Raman spectroscopy is commonly used to characterize carbonaceous materials, such as coal and graphite, due to its wide applicability in the detection of crystals, molecules, and amorphous structures [
1,
2,
3,
4]. It also has the advantages of facile sample preparation, low minimum sample quantity, high resolution, high sensitivity, and being non-destructive. Therefore, it can provide reliable information for the evaluation of the orderliness of carbon chemical structures [
5,
6,
7,
8,
9]. The half-width at maximum height, peak area ratio, peak position, and peak intensity of each peak obtained from the Raman peak fitting can be used to characterize the chemical structure of coal chars [
5,
10,
11,
12,
13,
14]. These characteristic parameters can also provide strong support for speculating on coal pyrolysis reaction surface adsorption mode, mechanical strength, and other properties [
1,
5,
7,
15].
Raman spectroscopy is widely used for analysis and research of coal char structure, precisely because this technique contains rich information characterizing coal char structure, although there have been a large number of studies with regard to utilization of Raman characteristic parameters to characterize the structure of coal chars. However, the Raman characteristic parameters selected to characterize the char structure are not always consistent [
5,
15,
16,
17]. For example, Sheng [
5] selected the ratios between the peak spectrum intensities I
G/I
All, I
D1/I
G, and I
D2/I
G; Zou et al. [
16] performed peak fitting of Raman spectra of coal chars and further processed peak areas (peak area ratios A
D1/A
G, A
G/A
All, and A
R/A
All) to construct the relationship between the coal char reaction performance and the ratios of Raman peak areas; Liu et al. [
17] used Raman characteristic parameters, including peak spectral intensity, total peak area, peak position, and peak half-width at maximum height. On the other hand, the mode for testing conditions and the peak deconvolution method of the original Raman peak spectrum influence the conclusions that have been studied. Vidano et al. [
18] studied the influence of four excitation wavelengths (488.0/514.5/568.2/647.1) on the Raman peak spectrum, where a progressive red-shift with increasing excitation wavelengths is evident for the D (1350 cm
−1) peak, whereas the positions of the G (1580 cm
−1) and D’ (1620 cm
−1) peaks are essentially invariant. Similar line-shift results were obtained for all of the samples investigated. Similarly, Sadezky et al. [
19] and Matthews et al. [
20] also arrived at the same conclusion. For the original Raman peak spectrum fitting research, Cuesta et al. [
21] studied the Raman peak spectra of samples with different graphitization degrees and found that more comprehensive information of coal char could be obtained by multi-peak fitting of Raman original peak spectrum. Currently, the most commonly used method for coal char original peak spectrum fitting is five-peak fitting, including D4 peak at about 1150 cm
−1, D1 peak at about 1350 cm
−1, D3 peak at about 1530 cm
−1, G peak at about 1580 cm
−1, and D2 peak at about 1620 cm
−1 [
17,
19,
20]. However, there is no clear specification for the setting of objective lens magnification and coal char particle size. In particular, the objective lens magnification and particle size employed by different researchers to detect the microstructure of coal chars differs greatly. For example, the selected objective lens magnifications are 10× [
22], 50× [
17,
23,
24,
25], and 100× [
26], and particle sizes of coal chars are 48–96 [
27], 74–105 [
17], 105–150, and 150 μm [
5], respectively. In addition, there are differences in Raman peak fitting methods for different carbonaceous materials. For example, it is more reasonable to adopt two-peak fitting for Raman peak spectrum of coke [
28], while it is more reasonable to adopt five-peak fitting for some structure features of lignite and char, as they remain under cover around the D and G bands because of their disordered natures, although the five-peak fitting method of char Raman spectroscopy is generally accepted. However, there is controversy in the selection of fitting method for the D3 peak. Wang et al. [
15] thought that Lorentz fitting was more reasonable, while Sadezky et al. [
19] believed that Gaussian fitting was more reasonable. In addition, when Raman spectroscopy is applied to coal char structure detection and performance evaluation, the effects of variation in these testing conditions on characterizing coal char chemical structure have yet to be investigated.
In addition, some studies [
29,
30] have shown that the Raman characteristic parameters that characterize the coal char structure obtained after multiple Raman spectra of the same coal char sample exhibit variability. Taking A
D1/A
G as an example, the difference between the extrema of multiple detections can be as high as, approximately, three [
29]. Rantitsch et al. used micro-Raman spectroscopy to measure and analyze the bulk cross-section of metallurgical coke with multiple data points. The standard deviations for the respective positions of the characteristic peak spectra from multiple measurements and the Raman characteristic parameters that characterize the structure differed, further demonstrating the non-uniformity of the metallurgical coke and the importance of multiple data points in the Raman spectroscopy detection process [
30]. Unfortunately, the majority of studies adopted the results of one-time experimental analysis for Raman characteristic parameters to characterize the char structure. There are few studies on the influence of the number of spectra of a site on the stability of Raman characteristic parameters. Previous lab research suggested repeated Raman detection on a variety of char samples (particle size < 75 μm, excitation wavelength 514 nm, spectral resolution 1 cm
−1, objective lens magnification 20×) and five-peak fitting. It is found that most of the Raman characteristic parameters of chars are quite different. Therefore, the accuracy of one-time Raman results predominantly used in studies of char structure analysis is questionable.
In this paper, the rationality of the two-peak deconvolution methods was evaluated by the value of goodness of fitting and a more reasonable fitting method for Raman peak spectrum was proposed. Subsequently, the influence of objective lens magnification and char particle size (<30, 61–75, 81–96, and 120–140 μm) on the intensity of the Raman peak spectrum, characteristic parameters, and their degree of stability are studied. As a result of the above, a method for obtaining average values (AD1/AG, AG/AAll, AD3/AAll and AD1/AAll) through multiple Raman measurements is adopted to investigate the influence of the number of measurements on the stability of Raman characteristic parameters. Lastly, using micro-Raman imaging technology, a full scan of the micro zones of the char is performed to clarify the reasons for the differences in variability of the Raman characteristic parameters for different chars.