Combustion Optimization in pulverized Coal fired Boilers

1. Combustion Optimization in pulverized Coal fired Boilers Ranjan Kumar NTPC Ltd 1st December,2017

2. PRESENTATION OUTLINE Basics of Combustion NOx Formation Mechanism Combustion Quality Assessment Strategy for NOx Reduction Effect of high PA flow and high VM on combustion Practices for Boiler Tuning Conclusion and Recommendations

3. Basics of Combustion The chemical reaction between the fuel and oxidant is called combustion.it is accompanied by the release of heat and usually by the emission of light in the visible region of the spectrum.

4. Basics of Combustion COAL PROPERTIES AFFECTING BOILER PERFORMANCE • Fixed carbon, • Volatile matter • Moisture • Ash • Calorific Value • Ignitability • Ash Composition

5. Basics of Combustion NOx optimization philosophy during combustion and post combustion has been adopted in this paper for overall combustion optimization of pulverized coal fired boilers. NO is the predominant compound found in NOx (NO + NO2) at the stack and typically accounts for 95% to 98% of the total NOx emitted from fossil fuel-fired boilers. Types of NOx : • Thermal NOx • Fuel NOX

6. NOx Formation Mechanism

7. NOx Formation Mechanism Thermal NOx : The formation rate of thermal NO is dependent on the reaction temperature, the local stoichiometry, and the residence time. Thermal NO formation mechanisms are well known, having been described by the extended Zeldovich mechanism: N + O2 ≡ NO + O N2 + O ≡ NO + N N + OH ≡ NO + H with the reaction rate increasing rapidly at high temperatures and becomes significant at temperatures in excess of 2700°F (1482°C). Thermal NO formation rates are also dependent on local stoichiometry and the associated oxygen availability. The residence time in high-temperature regions also contributes to the amount of thermal NO formed.

8. NOx Formation Mechanism • Fuel NOx : • Fuel NOx formation results from the oxidation of nitrogen compounds in the coal. • Nitrogen compounds are evolved during the initial coal particle heat up and devolatilization period in the near burner zone. • Fuel nitrogen compounds react with available oxygen to form NOx. • Generally, these volatile organic compounds react in a homogeneous gas phase to form NOx. • Fuel NOx formation is strongly dependent on local O2 concentrations and more weakly dependent on flame temperature than the thermal NOx mechanism. • Fuel NOx emissions are, however, a strong function of fuel/air mixing.

9. Combustion Quality Assessment Each kilogram of CO formed means a loss of 5654 kCal of heat (8084-2430)

10. Combustion Quality Assessment IMPACTS OF POOR COMBUSTION

11. Strategy for NOx Reduction Based on the mechanisms responsible for thermal NOx formation, three primary strategies can be employed to reduce thermal NOx: • Reduce the peak temperature • Reduce local oxygen concentrations at peak temperature • Reduce the residence time at peak temperature

12. Strategy for NOx Reduction Staged Combustion The predominant technique used for NOx formation control in coal-fired boilers is staged combustion. Staged combustion is effective in controlling fuel NOx by reducing the oxygen concentration in the initial combustion zone.

13. Strategy for NOx Reduction Staged Combustion • Salient points involved are: • Oxygen-rich conditions tend to drive the reactions toward NOx formation • fuel-rich conditions tend to drive the reactions toward the formation of molecular nitrogen (N2) • The reaction rates driving NOx and fuel nitrogen species to N2 are also strongly dependent on temperature and residence time • Sufficient residence time must be available in the second stage (e.g., OFA addition) to burn out the residual carbon • NOx variability is likely attributable to fuel/air distribution and changes in excess air level • adjust SOFA dampers to reduce CO at the expense of higher NOx levels

14. Strategy for NOx Reduction Staged Combustion

15. Strategy for NOx Reduction NOx control during combustion process • Windbox to furnace DP optimization as a function of total air flow

16. Strategy for NOx Reduction NOx control during combustion process • OFA damper position optimization

17. Strategy for NOx Reduction NOx control during combustion process • Fuel air damper position optimization If volatile matter of coal is less than 22%, fuel air damper should be fully closed; else its opening should be as per feeder speed functional curve provided by OEM.

18. Strategy for NOx Reduction NOx control during combustion process • Under fire air damper position Under fire air damper controls the unburned carbon in bottom ash.

19. Strategy for NOx Reduction NOx control during combustion process • Burner tilt optimization When operating at part load, burner tilt should be horizontal or some degree up from horizontal. Negative degree of burner tilt at part load increases unburned carbon in bottom ash. Burner tilt position (in degree) versus load

20. Strategy for NOx Reduction NOx control during combustion process • Fuel Biasing • furnace is divided into upper and lower combustion zones. • The lower zone is operated fuel-rich to reduce the NOX formation. • The upper zone is operated fuel-lean to complete CO burnout. • The total quantity of fuel supplied to the boiler remains the same as before fuel biasing. • The specific amount of fuel supplied to the upper burners depends on the unit load with more effective biasing possible at lower loads. • The extent of the fuel biasing that can be implemented depends on the design of the burners and the capacity of the pulverizers. • NOX reductions of up to 15% may be achieved with fuel biasing.

21. Strategy for NOx Reduction NOx control during combustion process • Coal mill outlet temperature • Mill outlet temperature (77°C – 82°C) for bituminous/sub-bituminous coal • Mill outlet temperature should be restricted around 67°C in case of high VM coal. • Primary air to fuel ratio • Air to fuel ratio should be maintained between 1.3 and 1.8 • This ratio is higher side for low coal feed rate and vice versa.

22. Strategy for NOx Reduction NOx control during combustion process • Excess air control • Excess air should not be more than 20% if load is more than 30% of TMCR. • If load is less than 30%, excess air may go up to 30%. • When oil gun is in service, higher excess air is required. • Coal mill fineness • 75% through 200 mesh • 99.5% through 50 mesh

23. Strategy for NOx Reduction NOx control during combustion process • Air-fuel mixture velocity through coal pipe • Mixture velocity should be greater than settling velocity • Mixture velocity should be maintained between 20 m/s and 25 m/s. • Tolerances for fuel and air balance • ±2% balance in clean airflow between each pulverizer’s fuel lines • ±5% balance in dirty airflow between each pulverizer’s fuel lines.

24. Strategy for NOx Reduction NOx control during combustion process • Furnace exit gas temperature control • By air staging • By fuel staging • FEGT temp and O2 monitoring • Low NOx Burner

25. Strategy for NOx Reduction NOx control during combustion process • Flame Stability • Increases with increasing VM • Increases with increasing coal fineness • Increases with mill outlet temperature  • The flame velocities are higher when the mixture is richer than stoichiometric. • The maximum flame temperature occurs in stoichiometric mixtures. But when the mixture is slightly fuel rich, the concentrations of free radicals - which play a significant role in flame propagation - are at a peak.   • If the gas velocity > burning velocity ==> blow off  • If the burning velocity > gas velocity ==> flash back

26. Strategy for NOx Reduction NOx control during combustion process • Optimized combustion parameters

27. Strategy for NOx Reduction NOx control during combustion process

28. Strategy for NOx Reduction

29. Strategy for NOx Reduction Post Combustion NOx control Introduction of SNCR Introduction of SCR Each of these technologies requires the introduction of a reagent, such as ammonia or urea that will “selectively” react with NOX. This reaction occurs in the presence of oxygen. The following simplified chemistry summarizes the reactions involved in the post combustion controls to convert NOX to elemental nitrogen:

30. Strategy for NOx Reduction Post Combustion NOx control

31. Effect of high PA flow and high VM on combustion • Poor flame stability at lower loads and flame ignition points unattached to burner nozzles. • Optimized primary airflow will maintain velocities at the coal nozzle tip within the best range of flame propagation speed for flame stability and improved combustion efficiency. • Higher dry gas losses are the result of increased tempering air usage. • Increased furnace exit gas temperatures. • High primary airflow increases the differential in velocity between the primary air/fuel mixture and the secondary (combustion) air. This stages or delays combustion that allows a large percentage of heat to be released above the burner belt.

32. Effect of high PA flow and high VM on combustion • A sudden dip in volatile matter value of the fuel will lower the flame temperature thereby dipping radiant heat transfer in furnace. This will increase the unburnt carbon in bottom ash. • When the Volatile matter of the coal is low, flame stability decreases (Continuous support of oil may be required for normal operation) • When the Volatile matter of the coal is high, pulveriser outlet temperature is to be decreased correspondingly which will directly reduce the boiler efficiency.

33. Practices for Boiler Tuning • Inspect the burner and combustion controls at least once every 24 months • Inspect the flame pattern and adjust it in accordance with best combustion engineering practice. • Evaluate windbox pressures and air proportions. Make adjustments and perform repairs to dampers, actuators, controls and sensors. • Inspect the system controlling the air-to-fuel ratio. Ensure that it is correctly calibrated and functioning properly. Such inspection should include calibrating excess O2 probes and/or sensors, and adjusting over fire air systems. • Optimize combustion to minimize generation of CO and NOX consistent with the manufacturer’s specifications. This includes burners, over fire air, firing system improvements, control systems calibration and combustion zone temperature profiles. Add-on controls such as SCR and SNCR should be optimized to minimize generation of NOX. • Maintain an on-site annual report detailing burner inspection and tuning information. This should include CO, NOX and O2 measurements before and after tuning, description of corrective actions taken, as well as type and amount of fuel used in the prior 12 months. • CEMS data should be validated on specified frequency and calibration of instrument should be done accordingly. • Inspect and calibrate all instrumentations involved.

34. Conclusion and Recommendations • Ensuring the basic combustion “principles” are followed is the first step in reducing NOx. • The increasing requirements for reduction on NOX emissions from combustion furnaces require the development of a control strategy to identify the most effective approach for each installation. • Fuel NOx accounts for approximately 80% of the total NOx emissions, air staging is the best technique to reduce overall NOx. • The characteristics of the furnace design, unit operating practices, and presence of any existing NOX controls should be considered in determining the most cost-effective solution. • Factors that limit the degree of staging are typically economizer outlet CO levels and fly ash unburned carbon levels. • Monitor FEGT continuously with a well calibrated and accurate device. • Use of PADO for online combustion optimization. (loss optimization) • The challenge, however, is not solely reducing NOX levels, but achieving the necessary emissions control while maintaining unit performance and safety.