A density separation method was used to reduce ash components, which form fireside deposits
during coal combustion, as well as trace elements, in a Greek lignite. Standard techniques were
applied to predict the slagging and fouling propensity of clean coal and to determine the trace
elements concentration in this. Deposition tendencies were estimated through the use of empirical
indices for lignitic type ashes. Significant reductions in ash content (up to 60%) were achieved
by dense liquid separation of the lignite. Coal cleaning diminished the slagging and fouling
potential of the feed, reflecting some alterations in the chemistry and mineralogy of the ash-forming components. The ash of beneficiated materials was richer in the basic oxides, while poorer
in quartz. Also, the ash fusion temperatures (initial deformation and hemispherical) were
significantly increased, though the contents of quartz and anhydrite remained high. Most trace
elements were found to be associated with the mineral phase. Removal efficiency was low by the
density separation method, but the environmental impact of these elements is anticipated to be
negligible.
The possibility of using commercial on-line analysis systems for monitoring the ash content of low-grade lignites was investigated by carrying out numerous bench-and pilot-scale trials in the mines of Public Power Corporation SA, Greece. Pilot-scale trials were based on a dual-energy γ-ray transmission analyzer, which was installed on the conveyor belt that transports lignite from the pit to the bunker of Kardia mine, Ptolemais. According to the obtained results, the accuracy of the on-line measurements was not adequate and did not allow lignite quality monitoring in real time. The deterioration of the on-line measurements' accuracy, compared to previous applications in other mining sites, was related to the intense variation of the lignite ash content and ash composition, which distorted the calibration of the analyzer. The latter is based on certain assumptions regarding the average atomic number of the organic and mineral matter contained in the lignite. Further experimental work is needed to investigate solutions for successful implementation of this method to lowgrade lignites that exhibit large variation in ash content and composition.
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