STOICHIOMETRY OF COMBUSTION

1 STOICHIOMETRY OF COMBUSTIONFUNDAMENTALS: moles and kilomolesAtomic unit mass: 1/12 126C ~ 10-27kgAtoms and molecules mass is defined in atomic unit mass: whichis defined in relation to the 1/12 of carbon : (Avogadro number) 1023atomsVolume of 1 mole (perfect gas) l (T= 0 oC, p = 1 atm)Kmole: 103molesMass of one mole (kmole) is a number of grams (kilograms) equal to the relative atomic REACTIONSC onsider a simple chemical reaction of two species A and B giving a single product CA + 2B C1 mol + 2 moles 1 molA, B and C are reactants. A and B are substracts. C is a product of reaction. STOICHIOMETRY of chemical reactions means that species react in exact S CONCENTRATIONS AND FRACTIONSC oncentration of i specie: (moles of ispecie) /volume (mass of ispecie)/volumeMole fraction: (moles of i specie)/(total number of moles of species)Mass fraction: (mass of ifraction)/(total mass)Fossil fuels are mainly compounds of carbon and hydrogen (hydrocarbons - CmHn).

2 The reaction of its oxidation can by written by the equation of STOICHIOMETRY :CmHn+ (m + n/4)O2 mCO2+ n/2H2O1 mol + (m + n/4)moles (m +n/2)molesThis is an equation of STOICHIOMETRY of OF HYDROCARBONS OXIDATION It is important that for one mole of fuel CmHnthere is necessary exactly:(m + n/4) moles of oxygen for complete OF OXIDIZERSIn COMBUSTION processes the oxidizer could be:1. Oxygen (O2)2. Air (21%O2+ 79%N2)3. Air enriched with oxygen (O2> 21%)4. Some compounds containing oxygen, like nitrogen oxides: N2 OCOMPOSITION OF FUEL MIXTUREWhen fuel and oxidizer composition in the mixture (fuel and oxidizer) results from the equation of stoichiometrywe say that the mixture is COMBUSTION of a stoichiometric mixture is complete in flue gas cannot be nor fuel neither OF COMBUSTIBLE MIXTURESLeanStoichiometricRichExcess of oxidizerStoichiometric content of fuel and oxygenExcess of fuelFUEL MIXTUREBURNING OF STOICHIOMETRIC METHANE MIXTURE IN AIRCH4+ 2O2+ CO2+ 2H2O + : 79% N2+ 21% O2 Attention: there is not fuelneither oxygenin flue OF STOICHIOMETRIC METHANE MIXTURE IN AIR: flue gas compositionA number of moles of flue gas N.

3 N=1 mole CO2+ 2 moles H2O + mole N2= molesAccording to the wet (water is steam/liquid) analysis of flue gas the concentration of the components is as follows:[CO2] =1mole CO2 = CO2vol.[H2O] =2 moles H2 =19% H2O vol.[N2] = mole N2 = N2vol. BURNING OF LEAN METHANE MIXTURE IN AIRCH4+ + CO2+ 2H2O + + : there is oxygen in flue : AIR EXCESS = 5%BURNING OF LEAN METHANE MIXTURE IN AIR: flue gas compositionA number of moles of flue gas Nequals: N=1 mole CO2+ 2 moles H2O + mole N2+ mole O2 =11 molesAccording to the wet (water is liquid) analysis of flue gas the concentration of the components is as follows:[CO2] =1mole CO2/11= CO2vol.[H2O] =2 moles H2O/11= H2O vol.[O2] = mole O2/11 = O2vol.[N2] = moles N2/10,52 = AIRThe theoretical air required to complete COMBUSTION of fuel results from the equation of STOICHIOMETRY of oxygen/fuel reaction.

4 Stoichiometric air means the minimum air in stoichiometric mixture. The stoichiometric air/fuel ratio (AFR) can be calculated from the reaction equation (g/g). For gas AFR is usually determined in m3 actual COMBUSTION air depends also on the assumed air excess (equivalence ratio or stoichiometric ratio).THE STOICHIOMETRIC RATIO ( )(WSP CZYNNIK NADMIARU POWIETRZA)Actual air =Stoichiometric air [02]AN EQUIVALENCE RATIO (WSP CZYNNIK EKWIWALENCJI)F/A =(F/A)s =1 F number of moles of fuel,A number of moles of - stoichiometricIn English more often than stoichiometric ratio the equivalence ratio is in RATIO (AFR) andFUEL/AIR RATIO (FAR)F/AAFR =A air mass-flow (A air), kg/sF fuel mass-flow (F fuel), kg/sForm the practical reasons in aviation (aircrafts) mostly in use is the ratio of air mass-flow to the fuel mass-flow determined as AFR (Air/Fuel Ratio)In similar way FAR (Fuel/Air Ratio) is RATIO/EQUIVALENCE RATIO AND TYPE OF MIXTURE < 1 = 1 > 1 >1 =1 <1 Lean(weak)

5 StoichiometricRichType of flame/type of mixtureactual - stoichiometric airn=stoichiometric airn=AIR EXCESS (n)( -1) 100%The relationship between the stoichiometric ratio and air excess nSTOICHIOMETRIC AIR/FUEL RATIO FOR SELECTED (approx. 82% CH4)COH2CH4C2H2C2H6C3H8 Natural gasStoichiometric air/fuel ratioAFR, m3/m3 GazEXAMPLES toichiometric air for methane burning (CH4) is m3/m3 Assuming: the rate feeding of methane V = 10 m3/h the rate feeding of airV = m3/hWe have: = = = ( ) 100% = 10%