FUELS and COMBUSTION-2

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WHAT IS COMBUSTION?

What are the various types of combustion?

There are three types of combustion:

—  Perfect

—  Complete 

—  Incomplete 

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.

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

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.

Combustion Equation

  • C   +   O2               CO2 + 33820KJ/K
  • 2H2  +   O2            2H2O + 143050KJ/KG
  • S   +    O2  SO2   + 9320KJ/KG

Stoichiometric or theoretical air

  • It is the ideal amount of air required for burning 1 kg of fuel.
  • 1 kg of coal requires ~ 7-8 kg of air.
  • 1 kg of fuel oil requires ~14.1 kg of air.
  • 1 m3 of natural gas requires ~ 9.5-10 m3 of air.

Calculation of Stoichiometric O2

  • C       +        O2        ®      CO2
  • 12      +       32       ®       44
  • 12 kg C requires 32 kg O2 to form 44kg CO2
  • 1kg C requires 32/12 kg ie. 2.67kg of O2

Calculation of Stoichiometric O2

  • 2H2     +        O2        ®      2H2O
  • 4        +       32        ®       34
  • 4kg H2 requires 32kg O2 to form 36kg CO2
  • 1kg H2 requires 32/4 kg ie. 8kg of O2

Calculation of Stoichiometric O2

  • S         +        O2        ®      SO2
  • 32        +       32        ®     64
  • 32kg S requires 32kg O2 to form 64kg SO2
  • 1kg S requires 32/32 kg ie. 1kg of O2

Calculation of Stoichiometric air

  • Total O2 required = 2.67%C+  8%H2+ %S
  • Total O2 required = 2.67%C+  8%H2+ %S – % O2 in fuel.
  • Air required = 100/23((2.67%C+  8%H2+ %S) – % O2 in fuel)

Optimising Excess Air

  • In practice Excess Air over Stoichiometric air is needed for complete combustion.
  • Less Air  – Incomplete combustion & Smoke
  • More Air – Heat loss through stack

Relation Between CO2, CO and O2 with Excess air

EXCESS AIR

Excess air- too high

  • Fire may be blown off due to high turbulence.
  • May upset in steam temp.-High.
  • High erosion.
  • Cold end corrosion.

Excess air-too low

  • Incomplete combustion.
  • Furnace hazards.
  • Upset in steam temp. – Low.
  • High flue gas velocity and high erosion.

FACTORS AFFECTING THE PERFORMANCE OF COMBUSTION

  • SURFACE CONTACT AREA OF FUEL WITH OXYGEN
  • AIR FUEL RATIO
  • RETENTION TIME
  • COMBUSTION CHAMBER TEMP.
  • TURBULANCE IN COMBUSTION CHAMBER
  • REMOVAL OF PRODUCTS OF COMBUSTION

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