POWER July 2015 - 52

ENVIRONMENTAL ISSUES
1. Stack testing equipment. Equipment in the " front " of the sampling train is used for
measuring filterable PM10 and PM2.5 via Method 201A, while equipment in the " back " of the
sampling train is used for measuring condensable PM (CPM) via Method 202. Courtesy: Zephyr
Environmental Corp.
Front of sampling equipment train
PM 10
sizer
Filter
Holder
Stack
wall
Heated
probe
Nozzle
PM 2.5
sizer
S-type
pitot tube
Orifice
Temperature
sensors
Recirculation
pump
Manometer
Dry gas
meter
Bypass
valve
Pump
alternate PM emission factors in their regulatory
analyses.
This article concisely summarizes results
from a prominent PM2.5 emission measurement
research program for gas-fired combustion
units and discusses how these results
have been successfully used to support development
and application of alternate PM
emission factors by both regulatory agencies
and permit applicants.
Description of Stack PM Emissions
PM emissions from gas combustion that enter
the atmosphere directly from the stack-
primary PM emissions-are composed of
two components: filterable PM and condensable
PM.
Filterable PM is PM that is captured on a
filter media closer to the front of the stack
test equipment assembly. The EPA defines
condensable PM as material that is a gas at
stack temperature, but which condenses into
a solid or liquid particle within a few seconds
of leaving the stack. Condensable PM
is captured on a separate, special filter and in
impingers closer to the rear of the stack test
equipment assembly. All emitted condensable
PM is in the PM2.5 size fraction, with
most of the mass being inorganic material,
such as sulfates/sulfuric acid.
Total PM is defined as the sum of the filterable
and condensable PM components. Applicants
must use total PM emissions in their air
permit applications for any required NNSR
or PSD applicability assessment and NAAQS
compliance demonstration (separately for
PM10 and PM2.5, as applicable). These emission
rates ultimately become emission limitations
in the permit for that source.
52
PM Emissions Testing/Measurement
Background
Subsequent to permit issuance, any required
demonstration of compliance with PM2.5
and/or PM10 emission limits requires the
use of EPA-approved (reference) test methods
to measure the in-stack filterable and
condensable PM components. The reference
test methods typically used to measure stack
PM2.5 emissions are Method 201A (for the
filterable PM2.5 component) and Method
202 (for the condensable PM component).
Figure 1 depicts the stack test equipment assembly
for Methods 201A and 202
Traditional PM Emission Factor
Resource: AP-42
The EPA's AP-42 document provides
guidance to the regulated community and
regulators alike on the estimation of PM
emissions from a wide variety of industrial
sources, including different types of gasfired
combustion units. It includes emission
factors for both the filterable and condensable
PM components, developed using the
results from older versions of EPA reference
test methods.
The EPA characterizes the quality of an
emission factor based on two measures: the
quality of test data the factor is based on and
whether the factor can be expected to be repUse
of AP-42 factors result in a significant
overestimation of PM emissions from
gas-fired combustion units, which leads to
many unnecessary permitting and regulatory
planning complications.
The evolution of the form of the PM
NAAQS over the years, especially as it
relates to size fractions, has required the
EPA to revise its reference test methods.
(For a brief description of PM test methods
and a history of method development,
see the online supplement " A Brief History
of In-Stack PM Measurement, " associated
with this issue in the archives at powermag.com.)
As described below, even with
the codified updates to the reference test
methods, the measurement of PM emissions
has continued to be problematic. This
www.powermag.com
resentative of the typical emission source.
The agency's assessment of emission factor
quality is typically identified with the factor
in AP-42 by an assigned quality rating.
Per the EPA, an " A " rated emission factor
is the best available factor. As the factor ratings
drop to as low as an " E " rating, the reliability
of the emission factor is considered a
poorer quality.
The AP-42 emission factor ratings for PM
emissions from natural gas-fired combustion
sources follow.
Boilers and Heaters (AP-42 Section
POWER | July 2015
Empty
impingers
Main
valve
Silica gel
impinger
Vacuum
gauge
Vacuum line
Back of sampling equipment train
(<30 C/85 F)
Condenser
Wash bath
CPM filter
(<30 C/85 F)
Temperature
sensor
Check
valve
is evidenced by the EPA's April 8, 2014,
memorandum providing interim guidance
on revised sample quality control methods
to improve the accuracy of condensable
PM measurements.
In reality, the reference test methods
were not designed for the level of detection
needed for gas-fired combustion units. Research
studies have shown that, for gas-fired
units, actual PM2.5 emissions are extremely
low-probably near ambient air background
concentrations of PM2.5 in many cases.
Such emissions levels are difficult to quantify
with high confidence using the reference
test methods. At such low PM levels,
any foreign matter (such as residue on improperly
cleaned glassware used to collect/
store samples) that is inadvertently included
in the analysis can significantly bias the test
results high, leading to overestimates in PM
emission rates.

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POWER July 2015

Table of Contents for the Digital Edition of POWER July 2015