POWER June 2012 - 42

AIR QUALITY
of emission source under specific operating
conditions.
Caution is required when selecting emission
factors from AP-42. These emission factors are
based on stack test results, but those test results
could be representative of either a very limited
set of emission units under specific operating
conditions or a very broad set of emission units
under different operating conditions.
Many permit applicants that use an AP-42
emission factor do not realize that data more directly
applicable to their specific emission sources
and operating conditions may be available for
evaluation as a more appropriate or representative
emission factor. In many instances, permit
applicants rely on vendor-provided (sometimes
referenced as " guaranteed " ) emissions data as
an alternative to AP-42. Such data may be more
representative of the particular emission unit
proposed for a project; however, such data may
not be specific enough for the operating conditions
associated with the proposed unit. To the
extent possible, an emissions factor should be
evaluated based on actual operating parameters
(such as emission controls on or off), operating
modes (for example, load level and startup/shutdown),
and fuel type associated with the testing
program(s) that generated the emission factor.
Permit applicants should carefully select
PM2.5 emission factors, particularly in light of
the historic limitations of RM 202. EPA AP-42
emission factors currently are based on test results
for the " old " version of RM 202; the use
of such factors may yield nonrepresentative,
overly conservative PM2.5 emissions estimates
due to the sulfate artifact issue discussed earlier.
Vendor-supplied emissions data and even emissions
data based on the recently revised RM 202
may appear to be more appropriate but should
still be closely scrutinized, especially data for
natural gas-fired combustion units.
Once the basis for the PM2.5 emission rates
is understood, complex regulatory assessments
such as NSR applicability evaluation, emissions
offsets quantification (for sources subject to
NNSR), and a NAAQS compliance demonstration
(for sources subject to PSD review) can be
conducted as accurately and effectively as current
understanding and technology will allow.
In addition, for existing, permitted sources of
PM10/PM2.5, understanding the basis for the
emissions data can be important when negotiating
revised permit limits that account for condensable
PM emissions.
NSR Applicability Evaluations
Sources to be permitted in an area that is in
attainment of the NAAQS for a particular
pollutant must be evaluated in terms of potential
applicability of PSD review rules for
that pollutant, while sources to be permitted
in an area that is in nonattainment of the
NAAQS for a particular pollutant must be
evaluated in terms of potential applicability
of NNSR rules for that pollutant.
For a new or modified source, projected
emissions are compared against regulatory
threshold levels in the NNSR or PSD review
rules to assess the applicability of such rules;
therefore, the representativeness of emission
2. Few counties not in compliance. This map shows counties designated nonattainment
for the PM2.5 NAAQS (1997 Standard and/or 2006 Standard). Nonattainment areas are
indicated by color. When only a portion of a county is shown in color, it indicates that only that
part of the county is within a nonattainment area boundary. Source: EPA
factors (which are used to calculate emissions)
can be critically important in determining
when an NNSR or PSD permit is
required. This is especially so for emission
factors for condensable PM, which now must
be included with emission factors for filterable
PM in calculating PM2.5 emission rates
for applicability assessments.
Both the NNSR and PSD permitting programs
contain requirements for PM2.5 that
may be challenging and onerous if projected
emissions exceed the regulatory threshold
levels. The potential ramifications of having
to comply with the requirements of NNSR
(emission offsets quantification) and PSD review
(NAAQS compliance demonstration) are
explored further in the following sections.
NNSR Requirement: Emission Offsets.
Under the CAA, all major sources and major
modifications at existing major sources within
a nonattainment area must obtain emissions
reductions to offset any emissions increases resulting
from the project that are at least equal to
the emissions increase. Figure 2 shows areas of
the U.S. that are in nonattainment of the 24-hour
PM2.5 NAAQS. Emission " offsets " are generally
purchased from another industrial source in
the same vicinity that has voluntarily overcontrolled
emissions or shut down emission units.
For direct PM2.5 emissions, the EPA established
that the ratio of total actual emissions
reductions to the emissions increase must be
at least 1:1, although a state may establish a
higher offset ratio in its state program (73 FR
28321). Additionally, states may adopt NNSR
rules that would allow direct PM2.5 emissions
to be offset with reductions in PM2.5 precursors
in accordance with offset ratios contained in an
approved SIP for the applicable nonattainment
area. The cost of offsetting emissions has increased
with the recent requirement to account
for the condensable PM component when calculating
PM2.5 emissions. An overly conservative,
unrepresentative PM2.5 emission rate can
result in additional increased costs to obtain the
required excessive number of emission offsets.
With limited availability of emission offsets,
project delays can result as applicants attempt to
locate and negotiate the acquisition of an excessive
number of offsets for a project.
Notably, the EPA will be proposing revisions
to the PM2.5 NAAQS in the summer
of 2012, and indications are that the EPA administrator
will propose to lower the annual
PM2.5 NAAQS from the current value of 15
μg/m3
to somewhere in the range of 11 to 13
μg/m3 (while most likely leaving the 24-hour
Designated nonattainment
1997 PM-2.5
Both 1997 and 2006 PM-2.5
2006 PM-2.5
NAAQS unchanged). With this tightening of
the annual NAAQS, more areas of the country
could be designated as PM2.5 nonattainment
areas, potentially increasing the number
of future permitting projects that will need to
obtain emission offsets.
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POWER June 2012

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