Growth Of Polycyclic Aromatic Hydrocarbons Research Articles

Experimental and modeling study of fuel-rich oxidation of gasoline surrogate components/dimethyl carbonate blends in an atmospheric flow reactor

Fuel-rich oxidation of blending fuels consisting of three typical gasoline surrogate components (n-heptane, iso-octane, and toluene) and dimethyl carbonate (DMC) was investigated in an atmospheric-pressure flow reactor at mean gas temperatures from 1000 to 1350 K, equivalence ratio of 9.0, and residence time of 1.2 s. Blending ratios of DMC were 35 and 70 % on a molar basis. The mole fractions of not only polycyclic aromatic hydrocarbons (PAHs) up to 3 ring structure but also small intermediate species from C1 to C5 contained in sampled gas were experimentally quantified. A kinetic model that includes not only oxidation mechanism of gasoline surrogate components and DMC but also PAH growth reactions was developed to reproduce the experimental data. The experimental and simulated results showed that the blending effect of DMC on PAH production was different according to gasoline surrogate components. By blending DMC with n-heptane and iso-octane, the mole fractions of PAHs were generally reduced with an increase of the DMC blending ratio in the temperature range studied here. On the other hand, when DMC was blended with toluene, PAH production in the blending fuels was larger than that in pure toluene at a temperature below 1150 K, while the mole fractions of PAHs in pure toluene were identical to or larger than those in the blending fuels at a temperature above 1200 K. Because a reactivity of DMC is higher than toluene, toluene conversion was facilitated at a low temperature, leading to the promoted PAH production in the blending fuels compared to pure fuel. However, such a promoting effect of DMC was diminished at an elevated temperature, explaining the identical or large PAH production in neat toluene compared to the blending fuels. On the other hand, n-heptane and isooctane can be easily reacted even at a low temperature, suggesting that the promoting effect of DMC cannot be observed. This study reasonably succeeded in explaining the different DMC blending effects on PAH formation depending on the gasoline surrogate components. Novelty and significance statementExperiments and kinetic modeling for the mixtures consisting of dimethyl carbonate (DMC) and gasoline surrogate components, such as n-heptane, iso-octane, and toluene, were conducted to reveal the blending effect of DMC on polycyclic aromatic hydrocarbon (PAH) formation under the fuel-rich environment. It was found that DMC had a dilution effect in n-heptane/DMC and iso-octane/DMC blends, resulting in a reduction in PAH production with an increase of a DMC blending ratio. On the other hand, while DMC had the dilution effect in the toluene/DMC blend at an elevated temperature similar to other blends, toluene conversion was facilitated in the presence of DMC at a low temperature, resulting in the enhanced PAH production in the toluene/DMC blends compared to pure toluene.

Compact and accurate chemical mechanism for methane pyrolysis with PAH growth

A reliable and compact chemical mechanism of gas-phase methane pyrolysis leading to formation of large polycyclic aromatic hydrocarbon (PAH) molecules has been developed. This model is designed for studies of carbon nanostructure synthesis such as carbon black and graphene flakes, including soot growth kinetics. Methane pyrolysis with carbon nanostructure synthesis is a two-stage process where conversion of CH4 to C2H2 precedes the growth of PAH molecules from acetylene. We present a single chemical mechanism that accurately describes both stages. We have constructed a compact and accurate chemical mechanism capable of modeling both stages of methane pyrolysis based on the ABF [1] mechanism which was expanded with the most prominent reaction pathways from the mechanism by Tao [2] for small PAH molecules and HACA pathways for larger PAH molecules, up to 37 aromatic rings. The resulting mechanism was validated through comparison to multiple available sets of experimental data. Good agreement with the experimental data for both processes was obtained. Performance of the mechanism was tested for pyrolysis of methane-rich mixtures under long residence times leading to abundant formation of PAH molecules. It is shown that the inclusion of larger PAH species (up to A37) in the chemical mechanism is important for accurate prediction of the fraction of carbon converted to PAH molecules and, correspondingly, the residual fraction of acetylene in the mixture. The mechanism file is available upon request.

Pyrolytic decomposition of methanol, ethanol, and butanol at various temperatures and residence times in a high-temperature flow reactor

The potential use of low-carbon renewable fuel molecules, such as short chain alcohols, as a substitute or extender for fossil fuels, offers an opportunity to address pollutant emissions from transport and the sustainability of energy sources. Moreover, it may help reduce pollutants detrimental to air quality and human health, of which particulate matter (PM) is a significant contributor. To this end, the pyrolysis of three renewable alcohol fuels, methanol, ethanol, and butanol, has been studied in a laminar flow reactor at a sample inlet temperature range of 869 0C to 1120 0C. All the tested fuels were injected into a nitrogen carrier gas stream at a fixed concentration of 10,000 ppm on a carbon atom basis. A range of pyrolysis temperatures and residence times were selected so as to gain insight into the pyrolytic decomposition of short chain alcohol fuels to intermediate species important to the subsequent formation of benzene rings, larger polycyclic aromatic hydrocarbons (PAH), and particulate matter. In this paper, the presence of ten intermediary species that may aid in the formation of benzene rings and the growth of PAHs were quantified, with an emphasis on probable routes of first ring formation from the alcohols of varying chain length. Gaseous samples were collected from the pyrolyser at different temperatures and residence times using a novel high-temperature air-cooled ceramic sampling probe. The identification and quantification of intermediate gaseous species were undertaken by Gas Chromatography-Flame Ionization Detection (GC-FID). During methanol pyrolysis, only a tenth of the supplied carbon was detected as hydrocarbon gaseous species (mainly methane), with no benzene found, highlighting that methanol pyrolysis does not readily result in polyaromatic hydrocarbons and soot. Ethanol and butanol pyrolysis produced substantial amounts of ethylene and acetylene, suggesting a role in benzene and soot formation. Uniquely, pyrolysis of the longest chain alcohol, butanol, produced appreciable levels of C3 and C4 hydrocarbons, in addition to C1 and C2 hydrocarbons, suggesting additional reaction pathways for benzene formation and growth in comparison with ethanol.

Phenalenyl growth reactions and implications for prenucleation chemistry of aromatics in flames.

The energetics and kinetics of phenalene and phenalenyl growth reactions were studied theoretically. Rate constants of phenalene and phenalenyl H-abstraction and C2H2 addition to the formed radicals were evaluated through quantum-chemical and rate-theory calculations. The obtained values, assigned to all π radicals, were tested in deterministic and kinetic Monte Carlo simulations of aromatics growth under conditions of laminar premixed flames. Kekulé and non-Kekulé structures of the polycyclic aromatic hydrocarbons (PAHs) evolving in the stochastic simulations were identified by on-the-fly constrained optimization. The numerical results demonstrated an increased PAH growth and qualitatively reproduced experimental observations of Homann and co-workers of non-decaying PAH concentrations with nearly equal abundances of even and odd carbon-atom PAHs. The analysis revealed that the PAH growth proceeds via alternating and sterically diverse acetylene and methyl HACA additions. The rapid and diverse spreading in the PAH population supports a nucleation model as PAH dimerization, assisted by the non-equilibrium phenomena, forming planar aromatics first and then transitioning to the PAH-PAH stacking with size.

A theoretical and modeling study of nitrogen chemistry in polycyclic aromatic hydrocarbons growth process

Ammonia blended with hydrocarbon fuels inhibits soot formation due to the chemical effect of NH3 in polycyclic aromatic hydrocarbons (PAHs) growth process. Nitrogenous species are highly concentrated intermediates in combustion of fuels doped with NH3, while their interaction with PAHs remains unclear for the lack of experimental data. In this work, hydrogen cyanide (HCN) reacts with benzene and naphthalene as isoelectronic species of C2H2 to complete the hydrogen-abstraction-acetylene-addition (HACA) chemistry reaction in PAHs growth process. The reaction pathways for HCN-benzene and HCN-naphthalene were investigated at G3(MP2, CC)//B3LYP/6-311G(d,p) level, and the rate coefficients were calculated using Rice-Ramsperger-Kassel-Marcus (RRKM) theory to analyze the yield distribution of various reaction products. Cyano-substituted species or N-PAHs may form in HCN-benzene and HCN-naphthalene reaction systems. The kinetic results show that the formation of cyano group dominates the HCN addition at high temperatures, which indicates that the Frenklach HACA route is more favorable in flame conditions. The branching ratios results show that HCN addition to benzene and naphthalene competes with C2H2 addition within the temperature range of 1000–2000 K. Besides, the yield of HCN reacting with naphthalene is considerable at low temperatures. The simulation results confirm the contribution of HCN addition to benzene by detecting the mole fraction of benzonitrile. Theoretical study and kinetic simulation identify the interaction between HCN and PAHs, blocking the formation of larger PAHs. These findings provided a deeper insight into the chemical effect of nitrogenous species on PAHs growth, which helps to explain the inhibitory effect of NH3 on soot formation.

Insight into the effect mechanism of sedimentary record of polycyclic aromatic hydrocarbon: Isotopic evidence for lake organic matter deposition and regional development model

Deciphering the temporal patterns of polycyclic aromatic hydrocarbons (PAHs) in sediment cores, and the effect mechanism of sedimentary organic matter (OM) and regional development model on PAHs are crucial for pollution control and environmental management. Herein, sediment core was collected from Chenhu international wetland in Wuhan, central China. Meanwhile, historical trend and source of PAHs and sedimentary OM were presented, respectively. Result demonstrated that the most significant growth of PAHs (increased by 158.8%) was attributed to the significant enhancement of traffic emission (5.57 times), coal combustion (4.59 times), and biomass burning (8.09 times). Similarly, the percentage of phytoplankton (stage Ⅲ: 37.9%; stage Ⅳ: 31.2%) and terrestrial C3 plants (stage Ⅲ: 24.6%; stage Ⅳ: 29.2%) to sedimentary OM hold the dominant position after the stage Ⅱ. The obvious shifts of historical trend and sources in PAHs were highly related to economic development models (r = 0.72, p < 0.001) and sedimentary OM (r = 0.82, p < 0.001). It demonstrated that eutrophication of lake accelerated the burial of PAHs. Redundancy analysis results suggested that TOC was dominating driver of sedimentary PAHs (16.56%) and phytoplankton occupied 9.58%. To further confirm the significant role of economic development models, three different historical trends of PAHs in different regions of China were presented. The result of this study provides the new insight into the geochemistry mechanism of lake sedimentary OM and PAHs. Meanwhile, the relationship of regional development model and sedimentary PAHs was highlighted in this study. Significantly, the main environmental implications of this study are as follows: (1) lake eutrophication of phytoplankton OM accelerated the burial of PAHs in lake sediment; (2) economic development models and energy structure significantly influence the sedimentary PAHs. This study highlights the coupling relationship between OM burial and PAHs sedimentation, and the importance of accelerating the transformation of economic energy structure.

Surfactant-enhanced anoxic degradation of polycyclic aromatic hydrocarbons (PAHs) in aged subsurface soil at high temperature (60 °C)

Thermally enhanced anoxic biodegradation is emerging as a promising method for removing PAHs from subsurface soil. However, some PAHs still remain in soil following remediation with thermally enhanced anoxic degradation due to low bioavailability of these residual PAHs. The effects of five surfactants (Tween 80, TX 100, Brij 30, SDS, and SDBS) on the desorption of PAHs, anoxic degradation of PAHs, and native bacteria in soil at high temperature (60 °C) were evaluated in this study. The desorption of PAHs in soil increased as surfactant concentration increased. Low doses of surfactants (0.08%, w/w) enhanced the growth of potential PAHs degrading bacteria and promoted the anoxic degradation of PAHs, whereas high doses of surfactants (0.3%–0.8%, w/w) displayed the opposite effect, and the degree of inhibition increased with increasing surfactant concentration. The results also indicated that the inhibitory effect of anionic surfactants (SDS and SDBS) on microbial growth and PAHs degradation is stronger than that of nonionic surfactants (Tween 80, TX 100 and Brij 30) at the same concentration. These results suggest a feasible way of enhancing the anoxic degradation of PAHs in soil where heat cannot be effectively utilized when in situ thermal desorption (ISTD) technology is used.

Experimental and kinetic modeling study of polycyclic aromatic hydrocarbon formation pathways in fuel-rich oxidation of 2-methylfuran using an atmospheric flow reactor

2-Methylfuran is one of promising biofuels that is expected to be an alternative fuel or fuel additive to conventional fuels for reducing greenhouse gas emissions from internal combustion engines. Although oxidation experiments under a wide range of conditions are required for a better understanding of 2-methylfuran combustion, the fuel-rich oxidation of 2-methylfuran has not been studied extensively. In this study, the fuel-rich oxidation of 2-methylfuran was investigated using an atmospheric flow reactor at temperatures of 1000–1350 K, equivalence ratios of 3.0–12.0, and residence times of 0.2–1.5 s. Chemical species such as the polycyclic aromatic hydrocarbons (PAHs) with one to six aromatic rings formed in the oxidation were sampled and quantified using gas chromatography mass spectrometry, while small intermediate products from C1 to C5 were quantified using gas chromatograph equipped with flame ionization detector. A kinetic model was developed by combining the existing reaction mechanisms for 2-methylfuran oxidation and PAH growth, which were constructed separately. The developed model was validated against the experimental results obtained in this study, showing that the simulated concentration profiles of the chemical species were in reasonable agreement with the measured data under the present experimental conditions. Kinetic analyses using the model were conducted to unravel the reaction pathways of PAHs as well as the main consumption pathways of 2-methylfuran. These results showed that 2-methylfuran underwent ring cleavage and CO/CH3CO removal, resulting in production of C2–C4 species. It was also found that the resonantly stabilized radicals, particularly the propargyl radical, were the key species responsible for PAH growth reactions, and the hydrogen-abstraction carbon-addition mechanism was important for the formation of several PAHs.

The use of an algal bioindicator in the assessment of different chemical remediation strategies for PAH-contaminated soils and sediments

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous petroleum-derived pollutants that tend to accumulate in soils and sediments. Among the in situ chemical oxidation technologies applied at PAH-contaminated sites are Fenton reagent (FN), persulfate ion (PS) and permanganate ion (PM). The natural phytoplankton of an aquatic environment is useful as a biological indicator of the treatment efficacy of pollutant degradation. The objective of this work was to assess the use of an algal species, native from Los Barreales Reservoir (Argentina), as an efficacy indicator of the chemical remediation of PAH-contaminated soils and sediments. Oxidations with FN, PS or PM were carried out on soils and sediments previously contaminated with two types of PAHs (phenanthrene (Ph) or Benzo(a)anthracene (Ba)), analysing pH, residual oxidant, PAH degradation and heterotrophic bacteria abundance in soils or sediments after 7 days. Then, a bioassay was carried out by incubating the obtained soils or sediments, and the native alga Scenedesmus quadricauda (Turpin, Brébisson), analysing its abundance after 7 days. S. quadricauda abundance was related to PAH degradation efficacy, residual oxidants and changes in system pH. In this sense, the application of PS produced the lowest growth of the indicator in assays both with soils and sediments. Fenton reaction results varied depending on the contaminating PAH, with the best percentages of PAH degradation and growth of S. quadricauda for Ba. PM oxidation resulted in the best development of the indicator. This study constitutes a first approach to the use of S. quadricauda as an efficiency indicator of soil/sediment chemical remediation.

Star Formation and Molecular Gas Diagnostics with Mid- and Far-infrared Emission

With the start of JWST observations, mid-infrared (MIR) emission features from polycyclic aromatic hydrocarbons (PAHs), H2 rotational lines, fine structure lines from ions, and dust continuum will be widely available tracers of gas and star formation rate (SFR) in galaxies at various redshifts. Many of these tracers originate from dust and gas illuminated by UV photons from massive stars, so they generally trace both SFR and gas to varying degrees. We investigate how MIR spectral features from 5–35 μm and photometry from 3.4–250 μm correlate with SFR and molecular gas. In general, we find MIR emission features (i.e., PAHs and H2 rotational lines) trace both CO and SFR better than CO and SFR trace one another. H2 lines and PAH features correlate best with CO. Fine structure lines from ions correlate best with SFR. The [S iii] lines at 18.7 and 33.5 μm, in particular, have a very tight correlation with SFR, and we use them to calibrate new single-parameter MIR tracers of SFR that have negligible metallicity dependence in our sample. The 17 μm/7.7 μm PAH feature ratio increases as a function of CO emission which may be evidence of PAH growth or neutralization in molecular gas. The degree to which dust continuum emission traces SFR or CO varies as a function of wavelength, with continuum between 20 and 70 μm better tracing SFR, while longer wavelengths better trace CO.

Role of ammonia addition on polycyclic aromatic hydrocarbon growth: A ReaxFF molecular dynamics study

Ammonia (NH3), one of the most promising carbon-free fuels, has received great research interest. In particular, NH3 is often blended with hydrocarbon fuels to achieve desired combustion characteristics. However, NH3 addition could affect soot formation, which has not been adequately understood. In this study, the effect of NH3 on the growth of polycyclic aromatic hydrocarbons (PAHs) is investigated with the reactive force field molecule dynamics (ReaxFF MD) simulations and quantum chemistry calculations. The simulation results indicate that NH3 addition slows down the growth of large carbon-containing species in the C2H4/O2 system. Novel path with HCN addition is discovered in the PAH growth, which inhibits the PAH growth compared to the conventional Hydrogen-Abstraction-Carbon-Addition (HACA) path via C2H2 addition. Moreover, quantum chemical calculations verified the rationality of this addition path and explained its inhibition on PAH growth by calculating the heat of reactions and reaction energy barriers. According to the present study, NH3 also has an inhibitory effect on both the HCN addition path and C2H2 addition path by providing H atoms to promote the generation of PAH radicals back to PAH molecules, and this effect is more significant for the C2H2 addition path. Results from the study provide a fundamental insight for the inhibition of PAH growth due to the NH3 addition from an atomistic insight, which helps to improve the understanding of the combustion of blends of NH3 and hydrocarbon fuels.

Polycyclic aromatic hydrocarbons production from the supercritical pyrolysis of n-decane, ethylcyclohexane, and n-decane/ethylcyclohexane blends

To investigate the effects of fuel composition on the production and growth of polycyclic aromatic hydrocarbons (PAH) at conditions relevant to the pre-combustion environment of fuels in future high-speed aircraft, we have conducted supercritical pyrolysis experiments in an isothermal, silica-lined stainless-steel flow reactor at 568 °C, 94.6 atm, and 133 s, with the model fuels n-decane and ethylcyclohexane—as well as n-decane/ethylcyclohexane blends in which the fraction of fuel carbon coming from ethylcyclohexane is 0.25, 0.50, and 0.75. High-pressure liquid-chromatographic analyses of the reaction products have led to the isomer-specific identification and quantification of 169 three- to nine-ring PAH—159 of which have never before been reported as products of a cyclic-alkane fuel. Quantification of the aliphatic products by gas chromatographic techniques reveals that n-decane produces mostly 1-alkenes and n-alkanes, whose radicals generate more radicals; whereas ethylcyclohexane produces mostly C5-ring and C6-ring alkanes and alkenes, whose radicals show a propensity for ring dehydrogenation. These contrasts in the aliphatic-product distributions have major impacts on PAH growth, which, in this reaction environment, occurs chiefly through the reactions of resonance-stabilized arylmethyl radicals with the C2-C4 1-alkenes. For fuel compositions rich in n-decane, all the ingredients necessary for these PAH-growth reactions to thrive are in place: methyl-substituted PAH, as sources of arylmethyl radicals; a radical-rich environment that fosters H abstraction and arylmethyl-radical formation; and C2-C4 1-alkenes, as growth species. However, an increase in the level of ethylcyclohexane in the fuel brings about substantial reductions in the levels of H-abstracting radicals and C2-C4 1-alkenes as well as an increase in the supply of “donated” hydrogen for stabilizing radicals. These factors combine to bring about marked reductions in PAH growth at fuel compositions rich in ethylcyclohexane, resulting in much lower production of the high-ring-number PAH that are the precursors to solids in the supercritical fuel-pyrolysis environment.

A theoretical study on the effect of C60 particles on the growth of coronene radical based on HACA pathway.

Soot is a pollutant caused by combustion and is harmful to the environment and human health. Polycyclic aromatic hydrocarbons (PAHs) are considered the precursors of soot, thus exploring the growth mechanism of PAHs is conducive to reducing soot release. The mechanism by which a pentagonal carbon ring triggers the formation of curved PAHs has been demonstrated but studies on subsequent growth of soot are rare due to the lack of a suitable model. Buckminsterfullerene (C60), as one of the products from incomplete combustion under specific conditions, is similar in structure to soot particles with a surface that can be treated as curved PAH. Coronene (C24H12) is a typical seven-membered fused-ring PAH. In this study, C60 was employed as a substitute for soot particles to investigate its effect on coronene growth reaction based on the hydrogen-abstraction/acetylene-addition (HACA) mechanism. Density functional theory (DFT) at the M062X/6-31G(d,p) level was adopted to investigate the potential energy surfaces (PESs) for these reactions. The high-pressure limiting rate constants for the relevant reactions were obtained based on transition state theory. The calculated results indicate that C60 is easily hydrogenated, providing new pathways for coronene growth. Soot particles have a certain effect on the growth of PAHs. This study provides favorable support for further understanding the effect of soot on the growth pathway of PAHs.

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.

Blending effect of methanol on the formation of polycyclic aromatic hydrocarbons in the oxidation of toluene

Methanol has been considered as a potential renewable liquid fuel and blending it with gasoline and diesel is an effective way to reduce greenhouse gas emissions from the transport sector. To understand the mixing effect of methanol on the formation of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs), the fuel-rich oxidation of toluene with and without methanol was studied using a flow reactor at atmospheric pressure, temperatures from 1050 to 1350 K, equivalence ratio of 9.0, and residence time of 1.2 s. The blending ratio of methanol was varied as 0% and 50% on a molar basis. Gas chromatograph mass spectrometer was employed to identify and quantify PAHs and OPAHs in gaseous products. A kinetic model on PAH growth up to five ring structures was used to investigate the blending effect on PAH and OPAH formation. Both experiment and modeling showed that PAH and OPAH production at lower temperatures was unexpectedly promoted in toluene/methanol oxidation compared with toluene oxidation, while their production in toluene oxidation was identical with or larger than that in toluene/methanol oxidation at elevated temperatures. In methanol oxidation, no PAHs were produced under the current experimental conditions. Kinetic analysis indicated that high methanol reactivity produced several radicals, such as OH, H, and HO2, which promoted toluene reactivity at lower temperatures, resulting in the enlargement of PAH and OPAH formation in toluene/methanol oxidation compared to neat toluene oxidation. When the temperature was increased, the effect of methanol blending was diminished based on the kinetic analysis. These results suggest that oxygenated fuels do not necessarily reduce PAH production, but promote it under some conditions.

Insight into the internal relevance of three-phase products in supercritical water gasification of coal

• Internal relevance of three-phase products of coal gasification was investigated. • Insight into the formation mechanism of products in the micro dimension. • Solid-liquid reaction model in coal particle surface was established. • Control steps for decreasing the irreversibility of reaction process was summarized. Supercritical water gasification (SCWG) is a promising technology for coal utilization and has attracted more attentions. This paper is aimed to obtain coal gasification behavior and the internal relevance of products from the perspective of micro level. Evolution characteristics of three-phase products were employed in high heating rate batch reactor. As a result, removal of functional groups in solid particles led to the decrease of the gasification reactivity of char (R coal > R 20s > R 60s > R 100s > R 1200s ). The hydrogen bonds formed by water forced the coal structure decomposition, in which the CO and CO 2 gas were generated from the deetherization and decarboxylation reaction. H 2 and CH 4 primarily came from the collision of free radicals and polycondensation reaction. Liquid intermediate components were detected by high-performance liquid chromatography quantitatively. The content of phenol and naphthalene was up to 280.8 ppm and 3647.4 ppm, and the mutual conversion mechanism of organic matters and polycyclic aromatic hydrocarbons (PAHs) was represented. Effectively controlling the ring growth of PAHs and the polycondensation of solid fragments is the primary goal to inhibit char formation, which is of great significance to decrease the irreversibility of chemical reaction process and realize the large-scale industrial production.

Effects of prenol and isoprenol addition on soot and PAH formation in ethylene/methane counterflow diffusion flames

Prenol has attracted much attention due to its high heating value and novel octane hyperboosting phenomenon; while its soot formation behavior is not studied yet. In this study, experimental and numerical analysis of the effects of prenol and isoprenol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene/methane flames is performed. Distribution of soot volume fraction and PAHs in the flames is measured via laser induced incandescence and laser induced fluorescence, respectively. The results show that both prenol and isoprenol addition enhances soot and PAH formation in ethylene/methane flames, and the prenol shows a stronger enhancing effect. Modeling of the sooting behavior is performed with a constructed gas-phase chemistry and a sectional method. The numerical results well capture the experimental observations and reveal that the increased concentrations of C2H2, C3H3 or C4H6 after prenol or isoprenol blending chemically promote benzene ring formation, subsequent PAH growth and soot formation processes. ROP (rate of production) analysis is performed to investigate the effect of prenol and isoprenol addition on rate-limiting ring formation process. In methane and ethylene flame, prenol or isoprenol blending promotes benzene formation in different ways, due to the discrepancies on the radical pool in two flames. Moreover, prenol also shows stronger effect on both of soot inception rate and surface growth rate than isoprenol, which is due to more PAHs and C2H2 formed in the prenol-doping flames. Reaction pathway analysis shows that decomposition of prenol generates C2H2 and C3H3 while isoprenol mainly generate C3H3 and almost no C2H2, the distinctive decomposition pathways of prenol results in its higher formation propensity of PAHs and soot.

Kinetic analysis of the pathways to naphthalene formation from phenyl + 1,3-Butadiyne reaction

Having a better understanding of polycyclic aromatic hydrocarbon (PAH) formation under flame conditions contributes to optimizing the fuel reforming process, where soot poisons the downstream catalyst. In this work, the phenyl + 1,3-Butadiyne reaction is systematically investigated to examine its contribution to naphthalene formation. The reaction potential energy surfaces were calculated using DFT/M06–2X/cc-pvtz and G4 methods. The temperature- and pressure-dependent reaction rate constants were calculated using RRKM theory with solving master equation. The results revealed that 2-naphthyl could be directly formed by phenyl + 1,3-Butadiyne reaction. With H assistance, naphthalene could be formed by the pathway of phenyl + 1,3-Butadiyne → C6H5CHCCCH (+H) → C6H5CHCHCCH (+H) →naphthalene +H. The proposed pathway is kinetically favorable, and featured by relatively low energy barrier. The importance of the proposed pathway reaction was confirmed in a premixed and a diffusion C2H4/O2/Ar flame simulations, where the enhancement of naphthalene by the investigated reactions is notable. The mole fraction of A2 is promoted by a factor of 10% in premix C2H4/O2/Ar flame and 30% in C2H4/O2/Ar counterflow flame, bringing the prediction results closer to the experimental results. The relative contribution of different reaction route to A2 formation is evaluated for HACA, cyclopentadienyl radical-cyclopentadienyl radical, phenyl-vinylacetylene[1], benzyl radical-propargyl radical, indene-CH2 and phenyl-1,3-Butadiyne routes in premixed and diffusion C2H4/O2/Ar flames. This work suggests that the PAH growth by 1,3-Butadiyne addition reaction is an effective pathway for A2 formation, which should be considered in future PAH mechanism.

PAH formation characteristics in hydrogen-enriched non-premixed hydrocarbon flames

The utilisation of hydrogen with conventional hydrocarbons offers an excellent opportunity to decarbonise current energy systems without significant hardware upgrades. However, this presents fresh scientific challenges, one of which is the difficulty in effective control of pollutant soot emissions due to complex reaction kinetics of hydrogen enriched flames. This paper focuses on polycyclic aromatic hydrocarbons (PAHs), which are the building blocks of soot and responsible for its carcinogenicity. Detailed understanding of the effect of H2 on the underlying processes of PAH formation and growth is important for the development of effective strategies to curtail PAH formation and hence, reduce soot emissions from combustion systems.In this study, an experimental methodology was employed to analyse PAH formation and growth characteristics of laminar inverse diffusion flames of various hydrocarbon fuels (alkanes and alkenes) enriched with H2 using simultaneous planar laser induced fluorescence (PLIF) imaging of PAHs and hydroxyl radicals (OH). OH PLIF was used to indicate peak temperature locations in the flame (flame front), while PAH PLIF was used to determine PAH formation characteristics. Methane (CH4) was also separately added to the same hydrocarbon fuels to study effects of carbon-bound hydrogen addition, in comparison to H2 addition. It was observed that only the addition of H2 to CH4 showed significant variation in the magnitude of PAH reduction levels as the length along the flame front, Lf increased. The results also showed that while the addition of H2 was more effective in reducing the rate of PAH fluorescence signal increase (indicative of concentration growth) when compared to CH4 addition, both fuels showed two distinct regions in the PAH growth curve; a steep growth region followed by a slower growth region. This is potentially indicative of the self-limiting nature of PAH formation and growth. The study concluded that the growth rate of PAHs lies within a narrow band irrespective of the fuel bonding, molecular structure and the H:C ratio of the fuel mixtures tested.

Accurate thermochemistry prediction of extensive Polycyclic aromatic hydrocarbons (PAHs) and relevant radicals

Polycyclic aromatic hydrocarbons (PAHs) are important intermediates to soot formation in combustion. A reliable database of their thermochemistry is required for the development of chemical kinetic models describing the gas-phase chemistry of hydrocarbon fuels. In this study, temperature-dependent thermodynamic properties are consistently determined using high-accuracy quantum chemistry calculations for an extensive set of PAH compounds. The developed database comprehensively consists of 125 C6-C18 PAH molecules and radicals, which are important and commonly included in chemical mechanisms studies. At the M06-2X/6-311++G(d,p) level of theory, geometry optimizations, vibrational frequency calculations, and dihedral angle scans are performed for all PAH species. The G3 method, together with the atomization reaction approach, is selected to derive the average atomization formation enthalpy. This method produces the most accurate thermochemistry quantities for PAHs, as demonstrated in a previous study. The entropy and heat capacity values are calculated using statistical thermodynamics in MultiWell. These results exhibit good agreement with the databases in literature. To examine the application of the group additivity (GA) method for PAHs, the Bland−Altman plot, a statistical analysis approach, is employed to visualize the agreement between the results from the quantum chemical calculations and GA methods. Two GA methods are examined and significant differences are found, which indicates that GA values of relevant groups need to be further updated. The database of thermodynamic quantities developed in this study are of particular value in modeling studies and important for exploring mechanisms of the PAH growth.