Performance Analysis of a Calcium Looping Process Integrating Biomass Sorption-Enhanced Gasification with CaCO₃-Based Methane Reforming

Abstract

The growing demand for sustainable energy solutions has led to significant interest in biomass gasification and methane reforming. To address this demand, a novel calcium looping process (CaLP) is proposed, which integrates biomass sorption-enhanced gasification (BSEG) with in situ calcium CaCO₃-based methane reforming (CaMR). This process eliminates the need for CaCO₃ calcination and facilitates the in situ utilization of CO₂. The effects of gasification temperature, steam flowrate into the gasifier α_G(H2O/C), reforming temperature, and steam flowrate into the reformer α_R(H2O/C) were systematically evaluated. Increasing the gasification temperature from 600 °C to 700 °C enhances CO and H₂ yields from 0.653 to 11.699 kmol/h and from 43.999 to 48.536 kmol/h, respectively. However, CaO carbonation weakens, reducing CaO conversion from 79.15% to 48.38% and increasing CO₂ release. A higher α_G(H2O/C) promotes H₂ yield while suppressing CO and CH₄ formation. In the CaMR process, raising the temperature from 700 °C to 900 °C improves CH₄ conversion from 64.78% to 81.29%, with a significant increase in CO and H₂ production. Furthermore, introducing steam into the reformer enhances H₂ production and CH₄ conversion, which reaches up to 97.30% at α_R(H2O/C) = 0.5. These findings provide valuable insights for optimizing integrated biomass gasification and methane reforming systems.

1. Introduction

The escalating global demand for clean and sustainable energy has catalyzed extensive research into alternative energy sources and sophisticated energy-conversion technologies. The efficient utilization of carbon-neutral fuels and carbon capture have both become important trends in the field of energy. Biomass, recognized as a renewable and carbon-neutral resource, has come to the fore as a promising alternative fuel [1,2].

Various methods exist for biomass utilization, among which gasification stands out as a compelling thermochemical conversion technique. This technology holds significant potential for the production of syngas, heat, renewable power, hydrogen, and chemicals [3,4,5], as it offers the potential to overcome the fluctuations in quality and calorific value of feedstocks. However, traditional biomass gasification faces challenges such as low hydrogen yield, tar formation, and carbon dioxide emissions [6]. Additionally, the syngas is always diluted by nitrogen. To address these issues, the integration of biomass steam gasification with calcium looping processes (CaLP) has attracted attention due to its potential to enhance hydrogen production and enable efficient carbon dioxide capture, forming sorption-enhanced gasification (SEG) [7,8,9,10]. The hydrogen concentration in syngas from SEG is usually higher than 70% [7]. In SEG, the CO₂ capture capacity as well as the stability of the bed material have a huge influence on the performance of the SEG process. Previously, different CO₂ sorbents were employed to investigate the gasification performance, such as the CaO, MgO, and Li₄SiO₄. It was found that SEG with a CaO sorbent can produce syngas with the highest hydrogen concentration [11]. Consequently, CaO-based sorbents have been predominantly utilized as the bed material in the SEG process, primarily due to their superior CO₂ capture capacity and economic viability. This process is shown in Figure 1a. Both the feedstock and steam are fed into the gasifier while the CO₂ sorbent (typically CaO) circulates into it simultaneously. During the biomass steam gasification process, CaO reacts with CO₂ to shift the equilibrium of the reactions to improve the hydrogen yield. The carbonation product, CaCO₃, is then calcined in a calciner for CaO regeneration.

Different sorbents of limestones are employed in a lab-scale dual interconnected fluidized bed (DIFB) reactor for biomass SEG (BSEG) [12]. In an experimental demonstration using a 30 kW_th bubbling fluidized bed reactor, the SEG was tested with municipal solid waste as feedstock. The results reveal that the SEG exhibited a commendable performance in preventing tar formation, maintaining a content of as low as 7 g/Nm³ [13]. In the same pilot plant, the SEG of sewage sludge was compared with steam–oxygen gasification, showing a high H₂ content of 70–73% [14]. Adjusting the steam-to-carbon ratio and the utilization of a high-sorption-capacity CO₂ sorbent are proven to be effective strategies for modifying the H/C ratio in syngas [15]. Mbeugang et al. conducted an experimental investigation into the SEG performance of cellulose within a fixed-bed reactor. Their findings suggest that the role of CaO as either a CO₂ absorbent or a catalyst is intrinsically linked to the gasification temperature [16]. In a bubbling fluidized reactor, the BSEG of sawdust was experimentally studied by Han et al. [17]. The highest concentration of hydrogen reached 62% and the optimal conditions were found at CaO/C = 1, H₂O/C = 2.18, and T = 740 °C.

In addition to experimental investigation, simulation provides a useful tool for system evaluation and optimization [18,19]. Pitkäoja et al. proposed a strategy for H₂O staging to tailor the gas composition of syngas derived from BSEG. Their research indicated that employing H₂O staging and cooling in a fluidized bed gasifier significantly boosted the carbonation of limestone and water–gas shift reactions [20]. A process simulation model for the SEG of corn stalk was established, in which the steam gasification and CO₂ sorption were separated in two stages to enhance the H₂ yield [21]. Although this increased the hydrogen yield, the reaction system was more complex. A modified thermodynamic equilibrium model was established for BSEG, which had high accuracy compared with the experimental results. Any increase in temperature, CaO/C, and steam/biomass helped to increase the H₂ concentration in the syngas [22]. Santos et al. evaluated the techno-economic feasibility of the SEG, demonstrating a higher H₂ production efficiency when using SEG compared to traditional steam gasification [23]. In a techno-economic assessment by Cormos, green hydrogen generation from BSEG obtained good results, with high efficiency, a low energy cost, few economic penalties, and negative CO₂ emissions [24]. As the SEG showed a favorable performance for hydrogen generation and carbon capture, it was integrated with other systems, such as thermal energy storage [25].

Furthermore, the conversion of CH₄, another greenhouse gas, has attracted much attention. The dry reforming of methane (DRM) utilizes the CO₂, converting it into CO and H₂, and thereby reducing the greenhouse gases into fuels [26,27]. While CO₂ capture using CaO can form CaCO₃, a method of calcium looping coupled with DRM has already been proposed and widely studied in the field of post-combustion CO₂ capture [26,28,29,30,31]. Zhang et al. developed an integrated process of CaLP and in situ DRM using Fischer–Tropsch, which reduces the heat energy demand, with a drop of 28.08% [32]. This demonstrates the advances of integrating process CaLP with DRM.

It was shown that the BSEG is beneficial in generating high-quality syngas with adjustable gas compositions while CaCO₃ is formed. For the regeneration of CaO, this work proposes coupling it with DRM, instead of the traditional calcination. The principle of the proposed CaLP, integrating BSEG and CaCO₃-based methane reforming (CaMR), is shown in Figure 1b. In contrast to conventional CaLP, which relies on energy-intensive high-temperature calcination for CaO regeneration, this study proposes an innovative integration of BSEG with CaMR. The novelty lies in utilizing CaCO₃ (generated during BSEG) as a feedstock for methane reforming, thereby eliminating the need for calcination while enabling in situ CO₂ utilization. This dual-function approach simultaneously regenerates CaO for CO₂ capture and converts methane into syngas, enhancing process efficiency and reducing the carbon footprint. Prior studies primarily focused on standalone BSEG or methane reforming; this work bridges the two processes, establishing a synergistic pathway for sustainable energy production. The integration of BSEG and CaMR introduces a closed-loop carbon-utilization strategy.

The global demand for rice is projected to increase by over 1.2% annually, in response to the escalating food requirements caused by worldwide population growth and economic development [33]. Typically, the processing of one ton of rice results in the generation of approximately 240 kg of husks [34]. Consequently, the production of rice husks is a significant global phenomenon. Its valorization aligns with the principles of circular economy principles. Thus, rice husk was chosen as the feedstock in this work. To evaluate the performance of the proposed process, a thermodynamic process model was established based on the Gibbs free energy minimization to analyze the effects of different operating parameters on product distribution in both the gasifier and the reformer. The results demonstrate the feasibility of this CaLP and offer a practical concept for the conversion of methane.

2. Model of the Calcium Looping Process

Rice husk is a globally abundant agricultural residue, particularly in rice-producing regions. Utilizing it aligns with the circular economy principles as it valorizes waste. Due to its abundance, low cost, and carbon neutrality, it has been used as an alternative fuel in many fields. Therefore, rice husk was used as the feedstock for the BSEG. The proximate and ultimate analyses of the rice husk were derived from the literature and are presented in

Table 1. The proximate and ultimate analyses of the rice husk [35].

Item	Proximate Analysis				Ultimate Analysis
	FC	Moisture	Volatiles	Ash	C	H	O	N	S
wt%	15.56	8.95	63.78	11.71	40.44	4.62	34.02	0.26	0

[35].

In this work, a thermodynamic analysis model is established to investigate the coupling cycle of BSEG and CaMR. As has been mentioned, CaO can play multiple roles in the biomass gasification process, namely as a catalyst for tar-cracking and as a CO₂ sorbent. It was used as the bed material for BSEG. The CaLP process integrating BSEG and CaMR is principally shown in Figure 2, and consists of a gasifier, reformer, and other components.

When the rice husk (RH) is fed into the gasifier, it goes through a fast pyrolysis step and the products are then gasified with the addition of the gasification agent. During the pyrolysis period, the volatiles are evaporated from the solid phase and some of them form tar. The reaction can be described as follows:

$$RH\to Char(C)+Tar+Gases(CO,H_2,H_{2}O,C{H}_4,C_2{H}_4,C_2{H}_6,C{O}_2)+Ash$$

(1)

Usually, the tar undergoes a second thermal decomposition period with the catalyst. As the CaO serves as the catalyst for tar-cracking, while the tar can be converted to syngas in the steam gasification period, the tar is regarded as having been totally converted into light hydrocarbons in this work.

The pyrolysis products then react with the gasification agent steam. The gasification period relates to many reactions and is quite complex. The gasification period should consist of the following reactions:

$$C+C{O}_2\to 2CO$$

(2)

$$C+H_{2}O\to CO+H_2$$

(3)

$$CO+H_{2}O\leftrightarrow C{O}_2+H_2$$

(4)

$$CO+3H_2\leftrightarrow C{H}_4+H_{2}O$$

(5)

$$C{O}_2+4H_2\leftrightarrow C{H}_4+2H_{2}O$$

(6)

$$C{H}_4\leftrightarrow C+2H_2$$

(7)

$$C{H}_4+C{O}_2\leftrightarrow 2CO+2H_2$$

(8)

Additionally, the light hydrocarbons (e.g., C₂H₄ and C₂H₆) can also react with steam:

$$C_2{H}_4+2H_{2}O\leftrightarrow 2CO+4H_2$$

(9)

$$C_2{H}_6+2H_{2}O\leftrightarrow 2CO+5H_2$$

(10)

When the CaO is introduced into the gasifier, it can react with CO₂ and weaken the CO₂ partial pressure to shift the equilibria. The CaO carbonation is given as follows:

$$CaO+C{O}_2\to CaC{O}_3$$

(11)

By shifting the equilibria, this reaction in gasifier is beneficial to enhance the purity of the H₂ in syngas and lowers the operating temperatures for RH gasification.

After BSEG, the solid phase is separated from the gaseous products in two separators. The CaCO₃ and unconverted CaO then flow into the reformer. Meanwhile, a stream of CH₄ is introduced into the reformer for the reforming reactions. In the reformer, the CaCO₃ can be decomposed into CaO and CO₂. The CO₂ can react with CH₄ for the dry reforming process. In addition, CaCO₃ may react with CH₄ directly for the in situ reforming. During this process, CaO is regenerated. After separation, the gaseous CaMR products are separated from the CaO. Then, the regenerated CaO is circulated into the gasifier for a whole CaLP integrating BSEG and CaMR.

3. Data Processing and Parameter Settings

3.1. Data Processing

In the CaLP process, the products of the reformer and gasifier are influenced by many operating parameters. Once the feeding rate of the RH is specific, the carbon content can be determined. In the gasifier, the CaO and the steam are introduced simultaneously with the RH. To evaluate the amount of steam, the equivalence ratio of steam to carbon in the gasifier α_G(H2O/C) is defined as follows:

$${\alpha }_{\mathrm{G}({\mathrm{H}}_2\mathrm{O}/\mathrm{C})}={\displaystyle \frac{n_{G,steam}}{n_{\mathrm{C}}}}$$

(12)

where n_G,steam is the molar flowrate of the steam flowing into the gasifier and the n_C is the molar flowrate of the carbon in the feedstock, which can be calculated according to the feeding rate of the RH and the ultimate analysis.

Correspondingly, the equivalence ratio of steam to carbon in the reformer α_R(H2O/C) is defined as follows:

$${\alpha }_{\mathrm{R}({\mathrm{H}}_2\mathrm{O}/\mathrm{C})}={\displaystyle \frac{n_{R,steam}}{n_C}}$$

(13)

where n_R,steam is the molar flowrate of the steam flowing into the reformer.

In the gasifier, the CaO reacts with CO₂ to realize its conversion. The CaO conversion is the ratio of converted CaO (CaCO₃) to the total CaO into the gasifier, namely,

$$\mathrm{CaO}\hspace{1em}conversion={\displaystyle \frac{{\mathit{n}}_{{\mathrm{CaCO}}_3}}{{\mathit{n}}_{\mathrm{CaO}}+{\mathit{n}}_{{\mathrm{CaCO}}_3}}}$$

(14)

in which n_CaO and n_CaCO3 are the molar flowrates of the CaO and CaCO₃ at the outlet of the gasifier, respectively.

In the reformer, the CH₄ is converted via reforming reactions. Its conversion is defined as the ratio of converted CH₄ to CH₄ in the reformer, which can be calculated by the following equation:

$${\mathrm{CH}}_4\hspace{1em}conversion=1-{\displaystyle \frac{{\mathit{n}}_{{\mathrm{CH}}_4,\mathit{o}\mathit{u}\mathit{t}}}{{\mathit{n}}_{{\mathrm{CH}}_4,\mathit{i}\mathit{n}}}}$$

(15)

where $n_{{\mathrm{CH}}_4,out}$ and $n_{{\mathrm{CH}}_4,in}$ are the molar flowrates of the CH₄ flowing out of and into the reformer, respectively.

3.2. Parameter Settings

In the process simulation, the feeding rate of the RH is specified as 1000 kg/h. From the ultimate analysis results, the corresponding carbon flowrate is approximately 33.7 kmol/h. In these cases, the molar flowrate of the CaO into the gasifier is set to the same as that of the carbon. Additionally, the molar flowrate of the CH₄ into the reformer is set as 30 kmol/h. Under the fundamental condition, the temperature of the gasifier is 650 °C while that of the reformer is 800 °C. The α_G(H2O/C) and α_R(H2O/C) are set as 1 and 0, respectively. That means the dominant reaction in the reformer is the dry reforming of CH₄ under the fundamental condition.

For the established thermodynamic process model, the pyrolysis products in the gasifier are obtained based on the component yields, calculated using the mass balance of different elements. The gasification products and the reformer products are calculated based on the principle of Gibbs free energy minimization. Both the phase equilibrium and chemical equilibrium are realized by determining the composition of a system that minimizes the Gibbs free energy, subject to constraints, material balances, and the specified temperature and pressure.

4. Performance of CaLP Integrating BSEG with CaMR

4.1. Effects of the Gasification Temperature

When the gasification temperature increased from 600 °C to 700 °C, the other parameters remained constant with those under the fundamental conditions. The products at the outlet of the gasifier are shown in Figure 3.

It is shown that when the gasification temperature increased from 600 °C to 700 °C, the flowrates of CO and CO₂ exhibited an increasing trend. Specifically, the CO at the outlet of the gasifier increased from 0.653 kmol/h at 600 °C to 11.699 kmol/h at 700 °C, while the CO₂ increased from 0.220 kmol/h to 3.440 kmol/h over the same temperature range. This indicates enhanced carbon monoxide and carbon dioxide production at higher temperatures. The increase in CO and CO₂ can be attributed to the endothermic nature of gasification reactions, such as the Boudouard reaction and char steam gasification, which are enhanced at elevated temperatures [36,37]. Additionally, the calcium oxide serves as a CO₂ sorbent. When the temperature increases, the carbonation reaction of CaO is weakened, which leads to an increased generation of CO₂. A higher CO₂ content can promote the char CO₂ gasification and CH₄ reforming reactions in the reaction system. Meanwhile, the H₂ in syngas showed a gradual increase, from 43.999 kmol/h at 600 °C to 48.536 kmol/h at 700 °C, suggesting a relatively stable but slightly improved hydrogen yield. This increase is likely due to the enhanced steam gasification and water–gas shift reactions that occur at higher temperatures, which are facilitated by the presence of CaO. The molar ratio of H₂/CO in syngas decreased from 67.38 to 4.15. The sorption of CO₂ by CaO shifts the equilibrium of these reactions toward H₂ production, contributing to the increase in hydrogen generation. However, from the perspective of the volume concentration, the H₂ concentration in the dry-basis syngas decreased from 86.15% to 73.56% as the temperature increased. However, the CH₄ flowrate declined from 6.108 kmol/h to 2.212 kmol/h when the temperature increased from 600 °C to 700 °C. This decrease can be explained by the endothermic methane reforming reactions, which become more dominant at higher temperatures, leading to the decomposition of methane into CO and H₂. This is in close alignment with the reforming property of CH₄ [38]. Additionally, the CaO may react with the CO₂ during the BSEG process, suppressing CH₄ formation. These trends highlight the temperature-dependent behavior of the BSEG process, with higher temperatures favoring the production of CO and H₂. The role of CaO as a CO₂ sorbent can enhance the gasification process and dramatically improve the overall hydrogen yield by shifting the reaction equilibria.

As for the solid-phase products, the CaO at the gasifier outlet increased steadily, from 7.027 kmol/h at 600 °C to 17.397 kmol/h at 700 °C, while the CaCO₃ decreased from 26.673 kmol/h to 16.303 kmol/h during the BSEG process. The carbonation reaction between CaO and CO₂ is exothermic and increasing the temperature shifts the equilibrium toward the reactants, reducing the formation of CaCO₃. During this process, the CaO conversion correspondingly decreased from 79.15% to 48.38%. This decrease in CaO conversion at higher temperatures aligns with the increase in CO₂ flowrates in the gasification products. As CaO captures less CO₂, more CO₂ is released into the gas stream and shifts the equilibrium of the gasification reactions toward CO formation. Though high temperatures are beneficial to gasification, they weaken the sorption reaction between CaO and CO₂, which negatively impacts the CO₂ capture and the generation of hydrogen-rich syngas.

The effects of the gasification temperature on the CaMR products are shown in Figure 4.

When the gasification temperatures changed, the CaCO₃ flowing into the downstream reformer varied from 26.673 kmol/h to 16.303 kmol/h and showed the ability to react with methane for reforming. When the gasification temperature increased from 600 °C to 700 °C, the flowrate of CO decreased from 51.103 kmol/h to 32.361 kmol/h, while H₂ decreased from 49.724 kmol/h to 32.222 kmol/h. This indicates that a higher CaCO₃ flowrate can increase CO and H₂ production in the reforming process. During this process, the CO₂ flowrate decreased from 0.777 kmol/h to 0.084 kmol/h as the gasification temperature increased. Similarly, the H₂O flow rate decreased from 0.690 kmol/h to 0.076 kmol/h. This suggests that a higher CaCO₃ flowrate results in more CO₂ and H₂O being produced from the CaCO₃ decomposition and/or the CH₄ reforming. Under the simulated conditions, the model predicts negligible carbon formation due to the suppression of methane cracking and the Boudouard reaction. Additionally, as seen in the calculation results, due to some side reactions, H₂O is produced. The absence of steam in the methane reforming system is beneficial to avoid the decomposition of the coke. However, the CH₄ flowrate at the reformer outlet increased from 4.792 kmol/h to 13.842 kmol/h, corresponding to a decrease in CH4 conversion in CaMR from 84.03% to 53.86%, as illustrated in Figure 4b.

During the CaMR process, the CaCO₃ may decompose at high temperatures to form CaO and CO₂. The produced CO₂ can then participate in the dry reforming of methane, which increases the production of CO and H₂. Thus, the CO and H₂ flowrate are positively related to the CaCO₃ flowing into the reformer. As the H₂O is generated, there must be some side reactions taking place in the reformer. It is clear that low flowrates of C₂H₄ and C₂H₆ are also produced in the reformer due to the side reactions, varying in from 0.390 mol/h to 5.691 mol/h and from 0.153 mol/h to 1.967 mol/h, respectively.

4.2. Effects of the Steam Flowrate into Gasifier

In the gasifier, the steam is an important gasification agent for the conversion of the rice husk. As has been mentioned in the previous section, the steam is involved in several relevant reactions in the BSEG process. When the equivalence steam-to-carbon ratio in the gasifier α_G(H2O/C) varies from 0.8 to 1.5, the products of BSEG are shown in Figure 5.

As the equivalence ratio α_G(H2O/C) increased from 0.8 to 1.5, the flowrates of CO and CH₄ decreased at the outlet of the gasifier. In these cases, the CO flowrate dropped from 3.736 kmol/h to 2.197 kmol/h while that of CH₄ declined from 7.011 kmol/h to 1.752 kmol/h. In contrast, the flowrate of H₂ dramatically increased from 37.305 kmol/h to 59.879 kmol/h, which correspond to the increase in H₂ concentration from 76.29% to 92.02% (in dry basis). Meanwhile, the CO₂ flowrate slightly increased from 0.753 kmol/h to 1.151 kmol/h. These trends indicate that an increase in steam availability suppresses CO and CH₄ formation while significantly promoting H₂ production. Additionally, the gradual increase in CO₂ and the sharp rise in H₂ suggest a shift in the dominant reaction pathways as the α_G(H2O/C) increases. These phenomena can be explained by the interplay of several key gasification reactions, which are influenced by the equivalence ratio α_G(H2O/C) and the presence of CaO. At a lower α_G(H2O/C), the primary reactions include the water–gas reaction and methane reforming reactions (dry reforming and/or steam reforming), which produce a large amount of H₂. These reactions are further enhanced by the presence of CaO, which captures CO₂ through the carbonation reaction, shifting the equilibrium of the water–gas shift and steam reforming reactions toward H₂ production. The decrease in CH₄ flowrate is a direct result of this enhanced reforming process. On the other hand, excess steam drives the water–gas shift reaction, converting CO into CO₂ and further increasing H₂ production. This explains the decline in CO and the rise in CO₂ and H₂ at a higher α_G(H2O/C). The steam in the gasifier has the dual roles of promoting both H₂ production and CO₂ generation, as well as showing the limitations of CaO in fully mitigating CO₂ release under high-steam conditions. These insights highlight the need to optimize this ratio to maximize H₂ production and minimize undesirable byproducts in the BSEG process.

When the α_G(H2O/C) increased from 0.8 to 1.5, the flowrate of CaO at the gasifier outlet gradually decreased from 11.547 kmol/h to 5.146 kmol/h, while that of CaCO₃ increased from 22.153 kmol/h to 28.554 kmol/h. This indicates that CaO is continuously consumed and converted into CaCO₃ during the BSEG process when more steam is introduced into the gasifier. Correspondingly, the CaO conversion rate increased from 65.74% to 84.73%. The presence of more steam in the gasifier can significantly affect CaO conversion via enhancing the reactions through CO₂ generation.

The effects of the equivalence ratio α_G(H2O/C) on the CaMR products are shown in Figure 6.

When the equivalence ratio α_G(H2O/C) varied from 0.8 to 1.5, different CaCO₃ flowrates were introduced into the reformer. The flowrates of CO, CO₂, H₂, and H₂O at the reformer outlet showed a consistent upward trend. Specifically, the CO flowrate increased from 43.435 kmol/h to 53.802 kmol/h and the H₂ flowrate increased from 42.898 kmol/h to 51.785 kmol/h. The flowrates of CO₂ and H₂O have approximately the same variation trends. Specifically, when the α_G(H2O/C) increased from 0.8 to 1.5, the CO₂ flowrate rises from 0.300 kmol/h to 1.149 kmol/h, and H₂O increased from 0.271 kmol/h to 1.009 kmol/h. However, the CH₄ flowrate decreased significantly from 8.414 kmol/h to 3.603 kmol/h, indicating higher CH₄ consumption at a higher α_G(H2O/C). In the CaMR, CaCO₃ may decompose at high temperatures to produce CaO and CO₂. The CO₂ participates released during the dry reforming of methane drive the production of CO and H₂. As the increase in α_G(H2O/C) results in an increase in the CaCO₃ flowrate into the reformer, more CO₂ is available for the dry reforming reaction, leading to higher CO and H₂ production and greater CH₄ consumption. Under these conditions, no steam is introduced into the reformer; however, steam is generated at the outlet. The reverse water–gas shift reaction may also contribute to the observed trends, as it further converts CO₂ and H₂ into CO and H₂O, particularly at higher CaCO₃ flowrates, due to the higher partial pressure of CO₂ in the reaction system. This can be demonstrated by the phenomenon that the increasing rate of CO is higher than that of H₂ at a higher α_G(H2O/C). As a comprehensive result, the CH₄ conversion in the reformer also increased in line with the α_G(H2O/C). When the α_G(H2O/C) increased from 0.8 to 1.5, the CH₄ conversion increased from 71.95% to 87.99%.

4.3. Effects of the Reforming Temperature

Under fundamental conditions, the CaO and CaCO₃ flowrates at the gasifier outlet were approximately 9.204 kmol/h and 24.496 kmol/h, respectively. They separated from syngas and flowed into the gasifier downstream. When the temperature of the reformer changed from 700 °C to 900 °C, the CaMR products obtained were as shown in Figure 7.

The results illustrate the effects of reforming temperature variations on the product distribution in the CaMR process. As the temperature increased from 700 °C to 900 °C, the flowrate of CO increased from 40.68 kmol/h at 700 °C to 48.82 kmol/h at 900 °C. This increase is consistent with the endothermic nature of the reforming reactions, which favor higher temperatures. As another product of the reforming reaction, the flowrate of H₂ increased from 37.05 kmol/h at 700 °C to 48.70 kmol/h at 900 °C. This increase is consistent with the enhanced reforming reactions at higher temperatures, as mentioned previously. The flowrate of CO₂ decreased from 3.25 kmol/h at 700 °C to 0.052 kmol/h at 900 °C. Additionally, the decomposition of CaCO₃ releases CO₂, but this is rapidly consumed in the reforming process. The flowrate of CH₄ decreased from 10.57 kmol/h at 700 °C to 5.61 kmol/h at 900 °C, indicating an increase in CH₄ conversion at higher temperatures. This reduction is due to the consumption of CO₂ and CH₄ in the reforming reactions, particularly the dry reforming of CH₄, which becomes more favorable at higher temperatures. The generation of H₂O should be caused by side reactions such as the reverse water–gas shift reaction. Its flowrate decreased from 1.81 kmol/h at 700 °C to 0.066 kmol/h at 900 °C. This reduction is due to the consumption of H₂O in the steam reforming reaction and the water–gas shift reaction. The increase in CO and H₂ production, along with the decrease in CH₄, CO₂, and H₂O, indicates that the reforming reactions favor higher temperatures, leading to more CH₄ being converted into syngas in CaMR. Interestingly, the side reactions can produce C₂H₄ and C₂H₆, as mentioned in the previous section. However, they show different variation trends. The flowrate of C₂H₄ increased from 0.244 mol/h to 4.312 mol/h with the increase in reforming temperature. The flowrate of C₂H₆ first decreased from 0.427 mol/h at 700 °C to 0.293 mol/h at 780 °C, then increased to 0.435 mol/h at 900 °C. Overall, the CH₄ conversion in CaMR increased from 64.78% to 81.29% as the temperature increased from 700 °C to 900 °C.

4.4. Effects of the Steam Flowrate into Reformer

It has been demonstrated that some side reactions in the reformer can produce H₂O. In reality, the addition of H₂O in the CH₄ reforming system is beneficial to avoid carbon deposition, changing it to a bio-reforming process. Therefore, a stream of steam was added to the reformer to investigate the effects of the steam flowrate on the CaMR process. The CaMR products obtained whenthe equivalence ratio α_R(H2O/C) increased from 0 to 0.5 are shown in Figure 8.

As illustrated in Figure 8, the flowrate of CO initially increased from 47.57 kmol/h at α_R(H2O/C) = 0 to a maximum of 50.23 kmol/h at α_R(H2O/C) = 0.2, after which it gradually decreased to 48.38 kmol/h when the α_R(H2O/C) reached 0.5. This trend is caused by the competing effects of steam reforming and the water–gas shift reaction. At low steam flowrates, steam reforming dominates, leading to an increase in CO production. However, as steam flow increases further, the water–gas shift reaction becomes more significant, consuming CO and producing CO₂. The steam reforming reaction produces H₂ as a primary product while the water–gas shift reaction also contributes to H₂ production. As they are enhanced, hydrogen generation is improved. It is shown that the flowrate of H₂ increased substantially from 46.69 kmol/h to 68.38 kmol/h when the α_R(H2O/C) rises from 0 to 0.5. The flowrate of CO₂ increased steadily from 0.49 kmol/h at α_R(H2O/C) = 0 to 5.31 kmol/h at α_R(H2O/C) = 0.5. This increase is due to the water–gas shift reaction, which converts CO and H₂O into CO₂ and H₂. It may also be affected by the competing reactions of steam reforming and CO₂ reforming. When the steam flowrate increases, CO₂ reforming should be weakened due to the increase in the partial pressure of steam. When the α_R(H2O/C) increased from 0 to 0.5, the flowrate of H₂O at the outlet increases from 0.44 kmol/h to 6.85 kmol/h. However, the rate of the H₂O increase in the outlet is lower than the rate of the increase in inlet steam, indicating that more H₂O is consumed during the reactions. This is consistent with the increased activity observed in both steam reforming and the water–gas shift reaction at higher steam flowrates. Correspondingly, the flowrate of CH₄ decreased from 6.43 kmol/h to 0.81 kmol/h as α_R(H2O/C) increased from 0 to 0.5, indicating improved CH₄ conversion with higher steam flowrates. This is expected because steam reforming is enhanced with an increase in steam availability, leading to the greater consumption of CH₄. In these cases, the generation of C₂H₄ and C₂H₆ is very low compared with that observed previously. As the methane reforming is enhanced, the CH₄ conversion increases from 78.56% to 97.30% in line with the increase in α_R(H2O/C).

The proposed integrated calcium looping process directly contributes to green hydrogen production by coupling biomass gasification with methane reforming. The integration of BSEG and CaMR eliminates energy-intensive calcination, enabling in situ CO₂ utilization and closed-loop carbon management. The SEG has a comparable economic feasibility with that of conventional steam gasification. If the cost of carbon capture is considered, SEG has significant advantages. While the CaMR is coupled, it reduces the CO₂ cost of methane reforming. For a detailed analysis of the techno-economic feasibility, further systemic work should be conducted.

5. Conclusions

This study presents a favorable integration of BSEG with CaMR within a novel CaLP. By replacing energy-intensive calcination with in situ CO₂ utilization, the system achieves closed-loop carbon management while maximizing hydrogen-rich syngas production. The key findings are summarized as follows:

(1)

Increasing the gasification temperature in BSEG enhances CO production and H₂ yield, which are driven by endothermic reactions. However, higher temperatures reduce CaO conversion, leading to a lower CH₄ conversion in the downstream reformer.

(2)

Increasing α_G(H2O/C) from 0.8 to 1.5 substantially improves H₂ production from 37.305 kmol/h to 59.879 kmol/h and increases CH₄ conversion from 71.95% to 87.99% in the reformer.

(3)

Increasing the reforming temperature from 700 °C to 900 °C increases CH₄ conversion from 64.78% to 81.29%, indicating the more efficient utilization of CO₂ in reforming reactions at higher temperatures. Meanwhile, as α_R(H2O/C) rises from 0 to 0.5, CH₄ conversion improves dramatically from 78.56% to 97.30% while the H₂ flowrate in the reformer increases from 46.69 kmol/h to 68.38 kmol/h.

This technology bridges biomass utilization, carbon capture, and methane conversion. Future work should focus on experimental validation and techno-economic feasibility assessments.