Technical Market Support » Thermal Coal
The objective of this report is to integrate the work undertaken in ACARP projects on ash fusibility conducted by ACIRL and the ICR. The objectives of the projects are to provide alternative procedures for the measurement of ash fusibility (ash fusion temperature - AFT) together with an adequate understanding of the mechanisms observed which can relate the test to coal performance in operating plant. Three tests have been considered:
- The ACIRL test was a visual observation of small ash pellets as they shrink during heating.
- The CSIRO test monitors shrinkage of loose ash in a thermomechanical analysis (TMA) instrument.
- The HRL test uses electrical conductivity as a fusion indicator.
The HRL test has been found to be unsatisfactory in that unreliable contact of the electrodes with the ash influences the result. The results of the other two tests are similar, with the CSIRO test essentially being a precise, instrumented version of the ACIRL test. Both these tests report progressive shrinkage of ash as it heats at similar rate to the AFT procedure.
The main emphasis of the ACIRL studies has been to establish the repeatability and reproductivity of the test. At ± 5ºC depending on frequency of measurements, this has been found to be substantially better than the AFT procedure. The CSIRO test should have better accuracy but this has yet to be established.
The accuracy indicated here, however, is of the temperature of particular shrinkage events. These events do not always correlate with the standard AFT's nor should they be expected to, given the uncertainties of the AFT procedure.
The issue then is the relating of the results from the ACIRL and CSIRO tests (which are provided as a continuous record of shrinkage against temperature) to both existing AFT's (which are based on four temperatures) and also to plant performances. Both projects have concentrated on this issue. The shrinkage traces have been explained in terms of events of liquid formation at the temperatures predicted by phase diagrams and are related to the ash chemistry. The results are understandable, in contrast to the lack of understanding related to the existing AFT's.
The CSIRO test has been used to provide a ranking of coal performance for slagging when compared against operating power plant data. However the development of correlations between plant performance and shrinkage data requires further work.
A theoretical basis for the temperatures when ash becomes sticky suggests that shrinkage levels of about 50% may be used in this correlation.
The ACIRL test demonstrated that it would be useful to rank the coals in terms of their performance for slagging when compared against operating power plant data.
The test successfully predicted the troublesome coals in the power stations.
The results show that the method is very sensitive and the smaller the recording interval, the greater the precision in recording real detail and not noise. Therefore the choice of recording frequency depends on the amount of detail required. For a routine test basis, recording every 10ºC is a recommended choice.
Heating rate within the limits studied was found not to affect the ash fusion behaviour in the ash furnace.
In the ACIRL improved ash fusion test, several distinct pathways were observed for 'pure' minerals. Some samples exhibited only minor movement at high temperatures. Other samples exhibited marked pillar shortening over a narrow temperature range. This was generally related to distinctive phase changes. Gradual pillar shortening over an extended temperature range was related to either the transformation of clays to mullite or glass or the dehydration of carbonates.
The quartz and apatite samples exhibited only minor movement at high temperatures
A new method for measurement of Ash Fusion Temperatures has been developed by ACIRL using essentially the same equipment as is used for the measurement of ash fusibility under Standards such as AS1038.15-87 and ASTM D1857-87. However, unlike the standard method the new method produces quantitative results of the progressive change during melting of the ash. Further, the new method has a much improved precision in determination of the temperatures at which these changes take place. Both the repeatability and reproducibility of the results are excellent and might be further improved by continuous video recording.
The CSIRO test, based on a TMA system, appears to provide accurate peak temperatures identifying rapid shrinkage events during ash heating which are related to the formation of eutectics identified of phase diagrams. The temperatures necessary for particular shrinkage levels, as well as the existence and magnitude of these peaks, provide alternative ash fusion temperatures. These temperatures may be related to plant design and to operational problems in existing plant. The development of slagging correlations with the present data set is inherently limited by the relatively small range of coals and power stations furnace slagging performances included in the study.
The ACIRL improved ash fusion test is a powerful tool for understanding the complex reactions that occur during the standard ash fusion test. This is particularly so when combined with XRD ash analysis and normative calculations. Whether these tools are the best combination or whether the improved ash fusion test should be combined with other tools such as thin-section analysis, SIRO-quant or QEM-SEM is still to be evaluated.
Initial work has shown that the improved ash fusion test has potential to rank the slagging ability of coal ashes, based on volume changes of the ash pillars. This work requires more investigation that will be undertaken in part in ACARP Project C5057 Practical Application of the Improved Ash Fusion Test
A commercial version of the CSIRO test, with all of its precision but lower costs, needs to be developed then introduced as a standard test, in parallel with the ash fusibility test. Its universal applicability, and hence acceptance, leading to its adoption as an ISO standard, needs to be proven by further testing on coals, both from Australia and overseas, with a wide range of ash chemistries. Funds to evaluate the procedures over a greater range of ash chemistries are currently being sought from ACARP.