Tar Cracking Reactions Research Articles

Characteristics and kinetics of in-situ catalytic cracking of biomass tar in a micro dual bed reactor

This research introduces a novel micro dual bed reactor, combining rapid pyrolysis in a top-down with a fluidized bed for biomass tar catalytic cracking assessment. The study initially examines the impact of reactor zones on biomass pyrolysis, followed by an exploration of thermal and catalytic cracking behaviors of in-situ biomass tars using three fluidizing media. It delves into the generation of cracking gas and tar cracking reaction kinetics, based on the evolutionary trends of the gas. The process involves two distinct stages (S1 and S2). In S1, olivine shows the highest tar conversion rates at 650 °C and 750 °C, while char peaks at 850 °C. In S2, char leads in conversion rate and reaction time, with a preference for CO and H2. Tar conversion rates at high temperatures follow the order char > olivine > sand, and reaction times vary as olivine > sand > char. Temperature affects tar cracking rates due to thermal and catalytic effects interplay. This study applies an integral method to solve the kinetics under varying conditions, providing insights into tar behaviors during both thermal and catalytic cracking. These findings support designing and optimizing tar removal units in biomass gasification, potentially reducing tar production and enhancing gas quality.

Conversion of Biomass-Derived Tars in a Fluidized Catalytic Post-Gasification Process

The present study deals with the development, characterization, and performance of a Ni-based catalyst over a ceria-doped alumina support as a post-gasification step, in the conversion of biomass-derived tars. The catalysts were prepared using the incipient wetness technique and characterized chemically and physically using NH3-TPD, CO2-TPD, H2-TPR, XRD, Pyridine-FTIR, N2 physisorption, and H2-Pulse Chemisorption. It was observed that the 5 wt% CeO2 reduced the strong and very strong acid sites of the alumina support and helped with the dispersion of nickel. It was noticed that the nickel crystallite sizes and metal dispersion remained unchanged as the nickel loading increased. The performance of the catalysts was studied in a mini-fluidized CREC Riser Simulator at different temperatures and reaction times. The selected tar surrogate was 2-methoxy-4-methylphenol, given its functional group similarities with lignin-derived tars. A H2/CO2 gas blend was used to emulate the syngas at post-gasification conditions. The obtained tar surrogate conversion was higher than 75%, regardless of the reaction conditions. Furthermore, the catalysts used in this research provided an enhancement in the syngas product composition when compared to that observed in the thermal experiments. The presence of hydrocarbons greater than CH4 (C1+) was reduced at 525 °C, from 96 ± 3% with no catalyst, to 85 ± 2% with catalyst and steam, to 68 ± 4% with catalyst and steam-H2/CO2. Thus, the catalyst that we developed promoted tar cracking, tar reforming, and water-gas shift reactions, with a H2/CO ratio higher than 3.8, providing a syngas suitable for alcohol synthesis.

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.

Experimental study of biomass waste gasification: Impact of atmosphere and catalysts presence on quality of syngas production

The presented research results the gasification process of biomass waste (brewery spend grain, wheat straw, hay, pine sawdust). Experimental investigations focused on determining the influence of gasification agent (CO2, steam, and steam and CO2 mixture) and the presence of a solid catalyst (MgO∙CaO, TiO2, CuO and SrO). Investigations were performed towards syngas production. A wide range of analyses and instrumental methods were used to determine the properties of gasification process products, including: GC, TGA, FTIR, SEM, BET. The main component of syngas obtained produced in atmosphere CO2 and steam mixture was hydrogen. The H2 concentration increased from 20% up to 44% in case of brewery spend grain. The presence of the catalyst in the gasification process favoured the tar cracking reaction. The amount of tar was reduced by more than 17% in case of brewery spend grain. As well as syngas composition was enriched with CH4, H2 and CO concentration.

Modeling pyrolysis kinetics of extracted biomass components with the bio-chemical percolation devolatilization model

Within the pyrolysis modeling of biomass, the solid is typically interpreted as a mixture of cellulose, hemicellulose, and lignin, where the components react independently before the products are linearly superimposed. This assumption applies to the bio-chemical percolation devolatilization model (Bio-CPD), which, as a network model, represents the physicochemical processes very realistically. A literature review regarding available structural and kinetic parameters for the Bio-CPD model revealed four hits. To investigate and evaluate the model predictivity, experiments from extracted lignocellulosic biomass components pyrolyzed separately in a fluidized bed reactor are used as reference data. Using single-component data simplifies the reaction chemistry by minimizing interactions between the components and best approximates the model assumptions. The comparison between experimentally obtained total volatile release rates and model predictions revealed the parameter set from Vizzini et al. (2008) for cellulose and the one from Sheng and Azevedo (2002) for hemicellulose as best-suitable candidates to predict the pyrolysis behavior for the boundary conditions of the fluidized bed reactor. For lignin, instead, the Genetti correlation (Genetti, Fletcher, and Pugmire, 1999) to estimate structural parameters based on the ultimate and proximate analysis led to better results than all other sets. Further, the analysis of light gas and tar fractions showed a high relevance to model secondary tar cracking reactions in the fluidized bed reactor system.

Modelling and verifying multi-path product generation pyrolysis of waste cabbage leave

This research employs a conventional pyrolysis mechanism model to determine the characteristics and kinetics of waste cabbage leave pyrolysis. Further analysis is conducted using the Multi-path Product Generation model to support the findings. The samples were pyrolyzed from ambient temperature to 1073.15 K at heating rates of 20, 30, 40, and 50 K/min. Average activation energies obtained for waste cabbage leave through the Kissinger-Akahira-Sunose, Flynn-Wall-Ozawa, and Starink methods ranged from 102.44 to 319.71 kJ/mol, 105.43–296.83 kJ/mol, and 115.36–366.19 kJ/mol, respectively. The average values across these methods were 183.97, 181.08, and 207.42 kJ/mol, displaying noticeable variation in activation energies depending on the method employed. Additionally, the model demonstrated: (i) pyrolysis attributes in multi-path reactions, (ii) product-generating regime development, and (iii) valuable insights of pyrolysis product yield and composition. The model comprises three primary devolatilization reactions (iChar, iGas, iTar), two tar cracking reactions (tar to char, tar to gas), and one secondary char reaction. The model parameters accurately align with the thermogravimetric curves at varying heating rates, which demonstrate the progression of multiple product pathways. Thus, this model can be effectively utilized for the simulation of multi-path pyrolysis processes.

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.

Validation of a biomass conversion mechanism by Eulerian modelling of a fixed-bed system under low primary air conditions

This work presents a three-dimensional Computational Fluid Dynamics study of a small-scale biomass combustion system operating with low primary air ratios. The Eulerian Biomass Thermal Conversion Model (EBiTCoM) was adapted to incorporate a pyrolysis mechanism based on the detailed Ranzi-Anca-Couce (RAC) scheme. Two scenarios were simulated using woodchips with 8% and 30% moisture content, and the results were validated against experimental data, including in-flame and bed measurements. The model accurately predicted bed temperature profiles and the influence of fuel moisture content on the pyrolysis and drying fronts, as well as on the distribution of volatiles and temperatures above the solid fuel bed. For the 8% moisture content case, the average gas temperature above the bed is approximately 700 °C, while for the 30% case, it drops to around 400 °C. The lower temperatures hinder the tar cracking reaction, resulting in a 25% higher tar content in the producer gas for the 30% moisture content fuel. The lower part of the bed consists of a thick layer of char undergoing reduction reactions, similar to that of an updraft gasifier. The developed model can accurately simulate biomass combustion systems with solid fuel beds consisting of numerous particles, while maintaining low computational requirements.

Prediction and optimization of syngas production from Napier grass air gasification via kinetic modelling and response surface methodology

In this research, a kinetic model was developed for Napier grass air gasification using Aspen Plus software and thereafter collated and validated with experimental results obtained from a lab-scale fluidized bed reactor. Herein, the model was further employed to investigate the effect of gasification operating parameters, including temperature (650–850) °C, equivalence ratio (0.2–0.4), and moisture content (0–18) wt.% on products’ yields and syngas quality. The model was segregated into four sections: the drying, devolatilization, combustion, and reduction sections. The outputs of tar, gas, and char from the devolatilization section were defined by external Microsoft Excel subroutine. Kinetic parameters were incorporated in the combustion and reduction sections to simulate tar cracking, oxidation, and reduction reactions. A good agreement was observed between the predicted and experimental results. Furthermore, response surface methodology (RSM) was employed to analyze the mutual effects of process variables and perform multi-objective optimization to maximize producer gas yield, higher heating value, carbon conversion efficiency, and cold gas efficiency by incorporating the predicted results from the developed kinetic model. The optimized syngas yield and HHV were 69.42 wt% and 8.14 MJ/Nm3 at 850 °C, 0.3021, and 15.69 wt% for temperature, ER, and moisture content, respectively.

Assessment of the inherent CaO in char on tar catalytic conversion by a micro fluidized bed reaction analyzer for biomass gasification

In this study, a newly-developed micro fluidized bed reaction analyser (MFBRA) was adopted to examine the effects of char’s inherent CaO on tar catalytic cracking from 1023 K to 1173 K. A simulated CaO-based char catalyst was prepared by impregnation method. For comparison, the blank experiments were also conducted by employing the demineralized char free of CaO. The tar cracking behaviour and reaction kinetics based on the gas products generation were systematically compared. The structure and reaction activity of char sample before and after experiment were also analyzed. The results shows that tar conversion processes can be divided into a fast cracking stage and a stable conversion stage. By increasing the reaction temperature, the tar conversion by char without CaO to the total gas products and its reaction rate changed from 41.15 % to 71.50 % and from 0.055 s−1 to 0.081 s−1, respectively; but for char with CaO, they improved from 57.54 % to 88.25 % and from 0.062 s−1 to 0.096 s−1, respectively. The presence of CaO dramatically enhanced tar conversion even at a low temperature of 1023 K, producing more effective gas components, such as H2, CO, CO2, and C2H6. For CH4 and C3H6, the promotion became obvious at high temperatures above 1123 K. During tar catalytic cracking, CaO suppressed carbon deposition on char surface to some extent. Meanwhile, the generation activation energy (Ea) of CH4, the total gas products, H2, CO2, CO, C2H6, and C3H6 decreased to a great extent, indicating the good catalytic activity of the inherent CaO in char.

Dual effect of CaO on waste PVC plastics pyrolysis: A kinetics study using Fraser-Suzuki deconvolution

Green and sustainable treatment of waste plastics emerged as one of the greatest challenging matters facing the world. In this work, the pyrolysis of chlorine-contained PVC plastics was investigated from a kinetic study perspective. The dual effects of calcium oxide (CaO) additive were examined. The Fraser-Suzuki deconvolution function was adopted to fit the differential thermogravimetric (DTG) curves into three pseudo-reactions. The apparent activation enegry (Ea), pre-exponential factor (A) and reaction mechanism models (f(a)) for each pseudo-reaction were identified. The results show that the use of CaO increased significantly the Ea value of the dehydrochlorination reaction but lowered that of the tar cracking reactions, indicating an apparent HCl release mitigation as well as tar compounds decomposition. The mechanism models of the three pseudo-reactions kept identical with or without CaO addition, which were A4 Avrami-Erofeev: ƒ(α)=4(1-α) [-ln(1-α)]3⁄4 for pseudo-reactions 1 and 3 and B1 Prout-Trompkins: ƒ(α)=α(1-α) for pseudo-reactions 2. The kinetic models of the pyrolysis of waste PVC and the mixtures were thus established. The obtained results could be used as a theoretical basis for the development and application of clean and efficient waste PVC plastics pyrolysis technology.

Performance of expanded perlite as granular bed filtration media: Effect on coal pyrolytic products

Granular bed filtration is considered one of the most promising engineering solutions to the dust removal from high temperature coal pyrolytic vapors. Expanded perlite as filtration media was studied in a lab-scale fixed bed reactor to understand its effect on pyrolysis products. Factors including filtration temperature, filtration time, dust accumulation and regeneration effect were investigated. The results show that there is around 12.9 % reduction in tar yield even under an optimized temperature of 550 °C after filtration bed. Dust accumulation would further promote tar cracking reactions. Coke deposition on the perlite increases with filtration time, which causes further reduction in tar yield. The redox cycle between filtration and regeneration significantly alters the properties of perlite. It is found that carbon deposition on perlite is alleviated in the initial cycles due to the formation of potassium carbonate. And the extent of tar cracking becomes weak. With the cycle numbers further increasing, the situation is inversed. Characterization techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) were applied to explore the mechanisms behind the findings. Potassium and transition metals such as iron contained in perlite may make significant contributions.

Catalytic pyrolysis of biogas residues with incineration bottom ash by TG-MS: Kinetics analysis and biochar stability

Catalytic pyrolysis is an efficient disposal technology for multiple solid wastes. In this work, biomass incineration bottom ash (ASH) was added into the pyrolysis of biogas residues (BR) to suppress the over reactions for high-quality biochar yield, and the pyrolysis behavior and non-condensable gases release behavior were investigated by Thermogravimetrice-Mass spectrometry (TG-MS). Kinetics of pyrolysis and carbon sequestration potential of biochar were studied to reveal the mechanism of pyrolysis and the properties of solid product. The addition of ASH showed an inhibitive effect on BR pyrolysis, while it favored the biochar formation and stability. The kinetics analysis by Starink method confirmed that average activation energy increased with ASH blending, where the highest average activation energy was 209.06 kJ/mol of catalytic pyrolysis with 10 wt% of ASH. Kinetics data by Coats-Redfern method showed that the first-order reaction function performed well fitting at low temperature region, while the second-order reaction fitted well at high temperature. MS analysis detected the main release of CO2 during catalytic pyrolysis. The ASH addition made a slight influence on thermal-decomposite reactions including tar cracking reactions. Composite biochar derived from BR pyrolyzed with 20 wt% of ASH had the preferable carbon sequestration potential of 27.29%. According to the expected output of biogas in 2030 by Chinese government, once all the biogas residue were pyrolyzed with 20 wt% of ASH to produce agricultural biochar, a potential of 8.11 million tons of equivalent carbon dioxide could be prevented to emmision and storaged in biochar as carbon sink. This work provides an important reference to understand fundamental characteristics of pyrolysis of BR with ASH.

Hydrogen-Rich Gas Production from Two-Stage Catalytic Pyrolysis of Pine Sawdust with Nano-NiO/Al2O3 Catalyst

Hydrogen production from biomass pyrolysis is economically and technologically attractive from the perspectives of energy and the environment. The two-stage catalytic pyrolysis of pine sawdust for hydrogen-rich gas production is investigated using nano-NiO/Al2O3 as the catalyst at high temperatures. The influences of residence time (0–30 s) and catalytic temperature (500–800 °C) on pyrolysis performance are examined in the distribution of pyrolysis products, gas composition, and gas properties. The results show that increasing the residence time decreased the solid and liquid products but increased gas products. Longer residence times could promote tar cracking and gas-phase conversion reactions and improve the syngas yield, H2/CO ratio, and carbon conversion. The nano-NiO/A12O3 exhibits excellent catalytic activity for tar removal, with a tar conversion rate of 93% at 800 °C. The high catalytic temperature could significantly improve H2 and CO yields by enhancing the decomposition of tar and gas-phase reactions between CO2 and CH4. The increasing catalytic temperature increases the dry gas yield and carbon conversion but decreases the H2/CO ratio and low heating value.

Simulation of air gasification of Napier grass using Aspen Plus

In this research, a thermodynamic equilibrium model was developed using Aspen Plus for parametric study of syngas composition and product yield from air gasification of Napier grass in a fluidized bed gasifier. Operating conditions of gasification encompassing temperature, pressure, equivalence ratio (ER), and moisture content were manipulated and studied with further validation against experimental data. Tar cracking reaction was considered with M-cresol and heptane as represented species. Conversion of homogenous reactions was empirically correlated in MATLAB and further utilized to adjust the composition of CO, H2 and CH4 for prediction of experimental data with better accuracy, where the average mean error ranges from 0.20 to 0.25. Maximum lower heating value (LHV) was attained at 750 °C, contributed by the highest CH4 composition. ER change had limited effect on bioliquid yield but showed a significant contribution in lowering biochar yield. Lowering moisture content improved syngas quality significantly through the promotion of CO, H2 and CH4 production, with minimum biochar and bioliquid yield. The optimized LHV were 7.69 MJ/Nm3 at T = 750 °C, ER = 0.2 and moisture content of 4.5 wt%.

The effect of steam concentration on hot syngas cleaning by activated carbons

Activated carbons are recognised as inexpensive, easily available, and efficient catalysts for tar cracking reactions at high temperatures. Their use in hot syngas cleaning is limited by the rapid deactivation resulting from the coke deposition, and the consequent masking phenomenon over the activated carbon surface. This study contributes to a better understanding of the problem and sheds light on possible solutions, by investigating how the temperature (750 °C–850 °C) and the steam concentration (0%–10%) can affect the extent of the water-gas and reforming reactions occurring between steam and the coke covering the catalyst surface. The activated carbons have been fully characterised before and after the tests utilizing naphthalene as a tar model compound, to study the evolution of their structure after long duration tests. Steam positively affects the naphthalene conversion efficiency, preserving the characteristics of the material. For example, at a temperature of 800 °C and a steam concentration of 7.5%, a naphthalene conversion of over 90% is achieved even after 7 h of testing. XRD and Raman spectroscopy analyses suggest that naphthalene cracking produces a coke layer that is more amorphous and, above all, more reactive towards the water gas reaction than the original activated carbon structure.

Experimental validation of complex mathematical model of screw reactor coupled with particle model describing pyrolysis of lignocellulosic biomass

As in every complex technological process design, pyrolysis reactor design and optimization require a mathematical model of sufficient complexity. Aside from properties of utilized feedstock, reaction temperature and residence time of solid and gaseous phase are the main parameters in pyrolysis reactor design. Screw pyrolysis reactor, also known as auger reactor, is a well-known and studied reactor type suitable for pyrolysis of heterogeneous materials. In this work, a complex mathematical model of a screw pyrolysis reactor coupled with a particle model is proposed. Primary pyrolysis reactions as well as secondary tar cracking reactions were taken into account. In addition, heat and mass transfer inside a solid particle and heat transfer between both phases and the reactor wall were considered. Experimental results obtained at a pyrolyzer temperature of 650, 700 and 750 °C and residence time of solid phase of 7, 9.6 and 15 min were compared with data predicted by the mathematical model. Both, tar and gas yield predicted by the model at 700 and 750 °C showed a good agreement with experimental data at all solid phase residence times. However, the model predicted higher gas production compared to the experimental one at 650 °C. The value of relative deviation of char yield remained under 2% under all pyrolysis conditions. The proposed model can be used for simulation and optimization of industry-scale screw pyrolyzers. However, its validity should be verified on wider operation temperature range.

Iron Ore Reduction by Biomass Volatiles

The use of biomass in different routes of ironmaking has been recently investigated. Although the volatile matter is the major constituent of the biomass, its contribution as reducing agent has not been explored in detail. This work aimed to investigate the reduction of iron ore by biomass volatiles and elucidate the steps involved, searching for optimization of its use in the ironmaking industry. Experiments with biomass and iron ore packed beds placed separately were carried out in the interval between 200 and 1000 °C in an infrared furnace. Iron ore reduction was observed at low temperatures (< 800 °C) and increased up to 1000 °C, where wustite, metallic iron and cementite were detected by XRD. Carbon deposition at low temperatures and carbothermic reduction at high temperatures were identified through carbon analysis and thermal profile measures. At low temperature the reduction occurred mainly by the non-condensable gases (CO, H2) from biomass pyrolysis and tar cracking reactions, while from 800 °C the reduction advanced by carbon deposited on the iron ore. The activation energy for the carbothermic reduction indicated the carbon gasification as the reaction-limiting step. The deposited carbon proved to be more reactive than other carbon sources commonly used in the ironmaking.

Simulation of the Fast Pyrolysis of Coffee Ground in a Tilted-Slide Reactor

The fast pyrolysis of coffee ground for bio-crude oil production was simulated in a tilted-slide reactor. The biochemical composition was derived by an extended biomass characterization method based on the elemental analysis. The simulation was performed in a steady-state and a Lagrangian multiphase model was adopted to describe the transport of sand and biomass particles together with a multistep kinetic mechanism for fast pyrolysis. When the secondary tar cracking reactions were not considered the volatile yield increased monotonically with temperature. The inclusion of secondary reactions could improve the prediction of volatile yield which turn to decrease at higher temperature. It was found that not only the maximum volatile yield but also the corresponding reactor temperature agreed well with the experimental results. At the temperature higher than 550 °C the trend of volatile yield is similar to that of experiment while it is larger at lower reactor temperature. The individual species yields were compared at various reactor temperatures and the pyrolysis processes were analyzed by tracking the reference components when they were decomposed along the distance. It was found that the reactor temperature should be above 500 °C for effective pyrolysis of all reference components of coffee ground.

Fluidized bed gasification of eucalyptus chips: Axial profiles of syngas composition in a pilot scale reactor

Air gasification tests have been carried out in a pilot scale bubbling fluidized bed gasifier, able to treat up to 100 kg/h of Eucalyptus wood chips, and operated, under chemical and thermal steady-state conditions, at different values of equivalence ratio. The results are reported in terms of the main process performance parameters and transfer coefficients, which indicate the fate of carbon in the different output streams (syngas, fines, tars). The results can be transferred to commercial scale reactors and confirm the technical feasibility of the air gasification of the biomass in the range 0.25–0.34 of equivalence ratio, yielding a good quality syngas, with a low heating value between 5000 and 6000 kJ/m3N,syngas. The measured axial syngas composition profiles along the reactor indicate a monotonic reduction of methane and carbon dioxide concentrations and an increase of that of hydrogen. This suggests that water gas shift and steam and dry reforming reactions play a significant role, together with a limited contribution of tar cracking reactions, probably enhanced by alkali and alkaline earth metallic species contained in the inorganic fraction of elutriated fines. Results can support the implementation of reliable numerical models of bubbling fluidized bed gasifiers and the optimisation of design/operating criteria.