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Monday, 17 October 2016
Thermal & chemical analyses of hydrothermally derived carbon materials from corn starch
Published Date
doi:10.1016/j.fuproc.2016.08.002
Open Access, Creative Commons license
Author
Min Zhao a
Bin Li a,,
Jia-Xiao Cai a
Chuan Liu b
K.G. McAdam b
Ke Zhang a,c,,
aZhengzhou Tobacco Research Institute of CNTC, Zhengzhou 450001, China
bGroup Research & Development, Regents Park Road, Southampton SO15 8TL, UK
cSchool of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
Received 29 December 2015. Revised 29 July 2016. Accepted 1 August 2016. Available online 6 August 2016. Highlights
•
Post-treatment increased aromatization degree of hydrothermally derived carbon.
•
Combustion behavior of HTC coal can be adjusted by thermal post-treatment.
•
Calculated kinetic parameters of samples agreed well with the reaction mechanism.
Abstract
Hydrothermal and pyrolytic carbonization were used in this study to prepare a range of carbonaceous materials from corn starch powder. The objective of the study was to investigate the relationship between microstructures of the hydrothermally carbonized materials synthesized by a two-step carbonization process and their thermophysical and thermochemical properties. Differences in thermal behavior, as observed by thermogravimetric measurements conducted in air, were investigated by supplementary studies using elemental and proximate analysis, Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS) characterisation and weight loss kinetic modeling. The results indicated that the degree of the aromatization of obtained materials with higher carbon contents could be increased by subsequent annealing at higher temperatures. The resulting materials with different carbon skeletons displayed different kinetic behavior upon heating. Kinetic modeling of the thermogravimetric data revealed a low temperature and a high temperature combustion region with different kinetics parameters, thus demonstrating the potential of hydrothermal carbonization (HTC) as a synthesis strategy for fine-tuned functional high-purity combustion fuel preparation.
Graphical abstract
Keywords
Hydrothermal carbonization
Thermogravimetric analysis
Kinetics
Microstructure
Pyrolysis
1 Introduction
Sustainable and green energy development is a global challenge. Charcoal can play an important role for sustainable energy consumption if it can be made economically through non-fossil fuel based routes. Conventional charcoal production usually involves thermochemical conversion of biomass (e.g., wood, coconut shell, or other agricultural byproducts) or coal [1] and [2]. Charcoals with more specific surface properties have been made via petrochemical route [3] or secondary treatments of charcoal with surface modification [4]. Research on nano-structured carbon-based materials biomedical and electronic applications has also attracted significant interests. In general, however, research is still needed to shift both the source materials and the energy used to more sustainable bases for economically viable charcoal materials. For this reason, different biomass have been used as source materials for carbonaceous fuels [5] and also for bioenergy [6] and [7].
In the last decade or so, hydrothermal carbonization (HTC) of biomass has attracted increasing interest in the field of carbon materials and biofuels [8]. Bergius first described the hydrothermal transformation of cellulose into coal-like materials in 1913. Early work mainly focused on the preparation of biofuels [9]. The full potential of hydrothermally carbonized materials has only been discussed in recent years through the work of Titirici and co-workers [10] and [11], in several fields such as catalysis, CO2 sequestration, agriculture and renewable energy products.
Typical energy contents of HTC materials have been measured to be between 25 and 35 MJ · kg− 1, which is about 40% higher than those of biomass precursors [12]. The HTC materials are expected to exhibit favourable behaviors with respect to combustion, gasification, and other thermal conversion processes. There have also been studies on combustion or gasification of carbon materials [13] and [14]. The most important issue in combustion or thermo-synthesis of carbon materials, such as fullerenes, nanotubes, graphite, and diamond, is the control of compositional, morphological and chemical nature of different carbon sources reacting with an oxidant agent [15] and [16]. However, as far as we are aware, there has been no systematic work on combustion characteristics of HTC materials.
The objective of this study was to conduct a feasibility of using corn starch as a model precursor, due to the defined mechanism under HTC conditions, to evaluate the link between various carbonized products as a combustion fuel. The combustion characteristics of different HTC intermediates from corn starch, its post-carbonization forms, and the carbon derived from pyrolysed corn starch were investigated to understand the relationship between the microstructural properties and combustion characteristics. A range of experimental techniques were employed such as scanning electron microscopy (SEM), N2-adsorption/desorption, Fourier-transform infrared spectroscopy (FTIR), elemental and proximate analysis of the materials, X-ray photoelectron spectroscopy (XPS) and non-isothermal thermogravimetrics analysis. In addition, kinetic analysis based on the Coats-Redfern model was performed on the HTC materials combustion processes [17] and [18].
2 Materials and methods
2.1 Materials
Corn starch powder was purchased from Aladdin Reagent Co. Ltd. (Shanghai, China) and was used as received. Deionized water used was from A. S. Watsons Group Ltd. (Shanghai, China). All other reagents used in the synthesis were of analytical grade.
2.2 Preparation of carbon materials
HTC material, its post-carbonization forms, and pyrolytic carbon were prepared and studied following the methods described below. For preparing the HTC materials, 70 mL deionized water was mixed with 6.3 g corn starch power and the solution was placed into a 100 mL Teflon lined stainless steel autoclave at room temperature. The autoclave was sealed and heated in an oven for 12 h at 180 °C. The HTC material obtained was separated from the remaining aqueous solution by centrifugation at 7000 rpm for 20 min and then washed 5 times with deionized water and ethanol. Finally, the derived material was dried in a vacuum oven at 80 °C for 12 h. The sample material thus prepared is labeled as “HTC-S”.
Pyrolysis was performed in a horizontal tube furnace with a 20 mL · min− 1 nitrogen flow rate at 350 °C, 550 °C, and 750 °C, respectively. The heating rate was 10 K · min− 1. The carbon source materials were the starch and its HTC form (HTC-S). All the samples were held at the final temperature for 4 h before being cooled to room temperature by the nitrogen flow. They were then homogenized by grinding with a mortar and pestle. The post-carbonization HTC-S is denoted as HTC-S-X and the pyrolysed starch as Py-S-X; in both cases, “X” is denoted as the carbonization temperature (350, 550, or 750 °C). For example, Py-S-550 is a pyrolysis-derived carbon material heated at 550 °C under nitrogen atmosphere.
2.3 Characterization
The morphology of the prepared samples was characterized using JEOL-6010LA scanning electron microscopy (SEM). Before imaging, sample powder was loaded onto Kapton carbon tapes and sputtered with Au. Images were acquired by setting an SEM working distance of 10 mm and accelerating voltage of 1.5 kV. In addition, the surface area, total pore volume, and average pore diameter of the synthesized samples were calculated by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The measurement was carried out at 77 K using the adsorption of nitrogen gas (Micromeritics, Model ASAP2020). Fourier transformed infrared (FTIR) spectroscopy measurements were carried out at room temperature in ambient atmosphere using a Nicolet 6700 FTIR spectrometer with the KBr pellet technique. FTIR spectra were recorded in the range of 4500–400 cm− 1. The spectral outputs were recorded in the transmittance mode as a function of wave-number and 124 scans were recorded with the resolution of 4 cm− 1. The X-ray photoelectron spectra (XPS) were recorded with an ESCALAB 250 (Thermo Electron). The X-ray excitation was provided by a monochromatic Al Kα (1486.6 eV) source. Data quantification was achieved by the advantage program.
Elemental chemical analysis of the samples were carried out in a CHONS Elemental Analyzer (Vario EI Cube, Elementar, Germany) to determine C and H contents in a stream of pure O2. The content of oxygen was calculated by subtracting the relative C and H fractions. Proximate analysis is based on thermogravimetry [19], containing moisture (M, wt.%), volatile matter (VM, wt.%), fixed carbon (FC, wt.%) and ash (A, wt.%) contents.
Channiwala and Parikh' correlation for estimating higher heat value (HHV, MJ · kg− 1) of solid fuels was applied in this study [20], i.e.:
where C, H, S, O, N, and A represent carbon, hydrogen, sulphur, oxygen, nitrogen, and ash contents of a fuel, respectively, in wt.%. Due to corn starch with few N and S elements, 0.1005S and − 0.0151N were ignored. The results were placed in Table 2and Table 3.
The combustion characteristics of the resultant carbon materials were analysed by monitoring their weight changes in air at a flow rate of 20 mL · min− 1 using a Netzsch STA 449F3 thermogravimetric analyzer. Each experiment was performed with ca. 9–13 mg of the sample loaded in α-Al2O3 crucible, and covering a temperature range from 40 to 850 °C at a heating rate of 5 K · min− 1. The heating rate of this order is generally considered able to ensure that no temperature gap exists between the sample and its surroundings. The isothermal temperature precision is 0.01 K and the weighing balance sensitivity is 0.1 μg.
2.4 Combustion kinetic model
The thermogravimetric (TG) data obtained were analysed using the following kinetic equation [21]:
equation1
where α is the fraction of the sample decomposed (%); t is the heating time (s); n is the order of reaction; K is the rate constant.
Let f (α) denote the reaction model, which contains the information regarding the reaction mechanism. A first-order kinetic reaction is the most frequently used assumption for solid fuel decomposition/combustion [22], [23] and [24]. Therefore, in present study was fitted to first-order reactions. The reaction equation can be described by the following:
equation2
The temperature dependence of K is expressed by the following Arrhenius expression [25]:
equation3
where E is the activation energy (kJ · mol− 1); A is the pre-exponential factor (s− 1); Ris the ideal gas constant (8.314 J · mol− 1 · K− 1); T is the temperature (K). Coats and Redfern developed an integral method based on the Arrhenius model, which can be applied to the TG data, assuming different order of reactions and introducing the constant heating rate β = dT/dt. This order is presumed to give the best square of the linear correlation coefficient. The kinetic parameters, E and A, can be calculated from the slope and the intercept of the straight line, respectively. In this paper, the least square regression method was used in drawing the straight lines to fit the experimental data. The final form of the equation [17], which was used for data analysis is as follows:
equation4
For easy calculation, Eq. (4) was simplified and translated by natural logarithm:
equation5
3 Results and discussion
3.1 Morphology and microstructure
The SEM morphologies of corn starch and the carbon samples prepared in this study are given in Fig. 1. Corn starch granules are irregular particle shape in Fig. 1(a). When corn starch was hydrothermally carbonized, as can be seen in Fig. 1(b), HTC-S consisted of agglomerated micro-spheres or granules with smooth surfaces and particle size between 2 and 5 μm. This HTC material has a similar morphology and particle size to those synthesized by HTC treatment with potato starch [26]. Fig. 1(c–e) show that following post-carbonization treatments at three different temperatures, the HTC products still maintained their smooth spherical shape but with an increased average particle size. When starch was pyrolysed directly at 550 °C under nitrogen, starch granules appeared to melt together during the process [27] and a foam network structure was observed (Fig. 1(f)). The surface area, total pore volume and average pore diameter were carried out by the BET and BJH methods by N2adsorption-desorption isotherm (Table 1). There was a significant increase in the BET surface area when HTC-S sample was subjected to further carbonization at 350 and 550 °C; the trend appeared to be stabilized from 550 to 750 °C. These differences in micro and texture structures, as a result of HTC vs pyrolysis, may affect subsequent mass (oxygen) transfer and surface reaction, hence influence the course and outcome of combustion behavior.
Fig. 1. SEM images of (a) corn starch, (b) HTC-S, (c) HTC-S-350, (d) HTC-S-550, (e) HTC-S-750, (f) Py-S-550.
Table 1. Surface area and pore characteristics of corn starch, HTC-S, its post-carbonization forms, and pyrolytic carbon material.
Sample
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
Starch
0.8
–
–
HTC-S
3
0.02
23
HTC-S-350
26
0.05
6
HTC-S-550
496
0.27
2
HTC-S-750
512
0.27
2
Py-S-550
437
0.24
2
3.2 Chemical characteristics
FTIR spectra were used to investigate the effects of the hydrothermal and pyrolytic treatment on the molecular carbon skeleton structures (Fig. 2). For this purpose, the spectra of starch and HTC-S may be used as references. The broad band around 3500 cm− 1 is attributed to the OH (hydroxyl or carboxyl) stretching vibration; while the peak at 1720 cm− 1 corresponds to CO, suggesting the abundant existence of these functional groups on the surface of HTC-S [28]. The small peak at 2910 cm− 1 is ascribed to stretching vibration of the aliphatic CH. Baccile et al. characterized scaffold of HTC material by using a combination of advanced solid-state 13C NMR techniques and described the scaffold as furan rings cross-linked by domains containing short aliphatic chains [29]. The bands at 1600 cm− 1 and in the region of 1000–1450 cm− 1 are therefore related to CC and COC stretching vibrations within furan rings. The FTIR spectrum of Py-S-550 around the bands at 3450, 1640, and 910 cm− 1 provides evidence for the presence of condensed polyaromatics containing randomly distributed oxygen functionalities [30]. The FTIR spectra of HTC-S-350, HTC-S-550, and HTC-S-750 are similar, especially between the latter two samples. When HTC-S was calcined at 350 °C under nitrogen, loss of the functional groups and aliphatic CH is evident from the weak peak intensity in FTIR spectrum of HTC-S-350 at 3500, 2900 and 1720 cm− 1. This is consistent with the elemental chemical composition change which shows reduced H and O contents and, correspondingly, increased carbon content (Table 2). At the two higher calcination temperatures, i.e., 550 and 750 °C, the COC vibration band in the region of 1400–1100 cm− 1 have diminished; whereas the growth of bands at 1600 cm− 1indicates progressive development of aromatization in the carbon skeleton through the post-carbonization treatment. These differences describe the possible structural changes that occur in the materials under both hydrothermal carbonization and pyrolysis at higher temperature, which are similar to those reported previously for the structures of HTC derived from glucose and starch [31].
Fig. 2. FTIR spectra of starch, HTC-S, HTC-S-X and pyrolytic carbon materials.
Table 2. Elemental analysis and HHV for starch, HTC-S, HTC-S-Xs, and Py-S-550Table 2a and Table 2b.
Sample
C (wt.%)
H (wt.%)
O (wt.%)
O/C
H/C
HHV (MJ · kg− 1)
Yield (%)
Starch
38.98 ± 0.52
7.04 ± 0.10
53.62 ± 0.62
1.06 ± 0.02
1.84 ± 0.00
14.96
–
HTC-S
62.08 ± 0.88
5.12 ± 0.02
32.80 ± 0.90
0.40 ± 0.02
1.00 ± 0.00
24.29
7.51
HTC-S-350
69.57 ± 0.92
3.68 ± 0.14
26.76 ± 1.04
0.29 ± 0.02
0.64 ± 0.00
28.56
56.87
HTC-S-550
78.49 ± 2.60
3.24 ± 0.02
18.27 ± 2.62
0.19 ± 0.04
0.50 ± 0.00
29.29
52.41
HTC-S-750
81.42 ± 0.46
2.21 ± 0.04
16.37 ± 0.50
0.15 ± 0.01
0.32 ± 0.00
29.30
43.52
Py-S-550
83.09 ± 2.04
3.09 ± 0.22
13.82 ± 2.26
0.13 ± 0.03
0.44 ± 0.00
31.19
13.89
a
O/C and H/C are given in molar ratio.
b
The yields of HTC-S and Py-S-550 are based on starch and the yields of HTC-S-X are based on HTC-S.
As noted above, the results of elemental and proximate analysis of the carbon samples were respectively compared in Table 2 and Table 3. From the elemental analysis, it can be seen that the carbon content increased from 62.1% for the HTC-S sample to higher contents for all the other tested materials, with Py-S-550 having the highest of 83.1%. This trend is accompanied by the decreasing oxygen and hydrogen contents. The O/C and H/C ratios for the HTC-S are higher than the other carbon samples due to the presence of oxygen-containing functional groups and cross-linked furan rings, which is consistent with FTIR spectrum. There was further reduction in the oxygen and hydrogen contents in the post carbonization samples. In particular, the HTC-S-750 had a relatively lower O/C ratio and the lowest H/C ratio, suggesting an increase in the degree of aromatization. Meanwhile, for HTC-S-X, the HTC-S-750 had the lowest volatile matter and the highest fixed carbon content. These variations are consistent with the formation of a well-condensed material [11]. The evolution of the O/C and H/C atomic ratios from HTC-S to HTC-S-X followed the dehydration, decarboxylation and internal structural rearrangement processes, similar to that previously observed for the pyrolysis process of saccharides [32]. For Py-S-550, the oxygen content and O/C ratio were the lowest. These observations appear to agree with the HTC and pyrolysed carbon chemical structures reported by Falco et al. [31], in which carbon structures evolve to similar forms when the post-carbonization temperature is above 350 °C.
Table 3. Proximate analysis for HTC-S, HTC-S-Xs, and Py-S-550.
Sample
M (wt.%)
VM (wt.%)
FC (wt.%)
A (wt.%)
HTC-S
0.89 ± 0.01
50.14 ± 1.23
47.96 ± 0.66
1.01 ± 0.01
HTC-S-350
0.55 ± 0.02
41.91 ± 0.98
56.03 ± 0.43
1.51 ± 0.00
HTC-S-550
0.73 ± 0.04
30.01 ± 1.04
67.48 ± 0.71
1.78 ± 0.02
HTC-S-750
0.81 ± 0.04
28.67 ± 0.63
68.63 ± 0.56
1.89 ± 0.00
Py-S-550
0.69 ± 0.02
28.89 ± 0.59
68.85 ± 0.30
1.57 ± 0.01
X-ray photoelectron spectroscopy (XPS) was conducted to confirm the chemical structure variations of different carbon samples. XPS of C1s ranging from 280 to 290 eV are shown in Fig. 3. The C1s strong peak at 284.5 eV, which is consistent with sp2graphite carbon [33]. The C1s spectrum is composed of a major peak at 285.4 eV, which is associated with the CC, CC and CH bonding, and three minor peaks at 286.5 eV, 287.8 eV, and 289.1 eV, which correspond to the CO, CO, and OCO, respectively. Based on the compositional information of carbon, it can be found that the treatment temperature has a significant effect on the carbon levels of the samples. The intensity peaks characteristic of the CC increased, but the characteristics of the CC, CO, CO, and OCO groups decreased, indicating that the post-carbonization treatment led to condensed polyaromatic structure. These results of XPS data verified the conclusion of carbon structures evolution to be reasonable and reliable.
Fig. 3. XPS C1s spectra of HTC-S, its post-carbonization forms, and pyrolytic carbon material.
3.3 Thermogravimetric (TG) analysis
Fig. 4 shows the typical TG and the derivative thermogravimetric (DTG) curves between 40 and 850 °C in air atmosphere. As the samples were heated in air, their overall behavior is examined for combustion properties. At the given heating rate (5 K · min− 1), the mass loss of HTC-S mainly occurred in the temperature region of 257–459 °C. As shown in Fig. 4b, after the evaporation of adsorbed water, the combustion of HTC-S occurring in the lower temperature range led to the first DTG peak around 286 °C, while the later DTG peak around 449 °C was due to oxidization occurring in the higher temperature region. For the TG profile of the post-carbonization samples (HTC-S-X), the lower temperature inflection point, started around 286 °C by HTC-S sample, either moved to higher temperature regions or disappeared completely. The higher temperature inflection point where the material lost most of its starting weight (around 449 °C for HTC-S) increased gradually with the increasing temperature of the post-carbonization. The HTC-S-550 and HTC-S-750 profiles presented single weight loss rate peak in the high temperature region and their maximum weight loss rate increased significantly. Shiori et al. investigated the thermal behavior of HTC derived from 2-furaldehyde and observed two main mass loss regions, an initial mass loss around ~ 300 °C corresponding to the desorption of structural H2O and CO2 and second peak at ~ 440 °C as result of oxidative decomposition of the carbonaceous framework [34]. This observation did not agree with the TG profiles of HTC-S-X and Py-S-550 samples prepared in this work. Previous work has shown that the HTC-S generally contains abundant oxygen-containing functional groups, sp3-hybridized alkyls carbons and conjugated furan carbons [10], [31], [35], [36] and [37]. According to the chemical characterization reported in these studies, the functional groups (e.g., hydroxyl, carbonyl, or carboxyl) and low energy chemical bonds were either eliminated or reduced, and the sp2-hybridized aromatic scaffold were extended during post-carbonization, resulting to the similar structure of pyrolysed carbon. In agreement with this, the mass loss of the HTC-S samples obtained in this work also displayed two clearly separate steps. The main mass loss in the lower temperature region was mainly caused by the decomposition of oxygen-containing functional groups, sp3-hybridized carbon motif and partial furan rings and may be existed partial volatile matter release and combustion. The second mass loss occurring in the higher temperature region can be ascribed to the oxidation of conjugated carbon atoms.
Fig. 4. (a) TG and (b) DTG curves of starch, HTC-S, HTC-S-X and pyrolytic carbon materials.
It has been reported that synthesis temperature, reaction time, and biomass precursors all significantly affect the chemical nature and morphology of the HTC materials [38]. For example, a higher temperature or longer reaction time increases the polyaromatic extent of the final HTC material [31]. Using the experimental data and these relationships in the literature between the microstructural properties and combustion characteristics mentioned above, these different TGA and DTG curves of the carbon materials derived under the various HTC materials process conditions may be explained, at least at a phenomenon logical level.
3.4 Combustion kinetic model
Typical kinetic plots for the low and high temperature reactions of the samples are shown in Fig. 5, and the supplementary data related to kinetic calculation were shown in supporting information. The least square correlation values varied between 0.982 and 0.997, indicating reasonable degree of fit. The HTC-S had lower activation energy (E) and pre-exponential factor (A) values compared with the HTC-S-X and Py-S-550 samples (Table 4). The apparent activation (E) values for the low and high temperature region varied from 69 to 198 and 84 to 166 kJ · mol− 1 for the HTC-S and HTC-S-X samples, respectively. The results suggest that, the E and A values increased with the increase of X values for the HTC-S-X samples. This appears to contradict with the results shown in Table 1, where the increasing calcination temperatures resulted in significant increase in the surface area and total pore volume, thus may be in benefit of surface reaction and gas transport during combustion. It can be concluded that the nature of the texture structures of the different samples do not affect the evolution of their combustion. The reactivity of carbon samples investigated in the present work is strongly affected by molecular chemical structures, which also indicates the first-order kinetic reaction assumption is reasonable. Abundant oxygen-containing functional groups, sp3-hybridized alkyls carbons and furan carbons were thought to favour the reduction in the lower temperature peak, which coincided with the E value of the HTC material. Among all the carbon materials, the E value of the HTCs was the lowest and the Py-S-550 the highest, indicating the possible range of energy utilization. These observations are in logical agreement with the fact that post-carbonization preferentially removed those more reactive species from the samples, thus increasing the energy required for further carbonization as the bond in the remaining structures would be increasingly aromatic.
Fig. 5. (a) The low temperature and (b) the high temperature region kinetic curves of HTC-S, its post-carbonization forms, and pyrolytic carbon material.
Table 4. Kinetic analysis for the HTC-S, its post-carbonization forms, and pyrolytic carbon material.
Sample
Low-temperature region
High-temperature region
E (kJ · mol− 1)
A (min− 1)
R2
E (kJ · mol− 1)
A (min− 1)
R2
HTC-S
69
2.81 × 105
0.997
84
1.12 × 105
0.984
HTC-S-350
198
7.62 × 1015
0.989
129
1.53 × 108
0.983
HTC-S-550
–
–
–
145
8.69 × 108
0.992
HTC-S-750
–
–
–
166
4.72 × 109
0.992
Py-S-550
–
–
–
193
1.48 × 1012
0.995
Typical kinetic plots for the low and high temperature reactions of the samples are shown in Fig. 5, and the supplementary data related to kinetic calculation were shown in supporting information. The least square correlation values varied between 0.982 and 0.997, indicating reasonable degree of fit. The HTC-S had lower activation energy (E) and pre-exponential factor (A) values compared with the HTC-S-X and Py-S-550 samples (Table 4). The apparent activation (E) values for the low and high temperature region varied from 69 to 198 and 84 to 166 kJ · mol− 1 for the HTC-S and HTC-S-X samples, respectively. The results suggest that, the E and A values increased with the increase of X values for the HTC-S-X samples. This appears to contradict with the results shown in Table 1, where the increasing calcination temperatures resulted in significant increase in the surface area and total pore volume, thus may be in benefit of surface reaction and gas transport during combustion. It can be concluded that the nature of the texture structures of the different samples do not affect the evolution of their combustion. The reactivity of carbon samples investigated in the present work is strongly affected by molecular chemical structures, which also indicates the first-order kinetic reaction assumption is reasonable. Abundant oxygen-containing functional groups, sp3-hybridized alkyls carbons and furan carbons were thought to favour the reduction in the lower temperature peak, which coincided with the E value of the HTC material. Among all the carbon materials, the E value of the HTCs was the lowest and the Py-S-550 the highest, indicating the possible range of energy utilization. These observations are in logical agreement with the fact that post-carbonization preferentially removed those more reactive species from the samples, thus increasing the energy required for further carbonization as the bond in the remaining structures would be increasingly aromatic.
4 Conclusions
In this study, the relationship between the microstructures synthesized by the HTC and pyrolysis-derived corn starch was systematically investigated. The differences in thermal behavior as observed by TG curves in air between the HTC and pyrolysis-derived carbonaceous materials were mainly due to their molecular chemical structures. The elemental analysis, FTIR, and XPS spectra obtained indicated that the aromatic degree of HTC materials can be tailored by subsequent post-carbonization with accompanying increase in carbon contents. These different carbon skeletons and their microstructures may be the reasons why these materials displayed different rate of combustion upon heating. Their ignition and combustion temperatures as measured in the TG curves indicate possibilities for different fuel source applications. The systematic links shown between the derived carbon materials and their laboratory processing parameters may allow for further processing optimization and potential pilot-scale production of these materials.
Nomenclature
HTC
hydrothermal carbonization
SEM
scanning electron microscopy
HHV
higher heat value
FTIR
Fourier-transform infrared spectroscopy
XPS
X-ray photoelectron spectroscopy
HTC-S
hydrothermally derived carbon materials from corn starch
HTC-S-350
post-carbonization of HTC-S at 350 °C under nitrogen atmosphere
HTC-S-550
post-carbonization of HTC-S at 550 °C under nitrogen atmosphere
HTC-S-750
post-carbonization of HTC-S at 750 °C under nitrogen atmosphere
Py-S-550
pyrolysis-derived carbon heated at 550 °C under nitrogen atmosphere
TG
thermogravimetric
DTG
derivative thermogravimetric
E
activation energy
A
pre-exponential factor
The following are the supplementary data related to this article.
Chemical structures of swine-manure chars produced under different carbonization conditions investigated by advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy
H (hydroxyl or carboxyl) stretching vibration; while the peak at 1720 cm− 1 corresponds to C
O, suggesting the abundant existence of these functional groups on the surface of HTC-S [28]. The small peak at 2910 cm− 1 is ascribed to stretching vibration of the aliphatic C
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