Summary	of	Interagency	Working	Comments	on	Draft	Language	under	

EO12866/13563	Interagency	Review.		Subject	to	Further	Policy	Review.

Page 455 (Redline/strikeout version).

In a separate submission reviewer provided EPA staff with data on 7 examples of modifications to fossil steam 

EGUs that resulted in unit capacity increases ranging from 8% to 20%. Reviewer hopes that these data will provide 

a rationale for strengthening the technical basis for the “large modification” threshold. Reviewer found the 

technical basis for the 10% “large modification” threshold to be unclear given the current explanation and 

examples that are included in the Preamble, and suggests clarifying. In addition, Reviewer would be happy to 

consult with EPA to determine and strengthen the technical basis for an appropriate “large modification” 

threshold. 

EPA Response: Yes, the information provided with the 7 examples of projects was received and is helpful. It will 

be included in the rulemaking docket. However, the EPA notes that these projects do not necessarily qualify as 

NSPS modifications as capacity increases do not necessarily always result in hourly increases in CO2 emissions. The 

EPA has added additional language to the preamble to further justify the distinction between “large modifications” 

and “small modifications”.

Reviewer also notes that the modifications undertaken at AmerenUE’s Labadie Plant were completed over an 11 

year period from 1992 to 2003, and that on an individual basis, each specific incremental modification likely 

resulted in a unit capacity change well below the 10% “large modification” threshold. As such, reviewer requests 

clarification as to whether a “large modification” must be completed in a particular period of time, and suggests 

that EPA clarify the time frame in which a modification must be completed in the Preamble. Reviewer also 

recommends limiting the actions that constitute a modification to a short time period, and revisiting the “large 

modification” threshold accordingly. 

EPA Response: The EPA included a description of the Labadie Plant projects (and those at the Arnot Plant) in order 

to show the “types of projects” that may potentially result in an increase in hour CO2 emissions. The EPA does not 

intend to imply that those projects would definitely trigger the “large modification” threshold. Owner/operators 

can often undertake other projects simultaneously that improve operational efficiency and can offset capacity 

upgrades such that there is no net increase in hourly CO2 emissions from the combined projects. Specific criteria 

for the NSPS modifications are provided in 40 CRF 60.14. There is no specified period in which a “large 

modification” must be completed. However, the modification must be demonstrated to be a single project and not 

the result of a series of separate projects that are undertaken over an extended time period. The owner/operators

are encouraged to consult with their permitting authority prior to undertaking potential modifications to ensure 

that they are aware of any changes in regulatory requirements resulting from the potential modification.

In addition, reviewer notes that changing coal types from bituminous to subbituminous coal was one of the 

modifications carried out at the AmerenUE Labadie Plant. Reviewer found it confusing that changing coal type was 

included in this example of a “large modification” in the Preamble. If reviewer’s understanding that a modification 

must be a physical change to the EGU that increases capacity and allows a unit to burn more fuel is correct, then 

changing coal type does not satisfy this criterion. As such, reviewer suggests that the reference to changing coal 

type be caveated with a clarification indicating that changing coal type does not constitute an EGU modification.

EPA Response: Such a caveat has been added to the preamble, stating that changing coal type, without a physical 

or operational change within the meaning of section 111(a)(4), does not constitute an EGU modification.