United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Technology Transfer
v>EPA Capsule Report
First Progress Report:
Physical Coal-Cleaning
Demonstration at
Homer City, Pennsylvania
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Technology Transfer
EPA 625/2-79-023
Capsule Report
First Progress
Physical Coal
Demonstration
Homer City, Pennsylvania
August 1979
Report:
Cleaning
at
This report was developed by the
Energy and Assessment Control Division
Industrial Environmental Research Laboratory
Research Triangle Park NC 2J7711
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"Spider" cyclone classifier
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1. Strategy
Regulations established under the
Federal Clean Air Act Amendments
have set primary and secondary
air quality standards. Under these
amendments specific standards
have been established to limit
sulfur dioxide ($02) emissions
from large stationary sources such
as coal-burning! boilers used to
generate electricity. The Clean Air
Act Amendments of 1977 (Public
Law 95-95) an(jl Environmental
Protection Agency (EPA)
regulations promulgated in
May 1979 require that all new
sources implement pollution
control technology to control SO2
emissions. Consequently, sulfur-
removing technologies such as
physical coal cleaning (PCC),
chemical coal cleaning, and flue
gas desulfurizaljion (FGD) are of
increasing interest.
Sulfur appears n coal in three
forms: mineral sulfur in the form
of pyrite, organically bound sulfur,
and sulfate sulfur in trace
quantities. The jsuccess of the
PCC process is because the pyritic
sulfur has a higher density than
the rest of the 6oal. Particle size
is a factor in the efficiency of
PCC. This process is capable of
removing 40 to;90 percent of
pyrite sulfur. Unfortunately, owing
to the strong chiemical bonding
between the sujfur and the coal,
the organic sulfur cannot be
removed by physical separation
techniques.
PCC is a viable 'technique in
the following applications:
Complying wi^h 862 emission
standards when the amount of
reduction required for
compliance is! moderate
Combining wjth FGD to lower
emission control costs
Producing several product
coals, each w|ith a different fuel
sulfur value
The Pennsylvania Electric
Company (Penelec) is using the
third application, producing
multiple produdt coals, at the
plant that it operates in Homer
City. Penelec, a subsidiary of
General Public Utilities
Corporation, and the New York
State Electric and Gas Corporation
jointly own the Homer City
Generating Station, a mine-mouth,
coal-fired generating station in
central Pennsylvania. The plant
went into commercial operation in
1969 with two 600-MW units
(Units 1 and 2). A 650-MW
unit (Unit 3) was recently put into
commercial operation. The station
uses coal from two dedicated
mines, Helen and Helvetia, and
from other mines nearby.
Pennsylvania's SC"2 emission
regulations for large existing
stationary sources outside air
basins permit a maximum S02
emission level of 4.0 lb/106 Btu
(7.2 g/106 cal). Normal run-of-
mine coal from Helen and Helvetia
cannot meet this requirement.
The EPA adopted revised air
pollution standards in May 1979
retaining the previous SOa
emissions ceiling of 1.2 lb/106
Btu (2.2 g/106 cal) and adding
the requirement to remove
90 percent of the sulfur dioxide
from high sulfur coals and
70 percent from coal with an SO2
emission rate less than 0.6 lb/106
Btu (1.1 g/106 cal). The allowable
SO2 emission rate from Unit 3 is
1.2 lb/106 Btu (2.2 g/106 Btu),
based on the sulfur content of the
Helen and Helvetia mined coal.
To comply with emission
standards for 862, the Homer City
owners planned to install an FGD
system on Unit 3 and to construct
a PCC plant to desulfurize coal
sufficiently to meet the
Pennsylvania regulations applying
to Units 1 and 2. The PCC plant,
designed and constructed by Heyl
& Patterson, Inc., is currently in
an early stage of operation. The
facility features dense medium
cyclone circuits that desulfurize
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the coal and optimize Btu recovery
from the raw coal feed. A
lime/limestone FGD system was
initially selected for Unit 3 and
was ordered in January 1975.
This plan was later discarded,
however, in favor of the strategy
of using a single coal preparation
plant to generate two coal product
streams capable of meeting the
emission regulations for both the
new and existing units. The
method was termed the
multistream coal cleaning system
(MCCS).
Indigenous coal deposits in the
Penelec service territory are
largely from the Allegheny series
of coal seams; they have long
been recognized for their
suitability for desulfurization by
coal cleaning because they
contain both a relatively low
percentage of chemically bound
organic sulfur and a high but
easily liberated pyritic sulfur.
Favorable research and
development findings on coal
washing encouraged consideration
of advanced preparation methods
as a possible alternative to FGD at
Homer City.
From a large research,
development, and engineering
evaluation program conducted
by the owners' consultants and
Heyl & Patterson, it was
concluded that an advanced, state-
of-the-art PCC plant to produce
two coal products simultaneously
could supply all necessary Homer
City coal for compliance without
supplemental FGD. The technology
would remove 40 to 50 percent
of the sulfur from the raw coal
and redistribute the remaining
sulfur content of the cleaned coal
so that SC>2 emissions of less than
4.0 lb/106 Btu (7.2 g/106 cal)
would result from Units 1 and 2
and less than 1.2 lb/106 Btu
(2.2 g/106 cal) from Unit 3 (see
Figure 1).
Low sulfur coal stream
(S02<1.2 lb/106 Btu)
Medium sulfur coal stream
(S02<4.0 lb/106 Btu)
New unit
Existing units
Burns low sulfur coal
Burns medium sulfur coal
Note.All units meet environmental standards; no scrubbers used.
Figure 1.
The Homer City Configuration
Economic studies indicated that
annual revenue requirements for
complying with the SOz emission
regulations by using clean coal
for the existing units and installing
an FGD system on Unit 3 would
exceed $20 million. In contrast,
annual revenue requirements for
compliance with 862 emission
regulations in all three units using
the MCCS approach would be less
than $14 million.
In August 1975, the Homer City
owners decided to proceed with
the design and construction of
new, dense medium cyclone
circuits to accomplish deep coal
cleaning to meet the NSPS for
Unit 3. The order for-the FGD
system was canceled.
The basic coal-cleaning technology
to be implemented at Homer City
is well established; however,
several new process design
features and innovations will be
used for the first time, including
the following:
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Raw coal crushing will be
controlled to minimize produc-
tion of the very fine coal that
is difficult to recover from the
plant process water.
Dense medium cyclones will be
used to separate coal and ash
at smaller particle sizes than
normally are used in commercial
practice.
Full-stream electromagnetic
separators will be substituted
for the drain-and-rinse screens
that are customarily used for
magnetite recovery.
The operating density range for
dense medium cyclones will be
to 110lb/ft3(1.8 Mg/m3)at
the upper end and to 81 Ib/ft3
(1.3 Mg/m3) at the lower end,
so that requisite redistribution
of sulfur is possible.
A scheme will be developed
to control the coal slurry
medium density at the
1.3-Mg/m3 level. This density
level is vital in obtaining high
yields from the deep cleaning
circuits while maintaining a
high efficiency of sulfur
separation from coal.
Scavenging equipment will be
incorporated in the fine-coal-
processing circuits to recover
approximately 95 percent of the
energy in the coal fed to the
preparation plant.
Efficient pollution control
devices and appropriate methods
of residue disposal will be
employed to minimize the
environmental impact of the
coal cleaning. The process
water will be recirculated
throughout the coal-cleaning
operation.
PCC implemented at Homer City
may gain wide utility application
as a means of controlling SC>2
emissions or as a supplement to
other control technologies. Further
application of PCC hinges on the
establishment of its technical
and economic feasibility and the
identification of specific locations
where it might be used.
Spiral classifier
The U.S. Environmental Protection
Agency (EPA), in partnership with
the Electric Power Research
Institute (EPRI) and the Homer
City owners, is sponsoring a
3-year project aimed at evaluating
PCC technology and at assessing
its applicability on a nationwide
scale. The project will mainly
study the Homer City coal-cleaning
plant and the effects of clean coal
burning on the operation and
economics of power generation in
all three units. Furthermore, the
project will evaluate the coal
reserves available to the
Homer City coal-cleaning facility
with respect to variability in
cleaning characteristics and the
effect of this variability on the
properties of the clean coal.
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2. Program
Homer City's MCCS is complex
and serves boilers with different
allowable sulfur oxide emission
rates, yet it demonstrates many of
the generally applicable aspects
of PCC that will prove to be
beneficial in other plants.
Wide application of PCC, as used
at Homer City, for full or partial
control of sulfur oxide emissions
from utility coal burners depends
on the resolution of several issues.
First, the workability and
practicability of the technology's
novel aspects (such as separation
of coal and refuse in dense
medium cyclones at a density of
1.3 Mg/m3) need to be
established. Second, the economic
feasibility of the concept in
comparison with other SOa control
alternatives requires a detailed
assessment. These issues must
be resolved successfully and the
amenability of the local coal
reserves to treatment by PCC
must be assessed in detail (taking
into account applicable sulfur
oxide emission regulations) before
PCC will be widely accepted as a
method of complying with SOa
emission regulations.
The test program has been divided
into six major tasks, excluding
those concerned with the
management, planning, and
control of the project:
PCC test and evaluation
Support studies for PCC test
and evaluation
Power plant test and evaluation
Coal reserve characterization
study
Ancillary environmental tests
Engineering studies
PCC Test and Evaluation
The coal-cleaning facility will be
completely characterized. The
quality and quantity of all major
streams will be measured and
analyzed. The performance of
individual cleaning equipment and
controls will be measured and
assessed. In view of the expected
variability of the sulfur and ash
contents of the raw coal feed, the
capability of the process to provide
a uniform and acceptable product
will be investigated. The efficiency
of schemes for magnetite recovery
and water recirculation will be
evaluated. Consumption of
magnetite, water, flocculants (for
thickener operation), and energy
will be determined.
To accomplish these tasks will
require sampling and material
flow measurement of process
streams containing large amounts
of coal and refuse that may or
may not be present in slurry form
with water and magnetite. The
solids flow rate is expected to
range from 100 to 1,000 tons/h
(90 to 900 Mg/h). Liquid or slurry
flow rates up to 15,000 gal/min
(0.95 m3/s) are anticipated. The
development of accurate flow
measurement, sampling, and
analytical schemes is an important
precursor to the plant evaluation.
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Vacuum filters
Information obtained under this
task will allow: determination of
the technical feasibility of PCC for
compliance with SOa emission
regulations and novel process
applications; gathering of sufficient
technical data on equipment
performance for use in
comprehensive computer modeling
of advanced coal cleaning, to be
carried out under the support
studies task; determination of the
quality and quantity of effluent
waste streams and emissions; and
determination of the economic
feasibility of PCC compared with
that of other control alternatives.
Support Studies for PCC Test
and,Evaluation
The U.S. Department of Energy
(DOE) now operjates a pilot plant
in Bruceton, Pennsylvania. It is
the main objective of the pilot
plant tests to provide technical
data on factors affecting
separation of ccal from refuse
in dense mediu n cyclones
under conditions used in the
Homer City cleaning plant.
Factors such as the magnetite
particle size, cyclone orifice size
ratios, cyclone inlet pressures,
dense medium viscosity, medium
density, and the proportion of
coal to dense medium (pulp
density) can all |3e varied easily in
the pilot plant afid their effects
on separation efficiency can be
determined. The resulting
information will] be used by the
operators of the Homer City plant.
Equipment performance data
obtained from the PCC evaluation
and the DOE pilot plant will
provide the information base
needed for ongoing computer
modeling, sponsored by EPA,
aimed at facilitating the design
and cost estimation of advanced
cleaning plants. The computer
model will be versatile enough
to specify the required processing
of any washable raw coal to
achieve a cleaned coal of a
certain quality.
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Power Plant Test and Evaluation
It is anticipated that burning coal
of uniform quality with reduced
sulfur and ash content will lead to
more economical and reliable
boiler operation. Some uncer-
tainties still exist, however,
concerning the effect of the altered
coal properties on the operation
of the boiler and the collection
efficiency of stack gas paniculate
control devices.
Preliminary laboratory tests have
revealed some problems with the
burning of cleaned Homer City
coal. The ash in the cleaned coal
seems to have a higher slagging
potential; that is, the molten ash
does not flow as readily as is
necessary for efficient boiler
operation. This characteristic leads
to undesirable deposits on heat
exchange surfaces; more frequent
load cutbacks are then required
for manual or automatic steam
cleaning. It is also anticipated that
the lower sulfur content of the
coal will cause the resistivity of
the ash particles in the flue gas
to increase, resulting in a drop in
collection efficiency in the
electrostatic precipitators (ESP's)
used for fly ash collection. This
efficiency loss may be offset by
the decreased dust loading to
which the units will be subjected
when firing very low ash coal.
The effects of burning the
deep-cleaned coal (versus those
of burning raw coal) on boiler
performance will be investigated
by monitoring and testing Unit 3
in the power-generating complex.
The tests will include sampling
and analyzing the fuel fly ash and
flue gas, measuring unit
efficiency, and monitoring boiler
maintenance and cleanup
operations. Data on the economics
of power generation using clean
coal also will be extracted. These
tests will point out any ash
slagging problems in burning the
deep-cleaned coal and their effects
on power costs.
The performance of the ESP's on
Units 1, 2, and 3 will be closely
monitored. Inlet and outlet fly ash
loadings and characteristics,
together with sampling of the flue
gas for QZ, COa, SC-2, and SOs
content, will provide data
necessary for evaluating the
performance of the collection
devices on flue gases resulting
from the combustion of:
High sulfur, high ash fuel
Medium sulfur, high ash fuel
Low sulfur, low ash fuel
The foregoing tests will determine
the effects of sulfur content of the
fuel on the performance of ESP's.
Coal Reserve Characterization
EPA is currently sponsoring a
program, conducted by the U.S.
Bureau of Mines and the
U.S. Geological Survey, aimed at
assessing the cleanability of U.S.
coals, including those of the Helen
and Helvetia mine reserves.
Extensive analysis of core samples
and samples obtained by mining
methods will yield significant
information concerning:
The geographic variability of
sulfur and ash in the available
seams
The variation in washability
characteristics within the seam
The relationship between
sulfur/ash content and mining
(or sampling) methods
(mechanized mining methods
cause the entrainment of more
refuse in the raw coal)
Information obtained under this
task will help establish an
economic mining or blending
scheme that would reduce the
variability of the sulfur in the feed
to the coal-cleaning plant. This
approach would minimize short-
term sulfur emissions from the
power plant and facilitate
equiprrtent and process control
in the coal-cleaning plant.
Ancillary Environmental Testing
Environmental impacts associated
with disposal of solid and liquid
wastes will be investigated.
Studies are concerned with
pollutants that may be considered
hazardous or toxic under the
provisions of the Water Pollution
Control Act (priority pollutants),
the Resource Conservation and
Recovery Act (hazardous wastes),
the 1977 Clean Air Act
Amendments (hazardous air
pollutants), or the Toxic
Substances Control Act.
The primary intent of the
environmental assessment
activities is to identify pollutants
that pose a threat to health or to
the ecology and to devise
cost-effective strategies for dealing
with them. These pollutants are
associated with various sources,
for example, the leachates
(contaminated water) from
collecting ponds, coal and residue
storage piles, and combustion ash
refuse disposal areas. The
leachates may be acidic and may
contain dissolved heavy metals,
sulfates of iron, and so forth. They
are generated by runoff and
percolation of rain water through
the residue piles and by seepage
of pond water. There is also a
potential for fugitive dust emission
from coal and ash piles.
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Leachates will be characterized
and their magnitudes measured by
drilling strategically located wells
that extend beyond the strata
underlying the piles and the
collecting ponds. The rate of flow
of ground water will be
determined and samples will be
collected and analyzed for
contaminants. Ambient air will be
sampled to determine the
concentration of fugitive dust.
The information obtained through
environmental testing will aid in
evaluating current waste disposal
methods and will help to
determine more efficient pollution
control measures. It will also be
helpful in comparing the
environmental impacts of power
generation using PCC with those
of power generation using other
control alternatives for sulfur
oxides.
Engineering Studies
Three projects will be undertaken
to enhance product quality control
and the efficiency of sulfur
removal and coal recovery at the
cleaning plant. These projects will
consist of:
Evaluating a process to recover
magnetite from power plant
fly ash
Developing and evaluating a
nuclear probe for the in-line
determination of sulfur and ash
in the product coal
Optimizing the raw coal
comminution (crushing) process
to maximize the release of pyrite
and minimize the production of
fine product
Magnetite Recovery. Section 3
describes the use of suspensions
of magnetite in water to effect the
separation of pyrite from coal at
the Homer City coal-cleaning
plant. Although the plant design
incorporates advanced schemes
for magnetite collection and
recovery, losses ranging between
2 and 5 Ib/ton 1(1 and 2.5 kg/Mg)
of product coal are anticipated.
These losses represent about
10 percent of the plant operating
cost of coal cleaning.
In addition to maximizing the
percentage of riagnetite recovery,
the Homer City owners have
performed prelininary studies on
the extraction cf iron oxides from
power plant fly lash. This may be
accomplished by dry magnetic
separation followed by a wet
separation and milling of the
resultant concentrate to the
desired particle size distribution.
To evaluate the suitability of this
magnetite for coal/refuse
separation, as darned out at
Homer City, some tests will be
made at the DOE pilot plant.
Full-scale testing in one of the
dense medium cyclones will be
conducted later
On-Line Sulfur and Ash Measuring
Instrument. Development of an
instrument capable of continuously
measuring the sulfur and ash
content of the product coal would
contribute significantly to solving
the problem of product quality
control at coal-cleaning plants.
Work sponsored by the Homer City
owners has resulted in the
development of a prototype sulfur
and ash meter. A high energy
neutron beam from the instrument
strikes the surface of a coal or
ash particle and results in the
emission of secjondary radiation.
This radiation is detected and
resolved electronically, indicating
the elemental constituents of the
irradiated particles.
The prototype meter is to be
installed at the Homer City plant.
Its ability to continuously and
accurately measure coal and ash
properties will be evaluated.
Comminution Studies. Controlled
crushing of raw coal is necessary
before washing to separate the
ash and pyrite present in the
mined coal, with two objectives:
to maximize the degree of
segregation of ash and coal
particles and to minimize the
production of fine particles (lower
than 100 mesh). Coal/refuse
separations at sizes below
100 mesh are costly and less
efficient.
Several comminution studies are
proposed for optimizing the
crushing operation at the
Homer City plant:
Washability tests of the natural
particle size fractions of the raw
coal will be performed.
Tests will be conducted of the
commercial crusher at the
Homer City plant. In these tests
operating conditions will be
varied to obtain different
proportions of the size fractions
of interest.
Chemical comminution of the
various sizes of coal in ammonia
will be performed to determine
the potential for improved
liberation of mineral matter from
the coal particles. It is thought
that this method of comminution
causes a separation of coal
and refuse along the surfaces of
the particles, which leads to
maximum segregation.
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3. The Coal-Cleaning
Plant
The Homer City plant contains two
parallel coal-cleaning circuits,
each capable of independent
production and each designed for
continuous operation. Provisions
for storage of a supply of each
product coal have been included in
the plant design, allowing either
or both circuits to be shut down
for maintenance without
interrupting power generation. As
stated earlier, the Homer City
Generating Station receives the
major part of its coal from the
nearby Helen and Helvetia mines,
and additional coal is shipped by
truck from other mines in
western Pennsylvania. Most of
the coal is characterized by a low
organic sulfur content (less than
0.6 percent) and a high ratio of
pyritic to organic sulfur (about 3:1).
The pyritic sulfur is liberated
relatively easily from the coal
matrix by crushing.
In the coal-cleaning plant, the
coal is processed to obtain two
clean coal products: one to meet
SOa emission regulations of
4.0 lb/106 Btu (7.2 g/106 cal) for
Units 1 and 2, and one to meet
the SC>2 emission regulations of
1.2 lb/106 Btu (2.2 g/106 cal)
for Unit 3. Coal refuse is also
generated; this material is
dewatered and discarded in a
refuse disposal area. Specifications
for the coal products and the
refuse material are shown in
Table 1.
The coal-cleaning circuits at
Homer City are shown
schematically in Figures 2 and 3.
Figure 2 represents the coarse-
cleaning circuit, which consists
of crushers, screens, dense
medium cyclones, and centrifuges.
The fine coal circuit, shown in
Figure 3, contains dense medium
cyclones, hydrocyclones, Deister
tables, spiral classifiers,
centrifuges, vacuum filters,
thickeners, and other equipment
necessary to the classification and
recovery of fine coal. Both figures
are greatly simplified for clarity
in illustration.
Coarse Coal Procedures
Run-of-mine coal is first crushed
in cage crushers to a top size of
1.25 inch (31.8 mm). This
operation is carefully controlled to
minimize the production of sizes
larger than 0.25 inch (6.35 mm)
and smaller than 100 mesh
(0.15 mm). In the crusher, the coal
is introduced into the center of
an open impeller that spins at
high speed and flings the coal
outward against round cage bars.
Table 1.
Homer City Plant Production Specifications
Item
Medium
sulfur
coal
Low
sulfur
coal
Refuse
Weight distribution (percent)
Enetgy distribution (percent)3 ...
Energy content (Btu/lb dry basis)
Ash content (percent)
Sulfur content (percent)
SC>2 emission factor (lb/106 Btu)
56.2
61.6
12,500
17.75
2.24
3.6
24.7
32.9
15,200,
2,84
0.88
1.2
19.1
5.5
3,400
69.69
6.15
65.9
aOverall plant Btu recovery is 94.5 percent, which includes 1 percent credit for -thermal
drying loss.
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To fine coal
circuit
Dense medium cyclone
1.3 specific gravity
Screen
3*
YZ
Dense medium cyclone
1.8 specific gravity
D inse medium cyclone
1.8 specific gravity
Deep clean
coal
I
Figure 2.
Coarse-Coal-Cleaning Circuit
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1
From
coarse coal
circuits
Classifier
Dense medium cyclone
1.3 specific gravity
V
Hydrocyclone
Figure 3.
Fine-Coal-Processing Circuit
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After crushing, the coal is
segregated into various size
fractions by using several
horizontal vibrating stainless steel
wedge-wire mesh screens. Coal of
a given size fraction is then
processed separately, using dense
medium cyclone circuitry. A
typical cyclone is shown
schematically in Figure 4. The
dense medium, which is a water
slurry containing very finely
divided magnetite, acts essentially
as a liquid in its action on larger
particles of coal and refuse.
Particles more dense than the
apparent density of the magnetite
slurry tend to sink, whereas
particles less dense than the
slurry will float. Pyrite and
ash-forming minerals, being
heavier per unit volume than the
dense medium, are selectively
concentrated in an underflow
fraction. Clean coal particles,
composed primarily of less-dense
carbon, tend to become
concentrated in an overflow
stream. The slurry is fed
tangentially into the cyclone, and
the joint action of gravity and
centrifugal force aids the
separation of dense and light
particles. Those particles which
tend to float in the medium move
toward the axis of the cone and
exit through a pipe fixed to the
base and jutting into the cone a
short distance toward the apex.
This pipe is called a vortex finder.
Particles that tend to sink in the
medium move toward the wall of
the cone and are carried toward
the apex. The stream exiting
tangentially from the top of the
cyclone is a slurry of clean coal,
while slurry leaving at the base
contains refuse. The clean coal
and refuse streams are passed
over screens and magnetic
separators to recover the
magnetite. The clean coal product
stream may then pass through
further separation stages, or it
may be dewatered and dried. The
refuse stream is dewatered before
disposal.
,- "7
1) Feed inlet
2 Overflow
Washed
4J Cylindrica
5 Conical
Replaceatl
7 J Vortex fin ier
Figure 4.
Dense Medium
Basket centrifuges operating on
a vertical axis arje used for final
dewatering of the product. The.
rapidly spinningpasket forces
water through the basket openings
and solids are trapped on the
basket surface. After discharge
from the centrifuge, the coal solids
are dried in a fluidized bed drier
where hot comfcjustion gases are
blown through a bed of coal
particles. The coal/gas mixture
behaves like a turbulent fluid
providing for excellent mixing of
particles and ensuring intimate
gas-solid contact.
Fine Coal Procedures
The unusual features of the
Homer City plant are found in the
circuits (Figure 3) that process coal
finer than 0.079 inch (2 mm).
One unusual feature is the
cleaning of coal as finely divided
as 100 mesh (0.15 mm); most
coal-cleaning plants segregate and
exclude particles smaller than
about 28 mesh (0.59 mm) in
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dense medium circuits. At
Homer City, the fine coal is
processed by first separating
particles smaller than 100 mesh
(0.15 mm) in cyclone classifiers.
A cyclone classifier operates in
essentially the same manner as
a dense medium cyclone, except
that the centrifugal force
segregates the fine coal particles
of 100 mesh (0.15 mm) or larger.
These larger coal particles flow
to the base of the classifying
cyclones, and the fine coal exits
from the top. Underflow from the
classifiers is then mixed with
dense medium, under automatic
control from complex
instrumentation.
The underflow mixture is then
treated in dense medium cyclones,
where almost half the coal is
removed as deep-cleaned product
at a separation density of about
1.3 Mg/m3. An extensive series
of preliminary tests has shown
that the coals from this coal
reserve region can be crushed so
that particles of 0.079 inch by
100 mesh (2 mm by 0.15 mm) are
to a large extent distinct particles
of either mineral or carbon, and so
that a sharp separation of pyritic
sulfur and ash can be effected if
close control of the cyclone
operating conditions is maintained.
The deep-cleaned product is
expected to contain consistently
less than 0.9 percent sulfur, and
more than two-thirds of the coal
burned in the new boiler will be
produced from these cyclones.
To clean particles as small as
100 mesh (0.15 mm) it has been
necessary to include another
uncommon feature at the
Homer City plant. Conventional
mechanical screening could not be
used to segregate magnetite for
recycling. Instead, both the
overflow and the underflow from
the dense medium cyclones are
passed through two banks of
magnetic separators. The cleaned
coal is subsequently separated
from water in spiral classifiers and
dewatered in two stages of
centrifugation.
Magnetic separators are large,
fixed permanent magnets inside a
rotating, nonmagnetic shell onto
which the medium is distributed.
The magnetite is then held to the
drum and is carried by rotation to
a recovery trough outside the
area of influence from the fixed
magnets.
The spiral classifiers accomplish
separation by a combination of
centrifugal and gravitational
forces. The classifier consists of a
spiral conduit through which feed
slurry is introduced. The slurry
flows down until the centrifugal
forces cause the heavier particles
to stratify. The two streams are
then separated to give a clean coal
product and a middlings product.
The rest of the plant consists of
equipment to reclaim more of the
middlings coal and to condition
the processing water for recycling.
Part of the remaining coal is
cleaned on Deister tables to
remove refuse after being
dewatered in hydrocyclones. A
Deister table consists of a large
flat surface that is inclined slightly
both front to back and left to
right. The smooth surface is
divided into narrow strips by riffles
that form a large number of
parallel channels approximately
0.5 inch (12.7 mm) deep by 0.5
inch (12.7 mm) wide. Coal
slurry is fed in and the table is
vibrated. Lighter and smaller
particles tend to flow
predominately with the water
flow; denser and larger pieces
move in the direction of stroke
until they fall over the edge of the
table into a collecting sluice.
A variety of centrifugal and static
thickeners is used to concentrate
suspended coal solids that remain
in the processing water. The finest
solids are separated by vacuum
filtration, and complete closed-loop
recycling of process water is
practiced during operation.
Some of the centrifuges are of the
basket type, described earlier, and
others are of the solid bowl type.
The solid bowl centrifuge contains
a horizontal, high speed, rotating
bowl. Segregated water overflows
at the top of the bowl while the
solids are removed via a helical
screw conveyor at the base.
Static thickeners are large tanks
that allow fine, suspended solids
in a slurry to settle by gravity. The
partially dewatered solids are then
removed by rakes from the bottom
of the tank.
Vacuum filters are mechanical
devices that remove water from
various thickener sludges by
applying a suction across the
filtering medium. The water is
drawn through the filter while
solids are trapped on the porous
metal filter medium.
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4. Other Coal-Cleaning
Applications
Successful implementation of the
MCCS concept at Homer City will
provide some coal-burning power
plants with an alternative SOa
control techniqup that may be
economically preferable to FGD or
the long-range transportation of
low sulfur coals.' For any specific
location, three criteria should be
met before the concept is applied:
Available coal! mine reserves
should be amenable to sulfur
removal by PGC methods. To be
amenable, the coal should have
a ratio of pyritlic to organic
sulfur on the order of 2:1 to 4:1 .
SO2 emissions resulting from
burning the clean coal should
meet Federal pr local
regulations. I
If coal cleaning is to be used as
a sulfur oxide [control measure,
the overall economics should be
proved more favorable than
those of currently available
sulfur oxide control alternatives.
I
The PCC concep|t also may be
used in combination with FGD and
other advanced SOa control
technologies, including fluidized
bed combustion land coal gasifica-
tion and liquefaction, to reduce
the cost of compliance with sulfur
oxide regulations. In these
applications, thei deep-cleaned coal
would be burned in new units for
which NSPS are| applicable, in old
units for which i|t is usually not
economically attractive to retrofit
FGD systems and for which
regulations requ re a reduction in
sulfur oxide emission, and in other
areas where the' need for
control is more critical. Clean coal
with a high sulfur content either
may be treated for sulfur removal
before combustion, as in
liquefaction and [gasification, or
may be subject to FGD after
combustion. It isj believed that
this scheme woiild effect
substantial savin'gs in capital and
operating costs compared with an
approach that uses the advanced
desulfurization technique
exclusively.
It is estimated that vast coal
reserves in which sulfur content
is mainly in the pyritic form (as is
true of the Homer City coal) exist
in the eastern Appalachian coal
basin, especially in central
Pennsylvania, Maryland, and
West Virginia. There are believed
to be 10 billion tons of washable
coals in central Pennsylvania
alone.
Aside from SOa control, coal
cleaning before combustion may
have other beneficial effects in
power generation. The resulting
improvement in coal quality may
increase boiler availability and
generating capacity, thereby
reducing operating and
maintenance costs of the boiler
and of any particulate and SOa
control systems downstream.
Increased boiler availability could
effectively reduce the necessary
generating reserve requirement of
existing electrical systems. It
would postpone the installation of
new and expensive generating
capacity. The PCC evaluation
program will help to resolve cost
uncertainties associated with the
combustion of cleaned coals.
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System for conveying coal to cleaning plant
This report was prepared for the
U.S. Environmental Protection
Agency by PEDCo Environmental,
Inc., in Cincinnati OH. Principal
contributors were Dr. Robin D.
Tems and Dr. Gerald A. Isaacs.
Mr. James D. Kilgroe, the EPA
Coal Cleaning Program Manager,
served as the technical coordinator
for the report. Mr. Kilgroe is with
the Industrial Environmental
Research Laboratory's Fuel
Process Branch in Research
Triangle Park NC. Technical
assistance was provided by the
Pennsylvania Electric Company.
The contact for further information
is:
James D. Kilgroe
Fuel Process Branch
Industrial Environmental Research
Laboratory
U.S. Environmental Protection
Agency
Research Triangle Park NC 27711
This report has been reviewed by
the Industrial Environmental
Research Laboratory, U.S.
Environmental Protection Agency,
Research Triangle Park NC, and
approved for publication. Approval
does not signify that the contents
necessarily reflect the views and
policies of the U.S. Environmental
Protection Agency, nor does
mention of trade names or
commercial products constitute
endorsement or recommendation
for use.
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