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

This 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 historical electricity 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 EGUs. 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 NOX nitrogen oxides

CO2 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 – NOX control

FGD flue gas desulfurization – SO2 and acid gases control SO2 sulfur dioxide

Definitions

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 amount 

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 starting 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) Rule. Several commenters advocated that the startup period should not end when the EGU

begins generating 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 that 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 electricity

(gross) output data from the EPA’s Clean Air Markets Database2for the types of EGUs identified by 

the commenters. Using these data, the EPA calculated 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 capacity) and for SO2 and NOX APCDs to begin reducing SO2 and NOX 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 

operation of their SO2 or NOX APCDs in the least amount of time following the start of generation.

The analysis focused on SO2 and NOX emissions because the EPA believes that emissions will be 

sufficiently stable and consistent at this time to accurately measure HAP emissions. In addition, (a) 

SO2 emissions serve as a surrogate for acid gas hazardous air pollutants (HAP), (b) FGD and SCR can 

impact mercury levels3and the effectiveness of mercury controls, and (c) changes in SO2 and NOX

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 temperature 

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 SO2 and NOX

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 determine 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 efficacy 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 docket ID EPA-HQ-OAR-2009-0234; 

full data are available from the Clean Air Markets Database [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 Most fossil fuelfired EGUs report hourly emissions (e.g., SO2, NOX, CO2), stack gas flow, and operations (e.g., 

operating time, heat input, gross electricity generation) data on a quarterly basis.5 These data were used 

to identify all startup events at 414 subcritical and supercritical EGUs with FGD and/or SCR APCDs 

and CFB boiler EGUs6during calendar years 2011 and 2012.

This 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 SO2 and NOX APCDs at the 

coal-fired EGU fleet generally and at the best performing 12 percent of EGUs. In light of the 

comments received and to facilitate this assessment, we examined operating data by boiler type (PC

supercritical, PC subcritical, and CFB boilers) and by APCD type. For SO2 emissions, we examined 

PC boilers with FGD and CFB EGUs. For NOX emissions, 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)8because 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 Part 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 SO2 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 

SO2 emission rates are based on PC EGUs that have installed SCR, and therefore are a part of the data 

set, but have not installed an FGD APCD. For NOX, “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 NOX controls 

such as low-NOX burners, overfire 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 operating 

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-factsheet.html
4

For purposes of conducting this analysis, we defined a startup 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 startup event, we 

calculated the following values:

 Number of non-operating hours prior to the startup 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 inherent during startup

4. Results

During calendar years 2011 and 2012, there were 9,719 distinct startup events (see Table 1)

10

– 9,467 

at PC EGUs and 252 at CFB EGUs. 

Table 1: Number of normal and failed starts 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 414 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 pipe. 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 events 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 1,612 369 1,981

 Supercritical w/ SCR 1,413 324 1,737

 Subcritical w/ FGD 4,827 1,335 6,162

 Subcritical w/ SCR 2,578 823 3,401

CFB EGU 208 44 252
5

4.1 Operations between 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 slowly 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 time 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 failed 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 lasted 10 hours or less. The failed 

starts that combusted fossil fuel for more than 10 hours generally followed longer periods of downtime

(e.g., extended maintenance events). The average time offline before such failed starts 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), the total SO2 emissions during failed 

starts in 2011 and 2012, combined, were 154 tons of SO2 and 404 tons of NOX. This represents less 

than 0.008 and 0.030 percent of total annual SO2 and annual NOX emissions, respectively, at these 

EGUs.

More than 97 percent of the “normal” starts13– 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 Operation. 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-CyclingOperations_3885.html)

12 It is difficult to estimate emissions from a single EGU with a shared 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 fossil fuel combustion during failed startup events

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 SO2 concentration for all startup events, including failed starts, from 

the initiation of fossil fuel combustion until the SO2 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 SO2 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, over 40 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 input 

0

5 0

100

150

200

250

300

350

400

1 3 5 7 9 1 1 1 3 1 5 1 7 1 9 2 1 2 3 >24

Number of failed starts

Hours of fossil fuel combustion

Cold

Warm

Hot

0

100

200

300

400

500

600

700

800

900

1 3 5 7 9 1 1 1 3 1 5 1 7 1 9 2 1 2 3 >24

Number of normal starts

Hours of fossil fuel combustion before generation

Cold

Warm

Hot
7

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 “fast 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 combustion, 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 SO2 concentration for all 

startup events, including failed starts, from the initiation of fossil fuel combustion until the SO2

concentration (parts per million (PPM)) exceeded 10 PPM. On average across both years, it took CFB 

boiler EGUs a little over 5 hours for SO2 concentrations to exceed 10 PPM.

Figure 3: Heat input per hour following 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 38 percent of nameplate capacity by hour 4. (Note: the yellow line is the average 

gross load as a percentage of nameplate capacity across all startup events at supercritical PC EGUs; the 

purple boxes and black whiskers are the quartile ranges.) Figure 5 shows 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 generation 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) and 

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 SO2 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 SO2

emission rates for the hours following the start of generation are shown in Figure 8. The average SO2

emission rates for normal starts at both dry FGD- and wet FGD-equipped supercritical PC EGUs are 

approximately 80 - 90 percent lower across every hour (0-24) than the average SO2 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 capable of operating and capturing SO2 emissions commensurate with

the start of electricity generation.

Table 2: Number of normal starts at supercritical PC EGUs by SO2 control type

SO2 control type Normal starts

Wet FGD 1,492

Dry FGD 120

Uncontrolled for SO2 190

Total 1,802
10

Figure 8: Average SO2 emission rates following start of generation at supercritical PC EGUs by SO2

control type

Figures 9 and 10 show the distribution of SO2 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 

figures show that average and median SO2 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 SO2 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 

SO2 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 SO2 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’ SO2 emission rates are relatively low and stable (see Figure 11 for wet FGD and Figure 12 for 

dry FGD). Both types of FGDs show declining average SO2 emission rates by the third hour after 

reaching 25 percent load. 

Figure 11: Average SO2 emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at supercritical PC EGUs with wet FGDs 
12

Figure 12: Average SO2 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 NOX emissions from supercritical PC EGUs with SCRs

To determine the time necessary to start SCRs, the EPA examined hourly NOX emissions at EGUs 

with and without SCR. Of the 1,802 normal startup events at supercritical PC EGUs, 78 percent were 

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

PC EGUs have low-NOX burners, over-fire air, and/or selective non-catalytic reduction installed. The 

average NOX emission rates for the hours following the start of generation are shown in Figure 13. The 

average NOX emission rates for SCR-equipped and non-SCR supercritical PC EGUs begin at 

approximately the same level but the rate for the SCR-equipped EGUs grows slower and begins to 

decline by hour 5. This indicates that, on average, SCR APCDs are able to begin controlling NOX

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 NOX control type

NOX control type Normal starts

SCR 1,413

non-SCR 389

Total 1,802
13

Figure 13: Average NOX emission rates following start of generation at supercritical PC EGUs by 

NOX control type

Figure 14 shows the distribution of NOX emission rates during normal starts at supercritical PC EGUs 

with SCR NOX 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 average and median NOX

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 controls NOX approximately 6

hours or less after the start of generation.

Figure 14: Average NOX emission rates following start of generation at supercritical PC EGUs

Figure 15 shows the distribution of NOX emission rates during normal starts after achieving 25 percent

of nameplate capacity at supercritical PC EGUs with SCR NOX 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 that average and median NOX 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 NOX 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 SO2 emissions from subcritical PC EGUs with FGDs

Of the 5,770 normal startup events at subcritical PC EGUs, 70 percent were at subcritical PC EGUs 

with wet FGD and an additional 14 percent were at subcritical PC EGUs with dry FGD (see Table 4). 

The average SO2 emission rates for the hours following the start of generation are shown in Figure 16. 

The average SO2 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 SO2 emission rates for 

normal starts at subcritical PC EGUs without FGDs (i.e., uncontrolled). The average SO2 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 SO2 emission rates for normal starts at subcritical PC 

EGUs without FGDs. 

Table 4: Number of normal starts at subcritical PC EGUs by SO2 control type

SO2 control type Normal starts

Wet FGD 4,024

Dry FGD 803

Uncontrolled for SO2 943

Total 5,770
15

Figure 16: Average SO2 emission rates following start of generation at subcritical PC EGUs by SO2

control type

Figures 17 and 18 show the distribution of SO2 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 emission rate across starts at subcritical PC 

EGUs with FGD; the red boxes and black whiskers are the quartile ranges.) The figures show that 

average and median SO2 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 SO2 emission rates following start of generation at subcritical PC EGUs with wet 

FGDs 
16

Figure 18: Average SO2 emission rates following start of generation at subcritical PC EGUs with dry 

FGDs 

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

EGUs with wet FGD have relatively low and stable average and median SO2 emission rates (see Figure 

19) while subcritical PC EGUs with dry FGD reduce average SO2 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 SO2 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 19: Average SO2 emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at subcritical PC EGUs with wet FGDs 
17

Figure 20: Average SO2 emission rates following gross load levels greater than or equal to 25 percent

of nameplate capacity at subcritical PC EGUs with dry FGDs 

4.2.1.4 NOX emissions from subcritical PC EGUs with SCRs

Of the 5,770 normal startup events at subcritical PC EGUs, nearly 47 percent were at subcritical PC 

EGUs with SCR (see Table 5). Nearly all of the remaining non-SCR subcritical PC EGUs have 

installed low-NOX burners, over-fired air, and/or selective non-catalytic reduction. The average NOX

emission rates for the hours following the start of generation are shown in Figure 21. The average NOX

emission rates for SCR-equipped and non-SCR subcritical PC EGUs 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 NOx control type

NOX control type Normal starts

SCR 2,578

non-SCR 3,192

Total 5,770
18

Figure 21: Average NOX emission rates following start of generation at subcritical PC EGUs by NOX

control type

Figure 22 shows the distribution of NOX emission rates during normal starts at subcritical PC EGUs 

with SCR NOX 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 average and median NOX

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 NOX 2 to 3 hours after the start 

of generation.

Figure 22: Average NOX 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 SO2 capture by adding lime or limestone to the bed of the boiler. For this reason, the EPA 

evaluated CFB boiler EGU starts 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 startup events at 

CFB boiler EGUs; the purple boxes and black whiskers are the quartile ranges.) We found 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 startup events, 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 SO2 emission 

rates during startup events at CFB boiler EGUs begin to decline at hours 4 to 6 following the start of 

electricity generation. (Note: the top and bottom 5 percent of emission rates are excluded from the 
20

chart; the yellow line is the average emission rate across startup events at CFB boiler EGUs with FGD; 

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

Figure 25: Average SO2 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 EGUs. To evaluate the startup time of the best performing 12 percent (i.e., the EGUs that 

were able to start operation 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 removed 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 were 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 SO2 and NOX APCDs were 

analyzed 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 APCDs
21

3. We ranked EGUs by the average time to initiate operation of SO2 APCDs and NOX 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 initiate 

operation of SO2 APCDs and NOX 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 SO2 and NOX 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 SO2 “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% 3.15 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 NOX “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, the results indicate that the average of the best 

performing 12 percent of EGUs initiate SO2 and NOX control within 4 hours after the start of 

electricity generation (see Figure 26). At this threshold, the EGUs that comprise the best performing 12

percent include a CFB, supercritical and subcritical pulverized coal boilers, and wet and dry FGDequipped EGUs (see Table 8).

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

Characteristic SO2 “top 12%” NOX “top 12%”

Supercritical 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 after the start of generation;

 can begin controlling SO2 and NOX 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 EGUs, this analysis shows these EGUs, 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 include 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.