1. Thermo chemistry of combustion
P M V Subbarao
Professor
Mechanical Engineering Department
Selection of Sufficient Air to use the Entropy Vehicles…..

2. Fuel Models A gravimetric analysis of fuels Dry Basis
As Received 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. Proximate Analyses Seeds
S. No.
Scientific Name
Total Solids
(%) M
Volatile Solids
(% of TS)
Non-volatile solids 1
Acacia nilotica 
88.48
11.52
91.59
8.41 2
Prosopis julifora
89.95
10.05
90.43
9.57 3
Albizia lebbeck
89.38
10.62
94.58
5.42 4
Leucaena
89.4
10.6
93.99
6.01 5
Cattle manure
18.43
81.57
77.97
22.03

4. Ultimate analysis Seeds
S. No.
Scientific Name
Carbon
(%)
Hydrogen
Nitrogen 1
Acacia nilotica
42.17
6.93
5.63 2
Prosopis julifora
40.73
6.94
5.97 3
Albizia lebbeck
47.31
6.81
7.08 4
Leucaena leucocephala
44.87
6.44
5.34 5
Cattle manure
35.54
4.51
1.71

5. Equivalent Chemical Formula
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.

6. Equivalent Chemical Formulae
S. No.
Scientific name
Chemical formula 1
Acacia nilotica
C3.51H6.93O2.22N0.40 2
Prosopis julifora
C3.39H6.94O2.26N0.43 3
Albizia lebbeck
C3.94H6.81O2.05N0.50 4
Leucaena leucocephala
C3.74H6.45O2.30N0.38 5
Cattle manure
C2.96H4.51O2.26N0.12

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

8. Stoichiometric Ideal Combustion

9. Philosophy of Combustion (Reaction)
It is spontaneous Combination of species, known as reactants to become products and release heat.
The first and foremost molecules of reactants react in infinitesimal (~zero) time.
It requires infinite time for last set of molecules of reactants to become products.
Humans depend on combustion, in spite of knowing that they generate pollutants.

10. Classification of Engineering Combustion Systems
External Combustion Systems: Only combustion of fuel with air occurs in these systems.
These systems transfer the thermal energy liberated due to combustion to surroundings thru various modes of heat transfer.
Process Heat Utilization Surroundings.
Power generating Water-steam Surroundings.
Air is just a source of oxygen.
Internal Combustion Systems: Thermal energy liberated due to combustion is used generate Mechanical Power.
Air is both working fluid and source of oxygen.

11. Furnace in A Modern Coal Fired Steam Power Plant

12. First Law Analysis of External Combustion System: SSSF
Many of the thermal power plants running on Ranke Cycles use an external combustions system known as Coal (fuel) Fired Steam generator.
First Law Analysis of a Combustion System (SSSF) in molar form :

13. First Law Analysis of A Furnace
First Law Analysis of a Furnace (SSSF) in molar form :

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

15. 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!!!

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

17. 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

18. Solid Residue & Unburnt Carbon
Solid fuels contain large amounts of non-combustible solid residue.
This is called as Ash.
In modern power plants this is lost as fly ash and bottom ash.
Unburnt carbon is lost with ash.
Ash sample is generally collected to assess the amount of carbon loss.
Combustible Solid Residue is defined as:

19. 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.

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

21. Optimization of Furnace Air

22. Danger of Deficient Air

23. Influence Unnecessarily Excess Air

24. Theoretical (100%) Air for Combustion
Perfect or stoichiometric combustion is the complete oxidation of all the combustible constituents of a fuel.
The oxygen consumed by a Perfect Combustion is known as 100 percent theoretical oxygen.
The air required by a perfect combustion is 100% theoretical air.
Excess air is any amount above that theoretical quantity.

25. Optimal Requirement of Excess Air for Combustion
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 required in any system depends on :
The physical State of the fuel in the combustion chamber.
For complete combustion, solid fuels require the greatest, and gaseous fuels the least, quantity of excess air.
Fuel particle (drop) size, or viscosity.
The proportion of inert matter (ASH) present in the fuel.
The design of furnace and fuel burning equipment.
Excess air requirement can be decreased:
By finely subdividing the fuel.
By producing high degree of turbulence and mixing.

26. Typical values of Excess Air vs Fuel
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