POWER July 2017 - 36

ENVIRONMENTAL ISSUES
Particle Sizing in Emissions
Samples by Scanning Electron
Microscopy
With low particulate loading common in stack emissions these days, particulate
matter measurement can be difficult using traditional techniques. Microscopical
measurement of particle-size distributions offers an alternative
because it can be performed on very light sample collections, allowing
particle-size determination of Environmental Protection Agency Method 5
samples that weigh as little as 0.25 milligrams.
Tim B. Vander Wood, PhD
M
any stacks now have such a low particulate
loading that the traditional
aerodynamic methods of particlesize
distribution (PSD) determination using
cascade impactors or cyclones may require
sampling for many hours in order to obtain
sufficient material mass for accurate weighing.
Even with long sampling times, total
particulate collections of only a milligram
(mg) are becoming common.
There are also some stack-sampling situations
where cascade impactors or cyclones
simply will not fit into the stack, may be degraded
by high temperatures, or are rendered
unusable by the presence of liquid droplets.
In such cases, the determination of PSDs in
the stack emissions for engineering studies
or compliance tests can be more easily accomplished
by sizing the particulate matter
(PM) collected by traditional methods, such
as the Environmental Protection Agency's
(EPA's) Method 5 Determination of Particulate
Matter Emissions from Stationary
Sources. Furthermore, the difficulties that
can be encountered with Method 201A-
Determination of PM10
and PM2.5
Emissions
from Stationary Sources (Constant Sampling
Rate Procedure) have led a number of states
and provinces to accept microscopy-based
particle sizing in cases where Method 201A
cannot be applied.
The most common analytical tool used to
measure the PSD of stack samples collected
on Method 5 or other filters is the scanning
electron microscope (SEM). A high-quality
standard SEM can be used to determine PSDs
down to about 0.3 micrometers (µm), or as
small as 50 nanometers (nm), if a specialized
field emission scanning electron microscope
is employed. Visible light microscopy may
be used if the minimum particle diameter of
36
concern is no less than 1 µm, and transmission
electron microscopy can be used if particle
diameters less than about 50 nm (0.05
µm) are of interest.
Particulate Matter Sampling
Considerations
PSD measurement by SEM is most often
used in conjunction with Method 5 (or 5B)
samples, and sometimes with Method 17
samples. For these types of samples, the filter
used in sampling determines the subsequent
SEM analytical approach.
The ideal filter is polycarbonate, which
comes in a variety of pore sizes. The most
common pore size used in stack sampling is
0.4 µm. Smaller pore sizes (0.2 µm and even
0.1 µm) are sometimes used.
Polycarbonate filters are preferred because
they are nearly featureless. The lack
of features means that automated image processing
analysis tools can be used to distinguish
particles from the filter background.
Unfortunately, polycarbonate filters are temperature
sensitive and will melt at 300F, so
care must be taken to prevent overheating
during collection.
Sampling flow rate can also be an important
consideration for polycarbonate filters,
particularly when smaller pore sizes
are
used, because filters can blank under heavy
load. Fortunately, very light sample loadings
are most appropriate for PSD measurement,
and samples experiencing increasing pressure
drops across the filter are already more
heavily loaded than is optimal.
A rule of thumb for appropriate sample
loading leads to an estimate of optimal sample
collection times of about 20% of the time
required to obtain a significant decrease in
flow rate or increase in pressure drop across
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1. Particles collected are visually
apparent. A secondary scanning electron
microscopy (SEM) image of stack particulate
matter on a polycarbonate filter is shown
here. Courtesy: MVA Scientific Consultants
2. Fibers trap particles. This SEM image
shows stack particulate matter collected
on a quartz fiber filter. Courtesy: MVA Scientific
Consultants
the filter. An underloaded filter is generally
preferable to an overloaded filter.
In sampling situations where a polycarbonate
filter cannot be used due to high temperatures,
quartz fiber filters are generally
POWER | July 2017
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POWER July 2017

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