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Chen, W., Shi, S., Nguyen, T., Chen, M., and Zhou, X. (2016). "Effect of temperature on the evolution of physical structure and chemical properties of bio-char derived from co-pyrolysis of lignin with high-density polyethylene," BioRes. 11(2), 3923-3936.


Bio-chars were produced by co-pyrolysis of lignin with high-density polyethylene at 350 °C, 450 °C, and 550 °C. X-ray diffraction (XRD), Raman spectroscopy, automated surface area and pore size analysis, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR) spectroscopy were performed on bio-char to reveal the effect of temperature on its physical structure and chemical properties. All of the bio-chars demonstrated a highly disordered, turbostratic structure and exhibited a wide pore distribution. Dramatic losses of carbonyl, hydroxyl, and C-H groups indicated the development of condensed aromatic structure in the bio-chars. Specifically, biochar produced at 450 °C showed the highest degree of aromaticity, which is the relative content of aromatic structure with small fused rings and free radical concentration. This structure has more potential application in composite production and as solid fuel for its combustion or gasification. Moreover, biochar produced at 550 °C had the greatest porosity development, favoring its use as a precursor for activated carbon production.

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Effect of Temperature on the Evolution of Physical Structure and Chemical Properties of Bio-char Derived from Co-pyrolysis of Lignin with High-Density Polyethylene

Weimin Chen, Shukai Shi, Thiphuong Nguyen, Minzhi Chen, and Xiaoyan Zhou *

Bio-chars were produced by co-pyrolysis of lignin with high-density polyethylene at 350 °C, 450 °C, and 550 °C. X-ray diffraction (XRD), Raman spectroscopy, automated surface area and pore size analysis, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR) spectroscopy were performed on bio-char to reveal the effect of temperature on its physical structure and chemical properties. All of the bio-chars demonstrated a highly disordered, turbostratic structure and exhibited a wide pore distribution. Dramatic losses of carbonyl, hydroxyl, and C-H groups indicated the development of condensed aromatic structure in the bio-chars. Specifically, biochar produced at 450 °C showed the highest degree of aromaticity, which is the relative content of aromatic structure with small fused rings and free radical concentration. This structure has more potential application in composite production and as solid fuel for its combustion or gasification. Moreover, biochar produced at 550 °C had the greatest porosity development, favoring its use as a precursor for activated carbon production.

Keywords: Co-pyrolysis; Lignin; High-density polyethylene; Aromatic structure; Chemical property

Contact information: College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China; *Corresponding author:


Lignin is the second most abundant natural polymer after cellulose, and it is the main byproduct of the pulp and paper industry (Kong et al. 2009). Its massive annual production worldwide yield reaches 50 million metric tons. However, lignin is usually considered as an energy source, with low-added value (Pinto et al. 2015). Only a minor portion of the available lignin is utilized by traditional management processes, such as burning as low-valued fuels, which cannot fundamentally solve the environmental and clean-energy recycling issues. The massive production of plastic as a municipal solid waste (MSW) has reached almost 20 million tons in China. Moreover, most plastic species cannot be naturally degraded, which has prompted an increased interest in thermal conversion, such as pyrolysis, which can convert feedstocks into clean fuels and recover chemicals (Ucar et al. 2014; Veksha et al. 2014).

Co-pyrolysis has been applied to produce bio-fuels in recent years, especially bio-gas and bio-oil (Suelves et al. 2002; Martinez et al. 2014). Plastic with higher hydrogen content could be used as a hydrogen donor in co-pyrolysis with biomass or coal of less hydrogen content, which could balance the carbon, oxygen, and hydrogen in the feedstock. This process could result in a strong upgrading effect on its derived product, such as bio-oil and bio-char (Shadangi and Mohanty 2015). Previous studies have demonstrated that co-pyrolysis of biomass with plastic results in an increase in bio-oil yield and an improvement in bio-oil quality (Onal et al. 2014). Moreover, co-pyrolysis of biomass with plastic could solve the problem with different components of MSW that are not easily separated from the post-consumer stream.

However, little work has focused on bio-char derived from co-pyrolysis. It has been demonstrated that bio-char obtained by co-pyrolysis of biomass with high-density polyethylene is upgraded, with the characteristics of lower oxygen-containing groups, increased aromatic structure, higher calorific value, and greater porosity development compared with biomass pyrolyzed alone (Chen et al. 2015).

Almost all carbonaceous biomass can be converted to bio-char using slow or fast pyrolysis. Bio-char can be used as a fuel cell catalyst for biomass gasification and/or applied in the field of catalyst support for tar cracking because of its pore texture (Sutton et al. 2001). The aromatic structure and large carbon content of bio-char favors its application as a precursor for activated carbon and supercapacitor production (Peng et al. 2013). Bio-char also can be used in composite materials, mostly as filler in polymer-biochar composites (Ahmetli et al. 2004). In addition, the surface chemical property and condensed aromatic structure in bio-char improves soil by enhancing soil fertility, soil microbial activity, and nutrient availability (Bornermann et al. 2007; Steinbeiss and Gleixner 2009). Because of its relatively low oxygen content, bio-char can be directly used in carbon fuel cells with a composite electrolyte of samarium-doped ceria and a eutectic carbonate phase (Yu et al. 2014).

In this study, lignin was selected as a bio-char precursor because of its high carbon content, aromatic structure, and abundant availability. High-density polyethylene was used as a hydrogen donor for co-pyrolysis. Co-pyrolysis derived bio-chars were produced at 350 °C, 450 °C, and 550 °C. X-ray diffraction and Raman spectroscopy were used to investigate crystalline and aromatic structural changes. The pore texture of co-pyrolysis-derived bio-char was revealed using an automated surface area and pore size analyzer and was confirmed by scanning electron microscopy. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis were performed to investigate the surface atomic components and chemical functional group changes. The chemical activity of bio-char was studied using electron spin resonance analysis.



Lignin was obtained from pulping black liquor (PBL), which was purchased from a local paper mill (Yonghong, Nanjing, China). The separation and purification of lignin from PBL was performed as described (Zhang et al. 2011). High-density polyethylene (HDPE) was obtained from shampoo bottles. Lignin and HDPE were ground by a grinder and then sieved to a particle size of less than 1 mm using mesh screen. Lignin and HDPE powders were mixed in a 50:50 (w/w) ratio and rolled for 12 h, then dried at 103 °C for 8 h for further carbonization.


Bio-char preparation

The co-pyrolysis reactor was previously described (Chen et al. 2015). It was operated under a 20 mL/min N2 flow with high purity (99.99%). Vacuum pump was used before each experiment started to ensure pure N2 atmosphere in the quartz tube. Approximately 6 g of mixed feedstock (lignin/HDPE weight ratio of 50:50) was placed in crucible for each experiment and heated up to a final pyrolysis temperature of 350 °C, 450 °C, or 550 °C at a heating rate of 10 °C/min and then incubated at the given temperature for 60 min. The co-pyrolysis derived bio-chars produced at 350 °C, 450 °C, or 550 °C were obtained from the crucible after pyrolysis was finished and were denoted as CB-350, CB-450, and CB-550, respectively.

Bio-char characterization

The crystalline structure of bio-char was analyzed using an Ultima-IV X-ray diffractor (Rigaku Corporation, Tokyo, Japan). The 2θ value for all samples was recorded from 5 ° to 65 °, with a step size of 0.02 °/s using Cu Kα radiation (40 kV, 200 mA).

The amorphous, crystalline, and aromatic nature of the bio-char samples was investigated using Raman spectroscopy with a DXR532 apparatus (Nicolet, Michigan, USA) under laser radiation operating at a wavelength of 532 nm, a spectral resolution of 4 cm-1, laser power of 10%, exposure time of 10 s, and a total of 30 acquisitions. The Raman spectra were collected between 800 and 1800 cm-1 and fitted using XPSPEAK Software (Version 4.1, Freeware Co., Hongkong, China).

The Brunauer-Emmett-Teller (BET) surface area and pore distribution were studied using an automated surface area and pore size analyzer (Micromeritics Co., Norcross, GA, USA). The N2adsorption isotherms were obtained at 77 K. The BET surface area for all samples was obtained using the BET standard method (Mushrif et al. 2008). Total pore volume was calculated by converting the amount of nitrogen gas adsorbed at a relative pressure of 0.99 to the volume of liquid adsorbed. Micropore volume was calculated using the t-plot method (Mushrif et al. 2008).

Surface morphology was investigated using scanning electron microscopy (SEM; Quanta 200, FEI Co., Hillsboro, OR, USA). Five kV voltage and high vacuum mode were used for all tests. All samples were gold-coated using a PECS coating machine (Gatan Inc., Pleasanton, CA, USA) to reduce the charging effects.

Chemical functional groups were analyzed using Fourier transform infrared (FT-IR) spectroscopy, performed in a NEXUS 870 (Nicolet Instrument Corp., Madison, WI, USA). One percent of the bio-char samples were carried using KBr discs. All samples were scanned and recorded in the wave number range from 400 to 4000 cm-1, with a resolution of 4 cm-1. The precision was no more than 0.01 cm-1.

Surface elemental composition and chemical functional groups were examined using X-ray photoelectron spectroscopy (XPS) in an AXIS Ultra DLD (Shimadzu, Tokyo, Japan). Survey scanning was performed for low-resolution spectra in the binding energy region from 0 to 1100 eV, and the C1s scan of high-resolution spectra from 277 to 396 eV were recorded with a pass energy of 10 eV and non-monochromatic Mg Kα and Al Kα X-radiations (hγ = 1253.7 eV and 1486.7 eV, respectively). The C1s spectra were deconvoluted into five components using XPSPEAK Software.

Chemical activity was analyzed using electron spin resonance (ESR) spectrometry (EMX-10/12, Bruker Corp., Hannover, Germany), operating at a microwave frequency of 9.8617GHz. Bio-char samples were placed in 4-mm borosilicate tubes and examined at room temperature during a scan time period of 4 min and a time constant of 0.25 s.


X-Ray Diffraction Analysis

X-ray diffraction was applied to study the short-range ordered structures and crystalline structures of bio-char samples (Liao et al. 2007) (Fig. 1). Peaks at approximately 23°, corresponding to the 002 plane reflection of graphite crystallite, were observed in all samples, implying that graphite-like structures were present in the bio-char. Furthermore, the broader width and shifted position of this peak compared with pure graphite indicated the highly disordered and turbostratic character of the bio-char structure. Moreover, the intensity of this peak decreased as the pyrolysis temperature increased, indicating that the bio-char structure was damaged at higher pyrolysis temperatures. Several new peaks appeared in CB-550, showing the presence of several minerals (Fan et al. 2015). This result implied that pyrolysis at higher temperatures results in higher ash content.

Fig. 1. X-ray diffraction of bio-char samples

Raman Spectroscopy

Raman spectroscopy was applied to investigate structure, specifically the amorphous, crystalline, and aromatic nature of co-pyrolysis-derived bio-char (Keown et al. 2007). Figure 2a shows the Raman spectrum of lignin and its derived bio-char samples. The band intensity initially decreased as the pyrolysis temperature increased, which was related to the loss of oxygen-containing groups in the bio-char structure, resulting in a decline in the Raman scattering ability. An obvious increase in the band intensity of CB-550 was observed. This result was attributed to the high relative content of minerals (confirmed by XRD analysis), especially Ca and Na, which passively impact the Raman peak intensity by accelerating the formation of condensed aromatic ring structures (Keown et al. 2007). Moreover, two broad peaks at 1340 cm-1 and 1580 cm-1 were observed, implying structural disorder and multiphase bio-char structure. The Raman spectra of those bio-char samples were fitted into 9 Gaussian bands from 800 cm-1 to 2000 cm-1 (Guo et al.2008; Zhang et al. 2011) (Fig. 2b). Only 5 bands (D, VR, VL, G, and GR) were predominant, representing the typical aromatic structures of those bio-chars. The band at 1590 cm-1 (G-band) was attributed to aromatic ring quadrant breathing (Li et al. 2006). The D band at 1300 cm-1referred to structures with more fused aromatic rings (≥ 6 rings) (Zhang et al. 2011), while the VR, VL, and GR bands at 1380 cm-1, 1460 cm-1, and 1540 cm-1, respectively, corresponded to the amorphous structure with smaller fused rings (3 to 5 fused aromatic rings) (Kim et al. 2011 and Guo et al. 2008).


Fig. 2. (a) Raman spectrum of lignin and its derived bio-char samples; (b) deconvolution for Raman spectrum of CB-350; (c) AG/Aall and A(VR+VL+GR)/AD of lignin and its derived bio-char samples

The band area ratio (AG/Aall) between the G-band and all 9 bands (SR, S, SL, D, VR, VL, G, GR, and GL) was calculated to reflect the evolution of aromaticity in the bio-char samples (Liu et al.2012). As illustrated in Fig. 2c, the AG/Aall value initially increased and reached the maximum at 450 °C. This result was attributed to the condensation of aromatic structures and the removal of oxygen-containing groups present in the crosslinking of carbon, yielding non-graphitizing carbon. An obvious decrease in AG/Aall was observed at 550 °C. This result was related to the opening of the benzene ring, beginning the conversion of benzene to a linear C-C chain.

The band area ratio (A(VR+VL+GR)/AD) between the sum of the VR, VL, and GR bands and the D band was used to compare the aromatic content containing small fused rings vs. more fused rings (Guo et al. 2008). This value was also used to evaluate the bio-char reactivity (McDonald-Wharry et al. 2013). The A(VR+VL+GR)/AD value of the bio-chars showed similar variation trends to AG/Aall and reached a maximum at 450 °C. According to Liu et al. (2008), the mass loss rate peak of lignin appears at 450 °C, indicating a strong volatile char interaction on the bio-char surface that favors the accumulation of small aromatic-fused rings. It should be noted that bio-char samples with a higher content of small aromatic-fused rings exhibited higher reactivity, favoring its combustion or gasification as solid fuel (McDonald-Wharry et al. 2013).


Fig. 3. (a) N2 adsorption isotherms of co-pyrolysis derived bio-char at 77 K; (b) BJH pore distribution of co-pyrolysis derived bio-char

Pore Texture Analysis

The N2 adsorption isotherms and pore size distributions of co-pyrolysis-derived bio-char samples are shown in Fig. 3. Bio-char samples demonstrated type IV isotherm curves, according to the BDDT classification system. A slight increase in adsorption volume initially appeared in the low relative pressure region, and a sharp increase was observed in the relative pressure region of 0.9 to 1.0, indicating a low level of microporosity in all samples (Mohan et al. 2014). Moreover, there was a gradual increase in absorption volume from 0.2 to 0.8 relative pressure, implying the presence of mesopores with a broad distribution (Mohan et al. 2014; Wang et al. 2014). As shown in Fig. 3b, pores in both bio-char samples demonstrated incomplete distribution near the lowest pore size limit, revealing the existence of micropores. Rapidly decreasing slopes were observed for all samples, and no peaks appeared at the entire pore diameter region, indicating the broad pore diameter distribution in the bio-char structure (Li et al. 2010). Thus, CB-550 exhibited greater porosity development than CB-450 and CB-350.

The BET surface area and pore properties of co-pyrolysis-derived bio-char samples are presented in Table 1. This data showed that pyrolysis at higher temperatures resulted in higher bio-char BET surface area, which was attributed to more volatile matter generated, leading to the formation of more pores. Moreover, CB-550 exhibited lower average pore width compared with CB-450 and CB350. This effect was related to the shrinking effect at higher temperatures, leading to the collapse of some macropores.

Table 1. BET Surface Area and Pore Properties of Bio-Chars

aCalculated from BET surface area and total pore volume

bCalculated by the t-plot method

Note: All data in Table 1 were calculated from adsorption isotherms


CB-350                                                        Lignin


CB-550                                                         CB-450

Fig. 4. Scanning electron microscopy of bio-char samples

SEM Analysis

Micrographs of co-pyrolysis-derived bio-char samples are presented in Fig. 4. Several cracks appeared on the surface of CB-350, which was attributed to the devolatilization of lignin structure with poor thermal stability. Pores with non-uniform size were observed in CB-450 and CB-550 because of the release of micromolecules from the lignin structure. Notably, the CB-550 pores were much smaller than those in CB-450. This result may be related to pore collapse caused by higher temperatures, which was confirmed in the pore distribution analysis.

FT-IR Analysis

The FT-IR spectra of lignin and its derived bio-char samples are presented in Fig. 5. As the pyrolysis temperature increased, the adsorption intensity at 3417 cm-1 decreased, which was assigned to OH groups in aromatic and aliphatic structures. This result was related to breaking hydrogen bonds, which released of alcohol and water. The adsorption peak at 2935 cm-1 and 1035 cm-1 corresponded to C-H in aliphatic and aromatic structure (Sharma et al. 2002); these peaks were greatly decreased after pyrolysis. The peak intensity of the adsorption bands at 1597 cm-1, 1265 cm-1, and 1210 cm-1 were attributed to C=O and C-O, respectively (Elmay et al. 2015) and were considerably decreased at 550 °C. Consequently, pyrolysis at 550 °C resulted in a dramatic loss in carbonyl, hydroxyl, and C-H groups, indicating the significant development of aromatic structures.

Fig. 5. FT-IR spectra of lignin and its derived bio-char samples

XPS Analysis

X-ray photoelectron spectroscopy was applied to study surface chemistry and to identify and quantify the basic elements (except hydrogen and helium) and functional groups (Sun et al. 2012) (Fig. 6; Table 2). As illustrated in Table 2, the O/C mass ratio decreased as the pyrolysis temperature increased, indicating the gradual development of condensed aromatic structures.

Fig. 6. Deconvolution of C1s spectra of co-pyrolysis-derived bio-char samples

Table 2. Atomic Element Composition and C1s Chemical Composition of Co-pyrolysis-Derived Bio-char Samples

According to Azargohar et al. (2014), the C1s spectra of the bio-char samples were deconvoluted into 5 Gaussian peaks corresponding to 5 carbon-related functional groups, namely C=C/C-C/CHx for aromatic/aliphatic carbon at 285 eV, -C-OR for ether and hydroxyl/phenol groups at 285 eV, C=O for ketone and quinone at 288 eV, -COO for carboxyl and ester at 289.2 eV, and -O-CO-O- for carbonate at 290.7 eV. C=C/C-C/CHx was the most dominant component of all samples, and it dramatically increased as the pyrolysis temperature increased. This increase was accompanied by a notable decrease in oxygen-containing groups (except COO), which agreed with the FT-IR analysis. These results were attributed to the removal of heteroatoms and aliphatic compounds during crosslinking, implying a considerable development of condensed polyaromatic skeletons in co-pyrolysis-derived bio-char and the carbon conversion into easy-graphitizing type, which was confirmed by Raman analysis. Thus, carbon enrichment in bio-char at higher pyrolysis temperatures favors its application as a precursor for activated carbon production.

ESR Analysis

Free radical formation is a significant step in bio-char evolution. Higher free radical concentration favors chemical activity (Liao et al. 2014). For example, radical centers in a substrate initiate the grafting of other polymers into a composite. In addition, radicals that form in the co-pyrolysis-derived bio-char favor crosslinking reactions, which enhance the physical and mechanical properties of composites (Zhou et al. 2013).

The g factor parameter identifies the type of free radical. The g factor and free radical concentration of lignin and co-pyrolysis-derived bio-char are presented in Table 3. A g factor greater than 2.0040 corresponds to oxygen-centered radicals, while a g factor less than 2.0030 represents carbon-centered radicals (aromatics). A g factor of 2.0030 to 2.0040 is attributed to mixed carbon- and oxygen-centered radicals (Di et al. 2006). The g factor decreased with increasing pyrolysis temperature to a minimum (2.0032) at 550 °C, indicating the conversion of free radicals from oxygen-centered radicals into a combination of carbon- and oxygen-centered radicals. A decrease in the g factor was related to the decomposition of oxygen-centered radicals with less thermal stability.

Radical concentration initially increased to a maximum at 450 °C, which reflected the breakdown of side chains and carbon-oxygen bonds in bio-char, as confirmed by FT-IR and XPS analysis. The radical concentration then decreased. Interactions between the free radicals generated from micromolecule structures at higher temperature may have been responsible for the decrease in radical concentration. It should be noted that higher free radical concentrations lead to higher chemical activity in bio-char.

Table 3. The g Factor and Free Radical Concentration of Lignin and Co-Pyrolysis-Derived Bio-Char Samples


  1. All bio-chars showed a high degree of disorder, turbostratic structure, and wide pore distribution.
  2. The removal of oxygen-containing groups and increased carbon density resulted in the development of condensed aromatic structures.
  3. Bio-char produced at 450 °C demonstrated the highest degree of aromaticity, relative content of aromatic structure with small, fused rings, and free radical concentration. This product has potential application in composite production and as solid fuel for its combustion or gasification.
  4. Bio-char produced at 550 °C exhibited the greatest porosity development and highest carbon enrichment, favoring its use as a precursor for activated carbon production.


The authors are grateful for support from the Doctorate Fellowship Foundation of Nanjing Forestry University, the National Natural Science Foundation of China (Grant No. 31400515), the National Natural Science Foundation of China (Grant No.31270606), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This paper was sponsored by the Qing Lan Project.


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Article submitted: December 24, 2015; Peer review completed: February 27, 2016; Revised version received and accepted: March 1, 2016; Published: March 14, 2016.

DOI: 10.15376/biores.11.2.3923-3936