Combustion of Coal in Boiler and

Fireball FormationPrepared By:

Tarun Dogra

Animesh Okhade

Zalak Shah

CombustionCombustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat.

Combustion Reactions

C+ O2 = CO2 + 8084 kCals/kg of carbon 2C + O2 = 2CO + 2430kCals/kg of carbon 2H2 + O2 = 2H2O + 28,922 kCals/kg of hydrogen S + O2 = SO + 2,224 kCals/kg of sulphur

Complete vs Incomplete CombustionComplete Combustion • Involves complete burning

of fuel.• It takes place when there is

constant and enough oxygen supply as well as sufficient temperature.

• In complete combustion limited number of products are formed in contrast to incomplete combustion. If case of hydrocarbon, only carbon dioxide and water is produced.

• Results in more energy.

Incomplete Combustion• Involves partial burning of

fuel.

• Takes place when there isn’t enough oxygen supply or temperature.

• In case of hydrocarbon, carbon monoxide and carbon particles are also produced other than carbon dioxide and water.

• Results in less energy.

Reasons for incomplete combustion Lack of Oxygen/Air Improper turbulence Improper fuel sizing Inadequate fuel flows Inadequate fuel velocities Improper temperatures

3T’s of Combustion

1. Temperature high enough to ignite the fuel,2. Turbulence vigorous enough for the fuel

constituents to be exposed to the oxygen of the air, and

3. Time long enough to assure complete combustion.

The three requirements are best met by pulverized coal, which is forced into the furnace by an air stream under high pressure and is ignited as it enters through a nozzle.

Why Use Air Instead of Pure Oxygen?

Excess AirTo ensure there is enough oxygen to completely react with the fuel, extra combustion air is usually supplied. This extra air, called “Excess Air”.• Reduces amount of carbon monoxide, unburned fuel, soot

and smoke in exhaust.• Prevents surface fouling from emissions.

Too much excess air can lower boiler efficiency.o Wastes fuel by absorbing combustion heat and transporting

it out the exhaust flue.o Consider, too, that nitrogen, which makes up about eighty

percent of the air, plays no role chemically to produce heat. It does, however, add significantly to the weight of gas that absorbs heat energy.

Various Elements of Coal

1. Volatile Matter

2. Fixed Carbon

3. Ash Content, Moisture and other minerals.

PulverizationPulverization process coverts coal into powdered form with particle size of about 75micron.Advantages: Increases the surface area of coal and hence fuel-air

contact area increases. Reduces amount of excess air required. Ability to fire varying quality of coal. Quick responses to changes in load. Free from clinker or slagging troubles. Large amount of heat release possible.

Pulverization

Disadvantages: Capital cost is high as it requires additional pulverization

system. Operating cost is also high. This system produces fly ash (fine dust), which requires

costly fly- ash removal equipment like ESP. Flame temperatures are high, therefore it is necessary to

provide water cooled walls for safety of furnace. The possibility of explosion is more as coal burns like a gas. The storage of powdered coal requires special attention

and high protection from fire hazards.

Combustion Efficiency & Optimization

Why do we need Efficient Combustion ?

Pulverized unburned coal may fall back into the boiler

Erosion of mill, pipes and burner components

Increased levels of Nitrogen oxides and other harmful gases

Flame instability

Combustion Efficiency

Combustion Efficiency = 100 - (Stack heat losses/Fuel heating value*100)

Note: Efficiency calculated in percentage.

It is a measure of how effectively energy from the fuel is converted into useful energy.

Factors affecting Combustion Efficiency Amount of excess air

Stack Gas exit temperature

Reheat Stages

Steam temp & pressure

Why calculate Combustion efficiency when alternates are

available ?

Heat losses during combustion

Stack Heat Losses

Incomplete Combustion

Inappropriate amount of excess air

Moisture stuck in coal particles

Radiation Losses

Combustion in a boiler

Step 1. Pulverization of Coal

Coal crushed to the size of 25mm reaches coal mill

Further crushed to the size of 75 micron before firing in the boiler

Coal pulverized to increase fuel-air contact area

Feeding rate of coal according to the boiler demand controlled by computers

Step 2. Boiler Start-up

Oils used for start-up

LDO (Light Diesel Oil)

HFO (Heavy Fuel Oil)

Step 3. Fireball Formation

Methods for NOx reduction in the boiler during combustion

NOx emission harmful to environment

SOFA technique used to reduce the emission levels of NOx

Reduces combustion efficiency but helps to maintain NOx emission levels

Fuel Firing System

• Provide controlled, efficient conversion of the chemical energy of the fuel into heat energy which in turn is transferred to heat absorbing surfaces.

• It introduces the fuel and air for combustion, mix these reactants, ignite the combustible mixture and distribute the flame envelope and products of combustion.

Characteristics of Ideal Firing System No excess oxygen or unburned combustibles in the end

products of combustion. A low rate of auxiliary ignition energy input to initiate

combustion. An economic reaction rate between fuel and oxygen

compatible with acceptable N0x & SOx formation. Effective Handling and disposal of solid fuel impurities. Wide and stable firing range. Fast responses to changes in firing rate. Low maintenance costs.

Firing System Concepts

In the first concept, the fuel and air are divided and distributed into many similar streams. Each stream is treated independently to provide multiple flame envelopes called multi flame envelope concept.

In the second concept a single flame envelope is produced, by providing interaction between all streams of air and fuel introduced into the furnace. This is called single flame envelope concept.

Single-flame vs Multi-flameSingle-flame• Provides interaction between all streams of fuel and air

introduced into the furnace and so precise subdivision of fuel and air at each point of admission is not required.

• Allows more time for contact between all fuel and air molecules.

• Mechanical turbulence is sustained throughout the furnace.

Multi-flame• Requires accurate subdivision fuel and air supplied to the

furnace. • Limits the opportunity for sustained mechanical turbulence.

Firing Systems

Direct Firing System

Indirect Firing System

Methods of Fuel Firing

Vertical Firing

Horizontal Firing

Tangential Firing

Vertical Firing

Components of Fuel Firing SystemIgnition System Provide ignition energy to the flammable mixture of fuel and air introduced to the furnace.

Auxiliary Ignition System For igniting the oil while starting the oil burners, ignitors are used in the firing system.

Components of Fuel Firing SystemOil Guns and Atomisers• Fuel oils like light diesel oil (LDO), heavy fuel oil (HFO) are

used in boilers as supplement fuel.• These fuel oils are burnt by spray combustion method

wherein the oil is split into fine droplets (atomised) and distributed into the furnace in a spray form in a controlled manner.

• Oil guns installed in windbox with atomiser mounted at their tips provide this oil spray to the furnace.

Components of Fuel Firing SystemFlame Scanners/Flame Detecting System- Must be reliable.- Sensible to discern the minimum flame envelope.- Reaction time must be minimum.

Flame Scanner Types- Ultraviolet Scanners- Visible Light Scanners- Infrared Scanners

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