Combustion Theory
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Combustion Theory (Definition, influencing factor etc.). www.powerpointpresentationon.blogspot.com. What is COMBUSTION ?. High speed, high temperature chemical reaction Rapid union of an element or compound with oxygen to liberate heat – controlled explosion

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Combustion theory definition influencing factor etc

Combustion Theory

(Definition, influencing factor etc.)

www.powerpointpresentationon.blogspot.com


Combustion theory definition influencing factor etc

What is COMBUSTION ?

  • High speed, high temperature chemical reaction

  • Rapid union of an element or compound with oxygen to liberate heat – controlled explosion

  • Combustion occurs when elements of fuel such as carbon and hydrogen combine with oxygen


Chemical reaction in combustion

Moisture (%)

Mineral matter (%)

Carbon (%)

Hydrogen (%)

Nitrogen (%)

Sulphur (%)

Oxygen (%)

GCV (Kcal/kg)

Chemical reaction in Combustion

C + O2  CO 2 + 8084 Kcals/kg of Carbon

2C + O2  2 CO + 2430 Kcals/kg of Carbon

2H 2 + O2  2H2O + 28,922 Kcals/kg of Hydrogen

S + O2  SO2 +2,224 Kcals/kg of Sulphur

  • Stoichiometric or theoretical air is ideal amount of air required for burning 1 kg of fuel

  • Ex:1 kg of fuel oil requires ~14.1 kg of air for complete combustion


3 t s of combustion

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.


3 t s of combustion1

3 Ts of Combustion

  • Ignition Time and Residence Time- Furnace volume to be large enough to give the mixture time for complete combustion.


3 t s of combustion2

3 Ts of Combustion

  • Ignition Temperature- Fuel-Air Mixture maintained at or above the Ignition Temperature


3 t s of combustion3

3 Ts of Combustion

  • Oxygen and Fuel thoroughly mixed.


What are the three types of combustion

What are the three types of combustion?

  • Perfect Combustion is achieved when all the fuel is burned using only the theoretical amount of air, but perfect combustion cannot be achieved in a boiler.

  • Good / CompleteCombustion is achieved when all the fuel is burned using the minimal amount of air above the theoretical amount of air needed to burn the fuel. Complete combustion is always our goal. With complete combustion, the fuel is burned at the highest combustion efficiency with low pollution. 

  • Incomplete Combustion occurs when all the fuel is not burned, which results in the formation of soot and smoke.


Combustion inefficiencies

Combustion Inefficiencies

Flue gas

H2O

Deficiency of Air

H2

CO

Air

Unburnt Fuel

Soot

O2

CO2

N2

N2

Fuel

=

+


Combustion inefficiencies1

Combustion Inefficiencies

Too much of Air

Flue gas

O2

H2O

Air

CO2

Excess O2

Excess O2

N2

N2

Fuel

=

+


Combustion inefficiencies2

Combustion Inefficiencies

Stochiometric Air

Flue

Air

H2O

O2

CO2

N2

N2

Fuel

=

+


Combustion theory definition influencing factor etc

Operating in this

Zone results in

Excess heat loss up the stack

Operating in this

Zone results in

wasted fuel

Zone of maximum

Combustion Efficiency

Decrease

Increase

Excess Air

Loss

Unburned

Fuel Loss


Relation between co 2 and excess air

Relation Between CO2 and Excess air


Relation between residual o 2 and excess air

Relation Between Residual O2 and Excess air


Effect of excess air on carbon di oxide

Effect of excess air on carbon di oxide


Combustion theory definition influencing factor etc

The recommended boiler exhaust velocity should be within limits indicated below:


Combustion of fuel oil

Combustion of Fuel oil

  • Viscosity of 100 Redwood/ secs at burners

  • Atomising air 1- 3 Kg/cm2 (about 2 % of total air requirement)

  • 14 Kg of air/kg fuel is required for complete combustion. Optimum efficiency with 10 % excess air

  • Flue gas should be analysed for CO2 or O2

  • Sulphur dewpoint at 160oC. Corrosion max at 30oC below dew point


Combustion of fuel oil contd

Combustion of Fuel oil (contd.)

  • Slightest damage to burner tip may increase fuel consumption by 10-15 % and hence worn out tips should be replaced immediately

  • Oil pressure at burner should be 17-20 Kg/cm2

  • Correct flame is normally short. Impingment on walls, tubes cause carbon formation

  • Too short a flame indicates high excess air and air supply to burners should be adjusted for light haze brown out of chimney


Burners

Burners

  • Burners convert fuel oil into millions of small droplets –process called atomization

  • High surface to volume ratio in oil to facilitate evaporation and combustion

  • 3 basic types of burners are pressure jet, air or steam blast burners and Rotary Cup

TURNDOWN ratio is the relationship between the maximum and minimum fuel input without affecting the excess air level is called ‘Turn-Down Ratio’.

For example, a burner whose maximum input is 250,000 Kcals and minimum rate is 50,000 Kcals, has a ‘Turn-Down Ratio’ of 5 to 1.


Pressure jet burner

Pressure Jet Burner

  • Simple, inexpensive and widely used

  • Oil pumped at pressure through a nozzle

  • Good efficiencies at lower loads

  • Low Turndown ratio of 2:1

  • High oil pre-heat required for atomization

  • Prone to clogging due to dirt in oil –requires fine filtration


Combustion theory definition influencing factor etc

Spray at 10 psi pressure

Spray at 100-psi pressure

Spray at 300-psi pressure


Air or steam blast burner

Air or Steam Blast Burner

  • High Turndown ratio of 4:1

  • Good control of combustion over wide range

  • Good combustion of heavier fuel Oil

  • Additional Energy required as steam or compressed air for atomization


Burner controls

Burner Controls

  • ON/OFF-Burner firing at either full rate or OFF

  • HIGH/LOW/OFF – Burner operates at slow firing rate and full firing rate as per load

  • MODULATING BURNER – Firing rate matches the boiler load


Coal combustion

Coal Combustion

  • 1 kg of coal requires 10 -12 kg of air for complete combustion

  • Primary air is supplied below the grate and Secondary air over the grate

  • Supply of PA and SA regulated with coal bed thickness

  • Secondary air provided to create good turbulence

  • Clinkers formed on combustion to be removed immediately


Properties of coal

Properties of Coal

Coal Classification

  • Three main coal classes: anthracite, bituminous, and lignite(Sub class- semi anthracite, semi bituminous, and sub bituminous)

  • Anthracite-oldest coal,hard coal composed mainly of carbon with little volatile content and practically no moisture.

  • Lignite -the youngest coal.

  • Chemical composition of coal has a strong influence on its combustibility.


Anthracite

Anthracite


Bituminous coal

Bituminous coal


Lignite

Lignite


Proximate analysis

Typical proximate analysis of various coals (in Percentage by weight)

Parameter

Indian Coal

Indonesian Coal

South African Coal

Moisture

5.98

9.43

8.5

Ash

38.56

13.99

18

Volatile matter

20.70

29.79

23.28

Fixed Carbon

34.69

46.79

51.22

Proximate Analysis


Significance of various parameters in proximate analysis

Significance of Various Parameters in Proximate Analysis

Fixed carbon:

Fixed carbon gives a rough estimate of heating value of coal

Volatile Matter:

Volatile matters are the methane, hydrocarbons, hydrogen and carbon monoxide, and incombustible gases like carbon dioxide and nitrogen found in coal. Thus the volatile matter is an index of the gaseous fuels present. Typical range of volatile matter is 20 to 35%.  

  • Proportionately increases flame length, and helps in easier ignition of coal.

  • Sets minimum limit on the furnace height and volume.

    Influences secondary air requirement and distribution aspects.


Combustion theory definition influencing factor etc

Ash Content:

Ash is an impurity that will not burn. Typical range is 0.5 to 40%

  • Reduces handling and burning capacity.

  • Increases handling costs.

  • Affects combustion efficiency and boiler efficiency

  • Causes clinkering and slagging.

    Moisture Content:

  • Moisture decreases the heat content per kg of coal. Typical range is 0.5 to 10%

  • Increases heat loss, due to evaporation and superheating

  • Helps, to a limit, in binding fines.

         Sulphur Content:

  • Typical range is 0.5 to 5% normally.

  • Affects clinkering and slagging tendencies,Corrodes chimney and other equipment such as air heaters and economisers,Limits exit flue gas temperature.


Ultimate analysis

Ultimate Analysis:

Useful to find the quantity of air required for combustion and the volume and composition of the combustion gases, calculation of flame temperature and the flue duct design etc


Storage handling of coal

Storage & Handling of Coal

Stocking of coal has its own disadvantages like build-up of inventory, space constraints, deterioration in quality and potential fire hazards. Other minor losses associated with the storage of coal include oxidation, wind and carpet loss.  

  • Minimise carpet loss and the loss due to spontaneous combustion.  

  • The measures to reduce the carpet loses are

    • Preparing a hard ground for coal to be stacked upon.

    • Preparing standard storage bays out of concrete and brick

  • In process Industry, modes of coal handling range from manual to conveyor systems. It would be advisable to minimise the handling of coal so that further generation of fines .


Preparation of coal

Preparation of Coal

  • Sizing of Coal

    • Proper coal sizing, with specific relevance to the type of firing system, helps towards even burning, reduced ash losses and better combustion efficiency.

      Conditioning of Coal

    •  Segregation of fines from larger coal pieces can be reduced to a great extent by conditioning coal with water. Water helps fine particles to stick to the bigger lumps due to surface tension of the moisture, thus stopping fines from falling through grate bars or being carried away by the furnace draft.

    • Blending of Coal

    • In case of coal lots having excessive fines, it is advisable to blend the predominantly lumped coal with lots containing excessive fines. Coal blending may thus help to limit the extent of fines in coal being fired to not more than 25%. Blending of different qualities of coal may also help to supply a uniform coal feed to the boiler.


Spontaneous combustion

Spontaneous Combustion


Lpg n gas

LPG & N.Gas

  • LPG is a predominant mixture of propane and Butane with a small percentage of unsaturates (Propylene and Butylene)  

  • LPG -gaseous at normal atmospheric pressure, but may be condensed to the liquid state at normal temperature, by the application of moderate pressures. Liquid LPG evaporates to produce about 250 times volume of gas.

  •  LPG vapour is denser than air

  • Natural Gas

  • Methane is the main constituent of Natural gas and accounting for about 95% of the total volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen.sulphur negligible. 

  • It is lighter than air and disperses into air easily in case


Gas burner

Gas Burner

  • Low pressure burners

  • Operate over range of 25-100 mm Water Column Gas Supply Pressure

  • High pressure burners operate over range of 120 – 1750 mm WC


What are the agro residues available and their properties

Ultimate analysis of typical agro residues

Deoiled Bran

Paddy Husk

Saw Dust

Coconut Shell

Moisture

7.11

10.79

37.98

13.95

Mineral Matter

19.77

16.73

1.63

3.52

Carbon

36.59

33.95

48.55

44.95

Hydrogen

4.15

5.01

6.99

4.99

Nitrogen

0.82

0.91

0.80

0.56

Sulphur

0.54

0.09

0.10

0.08

Oxygen

31.02

32.52

41.93

31.94

GCV (Kcal/kg)

3151

3568

4801

4565

What are the agro residues available and their properties ?


Thank you

Thank You!


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