Optimization of laser-induced breakdown spectroscopy for coal powder analysis with different particle flow diameters

• Tapered tube was designed for beam-focusing the coal particle flow as well as enriching the particles in laser focus spot.
• The characteristics of laser-induced plasma of coal particle flow were investigated carefully.
• An appropriate diameter of coal particle flow was proven to benefit for improving the performance of LIBS measurement.

The on-line measurement of coal is extremely useful for emission control and combustion process optimization in coal-fired plant. Laser-induced breakdown spectroscopy was employed to directly analyze coal particle flow. A set of tapered tubes were proposed for beam-focusing the coal particle flow to different diameters. For optimizing the measurement of coal particle flow, the characteristics of laser-induced plasma, including optical breakdown, the relative standard deviation of repeated measurement, partial breakdown spectra ratio and line intensity, were carefully analyzed. The comparison of the plasma characteristics among coal particle flow with different diameters showed that air breakdown and the random change in plasma position relative to the collection optics could significantly influence on the line intensity and the reproducibility of measurement. It is demonstrated that the tapered tube with a diameter of 5.5 mm was particularly useful to enrich the coal particles in laser focus spot as well as to reduce the influence of air breakdown and random changes of plasma in the experiment.