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Biofuels and Biomass
Two-stage hydrothermal liquefaction of sweet sorghum biomass - Part II: Production of upgraded biocrude oil Yang Yue, James R Kastner, and Sudhagar Mani Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00669 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018
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A schematic of two-stage HTL-HDO process for sweet sorghum biomass to produce upgraded biocrude oil 275x113mm (96 x 96 DPI)
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Two-stage hydrothermal liquefaction of sweet sorghum biomass -
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Part II: Production of upgraded biocrude oil
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Yang Yue, James R. Kastner and Sudhagar Mani*
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School of Chemical, Materials and Biomedical Engineering, University of Georgia, Athens, GA
5
*
Corresponding Author: Ph: (706) 542-2358; email:
[email protected] 6 7
ABSTRACT: The hydrothermal liquefaction (HTL) followed by hydro-deoxygenation (HDO)
8
of lignin-rich biomass has a potential to improve the yield and the quality of upgraded biocrude
9
oil. In this study, the lignin-rich biomass obtained from the low-temperature HTL (stage 1) of
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sweet sorghum bagasse was investigated to evaluate the biocrude oil yield and its compositions
11
using catalytic high-temperature HTL-HDO (stage 2) and compared with conventional, whole-
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stage HTL-HDO process. Biocrude oil yield was achieved up to 38% during a high-temperature
13
HTL, while 42% was obtained after catalytic (5% Ru/C) HDO process. Up to 96% hydrocarbon
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content of biocrude oil was achieved, which is twice higher than that of conventional whole-
15
stage HTL-HDO process. Aromatic hydrocarbons and long-chain alkanes were also dominant in
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the upgraded biocrude oil, which collectively accounted for 28% of total hydrocarbons. From
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raw sweet sorghum bagasse, the hydrocarbons yield from second-stage HTL-HDO process (11%)
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was substantially increased by 78% compared to that of whole-stage HTL-HDO process (7%).
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The upgraded biocrude oil could be directly processed or co-processed in the existing refinery to
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produce drop-in fuels.
21 22
Keywords: High-temperature HTL, lignin, biocrude oil, hydrodexoygenation,
23 24
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1. INTRODUCTION
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Lignocellulosic biomass is both sustainable and renewable energy resources for biofuel
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production due to its abundant availability and potential to reduce greenhouse gas emissions.
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Sweet sorghum, a typical C4 annual energy crop can be grown on an uncompetitive arable land.
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This energy crop has a short growth cycle, produces high biomass yield, requires low
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fertilization input and is tolerance to drought condition.1-3 Therefore, it can be an excellent
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biorefinery feedstock to produce diverse stream of biochemical and biofuels. Sweet sorghum
8
biomass also accumulates sugars in plant stalks, which can be squeezed to extract juices rich in
9
sucrose.4, 5 The sugar content of the juice varies from 9 to 15%, with the total juice yield of 3.6 to
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15.5 Mg ha-1. The bagasse, the byproduct of stalk after juice extraction represents approximately
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two-third of the dry biomass, and has limited value due to its low energy content and high stalk
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moisture content.6 However, the bagasse with high moisture content ranging from 60-80% (wet
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basis) could be an excellent feedstock for hydrothermal liquefaction (HTL) based biorefinery.
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Hydrothermal liquefaction (HTL) is a rapidly developing technology to convert wet biomass
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into biocrude oil using subcritical water as the solvent at moderate temperature (280-370 °C) and
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high pressure (10-25 M Pa).7 During HTL process, all the solid biomass is completely dissolved
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into mainly liquid and gaseous products. The liquid product can be further separated into
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aqueous and oil (biocrude oil) fractions. The unconverted solids and other inorganic minerals in
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the biomass are called residual solids or char. The HTL biocrude oil yield from lignocellulosic
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biomass ranged from 30 to 40%;8-12 this yield was impacted by various HTL parameters,
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including temperature, catalyst, and retention time. It was reported that the typical HTL
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temperature range was from 300 °C to 350 °C, to obtain a maximum biocrude oil yield using
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water as a solvent with or without catalysts.13-16 The utilization of an alkaline catalyst in the HTL
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process suppresses the formation of solid products, which would enhance the biocrude oil yield.9
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Karagöze et al. employed 0.94 M K2CO3 as a catalyst to HTL of wood and found that the
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biocrude oil yield increased from 17.8 to 33.7% and the solid residue yield was reduced from 42
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to 4%.17, 18 For most lignocellulosic biomass, the reported optimum holding time ranged from 15
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min to 60 min depending on the reaction temperature (250~375 °C) and heating rate (5-
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140 °C/min).7, 19, 20 Alkaline catalyst improved the processes of hydrolysis and decomposition
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and shortened holding time; however, the most important effect of alkaline catalyst was to
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increase biocrude oil yield.21 It was also reported by Boocock22 and Zhu13 that prolonged holding
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time suppressed the biocrude oil yield because the cracking reaction of biocrude oil or its
10
precursors promoted the formation of gases and chars through condensation and
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repolymerization reactions.23 The reduction in oil yield with longer holding time was also
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confirmed by the National Collaborative Research Infrastructure Strategy (NCRIS, University of
13
Sydney, Australia) through a continuous HTL process, which reported that shorter residence time
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with a higher heating rate benefited the higher biocrude oil yield.24
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The obtained biocrude oil through HTL contains a wide-range of highly-oxygenated small
16
molecular compounds as well as undesirable water, which resulted in poor combustion properties
17
due to low heating value, high acidity, high reactivity, and corrosion against combustor.25 Hydro-
18
deoxygenation (HDO) is, therefore, essentially required to improve biocrude oil quality by
19
catalytically removing oxygenated compounds from biocrude oil through hydrodeoxygenation
20
and decarboxylation reactions in the presence of hydrogen and heterologous catalysts at
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moderate temperature (250-450 °C) and high pressure (75-300 bar) before applying for drop-in
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fuels.26 The most commonly used HDO catalyst is ruthenium metal catalyst supported with
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carbon (Ru/C), which has high potential to increase biocrude oil yield (52%) and higher degree
2
of HDO (86%) among various transition metal catalysts at 350 °C for 4 h reaction time.27, 28
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One major issue concerning HTL of lignocellulosic biomass is that the presence of
4
hemicellulose in the biomass limits the biocrude oil yield and further impairs the downstream
5
hydro-deoxygenation (HDO) upgrading process due to high content of relatively small
6
oxygenates.29 Although the hemicellulose in the biomass accounts for approximately 20-40%,
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but it was reported to contribute only up to 5.3% of biocrude oil yield.30 The majority of the
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hemicellulose during HTL process were converted into aqueous compounds (up to 42%).30 Liu
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also reported the cross reactions occurred between hemicellulose derived intermediates and
10
lignin units, further promoted the formation of solid residues through repolymerization
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reactions.31 However, limited research was conducted on improving the HTL oil yield, while
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effectively utilizing all fractions of the lignocellulosic biomass into useful products. Therefore,
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we developed a two-stage HTL-HDO technology to comprehensively utilize lignocellulosic
14
biomass. In the first-stage HTL, hemicellulose in the biomass can be selectively hydrolyzed
15
under low-temperature HTL conditions and the remaining lignin-rich solid are prepared for
16
further conversion into upgraded biocrude oil (Fig. 1). The optimal first-stage HTL conditions
17
for sweet sorghum bagasse and the sugar yields were investigated in our previous study and
18
found that (i) about 94.1% of hemicellulose and 49.2% cellulose were respectively hydrolyzed
19
into C5 and C6 sugars platform with the concentration of 28.52 g/L; (ii) the co-product of lignin-
20
rich solid fraction, an ideal source of phenolics feedstock can be further hydrothermally liquefied
21
into high-quality biocrude oil.32 This article aims to investigate the feasibility of conversion
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pretreated lignin-rich biomass into high-quality biocrude oil. The main objectives of this paper
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were to experimentally investigate the high-temperature HTL-HDO of lignin-rich sweet sorghum
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bagasse to determine the biocrude oil yield compared with whole-stage HTL-HDO process and
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to evaluate the fuel characteristics and the compositions of second-stage HTL and HDO
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upgraded biocrude oil to produce drop-in biofuels.
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5 6 7 8 9
Fig.1 A schematic of two-stage HTL-HDO process for sweet sorghum biomass to produce upgraded biocrude oil.
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2. EXPERIMENTAL SECTION
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2. 1 Materials. Sweet sorghum bagasse used for the whole-stage HTL-HDO was obtained from
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the experimental research station in Fort Valley State University, Fort Valley, Georgia, USA.
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Sweet sorghum variety of EJ7281 (Ceres Inc., Thousand Oaks, California) was harvested in mid-
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October and the stalks were squeezed to extract sugar juices. The remaining solids, sweet
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sorghum bagasse was size reduced using a knife cutting mill (Retsch SM 2000, Germany) into a
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fine powder with an average size of 0.3 mm and then oven-dried at 105 °C overnight. The
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sample was stored in a desiccator for subsequent use. It composed of around 20% hemicellulose,
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36% cellulose and 18% lignin. The lignin-rich sweet sorghum bagasse used in the second-stage
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HTL-HDO was prepared from the first-stage HTL process using the optimal condition of 170 °C
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for 90 min based on our previous study.32 The lignin-rich biomass contained about 33% lignin
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and 43% cellulose with a negligible hemicellulose content. Reagent grade dichloromethane
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(DCM), magnesium perchlorate, and 5% ruthenium on carbon (Ru/C, particle size of 20 µm)
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were all purchased from Sigma-Aldrich.
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2.2 HTL and HDO Experiment. The stainless-steel reactor utilized for HTL and HDO was a
6
Parr 4598 batch micro-stirred reactors system (Parr Instrument Company, Moline, USA) with a
7
maximum temperature of 500 °C and pressure of 5000 psi. The vessel capability was 100 ml. A
8
consumable Parr Grafoil graphite gasket was used to avoid pressurized gas leakage during
9
reaction. A 4-blade impeller (0.81 in dia.) powered by a magnetic stirrer (model no. A1120HC6,
10
1/8 hp variable speed) provided agitation with a maximum speed of 750 rpm. The vessel was
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heated through a ceramic fiber external jacket (700 watts), and the processes of HTL and HDO
12
reaction were monitored with Parr Specview Software. The sketch of the experimental apparatus
13
is showed as Fig. 2. The average heating rate was 14 °C/min; the average cooling rate was
14
34.5 °C/min in a water bath. 3
He 2
4
Monitor 5 6 1
Micro-stirred reactor 15
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Fig. 2 A schematic graph of experimental micro-stirred reactor. 1. Helium cylinder, 2. Pressure gauge, 3. rotor, 4. Thermocouple, 5. Heating jacket, 6. Gas sample collector, 7. Liquid sample collector.
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About 5.0 g of dried sweet sorghum bagasse sample was placed into the vessel followed by
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the addition of 50 ml 0.94 M potassium carbonate solution or water for each HTL and control
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experiments. Helium was used to purge and displace the air in headspace until 500 psi was
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reached. The reaction was monitored through Parr Specview Software at a temperature of 350 °C
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and with an agitation of 300 rpm. After the holding time, the vessel was immediately cooled in a
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water bath until reaching room temperature. The gas sample was then collected from the
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headspace through a blowdown needle valve into a sealed-air sample bag. The vessel was
11
unloaded after releasing pressure through the blowdown valve. The liquid and solid products
12
were analyzed or immediately used for HDO experiment. Based on the previous studies,33-35
13
homogeneous alkaline catalyst, K2CO3 was used a catalyst in the present study; and three
14
different holding times (15, 30 and 60 min) were investigated for the whole-stage HTL to study
15
the effects on biocrude oil yield and its quality. The optimum condition of whole-stage HTL
16
process for the maximum biocrude oil yield was used to study the second-stage HTL. In the
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second-stage HTL, the lignin-rich sweet sorghum bagasse obtained from first-stage HTL
18
conditions (170 °C for 90 min) was used to determine the biocrude oil yield and its compositions.
19
HDO Experiment: Hydro-deoxygenation (HDO) experiment was carried out immediately
20
after HTL experiment in the same Parr reactor. About 1.5 g Ru/C was added as a catalyst into the
21
vessel. The properties of Ru/C catalyst have been reported in our pervious study with the surface
22
area of 714 m2/g, average pore size of 14.4 Å and pore volume of 0.52 cm3/g.36 Hydrogen was
23
used to purge the headspace under a pressure of 50 psi for 30 s three times and finally
24
pressurized until 500 psi was reached. The HDO process was conducted at 350 °C for 4 h with
25
an agitation rate of 500 rpm. After HDO reaction, the gas sample was collected as similar to the 7 ACS Paragon Plus Environment
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HTL procedure described previously. After unloading the vessel, the solid and liquid samples
2
were collected and stored in a refrigerator at 4 °C before separation for compositional analysis.
3
Three replicates of HTL and HDO operations were carried out and the average data was reported
4
with standard derivation. All the yields and compositions of HTL products were determined
5
based on mass basis.
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2.3 Separation and Extraction Procedures. About 40 ml DCM solvent was added into the
7
mixture of HTL or HTL-HDO liquid and solid products to extract biocrude oil (oil phase). The
8
mixture was then filtered with vacuum filtration through Whatmann #4 90mm filter paper. The
9
solid residue was rinsed with an additional 15 ml DCM to collect residual biocrude oil. The
10
rinsing solution was merged with the filtrate. The biocrude oil fraction in liquid products was
11
extracted with the combined DCM and decanted off from aqueous phase using a separatory
12
funnel. To remove residual moisture, 1.0 g anhydrone (magnesium perchlorate) was added into
13
the biocrude oil fraction. This anhydrone-biocrude oil mixture was then filtered again through
14
Whatmann #4 90mm filter paper, and the extracted de-watered biocrude oil fraction was further
15
transferred into a round-bottomed flask for rotary evaporation in a water bath of 36 °C for 60 min
16
under a vacuum of 2 mmHg. The DCM free biocrude oil or upgraded biocrude oil was then
17
collected and weighed before storing in a refrigerator for compositional analysis.
18
2.4 HTL and HDO Products Analysis. Major compounds in aqueous products were
19
determined with HPLC using a Coragel 94 column with the mobile phase of 4 mN sulfuric acid
20
at the flow rate 0.6 ml/min. The oven temperature was kept constantly at 60 °C. 5 µl sample was
21
injected for analysis.
22
The elemental composition of biocrude oil was measured using an elemental analyzer (LECO
23
CHNS 932, LECO Corporation, St. Joseph, MI), based on the procedure of ASTM D3176–89 in
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which oxygen content was obtained by difference.37 Moisture content of biocrude oil was
2
measured with Karl Fisher Titration.38
3
The higher heating value (HHV) of biocrude oil could be calculated based on the elemental
4
composition, using the Dulong Formula as
5
(MJ/kg) = 0.3383C + 1.422 (H − O/8) 39
6
The compositions of biocrude oil were analyzed with GC-MS using a HP-5ms Capillary
7
Column (30m x 0.25um x 0.25mm). The GC oven temperature was set to 40 °C initially and
8
hold for 4 min, then ramped at 5 °C /min until 275 °C and held consistently for 5 min. The Mass
9
Spec interface and inlet temperature were set at 280 °C and 260 °C, respectively. Helium was
10
employed as GC carrier gas at the flow rate of 0.8 mL/min. The split ratio was set to 50:1. The
11
electron impact (EI) ionization was set at 70 eV with a mass to charge range from 50 to 600 u.
12
Major compounds identified with GC-MS from biocrude oil and upgraded biocrude oil were
13
quantitatively analyzed with an internal standard method. Neat compounds of toluene, trimethyl
14
benzene, pentadecane, 2-methoxy phenol and 2-cyclopenten-1-one were used to estimate the
15
mass percentage of aromatic hydrocarbons, benzenes, alkanes, phenols and cyclic ketones,
16
respectively. DCM was used as a solvent to mix with the internal standard, 1-hexanol, at a
17
concentration of 1.04 g/L. The standard calibration curves were developed by calculating the
18
quantification factor of the slope obtained from concentration versus GC area of above neat
19
compounds with three replications. For biocrude oil and upgraded biocrude oil compositional
20
analyses, a standard stock solution was prepared immediately before GC-MS operation by
21
adding 10 µl 1-hexanol into 800 µl DCM. After vortexing, 35 µl standard stock solution was
22
mixed with 302 µl biocrude oil or upgraded biocrude oil sample. 1 µl mixed sample solution was
23
injected into GC-MS after filtration with a 0.45 µm filter. The solvent delay was setup for 3 min
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to ignore DCM. The compounds were identified by matching fragmentation patterns with
2
National Institute of Standards and Technology (NIST) mass spectral library. Each sample from
3
HTL and HDO experiments was repeated thrice. The average result was reported with standard
4
deviation.
5 6
3. RESULTS AND DISCUSSION
7
3.1 Whole-Stage vs Second-Stage HTL Experiment
8
3.1.1 HTL Biocrude Oil Yield. The biocrude oil yield increased up to 1.44 times in the presence
9
of K2CO3 catalyst due to decreased aqueous product and solid yields (Table 1). During HTL of
10
lignocellulosic biomass, K2CO3 catalyzed the hydrolysis of lignin and cellulose macromolecules
11
into oligomers and monomers; thus, promoted the following decomposition into a diversity of
12
secondary small-molecular compounds through dehydration, dehydrogenation, deoxygenation,
13
and decarboxylation reactions. Once produced on the basis of their activities, these small-
14
molecular compounds experienced rearrangement reactions of condensation, cyclization and
15
polymerization and formed the majority of biocrude oil.40 In addition, the solid product yield
16
decreased by 58%, while the gas product yield increased by 2.27-fold due to an alkali carbonate
17
effect proposed by Appell.41 This gasification effect was crucial for deoxygenation, but at the
18
cost of carbon loss.13 The yield trends of HTL with alkaline catalysts were similar to that of other
19
studies conducted elsewhere.7, 14
20 21 22 23
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Table 1. A summary of product yields from various HTL and HDO conditions Whole-stage HTL a Catalyst Holding time (min)
0.94 M K2CO3 15
30
60
Second-stage HTL b
Whole-stage HTL-HDO c
Second-stage HTL-HDO d
N/A
0.94 M K2CO3
5% Ru/C
5% Ru/C
30
30
240
240
Product Yields,% Gas
9.33 (0.67)
36.03 (1.33)
41.30 (1.33)
11.00 (1.17)
36.90 (2.07)
34.76 (1.79)
32.91 (2.25)
Solid
8.03 (0.00)
6.67 (0.67)
5.33 (0.67)
15.89 (1.28)
7.47 (1.60)
4.01 (0.70)
7.42 (1.84)
Biocrudeoil
17.30 (0.67)
25.30 (1.33)
21.33 (0.67)
10.38 (1.48)
37.83 (2.32)
27.81 (2.46)
41.70 (0.67)
Aqueous
65.34 (0.00)
32.00 (0.67)
32.04 (0.00)
62.80 (1.40)
17.83 (3.84)
33.42 (4.95)
17.97 (3.40)
2 3 4 5 6
Note: a, yield of whole-stage HTL was calculated on the basis of dried raw sweet sorghum bagasse; b, yield of second-stage HTL was calculated on the basis of first-stage HTL solid product; c, yield of wholestage HTL-HDO was calculated on the basis of solid and liquid products from whole-stage HTL; d, yield of second-stage HTL-HDO was calculated on the basis of solid and liquid products from second-stage HTL.
7
The effect of holding time on the biocrude oil yield of whole-stage HTL with alkaline catalyst
8
was also studied. Biocrude oil yield increased by 46% from 15 to 30 min residence time and then
9
decreased to 16% from 30 to 60 min. The maximum biocrude oil yield of 25% was obtained at
10
30 min. A prolonged retention time suppressed the biocrude oil yield as reported by Boocock19
11
and Zhu13. The decline in biocrude oil yield with longer time could be due to the condensation
12
and repolymerization reactions within the biocrude oil.23 Although the detailed mechanism of
13
biocrude oil decomposition are still unclear, our study confirmed an increase in gas yield with
14
longer reaction time. The result further confirmed that the use of 0.94 M K2CO3 had prevented
15
the repolymerization reaction between hemicellulose decomposition intermediates and lignin
16
unit derived from raw sweet sorghum bagasse. Therefore, the HTL condition of 350 °C, 30 min
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with 0.94 M K2CO3 was the most suited conditions for maximum biocrude oil yield and thus was
2
used as a basis for comparing the second-stage HTL to produce biocrude oil from lignin-rich
3
sweet sorghum bagasse.
4
The lignin-rich sweet sorghum bagasse had 94% increase in lignin and 18% increase in
5
cellulose content as compared with that of raw sweet sorghum bagasse. Table 1 summarized the
6
HTL products yield for second-stage HTL of lignin-rich feedstock. The biocrude oil yield of
7
second-stage HTL was increased by 49% as compared with that of whole-stage HTL. The
8
second-stage HTL biocrude oil yield of 38% was obtained from hemicellulose-free sweet
9
sorghum bagasse and the yield was higher than reported elsewhere for pine wood,17 birth wood,12
10
sawdust42 and cornstalks10. The second-stage HTL process also reduced the aqueous product
11
yield by 44% compared with that of whole-stage HTL. In addition, it was found that there was
12
no significant solid yield difference between the second-stage HTL and whole-stage HTL, which
13
confirmed that the potential formation of char through cross-reaction and repolymerization
14
between hemicellulose and lignin intermediates was suppressed from whole-stage HTL with the
15
presentence of K2CO3.17, 18
16
Table 2. Major compositions of biocrude-oil from whole-stage HTL Catalytic (K2CO3) whole-stage HTL condition Holding time (min)
15
Compounds
30
60
Mass%
Cyclic ketones
74.57
56.75
58.48
Cyclopentanone, 2,5-dimethyl-
4.23
2.78
4.72
2-Cyclopenten-1-one, 2,3-dimethyl-
5.11
2.51
1.64
2-Cyclopenten-1-one, 2,3,4-trimethyl-
31.57
21.95
15.72
2-Cyclopenten-1-one, 2,3,4,5tetramethyl-
33.66
29.51
36.40
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Phenolics
18.62
35.83
35.30
Phenol
0.88
1.66
-
Phenol, 2-methyl-
0.00
2.43
0.99
Phenol, 4-methyl-
3.21
5.21
3.06
Phenol, 2,4-dimethyl-
-
-
13.59
Phenol, 3,5-dimethyl-
8.54
10.65
-
Phenol, 2-ethyl-4-methyl-
1.84
1.46
2.16
Phenol, 2-ethyl-5-methyl-
-
3.38
-
Phenol, 4-ethyl-3-methyl-
-
-
4.05
1.29
2.52
-
-
-
3.67
2.86
8.51
7.78
6.81
7.43
6.22
4.61
7.43
5.07
1,3-Dimethyl-1-cyclohexene
0.93
-
-
Toluene
0.65
1.21
0.55
1H-Indene, 2,3-dimethyl-
2.22
1.32
2.24
-
4.05
1.56
0.81
0.85
0.71
2.20
-
1.15
Pentadecane
1.20
-
0.72
Heptadecane
1.00
-
0.43
Phenol, 2-methyl-5-(1-methylethyl)Thymol Other phenolics Hydrocarbons Aromatic hydrocarbons
1H-Indene, 2,3-dihydro-1,6-dimethylOther aromatic hydrocarbons Alkanes
1 2
3.1.2 HTL Fuel Properties. The elemental composition of biocrude oil from HTL experiments
3
is summarized in Table 3. In the presence of 0.94 M K2CO3, about 43% of carbon in the sweet
4
sorghum bagasse was converted into biocrude oil after 30 min, which is 2.45 times more than
5
that of non-catalytic biocrude oil yield. In absence of K2CO3, a major fraction of the carbon was
6
converted into carboxylic acid and ends up in the aqueous fraction of the biocrude oil.
13 ACS Paragon Plus Environment
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Energy & Fuels
1
The HHVs of biocrude oils ranged from 29.10 MJ/kg to 32.88 MJ/kg (Table 3), which almost
2
doubled that of the sweet sorghum bagasse feedstock of 16.30 MJ/kg.
3
Table 3. Fuel properties of biocrude oils obtained from different HTL conditions. Whole-stage HTL
Catalyst
4 5
0.94 M K2CO3
Secondstage HTL
Wholestage HTLHDO
Secondstage HTLHDO
Noncatalyst
0.94 M K2CO3
5% Ru/C
5% Ru/C
Benchmark Petroleum43
Holding time (min)
15
30
60
30
30
240
240
C [%]
72.81 (0.25)
69.62 (0.34)
73.96 (0.41)
69.13 (0.29)
79.30 (0.53)
86.87 (2.12)
88.22 (2.28)
83~86
H [%]
7.46 (0.02)
7.24 (0.01)
7.66 (0.08)
6.86 (0.06)
8.37 (0.06)
9.06 (0.31)
9.11 (0.20)
11~14
N [%]
1.36 (0.06)
1.35 (0.02)
1.30 (0.02)
1.24 (0.02)
0.63 (0.03)
1.28 (0.02)
0.61 (0.03)
