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<1 which means or this situation leads to incomplete combustion. POC will contain CO, smoke dust besides CO2 N2 .etc. ii.
This situations leads to complete combustion. iii.
Complete combustion but POC will contain excess O2 in addition to CO2 H2O, SO2 and N2 etc. Theoretically CO will be absent. Amount of POC depends on value of K. Increase in K beyond 1 increase the amount of POC Combustion efficiency Theoretically complete combustion is obtained by stoichiometric amount of air and POC should not contain CO. But in actual, combustion of any fuel does not occur with just stoichiometric amount of air. Excess air is required. Excess air depends on type of fuel. Normally solid fuels require more excess air than liquid fuels and gaseous fuels. Gaseous fuels require least amount of excess air. Mixing of fuel and air is important simply because 1 mole of oxygen is accompanied by 3.76 moles of N2. In an air + fuel mixture, statistically the probability of finding nitrogen in the neighborhood of carbon is more than oxygen. Thus mixing determines combustion efficiency, i.e. the ability of a device (used for mixing of fuel and air) to convert potential energy of fuel in to thermal energy. Ideally = 100% when thermal energy= Potential energy of the fuel. Inefficiency in combustion is due to poor mixing which may lead to formation of CO. Poor combustion efficiency leads to •
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Wastage of fuel Environmental pollution. Expensive to run. For any given type of fuel, some amount of CO is always observed in POC even at stoichiometric amount of air in all practical combustion systems. Presence of CO in POC denotes incomplete combustion and combustion efficiency will be low. So,excess air has to be used to increase combustion efficiency. Increase in excess air decreases drastically CO in POC but at the same time increases amount of POC due to increase in nitrogen and oxygen (at stoichiometric air no oxygen is present in POC). The additional nitrogen and oxygen in POC due to excess air will carry heat of combustion with them. Thus, control of excess air is important. The following plot illustrates the relationship between excess O2, CO and heat losses. Figure: Plot of variation of CO and heat losses with excess oxygen in POC. Note zero value of oxygen in POC denotes stoichiometric oxygen used for combustion X‐ axis on the figure is % O2 in POC. Theoretically percent oxygen in POC is zero at theoretically amount of air. Increase in excess air increases percent oxygen in POC (see black line in the figure). It can be seen in the figure that amount of CO (see green line) decreases drastically by using slight amount of excess air. Beyond around 1 to 2 % O2 CO in POC disappears completely. But increase in excess air at the same time increases O2 in POC as shown by (blue line). Heat losses are shown by the blue line. Heat losses decrease drastically with the excess air and become minimal at around 1% O2 which is due to decrease in CO. Beyond 1% O2 heat losses increases further because now nitrogen and oxygen in POC increases. Ref.: O.P. Gupta: elements of fuels. furnaces and refractories. Key words: Combustion, material balance, furnace, stoichiometry