Coal Preparation » Process Control
Coal contains water which can be defined in various ways, the most common of which are the laboratory determined moisture characteristics of moisture in the analysed sample, air dried/free moisture and equilibrium moisture. None adequately describe the water that can be removed from coarse coal in a coarse coal centrifuge.
Theoretical prediction of product moisture from a basket centrifuge have to date not been successful mainly due to the fact that the cake bed, the cake resistance and cake porosity change at a non-linear rate. The driving force will also increase with the radius of the centrifuge basket as it increases its taper from the feed to the discharge, therefore simple empirical approaches such as that taken by (Donnolly, 1992) have been used to estimate the final product moisture based on the size distribution, the mass fraction in each size distribution and an allocated moisture for each size distribution. This technique can give a close approximation however it does not take into consideration the coal type and other coal properties.
The impact of coal properties, surface area, surface topography, maceral composition, etc, on the basic water retention of wet coarse product coal processed through a coarse coal centrifuge can be characterised using a laboratory batch centrifuge developed in an ACARP project in 1996, these properties can be reliably used to predict the final product moisture expected from a coarse coal centrifuge. One of these properties the Non-Centrifugable Moisture Internal (NCMi) provides a method to characterise the coal seam and for predicting the final coarse coal centrifuge product moisture given a centrifuge feed size distribution.
This project's objective is to characterise Australian coals from various coal measures in terms of their centrifugal dewatering characteristic of NCMi and the relationship of the NCMi with rank as indicated by the dry ash free carbon content. A method to prepare bore cores for this test work and the effect of organic liquids on the NCM results were also examined. Several methods were also trialled in an attempt to replace the moisture after drying and the effect that these different methods had on the NCM was examined.
Fifty four coals of various ranks have been tested for NCMi to date, for this project ten low rank coals, including slim bore core samples and one high rank coal were tested, the relationship of the NCMi for all the coals tested to date and the rank of the coal as indicated by its Vitrinite has been shown in this report.
There can be a large variation in the NCMi of coals with similar ranks especially with lower rank coals. For this reason graphs will only give an indication of the range of NCMis that can be found at that particular rank. As the NCMi will be dependent not only on the coal rank but on the coal type and physical properties such as surface area, internal moisture (pore structure), ultrafines attached to the surface etc, there will still be a need to perform the test work to determine the actual NCMi of that particular coal.
The use of a water jig to treat the slim bore core samples so that they were not contaminated by organic solvent or allowed to dry out was also found to be a suitable method for the preparation of simulated plant products for this test work.
This test work also showed that it may be possible with low rank coals to replace water removed while drying by immersing in water under 4 Bar of pressure. For high rank coals while the experiments showed little difference between the wet NCMi and the NCMi produced after drying and using immersion techniques only one sample was processed and more work will need to be done in this area.