Showing papers by "Thomas H. Fletcher published in 2001"

13C NMR Analysis of Soot Produced from Model Compounds and a Coal

Abstract: Soot samples, including the associated organics, produced from an Illinois No. 6 coal (five samples) and two model compounds, biphenyl (three samples) and pyrene (two samples), have been studied by...

Nitrogen Transformations during Secondary Coal Pyrolysis

Abstract: A CO/H2/O2/N2 flame was operated under fuel-rich conditions in a flat flame reactor to provide a high-temperature, oxygen-free environment to study secondary reactions of coal volatiles. The distributions of fuel nitrogen in the devolatilization products of four coals, ranging from high volatile bituminous to lignite, were obtained at gas temperatures ranging from 1159 to 1858 K. It was found that the initial nitrogen released was contained almost exclusively in the tar for all coals. Release of nitrogen from the char as light gases started at a later stage than tar nitrogen release. During secondary reactions, the nitrogen contents in the coal tars were higher than the nitrogen contents in the parent coals at temperatures below 1300 K. A rapid decay in the tar nitrogen content was observed between 1300 and 1600 K, followed by a much slower decrease in nitrogen content at temperatures above 1600 K. Nitrogen release from the coal tar can be described with first-order kinetics using the same rate constant f...

Small-angle X-ray studies of soot inception and growth.

Abstract: The high spectral intensity of X-rays produced by the undulator at the Basic Energy Sciences Synchrotron Radiation Center of Argonne's Advanced Photon Source has allowed us to perform small-angle X-ray scattering (SAXS) studies of the initial distribution of soot particles formed by various fuels. SAXS provides an in situ probe of the morphology of soot in the region between 1 and 100 nm and complements the ex situ technique of electron microscopy. The basic aspects of SAXS and its potential are illustrated with measurement on a laminar flame of acetylene in air. The more complex fuel toluene has been studied in a flat-flame burner that supports a CH4/H2/air or CO/H2/air diffusion flame stabilized by N2 co-flow. This burner produces a nearly constant temperature region above the flame where the pyrolysis and combustion of the heavier fuels occurs. Kinetic information is obtained by performing measurements of the scattered intensity profile as a function of the height above the burner. These profiles have been reduced to give the mean radius and dispersion of a distribution of spherical particles. Mean radii between 0.8 and 18 nm have been observed. The smallest of these is a factor of ten smaller than previously detected with Lorentz–Mie scattering. Near 1550 K, the soot distribution found in toluene shows a distinct step behavior that is consistent with model calculations.

Simulation of Coal Pyrolysis in Plasma Jet by CPD Model

Abstract: Reaction of coal in a plasma jet is complex and extremely rapid, and acetylene and carbon monoxide are the main products in the pyrolysis gas. Coal pyrolysis is assumed as the first step reaction when coal is injected into hot plasma jet with initial average temperature of 3700 K. Chemical percolation devolatilization (CPD) is employed first to simulate this procedure in mechanism. The calculation results indicate coal pyrolysis rate in plasma jet is very fast and the retention time of coal staying in reactor is only several milliseconds. Comparing the calculation with experiment result, it was concluded that the CPD agree with the experiment well when the coal feed rate is larger than about 2.0 g s-1. As the coal feed rate was increased, the average temperature of coal particle during staying in reactor was reduced and the residual time became long, but it was not found that the residual time influenced the coal conversion evidently.

Transformations of Coal-Derived Soot at Elevated Temperature

Abstract: Coal pyrolysis experiments were performed in the post-flame region of a CH 4 /H 2 /air flat-flame burner operating under fuel-rich conditions, where the temperature and gas compositions were similar to those found in the near-burner region of an industrial pulverized coal-fired furnace. Volatiles released from the coal particles formed a cloud of soot particles surrounding a centrally fed coal/char particle stream. Soot samples were collected from the cloud at different residence times using a water-cooled, nitrogen-quenched probe. The soot samples were then analyzed for their elemental compositions of carbon, hydrogen, nitrogen, sulfur, and (by difference) oxygen plus inorganic matter. Soot from three parent coals (Pittsburgh #8, Illinois #6, and Utah Hiawatha) and two gaseous hydrocarbon fuels (propane and acetylene) were investigated at temperatures of 1650, 1800, and 1900 K. The results reveal that the yield of coal-derived soot decreases with increasing reactor temperature, even though the total volatiles yield increased only slightly with temperature. The coal-derived soot yield at each reactor temperature condition also increased slightly with residence time. The carbon content in the coal-derived soot decreased with increasing particle residence time (at a given reactor temperature) and with increasing reactor temperature (at a given residence time) for all three coals. Carbon content remained constant with residence time for the gaseous hydrocarbon-fuel-derived soot. It is suggested that the observed decrease in coal-derived soot yield with increasing temperature is due to reactions of radical species from the flame with the soot precursors (i.e., the tar molecules). The slight increase in coal-derived soot yield with increasing residence time is due to attachment of light gas species such as acetylene which are richer in hydrogen than the local soot particles. The different behavior of soot from coal and the gaseous hydrocarbon fuels is explained in terms of their different chemical structures; coal-derived soot molecules have more aliphatic attachments and heteroatoms than soot from acetylene or propane. Carbon/hydrogen ratios in the soot samples were observed to be significantly different for the different soot types depending on parent fuel.

Minimization of no emissions from multi-burner coal-fired boilers

Abstract: An initial testing campaign was carried out during the summer of 2000 to evaluate the impact of multiburner firing on NOx emissions. Extensive data had been collected during the Fall of 1999 and Spring of 2000 using a single pulverized-coal (PC) burner, and this data collection was funded by a separate Department of Energy program, the Combustion 2000 Low Emission Boiler System (LEBS) project under the direction of DB Riley. This single-burner data was thus available for comparison with NOx emissions obtained while firing three burners at the same overall load and operating conditions. A range of operating conditions were explored that were compatible with single-burner data, and thus the emission trends as a function of air staging, burner swirl and other parameters will be described below. In addition, a number of burner-to-burner operational variations were explored that provided interesing insight on their potential impact on NOx emissions. Some of these variations include: running one burner very fuel rich while running the others fuel lean; varying the swirl of a single burner while holding others constant; increasing the firing rate of a single burner while decreasing the others. In general, the results to date indicated that multiburner firing yielded higher NOx emissions than single burner firing at the same fuel rate and excess air. At very fuel rich burner stoichiometries (SR < 0.75), the difference between multiple and single burners became indistinguishable. This result is consistent with previous single-burner data that showed that at very rich stoichiometries the NOx emissions became independent of burner settings such as air distributions, velocities and burner swirl.