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 sp ecifications from Gas Turbine World 1 to estimate the CO 2 emissions rate for new natural gas-fired combined cycle facilities. We ad justed 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 measur e 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-combusti on controls (i.e., selective catalytic reduction) on efficiency is also relatively minor. To account for the increased pressure drop fr om 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 per cent 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 CO 2 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 approxima tely 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 per cent above the design rate. Even though most combined cycle power plants are intended to operate at high loads, part-load operation is inevita ble 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 exp ected annual operating heat rate to 11 percent above the design heat rate plus 125 Btu/kWh. Based on these assum ptions, 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 CO 2 /MWh to 480 kg

CO 2 ).

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 CO 2 /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 d esigns would not be affected by this standard. By only limiting the use of less efficient HRSGs, the proposed standard would still provi de options across the full spectrum of electric generation sizes of combined cycle facilities.

1 Gas Turbine World is an industry periodical th at provides information concerning size, performance specifications, and other characteristics for gas turbin es. Information was obtained from the ‘2011 Performance Specs, 27 th Edition’.