Shunchun Yao

Extracting Coal Ash Content from Laser-Induced Breakdown Spectroscopy (LIBS) Spectra by Multivariate Analysis

Laser-induced breakdown spectroscopy (LIBS) combined with partial least squares (PLS) analysis has been applied for the quantitative analysis of the ash content of coal in this paper. The multivariate analysis method was employed to extract coal ash content information from LIBS spectra rather than from the concentrations of the main ash-forming elements. In order to construct a rigorous partial least squares regression model and reduce the calculation time, different spectral range data were used to construct partial least squares regression models, and then the performances of these models were compared in terms of the correlation coefficients of calibration and validation and the root mean square errors of calibration and cross-validation. Afterwards, the prediction accuracy, reproducibility, and the limit of detection of the partial least squares regression model were validated with independent laser-induced breakdown spectroscopy measurements of four unknown samples. The results show that a good agreement is observed between the ash content provided by thermo-gravimetric analyzer and the LIBS measurements coupled to the PLS regression model for the unknown samples. The feasibility of extracting coal ash content from LIBS spectra is approved. It is also confirmed that this technique has good potential for quantitative analysis of the ash content of coal.

Experimental study on the characteristics of molecular emission spectroscopy for the analysis of solid materials containing C and N.

Solid materials with different structure containing C and N were analyzed by laser-induced breakdown spectroscopy (LIBS). Comparing the emission molecular species in different atmosphere (air and argon), it can be determined that whether the molecular species are directly vaporized from sample or generated through dissociation or the interaction between plasma and air molecules. The results showed that the characteristic of C2 bands emission is similar with that of neutral atomic carbon emission CI in different atmosphere (air and argon). While the characteristic of CN bands emission is more complicated and it has great relationship with the existence of CN radicals, the interaction between plasma and air ambient, and the recombination of excited partials.

Multi-elemental analysis of fertilizer using laser-induced breakdown spectroscopy coupled with partial least squares regression

Laser-induced breakdown spectroscopy (LIBS) has been applied for the multi-elemental analysis of fertilizer. With a set of 11 fertilizer samples containing different levels of phosphorus and potassium, it was identified that the line intensity of the analyte does not follow a straightforward correlation with the element concentration with the presence of matrix effects. Instead, the line intensity of a given analyte element is not only related to that analyte, but also to other elements present in the samples. Further analysis reveals the correlations among the line intensities of the main components. Based on the correlation analysis, a set of calibration models were generated for phosphorus and potassium with the method of partial least squares (PLS) analysis, which is known as a multivariate calibration method. The prediction accuracy and reproducibility of these PLS models were then validated using independent LIBS measurements. The results show that the predicted concentrations with these models provided by LIBS measurements are in good agreement with the reference concentrations, which confirms that the LIBS technique has good potential for the in situ rapid determination of the main elements present in fertilizer.

Analyzing unburned carbon in fly ash using laser-induced breakdown spectroscopy with multivariate calibration method

This paper presents an alternate calibration method for measurement of the unburned carbon (UC) in fly ash using laser-induced breakdown spectroscopy (LIBS). Fly ashes studied in this research were produced from pulverized coal in a muffle furnace and the UC was determined by a loss-on-ignition (LOI) test. We apply both univariate calibration and multivariate calibration methods to LIBS data to establish the calibration curve of the UC in measured samples, and the performance of these two approaches was compared. Our analysis shows that traditionally used univariate calibration in LIBS does not qualify quantitative analysis of fly ashes from different kinds of coal due to the presence of matrix effects. Instead, multivariate calibration has a better performance as the matrix effects can be taken into account with the influence of the spectroscopic signals of other components in fly ash. The correlation coefficients R2 of multivariate calibration reach 0.994 (fly ashes from one kind of coal) and 0.981 (fly ashes from different kinds of coal), which is significantly improved compared with univariate calibration.

Application of LIBS for direct determination of volatile matter content in coal

Volatile matter content is an important index of coal quality characteristics. Laser-induced breakdown spectroscopy (LIBS) is a promising analytical spectroscopy technique. In this paper, a broad set of coal samples with different amounts of volatile matter content were tested by LIBS. Analytical methods with partial correlation and principal component regression were used to extract related LIBS spectral information based on coal structure, which indicates a strong correlation with the content of volatile matter to establish the calibration model. The experimental results derived from the correlation are consistent with those from standard analysis procedures (thermo-gravimetric analysis, TGA). This demonstrates that LIBS is a viable approach to directly determine the volatile content rather than indirect ultimate analysis.

Experimental study of laser-induced breakdown spectroscopy (LIBS) for direct analysis of coal particle flow.

Laser-induced breakdown spectroscopy (LIBS) was employed to directly analyze coal particles in the form of descending flow. Coal-particle ablation was performed using a 1064 nm neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at atmospheric conditions. Spectral identification schemes were used to acquire spectra containing all the emission lines of the important elements in coal. These acquired spectra were classified as representative spectra. The background of the line emission plus three times the standard deviation of the background of the representative spectra was chosen as the threshold value. A method using a single line and a method using combined multiple lines (C, 247.8 nm; N, 746.8 nm; Si, 288.2 nm; and Ca, 396.8 nm) were compared to obtain the best results for the spectral identification of coal particle flow. The feasibility of rejecting the partial breakdown spectra was verified using quantitative analysis of fixed carbon in coal.

Correlation between grade of pearlite spheroidization and laser induced spectra

Laser induced breakdown spectroscopy (LIBS) which is used traditionally as a spectrochemical analytical technique was employed to analyze the grade of pearlite spheroidization. Three 12Cr1MoV steel specimens with different grades of pearlite spheroidization were ablated to produce plasma by pulse laser at 266?nm. In order to determine the optimal temporal condition and plasma parameters for correlating the grade of pearlite spheroidization and laser induced spectra, a set of spectra at different delays were analyzed by the principal component analysis method. Then, the relationship between plasma temperature, intensity ratios of ionic to atomic lines and grade of pearlite spheroidization was studied. The analysis results show that the laser induced spectra of different grades of pearlite spheroidization can be readily identifiable by principal component analysis in the range of 271.941?289.672?nm with 1000?ns delay time. It is also found that a good agreement exists between the Fe ionic to atomic line ratios and the tensile strength, whereas there is no obvious difference in the plasma temperature. Therefore, LIBS may be applied not only as a spectrochemical analytical technique but also as a new way to estimate the grade of pearlite spheroidization.

Correction of C–Fe line interference for the measurement of unburned carbon in fly ash by LIBS

For the serious interference between the Fe 247.98 nm and the C 247.86 nm lines in the analysis of fly ash when using a poor-resolution spectrograph, this paper presents a correction method that uses Fe lines to correct the C–Fe line interference. The integrated intensity of the spectral lines is chosen as the input intensity to obtain the intensity ratio of non-interference Fe lines and the Fe 247.98 nm line by LIBS measurements, and to obtain the C 247.86 nm line integrated intensity from the overlapping peak. Fe 248.33 nm, Fe 250.11 nm, and Fe 251.08 nm lines were used as correction lines to predict the unburned carbon (UC) content in fly ash. We applied multivariate calibration methods to establish the calibration curve of UC in the measured samples, and used quantitative analysis to further verify the effect of Fe lines as correction lines on the detection accuracy of UC in fly ash by LIBS. The results show that the correlation coefficients R2 of multivariate calibration all have improved based on Fe 248.33 nm, Fe 250.11 nm, and Fe 251.08 nm line correction. In terms of the overall evaluation from the calibration curve fitting and predictive accuracy of quantitative analysis, the Fe 248.33 nm line has the best performance as a correction line to correct C–Fe line interference when measuring UC content in fly ash. The correlation coefficients R2 of the multivariate calibration and the averaged prediction absolute error of the UC content are 0.998 and 0.002 wt%, respectively, which is significantly improved compared with that without correction.

Optimizing the binder percentage to reduce matrix effects for the LIBS analysis of carbon in coal

Quantitative analysis of elements by laser-induced breakdown spectroscopy (LIBS) is significantly affected by matrix effects in coal. Coal powder was mixed with the KBr binder and pressed into pellets to reduce matrix effects. Four groups of mixed-pressed pellet samples were prepared from nine different coal types and different percentages of the KBr binder (KBr accounts for 0 wt%, 30 wt%, 60 wt%, and 90 wt%, respectively). To optimize the percentages of the KBr binder in the mixed-pressed pellets, the influence of KBr binder on laser-induced plasma was investigated in detail. The results indicate that the plasma excitation temperature decreases with increase of the KBr binder concentration. The difference of the excitation temperature between the nine different coal samples was minimal when KBr accounts for 60 wt% in the mixed-pressed pellets. The relative standard deviation of the excitation temperature is 4.26%. The matrix changes from coal to KBr when the percentages of KBr are higher than 60 wt%, which was confirmed by scanning electron microscopy images of the ablated crater. Finally, Si and K were individually chosen as the internal calibration element to construct the calibration curves of carbon. Better results were obtained when Si I 288.16 nm was used as the internal standard. The correlation coefficients R2 of the four groups of mixed-pressed pellet samples are 0.835, 0.893, 0.983, and 0.903, respectively. Hence, the appropriate percentage of binders needs to be carefully confirmed to reduce matrix effects in quantitative analysis of coal by LIBS.

Optimizing critical parameters for the directly measurement of particle flow with PF-SIBS

A novel measurement technology named as particle flow-spark induced breakdown spectroscopy (PF-SIBS) was reported for real-time measurement of solid materials. Critical measurement parameters of PF-SIBS were optimized and a set of fly ashes with different carbon content were measured for evaluation of measurement performance. Four electrode materials, tungsten, copper, molybdenum and platinum, were compared in the aspects of signal stability, line interference and electrode durability. Less line interference and better signal stability were obtained with W and Cu electrode, while W electrode has better durability. Quartz sand with diameters from 48 μm to 180 μm were tested to investigate the influence of particle size. As the particle diameter increased, the intensity of Si 288.16 nm line decreased while that of ambient air constituents increased. To reduce the particle effect, the sum intensity from sample and ambient air were introduced to correct. The RSD of line intensity between the five diameters were reduced from 67.30% to 16.59% with Cu electrodes and from 63.21% to 13.64% with W electrodes. With the optimal measurement parameters and correction, fly ash samples with different carbon content were tested and the correlation coefficients R2 of multivariate calibration achieved 0.987.