Utilisation of FLY ASH

8/13/2019 Utilisation of FLY ASH

1/4

VSRD International Journal of Electrical, Electronics & Communication Engineering, Vol. III Issue XI November 2013 / 403

e-ISSN : 2231-3346, p-ISSN : 2319-2232 VSRD International Journals : www.vsrdjournals.com

RESEARCH ARTICLE

AN ANALYSIS FOR UNDERSTANDING OF UTILIZATION

AND CHARACTERISTICS OF POWER PLANT WASTAGE : FLY-ASH

1Mohammad Minhaj Jafri* and 2Pradeep Kumar1Research Scholar, 2Associate Professor, 1,2Department of Civil Engineering,

Harcourt Butler Technological Institute, Kanpur, Uttar Pradesh, INDIA.

*Corresponding Author: [email protected]

ABSTRACT

Structural development in developed as well as in developing country is very fast. In the development of any structure, two criteria is

most important i.e. strength and stiffness so to maintain the proper strength, cement is the most important constituent because thestrength of any structure will depend upon the quality of cement which is also important due to the safety of the structure. The qualityof the cement will depend upon the quality and properties of the fly ash so this work will show the importance and properties of the fly

ash for producing good quality of cement. Fly ash coming from the thermal power plant is an important constituent. Due to itscompressibility, fly ash behaves very much like a cohesive soil with respect to consolidation and due to age hardening or pozollanic

behavior; the shear strength of the fly ash can change.

Keywords : Fly ash, Structure, Cement, Power Plant, DoC.

1. INTRODUCTIONThe increasing demand for electrical energy has resultedin the construction of many coal fired power plants and asa result the production of power plant wastes has also

increased. The economic and environmentalconsiderations affecting the collection and disposal ofpower plant wastes have received much attention in

recent years. Coal combustion results in a residueconsisting of the inorganic mineral constituents in thecoal and the organic matter which is not fully burnt. The

inorganic mineral constituents, whose residue is ash,make up from 30 to 40% of the coal. During combustionthis ash is distributed into two parts bottom ash (collected

from the bottom of the boiler unit) and fly ash (most ofwhich is collected by air pollution control equipmentthrough which the stack gases pass). A third residue

vapors is that part of the coal which is volatilized in thefurnace. Fly ash makes up from 10 to 85% of the coal ashresidue and occurs as spherical particles, usually ranging

in diameter from 0.5 to 100 microns. Major constituents

of fly ash are silica, alumina, iron and calcium oxide withsmaller quantities of various oxides including

magnesium, sodium, titanium, potassium and sulphur. Itscolor varies from light tan to black depending uponcarbon content.

2. GRAIN-SIZEDISTRIBUTIONIn terms of typical soil grain-size analysis, most of ashparticles fall within the silt range, with small percentages

in the fine sand and clay sizes. The grain size distributionof fly ash provides considerable insight into its propertiesand behavior. The range of grain-size distributions for fly

ash is shown in Fig. 2.1, which also indicates therelatively uniform grain-size distribution of fly ash ascompared to several types of soils. Because of itsspherical shape small surface areas, and uniform silt sizeof individual particles, fly ash has no plasticity. A more

meaningful measure used to indicate the fineness of fly

ash is the Blaine Fineness. This usually ranges from 1700cm2/gm in fly ashes from mechanical collectors to 5400

cm2/gm in fly ashes from electrostatic precipitators.Another measure of fineness is surface area, which is theproduct of the Blaine Fineness and specific gravity and isusually specified in terms of cm2/cm3. It is likely that the

gradation and fineness are most influenced by the degree

of pulverization of the coal.

Fig. 2.1: Range of Typical Fly Ash Grain Sizes

3. CHEMICAL COMPOSITIONThe chemical composition of fly ash depends largely onthe geologic and geographic factors related to the coaldeposits, the combustion conditions and the removal

efficiency of air pollution control devices. Fly ash fromAmerican coals contains large quantities of silica (SiO2),

alumina (AI2O3), and ferric oxide (Fe2O3), with smallerquantities of various oxides and alkalis. The oxides of Si,Al, Fe and Ca comprise 95 to 99% of the composition ofash. The average values of different chemical constituentsin the Indian fly ashes fall within the range of the averagevalues reported for fly ashes produced in other countries.


2/4

Mohammad Minhaj Jafri and Pradeep Kumar VSRDIJEECE, Vol. III (XI) November 2013 / 404

Indian fly ashes are however, characterized by higher

contents of SiO2, AI2O3 and un-burnt fuel as determinedby loss on ignition (L.O.I.) and lower contents of Fe2O3and SO3. Indian fly ashes exhibit greater variation in their

composition partly due to variable quality of Indian coalsand partly due to lack of standardization in collection and

disposal plants.

4. FLY ASH CHARACTERISTICS Physical characteristics Specific gravity Permeability Shear strength CompressibilityPhysical Characteristics: Theash residue resulting from

the combustions of coal is primarily derived from theinorganic mineral matter in the coal. Different types of

coal produce different quantities of ash, depending uponthe concentration of mineral matter in the type of coal.Generally, ash makes up from 3 to 30% of the coal. Forthe majority of elements found in coal, most or their

quantity (95% or more) will be found in the ash fractionswhile the reminder (5% or less) will be discharged intothe atmosphere. The quantity of vapours produced

depends primarily on the temperature history of thecombustion gases and the concentrations and propertiesof the various elements in the coal. Fly ash is comprised

of very fine particles, the majority of which are glassyspheres, and rest of which are crystalline matter andcarbon. The ash varies in size as they are discharged from

the furnace from less than to 4 cm. in diameter. Finespherical particles generally vary in diameter from 0.5to 100 . This fraction spans a color range of light tan to

gray to black. Increased carbon content causes a darker-grey-black tone, while increased iron content tends toproduces a tan-colored ash. The pH of fly ash contacted

with water varies from 3 to 12.Cenospheres, the verylight weight, particles that float on ash pond surfaces, arean interesting fraction of the fly ash. These are silicate

glass spheres filled with nitrogen and carbon dioxidewhich vary from 20 to 200 in diameter. Particledensity ranges from 0.4 gm/cc to 0.8 gm/cc. These

particles may comprise as much as 5% by weight or 20%

by volume of the fly ash.

Specific Gravity: Specific gravity is frequentlydetermined in analyzing fly ash for chemical propertiesand for use as on additive in concrete. Fly ash is

characterized by low specific gravity, uniform gradationand lack of plasticity. The specific gravity of fly ash

particles varies with chemical composition and generallyvaries from about 2.1 to 2.6 with an average of about 2.4.Therefore fly ash fills tend to be less dense than thoseconstructed of natural soils. The reduction in density can

be an advantage in some applications where fly ash isused as a structural fill. One explanation for low specific

gravity is the fact that a high proportion of fly ashparticles are chemosphere or hollow particles.

Permeability: The coefficient of permeability of fly ash

depends upon its degree of compaction and the

pozzolanic activity. The range of coefficient ofpermeability for fly ash generally varies from 1x10-4 to5x10-4 cm/sec and from 3x10-2 to 9x10-2 cm/sec for

bottom ash. Low permeabilities lessen the probability ofextensive ground percolation and the consequent danger

of soluble material being leached out of the fill. Lowpermeabilities on the other hand, also mean a higherdegree of runoff; therefore precautions to prevent erosionof side slopes should be taken.

Shear Strength: Fig. 2.2 shows the general range of therelationship between the angle of internal friction and dry

unit weight for fly ash, It has been shown that fly ashpossesses significant cohesive strength due to capillarystresses in the pore water and that the shear strength of fly

ash can change significantly with time due to agehardening or pozollanic behaviour. Age hardening hasbeen correlated to the amount of free lime present in fly

ash. In some cases the strength increased as much as 5-8fold over a 3-month period. Fly ashes which have been

lagooned prior to compaction do not exhibit as much agehardening.

As with fly ash, the shear strength of bottom ash varieswith the degree of compaction. The angle of internalfriction for bottom ash in loose condition can vary from

38 to 42.5 averaging 41.

Compressibility: Fly ash behaves very much like acohesive soil with respect to consolidation. Laboratorytest results have indicated that compaction can

significantly reduce the compressibility of fly ash. Thispozzolanic behaviour tends to limit the extent of actualfield settlement in the long run. Partial saturation also

accounts for a considerable difference in compressibilitybehaviour. The field evidence to data suggests thatcompressibility or settlement is not a significant problem

in compacted fly ash.

5. EFFECTOFLIMEANDCEMENTSTABILIZATION

The addition of few percent of hydrated lime (up to 10%by weight) increased the compressive strength of the

compacted fly ashes more than 10-fold after one month ofmoist curing at 20C. The effect of pozzolanic activity forage hardening on fly ash compressibility is much more

evident in the case of fly ashes which have been mixedwith lime before compaction. Increase in the time of

curing before testing decreases settlement. Lime orcement treatment also decreases the permeability ofcompacted fly ash.

6. COMPACTION BEHAVIOURThe moisture density relationship for fly ash is similar tothat for cohesive soils. That is, for a given compactiveeffort the dry density increases with increasing moisture

content to a point of maximum dry density. Thecompacted dry densities of fly ash are commonly found

to be in the range of 1100-1520 kg/m3 and depend on themethod of its determination. Hopper and silo fly ashes


3/4


tend to have sharp, well defined points of maximum dry

density and optimum moisture content, with rapid declinein density values, on either side of optimum moisturecontent. Fly ash which has been exposed to large

quantities of moisture, such as lagoon ashes, tend to haveflatter moisture density curves with little change in dry

density occurring over a broad range of moisturecontents. Maximum dry densities of lagoon ashes tend tobe lower and occur at higher moisture contents than thosefor hopper and silo fly ashes. Stock piled fly ashes tend tohave intermediate values of maximum dry density. Whilethe dry density and compaction behaviour is of interest intesting and quality control, the wet density is needed in

conceptual design of fly ash fills. Calculation ofsettlement and slope stability are all determined using wetdensity values.

7. RESULTS AND DISCUSSIONThis work shows that fly ash coming from the thermalpower plant is an important constituent due to its

properties like compressibility, physical characteristics,shear strength, permeability. Due to its compressibility,fly ash behaves very much like a cohesive soil with

respect to consolidation and due to age hardening orpozollanic behavior; the shear strength of the fly ash canchange. In the pore water, fly ash possesses significant

cohesive strength due to capillary stresses. Thecoefficient of permeability of fly ash depends upon itsdegree of compaction and the pozzolanic activity. Fly ash

is characterized by low specific gravity, uniformgradation and lack of plasticity.

8. CONCLUSIONStudy of fly ash properties shows that the values of mostof the parameters investigated and reviewed throughliterature, fall within a relatively narrow range of values.

The principal chemical constituents are silica, aluminaand iron oxide with smaller amounts of calcium,magnesium, sulphur and titanium. Fly ash falls within the

silt range with small percentage in the fine sand.

9. REFERENCES[1] William G. Davids, Hui Zhang,(2008), Beam finite

element for nonlinear analysis of pressurized fabric beamcolumns, Journal of engineering structure, Vol.30pp.1969-1980.

[2] Yong-Hak Leea, Won-Jin Sungb, Tae-Hyung Leea, Kee-Won Seong, (2007) Finite element formulation of acomposite double T-beam subjected to torsion,Journal ofengineering structure, Vol.29 pp. 29352945.

[3] P. Nanakorn, L.N. Vu, (2006),A2Dfield-consistent beamelement for large displacement analysis using the totalLagrangian formulation Journal of finite element inanalysis and design, Vol.42 pp. 1240 1247.

[4] Mohamed Taktak, Fakhreddine Dammak, Said Abid,Mohamed Haddar, (2005), A mixed-hybrid finite element

for three-dimensional isotropic helical beam analysis,

International Journal of Mechanical Sciences, Vol.47 pp.209-229.[5] D. Zupan, M. Saje(2004),On A proposed standard set of

problems to test finite element accuracy: the twisted

beam, Journal of finite element in analysis and design,

Vol.40 pp.1445-1451.[6] Dr. V.N.S. Murthy, (2003), Soil Mechanics and

Foundation Engineering Vol.5, pp. 240-302.[7] Dr. K.R. Arora, (2003), Soil Mechanics and Foundation

Engineering Vol.6, pp. 440-477.[8] McLaren, R.J. and A.M. Digioia, (1987), The typical

engineering properties of fly ash, Geotechnical practice

for waste disposal, Vol.13, pp.87.[9] Dayal U., (1987), Design of fly ash disposal facility,

Indian Geotechnical Conference, Vol. 1, pp. 87.

[10]Digioia, Jr., A.M., J.F., Meyers, and J.E. Niece, (1977),Design and construction of fly ash disposal sites,Geotechnical Practice for Disposal of Solid WasteMaterials.


4/4


Documents

Utilisation of FLY ASH