As we explain below, IEPA employed the recommended top-down method for determining BACT limits, which employs a five-step analysis. Because a number of the pollutant control technologies at issue in this case are potentially applicable to control emissions of several pollutants, we have organized our analysis of Petitioners' arguments corresponding to the five steps of the top-down method. We begin with a brief background summary of the top-down method and then proceed to discuss all issues raised at step 1, followed by the issues Petitioners raise at step 2, and so forth.

BACT is a site-specific determination resulting in the selection of an emission limitation that represents application of control technology or methods appropriate for the particular facility. In re Cardinal FG Co., 12 E.A.D. 153, 161 (EAB 2005); In re Three Mountain Power, L.L.C., 10 E.A.D. 39, 47 (EAB 2001); In re Knauf Fiber Glass, GmbH, 8 E.A.D. 121, 128-29 (EAB 1999); see also In re CertainTeed Corp., 1 E.A.D. 743, 747 (Adm'r 1982) (“It is readily apparent * * * that***BACT determinations are tailor-made for each pollutant emitting facility."). BACT is defined by the statute in relevant part as follows:

The term "best available control technology" means an
emission limitation based on the maximum degree of re-
duction of each pollutant subject to regulation under this
chapter emitted from or which results from any major
emitting facility, which the permitting authority, on a
case-by-case basis, taking into account energy, environ-
mental, and economic impacts and other costs, determines
is achievable for such facility through application of
pro-
duction processes and available methods, systems, and
techniques, including fuel cleaning, clean fuels, or treat-
ment or innovative fuel combustion techniques for control
of each such pollutant.

CAA § 169(3), 42 U.S.C. § 7479(3).9

9 The definition set forth in the regulations is nearly identical:

Best available control technology means an emissions limitation * * *
based on the maximum degree of reduction for each pollutant *
which would be emitted from any proposed major stationary source or
major modification which the Administrator, on a case-by-case basis,
taking into account energy, environmental, and economic impacts and
other costs, determines is achievable for such source or modification
through application of production processes or available methods, sys-
tems and techniques, including fuel cleaning or treatment or innovative
fuel combustion techniques for control of such pollutant.

40 C.F.R. § 52.21(b)(12).

In the present case, IEPA followed the NSR Manual's guidance for determining BACT using an approach known as the "top-down" method. Project Summary at 5. "The NSR Manual is not a binding Agency regulation and, as such, strict application of the methodology described in the NSR Manual is not mandatory." In re Cardinal FG Co., 12 E.A.D. 153, 162 (EAB 2005). "However, a careful and detailed analysis of the criteria identified in the regulatory definition of BACT is required, and the methodology described in the NSR Manual provides a framework that assures adequate consideration of the regulatory criteria and consistency within the PSD permitting program." Id.; see also In re Steel Dynamics, Inc., 9 E.A.D. 165, 183 (EAB 2000) ("This top-down analysis is not a mandatory methodology, but it is frequently used by permitting authorities to ensure that a defensible BACT determination, involving consideration of all requisite statutory and regulatory criteria, is reached."). The NSR Manual summarizes the top-down method for determining BACT as follows:

The top-down process provides that all available control
technologies be ranked in descending order of control ef-
fectiveness. The PSD applicant first examines the most
stringent - or "top" - alternative. That alternative is estab-
lished as BACT unless the applicant demonstrates, and
the permitting authority in its informed judgment agrees,
that technical considerations, or energy, environmental, or
economic impacts justify a conclusion that the most strin-
gent technology is not "achievable" in that case.

NSR Manual at B.2; see also In re Haw. Elec. Light Co., 8 E.A.D. 66, 84-92 (EAB 1998).

The NSR Manual's recommended top-down analysis employs a five-step analysis. The first step requires the permitting authority to identify all "potentially" available control options. NSR Manual at B.5. Available control options are those technologies, including the application of production processes or innovative technologies, "that have a practical potential for application to the emissions unit and the regulated pollutant under evaluation." Id.

The second step is to eliminate "technically infeasible" options from the potentially available options identified at step 1. NSR Manual at B.7. This second step involves first determining for each technology whether it is "demonstrated," which means that it has been installed and operated successfully elsewhere on a similar facility, and if not demonstrated, then whether it is both "available" and "applicable." Technologies identified in step 1 as "potentially" available, but that are neither demonstrated nor found after careful review to be both available and applicable, are eliminated under step 2 from further analysis. Id.

In step 3 of the top-down method, the remaining control technologies are ranked and then listed in order of control effectiveness for the pollutant under review, with the most effective alternative at the top. Id. A step 3 analysis includes making determinations about comparative control efficiency among control techniques employing different emission performance levels and different units of measure of their effectiveness. Id. at B.22-25.

In the fourth step of the analysis, the energy, environmental, and economic impacts are considered and the top alternative is either confirmed as appropriate or is determined to be inappropriate. Id. at B.29. Issues regarding the cost effectiveness of the alternative technologies are considered under step 4. Id. at B.31-.46. The purpose of step 4 of the analysis is to validate the suitability of the top control option identified, or provide a clear justification as to why the top control option should not be selected as BACT. Id. at B.26.

Finally, under step 5, the most effective control alternative not eliminated in step 4 is selected and the permit issuer sets as BACT an emissions limit for a specific pollutant that is appropriate for the selected control method. Id. at B.53; see also In re Hillman Power Co., 10 E.A.D. 673, 677 (EAB 2002); In re Three Mountain Power, LLC, 10 E.A.D. 39, 42 n.3 (EAB 2001).

B. The Permit's BACT Limits for NOx, SO2 and PM

Petitioners contend that IEPA made numerous errors in setting the BACT limits for controlling emissions of NOx, SO2 and PM. Specifically, Petitioners question at BACT step 1 whether IEPA improperly excluded low-sulfur coal from the BACT analysis as a method for controlling SO2 emissions. As part of our discussion of this issue, we also consider the related argument regarding whether IEPA erred in rejecting a detailed consideration of "alternatives" to Prairie State's proposed Facility. At step 2, Petitioners argue that IEPA clearly erred by rejecting Integrated Gasification Combined Cycle as a method for controlling SO2 and NOx emissions without a detailed step 4 analysis. Petitioners have not raised any issues arising at step 3 of the top-down method. At step 4, Petitioners argue that IEPA improperly rejected the use of coal washing as an additional method for controlling SO2, NOx and PM emissions. Finally, at step 5, Petitioners argue that IEPA erred in establishing the numeric BACT limits appropriate for the technologies selected to control emissions of SO2, NOx and PM. We address each of Petitioners' arguments below.

1. BACT Step 1: Identifying Potentially Available Control Options
(Low-Sulfur Coal); and the Authority to Consider "Alternatives"
that Redefine the Source

Petitioners argue that IEPA improperly rejected consideration of low-sulfur coal as a method for controlling emissions of SO2 from the proposed Facility.

Petitioners state that IEPA improperly treated low-sulfur coal as "redefining” the Facility proposed by Prairie State and that IEPA's rationale violates the statutory requirement to consider "clean fuels." Petition at 33. Petitioners also argue that IEPA improperly rejected consideration of "alternatives" to the Facility proposed by Prairie State, including alternative power sources such as wind or solar power and natural gas or low-sulfur fuel, and the alternative of limiting the Facility's size or prohibiting construction of the Facility.

a. BACT Step 1 Analysis of Low-Sulfur Coal

In the first step of the NSR Manual's five-step top-down method for determining BACT, the permitting authority must confirm that the applicant has identified "all control options with potential application." NSR Manual at B.5. An incomplete BACT analysis, including failure to consider all potentially applicable control alternatives, constitutes clear error and, therefore, is grounds for remand. In re Knauf Fiber Glass, GmbH, 8 E.A.D. 121, 142 (EAB 1999); In re Masonite Corp., 5 E.A.D. 551, 568-69, 72 (EAB 1994); In re Brooklyn Navy Yard Res. Recovery Facility, 3 E.A.D. 867, 875 (Adm'r 1992).

In the present case, Petitioners argue that IEPA's BACT determination is fatally flawed because it did not identify in the first step, and therefore improperly excluded from BACT consideration, several control alternatives that Petitioners contend are "potentially" applicable to Prairie State's proposed Facility. Petitioners argue, among other things,10 that IEPA improperly excluded low-sulfur coal from the BACT analysis as an additional SO2 control alternative that may be used in combination with the add-on controls proposed by Prairie State." Petition at 31-32. According to Petitioners, the record shows that if Prairie State "were to use low-sulfur coal, it could achieve an SO2 emission limit as low as 0.05 to 0.06 lb/MMBtu." Id. at 32. In contrast, the Permit would limit SO2 emissions to 0.182 lb/MMBtu.12 Permit ¶ 2.1.2(b)(ii).

10 Petitioners also argue that IEPA failed to consider as separate and distinct control alternatives the following: magnesium-enhanced lime scrubbers, the Chiyoda CT-121 bubbling jet reactor, and certain specified scrubber design enhancements. Petition at 56-58. IEPA's response to comments, however, demonstrates that these "scrubbing" methods share the same fundamental control process and are not sufficiently distinct to warrant separation analysis throughout the BACT review. See Response to Comments at 49 ("The fundamental issues for wet srubbers is setting the SO2 emission rate of level of control efficiency that a scrubber must be designed to achieve."). Petitioners have not demonstrated clear error in this response to comments and, accordingly, we will consider issues relative to these scrubber technology variations at step 5 where IEPA considered the appropriate numeric emissions limit applicable to the selected control process.

11 This issue was raised in the public comments. Response to Comments at 23-26.

12 Under the Permit's limit, the total potential SO2 emissions would be 11,866 tons per year. Calculation Sheet at 1.

IEPA acknowledged that "the high sulfur content of the design coal results in an SO2 emission rate that is substantially higher than that of power plants that are designed to use a coal supply with a low or very low sulfur content." Calculation Sheet at 5. Nevertheless, IEPA has consistently articulated its view that requiring Prairie State to use low-sulfur coal would "redefine" the project, and that it cannot require Prairie State to build a project different from the one Prairie State has proposed.

Prior to the public comment period, IEPA explained its reasoning as follows:

With respect to alternate sources of coal, e.g., low-sulfur
western coal from Wyoming or Montana, the proposed
plant is being designed and developed to burn high-sulfur
Illinois coal, the locally available coal. It would be incon-
sistent with the scope of the project to use coal from other
regions of the country. Rather, the BACT determination
addresses the appropriate control technology for SO2
emissions associated with use of this coal at the proposed
plant.

Project Summary at 8. Later, in responding to public comments, IEPA further explained as follows:

The project that must be addressed when evaluating
BACT is the project for which an application has been
submitted, i.e., a proposed mine-mouth power plant. The
source of coal for which the plant would be developed is a
specific reserve of 240 million tons of recoverable coal,
which would meet the needs of the proposed plant for
more than 30 years. Accordingly, the use of a particular
coal supply is an inherent aspect of the proposed project.
To require an evaluation of an alternative coal supply, as
suggested by this comment, would constitute a fundamen-
tal change to the project.

Response to Comments at 23. IEPA also stated in its response to comments that the Board's prior decisions "support the principle that a permitting authority should consider BACT for the project for which an application has been submitted and not 're-define the source." Id. at 24.

Petitioners argue that IEPA's Response to Comments is clearly erroneous on the grounds that it allegedly contravenes the "plain language of the definition of BACT and previous Board decisions." Petition at 33. Specifically, Petitioners argue that IEPA's rejection of low-sulfur coal at step 1 of the top-down BACT