Article pubs.acs.org/EF
Upgrading of Crude Duckweed Bio-Oil in Subcritical Water Peigao Duan,* Yuping Xu, and Xiujun Bai College of Physics and Chemistry, Department of Applied Chemistry, No. 2001, Century Avenue, Henan Polytechnic University, Jiaozuo, Henan 454003, P. R. China S Supporting Information *
ABSTRACT: In the present work, crude bio-oil derived from the hydrothermal liquefaction (HTL) of duckweed (Lemna sp.) was treated in subcritical water at different reaction environment (H2,CO), temperature (330−370 °C), time (2,4 h), and Pt/C sulfide (Pt/C−S) catalyst loading (0−20 wt %), aiming to find how these parameters affect the products yield and properties of the treated oil. The results demonstrated that treating the crude duckweed bio-oil in subcritical water with or without catalyst under either H2 or CO environment effected several desirable changes in the oil. Compared to H2, using CO as initial gas led to treated oil with higher yield, lower viscosity, and higher hydrogen, and could also achieve larger energy recovery. Higher temperatures and longer reaction times produced treated oil with better quality but at the expense of reducing oil yield, respectively, due to the increased coke and gas formation. Larger catalyst loading was also favorable in realizing high quality treated oil, but it also promoted the production of coke and water-soluble material. During the treatment, the oxygenates in the crude duckweed bio-oil were more reactive than that of the nitrogenates, especially with catalyst. The higher heating values of the treated oils were estimated within the range 34.3−38.2 MJ/kg. CO2 was the dominant gas formed under either CO or H2 environment. Thus, this study suggested that the crude bio-oil from the HTL of duckweed can be effectively upgraded in subcritical water.
1. INTRODUCTION Hydrothermal liquefaction (HTL), which obviates the need for feedstock dewatering and drying, is one of the most promising paths to convert higher moisture biomass into a sustainable and energy-dense bio-oil.1,2 Of the different candidate feedstocks for HTL, aquatic biomass shows the highest potential for the bio-oil production due its high photosynthetic efficiency, fast growth rate, and large area-specific yield.3 Among the aquatic biomass, microalgae are one of the most promising feedstocks for bio-oil production because of their strong ability for lipid accumulation and noncompetition with arable land.4 However, challenges such as strain isolation, high cost of harvesting, nutrient sourcing, and commercialization are also encountered for the algal biofuels.5 Furthermore, an algal farming facility usually requires higher capital costs and more intensive care than that of the conventional agricultural farm, which delays the further industrialization of algal biofuels.6 In contrast, duckweed can overcome the shortcomings of microalgae and is now being tested as a viable means for biofuel production. Not only does it has the capability to be grown for industrial purposes, but also this plant can be easily harvested and dewatered due to its high surface area to volume ratio, making it ideal for mass production.7 Advantageously, it can adapt to a wide variety of geographic and climatic zones and double the mass within two days under optimal nutrient availability, sunlight, and water temperature.8 Compared to microalgae, duckweed can thrive on any additions of nutrients and accumulate large amount of starch, a key attribute for a potential biofuel. Duckweed usually contains high levels of moisture after its direct harvesting. Thus, HTL seems a preferred way to convert such biomass feedstock from an energy consumption point of view. HTL of duckweed is available in the literature.3,8,9 However, the bio-oils produced © 2013 American Chemical Society
from the HTL of duckweed mainly consisted of ketones and their derivatives, heterocyclic nitrogen-containing compounds, and fatty acids, and thus had high levels of nitrogen, oxygen, and sulfur.8 Therefore, additional upgrading such as deoxygenation, denitrogenation, and desulfurization of this liquefied crude oil is needed if one expects to produce a drop-in transportation fuel. The crude oil produced from the HTL of duckweed will be generated in an aqueous environment, and thus it may be beneficial from a process engineering perspective to upgrade the crude oil in that same environment (water). Water above its boiling point is known as subcritical or supercritical (above critical point, SCW) water which usually exhibits many desirable properties such as low dielectric constant, high diffusivity, and adjustable solvency, making it a potential medium for the bio-oil processing. Therefore, subcritical/SCW water treatment may be a path toward the crude duckweed oil upgrading. To date, SCW was mostly used for the upgrading of heavy oil and coal tar.10−15 All these previous studies suggested that SCW had many desirable effects on the treated oils. Duan et al.16 studied the SCW treatment of crude algal bio-oil under different reaction conditions. The results suggested that SCW alone could decrease the sulfur in the treated oil below the detection limit, and the high levels of N and O in the treated oil were also considerably lowered. Addition of a Pt/C catalyst led to the additional desirable effect of reduced oil viscosity and increased deoxygenation and denitrogeantion of the oil. Moreover, the coke, which was usually an inevitable byproduct in the heavy oil upgrading process, could also be effectively Received: March 18, 2013 Revised: July 22, 2013 Published: August 6, 2013 4729
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that heating rate is responsible for the relative proportion of oil from the liquefaction of biomass, but it is not so obvious.20 After the reactor had cooled, it was depressurized and opened. Dichloromethane was added to dissolve the tar-like material and separate it from the reaction mixture. The dichloromethane extract was vaporized by using a rotary evaporator to remove the solvent. The material left after the vaporizing the sovent is the crude duckweed biooil. Liquefaction experiments were repeated for 10 times by employing the same batch reactor under the identical reaction conditions, and the crude bio-oils obtained were collected together. The crude bio-oil yield was calculated as the total mass of crude oil divided by the total mass of duckweed powder loaded into the reactor. 2.2.2. Upgrading of Crude Duckweed Oil. The crude oil upgrading was performed in a 17.2 mL bath reactor as mentioned above. In a typical run, 1.5 g of crude duckweed bio-oil, 0.15 g of Pt/C−S (when used), and 3.5 mL of deionized water were added into the reactor in sequence. A magnetic stir bar was placed into the reactor. After sealing, the air remaining in the reactor was displaced by flushing the reactor head space with H2 for about 10 min. 6 MPa (room temperature) H2 was next further charged into the reactor. The amount of H2 was charged so that it is in large excess over the demand for the complete removal of heteroatoms from the crude duckweed oil. The loaded reactor was sonicated for about 5 min to effectively disperse the catalyst in the oil. For those reactions conducted under the CO environment, the same procedure was employed for the reactor loading, but by using CO to purge and charge the reactor. The loaded reactor was placed into a preheated molten-salts tank set at the desired temperature to start the upgrading reaction. The stir speed was set at 120 r/min. After a desired reaction time had elapsed, the reactor was removed from the molten-salts tank and immediately quenched in a cool water bath for about 10 min to stop the reaction. After cooling, the reactor was blow-dried by an electric hair dryer, and then depressurized. The gas product was collected for composition analysis. The gas production was estimated by using the weight of reactor after reaction subtracted the weight of reactor after venting plus the initial loading amount of CO or H2. The freely flowing materials in the reactor were dumped into a beaker, and the reactor was washed by dichloromethane three times. All the dichloromethane extracts were sonicated for about 3 min and filtered. After filtration, the filter paper (Whatman, grade 40) and the remained solid residue adhered together and were dried in an oven at 110 °C for 12 h and weighed. The noncatalyst part was defined as coke whose amount was calculated by removing the weight of catalyst from the solid residue. The separated liquid phase was transferred to a separatory funnel for aqueous and organic phase separation. The separated organic layer was dried by using 3 g of anhydrous magnesium sulfate, and the dichloromethane in the organic layer was removed by using a rotary evaporator. The remaining material is treated oil, and its amount was determined gravimetrically. Almost no dichloromethane residual was left in the treated oil which was confirmed by treating the same amount of pure dichloromethane to the same evaporation procedure as the dichloromethane extract. The separated aqueous phase was evaporated at 60 °C to get the water-soluble material. The yield of each fraction was calculated as the following equations:
suppressed in SCW. Hydrodenitrogenation of pyridine, a nitrogen-containing compound in the crude algal oil in SCW, suggested that the activity of a Pt/C catalyst was usually higher when using the sulfide form of this catalyst.17 Interestingly, the in situ generated hydrogenating species via the water−gas shift reaction by the addition of CO was even more effective than the externally added H2.18 HTL of duckweed suggested that the liquefied oil would be heavily decomposed and polymerized to char at SCW conditions.8 Thus, subcritical water rather than SCW seems more preferred for the crude duckweed oil upgrading. So far, however, there has been no study that examined the upgrading of crude bio-oil from the HTL of duckweed in subcritical water. This paper provides such report. In the present study, a liquefied bio-oil produced from the HTL of duckweed Lemna sp. was treated in subcritical water. Pt/C catalyst, which showed an excellent activity toward the deoxygenation and denitrogenation of crude algal oil,16 was selected and used in its sulfide form in order to increasing its tolerance to N and S in the oil. Effects of reaction environment (H2, CO), temperature (330−370 °C), time (2, 4 h), and Pt/C sulfide (Pt/C−S) catalyst loading (0−20 wt %) on the product distribution and the properties of the treated oil were determined.
2. EXPERIMENTAL SECTION 2.1. Materials. Duckweed Lemna sp. was gathered from the rice field in Hebei Province, North China. An earlier article8 gave its proximate and ultimate analysis along with other properties. The Pt/ C−S catalyst (5 wt % Pt, specific surface area = 820 m2/g, average particle size = 25 μm) was commercially available from Sigma-Aldrich. Both the duckweed and the catalyst were used as received. We have not prereduced this catalyst under H2 prior to the experiment because the reaction was performed in an oxidizing hydrothermal environment. In addition, previous study19suggested whether the prereduction or not of the catalysts has no obvious influence on the hydrocarbon yield from the hydrothermal deoxygenation of fatty acid. Self-prepared deionized water was used throughout the experiments. Carbon dioxide, hydrogen, argon, and carbon monoxide in higher purities (≥99.999%) were purchased from Changzhou Jinghua Standard Gas Ltd., East China. All other chemicals used in this study were obtained commercially and used as received. We used a custom-made high-pressure and corrosion-resistant batch reactor, which has a total internal volume of 164 mL, to produce the crude duckweed bio-oil. The reactor consists of a cylindrical body, an autoclave cover, and two semicircular flanges with 2 holes in each for M14 bolts. A pressure gauge is installed on the top cover to test the pressure of the system. The process temperature inside the reactor was measured by a thermocouple inserted into a thermowell which is inserted through the top cover. The reaction mixture was mixed by a magnetic stir bar. The same type of stainless steel batch reactor but with a volume of 17.2 mL was used for the upgrading experiments. The two reactors were conditioned by SCW at 400 °C for 4 h prior to their use. After this aging, potential catalytic wall effects might also play a role in producing the observed results. 2.2. Methods. 2.2.1. HTL of Duckweed. About 72 g of crude duckweed bio-oil was generated by liquefying 400 g of duckweed powder by using the batch reactor as mentioned above, which resulted in a bio-oil yield of 18 wt % on the dry base. After being conditioned, the batch reactor was washed by acetone and dried by high-pressure air. A 40 g portion of dry duckweed powder and 50 mL of deionized water were added into the reactor, respectively. The loaded reactor was placed in a preheated molten-salts (consists of potassium nitrate and sodium nitrate at a mass ratio of 5:4) tank set at 350 °C to initiate the reaction. The temperature was isothermally controlled to within ±2 °C by an Omega temperature controller. After about 30 min, the reactor was removed from the molten-salts tank and immersed in a cold-water bath to quench the reaction for about 10 min. It is noted
⎛ weight of treated oil ⎞ yield of treated oil (wt %) = ⎜ ⎟ × 100% ⎝ weight of crude oil loaded ⎠ (1) ⎛ ⎞ weight of coke yield of coke (wt %) = ⎜ ⎟ × 100% ⎝ weight of crude oil loaded ⎠
(2)
⎛ weight of gas − initial weight of H2 or CO ⎞ yield of gas (wt %) = ⎜ ⎟ weight of crude oil loaded ⎝ ⎠ × 100% 4730
(3)
dx.doi.org/10.1021/ef4009168 | Energy Fuels 2013, 27, 4729−4738
Energy & Fuels yield of water ‐ soluble material (wt %) ⎛ weight of water − soluble material ⎞ =⎜ ⎟ × 100% weight of crude oil loaded ⎝ ⎠
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(4)
The uncertainties were estimated by conducting the experiments with at least duplicate independent runs under nominally identical conditions. Uncertainties are reported as the experimentally determined standard deviations. 2.3. Analysis Methods. A gas chromatograph equipped with a thermal conductivity detector (TCD) was used to analyze the gas products. More details about the gas analysis are available in our previous publication.8 An Agilent Technologies 6890N GC and 5975 mass spectrometric detector were used to analyze the bio-oil (redissolved in dichloromethane). An Agilent HP-5MS nonpolar capillary column (30 m length, 0.25 mm i.d., 0.25 μm film thickness) separated the constituents. More detailed description about the GC− MS analysis can be found in ref 8. 13 C NMR, FT-IR, and elemental analyses were performed as previously described.8 The HHV of bio-oils and biomass feedstocks are estimated by using the following Dulong formula:
Figure 1. Comparison of product yields from the HTL of duckweed and microalgae.
higher heating value (MJ/kg)
compared to the microalgae, suggesting the duckweed oil can be more easily gasified than the algal oil. The high inorganic salts in the duckweed are also responsible for the higher gas yield because they can catalyze the gasification reaction.21 Therefore, lower temperature may be more suitable for the upgrading of crude duckweed oil. 3.2. Effect of Operating Conditions on the Product Yields. The crude duckweed oil was converted into four fractions such as treated oil, coke, gas, and water-soluble material after being treated in subcritical water. Upgrading experiments were performed at different reaction atmospheres, temperature, time, and Pt/C−S catalyst dosages in order to study the effect of these parameters on the product yields. All the results are presented in Table 1. For comparison purpose, Table 1 also includes the product yields from hydrothermal processing of crude algal oil at 400 °C for 2 h with added 6 MPa H2. To test whether CO is more effective or not than H2 on the upgrading of the liquefied duckweed oil, batch experiments were conducted at 350 °C for 2 h without and with 10 wt % catalyst under H2 and CO atmosphere, respectively. As inferred from Table 1, the reaction atmosphere markedly affects the product yields and mass balance. Regardless of the presence or not of a catalyst, the treated oil yield, mass balance, especially the gas yield produced under CO atmosphere, is always higher than that of material obtained under H2. One possible explanation for the higher oil yield under CO atmosphere is that the hydrogen produced from the water−gas shift reaction between CO and H2O is more reactive than the molecular hydrogen itself due to the stability of the H−H bond. This in situ generated hydrogen can effectively stabilize the bio-oil intermediates, thereby increasing the bio-oil yield. Similar results were observed in the HTL of biomass, in which using CO as initial gas led to higher oil yield in the process.22 The most likely reason for the higher gas yield under CO atmosphere might due to conversion of CO to CO2 and H2 from the water−gas shift reaction.18 Without catalyst, higher coke and lower water-soluble yields are obtained under CO atmosphere. With added catalyst, almost the same coke and water-soluble yields are observed no matter what the reaction atmosphere. Under either CO or H2 atmosphere, addition of catalyst promotes the formation of coke and water-soluble materials, which is consistent with a previous study on the
= 0.338C + 1.428(H − O/8) + 0.095S Perkin-Elmer Pyris 1TGA was employed to perform the thermogravimetric analysis (TGA). Bio-oil samples were heated from 35 to 750 °C with a heating rate of 10 °C/min. The nitrogen gas flow rate was 20 mL/min.
3. RESULTS AND DISCUSSION This section first compares the HTL behavior of duckweed and microalgae, then provides information on the products yield and properties of the treated oils obtained from the upgrading of crude duckweed oil under different reaction environment, temperature, time, and Pt/C−S catalyst loading, and subsequently gives a detailed properties analysis of three representative treated oils produced from the upgrading experiments. The final sections present results from the gasphase analysis and atomic and energy balance. 3.1. Product Yields from HTL of Duckweed and Microalgae. The HTL of duckweed and microalgae were respectively performed at 350 °C and 30 min and 350 °C and 60 min which correspond to the condition of producing the highest bio-oil yield of each feedstock. The liquefaction results of the duckweed are available in our previous work.8 Product yields (on dry base) from the HTL of duckweed and microalgae are compared in Figure 1. Clearly, the product yield from the HTL of duckweed shows much difference from that of microalgae. Lower oil yield is observed for the duckweed, likely due to the lower organic matter in the duckweed biomass. If the oil yield was calculated on the organic base, higher oil yield is also observed for the microalgae, suggesting the components in these two biomass feedstocks show different reaction activity. The major components of the duckweed and microalgae are lipid, protein, and carbohydrate. The microalgae contain larger amount of protein and lipid and smaller amount of carbohydrate than the duckweed. It seems that higher lipid and protein content is favorable in realizing higher oil yield. The duckweed contains significant amount of ash, and thus results in higher solid residue yield compared to the microalgae. The ash in the duckweed mainly consisted of inorganic salts which will remain in the aqueous phase after the HTL. Therefore, higher water-soluble yield is observed for the duckweed. Higher gas yield is also achieved for the duckweed although it contains fewer amounts of organic matters 4731
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Table 1. Effects of Operating Conditions on Product Yields expt conditions
treated oil (wt %)
gas (wt %)
Reaction Environment (350 °C, 2 h, 1.5 g oil, 6 MPa gas) CO 77.2 ± 0.5 13.0 ± CO 78.0 ± 0.8 11.2 ± H2 75.7 ± 0.7 0.8 ± H2 75.4 ± 0.6 0.6 ± Temperature (2 h, 1.5 g oil, 6 MPa CO) 400 °Ca (1 h, 6 MPa H2) 70.0 ± 3.5 5.0 ± 400 °C (1 h) 41.2 ± 0.2 10.4 ± 370 °C 52.9 ± 0.6 11.6 ± 360 °C 69.5 ± 0.5 13.2 ± 350 °C 77.2 ± 0.5 13.0 ± 330 °C 81.9 ± 0.4 13.6 ± Time (350 °C, 1.5 g oil, 6 MPa CO) 2h 77.2 ± 0.5 13.0 ± 4h 82.6 ± 1.6 15.8 ± 2h 78.0 ± 0.8 11.2 ± 4h 74.5 ± 0.8 18.8 ± Catalyst Loading (350 °C, 4 h,1.5 g oil, 6 MPa CO) 0 wt % 82.6 ± 1.6 15.8 ± 10 wt % 74.5 ± 0.8 18.8 ± 20 wt % 73.9 ± 0.4 19.6 ± a
coke (wt %)
water-soluble material (wt %)
mass balance
catalyst loading (wt %)
0.8 0.7 0.2 0.3
9.3 9.9 4.5 10.0
± ± ± ±
0.4 0.6 1.2 1.1
0.3 1.1 0.8 1.2
± ± ± ±
0.0 0.1 0.1 0.1
99.8 100.2 81.8 87.2
± ± ± ±
1.7 2.2 1.1 2.1
0 10 0 10
0.3 0.8 0.4 1.3 0.8 0.5
18.1 20.4 15.2 10.9 9.3 3.8
± ± ± ± ± ±
0.9 1.5 0.8 0.6 0.4 0.3
2.7 0.2 0.2 0.3 0.3 1.8
± ± ± ± ± ±
0.4 0.0 0.1 0.0 0.0 0.2
95.8 72.2 79.9 93.9 99.8 101.1
± ± ± ± ± ±
5.1 2.5 1.9 2.4 1.7 1.4
0 0 0 0 0 0
1.8 1.0 0.7 0.9
9.3 4.5 9.9 9.2
± ± ± ±
0.4 0.3 0.6 0.7
0.3 0.9 1.1 1.5
± ± ± ±
0.0 0.3 0.1 0.2
99.8 103.8 100.2 104.0
± ± ± ±
1.7 3.2 2.2 2.6
0 0 10 10
103.8 ± 3.2 104.0 ± 2.6 111.7 ± 3.3
0 10 20
1.0 0.9 1.2
4.5 ± 0.3 9.2 ± 0.7 15.0 ± 1.3
0.9 ± 0.3 1.5 ± 0.2 3.2 ± 0.4
Treated algal bio-oil.25
heavy oil upgrading.23 They attributed this increased coke formation with catalyst to two reasons: (1) The catalyst dispersed in the oil is helpful for the precipitation of solids. (2) Large amount of catalyst favored the hydrogenation of the oil and reduced the asphaltene stability, thus promoting coke formation. Comparatively lower gas yield is always achieved with added catalyst under either CO or H2 atmosphere, suggesting catalyst is favorable for the control of radical evolution and thus inhibiting the gas formation. Good mass balance is always achieved for the catalyzed reactions. Moreover, the treated oil produced under CO atmosphere has a lower viscosity and diesel-like odor compared to that of material obtained under H2. We, therefore, conclude that higher quality treated oil and good mass balance would be achieved with employing CO as the initial gas for the crude duckweed oil upgrading under hydrothermal conditions. It is suggested that temperature was always the most influential factor affecting the properties of the treated oil when doing treatment in SCW.24 Severe decomposition would occur for the crude duckweed oil at temperatures higher than 370 °C. Therefore, effect of temperature ranging from 330 to 370 °C on the product yield was examined for 2 h under 6 MPa CO and without catalyst. One additional experiment was also conducted at 400 °C (supercritical water condition) for 1 h. Table 1 shows all the results, which also include the results from treating the crude algal oil at 400 °C for 1 h.25 Obviously, temperature has significant influence on the product yield and mass balance. As temperature increases from 330 to 370 °C, both the treated oil yield and mass balance sharply decrease from 81.9 and 101.1 to 52.9 wt % and 79.9 wt %, respectively, while the coke yield significantly increases from 3.8 to 15.2 wt %. Treating the crude duckweed oil at 400 °C for 1 h further decreases the treated oil yield to 41.2 wt % and increases the coke yield to 20.4 wt %, respectively, indicating severe decomposition and polymerization of the oil intermediates occurred. At 400 °C, the mass balance is only 72.2 wt %, suggesting most of the lighter products produced from the cracking of oil were lost during the
operating procedure, especially during the solvent evaporation process. In contrast, 70.0 wt % treated oil and 18.1 wt % coke were observed when treating the crude algal oil at 400 °C for 1 h,25 indicating the components in the crude oil play a very important role in selecting the upgrading temperature due to their different reactivity. At more severe temperatures, as expected, the oil products would be thermally gasified and converted into light ends and gas, and thus would result in higher gas yield.1 Unexpectedly, the gas yield basically decreases with increasing temperature, suggesting that the consumption of CO was larger than that of the formation of other gases. That is, the CO was incorporated directly into the reaction. The water-soluble material also decreases with increasing temperature, which would convert into other products such as oil and/ or gas. Although the highest treated oil and lowest coke yield are obtained at 330 °C, the color and viscosity of the treated oil are very close to the crude feeding, suggesting that most of the components in the duckweed oil are unreactable at this temperature. At 360 °C or higher, lower treated oil and higher coke yield are always obtained due to the sever cracking and polymerization reactions. Therefore, from the yield and quality perspective, 350 °C is expected to be a more suitable temperature for the duckweed oil upgrading under hydrothermal conditions. That is, subcritical water is a more suitable medium for the upgrading of liquefied oil derived from the lignocellulose biomass. Effect of reaction time on the product distribution was determined at 350 °C without and with 10 wt % catalyst under 6 MPa CO. The results are also included in Table 1. Without catalyst, the yields of all fractions except coke increase as the reaction time increases from 2 to 4 h. Lower coke yield at a longer reaction time might due to the subsequent reaction of the coke. The mass balance increases with increasing reaction time, which is even larger than 100 wt % at a reaction time of 4 h, suggesting that CO was reacted and incorporated into the reaction products. Reactions between the oil and water are also responsible for the higher mass balance at a longer reaction 4732
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Table 2. Properties of Duckweed and Its Crude and Treated Oils and Microalgae and Its Crude and Treated Oils exptl conditions duckweed crude duckweed oil microalgae25 crude algal oil25 TBO (350 °C, no cat., 2 h, H2) TBO (350 °C, no cat., 2 h, CO) treated algal oil (400 °C, no cat., 1 h, H2)25 TBO (350 °C, 10% cat., 2 h, H2) treated algal oil (400 °C, 10% cat., 1 h, H2)25 TBO (350 °C, 10% cat., 2 h, CO) TBO (350 °C, no cat., 4 h, CO) TBO (350 °C, 10% cat., 4 h, CO) TBO (350 °C, 20% cat., 4 h, CO) a
C 30.3 74.2 46.8 75.1 74.3
± ± ± ± ±
H 0.1 0.5 0.3 0.2 0.6
4.3 8.2 6.9 9.9 8.2
± ± ± ± ±
N 0.1 0.0 0.2 0.1 0.2
2.1 4.6 8.4 7.3 3.1
± ± ± ± ±
Oa
S 0.8 ± 0.4 0.5 ± 0.1 0.6 ± 0.2
± ± ± ± ±
H/C 1.70 1.33 1.77 1.58 1.32
± ± ± ± ±
0.04 0.03 0.04 0.02 0.01
0.510 0.126 0.276 0.078 0.143
± ± ± ± ±
0.013 0.024 0.002 0.002 0.003
N/C 0.059 0.053 0.154 0.083 0.036
± ± ± ± ±
0.005 0.004 0.001 0.001 0.002
HHV (MJ/ kg) 12.8 34.6 22.7 38.1 34.3
± ± ± ± ±
0.2 0.1 0.1 0.1 0.1
0.2 ± 0.0
20.6 12.5 17.2 7.8 14.2
0.5 ± 0.1
13.4 ± 0.7
1.35 ± 0.05
0.135 ± 0.016
0.038 ± 0.002
34.8 ± 0.4
12.3 ± 0.1
1.47 ± 0.05
0.125 ± 0.002
0.059 ± 0.005
35.5 ± 0.1
11.5 ± 1.0
1.35 ± 0.02
0.112 ± 0.002
0.034 ± 0.001
36.2 ± 0.8
8.3 ± 1.5
1.52 ± 0.03
0.081 ± 0.003
0.052 ± 0.002
38.6 ± 0.3
b
0.5 0.7 0.2 0.1 0.9
O/C
0.2 0.2 0.4 0.1 0.6
74.4 ± 0.4
8.4 ± 0.2
3.3 ± 0.1
73.6 ± 0.0
9.0 ± 0.1
5.1 ± 0.0
76.7 ± 0.7
8.6 ± 0.1
3.0 ± 0.1
77.2 ± 1.2
9.8 ± 0.2
4.7 ± 0.1
76.1 ± 0.5
8.7 ± 0.1
2.8 ± 0.2
0.4 ± 0.0
12.0 ± 0.8
1.37 ± 0.02
0.118 ± 0.002
0.032 ± 0.001
36.0 ± 0.6
77.1 ± 0.7
8.9 ± 0.1
3.3 ± 0.1
0.4 ± 0.0
10.3 ± 0.9
1.39 ± 0.02
0.100 ± 0.003
0.037 ± 0.004
37.0 ± 0.6
77.9 ± 0.8
9.0 ± 0.1
2.7 ± 0.1
0.2 ± 0.0
10.2 ± 1.1
1.39 ± 0.03
0.098 ± 0.002
0.030 ± 0.001
37.4 ± 0.8
78.9 ± 0.6
9.2 ± 0.1
2.8 ± 0.3
0.1 ± 0.0
9.0 ± 1.0
1.40 ± 0.03
0.086 ± 0.001
0.030 ± 0.002
38.2 ± 0.8
0 0.2 ± 0.0 0
Calculated by difference. bNot provided.
doubles the crude algal oil. Usually, the higher the N in the starting biomass material, the higher the N in the crude bio-oil. Comparatively lower N in the crude duckweed oil is desirable as denitrogenation is the most difficult step in the subsequent upgrading process. The elemental composition of this crude duckweed oil is similar as reported in an early article.8 The HHV of the crude duckweed oil is estimated to be 34.6 MJ/kg, which is slightly lower than the crude algal oil but much higher than that of the biocrudes produced from the pyrolysis of terrestrial biomass.26 Without catalyst, the C and H content as well as the HHV of the treated oil produced under either CO or H2 environment for 2 h are very similar to those of the crude duckweed oil. However, moderately reduced N in the treated oil is observed, suggesting that subcritical water treatment alone (no added catalyst) showed some denitrogenation of the crude duckweed oil. The S content in the treated oil from processing in H2 is lower than that the treated oil produced in CO. The O content in the treated oil slightly increases after being treated under either CO or H2 environment. Although deoxygenation occurred in the upgrading, the reactions among CO, H2O, and the oil also took place and are predominant without catalyst, and thus increase the oxygen content in the treated oil. At a longer reaction time of 4 h and under CO environment, the water−gas shift reaction is promoted and increases the H2 content in the reaction system which is favorable for the deoxygenation. Therefore, both the C and H contents in the treated oil increase at a reaction time of 4 h, whereas the O content decreases and becomes lower than that of the crude feed. Therefore, the treated oil obtained at a longer reaction time has a higher-energy density because of its higher C and H content and lower O content than that of the oil produced at a shorter reaction time. The HHV of the treated oil produced at 4 h is around 37.0 MJ/kg, which is close to that of the diesel fuel (44.8 MJ/kg).27 The N content in the treated oil seems independent of the reaction time under either CO or H2
time. With added catalyst, similar yield trends as a function of reaction time for all fractions except treated oil are observed as the uncatalyzed reactions. The treated oil yield decreases with increasing reaction time. It is worth noting that a comparative lower bio-oil yield might be desirable if conditions also yielded a treated oil of higher quality (e.g., lower oxygen and nitrogen and higher HHV). The treated oil obtained at 4 h has lower viscosity, lighter color, and more diesel-like odor than that obtained at 2 h, suggesting longer reaction is helpful for producing treated oil with desirable properties. Effect of catalyst loading on the products yield was determined at 350 °C for 4 h under 6 MPa CO. Compared to that without catalyst, significant decrease in treated oil yield is observed with 10 wt % catalyst. Further increase in catalyst loading has less effect on the treated oil yield. However, higher catalyst loading always results in treated oil with higher quality such as lower viscosity and higher energy density. The gas yield always increases with increasing catalyst loading, suggesting the catalyst promoted the conversion of CO from the reaction such as water−gas shift. Accordingly, the mass balance also increases with increasing catalyst loading. The gas yield change is small with increasing catalyst loading from 10 to 20 wt %, however. Significant increase in coke and water-soluble yields are observed with increasing the catalyst loading, which is consistent with the results from the upgrading of crude algal oil catalyzed by Pt/γ-Al2O3 in SCW.25 3.3. Effect of Operating Conditions on the Treated Oil Composition. Table 2 compares the ultimate analysis and estimated HHVs of the crude oils from the HTL of duckweed and microalgae and treated oils from the hydrothermal upgrading of crude duckweed oil and crude algal oil, respectively. Compared to the crude algal oil, the crude duckweed oil was characterized in slightly lower C and H content due to its high ash content. The N in the crude duckweed oil likely originates from the protein in duckweed and is almost half of the crude algal oil while the O nearly 4733
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Figure 2. Total ion chromatograms for (a) crude duckweed oil; (b) treated oil produced at 350 °C, 2 h, 10% cat. 6 MPa H2; (c) treated oil produced at 350 °C, 2 h, no cat. 6 MPa CO; (d) treated oil produced at 350 °C, 2 h, no cat. 6 MPa H2.
density from the upgrading of crude algal oil in SCW. Both the N and S content in the treated oil are reduced compared to the uncatalyzed reaction, suggesting Pt/C−S also showed some but not obvious catalytic effect on the N and S removal. Slight lower S content is observed under H2 environment. We, therefore, conclude that the in situ generated H is more effective for the hydrogenation reaction and the externally charged H2 is more active for the deoxygenation, denitrogenation, and desulfurization. Possibly, cocharging of CO and H2 is an alternative way for the crude duckweed oil upgrading under hydrothermal conditions. This will be the ongoing work. The C and H in the treated oil increase while the N, O, and S content decrease as the reaction time increases from 2 to 4 h. Increasing the catalyst loading would also increase the C and H and decrease the O and S in the treated oil, although these changes are small, however. Higher H/C and lower O/C ratios are always achieved at a longer reaction time and higher catalyst loading. The highest HHV of 38.2 MJ/kg of treated oil is obtained at a reaction time of 4 h and catalyst loading of 20 wt %. Although the N, O, and S contents in crude duckweed oil are considerably reduced after catalytic upgrading, their levels are still higher than the requirement of ASTM.28 Therefore, additional treatments toward the deoxygenation and denitrogenation of the treated oil are also needed. 3.4. Comparison of the Molecular Composition of the Crude and Treated Oil. 3.4.1. GC−MS Analysis. The crude duckweed oil was a dark-brown and viscous material with a distinctive smoky odor. In contrast, most of the treated oils have a light-brown color, low viscosity, and diesel odor, especially those produced at higher temperatures, longer reaction times, and larger catalyst loadings. Gas chromatography and mass spectrometry were used to separate and indentify some of the components in the crude and treated oils. We set the inlet temperature of the GC at 325 °C; at this point
environment, suggesting denitrogenation in the absence of catalyst is more difficult than the deoxygenation. The same scenario was found in the crude algal oil upgrading in SCW.16,17,24,25 Under CO environment, increasing reaction time also decreases the S content in the treated oil. In contrast, complete sulfur removal was achieved for the crude algal oil upgrading in SCW.16,17,24,25 Therefore, it seems that SCW is more effective for the nitrogen and sulfur removal than the subcritical water. Recall, however, SCW also led to lower treated oil. The H/C molar ratio of the treated oil is higher than the crude oil and increases with increasing the reaction time under CO environment. The O/C ratio in the treated oil produced under either CO or H2 is higher than the crude oil and decreases with increasing reaction time, but the N/C ratio is lower than crude oil and decreases with increasing the reaction time. With added catalyst, the treated oil is characterized in higher C and H and higher HHV than that of oil produced without catalyst under either CO or H2 environment. The C and N content as well as the HHV of the treated oil produced in CO are very close to that of oil produced in H2 at otherwise identical conditions. Regardless of the presence or not of a catalyst, the treated oil from processing in CO has a higher H content than the treated oil formed in H2, suggesting in the situ generated hydrogen from the water−gas shift reaction is more active for the hydrogenation reactions than the externally supplied molecular H2. As expected, the O contents in the treated oils are all lower than that of the treated oil produced from the uncatalyzed reaction under either CO or H2 environment, suggesting Pt/C−S promoted the deoxygenation of the oil. Slight lower O in treated oil is observed in H2 than that of in CO, which might due to the participation of water and/or CO in the reaction because they are the only oxygen sources within the reaction system. Duan et al.25 also observed that the O in the treated oil increased with increasing the water 4734
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ketones in the treated oil produced in H2 significantly increases while it decreases under CO atmosphere compared to the crude duckweed oil. The total hydrocarbon content in the treated oil produced in CO is higher than the crude duckweed oil, which is also higher than that of treated oil produced in H2. The major hydrocarbons in the treated oil are long chain alkanes such as pentadecane and 2,6,10,14-tetramethylhexadecane. The treated oil produced in H2 contains some nitrogen-containing compounds such as pyrazine and its derivatives, which are not detected in the treated oil produced in CO. The treated oil produced without catalyst under CO atmosphere has the highest fatty acid content, which is also more viscous than that of the oil produced from the catalytic reaction. The peak region at around 43 min represents the fatty acid (palmitic acid). Conditions including being under highpressure CO or H2 and also including the Pt/C−S catalyst significantly reduced the fatty acids content in the treated oil. Certain amounts of amide such as hexadecanamide and 9octadecenamide were observed after an uncatalytic treatment of crude duckweed oil. No sulfur-containing compounds were found in the treated bio-oil, although the treated oil still contained a small amount of sulfur. Generally speaking, the amount of alkanes and aromatics increased, and the oxygenated-, nitrogen-, and sulfur-containing compounds decreased, in the treated oils after the crude duckweed oil was catalytically treated in subcritical water. However, additional or different treatment is necessary because the treated oil still contained undesirable amount of oxygenated compounds (phenol derivatives) and nitrogenated compounds (amide and indole derivatives). 3.4.2. TGA of Bio-Oils. As mentioned above, GC−MS analysis for the bio-oil has limitations due to its rich heavy compounds, which are unable to be detected by the GC−MS because of their poor volatility. Therefore, TGA was used to simulate a miniature distillation, aiming to provide an evaluation of the boiling range of bio-oils, although some thermal degradation is likely.29 Figure 3 shows the thermo-
about 50 wt % of the crude oil sample was volatilized. No less than 60 wt % of the overall mass of treated oil was volatilized after 325 °C, which was evaluated by performing a thermogravimetric analysis in a nitrogen atmosphere. Figure 2 compares the total ion chromatograms of the crude duckweed oil (a) and treated oils (b, c, and d) produced under different reaction conditions. All the oils were redissolved in dichloromethane at very close concentration. Clearly, the total ion chromatogram of the crude oil is different from those treated oils. The bio-oils likely contain others compounds that do not appear on the chromatogram because of their high boiling point. In addition, those low boiling point compounds would be lost during the solvent evaporation, and those remaining would become undetectable due to the solvent delay. Clearly, the peaks for the crude duckweed oil prior to 45 min are sparse. In contrast, the treated oil, especially produced under H2 environment or longer reaction times under CO environment, shows large peak density prior to 45 min. The major components of the treated oil obtained in H2 are different from that produced in CO. The main components of treated oil produced with and without catalyst under CO environment are also different. The primary characteristic in determining the retention time of a compound is the boiling point. That is, the lower the boiling point of the compounds, the earlier they will elute the column. Therefore, it seems that the treated oils from the catalyzed reaction contain more lowboiling species, thereby resulting in a lower viscosity than the treated oil produced from uncatalyzed reaction. In order to further understand the detailed properties of the crude duckweed oil and treated oils as shown in Figure 2, compound identification was carried out by using a mass spectral library and computer matching, and the results are listed in Table S1 (available as Supporting Information). “CBO” and “TBO” represent the crude duckweed oil and treated oil, respectively. Only those compounds with an area % in the total ion chromatogram exceeding 0.5% were listed for both the CBO and TBOs. The area % only represents the semiquantitative content of each component in the product oil because most of the peak identities are necessarily tentative. The major components of the crude duckweed oil mainly include fatty acid (n-hexadecanoic acid), phenol and its alkylated derivatives (4-methyl-phenol), long-chain hydrocarbons (2,6,10,14-tetramethyl-hexadecane,), piperidine derivatives (1-methyl-piperidine,), amide (n-hexadecanamide), and other N-containing compounds, which showed some differences from the crude duckweed oil obtained in our previous work.8 One very likely reason for the component differences is using different GC column (DB-5MS vs HP-5MS of present study) for the bio-oil separation. The major difference of the crude duckweed oil from the crude algal oil is its higher phenol and alkylated phenol derivatives which are usually derived from the lignin in the biomass. The compounds in the treated oils listed account for no less than 74% of the total peak area depending on the experimental conditions. As shown in Table S1, the phenolic compounds in the treated oil produced under CO environment, especially with added catalyst, are even higher than that of the crude duckweed oil, suggesting the presence of CO favored the production of phenolic compounds under hydrothermal conditions. The phenolic compounds in the treated oils are primarily phenol and alkylated phenols. In contrast, decreased phenolic compounds are observed when treating the oil in H2, likely due to their hydrogenation to benzene and benzene derivatives. The relative content of
Figure 3. TGA curves for crude oil, TBO-a, and TBO-b.
graphic curves of crude duckweed oil and treated bio-oils produced from TBO-a, 350 °C, 2 h, 10% Pt/C−S, and 6 MPa H2; and TBO-b, 350 °C, 4 h, 10% Pt/C−S, and 6 MPa CO, respectively. About 80% mass loss of crude oil and TBO-a was observed when heating the bio-oils from 35 to 750 °C. Larger mass loss was observed for the TBO-b than for TBO-a at the same catalyst loading. It is clear in the TGA curves of the 4735
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Absorbance peaks between 1650 and 1760 cm−1 represent the CO group stretching vibration in ketones, fatty acids, and fatty amides. However, comparatively weaker absorbance in this wavenumber range is observed for the treated oil, corresponding to the reduced fatty ketones, fatty acids, and fatty amide content. The crude duckweed oil also shows a strong intensity of alkyl C−H stretch bands between 1450 and 1360 cm−1, indicating large proportion of alkyl substituted compounds exist in the crude duckweed oil. 3.4.4. 13C NMR Analysis. Figure 5 displays the 13C NMR spectra for the crude (top) and treated (bottom) oil obtained at
treated oil that a low weight loss rate is observed from the temperature around 250 °C. Table 3 lists the boiling point distribution of the crude and two treated oils (TBO-a and TBO-b), which indicates they Table 3. Boiling Point Distribution of Crude and Treated Bo-Oils distillate range (°C)
CBO
TBO-a
TBO-b
35−150 150−200 200−250 250−300 300−350 350−400 400−450 450−500 >500
6.3 13.3 13.7 13.6 11.1 10.3 8.6 4.2 2.0
16.2 12.2 13.8 3.3 18.8 9.9 2.9 1.7 1.4
10.5 15.1 15.8 3.8 20.7 6.1 9.8 2.3 1.6
contain a large amount of high boiling fractions. The treated oil, especially TBO-b, has a larger distillation fraction below 250 °C than the crude duckweed oil. There are about 33.3%, 42.2%, and 41.4% fractions of crude oil, TBO-a, and TBO-b with a boiling point
