BCURA PROJECT B54 on Arching propensity in coal bunkers with non-symmetric geometries ...

106 Pages·2005·6.64 MB·English

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University of Edinburgh
Institute of Infrastructure and Environment
Silos and Granular Solids Research Group

BCURA PROJECT B54

on

Arching propensity in coal bunkers
with non-symmetric geometries

Final report

Project start date: 01.09.01 end date: 31.08.05

Project Officer: Mr M. Jones
UK COAL PLC
Harworth Park, Blyth Road,
Harworth, Doncaster DN11 8DB, UK
Tel:[phone] Fax: [phone]
Email: [email protected]

Investigators: Dr J. Y. Ooi and Prof. J. M. Rotter
Researcher: Dr Shiwen Wang

Project Manager: Dr J.Y. Ooi
SCHOOL OF CIVIL & ENVIRONMENTAL ENGINEERING
The University of Edinburgh
Edinburgh EH9 3JN, UK
Tel: [phone] Fax: [phone]
Email: [email protected]

CONFIDENTIAL

September 2005
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Contents

EEXXEECCUUTTIIVVEE SSUUMMMMAARRYY........................................................................................................................................................................................................................................44
11.. IINNTTRROODDUUCCTTIIOONN............................................................................................................................................................................................................................................................66
1.1 PROBLEM STATEMENT...................................................................................................................6
1.2 AIMS AND OBJECTIVES..................................................................................................................7
22.. RREEVVIIEEWW OOFF JJEENNIIKKEE AARRCCHHIINNGG TTHHEEOORRYY..................................................................................................................................................................77
2.1 EXAMINATION OF THE JENIKE FLOW FUNCTION TEST.....................................................................7
2.2 MECHANICAL MODELS FOR THE MECHANICAL BEHAVIOUR OF WET COAL.....................................8
2.3 MULTI-STRAND COMPUTATIONAL STRATEGY................................................................................8
33.. IINNDDUUSSTTRRIIAALL SSIITTEE SSUURRVVEEYY....................................................................................................................................................................................................................88
3.1 COAL PROPERTIES.........................................................................................................................9
3.2 BUNKER GEOMETRY......................................................................................................................9
3.3 LINING STRUCTURE AND WALL FRICTION....................................................................................10
3.4 FILLING SEQUENCE AND DISCHARGE AID.....................................................................................11
3.5 SOME INNOVATIONS....................................................................................................................11
44.. CCOOMMPPUUTTAATTIIOONNAALL MMOODDEELLLLIINNGG OOFF AA SSYYMMMMEETTRRIICC CCOONNIICCAALL HHOOPPPPEERR......................................1111
4.1 EXPLORATION OF CAM CLAY MODEL AND STRESS HISTORY IN COAL...........................................11
4.2 GEOMETRY OF THE HOPPER.........................................................................................................12
4.3 FE MESH AND MATERIAL PARAMETERS.......................................................................................12
4.4 BOUNDARY CONDITIONS AND LOADING.......................................................................................12
4.5 RESULTS AND DISCUSSIONS.........................................................................................................12
4.6 COMPARISONS WITH ANALYTICAL MODELS.................................................................................13
4.6.1 Summary of analytical models.......................................................................................................13
4.6.2 Comparisons..................................................................................................................................13
55.. SSTTRREESSSSEESS IINN TTHHEE CCOOAALL AABBOOVVEE AARRCCHHEEDD OOUUTTLLEETT OOFF SSYYMMMMEETTRRIICCAALL BBUUNNKKEERRSS1144
5.1 INTRODUCTION............................................................................................................................14
5.2 THE DETAILS OF THE MODEL.......................................................................................................15
5.3 RESULTS......................................................................................................................................15
5.3.1 Wall Pressure studies....................................................................................................................15
5.3.2 Comparisons for wall pressures predicted by theoretical and numerical models.........................17
5.3.3 Stress ratio affected by arch and hopper half angles....................................................................19
5.4 STRESSES ABOVE OUTLET USING CRITICAL STATE THEORY........................................................19
5.4.1 Case 1: Hopper with selfweight.....................................................................................................19
5.4.2 Case 2: Hopper with selfweight and surcharge............................................................................19
66.. SSTTRREESSSSEESS AABBOOVVEE AANN AARRCCHHEEDD OOUUTTLLEETT IINN UUNNSSYYMMMMEETTRRIICCAALL BBUUNNKKEERRSS..............................2200
6.1 THE DETAILS OF THE MODEL.......................................................................................................20
6.2 THE EFFECTS OF WALL FRICTION.................................................................................................20
6.3 THE EFFECTS OF UNSYMMETRICAL ARRANGEMENTS...................................................................21
6.4 PADDLE POSITION.......................................................................................................................21
6.5 THE EFFECTS OF OUTLET HEIGHT................................................................................................22
6.6 THE EFFECTS OF HOPPER HEIGHT................................................................................................22
6.7 REMARKS....................................................................................................................................22
77.. SSTTUUDDYY OOFF AA TTYYPPIICCAALL UUKK CCOOAALL BBUUNNKKEERR........................................................................................................................................................2233
7.1 INTRODUCTION............................................................................................................................23
7.1.1 Geometry.......................................................................................................................................23
7.1.2 Mesh..............................................................................................................................................23
7.1.3 Material Parameters.....................................................................................................................23
7.1.4 Loading and boundary condition..................................................................................................24
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7.2 RESULTS......................................................................................................................................24
7.2.1 Flow propensity analysis...............................................................................................................24
7.2.2 Wall Friction Effect.......................................................................................................................24
7.2.3 Paddle Feeder Position.................................................................................................................25
7.2.4 Progressive filling.........................................................................................................................26
7.3 DISCUSSION................................................................................................................................26
88.. CCOONNCCLLUUSSIIOONNSS..............................................................................................................................................................................................................................................................2277
RREEFFEERREENNCCEESS..........................................................................................................................................................................................................................................................................2299
AAPPPPEENNDDIIXX AA:: CCAAMM--CCLLAAYY MMOODDEELL............................................................................................................................................................................................3322
A.1 GENERAL FORMULATION............................................................................................................32
A.1.1 Cam clay model simplified into three dimensional model............................................................33
A.1.2 Cam clay model simplified into tri-axial compression model.......................................................33
A.2 HARDENING LAW.......................................................................................................................34
A.2.1 Exponential form...........................................................................................................................34
A.2.2 Piecewise linear form....................................................................................................................34
A.3 CRITICAL STATE LINE.................................................................................................................34
AAPPPPEENNDDIIXX BB SSTTUUDDYY OOFF SSTTRREESSSS HHIISSTTOORRYY BBYY TTRRIIAAXXIIAALL CCOOMMPPRREESSSSIIOONN........................................3366
B.1 GEOMETRY OF THE MODEL.........................................................................................................36
B.2 MESH AND MATERIAL PARAMETERS...........................................................................................36
B.3 BOUNDARY CONDITIONS AND LOADING......................................................................................36
B.4 HARDENING EFFECTS..................................................................................................................36
B.5 SOFTENING EFFECTS....................................................................................................................38
B.6 LOADING METHOD EFFECTS........................................................................................................38
B.7 ELEMENT EFFECTS......................................................................................................................38
B.8 LOADING PATH EFFECTS.............................................................................................................39
B.9 DISCUSSION AND REMARKS.........................................................................................................40
AAPPPPEENNDDIIXX CC SSUUMMMMAARRYY OOFF SSEEVVEERRAALL AANNAALLYYTTIICCAALL SSOOLLUUTTIIOONN..........................................................................4422
NNOOTTAATTIIOONN....................................................................................................................................................................................................................................................................................4433
FFIIGGUURREESS............................................................................................................................................................................................................................................................................................4444
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Executive summary
Coal handling problems can cause serious unanticipated economic losses when the coal does
not flow as expected from a bunker. A comprehensive survey (EPRI, 1995) on coal handling
problems concluded that plugged bunkers and feeders are the biggest handling problems in this
industry. In the UK, problems of coal arching and bridging in bunkers frequently occur at
some power stations and human intervention is needed to break up the arches. These
blockages can be very costly.

Arching or bridging at a bunker outlet occurs when the stresses in the coal near the outlet are
not sufficiently large to overcome the cohesive strength that permits an arch to form over the
outlet. The mechanics of the problem depend on two major considerations:
a) coal flow properties (characterised by the coal’s Flow Function) ;
b) the bunker design (geometry, hopper angles, wall friction, feeder arrangement etc.),
traditionally characterised by the Hopper Flow Factor.

The aim of this project is to examine the practical coal bunker geometries and to investigate
and quantify their propensity for arching. This was achieved through the use of modern
computational modelling techniques that can predict the performance of a coal of given
properties when placed in a bunker of given geometry and surface wall friction. The objectives
of the whole project were:

1. to conduct a survey of existing power station coal bunker geometries;
2. to develop computational models of typical existing coal bunker geometries and feeder
arrangements using a finite element method with appropriate constitutive models for the
coal and the coal-bunker interface;
3. to determine the stress history of the coal as it passes from the bunker to the outlet (so
evaluating the major consolidating stress applied to the coal);
4. to model the coal in various arched geometries (so evaluating the stress at incipient arch
collapse);
5. using this information, to extend the Jenike flow factor design approach to cover these
cases of non-standard bunkers;

The influence of coal flow properties on handlability has been extensively investigated by the
Edinburgh University Group recently through BCURA funding, leading to the successful
design and development of two industrial testers. However, it is not sufficient only to measure
the coal properties. Poor handling coal can pass easily through some bunkers, but even good
handling coal may block a bad bunker geometry. The key scientific information on bunker
outlets for arching prediction is only available for very simple conical and wedge shaped
hoppers, with a horizontal hole outlet. Such geometries are rarely found in UK power stations,
and the interpretation of current design methods to give appropriate predictions of arching
requires much speculative engineering judgment.

The most significant outcome of this project is the new design information that can be used to
eliminate arching problems in typical coal bunkers with the typical geometries and outlet
arrangements used in UK coal bunkers, which differ considerably from those considered in
Jenike’s classic arching theory. The new calculations permit the Jenike design method to be
extended to the hopper outlet forms used in power stations in the UK.
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This main work commenced with careful measurements of power station bunker geometries.
Then a computational finite element model was created and verified for the modelling of coal
in such bunkers. The model was first used to predict consolidation stress states when the
bunker was filled, considering a range of different material properties for the coal and different
wall friction coefficients for the hopper walls i.e. different hopper linings. The computational
model was then used in more difficult and extensive calculations to examine the stresses that
develop in different shapes of coal bridges or arches across the outlet, to determine the most
critical arch form and to deduce the conditions under which cohesive strength in the coal
would just cause a stable arch to form. These calculations led to different outcomes depending
on the geometry of the hopper sides, the hopper wall friction (with or without low friction
liners), and the size of the outlet. The calculations addressed all these items in a huge
parametric study, where all were systematically varied.

Not only are these calculations very extensive, yielding a huge mass of information, but they
are complicated to interpret, and much effort has been put into transforming them into design-
relevant information that can be used in the Jenike design method for these bunkers. The main
achievement of this project is the development of a method for assessing typical UK coal
bunker geometries according to a modified version of the Jenike method for predicting arching
across the outlet of a classic symmetrical bunker. This gives a rigorous basis for addressing
arching problems and evaluating arching propensity in coal bunkers. The results will be
beneficial to the owners and manufacturers of coal bunkers.
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1. Introduction
1.1 Problem statement
The bulk handling of coal is important in many industries, such as mining, power generation and steel
making. Problems in coal handling can seriously affect the reliable supply of coal and cause
unanticipated hidden costs. A comprehensive survey on coal handling problems concluded that
plugged bunkers and feeders are the biggest handling problems facing the industry. Another survey
from the North American Electric Reliability Council (NERC) showed that these problems are not rare
events (nearly 1000 events were reported on bunker flow problems in the period 1982-87 alone)
resulting in major costs to some plants (EPRI, 1995). In the UK, problems of coal arching in bunkers
occur frequently in some power stations, and regular human intervention (e.g. the use of air lances) is
needed to break up the arches.

Arching at a bunker outlet occurs when the stress field in the coal near the outlet is not sufficient to
break down an arch which is held together by the cohesive strength developed in the coal (Jenike,
1964). The mechanics of the problem depend on the coal flow properties (characterised by the coal’s
Flow Function) and the bunker design (geometry, hopper angles, wall friction, feeder arrangement etc.),
traditionally characterised by the Hopper Flow Factor.

Bunker design is normally performed using the Jenike method (Jenike, 1964; Rotter, 2001), which
includes some simplifying approximations, and which is difficult to apply to geometries where the flow
factor has not been rigorously evaluated. Until this project, flow factors only existed for symmetrical
conical or planar (wedge) bunkers (Fig. 1.1). The concept of arching used in the existing theory is
illustrated in Fig. 1.2. This project was not concerned with mechanical arching, since this is easily
prevented. Instead, it was exclusively concerned with cohesive arching.

The standard method of assessing arching potential (Rotter, 2001) relies on an analysis of stresses in the
hopper that relate only to these symmetrical geometries (Figs 1.3 & 1.4). British coal bunkers often
have more complicated geometries and outlet arrangements (Fig. 1.5), and the standard method cannot
be applied directly.

As each coal bunker is different, each currently needs an individual evaluation, and it is far from a
simple matter to determine which of several alternative remedies should be chosen to be the cheapest
reliable solution for handling problems. There was thus a need to develop new more accurate
predictions with a wider range of applicability. This has been achieved using modern nonlinear finite
element computer modelling.

The finite element method has been used with a variety of different constitutive material models to
predict pressures and flow in silos (e.g. Haussler and Eibl, 1984; Rombach and Eibl 1989; Ooi and
Rotter, 1990; Schwedes and Feise, 1993; Kolymbas, 1993; Ragneau et al, 1994; Ooi and She, 1997).
However almost all these studies focused exclusively on improving constitutive models and modelling
techniques, which were then applied to relatively simple geometries in symmetrical bottom-discharging
gravity flow silos. A numerical model of arching in a typical UK coal bunker with a twin horizontal
outlet and paddle feeder arrangement (e.g. Fig. 1.5) does not appear to have been attempted before the
present study.

The results of this study apply to most British coal bunkers. They have been calibrated against the
analytical studies of Jenike which were undertaken in the 1960s, which are still used throughout the
world as the best available information.

The outcome of the present study does not depend on the method by which the flow properties of the
coal are evaluated, since the calculations have adopted the Jenike philosophy. Thus, they are usable
with any reliable method for measuring the flow properties of the coals, including the Jenike Shear
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Cell, Schulze Annular Shear Cell, Edinburgh Cohesion Tester etc. One of the most direct benefits of
this project is that it should now be possible to re-evaluate each existing bunker and assign a maximum
unconfined strength for reliable flow in it.

The project has exploited recent research on coal handling and bunker design conducted at Edinburgh
and funded by BCURA, in which the interactions between stress history, moisture content and particle
size distribution as affected by segregation were found to have a major influence on coal handling
performance (Zhong et al, 2000; Rotter and Ooi, 2000; Zhong et al, 2005).
1.2 Aims and objectives
In this project, different practical bunker geometries were examined to investigate and quantify their
propensity to stop flowing. This was achieved through the use of modern computational modelling
techniques that can predict the performance of a coal of given properties when placed in a bunker of
given geometry and surface wall friction.

The objectives of the whole project were:
6. to conduct a survey of existing power station coal bunker geometries;
7. to develop computational models of typical existing coal bunker geometries and feeder
arrangements using a finite element method with appropriate constitutive models for the coal and
the coal-bunker interface;
8. to determine the stress history of the coal as it passes from the bunker to the outlet (so evaluating
the major consolidating stress applied to the coal);
9. to model the coal in various arched geometries (so evaluating the stress at incipient arch collapse);
10. using this information, to extend the Jenike flow factor design approach to cover these cases of
non-standard bunkers;
These objectives have all been satisfactorily achieved.
2. Review of Jenike arching theory
A review has been undertaken of the Jenike arching theory and its application in practice, including the
determination of the flow function by testing and the flow factor evaluation using classical mechanics
theory.

This work has led to a number of interim conclusions
a) The Jenike arching theory is relatively simple, so some modifications to it can be made even for
standard outlet geometries;
b) The application of the Jenike method depends heavily on the analysis that leads to the flow
factor for the hopper. It has been found that the method is rather variable in its accuracy when
different properties are assumed for the coal. New proposals for modifications to this classic
theory are still being developed and verified;
c) The application of the Jenike method leaves quite a lot to be desired. In particular, the stress
history of the solid that arches in the bunker is seriously over-simplfied because that was all
that could be done in the 1960s. The computer programs that have been used in this project
overcame this stress history difficulty and produced a much clearer image of true arching
situations. However, the full stress history was found to be very complicated indeed, and many
numerical problems had to be overcome in the search for sound solutions.
d) The Jenike method could not be applied to bunkers with the outlet geometries commonly used
in Britain (Fig. 1.5). This project has focussed on these other geometries.
2.1 Examination of the Jenike flow function test
The entire world bulk solids handling community depends on the Jenike flow function test for its
predictions of arching in all bunkers. This test is conducted in a shear cell, in which the stress state is
not well defined. Under this project, some work was undertaken to assess the real behaviour of coal
placed in this tester, so that this behaviour can be related to the conditions under which arching can
occur at a hopper outlet. However, the relationship proved to be more difficult to establish than was
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originally thought, and it was found that this part of the project could not be completed within the
timescale, so it was abandoned before it was complete.

Nevertheless the specific conclusions arising from this study include:
a) The Jenike tester is difficult to use and slow, and it is not easy to produce identical results with
it every time. For this reason, some doubt must be cast on many of the precise values of
properties that come from the test.
b) This tester is completely unsuitable for coal testing, because it cannot accommodate particles
larger than a few millimetres in size.
c) Several research groups world-wide have attempted to devise new testers that can reveal the
coal behaviour more accurately than the Jenike tester.
d) The task of relating measurements made in other testers, such as the Edinburgh Cohesion
Tester, to those of the Jenike cell is quite complicated, and requires a relatively complicated
numerical analysis to determine the relationships.
2.2 Mechanical models for the mechanical behaviour of wet coal
The numerical modelling by computer used in this project required that the behaviour of the coal should
be represented in an appropriate manner. The Jenike theory assumes that coal is a rigid-plastic material
obeying a simple frictional failure criterion at all points. Whilst the computer model could also
represent it approximately in this manner, it is a considerable oversimplification, and this idealisation
omits key aspects of the stress history arising from placement and flow of the coal.

Interim conclusions that have been reached in this project include:
a) A simple elastic model should be used for parametric explorations of a wide range of
geometries of both bunkers and arch shapes;
b) A simple elastic-plastic model using a Mohr-Coulomb or Drucker-Prager failure criterion
should be used to assess Jenike theory against more modern calculations.
c) It was thought that it would be very useful to conduct a few analyses using a full stress history
for the coal. However, this was found to be a very complicated task. The Modified Cam Clay
mathematical model was chosen as probably the most appropriate for this purpose, but it was
known to have difficulties when the stress level becomes very low, as in the bunker arching
problem. Although this full stress history study was attempted in this project, it ran into many
difficulties associated with the highly overconsolidated material and the prediction of very high
strength with very brittle behaviour.
d) The main predictive outcomes from this project were achieved with an elastic-plastic model
using a Drucker-Prager failure criterion. This was calibrated onto the unconfined compressive
strength of the coal.
2.3 Multi-strand computational strategy
The plan for computational work was set out as follows:

a) The different constitutive models were each used for the greatest applicable information.
b) Elastic stress calculations were used to determine stress states in a wide range of arch
geometries to ensure that the worst conditions were identified.
c) The classical bunker outlet geometry was used to calibrate the predictions.
d) The geometries of existing British bunker outlets were carefully measured and documented and
adopted into the calculation plan.
3. Industrial site survey
The first industrial surveys at West Burton and Drax Power Stations were carried out on 26th and 27th
July 2002.

West Burton Power Station then belonged to the London Power Company (LPC) (now owned by EDF
Energy Ltd). Besides West Burton Power station, LPC also owned Cottam Power station, Sutton Bridge
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Power station and several others. West Burton Power Station was opened on 25th April 1969. It is a
2000MW power station. The station comprises four coal-fired generating sets and two 20MW gas
turbines. The station can burn up to 19,000 tonnes of coal a day, so the bunkers must be capable of
handling at least 19,000 tonnes of coal within one day.

Drax is now the largest coal-fired power station in Western Europe. It stands in 1854 acres and is
situated in North Yorkshire on the south bank of the River Ouse, midway between Selby and Goole. It
can produce enough electricity - about 4000 MW- to meet the needs of approximately four million
people. The station was built in two 2000 MW stages. It has six 660 megawatt, coal-fired generating
units. The first stage of construction began in 1965 and was completed in 1974. The second stage began
in 1978 and was completed in 1986.

Both power stations are coal-fired plant and have Flue Gas Desulphurisation (FGD) equipment
associated with their generating units. The coal receiving bunker is typical and its filling and
discharging operation is important to avoid disruption of power generation.
3.1 Coal properties
The principal coal properties that cause the greatest concern to power stations include ash, sulphur,
moisture and volatile matter contents, heating value and grindability. A high moisture content will
lower the boiler efficiency, whilst ash, sulphur and nitrogen may contribute to air pollution, acid rain
and global warming.

As Power Station Fuel, coal should be easy to handle. This further requires that the coal should develop
only a small amount of cohesion when compressed in a bunker. A well designed bunker can handle
coals with very poor handling performance, but the challenge in this project is to recommend limits on
the cohesion potential of coals so that they can be guaranteed to flow through the existing bunkers
without modification.

Table 3.1 shows some typical properties of UK coal. These data come from one coal mine belonging to
RJB Mining (UK) Ltd (Zhong, 2001).

Table 3.1 Properties of typical UK coal
Particle size
Coals Moisture range Ash content range CV§
(mm)
Washed coal (46%) 8% ~ 12% 7% ~ 13% <10 Very high
Singles (15%) 6% ~ 9% 5% ~ 8% 10~50 29270
Filter cake (6.5%) 25% ~ 34% 8% ~ 15% <1 19450
Foreign fines (7.5%) 8% ~ 12% 4% ~ 18% <6 2460
U/T# fines (25%) 7% ~ 9% 28% ~ 50% 10~50 Varying
Final PSF* 8% ~ 12% 14.0% ~ 19.2% <50 24260

§CV --- Calorific Value (kcal/kg) #U/T --- Untreated fines *PSF --- Power Station Fuel
3.2 Bunker Geometry
Four types of coal receiving bunker were found in the power stations visited during this survey. Their
geometries are explained separately in the following.

Type A (Fig. 3.1a): This is a concrete planar hopper, 55.47m long and 6.7m high with hopper wall
slopes of 30° to vertical(Fig.3.1a). This bunker is very similar to bunkers at Fiddler’s Ferry (Rotter and
Ooi, 2000). The bunker is partitioned into eight separate compartments (Fig.3.1b). At the top of the
bunker, there are two railway tracks along the length of the bunker.

At the level of the two outlets, there is an inverted cone making an angle of 60° with the vertical on a
concrete platform. This structure is different from that of Fiddler’s Ferry. In Fiddler’s Ferry, a structure
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termed a “Coal Bunker Centre Extension Cone” (colloquially known as the “dog kennel”) was
constructed at a date after its initial bunker construction.

The two outlets are horizontal slots above a concrete table on which the coal was intended to lie at its
angle of repose. For the coal to lie on this table without spilling onto the conveyor, it must have an
angle of repose greater than the angle defined by the slope from lip of the concrete bunker wall to the
edge of the table. From the original design, it measured an angle of 50°. Thus, unless the coal had an
angle of repose in excess of 50°, it would continuously fall from the table onto the conveyor. Coals
have a wide range of repose angles, varying from about 35° for very free-flowing coals to as high as
65° for highly cohesive coals. Additional plates (Fig.3.1c) are bolted onto the external wall of bunker
on the outside to reduce the repose angle required of the coal and help to control the discharge of free
flowing granular coals. This is similar to that of Fiddler’s Ferry.

The outlet is served by two travelling feeding machines, each of which carries a pair of counter-rotating
paddle feeders which withdraw coal from the feeding platform onto the conveyor. Other dimensions
and comparisons for the bunker are listed in Table 3.2:

Table 3.2. Dimension comparisons between two receiving bunkers
Fiddler’s Ferry West Burton
Bunker width at the top 9.6 m 9.8 m
Bunker width at the bottom 2.5 m 2.8 m
Width of discharge opening 0.63 m 0.67 m
Bunker half angle 30° 30°
UHMWP sheets, good Stainless steel sheets, good
Bunker lining material
condition condition
60° slope on lower part and
Dog kennel (Inverted cone) 60° slope for the whole part
45° slope on the upper part

Type B: The second kind of bunker shown in Fig. 3.2 is the “so called” milling bunker. It is comprised
by two arrays of rectangular compartments. Each compartment comprises six rectangular cells with a
length of 45.72m and a width of 22.86m. Bunker structures, one typical cell and their dimensions are
shown in Fig.3.2. Each cell is consisted by a vertical rectangular silo and an unsymmetrical trapezoidal
hopper. The side walls of the trapezoidal hopper are inclined in different angles with vertical in two
directions. The hopper walls are very steep, with angles of 67° to 75° degrees to the horizontal. The
total height of each cell is 15.36m. Coals are discharged automatically when trains go through the
tracks.

Type C: Fig. 3.3 shows one type of concrete bunker with twin horizontal outlets. The whole structure
of the bunker is in the shape of a letter W. Paddle feeders and belt conveyors are used to transfer coal
from the hopper to the next process.

Type D: Fig.3.4 shows an all metal hopper found in Drax power station. This hopper is completed
encapsulated in the coal operation (Fig.3.4a). Vibration is applied at the outlet to assist the withdrawal
of coal from the bunker (Fig.3.4b)
3.3 Lining structure and wall friction
Three kinds of lining have been observed in coal bunkers in power industry. The first type is found at
West Burton power station, in an un-used bunker. There are no linings at all. Heavy wear scratches can
be observed on the inside of the bunker (Fig.3.5a). The second type is UHMWP linings bolted onto the
inside of bunkers. This kind of lining has been observed in our previous survey at Fiddler’s Ferry
power station (Rotter and Ooi, 2000). A layer of 10mm thick UHMWP sheets is bolted into the
concrete bunker as the lining. The surface of the UHMWP sheets is very smooth with low wall
frictions. This kind of lining is also cheaper and easier to install and replace. The main disadvantage
for this lining is that it is susceptible to wear and tear. There is also the possibility of ageing in the