Laser-induced macrofluorescence of coal: oxidation and macroalteration

Measuring the fluorescence decay of bulk coal samples with prolonged illumination may be a useful method for the on-line grading of coal. When large areas (≈0.02-0.12 cm2) of Goonyella and Upper Hunter coal samples were illuminated with a 442 nm laser the resulting macrofluorescence emission decreased during prolonged exposure (macroalteration). With a fresh Goonyella coal the 800 nm macroalteration in 5 min with a 1.5 mm diameter spot and 8 mW incident power was 30% in nitrogen and 55% in oxygen. Oxidized coals macroaltered less than fresh coals. Macroalteration was faster at near infrared emission wavelengths than at visible emission wavelengths.