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Final Transport Rule Response to Comment (RTC) Document

Docket ID No. EPA-HQ-OAR-2009-0491

Transport Rule IPM Assumptions Response to Comments

U.S. Environmental Protection Agency

Office of Air and Radiation

July 2011
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Background

IPM is a multi-regional, dynamic, deterministic linear programming model of the U.S. 

electric power sector. It provides forecasts of least cost capacity expansion, electricity dispatch, 

and emission control strategies while meeting energy demand and environmental, transmission, 

dispatch, and reliability constraints. IPM can be used to evaluate the cost and emissions impacts 

of proposed policies to limit emissions of sulfur dioxide (SO2), nitrogen oxides (NOX), carbon 

dioxide (CO2), and mercury (Hg) from the electric power sector. The IPM base case serves as the 

starting point against which policy scenarios such as the Transport Rule are analyzed and

compared. The model has over 1 million constraints and 13 million variables. EPA used the 

IPM model in both its proposed and final Transport Rule analysis, providing the public with the 

opportunity to review and provide input on the model at several points leading up to final rule 

promulgation. Specifically, the public had the opportunity to review the universal IPM 

assumptions (e.g., demand, fuel costs) and source-specific assumptions where the commenter 

may have superior knowledge (e.g., retrofit status at a particular source). EPA reviewed these 

comments received on model parameters, and made significant model updates before conducting 

final Transport Rule analysis.

EPA’s July 6, 2010 proposed Transport Rule was based on the EPA IPM v3.02 modeling 

platform. EPA had a sixty day comment period from the time which the rule was published in 

the Federal Register (closing October 1, 2010) in which it accepted feedback on this modeling 

platform. EPA noted at the time that it would be updating its modeling platform, and would 

release the updated platform for further review before final rule promulgation. Subsequently, 

EPA issued a Notice of Data Availability (NODA) on September 1, 2010 with the updated IPM 

v4.10 modeling platform. EPA had a 45 day comment period on this NODA which closed 

October 15, 2010.

During each of these comment periods, sources submitted data intensive comments with 

corrections and suggested updates to the IPM modeling platform to be used for the final 

Transport Rule. EPA reviewed these comments, and where technically substantiated, made 

updates to the IPM modeling platform used for the analysis in the Final Transport Rule. These 

updates included both universal comments affecting broad categories of generating units (e.g., 

change to modeling of removal efficiencies for projected FGD retrofits), and source specific 

comments (e.g., change to the FGD retrofit status at a particular plant). EPA focused specifically 

on incorporating comments that met four criteria: 1) had sufficient documentation and 

argumentation supporting them, 2) would have a significant impact on the model, 3) could be

reasonably incorporated into the model, and 4) EPA did not have significant data contradicting 

the commenter suggested value. This document and the accompanying spreadsheet file 

“NEEDS1Source-Specific Adjustments in EPA Base Case v.4.10_FTransport.xlsx” serve as a 

response to IPM source-specific comments2. It summarizes below the type of source specific 

 

1 NEEDS (National Electric Energy Data System), is a modeling input that contains data on all existing and planned 

electric generating units (EGUs) that factor into EPA’s IPM modeling.

2EPA’s response to the universal comments on IPM can be found in the separate report Documentation Supplement 

for EPA Base Case v.4.10_FTransport – Updates for Final Transport Rule (EPA 430-K-11-004, June 2011). These 

responses to universal comments include updates to modeling of cogeneration units, NOx Rates, SO2 removal rates, 

coal switching – bituminous to subbituminous, restrictions on coal choice in 2012, and waste coal cost.
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modeling comments received during the comment period and subsequent modeling updates 

made in response to these comments.3 It describes in general, the source-specific updates EPA 

made to its model regarding each parameter. However, it should be noted that comments varied 

in quality, specificity, context, and support, and the “general” response outlined below may not 

apply to all comments submitted in that parameter field. Each comment was evaluated against 

the four criteria outlined above.

The spreadsheet file that accompanies this document catalogues in detail the sourcespecific model adjustments that were made. The spreadsheet lists each unit, its NEEDs unique 

ID, plant name, plant type, the parameter on which a commenter submitted a correction, the 

original value of that parameter in the IPM version reviewed by commenter, the corrected value 

submitted by commenter, and the final value in EPA IPMv.4.10 used in EPA’s final Transport 

Rule analysis. See example in Table 1 below:

Table 1: Example IPM Comment as listed in 

Source-Specific Adjustment Table

Unique 

ID

Plant 

Name

Plant 

Type

Parameter 

Commented

NEEDS 

Value in the 

Commented 

Version

Value 

Proposed 

by the 

Commenter

Final 

NEEDS 

Value

2094_B_1 Sibley

Coal 

Steam

NOx 

PostCombControl No SNCR SNCR SNCR

“NEEDS Source-Specific Adjustments in EPA Base Case v.4.10_FTransport” is available in Excel format in the 

Transport Rule Docket.

Useful Links

The documentation for the v4.10 model (including the Documentation Supplement for 

EPA Base Case v.4.10_FTransport – Updates for Final Transport Rule, EPA 430-K-11-004, 

June 2011) is available at http://www.epa.gov/airmarkets/progsregs/epa-ipm/transport.html.

1) Source-Specific Comment descriptions and EPA General Response

EPA received significant source-specific comment. These comments could typically be 

divided into two classes; 1) comments on the source at the model input stage (e.g., NEEDS), 2) 

comments on the source at the model output stage (e.g., parsed files). In regards to the later, 

EPA generally did not make source-specific adjustments to its modeling output based on 

comment. For example, if a cogeneration source commented that it disagreed with IPM’s 2014 

projected dispatch for its unit because they felt the heat input should be value x whereas IPM 

projected value y, EPA did not adjust the value from y to x. Rather, EPA examined the model 

 

3 The table also includes some source-specific adjustments included in the v.4.10 F_Transport IPM modeling made 

in response to additional input received from sources through non-Transport Rule related channels (e.g., “Utility 

MACT ICR (ICR No. 2362.01: OMB Control Number 2060-0631)).
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output for this technology class as a whole for a system-wide bias, and, if identified, made 

adjustments to its modeling approach for the technology. These types of changes are reflected in

the universal adjustments noted above and discussed in the Documentation Supplement for EPA 

Base Case v.4.10_FTransport – Updates for Final Transport Rule. These universal adjustments

were EPA’s general approach to addressing source-specific comments on EPA’s projections. 

Again, EPA notes that it rarely made source-specific adjustments to projected values in response 

to comment. EPA also notes that in certain cases, EPA did not identify or find reasons to adjust 

its modeling approach. IPM is a least cost model with perfect foresight. As commenters noted, 

projections from this approach may vary from actual future values at particular sources as the 

model may not capture non-economic variables. Furthermore, IPM does not purport to be 100% 

accurate when projecting future operations at the source-level. The projections are reasonable 

approximations of what sources can be expected to do under typical economic constraints. 

However, there are inherent differences between predicted value and actual future values in any 

predictive model.

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In regards to source specific comments at the model input stage (i.e., NEEDS), EPA did 

make extensive source-specific updates to its IPM model. These comments were generally 

pointing out a discrepancy between a commenters understanding of a unit’s current 

configuration, and how the units configuration is depicted in EPA’s National Electric Energy 

Data System (NEEDS) database. NEEDS is meant to reflect the current configuration (or 

configuration as of December 31, 2011) of the U.S. Power Sector and is the primary input for 

unit level data in the IPM v.4.10 modeling. Since NEEDS is an approximate snap shot of the 

power sector, EPA was sensitive to comments that suggested this depiction was not consistent 

with the current configuration as observed by the commenter. EPA reviewed NEEDS in light of 

these comments and made updates accordingly. 

For the reasons stated above, EPA adjustments to source-specific assumptions in the IPM 

model generally occurred in the NEEDs database. By focusing on changes to model inputs, as 

opposed to outputs, EPA made changes to existing data instead of projected data. EPA believes 

these types of changes which focus on model inputs are more appropriate as they are generally 

fact based. EPA also notes that by making these source-specific adjustments to the modeling 

inputs and then re-running the IPM model, their impact on model projections is captured in the 

final Transport Rule analysis. 

Below, EPA highlights some of the key unit level parameters in NEEDS to which EPA 

made changes in response to comments received. The actual source-specific changes themselves 

can be seen in the accompanying table titled “NEEDS Source-Specific Adjustments in EPA Base 

Case v.4.10_FTransport” available in the Transport Rule Docket.

“BCGI” adjustments

 

4 Commenter’s noted that the residual, while typically minimized at regional and state levels as source-specific data 

is aggregated, could be significant at the source level. Therefore, in addition to making model updates, EPA revised 

its allocation methodology in the final Transport Rule to be based purely on historic data (as opposed to projected, 

source-specific model data used in proposal). By basing the allocation methodology in historic data in the final rule, 

EPA limits the degree to which source-specific modeling projections (and potential discrepancies between source 

projected and actual future data) impact a source’s allocation.
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NEEDS has a column indicating whether a unit is a boiler (B), generator (G), or 

committed unit (C). Some commenters submitted a correction to a unit’s classification in 

this data field. EPA reviewed and generally accepted these comments. The attached 

table indicates for which units these types of changes were made.

CAMD ID

NEEDS contains a field with a unit-level identifier assigned by EPA’s Clean Air Markets 

Division (CAMD) business system. Unlike other identification codes (e.g., ORIS codes), 

which are subject to change, once assigned to a unit, the CAMD database ID does not 

change. It is used primarily for internal tracking and data analysis purposes at EPA. 

EPA made several source-specific changes to this field that reflected assigning a number 

where one had not previously been assigned.

Capacity

Shows the net summertime dependable capacity (in MW) of the unit available for 

generation for sale to the grid. Net summer dependable capacity is the maximum 

capacity that the unit can sustain over the summer peak demand period reduced by the 

capacity required for station services or auxiliary equipment. To the extent that EPA 

received comment correcting these values, it generally accepted them and the source 

specific changes are reflected in the attached table as well as the final IPM modeling. In 

cases where the comment was unclear or data was provided without definitively 

suggesting a change, EPA exercised discretion in incorporating the comment. 

Cogen

This field indicates whether a unit is a cogenerator. A unit is considered a cogenerator 

for IPM purposes if it produces electricity and another form of useful thermal energy 

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

Some commenters suggested corrections to this field in NEEDS for a particular unit. 

EPA reviewed, and generally accepted these comments.

Dispatchable SCR

NEEDS contains a field that indicates a one-time option that allows certain generating 

units in the model to install a SCR retrofit at zero capital cost (see IPM documentation for 

more details). The “dispatchable” status of a retrofit in IPM modeling reflects that the 

control would only run if provided with appropriate economic incentive. Some

commenters suggested a change to the “dispatchable” status of their retrofit. For 

example, a source may have pointed out that its retrofit has to run regardless of economic 

incentive because of its permit limit or consent decree, and therefore should be 

considered “non-dispatchable”. In these cases, EPA reviewed the unit in question in 

concert with existing, non Transport Rule state or Federal requirements that may 

influence its dispatchable status. Upon review, EPA updated its “dispatchable” status to 

be consistent with the explanation as described in the IPM documentation.

Dispatchable FGD

NEEDS contains a field that indicates a one-time option that allows certain generating 

units in the model to install a FGD retrofit at zero capital cost (see IPM documentation 
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for more details). Some commenters suggested a change to the “dispatchable” status of 

their retrofit. For example, a source may have pointed out that its retrofit has to run 

regardless of economic incentive because of its permit limit or consent decree, and 

therefore should be considered “non-dispatchable”. In these cases, EPA reviewed the 

unit in question in concert with existing, non Transport Rule state or Federal 

requirements that may influence its dispatchable status. Upon review, EPA updated its

“dispatchable” status to be consistent with the explanation as described in the IPM 

documentation.

Firing

NEEDs contains a field, which applies only to boilers, which indicates the burner type 

and configuration. Some commenters suggested a correction to the burner status of their 

boiler. EPA generally accepted these comments.

Heat Rate

NEEDS contains a field that indicates the net heat input (in BTU) required to generate 1 

kilowatt hour of electricity. It is a measure of a generating unit’s efficiency. Some 

commenters submitted corrected heat rate values which EPA generally accepted. Note -

some heat rate values were flagged as accepted, but the final NEEDS value still remains 

different from that noted by commenter. This indicates that EPA accepted the comment, 

but still made necessary adjustments to translate it from the form in which it was 

presented by commenter to the form in which it is presented in NEEDS. One example 

would be making adjustments to account for net or gross heat rate, depending on how the 

value was presented in the comment.

Mercury Controls

NEEDS contains a field that indicates the mercury emissions control in existence at the 

unit. This field generally is blank, or has an “ACI” indicator to suggest that activated 

carbon injection is present at the unit. Some commenters submitted a correction to the 

status of a mercury control at their plant. EPA generally accepted these comments and 

adjusted the value in NEEDS accordingly.

Mercury Control Efficiency

NEEDS has an indicator for the removal efficiency of the mercury control device. Some 

commenters submitted a correction to this efficiency. EPA generally accepted these 

comments and updated the efficiency in the final version of NEEDS.

Modeled Fuels

NEEDS has an indicator that list the fuels that are reported to be combusted by the unit. 

Some commenters submitted a correction to the fuels listed. In some cases, they 

submitted an additional fuel, and in other cases noted that a fuel should be removed. 

EPA generally accepted these comments and updated the final NEEDS to reflect the type 

of fuels combusted by the unit according to the commenter.

New Unit Comment

NEEDS attempts to capture new units committed to come online in the near future. 

These are units that are typically already under construction. Some commenters noted 
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that certain new units were incorrectly left out of NEEDS. EPA reviewed and updated 

NEEDS where appropriate to reflect these new units.

NOX Control

NEEDS has an indicator for the combustion controls employed by a generating unit. 

Some commenters corrected the initial combustion control status listed in NEEDS. EPA 

generally accepted these comments, and updated NEEDs to reflect the comment.

NOX Rate Comments

NEEDS contains four indicators that relate to a unit’s NOX emission rate under various 

conditions. Many commenters submitted corrected data. However, EPA was already 

updating the NOX rates to reflect the most recent 2009 data (most recent at time of 

update), and therefore used the 2009 reported data in most cases instead of commenter 

specific data. These updates generally brought the rates closer to those values suggested 

by the commenter. EPA felt that using 2009 ETS data to update NOX rates as expressed 

in NEEDS was superior to relying on commenter data. It had the advantages of 

promoting consistency and accuracy among all sources, and the data is high quality as it 

is reported by the sources designated representative who testified to its accuracy and 

completeness. In instances where EPA did not have ETS data for particular units in 

NEEDS, it did accept commenter data on NOX emission rates if provided as it 

represented the best available data. 

NOx Post Combustion Control

NEEDs contains an indicator for the post combustion NOx control status at a unit (e.g., 

SCR, SNCR). Some comments submitted corrections to this status as initially proposed 

in NEEDS. EPA generally accepted these comments, and updated the final NEEDS to 

reflect the NOX post combustion status of the unit as recommended by the commenter. 

EPA notes in instances where commenter pointed out that the control was not 

present/complete at time of comment (e.g., October, 2010), but was expected to be 

completed by 2012, it was still reflected in NEEDS.

Online Year

NEEDS has an indicator that list the year in which the unit was commissioned. Some 

commenters submitted corrected dates. EPA generally accepted these comments. EPA 

notes that changes to the online year do not impact the projections, so the changes were 

primarily illustrative from a modeling standpoint. Even though these changes did not 

meet EPA criteria of “having a significant impact on the model” they were still generally 

included in the model due to their ease of implementation and their ability to improve the 

accuracy of certain analyses of model outputs.

ORIS

NEEDS contains a unique identifier for each power plant. Some commenters corrected 

the ORIS code for particular units. EPA generally accepted these comments.

Plant Name
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NEEDS identifies each plant with a plant name. Some commenters noted a correction to 

the name. EPA generally accepted and made these changes. However, EPA notes this 

does not impact modeling projections and is for identification and data analyses purposes 

only.

PM Control

NEEDS contains a field that indicates the presence of particular matter (PM) controls 

(e.g., baghouse, electrostic precipitator, etc). Some commenters provided a correction to 

the status of its PM controls has identified in NEEDS. EPA generally accepted these 

comments.

Retirement Year

NEEDs has an indicator for the year in which a unit is to be decommissioned (“9999” 

indicated no know retiring data). Some sources provided a corrected retirement date 

which EPA generally accepted and incorporated into its modeling. If the source reported 

the unit had already been retired at the time of comment, then EPA removed the unit 

from its NEEDS inventory of units. If the retirement was planned or announced for a 

future date according to the commenter, then EPA incorporated that date into its 

“retirement year” indicator in NEEDS, and the unit still appears in the NEEDS inventory.

However, the unit would not be available for dispatch in model projections for years on

or after the updated retirement year.

Scrubber Online Year

NEEDS provides the year in which a scrubber was installed at a unit. Some sources 

submitted corrected online years. EPA accepted this data and adjusted the final NEEDS 

to reflect the comment. However, EPA notes that an adjustment of the online year of an 

FGD in NEEDs does not impact the modeling projections provided the pre and post 

adjustment values are both dates earlier than 2012.

Scrubber Efficiency

NEEDS provides the scrubber efficiency for each existing FGD. EPA received many 

comments that suggested corrected values for these efficiencies. EPA made universal 

adjustments to its IPM assumed FGD removal efficiencies for existing FGDs in NEEDS 

by basing the values on historic removal efficiencies for the control as reported in EIA 

form 860 (2008) as described in the Documentation Supplement for EPA Base Case 

v.4.10_FTransport – Updates for Final Transport Rule of this document. This change 

was made in response to the comments on removal efficiency, and brought the efficiency 

used in NEEDS to, or closer to, levels suggested by the commenter. However, EPA 

notes that where EIA 860 reported values conflicted with those provided in comments, 

EPA generally relied on the EIA 860 reported values to promote consistent treatment of 

removal efficiencies among scrubbed units.

SO2 Permit Rate

NEEDS has an indicator for the SO2 emission rate limit that applies to the unit due to 

federal, state, or local emission regulations. Some commenters submitted corrections to 

permit limits listed in earlier versions of IPM. EPA reviewed and generally accepted 
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these corrections. The values in the final NEEDS file were updated to reflect the 

comments. The permit rates listed in the NEEDS file reflect data obtained from EIA 

form 757, reviews of state regulations, and/or comments/corrections submitted by 

stakeholders.

Unit ID

NEEDS contains a unit identifier assigned to each unit boiler at a given plant. Some 

commenters submitted the correction to this unit ID so it would be more consistent with 

how the unit was reporting or recognized at that facility. EPA generally accepted these 

comments. However, EPA notes that such changes have no impact on the model 

projections.

Wet Dry Scrubber

NEEDS has an indicator that suggest whether a scrubber is a wet scrubber or a dry 

scrubber. Some commenters suggested corrections to the wet/dry classification of a 

particular scrubber. EPA reviewed and generally accepted these comments.

Other Key IPM Source-specific & Non-NEEDS Parameters on Which EPA Accepted Comment

New Unit

Some sources noted that a particular unit had not been included in the inventory of EGUs 

listed in NEEDS. EPA reviewed, and where appropriate added unit level records to 

reflect these units. These adjustments are noted in the third table of this documents 

companion file. The file is titled “NEEDS Source-Specific Adjustments in EPA Base 

Case v.4.10_FTransport.xlsx”, and the third worksheet in the file is titled “New Unit 

Comments”

Dry Sorbent Injection (DSI)

Many commenters noted the presence of dry sorbent injection at a particular facility that 

was not correctly captured in NEEDS. The EPA IPMv.4.10_FTransport used for the 

final rule modeling does account for DSI in NEEDS and allow for DSI retrofit in the IPM 

outputs. DSI retrofit is labeled as such in the IPM output files. The IPM NEEDS file 

reflects the presence of DSI operationally, but not nominally. That is the DSI presence at 

a unit is indicated by the unit having a removal efficiency, but not having a “scrubber 

efficiency value” - typically 70% or less – but not having a scrubber reflected in the 

“wet/dry scrubber” field or in the “dispatchable scrubber” field. There is no field in this 

final Transport Rule NEEDS file that flags DSI at a unit, but it can be identified through 

the approach described above.

Primary Fuel and Prime Mover

EPA received comments on these two parameters that are factored into the IPM

modeling, and generally made adjustments to its model in instances where sufficient 

evidence was provided.

State Rules and Consent Decrees
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EPA updated the State Rules and Consent Decrees that are reflected in the final Transport 

Rule IPM modeling based on comments received. These updates often involve sourcespecific rate and/or control requirements that influence facility behavior in IPM modeling 

outputs. See IPM documentation referenced above for a complete list of such State Rules 

and Consent Decrees that are captured in the updated IPM modeling.

2) Tables of Source Specific IPM adjustments in NEEDS 

Spreadsheet file “NEEDS Source-Specific Adjustments in EPA Base Case 

v.4.10_FTransport.xlsx” contains three tables in different worksheets. The first, 

labeled “Comments” is presented in the format described above and contains the bulk 

of the source-specific adjustments reflected in the final Transport Rule IPM 

modeling. The second, titled “NOX Rate Comments”, reflects those comments that 

did result in adjustments to NOX rates. As noted above, such adjustments were 

typically only made when 2009 ETS data was not available. The third, titled “New 

Unit Comments”, reflects units that were added to the NEEDS inventory of EGUs 

based on comments. The second and third tables are presented in a format similar to 

the first, but with slight modifications based on the nature of the comment and 

subsequent adjustments.