119657695 Fuels and Combustion

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    1. FUELS AND COMBUSTION

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    Introduction to Fuels liquid, solid and gaseous fuels Criteria for fuel selection

    Availability

    Storage

    Handling

    landed cost of fuel

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    Properties of Liquid Fuels

    Specific gravity

    Viscosity

    Flash Point

    Pour Point

    Specific Heat

    Calorific Value

    Carbon Residue Water Content

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    Physical Properties Heating Value

    Analysis of Coal

    ultimate analysis

    proximate analysis

    Table 1.4 GCV for Various CoalsParameter Lignite

    (Dry

    Basis)

    IndianCoal

    IndonesianCoal

    South AfricanCoal

    GCV(kcal/kg)

    4,500* 4,000 5,500 6,000

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    Properties of Coal

    Anthracite, bituminous, and lignite

    Grade Calorific Value Range

    ( in kCal/kg)A

    BC

    DE

    FG

    Exceeding 6200

    5600 62004940 5600

    4200

    49403360 4200

    2400 33601300 2400

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    Storage, Handling and Preparation of Coal carpet loss

    spontaneous combustion

    http://images.google.co.in/imgres?imgurl=www.dfd.dlr.de/projects/coalfire/Overview/images/coal_process.jpg&imgrefurl=http://www.dfd.dlr.de/projects/coalfire/Overview/html/spontaneous_combustion.html&h=368&w=640&prev=/images%3Fq%3Dspontaneous%2Bcombustion%2Bof%2Bcoal%26svnum%3D10%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26sa%3DN
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    Preparation of Coal

    Sizing of CoalS. No. Types of Firing System Size (in mm)

    1. Hand Firing

    (a) Natural draft

    (b) Forced draft

    25-75

    25-40

    2. Stoker Firing(a) Chain grate

    i) Natural draft

    ii) Forced draft

    (b) Spreader Stoker

    25-40

    15-25

    15-25

    3. Pulverized Fuel Fired 75% below 75

    micron*

    4 Fluidized bed boiler < 10 mm

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    3 Ts of Combustion

    TIME

    All combustion requires sufficient Time which depends

    upon type of Reaction

    TEMPERATURE

    Temperature must be more than ignition temperature

    TURBULENCE

    Proper turbulence helps in bringing the fuel and air in

    intimate contact and gives them enough time to

    complete reaction.

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    Air for combustion

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    Combustion of Coal Excess air required for coal combustion

    depends on the type of coal firing equipment Hand fired boilers use large lumps of coal and hence need

    very high excess air.

    Stoker fired boilers use sized coal and hence require less

    excess air. Also in these systems primary air is suppliedbelow the grate and secondary air is supplied over thegrate to ensure complete combustion.

    Fluidised bed combustion in which turbulence is created

    leads to intimate mixing of air and fuel resulting in furtherreduction of excess air.

    The pulverized fuel firing in which powdered coal is firedhas the minimum excess air due to high surface area ofcoal ensuring complete combustion.

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    Coal Combustion

    Presence of silica,calcium oxide,magnesiumoxides etc. in ash

    lead to a lowfusiontemperature andClinker formation.

    Ash fusiontemperature aslow as 1100oC

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    Draft Natural Draft

    It is the draft produced by a chimney alone. It is caused by the

    difference in weight between the column of hot gas inside thechimney and column of outside air of the same height and crosssection

    Balanced Draft Forced-draft (F-D) fan (blower) pushes air into the furnace and

    an induced-draft (I-D) fan draws gases into the chimneythereby providing draft to remove the gases from the boiler

    Negative 0.05 to 0.10 in wg in boiler and slightly + ive inFurnace

    Induced Draft

    An induced-draft fan draws enough draft for flow into thefurnace, causing the products of combustion to discharge toatmosphere

    Forced Draft The Forced draft system uses a fan to deliver the air to the

    furnace, forcing combustion products to flow through the unitand up the stack

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    THANKS