Particle Size Distribution Of Fly Ash Research Articles

Measurements and Prediction of Ash Deposition in a Cyclone-Fired Boiler Operating under Variable Load Conditions

Measurements of ash deposition rates were made between the secondary superheater and reheater sections of a 450 MW cyclone-fired lignite boiler as the operational load varied from 33 to 100%. Significant reductions in deposition rates with a decrease in operational load were observed. To uncover the causative mechanisms behind these observations, operational data from the power plant were used to carry out computational fluid dynamic (CFD) simulations of the boiler. After ascertaining that the gas temperatures and velocities at various sections within the boiler were being represented adequately, decoupled simulations of the ash deposition process on the deposit probe were carried out using a finely resolved boundary layer mesh. Fly ash particle size distribution (PSD) and its concentration for the decoupled calculations were determined from stand-alone cyclone barrel simulations. The ash partitioning (mass %) between the fly ash and slag was found to be ~50:50, which was in line with previous field observations, and it did not vary significantly across different cyclone loads. The predicted PSD of the deposit ash was concentrated in the size range 10–30 microns, which was in agreement with cross-sectional images of the deposit obtained from the measurements. At lower loads, sharp variations in the deposition rates were predicted in the gas temperature range 950–1150 K. The particle kinetic energy—particle viscosity-based capture methodology utilized in this study in conjunction with appropriate ash compositions, ash viscosity models and gas temperature estimates can help estimate slagging propensities at different loads reasonably well in these systems.

Modelling Particle Size Distribution of Residual Fly Ash from Pulverized Biomass Combustion

The present work aims to develop a simple model for describing the particle size distribution (PSD) of residual fly ash from pulverized biomass combustion. The residual ash formation was modelled considering the mechanism of fragmentation and coalescence. The influences of particle shape and stochastic fragmentation on model description of the PSD of the fly ash were investigated. The results showed that biomass particle shape has a great influence on the model prediction, and a larger fragmentation number is required for cylindrical particles than that for spherical particles to get the same PSD of fly ash, and the fragment number of the particles increases with the shape factor increasing. For pulverized biomass with a wide size distribution, the model predicted ash PSD considering the stochastic fragmentation is very similar to that assuming uniform fragmentation. It implies that the simple model assuming uniform fragmentation is applicable for predicting fly ash size distribution in practical processes where biomass particles have a wide range of sizes. For the fuel with a narrower initial PSD, the stochastic fragmentation model generally predicts a coarser PSD of the residual ash than assuming uniform fragmentation. It means the stochastic fragmentation is of great influence to be considered for accurate description of ash formation from the fuel with a narrow PSD.

Effects of fly ash on the properties and microstructure of alkali-activated FA/BFS repairing mortar

This article presents the effects of fly ash (FA) on the properties and microstructures of alkali-activated FA/BFS repairing mortars. Firstly, the characteristics (chemical and mineral compositions, particle size distribution or specific surface area) of FAs were investigated. Two factors (the content of amorphous phases (CAP) and particle size distribution (PSD) or specific surface area) were defined as the two major variables and its effects on the properties (mechanical strength, flow value, and shrinkage) of alkali-activated FA/BFS repairing mortars were studied. The results indicated that the mechanical strength of alkali-activated FA/BFS mortar depended more on the content of amorphous phases since it facilitated the degree of geopolymerization reaction and the formation of products. IIFA/BFS exhibited the highest compressive, flexural, and interfacial flexural-tensile strength due to its higher content of amorphous phases. However, the particle size distribution of FA had little contribution on mechanical strength but effected the flow value and drying shrinkage more apparently. Additionally, it’s also found that except the mechanical interlocking and Van der Waals forces, the chemical bonding also presented between the repairing systems and substrate. That’s why the alkali-activated FA/BFS mortar exhibited higher bonding strength and was suitable for repairing concrete.

Optimization of fly ash particle size distribution for cementitious systems with high compactness

Compactness is of special importance for aggregates in concrete; however, the particle-size distribution (gradation) of powdery ingredients is commonly disregarded especially for pozzolanic materials such as fly ash. Without good gradation, powdery materials will result in higher void ratios, as in the case of aggregates, and the products obtained after hydration will still have some voids. Using a vacuum sieve, this study found the particle-size distribution of fly ash in accordance with the Dinger–Funk particle-size-distribution modulus, q, and explored the effects of various fly ash particle-size distributions on the compressive and flexural strengths of 7-, 28- and 90-day-old fly ash-blended cement mortars. After defining the optimal size distribution, the mechanical properties of cement mortars were assessed for several fly ash replacement levels. Results reveal that q of 0.4 yields the best mechanical property results. Further, the cement mortar with a 20% fly ash replacement level and a previously optimized particle-size distribution offered improved mechanical properties and high-compactness results over the control cement mortar after 90 days. Experimental results clearly indicate that only by properly adjusting the particle size distribution of fly ash used in the mixtures, better mechanical properties than the control mixtures without any fly ash addition can be achieved at a fly ash replacement ratio of 20%. The findings of current study are believed to greatly contribute to new lines of research on more effective replacement techniques for different pozzolanic materials without risking basic properties expected from cement-based materials.

The Physicochemical Properties and the Fate of Fly Ash along the Air Pollution Control Devices in Coal-Fired Power Plant

The fly ash at the exit of inlet and outlet of No.2 ESP and outlet of No.2 FGD in San-he Power Plant were sampled. The microstructure, elemental composition, particle size distribution and other physicochemical properties of the collected ash samples were tested and analyzed by a scanning electron microscope (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Mastersizer2000 laser particle size analyzer. The results showed that, ESP outlet fly ash particle size distribution curve was significantly moved to the left, the proportion of the fine ash at the outlet was significantly increased compared with that of at the inlet, the morphology and composition of ash at the outlet of ESP, FGD was significantly changed compared to the ash at the inlet of ESP, indicating that air pollution control equipment (APCDs) have a great impact on physicochemical properties and the fate of fly ash.

Effect of fly ash size fraction on the potential to neutralise acid mine drainage and rheological properties of sludge

AbstractLarge quantities of fly ash (FA) are generated annually in South Africa and most of it is disposed in landfills and ash dams. Previous studies indicated that FA can be used to treat acid mine drainage (AMD), another waste stream commonly found in the minerals industry. Moreover, encouraging results were obtained from the studies carried out on the sludge (referred as solid residues (SR) hereafter) recovered from FA-AMD treatment as a suitable mine backfill material. Particle size distribution (PSD) of FA plays an important role in the AMD treatment quality and viscosity of the SR that can be used directly for backfill purposes. In this study, we have investigated the effect of PSD of FA on AMD neutralisation and metal removal. This work also looked at the effect of PSD on the rheology of the SR. Neutralisation experiments indicated that the higher fines fraction enhances the treatment of AMD by reducing the time taken to increase the pH to a minimum of 7. Moreover, FA with higher fines fraction ha...

Influence of particle size distributions on yield stress and viscosity of cement–fly ash pastes

The rheological properties of blended cement-based materials depend strongly on mixture proportions and the characteristics of the components. In this study, design of experiments is used to investigate the influence of three variables (cement particle size distribution (PSD), fly ash PSD, and ratio of fly ash to cement) at each of four levels on the yield stress and viscosity of blended pastes. Both rheological parameters are seen to vary over several orders of magnitude for the evaluated design space. Physical characteristics of the powders, such as cement and total particle densities and total particle surface area, are computed for each mixture. A percolation-type relationship is observed between yield stress and cement particle (number) density. While neither apparent nor plastic viscosities were particularly well described by the commonly employed Kreiger–Dougherty equation, plastic viscosities were found to be linear functions of either total (cement+fly ash) particle surface area or total particle density.

Formation of fine particles from residual oil combustion: Reducing nuclei through the addition of inorganic sorbent

The potential use of sorbents to manage emissions of ultrafine metal nuclei from residual oil combustion was investigated by using an 82-kW-rated laboratory-scale refractory-lined combustor. Without sorbent addition, baseline measurements of the fly ash particle size distribution (PSD) and chemical composition indicate that most of the metals contained in the residual oil form ultrafine particles (∼0.1 μm diameter). These results are consistent with particle formation via mechanisms of ash vaporization and subsequent particle nucleation and growth. Equilibrium calculations predict metal vaporization at flame temperatures and were used to define regions above the dew point for the major metal constituents (iron [Fe], nickel [Ni], vanadium [V], and zinc [Zn]) where vapor-phase metal and solidphase sorbents could interact. The addition of dispersed kaolinite powder resulted in an approximate 35% reduction in the ultrafine nuclei as determined by changes to the PSDs as well as the size-dependent chemical composition.

Effect of urea on fly ash PCDD/F concentrations in different particle sizes

The effect of urea as an inhibitor for reducing polychlorinated dibenzo- p-dioxin and dibenzofuran (PCDD/F) concentrations in flue gases was studied in a pilot scale plant, together with the effect on the particle size distribution of these compounds. Total PCDD/F concentrations decreased by a maximum of 74%, the decrease being greatest for the most highly (octa-) chlorinated isomers. The PCDD/F reduction affected all the particle size classes when an adequate amount of urea was used (1% of the fuel input), which indicates that inhibition, unlike formation, is independent of the fly ash particle size distribution.

Selenium in onions grown in media amended with coal fly ashes collected with differing efficiencies

A study was conducted to learn if the extent of selenium uptake by onions grown on coal fly ash-amended growth media would be influenced by the particle size distribution of the fly ash. Two fly ashes having differing percentages of finely sized particles were used as plant growth amendments at percentages in the media to yield equal concentrations of selenium. Selenium concentrations in the harvested onion bulbs were found to be independent of fly ash particle size distribution.

Modelling of fouling and slagging in coal-fired utility boilers

In a current study, models are under development to predict the impact of ash deposition in a pulverized coal-fired boiler. These models are being incorporated into a coal quality expert software product being developed under a DOE Clean Coal Technology Program. Slagging and fouling are addressed by two separate submodules that interact with each other and with a boiler performance model to account for changes in coal input properties and boiler operation. The input to these models includes results from detailed analysis of coal mineralogy obtained from computer-controlled scanning electron microscopy, coal size-distribution data, boiler design details, and boiler operating conditions. The output from the models is an assessment of boiler performance associated with the use of the coal specified in the model. This paper describes the slagging and fouling algorithms, the rationale used in their development, and algorithm validation with laboratory, pilot, and field test data. Submodel validation for slag deposition growth and heat transfer was carried out using data from well-controlled, pilot-scale combustor testing. Composition of the different layers within the deposit was compared with the model prediction to validate the ash particle stickiness model. In addition, the measured temporal heat flux profile was used to check the consistency of the parameters describing the deposit growth submodel. The fouling submodel was developed using information from a bench-scale drop-tube furnace, along with data from other laboratory- and pilot-scale studies. The fly-ash particle-size distribution, as a function of distance from the deposition tube, was compared for both upstream and downstream deposits in bench- and pilot-scale studies.

Modelling of pulverized coal combustion with respect to fly ash particle size distribution

An approach is made to predict fly ash production and characteristics on the basis of: experimental information from a power plant tracking programme; and the results of a pulverized coal combustion model. Conclusions presented in this paper do not agree completely with previous experimental results obtained under laboratory simulated combustion conditions, or with generally accepted hypotheses about fly ash particle size configuration processes, showing the critical influence of individual particles temperature evolution on the final ash size distribution. The validity of a char fragmentation model, used extensively in fly ash size estimation, is also investigated. It is found that the presence of unburned particles in industrial fly ash must always be taken into account. It is also proved that model application must be restricted to a small number of cases that satisfy the global assumptions.

Percolation Model for Coal Char Particle Combustion and Fragmentation

Abstract A char combustion model based on percolation theory is developed. The concept of percolation is used to derive the time and position dependent transport and structural properties of the char particle during combustion. By adopting existing kinetic parameters for the surface reaction of the char particle, porosity and oxygen concentration profiles within the particle are calculated for progressive carbon conversion values. Assuming an initially uniform porosity of the char particle, and a Bethe lattice to describe the pore structure, a critical porosity is calculated at which the particle surface layer breaks into fragments. The effects of particle diameter, combustion temperature, pore structure, and fragmentation porosity upon particle reactivity and burnout times are evaluated to determine the sensitivities of the results to the assumption of initial and operating conditions. The impact of fragmentation on the fly ash particle size distribution during pulverized coal combustion is discussed.

Effect of fuel treatment on Coal-Water Fuel Combustion

Three methods of improving spray fineness by fuel treatment were investigated—1) the heating of the Coal Water Fuel (CWF) under pressure to produce steam as the pressure drops during passage of the CWF through the atomizer nozzle, 2) the absorption of CO2 gas in the CWF to produce a similar effect, and 3) the addition of a chemical additive which will cause microexplosions in the droplets upon heating. These treatments are expected to produce disruptive atomization, i.e., the disintegration of slurry droplets subsequent to their leaving the atomizing nozzle and, hence, to yield better burnout and finer fly ash particle size distribution upon combustion. The effects of disruptive atomization upon CWF spray size distribution were studied using a spray test chamber equipped with a laser diffraction particle size analyzer; the data were fitted to the Rosin-Rammler particle size distribution function. The combustion characteristics of the treated CWFs were investigated in the MIT Combustion Research Facility. The spray chamber tests established that thermally-assisted atomization produced reductions both in the mean droplet size and in the mass fraction of large particles in the spray. For fuel delivery temperatures up to 100°C this effect is attributable to lowered fuel viscosity, while further heating of the CWF (to 150°C in these experiments) produces disruptive atomization. In-flame measurements and high speed cine pictures made during combustion tests provided detailed information for comparisons of treated and untreated CWF. Thermally-assisted atomization was the most effective of the methods studied for improving carbon conversion efficiency and reducing fly ash particle size. CO2 and picric acid addition techniques showed substantial improvements but they were less effective.