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MEMORANDUM 

Date: November 6, 2012 

Subject: Startup and shutdown provisions 

From: William Maxwell, ESG/SPPD/OAQPS 

To: EPA-HQ-OAR-2009-0234 

The Administrator signed the “National Emission Standards for Hazardous Air Pollutants from 

Coal- and Oil-fired Electric Utility Steam Generating Units and Standards of Performance for 

Fossil-Fuel-Fired Electric Utility, Industrial-Commercial-Institutional, and Small IndustrialCommercial-Institutional Steam Generating Units,” comprising the NESHAP portion, known as 

the Mercury and Air Toxics Standards (MATS) and the Utility NSPS, on December 16, 2011, 

and the final rules were published in the Federal Register on February 16, 2012 (see 77 FR 

9304). We proposed different standards for startup and shutdown by our inclusion of certain 

default values, which applied only during startup and shutdown. 

In response to comments on the proposed rule, we finalized work practice standards to apply 

during startup and shutdown, as described further below. This technical support document 

describes the provisions for startup and shutdown promulgated in the final rule1 and changes to 

these provisions to address issues raised in the petitions for reconsideration of the final MATS 

rule and the Utility NSPS. 

I. MATS Proposed Rule 

In the final rule, we proposed different standards for startup and shutdown by our inclusion of 

the default values described below, which applied only during startup and shutdown. 

Specifically, we stated: 

To appropriately determine emissions during startup and shutdown and account 

for those emissions in assessing compliance with the proposed emission 

standards, we propose use of a default diluent value of 10.0 percent O2 or the 

corresponding fuel specific CO2 concentration for calculating emissions in units 

of lb/MMBtu or lb/TBtu during startup or shutdown periods. For calculating 

emissions in units of lb/MWh or lb/GWh, we propose source owners use an 

electrical production rate of 5 percent of rated capacity during periods of startup 

 

1 This document uses the terms “the final rule” and “the rule” to refer to MATS, though the 

Utility NSPS incorporates these same provisions with regard to the particulate matter standard 

for that rule. 
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or shutdown. We recognize that there are other approaches for determining 

emissions during periods of startup and shutdown, and we request comment on 

those approaches. We further solicit comment on the proposed approach 

described above and whether the values we are proposing are appropriate. 

We proposed application of the respective emission limits during periods of startup and 

shutdown and use of default values to calculate the emission limits. The standards that apply at 

all times other than startup and shutdown are production-based limits, which is why we proposed 

the default values. The default values were meant to account for the fact that during startup and 

shutdown events, production (in this case the generation of electricity) is by definition 

nonexistent. 

We received a variety of comments on the proposed rule concerning the application of the 

standards during startup and shutdown. Many commenters pointed to the lack of data in the 

record concerning emissions that occur during periods of startup and shutdown. Commenters 

also asserted that emissions during these periods can vary widely during the startup and 

shutdown of an EGU. Other commenters questioned the feasibility of collecting additional data 

during such periods and voiced concerns regarding the reliability of measurements obtained from 

EGUs during such periods. 

II. Final Rule 

In response to comments, we established work practice standards rather than numeric emissions 

standards for periods of startup and shutdown in the final MATS rule. See 77 FR 9380-82. 

The final rule required the owners and operators of EGUs to: (1) operate using “clean fuels,” 

which were listed as either natural gas or distillate oil, or a combination of those, for ignition 

during startup, (2) vent emissions to the main stack(s) and operate all control devices necessary 

to meet the normal operating standards under the final rule, with the exception of those 

technologies whose operation is temperature-dependent, when coal, solid oil-derived fuel, or 

residual oil is fired in the EGU during startup or shutdown, and (3) start their temperaturedependent control systems appropriately to comply with relevant standards applicable during 

normal operation. 

In the proposed reconsideration notice, the Agency is proposing work practice standards that are 

substantially the same as the work practice standards established in the final MATS. Specifically, 

sources must combust clean fuels during periods of startup and shutdown, and coal and oil may 

not be combusted in the EGU until certain air pollution controls are engaged. The EPA is, 

however, proposing to revise the definitions of “startup” and “shutdown,” to add a definition of 

“clean fuel” and expand the list of “clean fuels,” and to revise requirements related to the 

operation of controls. 
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A. Definitions 

We first address what constitutes startup and shutdown because definitions of these periods are 

essential to implementation of work practice standards. 

1. Startup and Shutdown Periods

In the final rule, we included the following definitions of startup and shutdown: 

Startup means either the first-ever firing of fuel in a boiler for the purpose of 

producing electricity, or the firing of fuel in a boiler after a shutdown event for 

any purpose. Startup ends when any of the steam from the boiler is used to 

generate electricity for sale over the grid or for any other purpose (including on 

site use). 

Shutdown means the cessation of operation of a boiler for any purpose. Shutdown 

begins either when none of the steam from the boiler is used to generate 

electricity for sale over the grid or for any other purpose (including on-site use) or 

at the point of no fuel is being fired in the boiler. Shutdown ends when there is 

both no electricity being generated and no fuel being fired in the boiler. 

Petitioners sought clarification regarding these terms as applied to EGUs and stated that the 

definitions should be revised to accommodate the operation of cogeneration units, and did not 

accurately reflect startup conditions for all affected units, particularly supercritical units. We are 

proposing to revise the definitions in this reconsideration notice as follows: 

Startup means the period in which operation of an EGU is initiated for any 

purpose. Startup begins with either the first-ever firing of fuel in an EGU for the 

purpose of producing electricity or useful thermal energy (such as heat or steam) 

for industrial, commercial, heating, or cooling purposes, or the firing of fuel for 

any purpose in an EGU after a shutdown event. Startup ends when the EGU 

generates electricity that is sold or used for any other purpose (including on site 

use), or the EGU makes useful thermal energy (such as heat or steam) for 

industrial, commercial, heating, or cooling purposes (16 U.S.C. 796(18)(A) and 

18 CFR 292.202(c)), whichever is earlier. 

Shutdown means the period in which cessation of operation of an EGU is initiated 

for any purpose. Shutdown begins when the EGU no longer generates electricity 

or makes useful thermal energy (such as heat or steam) for industrial, commercial, 

heating, or cooling purposes; or when no coal, liquid oil, syngas, or solid oilderived fuel is being fired in the EGU, whichever is earlier. Shutdown ends when 

the EGU no longer generates electricity or makes useful thermal energy (such as 
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steam or heat) for industrial, commercial, heating, or cooling purposes, and no 

fuel is being fired in the boiler. 

We maintain that the proposed revised definitions, including the addition of provisions for useful 

thermal energy,2 appropriately address the fact that EGUs can provide electricity both for sale 

and for on-site use, and cogeneration EGUs can also supply useful thermal energy (such as heat 

or steam) for industrial, commercial, heating, or cooling purposes. For this reason, we believe 

that EGUs should be considered to be operating normally at all times electricity is generated for 

sale or use, and/or useful thermal energy (such as heat or steam) is supplied for a useful purpose. 

Some petitioners have asserted that, by definition, a supercritical EGU can only achieve its 

normal operating conditions once the steam reaches supercritical conditions. Because this 

process requires transition from “subcritical” to “supercritical” conditions, a supercritical EGU 

has a dedicated startup system to separate steam from water until the unit reaches operating 

conditions at approximately 30 to 40 percent of full load. Through this comment, the petitioners 

seem to suggest that for supercritical EGUs the startup period ends only after the steam sent to 

the turbine has no water droplets, which, according to the petitioners, happens at approximately 

30 to 40 percent load. 

We believe our proposed revised definition of startup accounts for the operation of supercritical 

EGUs. Under the revised definition, the startup period begins with firing of clean fuel and ends 

with generation of electricity for any useful purpose or production of useful thermal energy (such 

as steam or heat), whichever comes first. Separation of steam and water, irrespective of the EGU 

type – subcritical or supercritical – occurs within this period.3 

Some petitioners asserted that the definition of startup period and the work practices in the final 

rule are not accurate because startup extends beyond first generation of electricity, because 

electricity generation does not mean that the EGU and emission controls have reached a safe or 

stable normal operating condition, because the concept of minimum stable load (at which point 

supplemental fuel is not needed) is relevant relative to efficient, stable, and safe operation of air 

pollution controls, and because operation of all control equipment should be tied to the end of the 

startup in the revised definition. As discussed below, we believe both the proposed revised 

definitions of startup and shutdown periods and the proposed revised work practices, which take 

into consideration potential operational constraints of air pollution control devices (APCDs), 

address these concerns. 

Per our proposed definition of startup period, this period ends when an EGU generates electricity 

 

2

 16 U.S.C. 796(18)(A) and 18 CFR 292.202(c). 

3

 We understand that a steam turbine should not be exposed to water droplets at its inlet at any 

load, to prevent blade erosion and other problems, and that boiler/turbine systems are designed to 

prevent this, and we do not consider the proposed definition to be problematic in this regard. 
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that is sold or used for any other purpose (including on-site use) or an EGU makes useful thermal 

energy (such as heat or steam) for industrial, commercial, heating, or cooling purposes, 

whichever is earlier. We disagree that the startup period extends beyond first generation of useful 

electricity, as we believe EGUs are operating in production (not startup) mode when they 

generate electricity or produce thermal energy for use. 

We also believe that the proposed APCD operation-related work practice requirements 

accommodate efficient, safe, and stable operation. Moreover, we have carefully designed our 

definitions and work practice requirements to provide flexibility to EGU owners/operators 

regarding operation of controls, and therefore we have not tied operation of these more 

condition-specific controls to loads or specific points in the startup period. We also understand 

that no technical barrier exists to burning clean fuels for longer portions of startup or shutdown 

periods. Hence, if needed, an EGU could burn clean fuels until the appropriate flue gas 

conditions are achieved to engage the required APCDs. 

While petitioners suggest that the concept of minimum stable load (at which point supplemental 

fuel is not needed) is relevant relative to efficient, stable, and safe operation of air pollution 

controls, they have not provided data supporting this claim. Moreover, if an EGU co-fires clean 

fuels with production fuels (coal, liquid oil, syngas, or solid oil-derived fuel) after the end of its 

startup period, its emissions will be lower than those generated when it fires only production 

fuels. Considering these factors, the EPA is not taking any actions relative to the concept of 

minimum stable load asserted by petitioners. 

 2. Clean Fuels

The final rule required use of “clean fuels,” listed as either natural gas or distillate oil, or a 

combination of those, for ignition during startup. Petitioners asserted that the finalized work 

practice standards should include synthetic natural gas, syngas, and ultra-low sulfur diesel 

(ULSD) as “clean fuels.” Specifically, petitioners referred to the use of synthetic natural gas or 

diesel fuel for some EGUs and syngas for startup of integrated gasification combined cycle 

(IGCC) EGUs affected by this rule. The EPA also understands that propane is used for startup at 

some EGUs. 

In the proposed reconsideration rule, we have proposed adding a definition of “clean fuels” and 

including synthetic natural gas that meets the specification necessary for that gas to be 

transported in/via a Federal Energy Regulatory Commission-regulated pipeline, synthesis gas 

(syngas)4

, propane, and ultra-low sulfur diesel (ULSD) to the list of clean fuels noted in the final 

rule (natural gas and distillate oil). We believe that HAP emission reduction benefits associated 

with burning of clean fuels warrant utilization of such fuels during startup and shutdown periods. 

 

4

 Synthesis gas (syngas) is gasifier product gas consisting mostly of hydrogen and carbon 

monoxide.
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We also note that addition of synthetic natural gas to the list of clean fuels provides the 

opportunity for conventional coal-fired EGUs to startup on synthetic natural gas if desired. Also, 

limiting the diesel fuel to the ULSD type will help minimize emissions during period of startup 

and shutdown. 

The proposed definition is below. 

Clean fuel means natural gas, synthetic natural gas that meets the specification 

necessary for that gas to be transported on a Federal Energy Regulatory 

Commission (FERC) regulated pipeline, propane, distillate oil, synthesis gas 

(syngas), or ultra-low-sulfur diesel (ULSD). 

B. Operation During Startup and Shutdown 

In the final rule, the EPA required sources to vent emissions to the main stack(s) and operate all 

control devices necessary to meet the operating standards that apply at all other times under the 

final rule (with the exception of dry scrubbers and selective catalytic reduction (SCR) systems) 

when coal, solid oil-derived fuel, or residual oil is fired in the EGU during startup or shutdown. 

The EPA stated that it is the responsibility of the operators of EGUs to start their dry scrubber 

and SCR systems appropriately to comply with relevant standards applicable during normal 

operation. 

Petitioners stated that IGCC EGUs are fired with syngas, not the fuels listed as “clean fuels” in 

the final rule. Petitioners also contended that the standards need to recognize operating 

conditions for fluidized bed combustion (FBC) EGUs that inject limestone for acid gas control, 

for most effective control of those gases, as well as operating conditions for selective noncatalytic reduction systems (SNCRs), selective catalytic reduction systems (SCRs), and other 

systems. We have considered these concerns, and in the proposed reconsideration notice are 

proposing to require EGU source owners or operators, when firing coal, solid oil-derived fuel, or 

residual oil in the EGU during startup and shutdown, to vent emissions to the main stack(s) and 

operate all control devices necessary to meet the operating standards that apply at all other times 

under the final rule (with the exception of limestone injection in FBC EGUs, dry scrubbers, 

SNCRs, and SCRs). Owners and operators of EGUs are responsible for starting limestone 

injection in FBC EGUs, dry scrubber, SNCR, and SCR systems as expeditiously as possible, but, 

in any case, when necessary to comply with other standards applicable to the source that require 

operation of the control devices. 

1. IGCC Units 

The EPA understands that at IGCC EGUs, syngas(primarily a mixture of carbon monoxide and 

hydrogen) is used as both a clean fuel and the production fuel. We understand that syngas is 

generated in the gasifier of an IGCC EGU and after the syngas achieves the desired site-specific 
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composition, it is fired in the combustion turbine as a clean and production fuel. The syngas fired 

in the combustion turbine is considered a clean fuel because it is cleaned to remove trace 

contaminants such as sulfur before its injection in the turbine. We also understand that during the 

startup and shutdown periods, some or all of the syngas produced may not be combusted in the 

turbine. In the proposed reconsideration rule, we are proposing two options for IGCC EGUs for 

handling syngas not fired in the combustion turbine: (1) syngas must be flared, not vented, or (2) 

syngas must be routed to duct burners, which may need to be installed, and the flue gas from the 

duct burners must be routed to the heat recovery steam generator. 

2. Operating Constraints of Certain APCDs 

In the proposed reconsideration rule, we also propose to revise the rule’s work practice 

requirements to recognize constraints of certain EGUs and APCDs. Specifically, after 

considering petitions that FBC EGUs that inject limestone for acid gas control need to be 

operated at temperatures at which limestone can calcine for most effective control of acid gases5,

we are proposing to revise the standards to allow limestone injection to start after appropriate 

temperatures as specified by the equipment manufacturer or installer have been attained in such 

EGUs. In addition, we acknowledge operating temperature constraints of SNCR systems, and we 

are proposing to revise the standards to allow such systems to start as soon as technically 

possible after the appropriate temperatures as specified by the equipment manufacturer or 

installer have been reached. We are also aware that SNCR and SCR systems with ammonia 

injection need to be operated within a prescribed and relatively narrow temperature window to 

provide nitrogen oxide (NOX) reductions6 and that dry scrubbers also need to be operated close to 

flue gas saturation temperature7

, and we have addressed these considerations in the proposed 

work practice requirements. 

Some petitioners have asserted that premature use of ACI, as well as trona and other alkaline 

sorbents that are heavier than activated carbon, can lead to deposits on particular matter (PM) 

control surfaces and potentially cause detrimental impacts on hardware and operation of these 

controls and create safety concerns, including fires. Petitioners suggested that sorbent use can 

only begin when a sufficiently high gas flow rate is achieved in the duct. The EPA has reviewed 

publicly available information and information submitted in comments to the rule, and we have 

not found any information supporting the petitioners’ assertions. Moreover, petitioners have not 

provided data substantiating their assertions. Therefore, we are not adding specific work practice 

requirements associated with sorbent (carbon or trona or other alkaline materials) injection. 

 

5

 Fluidized Bed Combustion. Simeon Oka, CRC Press, September 2003, pp. 527-528.

6

 NOX Emission Control Options For Coal-Fired Electric Utility Boilers. R.K. Srivastava, R.E. 

Hall, S. Khan, K. Culligan, and B. Lani, Journal of the Air & Waste Management Association, 

55, September 2005, pp. 1372-1374. 

7

 Flue Gas Desulfurization: The State of the Art. R.K. Srivastava and W. Jozewicz, Journal of the 

Air & Waste Management Association, 51, December 2001, p. 1680. 
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Further, as discussed in the preamble to the final rule, we understand that there may be concerns 

with acidic and tarry deposits on surfaces of PM controls (ESPs and baghouses) related to firing 

of heavy (residual) oil, but that such concerns do not exist when EGUs combust light (distillate) 

oil.8 Accordingly, with residual fuel oil firing, site-specific flue gas temperature and oxygen (O2) 

concentration thresholds may be relevant to minimize condensation of acids and tars and thereby 

minimize any potential for detrimental impacts on hardware and any safety concerns. The rule’s 

proposed work practice requirements provide flexibility to the operator to take appropriate sitespecific remedial measures, if needed, to address this issue. EGU owners or operators have 

several options to prevent detrimental impacts: (1) using clean startup fuels (e.g., natural gas, 

synthetic natural gas, syngas, ULSD, or distillate oil) until appropriate flue gas conditions have 

been reached and then subsequently switching to firing of residual oil; (2) pre-coating the PM 

control surfaces9

 with an alkaline powder (e.g., limestone); (3) installing chemically resistant 

bags10 in fabric filters (FF)s if applicable, or (4) using low-sulfur oils. 

The EPA also notes that there are currently many operational residual oil-fired EGUs that are 

started up with either natural gas or distillate oil. At these EGUs, the transition from the startup 

fuel, distillate oil or natural gas, to residual oil is already being practiced without unacceptable 

impacts on APCDs, including PM controls that are operated to meet applicable opacity limits.

Based on this experience and the options described above, those EGUs where residual oil is used 

for either a part of the startup period or as the main fuel will also be able to operate their PM 

controls to meet the rule’s work practice requirements. 

We are not aware of any operational constraints applicable to operation of wet scrubbers during 

startup that could cause detrimental impacts on wet scrubber hardware or could create safety 

concerns, and we did not receive comments on this aspect of wet scrubber operation. 

We also note that dry sorbent injection (DSI) can be applied across a very broad temperature 

range and will be engaged when residual oil or coal is fired in an EGU if necessary to comply 

with HCl requirements. Again, we did not receive comments on this aspect of DSI operation. 

In summary, based on the discussion above, we believe there are no constraints with engaging 

PM controls (ESPs and baghouses), wet scrubbers, and sorbent injection systems (ACI and DSI) 

when firing coal, solid oil-derived fuel, or residual oil in EGUs during startup and shutdown. 

Owners and operators of EGUs are responsible for starting limestone injection in FBC EGUs, 

 

8

 Coal is fired at sufficiently high temperatures that concerns with condensation causing acidic 

and tarry deposits are not relevant. 

9

 Coal Power, May 1, 2007: http://www.coalpowermag.com/plant_design/Coal-Plant-O-and-MRiver-Locks-and-Barges-Are-an-Aging-Workforce-Too_36.html 10 Neundorfer: Lesson #4, p.4-7, Table 4-1: 

http://www.neundorfer.com/FileUploads/CMSFiles/Fabric%20Filter%20Material[0].pdf
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dry scrubber, SNCR, and SCR systems as expeditiously as possible, but, in any case, when 

necessary to comply with other standards applicable to the source that require operation of the 

control devices.