Surface Of Fly Ash Research Articles

The effects of temperature and oxygen content on the PCDD/PCDFs formation in MSW fly ash

In this study, the effects of the temperature, oxygen content in the gas stream and carbon content in ash particles on PCDD/Fs formation on the fly ash surface were investigated. The optimum temperatures for dioxin formation were found at 350°C for boiler ash, 300°C for cyclone ash and 250°C for ESP ash, respectively. Preliminary results indicate that the optimum temperature will decrease as the particle size decreases. When the O 2 concentration is varied between 0% and 100%, the optimum oxygen content for PCDD/Fs formation is found to be at 7.5% for cyclone ash, and the PCDD/PCDF ratio increases with the increase of oxygen content. Dioxin formation is observed even for the gas containing no oxygen passed through the fly ash. Hence, other reacted routes that do not need O 2 for dioxin formation take place on fly ash. The carbon content in fly ash is varied between 0% and 20% in this study, and the results have indicated that the maximum dioxin formation is to be found at 5%. The precursors are not injected into the fly ash or gas stream in all formation experiments, however, dioxin is still formed in fly ash. Consequently, other chlorinated routes besides Deacon reactions may take place on the fly ash surface.

The partitioning of arsenic during pulverized coal combustion

An important issue facing the coal-fired electric utility industry is the environmental impact resulting from the release of arsenic contained in the coal. Depending on the forms of occurence in the coal and transformation mechanism involved, arsenic-containing compounds may exit the furnace in the vapor, as part of submicron size aerosol particles, or as part of larger, supermicron particles. The specific path by which arsenic transforms from the vapor phase to the condensed phase is important in assessing the toxicity of the waste product. This paper documents research exploring the partitioning of arsenic during the combustion of five well-characterized pulverized coals under commercially relevant conditions in a welldescribed laboratory combustion environment. Insight is gained into the timing of partitioning because this combustions system allows comparison of furnace vapor and particulate at different locations in the furnace. The partitioning of arsenic is governed primarily by the extent of arsenic volatilization during combustion. Volatilized arsenic will heterogeneously transform to both submicron and supermicron particles primarily by reaction with active cation sites. For many coals, arsenic appears to be highly reactive with active calcium sites to form calcium arsenates. The transformation process is typically dominated by surface reactivity rather than by gas film diffusion. Due to the similarities in the solubility characteristics of As 2 O 3 and calcium arsenate, leaching studies provide little insight into the form of arsenic present on fly ash surfaces.

Kinetics of MSWI Fly Ash Thermal Degradation. 2. Mechanism of Native Carbon Gasification

A generalized kinetic model for fly ash native carbon oxidation is developed. It is shown that the conversion of fly ash native carbon to CO{sub 2} is the result of two simultaneous processes taking place on fly ash surface: the first process (rate constant k{sub 2}) is the direct oxygen transfer from a metal oxide site to a vacant carbon active site leading to immediate carbon gasification; the second one is dissociative oxygen chemisorption (k{sub 1}) followed by C(O) complex intermediate uncatalyzed gasification (k{sub 3}). The model is validated using the kinetic data from model systems and those reported in Part 1 for four different fly ashes. The rate constants, k{sub 1}, k{sub 2}, and k{sub 3}, together with activation and thermodynamic parameters are calculated. For each fly ash, the rate determining step is the complex intermediate oxidation. The nature of the interaction between native carbon and the fly ash surface is the key factor for C and C(O) complex formation and gasification.

ＮａＯＨ／Ｎａ２ＣＯ３をアルカリ源として用いた石炭灰フライアッシュからのゼオライトの水熱合成

Zeolites have been synthesized through hydrothermal reaction of coal fly ash with alkaline solutions in order to facilitate effective use of the fly ash generated from the coal power stations. The properties of synthesized zeolite were examined by X-ray diffraction, and characterized by cation exchange capacity (CEC) and specific surface area. Each of NaOH solution or Na2CO3 solution was applied to test the effects of alkali type on hydrothermal reaction. The reaction conditions established in the present work ware the ratio of solid and liquid, 1 : 4 and reaction temperature, 393 K for the favored synthesis of zeolite in only few hours without any pretreatment. Only zeolite P was synthesized when 0.5 ～ 3.0 mol/dm3 NaOH was used, whereas both zeolite P and hydroxysodalite were produced in the case of NaOH solution of 4.0 mol/dm3. CEC value of 350 meq/100 g of the zeolite P synthesized with 2.0 mol/dm3 NaOH was sufficient for a cation exchanger. SEM photographs showed the growth of zeolite on the surface of the undissolved fly ash, resulting texture of amorphous fly ash covered with the crystallized zeolite. Sodium carbonate solution of 1.0 mol/dm3 alone failed the synthesis reaction of zeolite, whereas addition of more than 15% of NaOH proceeded the reaction. The thickness of zeolite layer depended on the amount of NaOH.

Thermal Formation of Polychlorinated Dibenzo-p-dioxins and -furans: Investigations on Relevant Pathways

ABSTRACT The recent status of the relevant formation of polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) in technical incineration processes is discussed. Within this discussion, the competition experiments for the alternative pathways, de novo, and precursor reactions are critically evaluated. The results suggest that the precursor mechanism is dominant under conditions of municipal waste incineration (MWI). Results of field observations support this primary hypothesis. Chloroaliphatic compounds such as dichloroacetylene are responsible for the formation of higher or perchlorinated aliphatic and aromatic compounds and precursors in thermal PCDD/F formation. The meaning and implications of indicator compounds for PCDD/F are also evaluated. Observed correlation among interrelated classes of chloroaromatic compounds can be explained by stationary reaction equilibrium in the postcombustion zone of incinerators in the presence of fly ash surfaces (i.e., by heterogeneous catalysis). Recent findings on th...

Photoreactions of ethanol and MTBE on metal oxide particles in the troposphere

Ethanol (EtOH) and methyl tert-butyl ether (MTBE) are finding increasing use as fuel additives because of governmental mandates, and the relative merits of each have therefore become the focus of intense debate. The ultimate fate of fugitive emissions of these species forms one aspect of this controversy, because EtOH has been implicated in reductions in air quality, while MTBE has been linked to human illness. Both species have been shown to photoreact rapidly on fly ash particles in urban atmospheres. This work reports kinetic studies as well as detailed surface and bulk characterizations of fly ash by XRD, XPS, and SEM/EDA. The active phases in fly ash are probably Fe and Ti oxides. Addition of water to the reactant gas in some cases more than doubled the reaction rate of EtOH, but hardly affected that rate of MTBE. To gain insight into the respective roles of Fe, Ti, and alkali metals, kinetics were also measured over alkali-modified TiO 2 (anatase). Addition of Na or Ca oxides shifted the reaction product of EtOH from total oxidation (CO 2) to partial oxidation (acetaldehyde).

97/00378 Fly ash—a potential heterogeneous catalyst in peroxidative degradation of aqueous dye solutions

The potentiality of fly ashes from coal-fired power plants has been investigated as heterogenous catalyst in oxidative decolourization of aqueous solutions of several reactive, acid and chrome dyes, using H 2 O 2 in the temperature range 300-323 K. It is observed that the catalytic activity of fly ashes depends on their physico-chemical characteristics and pH of the reaction medium. For an initial dye concentration of 100 mg/L and H 2 O 2 of 5.2 x 10 -2 kmol/m 3 , complete decolourization is possible within 1500, 900 and 600s of all reactive, acid and chrome dyes, respectively, at 313 K and pH 3.75 ± 0.15 using 2% (w/v) fly ash loading. The extent of deactivation of catalytic effect of fly ash depends on total contact time, characteristics of fly ash surface and specific dye used.

NEUTRALIZATION AND RELEASE OF METALS PROPERTIES OF FLY ASH

The neutralization property and the release of metals for three types of coal combustion ash were evaluated. Fly ash (FA), fluidized bed ash (FBA), and hydrated fly ash (HFA) were assessed for their variation in physical, chemical and mineralogical properties. Acidic solutions of different pH were used to equilibrate the ash in conservative and nonconservative systems. Release studies were conducted for the following metals: iron, manganese, zinc, magnesium, aluminum, calcium, cadmium, sodium, chromium, copper, and nickel. Data indicated that pH was a very significant factor for the release of metals from the fly ash surface. At high pH iron was the least released cation from fly ash, whereas calcium was the most desorbed cation. Comparison of the three ashes indicated that FA and FBA were excellent buffering materials. However in the case of FBA, there seemed to be a critical pH at which a drop in the proton titration curve was apparent, and large amounts of metals were released into solution. On the other hand, HFA showed lower buffering intensity compared to the other two fly ashes. Changes in pH with time were also studied since time had an impact on the titration curve at the equivalence point. Although pHmore » was increasing with time, it had no impact on the titration curve after neutralization had occurred. Comparison of fly ash with limestone for neutralization properties revealed that fly ash had less buffer intensity than limestone, but it gave solutions a higher initial pH.« less

Modeling Arsenic and Selenium Leaching from Acidic Fly Ash by Sorption on Iron (Hydr)oxide in the Fly Ash Matrix

The sorption of As and Se on iron (hydr)oxide in acidic fly ash is studied as a possible process controlling the leaching of these contaminants. Following controlled leaching experiments with an acidic fly ash, sorption of As and Se is investigated both on crystalline and amorphous iron (hydr)oxide, using partial chemical extractions and isotopic exchange experiments. Virtually all As and Se at the fly ash surface, which is potentially available for leaching, is shown to be associated with hydroxylamine extractable “amorphous” iron (hydr)oxides in the fly ash matrix. Isotopic exchange experiments suggest that at pH < 10 the oxyanions are partly coprecipitated with secondary-formed amorphous iron (hydr)oxide, a process reducing their availability. The leaching of As and Se from acidic fly ash is described by a simplified model of surface complexation on amorphous iron (hydr)oxide, taking the limited availability of iron-associated As and Se into account. This approach is shown to constitute a useful tool to model sorption control of contaminant leaching from waste materials.

ADSORPTION OF ACID MINE DRAINAGE METALS ON FLY ASH

Fly ash (FA), fluidized bed ash (FBA), and hydrated fly ash (HFA) were assessed for their sorptive properties of metals from acid mine drainage (AMD). Metals in sulfuric acid solutions at different pH conditions were equilibrated with fly ash for varied detention time. Preferential adsorption studies were conducted with respect to six major metals that were found in AMD: iron, manganese, zinc, aluminum, magnesium, and calcium. Comparison of the three fly ashes indicated that all had excellent sorptive properties. However, in the case of the most acidic solution (pH=1), no significant adsorption occurred. After the solution had dropped below a certain pH, adsorption decreased dramatically and release of metals into solution began. Among the six metals, iron seemed to be preferentially adsorbed on fly ash; the adsorption of calcium was not observed. Results indicated that pH was a very significant factor for the adsorption of metals on the fly ash surface. Data obtained from the use of real AMD samples for fly ash treatment were in agreement with experimental observations. Comparison of fly ash with two clays for sorptive properties revealed that fly ash was the best alternative for treatment of acid mine drainage. Treatment of AMD with clay resultedmore » in solutions of low pH and high concentrations of cations in solution.« less

Desorption behaviour of polychlorinated dibenzo-p-dioxins/dibenzofurans on a packed fly ash bed

When incinerator fly ash is heated at 398 °C under a stream of N 2 O 2 polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) concentrations are lowered. Dechlorination/dtx;omposition and desorption are competing processes. In this paper the desorption was studied: within PCDD/F homologues, all isomers desorb at an equal rate. Obviously, values of the activation energy of desorption from the fly ash surface ( ΔG # 398,des do not differ significantly between these isomers.

Influence of HCl and Cl2 on the Formation of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans in a Carbon/Fly Ash Mixture

Formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) from carbon on incinerator fly ash was investigated using HCI and Cl 2 as a chlorinating agent. The formation proceeds roughly at an equal rate with both gases. These results do not support the theory that chlorination reactions on the fly ash surface take place through free Cl 2 generated by the Deacon reaction. Once formed, PCDD/F remain adsorbed on the fly ash surface until ca. 350°C ; at ca. 400 °C, the major part of these compounds has evaporated from the fly ash to the gas phase.

Analysis of time scales pertinent to dioxin/furan formation on fly ash surfaces in municipal solid waste incinerators

Formation of polychlorinated dioxins and furans (PCDD/F) in municipal solid waste incinerators has been reported extensively in the literature. Of specific interest are potential heterogeneous pathways: low temperature catalytic formation over fly ash in air pollution control devices (APCDs'). An intuitive analysis of APCDs identifies at least two different particle residence times: short and long time scales. Therefore, the PCDD/F measured in the stack gases may include contributions from formation on both time scales. During the residence of particles in the system, a sequence of mass transfer, precursor adsorption and surface reaction should take place to form PCDD/F. An analysis suggests that the mass transfer of precursors (chlorobenzenes/chlorophenols) may not be limiting for high reaction selectivities (> 10%) while chemisorption is the more probable adsorption mechanism. Time scale estimates for surface reaction mechanisms indicate that both Eley-Rideal and Langmuir-Hinshelwood mechanisms are possible on the fly ash surface. In determining the relevance of each mechanism to incinerator conditions, several parameters, such as the degree of desorption of PCDD/F, extent of reentrainment of collected particles, particle collection efficiency, etc., play a key role; an approach to quantitative treatment of such parameters is the content of this communication.

Surface and bulk studies of leached and unleached fly ash using XPS, SEM, EDS and FTIR techniques

The effective chemical utilization of fly ash in environmental applications requires a detailed knowledge of the surface and bulk changes induced by leaching in acid solutions. The surface and bulk characteristics of fly ash from the combustion of Texas lignite have been examined by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR). The effects of leaching with acid solutions commonly used in environmental studies have been documented using these techniques. The results of these studies reveal that the fly ash particles are relatively resistant to either chemical or physical changes due to attack by acidic leaching solutions.

Fly ash as a sorbent for the removal of biologically resistant organic matter

Six different fly ashes, in both an untreated and an acidified form, were studied with respect to their ability to remove color and organic materials from a municipal waste treatment facility effluent. Color, fluorescence, and chemical oxygen demand were used to monitor the removal. The apparent predominant mechanism varies with the pH and the chemical characteristics of the ash. These mechanisms include carbon sorption, calcium precipitation of tannins and humics, sorption on the fly ash surface by silica, alumina, and/or iron oxide and, in the acidified situation, coagulation of colored colloids in the effluent dissolved from the fly ash. Some additional deleterious components are dissolved into the effluent during the treatment process. Many of these materials can, however, be removed by using a two-stage fly ash sorption process.Particularly of interest is the ability to remove over 90% of the boron from the effluent.

Effect of superplasticizer on the hydration of a mixture of white portland cement and fly ash

Hydration of a mixture of white portland cement with 25 wt. % fly ash in the presence and absence of 1% lomar-D has been investigated. Both fly ash and the superplasticizer act as a water reducer and increase the setting time. The compressive strength of white portland cement at 28 days is increased with the addition of fly ash. The strength is futher increased in the presence of a superplasticizer. Porosity measurements indicate that increase in strength is a result of decrease in porosity. It is found that after 7 days of hydration the reaction between CH and fly ash increases; however, the extent of this reaction is lower in the presence of a superplasticizer. The superplasticizer is probably adsorbed at the cement and fly ash surfaces.

Formation of polychlorinated dibenzo-p-dioxins/dibenzofurans in the carbon/fly ash system

The effect of the reaction time and temperature on PCDD/F formation on a fly ash surface was studied. Carbon was used as a source for de novo synthesis. There is a great rise in PCDD/F formation between 2 and 4 hours annealing time. This is probably due to changes in the fly ash surface. The optimum temperature for formation was found to be around 350 °C, which is higher than the 300 °C found in other publications. Whereas the congener distribution changes as function of both reaction time and temperature, the isomer distribution shows little reaction time or temperature dependence.

Surface catalyzed halogenation-dehalogenation reactions of aromatic bromine compounds adsorbed on fly ash

Brominated aromatic compounds like brominated benzenes, diphenylethers and dibenzodioxins adsorbed on the surface of fly ash from a municipal waste incininerator give mixed brominated/chlorinated und completely chlorinated aromatic compounds. These consecutive halogenation-dehalogenation reactions proceed by a nucleophilic mechanism, which is favoured by a high concentration of chloride on the fly ash. Results of kinetic and stereoselective behavior of these reactions will be discussed. The relevance of these results for PCDD/PCDF formation from bromine precursors in municipal waste incinerators will be discussed.

Mechanism of chlorination of aromatic compounds adsorbed on the surface of fly ash from municipal incinerators

Aromatic substrates were adsorbed on the surface of fly ash from a municipal incinerator. Treatment with gaseous hydrogen chloride gave aromatic chlorination. Relative yields and isomer distributions indicate that an electrophilic mechanism is likely for the chlorination reaction. Inverse addition experiments were used to show that a surface-bound chlorinating agent is produced by the interaction of hydrogen chloride with the fly ash surface. Recycling experiments indicate that the fly ash surface functions as a stoichiometric oxidant, and not a catalyst, in promoting aromatic chlorination by HCl. The yields of aromatic chlorination on three different fly ash samples were found to be quite different. A variety of physical and chemical characterizations revealed no simple relationship between properties and yields. It was noted that the reaction of hydrogen chloride with the fly ash surface led to release of metal ions from the surface upon solvent extraction. It was found that the release of iron from the surface corresponded to the changing yields noted.

Chlorine substitution reactions of polycyclic aromatic hydrocarbons on fly ash from coal-fired power plants

Naphthalene, anthracene, and biphenyl were individually adsorbed in fly ash from a coal-fired power plant and treated with HCl(g) in nitrogen at 150°C. Products from the reaction included mono- and poly-chlorinated cogeners of the parent PAH at total yields of ca. 9–15% for all products. Brominated aromatic products, observed in identical studies using municipal incinerator fly ash, were not detected in significant amounts. The results suggest that the absence of chlorinated compounds in coal combustion effluent can not be attributed to chemical properties of fly ash surfaces involved in heterogeneous gas-solid phase reactions. Alternate explanations should be sought in the low levels of HCl in the effluent stream or the chemistry of the combustion event.