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The characterization of aqueous products obtained from hydrothermal liquefaction of rice straw: focus on products comparison via microwave assisted and conventional heating Chong Liu, Qing Zhao, Yechun Lin, Yihuai Hu, Haiyan Wang, and Guichen Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03007 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017
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Energy & Fuels
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The characterization of aqueous products obtained from hydrothermal
2
liquefaction of rice straw: focus on products comparison via microwave assisted
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and conventional heating
4
Chong Liu†,‡,*, Qing Zhao‡, Yechun Lin†, Yihuai Hu†, Haiyan Wang† ,Guichen Zhang†
5
†
Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China
6
‡
School of Physical Electronics, University of Electronic Science and Technology of
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China, Chengdu, Sichuan 611731, China
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Abstract: This paper focuses on the comparison of aqueous products obtained from
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hydrothermal liquefaction (HTL) of rice straw via microwave (MW) assisted and
10
conventional treatment. A systematic investigation of HTL experiments of rice straw
11
via MW assisted and conventional heating treatment have been carried out, covering a
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broad but mild temperature range from 150 to 230 °C at 20 °C intervals. In addition,
13
different reaction times were studied when the HTL rection temperature was 210 °C.
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Comparing with conventional HTL, considerable aqueous products could be obtained
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for MW assisted HTL while consumes less time and what’s more, repolymerization
16
behavior could be efficiently decreased and high saccharide yields could be obtained
17
with MW assisted heating. Besides, HTL temperature appeared to be the dominant
18
factor, while the increased residence time slightly changed the content of Total
19
Organic Carbon (TOC), typical sugars and acids both for conventional and MW
20
assisted HTL heating.
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Key words: aqueous products; hydrothermal liquefaction; microwave assisted heating;
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conventional heating; rice straw
23 24
1.
Introduction
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Due to the increasement of world population and rapid evolvement of industries,
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energy demand is constantly increasing in recent decades1. As a complementary
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resource to fossil fuel, biomass has some advantages versus fossil fuel as it is
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renewable and CO2 neutral energy source2. However, an economic way of converting
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biomass to bio-energy has not yet been devised despite the fact that lingocellulosic
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biomass costs significantly less than crude oil. HTL is an attractive process to produce
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bio-oil from wet feedstock as it reduces the need for feedstock drying comparing
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pyrolysis3, 4. HTL utilizes water as the only solvent and reaction medium. The
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omission of expensive or hazardous chemicals during HTL process make it simple,
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cost effective and environmentally friendly5. These characteristics are in compliance
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with the principles of Green Chemistry6. The HTL is usually performed in water at
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temperature range of 150-374 °C under pressures of 4-22 MPa7, 8. In a typical HTL
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process, feedstock is converted into bio-crude oil, aqueous products, gaseous
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products9,
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composition strongly depends on parameters (temperature and residence time) of HTL
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and the great variety of biomass feedstock such as grasses and trees, and other sources
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of lingocellulosic biomass12.
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and solid residue while water as the solvent and reactant11. Products
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Besides, MW radiation has recently been shown to be energy efficient heating
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method comparing with conventional heating and it has become widely accepted as a
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mild processing. MW assisted hydrothermal treatment at the same temperature range
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could get considerable amound of products with a less residence time as MW
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irradiation is rapid and volumetric with the whole material heated simultaneously13.
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Besides, there is also a good evidence to suggest that it can cause specific molecular
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activations14, 15.
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HTL degradation takes place in water and degrade the feedstock into small
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components into the aqueous phase firstly. Therefore, aqueous products analysis at a
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mild HTL condition could help understand the initial HTL reaction mechanism.
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Despite the growing interest in the composition of HTL related process waters, so far
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no attempt was published that characterize the key substances in the aqueous phase to
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assess the stage of the HTL process heated by MW assist and conventional method.
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Meanwhile, rice straw composed of cellulose, hemicellulose and lignin, is one of
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typical agricultural residues biomass and has a high utilization potential16, 17. Annually,
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around 731 million tons of rice straw is produced by Asia alone10.
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The aqueous products of HTL are intrinsically related to the HTL conditions such
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as reaction temperature, residence time, particle size and the feedstock to water ratio18.
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Among these factors, rection temperature and residence time are generally viewed as
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two decisive parameters19, 20. Therefore, the present work aims to compare the HTL
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aqueous products of rice straw via MW assist and conventional heating at mild
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conditions. A systematic investigation of rice straw HTL processing has been carried
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out, covering a broad temperature range from 150 to 230 °C. In addition, when the
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temperature was 210 °C, different reaction times were studied through this two
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different heating methods. TOC, pH and typical sugars and acids were quantified for
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the analysis of the obtained aqueous products.
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2.
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2.1 Feedstock
Materials and methods
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Rice straw was milled and grounded to pass through a #20 mesh sieve and the
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particle size ranging between 0.2 and 1.0 mm. Rice straw was obtained from Shanghai
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Pudong district. The three main fractions of cellulose, hemicelluloses and lignin of the
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rice straw feedstock is shown in Table 1.
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Table 1 Chemical composition of rice straw (wt % dry matter)
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Cellulose
Hemicellulose
Lignin
32.18
18.88
24.20
2.2 HTL treatment
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As Fig. 1 depicts, HTL converted rice straw into a carbon-rich aqueous phase via
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MW assisted or conventional treatment. HTL samples were prepared by mixing
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milled 5 g rice straw feedstock with 100 ml deionized water. A range of MW assisted
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and conventional HTL experiments were carried out between 150 and 230 °C at 20 °C
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intervals within 10 min. The HTL conditions were shown in Table 2. The residence
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time is the time of HTL reaction under the preset temperature, excluding the time of
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heating and cooling. This slurry was heated without any catalysts and additives in a
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MW tube or conventional rector to designed temperatures.
84 85 86
Fig. 1. Schematic of MW assisted and conventional HTL of rice straw 2.2.1 Conventional HTL
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HTL of rice straw was performed in a 250 mL completely mixed stainless steel
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(316L) reactor containing a stirrer. The reactor was heated by a standard resistance
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heater with power 1500 W. The temperature of the reactor was controlled by a
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programmable temperature controller with a temperature detector in the reactor. The
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reactor was sealed and heated to the desired temperature after loading the rice straw
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slurry. After reaching the desired residence time, the reactor was removed from the
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heater and cooled rapidly by a fan. The solid and liquid products were collected after
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depressurization, and HTL aqueous phase were separated from solid products by a
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vacuum buchner funnel through 0.45 µm membranes. Aqueous phase was stored in
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refrigerator at 4 °C for further utilization. Longer residence time, as Table 2 shows,
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was designed considering thermal gradient of conventional heating.
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2.2.2 MW assisted HTL
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MW assisted HTL was performed in a 250 mL MW tube containing a magnetic
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coil and a stirrer. The power of MW magnetic coil is 1200 W. Temperature of the
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slurry during HTL was measured by a thermocouple. The temperature was then kept
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constant for designed residence time before a fan started to cool the samples. Products
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were treated using same steps as conventional HTL. Table 2 HTL conditions via MW assisted and conventional treatment
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Tempe rature (oC)
Rice straw (g)
Water (mL)
150
5
170
Heating time (min)
Residence time (min)
Cooling time (min)
MW
Conventi onal
MW
Conventi onal
MW
Convent ional
100
10
10
5
30
10
10
5
100
10
10
5
30
10
10
190
5
100
10
10
5
30
10
10
210
5
100
10
10
5
30
10
10
230
5
100
10
10
5
30
10
10
210
5
100
10
10
5
30
10
10
210
5
100
10
10
10
60
10
10
210
5
100
10
10
15
120
10
10
210
5
100
10
10
20
180
10
10
210
5
100
10
10
30
240
10
10
105 106
2.3 Analytical methods
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TOC of aqueous products obtained from HTL was analyzed by a TOC analyzer
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(TOC-L CPH, Shimadzu, Japan) and pH value was measured using a pH meter (FE20,
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Mettler Toledo, Switzerland). The concentration of typical sugars and acids in the
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aqueous products were measured by high performance liquid chromatography
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(HPLC). The aqueous phase was filtered by a membrane of 0.22 µm before the test.
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Sugars and organic acids in aqueous phase product were measured by the differential
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refraction detector of HPLC at 50 °C with a Hi-Plex H column. The mobile phase
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contained 0.005 mol/L of an aqueous sulfuric acid solution. The flow rate was 0.4
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mL/min and the temperature of the column was 55 °C. Calibration range was 0.5-2.5
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mg/ml. Sulfuric acid applied during the mobile phase were chromatographically pure.
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3.
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3.1 Characterization of conventional HTL aqueous products
Results
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It is not surprising that the HTL process was governed by rection temperature and
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residence time to a large extent, as shown in Fig. 2. As a primary indicator of the
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production of soluble organic compounds, the values of TOC showed versatile
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tendencies with the increasing of temperature and residence time. The rice straw
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leaded to a TOC between 2.01 and 5.71 g/L during HTL under the conditions applied.
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Meanwhile, a significant increase of the TOC concentrations between 150 and 190 °C,
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while a decreasing tendency was observed between 190 and 230 °C or residence time
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from 30 min to 120 min. The pH of conventional HTL aqueous decreased from 6.32
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to 3.95 and 4.51 to 3.86, respectively, for the increasing of both reaction temperature
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from 150 to 230 °C and residence time from 30 min to 240 min.
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7
6
6
TOC (g/L) and pH
TOC (g/L) and pH
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5 4 3
TOC pH
2
3
TOC pH
2
0 150
130
4
1
1
129
5
165
180
195
Temperature (
210
225
240
0
50
100
150
Time (min)
)
(A)
(B)
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Fig. 2. TOC and pH of the aqueous products obtained from conventional HTL of
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rice straw (A) different reaction temperature when residence time was 30 min; (B)
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different residence time when temperature was 210 °C
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Fig. 3 (A) (B) presents the contents of typical sugars formed during conventional
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HTL. As the temperature elevated from 150 to 210 °C, the concentrations of total
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sugars increased from 0.06 to 0.35 g/L and then decreased to 0.25 g/L at 230 °C for 30
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min. For all aqueous samples obtained from conventional HTL, levoglucosan, xylose
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and fructose were the most prevalent sugars especially when reaction temperature
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increased appropriately to 210 °C. Aqueous phase contained significant amounts of
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levoglucosan, xylose and fructose with value of 0.16 g/L and 0.14 g/L at 210 °C for
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30 min. However, all of the typical sugars concentrations dropped rapidly below 0.05
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g/L when residence time was increased to 240 min at 210 °C.
0.15
0.20 Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan
Typical sugars content (g/L)
Typical sugars content (g/L)
0.20
0.10
0.05
0.00 140
Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan
0.15
0.10
0.05
0.00 160
180
200
220
240
0
50
o
143 144
(A)
146 147
250
200
250
Lactic acid Formic acid Acetic acid Levulinic acid
2.0
1.5
1.0
0.5
0.0 140
200
2.5 Lactic acid Formic acid Acetic acid Levulinic acid
Typical acids content(g/L)
2.0
150
(B)
2.5
145
100
Time (min)
Temperature ( C)
Typical acids content(g/L)
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1.0
0.5
0.0 160
180
200
220
240
0
50
Temperature (oC)
100
150
Time (min)
(C)
(D)
Fig. 3. Typical sugars and acids contents of aqueous products obtained from ACS Paragon Plus Environment
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conventional HTL of rice straw (A) different reaction temperature when residence
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time was 30 min; (B) different residence time when temperature was 210 °C; (C)
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different reaction temperature when residence time was 30 min; (D) different
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residence time when temperature was 210 °C
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As illustrated of acids contents in the aqueous products in Fig. 3 (C) (D), the total
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acid concentrations of aqueous phase which led to an increase of acidity have showed
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a stable increase with the increasing of HTL temperature from 150 to 230 °C or
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residence time from 30 min to 240 min and this was consistent with pH values
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variation. The major acids of the aqueous products identified by HPLC were lactic
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acid, acetic acid, formic acid and levulinic acid. As temperature elevated to 230 °C,
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the contents of dominate acids of lactic acid and acetic acid were induced a significant
159
increasement to 1.86 and 1.66 g/L, respectively. Meanwhile, the total acids achieved
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to 6.05 g/L at 210 °C when residence time increased to 240 min.
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3.2 Characterization of MW assisted HTL aqueous products
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The aqueous characterizations of TOC and pH values for MW assisted HTL
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samples are shown in Fig. 4. The pH values decreased from 6.52 to 3.96 and from
164
4.15 to 3.68 at the designed HTL temperatures and residence time ranges. It is noticed
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that TOC concentration of aqueous samples increased from 2.24 to 4.18 g/L when
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HTL temperature increased from 150 to 230 °C. Meanwhile, TOC concentration
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increased from 3.71 to 4.28 g/L with the increasement of residence time from 5 to 10
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min and then decreased to 3.89 g/L when residence time was 30 min.
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6
6
5
5
TOC (g/L) and pH
7
4 3 2
TOC pH
0 140
4 3 TOC pH
2 1
1
0 150
160
170
180
190
200
Temperature (
170 171
210
220
230
240
5
10
15
20
25
30
Time (min)
)
(A)
(B)
172
Fig. 4. TOC and pH of the aqueous products obtained from MW assisted HTL of rice
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straw (A) different reaction temperature when residence time was 5 min; (B) different
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residence time when temperature was 210 °C
175 176
Fig. 5 presents the typical six sugars and four acids identified in the aqueous
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products of MW assisted HTL. The concentrations of six sugars and four acids ranged
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from 0.06 to 0.43 g/L and 0.56 to 4.73 g/L, respectively, when HTL temperature
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varied from 150 to 210 °C for 5 min. Meanwhile, increased yields of sugars from 0.43
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to 0.84 g/L and acids from 3.22 to 5.45 g/L were observed when residence time
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increased from 5 to 30 min at 210 °C.
0.25
0.20
0.5 Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan
Typical sugars content(g/L)
Typical sugars content(g/L)
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TOC (g/L) and pH
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0.15
0.10
0.05
0.00 140
Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan
0.4
0.3
0.2
0.1
0.0 160
180
200
220
240
0
5
o
182 183
10
15
Time (min))
Temperature ( C)
(A)
(B)
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25
30
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4.0
3.0
4.0
Lactic acid Formic acid Acetic acid Levulinic acid
2.5 2.0 1.5 1.0 0.5 0.0 140
Lactic acid Formic acid Acetic acid Levulinic acid
3.5
Typical acids content(g/L)
3.5
Typical acids content(g/L)
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3.0 2.5 2.0 1.5 1.0 0.5 0.0
160
180
200
220
240
0
5
o
184 185
10
15
20
25
30
Time (min)
Temperature ( C)
(C)
(D)
186
Fig. 5. Typical sugars and acids contents of the aqueous products obtained from MW
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assisted HTL of rice straw (A) different reaction temperature when residence time was
188
5 min; (B) different residence time when temperature was 210 °C; (C) different
189
reaction temperature when residence time was 5 min; (D) different residence time
190
when temperature was 210 °C
191
As Fig. 5 depicts, sugars increased significantly when MW temperature at around
192
210 °C. Xylose, fructose and levoglucosan were the most prevalent sugars especially
193
when increased the residence time appropriately (30 min). Particularly, glucose
194
concentration was highly increased to 0.16 g/L at 210 °C for 30 min. Meanwhile,
195
significant upwards trends especially acetic acid were observed at temperature range
196
of 150-230 °C or residence time range of 5-30 min. Formic acid concentrations have
197
displayed about a third to half of the acetic acid load which was consistent with
198
Becker’s results21. Other chemicals with low concentrations such as HMF,
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levoglucosenone, furfural and phenyl ethanol were also detected by HPLC, which
200
were shown in Table S6 and Table S7. Solid yields at different HTL conditions for
201
both heating methods were shown in Table S8.
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4 Discussion
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4.1 Typical prodcuts and HTL conditions
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As a primary indicator of the soluble organic compounds, the values of TOC
205
showed versatile tendencies with the increasing of temperature and residence time
206
both for conventional and for MW assisted HTL as as Fig. 2 and Fig. 4 show. There
207
was a maximum TOC concentration at 190 °C for conventional HTL or at 230 °C for
208
MW assisted HTL, which suggests that the soluble organics revealed a further
209
cleavage at appropriate higher temperature. TOC concentrations of aqueous phase
210
obtained from conventional HTL decreased with the increase of reaction temperature
211
from 190 to 230 °C or residence time from 30 to 120 min. It was also the same
212
tendency for MW assisted HTL when residence time increased from 10 to 30 min.
213
Akhtar et al. reviewed that repolymerization reactions lead to the decrease of TOC
214
value when increased rection temperature or residence time22. Therefore, the TOC
215
negative tendencies of aqueous products obtained from both heating methods were
216
possibly due to repolymerization reactions during HTL.
217
Significantly, xylose, fructose and levoglucosan had a relatively higher selectivity
218
for this mild HTL conditions at 210 °C for 20 min using MW assisted HTL and at
219
210 °C for 30 min using conventional HTL with maximum total sugurs of 0.75 g/L
220
and 0.35 g/L, respectively. The repolymerization behavior of the dissolved oligomers
221
or further decomposition to produce furfuals is the main challenge for efficient
222
monosaccharide yield at exorbitant temperature or residence time23, 24. As Fig. 3 (A)
223
(B) and Fig. 6 show, the total sugars expecially xylose, fructose and levoglucosan
224
decreased significantly when HTL temperature increased up to 230 °C or residence
225
time lasted for more than 60 min for convernational HTL. Longer residence time and
226
higher rection temperature promote acids production during HTL via conventional
227
heating method, which is consistent with Chen’s results10. Meanwhile, it is also the
228
same positive correlation for total acids content with residence time and rection
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temperature when HTL via MW assisted. Acids variation consistent with pH values,
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as Fig. 2 reveals, the more acids produced, the lower of the pH values were.
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There were intricate chemical rections during HTL of biomass including
232
hydrolysis, deoxygenation, cracking and repolymerization25. These reactions may
233
occur selectively depending on the reaction conditions and significantly promote the
234
formation of target product, while inhibiting other products. Typical chemicals such as
235
levoglucosan, glucose, xylose, fructose, acetic acid and formic acid were formed
236
during the mild HTL of rice straw, as Fig. 6 shows. Xylose and fructose which come
237
from hemicellulose24 had prominent concentrations in both conventional and MW
238
assisted HTL samples.
239
cellulose which releases glucose monomers occurred when HT temperature higher
240
than 150 °C, however, the yields of glucose were low as shown in Fig. 3 (A) (B) and
241
Fig. 5 (A) (B). This might well be the nature structure of rice straw which composed
242
of cellulose, hemicellulose and lignin. Cellulose is organized into microfibrils
243
surrounded by hemicellulose and encased inside a lignin matrix and even though
244
cellulose and hemi-cellulose have similar chemical compositions, cellulose is more
245
stable to be hydrolyzed to monosaccharide than hemicellulose27. Instead of glucose,
246
levoglucosan was also one of the main saccharides for both of the heating methods
247
present in Fig. 3 (A) (B) and Fig. 5 (A) (B). However, the destruction of monomer
248
sugars contributed to the formation of organic acids and furans when HT temperature
249
elevated to 210 °C as Fig. 6, Table S2 and Table S4 show.
According to the report by Savage et al.26, hydrolysis of
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250
Fig. 6. The hydrolysis reactions of cellulose and hemicellulose leading to the formation of
251
sugars and acids
252 253
4.2 Comparision of conventional and MW assisted HTL
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It was found that the HTL products including acids and sugars were more
255
sensitive to rection temperature than residence time, but different heating methods
256
like conventional or MW assisted heating still played an important role. For example,
257
when HTL temperature increased from 150 to 230 °C, both the primary parameters
258
like pH, TOC and the specific products like acids and sugars had tremendous changes
259
comparing with influence of residence time in spite of the multiply increasement of
260
reaction time as Fig. 3 (C) (D) and Fig. 5 (C) (D) present.
0.8 9
Total sugars concents (g/L)
Conventional MW 6
TOC (g/L)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0
140
261 262 263
160
180
200
220
240
0.6
Conventional MW
0.4
0.2
0.0
-0.2 140
160
180
200
220
240
Temperature (oC)
Temperature (oC)
(A)
(B)
Fig. 7. Total sugars and TOC of the aqueous products obtained from conventional
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HTL for 30 min and MW assisted HTL for 5 min (A) TOC of aqueous products (B)
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total sugars contents of aqueous products
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As Fig. 7 presents, it need only 5 min for MW heating to get considerable or even
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higher amount of sugars and it has taken 30 min for conventional heating HTL at
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210 °C although the TOC values were lower. MW has the potential to provide rapid
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method for HTL28 as it interacts directly with the materials by changing
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electromagnetic energy into heat transfer inside the dielectric materials29 instead of
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conducting heat from an external heat source. MW heating can overcome the
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problems of conventional heating method of thermal gradient and has a slight
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temperature difference between the surface and interior of the slurry. The
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repolymerization behavior or secondary reaction might be decreased and high
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saccharide yields were obtained at a even higher HTL temperature but short residence
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time which counteracted low TOC of the aqueous for MW assisted HTL. Therefore,
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we can conclude that MW assisted HTL is propitious to sugars production comparing
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with conventional heating at this mild HTL condition.
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4.
Conclusion
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The aqueous products obtained from MW assisted and conventional HTL of rice
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straw were compared in this study. Levoglucosan, glucose, xylose, fructose, acetic
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acid and formic acid were main chemicals formed during the mild HTL of rice straw.
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HTL temperature appeared to be the dominant factor, while the increased residence
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time slightly changed the content of TOC, typical sugars and acids both for
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conventional and MW assisted HTL heating. The results also indicate that MW
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radiation is an efficient method to decrease residence time to get considerable or even
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higher amount of saccharide yields as it could decrease repolymerization behavior or
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further decomposion.
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ASSOCIATED CONTENT
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Supporting Information
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Table S1-S8 can be found in supplementary information.
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AUTHOR INFORMATION
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Corresponding Author
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† Chong Liu
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Present Addresses
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† Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China,
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E-mail:
[email protected].
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Author Contributions
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The manuscript was written through contributions of all authors. All authors have
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given approval to the final version of the manuscript.
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ACKNOWLEDGMENT
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The authors thank anonymous reviewers for fruitful suggestions.
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ABBREVIATIONS
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HTL, hydrothermal liquefaction; MW microwave; HPLC high performance liquid
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chromatography; TOC , Total Organic Carbon.
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