co 2 mineralization—bridge between storage and utilization of co ...

CH04CH05-Geerlings ARI 15 February 2013 18:27

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CO2 Mineralization—BridgeBetween Storageand Utilization of CO2

Hans Geerlings1 and Ron Zevenhoven2

1Department of Chemical Engineering, Faculty of Applied Sciences, Delft University ofTechnology, 2628 BL Delft, The Netherlands; email: [email protected] and Flow Engineering Laboratory, Department of Chemical Engineering,Abo Akademi University, 20500 Abo/Turku, Finland; email: [email protected]

Annu. Rev. Chem. Biomol. Eng. 2013. 4:103–17

The Annual Review of Chemical and BiomolecularEngineering is online at chembioeng.annualreviews.org

This article’s doi:10.1146/annurev-chembioeng-062011-080951

Copyright c© 2013 by Annual Reviews.All rights reserved

Keywords:

carbon dioxide emissions mitigation, mineral sequestration, CO2 captureand sequestration, CCS, CO2 capture and use, CCU

Abstract

CO2 mineralization comprises a chemical reaction between suitable mineralsand the greenhouse gas carbon dioxide. The CO2 is effectively sequestered asa carbonate, which is stable on geological timescales. In addition, the varietyof materials that can be produced through mineralization could find appli-cations in the marketplace, which makes implementation of the technologymore attractive. In this article, we review recent developments and assessthe current status of the CO2 mineralization field. In an outlook, we brieflydescribe a few mineralization routes, which upon further development havethe potential to be implemented on a large scale.

103

Review in Advance first posted online on February 28, 2013. (Changes may still occur before final publication online and in print.)

Changes may still occur before final publication online and in print

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INTRODUCTION

The countering of global climate change, induced by extensive use of fossil fuels, is a major chal-lenge with a significant technological content. Since the beginning of industrialization in theeighteenth century, the CO2 concentration in the atmosphere has risen at an unprecedented rate.It is nowadays widely accepted that the large-scale release of CO2 into the atmosphere induces cli-matic changes (1) and many related environmental problems, such as acidification of the oceans (2).The world currently consumes more than 500 exajoules (EJ) of primary energy annually, roughly80% of which is provided by fossil fuels (3). Most scenarios indicate that global energy consump-tion will at least double during this century, driven by economic development and further growthof the world population. Although the share of fossil fuels in the energy mix might decline in thecoming decades as a consequence of the growing deployment of renewable energy, it is reasonableto assume that the total amount of energy provided by the burning of fossil fuels will rise duringthat period owing to higher overall demand. In such a world, CO2 capture and sequestration (CCS)must play an important role, because it opens up the possibility of decoupling the use of fossil fuelsfrom the emission of CO2 into the atmosphere. Nowadays, capture and sequestration of CO2 isgenerally understood as comprising capture of this greenhouse gas at an industrial site, its subse-quent transport, and finally its permanent subsurface storage in a geological formation. In our view,this scope of CCS is too narrow and should be broadened to also include themes like CO2 captureand use (CCU), in line with the definition of CCS by the Intergovernmental Panel on ClimateChange (IPCC) (4). Naturally, as is the case with CCS, CCU can make a significant contributiononly if the technologies can be deployed at a sufficiently large scale. Sufficiently large here meansthat the globally deployed capacity of such a technology grows at an annual rate corresponding toapproximately 10, and later well in excess of 100, million tons (Mt) of avoided CO2 emissions. Thisimplies that the capacity grows from approximately 10 Mt at this moment in time to 10 billion tons(Gt) of CO2 avoided per annum by the year 2050. A possible scenario is given by the InternationalEnergy Agency (5) and shown in Figure 1. Note that the CO2 emissions problem can only be solvedby a portfolio of measures, as indicated by the various wedges in Figure 1. Today, neither CCS

60

2010 2015 2020

BLUE Map emissions 14 Gt

WEO 2009 450 ppm case ETP 2010 analysis

2025 2030 2035 2040 2045 2050

55

50

45

40

35

30

Gt C

O2

Year

25

20

15

10

5

0

Baseline emissions 57 Gt

CO2 capture and sequestration (19%)

Renewables (17%)

Nuclear (6%)

Power generation efficiencyand fuel switching (5%)

End-use fuel switching (15%)

End-use fuel and electricityefficiency (38%)

Figure 1A CO2 emission scenario as provided by the International Energy Agency. In a business-as-usual scenario,the emissions could reach 57 billion tons (Gt) in 2050. To reduce the emission to a more acceptable andmuch lower level, large-scale implementation of a portfolio of solutions is required. Abbreviations: ETP,Energy Technology Perspectives; ppm, parts per million; WEO, World Energy Outlook.

104 Geerlings · Zevenhoven


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nor CCU is growing at a rate that would qualify them as large, according to the definition above.Nevertheless, several CCS and CCU routes hold the promise that they can indeed develop into arelevant technology deployed on a large scale. [Note that currently the abbreviation CCUS (carboncapture, use, and storage) is gaining ground in the United States and several other locations.]

The synthesis of so-called solar fuels from CO2 and water, driven by solar energy, is also agood example of a potentially significant CCU field, in view of the very large sizes of the marketsfor possible products like methane or liquid hydrocarbons. This field includes not only variousbiobased pathways but also efficient inorganic routes. As is very often the case, it is very difficult topredict which specific technologies from a large portfolio will in the end be deployable on a largescale. Therefore, it is important to not discard technologies at too early a stage. A striking exampleof the latter, in the context of so-called solar fuels, could be the direct capture of CO2 from the air(6, 7). Whereas capture from the air might not be feasible in certain circumstances with certaintechnologies (which may imply an energy penalty of several 100 kJ/mol CO2) (8, 9), this couldbe different in a broader view. A trivial example would be a process in which the captured CO2

is hydrogenated to methane through Sabatier’s reaction using solar energy–produced hydrogen.Methanation of CO2 is exothermic, and the heat of reaction could be integrated to drive (part of)the capture process in a CCU scheme.

What has been said for the solar fuels field also holds for CO2 mineralization, the subject of thisreview. CO2 mineralization, or mineral sequestration, is another potentially important example ofa CCU field, although it can equally be considered a CCS path. Mineral sequestration involves anexothermic chemical reaction between CO2 and basic minerals, which effectively stores the CO2

in the form of a carbonate. This carbonate can be a solid, but it can also be a dissolved (bi)carbonatein water, the former having the benefit of thermodynamic stability. In recent years, quite a largenumber of suitable minerals have been studied as a possible feedstock for CO2 mineralization. Inthese studies, a division can be made between basic industrial waste or by-product materials andsuitable, naturally occurring minerals. Mineralization of alkaline (i.e., basic) waste materials, whichare often very reactive toward CO2, can as such not be considered as a technology deployable on alarge scale. This is because these waste materials, such as those produced by the steel industry, arenot available in sufficiently large amounts, being on the order of a few to 100 Mt (for steelmakingslags, for example) produced annually. However, suitable, naturally occurring minerals, especiallythe magnesium silicates, are available extensively and on enormous scales (10) that may exceedthat of available fossil carbon. Unfortunately, these abundant natural minerals are less reactivetoward CO2 than most of the alkaline industrial waste streams.

Since the IPCC special report on CCS (4), many reviews of the CO2 mineralization field haveappeared in the literature (11–16). We do not intend to add yet another review to this list. We do,however, provide the reader with a summary of the current state of the art, without striving forcompleteness. Starting from that basis, we subsequently provide a view on some possibly significantdevelopments in the field. Several dozens of processes for large-scale CO2 mineral sequestrationusing magnesium oxide–based feedstock have been suggested, again mostly after the IPCC specialreport on CCS (4) was published. An overview of this is given in Table 1. Details are given inthe mentioned literature reviews; note that calcium oxide feedstock is excluded here, as it cannotoffer the capacity needed for multi-Mt CO2 per annum fixation.

A BRIEF REVIEW OF THE CURRENT STATEOF AFFAIRS FOR CO2 MINERALIZATION

The fixation of CO2 in (hydro-)carbonates offers a method for CO2 emissions reduction, apartfrom, as smaller-scale advantages, valorization or stabilization of waste streams and more advanced

www.annualreviews.org • CO2 Mineralization 105


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Table 1 Overview of carbonation processes using magnesium oxide–based feedstock

Feedstock Serpentinite, olivines, basalts, fly ashes, industrial wastes, and mining residues(tailings, red mud, asbestos)

Feedstock pretreatment Crushing, grinding, milling, thermal treatment, extensive grinding,mechanochemical treatment

Single-step processes(direct)

Gas-solid (mid-1990s)Aqueous solutions (100◦C–180◦C, 100–160 bar, added salts)In situ injection, e.g., in basalt deposits or asbestos waste piles

Multistep processes(indirect)

Aqueous using pretreated feedstock (100◦C–180◦C, 100–160 bar, added salts)Aqueous via dissolved magnesium (bi)sulphateAqueous or gas-solid with pretreated fly ashes

Reaction intermediate Activated serpentinite or olivinesDissolved magnesium salt (chloride, sulphate, others)

Carbon dioxide Preseparated CO2

Unseparated gasCO2 reacted with ammonia, giving ammonium (bi)carbonate

Temperatures Early gas-solid up to 500◦CAqueous processes up to 200◦CPretreatment temperatures up to 700◦CGas-solid carbonation of Mg(OH)2 up to 550◦CAs low as stack emission or ambient temperatures for some processes

Pressures (CO2) Early gas-solid up to 340 barBelow 200 bar for most pressures20–30 bar for Abo Akademi processQuite a few atmospheric processes

Reaction time Minutes for indirect process stagesHours for direct processes

Valuable products Magnesium (hydro)carbonates, silica, iron oxides, mixed constructionmaterials (direct processes)

Additives Strong acids (sulphuric acid, hydrochloric acid) or bases (NaOH)Weak acids [ammonium (bi)sulphate]Ammonia (possibly for CO2 scrubbing), citrate, oxalate, chelating agents

(EDTA), NaCl, NaHCO3, KHCO3

Energy input needs/tonCO2

∼2.5–10 GJ, heat (primarily) and electricity

Costs $/ton CO2 ∼15–»100 depending on solid product value and whether or not CO2

preseparation is avoidedDevelopments 1990–2005: lab-scale, few patents

2005–2013: scale-up developments, several patents per year

sustainable chemical processing. Although CO2 mineral sequestration using the world’s vast mag-nesium silicate resources was recognized as a CCS route in the early 1990s (17), most CCS effortsso far aim at underground storage of pressurized CO2, and CO2 mineral sequestration is typicallyreferred to as a CCU method. Recently, large-scale carbonation of minerals has been recognizedas an additional CCS route along with underground storage, which gives leakage-free, perma-nent storage that will not require poststorage monitoring. An indication of this development isfound in the sudden increase of CO2 mineralization routes for which patents have been filed orgranted since 2005 (15). Uses for the significant amounts of solid (hydro-)carbonate product and



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Mg5(CO3)4(OH)2.4H2O

Mg(OH)2

NH3 (+ SOX)MgCO3

MgSO4

(aq.) Flue gas– CO2

NH3

H2O

H2O

Flue gas

(Flue) gas– CO2 + H2O

CO2 (or flue gas)

AC/ABC

AC (aq.)Rock

Residues

Residues

ABS (aq.)

Aq.<

100˚C

Aq.<<

100˚C

~120˚C

400–440˚CSolid mix

AS

AS (aq.)

500˚C20-bar

CO2

Aq.<

100˚C

330˚C

10˚C

Rock

a

b

Figure 2Comparison of the (a) Nottingham and (b) Abo Akademi routes for magnesium silicate rock carbonation(dark arrows, solid material; light arrows, aqueous material; white arrows, gas). Abbreviations: ABC, ammoniumbicarbonate; AC, ammonium carbonate; ABS, ammonium bisulphate; AS, ammonium sulfate; Aq., aqueous.

by-products obtained are being recognized, ranging from land reclamation (an application thatSingapore is considering) to specialized, carbon-neutral products (see Reference 18). These in-creasingly patent-driven developments, which define an overlap between CCS and CCU, offer arange of opportunities that need further development in the potential markets.

After pioneering work on direct gas/solid carbonation of magnesium silicate rock with hot,pressurized CO2 and processes in which reactive magnesium was extracted (activated) with strongacids (11), a direct, aqueous solution process known as the ARC route was developed in the UnitedStates at the Albany Research Center (currently the National Energy Technology Laboratory,Albany) approximately a decade ago (4, 19, 20). Although the final performance assessment of theARC process suffered from an overestimation of energy-requirement costs (heat and power werecharged equally), it was considered insufficient when it came to carbonation conversion levelsand rates. Because it operates at 40–150 bar and 100◦C–185◦C in water with added NaCl andNaHCO3 salt, a major challenge is to prevent a passivating layer of silicate from building up.

Ongoing research and development on this direct method address improving aqueous rockdissolution through improved activation procedures using thermal, mechanical, and/or chemicalmeans. The chemical additives that have been used [e.g., sulphuric acid, citric acid, acetic acid +ethylenediaminetetraacetic acid (EDTA), orthophosphoric acid + oxalic acid + EDTA] are oftenhard to recover for reuse, which leads to deteriorating economics. In addition, the reaction timesrequired for reasonable conversion levels are still long and run into (many) hours (21, 22). Today,simpler and probably more effective varieties of the ARC route are under investigation in Australia(23, 24).

Alternative routes involve so-called indirect processing, which implies that magnesium isfirst extracted from the rock material, followed by carbonation of the intermediate magnesium



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compound and recovery of extraction chemicals. A major advantage of these routes is the muchfaster carbonation of magnesium compared with, for instance, that in the ARC route. In addi-tion, indirect processing yields separate and relatively pure product streams. On the negative side,the costs of energy-input needs and unrecovered chemicals make economically viable processing achallenge. Extensive use of chemicals and water (and, of course, energy) obviously will result in neg-ative life-cycle effects in many impact categories (25–27). Examples of indirect routes are the aque-ous process from Nottingham University (28) and the staged process from Abo Akademi University(AAU) (27, 29, 30), both of which use ammonium (bi-)sulphate to produce (water-soluble) MgSO4

(in water or in a solid/solid reaction, respectively). Figure 2 gives a schematic view of both routes.In the Nottingham route, ammonium bisulphate (ABS) is used to extract magnesium from

silicate rock in a hot (∼90◦C) aqueous solution during 1–3 h. The (dissolved) magnesium isthen carbonated at a similar temperature with ammonium (bi-)carbonate, which is produced byreacting NH3 with a humid, CO2–containing (flue) gas at 10◦C. Regeneration of the ABS forreuse requires a heat-driven conversion of the dissolved ammonium sulphate (AS) to NH3 (forCO2 capture reuse) and ABS at 330◦C. This latter step puts an energy penalty on the processon the order of 1.3 kWh = 4.7 MJ/kg CO2 at 330◦C [i.e., an exergy1 requirement of 2.4 MJ/kgCO2 (at Tsurroundings = 15◦C)]. (Note that the AS salt that will be regenerated to ABS is dissolvedin water.) The extraction, capture, and carbonation stages produce 1.5 kWh = 5.4 MJ/kg CO2

at 80◦C–90◦C (i.e., an exergy generation of 1.0–1.1 MJ/kg CO2) (31). A heat pump may be usedto raise the temperature of the generated heat to the level needed for the ABS salt regeneration.Under favorable conditions, 95.9% of the magnesium in serpentine is converted to hydromagnesite[Mg5(CO3)4(OH)2 · 4H2O].

At AAU, AS salt is used to extract Mg from silicate rock at 400◦C–450◦C in a solid-solid process.This first process step is followed by precipitation of Mg(OH)2 (and by-product FeOOH) inaqueous solution. The Mg(OH)2 subsequently is carbonated in a pressurized fluidized bed reactorat approximately 20-bar CO2 pressure and a temperature of approximately 500◦C. The aim of thelast step is to obtain valuable heat from the carbonation, which reaches 50%–65% conversion after∼10 min (particle size 75–212 μm), after which the conversion levels off as Mg(OH)2 carbonationbecomes competitive with calcination to MgO. The combined process steps require 30–60 min,with a typical energy-input requirement of 3 MJ/kg CO2 (exergy) using 3.1 kg Finnish serpentinite(∼83%-wt serpentine) per kg of CO2. [This is calculated using combined pinch analysis–exergyanalysis process integration (30)]. One of the remaining challenges is the production of Mg(OH)2

with the right properties of purity, surface area, and particle size that will subsequently yield ahigher final conversion in a bubbling fluidized bed reactor. Also, this version of the process stillneeds pure CO2. Direct application of the current route to flue gases is part of ongoing scale-up and optimization work (32). More details of the AAU process are given below, where manypossible developments of the mineralization field are explored.

Several routes nowadays consider capturing CO2 with chilled ammonia, which produces am-monium (bi-)carbonate that can react with MgSO4. This is part of the Nottingham route describedabove, but it is also part of a larger scheme developed in Australia (33), where long, pipeline-typereactors are used to transport ammonia, water, and activated rock slurry. The company ICS pushesthe technology toward scale-up and commercialization (34).

In most mineralization concepts, it is assumed that the minerals are transported toward thesource of CO2. Alternative concepts are also conceivable. For instance, Brent et al. (35) suggested

1The exergy (i.e., work potential) of heat Q with temperature T and surroundings temperature Tsurroundings is given byEx(Q) = Q · (1−Tsurroundings/T). Thus, the heat quality of different temperature levels is accounted for.



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minimizing costs and efficiency losses resulting from material transport by conveying (fossil)carbon fuel to a power plant that is sited on a mineral deposit suitable for CO2 mineralization.Obviously, the feasibility of such a scheme also depends on the transport costs of power.

Methods that use electricity for CO2 mineralization are challenging: Electrolysis of seawatercan give separate HCl (via Cl2) and NaOH solutions in water, which can be used to leach Mg fromsilicate rock and subsequently increase the pH to levels where carbonation is possible. Althoughenergy balance and efficiency calculations suggest (36, 37) that electrolysis steps make this approachhighly unfeasible, work in Singapore (38, 39) (giving separate streams of carbonate, silicate, andother materials) suggests that state-of-the-art membrane-electrodialysis equipment can providethe necessary efficiency.

More developed and closer to commercial application are process routes that utilize CO2 forthe production of calcium carbonates, including the highly valuable precipitated calcium carbonate(PCC). In the past decade, quite a few calcium-containing by-products, residues, and wastes havebeen investigated for the potential to bind CO2, which offers emissions-reduction potential whilereducing (costly) landfill and the use of natural resources. Slag residue from iron- and steelmakingand ashes from oil shale combustion are important examples of such calcium-based CCU sources.The Finnish Slag2PCC concept is a clear example that is being scaled up to 25 tons/h steelconverter slag processing, which would yield some 10 tons/h PCC (40).

In the United States, three ongoing projects under the Department of Energy Recovery Act,“Innovative concepts for beneficial reuse of carbon dioxide,” are based mainly on calcium and/orsodium (bi)carbonate (Alcoa Inc., Skyonic Corp.) materials or reacting calcium and other mineralsdissolved in seawater (Calera Corp.) with CO2. (Earlier, Calera more visibly included carbonationof magnesium in their processes.) Also in the United States, Caterpillar Inc. has patented a processroute on mineral sequestration (41); however, information on possible implementation has notbeen reported in the open literature. Finally, in South Africa, the enormous amounts of magnesiumsilicate–based mining tailings from platinum group metals and other ore-mining activities haveinitiated work in the direction of CO2 mineral sequestration (42).

POSSIBLE DEVELOPMENTS OF THE CO2 MINERALIZATIONFIELD: AN OUTLOOK

The development of CO2 mineralization into a large-scale sequestration technology necessitatesthe use of abundant magnesium silicates, such as olivine and serpentine, as feedstock. Unfortu-nately, the reactivity of these minerals with CO2 is fairly low, which leads to conceptual mineraliza-tion processes running at elevated temperatures and pressures, typically using chemical additivesto increase conversion rates. This would imply that a costly, energy-intensive CO2 capture stepis required as a part of any CO2 mineralization process. Mineralization processes that do notrequire a separate capture step are the holy grail of the field. To get around application of CO2

capture, one has to drastically improve the intrinsic activity of the magnesium silicate, possiblyaugmented by the use of long reaction times. An example of the latter is the spreading of crushedolivine in the open air on dedicated sites, as advocated by Schuiling & Krijgsman (43). Thoughthis geoengineering approach to the CO2 problem will not be effective for every site on earth (44),it might work in the wet tropics. An estimate based on the published weathering data of olivine(44) (see Figure 3) yields a conversion of approximately 50% after one year (50 micron particles,T = 25◦C). A reaction time of one year would be quite long in an industrial process. However,compared with the characteristic timescales of the earth’s climate system, one year is quite short.In our view, this route needs a dedicated field test to measure the weathering rate of olivine underactual conditions and to assess its manageability and controllability.



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Dis

soci

atio

n ra

te (m

ol m

–2 s

–1)

pH

10–6

10–7

10–8

10–9

10–10

10–11

10–12

0 2 4 6 8 10 12 14

Wogelius & Walther (1991);Fo91 at 25˚C

Pokrovsky & Schott (2000);Fo91 at 25˚C (CO2-free)

Pokrovsky & Schott (2000);Fo91 at 25˚C (CO2-rich)

Golubev et al. (2005);Fo100 at 25˚C

Golubev et al. (2005); Fo92 at 25˚C

Arithmetic mean of olivine dissolution rate at pH 8.2

SoilpH

RainwaterpH

SeawaterpH

Figure 3Available data on the dissolution rate of olivine [(Mg,Fe)2SiO4] in water as a function of pH. Thetemperature is 25◦C. Forsterite (Fo) is pure Mg2SiO4. In Fo91, 9% of the magnesium ions in Fo arereplaced by iron. (See References 50–52).

An effective way to drastically improve the reactivity of serpentine in CO2 mineralization isthermal activation. Thermal activation of serpentine at temperatures of approximately 600◦C–650◦C leads to decomposition of the mineral according to

2Mg3Si2O5(OH)4− → 3Mg2SiO4 + SiO2 + 4H2O with �H ∼= 150 kJ/mol∼= 550 kJ/kg serpentine.

The solid product, which is left after activation, is essentially amorphous (45). The activatedserpentine turns out to be much more reactive than its parent mineral or crystalline olivine, whichideally also has a composition Mg2SiO4 (see Figure 4) and can easily be crushed and ground. Theheat of reaction of the serpentine activation must be invested to avoid a separate CO2 capture step.Assuming that every magnesium atom would eventually react with one CO2 molecule, the energypenalty of activation translates into approximately 1.2 MJ/kg CO2, which should be comparedwith an energy requirement (as heat) of approximately 3 MJ/kg CO2 for a conventional, amine-based capture process. However, one must take into account that conventional CO2 capture runsat much lower temperatures, typically below 150◦C, than does serpentine activation. Although apractical process to activate (large amounts of) serpentine does not exist, it appears that no majortechnical hurdles prevent developing such a process. A process driven by solar heat could turn outto be the most sustainable solution. In addition to the activated mineral, the process also yieldswater as a by-product. Activation of 1 m3 of serpentine rock could yield, upon condensation,approximately 330 kg of water as well. (See Reference 46 for a detailed assessment of antigoriteserpentine heat treatment.) Below, we give a short description of three possible processes, which



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0

0.5

1

1.5

2

0 50 100 150 200

Reaction time (min)

Mg2+

con

cent

rati

on in

wat

er (g

lite

r–1)

Olivine

Serpentine

Activated serpentine

Figure 4A slurry of magnesium silicate (2 wt% solid, particle size approximately 25 micrometers) is subjected at roomtemperature to a stream of pure CO2. The pressure is 1 bar. The magnesium concentration is shown as afunction of reaction time for three magnesium silicates. The activated sample is much more reactive than theparent serpentine or olivine.

in principle allow for mineralization of CO2 from flue gas without a separate capture step. Allprocesses are still in the concept phase.

Ocean-Neutral Sequestration of CO2 Using Heat-Activated Serpentine

A significant fraction of the CO2 emitted to the atmosphere ends up in the earth’s oceans. Thisdissolved CO2 disturbs the natural acid-base equilibrium of the ocean surface waters through theformation of carbonic acid. The chemistry involved is well known and can be described as follows:

CO2(aq) + H2O ↔ H2CO3(aq) ↔ H+(aq) + HCO−3 (aq) ↔ 2H+(aq) + CO2−

3 (aq).

Dissolving additional CO2 in the ocean leads to formation of acid protons and thus lowersthe pH of the water. Obviously, this acidification problem does not arise if, together with CO2,a neutralizing alkaline (base) is added to the water. This is called ocean neutral sequestration, incontrast to ocean acidic sequestration (7). An example of ocean neutral sequestration is:

Mg2SiO4 + 4CO2(aq) + 4H2O → 2Mg2+(aq) + 4HCO−3 (aq) + H4SiO4(aq).

The fate of the orthosilicic acid, which occurs naturally in ocean waters and participates in thebiochemical cycle of certain algae, needs further study. At higher concentrations, the silicic acidwill easily polymerize to yield silica and water. To develop this concept into a practical process,a reactor that efficiently contacts flue gas with a slurry of heat-activated serpentine in seawaterneeds to be designed. The process yields a slurry of unconverted and possibly precipitated material,which can, for example, be used in land reclamation. It is not easy to estimate which part of theactivated serpentine will be dissolved by the CO2 to yield dissolved magnesium bicarbonate. Thiswill depend on the exact nature of the serpentine involved but also on the details of the fluegas–slurry contactor.

CO2 Mineralization Using Heat-Activated Serpentinein a Low-Temperature Gas/Solid Fluid Bed

In the chemistry of mineralization of CO2 with magnesium silicates, water plays an essential role.Without liquid water, carbonic acid will not form, and an aqueous-process mineralization reaction



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Figure 5The conversion of activated serpentine after 20 min in a gas solid fluidized bed as a function of reactiontemperature. The particles are fluidized by wet CO2 at ambient pressure.

will not proceed. For this reason, slurry reactors, which contact gaseous CO2 with an aqueousslurry of a suitable mineral, are often the reactor systems of choice. However, compared withslurry reactors, a comparable gas solid fluid bed enjoys a lower pressure drop, which is extremelyimportant in the case of flue gas conversion. Also, the mass transfer characteristics of fluidizedbed reactors are generally very good. For the case of flue gas mineralization, a gas solid fluidbed might be feasible if the incoming gas is cooled to temperatures close to the dew point ofwater. Under these circumstances, a thin film of liquid water will likely form around the mineralparticles in the fluid bed, and the mineralization reaction will likely take place. [A similar effectwas reported recently for gas/solid carbonation of MgO and Mg(OH)2 at elevated temperatures inpressurized reactors (47)]. To test whether this process could actually work, a simple experimenthas been performed (R. Bhardwaj, J.R. van Ommen, H.W. Nugteren, H. Geerlings, manuscript inpreparation). Heat-activated serpentine (particle size, approximately 50 micrometers) was fluidizedwith pure CO2 at a pressure of 1 bar in a glass reactor. Using external heating, the temperaturecould be set between room temperature and 90◦C. During the experiment, samples could be takenand the conversion of the mineralization reaction was measured by thermogravimetric analysis.Using pure CO2 to fluidize the mineral particles obviously results in a conversion close to zero.In a second experiment, the CO2 was passed over a thermostatic bath of water and subsequentlydirected to the fluid bed. In this case, an appreciable conversion of the mineral was obtained,as shown in Figure 5. The amount of moisture in the bed was rather difficult to control in theexperimental setup, and from time to time, a part of the bed seemed to run dry. In practice,the latter will not occur if a wet flue gas at dew point is being employed. All in all, the presentexperimental results give a first proof that the concept of mineralizing CO2 in a low-temperaturegas/solid fluidized bed could work if heat-activated material can be produced at low costs.

CO2 Mineralization Using Magnesium Silicates Applied Directly to Flue Gases

In those cases, in which the mineralization process operates at elevated pressures and thus flue gascannot be used directly, the optimal design could be a process with integrated CO2 postcombus-tion capture and mineralization steps. The processes mentioned above, which employ a variantof the chilled ammonia capture technology, provide an example of such integration. Naturally, itis also possible to integrate precombustion CO2 capture with mineralization. In these processes,



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Mg extraction

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> 20 bar, > 500˚CMg(OH)2MgSO4, etc. MgCO3

Heat

Figure 6The Abo Akademi University route for magnesium silicate carbonation via MgSO4 and Mg(OH)2.Abbreviation: AS, ammonium sulphate.

the CO2 is available at pressures of tens of bars, which could be used directly in, for instance, theARC or the AAU indirect-mineralization process. Alternatively, one could also consider a com-bustion process that runs at elevated pressures. In this concept, the flue gas is thus produced atthe desired pressure and is expanded after the mineralization process. Conventional combustionat ambient pressures, followed by a flue gas compression-expansion step, is also possible. TheAAU process, which is described below in more detail, can be integrated naturally with any of thecapture processes described above. A scheme of the AAU process is given in Figure 6. Advantagesare the production of separate (by-)product streams (including FeOOH, which can be of interestfor iron- and steelmaking) and the production of carbonation reaction heat at a useful tempera-ture. Carbonation of Mg(OH)2 at 500◦C and pressurized conditions (a pressurized fluidized bedreactor is used) gives pressurized steam as a reaction product. The carbonation step generatesapproximately 30% of the heat that is needed for the Mg(OH)2 production process. A series ofmore than 100 tests, mainly with a synthetic Mg(OH)2 sample (Dead Sea Periclase Ltd., particlesize 75–125 μm or 125–212 μm, internal surface <10 m2/g) gave carbonation conversion levelsthat, although they were reached within 10 min, seldom exceeded 40% (see Figure 7). Clearly,the carbonation reaction is competing with calcination, which gives the much less reactive MgO.(X-ray diffraction analysis also showed MgO · 2MgCO3 in the products under certain conditions)(47). The use of large amounts of steam to suppress calcinations would complicate the final goalof CO2 sequestration. Much better results were obtained by using serpentinite-derived Mg(OH)2

produced from Finnish, Lithuanian, and other rock after extracting Mg using AS salt at 400–450◦C, followed by sequential precipitation of FeOOH (pH 8–9) and Mg(OH)2 (pH 11.5) in anaqueous solution, and raising the pH with NH3 released during the preceding solid/solid reaction,according to Nduagu et al. (27, 48, 49). The produced Mg(OH)2 shows specific surfaces of 45–50m2/g and, as a result, higher final carbonation levels (which presumably are reached faster) (seethe green triangle points in Figure 7).



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MgCO3 5% steamexperiment

15% steamexperiment

Initial material97% Mg(OH)2

Figure 7Ternary diagram of Mg(OH)2 carbonation experiments performed using a pressurized fluidized bed reactorat various temperature (460◦C–580◦C) and pressure (10–75 bar) conditions. The circled dots representexperiments in which steam was added to the CO2 stream, and the red dots indicate experiments at highpressure (>40 bar). High surface area is Mg(OH)2 produced from serpentinite; most other tests werecompleted with Dead Sea Periclase Mg(OH)2 (47). The starting material [97% pure Mg(OH)2] and addedsteam levels are labeled. Abbreviations: PCO2 , CO2 pressure; VFB fluidization velocity.

Current work is evolving along the lines of (a) producing Mg(OH)2 from serpentinites andother silicate rock with suitable purity, particle size, and specific surface; (b) regenerating the ASsalt; (c) optimizing a flue gas compression/carbonator exit gas expansion turbo-set when applyingthe process to a flue gas without CO2 preseparation; and (d ) facing challenges related to scale-upof the process in general. To apply all of this in practice and go beyond lab-scale work, a designis being made for a demonstration (at several hundred kg CO2/h) process unit at a 200-tons-of-CaO/day lime kiln near Turku in Southwest Finland. Process integration with the existing heatrecovery system is of great importance for the overall process energy economy.

FINAL OUTLOOK

The CO2 mineralization field includes a relatively large number of distinct technological routes,all of which convert CO2 into a carbonate. As a part of a larger portfolio of CCS and CCUtechnologies, mineralization could make a significant contribution to mitigating climate change.To be able to make this contribution a few decades from now, the right decisions must be madetoday. First of all, some basic research is necessary, especially for those routes that omit a separateCO2 capture step. In general, the overall energy efficiency of the technology is crucial because,after all, the method offers a solution to what is basically an energy use–related problem. Next to



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this basic research, promising technologies must be piloted on a large scale. Learning by doingis the motto in this case, which is crucial to soon reach the desired (very) large scales. The pasthas, however, shown that mineral sequestration appears unattractive as a large-scale CCS optionfor the energy sector. Nonetheless, driven by the interest from minerals, metal, cement, andpapermaking industries, mineralization processes are being developed that use industrial wastesand by-products to produce valuable carbonates while fixing noticeable amounts of CO2.

As shown by Department of Energy–cofunded projects in the United States, demonstrationson the order of $10 million are a way to demonstrate these technologies, as are the Singaporianefforts toward land reclamation with carbonated rock and the work at a lime kiln in Finland. Inall cases, the value of the produced carbonates and other solid materials is of great importancefor the overall economics. Hopefully, the forthcoming years do indeed show a trend toward 1 MtCO2/year demonstrations, which would pave the way for a possible large-scale implementationof CO2 mineralization as a CCS option.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight be perceived as affecting the objectivity of this review.

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