Fly ash composition

Katrinak, K. A. Zygarlicke, C. J. 1995. Size-related variations in coal fly ash composition as determined using automated scanning electron microscopy. Fuel Processing Technology, 44, 71-79. 

Work is still in progress on determining the particle size distribution in the flue gases before and after the precipitator and in the stack. Also, studies on fly ash composition as a function of particle size are in progress. 

A monotonous decrease of melt flow index in a series of HDPE-wood flour-fly ash composite materials was observed [23]. However, this decrease has resulted due to a shift from a neat HDPE to 40% fly-ash-fllled HDPE to 40% wood-flour-filled HDPE. The data show that wood flour increased viscosity of the filled plastic much more compared with that of fly ash (Table 4.27). 

The above data show that fly ash was not beneficial for mechanical properties of the HDPE-wood flour-based composite. However, the flowability (MFI) of the molten composite was getting higher (viscosity was lower) when fly ash was replacing wood flour. Last but not the least, it was found that fly ash stabilized the HDPE-wood flour composition against heating. The onset of thermal decomposition for the 40% wood-flour-filled HDPE started at about 280°C, while that of 40% fly ash-filled HDPE started at approximately 490°C. The triple (HDPE-wood flour-fly ash) composite was between the two in this regard, showing a thermally more stable behavior than composites with wood flour alone [23]. 

McQuadePV, Head WJ, Anderson RB (1981) Investigation of the use of coal refuse - fly ash compositions as highway base course material. Federal Highway Administration, Report No FHWA/RD-80/129, Washington, DC,p 352... 

The phase composition of fly ash can be very different even in particular grains. Glass is a main constituent its content in the sihceous fly ash is generally higher than 80%. The crystalline phases are quartz, mrrlhte, hematite and magnetite. In Table 7.6 some examples of different fly ash composition are shown. 

Figure 9.1 Compressive strength of Portland-fly ash cements made with different additions of class F fly ash. Composition of ash Si02, 52.3% AI2O3,24.2% Fe203,10.0% CaO, 4.4%.

Mehta, P.K. (1986) Effect of fly ash composition on sulfate resistance of cement Journal of the American Concrete Institute 83,994-1000. 

Arrangements have been made to enable the installation of a fixed granular bed filter after the two cyclones in the third quarter of this year. At the same moment isokinetic sampling systems will be installed in the in-and outlet of the granular bed filter. The filter, which has a diameter of 0.4 m, will be able to clean a variable part (10-75 %) of the exhaust gas stream. It will be used to study the influence of several parameters (filter velocity, fly ash composition, electrostatic charging and polarization) on the collection processes. 

The high temperatures in the MHD combustion system mean that no complex organic compounds should be present in the combustion products. Gas chromatograph/mass spectrometer analysis of radiant furnace slag and ESP/baghouse composite, down to the part per biUion level, confirms this behef (53). With respect to inorganic priority pollutants, except for mercury, concentrations in MHD-derived fly-ash are expected to be lower than from conventional coal-fired plants. More complete discussion of this topic can be found in References 53 and 63. 

The choice of selected raw materials is very wide, but they must provide calcium oxide (lime), iron oxide [1309-37-1/, siHca, and aluminum oxide (alumina). Examples of the calcereous (calcium oxide) sources are calcium carbonate minerals (aragonite [14791-73-2] calcite [13397-26-7] limestone [1317-65-3] or mad), seasheUs, or shale. Examples of argillaceous (siHca and alumina) sources are clays, fly ash, mad, shale, and sand. The iron oxide commonly comes from iron ore, clays, or mill scale. Some raw matedals supply more than one ingredient, and the mixture of raw matedals is a function of their chemical composition, as deterrnined by cost and availabiHty. 

Compositions of high-alumina cement containing quartz or glass, calcium carbonate, microsilica, carbon black, iron oxide red mud or screened fly ash, and styrene-butadiene latex have been described [141,1803,1804]. 

Additives often form a problem in recycling processes. Material recycling is often not possible or only with a considerable loss of quality. Plastics recycling is notoriously difficult due to the mixed composition of the plastics waste stream. The recycled material can only be used in certain applications that do not demand a pure material. Recycling of the additives themselves is theoretically possible only for metals, but in practice this type of recycling is not feasible. The metals occur only in low concentrations. Recovery from fly ash and bottom ash is possible, but expensive in view of growing scarcity problems it may become a viable options for at least some metals. 

The surface layer composition may influence the effectiveness of pollution control devices. For example, it is apparent that a surface region highly enriched is alkali-alkaline earth sulfates may enhance the fly ash particle collection efficiency of electrostatic precipitators (11, 12, 51-53). 

Weissman SH, Carpenter RL, Newton GJ. 1983. Respirable aerosols from fluidized bed coal combustion Element composition of fly ash. Env Sci Tech 17 65-71. 

Pozzolanic S/S systems use portland cement and pozzolan materials (e.g., fly ash) to produce a strucmrally stronger waste/concrete composite. The waste is contained in the concrete matrix by microencapsulation (physical entrapment). It is a chemical treatment that uses commercially available soluble silicate solutions and various cementious materials such as cement, lime, poz-zolans, and fly ash. By addition of these reagents and rigorous mixing, the waste is fixed or stabilized. Contaminant mobility is reduced through the binding of contaminants within a solid matrix, which reduces permeability and the amount of surface area available for the release of toxic components. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

Figure 7. Comparison of mass and elemental composition of fly ash sampled in the stack of a coal-burning power plant, and bulk hopper fly ash, collected while warm on the same day, later resuspended at Davis...

Jager and Hanus (1980) found the order of reactivity for the reactions of PAHs adsorbed on several substrates with 1.3 ppm of N02 in air to be silica gel > fly ash > deactivated aluminum oxide > carbon. The qualitative composition of nitro-PAH products, however, was independent of the substrate. 

Fig. 2. Pseudotemary diagram showing projected compositions in mol% of coal combustion fly ashes and of phases present in ash. Polymorphs of dicalcium silicate include lamite. Bulk composition of fly ash lies within the field bounded by the shaded thick line.

A wide range of melt compositions undergo phase separation upon cooling. Two mechanisms are observed classical phase separation (dashed curve) as well as by spinodal (dot-dashed curve) mechanisms. Both the position of these regions and the onset temperature of phase separation reported in the literature vary (e.g., Roth et al. 1987), but there is no doubt that the phenomenon occurs amongst the glassy phase of aluminosilicate fly ash despite the presence of other oxides that tend to promote miscibility. 

Lime-rich fly ashes exhibit large variations in both chemistry and mineralogy. First, as with the mineral and chemical composition of all fly ashes, large variations may occur from grain to grain (Stevenson Huber 1987 McCarthy et al. 1987). This variation extends to the distribution... 

Fig. 3. Phase diagram of a portion of ihe Sitb-AbO% system, showing regions of phase separation of undercooled melts. The projected composition of Class-F fly ash glasses lies in the range of 10-4051 mol /t AI,Ot. Adapted from Roth et al. (1987).

In general, lack of equilibrium during the pyro-processing stage and heterogeneity of composition make it impractical to calculate the phase content of fly ash from its chemical analysis. Two other factors, common to all types of fly ash, further complicate evaluation of the potential to use fly ash. One is the presence of unburnt carbon, which should preferably not exceed a few wt% the other is the presence of alkali-sulphates (see Groppo et al., 2004). When used in cementitious formulations, free... 

A thorough understanding of the chemical and mineralogical composition of CCPs is necessary for proper management of these materials. This chapter will cover (1) the composition of coal (2) the formation of CCPs (3) the physical, chemical, and mineralogical characteristics of CCPs (4) characterization of North American fly ashes (5) hydrated minerals in fly ash/ water pastes (6) sulphur scrubbing products and (7) environmental impact of CCPs. 

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