MEMORANDUM

From: The U.S. Environmental Protection Agency

To: EGU NSPS Docket (EPA-HQ-OAR-2013-0495)

Date: September 2013

Subject: Standard of Performance for Natural Gas-Fired Combustion Turbines

Summary

To establish an appropriate, combustion turbine-based sta ndard, we reviewed the emi ssions rates of natural gasfired (non-CHP) combined cycle units used in the power sector that commenced operation on or after January 1, 2000 and that reported complete generation data to the EP A. Based on this analysis, over 96% of these units achieve the proposed standard s with no excess emissions.

Abbreviations

CHP combined heat and power

CO 2 carbon dioxide EGU electric generating unit EPA (U.S.) Environmental Protection Agency MMBtu million British thermal units (unit of energy) MW megawatt(s) - one million watts MWh megawatt hours(s) NGCC natural gas combined cycle

Introduction

The purpose of this study is to determine how existing natural gas 1 -fired combined cycle (NGCC) units have performed in comparison to the proposed standards. The study is based on hourly emission and operation data reported to the Environmental Protection Agency (E PA). Approximately 4,700 electricity generating units 2 are applicable to the Acid Rain or Cl ean Air Interstate Rule programs. Of these, some 1,000 are NGCC located in 41 states and encompass a diverse populat ion in capacity, years of service a nd configuration. This study also explores whether factors such as cycling, ambient te mperature, or elevation affect emission rates. NGCC technology performance and efficiency has improved over time. 3 Since this study is retrospective, i.e., measuring the performance of units that are less efficient than currently available on the market, it is inherently

conservative.

The term ‘unit’ here refers to the natural gas-fired co mbustion turbine, heat recove ry steam generator and duct burner (if any) together in various co nfigurations. They fall into two groups : single-shaft, wher e the turbine and heat recovery steam generator are in line, and multiple-shaft, where two or more turbines may be harnessed to a single steam generator. In either case, for this analysis both are counted as a single NGCC unit.

The analysis covers monthly emission and operation da ta for 2007 to 2011. Not all units ran for all 60 months

so the total of ‘un it operating months’ used in this analysis is 15,670 out of 18,420 possible total months. The study includes only those months where both CO 2 mass and gross MWh output we re greater than zero. All

1 Natural gas refers to the primary fuel as reported to the EPA and includes both ‘Natural Gas’ and ‘Pipeline Natural Gas’. ‘Process

Gas’ and ‘Other Gas’ units, typically associated with industrial production, are excluded.

2 An electricity generating unit may be referred to as a ‘facility’ in NSPS.

3 http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG

‐ WI/pnw_5pp_02.pdf See vintage heat rate improvement;

the annual average is assumed to improve ‐ 0.6% per year over 2002

‐ 25.emission and operation data us ed in this analysis are publically available for download, including unit megawatt capacity, fuel type and m onthly operating time, CO 2 mass and gross load in megawatt hours (MWh). 4 The publically available data is essentially the same data that would be used for compliance determinations since there is no bias adjustment in Appendix G of Part 75 5 for units using fuel meters to determine CO 2 emissions.

The unit population is filtered to in clude only those NGCC units that repor t natural gas or pipeline natural gas as primary fuel, are in the electric utility or small power producer source categories and have commercial operation dates on or after January 1, 2000. In addition, the study is limited to those units that reported gross output data from both the primary combustion tu rbine and steam generator during the study period. 6 The study excludes all units at plants where cogeneration is reported. 7

The resulting population of 307 NGCC units is heterogene ous in location, age, capacity and operating profile. They are located in 33 states and all 10 EPA regions as shown in Table 1. Both large and small states are represented, including California w ith 43 units and Rhode Island with 3. Table 2 summarizes the study population by the year they entered service betwee n 2000 and 2010. Although new NGCC units came online in 2011, none are included here as they l acked sufficient data to calculate a 12-month rolling average, the basis for determining the standard of performance. The average st udy unit has been in service for over 8 years. The study population includes 11 units that our research determined we re retrofits or conversions of simple cycle turbines to NGCC or that operated in single cycle mode fo r a period of time during commencement of operations.

Table 1: Study Units by EPA region and State

State EPA Region Unit

Count MW capacity
AL 4 20 6,613
AR 6 20 4,854
AZ 9 22 7,271
CA 9 43 11,818
CO 8 5 1,107

CT 1 2 590
FL 4 20 7,625
GA 4 8 2,840

ID 10 1 314
IL 5 8 2,454

IN 5 2 690
LA 6 7 2,225

MA 1 2 858

ME 1 3 906
MI 5 3 1,206
MN 5 6 1,993

MO 7 2 568
MS 4 14 4,773

NE 7 2 290

NH 1 2 580
NM 6 5 1,310
NV 9 8 2,584 Table 2: Stud y population b y y ear commercial operation commenced

Online Year Unit

Count MW Capacity
2000 22 5,994

2001 33 10,331

2002 71 20,471

2003 64 21,817

2004 20 6,158

2005 39 11,300

2006 9 3,131

2007 6 1,605

2008 26 9,062

2009 13 5,165

2010 4 1,315

4 Data is available at http://ampd.epa.gov/ampd/

5 See part 75 in Volume 40 of the Code of Federal Regulations (CFR) originally published in January, 1993.

6 Reporting output from the steam generator is optional. See 40 CFR part 75.

7 Excludes units at plants with any cogeneration or CHP noted in eGRID, Greenhouse Gas Reporting Program, CAMD, Ventyx (EIA) or

NEEDs databases.NY 2 9 3,006
OH 5 6 2,100
OK 6 17 5,723
PA 3 10 3,508

RI 1 3 921

TN 4 2 655
TX 6 33 9,893
UT 8 5 1,341
VA 3 5 1,652
WA 10 4 1,282
WI 5 8 2,799

In capacity, the units range from 60 to 451 MW with an average of 314 MW. The mean annual capacity factor during the study period was 36 percent, but ranged from a low of 33 percent in 2008 to a high of 38 percent in 2010 when annual generation peaked. Between 2007 and 2011, the annual CO 2 rate, expressed in pounds of

CO 2 per megawatt hour of gross output (lb CO 2 /MWh), was consistently in the range of 845 to 848 lb

CO 2 /MWh, while the count of operati ng units grew from 264 to 307.

Level of the Standard

The proposed rule establishes two sub categories for NGCC. Small units, with a gas turbine heat input capacity less than or equal to 850 MMBtu (approximately 70 MW 8 ), are required to meet an output-based performance standard of 1,100 lb CO 2 /MWh. Large NGCC units, with a heat input capacity greater than 850 MMBtu, are required to meet an output-bas ed standard of 1,000 lb CO 2 /MWh. Note that output is m easured as gross, not net, electrical output. Si nce the study does not include cogeneration units, it gives no co nsideration to steam output. Fourteen study units are in the small subcategory and 293 are in the large s ubcategory. As shown in Table 3, the small units average 70 MW capacity wh ile the large unit mean is 325 MW.

Table 3: Small and Large Subcategory Units

Subcategory Unit Count Average Capacity (MW)

Small 14 70 Large 293 325

All Units 307 314

Calculation of the Standard

Compliance with the standard in the proposed rule is determined by calculating the 12-operating month rolling average. To calculate compliance, a unit’s total CO 2 emissions over the twelve pr evious operating months is divided by the unit’s total electrical ener gy output over that period. The formula is:

A = C / M

A is the 12-month ro lling average (lb CO 2 /MWh).

C is the sum of CO 2 mass emissions for the past 12 operating months in pounds (lbs) M is the sum of gross output for the past 12 operating months in MWh

8 Capacity for combustion turbine engine only.A calculation result only becomes available once 12 mont hs of operating data are available. Since the study period is 60 months long the maximum number of rolling average results is 49. An ‘exceedance, ’ as used here, means that a unit exceeded the applicable subcategor y standard of performan ce for one 12-month rolling average. Since the calculation only includes operating mont hs, the actual time span covered may be more than a calendar year. For example, if a unit started operation in January 2007, but did not operate in March of that year, the first rolling average would not be available until January 2008. Unless stated otherwise, the term ‘CO 2 rate’ as used here means the 12-month rolling average CO 2 emission rate.

There are a number of advantages to using this approach. A 12-month ro lling average gives more weight to high-load hours and more accurately reflects overall environmental performance. In addition, because low-load hours do not factor as heavily into the calculated average, it reduces the impact of startup and shutdown periods when the unit may be operating less efficiently. A 12-mont h rolling average, as oppos ed to a calendar year average, uses 12 months of data and equitably addresse s new and retiring units regardless of what month they begin or cease operation.

Fleet Averages

EPA calculated the 12-month rolling averages for this study using hourly unit emissions data aggregated to the monthly level. The key parame ters examined are monthly CO 2 mass (lbs), gross output (MWh), and operating time (hours). The calculation is based on opera ting months, i.e., months in which both CO 2 mass emissions and

gross output are grea ter than zero. The fleet average monthly CO 2 rate 9 for all study units is 861 lb CO 2 /MWh with a distribution (Figure 1) that is slightly right-tailed due to four un its with high rates that exceed three

standard deviations from the m ean. The fleet average monthly CO 2 rate for the small un it subcategory over the five-year study period is 907 lb CO 2 /MWh. There is a wide range in th e monthly fleet rate among the small units, from a low of 858 (May, 2009) to a high of 993 (July, 2008) lb CO 2 /MWh. In contrast, the fleet average monthly rate for the large unit subcategory is 859 lb CO 2 /MWh and is consistent within a two percent range over the same period.

Figure 1: Unit Average CO 2 Rate Distribution 2007-2011

The 1,100 lb CO 2 / MWh standard for the small unit subcat egory is justified by both technological and operating characteristics. In genera l, smaller capacity NGCC units available on the market today are less efficient than the largest units. Manufacturers of the largest NGCC units now report achieving overall thermal efficiency of over 60 percent while the small units currently top out at approximately 54 percent. 10 This

9 Fleet average means calculating the composite 12

‐ month rolling average for the defined group of units then averaging the resulting

48 months.

10 Compare, for example, Siemens H frame to General Electric LM6000. 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007

600 700 800 900 1,000 1,100 1,200NormalDistribution

Unit Averge 12

‐ Month Rolling CO 2 Rate 2007

‐ 2011difference in efficiency is reflected in the average CO 2 rates noted above: the monthly CO 2 rate average for large units is 5% lower than the small units. The five -year average unit capacity factor for all units was 36 percent. As shown on Table 4, small un its operate at a lower average capacity factor, 27 percent. However, both categories include units operating acro ss a wide range of capacities duri ng the study period. The typical unit with a high capacity factor opera ted in 55 or more months (out of 60) in the study period. 11

Table 4: Small and Large Subcategory Capacity Factors (Percent)

Subcategory Average Capacity

Factor

2007-2011

Small units 27 Large units 36

All units 36

Exceedances

Out of 307 units, over 92 percent (284) had no exceedances during the five year study period. The 23 units with exceedances included one in the small unit subcategory a nd range from 80 to 423 megawatt capacity. They are

located at thirteen different faciliti es in ten states that span the count ry (AR, CA, FL, IL, MO, MS, NE, NY, TX, WA). Five units had a single exceedance; the rema ining 18 units ranged from 2 to 39 exceedances each. The units with exceedances are a diverse group by location and capacity but share some characteristics. They report lower utilization factors and fe wer operating months than the fleet av erage. In addition, the nineteen least-efficient units, i.e., those with highest average CO 2 rates over the study period, all had one or more exceedances. This analysis is conservative since 12-month averages where electric sales were not sufficient for the facility to be considered an EGU (i.e., selling one-third or more of its pot ential electric output and more than 219,000 MWh net-electric output to a ut ility distribution system) were not filtered out of the analysis.

Temperature

Combustion turbines may operate less efficiently at higher ambient temperatures , however, a comparison of winter monthly (December, January and February) to summer monthl y (June, July, August) rolling average CO 2 rates in this study found no di fference overall in the NGCC CO 2 emission rate. Both time periods had nearly identical fleet average CO 2 rates of 864 lb CO 2 /MWh. Almost half (144) of 301 units operating in both time periods, had lower rolling averag e rates in the summer months.

Using the actual monthly summer-winter monthly da ta (rather than the 12- month rolling average CO 2 rate) finds that 184 units (60% of 306 with data in both time periods) had lowe r rates in the wint er months. This supports the argument that operating in higher ambient te mperatures tends to degr ade efficiency. Looking at individual cases, two of the four units with the highest percentage difference in actual CO 2 rate, i.e., 2-4 % higher in summer months, had exceedances. However, th ose units are the exception. Over 93 percent of the units with higher actual monthly CO 2 rates in summer still met the standard with no exceedances. For the large majority of post-2000 NGCC units, the 12-month rolling average CO 2 rate avoids exceedances by smoothing out any increases in CO 2 rate that may occur due to ambient temperature.

Pollution Controls

11 Five

‐ year capacity factors include only years with MW generation reported.All the units in this study are assumed to be equipped with low NO X burners. Nearly 90 pe rcent (274 of 307) are also equipped with selective catalytic reduction (SCR) NO X controls. 12 We found no correlation between units with SCRs and average CO 2 rate. In fact, five of the 23 units with exceedances did not have an SCR during the study period.

Cycling

NGCC units can cycle frequently. An examination of starts , defined here as the ini tiation of fuel consumption, found that the average unit started 106 times per calendar year over the study period. Several units in this study cycled over 300 times per calendar year, i.e. the e quivalent of starting up ev ery weekday. NGCC units demonstrate that they are good at cyc ling efficiently. There is no correlati on between the number of starts and exceedances. Figure 2 plots the count of starts 2007-2011 (on the X axis) against the average CO 2 rate for each unit (on the Y axis). The CO 2 rates are evenly distribu ted around the mean and show limited correlation to the starts which range from 39 to 1,553. No attempt was made to account for more efficient units operating more consistently (and having less starts and stops) due to the lower incremental operating costs compared to less efficient units. Of the top 50 units with the most starts, only three had any exceedances. The average CO 2 rate of the top ten cycling units, measured by star t count over the study period, was 883 lb CO 2 /MWh, well below the performance standards. There is no co rrelation between unit capacity and the start count, i.e., units in the small subcategory had no more or less starts than the large units. The average capac ity of the top ten cycling units was 278 megawatts which underlines that bot h small and large NGCC units operate to meet dispatch requirements.

FIGURE 2: Unit St art Ups and Average CO 2 Rate 2007-2011

12 No study units had sulfur dioxide or particulate matter controls.Elevation

The elevation of the units in this study ranges from 1 to 5,276 feet above sea level. There is no correlation

between the elevation of a st udy unit and either the number of exceedances or the average CO 2 rate. None of the units with exceedances is located above 1,500 feet. Fifteen units operated at elevations above 4,200 feet and had no exceedances during the study period. The average CO 2 rate for the fifteen units at altitudes over 4,000 feet is

862 lbs CO 2 /MWh, which would indicate that new units situat ed at higher elevations can maintain a high standard of efficiency.

Figure 3: Unit Elevation and Average CO 2 Rate 2007-2011