Correction of pair production gauge ash determinations for changes in coal ash composition

The ash content of coal can be determined by a technique based on pair production (PP). The PP technique has been previously proved in the laboratory and in the field, and five commercial PP gauges are in routine use at coal washeries in Australia. However for some applications on coals of highly variable ash composition, there is a need for improved accuracy.

Calculations and experiments have been made to investigate two methods of improving PP gauge accuracy for coals with highly variable ash composition. The first combines the PP gauge measurement with measurements of the low-energy part of the PP gauge spectrum and the second is done in conjunction with separate low and high energy γ-ray transmission measurements. Calculations show that these methods can correct the PP gauge assay exactly when the concentration of a single high-Z element in the ash varies. When more than one element varies, the error reduction is less and depends on the elements and their variations. Experiments with a group of samples from Blackwater, Queensland showed that gauge accuracy improved by about 20% relative using the first method. Use of the second method on two groups of samples from Blackwater and South Africa produced about 15% improvement. There was no reduction in error when either method was used on a third group of samples from the Hunter Valley, New South Wales.