1



### Assessment of startup period at coal

- fired electric generating units

- Revised

U.S. Environmental Protection Agency, Office of Air and Radiation

November 2014

1. Purpose

Th is analysis explores the time and gross load levels ( hourly electricity generation as a percentage of nameplate capacity) necessary for coal

- fired electric utility steam generating units ( EGUs ) to engage and operate air pollution control devices ( APCDs )

. The analysis uses h istorical electric ity output , heat input, and emission data , along with EGU characteristics and APCD information from 2011 and 2012 as indicators to assess operation of APCDs at coal

- fired EGU s

. The analysis includes two parts

– ( a ) an analysis of all startup events at all coal

- fired EGUs, and ( b ) an analysis of startup events at the best performing 12 percent of coal

- fired EGUs. 1 These results facilitate the identification of the start of APCD operation.

1 Clean Air Act section 112(h)(1) requires work practice standards to be established “consistent with the provisions of subsections [112](d) or (f) of this sections.” The EPA interprets that provision as requiring work practice standards to be based on the performance of the best performing sources in the category or subcategory. For EGUs startup and shutdown, the EPA defines best performing EGUs by determining the EGUs that are able to bring their pollution controls on line the most efficiently

. See the preamble to the final rule and the response to comments for additional discussion on this issue. Abbreviations

APCD air pollution control device(s) MMBtu million British thermal units (unit of energy)

CEMS continuous emission monitoring system MW megawatt(s)

– one million watts

CFB circulating fluidized bed

– boiler type NO X nitrogen oxides

CO 2 carbon dioxide PC pulverized coal

– boiler type

EGU electric utility steam generating unit PPM parts per million

EPA (U.S.) Environmental Protection Agency SCR selective catalytic reduction

– NO X control

FGD flue gas desulfurization

– SO 2 and acid gases control SO 2 sulfur dioxide

De finitions

Emission rates : average mass emissions (in pounds) released per million British thermal unit (MMBtu) of heat input

Failed start : a startup event in which the EGU begins combusting fossil fuel and subsequently ceases combusting fossil

fuel without generating any electricity. Failed starts may be planned or unplanned, and often occur when bringing a plant online after a maintenance outage.

Normal start : a startup event in which the EGU begins combusting fossil fuel and generates some measurable amou nt of electricity before ceasing fossil fuel combustion. Startup event : initiation of fossil fuel combustion at an EGU following one or more hours of non

- operation (i.e., no combustion)

 Hot start: a A startup event in which the EGU was offline for 24 hours or less before starting to combust fossil

fuels

 Warm start: a A startup event in which the EGU was offline for 25

- 119 hours before starting to combust fossil

fuels

 Cold start: a A startup event in which the EGU was offline for 120 hours or more before star ting to combust

fossil fuels

a Hot, warm, and cold starts are defined using turbine metrics presented in Lefton and Hilleman, “Is your plant ready for cycling operations?” Power Magazine ; 2011.2

2. Introduction

The EPA received several comments concerning our definition of the end of startup in response to the proposed reconsideration of startup/shutdown issues for the Mercury and Air Toxics Standards (MATS) R ule. Several commenters advocated that the startup period should not end when the EGU

be gins generat ing electricity or useful thermal energy. Rather, commenters argued that startup should end at different times depending on whether the EGU was subcritical or supercritical , and on the types
of controls that were installed . Commenters stated th at some APCDs, such as selective catalytic reduction ( SCR )

, need up to 12 hours after electricity generation begins before they become operational. They also stated that circulating fluidized bed ( CFB ) EGUs become operationally stable only after they reach approximately 40 percent load.

The EPA examined available data concerning the types of EGUs on which the commenters focused: subcritical and supercritical EGUs with flue gas desulfurization ( FGD ) and SCR controls , and CFB

EGUs

. This assessment required an hour

- by

- hour analysis of startup events using emission

measurements (from continuous emission monitoring systems ( CEMS ) ) , heat input

, and electric ity (gross) output data from the EPA’s Clean Air Markets Database 2 for the types of EGUs identified by

the commenters. Using th e s e data, the EPA calculate d the average time , in hours, for specific types of

EGUs to achieve decile and quartile load bins (e.g., 10 percent

, 20 percent

, and 25 percent of nameplate capacit y) and for SO 2 and NO X APCDs to begin reducing SO 2 and NO X emission rates

, respectively. In addition, the EPA analyzed the time required for emissions to decline at the best performing 12 percent of coal

- fired EGUs

– EGUs that were able to, on an annual average, initiate operati on of their SO 2 or NO X APCDs in the least amount of time following the start of generation

. The analysis focused on SO 2 and NO X emissions because the EPA believes that emissions will be sufficiently stable and consistent at this time to accurately measure HAP emissions. In addition

, ( a)

SO 2 emissions serve as a surrogate for acid gas hazardous air pollutants (HAP)

, ( b) FGD and SCR can impact mercur y levels 3 and the effectiveness of mercury controls, and ( c) changes in SO 2 and NO X emissions is a measure that can indicate when APCDs are operational. This study does not include

assessments of PM control devices (e.g., baghouses, electrostatic precipitators) because hourly PM

data were not available ; however, comments and other information in the record demonstrate that the best performing EGUs are able to sufficiently warm the PM control devices to operational t emperature on clean fuels alone (i.e.

, within 1 hour of charging coal to the boiler)

.

This analysis provides information on the startup process and the time required for SO 2 and NO X APCDs to become operational at coal

- fired EGUs. While the actual decision of when to initiate an APCD is affected by a variety of factors, including the type of control device, local weather conditions, flue gas temperature, and safety concerns, this analysis is intended to de termine the average time required to initiate APCDs at all coal

- fired EGUs and also at the best performing coal

- fired EGUs

. The

EPA believes that the removal effic a cy of APCDs, as evidenced by hourly emission rates below uncontrolled levels, is an appropriate indicator of the time the APCDs are operating and provide an appropriate metric for defining the end of the startup period for the purpose of the MATS rule because we are confident HAP emissions can be accurately measured at this time

.

2 The aggregated data set used in this analysis are included in d ocket ID EPA

- HQ

- OAR

- 2009

- 0234 ; full data are available from the Clean Air Markets Da tabase [http://ampd.epa.gov/ampd].

3 Some formulations of catalyst are capable of enhancing the oxidation of mercury, promoting greater capture by downstream APCDs (e.g., wet scrubbers (FGD)).3

3. Data and methodology

The EPA collects the emission data analyzed in this paper under 40 CFR Part 75

. 4 M ost fossil fuel

fired EGUs report hourly emissions (e.g., SO 2 , NO X , CO 2 ) , stack gas flow, and operations (e.g., operating time, heat input, gross electricity generation) data on a qu arterly basis. 5 T hese data were used to identify all start up event s at 41 4 subcritical and supercritical EGUs with FGD and/or SCR APCDs

and CFB boiler EGUs 6 during calendar years 2011 and 2012

.

T his study is intended to assess commenters’ claims that there are performance differences among

combustion technologies and APCDs as they relate to startup events , and to identify the average number of hours after the start of generation that is necessary to startup SO 2 and NO X AP C Ds at the

coal

- fired EGU fleet generally and at the best performing 12 percent of EGUs

. In light of the

comments received and t o facilitate this assessment, w e examined o perating data by boiler type (PC supercritical, PC subcritical

, and CFB boiler s ) and by APCD type. F or SO 2 emission s, we examined

PC boilers with FGD and CFB EGUs

. For NO X emission s, we examined PC supercritical and PC

subcritical boilers with SCR.

We excluded cogeneration EGUs from this analysis because adequate steam production data were not available. In addition, because the focus of the analysis is on the time it takes to engage the identified
APCDs, coal

- fired EGUs without FGD and/or SCR APCDs as of January 1, 2011

, were excluded from the analysis. 7 Finally, we excluded data during operating hours with the most conservative substitute data (i.e., maximum potential concentration, maximum potential flow) 8 because these data do not
represent actual emissions.

4 Supercritical boiler type is drawn from EIA form 860 and EPA research. The analysis data set noted

above includes this field.

5 Sources report data at the monitor (stack) level but this study used data apportioned to the EGU. For

more information about Pa rt 75, see the Plain English Guide to the Part 75 Rule at www.epa.gov/airmarkets/emissions/docs/plain_english_guide_part75_rule.pdf

6 CFB boiler technologies are capable of controlling SO 2 by injecting limestone in the combustion bed. Per the definition of “dry flue gas desulfurization technology” in 40 CFR 63.10042, “[a]lkaline sorbent injection systems in fluidized bed combustors (FBC) or circulating fluidized bed (CFB) boilers...” are considered to be FGD technologies (APCDs)

. 7 When a comparison is made between “uncontrolled” and “controlled” EGUs, the uncontrolled data represent startup events at EGUs that did not have the relevant APCD. In other words, uncontrolled

SO 2 emission rates are based on PC EGUs that have installed SCR, an d therefore are a part of the data set, but have not installed an FGD APCD. For NO X , “non

- SCR” startup events are based on PC EGUs that have installed FGD but do not have an SCR. These EGUs may, however, have other NO X controls

such as low

- NO X burners, ove rfire air, and/or selective non

- catalytic reduction APCDs.

8 Part 75 requires the use of substitute data when a monitor is not working properly or has not been quality assured. If the monitor is reporting valid emission data for less than 80 percent of ope rating hours during the previous 8,760 hours (i.e., one year), substitute data equal to the maximum potential concentration or maximum potential flow are applied for any missing data or invalid data. This

“conservative” emission value is intended to ensure emissions are not underreported and to create an incentive for EGUs to properly operate, maintain, and quality assure their monitoring equipment and provide the most accurate and reliable results. See http://www.epa.gov/airmarkets/emissions/continuous

- fac tsheet.html4

For purposes of conducting this analysis, we defined a start up event as the initiation of fossil fuel combustion following one or more hours of non

- operation (i.e., no combustion ), which is consistent

with the final definition of startup in the MATS reconsideration rule

. For each start up event , we calculated the following values:

 Number of non

- operating hours prior to the start up event (i.e., hours between previous cessation of combustion and start of combustion).

 Number of hours between start of combustion and start of electricity generation. 9

 Gross electricity generation as a percent of nameplate capacity for each hour following start of electricity generation.

 Emission rates and heat input for each hour after start of combustion and start of electricity generation.

For the best performing 12 percent of EGUs, 2

- hour rolling average emission rates ( pounds per million British thermal units, lb/MMBtu) were calculated following the start of generation. A 2

- hour average was used to smooth out some of the variability inhere nt during startup

4

. Results

During calendar year s 2011 and 2012 , there were 9

, 7 19 distinct start up event s (see Table 1 ) 10

– 9,467

at PC EGUs and 252 at CFB EGUs.

Table 1 : Number of normal and failed start s by boiler and APCD types , years 2011 and 2012.

The average EGU had between 9 and 10 startup events per year during 2011

– 2012, but data from a small number of EGUs indicated significantly more startup events (e.g., the EGUs with the most startup events had over 100 startup events in 2011 and over 80 in 2012

. ) For the 41 4 coal

- fired EGUs in this analysis, the overall number of startup events remains reasonably consistent across both years.

9 Reporting instructions for Part 75 allow the use of default megawatt values, typically 1 or 2 MWh, when combustion is underway but gross load is zero. This is typically done for apportioning heat input

among EGUs sharing a common stack or pip e. Without the default value, the calculation would require dividing by zero and, therefore, result in an error. For this study, we conservatively set the start of electricity generation from the hour where gross load exceeded 2 MWh.

10 Because startup even ts are grouped by boiler and control, a startup event may be counted more than once. For example, each startup event at a PC EGU with an FGD and SCR would be counted as a startup event at an FGD

- equipped EGU and at an SCR

- equipped EGU. Boiler

- control Normal starts Failed starts Total starts

PC EGU 7

, 364 2,103 9

, 467 Supercritical w/ FGD 1

, 612 3 69 1

, 981 Supercritical w/ SCR 1

, 413 324 1

, 737

Subcritical w/ FGD 4,827 1,3 35 6, 162

Subcritical w/ SCR 2, 578 8 23 3, 401

CFB EGU 208 44 2525

4.1 Operations betw een start of combustion and start of generation

We analyzed emissions and operations data for each startup event from the start of fossil fuel combustion to the start of electricity generation. Specifically, we examined the length of time an EGU

combusts fossil fuel before initiating electricity generation, giving consideration to the period of time

the EGU was offline and whether or not the EGU successfully initiated electricity generation.

Generally, during startup of a coal

- fired boiler the operator s lowly heats the boiler to avoid problems with boiler expansion and overheating of equipment (e.g., reheaters, superheaters). 11 If the boiler is offline for a short time and does not experience significant temperature declines, the time between start of combustion and start of electricity generation may be very short. Generally, natural gas or fuel oil is combusted during this tim e to slowly raise the temperature in the boiler. Natural gas and oil are used because of their low ignition temperature and ignition stability.

Approximately 23 percent of the startup events examined in this study failed to generate electricity following the start of fossil fuel combustion. These f ailed starts can occur for a variety of safety and operating reasons. In general, these failed starts have a short duration

– the average failed start

combusted fossil fuel , including natural gas, oil, and coal, for less than 8 hours with a median of 4 hours. Figure 1 shows the distribution of hours of fossil fuel combustion during failed starts. Fossil fuel combustion during approximately 75 percent of the failed starts last ed 10 hours or less . The failed

starts that combust ed fossil fuel for more than 10 hours generally follow ed longer periods of downtime ( e.g., extended maintenance events)

. The average time offline be for e such failed sta rts is approximately

360 hours. Of the 413 EGUs in this study, 91 use complex (i.e., shared) stacks making it difficult to estimate emissions during startup from these EGUs

. 12 Of the 319 EGUs with simple (i.e., one stack for one boiler) or multiple stacks (i.e., multiple stacks for one boiler), t he tota l SO 2 emissions during failed

starts in 2011 and 2012, combined, were 154 tons of SO 2 and 404 tons of NO X . This represents less

than 0.0 08 and 0.0 30 percent of total annual SO 2 and annual NO X emissions, respectively, at these

EGUs

.

More than 97 percent of the “normal” starts 13

– a startup event in which an EGU begins combusting fossil fuel and subsequently generating electricity during at least one operating hour before the EGU ceases combusting fossil fuel

– in this database were at PC EGUs. Following the start of fossil fuel combustion, PC EGUs began generating electricity in a relatively short period of time. On average, the time between start of fossil fuel combustion and start of generation was less than 9 hours (see Figure 2).

11 Lefton, S.A., and Hilleman, D., 2011. Make Your Plant Ready for Cycling Oper ation. Power Magazine. August 1, 2011, in docket ID EPA

- HQ

- OAR

- 2009

- 0234

- 20380. (http://www.powermag.com/issues/features/Make

- Your

- Plant

- Ready

- for

- Cycling

Operations_3885.html)

12 It is difficult to estimate emissions from a single EGU with a sh ared stack or pipe because these

EGUs generally share a single monitoring system.

13 For the purpose of this analysis, we define a “normal” start as any startup event that results in electricity generation for more than one hour (i.e., not a “failed” start). This may not represent the “average” or “typical” startup event.6

Figure 1 : Duration of f

ossil fuel combustion during failed startup event s

Figure 2 : Duration of fossil fuel combustion prior to electricity generation during normal startup

events at PC EGUs

During startup and prior to the start of generation, PC EGUs are generally burning clean fuels (e.g., natural gas, number 2 fuel oil). To estimate the transition from clean fuels to the start of coal combustion, the EPA looked at SO 2 concentration for all startup events, including failed starts, from

the initiation of fossil fuel combustion until the SO 2 concentration (parts per million (PPM)) exceeded

10 PPM

– a reasonable threshold for the introduction of coal, number 6 fuel oil, or higher

- sulfur number 2 oil. On average across both years, it took a little over 9 hours for SO 2 concentrations to exceed 10 PPM, approximately the same length of time it took PC EGUs to start generating electricity.

Approximately 3 percent of the normal startup events in this analysis were at CFB boiler EGUs. For these startup events, the average time between start of fossil fuel combustion and start of generation was approximately 10 hours with a median of 8 hours, comparable to the study population as a whole.
However, o ver 4 0 percent of startup events at CFB boiler EGUs had extended periods (10

- 75 hours) of fossil fuel combustion before electricity generation commenced. As commenters noted, this may be

due to the time it takes to achieve and maintain stability of the “bed

. ” In general, the hourly heat input7

during these “slow to generate” startup events (greater than 10 hours between start of fossil fuel combustion and start of electricity generation) is considerably lower than the heat input during “f ast to generate” starts (less than or equal to 10 hours between start of fossil fuel combustion and start of electricity generation) (see Figure 3). In other words, if a CFB boiler started electricity generation in 10

hours of less after the start of combu stion, total heat input rose quickly. However, if the CFB boiler took a longer time to start electricity generation, the total heat input was lower, on average, and

increased at a slower rate.

Prior to the start of generation, CFB EGUs burn clean fuels for several hours. To estimate the transition from clean fuels to the start of coal combustion, the EPA looked at SO 2 concentration for all startup events, including failed starts, from the initia tion of fossil fuel combustion until the SO 2 concentration (parts per million (PPM)) exceeded 10 PPM. On average across both years, it took CFB

boiler EGUs a little over 5 hours for SO 2 concentrations to exceed 10 PPM.

Figure 3: Heat input per hour follow ing start of fossil fuel combustion at CFB boiler EGUs

4.2 Operations following the start of generation

4.2.1 Pulverized coal EGUs

Following the start of generation, both supercritical and subcritical PC EGUs increased generation rapidly, achieving higher loads within the first few hours. Figure 4 shows that across startup events at supercritical PC EGUs, generation averaged approximately 30 percent of nameplate capacity by hour 3

and approximately 3 8 percent of nameplate capacity by hour 4. (Note: t he yellow line is the average gross load as a percentage of nameplate capacity across all start up event s at supercritical PC EGUs ; the purple boxes and black whiskers are the quartile ranges.) Figure 5 show s that across startup events at subcritical PC EGUs, generation averaged approximately 33 percent of nameplate capacity by hour 2,

42 percent of nameplate capacity by hour 3, and 49 percent of nameplate capacity by hour 4.8

Figure 4 : Gross electricity generation as a percentage of nameplate capacity (MW) by hour following

start of generation at supercritical PC EGUs

Figure 5: Gross electricity generation as a percentage of nameplate capacity (MW) by hour following

start of generat ion at subcritical PC EGUs

During the majority of normal starts, supercritical (Figure 6) and subcritical (Figure 7) PC EGUs

achieved 20 percent and 25 percent of nameplate capacity within the first few hours after the start of generation

.9

Figure 6 : Hours after start of generation for supercritical PC EGUs to generate 20 percent (left) an d

25 percent (right) of nameplate capacity

Figure 7: Hours after start of generation for subcritical PC EGUs to generate 20 percent (left) and 25

percent (right) of nameplate capacity

4 .2.1 .1 SO 2 emissions from supercritical PC EGUs with FGDs

Of the 1

, 802 normal startup events at supercritical PC EGUs, over 80 percent occurred at EGUs with

wet FGD and an additional 6 percent were at EGUs with dry FGD (see Table 2). The average SO 2 emission rates for the hours following the start of generation are shown in Figure 8

. The average SO 2 emission rates for normal starts at both dry FGD

- and wet FGD

- equipped supercritical PC EGUs are approximately 80

- 90 per cent lower across every hour (0

- 24 ) than the average SO 2 emission rates for

normal starts at supercritical PC EGUs without FGDs (i.e., uncontrolled) . This indicates that both wet FGD and dry FGD APCDs are cap able of operating and capturing SO 2 emissions commensurate with the start of electricity generation.

Table 2: Number of normal starts at supercritical PC EGUs by SO 2 control type

SO 2 control type Normal starts

Wet FGD 1, 492 Dry FGD 120

Uncontrolled for SO 2 190

Total 1, 80210

Figure 8: Average SO 2 emission rates following start of generation at supercritical PC EGUs by SO 2

control type

Figure s 9 and 10 show the distribution of SO 2 emission rates during normal starts at supercritical PC EGUs with wet FGD ( Figure 9 ) and dry FGD ( Figure 10 ). (Note: the top and bottom 5 percent of

emission rates are excluded from the chart ; 14 the yellow line is the average emission rate across starts

at supercritical PC EGUs with FGD; the red boxes and black whiskers are the quartile ranges.) The figure s show that ave rage and median SO 2 emission rates are low at the start of generation for the majority of normal starts, indicating that both wet FGD and dry FGD are likely operating at the start of generation.

Figure 9 : Average SO 2 emission rates following start of generation at supercritical PC EGUs with wet

FGDs

14 A number of PC EGUs shut down in 2011 and 2012. Several startup events at these EGUs had high

SO 2 emissions for more than 24 hours after the start of generation indicating the FGD equipment was likely not in use. By excluding the top 5 percent of values, these outliers do not bias the analysis. For parity, we also excluded the bottom 5 percent

.11

Figure 10: Average SO 2 emission rates following start of generation at supercritical PC EGUs with

dry FGDs

Following gross load levels greater than or equal to 25 percent of nameplate capacity, supercritical PC

EGUs ’ SO 2 emission rates are relatively low and stable (see Figure 11 for wet FGD and Figure 12 for dry FGD). Both types of FGD s show declining average SO 2 emission rates by the third hour after reaching 25 percent load.

Figure 11: Average SO 2 emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at supercritical PC EGUs with wet FGDs12

Figure 12: Average SO 2 emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at supercritical PC EGUs with dry FGDs

4 .2.1 .2 NO X emissions from supercritical PC EGUs with SCRs

To determine the time necessary to start SCRs, the EPA examined hourly NO X emissions at EGUs

with and wi t hout SCR. Of the 1, 802 normal startup events at supercritical PC EGUs, 7 8 percent were

at supercritical PC EGUs with SCR (see Table 3) . Nearly all of the remain ing non

- SCR supercritical

PC EGUs hav e low

- NO X burners, over

- fire air, and/or selective non

- catalytic reduction installed . The

average N O X emission rates for the hours following the start of generation are shown in Figure 13

. The average NO X emission rates for SCR

- equipped and non

- SCR supercri tical PC EGUs begin at approximately the same level but the rate for the SCR

- equipped EGUs grow s slower and begins to

decline by hour 5

. This indicates that , on average, SCR APCDs are able to begin controlling N O X emissions within 4 to 6 hours following the start of electricity generation at supercritical PC EGUs

.

Table 3 : Number of normal starts at supercritical PC EGUs by N O X control type

N O X control type Normal starts

SCR 1, 413

non

- SCR 3 89

Total 1

, 80213

Figure 13 : Average N O X emission rates following start of generation at supercritical PC EGUs by

NO X control type

Figure 14 shows the distribution of NO X emission rates during normal starts at supercritical PC EGUs

with SCR NO X APCDs. (Note: the top and bottom 5 percent of em ission rates are excluded from the chart; the yellow line is the average emission rate across starts at PC EGUs with SCR; the orange boxes and black whiskers are the quartile ranges.) The figure shows that average and median NO X emission rates for the full range of normal starts at SCR

- equipped supercritical PC EGUs begin to

decline around hour 6 , indicating that , on average, SCR effectively control s NO X approximately 6

hours or less after the start of generation.

Figure 14 : Average NO X emission rates following start of generation at supercritical PC EGUs

Figure 15 shows the distribution of NO X emission rates during normal starts after achieving 25 percent of nameplate capacity at supercritical PC EGUs with SCR NO X APCDs. ( Note: the top and bottom 5

percent of emission rates are excluded from the chart; the yellow line is the average emission rate across starts at supercritical PC EGUs with SCR; the orange boxes and black whiskers are the quartile ranges.) The figure shows t hat average and median NO X emission rates at SCR

- equipped supercritical PC EGUs begin to decline around 2 hours after achieving 25 percent of nameplate electricity generating capacity.14

Figure 15: Average NO X emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at supercritical PC EGUs with SCRs

4.2. 1.3 SO 2 emissions from subcritical PC EGUs with FGDs

Of the 5,77 0 normal startup events at subcritical PC EGUs, 70 percent were at subcritical PC EGUs

with wet FGD and an additional 1 4 percent were at subcritical PC EGUs with dry FGD (see Table 4). The average SO 2 emission rates for the hours following the start of gene ration are shown in Figure 16

. The average SO 2 emission rates for normal starts at wet FGD

- equipped subcritical PC EGUs are approximately 75

- 80 percent lower across every hour (0

- 24) than the average SO 2 emission rates for

normal starts at subcritical PC EGUs without FGDs (i.e., uncontrolled). The average SO 2 emission rates for normal starts at dry FGD

- equipped subcritical PC EGUs are approximately 40

- 70 percent lower across every hour (0

- 24) than the average SO 2 emission rates for normal starts at subcritical PC

EGUs without FGDs.

Table 4: Number of normal starts at subcritical PC EGUs by SO 2 control type

SO 2 control type Normal starts

Wet FGD 4, 024 Dry FGD 803

Uncontrolled for SO 2 943

Total 5,77 015

Figure 16 : Average SO 2 emission rates following start of generation at subcritical PC EGUs by SO 2

control type

Figures 17 and 18 show the distribution of SO 2 emission rates during normal starts at subcritical PC EGUs with wet FGD ( 17 ) and dry FGD ( 18 ). (Note: the top and bottom 5 percent of emission rates are excluded from the chart; the yellow line is the average emis sion rate across starts at sub critical PC EGUs with FGD; the red boxes and black whiskers are the quartile ranges.) The figures show that

average and median SO 2 emission rates are low at the start of generation for the majority of normal starts, indicating that wet FGD are likely operating at the start of electricity generation and dry FGD begin controlling emissions within the first 3 to 4 hours after the start of electricity generation.

Figure 17 : Average SO 2 emission rates following start of generation at sub critical PC EGUs with wet

FGDs16

Figure 18 : Average SO 2 emission rates following start of generation at sub critical PC EGUs with dry

FGDs

Following gross load levels greater than or equal to 25 percent of nameplate capacity, su b critical PC

EGUs with wet FGD have relatively low and stable average and median SO 2 emission rates (see Figure

1 9 ) while subcritical PC EGUs with dry FGD reduce average SO 2 emission rates (see Figure 20 ) by over 30 percent in the first 3 hours following gross load levels of 25 percent of nameplate capacity

. Average and median SO 2 emission rates at dry FGD

- equipped subcritical PC EGUs begin declining within the first hour of achieving gross load equal to or greater than 25 percent of nameplate capacity.

Figure 1 9 : Average SO 2 emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at su b critical PC EGUs with wet FGDs17

Figure 20 : Average SO 2 emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at su b critical PC EGUs with dry FGDs

4.2.1 .4 NO X emissions from su b critical PC EGUs with SCRs

Of the 5,77 0 normal startup events at subcritical PC EGUs, nearly 47 percent were at su b critical PC

EGUs with SCR (see Table 5) . Nearly all of the remaining non

- SCR su b critical PC EGUs have

installed low

- NO X burners, over

- fired air, and/or selective non

- catalytic reduction. The average NO X emission rates for the hours following the start of generation are shown in Figure 2 1 . The average NO X emission rates for SCR

- equipped and non

- SCR su b critical PC EGU s begin at approximately the same

level but the rate for the SCR

- equipped EGUs begins to decline around hour 2

.

Table 5 : Number of normal starts at subcritical PC EGUs by NO
x control type

NO X control type Normal starts

SCR 2, 578

non

- SCR 3, 192

Total 5

, 77 018

Figure 2 1 : Average NO X emission rates following start of generation at subcritical PC EGUs by NO X

control type

Figure 2 2 shows the distribution of NO X emission rates during normal starts at su b critical PC EGUs

with SCR NO X APCDs. (Note: the top and bottom 5 percent of emission rates are excluded from the chart; the yellow line is the average emission rate across starts at PC EGUs with SCR; the orange boxes and black whiskers are the quartile ranges.) The figure shows that a verage and median NO X emission rates for the full range of normal starts at SCR

- equipped subcritical PC EGUs begin to

decline around hour 2 , indicating that SCR are likely starting to control NO X 2 to 3 hours after the start of generation.

Figure 2 2 : Average NO X emission rates following start of generation at subcritical PC EGUs

4

. 2.2 Circulating fluidized bed boiler EGUs

CFB boiler EGUs typically do not have post

- combustion FGD APCDs installed since they achieve significant SO 2 capture by adding lime or limestone to the bed of the boiler. For this reason, the EPA

evaluated CFB boiler EGU start s separately from PC EGUs.19

Figure 23 shows that across startup events at CFB boiler EGUs, generation averaged approximately 30

percent of nameplate capacity by hour 2 and 40 percent of nameplate capacity by hour 3

. (Note: the yellow line is the average gross load as a percentage of nameplate capacity across all start up event s at CFB boiler EGUs; the purple boxes and black whiskers are th e quartile ranges.) We f ou nd that CFBs

achieve 25 percent and 40 percent load bins, on average, as fast as subcritical and supercritical PC EGUs ( see Figures 4 and 5).

Figure 23 : Gross electricity generation as a percentage of nameplate capacity (MW) by hour following

start of generation at CFB boiler EGUs

During the majority of normal start up event s, CFB boiler EGUs achieved 20 percent and 25 percent of nameplate capacity within the first few hours following the start of electricity generation (see Figure 24).

Figure 24 : Hours after start of generation for CFB boiler EGUs to generate 20 percent (left) and 25

percent (right) of nameplate capacity

Because CFB boiler EGUs generally do not have separate FGD APCDs, there is no need to compare

uncontrolled and controlled emission rates. Figure 25 shows that average and median SO 2 emission rates during start up event s at CFB boiler EGUs begin to decline at hour s 4 to 6 following the start of electricity generation. (Note: the top and bottom 5 percent of emission rates are excluded from the20

chart; the yellow line is the average emission rate across start up event s at CFB boiler EGUs with FGD;

the red boxes and black whiskers are the quartile ranges.)

Figure 25 : Average SO 2 emission rates following start of generation at CFB boiler EGUs

5. Average of the best performing 12 percent of existing EGUs

CAA section 112 requires the EPA to establish standards based on the average of the best performing

12 percent of EGU s. To evaluate the startup time of the best performing 12 percent (i.e., the EGUs that were able to start operatio n of APCDs in the shortest time)

– the EPA refined the dataset and

established a two

- tier test to identify when controls started operation. First, startup events in which electricity generation lasted less than 4 hours before fossil fuel combustion ended were deleted from

the dataset. This remo ved 563 startup events. For the remaining 6,963 startup events, the EPA

calculated the 2

- hour rolling average emission rate (lb/ MMBtu )

. A 2

- hour average was used to smooth out some of the variability inherent during startup. These 2

- hour rolling averages w ere then subjected to two tests to determine when the controls were operational:

 Following the 2

- hour average maximum emission rate, at what 2

- hour averaging period does the emission rate decline by a predetermined threshold and stay below that threshold ?

 At what 2

- hour averaging period does the 2

- hour average emission rate fall and remain below

110 percent of an EGU’s annual average emission rate? If the rate is lower than 110 percent of the annual average emission rate, the control is assumed to be operational beginning in that 2

- hour averaging period. This test is particularly relevant for controls that initiate operation before the start of generation.

Calculating the time it took the best performing 12 percent of EGUs to meet one of these two tests required several calculations:

1. For each normal startup event, we identified the time (i.e., the 2 hour average) at which the EGU met one or both of the tests listed above. Startup events for SO 2 and NO X APCDs were

a nalyzed separately.

2. For each EGU, we averaged the time identified in step 1 for all the EGU’s startup events in 2011 and 2012. This provided an average time (i.e., 2 hour average) for the EGU to initiate operation of APCDs21

3. We ranked EGUs by the average tim e to initiate operation of SO 2 APCDs and NO X APCDs to identify the best performing 12 percent (i.e., the EGUs that had the lowest average time to initiate APCD operation).

4. Finally, we averaged the best performing 12 percent of EGUs’ average time to initiat e operation of SO 2 APCDs and NO X APCDs to calculate the end result.

Tables 6 and 7 show the results of the analysis with these two tests

– percent reduction threshold and

110 percent of annual average rate

– for SO 2 and NO X controls, respectively. The time to “controls on” varies for different percent reduction thresholds, ranging from approximately 3.0 to 4.5 hours.

Table 6: Average number of hours until SO 2 “controls on” for best performing 12 percent of EGUs

Emission de cline threshold Avg hours to “controls on” for top 12% Slope for all hours following “controls on” (95% CI)

10% 3.15 hours

- 3.88% (±2.4)

15% 3.51 hours

- 3.73% (±2.4)

20% 3.86 hours

- 3.62% (±2.4)

25% 4.20 hours

- 3.82% (±2.6)

30% 4.53 hours

- 3.04% (±2.6)

Table 7: Average number of hours until NO X “controls on” for best performing 12 percent of EGUs

Emission decline threshold Avg hours to “controls on” for top 12% Slope for all hours following “controls on” (95% CI)

10% 3.33 hours

- 10.35% (±0.8)

15% 3.60 hours

- 10.04% (±1.2)

20% 3.90 hours

- 7.70% (±2.2)

25% 4.08 hours

- 7.34% (±1.5)

30% 4.57 hours

- 6.06% (±1.6)

Using an emission decline threshold of 20 percent , t he results indicate that the average of the best performing 12 percent of EGU s initiate SO 2 and NO X control within 4 hours after the start of electricity generation (see Figure 26)

. At this threshold, the EGU s that comprise the best performing 12

percent include a CFB, supercritical and subcritical pulverized coal boilers, and wet and dry FGD

equipped EGU s (see Table 8).

Table 8: Number of EGUs in best performing 12 percent with different characteristics

Characteristic SO 2 “top 12%” NO X “top 12%” Super critical 10 9

Subcritical 33 18

Wet FGD 31 NA Dry FGD 12 NA

CFB 1 NA

6. Conclusion

In this analysis of supercritical and subcritical PC EGUs with FGD and/or SCR and CFB boiler EGUs,

the EPA examined several indicators that can aid in assessing the time required to achieve operating

benchmarks . These indicators show that , on average , all types of EGUs in this study :22

 can reach 25 percent of nameplate capacity in 3 hours or less a fter the start of generation ;

 can begin controlling SO 2 and NO X emissions 3 hours or less after reaching 25 percent of nameplate capacity or 6 hours or less following the start of electricity generation

.

Evaluating the best performing 12 percent of EGU s, this analysis shows these EGU s, on average :

 can achieve and maintain a 20 percent reduction below the maximum emission rate or maintain

an emission rate below 110 percent of the EGU ’s annual emission rate in 4 hours or less following the start of generation

.

 The best performing 12 percent of EGUs i nclude CFB, and supercritical and subcritical PC

EGUs.

We found no significant difference in performance related to startup events between the different boiler types and APCD technologies assessed in this analysis.