Abstracts of papers

Veranth, J. M., K. R. Smith, A. E. Aust, S. L. Dansie, J. B. Griffith, A. A. Hu, M. L. Huggins and J. S. Lighty (2000). “Coal fly ash and mineral dust for toxicology and particle characterization studies: Equipment and methods for PM2.5- and PM1-enriched samples.” Aerosol Science and Technology 32(2): 127-141.

Laboratory methods to produce particle samples from known, reproducible sources with sufficient mass to perform both detailed characterization and replicated in vitro toxicological assays are described. These samples are being used to study of the ability of inhalable particles to produce abnormal concentrations of intracellular iron, resulting in the production of reactive oxygen species in cultured airway epithelial cells. Bulk samples of size-fractionated particles from laboratory-generated coal fly ash and from simulated fugitive mining tailings and road dust were collected as surrogates for important sources of iron-bearing particles in the ambient air. An Andersen cascade impactor was used to produce particle samples enriched in three size ranges: over 10 µm, 10 to 2.5 µm, and less than 2.5 µm aerodynamic diameter. A multi-jet preseparator and rectangular slot virtual impactor are being used to produce a fraction enriched in particles below 1 µm. Data on the particle production conditions, production rates, and particle sample quality are provided to illustrate the feasibility of the experimental approach. The amount of iron mobilized from particles by a physiologically-relevant chelator does not correlate with the total iron. This supports the hypothesis that particle characteristics and iron speciation are important for the production of abnormally iron concentrations in cultured type A549 human airway epithelial cells. Comparison of results obtained with these surrogate particles to previous work with urban particulate standard reference materials (SRM 1648 and SRM 1649) suggests particle sources and size fractions that should be emphasized for detailed characterization of particle morphology and mineralogy.

Mobilization of iron from coal fly ash was dependent upon the particle size and the source of coal.

Smith, K. R., J. M. Veranth, J. S. Lighty and A. E. Aust (1998). “Mobilization of iron from coal fly ash was dependent upon particle size and source of coal.” Chemical Research in Toxicology 11(12): 1494-1500.

Particulate air pollution, including coal fly ash, contains iron, and some of the pathological effects after inhalation may be due to reactive oxygen species produced by iron-catalyzed reactions. The objective of this study was to determine whether iron, present in coal fly ash, was mobilized, leading to ferritin induction in human airway epithelial cells, and whether the size of the particles affected the amount of iron mobilized. Three types of coal were used to generate the three size fractions of fly ash collected. The Utah coal fly ash was generated from a bituminous b coal, the Illinois coal fly ash from a bituminous c coal, and the North Dakota coal fly ash from a lignite a coal. Three size fractions were studied to compare the amount of iron mobilized in human airway epithelial (A549) cells and by citrate in cell-free suspensions. The size fractions selected were fine (<2.5 microm) and coarse (2.5-10 micron) components of PM10, airborne particulate matter <10 microm in diameter, and the fraction greater than 10 micron. Coal fly ash samples were incubated with 1 mM citrate to determine if iron associated with coal fly ash could be mobilized. Iron was mobilized by citrate from all three size fractions of all three coal types to levels as high as 56.7 nmol of Fe/mg of coal fly ash after 24 h. With all three coal types, more iron was mobilized by citrate from the <2.5 microm fraction than from the >2.5 microm fractions. Further, the mobilized iron was in the Fe(III) form. To determine if iron associated with the coal fly ash could be mobilized by A549 cells, cells were treated with coal fly ash, and the amount of the iron storage protein ferritin was determined after 24 h. Ferritin levels were increased by as much as 11.9-fold in cells treated with coal fly ash. With two of the three types of coal studied, more ferritin was induced in cells treated with the <2.5 micron fraction than with the >2.5 microm fractions. Further, inhibition of the endocytosis of the coal fly ash by the cells resulted in ferritin levels that were near that of the untreated cells, suggesting that iron was mobilized intracellularly, not in the culture medium. The results of this study suggest that differences in particle size and speciation of iron may affect the release of iron in human airway epithelial cells.

Mobilization of Iron From Coal Fly Ash Was Dependent upon the Particle Size and Source of Coal: Analysis of Rates and Mechanisms

Veranth, J. M., K. R. Smith, A. A. Hu, J. S. Lighty and A. E. Aust (2000). “Mobilization of iron from coal fly ash was dependent upon the particle size and the source of coal: Analysis of Rates and Mechanisms.” Chemical Research in Toxicology 13: 382-389.

The observed iron mobilization rate from size-fractionated coal fly ash is consistent with the model predictions for a limiting case of mass transfer where the dominant resistance is diffusion through a layer of depleted solid on the surface of spherical particles. The mobilization of iron from coal fly ash under physiologically-relevant conditions in vitro was previously shown to depend on the size of the ash particles and on the source of the coal, and these in vitro measurements have been shown to correlate with indirect measurements of excess iron in cultured cells. Existing iron mobilization data was compared to mathematical models for mass transfer and chemical reaction in solid-liquid heterogeneous systems. Liquid-phase diffusion resistance can be ruled out as the rate limiting mechanism for iron mobilization as the model predictions for this case are clearly inconsistent with the measurements. Other plausible hypotheses, such as a rate limited by a heterogeneous surface reaction, cannot be conclusively ruled out by the available data. These mathematical analysis methods are applicable to the design of future experiments to determine the rate-limiting mechanism for the mobilization of iron and of other transition metals from both ambient air samples and from surrogates for major sources of particulate air pollution.

Mössbauer Spectroscopy Indicates that Iron in an Aluminosilicate Glass Phase Is the Source of the Bioavailable Iron from Coal Fly Ash.

Veranth, J. M., K. R. Smith, J. G. Rohrbough, F. Huggins, A. A. Hu, J. S. Lighty and A. E. Aust (2000). “Mossbauer Spectroscopy Indicates that Iron in an Aluminosilicate Glass Phase Is the Source of the Bioavailable Iron from Coal Fly Ash.” Chemical Research in Toxicology 13: 161-164.

Iron speciation by Mössbauer spectroscopy indicates that ferric iron in an aluminosilicate glass phase is the source of the bioavailable iron in coal fly ash, and that this iron species is associated with combustion particles, but not with fugitive dust derived from soil minerals. Urban particulate has been shown to be a source of bioavailable iron and have been shown to be able to induce the formation of reactive oxygen species in cell culture experiments. Laboratory generated coal fly ash and crustal dust have been studied as surrogates for two sources of metal-bearing particles in ambient air. As much as a 60-fold difference in iron mobilization by a citrate chelator was observed between fly ash and crustal dust samples with similar total iron content. The iron mobilization by citrate in vitro has been shown to correlate with indirect measures of excess iron in cell culture and with assays for reactive oxygen species generation. Mössbauer spectroscopy of Utah coal fly ash before and after treatment with the chelator desferrioxamine showed that the iron in a glass phase was preferentially removed. The removal of the glass-phase iron reduced the amount of iron that could be mobilized by a citrate chelator, and reduced the ability of the particles to induce interleukin-8 in cultured human lung epithelial cells (A549). Ferric iron in aluminosilicate glass is associated with particles formed at high temperature followed by rapid cooling. The observation that ferric iron in aluminosilicate glass is the source of bioavailable iron in coal fly ash provides a hypothesis that can guide further studies for the sources of bioavailable transition metals in ambient particulate.

Interleukin-8 levels in human lung epithelial cells are increased in response to coal fly ash and vary with the bioavailability of iron, as a function of particle size and source of coal.

Smith, K. R., J. M. Veranth, A. A. Hu, J. S. Lighty and A. E. Aust (2000). “Interleukin-8 levels in human lung epithelial cells are increased in response to coal fly ash and vary with bioavailability of iron, as a function of particle size and source of coal.” Chemical Research in Toxicology 13: 118-125.

Particulate air pollution contains iron, and some of the pathological effects after inhalation may be due to radical species produced by iron-catalyzed reactions. We tested the hypothesis that iron present in coal fly ash (CFA) could induce the expression and synthesis of the inflammatory cytokine interleukin-8 (IL-8). CFA, containing as much as 14% iron, was used as a model combustion source particle. Three coal types were used to generate three size fractions enriched in particles [submicron (<1 micrometer), fine (<2.5 micrometer), or coarse (2.5-10 micrometer]), as well as the fraction of >10 micrometer. Treatment of human lung epithelial (A549) cells for 4 h with CFA from Utah enriched in <1 micrometer particles (20 microgram/cm(2)) resulted in a 2.6-fold increase in mRNA levels for IL-8. IL-8 levels were increased in the medium by as much as 8-fold when cells were treated with the fraction enriched in the smallest size Utah CFA for 24 h. IL-8 production was completely inhibited when the CFA was pretreated with the metal chelator desferrioxamine B, suggesting that a transition metal was responsible for the induction, probably iron. Treatment with a soluble form of iron, ferric ammonium citrate (FAC), mimicked the IL-8 level increase observed with CFA. There was a direct relationship, above a threshold level of bioavailable iron, between the levels of IL-8 and bioavailable iron in A549 cells treated with CFA or FAC. Further, the relationship between IL-8 and bioavailable iron for CFA was indistinguishable from that for FAC. These results strongly suggest that iron can induce IL-8 in A549 cells and that iron was the likely component of CFA that induced IL-8. CFA-induced IL-8 production was inhibited by tetramethylthiourea or dimethyl sulfoxide, suggesting that radical species were involved in the induction. These results demonstrate that iron present in CFA may be responsible for production and release of inflammatory mediators by the lung epithelium through generation of radical species and suggest that iron may contribute to the exacerbation of respiratory problems by particulate air pollution.

Combustion Aerosols: Factors Governing their Size and Composition and Implications to Human Health

Lighty, J. S., J. M. Veranth and A. F. Sarofim (2000). “Combustion aerosols: Factors governing their size and composition and implications to human health.” Journal of the Air & Waste Management Association 50: 174-227.

Particulate matter (PM) emissions from stationary combustion sources burning coal, fuel oil, biomass, and waste, and PM from internal combustion engines burning gasoline and diesel are a significant source of primary particles smaller than 2.5 µm (PM_2.5) in urban areas. Combustion-generated particles are generally smaller than geologically produced dust, and have unique chemical composition and morphology. The fundamental processes affecting formation of combustion PM and the emission characteristics of important applications are reviewed. Particles containing transition metals, ultrafine particles, and soot are emphasized because these types of particles have been studied extensively, and the emissions are controlled by the fuel composition and the oxidant-temperature-mixing history from the flame to the stack. There is a need for better integration of the combustion, air pollution control, atmospheric chemistry, and inhalation health research communities. Epidemiology has demonstrated that susceptible individuals are being harmed by ambient PM. Particle surface area, number of ultrafine particles, bioavailable transition metals, PAH and other particle-bound organic compounds are suspected to be more important than particle mass in determining the effects of air pollution. Time- and size-resolved PM measurements are needed for testing mechanistic toxicological hypotheses, for characterizing the relationship between combustion operating conditions and transient emissions, and for source apportionment studies to develop air quality plans. Citations are provided to more specialized reviews, and the concluding comments make suggestions for further research.

Bioavailability Of Iron From Coal Fly Ash: Mechanisms Of Mobilization And Of Biological Effects

Ball, B. Ryan, K. R. Smith J. M. Veranth, and A. E. Aust (2000): “Bioavailability Of Iron From Coal Fly Ash: Mechanisms Of Mobilization And Of Biological Effects.” Inhalation Toxicology 12: 209-225.

Particulate air pollution contains iron that may be involved in the pathological effects after inhalation. This article reviews work demonstrating that ambient particulate samples (Standard Reference Material [SRM] 1648 and SRM 1649, from the National Institute of Science and Technology) contain iron that can be mobilized from the particle in vitro and inside human lung epithelial (A549) cells. The mobilized iron can then catalyze the formation of reactive oxygen species (ROS). Work is also reviewed on the generation and size fractionation of coal fly ash (CFA) from three commercially important coal types, as well as size fractionation of three types of noncombustion particles. The availability of iron from these particles to A549 cells was measured by citrate mobilization in vitro and induction of the iron storage protein ferritin in particle-treated cells. The amount of bioavailable iron decreased with increasing particle size. The ability of particles to induce synthesis of the proinflammatory cytokine interleukin-8 (IL-8) was also determined. As with the bioavailability of iron, there was an inverse correlation with size. Further work showed that iron in CFA is responsible for IL-8 induction. Mössbauer spectroscopy of a CFA sample before and after desferrioxamine B treatment to remove bioavailable iron showed that the bioavailable iron was associated with the glassy aluminosilicate fraction of the particle. In conclusion, this work shows that bioavailable iron is responsible for ROS production by SRMs and IL-8 induction by CFA in A549 cells. The source of this bioavailable iron in CFA is glassy aluminosilicates, which are found at higher levels in smaller sizes of CFA.

Workplace exposure to submicron particle mass and number concentrations from manual arc welding of carbon steel.

D. Stephenson, G. Seshadri and J. M. Veranth, American Industrial Hygiene Association Journal, 64:4 p516-521, 2003. [13]

Particle emissions from manual shielded metal arc welding of carbon steel were sampled in a

typical industrial maintenance and metal fabrication workplace environment. Particle number measurements over the size range from 14 nm to 10 mm using a scanning mobility particle sizer and an optical particle counter showed that welding produced an approximately lognormal particle mode with a 120 nm count median and a geometric standard deviation of 2.07. This study produced welding particle number concentrations on the order of 23105/cm3 in the building air 8.5 m away from the welding. Workplace exposure samples were below the current 8-hour American Conference of Governmental Industrial Hygienists mass concentration threshold limit value of 5 mg/m3. Submicron particles comprised 80% of the total aerosol mass collected by a cascade impactor during welding. The concentration of larger particles was indistinguishable from indoor background. Microscopy showed that the welding emissions are dominated by clusters formed from ,0.1 mm primary spheres. These data on the particles resulting from aerosol transformation by natural dilution inside an industrial building can be compared with laboratory-scale studies of welding particulate. The particle number characteristics observed in this study are significant because toxicological hypotheses suggest that number or surface area may be a better metric than mass when evaluating the health effects of fine particles.

Vaporization - Condensation Generation of Ultrafine Hydrocarbon Particulate Matter for Inhalation Toxicology Studies.

J. M. Veranth, R. Gelein and G. Oberdörster, Aerosol Science and Technology 37 pp. 603-609, 2003.

An evaporation/condensation particle generator produced 30–50 nm count median diameter particles from both pure hydrocarbons and from a complex mixture—used motor oil—at a concentration above 1x10⁶/cm³. The objective was to generate ultrafine aerosols for inhalation toxicology studies using specified organic components as surrogates for the particulate emissions generated by diesel internal combustion engines. This nanoparticle generation system, assembled from commercially available components, produced smaller particle size and higher particle number concentration than has been previously documented using Sinclair-La Mer condensation generator technology. The paper describes both the experiments used to design and characterize the particle generator and the operating conditions used for a specific inhalation experiment as an example of the system capability.

Inflammatory cytokines and cell death in BEAS-2B lung cells treated with soil dust, lipopolysaccharide, and surface-modified particles.

Veranth, J. M., C. A. Reilly, M. M. Veranth, T. A. Moss, C. R. Langelier, D. L. Lanza and G. S. Yost (2004). Toxicological Sciences 82: 88-96.

Cultured human lung epithelial cells (BEAS-2B) were treated in vitro with PM_2.5-enriched particles of soil-derived mineral dust from nine sites in the western United States. The particle samples simulate wind-blown dust and vehicle-generated emissions from unpaved roads. Five of the sites yielded relatively benign dust. Particles from three sites caused IL-6 release when cells were treated for 24 h at doses from 20 to 80 µg/cm², and particles from one site were highly cytotoxic. The particle components or characteristics that caused the IL-6 release were stable at temperatures below 150 °C, but were inactivated by treatment at 300 - 550 °C. The active factors were also associated predominantly with the insoluble fraction, and were [GSY1] partially attenuated by leaching with aqueous and organic solvents. The IL-6 release caused by the particles was much greater than the cytokine response to either lipopolysaccharide (LPS) or to surrogate particles of titanium dioxide mixed with LPS, suggesting that endotoxin was not a major factor in the inflammatory response. The release of IL-8 in response to particle treatment was qualitatively similar to the IL-6 response, but release of TNF- was not detected at the 24-h time point. The combined results support the hypothesis that some ambient dusts from geological sources can cause cell death and cytokine release in a lung cell line that is widely used as an in vitro model to study mechanisms of environmental respiratory injury.

Correlation of in vitro cytokine responses with the chemical composition of soil-derived particulate matter

Veranth, J. M., T. A. Moss, J. C. Chow, R. Labban, W. K. Nichols, J. C. Walton, J. G. Watson and G. S. Yost (2006). Environmental Health Perspectives 114(3): http://ehp.niehs.nih.gov/docs/2005/8360/abstract.html.

Human lung epithelial cells, type BEAS-2B, were treated with 10–80 µg/cm2 of PM2.5-enriched dust derived from soils and road surfaces in the western United States. Cell viability and cytokine secretion responses were measured at 24 h. Each dust is a complex mixture containing particles from different minerals mixed with biogenic and anthropogenic materials. The particle chemical composition was determined using methods based on the US EPA Speciation Trends Network (STN) and the National Park Service IMPROVE network. The functionally defined carbon fractions reported by the ambient monitoring networks have not been widely used for toxicology studies. The soil-derived PM2.5 particles from different sites showed a wide range of potency for inducing the release of the proinflammatory cytokines IL-6 and IL-8 in vitro. Univariate regression and multivariate redundancy analysis were used to test for correlation of viability and cytokine release with the concentrations of 40 elements, seven ions, and eight carbon fractions. The particles showed positive correlation between IL-6 release and the elemental and pyrolyzable carbon fractions, and the strongest correlation involving crustal elements was between IL-6 release and the aluminum/silicon ratio. The observed correlations between low-volatility organic components of soil- and road-derived dusts and the cytokine release by BEAS-2B cells are relevant for investigation of mechanisms linking specific air pollution particle types with the initiating events leading to airway inflammation in sensitive populations.

Acute pulmonary and systemic effects of inhaled coal fly ash in rats: comparison to ambient environmental particles.

Kevin R. Smith, John M. Veranth, Urmila P. Kodavanti, Ann E. Aust, Kent E. Pinkerton. Accepted Toxicological Sciences, 2006

Although primary particle emissions of ash from coal-fired power plants are well controlled, coal fly ash (CFA) can still remain a significant fraction of the overall particle exposure for some plant workers and highly impacted communities. The effect of CFA on pulmonary and systemic inflammation and injury was measured in male Sprague-Dawley rats exposed to filtered air or to CFA for 4 hours/day for 3 days. The concentration of CFA (PM_2.5) was 1400 µg/m³, of which 600 µg/m³ was PM₁. Animals were examined 18 and 36 hours post exposure. Chemical analysis of CFA detected silicon, calcium, aluminum, and iron, as major components. The total number of neutrophils in bronchoalveolar lavage fluid (BALF) following exposure to CFA was significantly increased along with significantly elevated blood neutrophils. Exposure to CFA caused slight increases in MIP-2, and marked increases in transferrin in BALF. IL-1β and total antioxidant potential in lung tissues were also increased in rats exposed to CFA. Histological examination of lung tissue demonstrated focal alveolar septal thickening and increased cellularity in select alveoli immediately beyond terminal bronchioles. These responses are consistent with the ability of CFA to induce mild neutrophilic inflammation in the lung and blood following short-term exposure at levels that could be occupationally relevant. However, when comparing the effects of CFA with those of concentrated ambient particles, CFA does not appear to have greater potency to cause pulmonary alterations. This study furthers our understanding of possible mechanisms by which specific sources of particulate air pollution affect human health.

Dust generator for inhalation studies with limited amounts of archived particulate matter.

Teague, S. V., J. M. Veranth, A. E. Aust and K. E. Pinkerton (2005). Aerosol Science and Technology 39(2): 85-91.

A novel design for a dry aerosol generator that efficiently produces a well-dispersed dust suspension using small quantities of a PM_2.5-enriched powder sample is described. The motivation to develop a highly efficient dry aerosol particle generator was to facilitate collaborative projects that combine in vitro cell culture experiments and multiday inhalation exposures using a single batch of well-characterized particles. Premixing of the test particles with larger diameter glass beads permits delivery of aerosol concentrations from 100 - 1000 µg/m³ to an exposure chamber using only milligram quantities of the test powder per hour. Examination of exposure chamber filter samples by scanning electron microscopy showed well-dispersed particles of the test powder free of glass spheres or fragments. Data are presented from experiments using coal fly ash as the test powder to illustrate the system performance.

Atmospheric Particle Transport

Vehicle-generated fugitive dust transport: Analytic models and field study.

J. M. Veranth, G. Seshadri and E. Pardyjak, Atmospheric Environment 37(16) pp. 2295-2303, 2003

Vehicle-generated road dust was measured under stable atmospheric conditions at a flat site in the Utah west desert,and the horizontal flux of the dust was calculated by integration of interpolation functions that are fit to the wind speed and dust concentration measurements at discrete heights. The site contained a uniformly spaced array of 2.5m high x 2.4m x 12.2m roughness elements (cargo containers) simulating an urban setting. The dust measurement objective was to test the hypothesis that near-source removal of vehicle-generated dust may account for part of the reported systematic discrepancy between emission inventories based on AP-42 factors and receptor-based source apportionment studies. The horizontal flux of dust decreased to less than 15% of the initial value while traveling between measurement planes 3 and 95m from the road, indicating significant near-source removal under the stable atmosphere and high surface roughness conditions of this field experiment. The magnitude of the flux was sensitive to assumptions about dust concentration and wind speed near the ground. The estimates for the flux at 3m, obtained by integrating alternative interpolation functions, spanned the AP-42 value. Time-averaged data for meteorology, boundary layer turbulence, and dust concentration are provided to allow using the field observations for future theoretical analysis or numerical modeling studies.

Combustion Engineering

Field Investigation of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln

Veranth, J. M., D. Gao and G. D. Silcox (1996). “Field Investigation of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln.” Environmental Science and Technology 30(10): 3053-3060.

Gas and bed temperatures were studied in a 4.4–meter–by–12–meter, co-current flow, slagging rotary kiln at a commercial hazardous waste incinerator. The visual observations used by the kiln operators to control the process are described. These observations were quantified using thermocouples, radiation pyrometers, and phase-change indicators. The objectives were to estimate the peak bed temperature and compare this to measurements at the kiln exit. The maximum bed temperature occurs toward the middle of this type of kiln and not at the discharge. The slag melting temperature and test pellets with known melting points indicate that the peak bed temperature can be 100 to 300 K higher than the kiln exit temperature reported by the permanent instruments at this facility. Both broad-band radiation pyrometers and thermocouples give a qualitative temperature indication that can be used for process control, but the readings depend on the sensor locations relative to the incompletely-mixed air and combustion products. Two-color radiation pyrometer measurements of surface temperature near the kiln exit are higher than actual temperature due to reflected radiation. (Figure 5)

Numerical Modeling of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln

Veranth, J. M., G. D. Silcox and D. Pershing (1997). “Numerical Modeling of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln.” Environmental Science and Technology 31: 534-2539.

The gas, wall, and bed temperatures in a hazardous waste incineration kiln were studied using a commercially available, CFD-based, reacting flow code, which included radiation heat transfer. The model was compared to field measurements made on a co-current flow, 35 MW slagging rotary kiln. Cases were run to determine the sensitivity of the predictions to changes in the model assumptions and to simulate the normal variation in combustion inputs. The model predictions of the peak bed temperature, of the axial temperature profile, and of the gas temperature at the exit-plane were consistent with the measurements at a full-scale waste incinerator during normal operation. The model and the field observations both indicate that the peak bed temperature occurs near the middle of the kiln and that the difference between the peak bed temperature and the exit-plane gas temperature depends on the inlet flows. The geometry of the transition between the kiln and the secondary combustion chamber and the fuel-to-air equivalence ratio have the greatest effect on the calculated temperature distribution. Modeling studies provide useful information such as the relationship between available measurements and the temperature at inaccessible locations inside a full-scale kiln. (Figure 3)

The Role of Research in Practical Incineration Systems - A Look at the Past and the Future

Lighty, J. S. and J. M. Veranth (1998). The Role of Research in Practical Incineration Systems — A Look at the Past and the Future. Twenty-Seventh Symposium (International) on Combustion, Boulder, Colorado, The Combustion Institute, Pittsburgh, PA., 1255-1273.

This paper will cover the changes in incineration technology, research, and regulations over the last 10 years. As research understanding has evolved, regulations have become more stringent, and the need for optimized technology has become more critical. Both hazardous waste and municipal waste incineration are discussed. The paper focuses on pollutant minimization, including dioxin and furan work, criteria pollutants, and the impact of solid waste; process upsets; and system simulations. These topics are presented as case studies and, in each area, future needs are discussed. Finally, future directions are addressed.

Sources of Unburned Carbon in the Fly Ash Produced from Low-NOx Pulverized Coal Combustion

Veranth, J. M., D. W. Pershing, A. F. Sarofim and J. E. Shield (1998). Sources of Unburned Carbon in the Fly Ash Produced from Low-NOx Pulverized Coal Combustion. 27th Symposium (International) on Combustion, Boulder, Colorado, The Combustion Institute, 1737-1744.

The unburned carbon in the fly ash produced from low-NOx pulverized coal combustion is shown to consist of a mixture of soot and coal char. The soot was identified by the presence of chains or aggregates of 10—50 nm diameter primary particles in electron microscope images of both laboratory samples and a sample of fly ash from a power plant operating low-NOx burners. Laboratory samples showed increasing carbon content with decreasing nitrogen oxide (NOx) concentration. The experiments included a high-NOx base case and four low-NOx cases consisting of 1) staged combustion with short (0.5 s) residence time 2) staged combustion with long (1.5 s) residence time, 3) a low-NOx burner with slow mixing, and 4) reburning using coal as the reburning fuel. Comparison of the base case which used premixed coal and air with the long residence time staged combustion case shows a decrease in the NOx from over 900 ppm to below 200 ppm and an increase in the carbon in the ash from 4% to over 30%. The fly ash from staged combustion was a mixture of large soot aggregates, porous char, and spherical particles of mineral ash, while the ash from reburning lacked the large aggregates. For all laboratory conditions the carbon content in the particle fraction with an aerodynamic diameter over 10 µm was higher than in the 1—2.5 µm diameter fraction. Both soot aggregates and char contributed to the high carbon in the large particle fraction. The difference in carbon burnout between the two staging conditions was consistent with published soot oxidation rates. Both char burnout and soot formation need to be considered in studies of the carbon content of pulverized coal fly ash.

Measurement of Soot and Char in Pulverized Coal Fly Ash

Veranth, J. M., T. H. Fletcher, D. W. Pershing and A. F. Sarofim (2000). “Measurement of Soot and Char in Pulverized Coal Fly Ash.” Fuel 79(9): 1067-1075.

The unburned carbon in the fly ash produced by low-NOx pulverized coal combustion has been shown by electron microscopy to be a mixture of porous coal char particles and aggregates of submicron particles, which are thought to be soot. A method for determining the mass of these two carbon morphologies by liquid-suspension gravity separation is described. The mass of soot and char was determined for both laboratory-scale and power plant fly ash samples. For low-NOx, staged, pilot-scale combustion of bituminous coal the soot in the furnace exit ash was estimated to be 0.2 to 0.6% of the fuel carbon, which was about 35% of the total unburned carbon.