Hall and Cutress [1] examined several methods to
measure cohesion and found the triaxial test the most sensitive to
changing material properties. As their aim was to measure flow
rates from rail wagons rather than when blockages occurred, they
developed the Durham Cone as a good practical measure of flow of
coal from rail wagons. The handling issue currently faced by the UK
and Australian coal industries is to determine when a coal will not
discharge consistently from a wagon and this is directly related to
cohesion.
Mikka and Smitham [2] used the Durham Cone to
examine the influences of moisture, fines and clays on the
handleability of coals. Their results on blends made with different
fines content showed that at low levels of fines the handleability
was not affected by the moisture content but as the fines increased
the handleability became very sensitive to moisture. They also
showed that the size distribution of the coal influenced how a
coal’s handleability was affected by fines and moisture
content. In comparing the prediction of handleability based on the
critical outlet diameter, as determined by the Jenike shear cell,
and based on the Durham Cone. The Durham Cone gave a better
prediction of the observed handleability for two coals.
Chambers and others [3], in ACARP project C6057,
examined in detail the effects of moisture and fines content on the
handleability of coal. They investigated the influence of particle
size and size distribution on moisture retention and unconfined
yield strength. As moisture is increased there is an increase in
unconfined yield strength to a maximum strength, then with further
increases in moisture there is a decrease in the unconfined
strength. The work was limited to two coals with different levels
of fines and moisture contents. They concluded that the most
appropriate test for handleability is the unconfined stress. They
used a prototype apparatus that is mechanically more complex but
similar in principle to the ECT.
An extensive study [4] was conducted by Casella
CRE Energy into the handleability of Australian PCI coals using the
Johanson Indicizer System (JIS) and ECT. The Johanson Indicizer
System (JIS) comprises three test machines; the Hang-up Indicizer,
the Hopper Indicizer and the Flow Rate Indicizer. As the JIS
requires a sample size of minus 6.7mm, both the JIS and ECT tests
were conducted on this size fraction. The results of the JIS and
ECT gave similar handleability ranking for the seven coals tested
over a range of moisture contents. When the coals were crushed to
GCI sizing no unconfined stress was measurable. An interesting
result of this research showed the sensitivity of unconfined stress
of some pulverized coals, as measured by ECT, to low moisture
levels. This possibly explains why some steel plants have
experienced blockages in
PCI
systems.
The Handleability Monitor [5] is a handleability
test developed by Nottingham University. The coal sample is pushed
through the venturi section by a hydraulic ram. The handleability
index is the maximum pressure obtained as the sample moves through
the venturi. According to the developers of this tester, the main
advantages are it can be used continuously and it can deal with
large particles.
Holuszko and others [6] conducted comparative
testing between the Handleability Monitor and the Durham Cone and
found, when tested under the same conditions, the tests were
comparable, though when measuring the influence of moisture the
Durham Cone was more sensitive. They observed the influence of size
distribution on the Durham Cone test results as noted by other
authors. They showed the wettability characteristics of the coal
influence handleability. They termed higher rank coals (low
equilibrium moisture) to be hydrophobic. These coals displayed no
handling problems up to the equilibrium moisture and then were more
prone to handling problems than the other coals tested.
Brown and Miles [7] reviewed the different tests
used to measure handleability. These tests are; Durham Cone, Jenike
shear cell, Handleability Monitor, Edinburgh Cohesion Tester and a
new method - avalanching technique. They concluded that the Durham
Cone and the Handleability Monitor were the only tests suitable for
the normal product sizing of -50mm. No comparative testing was
conducted in this review nor did the authors make reference to any
results from any other comparative study on handleability.
Arnold [8] summarised the research in the 1990's
to develop a coal Handleability Index (HI) for power stations. This
work was funded by the Electric Power Research Institute (EPRI).
The HI is based on the work of Jenike relating the critical opening
of a bin to unconfined yield strength (a material property) and bin
geometry.
Initially the unconfined strength was determined
by triaxial device where the consolidation of a column of coal was
achieved by pressurising a rubber boot around a column of coal
before the column was loaded to failure. To simplify the testing a
uniaxial device was designed. In the uniaxial device the coal was
placed in a PVC cylinder compressed from the top and then, after
rotating the cylinder, compressed again from the top. The PVC
cylinder was in three longitudinal sections for easy removal from
the coal column. To ensure the lever arm was level, before a load
was placed on the coal column, spacers were used to lift the coal
column. The load was applied to the column by placing coal in a
pail hung from the lever arm, the force required to collapse the
column was calculated from the weight of coal placed in the pail.
EPRI also evaluated the Durham Cone test and the Jenike Shear Cell
test, standard cell and modified cell for larger top size, and
found that the unconfined yield stress as measured by the triaxial
was the most reliable predictor of handleability.
A comparison between the HI determined by the
triaxial and the uniaxial methods showed that there was a very good
correlation between the different devices. Testing of the coal flow
from a two-ton bin for a wide range of moisture, ash and fines
contents showed the HI correctly indicated when a no flow condition
would occur. Based on the testing of 22 eastern bituminous US coals
a handleability classification system was proposed that relates
handleability to the percent passing 0.5mm and the moisture
content.
CSIRO [9] researched the causes of rail discharge
problems and identified that the loading of wagons was as an
important contributor to hangup in wagons and the position of first
impact is of principal importance in determining the location of
the consolidation in the wagons. Further research [10] showed that
the amount of jackhammering required during the unloading of a
train provided a simple method for quantifying unloading
performance, that is, when a coal would trend to be held-up in the
wagons. Information on jackhammering times was collected on six
coal products from two mines over a extended period of time. This
data showed that there were different unloading characteristics for
the different products, rail wagons used and the wagon loading
techniques. Auditing the forces during loading and transport to the
port showed that travel forces contributed less to consolidation of
the coal than did loading forces.
To clarify the influences of coal properties and
consolidation force a laboratory scale rail wagon was constructed
by CSIRO and a pilot scaled rail wagon by Queensland Rail. Around
40 Australian coals were tested in laboratory scale rig (Fig. 2) to
determine the influence of consolidating pressure on the amount of
coal discharged on opening the door of the rig (Fig. 3).
[1] Hall, D., Cutress, J., (1960), The effect of
fines content, moisture and added oil on the handling of small
coal, Journal of the Institute of Fuel, V. 33, 1960.
[2] Mikka, R., Smitham, J., (1985), Coal
handleability assessment, Third Australian Coal Preparation
Conference, Wollongong, Nov., 1985.
[3] Chambers, J., Liu, Y., Roberts, A., (2001),
Effect of various coal constituents on the storage and
transportation of coal, ACARP Project C6057.
[4] Black, N. Sullivan, K., Whitehouse, M.,
(2005), Improved measurement of handleability for quality control
of Australian coals in PCI and carbonisation applications,
ACARP
Project C11016, June, 2005.
[5] Brown, D., Atkin, B., (2000), The industrial
evaluation of a coal handleability monitor, UK Dept. Trade and
Industry, Report No. COAL R175, DTI/Pub URN 00/662, March,
2000.
[6] Holuszko, M., Laskowski, J., Brown, D.,
(2004), Handleability assessment of selected coals using Durham
Cone and Handleability Monitor, Coal Preparation, Vol 24,
2004.
[7] Brown, D., Miles, N., (2004), Assessment of
coal handleability, Coal Preparation, Vol 24, 2004.
[8] Arnold, B., (2004), Efficient handling of
coal for power plants: Development of a coal handleability index,
Coal Preparation, Vol 24, 2004.
[9] O’Brien, G., O'Brien, M., Firth, B.,
Nemeth, D., Graham, J., Gnanananthan, S., (2002), Investigation
into problems of discharging Queensland coals from bottom dump rail
wagons,
ACARP
Project C10061, November, 2002.
[10] O’Brien, G., Patterson, S.,O'Brien,
M., Graham, J., (2004), Improved coal wagon unloading by reducing
loading force,
ACARP
Project C12061, September, 2004.