Combustion Calculations

1
Combustion Calculations

• Calculate the theoretical flame temperature of
  combustion of hydrogen with theoretical amount of
  air assuming
• No dissociation of combustion product (i.e. H2O)
• Assuming 4 dissociation of H2O
• NCV of H2 10.16 MJ/m3
• Assume that both the hydrogen and air are dry at
  0 C
• Mean value of Cp (of combustion product H2O)

Temp C 2000 2100 2200
Cp kJ/m3C 1.643 1.651 1.657
2
Combustion Calculations

• H2 ½ O2 H2O
• Air required for 1 m3 of H2 2.38 m3
• Total flue gas N2 H2O 0.79 x 2.38 1
• 2.88 m3 / m3 of H2
• (a) Theoretical flame temp T (10.16 x
  1000)/(2.88 xCp)
• Assuming T2100 C (Cp1.651)
• Calculated T 2136 C which is quite close to
  assumed
• (b) T (10.16 x 1000 x (1-0.04))/(2.88 xCp)
• Assuming T2000 C calculated T ?
• Assuming T 2100 calculated T ?

3
Combustion Process

• The requirements for Combustion are
• Fuel ( Solid, liquid, gaseous already discussed)
• Oxygen (Normal source is air)
• The 3 Ts time
• temperature
• turbulence
• time sufficient time must be available for
  complete combustion
• Temperature The fuel/air mixture must be heated
  to ignition temperature to promote combustion
• Turbulence turbulent mixing is the best approach
  for combustion to complete

4
Major Efficiency Losses

• Gas exit temperature
• A reduction of 22 C in flue gas temperature
  results in 1 increase in efficiency
• If gases cooled below dew point, sulphuric acid
  will condense on the surfaces
• The acid dew point temperature limits the amount
  of heat which can be safely recovered
• Losses due to excess air
• To reduce the mass of flue gas, we must reduce
  excess air If too little air, then incomplete
  combustion (just enough air to burn all the fuel)

5
Burners for Gaseous Fuels

• There are two methods for burning gaseous fuels
• (i) The gas and air is pre-mixed and then fired (
  premix or inside mixing type of burners e.g.
  Bunsen burner)
• (ii) The gas and air flow separately and mix
  together as combustion proceeds ( Outside mixing
  type or diffusion flame burners)

6
Bunsen Burner

• The device is named after Robert Bunsen, the
  German chemist who introduced it in 1855.
• The kinetic energy of the gas is used to draw
  primary air from the atmosphere into a mixing
  tube which has burner head at its end
• The primary air gas mixture velocity is kept more
  than the flame speed
• Secondary air is supplied from atmosphere to the
  flame so that flame does not flash
  back/backfire/strike back down the bunsen tube

7
The parts of a Bunsen burner.
Chimney A mixture of air (containing oxygen) and
methane flows through here.
Air holes Will allow air to mix with fuel in
different proportions
Collar This can spin if the holes are to be
opened or closed.
Gas inlet Will be attached to the gas supply
with a rubber tube.
Base The Bunsen must also be placed on a heat
mat before it is lit.
8
Bunsen Burner

• If the velocity of the primary air gas mixture
  velocity is much greater than the flame speed the
  flame can be blown off the tube and the burner
  get extinguished
• With insufficient primary air supply the flame
  produced is long, lazy and luminous which gives
  low heat release

9
Flame stability
10
Reading assignment

• Burners for gaseous fuels from the book