Article pubs.acs.org/EF
Deoxy-liquefaction of Corn Stalk in Subcritical Water with Hydrogen Generated in Situ via Aluminum−Water Reaction Tianhua Yang,† Wenqi Zhang,† Rundong Li,*,† Bingshuo Li,‡ Xingping Kai,† and Yang Sun† †
Key Laboratory of Clean Energy of Liaoning, College of Energy and Environment, Shenyang Aerospace University, Shenyang 110136, China ‡ School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China ABSTRACT: To reduce the oxygen content and increase the higher heating value (HHV) of bio-oil, this paper proposed a new method for deoxy-liquefaction of corn stalk in subcritical water with hydrogen generated in situ via aluminum−water reaction. The effects of aluminum on the physicochemical properties of bio-oil were investigated. The bio-oil was analyzed by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), and gas chromatography−mass spectrometry (GC−MS). The results showed that, when the aluminum content was 30 wt %, the yield of bio-oil reached 26.54 wt %, the deoxidation ratio was 47.92%, and the calorific value increased from 28.86 to 33.77 MJ/kg. The viscosity, acid value, and water content were also significantly improved. FT-IR analysis showed that the types of functional groups in bio-oil were almost unchanged. GC−MS analysis showed that the main components of bio-oil were phenols, ketones, hydrocarbons, and indoles. In addition, the content of aliphatic hydrocarbons and aromatic hydrocarbons was as high as 36.18% when the aluminum content was 30 wt %.
1. INTRODUCTION Hydrothermal liquefaction (HTL) is one of the methods for thermal conversion of biomass into chemical material or highgrade energy sources. It has a low reaction temperature, high bio-oil yield, and extensive materials, and produces crude biooil (CBO) that has a high energy density and high separation efficiency, which is expected to become an alternative energy to fossil fuels after the upgrading.1 Water in the process of liquefaction can be used as a source of hydrogen to prevent the recombination of free radicals and to inhibit the formation of residue.3−5 Therefore, HTL is a promising thermochemical conversion technology. The main elements of bio-oil are carbon, hydrogen, oxygen, and nitrogen. The sulfur content is relatively low, and almost no metal elements are present, which is beneficial to bio-oil being a clean fuel to burn.6,7 However, bio-oil has a high oxygen content, and the H/C atomic ratio is much lower than that of oil, which leads to low HHV, high viscosity, and poor miscibility with fossil fuel.6,8,9,12 Therefore, the main objective of bio-oil upgrading is to reduce the oxygen content and improve the H/C ratio. At present, the upgrading of bio-oil in the laboratory mainly refers to petroleum refining technology, such as catalytic hydrogenation or catalytic cracking. Catalytic hydrogenation is the most common and effective way to upgrade bio-oil.12 Hydrogen is essential for catalytic hydrogenation. The addition of hydrogen or hydrogen donors helps to reduce the oxygen content of the bio-oil. Oxygen is mainly removed in the form of H2O.2 Sheu et al.10 have suggested that the supply of hydrogen can be diversified and that a compound can be used as a hydrogen donor if it contains a mobile C−H bond, like tetrahydronaphthalene, ethanol, or methanol. The catalytic hydrogenation of bio-oil includes direct hydrogen supply and in situ hydrogen supply. Direct hydrogen supply uses industrial hydrogen as a hydrogen donor, whereas in situ hydrogen supply © XXXX American Chemical Society
uses inorganic hydrogen donors such as inorganic salts and other organic hydrogen donors such as tetralin, and small molecule liquid alcohol.11,12 Compared with direct hydrogen supply, in situ hydrogen supply does not require much energy, and it avoids hydrogen storage and transportation and safety issues; therefore, it receives extensive attention. In bio-oil refining, the catalyst is indispensable, mainly for loading the transition metal or precious metal load-based catalyst.31,12 Recently, nonsupported catalysts such as dispersion catalysts (MoS, CoMoS2, etc.) and amorphous catalysts (Co-Mo-B, Co-Mo-O-B, etc.) have received extensive attention, as they have high dispersibility and high catalytic activity, and can reduce the effects of the carrier and inhibit coking. NiS-MoS is a bimetallic dispersion catalyst with high deoxidizing activity and has particularly strong activity and selectivity for hydrogenolysis of C−OH bonds in phenolic molecules as Mo-based dispersion catalyst.28 Hydrogen produced by metal hydrolysis is a promising way to store and transport energy. Active metals that can react with water to produce hydrogen include beryllium (Be), aluminum (Al), zinc (Zn), magnesium (Mg), calcium (Ca), lithium (Li), sodium (Na), and potassium (K). Al is the most abundant metal on earth. Under normal temperature, the aluminum hydrolysis reaction can not only produce hydrogen but also release energy, and the reaction product of aluminum has catalytic properties.17,18 The hydrolysis reaction of aluminum has been used for hydrogenation experiments of coal, asphalt, and model compounds under supercritical water conditions. Hydrogen produced during oxidation of aluminum is used for the hydrogenation of organic compounds.13−16 Fedyaeva et al.19 successfully upgraded bitumen to liquid paraffinic fuel with Received: June 26, 2017 Revised: August 4, 2017 Published: August 9, 2017 A
DOI: 10.1021/acs.energyfuels.7b01825 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Ultimate and HHV Analysis of Corn Stalk ultimate analysis (wt %)
proximate analysis (wt %)
C
H
N
S
O
HHV (MJ/kg)
moisture
ash
volatile
fixed carbon
41.4
5.3
0.84
0.14
52.3
15.0
6.82
5.53
75.57
12.08
The solid−liquid mixture was poured from the autoclave and filtered to obtain a water phase and a solid A. After washing the reactor wall, stirring and flushing with ethanol and acetone, the mixture was filtered, and the organic phase and solid B were obtained. The water phase was extracted with dichloromethane to obtain the dichloromethane-soluble fraction, which was then evaporated at 40 °C to give oil A. Solids A and B were rinsed with acetone and ethanol, and the rinsed liquid and organic phases were mixed and finally evaporated at 85 °C to obtain oil B. The sum of oil A and oil B was defined as the bio-oil obtained in this experiment. The solids were defined as residues, which were weighed after drying in an oven at 105 °C for 12 h. 2.4. Analytical Methods. 2.4.1. Physicochemical Properties. The water content of the oil was determined by an SYD-2122C Coulometric Karl Fischer Titrator (Shanghai Changji). The total acid number (TAN) was determined by an SYD-264B Automatic Total Acid Number Tester (Shanghai Changji), which follows the ASTM D664-7 with a potentiometric titration method (titration range: ≥0.05 mg KOH/g; minimum titration volume: 0.02 mL). The viscosity of the bio-oil was measured with a capillary-type viscometer (accuracy: ±0.01 s; detection range: 0.35−10 000 mm2/s; Schott Instruments, Germany). The elemental analysis of the oil was performed using a Vario EL cube Elemental Analyzer (Elementar Analysensysteme GmbH, Germany, accuracy: C, H, N, S ≤ 0.1%). The oxygen content was calculated from the C, H, N, and S difference. A van Krevelen diagram was produced to depict the DOD, an important parameter of the catalytic HDO reaction which was estimated using eq 2. The HHV was calculated using the Boie formula (eq 4)24
aluminum and zinc under supercritical water conditions. However, the hydroprocessing of an aluminum in situ hydrogen carrier for the liquefaction process of biomass is still relatively rare. Therefore, to reduce the oxygen content and increase the HHV of bio-oil, this paper proposed a new method for deoxyliquefaction of corn stalk in subcritical water with hydrogen generated in situ via aluminum−water reaction. In HTL of corn stalk, aluminum was used as hydrogen carrier, NiS-MoS as catalyst. This study also evaluated the effect of aluminum on the effective H/C ratio (H/Ceff) and degree of deoxygenation (DOD); analysis of the distribution and transformation of the primary components was also conducted.
2. EXPERIMENTAL SECTION 2.1. Material. Naturally dried corn stalks were collected from the suburbs of Shenyang and ground to a particle size of less than 0.25 mm (Cellulose: 38.4%, Hemicellulose: 27.73, Lignin: 15.78). Ultimate analysis and the proximate analysis of corn stalks is shown in Table 1. The reagents used in the experiment included methylene chloride, ethanol, and acetone aluminum powder (99%, 75 μm), all of which were of analytical grade and commercially available. High-purity nitrogen and deionized water were used in the experiment. NiS-MoS catalyst precursor was obtained from China National Offshore Oil Corporation Tianjin Chemical Research and Design Institute. The basic properties of the catalyst are shown in Table 2.
Table 2. Physicochemical Properties of Catalyst
Bio‐oil yield = (Wbio‐oil − WN − WS)/WCS × 100
(1)
where Wbio‑oil is the mass of bio-oil (g), WCS is the mass of corn stalk fed into the reactor (g), and WN (g) and WS (g) refer to the mass of nitrogen and sulfur derived from catalyst.
DOD(%) = (Ocrude‐bio‐oil − Oup‐bio‐oil )/Ocrude‐bio‐oil × 100%
(2)
where Oup‑bio‑oil and Ocrude‑bio‑oil are the oxygen contents of the CBO and UBO.
H/Ceff = (n(H) − 2n(O))/n(C)
(3)
where H/Ceff refers to effective H/C ratio, and n(H), n(O), and n(C) refer to the atomic number of H, O, and C, respectively.
HHV(MJ/kg) = 0.3516C(wt%) + 1.16225H(wt%) − 0.1109O(wt%) + 0.0628N (wt%)
2.2. Hydrogen Production by Hydrolysis of Aluminum. The experiments were performed in a 500 mL autoclave. Four different doses of aluminum powder (1.5, 3, 4.5, and 6 g) and 150 mL of deionized water were added to the autoclave. The reaction temperature was set at 370 °C, and the residence time was 60 min. After the reaction, the gas volume was collected and measured. 2.3. Catalytic Liquefaction and Product Separation. The experiment was performed in a 500 mL autoclave with a solid−liquid ratio of 1g/10 mL. First, 15 g of corn stalk powder, 0.3 g of catalyst, and different amounts of aluminum powder (0−40 wt %) were added, and the blank group did not contain aluminum or catalyst. Then, 150 mL of deionized water was added to the mixture, which was mixed well and sealed in the autoclave. Air was purged with high purity nitrogen (99.99%) for 5 min before each experiment. The magnetic stirrer was set to 300 r/min. The reaction temperature was set at 370 °C, and the reaction time was 1 h. After the reaction, the autoclave was cooled to room temperature, and the gas was collected and measured; then the autoclave was opened.24
+ 0.10465S(wt%)
(4)
where C, H, O, N, and S refer to the mass fraction, which was measured by elemental analysis. 2.4.2. Fourier Transform Infrared Spectroscopy (FT-IR). The functional groups of bio-oil were determined using a Fourier transform infrared spectrometer (spectral range: 7800−350 cm−1, resolution: 0.5 cm−1, Thermo Nicolet, U.S.) with an attenuated total reflectance (ATR) mode. The detection range was 4000−500 cm−1, and the resolution was 4 cm−1. A background scan was performed under ambient atmosphere prior to all analysis 2.4.3. Gas Chromatography−Mass Spectrometry (GC−MS). Quantitative analysis of the organic composition of each group biooil was performed using a gas chromatography−mass spectrometry (Agilent 6890N/5975, U.S.). The bio-oil was completely dissolved in acetone before analysis. The carrier gas was helium at a flow rate of 1.0 mL/min and a split ratio of 50:1. The temperature was maintained at B
DOI: 10.1021/acs.energyfuels.7b01825 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. XRD patterns of the solid residue after the reaction of the aluminum with water at 370 °C for 60 min; aluminum dosage: 6 g. 60 °C for 3 min and then heated to 280 °C at a rate of 10 °C/min for 20 min. In the Electron Ionization (70 eV) mode, the mass of the scan is in the range of 20−550 amu. The injection volume was 1 μL. 2.4.4. Online Infrared Syngas Analysis. The major gas components were determined using an Online Infrared Syngas Analyzer-Gasboard 3100 (Wuhan Cubic Optoelectronics Co. Ltd., accuracy ≤ ±1%, resolution: 0.01%, error ≤ 2%). Measurement range can be customized by the requirement. The unit can measure the concentration of CO, CO2, CH4, H2, and O2 simultaneously. The relative contents of the components were calculated by a normalization method. 2.4.5. X-ray Diffraction (XRD) Analysis. X-ray powder diffraction (XRD) was carried out to detect aluminum and aluminum oxides in the residue. The diffraction patterns were monitored on a PAN analytical diffractometer utilizing Cu as anode material with Kα (k = 1.54443 Å) radiation to create diffraction patterns from powder crystalline samples at room temperature. The spectra were scanned in the range 2θ = 20−90° at a rate of 2.0°/min.
3. RESULTS AND DISCUSSION 3.1. Hydrogen Production from Aluminum−Water Reactions. The XRD analysis of the product of aluminum and
Figure 2. Effect of aluminum dosage on bio-oil yield.
Table 4. Effect of Aluminum Dosage on the Gas Component gases (%)
Table 3. Relationship between Hydrogen Production and Aluminum Dosage aluminum (g)
hydrogen (mL)
efficiency (%)
pressure (MPa)
1.5 3.0 4.5 6.0
1820 3650 5520 7345
97.46 97.72 98.53 98.33
0.4 0.9 1.5 2.1
CO
CO2
CH4
CnHm
H2
blank 0 10 20 30 40
14.83 2.94 2.03 1.85 2.20 2.23
58.00 77.58 68.01 53.13 55.56 62.52
12.25 6.86 4.97 4.46 5.17 5.95
2.63 1.63 1.53 1.10 1.16 1.61
12.29 11.00 23.47 39.47 35.90 27.70
3.2. Effect of Aluminum Addition on Bio-oil Yield. The yield of bio-oil is shown in Figure 2. The yield of bio-oil first increased and then decreased with increasing aluminum content. When aluminum exceeded 30 wt %, the yield of biooil started to decrease. This indicated that, with the increase of aluminum, more hydrogen was produced and the bio-oil yield was increased, but too much aluminum would in turn reduce the yield. This is because relatively less aluminum can be fully mixed with water to fully react to generate more hydrogen and inhibit the formation of resin material, which increases the content of short chain normal paraffin (
