Jack B. Howard

Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways

The generation by combustion processes of airborne species of current health concern such as polycyclic aromatic hydrocarbons (PAH) and soot particles necessitates a detailed understanding of chemical reaction pathways responsible for their formation. The present review discusses a general scheme of PAH formation and sequential growth of PAH by reactions with stable and radical species, including single-ring aromatics, other PAH and acetylene, followed by the nucleation or inception of small soot particles, soot growth by coagulation and mass addition from gas phase species, and carbonization of the particulate material. Experimental and theoretical tools which have allowed the achievement of deeper insight into the corresponding chemical processes are presented. The significant roles of propargyl (C3H3) and cyclopentadienyl (C5H5) radicals in the formation of first aromatic rings in combustion of aliphatic fuels are discussed. Detailed kinetic modeling of well-defined combustion systems, such as premixed flames, for which sufficient experimental data for a quantitative understanding are available, is of increasing importance. Reliable thermodynamic and kinetic property data are also required for meaningful conclusions, and computational techniques for their determination are presented. Routes of ongoing and future research leading to more detailed experimental data as well as computational approaches for the exploration of elementary reaction steps and the description of systems of increasing complexity are discussed.

Coal devolatilization at high temperatures

Devolatilization of a lignite and a bituminous coal was studied at high temperatures under rapid heating conditions. Devolatilization rates were measured in a flow furnace designed to yield heating rates of 10 4 –2×10 5 K/s, temperatures of 1000–2100 K time resolution down to a few milliseconds product quenching rates of 10 6 K/s and good particle collection efficiencies. The volatile yields in this study were determined both by difference in the weights of coal fed and char collected and by use of ash as a tracer. Volatile yields of both coals increase significantly with temperature, from about 30% (d.a.f.) at 1260 K and 20ms to about 63% at 2100 K and 25ms. The maximum volatile yields were significantly in excess of the 46% value given by the ASTM proximate volatile test. The yields are even higher when an estimated correction is introduced for condensed material produced during devolatilization that is collected with the char residue. Correlation of the pyrolysis kinetics with a single Arrhenius type rate expression, assuming first order behavior with respect to volatile matter remaining in the char, yielded an apparent frequency factor of about 6.6×10 4 s −1 and an apparent activation energy of about 25 kcal/mole. These results are consistent with an extrapolation to higher temperatures of data previously reported in the literature for similar coals. An empirical model based on two competing overall reactions gave a better fit of the data over all of the conditions of the present study than was obtained using the single-reaction model.

Composition profiles and reaction mechanisms in a near-sooting premixed benzene/oxygen/argon flame

The structure of a near-sooting flat low-pressure laminar benzene-oxygen-argon flame was studied using a molecular beam mass spectrometer system. Mole fraction profiles of fifty-one species are presented and their mechanistic implications are discussed with regard to the decay of benzene and the formation of polycyclic aromatic hydrocarbons and soot. A mechanism is outlined for production of C 2 , C 3 , C 4 and C 5 species from initial attack of O atom on benzene. Trends observed as the fuel equivalence ratio is increased past the sooting limit suggest a sequential growth from polycyclic aromatic hydrocarbons of 120–210 amu to heavier species and then to soot. A critical step in mechanisms for rapid growth of aromatic structures by free radical addition reactions is the stabilization of the adduct by forming six-membered rings. A mechanism requiring methyl substitution on the aromatic ring is discussed.

Formation mechanisms of aromatic compounds in aliphatic flames

Abstract Possible mechanisms for the formation of aromatic compounds in flames of aliphatic fuels were assessed by comparing predicted formation rates against experimental values calculated from mole-fraction profiles of compounds in near-sooting, premixed flat flames of 1,3-butadiene at a pressure of 2.67 kPa. The data were obtained by molecular beam sampling with on-line mass spectrometry. Postulated mechanisms, consisting of rate-controlling addition of 1,3-butadienyl radical to acetylenes followed by cyclization and H-atom elimination, and having the following overall expression and rate parameters, are consistent with the data: Acetylenic Species Aromatic product E, kcal/mol Acetylene, C2H2 Benzene, C6H6 3.7 Methylacetylene, C3H4 Toluene, C6H5CH3 3.7 Diacetylene, C4H2 Phenylacetylene, C6H5C2H 1.8 Vinylacetylene, C4H4 Styrene, C6H5C2H3 0.6 where k is l/mol s. However, the measured rate of toluene formation differs from the prediction in a manner that may indicate an important role for another toluene-forming reaction and toluene conversion to benzyl radical. The postulated role of C4H5 in aromatics formation is consistent with previous flame data in which the formation behavior of another product of C4H5, namely, C4H4, mimics that of aromatics. Diels-Alder reactions of 1,3-butadiene and free-radical mechanisms involving 1,3-butadiene are shown to be too slow to account for the formation of aromatic species in this flame.

Formation and consumption of single-ring aromatic hydrocarbons and their precursors in premixed acetylene, ethylene and benzene flames

Kinetic modeling is becoming a powerful tool for the quantitative description of combustion processes covering different fuels and large ranges of temperature, pressure and equivalence ratio. In the present work, a reaction mechanism which was developed initially for benzene oxidation, and included the formation of polycyclic aromatic hydrocarbons was extended and tested for the combustion of acetylene and ethylene. Thermodynamic and kinetic property data were updated. If available, data were taken from the recent literature. In addition, density functional theory as well as ab initio computations on a CBS-Q and CBS-RAD level, partially published previously, were carried out. Quantum Rice–Ramsperger–Kassel analysis was conducted in order to determine pressure-dependent rate constants of chemically activated reactions. The model was developed and tested using species concentration profiles reported in the literature from molecular beam mass-spectrometry measurements in four unidimensional laminar premixed low-pressure ethylene, acetylene and benzene flames at equivalence ratios (ϕ) of 0.75 and 1.9 (C2H4), 2.4 (C2H2) and 1.8 (C6H6). Predictive capabilities of the model were found to be at least fair and often good to excellent for the consumption of the reactants, the formation of the main combustion products as well as the formation and depletion of major intermediates including radicals. Self-combination of propargyl (C3H3) followed by ring closure and rearrangement was the dominant benzene formation pathway in both rich acetylene and ethylene flames. In addition, reaction between vinylacetylene (C4H4) and vinyl radical (C2H3) contributed to benzene formation in the ϕ = 1.9 ethylene flame. Propargyl formation and consumption pathways which involve reactions between acetylene, allene, propyne and singlet and triplet methylene were assessed. Significant overpredictions of phenoxy radicals indicate the necessity of further investigation of the pressure and temperature dependence and the product distribution of phenyl oxidation. The possible formation of benzoquinones, the ratio of the ortho and para isomers and their degradation pathways are of particular interest.

The Physical Transformation of the Mineral Matter in Pulverized Coal Under Simulated Combustion Conditions

Abstract The physical transformation of the mineral matter in coal has been studied in a laboratory furnace using size-graded, pulverized samples of a lignite and a bituminous coal. The mineral matter is originally distributed in micron-size inclusions in the coal particles. The paper illustrates how the final particle size distribution of the ash produced at combustion temperatures of 1250 to 1830K is determined by a combination of agglomeration of fused mineral matter, cenosphere formation due to gas evolution and vaporization and recondensation of volatile constituents

Rapid devolatilization and hydrogasification of bituminous coal

Abstract Rapid devolatilization and hydrogasification of a Pittsburgh Seam bituminous coal were studied and an appropriate coal conversion (weight loss) model was developed that accounts for thermal decomposition of the coal, secondary char-forming reactions of volatiles, and homogeneous and heterogeneous reactions involving hydrogen. Approximately monolayer samples of coal particles supported on wire mesh heating elements were electrically heated in hydrogen, helium, and mixtures thereof. Coal weight loss (volatiles yield) was measured as a function of residence time (0–20 s), heating rate (65–10000 °C/s), final temperature (400–1100 °C), total pressure (0.0001–7 MPa), hydrogen partial pressure (0–7 MPa), and particle size (70–1000 μm). Volatiles yield under these conditions increases significantly with decreasing pressure, decreasing particle size, increasing hydrogen partial pressure and increasing final temperature, but only slightly with increasing heating rate. The data support the view that coal conversion under these conditions involves numerous parallel thermal decomposition reactions forming primary volatiles and initiating a sequence of secondary reactions leading to char. Intermediates in this char-forming sequence can escape as tar if residence time in the presence of hot coal surfaces is sufficiently short (e.g. low pressures and small particles well dispersed). Hydrogen at sufficiently high partial pressure can interrupt the char-forming sequence thereby increasing volatile yield. Rate of total product generation is largely controlled by coal pyrolysis while competition between mass transfer, secondary reactions, and rapid hydrogenation affects only the relative proportions of volatile and solid products formed.

Formation mechanism of polycyclic aromatic hydrocarbons and fullerenes in premixed benzene flames

A better understanding of the formation of polycyclic aromatic hydrocarbons (PAH) and fullerenes is of practical interest due to the apparent environmental health effects of many PAH and potential industrial applications of fullerenes. In the present work, a kinetic model describing the growth of PAH up to coronene (C24H12) and of C60 and C70 fullerenes is developed. Comparison of the model predictions with concentration profiles in a nearly sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio φ = 1.8, pressure = 2.67 kPa) measured by Bittner using a molecular beam system coupled to mass spectrometry shows reasonably good predictive capability for stable and radical intermediates and growth species up to C16H10 isomers. Cyclopentadienyl is found to be a key species for naphthalene formation. The further growth process is based on H abstraction and acetylene addition but also the contribution of small PAH is considered. Good to fair agreement between model predictions and experimental data for larger PAH including the different C16H10 isomers obtained by gas chromatography coupled to mass spectrometry and high performance liquid chromatography could be achieved for PAH in a sooting low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame (φ = 2.4, 5.33 kPa). C60 and C70 fullerenes are underpredicted, and possible reasons such as uncertainties in rate coefficients or the existence of other formation pathways are discussed. PAH depletion in the burnt gas is not reproduced by the model and is believed to involve supplementary sinks such as reactions involving PAH and growing soot particles.

Rapid devolatilization of pulverized coal

The rapid devolatilization of a lignite and a bituminous coal was studied by electrically heating in helium approximately monolayer samples of small particles supported on wire mesh heating elements. The samples were rapidly brought to a desired temperature, held there for a desired time, and then rapidly cooled. Devolatilization rates, measured by weighing samples before and after experiments of known duration, were determined as a function of residence time (0.05–20 sec), temperature (400–1100°C), heating rate (102–104°C/sec), pressure (0.001–100 atm), and particle size (50–1000 μm). Devolatilization kinetics were determined by non-isothermal techniques since substantial reaction occurred during heating even under the most rapid heating rates. Weight loss from both coals was essentially complete within a fraction to a few seconds depending upon temperature, and increased with increasing final temperature up to 900 to 950°C. Weight loss (corrected to its value at a fixed temperature) was found to be independent of pressure, heating rate and particle size for the lignite, i.e., it depended only on temperature and time; but for the bituminous coal it increased with decreasing pressure, decreasing particle size and, to a small extent, increasing heating rate. The general reaction scheme appears to involve thermal decomposition forming volatiles and initiating a sequence of secondary polymerization and char-forming reactions. The kinetics and yields of the primary decomposition are successfully described by a set of independent first-order parallel reactions represented by a Gaussian distribution of activation energies around a mean of 56 kcal/mole for the lignite, with a standard deviation of 11, and 51 kcal/mole for the bituminous coal at 69 atm and 70 μm particle diameter, with a standard deviation of 7. For the bituminous coal it was necessary in addition to allow for pressure- and particle-size-dependent secondary reactions representing competition between char-forming reactions and diffusional escape of volatiles. Attempts to correlate the data in terms of a single first-order reaction lead to an overall activation energy (∼10 kcal/mole) that is considerably lower than the mean activation energy of the multiple-reaction system, and to a different set of kinetic parameters for each set of experimental conditions. Conditions such as lower pressure, smaller particle size, and better particle dispersion which help to diminish the effect of secondary reactions appear to be more important than rapid heating in the production of volatile yields in excess of the volatile content obtained by proximate analysis.

Fullerenic carbon in combustion-generated soot

Soot samples collected as bulk solids and by thermophoretic sampling at different residence times in a fullerene-forming premixed benzene/oxygen flat flame (C/O=0.96, P=5.34 kPa, 10% argon, v=25 cm/s) were analyzed by high resolution electron microscopy. The samples contained soot particles that were composed to some extent of amorphous and fullerenic carbon (e.g., curved layers and fullerene-molecule-sized closed-shell structures). Qualitative and quantitative analyses of residence time-resolved samples showed that the length of curved layers increases and their radius of curvature decreases with increasing residence time in the flame. The number of closed-shell structures in the soot as well as the concentration of fullerene molecules in the gas phase increase with increasing residence time, consistent with fullerenes concentration increasing with residence time and with the consumption of fullerenes by reaction with soot. The data suggest that the formation of amorphous and fullerenic carbon occurs in milliseconds, with the fullerenic carbon becoming more curved as a soot particle traverses the length of the flame. Conversely, the formation of highly ordered carbon nanostructures, such as tubes and onions, appears to require much longer residence times, perhaps seconds or minutes depending on the temperature, in the flame environment.