Features. Coal Combustion: Boiler optimization increases fuel flexibility

Power, Jun 2008 by Storm, Stephen K, Inc, Storm Technologies, Lyons, Jack, Commission, Orlando Utilities

face= Italic; Burning spot market fuels can reduce plant fuel costs, but it can also introduce unexpected operational problems throughout the boiler island. Orlando Utilities Commission's Stanton Energy Center optimized its Unit 2 combustion system and improved O&M practices as part of a project to increase the unit's fuel flexibility without degrading reliability or heat rate. OUC's attitude: If you can measure it, you can manage it.face=-Italic;

By Stephen K. Storm, Storm Technologies Inc. and Jack Lyons, Orlando Utilities Commission

Spring training is when rookies and veterans alike are drilled on baseball's fundamentals--throwing, catching, and hitting--regardless of the number of games or titles won in past years. Similarly, superior power plant performance is only achieved by a motivated plant team that's well-schooled in the fundamentals of power plant operations and maintenance (O&M). One of those fundamentals is optimizing combustion.

The cost of fuel is by far the largest variable cost of operating a power plant. The typical coal-fired plant strives for the lowest possible heat rate for a given fuel supply in order to keep retail rates low. To lower their overall fuel costs, some utilities have adopted a fuel supply strategy based on spot market fuel purchases rather than the more conventional practice of hedging future costs with long-term fuel purchase contracts. This strategy makes more sense to accountants than to plant operators, and here's why.

Purchasing lower-quality fuels will decrease fuel costs only if your plant is capable of reliably and economically burning the wide range of fuels available on the spot market. Some less-desirable fuels will increase boiler slagging and fouling and cause other serious operational and reliability problems. The decision to increase spot market purchases must be carefully considered, because it will add more operating risk to a plant already stressed by an aging infrastructure, workforce reductions, and a stagnant O&M budget. Using spot market fuels might lower fuel costs, but if a plant isn't carefully optimized to handle a wider range of fuel types than were originally anticipated, the resulting increase in O&M costs could forestall lower power prices at the busbar.

face= Bold; Fuel flexibility consequences face=-Bold;

Orlando Utilities Commission's (OUC's) Stanton Energy Center (SEC), located about 13 miles southeast of Orlando, features two 450-MW coal-fired plants and a 656-MW natural gas-fired combined-cycle plant (Figure 1). SEC has elected to fire spot market fuels with an extremely wide variation in quality over the past few years while attempting to maintain historic levels of plant reliability.

The plant's challenge was to improve its systems and procedures to become fuel flexible enough to reliably burn a wide variety of fuels purchased on the spot market. This required SEC staff to quickly deal with the inevitable fuel-related problems--such as waterwall wastage, tube exfoliation and failures in the secondary superheater, and slagging and fouling in the furnace and convection section--before they became critical. The key to eradicating these problems, or at least making them manageable, has been SEC's long-term plant improvement program to optimize the performance and life of the boiler and key components for lower-quality fuels.

face= Bold; Combustion fundamentalsface=-Bold;

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-NOface= Subscript; xface=-Subscript; burners, OFA, or an SCR.)

The 13 essentials of optimizing combustion in coal-fired boilers have been discussed in an earlier article. (face= Bold; POWER,face=-Bold; 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