Carbon Capture Technologies

The International Energy Agency noted that in the long run, the CCS (Carbon Capture Storage) technology would significantly reduce CO₂ emissions in the world by approx. 20–28% by 2050, which will translate into environmental protection and ensure energy security for countries in which the production of electricity is mainly based on char [1]. The main technologies for capturing carbon dioxide from gas streams include:

• •

  CO₂ capture before the combustion process (pre-combustion),

• •

  capture after the combustion process (post-combustion), and

• •

  combustion of fuel in an oxygen atmosphere (oxy-fuel combustion).

In the CO₂ capturing after the combustion process, the solid fuel is gasified before the combustion process, producing synthesis gas (a mixture of hydrogen and carbon monoxide). Then, due to the conversion of CO with steam, a mixture rich in hydrogen and carbon dioxide is obtained, which is subject to the separation process, and the hydrogen can be burned in a gas turbine integrated with the steam turbine [2].

In the CO₂ capturing after the combustion process, the solid fuel is gasified before the combustion process, producing synthesis gas (a mixture of hydrogen and carbon monoxide). Then, due to the conversion of CO with steam, a mixture rich in hydrogen and carbon dioxide is obtained, which is subject to the separation process, and the hydrogen can be burned in a gas turbine integrated with the steam turbine [2].₂ separation after the combustion process. After the combustion process, the advantage of the CO₂ separation system is that the fuel does not require prior preparation, as in the case of char gasification, but is burned conventionally, and the capture systems can be connected to the existing installations [3]. CO₂ capture can occur through adsorption, absorption, cryogenic, and membrane separation, of which chemical absorption and adsorption methods are the most promising methods of CO₂ capture. Absorption methods for capturing CO₂ from flue gas are characterized by the high efficiency and purity of the obtained product. However, the industrial processes of CO₂ separation by absorption in amine solutions (monoethanolamine, methyl diethanolamine, diethanolamine), which are most often used as sorbents, have some disadvantages, such as: high heat of absorption, which is associated with high energy demand in the desorption process, high vapor pressure amines and the corrosivity of the reaction medium [4]. The system for removing CO₂ by the absorption method consists of two main components: an absorber and a regenerator. First, the flue gases entering the absorber are cooled down to approx. 323 K, and then they are cleaned of dust that can cause operational problems, and other pollutants are removed, such as SO_x, NO_x, HCl, Hg, because irreversible reactions occur when they come into contact with the absorbent.

The method of capturing CO₂ after the combustion process consists in separating carbon dioxide from other substances in the exhaust gas, such as nitrogen oxides, nitrogen or sulfur oxides. There are many methods of CO

Consequently leads to a loss of sorbent, which is associated with additional costs. On the other hand, CO₂ reacts with the sorbent in reversible reactions. On the other hand, the regenerator is designed to recover the absorbent through desorption, during which carbon dioxide is released in a high concentration [5].₂ absorbed in one cycle corresponds to the difference in the amount of carbon dioxide absorbed at different temperatures: higher for desorption and lower for adsorption. One of the drawbacks of the TSA method is the long time it takes to heat the bed during desorption. Reducing the time of this process can be done by increasing the contact area of the hot gas with the adsorbent. A method that allows shortening the time of raising the temperature of the adsorbent is the use of electric current (ESA - Electric Swing Adsorption). This method uses a low voltage current to heat the adsorbent. The disadvantage of this solution is the inability to use the waste heat as it was in the TSA method. In Pressure Swing Adsorption (PSA), the adsorption process is carried out at a higher pressure, while the desorption process takes place at a lower pressure. This method is used to separate exhaust gases, which are a mixture of gases, and enrich it with carbon dioxide. The differences between the sorption capacity or the rate individual gases (exhaust gases) adsorption flowing through the adsorber bed are used. The PSA method allows obtaining CO₂ with a purity of up to 99% with a separation efficiency of 90%. A modification of the PSA method is the Rapid Pressure Swing Adsorption (RPSA) method and its further modification, i.e. Ultra Rapid Pressure Swing Adsorption (URPSA) [7]. Another CO₂ separation method is Vacuum Swing Adsorption (VSA). Contrary to the PSA method, where the gases must be compressed before entering the column, when using the VSA installation, adsorption is carried out under atmospheric pressure. On the other hand, desorption is carried out by reducing the pressure to about 0.05 atm [8]. The advantage of the VSA method is the high purity of the obtained carbon dioxide, even 99%. A method combines PSA and TSA or PTSA (Pressure Temperature Swing Adsorption), where desorption is done by swapping both pressure and temperature. This method is much more efficient compared to PSA or TSA methods. In such a method, it is possible to obtain a high concentration of carbon dioxide (75–90%) with a recovery efficiency of 65–88.5% [9].

An alternative to absorption methods is, among others, adsorptive methods of carbon dioxide separation, which are one of the methods of purifying pollutants from waste gases. Adsorptive methods of capturing carbon dioxide consist of absorbing gas molecules on the surface and in the pores of a solid body. CO₂ from the flue gas is removed by adsorption at a given pressure and temperature and then recovered by desorption, which occurs due to changes in temperature or pressure conditions or both. A critical issue in the adsorption process is the appropriate selection of the adsorber. The adsorption plant can operate continuously, but at least two adsorption columns are needed for this. Various adsorption techniques can be distinguished, and these are the following methods: TSA, ESA, PSA, PTSA, RPSA, URPSA, VSA [6]. Temperature Swing Adsorption (TSA) makes use of the differences in absorbent absorbency at different temperatures. Thus, the amount of CO₂ absorbed in one cycle corresponds to the difference in the amount of carbon dioxide absorbed at different temperatures: higher for desorption and lower for adsorption. One of the drawbacks of the TSA method is the long time it takes to heat the bed during desorption. Reducing the time of this process can be done by increasing the contact area of the hot gas with the adsorbent. A method that allows shortening the time of raising the temperature of the adsorbent is the use of electric current (ESA - Electric Swing Adsorption). This method uses a low voltage current to heat the adsorbent. The disadvantage of this solution is the inability to use the waste heat as it was in the TSA method. In Pressure Swing Adsorption (PSA), the adsorption process is carried out at a higher pressure, while the desorption process takes place at a lower pressure. This method is used to separate exhaust gases, which are a mixture of gases, and enrich it with carbon dioxide. The differences between the sorption capacity or the rate individual gases (exhaust gases) adsorption flowing through the adsorber bed are used. The PSA method allows obtaining CO₂ with a purity of up to 99% with a separation efficiency of 90%. A modification of the PSA method is the Rapid Pressure Swing Adsorption (RPSA) method and its further modification, i.e. Ultra Rapid Pressure Swing Adsorption (URPSA) [7]. Another CO₂ separation method is Vacuum Swing Adsorption (VSA). Contrary to the PSA method, where the gases must be compressed before entering the column, when using the VSA installation, adsorption is carried out under atmospheric pressure. On the other hand, desorption is carried out by reducing the pressure to about 0.05 atm [8]. The advantage of the VSA method is the high purity of the obtained carbon dioxide, even 99%. A method combines PSA and TSA or PTSA (Pressure Temperature Swing Adsorption), where desorption is done by swapping both pressure and temperature. This method is much more efficient compared to PSA or TSA methods. In such a method, it is possible to obtain a high concentration of carbon dioxide (75–90%) with a recovery efficiency of 65–88.5% [9].₂ adsorption, such as zeolites [17], metal-organic frameworks (MOFs) [18], porous silica [19], and activated carbons (ACs) [20], [21]. However, despite that zeolite presents relatively higher packing densities than activated carbons, they are incredibly hydrophilic. They can lose their adsorption capacity with time because of preferential moisture adsorption. Among these adsorbents, activated carbon has drawn great attention recently because of its high adsorption capacity, low cost, availability, large surface area, easy to design pore structure, hydrophobicity (insensitiveness to moisture), readily controlled structure, good thermal and chemical stability and large efficiency and low energy requirements for regeneration [22].

Due to their multifunctionality, there is an expansion of the scope of their application, and an increase in consumption in existing industries [23]. With the gradual transition to a circular economy, it became relevant to obtain AC from various types of raw materials, when its main sources were anthracite and bituminous chars. The search for raw materials for their production is the task of cleaner production worldwide [24]. The choice of raw materials primarily depends on a specific region’s available mineral (brown char, peat) and biomass (coconut and walnut shells, rice, wheat, bamboo, cane, etc.) resources. Kazakhstan possesses significant reserves of unclaimed minerals and biomaterials that can be processed into AС. Biocarbons obtained based on walnut shells and apricot pits have already established themselves as a unique electrode material [25].

Due to their multifunctionality, there is an expansion of the scope of their application, and an increase in consumption in existing industries [23]. With the gradual transition to a circular economy, it became relevant to obtain AC from various types of raw materials, when its main sources were anthracite and bituminous chars. The search for raw materials for their production is the task of cleaner production worldwide [24]. The choice of raw materials primarily depends on a specific region’s available mineral (brown char, peat) and biomass (coconut and walnut shells, rice, wheat, bamboo, cane, etc.) resources. Kazakhstan possesses significant reserves of unclaimed minerals and biomaterials that can be processed into AС. Biocarbons obtained based on walnut shells and apricot pits have already established themselves as a unique electrode material [25].

Shungite mineral raw material has been studied by many scientists in various processes: catalysis [26], [27], photocatalysis [28], [29], adsorption [30], [31] and this carbon-mineral raw material can be attributed to a promising material for obtaining AC in Kazakhstan. By its nature, shungite is a graphene-like substance that includes various compounds [32]. The main studied shungite deposits in Kazakhstan are "Bolshevik" and "Bakyrchik" of the East Kazakhstan region, as well as "Koksu" of the Almaty region. According to the research results, the content of carbonaceous matter ranges from a few percent to about 30% by wt., the content of silicon dioxide reaches 60% by wt., significant amounts of aluminum oxide (up to 23% by wt.) are observed. The rocks are enriched in iron, titanium, alkali, and alkaline earth metals to a lesser extent. However, carbon, silicon dioxide and aluminum oxide [33], [34] are allocated as the main structural components. Due to the complex combination of complexes of bimetallic particles, nanoparticles, oxides and salts of various metals in the structure of shungite, the general electrochemical properties of the produced electrodes and catalysts are improved [35].

Shungite rocks of the East Kazakhstan region are extracted along the way with polymetallic ores and due to the lack of interest from companies (concentrated on the extraction of precious metals) developing these deposits, shungite is stored in dumps. Domestic scientists, working on solving this problem, have developed a technology for processing shungite rocks in order to obtain target products [36]. Based on the products of processing of shungite rocks, carbon-mineral sorbents were obtained, which are used in various sorption [37] and catalytic processes [38].₂) [42]. Significant influence on the properties of the obtained biocarbon has the raw material from which it comes and the preparation method. [43].

To obtain AС from biocarbons / hydrocarbons, chemical or physical activation is required. As carbon source various biomass [44] or waste [45], [46] are commonly applied. There is also interest in MOF as the carbon source for very microporous ACs synthesis [47]. Chemical activation is an attractive method for treating carbon materials due to lower heating temperatures, faster reaction times and higher carbon content. From neutral reagents, ZnCl₂ [48], KCl, K₂SO₄ are used [49]. From the alkaline reagents, the most effective are KOH [50], [51] and NaOH [52], of the acidic ones – H₃PO₄ [53], [54], [55]. One of the advantages of this method is the ability to control the size and number of pores. However, the most important advantage of physical activation is its environmentally friendly production without secondary waste compared to chemical activation [56]. The steam activation (SA) method with live steam is the simplest and effective method for the activation of adsorption centers. Water vapor activates the pores both on the surface of the sorbent and inside their structure. The specific surface area of biocarbon after activation increases several times. In order to get good CO₂ sorbents commercial activated carbons for water purification can be modified by KOH [57] or acids [58]. For example, Zhang et al. [59] studied the effects of steam activation on the pore structure and surface chemistry of activated carbon derived from bamboo. They concluded that the activation conditions significantly affected the surface area evolution, porosity, yield, and burn-off of AC. Li et al. [60] investigated the effect of steam on improvement in the surface properties of activated carbon. They concluded that surface properties such as surface area, pore distribution, and pore volume were improved by highly controlled steam pretreatment on AC materials.

The purpose of this work is preparation new inexpensive activated carbons based on raw biomass - corn cobs, grape seeds, pine cones and birch and carbon-mineral raw materials - shungite with a well-developed surface by thermal and hydrothermal carbonization and steam activation as a potentially promising CO₂ capture after the combustion process (post-combustion). Activated carbons obtained from this raw material have been poorly studied in the sorption of СО₂, and the production of activated carbons with the presented raw materials was carried out by thermal and hydrothermal carbonization and activation with steam. Physical activation, especially steam activation, is well studied and relates to the cleaner production of activated carbon, and the resulting AC can be widely used in many areas of industry. Therefore, in manufacturing, physical activation is preferable to chemical activation. In addition, activated carbons described in this article by steam activation were characterized by narrower and more extensive micropores. The adsorption isotherms of one-component CO₂ for porous carbons were measured at 273 K and 298 K up to 1 atm. In brief, this work presents the possibility of using the developed ACs as an inexpensive and highly efficient sorbent of CO₂ from air media, which is of significant interest to the environment.