United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/084 Apr. 1988
Project Summary
Electrostatic Precipitator Efficiency
on a Multiple-Hearth Incinerator
Burning Sewage Sludge
R. C. Adams, G. Bockol, J. A. Maddox, and E. V. Robb
A pilot-scale electrostatic precipitator
(ESP) was evaluated for its removal
performance of 23 metals and for sulfur
containing particles when fitted to a
multiple-hearth incinerator burning
sewage sludge. The small-scale ESP
was installed to take a slipstream of
about 3% of the total incinerator emis-
sions. Particle size fractions were col-
lected from the gas streams entering
and leaving the ESP. Each particle size
fraction was analyzed for 24 elemental
species and ESP performance was
evaluated for overall removal efficiency,
size fraction removal efficiency, and
selective removal of specific metals.
Total concentrations of each element in
the controlled emission stream were
determined as well as the proportionate
concentrations of species in the solid
and volatile states. Concentrations of
each metal in the emission stream were
compared with the concentrations in
the sludge residue.
To obtain comparisons of ESP per-
formance with a more typical emission
control device, the performance of the
incinerator's full-scale wet scrubber was
also evaluated.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
Is fully documented In a separate report
of the same title (see Prefect Report
ordering Information at back).
Introduction
The purpose of this research project
was to determine the paniculate removal
efficiency of an electrostatic precipitator
(ESP) emission control system on a
multiple-hearth furnace burning sewage
sludge. Of particular interest was the fate
of metals found in a city/industrial type
of sludge that was incinerated and sub-
jected to ESP air pollution control. A
pilot-scale ESP was temporarily fitted to
an existing multiple-hearth furnace burn-
ing sewage sludge. A slipstream of in-
cinerator exhaust gas, amounting to about
3% of the incinerator's emissions, was
taken from the top hearth of the incin-
erator. This afforded an opportunity to
compare ESP performance with the par-
ticulate removal performance of the in-
cinerator's wet scrubber.
The test was conducted at a treatment
plant that receives sewage sludge from
an industrialized urban area. The sludge
feed had been digested and dewatered.
Procedure
The full-scale, six hearth, multiple-
hearth incinerator was fitted with ducting
to take a slipstream of uncontrolled
emissions. The slipstream, amounting to
about 3% of the incinerator's emissions,
was fed to the ESP at temperatures
averaging 525°F. The top hearth tem-
perature of the incinerator averaged
743°F during ESP operation.
Emission tests were conducted simul-
taneously at inlet and outlet of the ESP
and at outlet of the wet scrubber. A
source assessment sampling system
(SASS) train was used to collect particle
size fractions of diameters of > 10
micron (ftm), 3 to 10 Aim, 1 to 3 /im, < 1
Aim. Impingerswere located downstream
of the particle sizing part of the train to
collect volatile metals. The impingers were
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immersed in an ice bath. A weak solution
of nitric acid was used to facilitate capture
of the metals.
Metals were analyzed by the inductively
coupled argon plasma method (ICAP). The
concentrations of 23 target metals plus
sulfur were determined in each particle
size fraction and in the impinger catches.
From these results, it was possible to
determine concentrations of each metal
in the controlled and uncontrolled emis-
sion streams, in each particle size fraction,
and in the impinger catches. Overall ef-
ficiencies of the emission control devices
were determined. Removal performance
of the individual metals as well as specific
particle fraction were also determined.
Results and Discussion
In addition to determining the relative
efficiency of the ESP and wet scrubber,
the mass of various metals discharged
and the distribution of metals among
various particle size fractions were also
made. Twenty-four elements were
selected as target for investigation (Al,
Sb, As, Ba, Cd, Ca, Cr, Co, Cu, Au, Fe, Pb,
Mg, Mn, Ni, P, Se, Ag, Na, S, Sn, Ti, V,
Zn). Concentrations in the sludge were
determined on a dry, volatile-free sludge
that had been heated to 550°C. Selenium
and silver were not detected in the sludge,
and gold was detected in only one of
three samples. Of the metals detected,
cobalt and gold had concentrations less
than 100/ig/g-
At the other end of the range, six non-
toxic elements exhibited concentrations
higher than 10,000 ^9/9 (calcium, iron,
aluminum, phosphorus, magnesium, and
sulfur). These six elements contributed
88.2% by weight of the species analyzed.
The remaining metals in the intermediate
concentration range (100 to 10,000 M9/g)
amounted to 11.7 percent by weight of
the species analyzed. The intermediate
group includes all of the metals except
silver and mercury that are identified as
hazardous in the RCRA regulations. Sec-
tion 261.0 (As, Ba, Cd, Cr, Pb, Se). Silver
was not detected in the sludge, and
mercury was not a target compound.
Rank order of the intermediate group by
concentration was as follows:
v-g/a
sodium
zinc
titanium
copper
barium
lead
nickel
78SO
7220
7070
5330
244O
992
840
chromium
arsenic
manganese
tin
cadmium
vanadium
antimony
777
729
443
441
233
212
120
Our analysis focused on this group of
metals.
Emission Concentration
Table 1 presents particle concentrations
at the incinerator, ESP, and scrubber
outlets, along with overall removal effici-
encies for the ESP and scrubber for the
three runs carried out. ESP concentra-
tions were high and efficiencies poor for
Runs 1 and 2. Purge air introduced to
cool the ESP electrode insulators ad-
versely affected the results for the three
runs. This air was shut off for Run 3 and
efficiency improved substantially. Removal
efficiencies were consistently good for
the scrubber for all three runs. For the
ESP, Run 3 showed the highest removals.
Particle Size Distributions
The SASS train aerodynamically sepa-
rates the particulate discharges into four
size fractions, thus giving the particle
size distribution of the discharges (Table
2). Table 2 also shows the calculated
removal efficiency for each fraction for
the two pollution control devices. Only
Run 3 results for the ESP are shown.
The results show marked differences
in performance of the two devices. The
ESP is not as efficient as the wet scrubber
in collecting coarse particles (>10 ^m)
but is much more efficient in collecting
fines (<1 /urn). This result is especially
significant because several troublesome
metals such as cadmium and lead are
capable of vaporizing in the incinerator
subsequently forming fine fumes that are
difficult to remove with wet scrubbers.
Elemental Concentrations in
Outlet Stream
The elemental concentrations were
determined in each SASS fraction for
each outlet stream. Individual as well as
total mass removal efficiencies can be
calculated for each metal. A useful
comparison to make is to determine the
"enrichment" for a given element in the
particles as they leave the incinerator
and the air pollution control device. The
enrichment from sludge to incinerator
(average concentration of an element in
particles divided by its volatiles-free con-
centration in the sludge) and from sludge
to particles collected at the control device
outlet (concentration at device outlet
divided by volatiles-free concentration in
the sludge) are presented in Table 3.
Problems with analyses of the samples at
the control device outlets invalidated
results for some metals that are not
shown. There appeared to be no problem
with results collected at the incinerator
outlet.
The enrichment at the incinerator outlet
shows substantial enrichment of cadmium
and less but still substantial enrichment
of lead and tin. Enrichments from sludge
feed to ESP outlets and from sludge feed
to scrubber outlets are markedly different.
Cadmium shows a reduction in concen-
tration (enrichment less than 1) for the
ESP and a very high enrichment for the
wet scrubber. Consideration of the per-
formance of these two devices indicate
that this is a reasonable expectation.
Typically, cadmium is enriched during
incineration because unlike most metals,
part of it is volatilized in the incinerator. It
then reforms as a very fine solid on
cooling. The ESP as noted above is very
effective in capturing fine particles so
very little cadmium escapes it. On the
other hand, the wet scrubber is inefficient
in capturing fine particles, so much of the
cadmium escapes. It then is highly con-
centrated in the particles captured at the
scrubber outlet. Lead and tin also show
high enrichment between feed sludge
and scrubber outlet, probably for the same
reason as cadmium.
Conclusions
1. The experimental program demon-
strated that an ESP performed at least
as well as a wet scrubber for removing
total particulatesfrom the uncontrolled
gaseous discharge from a sludge
incinerator.
2. The ESP was especially efficient in
collecting particles in the finest size
range (0.1 to 1.0 /urn) investigated.
Because of this feature, the ESP was
far more effective than the wet
scrubber in removing volatile metals,
(including cadmium and lead) from
the stack gases.
The full report was submitted in ful-
fillment of Contract No. 68-03-3148 by
Radian Corporation under the sponsorship
of the U.S. Environmental Protection
Agency.
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Table 1. Paniculate Mass Emission Concentrations and Removal Efficiencies
Mass Concentrations (g/dscm*) Removal Efficiency
Inlet
ESP
Scrubber
ESP
Scrubber
Run 1
Run 2
Run 3
Average
0.75
0.93
0.79
0.82
0.11
0.11
0.012
0.074
0.013
0.014
0.019
0.015
91.5
93.3
98.3
94.4
98.4
98.5
97.6
98.1
* Corrected to 12% 0,,
Table 2. Particle Mass Concentrations by SASS Size Fraction (g/dscm. 12% O2j
Incinerator Outlet
Run 1
Run 2
Run 3
Average
Probe and
>10 micron
Cyclone
0.47
0.43
0.40
0.43
>3 \im
Cyclone
0.23
0.45
0.30
0.33
>1 nm
Cyclone
0.012
0.014
0.022
0.016
Filter
Catch
0.037
O.O34
0.065
0.045
Total
0.75
0.83
0.79
0.82
ESP Outlet
Run 3
R.E.(%)*
0.011
97
0.00
100
0.00
100
0.0052
99
0.012
Scrubber Outlet
Run 1
Run 2
Run 3
Average
Ave R.E.(%)
0.0008
0.00/7
O.OO12
O.OO10
99.8
0.00
0.00
0.00
0.00
100
0.00
0.00
0.00
0.00
100
0.012
0.013
0.018
0.014
69.0
0.013
0.014
0.019
0.015
*R.E. — removal efficiency, calculated for each fracton from MASS flow rates in that fraction
entering and leaving in the gas stream from the control device.
Table 3. Average Enrichment Ratios:* at Incinerator, ESP, and Wet Scrubber Outlets
Metal
Antimony
Arsenic
Cadmium
Cobalt
Copper
Gold
Iron
Lead
Magnesium
Phosphorous
Tin
Titanium
Vanadium
Zinc
Incinerator
Outlet
1.56
1.27
13.72
1.08
1.14
.56
1.41
2.94
1.13
1.17
2.57
1.13
1.41
1.21
ESP
Outlet
0.45
1.05
0.53
5.43
0.07
ND
0.38
2.44
0.60
0.03
0.89
0.22
0.28
0.07
Scrubber
Outlet
6.15
3.46
127.13
2.48
8.12
14.8
4.96
35.97
0.91
0.57
85.63
2.81
9.30
7.98
* Enrichment ratio — the ratio of the concentration of a metal in the panicles to the volatiles-free
concentration of that metal in the sludge.
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/?. C. Adams, G. Bockol, J. A. Maddox, andE. V. Robb are with Radian Corporation.
Research Triangle Park, NC 27709.
Howard Wall and Joseph B. Farrell were the EPA Project Officers (see below).
The complete report, entitled "Electrostatic Precipitator Efficiency on a Multiple
Hearth Incinerator Burning Sewage Sludge," (Order No. PB 88-112 164/
AS; Cost: $19.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For further information, Joseph B. Farrell can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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