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Zhang, Y., Ma, Z., Zhang, Q., Wang, J., Ma, Q., Yang, Y., Luo, X., and Zhang, W. (2017). "Comparison of the physicochemical characteristics of bio-char pyrolyzed from moso bamboo and rice husk with different pyrolysis temperatures," BioRes. 12(3), 4652-4669.

Abstract

Bio-char pyrolyzed from biomass waste has been notably applied in various industries due to its versatile physicochemical characteristics. This paper investigated the difference of the properties of the bio-char derived from moso bamboo and rice husk under different pyrolysis temperatures (200 °C to 800 °C). As the temperature increased, the bio-char yield for both bamboo bio-char (BC) and rice husk bio-char (RHC) decreased, while the carbon element content and fixed carbon, the value of higher heating value (HHV) and pH increased for both BC and RHC. At 800 °C, BC had a higher HHV of 32.78 MJ/kg than RHC of 19.22 MJ/kg, while RHC had a higher yield of char (42.99 wt.%) than BC (26.3 wt.%) because of the higher ash content (47.51 wt.%) in RHC. SiO2 was the dominant component in the ash of RHC, accounting for 86.26 wt.%. The surface area (SBET) of RHC (331.23 m2/g) was higher than BC (259.89 m2/g). However, the graphitization degree of BC was higher than RHC at the same temperature. The systematic study on the evolution of the basic properties of BC and RHC will provide a good reference for their high value-added application.

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Comparison of the Physicochemical Characteristics of Bio-char Pyrolyzed from Moso Bamboo and Rice Husk with Different Pyrolysis Temperatures

Yimeng Zhang,a Zhongqing Ma,b,* Qisheng Zhang,a Jiayao Wang,b Qianqiang Ma,b Youyou Yang,c Xiping Luo, c and Weigang Zhang b,*

Bio-char pyrolyzed from biomass waste has been notably applied in various industries due to its versatile physicochemical characteristics. This paper investigated the difference of the properties of the bio-char derived from moso bamboo and rice husk under different pyrolysis temperatures (200 °C to 800 °C). As the temperature increased, the bio-char yield for both bamboo bio-char (BC) and rice husk bio-char (RHC) decreased, while the carbon element content and fixed carbon, the value of higher heating value (HHV) and pH increased for both BC and RHC. At 800 °C, BC had a higher HHV of 32.78 MJ/kg than RHC of 19.22 MJ/kg, while RHC had a higher yield of char (42.99 wt.%) than BC (26.3 wt.%) because of the higher ash content (47.51 wt.%) in RHC. SiOwas the dominant component in the ash of RHC, accounting for 86.26 wt.%. The surface area (SBET) of RHC (331.23 m2/g) was higher than BC (259.89 m2/g). However, the graphitization degree of BC was higher than RHC at the same temperature. The systematic study on the evolution of the basic properties of BC and RHC will provide a good reference for their high value-added application.

Keywords: Moso bamboo; Rice husk; Bio-char; Pyrolysis; Characteristics

Contact information: a: School of Materials Science and Engineering, Nanjing Forestry University, Nanjing, Jiangsu 210037, P. R. China; b: School of Engineering, Key Laboratory of Wood Science & Technology of Zhejiang Province, National Engineering & Technology Research Center of Wood-Based Resources Comprehensive Utilization, Zhejiang Agriculture & Forestry University, Lin’an, Zhejiang 311300, P. R. China; c: Zhejiang Agriculture & Forestry University, Lin’an, Zhejiang 311300, P. R. China; *Corresponding authors: mazqzafu@163.com; zhangwgzafu@163.com

INTRODUCTION

Biomass is a promising, eco-friendly, and renewable source for generating energy, fuels, and chemicals that could partially replace fossil fuels to reduce the pressure of environmental pollution problems (Ma et al. 2012, 2015a). China is rich in biomass resources, mainly from agriculture and forestry residues. Bamboo and rice husk (RH) are the two typical biomass species in China with the largest annual production of 7 million and 40 million tonnes in the world, respectively (Paethanom and Yoshikawa 2012; Chen et al. 2014a). Compared to woody biomass, bamboo and rice husk have the advantage of a short growth period for harvesting, with growth periods of 3 to 5 years and 6 months, respectively. However, during the process of bamboo scrimber manufacturing and rice milling, a large proportion of these types of waste are produced, occupying approximately 30% and 20% of the initial weight, respectively (Alvarez et al. 2014; Chen et al. 2015a). Currently, this waste is simply used as fuel for burning in the boiler or as fodder in the livestock breeding industry. Therefore, identifying how to increase value through additional utilization of this waste biomass is a strategic problem for the development of the bamboo manufacturing and rice milling industry.

Biomass pyrolysis is a thermochemical degradation process operating in an inert or very low stoichiometric oxygen atmosphere that converts solid biomass into three phase products, namely gaseous (combustible gas), liquid (bio-oil), and solid (bio-char) products (Yang et al. 2016; Chen et al. 2017a, 2017b). Compared to fast pyrolysis with a fast heating rate (generally over 300 °C/min) and short residence time (0.5 s to 10 s), the main product of slow pyrolysis with a moderate heating rate of 1 °C/min to 30 °C/min was the solid bio-char instead of the liquid bio-oil (Duman et al. 2011). Lower heating rates allow sufficient time for the molecule repolymerization reaction to occur from the three main components (cellulose, hemicellulose, and lignin), forming a polycyclic carbon structure and maximizing the yield of solid bio-char (Chen et al. 2012).

Bio-char as a recalcitrant carbonaceous material has been notably applied in some areas because of its versatile physicochemical characteristics. These promising potential applications include: energy production for a high heating value (Nanda et al. 2016), acid soil remediation because of its alkaline pH value and being rich in plant nutrients (Qian et al. 2015), carbon sequestration due to the high content of carbon (Rafiq et al. 2016), an excellent intermediate form for activated carbon production (Alvarez et al. 2014), or use as a catalyst precursor and fuel cell material for a strong pore structure and low electrical resistivity (Chen et al. 2015b). However, these applications are highly related to the basic properties of bio-char, which are significantly affected by the types of raw materials and the pyrolysis parameters (heating rate, solid residence time, and temperature, etc.) (Angin 2013a; Zhang et al. 2015). Thus, the study of the influence of the pyrolysis parameters on the basic properties of bio-char has attracted increasing attention.

Among various slow pyrolysis parameters (temperature, heating rate, and residence time), pyrolysis temperature is considered to be the most important parameter that significantly controls the yield and quality of the solid bio-char, including the chemical compositions, higher heating value (HHV), pH value, and pore structure (Kan et al. 2016; Tripathi et al. 2016). Using other biomass (not bamboo and RH), numerous tests have been performed on the effect of temperature on the properties of solid bio-char (Uçar and Karagöz 2009; Fu et al. 2011; Angin 2013a; Lee et al. 2013; Cimo et al. 2014; Luo et al. 2015; Zhang et al. 2015; Rafiq et al. 2016; Chen et al. 2016c). The basic evolution was that higher pyrolysis temperature would lead to lower bio-char yield and surface function groups content, higher HHV and pH value, and stronger pore structure. However, for the biomass of bamboo (Krzesińska and Zachariasz 2007; Kantarelis et al. 2010; Muhammad et al. 2012; Oyedun et al. 2013; Ren et al. 2013) and RH (Chen et al. 2014b; Yang et al. 2015; Zhang et al. 2016), most of the research was focused on the identification of pyrolysis behaviors and kinetics, and the components of volatiles (bio-oil and noncondensable gas) using thermogravimetric analysis-Fourier transform infrared spectrometry (TG-FTIR) and pyrolyzer-gas chromatography/mass spectrometer (Py-GC/MS). Furthermore, the amount of bio-char collected from this kind of analytical instrument was only at a milligram (mg) level, which was too small for further characterization of the other properties of bio-char. Therefore, a lab-scale tube furnace with a higher mass of bio-char at the gram (g) level was been employed in experiments for bio-char production. Using a tube furnace, Chen et al. (2014a) investigated the effect of bamboo pyrolysis temperature (300 °C to 700 °C) on the carbon and energy distribution of the products. The variation of surface function groups and yield of RH bio-char at the temperature range of 600 °C to 900 °C was performed by Fu et al. (2011). Liu et al. (2011) also reported RH bio-char as presenting high adsorption capacity on the phenols. Alvarez et al. (2014) claimed that high quality activated carbon and amorphous silica could be produced using RH bio-char material. However, concerning the promising application of bio-char from bamboo and RH, some other important performance factors (chemical composition, functional group, pore structure, and crystallographic structure) under a wider range of pyrolysis temperatures still have been overlooked.

A systematic investigation was carried out to determine the effect of the pyrolysis temperature (200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C) on the yield and properties of bamboo bio-char (BC) and rice husk bio-char (RHC) was studied. Then, the comparison of the properties of BC and RHC was performed using Fourier transform infrared spectrometry (FTIR), thermogravimetric analysis (TGA), an X-ray diffractometer (XRD), gas sorption analyzer, etc., to discuss their corresponding high value-added application in various industries.

EXPERIMENTAL

Materials

Five-year-old moso bamboo (Phyllostachys edulis) was harvested from the mountain in the Lin’an city of the Zhejiang Province in China. Then the outer and inner skin of bamboo was removed by a sander, and the middle part was kept for bio-char production. Another biomass, rice husk, was collected from a rice milling plant in the Jurong city of the Jiangsu Province.

The two types of biomass were first ground into powder, then screened using 160- and 200-mesh sieves. Finally, the powder with the particle size between 75 µm and 96 µm was kept and dried at 105 °C for 12 h for the pyrolysis experiment. The preparation process of bamboo and RH power is shown in Fig. 1.

Table 1. Contents of Cellulose, Hemicellulose, Lignin, and Extractives in the Dried Bamboo and Rice Husk

After drying, the contents of cellulose, hemicellulose, lignin, and extractives in the bamboo and RH were tested. The content of extractives was determined using benzene-ethanol extraction according to the standard of GB/T 2677.6-94. The content of acid-insoluble lignin was determined by the standard of GB/T 2677.8-94. The content of cellulose was determined by nitric acid-alcohol method. The content of holocellulose was determined by the sodium chlorite method according to the standard of GB/T 2677.10-95. Then the content of hemicellulose was determined by Eq. 1.

Hemicellulose (wt%) = Holocellulose (wt%) – Cellulose (wt%) (1)

The results are shown in Table 1.

Fig. 1. Schematic diagram of biomass pyrolysis experiment

Pyrolysis experiment

An electrical heating tube furnace (Nanjing BYT Company, Nanjing, China, model number TL1200) was used to perform the biomass pyrolysis experiment, and its schematic diagram is shown in Fig. 1. As shown, the inner diameter and the length of the quartz tube were 60-mm and 1000-mm, respectively. In each experiment two ceramic boats (containing about 5 g of biomass powder in each boat) were put in the center of the quartz tube. Then the vacuum pump was used to extract all the air in the quartz tube. Next, the carrier (high-purity N2) was injected into the quartz tube until it reached atmospheric pressure. Finally, the furnace was heated to the temperatures of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C at a fixed heating rate (10 °C/min) and a fixed flow rate (300 mL/min) of carrier gas. The terminal temperature was kept for 1 h. Once the temperature was cooled to room temperature using high-purity N2, the bio-char was taken out for calculation of the yield and further analysis of its physicochemical characteristics. The average yield of bio-char was obtained from the data from at least three experiments.

Methods

Characteristics of bio-char

The ultimate analysis was conducted using an elemental analyzer (Vario EL III, Elementar, Langenselbold, Germany) following the CHNS/O model. Other mineral element content in the ash was tested with X-ray fluorescence (Axiosm AX-Petro, Rotterdam, Netherlands). The proximate analysis of bio-char was tested according to ASTM D1762-84 (2013). The heating value of bio-char was tested using an automatic calorimeter (ZDHW-300A, Hebi Keda Instrument & Meters Co., LTD, China). The pH value was tested according to the Chinese national standard GB/T 12496.7 (1999). In each test, 2.5 g bio-char was mixed with 50 mL of deionized water to form the suspension. Then the suspension was heated to boiling, which was maintained for 5 min. Next, the suspension was filtered to remove the bio-char powder. Finally, the pH value of the filtrate was tested using a pH meter (pHS-25, Shanghai Leici Company, Shanghai, China) until it was cooled to the room temperature.

The surface chemical functional groups of the bio-char were tested using Fourier transform infrared spectrometry (Nicolet 6700, Thermo Fisher Scientific, Waltham, Massachusetts, USA). The thermal stability of the bio-char was analyzed using a thermogravimetric analyzer (TG209F1, Netzsch Instruments, Bavaria, Germany), and the terminal temperature was set as 1000 °C at a heating rate of 10 °C /min and a flow rate of high-purity nitrogen of 100 ml/min. The surface area and pore size distribution were obtained by using a Gas Sorption Analyzer (ASAP2020, Quantachrome Instruments Co., Ltd., Boynton Beach, Florida, USA). The specific surface area (SBET), micro pore volume (Vmic), and mesopore volume (Vmes) were determined by way of the Brunauer-Emmett-Teller (BET) method, the t-plot method, and the Barrett-Joyner-Halenda (BJH) method, respectively. The crystallographic structure was tested using an X-ray diffractometer (XRD 6000, Shimadzu, Kyoto, Japan).

RESULTS AND DISCUSSION

Biochar Yields, Proximate and Ultimate Analysis, pH Value

Table 2 shows the effect of pyrolysis temperature on the yield, proximate analysis, HHV, and pH of BC and RHC. The yield decreased as the temperature increased for both BC and RHC because more solid substance was converted into lower molecular components, such as gaseous bio-gas and liquid bio-oil (Zhang et al. 2015). It is worth noting that compared to temperatures over 400 °C, larger weight loss (66.93 wt.% for BC and 47.65 wt.% for RHC) was observed before 400 °C. This result might be explained by the thermal degradation behaviors of three main components (cellulose, hemicellulose, and lignin). The contents of three main components in bamboo and RH are shown in Table 1.

According to the thermogravimetric analysis (TGA) of the three-component biomass, shown in Fig. 2, the main degradation temperatures were less than 400 °C along with 92 wt.%, 62 wt.%, and 35 wt.% weight loss for cellulose, hemicellulose, and lignin, respectively. However, the yield of RHC was much higher than BC at the same temperature. This was attributed to the higher ash content and lower volatile content in RHC, based on the result of the proximate analysis.

The HHV of BC and RHC increased from 19.47 MJ/kg and 15.54 MJ/kg to 32.78 MJ/kg and 19.22 MJ/kg, respectively, as the temperature was raised from 200 °C to 800 °C. This might be attributed to an increase of the fixed carbon content in the bio-char. However, the HHV of BC was much higher than RHC at the same temperature. More importantly, the HHV of the samples of BC-400, -500, -600, -700, and -800 were all higher than the standard coal (29.31 MJ/kg) (Majumder et al. 2008), which made the BC become a high-quality solid fuel that could be substituted for coal used in a boiler or used for barbecue char.

Table 2. Effect of Pyrolysis Temperature on the Proximate Analysis, HHV, and pH Value of BC and RHC

Fig. 2. Thermogravimetric analysis of cellulose, hemicellulose, and lignin at the heating rate of 10 °C/min

Table 3 shows the content of elements in the ash for BC-800 and RHC-800. Comparison of the pH value of bio-chars can be investigated after determination of the content of the alkali and alkali earth metals in the ash. The pH value of BC and RHC rose from 5.46 and 6.14 to 10.18 and 10.08, respectively, as the temperature was raised from 200 °C to 800 °C. This might be attributed to the increasing content of ash, which is rich in alkali and alkali earth metals (e.g. K, Ca, Na, and Mg), as shown in Table 2. Therefore, due to the strong alkalinity of BC and RHC, it could be used as a soil amendment to neutralize acidic soil in agriculture.

As shown in Table 3, the content of SiO2 in the RHC-800 reached 86.26 wt.% which was much higher than BC-800. Based on the SEM images of RHC, Ma et al. (2015b) reported that the SiO2 mainly presented on the outside and internal surface of the RHC at an amorphous structure. Alvarez et al. (2014) claimed that RHC from pyrolysis was a potential material from which to extract high purity amorphous silica that could be used in the zeolite catalyst, silica gel, or glass.

Table 3. Mass Percentage of Elements in the Ash of BC-800 and RHC-800

n.d.a: not detected

Table 4. Effect of Pyrolysis Temperature on the Ultimate Analysis of BC and RHC

The effect of the pyrolysis temperature on the elemental content (C, H, O, and N) is shown in Table 4. The carbon and nitrogen content of the bio-chars increased when the temperature increased, while the hydrogen and oxygen content decreased. Meanwhile the atomic ratio of H/C and O/C gradually decreased. This result indicated that more aromatic and carbonaceous components formed in the BC and RHC (Chen et al. 2012).

However, the content of carbon element in the BC was much higher than that of RHC at the same temperature. For the sample of BC-800, the content of carbon would reach nearly 90 wt.%, which was twice the content of carbon in the sample of RHC-800 (43.95 wt.%). The BC had lower ash content than RHC. This was also valid evidence to indicate that BC had a higher HHV than RHC. Also, the results indicated that BC had the stronger capacity for carbon sequestration.

FTIR Analysis

Figure 3 shows the effect of the pyrolysis temperature on the chemical functional groups of the BC and RHC. The infrared spectra showed 5 principal bands. The band at the wavenumbers of 3600 cm-1 to 3200 cm-1 was the most remarkable, being attributed to the stretching vibration of O-H (Ma et al. 2015a). The absorbance peak located between 3000 cm-1 and 2700 cm-1was related to the stretching vibration of C-H, which mainly came from aliphatic –CH2 and alkanes –CH(Ma et al. 2016). The stretch band at 1705 cm-1 was presented as the stretching vibration of C=O from the carboxyl and carbonyl groups (Chen et al. 2015a). The absorbance peak between 1690 cm-1 to 1450 cm-1 was the stretching vibration of C=C benzene ring skeleton mainly from aromatics (Chen et al. 2014b). The absorbance peak between 1200 cm-1 to 1000 cm-1 was caused by the stretching vibration of C-O from phenols (Chen et al. 2012).

As shown in Fig. 3, the intensity of absorbance from all functional groups in BC and RHC gradually decreased as the temperature was increased from 200 °C to 800 °C. At the lower pyrolysis temperatures (200 °C and 300 °C), these kinds of functional groups were still clearly observable in the BC and RHC. However, as the temperature was increased over 400 °C, some characteristic functional groups, such as hydroxyl, carboxyl, carbonyl, and methoxyl, gradually fell off under thermal cracking and were transformed into gaseous components (CO, CO2, H2, and CH4,) and liquid components (acids, aldehydes, and phenols) (Ren et al. 2013). Finally, when the temperature reached 700 °C and 800 °C, the FTIR curves were almost flat. This finding was also supported by other researchers using pinewood (Luo et al. 2015), white ash (Chen et al. 2016d), and walnut (Zhao et al. 2016).

 

Fig. 3. The effect of pyrolysis temperature on the chemical functional groups

In Fig. 3 (b), due to the high content of SiO2 in the RHC, the asymmetric stretching vibration of Si-O-Si was observed at the wavenumbers between 1200 cm-1 to 1050 cm-1, and the characteristic absorbance peaks were at 1086 cm-1 and 1168 cm-1. The symmetric stretching vibration of Si-O-Si was between 820 cm-1 and 760 cm-1, and the characteristic absorbance peaks were at 795 cm-1 and 515 cm-1. These bands for Si-O-Si stretching vibration in RHC were stronger than BC, indicating that the content of SiO2 in the RHC was higher than BC.

TG Analysis

The thermal stability of the BC and RHC was tested using the thermogravimetric analyzer. Figure 4 shows the TG and DTG curves of BC (B-control, BC-200, -400, -600, and -800) and RHC (RH-control, RHC-200, -400, -600, and -800) at the heating rate of 20 °C/min with the 100 mL/min of high-purity nitrogen. The characteristic points of the TG and DTG curves are shown in Table 5. For both BC and RHC, the mass fraction of solid residues increased as the pyrolysis temperature increased, but RHC had a higher content of solid residues at the same temperature because of the higher ash content in RHC based on the proximate analysis in Table 2.

 

 

Fig. 4. Thermogravimetric analysis of BC (a1 and b1) and RHC (a2 and b2) from different pyrolysis temperatures: TG (a) and DTG (b) curves

The weight loss rate at the peak temperature decreased from 0.82 wt.%/°C to 0.011 wt.%/°C for BC and 0.57 wt.%/°C to 0.013 wt.%/°C for RHC. This result was caused by a decrease in the contents of degradable components in the bio-char, such as cellulose, hemicellulose, and lignin. During carbonization process of biomass, large amounts of functional groups fell off, and more components were polymerized into polyaromatic components with large molecular weight that were hard to degrade. However, BC had a higher weight loss rate than RHC because of the higher volatile content in BC. In addition, the peak temperature and the main weight loss region shifted toward the side of the higher temperature.

Overall, the thermal stability was considerably improved as the pyrolysis temperature increased, presenting a higher content of solid residues, a lower weight loss rate, and a higher peak temperature. This conclusion was also confirmed by Chen et al. (2016d).

Table 5. The Characteristic Point of TG/DTG Curves from BC and RHC

XRD Analysis

Figure 5 shows results of the XRD analysis of the BC and RHC from different pyrolysis temperatures. Three diffraction peaks are clearly shown in Fig. 5. At lower temperatures (control and 200 °C) for both BC and RHC, two sharp peaks at the 2θ of 16° and 22° were clearly observed, which represented the typical crystalline structure of the cellulose Ια (triclinic) and cellulose Ιβ (monoclinic), respectively (Wada et al. 2010). However, as the temperature continuously increased over 300 °C, the two sharp peaks gradually turned into one dispersion peak. This was caused by the thermal degradation of cellulose that was converted to amorphous carbon and aliphatic side chains.

 

Fig. 5. XRD analysis of BC (a) and RHC (b) from different pyrolysis temperatures

The two other peaks observed at the 2θ values of 26.6° (from RHC) and 44° (from BC) were the graphite (002) and (100) bands, respectively (Fu et al. 2011). Guerrero et al. (2008) reported that the (002) peak was attributable to the stacking of the graphitic basal planes of bio-char, while the (100) peak was related to the graphite-like atomic order within a single plane. As shown in Fig. 5 (b) for RHC, the intensity of the (002) peak was very weak. However the intensity of the (100) peak for BC was very strong, and it gradually increased as the pyrolysis temperature increased. This indicated that BC had a higher degree of graphitization than RHC. In addition, higher temperature leads to the formation of a graphite microcrystalline structure for BC (Yang et al. 2016). Then, the high graphitization degree resulted in high electrical conductivity in the bio-char. This also indicated that the BC was a more suitable electrode material for use in fuel cells than RHC.

Pore Structure Analysis

The specific surface area (SBET), pore volume (V), and pore size distribution were the key parameters to evaluate the pore structure of biochar. Figure 6 shows the nitrogen adsorption-desorption isotherms of BC and RHC from different pyrolysis temperatures. According to the IUPAC classification, the types of adsorption isotherms can be divided into five classifications: type I(Henry), type II (Langmuir), type III (Freundlich), type IV (Brunauer-Emmett-Teller), and type V ( stepped) . Then the type of adsorption isotherms of BC and RHC might be a mixture of types I and IV. Type I was attributed to the micropore structure, while the type IV was mainly caused by the mixture of micropore and mesopore structures. However, the adsorption-desorption isotherms of BC and RHC were not closed curves at the starting point of left side, which was significantly differentiated with the isotherms from activated carbon material (Angin et al. 2013b). This might be caused by the tar components remaining in the pore of bio-char and could not be completely desorbed by liquid nitrogen (Ma et al. 2015b).

The evolution of the pore structure of BC and RHC at various temperatures is shown in Table 6, and can be divided into three stages. For the first stage at the lower pyrolysis temperatures (200 °C to 500 °C), the value of SBET for both BC and RHC had a slight increase compared to the control sample (bamboo and RH). In this stage, the maximum value of SBET for BC and RHC was 57.09 and 39.05 m2/g, respectively. The total pore volume consisted mainly of the mesopore (2 nm to 50 nm) volume, and the micropore (0 nm to 2 nm) could not be detected. In this temperature range, many volatiles were released from the thermal degradation of the cellulose and hemicellulose. However, the pore structure did not change remarkably. In the second stage at the middle pyrolysis temperature (600 °C), during the continuous degradation of lignin, substantial variation related to the pore structure occurred. Additionally, more “2D structure(s) of fused rings” formed (Yang et al. 2016). The SBET for BC and RHC sharply increased to 127.41 m2/g and 61.26 m2/g, respectively. It was noteworthy that the micropore volume was detected, and the average pore size gradually decreased. This indicated that the micropore structure gradually replaced the mesopore structure as the temperature increased.

In the last stage, at high pyrolysis temperatures (700 °C to 800 °C), the “2D structure of fused rings” was transferred into the graphite microcrystalline structure. At this stage, the SBET for BC and RHC reached their maximum values of 259.89 m2/g and 331.23 m2/g, respectively; the rate of micropore volume (VMic/VTot) also reached its maximum values. However, compared with other bio-char, the maximum SBET of BC and RHC was slightly lower than pine nut shell bio-char (433.1 m2/g) (Chen et al. 2016a) and poplar wood bio-char (411.06 m2/g) (Chen et al. 2016b).

Table 6. Pore Structure Analysis of BC and RHC from Different Pyrolysis Temperatures

n.d.* : not detected

The differences in BET results might be caused by the different biomass materials and operating conditions (temperature, flow rate of carrier gas, residence time, etc.) of pyrolysis.

The increase of SBET and micropore volume in the BC and RHC was mainly attributed to two pathways: (1) the release of the volatiles from the biomass’ three components, especially from the degradation of benzene-ring linked compounds in the lignin; (2) The gap or crack from the formation process of the fused ring and the graphite microcrystals in the BC and RHC. Concerning the strong pore structure of BC and RHC, it could be a good adsorbent for use in sewage treatment.

 

Fig. 6. Nitrogen (N2) adsorption and desorption isotherms of BC and RHC from different pyrolysis temperatures

CONCLUSIONS

  1. The physicochemical characteristics of the bio-char pyrolyzed from moso bamboo and rice husk under different pyrolysis temperatures were investigated and compared. As the temperature was increased, the bio-char yield for both BC and RHC decreased, while the carbon element content and fixed carbon, the value of HHV, and pH increased for both BC and RHC.
  2. At the same pyrolysis temperature, the BC had a higher content of the carbon element and fixed carbon, and a higher value of HHV than RHC, which indicated that BC was a more suitable material for high-quality fuels and carbon sequestration carriers. However, the RHC had a higher ash content, containing 86.26 wt.% of SiO2, which is a material with potential for use in extracting high purity amorphous silica. The pH value of the bio-chars was higher than 10 for BC-800 and RHC-800.
  1. The graphitization degree of BC was higher than that of RHC. The SBET for BC-800 and RHC-800 reached their maximum values of 259.89 m2/g and 331.23 m2/g, respectively, which indicated that BC and RHC could be an excellent interim form for activated carbon production, or use as a catalyst precursor and fuel cell material.

ACKNOWLEDGMENTS

This research was supported by the Natural Science Foundation of Zhejiang Province (LQ17E060002), the Natural 948 Project (2014-4-29), Natural Science Foundation of China (L1422039), the Fund for Innovative Research Team of Forestry Engineering Discipline (101-206001000706), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors also acknowledge the Advanced Analysis & Testing Center of Nanjing Forestry University for its testing service.

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Article submitted: January 30, 2017; Peer review completed: April 13, 2017; Revised version received and accepted: May 1, 2017; Published: May 9, 2017.

DOI: 10.15376/biores.12.3.4652-4669