POWER July 2015 - 53

ENVIRONMENTAL ISSUES
1.4, last updated in 1998):
■ The filterable PM emission factor has a
" B " rating.
■ The condensable PM emission factor for
external combustion sources was based on
only four tests and has a " D " rating (below
average quality).
■ The total PM emission factor rate has a
" D " rating (below average).
Combustion Turbines (AP-42 Section
3.1, last updated in 2000):
■ The filterable, condensable, and total PM
emission factors, based on only three tests
conducted in the mid-1990s at one facility,
all have a " C " rating ( " average " ).
According to the EPA, the same emission
factors can be used to estimate PM2.5 or
PM10 because all PM from natural gas combustion
is smaller than 2.5 micrometers.
The rating information shows that the
overall quality of the AP-42 PM emissions
factors is considered average at best. It
should be noted that the ratings do not take
into account that the test results used to develop
these emission factors were from testing
that was performed based on the 1991
version of Method 202, before revisions to
improve accuracy of the method were promulgated
and when stack testers were likely
not fully aware of the issues associated with
measuring condensable PM from gas-fired
combustion units. Based on current understanding
of the reference test method inadequacies,
a current quality assessment would
be expected to result in an even lower rating
of the AP-42 factors.
The net effect of these inadequacies is
that the AP-42 emission factors for PM are
expected to yield significant overestimates
of emissions for a specific gas-fired unit,
especially as it relates to condensable PM
emissions.
Research on PM Measurement
Methods and Emissions
A comprehensive research program, cosponsored
by the New York State Energy
Research and Development Authority (NYSERDA),
the U.S. Department of Energy,
the California Energy Commission, General
Electric Energy and Environmental Research
Corp., Gas Research Institute, and American
Petroleum Institute (API), was conducted
between 2000 and 2004 to evaluate the
representativeness of PM test methods and
emission factors for gas-fired boilers, heaters,
combustion turbines, and engines ( " NYSERDA/API
studies " ). Source testing during
the program was conducted using EPA referJuly
2015 | POWER
ence test methods and the research-scale dilution
sampling (DS) method. This research
culminated in the publication of a series of
technical, topical reports in 2004 entitled
" Development of Fine Particulate Emission
Factors and Speciation Profiles for Oil- and
Gas-Fired Combustion Systems. "
Versions of the DS method have been used
for more than 40 years to collect emission
samples from various source types. It was
adopted as the regulatory standard method
for determination of PM emissions from
mobile sources such as heavy-duty diesel
engines. Dilution sampling typically involves
extracting a sample from the stack, diluting it
with purified ambient air or pure compressed
gases, and then obtaining samples with ambient
air collection and analysis methods.
Because the sample is cooled to ambient
temperatures, aerosol formation conditions
approximately simulate those in actual exhaust
plumes. The DS method was used to
more accurately characterize emission rates
and chemical speciation profiles for PM2.5
and gaseous precursors in the NYSERDA/
API studies.
For the NYSERDA/API studies, the duration
of each test run for all test methods was
six hours to help ensure the collection of sufficient
particulate mass to increase measurement
precision. Note that uncertainty in test
method results can be reduced by extending
the sampling time period-that is, increasing
the sampling volume-for each run from
the minimum-required one-hour run to a
multiple-hour run. As acknowledged by the
EPA, collecting sufficient weighable mass is
important for the methods to be precise, but
interestingly, the EPA leaves it up to regulatory
authorities to dictate the level of precision
needed.
In general, the DS method test results from
the NYSERDA/API studies indicated that
substantially all of the PM in the stack was
smaller than 2.5 micrometers. The PM2.5
minimum detection level associated with the
DS method is far lower than can be achieved
by the EPA reference test methods. This is
primarily because the DS method has less inherent
biases and greater analytical sensitivity.
Therefore, the DS method can accurately
measure the actual emissions, which are far
lower than the total PM measured by the EPA
reference test methods.
The NYSERDA/API studies results for
natural gas-fired boilers and heaters and turbines
are summarized in Figure 2. The EPA's
AP-42 emission factors are also shown for
comparative purposes. With regard to PM2.5
emission factors based on EPA test methods,
the factors from the NYSERDA/API studies
are significantly smaller than the AP-42 emission
factors. This is not surprising, given that
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2. PM2.5 emissions research findings.
The bar chart compares PM2.5 emission
rates from the NYSERDA/API research
studies with AP-42 emission factors for boilers/heaters
and turbines. Courtesy: Zephyr
Environmental Corp.
■ NYSERDA/API dilution SM ■ NYSERDA/API EPA
methods ■ AP-42 (EPA methods)
! " !#!!
0.0100
:@A)/B
B+48F-.
:@A)/B
)%C &2
! " !!#!
0.0010
C%IJ'
3)%C &
! " !!!#
0.0001
:; 7-+42<,6=2012<, :; >> ?8<5+.2,
NG CC turbines
NG boilers/heaters
testing for the NYSERDA/API studies was
conducted under very controlled conditions.
The total PM2.5 emission factors for the DS
method test results are about an order of magnitude
smaller than the total PM2.5 emission
factors for the EPA test method results from
the NYSERDA/API studies. This indicates
that the EPA test methods results are inherently
biased high, even when those methods
are performed with extended sampling times
under well-controlled conditions.
Applications of Alternate PM
Emission Factors
Soon after publication of the NYSERDA/API
studies, both regulatory agencies and the regulated
community used the study results as
a basis for alternate and more representative
PM emission factors for gas-fired combustion
sources. This section discusses examples
of the use of alternate PM emission factors
over the past 10 years.
Use of Alternate PM Emission Factors
by Regulatory Agencies. In August 2005,
the EPA stated that it intended to revise the
PM emissions for gas combustion units in the
final version of the 2002 National Emissions
Inventory (NEI). The reason for the adjustment
was that the agency believed that the
current AP-42 emission factors for condensable
PM emissions were too high.
The EPA based its emission factor adjustments
on the NYSERDA/API studies results
for gas-fired combustion units; however,
specific information on the assumptions and
methodology used in developing these adjustments
is not available. Adjustments were
made to emission factors for combustion
units such as boilers, turbines, and engines
used for a broad range of purposes, including
power generation and industrial processes.
At the request of the Region 5 states, the
53
PM2.5 emission rate (lb/MMBtu)
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POWER July 2015

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