United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-096 Aug. 1981
Project Summary
Modification of Optical
Instrument for In-Stack
Monitoring of Respirable
Particle Size
A. L. Wertheimer
A light scattering instrument for in-
situ measurements of particulates in
the 0.2 to 20 micrometer diameter
size range is described, and field test
results are presented. The instrument
is a modified version of a prototype
built during a prior EPA contract.
Number 68-02-2447. The upper limit
of the size response has been extended
from 10 to 20 micrometers, and several
component and packaging changes
have been incorporated to make the
unit more suited to stack particulate
survey applications. Low forward
angle and 90° polarization dependent
scattering is employed to make the
measurements.
The completed instrument was tested
at a coal-fired electric power generat-
ing facility. During the test a cascade
impactor was used as a referee device
and both instruments were run side by
side in the outlet duct of the electro-
static precipitator.
The results show an excellent cor-
relation between the two instruments
with regard to the identification of a
"\fjrn diameter peak in the particle size
distribution. A second peak around 20
/urn was defined by the optical instru-
ment, but could not conclusively be
confirmed through the impactor data.
The optical instrument handled well
during the field test and was delivered
to EPA for additional testing.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle 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
A prototype real-time in-situ monitor
was developed and constructed on EPA
Contract 68-02-2447 to measure particle
size distribution of respirable particles
in the 0.2 to 10 /t/m range. The purpose
of this project was to add a channel to
cover the 15 yum size range so as to
include the upper cut-off of the inhalable
particulate emissions from stationary
sources.
The addition of the large particle
channel required a series of changes in
the optical and electronic assemblies of
the original instrument. In the process
of incorporating these changes, the
latest available components were se-
lected and packaging improvements
were made, resulting in an instrument
optimally suited for survey work and
stack particulate analyses. The new
instrument measures the size distribu-
tion in the 0.2 to 20 yum range in five size
fractions, using a low power helium
neon laser light source.
The modified prototype instrument
was tested at a coal-fired electric gen-
-------
erating plant. Referee measurements
were made with a cascade impactor.
Both instruments reported a strong
peak in particle size around one /um in
diameter.
Procedure
Principles of Operation
The instrument was designed by
using simple diffraction theory for the
low angle forward scattered light, and
rigorous Mie theory for the light scattered
at 90° to the probe beam. By adding high
angle scattering capabilities, the use of
light scattering for particle analysis can
be extended to the sub-micrometer size
range.
The stack particulate monitor mea-
sures the light scattered by particles
passing through a 2.5 cm by 36 cm slot
at the end of a 152 cm (5 foot) long
probe. The light source is a 2 milliwatt
helium neon laser, which emits a co-
herent beam at 0.6328 //meters. The
scattered light signals are proportional
to the volumes of particulate material
present in each of five size fractions. Six
scattered light readings are taken at
precisely determined angles. The light
signals are acquired through fiber optic
cables and transmitted to detectors
located in the transceiver. A digital
microprocessor calculates a five chan-
nel, volume-by-size histogram, covering
the size range from 0.2 //m to 20 /um.
Modification of the Prototype
Modification of the original instru-
ment to add a 15/um channel involved a
number of significant changes. When
appropriate, these changes were made
so as to accommodate improvements
suggested from field trial experience
with the first unit. The pertinent aspects
of the new design are discussed in the
following paragraphs.
The xenon arc source was replaced by
a low power (2 milliwatt) helium-neon
laser, which provides better collimation
of the source, and eliminates a trouble-
some electrical transient starting prob-
lem. A slightly larger collection lens
system was designed to accommodate a
wider range of forward scattering angles.
However, the 90° collection system
used in the earlier unit remains the
same.
A beam alignment sensor was added
to the tip of the probe to monitor any
thermally induced shifts. Through ports
accessible from the rear of the probe,
the beam can be aligned in or out of the
stack by maximizing the reading on a
meter adjacent to the adjustment ports.
A Z-80 microprocessor system re-
placed the original 8008 based elec-
tronics. The new system allowed for
rapid and efficient implementation of
the hardware and software changes
required in modifying the unit. The new
electronics is much more compact than
the earlier version, and is combined
with a small digital printer in a 20 pound
transportable electronics console. A
second, smaller box, contains the elec-
tronics power supply, packaged sepa-
rately to avoid heat build-up on the
control console box.
A summary of operational character-
istics of the prototype is shown in Table
1. The measurement time can be set by
the user and ranges from 5 seconds to
12 minutes. Immediately following the
data collection, the size distribution is
printed out at the console.
Calibration
The calibration process involved
several steps and used a variety of
materials. To properly fill the sample slot
region under operating conditions sim-
ulating a flowing gas stream, an aerosol
test chamber was constructed in the
laboratory.
The major steps of the calibration
process are outlined here.
(1) During assembly, the light collect-
ing apertures were checked for
alignment and adjusted to insure
that the correct angles were being
measured.
• (2) Di-octyl phthalate (OOP), a trans-
parent liquid with an index of
1.49, was dispersed as a droplet
suspension in the aerosol test
chamber by a Phoenix Precision
Aerosol Generator. This created a
well-controlled size and loading of
particles in the 0.2 to 3 /um size
range. From the measured signal
levels and knowledge of the load-
ings, detector gain adjustments
were made to accommodate a
uniform distribution of particles at
40 parts per billion.
(3) The collection geometry and fiber
transmission product at each
angle was determined by measur-
ing fresh, filtered cigarette smoke.
Because the majority of the par-
ticulate volume is well below one
/um in diameter, the forward
scattering pattern does not change
with particle size. A correction
constant is thus defined for each
scattering angle, based on
difference between scattered light
strengths observed and those
predicted by theory.
Results
Laboratory Tests
As a check for consistency, the instru-
ment was then used to measure the
aerosol distributions employed to cali-
brate it. Figure 1 shows the filtered
cigarette smoke distribution, indicating
a large percentage of the material in the
0.3 yum size channel, while Figure 2
shows the measured and manufacturer's
specifications for the OOP aerosol
suspension. In both cases, agreement
between expectation and observation is
quite good.
To further check the performance and
calibration, two other materials were
run, burning red phosphorous, and solid
glass spheres. The red phosphorous is
used for tactical smoke screens, but no
referee data was available. The instru-
ment readings indicated roughly equal
amounts of material in the 0.3 and 1.0
/urn size channels. This is consistent^
with its intended tactical use since par-V^
tides in this size range are the most
efficient scatters per unit volume and
thus provide good obstruction.
The solid glass spheres, from Potters
Industries, Inc., were used to check
performance of the larger size channels.
The spheres are specified as "3 to 10
micron" size, but no additional data was
provided or available. No material is
reported in the 0.3 /um channel, as
expected, and most of the material is in
the 3.5 or 7.5 fjim region. The material
reported in the 15 //m channel may be
caused by clumping of the beads due to
electrostatic charges introduced in the
suspension process. Microscopic exam-
ination of a bead sample collected
during the test confirmed this, showing
occasional clumping.
Field Test Performance
During July, 1980, the prototype
instrument was tested at an east coast
coal-fired electric power generating
station. L&N personnel used the proto-
type instrument to measure particle size
distribution in a duct leading to the
smoke stack. Personnel from Northrop
Services, Inc., Environmental Science
(NSI-ES), participated in the tests,
taking data with a cascade impactor, /*
and provided the necessary data analysis V
-------
Table 1. Operational Characteristics of Stack Paniculate Monitor
Size Range (Particle Diameter)
Size Discrimination
Mode of Operation
Loading Range
Measurement Time
Duct Velocity
Duct Temperature
Instrument Temperature
Power Requirements
Probe Dimensions
Sample Slot Dimensions
Transceiver-Probe Assembly
Control Console
Electronics Power Supply
Blower
Probe Material
0.2 to 20.0 urn
Five volume fractions with centers at 0.3, 1,0,
3.5. 7.5, 15 urn
Low angle forward scattering and 90°
polarization dependent scattering
0.01 to 1.0 grams of material/meter3 (.023
to 2.3 grains/ft3) or 4 to 400 pans/billion by
volume (with s.g. of 2.5)
Signal integration time selectable from 5
seconds to 12 minutes (including a 6-minute
position)
1.5 to 18 meters/second (5-60 feet/second)
260° C maximum (500° F)
2° C to 43° C (35 to 110° F)
One 20A, 115 volt, 60 Hz outlet
Physical Specifications
152 cm long (60 inches) by 9 cm diameter
(3'A inches)
2.5 x 36 cm (1 x 14 inches)
203 x 25x25 cm, 31.8 kg (80 x 10 x 10 inches.
70 pounds)
38x41 x 25 cm. 9.1 kg (15 x 16 x 10 inches,
20 pounds)
23 x 41 x 25 cm, 6.4 kg (9 x 16 x 10 inches,
14 pounds)
74x48x 43 cm, 22.7kg (29 x 19 x 17 inches,
50 pounds)
Type 316 Stainless Steel (except for optical
components)
for that method. Six separate data sets
were collected over two days. One set of
data from each day is presented here.
All testing was performed at the
outlet of the electrostatic precipitators
and prior to the final exhaust fan. The
testing section was a vertical flow duct,
approximately 32 1 /2 ft. wide by 7 ft.
deep. Sampling ports are located hori-
zontally across the wide side of the duct.
Each port is a 6-inch diameter flanged
pipe, approximately 14 in. long. Two
adjacent ports were selected as test
points. A summary of the stack condi-
tions appears in Table 2.
All aerodynamic particulate sizing
was performed using a University of
Washington Mark III Cascade Impactor
and necessary support equipment. Prior
to actual source testing, all in-stack
atmospheric measurements necessary
for isokinetic and other calculations
were recorded. Velocity head and stack
differential pressure measurements
were peformed using a type "S" pitot.
In-stack temperatures were measured
using a thermocouple system attached
to the end of the pitot tube. Velocity
profile measurements were made up to
4.5 ft. into the duct at both test ports,
with the impactor sampling conducted
at the point of both average velocity and
close proximity to the optical instrument.
The point used for sampling was ap-
proximately the mid-point of the duct or
4 ft. from the lip of the port flange.
The impactors were preheated to
stack temperature before sampling to
avoid moisture condensation within the
impactor body. The duration of each test
was varied according to the stack opacity,
knowledge that this coal unit was within
particulate emissions standards, and
the visual inspection of the previous
80-
70
60-
30-
20-
10-
.3 1.0 3.5 7.5 15
It
Figure 1.
Calibration run
cigarette smoke.
using
impactor test. Sample runs varied from
20 to 40 minutes in length.
Several hours were required to make
the preliminary measurements before
the impactors were inserted.
The prototype optical particle size
monitor was prepared within approxi-
mately one hour. All electrical cables
were connected and the instrument
was turned on to warm-up the electron-
ics. The optical alignment of the unit
was adjusted using the external meter.
The stack velocity, measured for the
impactor runs, was used to set the
purge flow rate on the blower. To facili-
tate insertion and removal from the
stack during the tests, a suspension rail
designed and built previously for this
unit by NSI-ES was erected. A typical
sample run lasted 6 minutes, and sev-
eral runs were made during the impactor
sample collection period.
On the first sampling day the boiler
unit was operating at maximum output.
On the second day, the boiler was
operating at reduced output, and the
particulate emissions were distinctly
lower, dropping from around 0.02g/Nm3
the first day, to 0.007g/Nm3 the second,
as measured by the impactor.
Results for both optical and inertia!
instruments are shown in Figures 3 and
4, plotted as histograms of volume
fraction per unit log interval of panicle-
size. The optical data in each figure are
indicated by the cross hatched histogram,
while the impactor data are shown as
the heavier outlined histogram. Varia-
tions in the individual channel widths
-------
so-
so-
70-
60-
| 50-\
30-
20-
10-
Table 2. Stack Conditions During Field Test
Measured
Manufacturer's
Spec.
Figure 2.
.3 1.0 3.5 7.5 15
Calibration using dibutyl
phthalate aerosol from
Phoenix generator.
are due to the different principles
involved in measuring the particle
distribution.
The impactor data, provided by NSI-
ES, were derived by plate weighings and
computer assisted data reduction. A
material density of 2.5g/cm3 was as-
sumed, and the channel edges were
based on the aerodynamic separation
properties of the individual stages of the
impactor.
During each impactor run, continuous
optical data measurements were made.
The histograms shown are compiled
from the time weighted average of the
sequential optical data, which involved
from 5 to 8 optical runs, depending on
the length of the impactor run. The
boundaries of the optical histogram are
determined by the instrumental response,
as calculated from scattering theory.
Discussion
The optical and inertia! measurements
agreed in some significant respects. In
all runs both instruments reported a
significant size fraction to be around
one fjm in diameter with, in most cases,
substantial reductions in the amount of
material above the one urn size. The
optical instrument consistently indi-
cated a good deal of material in its
largest size channel, which made the
distribution appear bimodal. This could
not be definitely confirmed by the
impactor data available, although im-
pactor runs from some tests show a
Stack gas velocity:
Stack gas temperature:
Gas pressure:
Direction of flow:
45 to 50 feet/second
230 to 300° F
-2 inches of Hg
Vertical downward
leveling off of the distribution, and run 4
does indicate a secondary peak in its
largest particle channel.
The general agreement between the
two methods is good. The size response
question could well be resolved through
further testing at other sites. There
were some relatively minor technical
problems, but none that should prevent
the optical instrument from being used
in other field tests. At the conclusion of
this test, the prototype stack participate
monitor and its associated equipment
were turned over to the EPA.
Conclusions and
Recommendations
The primary goal of this work was to
modify and test a prototype optical stack
particulate monitor by the addition of a
channel responding to particles in the
15 fjm size range. This was successfully
accomplished. Tests in the laboratory
showed results that agreed with ex-
pected size distributions of several
40-
sample materials which were in the 0.2
to 20 yum size range of the instrument.
The field tests, conducted at a coal fired
electric utility plant, provided size distri-
bution data which were in excellent
agreement with results reported by a
referee inertia! impactor. An additional
advantage with the optical instrument is
that size distribution data are computed
and displayed immediately upon the
conclusion of the signal collection
sequence.
A secondary goal of this project was to
improve the reliability and portability of
the instrument to make it more suitable
for stack survey work. The modified
prototype unit is lighter and smaller
than the original, and the operational
improvements, such as ease of align-
ment and reliability of operation, were
demonstrated during the field trial.
There may be some value in develop-
ing an on-site technique to provide the
operator with a quick means of checking
the calibration of the unit. Although
Run No. 1
Impactor
Optical Sizer
.2
Figure 3. Particle size vs. volumetric concentration distribution during Day 1 of
the field test.
-------
c
01
se
50-
40-
30-
< 20-
<
10-
Run No. 4
Impactor
Optical Sizer
.5 J 2
Diameter, D^irri)
i
5
20
Figure 4. Particle size vs. volumetric concentration distribution during Day 2 of
the field test.
there are no moving parts in the proto-
type which would affect the calibration,
some form of indicating calibration
status is desirable.
Another area for future consideration
is modification of the electronics to
optimize the gain for loadings at or
below the originally specified range of
0.01 to 0.1 grams/meter3. This could be
done by changing the feedback resistors
at the detector board and trimming the
electrical offsets to lower values.
In its present form, however, this type
of instrument should prove to be very
useful for field survey work for analysis
of size distributions from stationary
sources. Recommendations for future
work involve additional field trials at
sites with different types of fuel, clean-
up devices, and loading conditions. To
gain confidence in this type of instru-
mentation, measurements with referee
sizing instruments should be taken in
parallel.
> U.S. GOVERNMENT PRINTING OFFICE 1W1-757-012/7282
------- |