United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory «/"t.-"
Research Triangle Park NC 27711 '/ f v
Research and Development
EPA-600/S2-84-096 July 1984
&ER& Project Summary
In Situ Field Portable Fine
Particle Measuring Device
Robert G. Knollenberg
An in situ fine particle measuring
device—the Fine Particle Stack
Spectrometer System (FPSSS) has
been developed. It is a laser-fed optical
system with detection by near-forward
light scattering. Sample volume is
established by a high-resolution optical
system viewing particle images'in a
dark field through a masked beam split-
ter. The FPSSS covers an 0.5 to 11.0
um size range with 60-channel resolu-
tion. Absolute theoretical accuracy is
+20% of size for completely unknown
refractive index. The instrument is
designed to operate continuously at in-
stack temperatures up to 250°C at flow
velocities up to 30 m/sec. It has been
laboratory characterized and field
tested on coal-fired power plants at
both the inlets and outlets of control
devices. Its performance indicates good
agreement with impactors and excel-
lent agreement with opacity meters in
computed mass loading and optical
opacity. Its size resolution is greater
than other currently known techniques.
Its eventual use will be directed at char-
acterizing particulate emissions of
stacks or other stationary sources and
qualitatively evaluating the perform-
ance and collection efficiencies of
particulate control devices now in
operation.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory. Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The development of the in situ Fine
Particle Stack Spectrometer System
(FPSSS) was intended to characterize the
particulate emissions of stacks and other
stationary sources and to qualitatively
evaluate the performance and collection
efficiencies of particulate control devices
now in use or under development. The
primary EPA design criteria that the
FPSSS had to satisfy were to provide in
situ high-resolution particle sizing over a
size range from 0.5 to 5.0 fim diameter,
with number densities in this range
above 104 cm 3 and integrated particulate
loading from 0.3 to 3.0 g-3. The FPSSS
was also required to operate at tempera-
tures from 20 to 250°C and at flow
velocities from 1 to 30 m sec~1. The full
report describes the design, develop-
ment, and testing of this new high-resol-
ution in-stack particle size spectrometer
system.
Approach
Probably the greatest desire of
research in performing particle size
measurements is to do so without
disturbing the particles; i.e., making the
measurements in,situ. In some respects
the concept of in situ sampling is
unattainable; one desires to minimize
sample perturbation. As far as single
particle sizing devices are concerned, this
ability to size particles in situ generally
requires additional imaging technology to
dimension sample volume.
The FPSSS design draws heavily from
previous work. In essence, the FPSSS is
the result of using existing technology
rather than the result of basic research
and development. It has a near-forward
light-scattering optical system with an
expanded 60-channel 0.4 to 11.0 um size
range divided into four subranges of 15
size channels each. The instrument is
capable of relatively accurate measure-
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ments at number densities up to 5 x 104
cm~3, without significant sensitivity to
refractive index. An optical velocimeter
has been designed and incorporated in
the FPSSS. The instrument can operate
continuously at 250°C temperatures
utilizing a water-cooled head design and
external heat exchanger.
Extensive theoretical modeling of
thermal and optical performance has
been utilized in configuring the FPSSS
probe head. Wind tunnel facilities played
an extremely important role in measuring
aerodynamic impacts of the FPSSS
sampling section. Calibration included
laboratory and wind tunnel tests on par-
ticulates having known or independently
verifiable size distributions. Field tests on
three operating coal-fired power plants,
conducted during the course of this work,
are discussed in detail.
Field Tests
Field tests were conducted at the
Colorado Public Service Company's
Valmont power plant in Boulder, CO, and
at Duke Power Company's Riverbend
Plant at Charlotte, NC. The Valmont plant
was the primary test site, with a variety of
short tests during early development as
well as final testing with comprehensive
comparisons.
Conclusions and
Recommendations
From the results of theoretical analysis
and laboratory and field testing, the
FPSSS is considered to be satisfactory for
in situ size distribution measurements in
a hot stack environment:
There remain a number of unanswered
questions that further use of the FPSSS
in the field should clarify. For instance,
there is the possibility that large particles
might be rejected more easily if
turbulence indicates significant trajec-
tory deviations from normal flow. Users of
the FPSSS in situations where large
particles are dominant would be wise to
rotate the probe head and determine if
any position (other than normal to the
flow) maximizes count rate at large sizes
and leave it in that position if it does.
The stability of the internal velocimeter
is questionable;its accuracy is probably
10-15%. It may well be the limiting factor
in the overall accuracy of the basic
measurement system. Unlike impactors,
which only measure velocity to attempt to
provide isokinetic matching but then
accurately meter the actual sample flow,
the FPSSS is an in situ device requiring//?
situ flow measurements as accurate as
the final desired result. Again,
improvements in the circuitry could
alleviate instrumental sources of error,
but little can be done if the laser beam
width is not stable. The laser beam width
is, of course, directly related to the transit
time and (thus) velocity computation.
Multimode lasers can undergo temporal
fluctuations in output beam diameter
amounting to at least 10%. Thus, it is
probably impossible to achieve better
accuracy without additional optical hard-
ware. For instance, a separate detector
(photodiode and beam splitter) could be
used to measure transit time across a
mask. As long as the mask is always
smaller than thejaser beam, fluctuations
in the laser beam diameter would be a
factor. Another attractive possibility is to
use a laser operating in TEMoo mode, a
doughnut-shaped beam profile where
size is invariant, ft is also a smaller beam
that could a I low for measurements at still
higher number densities without
changes to the optical system.
Finally, it should be recognized that the
FPSSS is new and highly sophisticated,
with more opportunities for failure than
with conventional techniques. It becomes
contaminated with time, requiring
cleaning. It is thus viewed primarily as a
research or investigative tool and not a
routine monitoring device. As is the case
for all measurement processes, an
experienced operator is the best
guarantee of successful use. The real
utility of a device like the FPSSS is not in
extending sensitivity to the smallest
particles or in having the ability to handle
the highest number densities. Rather, it
provides measurements of sufficient
spectral quality to gain insight into pro-
cesses that produce changes in particle
size and number density and it retains
reasonable verisimilitude in integrated
properties such as mass loading and
opacity.
R. G. Knollenberg is with Particle Measuring Systems. Inc.. Boulder. CO 80301.
D. Bruce Harris is the EPA Project Officer (see below).
The complete report, entitled "In Situ Field Portable Fine Particle Measuring
Device." (Order No. PB 84-199 793; Cost: $20.5O, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, MA 22161
Telephone: 7O3-487-465O
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park. NC 27711
•ti U.S. GOVERNMENT PRINTING OFFICE: 1984 — 759-O15/7738
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