Combustion Theory

(Definition, influencing factor etc.)

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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

•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

C + O2 → CO 2 + 8084 Kcals/kg of Carbon2C + 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

Moisture (%)

Mineral matter (%)

Carbon (%)

Hydrogen (%)

Nitrogen (%)

Sulphur (%)

Oxygen (%)

GCV (Kcal/kg)

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.

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

3 Ts of Combustion

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

3 Ts of Combustion

•Oxygen and Fuel thoroughly mixed.

3 Ts 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 / Complete Combustion 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.

What are the three types of combustion?

Combustion Inefficiencies

Fuel

N2

O2

N2

CO2

Unburnt Fuel

CO

H2

H2O

+ =

Deficiency of Air

Air

Flue gas

Soot

Combustion Inefficiencies

Fuel

N2

Excess O2

N2

CO2

Excess O2

H2O

+ =

Too much of Air

Air

Flue gas

O2

Combustion Inefficiencies

Fuel

N2

O2

N2

CO2

H2O

+ =

Stochiometric Air

Air Flue

Operating in this Zone results in wasted fuel

Zone of maximumCombustion Efficiency

Operating in this Zone results in Excess heat loss up the stack

Unburned Fuel Loss

Excess Air Loss

Decrease Increase

Relation Between CO2 and Excess air

Relation Between Residual O2 and Excess air

Effect of excess air on carbon di oxide

Carbon di oxide in flue gas (%) when excess air is (%)

0 10 20 40 100

Natural gas

12.0 10.7 9.8 8.3 5.7

Distillate oil15.2 13.8 12.5 10.7 7.4

Residual oil

15.8 14.1 12.9 11.0 7.6

Anthracite coal

19.8 18.0 16.5 14.1 10.0

Excess Air and O2 and CO2 in Flue Gas

Excess Air%

Carbon Dioxide - CO2 - in Flue Gas (% volume) Oxygen in Flue Gas forall fuels

(% volume)

Natural Gas

Propane Butane Fuel Oil Bitumino

us CoalAnthracite Coal

0 12,0 14,0 15,5 18,0 20,0 0,0

20 10,5 12,0 13,5 15,5 16,5 3,0

40 9,0 10,0 12,0 13,5 14,0 5,0

60 8,0 9,0 10,0 12,0 12,5 7,5

80 7,0 8,0 9,0 11,0 11,5 9,0

100 6,0 6,0 8,0 9.5 10,0 10,0

Combustion Efficiency (%)

Excess % Net Stack Temperature1) (oF)

AirOxyge

n200 300 400 500 600

9.5 2.0 85.4 83.1 80.8 78.4 76.0

15 3.0 85.2 82.8 80.4 77.9 75.4

28.1 5.0 84.7 82.1 79.5 76.7 74.0

44.9 7.0 84.1 81.2 78.2 75.2 72.1

81.6 10.0 82.8 79.3 75.6 71.9 68.2

Exhaust GasExhaust Velocity

m/s ft/s

Ducts at minimum load < 4.0 < 13

Stack at minimum load < 5.0 < 16

Boiler with one-step burner (on - off) 5.0 - 8.0 16 - 26

Boiler with two-step burner (high - low) 10 - 15 31 - 49

Boiler with modulating burner 15 - 25 49 - 82

To keep the surface free from soot the velocity should exceed

3.0 - 4.0 9.8 - 13

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

•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

•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

Combustion of Fuel oil (contd.)

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•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

Spray at 10 psi pressure Spray at 100-psi pressure

Spray at 300-psi pressure

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

•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 •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

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.

Properties of Coal

Anthracite

Bituminous coal

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

Fixed carbon: Fixed carbon gives a rough estimate of heating value of coalVolatile 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.

Significance of Various Parameters in Proximate Analysis

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:

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

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 .

Storage & Handling 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.

Preparation of Coal

Spontaneous Combustion

•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

LPG & N.Gas

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

Thank You!