Hydrothermal Carbonization of Corncob Residues for Hydrochar

Article pubs.acs.org/EF

Hydrothermal Carbonization of Corncob Residues for Hydrochar Production Lei Zhang,† Qiang Wang,† Baobin Wang,† Guihua Yang,*,† Lucian A. Lucia,†,‡ and Jiachuan Chen*,† †

Key Laboratory of Pulp & Paper Science & Technology of Ministry of Education, Qilu University of Technology, Jinan 250353, PR China ‡ The Laboratory of Soft Materials & Green Chemistry, Departments of Forest Biomaterials, Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States ABSTRACT: Upgrading corncob residues (CCR) to a high quality energy resource is an effective utilization of an underutilized industrial lignocellulose waste. A hydrothermal carbonization technique was therefore employed to generate a high heating value (HHV) hydrochar. Results showed that its HHV increased 47% after treatment at 230 °C for 1.5 h. Decreases in H/C and O/C verified that reductions in C and O reactions were occurring following hydrothermal carbonization. The chemical and thermal properties of the final hydrochar as analyzed by FT-IR, TG/DTG, and XRD analyses indicated that dehydration and decarboxylation were the predominant pathways for the C and O reductions. The present hydrothermal carbonization process is offered as a promising approach to upgrade CCR to a high heating value hydrochar under mild conditions.

1. INTRODUCTION

high high heating value were obtained from sewage sludge under temperatures of 180−250 °C at 0.5 h. Based on the discussed past successful research, hydrothermal carbonization was used in the current work to convert CCR into a high quality energy resource, i.e., hydrochar. The high heating values of hydrochars were used to evaluate its efficiency under different temperatures, while energy recovery efficiency was also used to optimize the process. The mechanism of hydrothermal carbonization by dehydration and decarboxylation was supported by analysis of FT-IR, TG/ DTG, and XRD data.

Producing renewable and sustainable energy resources addresses depletion of fossil fuels and associated air pollution issues.1−3 Waste lignocellulosic biomass is a potential candidate to provide energy from both cost and pollution management perspectives.4−6 Corncob residue (CCR) is a waste lignocellulose obtained from the furfural production process. It is estimated that about 23 million tons of CCR is produced annually in China.7 However, burning it to recover heat is the present utilization policy, a poor option because of the low heating value and substandard air emissions.8 Therefore, converting it into a higher value energy resource is a matter of pressing concern. In fact, CCR has been exploited for various applications. For example, CCR has been used to produce carbon precursors with comparable morphologies and chemical natures such as observed for pure glucose from the hydrothermal carbonization process.9 In another study, biochar produced from CCR under various conditions, such as slow pyrolysis, hydrothermal carbonization, and flash carbonization, has been investigated and applied for soil fertility.10 Hydrothermal carbonization, a simulated natural coalification process, is a promising alternative to consider.11,12 Traditionally, the temperature of hydrothermal carbonization is in the range of 150 to 350 °C (autogenous pressure) with water as the reaction medium to reduce oxygen and hydrogen content.13,14 This technology has several advantages, such as high conversion efficiency, elimination of an energy extensive drying process, and relatively low operation temperatures relative to other possible thermal methods. Sevilla and Fuertes15 studied carbon materials production from cellulose and found that agglomerate carbonaceous microspheres were formed. Liu et al.16 reported that a high heating value for coconut fiber and dead eucalyptus leaves was obtained at temperatures of 150−375 °C and 0.5 h. Furthermore, Kim et al.17 concluded that clean solid fuels with © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. CCR from furfural production process was provided by Shandong Longlive Biotechnology Company (China). Initially, the CCR was dried to constant weight under room temperature, milled to a size under 100 meshes (0.15 mm), and stored in a sealed bag. The chemical composition (mass basis) was as follows: cellulose =72.3%, lignin =16.2%, hemicellulose =7.2%, extractives =8.1% and ash =4.2% as determined according to TAPPI methods (T 201, T 222, T 223, T 204 and T 211, respectively). 2.2. Hydrothermal Carbonization Experiments. A laboratory scale 250 mL Parr stirred pressure reactor, equipped with a temperature controller and indicator module of pressure and stirrer rate, was used for hydrothermal carbonization. Initially, 5 g CCR (oven-dried) was weighed into the reactor that was charged with 50 mL distilled water. The reactor was sealed airtight and flushed with nitrogen gas three times. Afterward, the reactor was heated to the desired temperature and maintained for 1.5 h at a stirring rate of 150 ppm. On the other hand, the varied treatment times of 1, 2, and 6 h were also carried out at 210 °C. Once the reaction was completed, the reactor was quickly cooled down to room temperature. The hydrochar was recovered by filtration with G3 filter (sand core funnel) and ovendried at 105 °C for 24 h. Received: November 3, 2014 Revised: December 24, 2014

A

DOI: 10.1021/ef502462p Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels 2.3. Analytical Methods. The volatile matter (VM) content (wt %) was measured in an electric muffle furnace at 900 °C, and the ash content (wt %) was weighed after the char was burnt in oxygen. The fixed carbon (FC) content (wt %) was calculated based on eq 1. FC (%) = 100% − VM (%) − Ash (%)

promising lignocellulosic material for renewable energy schemes.19 However, CCR needs to be upgraded for highenergy recovery and minimum gases emission (CO2, SO2, etc.).8 Figure 1 illustrates a proposed process diagram of hydrothermal carbonization integrated with the furfural production process. The predrying of CCR according to the scheme is not necessary because water is the reaction medium. Also, gases, such as CO2, nitrogen oxides, and sulfur oxides, dissolve into water, forming corresponding acids and salts, eliminating air pollution treatment schemes. The recovered liquid contains sugar-derived compounds, and lignin-derived compounds may be potentially used for biochemicals and biomaterials. The hydrochar, produced from the simulated natural coalification process, can be used for energy generation in a more effective and environmentally friendly way. 3.2. Chemical Compositions. Chemical compositions including volatile matter (VM), ash, fixed carbon (FC), and elemental analysis, as well as high heating value (HHV), are listed in Table 1. Temperature plays an essential roleHHV increased rapidly with increasing temperature from 190 to 230 °C, followed by a further gradual increase at higher temperatures (>230 °C). This can be explained by increases in retained carbon, a high amount of energy. The HHV of hydrochar was slightly lower than that of hydrochar produced from wood, which was 29.17 MJ/kg after production at 255 °C and 60 min.20 However, it was comparable to that of hydrochar produced from coconut fiber and Eucalyptus leaves, which were 26.7 and 25.0 MJ/kg, respectively, produced at 250 °C and 30 min.16 Ash content of hydrochars increased with an increase in temperature from 190 to 370 °C, although with some irregularity, suggesting that the minerals in the ash were more stable than the organic compounds during the process of hydrothermal carbonization. Accordingly, hydrothermal carbonization may represent a successful way to upgrade CCR into high quality energy. Similarly, Liu et al.16 observed the increases in HHV from 18.4 kJ/mol to 30.6 kJ/mol under 375 °C and 0.5 h for coconut fiber. The effect of varied residence times on hydrothermal carbonization treatment are also carried out and list in Table 1. As seen, only a slight increase of HHV was found for residence times beyond 1.5 h, albeit much dramatically from a residence time of 1 h. The less significant effect of residence time was also observed by Gao et al. when studying the hydrothermal carbonization of water hyacinth.21 Table 1 also showed that the VM of the hydrochars decreased at increased temperatures due to decomposition of cellulose and part of the lignin. The fuel ratio (Figure 2B) clearly showed an increasing trend from 0.25 to 2.4 over 190− 370 °C, implying that the hydrochar can be an alternative fuel. The hydrochar yields as a function of temperature are shown in Figure 2A. The temperature has an important effect on the hydrochar yields. They decrease rapidly over 190−270 °C, followed by further slow decreases at higher temperatures. Liu et al.16 observed a similar trend when the hydrothermal carbonization of coconut fiber and eucalyptus leaves produced solid biochar fuel under 150−375 °C for 0.5 h. The Van Krevelen diagram in Figure 2C expresses the coalification reaction.22,23 The atomic H/C and O/C ratios decreased from 1.62 and 0.78 to 0.74 and 0.19, respectively, in the temperature range 190−370 °C. The reduced H/C and O/ C ratios confirmed the importance of dehydration and decarboxylation. Furthermore, the atomic ratios of lignite are

(1)

High heating value (HHV) was calculated based on the Channiwala and Parikh formula (eq 2),18 which has been generally accepted for its high accuracy, i.e., average absolute error of 1.45% and bias error of 0.00%. Additionally, the formula has considered the entire spectrum of fuels, e.g., gas, liquid, and solid.

HHV (MJ ·kg −1) = 0.3491C + 1.1783H + 0.1005S − 0.1034O − 0.0151N − 0.021A

(2)

where, C, H, S, O, N, and A represent carbon, hydrogen, sulfur, oxygen, nitrogen, and ash content in wt %, respectively. The energy densification and energy recovery efficiency (ERE) were calculated from eqs 3 and 4:17 energy densification =

ERE (%) =

HHV of hydrochar HHV of CCR

(3)

HHV of hydrochar × hydrochar weight × 100% HHV of CCR × CCR weight (4)

2.4. Characterization Methods. Elemental analysis of samples for carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S) were carried out on a Vario EL III Elementar (USA). The reflectance infrared spectra were carried out on Shimadzu FTIR spectrophotometer (Japan). Each spectrum was recorded over a wavenumber range of 500−4000 cm−1. Oven-dried KBr pellets were used for sample preparation. Thermal analyses of all samples were performed on a TGA Q 50 (USA). A 10 mg sample was placed in a sample pan after which the temperature was increased from ambient temperature to 700 °C. The heating rate was set at 20 °C/min under nitrogen with a flow rate of 30 mL/min. The crystallinity of samples was determined by X-ray diffraction (XRD) (D8-ADVANCE, Bruker, Germany) with Cu Kα radiation at 10 kV. The XRD data was collected from 10 to 80° at a scan rate of 2°/ min.

3. RESULTS AND DISCUSSION 3.1. Proposed Process for Hydrothermal Carbonization of CCR. CCR is a hemicellulose-depleted waste residue from the furfural production process, as shown in Figure 1. The lignin and cellulose remaining in CCR makes it a

Figure 1. Proposed integration of hydrothermal carbonization of CCR into the furfural production process. B

DOI: 10.1021/ef502462p Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Chemical Compositions of CCR and the Hydrochars Prepared at Different Temperatures and Times

a

Temp (°C)

VM (%)

Ash (%)

FC (%)

C (%)

H (%)

O (%)

N (%)

S (%)

HHV (MJ/kg)

CCR 190 210 230 250 270 290 310 330 350 370 210a 210b 210c

78.6 77.0 74.3 51.3 49.3 46.3 42.7 42.1 40.0 30.0 27.2 83.9 71.4 50.1

4.2 3.7 6.1 6.0 6.4 5.9 6.8 6.3 4.7 6.4 7.0 4.8 5.3 6.4

17.2 19.2 19.6 42.7 44.3 47.8 50.5 51.6 55.3 63.6 65.8 11.2 23.3 43.5

43.1 46.7 51.4 64.0 65.0 65.6 66.7 67.7 67.3 71.6 70.3 47.0 53.9 56.7

5.8 5.5 5.4 4.9 4.6 4.4 4.4 4.5 4.2 4.0 4.3 5.61 4.92 4.72

44.6 43.9 37.5 26.4 24.5 23.7 21.6 20.9 23.2 17.4 17.8 42.0 35.6 31.5

0.23 0.19 0.28 0.38 0.38 0.40 0.40 0.46 0.48 0.50 0.46 0.41 0.32 0.53

0.12 0.04 0.07 0.05 0.03 0.02 0.01 0.13 0.12 0.12 0.07 0.15 0.07 0.06

17.2 18.1 20.3 25.2 25.5 25.5 26.1 26.6 25.9 27.8 27.6 18.6 20.8 21.9

Treatment time: 1 h. bTreatment time: 2 h. cTreatment time: 6 h.

Figure 2. Hydrochar data as a function of temperature. (A) hydrochar yields, (B) fuel ratio (calculated by FC/VM), (C) Van Krevelen diagram, (D) energy recovery efficiency and energy densification.

The hydrothermal carbonization treatment of CCR was carried out for difference purposes as documented in the literature. In one study, hydrothermal treated CCR was used to produce electrode material, and the results showed that the obtained carbon exhibited comparable morphology and chemical nature as those from pure monosaccharides.9 In another study, the hydrothermal treated CCR was used to improve solid fertility based on the surface properties, such as high surface area, high pH, and increase of the cation exchange capacity of the soil.10 Furthermore, solid-state nuclear magnetic resonance has been employed to determine the chemical structure of hydrochars from hydrothermal carbonization of CCR, and it has been found that partial preservation of

shown in Figure 2C for comparison. Lower H/C and O/C ratios than lignite were obtained for samples at temperatures higher than 230 °C, suggesting that those hydrochars can be combusted alone or with coal. Hydrochars with H/C and O/C ratios close to lignite can be produced from sewage sludge by hydrothermal carbonization of 280 °C at 0.5 h.17 The energy recovery efficiency and energy densification were calculated and are shown in Figure 2D. The energy densification increased from 190 to 230 °C and then remained constant. However, the energy recovery efficiency (ERE), a parameter affected by the hydrochar yield and HHV, tended to decrease. The highest ERE corresponds to a temperature of 230 °C, the optimal temperature for hydrothermal carbonization. C

DOI: 10.1021/ef502462p Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

peak observed for CCR at around 300 °C is likely due to a high content of volatile matter. After hydrothermal carbonization, the DTG peak shifted to 380 °C for hydrochar obtained at 210 °C. Furthermore, a significant weight loss was no longer detected for CCR treated from 230 to 330 °C, which can be explained by a high content of fixed carbon. An improved thermal stability favored reduction of air pollution due to complete combustion. Román et al.25 also observed significant changes in the thermogravimetry of hydrochars produced from sunflower and walnut shells at 230 °C and 20 min. 3.3.3. XRD Analysis. CCR exhibited two characteristic peaks at 2θ of 16° and 22.7°, typical peaks of cellulose (Figure 5).

cellulose and lignin occurred in hydrochars, a finding that helps to unravel the transformation mechanism of HTC.24 3.3. Characterization of Hydrochar. 3.3.1. FT-IR Spectra. FT-IR spectra of CCR and hydrochars are shown in Figure 3.

Figure 3. FT-IR spectra of CCR and the hydrochars prepared at different temperatures.

The FT-IR spectra of hydrochars were different from that of CCR, confirming that a chemical transformation occurred during hydrothermal carbonization. The bands at 3500−3300 cm−1, 1710 cm−1, and 1000−1460 cm−1 are attributed to O−H stretching vibrations, CO vibrations, and C−O stretching vibrations, respectively. The decreases in these bands confirmed a dehydration during hydrothermal carbonization in agreement with evolution of the O/C and H/C atomic ratios based on the Van Krevelen diagram (Figure 2C). On the other hand, the bands at 1600 cm−1 and 875−750 cm−1 are from the CC vibrations and aromatic C−H bending vibrations. The increases in these bands revealed that an aromatization process occurred during hydrothermal carbonization, in accord with an increase in fixed carbon as shown in Table 1. A significant chemical difference between the raw material and hydrochar was also observed by Sevilla and Fuertes15 when using cellulose to produce carbon materials through hydrothermal carbonization. 3.3.2. Thermal Stabilities. The TG/DTG spectra of CCR and hydrochars are shown in Figure 4A and B, respectively. It can be seen that decomposition of CCR led to significant changes in the thermal properties of hydrochars. Increased residual weights of hydrochars were observed with increased temperature, illustrating better thermal stability. A sharp DTG

Figure 5. XRD patterns of CCR and the hydrochars prepared at different temperatures.

After hydrothermal carbonization at 210 °C, the hydrochar exhibited a similar XRD pattern to CCR, indicating that the cellulosic structure was preserved. Further increases to 230 and 330 °C caused the microcrystalline structure to disappear. These observations confirmed that the decomposition of the microcrystalline structure took place at 230 °C. Such an event was also reported by Sevilla and Fuertes15 when using raw cellulose bought from Aldrich after hydrothermal carbonization.

4. CONCLUSIONS Hydrothermal carbonization of CCR increased the hydrochar high heat value, providing direct evidence that it can be utilized as a high quality energy source in an environmentally friendly way. The optimum temperature was found to be 230 °C based on energy recovery efficiency and energy density. FT-IR, TG/

Figure 4. TG/DTC spectra of CCR and the hydrochars prepared at different temperatures. D

DOI: 10.1021/ef502462p Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

(9) Falco, C.; Sieben, J. M.; Brun, N.; Sevilla, M.; van der Mauelen, T.; Morallón, E.; Cazorla-Amorós, D.; Titirici, M. M. Hydrothermal Carbons from Hemicellulose-Derived Aqueous Hydrolysis Products as Electrode Materials for Supercapacitors. Chem. Sus. Chem. 2013, 6, 374−382. (10) Budai, A.; Wang, L.; Gronli, M. G.; Strand, L. T.; Antal, M. J.; Abiven, S.; Dieguez-Alonso, A.; Anca-Couce, A.; Rasse, D. P. Surface Properties and Chemical Composition of Corncob and Miscanthus Biochars: Effects of Production Temperature and Method. J. Agric. Food Chem. 2014, 62, 3791−3799. (11) He, C.; Giannis, A.; Wang, J.-Y. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: Hydrochar fuel characteristics and combustion behavior. Appl. Energy 2013, 111, 257− 266. (12) Parshetti, G. K.; Liu, Z.; Jain, A.; Srinivasan, M.; Balasubramanian, R. Hydrothermal carbonization of sewage sludge for energy production with coal. Fuel 2013, 111, 201−210. (13) Titirici, M. M.; Thomas, A.; Yu, S.-H.; Müller, J.-O.; Antonietti, M. A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization. Chem. Mater. 2007, 19, 4205−4212. (14) Wang, Q.; Li, H.; Chen, L.; Huang, X. Monodispersed hard carbon spherules with uniform nanopores. Carbon 2001, 39, 2211− 2214. (15) Sevilla, M.; Fuertes, A. B. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 2009, 47, 2281−2289. (16) Liu, Z.; Quek, A.; Kent Hoekman, S.; Balasubramanian, R. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 2013, 103, 943−949. (17) Kim, D.; Lee, K.; Park, K. Y. Hydrothermal carbonization of anaerobically digested sludge for solid fuel production and energy recovery. Fuel 2014, 130, 120−125. (18) Channiwala, S.; Parikh, P. A unified correlation for estimating HHV of solid, liquid and gaseous fuels. Fuel 2002, 81, 1051−1063. (19) Bu, L.; Tang, Y.; Gao, Y.; Jian, H.; Jiang, J. Comparative characterization of milled wood lignin from furfural residues and corncob. Chem. Eng. J. 2011, 175, 176−184. (20) Hoekman, S. K.; Broch, A.; Robbins, C. Hydrothermal carbonization (HTC) of lignocellulosic biomass. Energy Fuels 2011, 25, 1802−1810. (21) Gao, Y.; Wang, X.; Wang, J.; Li, X.; Cheng, J.; Yang, H.; Chen, H. Effect of residence time on chemical and structural properties of hydrochar obtained by hydrothermal carbonization of water hyacinth. Energy 2013, 58, 376−383. (22) Van Krevelen, D. Graphical-statistical method for the study of structure and reaction processes of coal. Fuel 1950, 29, 269−284. (23) Williams, A.; Jones, J.; Ma, L.; Pourkashanian, M. Pollutants from the combustion of solid biomass fuels. Prog. Energy Combust. Sci. 2012, 38, 113−137. (24) Calucci, L.; Rasse, D. P.; Forte, C. Solid-State Nuclear Magnetic Resonance Characterization of Chars Obtained from Hydrothermal Carbonization of Corncob and Miscanthus. Energy Fuels 2012, 27, 303−309. (25) Román, S.; Nabais, J.; Laginhas, C.; Ledesma, B.; González, J. Hydrothermal carbonization as an effective way of densifying the energy content of biomass. Fuel Process. Technol. 2012, 103, 78−83.

DTG, and XRD analyses confirmed that dehydration and decarboxylation reactions occurred in the hydrothermal carbonization to form the hydrochar.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86053189631168. E-mail: [email protected] (G. Yang). *Phone: +86053189631168. E-mail: [email protected] (J. Chen). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded with financial support from the National Science Foundation of China (Grant No. 31270627, 31370580, 31470602) and the Outstanding Young Scientist Award Fund of Shandong Province (BS2014SW013, BS2014NJ025).

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ABBREVIATIONS USED CCR = corncob residues HHV = high heating value VM = volatile matter FC = fixed carbon ERE = energy recovery efficiency C = carbon H = hydrogen S = sulfur O = oxygen N = nitrogen A = ash FT-IR = Fourier transform infrared TG = thermogravimetric XRD = X-ray diffraction

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DOI: 10.1021/ef502462p Energy Fuels XXXX, XXX, XXX−XXX