Detailed kinetic modeling of autocatalysis in methane pyrolysis

Effect of Hydrogen Addition on Coke Formation and Product Distribution in Catalytic Coupling of Methane

The effect of hydrogen addition on catalytic nonoxidative coupling of methane at 1000 °C was investigated. Experiments were performed at varying ratios between the catalyst and the postcatalytic volume to discern the effect of hydrogen on the catalytic reaction as well as on the gas-phase reaction. Adding 10% H2 decreases the methane conversion by a factor of 2, almost independent of the ratio between the catalyst and the postcatalytic residence time. The effect on the conversion is mostly determined by gas-phase chemistry. Hydrogen addition has no influence on the C2 hydrocarbon yield, whereas aromatic selectivity is significantly reduced. Changes in selectivity are attributed to changes in methane conversion. Quantitative determination of the amount of coke deposited on the catalyst reveals a decrease by 1 order of magnitude when dosing up to 10% H2, while carbon deposits-downstream of the catalyst bed are suppressed to a much lower extent. These results suggest a process in which the produced hydrogen is partly recycled, maximizing the carbon selectivity to C2 hydrocarbons while minimizing both aromatics and, most crucially, formation of coke on the catalyst as well as further deposits-downstream.

First principles reaction discovery: from the Schrodinger equation to experimental prediction for methane pyrolysis

Our recent success in exploiting graphical processing units (GPUs) to accelerate quantum chemistry computations led to the development of the ab initio nanoreactor, a computational framework for automatic reaction discovery and kinetic model construction. In this work, we apply the ab initio nanoreactor to methane pyrolysis, from automatic reaction discovery to path refinement and kinetic modeling. Elementary reactions occurring during methane pyrolysis are revealed using GPU-accelerated ab initio molecular dynamics simulations. Subsequently, these reaction paths are refined at a higher level of theory with optimized reactant, product, and transition state geometries. Reaction rate coefficients are calculated by transition state theory based on the optimized reaction paths. The discovered reactions lead to a kinetic model with 53 species and 134 reactions, which is validated against experimental data and simulations using literature kinetic models. We highlight the advantage of leveraging local brute force and Monte Carlo sensitivity analysis approaches for efficient identification of important reactions. Both sensitivity approaches can further improve the accuracy of the methane pyrolysis kinetic model. The results in this work demonstrate the power of the ab initio nanoreactor framework for computationally affordable systematic reaction discovery and accurate kinetic modeling.

Emission characteristics and influencing mechanisms of PAHs and EC from the combustion of three components (cellulose, hemicellulose, lignin) of biomasses

Biomass burning is an important source of polycyclic aromatic hydrocarbons (PAHs) and elemental carbon (EC), but the formation mechanisms are still unclear. Cellulose, hemicellulose, and lignin are the three major components of biomass. In this study, the three-components extracted from three typical biomass raw materials were used for laboratory combustion experiments, to investigate the differences in the emission factors and chemical compositions of PAHs and EC. The average emission factors of the 16 kinds of PAHs were showing as lignin (135 ± 180 mg/kg) > cellulose (97.8 ± 124 mg/kg) > hemicellulose (48.9 ± 65.2 mg/kg), and the average emission factors of EC presented in the descending order of cellulose (1.65 ± 3.02 g/kg), lignin (1.30 ± 1.04 g/kg), and hemicellulose (0.450 ± 0.480 g/kg), respectively. The proportion of naphthalene emitted from cellulose and hemicellulose combustion is higher, while fluoranthene and pyrene accounted significantly higher proportion for lignin. Moreover, the influence of ignition temperature and oxygen content on the emission characteristics of PAHs and EC were also discussed. The influence of ignition temperature on the emission of EC and PAHs is more significant compared to oxygen content, because it obviously promoted the PAHs and EC formations through resonance-stabilized hydrocarbon-radical chain reaction (RSR) pathway. However, correlation analysis combined with cluster analysis showed that the RSR-pathway probably had different effects on PAH growth for the three-components, as the indene-involved RSR-pathway were mainly related to 4-6 ring PAHs for cellulose and lignin (except fluoranthene and pyrene), but 2-4 ring PAHs for hemicellulose. We also found that the fitted results according to the proportion of three-components were significantly higher than the measured values of raw materials for indene, medium-molecular-weight PAHs, and soot-EC. These results presented the different formation pathways for medium-molecular-weight PAHs and the two EC components emitted by biomass combustion, which are worthy of further studies in exploring the generation mechanisms of PAHs and EC.

Influence of hydrocarbon feed additives on the high-temperature pyrolysis of methane in molten salt bubble column reactors

Molten salts are excellent heat transfer fluids and a potential reaction environment for methane pyrolysis in which solid carbon can be continuously produced and separated from the liquid phase. Significant...

Effect of ethane and ethylene on catalytic non oxidative coupling of methane

The effect of addition of ethane and ethylene (C2) on methane coupling at 1000 °C was investigated. A Fe/SiO2 catalyst was used to determine the contributions of catalytic as well as C2 initiated methane activation. The catalyst load as well as the residence times at 1000 °C downstream of the catalyst bed were varied. C2 addition significantly increases methane conversion rates, similarly for both ethane and ethylene, although ethylene is more effective when operating with long residence times in the post-catalytic volume. Methane activation via C2 addition proceeds dominantly in the gas-phase whereas catalytic C2 activation is negligible. The catalyst has no effect on methane conversion when the feed contains more than 2 vol% C2. Product selectivity distribution as well as total hydrocarbon yield at 10% conversion is not influenced by C2 addition, but is influenced by the amount of catalyst as well as residence time in the post-catalytic volume at high temperature. It is proposed that C2 impurities in natural gas change from a nuisance to an advantage by enhancing methane conversion and simplifying purification of the natural gas feed. A process is proposed in which ethylene is recycled back into the reactor to initiate methane coupling, leading to a process converting methane to aromatics.

Direct Evidence on the Mechanism of Methane Conversion under Non-oxidative Conditions over Iron-modified Silica: The Role of Propargyl Radicals Unveiled

Radical-mediated gas-phase reactions play an important role in the conversion of methane under non-oxidative conditions into olefins and aromatics over iron-modified silica catalysts. Herein, we use operando photoelectron photoion coincidence spectroscopy to disentangle the elusive C2+ radical intermediates participating in the complex gas-phase reaction network. Our experiments pinpoint different C2 -C5 radical species that allow for a stepwise growth of the hydrocarbon chains. Propargyl radicals (H2 C-C≡C-H) are identified as essential precursors for the formation of aromatics, which then contribute to the formation of heavier hydrocarbon products via hydrogen abstraction-acetylene addition routes (HACA mechanism). These results provide comprehensive mechanistic insights that are relevant for the development of methane valorization processes.

Effects of Different Conditions on Co-Pyrolysis Behavior of Corn Stover and Polypropylene

The pyrolysis behavior of corn stover and polypropylene during co-pyrolysis was studied using a tube furnace reactor. The effects of mixing ratio of corn stover and polypropylene, pyrolysis temperature, addition amount of catalyst (HZSM-5) and reaction atmosphere (N₂ and CO₂) on the properties of pyrolysis products were studied. The results showed that co-pyrolysis of corn stover and polypropylene can increase the yield of pyrolysis oil. When corn stover:polypropylene = 1:3, the yield of pyrolysis oil was as high as 52.1%, which was 4.5% higher than the theoretical value. With the increase of pyrolysis temperature, the yield of pyrolysis oil increased first and then decreased, and reached the optimal yield at 550 °C. The addition of catalyst (HZSM-5) reduced the proportion of oxygenates and promoted the generation of aromatic hydrocarbons. CO₂ has a certain oxidation effect on the components of pyrolysis oil, which promoted the increase of oxygen-containing aromatics and the reduction of deoxy-aromatic hydrocarbons. This study identified the theoretical basis for the comprehensive utilization of plastic and biomass energy.

Kinetics and mechanism of decomposition induced by solvent evolution in ICM-101 solvates: solvent-evolution-induced low-temperature decomposition

[2,2'-Bi(1,3,4-oxadiazole)]-5,5'-dinitramide (ICM-101), a high-energy-density material, was reported in recent years. ICM-101 is the first energetic material with the 2,2'-bi(1,3,4-oxadiazole) structure as the main ring structure. The molecular structure of ICM-101 shows excellent planar characteristics, providing a new option for the design of high-energy-density materials. However, during crystal preparation, ICM-101 easily interacts with solvents and forms the corresponding solvates. Interestingly, during thermal decomposition, when the solvent escapes from ICM-101 solvates, it induces the decomposition of ICM-101. In this study, the decomposition of ICM-101 induced by solvent evolution was evaluated in detail, and the decomposition kinetic equation was established. The mechanism of solvent-evolution-induced decomposition in ICM-101 solvates was further studied, and it was found that solvent evolution might produce defects in the crystals of ICM-101 solvates, and induce the decomposition of ICM-101 on the defects.

Pyrolysis of mixtures of methane and ethane: activation of methane with the aid of radicals generated from ethane

Inert CH₄ molecules can be activated and incorporated into pyrolysis products with the aid of radicals generated by pyrolysis of C₂H₆.

Theoretical study of sensitive reactions in phenol decomposition

The reactivity of phenol is of utmost importance in combustion systems.

Direct Non-Oxidative Methane Conversion in a Millisecond Catalytic Wall Reactor

Direct non-oxidative methane conversion (DNMC) has been recognized as a single-step technology that directly converts methane into olefins and higher hydrocarbons. High reaction temperature and low catalyst durability, resulting from the endothermic reaction and coke deposition, are two main challenges. We show that a millisecond catalytic wall reactor enables stable methane conversion, C₂₊ selectivity, coke yield, and long-term durability. These effects originate from initiation of the DNMC on a reactor wall and maintenance of the reaction by gas-phase chemistry within the reactor compartment. The results obtained under various temperatures and gas flow rates form a basis for optimizing the process towards lighter C₂ or heavier aromatic products. A process simulation was done by Aspen Plus to understand the practical implications of this reactor in DNMC. High carbon and thermal efficiencies and low cost of the reactor materials are realized, indicating the technoeconomic viability of this DNMC technology.

On the formation of cyclopentadiene in the C3H5˙ + C2H2 reaction

The reaction between the allyl radical (C3H5˙) and acetylene (C2H2) in a heated microtubular reactor has been studied at the VUV beamline of the Swiss Light Source. The reaction products are sampled from the reactor and identified by their photoion mass-selected threshold photoelectron spectra (ms-TPES) by means of imaging photoelectron photoion coincidence spectroscopy. Cyclopentadiene is identified as the sole reaction product by comparison of the measured photoelectron spectrum with that of cyclopentadiene. With the help of quantum-chemical computations of the C5H7 potential energy surface, the C2H2 + C3H5˙ association reaction is confirmed to be the rate determining step, after which H-elimination to form C5H6 is prompt in the absence of re-thermalization at low pressures. The formation of cyclopentadiene as the sole product from the allyl + acetylene reaction offers a direct path to the formation of cyclic hydrocarbons under combustion relevant conditions. Subsequent reactions of cyclopentadiene may lead to the formation of the smallest polycyclic aromatic molecule, naphthalene.

Fast co-pyrolysis of cellulose and polypropylene using Py-GC/MS and Py-FT-IR

This work features the production of C8–C20 long chain alcohols and hydrocarbons via fast co-pyrolysis of cellulose and polypropylene. Decrease in pyrolysis time and increase in H/O ratio and HHV of bio-oil are demonstrated.

Reactions of allylic radicals that impact molecular weight growth kinetics

The reactions of allylic radicals have the potential to play a critical role in molecular weight growth (MWG) kinetics during hydrocarbon oxidation and/or pyrolysis.

Coke Formation in Steam Reforming of Natural Gas over Rhodium/Alumina Catalysts: An Atomic Force Microscopy Study using the Oscillating Friction Mode

The initial stage of coke formation in steam reforming of natural gas over rhodium/alumina catalysts was studied microscopically. A well-defined model catalyst prepared by an aerosol technique was placed in a flow reactor to very mildly coke the catalyst sample. Therefore, a natural gas–steam mixture at steam-to-carbon ratios of unity was fed to the reactor operated for thirty minutes at atmospheric pressure and moderate temperatures of 650 ºC. Fresh and used catalyst samples were characterized by SEM-EDX and a recently developed AFM technique, the Oscillating Friction Microscopy (OFM), to analyze the friction characteristics of the sample. OFM combined with SEM-EDX allowed to distinguish between coke depositions, alumina support (Al2O3), and Rh particles and to locate the initial carbon deposition in the process. It was found that coke formation starts on the catalyst particle. The carbonaceous overlayer can be removed from the catalyst and the closely surrounding support by multiples scans with the AFM tip.

Theoretical study of the formation of naphthalene from the radical/π-bond addition between single-ring aromatic hydrocarbons

The experimental investigations performed in the 1960s on the o-benzyne + benzene reaction as well as the more recent studies on reactions involving π-electrons highlight the importance of π-bonding for different combustion processes related to PAH's and soot formation. In the present investigation radical/π-bond addition reactions between single-ring aromatic compounds have been proposed and computationally investigated as possible pathways for the formation of two-ring fused compounds, such as naphthalene, which serve as precursors to soot formation. The computationally generated optimized structures for the stationary points were obtained with uB3LYP/6-311+G(d,p) calculations, while the energies of the optimized complexes were refined using the uCCSD(T) method and the cc-pVDZ basis set. The computations have addressed the relevance of a number of radical/π-bond addition reactions including the singlet benzene + o-benzyne reaction, which leads to formation of naphthalene and acetylene through fragmentation of the benzobicyclo[2,2,2]octatriene intermediate. For this reaction, the high-pressure limit rate constants for the individual elementary reactions involved in the overall process were evaluated using transition state theory analysis. Other radical/π-bond addition reactions studied were between benzene and triplet o-benzyne, between benzene and phenyl radical, and between phenyl radicals, for all of which potential energy surfaces were produced. On the basis of the results of these reaction studies, it was found necessary to propose and subsequently confirm additional, alternative pathways for the formation of the types of PAH compounds found in combustion systems. The potential energy surface for one reaction in particular, the phenyl + phenyl addition, is shown to contain a low-energy channel leading to formation of naphthalene that is energetically comparable to the other examined conventional pathways leading to formation of biphenyl compounds. This channel is the first evidence of a reaction which involves an aromatic radical adding to the nonradical π-bond site of another aromatic radical which leads directly to a fused ring structure.

The role of the gas species on the formation of carbon nanotubes during thermal chemical vapour deposition

In this paper, we investigate the several roles that hydrogen plays in the catalytic growth of carbon nanotubes from the point of view of gas species, catalyst activation and subsequent interaction with the carbon nanotubes. Carbon nanotubes and nanofibres were grown by thermal chemical vapour deposition, using methane and a mixture of hydrogen and helium, for a range of growth temperatures and pre-treatment procedures. Long, straight carbon nanotubes were obtained at 900 °C, and although the growth yield increases with the growth temperature, the growth shifts from nanotubes to nanofibres. By introducing a helium purge as part of the pre-treatment procedure, we change the gas chemistry by altering the hydrogen concentration in the initial reaction stage. This simple change in the process resulted in a clear difference in the yield and the structure of the carbon nanofibres produced. We find that the hydrogen concentration in the initial reaction stage significantly affects the morphology of carbon fibres. Although hydrogen keeps the catalyst activated and increases the yield, it prevents the formation of graphitic nanotubes.

On the Combination Reactions of Hydrogen Atoms with Resonance-Stabilized Hydrocarbon Radicals

Procedures for accurately predicting the kinetics of H atom associations with resonance stabilized hydrocarbon radicals are described and applied to a series of reactions. The approach is based on direct CASPT2/cc-pvdz evaluations of the orientation dependent interaction energies within variable reaction coordinate transition state theory. One-dimensional corrections to the interaction energies are estimated from a CASPT2/aug-cc-pvdz minimum energy path (MEP) on the specific reaction of interest and a CASPT2/aug-cc-pvtz MEP for the H + CH3 reaction. A dynamical correction factor of 0.9 is also applied. For the H + propargyl, allyl, cyclopentadienyl, and benzyl reactions, where the experimental values appear to be quite well determined, theory and experiment agree to within their error bars. Predictions are also made for the combinations with triplet propargylene, CH2CCCH, CH3CCCH2, CH2CHCCH2, CH3CHCCH, cyclic-C4H5, CH2CCCCH, and CHCCHCCH.

Amorphous carbon contamination monitoring and process optimization for single-walled carbon nanotube integration

We detail the monitoring of amorphous carbon deposition during thermal chemical vapour deposition of carbon nanotubes and propose a contamination-less process to integrate high-quality single-walled carbon nanotubes into micro-electromechanical systems. The amorphous content is evaluated by confocal micro-Raman spectroscopy and by scanning/transmission electron microscopy. We show how properly chosen process parameters can lead to successful integration of single-walled nanotubes, enabling nano-electromechanical system synthesis.

Modelling soot and SOF emissions from a diesel engine

Modelling of soot and SOF emissions from a typical European turbocharged diesel engine has been made. The model consists of a detailed kinetic mechanism with 472 reactions (120 chemical species) and data from the thermodynamic diagnostic procedure of the combustion process of the engine. The forward kinetic constants were obtained from literature and the background constants from a self-developed non-linear fitting routine based on the Marquardt algorithm. The dilution and mixing processes inside the engine are represented by a simple Wiebe function. The system of ordinary differential equations is solved with the Rosenbrock method for rigid systems and using the interpolating Lagrange polynomials to calculate the heat capacity of each species at the corresponding temperature. The kinetic model has been implemented in Digital Visual Fortran 6.0. The model has been executed for five different fuels and three mixtures of biodiesel and reference diesel operating under three diverse conditions from the European transient urban/extraurban Certification Cycle and the results of soot and SOF predicted are compared with experimental data.