In Situ Online Detection of Lignite and Soot by Laser-Induced Breakdown Spectroscopy

Due to the shortage of high-quality coal resources, lignite has become the main coal used in our country. Lignite has a low degree of coalification, which produce a lot of black ash and carbon dioxide when burning. The metal ions contained in the soot harm human health, so it is very meaningful to carry out the research on lignite soot. Laser-induced breakdown spectroscopy (LIBS) is a fast and multi-element method, which is suitable for in situ online detection of soot. Three lignite samples (O, H, L) with different lead concentrations were prepared in this paper, where O was the original lead-free sample. Laser Induced-breakdown Spectroscopy (LIBS) was used for insitu online detections of lignite and soot. The experimental instruments are mainly composed of laser, spectrometer, reflector, focusing lens, trigger device, carrier platform and analysis system. First, the wavelength drift of the experiment was calibrated using a high-purity lead block, and the elemental composition of the lignite samples O, H, and L was analyzed. It was found that lignite O contained C, Si, Fe, Mg, Al, Ca, Sr, Na and other elements, while N, O, H Hp and other air elements were detected. In addition, there were 8 more spectral lines of lead in the lead-containing lignite spectrum. Finally, a spectrum identification table of the main elements in lignite was given. Then the lignite was ignited with 447 nm continuous light, and 1 064 nm pulse light was focused on the soot for in situ online detectings. The qualitative analysis of the soot spectrum found that the soot contained metal ions such as Mg, Ca, Al, Sr, and Pb, indicating that some metal ions in the lignite would be discharged into the air with the soot and endanger human health. By comparing the spectrum of lignite and soot, it was found that the signal-to-noise ratio of soot was bad, and the spectral line strength of all elements was much weaker than that of lignite. In addition, it was found that the relative intensity of the carbon atom spectral line in soot was the highest among all elements (no open fire) which proved the effectiveness of LIBS for detecting CO, In addition, the CN molecular spectrum in the experiment was analyzed, and the specific wavelength of the CN molecule was given. The rotation temperature of the CN molecule was 6 780 K and the vibration temperature was 7 520 K using the software LIFBASE fitting. At last, the lead concentration in the soot of samples H and L were analyzed, and the reference line (Ca II 363. 846 nm) was selected to normalize and compare the relative intensities of lead elements at 363. 956, 368. 346 and 405. 780 nm. It was found that the relative intensities of the three characteristic spectral lines had a good linear relationship with their actual lead concentration, which indicates that the LIBS technology is feasible for semi-quantitative analysis of heavy metal elements in lignite soot.