Combustion fundamentals
SEC’s Unit 2, the focus of this case study, is a Babcock & Wilcox (B&W) balanced-draft, “Carolina Radiant” boiler rated at 3,305,000 lb/hr steam flow with a 1,005F superheat/reheat temperature. The boiler is configured with 30 B&W DRB-XCL burners. There are three burner levels on the front wall and two levels on the rear wall. Six overfire air (OFA) ports are on the front and rear walls. Fuel is prepared for firing by five MPS 89N mills. Primary and secondary combustion air is heated by one Ljungstrom regenerative air heater. The unit is also equipped with a selective catalytic reduction system (SCR), electrostatic precipitator, and a wet limestone scrubber for emissions control. One unique feature of this plant is that pulverized coal is burned with methane gas drawn from an adjacent municipal landfill. (Unit 1 does not yet have ultra-low-NOx burners, OFA, or an SCR.)
The 13 essentials of optimizing combustion in coal-fired boilers have been discussed in an earlier article. (POWER, October 2006, “Apply the fundamentals to improve emissions performance,” p. 26.) These fundamentals have been used to successfully improve the performance of many coal-fired plants, including SEC. The success of SEC’s project meant applying these essentials to improve load response and increase fuel flexibility while maintaining plant heat rate and reliability.
The project was broken down into nine core parts:
- Primary airflow measurement
- Secondary airflow measurement
- Pulverizer performance
- Burner performance
- Forced-draft fan performance
- Control damper settings
- Air heater performance
- Induced-draft fan performance
- Coal feeder accuracy
These components and settings were rigorously performance-tested and evaluated. Then a comprehensive plan was devised for necessary improvements to equipment and operating processes.
Space doesn’t allow for a thorough discussion of each system, or the many tasks that were part of each system analysis, or how the individual system performance tests were conducted. But we can give an overview of several of the more important tasks and their results. Also note that many of these systems are highly interrelated, making a series of performance tests necessary to completely understand how one system interacts with others.
The combustion air systems are good examples of the evaluation process used on this project. The amount of air required for proper fuel combustion can be calculated directly from the coal’s chemical constituents. As the coal properties change, airflow requirements also change slightly. When more carbon and hydrogen are bound in the fuel, that increases the amount of air required to fully combust the fuel. Recall that the total air used for combustion is split into three streams for the staged combustion of the fuel: secondary air (typically 55% to 65% of the total airflow) and primary and overfire air (each 15% to 20% of the total airflow). A fuel-flexible combustion system must be able to quickly change the amount of air and the airflow split when a fuel change is made.
The primary air transports pulverized coal from the mill to the burner to begin the combustion process by vaporizing moisture in the coal before combustion. Adding any surplus primary air will have a negative effect on fuel fineness, distribution, and combustion at the burner, which can lead to decreased emissions performance. Possible downstream effects of a furnace tuned for too much primary air include:
- Heat rate penalty caused by tempering air bypassing the air preheater.
- Reduced furnace residence time for carbon burnout.
- Increased slagging propensity.
- High carbon loss.
- High steam temperatures.
- High spray flows.
- High primary airflow, which increases erosion and mill circulation.
- Reduced once-through grinding efficiency of the mill, thus reduced mill capacity.
Remember that the total air also includes any unheated tempering air that may or may not be measured after the air heater, and any furnace casing leaks (in or out, as this is a balanced-draft boiler) can significantly degrade the results. Leaks must be identified early and repaired quickly, or the test results will be skewed.
Email
Comments
Print
Reuse
