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Today in Energy

June 15, 2022

Most combined-cycle power plants have duct burners that add energy to the turbine exhaust

schematic of a combined-cycle power plant with duct burners and combined-cycle power plant capacity factors
Data source: U.S. Energy Information Administration, Annual Electric Generator Report and Power Plant Operations Report

Most combined-cycle power plants in the United States have duct burners that help them make additional use of combustion turbine exhaust gases. Combined-cycle power plants are the most prevalent technology used to generate electricity in the United States, and of the 278 gigawatts (GW) of combined-cycle power plants, about 75% (208 GW) have duct burners located between the combustion and steam turbine components.

Combined-cycle systems have multiple components; the primary parts are a combustion turbine, a heat recovery steam generator (HRSG), and a steam turbine. During a combined-cycle system’s electricity generating process, a fuel, most commonly natural gas, burns in the combustion turbine to generate electric power. Exhaust gas from the combustion turbine is directed to an HRSG, where steam is pressurized to generate additional electric power in a steam turbine.

Plants with duct burners can burn additional fuel to heat the combustion turbine’s exhaust gases, which allows the HRSG to increase or maintain steam production to adapt to operating conditions. For example, these plants can increase steam production at reduced combustion turbine loads and exhaust gases or in a wider range of ambient conditions, especially in the summer. Duct firing also has associated costs, such as fuel expense, maintenance, increased emissions, and lower fuel efficiency. Whether a plant makes use of its duct burners depends on a variety of factors such as electricity market price, emissions regulations, and the need for operational optimization and ramping. We collect duct burner data in our Annual Electric Generator Report and Power Plant Operations Report surveys under the steam turbine component of a combined-cycle power block.

In 2020, the U.S. combined-cycle fleet ran at 56% of the fleet’s maximum potential generation, or capacity factor. Combined-cycle plants with duct firing capability ran at a higher average capacity factor (59%) than plants without (47%).

Most regions in the United States have more capacity from combined-cycle plants with duct burners than without. The mid-Atlantic region, which includes the PJM Interconnection, has the largest amount of combined-cycle capacity, at 52 GW, and 81% of the combined-cycle capacity in that region has duct burners.

In both Florida and New England, a relatively low percentage of combined-cycle capacity has duct burners. However, Florida and New England have larger shares of fuel-switchable combined-cycle capacity, which allow operators to switch fuels, most commonly between natural gas and petroleum, when a fuel becomes limited or expensive. Like duct burners, fuel-switching capability allows operators to adapt to operating and market conditions.

U.S. combined-cycle capacity by region
Data source: U.S. Energy Information Administration, Annual Electric Generator Report

Principal contributor: Ray Chen