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Article

Study on the Effects of Tar Reforming and Steam Gasification of Keyaki Bark in Saitama Prefecture

by
Shangrong Wu
*,
Qingyue Wang
* and
Isobe Ryota
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(5), 2215; https://doi.org/10.3390/su17052215
Submission received: 21 January 2025 / Revised: 19 February 2025 / Accepted: 24 February 2025 / Published: 4 March 2025
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Abstract

:
Keyaki bark is an abundant untapped resource of biomass in Saitama Prefecture, Japan, for steam gasification and tar reforming. To optimize performance, raw bark underwent demineralization with HCl to remove native metals and calcium impregnation using Ca (OH)2. Gasification experiments were conducted at 900 °C using steam and CO2 as gasifying agents. The tar was reformed in a two-stage metal reactor, resulting in improved syngas yields. Results showed that demineralization enhanced gasification efficiency, producing higher hydrogen (H2) and carbon monoxide (CO) yields compared to untreated samples. Experiments have shown that steam gasification of bark char produced 142% more syngas compared to raw bark, with H2 yield increasing by 86% and CO yield by 250%. Additionally, the two-stage metal tube reactor generated 200% more syngas than raw bark gasification and 24% more than bark char gasification. Therefore, we confirmed the feasibility of using the two-stage metal tube reactor for tar reforming to enhance syngas production in steam gasification processes. Keyaki bark’s high carbon and low ash content make it a promising feedstock for sustainable energy production.

1. Introduction

Biomass represents a critical component in the pursuit of renewable energy sources, owing to its widespread availability, carbon neutrality, and significant potential to reduce reliance on fossil fuels [1,2]. Among biomass types, wood biomass garners significant attention for its high energy content and versatility in energy conversion technologies [3]. Concurrently, the global energy sector prioritizes sustainable solutions to meet rising energy demands while mitigating environmental consequences, highlighting the need for innovative pathways in biomass utilization [4,5]. Keyaki trees are commonly planted along streets in Japan, generating large numbers of pruned branches and amounts of bark waste each year. However, research on the reuse of this biomass remains limited. This study addresses a critical gap by investigating the potential of Keyaki bark as a feedstock for syngas production through steam gasification and tar reforming. Unlike conventional biomass gasification studies, which primarily focus on widely studied feedstocks like rice husks and wood chips, this research explores the untapped potential of urban tree waste. Furthermore, the study employs a two-stage metal tube reactor, which enhances syngas yield and tar decomposition efficiency, offering a more effective approach compared to traditional single-stage gasification. By optimizing gasification conditions and catalytic interventions, this work provides valuable insights into the sustainable utilization of urban biomass waste for renewable energy applications. This study focuses on the steam gasification and tar reforming of keyaki bark (Zelkova serrata), a locally abundant biomass in Saitama Prefecture. Keyaki bark was chosen for its lignocellulosic content and potential as a waste-to-energy feedstock, fostering circular economy principles [6,7]. Understanding its behavior under gasification conditions provides insights into its renewable energy applications [3].
Gasification converts biomass into syngas (H2, CO, CH4, CO2) via high-temperature reactions with gasifying agents like steam (H2O) and CO2 [8,9,10]. However, the generation of tars—complex hydrocarbon byproducts—remains a bottleneck, hindering efficiency [11,12,13]. Tar reforming, which breaks down tars into lighter gases, is essential, but requires expensive catalysts and robust reactor designs [14,15,16,17,18]. This study employs a two-stage metal reactor to address these challenges: The lower stage gasifies biomass, while the upper stage utilizes char-derived catalysts for tar reforming. This configuration optimizes syngas yield by enhancing tar breakdown through catalytic interactions [19,20].
Keyaki bark’s metal composition (e.g., Ca, Mg, K, Na) acts as a natural catalyst influencing reaction pathways, but also poses challenges like fouling and slagging [21]. The study systematically evaluates demineralization and catalytic enhancement via Ca (OH)2 impregnation to optimize performance [22]. Benchmarking against prior research, the demineralization method used in this study demonstrates efficient mineral removal under mild conditions, aligning with or surpassing results for similar biomass types [23]. These treatments are expected to enhance gasification and tar reforming [24]. Experimentation involved pre-treating raw keyaki bark by demineralization or calcium impregnation, followed by drying, grinding, and compositional analysis [25,26]. A custom-built two-stage reactor operated at 900 °C facilitated gasification and tar reforming under controlled conditions. Real-time monitoring of gas yields and compositions was performed using gas chromatography [27,28]. Results highlighted the superior catalytic activity of treated biomass in reducing tar and enhancing syngas production [24,29]. This research integrates material characterization, process optimization, and experimental validation for keyaki bark, emphasizing the innovative use of two-stage reactors for tar reforming [30]. By demonstrating the feasibility and benefits of these approaches, the study advances biomass energy conversion technologies, offering valuable guidance for renewable energy applications [31].
The primary objective of this research is to comprehensively evaluate the potential of waste biomass derived from keyaki bark for energy applications. This includes assessing its detailed metal content, such as calcium, magnesium, potassium, and sodium, and analyzing the effects of demineralization on its chemical and physical properties. The study further investigates the impact of adding catalytic calcium through Ca (OH)2 impregnation and performs thermogravimetric analyses to elucidate the material’s thermal decomposition behavior. Additionally, the research aims to optimize gasification and tar reforming processes using advanced experimental reactor systems, while evaluating and comparing the syngas production and yield under different configurations, including CO2 gasification, steam gasification, and two-stage tar reforming.

2. Materials and Methods

2.1. Materials

As part of a study on woody biomass collected through routine pruning, bark samples of Zelkova serrata were obtained from the Saitama University campus. The biomass feedstock was processed into a powder with a particle size of ≤250 µm using a Wonder Blender WB-1, a continuous mill (IKA MF10 basic), sieves (160–250 µm, Tokyo Screen Co., Ltd., Tokyo, Japan), and a sieve shaker (Retsch AS 200).

2.1.1. Metal Contents of Waste Biomass

To accurately assess the concentrations of various metals such as calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) within a given sample, we employed an advanced analytical technique utilizing an X-ray Analytical Microscope (XGT-5000, Horiba Ltd., Tokyo, Japan). This capability is particularly important in fields such as materials science, as well as in environmental analyses and biological studies where understanding the distribution and concentration of trace metals can reveal significant insights into the properties and behaviors of the sample material [32].

2.1.2. The Demineralization of Materials

The demineralization process plays a pivotal role in biomass conversion techniques such as steam gasification and tar reforming. Biomass inherently contains inorganic mineral elements, including calcium, potassium, magnesium, and sodium, which, while naturally occurring, can impede downstream gasification and reforming processes. These minerals can lead to tar formation through catalytic side reactions, result in the fouling of reactor surfaces, and alter the behavior of carbon reactivity. Thus, the removal of inorganic substances, referred to as demineralization, is a critical pretreatment step to enhance the efficiency and reliability of biomass utilization in gasification processes. In this study, the demineralization of Keyaki bark—a common waste biomass in Saitama Prefecture—was investigated to optimize the material for steam gasification and tar reforming [33,34].
The methodology for demineralization revolved around the use of a dissolving solvent selected for its high efficacy in extracting ionic compounds and metal salts from organic solids. Acid solutions, such as hydrochloric acid (HCl) and nitric acid (HNO3), are widely used in the field of biomass processing due to their strong affinity for mineral ions. For this study, a dilute HCl solution was employed as the primary solvent, a choice guided by its proven ability to remove alkaline earth metals without extensively degrading the structural integrity of lignocellulosic materials.
Charcoal materials were prepared by pyrolyzing raw bark under an argon atmosphere at 900 °C for 2 h. The resulting charcoal was subsequently washed five times with 50 mL of 36% hydrochloric acid (Wako Pure Chemical Industries, Ltd., Japan), followed by filtration under vacuum. The washed materials were then dried in a drying oven (Kosumosu SSN-113S, Isuzu, Yokohama, Japan) at 105 °C for 72 h to obtain demineralized charcoal.

2.1.3. The Addition of Catalytic Calcium Using the Impregnation Method with Ca (OH)2

The use of catalysts is a crucial strategy in improving the efficiency of biomass gasification and tar reforming processes [35]. Catalysts can significantly enhance reaction rates, improve gas yield, and reduce the production of undesirable by-products like tar. Among potential catalysts, calcium-based compounds have garnered attention owing to their cost-effectiveness, widespread availability, and environmental benignity. Calcium hydroxide Ca (OH)2 stands out as an excellent catalytic agent for biomass modification due to its strong basicity, thermal stability, and compatibility with impregnation methods. This study investigates the addition of catalytic calcium to Keyaki bark, a waste biomass from Saitama Prefecture, using Ca (OH)2 applied through the impregnation method.
The impregnation method presented in Figure 1 is a widely utilized technique for incorporating catalysts into biomass materials. This approach involves soaking biomass in a solution containing the catalyst precursor to enable its distribution within the material. In the context of this research, impregnating Keyaki bark with Ca (OH)2 entails soaking the biomass in an aqueous solution of calcium hydroxide, followed by controlled drying to ensure the even retention of calcium ions on the biomass structure. This technique offers several advantages: It is straightforward, scalable, and economical, making it particularly suitable for processing waste biomass on a large scale. Moreover, the use of Ca (OH)2 allows for the utilization of a nontoxic reagent that ensures minimal environmental impact.
Raw bark materials were immersed in a calcium hydroxide solution prepared by stirring 3.5 g 96.0% of Ca (OH)2 (Wako Pure Chemical Industries, Ltd., Osaka, Japan) in 300 mL of ultrapure water for 24 h. The bark was soaked in the solution for 10 h, followed by drying in a drying oven (Kosumosu SSN-113S, Isuzu, Yokohama, Japan) at 105 °C for 72 h to obtain catalyst-impregnated materials. The calcium catalyst-impregnated materials were then pyrolyzed under an argon atmosphere at 900 °C for 2 h to produce calcium catalyst-loaded charcoal materials.

2.1.4. Ultimate Analysis of the Waste Biomass

To conduct a complete and high-precision analysis during the study, two advanced instruments were employed to ensure accurate measurement and comprehensive assessment of the required parameters. The first device, a CHN-CORDER (Model MT-5, manufactured by Yanaco, Co., Ltd., Yanaco, Japan), is a sophisticated analytical instrument designed to determine the elemental composition of a sample—specifically, the carbon (C), hydrogen (H), and nitrogen (N) content [36]. The second instrument was a Microcomputer Han Display Sulfur Analyzer (HYDL-9, developed by Minsheng Ltd., Hebi, China), an advanced tool specifically engineered for the detection and quantification of sulfur (S) in various samples. Accurate sulfur analysis is a critical factor in industries such as petrochemicals, energy production, and materials science, as even trace amounts of sulfur can influence the quality, stability, and environmental impact of a product.

2.2. Thermogravimetric Analyses

The gasification behaviors of the sample were analyzed using thermogravimetry differential thermal analysis (TG-DTA, DTG-60, Shimazu Co., Ltd., Kyoto, Japan). Approximately 5 mg of the dried sample was placed in an alumina crucible and heated from room temperature to 900 °C at a steady rate of 100 °C/min. The process took place under a continuous carbon dioxide flow of 100 mL/min, with no holding time. A TG-DTA analysis was then performed to assess the impact of metal catalysts on the thermal decomposition behavior and weight loss characteristics of the samples.

2.3. Reactor System for Steam Gasification Experiments

The gasification experiments (Figure 2) were conducted using a fixed-bed reactor system designed for the controlled conversion of biomass into syngas. The system consists of several essential components, including the gasification chamber, heating system, feedstock feeder, product gas condenser, and gas collection unit. As shown in Figure 2 the gasification chamber is a cylindrical glass vessel with a 20 mm internal diameter and a length of 500 mm, capable of withstanding temperatures up to 1200 °C. It features an inlet for feedstock introduction and a thermocouple for real-time temperature monitoring. The heating system includes a high-temperature furnace with PID controllers (Temperature Control Unit, MODEL SU, CHINO Co., Ltd., Tokyo, Japan) to precisely regulate reaction temperatures.
For sample preparation, 1 g of material was placed on a heat-resistant alloy sample table within the fixed-bed reactor, preventing thermal degradation and clogging. Each experiment involved a single mixed raw material. During gasification, a flow meter controlled the gas flow rate, introducing Ar at 100 mL/min. The final temperature of 900 °C was reached at a heating rate of 10 °C/min and maintained until the reaction ended. Water was continuously supplied at 0.9 mL/min using a MICRO TUBE PUMP (Rikakikai Co., Ltd., Tokyo, Japan) throughout the gasification process. The fixed-bed reactor operated under consistent conditions, ensuring reliable experimental results.
After gasification, the filtered gas stream was directed to a condenser unit, which cooled the gases to ambient temperature. This shell-and-tube heat exchanger, equipped with a cooling water system, efficiently condensed tars and other volatile compounds. The gas collection unit consisted of gas-impermeable bags designed to capture non-condensable gases while preventing leakage and contamination. To determine gas composition, samples were periodically extracted and analyzed via gas chromatography using the GC-2014 (Shimadzu Co., Ltd., Kyoto, Japan).

2.4. Reactor System for CO2 Gasification Experiments

The gas components of pyrolysis and gasification reactions were conducted using a fixed-bed pyrolysis/gasification reactor (Figure 3) and analyzed using GC-2014 (Shimazu, Co., Ltd., Kyoto, Japan). The pyrolysis reaction was executed within an Ar atmosphere, in volving a temperature escalation from room temperature to 900 °C. Subsequently, the gasification process was initiated by switching to a CO2 atmosphere. During the gasification process, a flow meter was used to control the gas flow rate. CO2 was introduced at a flow rate of 100 mL/min, and the temperature was maintained at 900 °C until the reaction was complete.

2.5. Reactor of Two-Stage Tar Reforming

The reactor’s innovative two-stage configuration Figure 4 is designed not only to handle, but also to optimize tar decomposition and gasification, ensuring high efficiency and product quality [35]. Upper Stage: Focused on Tar Decomposition and Reforming. The upper stage of the reactor is dedicated to the critical task of breaking down tar, which is comprised of heavy hydrocarbons that are difficult to decompose under standard conditions. This stage is specifically tailored for thermal decomposition of tar at elevated temperatures, utilizing a highly controlled environment to ensure effective breakdown of these compounds. To further enhance the reforming process, the upper stage is equipped with char catalytic materials, which play a dual role. First, they act as catalysts, accelerating chemical reactions that convert tar into simpler, lighter molecules such as hydrogen, carbon monoxide, and methane. Second, char, which is highly porous and possesses excellent thermal stability, provides an effective surface for hosting reactions. The high temperature maintained in this zone is a vital factor in breaking apart the molecular structure of the heavier hydrocarbons, enabling their conversion into gaseous products. This meticulous design ensures that the majority of tars are decomposed before they can reach or interfere with other parts of the system, addressing one of the most critical challenges in biomass gasification and ensuring smooth downstream operation. Lower Stage: Pyrolysis and Gasification of Biomass. The lower section of the reactor is focused on the efficient pyrolysis and gasification of biomass feedstock, with the goal of converting raw organic material into a syngas—a mixture primarily consisting of hydrogen, carbon monoxide, methane, and lesser quantities of other gases. In this stage, the biomass undergoes pyrolysis, a thermochemical decomposition process that occurs in the absence of oxygen and at moderate temperatures. During pyrolysis, the biomass is broken down into its constituent components: biochar (solid residue), volatile tars, and syngas (a mixture of hydrogen, methane, and carbon monoxide). Following pyrolysis, gasification reactions come into play. This involves introducing limited amounts of a gasifying agent—commonly air, steam, or oxygen—to further react with the pyrolyzed material [37]. The result is a synthesis gas (syngas) enriched in valuable gases such as hydrogen, along with controlled quantities of methane and carbon monoxide. The lower stage is meticulously optimized to maximize this conversion efficiency while minimizing the production of tar and other undesirable byproducts [38]. Importantly, the design facilitates a smooth feed of gas and tar vapors from the lower stage to the upper stage, where any remaining tar is processed and reformed into useful gaseous products [39]. This complementary relationship between the two stages ensures that the entire system operates efficiently and cleanly, with minimal waste and high energy recovery.

3. Results and Discussions

3.1. Metal Analysis of Ca, Mg, K, Na Contents in Bark

The results presented in Table 1 indicate that calcium has the highest concentration, measured at 9.19 weight%, which is significantly greater than the other metals evaluated. Calcium ranged at high concentrations, indicating a potential catalytic advantage in gasification. Calcium can be added to the gasification reaction as a catalyst. These data highlight that calcium is the predominant metal in bark, suggesting its critical role in bark structure or function, while the low levels of magnesium, potassium, and sodium may indicate limited involvement in bark-related gasification processes.

3.2. The Ultimate Sample Analysis

For demineralized bark charcoal, the removal of inorganic impurities further increased the carbon content to 75.01%, 24% higher than untreated bark charcoal (Table 2). Hydrogen and nitrogen contents slightly decreased by 11% and 3%, respectively, while the ash content was significantly reduced by 76% compared to untreated bark charcoal. These results demonstrate the effectiveness of demineralization in reducing ash and enriching carbon content, indicating a highly successful demineralization process.
In contrast, the addition of calcium to bark charcoal induced significant compositional changes. Compared to the demineralized sample, the carbon content decreased to 53.53%, while the ash content increased markedly to 44.38% due to the incorporation of calcium-added additives. These results emphasize the influence of the demineralization and calcium enhancement processes on the sample’s elemental and mineralogical composition, with implications for their potential applications in energy and materials science contexts [40]. Overall, demineralized bark charcoal exhibited the highest carbon content, suggesting that its syngas yield in terms of CO and CO2 production during gasification is likely to be superior.

3.3. Reaction Behaviors with Different Samples at 900 °C

The DTG curves for calcium-added bark, demineralized bark, and untreated bark in Figure 5 and Figure 6 reveal that the gasification behavior of bark is strongly influenced by the presence of calcium. From the temperature-based DTG curve on the left, it is evident that calcium-added bark undergoes gasification at lower temperatures compared to untreated bark, indicating an enhanced reactivity and faster reaction rates due to the catalytic effect of calcium. Conversely, the demineralized bark demonstrates a shift in gasification to higher temperatures, revealing a significant reduction in reaction rates. Additionally, the DTG curve changes for demineralized materials under TG-DTA analysis were found to be negligible. These results confirm that the reactivity of bark and its gasification temperature are substantially affected by calcium content, with calcium acting as a catalyst to promote the decomposition reactions at lower temperatures.

3.4. Gas Amount and Yields

3.4.1. Gas Amount and Yields by CO2 Gasification

The results of the synthesized gas production amount and gas yield ratio, as represented in Figure 7, can be analyzed as follows: The amount of gas production (mmol/g-sample) followed the order demineralized bark char (70.81) > bark char (67.85) > calcium-added bark char (53.08). This trend aligns with the carbon (C) content in the samples, indicating that the carbon content significantly affects the amount of synthesized gas generated. On the other hand, the gas yield (%) did not exhibit a direct correlation with the calcium content in the samples. The gas yield values were bark char (64.39%) > calcium-added bark char (59.49%) > demineralized bark char (56.64%). This suggests that calcium has a limited or negligible impact on improving gas yield.

3.4.2. Gas Amount and Yields by H2O Gasification

The synthesis gas yield and output, as shown in Figure 8, demonstrate that under steam gasification, all tested samples exhibit superior synthesis gas production compared to CO2 gasification. Consistent with the behavior observed in CO2 gasification, the gas yield aligns with the order of carbon content in the samples, with demineralized bark char showing the highest-synthesis gas production. Specifically, steam gasification of demineralized bark charcoal resulted in a 29% increase in syngas production and a 25% increase in yield compared to CO2 gasification of the same sample. Therefore, we conclude that steam gasification is more favorable than CO2 gasification for enhancing the production and yield of syngas from bark biomass in gasification technology.

3.4.3. Gas Amount and Yields by Tar Reforming

Figure 9 illustrates the syngas yield from steam gasification of raw bark, showing the production of H2, CO, CH4, and CO2 as a function of temperature. The yields of these gases peak at different temperatures between 200 °C and 800 °C, followed by a significant decrease above 800 °C, indicating that the gasification process is nearly complete at this stage. At around 800 °C, the decrease in gas production could be attributed to the consumption of char through tar reforming or the depletion of biomass, leading to the initiation of reactions between steam and residual char. Among the four gases, H2 yield is the highest under steam gasification conditions.
Figure 9 and Figure 10 compare the yields of syngas components (CO, H2, CH4, and CO2) under different gasification conditions in a metallic reactor. Figure 9 depicts the temperature-dependent evolution of syngas from bark gasification, whereas Figure 10 presents the time-dependent syngas generation from the gasification of bark-derived char. In Figure 9, syngas production is influenced by temperature. H2 shows a pronounced peak at around 600 °C, indicating that high temperatures are favorable for hydrogen generation; this is likely due to the enhanced decomposition of volatile matter and water–gas reaction at elevated temperatures. CO production begins to rise around 400 °C, peaking slightly after H2, suggesting its formation is associated with secondary reactions such as the Boudouard reaction. CH4 production is minimal, with a small peak at lower temperatures, indicating its formation primarily during the volatilization stage. In contrast, Figure 10 illustrates that the gasification of bark char primarily yields H2 and CO, with their peaks appearing at approximately 15 min into the reaction. The dominance of these two gases is indicative of char’s reaction with steam and CO2 via water–gas and Boudouard reactions. Notably, H2 production surpasses that of CO, suggesting that the water–gas shift reaction may also be significant. CH4 and CO2 yields are relatively low, with their contributions tapering off quickly, implying limited volatilization and oxidation processes during char gasification.
The results of syngas yield illustrated in Figure 11 and Figure 12 demonstrate the gasification behavior in a two-stage metal reactor where the lower stage contains bark feedstock, and the upper stage utilizes bark-derived char as a catalyst. As shown in Figure 11, syngas production varies with temperature, with H2 and CO yields showing significant increases as temperature rises, showing two distinct peaks: the first between 400–600 °C and the second between 700–800 °C. The maximum yields are observed near 800 °C, indicating the enhancement of gas-phase reactions such as tar cracking, steam reforming, and the Boudouard reaction. Beyond 800 °C, the decrease in gas production could be attributed to the consumption of char through tar reforming or the depletion of biomass, leading to the initiation of reactions between steam and residual char. The yields of CH4 and CO2 are generally lower, reflecting their consumption in secondary reactions. As shown in Figure 12, the syngas yield as a function of reaction time reveals dynamic progression. H2 and CO production increases steadily, peaking at approximately 80–100 min, corresponding to the main gasification phase where the interaction between volatile compounds and catalytic char facilitates efficient gas conversion [41]. The subsequent decline in gas yields is likely due to the depletion of biomass and char in the system. These results highlight the catalytic activity of bark char in promoting efficient gasification and the temperature- and time-dependent behavior of gas production.
We show in Table 3 that the syngas yield from the steam gasification of 1 g of raw bark in a glass reactor was generally lower than that from the steam gasification of 1 g of bark char under the same conditions. Steam gasification of bark char produced 142% more syngas compared to raw bark, with H2 yield increasing by 86% and CO yield increasing by 250%. These results indicate that bark char is a more suitable feedstock for gasification experiments than raw bark. However, it is important to note that this advantage comes with an increased financial input due to the additional costs associated with the second stage of the process.
Additionally, in an ideal scenario, the syngas yield from the two-stage metal tube reactor designed for tar reforming could be considered the sum of the syngas yields from the steam gasification of 1 g of raw bark and 1 g of bark char. However, due to potential minor leakage in the two-stage metal reactor, the calculated syngas yield appeared slightly lower than the combined yields of raw bark and bark char gasification. Nevertheless, the syngas yield from the two-stage metal tube reactor was 200% higher than that of raw bark gasification and 24% higher than that of bark char gasification. Therefore, we confirmed the feasibility of using the two-stage metal tube reactor for tar reforming to enhance syngas production in steam gasification processes.

4. Conclusions

This study examines the steam gasification and tar reforming of Keyaki bark, a waste biomass prevalent in Saitama Prefecture, highlighting its potential as a renewable energy feedstock. Demineralization and thermogravimetric analyses provided key insights into optimizing gasification conditions, with the highest syngas yields observed at 900 °C. The two-stage tar reforming process has demonstrated significant potential in enhancing the practicality of industrial-scale biomass gasification. By effectively reducing tar content, this approach improves the quality and usability of the produced syngas, making it more suitable for downstream applications such as power generation, synthetic fuel production, and chemical synthesis. In industrial biomass gasification, high tar content remains a major challenge, leading to operational issues such as equipment fouling, pipeline clogging, and catalyst deactivation. The two-stage reforming process addresses this by integrating primary and secondary reforming stages to achieve more complete tar breakdown. The primary stage typically involves thermal cracking or catalytic conversion at moderate temperatures to decompose heavy tar compounds, while the secondary stage further reforms residual tar using advanced catalysts or high-temperature oxidation. The implementation of this process at an industrial scale enhances gasifier efficiency, reduces maintenance costs, and extends equipment lifespan. Moreover, the improved syngas quality enhances its viability for gas engines, turbines, and synthesis processes, increasing the economic and environmental feasibility of biomass gasification technologies. Overall, two-stage tar reforming represents a crucial advancement in biomass gasification, offering a practical solution for large-scale applications by optimizing syngas purity and ensuring stable operation in industrial settings.

Author Contributions

Conceptualization, S.W.; methodology, S.W.; software, S.W.; validation, S.W.; formal analysis, S.W.; investigation, S.W.; resources, S.W.; data curation, S.W. and I.R.; writing—original draft preparation, S.W.; writing—review and editing, S.W. and Q.W.; visualization, S.W.; supervision, Q.W.; project administration, Q.W.; and funding acquisition, Q.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially supported by the Basic Research (B) (Number. 22H03747, FY2022-FY2024) of the Grant-in-Aid for Scientific Research of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We also thank the Comprehensive Analysis Center for Science, Saitama University for allowing us to conduct some analyses and providing insight and expertise that greatly assisted the research.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are provided in the manuscript.

Acknowledgments

The authors would like to sincerely thank Weiqian Wang for their invaluable support and guidance throughout this study. Their insightful feedback and expertise greatly contributed to the improvement of this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Scheme of the catalyst addition method.
Figure 1. Scheme of the catalyst addition method.
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Figure 2. Reactor system for steam gasification experiments.
Figure 2. Reactor system for steam gasification experiments.
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Figure 3. Reactor system for CO2 gasification experiments.
Figure 3. Reactor system for CO2 gasification experiments.
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Figure 4. Reactor system for tar reforming.
Figure 4. Reactor system for tar reforming.
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Figure 5. Reaction behaviors with different samples at different times.
Figure 5. Reaction behaviors with different samples at different times.
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Figure 6. Reaction behaviors with different samples at 900 °C.
Figure 6. Reaction behaviors with different samples at 900 °C.
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Figure 7. Gas amount and yields by CO2 gasification.
Figure 7. Gas amount and yields by CO2 gasification.
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Figure 8. Gas amount and yields by steam gasification.
Figure 8. Gas amount and yields by steam gasification.
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Figure 9. Gas amount of bark with temperature.
Figure 9. Gas amount of bark with temperature.
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Figure 10. Gas amount of bark char with time.
Figure 10. Gas amount of bark char with time.
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Figure 11. Gas amount by tar reforming with temperature.
Figure 11. Gas amount by tar reforming with temperature.
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Figure 12. Gas amount by tar reforming with time.
Figure 12. Gas amount by tar reforming with time.
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Table 1. Metal analysis of Ca contents in bark.
Table 1. Metal analysis of Ca contents in bark.
Ca (Weight %)Mg (Weight %)K (Weight %)Na (Weight %)
Bark9.190.170.040.15
Table 2. The ultimate sample analysis.
Table 2. The ultimate sample analysis.
Ultimate Analysis (Weight %)
CHNOAsh
Bark Raw44.505.810.2538.6910.76
Bark Char60.411.220.941.7635.67
Demineralized Bark Char75.011.090.9114.318.68
Calcium-added Bark Char53.531.060.850.1844.38
Table 3. Comparison of bark raw/char/bark raw and char total/tar reform gasification.
Table 3. Comparison of bark raw/char/bark raw and char total/tar reform gasification.
H2 (mmol/g)CO (mmol/g)CH4 (mmol/g)CO2 (mmol/g)Syngas (mmol/g)
Bark Raw Gasification14.827.610.604.6322.43
Bark Char Gasification27.6026.640.853.0254.24
Bark Raw and Char Total42.4234.251.447.6576.67
Tar Reform Gasification40.9026.281.6412.1367.19
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Wu, S.; Wang, Q.; Ryota, I. Study on the Effects of Tar Reforming and Steam Gasification of Keyaki Bark in Saitama Prefecture. Sustainability 2025, 17, 2215. https://doi.org/10.3390/su17052215

AMA Style

Wu S, Wang Q, Ryota I. Study on the Effects of Tar Reforming and Steam Gasification of Keyaki Bark in Saitama Prefecture. Sustainability. 2025; 17(5):2215. https://doi.org/10.3390/su17052215

Chicago/Turabian Style

Wu, Shangrong, Qingyue Wang, and Isobe Ryota. 2025. "Study on the Effects of Tar Reforming and Steam Gasification of Keyaki Bark in Saitama Prefecture" Sustainability 17, no. 5: 2215. https://doi.org/10.3390/su17052215

APA Style

Wu, S., Wang, Q., & Ryota, I. (2025). Study on the Effects of Tar Reforming and Steam Gasification of Keyaki Bark in Saitama Prefecture. Sustainability, 17(5), 2215. https://doi.org/10.3390/su17052215

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