In Situ Online Detection of Straw Burning Smoke via Laser-Induced Breakdown Spectroscopy Technique

The burning of straw has great harm to the atmospheric environment. During the combustion process, heavy metal elements will float into the air along with the smoke, causing a certain degree of pollution to the atmospheric environment. Therefore, it is of great significance to detect the heavy metal elements that spread into the air with the smoke of straw combustion. Laser Induced breakdown Spectroscopy (LIBS) is used to detect the straw smoke, and to analyze the heavy metal. The experimental setup consists of Nd YAG single pulse laser (wavelength : 1 064 nm, laser energy : 290 mJ.pulse(-1), frequency : 10 Hz) , Avantes spectrometer (AvasSpec-ULS2048-4Channel-usb2. 0, spectral range : 200 similar to 890 nm, resolution : 0. 13 nm), reflector, focusing lens (focal length : 150 mm) , pulse delay generator. The delay time of spectrometer was set at 6 mu s. The laser beam is focused on the smoke by the lens, and generates high-temperature plasma. The LIBS spectrum signal is obtained by the spectrometer. The samples were mature rice stems and leaves in east China. For the LIBS detection of air, N, O, FL, and H-beta elements can be observed in the spectra. Under the same experimental conditions, the smoke of the burning straw was detected by LIBS and the spectrum demonstrates that the smoke consists of C, Mg, Ca, Mn, Na, K and other elements. The observation of the heavy metal element Mn verifies the feasibility of the LIBS detection of heavy metal elements in smoke of burning straw. At the same time, the molecular spectrum of CN radical was observed in the smoke spectrum. CO2 molecules generated in the combustion process of straw could react with N-2 in the air under the irradiation of laser to form CN molecules. Then the detection of the breathing of human by LIBS was carried out under the same condition, and CN emission bands were also observed in the obtained spectra. What's more, LIFEBASE software is used to fit the emission bands of the CN molecule. Moreover, the result shows that the vibration temperature and rotation temperature of CN molecules are 8 000 and 7 700 K, respectively. Straw samples containing Pb were prepared by using lead acetate solution. Two samples of the same quality were soaked in different concentrations of Pb solution and dried. Online in situ LIBS detection was carried out on the two samples, and the spectra were obtained. By comparing the straw samples containing Pb with the original straw samples, five spectral lines with wavelengths of 357. 261, 363. 898, 368. 370, 373. 945 and 405. 747 nm were found in the spectrum of the straw samples containing Pb. Compared with the NIST database, it is found that the five spectral lines are all characteristic spectral lines of Pb. Taking Ca II (393. 329 nm)as a reference line in the signal normalization process, the intensity of Pb line in the sample with the high concentration of Pb is significantly higher than that in the sample with the lower concentration of Pb. It verifies that the online in situ LIBS detection is promising to apply to the semi-quantitative analysis of heavy metals of straw combustion and soot.