THERMOCHEMISTRY OF COMBUSTION P M V Subbarao Professor Mechanical Engineering Department Efficient Use of Resource thruSufficient Supply of Oxygen…..

Fuel Models A gravimetric analysis of fuels As Received Basis Dry Basis Proximate Analysis Ultimate Analysis Proximate Analysis Ultimate Analysis C, M, VM & A C, M=0, VM & A C, H,O, S, & A C, H,O, S, & A

Fuel Model & Ideal Combustion Ultimate Analysis of dry (moisture free) fuel: Gravimetric Percentage of carbon : x --- Number of moles, X = x/12 Percentage of hydrogen : y --- Number of atomic moles, Y = y/1 Percentage of oxygen: k --- Number of atomic moles, K = k/16 Percentage of sulfur: z – Number of atomic moles, Z = z/32 Equivalent chemical formula : CXHYSZOK Equivalent Molecular weight : 100 kgs. Ideal combustion CXHYSZOK + 4.76(X+Y/4+Z-K/2) AIR → P CO2 +Q H2O + R N2 + G SO2 Air- Fuel Ratio:

Stoichiometric Ideal Combustion Mass of fuel = one kilo mole = 100 kg : Equivalent chemical formula. Chemically exact amount of air for ideal combustion of one kilo morel air. Stoichiometric air fuel ratio is the ratio of exact mass of air required to mass of fuel.

Possible Extent of Combustion/Reaction Combustion is a spontaneous & exothermic Reaction

Second law limit on possibility of extent of reaction Reactants → Products At any time a reactor contains a combination of reactants and products. A reaction is said to be complete when the entropy of an adiabatic furnace reaches its maximum value. The value of maximum entropy will vary with the pressure, temperature ….. of the reaction. A reaction system and parameters of reaction should be designed such the maximum entropy is obtained when the reaction is almost complete (>98%).

Entropy of An Adiabatic Reacting System Let n number of reactants (∑Ri)are reacting to form m number of products (∑ Pj). For an adiabatic system, the entropy of system is same as entropy of universe. The entropy of universe during an adiabatic combustion :

Generalized Theory of Extent of Reaction Possible P1,T1 P2,T2 P3,T3 Entropy of Universe Extent of Reaction

Mathematical Model for Completion of Reaction For every fuel, a designer should know all possible reactants !!! Some products will influence the efficiency of reaction. Few other may not influence the efficiency of reaction but severely affect the environment. The optimal parameters for efficient reaction may not be optimal for safe reaction !! This model is possible iff all possible intermediate species are known?!?!?!

The True Process of Methane Combustion Overall reaction for the combustion of methane is given by: True Reaction Mechanisms: True Reaction mechanisms are composed of a series of elementary processes (steps) which are indicative of the molecular encounters. A detailed kinetic mechanism revealed 213 elementary reactions and 48 species. Another critical review showed 325 elementary reactions. The first two steps in the combustion of methane are given by: Interestingly, mechanisms are never proven but rather postulated and checked for consistency with observed experimental kinetic data.

9-Step reduced mechanism for Methane Combustion

Model Testing for Determination of important species Furnace of a Steam Generator in Thermal Power Plant Water Flow Rate Air Flow Rate Flue gas Analysis Fuel Flow Rate

Results of Model Testing. For a given fuel and required steam conditions. Optimum air flow rate. Optimum fuel flow rate. Optimum steam flow rate. Optimum combustion configuration!!!

Stoichiometry of Actual Combustion at site For every 100 kg of Dry Coal. 4.76 + Moisture in fuel

Stoichiometry of Actual Combustion Conservation species: Conservation of Carbon: X = P+V+W Conservation of Hydrogen: Y = 2 (Q-MA) Conservation of Oxygen : K + 2 e (X+Y/2+Z-K/2) = 2P +Q +2R +2U+V Conservation of Nitrogen: 2 e 3.76 (X+Y/2+Z-K/2) = T Conservation of Sulfur: Z = R

Actual Air-Fuel Ratio For 100 kg of coal: Mass of air: e*4.76* (X+Y/2+Z-K/2) *28.96 kg. Mass of Coal: 100 kg. Extra/deficient Air: (e-1)*4.76* (X+Y/2+Z-K/2) *28.96 kg.

Recognition of Actual Air Fuel Ratio Define equivalence Ratio as the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio.

 

Danger of Deficient Air

Other Symptoms of Flue Gas

Influence Unnecessarily Excess Air

Excess Air for Combustion Perfect or stoichiometric combustion is the complete oxidation of all the combustible constituents of a fuel. Perfect combustion consumes exactly 100 percent of the oxygen contained in the combustion air. Excess air is any amount above that theoretical quantity. Commercial fuels can be burned satisfactorily only when the amount of air supplied to them exceeds that which is theoretically calculated. The quantity of excess air provided in any particular case depends on : The physical State of the fuel in the combustion chamber. Fuel particle size, or viscosity. The proportion of inert matter present. The design of furnace and fuel burning equipment.

Excess air requirement can be decreased: For complete combustion, solid fuels require the greatest, and gaseous fuels the least, quantity of excess air. Excess air requirement can be decreased: By finely subdividing the fuel. By producing high degree of turbulence and mixing. Fuels % Excess air Solid Coal(P) 15 -- 30 Coke 20 -- 40 Wood 25 -- 50 Bagasse 25 – 45 Liquid Oil 3 – 15 Gas Natural Gas 5 – 10 Refinery gas 8 – 15 Blast-furnace gas 15 – 25 Coke-oven gas 5 - 10