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Combustion Calculations

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

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Combustion Calculations

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  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):

  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 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= ?

  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

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