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Catalytic upgrading of water-soluble biocrude from hydrothermal liquefaction of Chlorella Zhen He, Donghai Xu, Shuzhong Wang, Hanfeng Zhang, and Zefeng Jing Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03823 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018
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Catalytic upgrading of water-soluble biocrude from hydrothermal liquefaction of Chlorella Zhen He, Donghai Xu * , Shuzhong Wang, Hanfeng Zhang, ZefengJing Key Laboratory of Thermo-Fluid Science & Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049, China
Abstract: Hydrothermal liquefaction of microalgae produces water-insoluble biocrude that spontaneously separates from aqueous phase by gravity. A small proportion of water-soluble biocrude can be obtained via organic solvent extraction from the aqueous phase. This work explored catalytic hydrothermal upgrading of the water-soluble biocrude with five varieties of catalysts (i.e., Pt/C, Pd/C, Ru/C, Pt/C+Pd/C and newly synthesized CoNiMoW/γ-Al2O3) for the first time. The results show that the upgraded oil by Pt/C had the highest yield and energy recovery but the second lowest quality in respect of elemental composition and heating value. Pd/C led to the highest heating value and the lowest yield and energy recovery of upgraded oil, as well as the largest yields of CH4 and C2H6 simultaneously. Both Pt/C and CoNiMoW/γ-Al2O3 performed well in converting high-boiling-point macromolecules into smaller molecular compounds in water-soluble biocrude upgrading. More than 70% of components in upgraded oils were in the distillation range of 150–350 °C, in accord with that of petroleum diesel. A tradeoff between bio-oil yield and quality requires be made for catalyst selection in water-soluble biocrude upgrading. From the perspective of relatively high yield and quality of upgraded oil,
*Corresponding author. Tel.: +86-29-82665749; fax: +86-29-8266-8703. E-mail address:
[email protected] (D.H. Xu). 1
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CoNiMoW/γ-Al2O3 was a good option in catalytic hydrothermal upgrading of water-soluble biocrude. Keywords: Microalgae, hydrothermal liquefaction, water-soluble biocrude, catalytic upgrading, bio-oil
1. Introduction Microalgae, which are photosynthetic single-celled organisms residing in both marine and fresh water environments, are an attractive renewable feedstock for biofuel production.1 Hydrothermal liquefaction (HTL) has a promising future in crude bio-oil (biocrude) production, and can chemically convert microalgae into biocrude, gas, solid residue and aqueous phase.2,3 Organic solvent extraction for biocrude recovery is extensively used in the lab for convenience, but undesired in large scale bio-oil production due to economic and environmental limitation. It is more feasible to collect water-insoluble biocrude that separates spontaneously from aqueous phase by gravity and centrifugation actions. Undoubtedly, recovering the water-soluble biocrude existing in the aqueous phase helps to increase total biocrude yield (by around 5 wt% in our previous reports3-5), dispose and reuse the aqueous phase in large scale production. However, the water-soluble biocrude has relatively high heteroatoms (i.e. N, O and S) content and low heating value in comparison to the water-insoluble biocrude.3-5 Catalytic hydrothermal upgrading is an effective approach of improving algal biocrude quality such as decreasing heteroatom content and increasing heating value. For instance, Pd/C and Pt/C can convert algal biocrude into a hydrocarbon fuel under hydrothermal conditions.6,7 Ru/C leads to 45.1 MJ/kg of heating value after catalytic hydrothermal upgrading of pretreated algal oil.8 Two 2
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component catalyst shows better performance in biorefinery than that of its single component.9 Nonetheless, to the best of our knowledge, prior works just focused on the upgrading of the combined
solvent-derived
total
biocrude
(water-insoluble
biocrude
+
water-soluble
biocrude).6,8,10-12 It is noteworthy that the components of the water-soluble biocrude are substantially different from those of the total biocrude,3,4 so probably leading to completely various catalytic performances even at the same hydrothermal upgrading conditions. Thus, special catalytic hydrothermal upgrading of the water-soluble biocrude is necessary if it is recovered after large scale algae HTL and expected to use as a transport fuel. However, now there is no any relevant research except our latest publication, which provides little information about hydrothermal upgrading of water-soluble biocrude at 400 °C, 60 min with H2 and Ru/C.13 Therefore, as an extension, this work deeply explored catalytic hydrothermal upgrading of water-soluble biocrude under hydrothermal conditions (400 °C) with four types of noble metal catalysts (i.e., Pt/C, Pd/C, Ru/C and Pt/C+Pd/C) and one newly synthesized multi-metal catalyst (CoNiMoW/γ-Al2O3). The corresponding yield, elemental composition, heating value, energy recovery, compound composition, boiling point distribution of upgraded bio-oil as well as gas products characterization were examined systematically. To the best of our knowledge, these details are documented for the first time. The information is valuable in determining how best to optimize the hydrothermal upgrading process of water-soluble biocrude derived from algae HTL. 2. Experimental section 2.1. Materials The tested microalgae (Chlorella) were purchased from Xi'an Jinheng Chemical Engineering Co., Ltd and used as received. Its detailed properties can be seen in Table 1. Noble metal catalysts 3
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(i.e., Pt/C (5 wt%), Pd/C (5 wt%), Ru/C (5 wt%) and Pt/C+Pd/C (2 wt% Pt + 8 wt% Pd)) were obtained from Shaanxi Kaida Chemical Engineering Co., Ltd. The materials used to synthesize CoNiMoW/γ-Al2O3,
including
Ni(NO3)2·6H2O,
Co(NO3)2·6H2O,
(NH4)6H2W12O40·XH2O,
(NH4)6Mo7O24·4H2O and γ-Al2O3, were purchased from Sinopharm Chemical Reagent Co., Ltd. with high purity (≥99.9%). Fresh deionized water was produced in the lab and used throughout. High purity (≥99.999%) of hydrogen and helium were gained from Baoguang Gas Co., Ltd. Table 1. Analysis of Chlorella.
Proximate biochemical composition
Elemental composition
(wt% as received)
(wt% daf)
Ash
6.5
C
52.31
Moisture
5.7
H
6.99
Carbohydrates
25
N
10.14
Proteins
60
S
0.86
Lipids
2.8
O
29.70
Note: daf = dry ash free. O was calculated by difference.
2.2. Experimental procedures 2.2.1. Algae HTL First of all, a 415 ml batch reactor was adopted to produce water-soluble biocrude for subsequent hydrothermal upgrading. In a typical run, 53.205 g algae and 212.8 ml deionized water were loaded into the reactor. After sealed, the reactor was heated up to 340 °C (corresponding to 22 MPa) from room temperature after 140 min (preheating time), and maintained there for 10 min (residence time at 340 °C). Then, the electrical heater was shut down to cool the reactor to room 4
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temperature naturally after 300 min (cooling time). After gas emission, the reactor was opened and 180 ml aqueous phase was firstly gained via a pipette, and 8.3 g solids and 15.0 g water-insoluble biocrude were then obtained, corresponding to 15.6 wt% of solid yield and 28.2 wt% of water-insoluble biocrude yield. Detailed procedures can be seen in our prior reports.3-5 The aqueous phase above was transferred to an Erlenmeyer flask, and then 70 ml dichloromethane (DCM) was added into the flask. A constant temperature oscillator (THZ-82) was used to fully mix them, and the DCM phase was then separated from the aqueous phase. A rotary evaporator (RE-82AA) was adopted to further remove the extraction solvent from the DCM phase so as to obtain the water-soluble biocrude. Ultimately, about 3.70 g water-soluble biocrude was gained from 212.82 g algae HTL by four independent runs. 2.2.2. Catalyst preparation and characterization In present work, CoNiMoW/γ-Al2O3 was synthesized by the wet impregnation of the support material γ-Al2O3 and aqueous solutions of active metal salts. Nominal catalyst precursors contained 10 wt% MoO3, 14 wt% WO3, 3 wt% NiO and 3 wt% CoO. Incipient wetness solids were dried in an oven at 100 °C for 10 h, then ground by using a mortar and pestle. The powder was ultimately collected in a quartz boat and then calcined in a muffle furnace at 400 °C for 4 h. The catalyst was activated before use by a mixture gas (2% hydrogen + 98% argon) reduction in a tube furnace (JGL1200-60) at 500 °C for 1 h. Its specific surface area was 133.72 m2/g, which was determined by the Brunner−Emmet−Teller (BET) measurement method based on nitrogen and helium adsorption by an automatic volumetric device V-sorb 2800P. Catalyst components were analyzed by X-ray diffraction recorded by scanning with 2°/S at the angle range from 5° to 75° on the D8 Advanced Bruker (Germany) 5
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X-ray diffractometer. Fig. 1 illustrates the presence of different active components in the CoNiMoW/γ-Al2O3 catalyst, including MoO3, WO3, NiO and CoO as expected initially. Mo-Ni and Mo-Co composite oxide components (i.e., NiMoO4 and CoMoO4) were discovered as well. 10000
∗−NiO ∆−CoO ◊−ΜοΟ3
ψ
ψ ψ
8000
ψ Ω−WO3
α−CoMoO4
Intensity
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6000
β−NiMoO4 Ψ
Ψ
4000
ψ−Αl2O3 ψ
Ψ
2000 0
◊ Ωα ◊
Ωβ
10
20
∆
30
Ψ
∗ 40 2θ
50
60
70
Figure 1. X-ray diffraction pattern of CoNiMoW/γ-Al2O3 catalyst.
2.2.3. Catalytic hydrothermal upgrading Catalytic hydrothermal upgrading of water-soluble biocrude was carried out in the 4.9 ml mini-batch reactor assembled from 316 stainless steel part connectors and caps together with a length of stainless steel tubing and a high-pressure gas valve. In each run, 0.20 g water-soluble biocrude, 0.05 g catalyst, and 0.10 ml deionized water were loaded into the reactor, so leading to 0.025 g/cm3 of water density at 400 °C, 25 wt% of catalyst loading relative to biocrude, and 1:2 of water/biocrude mass ratio. Air residue inside the reactor was purified by repeatedly vacuumizing and filling with helium (10 kPa) twice before upgrading experiment. Helium was also served as an internal standard gas for yield quantification of each gas. The reactor was then filled with hydrogen to 3.4 MPa and placed in a preheated salt bath (sodium nitrate and potassium nitrate at the mass ratio of 4:5) at 400 °C for water-soluble biocrude upgrading. Herein, the reactor heat-up time (from room temperature to 400 °C) was approximately 3 min. After 4 h, the reactor was instantly removed from the salt bath and quenched in an ambient-temperature water bath for 15 6
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min to stop reactions. Prior to gas phase analysis, the reactor was equilibrated at room temperature for at least 12 h. The reactor was then opened and added 3 ml DCM twice to sufficiently recover upgrading products. The catalyst was recovered by filtering all reactor contents on filter paper. The DCM phase was then separated from the aqueous phase, and further evaporated to remove DCM solvent to obtain upgraded bio-oil (including water-soluble and water-insoluble fractions). Three independent runs were conducted at the same conditions, and the results reported here are their average values, and the uncertainties of data are the sample standard deviations. 2.3. Analysis methods Gaseous products were analyzed by a gas chromatograph analyzer (GC, Beifen-Ruili, SP-3420A) equipped with a thermal conductivity detector. A 15-ft.×1/8-in. stainless steel column (2.1 mm inner diameter), packed with 60/80 mesh Carboxen-1000 (Supelco) separating each gas component with argon (20 ml/min) as carrier gas. The column was firstly held at 50 °C for 8.5 min, then ramped to 170 °C at 20 °C/min and kept there for 16 min. Subsequently, the temperature was increased to 190 °C at 30 °C/min and maintained there for 25 min, and finally ramped to 220 °C at 50 °C/min and kept there for 60 min. Elemental compositions (i.e., C, H, N and S) of bio-oil were determined by a cube CHNS elemental analyzer (Elementar Vario EL) with uncertainties of
