Thermo chemistry of combustion

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 Ultimate Analysis Proximate Analysis Proximate Analysis Ultimate Analysis C, M=0, VM & A C, M, VM & A C, H,O, S, & A C, H,O, S, & A

3. Proximate Analyses Seeds

4. Ultimate analysis Seeds

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

7. Ideal Combustion • Ideal combustion • CXHYSZOK + 4.76(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