Effect of the carbonization temperature on the properties of biochar produced from the pyrolysis of crop residues

Abstract

Biochar, a carbon-rich product, can be obtained from crop residues via pyrolysis. Its properties may vary widely depending upon the pyrolysis conditions and feedstock type. Physicochemical properties were studied for biochars produced from rice straw, wheat straw, corn stover, rape stalk, and cotton stalk pyrolyzed at 300 °C to 700 °C. At higher pyrolysis temperatures, the carbon content, pH, and electrical conductivity of the biochars slightly increased, while the O/C and H/C ratios decreased. The pH values had a strong negative linear correlation with the H/C ratio. Higher carbonization temperatures resulted in larger pores and increased the aromatic/aliphatic carbon ratio in the biochars. The oxygen functional groups in the biochars, such as -COOH and -OH, decreased with an increasing carbonization temperature. The combustion performance of the biochars varied with the carbonization temperature because of the differences in the physicochemical compositions of the biochars. Additionally, the crop residue types also influenced the physicochemical properties. The cotton stalk biochar had the highest fixed carbon content and lowest H/C ratio, and thus can be used as a solid biofuel. The rice straw biochar, which had the highest N and O contents, may be a potential soil amendment.

Full Article

Effect of the Carbonization Temperature on the Properties of Biochar Produced from the Pyrolysis of Crop Residues

Zhaoxia Liu,^1,a,b Wenjuan Niu,^1,a,* Heying Chu,^b Tan Zhou,^a and Zhiyou Niu ^a,*

Biochar, a carbon-rich product, can be obtained from crop residues via pyrolysis. Its properties may vary widely depending upon the pyrolysis conditions and feedstock type. Physicochemical properties were studied for biochars produced from rice straw, wheat straw, corn stover, rape stalk, and cotton stalk pyrolyzed at 300 °C to 700 °C. At higher pyrolysis temperatures, the carbon content, pH, and electrical conductivity of the biochars slightly increased, while the O/C and H/C ratios decreased. The pH values had a strong negative linear correlation with the H/C ratio. Higher carbonization temperatures resulted in larger pores and increased the aromatic/aliphatic carbon ratio in the biochars. The oxygen functional groups in the biochars, such as -COOH and -OH, decreased with an increasing carbonization temperature. The combustion performance of the biochars varied with the carbonization temperature because of the differences in the physicochemical compositions of the biochars. Additionally, the crop residue types also influenced the physicochemical properties. The cotton stalk biochar had the highest fixed carbon content and lowest H/C ratio, and thus can be used as a solid biofuel. The rice straw biochar, which had the highest N and O contents, may be a potential soil amendment.

Keywords: Biochar; Crop residues; Property; Carbonization temperature

Contact information: a: Key Laboratory of Agricultural Equipment in Mid-lower Yangtze River, Ministry of Agriculture, College of Engineering, Huazhong Agricultural University, 430070 Wuhan, P. R. China; b: College of Mechanical and Electronic Engineering, Tarim University, 843300, Alar, P. R. China; *Corresponding authors: nzhy@mail.hzau.edu.cn; niuwenjuan234@mail.hzau.edu.cn; 1: These authors contributed equally to this work

INTRODUCTION

Pyrolysis technology is an effective way to utilize crop residues and solve environmental pollution problems caused by the burning of crop residues in the field. Additionally, pyrolysis products can be used as alternative renewable energy sources to fossil fuels (Clare et al. 2015). In 2010, a total of 842 million metric tons of crop residues were produced in China, which mainly included rice straw, wheat straw, corn stover, rape stalk, and cotton stalk (Peng et al. 2014). Crop residues may be burnt in the fields or directly incorporated into soils by farmers to increase the organic carbon content (Villamil et al. 2015). The burning of crop residues is a major contributor to carbon dioxide emissions (Murali et al. 2010). The most efficient way to solve this problem may be the conversion of crop residues into biochars by pyrolysis and carbonization. The solid pyrolysis products of crop residues are biochars, which contain char, ash, and unchanged biomass materials. Biochar can be used as a fuel, for carbon sequestration, as activated carbon, and soil amendment to reduce greenhouse gas emissions (Lehmann et al. 2006; Wu et al. 2013). The pore structure and contents of nitrogen (N), phosphorus (P), and potassium (K) in biochar make it an effective material to improve soil quality (Sohi et al. 2009).

The physicochemical properties of biochars vary with the pyrolysis conditions and type of crop residue. Among the pyrolysis parameters, the carbonization temperature is thought to remarkably influence the final properties of the biochar because of the release of volatile compounds and decomposition of organic compounds (Zhang et al. 2015b). It has been previously reported that for wheat straw, the biochar yield, surface oxygen (O)-containing groups, and contents of hydrogen (H), N, and O decreased with an increasing carbonization temperature (Zhang et al. 2015a). Biochars have been generated from manure, crop straw, nutshell, wood, and grass under various experimental conditions (Cantrell et al. 2012; Jeong et al. 2016). Biochars obtained from manures have a high content of ash because of their high levels of inorganic compounds, while wood and grass biochars have a low nutrient content, are carbon-rich, and have a higher rate of CO₂ adsorption (Cantrell et al. 2012; Ghani et al. 2013). Crop residues are composed of multiple components, such as cellulose, lignin, and hemicellulose, and the properties of the resulting biochars can vary remarkably with the type of crop residue. Therefore, it is highly necessary to characterize the physical and chemical properties of biochars made from various crop residues.

The physicochemical characteristics of crop residue biochars have been studied by a few researchers (Fu et al. 2012; Wu et al. 2013), but only a couple crop residues and their pyrolysis products have been studied under various conditions (Lee et al. 2010; Zhang et al. 2015b). Therefore, it is important to clarify the properties of biochars produced from multiple crop residues under similar pyrolysis conditions. This study investigated the effects of the carbonization temperature and crop residue type on the physicochemical properties of the final biochars. The results are expected to facilitate the production of biochars from crop residues with higher additional values, and lead to the effective and proper utilization of crop residues for different purposes.

EXPERIMENTAL

Biochar Preparation

Rice straw, wheat straw, corn stover, rape stalk, and cotton stalk were collected from Wuhan, Hubei Province in China. The collected crop residue samples were processed according to ASTM E1757-01 (2015). The samples were dried in a forced-air drying oven at 45 ℃ for 48 h, and then fed into a ZM100 mill (Retsch GmbH & Company, Haan, Germany) and milled to pass through a 0.425-mm sieve. The samples were stored in bags prior to the pyrolysis analysis. Cellulose, hemicellulose, and lignin were measured according to the National Renewable Energy Laboratory method (Sluiter et al. 2010). The basic compositions of the crop residues are shown in Table 1. The contents of cellulose, hemicellulose, and lignin in crop residues are 36.04% to 41.69%, 11.48% to 14.41%, and 18.46% to 27.35%, respectively. The cellulose content is the highest in rice straw and the lowest in corn stover. Cotton stalk contains the highest lignin and the lowest hemicellulose.

Approximately 30.0 g of crop residue were loaded into a square crucible (120 mm × 80 mm × 40 mm) and then placed into a tube furnace. Nitrogen was passed through the furnace for 30 min to remove the air and provide an inert gas atmosphere for pyrolysis. The crop residues were pyrolyzed at 300 °C, 350 C, 400 C, 450 C, 500 C, 550 C, 600 C, 650 C, and 700 C. The heating rate was 10 C/min and the holding time was 30 min. Then, the crucible with the product was naturally cooled to room temperature inside of the furnace. The biochar was immediately weighed after it was removed from the furnace. Each experiment was performed in duplicate.

Table 1. Chemical Composition of the Five Crop Residues

VM – volatile matter; FC – fixed carbon; H/C – ratio of H to C; O/C – ratio of O to C

Biochar Characteristics

Biochar yield

A number of physicochemical properties of the obtained biochars were determined. Each resulting biochar was characterized for its basic properties, including the pH, electrical conductivity (EC), proximate composition, and elemental composition. The surface functional groups and porosities were investigated using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS).

The biochar yield (dry basis, %) was calculated using Eq. 1,

 (1)

where m_a refers to the weight (g) of the sample after pyrolysis and m_bis the weight (g) of the sample before pyrolysis.

Proximate and ultimate analyses

The proximate analysis was conducted according to ASTM D1762-84 (2001). The ash content in the biochar samples was determined by the combustion of the biochar at 750 C to a constant weight for 6 h. The volatile matters were analyzed at 900 C after 6 min of combustion in a muffle furnace. The fixed carbon content in the biochar was calculated with Eq. 2,

 (2)

The carbon (C), H, N, sulfur (S), and O contents were determined by an element analyzer (EA3000, EuroVector, Milan, Italy) viacombustion at 980 C using the CHNS and O methods.

pH, electrical conductivity and metals

The pH and EC of the biochars were measured with a suspension, which contained 0.40 g of dry biochar sample dispersed in 8 mL of deionized water, using a shaking table at a constant temperature of 25 C for 24 h. Metal analysis was performed for K, Na. The analysis was performed by using wet acid digestion (conc. H₂SO₄ + 30% H₂O₂) and determined by AA-6300C atomic absorption spectrometry (Shimadzu, Kyoto, Japan).

Fourier transform infrared spectra

The FTIR spectra of the biochars were obtained by a Vertex 70 FTIR spectrometer (Bruker, Ettlingen, Germany). All of the samples were dried at 105 C for 24 h before being homogenized into a fine powder using a ball mill. Then, the biochars were mixed with KBr at a fixed ratio to fabricate translucent discs.

Scanning electron micrographs and X-ray photoelectron spectra

The surface morphologies of the biochars were investigated using field-emission SEM (Nova NanoSEM 450, Eindhoven, Netherlands).The XPS spectra were obtained with an AXIS-Ultra DLD-600W spectrometer (Kratos Analytical Ltd, Shimadzu, Kyoto, Japan) using 1000 eV to 1500 eV of X-ray radiation as the XPS excitation source. The binding energy for C1s was assigned to 284.9 eV. The relative contents of C, O, and N on the surface of biochars were calculated from XPS data.

Thermogravimetric analysis

A thermogravimetric analyzer (SDT Q600 thermal analyzer, TA Instruments, New Castle, USA) was used to analyze the combustion characteristics. A 10-mg sample was accurately weighed in an alumina crucible and placed into the SDT Q600 analyzer with dry air as the carrier gas at a flow rate of 100 mL/min. The temperature was increased from room temperature to 900 C at a rate of 20 C/min. All of the analyses were conducted in triplicate. Meanwhile, the pyrolysis process of cellulose, hemicellulose and lignin in nitrogen atmosphere was analyzed. The carrier gas was high purity nitrogen (99.999 %), and the flow rate was 100 mL/min. The pyrolysis temperatures were from room temperature to 900 C.

RESULTS AND DICUSSION

Biochar Yields

The biochar yields of the five crop residues ranged from 30.51% to 75.66% (Fig. 1). For all of the crop residues, the biochar yield decreased sharply when the temperature was increased from 300 C to 400 C, and it decreased more slowly at temperatures of 450 C and above. From the TG-DTG curves of cellulose, hemicellulose, and lignin, the hemicellulose was decomposed from 200 C to 300 C. Cellulose was decomposed from 300 C to 380 C. The decomposition of lignin occurred from 200 C to 500 C (Fig. 1 c). The crop residues were decomposed, and some vapors, including CO₂, CO, H₂, CH₄, and C_nH_m, were released with an increase in the temperature, and also the yields of these gases increased with the increase of carbonization temperature (Antal 2003; He et al. 2017). The biochar yields at 400 C were approximately half of those obtained at 300 C.

Fig. 1. Biochar yields of dry basis (a) and dry ash free basis (b) at different carbonization temperatures, and TG-DTG curves (c) of cellulose, hemicellulose, and lignin

Some variations were observed in the biochar yields (dry basis) among different crop residues. The biochar yields may have varied depending on the different contents of cellulose, hemicellulose, lignin, and inorganic mineral components in the different crop residues (Niu et al. 2016). The biochar yield of the cotton stalk was highest at 300 C, which may have been because of the relatively higher lignification degree of the cotton stalk (Table. 1). The pyrolysis products of the lignin contained more residual solid char because of the large number of benzene rings in the lignin structure and a great number of benzene-containing radicals that were generated from pyrolysis further developed into polycyclic aromatic compounds and formed the char (Chen and Cai 2009). From 400 C to 550 C, the cotton stalk and rice straw had slightly higher biochar yields, while the rape stalk had the lowest biochar yield. However, the biochar yields (dry ash free basis) of the cotton stalk were the highest, while the corn stover biochar yields were the lowest. Interestingly, at 600 C to 700 C, the rice straw had the highest biochar yield (dry basis), but the differences in the biochar yields (dry ash free basis) at 600 C to 700 C were small. The highest yields (dry basis) of the rice straw biochars at the temperatures over 600 C were mainly because of the accumulation of inorganic mineral components.

Biochar Characterization

Proximate analysis

With an increasing carbonization temperature, the volatile matter contents of the biochars gradually decreased, while the ash content of the biochars increased (Table 2). Consistent with the volatile matter in the raw crop residues, the volatile matters in the corn stover and cotton stalk biochars were higher than those in the wheat straw and rice straw biochars. The rape stalk biochar had the highest volatile matter content (Tables 1 and 2). The highest ash content was observed in the rice straw biochar, which ranged from 13.77% at 300 C to 37.91% at 700 C. This may have been caused by the compositional changes that resulted from the interaction between the organic and inorganic components during rice straw pyrolysis (Tables 1 and 2) (Jindo et al. 2014). The lowest ash content was observed in the rape stalk biochar. The fixed carbon contents in this study ranged from 47.51% to 71.69% (Table 2). Generally, the fixed carbon contents of the biochars increased when the temperature was increased from 300 C to 600 C, and the biochars with a lower ash content had a higher fixed carbon content. The contents of fixed carbon between 600 C and 700 C were slightly decreased, and this may be caused by the increase of crystallinity and content of calcite in the biochars produced at 700 C (Yuan et al. 2011). The fixed carbon content in the rice stalk biochar was remarkably lower than that in the other biochars, which may have been because of its higher ash content (Table 2).

Table 2. Proximate and Ultimate Analyses of the Biochars

Rice STB, rice straw biochar; Wheat STB, wheat straw biochar; Corn STB, corn stover biochar; Rape STB, rape stalk biochar; Cotton STB, cotton stalk biochar. The values following the biochar labels denote the temperature at which the material was pyrolyzed

Ultimate analysis

An ultimate analysis of the biochars was performed, and the results are shown in Table 2. The pyrolysis resulted in an increase in the C and N contents compared with the corresponding crop residues (Tables 1 and 2). The C content in the biochars increased with an increasing carbonization temperature, while the N content decreased and the C/N ratios ranged from 20 to 200. The H and O contents decreased with an increasing carbonization temperature. The loss of H and O in the biochars was mainly attributed to the cleavage of oxygenated bonds during the heating process (Fu et al. 2012). The S contents in the wheat straw, rape stalk, and cotton stalk biochars increased noticeably with an increasing carbonization temperature, but those in the rice straw and corn stover biochars decreased. The decrease of S content in the biochars at 700 C was thought to be due to the decomposition or evaporation of alkali metal sulfates (Lith et al. 2006, 2008).

The H/C of the biomass without burning such as cellulose and lignin was about 1.5, while the H/C of black carbon was lower than 0.2. The H/C of the biochar prepared at the temperature above 400 C was ≤0.5 (Skjemstad and Graetz 2003). The decrease in H/C and O/C ratio was indicative of the formation of structures containing condensed carbons such as the aromatic rings. So the H/C and O/C was usually used to determine the degree of aromatics of biochars (Wu et al.2012). The H/C and O/C molar ratios of different biochars are shown in Table 2. Regardless of the type of biochar, the H/C and O/C molar ratios had similar values. Both the H/C and O/C values for all of the biochar types were slightly lower than those of the crop residues. The H/C and O/C ratios decreased with an increasing carbonization temperature. The decrease in the ratios was possibly caused by the loss of volatile organic compounds and an increase in dehydrogenation and deoxygenation reactions over the course of heating (Xiao et al. 2016). The molar O/C ratio of biochar could partially indicate its surface hydrophilicity (Chun et al. 2004). The lower O/C ratios of the biochars at higher carbonization temperatures implied higher hydrophobicity of the biochars.

By comparing the biochars from different crop residues, it was found that the rape stalk biochar had the highest C, H, and S contents, and the rice straw biochar had the highest N and O contents. The H/C ratio was the lowest in the cotton stalk biochar and the highest ratio was in the wheat straw biochar for the carbonization temperature of 300 C to 600 C. The H/C atomic ratios seemed to be stable at 600 C and 700 C in the rice straw, corn stover, and rape stalk biochars, whereas slight increases were observed in the wheat straw and cotton stalk biochars for the same conditions.

pH and electrical conductivity analysis

Figure 2 shows the pH and EC of the biochars. The pH values of the biochars in water ranged from slightly acidic (6.3) to alkaline (10.6). The pH increased considerably with the carbonization temperature from 300 C to 550 C, and then it slowly increased when the carbonization temperature was over 550 C.

Fig. 2. Impact of the pyrolysis temperature on the pH (a) and EC (b) of the biochars; and relationship between the H/C ratio and pH (c)

These changes might be associated with the dehydration of the crop residues and a progressive loss of acidic surface groups during thermal treatment.

The biochars produced at higher carbonization temperatures with a lower H/C ratio had a relatively higher pH value, which indicated that the pH of the biochar was related to the carbonization degree. The lower H/C ratio of 0.1 for lignite coke was in accordance with the dominance of graphite-like domains, and the lower H/C ratio at higher temperatures indicated the biochar had a higher carbonization degree (Knicker et al. 2005). The development of alkaline pH values was determined by the extent of the biochar carbonization that resulted from the concentration of basic inorganic components, such as inorganic carbonates, in the biochar at high temperatures and formation of insoluble salts during pyrolysis (Yuan et al. 2011). The carbonates in the biochar and organic anions on the biochar surface were the main alkaline components in the biochars. With an increasing pyrolysis temperature, the content of carbonates in the biochars increased, while the content of organic anions on the biochar surface decreased. The contribution of carbonates to the alkalinity of the biochars increased with the pyrolysis temperature (Yuan et al. 2011). The alkaline biochar produced under higher temperatures may be used to improve acidic soils (Venegas et al. 2015).

The pH values had a strong negative linear correlation with the H/C ratio of the same species of biochars, which has also been observed for other biochars (Yargicoglu et al. 2015). The correlation coefficient (r²) values of the curves were all above 0.90, except for the corn stover biochar curve (0.87). The maximum and minimum slopes of the fitting curve were assigned to the cotton stalk and rape stalk biochars, respectively.

Fig. 3. Contents of K (a) and Na (b) in the biochars prepared at different temperatures

The EC is an electrochemical property that can reflect the degree of salinity in the biochar. The biochars were characterized with a wide EC range from 2.34 mS/cm for the cotton biochar to 9.12 mS/cm for the wheat straw biochar (Fig. 2b). The EC of the biochars increased with the carbonization temperature first, and then tended to be steady when the carbonization temperature was over 500 C (Fig. 2b). The increase of EC was due to the loss of volatile compounds, which resulted in an increased mineral element concentration (Cantrell et al. 2012). These results were not consistent with those described by Pituello et al. (2015), who found that the biochars derived from sewage sludge, municipal organic waste, cattle manure, and silage digestates had higher EC values at low and high temperatures. The highest EC of wheat straw biochar was related to its highest K content (Fig. 3a), while the cotton biochars with the lowest K contents showed the lowest EC. The rape stalk biochars with higher K and Na contents showed a higher EC value than those of rice straw, wheat straw and cotton stalk biochars (Figs. 2b and 3b.). The results were consistent with that from Cantrell’s studies (Cantrell et al. 2012). The low EC values indicated that the biochars had a low salinity, and thus would not have negative effects on plants and soil organisms and can be safely used to improve the quality of soils (Buss et al. 2016).