Showing papers on "Fly ash published in 1995"

Effect of curing on the compressive strength, resistance to chloride-ion penetration and porosity of concretes incorporating slag, fly ash or silica fume

Abstract: This paper reports an investigation in which the performance of slag, fly ash, and silica fume concretes were studied under four different curing regimes. The water-cementitious materials ratio of all the concrete mixtures was kept constant at 0.50, except for the high-volume fly ash concrete mixture, for which the ratio was 0.35. The concrete specimens were subjected to moist curing, curing at room temperature after demoulding, curing at room temperature after two days of moist curing, and curing at 38 °C and 65% relative humidity. The compressive strength was determined at various ages, and the resistance to chloride-ion penetration was measured according to ASTM C 1202 at different ages up to 180 days. Mercury intrusion porosimetry tests were performed on the 28-day old mortar specimens for comparison purposes. The results indicate that the reduction in the moist-curing period results in lower strengths, higher porosity and more permeable concretes. The strength of the concretes containing fly ash or slag appears to be more sensitive to poor curing that the control concrete, with the sensitivity increasing with the increasing amounts of fly ash or slag in the mixtures. The incorporation of slag or silica fume, or high volumes of fly ash in the concrete mixtures, increased the resistance to chloride ions and produced concretes with very low permeability.

Resource Recovery of Waste Fly Ash: Synthesis Of Zeolite-like Materials.

Abstract: Since the main components of fly ash are SiO 2 and Al 2 O 3 , which exhibit a similar chemical structure of zeolite, fly ash was used in this study to synthesize zeolite-like materials. Different hydrothermal conditions were evaluated for achieving a maximum cation-exchange capacity of the synthesized zeolites. Experimental parameters included temperature, nature and molarity of caustic reagents, pressure, and reaction time. Tasks performed in this work included phase identification of treated fly ash by X-ray diffraction techniques, measurement of cation-exchange capacity and specific surface area of the synthesized zeolites, determination of elemental composition, and evaluation of metal adsorption onto the synthesized zeolites. Four zeolites were identified under various experimental conditions : zeolite P (2-4 N NaOH at 70-130 °C) ; analcime (2 N NaOH between 130 and 170 °C) ; hydroxy sodalite (4-10 N NaOH at 90-200 °C) ; and cancrinite (T> 200 °C). After NaOH treatment, there was a significant reduction in Si content, while Na increased as the reaction temperature or the strength of NaOH increased. The experimental conditions at 2 N NaOH, 110 °C, and 3.45 MPa for 1-day reaction resulted in treated fly ash with a cation-exchange capacity of 210 mequiv/100 g. Treated fly ash exhibited a strong affinity for Cu and Cd metal ions. At pH = 6, approximately 90% of Cu and 80% of Cd were removed by the zeolites synthesized under different conditions.

Evaporation of Heavy Metals during the Heat Treatment of Municipal Solid Waste Incinerator Fly Ash

Abstract: Thermal treatment is a promising way for the decontamination and inertization of residues from waste incineration. The evaporation of heavy metals thereby is of great significance. It is the goal of this work to investigate the fundamental aspects of the evaporation of heavy metals in the heat treatment process and to determinethe process parameters leading to complete evaporation of the relevant heavy metals. Evaporation experiments in different atmospheres were carried out with filter ash from municipal solid waste incineration. The quantities of the heavy metals Zn, Pb, Cd, and Cu evaporated as a function of time were measured at temperatures between 670 and 1300°C ; evaporation turned out to be most effective at temperatures just below the melting range ofthe residue (i.e., at 1000-1100°C) and decreased drastically above this temperature range. The amounts of evaporation (relative to the contents in untreated filter ash) at about 1100°C were 98-100% of Pb, Cd, and Cu and 50% of Zn in air and 98-100% of Pb, Cd, and Zn and 10% of Cu in argon atmosphere, respectively. Results of experiments using model systems indicate that the decrease in the Zn evaporation at high temperatures is caused by the formation of compounds like Zn 2 SiO 4 and ZnAl 2 O 4 . The results of the experiments in argon atmosphere are explained thermodynamically by the reductive potential of the carbon, contained in the residue.

Cation exchange properties of hydrothermally treated coal fly ash.

Abstract: Two samples of fly ash were treated for 2-48 h in 3.5 M NaOH at 100 °C. Powder XRD patterns of resulting products were obtained, and their CEC was determined. Zeolite P and/or hydroxysodalite formed during the treatment from the glassy part of fly ash, while quartz gradually dissolved and mullite remained stable. Approximately 50% of fly ash could be converted to zeolites with the CEC of resulting products reaching 2.5-3 mequiv/g. Concentrations of extractable B, Mo, and Se in fly ash considerably decreased upon treatment. Adsorption isotherms of lead on treated fly ash suggested that at low initial lead concentrations and at pH lower than 6 precipitation of lead compounds is not likely. A desorption experiment indicated incomplete reversibility at higher concentrations, suggesting that part of the adsorption may not have been cation exchange related. Treated fly ash displayed high selectivity for Pb 2+ > Sr 2+ > Cu 2+ > Cd 2+ > Zn 2+ > Cs + in competition with Na + , especially at low concentrations of these cations, and was effective in removing Pb and Zn from industrial wastewaters. It was not selective for Ni 2+ and UO 2 2+ . In a column test, 160 bed vols of NH 4 + -contaminated fish-pond water was filtered through treated fly ash until NH 4 + breakthrough occurred.

Viscosity of ash particles in combustion systems for prediction of particle sticking

Abstract: A new model has been developed for predicting the viscosity of individual coal ash particles in the range of 10 4 -10 8 Pa.s. Previous experimental work has demonstrated that there is a critical viscosity for adhesion of ash particles. At temperatures and velocities typically found in coal-fired utility boilers, this critical viscosity has been shown to lie between 10 5 and 10 7 Pa.s. Previous models of coal slag viscosity were derived from data at much lower viscosities (less than 10 2 Pa.s) and, while such models accurately predict viscosity of coal slag at high temperatures, they do not perform well in predicting the viscosity associated with particle adhesion to heat-transfer surfaces in utility boilers. Using viscosity data taken from the literature, a new model has been formulated. The data sets are described in detail as well as the coefficients needed to calculate viscosity. Finally, the results of the model are compared to results from previous coal slag viscosity models. The new model is shown to produce a better fit between calculated and measured values of viscosity in the range of 10 4 -10 8 Pa.s. Furthermore, the new model does a better job of predicting viscosity for certain compositions which are commonly found in ash particles.

A Laboratory Study of the Partitioning of Trace Elements during Pulverized Coal Combustion

Abstract: The extent of vaporization and subsequent recondensation of many trace elements during coal combustion is believed to depend upon the form of occurrence of the trace element in coal. To test this hypothesis, the forms of occurrence of the trace elements arsenic, mercury, selenium, zinc, chromium, and antimony were determined by successive chemical extractions of both a bituminous and a subbituminous coal. Well-controlled laboratory combustion tests were then conducted, and the concentrations of each element measured as a function of flyash particle size. Results indicated that those elements associated with the coal organic matrix, either ion-exchanged or covalently bound, and those associated with the reactive mineral pyrite were highly volatile. Arsenic, selenium, mercury, and antimony provide examples. In contrast, elements that were associated with silicate or oxide minerals (e.g., chromium) were relatively nonvolatile. For all elements, but particularly zinc, differences in form of occurrence between the two coals led to differences in fraction volatilized. For arsenic and selenium, the results indicated that the condensation pathway was dependent upon flyash chemistry. Using thermochemical equilibrium modeling to interpret the experimental data, it was concluded that arsenic vapors reacted to form calcium arsenates during combustion of the high-calcium subbituminous coal, but condensed as arsenic oxide during combustion of the low-calcium bituminous coal. Limited (calcium) selenate formation was also inferred from the data.

Coal Fly Ash as a Composting Material for Sewage Sludge: Effects on Microbial Activities

Abstract: Coal fly ash has a strong alkaline buffering capacity which may render heavy metals in sewage sludge unavailable and the aim of this project is to study the potential of coal fly ash as a co-composting material for sewage sludge by evaluating its effect on the microbial decomposition of sewage sludge Lagoon ash and fly ash at 0, 25, 5, 10, 20, and 50% w/w were mixed with dewatered sewage sludge and incubated at 25°C for 3 months Carbon dioxide evolution for all treatments followed the same trend except 50% w/w amendment for both ash residues When compared with the control (sludge only), higher levels of ash amendments had a lower CO2 evolution which may be due to the high entrained salt content and pH in both ash residues The present results indicate that fly ash has a more pronounced inhibition on sewage sludge decomposition while 25% w/w and 5% w/w lagoon ash amendments demonstrate no significant inhibition Therefore, lagoon ash would be a more suitable co-composting candidate for sewage sludge a

Use of ash derived from oil-palm waste incineration as a cement replacement material

Abstract: The production of palm oil creates large quantity of solid waste by-products. Empty oil-palm fruit bunches discharged from the mill are currently being disposed of by burning in an incinerator. After combustion, a substantial amount of highly caustic ash is produced which creates problems of disposal. This study examines the feasibility of using the ash as a cement replacement material. The experimental results indicate that blended cement containing ash derived from oil-palm waste shows satisfactory setting times and soundness test results. Workability of concrete incorporating the ash is categorized as good, and no segregation was observed. The effects of ash addition on concrete densities and water absorption are insignificant. Compressive strength of the concrete decreases with the ash content in the cement. Up to 10% by weight of ash addition, no adverse effect on the strength was observed for the cubes cured for 1 year. The results suggest that the ash could possibly be blended in small amounts with ordinary Portland cement for concrete making without detrimental effects on long-term strength property.

Abrasion resistance of high-strength concrete made with class c fly ash

Abstract: This work was performed to evaluate the abrasion resistance of concrete proportioned to have five levels of cement replacements (15, 30, 40, 50, and 70 percent) with one source of Class C fly ash. A reference concrete without fly ash was proportioned to have a 28-day compressive strength of 41 MPa. Concrete specimens were subjected to abrasion according to the American Society for Testing and Materials (ASTM) C 944 test method. In this work, all concretes made with and without fly ash passed the abrasion resistance requirements per ASTM C 779, Procedure C. Depth of wear values produced by the ASTM C 944 test were quite low for the strength levels tested in this work. An accelerated test method was developed and used to evaluate the abrasion resistance of high-strength concrete. This method used grinding wheels with smaller washers, and a standard "Ottawa sand" was applied to the surface being abraded at intervals of 1 minute. Accelerated test results revealed that the abrasion resistance of concrete having cement replacement up to 30 percent was comparable to the reference concrete without fly ash. Beyond 30 percent cement replacement, fly ash concrete displayed slightly lower resistance to abrasion relative to non-fly ash concrete.

Evaluation of fly ash as a soil amendment for the Atlantic Coastal Plain: I. Soil hydraulic properties and elemental leaching

Abstract: A major limitation to crop yields in the Atlantic Coastal Plain is drought stress caused by the low moisture-holding capacities of the coarse-textured soils common to the area. Because coal fly ash is comprised primarily of silt and clay-sized particles, it has the potential, if applied at high enough rates, to permanently change soil texture and increase moisture holding capacity. A series of soil column studies were conducted to evaluate the effects of high rates of fly ash on soil hydraulic properties and elemental leaching of trace metals and boron. Fly ash from two Delaware power plants (EM=Edgemoor and IR=Indian River) was incorporated in a Hammonton loamy sand (fine-loamy, siliceous, mesic, Typic Hapludults) at six rates (0, 5, 10, 20, 30, and 40%, by weight). The effect of fly ash on soil moisture holding capacity, hydraulic conductivity, and wetting front velocity was determined. Leachates from columns amended with 30% fly ash were analyzed for B, Cd, Ni, Pb, Cu, and Zn. Soil moisture holding capacity was increased from 12% in the soil alone to 25% in the soil amended with 30% fly ash. Boron and soluble salts leached rapidly from ash amended soils while only trace quantities of Cd, Ni, Pb, Cu, and Zn were detected in column leachates.

Characterization of Na-X, Na-A, and coal fly ash zeolites and their amorphous precursors by IR, MAS NMR and XPS

Abstract: By fusion with sodium hydroxide followed by a hydrothermal reaction, fly ash and Alenriched fly ash were converted into Na-X and Na-A zeolites, respectively. The authentic Na-X, Na-A and fly ash zeolites as well as their amorphous precursors have been characterized by IR, 29Si and 27Al MAS NMR, XPS/AES, TG, and comparative ion-exchange studies of Cs and K with Na in zeolite samples. It appears that the same structural unit with a terminal OH, such as a sodalite unit, was formed in the induction period of the hydrothermal reaction for Na-X and Na-A, and then cross-linked through D4R and D6R external linkages to build up the zeolite framework of Na-A and Na-X, respectively.

Highly Active Absorbent for SO2 Removal Prepared from Coal Fly Ash

Abstract: The absorbent for SO{sub 2} from the flue gas of a coal-fired electric power station was prepared from calcium oxide, calcium sulfate, and coal fly ash and examined for the relation between the desulfurization activity and the structure. The activity is closely related to the progress of the hydration reaction taking place during preparation procedures. The activity increased with the hydration time and reached a maximum activity in 12 h. The hydration resulted in the formations of ettringite and calcium silicate. By elevation of the temperature for drying the hydration products, the activity markedly increased up to 400 C, which was caused by the removal of water covering the calcium component in the ettringite. For the efficient removal of SO{sub 2}, the existence of NO in the flue gas is required. NO{sub x} plays a catalytic role for oxidation of SO{sub 2} to SO{sub 3} which reacts with CaO to form CaSO{sub 4} as a final product.

The Leaching of Major and Trace Elements from Coal Ash

Abstract: Most power stations currently operate wet ash disposal systems. However, this method of ash disposal is being subjected to increasing scrutiny as there is a potential for contamination of surface and groundwaters by trace elements leached from the ash (Carlson and Adriano, 1993). Very high liquid to solid ratios of 10:1 to 20:1 are typically used in ash sluicing systems (Chu et al., 1978). Consequently, large volumes of water containing elements dissolved from the ash are produced. The composition of the water in an ash pond will be determined by the ash itself, by the quality of water used for sluicing, and by the relationship of the ash pond to other components of the power station water circuit (Figure 12.1).

Role of Oxygen in Formation of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans from Carbon on Fly Ash

Abstract: Under an oxygen-deficient flow, no PCDD/F is formed from a carbon/fly ash mixture, but with a mixture of 1% O 2 in N 2 de novo synthesis of PCDD/F sets in. The O 2 concentration was varied 0-10%, formation was highest at 5-10% O 2 . The [PCDD] :[PCDF] ratio rises with increasing [O 2 ], but the average degree of chlorination of PCDD and PCDF does not depend on the O 2 concentration. Especially 3,4,6,7-substituted PCDD/F congeners are formed. Without a gas flow, carbon/fly ash mixtures can give PCDD/F formation too : the O 2 now enters the mixture by diffusion. This formation process can continue for several hours. Isomer distributions do not change with increased reaction times. Formation rates under conditions without a gas flow are higher than with a gas flow present, suggesting that O 2 diffusion into the fly ash bed is more efficient than the supply of O 2 through a gas flow.