3.1. Combustion behaviors analysis
Weight loss and rate of weight loss measurements are illustrated in Fig. 1, and the percentages of weight loss during the HTC reaction are shown in Table 1.
Table 1: Weight loss during three stages and residues of samples
Samples
|
Stage 1
|
Stage 2
|
Stage 3
|
C-200-6
|
3.4%
|
78.0%
|
8.5%
|
C-250-6
|
3.3%
|
39.9%
|
41.5%
|
C-250-12
|
2.6%
|
38.6%
|
41.7%
|
W-200-6
|
3.9%
|
65.3%
|
30.1%
|
W-250-6
|
2.6%
|
30.3%
|
54.7%
|
W-250-12
|
2.1%
|
25.4%
|
60.5%
|
C : Cellulose, W : Wood |
The first stage of TG-DTG (ambient to 100 ℃) is for the evaporation of moisture. Changes of TG-DTG are similar at the first stage, indicating a similar content of moisture in cellulose- and wood-derived. A plateau between 110 and 190 ℃ follows, where heating of molecules occurs without breaking chemical bonds.
The second stage is the combustion phase (200 - 400 ℃), including devolatilization and combustion of volatile matters (e.g., moisture and small organic molecules) and sample combustion [39]. Wood-derived hydrochar and cellulose-derived hydrochar obtained by HTC reaction at 200 ℃ lost most of the weight during the second stage. Wood- and cellulose-derived hydrochars obtained at 200 ℃ lost more weight than at 250 ℃. Moreover, at the same HTC condition, the weight loss of cellulose-derived hydrochars is higher than that of wood-derived hydrochar. It indicates these hydrochars obtained at 200 ℃ are mainly made of volatile matters, and the content of volatile matters in cellulose-derived hydrochar is higher than that of wood-derived hydrochar.
The third stage, i.e., the burn-out stage ( 450–550 ℃), is for the combustion of fixed carbon remaining after the preceding stages [40, 41]. Compared with the weight loss of hydrochars obtained at 250 ℃ in the third stage, the weight loss of hydrochars obtained at 200 ℃ is lower. That means more fixed carbon formed at 250 ℃. At the same HTC reaction condition, the wood-derived hydrochar lost more weight than cellulose-derived hydrochar. It indicates the content of fixed carbon in wood-derived hydrochars is higher than that in cellulose-derived hydrochars. When HTC reaction temperature is 250 ℃, the weight loss of the cellulose-derived hydrochars obtained at 6 h is similar to that of 12 h, meaning that fixed carbon does not change much with the increasing of time. While wood-derived hydrochar obtained in 12 h has significantly higher weight loss than that of 6 h. It means prolonging the reaction improves the content of fixed carbon for wood-derived hydrochar.
3.2. Structural properties of the hydrochars
Figure 2 and Fig. 3 are the SEM images of cellulose-derived and wood-derived hydrochar. Because of the incomplete hydrothermal reaction, an irregular shape is shown in the SEM image of cellulose-derived (Fig. 2a) and wood-derived (Fig. 3a) hydrochar produced at 200 ℃ and 6 h reaction time. Hydrochars have a compact structure without any pores proves cellulose and wood dissolve incompletely due to the low temperature of the HTC reaction. The microspheres formed on the surface of hydrochars because sectional cellulose was solubilized and/or hydrolyzed. These nano/microspheres were also generated by the decomposition of amorphous cellulose [42].
As shown in Fig. 2b and Fig. 3b, with 250 ℃ reaction temperature and 6 h reaction time, the surface of hydrochar became rougher as some pores and different sizes of small carbon microspheres were formed on the surface. One possible reason is that higher temperature facilitates the degree of solubility and/or hydrolysis of cellulose.
However, unlike cellulose-derived hydrochar, the microspheres formed on the surface of wood-derived hydrochar should be expected due to the conversion of hemicellulose, which is a fundamental component of wood [10]. The amount of sphere-like nano/microparticles of wood hydrochar is lower than that of cellulose-derived hydrochar under the same experimental conditions. It indicates that cellulose-derived hydrochar is much easier to be carbonized and/or hydrolyzed than wood-derived hydrochar.
It is illustrated in Fig. 2 (c-d) and Fig. 3 (c-d), after 12 h treatment, the dense structure shifted to a loose structure. Figure 2c shows that parts of hydrochar are fused and adhered between the microspheres at 250 ℃ after 12 h reaction time. The fragments and porosity of hydrochar increase due to gas emissions during devolatilization, and chemical bonds were broken. In Fig. 2d and S3d the degree of cellulose decomposition was more evident because the content of fixed carbon of cellulose-derived hydrochar is less than that of wood-derived hydrochar. Besides, the temperature can influence samples' structure by changing the properties of water, which easily permeates into the porous structure of hydrochars.
3.3 Chemical characterization
The elemental analysis of samples is shown in Table 2. Cellulose- and wood-derived hydrochars have a higher weight percentage of carbon and lower weight percentage of hydrogen and oxygen than their original substrates.
Table 2: The elemental analysis and atomic ratio of raw biomass and hydrochars.
|
Elemental analysis
|
atomic ratio
|
Samples
|
C (wt %)
|
H (wt %)
|
Oa (wt %)
|
H/C
|
O/C
|
Cellulose
|
39.3
|
6.61
|
54.09
|
1.98
|
1.03
|
C-200-6
|
41.60
|
6.03
|
52.37
|
1.74
|
0.94
|
C-250-6
|
62.48
|
4.79
|
32.73
|
0.92
|
0.39
|
C-250-12
|
63.30
|
4.50
|
32.20
|
0.85
|
0.38
|
Wood
|
46.4
|
6.05
|
47.55
|
3.86
|
3.86
|
W-200-6
|
57.80
|
5.92
|
36.28
|
1.23
|
0.47
|
W-250-6
|
70.80
|
5.01
|
24.19
|
0.85
|
0.26
|
W-250-12
|
71.90
|
4.93
|
23.17
|
0.83
|
0.24
|
a: The amount of oxygen was calculated according to O (wt %) = 100-C (wt %) -H (wt %) |
Higher HTC reaction temperature and longer reaction time can further increase the weight percentage of carbon and reduce hydrogen and oxygen weight. Normally, the H/C and O/C ratios were considered as indicators for the degree of carbonization of hydrochars. The H/C atomic ratios are 1.74 (200°C 6 h), 0.92 (250°C 6 h), 0.85 (250°C 12 h) for cellulose-derived hydrochar, and 1.23 (200°C 6 h), 0.85 (250°C 6 h) and 0.23 (250°C 12 h) for wood-derived hydrochar. The O/C atomic ratios are 0.94 (200°C 6 h), 0.39 (250°C 6 h) and 0.38 (250°C 12 h) for cellulose-derived hydrochar, and 0.47 (200°C 6 h), 0.26 (250°C 6 h), 0.24 (250°C 12 h) for wood-derived hydrochar. The higher temperature and longer reaction can decrease the H/C and O/C ratios. The reduction of H/C means more condensed aromatic structures were produced due to aromatization as fundamental components of hydrochar. Aromatization also improves the stability of hydrochar in wood-derived hydrochar. The Decarboxylation process causes the reduction of the O/C by removing water from the raw materials without changing any chemical composition and produce CO2, and CO [40, 43][43].
A Van Krevelen diagram is shown in Fig. 4. As a kind of low-rank coal, the atomic ratios of lignite are 0.8–1.3 (H/C) and 0.2–0.38 (O/C) [41], and the atomic ratio of synthetic hydrochars under 250°C 6 h or 12 h is almost identical to that of lignite.
The results of the FTIR measurements were shown in Fig. 5. The band at 3000–3700 cm-1 is assigned to the O-H stretching vibrations of hydroxyl or carboxyl groups [44]. Due to the dehydration reaction, the peak of O-H becomes weak with increasing HTC temperature. Furthermore, it was reported that O-containing functional groups could absorb heavy metal ions (Kang et al., 2012; Liu and Zhang, 2009).
The band at 2800–3000 cm-1 is assigned to the stretching vibrations of aliphatic C-H, indicating an aliphatic structure [45]. The unobvious curves ranging from 2800 to 3000 cm-1 are attributed to the asymmetric stretching vibration of -CH3 (2955 cm-1) and -CH2- (2922 cm-1), symmetric vibration of -CH3 (2871 cm-1) and -CH2- (2850 cm-1), and stretching vibration of -CH (2900 cm-1) [46]. The C-H vibration at around 2920 and 2850 cm-1 is related to asymmetric and symmetric methylene stretching groups present in all the wood components [47]. The band at 1700 cm-1 refers to C = O vibrations [43], while C = O belongs to the carboxyl group or carbonyl group, owing to the dehydration of hydroxyl [10]. The peak at 1620 cm-1 is assigned to C = C vibrations of aromatic structures [48]. The peaks at 1120 − 1050 cm-1 refer to the C − O bond.
The results show the peaks of C-O, C = O, and C-H in both cellulose-and wood-derived hydrochars decreased when reaction temperature increasing. That is because more bonds ( C-O, C = O, and C-H) breaks during hydrothermal treatment [10]. Gases such as CH4, C2H6, and C2H4 release when these bonds break and lead to the reduction of H/C and O/C ratio of hrdrichar.
XPS analyzed the hydrocarbon and oxygen-containing functional groups of hydrochars, and the results are shown in Fig. 6. For cellulose-derived and wood-derived hydrochar, two main peaks of C (C1s) at around 285 eV and O (O1s) at about 530 eV are usually observed in the XPS spectroscope [49]. The peaks at 284.6 eV were attributed to CHX and C-C/C = C, which belong to aliphatic/aromatic carbon groups. The peaks at 285.7 eV and 287.3 eV were contributed by hydroxyl groups (-COR) and carbonyl groups (C = O), respectively. The small peak observed at 289.2 eV can correspond to carboxylic groups, esters (-COOR). It was also shown in the XPS results that the oxygen-containing functional groups such as hydroxyl groups, carbonyl groups, and esters existed on the surface of hydrochar.
Fig. 7 is the results of the FTIR measurements indicate that types and positions of functional groups of hydrochars after adding heavy metals are almost the same as those of hydrochars without heavy metals. Also, the SEM images of cellulose-derived and wood-derived hydrochar with heavy metals has been taken and it does not show difference with the one without heavy metal.
The speciation behavior of gaseous zinc-containing species was detected in atmospheres at 1200°C by using MBMS and the results are shown in Figs. 8 and 9. No peaks are determined in Fig. 8 (a) and Fig. 9 (a). One possible reason is that this may imply the very low partial pressures of the species zinc and zinc compounds under the present conditions, which are already outside of the sensitivity range of the instrument. Another possible reason is that zinc ions and other ions in hydrochar form high temperature resistant compounds, such as silicate minerals under hydrothermal carbonization process.
Compare Figs. 8 (b) and 9 (b), 10 wt% zinc is used in hydrothermal carbonization process, there is an obvious peak of zinc. Possible reason is that part of the zinc ions has formed high temperature resistant compounds, and the remaining zinc ions are combined with functional groups. For example, when Zn(II) is introduced into the hydrothermal carbonization reaction, these metal ions can be sorbed by phenolic, carboxyl, ester, and alcohol groups on the surface of the precursor [50][51]. by ion exchange interaction, large amount of phenolic, carboxyl and alcohol groups which are active functional group formed on the surface of hydrochars. Due to higher amount of zinc used, in Fig. 8 (b) and 9 (b), both of them show the peaks of zinc species. pure Zn gaseous species include 64Zn+, 68Zn+.
The previous study found that the structure of benzoic acid which is adsorbed on (0001)-Zn decompose to benzene under 300 K [52] Therefore, it is supposed that the connection of Zn and carboxyl grounp would promote decarboxylation reaction during the hydrothermal process. Moreover, Vohs et al. [52] reported that aromatic alcohols, benzyl alcohol, and phenol can form highly stable alkoxide species on the (0001)-Zn surface below 875 K. However, under high temperature, these organic functional groups are burned and zinc ions evaporate.