Combustion Calculations Calculate the theoretical flame temperature of combustion of hydrogen with theoretical amount of air assuming (a)No dissociation of combustion product (i.e. H2O) (b)Assuming 4 % dissociation of H2O NCV of H2 = MJ/m3 Assume that both the hydrogen and air are dry at 0 C Mean value of Cp (of combustion product H2O): Temp C Cp kJ/m3C

Combustion Calculations H2 + ½ O2 = H2O Air required for 1 m3 of H2= 2.38 m3 Total flue gas= N2+ H2O= 0.79 x = 2.88 m3 / m3 of H2 (a) Theoretical flame temp T = (10.16 x 1000)/(2.88 xCp) Assuming T=2100 C (Cp=1.651) Calculated T= 2136 C which is quite close to assumed (b) T =(10.16 x 1000 x (1-0.04))/(2.88 xCp) Assuming T=2000 C calculated T = ? Assuming T =2100 calculated T= ?

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

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)

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)

Bunsen Burner The device is named after Robert Bunsen, the German chemist who introduced it in 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

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

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

Flame stability

Reading assignment Burners for gaseous fuels from the book