Characterization of Hydrochars Produced by Hydrothermal

Article pubs.acs.org/IECR

Characterization of Hydrochars Produced by Hydrothermal Carbonization of Lignin, Cellulose, D-Xylose, and Wood Meal Shimin Kang, Xianglan Li, Juan Fan, and Jie Chang* State Key Laboratory of Pulp and Paper Engineering, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China ABSTRACT: Hydrothermal carbonization of cellulose, lignin, D-xylose (substitute for hemicellulose), and wood meal (WM) was experimentally conducted between 225 and 265 °C, and the chemical and structural properties of the hydrochars were investigated. The hydrochar yield is between 45 and 60%, and the yield trend of the feedstock is lignin > WM > cellulose > Dxylose. The hydrochars seem stable below 300 °C, and aromatic structure is formed in all of these hydrochars. The C content, C recovery, energy recovery, ratio of C/O, and ratio of C/H in all of these hydrochars are among 63−75%, 80−87%, 78−89%, 2.3− 4.1, and 12−15, respectively. The higher heating value (HHV) of the hydrochars is among 24−30 MJ/kg, with an increase of 45−91% compared with the corresponding feedstock. The carbonization mechanism is proposed, and furfural is found to be an important intermediate product during D-xylose hydrochar production, while lignin hydrothermal carbonization products are made of polyaromatic hydrochar and phenolic hydrochar. The formation of microspheres on the surface of cellulose and WM hydrochars is discussed, and transformation of the hemicellulose should be the reaction for WM microsphere production.

1. INTRODUCTION Biomass is becoming a very important renewable energy source, and was predicted to account for over 70% of the total renewable energy supply by 2030.1 In past decades, much of the biomass was used as a fuel by direct combustion, which resulted in low energy recovery and emission of environmentally unfriendly gases such as CO2, SO2, etc. Recently, studies of biochar applications have become a popular topic.2−22 Compared to biomass feedstock, biochar possesses a higher heating value (HHV) and higher C content, and biochar can also lead to lower emission of greenhouse gases, e.g., CO2.2−5 Biochar has been investigated as a material for soil amendment,2,6 as a substrate in container nurseries,7 in carbon black,8 in carbon sequestration,4,5,9,10 in solid fuels,2,3,11,12 in adsorbents,13−15 and for some other special carbon-based materials.16−19 There were several carbonization methods for biochar production, including pyrolysis, gasification, hydrothermal carbonization, and flash carbonization.20 Pyrolysis is a well-developed technology, and much information has been obtained from gasification, while compared with those on pyrolysis and gasification, many fewer papers have been about hydrothermal carbonization.20 To avoid confusion, the “biochar” produced by hydrothermal carbonization in this paper is named “hydrochar”. However, hydrochar has a lower ash content, higher C recovery, and more surface oxygen-containing groups than the biochar produced by pyrolysis.21,22 Moreover, there are advantages in the hydrothermal carbonization of biomass: (1) the hydrothermal carbonization temperature is usually much lower than that of pyrolysis, gasification, and flash carbonization,20 (2) there is no necessary for predrying as water is used as the solvent, and (3) some gases, such as CO2, nitrogen oxides, and sulfur oxides, are dissolved in water, forming the corresponding acids and/or salts, making further treatment for air pollution possibly unnecessary. The purpose and possible application of hydrothermal carbonization of biomass are shown in Scheme 1. © 2012 American Chemical Society

Cellulose, hemicellulose, and lignin are the three main components in common wood biomass. Different biomasses have different cellulose, hemicellulose, and lignin weight ratios, and there are big differences among the structures of these components. Cellulose is a polysaccharide made up of glucose, and hemicellulose is a heteropolymer consisting of various monosaccharides, including xylose and pectinose, while lignin is a phenolic polymer. Different contents of the three components in biomass may affect its derived hydrothermal carbonization products. Recently, some experiments on hydrothermal carbonization of raw biomass and cellulose were conducted; however, few papers were focused on the characterization comparison of hydrochars from different biomass components and information on biomass component interactions during the hydrothermal carbonization was lacking. In this work, the object was to test and compare the hydrothermal carbonization effects on these three biomass components and the properties of hydrochars.

2. EXPERIMENTAL SECTION 2.1. Materials. Lignin (dealkali) was obtained from Tokyo Chemical Industry Co., Ltd., and microcrystalline cellulose was obtained from Sinopharm Chemical Reagent Co., Ltd., China. DXylose (Aladdin Chemistry Co., Ltd.) was used as a hemicellulose model in the study. Pine wood meal (WM) consisted of 26.1% lignin, 50.3% cellulose, and 22.3% hemicellulose on a dry basis. The WM was about 3 × 1 × 0.5 mm3 in size. The moisture contents in lignin, cellulose, WM, and D-xylose were 10.3, 2.6, 4.6, and 0%, respectively. 2.2. Operation Conditions. Hydrothermal carbonization experiments were conducted in a 250 mL, 1.5 kW heating power Received: Revised: Accepted: Published: 9023

March 1, 2012 May 20, 2012 June 10, 2012 June 10, 2012 dx.doi.org/10.1021/ie300565d | Ind. Eng. Chem. Res. 2012, 51, 9023−9031

Industrial & Engineering Chemistry Research

Article

Scheme 1. Purpose and Possible Application of Hydrothermal Carbonization of Biomass

pan and heated from room temperature to 700 °C with a heating rate of 15 °C min−1. The surface morphology of the sample was studied using an environmental scanning electron microscopy (SEM) system (S-3700N, Hitachi). 2.4. Hydrogen Ion Exchange Capacity and OxygenContaining Functional Groups. The hydrogen ion exchange capacity was determined through neutralization titration. The titration was carried out as follows: 50 mg of hydrochar and 20 mL of 2 mol·L−1 aqueous KCl vibrated in a shaker at a constant speed of 200 rpm for 24 h. The solids were filtered off and washed with 20 mL of water. The combined filtrate (filtrate produced by the solids filtration and the 20 mL washing water) was titrated with 0.02 mol·L−1 NaOH using phenolphthalein as indicator. Oxygen-containing functional groups (carboxylic, lactone, and phenolic) were determined by the Boehm titration method with different alkali solutions (NaOH, Na2CO3, and NaHCO3).22 Briefly, a given amount of hydrochar was added to the alkali solutions (0.02 mol·L−1) and the mixture was agitated in an agitating bed for 12 h. After the vibration and filtration, a certain amount of HCl solution was added to the solution. Then the supernatant was back-titrated with 0.02 mol·L−1 NaOH using phenolphthalein as indicator. 2.5. Calculations. The yield, WM prediction yield, C recovery, volatile matter recovery, fixed carbon recovery, energy recovery, and HHV improvement are calculated by eqs 1, 2, 3, 4, 5, 6, and 7, respectively.

stainless steel autoclave, which was loaded with 30 g of biomass sample and 90 mL of water. The autoclave was purged three times with nitrogen to remove air. The temperature was raised to the set values (225, 245, 265 °C), and the autoclave was kept at the reaction temperature for 20 h. The products were filtered under negative pressure through a preweighed Shuangquan brand quantitative filter paper. The solid products (hydrochars) were dried at 105 °C to constant weight in a vacuum-drying oven. The lignin, cellulose, WM, and D-xylose derived hydrochars produced at X °C (X = 225, 245, 265) are labeled LX, CX, WMX, and DX, respectively. In order to test the hydrothermal carbonization mechanism of D-xylose, the filtrates of D-xylose products were extracted by ethyl acetate and then detected by gas chromatography−mass spectrometry (GC−MS). 2.3. Analysis and Characterization. The HHVs were measured with a Microcomputer calorimeter (WGR-1). The C, H, N, and S elemental contents (weight percent) were determined with an Elementer Vario EL III instrument. The ash content was tested in an electric muffle furnace. A preweighed sample was set in a crucible, with the following temperature program: 25 °C → 120 °C (20 °C/min, hold 30 min) → 575 °C (8 °C/min, hold 4 h). Both the volatile matter and the fixed carbon contents were measured according to Chinese National Standards (GB/T 212-2008). Briefly, the weighed sample was set in a muffle furnace (absence of air) at 900 ± 10 °C for 7 min. The decreased weight was volatile matter content. The fixed carbon percentage content was calculated as 100% − volatile matter percentage content − ash percentage content. The functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR) on a Nexus 670 (Nicolet, USA). GC−MS analysis was tested on a Shimadzu QP 2010 Plus system equipped with an Rxi-5 ms column (30 m × 0.25 mm × 0.25 μm). The temperature of the injector was set at 255 °C. The column temperature program was 50 °C (hold 1 min) → 260 °C (10 °C/min, hold 6 min). X-ray diffraction (XRD) patterns were obtained using a Rigaku D/max- IIIA X-ray diffractometer. Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses were conducted under N2 flow by a TGAQ 5000 instrument. A sample was placed in a sample C recovery (%) =

volatile matter recovery (%) =

yield (%) =

hydrochar weight ·100% feedstock weight

(1)

yield WM prediction = a ·yield cellulose + b· yield lignin + c·yield D‐xylose

(2)

The yieldWM prediction is a predicted WM yield according to its three constituents. Also, a, b, and c are the initial fractions of cellulose, lignin, and hemicellulose, respectively, and yieldcellulose, yieldlignin, and yieldD‑xylose are the hydrochar yields of cellulose, lignin, and D-xylose, respectively.

percentage of C in hydrochar ·hydrochar weight ·100% percentage of C in feedstock ·feedstock weight

(3)

percentage of volatile matter in hydrochar·hydrochar weight ·100% percentage of volatile matter in feedstock ·feedstock weight

(4)

9024

dx.doi.org/10.1021/ie300565d | Ind. Eng. Chem. Res. 2012, 51, 9023−9031


fixed carbon recovery (%) =

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percentage of fixed carbon in hydrochar·hydrochar weight ·100% percentage of fixed carbon in feedstock ·feedstock weight

HHVm of hydrochar·hydrochar weight ·100% HHVm of feedstock ·feedstock weight

(6)

HHVm of hydrochar − HHVm of feedstock ·100% HHVm of feedstock

(7)

energy recovery (%) =

HHV improvement (%) =

Channiwala and Parikh23 have developed an empirical correlation to predict the HHV of raw biomass, as follows:

cellulose).24 One possible reason for the highest yield of lignin is due to its stable phenolic structure, which is beneficial for char formation as a result of a condensation reaction. Also, in the pyrolysis experiments, the comparison among the overall distributions of char yields of the three biomass components (lignin, cellulose, hemicellulose) also revealed that the char yield from lignin pyrolysis was the highest.25 The WM prediction value (yieldWM prediction) is lower than that of the experimental WM yield at all three temperatures, and the potential explanations are discussed as follows. (1) D-Xylose was used as a substitute for hemicellulose, which should not be the same as the hemicellulose in WM; moreover, the lignin and cellulose used in the experiments may not be similar to those in the WM. (2) Interactions of the components may occur under the hydrothermal conditions. As shown in Table 1, the C content, ratio of C/O, and ratio of C/H in all of these hydrochars are among 63−75%, 2.3−4.1, and 12−15, respectively, and the basic trend is that these values increased as temperature increased. The C recovery is among 80−87%, which is much higher than that in the biochar produced by high-temperature pyrolysis at 620 °C.2 The improvement in the C content is mainly because of deoxygenating reactions, and it was reported that both dehydration and decarboxylation occurred during hydrothermal carbonization.10,26 As shown in Table 2, the volatile matter and its recovery of these hydrochars are among 35−55 and 20−45%, respectively, and both these values become lower at higher temperatures. While the fixed carbon content is among 45−65%, it becomes higher at higher temperatures. However, very high fixed carbon recoveries (more than 100%) are obtained for all of these hydrochars except the lignin hydrochars, especially for D245 (476%). Considering the principle of mass conservation, the volatile matter should be a raw material for the fixed carbon production. However, the fixed carbon recovery of the hydrochars derived from the same feedstock is almost the same at all these temperatures, which indicates that the decrease of volatile matter at higher temperature is converted to other products, probably CO2 and other gases. The energy recovery is very high (78−89%), which may be partly explained by the high C recovery (Table 1). The HHVm of these hydrochars is among 24−30 MJ/kg, equivalent to the heating value of medium-rank and high-rank coals according to the Chinese National Standard (GB/T15224.3-2004). Compared with the corresponding feedstock, the HHVm of these hydrochars increases 45−91%, indicating that hydrothermal carbonization is an effective

HHVp (MJ ·kg −1) = 0.3491C + 1.1783H + 0.1005S − 0.1034O − 0.0015N − 0.0211A (8)

Equation 8 is used to predict the HHV, while HHVm means the measured value. In the HHVp calculation equation, C, H, S, O, N, and A represent the weight percentages of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash in biomass or hydrochars, respectively. The relative error is defined as eq 9. relative error (%) =

HHVp − HHVm HHVm

(5)

·100% (9)

3. RESULTS AND DISCUSSION 3.1. Yield and Chemical Properties of Hydrochar. As shown in Figure 1, yields of hydrochars decrease as the

Figure 1. Yields of hydrochars and WM prediction.

temperature increases. This decrease should have relationships with deoxygenating reactions (e.g., dehydration, decarboxylation) and volatile matter conversion, as the O and volatile matter contents become lower at higher temperatures (Tables 1 and 2). The yield trend is lignin > WM > cellulose > D-xylose; similar results of the residue yield trend were obtained in the hydrothermal liquefaction of biomass (lignin > sawdust > 9025



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Table 1. Proximate Analysis of Ash and Element in Biomass Feedstock and Hydrochars ash (%)

C (%)

H (%)

N (%)

S (%)

Oa (%)

C/O

C/H

1.24 1.31 1.40 1.73 1.45 1.47 1.48 1.54 −b − − − − − − −

45.02 67.55 69.86 74.22 45.36 63.95 66.15 68.43 42.37 66.40 69.70 72.10 39.88 68.65 69.78 72.80

6.70 5.60 5.41 5.54 5.07 5.21 5.01 4.65 6.54 5.11 4.99 5.05 6.88 4.66 4.69 4.93

0.47 0.35 0.39 0.37 0.57 0.57 0.50 0.54 − − − − − − − −

0.33 0.25 0.26 0.23 4.61 1.51 1.30 1.25 − − − − − − − −

46.24 24.94 22.69 17.91 42.94 27.30 25.55 23.59 51.09 28.49 25.31 22.85 53.24 26.69 25.53 22.27

0.97 2.71 3.08 4.14 1.06 2.34 2.59 2.90 0.83 2.33 2.75 3.16 0.75 2.57 2.73 3.27

6.7 12.1 12.9 13.4 8.9 12.2 13.2 14.7 6.5 13.0 14.0 14.3 5.8 14.7 14.8 14.7

WM WM225 WM245 WM265 lignin L225 L245 L265 cellulose C225 C245 C265 D-xylose D225 D245 D265 a

C recovery (%) 87.0 85.4 86.3 84.1 82.6 80.5 83.6 85.0 83.4 85.5 85.7 85.2

Oxygen content is estimated as follows: O = 100 − (C + H + N + S + ash). b“− ” means undetectable.

Table 2. Proximate Analysis of Volatile Matter, Fixed Carbon, and HHV in Biomass Feedstock and Hydrochars volatile matter (%) WM WM225 WM245 WM265 lignin L225 L245 L265 cellulose C225 C245 C265 D-xylose D225 D245 D265

87.31 51.31 48.89 47.15 59.75 44.42 41.66 36.24 93.77 54.28 45.88 43.36 94.23 45.82 43.93 42.19

volatile matter recovery (%) 34.1 30.8 28.3 44.4 39.5 32.6 30.9 25.3 22.7 24.2 22.8 20.9

fixed carbon (%) 12.40 47.38 49.71 51.12 38.80 54.11 56.86 62.22 6.23 45.72 54.12 56.64 5.77 54.18 56.07 57.81

fixed carbon recovery (%)

HHVm (MJ/kg) 18.29 27.12 28.39 29.57 18.59 24.41 26.93 27.41 17.23 25.83 26.73 26.99 15.54 26.69 27.64 29.71

221 220 216 83 83 86 391 449 445 466 476 468

method to get energy-dense hydrochar from biomass. Comparing HHVp with HHVm, the relative error is within 7%, indicating that the empirical correlation Channiwala and Parikh developed is also meaningful to predict the HHV of biomass hydrochar. The volatile matter content, the fixed carbon content, and HHV are important characterization parameters of hydrochars. Comparison of these parameters would be helpful for further selection of hydrochars for directed application studies. Therefore, the hydrochars produced at 265 °C are compared: the volatile matter content is WM265 > C265 > D265 > L265; on the contrary, the fixed carbon content is L265 > D265 > C265 > WM265, while the HHVm is D265 > WM265 > L265 > C265. 3.2. Physical Properties of Hydrochars. As shown in Figure 2, broad peaks locate between 10 and 30° (2θ) for D245, L245, and WM245, which can be ascribed to the diffraction of amorphous carbon,27,28 and indicating that these corresponding feedstocks have been carbonized as carbon. For C245, there is an absence of any obvious crystalline peak (such as the sharp crystalline cellulosic peak at 2θ = 22.7°13), indicating that the crystalline structure was destroyed and C245 contained mainly amorphous components.

energy recovery (%)

HHV improvement (%)

86.0 85.4 84.6

48.3 55.2 61.7

78.4 82.1 78.6

31.3 44.9 47.4

80.0 80.2 76.8

49.9 55.1 56.7

85.3 87.2 89.2

71.6 77.9 91.2

HHVp (MJ/kg)

rel error of HHVp (%)

18.84 27.61 28.43 30.59 17.81 25.98 26.47 27.03 17.22 26.27 27.61 28.78 16.53 26.71 27.27 28.94

3.01 1.81 0.14 3.45 4.20 6.43 1.71 1.39 0.058 1.70 3.29 6.63 6.37 0.075 1.34 2.59

Figure 2. XRD spectra of hydrochars produced at 245 °C.

The TG/DTG curves are shown in Figure 3; the majority of the weight loss of most of the hydrochars (except C225) occurs between 350 and 550 °C, and these hydrochars seem stable 9026



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Figure 3. TG/DTG curves of (A) cellulose hydrochars, (B) lignin hydrochars, (C) WM hydrochars, and (D) D-xylose hydrochars.

before 300 °C. According to the DTG curve peak and TG remaining weight, the thermal stability increases with increased reaction temperature. Lower volatile matter content and higher fixed carbon content at higher temperatures are possible factors affecting the thermal stability. As shown in Table 2, the volatile matter content is L265 < D265 < C265 < WM265, while the TG remaining weight yield is L265 (66.7%) > D265 (58.0%) > WM265 (54.2%) > C265 (53.0%) (Figure 3). Compared with C265, the higher TG remaining weight yield of WM265 is probably related to the higher ash content (Table 1), as ash is stable during the TG process. 3.3. Functional Groups on Hydrochars. FTIR spectra of the functional groups are shown in Figure 4. The FTIR spectra of hydrochars produced by the same feedstock are similar and they differ only in the intensity of some peaks, indicating the functional group composition is not changed in the reaction temperatures from 225 to 265 °C. These hydrochars possess OH groups (3500−3300 cm−1) as well as aliphatic C−H (3000− 2800 cm−1), though the intensities are much lower than their corresponding feedstocks, and the aliphatic C−H peak intensity of hydrochar in Figure 4 A−C seems decreased as reaction temperature increased. Compared with the feedstock, the FTIR spectral peak of hydrochars at 1120−1050 cm−1 is disappeared or decreased, which means that the C−O linkage is broken under these hydrothermal conditions. The C−O linkage should be an ether bond in D-xylose and cellulose, while for lignin and WM the C−O linkage is a methoxy group or an ether bond. Methoxy groups and ether bonds are easy to fracture during hydrothermal

treatment.29 Besides the lignin and WM FTIR spectra, benzene peaks around 1600 (1610), 1520, and 1440 cm−1 and an aromatic C−H peak around 790 (800) cm−1 are found in the D-xylose and cellulose FTIR spectra, which indicates that the lignin derived aromatic structure is stable, and aromatic structures containing hydrochars are formed from D-xylose and cellulose during the hydrothermal carbonization. Glucose is a monomer of cellulose, so such features would support the previously described conclusion of glucose hydrothermal (subcritical water) char, indicating aromatic compounds are produced and further forming the char.30 For the FTIR spectra of D-xylose and cellulose, there is a newly added or intensity increased peak in their corresponding hydrochars around 1700 (1710) cm−1, indicating that a CO bond was formed during the hydrothermal carbonization process. The CO may be a carboxyl group and/or a carbonyl group formed by dehydration of a hydroxyl group, as the hydroxyl group is abundant in D-xylose and cellulose body. Ion exchange capacity and O-containing functional groups are important for some special applications of hydrochar, e.g., application as an adsorptive material. It was reported that biochar showed improved sorption capability for heavy metal ions when the O-containing functional groups increased.19,31 However, the ion exchange capacity, OH groups, and the total O-containing functional groups of all of these hydrochars became lower as temperature increased (Table 3). From the points of view of ion exchange capacity and total O-containing functional groups, 9027



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Figure 4. FTIR spectra of (A) cellulose hydrochars, (B) lignin hydrochars, (C) WM hydrochars, and (D) D-xylose hydrochars.

Table 3. Ion Exchange Capacity and O-Containing Functional Groupsa

WM225 WM245 WM265 L225 L245 L265 C225 C245 C265 D225 D245 D265 a

ion exchange capacity (mmol of H+·g−1)

OH (mmol of H+·g−1)

lactone (CO) (mmol of H+·g−1)

carboxylic (−COOH) (mmol of H+·g−1)

total O-containing functional groups (mmol of H+·g−1)

0.060 0.046 0.023 0.017 0.011 0.009 0.079 0.070 0.067 0.062 0.060 0.055

0.72 0.61 0.56 1.18 1.13 0.83 1.01 0.94 0.65 0.67 0.62 0.54

0.26 0.33 0.17 0.24 0.22 0.25 0.24 0.19 0.12 0.24 0.24 0.10

0.092 0.096 0.092 0.10 0.19 0.27 0.32 0.30 0.34 0.26 0.24 0.34

1.07 1.04 0.82 1.52 1.54 1.35 1.57 1.43 1.11 1.17 1.10 0.98

The values are averages of two or three measurements, within a relative error of 12%.

surface topography of D225 is accumulated by microspheres with diameters of 1−5 μm. Furfural is found to be a major product of D-xylose in the aqueous phase (Figure 6), which was formed by the dehydration of D-xylose. Also, furfural was reported to be an

these results indicate that higher temperature is an adverse factor for adsorptive hydrochar production. 3.4. SEM and Mechanism Analysis. The SEM spectra of Dxylose and lignin derived hydrochars are shown in Figure 5. The 9028



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Figure 5. SEM spectra of D-xylose and lignin derived hydrochars.

through a water-solubility homogeneous reaction. Solid−solid conversion is the preferential reaction,34 and polyaromatic char is produced undergoing heterogeneous pyrolysis of the nondissolved lignin.32 However, as shown in Figure 5, there are no cracks and holes produced as a result of emission of volatile matter on the surface of L225, while some visible holes produced by gas emission were shown in the char through pyrolysis of lignin.35 This phenomenon maybe explained by carbonization of the dissolved lignin fragments and the surface fragments of nondissolved lignin which were exposed to water. These fragments would be decomposed to phenolics through hydrolysis and further form phenolic hydrochar through polymerization. Part of the phenolic hydrochar, especially these derived from the surface fragments of nondissolved lignin, would locate on the surface of nondissolved lignin and/or polyaromatic char, and then these cracks and holes were stuffed and/or covered. The proposed formation pathways of lignin hydrochars are shown in Scheme 3. The surface topography of cellulose hydrochars (C225, C245, and C265) in this experiment looks like a club (Figure 7), which indicates that the cellulose keeps the fiber skeleton after carbonization. However, according to a previous report, microspheres were produced by the hydrothermal conversion of cellulose.18 This difference should be caused by the different operating parameters, e.g., a much higher cellulose/water ratio in this experiment. A few microspheres are found on C225, while many microspheres are shown on C245, and many more microspheres locate on C265. These microspheres should be formed by polymerization of cellulose hydrolysis products, such as glucose.36 Glucose is a well-known carbon microsphere material.37,38 Hashaikeh et al.33 have reported that the dissolved temperature of cellulose was very high, 280−320 °C in their experiments (reaction time < 1 h). For our experiments, the reaction temperature is relatively low (below 280 °C), which may limit the solubility of cellulose, and the club surface topography of cellulose hydrochars also proves there should not be a complete degree of solubility of cellulose. However, the microspheres on the hydrochars indicate that there should be some solubilized and/or hydrolyzed cellulose. The possible reason for the microsphere formation is related to the much longer reaction time (20 h) in the experiments. Higher temperature at the long reaction time may result in somewhat a higher solubility and/or higher hydrolysis degree of cellulose. This may be the reason for increased microsphere content on hydrochars as the temperature increased from 225 to 265 °C. The surface topography of WM hydrochars (WM225, WM245, and WM265) retains the cellular appearance (Figure 8). However, unlike the cellulose hydrochars, the amount of

Figure 6. GC−MS analysis of D-xylose hydrothermal carbonization aqueous phase products produced at 265 °C.

important intermediate product for char formation as a result of polymerization.30 Brief formation pathways of D-xylose hydrochars are proposed in Scheme 2. Scheme 2. Proposed Formation Pathways of D-Xylose Hydrochar

Lignin is a phenolic polymer. A major number of lignin fragments are hard to dissolve and disperse into the aqueous phase when the hydrothermal temperature is not high enough (e.g., below 377 °C for a water density of 954 kg/m3),32 though a fraction of lignin can be dissolved in water at 200 °C.33 The highest temperature in this experiment is 265 °C, and usually the temperature of hydrothermal carbonization is lower than 300 °C, so it is difficult for most of the lignin to become hydrochar 9029



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Scheme 3. Proposed Formation Pathways of Lignin Hydrochar

cellulose, lignin, and D-xylose hydrochars, the particular surface topography of WM hydrochars indicates that interactions among cellulose, lignin, and hemicellulose occurred, which may also be a reason for the lower yieldWM prediction compared with the experimental yield of WM hydrochars.

4. CONCLUSION Hydrochars produced by hydrothermal carbonization of lignin, cellulose, D-xylose, and wood meal were characterized and tested. The yield, C recovery, energy recovery, and HHV of these hydrochars are among 45−60%, 80−87%, 78−89%, and 24−30 MJ/kg, respectively. Temperature is very important in the hydrothermal carbonization process. Higher temperatures generally accelerate the hydrothermal carbonization of biomass, resulting in lower yield, lower volatile matter content, lower ion exchange capacity, lower O-containing functional groups, and higher C content of hydrochars. These results would be significative for further application (e.g., solid fuel, adsorbent) studies of hydrochars. Hydrothermal carbonization mechanisms are proposed: furfural is an important intermediate product for Dxylose hydrochar formation; both polyaromatic hydrochar and phenolic hydrochar are produced in the hydrothermal carbonization of lignin; transformation of the hemicellulose should be the reaction for WM microsphere production.

Figure 7. SEM spectra of cellulose derived hydrochars.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 20 87112448. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Basic Research Program of China (973 Program) (No. 2010CB732205) and the National High Technology Research and Development Program of China (863 Program) (No. 2012AA051801).

Figure 8. SEM spectra of wood meal derived hydrochars.

microspheres on the WM hydrochars decreased as the temperature increased, and WM225 is covered by many microspheres. As just discussed, there should be only a little dissolved cellulose at 225 °C, while hemicellulose is much more reactive than cellulose, and hemicellulose can be dissolved in a hydrothermal condition at a temperature as low as 200 °C.33 Therefore, these microspheres on WM225 should be generated as a consequence of transformation of the hemicellulose rather than cellulose. There are few microspheres but many cracks on WM265; one possible reason is that at high temperature (265 °C) the pyrolysis of WM plays a major role. Compared with the

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REFERENCES

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Characterization of Hydrochars Produced by Hydrothermal

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