Mechanochemical carbon sequestration of solid feedstocks using multiple carbon sequestration reactors

Abstract

The present invention relates to a facility for the carbon sequestration of a solid material comprising at least 3 carbon sequestration reactors configured for contacting a solid material with a gas, wherein the interior of the carbon sequestration reactors is fluidly connected.

Description

MECHANOCHEMICAL CARBON SEQUESTRATION OF SOLID FEEDSTOCKS USING MULTIPLECARBON SEQUESTRATION REACTORSField of the invention

[0001] The present invention relates to a method for the mechanochemical sequestration or capture of carbon from CO2 in solid feedstocks. The invention further relates to a facility suitable for the mechanochemical sequestration of carbon dioxide in solid feedstocks and uses thereof.Background Art

[0002] Anthropogenic emissions have increased carbon dioxide concentrations in the atmosphere to at least 30% more than Pre-Industrial Era levels. Moreover, it is estimated that future global carbon dioxide emissions will increase from about 7.4 GcC (billions of tons of atmospheric carbon) per year, which was the level in 1997, to 26 GtC per year by 2100. The adverse environmental impact linked with increased carbon dioxide emissions has resulted in more stringent environmental standards regarding emissions of pollutants into the atmosphere.

[0003] The management of carbon dioxide emissions can be achieved by: increasing the efficiency of energy conversion, using low-carbon or carbon-free energy sources, as well as the capture and sequestration of carbon dioxide emissions. The sequestration of carbon dioxide, remains the preferred method for tackling antecedent carbon dioxide emissions.

[0004] Mineral carbonation is a sequestration process in which minerals, such as olivine and serpentine, or calcium-rich minerals, react with carbon dioxide to form geologically stable mineral carbonates. These processes mimic the naturally occurring weathering of rocks.

[0005] The mineral carbonation approach has several inherent advantages, including a vast natural abundance of raw materials and the permanent and benign storage of CO2 in solid form. The primary drawback to mineral carbonation has been the reaction kinetics, because these naturally occurring processes happen slowly over geological timeframes instead of industrial time scales. Previous studies have suspended minerals in aqueous carriers, because such systems have been shown to have faster kinetics than direct carbonation. The mineral aqueous mineral carbonation step involves the leaching of magnesium, calcium or another suitable metal or combination of metals and the subsequent reaction with dissolved hydrogen carbonate. A disadvantage of aqueous methods is that the resulting aqueous product or slurry needs to be used on-site to avoid expensive and energy-inefficient transportation or evaporation.

[0006] Sequestration employing solids has been achieved by crushing magnesium-rich minerals to a fine particle size to promote surface reactions that are known to control most mineral dissolution reactions. Such processes have been hampered by extensive comminution of the raw materials, high partial pressures and long reaction times, all of which render the process energy intensive. Other studies have also reported that preheating the serpentine minerals to about 650 °C and above 50 bars of pressure significantly increases their carbonation reactivity, probably due to dehydroxylation, increase in the surface area and destabilization of the crystal structure. However, this pre-treatment isvery energy intensive, requiring more than 200 kWh / ton of serpentine.

[0007] The main commercially available carbonation solutions are either autoclaves relying on high temperature and pressure combinations, or so-called curing chambers designed to cure CC>2-absorbing concrete admixtures after casting. Typically these are not capable of continuous reaction.

[0008] The present inventors have found that, once a carbon sequestration facility is installed at an industrial CO2 emission site such as a combustion plant or a cement kiln, it is difficult to achieve consistent reaction performance due to variations in the composition of the gas, in the properties of the feedstock (such as humidity, particle size) and / or in the type of feedstock used, particularly in case a continuous process facility is installed since these typically do not easily allow to increase e.g. the reaction time as in the case of a batch reactor. Indeed, it can be advantageous if the same carbon sequestration facility could be employed to process various different feedstocks (e.g. fly ash, slags, clays) depending on the commercial need to produce a certain carbonated material. It would also be advantageous to be able to flexibly control the extent of carbon sequestration of the same feedstock depending on the desired carbonated material properties since low carbonated or high carbonated versions of the same feedstocks may exhibit different performance properties when used in an application, such as when used as a filler in concrete.

[0009] Hence, there remains a need to develop a streamlined, low-temperature and energy-efficient process and related equipment which can capture carbon dioxide into CC>2-related functional groups across a wide variety of raw materials and which is performed on economically viable timeframes.

[0010] US2008277319A1 discloses a method for the sequestration and transformation of carbon dioxide generated by anthropogenic sources. The method comprises the steps of size-selecting mineral particles and combining it with water to form a slurry which is then reacted with carbon dioxide.

[0011] W02021087606A1 discloses ball milling methods for mechanochemical CO2 sequestration in mineral feedstocks.

[0012] It is an object of the present invention to provide an improved method and apparatus for sequestering carbon dioxide into solid feedstocks.

[0013] It is a further object of the present invention to provide a method and apparatus for sequestering carbon dioxide into solid feedstocks which is energy efficient and can preferably be operated as a continuous process.

[0014] It is a further object of the present invention to provide a method and apparatus suitable for the sequestration of carbon dioxide in a variety of solid feedstocks having different properties and reaction behavior.Summary of the Invention

[0015] Against this background, the present inventors have found a carbon sequestration facility and corresponding carbon sequestration methods and uses which allow the sequestration of CO2 to beachieved in an efficient manner for a large variety of feedstocks when considering such factors as energy consumption, extent of carbon sequestration, reactor throughput capacity, etc. The facility and corresponding carbon sequestration methods and uses can be operated in a continuous manner and allow for flexibly adapting the process to various conditions such as the nature of the feedstock used, the properties of the carbon-rich gas used, the extent of carbon sequestration desired, etc.

[0016] The facilities, methods and uses described herein are preferably for the carbonation of a solid material. Thus, the carbon sequestration described herein is preferably carbonation and the CO2- enriched material referred to herein is preferably carbonated material.

[0017] Hence, in a first aspect of the invention there is provided a facility for the carbon sequestration of a solid material comprising a sequence of carbon sequestration reactors 1 to n, each carbon sequestration reactor being configured for contacting a solid material with a gas and comprising a hollow housing, an inlet forming a passage through the wall of the housing and an outlet forming a passage through the wall of the housing; wherein n is an integer > 3; and wherein the interior of the housing of each of carbon sequestration reactors 1 to n-1 is fluidly connected, optionally via one or more first flow regulators, to the interior of the housing of the next carbon sequestration reactor.

[0018] In highly preferred embodiments of the invention, the facility is configured to convey solid material from an outlet of each carbon sequestration reactor 1 to n-1 to an inlet of the next carbon sequestration reactor in the sequence of carbon sequestration reactors. The housing of each of the carbon sequestration reactors is preferably substantially airtight.

[0019] The present inventors have found that it is advantageous and thus highly preferred if the facility is configured for providing a gas in carbon sequestration reactor i different from the gas in carbon sequestration reactor i-1 , wherein i is an integer from 2 to n. This can be achieved, for example, by employing a facility as described herein which further comprises one or more external gas inlets for providing gas to the sequence of carbon sequestration reactors, the one or more external gas inlets being placed downstream of carbon sequestration reactor 1. In this way, the facility can remain in continuous operation while the properties of the gas (such as temperature, humidity, CO2 content) can be adapted at one or more points in the process as is appropriate for that feedstock. For example some feedstocks will be fully carbonated before reaching the end of the sequence of carbon sequestration reactors, in which case an external gas inlet can be used to provide a gas having a higher temperature and / or lower humidity than the gas employed for carbon sequestration such that a drying operation can already be initiated inside the carbon sequestration reactors downstream of the external gas inlet. An external gas inlet can also be used to provide reagents for alternative chemical processing. This setup also allows the process conditions to be easily adapted based on monitoring of the reaction progress, or of other properties of the solid material obtained from the reactors, such as the moisture content. By using a sequence of distinct carbon sequestration reactors, as opposed to one large reactor, factory floor space can be used more efficiently, more flexibility in managing process conditions is possible as well as operation without using the full sequence of carbon sequestration reactors is enabled resulting in energy savings.

[0020] In another aspect of the invention there is provided a method for carbon sequestration usingthe facility described herein, comprising: i) providing a solid feedstock; ii) providing a carbon-rich gas comprising at least 0.5 vol% CO2; iii) feeding the solid feedstock and the carbon-rich gas to carbon sequestration reactor 1 ; iv) contacting the solid feedstock with the carbon-rich gas in the reactor to obtain CC>2-enriched material.

[0021] In highly preferred embodiments the method is provided wherein the CC>2-enriched material is recovered from carbon sequestration reactor 1 and fed to the next carbon sequestration reactor in the sequence of carbon sequestration reactors. Preferably the CC>2-enriched material is recovered from each of carbon sequestration reactors 1 to n-1 and fed to the next carbon sequestration reactor in the sequence of carbon sequestration reactors until it is recovered from carbon sequestration reactor n. The skilled person will understand that if the method is operated continuously, the CC>2-enriched material is at the same time recovered from each of carbon sequestration reactors 1 to n-1 and fed to the next reactor in the sequence of carbon sequestration reactors.Brief description of the figures

[0022] The present invention will now be described in more detail with reference to specific embodiments of the invention, given only by way of illustration, and with reference to the accompanying drawings.

[0023] Figures 1-3 show a screw 10 for conveying solid material along a longitudinal axis A of a hollow housing of a carbon sequestration reactor. The screw 10 comprises a longitudinal shaft 100 extending along the longitudinal axis A and a helical flighting 101 mounted to the longitudinal shaft 100. The longitudinal shaft 100 is supported at each end by respective bearings (not shown) and may be rotated using any method known in the art.

[0024] Figure 1 shows in a perspective view a screw of a carbon sequestration reactor according to an embodiment of the invention. The helical flighting 101 is formed by a series of blades, each blade being defined as a section of the helical flighting related to one complete helix turn (as shown in figure 3). The helical flighting may be made from one continuous metal sheet wound into multiple blades, or may be made from distinct blades or parts of blades which are joined e.g. by welding. Furthermore, the helical flighting 101 comprises at least one blade having a plurality of openings 102 extending through the blade. The plurality of openings 102 may be arranged symmetrically around the longitudinal axis A, preferably the plurality of openings 102 are provide at the same radial distance in the radial direction R from the longitudinal axis A. As shown in figure 3, the plurality of openings 102 may be identical. The screw 10 further comprises agitation means formed by a plurality of agitation elements 103 extending between the blades of the helical flighting 101 to substantially form a single, double or triple agitation helix H or a part thereof. Preferably the pitch of the agitation helix H is at least ten times the pitch of thehelical flighting 101 .

[0025] Figure 2 shows a side view of the screw shown in Figure 1 .

[0026] Figure 3 shows a front view of the screw shown in Figure 1 .

[0027] Figures 4-6 show a carbon sequestration reactor 1000 for the preparation of CC>2-enriched material. The carbon sequestration reactor 1000 has a hollow housing 1 , 11 and a screw 10 as shown in figures 1-3 mounted in the hollow housing 1 , 11. The screw 10 is supported at each end by respective bearings (not shown) and may be rotated using a drive unit 2 connected to one end of the screw, imparting rotational movement to facilitate material transport. A motor 3 is connected to the drive unit, providing the necessary power for the rotational movement of the screw shaft. A variable speed control mechanism may be integrated into the motor for adjusting the rotational speed to control the conveying rate (not shown).

[0028] Figure 4 shows in a perspective view a carbon sequestration reactor according to an embodiment of the invention. The carbon sequestration reactor 1 further comprises a gas inlet 12 forming a first passage through the wall of the housing 1 , 11 , a solid material inlet 13 forming a second passage through the wall of the housing 11 and a solid material outlet 14 forming a third passage through the wall of the housing 1 , 11. Notwithstanding the in- and outlets, the housing 1 , 11 is configured to be substantially airtight. The carbon sequestration reactor 1000 may further comprise a material chute 4, wherein the material chute 4 is connected to the material outlet 14 such that solid material can be removed from the reactor through solid material outlet 14. carbon sequestration reactor 1 is configured to convey solid feedstock from the solid material inlet 13 to the solid material outlet 14 while the gas comprising at least 0.5 vol% CO2 is present in, preferably flowing through the hollow housing 1 , 1 1.

[0029] Figure 5 shows a side view of the carbon sequestration reactor shown in Figure 4.

[0030] Figure 6 shows a front view of the carbon sequestration reactor shown in Figure 4 wherein part of housing was omitted to expose the screw positioned inside the reactor.

[0031] Figure 7 shows a variant of a carbon sequestration reactor similar to the one shown in Figure 4, but having multiple solid material inlets 13 and outlets 14, as well as having a gas inlet 12 and gas outlet 15. This carbon sequestration reactor is more suitable to be vertically stacked in accordance with some preferred embodiments of the present inventions. Depending on the embodiment of screw used, and thus the direction of solid material flow in the reactor, one of both solid material outlets, and / or one of both solid material inlets may be permanently closed during operation as it is unnecessary.

[0032] Figure 8 shows a facility for the carbon sequestration of a solid material comprising a sequence of 6 carbon sequestration reactors. The carbon sequestration reactors are vertically stacked, with the solid material outlet of carbon sequestration reactor 1000 being vertically positioned above (and thus substantially aligned with) the solid material inlet of carbon sequestration reactor 1001 , the solid material outlet of carbon sequestration reactor 1001 being vertically positioned above (and thus substantially aligned with) the solid material inlet of carbon sequestration reactor 1002, and so on. Depending on the embodiment of screw used, and thus the direction of solid material flow in the reactor, one of both solidmaterial outlets, and / or one of both solid material inlets on each of the reactors may be permanently closed during operation as it is unnecessary. However, having a solid material inlet and a solid material outlet at each end of the reactor allows for maximum flexibility in operation of different processes with the same carbon sequestration facility, for example by exchanging the agitator (e.g. screw) positioned inside.Description of embodiments

[0033] The expression “comprise” and variations thereof, such as, “comprises” and “comprising” as used herein should be construed in an open, inclusive sense, meaning that the embodiment described includes the recited features, but that it does not exclude the presence of other features, as long as they do not render the embodiment unworkable.

[0034] The expressions “one embodiment”, “a particular embodiment”, “an embodiment” etc. as used herein should be construed to mean that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of such expressions in various places throughout this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. For example, certain features of the disclosure which are described herein in the context of separate embodiments are also explicitly envisaged in combination in a single embodiment.

[0035] The singular forms “a,” “an,” and “the” as used herein should be construed to include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its broadest sense, that is as meaning “and / or” unless the content clearly dictates otherwise.

[0036] Whenever reference is made throughout this document to a compound which is a salt, this should be construed to include the anhydrous form as well as any solvates (in particular hydrates) of this compound, unless explicitly defined otherwise.

[0037] As used herein, the expression “wt.%” when used in the context of an ionic compound refers to the amount of the compound inclusive of its counterion(s).

[0038] The term “CC>2-enriched material” is used herein to refer to the solid feedstock after being contacted with the carbon-rich gas in one of the carbon sequestration reactors of the sequence of carbon sequestration reactors. Depending on the feedstock, the physical and / or chemical mechanism underlying the CO2 sequestration may be different. For example, in case the feedstock comprises a mineral the CO2 will typically be converted to carbonates (also referred to as carbon sequestration). In case the feedstock comprises elemental carbon (such as graphite) the CO2 will typically be bonded to the carbon in the form of various functional groups including carboxylates, epoxides, carbonyl groups, etc.

[0039] In accordance with the invention, the BET surface area as referred to herein is determined at atemperature of 77K using a sample mass of 0.1 -0.5g. The BET surface area as referred to herein is determined using nitrogen. A preferred analysis method to determine the BET surface area comprises heating samples to 400 °C for a desorption cycle prior to surface area analysis. A suitable and thus preferred analysis apparatus for determining the BET surface area is a Micromeritics Gemini VII 2390 Surface Analyzer preferably equipped with a Micromeritics FlowPrep 060 flowing-gas degassing unit.

[0040] TGA as used herein refers to Thermogravimetric Analysis, a technique known to the person skilled in the art. A preferred TGA setup to determine the CO2 content of the feedstocks and carbonated materials in the context of the present invention is a Setaram TAG 16 TGA / DSC dual chamber balance employing a 0.1-2 mg sample. In accordance with the invention, the TGA is performed under an inert atmosphere, such as nitrogen or argon.

[0041] In accordance with the invention, unless specified otherwise, the particle size distribution characteristics referred to herein such as D10, D50 and D90 as well as the specific surface area (unless explicitly mentioned to be BET surface area) are determined by measuring with a laser light scattering particle size analyzer utilizing the Mie theory of light scattering, such as the Brookhaven laser particle sizer, Model Microbrook 2000LD or another instrument of equal or better sensitivity and reporting the data using a volume equivalent sphere model. As is known to the skilled person, the D50 is the mass median diameter, i.e. the diameter at which 50% of a sample’s mass is comprised of smaller particles. Similarly, the D10 and D90 represent the diameter at which 10 or 90% of a sample’s mass is comprised of smaller particles.

[0042] The Total Carbon (TC) content referred to herein is preferably determined in accordance with the method described on Soil Sampling and Methods of Analysis, 2nd Ed., CRC Press (2008), p. 244 and further, incorporated herein by reference. The Total Carbon (TC) content is always expressed herein as wt.% based on the total weight of the composition being measured. For example, in case the total carbon content of the solid feedstock is referred to, it is based on the total weight of the solid feedstock, in case the total carbon content of the CO2 enriched material is referred to, it is based on the total weight of the CO2-enriched material.

[0043] The moisture content of the solid materials referred to herein (e.g. the solid feedstock provided in step (i) of the method) refers to the moisture content determined by oven-drying the material at 150 °C until constant weight and comparing the initial weight with the dry weight.

[0044] For the purposes of the present disclosure, ideal gas law is assumed such that the mol% of a gas is considered as equal to the vol%.

[0045] In the context of the present invention, reference may be made to the solid material comprised in the reactor during operation as “the solid feedstock”. The skilled person will understand that the solid material in the reactor converts from "solid feedstock” to “CO2-enriched material” as the reaction progresses and that reference to “the solid feedstock” once it has been fed to the reactor in step (iii) should be interpreted as referring to the material inside the reactor regardless of its actual stage of conversion to CO2-enriched material, unless specified otherwise.

[0046] The expressions “downstream” or “upstream” compared to a certain reference point (e.g. aspecific carbon sequestration reactor, such as carbon sequestration reactor 1) as used herein, denote the facility and / or process downstream or upstream from that reference point as seen with regard to the direction of solid material flow through the facility and / or process. Thus, the expression “downstream” of a certain reference point denotes a position away from that reference point in the same direction of solid material flow.The carbon sequestration facility

[0047] In a first aspect of the invention there is provided a facility for the carbon sequestration of a solid material comprising a sequence of carbon sequestration reactors 1 to n, each carbon sequestration reactor being configured for contacting a solid material with a gas and comprising a hollow housing, an inlet forming a passage through the wall of the housing and an outlet forming a passage through the wall of the housing; wherein n is an integer > 3; and wherein the interior of the housing of each of carbon sequestration reactors 1 to n-1 is fluidly connected, optionally via one or more first flow regulators, to the interior of the housing of the next carbon sequestration reactor.

[0048] Preferably, each carbon sequestration reactor is configured for contacting a solid material with a carbon-rich gas comprising at least 0.5 vol% CO2 as is described herein elsewhere. Preferably, each carbon sequestration reactor is configured for receiving a feed of carbon-rich gas comprising at least 0.5 vol% CO2 as is described herein elsewhere. In highly preferred embodiments the facility is adapted for recovering the CC>2-enriched material from carbon sequestration reactor 1 and feeding it to the next carbon sequestration reactor in the sequence of carbon sequestration reactors. Preferably the facility is adapted for recovering CC>2-enriched material from each of carbon sequestration reactors 1 to n-1 and feeding it to the next carbon sequestration reactor in the sequence of carbon sequestration reactors until CC>2-enriched solid material is recovered from carbon sequestration reactor n. The skilled person will understand that if the facility is configured for continuous operation, the facility is adapted for at the same time recovering CC>2-enriched material from each of carbon sequestration reactors 1 to n-1 and feeding it to the next reactor in the sequence of carbon sequestration reactors. Since the carbon sequestration is preferably carbonation, the CC>2-enriched material referred to throughout this document is preferably carbonated material.

[0049] n is preferably an integer within the range of 3-14, preferably within the range of 3-10, preferably within the range of 3-8. For example n may be 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13 or 14. n is preferably an even number, such that n is preferably 4, 6, 8 or 10. In case at least part of the reactors are vertically stacked, as is explained herein elsewhere, this allows two stacks of equal height to be made.

[0050] The flow regulators referred to in the context of the present invention are typically valves, which are known to the skilled person for regulating gas flow. Such flow regulators are typically and thus preferably operable from a completely closed to a completely open state, such that the gas flow which is being regulated by the flow regulator can be completely closed.

[0051] The one or more first flow regulators are provided for controlling the gas flow from the interior of the housing of a carbon sequestration reactor to the next carbon sequestration reactor. The fluidconnection between the interior of the housing of each of the carbon sequestration reactors can simply be achieved using ducting or conduits connecting each carbon sequestration reactor in the sequence of reactors to the next. The flow regulator can then simply be a valve positioned within said ducting or conduits. The skilled person will also understand that the outlets from each of carbon sequestration reactor 1 to n-1 may be used both as solid material outlet, and as gas outlet, such that if appropriate ducting from the outlet to the next reactor is used, a single outlet may provide both such a fluid connection from one reactor to the next as well as a means for recovering solid material form the reactor.

[0052] In highly preferred embodiments of the invention, the facility is configured to convey solid material from an outlet of each carbon sequestration reactor 1 to n-1 to an inlet of the next carbon sequestration reactor in the sequence of carbon sequestration reactors. Thus, the flow of solid material through the reactors is from 1 to n, in order. This can be achieved using various means known to the skilled person, for example solid material collected from an outlet of a carbon sequestration reactor may be deposited on a conveying means such as a conveyor belt or a screw conveyor, and thus transported to the inlet of the next carbon sequestration reactor. Intermediary equipment may of course be placed between the reactors, such as surge bins, hoppers, etc. which can help with material handling. The present inventors have found that the design of the facility of the invention, which uses a multitude of carbon sequestration reactors, advantageously allows at least part of the sequence of carbon sequestration reactors to be vertically stacked. Preferably, at least part of the reactors are stacked such that the outlet of one reactor is vertically substantially aligned with the inlet of the next reactor in the sequence. The outlet of the one reactor and the inlet of the next reactor may be directly and / or vertically coupled. In particular, solid material may be conveyed in a substantially vertical direction through the outlet of the one reactor and the inlet of the next reactor. Stacking of reactors has several advantages, including that the space occupied on the factory floor is minimized, and that the conveying of solid material from one reactor to the next may be partially or completely achieved by the action of gravity for at least some reactors. The present inventors have also found that the design of the facility of the invention, which uses a multitude of carbon sequestration reactors, advantageously allows flexibility in the installation of the facility on factory floor, such as by irregular stacking or placement on a factory floor, as compared to having one massive reactor. The carbon sequestration facility of the invention is particularly advantageous in case flue gas is used, as is described herein elsewhere, as the carbon sequestration facility would typically be retrofitted in an existing facility where flue gas is generated (such as a combustion plant of cement kiln) which has limited factory floor space and a lot of existing equipment which cannot easily be moved.

[0053] The carbon sequestration reactors are preferably configured to move the solid material from an inlet of the carbon sequestration reactor, to an outlet of the carbon sequestration reactor. This can be achieved using agitation means as will be described herein elsewhere in more detail.

[0054] The housing of each of the carbon sequestration reactors is highly preferably substantially airtight, as has been detailed herein elsewhere. This is important to guarantee workers safety in case the reactors operate at a pressure above atmospheric pressure, or to limit false air in the reactors incase the reactors operate at a pressure below atmospheric pressure.

[0055] The present inventors have found that it is advantageous and thus highly preferred if the facility is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor / -1 , wherein / is an integer from 2 to n. This can be achieved, for example, by employing a facility as described herein which further comprises one or more external gas inlets for providing gas to the sequence of carbon sequestration reactors, the one or more external gas inlets being placed downstream of carbon sequestration reactor 1. Thus, the one or more external gas inlets are configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor / -1 , wherein / is an integer from 2 to n. This can be achieved, for example, by a facility wherein an inlet of carbon sequestration reactor 1 is fluidly connected to a first gas source and the one or more external gas inlets are fluidly connected to a second gas source, the first gas source being different from the second gas source. As will be understood by the skilled person, such external gas inlets will typically be placed between reactors, but may also be provided on a reactor, by providing a gas inlet forming a passage through the wall of the housing of the reactor, as this will also affect the gas composition downstream of that reactor. Placement of the gas inlet between the reactors may be achieved by providing the external gas inlet on ducting or conduits fluidly connecting the interior of the housing of two carbon sequestration reactor in the sequence of reactors. For example, the external gas inlet may be formed by the joining of a ducting or conduit for external gas to the ducting or conduits fluidly connecting the interior of the housing of two carbon sequestration reactor in the sequence of reactors, by using e.g. a T type connection, which typically would be provided with a flow regulator. An example is a three-way valve, connecting the interior of the housing of two carbon sequestration reactor in the sequence of reactors with ducting or conduit for external gas. External gas as used herein means gas different from the gas in carbon sequestration reactor / -1 , wherein the external gas inlet is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor / -1 , and wherein / is an integer from 2 to n.

[0056] In summary, in preferred embodiments of the invention the facility further comprises one or more external gas inlets for providing gas to the sequence of carbon sequestration reactors, the one or more external gas inlets being placed downstream of carbon sequestration reactor 1 ; wherein at least one of the one or more external gas inlets forms a passage through the wall of the housing of carbon sequestration reactor / , wherein / is an integer from 2 to n; or wherein at least one of the one or more external gas inlets is provided between carbon sequestration reactors / -1 and / wherein / is an integer from 2 to n.

[0057] The present inventors have found that in these highly preferred embodiments wherein the facility is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor i-1 , wherein i is an integerfrom 2 to n, to optimize control overthe gas composition in carbon sequestration reactor / , it is preferred that carbon sequestration reactor / -1 comprises an airtight solid material outlet, or that carbon sequestration reactor / comprises an air-tight solid material inlet, wherein the air-tight solid material inlet or outlet is preferably is selected from a rotary valve, a trickle valve, butterfly valve, check valve, gate valve, globe valve, ball valve, preferably a rotary valve,or a trickle valve, highly preferably a rotary valve. In this way, solid material can be conveyed from carbon sequestration reactor / -1 to carbon sequestration reactor / without necessarily transferring the gas from carbon sequestration reactor / -1 to carbon sequestration reactor / . Indeed, if the interior of the housing of carbon sequestration reactor / -1 is fluidly connected to carbon sequestration reactor / via a flow regulator, and carbon sequestration reactor / -1 comprises an air-tight solid material outlet, or carbon sequestration reactor / comprises an air-tight solid material inlet, then complete control over the gas composition in carbon sequestration reactor / is achieved. The gas in carbon sequestration reactor / can be completely replaced with external gas, or by not completely closing the flow regulator a mixture of external gas and gas recovered from carbon sequestration reactor i-1 can be provided. Typically, no other solid material outlets are used than air-tight solid material outlets in order to maintain full control over the gas flows.

[0058] It is preferred that carbon sequestration reactor 1 comprises a substantially airtight solid material inlet and a gas inlet distinct from the solid material inlet, wherein the solid material inlet is preferably is selected from a rotary valve, a trickle valve, butterfly valve, check valve, gate valve, globe valve, ball valve, preferably a rotary valve, or a trickle valve, highly preferably a rotary valve. The airtight solid feedstock inlet enables continuous feeding of the solid feedstock to the sequestration process with minimal air entrainment. The sequestration of CO2 to obtain CC>2-enriched material can consequently be conducted in a continuous manner simply using an agitated bed, without a need for an energy- intensive fluidized bed reactor, such that low-energy consumption carbonation can be achieved. By separating the solid feedstock inlet from the carbon-rich gas inlet, the high gas speeds and volumes (and associated compressor energy consumption) which are required to suspend solid feedstock in a gas stream such as in a fluidized bed reactor can be avoided. Minimal air entrainment in the carbon sequestration apparatus enables consistent product throughput, optimal carbon sequestration kinetics, and CO2 sequestration performance thereby greatly enhancing process reliability and performance. The reactors placed after the first reactor do not necessarily need to employ an airtight solid feedstock inlet since the feedstock from the first reactor may be fed directly to the second reactor together with the (depleted) carbon-rich gas through the same outlet. However, as is explained herein before, in the highly preferred embodiments wherein the facility is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor i-1 , wherein i is an integer from 2 to n, to optimize control over the gas composition in carbon sequestration reactor / , it is preferred that carbon sequestration reactor / -1 comprises an air-tight solid material outlet, or that carbon sequestration reactor / comprises an air-tight solid material inlet.

[0059] The inventors have found that the need for adapting the gas composition in the sequence of carbon sequestration reactors is typically largest for the final reactors, as this is where one will want the ability to e.g. switch to a drying mode, or change reaction conditions. Thus, preferably the facility is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor i-1 , wherein i is an integer from n-2 to n, preferably from n-1 to n (and n is preferably >3, such as >4). Similarly, in preferred embodiments of the invention the facility is provided, which further comprises one or more external gas inlets for providing gas to the sequence of carbon sequestration reactors, the one or more external gas inlets being placed downstream of carbonsequestration reactor n-3, preferably downstream of carbon sequestration reactor n-2, more preferably wherein at least one of the one or more external gas inlets is placed between carbon sequestration reactors n-2 and n-1 or between carbon sequestration reactors n-1 and n, preferably wherein at least one of the one or more external gas inlets is placed between carbon sequestration reactors n-1 and n and wherein at least one of the one or more external gas inlets is placed between carbon sequestration reactors n-2 and n-1. For similar reasons, in preferred embodiments of the invention the facility is provided wherein no external gas inlets are placed between carbon sequestration reactors 1 and 2, preferably n > 3 and no external gas inlets are placed between carbon sequestration reactors 1 to 3.

[0060] In preferred embodiments of the invention, the facility comprises one or more external gas inlets for providing gas to the sequence of carbon sequestration reactors, the one or more external gas inlets being placed downstream of carbon sequestration reactor 1 as has been described herein earlier, and the facility comprises one or more second gas flow regulators configured for controlling the flow of gas provided to the sequence of carbon sequestration reactors via the one or more external gas inlets. In this way, maximum flexibility is achieved, since the process conditions can easily be adapted by using the flow regulator to achieve the desired gas flow through the external gas inlet.

[0061] The carbon sequestration facility of the present invention preferably comprises a sensor for determining a property of the solid material or the gas downstream of carbon sequestration reactor 1 , preferably for determining a property of the solid material or the gas in carbon sequestration reactor j or conveyed from an outlet of carbon sequestration reactor j, wherein j > 1 and wherein preferably j = n or j = n-1. A sensor can be provided for monitoring various solid material properties, such as a solid material property selected from moisture content, temperature, extent of carbonation (for example based on a spectroscopic measurement, such as an IR spectroscopic measurement) and / or particle size. A sensor can be provided for monitoring various gas properties, such as a gas property selected from temperature, humidity, and / or composition (for example, CO2 concentration or O2 concentration). The facility preferably further comprises means for adapting one or more properties of the gas downstream of carbon sequestration reactor 1 based on the determined solid material property and / or on the determined gas property, wherein the adapting means is preferably for adapting the temperature, humidity, and / or CO2 content the gas. This can easily be achieved by employing adapting means comprising a controller configured for regulating one or more gas flow regulators. For example, by regulating the first and second flow regulators described herein elsewhere, the relative amount of external gas supplied to the process can be controlled. Alternatively, or in addition, the adapting means may comprise a controller configured for operating a wetting means, such as a spraying bar, configured for wetting solid material. The wetting means may be placed upstream of the sequence of carbon sequestration reactors, between carbon sequestration reactors, or inside a carbon sequestration reactor.

[0062] As is explained herein elsewhere, the facility is preferably operated in continuous mode. In this context it is preferred that the facility, further comprises gas flow induction means configured for inducing a flow of gas from an inlet of carbon sequestration reactor 1 , through carbon sequestration reactors 2 to n-1 to an outlet of carbon sequestration reactor n, or configured for inducing a flow of gasfrom an inlet of carbon sequestration reactor n, through carbon sequestration reactors n-1 to 2, to an outlet of carbon sequestration reactor 1. Since according to highly preferred embodiments of the invention mentioned before the flow of material is from 1 to n, this means that the induced gas flow may be concurrent with or countercurrent to the flow of solid material. Preferably, the induced gas flow is concurrent with the flow of solid material. This allows optimal use of e.g. a drying gas introduced through an external gas inlet. The gas flow can be induced by an induction fan placed downstream of carbon sequestration reactor n.The carbon sequestration reactors

[0063] In accordance with preferred embodiments of the invention described herein before, the carbon sequestration reactors comprised in the sequence of reactors 1 to n comprise a substantially airtight hollow housing. As will be evident to the skilled person, in order to allow the reactor to function the reactor comprises one or more solid feedstock inlets forming a passage through the wall of the airtight housing, one or more outlets forming a passage through the wall of the airtight housing and one or more gas inlets forming a passage through the wall of the airtight housing. For carbon sequestration reactor 1 , as is explained before, the gas inlet is preferably distinct from the solid feedstock inlet, such that an air-tight solid feedstock inlet can be used. The solid feedstock inlet and the gas inlet each form a passage through the wall airtight housing for feeding respectively the solid feedstock and the carbon- rich gas to the inside of the airtight housing. In case of carbon sequestration reactor 1 , these are separate passages, while for other carbon sequestration reactors in the sequence of carbon sequestration reactors, the solid feedstock inlet and gas inlet may be the same passage. However, as was explained before, in the highly preferred embodiments wherein the facility is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor i-1 , wherein i is an integer from 2 to n, to optimize control over the gas composition in carbon sequestration reactor / , it is preferred that carbon sequestration reactor / -1 comprises an air-tight solid material outlet, or that carbon sequestration reactor / comprises an air-tight solid material inlet. Consequently, carbon sequestration reactor / preferably also has a gas inlet distinct from the solid material inlet.

[0064] During operation, the carbon-rich gas needs to be contained in the reactor, such that the housing needs to be substantially airtight. The inlets and outlets during normal operation will be either closed or operably coupled to the remainder of the plant, such that the atmosphere within the hollow housing can be controlled during operation of the reactor. By employing an airtight solid feedstock inlet on carbon sequestration reactor 1 , continuous feeding of the solid feedstock to the reactor is enabled with minimal air entrainment such that optimal, continuous operation of the sequence of reactors can be achieved. Using a substantially air-tight solid feedstock inlet also allows the solid-feedstock provided in step (i) to be stored under a regular air atmosphere until it is fed to the reactor in step (iii), for example simply using a hopper coupled to the reactor. This allows workers to easily supply additional solid feedstock (e.g. by filling the hopper) without risk of exposure to the carbon-rich gas.

[0065] Each carbon sequestration reactor in the sequence of carbon sequestration reactors 1 to ncomprises at least one outlet distinct from the solid feedstock inlet and the gas inlet which is suitable for removing solid material form the reactor (and may also be referred to herein as “solid material outlet”). Preferably carbon sequestration reactor n comprises at least one solid material outlet which is substantially air-tight, for example using a valve as is described herein elsewhere for the solid feedstock inlet. In this case, a distinct gas outlet forming a passage through the wall of the housing of carbon sequestration reactor n is necessary in case step (iv) is to take place under an induced flow of gas as described herein elsewhere. Such setups are advantageous because by separating the solids outlet from the gas outlet they allow control over the gas composition in further stages of the process, for example if the solid material is exposed to another carbon sequestration step or a drying step wherein a gas having a different temperature, humidity and / or CO2 content is used. As was explained before, in the highly preferred embodiments wherein the facility is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor i-1 , wherein i is an integer from 2 to n, to optimize control over the gas composition in carbon sequestration reactor / , it is preferred that carbon sequestration reactor / -1 comprises an air-tight solid material outlet. Alternatively, a solid material outlet may be provided which performs a dual function of solid material outlet and gas outlet, such that the CC>2-enriched material and the (depleted) carbon-rich gas are removed from a reactor through the same outlet and supplied to the next reactor in sequence, or later separated. In such a case, the solid feedstock and gas outlet may simply be an opening in the reactor housing at a position where CC>2-enriched material can fall out of the reactor under the influence of gravity, through the opening into a container (e.g. a container operably coupled to conveying means configured for conveying the solid material to the next carbon sequestration reactor in the sequence) or directly (optionally via one or more conduits) into the next carbon sequestration reactor or from carbon sequestration reactor n into a dryer placed in series with the sequence of reactors. The (depleted) carbon-rich gas may then be amongst others used in the next carbon sequestration reactor in the sequence, recirculated to the reactor it was recovered from or removed from the process, such as by venting to the atmosphere after an optional dedusting step. If the container is reversibly closable or disconnectable from carbon sequestration reactor n, for example using a closable flow regulator, the container can be periodically emptied while the sequence of reactors can stay in continuous operation.

[0066] The various carbon sequestration reactors comprised in the sequence of reactors 1 to n may be different reactors, or may be duplicates of the same reactor. For example, some reactors may be configured for size-reduction of the solid material, while other reactors may be configured to agitate the material without significant size-reduction. The reactors may be simple hollow vessels, but are preferably equipped with some form of mechanical agitation means.

[0067] In preferred embodiments of the invention at least one, preferably all of the carbon sequestration reactors comprises mechanical agitation means configured for contacting solid material during operating such that an agitated bed of solid material is created inside the reactor during operation. The carbon sequestration reactors are preferably configured to move the solid material from an inlet of the carbon sequestration reactor, to an outlet of the carbon sequestration reactor. This can be achieved in various ways known to the skilled person. For example, as is preferred, the mechanical agitation means comprised in a carbon sequestration reactor may be configured to move the solidmaterial from an inlet of the carbon sequestration reactor comprising the agitator to an outlet of the carbon sequestration reactor comprising the agitator.

[0068] The mechanical agitation means may be provided in the form of any means known to the skilled person capable of creating an agitated bed. According to preferred embodiments of the invention the reactors comprised in the sequence of carbon sequestration reactors do not comprise a fluidized bed reactor, which employs a high velocity gas stream to fluidize a solid bed. The present inventors have found that such fluidized bed reactors which have been suggested in the art for carbonation consume excessive amounts of energy (due to the high pressure drops employed in the reactor) for the rather inefficient carbon sequestration they can achieve. Examples of mechanical agitation means are lifting flights mounted in the interior of a rotary drum (e.g. attached to the housing of a rotary drum), milling media (such as inert balls or beads) inside a rotary drum, static paddles mounted inside a rotary drum, a rotary shaft-driven agitation apparatus positioned within the reactor housing (e.g. a screw or a mixer). In preferred embodiments of the invention, the mechanical agitation means is a rotary shaft-driven agitator such as a screw (preferably a shaftless screw, or a screw wherein the helical flighting is attached to the outer surface of a screw shaft), a plough mixer, a ribbon mixer or a paddle mixer (e.g. a pug-mill) , preferably a plough mixer, a ribbon mixer or a paddle mixer, more preferably a plough mixer or a paddle mixer. Step (v) preferably comprises rotating the rotary shaft-driven agitator (preferably the screw) at no more than 350 rpm, preferably no more than 300 rpm. Step (v) highly preferably comprises rotating the rotary shaft-driven agitator (preferably the screw) at a tip speed within the range of 0.1-0.3 m / s, preferably within the range of 0.4-0.2 m / s.

[0069] At least one, preferably all of the carbon sequestration reactors in the sequence of carbon sequestration reactors are preferably configured to move the feedstock from the solid feedstock inlet to a solid material outlet of the reactor. The solid feedstock inlet and outlet are preferably located at opposite ends of the reactor. Since during operation an agitated bed of solid material exists in the reactor, at least one, preferably all of the carbon sequestration reactors in the sequence of carbon sequestration reactors is preferably configured to move the particles comprised in the agitated bed from the inlet of the reactor to an outlet of the reactor. The skilled person will understand that during normal operation, wherein each reactor is continuously fed with solid feedstock, the agitated bed in each reactor is constantly present, but the individual particles comprised in the bed will move from the inlet to the outlet of each reactor in accordance with their average residence time in the reactor. In preferred embodiments, the rotary-shaft driven agitator comprises a rotary shaft placed approximately horizontally. In case of a shaftless screw, there is typically a piece of shaft supporting the screw which is placed along the screw axis for connecting the screw with the motor.

[0070] The mechanical agitation means preferably comprises a rotary shaft-driven agitation apparatus, preferably the mechanical agitation means comprises a screw positioned within the reactor housing, wherein the screw is supported for rotational movement around a first axis and wherein the screw comprises a helical flighting. The screw may be a shaftless screw, or a screw wherein the helical flighting is attached to the outer surface of a screw shaft. As will be understood by the skilled person a shaftless screw is a rotary shaft-driven agitator because a rotary shaft is still used to rotate the screw,even though the shaft does not extend axially along with the flighting. The screw may be single flight or double flight. The screw may have a constant pitch or a variable pitch. The screw may have a tapered flight. The screw may further comprise paddles mounted on the shaft between the flighting. In some particular embodiments of the invention, the screw comprises agitation means protruding from the surface of the helical flighting and one or more gas flow passages radially positioned between the first axis and the outer radius of the screw.

[0071] It is highly preferred that the screw comprises agitation means protruding from the surface of the helical flighting and one or more gas flow passages radially positioned between the first axis and the outer radius of the screw. The present inventors have found that such screws provide an optimized agitated bed and solid-gas interaction providing further improved carbon sequestration efficiency. The agitation means can comprise cut and folded flights. Preferably the agitation means comprise one or more lifting bars connecting two adjacent blades of the helical flighting. It is preferred that two adjacent blades of the helical flighting are connected to each other by two or more lifting bars and / or that a majority of the blades are connected to two adjacent blades by one or more lifting bars. The lifting bars may be closed or comprise one or more openings. The lifting bars are preferably positioned substantially perpendicular to the helical flighting. It is preferred that if the screw comprises helical flighting attached to the outer surface of a screw shaft (as opposed to a shaftless screw), the one or more gas flow passages are formed by one or more openings extending through a blade of the helical flighting.Further processing equipment

[0072] Various additional processing equipment may be comprised in the carbon sequestration facility, in order to provide a fully integrated process capable of carbonating various incoming material streams. Some non-limiting embodiments are described here.

[0073] The carbon sequestration facility of the present invention may further comprise a size-reduction apparatus placed upstream of the sequence of carbon sequestration reactors, configured for reducing the size of a solid material before it is fed to the sequence of carbon sequestration reactors. Such a size-reduction apparatus is preferably configured to subject the solid material to milling or crushing, preferably to ball milling, roller milling, basket milling, hammer milling, rotor-stator milling, pin milling, jet milling and / or rod milling, preferably to roller milling.

[0074] The carbon sequestration facility of the present invention may further comprise wetting means configured for wetting solid material. The wetting means may be placed upstream of the sequence of carbon sequestration reactors, between carbon sequestration reactors, or inside a carbon sequestration reactor.

[0075] The carbon sequestration facility of the present invention may comprise a first and a second size-reduction apparatus placed upstream of the sequence of carbon sequestration reactors, wherein the first size-reduction apparatus is placed upstream of the wetting means and the second size-reduction apparatus, and wherein the wetting means are configured for wetting solid material upstream of the second size-reduction apparatus and / or in the second size-reduction apparatus. Such a facility advantageously allows a first size-reduction and surface activation of solid feedstock, followed by wetting to achieve optimal carbon sequestration performance. However it was found that such wetting may lead to undesirable agglomeration, such that a second size-reduction apparatus which can deagglomerate the solid material during or after wetting is desirable.

[0076] The carbon sequestration facility of the present invention may comprise a dryer configured for drying the solid material obtained from the sequence of carbon sequestration reactors. The sequence of carbon sequestration reactors of the present invention allow, if desired, for drying to partially or completely take place in the carbon sequestration reactors as explained herein earlier. In such scenarios, the dryer can be skipped and thus significant energy saving can be achieved.

[0077] The carbon sequestration facility of the present invention may comprise a size-reduction apparatus placed downstream of the sequence of carbon sequestration reactors, configured for reducing the size of the solid material obtained from the sequence of carbon sequestration reactors or from the optional dryer. It was found by the present inventors that the carbon sequestration reaction may lead to significant agglomeration, depending on the solid material being carbonated. In this case it can be advantageous to employ a deagglomeration apparatus after the carbon sequestration reactors.

[0078] It is repeated that the functions described above may also be performed by the carbon sequestration reactors themselves, which may incorporate e.g. size-reduction means inside the reactor housing. However, the present inventors have found that in some advantageous setups a majority of the total number of carbon sequestration reactors as detailed herein earlier (e.g. 4-12 reactors, such as 6, 7, or 8 reactors) are configured to agitate the material without significant size-reduction and employ additional size-reduction equipment before and / or after the carbon sequestration reactors as is explained above. The size-reduction before and / or after the carbon sequestration reactors may also take place in the presence of (i.e. while contacting) a carbon-rich gas as described herein in order to maximize the total carbon uptake. This means that, for example if 6 carbon sequestration reactors without significant size-reduction are used, in conjunction with a first and a second size-reduction apparatus placed upstream of the sequence of carbon sequestration reactors and a drying stage and a size-reduction apparatus placed downstream of the sequence of carbon sequestration reactors, at least 10 apparatuses of the carbon sequestration facility operate under the carbon-rich atmosphere. In some embodiments of the facility of the invention, one or more carbon sequestration reactors of the sequence of carbon sequestration reactors 1 to n is configured for reducing the size of a solid material while one or more carbon sequestration reactors of the sequence of carbon sequestration reactors 1 to n is configured to agitate the material without significant size-reduction. Preferably a carbon sequestration reactor configured for reducing the size of a solid material is placed downstream of a carbon sequestration reactor configured to agitate the material without significant size-reduction, and preferably upstream of a carbon sequestration reactor configured to agitate the material without significant sizereduction. As significant agglomeration may occur in the carbon sequestration reactors which are configured to agitate the material without significant size-reduction, placing a carbon sequestrationreactor configured reducing the size of a solid material in between carbon sequestration reactors which are configured to agitate the material without significant size-reduction intermittent deagglomeration can be achieved which increases the efficiency of subsequent carbon sequestration steps.

[0079] In another aspect of the invention there is provided a combustion plant (such as a fossil fuel combustion, wood pellet combustion, biomass combustion or municipal waste combustion plant) or a cement plant comprising a carbon sequestration facility as described herein fluidly coupled to a flue gas outlet of the combustion area of the combustion plant or the calciner of a cement plant. Thus, the combustion plant or cement plant and the reactor are connected such that flue-gas from combustion or calcining can be provided to the sequence of carbon sequestration reactors.

[0080] In another aspect of the invention there is provided the use of the carbon sequestration facility described herein for the CO2-enrichment of a solid feedstock. In another aspect of the invention there is provided the use of the carbon sequestration facility described herein for carbon sequestration. In another aspect of the invention there is provided the use of the carbon sequestration facility described herein for the removal of CC>2 from a carbon-rich gas, such as a flue gas as described herein. Preferably the uses described herein comprise contacting a solid feedstock as described herein with a carbon-rich gas as described herein inside at least one, preferably all of the carbon sequestration reactors of the sequence of carbon sequestration reactors.Method for the sequestration of carbon in a solid feedstock

[0081] In another aspect of the invention there is provided a method for carbon sequestration using the facility described herein, comprising: i) providing a solid feedstock; ii) providing a carbon-rich gas comprising at least 0.5 vol% CO2; iii) feeding the solid feedstock and the carbon-rich gas to carbon sequestration reactor 1 ; iv) contacting the solid feedstock with the carbon-rich gas in the reactor to obtain CC>2-enriched material.

[0082] In highly preferred embodiments the method is provided wherein the CC>2-enriched material is recovered from carbon sequestration reactor 1 and fed to the next carbon sequestration reactor in the sequence of carbon sequestration reactors. Preferably the CC>2-enriched material is recovered from each of carbon sequestration reactors 1 to n-1 and fed to the next carbon sequestration reactor in the sequence of carbon sequestration reactors until it is recovered from carbon sequestration reactor n. The skilled person will understand that if the method is operated continuously, the CC>2-enriched material is at the same time recovered from each of carbon sequestration reactors 1 to n-1 and fed to the next reactor in the sequence of carbon sequestration reactors.

[0083] Preferably, the method comprises contacting the CC>2-enriched material with a carbon-rich gas comprising at least 0.5 vol% CO2 in each of reactors 2 to n.

[0084] In embodiments of the invention, the method comprises feeding the first carbon-rich gas to carbon sequestration reactor 1 and feeding a second carbon-rich gas comprising at least 0.5 vol% CO2 to the sequence of carbon sequestration reactors downstream of carbon sequestration reactor 1 ,preferably through one or more external gas inlets placed downstream of carbon sequestration reactor 1 . It is highly preferred that the humidity, CC>2-content and / or the temperature of the second carbon-rich gas is different from the first carbon-rich gas. Most preferably, at least the temperature of the second carbon-rich gas when fed to the sequence of carbon sequestration reactors is different from the temperature of the first carbon-rich gas when fed to carbon sequestration reactor 1 .

[0085] In embodiments of the invention, the method comprises mixing the gas provided through one of the one or more external gas inlets with gas obtained from carbon sequestration reactor k and providing the gas mixture to carbon sequestration reactor k+1 , wherein said mixing takes place before carbon sequestration reactor k+1 or in carbon sequestration reactor k+1, and wherein k is from 1 to n- 1.

[0086] In embodiments of the invention, the method comprises determining a property of the solid material or the gas downstream of carbon sequestration reactor 1 , preferably determining a property of solid material in carbon sequestration reactor j or conveyed from the outlet of carbon sequestration reactor j, wherein j > 1 and wherein preferably j = n orj = n-1 . Such a solid material property is preferably selected from moisture content, temperature, extent of carbon sequestration (for example based on a spectroscopic measurement, such as an IR spectroscopic measurement) and / or particle size. Such a gas property is preferably selected from temperature, humidity, and / or composition (for example, CO2 concentration or O2 concentration). The determination of the property may be performed by a worker taking a sample and analysing it, or by a sensor provided for determining the property. Carbon sequestration facilities in accordance with the invention which comprise a sensor for determining a property of the solid material or the gas downstream of carbon sequestration reactor 1 are described herein elsewhere. Preferably the method further comprises adapting one or more properties of the gas downstream of carbon sequestration reactor 1 based on the determined solid material property, wherein the adapting preferably comprises adapting the temperature, humidity, and / or CO2 content of the gas. Adapting one or more properties of the gas downstream of carbon sequestration reactor 1 based on the determined solid material property can advantageously be performed by feeding a second carbon- rich gas comprising at least 0.5 vol% CO2 to the sequence of carbon sequestration reactors through one or more external gas inlets placed downstream of carbon sequestration reactor 1 , wherein preferably the humidity, CC>2-content and / or the temperature of the second carbon-rich gas is different from the first carbon-rich gas. Such feeding of second carbon-rich gas can be performed by regulating one or more of the second gas flow regulators configured for controlling the flow of gas provided to the sequence of carbon sequestration reactors via the one or more external gas inlets. Carbon sequestration facilities in accordance with the invention which contain such second gas flow regulators have been described herein before.

[0087] In some embodiments of the invention, the method is provided wherein at least step (iv), preferably steps (iii)-(iv) are performed in continuous mode. The term continuous is to be interpreted as a method in which the production of the CC>2-enriched material is performed continuously by continuous carbon sequestration in step (iv). The continuous process thus continuously outputs CC>2-enriched material over a given period of time. This is in contrast to batch processing wherein there is nocontinuous output of CC>2-enriched material but rather a discrete portion (“batch”) which is treated for a time until the complete batch is fully processed all at once. Even if step (iv) is performed in continuous mode, some other steps may still be performed in batch mode. In the context of the present invention, if discrete portions of solid feedstock are continuously fed (according to a regular or irregular pattern) in step (iii) (e.g. by operating a valve such as a rotary valve at predetermined intervals), this is considered to be continuous since the input of feedstock and output of CC>2-enriched material is continuous over a given period of time. In some embodiments of the invention, the method is operated such that CC>2-enriched material is continuously recovered over a period of at least 1 minute, preferably at least 10 minutes, more preferably at least 30 minutes. In preferred embodiments of the invention, the method is operated such that the CC>2-enriched material is continuously recovered over a period of at least 1 hour, preferably at least 4 hours, more preferably at least 8 hours.

[0088] In some embodiments of the invention, the solid feedstock provided step (i) is the CC>2-enriched material recovered from carbon sequestration reactor n. In other words, a first portion of CC>2-enriched material can be recovered and submitted as part or all of the solid feedstock of step (i) such that multiple passes through the sequence of carbon sequestration reactors are achieved. Such recycling of the CC>2-enriched material from the output side of the sequence of reactors to the inlet side of the sequence of reactors can be performed multiple times to obtain the desired CO2 uptake.

[0089] As will be clear from the present description, step (iv) is a substantially dry process. While the present inventors have found that moisture control of the solid phase provides unique advantages, step (iv) is not performed on an aqueous solution or slurry. The present inventors have found that this greatly improves energy efficiency (as no water has to be removed afterwards) and imparts unique properties on the resulting mechanochemically carbonated solid feedstocks, yielding materials which are substantially different from e.g. aqueous carbonation materials.

[0090] The common assumption when working with flue gas for carbon sequestration with regard to moisture control is that the flue gas temperature should be kept above the dew point (preventing condensation). The present inventors have surprisingly found that it is the moisture content of the solid feedstock which is decisive for the overall efficiency of the operations.

[0091] The moisture content of the solid feedstock is expressed in wt.% and determined by total weight of the solid feedstock. The moisture content of the solid feedstock provided in step (i) is preferably more than 2 wt.%, preferably more than 5 wt.%, more preferably more than 10 wt.%. The present inventors have surprisingly found that the moisture content of the solid feedstock is preferably less than 35 wt.%, preferably less than 27 wt.%, most preferably less than 20 wt.%, since above these levels reduced efficiencies are observed, in particular in case the CO2-enrichment of step (iv) is a method wherein no significant size reduction occurs, as is described herein elsewhere. Depending on the feedstock, above this moisture content, a tendency to convert to a slurry may also be observed. The solid feedstock preferably has the moisture content specified herein at at least one point in time during step (iv), for example when step (iv) is initiated. In some embodiments, the moisture content of the feedstock is as specified herein throughout a majority or substantially all of step (iv).

[0092] In some embodiments, step (i) and / or step (iv) comprises wetting the solid feedstock to achievethe moisture content described herein earlier. Wetting the solid feedstock may be performed by any means known to the skilled person. Preferably, spraying an aqueous composition, such as water, on the solid feedstock; and / or contacting the solid feedstock with a humid gas such that it can increase the moisture content of the solid feedstock when it is contacted therewith. This humid gas may be a carbon- rich gas as is described in the context of step (ii). The present inventors have found that such surface wetting is advantageous for the CO2 sequestration efficiency. However even if the solid feedstock is wetted by spraying, if dry gas is used for step (iv), the solid feedstock may dry out quickly, such that additional moisture control steps are necessary to maintain the moisture content of the solid phase during step (iv). To reduce or eliminate the need for such additional moisture control steps, it is preferred that the humidity of the carbon-rich gas provided in step (ii) is controlled such that moisture content of the solid material during step (iv) remains at or above the moisture content of the solid feedstock at the start of step (iv). In all of the embodiments described herein, it is preferred that the carbon-rich gas has a relative humidity (RH) of at least 50%, preferably at least 70 %, more preferably at least 85 %, such as about 100 %. As will be clear from the above, it is preferred that the solid feedstock is maintained at a moisture content equal to or higher than the moisture content at the start of step (iv), throughout step (iv). As will be understood from the above, it is preferred that the surface of the solid material is kept substantially wetted throughout step (iv). For the purposes of the present disclosure, to maintain at a target moisture content includes such situations where the solid feedstock deviates from the target moisture content, such deviation is detected or calculated, and corrected. In general, maintaining a certain moisture content throughout a method step should be interpreted to mean that the moisture content is within the desired range during at least 50% of the duration of the step, preferably at least 80% of the duration of the step, in particular throughout the complete duration of the step. If the feedstock is wetted before entering the reactor, in other words, if step (i) comprises wetting the feedstock, step (i) may further comprise an optional step of deagglomerating the feedstock. Since some feedstocks have the tendency to agglomerate upon wetting, such a deagglomeration step can be used to achieve the preferred solid feedstock particle sizes described herein.

[0093] The carbon-rich gas of step (ii) of the method described herein may be any gas stream comprising significant amounts of CO2. It will be understood by the skilled person that the composition of the gas may change as the reaction progresses but that continuous replenishment may be provided by e.g. a continuous flow of fresh gas through step (iv) of the method. Coupled with a continuous supply of solid feedstock, this allows the method to operate in continuous mode. In view of the CO2 sequestration happening throughout the method, the gas will have a lower CO2 concentration after step (iv) of the method compared to the gas entering step (iv) of the method respectively.

[0094] In all embodiments of the present invention, the carbon-rich gas has a hydrocarbon content of less than 1 vol%, preferably less than 0.3 vol%, more preferably less than 0.05 vol%.

[0095] In embodiments of the method described herein the carbon-rich gas provided in step (ii) comprises a combustion flue gas, in particular a flue gas from fossil fuel combustion, wood pellet combustion, biomass combustion or municipal waste combustion. Fossil fuel combustion may be coal, petroleum coke, petroleum, natural gas, shale oil, bitumens, tar sand oil, or heavy oils combustion, orany combination thereof. The combustion flue gas may optionally have been treated to reduce the water content, the SO2 content, and / or the NOx content. In embodiments of the method described herein, the carbon-rich gas provided in step (ii) comprises a cement kiln flue gas. Typically cement kiln flue gas will comprise a fossil fuel combustion flue gas as cement kilns are typically heated by fossil fuel combustion, typically additionally enriched with CO2 originating from the limestone calcination happening inside the cement kiln for the purpose of cement manufacturing. The cement kiln flue gas may optionally have been treated to reduce the water content, the SO2 content, and / or the NOx content.

[0096] In embodiments of the method described herein the carbon-rich gas used in each of carbon sequestration reactors 1 to n consists essentially of a flue gas, optionally mixed with air and / or water. The flue gas is selected from a combustion flue gas and / or a cement kiln flue gas.

[0097] The CO2 concentration in the carbon-rich gas provided in step (ii) is preferably at least 2 vol%, more preferably at least 3 vol%. Typical CO2 concentrations for combustion flue gas and cement kiln flue gas are in the range of 1-20 vol%, such as 2-15 vol% or 2-5 vol% or 5-15 vol%, such that it is preferred that the carbon-rich gas provided in step (ii) has a CO2 concentration in the range of 1-20 vol%, such as 2-15 vol% or 2-5 vol% or 5-15 vol%. In alternative embodiments of the invention, the carbon-rich gas provided in step (ii) comprises at least 80 vol% CO2, preferably at least 95 vol% CO2.

[0098] As will be understood based on the present disclosure, contrary to e.g. carbonate looping reactors, it is preferred that each reactor in the series of 1 to n is fed with gas comprising a flue gas. Typically the reactors will be fed with flue gas or mixtures of flue gas with air.

[0099] In preferred embodiments of the method described herein, step (ii) comprises providing hot combustion flue gas or cement kiln flue gas with a temperature of more than 120 °C, preferably more than 150 °C, preferably more than 190 °C, and cooling the hot flue gas. The cooling can be effected by any cooling means known to the skilled person, including a heat-exchanger (e.g. with ambient air or coolant), passive cooling, and / or jacket cooling. With passive cooling is meant that the piping transporting the flue gas allows for sufficient heat-exchange with ambient air to cool the hot flue gas. In practice, this can be achieved by e.g. increasing piping length, decreasing piping diameter, not insulating the piping etc. Jacket cooling may comprise running a coolant fluid through a jacket placed around the piping transporting the flue gas. The coolant fluid used in the heat-exchanger or the jacket may be any known coolant, including regular water, or a coolant having a freezing point of less than -5 °C, preferably less than -10°C, (typically an aqueous coolant comprising one or more glycols and one or more anticorrosion agents). The cooling is preferably effected by a heat-exchanger and / or jacket cooling. The cooling is preferably performed to a temperature of below 120 °C, preferably below 100 °C, preferably below 90 °C, more preferably below 80 °C, most preferably below 70 °C. Afterthe cooling it may be desirable to perform a further step wherein the cooled flue gas is submitted to a liquid-gas separation step (e.g. using a dehumidifier, demisting pad, or similar equipment) to remove water droplets and / or decrease the moisture content of the cooled flue gas, or to perform a humidification step (depending on the humidity of the flue gas source and the composition of other gases mixed to prepare the carbon-rich gas). In some embodiments of the invention step (ii) comprises providing hot combustion flue gas or cement kiln flue gas with a temperature of more than 120 °C, and cooling thehot flue gas to a temperature of below 100 °C, wherein the cooled flue gas is submitted to a gas-liquid separation step, the liquid being collected and used for wetting the solid feedstock before or during step (iv). In any scenario it is preferred that the temperature of the gas employed in step (v) is at least 1 °C above the dew point inside the reactor wherein step (iv) is performed, preferably within 1-20 °C above the dew point, such that the moisture content of the solid phase can more easily be controlled.

[0100] In case the carbon-rich gas provided in step (ii) comprises a combustion flue gas or cement kiln flue gas, it is preferred that the carbon sequestration reactor 1 comprises a gas inlet which is fluidly connected to a combustion flue gas outlet of a combustion plant. In some embodiments, step (ii) comprises combustion of a fuel as described herein elsewhere, thereby obtaining a hot combustion flue gas with a temperature of more than 120 °C, preferably more than 150 °C, preferably more than 190 °C. In case the carbon-rich gas provided in step (ii) comprises a cement kiln flue gas, it is preferred that carbon sequestration reactor 1 comprises a gas inlet which is fluidly connected to a cement kiln flue gas outlet of a cement plant. In some embodiments, step (ii) comprises calcining limestone, thereby obtaining a hot flue gas with a temperature of more than 120 °C, preferably more than 150 °C, preferably more than 190 °C.

[0101] As with any chemical process, the appropriate reaction parameters such as residence time, temperature, pressure, etc. during step (iv) are highly dependent on the extent of CC>2-sequestration desired and can routinely be determined by sampling material on a regular basis and monitoring reaction progress e.g. via Total Carbon determination as explained herein.

[0102] In general, step (iv) can be performed at atmospheric pressure, below atmospheric pressure or above atmospheric pressure. Hence, in some embodiments step (iv) is performed at a pressure of more than about 100 kPa (e.g. at least 101.325 kPa). In some embodiments of the invention, step (iv) is performed at a pressure of more than about 150 kPa (e.g., at least 151 .988 kPa), preferably more than about 300 kPa (e.g., at least 303.975 kPa). In alternative embodiments of the invention step (iv) is performed at a pressure below atmospheric pressure, such as a pressure of less than around 100 kPa (e.g. less than 101 .325 kPa), such as less than 50 kPa or less than 10 kPa. It will be understood by the skilled person that (if not actively maintained) the pressure of the gas will change as the reaction progresses. In these embodiments it should be understood that the pressure inside the reactor is as specified herein when step (iv) is initiated. In some embodiments the pressure inside the reactor is as specified herein throughout a majority or substantially all of step (iv). In any case, the pressure preferably does not exceed 1013 kPa during step (iv), preferably the pressure does not exceed 1013 kPa at any point during the method described herein. The present inventors have found that such pressures are not required for the present methods, thereby resulting in significant energy savings compared to high-pressure carbonation methods. In all embodiments of the present invention, but in particular those wherein the carbon sequestration step (iv) is performed at a pressure below atmospheric pressure (such as a pressure of less than around 100 kPa (e.g. less than 101 .325 kPa)), it is preferred that each carbon sequestration reactor is such that the % of false air during step (iv) is less than 30%, preferably less than 15%, more preferably less than 5%.

[0103] ln embodiments of the method described herein, in order to stimulate carbon sequestration,step (iv) is performed at a temperature of less than 160 °C, preferably less than 120 °C, preferably less than 100 °C, preferably less than 90 °C, more preferably less than 85 °C. In preferred embodiments of the invention, step (iv) is performed at a temperature of at least 30 °C, preferably at least 35 °C, preferably at least 38 °C. In particularly preferred embodiments step (iv) is performed at a temperature within the range of 30-85 °C, preferably 30-70 °C, most preferably 40-70 °C, such as 50-60°C. These temperatures allow improved CO2-enrichment efficiencies to be achieved, in particular in case a slag as described herein is used as a solid feedstock. In preferred embodiments of the invention, no active heating is applied and any increase in temperature is attributed to the temperature of the incoming gas, friction resulting from the agitator or to exothermic reactions taking place during carbon sequestration. The temperature is preferably determined on the solid material during processing. In some embodiments, active cooling is applied during step (iv), for example by providing a cooling jacket around the housing of apparatus wherein step (iv) is performed. The coolant fluid used in the jacket may be any known coolant, including regular water, or a coolant having a freezing point of less than -5 °C, preferably less than -10°C, (typically an aqueous coolant comprising one or more glycols and one or more anticorrosion agents). In all embodiments of the invention it is preferred that the carbon-rich gas has a temperature within the range of 30-85 °C, preferably 30-70 °C, most preferably 40-70 °C, such as 50-60°C when it is contacted with the solid feedstock in step (iv).

[0104] The preferred reaction conditions described herein for step (iv), such as the gas stream composition, humidity, pressure, reaction temperature, are equally applicable to each subsequent carbon sequestration step which occurs in reactors 2 to n. In other words, it is preferred that the CO2- enriched material is contacted with a carbon-rich gas comprising at least 0.5 vol% CO2 in each of carbon sequestration reactors 2 to n at the same reaction conditions (e.g. the gas stream composition, humidity, pressure, and / or reaction temperature) as described herein for step (iv).

[0105] The low temperature requirement of the present process means that no fossil fuels are required, and electric heating means (or low caloric value green fuel sources) can realistically be used to supply heat in case the temperature of the gas feed is insufficient to reach the desired temperature. In this way, fossil fuels can be avoided throughout the whole production chain.

[0106] In some embodiments of the invention, step (iv) comprises contacting the feedstock with a catalyst, preferably a metal oxide catalyst, such as a transition metal oxide catalyst. Examples of suitable catalysts are selected from the group consisting of iron oxides, cobalt oxides, ruthenium oxides, titanium oxides, nickel oxides, aluminium oxides and combinations thereof. The catalyst may be provided as a pure oxide or as a mineral comprising the oxide, for example the aluminium oxides may be provided in the form of a zeolite. The catalyst should be suitable for increasing the CO2 sequestration of the feedstock. In some embodiments the catalyst is selected from metal oxides for which the ratio TCcatTCo is more than 1 .01 , preferably more than 1 .05 wherein TCcat is the increase the total carbon content of a feedstock when a CO2 sequestration capacity test is performed, wherein a 10 gram sample of solid feedstock is loaded into a pressure cell with 500 grams of stainless steel balls (diameter 12 mm) coated with the catalyst, followed by pressurization of the cell to 1 MPa, and rotation of the pressurized cell at 5000 RPM for 48 hours, wherein the test is performed at an ambient temperature of 20°C andno heating or cooling is applied, and wherein TCo is the increase the total carbon content of a feedstock when a CO2 sequestration capacity test is performed, wherein a 10 gram sample of solid feedstock is loaded into a pressure cell with 500 grams of uncoated stainless steel balls (diameter 12 mm), followed by pressurization of the cell to 1 MPa, and rotation of the pressurized cell at 5000 RPM for 48 hours, wherein the test is performed at an ambient temperature of 20°C and no heating or cooling is applied.

[0107] The catalyst may for example be mixed with the solid feedstock, or immobilized on an inner surface of the reactor employed in step (iv). The catalyst may also be immobilized on a porous support or bed which is fixed in the reactor of step (iv) in a way such that it allows contact with the solid feedstock.

[0108] The carbon sequestration methods described herein are preferably performed without employing a strong acid, preferably without employing any further oxidizing agent other than the gas provided in step (ii).

[0109] In accordance with preferred embodiments of the invention, the total power consumption represented by step (iv) is less than 60 Wh per kg of CC>2-enriched material obtained in step (iv), preferably less than 40 Wh, more preferably less than 30 Wh.

[0110] As was explained herein earlier, at least one, preferably all of the carbon sequestration reactors in the sequence of carbon sequestration reactors comprises mechanical agitation means positioned within the housing provided for creating an agitated bed of solid material inside the reactor during operation. Thus, step (iv) of the method preferably comprises operating at least one, preferably all of the carbon sequestration reactors in the sequence to provide an agitated bed of solid material. If the method is operated continuously, as is preferred, steps (iii) and (iv) are performed simultaneously, and step (iv) comprises operating at least one, preferably all of the carbon sequestration reactors in the sequence to provide an agitated bed of solid material.

[0111] According to preferred embodiments of the invention the reactors comprised in the sequence of carbon sequestration reactors do not comprise a fluidized bed reactor, which employs a high velocity gas stream to fluidize a solid bed. Thus, at any point during step (iv) of the method, the gas velocity inside each of the carbon sequestration reactors in the sequence of carbon sequestration reactors is lower than the minimum fluidization velocity for the solid feedstock, preferably lower than 80% of the minimum fluidization velocity for the solid feedstock.

[0112] Step (iv) preferably comprises operating the reactor such that individual particles of the solid feedstock move from the inlet to the outlet of the apparatus. If the method is operated continuously, as is preferred, steps (iii) and (v) are performed simultaneously, and step (v) comprises operating at least one, preferably all of the carbon sequestration reactors in the sequence of carbon sequestration reactors to provide an agitated bed of solid feedstock wherein individual particles move from the inlet to the outlet of the apparatus.

[0113] It is highly preferred that step (iv) does not result in significant size reduction. However the inventors have found that some agglomeration may take place. In preferred embodiments of the invention, the ratio of the D50 of the solid feedstock provided in step (i) to the D50 of the CC>2-enriched material obtained in step (iv), preferably obtained from carbon sequestration reactor n, is less than 1 .2,preferably less than 1 .1 .

[0114] The method described herein preferably comprises a further step: (v) deagglomerating the CO2- enriched material obtained in step (iv), preferably obtained from carbon sequestration reactor n, optionally after one or more further CO2-enrichment steps wherein the solid material is contacted with a carbon-rich gas comprising at least 0.5 vol% CO2. The preferred embodiments described herein relating to the gas composition, temperature, pressure, duration, etc. of step (iv) are also applicable to the optional one or more further CO2-enrichment steps. Step (vi) may be performed using the same apparatus as used in step (iv). However it is preferred that step (v) is performed using an apparatus distinct from the apparatus used in step (iv) since, as is explained herein earlier, the apparatus of step (iv) preferably does not effect significant size-reduction. In preferred embodiments of the invention, the ratio of the D50 of the solid feedstock provided in step (i) to the D50 of the deagglomerated CO2- enriched material obtained in step (v), is within the range of 0.8-1 .5, preferably within the range of 0.8- 1 .2, most preferably within the range of 0.9-1 .2.

[0115] In preferred embodiments, there is an induced flow of the carbon-rich gas through at least one, preferably all of the carbon sequestration reactors in the sequence during step (iv). Such induced flow may be caused by mechanical means, such as a fan or compressor. In preferred embodiments, the induced gas flow of the carbon-rich gas through the reactor is induced by an induction fan placed downstream of the reactor. The present inventors have found that employing a screw comprising gas flow passages as described herein earlier is especially advantageous when low pressures during step (iv) are employed as has been described herein earlier, preferably in combination with an induced flow since this still allows sufficient carbon sequestration to occur without requiring high energy consumption caused by intensive agitation or high pressures. Since according to highly preferred embodiments of the invention mentioned before the flow of material is from 1 to n, this means that the induced gas flow may be concurrent with or countercurrent to the flow of solid material. Preferably, the induced gas flow is concurrent with the flow of solid material. This allows optimal use of e.g. a drying gas introduced through an external gas inlet. The gas flow can be induced by an induction fan placed downstream of carbon sequestration reactor n.

[0116] In preferred embodiments, the rotary shaft-driven agitator in at least one, preferably all of the carbon sequestration reactors is rotated at a speed which produces an agitated feedstock bed reaching a height which is at least 15% higher, preferably at least 25% higher than the height of the feedstock bed when there is no rotation, the height of the bed being determined approximately in the centre of the surface of the powder bed and wherein the height of the bed when there is no rotation and the height of the agitated feedstock bed are determined on the same quantity of feedstock loaded in the agitation apparatus.

[0117] Depending on the feedstock used and the extent of carbon sequestration required, one or more reactors in the sequence of carbon sequestration reactors of the present invention may be employed for drying such that less or no further drying step outside the sequence of carbon sequestration reactors is required. Drying of the material inside the sequence of carbon sequestration reactors preferably comprises drying the CC>2-enriched material such that material having a moisture content of less than8 wt.% (by total weight of the dried CC>2-enriched material), preferably less than 5 wt.% (by total weight of the dried CC>2-enriched material), more preferably less than 2 wt.% (by total weight of the dried CO2- enriched material) is recovered from carbon sequestration reactor n. However, for some feedstocks, all carbon sequestration reactors will be employed for carbon sequestration and further drying outside the sequence will be required. Thus, the method of the present invention may comprise a further step (vi) of drying the CC>2-enriched material obtained in step (iv), preferably obtained from carbon sequestration reactor n, optionally after one or more further CO2-enrichment steps wherein the solid material is contacted with a carbon-rich gas comprising at least 0.5 vol% CO2, or of drying the deagglomerated material obtained in step (v), wherein the drying takes place outside of the sequence of carbon sequestration reactors. The preferred embodiments described herein relating to the gas composition, temperature, pressure, duration, etc. of step (iv) are also applicable to the optional one or more further CO2-enrichment steps. This avoids agglomeration / lumping of the material upon long-term storage. The deagglomeration of step (v) may be performed before, during or after the drying of step (vi). Drying may be performed by any means known to the skilled person. Drying preferably comprises drying the CO2- enriched material recovered in step (iv), preferably recovered from carbon sequestration reactor n, optionally after one or more further CO2-enrichment steps, wherein the solid material is contacted with a carbon-rich gas comprising at least 0.5 vol% CO2, or obtained from step (v) to a moisture content of less than 8 wt.% (by total weight of the dried CC>2-enriched material), preferably less than 5 wt.% (by total weight of the dried CC>2-enriched material), more preferably less than 2 wt.% (by total weight of the dried CC>2-enriched material). Drying in or after the sequence of carbon sequestration reactors is preferably performed by contacting the CC>2-enriched material with a drying gas stream (preferably a continuous flow of a drying gas stream) having a temperature of less than 400 °C, preferably less than 300 °C, preferably less than 250 °C and preferably having a relative humidity (RH) of less than 50%, preferably less than 30%, most preferably less than 10%. By keeping the temperature of the drying gas below the values mentioned above, loss of the sequestered CO2 from the CC>2-enriched material is avoided. When drying in the sequence of carbon sequestration reactors, the drying gas stream may be advantageously provided through the one or more external gas inlets.

[0118] As is common in the art, the (depleted) carbon-rich gas after having been submitted to the carbon sequestration process described herein will typically be vented to the atmosphere, optionally after one or more treatments which preferably includes dedusting (e.g. using a baghouse or cartridge collector). Typically, in case an industrial flue gas is used, the (depleted) carbon-rich gas after having been submitted to the carbon sequestration process described herein will be fed to the emission stack of the industrial site.Properties of the solid feedstock and of the obtained CO2-enriched material

[0119] The method ofthe present invention is not particularly limited with regard to the feedstock used. In order to achieve significant carbon sequestration, it is preferred that the solid feedstock provided in step (i) is a material which exhibits an increase in CO2 content of at least 1 %, preferably of at least 4% when submitted to a CO2 sequestration capacity test wherein a 10 gram sample of solid feedstock isloaded into a pressure cell with 500 grams of stainless steel balls (diameter 12 mm) coated with titanium dioxide, followed by pressurization of the cell to 1 MPa, and rotation of the pressurized cell at 5000 RPM for 48 hours, wherein the test is performed at an ambient temperature of 20°C and no heating or cooling is applied, wherein the CO2 content is determined as mass loss above 400°C measured by TGA employing a temperature trajectory wherein the temperature was increased from room temperature to 800°C at a rate of 10 °C / min and then decreased to room temperature at a rate of 10 °C / min or that the solid feedstock provided in step (i) is a material which exhibits an increase in total carbon content of at least 1 %, preferably of at least 4% when submitted to a CO2 sequestration capacity test wherein a 10 gram sample of solid feedstock is loaded into a pressure cell with 500 grams of stainless steel balls (diameter 12 mm) coated with titanium dioxide, followed by pressurization of the cell to 1 Mpa, and rotation of the pressurized cell at 5000 RPM for 48 hours, wherein the test is performed at an ambient temperature of 20°C and no heating or cooling is applied.

[0120] In preferred embodiments of the invention, the solid feedstock provided in step (i) comprises a mineral and / or elemental carbon. In some preferred embodiments, the feedstock comprises a carbonatable material preferably the feedstock comprises at least 0.1 wt.% (by total weight of the feedstock) carbonatable material, preferably at least 0.5 wt.% (by total weight of the feedstock) carbonatable material, most preferably at least 2 wt.% (by total weight of the feedstock) carbonatable material. The carbonatable material is preferably selected from calcium oxides, calcium hydroxides, magnesium oxides, magnesium hydroxides, calcium silicates, magnesium silicates, and combinations thereof.

[0121] The solid feedstock is typically a particulate or granulate material, such as a powder, preferably having the particle size distribution as described herein elsewhere. In some embodiments the solid feedstock provided in step (i) comprises one or more of coal combustion products (in particular fly ash or bottom ash), graphene, graphite, clay, shale, silicate minerals, glass (in particular soda-lime glass or borosilicate glass), iron or steel-making slags, wood, limestone, calcite, amosite, brucite, natural pozzolan, volcanic rock, biochar, pyroxenes, hydrous magnesium silicates, talc, serpentines, serpentinite, peridotite, olivine, borosilicate glass, volcanic ash, perlite, pumice, obsidian, scoria, tuffs (e.g. rhyolite tuffs, dacite tuffs, basaltic tuffs, trachytic tuffs, phonolitic tuffs, digenetic lithoid tuffs), andesite, clinoptiololite, heulandite, augite, apatite, titanite, biotite, hauynite, nosean, sodalite, magnetite, muscovite, chabazite, analcite, hematite, cristobalite, tripoli, kaolinite, illite, mica, hornblende, mordenite, andesites, basalt, diatomite, tripoli, cherts, dolomite, forsterite, monticellite, wollastonite, diopside, enstatite, lizardite, potassium and sodium feldspars, antigorite, chrysotile, opaline shales, hydraulic cement, slaked lime, concrete, mafic or ultramafic mining tailings and zeolites, preferably selected from serpentinite, peridotite, basalt, volcanic rock, perlite, zeolites, soda-lime glass, bottom ash, coal fly ash, oil fly ash, coal fly ash, subbituminous coal fly ash, anthracite coal fly ash, bituminous coal fly ash, blast furnace (BF) slag (such as air-cooled blast furnace (ACBF) slag or water cooled blast furnace slag), granulating blast furnace (GBF) slag, basic oxygen furnace (BOF) slag, ladle furnace basic slag (LS), electric arc furnace (EAF) flag, and mafic or ultramafic mining tails. The furnace slags may be from ferrous metal production, non-ferrous metal production or phosphorous production, preferably from ferrous metal production. Most preferred are the slag feedstocks recited herein. Inpreferred embodiments of the present invention, the feedstock is not pre-treated with acid.

[0122] In preferred embodiments of the invention, the method described herein is provided wherein the solid feedstock provided in step (i), has one, two, three or four, preferably four of the following characteristics:• a D10 within the range of 0.005 to 3 pm;• a D50 within the range of 0.1 to 100 pm, preferably within the range of 0.1 to 40 pm, more preferably within the range of 1-40 pm, most preferably within the range of 5-40 pm;• a D90 within the range of 0.5 to 300 pm;• a BET surface area within the range of 0.1 to 10000 m2 / g, preferably within the range of 0.5-2000 m2 / g.

[0123] In all embodiments it is preferred that the D90 is as specified above, since this is the most important size characteristic influencing overall performance.

[0124] Since the CO2-enrichment treatment of step (iv) preferably takes place without significant size reduction (maintaining an agitated bed without significant size reduction, as explained herein before), the CC>2-enriched material recovered in step (iv), preferably recovered from carbon sequestration reactor n, preferably complies with the same particle size and surface area characteristics. In other words, the CC>2-enriched material recovered in step (iv), preferably recovered from carbon sequestration reactor n, preferably has one, two, three or four, preferably four of the following characteristics:• a D10 within the range of 0.005 to 3 pm;• a D50 within the range of 0.1 to 100 pm, preferably within the range of 0.1 to 40 pm, more preferably within the range of 1-40 pm, most preferably within the range of 5-40 pm;• a D90 within the range of 0.5 to 300 pm;• a BET surface area within the range of 0.1 to 10000 m2 / g, preferably within the range of 0.5-2000 m2 / g.

[0125] As explained before, for some materials which are prone to agglomeration during step (iv), an additional deagglomeration step (v) may be employed. If a deagglomeration step (v) is employed, the CC>2-enriched material recovered in step (v) preferably complies with the particle size and surface area characteristics specified above.

[0126] ln some embodiments of the invention, step (i) comprises preconditioning a solid feedstock precursor by contacting it with a carbon-rich gas comprising at least 0.5 vol% CO2 (e.g. the carbon-rich gas of step (ii)) and / or size-reducing a solid feedstock precursor. The preconditioned and / or size- reduced material is then provided as the solid feedstock of step (i). Preconditioning of the solidfeedstock precursor may be performed without simultaneously effecting a size-reduction of the feedstock. For example, the feedstock precursor can be stored with a continuous flow of the gas, the feedstock precursor can be stored in a gas-tight container in the presence of the gas, or the solid feedstock precursor can be subjected to mild agitation which does not result in a significant sizereduction of the solid feedstock precursor, for example in a Continuously Stirred Tank Reactor (CSTR). Preconditioning of the solid feedstock precursor may also be performed simultaneously with sizereducing the solid feedstock precursor by subjecting the solid feedstock precursor to a size-reduction operation while contacting it with the carbon-rich gas. The solid feedstock precursor may also be subjected to distinct preconditioning and size-reduction operations. Size-reduction of the solid feedstock precursor may be performed by subjecting the solid feedstock precursor, optionally while contacting the carbon-rich gas, to grinding, milling, mixing, stirring (such as low-speed stirring or high-speed stirring), shearing (such as high-torque shearing), shaking, blending, pulverizing, powderizing, crushing, crumbling, a fluidized bed or ultrasonication, preferably grinding, milling, mixing, stirring (such as low- speed stirring or high-speed stirring), shearing (such as high-torque shearing), or ultrasonication. Preferably size-reduction of the solid feedstock precursor is performed milling or crushing, preferably ball milling, roller milling, basket milling, hammer milling, rotor-stator milling, pin milling, jet milling and / or rod milling, preferably ball milling or roller milling, most preferably roller milling. Preferably size-reduction of the solid feedstock precursor is performed while the solid feedstock precursor contacts the carbon- rich gas. In preferred embodiments of the present invention, the solid feedstock precursor which is fed to the size-reduction operation in order to provide the solid feedstock of step (i) has a D95 before the size-reduction which is less than 20 mm, preferably less than 15 mm, more preferably less than 10 mm, most preferably less than 4 mm. For example, the D95 may be in the range of 3-7 mm. In some embodiments of the invention, the D95 before the size-reduction is in the range of 1-20 mm, preferably in the range of 2-15 mm, more preferably in the range of 3-10 mm such as 3-7 mm. The D95 described in this paragraph concerns the mass-based D95 determined by sieve analysis.

[0127] Without wishing to be bound by any theory, the present inventors believe that the sequestration methods of the invention when applied to materials such as graphite, allow platelet-like morphology to be maintained. This is observed by platelet thickness being maintained. Hence, in embodiments of the invention (in particular those embodiments where the feedstock of step (i) comprises or is graphite), the CC>2-enriched material recovered in step (iv), preferably recovered from carbon sequestration reactor n, has a platelet morphology and has a platelet thickness of more than 10 nm, preferably more than 20 nm, more preferably more than 50 nm. The platelet thickness as referred to herein is preferably determined by calculating the average platelet thickness determined by Transmission Electron Microscopy on all particles for which the platelet thickness can be determined in a random field of view of 25x25 micron recorded from a random sample of a homogenized batch of material.

[0128] The method of the invention is preferably performed such that the ratio of the CO2 content of the CC>2-enriched material recovered in step (iv), preferably recovered from carbon sequestration reactor n, to the CO2 content of the solid feedstock of step (i) is at least 1.2:1 , preferably at least 2:1 , more preferably at least 3:1 , wherein the CO2 content is determined as mass loss above 400°C measured by TGA employing a temperature trajectory wherein the temperature was increased fromroom temperature to 800°C at a rate of 10 °C / min and then decreased to room temperature at a rate of 10 °C / min. Similarly, the method of the invention, in particular in case the feedstock is selected from non-carbonaceous materials such as the minerals, slags and glass described herein earlier, is preferably performed such that the ratio of the total carbon content of the second CC>2-enriched material recovered in step (iv), preferably recovered from carbon sequestration reactor n, to the total carbon content of the solid feedstock of step (i) is at least 1 .03:1 , preferably at least 1 .1 :1 , more preferably at least 1.2:1 .List of reference signs1. Housing2. Drive Unit3. Motor4. Outlet10. Screw11. Housing12. Gas Inlet13. Material Inlet14. Material Outlet15. Gas Outlet100. Screw Shaft101. Helical Flighting102. Opening in the Flighting103. Agitation Element1000. Carbon sequestration reactor1001. Carbon sequestration reactor1002. Carbon sequestration reactor1003. Carbon sequestration reactor1004. Carbon sequestration reactor1005. Carbon sequestration reactorA. Longitudinal axisH. Triple HelixR. Radial direction

Claims

Claims1. A facility for the carbon sequestration of a solid material comprising a sequence of carbon sequestration reactors 1 to n, each carbon sequestration reactor being configured for contacting a solid material with a carbon-rich gas comprising at least 0.5 vol% CO2 and comprising a hollow housing, an inlet forming a passage through the wall of the housing and an outlet forming a passage through the wall of the housing; wherein n is an integer > 3; wherein the interior of the housing of each of carbon sequestration reactors 1 to n-1 is fluidly connected, optionally via one or more first flow regulators, to the interior of the housing of the next carbon sequestration reactor; the facility is adapted for recovering CC>2-enriched material from each of carbon sequestration reactors 1 to n-1 and feeding it to the next carbon sequestration reactor in the sequence of carbon sequestration reactors until CC>2-enriched solid material is recovered from carbon sequestration reactor n; and wherein at least one of the carbon sequestration reactors comprises mechanical agitation means configured for contacting solid material during operating such that an agitated bed of solid material is created inside the reactor during operation.

2. The facility of claim 1 , wherein the facility is configured for providing a gas in carbon sequestration reactor / different from the gas in carbon sequestration reactor i-1 , wherein i is an integer from 2 to n.

3. The facility of claim 2, wherein the facility further comprises one or more external gas inlets for providing gas to the sequence of carbon sequestration reactors, the one or more external gas inlets being placed downstream of carbon sequestration reactor 1 .

4. The facility of claim 3, wherein at least one of the one or more external gas inlets forms a passage through the wall of the housing of carbon sequestration reactor / , wherein / is an integer from 2 to n; or wherein at least one of the one or more external gas inlets is provided between carbon sequestration reactors / -1 and / wherein / is an integer from 2 to n.

5. The facility of any one of claims 3-4 wherein the facility comprises one or more second gas flow regulators configured for controlling the flow of gas provided to the sequence of carbon sequestration reactors via the one or more external gas inlets.

6. The facility of any one of claims 2-5 wherein carbon sequestration reactor i-1 comprises an air-tight solid material outlet, wherein the air-tight solid material outlet is preferably is selected from a rotary valve, a trickle valve, butterfly valve, check valve, gate valve, globe valve, ball valve, preferably a rotary valve, or a trickle valve, highly preferably a rotary valve.

7. The facility of any one of the previous claims, wherein the facility comprises a sensor for determining a property of the solid material or the gas downstream of carbon sequestration reactor 1 , preferably for determining a property of the solid material or the gas in carbon sequestration reactor j or conveyed from the outlet of carbon sequestration reactor j, wherein j > 1 and wherein preferably j = n orj = n-1.

8. The facility of claim 7 further comprising means for adapting one or more properties of the gas downstream of carbon sequestration reactor 1 based on the determined solid material property and / or on the determined gas property.

9. The facility of claims 5 and 8 wherein the adapting means is for adapting the temperature, humidity, and / or CO2 content the gas and wherein the adapting means comprises a controller configured for regulating one or more second gas flow regulators.

10. The facility of any one of the previous claims, further comprising gas flow induction means configured for inducing a flow of gas from an inlet of carbon sequestration reactor 1 , through carbon sequestration reactors 2 to n-1 to an outlet of carbon sequestration reactor n, or configured for inducing a flow of gas from an inlet of carbon sequestration reactor n, through carbon sequestration reactors n-1 to 2, to an outlet of carbon sequestration reactor 1 .11 . The facility of any one of the previous claims, wherein at least two of the sequence of carbon sequestration reactors are vertically stacked on each other, preferably stacked such that a solid material outlet of a carbon sequestration reactor is substantially vertically positioned above the solid material inlet of the next carbon sequestration reactor.

12. A method for carbon sequestration using the facility of any one of claims 1-11 , comprising: i) providing a solid feedstock; ii) providing a carbon-rich gas comprising at least 0.5 vol% CO2; iii) feeding the solid feedstock and the carbon-rich gas to carbon sequestration reactor 1 ; iv) contacting the solid feedstock with the carbon-rich gas in the reactor to obtain CC>2-enriched material; wherein the CC>2-enriched material is recovered from each of carbon sequestration reactors 1 to n-1 and fed to the next carbon sequestration reactor in the sequence of carbon sequestration reactors until CC>2-enriched solid material is recovered from carbon sequestration reactor n; and wherein the CC>2-enriched material is contacted with a carbon-rich gas comprising at least 0.5 vol% CO2 in each of carbon sequestration reactors 2 to n; and wherein step (iv) of the method comprises operating at least one, preferably all of the carbon sequestration reactors in the sequence to provide an agitated bed of solid material.

13. The method of claim 12 which is a method for carbonation of a solid feedstock, and wherein carbonated solid material is recovered from each of carbon sequestration reactors 1 to n.

14. The method of claim 12 or 13, comprising feeding the first carbon-rich gas to carbon sequestration reactor 1 and feeding a second carbon-rich gas comprising at least 0.5 vol% CO2 to the sequence of carbon sequestration reactors through one or more external gas inlets placed downstream of carbon sequestration reactor 1 , wherein the humidity, CC>2-content and / or the temperature of the second carbon-rich gas is different from the first carbon-rich gas.

15. The method of any one of claims 12-14, wherein the method comprises determining a property of solid material downstream of carbon sequestration reactor 1 , preferably determining a property of solid material in carbon sequestration reactor; or conveyed from the outlet of carbon sequestration reactor conveyed j, wherein j > 1 and wherein preferably j = n orj = n-1; and wherein the method further comprises adapting one or more properties of the gas downstream of carbon sequestration reactor 1 based on the determined solid material property, wherein the adapting preferably comprises feeding a second carbon-rich gas comprising at least 0.5 vol% CO2 to the sequence of carbon sequestration reactors through one or more external gas inlets placed downstream of carbon sequestration reactor 1 , wherein the humidity, CC>2-content and / or the temperature of the second carbon-rich gas is different from the first carbon-rich gas, wherein the feeding of the second carbon-rich gas comprises regulating one or more second gas flow regulators configured for controlling the flow of gas provided to the sequence of carbon sequestration reactors via the one or more external gas inlets.

16. The method of any one of claims 12-15 wherein step (iv) is performed at a temperature of less than 160 °C.

17. The method of claim 16 wherein the CC>2-enriched material is contacted with a carbon-rich gas comprising at least 0.5 vol% CO2 in each of carbon sequestration reactors 2 to n at a temperature of less than 160 °C.

18. The method of any one of claims 12-15 wherein the carbon-rich gas used in each of carbon sequestration reactors 1 to n consists essentially of a flue gas, optionally mixed with air and / or water.

19. The method of claim 18, wherein the flue gas is selected from a combustion flue gas and / or a cement kiln flue gas.

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