The development of laser spark emission spectroscopy for the characterization of ash deposits

Abstract

Laser spark emission spectroscopy (LASS) is being developed as an in situ, real time monitor for elemental composition of ash deposits formed during coal combustion processes. The analytical technique uses a high-energy pulsed laser to form a high-temperature plasma from a small quanity of the surface material. Optical emission line intensities are then recorded and analyzed to characterize the elemental constituents of the material. Sensitivity and linearity of the method are demonstrated for binary sodium/calcium sulfate deposits prepared on metallic substrates. Observed intensity ratios for sodium and calcium are found to correlate very well with deposit composition. LASS has also been used to qualitatively characterize complex ash deposits prepared by the combustion of pulverized coals in a pilot-scale combustor. Differences in composition related to coal type are monitored by comparing relative emission intensities for calcium, aluminum and iron. Examination of the ash deposit at a location on the probe for two different orientations relative to the main combustion flow reveals changes in composition that can be related to differences in the mechanism of deposition. Several instrument-related issues remain to be addressed in the study of ash deposits including the following: factors which influence the choice of analytical wavelengths; self-absorption of some emission lines; preferential vaporization of atomic species during plasma formation; variations in energy coupling and resultant temperature of the laser-induced plasma; optimum apparatus configuration for detection and resolution of complex emission spectra; and methods of data analysis.