Basics of Combustion

1. Basics of Combustion Training on Technologies for Converting Waste Agricultural Biomass into Energy Organized by United Nations Environment Programme (UNEP DTIE IETC) 23-25 September, 2013 San Jose, Costa Rica Surya Prakash Chandak Senior Programme Officer International environmental Technology Centre Division of Technology, Industry and Economics Osaka, Japan

2. BASICS OF COMBUSTION • Combustion • Generation of heat through rapid chemical reactions of fuels is known as combustion • Products of Combustion - CO2 - H2O - NO2 - SO2 - CO, - HCs, - NOX, SOX, …. Complete Combustion Incomplete Combustion

3. BASICS OF COMBUSTION • Main parameters for proper combustion • Temperature: To initiate and sustain combustion • Turbulence: For proper mixing of fuel and air • Time: Sufficient for complete combustion 3T’s : Time, Temperature, Turbulence

4. BASICS OF COMBUSTION • Combustion • Flame of different fuels

5. BASICS OF COMBUSTION • Combustion Reactions • During combustion, molecules undergo chemical reactions. • The reactant atoms are rearranged to form new combinations (oxidized). • The chemical reaction can be presented by reaction equations. • However, reaction equations represent initial and final results and do not indicate the actual path of the reaction, which may involve many intermediate steps and intermediate products. • This approach is similar to thermodynamics system analysis, where only end states and not path mechanism are used.

6. BASICS OF COMBUSTION • Combustion Reactions • Types of combustion reactions: • Exothermic: Heat is released • Endothermic: Heat is absorbed

7. BASICS OF COMBUSTION +3000 C + 4H + 4O Break two “O=O” bonds + 988 kJ/mol Form two “C=O” bonds -1598 kJ/mol +2000 C + 4H + 2O2 CO2 + 4H + 2O Break four “C-H” bonds + 1644 kJ/mol +1000 Form four “O-H” bonds -1836 kJ/mol CH4 + 2O2 (Reactants) 0 Net energy change -802 kJ/mol Exothermic – gives off heat energy CO2 + 2H2O (Products) -1000 • Combustion Reactions Exothermic Endothermic

8. BASICS OF COMBUSTION • Combustion Reactions • Some fundamental reactions of combustion: • C + O2 CO2 + 33.8 MJ/kg-C • 2H2 + O2  2H2O + 121.0 MJ/kg-H • S + O2  SO2 + 9.3 MJ/kg-S • 2C + O2  2CO + 10.2 MJ/kg-C • Note: Above equations are in accordance with conservation of mass. For example consider the first reaction: • 1 kmol C + 1 kmol O2  1 kmol CO2, or • 12 kg C + 32 kg O2  44 kg CO2, or • 0 vol. C + 1 vol. O2  1 vol. CO2.

9. BASICS OF COMBUSTION • Combustion Reactions • In fuels, the combustion reactions are more complex than above: • In general, air is used in combustion than pure oxygen • Fuels consists of many elements such as C, H, N, S, O • In addition to complete combustions, fuels undergo incomplete combustions too. • Heat generation during combustion: • Combustion reactions together with enthalpies of components could be used to predict the net heat generation. • This needs identification of all the combustion products.

10. BASICS OF COMBUSTION • Composition of Air • On a molar (or volume) basis, dry air is composed of: – 20.9% oxygen O2 – 78.1% nitrogen N2 – 0.9% CO2, Ar, He, Ne, H2, and others • A good approximation of this by molar or volume is: 21% oxygen, 79% nitrogen • Thus, each mole of oxygen is accompanied 0.79/0.21 = 3.76 moles of nitrogen

11. BASICS OF COMBUSTION • Composition of Air • At ordinary combustion temperatures, N2 is inert, but nonetheless greatly affects the combustion process because its abundance, and hence its enthalpy change, plays a large part in determining the reaction temperatures. • - This, in turn, affects the combustion chemistry. • - Also, at higher temperatures, N2 does react, forming species such as oxides of nitrogen (NOx), which are a significant pollutant.

12. BASICS OF COMBUSTION • Stoichiometry and Air/Fuel Ratios • Oxidation all the elements or components in a fuel is known as complete combustion or “Stoichiometric Combustion”. • The amounts of fuel and air taking part in a combustion process are often expressed as the ‘air to fuel’ ratio: • Minimum amount of air (or oxygen) required to have a complete combustion is represented by Stoichiometric Ratio AFRstoich. • For a fuel CxHyOz

13. BASICS OF COMBUSTION • Stoichiometry and Air/Fuel Ratios • Eg: Combustion of Methane CH4 + 2(O2 + 79/21N2 )  CO2 + 2H2O + 158/21N2 Therefore, AFRStoich = (232 + 22879/21)/(12 + 41) = 17.16

14. BASICS OF COMBUSTION • Stoichiometry and Air/Fuel Ratios • In order to obtain complete combustion, supply of excess amount of air (or oxygen) is required in practice. • The amount of excess air required depends on the properties of the fuel and the technology of the combustion device. • Amount of excess air is usually represented by the equivalence ratio, φ, or the ‘lambda’ ratio λ:

15. BASICS OF COMBUSTION • Stoichiometry and Air/Fuel Ratios • Eg:

16. BASICS OF COMBUSTION • Combustion Reactions of Fuels • Complete combustion of hydrocarbons: • Incomplete combustion of hydrocarbons :

17. BASICS OF COMBUSTION • Estimation of Heating Values • Eg: Methane: CH4 + 2(O2 + 79/21N2 )  CO2 + 2H2O + 158/21N2 Enthalpies CH4 : -4.667 MJ/kg; O2 : 0.0; N2 : 0.0 CO2 : -8.942 MJ/kg; H2O : -13.423 MJ/kg (Gas) / -15.866 MJ/kg (Liquid) (i) Net Calorafic Value NCV = - (Hproducts – Hreactants)/mass of CH4 = - [{-8.94244 + -13.423218} – {-4.66716}]/16 = 50.125 MJ/kg (ii)Gross Calorafic Value GCV = - (Hproducts – Hreactants)/mass of CH4 = - [{-8.94244 + -15.866218} – {-4.66716}]/16 = 55.622 MJ/kg Note: NCV = GCV – (Mwater/Mmethane)hfg = 55.622 – (36/16)2.443 = 50.125 MJ/kg.