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Guo, S., Dong, X., Liu, K., Yu, H., and Zhu, C. (2015). "Chemical, energetic, and structural characteristics of hydrothermal carbonization solid products for lawn grass," BioRes. 10(3), 4613-4625.


Hydrothermal carbonization (HTC) of lawn grass was carried out at 200 °C and 240 °C for 30 to 180 min. The chemical, energetic, and structural characteristics of HTC solid residues were investigated. Results from HTC experiments indicate that solid mass yield of all solid residues was 31 to 50%. The hydrogen/carbon (H/C) and oxygen/carbon (O/C) atomic ratios of all solid residues were 1.17 to 1.64 and 0.45 to 0.65, respectively. The higher heating value (HHV) increased up to 20.54 MJ/kg with increasing HTC residence time at 240 °C for 180 min. Both XRD patterns and FTIR spectra show that differences occur with samples treated as compared to the raw material. Solid hydrochar exhibited higher ordered structure characteristics and was mainly derived from amorphous components degradation when the residence time was increased from 30 to 180 min at 200 °C, while hydrochar formed from cellulose components degradation with increased residence time at 240 °C. According to the results studied, it was found that prolonged residence time was favorable to the formation of hydrochar from lawn grass.

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Chemical, Energetic, and Structural Characteristics of Hydrothermal Carbonization Solid Products for Lawn Grass

Shuqing Guo,* Xiangyuan Dong, Kaituo Liu, Hailong Yu, and Caixia Zhu

Hydrothermal carbonization (HTC) of lawn grass was carried out at 200 °C and 240 °C for 30 to 180 min. The chemical, energetic, and structural characteristics of HTC solid residues were investigated. Results from HTC experiments indicate that solid mass yield of all solid residues was 31 to 50%. The hydrogen/carbon (H/C) and oxygen/carbon (O/C) atomic ratios of all solid residues were 1.17 to 1.64 and 0.45 to 0.65, respectively. The higher heating value (HHV) increased up to 20.54 MJ/kg with increasing HTC residence time at 240 °C for 180 min. Both XRD patterns and FTIR spectra show that differences occur with samples treated as compared to the raw material. Solid hydrochar exhibited higher ordered structure characteristics and was mainly derived from amorphous components degradation when the residence time was increased from 30 to 180 min at 200 °C, while hydrochar formed from cellulose components degradation with increased residence time at 240 °C. According to the results studied, it was found that prolonged residence time was favorable to the formation of hydrochar from lawn grass.

Keywords: Hydrothermal carbonization; Lawn grass; Residence time; Hydrochar

Contact information: Department of Energy & Environment Engineering, Zhongyuan University of Technology, Zhengzhou, Henan 450007, PR China; *Corresponding author:


In recent years, there has been increasing concern regarding the energy shortage and environmental issues arising from fossil fuel. Biomass is regarded as an alternative energy source due to its wide availability, renewability, and cleanness. However, effective treatment and conversion of biomass into energy requires additional research. To date, varieties of techniques have been applied in biomass treatment and conversion, such as combustion, pyrolysis, gasification (Yang et al. 2006; Wiedner et al. 2013; Chen et al. 2014). However, a complicated high temperature process for biofuels and bioproducts will undesirably be accompanied by a high cost.

In recent years, hydrothermal carbonization (HTC) has been paid considerable attention for biomass treatment (Funke and Ziegler 2010). Hydrothermal carbonization is a wet thermal conversion technique that is simple, cheap, and easy to perform with a mild reaction condition (180 to 250 °C) as compared to other techniques (Funke and Ziegler 2010). The biomass should be submerged to ensure that the hydrothermal reaction occurred during the whole reaction process. The final solid products of HTC are often called hydrochar. Hydrochar has been regarded potentially useful as a soil additive, for carbon sequestration, adsorbents, container nurseries, and even for fuels. (Steinbeiss et al. 2009; Roman et al. 2012; Wiedner et al. 2013). In recent years, considerable HTC research has been conducted on methods for obtaining hydrochar. Roman et al. (2012) studied walnut shell and sunflower stem by HTC, and found that coalification was promoted by increasing the calorific value of the solid hydrochar. Xiao et al. (2012) treated corn stalk and Tamarix ramosissima by HTC, revealing that the solid products mainly consisted of lignin with a high degree of aromatization and a large amount of oxygen-containing groups. Moreover, the hydrochar had a considerably higher heating value (HHV) than that of the raw material. Poerschmann et al. (2014) utilized wet biomass, brewer’s spent grain, to produce biocoal. Their experimental results showed that the energy densification of biocoals was increased compared to the raw material. The HHV data for HTC coals was similar to that of lignite. Gao et al. (2013) studied the chemical and structural properties of water hyacinth hydrochar with increasing time. They found that higher HHV and better structural characterization of hydrochar could be obtained with increased time. These studies on HTC indicated that the physicochemical characteristics of hydrochar were dependent on the property of feedstock. In addition, the process parameters such as reaction temperature and time had a significant effect on the HTC process (Funke and Ziegler 2010; Lu and Pellechia 2013; Gao et al 2013; Zhang et al. 2015). Biomass could undergo complicated chemical reactions under different process conditions including hydrolysis, dehydration, decarboxylation, and condensation polymerization during hydrothermal carbonization (Funke and Ziegler 2010; Sevilla et al. 2011; Kang et al. 2012). To understand the hydrothermal carbonization process for different types of feedstock under various reaction conditions, fundamental research on HTC still has further to go.

Grass is a class of biomass easily found in urban areas, and was reported previously to produce value added products such as activated carbon (Kalyani et al. 2013), nitrogen-doped carbon materials (Liu et al. 2012), and biogas (McEniry et al. 2014; Meyer et al. 2014). However, few studies have been done that provide valuable information regarding the characteristics of hydrochar from HTC lawn grass at different residence times. To achieve deeper insight into the influence of residence time on HTC process of lawn grass, the HTC experiments of lawn grass changing residence time at two reaction temperatures were carried out. To characterize the evolutions of hydrochar at different residence times, elemental analysis, X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy were used to study the chemical, energetic, and structural characteristics of lawn grass hydrochar.



Lawn grass used in this study was collected from the Zhengzhou urban area. The grass was dried by natural air for a few days, and then broken into 1- to 2-mm samples. The raw samples were sealed in a plastic container for further use. The lignocellulosic compositions (%) of the dried samples were analyzed by the Van Soest analysis method (Zhang 2003). Elemental analysis of the samples was conducted on an element analyzer (EA3000, EuroVector, Italy). Proximate analyses of dried samples were determined according to the standard GB/T 28731 (2012). The test results were shown in Table 1. Lawn grass contained high cellulose, hemicellulose contents, and low lignin content. Elemental analysis indicated that the feedstock was mainly composed of C and O. Proximate analysis presented a high volatile matter content of 79.89%.

Table 1. Composition, Ultimate Analysis, and Proximate Analysis of Lawn Grass


The hydrothermal experiment was carried out in a 2-L, 316 L stainless steel autoclave, and its maximum temperature and pressure were 350 oC and 22 MPa, respectively. The autoclave was heated by an electrical heater. To evenly heat the feedstock, the autoclave was equipped with a magnetic stirrer, and a maximum stirring speed was 1000 rpm. The hydrothermal processes were shown as following: first, 10 g of raw material and 300 mL of water were mixed and stirred evenly in the autoclave. Then, the autoclave was heated to 200 °C and 240 °C, respectively, then maintained for a desired residence time (30, 60, 90, 120, or 180 min). During the whole process, it was stirred at a speed of 200 rpm. After each residence time, the reactor was cooled to room temperature. The solid reaction product was collected by filtration, and then dried at 105 °C until a constant weight was achieved. The dried samples were then sealed in plastic container for further analysis.

The X-ray diffraction (XRD) patterns of the dried raw material and hydrothermally treated samples were measured using an X-ray analyzer (X’Pert PRO, PANalytical, Netherlands). The XRD spectra were obtained with a Cu target using kα radiation (λ = 0.1542 nm) generated from a 40-kV potential and 40-mA current. Intensities ranged from 5 to 50° with a 2 °/min continuous scan speed, and the scan step size was 0.03°. The chemical structure and characteristics of the dried samples functional groups were studied by Fourier transform infrared (FTIR) spectroscopy (Bruker Tensor 27, Bruker Optics, Germany). The IR spectra recorded varying wave number over the range of 400 to 4000 cm-1, with a resolution of 4 cm-1 and scanning speed of 32 mm/s.


Chemical and Energetic Characteristics

To evaluate the chemical and energetic characteristics of the dried samples, six equations, including those for solid mass yield (SY); C, H, and O fraction; normalized C, H, and O content; higher heating value (HHV); energy densification (ED); and energy retention efficiency (ERE) were defined as follows:







Solid mass yield is one of the key characteristics for HTC solid products. Figure 1 shows that the HTC solid mass yields vs. the residence time increased from 30 min to 180 min at 200 and 240 oC.

Solid mass yield

Carbon content

Fig. 1. Solid mass yield and carbon content for feedstock and hydrochar at 200 and 240 °C

It can be observed that with 30 min of HTC treatment, the solid mass yield sharply decreased to 50% (200 oC) and 38% (240 oC) because part of the raw material (i.e., hemicellulose, lignin, and cellulose) was hydrolyzed and solubilized into water or partitioned to gas and liquid phases (Bobleter 1994; Funke and Ziegler 2010). With increasing residence time from 30 to 180 min, the solid mass yield slowly decreased from 50 to 46% at 200 oC, and from 38 to 31% at 240 oC. The influence of the residence time on yield at both temperatures was similar and led to a slow decrease. However, the yield of hydrochar at 240 oC with residence time ranging from 30 min to 180 min was lower than those of all hydrochar at 200 oC. This is because the HTC reaction of lignocellulosic biomass was a relatively slow process, and the reaction rate was mainly governed by temperature (Funke and Ziegler 2010). The amorphous hemicellulose degradation process was almost completed at 200 oC. Meanwhile, the dissolved solid phase deposits (Reza et al. 2013) or polymerizes to form water insoluble polymer (Bobleter 1994; Funke and Ziegler 2010), which makes the solid product yield slowly decrease and level off as the residence time is increased. Meanwhile the cellulose slowly degraded at 240 oC (Funke and Ziegler 2010), which led to a lower yield of hydrochar.

The elemental composition of the samples can provide further insight into the chemical transformations of the samples. Carbon content for the untreated sample and HTC solid residues at 200 and 240 °C are shown in Fig. 1. It can be seen that the carbon content underwent a gradual increase. First, carbon content increased from 42.9 (untreated) to 46.15% (30 min treated at 200 °C) and to 50.63% (30 min treated at 240 °C), then a further increase to 49.1% (180 min treated at 200 °C) and to 55.43% (180 min treated at 240 °C), respectively. Figure 2 illustrates the C, H, and O fractions, which are the ratios of the mass of C, H, and O in the dried recovered solids to the corresponding mass of C, H, and O in the dried raw material.

Fig. 2. C, H, and O fraction for hydrochar at 200 and 240 °C

After 30 min HTC treatment, C, H, and O fractions sharply decreased to 53.85%, 43.4%, and 46.5% at 200 °C, and to 44.89%, 29.07%, and 33.27% at 240 °C, respectively. This indicates that nearly or over half of the C, H, and O mass in dried raw sample were lost, which was mainly due to the solubilization or partition to gas and liquid phases. With an increasing residence time, the O and H fraction decreased to 36% and 37.8% at 200 °C, and to 23.59% and 23.07% at 240 °C, respectively. However, the C fraction at both temperatures showed less significant difference with residence time increased. This indicates that the increase of the carbon content with increasing residence time at each temperature (Fig. 1) was a result of the decrease in the H and O mass loss. The change trend for C fraction with increasing the residence time may be a combination result of hemicellulose and cellulose degradation and deposition of dissolved compounds. Additionally, the H and O fraction at 240 °C became obviously lower than that observed at 200 °C. The results indicated dehydration and decarboxylation reaction during lawn grass HTC were also significantly influenced by temperature. The changes of the C, H, and O fraction coincided with the changes of solid mass yield with increased time at both temperatures.

Figure 3 illustrates the normalized C, H, and O contents, which was calculated by the mass of C, H, and O in the recovered solids divided by the mass of the dried initial feedstock. From Fig. 3, it can be seen that after 30 min of HTC treatment, the normalized C mass decreased from 42.85 to 23.08% at 200 °C, to 19.24% at 240 °C. However, no substantial change about the normalized C mass was observed with a further increase in the residence time at both experimental temperatures. A similar trend in normalized mass of H was observed. Normalized O mass first decreased from 43.37 to 20.16% at 200 °C and 14.47% at 240 °C, and then further decreased to 15.6% at 200 °C and 10.26% at 240 °C with increasing the residence time. These results show that after 30 min of HTC treatment, the decrease of the solid mass yield was caused by the mass loss of C, H, and O. Since C and H mass changed mildly with increasing residence time, it can be concluded that the solid mass yield decrease is mainly due to the O loss. This result is consistent with other literature (Reza et al. 2013). Therefore, some organic acid (e.g. levulinic acid and formic acid) were formed and dissolved into liquid and a small amount of CO2 released (Funke and Ziegler 2010; Reza et al. 2013). It is notable that the normalized C, H, and O contents at 240 °C were lower than those at 200 °C, which indicated the increased temperature promoted the chemical reaction.

Fig. 3. Normalized C, H, and O content for feedstock and hydrochar at 200 and 240 °C

Higher heating value (HHV, as defined in Eq. 5), and the atomic H/C and O/C were calculated based on elemental content. Their relationship with time is presented in Fig. 4. As expected, HHV increased from 17 (untreated) to 17.4 MJ/kg (30 min treated at 200 °C) and 18.24 MJ/kg (30 min treated at 240 °C). With increasing residence time, HHV reached 19.3 MJ/kg at 200 °C for 180 min, while, it achieved 20.54 MJ/kg at 240 °C for 180 min. This is because of the removal of low HHV components such as fragrances, oils, and hemicellulose at 200 °C and degradation and conversion of cellulose at 240 °C (Funke and Ziegler 2010; Reza et al. 2013). In contrast to HHV, H/C and O/C decreased as residence time increased. After HTC treatment, the H/C and O/C ratios were between 1.17 to 1.64 and 0.45 to 0.65 at 200 °C and 240 °C, respectively; this result is in accordance with a previous report (Wiedner et al. 2013). The high O/C ratio (0.4 to 0.6) may also indicate the presence of hydroxyl, carboxyl, and carbonyl groups (Wiedner et al. 2013). As shown in Fig. 4, the dehydration and decarboxylation pathways were marked. The reduction in H/C and O/C ratios followed these pathways. This indicates that dehydration and decarboxylation occurred during the HTC process as residence time and temperature were increasing (Funke and Ziegler 2010; Reza et al. 2014).

Fig. 4. HHV and Van Krevelen diagram of feedstock and hydrochar at 200 and 240 °C

Fig. 5. Energy densification and energetic retention efficiency for feedstock and hydrochar at 200 and 240 °C

Energy densification and energy retention efficiency are shown in Fig. 5. As can be seen, energy densification presented a gradual increase, which was similar to that of carbon content. It first increased from 1 (untreated sample) to 1.02 (30 min treated sample), and then further increased to 1.13 (180 min treated) at 200 °C. It increased from 1.07 to 1.21 over the range of residence time at 240 °C. Energy retention efficiency followed a similar trend with solid mass yield, which exhibited a tendency of slight decrease. When compared with that from 200 °C, the hydrochar from 240 °C showed higher ED and lower ERE.

Structural Characteristics

The XRD patterns of untreated and hydrothermal treated samples are shown in Fig. 6. It can be seen that the XRD diffraction peaks for the untreated sample were broad, which suggest that the untreated samples have disordered and amorphous structures. After 30 min of 200 °C HTC treatment, the XRD pattern peaked at 2θ values of 15-20o and 22.7 and was sharper than those of the untreated materials. At the same time, a weaker peak appeared at a 2θ value of 34o. As stated in the literature (Hashaikeh et al. 2007), all three diffraction peaks represented the microcrystalline cellulose structure. This indicates that the amorphous component of the lawn grass (mostly hemicellulose) was removed. With increasing residence time at 200 °C, the XRD diffraction peaks for 2θ values of 15 to 20, 22.7o, and 34 became sharper, but the changes were with a small difference. This suggests that the effect of residence time on HTC process was slow under the operating conditions (200 °C, 30 to 180 min). However, the hydrochar at 240 °C for 30 min showed a stronger peak at the three aforementioned diffraction angles. With the increase of residence time at 240 °C, XRD peaks corresponding to microcrystalline cellulose slowly declined, which indicated that the cellulose components began to degrade to form hydrochar and the structure of hydrochar had been changed.

For an in-depth analysis of the chemical variation of hydrochar with increasing time, the correlation between the crystallinity index and carbon content, oxygen content are presented in Fig.7 (a) and (b). Crystallinity index was calculated by the ratio of the difference between maximum 002 diffraction intensity at 2θ equal to 22.7o and the amorphous diffraction intensity at 2θ of 18o to maximum 002 diffraction intensity, which reflected the relative amount of crystalline cellulose in the total biomass (Liet al. 2011). Different trends in carbon and oxygen content changes with crystallinity index variations were observed at 200 and 240 °C. All P values (shown in Fig. 7) of fitting curves were less than 0.01, which showed that a highly significant correlation existed between carbon (oxygen) content and crystallinity index. At 200 °C, with time increasing, crystallinity index increased with increase of carbon content and decrease of oxygen content. However, a significant increase in crystallinity index was observed at 240 °C for 30 min (the highest black square point in Fig.7), and then an obvious decrease of crystallinity index was exhibited with variations of carbon and oxygen content as time increasing. This suggested that the feedstock solubilization and hydrochar formation at 200 °C mainly caused by amorphous components degradation. However, at 240 °C, hydrochar production followed by cellulose degradation and carbonization, which was characterized by significant declining of crystallinity index at times exceeding 30 min. Figure 7 distinctly shows the different pathways of carbonization with time variations at different temperatures, and it is particularly useful to understand hydrochar production during HTC. These results could further demonstrate the results of yield and elemental composition.

Fig. 6. X-ray diffraction pattern of feedstock and hydrochar at 200 and 240 °C

Fig. 7. Correlation between C/O Content and Crystallinity Index

Figure 8 depicts the FTIR spectra of untreated and hydrothermal treated samples. The wide band located at 3100 to 3700 cm-1 was associated with O–H vibration in hydroxyl or carboxyl groups. The bands located at 2921 and 2855 cm-1 were associated with aliphatic C–H stretching vibration and deforming vibration. The band at 1061 cm-1 was associated with the β-glycosidic bond in cellulose and hemicellulose (Liu et al. 2013). These bands were present both in the untreated sample and their HTC sample. However, compared with the untreated sample, some differences appeared after 30 min of HTC treatment at both temperatures. An absorption peak located at 1515 cm-1, which could be related to aromatic ring stretching vibration appeared. This could be attributed to the removal of the amorphous fraction (Nitsos et al. 2013), as indicated in Fig. 7. The absorbance peak at 1700 cm-1representing the C=O became stronger than the feedstock. This difference indicated that the chemical components of lawn grass started to degrade and simultaneously underwent the reaction of polymerization. However, band at 3100 to 3700 cm-1 of O-H vibration obviously decreased at 240 °C for 30 min, which might be caused by the dehydration reaction. At 240 °C, with increasing residence time, the absorbance intensity at 1700 cm-1 of C=O gradually increased. The band at 1030 to 1160 cm-1 associated with the cellulose (Liu et al. 2013) decreased over time, which was in agreement with XRD pattern.

After HTC treatment, lawn grass could be transformed into hydrochar with significantly increased HHV, increased carbon content, and decreased O/C and H/C ratios, especially at 240 °C, which provided potential application in some applications, such as co-combustion with other fossil fuel. However, the solid yield was relatively low. Recovery of liquid product should be considered in the application (Stemann et al. 2013).

Fig. 8. FTIR spectra of feedstock and hydrochar at 200 and 240 °C


  1. In the present study, lawn grass was converted to hydrochar by hydrothermal carbonization at 200 and 240 °C for 30 to 180 min. The mass yield of hydrochar varied from 50% at 200 °C for 30 min to 31% at 240 °C for 180 min. The C content and HHV of hydrochar increased from 42.9% and 17 MJ/kg (untreated) to 55.43% and 20.54 MJ/kg at 240 °C for 180 min, respectively. The hydrogen/carbon (H/C) and oxygen/carbon (O/C) atomic ratios of all solid residues were 1.17 to 1.64 and 0.45 to 0.65, respectively.
  2. Temperature had the main influence on yield, carbon content, and HHV of hydrochar. Longer residence time was favorable for the dehydration, decarboxylation, and polymerization reactions, which resulted in carbon content increasing and oxygen content decreasing.
  3. Based on X-ray diffraction, at a low temperature of 200 °C, with an increase of residence time from 30 to 180 min, the crystalline structure of hydrochar was gradually enhanced and the hydrochar production was mainly derived from the degradation of amorphous components. At the high temperature 240 °C, longer residence time promoted crystalline cellulose components in lawn grass to form hydrochar, so the peaks representing the crystalline cellulose became wider and weaker and crystallinity index decreased. The FTIR spectra further confirmed the results of XRD pattern.


This work was supported by the National Nature Science Foundation of China, Grant no. 51206194.


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Article submitted: November 26, 2014; Peer review completed: February 8, 2015; Revised version received and accepted: May 29, 2015; Published: June 5, 2015.

DOI: 10.15376/biores.10.3.4613-4625