John C. Mackie

Formation and fate of PAH during the pyrolysis and fuel-rich combustion of coal primary tar

The formation and fate of polycyclic aromatic hydrocarbons (PAH) during the pyrolysis and fuel-rich combustion of primary tar generated under rapid heating conditions have been studied. Experiments were performed using a quartz two-stage reactor consisting of a fluidized-bed reactor coupled to a tubular-flow reactor. Primary tar was produced in the fluidized-bed reactor by rapid coal pyrolysis at 600°C. The freshly generated tar was subsequently reacted in the tubular-flow reactor at 1000°C under varying oxygen concentrations covering the range from pyrolysis to stoichiometric oxidation. PAH species present in the tars recovered from the tubular-flow reactor were analyzed by high-performance liquid chromatography (HPLC). Twenty-seven PAH species, varying from 2-ring to 9-ring structures, were identified, including benzenoid PAH, fluoranthene benzologues and indene benzologues. The majority of PAH species identified from pyrolysis were also identified in the samples collected from oxidation experiments. However, three products, 9-fluorenone, cyclopenta[def]phenanthrene and indeno[1,2,3-cd]fluoranthene, were produced only during oxidizing conditions. The addition of a small amount of oxygen brought about measurable increases in the yields of the indene benzologues and 9-fluorenone, but the yields of all PAH products decreased at high oxygen concentrations, in accordance with their destruction by oxidation. Possible formation and destruction mechanisms of PAH under fuel-rich conditions have been discussed.

Partial Oxidation of Methane: The Role of the Gas Phase Reactions

Abstract There is considerable interest in laboratories around the world in developing processes for the direct conversion of methane to higher valued products. Direct routes to useful chemicals would avoid the energy intensive steam reforming step used in existing technology [l]. Methane, however, is rather an inert substance and, apart from its ready reaction with oxygen, undergoes few reactions, the reaction between methane and halogens, carried out photochemically [2,3] or thermally [4], being an exception. Whilst patents have been issued for utilization of the thermal reaction between methane and chlorine to produce ethane [5,6], environmental concerns might prevent the large-scale commercial development of a process involving chlorine.

Search for a Charge‐Transfer State in Crystalline Anthracene

In this paper we examine the calculation of the location of the charge‐transfer state in crystalline anthracene. Also reported are the results of spectroscopic experiments designed to find the transition from the ground state to the charge‐transfer state. No experimental evidence could be found for this transition, and it is thereby concluded that: (a) The CT state lies above the first exciton state in anthracene. (b) The polarization energy in the CT state is much less than that calculated on the basis of classical considerations. There is a brief discussion of the nature of the polarization in aromatic crystals, especially with respect to Conclusion (b).

Products from rapid heating of a brown coal in the temperature range 400–2300 °C

The devolatilisation behaviour of Yallourn brown coal was investigated under rapid heating conditions using two different flash pyrolysers: a fluid-bed reactor giving coal particle heating rates of 104 °Cs−1 with a gas residence time of about 0.5 s and a shock tube which generated heating rates of the order of 107 °Cs−1 and a 1 ms reaction time. Yields of products are reported covering pyrolysis temperatures in the range 400–2300 °C. Hydrocarbon gas yields reached maximum values which were remarkably similar for both reactors although occurring at different temperatures. Carbon oxide production was also similar for both reactors with CO yields reaching 30% wt/wt daf coal. These high yields of CO are very different from those reported for slow heating conditions. It appears that on flash heating, coal decomposition pathways change in a manner which increases CO yields at the expense of H20 and to a lesser extent C02, resulting in the volatilisation of additional carbon from the coal.

Coal flash pyrolysis: secondary cracking of tar vapours in the range 870–2000 K☆

Abstract Tar free from influences of the original coal or char was cracked in two reaction systems, one using tar vapour in tubular quartz reactors at 900–1400 K and reactor residence times of ≈ 0.2 and ≈ 1 s, the other using tar aerosol in a shock tube at 1100–2000 K and residence time ≈ 1 ms. In the latter system the aerosol evaporated rapidly after passage of the shock front and the tar behaved kinetically as a vapour. Yields of light hydrocarbons including C6H6 and CO from tar cracking were determined as a function of temperature. Maximum yields of individual species agreed well between the two reactors but the temperatures of maximum yield depended on residence time. Hydrocarbon yields on a daf coal basis were very similar to those obtained previously by direct flash pyrolysis of the same coal at 870–1270 K. Kinetic analyses of the formation of C2H4, C3H6 and C2H2 gave activation energies of 220–260 kJ mol−1, similar to those for formation of the same hydrocarbons from n-hexadecane. The precursors of the alkenes could be polymethylene groups in the tar. CH4, C6H6 and CO all had low activation energies of formation, reflecting the many different functional groups capable of eliminating these molecules.

Inhibition of Premixed Hydrogen-Air Flames by 2-H Heptafluoropropane

The results of an experimental and numerical modeling study on the inhibition chemistry of CF3CHFCF3 are presented. Burner-stabilized, laminar, premixed, hydrogen-air flat flames with 1.0 and 3.2 mol % inhibitor added were studied as a function of equivalence ratio. The measured concentration profiles of stable species in the postflame gases were modeled with recently developed chemical kinetic mechanisms, augmented with estimated rate and thermochemical data germane to the oxidation of the inhibitor. Agreement between experimental and modeled profiles is adequate for most measured species, thereby partially validating the kinetic data used. Calculated reaction fluxes for 3.2 mol % inhibitor revealed that suppression of H atoms is effected by conversion to HF through reactions with fluorinated species. In contrast, suppression of OH was found to be less effective.

Kinetics of pyrolysis of furan

The kinetics of pyrolysis of furan vapour diluted in argon have been studied behind reflected shock waves in a shock tube both by time-resolved infrared CO2 laser absorption spectrometry and by single pulse shock techniques of product analysis over the temperature range 1100–1700 K, at pressures of ca. 20 atm and at uniform residence times of ca. 300 µs. The rate of overall disappearance of furan, as measured by absorption spectrometry, was found to be first order in furan concentration, with a rate constant of koverall= 1015.3±0.3 exp[–326(±8) kJ mol–1/RT] s–1 in agreement with a previous determination by Lifshitz et al.(A. Lifshitz, M. Bidani and S. Bidani, J. Phys. Chem., 1986, 90, 5373). Principal products were carbon monoxide, C3H4(propyne and allene) and acetylene. Ketene was identified in the products by FTIR spectroscopy.A detailed chemical reaction model for the pyrolysis was developed and shown to give good predictions of the concentration profiles of furan and the major products. Modelling and thermochemical considerations led to the postulate that the initiation of pyrolysis takes place by C—O bond scission to a biradical which can undergo decomposition, via parallel reaction paths, to the observed products.

The pyrolysis of cyclopentadiene: quantum chemical and kinetic modelling studies of the acetylene plus propyne/allene decomposition channels

The thermal decomposition of cyclopentadiene to acetylene plus propyne or allene was studied using ab initio CASSCF, CASPT2, G2(MP2) as well as density functional methods. Three distinct reaction pathways were explored that involve 1,2-hydrogen transfers, yielding cyclic carbenes or allyl vinylidene as initial intermediates. After ring opening or a second 1,2-hydrogen transfer, the resulting open chain isomers dissociate ia 1,5 sigmatropic shifts or by CC bond fission. From the computed CASSCF/CASPT2 data the appropriate RRKM rate constants for these reactions were computed and incorporated into a kinetic model containing the appropriate parameters for the conventional model of acetylene production ia cyclopentadienyl, c-C5H5. The modelling studies, although not achieving quantitative agreement with experiment without some adjustment of the ab initio derived kinetic parameters, demonstrate that the proposed additional mechanism may be a major source of acetylene under a range of shock tube conditions. Consequently, the rate constant for the decomposition of c-C5H5 to C2H2 + C3H3 cannot be derived directly from existing experimental data.

Low temperature oxidation of linseed oil: a review

This review analyses and summarises the previous investigations on the oxidation of linseed oil and the self-heating of cotton and other materials impregnated with the oil. It discusses the composition and chemical structure of linseed oil, including its drying properties. The review describes several experimental methods used to test the propensity of the oil to induce spontaneous heating and ignition of lignocellulosic materials soaked with the oil. It covers the thermal ignition of the lignocellulosic substrates impregnated with the oil and it critically evaluates the analytical methods applied to investigate the oxidation reactions of linseed oil.Initiation of radical chains by singlet oxygen (1Δg), and their propagation underpin the mechanism of oxidation of linseed oil, leading to the self-heating and formation of volatile organic species and higher molecular weight compounds. The review also discusses the role of metal complexes of cobalt, iron and manganese in catalysing the oxidative drying of linseed oil, summarising some kinetic parameters such as the rate constants of the peroxidation reactions.With respect to fire safety, the classical theory of self-ignition does not account for radical and catalytic reactions and appears to offer limited insights into the autoignition of lignocellulosic materials soaked with linseed oil. New theoretical and numerical treatments of oxidation of such materials need to be developed. The self-ignition induced by linseed oil is predicated on the presence of both a metal catalyst and a lignocellulosic substrate, and the absence of any prior thermal treatment of the oil, which destroys both peroxy radicals and singlet O2 sensitisers. An overview of peroxyl chemistry included in the article will be useful to those working in areas of fire science, paint drying, indoor air quality, biofuels and lipid oxidation.

Theoretical study of unimolecular decomposition of catechol

This study develops the reaction pathway map for the unimolecular decomposition of catechol, a model compound for various structural entities present in biomass, coal, and wood. Reaction rate constants at the high-pressure limit are calculated for the various possible initiation channels. It is found that catechol decomposition is initiated dominantly via hydroxyl H migration to a neighboring ortho carbon bearing an H atom. We identify the direct formation of o-benzoquinone to be unimportant at all temperatures, consistent with the absence of this species from experimental measurements. At temperatures higher than 1000 K, water elimination through concerted expulsion of a hydroxyl OH together with an ortho H becomes the most significant channel. Rice-Ramsperger-Kassel-Marcus simulations are performed to establish the branching ratio between these two important channels as a function of temperature and pressure. All unimolecular routes to the reported major experimental products (CO, 1,3-C(4)H(6) and cyclo-C(5)H(6)) are shown to incur large activation barriers. The results presented herein should be instrumental in gaining a better understanding of the decomposition behavior of catechol-related compounds.