Other Tests
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.