1. Rationale for Combining the Subpart Da Category and a 

Component of the Subpart KKKK Category into a New Category for 

GHGs

The EPA is proposing to create a new subpart in 40 CFR part 

60 by combining the sources in subpart Da (the Da category) and 

components of sources in subpart KKKK (the KKKK category) ––

combined cycle units, but not simple cycle units -- for purposes 

of promulgating standards of performance for emissions of GHGs

from new sources. This new subpart will be numbered TTTT. At 

this time, the EPA is not proposing subcategories for new 

sources and is not combining the Da category and components of 

the KKKK category for conventional pollutants. 

CAA section 111 provides legal authority for combining the 

categories into a new category. Clean Air Act section 

111(b)(1)(A) provides:

The Administrator shall, within 90 days after December 

31, 1970, publish (and from time to time thereafter 

shall revise) a list of categories of stationary 

sources. He shall include a category of sources in 

such a list if in his judgment it causes or 

contributes significantly to air pollution which may 

reasonably be anticipated to endanger public health or 

welfare.

(Emphasis added.)
As quoted, this provision grants to the Administrator 

the authority to “revise” the list of categories. Combining 

categories, in whole or in part, is a form of “revis[ing]” 

the list of categories (along with taking other actions, 

such as adding more categories or delisting categories),

and accordingly is authorized. 

It is reasonable for the EPA to combine the Da category and 

the combined cycle component of the KKKK category at this time 

for purposes of GHGs, for several related reasons. First, both 

types of plants (boilers and combined cycle units) perform the 

same function, which is to generate electricity. It is sensible 

to treat as part of the same category units that generate 

electricity, regardless of their design or fuel type. Second, 

although it is true that natural gas-fired plants are inherently 

less emitting than coal-fired plants, all newly constructed

sources have options in selecting their design. As a result, 

prospective owners and operators of new sources could comply 

with the proposed emission standards by choosing to construct a 

NGCC facility. These two factors provide sufficient legal 

rationale for EPA to combine the Da category and the combined 

cycle component of the KKKK category for purposes of GHGs. 

A third factor lends further support. Combining the 

categories does not raise adverse policy concerns. Based on 

comments made during the listening sessions, we anticipate that 
some commenters may express concern that combining the 

categories and applying the NGCC standard may limit construction 

of new coal-fired power plants, which would have adverse 

implications for energy diversity. We do not believe that this 

action would have those effects. As discussed below, modeling of 

the power sector, which is based on economic factors, forecasts 

no new coal-fired construction through the modeling period, 

which lasts until 2020. Accordingly, economic conditions may be 

expected to be the main driver precluding, or at least limiting, 

construction of coal-fired EGUs. Even so, we recognize the 

possibility that a limited amount of new coal-fired construction 

may nevertheless occur. Today’s action would not foreclose 

construction of new coal-fired EGUs. Rather, the small number of 

new coal-fired EGUs that may be expected to be built in the 

foreseeable future may install CCS control equipment (if not at 

the time of construction, then relatively soon thereafter) and, 

by doing so, achieve the same emission rate (at least over time) 

as a natural gas-fired EGU. As discussed below, CCS is both 

feasible and sufficiently available, although not the least-cost 

choice, as evidenced by the fact that power companies have 

announced plans to incorporate CCS at seven already permitted 

coal-fired EGU construction projects in this country (as we 

discuss below in section V.B., concerning transitional sources). 

Programs exist that provide some funding for CCS through pilot 
or other demonstration programs, and we expect those to 

continue. In addition, we reasonably expect the costs of CCS to 

decline over time. As discussed below, we are not proposing 

that CCS does or does not qualify as the “best system of 

emission reduction” that “has been adequately demonstrated.”

Rather, the feasibility of CCS and its availability for the 

limited amount of new coal-fired construction that may be 

expected, means that this action to combine the categories and 

establish the NSPS at the proposed emissions limit will not have 

adverse effects on new coal-fired construction or, therefore, on 

energy diversity. In fact, this action could promote coal-fired 

construction by eliminating the current uncertainty about the 

prospect of GHG controls. 

On the other hand, at this time, we do not consider it 

appropriate to include simple cycle facilities as an affected 

source in the new 40 CFR part 60, subpart TTTT for GHG emissions 

from new facilities. The reason for this is that the function of 

a new simple cycle power plant is different than that of a new 

combined cycle plant or coal-fired plant. Combined cycle plants 

and coal-fired plants are typically designed to provide baseload or intermediate-load power, while simple cycle turbines are 

designed to provide peaking power. Because combined cycle power 

plants and coal-fired power plants both serve the same purpose 

and have design options to emit CO2 at similar levels, we believe 
it is appropriate to combine them. Because peaking turbines 

operate less and because it would be much more expensive to 

lower their emission profile to that of a combined cycle power 

plant or a coal-fired plant with CCS, the EPA does not believe 

it is appropriate to include them in this source category.

By the same token, at this time, we do not consider it 

appropriate to combine the Da category and the combined cycle 

component of the KKKK category for any pollutants other than 

GHGs, that is, for conventional pollutants. This is because 

although coal-fired EGUs have an array of control options for 

conventional pollutants to choose from, those controls generally 

do not reduce their conventional emissions to the level of 

conventional emissions from natural gas-fired EGUs.