## 3 Results and Discussion
### In Situ High-Temperature Study of Different Goals by RAMAN  
Figure 6a shows the in-situ temperature-dependent Raman spectra of coal C1 under an Ar atmosphere. The gas flow was monitored and controlled by the amount of bubbles from water--approximately 160 bubbles per minute. When the sample temperature was lower than 473 K, the sample was heated at a rate of 30 K per min. The heating rate was increased to 50 K per min when the sample temperature was higher than 473 K. The integral time and frequency were \(20\times 20\) and were increased to \(30\times 30\) at a temperature of 873 K. A yellow substance was observed at a temperature of approximately 673 K. This substance was believed to be tar oil [25]. The spectrum labeled as 873 K-2 appeared during the second recording, when the sample was reheated to 873 K after it was held at 873 K for 40 min. The sample was then cooled down at 50 K/min to room temperature. This cooling allowed for the cleaning of the silica glass prior to measurement. Figure 6b shows the in-situ temperature-dependent Raman spectra of coal C2 under an Ar atmosphere. The integral time and frequency were set at \(30\times 30\) s in the entire heating process. From Figure 6, it is clear that the peak position of the D band related to the low molecular weight carbon organics, while the intensity of the D band changed negligibly with the increasing temperature [26; 27]. This implies an increase in the graphitization degree along with the increase in temperature. The low molecular weight carbon organics condensed considerably, starting to devolatilize at 673 K. Some of these organics stayed on the coal samples in a liquid state for some time. Most heavy molecular weight carbon organics seemed to condense and devolatilize starting at a temperature of 873 K. This suggests that there could be more heavy molecular weight carbon organics condensing and devolatilizing or moving from the coal samples in the range of 873 K to 1273 K, disappearing rapidly at 1473 K.  
Figure 6: Temperature dependence of in-situ Raman spectra of C1 and C2 under an Ar atmosphere using a 355 nm excitation laser. By contrast, the G peak position showed a blue shift with an increase in temperature, and its intensity became weaker due to the decreasing devolatilized content in the samples. This demonstrated that the G peak position was affected by the devolatilization of the coal samples. Generally, the Raman band would shift to a lower wavenumber and become smaller in intensity with an increase in temperature if no phase transformation takes place. From Figure 6b, it is clear that the spectrum at 673 K seemed quite different from the corresponding spectrum of C1 in Figure 6a. This difference was the result of the appearance and extension of the Rayleigh line. During pyrolysis, small liquid molecules on the surface where the sampling point is focused can occasionally enhance Rayleigh scattering, but this phenomenon occurs differently each time. This finding may imply the presence of a liquid substance on the coal samples. A gas, liquid, and solid three-phase coexistence was reported during coal pyrolysis from 673 to 873 K [15; 28]. When the temperature exceeded the curing temperature of 873 K, an adhesion phenomenon seemed to occurr, resulting in the formation of tar oil or char [29; 30], which covered the silica glass. The samples then needed to be cooled down at 50 K per min, from 873 K to room temperature, which allowed the silica glass to be cleaned. Finally, the samples were reheated to 873 K for further observation. The effect of temperature on the peak position variations of the G band of the samples pyrolyzed under an Ar atmosphere is shown in Figure 7. Generally, the spectral lines show a linear decline trend as the temperature increases in the in-situ temperature-dependent Raman spectra [31; 32].