Fly Ash and Its Usage

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    ASH AND ITS USAGES

    What is Ash and how it is generated:

    Ash is the non-combustible mineral portion of coal. It is produced by coal-fired

    electric and steam generating plants. Typically, coal is pulverized and transported

    with hot primary air into the boiler's combustion chamber where it immediatelyignites, generating heat. Boiler tubes absorb heat from the combustion chamber.

    During this process ash in the coal comes out as residue. Coarse ash particles,

    referred to as bottom ash, fall to the bottom of the combustion chamber, while the

    lighter fine ash particles, termed fly ash, remain suspended in the flue gas. Prior to

    exhausting the flue gas, fly ash is removed by particulate emission control devices,

    such as electrostatic precipitators or fabric filters.

    Types of Ash

    Bottom Ash

    Bottom ash is a coarse, granular, incombustible byproduct that is collected from

    the bottom of furnaces that burn coal for the generation of steam, the production of

    electric power, or both. The grain sizes span from fine sand to fine gravel. This is

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    characterized by comparatively higher carbon content and coal size. This

    comprises of less than 20 % ash produced in a power station. It is good for fills,

    embankments and road construction.

    Fly Ash

    Fly ash, also, is produced from the combustion of coal in electric utility or

    industrial boilers. These are the finest of coal ash particles. It is called "fly" ash

    because it is transported from the combustion chamber by exhaust gases. Fly ash is

    the fine powder formed from the mineral matter in coal, consisting of the

    noncombustible matter in coal plus a small amount of carbon that remains from

    incomplete combustion. This kind of ash is collected from the different fields of

    electrostatic precipitators (ESP), an ash collection device. This is characterizes by

    comparatively lower carbon and higher fineness. Fineness of fly ash is more insubsequent fields of ESP compared to initial fields. This ash comprises of more

    than 80 % of the total ash produced. Fly ash is generally light tan in color and

    consists mostly of silt-sized and clay-sized glassy spheres. This gives fly ash a

    consistency somewhat like talcum powder. Properties of fly ash vary significantly

    with coal composition and plant-operating conditions.

    There are two basic types of fly ash:

    (i) Class F: Produced by burning anthracite or bituminous coal.

    (ii) Class C: Produced by burning lignite or sub-bituminous coal.

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    Pond Ash

    The slurry formed after mixing ash (both fly and bottom ash) with water is pumped

    to the nearby ash pond, wherein water is drained / decanted away. The ash stored

    in ash pond is called pond ash.

    Properties of Fly Ash

    Physical properties:

    Size and Shape:

    Fly ash is typically finer than Portland cement and lime. Fly ash consists of silt-sized particles which are generally spherical, typically ranging in size between 10

    and 100 micron. Fineness is one of the important properties contributing to the

    pozzolanic reactivity of fly ash.

    Particle size distribution:

    Fly ash consists of fine, powdery particles that are predominantly spherical in

    shape, either solid or hollow, and mostly glassy (amorphous) in nature. The

    carbonaceous material in fly ash is composed of angular particles. The particle sizedistribution of most bituminous coal fly ashes is generally similar to that of silt

    (less than a 0.075 mm or No. 200 sieve). Although sub-bituminous coal fly ashes

    are also silt-sized, they are generally slightly coarser than bituminous coal fly

    ashes.

    Colour:

    Fly ash can be tan to dark gray, depending on its chemical and mineral

    constituents. Tan and light colors are typically associated with high lime content. A

    brownish color is typically associated with the iron content. A dark gray to blackcolor is typically attributed to an elevated unburned carbon content. Fly ash color

    is usually very consistent for each power plant and coal source.

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    Chemical properties:

    The chemical properties of fly ash are influenced to a great extent by those of the

    coal burned and the techniques used for handling and storage. There are basically

    four types, or ranks, of coal, each of which varies in terms of its heating value, its

    chemical composition, ash content, and geological origin. The four types, or ranks,of coal are anthracite, bituminous, sub-bituminous, and lignite. In addition to being

    handled in a dry, conditioned, or wet form, fly ash is also sometimes classified

    according to the type of coal from which the ash was derived. The principal

    components of bituminous coal fly ash are silica, alumina, iron oxide, and calcium,

    with varying amounts of carbon, as measured by the loss on ignition (LOI). Lignite

    and sub-bituminous coal fly ashes are characterized by higher concentrations of

    calcium and magnesium oxide and reduced percentages of silica and iron oxide, as

    well as lower carbon content, compared with bituminous coal fly ash. Very little

    anthracite coal is burned in utility boilers, so there are only small amounts ofanthracite coal fly ash. American Association of State Highway Transportation

    Officials (AASHTO) M 295 [American Society for Testing and Materials (ASTM)

    Specification C 618] defines the chemical composition of Class C and Class F fly

    ash. Table below compares the normal range of the chemical constituents of

    bituminous coal fly ash with those of lignite coal fly ash and sub-bituminous coal

    fly ash. From the table, it is evident that lignite and sub-bituminous coal fly ashes

    have a higher calcium oxide content and lower loss on ignition than fly ashes from

    bituminous coals. Lignite and sub-bituminous coal fly ashes may have a higher

    concentration of sulfate compounds than bituminous coal fly ashes. The chief

    difference between Class F and Class C fly ash is in the amount of calcium and thesilica, alumina, and iron content in the ash. In Class F fly ash, total calcium

    typically ranges from 1 to 15 percent, mostly in the form of calcium hydroxide,

    calcium sulfate, and glassy components in combination with silica and alumina. In

    contrast, Class C fly ash may have reported calcium oxide contents as high as 30 to

    40 percent. Another difference between Class F and Class C is that the amount of

    alkalis (combined sodium and potassium) and sulfates (SO4) are generally higher

    in the Class C fly ashes than in the Class F fly ashes.

    Table- Oxides in Fly ash and Portland cement

    Compound Fly ash class F Fly ash class C Portland Cement

    SiO2 55 40 23

    Al2O3 26 17 4

    Fe2O3 7 6 2

    CaO(LIME) 9 24 64

    MgO 2 5 2

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    SO3 1 3 2

    Source: http://www.fhwa.dot.gov

    Pozzolanic properties:

    Fly ash is most commonly used as a pozzolan in PCC applications. Pozzolans are

    siliceous or siliceous and aluminous materials, which in a finely divided form and

    in the presence of water, react with calcium hydroxide at ordinary temperatures to

    produce cementations compounds.

    Usages of Ash

    1.Fly Ash in Cement

    Ash find its main and maximum use in manufacturing of cement. To understand

    the use of fly ash in cement,

    2.Fly Ash in Portland cement Concrete

    Fly ash is used in concrete admixtures to enhance the performance of concrete.

    Portland cement contains about 65 percent lime. Some of this lime becomes free

    and available during the hydration process. When fly ash is present with free lime,

    it reacts chemically to form additional cementitious materials, improving many ofthe properties of the concrete.

    3.Fly Ash in Stabilized Base Course

    Fly ash and lime can be combined with aggregate to produce a quality stabilized

    base course. These road bases are referred to as pozzolanic-stabilized mixtures

    (PSMs). Typical fly ash contents may vary from 12 to 14 percent with

    corresponding lime contents of three to five percent. Portland cement may also be

    used in lieu of lime to increase early age strengths. The resulting material is

    produced, placed, and looks like cement-stabilized aggregate base.

    4.Fly Ash in Flow-able Fill

    Flowable fill is a mixture of coal fly ash, water, and Portland cement that flows

    like a liquid, sets up like a solid, is self-leveling, and requires no compaction or

    vibration to achieve maximum density. In addition to these benefits, a properly

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    designed flowable fill may be excavated later. For some mixes, an optional filler

    material such as sand, bottom ash, or quarry fines, is added. Flowable fill is also

    referred to as controlled low-strength material, flowable mortar, or controlled

    density fill. It is designed to function in the place of conventional backfill materials

    such as soil, sand, or gravel and to alleviate problems and restrictions generally

    associated with the placement of these materials.

    5.Fly Ash in Structural Fills/Embankments

    Fly ash can be used as a borrow material to construct fills and embankments. When

    fly ash is compacted in lifts, a structural fill is constructed that is capable of

    supporting highway buildings or other structures. Fly ash has been used in the

    construction of structural fills/embankments that range from small fills for road

    shoulders to large fills for interstate highway embankments.

    6.Fly Ash in Soil Improvement

    Fly ash is an effective agent for chemical and/or mechanical stabilization of soils.

    Typical applications include: soil stabilization, soil drying, and control of shrink-

    swell.

    Benefits:

    Fly ash provides the following benefits when used to improve soil conditions:1. Eliminates need for expensive borrow materials.

    2. Expedites construction by improving excessively wet or unstable sub-grade

    3. By improving sub-grade conditions, promotes cost savings through reduction in

    the required pavement thickness.

    4. Can reduce or eliminate the need for more expensive natural aggregates in the

    pavement cross-section

    7. Fly Ash in Asphalt Pavements:

    Fly ash can be used as mineral filler in HMA paving applications. Mineral fillersincrease the stiffness of the asphalt mortar matrix, improving the rutting resistance

    of pavements, and the durability of the mix.

    8.Fly Ash in Bricks

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    9. Fly Ash in Asbestos sheets

    In the process of asbestos sheet manufacturing, certain percentage of ashis used as replacement of cement.

    10. Manufacture of Fertilizers

    Fly ash mixed with dewatered sewage sludge can be utilized in manufacture of

    fertilizer. M/s N-VIRO of USA has developed a process for production of N-VIRO

    soil which can be used in agricultural and horticultural purpose. Similarly, M/s.

    Roy Research and Technology has developed a fly ash based synthetic soil

    enriched with lignocelluloses nitrogen with all the bio character.

    11. Pond Ash for Embankments

    Pond ash (collected from ash ponds) exhibit good shear strength comparable to

    soils. Due to its good permeability, it can be used in rainy seasons. It is also easier

    to compact as its moisture density curve is more even.

    12. Pond Ash for roads

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