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This work focuses on the understanding of the oxidation of soot particles which were the result of using a JP-8 surrogate fuel in a two-stage burner. The two-stage system consists of an initial premixed burner where soot was generated with an air/fuel mixture, specifically a JP-8 surrogate ( m -xylene and n -dodecane), under a variety of conditions. Downstream, the soot-laden combustion gases were passed through a second, flat-flame burner where soot was burned out under fuel-lean or slightly fuel-rich conditions. Soot oxidation in the secondary burner was determined by investigating particle size distribution (PSD), flame temperature, gas-phase composition, soot surface area, and soot morphology and nanostructure as a function of the height above the second burner (HAB). Measurements of soot size distribution and number concentration as a function of the HAB under fuel lean ( Φ overall = 0.8) and slightly rich ( Φ overall = 1.14) conditions showed a decrease in particle mean diameter and a significant increase in number concentration in the region where O 2 concentration decreased. In this region, the effectiveness factor for O 2 was found to be 1, indicating the potential for internal oxygen diffusion and burning. This caused both the breakup of the bridges cementing primary particles and the rupture of the primary particles. Higher in the burner, where modeling suggested the presence of OH * , soot oxidation was attributed to OH * mechanisms which are faster as compared to O 2 oxidation.

The combustion behavior of single fuel particles was assessed in O2/N2 and O2/CO2 background gases, with oxygen mole fractions in the range of 20–100%. Fuels included four pulverized coals from different ranks (a high-volatile bituminous, a sub-bituminous and two lignites) as well as pulverized sugarcane-bagasse, a biomass residue. Particles of 75–90 lm were injected under laminar flow in a bench-scale, transparent drop-tube furnace (DTF), electrically-heated to 1400 K where, upon experiencing high heating rates, they ignited and burned. The combustion of individual particles was observed with three-color optical pyrometry and high-speed high-resolution cinematography to obtain temperature and burnout time histories. Based on combined observations from these techniques, a comprehensive understanding of the behaviors of these fuels was developed under a variety of conditions, including simulated oxy-fuel combustion. The fuels exhibited distinct combustion behaviors. In air, the bituminous coal particles burned in two distinctive modes; the volatiles burned in bright envelope flames surrounding the devolatilizing char particles followed by heterogeneous char combustion. The volatile matter of sub-bituminous coal particles burned either in subdued envelope flames, surrounding devolatilizing and occasionally fragmenting chars, or heterogeneously at the char surface. Lignite particles typically burned with extensive fragmentation, and their volatiles burned simultaneously with the char fragments. The volatiles of bagasse particles burned in spherical and transparent envelope flames. Increasing the oxygen mole fraction in N2, increased flame and char surface temperatures, and decreased burnout times; particles of all fuels burned more intensely with an increasing tendency of the volatiles to burn closer to the char surface. When the background gas N2 was substituted with CO2, the combustion of all fuels was distinctly less intense; at moderate O2 mole fractions (

Comparative combustion studies were performed on particles of pulverized coal samples from three different ranks: a high-volatile bituminous coal, a sub-bituminous coal, and two lignite coals. The study was augmented to include observations on burning pulverized woody biomass residues, in the form of sugarcane bagasse. Fuel particles, in the range of 75–90 μm, were injected in a bench-scale, transparent drop-tube furnace, electrically-heated to 1400 K, where they experienced high-heating rates, ignited and burned. The combustion of individual particles in air was observed with three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time–size histories. Based on combined observations from these techniques, in conjunction to morphological examinations of particles, a comprehensive understanding of the combustion behaviors of these fuels was developed. Observed differences among the coals have been striking. Upon pyrolysis, the bituminous coal chars experienced the phenomena of softening, melting, swelling and formation of large blowholes through which volatile matter escaped. Combustion of the volatile matter was sooty and very luminous with large co-tails forming in the wake of the particle trajectories. Only after the volatile matter flames extinguished, the char combustion commenced and was also very luminous. In contrast, upon pyrolysis, lignite coals became fragile and experienced extensive fragmentation, immediately followed by ignition of the char fragments (numbering in the order of 10–100, depending on the origin of the lignite coal) spread apart into a relatively large volume. As no separate volatile matter combustion period was evident, it is likely that volatiles burned on the surface of the chars. The combustion of the sub-bituminous coal was also different. Most particles experienced limited fragmentation, upon pyrolysis, to several char fragments, with or without the presence of brief and low-luminosity volatile flames; other particles did not fragment and directly proceeded to char combustion. Finally combustion of bagasse was once again very distinctive. Upon pyrolysis, long-lasting, low-luminosity, nearly-transparent spherical flames formed around slowly-settling devolatilizing particles. They were followed by bright, short-lived combustion of the chars. Both volatiles and chars experienced shrinking core mode of burning. For all fuels, flame and char temperature profiles were deduced from pyrometric data and burnout times were measured. Combustion rates were calculated from luminous carbon disappearance measurements, and were compared with predictions based on published kinetic expressions.

A preliminary analysis has been conducted of the performance of a Chemical Looping system with Oxygen Uncoupling (CLOU) with copper oxide as the oxygen carrier and coal approximated by carbon as the fuel. The advantages of oxygen uncoupling are demonstrated by providing the energy balances, the circulation rate of oxygen carrier, the oxygen carrier mass loadings, the carbon burnout and oxygen partial pressure in the fuel reactor. Experimental data on the cycling of cuprous oxide to cupric oxide and kinetics for the oxidation and decomposition reactions of the oxides were obtained for use in the analysis. For this preliminary study unsupported oxides were utilized. The decomposition temperatures were rapid at the high temperature of 950°C selected for the fuel reactor. The oxidation kinetics peaked at about 800°C with the decrease in rate at higher temperatures, a decrease which is attributed in the literature to the temperature dependence of the diffusional resistance of the CuO layer surrounding the Cu2 O; the diffusion occurs through grain boundaries in the CuO layers and the rate of diffusion decreases as a consequence of growth of CuO grains with increasing temperature. The analysis shows the advantages of CLOU in providing rapid combustion of the carbon with carbon burnout times lower than the decomposition times of the oxygen carrier. For the full potential of CLOU to be established additional data are needed on the kinetics of supported oxides at the high temperatures (>850°C) at which oxygen is released by the CuO in the fuel reactor.

The behavior of Se during coal combustion is different from other trace metals because of the high degree of vaporization and high vapor pressures of the oxide (SeO 2 ) in coal flue gas. In a coal-fired boiler, these gaseous oxides are absorbed on the fly ash surface in the convective section by a chemical reaction. The composition of the fly ash (and of the parent coal) as well as the time–temperature history in the boiler therefore influences the formation of selenium compounds on the surface of the fly ash. A model was created for interactions between selenium and fly ash post-combustion. The reaction mechanism assumed that iron reacts with selenium at temperatures above 1200 °C and that calcium reacts with selenium at temperatures less than 800 °C. The model also included competing reactions of SO 2 with calcium and iron in the ash. Predicted selenium distributions in fly ash (concentration versus particle size) were compared against measurements from pilot-scale experiments for combustion of six coals, four bituminous and two low-rank coals. The model predicted the selenium distribution in the fly ash from the pilot-scale experiments reasonably well for six coals of different compositions.

Activated carbon adsorbs mercury from combustion flue gas, and this is exploited for the control of mercury emissions from coal-fired boilers and incinerators. The reaction pathway for the adsorption of mercury on activated carbon appears to be complex. The form of mercury that is adsorbed on activated carbon is likewise unknown. Experiments were carried out on vaporization of mercury compounds on non-treated, non-halogenated activated carbon in order to identify characteristics of the mercury compound adsorbed on the carbon and to assess the thermal stability of mercury once adsorbed. In these experiments, a solid material was heated in a boat in flowing air in the Ohio Lumex pyrolysis furnace. As the temperature of the boat was ramped up, the amount of mercury transferred to the gas was measured. A transient heat-transfer model was developed and coupled with a mercury vaporization model to predict, respectively, the heat-up and evolution of mercury from samples in the pyrolysis furnace. The model was able to reproduce the trends in the experimental observations, particularly the location of the peaks. The activation energy of mercury desorption in the model was used to distinguish between two different mercury species. The model also illustrated the effect of the thermal profile on mercury evolution in the pyrolysis furnace.

The chemical speciation of iron in combustion-derived ash is an important factor in determining the likelihood of ash deposit formation and buildup. In this study, the transformation of iron were examined in ash produced by the combustion of a beneficiated Pittsburgh no. 8 bituminous coal under a range of oxygen concentration ranging from 0% to 100% O2 in a drop tube furnace. The speciation of iron was found to be strongly dependent upon ash particle size, with the lowest fraction of glassy state iron found in the largest ash particles. Both the fraction of iron in a glassy state and the ratio of Fe+++(glass)/Fe++(glass) increased with increasing O2 concentration in the furnace. For the submicron ash particles, about 10% of the iron is formed by direct disintegration of pyrite and pyrrhotite during combustion. Most of the iron is however, present as Fe+++(glass), which results from the vaporization, recondensation, and coagulation of iron and silicates. For ash particles with size between 1 and 9 microns, most of the ash derives from mineral coalescence within the reacting char, with additional contribution from extraneous minerals. The fraction of glassy iron in those particles is high because of the high contact probability between iron melt and silicates. For the coarsest ash particles with size greater than 9 microns, extraneous pyrite is changed into hematite, and iron in the core of the char is changed into a glassy state.

The Utah normal heptane mechanism compiled from submechanisms in the literature was extended into a detailed normal decane combustion mechanism, which is a subset of the Utah surrogate mechanisms. Few species have greater impact on the concentrations of other species than the allyl radical CH2CH═CH2. Reactions involving the allyl radical and its isomers determine the concentration levels of all olefins, most higher unsaturated species, and benzene. To correctly predict the concentration of benzene, the reaction rates involving allyl-radical-related species need to be accurate and the concentration profiles of these species need to be satisfactory. Kinetic rates found in the literature are compared in the current work for various reference reactions that involve the allylic radicals. The improvements in numerical predictions of unsaturated species are achieved after a rigorous study in finding reliable reaction rates and kinetic correlations between various species. Some of these rates are adopted as the g...

Conventional analytical methods such as 1 H NMR, vapor pressure osmometry (VPO) and elemental analysis were used to characterize the soot precursor material represented by the chloroform extractable fractions of the young soot gathered at different heights of an ethylene inverse diffusion flame in terms of average structural parameters. The results indicate that the soot soluble fraction obtained at a 6 mm height has a relatively large molecular weight and has long aliphatic chains which later disappear with an increase in height above the burner base, especially in the region where the temperature is high (1200 K). This behavior is also accompanied by an increase in the aromaticity (fa) of the samples. 2006 Elsevier Ltd. All rights reserved.

Reaction pathway analyses were conducted for three mechanisms (designated as the Pitsch, Utah, and Lawrence Livermore National Lab) for a normal heptane premixed flame (Φ = 1.9) and a normal heptane opposed diffusion flame, in order to identify the relative importance of the major fuel consumption pathways in the two flame classes. In premixed flames, hydrogen abstraction is found to be the major fuel consumption route although it is surpassed by thermal decomposition when the flame temperature exceeds 1400-1500 K. At the higher temperatures, however, little fuel remains in a premixed flame so that thermal decomposition provides a minor pathway for overall fuel decomposition. The principal abstractor is the hydrogen radical in all three mechanisms with the hydroxyl radical having a secondary role. In opposed diffusion flames, thermal decomposition competes with hydrogen abstraction in providing the major pathway for fuel consumption. Thermal decomposition becomes important when a large fraction of the fuel reaches the high-temperature zone in a flame. By understanding the relative importance of competing fuel consumption pathways, mechanisms can be tailored to each specific application by eliminating or lumping insignificant reactions. The results obtained in this study for n-heptane may be used to guide the reduction of existing mechanisms for a particular application or the generation of mechanisms for the combustion of larger paraffins that are major components of liquid aviation and transportation fuels.

In this work, we investigate the utility of the smoke lamp for evaluating the soot-reducing potential of additives, by comparing it to a more complex liquid-fed laminar diffusion flame. The additives, ferrocene (bis(cyclopentadienyl) iron-Fe(C 5H5)2), ruthenocene (bis(cyclopentadienyl)ruthenium-Ru(C 5H5)2), iron naphthenate (a 12 % iron salt of naphthenic acid, which is a mixture of fatty carboxylic acids, some of which may include a cyclopentane ring), and MMT (Methylcyclopentadienyl manganese tricarbonyl-CH 3C5H4Mn(CO) 3) are

Three different n-heptane mechanisms were used to simulate a fuel-rich normal heptane premixed flame in order to identify major reaction pathways for olefin formation and consumption and areas of u...

The approach of mechanism generation is the accepted one of assigning generic rates to reactions in the same class. The procedure has been successfully applied to higher paraffins that include detailed sub-models of n-hexane, cyclohexane, n-heptane, n-decane, n-dodecane, and n-hexadecane and semi-detailed sub-models of iso-octane and methyl cyclohexane, in addition to reactions of aromatic formation and oxidation. Comparison between predictions and experimental data were found to be satisfactory for n-heptane, iso-octane, n-decane and gasoline premixed flames. The mechanism was also able to reproduce the measured concentrations for a n-hexadecane experiment in a jet stirred reactor. The numerical accuracy in predicting the flame structures of soot precursors, including acetylene and benzene, is one of the major foci of this study. The predicted maximum concentrations of acetylene and benzene are within 20% for most flames in this study.

Vapor-phase arsenic in coal combustion flue gas causes deactivation of the catalysts used in selective catalytic reduction (SCR) systems for NO x control. A one-dimensional model has been developed to predict the behavior of arsenic in the postcombustion region of a coal-fired boiler as a function of gas residence time. The purpose of the model is to calculate the partitioning of arsenic between the vapor phase from volatilization and arsenic on the ash particles due to surface reaction and/or condensation at temperatures characteristic of SCR systems. The model accounts for heterogeneous condensation of arsenic on the fly ash, as well as surface reaction for two regimes: (1) the free molecular regime (submicrometer ash particles) and (2) the continuum regime (supermicrometer ash particles). All gas properties are computed as functions of gas temperature, pressure, and composition, which are allowed to vary. The arsenic model can be used to calculate the impact of coal composition on vapor-phase arsenic at SCR inlet temperatures, which will help utilities better manage coal quality and increase catalyst lifetimes on units operating with SCR. The arsenic model has been developed and implemented and was tested against experimental data for several coals.

Knowledge of the chemical structure of young soot and its precursors is very useful in the understanding of the paths leading to soot particle inception. This paper presents analyses of the chemical functional groups, based on FT-IR and 1H NMR spectroscopy of the products obtained in an ethylene inverse diffusion flame. The trends in the data indicate that the soluble fraction of the soot becomes progressively more aromatic and less aliphatic as the height above the burner increases. Results from 1H NMR spectra of the chloroform-soluble soot samples taken at different heights above the burner corroborate the infrared results based on proton chemical shifts (Ha, Hα, Hβ, and Hγ). The results indicate that the aliphatic β and γ hydrogens suffered the most drastic reduction, while the aromatic character increased considerably with height, particularly in the first half of the flame.

Soot buildup and its effects on heat transfer have been investigated as part of an effort to understand the thermal response of containers of high-energy materials immersed in fires. Soot deposition rates were measured for cooled and uncooled cylindrical containers immersed in a jet fuel pool fire. The soot buildup was measured at different time intervals with a wet film gage with an uncertainty of 20%. These rates were compared with those calculated by solving the boundary layer equations along the cylinder surface including the thermophoretic transport of soot particles. Thermophoresis was the dominant soot transport mechanism controlling the deposition of soot on the container wall and gave deposition rates in good agreement with the measured values. The soot buildup was found to have an important insulating effect on the heat transfer to the container. A soot deposit thickness of 1.2 mm resulted in as much as a 35% reduction in heat flux.

A theoretical method for predicting the smoke point of pure hydrocarbon liquids is presented. The method is based on a structural group contributions approach and does not require any experimental procedures or information of fuel properties, other than the molecular structure of the fuel molecules. The proposed correlation is presented in the form of a multivariable regression. The average deviation is only 1.3 TSI (threshold soot index) units for ∼70 compounds from low-sooting paraffins to highly sooting aromatics, and the average relative error is 9.08%. The results of three different sets of structural groups derived from the Quann and Joback group contribution methods are tested and compared. For a mixture with a defined composition, the estimation of smoke point is also discussed. The method is of potential value for the formulation of surrogate fuels of hydrocarbon mixtures, where matching the fuel's sooting tendency is important.

Carbonaceous particulate matter from diesel fuel combustion was studied with Scanning Transmission X-ray Microspectroscopy, a novel synchrotron radiation-based technique, which combines X-ray absorption spectroscopy and microscopy. Single soot particles were chemically identified with a spatial resolution of better than 100 nm. Near-edge X-ray Absorption Fine Structure spectra from the carbon K-absorption edge could be assigned to specific particle regions, making it possible to distinguish graphitic carbon in the soot particles from hydrocarbons, such as residual lubricating oil and diesel fuel and their reaction products.

Carbonaceous soot produced in a small diesel engine test facility was investigated with ultra-small-angle X-ray scattering. Three soot samples produced using a reference diesel fuel and the reference fuel plus two oxygenate additives were investigated. The presence of objects at three typical size ranges, i.e., aggregates, primary particles, and subunits, was observed. By studying soot powders and pellets from pressed powder, a separation of scattering contributions from aggregates and primary particles was possible. The scattering curves of soot from oxygenated diesel show significant differences between samples obtained under idle and load conditions. Soot from regular diesel fuel did not show such differences.

A three-dimensional computational fluid dynamics code was used to analyse the performance of 550MW pulverized coal combustion opposite a wall-fired boiler (of IEC) at different operation modes. The main objective of this study was to prove that connecting plant measurements with three-dimensional furnace modelling is a cost-effective method for design, optimization and problem solving in power plant operation. Heat flux results from calculations were compared with measurements in the boiler and showed good agreement. Consequently, the code was used to study hydrodynamic aspects of air–flue gases mixing in the upper part of the boiler. It was demonstrated that effective mixing between flue gases and overfire air is of essential importance for CO reburning. From our complementary experimental-numerical effort, IEC considers a possibility to improve the boiler performance by replacing the existing OFA nozzles by those with higher penetration depth of the air jets, with the aim to ensure proper mixing to achieve better CO reburning. Copyright © 2004 John Wiley & Sons, Ltd.

A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oxidation of chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide ( αNO). The results confirm previous findings by other investigators that αNO decreases as the weight of char burned increases. αNO for coal was the same as (at 4% O2 )o r lower than (at 20% O2) that for an equal mass of char during oxidation. Since coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichiometry surrounding the particle is responsible for the observed reduction in αNO as sample size increases. The analysis of the exhaust gases showed increases in HCN concentration and a decrease in CO2/CO ratio as sample size increased, suggesting that local stoichiometry influences αNO. Additional experiments showed that αNO decreased as the background NO concentration was increased, at rates that diminished as the oxygen concentration increased, independent of particle size. The steep reduction in NO production as the background NO concentration increased was explained by the destruction of NO in the gas phase.

The effect of benzene concentration in the initial fuel on the evolution of soot size distribution in ethylene/air and ethylene/benzene/air flat flames was characterized by experimental measurements and model predictions of size and number concentration within the flames. Experimentally, a scanning mobility particle sizer was used to allow spatially resolved and online measurements of particle concentration and sizes in the nanometer-size range. The model couples a detailed kinetic scheme with a discrete-sectional approach to follow the transition from gas-phase to nascent particles and their coagulation to larger soot particles. The evolution of soot size distribution (experimental and modeled) in pure ethylene and ethylene flames doped with benzene showed a typical nucleation-sized (since particles do not actually nucleate in the classical sense particle inception is often used in place of nucleation) mode close to the burner surface, and a bimodal behavior at greater height above burner (HAB). However, major features were distinguished between the data sets. The growth of nucleation and agglomeration-sized particles was faster for ethylene/benzene/air flames, evidenced by the earlier presence of bimodality in these flames. The most significant changes in size distribution were attributed to an increase in benzene concentration in the initial fuel. However, these changes were more evident for high temperature flames. In agreement with the experimental data, the model also predicted the decrease of nucleation-sized particles in the postflame region for ethylene flames doped with benzene. This behavior was associated with the decrease of soot precursors after the main oxidation zone of the flames.

The condensed solids from three aerosols resulting from heating anthracene in a drop-tube furnace with a helium atmosphere at temperatures of 1250, 1300, and 1400 K were studied by carbon-13 solid-...

The effect of boundary layer reactions on the conversion of char-N to NO, N 2 O, and HCN was studied by a coupled experimental/theoretical approach. In the experimental part, the evolution of HCN at conditions of a batch fluidized-bed reactor was studied. The results show that the char-N conversion to HCN is very low under normal oxidizing conditions. This explains why it was not detected by early studies of this system. The results also suggest that the release of HCN is related to gases released when the char is oxidized rather than to late devolatilization reactions. The product distribution is modeled using heterogeneous reactions on the char surface coupled with homogeneous reactions in the gas phase. Three cases were analyzed. In Case A, only the heterogeneous production of N 2 O and NO was considered. In Case B, HCN and NO heterogeneous production was considered. Finally, Case C combined cases A and B by allowing the heterogeneous production of NO, N 2 O, and HCN. Cases A and B predict most of the ...

Pathways for the growth of high-molecular-mass compounds are presented, showing how reactions between aromatic moieties can explain recent experimental findings. A fundamental molecular analysis of polycyclic aromatic hydrocarbon growth processes in combustion systems involving five-membered ring compounds is presented using quantum mechanical density functional methods. Higher aromatics are produced through a two-step radical-molecule addition reaction and the iteration of this mechanism followed by rearrangement of the carbon framework ultimately leads to high-molecular-mass compounds. The distinguishing features of the proposed model lie in the chemical specificity of the routes considered. Naphthalene and acenaphthylene are used as examples of the aromatic and cyclopentafused aromatic classes of compounds postulated to be of importance in molecular weight growth. These reaction pathways are analyzed with a view toward explaining recent experimental findings on H/C ratio, NMR, and LMMS of soot precursors.

Jet A and JP-8 are kerosene fuels used in aviation and consist of complex mixtures of higher order hydrocarbons, including alkanes, cycloalkanes, and aromatic molecules. The objectives of the current work are to develop a surrogate mixture to represent JP-8 fuels and to discuss a general detailed chemical kinetic model for jet fuels, which is suitable for future reduction. Asurrogate blend of six pure hydrocarbons is found to adequately simulate the distillation and compositional characteristics of a practical JP-8. A hierarchically constructed kinetic model already available for the oxidation of alkanes and simple aromatic molecules (benzene, toluene, ethylbenzene, xylene, etc.) is extended to include methylcyclohexane and tetralin as new reference fuel components. The kinetic model is validated through comparisons with experimental data for the pure components and it is also used to verify and predict the structures of laminar premixed flames of different pure components as well as conventional kerosene...

There is increasing evidence that HCN is an important intermediate to the production of N2O during the oxidation of char nitrogen at fluidized bed combustion conditions. The experimental evidence shows that N2O is formed primarily during oxidation so that the fundamental question is on which fuel N intermediate is formed during char consumption. This paper models first the conversion of char nitrogen assumed to be released as HCN, simulating the homogeneous nitrogen chemistry in the boundary layer of a single char particle. This pathway, which is based on 180 reversible gas-phase reactions and 33 chemical species, assumes that HCN is released at a rate proportional to carbon consumption. The oxidation of HCN occurs in the boundary layer of the particle. An alternative pathway assumes that during oxidation, NO is formed in addition to a cyano species by heterogeneous processes and then both NO and the cyano species react in the boundary layer. The temporal evolution of NO, N2O, and cyano compounds from bot...

Accumulation of toxic trace elements generated by coal-fired power stations presents a serious threat to the environment. Field testing and laboratory studies have revealed the existence of trace elements in submicron particles emitted from power stations. Arsenic, selenium, and antimony are present in the submicron particles presumably via a vaporization-condensation pathway although the volatilities of these elements are very different. Previous explanations include the volatility and the forms of occurrence of elements in coals. Based on well-controlled experimental studies for selected coals, this paper establishes the first quantitative physicochemical model for vaporization of arsenic, selenium, and antimony during coal pyrolysis and combustion. Advanced characterization methods found that the three elements are associated with pyrites in the coals burned for this study. The vaporization processes for these three elements consist of three consecutive processes: transport of molecules or atoms through the bulk pyrite liquid (melt) to the melt/gas interface, vaporization of elements at the surface of melts, and transport of molecules/atoms through the pores of the char to the atmosphere. The controlling step for vaporization of arsenic is diffusion through the melt. Diffusion processes in the melt and within the char pores together determine the vaporization rates for selenium and antimony.

Formation pathways for high-molecular-mass compound growth are presented, showing why reactions between aromatic moieties are needed to explain recent experimental findings. These reactions are then analyzed by using quantum mechanical density functional methods. A sequence of chemical reactions between aromatic compounds (e.g., phenyl) and compounds containing conjugated double bonds (e.g., acenaphthylene) was studied in detail. The sequence begins with the H-abstraction from acenaphthylene to produce the corresponding radical, which then furnishes higher aromatics through either a two-step radical‐molecule reaction or a direct radical‐radical addition to another aromatic radical. Iteration of this mechanism followed by rearrangement of the carbon framework ultimately leads to high-molecular-mass compounds. This sequence can be repeated for the formation of high-molecular-mass compounds. The distinguishing features of the proposed model lie in the chemical specificity of the routes considered. The aromatic radical attacks the double bond of five-membered-ring polycyclic aromatic hydrocarbons. This involves specific compounds that are exceptional soot precursors as they form resonantly stabilized radical intermediates, relieving part of the large strain in the five-membered rings by formation of linear aggregates. © 2001 by The Combustion Institute

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...

The past decade has seen a dramatic increase in the use of computational fluid dynamics (CFD) in the solution of problems related to the design and operation of pulverized coal-fired utility boilers. These tools have been increasingly used to simulate the performance of utility boilers, primarily for NOx control and associated problems of unburned carbon in fly ash and in water-wall corrosion. These models are extended to calculate the emissions of submicron particles by the vaporization and condensation of ash constituents. A single particle model for the vaporization of minerals in coal was first calibrated with results for vaporization of 14 coals in a laboratory reactor. The calibrated model was then applied in the simulation of the ash vaporization in a 500-MW opposed-wall fired boiler with 12 burners on each of the front and rear walls, and for cases before and after retrofitting the boiler with burner technology to reduce NOx emissions. The simulations showed that the ash vaporization occurred primarily during short intervals along particle trajectories when the particle temperatures increased above 1800 K. The ash vaporization decreased slightly on retrofitting the boiler, and the contributions by different burners to the total amount vaporized varied widely, particularly after the low-NOx retrofit.

Pore structure of coal has a great influence on its behavior during mining, preparation, and utilization. Characterization of the pore structure of twelve Turkish coals from different geographic locations and with carbon contents varying between 61 and 84% (on dry ash-free basis) was carried out using different techniques. The volume and area of macropores were determined by mercury intrusion porosimetry. Mesopore volumes and areas were determined by N2 gas adsorption at 77 K using the Barrett−Joyner−Halenda (BJH) method. Brunauer−Emmet−Teller (BET) areas were calculated using the same data. Micropore volumes and areas were determined by the application of the Dubinin−Radushkevich (DR) equation to the CO2 adsorption data at 298 K. True and apparent densities of coals were measured by helium and mercury displacement. Pore size distributions were evaluated using data thus obtained. Small-angle X-ray scattering (SAXS) technique was also employed to determine the surface area of some samples. The highest BET ...

The contribution of nitrogen present in the char on the production of nitrogen oxides during char combustion was analyzed. A literature review summarizes the current understanding of the mechanisms that account for the formation of NO and N2O from the nitrogen present in char. The review focused on: (1) the functionalities in which nitrogen is present in the coal and how they evolve during coal devolatilization; (2) the mechanism of nitrogen release from the char to the homogeneous phase and its further oxidation to NO; and (3) the reduction of NO on the surface of the char. The critical analysis of these three issues allowed identification of uncertainties and well-founded conclusions observed in the literature for this system. The existing models for the production of nitrogen oxides from char-N were also reviewed. A critical analysis of the assumptions made in these models and how they affect the final predictions is presented. Finally, a simplified version of these models was used to perform a parametric analysis evaluating the impact of several parameters on the total conversion of char-N to NO. These parameters include: (1) the rate of NO reduction on the char surface; (2) the rate of carbon oxidation; and (3) early vs. late nitrogen release during the char oxidation process. The results underscore the importance of the reaction of NO reduction on the char surface to the final conversion of char-N to NO.

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. The carbon is bimodally distributed with large soot aggregates mixed with the char in the particles larger than 10 μm and dispersed soot found with the submicron particles. A method for determining the mass of soot and char by liquid-suspension gravity separation was used with 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 0.2–0.6% of the fuel carbon, which was about 35% of the total unburned carbon.

The emissions of trace elements and mercury from stationary combustion sources are determined by the occurrence of these elements in fuels, the transformation of the elements into vapor and particles in furnaces, and the ability of the vapors and particles to penetrate the air pollution control devices (APCDs). For electrostatic precipitators (ESPs), in use at greater than 90% of coal-fired utility boilers in the US, the preferential escape of particles is in the 0.1–1.0 μm size. The major source of particles in this size range is the vaporization and condensation of the inorganic constituents in the parent fuel. Potentially toxic elements, although mainly confined to the particulate phase, may therefore show enhanced release to the environment as a result of preferential condensation on the surface of submicron particulate matter. Surface reaction with larger fly ash particles can reduce these emissions by redistributing the trace elements away from the difficult-to-capture submicron particulate. In contrast, mercury is the most volatile of the trace elements in coal and its escape is near complete when the mercury is in the elemental vapor form. Mercury emissions may be mitigated, however, by transformation to mercuric chloride, more readily captured either in scrubbers or by collection in the particulate form. In this paper, we present our recent research developments contributing to an improved understanding of the relative importance of factors determining trace element emissions. Significant progress that has been made in understanding and quantifying each of the processes governing the transformation of the inorganic constituents of fuels leading to promising results on the prediction of emissions given detailed characterization of fuels, the combustion conditions to which they are exposed, and the characteristics of APCDs.

Trace elements in coal have diverse modes of occurrence that will greatly influence their behavior in many coal utilization processes. Mode of occurrence is important in determining the partitioning during coal cleaning by conventional processes, the susceptibility to oxidation upon exposure to air, as well as the changes in physical properties upon heating. In this study, three complementary methods were used to determine the concentrations and chemical states of trace elements in pulverized samples of four US coals: Pittsburgh, Illinois No. 6, Elkhorn and Hazard, and Wyodak coals. Neutron Activation Analysis (NAA) was used to measure the absolute concentration of elements in the parent coals and in the size- and density-fractionated samples. Chemical leaching and X-ray absorption fine structure (XAFS) spectroscopy were used to provide information on the form of occurrence of an element in the parent coals. The composition differences between size-segregated coal samples of different density mainly reflect the large density difference between minerals, especially pyrite, and the organic portion of the coal. The heavy density fractions are therefore enriched in pyrite and the elements associated with pyrite, as also shown by the leaching and XAFS methods. Nearly all the As is associated with pyrite in the three bituminous coals studied. The sub-bituminous coal has a very low content of pyrite and arsenic; in this coal arsenic appears to be primarily organically associated. Selenium is mainly associated with pyrite in the bituminous coal samples. In two bituminous coal samples, zinc is mostly in the form of ZnS or associated with pyrite, whereas it appears to be associated with other minerals in the other two coals. Zinc is also the only trace element studied that is significantly more concentrated in the smaller (45 to 63 μm) coal particles.

Because mercury enters the food chain primarily through atmospheric deposition, exposure models require accurate information about mercury emission rates and mercury speciation from point sources. Since coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere, the speciation of mercury in coal-fired power plant flue gas is currently an active topic of research. We have demonstrated that the assumption of gas-phase equilibrium for mercury-containing species in coal-fired power plant exhaust is not valid at temperatures below approximately 800 K (500°C). Chlorine-containing species have been shown to be the most important for oxidation of elemental mercury in the post-combustion gases. The conversion of HCl to Cl2 in the flue gas of a coal-fired power plant is kinetically limited. Kinetic calculations of the homogeneous oxidation of elemental mercury by chlorine-containing species were carried out using global reactions from the literature. The levels of mercury oxidation, while of comparable magnitude to field observations, are still below the 40% to 80% oxidation typically observed in field measurements.

The carbonation rate of 15−20 μm, nonporous, calcium oxide crystals has been studied over a temperature range of 550−1100 °C and a CO2 pressure range of 1−11.7 atm. At temperatures greater than 600 °C, the carbonation rate decreases more rapidly with time than would be expected from diffusion through a uniform product layer and the activation energy is initially low but increases with conversion. The product layer consists of crystalline grains, and these product layer grains grow by coalescence from less than one μm diameter to the approximate dimension of the particle. The carbonation rate can be described by a model where CO2 pressure-independent grain boundary diffusion and diffusion through the carbonate crystals act in parallel. The relative importance of bulk diffusion through the product layer crystals increases with time relative to transport through the grain boundaries and has an effective activation energy of 57 kcal/mol. Diffusion through the crystal boundaries has low activation energy and l...

The evolution during oxidation of the internal structure of soot and carbon black particles was studied using high-resolution transmission electron microscopy (HRTEM) and an image analysis system. Increasing ordering to the carbon structure with increasing oxidation is observed. Measures of the increased order are increases in the fractional coverage of a cross section of the particles with a layered structure, a decrease in the mean interlayer spacing, and a decrease in the spread of the interlayer spacing. The changing structure of the carbon black impacts its properties, such as the rate of oxidation.

A model is developed for CO oxidation in the boundary layer of a single char particle. The model includes char oxidation and a 56 reaction gas phase kinetic scheme which is coupled with the diffusive properties of the 12 species involved in the CO oxidation. The model is compared with the experimental data of Tognotti and co-workers. The temperature reached on ignition, the resulting CO 2 /CO ratio, and the effect of changing water vapor concentrations are well described. Studies on the effect of water concentration show that significant CO oxidation at low temperatures requires a high surrounding water concentration. The presence of water vapor or hydrogen is found to be necessary to obtain a high degree of CO oxidation at low temperatures; however, there is a minimum temperature below which significant CO oxidation in the boundary layer does not occur irrespective of how high the water concentration is. In addition, CO oxidation is negligible even at a surface temperature as high as 2500 K when water and hydrogen are absent. The model has also been applied to predict the effects of changing parameters. Catalytic acceleration of the rate of carbon oxidation, for example by the addition of calcium, leads to both a high particle temperature overshoot and a significant increase in CO oxidation over the particle surface.

The paper provides an indirect evidence that HCN is not released during char oxidation since the devolatilization times do not affect the amount of char bound nitrogen converted to NO or N 2 O. Consequently, the nitric and nitrous oxides produced from the combustion of char nitrogen are not formed from the homogeneous oxidation reactions of HCN

The relative abundances of the six isomeric naphthyl-anthracenes, six isomeric bianthracenes and three isomeric binaphthalenes, as well as of the total secondary products have been measured for the pyrolysis of a 1 : 1 mole ratio mixture of anthracene and naphthalene. A residence time of 2 seconds and a temperature range of 1180−1500 K is used, and similar pyrolysis experiments are also performed using pure anthracene and pure naphthalene as fuels. Several polyarene products consisting of naphthyl and anthryl components have also been observed in chains up to at least 6 units long. The suggestion is made that thermodynamic equilibrium is not the governing factor in polyarene formation (a notion previously presented by Mulholland et al., 1993) as evidenced by the following observations. First, the relative abundances of the primary biarene products are found to conform roughly to those that can be calculated using simple kineticlstatistical and steric hindrance considerations. Second, the total ab...

The effects S02 and NO concentrations on the emissions of N20 and NO from a single particle of coal burning in a fluidized bed combustor is reported. The experiments were carried out in a laboratory scale quartz glass fluidized bed reactor in the temperature range 1023-1123 K. Since the combustion processes for the volatiles and char are consecutive, the effects of added NO and SO2 occurring during the volatiles combustion can be distinguished from those occurring during the subsequent char combustion. During the combustion of the volatiles, addition of NO (250 ppm) results in an increased formation of N20, whereas the net NO formation decreases. This effect is enhanced when both NO (250 ppm) and SO2 (900 ppm) are included in the fluidizing gas mixture. The effect of NO can be explained by the generally accepted gas phase mechanism of NO and N2O formation and destruction. The role of SO2 is less clear, but is likely to involve its impact on the radical concentrations. During the char combustion s...

Aromatics growth beyond 2-, 3-ring PAH is analyzed through a radical-molecule reaction mechanism which, in combination with a previously developed PAH model, is able to predict the size distribution of aromatic structures formed in rich premixed flames of ethylene at atmospheric pressure with C/O ratios across the soot threshold limit. Modeling results are in good agreement with experimental data and are used to interpret the ultraviolet absorption and the light scattering measured in flames before soot inception. The model shows that the total number concentration of high molecular mass aromatics and the different moments of the size distribution are functions of both the PAH and H-atom concentrations, two quantities which have different trends as functions of the residence time and the C/O ratio. Regimes of nearly stoichiometric or slightly rich premixed combustion are dominated by reactions between aromatics which lead to the formation of particles with sizes of the order of 3 to 4 nm. At higher C/O ratios the formation of nanoparticles is less efficient. Particles with sizes of the order of 2 nm are predicted in flames at the threshold of soot formation, whereas particles with sizes around 1 to 1.5 nm are predicted in fully sooting conditions.

A turbulent reacting flow model has been developed, based upon a stochastic modeling approach that is applicable over a wide range of Reynolds and Schmidt numbers. All relevant length scales are resolved in one dimension which allows for direct implementation of detailed chemical kinetic calculations and molecular transport. Experiments were performed in a turbulent plug flow reactor for validation and application of the model to investigate the combined effects of turbulent mixing and chlorine chemistry on the formation of products of incomplete combustion. Methyl chloride was injected into the plug flow reactor whose baseline flow was the hot-product stream of a Toroidal Jet-Stirred Combustor. The micromixing and chemical reaction of the injected material with the baseline flow was monitored as a function of distance from the point of injection. Measurements of pulsed laser Rayleigh scattering were used to determine the extent of mixing while species concentrations were measured via extractive ...

The amount and composition of products of incomplete combustion (PICs) in combustion systems is controlled by the complex interactions of turbulent mixing and chemical kinetics. The current paper addresses this problem by focusing on the effects of incomplete turbulent mixing on the extent of reaction and on the distribution of the local equivalence ratio, and investigating the contributions of chlorocarbon inhibition of chemical reaction to the emission of PICs. The experimental facility has been designed to simulate the conditions in actual incinerator systems; it consists of a Toroidal Jet Stirred Reactor (TJSC) followed by a Plug Flow Reactor (PFR). The experimental conditions allow one to investigate the combined effects of turbulent mixing and chemical kinetic inhibition on PIC emissions. To address these issues methyl chloride is injected into a baseline flow of hot products flowing in the PFR. Instantaneous temperature measurement, obtained by application of a laser Rayleigh scattering diagnostic, and stable species concentrations are then measured at various distances from the point of injection. The temperature probability density functions (pdfs) for fuel-lean and fuel-rich conditions show evidence of mixing constraints. Stable species concentration measurements prove that both mixing and chemical kinetic constraints are present in the PFR environment. Detection of aromatic species is interpreted as an effect of the variation of the local value of the equivalence ratio due to imperfect mixing. Finally, in order to model the combined chemical kinetic and mixing constraints and to determine which constraints are responsible for PIC emission, results from a highly simplified turbulent reacting flow model are presented. The model can predict the trend and magnitude of methyl chloride burnout and CO CO 2 ratio. Species, such as aromatic compounds, having a strong non-linear dependence on the equivalence ratio can provide insights on the mixing history and might serve as diagnostics of failure modes in incinerators.

The conditions that favor the formation of nitrous oxide from char under fluidized bed combustion conditions are shown to be the oxidation of the char to make the nitrogen bound in the heterocyclic rings accessible so that it can react heterogeneously either with oxygen to form NO or with nitric oxide to form N2O. This was demonstrated by measurement of the gas composition when a batch of coal particles was introduced into a fluidized sand bed electrically heated to 1023 K. Oxygen and nitric oxide were added to the fluidizing helium gas in varying concentrations. N2O was formed in amounts that increased with increasing NO concentration showing the importance of NO for N2O formation. The N2O concentration, however, fell to zero when the O2 supply was interrupted underlying the essential role of oxygen in freeing the organically bound nitrogen so that it can react with NO to form N2O.

The changes in reaction rate of a carbon char with conversion in the temperature range 500–1200 K are followed by using an electrodynamic balance (EDB). This device allows, in a temperature range wider than in other apparatus, in situ measurements of mass, diameter, density, surface area, rate of reaction, and temperature for a single, suspended submillimeter particle as the reaction takes place. A synthetic char (Spherocarb) has been used because of its low ash and volatile matter contents and its spherical shape. The results on oxidation kinetics obtained in an EDB are compared with low-temperature reaction rate data, obtained by conventional thermogravimetric apparatus on Spherocarb, and with the data obtained in an entrained-flow reactor at temperatures typical of pulverized-fuels combustors. The correlation between all the data in the temperature range 500–2300 K is examined. EDB data can be combined with entrained-flow data to depict a sharp change in reaction regime from a chemical kinetic to an internal diffusion control. A simple Thiele analysis, considering different classes of pores, yields results that suggest pores in the 100–1000 nm range are responsible for diffusional constraints that cause the kinetic data to depart from the chemical regime.

The conversion of fuel nitrogen to NO and N 2 O have been determined in a small-scale fluidized bed. Small batches of coal particles were used, permitting the separation of the formation reactions within individual particles from subsequent destruction or re-formation on other particles. From time-resolved measurements of the concentrations of NO, N 2 O, CO 2 , CO, and CH 4 , the instantaneous fractional conversions of coal nitrogen to NO and N 2 O as a function of fractional char burnout were obtained for bed temperatures between 975 and 1148 K. The conversion to N 2 O decreases with increasing temperature, whereas that to NO erhibits a maximum between 1023 and 1095 K. As a particle burns out, the instantaneous conversion to N 2 O decreases, whereas the reversed trend is seen for NO

The pyrolysis of pure anthracene at temperatures between 1200 and 1500 K produced all six bianthryl isomers whose relative yields appear to be related to steric factors. To evaluate the hypothesis that thermodynamic factors govern the product distribution of bianthryls in this system, the relative enthalpies and entropies of biaryl isomers were estimated by molecular orbital modeling, using the semiempirical AM1 (Austin Model 1). Computational analysis of several isomer sets demonstrates that the relative stabilities of a large number of biaryl isomers are determined largely by steric interactions caused by structural features defined as bays, coves, and fjords. These steric factors affect both the degree of biaryl twist in the preferred conformation and the freedom of internal rotation. Molecular orbital modeling supports the hypothesis that a thermodynamic distribution of bianthryl isomers is produced by anthracene pyrolysis.