Electronic–Oxygen Synergy at Ca-Fe Dual-Metal Interfaces for Selective Syngas Regulation in Biomass Chemical Looping Gasification

Abstract

This study reveals the efficient catalytic role of Ca-Fe-based oxygen carriers (Ca₂Fe₂O₅) in biomass chemical looping gasification. With oxygen carrier introduction, the CO yield doubled (0.13 Nm³/kg→0.26 Nm³/kg), with 76.10% selectivity. Steam co-feeding further increased the H₂ yield from 0.19 Nm³/kg to 0.72 Nm³/kg, significantly elevating the H₂/CO ratio to 2.62. Combined with density functional theory (DFT), the micro-mechanism of reduced oxygen carrier surfaces activating CO₂/H₂O was elucidated. CO₂ (adsorption charge −0.952 |e|) and H₂O (adsorption charge −0.612 |e|) chemically adsorb at the CaO(111)/Fe(110) interface, where Fe atoms (charges 0.433 |e|, 0.927 |e|) act as electron donors to drive efficient molecule activation. CO₂ undergoes single-step splitting (CO₂→CO* + O*), with the desorption energy barrier (E_a = 1.09 eV, 105.17 kJ/mol) determining the reaction rate. H₂O splits via two-step cleavage (H₂O→HO* + H*→2H* + O*), which is rate-limited by the first step (E_a = 0.42 eV, 40.52 kJ/mol). Simultaneously, the reduced oxygen carrier achieves oxidative regeneration through surface O* lattice incorporation. This work atomically reveals the “electron transfer–oxygen transport” synergy at the Ca-Fe bimetallic interface, establishing a theoretical framework for the directional regulation of the syngas composition and the design of high-performance oxygen carriers.

1. Introduction

The urgent need for global energy structure transformation has driven the rapid development of renewable energy technologies represented by biomass energy. As a carbon-neutral renewable energy source that can be directly converted into high-grade fuels, the efficient utilization of biomass energy holds strategic significance in achieving “dual-carbon” goals [1,2,3]. According to International Energy Agency (IEA) statistics, the global annual utilizable biomass resources exceed 1.3 × 10¹¹ tons of standard coal equivalents, but their energy conversion efficiency remains below 10% [4]. High-efficiency biomass gasification technology has become one of the focal points in international energy research under carbon neutrality objectives [5,6]. However, traditional gasification processes face bottlenecks, such as the insufficient secondary cracking of tar, low product selectivity (uncontrollable H₂/CO ratio) and inadequate carbon conversion rates [7,8,9]. These limitations reduce the system efficiency and increase the processing costs, severely constraining large-scale applications.

In recent years, chemical looping gasification technology has achieved the directional conversion of biomass into syngas (H₂/CO) through redox cycles of oxygen carriers [10,11,12]. A typical chemical looping gasification system consists of a fuel and air reactor, where the former facilitates biomass gasification. At the same time, the latter enables the combustion of incompletely converted biomass and oxygen carrier regeneration [12]. In chemical looping gasification systems, oxygen carriers (typically metal oxides like Fe₂O₃) utilize lattice oxygen instead of O₂ as the oxygen source for fuel reactors [13,14], thereby eliminating the oxygen preparation costs [15]. The incomplete oxidation of biomass with oxygen carriers produces CO, enhancing the gas’ calorific value. Metal oxide oxygen carriers also exhibit catalytic effects on biomass tar cracking [16,17,18], addressing traditional gasification’s shortcomings.

Building upon chemical looping gasification, recent advancements include chemical looping gasification–water splitting (CLGWS) and chemical looping gasification–CO₂ splitting (CLGCS) technologies [19,20,21]. These systems integrate splitting processes to convert CO₂/H₂O into CO/H₂, as illustrated in Figure 1. By introducing H₂O and CO₂ between the fuel and air reactors, the reduced oxygen carriers undergo oxidation to generate high-purity H₂ and CO. Subsequently, the partially oxidized oxygen carrier is completely regenerated in air reactors. Both chemical looping gasification–water splitting and chemical looping gasification–CO₂ splitting processes rely critically on the oxygen carrier performance [22,23,24,25], which directly determines the gas product quality and yield [26].

Recent studies reveal that composite oxygen carriers [27] can overcome the limitations of single-metal oxygen carriers, such as their poor cycling capacity, instability and low reactivity [28]. Ca-based oxygen carriers have gained attention in chemical looping gasification research due to their cost-effectiveness, ability to reduce the CO₂ yield and tar catalytic cracking properties [28,29,30,31,32]. Chan et al. [33] demonstrated that CaO/Fe₂O₃ oxygen carriers can enhance syngas production with high H₂/CO ratios through the co-gasification of biomass and polyethylene.

Performance analyses of Ca-Fe composite oxygen carriers show that Ca₂Fe₂O₅ significantly improves the steam conversion efficiency and H₂ yield during gasification [34]. Its moderate oxidation capacity facilitates high-quality syngas production [35].

Density functional theory (DFT) calculations have proven effective in investigating the geometric and electronic structures of metal oxide interfaces, providing atomic-level insights into their properties [36,37,38,39,40]. Feng et al. [37] employed DFT to explore the CH₄ conversion pathways in Ca₂Fe₂O₅ oxygen carriers, offering theoretical support for enhanced CH₄ conversion efficiency. DFT revealed that [39] Ni doping lowers the oxygen vacancy formation energy and enhances CO₂ adsorption, promoting CO₂ activation and splitting. Thus, DFT has enabled novel chemical looping gasification systems for CO₂ utilization.

This study investigates the direct/steam chemical looping gasification of Ca₂Fe₂O₅ oxygen carriers with Chlorella vulgaris (biomass) through comparative experiments with blank (SiO₂) and steam groups, analyzing the oxygen carrier regulation mechanisms through carbon conversion, H₂/CO selectivity and the gas calorific value. Using DFT simulations, we construct a CaO(111)/Fe(110) composite interface model to elucidate the complete adsorption–activation–dissociation process of CO₂/H₂O molecules on reduced Ca₂Fe₂O₅ oxygen carriers (dual-metal sites), including the adsorption energy, transition state geometries and electronic density of state variations. The findings can guide the development of highly stable and selective oxygen carriers in chemical looping gasification–water splitting and chemical looping gasification–CO₂ splitting processes.

2. Results and Discussion

2.1. Chemical Looping Gasification Experimental Results

The experimental conditions for chemical looping gasification are listed in

Table 1. Design of chemical looping gasification experiments.

	Biomass	Oxygen Carrier	Steam (mL/min)	Temperature (°C)	Time (min)
A1	Chlorella vulgaris	SiO2	/	800	30
A2	Chlorella vulgaris	Ca2Fe2O5	/	800	30
A3	Chlorella vulgaris	Ca2Fe2O5	0.04	800	30

, labeled as A1, A2 and A3. The chemical looping gasification gas production results for each group are shown in Figure 2.

Compared to A1 (without the oxygen carrier), A2 exhibited significant increases in the carbon conversion rate and total gas yield, indicating that adding the oxygen carrier promoted biomass gasification. Although the lower heating value (LHV) of the gas of A2 decreased due to reduced concentrations of high-calorific components such as CH₄ and C₂H_m, the yields of CO₂, H₂ and CO all showed positive growth. Notably, the CO gas yield remarkably increased from 0.13 Nm³/kg to 0.26 Nm³/kg, doubling in value. The CO selectivity of A2 improved to 76.10% relative to A1, demonstrating that CO₂ was selectively converted to CO under the influence of the oxygen carriers. Previous studies have confirmed that reduced-state Ca₂Fe₂O₅ can decompose CO₂ into CO [21].

When steam was introduced to A2 (forming A3), the carbon conversion rate and total gas yield continued to rise. The H₂ gas yield increased substantially from 0.19 Nm³/kg to 0.72 Nm³/kg, and the H₂/CO ratio significantly improved from 0.60 to 2.62. This indicates that oxygen carriers can effectively enhance H₂ production. Research by [20,35,39] revealed that reduced Ca₂Fe₂O₅ undergoes steam oxidation regeneration to its initial state while efficiently releasing H₂.

To further investigate the microscopic interaction mechanisms between reduced Ca₂Fe₂O₅ and H₂O/CO₂, this study employed DFT to calculate and simulate energy exchange and reaction pathways. Before the calculations, the reduced Ca₂Fe₂O₅ was characterized to ensure the reliability of the model construction.

2.2. Characterization of Oxygen Carriers

The physicochemical properties of the Ca₂Fe₂O₅ oxygen carrier before and after reduction were analyzed by XRD, as shown in Figure 3a,b. The reduction performance of Ca₂Fe₂O₅ was investigated through H₂-TPR testing, with the results shown in Figure 3c.

As shown in Figure 3a, the primary component of the fresh oxygen carrier was Ca₂Fe₂O₅. Figure 3c reveals two characteristic reduction peaks for Ca₂Fe₂O₅ at 610 °C and 860 °C. Combined with the phase analysis of the reduced sample by XRD (Figure 3b), the high-intensity characteristic peaks at 37.3° and 44.8° correspond to the (111) crystal plane of CaO and the (110) crystal plane of Fe, respectively. It is inferred that these two reduction peaks correspond to the two-step reduction process: Ca₂Fe₂O₅→FeO + CaO and FeO→Fe. These characterization results indicate that Ca₂Fe₂O₅ preferentially exposes the CaO(111)/Fe(110) crystal planes during reduction. The CaO(111)/Fe(110) composite interface was selected as the simplified computational model for reduced Ca₂Fe₂O₅ to balance computational efficiency and model representativeness.

2.3. CO₂ Adsorption and Splitting

Based on literature reports and the gasification experiments described in Section 2.1, the reaction between CO₂ (as a gaseous oxidizer) and the reduced oxygen carrier is the key step for CO generation. This study employs DFT simulations to investigate the CaO(111)/Fe(110) composite interface system, aiming to reveal the dynamic activation mechanism of CO₂ molecules at the interface.

The adsorption configurations of CO₂ on the CaO(111)/Fe(110) composite interface are shown in Figure 4a–h. Configurations (a)~(c) represent three adsorption configurations of CO₂ at Fe-Fe bridge sites. Configuration (d) corresponds to CO₂ adsorption at the O top site between Ca and Fe. Configuration (e) shows CO₂ adsorption at the Ca top site. Configuration (f) demonstrates CO₂ adsorption at the Ca-Ca bridge site. Configurations (g) and (h) illustrate CO₂ adsorption at the O top sites between Ca and Ca. The adsorption energies for (a)~(h) are −0.74, −0.90, −1.00, −0.52, −0.28, −0.34, −0.39, and −1.11 eV (−71.40, −86.84, −96.49, −50.17, −27.02, −32.81, −37.63, and −107.10 kJ/mol), respectively. A more negative adsorption energy (larger absolute value) indicates lower system energy and greater structural stability. Thus, configuration (c) is the most stable for CO₂ adsorption on Fe sites, and configuration (h) is the most stable on CaO sites. Configuration (h) corresponds to CO₂ chemisorption at alkaline CaO sites, forming CaCO₃. However, as shown in Figure 5, at 800 °C, the standard Gibbs free energy △G > 0 of CaCO₃, making it thermodynamically unstable. In contrast, the reaction between Fe atoms on CaO(111)/Fe(110) and CO₂ exhibits a negative △G (△G < 0), indicating that the reaction can proceed spontaneously. Consequently, configuration (c) becomes the optimal stable state. In this configuration, the C-O1 and C-O2 bond lengths measure 1.2863 Å and 1.2990 Å, respectively, exceeding the original C-O bond length of CO₂ (1.1768 Å) by 0.1095 Å and 0.1222 Å. This elongation unequivocally confirms CO₂ activation at Fe catalytic sites.

Figure 6 presents the geometry and charge redistribution analysis for CO₂ adsorption on CaO(111)/Fe(110). Figure 6a represents the most stable configuration (c) from Figure 4, where CO₂ chemisorbs atop an Fe atom, forming an Fe-C bond (1.9218 Å) with an adsorption energy of −1.00 eV (−96.49 kJ/mol). In the deformation charge density map, blue regions denote electron gains (acceptors), and pink regions denote electron losses (donors). The analysis reveals charge transfer during adsorption: C, O1 and O2 act as electron acceptors, while Fe1 and Fe2 act as electron donors. This indicates electron transfer from reduced Ca₂Fe₂O₅ to adsorbed CO₂. A Bader charge analysis (

Table 2. Bader charge analysis.

Configuration		C	O1	O2	Fe1O	Fe2C
a	Before adsorption	1.626	−0.812	−0.814	−0.057	0.172
	After adsorption	0.918	−0.907	−0.963	−0.109	0.503
b	Before adsorption	1.236	−1.087	−0.814	−0.057	0.172
	After adsorption	0.421	−0.974	−0.868	−0.605	0.433

) shows that CO₂ in configuration (a) gains −0.952 |e|, acting as an electron acceptor. In Figure 6b, configuration (b) represents CO₂* (after electron gain) forming CO* via O migration. Configuration (b) has an adsorption energy of −2.24 eV (−216.13 kJ/mol), indicating higher stability. The deformation charge density analysis shows C, O2 and Fe1 as electron acceptors and O1 and Fe2 as electron donors. The total charges of CO₂* before and after adsorption are −0.963 |e| and −1.421 |e|, respectively, confirming CO* as an electron acceptor.

To elucidate the electronic interactions between CaO(111)/Fe(110) and CO₂, the partial density of states (PDOS) of C, O and Fe atoms before and after CO₂ adsorption on CaO(111)/Fe(110) was calculated. The PDOS reveals the electronic structure and chemical bonding characteristics by decomposing the density of states into individual atomic orbitals (e.g., s, p and d orbitals). As shown in Figure 7, the bonding strength between C and O atoms is reflected by the overlapping of the peaks and their positions relative to the Fermi level (0 eV). Comparing the PDOS results before and after adsorption (Figure 7a,b), new peaks emerge at −8.16 eV and −9.93 eV, indicating the formation of new chemical bonds between Fe atoms and the C atom of CO₂ after adsorption. Figure 7b,d demonstrate significant peak overlap between C and Fe atoms at −8.16 eV and −9.93 eV post-adsorption. Comparing Figure 7e,f, as well as Figure 7g,h, new peaks at the same positions as in Figure 7b appear in Figure 7f,h. This suggests that interactions between Fe atoms and the O atoms of CO₂ create lower-energy orbitals, weakening the covalent bond between C and O. In Figure 7f,h, the p orbitals of O atoms near the Fermi level shift to lower-energy states. Similarly, the d orbitals of Fe in Figure 7b and the p orbitals of C in Figure 7d shift to lower energy levels. These observations imply that interactions between CO₂ and Fe atoms in CaO(111)/Fe(110) reduce the system’s energy, leading to the formation of new molecular orbitals.

Starting from the most stable adsorption configuration, the reaction pathways of CO₂ are shown in Figure 8. The initial state (IS) corresponds to the adsorbed CO₂. CO₂ dissociates to form CO* and O*, with CO* desorbing from the surface to ultimately form gaseous CO as the final state (FS). CO₂ adsorption on Fe sites is exothermic. Two possible reaction pathways for CO₂ splitting were identified. Both pathways are exothermic (−0.89 eV (−85.87 kJ/mol) and −1.76 eV (−169.82 kJ/mol)), with activation energies of 1.09 eV (−105.17 kJ/mol) and 1.82 eV (−175.60 kJ/mol), respectively. Thermodynamically, both paths are feasible, but Pathway 1 (lower activation energy) is kinetically favored and identified as the optimal pathway.

The energy profile of the optimal pathway, including intermediates, transition states and final states, is shown in Figure 9. The dissociation reaction produces CO* and O* adsorbed on the surface. The O atom adsorbs on an Fe site, forming an Fe-O bond (1.9332 Å), close to the Fe-O bond length in Fe₂O₃ (1.9440 Å) [41]. The reaction energy is −0.62 eV (−59.82 kJ/mol), corresponding to Fe oxidation. CO desorbs into the gas phase, achieving CO reduction and the oxidative regeneration of reduced Ca₂Fe₂O₅.

2.4. H₂O Adsorption and Splitting

The adsorption configurations of H₂O at the CaO(111)/Fe(110) composite interface are shown in Figure 10a–d. Configuration (a) represents H₂O adsorbed at the Fe site, (b) at the Ca top site, (c) at the Ca hollow site and (d) at the Ca bridge site. The most negative adsorption energy corresponds to the most stable configuration.

When H₂O adsorbs at the Fe site (Figure 10a), the adsorption energy is −0.47 eV (−71.40 kJ/mol). The most substantial adsorption occurs at the Ca bridge site (Figure 10d), with an energy of −0.79 eV (−76.22 kJ/mol). Configurations (b) and (c) exhibit adsorption energies of −0.38 eV (−36.66 kJ/mol) and −0.55 eV (−53.07 kJ/mol), respectively. Thermodynamic simulations (Figure 5) indicate that CaO and H₂O cannot spontaneously form Ca(OH)₂ above 550 °C. However, the reaction between Fe atoms on CaO(111)/Fe(110) and H₂O has a △G < 0, indicating that the reaction can proceed spontaneously. Thus, this study focuses on Fe sites as the primary adsorption sites. Figure 10a shows the most stable H₂O adsorption configuration, where the H-O1 and H-O2 bond lengths (0.9792 Å and 0.9805 Å) are 0.0065 Å and 0.0078 Å longer than that of gaseous H₂O (0.9727 Å), indicating molecular activation. Previous studies reported Fe-site adsorption energies of −0.38 eV [42] and −0.39 eV [43], while the composite interface shows enhanced adsorption (−0.47 eV (−71.40 kJ/mol)), demonstrating its superiority for H₂O adsorption.

The optimized structures and deformation charge density of the H₂O adsorption configurations on the CaO(111)/Fe(110) surface were analyzed, as shown in Figure 11. Figure 11a represents the most stable configuration (a) from Figure 10, where H₂O undergoes chemisorption atop an Fe atom, forming an Fe-O bond (2.2187 Å) with an adsorption energy of −0.47 eV (−71.40 kJ/mol). The deformation charge density in Figure 11a reveals no significant electron gain/loss in the adsorbed H₂O molecule before and after adsorption. Specific Bader analysis results (

Table 3. Bader charge analysis.

Configuration		H1	H2	O	Fe1	Fe2
a	Before adsorption	0.581	0.551	−1.132	0.019	−0.011
	After adsorption	0.329	0.231	−1.172	0.198	0.391
b	Before adsorption	0.513	0.551	−1.132	0.018	−0.011
	After adsorption	0.431	−0.467	−0.871	0.927	0.198

) indicate electron transfer between H₂O and Fe. During adsorption, H1, H2 and O act as electron acceptors, while Fe1 and Fe2 are electron donors. The adsorbed H₂O carries a charge of −0.612 |e|. Configuration (b) in Figure 11 represents the formation of OH* through H atom migration after electron acquisition. The Bader analysis shows H1, H2, Fe1 and Fe2 as electron donors, with O as the acceptor. The total charges of Fe1 before and after adsorption are 0.019 |e| and 0.927 |e|, respectively, confirming Fe1 as an electron donor. This indicates that reduced-state Ca₂Fe₂O₅ acts as the electron donor during the cleavage reaction, while H* serves as the acceptor. The reduced Ca₂Fe₂O₅ is oxidized by H₂O at high temperatures.

The PDOS of Fe atoms and H atoms in OH* on CaO(111)/Fe(110) before and after H₂O molecule adsorption were calculated. By comparing the PDOS in Figure 12 before and after adsorption, electronic resonances between O and Fe atoms occurred at energies of −22.27 eV and −10.19 eV, indicating the formation of covalent bonds between Fe and O atoms. From the comparison of Figure 12c,d, the P orbitals of the O atoms shifted to lower energy levels, demonstrating that the adsorption of H₂O on the Fe atoms of CaO(111)/Fe(110) becomes more stable through the formation of new molecular orbitals.

After H₂O is adsorbed on the CaO(111)/Fe(110) surface, the H₂O undergoes a dissociation reaction. From the above analysis, H₂O can be chemically adsorbed stably on the composite surface as the initial state (IS) for the H₂O reduction process. Figure 13 shows the reaction pathway and energy profile for H₂O splitting on the composite surface and the oxidation of the reduced Ca₂Fe₂O₅ state.

At the transition state TS1, there are two reaction pathways. The activation energies for Pathway 1 and Pathway 2 are 0.42 eV (40.52 kJ/mol) and 2.18 eV (210.34 kJ/mol), respectively. Pathway 1 is exothermic, with heat release of −0.94 eV (−90.70 kJ/mol), while Pathway 2 is endothermic, with heat absorption of 0.02 eV (1.93 kJ/mol). Therefore, Pathway 1 is the optimal reaction path. The initial dissociation of H₂O forms HO* and H*.

At transition state TS2, there are a total of four reaction paths. The adsorbed HO* undergoes further dissociation to form H* and O*. When HO* is adsorbed on the surface, four different adsorption sites exist, resulting in four distinct reaction pathways: Paths 1, 3, 4, and 5. The heat release values are −0.91, −1.23, −1.10, and −0.65 eV (−87.80,−118.68,−106.13, and −62.72 kJ/mol), respectively, all being exothermic reactions. Therefore, all pathways are thermodynamically favorable. The activation energies are 0.38, 1.15, 0.90, and 0.58 eV (36.66, 110.96, 86.84, and 55.96 kJ/mol), respectively. Path 1 has the lowest activation energy, making it the most favorable pathway. The reaction is most likely to proceed through Path 1, which can thus be considered the optimal reaction pathway. The activation energies for transition states TS1 and TS2 are 0.42 eV (40.52 kJ/mol) and 0.38 eV (36.66 kJ/mol), respectively. Since TS1 has higher activation energy than TS2, TS1 determines the dissociation rate of H₂O on the composite surface. H₂O dissociates into two H* and one O* on the composite surface. The O* adsorbs on Fe sites, forming Fe-O bonds with surface Fe atoms. The two H* combine to form adsorbed H₂, which desorbs from the surface to form gaseous H₂. As shown in Figure 14, the overall reaction energy is −0.41 eV (−39.56 kJ/mol), indicating that the H₂O reduction to form H₂ is an exothermic reaction.

3. Materials and Methods

3.1. Material Preparation

The Chlorella vulgaris (biomass) was provided by Xi’an Shengqing Biotechnology Co., Ltd. (Xi’an, China). First, the biomass was dried at 105 °C for 24 h. It was then sieved into particles with a size of less than 100 μm and stored in a sealed desiccator. The ultimate and proximate analyses of the biomass are shown in

Table 4. Proximate and ultimate analyses of biomass.

Proximate Analysis (wt.%, ad)				Ultimate Analysis (wt.%, ad)					Low Heating Value (MJ/kg)
Moisture	Volatiles	FC	Ash	C	H	N	S	O a
4.40	76.63	14.55	6.42	48.66	6.96	9.26	0.65	34.47	19.23

^a Difference calculation.

.

The Ca₂Fe₂O₅ oxygen carrier was prepared via the sol–gel method. Ca(NO₃)₂·4H₂O (analytical reagent), Fe(NO₃)₃·9H₂O (analytical reagent) and citric acid were dissolved in deionized water at a molar ratio of 1:1:2. The solution was stirred until a viscous substance formed. The obtained viscous material was dried at 105 °C for 24 h and then calcined in a muffle furnace at 900 °C for 4 h. Finally, the solid was ground and sieved to below 100 μm for characterization. SiO₂ (analytical reagent) was used as a control in the experiments.

3.2. Material Characterization

The phase composition of the oxygen carrier was determined using an X-ray diffractometer (XRD). A Rigaku Ultima IV X-ray diffractometer (Tokyo, Japan) was employed, with the experimental conditions consisting of a Cu target, a scanning rate of 2°/min and a collection range of 10~90°.

To verify the temperature-programmed reduction (TPR) performance of the oxygen carrier, hydrogen temperature-programmed reduction (H₂-TPR) experiments were conducted under a hydrogen reduction atmosphere using an Micromeritics Auto Chem II 2920 instrument (Norcross, GA, USA). Temperature-programmed analysis is a dynamic process that detects changes in surface chemical properties under different atmospheres at specific heating rates under inert gas protection. Typically, 50 mg of oxygen carrier was pretreated by heating to 150 °C under an inert atmosphere and maintained for 30 min. After pretreatment, the temperature was programmed to cool to 30 °C. Once the instrument baseline stabilized, the oxygen carrier was heated to 900 °C at 10 °C/min under a mixed atmosphere of H₂ (10 vol.%)/N₂ (50 mL/min flow rate).

3.3. Chemical Looping Gasification Experiment

Figure 15 shows the fixed-bed setup for the gasification experiments. It mainly consisted of a gas supply system, reaction system, absorption–condensation–drying system, gas collection system and detection system. Argon was used as the carrier gas to maintain the required inert atmosphere during gasification. Steam was continuously fed into the reactor via a constant-flow pump.

Before each experiment, the biomass and oxygen carrier were uniformly mixed at a specific mass ratio and loaded into a basket suspended in the upper part of the tubular furnace. The system was then sealed. When the furnace reached 800 °C, the basket was pulled to drop into the constant-temperature zone for reaction. In the blank experiments, SiO₂ replaced the oxygen carrier. Argon was purged into the furnace at 100 mL/min. Deionized water was pumped into the tubular furnace using the constant-flow pump, where it rapidly vaporized and was carried by the argon into the reaction zone. The reaction duration was set to 30 min. The generated gases were collected in gas bags and analyzed using gas chromatography.

3.4. Gasification Performance Evaluation Indicators

As the content of C₂ and alkanes and olefins was relatively low, the main gases studied included the following. The definitions and calculation formulas for the chemical looping gasification evaluation indicators are shown below.

The gas yield G_v (Nm³/kg) is calculated via Equation (1):

$$G_V=V_g/m_B$$

(1)

where V_g represents the volume of the produced gas under standard conditions (273.15 K, 101,325 Pa), Nm³; m_B represents the mass of biomass used in the gasification process, kg.

The lower heating value of the gas (LHV) (kJ/Nm³) is calculated via Equation (2):

$$LHV=126\cdot V_{\mathrm{CO}}+108\cdot V_{{\mathrm{H}}_2}+359\cdot V_{{\mathrm{CH}}_4}+635\cdot V_{{\mathrm{C}}_2{\mathrm{H}}_m}$$

(2)

$V_{\mathrm{CO}}$, $V_{{\mathrm{H}}_2}$, $V_{{\mathrm{CH}}_4}$ and $V_{{\mathrm{C}}_2{\mathrm{H}}_m}$ represent the volume fractions (%) of CO, H₂, CH₄ and C₂H_m.

The carbon conversion efficiency η_c (%) is calculated via Equation (3):

$${\eta }_c={\displaystyle \frac{12\times \left(V_{CO}+V_{C{O}_2}+V_{C{H}_4}+2V_{C_2{H}_{\mathrm{m}}}\right)\times G_v}{22.4\times \left(T_1/T\right)\times C\%}}$$

(3)

where T₁ represents the temperature (K) during gas concentration measurement; T represents the standard temperature (K); C % represents the carbon content in the biomass.

The gasification efficiency η (%) is calculated via Equation (4):

$$\eta ={\displaystyle \frac{LHV\times G_v}{Q_B}}$$

(4)

where Q_B represents the lower heating value of the biomass, kJ/Nm³.

The selectivity of CO is given by Equation (5):

$$CO\hspace{1em}selectivity={\displaystyle \frac{G_{CO}}{G_{CO}+G_{C{O}_2}}}\times 100\%$$

(5)

3.5. Computational Details

All calculations were implemented in CASTEP of Materials Studio 6.0. The exchange-correlation energy was treated using the Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBE) and the projector augmented wave (PAW) method [36,37,38,39,40]. Dispersion corrections and spin polarization effects were considered. The plane-wave cutoff energy was set to 400 eV. The CaO(111)/Fe(110) surface was constructed to simulate the reduced Ca₂Fe₂O₅ (as shown in Figure 16). The k-point sampling of the Brillouin zone was set to 1 × 1 × 1. A vacuum layer of 15 Å was introduced. Similarly, the molecular structures of CO₂, CO, H₂O and H₂ were optimized.

The fully linear synchronous transit (LST)/quadratic synchronous transit (QST) [44] search protocol was employed to establish pathways between the reactants and products, locating transition states through interpolated reaction path synchronous transition methods. This approach performs LST calculations and iteratively applies conjugate gradient optimization followed by QST processing until the simulation steps terminate. Multiple path optimizations were executed until achieving transition state structures, avoiding unrealistic reaction paths caused by speculative intermediates.

Surface adsorption energy E_ads:

E_ads = E_total − E_surface − E_species

(6)

E_total represents the total energy of the adsorbed surface species; E_surface denotes the energy of the clean surface without adsorption; E_species is the energy of the substance in the gas phase.

Activation energy E_a and reaction energy E_r:

E_a = E_TS − E_IS

(7)

E_r = E_FS − E_IS

(8)

E_TS is the energy of the transition state; E_IS is the energy of the initial state; and E_FS is the energy of the final state.

This study did not incorporate zero-point energy (ZPE) corrections, following the general approach commonly used in the dependency analysis of similar periodic systems. The calculated adsorption energy values are consistent with those reported in reference [38].

4. Conclusions

This study reveals the multiscale mechanism of Ca₂Fe₂O₅ oxygen carriers in biomass direct/steam chemical looping gasification. The results demonstrate that introducing an oxygen carrier significantly enhances the gasification performance, with the CO yield doubling (increasing from 0.13 Nm³/kg to 0.26 Nm³/kg). The H₂ yield increases from 0.19 Nm³/kg to 0.72 Nm³/kg. This indicates that reduced Ca₂Fe₂O₅ can regenerate to its initial state through oxidation by CO₂/H₂O at high temperatures, accompanied by efficient CO/H₂ release. Combined with DFT calculations, the reaction mechanisms of CO₂/H₂O at the active interface of the reduced oxygen carrier are systematically elucidated. CO₂/H₂O are stabilized via chemisorption at the CaO(111)/Fe(110) composite interface. Adsorbed CO₂ (total charge −0.952 |e|) and H₂O (total charge −0.612 |e|) act as electron acceptors, while surface Fe atoms (charges 0.433 |e|, 0.927 |e|) serve as electron donors, driving efficient reactant activation. CO₂ undergoes single-step splitting (CO₂→CO* + O*) to form adsorbed CO, with its desorption energy barrier (E_a = 1.09 eV, 105.17 kJ/mol) determining the reaction rate. H₂O splits in two steps (H₂O→HO* + H*→2H* + O*), with the first step dictating the reaction rate (E_a = 0.42 eV, 40.52 kJ/mol). Overall, CO₂/H₂O enable oxidative regeneration through surface reactions on reduced Ca₂Fe₂O₅ while supplying heat to the system. This work establishes a theoretical framework for the precision design of high-performance oxygen carriers and opens up new pathways for the selective regulation of biomass gasification products.