Elmer B. Ledesma

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 of the release of nitrogen from coals pyrolyzed in fluidized-bed reactors

Nitrogen release has been measured from a suite of Australian, German, and U.S. coals pyrolyzed under rapid heating conditions in two bench scale reactors. The first reactor was a single-stage fluidized bed reactor: the second was a two-stage reactor in which a fluidized bed was coupled with a tubular reactor. The two-stage reactor enabled cracking reactions of the volatiles to be studied in isolation from reactions of the char. The results show that the gas-phase N-containing species, HCN and NH 3 , are formed from both brown and bituminous coals. In addition, HNCO is also observed from bituminous coals: HNCO is a plausible precursor for NH 3 formation. At high temperatures low-volatile coals give lower yields of HCN and NH 3 than high-volatile coals. This observation, however, does not imply that tar cracking is the only source of HCN and NH 3 . In fact, the results obtained in the two-stage reactor suggest that a significant proportion of the HCN is released from the char and that tar cracking is not a significant source of NH 3 . A major part of the NH 3 is produced by interactions of N-containing species with the char. These results, together with previous studies of N release, demonstrate that there is no simple relationship between N functionality in coals and the composition and rate of release of the nitrogen.

An experimental and kinetic modeling study of the reduction of no by coal volatiles in a flow reactor

The reduction of NO by reactions with primary coal tar and other volatiles under fuel-rich conditions was investigated using a tubular-flow reactor coupled to a fluidized-bed reactor. The primary coal volatiles were generated at high heating rate (10 4 K s −1 ) conditions in the fluidized-bed reactor. The results were compared to those from experiments performed using the same experimental setup but with CH 4 instead of the primary coal volatiles. The experimental results show that reactions of coal volatiles result in greater NO reductions than reactions with CH 4 . In addition. NO reduction by coal volatiles occurs at much lower oxygen concentrations, indicating the higher reactivity of the coal volatiles compared to that of CH 4 . Inereases in the yields of HCN and HNCO were found to occur concurrently with the decrease in NO during the experiments with the coal volatiles, suggesting that the NO is reduced to these two species under the conditions employed in this study. Kinetic modeling of the experiments with coal volatiles was performed using a highly simplified model for the coal tars, in which the tar was modeled as structures representative of the major species known to be present in primary tar: n -heptane to represent the longchain aliphatics, toluene and phenol to represent the aromatics, and a pseudospecies, designated as tar-N, to represent the nitrogen-containing components in primary tar. Despite the simplified tar-nitrogen chemistry employed (the nitrogen in tar was assumed to evolye via the first-order global reaction, tar-N→HCN), the modeling results show reasonable predictions of the major gas-phase species. Yields of HNCO and HN 3 however are poorly predicted.

The formation of nitrogen species and oxygenated PAH during the combustion of coal volatiles

The combustion of coal volatiles produced by rapid pyrolysis was studied using a two-stage reactor consisting of a fluidized-bed reactor coupled to a tubular-flow reactor. Volatiles were generated in the fluidized-bed reactor under high heating rates and at 600°C such that the major volatile species produced were tars. The freshly generated tars were subsequently oxidized in the tubular-flow reactor at 900 and 1000°C. Fourier transform infrared (FTIR) analysis showed that, with an increase in oxygen concentration, the recovered tars exhibited and increased in the carbonyl, C=O, functionality. The position of the C=O peak and the presence of absorbances in the aromatic C−H out-of-plane deformation region in the FTIR spectra and GC/MS identification demonstrate that polycyclic aromatic ketones and aldehydes are significant oxygenated polycyclic aromatic hydrocarbons (OPAH) products from coal volatiles combustion. The results indicate that combustion processes are primarily responsible for OPAH formation. HNCO yield was found to increase rapidly with the addition of small amounts of oxygen. The results show that HCN oxidation is not primarily responsible for HNCO formation: reactions of other N-containing species are likely sources. The observation of HNCO suggests that previous measurements of NH 3 in coal combustion probably represent the sum of NH 3 and HNCO yields. The presence of hydrocarbon species (gases and tars) has a significant effect on fuel-N conversion. The experimental results clearly demonstrated that NO production increased significantly onee the concentration of hydrocarbons decreased.