Low energy mechanochemical carbon sequestration of solid feedstocks

Abstract

The present invention relates to a method for the mechanochemical sequestration or capture of carbon from CO2 in solid feedstocks which employ an air-tight solid feedstock dosing such that continuous operation is enabled, and uses thereof. The invention further relates to an apparatus suitable for the mechanochemical sequestration of carbon dioxide in solid feedstocks.

Description

LOW ENERGY MECHANOCHEMICAL CARBON SEQUESTRATION OF SOLID FEEDSTOCKSField of the invention

[0001] The present invention relates to a method for the mechanochemical sequestration or capture of carbon from CO2 in solid feedstocks and uses thereof. The invention further relates to an apparatus suitable for the mechanochemical sequestration of carbon dioxide in solid feedstocks.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] Hence, there remains a need to develop a streamlined, low-temperature and energy-efficient process 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.

[0009] 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.

[0010] W02021087606A1 discloses ball milling methods for mechanochemical CO2 sequestration in mineral feedstocks.

[0011] It is an object of the present invention to provide an improved process for sequestering carbon dioxide into solid feedstocks.

[0012] It is a further object of the present invention to provide a method for sequestering carbon dioxide into solid feedstocks which is energy efficient and can preferably be operated as a continuous process.

[0013] It is a further object of the present invention to provide an apparatus suitable for the sequestration of carbon dioxide in solid feedstocks.Summary of the Invention

[0014] The present inventors have found that the sequestration of CO2 can be achieved in an efficient manner for a large variety of feedstocks when considering such factors as energy consumption, carbon sequestration capacity, and reactor throughput capacity, if a reactor equipped with an agitator capable of creating an agitated bed of solid material inside the reactor in combination with an air-tight solid feedstock inlet is employed. 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. Additionally, it was found that the method can conveniently be coupled to existing flue gas streams, such as fossil fuel flue gas combustion streams or cement kiln flue gas, allowing point-source emissions to be captured without capital- and energy-intensive stand-alone capture processes. The method results not only in a reduction of CO2 concentration but also in SOXand / or NOXreduction, thereby providing further environmental benefits.

[0015] Hence, in a first aspect, the present invention provides a method for carbon sequestration comprising: i) providing a solid feedstock; ii) providing a carbon-rich gas comprising at least 0.5 vol% CO2; iii) feeding the solid feedstock to a reactor comprising a substantially airtight hollow housing and mechanical agitation means configured for contacting solid material during operation such that an agitated bed of solid material is created inside the reactor during operation, wherein the solid feedstock is fed to the reactor via a substantially airtight solid feedstock inlet forming a passage through the wall of the housing; iv) feeding the carbon-rich gas to the reactor via a gas inlet distinct from the solid feedstock inlet, the gas inlet forming a passage through the wall of the airtight housing; v) contacting the solid feedstock with the carbon-rich gas in the reactor to obtain CC>2-enriched material.

[0016] In preferred embodiments there is an induced flow of the carbon-rich gas through the reactor during steps (iv) and / or (v).

[0017] In preferred embodiments, step (v) of the method is performed in continuous mode.

[0018] In preferred embodiments, step (v) of the method comprises operating the reactor to provide an agitated bed of solid material.

[0019] In preferred embodiments, step (v) is performed-at a temperature of less than 120 °C, preferably less than 100 °C, preferably less than 90 °C, more preferably less than 85 °C, and / or-at a pressure of less than 200 kPa, preferably less than 120 kPa, more preferably less than 100 kPa.

[0020] In a further aspect of the invention, there is provided a reactor for the preparation of CO2- enriched material comprising a substantially airtight hollow housing, mechanical agitation means configured for contacting solid material during operation such that an agitated bed of solid material is created inside the reactor during operation, a substantially airtight solid feedstock inlet, a gas inlet distinct from the solid feedstock inlet and an outlet.

[0021] In another aspect of the invention there is provided a carbon sequestration facility comprising the reactor as described herein and a gas source comprising a gas comprising at least 0.5 vol% CO2, wherein the gas source is fluidly connected to a gas inlet of the reactor. The gas source is preferably a flue gas source. In some embodiments the gas source is a flue gas outlet of a combustion plant (such as a fossil fuel combustion, wood pellet combustion, biomass combustion or municipal waste combustion plant) or a cement plant, as described herein earlier.

[0022] 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 reactor as described herein fluidly coupled to a flue gas outlet of thecombustion 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 reactor.

[0023] In another aspect of the invention there is provided the use of the reactor described herein for the CO2-enrichment of a solid feedstock. In another aspect of the invention there is provided the use of the reactor described herein for carbon sequestration. In another aspect of the invention there is provided the use of the reactor described herein for the removal of CO2 from a carbon-rich gas, such as a flue gas as described herein.Brief description of the figures

[0024] 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.

[0025] 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.

[0026] Figures 1-3 show a preferred embodiment of a rotary shaft-driven agitator as described herein, which is a screw 10 for conveying solid material along a longitudinal axis A of a hollow housing of a reactor (not shown). 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.

[0027] Figure 1 shows in a perspective view a rotary shaft-driven agitator as described herein, which is a screw. 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. Preferable the pitch of the agitation helix H is at least ten times the pitch of the helical flighting 101 .

[0028] Figure 2 shows a side view of the screw shown in Figure 1 .

[0029] Figure 3 shows a front view of the screw shown in Figure 1 .

[0030] Figures 4-6 show a reactor 1000 for the preparation of CC>2-enriched material. The reactor 1has 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).

[0031] Figure 4 shows in a perspective view a reactor according to an embodiment of the invention. The 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 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. The 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 , 11.

[0032] Figure 5 shows a side view of the reactor shown in Figure 4.

[0033] Figure 6 shows a front view of the reactorshown in Figure 4 wherein part of housing was omitted to expose the screw positioned inside the reactor.Description of embodiments

[0034] 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.

[0035] 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.

[0036] 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 dictatesotherwise.

[0037] 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.

[0038] 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).

[0039] The term “CC>2-enriched material” is used herein to refer to the solid feedstock after being contacted with the carbon-rich gas in step (v). 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 carbonation). 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.

[0040] In accordance with the invention, the BET surface area as referred to herein is determined at a temperature 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.

[0041] 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.

[0042] 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.

[0043] 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 solidfeedstock, in case the total carbon content of the CO2 enriched material is referred to, it is based on the total weight of the CC>2-enriched material.

[0044] 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.

[0045] 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%.

[0046] 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 “CC>2-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 CC>2-enriched material, unless specified otherwise.Process description

[0047] In a first aspect the present invention provides a method for carbon sequestration comprising: i) providing a solid feedstock; ii) providing a carbon-rich gas comprising at least 0.5 vol% CO2; iii) feeding the solid feedstock to a reactor comprising a substantially airtight hollow housing and mechanical agitation means configured for contacting solid material during operation such that an agitated bed of solid material is created inside the reactor during operation, wherein the solid feedstock is fed to the reactor via a substantially airtight solid feedstock inlet forming a passage through the wall of the housing; iv) feeding the carbon-rich gas to the reactor via a gas inlet distinct from the solid feedstock inlet, the gas inlet forming a passage through the wall of the airtight housing; v) contacting the solid feedstock with the carbon-rich gas in the reactor to obtain CC>2-enriched material.

[0048] In some embodiments of the invention, the method is provided wherein at least step (v), preferably steps (iii)-(v) 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 (v). 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 no continuous 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 steps (iii)-(v) are 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 iscontinuous 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.

[0049] In some embodiments of the invention, the solid feedstock provided step (i) is the CC>2-enriched material obtained in step (v). 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 reactor are achieved. Such recycling of the CC>2-enriched material from the reactor output to the inlet of the reactor can be performed multiple times to obtain the desired CO2 uptake. Due to the small footprint on a factory floor and high product throughput requirements, the present method may also be performed using two or more reactors as described herein in series, wherein the CC>2-enriched material from a first reactor is provided as feedstock for a second reactor placed subsequent to the first reactor, with the caveat that the reactors placed in series 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.

[0050] As will be clear from the present description, step (v) is a substantially dry process. While the present inventors have found that moisture control of the solid phase provides unique advantages, step (v) 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.

[0051] 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.

[0052] 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 (v) 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 (v), for example when step (v) is initiated. In some embodiments, the moisture content of the feedstock is as specified herein throughout a majority or substantially all of step (v).

[0053] In some embodiments, step (i) and / or step (v) comprises wetting the solid feedstock to achieve the moisture content described herein earlier. Wetting the solid feedstock may be performed by anymeans 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 (v), 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 (v). 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 (v) remains at or above the moisture content of the solid feedstock at the start of step (v). 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 (v), throughout step (v). As will be understood from the above, it is preferred that the surface of the solid material is kept substantially wetted throughout step (v). 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.

[0054] 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 (v) 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 (v) of the method compared to the gas entering step (v) of the method respectively.

[0055] 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, or any 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.

[0056] 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.

[0057] 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. After the 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 the hot 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 (v). 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 (v) 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.

[0058] 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 reactor wherein step (v) is performed 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 the reactor wherein step (v) is performed 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.

[0059] As with any chemical process, the appropriate reaction parameters such as residence time, temperature, pressure, etc. during step (v) 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.

[0060] In general, step (v) can be performed at atmospheric pressure, below atmospheric pressure or above atmospheric pressure. Hence, in some embodiments step (v) 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 (v) 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 (v) 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 (v) is initiated. In some embodiments the pressure inside the reactor is as specified herein throughout a majority or substantially all of step (v). In any case, the pressure preferably does not exceed 1013 kPa during step (v), 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 step (v) 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 reactor is such that the % of false air during step (v) is less than 30%, preferably less than 15%, preferably less than 5%.

[0061] In embodiments of the method described herein, in order to stimulate carbon sequestration, step (v) is performed at a temperature of 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 (v) 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 (v) 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 (v), for example by providing a cooling jacket around the housing of apparatuswherein step (v) 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 (v).

[0062] 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.

[0063] In some embodiments of the invention, step (v) 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 and no 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.

[0064] The catalyst may for example be mixed with the solid feedstock, or immobilized on an inner surface of the reactor employed in step (v). The catalyst may also be immobilized on a porous support or bed which is fixed in the reactor of step (v) in a way such that it allows contact with the solid feedstock.

[0065] 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).

[0066] In accordance with preferred embodiments of the invention, the total power consumption represented by step (v) is less than 60 Wh per kg of CC>2-enriched material obtained in step (v), preferably less than 40 Wh, more preferably less than 30 Wh.

[0067] The reactor employed in the method of the present invention comprises a substantially airtight hollow housing and mechanical agitation means positioned within said housing. As will be evident tothe 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. 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. 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 continuous feeding of the solid feedstock to the reactor is enabled with minimal air entrainment such that optimal, continuous operation of the reactor 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.

[0068] In view of using an airtight solid feedstock inlet, the reactor comprises at least one gas inlet forming a passage through the wall of the airtight housing which is distinct from the solid feedstock inlet. At least one outlet is distinct from the solid feedstock inlet and the gas inlet and should be suitable for removing solid material form the reactor (and may also be referred to herein as “solid material outlet”). Preferably the reactor 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 airtight housing is necessary in case step (v) 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. 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 the reactor through the same outlet and 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 or into a further carbon sequestration apparatus or a dryer placed in series with the reactor. The (depleted) carbon-rich gas may then be amongst others used in the further carbon sequestration apparatus, 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. The further carbon sequestration apparatus preferably has the features of the reactor described herein, with the proviso that an airtight solids inlet and a distinct solids and gas inlet are not necessary if the gas obtained from the reactor is fed to the further carbon sequestration apparatus. If the container is reversibly closable or disconnectable from the reactor, for example using a closable flow regulator, the container can beperiodically emptied while the reactor can stay in continuous operation.

[0069] The reactor comprises mechanical agitation means positioned within the housing. The mechanical agitation means is provided for creating an agitated bed of solid material inside the reactor during operation. Thus, step (v) of the method preferably comprises operating the reactor to provide an agitated bed of solid material. If the method is operated continuously, as is preferred, steps (iii) and (v) are performed simultaneously, and step (v) comprises operating the reactor to provide an agitated bed of solid material. 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 the invention the reactor is not 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 carbon sequestration consume excessive amounts of energy (due to the high pressure drops employed in the reactor) for the rather inefficient carbon sequestration they can achieve. Thus, at any point during step (v) of the method, the gas velocity inside the reactor is lower than the minimum fluidization velocity for the solid feedstock, preferably lower than 80% of the minimum fluidization velocity for the solid feedstock. 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.

[0070] The reactor is 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, the reactor 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 the reactor is continuously fed with solid feedstock, the agitated bed in the apparatus is constantly present, but the individual particles comprised in the bed will move from the inlet to the outlet of the apparatus in accordance with their average residence time in the apparatus. Thus, step (v) 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 the reactor to provide an agitated bed of solid feedstock wherein individual particles move from the inlet to the outlet of the apparatus. 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 thescrew which is placed along the screw axis for connecting the screw with the motor.

[0071] In some embodiments of the invention, 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. 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.

[0072] 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.

[0073] In preferred embodiments of the invention, the airtight solid feedstock inlet 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.

[0074] It is highly preferred that step (v) 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 (v), is less than 1 .5, preferably less than 1 .3. In more preferred embodiments of the invention, the ratio of the D50 of the solid feedstock provided in step (i) to the D50 of the CO2- enriched material obtained in step (v), is less than 1 .2, preferably less than 1.1.

[0075] The method described herein preferably comprises a further step: (vi) deagglomerating the CC>2-enriched material obtained in step (v), 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 (v) 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 (v). However it is preferred thatstep (vi) is performed using an apparatus distinct from the apparatus used in step (v) since, as is explained herein earlier, the apparatus of step (v) 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 CC>2-enriched material obtained in step (vi), is within the range of 0.6-1 .8, preferably within the range of 0.7-1 .7, more preferably within the range of 0.7-1 .6. In more preferred embodiments of the invention, the ratio of the D50 of the solid feedstock provided in step (i) to the D50 of the deagglomerated CC>2-enriched material obtained in step (vi), 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.

[0076] In preferred embodiments, there is an induced flow of the carbon-rich gas through the reactor during steps (iv) and / or (v), preferably during steps (iv) and (v). 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 (v) 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.

[0077] In preferred embodiments, the rotary shaft-driven agitator 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.

[0078] The method of the present invention preferably comprises a further step (vii) of drying the CO2- enriched material obtained in step (v), 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 (vi). The preferred embodiments described herein relating to the gas composition, temperature, pressure, duration, etc. of step (v) 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 (vi) may be performed before, during or after the drying of step (vii). Drying may be performed by any means known to the skilled person. Drying preferably comprises drying the CC>2-enriched material recovered in step (v) , 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 step (vi) 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 CO2- enriched material), more preferably less than 2 wt.% (by total weight of the dried CC>2-enriched material). Drying 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 CO2- enriched material is avoided. The drying may be performed inside the same apparatus as step (v) or in a distinct apparatus. For example, the drying may be performed inside the same apparatus as step (v) by changing the humidity of the gas stream after completion of the CO2-enrichment of step (v) which is especially advantageous if the method is not operated in continuous mode. For example, the drying may be performed inside the same apparatus as step (v), by introducing a drying gas stream having a reduced humidity and / or an increased temperature at a point inside the reactor between the solids feedstock inlet and the solid material outlet, which is especially advantageous if the method is operated in continuous mode.

[0079] 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

[0080] The method of the 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 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, 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.

[0081] 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) carbonatablematerial. The carbonatable material is preferably selected from calcium oxides, calcium hydroxides, magnesium oxides, magnesium hydroxides, calcium silicates, magnesium silicates, and combinations thereof.

[0082] 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. In preferred embodiments of the present invention, the feedstock is not pre-treated with acid.

[0083] 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.

[0069] In all embodiments it is preferred that the D90 is as specified above, since this is the most important size characteristic influencing overall performance.

[0070] Since the CO2-enrichment treatment of step (v) preferably takes place without significant sizereduction (maintaining an agitated bed without significant size reduction, as explained herein before), the CC>2-enriched material recovered in step (v) preferably complies with the same particle size and surface area characteristics. In other words, the CC>2-enriched material recovered in step (v) 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.

[0084] As explained before, for some materials which are prone to agglomeration during step (v), an additional deagglomeration step (vi) may be employed. If a deagglomeration step (vi) is employed, the CC>2-enriched material recovered in step (vi) preferably complies with the particle size and surface area characteristics specified above.

[0085] In 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 solid feedstock 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 by 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 fedto 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.

[0086] 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 (v) 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.

[0087] 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 (v) 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 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. 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 (v) 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 .Reactor for the preparation of CO2-enriched material

[0088] In a further aspect of the invention, there is provided a reactor for the preparation of CO2- enriched material comprising a substantially airtight hollow housing, mechanical agitation means configured for contacting solid material during operation such that an agitated bed of solid material is created inside the reactor during operation, a substantially airtight solid feedstock inlet, a gas inlet distinct from the solid feedstock inlet and an outlet.

[0089] The solid feedstock inlet and the gas inlet each form a passage through the wall of the airtight housing for respectively feeding solid feedstock as described herein earlier and carbon-rich gas as described herein earlierto the inside of the airtight housing. The outlet is distinct from the solid feedstock inlet and the gas inlet and should be suitable for removing solid material from the reactor (and may also be referred to herein as “solid material outlet”). The outlet may also be substantially air-tight, for exampleusing a valve as is described herein elsewhere for the solid feedstock inlet. In this case, a distinct gas outlet is necessary in case step (v) of the method is to take place under an induced flow of gas as described herein elsewhere. Alternatively, the solid material outlet may be provided for the dual function of solid material outlet and gas outlet, such that the CC>2-enriched material and the (depleted) carbon- rich gas can be removed from the reactor through the same outlet.

[0090] The mechanical agitation means is provided for creating an agitated bed of solid feedstock inside the reactor during operation. The mechanical agitation means may be provided in the form of any means known to the skilled person capable of creating an agitated bed. 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). The mechanical agitation means is preferably 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.

[0091] The reactor is 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, the reactor is preferably configured to move the particles comprised in the agitated bed from the inlet of the apparatus to an outlet of the apparatus. 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.

[0092] In preferred embodiments of the invention, 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.

[0093] 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 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 ormore 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.

[0094] In preferred embodiments of the invention, the airtight solid feedstock inlet 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.

[0095] In another aspect of the invention there is provided a carbon sequestration facility comprising the reactor as described herein and a gas source comprising a gas comprising at least 0.5 vol% CO2, wherein the gas source is fluidly connected to a gas inlet of the reactor. The gas source is preferably a flue gas source. In some embodiments the gas source is a flue gas outlet of a combustion plant (such as a fossil fuel combustion, wood pellet combustion, biomass combustion or municipal waste combustion plant) or a cement plant, as described herein earlier.

[0096] 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 reactor 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 reactor.

[0097] In another aspect of the invention there is provided the use of the reactor described herein for the CO2-enrichment of a solid feedstock. In another aspect of the invention there is provided the use of the reactor described herein for carbon sequestration. In another aspect of the invention there is provided the use of the reactor described herein for the removal of CO2 from a carbon-rich gas, such as a flue gas as described herein.

[0098] Preferably the uses described herein comprise contacting a solid feedstock as described herein with a carbon-rich gas as described herein inside the reactor.List of reference signs1. Housing2. Drive Unit3. Motor4. Outlet10. Screw11. Housing12. Gas Inlet13. Material Inlet14. Material Outlet100. Screw Shaft101. Helical Flighting102. Opening in the Flighting103. Agitation Element1000. Screw conveyorA. Longitudinal axisH. Triple HelixR. Radial direction

Claims

Claims1 . A method for carbon sequestration comprising: i) providing a solid feedstock; ii) providing a carbon-rich gas comprising at least 0.5 vol% CO2; iii) feeding the solid feedstock to a reactor comprising a substantially airtight hollow housing and mechanical agitation means configured for contacting solid material during operation such that an agitated bed of solid material is created inside the reactor during operation, wherein the solid feedstock is fed to the reactor via a substantially airtight solid feedstock inlet forming a passage through the wall of the housing; iv) feeding the carbon-rich gas to the reactor via a gas inlet distinct from the solid feedstock inlet, the gas inlet forming a passage through the wall of the airtight housing; v) contacting the solid feedstock with the carbon-rich gas in the reactor to obtain CC>2-enriched material, wherein 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 (v), is less than 1 .5, preferably less than 1 .3.

2. The method of claim 1 wherein 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 (v), is less than 1 .2, preferably less than 1 .1 .

3. The method of any one of the previous claims, wherein the solid feedstock inlet of step (iii) is selected from rotary valve, a trickle valve, butterfly valve, check valve, gate valve, globe valve, ball valve, preferably a rotary valve or a trickle valve, preferably a rotary valve.

4. The method of any one of the previous claims, wherein the mechanical agitation means comprises a rotary shaft-driven agitation apparatus.

5. The method of any one of the previous claims, wherein there is an induced flow of the carbon-rich gas through the reactor during steps (iv) and / or (v).

6. The method of claim 5, wherein the induced gas flow of the carbon-rich gas through the reactor is induced by an induction fan placed downstream of the reactor.

7. The method of any one of the previous claims wherein steps (iii) and (v) are performed simultaneously, and wherein step (v) comprises operating the reactor to provide an agitated bed of solid material.

8. The method of any one of the previous claims wherein the mechanical agitation means is configured to move the feedstock from the inlet of the reactor to an outlet.

9. The method of any one of the previous claims, wherein the mechanical agitation means comprises a screw, a plough mixer, a ribbon mixer or a paddle mixer.

10. The method of claim 9 wherein mechanical agitation means comprises a plough mixer or a paddle mixer.

11. The method of any one of the previous claims, wherein the % of false air during step (v) is less than 30%, preferably less than 15%, most preferably less than 5%.

12. The method of any one of the previous claims wherein step (v) is performed-at a temperature of less than 120 °C, preferably less than 100 °C, preferably less than 90 °C, more preferably less than 85 °C, and / or-at a pressure of less than 200 kPa, preferably less than 120 kPa, more preferably less than 100 kPa.

13. The method of any one of the previous claims comprising a further step:(vi) deagglomerating the CC>2-enriched material obtained in step (v), 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.

14. The method of claim 13 wherein the ratio of the D50 of the solid feedstock provided in step (i) to the D50 of the deagglomerated CC>2-enriched material obtained in step (vi), is within the range of 0.6-1 .8, preferably within the range of 0.7-1 .7, more preferably within the range of 0.7-1 .6.

15. The method of claim 14 wherein the ratio of the D50 of the solid feedstock provided in step (i) to the D50 of the deagglomerated CC>2-enriched material obtained in step (vi), 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.

16. Use of a reactor• for the CO2-enrichment of a solid feedstock; or• for carbon sequestration; or• for the removal of CO2 from a carbon-rich gas, such as a flue gas; characterized in that the reactor comprises a substantially airtight hollow housing, mechanical agitation means configured for contacting solid material during operation such that an agitated bed of solid material is created inside the reactor during operation, a substantially airtight solid feedstock inletforming a passage through the wall of the housing, a gas inlet forming a passage through the wall of the housing distinct from the solid feedstock inlet and an outlet forming a passage through the wall of the housing.

17. Use of claim 16 wherein the mechanical agitation means comprises a rotary shaft-driven agitation apparatus.

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