Preparation and Characterization of Biochars from Waste Camellia

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Preparation and characterization of biochars from waste Camellia oleifera shells by different thermo-chemical processes Fangyu Fan, Yunwu Zheng, Yuanbo Huang, Yi Lu, Zhen Wang, Bo Chen, and Zhifeng Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00269 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Preparation and Characterization of Biochars from Waste Camellia Oleifera Shells by Different Thermo-chemical Processes Fangyu Fan †, ‡,*, Yunwu Zheng ‡, Yuanbo Huang ‡, Yi Lu ‡, Zhen Wang ‡, Bo Chen ‡, Zhifeng Zheng ‡, * †

The College of Forestry, Southwest Forestry University, Kunming, Yunnan, 650224,

China; ‡

Engineering Laboratory for Highly-Efficient Utilization of Biomass, Yunnan

Province; University Key Laboratory for Biomass Chemical Refinery and Synthesis, Yunnan Province; College of Materials Engineering, Southwest Forestry University, Kunming, Yunnan, 650224, China ABSTRACT: Waste Camellia oleifera shells (WCOSs) have potential to produce sustainable clean-green energy sources. In this paper, hydrothermal carbonization (HTC), torrefaction, and pyrolysis were applied to investigate the characterization of three types of biochar from WCOSs. The biochars were analyzed with the ultimate analysis, proximate analysis, hydrophobicity, Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR). The results showed that the biochars were able to replace the lignite or semi-anthracite coal because of the higher calorific value. Compared with raw WCOSs, the BET characteristics and hydrophobicity of the biochars were improved by thermo-chemical processes. Evolutions of WCOSs under different conditions, as determined by FTIR and XRD, showed that hemicelluloses and cellulose were decomposed through HTC, torrefaction, and pyrolysis, while the degradation of lignin

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occurred through pyrolysis. In addition, combustion behaviors of WCOSs and their biochars were significantly different. 1. INTRODUCTION China is the largest developing agricultural country in the world, which produced a huge variety of agricultural and forestry wastes every year.1-2 These wastes are commonly burned as fuel in the rural area, which could lead to serious air pollution and energy loss.3-4 The comprehensive utilization of agricultural and forestry wastes becomes an important issue. A good method is to convert the wastes into biochars as solid fuel via applying thermo-chemical technologies. Recently, many literatures have reported the utilization of agricultural and forestry wastes as solid fuel through thermo-chemical processes, such as corn straw,5-6 rice husk ,7 cotton stalk,6 sawdust,8 and oil palm shell.9 However, no report is available concerning the biochar derived camellia oleifera shells by applying thermo-chemical methods till now. Camellia oleifera is a kind of important woody oil crop in some southern provinces of China, such as Hunan, Jiangxi, Anhui, Yunnan, Guizhou. Currently, the planting area is about 4×106 hm2 and the yield reaches 5.6×106 tons every year, with approximately 0.54 ton of the shell wastes produced for per ton.10-11 Therefore, a large amount of WCOSs need to be properly handled. However, WCOSs were normally abandoned as wastes or used as fuel or fertilizer after treatments in traditional industries in China. In the last decade, there were many reports about the extraction and separation of biologically active substances from WCOSs.12-13 Recently, carbon materials derived


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WCOSs have been prepared via different methods. Zhai et al.10 researched nitrogen-doped porous carbon from WCOSs with enhanced the electrochemical performance. They found that the nitrogen-doped activated carbon showed nearly four times the capacitance (191 vs 51 F/g) comparing to the pure activated carbon. Li et al.14 studied a fabrication process of carbon microspheres with controllable porous structure by using WCOSs through HTC combined with the physical activation or KOH chemical activation technique. As a type of forestry waste, WCOSs were usually used as a landfill material and incinerated in an open atmosphere, which caused many environmental problems.15 In fact, WCOSs contain a lot of hemicellulose, cellulose, lignin,13, 15 and can be converted into biochar or bio-oil as a carbon-neutral fuel by thermo-chemical processes (i.e., HTC, torrefaction, and pyrolysis). Up until now, some researchers have investigated the utilization of WCOSs in energy field through thermo-chemical processes. Chen et al.16 analyzed feasibility and comparative studies of thermo-chemical liquefaction of WCOSs in different supercritical organic solvents to produce bio-oil. Zhang et al.17 researched thermo-chemical liquefaction based onWCOSs and production of vegetal polyalcohol. Wu et al.18 studied characteristics of bio-oil and biochar in a fluidized-bed fast pyrolysis system. In these literatures, there is no detailed analysis of the fuel characteristics of biochar. In order to analyze the properties of biochar in different conditions, different preparation methods (HTC, torrefaction, and pyrolysis) were applied to produce biochars, and fuel characteristics were analyzed in this paper. But to the best of our knowledge, there has been no report


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dealing with characteristics of biochar produced by HTC, torrefaction and pyrolysis from WCOSs. The above-mentioned three types of biochar are obviously different in characteristics because of differences of thermo-chemical conditions on temperature and surrounding for HTC, torrefaction and pyrolysis. In HTC, samples were treated with hot compressed subcritical water in the range of 180-260 °C for more than 5 min- 8 h.5, 19 In torrefaction, dry samples were treated in an inert gas environment in a temperature range of 200-350 °C for more than 30 min, while pyrolysis differed in a temperature range of 400-700 °C and for more than 10 min.20-22 Therefore, the objective of this work was to analyze fuel properties of biochars produced by HTC, torrefaction and pyrolysis in terms of proximate analysis, ultimate analysis, mass yield, energy yield, higher heating value (HHV), hydrophobicity. Moreover, BET, XRD, SEM, and FTIR were further analyzed to provide additional explanations for the evolution during thermo-chemical processes. In addition, combustion behaviors of WCOSs and their biochars were analyzed by thermogravimetric analysis (TGA). According to researches, it is possible to provide basic data for the utilization of biochar from WCOSs. 2. EXPERIMENTAL SECTION 2.1. Materials. Camellia oleifera shells were collected from Yunnan province, China. Fresh WCOSs were washed with deionized water three times, and were kept in an oven at 105 °C for 24 h in order to dry samples. Dried WCOSs were ground to


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shorter than 0.6 mm fractions as test samples. The proximate analysis, and ultimate analysis were shown in Table 1. 2.2. Hydrothermal Carbonization. The HTC biochar was prepared in a laboratory scale semi-batch 500 ml Parr autoclave reactor (USA). Approximately 50 g of WCOSs powder were combined with 400 ml deionized water in a batch reactor. The reactor was flooded with nitrogen under high-pressure to discharge oxygen from the reactor. This process was repeated 5 times. Subsequently, the reactor was heated to the desired temperature, and was maintained at 300 °C for 30 min. After the specified reaction time, the reactor was immediately immersed into water to cool down to room temperature so as to stop the reaction. Finally, the mixture inside the reactor was filtered through a glass filter to obtain the solid product (HTC biochar). The HTC biochar was dried at 105 °C for 24 h and preserved hermetically. 2.3 Torrefaction and Pyrolysis. Experiments of torrefaction and pyrolysis were carried out in a laboratory scale fixed bed reactor under atmosphere pressure. As shown in Fig.1, the reactor was made of quartz tube reactor with dimension of 550 mm length and 60 mm inner diameter, a horizontal tubular resistance furnace with a digital temperature controller for heating, a liquid nitrogen condenser used for collecting product, N2 gas cylinder, and a gas bag for gas collection. Before torrefaction or pyrolysis was conducted, approximately 30 g of WCOSs powder were kept in the quartz boat in the cooling zone of the reactor. The flow rate of nitrogen was kept at 100 ml/min to remove the oxygen so as to maintain an inert atmosphere in the reactor. When the temperature reached the desired value, the quartz


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boat with samples was pushed into the reacting zone for 30 min. At the end of each reaction, the quartz boat was pulled out to the cooling zone. At last, solid products were produced and analyzed. According to literatures,21, 23-25 temperatures of torrefaction and pyrolysis were chosen at 300 °C, and 600 °C, respectively. 2.4. Research Methodology. The ultimate analysis (C, H, N, and S) was analyzed using a Vario EL Ⅲ Elemental Analyzer (Elementar, Germany), and the oxygen content was calculated by difference. The proximate analysis was conducted using 5E-MAG6600 Automatic Proximate Analyzer (China). The mass and energy yields were calculated by Eq. (1) and Eq. (2) as follows: Mass yield (%) = (mbiochar/mWCOSs) × 100

(1)

Energy yield (%) = (HHVbiochar/HHVWCOSs) × mass yield

(2)

The hydrophobicity is expressed by the equilibrium moisture content (EMC).22, 26 The EMC was measured by placing 20.0 g of each sample in two separate large bottles containing saturated sodium chloride and potassium carbonate, at 30 °C for 48 h. The relative humidity (RH) was approximately 75%, and 42% in two bottles, respectively. The treated samples were dried in an oven at 105 °C for 24 h, and the EMC of the samples was calculated, using the weight differences between before and after treatment. BET characteristics of biochars were analyzed by using BET (Micrometrics ASAP2020, America).The X-ray diffraction (PANalytical B.V., X’Pert PRO, Netherlands) for the crystal structure of biochars was carried out to provide information of solid products. For SEM measurements, the samples were


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sputter-coated with Pt and examined with JSM-6490LV scanning electron microscope (JEOL, Japan).The functional groups were determined by FTIR spectra (Magna-IR 560 ESP Thermo Nicolet, America). The sample discs were prepared by mixing the dried samples with KBr powder at room temperature in a biochar/ KBr ratio of 1:200. Combustion behaviors of samples were evaluated by thermogravimetric analysis (NET-ZSCH, Germany) at a temperature range of room temperature to 900 °C, with a rate of 10 °C/min. The gas flow rate was maintained at 60 ml/min (N2: O2 = 4:1). The samples were grounded into powder and sieved to less than 100 µm in size. Approximately 10.00 mg of each sample was used for the experiment 3. RESULTS AND DISCUSSION 3.1. The Fuel Properties of Biochar. Table 1 shows that the experimental designs were employed to research the effects of three types of condition on the mass yield, energy yield, and HHV, which are in the range of 31.01-53.45%, 46.35-70.12%, and 18.21-27.22 MJ·kg-1, respectively. The mass yield and energy yield of the HTC biochar are the highest, but the HHV is the lowest. The HHV of the HTC biochar can be comparable with those of peat, methanol and Converse School–Sub C coal,27 meanwhile that of torrefaction and pyrolysis biochars are close to Russian semi-anthracite and South African bituminous coals.28 The results demonstrated that three types of biochar can be used as solid fuels. Differences by three types of thermo-chemical treatment are mainly due to the variation in the extent of degradation of hemicellulose and cellulose in the biomass. The degradation extent of components (i.e. hemicelluloses, cellulose, and lignin) depends on the conditions, such as


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temperature, residence time, and so on.7-8, 19, 29 The ultimate analysis of WCOSs and their biochars is summarized in Table 1. As expected, the C content is increased after thermo-chemical treatments, and the O and H content are decreased. Variation in H/C and O/C atomic ratios of samples can be used to evaluate the fuel properties of solid production. A better solid fuel can be found close to the origin and vice versa.29 The H/C and O/C atomic ratios of lignite, semi-anthracite and bituminous coals are also plotted in an Krevelen diagram (Fig.2) for comparison with WCOSs and their biochars. As can be seen from Fig.2, the H/C and O/C atomic ratios of the HTC biochar is close to that of lignite, semi-anthracite, while that of torrefaction biochar is close to bituminous coals. The H/C and O/C atomic ratios of pyrolysis biochar are the closest to the origin, which means that the pyrolysis biochar possess the best fuel properties. From the above, it can be seen that three types of thermo-chemical treatment can obviously elevate the fuel properties of WCOSs. 3.2. Hydrophobicity. It is well known that the hydrophobicity and low moisture content are desirable for solid fuel. One of disadvantages of biomass is that it is apt to absorb moisture from the air, especially dried biomass. Meanwhile, damp biomass can support fungal growth and be rotten, and cause self-heating because of the heat production from oxidation and microbial activities.22, 26 The thermo-chemical process can increase hydrophobicity of biomass and promote the storage stability due to increasing resistance to biological decay.25 As shown in table 2, the hydrophobicity of biochars is all improved, and HTC is considered to be


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the most effective way to improve the hydrophobicity of WCOSs, then followed torrefaction and pyrolysis. The absorption moisture content of biomass mainly depends on the composition and structure. The thermo-chemical treatment can reduce cellulose and hemicellulos that have a strong capacity for water absorption.30 As shown in table 2, the EMC of biochars decrease compare with that of WCOSs. In addition, pyrolysis contribute to the formation of micropore structure, which made the EMC of pyrolysis biochar higher than that of HTC and torrefaction biochars.25 3.3. BET Analysis. The surface areas of WCOSs and their biochars are shown in Table 3 at different conditions. The results of BET reveal that the BET surface area, total pore volume, and average pore diameter of biochars are improved compared with WCOSs. The increase in BET characteristics can attribute to the reaction temperature and surroundings, which cause the fibrous structure of WCOSs to be disintegrated and then produce the number of pores in biochars. However, the BET surface areas of biochars range from 2.016 m2/g (HTC biochar) to 16.630 m2/g (pyrolysis biochar). Similar results are found from the other two biochars.19, 25 Some researchers believed that the filling or blocking of pores by ash materials at high contents prevents the access of adsorbate gas, resulting in relatively low production of surface area.31 Nevertheless, biochars obtained from WCOSs with low ash content also had low surface area. The results confirm that the pores inside biochars are extremely small and dead-ended, preventing any access to the N2 gas. As shown in Table 3, the average pore diameter of biochars is extremely low, ranging from 1.655 nm (HTC biochar) to 6.811nm (pyrolysis biochar).


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3.4. XRD Analysis. XRD is applied to research the crystallinity and structure of biochars. As shown in Fig. 3, three types of biochar are different in crystallinity and structure, especially those of pyrolysis biochar are obviously different from those of the other two biochars. The HTC and torrefaction biochars exhibit characteristic peaks at 16-22°,32 while that of the pyrolysis biochar disappear. The results indicate that partial structures of hemicelluloses and cellulose are retained after HTC and torrefaction treatment. Meanwhile the microcrystallinity structure of pyrolysis biochar appears at 30-35°. The band of pyrolysis biochar is observed at 23° and 44°, which correspond to the diffuse graphite (002) and (100) bands, respectively.33 The broad (002) and broad (100) bands are attributed to the stacking of the graphitic basal planes of biochar crystallites, and graphite –like atomic order within a single plane, respectively. In addition, the broad (002) at 23° means that pyrolysis biochar has highly disordered structure. 3.5. SEM Analysis. The physical structure of biochars was shown in Fig. 4. As shown in Fig. 3, three types of biochar are different in physical structures, especially those of the HTC biochar are obviously different from those of the other two biochars and raw WCOSs. The HTC biochar exhibit a loose structure. The reason for this phenomenon is the degradation and polymerization of cellulose and hemicelluloses. Meanwhile, polymerization is not complete because of shortage of experimental time. The torrefaction and pyrolysis biochars display a compact structure with a layered structure. But there are large holes in the pyrolysis biochar because of decomposition of volatile matter.


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3.6. FTIR Analysis. The FTIR spectra of WCOSs and their biochars are shown in Fig. 5. The spectra show that thermo-chemical conditions have an obvious effect on the adsorption bands of WCOSs and their biochars, especially the pyrolysis biochar derived at 600 °C. The O-H (3380 cm-1) stretching vibration of pyrolysis biochar is weaker than that of torrefaction and HTC biochars, mainly because of the strong degree of deoxygenation and dehydration. The O-H stretching vibration of torrefaction and HTC biochars is slightly weaker than that of the WCOSs, which indicate that the effect of pyrolysis on biomass decomposition and carbonization is stronger than that of HTC and torrefaction. The same phenomenon also occurs to the -CH2, and -CH3 (2920 cm-1, 2860cm-1) stretching vibration, and there are almost no adsorption bands in the pyrolysis biochar. The reason is that the demethoxylation, demethylation, and dehydration of lignin during the preparation of pyrolysis biochar cause the reduction in unstable aliphatic compounds .34 Aromatic C=C, and C=O (1625 cm-1) stretching vibration is found in all samples, whereas their intensities are not the same. The intensities are as followed: WCOSs, HTC biochars, torrefaction biochars, and pyrolysisi biochars. The band at 1750 cm-1 is due to the stretching vibration of C=O in FTIR spectra of WCOSs, and disappears after thermo-chemical treatments. The band at 1060 cm-1 is due to the stretching vibration of C-O-C, and the change rule of the intensity was consistent with that of C=O bands (1625 cm-1). 3.7. Combustion Behavior. Fig. 6 shows the TGA and differential thermogravimetric (DTG) curves of WCOSs and their biochars. From curves, it can be seen that the combustion behavior of WCOSs changed significantly after HTC,


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torrefaction, and pyrolysis. Fig. 6 shows typical combustion characteristics of WCOSs and HTC biochars, with two main peaks during the entire combustion process. The first peak occurs between 200 °C and 360 °C, and resulted from the combustion of volatiles for WCOSs and HTC biochars. The second peak occurs between 400 °C and 580 °C, and resulted from the combustion of char. As far as torrefaction and pyrolysis biochars were concerned, there is only a peak which resulted from the combustion of fixed carbon due to its low volatile content. It is well known that about 80-90% of raw biomass is combusted in the form of the volatile matter.37 So raw biomass is ignited at lower temperature and reaches maximum weight loss during the combustion process. As illustrated in Fig. 6, the ignition temperature of WCOSs is lower than that of biochars. In addition, the combustion of WCOSs and HTC biochar exhibit two peaks in the DTG curves because of differences in reactivity between the volatile matter and the resulting char. In contrast to raw WCOSs and HTC biochar, combustion behaviors of torrefaction and pyrolysis biochars show a wider temperature range and higher temperature zone, with only one main DTG peak. The reason is that the volatiles matter, and fixed carbon content are not the same in four samples.38 From the curves, it can be seen that, the burnout temperature of torrefaction and pyrolysis biochars is higher than that of HTC biochars about 50 °C, and 70°C, respectively. This phenomenon is probably associated to the synergetic effects of the biochar and some inorganics. As shown in table 1, the ash of torrefaction and pyrolysis biochars is much larger than raw WCOSs and the HTC biochar. Therefore, combustion performance of the HTC biochar is better than torrefaction and pyrolysis


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biochars. 4. CONCLUSION Biochars derived WCOSs with improved fuel properties were successfully produced from HTC, torrefaction, and pyrolysis technologies. Pyrolysis biochar shows considerably superior physicochemical properties when compared to the raw and the other two biochars. Structures of WCOSs are destroyed effectively after different thermo-chemical treatments and the surface porosity was enhanced. As determined by FTIR and XRD, cellulose and hemicelluloses are decomposed, prior to lignin during the HTC and torrefaction process. The combustion behavior of WCOSs and their biochar is significantly different because of differences in the volatiles matter, and fixed carbon content. ■ AUTHOR INFORMATION Corresponding Authors *(F.F.) Tel./Fax: 0086-871-63864089. E-mail: [email protected] *(Z.Z) Tel./Fax: 0086-871-63864089. E-mail: [email protected] Notes The authors declare no competing financial interest ■ ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (No. 31670599, No.31160147), the 948 project of State Forestry Administration of China (No.2013-4-08), the Major Project of New Energy (No.5 [2015]), Yunnan Province, China, and the Major Project (No.ZD2014012) of Education Department of Yunnan Province, China


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Figure 1 The system of the fixed bed reactor. 1: N2 gas cylinder; 2: cooling zone; 3: digital temperature controller; 4: quartz boat; 5: horizontal tubular resistance furnace; 6: liquid nitrogen condenser; 7: liquid collector; 8: gas bag. Figure 2 Van Krevelen diagram of samples and different kinds of coal.

HTC Torrefaction

Pyrolysis

10

20

30

40

2θ（ °）

Figure 3 XRD spectra of WCOSs and their biochars.


50

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Figure 4 SEM image of WCOSs and their biochars. (a) HTC, (b) torrefaction, (c) pyrolysis, (d) raw WCOSs.

Pyrolysis Torrefaction HTC

Raw

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 5 FTIR spectra of WCOSs and their biochars.


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35 100 30

60

WCOSs HTC Torrefaction

25 20

Pyrolysis

DTG (%·min-1）

80

Mass /%

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Energy & Fuels

15 40

10 20 5 0

0

WCOSs HTC -20

Torrefaction Pyrolysis 200

400

600

-5 800

Temperature (Ⅲ )

Figure 6 TGA and DTG curves of WCOSs and their biochars. Table 1 Proximate, and ultimate analysis of WCOSs and their biochars (as dry basis). Proximate analysis Samples

Ultimate analysis

(%)

(%)

(%)

(%)

(%)

5.45

46.74

0.43

0.17

--

--

60.36

4.88

34.06

0.59

0.11

53.45

70.12

25.51

67.85

3.84

27.47

0.67

0.17

43.05

60.31

27.22

73.07

2.46

23.54

0.69

0.24

31.01

46.35

Ash

HHV

C

H

O

(%)

(%)

(%)

(MJ·kg-1)

(%)

(%)

WCOSs

74.68

21.95

3.37

18.21

47.21

HTC biochar

21.24

74.55

4.21

23.89

12.54

79.99

7.56

8.41

82.17

9.42

pyrolysis biochar

yield

S

FC

biochar

Energy

N

VM

Torrefaction

Mass yield

a

VM: Volatile matter, FC: Fixed carbon, HHV: higher heating value. a: Oxygen content was obtained by difference. Table 2 Hydrophobicity of WCOSs and their biochars. Percentage moisture content Samples RH 42%

RH 75%

WCOSs

6.87

8.12

HTC biochar

2.01

2.45

Torrefaction biochar

2.99

5.34

pyrolysis biochar

5.74

7.85


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Table 3 BET characteristics of WCOSs and their biochars. Samples

Av. Pore diameter(nm)

BET surface area (m2/g)

Total pore volume (cm3/g)

WCOSs

1.045

0.451

0.001

HTC biochar

1.655

2.016

0.005

Torrefaction biochar

1.575

3.216

0.006

pyrolysis biochar

6.851

16.630

0.032


Preparation and Characterization of Biochars from Waste Camellia

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