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Chapter 8

Modified Natural Gas-fired Stationary Combustion Turbines

Contents

8.1 General................................................................................................................................. 2

8.2 Identification of BSER......................................................................................................... 2

8.2.1 Actions to Trigger Modification ........................................................................................ 3

Modifications to an Older (e.g. Pre-2000) Combined Cycle Unit.......................................... 7

Upgrading a Simple Cycle Turbine to a Combined Cycle Unit ............................................. 8

8.3 Determination of the Level of the Standard....................................................................... 11
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8.1 GENERAL

Comment 8.1-1: Commenter 190 stated it is important that EPA not apply the modification 

provisions of Section 111(b) to existing NGCC units in a manner that results in unintended 

consequences. According to the commenter, some existing NGCC units may be constrained by 

current configurations or operating permit parameters that could be addressed by relatively minor 

modifications that have the potential to increase emissions on an hourly or annual basis. The 

commenter stated if the re-dispatch of these units is intended to displace higher emitting 

resources or to backstop renewable generation under a Section 111(d) program, these potential 

NGCC emission increases would be offset from the resources they replace. According to the 

commenter, the possibility that existing NGCC units could become subject to the 111(b) 

standards by increasing utilization in order to meet the 111(d) requirements could force these 

units to forego the opportunity to increase output or retire, rather than be subject to the 

modification provisions (a scenario contemplated by EPA, "[T]hese standards serve another 

important purpose that may affect a larger number of sources: Providing an incentive, and the 

information needed, for existing sources to structure their actions to achieve their operating and 

business goals without triggering the modification or reconstruction standards."). The commenter 

concluded that the inability to effectuate modifications to these existing NGCC units has the 

potential to result in stranded investments or require that additional fossil-fueled generation be 

constructed, resulting in both a significant cost to customers and an increase in overall emissions.

Response 8.1-1: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

8.2 IDENTIFICATION OF BSER

Comment 8.2-1: Commenter 187 suggested EPA that indicate the heat input standard is based 

on the on the LHV of the fuel, or adjust the heat input standard to a higher value to account for 

the HHV of the fuel. The commenter is concerned with the Agency's determination of BSER for 

modified stationary combustion turbines for several reasons. According to the commenter, it is 

not technically or economically feasible to upgrade older stationary combustion turbines to meet 

the carbon rates of new combustion turbines, and there is no information in the rule-making 

docket to support the achievability of the proposed standards for modified and reconstructed 

units. According to the commenter, this lack of an EPA technical support document to justify 

how all older stationary combustion turbines can be technically and economically upgraded 

during a modification to meet the emission rates of new units indicates the Agency itself is not 

sure they can be achieved. The commenter stated that because a standard of performance must be 

achievable, EPA should withdraw the Proposed Rule or, at a minimum, expressly exempt any 

modified combustion turbine that is technologically or economically incapable of meeting the 

performance standard. 

The commenter also identified a further defect in the proposed definition of BSER as "NGCC 

technology." According to the commenter, this definition fails to provide the level of detail 
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required under Section 111(a) of the Act, and without a clear definition of NGCC technology, 

sources cannot know what equipment would need to be upgraded if a unit is modified. Moreover, 

the commenter stated, requiring a simple-cycle combustion turbine to convert to an NGCC is 

forcing a change in the basic equipment, which is strictly not allowed under Section 111(b): 

"[N]othing in this section shall be construed to require, or to authorize [EPA] to require, any new 

or modified source to install and operate any particular technological system of continuous 

emission reduction to comply with any new source standard of performance." The commenter 

stated that simple-cycle combustion turbines serve a different purpose from combined-cycle 

units (i.e., fast response and ramp up and limited use), and if a simple-cycle must be converted to 

an NGCC, then the unit is not suitable for its intended purpose. The commenter stated that 

simple cycle turbines are primarily employed for their fast-start capability, and requiring a 

conversion to a unit that requires more time to start defeats the purpose of using such units and is 

unlawful.

Response 8.2-1: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-2: With respect to "modified" simple cycle combustion turbines, commenter 

0206 suggested that EPA drop the standard entirely, as the proposed standard unlawfully 

redefines the source and should be deleted.

Response 8.2-2: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-3: Commenter 0215 stated that EPA failed to distinguish among turbines that 

were designed and constructed over a period of decades, and thus objects to the Agency's 

determination of BSER. The commenter stated that the definition of "NGCC technology" has 

implications for evaluating the cost of technology and standard-setting and without further 

definition of this term, it is impossible to understand or comment on the proposal. [39476]

Response 8.2-3: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

8.2.1 Actions to Trigger Modification

Comment 8.2-4: Commenter 0227 stated they are unaware of any unit subject to Subpart KKKK 

that has been alleged to have been "modified" and that subsequently triggered the applicability of 

an NSPS, but some NGCC facilities have been retrofit with selective catalytic reactors (SCR) to 

control NOx emissions, which could have similar impacts as the control device installations that 

have occurred at Subpart Da units. The commenter stated that according to EPA's data in the 

companion proposal for existing sources, state-wide average CO2 emissions in 2012 for all 
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existing NGCC units in Subpart KKKK exceeded 1,000 lbs/MWh-net in several states. The 

commenter stated EPA has not exempted simple-cycle combustion turbines from the proposed 

standard, and not all simple-cycle facilities could accommodate a conversion to combined cycle 

operation, which would be necessary to achieve the proposed standards.

Response 8.2-4: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-5: Commenter 222 stated EPA must explicitly find that installation of control 

technology or changes to comply with EPA's environmental regulations do not trigger 

modification or reconstruction requirements under this Rule Proposal or Other Rule Proposals. 

The commenter supported EPA's finding that changes to install "add-on control equipment 

required to meet CAA requirements for criteria and hazardous air pollutants ... would be 

exempted from the definition of modification under the current NSPS regulations." The 

commenter noted that, however, EPA goes on to add that there are some actions that "could 

potentially trigger the modification provisions of CAA section 111(b);" for example, this would 

include changes in the particulate control equipment that removes limitations on the amount of 

coal that can be combusted. According to the commenter, EGU operators should not be 

penalized for installing newer, larger, and more efficient technologies that allow increased 

combustion of coal, as that would likely increase efficiency and lower the emissions rate. The 

commenter stated that any changes that lower the emission rate of CO2 and any type of heat rate 

improvement should be explicitly excluded from triggering modification requirements.

Response 8.2-5: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-6: Commenter 142 stated EPA failed to correctly estimate the number of 

modifications and therefore dramatically underestimates the cost of the rule. The commenter 

agreed with EPA's estimate that few units would be reconstructed; however, disagreed with 

EPA's estimate that few units would be modified. According to the commenter, EPA's basis for 

the estimate is flawed. The commenter stated EPA has not previously regulated CO2 in any 

NSPS, so any physical changes made to units in the past that resulted in increased CO2 emissions 

would not have been considered modifications, as the changes would not have resulted in an 

increase in emission of a pollutant to which a standard applied. According to the commenter, if 

the EPA is relying upon other pollutants as a surrogate to estimate the number of modifications, 

this approach is also flawed. The commenter stated that unlike CO2, other pollutants currently 

regulated under the NSPS typically rely upon add-on control devices to control those emissions. 

The commenter also stated that the NSPS defines an increase in emissions as an increase that 

occurs on an hourly basis. The commenter continued that if a unit has an add-on control device, 

the hourly emission rate of the pollutant subject to regulation can be mitigated by enhanced 

utilization of the control device, yet since no add-on control devices are currently used to control 

CO2 emissions, all changes that would result in an hourly increase in CO2 would be 

modifications. The commenter estimated that most existing combustion turbines will be modified 
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over the course of their useful life, and upgrades to existing turbines based upon advances in 

turbine technology that are designed to increase power output or improve the heat rate are 

common. Additionally, the commenter stated, Block 1 heat rate improvements required in 

section 111(d) could actually trigger a modification under section 111(b), because the hourly 

emission rate of CO2 could increase depending upon the heat rate improvements implemented. 

The commenter also stated heat rate improvement can be achieved when there is a greater 

increase in output (MW produced) relative to the increase in input (BTU, fuel).

Response 8.2-6: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-7: Commenters 0238 and 260 stated EPA should clarify that turbine projects do 

not constitute modifications within the NSPS context. The commenter cited EPA that it "expects 

few units would trigger either the modification or the reconstruction provisions." However, the 

commenter continued, the Agency explains that upgrades to a steam turbine that allow more coal 

to be combusted in the boiler "could potentially trigger the modification provisions of CAA 

section 111(b)." The commenter stated that EPA's statement is contradictory to NSPS 

applicability determinations issued by EPA for electric utility steam generating units. The 

commenter provided the following discussion: 

"According to NSPS applicability determinations, projects that solely involve the steam turbine 

do not constitute modifications as defined in 40 CFR section 60.14. For example, in an 

applicability determination issued to NRG Energy, EPA stated that '[t]he analysis of whether a 

modification occurs under the NSPS is dependent on physical and operational changes at the 

existing facility' and, 'a turbine is not part of the existing facility (i.e., the electric utility steam 

generating unit).' The Agency further explained that turbine changes would fall under the 

exemption from the modification definition for '[a]n increase in production rate of an existing 

facility, if that increase can be accomplished without a capital expenditure.' According to the 

Agency, an increase in turbine capacity that removes a bottleneck at the boiler 'would not trigger 

a capital expenditure at the boilers since the turbine is not part of the electric utility steam 

generating unit.'"

The commenters provided a diagram attached to an applicability determination for the Florida 

Electric Power Coordinating Group which clearly shows that the turbine is not part of the electric 

utility steam generating unit. The commenter concluded that given EPA's past applicability 

determinations on this issue, EPA should clarify in the preamble to the final rule that turbine 

upgrades at an electric utility steam generating unit would not be considered a modification 

under the NSPS provisions.

Commenter 0255 stated that turbine upgrades should not be considered a modification. The 

commenter stated that EPA expresses that few turbine projects would qualify as a modification 

or reconstruction under the proposed rule but acknowledges that some turbine projects may 

qualify. The commenter stated these statements are not consistent with EPA's historic 

applicability determinations where EPA states that the turbine is not part of the electric steam 
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generating unit. The commenter stated EPA should clarify in this rule that turbine upgrades do 

not constitute a modification under NSPS.

Commenter 0171 stated EPA indicates that upgrades to a steam turbine that allow more coal to 

be combusted in the boiler "could potentially trigger the modification provisions of CAA section 

111(b)". The commenter stated EPA's statement is contradictory to NSPS applicability 

determinations issued by EPA for electric utility steam generating units. The commenter stated 

according to NSPS applicability determinations, projects that solely involve the steam turbine do 

not constitute modifications as defined in 40 C.F.R. Section 60.14. The commenter stated EPA 

should clarify in final rule that turbine upgrades at an electric utility steam generating unit would 

not be considered a modification under the NSPS provisions.

Commenter 0238 also stated that as a practical matter, a determination that turbine projects 

constitute NSR modifications could have negative implications for EPA's Proposed Clean Power 

Plan. The commenter stated that Building Block I of EPA's Proposed Clean Power Plan assumes 

heat rate improvements from existing sources, which will include steam turbine upgrades that 

also might allow more coal to be combusted in the boiler. According to the commenter, any 

company contemplating efficiency improvements may be dissuaded from enacting such 

improvements, which would counteract the Agency's intended goal of lower GHG emissions. 

The commenter quoted EPA statements that its approach to pollution control and prevention 

which easily extends to efficiency gains to lower GHG emissions.

Response 8.2-7: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-8: Commenter 0190 stated that while it believes that only extremely unusual 

circumstances would cause the conversion of an existing coal-fueled boiler to natural gas to 

trigger the modification or reconstruction provisions under Section 111(b), EPA should provide 

additional clarification on this scenario. The commenter stated that as defined by Section 111, a 

modification only occurs if there is an associated emissions increase, noting that under a gas 

conversion, CO2 emissions are reduced by approximately 50% on a rate basis, and that 

frequently, capacity factor limitations are also imposed, effectively ensuring there is no 

emissions increase. The commenter stated that under the reconstruction provisions under Section 

111, the costs of the fixed capital components being replaced must exceed 50% of the costs of 

the fixed capital components of an entirely new comparable facility, and as a result, it is highly 

unlikely that the cost of a coal to natural gas conversion will trigger the reconstruction 

provisions.

The commenter provided additional information on operating units that have plans to utilize 

natural gas rather than coal at existing utility boilers, noting one will cease burning coal as a 

lower cost option to comply with EPA's Mercury and Air Toxics Standards, and another that will 

convert to natural gas as a lower cost option to achieve compliance with EPA's Regional Haze 

Rules. The commenter provided cost information and analysis in comparison to the proposed 

rule. The commenter noted that the planned coal-to-gas conversions will achieve significant 

reductions of CO2 and other emissions and it is critical that these and similar projects at existing 
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units are not disincentivized by EPA's treatment of CO2 emissions under a modification scenario. 

The commenter suggested that EPA should not subject these converted units to the NSPS under 

111(b). The commenter stated that while the new source standards have yet to be finalized, these 

converted units cannot achieve the proposed emission rate for natural gas fueled units of 1,000 

lb/MWh but they do provide significant CO2 benefits. For purposes of Section 111(d), the 

commenter suggested that converted units should be treated as "other gas" units in EPA's 

compliance calculation.

Response 8.2-8: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-9: Commenter 0291 stated that certain replacements or upgrades to steam turbines 

or particulate control devices should not trigger the NSPS modification rule, yet the Proposed 

Rule will nonetheless become the minimum emission limit that must be achieved by an EGU if it 

is to comply with the PSD New Source Review modification rules specifying BACT application. 

The commenter stated that EPA must consider the implications of its utility enforcement 

initiative in its discussion of the NSPS modification rule. The commenter stated that EPA's 

proposed BSER includes precisely the kinds of projects that are the subjects of its NSR suits -

"equipment upgrades [to] replace existing components with upgraded ones or a more extensive 

overhaul of major equipment (turbine or boiler)." The commenter stated that EPA must exclude 

from the NSPS for modified sources any unit that would likely to trigger NSR upon undertaking 

an HRI improvement. 

Response 8.2-9: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Modifications to an Older (e.g. Pre-2000) Combined Cycle Unit

Comment 8.2-10: Commenter 0131 expressed reservations about the ability of new NGCCs to 

meet the 1,000 lbs./MWh limit given the inclusion of all start-ups and shut downs and cycling to 

back up renewables. The commenter stated that EPA now proposes the same standard for 

modified and reconstructed NGCCs, and as EPA notes in its discussion, the vast majority of 

NGCCs have been built after 2000. According to the commenter, likely the only NGCCs that 

would be considered for modification or reconstruction would be those built before 2000. The 

commenter stated that bringing these machines to this standard would be cost prohibitive and 

they would likely be destined for replacement on an economic evaluation. According to the 

commenter, different standards would be appropriate for different technologies. The commenter 

stated that they again would support the application of the alternative standard of matching 

recent history as opposed to applying 111(b) standards.

Response 8.2-10: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 
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As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-11: Commenter 0215 stated that due to changes over time in the design efficiency 

of NGCC units, it is not possible to bring older combustion turbines up to the efficiency levels of 

modern units. The commenter stated that EPA acknowledges that there is no available method 

for reducing CO2 emissions from these units other than applying NGCC technology, and an 

NGCC unit that triggers reconstruction analysis but cannot meet the proposed NSPS with its 

current turbine does not have an option of installing add-on technology and would have to either 

entirely replace its existing facility with a new modern turbine (and likely a new heat recovery 

steam generator) or retire it altogether in order to comply. The commenter stated that because it 

would not be "technologically and economically feasible" for those units to comply with the 

Proposed Standards, they would not be considered "reconstructed" and would not be subject to 

the standard.

Response 8.2-11: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-12: Commenter 0215 disagreed with EPA's assumption that it is technically 

feasible to upgrade "older" (e.g., especially pre-2000) or even more recent NGCCs to meet the 

same CO2 emission rate as modern technology. The commenter stated that EPA's assumption 

that such an upgrade "is likely to be cost effective" is unsupported. The commenter disagreed 

that older units are capable of replacing sufficient equipment to match the emissions 

performance of a brand-new NGCC unit on a cost-effective basis, and stated that EPA has failed 

to provide any technical support for basing its Proposed Standards on such an assumption. The 

commenter stated that without a definition of "NGCC technology," neither an owner or operator 

nor a regulatory agency would know precisely what equipment would need to be upgraded if a 

unit has been determined to have been modified.

Response 8.2-12: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Upgrading a Simple Cycle Turbine to a Combined Cycle Unit

Comment 8.2-13: Commenter 199 stated that if required to convert simple cycle units using 

NGCC technology, many units would be forced to shut down. The commenter stated it is not 

technologically feasible to use NGCC technology on a simple cycle unit because doing so 

requires the addition of a steam generator and steam turbine, which increases the size and 

characteristics of the plant. The commenter also stated it is not economically feasible because 

when the characteristics of the plant change, the original purpose for constructing the unit 

(emergency and peak loading) is destroyed. The commenter continued that since NGCC is a 

cycling or base load energy resource, simple cycle units could possibly run into trouble if they 

had to replace multiple rows of turbine and compressor blades and/or have the generator 
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overhauled. The commenter stated that simple cycle units would not be able to achieve the 

standard as proposed. The commenter stated that a simple cycle unit's efficiency is 

approximately 50 to 60 percent lower than a NGCC unit and emits about 1,600 lb CO2/MWh, 

and the only way to reduce emissions to the levels EPA proposes would be to install NGCC 

technology. However, according to the commenter, requiring that technology as the BSER for 

these units is arbitrary and capricious.

Response 8.2-13: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-14: Commenter 214 stated EPA's BSER determination for modified and 

reconstructed simple-cycle units are flawed. According to the commenter, EPA has provided no 

supporting documentation identifying the technical feasibility of altering existing simple-cycle 

CTs to "modern high efficiency NGCC technology." The commenter stated that EPA must 

provide this documentation for all potentially affected sizes of simple-cycle combustion turbines. 

The commenter stated that EPA also did not provide the incremental costs incurred at modified 

or reconstructed simple-cycle CTs due to the installation of a HRSG, however, EPA 

acknowledges potential technical and cost concerns with simple-cycle CTs installing a HRSG. 

The commenter referenced the Agency concerns in stating: "if the owner/operator of a simple[-

]cycle turbine wishes to make a modification they could do so - without having to comply with 

the requirements of this proposal - by maintaining an average annual capacity factor of less than 

one-third." According to the commenter, EPA, however, cannot substitute an operational limit 

for an appropriate BSER analysis.

Commenter 0215 disagreed that NGCC is BSER for a simple cycle turbine and that it would be 

"technologically and economically feasible" to apply, except in very rare situations. The 

commenter stated that such a requirement would violate the statutory prohibition on requiring 

"any new or modified source to install and operate any particular technological system of 

continuous emission reduction to comply with any new source standard of performance." 

Response 8.2-14: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-15: Commenter 0165 stated EPA's BSER conclusion that BSER for a simple 

cycle combustion turbine is to upgrade the system to a combined cycle combustion turbine 

redefines the existing source. According to the commenter, the performance standards under 

Section 111 of the Clean Air Act are linked to the Best Available Control Technology 

requirements under the Prevention of Significant Deterioration program. The commenter stated 

that Congress specifically linked the two programs together by stating the Best Available Control 

Technology could not be less stringent than the performance standards established under Section 

111. Additionally, the commenter stated that the US Supreme Court discussed limitations on 

EPA's authority on applying "Best Available Control Technology" and noted the "control 

technology" could not require a "fundamental redesign" of the facility. According to the 
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commenter, requiring the simple cycle combustion turbine to be upgraded to a combined cycle 

combustion turbine is a "fundamental redesign" of the existing source.

Response 8.2-15: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-16: Commenter 0242 stated concerns regarding the ability for all types of units 

under all circumstances to be able to achieve NGCC cost-effectively. The commenter stated that 

issues may arise for modified sources that may make it cost-prohibitive as BSER to apply NGCC 

depending on the modification that occurs. The commenter provided the example that under this 

rule a simple cycle may trigger a modification based on the need for a simple upgrade, and in 

turn this rule would necessitate conversion to an NGCC which could make the project very cost 

prohibitive. The commenter stated they are particularly concerned in the context of an energy 

market that is drastically altered through the proposed rule for existing units. The commenter 

stated that simple cycle units will be utilized more frequently due to greater reliance on 

intermittent renewable resources and the lack of coal-fired capacity. According to the 

commenter, the conversion of a simple cycle is a complicated and expensive process that will not 

occur frequently, thus possibly leading to electricity shortages, and the U.S. EPA must afford 

states the ability in those circumstances to determine emission rates on a case-by-case basis.

Response 8.2-16: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-17: Commenter 0215 stated that although many simple cycle units have 

historically operated less than 10 percent of the year, there are cases where individual units may 

need to run much longer to maintain system reliability, either for short or prolonged periods. The 

commenter stated that converting to a combined cycle facility would not be cost-effective for 

most, if not all, simple cycle turbine owners, meaning that a simple cycle turbine that becomes 

subject to the NSPS for modified sources would be unable to operate, and system reliability 

would be jeopardized.

Response 8.2-17: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-18: Commenter 0176 stated that EPA acknowledges that it is obligated to 

consider environmental impacts that would flow from the implementation of a proposed NSPS, 

including impacts to scarce water resources, According to the commenter, NGSC units that are 

affected by this rule cannot meet the standard and must convert to a NGCC to meet the standard. 

The commenter stated that to the extent that the use of NGCC results in depletion of water 

resources, especially in water scarce areas, it must be considered by EPA when finalizing the 
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rule, and the fact that NGCC are water intensive provides additional support that a mandate to 

convert NGSC units to NGCC is not BSER.

Response 8.2-18: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-19: Commenter 0215 stated that EPA should create a separate category or 

subcategory for modified high capacity factor simple cycle turbines, determine an appropriate 

BSER that does not require conversion to NGCC, analyze emissions from units in the 

subcategory, consider all of the requirements of section 111, and propose a limit that is 

achievable. 

Response 8.2-19: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.2-20: Commenter 0149 had concerns regarding the ability of all types of units under 

all circumstances to be able to achieve NGCC cost-effectively. The commenter believes issues 

may arise for modified sources that may make it cost-prohibitive as BSER to apply NGCC 

depending on the modification that occurs. The commenter stated that under the proposed rule a 

simple cycle may trigger a modification based on the need for a simple upgrade, and this would 

necessitate conversion to an NGCC, which could make the project cost prohibitive. This was 

particularly concerning to the commenter in the context of an energy market that is drastically 

altered through the proposed rule for existing units. 

Response 8.2-20: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

8.3 DETERMINATION OF THE LEVEL OF THE STANDARD

Comment 8.3-1: Commenter 0150 stated EPA also proposes to establish standards identical to 

those for new units: 1,000 lb CO2/MWh-gross for large (more than 850 MMBtu/h) units and 

1,100 lb CO2/MWh-gross for small (850 MMBtu/h or less) units. According to the commenter, 

EPA has not provided any record evidence supporting the achievability of these standards, and as 

such, they are per se arbitrary and capricious.

Response 8.3-1: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).
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Comment 8.3-2: Commenters 0171 and 0254 stated that for modified NGCC, the proposed 

rule subjects them to the same standard as a new NGCC, without any data or explanation that 

this is appropriate. According to the commenters, EPA's unsupported conclusion is wrong -- the 

design of many units precludes their ability to meet the new-unit limit, if modified.

Commenter 0215 stated that existing sources generally cannot meet the same emission standard 

as a new, greenfield source and an analysis of existing units demonstrates that a substantial 

number of units cannot achieve the standards EPA has proposed. 

Response 8.3-2: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.3-3: Commenters 0195, 0211, 0259 and 0261 stated that EPA should not set 

identical standards for modified, reconstructed and new units given the technical limitations of 

modified and reconstructed units. 

Commenter 0195 also stated that considering that modified units do not experience the same 

degree of alteration as reconstructed units, the commenter recommends that EPA apply the 

highest limits included in the proposal, i.e., 1,100 lb CO2/MWh for large sources and 1,200 lb 

CO2/MWh for smaller sources.

Commenter 0259 also stated that given that modified units do not experience the same level of 

physical change as reconstructed units, EPA should finalize a higher range than that included in 

the proposal - 1,200 lb CO2/MWh gross for large modified CTs and 1,300 lb CO2/MWh gross for 

smaller modified CTs.

Commenter 0211 stated the Proposed Rule improperly imposes identical standards for newly 

constructed, modified, and reconstructed natural gas-fired stationary combustion turbines. The 

commenter stated existing sources generally cannot meet the same emission standard as a newly 

constructed source. The commenter stated modified or reconstructed NGCC units should be 

subject to an emission standard that is higher than newly constructed units. The commenter also 

supported the Proposed Rule's rejection of carbon capture and sequestration (CCS) as BSER. 

Commenter 0261 stated modified combustion turbines should not be forced to comply with the 

same standards as those for new units given that existing units were not designed with the 

technical standards and technology that apply to new units today. The commenter stated units 

that are modified cannot take advantage of the same design standards and technology that would 

allow new units to meet such stringent limits. The commenter stated that due to these technical 

restrictions, the limits for natural gas-fired stationary CTs should differentiate between, new, 

modified, and reconstructed units. The commenter also stated the overly stringent emission 

standards, could possibly preclude utilities from making modifications that would 

improve efficiency, which would reduce the heat rate (Btu/MWh) and the CO2 emissions rate 

(lbs/MWh).
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Response 8.3-3: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.3-4: Commenter 0257 stated that EPA should exempt certain operational modes 

from Subpart KKKK compliance. The commenter stated that EPA's regulations for modified and 

reconstructed NGCC units should expressly exclude two modes of operation that are not typical 

for NGCC combustion turbines. According to the commenter, many combustion turbines are 

permitted to fire oil as a back-up fuel for up to 30 days per year, especially during periods of 

curtailment of natural gas when supplies are short, and it is vital that this mode of operation be 

kept available to ail such units to mitigate risks to reliability. The commenter stated The Agency 

has provided no emissions data to justify any NSPS for combined cycle combustion turbines 

when they are firing oil. The commenter suggested that EPA should expressly exclude periods of 

operation when units permitted to do so fire oil as a back-up fuel. Alternatively, the commenter 

recommended EPA should further analyze CO2 emissions during oil-fired operations and 

propose an NSPS that adequately accounts for such periods or re-evaluate, develop and propose 

a separate standard for dual-fuel units. The commenter stated that modifying the standard in 

these ways would help accommodate partial load operations, specific unit design and operational 

site characteristics, as well as the use of backup fuel. 

Commenter 0257 also stated that some NGCC units that would be subject to the modified or 

reconstructed source NSPS have designs that allow the operators to bypass the steam cycle and 

operate in simple cycle mode when necessary. The commenter urged EPA to exempt simple 

cycle mode operations from compliance when an NGCC unit falls below the 33 percent capacity 

factor in that mode.

Response 8.3-4: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.3-5: Commenter 203 stated many of the same concerns expressed for reconstructed 

units apply to the Proposal for modified Subpart KKKK units. The commenter provided the 

following points:

- No technical support document in the rulemaking docket that specifically discusses modified 

Subpart KKKK EGUs or that contains any analyses of emissions data was found. In particular, 

analyses that would justify the setting of any emissions limit for modifications are absent. So as 

to comply with CAA 307(d)(3), EPA should withdraw its proposal until it has developed and 

included within the rulemaking docket the necessary supporting information;

- It is even more important for EPA to set a reasonable performance standard for modified 

Subpart KKKK units, as compared to reconstructed units, given the much higher likelihood that 

future projects could trigger the modification threshold. As there is no explicit requirement in the 

modification regulations that mirrors the requirement that reconstructions must be 

technologically and economically feasible to apply to a given project, a rate limit for 
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modifications equal to the same number for reconstructions arguably results in a more stringent 

limit for the more likely to occur modification. Given the scope and magnitude of a triggering 

project- much lower than a project costing 50% of a comparable new facility, and instead an 

increase in hourly emissions - if anything, it would seem that modification performance 

standards for the same source types should perhaps be somewhat less stringent that 

reconstruction standards. Both should be less stringent that performance standards for new 

KKKK units.

- Because it may be infeasible for EPA to properly set performance standards, it could set design, 

equipment, work practice or operational standards under 111 (h). Alternatively, here EPA is 

justified in declining to promulgate performance standards for modified or reconstructed Da or 

KKKK units, since EPA concludes in its RIA that the Proposal will produce no significant costs 

or benefits;

- EPA does not have the authority to set source-specific performance standards for Subpart 

KKKK units, and instead must hew to the governing parameters of 111 (b). Standards must be 

set for properly categorized (or subcategorized source types), and energy efficiency audits have 

no place in lieu of the required approach in 111 (b);

- Once EPA develops the required emissions analyses, it should set the standards at a level that 

provides for 99% compliance;

- Simple cycle units should be exempt from the rule in all cases. Simple cycle units cannot meet 

the same standards as NGCC units (see further discussion below);

- When setting the performance standards for modified Subpart KKKK units, EPA must account 

for the fact that no add-on equipment is currently available for CO2 control, within the 

parameters of the determined BSER. As previously discussed, unlike many criteria pollutants 

(NOx, SO2 or particulates), there is no add-on equipment currently available to reduce CO2

emissions. Coupled with the fact that all equipment degrades over time, becoming less efficient 

(thus emitting more CO2), if EPA sets CO2 performance standards for modified units too 

stringently, sources will have no margin for compliance. If a project must be undertaken for 

continued operations, but the project triggers a NSPS modification, the only choice may be a 

permanent shut down of the units. EPA should not establish a NSPS that results in this outcome 

for modified Subpart KKKK units and, in fact lacks the authority to set such a rigorous standard.

Response 8.3-5: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).

Comment 8.3-6: Commenter 0215 stated that about 10 percent of NGCC units that were 

designed and built with truly modern high temperature materials cannot meet a 1,000 lb 

CO2/MWh-gross limit and that only 96 percent of large (more than 850 MMBtu/h) units can 

meet a 1,100 lb CO2/MWh-gross limit [See: Smith Turbine Report at 2 & 7, Fig. 1; and 

Attachment A at 3]. The commenter stated that EPA has failed to provide any information to 

justify that modified units that were constructed in the 1990s can match the emissions 
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performance of new turbines with more efficient designs that were built up to 25 years later. The 

commenter stated that turbine overhauls may allow some older units to approach their original 

design efficiency but cannot bring these units up to the level of modern turbines. The commenter 

stated that an NSPS "establishes what every source can achieve, not the best that a source could 

do." The commenter stated that EPA should set a limit whereby at least 99 percent of modified 

units should be expected to comply.

Response 8.3-6: The EPA is withdrawing the proposed emission standard for modified 

stationary combustion turbines, as explained in Section IX.D.3.e of the preamble to the final rule. 

As a result, combustion turbines that conduct modifications will remain “existing sources” 

subject to 111(d).