Catalytic sorption-enhanced steam gasification of medium-density fiberboard (MDF) sludge for hydrogen production

Medium-density fiberboard (MDF) sludge is a pasty byproduct of the wood manufacturing industry generated during washing or filtration processes and poses significant environmental challenges. This study investigates the valorization of this material through catalytic sorption-enhanced steam gasification (SESG) in a fixed-bed reactor, integrating catalytic gasification with in situ CO 2 capture to produce hydrogen-rich gas. The effects of key operating parameters, including weight hourly space velocity (WHSV) (0.5–2.7 h -1), temperature (600 °C–650 °C), and steam-to-carbon (S/C) ratio (1.2–14 × stoichiometric), were systematically evaluated. Temperature strongly influenced gasification performance and CO 2 capture efficiency, with 650 °C identified as the optimal condition, yielding up to 88.9 vol% H 2. Variations in WHSV had a limited effect on hydrogen yield, indicating stable process performance across the investigated range and suggesting the potential to reduce reactor volumes without compromising conversion. H 2 yield increased with steam addition, reaching a maximum of 55.4% and an H 2 purity of 84.7 vol% at an S/C ratio of 8 × stoichiometric. Further increases in steam resulted in lower efficiency, attributed to reduced CO 2 capture due to shorter residence times and dilution effects. While elevated steam demand may introduce energy penalties, the results demonstrate that catalytic SESG is an effective route for hydrogen production from industrial waste. Under the identified optimal conditions (650 °C, S/C = 8 × stoichiometric, WHSV = 0.6 h -1), stable performance was achieved despite the heterogeneous nature of MDF sludge, highlighting its viability as a feedstock for low-carbon hydrogen production.

Medium-density fiberboard (MDF) is produced by compressing wood fibers with a resin binder under heat and pressure. It is widely used in construction, furniture, interior design, and packaging due to its resilience, uniform density, and cost-effectiveness. Global MDF production exceeds 120 million m 3 annually, with a market value of USD 46.6 billion in 2025, projected to reach USD 50,691.6 billion in 2035 (Mordor Intelligence, 2026; Towards Packaging). However, a significant drawback of MDF manufacturing is the generation of substantial waste streams, particularly MDF sludge. Current disposal practices for wood industry wastes, mainly incineration or landfilling, pose environmental challenges and contribute to greenhouse gas emissions, highlighting the urgent need for sustainable valorization routes (Kalak, 2023).

At the same time, global energy demand exceeded 650 EJ in 2024, with the energy sector remaining a primary source of CO 2 emissions (International Energy Agency, 2025a). Hydrogen has emerged as a promising clean energy carrier due to its high energy density and water-only combustion by-product (Wu et al., 2021). Global hydrogen production reached almost 100 Mt in 2024, up 2% from 2023. However, its production remains largely derived from fossil-based routes such as steam methane reforming and coal gasification, generating approximately 980 Mt of CO 2 in 2024 (International Energy Agency, 2025b). The strong carbon dependence of current hydrogen production underscores the need for renewable, low-carbon hydrogen pathways.

In this context, thermochemical conversion of biomass integrated with carbon capture, utilization, and storage (CCUS) technologies offers a promising strategy to decarbonize both the wood-processing industry and the energy sector (V Hanssen et al., 2020). In particular, biomass gasification coupled with in situ CO 2 capture can produce high-purity hydrogen while potentially achieving net-negative emissions (Rosa and Mazzotti, 2022). Consequently, MDF sludge represents not only a waste management challenge but also a valuable feedstock for sustainable hydrogen production. In the gasification process, biomass reacts with a gasifying agent (typically steam) at elevated temperatures to produce a syngas mixture mainly composed of H 2, CO, CH 4, and CO 2. The primary gasification reaction governs the process (Equation 1), producing H 2 and CO (Fermoso et al., 2012), and the water–gas shift (WGS) reaction (Equation 2), which converts CO into additional H 2 and CO 2 (Ebrahimi et al., 2020).

Hydrogen yield and purity can be significantly enhanced by integrating gasification with in situ CO 2 capture, in a process known as sorption-enhanced steam gasification (SESG). Various solid sorbents have been explored in sorption-enhanced processes, including alkaline ceramics, hydrotalcites, zeolites, resins, and calcium-based materials (Wang et al., 2021; Dou et al., 2016; Cai et al., 2024; Iliuta, 2017). Among them, CaO-based sorbents are particularly attractive due to their low cost, rapid carbonation kinetics, and natural abundance (Farooqi et al., 2025; Guo et al., 2024; Shokrollahi Yancheshmeh et al., 2016). The carbonation reaction (Equation 3) removes CO 2 from the gas phase, shifting the equilibrium of the gasification and WGS reactions toward higher hydrogen production, in accordance with Le Chatelier’s principle (Rydén and Lyngfelt, 2006).

Nickel-based catalysts are commonly used in reforming processes due to their high activity and affordability (Masoudi Soltani et al., 2021). However, they suffer from coke deposition, which progressively deactivates the catalyst. The addition of Co enhances resistance to coking and improves long-term stability (He L. et al., 2009), while small amounts of noble metals (e.g., Pd) further increase reducibility and thermal stability (Fermoso et al., 2014). Previous studies have shown that a Pd/Ni–Co hydrotalcite-like (HT) catalyst is highly effective for the sorption-enhanced reforming of biomass-derived feedstocks (Gil et al., 2016; Noor et al., 2014; García et al., 2021; Capa et al., 2020; Esteban-Díez et al., 2016). These studies have also demonstrated the beneficial effect of in situ CO 2 capture using CaO-based sorbents.

Despite substantial research on biomass gasification, investigations into MDF sludge as a feedstock remain limited. Işık et al. reported the gasification of MDF residue using oxygen as the gasifying agent. However, this material is a dry byproduct with low moisture content, typically generated during mechanical processing steps, and the process yielded relatively low H 2 purities (7.9%–9.7%, up to 20.4% on an N 2 -free basis) (Işık et al., 2023). In contrast, MDF sludge is a wet, pasty byproduct originating from washing or filtration processes, which presents additional challenges for thermochemical conversion but also remains largely unvalorized.

To the best of the authors’ knowledge, no previous studies have investigated the catalytic SESG of MDF sludge (MDFS-SESG), in which steam is used as the gasifying agent in combination with a catalyst and a CO 2 sorbent to enable in situ CO 2 capture and equilibrium shifting toward hydrogen production. This integrated approach represents a significant departure from conventional gasification processes and offers the potential for substantially enhanced hydrogen yields and purity. Accordingly, this study investigates the direct valorization of MDF sludge for renewable hydrogen production via catalytic SESG. Sensitivity analyses were performed to evaluate the effects of key operating parameters, including space velocity, temperature, and the steam-to-carbon ratio, on hydrogen yield, purity, and selectivity, demonstrating the feasibility and potential of integrating catalytic gasification with in situ CO 2 capture for the efficient valorization of industrial waste streams.

Biomass. MDF sludge generated during panel manufacturing was provided by Industrias del Tablero S.A. (INTASA) (A Coruña, Spain) (Industrias del Tablero S.A.). The raw residue presented a moisture content of 80.9 wt%. To ensure stable feeding and reproducible operation under the laboratory-scale conditions employed, the material was oven-dried at 110 °C for 24 h before the experiments, then ground in a Retsch mill, and sieved to obtain particles between 0.6 and 1.0 mm. The physicochemical characterization of the dried sludge was performed in accordance with ASTM standards (Table 1). Proximate analysis (moisture, ash, and volatile matter) was determined using a LECO TGA701 (ASTM D7582), and fixed carbon was calculated by difference. Ultimate analysis (C, H, and N) was carried out with a LECO CHN-2000 (ASTM D5373), while sulfur content was determined with a LECO S-144DR (ASTM D4239). Oxygen was obtained by difference. The higher heating value was measured using an IKA-Werke C400 adiabatic bomb calorimeter (ASTM D5865). Based on the ultimate analysis, the stoichiometric reaction for MDF sludge sorption-enhanced steam gasification (MDFS-SESG) (Equation 4) was derived as:

Compared to conventional lignocellulosic biomass, MDF sludge presents a higher ash content and, before drying, higher moisture levels, which may affect handling and processing. On a dry basis, its elemental composition and heating value are comparable to those of typical woody biomass, supporting its suitability for thermochemical conversion. Unlike dry MDF residues (e.g., sanding dust), MDF sludge is a pasty material with higher inorganic content, which may pose additional operational challenges. Nevertheless, its abundance and limited current valorization routes make it a promising feedstock for hydrogen production via gasification.

Catalyst. A Pd–Ni–Co hydrotalcite-like catalyst (1 wt% Pd, 20 wt% Ni, 20 wt% Co) was synthesized following the procedure described by Fermoso et al. (2014). The precursor material, Ni–Co HT, was prepared via co-precipitation, while Pd was added by incipient wetness impregnation. The catalyst was sieved to achieve a particle size of 212–500 μm. SEM-EDX analysis (Quanta FEG 650) confirmed the expected composition: 0.96% Pd, 21.73% Ni, and 21.21% Co. Nitrogen physisorption (Micromeritics ASAP 2420) at −196 °C showed a BET surface area of 150 m 2 g -1, a pore volume of 0.63 cm 3 g -1, and pore diameters in the 5–25 nm range.

Sorbent. Arctic dolomite supplied by Franzefoss Minerals A.S. (Norway) was used as a CaO precursor for CO 2 capture. X-ray fluorescence analysis showed a composition of 98.5 wt% CaMg(CO 3) 2 with no detectable sulfur. Before use, the material was sieved to 212–500 μm and calcined in air at 800 °C for 6 h (5 °C min -1). The calcined sorbent showed a BET surface area of 29 m 2 g -1 and a pore volume of 0.28 cm 3 g -1. The maximum CO 2 capture capacity of the fresh material was 0.46 g CO 2 g sorbent -1.

Experimental equipment. Experiments were conducted using a PID Eng&Tech fixed-bed system (Figure 1). MDF sludge was loaded into a nitrogen-purged hopper coupled with a 3/8″OD automated valve, allowing semi-continuous solid feeding. The material was discharged into a stainless-steel tubular reactor (2.15 cm i. d., 29 cm length) containing 1 g of catalyst and 10 g of sorbent. The reactor temperature was monitored using a K-type thermocouple positioned in the catalytic bed and controlled by an electric furnace enclosed in a hot box at 200 °C. TOHO TTM-005 PID controllers controlled the temperatures of both the reactor and the hot box. Steam was generated by vaporizing deionized water using a Gilson® 307 HPLC pump. N 2, O 2, and H 2 were fed through Bronkhorst® mass flow controllers.

Experimental procedure. Before each run, the catalyst bed was reduced under a 50/50 (vol%) H 2 /N 2 atmosphere at 670 °C for 10 h (2 °C min -1, 100 NmL min -1) (Fermoso et al., 2014). MDF sludge was fed semi-continuously using an automated valve. The feeding system was adjusted to achieve a target feeding rate of 10 mg min -1. The actual biomass feed rate for each experiment was determined from the difference between the initial and final biomass mass in the hopper and ranged from 8.4 to 11.8 mg min -1. On average, approximately 900 mg of biomass was fed per experiment. This feeding strategy enabled a pseudo-steady feeding regime. While this intermittent operation inherently introduces short-term fluctuations in the instantaneous biomass feed rate, stable average operating conditions were achieved. The MDFS-SESG process was initiated under the desired conditions of temperature, steam-to-carbon (S/C) ratio, and weight hourly space velocity (WHSV). A nitrogen flow (25 NmL min -1) was used as carrier and tracer gas. WHSV was defined as the ratio of the mass flow rate of MDF sludge to the mass of catalyst used (Equation 5). The S/C ratio was defined as the molar ratio of water to carbon in the feed (Equation 6). To facilitate comparison across different operating conditions, the S/C ratio was normalized to the stoichiometric value, giving the normalized S/C ratio, S/C norm (Equation 7). All experiments were carried out at atmospheric pressure.

After each test, the sorbent was regenerated at 775 °C under synthetic air (200 NmL min -1; 79 vol% N 2, 21 vol% O 2) until complete decarbonation, producing a CO 2 -enriched gas stream. The experimental conditions for all the experiments are summarized in Table 2.

Gas analysis. The reactor effluent passed through a thermoelectric condenser to remove excess steam and other condensable compounds, followed by a manganese perchlorate drying column for further moisture removal. Although a condensable fraction was collected downstream, it consisted predominantly of water with only trace amounts of highly diluted organic compounds. Due to the absence of a dedicated tar sampling and analysis system, reliable quantification of tar content was not feasible within the experimental setup used. Although a complete mass balance closure was not achievable due to the unquantified condensable fraction, the continuous measurement of the main gaseous products provides a consistent and representative partial carbon and hydrogen balance for the gas phase. The dry gas composition was measured online every 2.8 min using a dual-channel Varian® CP-4900 Micro-GC equipped with Molsieve 5 Å and PoraPlotQ (PPQ) columns and a thermal conductivity detector (TCD). Helium was used as the carrier gas. H 2, CO, CO 2, and CH 4 gases were quantified using calibration curves obtained with fixed gas mixtures.

Data processing. The composition of the product gas was calculated based on the nitrogen-free, dry composition of the gas effluent. The gas flow rates of the species generated during the experiments were then estimated using a nitrogen balance. Process performance was evaluated based on H 2 yield, H 2 selectivity, and outlet gas composition, calculated using Equations 8–10.

F i is the molar flow rate of species i in the outlet gas, F MDFS is the molar flow rate of MDF sludge fed to the reactor, and n is the stoichiometric number of moles of H 2 produced per mole of MDF sludge (10.37 mol H 2 per mol sludge, as derived from Equation 4). H 2 yield quantifies the fraction of hydrogen obtained relative to the theoretical stoichiometric maximum. In contrast, H 2 selectivity refers to the proportion of hydrogen atoms appearing as molecular H 2 relative to those in H 2 and CH 4. This definition excludes hydrogen atoms retained in unreacted steam or in hydrocarbons that subsequently condense, if formed. From the gaseous emissions results, the gas yield and lower heating value of the gas obtained (LHV gas) (He M. et al., 2009) were determined using Equations 11–13:

Q outlet-gas is the volumetric flow of the outlet gas (Nm 3 h -1), obtained by applying a balance to the inert gas (N 2), m biomass is the biomass inlet mass flow on a dry basis (kg h -1), and x i (vol%) represents the volumetric percentage of each component in the dry product gas.

Effect of the weight hourly space velocity (WHSV)

In this study, the effect of space velocity was investigated using the weight hourly space velocity (WHSV). Experiments were performed over a WHSV range of 0.5–2.7 h -1 at 625 °C, while maintaining comparable reactant ratios (S/C norm = 1.4–1.6) (Exps. 1–5 in Table 2). The WHSV range was selected based on values commonly reported for SESR processes, while also considering the operational limits of the experimental setup, which restrict the exploration of more extreme conditions. Space velocity is a key operating parameter in thermochemical conversion processes, as it determines the effective contact time between reactants and the catalyst. WHSV is inversely related to the gas–solid residence time, such that lower WHSV values correspond to longer residence times and an enhanced approach to equilibrium, resulting in higher conversion. Conversely, higher WHSV values reduce residence time, increasing throughput and enabling smaller reactor volumes, but potentially limiting conversion efficiency. Therefore, optimizing WHSV is essential to balance reaction kinetics, equilibrium constraints, and overall process productivity.

As shown in Figure 2a, variations in WHSV had no significant impact on H 2 yield within the investigated range, which remained close to 39%. A modest decrease in outlet H 2 concentration was observed with increasing WHSV, declining from 85.6 vol% at 0.5 h -1 to 80.9 vol% at 2.7 h -1 (Figure 2b). This can be attributed to minor changes in the relative distribution of gaseous products. Higher WHSV (shorter residence times) leads to small increases in CH 4, CO, and CO 2 concentrations, slightly diluting the hydrogen fraction. These variations suggest that while the main reactions proceed to a similar extent, shorter contact times may slightly limit the conversion of intermediate species through reforming and water–gas shift reactions. This trend is consistent with previous non-catalytic SESG studies using different biomass feedstocks. For example, Schmid et al. (Schmid et al., 2021) reported only minor reductions in outlet H 2 concentration when increasing the WHSV from 0.3 to 1.3 h -1 (expressed as fuel per bed mass) during steam–oxygen gasification of sewage sludge. Similarly, Roche et al. (Roche et al., 2014) observed comparable behavior under both air and air–steam atmospheres, with and without dolomite addition.

Across the tested WHSV range, overall process performance remained stable, with an average H 2 yield of 38.5% ± 1.4% and an H 2 selectivity of 74.3% ± 1.8%. The outlet gas composition also showed limited variation, averaging 83.5 ± 1.9 vol% H 2, 12.3 ± 1.4 vol% CH 4, 2.5 ± 1.2 vol% CO, and 1.2 ± 0.2 vol% CO 2 (Figure 2b). The stability of both yield and gas composition suggests that, under the studied conditions, reaction rates are sufficiently fast to sustain consistent conversion despite changes in residence time, with no evident kinetic or mass transfer limitations within the investigated WHSV range.

The weak dependence of process performance on WHSV indicates that the system likely operates close to thermodynamic equilibrium at 625 °C. Gas–solid contact times appear adequate, even at the highest WHSV tested, for the main gasification and water–gas shift reactions to proceed effectively. The nearly constant CH 4 concentration across the WHSV range supports this interpretation, suggesting that methanation and reforming reactions are governed primarily by equilibrium constraints rather than kinetic limitations. In addition, the consistently low CO 2 concentrations indicate that carbonation of the sorbent remains effective and is not adversely affected by reduced residence times, implying rapid CO 2 capture kinetics relative to gas-phase transport processes.

From a process design perspective, the limited sensitivity of hydrogen yield and purity to WHSV variations highlights the robustness of the catalytic MDFS–SESG system under the conditions studied. Higher space velocities could therefore be considered to reduce reactor volume or capital costs without significantly compromising hydrogen production, provided that near-equilibrium operation is maintained. Conversely, operation at lower WHSV values may offer marginal benefits in terms of residence time or sorbent utilization but does not substantially improve hydrogen yield. In addition, the limited sensitivity of hydrogen yield and purity to WHSV variations suggests a kinetically robust process that can accommodate moderate fluctuations in biomass feed rate, an essential advantage for scale-up, where feedstock heterogeneity and nonuniform flow patterns can be challenging to avoid. Overall, WHSV can be optimized primarily based on engineering and economic criteria, such as throughput, reactor sizing, and sorbent management, without detriment to process performance within the evaluated range.

Effect of temperature

Temperature is a key operating variable in SESG, as it directly influences gasification kinetics, reaction equilibria, and the sorbent’s CO 2 capture capacity (Teixeira et al., 2022). Increasing temperature generally enhances carbon conversion and reaction rates, favoring hydrogen production through endothermic gasification and reforming reactions. In contrast, the exothermic carbonation of CaO is thermodynamically favored at lower temperatures. Additionally, elevated temperatures may also promote sintering of CaO-based sorbents, negatively affecting their long-term stability. Consequently, selecting an optimal operating temperature requires balancing gasification efficiency and sorbent performance. In non-catalytic SESG systems operated at atmospheric pressure, optimal temperatures are typically reported to be 600 °C–700 °C (Teixeira et al., 2022; Guoxin and Hao, 2009; Li et al., 2023a). In this study, the effect of temperature on catalytic MDFS-SESG was evaluated at 600, 625, and 650 °C, while maintaining comparable operating conditions (WHSV = 0.50–0.67 h -1 and S/C norm = 1.2–1.6; Exps. 1, 6, and 7 in Table 2).

As shown in Figure 3a, increasing temperature led to a clear improvement in hydrogen production, with H 2 yield rising from 35.3% at 600 °C to 41.5% at 650 °C. H 2 selectivity and outlet H 2 concentration followed a similar trend, reaching 84.5% and 88.9 vol%, respectively, at 650 °C (Figures 3a,b). The increase in H 2 purity is primarily associated with a marked decrease in CH 4 concentration as temperature increases (Figure 3b), indicating enhanced methane reforming and cracking reactions. This decrease in CH 4 concentration is consistent with the temperature dependence of methane reforming (CH 4 + H 2 O ↔ CO + 3H 2), whose equilibrium conversion and kinetics are strongly favored at higher temperatures. The slight variations in CO and CO 2 concentrations can be explained by the combined effects of the water–gas shift reaction and the CaO carbonation equilibrium. While increasing temperature enhances reforming and gasification reactions, it simultaneously shifts the carbonation equilibrium toward CO 2 release, leading to the observed increase in CO 2 concentration. These results indicate that, at higher temperatures, the system approaches thermodynamic equilibrium, with reaction behavior increasingly governed by equilibrium constraints rather than kinetic limitations.

In contrast, CO 2 capture capacity decreased moderately with increasing temperature, as evidenced by an increase in outlet CO 2 concentration from 0.4 vol% at 600 °C to 1.7 vol% at 650 °C (Figure 3b). This behavior is consistent with Le Chatelier’s principle, whereby higher temperatures shift the carbonation equilibrium away from CaCO 3 formation. Nevertheless, the relatively low CO 2 concentrations observed across the temperature range indicate that the sorption-enhanced mechanism remained effective, enabling the production of high-purity hydrogen. From a practical standpoint, operation at approximately 650 °C represents a favorable compromise, maximizing H 2 yield and selectivity while maintaining acceptable CO 2 capture performance.

These observations are in good agreement with previous SESG studies. Magoua Mbeugang et al. (2021) identified 650 °C as the optimal temperature for hydrogen production from cellulose using CaO as a CO 2 sorbent, while Fermoso et al. (2012) reported effective hydrogen generation from chestnut wood sawdust within the 500 °C–650 °C range. Similarly, Parvez et al. (2021) and Fuchs et al. (2019) found that temperatures between 625 °C and 650 °C maximize hydrogen concentration in non-catalytic biomass SESG. At higher temperatures, these authors observed increased CO 2 and CO concentrations accompanied by reduced hydrogen purity, reflecting the diminished carbonation driving force. Together, these findings highlight the intrinsic trade-off in sorption-enhanced processes: higher temperatures favor endothermic hydrogen-producing reactions but simultaneously reduce CO 2 capture efficiency.

Temperature selection can also have important implications for thermal management at larger scales. Operation near 650 °C not only optimizes hydrogen yield but also promotes a favorable balance between endothermic reforming reactions and the exothermic carbonation of CaO, which provides partial in situ heat compensation. This intrinsic thermal coupling can help moderate temperature gradients and reduce external heat input requirements compared to conventional steam gasification. Accordingly, scale-up strategies should consider integrated heat management approaches, such as internal heat circulation, efficient heat recovery, and steam generation, to maintain stable operation, preserve sorbent performance, and minimize external energy demand.

Effect of steam-to-carbon ratio

The steam-to-carbon (S/C) molar ratio is a key parameter in SESG, as it directly affects gasification reactions, reaction equilibria, and CO 2 capture efficiency. In this study, the normalized S/C ratio (S/C norm) was used (Equation 7). Experiments were carried out at the previously identified optimal temperature (650 °C) and at a WHSV ≈0.5–0.6 h -1, with S/C norm values ranging from 1.2 to 14 times the stoichiometric requirement (Exps. 7–12 in Table 2).

As shown in Figure 4a, increasing the S/C ratio initially enhanced H 2 yield, reflecting that the greater steam availability promotes steam gasification (Equation 1) and the water–gas shift (WGS) reaction (Equation 2). Higher steam concentrations also favor methane reforming, leading to a reduction in CH 4 concentration (Figure 4b) and a corresponding increase in H 2 yield (García et al., 2021; Li et al., 2023b). A maximum H 2 yield of 55.4% was achieved at an S/C ratio of 8 times the stoichiometric value. Further increases in steam resulted in a decline in performance, consistent with trends reported by Zou et al. (2024) for SESG of corn cob. At elevated S/C ratios, the outlet CO 2 concentration increased markedly (Figure 4b), indicating a reduction in CO 2 capture efficiency and explaining the observed decrease in H 2 concentration. This decline remained moderate because, although CO and CO 2 concentrations increased, enhanced methane reforming under high steam conditions partially offset the loss in sorption efficiency by reducing CH 4 levels.

Several factors may contribute to the decrease in performance at high S/C ratios. First, although bulk hydration of CaO to Ca(OH) 2 is thermodynamically unstable at 650 °C (Lin et al., 2009), transient surface hydroxylation of CaO in the presence of excess steam cannot be excluded. While limited Ca(OH) 2 formation has been reported to temporarily enhance sorbent reactivity by increasing pore accessibility (Liu et al., 2018), prolonged exposure to high steam concentrations may accelerate sintering and densification, reducing the available active surface area and adversely affecting sorbent durability (Rong et al., 2013).

Second, increasing the S/C ratio substantially raises the total volumetric gas flow. At the lowest S/C tested (S/C norm = 1.2), the outlet flow rate was approximately 48 NmL min -1, whereas at S/C norm = 14, it increased to about 298 NmL min -1. This increase significantly reduces gas residence time within the catalytic bed, limiting the extent to which gasification and carbonation reactions can proceed and shifting the system away from near-equilibrium operation toward kinetically constrained behavior.

Third, the significant increase in steam fraction, from approximately 48% at S/C norm = 1.2 to 92% at S/C norm = 14, results in dilution of CO 2 and other reactive gases. Since carbonation is driven by CO 2 partial pressure, such dilution weakens the driving force for CO 2 capture. Excessive dilution may also shift gasification equilibria away from optimal hydrogen production, further reducing process efficiency.

Overall, while increasing the S/C ratio initially enhances hydrogen production, excessive steam addition may negatively affect sorbent performance, reduce residence time, and dilute reactive gases, ultimately lowering conversion efficiency. An optimal balance was observed at S/C norm = 8, which maximized H 2 yield while limiting the adverse effects associated with high steam flow rates and reduced CO 2 capture efficiency. These results indicate that the optimal S/C norm value of 8 represents a balance between enhanced reforming and water–gas shift reactions and the adverse effects of excessive steam, including CO 2 dilution and reduced carbonation driving force. At high steam concentrations, dilution and increased gas flow not only weaken CO 2 capture efficiency but also reduce effective residence time, shifting the system away from near-equilibrium conditions and limiting overall process performance. These results emphasize that overfeeding steam does not necessarily lead to improved hydrogen production and can instead compromise both process efficiency and sorbent durability.

From a process design perspective, selecting the S/C ratio has significant energy and economic implications. Although S/C norm = 8 yields the highest H 2 production under the studied conditions, supplying this excess steam may significantly increase parasitic energy demand associated with water evaporation and steam superheating, which is typically one of the main energy penalties in steam gasification systems. High steam fractions may also increase the load on downstream condensation and gas purification units. Consequently, the optimal S/C ratio for industrial applications is likely to represent a compromise between hydrogen yield, sorbent durability, energy consumption, and overall system integration. Heat integration is therefore crucial in sorption-enhanced processes to improve overall energy efficiency, as demonstrated in process simulation studies reported in the literature (Yan et al., 2020; Capa et al., 2023).

Gas yield and heating value of the product gas under optimal MDFS-SESG conditions

Table 3 summarizes the main efficiency indicators obtained for all the conducted MDFS-SESG experiments. The results show that under the optimal operating conditions identified in this study (Exp. 10; 650 °C, S/C norm = 8, and WHSV = 0.62 h -1), the catalytic MDFS-SESG process achieved a H 2 yield of 55.4% (Figure 4a) and a H 2 purity of 84.7% (Figure 4b), confirming the favorable balance among gasification, reforming, and in situ CO 2 capture.

The gas yield, defined as the volume of dry, N 2 -free product gas per unit mass of biomass, is strongly influenced by operating temperature and steam-to-biomass ratio (Wei et al., 2007). In steam–oxygen gasification of sewage sludge at 650 °C, Schmid et al. (2021) reported gas yields below 1.1 Nm 3 kg -1, increasing to approximately 1.45 Nm 3 kg -1 at 900 °C in the absence of a CO 2 sorbent. Similarly, He et al. (2009b) obtained 0.51 Nm 3 kg -1 during steam gasification of municipal solid waste (MSW) at 900 °C, which increased to 1.65 Nm 3 kg -1 with the addition of calcined dolomite as a catalyst. When limestone was employed as a CO 2 sorbent, gas yield increased from 0.55 to 0.95 Nm 3 kg -1 for steam gasification of wood pellets between 640 °C and 711 °C (Martínez et al., 2022), and from 0.38 to 0.62 Nm 3 kg -1 for MSW gasification between 630 °C and 700 °C (Martínez et al., 2020). In comparison, the present catalytic MDFS-SESG process achieved a gas yield of 1.55 Nm 3 kg -1 at only 650 °C (Exp. 10, Table 3). Although direct comparisons are limited by differences in feedstock composition and reactor configuration, this relatively high gas yield at moderate temperature indicates a strong synergistic effect between the Pd–Ni–Co catalyst and the dolomite-based CaO sorbent. Catalytic reforming and water–gas shift reactions are effectively coupled with in situ CO 2 capture, shifting reaction equilibria toward gas production and enabling high conversion without the need for the elevated temperatures typically required in conventional or non-catalytic gasification. The cold gas efficiency (CGE) follows a similar trend as the gas yield, indicating that the energy content of the produced gas increases in line with volumetric production, further confirming the effective conversion of MDF sludge into hydrogen-rich syngas under the tested conditions.

The lower heating value (LHV) of the product gas obtained in this work was 12.1 MJ Nm -3 (Exp. 10, Table 3). This value is comparable to those reported for steam gasification of MSW at 900 °C using calcined dolomite, where an LHV of 11.43 MJ Nm -3 was obtained for a gas containing approximately 55 vol% H 2 (He et al., 2009b). Despite similar LHV values, the gas produced via MDFS-SESG exhibits a substantially higher H 2 purity (84.7 vol%), reflecting the effectiveness of the sorption-enhanced configuration. The relatively moderate LHV is associated with the low CO content of the product gas, as CO has a higher LHV than H 2. Consequently, while maximizing LHV is not the primary outcome of the SESG process, the resulting gas composition is better aligned with downstream hydrogen separation and utilization, where high hydrogen purity is typically more critical than calorific value alone.

Summarizing, while direct comparisons with previous studies on catalytic SESG of MDF sludge are not available, the gas yield and LHV values reported here are consistent with or exceed those observed in related non-catalytic or conventional gasification processes, highlighting the synergistic effect of the catalyst and in situ CO 2 capture in enhancing hydrogen production.

This study investigated the catalytic sorption-enhanced steam gasification (SESG) of industrial medium-density fiberboard (MDF) sludge, focusing on the effects of weight hourly space velocity (WHSV), temperature, and steam-to-carbon (S/C) ratio on hydrogen production and CO 2 capture. Within the investigated WHSV range (0.5–2.7 h -1), space velocity had only a minor influence on hydrogen yield, selectivity, and outlet gas composition. H 2 yield remained close to 39%, with outlet H 2 concentrations consistently above 80 vol%, indicating that reaction rates were sufficiently fast to sustain stable performance over a wide range of residence times. This kinetic robustness suggests that catalytic MDFS-SESG can tolerate moderate fluctuations in biomass feed rate without compromising performance, which is advantageous for continuous, scalable operation using heterogeneous industrial residues.

Increasing the operating temperature from 600 °C to 650 °C led to higher hydrogen yields (from 35.3% to 41.5%) and increased hydrogen purity (up to 88.9 vol%), despite a moderate reduction in CO 2 capture efficiency. These results confirm that operating near 650 °C provides a favorable balance between enhanced gasification and reforming kinetics and adequate carbonation, in agreement with the commonly reported optimal temperature window for non-catalytic SESG of biomass. The S/C ratio showed a more pronounced effect on process performance. Hydrogen yield increased with steam addition, reaching a maximum of approximately 55% at S/C norm = 8 due to enhanced steam gasification, water–gas shift, and methane reforming reactions. Further increases in steam supply reduced hydrogen yield and purity due to shortened residence times, reactant dilution, and lower CO 2 capture efficiency, highlighting the need to balance steam availability with thermodynamic and kinetic constraints.

Under the identified optimal conditions (650 °C, S/C norm = 8, and WHSV = 0.62 h -1), the process achieved the highest H 2 yield of 55.4% and a high H 2 purity of 84.7 vol%. In addition, gas yield reached 1.55 Nm 3 kg -1 at relatively moderate operating temperatures, which compares favorably with non-catalytic SESG of lignocellulosic feedstocks and conventional steam gasification of industrial residues. These results indicate that catalytic SESG can achieve competitive performance and high hydrogen purity. Overall, the main novelty of this work is the demonstration of the effective technical feasibility of converting MDF sludge, a genuine and abundant industrial byproduct, into high-purity hydrogen (up to 90 vol%) via catalytic SESG. These findings highlight the potential of catalytic SESG for industrial waste valorization and low-carbon hydrogen production. Further development of this process will benefit from integrated assessments combining experimental performance with sorbent cyclic stability, energy integration, and downstream gas conditioning to define operating windows that are both technically and economically viable at larger scales.