Abstract
Five-year-old moso bamboo was torrefied under nitrogen and different oxygen concentrations of 3% to 9% and torrefaction temperatures of 200 °C to 300 °C. Mass yields of 31.7% to 96.6%, energy yields of 30.8% to 98.9%, and higher heating values (HHVs) in the range 18.8 to 27.1 MJ/kg were obtained. The torrefied sample was characterized by Fourier transform infrared spectrometry (FTIR). Under the different torrefaction temperatures and oxygen concentrations, hemicellulose and cellulose were thermally decomposed, which led to significant changes in the chemical functional groups of the raw and torrefied bamboo. The pyrolysis experiments on raw and torrefied bamboo were conducted using the pyrolyzer coupled with a gas chromatography/mass spectrometer (Py-GC/MS). According to the Py-GC/MS analysis, the pyrolytic bio-oil were mainly composed of acids, furans, phenols, ketones, aldehydes, esters, alcohols, and hydrocarbons. Higher torrefaction temperature reduced the relative contents of acids, ketones, furans, and aldehydes. However, lower torrefaction temperatures and moderate oxygen concentrations were optimal for the production of phenols and hydrocarbons.
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Effect of Torrefaction Temperature and O2 Concentration on the Pyrolysis Behaviour of Moso Bamboo
Yi Sun,a,b Yuquan Sun,b Wei Chen,b Shanshan Wang,a Guangyuan Liang,a Wenzhu Li,a Zhongqing Ma,a,* and Wenbiao Zhang a,*
Five-year-old moso bamboo was torrefied under nitrogen and different oxygen concentrations of 3% to 9% and torrefaction temperatures of 200 °C to 300 °C. Mass yields of 31.7% to 96.6%, energy yields of 30.8% to 98.9%, and higher heating values (HHVs) in the range 18.8 to 27.1 MJ/kg were obtained. The torrefied sample was characterized by Fourier transform infrared spectrometry (FTIR). Under the different torrefaction temperatures and oxygen concentrations, hemicellulose and cellulose were thermally decomposed, which led to significant changes in the chemical functional groups of the raw and torrefied bamboo. The pyrolysis experiments on raw and torrefied bamboo were conducted using the pyrolyzer coupled with a gas chromatography/mass spectrometer (Py-GC/MS). According to the Py-GC/MS analysis, the pyrolytic bio-oil were mainly composed of acids, furans, phenols, ketones, aldehydes, esters, alcohols, and hydrocarbons. Higher torrefaction temperature reduced the relative contents of acids, ketones, furans, and aldehydes. However, lower torrefaction temperatures and moderate oxygen concentrations were optimal for the production of phenols and hydrocarbons.
Keywords: Biomass; Oxygen torrefaction; Oxygen concentration; Fast pyrolysis
Contact information: a: School of Engineering, Zhejiang Provincial Collaborative Innovation Center for Bamboo Resources and High-Efficiency Utilization, Zhejiang A & F University, Hangzhou, Zhejiang 311300, China; b: Shangdong Institute for Product Quality Inspection, Jinan, Shandong 250102, China;
* Corresponding authors: zwb@zafu.edu.cn
INTRODUCTION
As traditional fossil energy reserves are gradually decreasing and global population continues to rise, biomass has been an important environmentally friendly renewable energy resource used in construction materials, furniture, and the energy field (Tomak et al. 2014; Sun et al. 2018). The lignocellulosic biomass of bamboo is a potential feedstock for producing chemicals and fuels by thermochemical and biochemical conversion technology due to its sustainability, high abundance, and carbon-neutral character (Saidur et al. 2011; Saidi et al. 2014; Ma et al. 2019a). However, the use of the lignocellulosic biomass of bamboo for the production of fuels by thermochemical conversion technology has several downsides, which include low energy density, high moisture and oxygen contents, and poor grindability (Lu et al. 2013; Tran et al. 2013; Xu et al. 2015). The thermochemical conversion technologies (torrefaction, pyrolysis, combustion, and gasification) have received attention because they can improve biomass quality by enhancing the chemical structure and physical properties.
Torrefaction was considered as a biomass feedstock pretreatment technology at temperatures between 200 °C and 300 °C under inert atmosphere developed for the lignocellulosic biomass (mainly composed of hemicellulose, cellulose, and lignin) to improve its quality for efficient energy conversion and optimize the lignocellulosic structure of biomass (Chen et al. 2016b). The thermal decomposition of lignocellulose involves a series of chemical reactions and transforming mechanisms (Turner et al. 2010; Rousset et al. 2012). Uemura et al. (2017) and Ma et al. (2019b) reported that the three components (hemicellulose, cellulose, and lignin) of lignocellulose are decomposed to form oxygen-containing products (CO2, CO, H2O, acids, and aldehydes) through dehydroxylation, decarbonylation, and decarboxylation reactions during the torrefaction process (Chen et al. 2015). Through these mechanisms, the lignocellulosic biomass transforms raw biomass into a material with higher caloric value, low oxygen content, hydrophobicity, and better grindability. However, the properties of torrefaction products are influenced by several factors, such as biomass composition, temperature, duration, heating rate, and atmosphere (Rodrigues and Rousset 2015).
Much work has been done to study how atmosphere influences torrefaction (Li et al. 2015b; Strandberg et al. 2015; Xu et al. 2017). Recently, several types of atmosphere have been studied to evaluate thermal degradation behaviour, and the most commonly reported method is the use of N2 for oxygen removal in biomass. Li et al. (2015b) found that the bamboo torrefied under carbon dioxide atmosphere was subjected to substantial changes in structure and decomposition of carbohydrates, and the carbon content of biomass was enhanced considerably. Ma et al. (2019b) and Chen et al. (2018b) investigated the influence of the ammonia torrefaction and fast pyrolysis processes on the production of N-containing chemicals. They concluded that the content of nitrogen increased from 0.03% to 7.59% in torrefied bamboo, the concentration of N-containing chemicals in bio-oil dramatically increased, and the highest content of N-containing chemicals was 27.5% in NH3 atmosphere when the pyrolysis temperature was 850 °C. NH3 caused the decomposition reactions and generated H radicals, NH2* radicals, and NH* radicals. In addition, NH3 reacted with –C=O and –OH to inhibit the generation of O-containing compounds, such as ketones, aldehydes, and acids, etc.
As a cheap and efficient gas in the atmosphere, oxygen has been widely used for fuel, in the energy field, and by thermochemical conversion technology to reduce operating costs and energy consumption. However, past research has mainly focused on the qualitative analysis of the products at different conditions and did not focus on the torrefaction and pyrolysis product quantitative and semi-quantitative analyses (Rousset et al. 2012; Conag et al. 2018). Thus, it is important to explore the effect of oxygen concentrations on the characteristics of torrefied products and perform quantitative analysis.
In this study, different oxygen concentrations in the process of bamboo torrefaction pretreatment were investigated to examine the thermal reactivity and quantitative analysis of torrefied bamboo products relative to the mass and energy yield of the torrefaction process. Moreover, the oxygen torrefied biomass was pyrolyzed in Py-GC/MS to produce high value-added chemicals and investigate the formation mechanism of O-containing compounds in bio-oil. These obtained results provided a useful and novel method to upgrade biomass via torrefaction and pyrolysis.
EXPERIMENTAL
Materials
Five-year-old moso bamboo (Phyllostachys edulis) was collected from a bamboo processing factory in Zhejiang Province (Jiande Muke Ecological Agricukture Co., Ltd., Jiande, China). The moso bamboo was milled into powder using a grinder (Wuyi Haina Instrument Technology Co., Ltd., Beijing, China). Bamboo powder with particle sizes between 75 µm and 150 µm was used for the experiments. The bamboo powder was dried at 80 °C for 6 h to remove water before the torrefaction experiment.
Oxygen Torrefaction Experiment
The torrefaction experiments with different oxygen concentrations were performed in the temperature-controlled tube furnace shown (Bo Yuntong Instrument Technology Co., Ltd., Nanjing, China) in Fig. 1. Approximately 5 g of bamboo powder was placed in a quartz crucible with fixed torrefaction temperatures of 200 °C, 250 °C, and 300 °C and a fixed heating rate of 10 °C ·min-1. The samples were held for 30 min. The mixed gas was used as carrier gas with a fixed flow rate of 200 mL·min-1. The gas mass flow controller was used to control the input of volume flow of nitrogen and oxygen to achieve different oxygen concentrations atmosphere, and the volume flow of nitrogen and oxygen was shown in the control panel, respectively. The torrefied solid product was further cooled in a desiccator for mass and other analysis. The torrefied liquid products were collected in the condenser using a cold trap and -20 °C ethanol, and the gaseous products were collected in a gas collection bag. The bamboo torrefied in nitrogen and 3%, 6%, and 9% oxygen concentration atmospheres at different temperatures were labeled as BTN-200, BTN-250, BTN-300, BTO-300-3, BTO-300-6, and BTO-300-9, respectively. To reduce experimental error, the experiment was repeated three times under the same conditions. The mass and energy yield of the torrefied solid product were calculated using Eqs. 1 and 2:
After the torrefaction experiments were accomplished, the liquid and gaseous products were collected from the condensate tube and gas collection bag, respectively. The mass yield of solid and liquid products was determined by weighing, and the mass yield of gaseous product was calculated according to Eq. 3:
Gaseous yield = 100% – Yield of solid (%) – Yield of liquid (%) (3)
Pyrolysis Experiment
The tube furnace (TL1200; Nanjing BYT Instrument Technology Co., Ltd., Nanjing, China) was used for torrefied bamboo pyrolysis, and its schematic diagram is shown in Fig. 1. The temperature-controlled tube furnace were composed of a mass flow controller, thermo-controller, tubular furnace, and condensation system. The mass flow controller was used to control the input of volume flow of nitrogen and oxygen to achieve different oxygen concentrations atmosphere, and the volume flow of nitrogen and oxygen was shown in the control panel, respectively. The thermo-controller was used to control fixed temperature and the temperature program settings. The tubular furnace was mainly used for torrefaction and pyrolysis experiments, and the condensation system was used for collecting the condensable gas. In each test, approximately 5 g of the raw or torrefied bamboo was placed in a ceramic boat, and then the ceramic boats were placed in the top of a quartz tube. First, the vacuum pump was used to exhaust the air in the quartz tube. Next, the high-purity N2 or mixed gas (high-purity N2 and high-purity O2) was injected into the quartz tube until the pressure reached atmospheric pressure. Finally, the furnace was heated from room temperature to target temperature (850 °C) at a heating rate of 10 °C·min-1 and a fixed flow rate (200 mL·min-1), and the ceramic boat was rapidly moved from the top to the center of tube furnace. The terminal temperature was maintained for 30 min. Once the temperature fell to room temperature, the solid product was taken out to calculate the yield and further analyze their physicochemical characteristics.
Fig. 1. The temperature-controlled tube furnace setup
Analysis Methods
The ultimate analysis of raw and torrefied bamboo was measured via an elemental analyzer (Vario EL III; Elementar Analysensysteme GmbH, Hanau, Germany) according to the CHNS model (Ma et al. 2019c). The percentage of oxygen present was determined by the balance. The higher heating value (HHV) was calculated according to Eq. 4 based on the elemental content data (Yin 2011; Dava et al. 2017). The symbols C, H, and O in Eq. 4 represent the weight percentages of carbon, hydrogen, and oxygen, respectively, in the solid.
HHV (MJ/kg) = 32.7934 + 0.0053C2 – 0.5321C – 2.8769H + 0.0608CH – 0.2401N (4)
A Fourier transform infrared spectrometer (Nicolet 6700; Thermo Fisher Scientific, Waltham, MA, USA) was used to characterize the chemical functional groups in moso bamboo during torrefaction. The mass ratio of torrefied bamboo to KBr was 100, and it was ground to pass through a 200-mesh screen. The FTIR spectra were recorded in the wavenumber range 4000 cm-1 and 500 cm-1 with a resolution of 4 cm-1 and a spectrum scan time of 8 s.
PY-GCMS was used for qualitative analysis of the liquid organic compounds from the pyrolysis process of the torrefied bamboo. The organic compounds released from the pyrolysis process were analyzed using a pyrolyzer (CDS 5200; Boside Technology Co., Ltd., Beijing, China) and a gas chromatograph/mass spectrometer (5977B–7890B; Agilent Technologies, Santa Clara, CA, USA). In each experiment, approximately 0.2 mg of raw or torrefied bamboo was placed into the quartz filler tube, and then the temperature of the pyrolyzer was increased to the target pyrolysis temperature (850 °C) at a heating rate of 20 °C·ms-1 and a fixed residence time of 20 s. The injector and transfer line temperature were maintained at 300 °C. The HP-5MS capillary column (30 m × 0.25 mm inner diameter × 0.25 μm film thickness) (Agilent Technologies, Santa Clara, CA, USA) was used for chromatographic separation. The GC oven temperature was increased to 40 °C for 3 min, raised to 290 °C at a heating rate of 10°C·min-1, and maintained for 3 min. The carrier gas was helium (99.999%) with a fixed flow rate of 3 mL·min-1. Furthermore, the mass spectra were obtained in the mass-to-charge ratio range of 50 to 400. Based on the NIST library and the relevant literature, each chromatographic peak was identified using Mass Hunter Workstation Software (Agilent Technologies, Santa Clara, CA, USA). The relative content of each compound was determined by the corresponding peak area. Details of the experimental information were reported by Ma et al. (2019a,c).
RESULTS AND DISCUSSION
Product Distribution
Figure 2 shows the product distribution of bamboo during torrefaction under a nitrogen and 9% oxygen atmosphere. The detailed of the mass yield of solid, liquid, and gas were as shown in Table 1. When the torrefaction temperature increased from 200 to 300 °C in nitrogen, the mass yield of solid gradually decreased from 96.6% to 47.2%, whereas the mass yield of liquid product remarkably increased from 1.39% to 20.6%. The mass yield of gas gradually increased from 1.97% to 32.2%.
Table 1. Details of the Mass Yield of Solid, Liquid, and Gas in Nitrogen and Different Oxygen Concentration Atmosphere during Torrefaction
Fig. 2. The mass yield of solid, liquid, and gas products from bamboo torrefaction at different temperature under a nitrogen and 9% oxygen concentration atmosphere
For the oxygen torrefied bamboo, the mass yields of solid, liquid, and gas showed similar trends to those in nitrogen. However, when the torrefaction temperature was increased from 200 °C to 300 °C under a 9% oxygen atmosphere, the mass yield of solid product decreased remarkably at each corresponding torrefaction temperature, and the mass yield of solids decreased from 95.3% to 31.7%. The mass yields of liquid and gas increased substantially, and the final yields at 300 °C were 32.3% and 36.0%, respectively. In the different oxygen concentration ranges of 3% to 9%, the solid yield gradually decreased as oxygen concentration increased, and the mass yields of liquid and gas showed opposite tendencies. This was mainly because many volatiles were previously released during the torrefaction process, and thermal cracking and oxidative reactions occurred under oxygen atmosphere (Bach et al. 2017; Chen et al. 2018a).
Ultimate Analysis, Mass, and Energy Yield
The data of the elemental, mass yield, energy yield, and calorific analysis of the raw and torrefied bamboo are shown in Table 2.
Table 2. Effect of Torrefaction Temperature and Oxygen Concentrations on Elemental Analysis, Mass Yield, and HHV of Raw and Torrefied Bamboo
As the torrefaction temperature increased from 200 °C to 300 °C, the mass yield gradually decreased from 96.6% and 96.6% to 47.2% and 31.7% in nitrogen and oxygen, respectively. The result showed that oxygen effectively influenced the solid mass yield. The energy yield of torrefied solid product gradually decreased from 98.9% to 30.8%. The mass yield and energy yield decreased as temperature and oxygen concentration increased, and previous reports observed a similar trend (Uemura et al. 2013; Chen et al. 2014). The HHV of the original bamboo was 18.8 MJ/kg, and it increased to 27.1 MJ/kg and 23.7 MJ/kg when bamboo was torrefied at 300 °C under nitrogen and 9% oxygen, respectively. However, the HHV of the torrefied bamboo was lower than that of torrefied bamboo at 250 °C under an oxygen atmosphere. This result indicated that a small amount of oxygen mainly provided extra energy by reducing the low-energy bonds and increasing the high-energy bonds (C−C) (Liu et al. 2013; Li et al. 2015).
Figure 3 shows elemental analysis results for the raw and torrefied bamboo. The carbon content increased from 47.5% to 68.9%, and the oxygen content decreased from 43.0% to 22.3%. In particular, the hydrogen content decreased dramatically from 6.35% to 2.89%. This result was attributed to the oxidative reaction in the torrefaction process under an oxygen atmosphere (Li et al. 2015a). Ma et al. (2019c) reported that oxygen was mainly released in the forms of H2O, CO2, and CO by dehydroxylation, decarbonylation, and decarboxylation reactions during the torrefaction process. The carbon, oxygen and hydrogen concentrations of the raw bamboo were 47.5%, 43.0%, and 6.35%, respectively. When the torrefaction temperature was at 200 °C and 250 °C and the oxygen concentrations increased from 3% to 9%, the carbon sharply increased from 48.8% to 67.9%, and the oxygen and hydrogen decreased from 41.6% and 6.28% to 24.2% and 3.22%, respectively. However, the contents of carbon and hydrogen gradually decreased from 69.3% and 5.17% to 68.9% and 2.89%, respectively, as the oxygen concentrations increased. However, oxygen content had the opposite trend, as it increased from 20.4% to 22.3% at 300 °C. Based on the results of the elemental contents, the oxygen concentrations affected the elemental distribution (Bilbao et al. 1997). Specifically, a lower oxygen concentration torrefaction pretreatment could effectively remove oxygen and improve the use potential of bamboo in the energy field.
Fig. 3. Diagram of C, H, and O analysis at different torrefaction temperatures and atmospheres
FTIR Analysis
Figure 4 shows the effect of torrefaction temperatures and oxygen concentrations on the chemical functional groups of the bamboo. Five remarkable characteristic infrared absorbance bands with absorbance at 3460 cm-1, 2819 cm-1, 1706 cm-1, 1680 cm-1 to 1440 cm-1, and 1190 cm-1 to 950 cm-1 were observed. The band at 3460 cm-1 was attributed to the stretching vibration of the O−H band in the H2O, which was mainly related to cracking from the hydroxyl linked on the phenolic and aliphatic structure or the combustion reaction between hydrogen and oxygen elements during the oxygen torrefaction process (Ma et al. 2018a, 2018b). The band at 2819 cm-1 was ascribed to the stretching vibration of C−H, which was derived from the methoxy groups, methyl, and the methylene in aliphatic or alkanes (Chen et al. 2016a). The band at 1706 cm-1 was attributed to the C＝O stretching vibration in the aldehydes, ketones, and organic acids that contained the carboxyl and carbonyl functional groups (Zhen-Yu et al. 2017). The absorbance peak in the range 1680 to 1440 cm-1 was caused by the stretching vibration of C＝C from the benzene ring in aromatics (Esteves et al. 2013). The band between 1190 cm-1 to 950 cm-1 was the fingerprint region, which was derived from C−O and C−H linked on the syringyl, guaiacyl, and hydroxyphenyl. Therefore, the characteristic absorbance bands between 1190 cm-1 and 950 cm-1 may have represented alcohols, esters, and ethers compounds (Ma et al. 2015; Lin et al. 2018).