United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/003 Apr. 1985
&EPA Project Summary
The Relationship of Fly Ash
Composition, Refractive
Index, and Density to
In-Stack Opacity
S. J. Cowen and D. S. Ensor
The refractive index, density, and
composition of fly ash from coal-fired
boilers was investigated. The goals were
to determine: (1) the interrelationship
of refractive index and composition,
and (2) the significance of ash properties
on in-stack plume opacity. A survey
was made of 14 ash samples repre-
senting a wide range of coals. Light
absorption was measured using the
Integrating Plate Method, which com-
pares light absorption through a clean
filter to that through a filter with a single
layer of aerosol. Ony absorption is meas-
ured, while scattered light is integrated
equally for both cases. This technique
requires fine particles (volume absorb-
ers) for easy interpretation of results.
The technique was calibrated using an
aerosol, methylene blue, with known
absorption characteristics. The real part
of the refractive index was measured by
an oil immersion technique. The real
refractive index and density were found
to be highly correlated with composition
with a multilinear regression equation.
The absorbing refractive index was well
correlated with ash carbon content.
The modeling of in-stack opacity
showed a weak dependence on ash
optical properties for the range of ashes
studied .The effect of the real part of the
refractive index on opacity tends to be
counterbalanced by particle density
effects. Furthermore, except for very
high carbon ashes, fly ash absorbs
relatively little light.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Opacity limitations of air pollution
emissions are an important aspect of
stationary source regulations. However,
the engineering approaches needed to
design control equipment to meet opacity
limits are still under development. Ensor
(1972) established the relationship of
plume opacity to mass concentration as a
function of log-normal particle size distri-
butions of the emission for ideal condi-
tions. More recently, it was determined
that the particle size distributions down-
stream of emission control equipment are
usually not log-normal. As a result,
development of calculator programs to
allow prediction of expected plume opac-
ity from particle size distribution data and
mass concentration was the topic of a
study by Cowen, Ensor, and Sparks
(1980). However, the secondary variables
affecting plume opacity (e.g., particle
density and refractive index) have not
been systematically measured under
conditions useful for plume opacity pre-
diction The usual practice has been to
assume nominal values of refractive index
and particle d&nsity.
The present research has two objec-
tives: (1) to measure refractive index and
particle density with supporting chemical
composition for a wide selection of ashes
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from utility boilersfiring bituminous coal,
and (2) to compute the sensitivity of
plume opacity to the variation of particle
refractive index and density using these
data. The survey nature of this study
allowed the use of bulk samples taken
from ash hoppers at utility plants.
In no previous investigations have all
the physical properties required for plume
opacity prediction been measured oh the
same sample of fly ash. Examples of
typical fly ash characterization research
include determination of the chemical
composition of ashes from the major coal
fields in the U.S. by the Bureau of Mines
(Gluskoter et al., 1977) and measure-
ments of composition and particle density
in size-segregated samples for a single
sample of fly ash by Fisher et al. (1978). It
should be noted that the refractive index
of a material is a complex number: the
real part describes the bending of light,
and the imaginary part describes the
absorption of light with subsequent con-
version to heat in the material. Two totally
different measurements are required to
determine this physical property. The
measurement of real refractive index is a
fairly common practice for mineral and
small particle identification (McCrone et
al., 1967). Limited measurements of
absorptive refractive index have been
reported for fly ash: Volz (1973) measured
absorption of fly ash in the infrared
wavelengths, and Nolan (1977) reported
the visible light absorption of ash samples
at a university heating plant boiler under
limited conditions with the Integrating
Plate Method (IPM). The IPM, developed
by Lin et al. (1973), involves measuring
light transmission through the particle
sample on a membrane filter and an opal
glass diffuser. The reduction in the light
transmission can be related to the ab-
sorption coefficient and the imaginary
refractive index. Thus, an important con-
tribution of the present research is that a
battery of previously developed analytical
methods were used on a fairly wide range
of ash samples. Many of the ash samples
were also used by Bickelhaupt (1979) to
develop mathematical relationships be-
tween electrical resistivity and chemical
composition.
Conclusions and
Recommendations
A survey of the refractive index, density,
and composition of 14 different fly ash
samples indicates significant interrela-
tionships between their physical and
chemical properties. A regression equa-
tion was developed with the specific
refractivity [(n'-1 )/p] as a function of the
major chemical components of the ash.
The regression explains 82 percent of the
variability of the data. The imaginary
refractive index was found to be corre-
lated to carbon content of the ash, which
explains 80 percent of the variability of
the data. The imaginary refractive exper-
iments were limited by the accuracy and
sample restrictions of the IPM. However,
modeling of opacity with typical fly ash
size distributions and the ranges of
refractive indices indicates that the ab-
sorbing refractive index is insignificant.
Although pure soot is an effective material
for attenuating light, it is usually not of
sufficient quantity in boiler emissions to
alter the particle size distribution for
maximum effect on plume opacity. Opac-
ity was more sensitive to particle density
than real refractive index for computa-
tions using the extremes of each.
The IPM is semiquantitative and pro-
vides results with sufficient accuracy to
fulfill the objectives of this study. How-
ever, the technique suffers from several
sources of error; e.g., an apparent absorp-
tion of known transparent materials from
scattered light, and alteration of the
reflection of the filter surface by the
aerosol sample. Because the method is
restricted to submicrometer particles, the
ash samples required size segregation.
Thus, the chemical composition of the
total sample may not be representative of
the samples used for the absorption
measurement. This problem may be
responsible for the scatter observed in
the imaginary index carbon correlations.
The errors in the method were minimized,
however, by the use of an empirical
correlation developed with a laboratory
aerosol.
Continued work would refine the rela-
tionship of fly ash specific refractivity to
chemical composition. The data base
should be expanded to include a wider
range of coal ranks and types such as
lignite and anthracite. The results of this
effort could lend support to studies cor-
relating mass emissions to plume opacity.
At present, additional work is not
justified on the aerosol-absorbing refrac-
tive index because the relatively large
size of fly ash tends to minimize its
effects. However, absorption studies may
be worthwhile for sources with submi-
crometer emissions. Identification of
these processes would be a useful appli-
cation of the techniques developed in this
study.
References
Bickelhaupt, R. E.(1979): ATechniquefor
Predicting Fly Ash Resistivity. EPA/
600/7-79/204 (NTIS PB 80-102379).
Cowen, S. J., D. S. Ensor, and L. E. Sparks
(1980): TI-59 Programmable Calculator
Programs for In-Stack Opacity, Venturi
Scrubbers, and Electrostatic Precipita-
tors. EPA/600/8-80/024 (NTIS PB 80-
193147).
Ensor, D. S. (1972): Smoke Plume Opacity
Related to the Properties of Air Pollutant
Aerosols. Ph.D. Dissertation, University
of Washington, Seattle, WA.
Fisher, G. L., B. A Prentice, D. Silberman,
J. M. Ondov, A. H. Biermann, R. C.
Ragaini, and A. R. McFarland (1978):
Physical and Morphological Studies of
Size Classified Coal Ash. Environ. Sci.
Technol., 72,447-451.
Gluskoter, H. J., R. R. Ruch, W. G. Miller,
R. A. Cahill, G. B. Dreher, and J. K.
Kuhn (1977): Trace Elements in Coal:
Occurrence and Distribution. EPA/
600/7-77/064 (NTIS PB 270922).
Lin, C., M. Baker, and R. J. Charlson
(1973): Absorption Coefficient of At-
mospheric Aerosol: A Method for
Measurement. Appl. Optics, 12, 1356-
1363.
McCrone, W. C., R. G. Draftz, and J. G.
Delly (1967): The Particle Atlas. Ann
Arbor Science Publishers, Ann Arbor,
Ml.
Nolan, J. L. (1977): Measurement of Light
Absorbing Aerosols from Combustion
Sources. M.S. Thesis, University of
Washington, Seattle, WA.
Volz, F E. (1973): Infrared Optical Con-
stants of Ammonium Sulfate, Sahara
Dust, Volcanic Pumice, and Fly Ash.
Appl. Optics, 12. 564-568.
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S. J. Cowen is with Atmospheric Research Group, Altadena, CA 91001. andD. S.
Ensor is with Research Triangle Institute. Research Triangle Park, NC 27709.
Leslie E. Sparks is the EPA Project Officer (see below).
The complete report, entitled "The Relationship of Fly A sh Composition, Refractive
Index, and Density to In-Stack Opacity," (Order No. PB 85-169 860/AS; Cost:
$10.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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