Fly Ash Sorbent Research Articles

Fly ash sorbent modified with KMnO4 for the separation of important radionuclides

AbstractThe main aim of this work was to develop a suitable sorbent for the separation and determination of 226Ra through 133Ba (radio tracer) in water samples using fly ash sorbent. After the modification with KMnO4 the effects of pH, competing ions, the possibility of elution, and the effect of water volume were tested. As a suitable eluent 6 mol/L HCl was chosen, while the sorbent worked best at pH 6–8. The developed method is advantageous for minimizing the time required for separation, the volume of chemicals used, and the waste generated after separation.

Impregnated fly ash sorbent for cesium-137 removal from water samples

Cesium-137 is an important indicator of radioactive pollution in the aquatic environment. The aim of this study was to increase the sorption capacity of fly ash by its modification using hexacyanoferrate impregnation. According to the results, the Langmuir model was more likely to be correct than the Freundlich model. The maximum sorption capacity qmax was 1.59 mmol/g, i.e. 210 mg Cs+ per 1 g of the prepared sorbent. There was no significant decrease in the sorption capacity in the presence of both monovalent and divalent competitive cations. At the volume of 1–42 L the yield of Cs sorption was above 90%.

Using modified fly ash for mercury emissions control for coal-fired power plant applications in China

More than 50% of the mercury emissions in China come from coal-fired power plants. Coal-fired power plants also consume more than 50% of the coal used in China, and this proportion is expected to increase according to Chinese energy planning agencies, despite recent actions aiming at more stringent environmental constrains. This study reports mercury emissions measurements of several power plants in China. The results show that the mercury concentration in the flue gas has variability between 0.19 and 11.30μg/Nm3. At the same time, the co-benefit of mercury removal by the existing pollutant control devices at those plants was highly variable, ranging from 17.8% to 96.7%. This motivated research on developing and testing a modified fly ash sorbent that would be competitive due to its low cost, in relative abundance, and adequate for convenient injection at a power plant. Full-scale injection and adsorption experiments were performed at a 300MW plant. It was found that the use of a modified fly ash provide an additional mercury emissions reduction, in addition to the inherent natural mercury capture by the host unit of 30% due to additional fly ash adsorption. Implementation of this additional control option would help achieve combined mercury removal efficiencies in the 75–90% range.

The Removal of Elemental Mercury by Potassium Permanganate-modified Rice Husk Ash Sorbents

The elemental mercury (Hg0) capture of potassium permanganate (KMnO4)-modified calcium-based with rice husk ash sorbent (CaO/RHA) was first tested to evaluate the effectiveness of Hg0 adsorption. The KMnO4 modified CaO/RHA performances were compared with that of modified Ca-based with fly ash sorbent (CaO/FA). The effects of supports, pore size distribution, oxidants loading, temperature, and gas mixture were also investigated to understand the mechanism in capturing Hg0. The CaO/RHA/KMnO4 sorbent could remove over 90% of Hg0 under the simulated flue gas.

Experimental and model investigation on the mass balance of a dry circulating fluidized bed for flue gas desulfurization system

A moderate temperature dry circulating fluidized bed flue gas desulfurization (CFB-FGD) process was developed using rapidly hydrated sorbent. This technique has the advantages of low cost, no water consumption, and a valuable dry product CaSO4. To keep the system operation stable, a mass balance model, based on cell model considering flow state, particle abrasion, particle residence time, particle segregation and desulfurization processes, was built to predict the system state and optimize the operating condition. Experimental studies were conducted on a pilot-scale CFB-FGD system with rapidly hydrated sorbent made from CFB circulating ash and lime (circulating ash sorbent) or coal fly ash and lime (coal fly ash sorbent). Calculated results were compared with experimental results and the relative error was less than 10%. The results indicated that feed sorbent mass, feed sorbent size, superficial gas velocity, particle abrasion coefficient and cyclone efficiency had significant influence on the mass balance of CFB system. The circulating ash sorbent was better than the coal fly ash sorbent, for providing higher desulfurization efficiency and being better for the CFB-FGD system to achieve mass balance.

Removal of elemental mercury by iodine-modified rice husk ash sorbents

Iodine-modified calcium-based rice husk ash sorbents (I 2/CaO/RHA) were synthesized and characterized by X-ray diffraction, X-ray fluorescence, and N 2 isotherm adsorption/desorption. Adsorption experiments of vapor-phase elemental mercury (Hg 0) were performed in a laboratory-scale fixed-bed reactor. I 2/CaO/RHA performances on Hg 0 adsorption were compared with those of modified Ca-based fly ash sorbents (I 2/CaO/FA) and modified fly ash sorbents (I 2/FA). Effects of oxidant loading, supports, pore size distribution, iodine impregnation modes, and temperature were investigated as well to understand the mechanism in capturing Hg 0. The modified sorbents exhibited reasonable efficiency for Hg 0 removal under simulated flue gas. The surface area, pore size distribution, and iodine impregnation modes of the sorbents did not produce a strong effect on Hg 0 capture efficiency, while fair correlation was observed between Hg 0 uptake capacity and iodine concentration. Therefore, the content of I 2 impregnated on the sorbents was identified as the most important factor influencing the capacity of these sorbents for Hg 0 uptake. Increasing temperature in the range of 80–140°C caused a rise in Hg 0 removal. A reaction mechanism that may explain the experimental results was presumed based on the characterizations and adsorption study.

Influence of hydration variables on the properties of South African calcium/siliceous-based material

This study presents findings from experiments on the preparation and characterization of locally available fly ash, quicklime and the CaO/fly ash sorbent, synthesized using the atmospheric hydration process. The CaO was obtained from calcination of limestone in a laboratory kiln at a temperature of 900°C. The sorbents were prepared under different hydration conditions: CaO/fly ash weight ratio (1°1 to 1°3), hydration temperature (55°C–75°C) and hydration period (4–8 h). Results show that the specific surface area of CaO/ fly ash sorbents (8.8–23.6 m2/g) was higher than that of the CaO (4.78 m2/g) at all preparation conditions. The SEM micrographs show that the prepared sorbent had a more porous structure than either the fly ash or the CaO. The X-ray diffraction (XRD) analysis shows the presence of complex compounds containing calcium silicate hydrate in the synthesized sorbents. This contributed to the high BET specific surface area. The Brunauer-Emmett-Teller (BET) specific surface area was found to decrease with increase in the amount of fly ash with the ratio of 1:1 (CaO/Fly ash) giving the highest value. It was also found that an increase in the hydration time resulted in an increased BET specific surface area, although there was only a slight effect on the same by an increase in temperature.

Development of kinetic model for the reaction between SO2/NO and coal fly ash/CaO/CaSO4 sorbent

Sorbents for semidry-type flue gas desulfurization (FGD) process can be synthesized by mixing coal fly ash, calcium oxide, and calcium sulfate in a hydration process. As sorbent reactivity is directly correlated with the specific surface area of the sorbent, reacting temperature, concentration of the reacting gas species and relative humidity, two major aim in the development of a kinetic model for the FGD process are to obtain an accurate model and at the same time, incorporating all the parameters above. Thus, the objective of this work is to achieve these two aims. The kinetic model proposed is based on the material balance for the gaseous and solid phase using partial differential equations incorporating a modified surface coverage model which assumes that the reaction is controlled by chemical reaction on sorbent grain surface. The kinetic parameters of the mathematical model were obtained from a series of experimental desulfurization reactions carried out under isothermal conditions at various operating parameters; inlet concentration of SO2 (500ppm⩽C0,SO2⩽2000ppm), inlet concentration of NO (250ppm⩽CO,NO⩽750ppm), reaction temperature (60°C⩽T⩽80°C) and relative humidity (50%⩽RH⩽70%). For a variety of initial operating conditions, the mathematical model is shown to give comparable predictive capability when used for interpolation and extrapolation with error less than 7%. The model was found useful to predict the daily operation of flue gas desulfurization processes by using CaO/CaSO4/coal fly ash sorbent to remove SO2 from flue gas.

Preparation and Characterization of CaO/CaSO4/Coal Fly Ash Sorbent for Sulfur Dioxide (SO2) Removal: Part I

A 2 level full factorial design of experiment was used to evaluate the significance of four sorbent preparation variables towards the structural properties of sorbent prepared from coal fly ash, calcium oxide (CaO) and calcium sulfate (CaSO4) for flue gas desulfurization. The structural properties studied were BET specific surface area and average pore size of the sorbent while the experimental sorbent preparation variables studied were hydration period (x 1), ratio of CaO to fly ash (x 2), amount of CaSO4 (x 3), and drying temperature (x 4). The surface area and average pore diameter of the sorbent obtained in this work range from 12.9 to 92.7 m2/g and 48.4 to 159.5 nm, respectively. The results revealed that there were significant influence of all the variables studied on the average pore diameter of the sorbent while only variables x 1, x 2, and x 3 had significant influence on the surface area of the sorbent. X-ray diffraction (XRD) analysis revealed that the main phases detected in the sorbent was calcium alumino silicate hydrate compound. Scanning electron microscope (SEM) analysis showed that the sorbent consists of irregular shape particles that have a high structural porosity.

Optimization of Process Parameters for the Preparation of CaO/CaSO4/Coal Fly Ash Sorbent for Sulfur Dioxide (SO2) Removal: Part II

A two-step optimization strategy was employed to optimize the surface area of sorbent prepared from coal fly ash, calcium oxide (CaO) and calcium sulfate (CaSO4) for flue gas desulfurization. In the first step, a 3 level full factorial design of experiment was used to develop a regression model equation to correlate the significant experimental sorbent preparation variables to the surface area of the resulting sorbent. The three experimental sorbent preparation variables studied are hydration period (x 1), ratio of CaO to fly ash (x 2) and amount of CaSO4 (x 3). In the subsequent step, response surface methodology was used to identify the experimental sorbent preparation variables that maximize the surface area of the sorbent. Through this two-step optimization strategy, it was found that at a hydration period of 10 hrs and drying temperature of 100°C, optimum surface area of 67.0 m2/g could be attained by using 5 grams of CaO, 13.7 grams of fly ash, and 7.4 grams of CaSO4 in the preparation mixture. The prediction was verified with experimental runs.

Modeling and Simulation of Flue Gas Desulfurization Using CaO/CaSO4/Coal Fly Ash Sorbent

Modeling and simulation of flue gas desulfurization over sorbent synthesized from CaO/CaSO4/coal fly ash in a fixed-bed reactor has been studied. A mathematical model was proposed based on the material balance for the gaseous and solid phase using partial differential equations to describe the adsorption of SO2 from a moving gas stream to the sorbent-bed of changing composition. The kinetic parameters of the mathematical model were obtained from a series of experimental desulfurization reactions carried out under isothermal conditions at various operating parameters; initial concentration of SO2 (500 ppm ≤ Co ≤ 2000 ppm), reaction temperature (333 K ≤ T ≤ 373 K) and relative humidity (40% ≤ RH ≤ 70%). The partial differential equations were solved using a finite difference method. The model was found to give a very good description of the experimental data with an error less than 10%. The validated model was then used to predict the reactor performance under different modes of operation. It was found that higher relative humidity in the feed gas and higher reaction temperature increases the desulfurization activity of the sorbent. On the other hand, higher initial concentrations of SO2 reduce the desulfurization activity of the sorbent.

Effect of [formula omitted] ash weight ratio on the kinetics of the reaction of [formula omitted] ash sorbents with [formula omitted] at low temperatures

Ca ( OH ) 2 / fly ash sorbents were prepared with different weight ratios. Their reactions with SO 2 have been studied under the conditions similar to those in the bag filters of the spray-drying flue gas desulfurization system. The sorbent reactivity increased in general with decreasing Ca ( OH ) 2 / fly ash weight ratio. The reaction kinetics was well described by the surface coverage model which assumes that the sorbent was made up of plate grains and the rate was controlled by the chemical reaction on the grain surface and takes into account the surface coverage by products. For sorbents with Ca ( OH ) 2 / fly ash weight ratios than 10/90, the effect of weight ratio on the reaction was entirely represented by the effects of the intial specific surface area and the Ca content of the sorbent.

Kinetic Model for the Reaction of Ca(OH)2/Fly Ash Sorbents with SO2 at Low Temperatures

Sorbents prepared with different Ca(OH)2/fly ash weight ratios were reacted with SO2 under conditions similar to those in the bag filters in the spray-drying flue gas desulfurization system. The reaction kinetics was well described by a generalized surface coverage model developed in this study. The model assumes that the sorbent was made up of plate grains and the rate was controlled by the chemical reaction on the grain surface and takes into account the variation in sorbent Ca molar content (M-1) and the surface coverage byproducts. The effect of Ca(OH)2/fly ash weight ratio on the reaction was represented by the effects of the initial specific surface area (Sg0) and M-1 of the sorbent on the kinetic parameters. Under the same reaction conditions, the sorbents with weight ratios ≥ 30/70 had about the same initial reaction rate per unit surface area of the sorbent; their ultimate conversions increased linearly with increasing Sg0, being independent of M-1. The effects of relative humidity, SO2 concentration, and temperature on the reaction were unaffected by the Ca(OH)2/fly ash ratio. The results of this study are useful to the design and operation of the dry and semidry processes using Ca(OH)2/fly ash sorbents to remove SO2 from the flue gas.

Interaction of Cadmium and Flyash Sorbents in an Experimental Waste Incineration Furnace—Flue Gas System

The interaction of cadmium and different flyash sorbents has been examined in an experimental reactor under a variety of conditions simulating those found in waste incinerator furnace—flue gas systems. The experimental system consisted of two stainless steel reactors independently heated by electrical furnaces, enabling a temperature regime of between 100 and 1000°C to be investigated. Cadmium was vaporized in the furnace section of the reactor and the cadmium species formed were passed to the flue gas section where interaction with various flyash sorbents was investigated. The sorbents examined were three different aluminium oxides, silicon oxide and two aluminium silicates and the adsorption of cadmium species under various temperatures and flue gas compositions was determined. Flyash analysis consisted of a microwave extraction—ICP procedure to determine the adsorbed cadmium. In addition, SEM-EDX analysis of the surface of the flyash sorbents was used to characterize the surface adsorption. The results showed that at lower temperatures the cadmium species were adsorbed onto the flyash through physical adsorption whereas at temperatures above 765°C chemical adsorption occurred. SEM-EDX analysis of the surface showed that discrete particles of cadmium metal were produced in the flue gas system and were deposited on the flyash. The particle size of the flyash samples was found to influence the sorption of the cadmium. The presence of HCl and O2 in the flue gas altered the sorption capacity of the cadmium.

99/02784 Determination of the reactivity of Ca(OH) 2—fly ash sorbents for SO 2 removal from flue gases

99/02784 Determination of the reactivity of Ca(OH) 2—fly ash sorbents for SO 2 removal from flue gases

Determination of the reactivity of Ca(OH) 2–fly ash sorbents for SO 2 removal from flue gases

Ca(OH) 2 was activated towards SO 2 by hydrating it with various fly ashes collected from three different coal-fired power plants in Turkey. Reactive Ca(OH) 2–fly ash sorbents were prepared using both atmospheric and pressure hydration techniques. The hydrated sorbents were analyzed using mercury porosimeter, thermogravimetric analyzer and X-ray diffractometer. The relative surface area increments of sorbents are influenced by the hydration conditions and the source of the fly ash. The reactive species formed during hydration as detected with X-ray diffraction measurements, were found to be responsible for these surface area increments. Thermogravimetric measurements showed that decreasing the Ca(OH) 2 content of the hydrated sorbent which was presumably converted to Ca-containing reactive species caused an increase in the surface area of the sorbent. It was concluded that the reactivity of hydrated sorbents towards SO 2 closely related to the sorbent surface area and the presence of the reactive species in the sorbent.

Utilization of highly improved fly ash for SO2 capture.

A fly ash of low calcium content (1.67 wt% CaO) was ground (to particle sizes ofunder 5 μm) and treated with slaked lime (Ca(OH)2) to produce an improved sorbent for high-temperature dry sorbent injection. Four samples were treated at 350 K for 8 hours with different Ca(OH)2 concentrations (6.6–28.6 wt%). The specific pore surface area and pore volume were extensively characterized before and after treatment. Sulfation tests were performed by exposing the treated samples to a 0.31 vol% SO2 containing atmosphere at 1123 K in a thermo-gravimetric analyzer (TGA). The TGA experimental conditions were carefully controlled to closely simulate those encountered in an atmospheric fluidized bed combustor (AFBC).The treated fly ash sorbents showed higher SO2 sorptivity compared to untreated sorbents. The sorption capacity was found to increase as the Ca(OH)2 concentration increased in the range of interest. Up to 92% CaO conversion could be achieved when the 28.6% Ca(OH)2 fly ash sorbent was sulfated for 1 hour. Treatment of fly ash with Ca(OH)2 enhanced the specific surface area (by about 5 to 8 times), which was principally responsible for the high SO2 capture.