Reverse Chemical Looping Hydrogen Production from Pine Biomass with Ca–Fe–Ni Composite Oxygen Carriers

Abstract

Reverse chemical looping pyrolysis (RCLPy) utilizes a reduced oxygen carrier to extract oxygen from the biomass feedstock during the pyrolysis stage and transfer it for the subsequent gasification stage. This decoupled mechanism enables efficient in situ utilization of oxygen and hydrogen inherent in the biomass to produce a hydrogen-rich syngas. In this work, Ca–Fe–Ni composite oxygen carriers for RCLPy were synthesized and their impact on the hydrogen production was investigated and optimized. The results demonstrate that the reduced Ca–Fe–Ni oxygen carrier exhibited both excellent deoxygenation and catalytic cracking capability, significantly promoting the generation of hydrogen and CO. Specifically, the reduced CaFeNi15 oxygen carrier decreases the CO₂ content in the pyrolysis gas from 40.4 vol.% without an oxygen carrier to 6.89 vol.% and with a hydrogen yield of 280.2 mL·g⁻¹ biomass and has a total hydrogen production of 318 mL·g⁻¹ biomass during the whole pyrolysis–gasification process. These findings underscore the advantages of the RCLPy process in utilizing inherent biomass hydrogen for high-purity syngas production. Future efforts should focus on developing oxygen carriers with enhanced long-term cyclic stability.

1. Introduction

In recent years, the excessive consumption of fossil fuels has led to global energy shortages and a year-by-year increase in carbon dioxide emissions, which in turn has exacerbated climate change and associated environmental issues [1,2]. To reduce dependence on fossil fuels and address these challenges, it is imperative to develop clean and sustainable energy alternatives [3,4]. Hydrogen energy, characterized by its high energy density and zero-pollution nature, has attracted considerable attention as an excellent energy carrier. However, conventional hydrogen production methods, such as water electrolysis and coal conversion, suffer from high energy consumption and significant carbon emissions. Therefore, identifying suitable feedstocks for hydrogen production is of paramount importance. Biomass, as the only renewable organic carbon source on the earth, offers abundant resources and wide availability [5,6,7]. China, in particular, possesses substantial biomass resources, making biomass the fourth largest energy source after coal, oil, and natural gas. Among various biomass feedstocks, forest residues are especially promising, and converting them into hydrogen energy presents significant economic and environmental benefits [8,9].

Thermochemical conversion is the most commonly employed route for hydrogen production from biomass, with biomass gasification and bio-oil reforming being the primary methods. These processes utilize gasifying agents to supply oxygen and heat, thereby converting biomass into hydrogen [10,11,12,13,14]. However, the inherent properties of biomass, including high volatility, high moisture content, and high oxygen content, often lead to elevated levels of tar and carbon dioxide in the pyrolysis–gasification products, which compromise product purity and hydrogen yield. Consequently, processes such as catalytic deoxygenation and post-treatment are typically required to improve product quality and enhance hydrogen enrichment and utilization efficiency [15,16,17]. Although these approaches offer advantages such as high process maturity and stable material and process systems, they face challenges including tar formation, fluctuations in product purity, catalyst contamination, and high maintenance costs [18,19,20].

Chemical looping conversion technology has emerged as an efficient and environmentally friendly energy conversion method [21,22,23]. Its fundamental principle involves the use of oxygen carriers to perform redox cycles, avoiding direct contact between biomass and external oxygen sources. Biomass chemical looping hydrogen production includes biomass chemical looping gasification (CLG) and bio-oil reforming for hydrogen production (CLR) [24,25,26,27]. As for the CLG process, the pyrolysis, reforming, and gasification reaction are all concentrated within a single gasification reactor, making it difficult to control individual stages. As for the CLR process, the quality of bio-oil is a critical factor affecting hydrogen production. Bio-oil derived from conventional pyrolysis typically has a high oxygen content and poor quality, resulting in low hydrogen yield during reforming. Moreover, both CLG and CLR suffer from low biomass utilization efficiency.

To address these limitations, our research group has introduced the concept of chemical looping staged conversion (CLSC), which decouples the gasification process into two distinct stages conducted at different temperatures, i.e., pyrolysis stage and gasification stage [28,29,30]. This decoupling enables precise control of pyrolysis conditions, optimization of product quality, and ultimately, the production of higher-quality bio-oil and syngas. By utilizing an oxidized oxygen carrier for biomass pyrolysis, namely chemical looping pyrolysis (CLPy), the pyrolysis efficiency of solid biomass is enhanced significantly. However, oxygen contained inherently in biomass and an oxygen carrier leads to excessive oxygen incorporation into the gas and liquid phases. The obtained bio-oil still requires further upgrading [31,32,33]. Furthermore, by altering the direction of the oxygen transfer, reverse chemical looping pyrolysis (RCLPy) was proposed, which utilizes a reduced oxygen carrier (Re–OC) to extract oxygen from the biomass feedstock during the pyrolysis stage and transfer it for the subsequent gasification stage [34]. Re–OC can not only decrease the oxygen content of bio-oils in situ via its oxidation reaction but also provide H₂ for the in situ upgrading of bio-oils through hydrogen generation reactions between Re–OC and small-molecule oxygen-containing compounds. Furthermore, through the integration of oxygen carrying and catalytic cracking, this decoupled mechanism facilitates the efficient in situ utilization of oxygen and hydrogen inherent in the biomass for the production of a hydrogen-rich syngas. The schematic diagram of RCLPy for hydrogen production is presented in Figure 1. First, under pyrolysis conditions, biomass is directly decomposed into H₂-rich syngas via the deoxygenation of the reduced oxygen carrier. Next, the oxidized oxygen carrier is reduced by the mixed char to achieve the regeneration of the reduced oxygen carrier at a higher temperature, accompanied by the production of clean syngas free of tar. This hydrogen production method still needs to be further verified.

In this study, we investigate the performance of RCLPy for hydrogen production using pine biomass as the feedstock. The deoxygenation capability, redox cycling performance, and catalytic hydrogen production activity of a Ca–Fe–Ni composite oxygen carrier are systematically evaluated through multiple pyrolysis–gasification cycles. The findings of this research are expected to open new avenues for efficient hydrogen production by enhancing the utilization of hydrogen inherently present in biomass.

2. Results and Discussion

2.1. Characterization of the Re–CaFeNix Oxygen Carrier

Figure 2 presents the XRD patterns of Re–CaFeNix oxygen carriers with different NiO doping amounts to explore the structural evolution induced by nickel incorporation. As a reference, the XRD pattern of the undoped reduced Ca–Fe oxygen carrier reveals three dominant crystalline phases: metallic iron (Fe, PDF# 04-003-7116), calcium oxide (CaO, PDF# 04-004-8985), and a portion of the unreduced Ca₂Fe₂O₅ (PDF# 01-075-7773). This observation aligns well with a previous report on Ca–Fe-based oxygen carriers, primarily because of its relatively high thermodynamic stability [35]. Both CaO and Fe can remove oxygen and generate hydrogen through their oxidation reactions, returning to their oxidation state Ca₂Fe₂O₅ [29]. Additionally, they can participate in catalytic reactions to crack the biomass and its products [36]. Upon the introduction of NiO, a new series of diffraction peaks emerges at approximately 43.7°, 51.0°, and 74.9°, which are well-matched with the characteristic reflections of the Ni–Fe alloy (PDF# 04-009-3507), confirming the successful formation of a bimetallic phase during the high-temperature reduction step. Importantly, no diffraction signals related to Ca₂Fe₂O₅ are detectable in any of the Ni-doped oxygen carriers, indicating that NiO doping improves the reducibility of the Ca–Fe composites. Moreover, the addition of NiO leads to a reduction in the intensity of the CaO and Fe peaks. When the doping amount exceeds 15%, the intensities of both the CaO and Fe peaks decrease significantly. This is likely due to the interaction between Fe and Ni and the subsequent coverage of the CaO and Fe peaks by them.

2.2. Effect of the NiO Doping Amount on Hydrogen Production

Due to the favorable catalytic activity of Ni in the cracking of biomass, as well as its excellent performance in tar reforming, Ni was selected as a dopant for the Ca₂Fe₂O₅ oxygen carrier at five different doping levels, namely 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, and 25 wt.% to systematically evaluate the effect of Ni content on the pyrolysis–gasification performance. The effect of NiO doping amount on the distribution of gas, liquid, and solid products is presented in Figure 3. For comparison, the reduced Ca₂Fe₂O₅ without NiO doping exhibited high solid and liquid yields but a low gaseous yield, indicating its relatively weak cracking capability. This suggests that the unmodified Ca₂Fe₂O₅ oxygen carrier is less effective in promoting the conversion of biomass pyrolysis intermediates into gaseous products. Upon NiO doping, a significant increase in gaseous yield was observed, while the liquid yield initially decreased and then increased with increasing NiO content. This trend indicates that an appropriate amount of NiO doping enhances the cracking of bio-oil into light gases, whereas excessive NiO may lead to adverse effects such as particle agglomeration or surface coverage. Among the five doping levels, the Re–CaFeNi15 oxygen carrier demonstrated the best performance, achieving the highest gaseous yield of 55.0 wt.% and the lowest liquid yield of 26.3 wt.%. Meanwhile, a solid yield of 18.6 wt.% was also observed, which is primarily attributed to the transfer of oxygen from the biomass to the reduced oxygen carrier via oxidation reactions. This oxygen transfer reduces the oxygen content in the pyrolyzed products, which further contributes to the generation of hydrogen.

The effect of the NiO doping amount on the relative contents of gaseous products is depicted in Figure 4a. For the unmodified Ca₂Fe₂O₅ oxygen carrier, a high content of both CO and H₂ was detected, which can attributed to the outstanding selective oxidation of the reduced Ca₂Fe₂O₅, a state mainly composed of CaO and Fe. Upon the addition of 5 wt.% NiO, the relative content of CO increased significantly because of the catalytic cracking of pyrolyzed volatiles over Ni-active sites. Conversely, the H₂ content showed a relative decline. As the NiO doping amount further increased, the CO content showed a slight decreasing trend, while the H₂ content remained relatively stable at approximately 36 vol.%. The hydrogen yield and hydrogen production efficiency as functions of NiO-doping amount are shown in Figure 4b. Both parameters exhibited a clear trend of first increasing and then decreasing as the NiO doping amount increased from 5 wt.% to 25 wt.%. At the optimal doping level of 15 wt.%, a hydrogen yield of 237.6 mL·g⁻¹ biomass and a hydrogen production efficiency of 50% were achieved. This indicates that more than half of the hydrogen element inherently present in the biomass was released in the form of molecular hydrogen rather than as steam or other hydrogen-containing compounds. This phenomenon underscores a key advantage of the reverse chemical looping process over conventional thermochemical conversion methods, namely the enhanced in-situ utilization of hydrogen intrinsic to the biomass.

2.3. Effect of Re–OC/B Mass Ratio on the Hydrogen Production

Based on the results discussed above, the reduced oxygen carrier doped with 15 wt.% NiO was selected for further investigation. The effect of the reduced oxygen carrier-to-biomass (Re–OC/B) mass ratio on the distribution of gas, liquid, and solid products is illustrated in Figure 5. As the Re–OC/B mass ratio increases, the liquid yield decreases substantially, while the solid yield exhibits a slight increasing trend. The gaseous yield initially increases and then decreases, reaching a maximum of 55.0 wt.% at an Re–OC/B mass ratio of 2:1. In the absence of an oxygen carrier, the liquid yield is as high as 47.5 wt.%, which is consistent with reported values for direct biomass pyrolysis [28]. With the introduction of the oxygen carrier, volatiles undergo an enhanced decomposition into small molecules, leading to a marked reduction in both liquid and solid yields, accompanied by a corresponding increase in gas yield. A higher Re-OC/B mass ratio provides more reduction sites and catalytic sites, thereby facilitating oxygen transfer from volatiles to the reduced oxygen carrier and promoting the cracking of intermediates into small gaseous molecules. At an Re–OC/B mass ratio of 1:1, the available catalytic site appears insufficient to crack the liquid products into gaseous components. When the Re-OC/B mass ratio exceeds 2:1, the liquid yield stabilizes, whereas the gaseous yield begins to decline, indicating sustained deoxygenation capacity. Considering both deoxygenation and catalytic performance, the optimal Re-OC/B mass ratio is determined to be 2:1.

The effect of the Re-OC/B mass ratio on the relative contents of gaseous products is shown in Figure 6a. In the absence of the reduced oxygen carrier, the relative content of CO₂ is as high as 40.4 vol.%, while that of H₂ is as low as 12.7 vol.%. During conventional pyrolysis, most of the oxygen contained in biomass is released in the form of CO₂ and H₂O, which significantly limits the utilization efficiency of both the oxygen and hydrogen elements. With the introduction of the reduced oxygen carrier, the relative contents of CO₂ decreased sharply to 6.89 vol.%, while that of H₂ increased dramatically, reaching 45.5 vol.% at Re–OC/B mass ratio of 4:1. Under this condition, the total contents of syngas (CO + CH₄ + H₂) reached as high as 91.0 vol.%. Moreover, the increase in CO content was likely due to the partial deoxygenation of CO₂, while the decrease in CH₄ content can be attributed to its catalytic reforming over the active sites of the reduced oxygen carrier. The hydrogen yield and hydrogen production efficiency as functions of the Re–OC/B mass ratio are shown in Figure 6b. Both parameters exhibited a clear trend of first increasing and then decreasing as the Re–OC/B mass ratio increased from 0 to 4:1. The hydrogen yield and the hydrogen production efficiency increased significantly from 35.8 mL·g⁻¹ biomass and 7.6% without an oxygen carrier to 280.2 mL·g⁻¹ biomass and 59.7% with an Re–OC/B mass ratio of 4:1, respectively. The above results demonstrate that under optimized conditions, the majority of the hydrogen and carbon elements inherent in the biomass can be efficiently released in the form of H₂ and CO, respectively, highlighting the effectiveness of the reverse chemical looping process in promoting the production of high-quality syngas.

2.4. Gasification Performance of Pyrolysis Char

Further elevation of the reaction temperature not only promotes deep cracking of biomass but also initiates the chemical looping gasification (CLG) reaction. At a typical CLG temperature of 850 °C, the oxidized reduced oxygen carrier (Re–OC) could release oxygen via reactions with biomass. To regenerate the reduced oxygen carrier, the gasification stage was conducted between the oxygen carrier and bio-char, both with and without steam. The gaseous composition of the gasification products under different steam-to-biomass (S/B) mass ratios is presented in Figure 7a. In the absence of steam, CO accounted for 54.3 vol.% of the gasification gas, primarily originating from the further decomposition of char and the reaction between the oxygen carrier and char. Upon the introduction of steam as a gasifying agent, the relative content of H₂ in the gasification gas increased substantially, rising from 62.1 vol.% at an S/B mass ratio of 1:3 to 70.8 vol.% at an S/B mass ratio of 2:3. This enhancement can be attributed to two concurrent mechanisms: the char–steam gasification reaction and the reaction between unreacted oxygen carriers and steam. Notably, steam simultaneously provides an external source of both hydrogen and oxygen, facilitating hydrogen generation and the gasification of carbon derived from char. The yields of CO and H₂, along with the total carbon balance of gaseous products under different S/B mass ratios, are presented in Figure 7b. At an S/B mass ratio of 2:3, the yields of H₂ and CO during the gasification stage reached 721 mL·g⁻¹ biomass and 217 mL·g⁻¹ biomass, respectively, which are 18 times and 2.3 times higher than those achieved without a gasifying agent. The removal of hydrogen and oxygen elements during the pyrolysis stage leaves a greater proportion of carbon available for gasification, thereby generating more syngas.

It is particularly noteworthy that, because the oxygen carrier has already fixed a substantial amount of oxygen from the biomass via deoxygenation reactions during the pyrolysis stage, the external oxygen demand in the gasification stage is significantly reduced. Consequently, only a relatively low S/B mass ratio is required to achieve complete gasification of the pyrolysis char, thereby improving the overall material and energy utilization efficiency of the entire process. Based on the carbon balance, an S/B mass ratio of 1:2 is sufficient to meet the gasification requirement. The further increase in H₂ production observed at an S/B mass ratio of 2:3 is attributed to the reaction between the metal species in the oxygen carrier and steam. In the subsequent research, an S/B mass ratio of 1:3 was employed.

2.5. Comparison of Different Chemical Looping Processes

To elucidate the advantages of the RCLPy process, two typical chemical looping processes for hydrogen generation, namely CLG and CLPy, were investigated for comparison. The CLG process, employing the oxidized CaFeNi15 oxygen carrier, was carried out at a gasification temperature of 850 °C without the use of a gasifying agent. In contrast, the CLPy process utilized the Ox–CaFeNi15 oxygen carrier at the same pyrolysis temperature as that of RCLPy. The performance of these three processes was evaluated in terms of both gaseous product distribution and hydrogen production performance. As shown in Figure 8a, both CLG and CLPy, which employ an oxidized oxygen carrier for biomass gasification and pyrolysis, respectively, produced higher amounts of CO₂ and lower amounts of H₂ compared to RCLPy. This observation can be attributed to the combined effects of oxidation and catalytic cracking induced by the oxygen carrier. In contrast, the RCLPy process exhibited a markedly different product distribution. Specifically, the relative content of CO₂ decreased sharply to 8.0 vol.% in RCLPy, down from 46.7 vol.% in CLG and 35.8 vol.% in CLPy. Meanwhile, the H₂ content increased remarkably to 40.5 vol.%, compared to 16.6 vol.% for CLG and 19.6 vol.% for CLPy.

Figure 8b further highlights the superior hydrogen production performance of RCLPy. The hydrogen yield achieved by RCLPy reached 318 mL·g⁻¹ biomass, substantially higher than the 158.4 mL·g⁻¹ biomass obtained from CLG and the 194.4 mL·g⁻¹ biomass from CLPy. Correspondingly, the hydrogen production efficiency of RCLPy reached 67.6%, far exceeding the efficiencies of CLG (33.7%) and CLPy (41.3%). These results clearly demonstrate that the RCLPy process significantly outperforms conventional chemical looping approaches in terms of both hydrogen yield and efficiency, owing to its unique deoxygenation mechanism that promotes the in situ utilization of hydrogen inherent in the biomass.

2.6. Pyrolysis–Gasification Cyclic Reaction Performance

To evaluate the catalytic activity and structural stability of the reduced CaFeNi15 oxygen carrier over multiple reaction cycles, ten consecutive pyrolysis–gasification cyclic experiments were conducted. The hydrogen production performance as a function of cycle number is presented in Figure 9. As the number of cycles increased, the yields of both CO₂ and CO exhibited a gradual upward trend. In contrast, the H₂ yield first increased and then decreased, reaching its highest value at the second cycle. This observation suggests that the deoxygenation capability of the oxygen carrier gradually weakened with repeated cycling, as evidenced by the simultaneous decrease in H₂ yield and increase in CO₂ yield. Meanwhile, the progressive increase in CO yield may indicate an enhancement in the catalytic performance of the Re–CaFeNi15 oxygen carrier, potentially due to the activation of certain surface sites or structural rearrangements induced by repeated redox cycling. Overall, these results demonstrate that while the CaFeNi15 oxygen carrier maintains reasonable catalytic activity over ten cycles, its deoxygenation performance shows signs of degradation, highlighting the need for further optimization to improve long-term cyclic stability.

The XRD patterns of the Re–CaFeNi15 oxygen carrier after the first pyrolysis, first gasification, fifth pyrolysis, fifth gasification, tenth pyrolysis, and tenth gasification cycles are presented in Figure 10. Compared with the fresh reduced oxygen carrier, the XRD patterns reveal that the intensity of the Fe phase was significantly weakened, while that of the Fe–Ni alloy phase increased markedly. The Fe–Ni alloy is known to facilitate both the decomposition of biomass and the catalytic cracking of bio-oil into light gases, thereby contributing to enhanced gas yield and hydrogen production. After the first, fifth, and tenth pyrolysis cycles, the Ca₂Fe₂O₅ phase was detected, and its diffraction peak intensity increased progressively with the number of cycles. The formation of the Ca₂Fe₂O₅ phase during the pyrolysis stage indicates the transfer of oxygen from the biomass to the reduced oxygen carrier, reflecting the deoxygenation function of the oxygen carrier. In contrast, after the first, fifth, and tenth gasification cycles, the Ca₂Fe₂O₅ phase disappeared, which can be attributed to the release of lattice oxygen for the oxidation of char during the gasification stage. The redox transformation of Ca₂Fe₂O₅ throughout the pyrolysis and gasification stages demonstrates that the reduced oxygen carrier possesses excellent redox cycling performance. These results further confirm the feasibility of the RCLPy process for sustainable hydrogen production from biomass.

The surface morphologies of the reduced CaFeNi15 oxygen carrier after the first pyrolysis, first gasification, fifth pyrolysis, fifth gasification, tenth pyrolysis, and tenth gasification cycles are presented in Figure 11. The fresh oxygen carrier in its oxidized state exhibited a compact structure composed of densely packed particles. After reduction with H₂, a uniform dispersion of Fe and Fe–Ni alloy particles appeared on the surface, accompanied by an increase in porosity. Following the first pyrolysis cycle, the oxygen carrier surface became compact, which can be attributed to partial oxidation of the oxygen carrier and surface coverage by residual char. After the first gasification cycle, the surface morphology transformed into a porous structure, with Fe and Fe–Ni alloy particles uniformly distributed across the surface. This porous morphology was largely maintained after the fifth pyrolysis and fifth gasification cycles, indicating reasonable structural stability during the initial cycles. However, after the tenth pyrolysis and tenth gasification cycles, significant morphological changes were observed. The porous structure and uniform particle distribution disappeared, likely due to both the accumulation of unconverted carbon on the surface and the agglomeration of metal particles. These morphological deteriorations are consistent with the observed decline in hydrogen yield and deoxygenation performance after multiple cycles, as discussed previously. The degradation of the oxygen carrier structure underscores the need for further optimization of oxygen carrier design to enhance long-term cyclic stability.

3. Materials and Methods

3.1. Material Preparation and Characterization

Ca–Fe–Ni composite oxygen carriers were prepared via a mechanical grinding and calcination method. The precursors, including CaCO₃, Fe₂O₃, and NiO, were mixed at the desired molar ratio and ground for 60 min. The obtained mixture was then calcined in a muffle furnace at 300 °C for 2 h, followed by further calcination at 900 °C for 12 h. After cooling to room temperature, the samples were crushed, sieved, and labeled. The molar ratio of Ca and Fe was set as 1:1 to obtain Ca₂Fe₂O₅, owing to its excellent redox performance [35]. NiO was added at mass ratios of 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.% and 25 wt.% relative to the prepared oxygen carrier, and the resulting samples were designated as Ox–CaFeNix (x = 5, 10, 15, 20 and 25). Prior to the experiments, the as-prepared composite oxygen carrier was pre-reduced under a H₂ atmosphere of 100 mL/min at 850 °C for half an hour to obtain their reduced forms, which were correspondingly denoted as Re–CaFeNix (x = 5, 10, 15, 20 and 25). Quartz sand was used instead for comparison experiments.

The proximate, ultimate and component analyses of pine wood biomass are summarized in

Table 1. Proximate, ultimate and component analysis of pine wood biomass.

Item	Composition	Content
proximate analysis, wt.%	Moisture	1.21
	Volatile	77.71
	Ash	1.68
	Fixed carbon	19.39
ultimate analysis, wt.%	Carbon	44.78
	Hydrogen	4.2
	Nitrogen	3.45
	Oxygen *	47.57
component analysis, wt.%	cellulose	34.63
	hemicellulose	9.68
	lignin	24.42

* calculated by difference.

. The elemental composition of the biomass was determined using an elemental analyzer (Elementar UNICUBE, Langenselbold, Germany), and the oxygen content was calculated by difference. The proximate analysis was performed using a thermogravimetric analyzer (NETZSCH STA409PC, Selb, Germany). Component analysis was conducted in accordance with the Chinese energy industry standard NB/T 34057.5-2017 [37].

The phase composition of the fresh, reduced, and spent composite oxygen carriers was analyzed by an X-ray diffractometer (XRD, Rigaku SmartLab SE, Akishima, Japan) with Cu Kα radiation. The diffraction angle (2θ) ranged from 10° to 80°. All XRD patterns were analyzed using Jade 9 software. The surface morphology of the composite oxygen carriers was characterized using a scanning electron microscope (SEM, Zeiss Sigma 500, Jena, Germany).

3.2. Experimental Setup and Evaluation

Pyrolysis–gasification experiments were conducted in a fixed-bed reactor. The detailed procedure was as follows. (1) In the pyrolysis stage, the quartz reactor was firstly heated to the specified temperature (600, 650, 700, 750, 800 °C) under an argon flow of 100 mL/min. Once the temperature became stable, the mixed sample consisting of 3 g biomass and 3–12 g oxygen carrier, which was placed at the upper end of the reactor, was swiftly pushed into the heating zone. Liquid products were collected using an absorption bottle filling with isopropanol. The pyrolysis gases were measured with a wet gas meter and subsequently collected in a gas bag for gas chromatography (GC) analysis. The entire pyrolysis process endured for 30 min. The yield of the liquid phase was determined by weighing, the yield of the gas phase was calculated based on the gas volume and composition, and the yield of the solid phase was obtained by the difference method. (2) Next was the gasification stage, where after pyrolysis, the mixed sample was pulled from the heating zone, and the reactor was further heated to 850 °C. When the temperature reached stability, the pyrolyzed sample was reintroduced into the heating zone. The volume of the gas was measured using a wet gas meter and then collected in a gas bag for GC analysis. The gasification experiment lasted for 40 min. After the completion of the experiment, the reaction products were removed from the heating zone, the furnace heating was stopped, and the sample was cooled to room temperature under an argon atmosphere. (3) For the cyclic experiment tests, another 3 g of fresh pine biomass was added to the post-reaction solid mixture, and the pyrolysis–gasification procedure described above was repeated.

The relative contents of each component (X_i) in the pyrolysis gas and gasifying gas were determined by Equation (1), and the yield of hydrogen (Y_H2) and hydrogen production efficiency (η_H2) was calculated by Equations (2) and (3), respectively.

$$X_{\mathrm{i}}={\displaystyle \frac{C_i}{{\displaystyle \sum C_i}}}{ , \mathrm{i}=\mathrm{CO}}_2{, \mathrm{CO}, \mathrm{CH}}_4{, \mathrm{H}}_2{, \mathrm{N}}_2{, \mathrm{C}}_{\mathrm{n}}$$

(1)

$$Y_{{\mathrm{H}}_2}={\displaystyle \frac{V\times X_{{\mathrm{H}}_2}\times M_{{\mathrm{H}}_2}}{22.4\times m_{biomass}}}$$

(2)

$${\mathrm{\eta }}_{{\mathrm{H}}_2}={\displaystyle \frac{Y_{{\mathrm{H}}_2}}{Y_{H_2,biomass}}}$$

(3)

4. Conclusions

In this study, Ca–Fe–Ni composite oxygen carriers were successfully synthesized and applied in the reverse chemical looping pyrolysis (RCLPy) of pine biomass. The main conclusions are summarized as follows: (1) the reduced Ca–Fe–Ni oxygen carrier exhibited strong deoxygenation performance, reducing the CO₂ content in pyrolysis gas from 40.4 vol.% to 6.89 vol.%, with the total contents of syngas (CO + CH₄ + H₂) reaching as high as 91.0 vol.%. (2) Ni doping substantially enhanced catalytic cracking via the formation of a Ni–Fe alloy, with the Re–CaFeNi15 showing the best performance of 55.0 wt.% gaseous yield and hydrogen production efficiency up to 67.6%. (3) Compared to conventional chemical looping gasification (CLG) and chemical looping pyrolysis (CLPy) using an oxidized oxygen carrier for biomass conversion, the RCLPy process produces nearly twice the hydrogen yield and hydrogen production efficiency owing to its unique deoxygenation mechanism that promotes in situ utilization of inherent biomass hydrogen. Overall, the RCLPy process using reduced Ca–Fe–Ni composite oxygen carriers offers a promising route for efficient hydrogen production with high syngas purity and low CO₂ emissions. Future efforts should focus on developing oxygen carriers with enhanced cyclic stability.