Flow Reactor Pyrolysis Research Articles

A comparative pyrolysis study of tetraethoxysilane and tetraethoxymethane: Insight into congener substitution effects on pyrolysis chemistry of siloxane flame synthesis precursors

The pyrolysis of siloxane precursors, such as tetraethoxysilane (TEOS), is commonly employed in the flame synthesis and chemical vapor deposition of silica nanoparticles. In this work, the flow reactor pyrolysis of TEOS is studied using gas chromatography (GC) and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), and the congener substitution effects from the central C atom to the central Si atom are investigated through comparison with its hydrocarbon counterpart tetraethoxymethane (TEOM). Pyrolysis models of TEOS and TEOM are constructed and validated against the measured results. Modeling analysis, including rate of production analysis and sensitivity analysis, provides insights into chemistry in fuel consumption and product formation. In contrast, the observations of silicon-containing products in SVUV-PIMS experiments provide evidence for crucial decomposition pathways of TEOS. It is observed that TEOS exhibits significantly higher stability than TEOM under pyrolysis conditions. The most abundant products in the pyrolysis of TEOS and TEOM are ethylene and ethanol, and TEOS produces more hydrocarbon products than TEOM. The lower pyrolysis reactivity of TEOS is attributed to the slower unimolecular decomposition reaction which dominates the decomposition of TEOS than TEOM. This can be explained by the hindrance of the extremely strong Si-O bond resulting from the significantly different electronegativity between Si and O. The higher initial decomposition temperature of TEOS enhances the contribution of other decomposition channels, such as C-C bond dissociation and H-abstraction reactions, in TEOS consumption. This leads to the abundant formation of hydrocarbon productions such as methane, ethane, and acetaldehyde. As the main pyrolysis product, the ethanol produced by TEOM pyrolysis is four times that of TEOS due to the congener substitution effects. The ethanol formation pathway in TEOS pyrolysis is different from that in TEOM pyrolysis, which is mainly formed via a newly proposed multi-step mechanism, resulting in a lower yield in TEOS pyrolysis.

Exploration on the combustion chemistry of p-xylene: A comprehensive study over wide conditions and comparison among C8H10 isomers

Pyrolysis and oxidation of p-xylene over wide conditions were studied in this work in order to provide a comprehensive insight into the combustion chemistry of p-xylene. Concerning this goal, the pyrolysis of p-xylene was investigated in a flow reactor at pressures from 0.04 to 1 atm over the temperature range of 1000–1600 K. Key radicals, isomers and PAH products including three xylyl isomers, benzyl, xylylene isomers, styrene, benzene, fulvene, indene, naphthalene and phenanthrene, were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry. The oxidation of p-xylene was studied in a jet-stirred reactor at 10 atm, 800–1300 K and equivalence ratios of 0.5–2.0 with a residence time of 0.5 s. Oxidation products were quantified by using gas chromatography combined with mass spectrometry and Fourier transform infrared spectrometry. A kinetic model for p-xylene combustion over wide conditions was developed based on our previously proposed p-xylene oxidation model and validated against the new experimental measurements in this work and literature reported experimental data, including flow reactor oxidation, laminar premixed flame, ignition delay time and laminar burning velocity. In the flow reactor pyrolysis, p-xylene mainly undergoes unimolecular decomposition to yield p-xylyl, which mainly suffers the H-loss reaction to form p-xylylene and isomerization reaction to form o-xylyl. The resonantly stabilized fulvenallenyl and benzyl radicals play an important role in the formation of bicyclic and tricyclic PAHs including indene, naphthalene, and phenanthrene. In the JSR oxidation, p-xylene is predominantly consumed via H-abstraction by oxygenated radicals such as OH, O, and HO2 forming p-xylyl, which can be oxidized by HO2 and eventually converted to p-methylbenzaldehyde. As the most abundantly produced oxygenated aromatics, further consumption of p-methylbenzaldehyde results in the formation of other observed oxygenated aromatics such as cresol, benzofuran, and benzaldehyde. The combustion chemistry of three C8H10 isomers, i.e., p-xylene, o-xylene, and ethylbenzene, were also compared in terms of pyrolysis and oxidation fuel reactivity, and PAHs formation and growth, showing remarkable fuel isomeric effects.

Exploring flow reactor pyrolysis of branched OMEs: Insights into multi-sidechain effects on pyrolysis chemistry of CH3OCH3-n(OCH3)n (n = 1–3)

Oxymethylene ethers (OMEs) have received great attention due to their carbon-neutral nature and advantageous properties as alternative transportation fuels or fuel additives. Previous studies focused on the combustion of linear OMEs with the chemical formula of CH3O(CH2O)mCH3 (m ≥ 0), while a deep understanding of the combustion of branched OMEs with the chemical formula of CH3OCH3-n(OCH3)n (0 ≤ n ≤ 3) is still absent. In this work, the flow reactor pyrolysis of dimethoxymethane (DMM, n = 1), trimethoxymethane (TMM, n = 2), and tetramethoxymethane (TeMM, n = 3) is investigated using gas chromatography, with special attention given to the multi-sidechain effects on the pyrolysis chemistry. A pyrolysis model of branched OMEs is developed and validated against the new pyrolysis data. Modeling analysis is performed to provide insights into the pyrolysis chemistry of the three branched OMEs and the multi-sidechain effects. Unimolecular decomposition and H-abstraction reactions are found to be two major types of fuel decomposition pathways, with the former dominating TMM decomposition, the latter dominating DMM decomposition, and both contributing almost equally to TeMM decomposition. Sensitivity analysis shows that CO bond dissociation is the most sensitive reaction to fuel decomposition in DMM pyrolysis, while methanol elimination is the most sensitive reaction in TMM and TeMM pyrolysis. Among the three fuels, DMM has the lowest pyrolysis reactivity and produces the least abundant methanol. In contrast, TMM is observed with the highest pyrolysis reactivity and produces the most abundant methanol. Theoretical calculations reveal that TMM and DMM have the lowest and highest energy barriers of methanol elimination reactions, which determines the order of their pyrolysis reactivities and methanol formation trends. As a result, DMM pyrolysis occurs at higher temperature regions where the CO bond dissociation and H-abstraction reactions are preferred, while TMM pyrolysis occurs at lower temperature region where methanol elimination dominates.

Experimental and theorical study on the pyrolysis behaviors of 2,3,3,3-tetrafluroropropene

The pyrolysis behaviors of 2,3,3,3-tetrafluroropropene (R1234yf) had been first studied in a flow reactor pyrolysis apparatus coupled with the synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography. The pyrolysis temperature, reaction products and intermediates were measured at atmospheric pressure. The experimental results revealed that the initial pyrolysis temperature of R1234yf was 1040K and the thermal decomposition was accelerated with the increase of temperature. It was found that the pyrolysis reaction of R1234yf will be replaced by a self-polymerization reaction at 1160 K. In order to explore the chemical process of R1234yf pyrolysis, a detailed kinetic simulation was performed and compared with experimental results. The calculated results indicated that the unimolecular decomposition paths took a dominant role in the whole pyrolysis processes. The intermediates of C2H2, C2HF, CF2, CHF3, CF4 and CF3CF3 were detected in the pyrolysis experiments. Moreover, the distribution characteristics and sensitivity analysis of the vital fluorinated species of CF2, C2H2, CF4, CF3CF3 and HF were discussed. All of these founding could provide reliable data for the development of pyrolysis models and guidance for the use of R1234yf in the refrigeration system.

Experimental study on n-hexane pyrolysis initiated by 1-nitropropane in a low-pressure flow reactor

In order to investigate the species pool and primary channels of initiated pyrolysis of n-hexane, the low-pressure (30 Torr, 4.0 kPa) flow reactor pyrolysis of n-hexane initiated by 5% 1-nitropropane (1-NP) was studied experimentally in a wide range of temperature from 623 K to 1223 K. Combined with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectra scanned at photon energies ranging from 9.0 to 14.7 eV and near-threshold photoionization efficiency (PIE) spectra, 29 pyrolysis species, covering C1–C6 hydrocarbons and several pyrolysis species containing O or N atoms, were identified. Based on the ion signal intensities of the visible peaks in the mass spectra, the mole fraction profiles of all the experimental species were evaluated. The experimental results suggested that the initial pyrolysis temperature of n-hexane was reduced by approximately 275–325 K. On the basis of the mole fraction profiles and the initial formation temperatures reported in this work and rate coefficients proposed in previous studies, the 1-NP-initiated pyrolysis mechanisms of n-hexane were discussed. It was suggested that n-hexane was dominantly consumed by the H-abstraction reactions by the attack of OH radical in this work. The 1-NP-initiated pyrolysis of n-hexane mainly proceeded via reactions to produce C3H6 + C3H7, or 1-C4H8 + C2H5, or 1-C5H10 + CH3.

High-temperature thermal decomposition of triphenyl phosphate vapor in an inert medium: Flow reactor pyrolysis, quantum chemical calculations, and kinetic modeling

High-temperature thermal decomposition of triphenyl phosphate vapor in an inert medium: Flow reactor pyrolysis, quantum chemical calculations, and kinetic modeling

Towards a predictive kinetic model of 3-ethyltoluene: Evidence concerning fuel-specific intermediates in the flow reactor pyrolysis with insights into model implications

To reveal insights into high temperature kinetics of dialkylaromatics, a pyrolysis investigation of 3-ethyltoluene in a flow reactor together with its reaction kinetics are presented in this work. Concentrations and chemical structures of specific species covering temperature range from 796 to 1383K at the pressure of 30 and 760 Torr were recorded and quantified by using synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry (VUV-PI-MBMS). Important C8 and C9 fuel-specific intermediates relevant to primary decomposition of 3-ethyltoluene and isomerization of methylbenzyl and ethylbenzyl radicals were detected and identified. The kinetic model interpreting high temperature pyrolysis chemistry of 3-ethyltoluene was developed and reasonably predicted the measurements in this work. The model analyses reveal that the methyl-dissociated reaction from the ethyl group of 3-ethyltoluene is dominant in the fuel decomposition at low pressure, while the fuel is mainly consumed by hydrogen abstraction reactions at atmospheric pressure. The experimental observations of three methylbenzyl isomers, o-xylylene, p-xylylene, styrene and benzocyclobutene provide evidence for the relationships between products involving isomerization of methylbenzyl radicals, formation of xylylenes and decomposition of o-xylylene. The fuel structure effects of 3-ethyltoluene and m-xylene are revealed by comparing the pyrolysis behaviors in both cases. It has been found that the m-methylbenzyl-generating channel in the 3-ethyltoluene pyrolysis improved the reaction reactivity initially. Furthermore, the fuel with longer substituent ethyl group facilitates the formation of cycloalkenes and aromatics.

Unraveling combustion chemistry of tetramethoxysilane in flow reactor pyrolysis and laminar flame propagation

Tetramethoxysilane (TMOS, Si(OCH3)4) is a widely used precursor in the vapor-fed aerosol flame synthesis (VAFS) of silica nanoparticles. This work reports the first attempt to investigate the flow reactor pyrolysis and laminar flame propagation of TMOS. Pyrolysis species were detected using gas chromatography (GC) at 1.09 atm and 930–1250 K, such as methane, methanol, ethylene, ethane and carbon monoxide, and their mole fraction profiles versus the heating temperature were also measured. The laminar flame propagation of TMOS/air mixtures was investigated in a constant-volume cylindrical combustion vessel at the initial pressure (Pu) of 1 atm, initial temperature (Tu) of 373 K and equivalence ratios from 0.7 to 1.4. TMOS has generally lower laminar burning velocities (LBVs) than many hydrocarbon fuels due to the high oxygen content and consequently low adiabatic flame temperature of TMOS. A kinetic model of TMOS combustion was developed and validated against both the new data in this work and the shock tube pyrolysis data in literature. Rate of production (ROP) analysis and sensitivity analysis were performed to provide insight into the key pathways in TMOS decomposition. In the flow reactor pyrolysis, the methanol elimination reaction and the H-abstraction reactions dominate the decomposition of TMOS. Among the primary decomposition products, CH2OSi(OCH3)2 mainly isomerizes to CH3OSi(=O)OC2H5, while CH2OSi(OCH3)3 and OSi(OCH3)3 mainly decompose via β-Si-O scission reactions producing OSi(OCH3)3 and OSi(OCH3)2, respectively. The primary decomposition pathways of TMOS can all eventually lead to the formation of SiO2. In the laminar flame propagation, the modeling analyses indicate formaldehyde and methyl radical are abundantly produced, making many reactions involving the two intermediates influential on the laminar flame propagation of TMOS.

Investigation on n-pentylbenzene combustion at various pressures: Insight into effects of side-chain length on alkylbenzene combustion

This work investigated the flow reactor pyrolysis of n-pentylbenzene using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 0.04 and 1 atm, and measured the laminar burning velocities (LBVs) of n-pentylbenzene in a high-pressure constant-volume cylindrical combustion vessel at 473 K, 1–10 atm and equivalence ratios (ϕ) of 0.7–1.5. A kinetic model of n-pentylbenzene combustion was developed from our previous models of n-butylbenzene and validated against the present experimental data. The unimolecular decomposition reactions and H-abstraction reactions are the dominant consumption pathways of n-pentylbenzene under pyrolysis conditions. Aromatic intermediates with unsaturated side-chains play crucial roles in indene and naphthalene formation, while phenyl and benzyl are precursors of unfused polycyclic aromatic hydrocarbons (PAHs) which in turn convert to fused tricyclic PAHs. In the laminar flame propagation, small species reactions are crucial under lean conditions, while chain-termination reactions, especially fuel-specific reactions, show important inhibition effects on the laminar flame propagation under rich conditions. Furthermore, the effects of side-chain length on the pyrolysis and laminar flame propagation of alkylbenzenes were also investigated for toluene, ethylbenzene, n-propylbenzene, n-butylbenzene and n-pentylbenzene. It is concluded that alkylbenzene smaller than n-butylbenzene tends to decompose at low temperature as the side-chain length increases, while n-butylbenzene and n-pentylbenzene have very close decomposition temperature regions. For indene and naphthalene formation, decomposition/cyclization reaction sequences of C9 and C10 alkenylbenzenes have rapidly increasing importance in the pyrolysis of large alkylbenzenes. Benzyl is concluded to play an important role in inhibiting laminar flame propagation of alkylbenzenes, which is an important reason for the observations that toluene and ethylbenzene have the lowest and highest LBVs, respectively.

Experimental study of nitropropane pyrolysis with molecular-beam mass spectrometry and tunable synchrotron VUV photoionization. Part II. The flow reactor pyrolysis of 2-nitropropane

An experimental study of 2-nitropropane (2-NP) pyrolysis was performed in a flow reactor at low pressure (4.0 kPa) and in the temperature range of 523–923 K. The pyrolysis species were detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) techniques. Species from m/z = 15 to 87, such as radicals of CH3, C2H5, C3H5 and C3H7; isomers of C3H4 (allene and propyne), C5H8 (1,3-pentadiene and cyclopentene), were identified from near-threshold photoionization efficiency (PIE) spectra. Four nitrogenous intermediates never reported in the homogeneous pyrolysis of 2-NP (i.e., 2-propen-1-imine, 2-propen-1-amine, acetamide and 2-nitropropene) were identified in this work. The mole fraction profiles of the pyrolysis species were evaluated at different temperatures. Based on the experimental findings, the pyrolysis pathways of 2-NP were discussed.

Exploring pyrolysis and oxidation chemistry of o-xylene at various pressures with special concerns on PAH formation

This work reports an experimental and kinetic modelling investigation on flow reactor pyrolysis and jet-stirred reactor oxidation of o-xylene. The flow reactor pyrolysis experiments were conducted over 1050–1600 K at 0.04 and 1.0 atm. Key products related to fuel decomposition such as o-xylyl radical, o-xylylene, benzocyclobutene and styrene, as well as polycyclic aromatic hydrocarbons (PAHs), were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry. The jet-stirred reactor oxidation experiments were performed at 10 atm, 800–1200 K, a residence time of 0.5 s and equivalence ratios of 0.5, 1.0 and 2.0. Speciation were conducted by using gas chromatography and Fourier transform infrared spectrometry. A kinetic model of o-xylene was developed from our previous models of aromatic fuels and validated against both present data and experimental data in literature. Styrene is observed as the dominant product in the pyrolysis of o-xylene and it is mainly produced from the isomerization of o-xylylene directly or via benzocyclobutene as an intermediate species. C9 PAHs (indenyl, indene and indane), phenanthrene and its methyl derivatives were observed as the abundantly produced bicyclic and tricyclic PAHs. The main formation pathways of bicyclic and tricyclic PAHs are found to be different at low and atmospheric pressures, depending on the major precursors produced. Particularly, the self-combination reactions of o-xylyl and the addition reaction of o-xylyl with benzyl and subsequent stepwise H-loss/cyclization reactions are found to be the main sources of methylphenanthrene and dimethylphenanthrene. In the JSR oxidation of o-xylene, toluene and benzene were observed as the abundantly produced aromatic products, while o-methylbenzaldehyde was among the most abundantly produced oxygenated aromatics. modelling analysis reveals that o-xylyl radical is also the dominant fuel consumption product. Its consumption mainly proceeds through the oxidation by HO2 and finally produces o-methylbenzaldehyde. Further decomposition reactions of o-methylbenzaldehyde contribute to the formation of other major oxygenated aromatics such as cresol and benzofuran. Indene and naphthalene were also observed in the oxidation of o-xylene, which are mainly produced from the stepwise H-loss/cyclization reaction sequence of o-methylethylbenzene and decomposition of dibenzofuran, respectively.

Experimental study of nitropropane pyrolysis with molecular-beam mass spectrometry and tunable synchrotron VUV photoionization. Part I. The flow reactor pyrolysis of 1-nitropropane

An experimental study of 1-nitropropane (1-NP) pyrolysis was performed in a flow reactor at low pressure (4.0 kPa) in the temperature range of 623–973 K. The pyrolysis species was detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS). Species from m/z = 15–87 were identified from near-threshold photoionization efficiency (PIE) spectra, such as radicals of CH3, C2H5, C3H5 and C3H7; isomers of C2H4O (ethenol and acetaldehyde), C3H6O (acetone and propanal), C4H10 (butane and isobutane), HNC (hydrogen isocyanide), HCN (hydrogen cyanide)), C3H5N (ethyl isocyanide and species with unidentified structure) and C3H5NO (methoxyacetonitrile and species with unidentified structure). Their mole fraction profiles were evaluated at different temperature. Based on the mole fractions of species measured at the minimum temperature of 1-NP pyrolysis, some new channels of 1-NP primary pyrolysis were speculated, such as the channels involving O-transfer from N to C-atoms in different positions, deoxygenation, dehydrogenation and OH-abstraction. Based on the mechanisms proposed in the literature, the species produced from secondary or subsequent reactions were also discussed. These results are valuable to a further quantum mechanical calculation for a better understanding of 1-NP chemistry.

Probing pyrolysis chemistry of 1-heptene pyrolysis with insight into fuel molecular structure effects

The pyrolysis of 1-heptene was studied in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry at 0.04 and 1 atm and in a jet-stirred reactor using gas chromatography at 1 atm. Flow reactor pyrolysis products, including the allyl radical, cycloalkenes and aromatics, were identified and quantified. Alkenes are found to be the dominant product family, among which ethylene is the most abundant product. A detailed intermediate-to-high temperature model of 1-heptene was developed and validated against the new pyrolysis data in this work, as well as previous data of 1-heptene combustion in literature over a wide range of pressures, temperatures and equivalence ratios. Rate of production analysis and sensitivity analysis were performed to reveal the key pathways in fuel decomposition and product formation. The allylic CC bond dissociation reaction is concluded as the most important pathway in 1-heptene decomposition. Reactions of allyl, propargyl and cyclopentadienyl radicals play important roles in the formation of cycloalkenes and aromatics. Furthermore, comparative pyrolysis experiments of 1-hexene and n-heptane were also performed in the jet-stirred reactor at 1 atm using gas chromatography to explore fuel molecular structure effects on pyrolysis reactivity and product distributions among 1-alkene and n-alkane fuels. The comparison between 1-heptene and 1-hexene pyrolysis demonstrates that similar fuel molecular structure results in the similarities in primary fuel decomposition pathways and pyrolysis reactivity. Ethylene is the most abundant product in both 1-alkene pyrolysis, and the feature in 1-heptene molecular structure leads to enhanced formation of ethylene in its pyrolysis. The abundant formation of ethyl and methyl radicals leads to higher production of 1-pentene and 1-butene in 1-heptene and 1-hexene pyrolysis, respectively. The comparison between 1-heptene and n-heptane pyrolysis reveals that the existence of CC double bond enhances the pyrolysis reactivity of 1-heptene. Different from 1-heptene consumption, n-heptane consumption is dominantly controlled by H abstraction reactions instead of unimolecular decomposition reactions. Propene and 1-butene are prone to be produced in 1-heptene pyrolysis, while 1-hexene has higher mole fractions in n-heptane pyrolysis.

Exploring combustion chemistry of ethyl valerate at various pressures: Pyrolysis, laminar burning velocity and kinetic modeling

In this work, pyrolysis experiments of ethyl valerate were performed in a flow reactor over 705–1051 K at low and atmospheric pressures and in a jet-stirred reactor over 633–1013 K at near-atmospheric pressure. Products were measured with synchrotron vacuum ultraviolet photoionization mass spectrometry in the flow reactor pyrolysis and gas chromatography in the jet-stirred reactor pyrolysis. Valeric acid and ethylene were observed as the most abundant pyrolysis products in both experiments. Laminar burning velocities of ethyl valerate/air mixtures were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 443 K and initial pressures of 1–10 atm. A kinetic model of ethyl valerate combustion incorporated with recent theoretical progress was developed to predict the new experimental data in this work, as well as the speciation data under flame conditions and laminar burning velocities at different initial temperatures and pressures in literature. Experimental observations and modeling analyses both confirm the significant role of the intramolecular elimination reaction of ethyl valerate producing valeric acid and ethylene. In particular, this reaction has exclusive significance in decomposition of ethyl valerate under pyrolysis conditions, indicating pyrolysis experiments can provide crucial constraints for its rate constant. Subsequent decomposition reactions of valeric acid at higher temperatures enrich the intermediate pool, especially radicals, and can continue producing ethylene to make its mole fraction keep growing under the investigated temperature ranges in the jet-stirred reactor pyrolysis. Under the flame propagation conditions, C0C1 reactions have the highest sensitivity coefficients to the flame propagation, while ethylene- and vinyl-involved reactions also play important roles due to the abundant production of ethylene.

Exploring combustion chemistry of 1‐pentene: Flow reactor pyrolysis at various pressures and development of a detailed combustion model

AbstractThe pyrolysis of 1‐pentene was investigated in a flow reactor over temperatures of 900–1300 K and pressures of 0.04 and 1 atm using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV‐PIMS). Pyrolysis products, especially radicals and cyclic compounds, were identified, and their mole fraction profiles were quantified. A detailed kinetic model of 1‐pentene combustion was developed and validated against the new data in this work. Modeling analysis including rate of production analysis and sensitivity analysis was performed to reveal the key pathways in the decomposition of 1‐pentene and the formation of pyrolysis products. Due to the presence of CC double bond in 1‐pentene molecule, the allylic CC bond dissociation reaction and the retro‐ene reaction play important roles in the formation of dominant pyrolysis products such as small alkenes. Besides, the formation of cyclic products including 1,3‐cyclopentadiene, fulvene, benzene, and toluene is dominated by the reactions of C3 intermediates with C2–C4 intermediates. The model was further validated against the previous data of 1‐pentene combustion, including the ignition delay times and laminar burning velocities over a wide range of pressures, temperatures, and equivalence ratios. Modeling analysis shows that C0–C3 reactions and fuel‐specific reactions are important for the ignition process of 1‐pentene, while C0–C2 reactions play significant roles in the laminar flame propagation of 1‐pentene.

Flow reactor pyrolysis of iso-butylbenzene and tert-butylbenzene at various pressures: Insight into fuel isomeric effects on pyrolysis chemistry of butylbenzenes

This work reports an experimental and kinetic modeling investigation on the flow reactor pyrolysis of iso-butylbenzene and tert-butylbenzene at 0.04 and 1 atm. Pyrolysis products were detected and identified using synchrotron vacuum ultraviolet photoionization mass spectrometry, and their mole fractions versus heating temperature were measured. High-pressure-limit and pressure-dependent rate constants of unimolecular decomposition reactions of iso-butylbenzene were calculated in this work using the same method as the theoretical calculation investigation on similar reactions of n-butylbenzene, sec-butylbenzene and tert-butylbenzene reported by Belisario-Lara et al. [J. Phys. Chem. A, 122 (2018) 3980–4001]. Furthermore, a pyrolysis model of four butylbenzene isomers was developed from our previous models of n-butylbenzene and sec-butylbenzene and validated by the present experimental data. Modeling analysis was performed to reveal key pathways in fuel decomposition and polycyclic aromatic hydrocarbon (PAH) formation of iso-butylbenzene and tert-butylbenzene. The dominant decomposition reactions of iso-butylbenzene and tert-butylbenzene under pyrolysis conditions are benzylic CC bond dissociation reactions, while the major products in pyrolysis process are produced by β-scission reactions of primary radical products. Fuel-specific pathways are found to strongly affect the formation of PAHs, especially for indene, naphthalene and phenanthrene. Fuel isomeric effects on the pyrolysis of four butylbenzene isomers were also analyzed with special concerns on the fuel decomposition and PAH formation processes. Different sidechain structures result in different distributions of major products, while different formation pathways and concentrations of precursors lead to different formation tendency of typical PAHs.

Experimental and kinetic modeling investigation on sec-butylbenzene combustion: Flow reactor pyrolysis and laminar flame propagation at various pressures

This work reports the investigation on pyrolysis and laminar flame propagation of sec-butylbenzene. The pyrolysis experiments were performed in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry from 780 to 1166 K at 0.04 and 1 atm. The pyrolysis products were identified and their mole fraction profiles versus the heating temperature were evaluated. The laminar burning velocities of sec-butylbenzene/air mixtures were measured at the initial temperature of 423 K and initial pressures of 1–10 atm in a high-pressure constant-volume cylindrical combustion vessel with the equivalence ratio from 0.7 to 1.5. Furthermore, a kinetic model was developed to predict the pyrolysis and laminar flame propagation of sec‑butylbenzene. Validation of the present model was performed against the new experimental data in this work. Rate of production analysis and sensitivity analysis were performed to provide insight into the chemistry in fuel decomposition and polycyclic aromatic hydrocarbons (PAHs) formation. In the flow reactor pyrolysis, the consumption of sec‑butylbenzene is mainly controlled by the unimolecular decomposition reactions and H-atom abstraction reactions at both low and atmospheric pressures. Fuel specific pathways through propenylbenzene and α-methylstyrene become the dominant formation pathways of indene and naphthalene. In the laminar flame propagation, the laminar burning velocity of sec‑butylbenzene is sensitive to the reactions of both small species and fuel-relevant intermediates under all investigated conditions. In particular, the pyrolysis reactions in the fuel sub-mechanism play inhibition effects on the laminar flame propagation of sec‑butylbenzene under rich conditions. The laminar burning velocity of sec‑butylbenzene is also compared with benzene, toluene and ethylbenzene under same initial conditions. Both the thermodynamic and kinetic effects are responsible for the difference in laminar burning velocities of these aromatic fuels.

Experimental and kinetic modeling study on flow reactor pyrolysis of iso-pentanol: Understanding of iso-pentanol pyrolysis chemistry and fuel isomeric effects of pentanol

Flow reactor pyrolysis of iso-pentanol was conducted at 30 and 760 torr using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Pyrolysis products, especially radicals and enols, were identified and quantified. Olefins and oxygenated products (mainly aldehyde and enol) were found to be the two major product families in the pyrolysis of iso-pentanol. A kinetic model of iso-pentanol pyrolysis was developed and validated against the new experimental data. The rate of production (ROP) analysis and sensitivity analysis were conducted to understand the pyrolysis chemistry of iso-pentanol. The contribution of unimolecular decomposition reactions to the consumption of iso-pentanol is far less than that of H abstraction reactions at both pressures, which is similar to the situation in 2-methyl-1-butanol pyrolysis, but different from that in n-pentanol pyrolysis. Fuel isomeric effects on the pyrolysis chemistry of pentanol isomers were also investigated. Mole fraction profiles of fuels and major products were measured for other two typical pentanol isomers, i.e. n-pentanol and 2-methyl-1-butanol under the same pyrolysis conditions, while our previous models of n-pentanol and 2-methyl-1-butanol were used for the simulation. The initial decomposition temperatures of two branched pentanol isomers are slightly lower than that of n-pentanol at both pressures. The branched fuel structures strongly influence the molecular structures and formation pathways of fuel decomposition products such as C4 and C5 olefins, as well as the formation of benzene and fulvene.

Experimental and kinetic modeling study of n-propanol and i-propanol combustion: Flow reactor pyrolysis and laminar flame propagation

Flow reactor pyrolysis and laminar flame propagation are investigated for n-propanol and i-propanol, which are the smallest alcohol fuels with isomeric structures. Pyrolysis products of propanol isomers at 0.04–1 atm are detected using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Experimental observations demonstrate ethylene and propene are respective dominant hydrocarbon products in the n-propanol and i-propanol pyrolysis, while the most abundant oxygenated products are formaldehyde, acetaldehyde and ethenol in the n-propanol pyrolysis and acetone and acetaldehyde in the i-propanol pyrolysis. Higher concentrations of aromatic and oxygenated pollutants are observed in the i-propanol pyrolysis. The laminar burning velocities of propanol isomers are measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 423 K, pressures of 1–10 atm and equivalence ratios of 0.6–1.5. A general trend that n-propanol has much faster LBVs than i-propanol is noticed under all investigated conditions. A kinetic model of propanol isomers is developed and validated against the present experimental data, as well as other experimental data in literature covering wide ranges of temperatures, pressures and equivalence ratios. Rate of production analysis and sensitivity analysis are performed together to provide insight into the fuel isomeric effects on key reaction pathways and fuel reactivities. In the flow reactor pyrolysis, different dominant primary decomposition pathways of n-propanol and i-propanol lead to the differences in both molecular structures and concentration levels of pyrolysis products. In laminar flame propagation, different radical pool distributions of propanol isomers are illustrated and found to be largely influenced by fuel isomeric structures. The linear structure of n-propanol promotes the formation of more active radicals like formyl, vinyl and ethyl, while the branched structure of i-propanol facilitates the production of more stable radicals including methyl and allyl. Thus, the promoted chain-branching in n-propanol flames and enhanced chain-termination in i-propanol flames can explain the higher laminar burning velocities and reactivities of n-propanol than that of i-propanol.

Experimental and kinetic modeling investigation on anisole pyrolysis: Implications on phenoxy and cyclopentadienyl chemistry

In this work, the flow reactor pyrolysis of anisole was studied at pressures of 0.04 and 1 atm and temperatures from 850 to 1160 K. Comprehensive speciation was achieved using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A detailed kinetic model for anisole combustion was developed and validated against experimental results in the present work. Fuel decomposition and aromatics formation processes were investigated based on modeling analyses. The results show that the dominant decomposition pathway of anisole is the unimolecular OCH3 bond dissociation reaction at both pressures, while the role of bimolecular reactions becomes significant at 1 atm. At lower temperatures, phenoxy radical is mainly consumed via the reactions with methyl radical, producing methylcyclohexadienone. At higher temperatures, it is mainly consumed via the unimolecular decomposition reaction producing cyclopentadienyl. Cyclopentadienyl is responsible for the abundant production of aromatic products such as benzene, toluene, styrene and naphthalene. Furthermore, the bimolecular reactions of anisole also contribute to the formation of aromatic products at lower temperatures. Possible formation pathways of oxygenated aromatics such as benzofuran and dibenzofuran were also analyzed in this work. The present model was also validated against literature experimental data of anisole combustion, including global combustion parameters like ignition delay times and speciation profiles in flow reactor pyrolysis and jet stirred reactor pyrolysis and oxidation.