3.1 Thermogravimetric analysis of the sorghum grain-containing carbon materials in different proportions
Fig. 2 shows the TG and DSC figures of the mixed carbon materials containing S-DG and PC at the two different proportions (20:80 and 50:50). Compared to the TG curve of the PC in Fig. 2a (black curve), the TG curves of the mixed carbon materials showed that the weight loss rate increased with the increase of biomass mass ratio (red and pink curves for the 20:80 and 50:50 ratios, respectively). For unadulterated PC, only minimal mass loss below 953°C was observed, which was also consistent with the DSC curve of PC in Fig. 2b (black curve). Below 100°C, the mass loss from the PC was associated with evaporation of the adsorbed water. When the PC was mixed with the S-DG, the weight loss rate of the mixed carbon samples gradually increased as the proportion of the S-DG increased. The initial temperature (250°C) of pyrolysis of the TG curve when the ratio of S-DG to PC was 50:50 was significantly lower than that when the ratio was 20:80 (302°C). This means that adding biomass can earlier the starting temperature at which the pyrolysis began. However, it can be seen from the DSC curve in Fig 2b (pink and red curve) that as the quality of DGs increased, the DSCmax curve gradually shifted to the right, the Tmax corresponding to DSCmax gradually increased, and the DSCmax value decreased.
The thermodynamic parameters of the mixed carbon sample containing S-DG and PC in different proportions are shown in Table 3. As the mass ratio of DG to PC increased from 20% to 50%, the maximum peak temperature increased from 1126.5°C to 1299.7°C, which was 104.6°C and 177.8°C higher, respectively, than the highest temperature of the unadulterated PC sample. Furthermore, the DSCmax value decreased from 9.673 mW/mg to 2.902 mW/mg. Although the ash content in PC is low, the distillery grains contain high contents of Ca (0.49%), Mg (0.32%), K (1.24%), and Si (0.61%). When the pyrolysis temperature exceeds 1000°C, vitreous K, such as K2SiO3, may form in the molten mixture of the distillers grains and the PC and either migrate from DGs to the surface of PC or evaporate and then deposit onto the surface of the PC, covering many of the pores on the surface and causing the PC cracking temperature to increase [17]. In addition, alkali metals may form inactive minerals under co-reaction with alumina, such as potassium aluminosilicates (e.g. KAlSi3O8 and KAlSiO4), resulting in reduced reaction rates [18]. The increase in the maximum peak temperature was also attributed to the degree of graphitization of the carbon materials [19, 20]. As shown in Fig 2b, the DSC peak curves increased to higher temperature with the increase in the proportion of DG. This result indicated that the high content of alkali metals in the biomass reacted with the organic structure of PC, resulting in structural changes of PC, which will be discussed later. As the amount of volatile compounds during heating increased, a large number of cations (such as K+, Mg+, Al+, Ca+) present on the PC surface might have adsorbed O and H plasma particles, promoting the reorganization and ordering of the carbon structural on the PC surface.
Table 3. Thermodynamic parameters of the carbon materials with different mixing ratios of biomass feedstock and PC
Sample
|
Mass ratio
|
Time(days)
|
Tmax(°C)
|
DSCmax(mW/mg)
|
S-DG:PC
|
0:100
|
|
1121.9
|
9.673
|
20:80
|
> 1 month
|
1226.5
|
4.123
|
50:50
|
1299.7
|
2.902
|
100:0
|
1010.7
|
1.168
|
3.2 Thermogravimetric analysis of the corn grain-based mixed carbon samples
The thermogravimetric curves of the mixtures of C-DG and PC at different ratios are shown in Fig. 3. The thermodynamic parameters of the mixed carbon sample containing C-DG and PC in different proportions are shown in Table 4. The changes in the TG and DSC curves were similar to those of the TG and DSC curves of the sorghum-based mixed carbon materials. As the mass ratio of DG to PC in the samples increased from 20:80 to 50:50, the maximum peak temperature increased from 1209.7°C to 1268.9°C, which was 87.8°C and 147°C higher, respectively, than the maximum temperature of the unadulterated PC. Comparably, the DSCmax decreased from 9.673 mW/mg to 1.512 mW/mg. The C-DG biomass also contained a high content of alkali metals, such as Ca (0.42%), Mg (0.39%), K (1.80%), and Si (0.37%). The reasons that influenced the transition of the DSCmax to higher temperature as the percentage of C-DG increased from 20% to 50% were similar to those described in Section 3.1.
Table 4. Thermodynamic parameters of the carbon materials with different mixing ratios of biomass feedstock and PC
Sample
|
Mass ratio
|
Time(days)
|
Tmax(°C)
|
DSCmax(mW/mg)
|
C-DG:PC
|
0:100
|
|
1121.9
|
9.673
|
20:80
|
> 1 month
|
1209.7
|
5.190
|
50:50
|
1268.9
|
1.512
|
100:0
|
1009.6
|
2.328
|
3.3 Comparison of the experimental and theoretical thermogravimetric analysis values of the sorghum grain-based mixed carbon samples
Fig. 4 compares the experimental and theoretical pyrolysis values after mixing S-DG and PC with different mass ratios. The figure shows that, as the mass ratio of S-DG increased, the difference between the two changed. The synergy factor (SF) formula proposed in related studies was used to estimate the synergy effect of the two components in this study [21, 22]. When the mass ratio of the mixed carbon material containing S-DG was 20:80, the calculated SF was 0.455, while the calculated SF was 0.508 when the mass ratio was 50:50, indicating that the synergistic effect gradually increased as the mass ratio of S-DG increased. This was mainly due to the generation of different types of enzymes (such as amylase, protease, cellulase) during the brewing process of sorghum, which promoted the degradation of cellulose and starch into glucose and other monosaccharides, and the corresponding enzymes of these monosaccharides would promoted their generate ethanol and CO2 [23, 24]. During pyrolysis, a large number of volatile organic compounds, such as CO2, CH4, ketones, aldehydes, acids, and amines were cracked, causing them to interact with the non-volatile organic and/or inorganic substances in the PC to promote the cracking of the PC. The highly abundant alkali metals and alkali earth metals in the biomass ash exhibited a catalytic effect on the pyrolysis of the PC, and they further promoted the synergistic effect between S-DG and PC.
3.4 Comparison of the experimental and theoretical thermogravimetric analysis values of the corn grain-based mixed carbon samples
Fig. 5 compares the experimental and theoretical pyrolysis values after mixing C-DG and PC with different mass ratios. There was an obvious synergistic effect between C-DG and PC. The calculated SF was 0.459 when the mass ratio of C-DG to PC was 20:80, while the calculated SF increased to 0.495 when the mass ratio was 50:50, indicating that the synergistic effect also gradually increased as the mass ratio of C-DG to PC increased. The elemental composition of alkali metals and alkaline earth metals in the C-DG is similar to that in the S-DG; therefore, the reasons for the influence of C-DG on the PC were also similar to S-DG.
3.5 Kinetics analysis
3.5.1 Kinetics analysis of the sorghum grain-based mixed carbon materials
The Coats-Redfern method was used to calculate the relative relationship between Ln[g(x)/T2] and 1/T in order to study the kinetics of the carbon reaction reaction at high temperatures. Fig. 6 shows the curve generated by plotting the ln[g(x)/T2] values against 1/T calculated by Equation (1). After fitting the data to a linear regression model, the resulting regression coefficient (R2) was 98–99%, demonstrating a high correlation between the two kinetics parameters. The kinetic parameters calculated from the linear regression equation are shown in Table 5. When the DG ratios increased from 20% to 50%, the activation energies of the pyrolysis of S-DG in the mixed samples were 294.69 kJ/mol and 194.75 kJ/mol, respectively, which corresponded to a 33.91% decrease in the activation energy as the S-DG content increased from 20% to 50%. In addition, the activation energies of the pyrolysis of C-DG were 304.66 kJ/mol and 198.95 kJ/mol, respectively, which corresponded to a 34.70% decrease. These results indicated that the addition of either S-DG or C-DG could reduce the activation energy of the pyrolysis of the carbonaceous reductant and increase the reaction reaction rate.
Table 5. Kinetic parameters of PC and biomass co-reaction process
Sample
|
percentage
|
R2
|
Ea(kJ/mol)
|
A0(min-1)
|
S-DG
|
20:80
|
0.99792
|
294.69
|
8.43×1011
|
50:50
|
0.98477
|
194.75
|
2.68×107
|
C-DG
|
20:80
|
0.99929
|
304.66
|
3.89×1012
|
50:50
|
0.98612
|
198.95
|
4.32×107
|
3.6 FT-IR analysis
To understand the structural changes of DG before and after fermentation, Fourier transform infrared spectroscopy (FT-IR) was used to analyze the functional group composition of the biomass (Fig. 7). Overall, the intensities of the absorption peaks decreased after fermentation, which indicated that the organic matter in the biomass degraded and caused many of the polymeric structures to change. The FT-IR spectrum of S-DG features an absorption band at 3415 cm–1, which corresponded to the stretching vibrations of O-H and N-H bonds [25, 26]. Additional absorption bands at 2924 cm–1 and 2854 cm–1 were attributed to the C-H stretching vibrations of aliphatic -CH2- and -CH3 groups, respectively [27]. The C=O stretch of carboxyl, aldehyde, and ketone functional groups was observed at 1745 cm–1. The absorption bands in the region of 1610–1630 cm–1 were likely due to insignificant bimodal stretching vibrations, mainly those of C=C bonds in the aromatic rings [28]. The absorption band at 1457 cm–1 was the stretching vibration of aliphatic -CH3 groups. Finally, the absorption peak at 1033 cm–1 represented the stretching vibrations of aliphatic C-O bonds in the carbohydrate structures. Comparably, the changes in the FT-IR spectrum of C-DG before and after fermentation was essentially the same as the FT-IR spectrum of the S-DG, which indicated that the structure of C-DG had also has changed greatly as a result of fermentation.
3.7 Raman spectroscopy
3.7.1 Raman spectra of sorghum grains materials
The Raman spectra of the two S-DG and PC mixed carbon materials with different proportions in the range of 800–2000 cm–1 are shown in Fig. 8. The spectra featured two peaks in the range of 1100–1700 cm-1, which corresponded to the D and G peaks, respectively. The peak G represents the in-plane stretching vibration of the sp2-hybridized C atoms, and its value is proportional to the graphitization degree [19, 29, 30]. D1 represented disordered graphitic lattice [31]. The specific parameters of the Raman spectra of the two S-DG and PC mixed carbon materials are shown in Table 6. As shown in the table, the ID1/IG and IG/IA values of the 20:80 sample were 2.07 and 0.31, respectively. For the 50:50 sample, the ID1/IG and IG/IA values were 1.17 and 0.59, respectively. These results indicated that not only was the graphitization degree higher as the increase in the proportion of S-DG in the mixed carbon samples, but also the reactivity in reaction process became worse. This can also be seen from the DSC curve change of mixed carbon materials in Fig. 2. Therefore, it was determined that the reaction of the 20:80 mixed carbon material containing S-DG was better than the 50:50.
Table 6 Raman spectrum peak fitting data
Sample
|
Area/104
|
ID1/IG
|
IG/IA
|
ID1
|
IG
|
IA
|
20:80
|
1.14
|
0.55
|
1.78
|
2.07
|
0.31
|
50:50
|
1.90
|
1.62
|
2.73
|
1.17
|
0.59
|
3.7.2 Raman spectra of corn grains materials
Fig. 9 shows the Raman spectra of the C-DG and PC mixed carbon materials with different ratios, and Table 7 presents the specific parameters of the Raman spectra. As shown in the table, the ID1/IG value of the 20:80 sample was 1.75 and the IG/IA value was 0.43. For the 50:50 sample, the ID1/IG value was 2.88, and the IG/IA value was 0.27. These results showed that the graphitization degree of the C-DG-containing samples decreased with the increasing proportion of C-DG. Compared to the S-DG mixed carbon materials, the disorder degree (ID1/IG value) of the C-DG carbon materials was approximately 59% higher, which was conducive to improving the reactivity of carbon materials.
Table 7 Raman spectrum peak fitting data
Sample
|
Area/104
|
ID1/IG
|
IG/IA
|
ID1
|
IG
|
IA
|
20:80
|
1.38
|
0.79
|
1.82
|
1.75
|
0.43
|
50:50
|
2.68
|
0.93
|
3.42
|
2.88
|
0.27
|
3.7.3 SEM analysis of mixed carbon materials
Scanning electron microscopy (SEM) was employed to understand the changes in the apparent morphology of the mixed carbon materials after adding the distillers grains. As shown in Fig. 10, as the proportion of DG increased, the surface of the bituminous coal became etched first, causing many pores to appear, which was accompanied by fragmentation. These structural changes to the PC were beneficial to increasing the co-reaction reaction rate.
3.8 Carbothermal reduction experiments of the mixed carbon materials with silica
SEM and XRD were used to analyze the products of the smelting of the mixed carbon materials and silica at high temperatures. As shown in Fig. 11, the SEM images featured a large number of SiC particles that had grown on the surface of the carbon, which suggested that the mixed carbon materials and silica exhibited an agglomeration effect. However, the content of SiC particles in the C-DG samples were higher, and their growth was faster. The XRD spectra of that the smelting products showed that both of them (S-DG/PC and C-DG/PC respectively reduced silica) contained a large number of weak SiO2 diffraction peaks. However, Si diffraction peaks were not detected, which might have been attributed to a combination of experimental equipment (vacuum induction furnace) and the heating temperature.
To understand whether impurities affected the pyrolysis of the mixed carbon materials during the pyrolysis process, XRD was used to analyze the carbothermal reduction products. As shown in Fig. 12, the XRD spectra of the S-DG and C-DG products contained diffraction peaks corresponding to K2SiO3, KAlSi3O8, KAlSiO4, K(SiAl)O8, and K2SiO9. Because K2SiO3 can crystallize on the carbon surface, it will affect the reaction rate and cause the pyrolysis temperature to increase. However, potassium aluminum silicate is an inactive and stable substance that has no catalytic effect on the reaction reaction process; instead, it promotes the reduction of K+ content in the carbon materials and affects the pyrolysis reaction process [32-34].