The removal of atmospheric carbon dioxide

Abstract

There is provided herein a system for performing the removal of atmospheric carbon dioxide, the system comprising: a container comprising air-permeable sidewalls; and a plurality of agglomerates comprising calcium hydroxide (Ca(OH)2) particles within the container.

Description

[0001]THE REMOVAL OF ATMOSPHERIC CARBON DIOXIDE Field of the invention The present invention relates to the removal of atmospheric carbon dioxide using calcium hydroxide agglomerates. Background to the invention Global atmospheric CO2 levels in 2019 were 410ppm, higher than at any point in at least the past 800,000 years. Today, global anthropogenic CO2 emissions exceed 35 billion tonnes per year. Over the last decade growth in atmospheric CO2levels has averaged 2.3 ppm per year and shows no sign of slowing. The Intergovernmental Panel on Climate Change (IPCC) have reached a troubling conclusion: to avoid catastrophic climate change the increase in average global temperature above pre- industrial levels must be kept below 1.5 °C. There is growing scientific consensus that humans have already exceeded safe levels of CO2in the atmosphere. Every scenario to avoid a 1.5 °C rise involves removing CO2 from the atmosphere. The low-end IPCC estimate requires the removal of 100 billion tonnes of CO2 per year by 2100, three times more than current emissions. Carbon dioxide removal (CDR) technologies include Negative Emissions Technologies (NETs) and Direct air capture (DAC). NETs take CO2that is already in the atmosphere out of the atmosphere. However, NETs have problems with scalability and cost. DAC is the use of chemical or physical processes to extract carbon dioxide directly from the ambient air. If the extracted CO2is then sequestered in safe long-term storage (called direct air carbon capture and sequestration (DACCS)), the overall process will achieve carbon dioxide removal and be a "negative emissions technology" (NET). The carbon dioxide (CO2) is captured directly from the ambient air; this is contrast to carbon capture and storage (CCS) which captures CO2from point sources, such as a cement factory or a bioenergy plant. After the capture, DAC may generate a concentrated stream of CO2 for sequestration or utilisation or production of carbon- neutral fuel. Carbon dioxide removal is achieved when ambient air makes contact with chemical media, typically an aqueous alkaline solvent or sorbents. These chemical media are subsequently stripped of CO2 through the application of energy (namely heat), resulting in a CO2stream that can undergo dehydration and compression, while simultaneously regenerating the chemical media for reuse. In contrast to carbon capture and storage (CCS) which captures emissions from a point source such as a factory, DAC reduces the carbon dioxide concentration in the atmosphere as a whole. Typically, CCS is recommended for large and stationary sources of CO2rather than distributed and movable ones. On the contrary, DAC has no limitation on sources. At present, current CDR solutions are not commercially viable options due to high operating expenses and energy usage. Part of the problem is that CO2is very dilute (approx. 0.04%) in the atmosphere and so CDR processes must move huge quantities of air to capture CO2 resulting in large plant footprints and consequently high capital expenditure. Scale-up is therefore impractical and not economically viable. CCS technologies which capture CO2from point sources may use chemical sorbents such as calcium oxide (CaO). CaO may also be referred to as lime. CaO may be used to capture CO2via the following carbonation process: CaO + CO2 ĺ CaCO3 However, the reaction kinetics of absorbing CO2 using CaO are too slow for this to be a viable option for the removal of atmospheric CO2on a commercial scale. Calcium hydroxide (Ca(OH)2) is another sorbent material that can react with carbon dioxide to form calcium carbonate. This is shown by the equation below: Ca(OH)2+ CO2ĺ CaCO3+ H2O Powdered Ca(OH)2 is produced by adding a stoichiometric volume of water to CaO (‘hydration’, if excess water is used this is referred to as ‘slaking’). CaO + H2O ĺ Ca(OH)2 This hydration reaction is exothermic and resulting in the breakdown of CaO to a fine powder of Ca(OH)2. Ca(OH)2may provide faster reaction kinetics than CaO, however, the innately small particle size (~2 μm) of Ca(OH)2 can make it challenging to work with due to poor flowability and dust issues. Accordingly, due to these problems Ca(OH)2 has not yet been recognised as being a commercially viable option for use in CDR technologies. Present technologies may seek to carbonate CaO or Ca(OH)2via spreading thin layers across land. However, these technologies encounter issues with losses of materials (e.g., CaO) to the environment when wind entrains the fine particles, this may be especially apparent when the material is being initially handled. The loss of material via dust has a financial penalty but also poses environmental, occupational, and public health hazards. Environmental hazards include smothering of vegetation, alkalinisation of water ways, air pollution to surrounding land, vegetation, waterways, nearby residents, and workers. Occupational hazards relate to disease caused by dust for example, pneumoconiosis. This hazard may be more serious when reactivity of Ca(OH)2is considered. Finally, public health hazards, in the form of domestic water source contamination, food security issues and nearby residents are also possible due to dust exposure. There therefore remains a need for improved carbon dioxide removal technologies. Statement of the invention The present invention seeks to address the problems outlined above with the atmospheric removal of CO2by using calcium hydroxide agglomerates. The captured CO2 would otherwise continue to be present in the atmosphere. A first aspect of the present invention is a system for the removal of atmospheric carbon dioxide, the system comprising: a container comprising air-permeable sidewalls; and a plurality of agglomerates comprising calcium hydroxide (Ca(OH)2) particles within the container. A second aspect of the invention is method of the removal of atmospheric carbon dioxide, the method comprising the carbonation of agglomerates comprising calcium hydroxide (Ca(OH)2) particles. A third aspect of the invention is a method of the removal of atmospheric carbon dioxide, the method comprising a step of using the system as described in the first aspect. A fourth aspect of the invention is the use of the system as described in the first aspect for the removal of atmospheric carbon dioxide. A fifth aspect of the invention is method of forming agglomerates comprising calcium hydroxide (Ca(OH)2) particles for the removal of atmospheric carbon dioxide, the method comprising: a. combining a first mixture comprising powdered particles of calcium hydroxide with a second mixture comprising water to form a third mixture b. agitating the third mixture in order to bind the powder particles together into agglomerates of calcium hydroxide c. optionally, adding more water to the agitated third mixture if non- agglomerated calcium hydroxide remains. The method of the fifth aspect may further comprises: d. optionally, adding more calcium hydroxide to the agitated third mixture; and e. optionally, drying the agglomerates of calcium hydroxide. The present invention advantageously exploits the favourable reaction kinetics of the carbonation of Ca(OH)2 in order to provide a system which is commercially viable to remove CO2from the atmosphere on a large and industrial scale. In particular, the present invention advantageously uses a plurality of calcium hydroxide agglomerates which overcome the flowability and dust issues experienced with Ca(OH)2powder particles. The formation of calcium hydroxide agglomerates advantageously means that the high surface area of the powder particles used to make the agglomerates is maintained. When the agglomerate is formed the strength of bonds between individual Ca(OH)2 powder particles is enough to exceed subsequent forces which may act to break these bonds. This creates larger assemblage of Ca(OH)2 powder particles which are joined at the corners and edges of particles so that the total surface area of the agglomerate is approximately equal to the sum of the surfaces of the individual particles. Advantageously, the system of the present invention uses calcium hydroxide formed from the hydration of flash calcined lime. Advantageously, the system of the present invention uses calcium hydroxide formed from the hydration of high surface area lime. This improves the reaction kinetics. Advantageously, the calcium hydroxide used is formed from the hydration of zero or low carbon lime, preferably zero carbon lime (e.g., carbon neutral lime). This means that the CDR process of the present invention is a net zero process. Advantageously, the system of the present invention may comprise a number of mechanisms to minimise the diffusion limitations of air through the system. Advantageously, the system of the present invention comprises a humidification system. This is because having water present in the agglomerates can act as a catalyst to the carbonation reaction that takes place on the surface of the Ca(OH)2 agglomerates. Brief description of the drawings Examples of the present invention will now be described in detail with reference to the accompanying figures, in which: Figure 1 is a graph showing the particle size distribution of Ca(OH)2 powder performed using a Malvern Mastersizer. Figure 2 is a graph showing the particle size distribution of agglomerated Ca(OH)2performed using sieve analysis. Figure 3 is a graph showing the carbonation reaction extent of three agglomerate sizes and Ca(OH)2powder. Figures 4a and 4b are example containers of the present invention. Figure 5 is a graph showing the showing the particle size distribution of agglomerated Ca(OH)2performed using sieve analysis. Figure 6 is a graph showing the carbonation reaction extent of five agglomerate sizes. Detailed Description Unless indicated otherwise, all technical and scientific terms used herein will have their common meaning as understood by one of ordinary skill in the art to which this invention pertains. The term “comprising” or variants thereof will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The term “comprising” or “comprises” will include references to the component consisting essentially of (such as consisting of) the relevant features. The term “consisting” or variants thereof is to be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step or group of elements, integers or steps. “wt%” is a common abbreviation in the art to mean the “weight %” with respect to the total weight of the article / material referred to. As used herein, the term “calcium hydroxide agglomerates” is used to refer to agglomerates comprising calcium hydroxide particles. A first aspect of the present invention is a system for performing the removal of atmospheric carbon dioxide, the system comprising: a container comprising air-permeable sidewalls; and a plurality of agglomerates comprising calcium hydroxide (Ca(OH)2) particles within the container. In an embodiment, the system for performing the removal of atmospheric carbon dioxide, comprises: a container comprising air-permeable sidewalls; and a plurality of agglomerates of calcium hydroxide (Ca(OH)2) particles within the container. A collection of particles together may be referred to as a powder. The term “agglomerate” refers to a collection of particles that are associated with one another via bonds which are enough to exceed subsequent forces which may act to break these bonds. This creates larger assemblage of particles, specifically, joined at the corners and edges of particles so that the total surface of the agglomerate is equal to the sum of the surfaces of the individual particles. The bonds between particles can be created through physical or chemical forces either inherent in the particle or forced through some process such as compression, or with using binders which act to create bridges between particles. It is documented that Ca(OH)2 particles tend to agglomerate when dry, but enhanced by the addition of water. This means their agglomeration can be a result of both inherent forces of the particle (due to its cohesivity) and by creating liquid bridges between particles. The term “plurality of agglomerates” and “plurality of agglomerated particles” indicates that there are many agglomerates present, which may be of different shapes and sizes. The term “air-permeable sidewalls” means that at least one wall or surface of the container allows air to pass through it. The purpose of the air-permeable sidewall is so that air from the atmosphere may pass into the container and contact the calcium hydroxide material contained within the container. Structure of the container The person skilled in the art will appreciate that the container may be any shape that is suitable for holding a plurality of agglomerated calcium hydroxide (Ca(OH)2) particles within it and allow air from the environment outside the container to pass into the container. On contact with the calcium hydroxide material, CO2within the air may react with the calcium hydroxide in a carbonation reaction to form calcium carbonate (CaCO3). The container may be a shape which maximises the amount of agglomerates that may be in contact with the air. For example, the skilled person will appreciate that the sides of the container may be corrugated in order to increase the surface area of the container that may be in contact with the air. Any other means known to the person skilled in the art to increase the surface area of the container may be used. In an embodiment, the container may comprise only one sidewall. Accordingly, this sidewall will be an air-permeable sidewall. As used herein, a container comprising air-permeable sidewalls means that the container comprises at least one air-permeable sidewall, for example at least two air- permeable sidewalls, at least three air-permeable sidewalls, at least four air-permeable sidewalls, at least five air-permeable sidewalls, at least six air-permeable sidewalls, at least seven air-permeable sidewalls, or at least eight air-permeable sidewalls. All sidewalls of the container may be an air-permeable sidewall. All sidewalls of the container apart from one wall may be an air-permeable sidewall. In an embodiment the container may comprises more than one sidewall (e.g., when the container is not a sphere). For example, the container may be a cube shape. Any one or more of the faces may be an air-permeable sidewall. For a cylinder (which is formed of three faces, two circular and one rectangular), the rectangular face may be an air-permeable side wall. Preferably, the rectangular face and one of the circular faces may be an air-permeable side wall. Preferably, the rectangular face and two of the circular faces may be an air-permeable side wall. When the shape of the container is such that the length in one axis is substantially greater than in a second axis (for example, a cuboid or a cylinder) additional supports may be required in order to stabilise the structure. The overall dimensions of the container may be dictated by engineering limitations. By way of example, when the container is a cuboid structure, when the height of the container is increased, the container may require additional supporting beams in order to hold the container upright. Alternatively, or additionally, the container may also require foundations in order to distribute the weight of the container across the face which is in contact with the ground. The foundations and / or supporting structures may be removable, such that the container may be moved to a different location. This may be beneficial to allow the container to be filled / emptied of agglomerate material at a different location to where the CDR is taking place. The container may also be comprised of multiple (e.g., more than one) smaller containers in a modular fashion. This design may be favoured when it is desirable to stack or move the container. Accordingly, in an embodiment, the system for the removal of atmospheric carbon dioxide comprises: at least one container comprising air-permeable sidewalls; and a plurality of agglomerates comprising calcium hydroxide (Ca(OH)2) particles within the container. Suitably, the system for the removal of atmospheric carbon dioxide comprises at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11, at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19 or at least 20 containers. In a particular embodiment, when more that one container is present, the containers may be bundled together, as demonstrated in Figures 4a and 4b. The skilled person will appreciate that containers of the same or different shapes may be bundled together. Characterisation of the agglomerates Compared to the corresponding particles (i.e., the particles that make up the agglomerates) agglomerates are larger with a higher bulk density. Agglomerates have a higher bulk density as the individual particles that make up the granule can pack more efficiently compared to the corresponding particles and thus, there is more Ca(OH)2in an agglomerate compared to a sample of powder of the same volume. Without wishing to be bound by theory, when the particles / agglomerates are all taken to be equal volume spheres, the bulk density of agglomerates in a defined container, will be higher than the bulk density of the particles. In an embodiment, the agglomerates comprising Ca(OH)2 particles have a bulk density of greater than 0.5 g / cm3. Preferably, the bulk density is greater than 0.5 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising Ca(OH)2 particles have a bulk density of greater than 0.6 g / cm3. Preferably, the bulk density is greater than 0.6 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising Ca(OH)2 particles have a bulk density of greater than 0.7 g / cm3. Preferably, the bulk density is greater than 0.7 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising Ca(OH)2particles have a bulk density of greater than 0.8 g / cm3. Preferably, the bulk density is greater than 0.8 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising Ca(OH)2particles have a bulk density of greater than 0.9 g / cm3. Preferably, the bulk density is greater than 0.9 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising Ca(OH)2particles have a bulk density of greater than 1.0 g / cm3. Preferably, the bulk density is greater than 1.0 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising Ca(OH)2particles have a bulk density of greater than 1.2 g / cm3. Preferably, the bulk density is greater than 1.2 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising Ca(OH)2 particles have a bulk density of greater than 1.3 g / cm3. Preferably, the bulk density is greater than 1.3 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates comprising calcium hydroxide particles have a median particle size distribution of greater than 10 μm. Preferably, the median particle size distribution is greater than 10 μm when measured using sieve analysis. As used herein, the term “median particle size distribution” is the particle size at which half of the particles are smaller and half are larger. In an embodiment, the agglomerates comprising calcium hydroxide particles have a median particle size distribution of greater than 100 μm. Preferably, the median particle size distribution is greater than 100 μm when measured using sieve analysis. In an embodiment, the agglomerates comprising calcium hydroxide particles have a median particle size distribution of greater than 250 μm. Preferably, the median particle size distribution is greater than 250 μm when measured using sieve analysis. In an embodiment, the agglomerates comprising calcium hydroxide particles have a void space fraction of less than 50%, when measured using the following equation: ^^^^ ^^^^^ ^^^^^^^^ = ^^^^^ ^^^^ ^^^^^^^^^^^ ^^^^^^^^ ^^^^^^In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which have a surface area of greater than 20 m2 / g. Preferably, the surface is greater than 20 m2 / g when measured using N2 BET porosimetry. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which have a surface area of greater than 25 m2 / g. Preferably, the surface is greater than 25 m2 / g when measured using N2 BET porosimetry. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which have a surface area of greater than 30 m2 / g. Preferably, the surface is greater than 30 m2 / g when measured using N2 BET porosimetry. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which is formed from high surface area lime. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which is formed from flash calcined lime. As used herein, “flash calcined lime” refers to lime (CaO) formed in a flash calcination process. Flash calcined lime typically has a higher surface area than lime formed from a non-flash calcination process. In a preferred embodiment, CaO used in the process for forming Ca(OH)2 is low carbon lime, preferably zero carbon lime (e.g., carbon neutral lime). This means that the CDR process of the present invention is a carbon neutral or carbon negative process. CaO is formed in the following calcination process: CaCO3^^Ʃ^ĺ CaO + CO2Accordingly, for the CDR process of the present invention to be carbon neutral or carbon negative the CO2produced in the manufacture of CaO should be less than or equal to the CO2captured by the CDR process using the Ca(OH)2agglomerates. Calcium hydroxide is formed from the hydration of CaO. The calcium hydroxide used in the present invention may be formed from calcium oxide which has a surface area of greater than 5 m2 / g, preferably, 10 m2 / g, more preferably 15 m2 / g. Preferably the surface area is greater than 5 m2 / g, preferably, 10 m2 / g, more preferably 15 m2 / g when measured using N2 BET porosimetry. In an embodiment, the agglomerates comprising calcium hydroxide particles (which may be referred to simply as the “agglomerates”) may be substantially spherical (or referred to as spherical). Thus, in a particular embodiment, the agglomerates of the invention may have an aspect ratio (or an average aspect ratio) of about 1:1. In further embodiments, the agglomerates of the invention may be of amorphous shape. The skilled person will understand that the term mean particle size, as used herein, will refer to the mean diameter of the agglomerates at the greatest point thereof (e.g. in the case of non-spherical particles, the longest length thereof; or in the case spherical particles, the diameter thereof.), which may be measured using techniques well-known to those skilled in the art. For example, sieve analysis or visual image processing. The agglomerates may be separated into size fractions via a series of stainless-steel sieves on a mechanical shaker with different mesh sizes. For example, the size of the holes in the mesh may be 5.6, 4.75, 3.35, 2 and 0.85 mm. In particular embodiments, such as those in which the agglomerates are spherical, the size of agglomerates may be defined by reference to the diameter thereof. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 9 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 8 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 7 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 6 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.75 mm to 9 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.75 mm to 8 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.75 mm to 6 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.75 mm to 9 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.75 mm to 8 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm, and at least 50% of the materials have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight has a size distribution of from 0.5 mm to 6 mm, and at least 50% of the materials has a size distribution of from 1 mm to 4 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight has a size distribution of from 0.5 mm to 6 mm, and at least 50% of the materials has a size distribution of from 1 mm to 3 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 0.5 mm to about 9 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 0.5 mm to about 6 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 0.75 mm to about 6 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 1 mm to about 7 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 2 mm to about 6 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 3 mm to about 5 mm. For the avoidance of doubt, as used herein, the term “agglomerate material” refers to the agglomerates comprising calcium hydroxide particles. The agglomerates used in the present invention comprise water. Without wishing to be bound by theory, the presence of water in the agglomerates is thought the catalyse the reaction with CO2. This is because the carbonation reaction is thought to proceed in a water microlayer on the surface of the agglomerate where dissolved CO2from the air is able to react with dissolved calcium ions from the agglomerate surface. Arrangement of the agglomerates within the container It is important that air flow within the container does not become rate limiting. The air flow is important to deliver CO2 to the reactive surface of the Ca(OH)2 agglomerates. When larger agglomerates or particles are used, there are larger air spaces in between each agglomerate / particle. This means that air may move through the agglomerates more easily. When agglomerates are packed together, for example in a container, the greater the depth of the material the more tortuous the air flow path. This is because the air has to flow over a greater number of agglomerates in order to reach the agglomerates further from the source of air. Therefore, when the depth of material is greater, there is a greater the inhibition of the air flow and so a limited rate of reaction. In a preferred embodiment, the agglomerates are arranged within the container to intentionally increase airflow through the aggregates. Increasing the airflow throughout the aggregates, particularly while exposing the aggregate to an atmosphere comprising CO2, increases the rate of carbonation. Once pulled from the atmosphere, permanent storage would be required to stop CO2leaking back into the environment. Carbonation of the agglomerates may be used to store this CO2. In an embodiment, the plurality of agglomerates are arranged within the contain such that the size of the agglomerates decreases from the outer portion of the container to the inner portion of the container. In an embodiment, agglomerates of a plurality of sizes may be packed randomly within the container. In an embodiment, the agglomerates may be packed within the container to ensure a maximum packing density to ensure the most efficient use of the space with minimal diffusion barriers due to reduced or low airflow over the agglomerates. The agglomerates may be arranged inside the container so that larger agglomerates (with larger interstitial spaces) are present in the outer portion of the container and smaller agglomerates are nearer to the centre of the container. By separating the sizes of agglomerates, the packing density is reduced, and the slower reacting larger agglomerates being aided by a greater exposure to air from the atmosphere outside of the container. The outer portion of the container refers to the area of the container that is closest to the air-permeable barrier, such that the agglomerates are closer to the air containing CO2. The inner portion of the container refers to the area of the container furthest from the air-permeable barrier. In such an embodiment, there may be two categories of agglomerate sizes: large and small. The large and small categories may be identified to be agglomerates which fall within a predetermined particles size range. In an embodiment, there may be three categories or more of predetermined agglomerates sizes (e.g., four, five, six, seven, eight, nine or ten). When there are more than two categories or agglomerate sizes arranged within the container, it may be described that there is a gradient of agglomerate sizes within the container. That is, the larger particles are present in the outer portion of the container with the agglomerate size gradually getting smaller towards the centre of the container. The arrangement of the agglomerates within the container may be controlled by the order in which the agglomerates are added into the container (e.g., small particles first, followed by gradually larger particles). The method of packing the agglomerates may be tailored to be most effective depending on the shape of the container. Alternatively, or additionally, the container may comprise one or more barrier(s) to control the distribution of agglomerates throughout the container. The skilled person will appreciate that the barrier(s) should also be made of an air-permeable material. The barrier(s) should not unduly impact the flow of air through the container. The air-permeable material The purpose of the air-permeable material that is present as one or more of the container sidewalls, as well as that present as an internal barrier, is to allow air to flow into the container while preventing the loss of the agglomerate from the container. Accordingly, the air-permeable material typically has an aperture size of from 0.4mm to 0.7mm. The skilled person will appreciate that the air-permeable material may be made of any material that does not react with the atmosphere (e.g., air) or corrode on contact with calcium hydroxide, calcium carbonate, or any other chemical composition that would be formed as an intermediate or otherwise during the contact of calcium hydroxide with ambient air. Such a material may include a corrosive material coated with an inert material. In an embodiment, the air-permeable material is a mesh. Examples of mesh include a polymeric mesh, a steel mesh or a stainless steel mesh. The mesh size will be chosen to retain all of the agglomerate material within it. In an embodiment, the air-permeable material may be hydrophilic so as to pull moisture into the container from the outside atmosphere. Any such material should preferably be selected with an appropriate carbon footprint, so that any CO2captured using the system of the present invention is not negated by the air-permeable material. In an embodiment, the air-permeable material may have a geometry such that the air is channelled into the structure of the container. This may be done, for example, by utilising the venturi effect. Humidification system As already discussed, water that is present in the agglomerates is thought to act as a catalyst to the ambient carbonation reaction. This is turn leads to a higher rate and extend of reaction. Accordingly, it is therefore desirable to control the humidity of the environment of the container comprising the Ca(OH)2agglomerates. It will be appreciated that attempting to regulate the relative humidity of large spaces can be energy intensive. Accordingly, it is advantageous that moisture is retained in the Ca(OH)2 agglomerates that are used in the system of the present invention. In an embodiment, the system of the present invention therefore comprises a humidification system. This humidification system may alternatively be described as a localised humidification system. The purpose of the humidification system is to ensure that an appropriate amount of moisture is present in the agglomerate. That is, the humidification system replaces the water that was present in the agglomerates as a result of the manufacturing process that may evaporate over time. In an embodiment, the humidification system may be a passive humidification system. As used herein, “passive humidification system” means that the humidification does not require assistance from electrical or other non-renewable energy sources. In an embodiment, the system of the present invention comprises a water storage system which acts as the humidification system. The water storage system may be a source of water that is in close enough proximity to at least a portion of the agglomerates held within the container such that when the water evaporates it contacts a portion of the agglomerates. In an embodiment, the water storage system may be an evaporation tray located at the bottom of the container. The evaporation tray may comprise a medium with a high surface area to promote evaporation. For example, the evaporation tray may be a shallow tray comprising pebbles and / or a porous plastic or metallic foam. In an embodiment, the humidification system may be an internal column comprising water-permeable walls. In an embodiment the internal column comprises water-permeable walls, wherein the water-permeable walls allow water to move from within the column to within the container so as to contact the agglomerates contained within. In an embodiment, the column may run through the entire length of the container. In an embodiment, there is an opening at the top and bottom of the container to allow water to pass through. As the water passes through the column, water contacts the water-permeable walls of the column. In an alternative embodiment, the column only passes part way through the container. In this embodiment, there is only one opening to allow water into the column. This opening is preferably at the top of the container. The closed bottom of the column may then act as a water storage system (e.g., an evaporation tray). Advantageously, this embodiment comprises two ways of water reaching the agglomerates (i) by moving through the water-permeable walls as water first enters the column; and (ii) when excess water stored in the water storage system evaporates and travels up into the container. The water may evaporate directly into the container and contact the agglomerates and / or may evaporate into the column and through the water-permeable walls of the column. The water-permeable walls of the column are made of a material that allows water to pass into the container where the agglomerates are held. In an embodiment, the container may be clad in a material that may be wetted by rainwater or otherwise. Preferably, the internal column is clad in the wet material so as not to adversely affect the air flow from the external environment to within the container. In an embodiment, the water-permeable material may be a meshed material. For example, the water permeable material may be the same material as the air-permeable material used for the external container sidewalls. In an embodiment, the water permeable material may be a textile material or a ceramic material. In an embodiment, the humidification system comprises a means for directly contacting water to a portion of the agglomerates, such as a pouring, sprinkling, spraying and / or misting water onto the agglomerates. In an embodiment, the water utilised in the humidification system is rainwater. The system of the present invention is therefore arranged such that rainwater may enter the humidification system, when the system for performing the removal of atmospheric carbon dioxide is placed outside. Mechanisms for increasing airflow As has already been mentioned, it can be beneficial to increase the airflow throughout the aggregate. In addition to, or instead of, the arrangement (e.g., packing) of the agglomerates within the container, this may be achieved by structural features present in the container. In an embodiment, the container comprises one or more mechanism(s) for increasing airflow through the container (e.g., channels through the container and / or wind tunnels on the surface of the air permeable sidewalls). In an embodiment, the container comprises channels through the container. In an embodiment, the channels may be horizontal channels with walls made from the same air-permeable material used on the sidewalls of the container. In an embodiment, when the container is comprised of more than one module (e.g., comprised of more than one container), the channels may be formed from gaps between stacked modules. In an embodiment, the container may comprise channels which are stacked as a spiral throughout the column. Advantageously, these channels may prevent breakage of material when it is loaded from the top and will evenly distribute the aeration channels. While one of more channels present in the container may aid in the airflow throughout the container in order to increase the carbonation of the agglomerates, such channels may also the wind resistance experienced by the container. This will be particularly important for larger containers. In an embodiment, the mechanism for increasing airflow may be a passive system. As used herein, a passive mechanism means that the means for increasing airflow does not require assistance from electrical or other non-renewable energy sources. Channels and wind tunnels are examples of passive systems. In an embodiment, the mechanism for increasing airflow may comprise a mechanical means. As used herein, mechanical means is used to refer to a non-passive system. For instance, the system may comprise one or more fan, air-suction means or vacuum in order to force air through the container. These systems may require assistance from electrical or other non-renewable energy sources. In an embodiment, the mechanism for increasing airflow may comprise a combination of passive and non-passive systems. A method for the removal of atmospheric CO2A second aspect of the invention is a method of the removal of atmospheric carbon dioxide, the method comprising the carbonation of agglomerates comprising calcium hydroxide (Ca(OH)2) particles. In an embodiment, the agglomerates have a bulk density of greater than 1.0 g / cm3. Preferably, the bulk density is greater than 1.0 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates have a bulk density of greater than 1.2 g / cm3. Preferably, the bulk density is greater than 1.2 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates have a bulk density of greater than 1.3 g / cm3. Preferably, the bulk density is greater than 1.3 g / cm3when measured by mercury porosimetry at 0.32 psia. In an embodiment, the agglomerates have a median particle size distribution of greater than 10 μm. Preferably, the median particle size distribution is greater than 10 μm when measured using sieve analysis. In an embodiment, the agglomerates have a median particle size dis tribution of greater than 100 μm. Preferably, the median particle size distribution is greater than 100 μm when measured using sieve analysis. In an embodiment, the agglomerates have a median particle size distribution of greater than 250 μm. Preferably, the median particle size distribution is greater than 250 μm when measured using sieve analysis. In an embodiment, the agglomerates comprising calcium hydroxide particles have a void space fraction of less than 50%. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which have a surface area of greater than 20 m2 / g. Preferably, the surface is greater than 20 m2 / g when measured using N2 BET porosimetry. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which have a surface area of greater than 25 m2 / g. Preferably, the surface is greater than 25 m2 / g when measured using N2 BET porosimetry. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which have a surface area of greater than 30 m2 / g. Preferably, the surface is greater than 30 m2 / g when measured using N2 BET porosimetry. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which is formed from high surface area lime. In an embodiment, the agglomerates comprising calcium hydroxide are formed from particles of calcium hydroxide which is formed from flash calcined lime. In an embodiment, the calcium hydroxide which forms the agglomerates is formed from calcium oxide which has a surface area of greater than 5 m2 / g, preferably greater than 10 m2 / g, more preferably greater than greater than 15 m2 / g. Preferably the surface area is greater than 5 m2 / g, preferably, 10 m2 / g, more preferably 15 m2 / g, when measured when measured using N2 BET porosimetry. In particular embodiments, such as those in which the agglomerates are spherical, the size of agglomerates may be defined by reference to the diameter thereof. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 9 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 8 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 7 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm. In a more particular embodiment, at least 50% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 9 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 8 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 7 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 6 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.75 mm to 9 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.75 mm to 8 mm. In a more particular embodiment, at least 95% of the agglomerate material by weight have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 75% of the agglomerate material by weight have a particle size of from 0.5 mm to 6 mm, and at least 50% of the materials have a particle size of from 0.75 mm to 4 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight has a size distribution of from 0.5 mm to 6 mm, and at least 50% of the materials has a size distribution of from 1 mm to 4 mm. In a more particular embodiment, at least 85% of the agglomerate material by weight has a has a size distribution of from 0.5 mm to 6 mm, and at least 50% of the materials has a size distribution of from 1 mm to 3 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 0.5 mm to about 9 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 0.5 mm to about 6 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 0.75 mm to about 6 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 1 mm to about 7 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 2 mm to about 6 mm. In a more particular embodiment, the agglomerate material has a mean particle size distribution of from about 3 mm to about 5 mm. A method of forming calcium hydroxide agglomerates The addition of water to a powder for agglomeration purpose can be referred to as a wet agglomeration technique. Wet agglomeration involves the addition of a binding liquid (e.g. water) to the powder, this mixture is then agitated resulting in agglomerates which are then usually dried. In an aspect of the invention, there is a method of forming calcium hydroxide agglomerates for the removal of atmospheric CO2, the method comprising: a. combining a first mixture comprising a powder particles of calcium hydroxide with a second mixture comprising water to form a third mixture b. agitating the third mixture in order to bind the powder particles together into agglomerates of calcium hydroxide c. optionally, adding more water to the agitated third mixture if non- agglomerated calcium hydroxide remains. In an embodiment, the method further comprises the step (d) of adding more calcium hydroxide to the agitated third mixture. Preferably, the calcium hydroxide is added as a light dusting to the agitated third mixture. Advantageously, this step is carried out in order to prevent the agglomerates sticking together and to make the agglomerates easier to handle and / or process. In an embodiment, the method further comprises the step (e) drying the agglomerates. In a particular embodiment, the agglomerates are dried prior to being put into the container comprising air-permeable sidewalls. In a more particular embodiment, the agglomerates may be air dried at room temperature or dried in a drying oven. In an embodiment, amount of water in the third mixture of step a. is greater than 1% by weight, preferably greater than 5% by weight, more preferably greater than 10% by weight of the total weight of the third mixture. A final “dusting layer” of dry powder can be added to the mixture to achieve free flowing and fully separated agglomerates. In a preferred embodiment, the second mixture comprising water, as described in step a), is rainwater. Examples Example 1: Method of making calcium hydroxide agglomerates 1. 2 kg of Ca(OH)2powder was added to a R02 Laboratory Mixer vessel. 2. Water was added to the Ca(OH)2 powder over a period of 40 seconds at a mixing speed of 75 Hz. 3. The mixture formed in step 2 was allowed to mix for a further 20 seconds for a total duration at 75 Hz of 60 seconds. 4. The mixer speed was then dropped to 30 Hz and mixing was continued for 60 seconds. 5. The mixer speed was then dropped to 10 Hz and at the same time a final 200g of Ca(OH)2was added as a drying / dusting layer to free up the granules by giving them a dry surface. 6. This was mixed for a final 60 seconds at 10 Hz in total. Example 2a: Characterisation of calcium hydroxide agglomerates The particle size distribution of the agglomerates formed as described in example 1 was measured. The particle size distribution of agglomerated Ca(OH)2 was performed using sieve analysis. The results are shown in Figure 2. In order to demonstrate that agglomerates were formed, the particle size distribution of Ca(OH)2 powder was performed used a Malvern Mastersizer. The results are shown in Figure 1. Figure 1 shows the mean particle size of Ca(OH)2powder to be ~2 microns whereas Figure 2 shows the mean particle size of agglomerates to be 2 mm. This highlights the main difference in Ca(OH)2 before and after agglomeration. 2b: Further characterisation of calcium Further characterisation of calcium hydroxide agglomerates was carried out using sieve analysis. This characterisation is shown in Figure 5. 3a: Use of agglomerates in the removal of CO2The Ca(OH)2 agglomerates were separated into three size ranges, based on particle size distribution: x Small: >0.75 mm to <1 mm x Medium: >1 mm to <2 mm x Large: >2 mm to <3.35 mm The Ca(OH)2 powder had a mean particle size distribution of 2 μm. The reaction benefits of the agglomerates of the three size fractions were tested. These results are shown in Figure 3. Figure 3 shows the carbonation reaction extent of each agglomerate size and Ca(OH)2powder. The three size fractions of agglomerates achieve a higher reaction extent (>10%) at a higher rate than the powdered Ca(OH)2. Reaction extent was measured using thermogravimetric analysis (TGA). In the present example, TGA was carried out using the following method: 1. The moisture in a sample is calculated using a moisture balance. 2. A tared crucible is filled with ~2 g of material and the mass recorded. 3. The crucible is placed in an oven and heated to 520 °C for 4 hours then allowed to cool. 4. The mass of the crucible and material is recorded. 5. The crucible is then heated to 1000 °C for 8 hours then allowed to cool. 6. The mass of the crucible and material is recorded. From the change of mass at each step, the mass ratio of CaO, Ca(OH)2, CaCO3 and water can be determined. This is then converted to a “carbonation extent”, that is, the amount of CaO or Ca(OH)2that has reacted with CO2to form CaCO3. Example 3b: Use of agglomerates in the removal of atmospheric CO2 Further tests were carried out on agglomerates as formed according to Example 1. Ca(OH)2 agglomerates of five different size ranges were tested: x 0.5 to 2.0mm x 2.0 to 4.0mm x 4.0 to 5.0mm x 5.0 to 9.0mm x > 9.0 mm The results are shown in Figure 6. Here it can be seen that the rate of reaction of Ca(OH)2 agglomerates with a mean particle size of up to 9mm reach the final carbonation extent quickly (e.g., within around 20 days), compared to agglomerates with a mean particle size of greater than 9mm.

Claims

Claims 1. A system for the removal of atmospheric carbon dioxide, the system comprising: a container comprising air-permeable sidewalls; and a plurality of agglomerates comprising calcium hydroxide (Ca(OH)2) particles within the container.

2. A system according to claim 1, wherein the agglomerates have a median particle size distribution of greater than 10 μm.

3. A system according to any one of the preceding claims, wherein the agglomerates have a median particle size distribution of greater than 100 μm.

4. A system according to any one of the preceding claims, wherein the agglomerates have a median particle size distribution of greater than 250 μm.

5. A system according to any one of the preceding claims, wherein the agglomerates have a void space fraction of less than 50%.

6. A system according to any one of the preceding claims, wherein the agglomerates have a bulk density of greater than 0.5 g / cm3.

7. A system according to any one of the preceding claims, wherein the agglomerates have a bulk density of greater than 0.7 g / cm3.

8. A system according to any one of the preceding claims, wherein the agglomerates have a bulk density of greater than 1.0 g / cm3.

9. A system according to any one of the preceding claims, wherein the agglomerates have a bulk density of greater than 1.2 g / cm3.

10. A system according to any one of the preceding claims, wherein the agglomerates have a bulk density of greater than 1.3 g / cm3.

11. A system according to any one of the preceding claims wherein the calcium hydroxide particles have a surface area of greater than 20 m2 / g.

12. A system according to any one of the preceding claims, wherein the calcium hydroxide particles have a surface area of greater than 25 m2 / g.

13. A system according to any one of the preceding claims, wherein the calcium hydroxide particles have a surface area of greater than 30 m2 / g.

14. A system according to any one of the preceding claims, wherein the calcium hydroxide is formed from high surface area lime.

15. A system according to any one of the preceding claims, wherein the calcium hydroxide is formed from calcium oxide which has a surface area of greater than 5 m2 / g.

16. A system according to any one of the preceding claims, wherein the calcium hydroxide is formed from calcium oxide which has a surface area of greater than 10 m2 / g.

17. A system according to any one of the preceding claims, wherein the calcium hydroxide is formed from calcium oxide which has a surface area of greater than 15 m2 / g.

18. A system according to any one of the preceding claims, wherein the at least 50% of the agglomerates have a particle size of from 0.5 mm to 9 mm.

19. A system according to any one of the preceding claims, wherein the at least 50% of the agglomerates have a particle size of from 0.5 mm to 6 mm.

20. A system according to any one of the preceding claims, wherein at least 50% of the agglomerates have a particle size of from 0.75 mm to 6 mm.

21. A system according to any one of the preceding claims, wherein at least 50% of the agglomerates have a particle size of from 0.75 mm to 4 mm.

22. A system according to any one of the preceding claims, wherein at least 75% of the agglomerates have a particle size of from 0.5 mm to 6 mm.

23. A system according to any one of the preceding claims wherein the plurality of agglomerates are arranged within the container such that the size of the agglomerates decreases from the outer portion of the container to the inner portion of the container.

24. A system according to any one of the preceding claims wherein the system comprises a humidification system.

25. A system according any one of the preceding claims, wherein the container comprises one or more mechanism(s) for increasing airflow through the container (e.g., channels through the container and / or wind tunnels on the surface of the air permeable sidewalls).

26. A method of the capture of atmospheric carbon dioxide, the method comprising the carbonation of agglomerates comprising calcium hydroxide (Ca(OH)2) particles.

27. A method according to claim 26, wherein the agglomerates have a median particle size distribution of greater than 10 μm, preferably greater than 100 μm, more preferably greater than 250 μm.

28. A method according to claim 26 or claim 27, wherein the agglomerates have a void space fraction of less than 50%.

29. A method according to any one of claims 26 to 28, wherein the agglomerates have a bulk density of greater than 1 g / cm3, preferably greater than 1.2 g / cm3, more preferably greater than 1.3 g / cm3.

30. A method according to any one of claims 26 to 29, wherein the calcium hydroxide particles have a surface area of greater than 20 m2 / g, preferably greater than 25 m2 / g, more preferably greater than 30 m2 / g.

31. A method according to any one of claims 26 to 30, wherein the calcium hydroxide particles are formed from high surface area lime.

32. A method according to any one of claims 26 to 31, wherein the calcium hydroxide particles are formed from calcium oxide which has a surface area of greater than 5 m2 / g, preferably greater than 10 m2 / g, more preferably greater than greater than 15 m2 / g.

33. A method according to any one of claims 26 to 32, wherein the agglomerates have a moisture content of greater than 1% by weight, preferably greater than 5% by weight, more preferably greater than 10% by weight.

34. A method according to any one of claims 26 to 33, wherein at least 50% of the agglomerates have a particle size of from 0.5 mm to 6 mm.

35. A method according to any one of claims 26 to 34, wherein at least 50% of the agglomerates have a particle size of from 0.75 mm to 4 mm.

36. A method according to any one of claims 26 to 35, wherein at least 75% of the agglomerates have a particle size of from 0.5 mm to 6 mm.

37. A method of the removal of atmospheric carbon dioxide, the method comprising a step of using the system as described in any one of claims 1 to 25.

38. Use of the system as described in any one of claims 1 to 25 for the removal of atmospheric carbon dioxide.

39. A method of forming agglomerates comprising calcium hydroxide (Ca(OH)2) particles for the removal of atmospheric carbon dioxide, the method comprising: a. combining a first mixture comprising powdered particles of calcium hydroxide with a second mixture comprising water to form a third mixture b. agitating the third mixture to bind the powder particles together into agglomerates of calcium hydroxide c. optionally, adding more water to the agitated third mixture if non- agglomerated calcium hydroxide remains.

40. A method according to claim 39, wherein the method further comprises a step of adding more calcium hydroxide to the agitated third mixture.

41. A method according to claim 39 or claim 40, wherein the method further comprises a step of drying the agglomerates.

42. A method according to any one of claims 39 to 41 wherein the amount of water in the third mixture of step a. is greater than 1% by weight, preferably greater than 5% by weight, more preferably greater than 10% by weight of the total weight of the third mixture.

43. A method according to any one of claims 39 to 42 wherein the second mixture comprising water is rainwater.

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