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

Combustion Generation of heat through rapid chemical reactions of fuels is known as combustion Products of Combustion -CO 2 -H2O-H2O -NO 2 -SO 2 -CO, -HCs, -NO X, SO X, …. BASICS OF COMBUSTION Complete Combustion Incomplete 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 BASICS OF COMBUSTION 3T’s : Time, Temperature, Turbulence

Combustion Flame of different fuels 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. BASICS OF COMBUSTION

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

Combustion Reactions BASICS OF COMBUSTION Exothermic Endothermic C + 4H + 4O Break two “O=O” bonds kJ/mol C + 4H + 2O 2 Break four “C- H” bonds kJ/mol CH 4 + 2O 2 (Reactants) Form two “C=O” bonds kJ/mol CO 2 + 4H + 2O Form four “O-H” bonds kJ/mol CO 2 + 2H 2 O (Products) Net energy change -802 kJ/mol Exothermic – gives off heat energy

Combustion Reactions Some fundamental reactions of combustion:  C + O 2  CO MJ/kg-C  2H 2 + O 2  2H 2 O MJ/kg-H  S + O 2  SO MJ/kg-S  2C + O 2  2CO 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. 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. BASICS OF COMBUSTION

Composition of Air On a molar (or volume) basis, dry air is composed of: –20.9% oxygen O 2 –78.1% nitrogen N 2 –0.9% CO 2, Ar, He, Ne, H 2, 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 BASICS OF COMBUSTION

Composition of Air At ordinary combustion temperatures, N 2 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, N 2 does react, forming species such as oxides of nitrogen (NOx), which are a significant pollutant. 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 AFR stoich. For a fuel C x H y O z BASICS OF COMBUSTION

Stoichiometry and Air/Fuel Ratios Eg: Combustion of Methane CH 4 + 2(O / 21  N 2 )  CO 2 + 2H 2 O / 21  N 2 Therefore, AFR Stoich = (2   28  79/21)/(  1) = BASICS OF COMBUSTION FuelPhaseAFR Stoich Very light fuel oilliquid14.27 Light fuel oilliquid14.06 Medium heavy fuel oilliquid13.79 Heavy fuel oilliquid13.46 Generic Biomasssolid 5.88 Coal Asolid 6.97 LPG (90 P : 10 B)gas15.55 Carbonsolid11.44

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: BASICS OF COMBUSTION

Stoichiometry and Air/Fuel Ratios Eg: BASICS OF COMBUSTION FuelType of Furnace or Burners Excess air % by weight Pulverized Coal  Completely water-cooled furnace for slag-tap or dry-ash- removal  Partially water cooled furnace for dry-ash-removal 15 – Crushed coal Cyclone furnace – pressure or suction Coal  Spreader stroker  Water-cooled vibrating grate stroker  Chain-grate and traveling grate strokers  Underfeed stroker 30 – – Fuel oil  Oil burners, register type  Multi-fuel burners and flat-flame 5 – Acid sludge Cone and flat-plate-type burners, steam-atomized Natural coke ovens and refinery gas  Register-type burners  Multi-fuel burners 5 – Blast furnace gas Intertube nozzle-type burners Wood Dutch oven and Hofft-type35 – 50 Bagasse All furnaces Black liquor Recovery furnace for kraft and soda-pulping processes5 - 7

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

Estimation of Heating Values Eg: Methane: CH 4 + 2(O / 21  N 2 )  CO 2 + 2H 2 O / 21  N 2 Enthalpies CH 4 : MJ/kg; O 2 : 0.0; N 2 : 0.0 CO 2 : MJ/kg; H 2 O : MJ/kg (Gas) / MJ/kg (Liquid) (i) Net Calorafic Value NCV = - (H products – H reactants )/mass of CH 4 = - [{   2  18} – {  16}]/16 = MJ/kg (ii) Gross Calorafic Value GCV = - (H products – H reactants )/mass of CH 4 = - [{   2  18} – {  16}]/16 = MJ/kg Note: NCV = GCV – (M water /M methane )  h fg = – (36/16)  = MJ/kg. BASICS OF COMBUSTION