Comparison of fuel properties of nano-emulsions of diesel fuel


2 days ago - Solketal chemically derived from bio-glycerol was shown in this study to improve the fuel characteristics of ultra-low sulfur diesel...
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Comparison of fuel properties of nano-emulsions of diesel fuel dispersed with solketal by microwave irradiation and mechanical homogenization methods Cherng-Yuan Lin, and Shih-Ming Tsai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01466 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Energy & Fuels

Comparison of fuel properties of nano-emulsions of diesel fuel dispersed with solketal by microwave irradiation and mechanical homogenization methods Cherng-Yuan Lin, Shih-Ming Tsai Department of Marine Engineering, National Taiwan Ocean University, Keelung 202, Taiwan, ROC

ABSTRACT Solketal chemically derived from bio-glycerol was shown in this study to improve the fuel characteristics of ultra-low sulfur diesel. Either microwave irradiation or mechanical homogenization method was applied to prepare the nano- or micro-emulsions of solketal-in-ultra-low sulfur diesel. A nonionic surfactant mixture was used to reduce the surface tension of the interphase among the components to facilitate emulsion formation. The fuel properties of those emulsions were analyzed and compared. The experimental results show that a weight fraction of solketal lower than 5 wt. % along with a surfactant mixture amounting to 15 wt. % would produce a nano-emulsion that contains a mean droplet size of the dispersed phase in the range of nanometers. The nano-emulsion with 3 wt. % solketal as a combustion improver for diesel fuel, prepared by a microwave-irradiating reactor, appeared to have the highest amount of heat release, combustion efficiency, and the lowest carbon residue due to the increase in the combustion efficiency and was thus considered to be the optimum composition for acquisition of superior fuel properties. In addition, the nano- and micro-emulsions formed by the mechanical homogenizer had a lower cold filter plugging point and a higher flash point. Higher solketal content in the emulsions caused an increase in the dispersed droplet size and reductions in the amount of heat release, carbon residue, the flash point, and the cold filter plugging point. KEYWORDS: Fuel property; nano-emulsion; glycerol acetonide; microwave irradiation; combustion efficiency.

1. Introduction Glycerol is the major by-product of industrial transesterification reaction for Corresponding

author: Professor; tel/fax: +886 2 2462 2307.

E-mail address: [email protected] (C.-Y. Lin). 1 ACS Paragon Plus Environment

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biodiesel production. With the rapid global development of biodiesel, as much as 3 million tons of crude glycerol is produced each year.1 Proper treatment with the use of physical purification or chemical conversion approaches would promote the economic value of such an abundance of crude glycerol. For example, a few fuel additives or industrial chemicals are produced via chemical conversion from bio-glycerol.2 Solketal, also known as glycerol acetonide, is one of the significant chemical derivatives from bio-glycerol. Solketal has been used as an engine fuel additive to improve the fluidity of fuel feeding systems, particularly in frigid zones or during cold weather.3 The oxygen content of glycerol acetonide, C6H12O3, is as high as 36.4 wt.%, so glycerol acetonide might also be used as an adequate oxygenate such as ETBE (ethyl tert-butyl ether) for fossil fuel. Nobre et al.4 evaluated the antioxidant activity for retarding the oxidative damage of solketal and other chemical derivatives from glycerol. Mota et al.,5 after mixing solketal with gasoline and ethanol, observed that gasoline’s distillation curves were not altered significantly. In addition, the formation and accumulation of wax colloids in gasoline were reduced with the addition of solketal. However, solketal has not yet been used as a combustion improver in petroleum-derived fuel. As solketal and diesel fuel are hydrophilic and hydrophobic liquids, respectively, they are immiscible. Emulsifying methods were applied to form emulsions in which many dispersed solketal droplets were distributed within the continuous phase of liquid diesel fuel. An adequate surfactant mixture was generally used to reduce the surface tension between those two phases.6 A micro-explosion might occur during the burning process of the diesel emulsion, leading in turn to an increase in the surface-to-volume (S/V) ratio of the atomized spray, the burning rate, and the combustion efficiency of the emulsion fuel.7 Microwave irradiation could incur violent rotation of polar compounds to facilitate the formation of emulsions.8 Microwave irradiation or a mechanical homogenization approach was used in this study to increase the contact frequency and surface areas among the various components in an emulsion, resulting in a reduction in the droplet sizes of the dispersed phase and enhancement of emulsification stability. The fuel properties of nano-emulsions would be influenced accordingly by the emulsion structure, particularly the droplet size and the distribution of the dispersed phase. The fuel properties, such as the heating value, the cold filtering plugging point (CFPP), and the carbon residue of the nano-emulsions of dispersed solketal droplets in the continuous diesel phase, have not yet been studied. Moreover, the effects of the solketal content and emulsification methods on the physical structure of emulsions produced by microwave irradiation were first investigated in this study. The potential of 2 ACS Paragon Plus Environment

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solketal was also evaluated in applications for improvement of the fuel properties of nano-emulsion of diesel fuel.

2. Experimental details Mechanical homogenization or microwave irradiation method was applied to produce diesel emulsions distributed with dispersed droplets of solketal. The fuel properties of those micro- or nano-emulsions were then analyzed. The details of the experiment are explained below. 2.1 Emulsion materials Glycerol acetonide, also known as solketal, is a commercial product (serial number SL-191) of Rhodia Inc. 9 The chemical purity of solketal (C6H12O3) is 99.8 wt.%. The chemical structure of solketal is illustrated in Fig. 1. Solketal is immiscible with liquid diesel fuel but soluble in water. Its melting point and boiling point are −26°C and 191°C.9 The kinematic viscosity and specific gravity of solketal measured at 40 °C were 5.32 mm2/s and 1.058, respectively. Solketal was dispersed into nano- or micrometer sized droplets for distribution within the continuous diesel phase. Solketal is a colorless and transparent liquid. Ultra-low sulfur diesel (ULSD) with a sulfur content of less than 10 ppm, provided by CPC Corporation in Taiwan, was used in the continuous phase of those emulsions. Its kinematic viscosity and specific gravity measured at 40 °C were 3.62 mm2/s and 0.825, respectively. Diesel fuel like ULSD which are distillates from crude petroleum oil is mainly composed of many various hydrocarbon compounds, but also contains metallic components, ash, residual and other impurities. Hence, the materials of ULSD are difficult to be well-defined and vary from batch to batch. A surrogate or even a few surrogate compounds for ULSD is generally not able to represent the practical compositions and fuel properties of ULSD. The representative fuel properties of the emulsion components, including ULSD and solketal, are thus shown in Table 1 for reference when the properties of those emulsions prepared with ULSD are discussed. A nonionic surfactant mixture of Tween 80 and Span 80, with hydrophile-lipophile balance (HLB) values of 15.0 and 4.3, respectively,10 was used for the preparation of diesel emulsions by virtue of their reduction capability for surface tension among various phases in the emulsion. The surfactants were supplied by First Chemical Company in Taiwan. The typical properties of surfactants Tween 80 and Span 80 are shown in Table 2.10 The heating values of Span 80 and Tween 80 are 35.23 MJ/kg and 29.77 MJ/kg, respectively. By adjusting the weight proportion of those two surfactants, the combined 3 ACS Paragon Plus Environment

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HLB value of the mixture was set to 10 for each emulsion in this study. 2.2 Formation of emulsions by various methods The nano-emulsions of diesel fuel were prepared either by microwave irradiation or with a mechanical homogenization machine. Because microwaves are a kind of high-frequency magnetic wave, only adequate input work could render formation of emulsions. The input work for preparing the emulsions, which was adjusted by the operating time, remained the same for the emulsifying machines so that the fuel properties of those emulsions could be compared. The combined HLB of the surfactant mixture of Span 80 and Tween 80 was set to 10. The weight proportion of the surfactant mixture in the emulsion was controlled at 15 wt. %. The quantity of solketal, which acted as the dispersed phase in the emulsion, ranged from 1 to 9 wt. %. The emulsions were then prepared either by mechanical homogenization or microwave-assisted methods, as described below. For preparation of emulsions by a mechanical homogenizer, a magnetic stirrer (SP47235-60 model, Barnstead/Thermolyne Inc., USA) was first used at a speed of 700 rpm to stir ULSD and the surfactant mixture in a beaker for 5 min. A peristaltic pump (Masterflex L/S model, Cole-Palmer Inc., USA) was then used to supply solketal to the beaker. The emulsion mixture was then stirred by a mechanical homogenizer (T50 model, IKA Inc., Germany) at 3000 rpm for 5 min to complete the emulsion preparation. When using a microwave-irradiating reactor to prepare the emulsions, the processes for preparing the emulsion mixture composed of ULSD, the surfactant mixture, and solketal were the same as those used in the previous method of mechanical homogenization. However, instead of a mechanical homogenizer, a microwave reactor (Ym3101cb model, Teco Inc., Taiwan) was used to irradiate the emulsion mixture at an operating power of 0.1 kW for 30 s to constitute the formation of the diesel emulsion.

2.3 Analysis of fuel properties of emulsions The flash point is one of significant indicators of a liquid fuel’s safety extent, particularly during its storage and transportation. The flash point of a liquid fuel is the lowest temperature at which fuel vapor can ignite in air in presence of an ignition source. A Pensky-Marten closed-cup flash point tester (D93 model, Shin Kwang Inc., Taiwan) was used to measure the flash points of the emulsions in this study. A closed-bomb oxygen calorimeter (6772 model, Parr Inc., USA) was used to measure the amount of heat release from complete combustion of the diesel emulsion fuel. The combustion 4 ACS Paragon Plus Environment

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product including carbon dioxide and water was cooled to the initial temperature of the reaction. The water vapor formed from the combustion was thus condensed to the liquid state. This implies that high heating value of the test fuel was recorded by the calorimeter.11 Greater heat release indicates lower consumption of the test fuel required to attain the same power output and is thus favorable. The carbon residue referred to is the weight fraction of the carbon residue left within the crucible of the closed-bomb oxygen calorimeter after the burning process to the initial weight of the emulsion sample. The CFPP is the temperature at which sufficient wax crystals precipitate from the liquid fuel and agglomerate to clog a specific fuel filter for CFPP measurement. A liquid fuel with a lower CFPP implies superior fluidity and is thus preferable, especially in cold-weather regions. An automatic CFPP tester (B10/-40 model, Firstek Inc., Taiwan) was used to measure the CFPP of the emulsions according to the ASTM D6371-17 standard test method.12 The mean droplet size of the emulsions of solketal-dispersed-in ULSD was analyzed by a particle size analyzer (Mastersizer 2000 model, Malvern Panalytical Ltd., UK) together with a Zetasizer (Nano ZS model, Malvern Panalytical Ltd., UK). Emulsion fuel is atomized into many tiny droplets by a fuel nozzle embedded inside a combustion chamber. The burning of such atomized emulsion droplets is subjected to secondary droplet disruption after the droplets are vaporized and explode outwards after they receive sufficient combustion heat. This process is referred to as secondary emulsion-atomization or the micro-explosion phenomenon.8 More heat may be released through the micro-explosion process, resulting in greater combustion efficiency and reduced emission of particulate matter (PM). A higher extent of complete combustion, higher combustion efficiency, and in turn more heat release resulted due to the occurrence of a micro-explosion phenomenon during the burning of an emulsion fuel. The emulsification characteristics such as emulsion stability and micro-explosion might affect the actual amount of heat release from the fuel burning.13-14 The combustion efficiency (CE) is considered in this study to identify the effects of occurrence of micro-explosion, emulsion stability, and other relevant emulsification characteristics on the actual heat release from burning of the nano- or micro-emulsions of solketal-in-ultra-low sulfur diesel. The combustion efficiency (CE) is defined to measure the actual amount of heat release from burning of the emulsion fuels prepared by different emulsification methods corresponding to the combined higher heating value of the emulsion components. CE is formulated as the ratio of the actual amount of heat release through the emulsion burning by experimental measurement to the combined amount of higher heat release of the emulsion components. 5 ACS Paragon Plus Environment

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CE can be expressed as below: CE = actual amount of gross heat release from the emulsion burning / total amount of higher heat release of emulsion components in which the denominator is calculated based on the corresponding contribution of the amount of higher heat release of each component, including ULSD, surfactant mixture, and solketal, on a mass basis in the emulsion.

3. Results and discussion The nano-emulsions were formed with the use of an emulsifying method of microwave irradiation or mechanical homogenization. The fuel properties of those emulsions of solketal-in-diesel were compared. A nonionic surfactant mixture was used to assist the formation of nano-emulsions. The input energy of those two emulsifying machines was set the same for preparation of emulsions with various weight proportions of solketal, ULSD, and the surfactant mixture. The mean values of the data of fuel properties were recorded by at least three experimental repetitions. The experimental uncertainties of the amount of heat release, carbon residue, flash point, and CFPP of the emulsions prepared by the microwave reactor were ±0.87%, ±16.70%, ±0.68%, and ± 2.89%, respectively. The corresponding uncertainties of the emulsions formed by the mechanical homogenizer were ±0.71%, ±13.80%, ±1.09%, and ±4.35%, respectively. The representative fuel properties of the components including the ULSD and solketal in the micro- or nano-emulsions are given in Table 1. The amount of heat release of the ULSD was 46.26 MJ/kg, which is about 3.09 times of that of solketal (14.99 MJ/kg). In contrast, the carbon residue of the ULSD was only 27% of the latter component. A larger carbon residue from burning liquid fuel might cause a more serious carbon deposit inside the engine combustor. The flash point of solketal was shown to be higher than that of the ULSD, which indicates solketal is safer during storage and transportation periods. The temperature at which the kinematic viscosity and specific gravity were measured was 40 °C. The ULSD is equivalent to ASTM No. 2D diesel fuel. The limits of kinematic viscosity range between 1.9 and 4.1 mm2/s at 40 °C based on ASTM D975 specification. Hence, the kinematic viscosity of the ULSD is reasonable and lies within the specification limits of ASTM D975. In addition, solketal is a pure compound, and colorless and transparent liquid. The fluidity extent of a liquid decreases with the decrease of its operating temperature. When the environment was decreased to -4 °C, the solketal liquid was not able to flow through but plugged the fuel filter so this was recorded as the CFPP for solketal in this study. 6 ACS Paragon Plus Environment

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3.1 Amount of heat release Fuel with a higher heating value per unit mass would release more thermal energy by conversion of the fuel’s chemical energy. The effects of the solketal content and the emulsifying methods on the amount of heat release of the diesel emulsions are shown in Fig. 2. The emulsions that contained solketal less than 5 wt. % prepared by the microwave reactor were found to have higher amounts of heat release than those prepared by the mechanical homogenizer, probably due to the occurrence of a larger extent of micro-explosion from larger droplets in the former emulsions. A micro-explosion of emulsion frequently arises from volumetric expansion through rapid vaporization of dispersed solketal droplets after they absorb sufficient combustion energy for latent heat. The atomized liquid spray would be further broken up to enhance burning efficiency through micro-explosion process, leading to an increase in heat release from burning of the emulsion fuel.15 The diesel emulsion containing 3 wt. % solketal prepared by the microwave-irradiating reactor was found to have the highest amount of heat release in Fig. 2, primarily because of its relatively larger number of dispersed droplets and wider distribution of the solketal droplets within the continuous diesel phase among those emulsions. The amount of heat release of the emulsions containing solketal greater than 3 wt. % decreased significantly with the increase in the solketal content in that the heating value of solketal (14.99 MJ/kg) is only about 32% of that of diesel fuel (46.26 MJ/kg). Moreover, as the solketal content in the emulsions exceeded 5 wt. %, the mean droplet size of the dispersed phase increased to approach the micrometer (µm) range. As the solketal content in the emulsions approached 7 wt. %, the curve trend of the amount of heat release with the weight of solketal appeared to reverse with that of the nano-emulsions. The micro-emulsions that contained micrometer-sized droplets had faster separation and precipitation rates from the emulsification layers. Because solketal had a higher specific gravity (1.058 at 40 °C) and a lower heating value (14.99 MJ/kg) than those of diesel fuel, a lower amount of heat release was found for the diesel emulsion with the highest weight proportion of solketal, as shown in Fig. 2. 3.2 Carbon residue Carbon residue is defined as the weight percentage of the unburnt carbon residue that remains after the combustion process to the initial weight of the liquid fuel before being burnt.16 The thermal conductivity coefficient of the unburnt carbon residue is comparatively lower than the metallic parts of the interior engine cylinder. The excessive accumulation of unburnt carbon residue would therefore result in poor heat transfer from 7 ACS Paragon Plus Environment

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the engine cylinder to the surrounding area and in turn establish a high temperature at the deposit of the unburnt carbon residue. Abnormal engine knocking, intense fluctuation in burning, and abrupt engine vibration may be induced afterwards, 17 which implies that a lower amount of carbon residue produced from fuel burning is favorable for practical engine operation. The effects of the solketal content on the formation of carbon residue from burning emulsions prepared by various emulsification methods are shown in Fig. 3. Solketal and the surfactants Span 80 and Tween 80 appeared to have higher carbon residue than ULSD as shown in Tables 1 and 2, leading to somewhat high carbon residue after burning the emulsions. In addition, the data in Fig. 3 seemed a little scattered, which indicated the carbon residues left after the burning of the emulsions were not correlated well with the emulsification methods. However, the carbon residue was still influenced by the weight fraction of solketal in the nano- or micro-emulsions. For example, the diesel emulsion that contained 3 wt. % solketal by the microwave-assisted emulsifying method was observed to have the lowest carbon residue among those in Fig. 3. In contrast to Fig. 2, the same emulsion (with 3 wt. % solketal) prepared by the same emulsification method (microwave-assisted) also had the greatest heat release from burning the emulsion fuel. Hence, the addition of an adequate proportion (3 wt. %) of solketal to diesel fuel as a combustion improver could effectively enhance the combustion extent of the emulsion fuel, resulting in the greatest amount of heat release in Fig. 2, together with the lowest carbon residue in Fig. 3. In comparison with the above emulsions with 3 wt. % solketal, the diesel fuel emulsions with 9 wt. % solketal appeared to have the highest carbon residue in Fig. 3, along with the lowest amount of heat release in Fig. 2 because the dispersed solketal droplets with mean micrometer (µm) sizes rather than nanometer (nm) sizes were formed for the latter emulsions (with 9 wt. % solketal). Inferior oxidation stability, a lower extent of complete combustion, and a much lower amount of heat release were thus produced for the micro-emulsions that contained solketal greater than 5 wt. %. In consequence, the emulsion with 9 wt. % solketal was shown to have the worst combustion quality and thus the lowest amount of heat release and the highest carbon residues in Figs. 2 and 3, respectively. 3.3 Combustion efficiency (CE) During the burning process of diesel fuel emulsions, the micro-explosion phenomenon, in which the dispersed droplets are vaporized and expand outwards after they absorb sufficient thermal energy from high-temperature surrounding, might occur.18 The atomized liquid fuel droplets would be further broken into even smaller droplets, 8 ACS Paragon Plus Environment

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leading to enhancement of the combustion extent of the emulsion fuel and in turn increase of the formation of water product. The water vapor is condensed to its liquid state in the closed-bomb oxygen calorimeter (6772 model, Parr Inc., USA) during the measurement of the amount of heat release.11 Hence, larger amount of gross heat release from burning such emulsion fuel was observed. The definition of the combustion efficiency (CE) of an emulsion fuel is proposed in this study to consider the effect of the emulsification characteristics on the amount of heat release from the burning of such an emulsion fuel. The CE is defined as the ratio of actual amount of heat release of an emulsion fuel (by experimental measurement) to the cumulative summation of the respective higher heating values of its components, such as diesel fuel, solketal, and surfactant, multiplied by their corresponding weight percentages (by direct calculation). An emulsion with a CE value larger than 1 might be attributed to its superior emulsification characteristics such as the occurrence of micro-explosion, more formation of water product from combustion, and in turn larger contribution of latent heat of water vaporization to the actual amount of heat release in the numerator of CE formula. An emulsion with a larger CE value is thus favorable as fuel of a combustor. The droplet size of the dispersed solketal phase less than 5 wt. % in the emulsions was found to be within the nanometer (nm) range and had a CE value greater than 1, as shown in Fig. 4. Moreover, the emulsion with 3 wt. % solketal prepared with the microwave-assisted emulsifying method was shown to have the highest CE value (1.05) among the emulsions in Fig. 4. This implied that 5% more latent heat of water product was released from burning such a nano-emulsion probably due to the chemical composition of the emulsion fuel and occurrence of micro-explosions of the dispersed droplets, which enhanced the water formation in the combustion product. Hence, the quantity of burned diesel fuel could be reduced by 5 wt. %. It is assumed that a medium-sized power plant burns 3000 metric tons of ULSD per month and equivalently costs about 3 million USD every month if the prices of diesel fuel and solketal are 1000 USD/ton and 3000 USD/ton, respectively. Since 5 % more heat is released if the nano-emulsion which contains 97 wt. % ULSD together with 3 wt.% solketal is used as the alternative fuel to the neat ULSD, only 95% the original ULSD quantity is required for the nano-emulsion to attain the same amount of heat release. It will therefore cost about 3.02 million USD per month for the fuel expenditure of the nano-emulsion due to somewhat higher cost of solketal than ULSD. This means the fuel cost of the nano-emulsion was slightly higher than that of the neat ULSD case by 0.6 %. Moreover, the emulsions that contained solketal less than 5 wt. % were found to produce dispersed droplets which average size was within the nanometer range by either 9 ACS Paragon Plus Environment

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the emulsifying method. The nano-emulsions prepared with the microwave-irradiation method appeared to have greater heat release, as shown in Fig. 2, together with higher CE values, as shown in Fig. 4, than those prepared with the mechanical homogenization method. In contrast, the emulsions that contained solketal greater than 5 wt. % were prone to producing micro-emulsions in which most dispersed droplets were micrometer (µm)-sized. The higher separation rate of dispersed droplets from their emulsification layer were observed for the micro-emulsions (with solketal greater than 5 wt. %), particularly for those prepared with the microwave-irradiation method. It followed that more incomplete burning, a lower amount of heat release, and a lower CE value were found for those micro-emulsions than for those prepared with mechanical homogenization in Figs. 2 and 4. 3.4 Flash point The flash point of a liquid fuel is defined as the lowest temperature at which flame is produced but is soon extinguished after the fuel is vaporized and mixes with the surrounding air to form a combustible mixture.19 Hence, a higher flash point generally indicates greater safety in handling and storage of a liquid fuel, which is thus more favorable. The effects of the weight percentage of solketal and emulsification methods on the flash points of the micro- and nano- emulsions are shown in Fig. 5. The flash point was observed to be reduced significantly with an increase in the solketal content because the size of the dispersed droplet increases with the increase in the emulsion’s solketal content. In addition, a larger droplet size caused a faster separation rate from the emulsification layer20 to precipitate down to the emulsion bottom because solketal has a higher density than diesel fuel. As a consequence, a higher fraction of diesel fuel, whose flash point (55°C) is comparatively lower than that of solketal (91°C), accumulated at the upper layer of the emulsion, leading to a lower flash point of the emulsion fuel. Hence, the increase in the weight fraction of solketal in the emulsion lowered its flash point in Fig. 5. In addition, the flash points of the emulsions prepared with the mechanical homogenizer were significantly higher than those prepared with the microwave-assisted reactor, as shown in Fig. 5, because of the smaller mean droplet size and more uniform distribution of the dispersed solketal phase within the continuous phase of diesel fuel prepared with the former emulsification method. Greater emulsification stability, and thus higher flash points in Fig. 5 of the emulsions prepared by the mechanical homogenizer, was therefore observed. 10 ACS Paragon Plus Environment

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3.5 CFPP The CFPP is one of representative indicators for the fluidity of a liquid fuel. CFPP is defined as the temperature below which sufficient wax crystals are precipitated away from liquid fuel to plug the fuel filter. Therefore, a lower CFPP is preferable, particularly during cold weather or in frigid regions. Figure 6 revealed the influences of the solketal content and emulsification methods on the CFPP of the nano- or micro- emulsions. The weight percentage of the surfactant was kept constant (15 wt. %) for all emulsions in this study. The hydrophilic side of the surfactant acted to suppress the growth of the critical size of emulsion micelles. As a consequence, the CFPP of the emulsion fuel was influenced by the type and content of the surfactant used to facilitate the formation of the emulsion.21 The emulsions produced with the mechanical homogenizer were shown to have lower CFPPs than those with the microwave-assisted reactor because the dispersed solketal droplets formed by the former method were significantly smaller and therefore more apt to move through the tested fuel filter, resulting in a lower CFPP. In addition, the increase in the solketal content was found to significantly reduce the CFPP of the emulsions because the emulsification stability decreased as the solketal content in the emulsion increased. A higher separation rate of the dispersed solketal droplets thus occurred so that a higher solketal content, for which the specific gravity (1.058 at 40 °C) is higher than that of diesel fuel (0.825 at 40 °C), precipitated down to the emulsion bottom. As solketal has a lower CFPP (-4 °C) and solketal droplets are not susceptible to crystalizing together, even at lower temperatures,22 the higher solketal content caused the lower CFPP of the emulsion as shown in Fig. 6.

4. Conclusions The fuel properties of nano- and micro-emulsions of solketal-in-diesel prepared with various emulsification methods were investigated in this experimental study. Solketal chemically derived from bio-glycerol was scattered into many nano- or micro-meter sized droplets within the continuous phase of ULSD. The potential of solketal to be applied in diesel fuel to improve the fuel characteristics was also evaluated. The study’s major experimental results are summarized below. (1) The nano-emulsions of diesel fuel were only formed when solketal (less than 5 wt. %) was added and scattered to constitute the dispersed phase. A further increase in the solketal content to over 5 wt. % raised the mean droplet size to form micro-emulsions instead. Nano-emulsions had much superior oxidation stability, extent of complete 11 ACS Paragon Plus Environment

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(2)

(3)

(4)

(5)

combustion, and heat release than the micro-emulsions. The nano-emulsions containing 3 wt. % solketal in the dispersed phase prepared with the microwave-irradiation reactor had the greatest heat release and the highest combustion efficiency (CE) value due to the occurrence of micro-explosion and the increase in the burning efficiency along with the lowest carbon residue among those emulsions. Such nano-emulsions are suggested to have the optimum composition to obtain superior fuel properties. The micro-emulsions containing more than 5 wt. % solketal in the dispersed phase prepared with the mechanical homogenization method had greater heat release and a higher CE value together with less carbon residue than those prepared with the microwave-irradiation reactor. The nano- and micro- emulsions prepared with the mechanical homogenizer appeared to have higher flash points and lower CFPPs than those prepared with the microwave-irradiation reactor. An increase in the solketal content reduced the amount of heat release and carbon residue and lower the flash point and the CFPP while increasing the dispersed droplet size of those emulsions of solketal-in-diesel.

Authors’ contribution C. Y. Lin, the principal investigator and corresponding author, directed the research and wrote the manuscript. S.M. Tsai assisted with the experiment. All authors gave final approval for the publication.

Notes: The authors declare no competing financial interest.

Acknowledgements The authors gratefully acknowledge the financial support from Ministry of Science and Technology of Taiwan, ROC under contract No. MOST 102-2221-E-019-039-MY3 and MOST 105-2221-E-019-066.

Funding We received funding from Ministry of Science and Technology of Taiwan, ROC (MOST 102-2221-E-019-039-MY3 and MOST 105-2221-E-019-066).

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References (1). Ciriminna, R.; Pina, C. D.; Rossi, M.; Pagliaro, M. Understanding the glycerol market, European Journal of Lipid Science and Technology 2014, 116(10), 1432-1439. (2). Mize, H. E.; Lucio, J.; Fhaner, C. J.; Pratama, F. S.; Robbins, L. A.; Karpovich, D. S. Emulsions of crude glycerin from biodiesel processing with fuel oil for industrial heating, Journal of Agricultural and Food Chemistry 2013, 61(6), 1319-1327. (3). Nanda, M. R.; Yuan, Z.; Qin, W.; Ghaziaskar, S.; Poirier, M.; Xu, C. Thermodynamic and kinetic studies of a catalytic process to convert glycerol into solketal as an oxygenated fuel additive, Fuel 2014, 117, 470-477. (4). Nobre, P. C.; Borges, E. L.; Silva, C. M.; Casaril, A. M.; Martinez, D. M.; Lenardão, E. J.; Perin, G. Organochalcogen compounds from glycerol: Synthesis of new antioxidants. Bioorganic & Medicinal Chemistry 2014, 22(21), 6242-6249. (5). Mota, C. J.; da Silva, C. X.; Rosenbach Jr N.; Costa, J.; da Silva, F. Glycerin derivatives as fuel additives: the addition of glycerol/acetone ketal (solketal) in gasolines, Energy & Fuels 2010, 24(4), 2733-2736. (6). Lin, C. Y; Chen, L. W. Emulsification characteristics of three- and two-phase emulsions prepared by the ultrasonic emulsification method, Fuel Processing Technology 2006, 87(4), 309-317. (7). Park, S.; Woo, S.; Kim, H.; Lee, K. The characteristic of spray using diesel water emulsified fuel in a diesel engine, Applied Energy 2016, 176, 209-220. (8). Lin C. Y.; Tsai, C. T. Emulsification characteristics of three-phase emulsion of biodiesel-in nitromethane-in-diesel prepared by microwave irradiation, Fuel 2015, 158, 50-56. (9). Rhodia Company, Technical data sheet for Augeo SL-191 product, Rhodia Company, Brussels, Belgium, 2016. (10). Koneva, A. S.; Safonova, E. A.; Kondrakhina, P. S.; Vovk, M. A.; Lezov, A. A.; Chernyshev, Y. S.; Smirnova, N. A. Effect of water content on structural and phase behavior of water-in-oil (n-decane) microemulsion system stabilized by mixed nonionic surfactants Span 80/Tween 80. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2017, 518, 273-282. (11). Parr Instrument Company, Introduction to Bomb Calorimetry 2013, Moline, Illinois, USA. (12). ASTM International, ASTM D6371-17, Standard Test Method for Cold Filter Plugging Point of Diesel and Heating Fuels, West Conshohocken, PA, 2017, www.astm.org. 13 ACS Paragon Plus Environment

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(13). Califano, V.: Calabria, R.; Massoli, P. Experimental evaluation of the effect of emulsion stability on micro-explosion phenomena for water-in-oil emulsions. Fuel 2014, 117, 87-94. (14). Basha, J. S.; Anand, R. B. An experimental study in a CI engine using nanoadditive blended water–diesel emulsion fuel. International Journal of Green Energy 2011, 8(3), 332-348. (15). Ithnin, A. M.; Ahmad, M. A.; Bakar, M. A. A.; Rajoo, S.;Yahya, W. J. Combustion performance and emission analysis of diesel engine fuelled with water-in-diesel emulsion fuel made from low-grade diesel fuel, Energy Conversion and Management 2015, 90, 375-382. (16). Adebayo G. B.; Ameen, O. M. Physico-chemical properties of biodiesel produced from Jatropha curcas oil and fossil diesel, Journal of Microbiology and Biotechnology Research 2017, 1(1), 12-16. (17). Cheikh, K.; Sary, A.; Khaled, L.; Abdelkrim, L.; Mohand, T. Experimental assessment of performance and emissions maps for biodiesel fueled compression ignition engine, Applied Energy 2016, 161, 320-329. (18). Khan, Y. M.; Abdul Karim, Z. A.; Aziz, A. R. A.; Tan, I. M. Experimental study on influence of surfactant dosage on micro explosion occurrence in water in diesel emulsion, Applied Mechanics and Materials 2016, 819, 287-291. (19). Lin, C. Y.; Tsai, C. T.; Chen, L. W. Comparison of the fuel properties of nitromethane emulsions in diesel and biodiesel assisted by microwave irradiation and magnetic stirring, Journal of Dispersion Science and Technology 2016, 37(9), 1334-1340. (20). Neves, M. A.; Wang, Z.; Kobayashi, I.; Nakajima, M. Assessment of oxidative stability in fish oil ‐ in ‐ water emulsions: effect of emulsification process, droplet size and storage temperature, Journal of Food Process Engineering 2017, 40, 1. (21). Wang, Y.; Ma, S.; Zhao, M.; Kuang, L. Improving the cold flow properties of biodiesel from waste cooking oil by surfactants and detergent fractionation, Fuel, 2011, 90(3), 1036-1040. (22). Melero, J. A.; Vicente, G.; Morales, G.; Paniagua, M.; Bustamante, J. Oxygenated compounds derived from glycerol for biodiesel formulation: Influence on EN 14214 quality parameters, Fuel 2010, 89(8), 2011-2018.

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Energy & Fuels

Table 1. Properties of components in the emulsions. Property

Component Solketal

ULSD

Amount of heat release (MJ/kg)

14.99

46.26

Carbon residue (wt. %)

1.81

0.49

Kinematic viscosity (mm2/s) at 40℃

5.32

3.62

Specific gravity at 40℃

1.058

0.825

CFPP (℃)

-4

-3

Flash point (℃)

91

55

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Table 2. Properties of surfactants Span 80 and Tween 80.10 Property

Span 80

Tween 80

HLB value Chemical formula Molecular weight (g/mol)

4.3 ± 1.0 C24H44O6 428.60

15.0 ± 1.0 C64H124O26 1310

0.17

0.18

1.05 186.2 35.23

1.08 148.9 29.77

2.87

2.48

Chemical structure

Thermal conductivity (W/m·K) Specific gravity Flash point (℃) Amount of heat release (MJ/kg) Carbon residue (wt. %)

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CH3

O

CH3

O

CH2OH

Figure 1. Chemical structure of solketal.

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Nano-emulsion

Figure 2. Effects of dispersed solketal content and emulsification methods on the amount of heat release of the nano- or micro-emulsions.

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Nano-emulsion

Figure 3. Effects of dispersed solketal content and emulsification methods on the carbon residue of the nano- or micro-emulsions.

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Nano-emulsion

Figure 4. Effects of dispersed solketal content and emulsification methods on the combustion efficiency of the nano- or micro-emulsions.

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Nano-emulsion

Figure 5. Effects of dispersed solketal content and emulsification methods on the flash point of the nano- or micro-emulsions.

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Nano-emulsion

Figure 6. Effects of dispersed solketal content and emulsification methods on the cold filter plugging point (CFPP) of the nano- or micro-emulsions.

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