Technical Market Support » Thermal Coal
The main objective of the present report is to investigate the practical applicability of the Improved Ash Fusion Test for a suite of selected coals that were previously investigated in ACARP Project C3097 Evaluate Combustion Behaviour of Australian Export and Overseas Low Rank Coal Blends.
The issue was to relate the results from the Improved Ash Fusion Test (which are provided as a continuous record of shrinkage against temperature) to plant performances from the combustion testing and if possible, existing ash fusion temperatures.
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. There is only very poor correlation between the Improved Ash Fusion Test results and the standard ash fusion test results. The extreme subjectivity of many of the standard ash fusion test results makes comparison meaningless. It is demonstrated that the standard ash fusion test results do not discriminate effectively between the ashes in this study, whereas the Improved Ash Fusion Test produces an evaluation that is equal to or perhaps better than the visual and tactile evaluation of the ash on the slagging panels in the Boiler Simulation Furnace.
The Improved Ash Fusion 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 combustion testing.
Although the Improved Ash Fusion Test is much closer to a correlation with the practical outcomes in the Boiler Simulation Furnace it is still necessarily imperfect. It does not take into account the time frame with which reactions take place in commercial Pulverised Fuel fired boilers. It also does not take into account the continuously variable conditions experienced in the turbulent combustion volume and on the slagging surfaces.
The standard ash fusion test is highly subjective and there is little scope of improving that subjectivity either through computerisation or training. Also the standard ash fusion test lacks some crucial details which are evident in the Improved Ash Fusion Test.
As the Improved Ash Fusion Test slagging index appears to do just as well as the qualitative Combustion Test Slagging Index and with much better than the Improved Ash Fusion Test which showed no correlation at all, then its recommendation seems quite warranted.
The corollary derived from this study is that as the standard ash fusion test continues to give no meaningful results, then it should be phased out of use.