POWER June 2012 - 40

AIR QUALITY
pecially with regard to the required analyses
for sources subject to NNSR and PSD review.
Evolution of PM Test Methods
Evolution of the form of the PM NAAQS over
the years has necessitated periodic development
of new or revised stack test reference methods
(RMs) by the EPA to provide representative
emissions measurements. The accuracy of measured
emission rates needed for compliance assessment
is highly dependent on the adequacy
of the RMs. However, as described below, the
measurement of condensable PM emissions
continues to be a bit more problematic than the
measurement of filterable PM emissions.
The history of these PM measurement methods
begins in 1971 with promulgation of EPA
RM 5, following on the heels of promulgation
of the TSP NAAQS earlier that year. In 1978,
the EPA promulgated RM 17 as an alternative
method for measuring PM, where the stack gas
PM concentration is known to be independent
of temperature. Both RMs 5 and 17 rely on a
filter media to capture PM for emissions quantification,
thus measuring filterable PM.
With promulgation of the PM10 NAAQS
in 1987, a new method was needed to quantify
PM10 emissions specifically because RMs 5 and
17 provided a PM measurement that included all
size fractions. Therefore, in April 1990, the EPA
promulgated RM 201/201A as the stack test
method for filterable PM10 (see 40 Code of Federal
Regulations [CFR] Part 51, Appendix M).
The EPA recognized that condensable PM
emissions can be a significant fraction of the
actual total PM10 emissions from combustion
sources that was to be measured. As a result, in
December 1991, the EPA promulgated RM 202
(see 40 CFR Part 51, Appendix M). RM 202 is
typically used in conjunction with RM 5 or RM
201A to measure total (filterable plus condensable)
PM. The RM 202 apparatus actually is
the " back half " of the RM 5 or 201A sampling
equipment train.
In the originally promulgated version of RM
202, exhaust gases were bubbled through waterfilled
impingers, and the solution was analyzed
in a laboratory to quantify the organic and inorganic
PM fractions (typically, for combustion
sources, the inorganic fraction predominates,
with sulfates constituting most of this fraction).
However, numerous studies conducted
both inside and outside the EPA indicated that
noncondensable gases such as SO2
could react
in the water solution to form condensable PM
(sulfates) that would not have otherwise formed
in the stack. The formation of this sulfate artifact
or " pseudo-particulate " resulted in elevated
RM 202 stack test results. In some cases, when
combined with the filterable PM test results,
the stack test results yielded total PM10/PM2.5
emission rates that exceeded permit limits.
(Note that, in many instances, permit limits were
40
Orifice
Bypass
valve
Manometer
Dry gas
meter
based on filterable PM10/PM2.5 emission rates
only.) Thus, many sources were required to reopen
permits to revise PM emission limits and/
or conduct further compliance demonstrations,
at significant time and cost to the permittees.
The promulgation of the more restrictive
PM2.5 NAAQS in 2006, a revision of the
PM2.5 NAAQS released in 1997, put increased
emphasis on improving stack gas PM measurement
methods for the smaller size fraction. As a
result, in December 2010, the EPA promulgated
revised versions of RMs 201A and 202. Changes
to RM 201A required mainly hardware additions,
most notably a PM2.5 cyclone and a
PM2.5 filter after the PM10 cyclone to enable
the measurement of filterable PM2.5, in addition
to filterable PM10. RM 202 was modified
to incorporate a condenser and " dry impinger "
setup in the back half of the sampling train. The
equipment used in this test method is illustrated
in Figure 1. RM 202 sample recovery and processing
procedures also were revised, mainly
to incorporate existing procedures in the RM
that had been optional. EPA research studies
showed that these revisions to RM 202 would
reduce sulfate artifact formation by at least 90%
(75 FR 80118).
Despite the EPA's recent efforts to improve
the precision of RM 202, questions remain as
to the ability of the method to produce representative
PM measurements, especially for natural
gas-fired combustion sources. This issue is
critically important given the recent favoring of
gas-fired combustion turbines for baseload generation
and for new power plants.
Test results for natural gas-fired units indicate
that nearly all of the PM (filterable and
condensable) in the exhaust gas is smaller than
PM2.5. Comprehensive studies cosponsored by
federal and state government agencies and industry
have shown that actual emissions from
natural gas-fired units are extremely low, probably
near ambient background concentrations
of PM2.5 in many cases. Such levels are below
the estimated minimum detection limits and
lower quantification limits of not only RM 202,
but RM 5 and RM 201A as well; therefore, total
PM emission levels are difficult to quantify with
any confidence.
Increasing method sampling run times to
several hours (to collect more PM mass) may
help to improve the representativeness of the
test results; however, this is not a cure-all solution,
and significant uncertainty remains with
these RMs.
PM2.5 Emission Factors
Stack test results, while serving as an important
means of demonstrating compliance with
permit emission limits for existing sources,
also form the basis of emission factors used
in the permitting of new sources. As such,
test results are also a key source of uncertainty
in any permitting-related requirement
that relies on emission factors (for example, a
NAAQS compliance demonstration).
The basis for the selection of PM2.5 emission
factors used by permit applicants should
be thoroughly understood before proceeding
with regulatory assessments involving the
use of such factors. Emissions can either be
based on readily available, accepted emission
factor references, such as AP-42, or be based
on actual stack test results for a specific type
1. Chemistry lesson. This illustration shows the EPA Reference Method 202 sampling
components required to measure condensable PM. Source: EPA
CPM filter
(≤30C/85F)
Condenser
Thermocouple
Temperature
sensor
Check valve
Water bath
(≤30C/85F)
Ice
bath
Temperature
sensors
Recirculation
pump
Empty
impingers
Main
valve
Pump
Silica gel
impinger
Vacuum
gauge
Vacuum
line
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POWER | June 2012
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POWER June 2012

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