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

2. 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

3. 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 (X+Y/4+Z-K/2) AIR → P CO2 +Q H2O + R N2 + G SO2
Air- Fuel Ratio:

4. 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.

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

6. 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%).

7. 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 :

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

9. 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?!?!?!

10. 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.

11. 9-Step reduced mechanism for Methane Combustion

12. 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

13. 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!!!

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

15. 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

16. 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.

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

19. Danger of Deficient Air

20. Other Symptoms of Flue Gas

21. Influence Unnecessarily Excess Air

22. 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.

23. 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)
Coke
Wood
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