Measurement and Control of the Reflux Classifier

The purpose of this investigation was to establish the optimum control regime of the Reflux Classifier for coal sourced from various seams. Experiments were carried out at a CHPP in the lower Hunter Valley using an RC300 with six pressure probes installed along the vertical section and fitted with the underflow buffer system developed during this study. Using the readings from the six pressure probes, information about the bed density profile was obtained and the effect of applying different control regimes to different coal feeds was evaluated.

Examination of the individual probe readings and the bed density profiles revealed that insufficient fluidisation could be readily identified by assessing the functionality of the probe at the lowest elevation (Probe 6). At low fluidisation rates, the functionality of this probe became compromised such that any control pair, for example Probe Pair (PP) 1 - 6, using this probe was unable to provide sensible control. However, it was also found that by switching the control pair to exclude Probe 6, for example to PP 1 - 5, adequate control could be reinstated. Therefore, it was recommended that two probes be installed at a low elevation to monitor for insufficient fluidisation and provide a control option during low fluidisation.

An investigation into the influence of the location of the control probe pair on the separation performance of the Reflux Classifier revealed that PP 1 - 6, which spanned the entire fluidisation bed, provided sensible and stable control for a variety of different coal types. Whereas control using information from only the upper part (PP 1 - 3) or lower (PP 5 - 6) part of the bed was less reliable. The performance of the latter two control pairs was dependent upon whether they could adequately detect the movement of the bed during underflow discharge. For example, PP 1 - 3 performed very well when applied to thermal coal as the top of the dense mineral bed was located between the two probes. However, this pair performed poorly when applied to coking coal as both probes were located above the top of the bed. PP 5 - 6, being located at the lower end of the bed, effectively displayed the inverse performance of that observed for PP 1 - 3. As a result of these findings, it is recommended that the default control for the Reflux Classifier involve one probe at the base of the fluidisation bed and one probe at the top of the fluidisation bed, such that a broad range is covered. In some instances it may be appropriate to install another probe at a third location at a high level for a thermal coal or a low level for a coking coal.