Coal Flow Ability Report - Bunker Coal Flow Study for Anpara-D
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Transcript of Coal Flow Ability Report - Bunker Coal Flow Study for Anpara-D
Job no: S/BHEL- Noida/Flow Lab/16/2010-11 Confidential Not For Publication
Draft Report
Bunker Coal Flow Study for
2x500 MW Thermal Power Station of Anpara-D Project, U.P.
Sponsored by
M/s BHEL, Noida
December, 2010
Prepared by Research & Development Centre
NMDC Ltd (A Government of India Enterprise)
Uppal Road, Hyderabad - 500 007 India
Bunker coal flow study – TPS Anpara-D
I N D E X
Sl.no. Contents Page no.
Executive Summary 2
Glossary of Terms 5
Symbols and Abbreviations 7
List of Figures 8
List of Tables 9
1. INTRODUCTION 10
2. OBJECTIVE 14
3. EXPERIMENTAL WORK 15
3.1 SAMPLE PREPARATION 15
3.2 MOISTURE DETERMINATION 16
3.3 BULK DENSITY 16
3.4 BULK DENSITY VARIATION WITH CONSOLIDATION 17
STRESS
3.5 SHEAR TESTS 18
3.5.1 RING SHEAR TESTER 18
4. EXPERIMENTAL DATA GENERATED 21
5. RESULTS & DISCUSSION 27
6. RECOMMENDATIONS 30
R&D Centre, NMDC Ltd Page 1
Bunker coal flow study – TPS Anpara-D
Executive Summary
M/s. BHEL,Noida vide work order no. PW/PE/PG/ANP/P-307/10, dt:16.06.2010
have awarded the work to R&D centre, NMDC Ltd. for comprehensive flowability
studies on the coal sample of the proposed 2 x 500 MW Anpara-D thermal power
station, UP, to provide relevant parameters for the design of reliable gravity flow coal
silos. The silos are required to promote Mass Flow without choking and rat holing
problems. Accordingly approx. 400kg of coal sample was received from TPS Anpara at
R&D Centre of NMDC on 15.09.2010.
A representative sample was drawn from the lot to establish the size analysis. The coal
sample has been crushed to -5mm size and homogeneously mixed. The fine sized coal
of typically less than 2.36mm size, which is primarily responsible for flow related
problems has been screened and was used for conducting the shear tests. The
flowability characteristics of coal and its interaction with five different liners i.e. Stainless
steel SS409M(2D Finish), SS304(2B finish), IS 2062, Mild steel(rusted) and UHMWPE
(ultra high molecular weight polyethylene) were established using Ring Shear Tester at
four different moisture levels of coal by physically altering the moisture content. As most
coal samples exhibit high yield strength between 55% to 85% saturation moisture, it
was decided to conduct tests on coal sample at 55%, 65%, 75% and 85% saturation
moisture levels (SML). The corresponding moisture content (mc) of each saturation
moisture level for coal is respectively 16.4%, 19.4%, 22.3% and 25.3%. The coal
sample exhibited highest cohesive strength at 25.3% mc which is considered the critical
moisture. The undisturbed storage time tests were conducted at 25.3% mc for storage
up to 24 hours and 72 hours to establish the effect of undisturbed coal storage in the
bunker/silo. The shear test data has been processed to establish the minimum slopes
and outlet sizes required to generate Mass Flow in coal bunkers.
The following are the salient points.
• The results of testing indicate that the tested coal is compressible and has
moderate angles of internal friction and moderate bulk strength. Based on the
Jenike’s classification, the coal can be classified cohesive at 16.4% mc and
very cohesive thereafter.
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Bunker coal flow study – TPS Anpara-D
• The flow function curves indicate that coal exhibits highest yield strength at a
moisture content of 25.3%. Hence the storage time tests were conducted at
25.3%, which is considered the critical moistures. There is a significant
increase in the bulk strength of coal after 24hours and 72 hours of storage.
• The coal at 25.3%mc, requires a critical (minimum) outlet dia. of 0.70m to
prevent ‘cohesive arching’ at instantaneous condition (without storage) in
case of conical hoppers.
• The storage time test at 25.3%mc indicate that the critical (minimum) outlet
dia. required is 1.0m for 24 hours and 1.26m for 72 hours of storage to
prevent the formation of cohesive arching above the outlet in the conical
hoppers.
• The minimum conical hopper slope (with horizontal) required for Mass Flow
at a typical outlet dia. of 0.914m (0.914m was chosen out of general practice in coal bunkers of 500MW thermal power plant) is 73.50 with stainless steel SS304 (2B finish) liner and 71.30 with SS409M (2D finish) to handle coal at all moisture levels. The minimum slopes for other outlet
dimensions are also presented in the report.
• The slope of the hopper (with horizontal) decreases normally with the
increase in hopper outlet size for different liners. The minimum slope required
to promote Mass Flow against different outlet sizes has been provided in
tabular form in the report which may be utilised in the functional design
process.
• The typical outlet dimension of 0.914m (dia) for the coal bunkers is sufficient
to prevent cohesive arching above the bunker outlet while handling tested
coal at all moisture levels and undisturbed storage in the bunker for less than
24 hours. However, upon extended undisturbed coal storage beyond 24
R&D Centre, NMDC Ltd Page 3
Bunker coal flow study – TPS Anpara-D
hours, the coal is likely to form an Arch at the outlet leading to no flow
condition.
• If prolonged storage of coal in silo beyond 24 hours is expected, then Arching
can be avoided by locating a shut off gate at hopper sectional dia of 1.26m
instead of placing at the 0.914m dia. During the shut down, the shut off gate
should be closed and coal below the gate is to be emptied by running through
the system. In such an arrangement, the effective outlet size would be 1.26m.
This will facilitate to initiate the coal flow satisfactorily from the silo after
extended period of undisturbed storage up to 72 hours.
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Bunker coal flow study – TPS Anpara-D
GLOSSARY OF TERMS
Cohesive Arching:
A no – flow condition caused by bridging of the material over the hopper outlet
Bin:
Usually vertical section of a storage container (Sometime used synonymous to a
Bunker/Silo)
Bunker:
Storage container having both vertical and convergent sections
Expanded Flow:
Combination of Mass Flow in converging section and a Funnel Flow bin on top
Flow Function:
Plot of unconfined yield stress versus major consolidation stress for specific bulk
solid. It is a bulk solid parameter
Flow Factor:
It is a flow channel parameter. Flow factor is the ratio of major consolidation
stress in a bulk solid flowing in a channel to the major principal stress that would
cause it to cease flowing. The value of flow factor depends on the geometry of
the hopper, especially on the slope of the channel walls, the angle of wall friction
and the effective angle of friction.
Funnel Flow:
A flow pattern in which the material flows primarily in the central part of the bin or
hopper in the form of a funnel
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Bunker coal flow study – TPS Anpara-D
Gravity Flow:
The flow of a bulk material is induced by gravity alone
Hopper:
Converging section of a storage container
Instantaneous condition:
No storage at rest (Filling of bunker followed by extraction)
Mass Flow:
A flow pattern in which all the material in the bin or hopper is in motion and flow
occurs along the walls of bin or hopper
Rathole/Piping:
A restricted flow condition in which the material flow is limited to Vertical central
cylindrical core above the hopper outlet
Plane Flow:
A flow pattern characterized by flow trajectories that are symmetric about the
vertical plane through the longitudinal axis of the outlet slot
Silo:
Tall storage container, usually with a centrally located opening
Time storage:
Bulk solid stored undisturbed in the bunker for specified time
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Bunker coal flow study – TPS Anpara-D
SYMBOLS AND ABBREVIATIONS
Dc: Minimum diameter of circular discharge opening for a Mass Flow Silo, m
Ds: Minimum side of a square discharge opening for a Mass Flow Silo, m
Dp: Minimum width of a slot discharge opening for a Mass Flow Silo, m
Df: Critical rathole dimension (Funnel Flow), m
θc: Minimum angle (from horizontal) for a conical hopper walls and end walls
of a transition hopper for Mass Flow, Degrees
θp: Minimum angle (from horizontal) for a wedge shaped (plane flow) hopper
and side of transition hopper for Mass Flow, Degrees
FF: Flow Function
FFt: Time Flow Function
σ (Sigma): Normal stress, pa (Pascal)
σ1: Major consolidation stress, pa
FC: Unconfined Yield Stress, pa
δ (Delta): Effective angle of friction, Degrees
φ x (Phix): Kinematic angle of wall friction, Degrees
φ(Phi): Angle of internal friction, Degrees
sml: Saturation moisture level
mc: Moisture content
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Bunker coal flow study – TPS Anpara-D
LIST OF FIGURES
Sl. no.
Contents
Page no.
1 Mass Flow and Funnel Flow Patterns 12
2 Expanded Flow Bin 13
3 Variation of bulk density with major consolidation stress 17
4 Jenike – Schulze Ring Shear Tester
19
5 Photographs of Liners Tested 20
6 Typical treatment of yield Locus (25.3%mc, Level-2) 21
7 Kinematic angle of wall friction (16.4% & 19.4%mc) 22
8 Kinematic angle of wall friction (22.3% & 25.3%mc) 23
9 Flow functions 24
10 Flow function and Time Flow function (25.3%mc, 24hrs and 72hrs) 25
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Bunker coal flow study – TPS Anpara-D
LIST OF TABLES
Sl. no.
Contents
Page no.
1 Size analysis of as received sample 15
2 Percent moisture content with respect to saturation
moisture level 16
3 Bulk density and Angle of repose 17
4 Flowability parameters 26
5 Minimum outlet dimension to prevent cohesive arching at critical moisture 32
6 Minimum hopper slopes for Mass Flow at a given outlet dimension
33
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Bunker coal flow study – TPS Anpara-D
1. INTRODUCTION
1.1 Gravity Flow Of Bulk Solids
Gravity forces in general, are utilized wherever possible to cause the flow of bulk
solids in bins, hoppers and stockpiles. Earlier, the designs for such systems were
based on the angle of repose concept of the material. However, this parameter does
not take into account the consolidation loads experienced by the bulk solid when
stored and extracted from bunkers. The cost of Bulk material handling operations is
very substantial and for this reason handling and storage facilities should be
designed to gain maximum reliability and efficiency.
Advances in this field have shown conclusively various factors other than angle of
repose, which influence greatly in establishing optimum flow condition for the
material with respect to bunker geometry and liner selection. The common
problems associated with material flow are segregation, flow blockage due to
arching, Rat holing, wall failures etc. These problems are in turn related to factors
like Effective angle of friction between particles, Kinematic angle of wall friction,
Angle of internal friction etc. Now a days, increased awareness amongst material
handling experts has emerged to consider various flow parameters like moisture
effect, liner effect, storage effect, bunker geometry, effect of wall pressure in
hopper, flow path and velocity fields etc., while designing the geometry of the
system as against conventional approach of angle of repose.
The design of storage bins for bulk solids involves
1. Determination of the strength and flow properties of the bulk solids for the
worst likely conditions expected to occur in practice. 2. Determination of the bin geometry to give the desired capacity, and reliable,
predictable gravity flow. 3. Estimation of loadings exerted on the bin walls and the feeder under
operating conditions.
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Bunker coal flow study – TPS Anpara-D
1.2 Modes of Flow In order to appreciate the problems encountered with the bunker operation it is
important to know the Flow patterns that occur in bunkers during gravity flow.
There are two basic modes of flow, Mass Flow and Funnel Flow
1.2.1 Mass Flow Mass Flow pattern describes a condition in which all the material in the bin is in
motion whenever any of it is drawn out (Fig.1). It is not necessary that the velocity
across the cross section is constant only that all the material will be in motion.
Mass Flow bins require more headroom than the Funnel flow systems because
the hopper walls have to be smooth and steep. The flow pattern sequence is
“first-in, first-out”, Rat holes do not develop and fine powdery materials will have
time to deaerate after charged in to the bin. Material bulk density at the outlet is
relatively constant and segregation is minimized because particles at the centre
and sidewalls of the bin are discharged simultaneously. Mass Flow bins are
especially suitable for cohesive solids (including many fine powders) that degrade
with time, and where segregation should be minimized. But the disadvantage
associated with Mass Flow is wear of bin and hopper walls when handling
abrasive bulk solids.
1.2.2 Funnel Flow Funnel flow (or Core flow) on the other hand occurs when the bulk solid sloughs
off the surface and discharges through a vertical channel, which forms within the
material in the bin (Fig.1). This mode of flow occurs when the hopper walls are
rough and slopes are shallow.
It follows the “first-in, last-out” sequence of flow pattern. Flow rate tends to be
erratic with varying feed bulk density. Stable rat holes can form if the stored
material develops enough cohesive strength, resulting in severe loss of the ‘live
capacity’, besides pseudo-stable rat holes may develop causing erratic flow. Most
fine powders exhibit flushing in a Funnel flow bunker system, because they can
support a stable rat-hole. Funnel flow bunkers also exhibit the problems of
segregation.
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Bunker coal flow study – TPS Anpara-D
Fig.1 Mass Flow and Funnel Flow Patterns
1.2.3 Expanded Flow Bin Where large quantities of the bulk solid are to be stored, the expanded-flow bin
(Fig.2) is often an ideal solution. This bin combines the storage capacity of the
Funnel flow bin with the reliable discharge characteristics of the Mass Flow
hopper. It is necessary for the Mass Flow hopper to have a diameter at least
equal to the critical pipe or rathole dimension Df at the transition with the Funnel
flow section of the bin. This ensures that the flow of material from the Funnel flow
or upper section of the bin can be fully expanded into the Mass Flow hopper. The
Expanded flow bin concept may also be used as advantage in the case of bins or
bunkers with multiple outlets.
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Bunker coal flow study – TPS Anpara-D
Fig.2 Expanded Flow Bin
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2. OBJECTIVE The objective of the assignment is to establish the flow properties of coal sample for the
proposed 2 x 500 MW Anpara – D thermal power station, UP, to provide relevant
parameters for the design of reliable gravity flow coal silos based on the test data
obtained at different moisture contents along with storage time effect up to 72 hours.
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Bunker coal flow study – TPS Anpara-D
3. EXPERIMENTAL WORK 3.1 Sample Preparation
Approximately 400Kg of coal sample has been received from the Thermal power
plant, Anpara, U.P. The sample has been uniformly mixed and representative
sample was drawn from the lot to establish the size analysis. The as received
sample was coarse in size and containing lumps as large as 40mm. The size
analysis of as received sample is presented in Table-1. About 100 Kg of
representative sample has been further drawn from the lot which was subjected
to stage crushing and reduced to size below 5mm. The crushed sample was
screened through an 8 mesh (2.36mm) aperture screen. About 60 kg of
representative sample of -2.36 mm was cut from the lot and the same is used for
shear testing to generate flowability test data.
TABLE-1 SIZE ANALYSIS OF AS RECEIVED SAMPLE
Nominal Screen aperture Size
Tyler Mesh no. mm
Cumulative weight percent passing
--- 40 94.59 --- 20 85.51 --- 10 72.74 --- 3 51.78 18 1 33.89 20 0.833 30.50 28 0.589 26.09 35 0.417 21.40 48 0.295 17.45 65 0.208 13.68
100 0.147 11.45 150 0.104 11.29 200 0.074 9.93 250 0.061 9.66
325 0.043 9.46
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Bunker coal flow study – TPS Anpara-D
3.2 Moisture Determination
Moisture content was determined on -8# (-2.36mm) size fraction by drying small
quantity of samples at 107o C until dry in a forced convection oven. The loss in
weight of each sample divided by its original (wet) weight before drying is
denoted as moisture content.
After determining the moisture content of the air dried coal, the saturation
moisture level of coal is also established by gradually adding small quantities of
water to a known quantity of coal sample until the coal reaches a 100%
saturation level. The total quantity of water added is noted. The moisture of the
resultant sample at 100% saturation is determined.
The percentage moisture of the coal with reference to various saturation levels
are shown in Table-2.
TABLE- 2
PERCENT MOISTURE CONTENT WITH RESPECT TO SATURATION MOISTURE LEVEL
Moisture level
Moisture content (%)
Air dried sample 8.68
55% saturation moisture level 16.4
65% saturation moisture level 19.4
75% saturation moisture level 22.3
85% saturation moisture level 25.3
100% saturation moisture level 29.8
3.3 Bulk Density
The bulk density of the as received coal sample is determined along with repose
angle and is shown in Table-3.
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Bunker coal flow study – TPS Anpara-D
TABLE-3 BULK DENSITY AND ANGLE OF REPOSE
Sample Size of coal Bulk density (kg/m3)
Angle of Repose (deg)
Condition
Coal As received 977 35.50 10.25% moisture
3.4 Bulk Density Variation with Consolidation Stress
The bulk density is an important parameter in calculation of bunker/silo loads,
bunker capacities, opening sizes and material flow rates. The bulk density
variation with major consolidation load at different moistures is established using
Ring shear tester and is presented in Fig.3.
700
800
900
1000
1100
1200
1300
0 10000 20000 30000 40000 50000
Major Consolidation Stress, Pa
Bul
k D
ensi
ty, K
g/m
3
16.4% mc19.4% mc22.3% mc25.3% mc
Fig. 3 Variation of bulk density with consolidation stress
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Bunker coal flow study – TPS Anpara-D
3.5 Shear Tests The influence of moisture content on the flowability of coal is very significant. For
most bulk materials like coal, the bulk strength tends to increase (flowability
drops down) with increased moisture content, reaching a peak between 55% to
85% saturation. Beyond this peak value the bulk strength generally reduces with
further increase in moisture content. To find the moisture level at which the coal
attains maximum bulk strength, shear tests were conducted at moisture contents
16.4%, 19.4%, 22.3%, and 25.3%mc, by altering the moisture content of
received sample. The above moistures correspond to 55%, 65%, 75% and 85%
saturation respectively. The methodology of shear testing is based on the special
procedure of compacting the coal sample at different specified moisture levels to
obtain packing conditions expected in the bunkers/silos and then subjecting the
sample for shear testing using Ring shear tester.
3.5.1 Ring Shear Tester Ring shear tester (RST-01.pc) (Fig.4) is used to evaluate Effective angle of
friction and the Flow function (FF) of the coal sample at various moisture levels.
The standard shear cell is homogenously filled with the sample of -2.36mm size
by avoiding large voids and the excess material is scraped off in level with the
top of the shear cell. It is carefully placed on the driving axle of the ring shear
tester and the sample is subjected to shearing (Fig.4). Bunker storage time tests
were carried out in a Consolidation Test Bench to evaluate undisturbed storage
effect for 24 and 72 hours.
The wall friction tests were also carried on Ring Shear tester using the wall liners
Mild steel (rusted), SS304 (2B), IS2062, SS409M (2D) and UHMWPE. The liners
(Fig.5) were cut to the required shape and dimensions and placed in the
appropriate shear cell. The sample to be tested is homogeneously filled up to the
top of the shear cell. The cell is placed on the driving axle of the ring shear tester
and the sample is subjected to shearing against the wall liner under different
stress conditions.
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Bunker coal flow study – TPS Anpara-D
Fig.4 Jenike-Schulze Ring Shear Tester (RST-01.PC)
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Bunker coal flow study – TPS Anpara-D
SS304(2B) Mild Steel (rusted)
UHMWPE SS409M(2D)
IS 2062
Fig.5 Photographs of Liners Tested
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Bunker coal flow study – TPS Anpara-D
4. EXPERIMENTAL DATA GENERATED
The interactions of coal flowing within itself and against different bunker wall/
liners are determined from the shear test data. The data generated at 16.4%,
19.4%, 22.3%, and 25.3%mc for coal sample was analyzed using RST-
CONTROL 95 software for plotting yield loci and constructing Mohr stress circles
to evaluate the relevant flowability parameters which forms the basic design
criteria for suggesting bunker/silo configuration for Mass Flow. Typical treatment
of yield loci, wall friction curves (phiX) and flow function (FF) curves are
presented in Fig. 6 to 10. The flowability parameters determined are presented in
Table-4.
Fig. 6 Typical treatment of yield Locus (25.3 %mc, Level-2)
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Bunker coal flow study – TPS Anpara-D
Fig.7 Kinematic angle of wall friction (16.4%&19.4%mc)
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Bunker coal flow study – TPS Anpara-D
Fig.8 Kinematic angle of wall friction (22.3%&25.3% mc)
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Bunker coal flow study – TPS Anpara-D
02000400060008000
10000120001400016000180002000022000
0 10000 20000 30000 40000 50000
Major Consolidation Stress, Pa
Unc
onfin
ed Y
eild
Stre
ss, P
a
16.4% mc19.4% mc22.3% mc25.3% mc
Fig.9 Flow functions
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Bunker coal flow study – TPS Anpara-D
Fig.10 Flow function and Time Flow functions (25.3%mc, 24hrs and 72hrs)
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Bunker coal flow study – TPS Anpara-D
R&D Centre, NMDC Ltd Page 26
TABLE-4 FLOWABILITY PARAMETERS
Kinematic angle of wall friction, deg, Phix
Moisture content
Effective angle of friction,
deg,
δ IS2062 Mild Steel(rusted) SS304(2B) SS409M(2D) UHMWPE
16.4% 49Arctan[71/Sigmaw
+Tan(18.1)]
Arctan[173/Sigmaw
+Tan(20.3)]
Arctan[198/Sigmaw
+Tan(18.1)]
Arctan[74/Sigmaw
+Tan(17.0)]
Arctan[10/Sigmaw
+Tan(14.5)]
19.4% 53Arctan[170/Sigmaw
+Tan(18.7)]
Arctan[269/Sigmaw
+Tan(20.6)]
Arctan[519/Sigmaw
+Tan(18.2)]
Arctan[407/Sigmaw
+Tan(17.4)]
Arctan[25/Sigmaw
+Tan(19.2)]
22.3% 54Arctan[448/Sigmaw
+Tan(16.7)]
Arctan[554/Sigmaw
+Tan(19.1)]
Arctan[586/Sigmaw
+Tan(16.3)]
Arctan[536/Sigmaw
+Tan(15.9)]
Arctan[328/Sigmaw
+Tan(19.4)]
25.3% 56Arctan[431/Sigmaw
+Tan(17.5)]
Arctan[575/Sigmaw
+Tan(19.6)]
Arctan[527/Sigmaw
+Tan(16.6)]
Arctan[449/Sigmaw
+Tan(17.7)]
Arctan[379/Sigmaw
+Tan(18.6)]
Bunker coal flow study – TPS Anpara-D
5. RESULTS & DISCUSSION
5.1 Assessment of Coal Flowability The Flowability of coal at different moisture levels is characterised by their flow
function curves (Fig.9). Based on these curves along with wall yield loci, an
assessment of flowability of coal tested is given below.
• The results of testing indicate that the tested coal is compressible and has
moderate angles of internal friction and moderate bulk strength. Based on the
Jenike’s classification, the coal can be classified cohesive at 16.4% mc and
very cohesive thereafter.
• The flow function curves indicate that the coal exhibits highest yield strength
at a moisture content of 25.3% particularly at low consolidation stresses
(below 12 Kpa). Hence the storage time tests were conducted on coal at
25.3%, which is considered the critical moisture. There is a significant
increase in the bulk strength of coal after 24 hours and 72 hours of
undisturbed storage (Fig.10).
• Majority of the wall yield loci are not passing through origin and exhibit some
cohesion/adhesion particularly at elevated moisture contents. In other words,
the wall friction angle depends on normal stress (Fig.7&8). It implies that in
such cases, the minimum slope of the hopper to promote Mass Flow will vary
with the outlet dimension of the hopper.
• The average effective angle of internal friction (delta) varies from 490 to 560
depending on the moisture content of coal (Table-4). This forms one of the
factors for further evaluation of design parameters.
5.2 Mass Flow Design Parameters Based on the Jenike theory, the Mass Flow bin design parameters like minimum
slope (θc or θp) and outlet dimension (Dc or Dp) to prevent cohesive Arching in
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Bunker coal flow study – TPS Anpara-D
Hoppers were established using the flow parameters like effective angle of
friction, wall angle of friction and flow function. The results are presented in
Table-5&6. The minimum slope of hopper for Mass Flow varies with outlet size
(slope decreases with increase in outlet size) in some cases, which is due to the
cohesion/adhesion exhibited by bulk solids against the liner tested. The variable
slopes with different outlets are given in Table-6. This may be used in the bunker
design process. The above given results are minimum dimensions and hence
steeper angles and larger outlets than given above are permitted. Some of the
salient points are as follows.
• The coal at 25.3%mc, requires a critical (minimum) outlet dia. of 0.70m to
prevent ‘cohesive arching’ at instantaneous condition (without storage) in
case of conical hoppers (Table-5).
• The storage time test at 25.3% mc for 24hrs and 72hrs of storage indicate
that there is significant effect of storage on the coal flowability. The
storage time test at 25.3%mc indicate that the critical (minimum) outlet dia.
required is 1.0m for 24 hours and 1.26m for 72 hours of storage to
prevent the formation of cohesive arching above the outlet in the conical
hoppers (Table-5).
• The minimum conical hopper slope (with horizontal) required for Mass Flow
at the outlet dia. of 0.914m (0.914m was chosen out of general practice in
coal bunkers of 500MW thermal power plant) is 73.50 with stainless steel SS304(2B) liner, 71.30 with SS409M(2D finish) to handle coal at all moisture levels (Table-6).
• The Mass Flow slopes for a wedge shaped (slot outlet) hopper is 100 less
(minimum, varies depending on the outlet size and wall friction) compared to
a conical hopper and the minimum outlet dimension is typically half that of the
conical hopper. The Mass Flow slopes and minimum outlet dimensions for a
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Bunker coal flow study – TPS Anpara-D
wedge shaped hopper are also presented in Table-5&6 as a part of this
study.
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Bunker coal flow study – TPS Anpara-D
6. RECOMMENDATIONS The following recommendations have been proposed based on the test results.
• It is recommended that the proposed coal bunkers of the thermal power plant be
designed to promote Mass Flow. The design parameters presented in the report
should be followed to ensure Mass Flow of coal.
• For a typical case of a conical hopper with outlet dimension of 0.914m, the
minimum slope (with horizontal) should be 71.30 with SS409M (2D) liner in the
converging hopper portion to promote Mass Flow. Whereas the SS304 (2B) liner
is calling for a minimum slope of 73.50.
• The minimum outlet size required to prevent cohesive arching in case of a
circular outlet (dia) is 1.0 m and 1.26m to initiate coal flow after 24 and 72 hours
of undisturbed storage at critical moisture.
• The typical outlet dimension of 0.914m (dia) for the coal bunkers is sufficient to
prevent cohesive arching above the bunker outlet while handling tested coal at all
moisture levels and undisturbed storage in the bunker for less than 24 hours.
However, upon extended undisturbed coal storage beyond 24 hours, the coal is
likely to form an Arch at the outlet leading to no flow condition.
• This problem can be overcome by any one of the following options. 1) To Increase the outlet size to the required 1.26m dia. at the feeder
interfacing. This needs change of gravimetric feeder design which is
however generally not preferred due to increase in belt width. 2) To locate a shut off gate at hopper sectional dia of 1.26m instead of placing
at the 0.914 m dia. During the shut down, the shut off gate should be closed
and coal below the gate is to be emptied by running through the system. In
such an arrangement, the effective outlet size would be 1.26m. This will
facilitate to initiate the coal flow satisfactorily from the silo after extended
period of undisturbed storage up to 72 hours. This option is the most feasible
one in the present case.
R&D Centre, NMDC Ltd Page 30
Bunker coal flow study – TPS Anpara-D
• If a different liner like UHMWPE and geometric shape is chosen by any reason,
the design parameters presented in the report for the liners concerned should be
followed strictly to ensure Mass Flow.
R&D Centre, NMDC Ltd Page 31
Bunker coal flow study – TPS Anpara-D
TABLE-5
MINIMUM OUTLET DIMENSION TO PREVENT COHESIVE ARCHING AT CRITICAL MOISTURE
Sample Moisture Content Condition Dc
(m) Ds (m)
Dp (m)
Instantaneous 0.70 0.63 0.35
24 Hours storage 1.0 0.9 0.5
Coal
25.3%
72 Hours Storage 1.26 1.14 0.63
R&D Centre, NMDC Ltd Page 32
Bunker coal flow study – TPS Anpara-D
TABLE-6 MINIMUM HOPPER SLOPES FOR MASS FLOW AT A GIVEN OUTLET DIMENSION
Mass Flow Hopper slope (degrees, with horizontal)
IS 2062 MS (Rusted)
SS304 (2B)
SS409M (2D) UHMWPE
Moisture Content
(%)
Outlet Dimension
(Dia or Width)
(m) θc θp θc θp θc θp θc θp θc θp
0.5 66.7 54.3 72.2 58.8 70.5 56.4 65.6 52.9 60.7 48.4
0.914 65.8 53.8 70.0 57.6 67.8 54.9 64.6 52.7 60.5 48.4
1.5 65.3 53.5 68.9 57.0 66.6 54.2 64.1 52.0 60.5 48.3
2.0 65.2 53.4 68.5 56.8 66.1 53.9 63.9 51.9 60.4 48.3
16.4
2.5 65.1 53.3 68.3 56.6 65.8 53.8 63.8 51.9 60.4 48.3
0.5 70.9 57.1 75.8 61.1 80.5 62.6 76.6 59.7 67.0 55.1
0.914 68.6 55.7 72.2 59.1 73.5 58.4 71.0 56.4 66.7 54.9
1.5 67.4 55.0 70.6 58.1 70.4 56.5 68.4 54.8 66.5 54.8
2.0 67.0 54.8 69.9 57.7 69.1 55.7 67.4 54.2 66.4 54.8
19.4
2.5 66.7 54.6 69.5 57.4 68.3 55.3 66.8 53.8 66.4 54.7
0.5 76.9 59.6 81.9 64.1 80.3 61.5 78.6 60.2 76.2 60.7
0.914 70.8 55.9 74.8 59.8 72.4 56.7 71.3 55.8 71.8 58.1
1.5 68.1 54.2 71.7 57.8 69.0 54.6 68.1 53.8 69.8 56.9
2.0 67.0 53.6 70.4 57.0 67.6 53.7 66.8 53.0 69.0 56.4
22.3
2.5 66.3 53.2 69.6 56.6 66.7 53.2 66.0 52.3 68.6 56.1
0.5 75.5 59.2 80.7 63.7 76.9 59.6 76.1 59.7 75.3 59.7
0.914 70.5 56.2 74.5 59.9 70.9 55.9 71.0 56.6 71.0 57.2
1.5 68.3 54.9 71.7 58.2 68.2 54.3 68.5 55.2 69.0 56.0
2.0 67.4 54.3 70.5 57.5 67.1 53.6 67.7 54.6 68.2 55.5
25.3
2.5 66.8 54.0 69.8 57.0 66.4 53.2 67.1 54.3 67.8 55.2
R&D Centre, NMDC Ltd Page 33
Bunker coal flow study – TPS Anpara-D
Blank Page for Notes
R&D Centre, NMDC Ltd Page 34