## 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 th e 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 CO 2 emissions. The EPA has added a dditional 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 ho ur CO 2 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 capaci ty upgrades such that there is no net increase in hourly CO 2 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 the ir permitting au thority prior to undertaking potential modification s to ensure that they are aware of any changes in regulatory requirements resulting from the potential modification

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In addition, reviewer notes that changing coal types from bituminous to subbituminous c oal 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 revi e wer’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 clarificati on 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.