Biochar Synergistic New Ammonia Capture of CO₂ and High-Value Utilization of Intermediate Products

Abstract

In the face of global warming and the urgent need for CO2 reduction, carbon capture, utilization, and storage, technology plays an important role. Based on the traditional liquid-phase and solid-phase CO2 capture technologies, the liquid-phase ammonia and biochar CO2 capture technologies are reviewed with emphasis. A multiphase carbon capture technology that uses biochar to enhance the mass transfer-crystallization process of the new ammonia CO2 capture technology is proposed. High CO2 capture efficiency, limited ammonia escape, and low system energy consumption can be achieved through the orderly construction of three-dimensional graded pore channels and the directional functionalization of biochar. The intermediate products of CO2 captured by the ammonia process and the special agricultural waste rice husk components were considered. The use of rice husk-based biochar for CO2 capture by synergistic new ammonia method and the process regulation of intermediate products to prepare nano-silica to achieve high-value utilization of interstitial products of carbon capture. This technology may be important to promote the development of CO2 capture technology and CO2 reduction.

1. Introduction

1.1 Current status of CO₂ emissions and CCUS technology

Carbon is cycled between different sources (atmosphere, ocean, terrestrial biota, and marine biota) in the form of carbon dioxide, carbonates, and organic compounds. Human activities have disrupted the balance of this cycle, and a large amount of CO₂ emissions has led to an increasingly serious greenhouse effect. Global climate change has caused widespread concern in the international community. According to the report of the International Energy Agency (IEA) [1], to achieve the target of global average temperature increase within 2°C above the pre-industrial level by 2100 and to try to limit it to 1.5°C, direct CO₂ emissions from industrial production need to be reduced by about 30%, and CO₂ emissions per unit of GDP need to be reduced by about 60% by 2050 compared with the current level. However, as things stand today, global CO₂ emissions from energy combustion and industrial processes will rebound in 2021 to the highest annual level ever recorded (Figure 1(a)). Emissions increased by 6% compared with 2020 (Figure 1(b)). The largest increase in CO₂ emissions by sector in 2021 is from electricity and heat production, accounting for 46% of global emissions (Figure 1(d)). Coal accounts for more than 40% of the increase in total global CO₂ emissions, a record high (Figure 1(c)). As the most important coal-consuming industry, coal-fired power plants are the most important source of CO₂ emissions. Hence, the research on CO₂ reduction technology for coal-fired power plants has profound significance.

Figure 1.

(a) CO₂ emissions from energy combustion and industrial processes, 1900–2021, (b) annual change in CO₂ emissions from energy combustion and industrial processes, 1900–2021, (c) change in CO₂ emissions from fossil fuels, 2019–2021, relative to 2019 levels, (d) annual change in CO₂ emissions by sector, 2020–2021 [1].

Carbon Capture, Utilization, and Storage (CCUS) technology are considered the most economical and feasible way to reduce greenhouse gas emissions and mitigate global warming on a large scale in a short period. CCUS technology captures CO₂ from large point sources such as power plants or directly from the atmosphere. The captured CO₂ will be compressed and transported for various applications or injected into deep geological layers for permanent storage. As early as 2005, the Intergovernmental Panel on Climate Change (IPCC) identified CCUS as a key technology in mitigating the greenhouse effect [2]. Today, strengthened climate goals and new investment incentives have created unprecedented momentum for CCUS, and many countries have taken steps to develop CCUS technologies [3, 4, 5, 6, 7]. Projections indicate [8] that the least-cost pathway to “≤2°C” is to capture and sequester about 4 billion tons of CO₂ per year by 2040 and that the current CO₂ capture capacity is still far from the required amount, making CO₂ capture technology critical in the overall carbon reduction and CCUS system.

1.2 CO₂ capture

1.2.1 CO₂ capture technology

There are four main CO₂ capture technology routes: pre-combustion capture, oxygen-enriched combustion, post-combustion capture, and chemical loop combustion. In pre-combustion capture technology, fossil fuels are converted to a syngas of carbon dioxide and hydrogen before combustion using gasification or reforming technology so that the “carbon” in the fuel does not participate in the combustion process [9]. Oxyfuel combustion uses oxygen instead of air for combustion and can be used without considering the separation of nitrogen and carbon dioxide, a technically feasible process [10]. Post-combustion capture technologies remove CO₂ from the flue gas after combustion has occurred. In recent years, chemical loop combustion (CLC) has also been developed. It uses metal oxides to transport the oxygen required for combustion to prevent direct contact between fuel and air, with its inherent CO₂ capture capability [11]. Of the above capture technologies, post-combustion CO₂ capture is the most mature and most thorough and is the preferred option for retrofitting existing power plants.

1.2.2 Post-combustion CO₂ capture

Post-combustion CO₂ capture technologies mainly include adsorption, absorption, membrane separation, and low-temperature distillation. Low-temperature distillation is a method of separation using the difference in boiling point or volatility of each component gas in the gas mixture. This method has high CO₂ separation efficiency and purity and can directly produce liquid CO₂ for storage and transportation [12]. The absorption method includes chemical absorption and physical absorption. Physical absorption involves using a physical solvent to dissolve a component gas. The solubility increases with increasing pressure and decreasing temperature; therefore, the optimal conditions for the CO₂ absorption process are high pressure and low temperature [13]. The chemical absorption method uses an alkaline absorber to contact and react with CO₂ in the flue gas to remove CO₂. The salts generated by the reaction will decompose and release CO₂ under certain conditions, thus removing and enriching CO₂ from the flue gas [14]. The principle of membrane separation is that different components pass through the membrane with different selectivity. The membrane allows only specific gases to pass through, thus achieving separation and enrichment. The performance of the membrane system is influenced by the flue gas conditions [15], the enriched CO₂ concentration is low, and the separation conditions are demanding. Adsorption can be divided into physical adsorption and chemisorption, with physical adsorption having a weak binding force, a relatively small heat of adsorption, and easy desorption. On the other hand, chemisorption is caused by chemical bonding between the adsorbent and the adsorbent, the adsorption is often irreversible, and the heat of adsorption is usually larger [16]. Adsorption differs from the absorption process in that the adsorption efficiency is mainly influenced by the specific surface area, selectivity, and regeneration characteristics. Table 1 compares the above four post-combustion CO₂ capture technologies, and all of these methods inevitably have various problems. Therefore, the development of new ammonia decarbonization technology will become the main theme of CO₂ capture technology. However, its ammonia escape problem also needs to be further strengthened. The study of solid-phase adsorption combined with ammonia liquid-phase absorption to achieve two-phase synergistic CO₂ capture will have far-reaching significance in the future.

Technology	Advantage	Disadvantages	Reference
Low temperature	Technology maturity	Only for high CO2 concentration, low temperature, high energy consumption	[17, 18]
Adsorption	Reversible process, recyclable adsorbent, high adsorption efficiency	Requires high-temperature adsorbent and high energy for desorption	[19, 20]
Membrane separation	High separation efficiency	Operational problems include low flux and scaling	[21]
Absorbent	High absorption efficiency, renewable absorbent, mature process	Absorption efficiency depends on CO2 concentration, high energy consumption for absorber regeneration	[22]

Table 1.

Comparison of different CO₂ capture technologies.

2. Ammonia-based liquid-phase CO₂ capture technology

2.1 Liquid-phase chemical absorption of CO₂

The commonly used absorbents for chemical absorption targeting CO₂ capture are monoethanolamine (MEA), ammonia, and potassium carbonate. The CO₂ capture efficiency of MEA is very high, but it has high regeneration energy, a high corrosion rate, and is susceptible to oxidative degradation. The high energy consumption of CO₂ capture using aqueous amines is also one of the main drawbacks that limit its wide application. Non-aqueous absorbents have an absorption capacity comparable to aqueous MEAs and higher desorption efficiency, leading to a larger cycle capacity and nearly half the energy consumption (Figure 2(a)). Bougie et al. [27] investigated new non-aqueous MEA absorbers that greatly reduced energy consumption and improved CO₂ absorption kinetics. The regeneration of MEAs is also a major challenge, with approximately 80% of the total energy consumption in the CO₂ capture process occurring in the solvent regeneration process [28]. Many studies have shown that carbonate solutions can be used for CO₂ uptake, and K₂CO₃ solutions have higher capture capacity than other carbonate solutions and are more commonly used in industry [29]. Although carbonate solutions have been extensively studied, the kinetics and thermodynamics of their absorption solutions still need to be investigated, and K₂CO₃ solvents may be subject to corrosion due to flue gas contaminants and solvent degradation.

Figure 2.

(a) Comparison of aqueous and non-aqueous MEA absorbents [23], (b) Schematic diagram of NH₃-CO₂-H₂O three-phase system [24], (c) Applicable range of different CO₂ capture technologies (based on operating pressure and CO₂ concentration) [25], (d) Reaction mechanism of CO₂ absorption by ammonia method [26].

2.2 Absorption of CO₂ by ammonia

2.2.1 Ammonia CO₂ capture technology

Figure 2(c) shows that reliable absorbents for low concentration CO₂ capture without pressurization are amine-based and ammonia-based CO₂ capture technologies. Ammonia-based CO₂ capture is considered a viable carbon capture technology due to the high corrosiveness of MEA and regeneration problems over conventional amine-based CO₂ capture technologies in terms of technical and economic advantages. The CO₂-NH₃-H₂O system (Figure 2(b)) thermochemical properties have been reasonably well explained in recent studies. Although ammonia is the simplest amine, its interaction with CO₂ is quite complex, involving gas-liquid-solid three-phase reactions, making the application of CO₂-NH₃-H₂O systems in CO₂ capture poses some challenges. Thomsen and Rasmussen [30] developed a thermodynamic model with a temperature. The model can be used not only for gas-liquid systems but also for gas-liquid-solid equilibria, including forming NH₄HCO₃, (NH₄)₂CO₃-H₂O, and NH₂COONH₄. Que and Chen [31] developed an electrolyte NRTL activity coefficient model that can well represent the thermodynamic properties of the NH₃-CO₂-H₂O system when the CO₂ loading reaches a consistent level. The availability of these models allows to reliably calculate the thermochemical properties of the CO₂-NH₃-H₂O system under various conditions and to assess the energy performance of the capture process [32]. The uptake of CO₂ by ammonia is a relatively slow process; therefore, it is important to understand the reaction mechanisms/kinetics involved in the uptake chemistry. The most important reaction in the presence of free ammonia is the reaction of NH₃ with CO₂, and the reaction scheme is shown in Figure 2(d). The equilibrium constant of carbamate of MEA is much higher than that of ammonia, and the yield of ammonia-derived carbamate is lower than that of the equivalent monoethanolic ammonium carbamate, indicating that ammonia possesses a higher CO₂ capture capacity.

2.2.2 Ammonia escape

Ammonia CO₂ capture technology has many advantages, but it also has drawbacks in current applications: (1) low CO₂ absorption rate; (2) serious ammonia escape; and (3) high energy consumption for desorption and regeneration. The high volatility of ammonia is the main drawback of ammonia CO₂ capture technology. The concentration of NH₃ escaping from the emission gas of this technology is usually above 10,000 ppm [33], which is much higher than the emission standard of 50 ppm. The high NH₃ escape rate also decreases the concentration of NH₃ in the solution, which reduces the CO₂ absorption capacity [34]. Therefore, it is imperative to develop effective methods to suppress ammonia leakage or recover the leaked ammonia. The use of acid washing, membrane technology, and additives are common strategies to control ammonia escape.

2.2.3 Ammonia-ethanol mixture absorber

To better solve the above problems, many scholars have proposed the modification of CO₂ absorption by ammonia solution using additives, which can inhibit not only NH₃ escape but also improve CO₂ absorption performance. Many scholars have studied the CO₂ capture performance of ammonia with additives [35, 36, 37, 38, 39, 40, 41], among which Gao and Zhang et al. [39, 41] have shown significant advantages in various aspects of using ethanol as an additive. Ammonia and additives can, to some extent, promote each other to improve the CO₂ uptake rate of ammonia [42]. However, a slight contradiction emerged between the hybrid absorber improving the absorption rate and inhibiting ammonia escape [43]. The additive mainly binds the free ammonia in the ammonia solution by hydrogen bonding and thus inhibits ammonia escape. However, the additive cannot achieve effective ammonia release when this hybrid absorber absorbs CO₂, which will reduce the liquid-phase partial pressure of free ammonia and adversely affect the absorption process. The advantages of an “ammonia-ethanol adsorbent mixture” for CO₂ absorption and capture are significant [41]. However, many aspects still need to be improved. It is urgent to develop a new ammonia carbon capture technology based on this idea to maintain its advantages and avoid its shortcomings.

3. Biochar-new ammonia synergistic carbon capture

3.1 Solid-phase adsorption CO₂ capture

The adsorption of CO₂ by porous carbon materials is an exothermic process, with the heat of adsorption of physical adsorption processes ranging from −25 to −40 kJ/mol [44], and the amount of adsorption is directly related to the porous structure of the adsorbent and the active functional groups on the surface. The molecular kinetic diameter of CO₂ is 0.33 nm, so micropores (<1 nm) are the main sites for CO₂ adsorption (Figure 3(a)). Still, only micropores cannot achieve high adsorption capacity, and a suitable pore structure is required [45]. Macropores and mesopores act as channels for diffusive CO₂ transport and can facilitate CO₂ adsorption in micropores. CO₂ being polar and acidic molecules, basic and polar functional groups (e.g., pyridine, pyrrole nitrogen) also plays an important role in adsorption [45]. Therefore, when selecting CO₂ adsorbent, the economy and reliability should be satisfied. The adsorbent’s pore structure and surface functional groups should be considered to ensure that the distribution of the two reaches a certain balance. Too much pursuit of one side will lead to the deterioration of the other side, resulting in a worse adsorption effect. (Figure 3(b–e)) [47].

Figure 3.

(a) Molecular dynamics and quantum chemical simulation of CO₂ adsorption by porous carbon materials [45], (b) various structures in biochar micropores that effectively enhance CO₂ adsorption [46], (c) effect of adsorbent porosity and chemical properties on CO₂ adsorption performance [47], (d) correlation between CO₂ adsorption, micropore, and mesopore volumes at 25°C and 5 bar [48], (e) correlation between different pore ratios and relative CO₂ adsorption [45].

Non-carbon-based solid adsorbents, mainly MOF and zeolite, are well studied and widely used. Almost all metals and a large amount of organic matter can make MOF, which is widely used in adsorption due to its extremely high porosity and specific surface area. When the partial pressure of CO₂ is low (<0.2 bar), the adsorption capacity of MOF is poor [49], and impurity gases replace the skeletal ligands during the CO₂ capture process, leading to degradation of MOF and a decrease in the capture capacity. Zeolites have a regular pore size of 0.5–1.2 nm [50] and have been widely investigated for CO₂ capture due to the strong electrostatic interaction between CO₂ and alkali metal cations in the zeolite skeleton [51]. Siriwardane et al. [52] showed that natural zeolites with high sodium content exhibited high CO₂ adsorption capacity. However, the electrostatic interaction between CO₂ and alkali metal cations in the zeolite skeleton is reduced by water [53], and therefore only in dry gas streams is CO₂ separation effective. Among the carbon-based materials, activated carbon is one of the most commonly used adsorbents in industry. It is less costly than other adsorbents [54], but its adsorption capacity is only comparable to that of zeolites at high CO₂ pressure [55], and the heat of adsorption is lower than that of zeolites. By introducing impurity atoms or acid-base sites, activated carbon can appropriately improve adsorption selectivity and adsorption capacity. As a new carbon-based material, carbon nanotubes have also received attention in gas adsorption [56, 57].

3.2 Biochar adsorbent

The raw materials of biochar are widely sourced, and the cost is lower than other adsorbents. The biochar prepared from different raw materials is different due to their intrinsic elemental composition ratio and structure. Biochar prepared from raw materials with high strength and carbon content, such as wood chips, coconut shells, date kernels, and rice husks, has a more desirable CO₂ adsorption capacity [58]. During preparation, the carbonization temperature affects the structure, surface functional groups, and elemental composition of the final material and 500–800°C is considered the optimal temperature range for carbonization [59]. Thermal degradation of biomass at high temperatures in limited or complete anoxia is central to biomass conversion into porous carbon. Most biomass consists of lignin, cellulose, and hemicellulose, prepared under different pyrolysis and activation conditions to obtain different pore structures, group ratios, and surface chemistry (Figure 4(a and b)).

Figure 4.

(a) Dynamic molecular structure of biochar derived from plant biomass [60], (b) Infrared spectra of biochar after heat treatment at different temperatures and comparison of XPS spectra of raw biochar and biochar after heat treatment at 300°C [61], (c) SEM images of different stages of biochar preparation [62], (d) Mechanism of biochar pore classification and group functionalization [2].

3.3 Biochar modification

The adsorption of CO₂ by biochar is highly dependent on the pore structure and surface physicochemical properties, and the optimal pore size is about twice the kinetic diameter of CO₂ molecules. However, the CO₂ adsorption capacity of directly carbonized biochar is low. The authors’ previous studies [45] have been conducted to enhance the CO₂ adsorption capacity of biochar by sequential construction of pore channels and surface functionalization modification through activation (physical and chemical activation). Molecular dynamics simulations were also performed by clearly modeling the hierarchical pore channels to explain the experimental phenomena from a microscopic perspective. The mechanism is shown in Figure 4(d). On the other hand, the carbonization temperature plays a key role in controlling activated porous biochar’s functional groups and specific surface area. Therefore, the reasonable selection of the amount of activator and carbonization temperature becomes a necessary part of preparing activated porous biochar materials.

3.3.1 Biochar pore hierarchy construction

According to the International Union of Pure and Applied Chemistry (IUPAC) standards, biochar is classified as macroporous, mesoporous, and microporous. Usually, the pore size of macroporous exceeds 50 nm, mesoporous is 2–50 nm, and microporous is less than 2 nm. For the CO₂ adsorption process, macropores and mesopores help diffusive transport of CO₂ molecules, while micropores provide adsorption sites as direct storage sites for CO₂. Therefore, a reasonable construction of graded pores can effectively enhance the CO₂ capture performance of biochar. Lingyu et al. [46] prepared biochar from seven types of straw and wood biomass to study their CO₂ adsorption performance and found that wood biochar has better pore structure than straw biochar with 2.73–4.40 times larger specific surface area, and biochar with super pore structure has higher CO₂ adsorption capacity. Capacity was higher, and good pore structure played a crucial role in the CO₂ adsorption. Avanthi et al. [63] prepared biochar using pine sawdust and steam activated it at the same temperature for 45 min after completion of pyrolysis. Due to the high surface area and microporosity, pine sawdust biochar showed significantly higher CO₂ adsorption capacity than paper mill sludge biochar, which may be due to the Steam activation increased the microporosity, surface area, and oxygen-containing basic functional groups. In this paper, we summarized the literature that studied the CO₂ adsorption capacity of biochar with different pore structures in recent years, and the relationship between their specific surface area, pore-volume, and biochar CO₂ capture capacity is shown in Figure 5.

Figure 5.

Relationship between specific surface area and pore volume of biochar and CO₂ adsorption capacity [64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74].

3.3.2 Biochar functionalization construction

The adsorption of CO₂ on the biochar surface is influenced by the chemical properties of the biochar surface. Many studies have shown that the introduction of basic nitrogen functional groups can increase the alkaline sites on biochar and enhance the adsorption of acidic CO₂ [75]. Nitrogen-containing functional groups are the main contributors to the alkalinity of the biochar surface, and activation in different nitrogen-containing reagents was performed to introduce nitrogen-containing functional groups to the biochar surface. The commonly used activation reagents are KOH, NaOH, CO₂, and K₂CO₃. Activation of biochar with KOH or NaOH can dissolve compounds such as ash, lignin, and cellulose, thus increasing the O content and surface alkalinity of biochar. Some new activation reagents such as NaNH₂, CH₂COOK, and H₂SO₄ have been gradually investigated. He et al. [76] prepared activated carbon by KOH activation using rice husk as raw material and modified biochar with chitosan as a nitrogen source. They found that the modified AC exhibited better CO₂ adsorption performance in comparison. Yang et al. [77] prepared N doped porous carbon, and the CO₂ adsorption capacity could reach 6.33 mmol/g at 273.15 K and 100 kPa, which was significantly higher than most of the carbon-based adsorbents reported in the literature due to the introduction of nitrogen-containing functional groups that increased the CO₂ adsorption sites. In addition, unlike the acid-base interactions between CO₂ and biochar surfaces, it has been shown that the presence of oxygen-containing acidic functional groups such as hydroxyl and carboxyl groups also promotes hydrogen bonding between CO₂ molecules and carbon surfaces, thus increasing CO₂ adsorption on carbon-containing surfaces [78]. Ma et al. [79] synthesized a series of carbon materials with different functional group contents. The experimental results showed that introducing oxygen functional groups into the carbon framework can again improve CO₂ capture efficiency in N-doped porous carbon. According to the theoretical calculations (Figure 6), the carbon framework with high oxygen content further enhanced the hydrogen bonding and electrostatic interaction for CO₂ adsorption. Wu et al. [80] prepared biochar from corn kernels by KOH activation, and the samples possessed a very high number of oxygen functional groups (45.5%) and exhibited a large CO₂ adsorption capacity. The presence of alkali and alkaline earth metal (AAEM) elements such as Na, K, Ca, and Mg can also promote the formation of basic sites, which have a strong affinity for CO₂ with acidic properties [81]. Therefore, the presence of biochar’s AAEM elements may enhance the CO₂ adsorption capacity of biochar, and the introduction of alkaline metal sites in the biochar skeleton may also enhance the CO₂ adsorption of biochar in the order of Mg > Al > Fe > Ni > Ca > Raw biochar > Na [75].

Figure 6.

(a) modification of rice husk-based biochar and CO₂ adsorption capacity [76], (b) preparation of N-doped porous carbon from chitosan and NaNH₂ for CO₂ adsorption [77], (c) hydrogen bonding between CO₂ and functionalized biochar surface [79], (d) adsorption energy of different functional groups of biochar for CO₂ [45].

3.4 “Biochar-new ammonia” CO₂ capture system

The excellent CO₂ adsorption performance and low regeneration energy consumption of biochar are closely related to its well-developed specific surface area, three-dimensional through-gradient pore structure, and unique oxygen/nitrogen surface chemistry. Dagaonkar et al. [82] estimated the effective diffusion coefficient of CO₂ within biochar to be 9.645 × 10⁻⁷ m² s⁻¹. Suppose biochar particles are used as a modified material to enhance the mass transfer properties of the liquid phase. In that case, their stronger CO₂ diffusion properties can be fully utilized to improve the overall reaction rate of the carbon capture system. Biochar has also been used to adsorb ammonia nitrogen, and more than half of the mass of ammonia nitrogen adsorbed was completed within 2 h [83]. In view of this, biochar can be applied in ammonia water CO₂ absorption systems to achieve effective inhibition of ammonia escape through the sequestration of free ammonia by its active surface groups.

By combining the respective development potentials of biochar adsorbent and ammonia-ethanol absorber, biochar adsorbent was cross-linked with ammonia-ethanol absorber to realize the functionalized cross-linking of the biochar-enhanced new ammonia carbon capture mass transfer-crystallization process (Figure 7(a)). This system transforms the carbon capture process from the traditional ammonia carbon capture gas-liquid two-phase reaction to a gas-liquid-solid three-phase process. The system can achieve CO₂ adsorption and enrichment in micropores, CO₂ diffusion and transport in mesopores/macropores, and dynamic sequestration of free ammonia by regulating the hierarchical structure of biochar nanopores and the orderly grouping of active functional groups on the surface. The cross-scale multiphase system processes, such as functionalization, pore gradation, biochar/NH₄HCO₃ dissolution and crystallization, and adsorption/absorption coupling, are cross-linked by crystal regeneration instead of liquid-rich regeneration. The synergistic effect of “graded adsorption—efficient absorption—dissolution crystallization—crystal regeneration” in the system is accomplished. The multiple goals of ammonia carbon capture, such as increasing absorption rate, suppressing ammonia escape, and reducing system energy consumption, are achieved. This process can improve a series of shortcomings of ammonia CO₂ capture and overcome the shortcomings of biochar adsorbents. The synergy of the CO₂ capture process in the solid-liquid system of “biochar-ammonia-ethanol” is achieved by “taking the advantages of each and avoiding the shortcomings.”

Figure 7.

(a) Functionalized cross-linking of biochar-enhanced novel ammonia-based carbon capture mass transfer-crystallization processes, (b) Enhanced mass transfer mechanism of biochar in ammonia-ethanol mixed absorbent [2].

3.4.1 Biochar efficiency transfer

The absorption of CO₂ by ammonia is a typical non-homogeneous reaction process in which CO₂ in the gas phase is first dissolved in the absorption solution and then reacts with NH₃ in the liquid phase. Therefore, the absorption rate is controlled by the “chemical reaction in the liquid phase” and the “mass transfer characteristics between gas and liquid.” The generation and hydrolysis of carbamate in the reaction process is the most important factor affecting the chemical reaction rate, roughly divided by the carbonation degree of ammonia absorption CO₂ solution ≈0.5, as shown inFigure 7(a). The liquid membrane mainly controls the mass transfer resistance of ammonia absorption CO₂ reaction process, when the hydrolysis of ammonium carbamate mainly controls the carbonation degree >0.5, ammonia absorption CO₂, so that the liquid phase carbon capture rate is significantly reduced, and this process has been the bottleneck to improve the absorption rate in the later stage of the reaction in the traditional process. This process has been the bottleneck to improving the absorption rate in the later stage of the reaction in the traditional process. The key to reducing the liquid film mass transfer resistance in the process of CO₂ adsorption by ammonia and improving the low CO₂ absorption rate is to get rid of the influence of carbonation degree on the regeneration energy consumption and to control the CO₂ absorption reaction by ammonia only in the rapid generation phase of ammonium carbamate with carbonation degree <0.5. The mechanism of mass transfer characteristics of the new ammonia carbon capture process with biochar efficiency enhancement is shown in Figure 7(b). Using the highly efficient adsorption performance of biochar hierarchical pore channels, the initial rapid CO₂ sequestration is completed, and the biochar is used as a carrier to bring CO₂ into the ammonia absorption system. Subsequently, the transfer of CO₂ from solid particles’ adsorption space to the ammonia liquid phase’s absorption space is further realized. The release of CO₂ from biochar and the absorption of CO₂ by ammonia is completed, which greatly increases the contact time between CO₂ and ammonia liquid phase, thus realizing the reduction of liquid film resistance and prolonging the residence time of CO₂ in the solid-liquid phase system to increase the material transfer and chemical absorption rate in the ammonia liquid phase system. The liquid-liquid phase ammonia system can be used to increase the rate of material transfer and chemical absorption.

3.4.2 Limiting ammonia escape

The ammonia escape process is shown in Figure 8. Among many parameters affecting ammonia escape, the temperature is one of the most sensitive [84]. From the ammonia escape point of view, the absorption temperature should be as low as possible, requiring a large amount of energy to maintain cold ammonia. For the solid-liquid two-phase CO₂ capture system, the pore surface functional groups in the solution permeable region of biochar/macropore can undergo cation exchange with NH⁴⁺ in solution [85], which promotes the reverse migration of the hydrolyzing process of ammonia monohydrate. At the same time, the free ammonia in the liquid phase was held by the van der Waals force and chemical hybrid force [86] so that the production of free ammonia in the liquid phase could be effectively controlled. Therefore, the hierarchical functionalized construction of biochar pore structure ensures the hierarchical adsorption of CO₂/NH₃ by biochar particle pore, improves the material transfer and chemical absorption rate in the ammonia liquid phase system, and makes the dynamic balance of NH₃ adsorption and fixation in unsaturated solution impregnation space present in biochar pore. To a great extent, the effective concentration of free ammonia that can participate in the reaction in the liquid phase system is ensured, the dynamic partial pressure of free ammonia in the liquid phase is maintained, and the ammonia escape is limited.

Figure 8.

Ammonia escape mechanism.

3.4.3 Dissolution crystallization instead of rich liquid regeneration

The traditional ammonia-rich liquid thermal regeneration process is the largest energy-consuming part of the whole ammonia carbon capture process. The regeneration energy consumption is mainly composed of three parts: the sensible heat of rich liquid warming, the latent heat of vaporization, and the heat absorption of regeneration reaction, of which 50–70% of the energy is consumed in the warming and vaporization of rich liquid solvent [87]. The solubility of the product of the reaction process of CO₂ absorption by ammonia is known: ammonium carbamate is soluble in water and ethanol; ammonium bicarbonate is soluble in water-insoluble in ethanol. The main mechanism of solvation crystallization is to use the different chemical structures of the main solvent molecules and solvating agent molecules to make a difference in the microscopic forces between the ions of the substances to be separated and to change the macroscopic properties of the mixed solvent by changing the microscopic forces of the particles in the solution, thus greatly reducing the solubility of the solute, and using the solubility difference as the driving force to make the solute continuously precipitate out of the liquid phase in the form of crystals, so that the solvent and the solute are separated. The solvent and solute are separated. In the “biochar-ammonia-ethanol” carbon capture system, the crystallization process is strengthened by the solvation and precipitation method, and the regeneration of crystals replaces the regeneration of carbon-rich liquid, which greatly reduces the energy consumption of regeneration. The biochar functionalized Meso-/macropore pores ensure the NH₃/NH⁴⁺ concentration in the liquid phase. The pores’ active surface structure provides nucleation sites for the crystallization process, which accelerates the formation and growth of carbonated liquid solvation crystals in the liquid phase system. The dynamic balance between the crystallization process’s residence time and the biochar’s saturation time for efficient adsorption can provide a stable CO₂ adsorption-absorption-crystallization series process.

3.5 High-value utilization of carbon capture products

The main crystallization product of the novel ammonia CO₂ capture technology described above is ammonium bicarbonate, which is widely used in agriculture, food, pharmacy, and ecological management, but its utilization process’s complexity and economics have prevented its use widespread development. Therefore, further optimization of the new ammonia CO₂ capture technology and high-value utilization of the intermediate product ammonium bicarbonate have also become key issues. Rice husk is widely available, and its internal structure has a lignocellulose-SiO₂ crossover network, and the SiO₂ in it can be dissolved to construct pore channels of a specific size. With this unique structural advantage, rice husk is the best raw material for preparing graded porous carbon [88]. The particle size of SiO₂ in rice husk is mainly concentrated in the range of 8–22 nm, with a small fraction of SiO₂ in the range of 1–7 nm [89], indicating that SiO₂ in rice husk can be used as a natural template to induce mesopore generation in situ after solubilization. The chemical activation of agricultural waste rice husk as a raw material enables the orderly construction of high-quality rice husk-based biochar with a hierarchical pore structure and high specific surface area. Combined with the new ammonia carbon capture technology, the rice husk-based biochar-ammonia-ethanol system was constructed, and the nano-silica carbon black was produced by the acid-base neutralization and redecomposition reaction between NH₄HCO₃, an intermediate product of the new ammonia carbon capture, and silicate, an intermediate product of the rice husk-based biochar (Figure 9). This route greatly solved the problems of carbon capture product consumption and agricultural waste pollution and produced high-value products of rice husk-based biochar carbon and nano-silica at the same time.

Figure 9.

Technology roadmap for high-value utilization of process products from rice husk-based biochar-new ammonia-based carbon capture technology.

4. Summary and outlook

CO₂ capture is a crucial part of CCUS technology, and absorption and adsorption have been widely studied as the main means of CO₂ capture. The mainstream CO₂ liquid-phase chemical absorption method is difficult to avoid high regeneration energy consumption, degradation problems, and high corrosiveness. In contrast, the “ammonia-ethanol” system effectively avoids these problems but still has serious ammonia escape problems and crystallization control difficulties, and the technology needs to be improved. Biochar has excellent CO₂ adsorption performance due to its specific surface area, three-dimensional through-gradient pore structure, and unique oxygen/nitrogen surface chemistry. However, it still has many problems, such as poor CO₂ selectivity, limited adsorption capacity, high cost, and short service life. Combining the above-mentioned new ammonia carbon capture technology, the carbon capture process is transformed from the traditional ammonia carbon capture gas-liquid two-phase reaction to a gas-liquid-solid three-phase process, which maximizes the efficiency of CO₂ capture by graded adsorption of biochar and efficient absorption of ammonia-ethanol solution:

1. Enhancement of mass transfer between solid and liquid phases to improve the carbon capture rate;

2. Biochar hierarchical pore channel fixation of CO₂/NH₃ to achieve a “win-win” situation of enhancing liquid phase absorption and suppressing ammonia escape;

3. Enhancement of dissolution and crystallization of carbonized liquid to replace rich liquid regeneration with crystal regeneration to reduce energy consumption.

In order to realize the high-value utilization of intermediate products, we propose a system of rice husk-based biochar—new ammonia method—process product resource synthesis—process regulation, which provides new ideas and directions for the CO₂ capture industry.