reduce the CO2 emissions from power plants.After the research about the use of numerous geological forms as storage sites for mineral carbonation, we tried to find bibliographically such geological forms.We found that there are several sites with basalts in Greece that could be potential CO2 storage sites for mineralization such as in Othris Mountains in Central Greece, in the lower unit of the Pindos ophiolitic belt in Serbo-Macedonian (Volvi and Therma bodies) and in western Rodopi (Rila mountains) massifs of Bulgaria and Greece, finally basalts are found in Greek islands but this could be an infeasible solution because of the difficulties and high cost in CO2 transportation.Moreover there are large ultramafic bodies around the continental Greece which could be possible storage sites.In Vrinera, East Othris ophiolitic unit the ultramafic rocks consist of serpentinized harzburgites and are below gabbros and diorites.The ophilithic units of Eretria, Aerino, Velestino consist mainly of serpentites the same case in the southern part of Aerino.Furthermore, the Pindos ophiolite complex in NW Greece is mainly comprised of large harzburgite-dunite masses > 1000 Km 2 in the mantle peridotites.
As we mentioned in previous part of this paper based on bibliography all these geological forms under specific conditions could be the appropriate areas for mineral carbonation.
However, taking into account the large cost of CO2 transportation and also that the CCS technology sites around Europe are away from Greece, we tried to find capable geological forms near the lignite fired power plant area in Western Macedonia, Greece.After the bibliographical research we concluded that the mountain Pindos ophiolite complex and mainly the Vourinos ophiolite complex which are very close to the power station consist of harzburgite-dunite masses in the mantle peridotites and dunite surrounded by serpentinized harzburgites with some lenses of serpentinized dunite, respectively.These natural minerals are rich in oxides and hydroxides of Ca, Mg and Fe and could be the perfect materials for mineral carbonation.
In any case, this is a potential proposal which is based only in bibliographic research about the geology of the surrounding area and is trying to give some ideas to scientific community.A detailed research about the geology, the chemical and hydrodynamic characteristic of the underground should be conducted compared to financial study in order to ensure that the proposed solution is economically and technologically viable.This could be a future research.
formations and 15% in depleted oil and gas formations.The two of the three plants in operation (Snohvit in Norway and Sleipner in Norway) use as storage site saline formations and the Offshore Netherlands in Netherlands uses depleted oil and gas formations.It is of interest that all the pilot plants utilize the captured CO2, except for the Lacq CS Pilot in France which stores it in depleted oil and gas formation and the Carbfix in Iceland which uses mineral carbonization technique. Table Other possible minerals for storage are serpentine and harzburgite as indicated by Dichicco 2015 29 , Zevenhoven 2013 55 Veetil 2014 56 , Krevor 2011 57 , Turvey 2017 58 , Klein 2011 59 and Koukouzas 2009 30 and they indicated that basalt rocks, under specific conditions, are the most capable and safe sites.

Table 2 . The commercial scale projects of CCS technologies around the Europe 36
(46%) of these projects are cancelled/dormant or finished, only 3 operate, 21 are in Pilot phase and 18 are in planning/speculative or in design (Table biggest source of CO2 in Greece is the lignite fired power plants in western Macedonia.Greece ranks second in European Union and sixth worldwide in the terms of lignite production.Today, the 8 PPC lignite power plants represent 42% of the country's total installed capacity and generate nearly 56% of the country's electrical energy according to the website of the Public Power Corporation S.A. Hellas.The use of this important energy source is facing a challenge due to the vast amounts of CO2 emitted in the atmosphere during the lignite combustion.The CO2 emissions from fuel combustions in Greece, was found 64.6 Mt 2 including a high amount from lignite fired power plants.The reduction of CO2 emissions in the atmosphere is one of the highest challenges that the scientists have to face.The Paris agreement goal is keeping Greece has several industries that produce high amounts of CO2 (the total CO2 emissions from Greece in 2016 were 67870 kt according to world data atlas) and the mineral carbonation technology would be a sustainable solution in this problem taking into account that there are the appropriate geological forms capable for permanent and safe storage.We have already indicated that potential sites for CO2 storage exist in continental part of our country as Orthis mountain in central Greece, Western Rodopi in Northern Greece, Pindos in NW Greece, Vourinos in Western Macedonia and also Koziakas in western Thessaly.Pindos ophiolite and mainly the Vourinos ophiolite complex (which extend SW of Kozani covering an area of 450 Km 2 ) is very close to the power station and they comprise of harzburgite-dunite masses in the mantle peridotites and dunite surrounded by serpentinized harzburgites with some lenses of serpentinized dunite, respectively.These natural minerals are rich in oxides and hydroxides of Ca, Mg and Fe representing the perfect materials for mineral carbonization.Mineral carbonization is a CO2 storage permanent and environmental safe technology which do not incur longterm liability (avoiding the challenge of degrading the environment) or monitoring obligations.Taking in to account that these two areas is very close to the power stations limiting the CO2 transportation cost this method could be a potential technique for reducing the CO2 emissions fulfilling the goals of the Paris Agreement.According to Carbon Capture and Storage Association there are 78 commercial scale CCS projects around the Europe and they are in various stages of development.Only 3 of them operate and 21 are in Pilot phase.However the only project uses mineral carbonation technique and has injected about 10000t CO2 in basaltic rocks during 2017 is Carbfix in Iceland.It has been estimated that the percentage of CO2that is mineralized as carbonate in the basalt rocks in Carbic project is found to be almost complete ( 95%) in 2 years.As a result, this technique is very promising.
The Carbon Capture and Storage (CCS) is one of the most promising approaches to reduce the CO2 emissions.The CO2 storage being the third step of the CCS process chain is playing a very important role in this technology.The most suitable CO2 storage site is established in basins where exists rocks containing the appropriate porosity and are near to power stations or industries in order to avoid transportation cost.The power stations in our country are placed mainly in Ptolemais-Amynteo lignite centre (Western Macedonia, Northern Greece).After a bibliographic research that has been conducted in the near region, it was indicated that the mountain