Thermodynamic Analysis of Composite Metal Oxygen Carriers for Biomass Chemical Looping Gasification Coupled with CO₂ Splitting

Abstract

Biomass chemical looping gasification coupled with CO₂ splitting (BCLGCS) presents a promising carbon-negative route for simultaneous syngas production and CO₂ utilization, where the selection of oxygen carriers (OCs) is critical. Compared to single-metal oxides, composite metal OCs offer thermodynamic advantages. This study aims to evaluate the thermodynamic performance of composite metal OCs (LaFeO₃, BaFeO₃, CaFe₂O₄, and Ca₂Fe₂O₅) in BCLGCS to overcome the thermodynamic limitations of conventional biomass-CO₂ gasification. Gibbs free energy minimization calculations were performed to predict gas compositions and oxygen carrier phase transformations under varying operating conditions. Results show that steam addition promotes gasification by increasing H₂ content and lowering required temperatures, but substantially reduces CO₂ conversion in the splitting reactor by consuming residual char. Ca₂Fe₂O₅ demonstrates superior adaptability with tunable H₂/CO ratios, while LaFeO₃ requires high OC loading and BaFeO₃ undergoes deactivation via BaCO₃ formation. This work reveals inherent thermodynamic conflicts between gasification and CO₂ splitting steps, indicating that the optima for syngas production and CO₂ utilization are mutually exclusive, an insight not previously quantified in BCLGCS literature. The findings provide theoretical guidance for designing carbon-tolerant OCs and optimizing process parameters, advancing BCLGCS toward practical carbon-negative applications.

1. Introduction

The persistent reliance of global economic development on fossil fuels has driven a continual increase in greenhouse gas emissions, primarily carbon dioxide (CO₂), making climate change one of the most critical challenges of the 21st century [1,2]. Consequently, the development and utilization of renewable energy, along with the advancement of efficient and cost-effective carbon emission reduction technologies, have become pressing priorities for addressing resource, energy, and environmental constraints [3]. As the only carbon-neutral renewable energy source, biomass offers distinctive advantages including resource renewability, sustainable supply, and storable energy, and is regarded as the world’s fourth-largest energy source after coal, oil, and natural gas [4]. Meanwhile, beyond its role as a primary greenhouse gas, CO₂ is also an abundant and renewable carbon feedstock. The synergistic utilization of biomass and CO₂ can not only diminish societal dependence on fossil fuels, accelerate the transformation of modern energy systems, and support a green recovery of the post-pandemic global economy, but also contribute to carbon sequestration and emission reduction, thereby mitigating global climate change.

Various thermochemical and biochemical routes exist for biomass conversion, each with distinct product spectra and operational constraints. Anaerobic fermentation converts wet biomass to biogas (CH₄/CO₂) via microbial digestion, offering low energy input and high moisture tolerance, yet suffering from slow reaction rates, limited product flexibility, and incomplete lignin utilization [5]. Pyrolysis produces bio-oil, biochar, and syngas through thermal decomposition in the absence of oxygen [6]. While enabling liquid fuel production, it generates high tar content requiring costly upgrading and exhibits complex product distribution control [7]. Among various biomass utilization technologies, biomass gasification for syngas production offers notable advantages in terms of energy efficiency and production cost, making it an attractive approach for the synergistic utilization of biomass and CO₂ [8]. Employing CO₂ as a gasification medium not only enables negative CO₂ emissions but also leverages its carbon content to enhance syngas yield. However, the thermodynamic stability and kinetic inertness of CO₂ often result in incomplete conversion of the feedstock. This leads to elevated tar and residual CO₂ in the syngas, which represents a major barrier to the widespread application of biomass CO₂ gasification. Specifically, soot particles formed during tar combustion complicate dust removal, while condensed liquid tar at low temperatures can cause pipeline blockage and contamination, threatening the safe and stable operation of downstream equipment [9]. Meanwhile, excessively high CO₂ content degrades syngas quality, reduces target product yields, and deactivates catalysts in subsequent synthesis processes [10]. CO₂ also dilutes the H₂/CO ratio markedly, further restricting syngas applicability in energy and chemical industries. Although intensified reaction conditions, such as high temperature, high pressure, or supercritical CO₂, can partially improve the conversion of biomass and CO₂, they often compromise overall system efficiency [11]. Therefore, CO₂-enhanced biomass gasification remains constrained by challenges including incomplete feedstock conversion, inferior syngas quality, and unsatisfactory process economics.

Recently, a biomass chemical looping gasification route coupled with CO₂ splitting (BCLGCS) has been proposed to overcome the bottlenecks of conventional biomass CO₂ gasification for producing high-quality syngas. This approach transforms the conventional single-step process into two coupled sub-processes: biomass chemical looping gasification and CO₂ deoxygenation. Via the orderly exchange of lattice oxygen among biomass, oxygen carrier (OC) and CO₂, it achieves in situ directional conversion of biomass tar into small-molecule syngas and highly efficient deoxygenation reduction of CO₂ to CO, while maintaining satisfactory system economics [12,13,14]. As illustrated in Figure 1, the BCLGCS system consists of a gasification reactor, a CO₂ splitting reactor and an air reactor. Biomass undergoes partial oxidation by lattice oxygen (O_latt) to produce syngas in the gasification reactor, while tar is catalytically reformed into light gases over the active surface species of the OC. In the CO₂ splitting reactor, the OCs with oxygen vacancies (Ovs) and residual char (RC) react with CO₂ to produce CO. Finally, the reduced oxygen carrier is fully regenerated in the air reactor, releasing the heat required for both the gasification and splitting stages. The BCLGCS syngas production scheme offers distinct advantages: (i) sequential lattice oxygen exchange circumvents the thermodynamic constraints of conventional biomass-CO₂ gasification, achieving higher tar conversion and CO₂ utilization [15]; (ii) the exothermic oxidation reaction enables autothermal operation of the entire system; and (iii) blending hydrogen-rich syngas with CO allows precise H₂/CO ratio control, expanding the applicability of the product gas in downstream energy and chemical applications.

Rational design and activity tuning of OCs are critical for syngas generation, in situ tar reforming, and CO₂ splitting. Low OC activity limits lattice oxygen release, resulting in poor biomass conversion and high tar content in the gasification reactor. The reduced OC with few oxygen vacancies, further obstructing CO₂ activation and autothermal operation. Excessively high activity, by contrast, causes over-oxidation of the syngas, reducing its yield and quality, while the reduced carrier maintains a high thermodynamic oxygen partial pressure that hinders lattice oxygen exchange and CO₂ deoxygenation [16]. While BCLGCS has been experimentally demonstrated, existing studies have predominantly focused on the performance of specific oxygen carriers (e.g., NiFe₂O₄ [17], LaFeO₃ [18], LaMO₃ [19]) under limited operating conditions. The synergistic thermodynamic mechanisms governing BCLG and CO₂ splitting within the biomass-OC-CO₂ multicomponent system remain inadequately understood. This study addresses this knowledge gap by leveraging the thermodynamic equilibrium results. Through Gibbs free energy minimization, a comprehensive thermodynamic analysis of BCLG and CO₂ splitting for four composite oxygen carrier systems was performed. The gas product compositions and OC phase transformations across various reaction conditions were precisely predicted, thereby delineating the thermodynamic window for maximizing syngas yield and CO₂ conversion. This work provides a critical theoretical complement to existing experimental studies, offering direct guidance for overcoming the thermodynamic limitations of conventional gasification and optimizing process parameters. The findings serve as a valuable reference for BCLGCS toward practical application.

2. Materials and Methods

In the thermodynamic equilibrium calculations, sawdust was used as the feedstock.

Table 1. The proximate and ultimate analyses of sawdust (wt.%, air dry basis).

	Proximate Analysis				Ultimate Analysis					LHV
	Moisture	Volatile	Fixed Carbon	Ash	C	H	O	N	S	(MJ/kg)
Sawdust	9.2	77.6	12.6	0.6	45.09	4.3	40.2	1.93	0	17.75

presents the proximate and ultimate analysis results of the biomass. The primary elemental constituents of sawdust are C, H, and O, while the contents of N and S are negligible. In thermodynamic modeling of BCLG, it is common practice to simplify the feedstock composition to C, H, and O only. The simplified formula CH_1.144O_0.669 was derived from ultimate analysis following the methodology in the reference [20], which demonstrated that thermodynamic results using C-H-O simplification show minimal deviation (<3%) from those including minor elements for biomass gasification equilibrium calculations.

In order to theoretically screen the OCs suitable for BCLGCS process, Ellingham diagrams for common OCs and equilibrium yield diagrams for the CO/CO₂ system were drawn in Figure 2. It can be seen from Figure 2a that high-activity Cu-based OCs release molecular oxygen even at moderate temperatures, hindering selective biomass partial oxidation to syngas. Fe- and Ni-based single-metal oxides exhibit strong lattice oxygen release and effective tar cracking capability, yet suffer from syngas over-oxidation, diminished mechanical strength at elevated temperatures, and coking deactivation [21]. Composite metal carriers such as CaFe₂O₄, Ca₂Fe₂O₅, BaFeO₃ and LaFeO₃ possess moderated lattice oxygen release, demonstrating superior compatibility with biomass chemical looping gasification. Figure 2b demonstrates that higher equilibrium yields of the CO/CO₂ correspond to stronger thermodynamic driving force of OC for CO₂ splitting and lattice oxygen transfer. Apparently, composite metal OCs demonstrate high compatibility with CO₂ deoxygenation. Therefore, a comprehensive thermodynamic analysis of BCLG and CO₂ splitting for four oxygen carrier systems: LaFeO₃ (LF), BaFeO₃ (BF), CaFe₂O₄ (CF2), and Ca₂Fe₂O₅ (C2F2) was performed in this work.

In thermodynamic equilibrium calculations, the Gibbs free energy minimization approach was employed by using HSC Chemistry 6.0 software. The total Gibbs free energy of a system is expressed as a linear combination of the chemical potentials of its constituent components:

$$G=\sum _{i=1}^N{S}_i{G}_i$$

(1)

The Gibbs free energy depends solely on the amounts of each component present. The method involves determining the composition that corresponds to the minimum Gibbs free energy. It should be noted that the Gibbs free energy minimization method assumes that the system reaches a state of minimum Gibbs free energy, where all possible reactions have proceeded to completion. While real BCLGCS systems are indeed kinetically constrained, equilibrium calculations provide thermodynamic boundaries that are invaluable for identifying optimal oxygen carriers and operating windows without exhaustive experimental campaigns, revealing intrinsic thermodynamic conflicts that persist regardless of kinetics, and establishing theoretical maximum yields against which kinetic models and experiments can be compared. To ensure transparent application of this methodology, the underlying assumptions and their practical implications were stated as follows: (i) all reactions proceed to completion, with no kinetic barriers to reaction rates or diffusion limitations; (ii) the gas and solid phases are perfectly mixed with uniform temperature and composition, and particle-scale gradients are neglected; (iii) no mass loss or environmental interaction occurs during the calculation and elemental conservation is strictly maintained; (iv) gas-phase species follow ideal gas law and solid solutions are treated as mechanical mixtures of pure phases. The advanced gasification modeling that goes beyond equilibrium assumptions could be seen other work [22]. Furthermore, during the solution process, the amounts of components are constrained by the material (element) conservation equations [23]. The gaseous and solid species considered in the products for four OCs system were listed in

Table 2. The gaseous and solid species considered in the products for different OCs system.

	Species
LaFeO3	C(s), C(g), C2(g), C3(g), C4(g), C5(g), CO(g), CO2(g), CH(g), CH2(g), CH3(g), CH4(g), C2H2(g), C2H4(g), C2H6(g), H(g), H2(g), H2O(g), H2O(l), O(g), O2(g), Fe, FeO, Fe3O4, Fe2O3, FeCO3, Fe3C, LaFeO3, La2O3, LaC2, La
BaFeO3	C(s), C(g), C2(g), C3(g), C4(g), C5(g), CO(g), CO2(g), CH(g), CH2(g), CH3(g), CH4(g), C2H2(g), C2H4(g), C2H6(g), H(g), H2(g), H2O(g), H2O(l), O(g), O2(g), Fe, FeO, Fe3O4, Fe2O3, FeCO3, Fe3C, BaFeO3, BaCO3, BaO, BaO2, Ba2O, BaFe2O4, BaC2, Ba
CaFe2O4, Ca2Fe2O5	C(s), C(g), C2(g), C3(g), C4(g), C5(g), CO(g), CO2(g), CH(g), CH2(g), CH3(g), CH4(g), C2H2(g), C2H4(g), C2H6(g), H(g), H2(g), H2O(g), H2O(l), O(g), O2(g), Fe, FeO, Fe3O4, Fe2O3, FeCO3, Fe3C, CaFe2O4, Ca2Fe2O5, CaCO3, CaO, CaO2, CaC2, Ca

.

In the BCLGCS process, the carbon conversion ratio (η_C,con) and the CO₂ conversion ratio (η_CO2,con) are two key indicators for evaluating syngas quality and the performance of the CO₂ splitting reactor. The carbon conversion ratio is defined as the fraction of carbon in the feedstock that is converted into carbonaceous gases, while the CO₂ conversion ratio is defined as the proportion of CO₂ consumed relative to the initial amount fed into the splitting reactor. The high-quality syngas means that it has low contents of CO₂ and CH₄, high calorific value, and the H₂/CO ratio is suitable for subsequent chemical synthesis processes. This framework will serve as the basis for evaluating syngas quality in subsequent sections of this work.

$${\eta }_{C,con}={\displaystyle \frac{{\displaystyle \sum n_{C,gaseous}}}{n_{C,biomass}}}\times 100\%$$

(2)

$${\eta }_{C{O}_2,con}={\displaystyle \frac{n_{C{O}_2,initial}-n_{C{O}_2,final}}{n_{C{O}_2,initial}}}\times 100\%$$

(3)

3. Results

3.1. Biomass Gasification Process

3.1.1. Effect of OC Loading and Gasification Temperature

As illustrated in Figure 3 and Figure 4, gas yield distributions of CO, H₂, CO₂, CH₄, and syngas as well as carbon conversion ratio are strongly influenced by both OC loading and temperature. Irrespective of OC type and amount, a consistent trend across all OCs is the pronounced promotion of syngas yield and carbon conversion with increasing temperature, particularly above 800 °C. This is primarily attributed to the enhanced endothermic reactions, such as steam and dry reforming of methane ((R1) and (R2)), water-gas shift (WGS, (R3)), and improved carbon conversion at elevated temperatures [24]. The thermodynamic parameters of these reactions can be seen in

Table 3. Thermodynamic parameters of key reactions in this work.

Reactions	ΔH900°C (kJ/mol)	ΔS900°C (J/mol·K)	ΔG900°C (kJ/mol)
${\mathrm{CH}}_4{+\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CO}+3\mathrm{H}}_2$	225.695	252.393	−70.401
${\mathrm{CH}}_4{+\mathrm{CO}}_2\rightleftharpoons {2\mathrm{CO}+2\mathrm{H}}_2$	258.827	282.827	−72.972
${\mathrm{CO}+\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CO}}_2{+\mathrm{H}}_2$	−33.132	−30.434	2.571
${\mathrm{C}+\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CO}+\mathrm{H}}_2$	−583.537	−15.329	−565.553
${\mathrm{C}+\mathrm{CO}}_2\rightleftharpoons 2\mathrm{CO}$	−550.405	15.104	−568.125
${2\mathrm{LaFeO}}_3\rightleftharpoons {\mathrm{La}}_2{\mathrm{O}}_3{+\mathrm{Fe}}_2{\mathrm{O}}_3$	140.547	−33.271	179.579
${\mathrm{BaO}+\mathrm{CO}}_2\to {\mathrm{BaCO}}_3$	−236.266	−133.905	−79.176
${\mathrm{CaCO}}_3\to {\mathrm{CaO}+\mathrm{CO}}_2$	165.503	142.769	−1.986
${\mathrm{La}}_2{\mathrm{O}}_3{+2\mathrm{Fe}+3\mathrm{CO}}_2\to 3{\mathrm{CO}+2\mathrm{LaFeO}}_3$	−102.942	43.710	−154.221
${\mathrm{BaO}+\mathrm{Fe}+2\mathrm{CO}}_2\to {2\mathrm{CO}+\mathrm{BaFeO}}_3$	−49.563	−171.133	151.203
${\mathrm{CaO}+2\mathrm{Fe}+3\mathrm{CO}}_2\to 3{\mathrm{CO}+\mathrm{CaFe}}_2{\mathrm{O}}_4$	1.544	11.163	−11.552
${2\mathrm{CaO}+2\mathrm{Fe}+3\mathrm{CO}}_2\to 3{\mathrm{CO}+\mathrm{Ca}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$	−9.403	29.068	−43.504

. Meanwhile, CO and H₂ yields increase significantly, while CH₄ decreases due to reforming reactions. CO₂ yield generally shows a declining trend at higher temperatures, except for cases with excessive OC addition where complete oxidation is favored [25].

$$\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{CO}+3{\mathrm{H}}_2$$

(R1)

$$\mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{O}}_2\rightleftharpoons 2\mathrm{CO}+2{\mathrm{H}}_2$$

(R2)

$$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2$$

(R3)

$$2\mathrm{C}+3{\mathrm{O}}_{\mathrm{latt}}\rightleftharpoons \mathrm{CO}+\mathrm{C}{\mathrm{O}}_2$$

(R4)

$$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{CO}+{\mathrm{H}}_2$$

(R5)

$$\mathrm{C}+\mathrm{C}{\mathrm{O}}_2\rightleftharpoons 2\mathrm{CO}$$

(R6)

Among the OCs, the perovskite-type LF demonstrates exceptional oxidation resistance, maintaining high syngas yields (1.48 kmol/kmol bio at 2 kmol/kmol bio OC loading, 1000 °C) even at elevated OC loading (Figure 3a). The minimal decrement in CO yield underscores its moderate oxygen uncoupling capability, which prevents over-oxidation while sustaining a favorable CO/H₂ ratio of about 1.6. However, the excessively stable crystal structure of LF leads to slow oxygen release kinetics [26]. Under conditions of 1.0 kmol/kmol·bio OC addition and a gasification temperature of 900 °C, the carbon conversion rate remains limited to only 80.29% (Figure 4a). Consequently, a substantial amount of OC is required to achieve complete biomass conversion in practice, representing a major limitation for its scalable engineering application. By contrast, another perovskite BF exhibits BF relatively high reactivity at 0.1 kmol/kmol·bio loading, but suffered severe deactivation when the loading exceeded 1.0 kmol/kmol·bio (Figure 3b and Figure 4b). Under conditions of 1.0 kmol/kmol·bio loading and 900 °C, syngas yield and carbon conversion were drastically reduced to 0.49 kmol/kmol·bio and 32.68%, respectively. This pronounced suppression reflects the structural instability of BaFeO₃ and thermodynamically favorable formation of carbonates (BaCO₃), which passivate active sites [27,28]. The data indicate that BF transitions from a gasification promoter at 0.01 kmol/kmol·bio to an inhibitor at 1.0–2.0 kmol/kmol·bio, making it unsuitable for syngas generation under high loading conditions. In comparison, the calcium ferrites, CF2 and particularly C2F2, exhibited markedly superior performance compared to the perovskite-type OCs (Figure 3c,d and Figure 4c,d). They achieved higher syngas yields (exceeding 1.8 kmol/kmol·bio) and significantly enhanced H₂/CO ratios, especially within a moderate temperature window of 800–1000 °C. This enhanced hydrogen production is attributed to the potent catalytic effect of in situ generated CaO from the reduction of calcium ferrites, which actively promotes the WGS reaction (R3) [29]. Conclusively, among four OCs, C2F2 demonstrates favorable thermodynamic characteristics under equilibrium conditions, suggesting potential for further kinetic and experimental validation. It offered the highest syngas productivity with a broader optimal operational window. Moreover, the ability to tune syngas composition from H₂/CO of ~1.0 at low loading to ~0.5 at high calcium ferrite loading demonstrates superior process flexibility compared to conventional gasification. Considering both conversion efficiency and cost-effectiveness, the optimal OC loading is identified as 1.0 kmol/kmol·bio for LF, and 0.1 kmol/kmol·bio for BF, CF2, and C2F2.

3.1.2. Effect of Steam Amount

In the BCLG process, an appropriate amount of gasification agents such as steam or CO₂ is usually introduced into the gasification reactor to improve the quality of syngas [21]. At optimal OC loading mentioned above, the steam amount was varied to identify optimal conditions balancing gasification efficiency against CO₂ splitting performance. As seen in Figure 5, rising steam input consistently promotes H₂ and syngas yields while suppressing CO₂ and CH₄ formation across all four OCs. This is attributed to the enhanced WGS and methane steam reforming reactions under thermodynamic control, which shifts the product distribution toward more H₂-rich syngas [30]. Carbon conversion stabilizes when the steam amount reaches a certain level, indicating that the system approaches full carbon conversion under sufficient steam supply via (R5) and (R6). Among the four OCs, LF offers the highest ultimate syngas yield but requires substantial steam (0.5 kmol/kmol·bio) to maximize CO, reflecting its robust perovskite framework that tolerates higher steam partial pressures without structural degradation. However, other three OCs emerges as steam-responsive materials, achieving the highest carbon conversion and syngas production at approximately 1.0 kmol/kmol·bio steam. These findings reveal that steam addition is a powerful lever for product distribution control, but the optimal steam amount must be individually calibrated to balance CO maximization against H₂ enrichment. In the present case study at 900 °C, the optimal amounts of steam were 0.5 kmol/kmol·bio for LF, 1.0 kmol/kmol·bio for BF, and 0.9 kmol/kmol·bio for both CF2 and C2F2.

To identify the thermal window for maximum syngas quality under steam-assisted conditions, the effect of temperature on gas yield and carbon conversion for four OCs at each optimal steam amount was explored. The temperature at which syngas yield and carbon conversion reach a stable plateau can be defined as the minimum temperature for efficient gasification under the given conditions. It can be seen from Figure 6 that this temperature is approximately 800 °C for LF, whereas for BF, CF2 and C2F2 it is slightly slower, around 750 °C. Above these temperatures, syngas yields remain nearly constant, indicating that further energy input does not substantially improve gas production but may increase operational costs.

3.1.3. Phase Changes of OCs During CLG

Since the OCs in the CO₂ splitting reactor are supplied from gasification reactor, their reduction degree in the latter determines the CO₂ splitting performance. Therefore, under the optimal loading of each OC, the variation in phase composition of OCs with gasification temperature was further investigated under two different operating conditions (with and without steam), with results illustrated in Figure 7. Under steam-free conditions (Figure 7a), LF exhibits remarkable structural robustness, retaining its perovskite framework up to 750 °C before progressive reduction yields La₂O₃, FeO and metallic Fe. This indicated the partial structural collapse via reactions (R7) and (R8) [31,32]. Furthermore, as lattice oxygen is released and products are generated, the coke content decreases rapidly above 600 °C, which is primarily attributed to the char gasification reactions (R4)~(R6). Steam addition (Figure 4b) lowered the reduction extent of the LF. The apparent decomposition occurs only above approximately 800 °C. This is likely because steam primarily acts as an oxidant participating in the gasification of sawdust and replenishes the oxygen vacancies formed after the loss of lattice oxygen in LF. Meanwhile, the coke content decreases markedly and is almost completely consumed above 750 °C, owing to the enhancement of char gasification by steam (R5).

Similar phenomena were also observed for BF, CF2, and C2F2. The difference is that for BF, BaCO₃ remains the main component regardless of the presence or absence of steam. This may be due to the fact that BaO formed after BF decomposition (R9) tends to react with CO₂ to form carbonate (R10) [33]. The stable structure and low reactivity of BaCO₃ thus lead to the decrease in syngas yield at high OC loadings (see Figure 3b). Moreover, steam promotes the oxidation of Fe to FeO, making FeO the predominant iron oxide in BF after reaction. For both CF2 and C2F2, Fe and CaO were the predominant phases of OCs after gasification above 800 °C in the absence of steam, due to the decomposition of CaCO₃ formed above 700 °C under high-temperature conditions [34]. With the introduction of steam, the reduction extent of OCs decreased, and CF2, C2F2, and FeO became the major components.

$$2{\mathrm{LaFeO}}_3\rightleftharpoons \mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3+\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$$

(R7)

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{Sawdust}\to \mathrm{Fe}+\mathrm{FeO}+\mathrm{syngas}+\mathrm{char}+\mathrm{tar}$$

(R8)

$$2{\mathrm{BaFeO}}_3\rightleftharpoons 2\mathrm{BaO}+\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+{\mathrm{O}}_{\mathrm{latt}}$$

(R9)

$$\mathrm{BaO}+\mathrm{C}{\mathrm{O}}_2\to {\mathrm{BaCO}}_3$$

(R10)

$${\mathrm{CaCO}}_3\to \mathrm{CaO}+\mathrm{C}{\mathrm{O}}_2$$

(R11)

3.2. CO₂ Splitting Process

When considering the CO₂ splitting reaction in isolation, the reactions between the reduced phases of four OCs and CO₂ are theoretically described by reactions (R12)~(R15). The theoretical equilibrium compositions for the CO₂ splitting based on the stoichiometric ratios of these reactions are calculated and presented in Figure 8. Among four OCs, the LF-based reaction exhibits the most favorable thermodynamics, achieving 97.5% CO₂ conversion at 1000 °C with a steep activation threshold above 600 °C (Figure 8a). CO yield increases monotonically to 0.97 kmol/kmol·bio at 1000 °C, while residual CO₂ diminishes to 0.03 kmol/kmol·bio, reflecting strong forward reaction spontaneity. The strongly exothermic nature of LF formation facilitates the efficient reduction of the CO₂, which is otherwise thermodynamically less favorable at lower temperatures [35]. However, a critical drawback emerges below 850 °C range, where carbon deposition is remarkably evident. This carbon formation arises from the reverse Boudouard reaction (R6) competing with the desired oxidation pathway, favored at intermediate temperatures where CO concentration is substantial but thermal energy is insufficient to prevent disproportionation [36]. Above 850 °C, carbon gasification becomes dominant, reducing solid carbon to less than 0.04 kmol/kmol·bio. Figure 8b confirms that the BF-based system exhibits less favorable thermodynamics for CO production, attributed to the formation of BaCO₃. This suggests that BaO primarily functions as a CO₂ carrier rather than an active redox partner, since Ba-containing perovskites exhibit strong CO₂ adsorption on surface Ba sites, forming stable carbonates that inhibit oxygen exchange [37]. This could potentially be mitigated through kinetic control or structural modification. The calcium-based systems demonstrate stoichiometry-dependent performance. The single CaO system achieves 77.9% maximum conversion (Figure 8c), while the 2CaO system exhibits a 100% CO₂ conversion below 650 °C and decreases to 81.2% at 1000 °C (Figure 8d), reflecting the enhanced thermodynamic driving force associated with C2F2 formation. However, both systems exhibit apparent carbon deposition below 850 °C, indicating that the Fe-Ca-O phase assemblage cannot fully suppress the Boudouard side reaction [38].

$$\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3+2\mathrm{Fe}+3\mathrm{C}{\mathrm{O}}_2\to 3\mathrm{CO}+2\mathrm{LaFe}{\mathrm{O}}_3$$

(R12)

$$\mathrm{BaO}+\mathrm{Fe}+2\mathrm{C}{\mathrm{O}}_2\to 2\mathrm{CO}+\mathrm{BaFe}{\mathrm{O}}_3$$

(R13)

$$\mathrm{CaO}+2\mathrm{Fe}+3\mathrm{C}{\mathrm{O}}_2\to 3\mathrm{CO}+\mathrm{CaF}{\mathrm{e}}_2{\mathrm{O}}_4$$

(R14)

$$2\mathrm{CaO}+2\mathrm{Fe}+3\mathrm{C}{\mathrm{O}}_2\to 3\mathrm{CO}+\mathrm{C}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_5$$

(R15)

Theoretically, the conversion of CO₂ splitting in the four reduced OC systems can reach at least 50%. However, as shown in the results mentioned above, the actual composition of the OC after reduction in the practical CLG process is unlikely to match the theoretical one. Furthermore, the RC during gasification influences the CO₂ splitting reaction. The equilibrium composition also differs under steam-present and steam-absent atmosphere, and the reduced state of the OCs under actual operating conditions is even more complex. Therefore, phase equilibrium calculations for CO₂ splitting were performed utilizing the equilibrium composition of the reduced OC under two gasification conditions (with/without steam) as the initial composition for the CO₂ splitting reaction. The case without steam but considering RC is designated as Case 1, the case without steam and without considering RC as Case 2, and the case with steam where the RC is completely reacted as Case 3. The initial OC compositions are summarized in

Table 4. The initial OC compositions considered in CO₂ splitting reaction (unit: kmol/kmol·bio).

		Theoretical Initial Compositions	Case 1	Case 2	Case 3
CO2		1.000	1.000	1.000	1.000
LF	LaFeO3		0.877	0.877	0.986
	La2O3	0.333	0.061	0.061	0.007
	FeO		0.017	0.017	0.014
	Fe	0.667	0.106	0.106
	C		0.197
BF	BaCO3		0.097	0.097	0.100
	BaO	0.500	0.003	0.003
	Fe3O4				0.005
	FeO		0.001	0.001	0.085
	Fe	0.500	0.099	0.099
	C		0.253
CF2	Ca2Fe2O5				0.018
	CaFe2O4				0.004
	CaCO3		0.002	0.002	0.032
	CaO	0.333	0.097	0.097	0.028
	Fe3O4				0.011
	FeO		0.006	0.006	0.123
	Fe	0.667	0.193	0.193
	C		0.109
C2F2	Ca2Fe2O5				0.032
	CaFe2O4				0.005
	CaCO3		0.004	0.004	0.067
	CaO	0.667	0.195	0.195	0.064
	Fe3O4				0.004
	FeO		0.012	0.012	0.114
	Fe	0.667	0.187	0.187
	C		0.113

.

For LF in Case 1 (Figure 9(a1)), the CO₂ conversion increases monotonically from 11.3% at 500 °C to 36.3% at 675 °C and remains stable up to 1000 °C, accompanied by continuous consumption of RC via the Boudouard reaction (R6). When char is excluded from the initial composition (Case 2, Figure 9(a2)), the CO₂ conversion rises only modestly, reaching 16.5% at 600 °C. This suggests that RC acts as a primary source for CO production, shifting the reaction equilibrium toward enhanced CO₂ reduction. In contrast, under steam-added gasification conditions (Case 3, Figure 9(a3)), the CO₂ conversion in the splitting step reaches a maximum of merely 0.68% at 500 °C and subsequently declines with increasing temperature. This decrease is attributed to the complete consumption of RC by steam and the lowered reduction extent of the OC. The former is the dominant factor suppressing CO₂ conversion, indicating that, from a thermodynamic perspective, most of the CO₂ in the splitting reactor is consumed through the RC gasification reaction. A similar trend is observed for the BF oxygen carrier (Figure 9(b1–b3)). Although the CO₂ conversion under Case 3 (Figure 9(b3)) is slightly higher than that of LF, it remains limited to only about 2%. This indicates that the non-stoichiometry of the Ba-Fe-O system is inadequate for efficient CO₂ activation [39]. For both CF2 and C2F2 (Figure 9(c1–c3,d1–d3)), the CO₂ conversion in both Case 1 and Case 2 increases with temperature below 700 °C, followed by a decrease above this point. The initial rise is attributed to char gasification, while the subsequent decline results from the decomposition of formed CaCO₃, which releases CO₂ and reduces the net conversion. A parallel trend is observed in Case 3 at temperatures exceeding 700 °C. Notably, negative CO₂ conversion occurs above 900 °C for C2F2 (Figure 9(d3)). This indicates that, not only is the initially absorbed CO₂ released, but additional CO₂ is also produced from the decomposition of incoming carbonates derived from the gasification reactor.

4. Discussion

The optimal operating conditions for sawdust gasification and CO₂ slitting steps are summarized in

Table 5. Optimal operation parameters of BCLGCS process for various OCs.

OCs	Gasification Process				CO2 Splitting Process
		Steam-Free Condition	Steam-Added Condition
	OC Amount (kmol)	Gasification Temperature (°C)	Gasification Temperature (°C)	Steam Amount (kmol)	CO2 Splitting Temperature (°C)
LF	1.0	900	800	0.5	675–725
BF	0.1	900	750	1.0	700–725
CF2	0.1	900	750	0.9	675
C2F2	0.1	900	750	0.9	675

. In the absence of steam during gasification, a higher gasification temperature (~900 °C) is required to achieve greater carbon conversion and syngas yield for all four OCs. Steam addition significantly enhances gasification efficiency and syngas quality, allowing the gasification temperature to be lowered appropriately. The optimal temperature for the CO₂ splitting step should not be excessively high, primarily to prevent the decomposition of carbonates at elevated temperatures. While experimental research on BCLGCS remains in its early stages, available experimental results can substantiate our simulation findings. For instance, Li et al. [18] experimentally investigated LF with a fragmented flaky structure in BCLG coupled with CO₂/H₂O splitting. Their study demonstrated that steam addition notably increases gas yield and H₂ production, consistent with our thermodynamic prediction that steam promotes the WGS reaction and lowers the optimal gasification temperature for LF from 900 °C to 800 °C. Moreover, XRD characterization of reduced LF in their work identified La₂O₃ and Fe⁰ as the dominant phases, matching our equilibrium predictions in Figure 7a, where La₂O₃, FeO, and Fe form above 750 °C.

Although separate evaluation of the two steps yields respective optimal operating parameters, a key trade-off emerges when coupling the two reactors. While the presence of char can thermodynamically promote CO₂ conversion via the Boudouard reaction, it reduces gasification efficiency, lowers syngas yield and quality, may inhibit OC activity, and introduces operational complexities [40,41]. Although adding steam during gasification promotes in situ gasification of char to produce syngas [27,42], the consumption of RC fundamentally undermines the driving force for CO₂ splitting, making it incompatible with the production of high-purity CO. This thermodynamic analysis suggests that future experimental efforts should prioritize carbon-tolerant OC structures to balance conversion efficiency with material stability. Moreover, in practical applications, enhancing the catalytic performance of OCs through structural modifications, such as doping [19,43], defect engineering [44,45], and surface/interface tailoring [18,46], represents an effective pathway to kinetically improve the efficiency of CO₂ splitting. Sun et al. [47] confirmed C2F2 as the most promising basis material for CO₂ splitting. With 50 mol% Ce-doping, the maximum CO₂ conversion could be increased by 426%, whereas the CO yield (546 μmol·min⁻¹·g⁻¹) increased by 53.6 times compared with Fe·CaO without Ce-doping at 700 °C. Their results further validate the reliability of our thermodynamic predictions.

5. Conclusions

This study conducted a comprehensive thermodynamic analysis of biomass chemical looping gasification integrated with CO₂ splitting (BCLGCS) using Gibbs free energy minimization. LaFeO₃ (LF), BaFeO₃ (BF), CaFe₂O₄ (CF2) and Ca₂Fe₂O₅ (C2F2) were evaluated as OCs with respect to syngas production, carbon conversion, and CO₂ splitting efficiency under various operating conditions. Steam addition enhances gasification by promoting the WGS reaction and lowering the minimum temperature from 900 °C to 750–800 °C, yet simultaneously consumes residual char through gasification reactions, thereby eliminating the Boudouard driving force essential for CO₂ splitting. This inherent thermodynamic conflict indicates that the optima for syngas generation and CO₂ utilization are mutually exclusive. Among the four OCs evaluated, C2F2 emerges as the most promising candidate due to in situ CaO generation, which catalyzes WGS and enables tunable H₂/CO ratios. Conversely, LF requires impractically high loading attributable to its stable perovskite framework and sluggish oxygen release, while BF undergoes severe deactivation via thermodynamically favored BaCO₃ formation. CO₂ splitting is primarily driven by RC rather than OC re-oxidation, with direct OC contribution remaining minimal. These insights establish theoretical boundaries to guide carbon-tolerant OC design and process integration toward practical carbon-negative applications.