United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/005 May 1987
&EPA Project Summary
Bench-Scale Performance
Testing and Economic
Analyses of Electrostatic
Dry Coal Cleaning
Stanley R. Rich
This study presents the results of pre-
liminary performance evaluations and
economic analyses of the Advanced
Energy Dynamics, Inc. (AED) propri-
etary fine coal cleaning process. Coal is
destined to play a dominant role in the
national energy supply mix during the
next century. This fact, coupled with
the economic and environmental dis-
advantages of uncleaned coal, has gen-
erated intense interest in novel tech-
niques for coal beneficiation. The AED
"FC" Process relies on the substantial
differences in electrical conductivity
which exist between the organic coal
matrix and the inorganic inclusions in
the coal to separate these impurities
from the coal. It also takes advantage
of the additional liberation associated
with pulverization of the coal used in
most boilers. The electrostatic separa-
tion process is effected on a rotating
drum (roll) separator which can be
placed between the pulverizer and the
boiler at a power plant. It can be retro-
fitted to existing boilers.
This report covers work accomplished
jointly by Versar, Inc. and AED. Grab
samples of feed and product coal were
obtained from 25 operating physical
coal cleaning (PCC) plants by Versar.
Samples of PCC plant feed coal in a
run-of-mine (ROM) condition were pro-
cessed by Versar and a portion was
provided to AED for testing on the AED
bench-scale separator (Tech 1) Process.
Comparisons of performance and costs
between the AED Tech 1 Process and
the PCC plants have been developed.
The results show that the AED Tech 1
Process exhibits superior sulfur removal
performance at equivalent cost and
energy recovery levels. The design and
scope of the project did not permit a
complete evaluation of the ash removal
capability of the process. Overall, ash
removal results indicate that the AED
Tech 1 Process did not perform as well
as the PCC plants; however, the data
obtained offer the expectation of im-
proved ash removal with further work.
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 In-
formation at back).
Introduction
As the most abundant fossil fuel re-
source in our Nation, coal is destined to
play an increasingly significant role in
the energy future of the U.S. Unfortunate-
ly, coal is generally also the dirtiest fuel
in our energy supply mix. The presence
of substantial amounts of inorganic
material in coal has negative impacts on
the availability and lifetime of coal-fired
steam electric plants due to problems of
corrosion and slag formation. Further-
more, the stack gas from such facilities
contains high levels of air pollutants
which, to satisfy national policy, most be
removed by complex and expensive
cleaning techniques. Thus, for both
economic and environmental reasons,
both the government and the private
sector have a continuing interest in novel
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processes which offer the promise of
being able to upgrade the fuel quality of
available coal. AED has developed a novel
fine coal cleaning process (Tech 1) which
has shown encouraging results in a
number of preliminary tests. This process
removes inorganic matter (largely ash-
forming minerals, including pyrite) from
pulverized coal by a patented electrostatic
separation process. The AED Process can
be retrofitted to existing pulverized-coal-
burning facilities and may, if its initial
promise continues to be borne out, have
substantial advantages, compared both
to conventional coal cleaning and to stack
gas cleanup technologies.
The AED Process has its foundation in
research conducted during the late 1970s
which suggested that the American
Society for Testing and Materials (ASTM)
method for determining organic sulfur in
coals consistently overestimated the or-
ganic sulfur level (hence underestimating
the potential of physical coal cleaning
(PCC) to remove sulfur from coal). Micro-
scopic studies of coal suggested that the
ASTM method substantially understates
the pyritic sulfur levels in most coal.
Findings, using electron microscopy,
were:
A substantial quantity of pyrite is in
noncrystal form, from colloidal size
up to approximately 5 ^m.
Another group of pyrite inclusions
(called framboids) is in the 20 to 50
/urn size range, formed by aggregates
of the monocrystals.
A third class of pyrites is larger
inclusions, basically rock fragments,
sometimes referred to as "second-
ary" pyrite.
Monocrystals and framboids are so
small that the ASTM tests have largely
classified them as "organic" sulfur (thus
assumed to be removable only by chemical
means). But when the coal is finely
pulverized, these small pyrite particles
can be largely separated from the coal
matrix (liberated) and thus more remov-
able than conventional wisdom has
dictated (see Figure 1). Thus, coal-clean-
ing systems capable of treating pulverized
coal would generally have access to larger
quantities of pyrite than the conventional
coal-cleaning processes. Since most coal
used to produce steam is pulverized before
combustion, AED saw the possibility of a
process which could use the increased
liberation of pulverization and could be
retrofitted to existing systems. In addition,
AED sought a process concept that would
remove as much pyrite and ash as possible
after liberation in the pulverizing process
Large particles/lump:
Minerals encapsulated
The average size particle
treated by conventional
coal washing does not liberate
impurities.
Pulverized coal:
Minerals liberated
T«V
fxit The average size particle
handled by the A£D system
provides substantial liberation.
Kev: I I - impurities
- pure coal
Figure 1. Theory of liberation operation.
without the use of water, chemicals, or
other additives.
Electrostatic separation using the "roll"
separator was selected as the technology
for further development. This process
uses as the principle of separation the
differences in electrical conductivity
(roughly 4 to 10 orders of magnitude)
between the organic matrix and the in-
organic inclusions in coal. Electrostatic
separation for coal beneficiation had been
studied earlier by a number of researchers
in the U.S. and abroad. In these studies,
it was found that, although some separa-
tion could be achieved, the clouds of fine
coal dust formed made the process im-
practical and inefficient.
A model electrostatic cleaning system
was constructed at AED in early 1979. It
duplicated the results of earlier investiga-
tions where clouds of coal dust did indeed
leave the rotating drum, /endering coal
cleaning relatively inefficient. Furthe
work showed that the reason for the los
of coal from the drum was the present
of a boundary layer of air which prevente
the coal from contacting the surface i
the drum. Efforts to use this informatic
have resulted in major improvement
the technology. In the AED fine-co
separator, the boundary layer is stripp*
from the rotating roll by a doctor blat
(usually employed in the removal of solii
and liquids from rotating rolls, but in tr
case used to remove a gas layer). As
result, the coal particles are deposited <
the rotating roll immediately followii
the removal of the boundary layer, a
are pressed onto the roll by the ne
boundary layer formed downstream
the doctor blade. This innovation in tec
nology has resulted in issuance of U
Patent No. 5,325,820 to AED. Figure
schematically represents the principle
operation for the process.
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Feed
Doctor Blades
Corona
High Voltage
Attracting
Electrodes
Product
Figure 2.
Re/ects
Principle of operation of electrostatic roll separator
Project Approach
The premise of the AED Process con-
cept that separation of small inorganic
inclusions from the coal matrix will en-
hance coal quality is undoubtedly true,
to a degree. The question is' to what
degree? There is a great deal of variability
in the nature of the inorganic contamina-
tion of coals by type, between coal
seams, and even within a single seam
Hence, since no data base existed which
could allow an evaluation of this new
technology by addressing the validity of
the fundamental premise, it was decided
that two goals for this project were'
To develop a broader information
base on the performance of the AED
Process on various coals
To provide information which would
allow comparison of the AED bench-
scale fine coal cleaning process
(Tech 1) with conventional PCC
technologies.
The ultimate objective was to provide a
basis for assessing the merits of the
technology in the context of other coal
cleaning techniques. The project was a
joint effort by Versar, AED, and EPA. The
program involved comparison of the per-
formance of the AED Tech 1 Process with
PCC plants presently in operation.
Representative coal seams in the U.S.
were identified. Twenty-five PCC plants
cleaning coal from these seams were
then selected (see Figure 3). The feed
and product coal at these plants were
grab sampled and then split into various
fractions. A fraction of each feed coal
sample was sent to AED to be tested in
the AED Tech 1 Process. Another fraction
of each feed coal sample and each pro-
duct coal sample was analyzed at Versar's
coal laboratory for ash, sulfur content,
and heating value
Twenty-seven feed coal samples were
delivered to AED for processing. Note
that these were run-of-mme (ROM) coals
destined for treatment in a PCC plant As
such they were not typical of the coals
which would be fed to the pulverizer of a
coal-fired boiler. Boiler feed coals have
usually been through a preliminary
cleaning (to remove large inorganic in-
clusions incidental to the mining opera-
tion) before shipping them to the boiler
AED did not attempt such a step. The
sample coals were pulverized by AED in a
small hammermill to pulverized coal (p c.)
size (60 to 80 percent, <200 mesh) before
processing. Thus, any larger inclusions
(rocks and pebbles) were pulverized and
sent through the process. The pulverized
sample was separated into three fractions:
ultrafme (<20 ^m), very fine (53 x 20
/urn), and fine 246 x 53 ^m). The ultrafme
fraction was not processed, but was in-
corporated with the product from the
(separate) processing of the other two
fractions. These latter fractions were
processed on the Tech 1 Unit at a feed
rate of 125 kg/hr (275 Ib/hr). Drum
rotation rate and electrode voltage were
constant for all runs. Measurements were
made of the total sulfur, ash, and heating
value for each fractional product as well
as the sulfur, ash, heating value, yield by
weight, and energy recovery for the total
product of the Tech 1 Process.
Findings
Because the field samples obtained
were grab samples and not time-aver-
aged, it has not been feasible to conduct
a detailed assessment of the performance
of the AED Tech 1 Process or the in-
dividual PCC plants Such assessments
require the continuous processing (and
sampling) of fairly large (several tons per
hour) quantities of coal over a period of
days clearly beyond the scope of this
project. A qualitative comparison was
made by simply counting the number of
times the Tech 1 Process performed
significantly better or worse than the
appropriate PCC plant. The Tech 1 Process
exhibited a much better capability of
removing sulfur than the coal washing
plants from which the coals were obtained
(see Table 1). The sulfur removal was
greater for the high- to moderate-sulfur
coals than for the very-low-sulfur coals.
Ash removal capability of the Tech 1
System was found to be lower than the
ash removal capability of the conventional
coal preparation plants (see Table 2) This
relatively low overall ash removal reflects
the very high mineral content of the ROM
feed coals. Much of this mineral matter is
probably rough rock inclusions from the
mining process, usually removed at the
mine before shipping to any customer. In
the fine sub-fraction of the process, where
ash particles are larger than 53 pm (270
mesh) in average diameter, the Tech 1
Process removed slightly more ash than
did conventional coal washing. This frac-
tion of the ash particles is responsible for
most slagging m boilers.
Cost evaluations of PCC were performed
using the actual process flow diagrams
from six PCC plants visited during the
sampling period These flow diagrams
were modified and process equipment
was sized to provide a common basis for
the designs The results of these cost
-------
Illinois No. 2
IG-3)
Illinois No. 5
(G-5) Illinois No. 8
(G-41
Ohio No. 6
(B-2)
No. 8.8A & 11
Upper Freeport/
Lower Kittanning
(B-5)
(B-3) \ Upper Freeport
Pittsburgh No. 8
IB-4) (B-1) (B-7)
See Veer
(G-2)
Kansas
'#
Fleming ~^~
(G-1)
Pennsylvania^ Undisclosed
(B-5)
Illinois No. 6
(Ft-6)
Lower Kittanning
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Table 1. Comparison Of Sulfur Content In Process Product Streams
Versar
Sample
Designation
B-1
B-2
8-3(8/11)
B-3(1 1)
B-4
B-5
B-6(UF)
B-6(LK)
B-6(MIX)
B-7
B-8(HI
B-8(V)
B-9
R-1
R-2
R-3
R-4
R-5
R-6
R-7
R-8
R-9
G-1
G-2
G-3
G-4
G-5
G-6
Average
Observed
FeedS
(wt.%)
3.98
3.16
3.41
2.25
3.92
2.36
1.63
3.96
2.06
4.01
1.84
1.76
1.08
1.16
3.68
2.48
0.89
3.54
4.11
0.56
0.52
4.40
6.78
4.61
4.04
2.87
5.66
PCC
Process
Product
(wt.%)
2.81
2.48
2.61
N.A.
3.55
2.22
N.A.
N.A.
1.47
1.87
N.A.
N.A.
1.48
0.86
1.43
3.43
1.60
0.83
3.35
3.71
0.80
0.35
3.67
N.A.
3.96
3.12
2.97
5.20
AED Process AED Process
Total Product Fine Product
fwt. %) Comparison (wt. %) Comparison
2.73
2.44
2.66
1.76
3.22
1.72
1.15
2.91
1.20
2.58
1.31
1.31
0.81
0.93
3.25
1.45
.76
2.60
3.39
0.59
0.57
3.72
3.96
3.41
3.05
2.25
4.33
o 2.67 +
o 2.17 +
o 2.50 +
? 1.61 ?
+ 3.42 o
+ 1.69 +
? 1.00 ?
? 2.29 ?
+ 1.04 +
? 2.24 ?
? 1.17 ?
+ 1.31 +
o 0.78 +
+ .99 +
+ 3.16 +
+ 1.41 +
o 0.76 o
+ 2.53 +
+ 3.17 +
+ 0.69 o
0.59
o 3.20 +
? 3.57 ?
+ 3.23 +
o 2.97 +
+ 2.26 +
+ 3.73 +
S-1
2.48
1.30
1.53
1.14
Comparison Key: - AED Process Significantly Worse
o No Significant Difference
+ AED Process Significantly Better
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Table 2. Comparison Of Ash Content In Process Product Streams
Sample
Designation
B-1
B-2
8-3(8/11)
B-3 (1 1)
B-4
B-5
B-6 (UF)
B-6 (LK)
B-6 (MIX)
B-7
B-8 (H)
B-8 (V)
B-9
R-1
Ft-2
R-3
R-4
R-5
R-6
R-7
R-8
R-9
G-1
G-2
G-3
G-4
G-5
G-6
Average
Observed
Feed Ash
(wt. %)
15.6
24.0
22.4
21.1
41.3
32.0
254
22.5
N.A.
275
28.5
21.5
27.6
32.1
34.2
31 7
24.
25.2
25.8
23.8
39.6
23.8
37.9
23.9
27.6
377
40.6
25.1
PCC
Process
Product
(wt. %)
8.39
12.4
13.3
N.A
13.2
20.2
N.A.
N.A.
101
12.0
N.A.
N.A.
98
12.0
109
10.2
127
16.1
13.0
113
7.3
5.9
11.9
N.A.
8.10
12.1
14.0
145
AED Process AED Process
Total Product Fine Product
(wt. %) Comparison (wt. %) Comparison
104
136
17.0
16.4
26.9
25.6
19.4
21.6
N.A
21.2
18.7
17.2
23.3
23.2
262
20.9
10.3
17.2
14.1
14.9
24.1
17.8
24.1
15.0
13.2
25.9
30.2
20.2
82 o
o 10.8 +
13.7 o
? 13.3 ?
9.9 +
196 o
?(-) 17.6 ?(-)
?(-) 21.2 ?(-)
13.0 o
? 13 5 ?
? 14.0 ?
21.6
104 +
17.1
10.2 o
+ 7.5 +
o 8.8 +
o 10. 1 +
9.8 +
142
12. 1
11.0 o
? 8.4
96
13.9
13.3 o
11.2 +
S-1
32.2
9.2
24.2
15.8
Comparison Key - AED Process Significantly Worse
o No Significant Difference
+ AED Process Significantly Better
S. R. Rich is with Advanced Energy Dynamics, Inc.. Natick, MA 01760.
James D. Kilgroe is the EPA Project Officer (see below).
The complete report, entitled "Bench-Scale Performance Testing and Economic
Analyses of Electrostatic Dry Coal Cleaning," (Order No. PB 87-168 407/
AS; Cost: $18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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
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Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-87/005
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