chemical fuels

• Fuel


  Any substance used to produce heat or power by combustion
• Combustion


  Any chemical process accompanied by the evolution of light and heat
• Combustion


  1. Fuel + oxygen

  2. Combustion products + Heat
• Combustion reactions


  • C (s) + O2 (g) → CO2 (g) + ΔH = -94.1 Kcal/mol
  • H2 + 1/2O2 → H2O + ΔH = -67.5 Kcal/mol
• During the process of combustion, the atoms of carbon, hydrogen etc. combine with oxygen with the simultaneous liberation of heat at a rapid rate
• Energy is liberated due to rearrangement of valence electrons
• Classification of Fuels


  • Based on occurrence
  • Based on physical state
• Units of heat


  • Calorie
  • Kilo Calorie
  • British thermal unit
  • Centigrade heat unit
• Calorie


  Amount of heat required to raise the temperature of 1gm of water by 1°C
• Kilo Calorie


  1 kcal = 1000 cal
• British thermal unit (B. Th. U. or B.T.U)


  Amount of heat required to raise the temperature of 1 pound of water through 1°F
• Centigrade heat unit (C.H.U)

  Amount of heat required to raise the temperature of 1 pound of water through 1°C
• Interconversion of the various units of heat

  1. 1 Kcal = 1000 cals = 3.968 B.T.U = 2.2 C.H.U

  2. 1 B.T.U. = 252 Cal = 0.252 kcal
• Calorific value


  Total quantity of heat liberated when a unit mass of a fuel is burnt completely
• Units of Calorific value

  1. 1 cal/g = 1 Kcal/Kg = 1.8 B.Th.U./lb

  2. 1 Kcal/m3 = 0.1077 B.Th. U./ft3

  3. 1 B.Th.U./ft3 = 9.3 Kcals/m3
• Gross Calorific Value

  Total amount of heat generated when a unit quantity of fuel is completely burnt in oxygen and the products of combustion are cooled down to the room temperature
• Net Calorific Value

  Net heat produced when a unit quantity of fuel is completely burnt and the products of combustion are allowed to escape
• Relationship between Gross and Net Calorific Value

  LCV = HCV - Latent heat of condensation of the water vapors formed

  LCV = HCV - (weight of hydrogen × 9 × latent heat of steam)
• 1 part by weight of hydrogen gives 9 parts by weight of water
• Latent heat of steam

  587 Cal/g (or Kcal/Kg) or 1060 B.Th.U./lb of water vapor produced
• Calculation of Net Calorific Value

  Net C.V. or L.C.V = Gross C.V. - 0.09 × H × 587

  Where H = % of hydrogen in the fuel
• Bomb calorimeter


  • Used for determination of calorific value of solid and non volatile liquid fuels

  Principle: A known mass of the solid sample is burnt in excess oxygen. The surrounding water and the calorimeter absorb the heat liberated.
• Working of Bomb calorimeter


  A known mass of the solid fuel is placed in a crucible inside the bomb

  The bomb is filled with oxygen at 25-30 atm pressure

  The fuel is ignited and the temperature rise of the water is measured
• Calculations using Bomb calorimeter


  Heat liberated by the fuel = Heat gained by water and calorimeter

  Gross calorific value = (Heat gained by water and calorimeter) / (Mass of fuel sample)
• Net Calorific Value


  Gross Calorific Value - Latent heat of water formed
• Corrections in Bomb calorimeter

  • Acid Correction

  Cooling Correction

  Fuse wire Correction

  Cotton thread Correction
• Gross calorific value = Calculated calorific value - Corrections
• Boy's calorimeter is used for determination of calorific value of gases and volatile liquid fuels
• Boy's calorimeter


  • Principle: Burn the gas at a known constant rate in a vessel under such conditions that the entire amount of heat produced is absorbed by water which is also flowing at a constant rate.

  Construction: Combustion chamber surrounded by water tube made of copper with two thermometers T1 and T2 attached. A burner in the chamber connected to a gas tube.
• Working of Boy's calorimeter

  Water at a constant rate is allowed to pass through the water tube

  The temperatures of incoming and outgoing water are recorded by thermometers T1 and T2

  The gaseous fuel is burnt and the heat is absorbed by the flowing water
• Latent heat of steam

  580 calories / g
• Boy's calorimeter


  • Determination of calorific value of gases and volatile liquid fuels
• Principle of Boy's calorimeter

  1. Burn the gas at a known constant rate in a vessel under such conditions that the entire amount of heat produced is absorbed by water which is also flowing at a constant rate

  2. Heat liberated by the fuel is equal to the heat absorbed by the water and the calorimeter
• Construction of Boy's calorimeter

  • Combustion chamber surrounded by water tube made of copper with two thermometers T1 and T2 attached
  • Burner in the chamber, connected to a gas tube through which a known volume of gaseous fuel is burnt at a known pressure at a rate of 3-4 l/min
• Working of Boy's calorimeter

  1. Water at a constant rate is allowed to pass through the water tube

  2. Temperatures of incoming and outgoing water are recorded by thermometers t1 and T2 respectively

  3. Gaseous fuel is burnt, water passing through the tubes takes all the heat produced

  4. Water is collected in a measuring jar

  5. Whole set up is enclosed in an insulated container
• Calculations for Boy's calorimeter
• Heat absorbed by circulating water


  W (t2-t1)
• Heat produced by combustion of fuel

  V θ
• Assuming no heat loss, V θ = W (t2-t1)
• HCV


  θ = Kcal/m3