Mixing Properties of Emulsified Fuel Oil from Mixing Marine Bunker-C Fuel Oil and Water

Abstract

Alternative marine fuels are needed to help reduce the exhaust emissions of ships. In this study, we performed an analysis to verify the potential applicability of a fuel based on Bunker-C oil, a low-grade marine heavy oil, as a novel alternative marine fuel. Bunker-C oil and water were mixed in the presence of a 0.8–1.2% emulsifier in four steps from 0% to 25% to produce a special type of emulsified fuel oil. Confocal microscopy images of samples after stabilization for approximately three days at room temperature showed no variation in the pattern at the 0% condition with no water, but a relatively homogenous mixed state of water droplets was found across all domains at the 5–25% conditions. The open-source software Image-J indicated the extraction of 166, 3438, and 5636 water droplets with mean diameters of 1.57, 1.79, and 2.08 μm, as well as maximum diameters of 7.31, 21.41, and 25.91 μm, at the 5%, 15%, and 25% conditions, respectively. For all three conditions, the mean particle diameter was approximately 2 μm, below the 20 μm reported in previous studies, with uniform distributions. This suggests that the mixed state was adequately homogenous.

1. Introduction

Air pollution has been recognized as a serious environmental problem across the world; exhaust emissions and pollutants generated from fossil fuels through devices that require heat energy, such as ships, power plants, and automobiles, have increased to serious levels [1]. Although fossil fuels account for approximately 80% of total global energy supply, their production is predicted to fall rapidly, with 2020 as a turning point [2]. In the case of oil, approximately 65% of buried resources are produced primarily by certain countries located in and around the Middle East [3,4], resulting in a high dependency on imported oil in countries without oil production and potentially causing an increased imbalance between the supply and demand [5].

As of 2008, the International Maritime Organization (IMO) aims to reduce total CO₂ emissions by approximately 70% by 2050 to reduce the greenhouse gases generated from ships [6]. For this, an Energy Efficiency Design Index (EEDI) has been set for each type of ship, and its application is strictly mandated [7]. It is noteworthy that the fuels used in ships, compared to the road transport fuels generally used in automobiles, are low-grade fuels with high viscosity containing pyrolyzed fuel oil, which entails a greater release of harmful pollutants during combustion [8]. Efforts have been made to develop various alternative fuels to reduce the quantity of harmful substances and greenhouse gases released from exhaust emissions. To meet emission regulations, there is a field of post-treatment devices in which an independent exhaust gas-reducing device is installed on the conventional combustion system, as well as a field of pretreatment methods aiming to improve the fuel itself through the use of alternative energy [5].

In terms of independent posttreatment devices, the currently most widely known approaches to reducing exhaust gases are the exhaust gas recirculation, selective catalytic reduction, and diesel particulate filter methods [9]. The corresponding posttreatment reduction devices, however, are unable to improve fuel consumption. Furthermore, they entail a variety of problems that are difficult to resolve, such as requiring structural modification of complex mechanical devices, as well as periodic maintenance and management that hinder cost stabilization [3].

Hence, the demand for alternative energy produced through a pretreatment process, without requiring any system or structural modifications of the current mechanical devices, is high. Conventional approaches include biodiesel combining petroleum oil fuel and plant-based oil, dimethyl ether fuel, and emulsified oil [10,11]. Emulsion, in particular, is an environmentally friendly fuel technology used to reduce exhaust gases from combustion devices.

The emulsion is a state of dispersion of one liquid throughout another liquid in a mixture of two different liquids. The dispersed liquid maintains a notably small particle size; if the two liquids are set as water and oil, one of the two insoluble liquids takes the form of micro-particles to disperse throughout the other liquid. An emulsified fuel oil is defined as a fuel that maintains such a mixed state of two different liquids [12].

In general, there are two broad types of emulsified fuel oil that maintain a mixed state of water and heavy oil. One is the water-in-oil (W/O) type, where water is dispersed as tiny droplets throughout oil, and the other is the oil-in-water (O/W) type, where the oil takes the form of droplets dispersed in water [3]. To differentiate the two types, a small sample of the emulsified fuel oil is dropped on a slide under a microscope, to which water or oil is applied. When the sample mixes freely with water, the type is O/W, and when the sample mixes freely with oil, the type is W/O [13].

In the W/O type, the oil contains approximately 0–50% water, and the external surface of water droplets in a micro-particle form are surrounded by oil. These micro-particles are found dispersed in the continuous bulk of oil with sizes of approximately 20–100 μm; these droplets of water are evaporated first upon combustion to cause micro-explosions, and thus, the temperature of the combustion chamber can be reduced by hampering the generation of NOx in high-temperature domains [3,13].

In the O/W type, the water contains approximately 0–49% oil. The W/O type suddenly transitions into the O/W type when the content of water exceeds 51%. As the external surfaces of the oil droplets dispersed as micro-particles are surrounded with water, combustion is nearly impossible. The high content of water may result in a partial effect of fluidity, but owing to the much less stable combustion and ignition compared to the W/O type, the O/W type is rarely used as fuel. Figure 1A,B illustrates the two types according to the emulsion mixing ratio [3,12].

This method is characterized by the mixing of fossil fuels with water and an emulsifier [12] with known positive effects in improving exhaust emissions, mainly through the physical activity of water added to the fuel system, and many studies are ongoing across various countries [14].

Fundamentally, ship fuel itself contains low-quality impurities, so it is necessary to deal with the sediment generated during long-term storage, and related research is being actively conducted [15,16].

In the field of alternative fuel development, studies have applied ammonia or hydrogen fuel that does not include carbon, similar to the methods involving fuel mixtures [17]. The emulsified fuel produced through the mixing of fuels with different substances has the advantage of allowing ready application without any modification of the mechanical structures, while the output is comparable to that of conventional fuels and the exhaust gas pollutants can be reduced [18].

However, since the combustion performance of emulsified fuel may be partially deteriorated due to the mixing of water, it is judged that it is necessary to conduct a prior study more suitable for an auxiliary marine boiler that produces steam rather than an engine that shows the ship’s direct mechanical output. In a prior study by the present authors, a special type of emulsified fuel was formed by mixing water with a low-grade Bunker-C heavy oil used mainly as marine fuel, to be applied in a marine boiler. The results indicated reductions in nitrogen oxides by 31.41% and sulfur oxides by 37.47% under the condition of 4% standard oxygen concentration, with a 14.3% reduction in the exhaust gas temperature [5]. In addition, when applying emulsified fuel oil with 10.6% water in a mixture with Bunker-C oil, studies have shown approximately 30% reductions in nitrogen and sulfur oxides in a small, dry-type, fire-tube boiler used on land in lieu of a marine boiler [1,18].

In this study, therefore, a special emulsified fuel combining water and Bunker-C oil, a low-grade heavy oil used as marine fuel, was prepared using a real-time production device under four water content conditions: 0%, 5%, 15%, and 25%. The goal was to analyze the state of homogeneous mixing and verify the potential applicability of varying forms of this alternative fuel.

2. Materials and Methods

2.1. Definition of Emulsified Fuel Oil

Figure 2 presents a visual comparison of the emulsifier, water, and low-grade Bunker-C heavy oil for marine fuel [19]. As shown, the emulsified fuel oil, in which the heavy oil is mixed with water and the emulsifier, shows a relatively light brown color, and the color characteristically lightens compared to heavy oil as the water content increases. In addition, the emulsified fuel oil combining water and heavy oil can easily undergo oil–water separation; the properties of the two different liquids pose considerable challenges to mixing and maintenance of the mixed state as an unstable system is formed [12].

Thus, an emulsifier should be added during the mixing process to maintain the mixed state of two liquids; well-known emulsifiers include surfactants. The concentration of the emulsifier used in this study was set to approximately 0.8–1.2%; this is an adequate level in relation to water, as shown in a previous study where an independent carbide solution emulsifier was developed at a professional manufacturing company. The set concentration ensures that the most stable state is maintained after mixing [12]. The emulsifier also prevents the emulsified fuel from rapidly separating through purely physical means such as mixing, stirring, or shock. Furthermore, it lowers the energy required for separation to promote emulsion using various mechanical methods, while simultaneously enhancing the storage stability [20].

2.2. Production of Emulsified Fuel Oil

The emulsified fuel oil to be used in the analysis was prepared through a series of steps including filtration, emulsifier mixing, preheating (about 60 °C), homogenization (10,000–30,000 rpm), and ultrasonic fragmentation, using a specialized plant facility developed to allow one-pass production [19]. The conditions in this study included water contents of 0%, 5%, 15%, and 25%.

Table 1. Properties of emulsified oil material.

Specification	Test Method	EM0	EM5	EM15	EM25
Water content (Vol, %)	KS M ISO 3733:2008	0.6	5.0	15.0	25.0
Sulfur content (m/m, %)	KS M ISO 2414:2011	0.28	0.26	0.23	0.20
Flashpoint (°C)	KS M ISO 2592:2007	172	103	103	101
Viscosity (@50 °C, mm2/s)	KS M ISO 3104:2008	60.61	73.32	97.32	144.80
Specific Gravity @15/4 °C	KS M ISO 12185:2003	0.9197	0.9190	0.9270	0.9382

lists the results of analyzing the compositions of the produced samples [3,12]. The production device uses a single, real-time processing step of mixing water and oil to minimize the production time and physical processing. The quality of the emulsified fuel oil produced in the plant facility satisfied the strict criteria set by the Petroleum and Alternative Emulsification Fuel Business Act—No oil–water separation should occur in the upper and lower layers for approximately 90 days at a temperature above 40 °C—to verify that the production device had stable storage [21]. Figure 3a–g compare the actual emulsified fuel oil samples, produced with varying concentrations of 0–25%, and the marine fuel oil. Compared to light oil such as diesel oil and MGO, the color of the emulsified fuel oil was a much darker shade of brown, while no difference in color could be visually detected compared to the common Bunker-C oil with 0% water, with close proximity to general low-grade heavy oil.

2.3. Analytical Methods

Confocal microscopy was used to analyze the stability of the mixture of water and Bunker-C oil. The samples prepared using the production device were left to stabilize at room temperature for approximately three days prior to sampling. In addition, different types of microscopy, including fluorescent and scanning electron microscopy, were attempted; however, most were shown to be unsuitable for this imaging task owing to the high viscosity preventing clear distinctions between the water and oil particles in the viscous liquid state of the samples. Instead, confocal microscopy was selected for this study; this approach is based on the principle in which contact with constant-wavelength light irradiated using a laser causes excitation, and the emitted light passes through the focal point to reach the detection plate [3]. In addition, all parts other than the focal plane were kept out of images by unifying the focus of samples and that of detection holes using the light from the point source. This removed the blurring phenomenon observed in conventional fluorescent microscopy, thus comparatively increasing the resolution on the focal plane. Confocal microscopy was first introduced by Marvin Minsky, a researcher at Harvard University, in 1957. This method has been practically applied since the late 1970s in the field of bioscience [22]. By using a laser as the light source, high resolutions can be achieved, and by visualizing the amount of light on the screen through the 2D scanning of samples, 2D surface images can be obtained for the high-resolution measurement while retaining the characteristics of optical microscopy. Hence, confocal microscopy can be applied in most fields involving micrometer units, including gel surfaces, semi-conductors, and glass substrates [23].

Figure 4 shows the process of direct measurements by the present investigators, from the process of the uniform application of emulsion samples in varying compositions on an extremely thin slide cover glass, to the process of the optimization of images and the resolution through confocal microscopy.

3. Results

3.1. Distribution of Water Droplets

Samples of the four conditions of water content applied to the fuel prepared in this study (0%, 5%, 15%, and 25%) were, respectively, named as EM0, EM5, EM15, and EM25, and the distributions were analyzed. The obtained images are presented in Figure 5. An analysis of the confocal microscopy images showed that the distribution of water droplets became distinctly wide as the water content increased, with uniformity easily verified by visual examination. In particular, for EM0, a simple single-color pattern was observed with no water droplets. The whitish granules in the images of emulsified fuel oil containing water can be readily interpreted as water in a mixed state. Notably, the number of water droplets rapidly increased beginning from EM15, while EM25 displayed a uniform distribution and the highest number of water droplets across all domains, indicating a relatively homogenous mixture of water and oil.

3.2. Analysis Using Image-J Open-Source Software

Image-J 1.52r was selected as the software to analyze the mixed state of water and heavy oil, considering various factors including the number, form, area, and size of particles in the image file [24]. This software is a web-based, open-source image analysis tool with free access provided to researchers worldwide [25]. Image-J also allows a relatively easy means of manipulation and a high level of compatibility, so it is applied in diverse fields such as bioscience, nanotechnology, and cellular analysis where specific states can be defined based on image files [26].

Table 2. Operation orders for Image-J 1.52r software.

List	Specification	Unit
Set scale range	0–2400	%
Image size	512:512	pixel
Known distance	100	μm
Pixel aspect ratio	1.0	-
Unit of length	-	μm
Threshold	200:255	-
Droplet size	0.1–infinity	μm

presents the operation orders for Image-J. For EM0, analyzing the images using the described methods indicated no water droplets and a pure state of Bunker-C oil, so EM0 was excluded from further analyses.

For EM5 in the set analytic conditions, 166 droplets were detected in total. For EM15 and EM25, 3438 and 5636 droplets were detected, respectively. The number of water droplets increased as the water content increased. Figure 6 describes the EM15 condition as a simplified example of the analysis results using Image-J in sequence: (a) is the original image, (b) shows the detection of particles according to the concentration ratio, (c) shows the enumeration of the total integrated particles per image, and (d) shows the particle exteriors for the measurement of the circumference and diameter of the extracted particles.

3.3. Characteristics of Droplet Micro-Particles

Using the data obtained from the software, the water droplet micro-particles in the mixture with Bunker-C oil were analyzed in terms of mean diameter and distribution, and the results are summarized in

Table 3. Comparison of water droplet analysis results.

	Item	EM0	EM5	EM15	EM25
Measurement
Number (Z)	Effective	-	166	3438	5636
	Missing	-	5470	2198	0
Minimum (μm)		-	0.90	0.90	0.90
Maximum (μm)		-	7.31	21.41	25.01
Median (μm)		-	0.90	1.35	1.35
SD		-	1.07838	1.49292	1.96007
Average (μm)		-	1.57	1.79	2.08

.

The water droplet size increased slightly as the water content increased. This result is attributed to the activated levels of flocculation, in which droplets combine with one another via collision, and of coalescence, in which adsorption layers disjoin and join as water droplets collide with one another, as a result of the increased water content per volume of fuel [13,27]. In addition, the mean diameters of all three samples containing water were approximately 2 μm, below the 20–100 μm reported across studies on the production of emulsified fuel oil. This implies that the mixed state was adequately homogenous. Figure 7a–c present graphs of water droplet diameters according to the water content [3]. Figure 8 compares the mean and maximum diameters, considering the standard deviation.

4. Discussion

This study was conducted to produce emulsified fuel oil containing 0–25% mixtures of water and Bunker-C oil and to analyze the mixed state using confocal microscopy and Image-J. The key results are as follows:

Analyzing the sample images through confocal microscopy showed that there was no variation in the pattern for the EM0 condition with no water, whereas a rapid increase in the water droplet micro-particles was readily detected as the water content increased in visual examinations of the EM5–25 conditions.

Analyzing the extracted data of the Image-J software showed that there were 166, 3438, and 5636 water droplets in the EM5, EM15 and EM25 images, respectively, and the number of extracted water droplets rapidly increased as the water content increased.

The mean diameters of water droplets were 1.57 μm, 1.79 μm, and 2.08 μm for the EM5, EM15, and EM25 conditions, respectively; all were close to 2 μm, suggesting a relatively homogenous and adequate state of mixing.

The maximum diameters of water droplets were 7.31 μm, 21.41 μm, and 25.91 μm for the EM5, EM15 and EM25 conditions, respectively, which can be attributed to flocculation and coalescence via increased collisions across water droplets within a given volume in line with increased water content.

Therefore, in the combustion test using 0–25%, which is the maximum range of emulsified fuels performed by this research team in the past [1,3,5,18], it was judged that the homogeneous state of water and Bunker-C had some effect in reducing exhaust gas emissions.

The mixing of water and low-grade marine Bunker-C heavy oil in this study created a physically unstable system, in which gradual oil–water separation is expected as the storage or preservation time increases. Thus, to ensure long-term storage of the emulsified fuel oil after production, a mixing tank that allows constant operation should be developed to maintain the homogenized and atomized states and preserve the mixture even after a unit of time has passed. In a follow-up study, such a tank device will be developed.