ReaxFF MD Simulations Research Articles

Insights into the pyrolysis mechanisms of epoxy resin polymers based on the combination of experiments and ReaxFF-MD simulation

Epoxy resin offers promising properties in composites, while its cross-linked structure prevents the disposal of waste composites. Pyrolysis is an essential technique for the breakdown of epoxy resin polymers, but the fundamental reaction mechanisms need to be clarified. In this study, the combination of experimental studies and the reactive force field molecular dynamic (ReaxFF-MD) simulation were performed to determine the formation mechanisms of pyrolysis products considering the secondary reactions of intermediates. The pyrolysis process was divided into the cleavage and polycondensation stages based on the carbon atom numbers of the largest cluster. The cleavage stage began with the removal of side-chain substituents, followed by the breakdown of the primary epoxy resin chain, resulting in the generation of pyrolysis gas products, of which the OH, C-C-O, and CH3 groups were identified as the critical intermediates in the creation of H2O, CO, and CH4, respectively. Meanwhile, the simultaneous evolution of liquid pyrolysis products (bisphenol A, phenol, and benzene) was established. After the epoxy resin chains were completely cracked, the unsaturated hydrocarbon intermediates underwent the dehydrogenation and polymerization reactions, causing the formation of pyrolysis char and the continuous generation of H2, H2O, and CO. The findings could provide some guidance to selectively regulate the pyrolysis of epoxy resin matrix composites.

Dynamic structure transformation of char precursors during co-pyrolysis of coal and HDPE by using ReaxFF MD simulation and experiments

Understanding the effects of co-pyrolysis synergies between low-rank coal and high-density polyethylene (HDPE) on the carbon structure evolution in char and the relationship between char structure and chemical bonds evolution is very needed. A combination of ReaxFF MD and experimental approaches provides an opportunity for fully understanding the dynamic profiles of char structures and coking trends during co-pyrolysis process of coal and HDPE. In this work, consistent conclusions were obtained from fixed-bed experiments and ReaxFF MD simulations in product distribution during both individual coal pyrolysis and co-pyrolysis at different temperatures. The ReaxFF MD simulation results reveal that adding HDPE partially inhibited the coking process of coal char. The RDF results indicated higher order degree and aromaticity exist in coal/HDPE(7:3)-Char when compared to those in coal-Char, which agrees well with the findings from XRD and FTIR approaches. By analyzing the evolution of C=C, C=O, C-O, C(sp2)-C(sp3), C(sp3)-C(sp3) and C(sp2)-H bonds in C40+ fragments during secondary simulations of co-pyrolytic char, it was observed that the structural order and aromaticity of co-pyrolytic char increase with temperature, and the oxygen-containing bonds are the initial recombination sites for char formation. These findings provide rich theoretical information to complement the understanding of char structure in co-fed systems and the interaction between coal and HDPE blend.

Combustion and interaction mechanism of 2,3,3,3-tetrafluoropropene/1,1,1,2-tetrafluoroethane as an environmentally friendly mixed working fluid

Organic Rankine cycle (ORC) is an effective way to improve energy efficiency, and 2,3,3,3-tetrafluoropropene (R1234yf) is considered as a promising working fluid for ORC. However, the development of R1234yf in ORC is limited by its flammability, and the addition of 1,1,1,2-tetrafluoroethane (R134a) is an effective method to inhibit its flammability. In this paper, the combustion and interaction mechanism of R1234yf/R134a were investigated. Firstly, the combustion products of R1234yf/R134a were measured using an improved experimental device, the results showed a substantial reduction in the concentrations of CO, CO2, and HF with an increasing proportion of R134a. Then, the combustion mechanism of R1234yf/R134a was studied by ReaxFF-MD simulation. The initial decomposition time of R134a was found to be later than that of R1234yf, and a reduction in the production rate of combustion products was achieved with the addition of R134a. Furthermore, the combustion reaction was promoted by the increase in temperature. Finally, the main reaction pathways of working fluids were analyzed. The abstraction reaction of R134a with radicals and the scavenging of active radicals by flame retardant radicals were the key to inhibit the combustion reaction. The results have universal guiding significance for the safe application of R1234yf/R134a.

Investigating the pyrolysis mechanisms of three archetypal ablative resins through pyrolysis experiments and ReaxFF MD simulations

Boron (B-PF), high carbon (HC-PF), and silicone phenolic resins (Si-PF) are typical structures of high-temperature resistant resins with exceptional ablation resistance. Nevertheless, the mechanism of their pyrolytic behavior under ultra-high temperatures remains unclear. Herein, the pyrolysis processes and mechanisms of the three resins are investigated in-depth through experiments and ReaxFF molecular dynamics simulations. According to the experimental results, the final residual carbon rate of the resin shows B-PF > HC-PF > Si-PF. The main products of resin pyrolysis are H2, CH4, CO, small molecular hydrocarbons, and phenolic compounds. The simulation results show that the different types of phenolic resins have consistent pyrolysis products. The main small molecule products are H2, H2O, CO and C2H2. Moreover, these reaction paths are monitored and analyzed. The hydroxyl groups on the naphthalene ring in the high-carbon phenolic make the ring more active and susceptible to decarbonization processes, releasing more carbon monoxide (CO). When silicone resins undergo pyrolysis at high temperatures, the resulting H2O reacts with fragments of silicon-containing molecules to form new silicon oxides upon cooling. The B-O bond in boron phenolic undergoes reorganization upon breakage to form a B-containing five-membered ring, a factor that affects its thermal stability. This research reveals the pyrolytic properties of various phenolic resins and provides a theoretical basis for further research into the pyrolytic behavior of different resins.

Insight into hydrogen migration and redistribution characteristics during co-pyrolysis of coal and polystyrene

The hydrogen migration and redistribution features in products during co-pyrolysis of coal and polystyrene (PS) were explored using the reactive molecular dynamics (ReaxFF MD) simulations. The weight loss parameters of coal and PS pyrolysis derived by ReaxFF MD simulations agreed well with thermogravimetric (TG) experimental data. The hydrogen transfer route in the co-pyrolysis products was investigated and the hydrogen synergy mechanisms were also elucidated. The results show that the synergistic effect of coal and PS co-pyrolysis can promote char conversion to volatiles, but this effect diminishes from 20.7% to 5.3% (seen from the char yield) as the proportion of PS increases. It is discovered that a large amount of hydrogen migrates from char to volatile in coal, whereas hydrogen migrates from heavy tar to light tar in PS. The content of total hydrogen in heavy tar decreases from 21.47% to 14.06% as PS content increases, whereas hydrogen in light tar increases from 40.27% to 77.76%, indicating an improvement in co-pyrolysis tar quality. The variation of H and O in co-pyrolysis char demonstrates that char formation is highly dependent on the hydrogen/oxygen content. Different levels of hydrogen content cause changes in the dominant reaction and its reaction time in the various char reaction stages, affecting the formation of volatiles.

The effect of glycerol additive on high value-added chemicals from tobacco waste pyrolysis

It is of significant importance to understand the role of glycerol additive for the improvement of the energy recovery during tobacco-waste pyrolysis. Utilizing ReaxFF-based molecular dynamic simulation, herein, we systematically studied the pyrolysis mechanism of tobacco with different concentrations of glycerol (5%, 10%, 16.5%, 20%, 25%) at different temperatures (1800 K, 2000 K, 2300 K, 2500 K, 2800 K). The calculation shows that a temperature of 2000 K is the optimal temperature for producing methanol and ethanol. Furthermore, the concentration of glycerol had a significant effect on promoting methanol production at high temperatures. Methanol's main generation pathway was influenced by temperature and glycerol concentration, while the primary pathway of ethanol production was independent of them. Moreover, a pyrolysis reaction framework was created for methanol and ethanol production, and their main formation reactions were validated. Finally, a chemical kinetic analysis of the pyrolysis process confirmed the accuracy of the ReaxFF MD simulations.

Atomic oxygen degradation of a fluorine-containing colorless polyimide

Colorless polyimides (PIs) are regarded as the most feasible lens materials deployed in aerospace optical systems due to their light weight, flexibility, transparency, and excellent mechanical properties. However, durability towards atomic oxygen (AO) exposure is a chief concern for externally deployed colorless PI films in low Earth orbit (LEO) environments. In this work, we investigated the degradation behavior of a fluorine-containing colorless PI film with a combination of ground-based AO exposure experiments and reactive molecular dynamics (ReaxFF-MD) simulations. The fluorine-containing colorless PI showed linear erosion dynamics and a constant erosion yield (Ey) of ∼1.4 × 10−24 cm3 atom−1. Microscopic investigations demonstrated an increased surface roughness and shag-like appearance after continuous AO exposure. In addition, spectroscopic transformations resulted from elimination and oxidation processes occurring at specific reaction active sites (C=O, CN, CH, and CF, and CC) and the release of volatile substances. Moreover, the exposed fluorine-containing colorless PI films exhibited gradually deteriorating visible light transmission with increased AO fluence. Furthermore, the ReaxFF-MD simulations demonstrated physical sputtering and bombardment-enhanced chemical reaction models. Considering the limited-scale slab and the system thermal dissipation, the experimental and ReaxFF-MD computational results were largely consistent.

ReaxFF-MD simulation investigation of the degradation pathway of phenol for hydrogen production by supercritical water gasification

Wastewater from the thermochemical conversion of coal and biomass contains a significant amount of phenolic structures compounds. The degradation of these phenolic compounds to hydrogen-rich gasses can prevent environmental pollution and save energy. Supercritical water (SCW) gasification of phenol is experimentally studied and a reactive force field molecular dynamics (ReaxFF-MD) simulation is conducted to investigate the catalytic mechanism of Ni/Al2O3 in the phenol degradation. The experimental results indicate that Ni/Al2O3 facilitates the conversion of phenol to 1-ethoxy butane via ring opening, which is a crucial step for complete gasification. The ReaxFF-MD simulation demonstrated that Ni facilitates the formation of H3O free radicals and Ni-phenol intermediates. H3O free radicals can be decomposed into H2 and OH free radicals. Both the generated OH free radical and Ni-phenol intermediate promote the ring-opening reaction of phenol. Ni promotes the direct decomposition of phenol into C1, C2, and C3 fragments, which is beneficial for further complete gasification.

Investigation of Petroleum Coke Gasification with CO2/H2O Mixtures and S/N Removal Mechanism via ReaxFF MD Simulation.

The removal of environmentally harmful S/N is crucial for utilization of high-S petroleum coke (petcoke) as fuels. Gasification of petcoke enables enhanced desulfurization and denitrification efficiency. Herein, petcoke gasification with the mixture of two effective gasifiers (CO2 and H2O) was simulated via reactive force field molecular dynamics (ReaxFF MD). The synergistic effect of the mixed agents on gas production was revealed by altering the CO2/H2O ratio. It was determined that the rise in H2O content could boost gas yield and accelerate desulfurization. Gas productivity reached 65.6% when the CO2/H2O ratio was 3:7. During the gasification, pyrolysis occurred first to facilitate the decomposition of petcoke particles and S/N removal. Desulfurization with the CO2/H2O gas mixture could be expressed as thiophene-S → S → COS → CHOS, thiophene-S → S → HS → H2S. The N-containing components experienced complicated mutual reactions before being transferred into CON, H2N, HCN, and NO. Simulating the gasification process on a molecular level is helpful in capturing the detailed S/N conversion path and reaction mechanism.

Molecular representation and atomic-level coking evolution investigation of modified coal tar pitch via 13C NMR, MALDI-TOF-MS, SAXS, and ReaxFF MD

Modified coal tar pitch (MCTP) is a high-quality feedstock for high value-added (HVA) carbon materials. The coking process of MCTP is essential for the preparation of the carbon products and determines the product performance. Here, the MCTP molecular representation (C93H59NO) created by computational modeling strategy duplicates the microstructure and composition data from XRD, 13C NMR, FT-IR, XPS, and MALDI-TOF-MS analysis. The mechanism of volatile removal and coking evolution for MCTP was revealed via the obtained model, reactive force field molecular dynamics (ReaxFF MD) simulations, TG-MS, XRD, and SAXS. The removal of volatiles during the MCTP coking process was caused by the breaking of aromatic rings at the edge of MCTP molecules and the generation of radicals. The main volatile products are H2, H2O, CO, CH4, and C2H4, and the volatiles were mainly removed in the temperature range of 430–900 K. For the coking evolution, the XRD and SAXS analysis showed the crystalline and graphitization degree of pitch coke samples were destructed slightly below 973 K and improved above 973 K. The true density (finally 2.2 g/cm3), RDF critical peak intensity, sp2 hybridized bond proportion (finally 41.4%) and six-membered ring proportion (finally 63.3%) for the pitch coke nucleus underwent the decline and then increase during the ReaxFF MD simulations. The patterns of MCTP coking evolution obtained by the detection methods (XRD & SAXS) and ReaxFF MD were demonstrated mutually. The coking evolution could be divided into two stages: the growth and order of the coking nucleus. The destruction of the aromatic moieties in MCTP molecules and the cross-link of the aliphatic chains resulted in the primary nucleation. The condensation and aromatization proceeded within the primary nucleus to finally form the crystalline nucleus of pitch coke. This work provides theoretical guidance for the quality improvement of HVA carbon products.

Thermal runaway mechanisms and kinetics of benzene nitrification system based on microcalorimetry experiments and ReaxFF molecular dynamics simulation

The thermal runaway reaction mechanisms and kinetics of nitrification process under abnormal conditions remain incompletely understood. In this study. C600 microcalorimetry experiments combined with GC/MS characterization and IR analysis of the intermediates indicated that the secondary thermal runaway of benzene nitrification system was preferentially initiated by the decomposition of nitrobenzene sulfonated by-products rather than that of nitrobenzene products. This was further verified by the ReaxFF MD simulations and DFT calculations of the microscopic decomposition thermal runaway mechanisms of typical product and by-product. Furthermore, the model-free kinetic models were compared and evaluated from the perspective of reaction mechanism. The results showed that although the models of Kissinger, FWO and Friedman could obtain feasible kinetic parameters, Friedman model had a good match with the microscopic kinetics and thermodynamic mechanisms, and could reasonably approximate the intrinsic thermokinetics. Finally, the thermal hazards under adiabatic conditions with different thermal inertia factors (φ) were discussed, the results provide a reference for the optimization of safety and economy.

Study on the molecular structure model of tar-rich coal and its pyrolysis process

With the continuous consumption of medium/high-rank coal and increasing energy shortage, the conversion and utilization of low-rank coal is gaining attention. A rational molecular model of coal combined with reaction molecular dynamics (ReaxFF MD) is becoming a promising way to explore coal conversion processes at the molecular level. In this work, a molecular structure model of tar-rich coal from northern Shaanxi, China, with the molecular formula C355H332O57N4S was proposed by combining experimental characterization tests and theoretical computational analysis. In addition, the molecular model was validated by calculating FTIR and 13C NMR spectra with quantum chemistry. Furthermore, complex multi-molecule systems were constructed based on the single-molecule model, the effect of heating rates on the pyrolysis process was investigated by ReaxFF MD simulations, and the evolution of pyrolysis process and products were analyzed in detail. The results show that a slower heating rate helps to obtain more gas products, inhibit the condensation reaction between tar fragments, and promote their cracking, more light tars could be obtained. The combination of experimental characterization and theoretical calculations provides a comprehensive understanding of the molecular structure and pyrolysis processes of tar-rich coal, and provides a basis for an in-depth study of the structure of low-rank coal and its clean utilization technologies.

Atomistic insights into bias-induced oxidation on passivated silicon surface through ReaxFF MD simulation

The study investigated the bias-induced oxidation through ReaxFF molecular dynamics simulations in order to bridge the knowledge gaps in the understanding of physical–chemical reaction at the atomic scale. Such an understanding is critical to realise accurate process control of bias-induced local anodic oxidation nanolithography. In this work, we simulated bias-induced oxidation by applying electric fields to passivated silicon surfaces and performed a detailed analysis of the simulation results to identify the primary chemical components involved in the reaction and their respective roles. In contrast to surface passivation, bias-induced oxidation led mainly to the creation of Si–O–Si bonds in the oxide film, along with the consumption of H2O and the generation of H3O+ in the water layer, whereas the chemical composition on the oxidised surface remained essentially unchanged with a mixture of Si–O–H, Si–H, Si–H2, H2O–Si and Si–O–Si bonds. Furthermore, parametric studies indicated that increased electric field strength and humidity did not significantly alter the surface chemical composition but notably enhanced the bias-induced oxidation, as indicated by the increased number of Si–O–Si bonds and oxide thickness in simulation results. A good agreement is achieved between the simulation and experimental results.

Reactive behaviors and mechanisms of cellulose in chemical looping combustions with iron-based oxygen carriers: An experimental combined with ReaxFF MD study

Biomass energy is one of the important and feasible renewable energy resources. Chemical looping combustion (CLC) can meet the requirements of efficient CO2 capture with low energy consumption. The combination of the two factors is intuitive and important for realizing carbon neutral energy processing and conversion. Cellulose is the component with the highest content in biomass. However, there is still a lack of clear understanding of the thermal conversion behavior and microscopic reaction mechanism of cellulose in CLC. In this work, the behaviors and mechanisms of cellulose in CLC with iron-based OCs were studied systematically based on the comprehensive technics of thermo-gravimetric analysis (TGA) experiments, kinetic models and molecular dynamic simulations. The reaction behaviors, kinetic mechanism, and complex reactive networks of cellulose under complex environmental interface conditions of CLC were revealed. Three different stages were revealed for thermal weight loss and reaction behavior of cellulose in CLC via TGA. The effects of different conversion rates and reaction stages on activation energy during CLC were described based on two model-free integration methods. The dynamic evolution of the CLC process was discussed by ReaxFF MD (reactive force field molecular dynamics) simulations. A complex reaction network of cellulose depolymerization and CO2 release in CLC was obtained.

Kinetic Mechanism for Simulating the Temperature and Pressure Effect on the Explosive Decomposition of Acetylene by ReaxFF Molecular Dynamics

AbstractIn the utilization of acetylene as the raw material in chemical engineering, understanding its pyrolysis mechanism is a vital issue in avoiding its explosion. The ReaxFF MD simulation was adopted to investigate the pyrolytic behavior of acetylene at various temperatures and pressures. The simulation results revealed that the pyrolysis mechanism of acetylene could be divided into three temperature ranges: (i) T<1200 K, where homogenous molecular polymerization of acetylene occurs, producing polymerization products primarily C4H4 and C6H6; (ii) 1200<T<1800 K, where the free radical path competes with the molecular path, and free radicals such as C2H3⋅ and C4H3⋅ participate in the polymerization reaction; (iii) T>1800 K, where the acetylene molecule is cracked, and H⋅ and C2H⋅ drive the formation of polyacetylene and hydrogen. Finally, a reaction network for the pyrolysis of acetylene to yield compounds below C10 is generated through the identification, quantification, and evaluation of the reaction trajectory. The kinetic model of acetylene pyrolysis has been dramatically improved and supplemented, which is in satisfactory agreement with experimental values.

A study of polyethylene glycol terephthalate (PET) pyrolysis mechanisms using reactive molecular dynamic simulations

ABSTRACT In the present work, the pyrolysis of polyethylene glycol terephthalate (PET) was analyzed with reactive molecular dynamics (ReaxFF MD) simulations. The results of non-isothermal pyrolysis simulation were compared with those from thermogravimetric (TG) experiments to determine the simulation temperature for ReaxFF MD. It was indicated that the weight loss curve in ReaxFF MD simulation at 500–2500 K was agreed well with that of TG experimental data at 400–900 K. In isothermal pyrolysis simulation at 1900 K, the final char, tar, and gas yields were 7%, 65%, and 28%, respectively. The activation energies for the conversion of PET to tar and gas were 231 and 143 kJ/mol, respectively. CO2, CO, CH2O and C2H4 are the major gaseous products, and their formation pathways are revealed by analysis of simulation trajectories. The migration and transformation of oxygen during PET pyrolysis at 1900 K were also examined. About 5%, 53%, and 42% of the oxygen were converted into char, tar, and gas, respectively. Among all the gas products, 50%, 35.6%, 5.5%, and 1.2% of oxygen was in CH2O, CO2, CO, and H2O, respectively. The oxygen in C5–15, C16–25 and C26–40 species accounted for 51.76%, 27.46%, and 20.78% of the total oxygen in tar, respectively. The results indicated that most of the oxygen in tar products was in light tars. Finally, the development of char is analyzed. It is found that the final O/C and H/C ratios of char were 0.3 and 0.61, respectively. The maximum pore diameters of char at char conversions of 10%, 30%, 50%, 70%, and 90% were 7.24, 9.85, 6.01, 4.51, and 4.43 Å, respectively.

Oxidation decomposition mechanism of hexamethyldisiloxane

Siloxane working fluid has good environmental performance and thermodynamic performance, widely used in high temperature heat source ORC system. During the filling process of working fluid and operation in ORC system operation, air may leak into the system. The reaction of siloxane working fluid with oxygen in high temperature environment will produce abrasive silica, which will cause serious damage to the system. The oxidative decomposition mechanism of hexamethyldisiloxane (MM) was analyzed by density functional theory (DFT) and ReaxFF molecular dynamic simulation. It is found that the initial reactions of MM oxidation decomposition can be divided into two categories. The first type is the thermal decomposition of MM itself. The second one is the collision reaction between MM molecule and O2 molecule. Comparing the two kinds of chemical reaction barriers, MM is more likely to react with O2. Pathways 2–3 involving the collision between O2 molecule and Si atom are the optimal pathway of initial reaction kinetics, and its reaction energy barrier is 204.1 kJ/mol. It was observed from ReaxFF MD simulations that the main products of MM oxidative decomposition were CH4, CH2O, CO, CO2, SiO and amorphous silica. In the process of oxidative decomposition, small free radicals such as CH3, O, H, OH played a very important role in the oxidative decomposition of working fluids, which greatly promoted the decomposition of working fluids and the formation of main products.

Insight into pyrolysis behavior of silicone-phenolic hybrid aerogel through thermal kinetic analysis and ReaxFF MD simulations

Phenolic aerogel matrix nanocomposites are modified to improve their thermal stability and oxidation resistance, while the pyrolysis behavior will become more complex. Herein, the pyrolysis behavior of silicone-phenolic hybrid aerogel, as the pyrolyzable component of the nanocomposites, was investigated by thermal analysis experiments (TG-MS, TG-FTIR, and Py-GC/MS), kinetic studies (isoconversional methods and distributed activation energy model (DAEM)), and reactive force field molecular dynamics (ReaxFF MD) simulations. Experimentally, it was found that the main product of silicone-phenolic hybrid aerogel during the first decomposition stage is CH4, which was also validated by ReaxFF MD simulation. The main product CH4 was predominantly formed by hydrogen extraction reaction from silicone aerogel. The main decomposition products and pyrolysis kinetics of silicone-phenolic hybrid aerogel were analyzed by a combination of the multi-DAEM and ReaxFF MD simulations. The five-pseudo-components DAEM was utilized to reveal the decomposition of silicone-phenolic hybrid aerogel in five stages, which were in good agreement with the experimental conversion rates. The rearrangement of the Si-O-Si backbone of silicone aerogel at elevated temperatures forms silica-like species with antioxidant performance. This study can provide reliable guidance for the characterization and design of phenolic aerogel matrix nanocomposites and the selection of antioxidant modifiers.

Exploring depolymerization mechanism and complex reaction networks of aromatic structures in chemical looping combustion via ReaxFF MD simulations

Uncovering the reaction mechanism of various fuels to chemical looping combustion (CLC), especially depolymerization of solids in a complex environment, is an important scientific issue. Aromatic structures are the backbone of solid fuels such as coal and biomass. However, their depolymerization mechanisms during CLC remain unclear, and few complex reaction networks have been established to date. This work presents molecular insights into reaction behaviors and mechanism for several aromatic structures in the fuel reactor of CLC. The distribution and evolution of reactants, important intermediates, and products in the CLC are studied in details by means of reactive force field molecular dynamics (ReaxFF MD) simulations. It is found that the reaction process of aromatic fuels reacted with Fe2O3 as oxygen carriers in CLC can be divided into four stages. The decomposition rate of fuel molecules is relevant to the reaction temperature and aromatic sizes. The higher of reaction temperature and the smaller aromatic sizes, the faster the conversion of fuel molecules. The amount of CO and H2O produced gradually increases during CLC process, and however shows a decreasing trend in the later stages. The higher the CLC temperature is, the more favorable the oxygen releases from the oxygen carrier lattice are, so that the fuel molecules can burn fully to produce more CO2. The fuel molecular structure has less effect on generation time of CO molecule, while more on that of H2O molecule. The initial reactions of aromatic structures in CLC involve competition between thermal cracking and oxidation reactions. Fuel molecules generally go through a thermal ring-opening step first, followed by continuous bonding dissociation and oxidation to produce numerous structural fragments. The reactions to aromatic structures become more complex with an increasing number of aromatic rings and a wider variety of intermediates. By visualizing the ReaxFF MD results, complex reaction networks of different aromatic molecules in CLC are systematically established.

Insight into gaseous product distribution of cross-linked polyethylene pyrolysis using ReaxFF MD simulation and TG-MS

Cross-linked polyethylene (XLPE) has been widely used as the insulation material of power cables due to the great electrical and thermal properties. However, the material deteriorated when subjected to excessive thermal stress, leading to performance reduction and even failure of the cables. In this paper, we proposed a novel thermal fault detection method in power cables by exploring the gaseous product distribution of XLPE pyrolysis. Firstly, the gas generation characteristics of the XLPE pyrolysis was investigated using large scale ReaxFF molecular dynamics simulation. The simulation results showed that the major gaseous products included H2, CH4, C2H6, C2H4, C2H2, CO. The pyrolysis kinetics as well as the formation mechanism of the gases was illustrated using the real-time trajectory obtained by molecular dynamic calculation. Furthermore, the influence of temperature on the gas product distribution was characterized. Higher temperature increased the yield of H2, C2H4, C2H2, but decreased that of CH4. The yield of C2H6 and CO almost kept constant as temperature changing. The influencing mechanism of temperature was reasonably explained by the proposed gas formation mechanism. On the basis of investigation on XLPE pyrolysis, a three ratios technique was proposed for the detection of thermal faults in power cables. Finally, the chemical composition of the gas product was validated by thermogravimetric and mass spectrometric simultaneous experiment. The experimental results exhibited good agreement with the numeric simulation.