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The disposal or use of coal fly ash in ponds, landfills or in construction of roads is a great concern, because of its leaching behavior. It has problem especially when elements from fly ash get leached and contaminate or degrade the quality of drinking water. The aim of this study is to investigate leaching behavior of metals from northeastern coal fly ash. Serial batch leaching test for elements namely Fe, Mn, Zn, Cu and Pb has been carried out with liquid to solid (L/S) ratio at 4 and 12. The pH of the leachate was weekly alkaline in nature, ranging from 7.39 to 7.90. Concentration of Fe and Mn in the leachate is higher than the maximum contaminant level of drinking water. However, concentration of elements in the leachate from the northeastern fly ash is less than that of the Gondwana fly ash. This is possibly due to the different physico-chemical properties of the fly ash.
What is fly ash?
Coal is the only source till now widely spread all over resource contributing for maximum energy generation. Fly ash is generally the by-product produced due to the combustion process. It is actually captured from the chimneys of coal fired power plants. The bottom ash and fly ash jointly are known as coal ash, the bottom ash is generally removed from the bottom of coal furnaces.
The component of fly ash varies significantly, which depends on the makeup of coal being burned. But all the fly ash includes some amount of silicon dioxide (SiO2) both amorphous and crystalline and calcium oxide (CaO). Coal contains various trace elements in various quantities and during combustion process of coal they all get enriched as a result of carbon loss as carbon dioxide and trace elements get associated on the surface of ash particles due to evaporation and condensation. The characteristics of the coal used ant the type of installations used for the generation of solid combustion waste (fly ash) have a direct influence on chemical and mineralogical composition of coal .The physical and chemical properties of this industrial waste product, in general, are quite variable, as they are influenced by coal source, particle size, type of coal burning process, and degree of weathering.
Origin of fly ash
The fly ash produced from burning pulverized coal in a coal-fired boiler is a fine-grained, powdery particulate material that is carried off in the flue gas and usually collected by means of electrostatic precipitators, bag houses, or mechanical collection devices such as cyclones.
In general, there are three types of coal-fired boiler furnaces used in the electric utility industry. They are referred to as dry-bottom boilers, wet-bottom boilers, and cyclone furnaces. The most common type of coal burning furnace is the dry-bottom furnace.
When pulverized coal is combusted in a dry-ash, dry-bottom boiler, about 80 percent of all the ash leaves the furnace as fly ash entrained in the flue gas. When pulverized coal is combusted in a wet-bottom (or slag-tap) furnace, as much as 50 percent of the ash is retained in the furnace, with the other 50 percent being entrained in the flue gas. In a cyclone furnace, where crushed coal is used as a fuel, 70 to 80 percent of the ash is retained as boiler slag and only 20 to 30 percent leaves the furnace as dry ash in the flue gas. A general flow diagram of fly ash production in a dry-bottom coal-fired utility boiler operation is presented in Figure 1.1.
Figure 1.1: Generation of fly ash
Ash may contain As ,B, Be, Ca, Cd, Cr, Fe, Hg, Mg, Mo, Na, Ni, Pb, Ra, Se, Th, U,V, Zn, etc either on the surface of the ash particles or on the aluminosilicate matrix phase or in both as a consequence of condensation during combustion. While transporting, disposal and storage of fly ash, the residues of from coal combustion undergo a process called leaching due to rainfall or by weathering, by which an undesirable components get leached of into ground water as well as on surface water, creating pollution. These leached out components in higher concentration than drinking water can cause contamination in drinking standards.
Hence, it is important to predict the leaching behavior of the fly ash for the prevention of effects to the environment, especially for the aquatic environment, when ash is majorly disposed off in water. The toxic elements leached out from the fly ash can contaminate the soil, ground water and surface water. Therefore effective water management plans are required for the fly ash disposal..
The specific objective of this study is
To study the leaching behavior of fly ash
To compare the leaching study with Gondwana fly ash
To achieve the goal of the study, I have followed the following steps:
Literature review: I have gone through different books, journals and magazines to gain knowledge about the generation, production statistics of fly ash. I have also gone through literature on the different leaching tests such as batch and column tests.
Sample collection: I have collected samples from Naogaon Paper Mill, Shillong, Meghalaya, India.
Experiments carried out
Leaching test (Serial Batch test)
Elements in leachate: AAS (Atomic Absorption Spectrophotometer
Morphology: SEM (Scanning Electron Microscopy)
Mineralogy: XRD (X-Ray Diffractometer)
LOI (Loss on Ignition)
Composition of Fly ash: XRF.
results and discussion
COMPOSITION OF FLY ASH
HAZARD FROM FLY ASH
Coal is the only source till now widely spread all over resource contributing for maximum energy generation. While transporting, disposal and storage of fly ash, the residues of from coal combustion undergo a process called leaching due to rainfall or by weathering, by which an undesirable components get leached of into ground water as well as on surface water, creating pollution. These leached out components in higher concentration than drinking water can cause contamination in drinking standards. Hence, it is important to predict the leaching behavior of the fly ash for the prevention of effects to the environment, especially for the aquatic environment, when ash is majorly disposed off in water. Leaching studies are important in predicting the environmental impact associated with ash pond disposal techniques.
PRODUCTION OF FLY ASH
The process of coal combustion results in fly ash. The problem with fly ash lies in the fact that not only does its disposal require large quantities of land, water, and energy, its fine particles, if not managed well, by virtue of their weightlessness, can become airborne.
Around 110 million tonnes of fly ash was generated in 2008.
Expected to increase by 120 million tonnes by 2012.
65 acres of land occupied by ash ponds.
Table 2.1: Production of fly ash worldwide (Dhadse et al., 2008)
Composition of fly ash
The chemical properties of fly ash are generally influenced to a great extent by the chemical content of the coal burned, the air pollution control strategy at the power plant, and the techniques used for handling and storage.
Table 2.2 summarizes the normal range of chemical constituents of fly ashes from bituminous coal, lignite coal, and sub bituminous coal. Lignite and sub bituminous coal fly ashes have higher calcium oxide content than fly ashes from bituminous coals. Lignite and sub bituminous coal fly ashes may have a higher amount of sulfate compounds than bituminous coal fly ashes.
Table 2.2: Normal range of chemical composition for fly ash produced from different coal types (expressed as percent by weight).
Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash. The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e., anthracite, bituminous, and lignite).
Not all fly ashes meet ASTM C618 requirements, although depending on the application, this may not be necessary. Ash used as a cement replacement must meet strict construction standards, but no standard environmental standards have been established in the United States. 75% of the ash must have a fineness of 45Â Âµm or less, and have carbon content, measured by the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be fewer than 6%.
The reuse of fly ash as an engineering material primarily stems from its pozzolanic nature, spherical shape, and relative uniformity. Fly ash recycling, in descending frequency, includes usage in:
Portland cement and grout
Embankments and structural fill
Waste stabilization and solidification
Raw feed for cement clinkers
Stabilization of soft soils
Road sub base
Flow able fill
Mineral filler in asphaltic concrete
Other applications include cellular concrete, geopolymers, roofing tiles, paints, metal castings, and filler in wood and plastic products
For agriculture purpose
HOW FLY ASH IS HAZARDOUS?
Fly ash is a very fine powder and tends to travel far in the air. When not properly disposed, it is known to pollute air and water, and causes respiratory problems when inhaled. When it settles on leaves and crops in fields around the power plant, it lowers the yield.
It is a very difficult material to handle in dry state because it is very fine and readily airborne even in mild wind.
It disturbs the ecology of the region, being a source of soil, air and water pollution.
Long inhalation of fly ash causes silicosis, fibrosis of lungs, bronchitis, pneumonitis etc.
Flying fine particles of ash posses problems for people living near power stations
Eventual settlement of fly ash particles over many hectares of land in the vicinity of power station brings about perceptible degeneration in soil characteristics.
WHY NEED OF LEACHING STUDY?
While transporting, disposal and storage of fly ash, the residues of from coal combustion undergo a process called leaching due to rainfall or by weathering, by which an undesirable components get leached of into ground water as well as on surface water, creating pollution. These leached out components in higher concentration than drinking water can cause contamination in drinking standards.
Hence, it is important to predict the leaching behavior of the fly ash for the prevention of effects to the environment, especially for the aquatic environment, when ash is majorly disposed off in water
Leaching studies are important in predicting the environmental impact associated with ash pond disposal techniques.
MATERIAL AND METHODS
MATERIALS AND METHODS
Ash samples were collected simultaneously from Naogaon paper mill, Shillong, Meghalaya, India. The ash samples were dried before leaching and analysis. To understand the short term leaching effects serial Batch leaching test was carried out. The leaching test was carried out for different L/S ratios, 4 and 12. For L/S ratio 4 the known amount of sample taken was 12.5gm, and for L/S ratio 12 the known amount of sample taken was 5gm. The solution was made acidic in nature of pH 5.6. Both the L/S ratios were then stirred after every 1hr, continuing for 24 hrs. After 24 hrs the filtrations was carried out, and the lechate was added with some drops of acid for preserving the metals, and then stored in tightly capped acid washed bottles. For each next step ,additional 60 ml of solution was added along with the filter paper from the last step .Each leaching stage was carried out for 24 hrs .Similarlily the above mentioned procedure was followed for 4 days .Then the filtrate samples of all days were analyzed. The bottles were filled and rinsed with HCL and then with distill water before using in leaching test.
Element concentrations in the leachates were determined by AAS-200 Perkin Almer, using standard techniques. X-ray diffraction (XRD) of fly ash was carried out using a Phillips diffractometer, using CuKÎ± radiation.
Scanning electron micrographs of fresh and leached fly ash samples were obtained with a JEOL JSM-6480LV, a scanning electron microscope (SEM).
LOI (loss on ignition) was also calculated. First, the weight of crucible was measured and then a known weight of fly ash was taken in crucible. Then the crucible was covered with lid, and it was inserted in Muffle furnace for half an hour at 5500C. After ignition, the weight of crucible was measured along with sample. From the difference between weight of crucible with fly ash before ignition and after ignition was calculated, and was divided by sample taken.
RESULT AND DISCUSSION
MINERALOGY OF FLY ASH
XRD of fly ash samples confirm the presence of quartz, anhydrite, and hematite as the dominant phases, calcite present in trace. The presence of large humps and irregular background in diffractograms indicated the presence of glassy or amorphous constituents in the ash. The principal component of fly ash comprising of quartz, anhydrite and hametite along with minor phases of aluminosilcate in weathering environment may form clay minerals (Bradely et.al 1999) and elements may be released into solution and can contaminate groundwater (Spears et.al 1994).
Figure 4.1: XRD pattern of fly ash
The scanning electron micrograph of fly ash shows the presence of solid spherical cenospheres. The occurrence of rod shaped anhydrite crystal is also confirmed under SEM. In all the micrographs, the preleached fly ash spherical cenospheres were observed to be possessing relatively smooth surfaces compared to the postleached ash spherical cenospheres (Figure 4.2). EDX spectra also indicate the presence of high carbon and oxygen content and are due to the presence of high concentration of unburnt carbon (Figure 4.4). The unburnt carbon is clearly distinguishable under SEM due to very high porous structure (Figure 4.3).
Figure 4.2: SEM photomicrographs of cenospheres of fly ash before leaching (A) and after leaching (B).
Figure 4.3: Porous structure of the unburnt carbon in the fly ash
Figure 4.4: EDX spectra of the fly ash sample
Comparison of composition of Sub-bituminous coal fly ash
Fly ash of present study is compared with the fly ash from Gondwana fly ash (Table 4.1).It was observed that the major and minor elements concentration of present study fly ash was much higher than that of Gondwana fly ash, but the leached out elements concentration was higher in Gondwana fly ash than present study fly ash(Table 4.2). The LOI (loss on ignition) of present study was also much higher than the Gondwana because of presence of high unburned carbon in fly ash(Figure 4.3). The sulphur and Fe content is also high in present study, since the fly ash is produced from the high sulphur feed coal of northeast India. All the minor elements concentration in present study fly ash is higher than Gondwana leaving Mn, which is higher than Gondwana.
Table 4.1: Comparison of Composition of fly ash
Gondwana fly ash*
Present study fly ash
*Prahraj et.al. (2002)
Table 4.2: Concentration of elements in fly ash leachate at different L/S ratios (all values in ppm)
Present study (Northeastern fly ash)
Gondwana fly ash (Prahraj et al.,2002)
Maximum permissible limit (USEPA)
pH of solution
The pH of the leachate was ranging from 7.39 to 7.70 for L/S ratio 4 and for L/S ratio 12 it ranged from 7.43 to 7.90. Alkaline pH is due to the storage and use of fly ash with Lime in Paper mill. The pH of solution for L/S ratio 4 decreased with increase in contact time while for L/S ratio 12 the vice-versa occurred. The pH of solution was weakly alkaline in nature.
Table 4.3: pH of solutions in different days with L/S ratio 4 and 12
Table 4.4: Maximum Contaminant level for different elements for drinking water prescribed by the USEPA (Unit: ppm).
Maximum contaminant level
Figure 4.5 Comparison of Fe concentration Lechate of Gondwana and present study
Figure 4.6 Comparison of Zn concentration Lechate of Gondwana and present study
Figure 4.7: Comparison of Cu concentration Lechate of Gondwana and present study
Figure 4.8: Comparison of Mn concentration Lechate of Gondwana and present study
Serial Batch leaching test was carried out in this study. The serial batch leaching was carried out in the present study.
Fly ash has high LOI (loss on ignition) due to the presence of high unburned carbon content.
It was observed that Fe and Mn in the Lechate exceeded the maximum permissible limit of drinking water prescribed by WHO in some cases. Concentration of Fe and Mn increased with increase in contact time, whereas the values of Zn and Cu decreased with increase in contact time and they were under the maximum permissible limits.
The leachates had alkaline pH throughout the experiment because of storage of fly ash along with lime in paper mill industry.
Concentration of elements in the leachate was less in comparison to the Gondwana coal fly ash due to presence of very high unburnt carbon.