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The combustion of coal continuously emits carbon dioxide (CO2) into the atmosphere, and the increase in CO2 concentration will exacerbate the greenhouse effect. At the same time, toxic substances from coal combustion, such as sulfur dioxide, are emitted into the atmosphere, causing air pollution. According to statistics, coal combustion accounts for 45% of the source of carbonaceous particulate matter in the mainland. The large amount of soot produced by coal combustion will not only reduce atmospheric visibility, but also the heavy metal particles contained in soot will endanger human health. Because heavy metals are non-degradable, they can be enriched in organs after entering the human body, causing various diseases.
The traditional methods for the detection of heavy metals mainly include atomic absorption spectrometry (AAS) and inductively coupled plasma mass spectrometry (ICP-MS). Laserinduced-breakdownspectroscopy (LIBS) focuses a high-power laser beam on the surface of a sample to study substances by analyzing the resulting microplasma. LIBS is used in a variety of fields because it is fast and multi-element analysis without the need to prepare a large number of samples.
At this stage, the research on coal usually adopts the method of offline analysis, the research object is mostly coal ash, which requires the preparation of samples and long-term analysis, and the online detection of lignite soot has not been reported in the literature. LIBS technology is very promising for the in-situ on-line detection of atmospheric pollutants, but the soot itself is thin and complex, and it is very challenging to use LIBS technology for in-situ on-site detection of gas pollutants and particulate matter therein. Therefore, the spectra of lignite and leaded lignite were analyzed, and the soot generated by the two was detected online in situ, which provided an experimental basis for the treatment of lignite soot.
Experimental part
The lignite sample came from Shanxi, China, and is an irregularly shaped solid. In order to study the heavy metal element pollution in soot, representative lead elements were selected. A certain amount of (CH3COO)2Pb·3H2O solid samples were weighed, and lead acetate solutions with concentrations of 1.2% and 0.01% were prepared using distilled water. Then the selected lignite sample is immersed in it, so that the lead on the surface of the sample is as uniform as possible, and the sample is taken out after 30min, and the sample is naturally air-dried in the air at room temperature. The original lignite samples are represented by O, the lignite samples immersed in 1.2% and 0.01% lead acetate solutions are represented by H and L, respectively, and the soot they produce is denoted by O', H' and L', respectively.
A schematic diagram of the experimental setup is shown in Figure 1. It is mainly composed of laser (ContinuumNd∶YAG pulsed laser, wavelength 1064nm, laser energy 290mJ·pulse-1, frequency 10Hz) and spectrometer (AvasSpec-ULS2048-4Channel-usb2.0, spectral detection range 240~890nm), mirror, focusing lens (f=150mm), trigger device, carrier platform and analysis system. The laser trigger delay time and the spectrometer delay settings are 2.5 and 6 μs, respectively.
A schematic diagram of the device for detecting lignite is shown in Figure 1(a). The laser optical path is changed by three mirrors, and then the focusing lens (f=150mm) is focused vertically to the lignite sample surface, forming a focal spot with a diameter of about 100 μm on the sample surface and generating a high-temperature plasma. A schematic diagram of the device for the soot detection experiment is shown in Figure 1(b). On the basis of the original experiment, a high-energy continuous laser LaserII. (PGL-III.-C PORTABLE LASER, with a wavelength of 447nm and an output power of about 1.154W) was focused on the surface of lignite to make it burn, and then the pulsed laser LaserI (wavelength of 1064nm) was focused on the generated soot. The laser plasma signal is collected by the detector, coupled to the optical fiber and transmitted to the spectrometer.
Figure 1
Qualitative analysis of primitive lignite and leaded lignite spectra
To calibrate for wavelength drift, a higher purity lead block (98% purity) is used as a reference sample. The LIBS spectra of the lead block are compared with the characteristic spectral lines in the NIST database [7] to correct for wavelength drift errors. Figure 3(c) shows the spectral pattern of the lead block in the 240~450nm band after calibration, and the characteristic wavelength of some lead elements is marked in the figure. As can be seen from Figure 3(c), the highest spectral intensity is the characteristic line (PbI.405.78nm) located at 405.78 nm.
Figure 2 is the spectrum of lignite sample O at 240~880nm. Careful identification and analysis, the characteristic peaks of C, Si, Fe, Mg, Al, Ca, Sr, Na were observed, among which, the spectral line of Ca element was strong, followed by Sr, Al, Mg and other metal elements. In addition, the presence of elements such as N, O, Hα and Hβ in the air was detected. After sorting, the spectral line identification table of lignite sample O is given, see Table 1. As shown in Figure 3, comparing the spectra of sample O and sample H within 240~450nm, it was found that the 8 additional characteristic lines in sample H belonged to lead (261.417, 266.315, 282.318, 283.305, 287.331, 363.956, 368.346 and 405.780nm).
| Element | Characteristicspectrallines/nm |
| Fe | 248.327,248.814,249.064,259.939,302.079,371.993,373.486,373.713,374.561,374.826,374.948,375.823,438.354 |
| C | 247.856 |
| Yes | 250.689,251.431,251.611,251.920,252.410,252.850 |
| Mg | 279.552,280.270,285.166 |
| To the | 308.215,309.271,394.400,396.152 |
| Like | 315.886,317.933,393.366,396.846,443.496,445.478 |
| Sr | 407.771,421.552,460.733 |
| On | 588.995,589.592 |
Figure 2
In-situ on-line detection of lignite soot and qualitative analysis of characteristic spectra
Figure 4 shows the soot spectra (O',H',L') produced by the three samples. Comparing the spectra of lignite and soot, it is found that the signal-to-noise ratio of soot is significantly lower than that of lignite, because the soot itself is very thin, which adds great challenges to LIBS detection and requires the instrument to have a higher signal-to-noise ratio. In soot H', lead is clearly seen. Compared with the spectrum of H', the lead line in L' is only more obvious at 405.780 nm.
Qualitative analysis of the spectrum of soot produced by leaded lignite shows that soot contains some metallic elements, including Mg, Ca, Al, Sr, Pb. It shows that if there are heavy metal elements in lignite, such as lead, then these metal ions will enter the air with the emitted soot, which is not conducive to human health. In addition, the spectra of sample H and its soot H' at 240~350nm were compared, see Figure 5. After comparison, it was found that the spectral line strength of all elements in the soot was much weaker than in lignite, and the relative intensity of the carbon atom spectral line in the soot was the highest among all elements, proving that a certain amount of CO2 gas was produced during the combustion of lignite. Instead of using an open flame, the experiment was replaced by a laser, so the increase in carbon content came all from the combustion of lignite itself. This proves that the application of LIBS technology in the in-situ on-line detection of gases has a very bright future.
In addition, CN molecular spectra were detected in LIBS spectra of lignite and soot. Since there is no CN component in the structure of lignite and soot, the observed CN molecules are formed by multiple reactions between ignited carbon and nitrogen in the air in plasma plumes. The emission spectrum of CN is distributed in 355~360, 384~389 and 413~422nm, corresponding to Δν=+1, Δν=0 and Δν=-1, respectively. Figure 6 shows the emission spectrum of sample O in air, and Table 1 shows the peak wavelength of the experimental CN molecule emission spectrum, which is close to the results of the CN molecule study in the literature. In addition, the spectrum of CN molecules detected by the experiment was fitted with software LIFBASE, and the rotation temperature of CN molecules was 6780K and the vibration temperature was 7520K.
Figure 3
Figure 4
Figure 5
| Vibrational Band | Wavelength /nm | Vibrational Band | Wavelength /nm |
| (0-0) | 388.22 | (3-2) | 358.28 |
| (1-1) | 387.02 | (0-1) | 421.50 |
| (2-2) | 386.12 | (1-2) | 419.61 |
| (3-3) | 385.41 | (2-3) | 417.95 |
| (4-4) | 385.01 | (3-4) | 416.66 |
| (1-0) | 359.00 | (4-5) | 415.67 |
| (2-1) | 358.53 | (5-6) | 415.14 |
Figure 6
Semi-quantitative analysis of lead in soot
Two lignite samples were immersed in different concentrations of (CH3COO)2Pb·3H2O solution to make leaded lignite samples H and L at different concentrations, and the specific preparation method is shown in 2.1. Lignite is irradiated with a laser (@1064nm) to obtain spectra of two soots (H', L') with different concentrations of lead. The content of lead in soot was semi-quantitatively analyzed, and it was seen from Figure 4 that the characteristic lines of lead 363.956, 368.346 and 405.780nm were common to both soot, and there was no interference from other lines nearby, so these three lines were selected for analysis.
Calcium (CaII.363.846nm) was selected as the reference line to make the spectral intensity consistent there, and the relative intensity of the characteristic spectra of lead at 363.956, 368.346 and 405.780nm was compared after normalization. As shown in Figure 7, it can be found that the relative strength of these three characteristic lines has a good linear relationship with the actual lead concentration in the sample, which proves the practicability of LIBS technology for the semi-quantitative analysis of heavy metal elements in lignite soot. Only the rough level of elemental content is reflected here, and a large number of experiments and more data are required to accurately analyze the content.
Figure 7
Using laser-induced breakdown technology spectroscopy to carry out on-line in-situ detection of lead-containing lignite and its soot, it was found that the spectrum of lignite contained C, Si, Fe, Mg, Al, Ca, Sr, Na, N, O, Hα, Hβ and other elements, and there were 8 more lead lines in lead-containing lignite, and metal ions such as Mg, Ca, Al, Sr, Pb and other metal ions existed in the spectrum of soot, indicating that some metal ions in lignite would enter the air with soot, which was not conducive to human health. The spectral comparison of lignite and soot showed that the signal-to-noise ratio of soot was worse, and the spectral line intensity of the elements was lower. In addition, it was found that the relative strength of carbon in soot is higher than all elements, proving that the CO2 produced during combustion is detected.
In addition, the specific wavelength of the CN molecule in the experiment is given, and the rotation temperature of the CN molecule is 6780K and the vibration temperature is 7520K by software fitting. The analysis of lead content in lignite soot showed that the relative strength of lead had a good linear relationship with the actual lead content of the sample, which proved that the LIBS technology was feasible for semi-quantitative analysis of heavy metal elements in lignite soot. In summary, LIBS technology provides an effective method for the in-situ online detection of harmful gases in the atmosphere and the heavy metal ions in them.