MEMORANDUM 

From: The Office of Air Quality Planning and Standards 

 U.S. Environmental Protection Agency 

To: EGU NSPS Docket (EPA-HQ-OAR-2011-0660) 

Date: April, 2012 

Subject: Design Data for New Combined Cycle Facilities 

Introduction 

The EPA used design performance specifications from Gas Turbine World1

 to estimate the CO2 emissions rate for 

new natural gas-fired combined cycle facilities. We adjusted the design value emission rates to account for 

operation at non-design ambient temperatures, the use of post-combustion controls, normal degradation of 

combustion turbine components, and operation at part load, all of which reduce turbine performance. 

Specifically, the impact of temperature on the efficiency of combined cycle power plants is relatively minor, and 

we increased the design heat rate (which is a measure of overall efficiency, calculated by the amount of Btu 

needed to generate a kilowatt hour of energy) by 1 percent to account for operations at other than design 

conditions. The impact of post-combustion controls (i.e., selective catalytic reduction) on efficiency is also 

relatively minor. To account for the increased pressure drop from the catalyst we increased the design heat rate by 

125 Btu/ kWh. Degradation is a more important factor as the heat rate of the facility will gradually deteriorate 

between overhauls. Although generally estimated at less than 3 percent over the life of the facility, we 

conservatively estimated a 5 percent increase in heat rate due to degradation to account for adverse conditions and 

different turbine designs. 

Operational load is one of the primary factors in the actual operating efficiency and CO2 emissions rate of a 

combined cycle power plant. Combined cycle power plants are most efficient when operating at full load. At 80 

percent load, the actual heat rate only increases by approximately 1 percent relative to the design basis heat rate. 

However, below 80 percent load, the heat rate begins to increase at a more significant pace. At 60 percent load, 

the actual heat rate increases by approximately 10 percent above the design rate. Even though most combined 

cycle power plants are intended to operate at high loads, part-load operation is inevitable with variable demand. 

We selected a 5 percent heat rate increase relative to the design rate to account for part-load conditions. 

The overall impact of these adjustments increases the expected annual operating heat rate to 11 percent above the 

design heat rate plus 125 Btu/kWh. Based on these assumptions, the range of operating emission rates for the 

eighty-nine facilities listed in Gas Turbine World is 820 to 1,060 lb/MWh /MWh (370 kg CO2/MWh to 480 kg 

CO2). 

Ninety percent of these available new combined cycle designs could reasonably be expected to achieve an 

emissions rate of 1,000 lb/MWh (460 kg CO2/MWh) over the life of the facility. This proposed standard would 

only limit the use of available combined cycle facilities with relatively inefficient HRSG designs. Similar designs 

using the same combustion turbine engine with a more efficient HRSG would be able to comply with the 

standard. As such, selection of combustion turbine engine designs would not be affected by this standard. By only 

limiting the use of less efficient HRSGs, the proposed standard would still provide options across the full 

spectrum of electric generation sizes of combined cycle facilities. 

 

1

 Gas Turbine World is an industry periodical that provides information concerning size, performance 

specifications, and other characteristics for gas turbines. Information was obtained from the ‘2011 Performance 

Specs, 27th Edition’.