Construction and reaction characteristics of pyrochlore-supported Ni catalysts for tar steam reforming

Tar is a common by-product during the gasification of biomass and its presence largely limits the subsequent application of syngas generated. Although biomass tar could be converted into hydrogen-rich syngas by catalytic steam reforming, the frequently adopted high-activity and low-cost Ni catalysts suffer from the problem of easy deactivation as a result of carbon deposition, and more efficient and stable catalyst needs to be developed for tar removal in biomass gasification. In the work, various Ni/pyrochlore catalysts characterized with redox properties were constructed and further modified through partial replacement of A-site in support, and their reaction characteristics in toluene steam reforming were comprehensively investigated. Results show that catalysts of Ni/La₂Ce₂ and Ni/Y₂Ce₂ have good catalytic performance due to the strong interaction between Ni and pyrochlore. Although a small amount doping of Sr in A-site is observed to decrease Ni/pyrochlore interaction, the great promotion in surface oxygen mobility make Ni/La_1.8Sr_0.2Ce₂ possess the best reactivity among all catalysts studied, and the optimum operating conditions is determined to be 650 °C and S/C = 2. Moreover, Ni/La_1.8Sr_0.2Ce₂ is found to be very stable during toluene steam reforming, which is proved to be a result of the superior capability in resisting coke formation.     

The performance of nickel-loaded lignite residue for steam reforming of toluene as the model compound of biomass gasification tar

Tars should be removed from biomass gasification systems so as not to damage or clog downstream pipes or equipment. In this paper, lignite insoluble residue (LIR) after extraction of humic acids was used as the support to prepare a nickel-loaded LIR (Ni/LIR) catalyst. This novel catalyst Ni/LIR was tested in steam reforming of toluene as a model compound of biomass tar conducted in a laboratory-scale fixed bed reactor. When compared to the reactions without catalyst or with Ni/Al₂O_3, Ni/LIR was confirmed as an active catalyst for toluene conversion at a relatively low temperature of 900 K. The investigated reforming parameters during the experiments in this research were selected as reaction temperature at a range of 850–950 K, steam/carbon molar ratio at a range of 2–5 mol/mol, and a space velocity from 1696 to 3387 h^?1. It was concluded that, under optimum conditions, significant amount of syngas yields, acceptable Ni/LIR consumption and more than 95% of toluene conversion can be obtained from the biomass Ni/LIR catalytic gasification system.     

Effect of phenols extraction on the behavior of Ni-spinel derived catalyst for raw bio-oil steam reforming

Alkyl-phenols and hydroxy- or methoxy-phenols (e.g., catechols, guaiacols and syringols) tend to polymerize into carbonaceous structures, causing clogging of reaction equipment and high coke deposition during bio-oil steam reforming (SR). In this work, removal of these phenolic compounds from raw bio-oil was addressed by accelerated aging and liquid-liquid extraction methods. The solvent-anti-solvent extraction with dichloromethane and water was suitable for obtaining a treated bio-oil appropriate for SR. The effect that phenols extraction has on the stability and regenerability of a NiAl₂O₄ spinel catalyst was studied by conducting reaction-regeneration cycles. Operating conditions were: 700 °C; S/C, 6; space-time, 0.15 g_catalysth/g_bio-oil (reaction step), and in situ coke combustion at 850 °C for 4 h (regeneration step). Fresh, deactivated and regenerated catalyst samples were analyzed by temperature programmed oxidation (TPO), temperature programmed reduction (TPR) and X-ray diffraction (XRD). Stability of the Ni-spinel derived catalyst was significantly improved by removing phenols due to attenuation of both coke deposition and Ni sintering. Regenerability of this catalyst was also slightly improved when reforming the treated bio-oil.     

Hydrogen production by steam reforming of bio-oil/bio-ethanol mixtures in a continuous thermal-catalytic process

The feasibility of the steam reforming of bio-oil aqueous fraction and bio-ethanol mixtures has been studied in a continuous process with two in-line steps: thermal step at 300 °C (for the controlled deposition of pyrolytic lignin during the heating of the bio-oil/bio-ethanol feed) followed by steam reforming in a fluidized bed reactor on a Ni/α-Al₂O₃ catalyst. The effect of bio-ethanol content in the feed has been analyzed in both the thermal and reforming steps, and the suitable range of operating conditions (temperature and space-time) has been determined for obtaining a high and steady hydrogen yield. Higher ethanol content in the mixture feed improves the reaction indices and reduces coke deposition. Operating conditions of 700 °C and space-times higher than 0.23 g_catalyst h (g_bio-oil+EtOH)⁻¹ are suitable for attaining almost fully conversion of oxygenates (bio-oil and ethanol) and hydrogen yields above 93%, with low catalyst deactivation.     

Carbon deposition behavior in steam reforming of bio-oil model compound for hydrogen production

Catalyst deactivation caused by coke formation is a bottleneck in steam reforming of bio-oil for hydrogen production. The investigation of carbon deposition behavior can make a contribution to the improvement of catalyst and the knowledge of reaction mechanism. In this paper, m-cresol (C₇H₈O, one of the organic compounds present in bio-oil) was chosen as model compound. The experiment was carried out on a commercial Ni/MgO catalyst. As a comparative test, m-cresol decomposition showed carbon deposition can be formed more easily under higher temperature. In steam reforming process, for the competition of carbon deposition and carbon elimination, a peak value of coking formation rate was obtained in a broad range of temperature (575–900 °C). The increase of steam to carbon ratio can favor the carbon elimination. Final coking formation rate curve was determined under optimal reaction conditions and the results showed the severity of carbon deposition maintained a very low level during the entire reaction time. Based on the distribution of reforming products, high temperature and sufficient water feeding can favor the carbon conversion from solid and liquid phase to gaseous phase. Unreacted m-cresol is the main organic compound detected in liquid condensate. Some secondary reactions can be deduced through the other compounds detected. The carbon deposition state on catalyst surface can be in the form of nanofiber, but their concrete shapes can be different due to different reaction conditions.     

Hydrogen from aqueous fraction of biomass pyrolysis liquids by catalytic steam reforming in fluidized bed

Sustainable pathways for producing hydrogen as a synthesis intermediate or as a clean energetic vector will be needed in the future. Renewable biomass resources should be taken into account in this new scenario. Processing through a pyrolysis step, optimized to high liquid production (bio-oil), increases the energy bulk density of biomass for transportation. Steam reforming of the aqueous fraction is an alternative process that increases the hydrogen content of the syngas. However, the thermochemical conversion of organic compounds derived from biomass involves drawbacks such as coke formation on the catalysts. This work studies the performance of Ni-Al catalysts modified with Ca or Mg in the steam reforming of the aqueous fraction of pyrolysis liquids and the resulting coke deposits. The catalyst composition influenced the quantity and type of coke deposits. Calcium improved the formation of carbonaceous products leading to lower H₂/CO ratios while magnesium improved the WGS (water gas shift) reaction. The strategy of reducing the space velocity resulted in a low coke removal although the addition of small quantities of oxygen decreased the coke content of the catalyst by more than 50% weight. Greater efficiency and further catalyst development are needed to improve the energetic requirements of the process.     

CO2 reforming of methane combined with steam reforming or partial oxidation of methane to syngas over NdCoO3 perovskite-type mixed metal-oxide catalyst

CO₂ reforming with simultaneous steam reforming or partial oxidation of methane to syngas over NdCoO₃ perovskite-type mixed metal oxide catalyst (prereduced by H₂) at different process conditions has been investigated. In the simultaneous CO₂ and steam reforming, the conversion of methane and H₂O and also the H₂/CO product ratio are strongly influenced by the CO₂/H₂O feed-ratio. In the simultaneous CO₂ reforming and partial oxidation of methane, the conversion of methane and CO₂, H₂ selectivity and the net heat of reaction are strongly influenced by the process parameters (viz. temperature, space velocity and relative concentration of O₂ in the feed). In both cases, no carbon deposition on the catalyst was observed. The reduced NdCoO₃ perovskite-type mixed-oxide catalyst (Co dispersed on Nd₂O₃) is a highly promising catalyst for carbon-free CO₂ reforming combined with steam reforming or partial oxidation of methane to syngas.     

Steam reforming of gasification-derived tar for syngas production

In this study, the steam reforming of tar was catalyzed by dolomite, Ni/dolomite, and Ni/CeO₂ for syngas production under different reaction temperature and weight hourly space velocity (WHSV, h⁻¹). The tar was the major side product from the biomass gasification.     

Catalyst modification strategies to enhance the catalyst activity and stability during steam reforming of acetic acid for hydrogen production

A high energy content (∼122 MJ/kg H₂) and presence of hydrogen-bearing compounds abundance in nature make hydrogen forth runner candidate to fulfill future energy requirements. Biomass being abundant and carbon neutral is one of the promising source of hydrogen production. In addition, it also addresses agricultural waste disposal problems and will bring down our dependency on fossil fuel for energy requirements. Biomass-derived bio-oil can be an efficient way for hydrogen production. Acetic acid is the major component of bio-oil and has been extensively studied by the researchers round the globe as a test component of bio-oil for hydrogen generation. Hydrogen can be generated from acetic acid via catalytic steam reforming process which is thermodynamically feasible. A number of nickel-based catalysts have been reported. However, the coke deposition during reforming remains a major challenge. In this review, we have investigated all possible reactions during acetic acid steam reforming (AASR), which can cause coke deposition over the catalyst surface. Different operating parameters such as temperature and steam to carbon feed ratio affect not only the product distribution but also the carbon formation during the reaction. Present review elaborates effects of preparation methods, active metal catalyst including bimetallic catalysts, type of support and microstructure of catalysts on coke resistance behavior and catalyst stability during reforming reactions. The present study also focuses on the effects of a combination of a variety of alkali and alkaline earth metals (AAEM) promoters on coke deposition. Effect of specially designed reactors and the addition of oxygen on carbon deposition during AASR have also been analyzed. This review based on the available literature focuses mainly on the catalyst deactivation because of coke deposition, and possible strategies to minimize catalyst deactivation during AASR.     

Desulfurization and tar reforming of biogenous syngas over Ni/olivine in a decoupled dual loop gasifier

For the production of bio-SNG (substitute natural gas) from syngas of biomass steam gasification, trace amounts of sulfur and tar compounds in raw syngas must be removed. In present work, biomass gasification and in-bed raw gas upgrading have been performed in a decoupled dual loop gasifier (DDLG), with aggregation-resistant nickel supported on calcined olivine (Ni/olivine) as the upgrading catalyst for simultaneous desulfurization and tar elimination of biogenous syngas. The effects of catalyst preparation, upgrading temperature and steam content of raw syngas on sulfur removal were investigated and the catalytic tar reforming at different temperatures was evaluated as well. It was found that 850 °C calcined Ni/olivine was efficient for both inorganic-sulfur (H₂S) and organic-sulfur (thiophene) removal at 600–680 °C and the excellent desulfurization performance was maintained with wide range H₂O content (27.0–40.7%). Meanwhile, tar was mostly eliminated and H₂ content increased much in the same temperature range. The favorable results indicate that biomass gasification in DDLG with Ni/olivine as the upgrading bed material could be a promising approach to produce qualified biogenous syngas for bio-SNG production and other syngas-derived applications in electric power, heat or fuels.     

Biomass to hydrogen-rich syngas via catalytic steam reforming of bio-oil

Hydrogen-rich syngas production from the catalytic steam reforming of bio-oil from fast pyrolysis of pinewood sawdust was investigated by using La_1−xK_xMnO₃ perovskite-type catalysts. The effects of the K substitution, temperature, water to carbon molar ratio (WCMR) and bio-oil weight hourly space velocity (W_bHSV) on H₂ yield, carbon conversion and the product distribution were studied in a fixed-bed reactor. The results showed that La_1−xK_xMnO₃ perovskite-type catalysts with a K substitution of 0.2 gave the best performance and had a higher catalytic activity than the commercial Ni/ZrO₂. Both high temperature and low W_bHSV led to higher H₂ yield. However, excessive steam reduced hydrogen yield. For the La_0.8K_0.2MnO₃ catalyst, a hydrogen yield of 72.5% was obtained under the optimum operating condition (T = 800 °C, WCMR = 3 and W_bHSV = 12 h⁻¹). The deactivation of the catalysts mainly was caused by coke deposition.     

Combined steam and dry reforming of methane in narrow channel reactors

The performance of methane reforming reactions in narrow channel reactors has been investigated. Two types of reactors (Diffusion Bonded Reactor and Demountable Reactor) were utilized and two forms of catalysts were prepared by the sol-gel method with different additives. The sol-gels were prepared to have desirable rheological properties for coating onto stainless steel substrates, which after calcining formed an adherent thin catalyst layer. Employing the catalyst as a thin layer (<50 μm) coated on the channel surface reduces mass and heat transfer restrictions compared with pellet catalysts and can improve the effectiveness factor. Carbon deposition is known to be rapid in the case of the CO₂ reforming alone. In this study, carbon deposition was reduced drastically when CO₂ reforming is carried out simultaneously with the steam reforming reaction in narrow channels coated by thin layers of catalyst (≤50 μm) prepared using the sol-gel method. It has been shown that the stability and coking resistance of Ni/Al₂O₃ catalyst are increased by the addition of Ba, Cr and La₂O₃ in combined steam reforming of methane with carbon dioxide reforming. This process is an attractive approach for improving catalyst stability and offers the possibility of obtaining H₂/CO ratio close to 2, which is suitable for Fischer–Tropsch and methanol synthesis.     

Synthesis of Ni@Al2O3 nanocomposite with superior activity and stability for hydrogen production from plastic-derived syngas by CO2-sorption-enhanced reforming

Catalytic dry (CO₂) reforming of plastic-derived syngas is a promising method of producing hydrogen-rich syngas and reducing greenhouse gases. The development of catalysts with high activity and stability is critical for this reaction. In this study, we fabricated core-shell structured Ni@Al₂O₃ catalysts with different shell thicknesses using advanced polyol and sol-gel methods. The effects of different Al/Ni ratios on the activity and stability of the catalysts in the CO₂ reforming reaction were investigated. The main challenge for CO₂ reforming of methane is carbon deposition. In the developed catalysts, the mesoporous Al₂O₃ coating outside the Ni core enhances the stability. However, the interaction between the core and the shell strongly affects the catalyst activity and product selectivity in the reaction. The catalyst with an Al/Ni ratio of 2 exhibited the highest methane conversion of up to 88% and the lowest carbon deposition, compared to the congeners with Al/Ni ratios of 1 and 3.     

Reaction pathway for ethanol steam reforming on a Ni/SiO2 catalyst including coke formation

The effect operating conditions (temperature, space time, steam/ethanol molar ratio, ethanol partial pressure and time on stream) have on the activity and stability of a Ni/SiO₂ catalyst for H₂ production by ethanol steam reforming has been studied in a fluidized bed reactor. This catalyst allows obtaining total conversion above 500 °C, with a steam/ethanol molar ratio of 6 and a space time of 0.138 g_catalysth/g_ethanol. Catalyst deactivation in the 300–500 °C range is due to coke deposition, whose nature (determined by TPH and TPO analysis) mainly depends on reaction temperature. The coke deposited at 300 °C is amorphous and blocks metallic sites, whereas at higher temperatures the coke is mainly filamentous and, although its content increases as reaction temperature is raised to 500 °C, it has a low effect on catalyst deactivation because it does not block metal sites. Above 600 °C the decrease in coke content due to gasification is noticeable, although at this temperature an incipient Ni sintering is observed, which is significant at 700 °C.     

Effect of W incorporation on the product distribution in steam reforming of bio-oil derived acetic acid over Ni based Zr-SBA-15 catalyst

Zirconia incorporated SBA-15 type mesoporous material was synthesized following a one-pot hydrothermal route, characterized and used as the catalyst support in the synthesis of Ni and bi-metallic Ni–W based catalysts. Performances of these catalysts were tested in steam reforming of AcOH. Catalytic activity tests proved that the performances of SBA-15 and Zr-SBA-15 supported Ni based catalysts were highly stable and they also showed very high activity in steam reforming of acetic acid, giving complete conversion at temperatures over 700 °C. Product distributions were shown to be strongly influenced by the composition of the catalyst. In the case of 5Ni@Zr-SBA-15, syngas produced at 750 °C contained about 54% H₂, 22% CO, 20% CO₂ and 4% CH₄. These results indicated that decarboxylation reaction of AcOH to CH₄ and CO₂ was minimized over this catalyst. Results were considered to be highly promising for the production of hydrogen rich syngas. It was most interesting to observe that modification of this catalyst by the addition of tungsten caused significant changes in the product distribution. For instance, syngas produced over 5Ni-50W@Zr-SBA-15 at the same reaction conditions, contained equimolar quantities of H₂ and CO (about 47.5% each) with very small amounts of CO₂ and CH₄ (about 3% and 2%, respectively). Production of a syngas with such a composition was considered to be highly attractive from the point of view of a resource gas for dimethyl ether and Fischer-Tropsch synthesis.     

Kinetic study of the catalytic reforming of biomass pyrolysis volatiles over a commercial Ni/Al2O3 catalyst

An original kinetic model has been proposed for the reforming of the volatiles derived from biomass fast pyrolysis over a commercial Ni/Al₂O₃ catalyst. The pyrolysis-reforming strategy consists of two in-line steps. The pyrolysis step is performed in a conical spouted bed reactor (CSBR) at 500 °C, and the catalytic steam reforming of the volatiles has been carried out in-line in a fluidized bed reactor. The reforming conditions are as follows: 600, 650 and 700 °C; catalyst mass, 0, 1.6, 3.1, 6.3, 9.4 and 12.5 g; steam/biomass ratio, 4, and; time on stream, up to 120 min. The integration of the kinetic equations has been carried out using a code developed in Matlab. The reaction scheme takes into account the individual steps of steam reforming of bio-oil oxygenated compounds, CH₄ and C₂-C₄ hydrocarbons, and the WGS reaction. Moreover, a kinetic equation for deactivation has been derived, in which the bio-oil oxygenated compounds have been considered as the main coke precursors. The kinetic model allows quantifying the effect reforming conditions (temperature, catalyst mass and time on stream) have on product distribution.     

Intensified bio-oil steam reforming for high-purity hydrogen production: Numerical simulation and sorption kinetics

This work proposes the production of high-purity hydrogen by an intensified non-isothermal sorption-enhanced bio-oil steam reforming (SEBOSR) process, by combining the bio-oil steam reforming over a Ni/La₂O₃-αAl₂O₃ catalyst and in-situ CO₂ adsorption over Li₂CuO₂. The kinetics of CO₂ adsorption on Li₂CuO₂ was studied experimentally and applied to assess the performance of SEBOSR in a fixed bed reactor via a non-isothermal mathematical model. Model simulations show that the prebreakthrough stage of the SEBOSR process, which corresponds to high purity H₂ production, can be extended by increasing the adsorbent loading and the S/C ratio, as well as by decreasing the inlet gas velocity. Increasing inlet temperature generates longer prebreakthrough step times but leads to a reduction in hydrogen purity. This intensified process allows to diminish the catalyst deactivation, which ultimately only occurs in the inlet region of the packed bed to some extent. In addition, SEBOSR indirectly uses sustainable CO₂-neutral biomass as a source of hydrogen; highly pure and renewable H₂ can be produced in one step (without the need of additional gas purification), via a process with enhanced thermal efficiency.     

Sequence of Ni/SiO2 and Cu/SiO2 in dual catalyst bed significantly impacts coke properties in glycerol steam reforming

Coke formation is a major challenge in steam reforming reactions. In addition to development of robust catalyst for tackling coking, in this study we explored the approach of using dual catalyst bed with the catalyst on top as the guard or sacrifice catalyst while with the bottom catalyst to catalyze the steam reforming. The rationale is that some oxygen-containing reactants are prone to polymerize on heating, and the polymeric coke could directly fall on surface of catalyst and leads to the rapid deactivation. Hence, glycerol was selected as the reactant for steam reforming in the catalyst bed with the Cu/SiO₂ placed on the top of Ni/SiO₂ catalyst. Our results demonstrate that first contact of glycerol to Cu/SiO₂ on top changed abundance/type of small intermediates and the π-conjugated oligomers reached the Ni/SiO₂ catalyst, rendering the Ni catalyst with a higher resistivity towards coking and deactivation. In addition, the carbon nanotube form of coke over Ni/SiO₂ was thinner in wall thickness and larger in inner diameter of the cavity due to the impact of D-Cu/SiO₂. Substantial polymeric coke with amorphous structure and low thermal stability formed over Cu/SiO₂ via polymerisation of reaction intermediates. The characterization (in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)) for the glycerol steam reforming indicated that the Cu/SiO₂ and Ni/SiO₂ catalyst induced the formation of the very different functionalities of the reaction intermediates.     

Synergistic catalysis of bi-metals in the reforming of biomass-derived hydrocarbons: A review

Biomass such as ethanol and glycerol has emerged as an alternative feedstock for hydrogen (H₂) production in recent years. Ethanol, which is high in H_2, can easily be derived from renewable biomass sources, whereas; glycerol is a by-product of biodiesel expected to be surplus in the coming years. Several catalytic reforming routes involving biomass such as steam, CO₂, auto thermal, partial oxidation and aqueous-phase reforming can produce syngas or H₂. Bimetallic catalysis is one of the potential solutions to reduce carbon formation and catalysts deactivation in reforming processes since it can produce more stable catalysts from the synergistic effect of the combined metals. There are many reviews on catalyst designs and reaction pathways reported in the literature; nevertheless, comparative literature is lacking on the metal configuration of bimetallic catalyst in biomass reforming particularly for ethanol and glycerol reforming reactions. Therefore, studies linked with the synergistic effects of various bi-metal combinations of catalysts used in biomass reforming processes have been reviewed in the paper. Moreover, the study provides data for the application of bimetallic catalyst for industrial biomass processes.     

The influence of shell thickness on coke resistance of core-shell catalyst in CO2 catalytic reforming of biomass tar

The development of catalysts with resistance to sintering and coke deposition is the key to tar cracking during biomass gasification technology. In this work, the core-shell catalysts with mesoporous and microporous silica-coated nickel nanoparticles were prepared for the CO₂ reforming of toluene as a model compound for biomass tar. The influence of thickness of SiO₂ shell layer on catalyst activity, coke and sintering resistance of the catalysts in the CO₂ reforming of toluene was explored. Appropriate increasing in the thickness of the silica shell can significantly increase the specific surface area, pore volume and the interaction between core and shell of the catalysts, which can further improve the reactivity and coke resistance ability. However, excessive increase in shell thickness can lead to a drastic decrease in the specific surface area and pore volume of the catalyst, resulting in significant coke deposition. The Ni@SiO₂-4 catalyst showed the highest catalytic activity of toluene conversion of around 50% within 300 min, stability H₂/CO ratio of 0.25～0.3 and durability of 26 h lifetime in the CO₂ reforming of toluene. Overall, the optimization of the silica shell thickness can improve the reactivity and coke resistance ability of Ni@SiO₂.