Edinburgh Cohesion Tester
Starting in 1996 the Edinburgh Cohesion Tester (ECT) was developed over several years with funding from the British Coal Utilisation Research Association (BCURA) and UK Department of Trade and Industry to enable rapid assessments of a coal's handling characteristics. Several UK coal producers also assisted with the evaluation of the ECT in the UK.  ECT Testing can be carried out in Australia through  CoalTech.
As shown by many researchers the unconfined compression strength is a measure of the cohesive strength of a coal and is the property of the coal that determines a coal’s handleability characteristics. The ECT has been devised to measure directly the cohesion developed in a coal under a known consolidation pressure (Figure 1).
Figure 1 Edinburgh Cohesion Tester
Figure 1(a) shows the removable three-piece mould into which coal is placed when a uniaxial consolidation pressure is applied. Figure 1(b) shows the consolidated column of coal during loading to measure the unconfined compression strength by a load cell.  The maximum load applied by the jack is recorded by the digital display. The ECT uses similar principles as the triaxial and uniaxial testers developed in the early 1990's by Electric Power Research Institute (EPRI) of the US [1].  The same method was later adopted for  the tester developed by Newcastle University [2].
The ECT has been used by Edinburgh University to undertake extensive studies of many coals to discover key phenomena relating to handleability. The tester has been improved over the years of research with several new features, which significantly improve the test procedure and results, and substantially overcome the main problems encountered in other similar testers. These features include:
  • a three piece split mould used to minimise the disturbance while setting up the sample,
  • the mould is coated to reduce wall friction during vertical consolidation,
  • support of the mould on a spring loaded base to effectively consolidate the coal from top and bottom simultaneously and also to minimise wall friction effects,
  • sample height correction to avoid interference with the initial sample (cutting or adding material after consolidation), and
  • a digital display with a peak hold to record the maximum vertical force required to fail the sample.
A patent application for the novel features of this tester has been filed by Edinburgh University.
Predictions obtained with the ECT were compared with the results obtained using the Durham Cone [3,4]. Comparisons were made both in the laboratory and in field trials. The field evidence used was the time taken to unload a train and a detailed pro-forma report on each trainload of any observations of coal bridging in the receiving bunker or action required to aid discharging (air lances etc). These tests showed that the Durham Cone can distinguish between very good and very bad handling coals, but was erratic when coals close to the limiting acceptable handleability were tested, thus explaining why handleability problems still exist despite the thirty-year extensive usage of the Durham Cone.  The ECT results at the colliery successfully identified all 6 consignments out of a total of 50 consignments in the trial that were later reported to be problematic at the receiving power station. This allowed a much improved ranking of the coals and therefore gave a better indication of the possible handling problems of marginal coals.
Figure 2 shows that the ECT measurements for two problematic consignments, which indicates that handling problems in the receiving bunkers occurred when the ECT measured unconfined strength was around 3 kPa.
Edinburgh University found that handleability is very sensitive to moisture content in a highly non-linear manner.  Each coal has a critical moisture content at which its handleability reaches its worst condition. The critical moisture content may be low, and the coal may appear relatively dry.  The handleability of a coal is characterised by its Stress-Moisture-Cohesion (SMC) function that describes the effects of both stress and moisture on the particular coal.  This function is obtained by progressive wetting of the dry coal.
Zhong and others [5] devised a mathematical model to predict the resultant cohesion of a coal blend consisting of three source materials whose SMC functions are known. The non-linear character of the handleability of blends was successfully taken into account by using a blending factor, which can be found experimentally. The predicted cohesions of trial blends were found to be in very good agreement with experiments.
The speed of testing of the Edinburgh Cohesion Tester permits a substantial increase in the available information on the sensitivity of coal handleability to different key coal quality parameters. This tester can be easily transported and used by other laboratories. Repeatable tests can be achieved with only 15 minutes of operator training.
An agreement between The University Court of the University of Edinburgh and CoalTech Pty Ltd allowed CoalTech to import into Australia a manual Edinburgh Cohesion Tester.  Edinburgh University assisted with the development of the test procedures and interpretation of the results in a recent ACARP project. This project found that all coals tested had unique SMC functions and that the size distribution (not just the fines content) and ash content significantly influenced the handleability of the coals. The ECT is a superior test to the Durham Cone for investigating the causes of handleability problems as it gives a better discrimination between coals that may or may not have a handleability problem.
[1] Arnold, B., (2004), Efficient handling of coal for power plants: Development of a coal handleability index, Coal Preparation, Vol 24, 2004.
[2] Chambers, J., Liu, Y., Roberts, A., (2001), Effect of various coal constituents on the storage and transportation of coal, ACARP Project C6057.
[3] Ooi, J.Y., Rotter, J.M., Lahlouh, E.H. and Zhong, Z. (1998), Blind  trial on coals for rapid handling assessment, Proc., 6th Int. Conf. on  Bulk Materials Storage Handling and Transportation, I. E. Australia,  September, Wollongong, Australia.
[4] Zhong, Z., Rotter, J.M., Ooi, J.Y. and Armstrong, B. (2001), Rapid  assessment of handling performance for coals, Proc., 18th Int. Coal  Preparation Exhibition & Conf., Kentucky, USA, May, 47-61.
[5] Zhong, Z., Ooi, J.Y., Rotter, J.M. (2005), Predicting the handleability of a  coal blend from measurements on the source coals, Fuel,  Volume 84, Issue 17, December 2005, Pages 2267-2274
Influence of Moisture
The influence of moisture and stress is characterised by the Stress-Moisture-Cohesion (SMC) function.  From this function the maximum unconfined stress of a coal sample can be determine. The critical moisture is the moisture at which this maximum occurs. The maximum unconfined stress and the critical moisture are the most important handling properties of a coal product and the ECT allows these to be measured effectively.
In the recent ACARP project 15070 19 Australian coals were tested and it was found that all coals tested had unique SMC functions as shown in Figure 1.
Figure 1  SMC function for the coals tested in ACARP project
Influence of Size
Figure 1 shows the how the unconfined strength of one coal behaves for a wide range of top size and shows both the decreasing unconfined strength with decreasing top size for the large top sizes and more rapid increase unconfined strength with further reduction in top size.  The differing behaviour when coarse coal is removed and when fine coal is added was also found in the ACARP project 15070 .
As shown in Figure 2, all parameters relating to the size distribution of the coal have a significant influence of the maximum unconfined strength. One must remember for a fixed top size, whether 19mm as for Figure 2 or for normal product top size of 40 to 50mm, all these sizing parameters are closely related.
The Rosin Rammler “n” and the fines content have the best linear fit to maximum unconfined strength with r squared’s of 0.66 and 0.63 respectively.

Figure 1 Influence of top size for one coal (testing carried out by Edinburgh University)
Figure 2 Influence of size distribution on maximum unconfined strength for 19 coals, ( ACARP project 15070)
Influence of Ash Content
The dependence of cohesion strength on consolidation stress and moisture content is the stress- moisture-cohesion (SMC) function and depends on the size distribution, ash content (especially clays) and the surface properties of the coal particle (rank related). Several authors have examined the influence of ash, in particularly non-swelling and swelling clays, on handleability. Figure 1 shows how ash influence the  unconfined strength of 19 coals tested in ACARP project 15070.  There is a general trend for higher ash contents to lead to higher unconfined strengths, that is poorer handleability. The data only showed weak linear correlations of ash (%d) with the unconfined strength on as received sample and with maximum unconfined strength  with r squared's of 0.52 and 0.58 respectively.
Figure 1  Influence of ash on unconfined strength
Black and others (2005) [ ACARP Project C11016] conducted ECT testing on 7 pulverised coals to determine handleability in PCI distribution systems.  For the seven coals tested by Black and others the size distribution of the PF was similar and all coals had no unconfined cohesion strength at the moisture content after milling.  Figure 2 shows the SMC function for two coals of different rank (Aus L16 & Aus M17) all other coals were only tested at two moisture levels.
At higher moisture contents, one could be expected the cohesion strength of the PF at different moistures would follow similar SMC function due to the similar size distribution. This holds true for five of the coals at a consolidation of 70 kPa, as shown in Figure 2, whereas coals L15 and L18 do not follow the trends of the other coals. Mikka and Smitham [1] showed that in coarse coals clays could contribute to handleability problems. Clays in the PF may also be the cause of the higher unconfined strength of coals L15 and L18 at higher moistures levels.
Figure 1  ECT test results showing the influence of PF moisture on unconfined strength for different consolidation pressures, only coals Aus L16 and Aus M17 where tested at a range of moistures.   Data from Black and others (2005).
[1] Mikka, R., Smitham, J., (1985), Coal handleability assessment, Third Australian Coal Preparation Conference, Wollongong, Nov., 1985.