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Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar
Journal Pre-proofs Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar Xiaoxiao Zhang, Peizhen Zhang, Xiangru Yuan, Yanfei Li, Lujia Han PII: DOI: Reference:
S0960-8524(19)31548-2 https://doi.org/10.1016/j.biortech.2019.122318 BITE 122318
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Bioresource Technology
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Please cite this article as: Zhang, X., Zhang, P., Yuan, X., Li, Y., Han, L., Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122318
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Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar
Xiaoxiao Zhang, Peizhen Zhang, Xiangru Yuan, Yanfei Li, Lujia Han*
*
Corresponding author: Tel.: +86-10-6273-6313; Fax: +86-10-6273-6778; E-mail address: [email protected] (L. Han)
1
Laboratory of Biomass and Bioprocessing Engineering, College of Engineering, China Agricultural University, Box 191, Beijing 100083, China
2
Abstract The aim of this study was to evaluate how pyrolysis temperature influences the yield and physicochemical properties of biochar. We produced biochar from four feedstocks (wheat straw, corn straw, rape straw, and rice straw) pyrolyzed at 300, 400, 500, and 600 °C for 1 h, respectively. The results showed that all biochar yields decreased consistently with increasing temperature during pyrolysis and showed a steady decrease over 400 °C. Rice straw derived biochar had high yield superiority due to its higher content of ash. Pyrolysis temperature has significant effects on the properties of biochar; demonstrating a negative relationship with H, O, H/C, O/C, (O+N)/C, and functional groups, whilst having a positive relationship with C, ash, pH, electrical conductivity, and surface roughness. Higher pyrolysis temperature was beneficial to the formation of a more recalcitrant constitutions and crystal structure, making it available for material application. Keywords: pyrolysis temperature; crop residue; biochar yield; physicochemical properties
3
1.
Introduction The annual yield of crop residues is 600–800 Tg in China (Chen et al., 2019), of
which wheat straw, corn straw, rape straw, and rice straw account for a large proportion. The burning of large quantities of underutilized crop residues releases harmful matter, which not only contributes to air pollution and climate change, but also seriously affects human health. Biochar is derived from biomass (e.g., crop residues) under low oxic or anoxic conditions with the potential to sequestrate carbon. Hence, the use of crop residues as a raw material for biochar preparation can effectively relieve the pressures of straw treatment and environmental pollution. Biochar, which has a developed pore structure, stable aliphatic chain structure, and high mineral content (Cha et al., 2016), has the potential to control water pollution, mitigate greenhouse gas emissions, and remediate soils (Ahirwar et al., 2018; Pukalchik et al., 2018; Trakal et al., 2018). The potential to utilize biochar for various applications is related to its properties; for example, biochar with a microporous structure, active surface functional groups, high pH, and cation exchange capacity is preferred for immobilization of heavy metals (Xie et al., 2015). Biochar with high porosity and plenty of liming and fertilizer-related elements (e.g., N, P, and K) is preferred for improving soil properties (Ding et al., 2016). The economic benefits also need to be considered in biochar industrial production, specifically related to biochar yield. Extensive researches have shown that pyrolysis temperature influence the physicochemical properties of biochar. Pyrolysis temperature is a major contributor to 4
carbon loss during the preparation of biochar (Shaaban et al., 2018), and has an important effect on biochar yield (Uchimiya et al., 2011). Biochar prepared at different pyrolysis temperatures has different degrees of carbonization and highly ordered aromatic structure forms when the temperature exceeds 400 ℃ (Kim et al., 2012). Pyrolysis temperature can also change the surface roughness, micromorphology, and porous structure of biochar (Gupta et al., 2019; Wang et al., 2019). Biochar pyrolyzed at different pyrolysis temperatures has different electrical conductivities (EC), pH, P, and N (Chan et al., 2008; Meier et al., 2017).Some studies have indicated strong relationships between the properties and feedstock types of biochar. Compared with wood char, the biochar derived from crop residues has a more developed pore structure because of the lower lignin content (Sun et al., 2014). Correlations between ash content and carbon content, pH, and K content have been reported (Windeatt et al., 2014).To date, studies which explore the properties of biochar produced from different crop residue feedstock are limited. Most studies only focused on the effects of pyrolysis temperature on one or two crop residue biochar and characterized the properties of biochar with a certain application (O'Toole et al., 2013; Cabeza et al., 2018; Purakayastha et al., 2015). The different pyrolysis parameters used in these studies also makes it difficult to compare the effects of pyrolysis temperature on different crop residue biochar. Therefore, there is a need to characterize different crop residue biochar under uniform process conditions in order to further understand how pyrolysis temperature influence the properties of crop residue biochar. The aim of this study was 5
to investigate the effects of different pyrolysis temperatures on the yield and physicochemical properties of biochar derived from various crop residues. This is of great importance for understanding the pyrolysis conversation mechanism of crop residue biochar and promoting the scientific preparation and utilization of crop residue biochar. 2.
Materials and Methods
2.1 Crop residue material The wheat straw and corn straw were supplied by the Shangzhuang experimental station of the China Agricultural University. Rape straw and rice straw were collected from Anhui and Guangxi, respectively. The four different types of straw collected did not contain roots, and needed air-drying. Thereafter, they were crushed using a high-speed grinder (RT-34, Taiwan RongCong Precison Technology Co., China) to pass through a 20-mesh sieve. Samples were sealed in self-sealed bags for later use. All straw samples were marked as CK (straw without pyrolysis treatment). 2.2 Biochar preparation 2.2.1
Determination of pyrolysis temperature The pre-experiment was carried out by a thermogravimetric analyzer (SDTQ600,
TA Instruments, USA) to determine the temperature range for pyrolysis. Experiments were set at a linear heating rate of 10 °C/min within the temperature range of 0–1000 °C and a steady nitrogen flow rate of 100 mL/min. The largest weight loss rate was observed at 300 °C, and weight loss rate slowed down at 600 °C. Therefore, the 6
temperature range of 300–600 °C was selected for pyrolysis. 2.2.2 Preparation of straw biochar The straw samples were dried at 60 °C for 48 h before pyrolysis. Pyrolysis experiments were performed in a tube furnace (GSL-1100X, Hefei Kejing Materials Technology Co. Ltd., China) with sample masses of about 10 g. Straw samples were fed into the quartz boats using precise weighing and pyrolyzed at 300, 400, 500, and 600 °C for 60 min under N2 flow (99.999%) at a flow rate of 100 mL/min. The carrier gas velocity was controlled by a D07-7B mass flow controller (Beijing Seven Star Electronics Co., Ltd., China) and pre-ventilation time was set for 15 min to clear the air in the tube furnace. The resulting chars were allowed to cool to ambient temperature while maintaining the N2 flow. Following pyrolysis, the total mass of quartz boats and biochar were accurately weighed. Biochar was stored in sample bottles for subsequent analysis. The pyrolysis experiment was repeated three times for each straw type. The yield of biochar was calculated using the following equation (Narzari et al., 2017): Biochar yield (%) = (𝑊𝑇 ― 𝑊𝐵)/𝑊0 × 100%
(1)
where WT was the total mass (g) of the quartz boat and biochar after pyrolysis, WB and W0 were the mass (g) of the quartz boat and straw samples prior to pyrolysis, respectively. The obtained biochar was denoted as WB (wheat straw biochar), CB (corn straw biochar), RIB (rice straw biochar), and RB (rape straw biochar). Biochar produced at different temperatures was abbreviated as XX300, XX400, XX500, and XX600 based 7
on the corresponding materials (XX) (e.g., WB300, standing for wheat straw biochar derived at 300 °C). 2.3 Characterization of biochar The ash content was analyzed using a fully automatic measuring industrial analyzer (YX-GYFX 7705B, U-Therm Instrument Manufacturing Co., Ltd., China). The experimental method was based on the preset procedure i.e., 0.5–0.6 g of sample was dried in a crucible at 105 °C to a constant weight, and then heated to 575 °C until the sample reached a constant weight again and the ash was obtained in the end. C, H, N, and S content of the biochar were determined using an elemental analyzer (Vario Macro Elementar, Germany), and O was calculated by the subtraction method (Narzari et al., 2017). EC and pH were measured in deionized water following 48 h of shaking (1:20, wt/v) using a conductivity meter (FE30, Mettler-Toledo Group, USA) and a pH meter (FE20, Mettler-Toledo Group, USA), respectively.All the above experiments were repeated in triplicate. Fourier transform infrared spectroscopy (FTIR; Spectrum 400, Perkin Elmer, USA) was used to characterize the functional groups of biochar with the KBr method (1:100, wt/wt). The biochar samples and KBr were dried at 105 °C for 8 h before the test. Tableting operation was performed under infrared lamp irradiation. All FTIR spectra were collected in the mid-infrared range from 4000 to 400 cm-1 using a 4 cm−1 spectral resolution and 64 accumulations for each collection by the array, and were corrected by a pure KBr pellet (Cao et al., 2019). 8
Biochar was characterized by a polycrystal diffractometer (XD3 X-ray, Persee Ltd., China) that could provide information on the structure or morphology of their internal atoms or molecules for studying the short-range ordered structure of biochar and the crystal structure of minerals (Azargohar et al., 2014). The measurements were conducted using Cu-Kα radiation operating at 40 kV and 30 mA, 2θ scale of 5–60° at a step size of 0.02°, and a scanning speed of 2°/min (Fidel et al., 2017; Uzi et al., 2019). The surface morphology of biochar was analyzed by scanning electron microscopy (SEM; SU3500, Hitachi Ltd., Japan). The straw biochar samples were fixed on black conductive adhesive and coated with Pt to enhance the conductivity of the samples. Images were captured at 1000 magnification operated in high vacuum mode with 15 kV accelerating voltage. 2.4 Statistical analysis The software SPSS (IBM SPSS statistics 25) was used to analyze variance (ANOVA) and correlation relationship (Pearson). Bonferroni post-hoc t-tests (1% significance level) (Mandal et al., 2018) were carried out to identify significant differences between different biochars, and a p value of <0.01 was considered statically significant. The fitting analysis of sample data was conducted by MATLAB (MathWorks, MATLAB 2016). Adjusted R-square (R2) and root mean square error (RMSE) were chosen to evaluate goodness of fit. When R2 tended to 1 and RMSE values were smaller (approaching 0), the fitting effect was considered to be better. The X-ray diffraction (XRD) patterns of the samples were obtained by MDI Jade 6.0 software, and 9
the FTIR of the samples was analyzed by OPUS 7.2. 3.
Results and Discussion
3.1 Effect of pyrolysis temperature on biochar yield The yields of the different straw biochars decreased markedly with the increase in temperature (Fig. 1). The maximum weight losses of straw biochars occurred in the range of 300–400 °C due to water removal and the decomposition of complex components (such as cellulose and hemicellulose) (He et al., 2018b; Liu et al., 2018). As the temperature exceeded 400 °C, the yield showed a more steady decrease. This change may be attributed to the most of volatile matter exhalation and lignin decomposition below 400 °C (Shen et al., 2019; Suliman et al., 2016). The effects of pyrolys-is temperature on the yields of different straw biochars were similar. Table 1 shows how the yield of each straw biochar decreased significantly with the increase in temperature (p < 0.01). For example, the yield of the rice straw biochar decreased from 51.36 to 32.75% when temperature increased from 300 to 600 °C. In addition, the yield of the straw biochar was related to the source of the straw feedstock. The yield of the rice straw biochar was significantly higher than that of the other three straw biochars (p < 0.01), while the rape straw biochar had the lowest yield among all straw biochars (He et al., 2018b). This may be due to the high ash content of the rice straw and the high volatile content of the rape straw. Biochars derived at 300 °C showed significant differences among the four straw biochars (p < 0.01). There was no significant difference in biochar yield between wheat straw and maize straw when 10
pyrolysis temperature exceeded 400 °C. In addition, the rice straw biochar lost its advantage in yield at 600 °C, which may indicate that the degree of carbonization of the rice straw biochar was relatively high at this pyrolysis temperature. For further exploring the effects of pyrolysis temperature on straw biochar yield further, the quantitative relationship between biochar yield and pyrolysis temperature was analyzed using the fitting curve. Figure 2 shows the non-linear decreasing relationship between the yields of straw biochars and pyrolysis temperature, which was similar to previous studies (Zhang et al., 2017; Zhao et al., 2018). The biochar yield was a function of pyrolysis temperature: Yield = -23.04 ln(x) + 176.8 (R2 = 0.85, RMSE = 2.57). According to the equation, the yields of straw biochars would show a more steady decrease as the pyrolysis temperature continued increasing. 3.2 Effect of pyrolysis temperature on biochar physicochemical properties 3.2.1 Surface morphology The morphology of biochars changed at different temperatures. The physical structure of the wheat straw biochar appeared to show no obvious damage at a pyrolysis temperature of 300 °C. The surface of the wheat straw biochar produced at 400 °C was in a molten state, whereas it appeared cracked and collapsed as temperatures increased from 400 to 600 °C, resulting in the destruction of its pore structure. The surface roughness of the corn straw biochar increased significantly with the increase in temperature (Luo et al., 2018), and its surface structure was seriously damaged and formed an obvious flaky structure when the pyrolysis temperature exceeded 500 °C. 11
The rape biochar derived at 300 °C showed a few pores, and pore volume increased significantly at 400 °C, but the surface structure of the biochar melted and deformed at temperatures over 500 °C. High pyrolysis temperatures seriously damaged the surface and pore structure of straw biochar. The rice straw biochar showed a smooth surface and tubular structure below 400 °C; however, surface deformation, increases in pore quantity, and decreases in pore volume were apparent above this temperature, which may be due to the collapse of the internal pore structure (Fu et al., 2012). 3.2.2 Chemical characteristics Table 2 provides the chemical characteristics of the straw biochars and straw feedstock. The C content of the four straw biochars increased significantly to 56.49–67.85 wt% (p < 0.01) compared with the feedstock (42.12–45.53 wt%), indicating that straw biochars had good carbon sequestration ability. The H and O content in the straw biochars decreased due to pyrolysis. Although the N content of the four straw biochars was higher than that of the raw materials, agronomic application of biochar may be needed to supply additional nitrogen. The elemental composition of the straw biochars varied significantly (p < 0.01). The C content in the wheat straw biochar derived at varied temperatures showed significant differences (p < 0.01), and decreased after the first increase in temperature. In contrast, the C content in the other three straw biochars revealed an increasing trend with increases in temperature. The secondary cracking of C in the wheat straw biochar may lead to the decrease in C content (He et al., 2018b). The H content in all the four 12
straw biochars decreased significantly at 300 to 600 °C (p < 0.01), and this trend was consistent with the O content in the straw biochars. The decrease in H and O may be related to the loss of volatile matter. The N content of the wheat straw biochar and corn straw biochar decreased significantly at 300–400 °C and 500–600 °C (p < 0.01), respectively, but was steady in both biochars at 400–500 °C. This indicated that a stable C-N heterocycle might form at 400 °C and crack after 500 °C (He et al., 2018b). The N content of rape straw and rice straw biochar derived at different pyrolysis temperatures did not change significantly (p < 0.01). In addition, the pyrolysis temperature had no significant effect on the S content of straw biochars, which may be due to the relatively stable S functional groups in straw biochars at 300–600 °C. There was a significant difference in N content except C, H, and S (p < 0.01). The content sequence of N in the various straw raw materials was as follows: corn straw > rice straw > wheat straw > rape straw. The ash content of each straw biochar was higher than that of the corresponding raw material, and there were significant differences between the rice straw biochar and other three straw biochars (p < 0.01, Table 2). The ash content in the rice straw raw material was significantly higher than that in the other three types of straw materials (p < 0.01). This indicated that the ash content of straw biochars correlated with the composition of their raw materials. Table 1 shows the significant effect of pyrolysis temperature on the ash content of straw biochars (p < 0.01). The ash content of the four straw biochars increased with the increase in pyrolysis temperature, indicating the 13
formation and accumulation of mineral elements in the straw biochars during pyrolysis (Waqas et al., 2018). As shown in Table 2, the ash content of all straw biochars increased sharply between 300–500 °C, followed by slight increases when the pyrolysis temperature exceeded 500 °C. This agrees with He et al. (2018b) who found that ash content may change significantly within 300–450 °C, and show less change over 450 °C. Compared with the straw raw materials, the biochars were alkaline and suitable for soil acidity improvements. The pH of each straw biochar increased with the increase in pyrolysis temperature, and there were significant differences in biochars derived at different temperatures (p < 0.01). The pH values of the biochars (7.75 to 10.83) were slightly higher than a previous study (6.3–10.6) (Liu et al., 2018), which may be related to the species of straw used. At the same pyrolysis temperature, the pH values of the rape straw biochar were significantly lower than the other three straw biochars (p < 0.01). In general, temperature can influence the pH by affecting the ash content of alkaline minerals (e.g., Na and K) and acidic functional groups such as phenol and carboxyl in biochar (Zhao et al., 2018). Table 2 shows that the higher relative temperature (over 500 °C) had a slower increasing rate of pH; this could be attributed to the prominent changes in acidic functional groups and inorganic alkaline substances below and above 500 °C, respectively (Liu et al., 2014). The EC of straw biochars was significantly higher than that of the corresponding raw materials, and showed a significant increasing trend with increasing temperatures (p 14
< 0.01), which were similar to the results obtained in other studies (Kloss et al., 2012; Mukherjee et al., 2011). The increase in EC could be attributed to the increase in soluble salt content and the decrease in acidic functional groups in straw biochar as temperature rises (Azargohar et al., 2014; Narzari et al., 2017). The EC value can be used as a basis for the evaluation of the salt content and soluble alkaline cations content of biochar (Irfan et al., 2016). Table 1 shows that there were significant differences between the rape straw biochar and other three straw biochars (p < 0.01). The wheat straw and rice straw biochars had the lowest EC, whereas the rape straw biochar had the highest EC (7.44–10.65 mS/cm). This could be attributed to a higher content of K and Na in rapeseed straw biochar (Liu et al., 2018). Higher salinity and soluble cations have negative effects on plant growth and soil organic matter (Buss et al., 2016). It can be inferred that rape straw biochar should be considered for soil amendment due to soil salinization. 3.2.3 Constitution structure The ratios of H/C and O/C can indicate the aromaticity of biochar to a certain extent, and the ratios of O/C and (O+N)/C can indicate the polarity of biochar (Cantrell et al., 2012). In the present study, the H/C and O/C of the corn straw biochar, rape straw biochar, and rice straw biochar decreased with increases in pyrolysis temperature, and the ratios of H/C and O/C in straw biochars pyrolyzed at 300–600 °C were less than 0.6 and 0.4, respectively (Zhao et al., 2018). This indicated that the aromatic structure forms at increasing temperatures due to the decomposition of cellulose and other 15
macromolecules in biochar (Cao et al., 2013). The ratio of H/C decreased faster than that of O/C with increasing temperatures, indicating that H is more easily lost at lower pyrolysis temperatures than O. In addition, as the pyrolysis temperature increased, the O/C and (O+N)/C of the different straw biochars decreased. This indicated that straw biochars prepared at higher pyrolysis temperatures have less polar functional groups and stronger hydrophobicity (Wang et al., 2016). Furthermore, there were small differences in O/C and (O+N)/C of the straw biochars, while there were large differences in the H/C. It is apparent that the different characteristics of the straw biochars may be related to the change in H during pyrolysis. Changes in the FTIR spectrum can reflect the effects of pyrolysis temperature on the functional groups. The decrease in the O-H stretching vibration (3500–3200 cm-1) showed that the bound water of straw biochars disappeared when the pyrolysis temperature exceeded 500 °C (Cantrell et al., 2012; Keiluweit et al., 2010). The absorption peak of straw biochars at 2926 cm-1 revealed the presence of C-H stretching vibration in the aliphatic group (2930–2850 cm-1), but this was lost at temperatures over 500 °C (Mandal et al., 2018). This indicated a decrease in the aliphatic group and the formation of a dense ring structure in straw biochars (Wang et al., 2013). In the present study, the C-H stretching vibration (2919 cm-1) of the wheat straw biochar was observed at 300 °C and 400 °C, indicating the existence of cellulose and hemicellulose in the wheat straw biochar (Waqas et al., 2018) in this pyrolysis temperature range. With the increase in pyrolysis temperature, the absorption peak strength of the straw biochar in 16
the range of 1600–1580 cm-1 (aromatic ring C=C stretching vibration and conjugated ketone and quinone C=O stretching vibration) decreased significantly (Wang et al., 2013). Concurrently, bands related to C=C stretching vibration in lignin (1510–1512 cm-1) and C-H stretching vibration in cellulose and other biopolymers (1455–1439 cm-1 and 1376–1382 cm-1) diminished at temperatures over 400 °C (Shen et al., 2019; Wang et al., 2013). Thus, it was apparent in the present study that a higher pyrolysis temperature may lead to the decomposition of polymers in the biochars. In addition, the four straw biochars derived at 600 °C lost most of the absorption peaks, except for bands in 680–850 cm-1, which was related to the monocyclic and polycyclic aromatic structures (Azargohar et al., 2014). This may imply that biochars derived at 600 °C have high stability. According to XRD spectra analysis, similarities were noted among the four straw raw materials. The peaks at 16° (2θ) and 22° (2θ) in the raw materials indicated the existence of a cellulose crystal structure (Keiluweit et al., 2010). Although all the straw biochars had similar patterns, there were also some differences between them. The wheat straw biochar and rice straw biochar showed a loss of peak intensity at 16° (2θ) and a gain of peak intensity at 22° (2θ) with an increase in pyrolysis temperature. This indicated that the crystalline cellulose of the wheat straw biochar and rice straw biochar had higher thermal stability than that of the other two biochars. The XRD patterns revealed that there were some potassium salts (2θ = 28° and 2θ = 40°) in all the straw biochars (Gao et al., 2018) and no significant differences were observed with the 17
changes in temperature. When the temperature exceeded 300 °C, the characteristic peak at 26.6° of the wheat straw biochar and corn straw biochar were attributed to SiO2 (Zhang et al., 2015), and the characteristic peak at around 29.6° of the rape straw biochar and rice straw biochar were attributed to calcite (He et al., 2018a). That peak intensity increased with increases in temperature, suggesting that increasing pyrolysis temperature leads to the formation of a large and orderly crystal structure (Zhao et al., 2017). In addition, the peak at around 30° of the rape straw biochar and corn straw biochar showed the presence of calcium and magnesium. 3.3 Correlation between yield and physicochemical properties The correlation analysis between biochar yield and physicochemical properties of all straw biochars is tabulated (Table 3). Figure 3 shows the quantitative relationships between biomass yield and physicochemical properties of the straw. There was a significant correlation between the yields of straw biochars and C (p < 0.01), and the yield decreased linearly with decreases in C (He et al., 2018b). There were positive correlations between the yield of the straw biochars and H, O (p < 0.01). Compared with C content, H and O provided a better linear fitting degree with the yield of straw biochars (Fig. 3). This indicated that the reduction in biochar yield was caused by the thermal decomposition of organic compounds containing H and O. The biochar yield was negatively linearly correlation with pH and EC. When the yield of straw biochars was less than 30%, their pH showed less change with the same change in temperature. According to the relationship between pyrolysis temperature and biochar yield, the 18
range of temperatures in which the pH of the straw biochar showed considerable change was below 500 °C, which was consistent with previous results (Zhang et al., 2017). The yield of straw biochars was significantly correlated with O/C, H/C, and (O+N)/C (p < 0.01), revealing a good linear relationship. The low yield of straw biochar with strong aromaticity and hydrophobicity may be related to the formation of a stable graphitized crystal structure (Jindo et al., 2014) and the precipitation of water-soluble substances. The quantitative relationships between the different biochar properties were further studied. There were significant linear correlations between C and (O+N)/C, EC (p < 0.01), respectively. The H and O had a highly significant positive correlation (p < 0.01). Both H and O had strong positive correlations with O/C, H/C, and (O+N)/C, and had strong negative correlations with pH and ash (p < 0.01). In addition, H and O showed strong positive linear correlations with O/C, H/C, and (O+N)/C, indicating that the precipitation of H and O in the chemical bonds of the straw samples during pyrolysis had significant impacts on the increase in aromaticity and hydrophobicity in the straw biochars. Ash also had a significant positive correlation with pH (p < 0.01), but the linear fitting degree was lower than that of H and O. This showed that the loss of functional groups containing H and O played an import role in enhancing biochar basicity at pyrolysis temperatures of 300–600 °C. H/C and O/C showed strong negative correlations with pH (p < 0.01), which indicated that the alkalinity of the straw biochar was related to the formation of the aromatic structure during pyrolysis (Liu et al., 2018). In addition, there were significant negative correlations between N and EC, ash and H/C, 19
O+N/C, and EC and (O+N)/C (p < 0.01), while EC had a significant negative correlation with O/C and H/C (p < 0.05). 4.
Conclusions Biochar yields decreased with increasing temperature, but tended to reduce slowly
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Characteristics, and Ecological Risks of Biochars. J. Therm. Sci. 28, 755-762. Waqas, M., Aburiazaiza, A.S., Miandad, R., Rehan, M., Barakat, M.A., Nizami, A.S., 2018. Development of biochar as fuel and catalyst in energy recovery technologies. J. Clean. Prod. 188, 477-488. Windeatt, J.H., Ross, A.B., Williams, P.T., Forster, P.M., Nahil, M.A., Singh, S., 2014. Characteristics of biochars from crop residues: Potential for carbon sequestration and soil amendment. J. Environ. Manage. 146, 189-197. Xie, T., Reddy, K.R., Wang, C., Yargicoglu, E., Spokas, K., 2015. Characteristics and Applications of Biochar for Environmental Remediation: A Review. Crit. Rev. Env. Sci. Tec. 45, 939-969. Zhang, H., Chen, C., Gray, E.M., Boyd, S.E., 2017. Effect of feedstock and pyrolysis temperature on properties of biochar governing end use efficacy. Biomass Bioenerg. 105, 136-146. Zhang, J., Lü, F., Zhang, H., Shao, L., Chen, D., He, P., 2015. Multiscale visualization of the structural and characteristic changes of sewage sludge biochar oriented towards potential agronomic and environmental implication. Sci. Rep. 5, 9406. Zhao, B., Nan, X., Xu, H., Zhang, T., Ma, F., 2017. Sulfate sorption on rape (Brassica campestris L.) straw biochar, loess soil and a biochar-soil mixture. J. Environ. Manage. 201, 309-314. Zhao, B., O'Connor, D., Zhang, J., Peng, T., Shen, Z., Tsang, D.C.W., Hou, D., 2018. Effect of pyrolysis temperature, heating rate, and residence time on rapeseed 27
stem derived biochar. J. Clean. Prod. 174, 977-987.
28
Figures captions Fig. 1 Yields of straw biochars pyrolysed at 300 °C, 400 °C, 500 °C, and 600 °C. Fig. 2 Effects of pyrolysis temperature on the yields of straw biochars. Fig. 3 Regression analysis of biochar yield and physicochemical properties
29
TABLE AND FIGURES
Table 1 Yields of straw biochars. Different letters a,b, c, and d (A, B, C, and D)in the same column (line) show statistical differences (p<0.01). Pyrolysis temperature (℃)
Wheat straw
Corn straw
Rape straw
Rice straw
300
46.96±0.25dC
45.84±0.17dB
44.32±0.15dA
51.36±0.06dD
400
35.76±0.11cB
35.85±0.24cB
34.48±0.20cA
38.56±0.05cC
500
32.49±0.18bB
33.07±0.23bB
31.17±0.20bA
34.42±0.11bC
600
31.55±0.28aBC
30.87±0.12aB
29.27±0.07aA
32.75±0.08aC
30
Table 2 Chemical properties of all straw biochars. Different letters a,b, c, and d (A, B, C, and D)in the same column (line) show statistical differences (p<0.01).
31
Pyrolysis temperature (℃)
Wheat straw
Corn straw
Rape straw
Rice straw
C(wt%)
CK
45.53±0.19aC
44.53±0.31aB
44.63±0.19aB
42.12±0.10aA
300
61.48±0.43bB
61.20±0.43bB
61.80±0.18bB
56.49±0.20bA
400
64.18±0.37cB
63.36±0.70cB
63.74±0.45cB
56.42±0.07bA
500
67.39±0.32eC
65.08±0.59cB
66.96±0.33dC
59.59±0.49cA
600
65.34±0.53dB
67.48±0.64dBC
67.85±0.72dC
61.30±0.29dA
CK
3.56±0.25eA
5.31±0.20eB
4.89±0.05eB
4.16±0.11eA
300
2.73±0.07dA
3.68±0.09dB
3.54±0.20dB
2.95±0.19dA
400
1.78±0.03cAB
1.96±0.02cB
1.91±0.20cB
1.35±0.10cA
500
1.01±0.06bC
0.77±0.04bBC
0.87±0.05bBC
0.47±0.02bA
600
0.52±0.03aB
0.18±0.01aA
0.18±0.02aA
0.12±0.02aA
CK
0.88±0.02aA
1.69±0.03aC
0.64±0.10aA
1.24±0.08aB
300
1.40±0.06cB
2.93±0.06dD
1.02±0.07bA
2.15±0.06bC
400
1.36±0.08bcB
2.52±0.07cD
0.98±0.04bA
1.99±0.06bC
500
1.38±0.14bcB
2.43±0.04cD
0.88±0.11bA
1.90±0.05bC
600
1.10±0.01abA
2.12±0.01bB
0.90±0.10bA
2.01±0.11bB
CK
0.36±0.05aA
0.71±0.11cB
0.55±0.03aAB
0.75±0.12bB
300
0.39±0.04aA
0.54±0.02bcAB
0.59±0.04aB
0.45±0.08aAB
400
0.47±0.03aA
0.51±0.02bA
0.68±0.03aB
0.44±0.01aA
500
0.48±0.02aA
0.39±0.01abB
0.64±0.04aC
0.34±0.02aB
600
0.45±0.01aB
0.24±0.02aA
0.64±0.07aC
0.29±0.02aA
CK
42.53
41.18
42.34
41.22
300
19.61
17.39
17.95
17.73
400
13.93
13.46
13.48
13.71
500
7.35
11.36
9.46
8.27
600
10.77
8.98
7.89
5.71
CK
7.14±0.15aB
6.58±0.06aA
6.95±0.15aAB
10.51±0.01aC
300
14.39±0.53bA
14.26±0.06bA
15.10±0.06bA
20.23±0.12bB
400
18.28±0.32cA
18.19±0.09cA
19.21±0.05cB
26.09±0.30cC
500
22.39±0.32dC
19.97±0.12dA
21.19±0.10dB
29.43±0.36dD
600
21.82±0.29dAB
21.00±0.19eA
22.54±0.02eB
30.57±0.32eC
CK
5.49±0.07aB
5.33±0.02aB
6.15±0.01aC
4.22±0.09aA
300
7.98±0.02bB
8.28±0.02bC
7.75±0.03bA
8.29±0.04bC
400
9.06±0.02cA
9.83±0.02cD
9.18±0.03cB
9.33±0.02cC
500
10.37±0.01dC
10.22±0.01dB
9.95±0.02dA
10.23±0.04dB
600
10.83±0.03eB
10.26±0.01dA
10.28±0.03eA
10.75±0.03eB
CK
3.48±0.07aB
4.59±0.14aC
2.47±0.07aA
3.28±0.07aB
300
4.12±0.04bA
5.00±0.18aB
7.44±0.02bC
3.88±0.11aA
400
5.16±0.10cA
5.65±0.09bA
8.70±0.05cB
5.25±0.24bA
500
6.53±0.08dA
7.00±0.09cB
9.74±0.01dB
6.29±0.20cA
600
6.91±0.11eA
7.67±0.06dB
10.65±0.29eC
6.67±0.22cA
H (wt%)
N (wt%)
S (wt%)
O1 (wt%)
Ash (wt%)
pH
EC (mS/cm)
Note: 1O % = 100% - (C % - N % + H % + S % + Ash %)
32
Table 3 The correlation analysis between biochar yield and physicochemical properties Yield
C
H
N
S
O
pH
Yield
1.00
C
-0.71A
1.00
H
0.88A
-0.47 1.00
N
0.30
-0.40 0.18
1.00
S
0.03
0.21
-0.50a 1.00
O
0.88A
-0.51a 0.92A
0.15
0.23
1.00
pH
-0.89A
0.50a
-0.05
-0.29
-0.93A 1.00
EC
-0.71A
0.69A -0.52a
Ash
-0.51a
-0.14
-0.75A
O/C
0.92A
-0.61a
0.91A
H/C
0.91A
-0.52a
1.00A
(O+N)/C
0.93A
-0.66A 0.90A
0.35
-0.94A
EC
Ash
O/C
H/C
(O+N)/C
-0.57a 0.46
-0.57a 0.46
1.00
0.01
-0.41
-0.77A
0.70A
0.19
1.00
0.19
0.99A
-0.93A
-0.62a
-0.69A
1.00
0.20
0.33
0.92A
-0.94A
-0.54a
-0.73A
0.92A 1.00
0.34
0.10
0.97A -0.90A -0.69A
-0.65A
0.99A 0.92A 1.00
0.19
a
Correlation is significant at the 0.05 level (2-tailed).
A
Correlation is significant at the 0.01 level (2-tailed).
33
60
Wheat straw
Corn straw
Rape straw
Rice straw
Yield (%)
45
30
15
0
300
400 500 Temperature (℃)
600
Fig. 1 Yields of straw biochars pyrolysed at 300 ℃, 400 ℃, 500 ℃, and 600 ℃.
34
Fig.2 Effects of pyrolysis temperature on the yields of straw biochars.
35
Fig.3 Regression analysis of biochar yield and physicochemical properties
36
Declaration of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the manuscript entitled “Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar”.
37
Highlights
Pyrolysis temperature had similar effects on four different crop residues biochar
The maximum losses rate in biochar was observed below 400 °C
Rice straw biochars have high yield superiority
Higher temperature biochars had high stability and an orderly crystal structure
38
S0960-8524(19)31548-2 https://doi.org/10.1016/j.biortech.2019.122318 BITE 122318
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
18 September 2019 18 October 2019 19 October 2019
Please cite this article as: Zhang, X., Zhang, P., Yuan, X., Li, Y., Han, L., Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122318
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Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar
Xiaoxiao Zhang, Peizhen Zhang, Xiangru Yuan, Yanfei Li, Lujia Han*
*
Corresponding author: Tel.: +86-10-6273-6313; Fax: +86-10-6273-6778; E-mail address: [email protected] (L. Han)
1
Laboratory of Biomass and Bioprocessing Engineering, College of Engineering, China Agricultural University, Box 191, Beijing 100083, China
2
Abstract The aim of this study was to evaluate how pyrolysis temperature influences the yield and physicochemical properties of biochar. We produced biochar from four feedstocks (wheat straw, corn straw, rape straw, and rice straw) pyrolyzed at 300, 400, 500, and 600 °C for 1 h, respectively. The results showed that all biochar yields decreased consistently with increasing temperature during pyrolysis and showed a steady decrease over 400 °C. Rice straw derived biochar had high yield superiority due to its higher content of ash. Pyrolysis temperature has significant effects on the properties of biochar; demonstrating a negative relationship with H, O, H/C, O/C, (O+N)/C, and functional groups, whilst having a positive relationship with C, ash, pH, electrical conductivity, and surface roughness. Higher pyrolysis temperature was beneficial to the formation of a more recalcitrant constitutions and crystal structure, making it available for material application. Keywords: pyrolysis temperature; crop residue; biochar yield; physicochemical properties
3
1.
Introduction The annual yield of crop residues is 600–800 Tg in China (Chen et al., 2019), of
which wheat straw, corn straw, rape straw, and rice straw account for a large proportion. The burning of large quantities of underutilized crop residues releases harmful matter, which not only contributes to air pollution and climate change, but also seriously affects human health. Biochar is derived from biomass (e.g., crop residues) under low oxic or anoxic conditions with the potential to sequestrate carbon. Hence, the use of crop residues as a raw material for biochar preparation can effectively relieve the pressures of straw treatment and environmental pollution. Biochar, which has a developed pore structure, stable aliphatic chain structure, and high mineral content (Cha et al., 2016), has the potential to control water pollution, mitigate greenhouse gas emissions, and remediate soils (Ahirwar et al., 2018; Pukalchik et al., 2018; Trakal et al., 2018). The potential to utilize biochar for various applications is related to its properties; for example, biochar with a microporous structure, active surface functional groups, high pH, and cation exchange capacity is preferred for immobilization of heavy metals (Xie et al., 2015). Biochar with high porosity and plenty of liming and fertilizer-related elements (e.g., N, P, and K) is preferred for improving soil properties (Ding et al., 2016). The economic benefits also need to be considered in biochar industrial production, specifically related to biochar yield. Extensive researches have shown that pyrolysis temperature influence the physicochemical properties of biochar. Pyrolysis temperature is a major contributor to 4
carbon loss during the preparation of biochar (Shaaban et al., 2018), and has an important effect on biochar yield (Uchimiya et al., 2011). Biochar prepared at different pyrolysis temperatures has different degrees of carbonization and highly ordered aromatic structure forms when the temperature exceeds 400 ℃ (Kim et al., 2012). Pyrolysis temperature can also change the surface roughness, micromorphology, and porous structure of biochar (Gupta et al., 2019; Wang et al., 2019). Biochar pyrolyzed at different pyrolysis temperatures has different electrical conductivities (EC), pH, P, and N (Chan et al., 2008; Meier et al., 2017).Some studies have indicated strong relationships between the properties and feedstock types of biochar. Compared with wood char, the biochar derived from crop residues has a more developed pore structure because of the lower lignin content (Sun et al., 2014). Correlations between ash content and carbon content, pH, and K content have been reported (Windeatt et al., 2014).To date, studies which explore the properties of biochar produced from different crop residue feedstock are limited. Most studies only focused on the effects of pyrolysis temperature on one or two crop residue biochar and characterized the properties of biochar with a certain application (O'Toole et al., 2013; Cabeza et al., 2018; Purakayastha et al., 2015). The different pyrolysis parameters used in these studies also makes it difficult to compare the effects of pyrolysis temperature on different crop residue biochar. Therefore, there is a need to characterize different crop residue biochar under uniform process conditions in order to further understand how pyrolysis temperature influence the properties of crop residue biochar. The aim of this study was 5
to investigate the effects of different pyrolysis temperatures on the yield and physicochemical properties of biochar derived from various crop residues. This is of great importance for understanding the pyrolysis conversation mechanism of crop residue biochar and promoting the scientific preparation and utilization of crop residue biochar. 2.
Materials and Methods
2.1 Crop residue material The wheat straw and corn straw were supplied by the Shangzhuang experimental station of the China Agricultural University. Rape straw and rice straw were collected from Anhui and Guangxi, respectively. The four different types of straw collected did not contain roots, and needed air-drying. Thereafter, they were crushed using a high-speed grinder (RT-34, Taiwan RongCong Precison Technology Co., China) to pass through a 20-mesh sieve. Samples were sealed in self-sealed bags for later use. All straw samples were marked as CK (straw without pyrolysis treatment). 2.2 Biochar preparation 2.2.1
Determination of pyrolysis temperature The pre-experiment was carried out by a thermogravimetric analyzer (SDTQ600,
TA Instruments, USA) to determine the temperature range for pyrolysis. Experiments were set at a linear heating rate of 10 °C/min within the temperature range of 0–1000 °C and a steady nitrogen flow rate of 100 mL/min. The largest weight loss rate was observed at 300 °C, and weight loss rate slowed down at 600 °C. Therefore, the 6
temperature range of 300–600 °C was selected for pyrolysis. 2.2.2 Preparation of straw biochar The straw samples were dried at 60 °C for 48 h before pyrolysis. Pyrolysis experiments were performed in a tube furnace (GSL-1100X, Hefei Kejing Materials Technology Co. Ltd., China) with sample masses of about 10 g. Straw samples were fed into the quartz boats using precise weighing and pyrolyzed at 300, 400, 500, and 600 °C for 60 min under N2 flow (99.999%) at a flow rate of 100 mL/min. The carrier gas velocity was controlled by a D07-7B mass flow controller (Beijing Seven Star Electronics Co., Ltd., China) and pre-ventilation time was set for 15 min to clear the air in the tube furnace. The resulting chars were allowed to cool to ambient temperature while maintaining the N2 flow. Following pyrolysis, the total mass of quartz boats and biochar were accurately weighed. Biochar was stored in sample bottles for subsequent analysis. The pyrolysis experiment was repeated three times for each straw type. The yield of biochar was calculated using the following equation (Narzari et al., 2017): Biochar yield (%) = (𝑊𝑇 ― 𝑊𝐵)/𝑊0 × 100%
(1)
where WT was the total mass (g) of the quartz boat and biochar after pyrolysis, WB and W0 were the mass (g) of the quartz boat and straw samples prior to pyrolysis, respectively. The obtained biochar was denoted as WB (wheat straw biochar), CB (corn straw biochar), RIB (rice straw biochar), and RB (rape straw biochar). Biochar produced at different temperatures was abbreviated as XX300, XX400, XX500, and XX600 based 7
on the corresponding materials (XX) (e.g., WB300, standing for wheat straw biochar derived at 300 °C). 2.3 Characterization of biochar The ash content was analyzed using a fully automatic measuring industrial analyzer (YX-GYFX 7705B, U-Therm Instrument Manufacturing Co., Ltd., China). The experimental method was based on the preset procedure i.e., 0.5–0.6 g of sample was dried in a crucible at 105 °C to a constant weight, and then heated to 575 °C until the sample reached a constant weight again and the ash was obtained in the end. C, H, N, and S content of the biochar were determined using an elemental analyzer (Vario Macro Elementar, Germany), and O was calculated by the subtraction method (Narzari et al., 2017). EC and pH were measured in deionized water following 48 h of shaking (1:20, wt/v) using a conductivity meter (FE30, Mettler-Toledo Group, USA) and a pH meter (FE20, Mettler-Toledo Group, USA), respectively.All the above experiments were repeated in triplicate. Fourier transform infrared spectroscopy (FTIR; Spectrum 400, Perkin Elmer, USA) was used to characterize the functional groups of biochar with the KBr method (1:100, wt/wt). The biochar samples and KBr were dried at 105 °C for 8 h before the test. Tableting operation was performed under infrared lamp irradiation. All FTIR spectra were collected in the mid-infrared range from 4000 to 400 cm-1 using a 4 cm−1 spectral resolution and 64 accumulations for each collection by the array, and were corrected by a pure KBr pellet (Cao et al., 2019). 8
Biochar was characterized by a polycrystal diffractometer (XD3 X-ray, Persee Ltd., China) that could provide information on the structure or morphology of their internal atoms or molecules for studying the short-range ordered structure of biochar and the crystal structure of minerals (Azargohar et al., 2014). The measurements were conducted using Cu-Kα radiation operating at 40 kV and 30 mA, 2θ scale of 5–60° at a step size of 0.02°, and a scanning speed of 2°/min (Fidel et al., 2017; Uzi et al., 2019). The surface morphology of biochar was analyzed by scanning electron microscopy (SEM; SU3500, Hitachi Ltd., Japan). The straw biochar samples were fixed on black conductive adhesive and coated with Pt to enhance the conductivity of the samples. Images were captured at 1000 magnification operated in high vacuum mode with 15 kV accelerating voltage. 2.4 Statistical analysis The software SPSS (IBM SPSS statistics 25) was used to analyze variance (ANOVA) and correlation relationship (Pearson). Bonferroni post-hoc t-tests (1% significance level) (Mandal et al., 2018) were carried out to identify significant differences between different biochars, and a p value of <0.01 was considered statically significant. The fitting analysis of sample data was conducted by MATLAB (MathWorks, MATLAB 2016). Adjusted R-square (R2) and root mean square error (RMSE) were chosen to evaluate goodness of fit. When R2 tended to 1 and RMSE values were smaller (approaching 0), the fitting effect was considered to be better. The X-ray diffraction (XRD) patterns of the samples were obtained by MDI Jade 6.0 software, and 9
the FTIR of the samples was analyzed by OPUS 7.2. 3.
Results and Discussion
3.1 Effect of pyrolysis temperature on biochar yield The yields of the different straw biochars decreased markedly with the increase in temperature (Fig. 1). The maximum weight losses of straw biochars occurred in the range of 300–400 °C due to water removal and the decomposition of complex components (such as cellulose and hemicellulose) (He et al., 2018b; Liu et al., 2018). As the temperature exceeded 400 °C, the yield showed a more steady decrease. This change may be attributed to the most of volatile matter exhalation and lignin decomposition below 400 °C (Shen et al., 2019; Suliman et al., 2016). The effects of pyrolys-is temperature on the yields of different straw biochars were similar. Table 1 shows how the yield of each straw biochar decreased significantly with the increase in temperature (p < 0.01). For example, the yield of the rice straw biochar decreased from 51.36 to 32.75% when temperature increased from 300 to 600 °C. In addition, the yield of the straw biochar was related to the source of the straw feedstock. The yield of the rice straw biochar was significantly higher than that of the other three straw biochars (p < 0.01), while the rape straw biochar had the lowest yield among all straw biochars (He et al., 2018b). This may be due to the high ash content of the rice straw and the high volatile content of the rape straw. Biochars derived at 300 °C showed significant differences among the four straw biochars (p < 0.01). There was no significant difference in biochar yield between wheat straw and maize straw when 10
pyrolysis temperature exceeded 400 °C. In addition, the rice straw biochar lost its advantage in yield at 600 °C, which may indicate that the degree of carbonization of the rice straw biochar was relatively high at this pyrolysis temperature. For further exploring the effects of pyrolysis temperature on straw biochar yield further, the quantitative relationship between biochar yield and pyrolysis temperature was analyzed using the fitting curve. Figure 2 shows the non-linear decreasing relationship between the yields of straw biochars and pyrolysis temperature, which was similar to previous studies (Zhang et al., 2017; Zhao et al., 2018). The biochar yield was a function of pyrolysis temperature: Yield = -23.04 ln(x) + 176.8 (R2 = 0.85, RMSE = 2.57). According to the equation, the yields of straw biochars would show a more steady decrease as the pyrolysis temperature continued increasing. 3.2 Effect of pyrolysis temperature on biochar physicochemical properties 3.2.1 Surface morphology The morphology of biochars changed at different temperatures. The physical structure of the wheat straw biochar appeared to show no obvious damage at a pyrolysis temperature of 300 °C. The surface of the wheat straw biochar produced at 400 °C was in a molten state, whereas it appeared cracked and collapsed as temperatures increased from 400 to 600 °C, resulting in the destruction of its pore structure. The surface roughness of the corn straw biochar increased significantly with the increase in temperature (Luo et al., 2018), and its surface structure was seriously damaged and formed an obvious flaky structure when the pyrolysis temperature exceeded 500 °C. 11
The rape biochar derived at 300 °C showed a few pores, and pore volume increased significantly at 400 °C, but the surface structure of the biochar melted and deformed at temperatures over 500 °C. High pyrolysis temperatures seriously damaged the surface and pore structure of straw biochar. The rice straw biochar showed a smooth surface and tubular structure below 400 °C; however, surface deformation, increases in pore quantity, and decreases in pore volume were apparent above this temperature, which may be due to the collapse of the internal pore structure (Fu et al., 2012). 3.2.2 Chemical characteristics Table 2 provides the chemical characteristics of the straw biochars and straw feedstock. The C content of the four straw biochars increased significantly to 56.49–67.85 wt% (p < 0.01) compared with the feedstock (42.12–45.53 wt%), indicating that straw biochars had good carbon sequestration ability. The H and O content in the straw biochars decreased due to pyrolysis. Although the N content of the four straw biochars was higher than that of the raw materials, agronomic application of biochar may be needed to supply additional nitrogen. The elemental composition of the straw biochars varied significantly (p < 0.01). The C content in the wheat straw biochar derived at varied temperatures showed significant differences (p < 0.01), and decreased after the first increase in temperature. In contrast, the C content in the other three straw biochars revealed an increasing trend with increases in temperature. The secondary cracking of C in the wheat straw biochar may lead to the decrease in C content (He et al., 2018b). The H content in all the four 12
straw biochars decreased significantly at 300 to 600 °C (p < 0.01), and this trend was consistent with the O content in the straw biochars. The decrease in H and O may be related to the loss of volatile matter. The N content of the wheat straw biochar and corn straw biochar decreased significantly at 300–400 °C and 500–600 °C (p < 0.01), respectively, but was steady in both biochars at 400–500 °C. This indicated that a stable C-N heterocycle might form at 400 °C and crack after 500 °C (He et al., 2018b). The N content of rape straw and rice straw biochar derived at different pyrolysis temperatures did not change significantly (p < 0.01). In addition, the pyrolysis temperature had no significant effect on the S content of straw biochars, which may be due to the relatively stable S functional groups in straw biochars at 300–600 °C. There was a significant difference in N content except C, H, and S (p < 0.01). The content sequence of N in the various straw raw materials was as follows: corn straw > rice straw > wheat straw > rape straw. The ash content of each straw biochar was higher than that of the corresponding raw material, and there were significant differences between the rice straw biochar and other three straw biochars (p < 0.01, Table 2). The ash content in the rice straw raw material was significantly higher than that in the other three types of straw materials (p < 0.01). This indicated that the ash content of straw biochars correlated with the composition of their raw materials. Table 1 shows the significant effect of pyrolysis temperature on the ash content of straw biochars (p < 0.01). The ash content of the four straw biochars increased with the increase in pyrolysis temperature, indicating the 13
formation and accumulation of mineral elements in the straw biochars during pyrolysis (Waqas et al., 2018). As shown in Table 2, the ash content of all straw biochars increased sharply between 300–500 °C, followed by slight increases when the pyrolysis temperature exceeded 500 °C. This agrees with He et al. (2018b) who found that ash content may change significantly within 300–450 °C, and show less change over 450 °C. Compared with the straw raw materials, the biochars were alkaline and suitable for soil acidity improvements. The pH of each straw biochar increased with the increase in pyrolysis temperature, and there were significant differences in biochars derived at different temperatures (p < 0.01). The pH values of the biochars (7.75 to 10.83) were slightly higher than a previous study (6.3–10.6) (Liu et al., 2018), which may be related to the species of straw used. At the same pyrolysis temperature, the pH values of the rape straw biochar were significantly lower than the other three straw biochars (p < 0.01). In general, temperature can influence the pH by affecting the ash content of alkaline minerals (e.g., Na and K) and acidic functional groups such as phenol and carboxyl in biochar (Zhao et al., 2018). Table 2 shows that the higher relative temperature (over 500 °C) had a slower increasing rate of pH; this could be attributed to the prominent changes in acidic functional groups and inorganic alkaline substances below and above 500 °C, respectively (Liu et al., 2014). The EC of straw biochars was significantly higher than that of the corresponding raw materials, and showed a significant increasing trend with increasing temperatures (p 14
< 0.01), which were similar to the results obtained in other studies (Kloss et al., 2012; Mukherjee et al., 2011). The increase in EC could be attributed to the increase in soluble salt content and the decrease in acidic functional groups in straw biochar as temperature rises (Azargohar et al., 2014; Narzari et al., 2017). The EC value can be used as a basis for the evaluation of the salt content and soluble alkaline cations content of biochar (Irfan et al., 2016). Table 1 shows that there were significant differences between the rape straw biochar and other three straw biochars (p < 0.01). The wheat straw and rice straw biochars had the lowest EC, whereas the rape straw biochar had the highest EC (7.44–10.65 mS/cm). This could be attributed to a higher content of K and Na in rapeseed straw biochar (Liu et al., 2018). Higher salinity and soluble cations have negative effects on plant growth and soil organic matter (Buss et al., 2016). It can be inferred that rape straw biochar should be considered for soil amendment due to soil salinization. 3.2.3 Constitution structure The ratios of H/C and O/C can indicate the aromaticity of biochar to a certain extent, and the ratios of O/C and (O+N)/C can indicate the polarity of biochar (Cantrell et al., 2012). In the present study, the H/C and O/C of the corn straw biochar, rape straw biochar, and rice straw biochar decreased with increases in pyrolysis temperature, and the ratios of H/C and O/C in straw biochars pyrolyzed at 300–600 °C were less than 0.6 and 0.4, respectively (Zhao et al., 2018). This indicated that the aromatic structure forms at increasing temperatures due to the decomposition of cellulose and other 15
macromolecules in biochar (Cao et al., 2013). The ratio of H/C decreased faster than that of O/C with increasing temperatures, indicating that H is more easily lost at lower pyrolysis temperatures than O. In addition, as the pyrolysis temperature increased, the O/C and (O+N)/C of the different straw biochars decreased. This indicated that straw biochars prepared at higher pyrolysis temperatures have less polar functional groups and stronger hydrophobicity (Wang et al., 2016). Furthermore, there were small differences in O/C and (O+N)/C of the straw biochars, while there were large differences in the H/C. It is apparent that the different characteristics of the straw biochars may be related to the change in H during pyrolysis. Changes in the FTIR spectrum can reflect the effects of pyrolysis temperature on the functional groups. The decrease in the O-H stretching vibration (3500–3200 cm-1) showed that the bound water of straw biochars disappeared when the pyrolysis temperature exceeded 500 °C (Cantrell et al., 2012; Keiluweit et al., 2010). The absorption peak of straw biochars at 2926 cm-1 revealed the presence of C-H stretching vibration in the aliphatic group (2930–2850 cm-1), but this was lost at temperatures over 500 °C (Mandal et al., 2018). This indicated a decrease in the aliphatic group and the formation of a dense ring structure in straw biochars (Wang et al., 2013). In the present study, the C-H stretching vibration (2919 cm-1) of the wheat straw biochar was observed at 300 °C and 400 °C, indicating the existence of cellulose and hemicellulose in the wheat straw biochar (Waqas et al., 2018) in this pyrolysis temperature range. With the increase in pyrolysis temperature, the absorption peak strength of the straw biochar in 16
the range of 1600–1580 cm-1 (aromatic ring C=C stretching vibration and conjugated ketone and quinone C=O stretching vibration) decreased significantly (Wang et al., 2013). Concurrently, bands related to C=C stretching vibration in lignin (1510–1512 cm-1) and C-H stretching vibration in cellulose and other biopolymers (1455–1439 cm-1 and 1376–1382 cm-1) diminished at temperatures over 400 °C (Shen et al., 2019; Wang et al., 2013). Thus, it was apparent in the present study that a higher pyrolysis temperature may lead to the decomposition of polymers in the biochars. In addition, the four straw biochars derived at 600 °C lost most of the absorption peaks, except for bands in 680–850 cm-1, which was related to the monocyclic and polycyclic aromatic structures (Azargohar et al., 2014). This may imply that biochars derived at 600 °C have high stability. According to XRD spectra analysis, similarities were noted among the four straw raw materials. The peaks at 16° (2θ) and 22° (2θ) in the raw materials indicated the existence of a cellulose crystal structure (Keiluweit et al., 2010). Although all the straw biochars had similar patterns, there were also some differences between them. The wheat straw biochar and rice straw biochar showed a loss of peak intensity at 16° (2θ) and a gain of peak intensity at 22° (2θ) with an increase in pyrolysis temperature. This indicated that the crystalline cellulose of the wheat straw biochar and rice straw biochar had higher thermal stability than that of the other two biochars. The XRD patterns revealed that there were some potassium salts (2θ = 28° and 2θ = 40°) in all the straw biochars (Gao et al., 2018) and no significant differences were observed with the 17
changes in temperature. When the temperature exceeded 300 °C, the characteristic peak at 26.6° of the wheat straw biochar and corn straw biochar were attributed to SiO2 (Zhang et al., 2015), and the characteristic peak at around 29.6° of the rape straw biochar and rice straw biochar were attributed to calcite (He et al., 2018a). That peak intensity increased with increases in temperature, suggesting that increasing pyrolysis temperature leads to the formation of a large and orderly crystal structure (Zhao et al., 2017). In addition, the peak at around 30° of the rape straw biochar and corn straw biochar showed the presence of calcium and magnesium. 3.3 Correlation between yield and physicochemical properties The correlation analysis between biochar yield and physicochemical properties of all straw biochars is tabulated (Table 3). Figure 3 shows the quantitative relationships between biomass yield and physicochemical properties of the straw. There was a significant correlation between the yields of straw biochars and C (p < 0.01), and the yield decreased linearly with decreases in C (He et al., 2018b). There were positive correlations between the yield of the straw biochars and H, O (p < 0.01). Compared with C content, H and O provided a better linear fitting degree with the yield of straw biochars (Fig. 3). This indicated that the reduction in biochar yield was caused by the thermal decomposition of organic compounds containing H and O. The biochar yield was negatively linearly correlation with pH and EC. When the yield of straw biochars was less than 30%, their pH showed less change with the same change in temperature. According to the relationship between pyrolysis temperature and biochar yield, the 18
range of temperatures in which the pH of the straw biochar showed considerable change was below 500 °C, which was consistent with previous results (Zhang et al., 2017). The yield of straw biochars was significantly correlated with O/C, H/C, and (O+N)/C (p < 0.01), revealing a good linear relationship. The low yield of straw biochar with strong aromaticity and hydrophobicity may be related to the formation of a stable graphitized crystal structure (Jindo et al., 2014) and the precipitation of water-soluble substances. The quantitative relationships between the different biochar properties were further studied. There were significant linear correlations between C and (O+N)/C, EC (p < 0.01), respectively. The H and O had a highly significant positive correlation (p < 0.01). Both H and O had strong positive correlations with O/C, H/C, and (O+N)/C, and had strong negative correlations with pH and ash (p < 0.01). In addition, H and O showed strong positive linear correlations with O/C, H/C, and (O+N)/C, indicating that the precipitation of H and O in the chemical bonds of the straw samples during pyrolysis had significant impacts on the increase in aromaticity and hydrophobicity in the straw biochars. Ash also had a significant positive correlation with pH (p < 0.01), but the linear fitting degree was lower than that of H and O. This showed that the loss of functional groups containing H and O played an import role in enhancing biochar basicity at pyrolysis temperatures of 300–600 °C. H/C and O/C showed strong negative correlations with pH (p < 0.01), which indicated that the alkalinity of the straw biochar was related to the formation of the aromatic structure during pyrolysis (Liu et al., 2018). In addition, there were significant negative correlations between N and EC, ash and H/C, 19
O+N/C, and EC and (O+N)/C (p < 0.01), while EC had a significant negative correlation with O/C and H/C (p < 0.05). 4.
Conclusions Biochar yields decreased with increasing temperature, but tended to reduce slowly
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Figures captions Fig. 1 Yields of straw biochars pyrolysed at 300 °C, 400 °C, 500 °C, and 600 °C. Fig. 2 Effects of pyrolysis temperature on the yields of straw biochars. Fig. 3 Regression analysis of biochar yield and physicochemical properties
29
TABLE AND FIGURES
Table 1 Yields of straw biochars. Different letters a,b, c, and d (A, B, C, and D)in the same column (line) show statistical differences (p<0.01). Pyrolysis temperature (℃)
Wheat straw
Corn straw
Rape straw
Rice straw
300
46.96±0.25dC
45.84±0.17dB
44.32±0.15dA
51.36±0.06dD
400
35.76±0.11cB
35.85±0.24cB
34.48±0.20cA
38.56±0.05cC
500
32.49±0.18bB
33.07±0.23bB
31.17±0.20bA
34.42±0.11bC
600
31.55±0.28aBC
30.87±0.12aB
29.27±0.07aA
32.75±0.08aC
30
Table 2 Chemical properties of all straw biochars. Different letters a,b, c, and d (A, B, C, and D)in the same column (line) show statistical differences (p<0.01).
31
Pyrolysis temperature (℃)
Wheat straw
Corn straw
Rape straw
Rice straw
C(wt%)
CK
45.53±0.19aC
44.53±0.31aB
44.63±0.19aB
42.12±0.10aA
300
61.48±0.43bB
61.20±0.43bB
61.80±0.18bB
56.49±0.20bA
400
64.18±0.37cB
63.36±0.70cB
63.74±0.45cB
56.42±0.07bA
500
67.39±0.32eC
65.08±0.59cB
66.96±0.33dC
59.59±0.49cA
600
65.34±0.53dB
67.48±0.64dBC
67.85±0.72dC
61.30±0.29dA
CK
3.56±0.25eA
5.31±0.20eB
4.89±0.05eB
4.16±0.11eA
300
2.73±0.07dA
3.68±0.09dB
3.54±0.20dB
2.95±0.19dA
400
1.78±0.03cAB
1.96±0.02cB
1.91±0.20cB
1.35±0.10cA
500
1.01±0.06bC
0.77±0.04bBC
0.87±0.05bBC
0.47±0.02bA
600
0.52±0.03aB
0.18±0.01aA
0.18±0.02aA
0.12±0.02aA
CK
0.88±0.02aA
1.69±0.03aC
0.64±0.10aA
1.24±0.08aB
300
1.40±0.06cB
2.93±0.06dD
1.02±0.07bA
2.15±0.06bC
400
1.36±0.08bcB
2.52±0.07cD
0.98±0.04bA
1.99±0.06bC
500
1.38±0.14bcB
2.43±0.04cD
0.88±0.11bA
1.90±0.05bC
600
1.10±0.01abA
2.12±0.01bB
0.90±0.10bA
2.01±0.11bB
CK
0.36±0.05aA
0.71±0.11cB
0.55±0.03aAB
0.75±0.12bB
300
0.39±0.04aA
0.54±0.02bcAB
0.59±0.04aB
0.45±0.08aAB
400
0.47±0.03aA
0.51±0.02bA
0.68±0.03aB
0.44±0.01aA
500
0.48±0.02aA
0.39±0.01abB
0.64±0.04aC
0.34±0.02aB
600
0.45±0.01aB
0.24±0.02aA
0.64±0.07aC
0.29±0.02aA
CK
42.53
41.18
42.34
41.22
300
19.61
17.39
17.95
17.73
400
13.93
13.46
13.48
13.71
500
7.35
11.36
9.46
8.27
600
10.77
8.98
7.89
5.71
CK
7.14±0.15aB
6.58±0.06aA
6.95±0.15aAB
10.51±0.01aC
300
14.39±0.53bA
14.26±0.06bA
15.10±0.06bA
20.23±0.12bB
400
18.28±0.32cA
18.19±0.09cA
19.21±0.05cB
26.09±0.30cC
500
22.39±0.32dC
19.97±0.12dA
21.19±0.10dB
29.43±0.36dD
600
21.82±0.29dAB
21.00±0.19eA
22.54±0.02eB
30.57±0.32eC
CK
5.49±0.07aB
5.33±0.02aB
6.15±0.01aC
4.22±0.09aA
300
7.98±0.02bB
8.28±0.02bC
7.75±0.03bA
8.29±0.04bC
400
9.06±0.02cA
9.83±0.02cD
9.18±0.03cB
9.33±0.02cC
500
10.37±0.01dC
10.22±0.01dB
9.95±0.02dA
10.23±0.04dB
600
10.83±0.03eB
10.26±0.01dA
10.28±0.03eA
10.75±0.03eB
CK
3.48±0.07aB
4.59±0.14aC
2.47±0.07aA
3.28±0.07aB
300
4.12±0.04bA
5.00±0.18aB
7.44±0.02bC
3.88±0.11aA
400
5.16±0.10cA
5.65±0.09bA
8.70±0.05cB
5.25±0.24bA
500
6.53±0.08dA
7.00±0.09cB
9.74±0.01dB
6.29±0.20cA
600
6.91±0.11eA
7.67±0.06dB
10.65±0.29eC
6.67±0.22cA
H (wt%)
N (wt%)
S (wt%)
O1 (wt%)
Ash (wt%)
pH
EC (mS/cm)
Note: 1O % = 100% - (C % - N % + H % + S % + Ash %)
32
Table 3 The correlation analysis between biochar yield and physicochemical properties Yield
C
H
N
S
O
pH
Yield
1.00
C
-0.71A
1.00
H
0.88A
-0.47 1.00
N
0.30
-0.40 0.18
1.00
S
0.03
0.21
-0.50a 1.00
O
0.88A
-0.51a 0.92A
0.15
0.23
1.00
pH
-0.89A
0.50a
-0.05
-0.29
-0.93A 1.00
EC
-0.71A
0.69A -0.52a
Ash
-0.51a
-0.14
-0.75A
O/C
0.92A
-0.61a
0.91A
H/C
0.91A
-0.52a
1.00A
(O+N)/C
0.93A
-0.66A 0.90A
0.35
-0.94A
EC
Ash
O/C
H/C
(O+N)/C
-0.57a 0.46
-0.57a 0.46
1.00
0.01
-0.41
-0.77A
0.70A
0.19
1.00
0.19
0.99A
-0.93A
-0.62a
-0.69A
1.00
0.20
0.33
0.92A
-0.94A
-0.54a
-0.73A
0.92A 1.00
0.34
0.10
0.97A -0.90A -0.69A
-0.65A
0.99A 0.92A 1.00
0.19
a
Correlation is significant at the 0.05 level (2-tailed).
A
Correlation is significant at the 0.01 level (2-tailed).
33
60
Wheat straw
Corn straw
Rape straw
Rice straw
Yield (%)
45
30
15
0
300
400 500 Temperature (℃)
600
Fig. 1 Yields of straw biochars pyrolysed at 300 ℃, 400 ℃, 500 ℃, and 600 ℃.
34
Fig.2 Effects of pyrolysis temperature on the yields of straw biochars.
35
Fig.3 Regression analysis of biochar yield and physicochemical properties
36
Declaration of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the manuscript entitled “Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar”.
37
Highlights
Pyrolysis temperature had similar effects on four different crop residues biochar
The maximum losses rate in biochar was observed below 400 °C
Rice straw biochars have high yield superiority
Higher temperature biochars had high stability and an orderly crystal structure
38