Mechanical Fatigue

Mechanical wear on components is accelerated with use of LBNs. In a coal-fired unit, the coal flows through a small concentric tube into the burner. Poor distribution in the injection ports and tubes results in concentrated streams of coal called roping. Coal roping compromises low NOX formation by preventing a good distribution of the coal into the firing zone. In addition, coal roping can wear flame tips, swirl vanes, and other components.

7-16 LOW NOX BURNERS

Adjustments of the air and fuel mixing should be made to prevent roping.

Recent improvements in flame scanners may allow improved

performance of LNBs and other combustion modification techniques in the future. Previously, these scanners were used to rapidly shut down fuel flow to a burner in the event that the flame was

extinguished. Scanners are also being developed to monitor the average flame intensity, flame ignition point, stability at the ignition point, and size of the fuel stream.⁴ These data can be used to optimize the burner performance and to identify burners that are not operating properly.

LOW NOX BURNERS 7-17

Review Exercises

1. Why are nitrogen-containing fuels used with staged air burners?

(Select all that apply.)

a. The primary zone is fuel-lean.

b. The primary zone is fuel-rich.

c. Oxygen is limited in the primary zone.

d. Oxygen is limited in the secondary zone.

e. None of the above

2. What is the best choice for a small gas-fired boiler?

(Select all that apply.)

a. Controlled flow split burner b. Dual register LNB

c. Distributed mixing burner d. Directional flame burner e. None of the above

3. What are the techniques applied for controlling NOX with the staged fuel burner design?

(Select all that apply.)

a. Reducing the peak flame temperature

b. Increasing the residence time at reduced conditions c. Recirculating the flue gas

d. Limiting the oxygen available during combustion e. None of the above

4. Which of the following could result in reduced combustion efficiency?

(Select all that apply.)

a. Mixing of the fuel and air in a slow controlled pattern b. High LOI levels

c. High flame temperatures

d. Recirculation of combustion gases e. None of the above

5. What is the typical control efficiency for LNBs?

a. 75% to 95 % b. 50% to 75 % c. 20% to 50 % d. 5% to 20 %

6. Which of the following produces the shortest flame?

a. Staged air burner b. Staged fuel burner c. Tangentially-fired LNB

d. All are approximately the same.

e. Depends on the model of low NOX burner

7-18 LOW NOX BURNERS

7. Staged air burner designs may produce a(n) ______________ in comparison to conventional burners?

(Select all that apply.) a. Short wide flame b. Elongated flame c. Divided flame d. Swirling flame e. None of the above

8. Swirl vanes are used in certain low NOX burner designs in order to create eddies in the burner flame. What is the result of these eddies?

(Select all that apply.)

a. Recirculation combustion products b. A localized reduction of available oxygen c. A reduction of the peak flame temperature d. Increased static pressure drop of the boiler e. None of the above

9. Which of the following techniques reduce NOX formation with the application of LNBs to tangentially-fired boilers?

(Select all that apply.)

a. Pre-mixing of the fuel and air b. Addition of swirl vanes

c. Increased turbulent mixing of air and fuel

d. Creating a more distinct separation between the fuel-rich and air-rich zones

e. None of the above

10. Roping is a result of which of the following?

a. Elongated flames

b. Poor coal distribution in injection ports and tubes c. Swirl vanes

d. Tangentially-fired burners e. None of the above

LOW NOX BURNERS 7-19

References

1 Vatsky, J. October 7-10, 1991. Field Experience in Over 7,000 MWe of Low NOX Burner Retrofits. Presented at the Joint Power Generation Conference. San Diego, CA.

2 U.S. Environmental Protection Agency. February 1992. Summary of NOX Control Technologies and their Availability and Extent of

Application. EPA-450/3-92-004.

3U.S. Environmental Protection Agency. July 1986. Nitrogen Oxides Control for Stationary Combustion Sources. EPA/625/5-86/020.

4Collins, S. October 1993. Advanced Flame Monitors Take on Combustion Control. Power.

NOX REBURNING 8-1

8 NO _X Reburning

STUDENT OBJECTIVES

When you have mastered the material in this chapter, you should be able to do the following:

1. Explain the principles of reburning and name two source categories where it is currently applied.

2. Briefly describe combustion conditions in the primary, reburn, and burnout zones and how they result in lowering NOX

emissions.

3. State the requirements for good reburn fuels and name at least five appropriate reburn fuels.

4. State the relationship of the stoichiometric ratio to NOX emissions.

5. Identify at least three disadvantages or limitations of reburn technology as a stand-alone technology for NOX control.

8-2 NOX REBURNING

NOX REBURNING 8-3

8 NO _X Reburning

8.1 INTRODUCTION

Reburning is a NOX staged combustion control technique that suppresses the formation of NOX in burners and then provides for additional NOx

reduction in the furnace area of the boiler

The technology was conceived by Dr. Yost Weant, now with the University of Arizona. He applied the reburning concept on a bench scale in the late 1960s. Based on his work, he developed the process and presented a paper in 1973 at the Combustion Institute.

Reburning was first laboratory-tested in the mid 1970s by the Shell Development Company.¹

Some Japanese researchers applied the concept to bench scale and pilot scale boilers and presented the results of their research at an information exchange conference in Tokyo in 1981.

In the mid-1980s, reburn technology was applied commercially on large-scale boilers in Japan.

At the same time EPA began conducting in-house research on reburn technology, primarily focusing on oil and gas as reburn fuels. Later, EPA funded extramural research on the use of coal as a reburn fuel.

Reburning technology is now in use in the U.S., Europe, and Japan for electric generating units and some municipal waste incinerators. NO_x reburning is also called reburn, in-furnace NO_X reduction, and staged fuel injection.

Reburning is applicable to most types of boilers, including cyclone-fired boilers. NO_X reductions of 50% to 70% at full boiler load have been demonstrated with gas reburning on coal- and oil-fired boilers.² In most applications a reduction of more than 60% has been achieved with

minimal impacts on boiler performance and durability.³ Table 8-1 displays reburn performance data for coal-fired boilers in the U.S. as of 1999.¹

8-4 NOX REBURNING

Table 8-1

Reburning Application of Coal-Fired Boilers in the United States¹

NOX Emissions (lb/MMBtu)