EP4648882A2

EP4648882A2 - Process for capturing carbon dioxide and composite material used therein

Info

Publication number: EP4648882A2
Application number: EP24703859.9A
Authority: EP
Inventors: Bruno VODOPIVEC
Original assignee: Fervo Srl
Current assignee: Fervo Srl
Priority date: 2023-01-10
Filing date: 2024-01-10
Publication date: 2025-11-19
Also published as: WO2024150151A2; WO2024150151A3; IT202300000171A1

Abstract

The invention relates to a process for capturing CO₂ comprising the step of contacting a gaseous stream comprising CO₂ with a composite material comprising a carbonaceous solid substrate and a reactive material comprising particles of potassium carbonate (K₂CO₃), the composite material having a surface area of 300 m²/g or greater, the gaseous stream having a CO₂ content of 200 to 1000 ppm and a relative humidity of 10 to 90%. The invention further relates to a composite material comprising a carbonaceous solid substrate and a reactive material comprising particles of K₂CO₃, the carbonaceous solid substrate having a surface area of 300 m²/g or greater. The invention also relates to a process for the preparation of a composite material and a process for the preparation of a carbonaceous solid substrate and to the composite material and the carbonaceous solid substrate obtainable through the respective processes.

Description

PROCESS FOR CAPTURING CARBON DIOXIDE AND COMPOSITE

MATERIAL USED THEREIN

The present invention generally refers to the environment l engineering sector, in particular to processes for the capture of environmental carbon dioxide .

In particular, the invention concerns a process for the capture of environmental carbon dioxide through the use of potassium carbonate. The invention also concerns a composite material comprising potassium carbonate for the capture of carbon dioxide.

As is known, international scientific authorities agree on the fact that the climate change that has been occurring for several decades is associated with the accumulation of carbon dioxide in the atmosphere.

The main known methods for capturing atmospheric carbon dioxide fall into two categories: physical absorption, i.e. trapping CO2 in porous materials, for example zeolites or porous membranes; and chemical absorption, in which CO2 is captured thanks to a chemical reaction with a specific substrate.

For example, a CO2 separation technology is known that uses a gas-selective membrane, as described in EP 2 435 167 (Corning Inc) which describes a method for producing a hybrid composition membrane which comprises the steps of preparing a sol comprising at least one poly ( amino-alcohol ) , and at least one aminoalkyl alkoxysilane; casting the sol onto a surface, and drying the sol. However, this technology does not guarantee good quality of the extracted gas and requires an entering gas having a low partial pressure of CO₂.

A known alternative technology to gas-selective membranes is based on chemical processes of absorption of CO2 in the liquid phase, which mainly exploit the acidic characteristic of carbon dioxide. The absorbent molecules can be organic amines as described in EP 2 514 509 (Toshiba KK) , which describes a CO2 capturing system comprising a CO2 capturing module in which a CCg-cont aining gas comes into contact with an amine- containing solution which captures CO2. However, this system poses the problem of the environmental dispersion of amines, which have a medium-high toxicity, and therefore the need to guarantee perfect confinement of the vapors, which can be problematic considering the high temperatures required by the process .

Ionic liquids have recently been introduced for the continuous absorption of carbon dioxide. These are organic salts in the liquid phase, and some of them have a specific reactivity with carbon dioxide through the formation of carbamic acid or by dipolar interaction. See, for example, WO 2012/033991 (ExxonMobil Research and Engineering Co) which describes a cyclic process for separating CO2 from a gas stream, which comprises the steps of contacting the gas stream with a sorbent comprising a solution of an amine sorbent in an ionic liquid to absorb CO2 in solution so as to produce an enriched solution containing absorbed CO2, and treating the enriched solution under conditions sufficient to cause desorption of at least a portion of the CO2 and regenerating the amine. However, the known ionic liquids used for carbon dioxide capture are typically complex molecules, so much so that their synthesis is expensive, just as their use is not very attractive for an industrial application. Additionally, many ionic liquids have high viscosities, which limits their efficiency .

Like gas-selective membranes having amines as absorbent molecules, ionic liquids can also be heated, promoting the release of CO2 previously absorbed by carbamic acid. However, such process requires considerable energy consumption. It follows that the stripping process requires the introduction of heat to help cleave the bond between the carbonyl carbon and the nitrogen of the carbamate. Conversely, in the absorption step, the carbamate formation reaction produces heat which, if not removed from the system by an appropriate cooling system, reduces the efficiency of the reaction.

In this context, systems are also known that exploit the combination of a basic organic molecule capable of absorbing CO2 with a non-polar solvent. In fact, the presence of non-polar molecules mixed with some types of absorbent molecules favors the release of carbon dioxide. This phenomenon is caused by the change in polarity of the absorbing molecule bound to CO2 compared to the free state, i.e. the initial state in the original form not bound to CO2. This phenomenon is known as polarity swing-assisted regeneration (PSAR) and is described for example in the patent WO 2013/032565 (Battelle Memorial Institute) , which describes a PSAR process characterized by the steps of contacting a switchable ionic liquid with an antisolvent at a selected desorption temperature to release at least one chemically bound gas from the aforementioned liquid, thus regenerating the liquid. However, such system involves the very use of a nonpolar solvent (antisolvent) , which is a non- biodegradable hydrocarbon.

Furthermore, the capture of carbon dioxide from the air is a process that is limited by the low concentration of this gas in the air, typically of around 500 ppm, which therefore involves the treatment of large volumes of air with consequent high energy expenditure .

The technical problem underlying the present invention is therefore to provide a process for the capture of CO2 through chemical reaction with a reactive material, which overcomes the drawbacks of the prior art, in particular which is particularly efficient and capable of capturing considerable quantities of CO2 per gram of reactive material.

A further technical problem underlying the present invention is that of providing such a process that has long-lasting efficiency.

Such problem has been solved according to the invention by a process for capturing CO2 comprising the step of contacting a gaseous stream comprising CO2 with a composite material comprising a carbonaceous solid substrate and a reactive material comprising particles of potassium carbonate (K2CO3) , said composite material having a surface area of 300 m²/g or greater, said gaseous stream having a CO2 content from 200 to 1000 ppm and a relative humidity from 10 to 90%.

The process of the invention is therefore a process of direct chemical capture of CO2 through a heterogeneous phase reaction, which occurs at the solid-gas interface.

Preferably, the carbonaceous solid substrate is a porous carbonaceous solid substrate.

Preferably, the composite material is a porous composite material.

Preferably, in such a composite material, the particles of reactive material are distributed on the surface of the carbonaceous solid substrate. In particular, the particles of reactive material are distributed uniformly on the surface of the carbonaceous solid substrate, also inside the pores.

The carbonaceous solid substrate can be combined with the reactive material by producing a suspension of the carbonaceous substrate in an aqueous solution of potassium carbonate, followed by filtration and drying of the carbonaceous substrate on which the K2CO3 particles are deposited.

Preferably, the CCg-reactive material further comprises K2O (potassium oxide) and/or KHCO3 (potassium bicarbonate) .

Preferably the composite material is in the form of particles having particle size from 0.05 to 5 mm, more preferably from 0.1 to 3 mm, even more preferably from 0.2 to 1.5 mm, most preferably from 0.5 to 0.8 mm.

The particle size of the particles is measured by sieving, for example according to the procedure of the UNI EN 933-2:2020 standard, "Tests for geometrical properties of aggregates - Part 2: Determination of particle size distribution - Test sieves, nominal size of apertures". Preferably, the carbonaceous solid substrate is activated carbon, more preferably obtained from a substrate of vegetal origin chosen from coffee grounds (i.e. seeds of species of the Coffea genus, for example Coffea arabica and Coffea robusta) , corn cob (Zea mays) , almond husk (edible seed of the Prunus amygdalus or Prunus dulcis plant) , walnut shells (dry fruit of the plant of the genus Juglans, for example J. regia and J. nigra) , hazelnut shells (fruit of the Corylus avellana plant) . Alternatively, it is also possible as a starting substrate to use chlorine-free plastic materials, such as for example polystyrene, polystyrol, and polyethylene.

In a preferred embodiment, the activated carbon is obtained from coffee grounds (i.e. spent coffee powder obtained from the preparation of a coffee beverage by extraction with hot water) by a washing and pyrolysis process .

Preferably, the composite material has an average pore size from 0.5 nm to 10 pm, more preferably from 1 nm to 5 nm, even more preferably from 2 nm to 4 nm.

The average pore size can be measured according to methods known in the field, for example by nitrogen absorption according to the BET method (ISO 9277:2010) .

In fact, the contact surface of the reactive material increases as the porosity of the composite material, carrying the reactive material, increases.

The porosity of the composite material is therefore important to control the efficiency of the CO2 capture reaction and therefore of the process.

Preferably, in the composite material, the weight ratio of carbonaceous solid substrate to K2CO3 is from 2:1 to 1:10, more preferably from 1:1 to 1:5, even more preferably from 1:1 to 1:3, most preferably about 1:2.

Preferably, the surface area of the composite material is from 400 to 2000 m²/g, more preferably from 500 to 1500 m²/g, even more preferably from 700 to 1000 m²/g .

The surface area of the carbonaceous solid substrate can be measured according to methods known in the field, such as by nitrogen absorption according to the BET method (ISO 9277:2010) .

The gaseous stream can be produced by any known method, for example by a fan. It is useful to note that the stream pressure can be maintained within values slightly higher than atmospheric pressure, for example no more than 4 mbar higher than atmospheric pressure.

Preferably, the gaseous stream consists of air, more preferably ambient air.

Preferably, the gaseous stream has a CO2 concentration from 300 to 800 ppm, more preferably from 400 to 700 ppm, even more preferably from 450 to 600 ppm, most preferably from 480 to 550 ppm (ppm: parts per million expressed in weight) .

The measurement of the CO2 concentration can be performed according to methods known in the field, such as through an infrared detector with an operating range between 0 and 10000 ppm. An infrared detector suitable for this purpose is for example the Testo 440 CO2 kit with Bluetooth® distributed by Testo SE & Co KgaA (Titisee-Neustadt, Germany) .

Preferably, the gaseous stream has a relative humidity from 30% to 70% more preferably from 40% to 50% . Preferably, the gaseous stream has a temperature of 5°C to 80°C, more preferably 10°C to 70°C, most preferably 20°C to 40°C.

Preferably, the flow rate of the gaseous stream through the composite material is from 30 to 1000 m³ h~ ^x, more preferably from 70 to 500 m³ h^-1, even more preferably from 150 to 300 m³ tr¹ per square meter of composite material section.

In fact, a flow rate that is too high has the drawback of causing the gaseous stream to remain in contact with the composite material for too short a time. On the contrary, a flow rate that is too low is inefficient .

The composite material can furthermore comprise at least one inert mineral material having lamellar structure, in an amount, as a percentage by weight over weight, from 1 to 30% (w/w) , more preferably from 10 to 20% (w/w) , with respect to the amount of K2CO3.

Preferably, the mineral material is selected from perlite, vermiculite, expanded graphite, mica and mixtures thereof, more preferably perlite and/or vermiculite .

It has in fact been found that such a material prevents the formation of aggregates of the composite material particles which can reduce the CO2 absorption efficiency and the duration of the effectiveness of the composite material.

Furthermore, it has been found that, in the event that the incoming gaseous stream is too dry, generally lower than 30% relative humidity, the presence of such a hydrophobic material, such as for example vermiculite, mica and graphite, has the effect of promoting the flow of humidity and its distribution throughout the material, thus improving performance. In fact, low flow humidity prevents the correct execution of the reactions that lead to the capture of CO2.

The choice of the aforementioned mineral material depends on the operating conditions and can be determined by the person skilled in the art based on the individual specific application.

The process of the invention preferably allows to capture a quantity of CO2 of at least 100 g/Kg of K2CO3, more preferably from 100 to 200 g/Kg of K2CO3, even more preferably from 110 to 180 g/Kg of K2CO3, even more preferably from 130 to 170 g/Kg of K2CO3.

It has been found that thanks to the process of the present invention it is possible to capture substantially all the CO2 present in 150-200 m³ of air with only 1 Kg of K2CO3.

Preferably, the process further comprises a regeneration step of the composite material by heating. Such step allows the captured CO2 to be reformed.

Preferably, the regeneration step comprises a step of heating the composite material which has captured the CO2 at a temperature of from 100°C to 250°C, more preferably from 110°C to 200°C, even more preferably from 140°C to 160°C.

Preferably, the heating step is carried out until the desired CO2 recovery is achieved.

Preferably, during the heating step a reduced pressure is applied in order to facilitate the release of CO2 from the composite material.

Preferably, the pressure is reduced to values of 700 mbar or lower, more preferably from 50 to 500 mbar, even more preferably from 75 to 200 mbar.

Preferably, the heating step is preceded by a heating step from 35 to 70 °C, more preferably from 40 to 60 °C, preferably at reduced pressure, as described above .

Preferably, the aforementioned heating step at a temperature of from 100°C to 250°C is followed by a cooling step of the composite material at a temperature of from 30 to 80°C, more preferably from 50 to 70°C, even more preferably from 55 to 65 °C.

Preferably, the cooling step is further followed by a pressurization step, more preferably until the ambient pressure is reached, preferably with the introduction of air.

Preferably, the process is repeated cyclically with the alternation of a CO2 absorption step and a regeneration step.

Preferably, the composite material is obtainable by the process for the preparation of a composite material described below or by the process for the preparation of a composite material from coffee grounds described below.

Preferably, the carbonaceous solid substrate is obtainable by the process for the preparation of a carbonaceous solid substrate described below.

It has been surprisingly found that the process of the present invention has the effect of increasing, with use, the surface available on the composite material for the reaction with the CO2 in the heterogeneous phase, and therefore the efficiency of the reaction itself which can increase up to as much as five times. In particular, this effect is more evident following the first cycles of capture (or absorption) and regeneration, about ten cycles, and gradually decreases as such cycles increase, until a plateau value is reached.

Without wishing to be bound by any theory, it is believed that this effect is due to the chemical reactions that allow the capture of CO2 by potassium carbonate .

The capture of carbon dioxide by potassium carbonate is believed to occur according to the following reaction, which is moderately exothermic: K2CO3 + CO2 + H₂O 2KHCO₃ (1)

The reaction can be divided into two steps: K2CO3 + 1.5 H₂O K2CO3 (1.5H₂O) (la)

K2CO3 (1.5H₂O) + CO₂ - 2KHCO3 + 0.5H₂O (lb) It can be deduced that the presence of water is essential to trigger the CO2 capture reaction. A gaseous stream that is too dry would not produce any reaction .

Furthermore, the first reaction intermediate, i.e. the hydrated carbonate K2CO3 (1.5 H2O) , favors, thanks to a condition of partial solubility of the carbonate in water, an increase in the porosity of the salt surface. In practice, every time hydrated carbonate and subsequently anhydrous bicarbonate are formed, pores with dimensions smaller than 50 nm, even smaller than 2 nm, are created, which increase the contact surface between gas and salt. With each capture and release cycle, the CCy-reactive material therefore improves its performance .

In the regeneration step the composite material is heated, so that the bicarbonate KHCO3 decomposes and produces carbon dioxide as it transforms back into carbonate according to the reaction:

2KHCO₃ K2CO3 + CO2 + H₂O

The present invention also refers to a composite material comprising a carbonaceous solid substrate and a reactive material comprising particles of K2CO3, said carbonaceous solid substrate having a surface area of 300 m²/g or greater.

According to another aspect, the present invention refers to a process for the preparation of a composite material comprising the steps of: a) providing a carbonaceous solid substrate, preferably activated carbon; b) dissolving potassium carbonate in an aqueous solvent, preferably water, thus obtaining a potassium carbonate solution; c) suspending the carbonaceous solid substrate in the potassium carbonate solution and stirring; d) drying the suspension; e) crushing and sieving the material thus obtained to obtain a predetermined particle size; and f) subjecting the composite material thus obtained to pyrolysis.

Preferably, the carbonaceous solid substrate of step (a) is activated carbon, more preferably obtained from a substrate of vegetal origin chosen from coffee grounds (i.e. seeds of species of the Coffea genus, for example Coffea arabica and Coffea robusta) , corn cob (Zea mays) , almond husk (edible seed of the Prunus amygdalus or Prunus dulcis plant) , walnut shells (dry fruit of the plant of the genus Juglans, for example J. regia and J. nigra) , hazelnut shells (fruit of the Corylus avellana plant) , more preferably coffee grounds. Alternatively, it is also possible as a starting substrate to use chlorine-free plastic materials, such as for example polystyrene, polystyrol and polyethylene.

In a preferred embodiment, the carbonaceous solid substrate is obtained from coffee grounds according to the process for the preparation of a carbonaceous solid substrate described below.

Preferably, potassium carbonate is obtained by heating potassium bicarbonate, for example at a temperature from 150°C to 250°C for 2-4 hours, more preferably 180-220°C for 150-210 minutes.

Preferably, the carbonaceous solid substrate is combined with the potassium carbonate solution in a weight ratio of potassium carbonate (in dry weight) to carbonaceous solid substrate from 0.3 to 3, more preferably of 2.

Preferably, the stirring in step (c) is carried out for 2-10 hours, more preferably 5-9 hours, even more preferably 5-7 hours.

Preferably, the drying step (d) is performed by exposure to a temperature from 100°C to 140°C, more preferably from 100 to 120°C, for 2-12 hours, more preferably 3-10 hours, even more preferably about 4 hours; and optionally subsequently at a temperature from 150°C to 180°C, more preferably about 170°C, for 2-4 hours, more preferably about 3 hours.

Preferably, the composite material thus obtained in step (e) is subjected to at least one residual humidity removal step. Preferably such step is carried out by heating at a temperature of from 180 to 220°C for 1-3 hours, more preferably from 190 to 210°C for 90-150 minutes. This step has the aim of eliminating any residual humidity possibly present in the pores of the composite material.

Preferably, the composite material is subjected to a potassium carbonate melting step. Preferably, such step is carried out by heating the composite material at from 350 to 450°C for 30-90 minutes, more preferably from 380 to 420°C for 45-75 minutes. This step has the aim of allowing the distribution of the carbonate over the entire available surface of the carbonaceous substrate .

Preferably the potassium carbonate melting step is carried out following the aforementioned residual humidity removal step.

Preferably, in step (f) , the composite material is subjected to pyrolysis at a temperature of from 700 to 1000°C for 2-4 hours, preferably from 800 to 950°C for 150-210 minutes. This step has the aim of allowing the formation of further pores. In fact, the carbonate loses a molecule of CO2 according to the reaction:

The potassium oxide formed is very reactive and contributes to increasing the porosity of the composite material .

Preferably, the pyrolysis step is carried out after the aforementioned residual humidity removal step or after the aforementioned potassium carbonate melting step . The pyrolysis step is followed by the passage of air into the composite material, to make the potassium oxide react with the water and the CO2 present in the air with the formation of potassium bicarbonate according to the following reactions:

K₂O + H₂O — > 2 KOH

2 KOH + 2 CO2 — > 2 KHCO3

Such reaction is then followed by a heating step of the composite material, preferably in vacuum, at 150-210 °C, more preferably from 160 to 200 °C, with the formation of potassium carbonate according to the reaction :

2 KHCO3 + HEAT — > K2CO3 + CO2 + H₂O

Preferably, at least one inert mineral material having lamellar structure is added to the composite material thus obtained, more preferably by mixing.

Preferably, the mineral material is selected from perlite, vermiculite, expanded graphite, mica and mixtures thereof, more preferably perlite and/or vermiculite, preferably in particulate form.

The present invention also refers to a composite material obtainable according to the aforementioned process for the preparation of a composite material described above.

According to another aspect, the present invention refers to a process for the preparation of a carbonaceous solid substrate comprising the steps of: a) providing coffee grounds; b) subjecting the coffee grounds to washing, preferably for 40-80 minutes, more preferably about 60 minutes; c) filtering and drying the coffee grounds thus obtained, wherein preferably the drying is carried out at 100-140°C, more preferably 120°C, preferably for 2-6 hours, more preferably for 2 hours and 30 minutes - 5 hours and 30 minutes, even more preferably for 2 hours and 30 minutes - 4 hours; d) pyrolyzing the coffee grounds thus obtained, preferably in a nitrogen flow, preferably at 350-450°C, more preferably at about 400°C, preferably for 1-5 hours, more preferably for 90 minutes-4 hours and 30 minutes, even more preferably for 2-3 hours; e) cooling and dispersing in water the coffee grounds thus obtained; f) adding to the dispersion thus obtained a quantity of K2CO3, preferably under stirring for a period of 5-9 hours, more preferably 5 hours and 30 minutes-8 hours and 30 minutes, even more preferably 5 hours and 30 minutes-7 hours ; g) drying the mixture thus obtained at from 100 to 120°C and pyrolyzing at 700-1000°C, more preferably 800-950°C, even more preferably 800-900°C, for 2-4 hours, more preferably 150-210 minutes; h) cooling the pyrolyzed mixture and washing with water in order to remove excess K2CO3; i) filtering and drying the product thus obtained .

Preferably, in step (b) and (h) , the step of subjecting the coffee grounds to washing is carried out with water, more preferably at a temperature from 50 to 100 °C, even more preferably from 70 to 100 °C.

Preferably, step (c) is followed by a washing step of the coffee grounds thus obtained.

Preferably, in step (g) , the mixture is left to dry for a period of time from 4 to 12 hours, more preferably 4 hours and 30 minutes-about 11 hours, even more preferably from 4 and 30 minutes to 6 hours.

Preferably, following the pyrolysis of step (d) and step (g) , the pyrolysed coffee grounds and the pyrolysed mixture, respectively, are cooled until a temperature from 10 to 40 °C, more preferably 20-30 °C, is reached.

Preferably, in step (f) of adding to the dispersion a quantity of K2CO3, the quantity by weight of K2CO3 is from 0.5 to 2 times, more preferably from 0.7 to 1.5 times, even more preferably from 0.9 to 1.2 times, most preferably equal to the amount by weight of the pyrolyzed coffee grounds (weight ratio 1:1) .

Preferably, step (f) is followed by filtration of the dispersion thus obtained.

Preferably, in step (h) , the washing comprises a step of subjecting the pyrolyzed material to ultrasound .

The present invention also refers to a carbonaceous solid substrate obtainable according to the aforementioned process for the preparation of a carbonaceous solid substrate described above.

The present invention also refers to a process for the preparation of a composite material from coffee grounds which comprises the steps of: a) providing coffee grounds; b) subjecting the coffee grounds to washing, preferably for 40-80 minutes, more preferably about 60 minutes; c) filtering and drying the coffee grounds thus obtained, wherein preferably the drying is carried out at 100-140°C, more preferably 120°C, preferably for 2-6 hours, more preferably for 2 hours and 30 minutes-5 hours and 30 minutes, even more preferably for 2 hours and 30 minutes-4 hours; d) pyrolyzing the coffee grounds thus obtained, preferably in a nitrogen flow, preferably at 350-450°C, more preferably at about 400°C, preferably for 1-5 hours, more preferably for 90 minutes-4 hours and 30 minutes, even more preferably for 2-3 hours; e) cooling and dispersing in water the coffee grounds thus obtained; f) adding to the dispersion thus obtained a quantity of K2CO3, preferably under stirring for a period of 5-9 hours, more preferably 5 hours and 30 minutes-8 hours and 30 minutes, even more preferably 5 hours and 30 minutes-7 hours ; g) drying the mixture thus obtained at from 100 to 120°C and pyrolyzing at 700-1000°C, more preferably 800-950°C, even more preferably 800-900°C, for 2-4 hours, more preferably 150-210 minutes; h) cooling the pyrolyzed mixture; and i) crushing and sieving the material thus obtained to obtain a predetermined particle size .

Preferably, in step (g) , before pyrolysis, the coffee grounds are subjected to at least one residual humidity removal step. Preferably such step is carried out by heating at a temperature of from 180 to 220°C for 1-3 hours, more preferably from 190 to 210°C for 90-150 minutes.

Preferably, the coffee grounds are subjected to a potassium carbonate melting step. Preferably, such step is carried out by heating from 350 to 450°C for 30-90 minutes, more preferably from 380 to 420°C for 45-75 minutes. Preferably the potassium carbonate melting step is carried out following the aforementioned residual humidity removal step.

It is in fact possible to carry out a process for the preparation of a composite material using coffee grounds as a carbonaceous solid substrate by omitting the final washing of step (h) and the subsequent step (i) from the process for the preparation of a carbonaceous solid substrate described above and then proceeding directly to crushing without the need to reapply the carbonate.

Further process preferences are thus described in the processes described above for the preparation of a composite material and for the preparation of a carbonaceous solid substrate.

The present invention also refers to a composite material obtainable according to the aforementioned process for the preparation of a composite material from coffee grounds described above.

Brief description of the Figures Figure 1 shows the macroscopic appearance of pure potassium carbonate and of the composite material prepared in Example 2.

Figure 2 shows: 2a) the electron microscope image (5000x) of the carbonate surface after 10 cycles of absorption and release; 2b) graph comparing the CO2 absorption of the same material (potassium carbonate) at the first cycle and after the tenth cycle.

Figure 3 shows a graph comparing the CO2 absorption of the composite material at different granularities .

Figure 4 shows electron microscopy images of carbonaceous material before pyrolysis (4a) , pyrolyzed for 3 hours at different temperatures: 700°C (4b) ,

800°C (4c) , and 900°C (4d) .

Figure 5 shows the surface of pure potassium carbonate and of the composite material under an optical microscope (50x) .

Figure 6 is a graph showing the CO2 absorption efficiency of pure potassium carbonate and potassium carbonate when associated with activated carbon.

Figure 7 shows SEM electron microscope images of samples A and B of Example 4, mapping to the element potassium by EDX microanalysis. In particular: (a) Sample A; (b) Sample B; (c) Sample A at 750 Ox magnification; (d) Sample A, potassium mapping; (e) Sample B 1300x magnification; (f) Sample B, potassium mapping .

Figure 8 is a graph showing a comparison in the absorption of CO2 from the air for the two samples A and B .

Detailed description of the invention The invention will now be further described with reference to embodiment examples provided for illustrative and non-limiting purposes.

EXAMPLE 1

Preparation of activated carbon from coffee grounds

0.5 kg of coffee grounds were washed for 60 minutes in hot water at a temperature of 80 °C. The dispersion thus obtained was then filtered and dried in an oven at 120°C for three hours. The washed coffee grounds were then pyrolyzed in nitrogen flow at 400°C for 2 hours, eliminating all volatile components. It was then cooled to room temperature (25°C) and the carbonaceous product was dispersed in water. A quantity by weight of K2CO3 equal to the weight of the pyrolysed coffee grounds was then added and stirring was carried out for 6 hours. The mixture was then filtered and dried in an oven at 120°C for 5 hours. The product was then pyrolyzed at 800°C for 3 hours. It was cooled to room temperature (25 °C) and the potassium carbonate was washed with water. The filtered and dried product represented the carbonaceous solid to be used as a substrate. The yield of the process was 30% with respect to the weight of the washed coffee grounds.

EXAMPLE 2

Preparation of the composite material

0.5 kg of potassium bicarbonate (KHCO3) was heated in an oven at 200°C for 3 hours. This heating transformed the bicarbonate into carbonate due to the loss of a molecule of CO2 and a molecule of water. The potassium carbonate thus obtained was dissolved in water. The activated carbon (AC) of Example 1 was added at a weight ratio of the K2CO3/AC concentrations of 2. The mixture was stirred for 6 hours. The product was then placed in an oven at 120°C for 4 hours and at 170°C for 3 hours. The material was then crushed and sieved to obtain a powder with a granularity between 0.5 and 0.8 mm. Material with different granularity was also prepared, obtaining the following granularities: a) between 0.5 and 0.8 mm; b) between 1.0 and 1.2 mm; c) between 2.0 and 2.5 mm.

Figure 1 shows the appearance of K2CO3 in granules (Figure 1A) and K2CO3 on activated carbon from coffee grounds after crushing (Figure IB) .

The material obtained was placed in an oven and heated according to the following sequence:

1. 200°C for 2 hours, for eliminating humidity residues present in the pores;

2. 400°C for 1 hour, for allowing the carbonate to melt, penetrating into all the pores;

3. 700-900°C for 3 hours: pyrolysis, with the formation of pores and reaction of potassium carbonate with the formation of K2O distributed on the surface of the composite material. In particular, three different temperatures were used: 3a) 700 °C, 3b) 800 °C and 3c) 900 °C.

Finally, the composite material thus formed was placed in contact with the air, thus forming potassium bicarbonate (KHCO3) thanks to the water and CO2 present in the air and heating the composite material to 180 °C under vacuum with the formation, from the potassium bicarbonate, of K2CO3 reactive for the absorption of

CO₂. The surface area of the composite material thus formed was measured by nitrogen absorption according to the BET method (ISO 9277:2010) and was found to be 800 m²/g .

The average pore size of the composite material thus formed was measured by nitrogen absorption according to the BET method (ISO 9277:2010) and was found to be 3 nm.

EXAMPLE 3

Absorption and release test

A tubular pyrex reactor having a diameter of 5 cm and a useful surface area of 18 cm² was set up. The reactor was thermostated at 40°C in the capturing step and at 200°C in the regeneration step. The incoming air flow was generated by a fan. The flow rate entering and exiting the reactor was measured by flow meters. Depending on the test, the material in the reactor was the composite material obtained as described in Example

2 or K2CO3 alone (comparison) . The amount of composite material or K2CO3 alone was 150 g, which corresponded to a thickness in the reactor of 10 cm.

In the case where the reactor was operated with carbonate alone, the latter was prepared as follows. Potassium bicarbonate was dissolved in water. The solution was heated under stirring until a mixture with the consistency of honey was obtained. This was dried in an oven at 130°C until the anhydrous salt was obtained. The salt in porous blocks was ground and passed through a sieve to obtain the desired particle size .

The characteristics of the treated air flow were as follows : 1. CO₂ concentration: about 500 ppm;

2. Operating pressure: 2 mbar above atmospheric pressure ;

3. Flow: 200 m³ Fr¹ per square meter of composite material section;

4. Humidity: 50%.

The measurement of the CO₂ concentration was made with an infrared detector with an operating range between 0 and 10, 000 ppm (Testo 440 CO2 kit with Bluetooth® distributed by Testo SE & Co KgaA (Titisee- Neustadt, Germany) ) . The instrument was also equipped with a humidity detector, calculated as a relative percentage .

Once the absorption step was completed, the saturated material was placed in an electrically heated reactor. Using a vacuum pump, the pressure inside the reactor was reduced to 100 mbar (absolute pressure or one tenth of the external pressure) . The temperature was brought to 50 °C for 1 hour to eliminate the air and water trapped in the pores. The temperature was brought to 160°C for 3 hours to release the CO2. The internal environment was then cooled to a temperature of 60°C. The pressure inside the reactor was then brought back to ambient values (1000 mbar) with the introduction of air.

The passage of a flow of ambient air as described above was repeated.

The absorption and release cycle was repeated ten times .

Porosity of potassium carbonate: From a microscopic analysis on samples of pure potassium carbonate that had undergone ten cycles of absorption and release, it was possible to observe the appearance of porosity of K2CO3 which justified the improvement in absorption performances. Figure 2a shows a SEM-BSE (Back Scattered Electron) electron microscope image showing the surface porosity of the material and Figure 2b is a graph showing CO2 capture over time (solid line: first cycle, dashed line: tenth cycle) . From the calculation of the integral, an increase of 25-30% was found in the CO2 absorption capacity between the first cycle and the tenth cycle.

Effect of the size of the K2CO3 particles: Figure 3 shows the capture curves over time for different particle sizes: a) between 0.5 and 0.8 mm; b) between 1.0 and 1.2 mm; c) between 2.0 and 2.5 mm.

From these curves it can be deduced that the larger the size of the grains, the lower the overall performance of the material, however with a less marked decrease in the capture curve, probably due to a diffusive process following the partial solubilization of the surface material.

Effect of the pyrolysis temperature of the composite material on the K2CO3 particles:

As mentioned above, in the preparation of the composite material according to Example 2, three different pyrolysis temperatures were compared using potassium carbonate as the reactive molecule. The temperatures used were 700 °C, 800 °C and 900 °C.

Figure 4 (SEM (electron microscope) images) shows the variation of pore sizes in pyrolysis at different temperatures. By increasing the temperature and pyrolysis time between 700 and 900 °C (figures 4b (700 °C) , 4c (800 °C) and 4d (900 °C) ) , it was observed that, with increasing temperature of pyrolysis, small pores decreased while larger ones increased compared to the sample before pyrolysis (fig. 4a) .

In Figure 5 the porosity distribution of an AC- K2CO3 composite material prepared according to Example 2 can be evaluated in comparison with pure K2CO3 granules (prepared as described in Example 3) of the same dimensions (photographs at 4x magnification) . As can be seen, the porosity of the granules of composite material (Figure 5a) subjected to pyrolysis is clearly visible, while the granules of pure potassium carbonate (which have not yet undergone any absorption and release cycle) do not present visible porosity.

A comparison test was carried out between the composite material of Example 2 and the same quantity of calcium carbonate alone (again according to Example 3) . The materials were subjected to ten adsorption and release cycles as described above.

Figure 6 shows the comparison of the CO2 capture curves over time between pure potassium carbonate (solid line) and that supported on activated carbon (dashed line) . As can be seen, the presence of activated carbon significantly improves both total capture and capture efficiency.

EXAMPLE 4

Comparison of chemical-morphological analyses and absorption test

1000 g of coffee grounds were dispersed (washed) in water at room temperature and the mixture was brought to the boil and kept boiling for about 1 hour, followed by filtration. The boiling and filtering steps were repeated a total of three times. After the last filtration, the coffee grounds were left to dry in an oven at 120°C for 5 hours.

500 g were taken from the washed and dried coffee grounds and pyrolyzed under nitrogen flow (3 liters/min) at 400°C for 4 hours. This was followed by cooling to room temperature (25 °C) .

After pyrolysis, a weight loss of 56% was observed (220 g of product obtained)

The cooled and pyrolysed material was dispersed in 100 ml of water and potassium carbonate (K2CO3) was added in a 1:1 weight ratio, i.e. 220 g of pyrolysed material and 220 g of K2CO3.

The dispersion was stirred while heating at 100°C under reflux for 8 hours. The mixture was then placed in an oven overnight at 110°C.

The dry mixture thus obtained was pyrolyzed at 900°C in a nitrogen flow (3 liters/min) for 3 hours. The pyrolyzed material was allowed to cool under a nitrogen flow until it reached room temperature. After cooling, a weight loss of 20% was observed (352 g remained) .

The pyrolyzed material was dispersed (washed) in 200 ml of water at room temperature, heated at 80°C for 120 minutes and placed in an ultrasonic bath for 30 minutes. The mixture was filtered and dried in an oven for 2 hours at 120°C. The weight of the mixture thus obtained was 132 g and this demonstrates that all the potassium carbonate had been washed. The material was dispersed in 100 ml of an aqueous solution of potassium carbonate (K2CO3) containing 200 g of K2CO3. The dispersion was stirred while heating at 100°C under reflux for 8 hours.

The mixture was then placed in an oven overnight at 110 °C.

The mixture was then divided into two parts of equal weight: sample A (according to the invention) and sample B (comparative) .

Samples A and B, separately, were placed in a Pulverisette 16 hammer shredder (Fritsch GmbH, Germany) and subsequently sieved with a mechanical vibration sieve (IG/3 Export) to isolate the fraction between 20 and 30 mesh (500 -800 pm) .

Sample B was therefore used as is for the chemical-morphological analyses and for the absorption tests .

Sample A was pyrolyzed at 900°C in nitrogen flow (3 liters/min) with the following temperature ramp:

25°C - 200°C at 10°C/min

200°C for 120 min

200°C - 400°C at 10°C/min

400°C for 60 min

400°C - 900°C at 10°C/min

900°C for 180 min

900°C - 25°C overnight SEM electron microscope analysis of samples A and B was carried out by performing a map on the element potassium using EDX microanalysis. The results are shown in Figure 7.

Obvious differences in the distribution of the CO2~reactive material can be observed both in the high magnification images (Figures 7 (c) and 7 (e) ) and in the chemical map (on the element potassium) (Figures 7 (d) and 7 (f) ) . The reactive material is given by potassium carbonate (K2CO3) which can of course be in a mixture with potassium bicarbonate (KHCO3) . In sample B, macrostructures of K2CO3 are observed while in sample A the potassium carbonate is dispersed in a nanometric and uniform form throughout the material. In particular, in the case of sample A, the potassium carbonate is also dispersed within the pores, therefore covering a greater surface area compared to sample B. This difference in distribution corresponds for sample A to a greater efficiency of the material in capturing CO2 from the air, as demonstrated below.

Sample A and sample B were each placed in a tubular absorption reactor having a glass partition on the bottom (for uniform distribution of the air) and the material, for both samples, was heated to 200°C with a constant nitrogen flow of 8 litres/min. The gas coming out of both tubes was analyzed with an infrared detector Testo 440 CO2 kit with Bluetooth® distributed by Testo SE & Co KgaA (Titisee-Neustadt, Germany) and the material preparation step was interrupted at the moment where the detector measured an output CO2 level of zero. For both samples, the material was then cooled again under a nitrogen flow to 35°C and once the temperature had been reached the nitrogen flow was replaced with an air flow at 8 litres/min (which corresponds to 240 m³ Fr¹ per square meter of section of composite material) for a total time of 27 hours. The concentration of carbon dioxide in the air and exiting the absorption reactor was measured with an infrared detector Testo 440 C02 kit with Bluetooth® distributed by Testo SE & Co KgaA (Titisee-Neustadt, Germany) .

The results of these measurements are shown in Figure 8.

By integrating the areas under the two curves it was possible to calculate for sample A a quantity of CO2 of 8.7 grams, which corresponds to an absorption capacity of the material of 116 g CO2 /kg of K2CO3. For sample B, the absolute quantity of CO2 absorbed was 7.06 grams, which corresponds to an absorption capacity of 94 g CO₂/kg K2CO3.

Note that, in order to have a linear and comparable trend, the CO2 value detected at the various points was normalized with the first value initially detected .

Claims

1. Process for capturing CO2 comprising the step of contacting a gaseous stream comprising CO2 with a composite material comprising a carbonaceous solid substrate and a reactive material comprising particles of potassium carbonate (K2CO3) , said composite material having a surface area of 300 m²/g or greater, said gaseous stream having a CO2 content from 200 to 1000 ppm and a relative humidity from 10 to 90%.

2. Process according to claim 1, wherein said composite material is in the form of particles having particle size from 0.05 to 5 mm, preferably from 0.1 to 3 mm, more preferably from 0.2 to 1.5 mm, even more preferably from 0.5 to 0.8 mm .

3. Process according to claim 1 or 2, wherein said composite material has an average pore size from 0.5 nm to 10 pm, more preferably from 1 nm to 5 nm, more preferably from 2 nm to 4 nm.

4. Process according to any one of the preceding claims, wherein the weight ratio of carbonaceous solid substrate to K2CO3 is from 2:1 to 1:10, preferably from 1:1 to 1:5, more preferably from 1:1 to 1:3, even more preferably about 1:2.

5. Process according to any one of the preceding claims, wherein said surface area of said composite material is from 400 to 2000 m²/g, preferably from 500 to 1500 m²/g, more preferably from 700 to 1000 m²/g.

6. Process according to any one of the preceding claims, wherein said gaseous stream has a CO2 concentration from 300 to 800 ppm, preferably from 400 to 700 ppm, more preferably from 450 to 600 ppm, even more preferably from 480 to 550 ppm .

7. Process according to any one of the preceding claims, wherein said gaseous stream has a relative humidity from 30% to 70%, preferably from 40% to 50%.

8. Process according to any one of the preceding claims, wherein the flow rate of said gaseous stream through the composite material is from 30 to 1000 m³ tr¹, more preferably from 70 to 500 m³ tr¹, even more preferably from 150 to 300 m³ tr¹ per square meter of composite material section.

9. Process according to any one of the preceding claims, wherein said composite material further comprises at least one inert mineral material having lamellar structure, in an amount, as a percentage by weight over weight, from 1 to 30% (w/w) , preferably from 10 to 20% (w/w) , with respect to the amount of K2CO3.

10. Process according to any one of the preceding claims, which further comprises a regeneration step of said composite material by heating.

11. Process according to claim 10 wherein said regeneration step comprises a step of heating the composite material which has captured the CO2 at a temperature from 100°C to 250°C, preferably from 110°C to 200°C, more preferably from 140°C to 160°C.

12. Process according to any one of the preceding claims comprising a composite material according to claim 17.

13. Process according to any one of the preceding claims comprising a carbonaceous solid substrate according to claim 19.

14. Process according to any one of claims 1-12, comprising a composite material according to claim 21.

15. Composite material comprising a carbonaceous solid substrate and a reactive material comprising particles of K2CO3, said carbonaceous solid substrate having a surface area of 300 m²/g or greater.

16. Process for the preparation of a composite material comprising the steps of: a) providing a carbonaceous solid substrate, preferably activated carbon; b) dissolving potassium carbonate in an aqueous solvent, preferably water, thus obtaining a potassium carbonate solution; c) suspending said carbonaceous solid substrate in said potassium carbonate solution and stirring; d) drying the suspension; e) crushing and sieving the material thus obtained to obtain a predetermined particle size; and f) subjecting the composite material thus obtained to pyrolysis.

17. Composite material obtainable according to the process of claim 16.

18. Process for the preparation of a carbonaceous solid substrate comprising the steps of: a) providing coffee grounds; b) subjecting said coffee grounds to washing, preferably for 40-80 minutes, more preferably about 60 minutes; c) filtering and drying the coffee grounds thus obtained, wherein preferably the drying is carried out at 100-140°C, more preferably 120°C, preferably for 2-6 hours, more preferably for 2 hours and 30 minutes-5 hours and 30 minutes, even more preferably for 2 hours and 30 minutes-4 hours; d) pyrolyzing the coffee grounds thus obtained, preferably in a nitrogen flow, preferably at 350- 450°C, more preferably at about 400°C, preferably for 1-5 hours, more preferably for 90 minutes-4 hours and 30 minutes, even more preferably for 2-3 hours; e) cooling and dispersing in water the coffee grounds thus obtained; f) adding to the dispersion thus obtained a quantity of K2CO3, preferably under stirring for a period of 5-9 hours, more preferably 5 hours and 30 minutes-8 hours and 30 minutes, even more preferably 5 hours and 30 minutes-7 hours; g) drying the mixture thus obtained at from 100 to 120°C and pyrolyzing at 700-1000°C, more preferably 800- 950°C, even more preferably 800-900°C, for 2-4 hours, more preferably 150- 210 minutes; h) cooling the pyrolyzed mixture and washing with water in order to remove the excess K2CO3; i) filtering and drying the product thus obtained.

19. Carbonaceous solid substrate obtainable according to the process of claim 18.

20. Process for the preparation of a composite material from coffee grounds comprising the steps of: a) providing coffee grounds; b) subjecting said coffee grounds to washing, preferably for 40-80 minutes, more preferably for about 60 minutes ; c) filtering and drying said coffee grounds thus obtained, wherein preferably the drying is carried out at 100-140°C, more preferably 120°C, preferably for 2-6 hours, more preferably for 2 hours and 30 minutes-5 hours and 30 minutes, even more preferably for 2 hours and 30 minutes-4 hours; d) pyrolyzing the coffee grounds thus obtained, preferably in a nitrogen flow, preferably at 350- 450°C, more preferably at about 400°C, preferably for 1-5 hours, more preferably 90 minutes-4 hours and 30 minutes, even more preferably for 2-3 hours; e) cooling and dispersing in water the coffee grounds thus obtained; f) adding to the dispersion thus obtained a quantity of K2CO3, preferably under stirring for a period of 5-9 hours, more preferably 5 hours and 30 minutes-8 hours and 30 minutes, even more preferably 5 hours and 30 minutes-7 hours; g) drying the mixture thus obtained at from 100 to 120°C and pyrolyzing at 700-1000°C, more preferably 800- 950°C, even more preferably 800-900°C, for 2-4 hours, more preferably 150-210 minutes; h) cooling the pyrolyzed mixture; and i) crushing and sieving the material thus obtained to obtain a predetermined particle size.

21. Composite material obtainable according to the process of claim 20.