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Comparative production of biochars from corn stalk and cow manure
Bioresource Technology 291 (2019) 121855
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Comparative production of biochars from corn stalk and cow manure a
b
Ziyun Liu , Yuanhui Zhang , Zhidan Liu
a,⁎
T
a Laboratory of Environment-Enhancing Energy (E2E), Key Laboratory of Agricultural Engineering in Structure and Environment, Ministry of Agriculture, College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China b Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
G R A P H I C A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal carbonization Lignocellulosic waste Hydrochar Physicochemical property Solid biofuel
The aim of the present work was to compare corn stalk (CS) and cow manure (CM) for hydrochar production at different reaction temperatures (180–260 °C) and retention times (1–4 h). CM and CS resulted in hydrochars with significantly different physicochemical properties; however, both led to similar yields (30–65%). CM-derived hydrochar had a lower carbon content but a higher nitrogen and ash content than CS-derived hydrochar. CMderived hydrochar demonstrated potential as a soil amendment due to its higher content of nitrogen, the presence of surface functional groups and higher specific surface area in comparison to CS-derived hydrochar. In comparison, CS-derived hydrochar demonstrated suitability as a solid fuel due to its high heating value and low ignition temperature. This study revealed that the composition of lignocellulose significantly impacted the properties and thus potential applications of hydrochar.
1. Introduction In recent years, agricultural wastes, including livestock manure and crop straws, have attracted increasing attentions among researchers due to their abundant distribution, carbon neutrality and ability to sidestep competition with food for land use. Thus, agricultural wastes have the potential to serve as a carbon source for the production of biomaterials and bioenergy. The quantity of livestock manure in China amounts to ⁎
250 million tons annually, due to the rapid development of the livestock and poultry industry (Yang et al., 2017). In addition, lignocellulosic wastes are produced at a volume of about 200 billion tons per year around the world (Monlau et al., 2012). Hydrothermal carbonization (HTC) is a thermochemical conversion technology that can convert agricultural biomass into hydrochar and other by-products (synthesis gas, aqueous phase, etc.). HTC is generally conducted at a temperature ranging from 150 to 350 °C, at a certain retention time
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.biortech.2019.121855 Received 30 May 2019; Received in revised form 18 July 2019; Accepted 20 July 2019 Available online 22 July 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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longer than 1 h and at a self-generated pressure. Agricultural wastes are rich in organic polymers like lignin, cellulose, hemicellulose, protein and lipid (Wu et al., 2018). Previous studies have speculated the production mechanism of hydrochar and by-products during HTC (Liu et al., 2018b; Zhu et al., 2015). The hydrolysis and cleavage of biomass are facilitated by subcritical water which can act as both reagent and a catalyst (Gao et al., 2016). The incorporation of water triggers a series of parallel reactions, such as the removal of hydroxyl groups via dehydration, the loss of carboxyl and carbonyl groups via decarboxylation and the cleavage of ester and ether bonds via hydrolysis of biomass components (Liu et al., 2017). Lignin, a recalcitrant component of biomass, has a complex and branched structure composed of hydroxyphenol, guaiacol and syringol connected by ether linkages (e.g. β–O–4) and CeC bonds (e.g. β–β, β–5). During the HTC process, it is difficult for lignin to undergo hydrolysis and depolymerization of lignin (Mouthier et al., 2018; Zhou et al., 2018). In comparison, it is easier to decompose carbohydrates, lipid and proteins under identical conditions. Specifically, the degradation products of carbohydrates may react with lignin-derived phenols and form the dispersed char that is adsorbed to the initial skeleton of lignin (Mouthier et al., 2018; Zhou et al., 2018). Hydrochar from biomass HTC has previously been investigated as a fuel source (Wang et al., 2018b), catalyst precursor (Yan et al., 2014), soil amendment (Eibisch et al., 2013), contaminant adsorbent (Tan et al., 2015), and capacitor (Yang et al., 2019). The varied application of hydrochar is mainly based on its carbon sequestration ability, oxygenated functional groups (OFGs) and various porous structures, which is benefited from the presence of various biomass components. For instance, Guo et al. found the grape marc hydrochar exhibited a significantly higher carbon content than hydrochar derived from corn stalk and wood, which could be partly attributed to the lower cellulose content and the higher C content of grape marc feedstock (Guo et al., 2016). Another study conducting co-HTC of a food waste (FW)-woody biomass blend indicated that the hydrochar higher heating value (HHV) increased as the ratio of FW increased (Wang et al., 2018b). Additionally, most of the nitrogen and chlorine elements in the biomass migrated into the aqueous phase, indicating that the emission of NOx and SOx decreased (Wang et al., 2018c). Liu et al. developed a facile strategy to prepare the catalyst support for nanoscale zerovalent iron using pinewood sawdust-derived hydrochar, owing to the absorption capacity and porous structure of the hydrochar (Liu et al., 2016). The mesoporous structure of hydrochar had a strong correlation with the presence of OFGs, which favored the adsorption ability of the contaminants (Jain et al., 2015). Hence, it is of great significance to prepare hydrochar with varied physicochemical properties aiming at different application purposes. Recently, a couple of substrates have been used as a feedstock for hydrochar production via HTC (Liu et al., 2017; Mouthier et al., 2018). Smith et al. found that willow-derived hydrochar had a HHV of 25 MJ/ kg which was higher than the HHV of the hydrochar derived from macroalgae (16.8 MJ/kg) under the same HTC conditions. The disparity between these two values may be attributed to the higher ash content of the macroalgae (Smith et al., 2016). Obviously, the characteristics of hydrochar are largely affected by the biomass components. In addition, the processing parameters, e.g., temperature, retention time, pH, catalyst, reaction solvent and solid load, also play significant roles in determining the properties of hydrochar which in turn affect its application prospects (Guo et al., 2016; Simsir et al., 2017). However, limited information is available comparing different agricultural wastes for hydrochar production at various reaction temperatures and retention times. Cow manure (CM) and corn stalk (CS) are two typical agricultural wastes. CS is mainly comprised of carbohydrates and lignin, whereas CM consists of carbohydrates, lignin, protein, lipid and an abundance of ash. Thus, the HTC process of these two typical agricultural wastes was compared at different reaction conditions in this study. The main
Table 1 Characteristics of the feedstock.a CM
CS
Biochemical compositions analysis (wt%) Cellulose 19.12 ± 0.12 Hemicellulose 22.25 ± 1.18 Lignin 12.70 ± 0.89 6.77 ± 0.18 Crude proteinb Crude lipid 5.43 ± 0.45
28.93 ± 0.36 28.26 ± 3.48 9.78 ± 0.10 ND ND
Elemental analysis (wt%) Carbon (C) Hydrogen (H) Nitrogen (N) Sulfur (S) Oxygen (O)c
43.18 ± 0.18 5.92 ± 0.41 1.08 ± 0.03 0.25 ± 0.09 49.57 ± 0.13
44.96 ± 0.03 6.34 ± 0.45 0.49 ± 0.03 0.08 ± 0.07 48.13 ± 0.34
Proximate analysis (wt%) Ash Volatile matters (VM) Fixed carbond (FC)
18.30 ± 0.38 83.67 ± 0.55 0.93
5.65 ± 0.08 71.43 ± 2.22 21.59
a b c d
Dry basis. Calculated by the content of N (The wt.% protein = the wt.% N × 6.25). Calculated by difference of C, H, N and S. Calculated by difference of ash and VM.
objectives of this study are to: (1) Investigate the influence of temperature (180–260 °C) and retention time (1–4 h) on hydrochar production from CM and CS; (2) Compare the physicochemical properties of CM- and CS-derived hydrochar based on their porous structure, OFGs and surface morphology; (3) Discuss the application potential of CMand CS-derived hydrochar. 2. Experimental section 2.1. Feedstock CM and CS were collected in Lianyungang (Jiangsu, CHN) and Beijing (CHN), respectively. The basic physicochemical properties including biochemical, elemental and proximate compositions of CM and CS are shown in Table 1. 2.2. HTC procedure HTC experiments were performed in a stainless-steel cylindrical reactor (Parr 4848, USA) with a working volume of 100 mL. The raw materials and deionized water were added at a solid content of 15 wt%, and they were then mixed for 30 min before heating. The closed reactor was purged with nitrogen to replace the air. The system was then heated to the desired temperature (180, 200, 220, 240 and 260 °C) using a cylindrical jacket coupled with electric heating. During the reaction, the pressure ranged from 8.27 to 30.34 bar. Once the designed temperature was reached, the retention time was maintained for 1, 2, 3 and 4 h, respectively. The reactor was cooled down to ambient temperature with the assistance of an electric fan after the reaction completed. The produced gas was collected by gas bags, and the hydrochar and liquid product were subsequently separated via vacuum filtration. After drying at 105 ± 5 °C for 24 h, the hydrochar was stored in enclosed plastic bottles before further characterization. The aqueous phase was further filtered using a 0.45 μm membrane and stored in a refrigerator at 4 °C. The obtained hydrochars were labeled as “CM-m-n” and “CS-mn” in which “m” represented the reaction temperature (180–260 °C) and “n” indicated the retention time (1–4 h). 2.3. Characterization For the solid analysis, the elemental composition (C, H, N and S) of the feedstock and hydrochar were determined using an elemental 2
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analyzer (Vario EL III, GER). The HHV was calculated according to Dulong’s formula (Qi et al., 2018). The determination of the content of ash and volatile matter (VM) were conducted in a muffle furnace at 550 °C for 4 h and 900 °C for 7 min, respectively. The content of fixed carbon was calculated by difference, according to standard methods (GB/T 28731-2012, China). The proximate analysis and determination method for the biochemical composition of the feedstock was conducted according to the method utilized in a previous study (Liu et al., 2018b). The weight content (wt.%) of protein was estimated by multiplying the nitrogen (N) content (wt.%) by 6.25 (Liu et al., 2017; Wu et al., 2018). The surface area (SBET), total pore volume (Vpore) and mean pore size (Mpore) were measured by N2 adsorption-desorption isotherms (ASAP2460, USA). SBET was calculated based on the BET method. The surface morphologies of the feedstock and hydrochar were observed by a scanning electron microscope (SEM), (JSM-7001F, JPN) at an accelerating voltage of 20 kV. Fourier transform infrared spectroscopy (FTIR) (VERTEX 70X, GER) was performed to examine the surface functional groups with wavenumbers of 4000–400 cm−1 at a resolution of 2 cm−1. A thermogravimetric analyzer (TGA-1000B, CHN) was used to analyze the combustion behavior of the hydrochar. a non-isothermal combustion procedure was performed by increasing the temperature from 30 °C to 800 °C at a heating rate of 20 °C/min. The carrier gas was high purity air with a flow rate of 120 mL/min. The organic compounds in the aqueous phase were determined by a gas chromatography-mass spectrometer (GC–MS), (Shimadzu QP2010, JPN) equipped with a DB-5 column. Before GC–MS analysis, the organics in the aqueous phase were extracted by ether (Si et al., 2016). The composition of the gaseous product was analyzed using gas chromatography (GC), (SP-6890, CHN). Quantitative analysis of the volatile fatty acids (VFAs) in the aqueous phase was performed using highperformance liquid chromatography (HPLC-10A), (Shimadzu, JPN). 2.4. Calculation Fig. 1. The yields, elemental and proximate compositions of the hydrochar from different HTC conditions. (a) different reaction temperature, (b) different retention time.
The hydrochar yield and energy recovery (ER) were calculated according to the following equations:
Hydrochar yield =
ER =
Mass of hydrochar × 100% Mass of feedstock
Yield of hydrochar × HHV of hydrochar × 100% HHV of feedstock
CS at temperature below 240 °C. Correspondingly, higher aqueous and gaseous yields were obtained by CM in comparison to CS. This might be associated with the higher content of ash in CM which provides an alkaline environment causing crude fat to undergo saponification rather than hydrolysis (Chen et al., 2014). Humic acid and bitumen-based molecules were formed via condensation and polymerization of carbohydrate molecular fragments and then absorbed by hydrochar (Funke et al., 2010), eventually contributing to the higher yield of hydrochar. The retention time had no significant effects on the yield of hydrochar compared to the reaction temperature. The hydrochar yields of CM and CS were within the range of 31.00–37.00% and 30.94–38.43%, respectively, when the retention time increased from 1 h to 4 h. The elemental distribution of CS-derived hydrochar showed an increase in the C content and decrease in the O content as the temperature increased. In particular, the highest C content (51.41%) and the lowest O content (40.61%) in the hydrochar were achieved at 260 °C (Fig. 1). An elemental distribution trend was also observed in CS-derived hydrochar. The highest C content (71.51%) and the lowest O content (22.61%) in the hydrochar were achieved at the same temperature. The lower C content in CM-derived hydrochar might be due to the conversion of crude lipid and protein into water-soluble products. This result was also supported by the much higher TOC concentration in the aqueous phase of CM in comparison to that of CS. In addition, more C in CM migrated into the aqueous phase during HTC. The N content tended to increase in hydrochar with an increased temperature and retention time. N-containing derivatives tended to migrate into the aqueous phase during HTC (Wang et al., 2018c), implying that N-containing
(1)
(2)
3. Results and discussion 3.1. HTC of CM and CS at different conditions Fig. 1 depicts the yields of hydrochar produced from CM and CS at reaction temperatures ranging from 180 °C to 260 °C and retention times ranging from 1 h to 4 h. The hydrochar yield of CM significantly reduced from 50.71% to 35.30% when the temperature rose from 180 °C to 260 °C. CS demonstrated a similar hydrochar yield trend, in which the hydrochar yield reduced from 64.85% to 33.29% throughout the same temperature range. A higher temperature resulted in a decrease in the hydrochar yield coupled with enhancement of the gaseous and aqueous yields. The aqueous yields increased from 27.63% and 24.18% to 52.16% and 34.90% for CM and CS, respectively. Meanwhile, gas content increased from 1.98% and 2.40% to 9.78% and 12.16% for CM and CS, respectively. This result was possibly due to the degradation of carbohydrates into water-soluble organics or gas at a temperature up to 200 °C (Funke et al., 2010; Simsir et al., 2017). The initial skeleton of “polymerized form hydrochar”, originating from lignin, contains a significant number of C]C bonds and is generally more stable and harder to hydrolyze and crack (Mouthier et al., 2018). In comparison, CM achieved 1.02–14.14% lower hydrochar yields than 3
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produced from HTC of polysaccharides and can further react with soluble phenols from lignin to form dispersed char which can cover the surface of hydrochar (Ryu et al., 2010). In addition, bio-oil could be absorbed to the surface of hydrochar since it was not washed with organic solvents, which could also lead to a decrease in the surface area of hydrochar (Zhu et al., 2017). Additional studies are needed in order to propose a suitable washing solvent for the subsequent utilization of hydrochar. CM-derived hydrochar obtained the highest SBET (6.114 m2/ g) and Vpore (0.021 cm3/g) at a temperature of 180 °C and a retention time of 3 h. The highest Mpore of 3.061 nm was achieved at CM-260-3, indicating the enlargement of the pore size and the presence of mesopores which could be favorable for use as a support or absorbent. The SBET and Mpore of CS-derived hydrochar were lower than previously reported values (6.37 m2/g and 22.55 nm), while Vpore was higher than previously reported literature values (0.001 cm3/g) (Liu et al., 2018a). This difference may have resulted from the lower reaction temperature (200 °C) and longer retention time (5 h) used in this study compared to previous values in the literature. The specific surface area and microporous nature of CM-derived hydrochar may lead to the mitigation of tetracycline resistance genes and the disappearance of human pathogenic bacteria (HPB). In particularly, CM-180-3 shows particular promise towards promoting food safety due to its potential to serve as a soil amendment (Chen et al., 2018).
heterocycles could be further repolymerized and form polycyclic aromatic hydrocarbons that were absorbed on the surface of the solid product (Wu et al., 2018). CM-derived hydrochar is inappropriate to be fuel since it has a high content of N (2.61–3.18%) and would subsequently result in the emission of NOX. However, CM-derived hydrochar is attractive for use as a soil amendment owing to its enriched nutrient content (Eibisch et al., 2013). The proximate analysis showed that the content of VM in CM-derived hydrochar significantly reduced from 83.67% to 50.15% as the temperature increased (Fig. 1). The reduction of VM might be due to the hydrolysis and decomposition of the components in the raw biomass, as previously discussed, and the additional production of gaseous and aqueous products (Kalderis et al., 2014). At the same time, the content of ash increased from 16.49% to 32.84% in CM-derived hydrochar. In comparison, a lower ash content (3.00–5.06%) and VM content (78.82–50.26%) were observed in CS-derived hydrochar, and it was evident that the ash content was not heavily impacted by the HTC conditions. This was consistent with the result of a previous study (Xiao et al., 2012). The contents of ash, FC and VM in hydrochar remained stable after 3 h, suggesting that sufficient carbonization was achieved over a time of 3 h and at a temperature of 240 °C. The lower yield and C content in conjunction with the higher content of VM and ash of CMderived hydrochar indicated its lower carbonization degree, which owing to the higher content of proteins and lipids, and lower content of carbohydrates in raw CM.
3.2.3. Surface functional groups FTIR spectra further revealed the structural evolution of the feedstock and hydrochar. The peak at 1064 cm−1 in CS-180-3 corresponding to CeO tended to decline as the temperature increased, which was attributed to the degradation of cellulose, hemicellulose and the cracking of their respective β-glycosidic bonds (Liu et al., 2013). On the contrary, the intensity of the peak demonstrated an opposite trend. The enhancement might be due to the CeN mode of the stretching vibration of N-containing aromatic hydrocarbons in CM-derived hydrochar. Peaks at 1420 cm−1 and 1590 cm−1 corresponding to aromatic C]C bonds and aromatic CeH bonds slightly increased as the temperature and retention time increased, demonstrating the development of aromatization (Wang et al., 2018a). The increasing aromatized hydrochar structure was mainly formed by the aromatization of carbohydrates and molecules obtained via dehydration of lignin (Funke et al., 2010). The peaks around 1600–1800 cm−1 were attributed to the C]O stretching vibration of carboxylic acids, ketones, esters aldehydes and/or phenol compounds (Chen et al., 2018). The band intensities of hydrochar were obviously strengthened compared to the raw materials, which may be due to the absorbed O-containing derivatives on the surface of hydrochar. The stretching vibration between 2800 and 3000 cm−1 indicated the presence of aliphatic and aromatic CeH. The peak intensity in CSderived hydrochar dropped as the temperature and retention time increased, suggesting the promoted degree of carbonization and the separation of the methoxyl functional group from the aromatic ring structure of lignin (Lin et al., 2019). CM-derived hydrochar showed an inverse trend, and the increased intensity of the peaks may be attributed to the decomposition of lipid components. A broad band was observed at 3200–3600 cm−1 and was ascribed to hydroxyl eOH, carboxyl eCOOH and phenolic eOH functional groups (Ngaosuwan et al., 2016). The hydrochar peaks were less intense compared to the raw feedstock, probably due to the promoted dehydration and condensation polymerization reactions during HTC. Condensation polymerization was enhanced at increased temperatures and retention times (Yu et al., 2008). The coexistence of the eCOOH and eOH functional groups have been confirmed to be responsible for the hydrophilic nature of hydrochar. Both the presence of functional groups and the pore structure properties influence the adsorption capacity of hydrochar, but the former plays a larger role than the latter (Tan et al., 2015).
3.2. Physicochemical characterization of CM- and CS-derived hydrochar 3.2.1. Surface morphology SEM images illustrate the changes in surface morphology of the raw biomass and the resulting hydrochar. Continuous, compact and flat rough surfaces were observed in raw CM and CS. As for CM, its surface became more rough and uneven and demonstrated a multi-porous and irregular structure after HTC. More microspores appeared when the temperature increased from 180 °C to 260 °C. As for CS, an irregular surface was observed after HTC, which was consistent with the removal of hemicellulose from the cell wall (Lin et al., 2019). Pores with different sizes (1.0–4.5 μm) were found on the surface of the hydrochar. The size of the pores in the surface of CS-derived hydrochar expanded as the temperature increased. 3.2.2. Pore size and distribution The pore size and distribution were analyzed to further reveal the physicochemical characteristics of hydrochar (Table 2). SBET increased as the temperature and retention time increased during HTC of CS. The maximum values of Vpore (0.017 cm3/g) and SBET (4.256 m2/g) for CSderived hydrochar were achieved for CS-derived hydrochar at 260 °C and 3 h. In general, CM-derived hydrochar had a higher SBET (2.199–6.114 m2/g) and Vpore (0.010–0.021 cm3/g) than CS-derived hydrochar (2.236–4.256 m2/g and 0.005–0.017 cm3/g). This was mainly due to the lower polysaccharides content of CM. Furans can be Table 2 Texture properties of different hydrochar. Sample
SBET (m2/g)
Vpore (cm3/g)
Mpore (nm)
CM-180-3 CM-240-1 CM-240-3 CM-240-4 CM-260-3 CS-180-3 CS-240-1 CS-240-3 CS-240-4 CS-260-3
6.114 2.790 3.760 3.549 2.199 3.045 2.236 3.125 3.579 4.256
0.021 0.010 0.014 0.014 0.011 0.007 0.009 0.005 0.007 0.017
1.432 1.425 1.407 1.422 3.061 1.439 1.433 1.423 1.456 1.421
4
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Fig. 2. The analysis of aqueous phase obtained from HTC of CM and CS. (a) organic composition, (b) organic acids concentration.
3.3. Characterization of HTC by-products 3.3.1. The characteristics of the aqueous phase Fig. 2 shows the composition of the aqueous products based on qualitative analysis conducted via GC–MS. The CM-derived aqueous phase mainly consisted of phenols, ketones, acids and N-containing compounds. A limited amount of benzenes, alkanes and aldehydes were detected. N-containing compounds were not detected in the CS aqueous phase. The content of aromatic phenols, such as phenol and cresol, increased from 5.39% to 25.67% as the temperature increased in the CM aqueous phase. The phenolic content exhibited an initial increase and a subsequent decline as the retention time reached up to 4 h. A maximum phenol content of 44.88% was achieved at 240 °C and 3 h. The phenolic content was higher in the CS aqueous phase which increased from 25.81% to 68.73% as the temperature increased. The formation of phenols was in accordance with the degradation of unstable ether bonds connecting lignin monomers under hydrothermal conditions (Kang et al., 2011). In addition, the content of aldehydes, including furfural and 5-hydroxymethylfurfural (5-HMF), decreased from 39.91% to 3.62% as the temperature increased in the CS aqueous phase. Aldehydes were probably produced from the dehydration of fructose, and these compounds can be subsequently converted into acids by hydrolysis. In the CS aqueous phase, the content of ketones, acids and aldehydes decreased when the temperature exceeded 260 °C, whereas the content of phenols and benzenes derivatives increased significantly, indicating the enhanced presence of the aromatization reaction. The acids content substantially decreased, but the ketones content increased in the CM aqueous phase. CM resulted in a higher content of acids than CS after HTC, potentially due to the hydrolysis of lipids intro free fatty acids and glycerol (Li et al., 2014). N-containing
Fig. 3. Energy calculation of CM- and CS-derived hydrochar. (a) HHV, (b) ER.
Fig. 4. Van Krevelen diagram of CM- and CS-derived hydrochar.
heterocycles were detected in the CM aqueous phase owing to the conversion of proteins. The organic acids present within the aqueous phase may be further converted into biogas through subsequent treatment via anaerobic 5
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Fig. 5. TG and DTG curves of the feedstock and their hydrochar. (a) feedstock, (b) CM- and CS-derived hydrochar from different temperature, (c) CM- and CS-derived hydrochar from different retention time. Table 3 Combustion characteristics parameters of the feedstock and hydrochar. Sample
Ti (°C)
Tm (°C)
Tb (°C)
Residue (%)
Sample
Ti (°C)
Tm (°C)
Tb (°C)
Residue (%)
CM CM-180-3 CM-200-3 CM-220-3 CM-240-3 CM-260-3 CM-240-1 CM-240-2 CM-240-4
242.07 232.00 262.40 266.54 316.53 361.45 326.88 288.01 342.52
329.27 365.19 372.58 356.55 488.98 509.81 513.70 561.17 526.66
592.73 573.03 582.46 597.12 694.01 673.52 691.25 641.49 685.61
22.08 34.33 42.01 44.16 45.55 55.94 38.34 50.72 55.70
CS CS-180-3 CS-200-3 CS-220-3 CS-240-3 CS-260-3 CS-240-1 CS-240-2 CS-240-4
248.56 257.04 261.74 229.51 213.73 332.83 334.81 275.01 293.67
321.04 341.46 378.69 369.22 494.94 508.11 496.76 481.59 502.83
542.64 584.67 553.01 612.52 649.85 770.46 640.87 606.16 641.85
30.53 30.10 31.95 37.86 34.92 30.21 32.75 26.31 37.55
lactic acid concentration decreased by 59.79% in the CM-derived aqueous phase. In contrast, the concentration of butyric acid was less affected by the HTC conditions. Hence, the CM- and CS-derived aqueous phase produced at 180 °C and 3 h contained an abundance of volatile organic acids which could be used as an organic source for subsequent biogas production.
digestion (Si et al., 2019). The concentration of total volatile fatty acids (VFAs) decreased from 10.27 g/L to 1.21 g/L (CS-derived aqueous phase), and decreased from 8.67 g/L to 4.71 g/L (CM-derived aqueous phase) as the temperature increased. Hence, the formation of acids is more favorable at lower temperatures (Simsir et al., 2017). The concentrations of formic acid and lactic acid decreased by 94.10% and 85.58%, respectively, in the CS-derived aqueous phase. Similarly, the 6
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the retention time had no significant influence on the Tm. The residue content ranged from 26.31% to 37.86% (CS-derived hydrochar), and 34.33% to 44.94% (CM-derived hydrochar) as the temperature increased. CM-derived hydrochar exhibited an increased residue content as the temperature and retention time increased, which was consistent with the increased ash content based on the proximate analysis. Considering the combustion properties (e.g., HHV, ER and TG curves) of hydrochar and the HTC process cost, it is clear that CS-derived hydrochar produced at 240 °C for 1 h is more favorable for use as a solid fuel.
3.3.2. Composition of gas products The gas products after HTC mainly consisted of CO2, H2 and CH4. The total gas volume of CM was slightly lower than that produced by CS, owing to the lower content of cellulose and hemicellulose in CM compared with CS (Kalderis et al., 2014; Liu et al., 2018b). CM resulted in a lower content of H2 and CH4 than CS during the HTC process. Retention time had no significant impact on the gas composition. 3.4. Fuel properties of CM- and CS-derived hydrochar
4. Conclusion
3.4.1. HHV and ER The HHV and ER of the feedstock and hydrochar were characterized in order to further assess the fuel properties after HTC (Fig. 3). CSderived hydrochar had a higher HHV (18.26–26.63 MJ/kg) than CMderived hydrochar (12.92–17.00 MJ/kg). The HHV increased as the temperature increased, which corresponded to the significant decrease in the O content. Specifically, the HHV of CS-240-1 (25.15 MJ/kg) and CS-260-3 (26.63 MJ/kg) were higher than that of commercial lignite (25.0 MJ/kg) (Gao et al., 2016), implying that hydrochar has the potential to be utilized as a fuel source. The ER of CS- and CM-derived hydrochar exhibited a decreasing trend as the temperature increased. As the retention time increased from 1 h to 4 h, the ER of CM-derived hydrochar as a function of the HHV correspondingly increased from 27.47% to 40.76%. The HHV of CM-derived hydrochar was lower than that of CS-derived hydrochar, which could be due to the higher content of ash in CM (Fig. 1). The HHV of CS-260-3 obtained in this study exhibited higher values than most of the values presented in the literature for CS-derived hydrochar (15.6–29.79 MJ/kg) and pyrochar (16.95–28.66 MJ/kg) (Zhu et al., 2016; Guo et al., 2015; Intani et al., 2018; Qi et al., 2018; Salema et al., 2017). The coalification degree of hydrochar is illustrated in Fig. 4. Overall, the H/C and O/C content of hydrochar shifted from the upper right to the lower left and followed vector directions as the temperature increased. The increased temperature caused a decrease in the H/C and O/C atomic ratios, suggesting enhanced decarboxylation, dehydration and condensation reactions (Wataniyakul et al., 2018). The H/C ratio of CS and CM-derived hydrochar decreased to 0.80 and 1.07, respectively after HTC. The O/C content also decreased to 0.24 and 0.59, respectively, corresponding to the dehydration and decarboxylation pathways, which was in agreement with the increase in the HHV. In addition, demethanation occurred during the HTC of CM. The ratios of H/C and O/C increased as the retention time increased for CS-derived hydrochar. However, the O/C ratio decreased with the occurrence of dehydration reactions. The position of the CS-derived hydrochar produced at 240–260 °C was within the typical range of peat and lignite.
This study gives full comparisons of the product distribution and hydrochar properties of CM and CS processed via HTC. CM-derived hydrochar exhibited an enriched concentration of nutrients, abundant ORGs on the surface and a microporous structure, which may increase the nutrition level of the hydrochar and be appropriate as a soil amendment. In comparison, CS-derived hydrochar resulted in a higher yield with a higher carbon content than CM-derived hydrochar. CSderived hydrochar had a higher HHV, lower ratios of O/C and H/C and a better combustion behavior compared with CM-derived hydrochar, indicating the potential for CS-derived hydrochar to be used as an attractive fuel source. Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2016YFD0501402), the National Natural Science Foundation of China (51861125103) and Beijing Dairy Industry Innovation Team (BAIC06-2019). We thank Dr. Buchun Si (China Agricultural University) and Jamison Watson (University of Illinois at Urbana-Champaign) for improving the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.121855. References Chen, J., Wang, L., Zhang, B., Li, R., Shahbazi, A., 2018. Hydrothermal liquefaction enhanced by various chemicals as a means of sustainable dairy manure treatment. Sustainability-Basel 10 (1), 230. Chen, W., Zhang, Y., Zhang, J., Schideman, L., Yu, G., Zhang, P., Minarick, M., 2014. Coliquefaction of swine manure and mixed-culture algal biomass from a wastewater treatment system to produce bio-crude oil. Appl. Energ. 128, 209–216. Eibisch, N., Helfrich, M., Don, A., Mikutta, R., Kruse, A., Ellerbrock, R., Flessa, H., 2013. Properties and degradability of hydrothermal carbonization products. J. Environ. Qual. 42 (5), 1565–1573. Funke, A., Ziegler, F., Berlin, T., 2010. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuel. Bioprod. Bior. 4, 160–177. Gao, P., Zhou, Y., Meng, F., Zhang, Y., Liu, Z., Zhang, W., Xue, G., 2016. Preparation and characterization of hydrochar from waste eucalyptus bark by hydrothermal carbonization. Energy 97, 238–245. Guo, S., Dong, X., Wu, T., Shi, F., Zhu, C., 2015. Characteristic evolution of hydrochar from hydrothermal carbonization of corn stalk. J. Anal. Appl. Pyrol. 116, 1–9. Guo, S., Dong, X., Wu, T., Zhu, C., 2016. Influence of reaction conditions and feedstock on hydrochar properties. Energ. Convers. Manage. 123, 95–103. Intani, K., Latif, S., Cao, Z., Müller, J., 2018. Characterisation of biochar from maize residues produced in a self-purging pyrolysis reactor. Bioresour. Technol. 265, 224–235. Jain, A., Balasubramanian, R., Srinivasan, M.P., 2015. Production of high surface area mesoporous activated carbons from waste biomass using hydrogen peroxide-mediated hydrothermal treatment for adsorption applications. Chem. Eng. J. 273, 622–629. Kalderis, D., Kotti, M., Méndez, A., 2014. Characterization of hydrochars produced by hydrothermal carbonization of rice husk. Solid Earth 5 (1), 477–483. Kang, S., Li, X., Fan, J., Chang, J., 2011. Characterization of hydrochars produced by hydrothermal carbonization of lignin, cellulose, d-xylose, and wood meal. Ind. Eng. Chem. Res. 51 (26), 9023–9031. Li, H., Liu, Z., Zhang, Y., Li, B., Lu, H., Duan, N., Liu, M., Zhu, Z., Si, B., 2014. Conversion efficiency and oil quality of low-lipid high-protein and high-lipid low-protein
3.4.2. Combustion behavior TGA was utilized to determine the combustion behavior of the raw feedstock and the hydrochar (Fig. 5). The combustion parameters derived from the profiles were listed in Table 3. Two major degradation stages were found in the raw feedstock, while only one sharp peak was detected for hydrochar. The first stage of the feedstock occurred between 30 and 150 °C, corresponding to the evaporation of moisture (Gao et al., 2016). The main weight loss occurred at the second stage, mainly owing to the devolatilization and combustion of the samples. The ignition temperature (Ti) of CM-derived hydrochar increased from 232.00 °C to 361.45 °C, which was positively correlated with an increase in the reaction temperature. Similarly, the Ti of CS-derived hydrochar increased from 248.56 °C to 332.83 °C. This result was due to the enhanced destruction of the solid matrix of easily liberated carbon structures (Lucian et al., 2018), and this was also due to the reduced VM content of the hydrochar (Wang et al., 2018b). A number of peaks in the DTG curves were due to the hydrolysis of distinct components in the raw biomass which was accelerated by the increase in the HTC reaction severity (Funke et al., 2010). A higher maximum weight loss rate (Tm) was also observed with an increase of the temperature. In comparison, 7
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microalgae via hydrothermal liquefaction. Bioresour. Technol. 154, 322–329. Lin, Q., Zhang, C., Wang, X., Cheng, B., Mai, N., Ren, J., 2019. Impact of activation on properties of carbon-based solid acid catalysts for the hydrothermal conversion of xylose and hemicelluloses. Catal. Today 319, 31–40. Liu, Y., Ma, S., Chen, J., 2018a. A novel pyro-hydrochar via sequential carbonization of biomass waste: preparation, characterization and adsorption capacity. J. Clean. Prod. 176, 187–195. Liu, Y., Yao, S., Wang, Y., Lu, H., Brar, S.K., Yang, S., 2017. Bio- and hydrochars from rice straw and pig manure: Inter-comparison. Bioresour. Technol 235, 332–337. Liu, Z., Li, H., Zeng, J., Liu, M., Zhang, Y., Liu, Z., 2018b. Influence of Fe/HZSM-5 catalyst on elemental distribution and product properties during hydrothermal liquefaction of Nannochloropsis sp. Algal Res. 35, 1–9. Liu, Z., Quek, A., Kent Hoekman, S., Balasubramanian, R., 2013. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 103, 943–949. Liu, Z., Zhang, F., Hoekman, S.K., Liu, T., Gai, C., Peng, N., 2016. Homogeneously dispersed zerovalent iron nanoparticles supported on hydrochar-derived porous carbon: simple, in situ synthesis and use for dechlorination of PCBs. ACS Sustain. Chem. Eng. 4 (6), 3261–3267. Lucian, M., Volpe, M., Gao, L., Piro, G., Goldfarb, J.L., Fiori, L., 2018. Impact of hydrothermal carbonization conditions on the formation of hydrochars and secondary chars from the organic fraction of municipal solid waste. Fuel 233, 257–268. Monlau, F., Sambusiti, C., Barakat, A., Guo, X.M., Latrille, E., Trably, E., Steyer, J., Carrere, H., 2012. Predictive models of biohydrogen and biomethane production based on the compositional and structural features of lignocellulosic materials. Environ. Sci. Technol. 46 (21), 12217–12225. Mouthier, T., Appeldoorn, M.M., Pel, H., Schols, H.A., Gruppen, H., Kabel, M.A., 2018. Corn stover lignin is modified differently by acetic acid compared to sulfuric acid. Ind. Crop. Prod. 121, 160–168. Ngaosuwan, K., Goodwin, J.G., Prasertdham, P., 2016. A green sulfonated carbon-based catalyst derived from coffee residue for esterification. Renew. Energ. 86, 262–269. Qi, J., Zhao, J., Xu, Y., Wang, Y., Han, K., 2018. Segmented heating carbonization of biomass: Yields, property and estimation of heating value of chars. Energy 144, 301–311. Ryu, J., Suh, Y., Suh, D.J., Ahn, D.J., 2010. Hydrothermal preparation of carbon microspheres from mono-saccharides and phenolic compounds. Carbon 48 (7), 1990–1998. Salema, A.A., Afzal, M.T., Bennamoun, L., 2017. Pyrolysis of corn stalk biomass briquettes in a scaled-up microwave technology. Bioresour. Technol. 233, 353–362. Si, B., Li, J., Zhu, Z., Zhang, Y., Lu, J., Shen, R., Zhang, C., Xing, X., Liu, Z., 2016. Continuous production of biohythane from hydrothermal liquefied cornstalk biomass via two-stage high-rate anaerobic reactors. Biotechnol. Biofuels 9 (1), 254–269. Si, B., Yang, L., Zhou, X., Watson, J., Tommaso, G., Chen, W., Liao, Q., Duan, N., Liu, Z., Zhang, Y., 2019. Anaerobic conversion of the hydrothermal liquefaction aqueous phase: fate of organics and intensification with granule activated carbon/ozone pretreatment. Green Chem. 21 (6), 1305–1318. Simsir, H., Eltugral, N., Karagoz, S., 2017. Hydrothermal carbonization for the preparation of hydrochars from glucose, cellulose, chitin, chitosan and wood chips via lowtemperature and their characterization. Bioresour. Technol. 246, 82–87.
8
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Comparative production of biochars from corn stalk and cow manure a
b
Ziyun Liu , Yuanhui Zhang , Zhidan Liu
a,⁎
T
a Laboratory of Environment-Enhancing Energy (E2E), Key Laboratory of Agricultural Engineering in Structure and Environment, Ministry of Agriculture, College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China b Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
G R A P H I C A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal carbonization Lignocellulosic waste Hydrochar Physicochemical property Solid biofuel
The aim of the present work was to compare corn stalk (CS) and cow manure (CM) for hydrochar production at different reaction temperatures (180–260 °C) and retention times (1–4 h). CM and CS resulted in hydrochars with significantly different physicochemical properties; however, both led to similar yields (30–65%). CM-derived hydrochar had a lower carbon content but a higher nitrogen and ash content than CS-derived hydrochar. CMderived hydrochar demonstrated potential as a soil amendment due to its higher content of nitrogen, the presence of surface functional groups and higher specific surface area in comparison to CS-derived hydrochar. In comparison, CS-derived hydrochar demonstrated suitability as a solid fuel due to its high heating value and low ignition temperature. This study revealed that the composition of lignocellulose significantly impacted the properties and thus potential applications of hydrochar.
1. Introduction In recent years, agricultural wastes, including livestock manure and crop straws, have attracted increasing attentions among researchers due to their abundant distribution, carbon neutrality and ability to sidestep competition with food for land use. Thus, agricultural wastes have the potential to serve as a carbon source for the production of biomaterials and bioenergy. The quantity of livestock manure in China amounts to ⁎
250 million tons annually, due to the rapid development of the livestock and poultry industry (Yang et al., 2017). In addition, lignocellulosic wastes are produced at a volume of about 200 billion tons per year around the world (Monlau et al., 2012). Hydrothermal carbonization (HTC) is a thermochemical conversion technology that can convert agricultural biomass into hydrochar and other by-products (synthesis gas, aqueous phase, etc.). HTC is generally conducted at a temperature ranging from 150 to 350 °C, at a certain retention time
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.biortech.2019.121855 Received 30 May 2019; Received in revised form 18 July 2019; Accepted 20 July 2019 Available online 22 July 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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longer than 1 h and at a self-generated pressure. Agricultural wastes are rich in organic polymers like lignin, cellulose, hemicellulose, protein and lipid (Wu et al., 2018). Previous studies have speculated the production mechanism of hydrochar and by-products during HTC (Liu et al., 2018b; Zhu et al., 2015). The hydrolysis and cleavage of biomass are facilitated by subcritical water which can act as both reagent and a catalyst (Gao et al., 2016). The incorporation of water triggers a series of parallel reactions, such as the removal of hydroxyl groups via dehydration, the loss of carboxyl and carbonyl groups via decarboxylation and the cleavage of ester and ether bonds via hydrolysis of biomass components (Liu et al., 2017). Lignin, a recalcitrant component of biomass, has a complex and branched structure composed of hydroxyphenol, guaiacol and syringol connected by ether linkages (e.g. β–O–4) and CeC bonds (e.g. β–β, β–5). During the HTC process, it is difficult for lignin to undergo hydrolysis and depolymerization of lignin (Mouthier et al., 2018; Zhou et al., 2018). In comparison, it is easier to decompose carbohydrates, lipid and proteins under identical conditions. Specifically, the degradation products of carbohydrates may react with lignin-derived phenols and form the dispersed char that is adsorbed to the initial skeleton of lignin (Mouthier et al., 2018; Zhou et al., 2018). Hydrochar from biomass HTC has previously been investigated as a fuel source (Wang et al., 2018b), catalyst precursor (Yan et al., 2014), soil amendment (Eibisch et al., 2013), contaminant adsorbent (Tan et al., 2015), and capacitor (Yang et al., 2019). The varied application of hydrochar is mainly based on its carbon sequestration ability, oxygenated functional groups (OFGs) and various porous structures, which is benefited from the presence of various biomass components. For instance, Guo et al. found the grape marc hydrochar exhibited a significantly higher carbon content than hydrochar derived from corn stalk and wood, which could be partly attributed to the lower cellulose content and the higher C content of grape marc feedstock (Guo et al., 2016). Another study conducting co-HTC of a food waste (FW)-woody biomass blend indicated that the hydrochar higher heating value (HHV) increased as the ratio of FW increased (Wang et al., 2018b). Additionally, most of the nitrogen and chlorine elements in the biomass migrated into the aqueous phase, indicating that the emission of NOx and SOx decreased (Wang et al., 2018c). Liu et al. developed a facile strategy to prepare the catalyst support for nanoscale zerovalent iron using pinewood sawdust-derived hydrochar, owing to the absorption capacity and porous structure of the hydrochar (Liu et al., 2016). The mesoporous structure of hydrochar had a strong correlation with the presence of OFGs, which favored the adsorption ability of the contaminants (Jain et al., 2015). Hence, it is of great significance to prepare hydrochar with varied physicochemical properties aiming at different application purposes. Recently, a couple of substrates have been used as a feedstock for hydrochar production via HTC (Liu et al., 2017; Mouthier et al., 2018). Smith et al. found that willow-derived hydrochar had a HHV of 25 MJ/ kg which was higher than the HHV of the hydrochar derived from macroalgae (16.8 MJ/kg) under the same HTC conditions. The disparity between these two values may be attributed to the higher ash content of the macroalgae (Smith et al., 2016). Obviously, the characteristics of hydrochar are largely affected by the biomass components. In addition, the processing parameters, e.g., temperature, retention time, pH, catalyst, reaction solvent and solid load, also play significant roles in determining the properties of hydrochar which in turn affect its application prospects (Guo et al., 2016; Simsir et al., 2017). However, limited information is available comparing different agricultural wastes for hydrochar production at various reaction temperatures and retention times. Cow manure (CM) and corn stalk (CS) are two typical agricultural wastes. CS is mainly comprised of carbohydrates and lignin, whereas CM consists of carbohydrates, lignin, protein, lipid and an abundance of ash. Thus, the HTC process of these two typical agricultural wastes was compared at different reaction conditions in this study. The main
Table 1 Characteristics of the feedstock.a CM
CS
Biochemical compositions analysis (wt%) Cellulose 19.12 ± 0.12 Hemicellulose 22.25 ± 1.18 Lignin 12.70 ± 0.89 6.77 ± 0.18 Crude proteinb Crude lipid 5.43 ± 0.45
28.93 ± 0.36 28.26 ± 3.48 9.78 ± 0.10 ND ND
Elemental analysis (wt%) Carbon (C) Hydrogen (H) Nitrogen (N) Sulfur (S) Oxygen (O)c
43.18 ± 0.18 5.92 ± 0.41 1.08 ± 0.03 0.25 ± 0.09 49.57 ± 0.13
44.96 ± 0.03 6.34 ± 0.45 0.49 ± 0.03 0.08 ± 0.07 48.13 ± 0.34
Proximate analysis (wt%) Ash Volatile matters (VM) Fixed carbond (FC)
18.30 ± 0.38 83.67 ± 0.55 0.93
5.65 ± 0.08 71.43 ± 2.22 21.59
a b c d
Dry basis. Calculated by the content of N (The wt.% protein = the wt.% N × 6.25). Calculated by difference of C, H, N and S. Calculated by difference of ash and VM.
objectives of this study are to: (1) Investigate the influence of temperature (180–260 °C) and retention time (1–4 h) on hydrochar production from CM and CS; (2) Compare the physicochemical properties of CM- and CS-derived hydrochar based on their porous structure, OFGs and surface morphology; (3) Discuss the application potential of CMand CS-derived hydrochar. 2. Experimental section 2.1. Feedstock CM and CS were collected in Lianyungang (Jiangsu, CHN) and Beijing (CHN), respectively. The basic physicochemical properties including biochemical, elemental and proximate compositions of CM and CS are shown in Table 1. 2.2. HTC procedure HTC experiments were performed in a stainless-steel cylindrical reactor (Parr 4848, USA) with a working volume of 100 mL. The raw materials and deionized water were added at a solid content of 15 wt%, and they were then mixed for 30 min before heating. The closed reactor was purged with nitrogen to replace the air. The system was then heated to the desired temperature (180, 200, 220, 240 and 260 °C) using a cylindrical jacket coupled with electric heating. During the reaction, the pressure ranged from 8.27 to 30.34 bar. Once the designed temperature was reached, the retention time was maintained for 1, 2, 3 and 4 h, respectively. The reactor was cooled down to ambient temperature with the assistance of an electric fan after the reaction completed. The produced gas was collected by gas bags, and the hydrochar and liquid product were subsequently separated via vacuum filtration. After drying at 105 ± 5 °C for 24 h, the hydrochar was stored in enclosed plastic bottles before further characterization. The aqueous phase was further filtered using a 0.45 μm membrane and stored in a refrigerator at 4 °C. The obtained hydrochars were labeled as “CM-m-n” and “CS-mn” in which “m” represented the reaction temperature (180–260 °C) and “n” indicated the retention time (1–4 h). 2.3. Characterization For the solid analysis, the elemental composition (C, H, N and S) of the feedstock and hydrochar were determined using an elemental 2
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analyzer (Vario EL III, GER). The HHV was calculated according to Dulong’s formula (Qi et al., 2018). The determination of the content of ash and volatile matter (VM) were conducted in a muffle furnace at 550 °C for 4 h and 900 °C for 7 min, respectively. The content of fixed carbon was calculated by difference, according to standard methods (GB/T 28731-2012, China). The proximate analysis and determination method for the biochemical composition of the feedstock was conducted according to the method utilized in a previous study (Liu et al., 2018b). The weight content (wt.%) of protein was estimated by multiplying the nitrogen (N) content (wt.%) by 6.25 (Liu et al., 2017; Wu et al., 2018). The surface area (SBET), total pore volume (Vpore) and mean pore size (Mpore) were measured by N2 adsorption-desorption isotherms (ASAP2460, USA). SBET was calculated based on the BET method. The surface morphologies of the feedstock and hydrochar were observed by a scanning electron microscope (SEM), (JSM-7001F, JPN) at an accelerating voltage of 20 kV. Fourier transform infrared spectroscopy (FTIR) (VERTEX 70X, GER) was performed to examine the surface functional groups with wavenumbers of 4000–400 cm−1 at a resolution of 2 cm−1. A thermogravimetric analyzer (TGA-1000B, CHN) was used to analyze the combustion behavior of the hydrochar. a non-isothermal combustion procedure was performed by increasing the temperature from 30 °C to 800 °C at a heating rate of 20 °C/min. The carrier gas was high purity air with a flow rate of 120 mL/min. The organic compounds in the aqueous phase were determined by a gas chromatography-mass spectrometer (GC–MS), (Shimadzu QP2010, JPN) equipped with a DB-5 column. Before GC–MS analysis, the organics in the aqueous phase were extracted by ether (Si et al., 2016). The composition of the gaseous product was analyzed using gas chromatography (GC), (SP-6890, CHN). Quantitative analysis of the volatile fatty acids (VFAs) in the aqueous phase was performed using highperformance liquid chromatography (HPLC-10A), (Shimadzu, JPN). 2.4. Calculation Fig. 1. The yields, elemental and proximate compositions of the hydrochar from different HTC conditions. (a) different reaction temperature, (b) different retention time.
The hydrochar yield and energy recovery (ER) were calculated according to the following equations:
Hydrochar yield =
ER =
Mass of hydrochar × 100% Mass of feedstock
Yield of hydrochar × HHV of hydrochar × 100% HHV of feedstock
CS at temperature below 240 °C. Correspondingly, higher aqueous and gaseous yields were obtained by CM in comparison to CS. This might be associated with the higher content of ash in CM which provides an alkaline environment causing crude fat to undergo saponification rather than hydrolysis (Chen et al., 2014). Humic acid and bitumen-based molecules were formed via condensation and polymerization of carbohydrate molecular fragments and then absorbed by hydrochar (Funke et al., 2010), eventually contributing to the higher yield of hydrochar. The retention time had no significant effects on the yield of hydrochar compared to the reaction temperature. The hydrochar yields of CM and CS were within the range of 31.00–37.00% and 30.94–38.43%, respectively, when the retention time increased from 1 h to 4 h. The elemental distribution of CS-derived hydrochar showed an increase in the C content and decrease in the O content as the temperature increased. In particular, the highest C content (51.41%) and the lowest O content (40.61%) in the hydrochar were achieved at 260 °C (Fig. 1). An elemental distribution trend was also observed in CS-derived hydrochar. The highest C content (71.51%) and the lowest O content (22.61%) in the hydrochar were achieved at the same temperature. The lower C content in CM-derived hydrochar might be due to the conversion of crude lipid and protein into water-soluble products. This result was also supported by the much higher TOC concentration in the aqueous phase of CM in comparison to that of CS. In addition, more C in CM migrated into the aqueous phase during HTC. The N content tended to increase in hydrochar with an increased temperature and retention time. N-containing derivatives tended to migrate into the aqueous phase during HTC (Wang et al., 2018c), implying that N-containing
(1)
(2)
3. Results and discussion 3.1. HTC of CM and CS at different conditions Fig. 1 depicts the yields of hydrochar produced from CM and CS at reaction temperatures ranging from 180 °C to 260 °C and retention times ranging from 1 h to 4 h. The hydrochar yield of CM significantly reduced from 50.71% to 35.30% when the temperature rose from 180 °C to 260 °C. CS demonstrated a similar hydrochar yield trend, in which the hydrochar yield reduced from 64.85% to 33.29% throughout the same temperature range. A higher temperature resulted in a decrease in the hydrochar yield coupled with enhancement of the gaseous and aqueous yields. The aqueous yields increased from 27.63% and 24.18% to 52.16% and 34.90% for CM and CS, respectively. Meanwhile, gas content increased from 1.98% and 2.40% to 9.78% and 12.16% for CM and CS, respectively. This result was possibly due to the degradation of carbohydrates into water-soluble organics or gas at a temperature up to 200 °C (Funke et al., 2010; Simsir et al., 2017). The initial skeleton of “polymerized form hydrochar”, originating from lignin, contains a significant number of C]C bonds and is generally more stable and harder to hydrolyze and crack (Mouthier et al., 2018). In comparison, CM achieved 1.02–14.14% lower hydrochar yields than 3
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produced from HTC of polysaccharides and can further react with soluble phenols from lignin to form dispersed char which can cover the surface of hydrochar (Ryu et al., 2010). In addition, bio-oil could be absorbed to the surface of hydrochar since it was not washed with organic solvents, which could also lead to a decrease in the surface area of hydrochar (Zhu et al., 2017). Additional studies are needed in order to propose a suitable washing solvent for the subsequent utilization of hydrochar. CM-derived hydrochar obtained the highest SBET (6.114 m2/ g) and Vpore (0.021 cm3/g) at a temperature of 180 °C and a retention time of 3 h. The highest Mpore of 3.061 nm was achieved at CM-260-3, indicating the enlargement of the pore size and the presence of mesopores which could be favorable for use as a support or absorbent. The SBET and Mpore of CS-derived hydrochar were lower than previously reported values (6.37 m2/g and 22.55 nm), while Vpore was higher than previously reported literature values (0.001 cm3/g) (Liu et al., 2018a). This difference may have resulted from the lower reaction temperature (200 °C) and longer retention time (5 h) used in this study compared to previous values in the literature. The specific surface area and microporous nature of CM-derived hydrochar may lead to the mitigation of tetracycline resistance genes and the disappearance of human pathogenic bacteria (HPB). In particularly, CM-180-3 shows particular promise towards promoting food safety due to its potential to serve as a soil amendment (Chen et al., 2018).
heterocycles could be further repolymerized and form polycyclic aromatic hydrocarbons that were absorbed on the surface of the solid product (Wu et al., 2018). CM-derived hydrochar is inappropriate to be fuel since it has a high content of N (2.61–3.18%) and would subsequently result in the emission of NOX. However, CM-derived hydrochar is attractive for use as a soil amendment owing to its enriched nutrient content (Eibisch et al., 2013). The proximate analysis showed that the content of VM in CM-derived hydrochar significantly reduced from 83.67% to 50.15% as the temperature increased (Fig. 1). The reduction of VM might be due to the hydrolysis and decomposition of the components in the raw biomass, as previously discussed, and the additional production of gaseous and aqueous products (Kalderis et al., 2014). At the same time, the content of ash increased from 16.49% to 32.84% in CM-derived hydrochar. In comparison, a lower ash content (3.00–5.06%) and VM content (78.82–50.26%) were observed in CS-derived hydrochar, and it was evident that the ash content was not heavily impacted by the HTC conditions. This was consistent with the result of a previous study (Xiao et al., 2012). The contents of ash, FC and VM in hydrochar remained stable after 3 h, suggesting that sufficient carbonization was achieved over a time of 3 h and at a temperature of 240 °C. The lower yield and C content in conjunction with the higher content of VM and ash of CMderived hydrochar indicated its lower carbonization degree, which owing to the higher content of proteins and lipids, and lower content of carbohydrates in raw CM.
3.2.3. Surface functional groups FTIR spectra further revealed the structural evolution of the feedstock and hydrochar. The peak at 1064 cm−1 in CS-180-3 corresponding to CeO tended to decline as the temperature increased, which was attributed to the degradation of cellulose, hemicellulose and the cracking of their respective β-glycosidic bonds (Liu et al., 2013). On the contrary, the intensity of the peak demonstrated an opposite trend. The enhancement might be due to the CeN mode of the stretching vibration of N-containing aromatic hydrocarbons in CM-derived hydrochar. Peaks at 1420 cm−1 and 1590 cm−1 corresponding to aromatic C]C bonds and aromatic CeH bonds slightly increased as the temperature and retention time increased, demonstrating the development of aromatization (Wang et al., 2018a). The increasing aromatized hydrochar structure was mainly formed by the aromatization of carbohydrates and molecules obtained via dehydration of lignin (Funke et al., 2010). The peaks around 1600–1800 cm−1 were attributed to the C]O stretching vibration of carboxylic acids, ketones, esters aldehydes and/or phenol compounds (Chen et al., 2018). The band intensities of hydrochar were obviously strengthened compared to the raw materials, which may be due to the absorbed O-containing derivatives on the surface of hydrochar. The stretching vibration between 2800 and 3000 cm−1 indicated the presence of aliphatic and aromatic CeH. The peak intensity in CSderived hydrochar dropped as the temperature and retention time increased, suggesting the promoted degree of carbonization and the separation of the methoxyl functional group from the aromatic ring structure of lignin (Lin et al., 2019). CM-derived hydrochar showed an inverse trend, and the increased intensity of the peaks may be attributed to the decomposition of lipid components. A broad band was observed at 3200–3600 cm−1 and was ascribed to hydroxyl eOH, carboxyl eCOOH and phenolic eOH functional groups (Ngaosuwan et al., 2016). The hydrochar peaks were less intense compared to the raw feedstock, probably due to the promoted dehydration and condensation polymerization reactions during HTC. Condensation polymerization was enhanced at increased temperatures and retention times (Yu et al., 2008). The coexistence of the eCOOH and eOH functional groups have been confirmed to be responsible for the hydrophilic nature of hydrochar. Both the presence of functional groups and the pore structure properties influence the adsorption capacity of hydrochar, but the former plays a larger role than the latter (Tan et al., 2015).
3.2. Physicochemical characterization of CM- and CS-derived hydrochar 3.2.1. Surface morphology SEM images illustrate the changes in surface morphology of the raw biomass and the resulting hydrochar. Continuous, compact and flat rough surfaces were observed in raw CM and CS. As for CM, its surface became more rough and uneven and demonstrated a multi-porous and irregular structure after HTC. More microspores appeared when the temperature increased from 180 °C to 260 °C. As for CS, an irregular surface was observed after HTC, which was consistent with the removal of hemicellulose from the cell wall (Lin et al., 2019). Pores with different sizes (1.0–4.5 μm) were found on the surface of the hydrochar. The size of the pores in the surface of CS-derived hydrochar expanded as the temperature increased. 3.2.2. Pore size and distribution The pore size and distribution were analyzed to further reveal the physicochemical characteristics of hydrochar (Table 2). SBET increased as the temperature and retention time increased during HTC of CS. The maximum values of Vpore (0.017 cm3/g) and SBET (4.256 m2/g) for CSderived hydrochar were achieved for CS-derived hydrochar at 260 °C and 3 h. In general, CM-derived hydrochar had a higher SBET (2.199–6.114 m2/g) and Vpore (0.010–0.021 cm3/g) than CS-derived hydrochar (2.236–4.256 m2/g and 0.005–0.017 cm3/g). This was mainly due to the lower polysaccharides content of CM. Furans can be Table 2 Texture properties of different hydrochar. Sample
SBET (m2/g)
Vpore (cm3/g)
Mpore (nm)
CM-180-3 CM-240-1 CM-240-3 CM-240-4 CM-260-3 CS-180-3 CS-240-1 CS-240-3 CS-240-4 CS-260-3
6.114 2.790 3.760 3.549 2.199 3.045 2.236 3.125 3.579 4.256
0.021 0.010 0.014 0.014 0.011 0.007 0.009 0.005 0.007 0.017
1.432 1.425 1.407 1.422 3.061 1.439 1.433 1.423 1.456 1.421
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Fig. 2. The analysis of aqueous phase obtained from HTC of CM and CS. (a) organic composition, (b) organic acids concentration.
3.3. Characterization of HTC by-products 3.3.1. The characteristics of the aqueous phase Fig. 2 shows the composition of the aqueous products based on qualitative analysis conducted via GC–MS. The CM-derived aqueous phase mainly consisted of phenols, ketones, acids and N-containing compounds. A limited amount of benzenes, alkanes and aldehydes were detected. N-containing compounds were not detected in the CS aqueous phase. The content of aromatic phenols, such as phenol and cresol, increased from 5.39% to 25.67% as the temperature increased in the CM aqueous phase. The phenolic content exhibited an initial increase and a subsequent decline as the retention time reached up to 4 h. A maximum phenol content of 44.88% was achieved at 240 °C and 3 h. The phenolic content was higher in the CS aqueous phase which increased from 25.81% to 68.73% as the temperature increased. The formation of phenols was in accordance with the degradation of unstable ether bonds connecting lignin monomers under hydrothermal conditions (Kang et al., 2011). In addition, the content of aldehydes, including furfural and 5-hydroxymethylfurfural (5-HMF), decreased from 39.91% to 3.62% as the temperature increased in the CS aqueous phase. Aldehydes were probably produced from the dehydration of fructose, and these compounds can be subsequently converted into acids by hydrolysis. In the CS aqueous phase, the content of ketones, acids and aldehydes decreased when the temperature exceeded 260 °C, whereas the content of phenols and benzenes derivatives increased significantly, indicating the enhanced presence of the aromatization reaction. The acids content substantially decreased, but the ketones content increased in the CM aqueous phase. CM resulted in a higher content of acids than CS after HTC, potentially due to the hydrolysis of lipids intro free fatty acids and glycerol (Li et al., 2014). N-containing
Fig. 3. Energy calculation of CM- and CS-derived hydrochar. (a) HHV, (b) ER.
Fig. 4. Van Krevelen diagram of CM- and CS-derived hydrochar.
heterocycles were detected in the CM aqueous phase owing to the conversion of proteins. The organic acids present within the aqueous phase may be further converted into biogas through subsequent treatment via anaerobic 5
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Fig. 5. TG and DTG curves of the feedstock and their hydrochar. (a) feedstock, (b) CM- and CS-derived hydrochar from different temperature, (c) CM- and CS-derived hydrochar from different retention time. Table 3 Combustion characteristics parameters of the feedstock and hydrochar. Sample
Ti (°C)
Tm (°C)
Tb (°C)
Residue (%)
Sample
Ti (°C)
Tm (°C)
Tb (°C)
Residue (%)
CM CM-180-3 CM-200-3 CM-220-3 CM-240-3 CM-260-3 CM-240-1 CM-240-2 CM-240-4
242.07 232.00 262.40 266.54 316.53 361.45 326.88 288.01 342.52
329.27 365.19 372.58 356.55 488.98 509.81 513.70 561.17 526.66
592.73 573.03 582.46 597.12 694.01 673.52 691.25 641.49 685.61
22.08 34.33 42.01 44.16 45.55 55.94 38.34 50.72 55.70
CS CS-180-3 CS-200-3 CS-220-3 CS-240-3 CS-260-3 CS-240-1 CS-240-2 CS-240-4
248.56 257.04 261.74 229.51 213.73 332.83 334.81 275.01 293.67
321.04 341.46 378.69 369.22 494.94 508.11 496.76 481.59 502.83
542.64 584.67 553.01 612.52 649.85 770.46 640.87 606.16 641.85
30.53 30.10 31.95 37.86 34.92 30.21 32.75 26.31 37.55
lactic acid concentration decreased by 59.79% in the CM-derived aqueous phase. In contrast, the concentration of butyric acid was less affected by the HTC conditions. Hence, the CM- and CS-derived aqueous phase produced at 180 °C and 3 h contained an abundance of volatile organic acids which could be used as an organic source for subsequent biogas production.
digestion (Si et al., 2019). The concentration of total volatile fatty acids (VFAs) decreased from 10.27 g/L to 1.21 g/L (CS-derived aqueous phase), and decreased from 8.67 g/L to 4.71 g/L (CM-derived aqueous phase) as the temperature increased. Hence, the formation of acids is more favorable at lower temperatures (Simsir et al., 2017). The concentrations of formic acid and lactic acid decreased by 94.10% and 85.58%, respectively, in the CS-derived aqueous phase. Similarly, the 6
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the retention time had no significant influence on the Tm. The residue content ranged from 26.31% to 37.86% (CS-derived hydrochar), and 34.33% to 44.94% (CM-derived hydrochar) as the temperature increased. CM-derived hydrochar exhibited an increased residue content as the temperature and retention time increased, which was consistent with the increased ash content based on the proximate analysis. Considering the combustion properties (e.g., HHV, ER and TG curves) of hydrochar and the HTC process cost, it is clear that CS-derived hydrochar produced at 240 °C for 1 h is more favorable for use as a solid fuel.
3.3.2. Composition of gas products The gas products after HTC mainly consisted of CO2, H2 and CH4. The total gas volume of CM was slightly lower than that produced by CS, owing to the lower content of cellulose and hemicellulose in CM compared with CS (Kalderis et al., 2014; Liu et al., 2018b). CM resulted in a lower content of H2 and CH4 than CS during the HTC process. Retention time had no significant impact on the gas composition. 3.4. Fuel properties of CM- and CS-derived hydrochar
4. Conclusion
3.4.1. HHV and ER The HHV and ER of the feedstock and hydrochar were characterized in order to further assess the fuel properties after HTC (Fig. 3). CSderived hydrochar had a higher HHV (18.26–26.63 MJ/kg) than CMderived hydrochar (12.92–17.00 MJ/kg). The HHV increased as the temperature increased, which corresponded to the significant decrease in the O content. Specifically, the HHV of CS-240-1 (25.15 MJ/kg) and CS-260-3 (26.63 MJ/kg) were higher than that of commercial lignite (25.0 MJ/kg) (Gao et al., 2016), implying that hydrochar has the potential to be utilized as a fuel source. The ER of CS- and CM-derived hydrochar exhibited a decreasing trend as the temperature increased. As the retention time increased from 1 h to 4 h, the ER of CM-derived hydrochar as a function of the HHV correspondingly increased from 27.47% to 40.76%. The HHV of CM-derived hydrochar was lower than that of CS-derived hydrochar, which could be due to the higher content of ash in CM (Fig. 1). The HHV of CS-260-3 obtained in this study exhibited higher values than most of the values presented in the literature for CS-derived hydrochar (15.6–29.79 MJ/kg) and pyrochar (16.95–28.66 MJ/kg) (Zhu et al., 2016; Guo et al., 2015; Intani et al., 2018; Qi et al., 2018; Salema et al., 2017). The coalification degree of hydrochar is illustrated in Fig. 4. Overall, the H/C and O/C content of hydrochar shifted from the upper right to the lower left and followed vector directions as the temperature increased. The increased temperature caused a decrease in the H/C and O/C atomic ratios, suggesting enhanced decarboxylation, dehydration and condensation reactions (Wataniyakul et al., 2018). The H/C ratio of CS and CM-derived hydrochar decreased to 0.80 and 1.07, respectively after HTC. The O/C content also decreased to 0.24 and 0.59, respectively, corresponding to the dehydration and decarboxylation pathways, which was in agreement with the increase in the HHV. In addition, demethanation occurred during the HTC of CM. The ratios of H/C and O/C increased as the retention time increased for CS-derived hydrochar. However, the O/C ratio decreased with the occurrence of dehydration reactions. The position of the CS-derived hydrochar produced at 240–260 °C was within the typical range of peat and lignite.
This study gives full comparisons of the product distribution and hydrochar properties of CM and CS processed via HTC. CM-derived hydrochar exhibited an enriched concentration of nutrients, abundant ORGs on the surface and a microporous structure, which may increase the nutrition level of the hydrochar and be appropriate as a soil amendment. In comparison, CS-derived hydrochar resulted in a higher yield with a higher carbon content than CM-derived hydrochar. CSderived hydrochar had a higher HHV, lower ratios of O/C and H/C and a better combustion behavior compared with CM-derived hydrochar, indicating the potential for CS-derived hydrochar to be used as an attractive fuel source. Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2016YFD0501402), the National Natural Science Foundation of China (51861125103) and Beijing Dairy Industry Innovation Team (BAIC06-2019). We thank Dr. Buchun Si (China Agricultural University) and Jamison Watson (University of Illinois at Urbana-Champaign) for improving the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.121855. References Chen, J., Wang, L., Zhang, B., Li, R., Shahbazi, A., 2018. Hydrothermal liquefaction enhanced by various chemicals as a means of sustainable dairy manure treatment. Sustainability-Basel 10 (1), 230. Chen, W., Zhang, Y., Zhang, J., Schideman, L., Yu, G., Zhang, P., Minarick, M., 2014. Coliquefaction of swine manure and mixed-culture algal biomass from a wastewater treatment system to produce bio-crude oil. Appl. Energ. 128, 209–216. Eibisch, N., Helfrich, M., Don, A., Mikutta, R., Kruse, A., Ellerbrock, R., Flessa, H., 2013. Properties and degradability of hydrothermal carbonization products. J. Environ. Qual. 42 (5), 1565–1573. Funke, A., Ziegler, F., Berlin, T., 2010. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuel. Bioprod. Bior. 4, 160–177. Gao, P., Zhou, Y., Meng, F., Zhang, Y., Liu, Z., Zhang, W., Xue, G., 2016. Preparation and characterization of hydrochar from waste eucalyptus bark by hydrothermal carbonization. Energy 97, 238–245. Guo, S., Dong, X., Wu, T., Shi, F., Zhu, C., 2015. Characteristic evolution of hydrochar from hydrothermal carbonization of corn stalk. J. Anal. Appl. Pyrol. 116, 1–9. Guo, S., Dong, X., Wu, T., Zhu, C., 2016. Influence of reaction conditions and feedstock on hydrochar properties. Energ. Convers. Manage. 123, 95–103. Intani, K., Latif, S., Cao, Z., Müller, J., 2018. Characterisation of biochar from maize residues produced in a self-purging pyrolysis reactor. Bioresour. Technol. 265, 224–235. Jain, A., Balasubramanian, R., Srinivasan, M.P., 2015. Production of high surface area mesoporous activated carbons from waste biomass using hydrogen peroxide-mediated hydrothermal treatment for adsorption applications. Chem. Eng. J. 273, 622–629. Kalderis, D., Kotti, M., Méndez, A., 2014. Characterization of hydrochars produced by hydrothermal carbonization of rice husk. Solid Earth 5 (1), 477–483. Kang, S., Li, X., Fan, J., Chang, J., 2011. Characterization of hydrochars produced by hydrothermal carbonization of lignin, cellulose, d-xylose, and wood meal. Ind. Eng. Chem. Res. 51 (26), 9023–9031. Li, H., Liu, Z., Zhang, Y., Li, B., Lu, H., Duan, N., Liu, M., Zhu, Z., Si, B., 2014. Conversion efficiency and oil quality of low-lipid high-protein and high-lipid low-protein
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