Keywords

1 Introduction

The increasing emission of greenhouse gases, especially CO2, is responsible for various unwanted environmental changes such as the rise of sea level, drought, desertification, global warming and ocean acidification. Based on NOAA’s Global Monitoring Lab’s observation, atmospheric carbon dioxide concentration sharply increased from around 300 ppm in the 1960s to more than 400 ppm in the 2020s. The International Panel on Climate Change (IPCC) further predicts that atmospheric CO2 concentration will reach 570 ppm by 2100, resulting in a mean temperature increase of 1.9 °C [1]. This will lead to a significant impact on the global environment and further exacerbate adverse conditions. Thus, developing sustainable technologies for mitigating CO2 emissions to overcome global warming has become a priority as stated in the fifth assessment report of the IPCC. Carbon capture has been highlighted as a straightforward means that can directly decrease the distribution of CO2 in the atmosphere if appropriate absorbent and scalable techniques are employed [2].

In 2005, IPCC identified three possible intervention phases during the process of carbon emissions by significant stationary sources: precombustion carbon capture, oxy-fuel process, and post-combustion carbon capture [3]. Among them, post-combustion capture is the only technology that can be applied to existing large point sources (e.g., power plants and cement manufacturing plants) and therefore is a promising approach in the short and medium term with low technological risk. More specifically, CO2 separation technologies involve most commonly four processes, (1) absorption; (2) adsorption; (3) gas separation membranes; and (4) cryogenic distillation [3]. Adsorption, due to the low energy consumption, easy regeneration of adsorbent, adaptability at a wide range of temperatures and pressures, as well as lack of unfavourable by-product formation, has been considered a feasible process for CO2 capture at an industrial scale. Materials used for adsorption sit at the heart of any adsorption process. Zeolites, activated carbons, porous silicas, engineered carbon nanomaterials and metal-organic frameworks have been studied for carbon capture in the last few years [4]. However, none of the above-mentioned materials can be practically applied for CO2 capture without improving their selectivity of CO2 adsorption in cost-effective ways via appropriate engineering processes.

Biochar is defined as a carbon-rich, porous solid produced by the thermal decomposition of biomass in a process with the absence or limited access to oxygen at moderate temperatures (normally below 700 °C) [5]. As biochars can be prepared from abundant biomass such as crop residues, wood wastes, animal manure, food wastes, municipal solid wastes, and sewage sludge, it is sustainable and widely accessible to end users at low cost. Given the variety of raw materials and tuneable preparation conditions, biochars can be purposely prepared with abundant surface functional groups (C−O, C=O, −COOH, and −OH, etc.), which provide modifiable sites for the synthesis of various functionalized carbon materials, making biochar a versatile precursor amenable for many uses. Hence, engineering biochars is more promising than developing other carbon-based materials. For instance, compared with activated carbon, biochar exhibits significant advantages in energy demand and production cost (Table 8.1),

Table 8.1 Main distinctions between biochar and activated carbon (Adapted from [6,7,8])
Full size table

In the past few years, the great potential of using biochars and related engineered materials for environmental applications has garnered much attention. Carbon capture by biochars as well as enhancing the adsorption capacity by structural modification and surface functionalization have been of great interest to the research community. In this chapter, the advantages of using biochar and related engineered materials for CO2 adsorption and separation have been evaluated. The factors related to the CO2 uptake of biochar as well as the engineering strategies targeting CO2 separation are also discussed. Upon overview and discussion of the recent advances, the chapter concludes with key challenges and perspectives to advise development towards a sustainable approach to reducing carbon emissions.

2 Recent Advances in Using Biochar as an Adsorbent for Carbon Capture

Biochar is a low-cost and sustainable adsorption medium generated from natural biomass or agricultural waste. In industrial applications, biochars can be produced as a major product or by-product, accompanied by various liquid and gas flows (e.g., syngas and bio-oil). At present, the estimated amount of biochar produced by fast pyrolysis of biomass is 220 million tons/year (with an estimated 10% of biochar). With the continuously growing demand for biorefinery products, this amount is expected to increase sharply in the next 30 years [9]. Table 8.2 presents an overview of the production capacity of some significant types of biomass or biochars throughout the world. Due to the wide availability of biomass, biochars are about 10 times cheaper than other CO2 adsorbents [10]. However, raw biochars have limited CO2 adsorption capacity and various modification techniques are required to improve the performance of biochars for carbon capture.

Table 8.2 Production capacity of selected types of biomass/biochar (Adapted from [11])
Full size table

Many studies have demonstrated that having a microporous structure allows for more active sites on the biochar; enhancing the CO2 adsorption capacity. For instance, a novel activated carbon prepared from bamboo by KOH activation with enhanced specific surface area (SSA) showed a 15% CO2 adsorption capacity up to 1.50 mmol g−1 at 25 °C [12]. In addition, biochars prepared from food waste, wood waste gasification and KOH + CO2 activation showed that the development of a microporous structure contributed to a high CO2 adsorption capacity (qmax = 3.23 mmol g−1 at 25 °C) [13]. The CO2 adsorption performances of wood pellet-derived biochar with steam and KOH activation were compared [14]. The KOH-activated biochar exhibited a higher CO2 uptake (50.8 mg g−1) than that of steam-activated biochar (38.3 mg g−1) which supports the statement that a higher percentage of micropores contributes to higher CO2 uptake. It was also found that carbon capture from microporous biochar produced via KOH activation may be lower in high humidity conditions since water molecules occupy available micropore sites, acting as a competitive inhibitor. Thus, it is important to remove water from the gas stream before the CO2 adsorption process, while it is even more preferred to enhance the stability and adsorption efficiency of biochar in conditions with high relative humidity.

It has been widely accepted that surface functionalization of biochars will improve the capture of acidic CO2. Specifically, biochars with N-functional groups provide active sites for the uptake of CO2 through both chemical and physical adsorption. For instance, pyridinic-N, pyrrolic-N, and primary, secondary and tertiary amines can all react with CO2. The presence of these N-functional groups (Fig. 8.1) increases the alkalinity of biochar, promoting Lewis acid-base interactions, thus enhancing the CO2 adsorption [15]. Coffee grounds were used as the raw material for the preparation of biochars for CO2 adsorption through ammoxidation and KOH activation. The biochars with nitrogen-doping by melamine and KOH activation exhibited high microporosity and developed more pyrrolic nitrogen as active adsorption sites contributing to high CO2 uptake (2.67 mmol g−1 at 35 °C). In addition, the prepared biochar showed high stability (total decrease of ca. 6% in 10 cycles) and high selectivity of CO2 over N2 (CO2/N2 = 74.2) [16]. This excellent performance was attributed to a high proportion of micropore volume (Vmicro/Vtotal = 82%), increased nitrogen doping (5.1 wt%) and specific pyrrolic nitrogen (active adsorption sites).

Fig. 8.1
A structure and an X P S profile. a. The 2 D structure of the carbon sheet attached to nitro, nitroso, amine, pyrrolic, pyridonic, and pyridinic groups. b. The profile plots intensity versus energy level. 6 lines from N 1 to N 6 follow a left-sewed distribution. N 1 line has the highest peak.

(a) Schematic of nitrogen functionality on carbon surface and (b) XPS peak deconvolution results of nitrogen (N-1 s). Reprinted with permission from [17]. Copyright © 2022, Elsevier

Full size image

Lignin was used as a precursor for the preparation of hierarchical porous carbon by KOH activation and NH3 was selected as the source for N-doping [17]. The obtained biochar showed a large SSA (1631–2922 m2 g−1) and a highly developed porous structure with both micropores and mesopores. NH3 activation introduced basic functional groups including mainly pyridinic as well as amino and pyrrolic/pyridonic type of functionality with a total nitrogen content of 5.6–7.1 at.%. All these properties are beneficial for CO2 adsorption (5.48 mmol g−1 at 25 °C) and separation (CO2/N2 = 37–433), as well as a consistent working capacity (up to 10 cycles).

Recently, a simple method to prepare N-doped biochar for carbon capture was reported where the importance of amine and nitrile content was highlighted [18]. Bagasse and hickory chips were used as the raw materials and processed with pyrolysis at 450 or 650 °C under an N2 atmosphere followed by ball milling with NH3·H2O. The dehydration of oxygen-containing functional groups including carboxyl and hydroxyl transferred to nitrogen species such as –NH2 and C ≡ N on the surface of the prepared biochar. Meanwhile, a lower pyrolysis temperature (450 °C) was found to contribute to doping more nitrogen on the biochar surface compared to a higher pyrolysis temperature (600 °C). One possible explanation is that low pyrolysis temperature may result in higher concentration of oxygen-containing functional groups, thus more nitrogen-containing functional groups can be retained. Nevertheless, biochars (pyrolysis at 450 °C) with nitrogen-containing functional groups enhanced the CO2 uptake by 31.6–55.2% compared with those of green and ball-milled biochars.

Recent efforts have also evaluated biochars as a potential adsorbent for cyclic adsorption processes. Karimi et al. [11] comprehensively reviewed studies on cyclic adsorption processes (e.g., pressure/temperature/vacuum swing adsorption) which highlighted the application of biochars as a raw material used for engineered adsorbents in large-scale applications of gas adsorption and separation. Biomass waste (olive and cheery stone) was used as the raw material for preparing adsorbent for the separation of CO2 and CH4 [19]. The precursor was calcinated with CO2 in the atmosphere at a high temperature followed by loading in a bench-scale pressure swing adsorption (PSA) setup equipped with a fixed-bed adsorption column. The result showed that the olive stone-derived adsorbent was comparable with the commercial carbon adsorbent and presented a high sorbent selection value for efficient separation of CO2/CH4 gas mixture at given conditions. Karimi et al. [20] evaluated municipal solid wastes-derived biochars for carbon capture. The samples were activated using H2SO4 and thermal treatment at 400 °C and 800 °C, respectively. It has been found that the combination of H2SO4 treatment and activation at 800 °C contributed to the best CO2 adsorption capacity with an uptake (2.6 mmol g−1 at 40 °C and 2.5 bar) comparable with commercial carbon materials. The following scale-up process carried out in a conceptual PSA unit confirmed that municipal solid wastes derived biochars exhibited excellent performance as a source of material for carbon capture under conditions of post-combustion operation.

3 Key Engineering Strategies Targeting Biochar for Carbon Dioxide Adsorption and Separation

The CO2 adsorption capacity of biochar is the amount of CO2 uptake per unit weight of biochar, which mainly depends on its physio-chemical properties including composition, SSA, pore volume, pore size, surface functional groups, presence of alkali and alkali earth metals and hydrophobicity. These properties of biochar are significantly influenced by the type of feedstocks and the thermal treatment conditions as well as the use of additives and activation methods, which are further discussed in the following subsections.

3.1 Strategies for Modifying the Physical Properties of Biochar

The van der Waals force and pore filling mechanism play important roles between CO2 molecules and the solid phase of biochar [21, 22], thus increasing the SSA and pore volume as well as optimising the pore size and distribution (hierarchical pore structure) are the main strategies for modifying the physical properties of biochar.

3.1.1 Increasing the Specific Surface Area

Increasing the SSA means providing biochars with a wider surface for active sites responsible for carbon capture, thus contributing to a larger adsorption capacity. A positive relationship can be seen between the SSA (R2 = 0.6475), micropore area (R2 = 0.9032), and the CO2 adsorption capacity of biochars (Fig. 8.2). Many reports have concluded that a high SSA of adsorbents presents a superior CO2 adsorption capacity.

Fig. 8.2
2 scatter plots of C O 2 adsorption capacity versus surface and micropore areas. a. The plots are distributed on either side of the line, increasing from (0, 32) to (530, 75). b. The plots are distributed on either side of the line, increasing from (100, 20) to (630, 102). Values are estimated.

Relationship between the SSA (a), micropore area (b), and CO2 adsorption capacity of biochar. Reprinted with permission from [26]. Copyright © 2022, Elsevier

Full size image

Creamer et al. [23] investigated the effect of SSA of biochar on CO2 adsorption performance. Sugarcane bagasse and hickory wood were used as the precursor for the preparation of biochars at higher pyrolysis temperatures with a large SSA and excellent CO2 capture performance (73.50 mg g−1 and 61.0 mg g−1 at 25 °C). Physical adsorption played an important role by bonding CO2 to the surface of biochar, thus SSA was an important contributor to CO2 adsorption. Huang et al. [24] prepared biochars by microwave pyrolysis of rice straw for CO2 capture and reported the high correlation (correlation coefficient: 0.84) of the SSA with the CO2 uptake of biochar. The prepared biochar exhibited a CO2 adsorption capacity of up to 80 mg g−1 at 20 °C. Sun et al. [25] prepared engineered biochar from wood waste by pyrolysis in molten salts. It was found that high temperature (800 °C) and ternary carbonate contributed to the largest SSA (476 m2 g−1) and highest CO2 capacity (4.5 mmol g−1, 0 °C, 100 kPa) of the resultant biochar. The high selection parameter (S) in both PSA and vacuum swing adsorption (VSA) processes and high recovery rate (89%) of the molten salt showed great potential for repurposing biowaste as green and efficient adsorbents for carbon capture.

3.1.2 Increasing Pore Volume and Optimising Pore Size

During biomass pyrolysis, the release of volatile organic matter causes the formation of porous structures in the resulting biochar. This behaviour is related to the above-mentioned SSA. In addition, the specific pore structure (e.g., volume and size) can also alter the carbon capture capacity. The CO2 uptake was found to closely follow a linear correlation with the size of pores ≤1.5 nm at the ambient pressure [27]. At 1 bar, pores <0.8 nm contributed the most to the CO2 uptake and at 0.1 bar pores ≤0.5 nm were preferred. The coefficients of determination between the CO2 uptake at a certain pressure and the cumulative pore volume of pores less than or equal to a certain pore diameter are shown in Fig. 8.3a. It agrees well with the CO2 adsorption performance of bamboo-derived activated carbon [28], where the volume of micropores in the range of 0.33–0.82 nm demonstrates a positive correlation with CO2 adsorption in Fig. 8.3b.

Fig. 8.3
A contour and scatter plot. a plots pore diameter versus pressure. The plot has regions with various values between 0.50 and 0.95. b plots C O 2 adsorbed versus pore volume. Plots are distributed on either side of the line, increasing from (0.21, 4.2) to (0.37, 7.0). An insert with a porous surface.

(a) Coefficient of determination between CO2 uptake between 0.1 bar and 1.0 bar (x-axis) and the volume of pores smaller to threshold size (y-axis) and (b) volume of micropores in the range of 0.33–0.82 nm has a good linear relationship with CO2 adsorption on bamboo-derived activated carbon. Reprinted with permission from [27]. Copyright © 2022, RSC. Reprinted with permission from [28]. Copyright © 2023, John Wiley and Sons

Full size image

The type of biomass or feedstock is one of the main factors that may influence the pore volume and pore size of biochar. Specifically, biochars produced from biomass materials that are high in cellulose are known to develop more pores that are less than 2 nm during the pyrolysis, while pyrolysis of biomass materials that are high in lignin results in pores larger than 50 nm [29]. In addition, higher pyrolysis and activation temperatures result in the widening of the pore size [30]. It is worth noting that over-pyrolysis and activation of biochar may lead to excess widening of the pore which may be detrimental to the CO2 adsorption capacity as micropores are more desirable for gas adsorption.

Igalavithana et al. [31] prepared different types of biochars from pine sawdust and paper mill sludge and investigated the effect of porous structure on carbon capture performance. It was found that the SSA and microporosity, especially the micropores of 0.5 nm diameter played an important role in CO2 adsorption (positively correlated to carbon capture with r = 0.960 and p = 0.039). The study demonstrated that the feedstock, pyrolysis temperature and activation method (steam activation) had a great effect on the textural, surface, and structural properties of biochar as well as the resulting carbon capture performance.

3.1.3 Developing a Hierarchical Pore Structure

The International Union of Pure and Applied Chemistry defines pores with a diameter > 50 nm as macropores, those with a diameter between 2 nm and 50 nm as mesopores, and those with a diameter of <2 nm as micropores [32]. Micropores with a pore size of <1 nm predominantly affect CO2 adsorption. However, hierarchical structures with balanced and cross-linked macro-, meso- and micro-porosity also play a significant role in the adsorption process [33]. Feng et al. prepared biochars from corn straw with a hierarchical-functionalized structure and explored its adsorption properties of CO2 with both experimental data analysis and molecular dynamics simulation [34]. The results showed strong evidence of a link between one-step activation with H2O and the construction of functional groups on the biochar surface, however, the two-step activation with H2O and NH3 was beneficial to the enhancement of pore formation and aromatization of biochar. The adsorption sites of CO2 were mainly generated from micropores with a diameter of less than 0.7 nm, while the mesopores provided a significant contribution to the total adsorption capacity due to the creation of a concentration gradient for diffusion based mass transfer. In another study, hierarchically porous carbon was prepared from an ocean pollutant (Enteromorpha prolifera) via hydrothermal carbonization, KOH activation and HF washing [35]. The intrinsic sponge structure of the precursor, hydrolysis of hemicellulose, carbon gasification, KOH activation and inorganic compounds together contributed to the hierarchical carbon with micropores, mesopores and macropores (Fig. 8.4). As a result, the diffusion of CO2 into the bulk carbon skeleton was accelerated by the presence of meso- and macropores in the adsorbent, thus improving the kinetics of adsorption.

Fig. 8.4
A schematic diagram presents the formation of porosity in the carbon compound. The compound attached to metal, nitrogen sites, oxygen groups, and pore induced sites undergoes K O H activation and H F etching, forming biochar. 2 microscopic images of the biochar with a porous surface.

Diagram of porosity formation and enhanced CO2 capacity of KOH activated and hydrothermally treated ocean waste-derived biochar. Reprinted with permission from [35]. Copyright © 2022, Elsevier

Full size image

3.2 Strategies for Functionalizing Biochar for High-Performance CO2 Adsorption and Separation

The uptake of CO2 by biochar surface is also influenced by the functionality (chemical properties) of the biochar including alkalinity, presence of surface functional groups, mineral composition, hydrophobicity, etc. Thus, the CO2 adsorption capacity of biochar can be optimised by introducing functional groups and elements with a strong affinity for acidic gas.

3.2.1 Introducing Basic Functional Groups

The reaction between the acidic CO2 molecules and basic nitrogen-containing functional groups is well accepted as the main contribution to CO2 capture. Pyridone, pyrrole, amine, pyridine-N-oxide, quaternary-N, cyanide, and pyridine groups with different basic strengths are common N species on the carbon surface. Among them, pyridone, pyridine and pyrrole are well reported to enhance CO2 adsorption due to various interactions including hydrogen bonding and Lewis acid-base interactions [36, 37]. Figure 8.5 summarises different methods (agents) for introducing various N-containing functional groups.

Fig. 8.5
A cyclic diagram exhibits the methods for the introduction of N functional groups. It includes pyrrole and pyridines for nitration, amino groups, nitro and amino groups for nitration and reduction, nitro groups for oxidation, and nitro and amino groups for oxidation and nitration.

Different methods for introducing various N-containing functional groups. Reprinted with permission from [38]. Copyright © 2022, Elsevier

Full size image

The direct pyrolysis of nitrogen-rich precursors is a sustainable method for doping nitrogen atoms into the carbonaceous matrix of biochar, which is conducive to the application of the carbon capture [39, 40]. Nitrogen-doped microporous carbon was prepared from chitosan and K2CO3 as a nitrogen-containing precursor and activation agent, respectively [41]. The high CO2 uptake by the porous carbon was attributed to the large pore volume, well-defined microporosity and high nitrogen content. In the XPS analysis of N-containing functional groups, the dominant peak belonged to pyridone, and pyridine contributed less. As for pyridone nitrogen, the p orbital in –OH produces a p–π conjugation effect with the π bond and as consequence, the Lewis basicity of the nitrogen located in the ortho-position of –OH with a stronger negative charge is enhanced. Thus, pyridonic nitrogen was attributed as the main active site for CO2 capture. Density functional theories calculation also confirmed that the pyridone groups exhibited the maximum energy of adsorption contributed by the H bond between the H atom of the –OH and the O atom of the CO2 with high electronegativity [37].

Sawdust was impregnated with a urea phosphate solution and kept for 24 h, then carbonized at a high temperature (600–900 °C) under N2 followed by hot water washing and drying [42]. The obtained N-doped biochar exhibited a large SSA (1189 m2 g−1), high micropore volume (0.433 cm3 g−1), high nitrogen content (4.24 at.%) with the main basic functional groups including pyridinic and pyrrolic N, and decent CO2 adsorption capacity (1.34 mmol g−1 at 30 °C and 1 bar). These results confirmed that the adsorption of CO2 was related to the synergistic effect of SSA, micropore volume and N-containing functional groups [43]. Specifically, due to the strong polarity and improvement for acid-base reaction, pyridinic N was more conducive to CO2 adsorption than other types of N species. On the other hand, pyrrolic N can promote the hydrogen-bond interaction between the biochar surface and CO2 (Fig. 8.6). In addition, a high pyrolysis temperature (900 °C) was found to transfer some nitrogen groups into aromatic nitrogen-containing groups and weaken the alkalinity of the biochar.

Fig. 8.6
A process flow diagram explains the synthesis of N doped biochar. The biomass undergoes the co-pyrolysis process that includes decomposition, N doping, and pore formation, resulting in N doped porous biochar that results in carbon capture with acid base and hydrogen binding interactions.

One-step method for preparation of microporous N-doped biochar from plant biomass using urea phosphate (N-5: pyrrolic N; N-6: pyridinic N). Adapted from [42]. Copyright © 2022, Elsevier

Full size image

CO2 adsorbent was prepared from biomass waste by three ammoxidation methods and KOH activation (400 °C for 1 h and 600 °C for 1 h under an N2 atmosphere) [16]. In particular, the process of using melamine contributed to the smallest SSA (990 m2 g−1) but the highest nitrogen content (5.1%) and the largest CO2 capacity (117.5 mg g−1 at 35 °C) of the resultant biochar. The study suggests that N-functional groups (mainly pyrrolic nitrogen) rather than the SSA are considered to be the dominant factor for CO2 adsorption. The high value (49.9 kJ mol−1) of adsorption enthalpy of biochar doped with pyrrolic nitrogen indicates that the interaction (e.g., hydrogen bonding or intermolecular force) between adsorbent and CO2 was enhanced, resulting in the improved carbon capture performance.

Xie et al. [44] investigated the bean dregs-based porous carbon for CO2 capture. A significant increase of C and N contents in the hydrothermally treated biochar was found and the pyridinic-N and pyrrolic-N accounted for more than 60% of the total N-containing groups. As a result, the isosteric heat of adsorption at the low CO2 uptake stage was much higher than the value of interaction between CO2 and pure carbon-based materials, indicating that the adsorption of CO2 on N-rich biochar is not completely physical and may occur via intermolecular forces and covalent bonds. Nuclear magnetic resonance spectroscopy data verifies that interactions between N-containing species and CO2 result in the formation of new chemical bonds (Fig. 8.7).

Fig. 8.7
2 reversible reactions. a. 2 reactions. 2 reactants react to yield the product, which further reacts with B to form another product. b. 3 reactions. Pyridine-N reacts with H 2 O to yield the product, which further reacts with O C O O H to yield the final product.

Adsorption mechanism of CO2 on (a) pyrrolic-N and (b) pyridine-N. Reprinted with permission from [44]. Copyright © 2023, John Wiley and Sons

Full size image

Apart from pyridone, pyridine and pyrrole, other N-containing groups including Amine-N (− NH2 and C–N) and N=N [45, 46] were also investigated for their performance of carbon capture. It was found that all these functional groups presented the ability to interact with CO2, and most of them can be regenerated except for the C-N.

3.2.2 Introducing Oxygenated Functional Groups

It has been reported that hydrogen bonding plays a critical role in the interaction between adsorbate and adsorbent when oxygenated functional groups are presented on the surface of the adsorbent [38, 47]. Although most oxygen-containing functional groups (e.g., –COOH, –OH, C=O, phenolic, lactone, and peroxide groups) are acidic which would inhibit the capturing of acidic CO2, following studies found that the adsorption of CO2 may be enhanced by the functional groups including –OH and –COOH, when they were presented on the carbon skeleton. Xing et al. [48] reported the influence of introducing O-containing functional groups to the adsorbent matrix (by chlorination and followed by HNO3 oxidation) on the CO2 adsorption capacity of carbide-derived carbon (CDC). It was found that the CO2 adsorption capacity per unit SSA was positively related to the oxygen content of the carbons, suggesting that the CO2 uptake was significantly facilitated by the doping of O-containing groups. Quantum chemical calculation further supports that the introduction of oxygen atoms into the carbon surface promotes hydrogen bonding between the CO2 molecules and the carbon surface (Fig. 8.8). In addition, the binding energy difference between oxygenated and non-oxygenated complexes roughly agreed with the difference in CO2 adsorption heat between the pristine CDC and oxygenated CDC. The findings indicate that acidic O-containing groups of carbon adsorbents can also facilitate CO2 adsorption.

Fig. 8.8
2 models and a 3 D grouped bar graph. a and b. The ball and stick model of O C S M and C S M. c plots hydrogen bond energy versus adsorption site. The values are plotted for 2 models. O C S M has the highest value for sites 3 and 5 at 14.5. C S M has the lowest value for site 5 at 1.

Theoretical models for (a) oxygen-containing carbon surface and (b) pure carbon surface (red ball: oxygen atom; grey ball: carbon atom; small white ball: hydrogen atom). (c) Hydrogen bond energies at different adsorption sites. Reprinted with open access from [48] under the terms of the creative commons attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0)

Full size image

3.2.3 Loading Alkaline and Alkaline Earth Metals

It has been demonstrated that the carbon frameworks with open metal sites tend to have a high affinity for CO2 [49]. The basic nature of metal oxide and carbon framework may improve the selectivity of adsorbent on CO2 and suppress the adsorption of other gases such as N2 and CH4. This is because metal oxyhydroxides tend to bond with acidic CO2 when polar surfaces are in contact. Creamer et al. [50] prepared biochar/metal (Al, Mg and Fe) oxyhydroxide composites using cottonwood as the precursor and evaluated their capacity to uptake CO2. The treatment with a sequence of metal ions impregnation and pyrolysis at 600 °C prepared biochar loaded with metal oxyhydroxide nanoparticles. The CO2 uptake by biochar/metal oxyhydroxide composites was attributed to the surface adsorption mechanisms contributed by both carbon matrix and metal oxyhydroxide particles. The high CO2 uptake by AlOOH–biochar (71 mg g−1 at 25 °C) is roughly equal to commercial adsorbents, and together with the low-cost regeneration at 120 °C (90% and 99% desorption) suggests that metal oxyhydroxide–biochar composites can be used for carbon capture with high efficiency at a low cost. Lahijani et al. [51] further investigated the impact of pyrolysis temperature (500 °C, 700 °C and 900 °C) and type of metal oxide (Mg, AL, Fe, Ni, Na and Ca) on the carbon capture ability of the modified biochar prepared from walnut shell. It was found that increasing the pyrolysis temperature was effective to enhance the SSA and porosity of the biochar. In particular, the largest SSA (397 m2g−1) and highest pore volume (0.198 cm3 g−1) were obtained by preparing the sample at 900 °C. Meanwhile, the loading of basic metal sites onto the metal/biochar composite surface enhanced the adsorption of acidic CO2 gas in the order of Mg > Al > Fe > Ni > Ca > green-biochar > Na. In addition, Mg-loaded biochar exhibited high stability and reusability upon 10 cycles of CO2 adsorption and desorption with almost no compromise in its carbon capture performance.

3.3 Summary

In addition to the above-mentioned strategies to engineer biochars for improved CO2 adsorption and separation, studies have indicated that biochars with hydrophobic and non-polar characteristics may enhance CO2 uptake by restraining the interference of H2O (polar molecule) [8, 52]. The carbonization process can reduce the concentration of O, H, N and S in the biochar matrix, thus increasing the degree of hydrophobicity by increasing the degree of aromaticity, resulting in a high potential to adsorb CO2 when competing with water molecules in humid conditions [29]. In summary, CO2 adsorption on biochars is a complex interaction that may be affected by multiple factors including van der Waals forces, hydrogen bonds, micropore filling phenomenon as well as Lewis acid-base reaction when functional groups (N and O) presented on the surface of adsorbents. Therefore, tuning the properties of biochars and developing their surface functionality to enhance the above-mentioned interactions is critical to engineering biochars with improved CO2 adsorption capacity and selectivity.

4 Challenges and Perspectives

The use of biochars as a widely accessible and low-cost adsorbent for CO2 capture promises chances for valuable application of biomass resources and an alternative approach for reducing carbon emissions, thus contributing to the achievement of our sustainable development goals. As an emerging research area of interest, the practical use of biochars for carbon capture is yet to be realised with efforts addressing challenges in all aspects from the initial biochar preparation and engineering to the application of CO2-saturated biochar in real practices. Based on the above overview, some key challenges and corresponding perspectives are given as follows.

Biochar is a complex material with variants in raw materials and preparation methodologies. This provides possibilities for preparing versatile products for multiple purposes including CO2 adsorption. However, there is a key challenge to developing standardised protocols for biochar preparation and engineering towards targeted purposes. Addressing this challenge will require a fundamental understanding of the correlations among feedstock of biomass, engineering strategies (e.g., activation and modification), the physiochemical properties of biochar and the CO2 uptake capability. The emerging application of artificial intelligence (AI) techniques such as machine learning in biomass valorization is promising to assist in identifying these correlations based on a considerable amount of experimental data.

Overall, achieving ideal adsorption performance will require engineering biochar by both physical and chemical means. This also means extra costs and the possible use and/or emission of toxic chemicals. From the perspective of life-cycle analysis, the process of biochar preparation and engineering is carbon emissive, despite the facts (1) producing and storing biochar is considered a negative emission process [53] and (2) the adsorption and storage of CO2 in biochar further contribute to emission reduction. Life-cycle analysis is thus highly recommended to detail the emission portfolios.

Enabling and promoting the practical application of biochar as a CO2 adsorbent also expects a shift from Carbon Capture & Storage to Carbon Capture & Utilization, whilst re-utilizing engineered biochar for carbon capture in a circular manner. This calls for efforts in (1) improving the reusability of biochar, and (2) developing proof-of-concept of integrated biochar carbon capture and CO2 utilization or valorization. Recent drastically progressing technologies such as electrochemical CO2 reduction [54] and the use of CO2 for algae cultivation [55] have provided wide opportunities for the post-adsorption valorization of CO2.

5 Conclusions and Future Outlook

Engineered biochars with their enhanced inherent properties are promising sustainable materials for CO2 capture. Specifically, the specific surface area, micropore area and volume, hierarchical pore structure, presence of functional groups (e.g., N-containing functional groups) and hetero elements are great contributing factors to CO2 adsorption capacity. Thus, strategies of biochar modification through physical and chemical processes will significantly improve the aforementioned properties, thereby enhancing the CO2 uptake of biochar.

On the other hand, the practical use of biochar is yet to be realised in all aspects from the initial biochar preparation and engineering to the application of CO2 capture in large-scale production. Hence, future studies should focus on developing standardised biochar preparation and engineering protocols towards specific carbon capture scenarios with a complete life-cycle analysis. The high-value utilization of biochar as byproducts from biorefinery or lignin residue after valerization of cellulose and hemicellulose is suggested to increase the overall competitiveness of bioenergy. Novel technologies and biochar-based materials such as nano-biochar composites and biomass-derived carbon foam should be developed to enhance the CO2 adsorption capacity of biochar. Study on the adsorption and reaction mechanism of CO2 with specific functional groups of biochar is preferred as it not only helping understand the adsorption process but also paving the way for CO2 conversion on carbon capture and utilization. Moreover, industrial-scale application of biochar for CO2 capture should also be highlighted as well as developing proof-of-concept of integrated biochar carbon capture and CO2 utilization.