Production Of Polycyclic Aromatic Hydrocarbons Research Articles

06/02096 Two-stage desulfurization process for hot gas ultra cleanup in IGCC: Park, N. -K. et al. Fuel, 2006, 85, (2), 227–234.

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.

The formation of polycyclic aromatic hydrocarbons from the supercritical pyrolysis of 1-methylnaphthalene

In order to better understand the pyrolytic reactions leading to the formation of polycyclic aromatic hydrocarbons (PAH) and carbonaceous solids under supercritical conditions, we have pyrolyzed the model fuel 1-methylnaphthalene (critical temperature, 499 °C; critical pressure, 36 atm) in an isothermal silica-lined stainless-steel reactor coil at 585 °C, 110 atm, and 140 s. Analysis of the reaction products by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection and mass spectrometric detection has led to the identification of 37 individual 2- to 7-ring PAH—fifteen of which have never before been reported as products of 1-methylnaphthalene pyrolysis. The absence, among the reaction products, of single-ring aromatics and acetylene indicates that there is no aromatic ring rupture in this reaction environment, and the structures of each of the 5- to 7-ring PAH products reveal the intactness of the two 2-ring naphthalene units required in their construction. Proposed reaction pathways involving species plentiful in the reaction environment—1-naphthylmethyl radical, methyl radical, 1-methylnaphthalene, naphthalene, and 2-methylnaphthalene—account for the formation of the observed 5- to 7-ring PAH products. These reaction pathways, along with consideration of bond dissociation energies and relative abundances of reactant species, account for the extremely high product selectivity exhibited by the observed product PAH. The detection of seven 8- and 9-ring PAH, each requiring construction from three naphthalene or methylnaphthalene units, provides evidence that the types of reaction mechanisms outlined here—for the combination of two naphthalene entities to form 5- to 7-ring PAH—are also likely to apply to the combination of three and more such entities in the formation of larger-ring-number PAH and eventually carbonaceous solids.

Polycyclic aromatic hydrocarbons in flames, in diesel fuels, and in diesel emissions

Numerous chemical analyses of gaseous and particulate samples from laboratory flames provide a library of data on the polycyclic aromatic hydrocarbon (PAH) species found in diverse flame types burning fuels consisting of pure gaseous hydrocarbons. The diesel fuels utilized by the more complex combustion in compression ignition engines are composed of thousands of hydrocarbon species. Mass spectrometry by the laser microprobe and gas chromatography were used in a complementary manner to distinguish the PAHs originating in the fuel from those produced by engine combustion. The C x H y PAH products of premixed and diffusion flame processes, which also occur in the unsteady diesel combustion, range in mass from 128 u (two rings, x = 10 , y = 8 ) to beyond 350 u (eight rings, x = 28 , y = 14 ). Graphs of the number of hydrogen atoms y vs the number of carbon atoms x for the species found by many investigators of laboratory flames show these pyrogenic PAHs to lie on or near the staircase curve that describes the most stable, pericondensed, benzenoid PAHs. In contrast, samples of diesel fuels from the United Kingdom and the United States contain petrogenic alkyl-PAHs with high hydrogen contents. Samples of diesel particulate emissions typical of the 1990s from two different sources display the full mass range of PAHs from 128 to 350 u, including both the benzenoid PAHs and the alkyl-PAHs. Thus diesel emissions, in general, may contain petrogenic fuel components ranging up to 206 u and also the combustion-generated four- to seven-ring species in the 228 to 302 u mass range that have greater carcinogenic potency. The absence of petrogenic components larger than 206 u facilitates their detection and delineation from pyrogenic PAHs by methods of chemical analysis.

Polyynes and cyanopolyynes synthesis from the submerged electric arc: about the role played by the electrodes and solvents in polyynes formation

The products of the electric arc between graphite electrodes have been investigated by high performance liquid chromatography-diode-array detector (HPLC-DAD) analysis in various media: distilled water, liquid nitrogen, methanol, ethanol, n-hexane and benzene. In distilled water, hydrogen capped polyynes H–(CC) n –H were the unique products demonstrating that carbon is supplied by the graphite electrodes while hydrogen is supplied by the solvent plasmalysis (in this case water plasmalysis). Arcing graphite electrodes in liquid nitrogen produces cyanopolyynes: NC–(CC) n –CN demonstrating that in this case the end groups of the polyyne chains are supplied by molecular nitrogen plasmalysis caused by the electric arc. Graphite arcing in methanol and ethanol produces very clean solutions (by-products negligible or absent) of hydrogen-capped polyynes with C 8H 2 as the main product accounting for more than 70 mol percent of the total polyyne concentration. By replacing graphite electrodes with titanium electrodes in methanol or in ethanol, polyynes are not formed at all; only trace amounts of polycyclic aromatic hydrocarbons (PAHs) were detected. When arcing with graphite electrodes is conducted in n-hexane or in benzene, polyyne formation is accompanied by a significant production of PAH, especially in benzene. These results have been rationalized in terms of carbonization or coking tendency of a given solvent. The effect of using titanium electrodes in place of graphite electrodes has been investigated also in n-hexane and in benzene as well as the effects of very high electric current intensity employed to ignite and sustain the submerged electric arc.

Assessment of the toxicity of anthracene photo-modification products using the topminnow ( Poeciliopsis lucida) hepatoma cell line (PLHC-1)

Many polycyclic aromatic hydrocarbons (PAHs) are acutely toxic to fish and other aquatic organisms in the presence of environmentally realistic intensities of solar ultraviolet radiation (SUVR). In this study, the cytoxicity/phototoxicity of anthracene photo-modified products to aquatic animals was assessed based on in vitro toxicity assays using a fish hepatoma cell line (PLHC-1 cell line). The results from this study showed that pre-exposure of anthracene/cell culture media to SUVR caused a significant amount of photo-modification and reduced the phototoxicity of parent anthracene compound. SUVR pre-exposed anthracene did not induce cytotoxicity in the absence of SUVR. It was shown that in comparison with anthracene, two major anthracene photo-modified products (anthraquinone, 1,2-dihydroxyanthraquinone) were significantly less phototoxic to PLHC-1 cells. Also in the absence of SUVR, these chemicals did not induce any detectable cytotoxicity to PLHC-1 cells. In conclusion, this study demonstrated that photo-modification reduced the phototoxicity of anthracene to PLHC-1 cells because major anthracene photo-modified products are not phototoxic. Also it was found that these photo-modified anthracene products were not cytotoxic in the absence of SUVR. This study indicated the need for separate toxicity assessments of individual photo-modified PAH products in both animal and plant models.

Models for Polar Haze Formation in Jupiter's Stratosphere

We present coupled chemical–microphysical models for the formation, growth, and physical properties of the jovian polar haze based on a gas-phase photochemical model for the auroral regions developed by A. S. Wong et al. (2000, Astrophys. J. 534, L215–217). In this model, auroral particle precipitation provides an important energy source for enhanced decomposition of methane and production of benzene and polycyclic aromatic hydrocarbons (PAHs). We find that at high altitude, A 4 (pyrene, a hydrocarbon consisting of four fused aromatic rings) should homogeneously nucleate to form tiny primary particles. At lower altitudes, A 3 (phenanthrene) and A 2 (naphthalene) heterogeneously nucleate on the A 4 nuclei. These particles subsequently grow by additional condensation of A 2 on the nucleated particles and by coagulation and eventually sediment out to the troposphere. We run different cases of the aerosol microphysical model for different assumptions regarding the fractal dimension of aggregate particles formed by the coagulation process. If coagulation is assumed to produce spherical particles (of dimensionality 3), then their mean radius at altitudes below the 20-mbar pressure level is computed to be approximately 0.1 μm. If coagulation produces fractal aggregates of dimension 2.1, then their equivalent mean radius below the 20-mbar level is much larger, of order 0.7 μm. Aggregates with fractal dimensions between 2.1 and 3 form with equivalent mean radii between 0.1 and 0.7 μm. In every case, mean particle radius is found to decrease with increasing altitude, as expected for a system approximately in sedimentation–coagulation equilibrium. The predicted range of altitudes where aerosol formation occurs and the mean size to which particles grow are found to be generally consistent with observations. However, our calculations cannot presently account for the large amount of total aerosol loading inferred by M. G. Tomasko et al. (1986, Icarus 65, 218–243). We suggest that the primarily neutral chemical pathway to heavy hydrocarbon and PAH formation proposed by Wong et al. (2000) may proceed too slowly to produce a sufficient amount of condensible material. Inclusion of ion and ion–neutral reactions in the chemical scheme could potentially lead to the prediction of higher PAH production rates, higher nucleation rates, and greater aerosol loading, producing better agreement with the observations.

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.

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

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 component's 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 1500 K. The five new CP-PAH, along with five previously identified CP-PAH, together account for approximately a quarter, by number, of the anthracene product PAH identified and quantified to date. At the highest temperatures, the CP-PAH as a whole account for almost half of the mass of the “thermally produced” P \\H, the dominant PAH under the conditions at which soot is the major product. The results show some consistency with a CP-PAH formation mechanism involving acetylene atldition to aryl radicals, followed by cyclization of the resulting ethynyl-PAH. Further research is needed, however, to better elucidate the governing reaction mechanisms.

Soot and PAH production from spray combustion of different hydrocarbons behind reflected shock waves

The combustion/pyrolysis of three chemically different hydrocarbons, toluene, n-heptane, and propanol-1, has been investigated in reflected shock waves. The formation of soot and polycyclic aromatic hydrocarbons (PAHs) has been studied, and the factors which influence the production of these compounds during liquid spray combustion have also been investigated at temperatures between 1000 and 3000 K and pressures of 2–25 bar. The results indicate that formation of soot and PAH is significantly dependent on the chemical properties of fuel, temperature, and oxygen concentration, but the pressure dependence is found to be minimal in the pressure range less than 25 bar. The results are instrumental in adding to our understanding of the soot and PAH formation chemistry.

Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Premixed Propane Flame

Experimental and detailed chemical kinetic modeling has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, propane-oxygen-argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer (GC/MS) technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the propane flame. Benzene formation was shown to be dominated by the propargyl recombination reaction. A secondary benzene formation pathway occurred from the reaction sequence of allyl plus propargyl leading to fulvenc and H-atoms whereupon the fulvene is converted to benzene by H-atom catalysis. Large negative sensitivity coefficients were calculated for the H2CCCH + H↔C3,H2 +2 reaction in the propargyl, benzene, and naphthalene sensitivity analysis study. This result implicates propargyl consumption by H-atoms as an important reaction step that limits aromatic and PAH growth Naphthalene formation through the reaction step of cyclopentadienyl self-combination and phenanthrene formation from indenyl and cyclopentadienyl combination were shown to be plausible global reaction steps for PAH production.

Influence of Coal Type and Operating Conditions on the Formation of Incomplete Combustion Products, Pilot Plant Experiments

ABSTRACT This experimental study was mainly aimed to evaluate the influence of the coal type on the production of Polycyclic Aromatic Hydrocarbons (PAHs) and other trace organics from pulverized coal combustion. The behaviour 7 of coals, having different characteristics and geographical origin, was tested in a small scale furnace (50kWt), under relatively good practical combustion conditions. As regards fuel carbon conversion, some observations were also made when the combustion conditions got worse, slightly differing from those set, or becoming definitely poor. A detailed chemical characterization of the Products of Incomplete Combustion (including unburnts, the total Organic Material Extractable by methylenchloride (EOM), PAHs from naphthalene to coronene and n-alkanes (n-A) from C12 to C36) was performed on solid and gaseous emissions. The fluorescence emission spectra of the integral methylenchloride extracts, in the range from 270 to 600 nm, were also studied. The experimental data concerning stack ...

Polycyclic aromatic hydrocarbons from the high-temperature pyrolysis of pyrene

The formation of polycyclic aromatic hydrocarbons (PAHs) from solid fuels is a complicated phenomenon involving many physical and chemical processes. In order to gain a better understanding of the types of reactions taking place during high-temperature PAH production, we carried out a pure compound pyrolysis program between 1200 and 1500 K. In this work, which deals with pyrene pyrolysis in a laminar flow drop tube furnace, we identified four classes of condensed phase reaction products—bipyrenes, their condensation or cyclodehydrogenation products, pyrene fragmentation products and soot. Statistical and steric factors—accounted for qualitatively by the molecular topology of the reaction products—were used to rationalize the relative abundance of the different products in the first two categories. The absence of PAH species that might arise from a sequential acetylene addition pathway (like benzo[e]pyrene, benzo[ghi]perylene, coronene, etc.) leads us to conclude that in our system the most important molecular weight growth channel involves direct polymerization of aromatics. Fragmentation products like triphenylene, cyclopenta[hi]acephenanthrylenes, and cyclopenta[cd]pyrene were also detected; however, based on evidence adduced from parallel studies, we think that high amounts of CPP in this and other studies result from isomerization rather than acetylene addition.

Production of Polycyclic Aromatic Hydrocarbons in Chlorine Containing Environments*

Abstract The factors affecting the formation and growth of polycyclic aromatic hydrocarbons (PAHs) in chlorine-containing environments are investigated. The experimental results obtained in shock-tube pyrol-ysis of chlorinated C1, and C2, hydrocarbons are re-analyzed in terms of Cl-to-H molar ratio of the mixture and it is found that the sooting tendency and hence PAH production are increased with the increase in [C1]/[H] ratio. Based on the analysis of the reaction kinetics and results or detailed modeling, two basic factors are suggested to be responsible for this phenomenon: (1) the enhanced, chlorine-catalized molecular degradation that promotes the formation of aromatic-ring compounds; and (2) the large concentration of CI atoms that accelerates the abstraction of aromatic H from stable PAH molecules, which activates them for further growth. The modeling results also provide an evidence against existence of a correlation between PAHs and CO in flame environments.

Fast, Repetitive GC/MS Analysis of Thermally Desorbed Polycyclic Aromatic Hydrocarbons (PAHs) from Contaminated Soils

Abstract A syslem for on-line analysis of organic vapors by short column gas chromatography/mass spectrometry (GC/MS) has been used to monitor products from a thermal soil desorption reactor. The system consists of a unique air sampling inlet with a 1 meter long capillary column coupled directly to a modified Ion Trap Mass Spectrometer (Finnigan MAT) with demonstrated detection limits for alkylbcn-zencs in the low ppb range. In this work the mobile instrument is used for repetitive GC/MS and GC/MSn (tandem MS) analysis at 30 to 60s intervals of PAH products from coal tar contaminated soils in a bed characterization reactor. Results for naphthalene through dibenzanthracenes arc compared to conventional. more detailed GC/MS analyses of extracts from soil before and after thermal treatment.

Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes

Production of polycyclic aromatic hydrocarbons in carbon-rich circumstellar envelopes was investigated using a kinetic approach. A detailed chemical reaction mechanism of gas-phase PAH formation and growth, containing approximately 100 reactions of 40 species, was numerically solved under the physical conditions expected in cool stellar winds. The chemistry is based on studies of soot production in hydrocarbon pyrolysis and combustion. Several first-ring and second-ring cyclization processes were considered. A linear lumping algorithm was used to describe PAH growth beyond the second aromatic ring. PAH production using this mechanism was examined with respect to a grid of idealized constant velocity stellar winds as well as several published astrophysical models. The basic result is that the onset of PAH production in the interstellar envelopes is predicted to occur within the temperature interval of 1100 to 900 K. The absolute amounts of the PAHs formed, however, are very sensitive to a number of parameters, both chemical and astrophysical, whose values are not accurately known. Astrophysically meaningful quantities of PAHs require particularly dense and slow stellar winds and high initial acetylene abundance. It is suggested that most of the PAHs may be produced in a relatively small fraction of carbon-rich red giants.

Organic geochemical evidence for global fires at the Cretaceous/Tertiary boundary

MANY hypotheses have been advanced to explain the mass extinction at the Cretaceous/Tertiary (K/T) boundary1–3. Recently, Wolbach et al. 4 suggested that massive forest fires were triggered by the impact of a meteorite, and cite as evidence the presence of elemental carbon (mainly soot) from K/T boundaries5. Almost all of the airborne polycyclic aromatic hydrocarbons (PAHs) generated by pyrosynthesis are adsorbed, through hydrogen bonding, on the surface of soot, the participate fraction from combustion6,7. Although soot itself is a polymer of polybenzenoid radicals, early termination of polymerization leads to enhanced PAH production. Pyrosynthesis of PAHs is thus favoured by a chemically reducing atmosphere. If there were wildfires, a group of high-molecular-weight parent PAHs characteristic of combustion, predominating over their alkyl homologues8,11, should be present in K/T boundary samples known to contain soot4,5. Here we compare K/T samples from New Zealand, Italy and Denmark to those from above and below the boundary, and find enhanced PAH contents and distribution profiles that reflect a pyrolytic origin. The data thus provide the first detailed organic-molecular evidence for the combustion source of organic carbon at the K/T sites.

The environmental load and cycle of polycyclic aromatic hydrocarbons

Many polycyclic aromatic hydrocarbons (PAH) are known to be carcinogenic to animals and probably to man. Therefore, a better understanding of their formation, behavior and fate in the environment is necessary. Of the PAH carcinogens, 3,4-benzpyrene (BaP), which constitutes only about 1 % – 20 % of the group, is one of the most potent and ubiquitous, and therefore the most widely studied. Although the risk from carcinogenic PAH cannot yet be estimated, synergistic effects with other substances have also to be considered. The formation of PAH in the environment is due to an endogenic synthesis by microorganisms, phytoplankton, algae and highly developed plants, which provide the natural background, and to a second process which is connected with man-controlled high temperature pyrolitic reactions, open burning, and natural volcanic activities. Photooxidation is the major decomposition process by which PAH are reduced, biodegradation by microorganisms being a secondary reduction mechanism. The worldwide production of PAH by biosynthesis cannot be estimated on the basis of the limited data available at present. However, the global emission of BaP derived from exogenous sources has been estimated at about 5000 tons per year, a quantity which appears to exceed that formed endogenically. It may be assumed, that until the beginning of this century there still existed a natural balance between the production and degradation of PAH, which kept the natural background concentration low and fixed. However, with the ever-increasing industrial development in many parts of the world the natural balance is being disturbed and the production rate of PAH is constantly rising, while their rate of decomposition remains more or less constant. In view of the carcinogenic characteristics of many PAH compounds, their concentration in the environment should be considered alarming and efforts should be made to reduce or even eliminate them wherever possible. At the same time, an environmental programme should be developed for monitoring the PAH compounds and the various parameters affecting their formation, retention and destruction.