Mechanisms of aliphatic hydrocarbon formation during co-pyrolysis of coal and cotton stalk

Pyrolysis experiments were carried out in a tubular furnace. The characteristics of pyrolysis tar were analyzed by GC/MS. The results indicated that the aliphatic hydrocarbon yield derived from co-pyrolysis tar of cotton stalk and Shenmu coal was obviously higher than that of Shenmu coal pyrolysis under optimum condition. Moreover, microcrystalline cellulose was selected as a model compound and the copyrolysis tar of microcrystalline cellulose and Shenmu coal was analyzed for comparison. Base on the experimental results, it was indicated that the alkyl radicals generated from pyrolysis were converted to aliphatic hydrocarbons by radical reactions. Furthermore, the mechanisms of aliphatic hydrocarbon formation were discussed during co-pyrolysis of cotton stalk and Shenmu coal.     

Combustion modeling and kinetic rate calculations for a stoichiometric cyclohexane flame. 1. Major reaction pathways

The Utah Surrogate Mechanism was extended in order to model a stoichiometric premixed cyclohexane flame (P = 30 Torr). Generic rates were assigned to reaction classes of hydrogen abstraction, beta scission, and isomerization, and the resulting mechanism was found to be adequate in describing the combustion chemistry of cyclohexane. Satisfactory results were obtained in comparison with the experimental data of oxygen, major products and important intermediates, which include major soot precursors of C2-C5 unsaturated species. Measured concentrations of immediate products of fuel decomposition were also successfully reproduced. For example, the maximum concentrations of benzene and 1,3-butadiene, two major fuel decomposition products via competing pathways, were predicted within 10% of the measured values. Ring-opening reactions compete with those of cascading dehydrogenation for the decomposition of the conjugate cyclohexyl radical. The major ring-opening pathways produce 1-buten-4-yl radical, molecular ethylene, and 1,3-butadiene. The butadiene species is formed via beta scission after a 1-4 internal hydrogen migration of 1-hexen-6-yl radical. Cascading dehydrogenation also makes an important contribution to the fuel decomposition and provides the exclusive formation pathway of benzene. Benzene formation routes via combination of C2-C4 hydrocarbon fragments were found to be insignificant under current flame conditions, inferred by the later concentration peak of fulvene, in comparison with benzene, because the analogous species series for benzene formation via dehydrogenation was found to be precursors with regard to parent species of fulvene.     

Mechanistic studies of the pyrolysis of 1,3-butadiene, 1,3-butadiene-1,1,4,4-d4, 1,2-butadiene, and 2-butyne by supersonic jet/photoionization mass spectrometry

The thermal decomposition of 1,3-butadiene, 1,3-butadiene-1,1,4,4-d(4), 1,2-butadiene, and 2-butyne at temperatures up to 1520 K was carried out by flash pyrolysis on a approximately 20 mus time scale. The reaction products were isolated by supersonic expansion and detected by single-photon (lambda = 118 nm) vacuum-ultraviolet time-of-flight mass spectrometry (VUV-TOFMS). Direct detection of CH(3) and C(3)H(3), as well as C(3)H(4), C(4)H(4), and C(4)H(5) products, provides insight into the initial steps involved in the complex pyrolysis of these C(4)H(6) species below T = 1500 K. The similar pyrolysis product distributions for the C(4)H(6) isomers on such a short time scale support the previously proposed mechanism of facile isomerization of these species. Isomerization of 1,3-butadiene to 1,2-butadiene and subsequent C-C bond fission of 1,2-butadiene to produce CH(3) and C(3)H(3) (propargyl) are most likely the primary initial radical production channel in the 1,3-butadiene pyrolysis.     

PAH formation under single collision conditions: reaction of phenyl radical and 1,3-butadiene to form 1,4-dihydronaphthalene

The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry.     

Propyne and 1,3-butadiene adsorption and co-adsorption over palladium catalysts

The adsorption of propyne, 1,3-butadiene and the co-adsorption of propyne and 1,3-butadiene has been studied over a series of palladium catalysts. The adsorption of propyne shows a “secondary” adsorption region typical of hydrocarbon adsorption over supported metal catalysts. Hydrogenation occurs via the hydrogen associated with the β-palladium hydride formed during reduction. A support effect is seen with the titania support reducing the stability of the palladium hydride resulting in a lower hydrogen concentration in the titania supported hydrides. The behaviour of 1,3-butadiene is similar to that found with propyne. The catalyst with the lowest dispersion, as measured by carbon monoxide adsorption, has the highest butadiene adsorption and vice versa, whereas propyne adsorption followed the same trend as carbon monoxide. This behaviour is linked to the mode of adsorption and site requirements. Sequential adsorption revealed that the catalysts had sites that were specific to each adsorbate. Co-adsorption revealed a reduction in adsorption for both gases but with a larger reduction for 1,3-butadiene due to site requirements.     

自由落下床中生物质与煤共热解的协同效应对焦油组成的影响

利用溶剂萃取-柱层析方法,将自由落下床中豆秸与大雁褐煤共热解以及单种原料热解的液体产品分为沥青烯、酚类、脂肪烃类、芳香烃类和极性物等组分。结果表明,共热解的沥青烯产率为11.4%,低于根据煤和生物质单独热解的质量加权平均计算值19.0%,且芳香性增大;与计算值相比,低分子量的酚类、甲基苯酚、二甲基苯酚及其衍生物的含量提高了5%;而且长侧链的脂肪烃含量减少。共热解焦油的芳香类组分中十氢萘的质量分数是43.37%,但其在单一原料热解焦油中并没有被检测到。热解油分析结果表明,自由落下床生物质与煤快速共热解过程中存在协同效应,其主要原因是,发生氢解和加氢反应。煤与生物质共热解有利于产生低分子量的化合物,改善油品的质量。     

Pyrolysis of methyl tert-butyl ether (MTBE). 2. Theoretical study of decomposition pathways

The thermal decomposition pathways of MTBE have been investigated using the G3B3 method. On the basis of the experimental observation and theoretical calculation, the pyrolysis channels are provided, especially for primary pyrolysis reactions. The primary decomposition pathways include formation of methanol and isobutene, CH4 elimination, H2 elimination and C-H, C-C, C-O bond cleavage reactions. Among them, the formation channel of methanol and isobutene is the lowest energy pathway, which is in accordance with experimental observation. Furthermore, the secondary pyrolysis pathways have been calculated as well, including decomposition of tert-butyl radical, isobutene, methanol and acetone. The radicals play an important role in the formation of pyrolysis products, for example, tert-butyl radical and allyl radical are major precursors for the formation of allene and propyne. Although some isomers (isobutene and 1-butene, allene and propyne, acetone and propanal) are identified in our experiment, these isomerization reaction pathways occur merely at the high temperature due to their high activation energies. The theoretical calculation can explain the experimental results reported in part 1 and shed further light on the thermal decomposition pathways.     

Development of a detailed pyrolysis mechanism for C1–C4 hydrocarbons under a wide range of temperature and pressure

A model of core mechanism of hydrocarbon pyrolysis with good predictive ability is crucial to the development of active cooling technology for advanced aeroengines. In this work, a detailed core kinetic model of pyrolysis of C₁–C₄ hydrocarbon fuels is developed through the combination of a series of potential energy surfaces and validated against a series of experimental results. The kinetic model contains 103 species and 1290 reactions, and most of the kinetic and thermochemical parameters are compiled from recent highly accurate quantum chemical calculations without modification. The pressure-dependent rate constants are considered for the dissociation/association reactions, isomerization reactions, and chemically activated reactions. Simulation results for various alkanes (methane, ethane, propane, n-butane, isobutane), alkenes (ethylene, propene, 1-butene, 2-butene, isobutene, allene, 1,3-butadiene), and alkynes (acetylene, propyne, vinylacetylene) indicate that the major product distributions at various temperatures (800-2300 K) and pressures (0.8-10 atm) can be predicted well by the developed core kinetic model. Thus, the developed pyrolysis mechanism for C₁–C₄ hydrocarbons can be used as a cornerstone to develop the pyrolysis mechanisms of larger hydrocarbon fuels and thus support the development of thermal management in advanced aeroengines.     

Decomposition and hydrocarbon growth processes for esters in non-premixed flames

Biomass fuels are a promising renewable energy source, and so, the mechanisms that may produce toxic oxygenated byproducts and aromatic hydrocarbons from oxygenated hydrocarbons are of interest. Esters have the form R-(C=O)-O-R' and are components of biodiesel fuels. The five specific esters studied here are isomers of C5H10O2. The experiments were performed in atmospheric pressure coflowing methane/air non-premixed flames. A series of flames were generated by separately doping the fuel mixture with 5,000 ppm of each ester. This concentration is sufficiently large to produce measurable changes in intermediate hydrocarbon concentrations, yet small enough to not disturb the overall flame structure. Since the overall structure is not perturbed, the measured changes in the intermediate hydrocarbons can be directly attributed to the reactions of the esters. Analysis of these changes reveals that unimolecular six-centered dissociation is the primary decomposition pathway for the three esters with molecular arrangements capable of undergoing that mechanism. The remaining two esters exhibited decomposition rates and products that are consistent with simple fission as the dominant decomposition mechanism, though we do not exclude other pathways from playing a significant role in their decomposition. All of the esters produce aromatic hydrocarbons at higher rates than the undoped fuel, and the molecular arrangement of the ester isomers plays a role in the degree of aromatic formation. Isomer variations also influence the type and quantity of toxic oxygenates that are produced in the flames.     

Investigation of thermal transformations of propylene and its role in the formation of benzene by pyrolysis of n-hexane

Conclusions We investigated the thermal transformations of propylene and mixtures of n-hexane and propylene at 800C by use of radioactive hydrocarbons labeled with¹⁴C. The data obtained lead to the conclusion that on pyrolysis of n-hexane benzene is formed by the interaction of propylene with 1,3-butadiene or by intermediate formation of 1,3-butadiene.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1556–1560, July, 1984.     

Photoionization Mass Spectrometric and Kinetic Modeling of Low-pressure Pyrolysis of Benzene (cited: 3)

Pyrolysis of benzene at 30 Torr was studied from 1360 K to 1820 K in this work. Synchrotron vacuum ultraviolet photoionization mass spectrometry was employed to detect the pyroly-sis products such as radicals, isomers and polycyclic aromatic hydrocarbons, and measure their mole fraction profiles versus temperature. A low-pressure pyrolysis model of benzene was developed and validated by the experimental results. Rate of production analysis was performed to reveal the major reaction networks in both fuel decomposition and aromatic growth processes. It is concluded that benzene is mainly decomposed via H-abstraction reaction to produce phenyl and partly decomposed via unimolecular decomposition reac-tions to produce propargyl or phenyl. The decomposition process stops at the formation of acetylene and polyyne species like diacetylene and 1,3,5-hexatriyne due to their high thermal stabilities. Besides, the aromatic growth process in the low-pressure pyrolysis of benzene is concluded to initiate from benzene and phenyl, and is controlled by the even carbon growth mechanism due to the inhibited formation of C5 and C7 species which play important roles in the odd carbon growth mechanism.     

Ab initio G3-type/statistical theory study of the formation of indene in combustion flames. I. Pathways involving benzene and phenyl radical

Ab initio G3(MP2,CC)//B3LYP calculations of the potential energy surface (PES) for the formation of indene involving hydrocarbon species abundant in combustion, including benzene, phenyl, propargyl, and methyl radicals, and acetylene, have been performed to investigate the build-up of an additional cyclopenta moiety over the existing six-member aromatic ring. They were followed by statistical calculations of high-pressure-limit thermal rate constants in the temperature range of 300-3000 K for all reaction steps utilizing conventional Rice-Ramsperger-Kassel-Marcus (RRKM) and transition-state (TST) theories. The hydrogen abstraction acetylene addition (HACA) type mechanism, which involves the formation of benzyl radical followed by addition of acetylene, is shown to have low barriers (12-16 kcal/mol) and to be a viable candidate to account for indene formation in combustion flames, such as the 1,3-butadiene flame, where this mechanism was earlier suggested as the major indene formation route (Granata et al. Combust. Flame 2002, 131, 273). The mechanism of indene formation involving the addition of propargyl radical to benzene and rearrangements on the C9H9 PES is demonstrated to have higher barriers for all reaction steps as compared to an alternative pathway, which starts from the recombination of phenyl and propargyl radicals and then proceeds by activation of the C9H8 adducts by H abstraction or elimination followed by five-member ring closure in C9H7 and H addition to the 2-indenyl radical. The suggested pathways represent potentially important contributors to the formation of indene in combustion flames, and the computed rate constants can be utilized in kinetic simulations of the reaction mechanisms leading to indene and to higher cyclopentafused polycyclic aromatic hydrocarbons (CP-PAH).     

生物质和废塑料混合热解协同特性研究

选取聚丙烯(PP)和竹屑作为废塑料与生物质的典型代表,在热重分析仪和固定床台架上研究了塑料掺混比例对混合热解失重特性、动力学机理、产物分布行为等特性的影响,并分析了混合热解时生物质和废塑料间的协同作用机制。结果表明,随着塑料掺混比例的增加,混合热解终止温度由501℃降低至471℃,主要热解温度区间缩短;混合热解所需活化能呈现先减小后增大的趋势,在塑料掺混比例为0.25时取得最小值。通过对比实验数据和理论数据发现,生物质与废塑料混合热解具有很强的协同作用:该协同作用降低了生物质反应所需能量,增加了废塑料反应所需能量,降低了混合热解过程的总活化能;此外,协同作用促进大分子挥发分转化为小分子气体,促进芳烃、烷烃等烃类生成,抑制CO_2、苯酚、羧酸、呋喃和酮类等含氧物质生成。     

Analysis of low molecular weight hydrocarbons including 1,3-butadiene in engine exhaust gases using an aluminum oxide porous-layer open-tubular fused-silica column.

A method for the quantitative analysis of individual hydrocarbons in the C1-C8 range emitted in engine exhaust gases is described. The procedure provides base-line or near base-line resolution of C4 components including 1,3-butadiene. With a run time of less than 50 min, the light aromatics (benzene, toluene, ethyl benzene, p- and m-xylene, and o-xylene) are resolved during the same analysis as aliphatic hydrocarbons in the C1-C8 range. It is shown that typical 1,3-butadiene levels in engine exhaust are about 5 ppm at each of two engine conditions. Aromatic hydrocarbon levels show a dependence on engine operating conditions, benzene being about 20 ppm at high speed and about 40 ppm at idle.     

Mo和Ni改性的HZSM-5催化剂对煤热解焦油的改质

考察了Mo和Ni改性的HZSM-5催化剂对煤热解焦油的改质性能,分析了催化改质前后焦油中轻质芳烃分布的变化规律。结果表明,经HZSM-5催化剂褐煤(XM)热解轻质芳烃总量的增加率为220%,这与煤热解产物在HZSM-5催化剂中发生烯烃和烷烃的芳构化以及酚羟基脱除等作用有关。负载活性金属Mo和Ni后,可以有效促进轻质芳烃的生成;Ni对焦油中带脂肪侧链化合物具有更强的裂解作用,而Mo则有利于带侧链化合物如甲苯和二甲苯的形成。焦煤(FX)热解过程中轻质芳烃的释放量分别是XM煤和年轻烟煤(PS)的2.2和2.4倍。经催化改质后,XM煤产物中轻质芳烃产率明显大于PS煤,并接近FX煤;这主要是因为XM煤结构中含有较多的含氧官能团和脂肪结构,在HZSM-5作用下可催化形成轻质芳烃。     

Thermal decomposition of poly(butylene terephthalate)

Detailed results of the overall thermal degradation of poly(butylene terephthalate) are reported. Laser microprobe analysis and dynamic mass spectrometric techniques were used to identify the primary volatile degradation products and initial pyrolysis reactions that control polymer degradation. A complex multistage decomposition mechanism was observed which involves two major reaction pathways. Initial degradation occurs by an ionic decomposition process that results in the evolution of tetrahydrofuran. This is followed by concerted ester pyrolysis reactions that involve an intermediate cyclic transition state and yield 1,3-butadiene. Simultaneous decarboxylation reactions occur in both decomposition regimes. Finally, the latter stages of polymer decomposition were characterized by evolution of CO and complex aromatic species such as toluene, benzoic acid, and terephthalic acid. Activation energies of formation for the main pyrolysis products were determined from the dynamic measurements of the major ion species and indicate values of E = 27.9 kcal/mole for the production of tetrahydrofuran and E = 49.7 kcal/mole for the production of butadiene.     

Effect of metal oxides on the evolution of aromatic hydrocarbons in the thermal decomposition of PVC

The thermal decomposition of poly(vinyl chloride) (PVC) mixed with several metal oxides was investigated by direct pyrolysis in a mass spectrometer (MS) and flash pyrolysis–gas chromatography. Our results show that the thermal decomposition of PVC occurs in two stages. Unsubstituted aromatic hydrocarbons (benzene, naphthalene, and anthracene) are evolved mainly in the first stage, alkyl-aromatics (e.g., toluene) in the second. Although the addition of some metal oxides results in an overall suppression of aromatic hydrocarbons, the unsubstituted aromatics are much more suppressed with respect to alkyl-aromatics. Furthermore, the formation of ZnCl₂ and SnCl₄ was revealed by the mass spectra of PVC–metal oxide pyrolysates. This suggests that, at least in these two cases, metal chlorides are responsible for aromatic hydrocarbon suppression. With this information a detailed reaction mechanism could be formulated for the thermal degradation of PVC.     

Identifizierung von Pyrolysebeiprodukten in Fließbettruß und Pyrokohlenstoff

In the frame of further development of fluidized bed coated particle fuel for high temperature reactors, both the pyrocarbon desposited on fuel kernels and the soot obtained besides, were investigated to their pyrolysis by-products. Target of the study was to improve the understanding of the mechanisms in pyrocarbon desposition on fuel kernels during the thermal decomposition of lower hydrocarbons. This study was on products formed in acetylene and propylene pyrolysis, the extraction being performed by various methods. The extracts were separated by gas-chromatographic and mass-spectrometric methods. 21 Polycyclic aromatic hydrocarbons could be identified in the soot. Beyond those the pyrocarbon containes polycycles of even higher molecular weights.     

Production of High-Density Jet and Diesel Fuels by Hydrogenation of Highly Aromatic Fractions

Physicochemical characteristics and hydrocarbon composition of highly aromatic wastes (light gas oil from catalytic cracking, pyrolysis tar, coal tar, coal gasification tar) as a feedstock for producing high-density jet fuels are considered. The hydrogenation reactions of polycyclic aromatic hydrocarbons, including mixtures of hydrocarbons with different numbers of rings, are described. Catalysts for hydrogenation of highly aromatic waste to obtain fuel fractions are considered. Particular attention is paid to catalyst deactivation in the course of processing of this feedstock. A separate section deals with the choice and implementation of procedures for processing highly aromatic feedstock to obtain jet and diesel fuels.     

Antiradical activity of dioxolane derivatives

1,3-Benzodioxoles synthesized by condensation of 3,6-di-tert-butylbenzene-1,2-diol with carbonyl compounds showed antiradical activity due to their ability to undergo one-electron oxidation with formation of stable radical cations. On this basis, the antiknock effect of their structural analogs, 1,3-dioxolanes derived from vicinal diols, was interpreted in terms of oxidation of these compounds with active radicals generated from fuel hydrocarbons to produce more stable radical or radical ion species, depending on the fuel composition. The formation of radical species was detected in model oxidation reactions of 2,2-dimethyl-1,3-dioxolane and 2,2-dimethyl-1,3-dioxolan-4-ylmethanol with radicals generated by photolysis of iron(III) chloride and benzoyl peroxide.