Lecture-9 Principles of combustion. Contents of the lecture. What is combustion.
Objectives of combustion. Stoichiometric amount of air. Air/fuel ratio and relation
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Lecture‐9 Principles of combustion Contents of the lecture What is combustion Objectives of combustion Stoichiometric amount of air Air/fuel ratio and relation with POC Combustion efficiency What is Combustion? All fossil fuels contain combustible and incombustible components (also known as diluents) Fossil fuel Combustible components
Incombustible or diluents
Solid fuels: C,S,H
N,O, ash and moisture
Liquid fuels C,H,S
N,O
Gaseous fuels H2, CO, N2, CO2, O2, SO2 hydrocarbons, NH3 and H2S In the table C is carbon, S is sulphur, H is hydrogen, N is nitrogen, O is oxygen. Note that in the solid fuels they are present in the elemental form. Combustion is a fast chemical reaction between combustible component(s) and an oxidizing agent. Mostly air is used for combustion. Dry air contains 21% O2 and 79% N2 and so 1 mole of O2 carries with it 3.76 moles of N2. Combustion equations with air, when complete combustion takes place are C+ (O2+3.76 N2) = CO2+3.76 N2 (1) S + (O2+3.76 N2) = SO2+3.76 N2 (2) H2 + (O2+3.76 N2) = H2O+1.88 N2 (3) CH4 +2 (O2+3.76 N2) = CO2+2H2O+7.52 N2 (4)
Similarly, for other hydrocarbons combustion reactions can be written. Note that complete combustion refers to conversion of C,S,H2 or CH4 into CO2, SO2 and H2O.
Objective of combustion All fuels whether naturally occurring or synthetically prepared contain potential energy. Potential energy of the fuel, on combustion with air is released in products of combustion (here after termed POC) at the temperature which is termed flame temperature. POC transfer their heat to sink (sink could be furnace chamber, charge materials etc) and then exit the system. Below are given the products of combustion which can be obtained when air is used for combustion of fossil fuels: POC Release of potential energy
Complete combustion CO2 H2O SO2 O2 N2 Maximum
Incomplete combustion CO H2O H2 SO2 O2 N2 Unburnt carbon Soot Lower than that of complete combustion due to CO, H2, unburnt C and soot in POC.
Objective of combustion is to attain complete combustion with stoichiometric amount of air as required in equations 1 to 4. Complete combustion is also termed as perfect combustion.
Stoichiometric amount of air: Theoretically complete combustion can be obtained when stoichiometric amount of air is used. Stoichiometric amount (also termed theoretical air or air for complete combustion) of air can be calculated by considering the products of combustion obtained on complete combustion. In general any balanced chemical equation (mole input = moles output) can be used to calculate stoichiometric amount of air. In combustion equations 1 to 4, we note that 1mole of C requires 1 mole of O2 or 4.76 moles of air to give 1 mole of CO2 and 3.76 moles of N2. Similarly 1 mole of H2 requires mole of O2 or 2.38 moles of air to give 1 mole H2O and 1.88 mole of N2. In several other metallurgical processes like roasting (conversion of metal sulphide to oxide, reduction of oxides, oxidation of impurities etc,) air or pure oxygen is used. In all these cases it is often required to calculate stoichiometric amount of air. For example consider roasting of sulphide;
ZnS + 1.5O2 = ZnO + SO2 5) PbS + 1.5O2 = PbO + SO2 or in general
6)
MS + 1.5O2 = MO + SO2 7) M stands for metal. In all the above equations the amount of air can be easily calculated following the stoichiometry of the reactions. Let us calculate stoichiometric amount of air for combustion of solid fuel of composition 84%C, 5%H, 5% moisture and 6% ash, per kg of coal. Following the stoichiometry of combustion, the amout of air would be = 10.12m3 (1atm,273 K)/kg coal. Note 1 kg mole= 22.4 m3 (1atm,273 K).
In the above example if actual amount of air is 0.5 moles then We can also call‐ that 110.62% theoretical air is used for combustion. Note: Complete combustion can occur only when amount of air is equal to or greater than stoichiometric air. When excess air is used, POC will contain O2 in addition to CO2, H2O, N2 and SO2.
Importance of Air/ fuel ratio For a given type of fuel, air/fuel ratio controls the combustion and amount of POC. Consider combustion of fuel of amount Let
with air
which produces POC
when
is constant.
Stoichiometric amount of air Theoretically if i.
K