Mary J. Wornat

Structural and compositional transformations of biomass chars during combustion

Abstract In an investigation of the physical and chemical transformations of biomass chars during combustion, we have subjected two chars, produced from the pyrolysis of pine and switchgrass, to combustion at 1600 K in a laminar flow reactor. In order to obtain time-resolved data on the structural and compositional transformations of the biomass chars. samples are extracted from the reactor at different residence times and subjected to a variety of analytical techniques: elemental analysis, scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction analysis, and high-resolution transmission electron microscopy. The results point to several changes in both the organic and inorganic constituents of the chars. The early stages of conversion are characterized by devolatilization, which leads to the removal of amorphous material and the release of oxygen- and hydrogen-rich gases. After devolatilization, combustion is accompanied by: vaporization of some metals (particularly Na and K), surface migration and coalescence of inorganic material, and the incorporation of metals (particularly Ca) into silicate structures. The latest stages of combustion reveal the transformation of inorganic constituents from amorphous phases to crystalline forms. Some short-range order appears in the carbon-rich portions of the chars as combustion proceeds, but the high levels of oxygen originally present in these chars foster cross-linking, which limits the extent of order ultimately attained. The transformations of the biomass chars are compared with those of coal chars, and the implications of these observations—with respect to reactivity and ash behavior—are discussed.

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.

An experimental study on the thermal decomposition of catechol

In order to better elucidate the role of thermal decomposition products in the formation of polycyclic aromatic hydrocarbons (PAH) from complex fuels, we have performed pyrolysis experiments in a tubularflow reactor, using the model fuel catechol (ortho-dihydroxybenzene), a phenol-type compound representative of structural entities in biomass, coal, and wood. Catechol pyrolysis at temperatures of 700–1000 C and a residence time of 0.4 s produces a range of C1–C6 products, which have been analysed by nondispersive infrared analysis and by gas chromatography with flame-ionization detection. Quantification of product yields versus temperature reveals that the major products are CO, acetylene, 1,3-butadiene, phenol, cyclopentadiene, benzene, and ethylene; minor products are methane, ethane, propyne, propadiene, and propylene. CO is the highest yield catechol pyrolysis product at all temperatures. Among the hydrocarbons, 1,3-butadiene is the highest yield product at temperatures up to 800 C; above 800 C, acetylene is. The structural features of catechol and the experimental product yield data—considered with the established reactions for phenol decomposition—suggest that the major products of catechol decomposition come from the following routes: (1) phenol and benzene from H displacement of OH on catechol and phenol, respectively, (2) cyclopentadiene from unimolecular decomposition of the phenoxy radical, and (3) 1,3-butadiene, acetylene, and CO from decomposition of the hydroxy-substituted phenoxy radical (but with different oxygenated C5 intermediates). The remaining C1–C3 products appear to arise chiefly from the decomposition of key radicals such as cyclopentadienyl, propargyl, and 1,3-butadienyl. The results presented in this work, in concert with those from a complementary study of the C7–C28 catechol products, provide the basis for the development of a detailed kinetic model for both pyrolytic catechol decomposition and PAH formation and growth.

Single-particle combustion of two biomass chars

In order to assess the combustion reactivities of chars produced from the pyrolysis of woody and herbaceous biomass, we have subjected particles of Southern pine and switchgrass chars to two sets of combustion experiments. In the first, a dilute stream of biomass char particles (nominal size, 75–106 μm) is burned in a laminar flow reactor at 12 mole % O 2 and a mean gas temperature of ∼1600 K. In situ optical measurements reveal that at a given residence time in the early stages of char conversion, biomass char particles burn over a much wider temperature range (∼450 K) than coal particles (∼150 K) and that the biomass char particle temperatures span the entire range of the theoretical limits (from the slowest burning inert particles to the fastest burning diffusion-controlled particles). As biomass char conversion proceeds, however, mean particle temperatures decrease, and particle temperature distributions narrow-consequences of the preferential removal of more reactive carbon as well as a number of physical and chemical transformations of the inorganic constituents of the chars (vaporization, surface migration and coalescence, and incorporation into silicate structures). Kinetic parameters for median particles taken at the early stages of char conversion indicate that biomass chars are somewhat less reactive than low-rank lignites and subbituminous coals, somewhat more reactive than high-rank low-volatile bituminous coals, and comparable in reactivity to high-volatile bituminous coals. In the second set of combustion experiments, individual biomass char particles are suspended on an inert mesh and suddenly subjected to a hot 6 mole % O 2 environment. Video images of reflected light and near-infrared emission for a number of pine and switchgrass char particles demonstrate the heterogeneity of the biomass chars in terms of both the sequence of morphological changes and the temperature-time histories of the particles as they undergo combustion.

Application of microscopy to the investigation of brown coal pyrolysis

Abstract To examine the influence of calcium on the mechanisms of brown coal pyrolysis and gasification, the morphology of chars from raw and calcium-exchanged Yallourn brown coal was analysed. The chars were obtained by slow pyrolysis in a thermo gravimetric analyser and rapid pyrolysis in fluidized bed reactors operating at atmospheric pressure and at 1.1 MPa. They were examined by optical microscopy to determine reflectance and the percentage of particles that had become plastic during pyrolysis. In addition to confirming calciums inhibiting effect on tar yield, the results from the rapid pyrolysis experiments show that in the presence of calcium, char reflectivity decreases, char H/C ratio increases, and the proportion of particles going through a plastic stage decreases. Calciums inhibition of plasticity development is augmented by high pressure in the fluidized bed reactor. The effects appear to be attributable to the action of carboxylate calcium as a cross-linking agent, leading to the formation of a tighter char structure which traps the organic material that would otherwise be liberated as tar. The presence of Ca also increases the H/C ratio of the chars produced by slow pyrolysis, but the mechanism of pyrolysis differs, since in slow pyrolysis none of the particles showed evidence of plasticity. In slow pyrolysis, calciums influence on char reflectivity depends on the holding temperature, since temperature determines the extents of both coal devolatilization and catalytic transformations. The roles of calcium in these processes and their influence on optical anisotropy and reflectance are discussed.

Proton magnetic resonance thermal analysis of a brown coal: effects of ion-exchanged metals

Abstract Four versions of a Yallourn brown coal, each distinguished by the form of its carboxyl groups (acidic or cation-exchanged with Na, Ca or Ba), were subjected to proton magnetic resonance thermal analysis from 25 to 575°C. Analysis of the profiles of two second-moment parameters, calculated from the transverse relaxation signals of each coal, indicates the following: (1) Upon heating, cation-exchanged coals attain a lower extent of fusion (i.e. they are more rigid structures) than the acid-form coal—the two divalent cations Ba and Ca having a greater effect on fusion reduction than the monovalent cation Na. (2) The matrix densities of the coals follow the order: acid-form

Formation pathways of ethynyl-substituted and cyclopenta-fused polycyclic aromatic hydrocarbons

Two novel classes of polycyclic aromatic hydrocarbons (PAH), those with ethynyl substituents (ethynyl-PAH) and those with externally fused five-membered rings (cyclopenta-fused PAH or CP-PAH), have recently been identified in the products of a variety of fuels and combustion/pyrolysis environments. However, the recently developed capacity for identifying these compounds has raised new questions about preferential reaction pathways. Specifically, across various fuels and operating conditions, experimentally observed products are (1) CP-PAH, which result from C 2 H 2 addition to an aryl radical, followed by cyclization to a cyclopenta ring and (2) ethynyl-PAH, which result from C 2 H 2 addition to locations on the aryl radical where cyclization is not possible. We have never observed ethynyl-PAH resulting from C 2 H 2 addition to an aryl radical at a point where cyclization into a five-membered ring is possible. To explain this behavior, we have performed AM1 semiempirical quantum chemical computations with group correction in order to examine the potential energy surfaces of the reaction pathways that lead to ethynyl-PAH and CP-PAH. We have performed computations for the parent aryl radical, possible ethynyl-PAH products, possible CP-PAH products, as well as intermediates and transition states, for C 2 H 2 addition to naphthalene, anthracene, phenanthrene, acenaphthylene, fluoranthene, and pyrene. Possible CP-PAH products are acenaphthylene, aceanthrylene, acephenanthrylene, pyracylene, cyclopenta[ cd ]fluoranthene, and cyclopenta[ cd ]pyrene. In all cases, we have found that, although energy differences between ethynyl-PAH isomers are very small (∼1 kcal/mol), the experimentally observed ethynyl-PAH is always the lowest energy isomer. Furthermore, the observed preference for cyclization to CP-PAH over formation of an ethynyl-PAH can be explained by the significantly lower energy barrier (23 vs. 36 kcal/mol) for the cyclization reactions. Finally, we have determined that, while not prohibited, the isomerization of ethynyl-PAH to CP-PAH requires significantly higher energy than the aryl-vinyl cyclization reactions, and therefore is not expected to make a significant contribution to the product distribution. These results are sufficiently consistent that the computation of reaction pathway energy surfaces can be used to identify likely ethynyl-PAH and CP-PAH products from the addition of C 2 H 2 to much larger parent PAH.

The Identification of New Ethynyl-Substituted and Cyclopenta-Fused Polycyclic Aromatic Hydrocarbons in the Products of Anthracene Pyrolysis

Abstract The recent synthesis of new reference standards of polycyclic aromatic hydrocarbons (PAH) has enabled us to identify six new PAH species among the products of anthracene, pyrolyzed in argon at temperatures of 1300 to 1500 K. The anthracene product samples are analyzed by high performance liquid chromatography (HPLC) with diode-array ultraviolet-visible (UV) detection, and the identifications are made by matching each product components HPLC elution time and UV absorption spectrum with those of the corresponding reference standard. The newly identified PAH products include 1-ethynylacenaphthylene (C14H8) as well as five cyclopenta-fused PAH (CP-PAH): cyclopenta[cd]fluoranthene (C18H10); dicyclopenta[cd, mn]pyrene, dicyclo-penta[cd,fg]pyrene, dicyclopenta[cd,jk]pyrene (C20H10); and benzo[ghi]cyclopenta[cd]-perylene (C24H12). Undetectable at temperatures < 1300 K, the yields of the newly identified CP-PAH rise quickly with temperature above 1350 K, levelling off somewhat at temperatures approaching...

The Identification of Cyclopenta-Fused and Ethynyl-Substituted Polycyclic Aromatic Hydrocarbons in Benzene Droplet Combustion Products

Abstract In order to investigate new aspects of polycyclic aromatic hydrocarbon (PAH) growth and soot formation, we have synthesized special reference standards of cyclopenta-fused PAH (CP-PAH) and ethynyl-substituted PAH. We have identified several of these CP-PAH and ethynyl-PAH in benzene droplet combustion products, using high pressure liquid chromatography (HPLC) and ultraviolet-visible (UV) absorption spectroscopy. Although one CP-PAH identified in these products - acenaphthylene - has previously been identified as a product of a variety of combustion systems, we have identified six additional CP-PAH and two ethynyl-PAH which have never before been unequivocally identified as the products of benzene pyrolysis or combustion: acephenanthrylene, aceanthrylene, cyclopent[hi]acephenanthrylene, cyclopenta[cd]fluoranthene, cyclopenta[cd] pyrene, dicyclopenta[cd, jk]pyrene, 2-ethynylnaphthalene, and 1-ethynylacenaphthylene. We present the corresponding UV absorption spectra obtained from the HPLC analysis of benzene droplet combustion products, and compare them to the UV absorption

Cyclopenta-fused polycyclic aromatic hydrocarbons from brown coal pyrolysis

To examine certain aspects of coal tar composition, we have pyrolyzed acid-washed Yallourn brown coal under nitrogen at temperatures of 600 to 1000°C in a fluidized-bed reactor. Analysis of the product tar by reverse-phase high-performance liquid chromatography with diode-array ultraviolet-visible absorption detection reveals that the tars are composed of a large number of polycyclic aromatic compounds, many of which are polycyclic aromatic hydrocarbons (PAH) with peripherally fused cyclopenta rings (CP-PAH). Among PAH, CP-PAH are of particular interest because of their proneness to oxidation in the en vironment, their relatively high biological activity, and their postulated role in soot formation. Of the 10 CP-PAH identified in our tar samples, 4 of the most abundant are acenaphthylene (C12H8), acephenanthrylene and aceanthrylene (C16H10), and cyclopental [cd]pyrene (C18H10)—all of which have been detected previously in products of coal pyrolysis and/or combustion. The recent synthesis of several new CP-PAH reference standards, however, has enabled us to also identify, in the brown coal tars, six additional CP-PAH-cyclopent[hi]acephenanthrylene and cyclopenta[cd]fluoran thene (C18H10), dicyclopenta[cd, mn]pyrene and dicyclopental[cd, jk]pyrene (C20H10), benzo[ghi]cyclopenta[cd]perylene (C24H12), and cyclopenta[bc]coronene (C26H12)—none of which has ever before been identified in coal products. The mass fractions of individual CP-PAH span a range of four orders of magnitude—from 0.000062 for cyclopenta[bc]coronene to 0.265 for acenaphthylene in the 1000°C tar smaple. Accounting for approximately one-third of the mass of the tar produced at 1000°C, the CP-PAH yields show a monotonic increase with pyrolysis temperature—confirming that the CP-PAH are not primary products of coal devolatilization but instead result from secondary pyrolytic reactions in the gas phase. Possible reaction mechanisms are explored.