As market conditions and environmental requirements increasingly push generation asset owners to deploy fossil units in cyclic operation, fuel flexibility provide one plausible pathway to continued viability. The Orlando Utilities Commission has hit on a way to blend cycling at its Stanton Energy Center into a broader effort to expand fuel flexibility and improve economics by capitalizing on low gas prices. A key element of this transition involved equipping the two coal-fired boilers at Stanton with igniter systems that could effectively accommodate various fuel firing configurations
Oil and gas igniters continue to have wide applicability for generating heat energy necessary to safely ignite the main boiler fuel. The igniter serves three basic purposes: warm-up of the furnace, ignition (light-off) of the main fuel, and load stabilization. Matching a specific igniter type with the anticipated duty cycle helps ensure safe and stable furnace operation while maintaining reliability and potentially reducing fuel costs.
The U.S. National Fire Protection Association (NFPA) Regulation 85 establishes standards to prevent explosions in pulverized fuel-fired multiple burner boiler furnaces. These standards apply to the full range of large boiler installations and pulverized fuel systems in the United States and encompass boiler design, installation, operation, maintenance, and training. Specific topics covered in the 85 provisions include combustion equipment, draft control, safety interlocks, alarms and trips, and associated controls.
The NFPA 85 provisions classify igniters into three classes (Class 1, 2 or 3) according to their intended operation:
Class 1 Igniters Provide Maximum Flexibility
Class 1 oil or gas igniters are the most versatile igniters in pulverized coal applications because they can provide burner ignition, stabilization, and boiler warm-up. For pulverized fuel combustion, gas igniters are typically preferred over oil igniters because oil atomization is not required; this advantage is amplified with the low gas prices in today’s energy markets. Class 1 igniters typically use a high-energy spark ignition (HESI) rod, a fuel gun, and a guide tube. Some designs also feature a steel diffuser attached to the tip of the fuel gun guide tube to assist in providing a stable flame (Figure 1).
Many fuel guns have a retraction assembly that moves the gun into firing position and back into the guide tube when not in service. This feature extends the lifetime of the igniter by limiting exposure of the tip components to flame conditions once ignition and stabilization have occurred. This feature is particularly attractive for dual fuel (oil/gas) igniter applications because of the variation in combustion characteristics and combustion products near the tip.
High turndown ratios are increasingly important for today’s igniters. With low gas prices and the emergence of renewable energy, coal-fired power plants are routinely being called on to cycle according to electricity demand curves. Igniters, therefore, must be able to function effectively during continuous, intermittent, and interrupted operational modes. Igniter turndown ratios of at least 2:1 are required for efficient igniter fuel utilization during load following. For continuous service applications, the igniter fuel can account for a significant portion of total Btu input.
Fuel Flexibility at Stanton
The Orlando Utilities Commission (OUC) is well positioned to take advantage of changing fuel costs and availability conditions in producing power for its 200,000 customers in central Florida. Its Stanton Energy Center in Orange County produces electricity using coal, natural gas, landfill gas, and solar power, making it Florida’s most diverse power generating facility. The site can produce 950 MW of power using two natural gas-fired combined cycle units (Units A&B) and 900 MW using two B&W 450 MW opposed-fire boilers (Units 1 & 2). The main fuel for Units 1 & 2 is pulverized coal, but the boilers can co-fire natural gas as well as landfill gas delivered by a five-mile pipeline from the Orange County Landfill. The landfill gas provides up to 22 MW of additional power, enough to displace up to 3% of the fossil fuel consumed by the boilers.
The Stanton Energy Center also is home to Orange County’s first solar farm – a 22-acre, 5.9 MW photovoltaic array that can generate enough energy to power more than 600 homes. This level of fuel diversity makes the Stanton Energy center one of the nation’s most reliable and environmentally responsible power generation stations.
Because of low natural gas prices and increasing environmental concerns surrounding coal, OUC’s planning has focused on reducing coal use. Both coal-fired units at Stanton are cycled to reduce load during the evening hours. While each unit operates five pulverizer mills during the higher daytime demand hours to generate 450 MW each, the plant reduces the number of mills in operation each night down to one, reducing generation to 90-120 MW. Landfill gas is typically co-fired with the coal in both full load and reduced load operation.
Each coal-fired unit has 30 pulverized coal burners arranged in five rows with six burners per row. Each burner has a 20 MMBtu/h natural gas retractable fuel gun and diffuser attached to the tip of the gun guide tube. This gas gun is ignited by a retractable high energy spark igniter (HESI). The gun is typically moved into firing position for ignition, warm-up, and stabilization, and then retracted when not in service.
To take full advantage of low natural gas prices and to reduce coal consumption, Units 1 & 2 at Stanton can operate with the gas igniters in service 24 hours per day, seven days per week. When co-firing natural gas, the gas igniter guns are not retracted and remain in firing position. Co-firing with natural gas during low-load operation provides about 50% of the heat input, resulting in a considerable reduction in coal consumption. The thirty 20-MMBtu/h gas igniters can add up to 600 Btu/h heat input per unit. Notably, since 2008, OUC’s annual coal consumption has dropped from 78% to 28%, while the use of igniter gas has increased from essentially 0% to 3%.
Matching Igniter to Application
The fuel flexibility at the Stanton Energy Center presented some challenges in fine-tuning the igniter systems. Following start-up and during the initial campaigns, OUC found that the stainless steel diffuser cones attached to each gun guide overheated and distorted, preventing full extension of the sparker rods into the boiler. Without full extension, the igniters failed to light off. The overheating may have been due to continuous igniter operation when co-firing natural gas.
Working with the igniter supplier, Forney Corp., OUC addressed the diffuser overheating issue by changing the material of construction from stainless steel to Inconel. As shown in Figure 3, the Inconel diffuser cones have remained intact with no distortion since installation. Minor ash accumulation has been easily removed by maintenance personnel using a wire brush.
The Bottom Firing Line
Changing market conditions are forcing generation asset owners to evaluate and implement alternate operational modes to remain relevant in the dispatch curve while continuing to generate revenue in a cost-effective manner. Fuel flexibility is one plausible path toward continued viability and, as demonstrated at the Stanton Energy Center, attention to key issues such as igniter system turndown and materials of construction cannot be overlooked.
— Robert Parent (firstname.lastname@example.org) is a VP of Sales for Forney Corp.
— James Czarniecki PE is a Project Engineer for Orlando Utilities Commission