Tar Model Compound Research Articles

Steam reforming of toluene as model compounds of biomass tar by Fe–K-based biochar nano-catalysts at mild temperatures

Development of efficient catalysts for steam reforming of biomass tar is crucial for the widespread application of biomass gasification. In this work, a novel biochar nano-catalyst (BN) is conveniently prepared via one-step catalytic pyrolysis, using K and Fe to adjust the surface structure of the biochar, thereby enhancing the activity for toluene reforming. Fe–K/BN is tested in a fixed-bed reactor for steam reforming of toluene, a model compound of biomass tar, achieving a 100% toluene conversion rate at 650 °C. Fe–K/BN is activated in situ with steam to increase their specific surface area and enhance the O-containing functional groups on the surface, improving toluene adsorption and reforming. Finally, the stability of Fe–K/BN is tested in the simulated syngas, maintaining a 100% toluene conversion rate during a 26-h continuous catalytic reforming experiment. This work provides a promising strategy for the design of an active and stable catalyst for toluene reforming, which has potential for application of tar steam reforming during biomass gasification.

Dry reforming of toluene for syngas production over Ni-based perovskite-type oxides

Developing efficient Ni-based catalysts and understanding the mechanism of tar removal are crucial for upgrading biomass gasification technology. Literature has identified the superior performance of Ni-based catalysts in reforming tar model compounds like toluene under a steam atmosphere. In this work, we have synthesized La1-xCexNiO3 catalysts and examined their activity with benchmark A2BO4-type perovskites in dry reforming of toluene (DRT). Catalytic experiments revealed that La0.25Ce0.75NiO3 catalysts exhibited superior activity and stability regarding toluene conversion (85.4%) compared to La0.5Ce1.5NiO4. The structural study, conducted through various techniques, highlighted the easier reducibility of Ni from the ABO3 perovskite lattice, leaving higher oxygen vacancies, and higher basicity for reactant adsorption in DRT. Besides, in-situ DRIFTS experiments confirmed the *CHO-participated pathway of syngas generation from DRT. The findings suggested potential pathways for designing catalysts to support biomass gasification while contributing to global carbon reduction.

Biomass gasification tar removal using dielectric barrier discharge reactor: Effect of reactor geometry and carrier gases

This study investigates the impact of reactor geometry (varying external electrode length) of Dielectric Barrier Discharge (DBD) reactors on the decomposition of toluene, a model compound for biomass gasification tar, using different carrier gases and various power levels. Results reveal that toluene decomposition is higher at longer electrode lengths (30 mm) at all power levels tested. Specifically, the toluene decomposition in H2 carrier gas at 30 mm electrode length increased from 67.2 % to 97.5 % with rising power from 5 to 40 W, while it ranged from 52 % to 97.4 % at 15 mm electrode length. The decomposition of toluene was found to be higher in N2 carrier gas than in H2 carrier gas at both discharge lengths. At 30 mm external electrode and with rising power from 5 to 40 W, toluene decomposition ranged from 90.5 % to 98.7 %. Similarly, when the electrode length was reduced from 30 to 15 mm for N2 carrier gas, the decomposition of toluene ranged from 74 % to 97.9 %. Thus, the results indicate that the decomposition of toluene is affected by both the electrode length and the nature of the carrier gas. The effect of electrode length was significant at lower power levels, and the difference between the conversion at both electrode lengths nearly disappeared at higher power levels.

A DFT study on catalytic cracking mechanism of tar by Ni or Fe doped CaO during biomass steam gasification

The presence of byproduct tar seriously reduces the efficiency and syngas quality in the biomass gasification process. The current understanding of CaO-catalyzed tar cracking mainly depends on experimental findings and conjecture about CaO properties. However, there is a significant lack of simulation studies that explore the conversion mechanism at the molecular level. In this work, the reaction mechanism of tar catalytic cracking on Ni or Fe doped CaO was studied using density functional theory simulation. The benzene, phenol and toluene were chosen as the tar model compounds to determine the influences of Ni and Fe on the adsorption characteristics of CaO surface. CaO doped with Ni and Fe increases its electron affinity and electron conductivity from 4.99 to 9.21/5.58 eV-1. It also enhances the interactions between the d-orbitals of Ni/Fe atoms and the p-orbitals of O atoms. The C-H activation of ·CH3 is the most kinetically and energetically feasible first step. The Ni-doped CaO and Fe-doped CaO decrease the energy barriers by 15.7 % and 21.7 %, respectively, compared with undoped CaO. Ni enhances the surface adsorption capacity for ·H radicals to form OH* at the O top site, whereas Fe promotes the stabilization of ·H radicals at the Ca-O vacancy. Through the enhancement of chemical adsorption capacity, improvement of electronic properties, introduction of electron reorganization and orbital hybridization, low-energy shift of energy levels, and generation of new energy states (from -0.85 to -1.64/-3.05 eV), the surfaces of Ni-doped CaO and Fe-doped CaO provide a favorable electronic environment for the tar catalytic cracking, which improves the reaction efficiency and selectivity.

Tar microwave reforming over different biochar-based Ni catalysts by experiments and DFT

In order to remove tars generated during biomass pyrolysis. In this study, tar steam reforming was investigated using toluene as a tar model compound and a biochar-based nickel catalyst under microwave conditions. Based on the catalyst characterization results, density functional theory (DFT) calculations were performed to investigate the effect of different forms of nickel presence in the catalyst on toluene cracking. The experimental and simulation results indicate that the formation of Ni3S2 negatively affects the cracking of toluene. The catalyst prepared by Ni(NO3)2 impregnation demonstrated relatively good catalytic activity and stability, with 81.6 % conversion even after 190 min of pyrolysis. The ratio of water to carbon (S/C) has a significant impact on the steam reforming of toluene. A high S/C can lead to the destruction of the catalyst carbon matrix, resulting in catalyst deactivation. Conversely, a low S/C can cause a slow increase in the catalytic effect of the catalyst. The catalytic effect of the Ni(NO3)2 catalytic remains good even without the addition of water, with 78.6 % hydrogen conversion remaining at the end of 190 min pyrolysis. The addition of water molecules will inhibit the production of CH4.

Microwave-Assisted dry reforming of toluene as a model tar compound using low-cost iron catalyst for syngas clean-up

Gasification of waste feedstock such as biomass, waste plastics suffers from high tar yields during hydrogen-rich syngas production. The presence of tars result in lower quality syngas, lower syngas yields, reactor blockage, reactor down time, and costly maintenance. Therefore, removal of tar from syngas during gasification is essential. Catalytic reforming of tars via in-situ syngas cleanup is an effective way of mitigating tars. This work explores the possibility of microwave-assisted catalytic dry reforming of tars for syngas production. Due to its chemical complexity, toluene, which is one of the main tar constituents, could be used as a model tar compound. Toluene dry reforming was studied using the Fe/Al2O3 catalyst under CO2 under the temperature range of 400–700 ℃. The toluene reforming reaction was conducted using microwave and conventional thermal reactors. Under microwave irradiation, CO2 and toluene conversions are boosted to 80% at 500 ℃. Hydrogen and carbon monoxide yields were approximately five times and ten times higher in the microwave reactor at 500 ℃, respectively, compared to the productions obtained in the conventional fixed-bed reactor at 700 ℃. Filamentous carbon was also produced as a valuable side product to improve the economy of this process and such value-added carbon was only observed in the microwave reactor. Three reaction pathways were observed during microwave reaction: the toluene decomposition produces an initial hydrogen and carbon deposit on the catalyst; the formation of methane and benzene suggests toluene hydrodemethylation as a secondary reaction; and toluene hydrogenolysis forms light alkanes such as methane, and through reforming reaction under CO2 to syngas.

Insights into the adsorption and decomposition mechanisms of tar typical model compounds (benzene, toluene and phenol) on Ni(111) surface by DFT calculations

Studying the adsorption and decomposition of tar-typical model compounds on nickel surfaces is necessary to guide catalyst design. This study investigates the adsorption and decomposition mechanisms of three typical tar model compounds (benzene-benzene ring without branched chain, toluene-benzene ring with methyl group and phenol-benzene ring with hydroxyl group) on the Ni(111) surface using density functional theory calculations. The results showed that the most stable adsorption site was Bridge0 for benzene and Bridge0-F for toluene and phenol. The adsorption energies were −0.89 eV, −0.76 eV and −0.71 eV, respectively. During decomposition, benzene will first be dehydrogenated (energy barrier of 1.57 eV) and the benzene ring will be opened after the removal of the first H atom (2.10 eV). The rate-limiting step in benzene decomposition is the first dehydrogenation step (1.57 eV). Toluene decomposes like phenol by dehydrogenating functional groups before breaking the benzene ring. The rate-limiting step in the decomposition of toluene is the ring opening of the benzene ring (1.67 eV). The dehydrogenation of the benzene ring is the rate-limiting step in the decomposition of phenol (1.74 eV). This occurs after the removal of the hydrogen atom from the hydroxyl group.

Waste-derived catalysts for tar cracking in hot syngas cleaning

Catalytic tar cracking is a promising technique for hot syngas cleaning unit in gasification plants because it can preserve tars chemical energy, so increasing the syngas heating value. The cost associated with catalyst preparation is a key issue, together with its deactivation induced by coke deposition. Iron is a cheap and frequently used catalyst, which can also be found in some industrial wastes. The study aims to assess the catalytic efficiency for tar cracking of two waste-derived materials (red mud and sewage sludge) having high content of iron. The catalysts were supported on spheres of γ-Al2O3, and their efficiency was compared to that of a pure iron catalyst. The role of support was investigated by testing pure red mud, with and without the support. A series of long-term tests using naphthalene as tar model compound were carried out under different values of process temperatures (750 °C-800 °C) and steam concentrations (0 %-7.5 %). The waste derived catalysts showed lower hydrogen yields compared to pure iron catalyst, due to their lower content of iron. On the other hand, the conversion efficiencies of all the tested catalysts resulted rather similar, since the Alkali and Alkaline-Earth Metallic species present on the surface of waste-derived catalyst help in preventing coke deposition. The iron oxidation state appears to play an important role, with reduced iron more active than its oxidised form in the tar cracking reactions. This indicates the importance of tuning steam concentration to keep constant the reduced state of iron while limiting coke deposition.

Evaluation of a novel nickel ceramic filter prepared by urea precipitation method for removal of tars from biomass syngas using naphthalene as tar model compound

Gasification has been shown to be an effective thermal conversion process for waste biomass in order to create fuel, specialty chemicals, and hydrogen. Gasification is likely to take on a major role in the near future in the burgeoning hydrogen economy. In order to improve the commercial viability of gasification, catalytic tar reforming has come to the forefront as one of the most promising methods of reducing the tar content that has been detrimental to downstream equipment in the process. The use of catalytic ceramic filters has presented promising potential for this application that can serve dual role of tar and particulate removal. In this study, a nickel-based ceramic filter has been prepared using urea vacuum impregnation technique and evaluated for tar removal efficiencies. The urea technique was more effective than traditional impregnation techniques at uniformly dispersing nickel across the surface of the ceramic filter, driving the nickel particles to penetrate the pores of the support, thus aiding in improved tar removal capabilities. Through steam reforming studies using a naphthalene simulant tar molecule, the 15 wt% nickel filter was able to reduce the naphthalene content by 97% at temperature of 750 °C over 2 h. An activation energy of 126.4 kJmol was achieved for steam reforming using the urea vacuum impregnated catalytic filter compared to 136.4 kJmol using the incipient wetness impregnated filter.

Preparation of Ni-based catalyst from incineration bottom ash for steam reforming of toluene as a model tar compound

Ni-based catalysts are anticipated to emerge as the promising candidates for the reforming of biomass tar, owing to their commendable combination of high activity and low cost. However, waste-derived Ni-based catalysts exhibit inadequate stability in high-temperature reactions due to the sintering of catalyst. In this work, therefore, CaO was extracted from municipal solid waste (MSW) incineration bottom ash (IBA) as the support and the slag was prepared by melting the residues (molten slag, MS) as an inert stabilizer to enhanced the stability of catalysts. Toluene was chosen as the model compound for biomass tar to evaluate the performance of IBA-derived Ni-based catalyst in the steam reforming process. The toluene conversion of catalysts exhibited a volcano-type variation as the MS addition increased from 5 % to 20 %. Ni/MS15Ca85 exhibited excellent activity (83.87 %) at 850 °C, steam-to-carbon ratio (S/C) of 3, and gas hourly space velocity (GHSV) of 23040 h−1, and displayed no significant deactivation after 24 h (GHSV of 45700 h−1). The introduction of MS promoted the reduction of NiO, which improved the catalyst activity by forming the Ni-rich Ni-Fe alloy. Meanwhile, MS promoted the increase of lattice oxygen species (OL) content to enhance the oxidation of carbon. Furthermore, Ca2SiO4 and Ca12Al14O33 formed by the reaction of SiO2 and Al2O3 in MS with CaO contributed to mitigating the aggregation of CaO particles and maintaining the porous structure of catalysts. This work provides a novel approach to improve the catalytic performance of IBA-derived Ni-based catalyst for steam reforming of toluene (SRT) by realizing the precise regulation of each component in IBA and elimination of secondary residue disposal that was rarely considered in previous studies.

Highly Efficient Transformation of Tar Model Compounds into Hydrogen by a Ni-Co Alloy Nanocatalyst During Tar Steam Reforming.

Hydrogen (H2) production from coal and biomass gasification was considered a long-term and viable way to solve energy crises and global warming. Tar, generated as a hazardous byproduct, limited its large-scale applications by clogging and corroding gasification equipment. Although catalytic steam reforming technology was used to convert tar into H2, catalyst deactivation restricted its applicability. A novel nanocatalyst was first synthesized by the modified sol-gel method using activated biochar as the support, nickel (Ni) as the active component, and cobalt (Co) as the promoter for converting tar into H2. The results indicated that a high H2 yield of 263.84 g H2/kg TMCs (Tar Model Compounds) and TMC conversion of almost 100% were obtained over 6% Ni-4% Co/char, with more than 30% increase in hydrogen yield compared to traditional catalysts. Moreover, 6% Ni-4% Co/char exhibited excellent resistance to carbon deposition by removing the nucleation sites for graphite formation, forming stable Ni-Co alloy, and promoting the char gasification reaction; resistance to oxidation deactivation due to the high oxygen affinity of Co and reduction of the oxidized nickel by H2 and CO; resistance to sintering deactivation by strengthened interaction between Ni and Co, high specific surface area (920.61 m2/g), and high dispersion (7.3%) of Ni nanoparticles. This work provided a novel nanocatalyst with significant potential for long-term practical applications in the in situ conversion of tar into H2 during steam reforming.

The effect of CO2 and a promoter over a Ni-based catalyst on the gas production of toluene as a model tar compound

Catalytic cracking of biomass tar can increase the gas production of biomass gasification and reduce tar content. Taking tar model compound (toluene) as the research object, HZSM-5 as the catalyst carrier, the influence of different Ni loadings, promoters, and CO2 atmosphere on the gas production trend of toluene catalytic cracking was studied. The crystal structure, specific surface area, microstructure and carbon deposition of the catalyst were investigated. When the Ni/HZSM-5 load was 8 wt%, the H2 instantaneous gas production reached highest value. When the load was increased to 10 wt%, catalyst deactivation was delayed, leading to an increase in gas production. With the addition of Ca, Mn and Fe as promoters, Ca decreased the catalytic activity of Ni/HZSM-5, leading to a decrease in H2 instantaneous gas production; however, both Mn and Fe promoted the catalytic activity of Ni/HZSM-5, with Fe resulting in the highest H2 instantaneous gas production. When 20% CO2 was introduced, Ni-Mn/HZSM-5 had the highest catalytic activity and produced the highest H2 content. The characterization of the catalysts showed the addition of CO2 effectively inhibited the formation of carbon deposits, which was beneficial to prolong the catalytic effect and increase gas production.

Steam reforming of aromatics mixture as a model tar over Ni/Al2O3 structured catalyst

The Ni/Al2O3 structured catalyst shows stable performance for steam reforming of toluene and naphthalene mixture feeds for 50 hours under optimal conditions, exhibiting remarkable durability in steam reforming of model tar compounds.

Transient profiles between steam and cyclohexane as tar model compound on Ni-Mn/SBA-15 catalyst in biomass steam reforming at low temperature

Abstract The role of Mn in steam reforming of cyclohexane as a tar model compound from biomass on Ni-Mn/SBA-15 catalyst was examined using transient technique at 773 K and temperature programmed reduction (TPR) method. According to experimental results, it was expected that the addition of a small amount of Mn into Ni might strengthen the interaction between Mn and Ni surfaces, accelerate the dehydrogenation of C6H12, and inhibit the dissociation of C–C bonds in the adsorbed hydrocarbons.

Highly efficient steam reforming of biomass tar model compound using a high-entropy alloy-based structured catalyst

The development of efficient Ni-based catalysts for steam reforming of biomass tar is crucial for the widespread application of biomass gasification. Multimetal high-entropy alloys (HEAs) have emerged as promising catalysts that can potentially replace conventional metallic materials in various applications. In this study, we investigate NiFeCoMnCu-HEA catalysts supported on the surface of wood carbon (WC) for highly efficient steam reforming of toluene, a model compound for biomass tar. The synergistic effect between Ni and the other four metals in the HEA composition inhibits coke formation during the reaction process, while the appropriate mole ratio of Ni: (Fe + Co + Mn + Cu) reduces the particle size of HEA nanoparticles. Furthermore, the unique structure of wood carbon, with its long, interconnected, yet distorted microchannels, enhances the dispersion of active sites and improves mass transfer efficiency. As a result, the NiFeCoMnCu-HEAs/WC catalyst exhibits outstanding catalytic activity, achieving over 95% toluene conversion for 48 h at 650 °C. This performance surpasses that of the compared catalysts prepared in this work, namely NiFeCoMn/WC (39%), NiFeCoCu/WC (31%), and NiFeCo/WC (20%), underscoring the significance of the HEA structure. The findings also offer valuable insights into enhancing the performance of other nanostructures in the steam reforming of biomass tar.

The mechanism of photothermal catalytic reforming of biomass tar: An exploratory study based on density functional theory

Tar removal is a hot topic in the biomass gasification field. Photothermal catalytic reforming (PCR) is an emerging approach that shows high efficiency, but the mechanism and tar cracking reaction pathway under PCR are still not clear. In this paper, as a widely recognized catalyst that has both thermal and photo-catalysis effects, Ni/TiO2 was investigated and the model was calculated by density functional theory (DFT). Toluene was selected as the model tar compound, and the cracking performances were analyzed in both PCR and conventional heating conditions. It was found that H2O and toluene can be stably adsorbed on the catalyst surface but the adsorption sites are different, which provides support for the comparisons of toluene cracking pathways. The energy barriers of CC bond breaking reactions were calculated and the toluene cracking pathway showed significant changes under PCR, due to the formation of oxygen vacancy and hydroxyl radical. Toluene mainly occurs in the dissociation reaction of methyl group under conventional conditions, but toluene tends to open aromatic ring to form chain hydrocarbons under PCR conditions. It indicates that PCR shows greater activation on the aromatic ring which is crucial for tar cracking. This study confirms the change of the toluene cracking pathway under photothermal conditions by the DFT calculation method and also provides guidance for the optimization of the tar removal process and photothermal catalyst preparation.

Numerical simulation on tar cracking behavior under the Brown’s Gas atmosphere

The Chemkin® calculation software platform was used in this paper, using benzene, toluene, and phenol as tar model compounds, and the cracking law of Brown’s gas to tar model compounds was studied by sensitivity analysis at the temperature of 892.5 K and 1130.5 K, respectively. The results showed that the active free radicals of -H and -O in Brown’s gas promote the breaking of methyl bonds and hydrogen bonds, and the formation of phenoxy ions. With the increasing temperature, the secondary products also promote the tar components cracking. Brown’s gas plays an obvious catalytic role in tar cracking, reducing the activation energy of the reaction, and resulting in a decrease in cracking reaction temperature. In addition to active free radicals of Brown’s gas involved in tar cracking, the phenoxy ions also indirectly affect the tar cracking rate.

Chemical looping gasification of benzene as a biomass tar model compound using hematite modified by Ni as an oxygen carrier

The effective removal of tar is a major challenge for biomass gasification. Schemes based on chemical looping gasification (CLG) offer a promising alternative for converting biomass into syngas with low tar content. The present work examined the influence of a Ni-modified oxygen carrier (OC) on the benzene conversion rate, hydrogen yield, carbon deposition rate, and yield of combustible gas products under various experimental conditions in the CLG of benzene as a model biomass tar compound. After modification, the NiFe2O4 OC exhibited good catalytic and oxidation performance. With the increase in reaction temperature and Ni loading, the hydrogen yield, benzene conversion rate, and carbon deposition rate all increased gradually. With the increase of weight hourly space velocity (WHSV), the benzene conversion rate decreased from 89.75% to 81.1%. which indicated that the oxidation capacity of the OC to benzene decreased. Meanwhile, the addition of steam significantly eliminated the carbon deposition, which decreased to 2.62% at an S/C ratio of 1.48. During the long-term experiment, the oxidation activity and catalytic activity of the OC each exhibit a slight decreased, which was possibly due to the sintering and agglomeration of the OC. Finally, the cracking mechanism of the tar model compound benzene was proposed.

Effects of K doping over ordered mesoporous nickel-alumina catalysts on toluene steam reforming

Nickel-based catalysts have been broadly studied for the steam reforming of coal/biomass tar due to their inexpensiveness and good activity. Nevertheless, their industrial application has long been restricted by the deactivation induced by carbon deposition and Ni sintering in the reforming process. Herein, ordered mesoporous nickel-alumina catalysts with varying K contents were prepared. Their performance in steam reforming of toluene as a model compound of tar was evaluated to reveal the effects of K doping. Among them, the catalyst with 2 wt% K loading displayed the best catalytic activity and stability. Experimental and theoretical calculation results suggested that K doping enhances the water adsorption capability of the catalysts, which thereby reduces the amount and graphitization degree of carbon deposits. Moreover, the K doping reduces the acid amount of the support and thereby inhibits the formation of carbon deposits as well. However, K doping does not affect the reaction pathway of the catalysts in toluene steam reforming, and the main reaction intermediates detected with in-situ DRIFTS remain the carbonate groups. Furthermore, the confinement effects of the ordered mesoporous structures inhibit the sintering of Ni particles and the formation of carbon deposits. In addition, the influence of different spatial positions of K on the catalysts was explored. It was found that the K introduction via the one-pot solvent evaporation induced self-assembly method can suppress the loss of K from the catalysts.

Hydrogen Production from Catalytic Pyrolysis of Phenol as Tar Model Compound in Magnetic Field

Tar conversion during biomass pyrolysis is essential for hydrogen production. In this study, phenol and 10 wt.% Ni/CaO-Ca12Al14O33 were used as the tar model compound and catalyst, respectively. The purpose of the present investigation was to analyze the influence of varying magnetic field strength (ranging from 0 to 80 mT), reaction temperature (ranging from 550 to 700 °C), and carrier gas velocity (ranging from 20 to 30 mL/min) on the catalytic pyrolysis outcomes obtained from phenol. The findings indicated that the conversion rate of phenol and H2 output exhibited an increase with an escalation in magnetic field strength and reaction temperature but demonstrated a decrease with an upsurge in the carrier gas velocity. The ideal conditions for achieving the maximum phenol conversion (91%) and H2 yield (458.5 mL/g) were realized by adjusting the temperature to 650 °C, retaining the carrier gas velocity at 20 mL/min, and elevating the magnetic field intensity to 80 mT. These conditions resulted in a considerable increase in phenol conversion and H2 yield by 22.2% and 28.2%, respectively, compared with those achieved without magnetism. According to the kinetic calculations, it was indicated that the inclusion of a magnetic force had a beneficial effect on the catalytic efficacy of 10 wt.% CaO-Ca12Al14O33. Additionally, this magnetic field was observed to lower the activation energy required for the production of H2 when compared with the activation energy required during phenol catalytic pyrolysis. This consequently resulted in an enhancement of the overall efficiency of H2 production.