Thermo-Chemical Analysis of Steam Generator Operation & Performance

1. Thermo-Chemical Analysis of Steam Generator Operation & Performance By P M V Subbarao Associate Professor Mechanical Engineering Department I I T Delhi A Mathematical Model for Fuel and Its Utilization…..

2. Operational Issues Design Issues Thermal structures Slagging. Fouling &Erosion Combustion System Thermal Performance Fuel Preparation system Combustion performance Fuel Handling system Fuel & Air

3. Analysis of Coal • Proximate Analysis & Ultimate Analysis. • Proximate analysis - to determine the moisture, ash, volatiles matter and fixed carbon • Ultimate or elementary analysis - to determine the elemental composition of the coal • The Energy content -- CFRI Formulae -- • Low Moisture Coal(M < 2% ) -- CV (Kcal/kg) = 71.7 FC + 75.6 (VM-0.1 A) - 60 M • High Moisture Coal(M > 2%) -- CV(kcal.kg) = 85.6 {100 - (1.1A+M)} - 60 M • Where, M, A, FC and VM denote moister, ash , fixed carbon and Volatile mater (all in percent), respectively.

4. Fuel Model • Ultimate Analysis of fuel: Gravimetric • For a given mass of fuel : Say 100 units. • Estimate : • Percentage of Carbon in all forms: x • Percentage of Hydrogen in all forms: y • Percentage of Oxygen in all forms: k • Percentage of Sulfur in all forms: z • Percentage of Ash : A

5. Fuel Model

7. Chemical Model for Fuel & Ideal Combustion • A mixture of many chemical compounds and elements. • No explicit chemical formula to represent the chemical boding. • An equivalent chemical Formula is required for Combustion analysis. • Number of kmoles of combustible carbon, X = x/12 • Number of atomic kmoles combustible hydrogen : Y • Y = Total number of atomic kmoles of hydrogen – Number of atomic kmoles of hydrogen in the form of water. • Y : (y-M/9)/1 • Number of atomic kmoles combustible oxygen: K = (k-8M/9)/16 • Number of atomic kmoles combustible sulfur: Z = z/32 • Equivalent chemical formula (ECF) or Representative Chemical Formula : CXHYSZOK • Equivalent Molecular weight of fuel : 100 kgs. • Ideal combustion • CXHYSZOK + 4.76 (X+Y/4+Z-K/2) AIR→ P CO2 +Q H2O + R N2 + G SO2

8. Stoichiometry of Actual Combustion • CXHYSZOK +e 4.76 (X+Y/4+Z-K/2) AIR→ P CO2 +Q H2O +R SO2 + T N2 + U O2 + V CO • Conservation species: • Conservation of Carbon: X = P+V • Conservation of Hydrogen: Y = 2 Q • 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

9. First Law Analysis of Furnace at Site • CXHYSZOK +e 4.76 (X+Y/4+Z-K/2) AIR + Moisture in Air + Ash & Moisture in fuel→ P CO2 +Q H2O +R SO2 + T N2 + U O2 + V CO + W C + Ash • S Q + n air hair + n fuel hfuel = n fluegas hfluegas + S W • Unburned carbon losses. • Incomplete combustion losses. • Loss due to ash. • Loss due to moisture in air. • Loss due to moisture in fuel. • Loss due to combustion generated moisture. • Loss due hot exhaust gases.

11. 13 Essentials of Optimum Combustion Fuel Preparation 1. Fuel feed quality and size shall be consistent. 2. Fuel feed shall be measured and controlled as accurately as possible. Load cell equipped gravimetric feeders are preferred. 3. Fuel line fineness >75% passing a 200-mesh screen, and 50 mesh particles <0.1%. Distribution to Burners 4. Primary airflow shall be accurately measured and controlled to ±3% accuracy. 5. Primary air to fuel ratio shall be accurately controlled when above minimum. 6. Fuel line minimum velocities shall be 16 m/sec. 7. Fuel lines shall be balanced by “Clean Air” test to within 2% of average. 8. Fuel lines shall be balanced by “Dirty Air” test to within 5% of average. 9. Fuel lines shall be balanced in fuel flow to within 10% of average.

12. Combustion 10. Over-fire air shall be accurately measured and controlled to ±3% accuracy. 11. Furnace exit shall be oxidizing; 3% oxygen is preferable. 12. Mechanical tolerances of burners and dampers shall be ±1/4’’. 13. Secondary air distribution to burners shall be within 5-10% of average.

13. Burner In Good Conditions

14. Burner After some Period

15. Model Testing for Determination of important species Water Flow Rate Air Flow Rate Flue gas Analysis Fuel Flow Rate

16. 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!!!

17. Stoichiometry of Actual Combustion at site • For every 100 kg of Coal.

18. 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/4+Z-K/2) = 2P +Q +2R +2U+V • Conservation of Nitrogen: 2 e 3.76 (X+Y/4+Z-K/2) = T • Conservation of Sulfur: Z = R

19. Actual Air-Fuel Ratio • For 100 kg of coal: • Mass of air: e*4.76* (X+Y/4+Z-K/2) *28.89 kg. • Mass of Coal: 100 kg. • Excess Air: (e-1)*4.76* (X+Y/4+Z-K/2) *28.89 kg.

20. Optimization of Furnace Parameters.

22. Optimization of Excess Air PPM of CO in Flue Gas Percentage of Oxygen in Flue Gas, %

25. 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.

26. 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.