Computer-controlled Scanning Electron Microscopy Analysis Research Articles

Effects of co-combustion Fe-rich mediocre ash coal on slag deposition of Ca-rich Zhundong coal in a 0.4 MW pilot-scale facility

A type of coal with low calorific value and high ash content, locally known as mediocre ash coal (MAC), has been discovered in the interlayer of stratified coals in Jiang Jun Temple area Zhundong district, Xinjiang, China. In order to investigate slag deposition on the convective heating surface during co-combustion of MAC and Wucaiwan Ca-rich Zhundong Coal (WCW), a 0.4 MW pilot-scale facility, with tube panels #1 and #2 located in horizontal flue gas ducts, was constructed. The tube panels were designed to simulate the superheater and reheater of the Π-type boiler. FactSage was utilized to simulate the material balance, whereas the classic models were employed to calculate the slag viscosity. Combustion tests were conducted while maintaining flue gas temperatures (FGT) of 1050∼1100 °C and 900∼950 °C at tube panels #1 and #2, respectively, which were consistent with those observed during sole combustion of WCW. The slags collected from tube panels were analyzed by X-ray fluorescence, X-ray diffraction and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Detector. Furthermore, Computer-Controlled Scanning Electron Microscopy analysis was used to obtain a quantitative characterization of the symbiotic relationship among mineral elements. The study of co-combustion MAC led to the following conclusions: The liquid phase ratio of slag decreased by 44% compared to sole combustion of WCW at 1100 °C. The viscosity was higher than that of sole combustion of WCW from 900 to 1650 °C. Compared to the sole combustion of WCW, the average S, Ca, and Na contents in the slag obtained on the two tube panels decreased by more than 85%, 40%, and 43.5% respectively. Sulfate formation in the slag on the tube panel was significantly reduced at medium to low FGT, and no Na2SO4 and CaSO4 compounds could be detected in the slag. The interaction between Fe-bearing minerals and Si- and Al-bearing minerals during combustion was evidently stronger than that between Ca-bearing minerals and Si- and Al-bearing minerals during combustion. The interaction of Fe with Ca/Na during medium to high FGTs led to a decrease in Si/ Al consumption, thereby increasing their availability for reacting with Ca/Na at medium to low FGTs. As a result, the formation of Na2SO4 and CaSO4 was inhibited.

Effects of torrefaction and water-washing on the mineral transformation behavior during co-combustion of straw and coal: A CCSEM analysis

Biomass can be torrefied to improve the fuel quality prior to its utilization. In addition, a large number of soluble alkali metals can be removed by water washing, to solve slagging and other ash-related problems. In this study, computer-controlled scanning electron microscopy (CCSEM) was used to analyze the mineral composition and elemental distribution of straw, torrefied straw char, and torrefied-water washed straw char. The mineral transformation behavior and the distribution of main elements, when these biomass species were individually co-combusted with coal, were also studied in detail. The comparison of the mineral composition of bulk ash obtained from torrefied straw combustion and straw combustion revealed that the quartz phase decreased and the aluminosilicates of K-, Na-, and Ca–Fe–K increased in the former compared to that in the latter. After the torrefied straw was washed with water, the quartz and Si-Rich phases increased. K aluminosilicate decreased and the quartz phase increased when the torrefied sample was co-combusted with coal at both mixing ratios of 1:1 and 1:4 compared to straw/coal co-combustion. When the torrefied-water washing sample and coal were co-combusted at a 1:1 ratio, the quartz phase decreased, and the Si-Rich and mullite phases increased. When the straw/torrefied char/torrefied-washed char was co-fired with coal at the 1:4 ratio, the amount of K aluminosilicate was less compared to that observed at the 1:1 ratio. It can be concluded that both torrefaction and torrefaction-washing can reduce the content of low-melting-point aluminosilicates in the mineral composition of the studied biomass after co-combustion, which can be beneficial to alleviate the ash slagging problem.

Occurrence and characteristics of abundant fine included mineral particles in Collie coal of Western Australia

Collie coal is the only coal being mined in Western Australia and plays an important role in supplying cheap energy to the State’s energy mix. This study reports a systematic investigation into the occurrence and characteristics of the fine mineral matter in Collie coal that is crushed to the size of <212 µm and density separated. The results show that 94.1 wt% of the mineral matter in the whole Collie coal is present in the three density fractions of 1.2–1.4, 1.4–1.6 and ≥2.0, including 26.8 and 58.6 wt% as included mineral matter in two density fractions with specific gravities of 1.2–1.4 and 1.4–1.6, and 8.7 wt% as excluded mineral matter in the density fraction of ≥2.0. Further analysis of these key Collie coal density fractions using computer-controlled scanning electron microscopy (CCSEM) shows that 54.13 wt% of the total mineral matter in the whole Collie coal are fine mineral particles of sizes <10 μm. These included fine mineral particles (<10 μm) are distributed as 20%:80% in the two coal density fractions of 1.2–1.4 and 1.4–1.6, which contribute to 10.64 and 43.49 wt% of the total mineral matter in the whole coal, respectively. CCSEM analysis also shows that the key minerals in the fine mineral particles are quartz, kaolinite, K-, Ca- or Fe-Al silicates and pyrite-related minerals. However, large proportions of these included fine mineral particles have “unclassified” mineral phases. Such unclassified particles contribute to 43.88 and 36.84 wt% of the fine mineral particles with size <10 µm present in the coal density fractions of 1.2–1.4 and 1.4–1.6 (equivalent to 4.67 and 16.02 wt% of the total mineral matter in the whole coal), respectively. The chemistry data show that these individual fine mineral particles (<10 µm) contain mixed compositions of Si + Al, Fe + Ti and other elements such as Ca, K, S and P, suggesting that one single fine mineral particle contains multiple mineral phases.

Enhancing the interpretation of in vitro bioaccessibility data by using computer controlled scanning electron microscopy (CCSEM) at the individual particle level

The adverse health effects resulting from exposure to contaminated soil on internally displaced populations in Mitrovica, Kosovo can be determined by how the potentially harmful elements are bound in the soils. Certainly this was the case for Pb, present at concentrations ranging from 624 to 46,900 mg/kg, and at bioaccessibilities ranging <5% to nearly 90%. To assess why the soil Pb might differ so markedly in terms of its bioaccessibility, computer controlled scanning electron microscopy (CCSEM) was employed to determine how the Pb was associated with other elements at the individual particle (IP) level in soils from the area. It was found that the Pb-bearing particle types were, for the most part, different in each sample. We consider these differences as the main control on Pb bioaccessibility in these soils. Pb solubility at the IP level was evaluated by examining Pb-particles from these soils in the electron microscope before and after successive immersions in a simulated gastric fluid. This analysis (differential IP analysis) confirmed the CCSEM characterization that Pb associated with other higher atomic number elements (Fe, Zn, Cu and Ni) was less soluble than when it was present as isolated phases (e.g., as carbonate) or when it was bound with lower atomic number elements (Na, Al, Si, K, Ca). The heterogeneity in solubility and composition of the Pb-particles suggested that the Pb originated from a range of different anthropogenic activities. The nature of these different anthropogenic activities created the wide differences in Pb-bioaccessibilty by producing Pb bound in many different forms in the soil particles. This type of Pb-particle characterization highlights the role CCSEM analysis, and IP acid extraction, can play in providing supporting evidence alongside bioaccessibility data for applications in human health risk assessment and management of contaminated soil.

Ash transformation by co-firing of coal with high ratios of woody biomass and effect on slagging propensity

The co-firing of coal with biomass is a promising method for reducing net CO2 emissions from existing coal-fired power plants, as well as for the utilization of forest resources. This present study examined the effect of the co-firing of coal with woody biomass on the produced combustion ash and slagging propensity under different conditions representative of the wall furnace region of pulverized-coal boilers. The slagging tests were conducted in a drop-tube furnace by inserting a water–air-cooled deposition probe to the point where the inner furnace temperature was 1300°C. Bituminous coal was mixed with up to 70% (energy basis) of four different Japanese woody biomasses, namely; sakura wood (Prunus spp.), sugi (Cryptomeria japonica), nara (Japanese oak), and bark of sugi respectively. For comparison purposes, pure coal firing was also performed. The collected combustion ashes and ash deposits were characterized by computer-controlled scanning electron microscopy analysis while the slagging propensity was evaluated by determining the ratio of the deposited ash to the fuel ash, the so-called ash deposition ratio. The results showed no increase in the ash deposition ratio or significant change in the properties of the ash for co-firing using up to 50% of the low-ash (0.4wt%) biomasses (sugi and sakura). In contrast, there was significant change in the properties of the ash and an increase in the ash deposition ratio with increasing biomass ratio for co-firing using the high-ash (>1.0wt%) biomasses (nara and bark of sugi). The significant transformation of ash with regard to its morphology and chemical composition, together with the formation of eutectic calcium mineral mixtures in the deposit layer enhanced the slagging propensity during the co-firing process, particularly when using the high-ash biomasses. The use of a low-ash biomass was found to be suitable for preventing severe slagging during the co-firing process.

Heterogeneity of Coarse Particles in an Urban Area

The variation in composition and concentration of coarse particles in Rochester, a medium-sized city in western New York, was studied using UNC passive samplers and computer-controlled scanning electron microscopy (CCSEM). The samplers were deployed in a 5 × 5 grid (2 km × 2 km per grid cell) for 2-3 week periods in two seasons (September 2008 and May 2009) at 25 different sites across Rochester. CCSEM analysis yielded size and elemental composition for individual particles and analyzed more than 800 coarse particles per sample. Based on the composition as reflected in the fluoresced X-ray spectrum, the particles were grouped into classes with similar chemical compositions using an adaptive resonance theory (ART) network. The mass fractions of particles in the identified classes were then used to assess the homogeneity of composition and concentration across the measurement domain. These results illustrate how particle sampling using the UNC passive sampler coupled with CCSEM/ART can be used to determine the concentration and source of the coarse particulate matter at multiple sites. The particle compositions were dominated by elements suggesting that the major particle sources are road dust and biological particles. Considerable heterogeneity in both composition and concentration were observed between adjacent sites as indicated by cofficient of divergence analyses.

Characterization of Ash Deposition and Heat Transfer Behavior of Coals during Combustion in a Pilot-Scale Facility and Full-Scale Utility

Experimental measurements as well as theoretical models were used to investigate the impact of mineral matter of three coals on ash deposition and heat transfer for pulverized coal fired boilers. The ash deposition experiments were conducted in a pulverized fuel combustion pilot-scale facility and a full-scale unit. A mathematical model with input from computer-controlled scanning electron microscopy analysis of coal minerals was used to predict the effect of ash deposition on heat transfer. The predicted deposit thickness and heat flux from the model are shown to be consistent with the measurements in the test facility. The model differentiates the coals according to the deposits they form and their effect on heat transfer. The heat transfer predictions in the full-scale unit were found to be most suitable for the water wall under the furnace nose. The study demonstrates that the measurements in a full-scale unit can differ significantly from those in pilot-scale furnaces due to soot-blowing operations.

Determination of index of refraction and size of supermicrometer particles from light scattering measurements at two angles

A twin angle optical particle counter (TAOPC) that measures forward scattering at 40° and 74° was developed to determine the index of refraction of atmospheric particles. An error analysis indicates that measurement uncertainties for size at 40° are between 4 and 10% for particles &gt;1.5 μm and between 5 and 15% at 74°. For the index of refraction determination, the measurement uncertainties lead to index of refraction uncertainties between 1 and 2.5%. The instrument was tested on nonabsorbing spherical particles of known composition and size in the laboratory. The majority of the estimated indices of refraction were within ±1% of the expected indices, and size determination was within acceptable error. The instrument was also tested on non‐spherical absorbing particles to determine the complex index of refraction of ambient mineral dust particles collected in Laramie, Wyoming, in February 2006. The index of refraction was determined with the particle number ratio approach and was estimated to be in the range 1.60–1.67 for the real part and 0.009–0.0104 for the imaginary part. Simultaneously with the TAOPC measurements, particles were collected on polycarbonate filters for computer‐controlled scanning electron microscopy (CCSEM) analysis. Index of refraction calculated from this analysis was in the range 1.61–1.66 for the real part and 0.008–0.012 for the imaginary part. Particles were also collected for longer periods on two different filter pack systems in February 2006. Estimates of index of refraction from these measurements compared well with the CCSEM analysis.

Fully automated analysis of grain chemistry, size and morphology by CCSEM: examples from cement produc tion and diamond exploration

Computer-controlled scanning electron microscopy (CCSEM) combines the advantages of energy dispersive X-ray spectrometry (EDX) with those of digital image analysis of back-scattered electron (BSE) micrographs. CCSEM analysis of a wide range of geological or non-geological materials has been introduced at the Geological Survey of Denmark and Greenland (GEUS) as a fast and reliable method to determine both the chemistry of individual grains and bulk samples. The chemical analysis is combined with measurements of the two-dimensional size and morphology of every single grain. The CCSEM technique was developed in the early 1980s for characterisation of coal minerals (Huggins et al. 1980; Lee &amp; Kelly 1980) and studies of synthetic crystals for super-conductors and catalysts (Lin &amp; Barnes 1984). Soon it found a broader application in the study of dust particles and fibres in lung tissue of mine workers (Friedrichs 1987), in the analyses of aerosols for air quality control and source emission characterisation (e.g. Heasman &amp; Watt 1989) and the degree of sintering and consolidation of coal ash deposits (e.g. Huffman et al. 1994). CCSEM has been used in the earth sciences for the determination of the sediment budget of a lake (Yin &amp; Johnson 1984), for the characterisation of soil and dust (Pirrie et al. 2004), for provenance analysis of ilmenite-bearing beach sands (Knudsen et al. 2005; Bernstein et al. 2008), and provenance studies on sandstones in oil-bearing basins (Frei et al. 2005). Other areas where CCSEM has been applied range widely and include characterisation of small inclusions, e.g. impurities in metal alloys or steel (Schwoeble et al. 1988), analyses of gun-shot residues (e.g. Steffen et al. 2007), and analyses of bladder stones obtained from a skeleton found in a Mesolithic cave-tomb (D’Alessio et al. 2005). In this paper, we demonstrate the benefits of the method with examples from the cement industry and from diamond prospecting.

Differences in estimates of size distribution of beryllium powder materials using phase contrast microscopy, scanning electron microscopy, and liquid suspension counter techniques

Accurate characterization of the physicochemical properties of aerosols generated for inhalation toxicology studies is essential for obtaining meaningful results. Great emphasis must also be placed on characterizing particle properties of materials as administered in inhalation studies. Thus, research is needed to identify a suite of techniques capable of characterizing the multiple particle properties (i.e., size, mass, surface area, number) of a material that may influence toxicity. The purpose of this study was to characterize the morphology and investigate the size distribution of a model toxicant, beryllium. Beryllium metal, oxides, and alloy particles were aerodynamically size-separated using an aerosol cyclone, imaged dry using scanning electron microscopy (SEM), then characterized using phase contrast microscopy (PCM), a liquid suspension particle counter (LPC), and computer-controlled SEM (CCSEM). Beryllium metal powder was compact with smaller sub-micrometer size particles attached to the surface of larger particles, whereas the beryllium oxides and alloy particles were clusters of primary particles. As expected, the geometric mean (GM) diameter of metal powder determined using PCM decreased with aerodynamic size, but when suspended in liquid for LPC or CCSEM analysis, the GM diameter decreased by a factor of two (p < 0.001). This observation suggested that the smaller submicrometer size particles attached to the surface of larger particles and/or particle agglomerates detach in liquid, thereby shifting the particle size distribution downward. The GM diameters of the oxide materials were similar regardless of sizing technique, but observed differences were generally significant (p < 0.001). For oxides, aerodynamic cluster size will dictate deposition in the lung, but primary particle size may influence biological activity. The GM diameter of alloy particles determined using PCM became smaller with decreasing aerodynamic size fraction; however, when suspended in liquid for CCSEM and LPC analyses, GM particle size decreased by a factor of two (p < 0.001) suggesting that alloy particles detach in liquid. Detachment of particles in liquid could have significance for the expected versus actual size (and number) distribution of aerosol delivered to an exposure subject. Thus, a suite of complimentary analytical techniques may be necessary for estimating size distribution. Consideration should be given to thoroughly understanding the influence of any liquid vehicle which may alter the expected aerosol size distribution.

Anthropogenic vanadium-chromium-iron and cerium-lanthanum-iron particles in settled urban house dust: CCSEM identification and analysis

There is continuing concern over the health threat posed by theinhalation of ambient, transition-metal-containing particulatematter. Here, we report on the collection of two types ofanthropogenic, airborne particles, composed of iron with vanadiumand chromium (FeVCr) and iron with the rare earth metals ceriumand lanthanum (FeCeLa), respectively. Bi-weekly deposition platesamples were collected on indoor window sills of residentiallocations in inner-city Syracuse, NY. Sampling was continuous ateach location for two months during the spring of 1999. Themetal-bearing particles were identified, analyzed, and sizedin situ on each deposition plate using computer controlledscanning electron microscopy (CCSEM). Approximately 17% of thoseanalyzed by CCSEM fell into the inhalable size fraction. Particle loading varied between locations and changed over time.This was probably a function of various factors affectingparticle deposition rates. Notwithstanding, the plate loadingsfor the FeVCr-bearing particles tended to peak during the middleof the sampling period, suggesting that a single source/processwas operating to produce the FeVCr particles. The large sizeattained by some of these particles (greater than 50 μm inaggregates) suggest that long range transport was probably notinvolved.

Evaluation of Computer-Controlled Scanning Electron Microscopy Applied to an Ambient Urban Aerosol Sample

Concerns about the environmental and public health effects of particulate matter (PM) have stimulated interest in analytical techniques capable of measuring the size and chemical composition of individual aerosol particles. Computer-controlled scanning electron microscopy (CCSEM) coupled with energy-dispersive X-ray analysis (EDX) allows automated analysis of particle size, chemistry, and particle classification. In combination with manual SEM and bulk analytical techniques such as X-ray fluorescence, CCSEM can be a valuable tool for characterizing individual ambient particles and determining sources of ambient PM. The goal of this study was to examine several issues related to the quality and validity of CCSEM data. These included the stability of unattended CCSEM for multihour runs, the number of particles that must be analyzed in order to yield representative results, and errors associated with CCSEM. CCSEM was applied to the analysis of a 24 h ambient particle sample collected in Baltimore, MD. The coarse-fraction sample (PM10-2.5) was collected with a dichotomous sampler on a polycarbonate filter. A total of 2819 particles in 78 randomly selected fields of view were analyzed by CCSEM during an unattended 8 h run. Particle diameter, aspect ratio, particle location, X-ray counts for 20 elements, and digital images of each particle and its field of view were stored. The average number of particles per field (N/F), average particle diameter (Dave), average mass loading per field (Mave), and average particle composition were calculated for subsets of the data and compared against results for the full data set in order to assess the stability of the CCSEM analysis over time and the number of particles needed to obtain representative results. These comparisons demonstrated excellent stability of CCSEM over the 8 h run. Physical properties (represented by N/F, Dave, and Mave) of the sample were well characterized by analyzing approximately 360 particles. Chemical properties of the sample (average elemental composition and major chemical class abundances) converged to within a few percent of their final values after analyzing about 1000 particles. However, for many purposes several hundred particles may provide adequate characterization. Convergence of minor class abundances was limited by statistical fluctuations as the number of particles populating a class became very small. Manual review of the CCSEM data identified errors associated with CCSEM due to missed particles, overlapping particles, contrast artifacts, sizing errors, and heterogeneous particles. Most errors could be corrected or eliminated during manual off-line review of the data or avoided by maintaining a proper particle loading on the filter.

Evaluation of Computer-Controlled Scanning Electron Microscopy Applied to an Ambient Urban Aerosol Sample

Concerns about the environmental and public health effects of particulate matter (PM) have stimulated interest in analytical techniques capable of measuring the size and chemical composition of individual aerosol particles. Computer-controlled scanning electron microscopy (CCSEM) coupled with energy-dispersive X-ray analysis (EDX) allows automated analysis of particle size, chemistry, and particle classification. In combination with manual SEM and bulk analytical techniques such as X-ray fluorescence, CCSEM can be a valuable tool for characterizing individual ambient particles and determining sources of ambient PM. The goal of this study was to examine several issues related to the quality and validity of CCSEM data. These included the stability of unattended CCSEM for multihour runs, the number of particles that must be analyzed in order to yield representative results, and errors associated with CCSEM. CCSEM was applied to the analysis of a 24 h ambient particle sample collected in Baltimore, MD. The coarse-fraction sample (PM10-2.5) was collected with a dichotomous sampler on a polycarbonate filter. A total of 2819 particles in 78 randomly selected fields of view were analyzed by CCSEM during an unattended 8 h run. Particle diameter, aspect ratio, particle location, X-ray counts for 20 elements, and digital images of each particle and its field of view were stored. The average number of particles per field (N/F), average particle diameter (Dave), average mass loading per field (Mave), and average particle composition were calculated for subsets of the data and compared against results for the full data set in order to assess the stability of the CCSEM analysis over time and the number of particles needed to obtain representative results. These comparisons demonstrated excellent stability of CCSEM over the 8 h run. Physical properties (represented by N/F, Dave, and Mave) of the sample were well characterized by analyzing approximately 360 particles. Chemical properties of the sample (average elemental composition and major chemical class abundances) converged to within a few percent of their final values after analyzing about 1000 particles. However, for many purposes several hundred particles may provide adequate characterization. Convergence of minor class abundances was limited by statistical fluctuations as the number of particles populating a class became very small. Manual review of the CCSEM data identified errors associated with CCSEM due to missed particles, overlapping particles, contrast artifacts, sizing errors, and heterogeneous particles. Most errors could be corrected or eliminated during manual off-line review of the data or avoided by maintaining a proper particle loading on the filter.

Classification of single particles by neural networks based on the computer-controlled scanning electron microscopy data

The identification of sources of particles found in chemical process equipment such as photographic printer cabinets and their quantitative apportionment to those sources could lead to effective control strategies that would improve productivity and customer satisfaction with the service. Computer-controlled scanning electron microscopy (CCSEM) has proven to be a powerful tool in the characterization of individual particles. Thus, in this paper the samples of particles taken from experiments examining the formation of particles when cutting photographic paper and particles collected in a printer cabinet have been characterized by using CCSEM analysis. The obtained data have been analyzed by using two different neural networks, namely, the adaptive resonance theory based neural network (ART-2a) and Kohonen neural network. Both neural networks can be used to perform an unsupervised pattern recognition examination of which particles should be grouped together. The results show that they are generally able to extract the main particle groups present in the data set. The produced particle groups are almost homogeneous based on the major chemical elements. From the general size, shape and density parameters provided by the CCSEM analysis, the volume and mass of each particle were estimated. Then the mass fractions of each particle class produced by the neural networks were calculated. Based on the mass conservation principle and the resulting mass balance, the particle class balance model has been used to discern particles from different sources and apportion the corresponding source contributions.

97/02183 Some investigations on heat transfer in a hot circulating fluidized bed

The co-firing of coal with biomass is a promising method for reducing net CO2 emissions from existing coal-fired power plants, as well as for the utilization of forest resources. This present study examined the effect of the co-firing of coal with woody biomass on the produced combustion ash and slagging propensity under different conditions representative of the wall furnace region of pulverized-coal boilers. The slagging tests were conducted in a drop-tube furnace by inserting a water–air-cooled deposition probe to the point where the inner furnace temperature was 1300 °C. Bituminous coal was mixed with up to 70% (energy basis) of four different Japanese woody biomasses, namely; sakura wood (Prunus spp.), sugi (Cryptomeria japonica), nara (Japanese oak), and bark of sugi respectively. For comparison purposes, pure coal firing was also performed. The collected combustion ashes and ash deposits were characterized by computer-controlled scanning electron microscopy analysis while the slagging propensity was evaluated by determining the ratio of the deposited ash to the fuel ash, the so-called ash deposition ratio. The results showed no increase in the ash deposition ratio or significant change in the properties of the ash for co-firing using up to 50% of the low-ash (0.4 wt%) biomasses (sugi and sakura). In contrast, there was significant change in the properties of the ash and an increase in the ash deposition ratio with increasing biomass ratio for co-firing using the high-ash (>1.0 wt%) biomasses (nara and bark of sugi). The significant transformation of ash with regard to its morphology and chemical composition, together with the formation of eutectic calcium mineral mixtures in the deposit layer enhanced the slagging propensity during the co-firing process, particularly when using the high-ash biomasses. The use of a low-ash biomass was found to be suitable for preventing severe slagging during the co-firing process.

97/02094 Development of a stack plume opacity index for subbituminous coal-fired utility boilers

Powder River Basin subbituminous coals were burned using conventional and low-NO{sub x} combustion conditions in a drop-tube furnace equipped with a multicyclone ash collection device. Fine ash fractions (< 2 {micro}m in diameter) collected during the tests were analyzed using computer-controlled scanning electron microscopy (CCSEM). Advances in particulate sample preparation methods enabled the CCSEM analysis of individual ash particles with submicron diameters as small as 0.1 {micro}m. The fine ash samples produced from the conventional combustion of coal consisted of discrete spherical particles, whereas particle agglomerates were characteristic of the low-NO{sub x} ash samples. Particle-size distributions of the low-NO{sub x} fine ash fractions were coarser because of the agglomeration. Theoretical light-scattering calculations indicate that for a given coal, the ash produced in low-NO{sub x} conditions causes less opacity as compared to conventional combustion conditions. The following phases were abundant in the ashes: Ca aluminosilicate, Ca aluminate, aluminosilicate, silica, (Ca, Mg)O, CaSO{sub 4}, Na{sub 2} SO{sub 4}, and (Na, K)Cl. Primary mechanisms that produced the fine ash include the thermal metamorphism of small (0.1 to 5 {micro}m) mineral grains and the vaporization and subsequent condensation of organically bound Na, Mg, and Ca, Empirical equations for estimating the concentration of fine ashmore » produced from burning subbituminous coals were formulated into an opacity index based on CCSEM coal mineral and fine ash analyses and on drop-tube furnace testing results. The effects of ash electrical resistivity on electrostatic precipitator collection efficiency are also considered in the index.« less