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Article

Steam Reforming of High-Concentration Toluene as a Model Biomass Tar Using a Nickel Catalyst Supported on Carbon Black

by
Soohyun Kim
1,†,
Sangjun Yoon
2,†,
Zulqarnain
1,3,
Jiho Yoo
1,*,
Hyuk Im
1,
Sangdo Kim
1,
Donghyuk Chun
1,
Hokyung Choi
1 and
Jeonghwan Lim
1
1
Clean Air Research Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of Korea
2
Clean Fuel Research Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of Korea
3
Department of Energy Engineering, University of Science and Technology (UST), Daejeon 34113, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Energies 2025, 18(2), 327; https://doi.org/10.3390/en18020327
Submission received: 16 December 2024 / Revised: 3 January 2025 / Accepted: 13 January 2025 / Published: 13 January 2025
(This article belongs to the Topic Energy Storage and Conversion: From Materials to Technologies)
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Versions Notes

Abstract

:
Biomass tar, an inevitable byproduct of biomass pyrolysis and gasification, poses a significant challenge due to its tendency to condense in pipelines, causing clogging and operational issues. Catalytic steam reforming can convert tar into syngas, addressing the tar issue while simultaneously producing hydrogen. However, the reforming catalyst is highly susceptible to deactivation by coking, especially when dealing with highly concentrated polymeric hydrocarbons such as tar. This study focused on enhancing the durability of tar-reforming catalysts. Nickel-based catalysts were prepared using carbon supports known for their high coking resistance, such as carbon black (CB), activated carbon (AC), and low-rank coal (LRC). Their performance was then tested for the steam reforming of high-concentration toluene, a representative tar. All three carbon supports (CB, AC, LRC) showed high catalytic performance with NiMg catalysts at 500 °C. Among them, the mesoporous CB support exhibited the highest stability when exposed to steam, with NiMg on CB (NiMg/CB) remaining stable for long-term continuous operation without any deactivation due to coking or thermal degradation.

1. Introduction

Climate change is a matter of human survival. As of January 2024, approximately 140 countries have set net-zero targets [1,2]. A clean energy source, such as hydrogen, is essential in addressing climate change [3]. Currently, hydrogen is primarily produced through the steam reforming of natural gas, a process that inevitably generates CO2 emissions [4]. Biomass gasification and pyrolysis technologies have been actively developed for the production of zero-emission hydrogen (H2) [5]. The International Energy Agency (IEA) reported that bioenergy will account for 14.3% of total electricity generation in the carbon-neutral era, which is a significant increase from its 2.2% share in 2019 [6].
Thermochemical biomass conversion, however, releases large amounts of sticky tar, which is a mixture of relatively heavy oxygenated hydrocarbons [7,8,9]. It condenses and coalesces after leaving the reactors (gasifier and pyrolyzer), causing major problems such as pipeline plugging and downstream fouling [10,11]. Tar is typically removed using spray towers and scrubbers [12]. However, this wet process requires complex equipment cleaning and treatment of large volumes of wastewater, and does not recover the energy value of the tar [13]. Additionally, the pyrolytic decomposition method can be used to break down tar at >1000 °C. However, these high temperatures can damage the equipment [14,15,16]. There has been significant interest in steam reforming, which converts biomass tar into hydrogen, enabling the tar to become a source of renewable energy rather than being removed [17].
Steam reforming of toluene, a representative tar, produces H2 and CO (Equation (1)). Subsequent reactions, such as the water–gas shift reaction (Equation (2)), the Boudouard reaction (Equation (3)), and methanation (Equation (4)) occur simultaneously [18,19].
C7H8 + 7H2O → 11 H2 + 7CO
7CO + 7H2O → 7H2 + 7CO2
2CO → CO2 + C
CO + 3H2 → CH4 + H2O
The steam reforming reaction is typically facilitated by a catalyst. Nickel is a commercially preferred active metal due to its oxidation stability and cost-effectiveness, compared to other transition metals (such as Co and Fe) and noble metals (such as Rh, Ru, Pd, and Pt) [15]. However, nickel catalysts can be deactivated by coking, especially in the steam reforming of high-molecular-weight hydrocarbons, such as tar [20,21]. Various promoters, such as Mg, Ce, La, and K, have been reported to enhance the activity and stability of the Ni catalyst at <700 °C [22,23]. However, coking still occurs, requiring periodic regeneration of the catalyst through coke oxidation, followed by reductive activation of nickel [20]. This reduces process efficiency and causes thermal deactivation, which shortens the catalyst’s lifespan.
Carbon supports, such as activated carbon (AC), carbon black (CB), biochar, and low-rank coal (LRC), are known to provide better coking resistance than metal oxides (MOx) like alumina and silica, as they can prevent the formation of metal carbides (coke intermediates) and suppress the polymerization of monomeric Cα to polymeric Cβ [24,25,26]. Carbon materials also offer several other advantages as supports. Their micropore structure and surface composition can be easily optimized [27,28]. They are highly thermally conductive, readily oxidized (facilitating the reduction of NiO to Ni metal), and hydrophobic, which enhances hydrocarbon adsorption [29]. Nickel dispersed on carbon supports has shown high tar-reforming activity, partly due to improved nanoscale metal dispersion [30,31,32]. Carbon-supported Ni catalysts are well-known for their high catalytic performance. However, they should be maintained at relatively low temperatures to prevent a reaction with steam at higher temperatures. Therefore, achieving sufficiently high catalytic activity in lower temperature ranges, without compromising stability, becomes essential.
In this study, Ni-based catalysts were prepared using three different carbon supports (CB, AC, and LRC) and tested for the steam reforming of highly concentrated toluene, a representative biomass tar compound [33]. To further improve catalytic behavior, carbon-supported Ni catalysts were modified with promoters (Mg, Ce, La, and K). Additionally, the effects of key reaction parameters (temperature, steam to carbon ratio (S/C), and gas hourly space velocity (GHSV)) were also investigated. Finally, the stability of a selected catalyst was evaluated during 50 h of continuous toluene steam reforming.

2. Materials and Methods

2.1. Carbon Support

Carbon black (CB), activated carbon (AC), and low-rank coal (LRC) were selected as catalyst supports. CB (MH-00, Toyo Tanso, Tokyo, Japan) was treated with 30 vol% nitric acid at 60 °C for 3 h to improve its hydrophilicity and enhance metal dispersion [34]. AC (Shin-Kwang Chemical Industry Co., Ltd., Yangsan-si, Republic of Korea) was prepared by chemical activation of a coconut shell. The feedstock was mixed with KOH flake (1:3 ratio) and heat-treated at 800 °C for 6 h. LRC was sourced from Indonesia (Eco coal) [24].

2.2. Catalyst Preparation

Nickel (II) nitrate hexahydrate (Ni(NO3)2∙6H2O, Samchun Chemicals, Seoul, Republic of Korea) was thoroughly dissolved in distilled water. It was then mixed with a carbon support using the incipient wetness impregnation method. After drying in an oven at 107 °C for 17 h, it was calcined in an N2 environment at 600 °C for 30 min. The above procedure was repeated to disperse magnesium nitrate (Mg(NO3)2, Samchun Chemicals, Seoul, Republic of Korea), cerium(III) nitrate hexahydrate (Ce(NO3)3∙6H2O, Samchun Chemicals, Seoul, Republic of Korea), lanthanum(III) nitrate hexahydrate (La(NO3)3∙6H2O, Samchun Chemicals, Seoul, Republic of Korea), and potassium nitrate (KNO3, Samchun Chemicals, Seoul, Republic of Korea) promoters. The catalysts prepared are listed in Table 1.

2.3. Catalyst Test

Steam reforming of tar was conducted in a fixed-bed reactor [31]. Gas chromatography (GC) with a thermal conductivity detector (Agilent 6890N) was used to quantify the products (H2, CH4, CO, and CO2). The 1/2″ quartz tube reactor had a frit in the center to allow only gaseous materials to pass through. The catalyst (0.1 g) was placed in a reactor and a thermocouple was positioned directly above the catalyst. Steam and toluene were injected using a syringe pump, heated to 200 °C using heating tape, and then N2 was injected as a carrier. Since the concentration of tar released from the biomass gasification reactor ranges from 0.3 to 100 g/Nm3, and the highest tar concentration from the downdraft biomass gasifier is around 30 g/Nm3, toluene, at a concentration of 30 g/Nm3 (7766 ppm), was used as a representative tar in this experiment [35]. Reaction residues at the outlet of the reactor were removed using an oil trap and a chiller maintained at 2 °C, ensuring that only the product gases reached the GC. The reactions were carried out in varying temperatures (450–600 °C), steam to carbon ratios (S/C, 2–20), and gas hourly space velocities (GHSV, 6000–25,000 h−1). Equation (5) and Equation (6) were used to calculate the carbon conversion and H2 yield, respectively.
Carbon conversion (%) = ([CO2]out + [CO]out + [CH4]out)/7[C7H8]in × 100
H2 yield (%) = [H2]out/18[toluene]in × 100

2.4. Carbon Support Stability Test

The chemical stability of each carbon support (CB, AC, and LRC) was evaluated at 350–550 °C. Under the same conditions and apparatus as steam reforming, the product gases were monitored with steam as the sole feedstock, with no toluene introduced. The temperature was initially set to 350 °C and increased in 50 °C increments. At each temperature step, the reaction was maintained for 30 min, while monitoring H2 concentrations.

2.5. Catalyst Characterization

Thermogravimetric analysis (TGA) was performed in the temperature range of 30–800 °C at a heating rate of 10 °C/min under a flow of 100 cc/min N2 (TASDT 600, TA Instruments, New Castle, DE, USA). X-ray diffraction (XRD) was conducted using a D/Max 2500PC instrument (60 kV, 300 mA, Rigaku, Tokyo, Japan) in the 2θ range of 20–70°. BET analysis was carried out using an ASAP 2420 (Micromeritics, Norcross, GA, USA). H2 temperature-programmed reduction (TPR) was performed using an AutoChem 2920 V5.02 instrument (Micromeritics, Norcross, GA, USA). The TPR procedure involved initial heating to 100 °C under Ar, followed by the replacement of Ar with hydrogen gas, which was held for 30 min. The sample was then cooled to 50 °C, and then heated from 50 to 900 °C at a rate of 10 °C/min. Transmission electron microscope images were obtained using a FEI Tecnai G2-20 S-twin 200-kV LaB6 (FEI, Hillsboro, OR, USA).

3. Results and Discussion

3.1. Performance of Carbon Supports (CB, AC, LRC) for NiMg Catalysts

Steam reforming catalysts were prepared on three different carbon supports: CB, AC, and LRC. As shown in Table 1, proven Ni (15 wt%) was used as an active metal, and Mg (3 wt%) as a promoter. Both were uniformly dispersed on the carbon supports [36]. The catalytic activities of NiMg/CB, NiMg/AC, and NiMg/LRC were evaluated for the steam reforming of 30 g/Nm3 toluene at 500 °C with S/C = 6 and GHSV = 15,000 h−1. As shown in Figure 1, the AC-supported catalyst exhibited the highest carbon conversion (95%) and H2 yield (79%). The performances of NiMg/CB and NiMg/LRC were similar, with both showing approximately 87% carbon conversion and around 70% H2 yield. The relative amounts of C1 gases (CO, CH4, and CO2) varied depending on the type of support. The AC catalyst produced approximately 8.5% CO, less than 1% CH4, and about 91% CO2, whereas the CB and LRC catalysts each produced around 8% CO, 5–8% CH4, and 84–86% CO2. This difference suggests that the hydrogenation reaction of CO (Equation (4)) may be influenced by the type of carbon support.
Table 2 shows the pore characteristics of CB, AC, and LRC. CB and AC had similar specific surface areas (SBET = 1534 and 1515 m2/g, respectively), but their pore sizes differed. CB had approximately 18% micropores and 82% mesopores, whereas AC consisted mainly of micropores (~90%) (Table 2). The average pore size and pore volume (Vtotal) of CB were 4.7 nm and 1.87 cm3/g, respectively, and those of AC were 0.96 nm and 0.73 cm3/g, respectively. LRC, on the other hand, had much lower SBET (~4 m2/g) and larger pore size (~28 nm). LRC contains a significant amount of oxygen functional groups, and metal nanodispersion is easily achieved through ion exchange at these sites, increasing the number of active sites [31]. The comparable activities of NiMg/CB and NiMg/LRC indicate that, in addition to pore structure, which is critical for adsorption/desorption and diffusion, other factors such as the metal dispersion and the defect structure of the support may also influence its catalytic activity [37].
Thermogravimetric analysis (TGA) in N2 was employed to assess the thermal stability levels of raw CB, AC, and LRC. As shown in Figure 2a, CB and AC demonstrated minimal weight loss (<3%) up to 600 °C, likely due to the release of volatiles and adsorbed trace gases. In contrast, LRC showed a gradual weight loss with increasing temperature, resulting in a total weight loss of approximately 30% at 600 °C. However, LRC-supported catalysts are expected to be more stable than raw LRC, as much of their volatile matter is evaporated during calcination.
Steam gasification of raw CB, AC, and LRC was conducted at temperatures ranging from 350 to 550 °C. As shown in Figure 2b, AC and LRC were stable below 400 °C, but reacted with steam at ≥450 °C, producing H2, with the maximum H2 release occurring at 500 °C (28,000 ppm H2 from AC and 11,400 ppm from LRC). AC exhibited thermally unstable behavior, partly due to its high specific surface area, which may enhance steam contact. LRC, being a less carbonized material, can undergo gasification at lower temperatures [38,39]. On the other hand, raw CB was stable up to 550 °C and, therefore, was chosen as the carbon support for subsequent tests.
XRD patterns for Ni dispersed on three carbon supports are shown in Figure 3. Most of the Ni in Ni/AC and Ni/LRC existed in its metallic form (2θ = 44.5° and 51.8°) after calcination in N2, which was likely due to the oxidation of the carbon supports consuming oxygen from NiOx [40]. This can enhance process efficiency by eliminating the need for the H2 reduction step to convert NiO to metallic Ni. CB has a high degree of carbonization, as it was heat-treated at relatively high temperatures (>1300 °C), resulting in low oxidation reactivity [41]. Consequently, while some of the NiOx was reduced, a significant portion remained as NiO (2θ = 37.3°, 43°, and 62.9°) after calcination at 600 °C for 30 min in N2. When Mg or Ce promoters were co-dispersed with Ni (NiMg/CB and NiCe/CB), both Ni metal and NiO phases were still detected. In this case, a relatively higher ratio of Ni metal was observed compared to that of Ni/CB, likely due to the enhanced oxidation of CB facilitated by Mg or Ce [42].

3.2. Effects of Promoters (Mg, Ce, La, and K) on Ni/CB Catalyst

The effects of promoters (Mg, Ce, La, and K) on the Ni/CB catalyst were comparatively evaluated in the 450–600 °C range (Figure 4). Catalytic activity was measured at S/C = 6 and GHSV = 15,000 h−1, with the temperature increasing from 450 °C to 600 °C in 50 °C intervals. All catalysts showed an increase in H2 yield with increasing temperature from 450 to 550 °C. Among these, NiK/CB showed the highest H2 yield, followed by NiCe/CB, NiMg/CB, and NiLa/CB. The addition of promoters enhanced the catalytic ability to cleave C–C and C–H bonds, thereby improving reactivity [43]. The relatively high H2 yield of NiCe/CB was likely due to the enhanced water–gas shift reaction promoted by Ce (Equation (2)) [44]. However, the activities of all of the catalysts, except NiMg/CB, decreased significantly with a further temperature increase to 600 °C. In particular, NiK/CB exhibited a much lower H2 yield at 600 °C (~68%) compared to 450 °C. Increasing reactivity with temperature was expected, as steam reforming is an endothermic reaction. NiCe/CB and NiK/CB exhibited carbon conversion greater than 100% at 550 °C (125% for NiCe/CB and 133% for NiK/CB). This was likely due to the reaction of the CB support with steam, which might reduce the number of active sites by causing the agglomeration of metals dispersed on the support [45]. On the other hand, NiMg/CB exhibited lower activity than NiCe/CB and NiK/CB at temperatures below 550 °C, but showed no decrease in activity at 600 °C. The dispersed promoters, along with Ni, appeared to activate the steam gasification of CB, with the extent of activation varying depending on the type of dispersed metal, since no gasification of raw CB was observed up to 550°C during the reaction with steam. (Figure 2b). It is well known that the steam gasification of carbon materials is enhanced by K, which, in turn, increases H2 production [46,47]. According to the thermal stability behavior observed in Figure 2b and Figure 4, CB-supported catalysts should be used at relatively low temperatures (<500 °C).
As shown in Table 3, the total surface area (SBET) of the samples ranged from 381 to 903 m2/g. The loading of Ni and the promoters reduced the SBET value by more than 40% compared to raw CB (1534 m2/g), primarily due to pore blockage from the dispersed metals. A large specific surface area is generally advantageous for metal dispersion, providing better catalytic activity [48]. In addition, the percentage of surface area covered by mesopores decreased in all of the samples, from about 82% in raw CB to about 72% for NiMg/CB, 66% for NiCe/CB, 55% for NiLa/CB, and 81% for NiK/CB. Mesoporous supports are known to enhance reactant diffusion and adsorption, thereby increasing catalytic reactivity [49,50]. Total pore volume (Vtotal) also decreased to 0.54–1.01 m2/g, compared to that of raw CB (1.87 m2/g). Average pore size showed similar values to that of raw CB (3.7–7.5 nm).
H2-TPR was carried out to investigate the reducibility of nickel with promoters. The prepared catalysts consumed considerable amounts of H2 between 250 °C and 600 °C. As shown in Figure 5, three reduction peaks appeared in NiMg/CB, with maxima at 290 °C, 321 °C, and 468 °C. The first and second peaks were probably due to surface NiOx or NiOx sitting on highly defected CB regions [31,51]. The third broad peak, with a maximum at 468 °C, seemingly corresponded to well-dispersed NiOx particles deposited on the support in various environments [52]. Graphitic carbon in CB might donate a π-electron to neighboring NiOx, facilitating the reduction of NiOx to Ni [41]. When the other promoters (Ce, La, and K) were loaded, their TPR profiles generally resembled that of NiMg/CB, showing 2–3 relatively sharp overlapping peaks below 350 °C, followed by a broad peak with a maximum around 450 °C. The Ce-promoted catalyst showed a reduction peak with a maximum at a lower temperature (210 °C). This observation was consistent with the XRD pattern, which revealed a higher ratio of metallic Ni for the Ce-promoted sample compared to the Mg-promoted one (Figure 3). NiK/CB showed a similar TPR profile to NiMg/CB. However, a reduction peak appeared at a lower temperature (230 °C) in addition to a doublet near 300 °C, suggesting that K provided a more diverse dispersion of Ni compared to the Mg promoter. The promoters generally shifted the Ni reduction peaks to lower temperatures by modifying the bonding between CB and Ni, which can enhance the activation of the toluene steam reforming reaction [53,54]. Ce and La have strong oxygen storage capacities, providing oxygen transfer pathways and, thus, facilitating the reduction of nickel oxides. This resulted in a shift of the reduction peak to lower temperatures compared to NiMg/CB.

3.3. Effects of Steam to Carbon Ratio (S/C) and Space Velocity (GHSV) on NiMg/CB

The steam reforming of toluene using NiMg/CB was carried out at 500 °C, with variations in the steam to carbon ratio (S/C = 2–20) and gas hourly space velocity (GHSV = 6000–25,000 h−1). Figure 6a shows the effect of S/C with GHSV = 15,000 h−1. Increasing S/C resulted in a relatively steep increase in H2 yield in the low S/C range (2–6). The H2 yield increased, from 45% at S/C = 2 to approximately 70% at S/C = 6. However, when S/C was increased from 8 to 20, the H2 yield showed a modest increase, from 71% to 78%. The general trends in carbon conversion and H2 yield with changes in S/C were similar. However, carbon conversion did not show any systematic change when increasing S/C from 8 to 20, remaining steady at 79–83%. This was likely due to excessive steam, which not only wastes energy, but also inhibits the steam reforming reaction by saturating the catalyst surface with steam, thereby obstructing the catalytic process [55].
The relative amounts of CO and CH4 production decreased with increasing S/C (Figure 6a). The CO ratio was approximately 16% at S/C = 2, gradually decreasing as S/C increased, eventually dropping to <4% when S/C exceeded 12. Steam is the primary reactant in the reforming process, and its concentration plays a crucial role in controlling syngas output [56]. In general, increasing the amount of steam can enhance the H2 production rate in syngas due to the dominance of the steam reforming reaction (Equation (1)) and the water–gas shift reaction (Equation (2)), which reduce CO production. In addition, the CH4 ratio decreased, from 20% at S/C = 2 to 9% at S/C = 6, and dropped to less than 2% at S/C > 10. This trend suggests that a higher S/C facilitates steam reforming and reduces methane formation, which is consistent with the increase in hydrogen production observed under higher S/C conditions. Under low S/C conditions, the CO concentration is expected to be high, and thus CO methanation (Equation (4)) was probably increased accordingly, consistent with the higher CH4 ratio, as CO methanation became more dominant with abundant CO and insufficient steam [57]. S/C = 6, which showed the highest carbon conversion, was selected as the condition for the following long-term stability tests.
Figure 6b shows the effect of GHSV (6000–25,000 h−1) on the steam reforming of toluene at 500 °C and S/C = 6. Both carbon conversion and H2 yield were inversely correlated with GHSV. They decreased slightly with increasing GHSV in the range of 6000–15,000 h−1. Carbon conversion was 100% at 6000 h−1, 91% at 8500 h−1, and 87% at 15,000 h−1, but it dropped to 68% at 25,000 h−1. In addition, H2 yield was 73% at 6000 h−1, 68% at 8500 h−1, and 70 at 15,000 h−1, and it dropped to 57% at 25,000 h−1. As expected, high space velocity makes the conversion of toluene into syngas more difficult, owing to insufficient contact time between the reactants and the catalyst, and the opposite was observed for low GHSV [58]. While low GHSV may result in a higher conversion rate, high GHSV can increase productivity by increasing reactant throughput. Therefore, the optimal GHSV should be determined based on the trade-off between conversion efficiency and productivity. A GHSV of 15,000 h−1 was selected as a compromise, and a long-term test was conducted under the same conditions. Similarly, the relative ratios of CO (~8%), CH4 (~10%), and CO2 (~82%) remained consistent in the range of 6000–15,000 h−1, but an increased CO2 ratio (~88%) was observed at 25,000 h−1.

3.4. Continuous Toluene Steam Reforming for 50 H

The long-term stability of NiMg/CB catalysts was evaluated during a 50 h continuous run of toluene (30 g/Nm3) steam reforming at 500 °C, S/C = 6, and GHSV = 15,000 h−1. Figure 7 shows the product concentrations over the course of the 50 h reaction. The steam reforming process produced approximately 82,000 ppm H2, 3800 ppm CO, 2100 ppm CH4, and 49,000 ppm CO2, with no sign of production decline throughout the 50 h continuous run. The relative ratio of C1 gases was approximately 7% CO, 4% CH4, and 89% CO2, and this ratio remained consistently similar throughout the 50 h run, indicating no deactivation associated with either toluene steam reforming (Equation (1)) or water–gas shift reactions (Equation (2)).
Figure 8 presents Transmission electron microscopy (TEM) images of the NiMg/CB catalyst, taken before and after the 50 h continuous run. The images indicated that Ni was relatively evenly distributed on the CB support, appearing in the form of dark, spherical shapes. Mg particles were found to be significantly smaller than Ni particles and were typically dispersed alongside Ni particles on the CB support. Neither the fresh NiMg/CB catalyst nor the used catalyst showed significant changes in Ni particle size after 50 h of continuous operation. The average particle size from each sample was similar, approximately 20 nm, indicating that thermal degradation was minimal after 50 h of continuous operation. Coking is a common issue in steam reforming, and it can be more severe, particularly when dealing with highly concentrated hydrocarbons [24]. The TEM image of the fresh NiMg/CB catalyst showed no coking. However, after the 50 h reaction, numerous tube-like cylindrical structures, likely multi-walled carbon nanotubes (MWCNTs), were observed (Figure 8b). Coking is known to be initiated by the dissociation of carbon bonds, which is followed by steam reforming on catalytically active sites, such as the Ni surface [59]. Carbon polymerization possibly leading to coking is favored when there is an excess of carbon generated through processes like carbon dissociation and the Boudouard reaction (Equation (3)). Polymeric carbons (Cβ) may accumulate on the catalyst surface, potentially obstructing the active sites of the catalyst [59]. However, the formation of polymeric carbons (Cβ) on a carbon support, such as CB, is relatively difficult due to the shifting of thermodynamic equilibrium [60]. This shift can suppress coke formation, as the carbon support can stabilize the carbon species, preventing polymerization and, thus, catalytic deactivation [60]. In addition, the filamentous coke observed on NiMg/CB after the 50 h run can be less detrimental to its activity than amorphous coke, because filamentous coke, such as MWCNTs, cannot cover the active sites as effectively as amorphous coke [61].

4. Conclusions

The thermochemical conversion of renewable biomass typically releases highly concentrated, sticky tar, which can cause operational issues such as plugging and fouling, thereby reducing process efficiency. The steam reforming of tar offers a solution by converting tar into hydrogen energy. However, when treating high-concentration tar (1–100 g/Nm3), catalysts are prone to frequent deactivation due to coking. This study aimed to enhance the catalytic activity and long-term stability of the catalysts for the steam reforming of biomass tar. Ni catalysts were synthesized using carbon supports (carbon black (CB), activated carbon (AC), and low-rank coal (LRC)) that are known for high coking resistance, and their performances were evaluated. These catalysts were evaluated with highly concentrated toluene (30 g/Nm3, 7766 ppm) as a biomass tar model compound. All three catalysts (NiMg/CB, NiMg/AC, and NiMg/LRC) exhibited satisfactory catalytic activity (carbon conversion >85% and H2 yield >70%) at 500 °C, S/C = 6, and GHSV = 15,000 h−1. However, the NiMg catalysts supported on AC and LRC were unstable when exposed to steam at elevated temperatures (450–550 °C). In contrast, NiMg/CB, synthesized with carbon black (CB), demonstrated relatively high stability, maintaining consistent catalytic performance without any decline during 50 h of continuous steam reforming, which was most likely due to minimal thermal degradation and the absence of amorphous coke formation. Further research is required to improve the steam reforming activity of CB-supported catalysts under lower temperature conditions (<500 °C).

Author Contributions

Conceptualization, S.Y., J.Y., S.K. (Sangdo Kim), D.C., H.C. and J.L.; methodology, S.K. (Soohyun Kim), S.Y., Z., J.Y., H.I., S.K. (Sangdo Kim), D.C., H.C. and J.L.; formal analysis, S.K. (Soohyun Kim) and H.I.; investigation, S.K. (Soohyun Kim); resources, J.Y.; data curation, S.Y.; writing—original draft preparation, S.K. (Soohyun Kim); writing—review and editing, S.Y. and J.Y.; visualization, S.K. (Soohyun Kim); supervision, S.Y. and J.Y.; project administration, J.L.; funding acquisition, S.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Research Foundation of Korea ERC Center, grant number NRF-2022R1A5A1033719.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

ACActivated carbon
BETBrunauer–Emmett–Teller
CBCarbon black
CαMonomeric carbon
CβPolymeric carbon
GCGas chromatography
GHSVGas hourly space velocity
IEAInternational energy agency
LRCLow-rank coal
S/CSteam to carbon ratio
TEMTransmission electron microscopy
TGAThermogravimetric analysis
TPRTemperature-programmed reduction
XRDX-ray diffraction

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Figure 1. Performance of NiMg/CB, NiMg/AC, and NiMg/LRC catalysts for steam reforming of 30 g/Nm3 toluene (500 °C, S/C = 6, and GHSV = 15,000 h−1).
Figure 1. Performance of NiMg/CB, NiMg/AC, and NiMg/LRC catalysts for steam reforming of 30 g/Nm3 toluene (500 °C, S/C = 6, and GHSV = 15,000 h−1).
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Figure 2. (a) TGA profiles of raw CB, AC, and LRC under N2 and (b) H2 production during steam gasification of raw CB, AC, and LRC.
Figure 2. (a) TGA profiles of raw CB, AC, and LRC under N2 and (b) H2 production during steam gasification of raw CB, AC, and LRC.
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Energies 18 00327 g003
Figure 3. XRD patterns of the Ni/CB, Ni/AC, Ni/LRC, NiMg/CB, and NiCe/CB catalysts.
Figure 3. XRD patterns of the Ni/CB, Ni/AC, Ni/LRC, NiMg/CB, and NiCe/CB catalysts.
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Figure 4. H2 yield of NiMg/CB, NiCe/CB, NiLa/CB, and NiK/CB catalysts for steam reforming of 30 g/Nm3 toluene (450–600 °C, S/C = 6, and GHSV = 15,000 h−1).
Figure 4. H2 yield of NiMg/CB, NiCe/CB, NiLa/CB, and NiK/CB catalysts for steam reforming of 30 g/Nm3 toluene (450–600 °C, S/C = 6, and GHSV = 15,000 h−1).
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Figure 5. TPR profiles of the NiMg/CB, NiCe/CB, NiLa/CB, and NiK/CB catalysts.
Figure 5. TPR profiles of the NiMg/CB, NiCe/CB, NiLa/CB, and NiK/CB catalysts.
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Figure 6. Steam reforming of 30 g/Nm3 toluene using NiMg/CB at 500 °C: (a) effect of S/C (2–20) and (b) effect of GHSV (6000–25,000 h−1).
Figure 6. Steam reforming of 30 g/Nm3 toluene using NiMg/CB at 500 °C: (a) effect of S/C (2–20) and (b) effect of GHSV (6000–25,000 h−1).
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Figure 7. Long-term (50 h) continuous run of 30 g/Nm3 toluene steam reforming over NiMg/CB at 500 °C, S/C = 6, and GHSV = 15,000 h−1.
Figure 7. Long-term (50 h) continuous run of 30 g/Nm3 toluene steam reforming over NiMg/CB at 500 °C, S/C = 6, and GHSV = 15,000 h−1.
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Figure 8. TEM images of (a) fresh NiMg/CB and (b) NiMg/CB after 50 h continuous run.
Figure 8. TEM images of (a) fresh NiMg/CB and (b) NiMg/CB after 50 h continuous run.
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Table 1. List of Ni-based catalysts on carbon supports.
Table 1. List of Ni-based catalysts on carbon supports.
CatalystCarbon SupportMetal Loading (wt%)
NiMg/CBCarbon black (CB)Ni 15, Mg 3
NiMg/ACActivated carbon (AC)Ni 15, Mg 3
NiMg/LRCLow-rank coalNi 15, Mg 3
NiCe/CBCBNi 15, Ce 3
NiLa/CBCBNi 15, La 3
NiK/CBCBNi 15, K 3
Table 2. Pore characteristics of the carbon supports.
Table 2. Pore characteristics of the carbon supports.
SupportSBET (m2/g)Smic (m2/g)Vtotal (cm3/g)Vmic (cm3/g)Average Pore Size (nm)
CB15342761.870.354.7
AC151513640.730.560.96
LRC4-0.03-28.2
SBET: total surface area, Smic: micropore surface area, Vtotal: total pore volume, Vmic: micropore volume.
Table 3. Surface areas and pore characteristics of the catalysts.
Table 3. Surface areas and pore characteristics of the catalysts.
CatalystSBET (m2/g)Smic (m2/g)Vtotal (cm3/g)Vmic (cm3/g)Average Pore Size (nm)
NiMg/CB7352030.850.134.6
NiCe/CB9033111.010.234.4
NiLa/CB5772610.540.113.7
NiK/CB381740.720.037.5
SBET: total surface area, Smic: micropore surface area, Vtotal: total pore volume, Vmic: micropore volume.
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Kim, S.; Yoon, S.; Zulqarnain; Yoo, J.; Im, H.; Kim, S.; Chun, D.; Choi, H.; Lim, J. Steam Reforming of High-Concentration Toluene as a Model Biomass Tar Using a Nickel Catalyst Supported on Carbon Black. Energies 2025, 18, 327. https://doi.org/10.3390/en18020327

AMA Style

Kim S, Yoon S, Zulqarnain, Yoo J, Im H, Kim S, Chun D, Choi H, Lim J. Steam Reforming of High-Concentration Toluene as a Model Biomass Tar Using a Nickel Catalyst Supported on Carbon Black. Energies. 2025; 18(2):327. https://doi.org/10.3390/en18020327

Chicago/Turabian Style

Kim, Soohyun, Sangjun Yoon, Zulqarnain, Jiho Yoo, Hyuk Im, Sangdo Kim, Donghyuk Chun, Hokyung Choi, and Jeonghwan Lim. 2025. "Steam Reforming of High-Concentration Toluene as a Model Biomass Tar Using a Nickel Catalyst Supported on Carbon Black" Energies 18, no. 2: 327. https://doi.org/10.3390/en18020327

APA Style

Kim, S., Yoon, S., Zulqarnain, Yoo, J., Im, H., Kim, S., Chun, D., Choi, H., & Lim, J. (2025). Steam Reforming of High-Concentration Toluene as a Model Biomass Tar Using a Nickel Catalyst Supported on Carbon Black. Energies, 18(2), 327. https://doi.org/10.3390/en18020327

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