CO₂-looping in biomass pyrolysis or gasification

Abstract

Thermochemical technologies for converting biomass into energy or chemicals mainly consist of combustion, pyrolysis, and gasification. This review summarizes the recent advances in biomass pyrolysis and gasification by using CO₂ as a reaction medium. The CO₂-looping pyrolysis or gasification of biomass is compared with conventional processes. In the integrated valorization of biomass by pyrolysis or gasification, CO₂ can play a vital role in each stage, mainly including biomass pyrolysis, biomass/biochar gasification, biochar activation, and tar cracking/reforming. CO₂ as a reaction medium can significantly improve the thermal efficiency of biomass pyrolysis. Pyrolysis in CO₂ results in deep decomposition of biomass compared to pyrolysis in N₂. Also, CO₂ has an affinity to react with hydrogenated and oxygenated groups, leading to biochar with a higher specific surface area (S_BET). Thus, exploiting CO₂ as a reaction medium in biomass pyrolysis provides an attractive option for enhanced generation of syngas and tuned adsorption capability of biochar. In addition, the CO₂ pyrolysis of biomass can enhance the thermal cracking of harmful organic compounds, thus suppressing of the formation of benzene derivatives (e.g., volatile organic compounds) and polycyclic aromatic hydrocarbons. In general, CO₂ gasification of biomass serves a dual purpose of reducing pollution and generating syngas. In addition, introducing CO₂ with steam as a gasifying agent can enhance CO production. The internal mineral species (e.g., K, Ca) in biomass can also improve the reactivity of char gasification and tar reforming in the entire gasification processes.

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1. Introduction

Global environmental issues such as climate change triggered by anthropogenic greenhouse gases have prompted unprecedented researches associated with carbon capture and storage and renewable energies to alleviate various global environmental problems.^1–4 Harnessing biofuels has been paid much attention due to its intrinsic carbon neutrality, along with political support such as renewable fuel standard that expedited the rapid employment of biofuels. In parallel, the concept of biorefining has been developed since our demand for carbon to sustain the current industrial infrastructure still continues to grow.^2,4 Despite significant technical advances, biomass as an energy feedstock has been considered as quite challenging due to its heterogeneous matrices and highly functionalized structures.^1,5 In this case, concrete technical methodologies for producing fuels from biomass have not been established. In general, biomass can be converted to biofuels via various pathways such as biochemical (e.g., cellulosic-ethanol production) or thermochemical processes (e.g., gasification, pyrolysis). However, pyrolysis or gasification has been recognized as one of the promising techniques for efficiently transforming biomass into various fuels or chemicals.^6,7

1.1 Biomass pyrolysis

Biofuels derived from biomass pyrolysis have technical advantages over those from conventional biochemical conversion since the entire biomass can be used as the feedstock (rather than simple sugars) in only a few seconds (rather than hours or days).⁸ Commonly, biomass pyrolysis is induced by the heat transported from the surrounding gas to the fuel particle, causing thermal decomposition of it into a huge number of products. As illustrated in Fig. 1, during transient heating, temperature increases locally, leading to the evaporation of moisture (i.e. drying stage) and then to the progressive release of pyrolytic volatiles (i.e. primary pyrolysis stage). The primary volatiles (denoted by “1”) are produced from the thermal scission of chemical bonds in the individual biomass components, including cellulose, hemicellulose, lignin and extractives, and comprise permanent gas species (e.g., CO₂, CO, CH₄) and condensates at ambient conditions (several organics and water). Although each of the biomass components decomposes at faster rates in different temperature ranges, the overall primary pyrolysis stage is complete at relatively low temperatures (<500 °C), yielding a carbon-rich non-volatile solid (i.e. char). The char contains a significant part of the mineral matter of the parent fuel. Nevertheless, if the fuel is converted at higher temperatures, some of the primary volatiles released inside the particle can further participate in a variety of secondary reactions to form product “2”.

Fig. 1 Thermal conversion of solid biomass particle under inert atmosphere: drying, primary pyrolysis and secondary pyrolysis. The arrows indicate the main routes for the formation of products.⁹

Serial and parallel reactions either heterogeneously or homogeneously (e.g., cracking, reforming, dehydration, condensation, polymerization, oxidation and gasification) can occur.^10,11 The distinction between intraparticle/primary pyrolysis and extraparticle/secondary pyrolysis is not perfect as the secondary reactions of volatiles can occur both in the pores of the particles and/or in the bulk gas. Thus, the primary and secondary reactions can occur simultaneously in different parts of a fuel particle. The char derived from the primary pyrolysis stage can be active during the secondary reactions, namely by catalytic conversion of organic vapors into light gases (cracking reactions) and secondary char (polymerization reactions). In addition, the char can be converted into gases by gasification reactions with H₂O and CO₂. Nevertheless, the rates of char gasification with H₂O and CO₂ are orders of magnitude lower than those of primary pyrolysis, so the conversion of char is limited during the release of volatiles. In comparison, the secondary conversion of the primary volatiles is a rapid process, mainly depending on the operational conditions, and it can exert a major influence on the ultimate composition and yields of the volatiles. In addition, primary fragmentation and shrinkage of the particles can occur in parallel with the above physicochemical processes. The composition of pyrolytic volatiles mainly includes H₂O, CO₂, CO, H₂, CH₄, other non-condensable hydrocarbons and condensable organic compounds (i.e. tar).

Pyrolysis can be considered as a sustainable biomass valorization method for energy production and carbon capture and sequestration.¹² Pyrolysis defined as the thermal decomposition at an intermediate temperature (500–900 °C) under limited O₂ can convert biomass into bio-oil, gases and biochar.¹³ The yields and properties of the pyrolytic products are influenced by many parameters including residence time, temperature, and heating rate. Based on the heating rate and reaction time, the pyrolysis process could be classified into three types, slow, fast and flash pyrolysis (as shown in Table 1).^13–15 Slow pyrolysis can provide great environmental benefits since it produces more biochar, possibly applied for soil reclamation and carbon sequestration. However, fast pyrolysis has a better economic return because of the valuable biofuel production (e.g., bio-oil, syngas).¹⁶ In addition, pyrolysis is the initial chemical step in all biomass gasification and combustion processes.¹⁷

Table 1 Main features of slow, fast and flash pyrolysis

	Slow pyrolysis	Fast pyrolysis	Flash pyrolysis
Heating rate (°C min^−1)	5–7	300–800	>1000
Pyrolysis temp. (°C)	500–1200	600–900	600–1200
Feedstock size (mm)	5–50	<3	<0.5
Feedstock residence time	>1 h	0.5–10 s	<2 s
Pyrolysis setup	Fixed bed	Fixed or fluidized bed	Fluidized or entrained flow bed
Main products	Char	Oil	Gas

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In general, the mechanism for pyrolysis of lignocellulosic biomass can be divided into the mechanisms of three individual components.¹⁸ The decomposition of cellulose, hemicelluloses, and lignin depends mainly on heating rate, temperature, reactor type, residence time, and particle size. In biomass pyrolysis, hemicelluloses decompose initially at 470–530 K, whereas cellulose breaks down at 510–620 K, and decomposition of lignin occurs at 550–770 K.¹⁴ The mechanism of cellulose pyrolysis can be characterized by depolymerization.¹⁹ Two basic reactions occur in the pyrolysis: one is the decomposition and charring of the cellulose in slow pyrolysis;²⁰ the other is rapid volatilization with levoglucosan formation in fast pyrolysis.²¹ The cellulose is initially depolymerized into oligosaccharides, and then the glucosidic bonds of the oligosaccharides are broken to form D-glucopyranose, undergoing an intramolecular rearrangement to form levoglucosan (LG).²² The LG decomposes via three ways (i.e., C–O scission, C–C scission, and LG dehydration) to form the small-molecular-weight compounds.^23–31 At 667–1327 K, the dehydration, with the formation of a H₂O molecule composed of an OH group from C2 and an H atom from C3, is a preferred pathway for LG decomposition (Fig. 2), whereas the C–O bond scission mechanism agrees better with that of C–C bond scission. The pathway of C–O bond scission starts with the breakage of C6–O1 and C1–O5 bonds to form CO simultaneously (Fig. 2). The C–C bond scission pathway has the highest energy barrier among the three LG decomposition pathways (Fig. 2).³²

Fig. 2 Levoglucosan (LG) decomposition mechanism in the pyrolysis of cellulose: LG dehydration, C–O bond scission, and C–C bond scission.³²

The pyrolysis mechanism of hemicelluloses is very similar to that of cellulose, which starts with the depolymerization of polysaccharide chains to form oligosaccharides, following the cleavage of the xylan chain in the glycosidic linkage and rearrangement of the produced molecules to afford 1,4-anhydro-D-xylopyranose.³³ After that, 1,4-anhydro-D-xylopyranose is further decomposed to form small-molecular-weight compounds, e.g., furfural and two- and three-carbon fragments.^34–36 More details of hemicellulose pyrolysis have been reviewed in terms of experimental studies on various hemicellulose feedstocks and reactors, reaction mechanisms for pyrolysis of hemicellulose and its model compounds unraveled by computational efforts, and global kinetic modeling as well as mechanistic modeling of hemicellulose pyrolysis.³⁷

The structure of lignin is more complex than those of cellulose and hemicellulose, leading to a complex decomposition mechanism.^38–41 Lignin pyrolysis consists of drying, fast degradation, and slow degradation stages. Phenolic compounds, i.e., guaiacol type, phenol (P-) type, syringol (S-) type, and catechol (C-) type, are the main products of lignin pyrolysis.⁴⁰ The dominant pathway of free radical reaction has been widely regarded as one of the most important mechanisms for lignin pyrolysis.^42–45 As shown in Fig. 3, in the pyrolysis process, the free radicals are generated in the breaking of the β-O-4 linkage of the lignin molecules, which is the initial step for the reaction of free radical chain.^43,46 The radicals can capture the protons from other species with weak C–H or O–H bonding (e.g., C₆H₅–OH) and form decomposed products (e.g., vanillin and 2-methoxy-4-methylphenol). Thereafter, the radicals can be passed to other species for chain propagation. Finally, the chain reactions can be terminated, once a stable compound is formed by the collision of two radicals.

Fig. 3 Lignin decomposition mechanism in pyrolysis process.⁴²

Besides cellulose, hemicelluloses, and lignin, inorganic species can significantly influence the pyrolysis of biomass. For example, K and Cl can vaporize at a relatively low temperature, whereas Ca and Mg are ionically or covalently bound with organic molecules and vaporize at high temperatures.⁴⁷ P, S, and N are covalently bound with complex organic compounds within the cells of plants⁴⁸ that can be decomposed at low pyrolysis temperatures. Biomass pyrolysis can also be regarded as an autocatalytic process, since some inorganic constituents in biomass, especially alkali and alkaline earth metals (such as K, Ca, and Mg), have significant catalytic effects on biomass pyrolysis.⁴⁹ K catalyzes the secondary cracking of the pyrolytic volatiles,¹³ producing more gas compounds (e.g., CO, H₂, CH₄, CO₂), and further resulting in biochar cracking. As for lignocellulosic biomass, the autocatalysis of the inherent inorganic species alone is insufficient to lead to bio-oil and biochar with the desired properties. Further studies are required to develop efficient catalysts for the selective conversion of biomass to bio-oil with high heating values and biochar with the desired functional groups and porous structure. The use of efficient catalysts may allow the reduction of the pyrolysis temperature and residence time and improve the qualities of both bio-oil and biochar. More recently, Liu et al.⁵⁰ critically reviewed the fates of the main chemical elements (C, H, O, N, P, Cl, S, and metals) in biomass during the pyrolysis process.

The integration of various technologies can work as leverage in promoting a “circular economy”, aiming at improving both resource use and operation efficiency. Moreover, using an integrated process can overcome defects in each individual process. Thus, the integrated process of anaerobic digestion with pyrolysis can contribute to a new highly effective conversion.⁵¹ The integration of anaerobic digestion with pyrolysis can allow the bio-conversion of the aqueous phase of pyrolysis liquor derived from the pyrolysis of solid digestate.⁵² In addition, biochar has the ability to catalyze anaerobic digestion by means of mitigating mild ammonia inhibition, supporting archaeal growth and the methanization of the biochar labile carbon.⁵³ Therefore, the integrated process of anaerobic digestion with pyrolysis will open up innovative pathways for efficient valorization of lignocellulosic biomass.⁵⁴ In general, lignocellulosic biomass through anaerobic digestion can be converted into biogas (mainly CH₄ and CO₂). After pyrolysis, biomass can be degraded into syngas (mainly H₂ and CO₂), bio-oil and biochar. Through such integration, resource use efficiency can be significantly improved. On the one hand, after anaerobic digestion, more energy in the feedstocks is transferred into pyrolysis volatiles and biogas.⁵⁵ Biochar derived from anaerobic digestate pyrolysis has lower heating value than biochar from pyrolysis of raw feedstocks. Besides, the biochar can fix heavy metals and nutrient elements such as P, K and Ca in digestate, preventing them from being released into aqueous phase.⁵⁶ In the anaerobic digestion process, some recalcitrant components of lignocellulose biomass (e.g., lignin) will prevent anaerobic microorganisms from degrading substrate. Thus, the anaerobic digestate still contains considerable energy.⁵⁷ In this case, pyrolysis of anaerobic digestate not only reduces anaerobic digestate cost management, but further uses anaerobic digestate to recover higher bioenergy and produce more products including biochar, bio-oil and syngas.⁵⁸ Through the life cycle assessment of lignocellulosic biomass pyrolysis integrated with anaerobic digestion, it was found that greenhouse gas emissions can be significantly reduced in the integrated process without worsening the abiotic resources depletion.⁵⁹ In summary, the integration of anaerobic digestion with pyrolysis can offer further potentially synergistic combinations including use of anaerobic digestate for pyrolysis,⁶⁰ biomethanation of syngas⁶¹ or use of pyrochars in anaerobic digestion.⁶² From an ecosystem perspective, functions carried out by two soil amendments (digestate or pyrochar) are different. While digestate is rich in degradable organic matter that will be slowly decomposed by soil microbes, pyrochar has very poor, easily degradable organic matter and its chemical form is particularly stable. Digestate gradually releases plant nutrients under microbial mineralization and its degradable organic matter is the primary source to start up the ecological “debris chain” (Fig. 4). Conversely, pyrochar can play an important role in carbon capturing and its long-term sequestration into soil, thus contributing to climate change mitigation.⁶³

Fig. 4 Integration of anaerobic digestion with pyrolysis for bio-waste valorization and the final closing loop with the return of the organic and mineral by-products to agricultural soil (as soil amendment and/or fertilizer).⁶³

The chemical mechanism of the integration of pyrolysis with anaerobic digestion for valorization of lignocellulosic biomass is illustrated in Fig. 5.⁶⁴ The complexity of the metabolic pathways leading to biogas from biopolymers (namely cellulose, lignin, and hemicellulose) can be classified into four sequential steps: degradation of biopolymers into smaller molecules like monomers via hydrolysis, which are fermented principally into volatile fatty acids via acidogenesis, further digested into CH₃COOH, CO₂ and H₂ (acetogenesis) that are converted into the final products CH₄ and CO₂ (methanogenesis). Upon pyrolysis, biopolymers are converted into gas (syngas), liquid (bio-oil) and solid (biochar). Herein, two main configurations can be identified in the integrated process of pyrolysis with anaerobic digestion. On the one hand, pyrolysis products are upgraded into biogas via anaerobic digestion, and pyrolysis is applied “upstream” to break down hemicellulose and lignin into smaller compounds accessible to biomethanation; on the other hand, anaerobic digestion products are upgraded via pyrolysis. Specifically, “downstream” pyrolysis can convert residual solid digestate from biogas plants into fuels and materials.

Fig. 5 Schematic of chemical conversion in the integration of pyrolysis and anaerobic digestion.⁶⁴

1.2 Biomass gasification

Biomass gasification is an intricate process involving drying the feedstock followed by pyrolysis, partial combustion of intermediates, and finally gasification of the resulting products. It is performed in the presence of a gasifying medium (e.g., air, O₂, H₂O or CO₂), inside a gasifier. Biomass gasification reduces the C/H mass ratio resulting in increased calorific content of the product on account of enhanced H₂ fraction.⁶⁵ The gasifying medium plays a vital role in converting char and heavy hydrocarbons to low-molecular-weight gases, e.g., CO and H₂. The quality and properties of the product mainly depend on the feedstock types and size, gasifying agent, temperature and pressure inside the reactor, design of gasifier and the presence of catalyst and sorbent, etc.⁶⁶ There are many useful products from biomass gasification including syngas, heat, power, biofuels, fertilizer and biochar. Syngas can be further processed by the Fischer–Tropsch process into methanol, dimethyl ether and other chemical feedstocks.

Over the past decade, biomass gasification has been developed to obtain value-added products such as syngas, H₂, CH₄ and chemical feedstocks.⁶⁷ These gases can also be produced from biomass via biochemical routes. Thermochemical pathways have an edge over the other routes, as biochemical processes have issues with treating biomass rich in lignocellulose.^68,69 In addition, they operate relatively slowly in batch mode and produce a dilute product stream, with large amounts of water recirculating. However, the thermochemical route can accommodate a more diverse range of biomass⁷⁰ with a higher efficiency and a lower cost.⁷¹ One of the main limitations is the small range of products.^70,71 At present, cutting-edge, innovative and economical techniques with high efficiencies are still urgently required for biomass gasification.⁷² Silkarwar et al.⁷³ provided an assessment of the fundamentals such as feedstock types, the impacts of operating parameters, tar formation/cracking, and modelling methods for biomass gasification. Herein, various mechanisms for biomass gasification are comparatively discussed. The gasification process is an alternative thermochemical method that can convert biomass into gaseous products (e.g., syngas) at relatively high temperatures. The gasification routes are shown in Fig. 6A. The related reactions including heterogeneous and homogeneous ones in biomass gasification are depicted in Fig. 6B. Generally, processes in biomass gasification include drying and devolatilization, volatile and char combustion/gasification, and tar reforming with CO₂ and steam.⁷⁴

Fig. 6 Biomass gasification routes (A) and reactions (B).⁷³

Gasification is the process of partial combustion of biomass to produce the valuable syngas (e.g., CO, H₂, CH₄, and CO₂) along with tar (i.e. condensable organic compounds) and other contaminants (e.g., NH₃, H₂S, and HCN).^74,75 Gasification consists of three major steps: drying, devolatilization (pyrolysis), and gasification. Extensive research has been done to understand the effect of steam, air, oxygen, and their combinations on the process.^76–80 However, the role of CO₂ as an oxidizing agent has been explored in only a limited number of studies. Biomass gasification with CO₂ as a gasifying agent is a promising way to relieve energy shortages and minimize CO₂ emission. In Section 3, CO₂ gasification of biomass will be further discussed.

1.3 Biomass tar

Condensable organic compounds referred to as “tar” are inevitably generated along with the producer gas in biomass pyrolysis or gasification. Tar is regarded as one of the main bottlenecks, which limits the commercialization of biomass pyrolysis and gasification technology. Tar can condense and polymerize to form more complex structures (e.g., coke). Tar represents a complex mixture of more than 100 compounds, including benzene, phenols, N-heterocyclic compounds, and polycyclic aromatic hydrocarbons (PAHs),^81,82 which can be condensed with small particle impurities to form complex structures in downstream equipment, resulting in the mechanical breakdown of the entire pyrolysis/gasification system, or deactivation of catalysts.⁸³ Furthermore, various aromatic and N-heterocyclic compounds in tars are toxic and act as environmental hazards. Tar can be divided into primary, secondary and tertiary tars.^84,85 The tar yield of biomass gasification varies from 0.5 to 100 g m⁻³, mainly depending upon the reactors, operating conditions, and feedstock types.⁸⁶ Tars are formed in the decomposition of cellulose, hemicelluloses, and lignin. The main components of primary tar include oxygenated hydrocarbons, and with increasing temperature, the oxygenated hydrocarbons are initially converted to light alkanes, olefins, and aromatics, and then transformed to higher hydrocarbons and larger PAHs with a further increase in temperature (Fig. 7).⁵⁰

Fig. 7 Formation of tar compounds at different reaction temperatures.⁵⁰

Many approaches can be used for tar removal in the biomass pyrolysis/gasification process. Generally, these methods can be divided into two types: (1) physical methods using absorption (e.g., wet scrubbers) and adsorption (e.g., carbon materials) processes; (2) thermochemical cracking/reforming with catalysts.^74,84,87 Among all these methods, thermochemical methods have received the most interest since tar can be transformed into useful gas products. Biochar has a high potential to be used as catalysts or supports for tar conversion.^83,88 In general, catalytic conversion of tar is significantly influenced by tar components, thermal cracking/reforming conditions, and catalysts performance. Steam reforming of tar has been extensively considered, because more value-added H₂ is generated through water gas and water gas shift reactions.⁸⁴ As for the biomass gasification technology, steam or CO₂ gasification agent can provide a necessary environment for tar conversion, and ensure that catalytic deactivation of biochar cannot be caused by the carbon deposition of tar during the reaction process. In the scope of this review, CO₂ reforming of tar with catalysts will be further discussed in Section 5.

1.4 Biochar

The overall mechanism of biochar formation is characterized by the pyrolysis mechanisms of the biomass components of cellulose, hemicellulose, and lignin. As illustrated in Fig. 8, cellulose is initially depolymerized into oligosaccharides followed by the cleavage of the glucosidic bond to produce D-glucopyranose, which undergoes an intramolecular rearrangement to produce levoglucosan. Levoglucosan can form levoglucosenone via a dehydration process, which further proceeds through dehydration, decarboxylation, aromatization, and intramolecular condensation, to form solid biochar. Alternatively, levoglucosan undergoes a series of rearrangement and dehydration processes to form hydroxymethylfurfural, which further decomposes to produce more bio-oil and syngas. It can also undergo polymerization, aromatization and intramolecular condensation reactions to form biochar. The pyrolysis of hemicellulose is similar to that of cellulose. Compared to cellulose and the hemicelluloses, the structure of lignin is more complex, and this leads to a complex decomposition mechanism. As shown in Fig. 8, free radicals are generated by the breaking of the β-O-4 lignin linkage, suggesting the initiating step in the free radical chain reaction. The radicals can capture the protons of other species with weak C–H or O–H bonds (e.g., C₆H₅–OH) and form decomposed products such as vanillin and 2-methoxy-4-methylphenol. Later in the reaction timeline, the radicals are passed onto other species for further reaction, leading to chain propagation. Finally, the chain reactions are terminated when two radicals collide with each other to form stable compounds.⁸⁹ The biochar can be fabricated into various functional materials.⁸⁹ Particularly, the biochar can work as catalysts or supports for tar conversion in the biomass pyrolysis or gasification processes.^90,91

Fig. 8 Biochar formation mechanism: cellulose, hemicellulose and lignin pyrolysis.⁸⁹

1.5 Purpose of this work

Thermochemical processes such as pyrolysis can effectively convert lignocellulosic biomass into value-added energy (e.g., syngas, bio-oil) and materials (e.g., biochar). CO₂-assisted technologies for biomass processing can be designed to deliver optimal performance with maximal energy savings, so the biorefinery concept assumes the integrated valorization of biomass toward energy, materials, and chemicals.⁸⁶ It can be found that CO₂ acting as an agent participates in each stage (e.g., hydrolysis, pyrolysis, and gasification) through the entire thermochemical processes. High-pressure CO₂ can act as an interesting solvent, which can be used in a great variety of reactions within the biorefinery concept starting from its use in direct biomass processing to its use as a catalyst for dehydration of biomass-derived carbohydrates along with its use in specific applications for biomass intermediary compounds.⁹² As for CO₂ used in pyrolysis and gasification of lignocellulosic biomass, several works have been conducted, but there is still a lack of a comprehensive article on the fundamentals in this field. The purpose of this work is to evaluate the CO₂ functions in the entire processes of biomass pyrolysis and gasification, mainly including biomass pyrolysis and gasification, biochar gasification, tar cracking/reforming and gas upgrading (as shown in Fig. 9). The research advances and challenges will be discussed. Furthermore, the environmental implications of CO₂-assisted biomass pyrolysis and gasification will be highlighted. In addition, the future development trends and challenges will be presented.

Fig. 9 Schematic diagram of CO₂-looping pyrolysis or gasification of biomass.

2. CO₂ pyrolysis of biomass

The thermochemical process of pyrolysis and gasification has been recognized as one of the promising techniques for biomass valorization, but one of the main limitations is that it involves very high temperature, and therefore is quite energy-intensive. To make it a more viable option in energy industry, technical advancements for enhancing the thermochemical efficiency are greatly required. Preferably, exploring the ways to use CO₂ as a reaction medium and initial feedstock in the thermochemical process would be a desirable scenario that conforms to current carbon management strategies.^93,94

In several works, it has been demonstrated that pyrolysis of carbonaceous materials (e.g., biomass) under CO₂ atmosphere has enhanced thermal efficiency compared to that under N₂ atmosphere.^95–106 Zhang et al.¹⁰⁷ also studied fast pyrolysis of biomass in a fluidized bed gasifier at 550 °C under N₂, CO₂, CO, CH₄ and H₂ atmospheres. It was found that the liquid yield, composition and higher heating value mainly depend on the composition of the pyrolysis bath gas. Pyrolysis in CO₂ can produce less char than that in others. The CO₂ yield decreased compared to the yield obtained under N₂. With regard to the liquid product distribution, the CO₂ atmosphere led to the highest yield of acetic acid compared to the others. The acid products yield under N₂ was 9% while it increased to 16% under CO₂. In addition, the ketones yield increased slightly from 15% to 17% while the phenol yield decreased from 33% to 26%. Two possible mechanisms were proposed: the CO₂ reacted either with the active volatiles or with the biochar. The former was more plausible in view of the pyrolysis temperature.

Biomass pyrolysis in the presence of CO₂ can cause deep decomposition compared to pyrolysis in pure N₂, possibly due to either char gasification by CO₂ or the fact that CO₂ hinders polymerization reaction and secondary char formation by reacting or cracking tar compounds that may lead to its formation. Moreover, CO₂ has an affinity to react with hydrogenated and oxygenated groups, leading to a carbon-rich char. The possible mechanisms in biomass pyrolysis with the CO₂ insertion are proposed in Fig. 10. In summary, introducing CO₂ in the pyrolysis medium alongside N₂ can enhance CO production as a result of homogeneous and heterogeneous reactions of CO₂ with gases, tars and char.⁹⁵

Fig. 10 Possible mechanisms of biomass pyrolysis with CO₂ insertion.⁹⁵

The pyrolysis of biomass can also be conducted after the initial biorefinery processes. As for the production of bioethanol, lignocellulosic biomass is generally subjected to various pre-treatment methods such as mechanical comminution, acid pre-treatment, ammonia fiber explosion, alkaline extraction, and ionic liquids pre-treatment.^108–113 The pre-treated biomass can be hydrolyzed to sugars using cellulose enzymes at 45–50 °C under pH 5.0.¹¹⁴ After that, sugars are converted into bioethanol with 20–50% yield via fermentation with bacteria, yeast, or fungi.^110,115 Furthermore, the solid residue after cellulosic ethanol production can be considered for energy recovery by thermal processing such as pyrolysis or gasification. Kim et al.¹⁰² studied the pyrolysis of wastes generated through saccharification of oak tree using CO₂ as reaction medium. The production of bioethanol was evaluated via saccharification and fermentation of oak tree pretreated with H₂SO₄, NH₃, or NaOH using a yeast (Pichia stipites). Then, the effects of CO₂ on pyrolysis of biomass residues were studied. The effect of CO₂ was most noticeable in syngas, as the ratio of CO and H₂ exhibited a 20- to 30-fold increase at >550 °C. Particularly, the CO/H₂ ratio of bio-waste pyrolysis in CO₂ is nearly 1100% of that of pyrolysis in N₂ at 720 °C. In addition, the use of CO₂ as pyrolysis medium does not enhance syngas production but reduces tar formation by thermal decomposition of volatile organic compounds (VOCs) and reaction between VOCs and CO₂.

Lee et al.¹⁰⁶ also studied the susceptibility of CO₂-assisted pyrolysis of various biomasses, including macroalgae (red seaweed), microalgae (C. vulgaris), major constituents of lignocellulosic biomass (cellulose, lignin, hemicellulose), and lignocellulosic biomass (spent coffee ground, corn stover, oak wood). From TGA, the thermal degradation rate of nine different samples in N₂ and CO₂ is nearly identical, which can be evidenced by the thermal decomposition rate (shown as the slope in Fig. 11), implying that the CO₂ co-feed impact on physical aspects such as onset and end temperature of thermal decomposition of biomass is negligible. And the final mass conversion (residual mass at 800 °C) of biomass and coal is nearly the same, suggesting the rate of volatilization via thermolysis (bond dissociation) is not affected by CO₂ even at different heating rates and for different types of biomass, since the different rate of volatilization induced by CO₂ in pyrolysis of biomass may be reflected by the different thermal degradation rate. Besides, the expected Boudouard reaction is not initiated, which can be explained by the heterogeneous reaction (i.e. slow reaction kinetics between solid phase carbon and gaseous phase of CO₂), since the Boudouard reaction is thermodynamically favored at temperatures higher than 720 °C.^97,99 Therefore, it also suggests that the reaction kinetics of the Boudouard reaction is highly contingent on the structural biochar matrix.

Fig. 11 Representative thermograms of various biomasses subjected to slow pyrolysis (20 °C min⁻¹) and fast pyrolysis (400 °C min⁻¹) in N₂ and CO₂ environments.¹⁰⁶

However, the morphology of biochar generated from pyrolysis of each biomass in N₂ and CO₂ was noticeably different (Fig. 12a and b). Biochar generated from pyrolysis of biomass in the CO₂ atmosphere could develop more pores, thus slightly increasing the specific areas of the biochar.^105,106 Furthermore, the differential scanning calorimetry (DSC) curves in N₂ and CO₂ were quite different in the temperature regime indicating volatilization. An exothermic reaction from 250 to 500 °C in CO₂ pyrolysis of oak wood noticeably occurred, suggesting several chemical aspects induced by CO₂. For example, carbonization and volatilization in CO₂ pyrolysis of oak wood occurred independently since the thermal degradation rate in Fig. 11 and 12c in CO₂ was nearly the same. In other words, morphology modification induced by CO₂ did not influence the rate of volatilization via bond scission.¹⁰⁶ In addition, the biochar derived from pyrolysis of biomass in CO₂ environment showed a comparatively high surface area and more porous structure as adsorbent materials.¹⁰¹

Fig. 12 SEM images of biochar generated from pyrolysis of oak wood (a) in N₂ and (b) in CO₂, and (c) TGA and DSC curves of oak wood pyrolysis in N₂ and CO₂.¹⁰⁶

The gas evolution of syngas follows the very typical pyrolytic patterns of biomass in CO₂ atmosphere: the concentration of H₂ is proportional to pyrolytic temperatures since the thermal cracking via dehydrogenation occurs predominantly at high pyrolytic temperatures.⁹⁷ These typical behaviors provide a favorable condition for forming high content of aromatic chemical species in the oil.¹¹⁶ Similarly, dehydrogenation is also favorable for carbonization.¹¹⁶ In these respects, the concentration of CO is substantially lower than that of H₂ (as shown in Fig. 13). However, the gas evolution of major gases is discrepant with the typical pyrolytic patterns.¹¹⁷ The concentration of CO is rapidly increased at above 500 °C. Indeed, this enhanced generation of CO is initiated at >450 °C, but its magnitude is not comparable to that which occurs at >500 °C. For example, the enhanced generation of CO in CO₂ can reach up to 1200% at 600 °C for oak wood (Fig. 13a). As for syngas production from biomass pyrolysis at higher temperatures, CO₂ can enhance the generation of syngas (e.g., CO, H₂) via the thermal cracking of VOCs and the reaction between VOCs and CO₂. The identified roles of CO₂ lead to the enhanced generation of CO and the subsequent reduction of condensable tar.¹⁰⁶ Besides, at lower temperatures (<450 °C), the gas yield can be slightly reduced while the biochar and bio-oil yields marginally improve in CO₂.¹¹⁸ Nevertheless, the Boudouard reaction and VOCs or tar cracking occur with difficulty at much lower temperatures. Through the study of the CO₂-assisted co-pyrolysis of coal and biomass,^119,120 the influence of CO₂ could effectively occur at above 550 °C, which leads to VOC and tar reduction, thus enhancing generation of syngas and modifying the ratio of CO/H₂.

Fig. 13 Concentration profiles of H₂ and CO in N₂ (black line) and CO₂ (red line) evolved from pyrolysis of (a) oak wood, (b) spent coffee ground (SCG), and (c) red seaweed.¹⁰⁶

As a major product, bio-oil produced from pyrolysis of biomass is a complex assemblage of chemicals, mainly containing organic acids, aldehydes, ketones, furans, sugar-based components, phenolic compounds, etc.^121,122 Miscellaneous oxygenates, sugars, and furans are generated from cellulose and hemicellulose. Esters, acids, alcohols, ketones, and aldehydes are produced from the decomposition of the miscellaneous oxygenates, sugars, and furans. Guaiacols and syringols come from lignin.¹²³ The compositions of bio-oils from the pyrolysis of oil palm fibers (OPF) and oil palm fiber pellets (OPFP) at 450 °C are shown in Fig. 14a–d. In general, the chromatograms in the figure are similar to each other, indicating that the carrier gas and biomass pattern (i.e. needle-shape fibers or pellets) have no remarkable effects on the components in bio-oils produced from pyrolysis of biomass at relatively low temperatures (i.e. 450 °C). However, chemical components in tar generated from pyrolysis of woody biomass at 650 °C in CO₂ were less than those in N₂ (Fig. 14e), which is attributed to the enhanced thermal cracking induced by CO₂.¹²⁴

Fig. 14 GC-MS chromatograms of bio-oils from the pyrolysis of (a) OPF in N₂, (b) OPFP in N₂, (c) OPF in CO₂, and (d) OPFP in CO₂ at 450 °C;¹⁰³ (e) of bio-oil generated from pyrolysis of woody biomass in N₂ and CO₂ at 650 °C.¹²⁴

The presence of CO₂ could enhance VOCs/tar cracking or bio-oil upgrading in the pyrolysis of biomass, but the catalysts used for catalytic pyrolysis (in situ) or catalytic reforming (ex situ) are still required to meet the demands of practical use.^125–127 Cho et al.^128,129 studied the pyrolysis of metal (e.g., Fe³⁺, Co²⁺) impregnated biomass using CO₂ as a reaction medium. Generally, metal species loaded in the biomass have catalytic effects in the pyrolysis process. For example, the yield of CO at above 620 °C was enhanced via the catalytic effect of iron oxide (e.g., Fe₃O₄) formed during pyrolysis of spent coffee ground (SCG) in CO₂. Moreover, evolution of organic compounds in liquid products (i.e. tar) was mitigated in CO₂. In particular, caffeine in SCG was almost decomposed due to the synergistic effect of Fe₃O₄ and CO₂.¹²⁸ Furthermore, value-added biochar composites (e.g., biochar-Co) could be synthesized via one-step pyrolysis for other applications including catalysis and adsorption.^129,130 More details can be found in some reviews^89,131 on catalytic pyrolysis for syngas and biochar functionalized materials production.

3. CO₂ gasification of biomass

As discussed above, gasification consists of three major steps: drying, devolatilization (pyrolysis), and gasification. Extensive research has been conducted on the effect of steam, air, oxygen, and their combinations in the gasification process.^132,133 However, CO₂ as an oxidizing agent has been explored for biomass gasification in only a limited number of studies.^134–139 From the viewpoint of CO₂ consumption and CO yield enhancement, the utilization of CO₂ is more preferable as a gasifying agent. Besides, CO₂ is a major component of flue gases from many industries. It is a major cause of global warming and leads to various health issues. Significantly, CO₂ exists in the gaseous products from biomass gasification. Thus, recycling CO₂ for biomass gasification serves a dual purpose of reducing pollution and generating syngas. A major drawback is an external source of heat that is constantly required to maintain the gasification temperature, since the heat is not supplied by partial combustion of biomass in air or oxygen.¹⁴⁰

Generally, biomass pyro-gasification is operated with steam or air as gasifying medium but can also be performed using CO₂. It was proved that introducing CO₂ with steam as a gasifying medium leads to an enhanced CO production.¹⁴¹ In the gas phase, CO₂ can potentially react with hydrocarbons, such as methane (CH₄), via a dry reforming reaction (1):

CO₂ + CH₄ ↔ 2H₂ + 2CO, ΔH_{298 K} = +246.9 kJ mol⁻¹ (1)

CO₂ can also react with H₂ according to the reverse water gas shift reaction (2):

CO₂ + H₂ ↔ H₂O + CO, ΔH_{298 K} = +41.2 kJ mol⁻¹ (2)

Finally, in a biomass gasifier, CO₂ can react with the carbon of the char formed by the pyrolysis step, via the heterogeneous Boudouard reaction (3):

CO₂ + C ↔ 2CO, ΔH_{298 K} = −179.5 kJ mol⁻¹ (3)

Renganathan et al.¹⁴² performed a thermodynamic analysis of biomass gasification by CO₂ or mixtures of CO₂ with steam or O₂ and identified a universal optimal operating temperature of 850 °C for minimum energy input. Also, it was found that the use of CO₂ in biomass gasification in a fluidized bed gasifier increased substantially the carbon and energy conversion efficiency and decreased the amount of tars in the produced gas. The highest cold gas efficiency was achieved when biomass was gasified with CO₂.¹³⁵ In the case of rice straw gasification, the effect of different gasification agents H₂O, CO₂, O₂ and N₂ on the thermal efficiency was studied and it was found that the introduction of CO₂ has a positive effect on the thermal efficiency of a gasifier at temperatures of 850 °C and above.^141,143 In addition, Parvez et al.¹⁴⁴ simulated the air-, steam- and CO₂-enhanced gasification of rice straw by using Aspen Plus™ and compared them in terms of their energy, exergy and environmental impacts. It was found that the addition of CO₂ had less influence on syngas yield compared with gasification temperature. At lower CO₂/biomass mass ratios (<0.25), CO₂-enhanced gasification had a lower gasification system efficiency (GSE) than air gasification (<22.1%). However, at higher CO₂/biomass ratios, CO₂-enhanced gasification showed higher GSE than conventional gasification. The GSE of CO₂-enhanced gasification increased to 58.8% as the CO₂/biomass ratio was raised to 0.87. It was also found that chemical exergy was 2.05–4.85 times higher than physical exergy. The syngas exergy increased with CO₂ addition, mainly due to the increase in physical exergy. The maximum exergy efficiency occurred at 800–900 °C. For CO₂-enhanced gasification, exergy efficiency was more sensitive to temperature than to CO₂/biomass ratio. In addition, CO₂-enhanced gasification can result in significant environmental benefits compared with steam gasification.

The major effects of CO₂ on biomass pyrolysis are the increase of syngas yield and modification of the biochar textural properties. However, the char reactivity to O₂, H₂O and CO₂ was practically the same as for char prepared in N₂.⁹⁵ Furthermore, Guizani et al.¹³⁹ studied the whole woody biomass gasification process in a CO₂ atmosphere at 850 °C. It was found that although the CO₂ is present inside the particle during the pyrolysis stage, it has no noticeable impact, neither on the reaction rate nor on the char yield due to the relatively low temperature. The CO₂ char gasification is the rate limiting step of the pyro-gasification reaction as its duration is near to 95% of the entire biomass conversion time.¹³⁹

As we know, gasification of biomass often needs higher temperatures (normally above 800 °C). In particular, an external source of heat is constantly required to maintain the gasification temperature for CO₂ gasification. Therefore, Sun et al.^145,146 reported the integrated CO₂ gasification of sludge using waste heat from hot steel slags. Here, the steel slags played two important roles of catalyst (strong crystallization ability due to high basicity: mass ratio of CaO to SiO₂) and heat source. Significantly, the steel slag could inhibit pollutants such as SO₂, NO_x from being released from the decomposition of sludge.^145,146 Besides, an integrated concept was proposed for industrial applications, mainly composed of a rotary cup atomizer (RCA) system, where the molten slags were granulated into small particles at 1550–1000 °C, and a sludge gasification system, where CO₂ gasification of sludge occurred using hot slags at 1000–500 °C (Fig. 15).¹⁴⁵

Fig. 15 CO₂ gasification of sludge using hot slags.¹⁴⁵

To enhance the reactivity of biomass gasification in CO₂, some researchers also studied the co-gasification of biomass and coal.^147,148 It was found that synergy occurs when ashes are formed, followed by an almost complete gasification of biomass. Potassium (K) species in biomass ash play a catalytic role in promoting gasification reactivity of coal char, which is a direct consequence of synergy during co-gasification. The transfer of K from biomass to the surface of coal char occurs during co-pyrolysis/gasification. Biomass ash rich in silica can eliminate synergy in coal/biomass blends but not to the extent of inhibiting the reaction rate of the blended chars to make it slower than that of separated ones. In general, the best result in terms of synergy was the combination of low-ash coal and K-rich biomass.¹⁴⁷ Kramb et al.¹⁴⁹ also investigated the effects of Ca and K on CO₂ gasification of birch wood in a fluidized bed. The CO₂ gasification was performed after pyrolysis in N₂. In this work, K in the biomass had a significant influence on the structure of the resulting char. However, K did not show a remarkable catalytic effect on the overall gasification reaction rate with CO₂ due to the formation of a coke layer on the char surface. In contrast, Ca can increase the char conversion rate, which is likely the primary active catalyst in gasification of birch wood with CO₂.

Entrained flow gasification, which plays an important role for large-scale utilization of carbonaceous materials, is typically performed at high temperatures (1200–1500 °C), normally higher than the ash fusion temperatures of the feedstocks.¹⁴⁸ Although such high-temperature gasification is favorable for high carbon conversion and tar reduction, it may bring technological problems associated with ash melting such as sintering, agglomeration, slagging or corrosion during thermo-chemical reaction of biomass and coal.^150,151 “Unburnt carbon”, called ungasified carbon, is formed in the char-slag/ash transition process as some unburnt carbon is encapsulated by the slag and excluded from slag discharge system, which is unfavorable for large-scale gasification. The process of solid particles transforming into slag is a crucial step for the stable operation of industrial gasifiers. Therefore, it is necessary to study in depth the particle fate and interactions between char matrix and minerals during the processes of char-slag/ash transition so as to further improve the gasification efficiency of biomass in gasifiers.

The char-slag/ash transition mechanisms were usually associated with biomass type, reaction temperature, etc.¹⁵² Moreover, the variations of morphological characters and reactivities of different biomasses at different temperatures have been studied by many researchers. Coda et al.¹⁵³ analyzed the slagging behavior of woody ash in entrained-flow gasification conditions and found that a molten slag of woody ash was not prone to form at typical gasification temperatures of 1300–1500 °C, in spite of the presence of a relatively high content of low-melting alkaline species. It was concluded that the formation of high-temperature-melting species and the vaporization of low-melting alkali metal species during the char–slag transition process contributed to the poor slagging problem of woody ash in the reaction conditions close to those of industrial gasification. Metallic compounds in ash always play a vital role in the processes of char-slag/ash transition.¹⁵⁴ For instance, alkali and alkaline earth metals (AAEM) in biomass ash, such as K, Na, Ca, had positive effects on biomass gasification.^155–157 Ding et al.¹⁴⁸ studied the characteristics of the morphological changes and interactions in char, slag and ash during CO₂ gasification of rice straw and lignite. Rice straw particles exhibited shrinkage of particle form at the reaction temperature (1000 °C). And the variation of the existing state of K along with char–slag/ash transition can well explain the reactivity differences between raw char and demineralized char.

AAEM (e.g., K) in biomass has been considered as a natural and economical catalyst for thermochemical processes such as pyrolysis and gasification. Co-gasification of biomass and non-biomass feedstocks has been widely studied to investigate their interaction, including synergistic effects, inhibition effects, and no interaction. It was found that co-gasifying switchgrass (high K content) with coal char (high ash content) showed an inhibition effect during CO₂ gasification.¹⁵⁸ This was attributed to sequestration of the mobile alkali elements by the reaction with aluminosilicate minerals in coals to form inactive alkali aluminosilicates, such as KAlSi₃O₈ and KAlSiO₄. K was not accessible to accelerate the gasification reaction. The catalytic activity was evident as excess alkali (K/Al > 1) present in the feed mixture to satisfy the stoichiometric requirements of these deactivation reactions.¹⁵⁸ Therefore, fossil fuels with low ash contents have a high potential to be co-gasified with a K-rich biomass to benefit from the synergistic effect. Fernandes et al.¹⁵⁹ quantified the synergistic/antagonistic behavior occurring in the co-gasification of non-biomass feedstocks (ash-free coal and fluid coke) with K-rich biomass (switchgrass). The acceleration of the gasification rate of the non-biomass feedstock followed a linear function of the switchgrass conversion. And the inhibition of the switchgrass conversion did not show a clear trend because it depends upon the non-biomass feedstock and temperature. The K in switchgrass acts as a catalyst significantly increasing the gasification rates for the fluid coke or ash-free coal in the mixture.¹⁵⁹ The K catalyst can take oxygen from the reaction gas (step 1 in Fig. 16) and transfer it to the surface where oxygen reacts with carbon to form carbon monoxide (step 2). Step 3 is site regeneration, which requires a certain K mobility (intra- and interparticle), designated non-specifically as K∼. Note that K–C, K∼, and CO–K⁺ represent generalized sites (i.e., reduced, reduced, and oxidized, respectively) with the required K–C contact but an unknown stoichiometry. Once biomass-based carbon is converted, K can either move to the next C within the biomass or move to the non-biomass feedstock (step 3 or 3′ in Fig. 16, respectively).¹⁵⁹

Fig. 16 Simplified mechanism of K-catalyzed CO₂ gasification (oxygen transfer cycle), including intra- and interparticle K transfer.¹⁵⁹

In summary, only unreacted AAEM can act as a catalyst for enhancing coal gasification. And coal minerals can increase the reactivity of coal. However, when they react with K in biomass, K is deactivated causing an inhibition effect. A schematic of switchgrass mineral (e.g., K₂CO₃) two-fold effects on coal gasification is displayed in Fig. 17.¹⁶⁰ It was also demonstrated that K-rich biochar can act as a catalyst for tar reduction in biomass gasification.¹⁶¹ CO₂ reforming of tar with biochar-based catalysts will be discussed in Section 5.

Fig. 17 Schematic of two-fold effects of ash minerals (e.g., K₂CO₃) in switchgrass on coal gasification. Crystal elements: red: oxygen; white: hydrogen; pink: potassium; yellow: silicon; cyan: aluminum.¹⁶⁰

Many studies, both experimental and theoretical, suggested that catalytic activity of alkali metal salts can be well explained by RedOx (reduction–oxidation) cycles of M₂CO₃, M(g), M₂O, and MOH.^162,163 Notably, black liquor can be gasified in an entrained-flow gasifier at relatively low temperatures (i.e. 1000–1100 °C) due to the catalytic activity of alkali metal salts.^164–166 These studies also showed very low tar concentrations in the product gas (10–200 ppm of C₆H₆). Considering the ability to recover alkali metal salts as an aqueous solution at the outlet of the gasifier, alkali-catalyzed gasification of biomass can be an alternative to produce clean product gas from the gasifier (as shown in Fig. 18). Recent studies have also proved that woody biomass impregnated with solutions of alkali metal salts showed an increase of char reactivity.^167,168 Furthermore, Umeki et al.¹⁶⁹ demonstrated that tar and soot can be reduced simultaneously using the catalytic activity of alkali metal species. Raw and alkali-impregnated sawdust were gasified in a laminar drop-tube furnace at 900–1400 °C in an N₂–CO₂ mixture. At 900–1100 °C, char, soot and tar decreased with an increase of temperature for raw and alkali-impregnated sawdust. In addition, K can inhibit the growth of PAHs and promote the decomposition of tar (with 1–2 aromatic rings). The catalytic activity of K for tar decomposition is present in both solid and gas phases. Since alkali salts can be recovered from the product gas as an aqueous solution, alkali-catalyzed gasification of woody biomass can be a promising process to produce clean syngas from the entrained-flow gasification process at a relatively low temperature.

Fig. 18 Alkali-catalyzed entrained-flow gasification of biomass along with catalyst recycling.¹⁶⁹

As discussed above, K₂CO₃ as a carbonate salt shows a considerable catalytic activity for gasification of biomass or char. It was also reported that the stability of the carbonate salt can be maintained up to high temperatures of about 1073 K under CO₂ atmosphere.¹⁷⁰ Moreover, CO₂ could reverse the salt oxides to liquid salt as in eqn (4). Thus, CO₂ is expected to be a suitable gasifying agent for gasification with the molten salt. However, the CO₂ gasification of char (eqn (5)) often requires slightly more energy input than steam gasification of char (eqn (6)). Therefore, additional catalytic enhancement by adding a heterogeneous catalyst to the molten salt would not increase syngas production rates and yields but would enable the process to operate at a lower temperature with smaller heat losses and less heat recovery requirement, thereby making the whole process more economical for large-scale construction. Regarding the heterogeneous metal catalysts for biomass gasification, noble metals such as Pt, Pd, and Rh show high reactivity but incur prohibitively high costs. Meanwhile, reasonably priced nickel catalysts are known to exhibit excellent catalytic enhancement of H₂ production in many processes, such as biomass gasification,¹⁷¹ tar cracking/reforming,¹⁷² and CH₄ reforming.¹⁷³ In fact, nickel is not only selective for H₂ production but also active for the enhancement of carbon conversion.^174,175 Ratchahat et al.¹⁷⁶ investigated the catalytic enhancement of syngas production rate and yield using a combined ternary eutectic carbonate salt–Ni/Al₂O₃ (MS–Ni) medium for CO₂ gasification of cellulose, biomass wastes (chopstick, newspaper, sawdust), and sawdust char in a lab-scale reactor. The Ni/Al₂O₃ catalyst in MS–Ni medium can be active for H₂ production and enhance CO production.

M₂O + CO₂ ↔ M₂CO₃ (M = Li⁺, Na⁺, and K⁺) (4)

C + CO₂ ↔ 2CO, ΔH⁰ = +172.5 kJ mol⁻¹ (@293 K) (5)

C + H₂O ↔ CO + H₂, ΔH⁰ = +130 kJ mol⁻¹ (@293 K) (6)

In MS–Ni, char could react with CO₂ to CO via the Boudouard reaction. Char was also decomposed by the C–C cleavage reaction on the surface of the nickel catalyst. Reforming reactions of CH₄ and hydrocarbons were enhanced by the nickel catalysts. Meanwhile, volatile gases could be dissociated in the molten salt (as illustrated in Fig. 19).¹⁷⁶

Fig. 19 Mechanism of CO₂ gasification of biomass in molten salt–Ni/Al₂O₃ (MS–Ni).¹⁷⁶

Biomass gasification is a generic term encompassing several reactions occurring during biomass conversion, including drying, pyrolysis and residual char gasification steps. Char gasification is the rate limiting step in biomass gasification reactors. As for the entire gasification process of biomass, biochar gasification generally follows with the biomass pyrolysis process. For instance, char gasification occurs in the reduction zone in a downdraft gasifier (as illustrated in Fig. 20).¹⁷⁷ Biochar is a porous, carbonaceous, non-organized material, which contains mainly C and, in lower proportions, O, H, N and minerals such as K, Ca, Na, Si and Mg. In the gasifiers, the char gasification reaction occurs with O₂, H₂O, CO₂, and H₂via the reactions of combustion, steam gasification, Boudouard reaction, and methanation, respectively. The gasification reaction is a heterogeneous reaction involving reactant gas diffusion inside the char, reaction on the char active sites and diffusion of the syngas out of the particle. The reaction can be also catalyzed in the presence of minerals such as the above mentioned K.¹⁷⁸

Fig. 20 Downdraft “Imbert-style” gasification.¹⁷⁷

The char porosity, its structural features, the nature of the surface functional groups as well as the presence of catalytic minerals affect its reactivity toward the reactant gases. These different char properties affecting the reactivity are classified in three categories: (1) the char textural properties of the porosity and pore size distribution; (2) the char structural properties of the carbonaceous structure and graphitization (ordering); (3) the char chemical properties of the surface functional groups as well as the catalytic mineral species. Based on reported works, the char reactivity is highly conditioned by its textural, structural and chemical properties. These characteristics are also highly coupled which makes the task of understanding the gasification reaction mechanisms even more difficult. Thus, several studies focus on the modeling of the char gasification reaction to determine the reaction kinetic parameters.^179,180 Char gasification models are often semi-empirical ones, as they include a term accounting for the changes in the different char properties during the gasification,¹⁸¹ which reflect the ambiguity of this issue. To understand the phenomenology of char gasification, Guizani et al.¹⁸² further studied the char surface chemistry, structural and textural properties as well as mineral species behavior during gasification in CO₂, H₂O and CO₂/H₂O mixture. Fig. 21 shows the evolution of the char reactivity with the concentrations of K, Ca and Mg in different gasification atmospheres. Near-linear correlations were found between the char reactivity and the molar concentrations of these species in different atmospheres. The alkali metals (e.g., K) are more effective than Ca.¹⁸³ As for Si-rich biomasses (e.g., rice husks, bagasse), much lower reactivities were observed. It was assumed that the formation of alkali silicates at low temperatures curtailed the catalytic action of K. In this work, quite interesting potential synergy was proved between CO₂ and H₂O during gasification as the presence of CO₂ induced the departure of Si from the char, which is an inhibitor in steam gasification.¹⁸² Noteworthy, H₂O and CO₂ gasification reactions have different pathways.^182,184 The principal characteristics and differences between CO₂ and H₂O gasification reactions are summarized in Table 2. In mixed atmosphere gasification, these two molecules cannot react independently because there are likely several competition and synergy interactions that lead to an apparent additive law of reactivity. The separate active site reaction mechanism is rather a sum of a more complicated synergy and inhibition interactions. It would be only a fortuitous correct mathematical representation of the char reactivity in mixed CO₂–H₂O. In addition, Chang et al.¹⁸⁵ proved that intermittent addition of steam could increase the gasification reactivity of palm kernel shell biochar with CO₂, which was influenced by the formation of char pore structure and size.

Fig. 21 Relationship between the char reactivity and molar concentration of K, Ca and Mg in the char (mol%) during the gasification with H₂O, CO₂ and their mixtures.¹⁸²

Table 2 Main differences between the gasification reactions in CO₂ and H₂O¹⁸²

	H2O gasification	CO2 gasification
SFG	(1) Less ethers, anhydrides and phenols since the early gasification stages; (2) differences in peak positions compared to CO2 gasification	(1) More CO emitting FG; (2) much less hydrogenated char
Structure	Preferential removal of amorphous carbon forms	Lower selectivity towards the carbon forms
Texture	(1) Higher TSA at equivalent X and less damaged surface; (2) development of mesoporosity besides microporosity; (3) small micropore size distribution; (4) preferential development of 1 nm micropores	(1) Lower TSA and more damaged char surface; (2) higher proportion of micropores; (3) large micropore size distribution
Minerals	(1) Better retention of AAEM species; (2) higher retention of Si and Al	(1) Lower retention of AAEM species; (2) lower retention of Si and Al

---

In respect of char gasification in CO₂, many experimental and modelling works have been performed.^186–192 Since the gasification reactivity of char is mostly determined by its carbonaceous structure, four kinetic models can be applied to describe the CO₂ gasification behavior of chars: the volumetric model (VM), the grain model (GM), the random pore model (RPM) and the modified random pore model (MRPM). The RPM and MRPM can well describe the reactivity of different chars. However, when the peak gasification rate appears in high conversion range, the MRPM can perform better.¹⁸⁸ Seo et al.¹⁹³ studied the gasification of biochars with CO₂ at different temperatures. It was found that the experimental data agreed well with the RPM. Fermoso et al.¹⁹⁴ studied the effect of gasification temperature, pressure and CO₂ concentration during gasification of biochars. It was found that among VM, GM and RPM, only the RPM accurately predicted the conversion of different char which can be well fitted by the Langmuir–Hinshelwood model. In general, RPM could well describe the gasification curve of char under CO₂ atmosphere, but when peak reaction rate appears at high conversion range over 0.393, the discrepancy between experiment data and model calculation is significant. Therefore, based on RPM, many further investigations were carried out. Zhang et al.¹⁹⁵ evaluated a semi-empirical kinetic model to reconcile with gasification reactivity profiles of biochars. It was found that the fitting parameters introduced in the MRPM well predicted the amount of active K in biochar. Zhang et al.¹⁹⁶ developed a MRPM, which could be reduced to a traditional VM, GM, hybrid model and RPM by varying the model parameters. Alvarez et al.¹⁹² studied the CO₂ gasification kinetics of rice husk char. Rice husk char gasification has a complex kinetic behavior, with the reactivity being strongly dependent upon the temperature and char conversion. The experimental results are fitted to four different kinetic models (namely homogeneous, nth order, RPM and MRPM). The RPM provides the best fit to the experimental evolution. In addition, the catalyst/biomass integration concept is demonstrated to promote the reduction of char residence time and energy consumption via the catalysis of the rate limiting step of the entire biomass gasification process.¹⁹⁷ Both the in situ generation of char/Ni⁰ catalyst and the CO₂ gasification of the char/Ni⁰ can occur at temperatures as low as 500 °C.^197,198

4. CO₂ activation of biochar

Biochar, a fine-grained, highly porous charcoal substance, is typically a byproduct from pyrolysis and gasification of biomass. Biochar can be used as an energy source due to its excellent fuel properties such as high energy density and good grindability.^199,200 Biochar has also been used as an effective adsorbent for application in environmental remediation.^201,202 In addition, there has been great interest in biochars as soil amendments to improve soil fertility and to increase soil carbon sequestration. In its application as a soil amendment and adsorbent, the effectiveness of biochar has been limited by its low surface area. Many researchers have attempted to produce activated carbons from biomass. The production of activated carbons has generally used two activation methods: physical and chemical activation. Physical activation uses oxidizing gases such as air, CO₂, and steam,²⁰³ but chemical activation uses chemicals such as alkali hydroxides (NaOH/KOH),^204,205 inorganic acids (H₃PO₄, HCl and H₂SO₄)²⁰⁶ or ZnCl₂.²⁰⁷ In general, the carbons produced are mainly microporous with high specific surface areas (up to 3000 m² g⁻¹). However, the difficulties associated with the recovery of the activating agent as well as the requirement for a stringent wash of the adsorbents produced to avoid the corrosion of the equipment and interferences in the adsorption process would increase the overall cost of the process and hinder its implementation at large scales.²⁰⁸ Alvarez et al.^203,209 studied the physical activation of rice husk pyrolysis char for the production of high surface area activated carbons. Thermal activation is known to be more environmentally friendly and less costly than chemical methods. This carbonaceous precursor is subjected to partial gasification by using steam and CO₂ as activating agents.

The mechanism of pore development in activated carbon could be summarized as two main antagonistic contributions to specific surface area (S_BET), namely the formation and disappearance of micropores (as shown in Fig. 22).²¹⁰ On the one hand, appropriate conditions (e.g., mass ratio) are favorable for the development of pore structure.²¹¹ Under this condition, the formation rate of micropores is greater than the disappearance rate, increasing the S_BET of activated carbon. When the formation and deformation rates are equal, pore development can reach a state of dynamic equilibrium with maximum S_BET. On the other hand, extreme conditions can accelerate the widening of micropores.²¹⁰ Many micropores are destroyed through collapsing or merging into mesopores and macropores, leading to a reduction of S_BET.²¹² In addition, it is demonstrated that the gasifying agent (e.g., CO₂, steam) has a profound influence on the activated carbon porosity development. First, steam is more reactive and can produce activated carbon with greater N₂ adsorption capacity. Second, steam can produce more mesopores with the increase of activation time. While steam generates micro-, meso-, and macropores from the early stages of the process, CO₂ can produce highly microporous carbon, with broadening of the microporosity only for long activation times.²¹³

Fig. 22 Possible mechanism of char activation.²¹²

The effects of steam and CO₂ activation on the nitrogen adsorption isotherms and pore size distribution (PSD) are shown in Fig. 23. CO₂ can favor the development of new fine porosity followed by pore enlargement, while steam causes widening of micropores from earlier activation stages.^214,215 The wide PSD of the activated carbons obtained by using both steam and CO₂ is also related to the presence of a well-developed porous structure in the precursor. Therefore, steam can generate new very small pores in the initial stages of activation, which subsequently form larger pores and finally porous structure destruction. However, CO₂ creates new micro- and mesopores for longer treatments.^203,209

Fig. 23 Adsorption–desorption isotherms and pore size distribution of activated carbons obtained at different burn off levels with (A) H₂O²⁰⁹ and (B) CO₂.²⁰³

5. CO₂ reforming of tar

Biomass tar removal has become one of the main challenges for biomass gasification.^216,217 Among several methods for tar elimination, catalytic reforming is considered to be the most promising for large-scale applications, because of the high reaction rate, high reliability and increased quantity of usable gases.⁸⁷ Efficient catalytic conversion of tar by inert carbon-based catalysts is an attractive method for commercializing this technique.⁸³ In particular, pyrolysis biochar is widely used as a low-cost catalyst in the cracking/reforming of tar in the process of biomass gasification.⁸³ During catalytic reforming of tar over biochar, even after the loss of catalytic activity through coking, the char samples can still be directly combusted, so as to recover the chemical energy of the catalyst, thus avoiding any reprocessing as a result of deactivation. Additionally, as for biomass gasification, steam or CO₂ gasification agent provides a necessary reaction environment for the tar conversion, and ensures that catalytic deactivation of biochar cannot be caused by the carbon deposition of tar. The biochar catalytic activity in tar reforming is significantly affected by the AAEM species in the biochar.^218–220 Steam reforming of tar has been widely used, since it produces syngas with high yield of H₂ due to the water–gas-shift reaction.²²¹ Nevertheless, few works on CO₂ reforming of tar have been reported.^222–225 Feng et al.^226,227 studied the catalytic effects of AAEM (e.g., K, Ca) in biochars on tar reforming. Moreover, the effects of H₂O and CO₂ on homogeneous conversion and heterogeneous reforming of biomass tar over biochar were studied. Fig. 24 shows that conversion of tar compounds with K-loaded char is significant in H₂O or CO₂.

Fig. 24 Decomposition rates of tar compounds (A) toluene, (B) phenol, and (C) naphthalene, and (D) mechanism of tar catalytic reforming with H-form/K-loaded/Ca-loaded chars in H₂O/CO₂ at 800 °C.²²⁶

As illustrated in Fig. 24D, the active oxygen atoms (O˙) for oxidative decomposition of hydrocarbons and intermediate products are mainly generated by the reaction CO₂ + e* = CO + O˙ + e. Active OH free radicals (˙OH) can be formed by replacing the hydrogen atoms in hydrocarbons with oxygen atoms. In a steam atmosphere, O and OH free radicals can be formed by ionization of H₂O (H₂O = ˙H + ˙OH). Fracture of OH can form new H and O free radicals. However, the process is not strong. The H/O/OH atoms in the gas phase exist in radical form. These free radicals play a vital role in transforming AAEM species from metallic to radical forms. The CM–Ca and CM–K bonds may not be stable at high temperatures in H₂O or CO₂ and can be broken to generate active sites together with the release of O-containing species (e.g., CO_x) or aliphatic materials (e.g., CH₄). Some of the AAEM species may leave the biochar particles (reactions (7) and (8)). New CM–Ca and CM–K bonds can be formed through recombination (reactions (11) and (12)). The CM–K and CM–Ca bonds can be continuously broken and reformed. Repeated CM–AAEM bond forming and bond breaking increases the concentration of active sites in the biochar particles, providing more chances for combination and cracking of tar.

(CM–Ca–CM) = (–CM) + (–Ca–CM) (7)

(–Ca–CM) = (–CM) + –Ca (8)

(CM–K) = (–CM) + K (9)

(–CM) = (–CM′) + gas (10)

(–CM′) + (–Ca–CM) = (CM′–Ca–CM) (11)

(–CM′) + K = (CM′–K) (12)

During volatilization, AAEM species leave the gas phase as K⁺ and/or Ca²⁺ radicals. The AAEM radicals can combine with activated tar fragments in the gas phase formed via dealkylation and depolymerization of tar. Due to the low stability of the species formed by combination, the H/O/OH free radicals can easily exchange with the AAEM species from the activated tar fragments to form light tar components. The presence of K and Ca, which lead to repeated bond forming and bond breaking of species from AAEM and tar fragments, causes tar conversion. After combination with free radicals, a series of bond-breaking and ring-opening reactions result in gradual tar decomposition and the formation of light hydrocarbons and small-molecule gases.²²⁸ The cracking and transformation of tar are promoted by reactions (7)–(12). Based on the free-radical theory,²²⁹ catalytic tar cracking occurs on the catalyst surface, and the formation of activated tar fragments in space could promote chemical reactions between tar fragments and H/O/OH free radicals. The reactants combined on the char active sites (AAEMs and some defects in the carbon structure) can easily participate in reforming reactions.²³⁰ For a biochar surface, the activated tar fragments can be combined on the active sites of the char matrix and take part in further reforming reactions. In sum, the pathways for H₂O or CO₂ reforming of tar by K and Ca in biochar can include direct homogeneous reforming and gasification on the biochar surface.

Furthermore, Feng et al.²²⁷ used H-form biomass (acid-pretreated biomass) with little AAEM to provide real tar components. For tar homogeneous reforming, temperatures of 700–900 °C are needed for tar reforming in the presence of H₂O/CO₂ (Fig. 25A). This process is considered to be a two-stage process. The first stage involves the decomposition and transformation of the active heteroatom-containing groups in tar, with the decomposition of dealkylated side chains, hydrocarbon molecular cyclization, aromatization reactions, etc. The products include short-chain aliphatic hydrocarbons, oxygen-containing small-molecule gases, and single-ring aromatic hydrocarbons. The second stage involves the reforming of tar, the dehydrogenation of cyclization products, the addition of acetylene and the growth, recombination and isomerization of aromatics. The two processes constitute the basis of the tar homogeneous reaction. The presence of reforming agents (i.e. H₂O, CO₂) can promote or inhibit the pyrolysis conversion of tar. The addition of H₂O or CO₂ can promote the generation of active free radicals such as ˙O, ˙OH, and ˙H. These free radicals can react with the active free tar fragments generated from the first stage of thermal decomposition, which suggests the importance of the H₂O and CO₂ reforming agents in the homogeneous conversion of biomass tar (as illustrated in Fig. 25B).

Fig. 25 (A) Yields and (B) homogeneous conversion mechanism of biomass tar in Ar, H₂O and CO₂ at 500–900 °C.²²⁷

For heterogeneous reforming, the tar yield was significantly reduced with the biochar catalyst, being lower than that of homogeneous reforming. Moreover, CO₂ reforming showed a similar tar reduction to H₂O reforming during heterogeneous reforming over the biochar catalyst (Fig. 26A and B). The heterogeneous reforming mechanism of tar over biochar in the presence of H₂O and CO₂ at 800 °C is illustrated in Fig. 26C. H₂O and CO₂ dissociate in space to form many H/O/OH radicals, which play an important role in tar reforming. Tar, through the biochar layer, is adsorbed onto the acid–base active sites (oxygen-containing functional groups and AAEM catalysts). The attraction effect of the biochar matrix involves an electron pair shift in the tar molecules, which promotes the tar molecules to break at high temperatures. The tar adsorbed on the catalyst surface will catalytically crack to form free radicals. The chemical reactions between these free radicals permit new products. H₂O and CO₂ as the reforming agents in the biochar matrix result in the fragmentation of the smaller aromatic rings. The empty active sites, formed by bond cleavage, are gradually occupied by H/O/OH radicals, forming active groups (e.g., O-containing functional groups). In the presence of H₂O/CO₂, a significant amount of H/O/OH radicals in the vicinity can enter into the biochar structure. The AAEM catalysts migrate at different rates and transformation occurs from the biochar matrix onto the gas–solid interface or the gas phase. As the AAEM species are bonded with the C element on the char surface by the O element,²²⁶ H₂O and CO₂ react with these C elements resulting in AAEM–O bond cleavage followed by precipitation. The valence state of Ca results in a stronger bonding interaction with the biochar compared with K. Additionally, Ca migration and precipitation are more difficult than those of K. When tar adsorbs and then cleaves the AAEM–O bond and functional group bond on the biochar surface, an aromatic fragmented radical is formed as other free radicals are encountered. Also, the active AAEM species in the vicinity will continue to occupy active sites on the tar fragment groups, thus inhibiting their secondary polymerization. Furthermore, the H/O/OH radicals are exchanged to the AAEM, increasing the possibility for reforming of tar macromolecules. After the reaction, gas and light tars (C_nH_y/CO/H₂) are formed, thus realizing the H₂O or CO₂ heterogeneous reforming of biomass tar over the biochar catalyst.²²⁷

Fig. 26 Tar yield during (A) H₂O and (B) CO₂ heterogeneous reforming on biochar. (C) Heterogeneous reforming mechanism of biomass tar over biochar in H₂O and CO₂ at 800 °C.²²⁷

6. Conclusions and prospective

Thermochemical processes such as pyrolysis or gasification can decompose carbon feedstocks in a controlled oxygen environment to transform them into gaseous (low molecular weight gases), liquid (high molecular weight compounds obtained after condensation), and solid (biochar) products. For example, pyrolysis is generally conducted in an inert environment (e.g., Ar, N₂), and gasification is performed in a partial-oxidation environment (e.g., air, steam). In particular, steam gasification has attracted more attention for converting biomass into valuable syngas with high concentration of H₂. This review summarized the recent progress in biomass pyrolysis and gasification using CO₂ as a reaction medium. The CO₂-looping pyrolysis and gasification of biomass were compared with conventional processes (e.g., N₂ pyrolysis, steam gasification). In the integrated valorization of biomass by pyrolysis or gasification, CO₂ can play a vital role in each stage mainly including biomass pyrolysis, biomass and biochar gasification, biochar activation, and tar cracking/reforming.

CO₂ as a reaction medium can significantly improve the thermal efficiency in the pyrolysis of biomass. Biomass pyrolysis in the presence of CO₂ can result in deep decomposition compared to pyrolysis in pure N₂, possibly due to either char gasification by CO₂ or the fact that CO₂ hinders polymerization reactions and secondary char formation by reacting or cracking tar compounds that may lead to its formation. Moreover, CO₂ has an affinity to react with hydrogenated and oxygenated groups, leading to a carbon-rich char with a high specific surface area. Thus, exploiting CO₂ as a reaction medium in biomass pyrolysis would provide an attractive option for the enhanced generation of syngas and tuned adsorption capability of biochar. In addition, CO₂ biomass pyrolysis can enhance the thermal cracking of harmful organic compounds, thereby suppressing the formation of benzene derivatives (e.g., VOCs) and PAHs.²³¹

Biomass gasification generally uses air, steam, oxygen, and their mixtures as gasifying agents to produce syngas at a desirable H₂/CO ratio. Air is the cheapest gasifying agent as it is readily available at almost no cost but the syngas product has a very low heating value. The use of oxygen, in lieu of air, increases heating value of syngas product at the cost of oxygen purifying facility. Steam increases the yield of H₂ production, but its use requires more heat input compared to air and oxygen gasification. CO₂ as a candidate gasifying agent has the potential to further reduce CO₂ emission. Also, the use of CO₂ as a gasifying agent offers several other advantages, including (1) producing a more reactive char resulting in efficient gasification and reduction in the residual char, (2) being a less corrosive gasification medium compared with steam, and (3) offering more flexibility in syngas production suitable for various downstream applications. In general, CO₂ gasification of biomass serves a dual purpose of reducing pollution and generating syngas. A major drawback is an external source of heat that is constantly required to maintain the gasification temperature, since heat is not supplied by partial combustion of biomass in air or oxygen. However, it has been proved that introducing CO₂ with steam as a gasifying agent can lead to enhanced CO production. The inert mineral species (e.g., K, Ca) in biomass/biochar can also improve the reactivity of char gasification and tar reforming in the entire gasification processes. The technologies of pyrolysis or gasification of biomass using CO₂ have attracted great attention, but they have still not been put into commercial applications. Future works should focus on the life cycle assessment of the integrated processes, including biomass pyrolysis for bio-oil and syngas production with activated carbon production (e.g., CO₂ pyrolysis of biomass, CO₂ activation of biochar) and biomass gasification for syngas production with tar reforming (e.g., CO₂ gasification of biomass/biochar, CO₂ reforming of tar). In addition, the CO₂-looping technologies still face some challenges including higher cost of purified and compressed CO₂. Actually, syngas including CO₂ is required to be separated prior to its applications, so CO₂ from syngas can be considered. Meanwhile, unpurified CO₂ can be employed in real applications, so reducing the costs. The suitable mix ratio of CO₂/N₂ can be evaluated in different biomass pyrolysis or gasification cases. Finally, CO₂ can be also captured from industrial flue gases, such as biomass gasification power plants.

Acknowledgements

This work is financially supported by the Startup Fund for Introducing Talent at NUIST (Grant 2243141501046), the National Science Foundation of China (Grant 91544220 and 21607079), and the National Science Foundation of the Jiangsu Higher Education Institutions (Grant 2151071609501).

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