Upgrading Sewage Sludge Liquefaction Bio-Oil by Microemulsification

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Upgrading sewage sludge liquefaction bio-oil by microemulsification: the effect of ethanol as polar phase on solubilization performance and fuel properties Xiaowei Ding, Xing-zhong Yuan, Lijian Leng, Huajun Huang, Hou Wang, Jianguang Shao, Longbo Jiang, Xiaohong Chen, and Guangming Zeng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02269 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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Upgrading sewage sludge liquefaction bio-oil by microemulsification:

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the effect of ethanol as polar phase on solubilization performance and

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fuel properties

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Xiaowei Ding a, b, Xingzhong Yuan a, b, *, Lijian Leng a, b, Huajun Huang c,

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Hou Wang a, b, Jianguang Shao a, b, Longbo Jiang a, b, Xiaohong Chen d,

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Guangming Zeng a, b

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a

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410082, P.R. China

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b

College of Environmental Science and Engineering, Hunan University, Changsha

Key Laboratory of Environment Biology and Pollution Control (Hunan University),

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Ministry of Education, Changsha 410082, P.R. China

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c

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Nanchang 330045, P.R. China

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d

School of Land Resources and Environment, Jiangxi Agricultural University,

School of Business, Central South University, Changsha 410083, P.R. China

Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, P.R. China. Tel.: +86–731–88821413; Fax: +86–731–88823701; E-mail address: [email protected] (X.Z. Yuan).

*

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ACS Paragon Plus Environment

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Abstract: Sewage sludge liquefaction bio-oil is capturing extensive attention for its

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sustainability and easy availability. However, some limitations of bio-oil such as high

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viscosity and poor cold flow properties hamper its direct application in engine. Two

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microemulsions including ethanol-in-diesel (M1) and diesel microemulsion (M2)

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were produced to upgrade bio-oil. Adding ethanol in M1 facilitated bio-oil

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solubilization and improved fuel properties of bio-oil. Because of low viscosity and

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low pour point of ethanol, M1 with ethanol as polar phase produced a more promising

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bio-fuel with acceptable viscosity and better cold flow properties compared with M2.

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Through microemulsification in diesel, hydrophilic and hydrophobic bio-oil

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components may penetrate into polar core and continuous phase, respectively, while

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amphiphilic components mainly located at interface. The hydrogen-bonding between

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ethanol and hydrophilic bio-oil components could enhance solubilization and improve

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stability. Besides polar phase ethanol could also act as solvent to dissolve more

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less-hydrophilic bio-oil components into polar core.

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Keywords: Sewage sludge (SS); Liquefaction bio-oil; Ethanol; Microemulsion; Fuel

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upgrading

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Nomenclature M1

Ethanol-in-diesel microemulsion

M2

Diesel microemulsion

BHMBs

Bio-oil hybrid diesel microemulsion based bio-fuels

BEMB

Bio-oil/ethanol-in-diesel microemulsion based bio-fuel

BDMB

Bio-oil-in-diesel microemulsion based bio-fuel

SS

Sewage sludge

MEs

Methyl esters

PP

Pour point

TAN

Total acid number

TG

Thermogravimetry

DTG

Differential thermogravimetry

DTGmax

The maximum degradation rate

Ea

Activation energy

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1. Introduction

Facing with the exhaustion of petroleum and the stringent regulations upon

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environment, renewable energy resources are attracting mounting attention as they

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can solve the two aforementioned problems.1-3 Sewage sludge (SS), a kind of

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renewable wastes, could not be used safely as fertilizer because of organic and

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inorganic pollutants in it.4, 5 However, conversion of SS to bio-oil can not only

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dispose of bulky wastes but also recover rich energy in SS.6 Liquefaction bio-oil

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derived from SS has been extensively studied and considered as one of the most

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feasible alternative fuels.7-12 Because of some shortcomings of bio-oil such as high

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viscosity13, 14 and aging15, bio-oil upgrading process becomes inevitable before using

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it in diesel engine.

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Among present upgrading processes including hydrodeoxygenation,

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emulsification, catalytic cracking and solvent addition, emulsification is distinguished

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for its simple procedure and low chemicals consumption. Compared with emulsion,

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microemulsion shows more superiorities, since it can form spontaneously and perform

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long shelf-life.2, 16-18 Moreover, microemulsion with lower activation energy could

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ignite more easily than emulsion.19 Despite such advantages of microemulsion, there

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is only limited information concerning microemulsifying bio-oil with other fuels.

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Before emulsifying bio-oil, pre-treatment processes are often conducted to remove

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bio-oil heavy components.20, 21 In the present study, we intend to microemulsify

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bio-oil directly in diesel with the help of ethanol without any need of pre-treatment. In 4


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this way, economically speaking, it cut down on production cost; technologically, it

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facilitated the application of bio-oil in engine.

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Ethanol is a promising bio-fuel among all the renewable fuel resources because

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of its availability in large volume. The unique physicochemical characteristics of

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ethanol make it used widely as a fuel directly or as fuel additive.22 The individual

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application of ethanol in engine could reduce torque output and power, as a result,

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ethanol is often blended with other fuels as additive.23 As a cosolvent, ethanol was

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often blended with bio-oil to reduce viscosity and remain homogeneity.24 Interestingly,

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adding ethanol in microemulsion was proved to increase the film flexibility and

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extend the homogeneous microemulsion area.25 It was found that introducing ethanol

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with microemulsion-forming could enhance the flexibility of application of ethanol in

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diesel engine compared with blend-forming.26 Due to the polarity of ethanol, it could

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be used in place of water as the polar phase to formulate ethanol-in-oil

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microemulsion.27-29 Furthermore, unlike water which may increase the viscosity of

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dispersion, ethanol could make an obvious improvement on viscosity.25, 30 The higher

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energy content of ethanol than water might be an added advantage. Meanwhile, its

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good cold flow properties such as low pour point (PP) could accelerate the application

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of blended fuels in cold areas.31 Because of high latent heat of vaporization of ethanol,

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it could delay combustion with large amount of heat released in the expansion stroke,

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thus improving brake specific fuel consumption (BSFC) and brake thermal efficiency

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(BTE).32 As an oxygenated fuel, ethanol could promote complete combustion and 5


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reduce NOx, PM and PAHs emissions.33 Overall, adding ethanol can not only

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introduce desirable fuel properties but also make the most of energy in blended fuels

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and abate pollutant emissions. Given the above advantages of ethanol, it was used as

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polar phase here to formulate microemulsion for bio-oil solubilization and upgrading.

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To the knowledge of the authors, the application of ethanol for solubilizing bio-oil

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into other fuels via microemulsification was rarely involved.

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In this study, with the goal of upgrading bio-oil by microemulsification,

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ethanol-in-diesel microemulsion (M1) was firstly used to solubilize bio-oil and

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circumvent the undesirable properties such as higher viscosity, low stability and poor

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cold flow properties which often occur with the commonly used diesel microemulsion

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(M2).34-36 There were four objectives as follows: (1) to prepare two microemulsion

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dispersions, i.e., M1 and M2 (without additional polar phase) for bio-oil solubilization,

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the produced bio-fuels were termed as bio-oil/ethanol-in-diesel microemulsion based

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bio-fuel (BEMB) and bio-oil-in-diesel microemulsion based bio-fuel (BDMB),

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respectively; (2) to study the physicochemical and thermochemical characteristics of

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bio-oil hybrid diesel microemulsion based bio-fuels (BHMBs) including BEMB and

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BDMB; (3) to elucidate the solubilization mechanism of ethanol-in-diesel

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microemulsion and evaluate fuel properties of corresponding bio-fuels.

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2. Materials and methods

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2.1. Materials 6


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Diesel (0#) was purchased from a local petrol station in Changsha, China. Span

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80 (sorbitan monooleate, purity > 98%) was the surfactant used with a HLB

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(hydrophilic-lipophilic balance) value of 4 ±1. Five alcohols (n-butanol, n-pentanol,

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n-hexanol, n-heptanol and n-octanol) were used for co-surfactant optimization.

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Different proofs of ethanol (200º–170º proofs, 0-15vol.% water content) were used as

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polar phase.

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2.2. Liquefaction bio-oil solubilization

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2.2.1. Bio-oil production

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SS was collected from a local wastewater treatment plant. Through drying and

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grinding, 12 g dry SS and 150 mL methanol were blended in an autoclave (GSHA-0.5,

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China), which was heated to 300 oC. After 20 min retention at 300 oC, the products

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were rinsed by ethyl acetate and filtered to separate solid and liquid. The solvent

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(methanol and ethyl acetate) and water were removed from bio-oil by vacuum-rotary

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evaporation at reduced pressure 50 oC and 90 oC, respectively.

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2.2.2. Bio-oil solubilization

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Initially, 5.0 mL diesel, surfactant and co-surfactant were blended in a 10 mL

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centrifuge tube to form an amphiphile solution. Then water or different proofs of

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ethanol was added and the blend was hand-shaken vigorously until the homogeneous

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microemulsion was obtained, which took no more than 2 min. Next, 1.0 g bio-oil was

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mixed with the microemulsion. The tube was hand-shaken vigorously for 1 min and 7


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then was placed in a water bath shaker for 20 min at 150 rpm to allow the

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solubilization of bio-oil reach the equilibrium. Through centrifugation, the precipitate

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formed at the bottom (the bio-oil-rich phase), was the fraction which cannot be

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solubilized in microemulsion. While the supernatant, BHMBs, were subjected to

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further investigations.

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Two parameters were used to evaluate the bio-oil solubilization performance of

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microemulsion. One parameter µ (g/g) was defined as the solubilized bio-oil mass of

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unit mass of bio-oil. The other E (g/mol), indicating the solubilization efficiency of

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surfactant, was defined as the bio-oil mass solubilized in per mole of surfactant.

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2.3. Bio-oil hybrid diesel microemulsion based bio-fuels characterization

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2.3.1. Physicochemical and thermochemical properties

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Elemental, GC-MS and thermogravimetric analyses were used to investigate the

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physicochemical and thermochemical properties of BHMBs. The detailed information

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about analytical methods could be found in S1 in Supporting Information.

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2.3.2. Fuel properties

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Fuel properties of samples such as the viscosity, PP and total acid number (TAN)

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were determined. More measurement information can be found in S2 in Supporting

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Information.

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3. Results and discussions

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3.1 Bio-oil solubilization

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3.1.1. Effect of surfactant concentration

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Above critical micelle concentration (CMC, the concentration of surfactant in the

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bulk of continuous phase where micelles start forming), microemulsion could form

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spontaneously when bringing all the components together. The CMC of Span 80 in

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diesel was certified as 0.15 M previously.36 Fig. 1 shows that the solubilized bio-oil

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amount increased continuously with surfactant concentration. However, the

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solubilization efficiency of surfactant (E) had a reverse trend along the concentration

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range except a slight increase at 0.30 M. More and smaller reverse micelles may be

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formulated at higher surfactant concentrations,37 as a result, the total solubilized

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bio-oil amount of microemulsion could increase while the solubilized amount of per

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mole surfactant reduced. Consequently, surfactant concentration of 0.30 M was used

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in later experiments.

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3.1.2. Effect of co-surfactant type and C/S mass ratio

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Fig. 2 shows that adding co-surfactant favored the solubilization of bio-oil

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compared with the case without co-surfactant. Co-surfactant could help form a

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structured and stable microemulsion by reducing rigidity of interface and improving

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flexibility of the interfacial film.38 Emax (the maximum solubilization efficiency of

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surfactant in each C/S ratio, Ci represents alcohol with carbon number of i, i=4, 5, 6, 9


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7, 8) reached its highest value at C/S ratio 0.9 with n-octanol as co-surfactant. It may

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be due to that N-octanol, with the most negative ∆Gt o (the standard Gibbs free energy

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of transfer of co-surfactant from oil to interface) value and negative entropic change,

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could easily locate at the interface as co-surfactant.39 At a fixed surfactant

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concentration, excess co-surfactant molecules may pack themselves between

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surfactant and edge off surfactant from the surface. This could reduce the amount of

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surfactant at the surface and restrain solubilization capacity of microemulsion. Based

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on these results, n-octanol was chosen as co-surfactant with the C/S ratio at 0.9.

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3.1.3. Effect of polar phase type

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To investigate the effect of polar ethanol on bio-oil solubilization, different

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proofs of ethanol (or pure water) were investigated and the phase behaviors were

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recorded. Fig. 3(a) shows that apart from dispersions with 200° proof ethanol as polar

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phase, all the other dispersions had two phases divided by an obvious boundary. The

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upper was homogeneous microemulsion. The lower was heterogeneous emulsion

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which may be induced by greater polarity difference between hydrous ethanol and

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diesel.33 Dispersion with 200° proof ethanol as polar phase could formulate a single

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microemulsion phase. Given stronger stabilities of microemulsion, using it in engine

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can eliminate phase separation problem intrinsically associated with emulsion. Bio-oil

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solubilization capacities of different microemulsions were also studied. Fig. 3(b)

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displays that adding ethanol in microemulsion promoted the solubilization of bio-oil.

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The solubilized bio-oil amount increased with increasing ethanol proportion in polar 10


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phase. When 200° proof ethanol was used as polar phase, the solubilized bio-oil

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amount reached the maximum. There are many kinds of compounds contained in

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bio-oil such as alcohols, phenols, nitrogenous compounds and esters,12 and some of

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them were not miscible with water but miscible with ethanol. Ethanol is a commonly

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used solvent for blending with bio-oil.40 They could mix well and form a stable blend

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without adding additives.41 Therefore, the microemulsion with higher proofs of

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ethanol as polar phase could solubilize more bio-oil. Considering phase behavior and

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solubilization performance, 200° proof ethanol was a suitable polar phase. In later

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sections, ethanol all refers to 200° proof ethanol unless otherwise noted.

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3.1.4. Effect of polar phase amount

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Different ethanol volumes from 0.1 to 2.0 mL were added in 5 mL diesel to

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select an appropriate one for bio-oil solubilization. Fig. 3(c) displays that the

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microemulsions with ethanol as polar phase solubilized more bio-oil than M2

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(without adding polar phase). Polar bio-oil has difficulties in directly blending with

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diesel while could easily mix with polar solvent ethanol.42 Increasing ethanol dose

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enhanced the solubilization of bio-oil at certain volume range. Above 1.0 mL of

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ethanol, the solubilized bio-oil amount experienced reduction. Ethanol may compete

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with bio-oil in solubilization process and it might take priority of being dissolved in

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microemulsion compared with bio-oil.20 When ethanol accounted for a great

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percentage, the amphiphiles were mostly consumed to solubilize ethanol. The bio-oil

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solubilization was hindered as a result of the solubilization capacity saturation of 11


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amphiphiles. In terms of the structure of the interfacial layer, when ethanol

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concentration increased, the surfactant content at the interface became lower than that

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in diesel.43 The amount of effective amphiphiles adsorbing at the interface reduced.

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Meanwhile, molecular association between surfactant and co-surfactant was weaker.

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Thus, the solubilization capacity of microemulsion would decline.43 To ensure strong

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solubilization capacity, the ethanol volume of 1.0 mL was chosen.

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3.2. BHMBs characterization

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3.2.1. Elemental composition

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The elemental compositions of diesel, bio-oil and two BHMBs are tabulated in

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Table 1. Because of high nitrogen content, direct combustion of bio-oil may have a

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potential of emitting more NOx. The lower nitrogen content in BHMBs indicates that

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microemulsification of bio-oil with diesel could significantly dilute nitrogen in fuel

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system. Adding ethanol not only dilutes nitrogen further but also introduces OH

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radicals. OH radicals may reduce soot emissions by oxidizing soot precursors.44

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Compared with BDMB, BEMB might reduce PAH as ethanol dilute aromatic content

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in original diesel and facilitate complete combustion.33 Furthermore, additional

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oxygenates of BEMB may shorten ignition delay due to a more rapid and higher

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production of H2O2.45 Higher H/C and lower O/C of BHMBs both improve calorific

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value compared with bio-oil.

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3.2.2. Chemical composition

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The chemical composition of BHMBs was obtained through GC-MS analysis 12


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and tabulated in Table S1. Bio-oil mainly contained esters (especially methyl esters)

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and included some N/P-containing compounds, alcohols, phenols, acids and

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hydrocarbons (Fig. 4(a)). Esters were dominantly dissolved in microemulsion

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followed by alcohols/phenols (BEMB) or N/P-containing compounds (BDMB) (Fig.

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4(b)). Fig. 4(c) shows that esters were concentrated through microemulsification

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compared with crude bio-oil, while other compounds were diluted. N/P-containing

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compounds may impede the development of bio-oil as transport fuel because of

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potential NOx emissions. Interestingly, M1 had lower solubilization capacity for

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N/P-containing compounds than M2 (Fig. 4(c)). As seen in Table 1, the content of

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nitrogen in BEMB was lower than that in BDMB. Overall, ethanol in M1 could not

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only dilute nitrogen but also reduce the solubilization of N-containing compounds.

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These results demonstrate that adding ethanol in microemulsion was favorable for

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solubilizing and upgrading bio-oil.

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3.2.3. Thermochemical properties

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Thermogravimetry (TG) and differential thermogravimetry (DTG) curves of

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samples were plotted by thermogravimetric analysis and shown in Fig S1. Table 2

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shows that the solubilization of bio-oil had little influence on Tmax while reducing

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DTGmax of M1 and M2 after microemulsification. It indicates that bio-oil may retard

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degradation. The reduction of DTGmax was higher for M1 (0.11 mg/min) than that of

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M2 (0.05 mg/min). Ethanol in M1 may help solubilize more bio-oil and the

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solubilized bio-oil could significantly retard degradation of fuels. The lower 13


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degradation rate could reduce peak combustion temperature and abate NOX

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emissions.46 The activation energy (Ea) was appropriate to evaluate reactivity in the case of

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mixtures of renewable and petroleum fuel.47 It is closely related to the ignition delay.

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The lower the Ea, the shorter the ignition delay.19 It could be calculated according to

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the equation below:48

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 ART 2  2 RT ln [ − ln(1 − x)] = ln  1 − Ea  BEa 

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x=

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Where W0, Wt and W∞ are the original, instantaneous and final weights of the sample,

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respectively. The slope of the plot ln [-ln (1–x)] versus 1/T produces the Ea.

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  Ea  −   RT

(1)

W0 - Wt W0 - W∞

(2)

Table 2 displays that bio-oil had higher Ea than diesel. It means that bio-oil was

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more difficult to ignite due to prolonged ignition delay. Additional energy may be

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required to facilitate the ignition of bio-oil at the beginning of combustion.49

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Compared with diesel, M1 and M2 showed reduced Ea. After microemulsification

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with bio-oil, the Ea even remained lower than that of diesel. Therefore,

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microemulsification could effectively reduce the Ea of bio-oil and improve its

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combustion quality. The unique structure of microemulsion may facilitate ignition by

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exerting stimulative effects on cracking/combustion.50 Adding ethanol in

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microemulsion reduced the Ea of M1 compared with M2 (Table 2). Ethanol could

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ignite easily and thus cause rapid rise in temperature due to its high flammability.51 14


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The presence of ethanol in microemulsion could offset the increase of Ea caused by

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the solubilized bio-oil. As a result, BEMB had lower Ea than BDMB. It manifests that

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ethanol in M1 makes it superior to upgrade bio-oil concerning the ease of ignition.

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3.3. Solubilization mechanism

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In the present study, reverse micelle microemulsion (ethanol-in-diesel) was

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prepared with bio-oil entering into the internal and/or the external of microemulsion.

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Bio-oil contains hydrophilic, amphiphilic and hydrophobic components. The locations

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of these components in microemulsion depend on many factors such as their

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physicochemical properties, microemulsion composition and thermodynamic

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conditions of the interfaces.52 After microemulsification, there may be several ways

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for solubilizates from bio-oil to co-exist with microemulsion system. As shown in Fig.

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5(a), reverse micelle microemulsion can be divided into four regions including oil

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continuous phase (diesel), the interfacial layer consisting of palisade layer (surfactant

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tails) and inserted oil molecules, transition region and hydrophilic core.

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According to the composition of bio-oil, there might be three approaches for bio-oil components to be solubilized in microemulsion.

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I: hydrophilic bio-oil components like alcohols, phenols and acids might

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preferentially penetrate into hydrophilic core in terms of “likes dissolve likes”

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principle. Other bio-oil fractions such as hydrophobic hydrocarbons and amphiphilic

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esters had difficulty in entering into hydrophilic core, since they are less-hydrophilic.

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Ethanol as an organic solvent could dissolve many kinds of compounds, as a result, 15


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the above less-hydrophilic fractions may be solubilized in ethanol core of BEMB (Fig.

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5(b)). In this case, other than as polar phase, ethanol could also function as a solvent.

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As for BDMB, it may only entrap hydrophilic bio-oil components into hydrophilic

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core because of the absence of any solvent in core.

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II: hydrophobic bio-oil components such as hydrocarbons were likely to be

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solubilized in diesel (Fig. 5(c)). This could also be explained by “likes dissolve likes”

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principle. In this way, the solubilizates could easily blend with diesel without the

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expense of surfactant.

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III: amphiphilic bio-oil components such as methyl esters (MEs) had a complex

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solubilization pattern due to their unique properties. Here it was divided into three

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substrates as illustrated in Fig. 6 and interpreted as follows.

287

(a): It was reported that MEs could serve as surfactant.53 MEs of bio-oil could

288

capture free surfactant/co-surfactant molecules spreading at the air/oil surface or in

289

diesel to form new reverse micelles (Fig. 6(a)). Therefore, more solubilization media

290

were available to solubilize bio-oil. Ethanol is a hydrogen-bond donating solvent.

291

The hydrogen donors groups (– OH) in ethanol could strongly interact with the

292

hydrophilic and oxygenated bio-oil components, while with the aid of Van der Waals

293

forces, the nonpolar hydrocarbon chains have good affinity with the hydrophobic

294

bio-oil components and diesel. Consequently, the addition of ethanol in BEMB could

295

enhance the solubilization for bio-oil and make the microemulsion more stable.

296

(b): Amphiphilic bio-oil components may also interact with the interfacial layer 16


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Energy & Fuels

297

(Fig. 6(b)). MEs from bio-oil could co-adsorb with Span 80 at the interface as

298

surfactant. Medium-chain alcohols from bio-oil could insert themselves between

299

surfactant molecules acting as co-surfactant. Consequently, a mixed interfacial film

300

could formulate which was easy to bend.54 Part of esters such as ethyl oleate was

301

found located at the surfactant tails.55 Likewise, MEs could be entangled in the

302

palisade layer with hydrophobic chain intertwining with surfactant tails (Fig. 6(b)). In

303

this case, amphiphiles could serve as stabilizers and solubilizates simultaneously.

304

(c): Like surfactant molecules, linker molecules (including lipophilic and

305

hydrophilic linkers) also could self-assemble in microemulsion.56 The partition of

306

lipophilic linkers in palisade layer could enhance the solubilization of polar oil (fatty

307

acid esters). Long chain alcohols, acids, phenols and some of esters from bio-oil could

308

act as lipophilic linkers to facilitate the solubilization of bio-oil.59 Esters with a short

309

lipophilic moiety and carboxylic compounds contained in bio-oil may serve as

310

hydrophilic linkers and co-adsorb with surfactant at the interface.57, 58 The

311

combination of lipophilic and hydrophilic linkers would offer assembled-surfactant

312

and exert a synergism on solubilization performance (Fig. 6(c)).57

313

3.4. Fuel properties

314

Fuel properties of samples are displayed in Table 3. Density and viscosity are

315

closely related with spray characteristics of fuel. The higher the viscosity, the more

316

likely the fuel could induce engine durability problems.59 Bio-oil produced here was 17


Energy & Fuels

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

317

highly viscous (266.84 cSt). The viscosity value was reduced by two orders of

318

magnitude after microemulsification. BEMB had lower viscosity (4.17 cSt) than

319

BDMB (6.98 cSt). PP provides a reliable reference to the cold flow properties of fuel.

320

Bio-oil had poor cold flow properties (PP = 1 oC), and this limits its popularization in

321

cold areas. After being microemulsified with diesel, PP of bio-oil was reduced. As a

322

result, the cold flow properties of bio-oil were improved. Particularly, BEMB showed

323

even lower PP (-18 oC) than diesel (PP = -12 oC), while BDMB had higher PP (-11 oC)

324

than diesel.

325

More obvious improvements on viscosity and cold flow properties occurred in

326

BEMB. They could be ascribed to the addition of ethanol. Ethanol was commonly

327

considered as viscosity modifier to reduce the viscosity of viscous fuel.31 By

328

controlling ethanol addition, the viscosity could drop to an acceptable level. With

329

ethanol introduced in microemulsion, BEMB could greatly reduce the viscosity of

330

bio-oil. Due to the extremely low PP of ethanol (-117.3 oC),31 adding it into M1 could

331

be the main reason for which BEMB significantly reduced PP of bio-oil. Thus, the

332

usage of BEMB in engine was more temperature-robust for better cold flow properties.

333

Although the calorific value of ethanol was only 26.7 MJ/kg,32 it did not compromise

334

the calorific value of BEMB. It may be due to that the relatively low ethanol content

335

in microemulsion could not make great negative contributions to the total calorific

336

value. Moreover, as an oxygen-rich fuel, ethanol could promote combustion and help

337

release more energy contained in diesel. As a result, the effect of low calorific value 18


Page 18 of 35

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Energy & Fuels

338

of ethanol on combustion could be offset to some extent.

339

High carbon residue represents the potential of depositing in combustion

340

chamber and damaging engine.60 Bio-oil had a high carbon residue (6.70%). Both

341

BEMB and BDMB reduced carbon residue, while the former had a greater reduction.

342

Oxygen in ethanol could promote clean combustion33 thus further reducing carbon

343

residue of BEMB compared with BDMB. Therefore, BEMB displayed more

344

improvements on combustion quality and engine durability than BDMB. Dilution of

345

bio-oil with diesel cause an expected reduction of sulfur content and the resultant

346

bio-fuels met the standard set for diesel in China (GB 252-2011).

347

High acidity, sediment and phase separation of fuel would damage engine for the

348

formation of corrosion and deposit. Considering long-term application of BHMBs in

349

engine, the corrosivity and stability of fuel deserve attention. Through

350

microemulsification with diesel, the TAN of bio-oil reduced. The stability results

351

show that sediment occurred in BDMB and bio-oil, whereas neither sediment nor

352

phase separation was observed in BEMB. The mutual reactions such as

353

polymerization and esterification between bio-oil components could induce aging

354

and instability.16 Ethanol is often used to blend with bio-oil for higher stability,24

355

consequently, BEMB with ethanol added could mitigate this undesirable instability

356

phenomenon, unveiling its stronger compatibility with engine than BDMB. Coupling

357

these results, BEMB may have a long-term application in engine without severe

358

failure. 19


Energy & Fuels

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

359

360

4. Conclusions

Ethanol-in-diesel microemulsion was prepared here to solubilize and upgrade

361

bio-oil. Ethanol could not only favor the bio-oil solubilization but also circumvent its

362

disadvantages. With ethanol as polar phase, BEMB reduced the viscosity and PP of

363

bio-oil from 266.84 cSt and 1 oC to 4.17 cSt and -18 oC, respectively, and the carbon

364

residue and TAN of bio-oil also decreased. Moreover, the ignition quality of bio-oil

365

improved due to reduced Ea after adding ethanol. Ethanol could not only dissolve

366

more less-hydrophilic bio-oil components in polar core, but also solubilize more

367

hydrophilic bio-oil components through the hydrogen-bonding. The interaction

368

between ethanol-in-diesel microemulsion and bio-oil would offer high stability to the

369

system. Further and deeper investigations are warranted in application to engine.

370

Acknowledgments

371

The authors gratefully acknowledge the financial support provided by the

372

National Natural Science Foundation of China (No. 21276069, 71431006, 51521006);

373

the Specialized Research Fund for the Doctor Program of Higher Education, China

374

(No. 20120161130002); and the Key Project of Philosophy and Social Sciences

375

Research, Ministry of Education, P.R. China (No. 13JZD0016).

376

References

377 378 379

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(2) Xiu, S.; Shahbazi, A. Bio-oil production and upgrading research: A review. Renewable and Sustainable Energy Reviews 2012, 16, (7), 4406-4414. (3) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chemical Reviews 2006, 106, (9), 4044-4098. (4) Shao, J.; Yuan, X.; Leng, L.; Huang, H.; Jiang, L.; Wang, H.; Chen, X.; Zeng, G. The comparison of the migration and transformation behavior of heavy metals during pyrolysis and liquefaction of municipal sewage sludge, paper mill sludge, and slaughterhouse sludge. Bioresource technology 2015, 198, 16-22. (5) Xiao, Z.; Yuan, X.; Jiang, L.; Chen, X.; Li, H.; Zeng, G.; Leng, L.; Wang, H.; Huang, H. Energy recovery and secondary pollutant emission from the combustion of co-pelletized fuel from municipal sewage sludge and wood sawdust. Energy 2015, 91, 441-450. (6) Malins, K.; Kampars, V.; Brinks, J.; Neibolte, I.; Murnieks, R.; Kampare, R. Bio-oil from thermo-chemical hydro-liquefaction of wet sewage sludge. Bioresource technology 2015, 187, 23-9. (7) Yuan, X.; Leng, L.; Huang, H.; Chen, X.; Wang, H.; Xiao, Z.; Zhai, Y.; Chen, H.; Zeng, G. Speciation and environmental risk assessment of heavy metal in bio-oil from liquefaction/pyrolysis of sewage sludge. Chemosphere 2015, 120, 645-52. (8) Leng, L.; Yuan, X.; Shao, J.; Huang, H.; Wang, H.; Li, H.; Chen, X.; Zeng, G. Study on demetalization of sewage sludge by sequential extraction before liquefaction for the production of cleaner bio-oil and bio-char. Bioresource technology 2016, 200, 320-7. (9) Leng, L.; Yuan, X.; Huang, H.; Jiang, H.; Chen, X.; Zeng, G. The migration and transformation behavior of heavy metals during the liquefaction process of sewage sludge. Bioresource technology 2014, 167, 144-50. (10) Huang, H.-j.; Yuan, X.-z. Recent progress in the direct liquefaction of typical biomass. Progress in Energy and Combustion Science 2015, 49, 59-80. (11) Huang, H.-j.; Yuan, X.-z.; Li, B.-t.; Xiao, Y.-d.; Zeng, G.-m. Thermochemical liquefaction characteristics of sewage sludge in different organic solvents. Journal of Analytical and Applied Pyrolysis 2014, 109, 176-184. (12) Leng, L.; Yuan, X.; Chen, X.; Huang, H.; Wang, H.; Li, H.; Zhu, R.; Li, S.; Zeng, G. Characterization of liquefaction bio-oil from sewage sludge and its solubilization in diesel microemulsion. Energy 2015, 82, 218-228. (13) Huang, H.-j.; Yuan, X.-z.; Zhu, H.-n.; Li, H.; Liu, Y.; Wang, X.-l.; Zeng, G.-m. Comparative studies of thermochemical liquefaction characteristics of microalgae, lignocellulosic biomass and sewage sludge. Energy 2013, 56, 52-60. (14) Wang, Y.; Chen, G.; Li, Y.; Yan, B.; Pan, D. Experimental study of the bio-oil production from sewage sludge by supercritical conversion process. Waste management 2013, 33, (11), 2408-15. (15) Zhang, M.; Liaw, S. B.; Wu, H. Bioslurry as a Fuel. 5. Fuel Properties Evolution and Aging during Bioslurry Storage. Energy & Fuels 2013, 27, (12), 7560-7568. (16) Chen, D.; Zhou, J.; Zhang, Q.; Zhu, X. Evaluation methods and research 21


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(31) Li, D.-g.; Zhen, H.; Xingcai, L.; Wu-gao, Z.; Jian-guang, Y. Physico-chemical properties of ethanol–diesel blend fuel and its effect on performance and emissions of diesel engines. Renewable Energy 2005, 30, (6), 967-976. (32) Chang, Y.-C.; Lee, W.-J.; Lin, S.-L.; Wang, L.-C. Green energy: Water-containing acetone–butanol–ethanol diesel blends fueled in diesel engines. Applied Energy 2013, 109, 182-191. (33) Lee, W.-J.; Liu, Y.-C.; Mwangi, F. K.; Chen, W.-H.; Lin, S.-L.; Fukushima, Y.; Liao, C.-N.; Wang, L.-C. Assessment of energy performance and air pollutant emissions in a diesel engine generator fueled with water-containing ethanol–biodiesel–diesel blend of fuels. Energy 2011, 36, (9), 5591-5599. (34) Chiaramonti, D.; Bonini, M.; Fratini, E.; Tondi, G.; Gartner, K.; Bridgwater, A. V.; Grimm, H. P.; Soldaini, I.; Webster, A.; Baglioni, P. Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines—Part 1 : emulsion production. Biomass and Bioenergy 2003, 25, (1), 85-99. (35) Martin, J. A.; Mullen, C. A.; Boateng, A. A. Maximizing the Stability of Pyrolysis Oil/Diesel Fuel Emulsions. Energy & Fuels 2014, 28, (9), 5918-5929. (36) Wang, X.-l.; Yuan, X.-z.; Huang, H.-j.; Leng, L.-j.; Li, H.; Peng, X.; Wang, H.; Liu, Y.; Zeng, G.-m. Study on the solubilization capacity of bio-oil in diesel by microemulsion technology with Span80 as surfactant. Fuel Processing Technology 2014, 118, 141-147. (37) Zhong, H.; Yang, L.; Zeng, G.; Brusseau, M. L.; Wang, Y.; Li, Y.; Liu, Z.; Yuan, X.; Tan, F. Aggregate-based sub-CMC Solubilization of Hexadecane by Surfactants. RSC advances 2015, 5, (95), 78142-78149. (38) Chiappisi, L.; Noirez, L.; Gradzielski, M. A journey through the phase diagram of a pharmaceutically relevant microemulsion system. Journal of colloid and interface science 2016, 473, 52-59. (39) Wang, F.; Fang, B.; Zhang, Z.; Zhang, S.; Chen, Y. The effect of alkanol chain on the interfacial composition and thermodynamic properties of diesel oil microemulsion. Fuel 2008, 87, (12), 2517-2522. (40) No, S.-Y. Application of bio-oils from lignocellulosic biomass to transportation, heat and power generation—A review. Renewable and Sustainable Energy Reviews 2014, 40, 1108-1125. (41) Nguyen, D.; Honnery, D. Combustion of bio-oil ethanol blends at elevated pressure. Fuel 2008, 87, (2), 232-243. (42) Demirbas, A. Competitive liquid biofuels from biomass. Applied Energy 2011, 88, (1), 17-28. (43) Bardhan, S.; Kundu, K.; Saha, S. K.; Paul, B. K. Effects of water content and oil on physicochemical and microenvironmental properties of mixed surfactant microemulsions. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2014, 450, 130-140. (44) Mathis, U.; Mohr, M.; Kaegi, R.; Bertola, A.; Boulouchos, K. Influence of Diesel Engine Combustion Parameters on Primary Soot Particle Diameter. Environmental Science and Technology 2005, 39, (6), 1887-1892. 23


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509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551

(45) Hoon Song, K.; Nag, P.; Litzinger, T. A.; Haworth, D. C. Effects of oxygenated additives on aromatic species in fuel-rich, premixed ethane combustion: a modeling study. Combustion and Flame 2003, 135, (3), 341-349. (46) Husnawan, M.; Masjuki, H. H.; Mahlia, T. M. I.; Saifullah, M. G. Thermal analysis of cylinder head carbon deposits from single cylinder diesel engine fueled by palm oil–diesel fuel emulsions. Applied Energy 2009, 86, (10), 2107-2113. (47) Conconi, C. C.; Crnkovic, P. M. Thermal behavior of renewable diesel from sugar cane, biodiesel, fossil diesel and their blends. Fuel Processing Technology 2013, 114, 6-11. (48) Wan Nik, W. B.; Ani, F. N.; Masjuki, H. H. Thermal stability evaluation of palm oil as energy transport media. Energy Conversion and Management 2005, 46, (13-14), 2198-2215. (49) Crnkovic, P. M.; Koch, C.; Ávila, I.; Mortari, D. A.; Cordoba, A. M.; Moreira dos Santos, A. Determination of the activation energies of beef tallow and crude glycerin combustion using thermogravimetry. Biomass and Bioenergy 2012, 44, 8-16. (50) Kok, M. V. Clay concentration and heating rate effect on crude oil combustion by thermogravimetry. Fuel Processing Technology 2012, 96, 134-139. (51) Kumar, S.; Cho, J. H.; Park, J.; Moon, I. Advances in diesel–alcohol blends and their effects on the performance and emissions of diesel engines. Renewable and Sustainable Energy Reviews 2013, 22, 46-72. (52) Bera, A.; Ojha, K.; Kumar, T.; Mandal, A. Water solubilization capacity, interfacial compositions and thermodynamic parameters of anionic and cationic microemulsions. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2012, 404, 70-77. (53) Fernando, S.; Hanna, M. Development of a novel biofuel blend using ethanol-biodiesel-diesel microemulsions: EB-diesel. Energy & Fuels 2004, 18, (6), 1695-1703. (54) Barth, A.; Prevost, S.; Popig, J.; Dzionara, M.; Hedicke, G.; Gradzielski, M. Solubilisation of different medium chain esters in zwitterionic surfactant solutions--effects on phase behaviour and structure. Journal of colloid and interface science 2011, 364, (1), 148-56. (55) Kaur, G.; Chiappisi, L.; Prevost, S.; Schweins, R.; Gradzielski, M.; Mehta, S. K. Probing the microstructure of nonionic microemulsions with ethyl oleate by viscosity, ROESY, DLS, SANS, and cyclic voltammetry. Langmuir : the ACS journal of surfaces and colloids 2012, 28, (29), 10640-52. (56) Acosta, E. J.; Harwell, J. H.; Sabatini, D. A. Self-assembly in linker-modified microemulsions. Journal of colloid and interface science 2004, 274, (2), 652-64. (57) Acosta, E.; Tran, S.; Uchiyama, H.; Sabatini, D. A.; Harwell, J. H. Formulating chlorinated hydrocarbon microemulsions using linker molecules. Environmental science & technology 2002, 36, (21), 4618-4624. (58) Acosta, E.; Uchiyama, H.; Sabatini, D. a.; Harwell, J. H. The role of hydrophilic linkers. Journal of Surfactants and Detergents 2002, 5, (2), 151-157. (59) Kibbey, T. C. G.; Chen, L.; Do, L. D.; Sabatini, D. A. Predicting the 24


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temperature-dependent viscosity of vegetable oil/diesel reverse microemulsion fuels. Fuel 2014, 116, 432-437. (60) Lin, Y.-S.; Lin, H.-P. Study on the spray characteristics of methyl esters from waste cooking oil at elevated temperature. Renewable Energy 2010, 35, (9), 1900-1907.

25


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Page 26 of 35

Table 1. Elemental analysis Elemental composition (wt. %)

Diesel

BEMB

BDMB

Bio-oil

C (wt.%)

77.92

69.69

73.79

65.14

H (wt.%)

11.66

9.77

10.39

8.79

O (wt.%)

10.02

19.14

14.33

19.17

N (wt.%)

0.40

1.40

1.49

6.90

H/C

1.80

1.68

1.69

1.62

O/C

0.10

0.21

0.15

0.22

26


Page 27 of 35

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

Energy & Fuels

Table 2. The decomposition characteristics and activation energy of samples Diesel

M1

M2

BEMB

BDMB

Bio-oil

Span 80

DTGmax (mg/min)

0.58

0.37

0.36

0.26

0.31

0.44

0.42

Tmax (oC)

201

178

183

177

181

229

388

Activation energy (kJ/mol)

42.64

36.43

38.30

36.65

38.66

47.48

45.52

Tmax: the temperature where DTGmax occurs.

27


a

0.9070 6.98

Density (g/cm3, 25 oC) 1.1674 Viscosity (cSt, 40 oC)

28


6.70

0.1601

0.155

Sediment

Carbon residue (wt.%)

Sulfur content (wt.%)

TAN (mg KOH/g)

Stability

Sediment

0.084

0.0255

The maximum mass percentage set by GB 252-2011 (China).

(Obtained in 90 days)

-11

1

Pour point (oC)

1.04

42.97

Calorific value (MJ/kg) 37.00

266.84

BDMB

Bio-oil

Type of fuel

nor separation

Neither sediment

0.122

0.0242

0.56

-18

41.50

4.17

0.8885

BEMB

-

0.059

< 0.035 a

< 0.3 a

-12

45.73

3.13

0.8515

Diesel

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

Table 3. Fuel properties of samples

Energy & Fuels Page 28 of 35

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Energy & Fuels

558

Graphical abstract

559 560 561

562 563 564 565 566 567

29


Energy & Fuels

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

568 569 570

Fig. 1. Effect of surfactant concentration on bio-oil solubilization capacity of

571

microemulsion. N-octanol acted as co-surfactant and C/S ratio was 0.9. The volume of

572

200° proof ethanol was 1.0 mL.

30


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Energy & Fuels

573 574 575

Fig. 2. Effect of co-surfactant type and C/S ratio on bio-oil solubilizaion capacity of

576

microemulsion. The C/S ratio (w/w) ranged from 0 to 1.5. The concentration of Span

577

80 and volume of 200° proof ethanol were fixed at 0.30 M and 1.0 mL, respectively.

31


Energy & Fuels

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

578 579 580

Fig. 3. Effect of polar phase: (a) Effect of polar phase type on phase behavior of

581

bio-oil hybrid bio-fuel; (Each polar phase volume was 1.0 mL to better clarify the

582

effect of ethanol.) (b) Effect of polar phase type on bio-oil solubilization capacity of

583

microemulsion; (Each polar phase volume was 0.1 mL because excess water could not

584

be dissolved completely thus cannot get microemulsion.) (c) Effect of ethanol volume

585

on bio-oil solubilization capacity of microemulsion. The concentration of Span 80 and

586

C/S ratio were 0.30 M and 0.9, respectively. N-octanol was co-surfactant.

32


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Page 33 of 35

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Energy & Fuels

587 588

Fig. 4. Distribution characterization: (a) Distribution of compounds in bio-oil; (b)

589

Distribution of bio-oil compounds in total solubilized bio-oil after

590

microemulsification; (The percentage was got through dividing the peak area of each

591

kind of bio-oil compound by total peak area of solubilized bio-oil in BEMB/BDMB.)

592

(c) Distribution change of bio-oil compounds between bio-oil and BEMB/BDMB

593

after microemulsification. (The ratio of each bio-oil compound proportion in total

594

solubilized bio-oil dissolved in BEMB/BDMB to that of the same compound in

595

liquefaction bio-oil represents distribution change. e.g. DC (A) represents the

596

distribution change of compound A, Percentage A in (a/b) represents the percentage

597

value of A in Fig. 4(a/b). DC (A) =

Percentage A in (b) × 100% ) Percentage A in (a) 33


Energy & Fuels

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598 599 600

Fig. 5. Reverse micelle structure and solubilization of hydrophilic and hydrophobic

601

components: (a) The structure of reverse micelle microemulsion; (b) Hydrophilic

602

components of bio-oil were solubilized in hydrophilic core with some solubilization

603

of hydrophobic and amphiphilic components; (c) Hydrophobic components of bio-oil

604

were solubilized in hydrophobic oil phase.

605 606 607 608

34


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Energy & Fuels

609 610 611

Fig. 6. Solubilization of amphiphilic components: (a) Amphiphilic components of

612

bio-oil captured free surfactant molecules to form new reverse micelles; (b)

613

Interaction of amphiphilic components of bio-oil with the established reverse micelles;

614

(c) Self-assembly of linker molecules derived from bio-oil.

35


Upgrading Sewage Sludge Liquefaction Bio-Oil by Microemulsification

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