Abstract
The release characteristics of polycyclic aromatic hydrocarbons during the pyrolysis of biomass remain poorly understood. This study investigated the effects of pyrolysis temperature on the polycyclic aromatic hydrocarbon release characteristics by analyzing both the transient vapor products and the physicochemical properties of corresponding biochar from the pyrolysis of corn stover pellets. The results revealed that the transient volatile compounds mainly included phenols, ketones, acids, furans, aldehydes, substances containing benzene ring, polycyclic aromatic compounds, and gaseous products. A range of 2 to 4 ring polycyclic aromatic hydrocarbons were generated at 400 to 700 °C with the peak at 560 °C, and the sum of relative content of polycyclic aromatic hydrocarbons ranged from 0.23% to 40.36%. For the biochar, the carbonization stage (400 to 700 °C) of corn stover pellets was further divided into three evolutionary stages, including the preliminary carbonization stage (380 to 480 °C), amorphous carbon structure stage (480 to 600 °C), and the stage of dehydrogenation and growth of aromatic rings (600 to 700 °C).The relationship between polycyclic aromatic hydrocarbon release in volatile compounds and H/C ratio of the biochar could be described by a power function.
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Effects of Pyrolysis Temperature on the Release Characteristics of Polycyclic Aromatic Hydrocarbons during Pyrolysis of Corn Stover Pellet
Lijie Li,a,b Wenjuan Niu,a Hongbin Cong,b Haibo Meng,b,* Zhiyou Niu,a,* Xiuli Shen,b Licong Cao,b and Xianrui Kong a
The release characteristics of polycyclic aromatic hydrocarbons during the pyrolysis of biomass remain poorly understood. This study investigated the effects of pyrolysis temperature on the polycyclic aromatic hydrocarbon release characteristics by analyzing both the transient vapor products and the physicochemical properties of corresponding biochar from the pyrolysis of corn stover pellets. The results revealed that the transient volatile compounds mainly included phenols, ketones, acids, furans, aldehydes, substances containing benzene ring, polycyclic aromatic compounds, and gaseous products. A range of 2 to 4 ring polycyclic aromatic hydrocarbons were generated at 400 to 700 °C with the peak at 560 °C, and the sum of relative content of polycyclic aromatic hydrocarbons ranged from 0.23% to 40.36%. For the biochar, the carbonization stage (400 to 700 °C) of corn stover pellets was further divided into three evolutionary stages, including the preliminary carbonization stage (380 to 480 °C), amorphous carbon structure stage (480 to 600 °C), and the stage of dehydrogenation and growth of aromatic rings (600 to 700 °C).The relationship between polycyclic aromatic hydrocarbon release in volatile compounds and H/C ratio of the biochar could be described by a power function.
DOI: 10.15376/biores.18.1.2112-2136
Keywords: Corn stover pellet; Pyrolysis; Pyrolysis temperature; Polycyclic aromatic hydrocarbons; Transient vapor products
Contact information: a: Key Laboratory of Agricultural Equipment in Mid–lower Yangtze River, Ministry of Agriculture, College of Engineering, Huazhong Agricultural University, Wuhan, Hubei 430070, P. R. China; b: Key Laboratory of Energy Resource Utilization from Agriculture Residue, Ministry of Agriculture and Rural Affairs, Academy of Agricultural Planning and Engineering, Beijing, 100125, P. R. China; *Corresponding author: newmhb7209@163.com; nzhy@mail.hzau.edu.cn
INTRODUCTION
Renewable energy is of growing importance in addressing the environmental concerns over fossil fuel usage, as it has been widely promoted as an alternative to fossil fuel and a potential strategy to tackle climate changes at the global and national scales (IEA 2018). Energy from crop residues is considered as a form of renewable energy with low cost and rich reserves. Corn is a typical food crop widely planted in the world. Accordingly, a large quantity of corn stover is produced every year, and it is rich in cellulose, hemicellulose, and lignin (Han et al. 2021). The high carbon content (over 40%) in corn stover makes it a valuable resource of renewable and clean energy (Niu et al. 2016).
Pyrolysis is an advanced technology of thermal treatment. Pyrolysis of corn stover has received increasing attention and is regarded as an effective strategy to reduce the carbon dioxide concentration from the perspective of global carbon cycle (Yang et al. 2021). Moreover, the preparation of corn stover pellets can reduce the volume of accumulation and cost of transportation. In addition, the biochar derived from pelleted corn stover can alleviate dust emissions caused by biochar powder during entry into soils (Gelardi et al. 2019; Xie et al. 2021). However, the pyrolysis of biomass is generally accompanied by the production of polycyclic aromatic hydrocarbons (PAHs), which is mainly distributed in liquid bio-oil, accounting for 62% to 92.7% of the total PAHs (Fagernäs et al. 2012; Zhao et al. 2020), causing the risk of carrying PAHs for the biochar (Odinga et al. 2021) and gas products (Zhao et al. 2020). In this study, PAHs generally refer to the sixteen PAHs listed by US EPA (Zhou et al. 2014a).
As undesired components in the pyrolysis products, PAHs are carcinogenic and/or mutagenic. The presence of PAHs has a negative effect on the utilization of pyrolysis products. For example, PAHs in biochar are dangerous components for soil improvement, feed additives, or cooking (Lehmann et al. 2015; Wang et al. 2019). Many countries or organizations have set certain limits about PAHs in biochar, especially the International Biochar Initiative (IBI) and European Biochar Certificate (EBC). In addition, during the utilization of bio-oil in cogeneration unit, PAHs pose threats to the environment and the operator’ health, and the presence of PAHs is not conducive to the upgrading of bio-oil (Stark and Ghoniem 2017).
For the pyrolysis-based poly-generation systems (Cong et al. 2018), the pyrolysis of biomass is expected to produce not only cleaner noncondensable gases, but also cleaner biochar. Although some studies have reported on the factors affecting the release of PAHs during biomass pyrolysis, such as the residence time of hot volatile compounds (Buss et al. 2016), biomass feedstock type (Krzyszczak et al. 2021; Zhou et al. 2014a), biomass particle size (Li et al. 2016; Stark and Ghoniem 2017), pyrolysis equipment (Weidemann et al. 2018), and catalysts (Cho et al. 2020; Muneer et al. 2019), they have only focused on the engineering conditions of pyrolysis, while the formation and release mechanisms of PAHs based on pyrolysis process have been largely ignored. Furthermore, many studies have indicated the presence of PAHs in the biochar mainly for the following reasons. The formation of biochar skeleton structure produces PAHs in the biochar, and the PAHs in the tar are adhered to the surface and pore of the biochar, which also contributes to the accumulation of PAHs (Fagernäs et al. 2012; Lehmann et al. 2015; Zhao et al. 2020). Therefore, understanding the formation and distribution mechanisms of PAHs during the pyrolysis of biomass will help to take more targeted measures to inhibit the formation of PAHs in the pyrolysis products. However, little is known about the release characteristics of PAHs at different temperatures during the pyrolysis process. Furthermore, the relationship between the release amount of PAHs in the volatile compounds from the pyrolysis of biomass and the aromaticity of biochar remains poorly understood.
Previous studies of PAHs in biomass pyrolysis products mostly adopted tubular furnace, batch pyrolysis reactor, and continuous pyrolysis reactor (Buss et al. 2016; Zhao et al. 2020). Due to the limitations of experimental equipment architecture and gas flow characteristics, volatile components could not leave the reaction system quickly, resulting in the secondary reaction of the volatile components (Legarra et al. 2019). Therefore, the obtained PAHs were produced under the combined action of primary reaction and secondary reaction, which largely hinders the dissection of the specific PAH formation mechanism, particularly during the primary reaction. The main reason is that it is extremely difficult to quickly remove the volatile components from the reaction system. In addition, bio-oil aging also makes it difficult to obtain primary products (Wang and Ben 2020). Therefore, a special design of the experimental equipment is highly necessary.
In this study, a thermogravimetric analyzer coupled with gas chromatography and mass spectrometry system (TG–GC/MS) was specially assembled to investigate the release characteristics of PAHs during pyrolysis process of corn stover pellets. The species and relative contents of volatile components, especially PAHs from the pyrolyzate plume, were semi-quantitatively analyzed during the pyrolysis process of corn stover pellets from 200 to 700 °C, and the physicochemical characteristics of biochar produced were analyzed in the pyrolysis process. The formation and evolution mechanisms of PAHs in the transient vapor products was investigated, as well as the relationship between the release amount of PAHs and the aromaticity of biochar during the pyrolysis of corn stover pellets.
EXPERIMENTAL
Materials
Sample collection and preparation
Corn stover (CS) was collected from Harbin, Heilongjiang province, China. It was ground by a FQ–320 grinder (FQ–320, Zhengzhou Yunkai mechanical equipment Co., Ltd, Zhengzhou, China) after natural drying, followed by oven drying at 105 °C for 12 h until it reached a constant mass. The milled corn stover was collected, and 293.78±2.31mg of milled corn stover sample was held for 5 s at 8 MPa and pressed into pellets (diameter 13 mm, height 1.1mm) by tablet press. The corn stover pellet samples and tablets were stored in a drying dish.
Methods
Proximate and ultimate compositions
Proximate analysis of corn stover pellets was conducted according to ASTM D3302/D3302M (2015), ASTM 3174(2012), and ASTM3175(2011) (1984). The ash content of the biochar was determined with a thermogravimetric analyzer (TG STA449Fs, NETZSCH, Selb, Germany). Ultimate analysis of biochar was conducted by an elemental analyzer (Vario EL III, Elementar, Langenselbold, Germany) to determine the C, H, N, and S contents. The O content was obtained by subtracting the C, H, N, S, and ash contents from 100%. The results are shown in Table 1.
Determination of cellulose, hemicellulose, and lignin
Cellulose, hemicellulose, and lignin in the corn stover pellets were determined by the Van Soest’s washing fiber analysis method according to NY/T 3494 (2019).
TG–FTIR- MS
The pyrolysis vapor on-line analysis of the samples was carried out by using a synchronous thermal analyzer (SDT Q600, TA Instruments, Newcastle, DE, USA), a Fourier transform infrared spectrometer (Nicolet i50, Thermo Fisher, Waltham, MA, USA), and a fast process mass spectrometry (MS, AMETEK, Philadelphia, PA, USA). The bio-crudes were detected by a Fourier transform infrared spectrometer, while a fast process mass spectrometry was employed to analyze the gaseous products (CO, CH4, H2, and C2H2). The test conditions were as follows: the carrier gas was 99.999% high-purity nitrogen, and the gas flow rate was 100 mL/min. The heating rate was 20 °C/min, and the temperature was from 25 to 900 °C. The temperature of the transfer tube and detector was 220 °C, and the scanning range was 4000 to 400 cm-1.
TG–GC/MS
Pyrolysis experiments were performed on the thermogravimetric analyzer (TG STA449Fs, NETZSCH, Selb, Germany) coupled with gas chromatography–mass spectrometry (GC 7890B and MS 5977B, Agilent, Hercules, CA, USA), and the schematic diagram of the TG–GC/MS reaction system is shown in Fig. 1. The reaction temperature was set to 250, 280, 320, 380, 400, 440, 480, 500,520, 560, 600, 660, and 700 °C, respectively. The temperature of the pyrolysis vapor products transfer tube was kept at 295 °C to prevent product condensation. About 1 mL of high temperature (300 °C) volatile sample was picked by the loop in the high temperature injection valve, carried by helium gas into GC, and then detected by mass spectrometry. It took about 0.5 s for the detection of the products after being released in an oxygen free environment. Therefore, the pyrolysis vapor products were instantly sampled and analyzed once generated to minimize the potential contribution of secondary gas phase reactions to the formation of PAHs. The heated furnace was continuously purged with a 100 mL/min helium gas flow. When the aim temperature was reached, the pyrolysis vapor from the reactor of the thermogravimetric analyzer was extracted by a peristaltic pump and carried by helium gas through a stainless steel pipe (inner diameter 0.75 mm, with passivated inner surface). The biochar from every test was designated as CS200, CS250, CS280, CS320, CS380, CS400, CS440, CS480, CS500, CS520, CS560, CS600, CS660, and CS700, respectively.
The GC/MS was fitted with a 30 m × 250 μm × 0.25 μm HP–5MS UI equivalent column. The GC oven was initially heated at 40 °C for 1 min, and the temperature was subsequently increased to 100 °C at 1.5 °C/min and held for 2 min, increased to 150 °C at a rate of 5 °C/min and held for 1 min, increased to 230 °C at a rate of 20 °C/min and held for 1 min, and then increased to 310 °C at a rate of 20 °C/min and held for 5 min (total analysis time of 68 min). The manifold was at 320 °C and the ion trap temperature was held at 300 °C. The GC/MS results were compared with the NIST 17 database.
The 16 mixed standard PAHs dissolved in hexane used for calibration were purchased from TanMo Quality Testing Technology Co., Ltd. (Beijing, China). PAHs in the products were pre-calibrated by injection of the 16 mixed standard PAHs dissolved in hexane. The GC/MS analysis spectra of US EPA 16PAHs are shown in Fig. S1, and their retention time is shown in Table S1. The product components were semi-quantified based on ratio of peak area of the target substance to the initial mass of loaded sample.
(1)
(2)
In the equations, Ii, represents the ratio of peak area of the target substance to the initial mass, namely, the absolute yield, mg-1; Ai represents the peak area of the target substance, dimensionless; AT represents the sum of peak areas of all released compounds, dimensionless; and m0 represents the initial mass of the loaded sample, mg.
Fig. 1. Schematic diagram of the TG−GC/MS reaction system
FTIR analysis
Biochar mentioned in this paper refers to the residue generated by pyrolysis of corn straw pellets at different temperatures on TG-GC/MS. The surface functional groups of crop residue and biochar were detected by using FTIR. The sample was mixed with KBr at the ratio of 1:100 and then made into tablets before scanning with an FTIR spectrometer.
Raman analysis
The structural features of biochar were characterized by Raman spectroscopy (Renishaw inVia Raman microscope, London, UK) with a laser at a wavelength of 785 nm. Detailed parameters are shown in the Supplementary Information. Raman spectra of the biochar were fitted by using 10 bands according to the method proposed by Li et al. (2006). The D band was assigned to C–C between aromatic rings and aromatics with not less than 6 rings, and the GR, VL, and VR bands were assigned to aromatics with 3 to 5 rings and amorphous carbon structures. The band area ratio was taken as a brief measure of the ratio between the large aromatic ring systems and the aromatic ring systems typically found in amorphous carbon (Li et al. 2006).
(3)
In Eq. 3, SD means the area of the D band, and 
represents the total area of the GR, VL, and VR bands.
RESULTS AND DISCUSSION
Characteristics of corn stover
Composition and higher heating value (HHV) of corn stover pellets are shown in Table 1. The main components of corn stover pellets are cellulose, hemicellulose, and lignin, of which the content of cellulose is the highest, up to 39.32±0.94%. The results of approximate analysis show that volatile matter was much greater than ash and fixed carbon. The results of elemental analysis showed that C was the most abundant element, followed by O.
Table 1. Composition and Higher Heating Value (HHV) of Corn Stover Pellets
TG−FTIR–MS Analysis of the Pyrolysis of Corn Stover Pellets
The TG/DTG curves of the pyrolysis of corn stover pellets and the FTIR analysis results of pyrolysis volatile compounds are shown in Fig. 2. The weight loss process of corn stover pellets can be divided into three stages. These are: (1) a drying stage, at which the evaporation of water resulted in a slight weight loss; (2) a volatile matter escaping stage, at which the main released substances were water, carbon dioxide and acetic acid before 250 °C (Fig. 2b); and (3), a carbonization stage. During the second stage, the cellulose, hemicellulose, and lignin in the corn stover pellets were gradually decomposed, resulting in the release of small molecular substances and formation of volatile components, including acetic acid, alkanes, and furans (Fig. 2b). The maximum weight loss rate of corn stover pellet was obtained (Fig. 2a), and the total absorbance of volatile compounds was the highest at 320 °C. With increasing pyrolysis temperature, the total absorbance of volatile compounds at different temperatures after 320 °C decreased. The volatile compounds released from the pyrolysis mainly included CO2 (2390 to 2280 cm-1), CO (2173 to 2120 cm-1), CH4 (2960 cm-1), and alkanes (2928 cm-1). The easily condensable substances included H2O (3357 cm-1), carbonyl compounds C=O (1760 to 1660 cm-1), furans (1240 to 1160 cm-1), and acid RCOOH (1300 to 1200 cm-1). The carbonyl C=O stretching signal was the strongest, and the DTG curve formed the maximum peak at this stage (Fig. 2a). The main reason might be that cellulose and hemicellulose were decomposed at 250 to 440 °C and 250 to 400 °C, respectively, and lignin was decomposed at 200 to 500 °C, accompanied by a series of dehydrogenation and decarboxylation reactions (Liu et al. 2019). In the carbonization stage, the aromatic rings in the biochar were further condensed and the side chains were broken, accompanied by a slight weight loss and the release of a relatively high amount of H2.
The composition of main non-condensable gases during corn stover pellet pyrolysis was detected (Figs. 2b, 2c). The highest peaks of CO and CO2 release were observed at 320 °C, the temperature corresponding to the maximum weight loss. The release of CO2 could be ascribed to the cracking of functional groups of carboxyl (C=O) and COOH during the pyrolysis process. CH4 was released at below 300 °C and reached peaks at 380 °C and 560 °C, respectively (Figs. 2b, 2c). CH4 was generated through the decomposition of methoxyl mainly from hemicellulose, cellulose, and lignin in corn stover pellets, and lignin had the highest methoxyl content (Yang et al. 2020a). A small amount of C2H2 was released from 400 °C to 600 °C (Fig. 2c). The production of C2H2 was due to the decomposition of C2H4 and certain aromatic tar components (Font Palma 2013). A small amount of H2 was produced between 300 and 600 °C, and the production increased rapidly after 600 °C. The production of H2 could be attributed to the dehydrogenation polymerization of aromatic rings (Yang et al. 2020b).
Fig. 2. TG/DTG curves (a), FTIR analysis of volatile compounds (b) and the main gas composition (c) during the pyrolysis of corn stover pellets
TG−GC/MS of the Pyrolysis of Corn Stover Pellets
The overall product distribution during the pyrolysis of corn stover pellet at different temperatures is presented in Fig. 3. Volatile compounds were mainly released at 250 to 700 °C during the pyrolysis process of corn stover. Acids, aldehydes, ketones, and phenols were mainly generated at the early stage of the pyrolysis. The acids appeared at the temperature between 250 and 400 °C, including acetic acid and a small amount of formic acid and propionic acid. The ketones were mainly generated in the temperature range between 250 and 500 °C, including acetone, 1-hydroxy-2-propanone, and 2-cyclopenten-1-one. Furans were released from 250 to 480 °C, including 3-furaldehyde and dibenzofuran. Phenols were produced at 250 to 600 °C, including phenol, 5-ethenyl-2-methoxy-phenol, 2-methoxy-phenol, 2-methoxy-phenol, catechol, and p-cresol. Benzenes and polycyclic aromatic compounds (PACs) were generated in the later pyrolysis stage. Benzenes were produced at the temperature from 250 to 700 °C, including benzene, homologues of benzene, and some other substances containing benzene ring (except for benzofuran, phenol, and PACs). PACs were produced at temperatures from 380 to 700 °C, including eight PAHs, a large number of derivatives of naphthalene, fluorine, phenanthrene, and a few derivatives of anthracene and pyrene. The formation of PACs occurred in the period during which the weight loss of the corn stover pellets increased from 59.3% to 69.2% (Fig. 3a), and the relative content of PACs reached 47.2% at 600 °C (Fig. 3b). In addition to the volatile compounds described above, sugars, short chain hydrocarbons, pyrrole, and indole were also produced during the pyrolysis process of corn stover pellets.
Fig. 3. Distribution of transient volatile compounds of corn stover pellets during pyrolysis at different temperatures
The release intensity of phenols reached the maximum at the temperature between 320 and 380 °C, followed by rapid decreases, while that of PACs began to increase from 380 to 480 °C (Fig. 3a). The release intensity of phenols and PACs showed opposite trends in the same temperature range of 380 to 600 °C (Fig. 3a), indicating that the production of PAHs might be attributed to the deep decomposition of lignin structure (Zhou et al. 2014). Previous studies reported that PACs were detected at 350 °C (Lee et al. 2008; Zhao et al. 2020), and the volatile compounds from biomass in the tube furnace might stay in the reactor for a certain time, between half an hour and 2 hours generally (Li et al. 2016; Zhao et al. 2020). However, in this study, the reason for the detection of PACs at higher temperature may be that biochar did stay not longer than about 1 minute at any temperature due to the relatively sufficient carrier gas flow. The volatile compounds might be quickly removed by the peristaltic pump used in this study, resulting in a very short residence time, which would minimize the secondary reaction of volatile compounds so as to inhibit the formation of PAHs (Wong et al. 2019).
Effects of Pyrolysis Temperature on PAH Monomer and Homologues
PAH monomer
Figure 4 shows the total amounts of generated PAHs in the volatile compounds from the pyrolysis of corn stover pellets at different temperatures. With the increase of temperature from 400 to 700 °C, the release intensity of PAHs firstly increased and then decreased. These PAHs contained a series of 2 to 4 ring compounds, including naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, fluoranthene, and pyrene. Naphthalene, phenanthrene and fluorene were the major contributors to the release intensity and relative content of total PAHs, followed by anthracene, acenaphthylene, acenaphthene, fluoranthene, and pyrene. At 400 °C, fluorene was the most abundant PAH (up to 0.35 %), and naphthalene was always the most abundant PAH (up to 32.90 %) in the transient volatile compounds between 440 and 700 °C. The relative content of 3-ring PAHs reached up to 15.22 % at 600 °C, and that of 4-ring PAHs was lower than 0.2 % before 560 °C and became 0.27 % at 600 °C. High temperature is more beneficial for the formation and release of middle-molecular-weight components (Zhao et al. 2020). These observations are also in accordance with the evolution profiles of PAHs from McGrath et al. (2007), in which the PAHs in tar reached the highest value at the temperature between 500 and 550 °C, and then decreased sharply (McGrath et al. 2007).
With the pyrolysis temperature increasing from 520 to 560 °C, the relative content of PAHs rose rapidly from 6.15% to 16.36% (Fig. 4), and then the release intensity of PAHs reached the highest at 560 °C. With further increases of temperature, the relative content of PAHs reached the peak at 600 to 660 °C. Therefore, considering the release of PAHs, 560 °C could be a landmark temperature for the pyrolysis process of corn stover. The production of bio-oil at temperatures lower than 560 °C may be conducive to the further upgrading and transformation of the bio-oil. Therefore, the discovery of this temperature explains the optimization mechanism of bio-oil.
In previous studies, PAHs with larger ring sizes were detected (Xiong et al. 2020; Zhao et al. 2020), but they were not detected in this study, which does not mean that they were not produced. No detection of such PAHs may be related to the vapor pressure of the PAHs with larger ring sizes and the limitation of online monitoring (Tsekos et al. 2020).
Fig. 4. Total amount of the generated PAHs during the pyrolysis of corn stover pellets
The evolution of PAHs in volatile compounds with pyrolysis temperature during pyrolysis of corn stover pellets is shown in Fig. 5. The evolutionary profiles of each PAH monomer were different along with temperature. The temperature for the peak production of these eight PAHs followed the order of acenaphthene (480 °C), fluorene (480 °C), acenaphthylene (500 °C), naphthalene (560 °C), phenanthrene (560 °C), anthracene (560 °C), fluoranthene (600 °C), and pyrene (600 °C). Acenaphthene was the first species to disappear, and a small amount of other species were released until 700 °C (Figs. 5a-h). Acenaphthylene, acenaphthene, and fluorene were mainly produced at relatively lower temperature, and five other species were produced at relatively higher temperature (Fig. 5). With the increase in pyrolysis temperature, the release of 2-ring PAHs decreased and that of 3-ring and 4-ring PAHs increased, which may be related to the different formation mechanism of each PAH monomer.
The release intensity peak of naphthalene appeared at 560 °C. The relative content of naphthalene rapidly increased from with increasing temperature from 400 to 660 °C, and then it decreased to 0.11% with further increases in temperature from 660 to 700 °C (Fig. 5a). Two possible ways may be used to explain naphthalene formation. In the first pathway, naphthalene was primarily evaporated from biochar according to low temperature PAH formation mechanism (Eq. 4) (McGrath et al. 2007; Friederici et al. 2021). In the second pathway, phenol as a PAH precursor was transformed to cyclopentadiene, and naphthalene was formed from the cyclopendienyl radicals (Sharma and Hajaligol 2003). However, the decrease in naphthalene may be due to the formation of fluoranthene and anthracene after 660 °C (Eq. 5) (Zhou et al. 2015). Acenaphthene just appeared at the temperature between 400 and 600 °C, and the release intensity dropped rapidly after 480 °C (Fig. 5c). However, acenaphthylene began to be produced at 480 °C, and its release decreased with increasing temperature after 500 °C (Fig. 5d). In view of the emergence of acetylene at 400 to 600 °C (Fig. 2c), it could be inferred that acenaphthene might come from hydrogen abstraction acetylene addition (HACA) pathway (Eq. 5). The reduction of acenaphthene release suggested that acenaphthene might be an intermediate for the formation of other PAHs. For example, acenaphthylene could be obtained by dehydrogenation and fluoranthene could be obtained by acetylene addition (Liu et al. 2017).
(4)
(5)
The release intensity of phenanthrene and anthracene was similar within the same temperature range. Their release intensity increased from 440 to 560 °C, reached the maximum at 560 °C, and decreased from 600 to 700 °C. Their relative content reached the maximum at 600 °C, which was 12.10% and 2.09%, respectively. Fluorene began to be produced at 400 °C, and it reached a maximum at 480 °C, followed by gradual decreases (Fig. 5e). The formation of fluorene may be due to the addition of biphenyl and methyl radical (Eq. 6), and previous studies have suggested that lignin could produce higher concentrations of fluorine (Zhou et al. 2015).
(6)
The release profiles of fluoranthene and pyrene were similar, both of which were produced at 480 °C (Figs. 5f−g), increased slowly from 480 to 600 °C, and then reached a maximum at 600 °C (Figs. 5f−g). Considering that fluoranthene and pyrene were mainly produced at high temperature, they might be mainly evaporated from biochar. Another possible formation path of acenaphthylene, acenaphthene and fluoranthene may be that they were formed during the solid-phase charring process (Lee et al. 2008), because their methyl substitutes were also not detected in the volatile components.
Fig. 5. Temperature evolution of PAHs in volatile compounds of corn stover pellets in pyrolysis
Effects of pyrolysis temperature on PAH homologues
The related alkylated and hydrogenated PAHs were also determined, as a way to gain additional insights into the formation mechanisms and pathways of PAHs. Figure 6 shows the relative contents of naphthalene and phenanthrene homologues during the pyrolysis of corn stover pellets at different temperature. The 1-methynaphthalene and 2-methynaphthalene both increased at the temperature from 380 to 480 °C, and then they decreased afterwards (Fig. 6a), which is consistent with the results of Zhou et al. (2015). The results indicate that methylnaphthalene may be converted into naphthalene through demethylation (Eq. 7). The dimethyl-substituted naphthalene contained ortho and meta isomers, all of which first increased from 400 °C, reached the highest value at 480 °C, and then decreased until 600 °C (Fig. 6a). The trimethyl naphthalene included 3 isomers, all of which first increased from 400 °C, reached the peak at 480 °C, and then decreased until 560 °C (Fig. 6a). The production of polymethyl-substituted naphthalene was because the substitution sites of naphthalene are more active, and the H atom is easily replaced by methyl. 1,2-dihydro-naphthalene and 1,2-dihydro-4-methyl-naphthalene were detected at 380 °C, and disappeared at 440 and 520 °C, respectively (Fig. 6b). Derivatives of fluorene were 9-methyl-9H-fluorene from 400 to 600 °C and 9,9-methyl-9H-fluorene from 400 to 560 °C (Fig. 6c), which might be attributed to the activation of biphenyl after being attacked by methyl radical. The methyl substituents of phenanthrene mainly included methyl phenanthrene between 400 and 600 °C and dimethyl phenanthrene between 500 and 560 °C (Fig. 6d).
(7)
It is interesting to note that a large number of derivatives of naphthalene, fluorine, and phenanthrene, as well as a small amount of derivatives of anthracene and pyrene were detected. However, the homologues and derivatives of acenaphthylene, acenaphthene, and fluoranthene were not detected. The peak of PAH derivatives released from corn stover pellet pyrolysis vapors appeared before that of corresponding PAH monomers (Figs. 5 and 6), which may be ascribed to methyl cracking at higher temperature. The methyl substituents of PAHs may be the precursor of PAHs (Liu et al. 2017). In addition, a small amount of oxy-PAHs have been found in the pyrolysis vapors, such as 1-naphthalenol, 5,8-dihydro-1-naphthalenol, and other homologues, but when the pyrolysis temperature was higher than 500 °C, they were not detected.
