Influence of Fuel Additive in the Formulation and Combustion

Article pubs.acs.org/EF

Influence of Fuel Additive in the Formulation and Combustion Characteristics of Water-in-Diesel Nanoemulsion Fuel B. S. Bidita,† A. R. Suraya,*,†,‡ M. A. Shazed,† M. A. Mohd Salleh,†,‡ and A. Idris† †

Department of Chemical and Environmental Engineering, Faculty of Engineering, University Putra Malaysia, Selangor 43400, Malaysia ‡ Nanomaterials and Nanotechnology Group, Materials Processing and Technology Laboratory, Institute of Advanced Technology, University Putra Malaysia, Selangor 43400, Malaysia ABSTRACT: A comprehensive experimental study was accomplished to assess the influence of fuel additive in the formulation of water-in-diesel (W/D) nanoemulsions using surfactant in two ways, with and without including fuel additive, and a comparison is made with neat diesel. A range of surfactant concentration (0.25% to 0.40% v/v) was used with varying water concentration (0.7% to 1% v/v) to prepare W/D nanoemulsion fuel. High energy emulsification process was employed for this purpose and attempts to compare between both ways, physical and experimental observations were considered. The destabilization methods, mainly Oswald ripening, was discussed to investigate the stability. The influence of fuel additive over the characteristics of nanoemulsion such as droplet size, stability, viscosity, emulsion calorific value was studied in detail. The droplet size of W/D nanoemulsion fuel was found to be in the range approximately from 2 to 200 nm in both procedures. The engine test bed was utilized to combust the emulsion at the speed of 2600 rpm using power 2.2 KW with 50% load in order to investigate the brake specific fuel consumption (BSFC), brake thermal efficiency, exhaust mass flow rate, exhaust gas temperatures, emission gases concentrations such as carbon dioxide (CO2), carbon monoxide (CO), ammonia (NH3), and nitrogen oxides (NOx). It has been noted that the W/D nanoemulsion including fuel additive is able to considerably reduce the BSFC and exhaust mass flow rate compared to neat diesel. Brake thermal efficiency was increased by using W/D nanoemulsion including fuel additive. In addition, exhaust emissions of CO2, NH3, and NOx have been reduced using fuel additive mixed W/D nanoemulsion fuel. In contrast to the neat diesel, W/D nanoemulsion offered significantly less exhaust temperature and emission gas concentrations. The comprehensive study of the emission results recommends that the potential W/D nanoemulsions can be considered as an alternative to the conventional diesel fuel in order to reduce the environmental contaminations.

1. INTRODUCTION Nanoemulsions are a segment of multiphase colloidal dispersions that can possess long-term persistence.1,2 Kinetic stability of nanoemulsions makes them unique, and sometimes, they are referred to as approaching thermodynamic stability.3−5 The droplets of nanoemulsions comprise of size in the range from 50 to 200 nm (transparent or translucent) or from 50 to 500 nm (milky type) length scale.6−8 On account for having small droplet size, it is remarkable that they are not subject to gravity-driven separation due to the density differences of the two phases.9 Based upon the dispersion mechanism, emulsions usually exhibit two forms: water-in-oil (W/O) with dispersed water droplets in the oil phase and oil-in-water (O/W) with dispersed oil droplets in water.10 The term nanoemulsion has attracted noteworthy attention in the field of emulsion fuel, as it demonstrates a possibility to utilize them as the potential energy source in the internal combustion engine providing less environmental pollution rather than other conventional fuels. To date, waterin-diesel (W/D) nanoemulsions are the thriving outcome of years of immense efforts intended to develop an alternative for traditional diesel fuel. The preparation of W/D nanoemulsion comprises the mixture of water with specific surfactant, and diesel fuel. The surfactants are used to stabilize the nanoemulsion so that the finely dispersed water droplets remain in suspension within the diesel fuel. In contrast to the conventional microemulsion fuel, formulation © XXXX American Chemical Society

of nanoemulsion requires extreme shear rupturing whereas microemulsion phases are formed by self-assembly.11,12 In microemulsion, the diameter of water droplets is greater than 1 μm whereas in nanoemulsion, the size of water particles is in nanoscale. Due to smaller droplet size, nanoemulsion contains more water particles with more surface area than microemulsion,13 and consequently, contributes to enrich fuel efficiency as well as increase combustion competence substantially. However, the means of preparation have noteworthy influence on nanoemulsion stability based on preparation methods.14,15 The nanosize of droplets can be found either by high energy emulsification methods (e.g., by high-shear stirring, high-pressure homogenizers, or ultrasound generators),16 or by low-energy emulsification methods (e.g., phase inversion temperature (PIT)).17,18 An immense control of the droplet size and a large variety of composition is allowed by high-energy emulsification method while the energy stored in the system is used to promote the formation of small droplets in low energy emulsification method.2 Accordingly, stability studies of nanoemulsion comprise the investigations on Ostwald ripening and coalescencetwo processes that are responsible for the destabilization of nanoemulsion fuel.19 Coalescence acts as a Received: January 23, 2014 Revised: May 5, 2014

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dx.doi.org/10.1021/ef5002259 | Energy Fuels XXXX, XXX, XXX−XXX

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source; concurrent oxidation of hydrocarbons as well as the lessening of oxides of nitrogen, thus reducing emissions, especially in the stoichiometric conditions.39 Additionally, being a rare earth material CeO2 attributes two significant properties: the high mobility and storage capacity of oxygen within the lattice and the ease with which cerium changes between Ce3+ and Ce4+ states. These properties, together with the abundance of cerium on Earth, make CeO2 a low-cost high effective alternative to other noble metal oxide catalysts.47,48 Therefore, experimental investigations have been performed to evaluate the effect of CeO2 additive in diesel fuel and biodiesel on the engine performance and emissions.39,40 However, no report has been found in the field of W/D nanoemulsion where CeO2 was utilized as fuel additive to investigate the engine exhaust gas emissions. In the context of this work, the involvement of CeO2 as a fuel additive in the preparation of W/D nanoemulsions fuel and consequences of fuel additive based emulsified fuel on engine performance as well as exhaust emissions concentration is presented. An attempt has been made to compare characteristics of W/D nanoemulsions between two schemes, with and without fuel additive, by means of ultrasonication (high energy emulsification method). In this regard, varying amounts of surfactant (0.25%, 0.30%, 0.35%, 0.40% v/v) in combination with a specific range of water content (0.7%, 0.8%, 0.9%, 1.0% v/v) was utilized to formulate W/D nanoemulsion and the compatibility was analyzed by combusting them using a diesel engine. The physical characteristics such as droplet size, distribution behavior, viscosity and the chemical and mechanical characteristics such as calorific value, BSFC, brake thermal efficiency were examined. The exhaust temperature in addition to emission concentrations of CO2, CO, NH3, and NO was also measured. The comparison between the two schemes in terms of their effect on the physical and chemical properties as well as influence on engine emissions has also been discussed. In addition, the comparison between two schemes has been made on the basis of physical, chemical, and emission characteristics of neat diesel.

driving force to change droplet size with time whereas in Ostwald ripening small particles dissolve and form larger particles over time by following the mechanism of molecular diffusion.20,21 Diesel engine provides better thermal efficiency of the two main types of internal combustion engines. Presently diesel engine is the most efficient combustion engine which dominates most nonroad equipment including construction, agricultural, marine vessels, and locomotives.22,23 Although the operational advantages of diesel engine are apparent, pollution caused by diesel engines in the form of obnoxious odor, gas pollutants, and particulate matter (PM) to the atmosphere is becoming one of major public attentions. Particulate emissions caused by diesel engine consist of a complex mixture of engine’s oils, sulfates, and inorganic materials. They are toxic and play a vital role in lung related illness.24,25 Environmental pollution caused by combustion processes of diesel engine makes it crucial that solutions are found for their reduction.26 Recently, more research, advanced technologies and development of interest are required to control diesel engine exhaust emissions due to the enforcement of increasingly stringent emission regulations.27−31 It has been found that the addition of water into diesel engine has numerous potential benefits.32 Water-based emulsified diesel is a suitable alternative renewable fuel since any prior or post modification is not required in the existing engine.22,33 Several studies have demonstrated that nanoemulsions can be an environmental substitute for traditional diesel fuel, which typically contains 5−20% of water, surfactant, and the base fuel, such as diesel.22 Researchers have also found that the mean sizes of the water droplets in the nanoemulsions appeared in the range 19.3− 39 nm, based on the water content as well as the concentration of the blend emulsifiers.2 It has been reported that the emissions of nitric oxide (NO) were less, when the nanoemulsions were utilized as the engine fuel. In fact, reduction in calorific value of nanoemulsions with less exhaust gas temperature was reported by the researchers. An increase in brake specific fuel consumption (BSFC), and reduction in exhaust temperature and NOx, was found with increasing water percentage in another study.34 Although the addition of water in the diesel fuel has some positive effects on the environmental pollution, it is also important to utilize suitable percentage of water in the diesel fuel, since too much water in the fuel can be sucked in the fuel line, which can cause starting and performance problems.35 Despite the fact that emulsion fuel was commercialized as a new type of fuel about 10 years ago in the U.S. and Europe, it has not achieved widespread usage. The possible reason may be due to too much water content, which is a drawback for combustion because of the latent heat loss that becomes too large for the fuel to burn completely, which results in the formation of larger amount of CO in addition to engine problems.36,37 A number of experimental studies have demonstrated that the addition of fuel additives can increase combustion engine efficiency and reduce emissions.38−42 Common metal oxides such as cerium, copper, iron, and cobalt are considered as fuel additives. An effective approach for improving diesel combustion, reducing fuel consumption and lowering engine emissions was found by the addition of metal-based combustion catalysts to diesel fuel.43−45 By using homogeneous combustion catalysts, brake BSFC was decreased under all test conditions.46 However, researchers have found that substantial changes occurred in the particles augmentation mode by the addition of cerium oxide (CeO2). Without being influenced by the dosing level of CeO2, the kinetics of oxidation increased considerably.40,41 Particularly, CeO2 nanoparticles impose the capability to act as oxygen buffer

2. MATERIALS AND METHODS 2.1. Materials. Water-in-diesel (W/D) nanoemulsion fuel was prepared by pouring water (dispersed phase) into diesel (continuous phase) at 25 °C. The surfactant used in these experiments was Triton X-100 (Scharlab S.L., Spain). As received cerium(IV) oxide (Acros Organics from Fischer Scientific, U.S.A.) was used as the fuel additive where applicable. Commercial diesel fuel (Petroleum Nasional Bhd., Malaysia) was utilized as the continuous phase of the nanoemulsion. 2.2. Preparation and Characterization of W/D Nanoemulsions. 2.2.1. Preparation of W/D Nanoemulsions. The preparation of W/D nanoemulsions was conducted by using an ultrasonic processor (Cole-Parmer 500-W Ultrasonic Homogenizer, 115 VAC) in two ways. In Scheme 1, surfactant was added in different percentages (0.25% to 0.40% v/v with 0.05% v/v increment) with varying amount of water (0.7% to 1% v/v by an increment of 0.1% v/v) into diesel fuel. Each surfactant concentration was used with different percentages of water at constant amplitude of 30% for 10 min. In Scheme 2, the cerium(IV) oxide was utilized as the fuel additive into diesel fuel; further mixing of additive and fuel was executed by means of an ultrasonication, applying constant agitation time of 30 min to produce a uniform suspension. The dosing level of cerium oxide was maintained at 80 ppm. The water with surfactant was then added into fuel additive mixed diesel fuel; thus W/D nanoemulsion fuel was formulated by using ultrasonic processor. Afterward, the same procedure was repeated as described in Scheme 1. All experiments were carried out at 25 °C. 2.2.2. Droplet Size Distribution Analysis. The mean droplet size distribution was analyzed via dynamic light scattering equipment (Zetasizer Nano-ZS 90, Malvern, UK) at 25 °C to examine the stability of the prepared W/D nanoemulsion. Transmission electron microscope B


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Scheme 1. Schematic Flow Diagram Representing the Preparation Method of W/D Nanoemulsion via Scheme 1

Scheme 2. Schematic Flow Diagram Representing the Preparation Method of W/D Nanoemulsion via Scheme 2

C


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Figure 1. Physical appearance of (a) neat diesel, and W/D nanoemulsion fuels prepared via (b) scheme 1 and (c) scheme 2. (TEM) was used to analyze the droplet diameter of prepared W/D nanoemulsion. 2.2.3. Viscosity Measurement. Glass capillary viscometer was utilized to measure kinematic viscosity in accordance with water bath (CANNON CT-500/518). Temperature was maintained within the range from 20 to 100 °C according to ASTM D445. 2.2.4. Calorific Value Measurement. In order to calculate calorific value, the value for heat of combustion is required which was determined by using an oxygen bomb calorimeter. The measurement was followed by a substitution procedure in which the heat obtained from the W/D nanoemulsion was compared with the heat obtained from combustion of a similar amount of benzoic acid whose calorific value was known. The gross heat of combustion (Hg) is determined as follows:

of fuel additives on the physical appearance of W/D nanoemulsion fuels. 3.1.2. Nanoemulsion Fuel Stability. The major concern in the preparation of W/D nanoemulsion fuel is the selection of appropriate surfactant with fuel additive. Surfactant exhibits a good influence on emulsification process to emulsify the ingredients; the usage of fuel additives with surfactant can give better combustion performance. It is important to consider some factors such as emulsifying agent, water content, mixing speed, mixing time, viscosity, and temperature for the stability of nanoemulsion fuel. In this work, the preparation of W/D nanoemulsion fuel was performed using various surfactant concentration (0.25%, 0.30%, 0.35%, 0.40% v/v) with a range of water percentages (0.7%, 0.8%, 0.9%, 1.0% v/v). Physical analysis was performed to study the stability of nanoemulsion fuel prepared by both schemes. In scheme 1, the nanoemulsion exhibited stability for more than 2 weeks (16 days) without any phase separation. On the other hand, scheme 2 showed one layer of emulsion in less than 2 weeks (13 days). Although nanoemulsion was more stable in scheme 1 than in scheme 2, the overall stability of W/D nanoemulsions was satisfactory in both schemes indicating good quality of nanoemulsion fuel. Previous work on nanoemulsions either did not disclose their stability inside the engine.2,22 The relatively long period of 13−16 days of stability for our samples indicates that it is very promising for them to be used in a diesel engine. Note that we have also observed that the nanoemulsions can be stable for more than one month if stored at a relatively lower temperature ranged from 15 to 17 °C. The effects of surfactant on nanoemulsion stability are currently being investigated. To express Ostwald ripening rate (ω), the Lifshitz− Slezovand−Wagner (LSW) theory49,50 is used:

Hg = (temperature rise) (energy equiv. of standard benzoic acid) /W (wt of W/D nanoemulsion fuel)

(1)

By multiplying Hg by 1.8, the heat of combustion is converted to calorific value (Btu/pound), Calorific value = Hg × 1.8

(2)

2.3. Combustion Analysis of Prepared W/D Nanoemulsions. An engine test bed (Model SOLTEQ TH03) and the diesel engine (Model YANMAR L48N) were used to combust the nanoemulsion fuel in order to analyze the BSFC, brake thermal efficiency, exhaust gas mass flow rate, exhaust temperature, and exhaust gas emissions. The tests were conducted at engine speed of 2600 rpm with the power of 2 KW at 50% load. Two gas monitors were used to measure the exhaust gas component concentrations: MultiRae IR (PGM-54, RAE Systems, U.S.A.) for CO and CO2and Vrae (PGM-7800 and 7840, RAE Systems, U.S.A.) for NH3. An optimum surfactant concentration was found based on maximum reduction of exhaust gas emission from W/D nanoemulsion fuel. By using an optimum surfactant concentration, NOx emissions were determined with the help of a gas analyzer (Horiba, U.S.A.). The combustion characteristics of neat diesel were utilized to compare with the results found using emulsified fuels.

ω = dr 3/dt

(3)

Here, r is the average radius of the droplet and t is the storage time. A small mean droplet size and a large Ostwald ripening rate is required in order to formulate a stable emulsion.2 Based on eq 3, a graph of r3 vs time was plotted whereby a linear relationship between the cube of the droplet radius and time was found, as shown in Figure 2. The slope of each straight line in Figure 2a indicates Ostwald ripening rate. The Ostwald ripening rate, ω, as depicted from the plot was 3.57 × 10−29 m3 s−1 and 2.05 × 10−29 m3 s−1 for schemes 1 and 2, respectively. Figure 2b shows the droplet size distribution of nanoemulsions with respect to intensity. To compare the stability of W/D nanoemulsions prepared by both schemes, the highest percentage of surfactant (0.40%) with highest amount of water (1%) was chosen.

3. RESULTS AND DISCUSSION 3.1. Influence of Fuel Additive on W/D Nanoemulsions Properties. 3.1.1. Physical Properties. Neat diesel fuel appears as a translucent light brown liquid. However, W/D nanoemulsion fuels appeared as an opaque liquid with straw-like color for both of the schemes used without and with fuel additive. Figure 1 shows the physical appearance of W/D nanoemulsions in contrast to the conventional diesel fuel. Although the preparation procedures for the two schemes were different to some extent, almost the same physical appearance was obtained in the nanoemulsions prepared by both schemes. Therefore, it can be said that there is no influence D


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The features of nano droplet size achieved by both schemes were also observed by TEM images at day 1 as shown in Figure 3. The smallest droplet size found was 15.15 nm for scheme 1, which is represented in Figure 3a. For scheme 2, the smallest droplet size found was 12.56 nm as represented in Figure 3b. Comparing both schemes, although the initial droplet size involved in scheme 1 was greater than that in scheme 2, better stability remained in scheme 1 (an increase about 24% in droplet size) rather than scheme 2 (approximately 49.5% increase in droplet size). The kinetics of nanoemulsions was decreased due to the increase of droplet sizes in both schemes, which was attributed to Ostwald ripening.2 Thus, CeO2 played a role on the size and the stability of the droplets formed during the emulsification process. 3.1.3. Viscosity. The viscosity of neat diesel was found to be 6.5 cst/s by using the glass capillary viscometer procedure. Figure 4 shows plots of viscosity against water content with varying surfactant concentration. It can be seen that as the amount of water and surfactant content increased, the viscosity of the emulsion fuel decreased in both schemes compared to neat diesel, which is favorable for combustion. The decrease of viscosity may be due to the disintegration of aggregates of surfactant molecules, which results from release of the oil phase entrapped within the interstices of the aggregates and, therefore, fluidity was increased. In addition, the adsorption of surfactant should also affect the species distribution at the interface. The interface has no sharp boundaries and is rather defined as a region over which the density and local pressure varies, such that part of oil molecules comprising the interface was replaced by inverting surfactants.51 The viscosity reduction was found higher in Figure 4b than in Figure 4a. This is because additive compound may have a significant role in viscosity reduction. Smaller molecules or molecules with the correct physical and chemical characteristics to fit into the gaps between the agglomerates can reduce the viscosity to a greater extent. This is caused by deeper penetration, which induces greater breakup of the agglomerates.52 However, it is important to ensure suitable viscosity for fuel atomization since the diesel engines are primarily lubricated by fuel itself. Fuel viscosities for high speed engines range from 1.8 to 5.8 cst/s (ASTM D-2 on Petroleum Products and Lubricants). In the context of this study, the lowest viscosity for schemes 1 and 2 is 5.72 cst/s and 5.65 cst/s, respectively.

Figure 2. Representative plots of (a) the linear relationship of the droplet size (r3) as a function of time, (b) size distribution by intensity for 0.40% surfactant with 1% water in W/D at day 1 prepared via both schemes.

In scheme 1, the smallest initial droplet size in the nanoemulsion was 1.74 nm. An increase in droplet size was observed up to 43.77 nm after 312 h; indicating influence of Ostwald ripening in which smaller droplets assemble to form a larger droplet. In the case of Scheme 2, the same surfactant/water composition was used with the addition of 80 ppm cerium oxide. Initially a droplet size of 1.46 nm was obtained but gradually the size increases up to 74 nm after 312 h due to the influence of Ostwald ripening. At ambient temperature, the nanoemulsion fuel in both schemes can be stable for almost 2 weeks. We have also observed that it can be stable for more than one month if stored at a lower temperature ranging from 15 to 17 °C.

Figure 3. Representative TEM images (taken on day 1) of W/D nanoemulsions prepared via (a) scheme 1, (b) scheme 2. E


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Figure 5. Different calorific value as a function of water content obtained from nanoemulsion prepared via (a) scheme 1 and (b) scheme 2.

Figure 4. Comparative viscosity measurements of W/D nanoemulsion with different surfactant concentration prepared via (a) scheme 1 and (b) scheme 2.

Although the nanoemulsion fuel exhibits lower viscosity compared to diesel fuel, the viscosities remain within this range, having no significant effect on fuel lubricity property. 3.1.4. Calorific Value. Calorific value is normally considered as the amount of heat produced from the complete combustion of a specific amount of fuel. The calorific value for neat diesel was found to be 19 340 Btu/lb. Figure 5 shows plots of calorific value against water content with varying amounts of surfactant concentration. Parts a and b of Figure 5 show the corresponding calorific values for nanoemulsion fuel prepared via schemes 1 and 2, respectively. It can be seen that as the amount of water and surfactant content increased, the calorific value for both schemes decreased as expected. Therefore, the lowest calorific value was found from the highest percentage of water (1.0%) with the highest amount of surfactant (0.40%) used. The reduction in calorific value is attributed to the vaporization of the water phase in the nanoemulsion. Using eqs 1 and 2, it was found that the lowest calorific value was 16 350 Btu/lb and 15 890 Btu/lb for schemes 1 and 2, respectively. The results indicated that the fuel additive was able to decrease the calorific value due to its chemical activity as acting to release oxygen in the presence of reductive gases, as well as to remove oxygen by interaction with oxidizing species. Consequently, the amount of heat produced from complete combustion gradually decreased; hence, calorific value also decreased. 3.2. Effect of Fuel Additive on Combustion. 3.2.1. Brake Specific Fuel Consumption (BSFC). Figure 6 shows plots of BSFC against water content with varying amounts of surfactant concentration. Parts a and b of Figure 6 show the corresponding BSFC for nanoemulsion fuel prepared via schemes 1 and 2, respectively. According to the experimental observation, the BSFC of neat diesel was found to be 0.314 kg/KWh. Although there are a lot of fluctuations in the data, BSFC of nanoemulsion fuel in both schemes are generally lower than that of neat diesel. The highest reduction was found to be 23% for scheme 1,

Figure 6. Different brake specific fuel consumption rate as a function of water content obtained from nanoemulsion prepared via (a) scheme 1 and (b) scheme 2.

whereas it was 24% for scheme 2 compared to neat diesel. The water percentage of 0.8% incorporated with 0.4% surfactant concentration in scheme 2 exhibited the highest reduction in fuel consumption. This decrease is possibly due to finer dispersion of diesel fuel incorporated with 0.8% water which leads to higher contact with the air during the burning process.37 The BSFC was found to increase for 1% water content most probably due to the decrease in calorific value for the highest amount of water. F


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Apart from the calorific value the final outcomes of diesel engine operated by an emulsion fuel are dependent some other factors, such as injection timing, charge-air cooling, swirl, fuel atomization, etc.53 Although the calorific value and water amount were not much different between 0.7 and 0.8% water containing emulsion, the decrease in BSFC may raise due to this additional parameters. 3.2.2. Brake Thermal Efficiency. Figure 7 shows plots of brake thermal efficiency of different W/D nanoemulsions against

Figure 8. Exhaust gas mass flow rate as a function of water content obtained from W/D nanoemulsions prepared via (a) scheme 1 and (b) scheme 2.

amount of surfactant (0.40%) in Figure 8a, whereas in Figure 8b, 5% reduction in exhaust gas mass flow rate was found for the same ratio compared to neat diesel. This is possibly due to the reaction temperature, engine power, etc., which might have influenced the reduction of exhaust gas mass flow rate of the W/D nanoemulsion fuel. According to the mass conservation of chemical reaction (combustion), the exhaust gas flow rate will reduce only in case of increasing solid/liquid product in exhaust pipe. In this context, water remains as the liquid byproduct lowering the exhaust gas flow rate. Mass flow rate calculations for the combustion of nanoemulsion fuel are discussed below. Initially, mass flow rate of combustion of nanoemulsion without fuel additive was considered with two different water contents. For 0.25% surfactant with 0.7% water,

Figure 7. Different brake thermal efficiency as a function of water content obtained from W/D nanoemulsion prepared via (a) scheme 1 and (b) scheme 2.

water content with varying amounts of surfactant concentration. Parts a and b of Figure 7 show the corresponding brake thermal efficiency for nanoemulsion fuel prepared via scheme 1 and scheme 2, respectively. In spite of having some anomalies, there is an overall increase in the brake thermal efficiency of nanoemulsion fuel prepared by both schemes compared to that of neat diesel. It has been observed that the surfactant concentration of 0.40% showed the maximum thermal efficiency of 37.1% for scheme 1 while it was 37.2% for scheme 2. The brake thermal efficiency of neat diesel was 34.2%. Although calorific value of nanoemulsion fuel prepared by both schemes was lower than neat diesel, some other factors such as reaction time, oxygen content, engine performance, etc. may have influenced the increase of thermal efficiency. 3.2.3. Exhaust Gas Mass Flow Rate. Figure 8 shows plots of the exhaust gas mass flow rate of different W/D nanoemulsions against water content with varying amounts of surfactant concentration. Parts a and b of Figure 8 show the corresponding brake thermal efficiency for nanoemulsion fuel prepared via schemes 1 and 2, respectively. The exhaust gas mass flow rate of neat diesel was 78 g/min. Overall, there is a reduction of exhaust gas mass flow rate in both schemes compared to neat diesel. A reduction of 3% in exhaust gas mass flow rate was achieved using the highest amount of water (1.0%) with the highest

C12H23 + 11.3H2O + 18.75(O2 + 3.76 N2) + 0.006(Triton X-100) 2947 g/h

=

12CO2 + 22.8H2O + O2 + 70.5N2 + 0.006(Triton X-100) 2947 g/h

Since 1850 g/h of nanoemulsion fuel enter into the engine, the exhaust flow rate is (1850 + 2947) g/h or 80 g/min for nanoemulsion with 0.25% surfactant with 0.7% water. For 0.25% surfactant with 1.0% water, C12H23+ 7.9H2O+ 18.75 (O2 + 3.76N2) + 0.006(Triton X-100) 2833 g/h

=

12 CO2 + 19.4H2O + O2 + 70.5N2 + 0.006(Triton X-100) 2833 g/h

Since 1800 g/h of nanoemulsion fuel enter into the engine, the exhaust flow rate is (1800 + 2833) g/h or 77.2 g/min for nanoemulsion with 0.25% surfactant with 1.0% water. Now, mass flow rate of combustion of nanoemulsion with fuel additive was considered with two different water contents. G


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For 0.25% surfactant with 0.7% water, 2CeO2 + C12H23 + 11.3H2O + 18.75(O2 + 3.76 N2) + 0.006(Triton X-100) 3275 g/h

=

Ce2O3 + 12CO2 + 22.8H2O + O2 + 70.5N2 + 0.006(Triton X-100) 3275 g/h

Since 1500 g/h of nanoemulsion fuel enter into the engine, the exhaust flow rate is (1500 + 3275) g/h or 79.5 g/min for nanoemulsion with 0.25% surfactant with 0.7% water. For 0.25% surfactant with 1.0% water, 2CeO2 + C12H23 + 7.9H2O + 18.75(O2 + 3.76 N2) + 0.006(Triton X-100) 3275 g/h

=

Ce2O3 +12CO2 + 19.4H2O + O2 + 70.5N2 + 0.006(Triton X-100) 3275 g/h

Since 1500 g/h of nanoemulsion fuel enter into the engine, the exhaust flow rate is (1500 + 3161) g/h or 77 g/min for nanoemulsion with 0.25% surfactant with 1.0% water. 3.2.4. Combustion Characteristic of Neat Diesel. Neat diesel was initially characterized through an engine test bed in terms of its exhaust temperature and emission concentrations in the perspective of analyzing combustion characteristics. Figure 9

Figure 10. Exhaust gas temperature of W/D nanoemulsions prepared via (a) scheme 1 and (b) scheme 2.

Figure 11. Schematic illustration of combustion mechanism of scheme 1 inside the fuel chamber of the diesel engine.

Figure 9. Emission characteristics of neat diesel obtained from diesel engine test bed.

atomized spray droplets.55 Figure 11 shows the schematic illustration of combustion mechanism in the prepared W/D nanoemulsion using surfactant without fuel additive. In Figure 11, owing to the difference in boiling points in the secondary explosion, water droplets explode first.56,57 Consequently, the oil phase is particulated due to shearing effects and a force blending with the air occurs leading to more efficient and complete combustion. In the combustion chamber, the carbon reacts with the oxygen in the water phase which considerably reduces the amount of air required from the outside. These particular water−gas reactions are shown in eqs 4 and 5. Consequently, the heat efficiency is improved as there is no cooling in the internal chamber.

shows the results obtained, which will be further used to be compared with the nanoemulsified fuels. As shown in Figure 9, the exhaust temperature of neat diesel was found to be 290 °C. In addition, the emission concentration of CO2, CO, NH3, and NO was obtained as 21784 ppm, 766, 109, and 276 ppm, respectively. In the combustion chamber of the diesel engine, diesel fuel produces power due to its atomization and mixing with air. Therefore, pressure is produced which contributes to the rapid increase of temperature and emission concentrations of exhaust gases.54 3.2.5. Exhaust Temperature of W/D Nanoemulsion. The maximum combustion temperature was found to be 1965 and 1948 °C for schemes 1 and 2, respectively. The maximum combustion temperature for neat diesel was 2150 °C. Parts a and b of Figure 10 show the variation of exhaust gas temperature for different ratio of water/surfactant/diesel obtained from schemes 1 and 2, respectively. The general observation was governed by an overall decrease in exhaust temperature obtained for all W/D nanoemulsions compared to the neat diesel in both schemes. The overall decrease in exhaust temperature was because the water phase in the diesel served as a heat sink, which is described below. Emulsions with the greater water content produced a larger heat sink, a lower flame temperature, and a shorter burning time due to microexplosion phenomenon from

C + H 2O = CO + H 2 − 131 kJ

(4)

1 O2 = CO2 + 283 kJ (5) 2 The volatility of water serves to improve fuel atomization. When heat sink occurs, the water content of the inner phase has partially absorbed the calorific heating value of the emulsions, thereby decreasing the burning gas temperature inside the combustion chamber. In fact, water degradation of fuel molecules caused by high combustion temperature to form short chain CO +

H


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Figure 12. Schematic illustration of combustion mechanism of scheme 2 inside the fuel chamber of the diesel engine.

alcohol and hydrocarbons, which further produce CO2 and H2O due to continuity of the combustion process. ΔT

R−R′

O2

H 2O ⎯→ ⎯ OH + H → R − OH + H − R′ → CO2 + H 2O

(6)

Generally, the heat of hot exhaust gases is not transferred to do useful work, such as producing steam. Lower fuel efficiency is caused by this heat loss. Because the heat content of the exhaust gas is proportional to its temperature, the fuel efficiency drops as the temperature increases. A reduction of 36% in exhaust temperature was achieved using the highest amount of water (1.0%) with the highest amount of surfactant (0.40%) in Figure 7a, whereas in Figure 7b, 40% reduction in exhaust temperature was found for the same ratio. This may be due to cerium oxide as a fuel additive lowers the combustion temperature and consequently enhances hydrocarbon oxidation. Besides, as the percentage of water and surfactant increases, the exhaust gas temperature decreases. Figure 12 shows an illustration of the reaction mechanism inside the combustion chamber which contains cerium oxide as the fuel additive. Cerium oxide being oxygen storing diesel additive, causes the oxidization of excess hydrocarbons into CO2 as there is a transitory lack of oxygen in the exhaust mixture. During that process, CeO2 is reduced to Ce2O3 through supplying oxygen in the reaction that is reoxidized later immediately there is sufficient oxygen yet again in the exhaust gases. Simultaneously, the expansion/explosion of water droplets occurs and follows the same mechanism as described in the mechanism of Figure 11. Thus, cerium oxide particles have potential for the reduction of fuel consumption as well as emission of soot particles of diesel engines. 3.2.6. Exhaust Emission Gas Concentration. Figure 13 shows the emission concentrations of CO2 produced from the combustion of different water/surfactant/diesel ratios. About 79% decrease in CO2 emissions was attained in Figure 13a for the highest amount of water (1.0%) with the highest amount of surfactant (0.40%) while in Figure 13b, the decrease in CO2 emissions was 85% of the same water and surfactant ratio. This reduction is possibly due to the absorption of CO2from the system by CeO2. CeO2 is usually a hygroscopic element with the ability to absorb moisture from the surroundings which also contains a small quantity of CO2. Overall, a reduction in CO2 emissions was observed for both schemes in comparison to neat diesel. It is highly likely that the microexplosion effects during the combustion led to an enhanced combustion efficiency.

Figure 13. Emission concentration of CO2 produced through the combustion process of nanoemulsion fuel prepared via (a) scheme 1 and (b) scheme 2.

The most possible reason for the reduction of CO2 emission of nanoemulsion fuel in both schemes was its lower fuel consumption and lower exhaust gas mass flow rate compared to that of neat diesel. Besides, fuel efficient engines, high performance drivelines and hydraulic systems are also considered as essential elements in the quest for lower CO2 emissions. Figure 14 shows the emission concentrations of CO produced from the combustion of different water/surfactant/diesel ratios. A general reduction in CO emissions was observed using W/D nanoemulsions compared to neat diesel in both figures. The largest and relatively uniform decrease was acquired using a surfactant concentration of 0.40% with 1.0% water content. In Figure 14a, a decrease of approximately 88% compared to the neat diesel was achieved, but in Figure 14b, a decrease of only 72% was obtained. This is possibly due to the lean air/fuel mixtures are not combusted completely owing to flame quenching and the rich mixtures do not find oxygen for its entire I


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Figure 14. Emission concentration of CO produced through the combustion process of W/D nanoemulsion fuel prepared via (a) scheme 1 and (b) scheme b.

Figure 15. Emission concentration of NH3 produced through the combustion process of nanoemulsion fuel prepared via (a) scheme 1 and (b) scheme 2.

longer in the atmosphere if it reacts with other chemicals and is formed into a particle. Further oxidation of NH3 can produce NOx, which is also responsible for environmental pollution. It can react with other ambient gases and particles, including nitric and sulfuric acids, which is formed from NOx and SOx, produced by combustion processes. Besides, ammonia gas and particulates have an impact on human and animal health and cause environmental degradation. If inhaled, the fine particulate forms of ammonia pose a risk to human and animal health. These particles can pass through deep into lung tissue and cause a variety of respiratory problems.62 Overall, W/D nanoemulsions with fuel additive can reduce NH3 emission substantially, which enhances the combustion efficiency. Figure 16 shows the emission concentrations of NO produced from the combustion of W/D nanoemulsion fuels prepared via

combustion.58 Instead, carbon monoxide (CO) is formed by the combination of some oxygen with carbon.59 The combustion of emulsified fuels produced lower CO emissions as compared to neat diesel, most likely because of the better combustion of the W/D nanoemulsion than neat diesel. The CO concentration can be related to the exhaust temperature of engine test bed. The latent heat of water will cool the system due to the evaporation of water, and the diesel engine temperature becomes lower as the water percentage increases which results in a lower peak combustion temperature. The CO oxygenating rate, KCO, is shown in eq 7:60 CO + OH ⇆ CO2 + H; K CO = 6.76 × 1010 exp[T /1102] cm 3/g mol

(7)

According to this expression, when the temperature, T, is reduced, KCO is also reduced. Furthermore, the effect of the presence of water in the fuel will increase oxygen availability thus lowering CO emissions. NH3 has an ability to react with the atmospheric gases to produce particulate matter (PM), such as ammonium nitrate (NH4NO3) or ammonium sulfates (NH4SO4, NH4(SO4)2). Analysis of ambient PM indicates that ammonium can be composed from 14.0 to 17.0% of the total fine PM (PM2.5) at various locations.61 The emission concentrations of NH3 produced from the combustion of different water/surfactant/diesel ratios are shown in Figure 15. Similar to CO2 and CO, an overall reduction of NH3 emissions was found for the nanoemulsions compared to neat diesel. In Figure 15a, a decrease of 42% in NH3 emissions was found whereas in Figure 15b about 89% decrease was obtained. This behavior may be because of multifaceted interaction of cerium oxide with some factors such as combustion temperature, reaction time, and the oxygen content. However, it is important to reduce NH3 emissions because gaseous ammonia can last

Figure 16. Emission concentration of NO produced through the combustion process of nanoemulsion fuel prepared via schemes 1 and 2.

both schemes. A reduction of NO emission was found to be 31% using the nanoemulsion prepared by scheme 1 with highest amount of water (1.0%) with the highest amount of surfactant (0.40%). However, W/D nanoemulsion prepared by scheme 2 shown 61% reduction in NO emission for the same ratio compared to that of neat diesel as expected because the reduction J


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is much accepted in terms of reduced exhaust emissions from diesel engine during the combustion of W/D nanoemulsions.

of NO in the exhaust emissions is one of the general requirements of emulsified fuel.63 A larger reduction of NO was achieved in the presence of CeO2. This is because of the high thermal stability of Ce2O3, which can be formed from the oxidation of hydrocarbons. Besides, soot remains active after the enhancement of the initial combustion cycle and further oxidized to CeO2 through the reduction of NO, which is shown as follows:39 1 N2 2

Ce2O3 + NO → 2CeO2 +

4. CONCLUSIONS In this work, W/D nanoemulsions were formulated with and without the presence of cerium oxide as fuel additive by the aid of an ultrasonic processor. The stability of the nanoemulsion was observed up to 16 days after which the stability deteriorated most probably due to Ostwald ripening. The nanoemulsion appeared to be homogeneous with a range of water droplet size from 2 nm up to 200 nm. Glass capillary viscometer was utilized to measure kinematic viscosity, and a decreasing trend in viscosity was found in fuel additive rich W/D nanoemulsion fuel rather than the additive free fuel. The heat of combustion of W/D nanoemulsions was determined by using an oxygen bomb calorimeter, and the fuel additive was able to decrease the calorific value due to its chemical activity as acting to release oxygen. In both schemes, the highest amount of surfactant (0.40%) led to the most favorable combustion. The nanoemulsions incorporated by fuel additive exhibited with comparatively lower BSFC, exhaust gas flow rate and exhaust gas temperature but higher thermal

(8)

Therefore, NOx emission was decreased by the addition of CeO2, which is as expected. In the context of exhaust gas emission, CeO2 additive was additionally added in the diesel in order to study its effect on combustion mechanism. CeO2 is oxygen storing diesel additive which causes the oxidization of excess hydrocarbons into CO2 as there is a transitory lack of oxygen in the exhaust mixture. Since the exhaust gas emission was found to be lower in the presence of CeO2, complete combustion reaction of diesel fuel using this fuel additive is discussed as follows: 2CeO2 [(140 × 2) + (16 × 4)] g/mol

+

C12H23 [(12 × 12) + (1 × 23)] g/mol 1063 g/mol

+

17.25O2 = [17.25 × 16 × 2] g/mol =

Ce2O3 [(140 × 2) + (16 × 3)] g/mol

+

12CO2 + 11.5H2O [12(12 + 16 × 2)] [11.5(1 × 2 + 16)] g/mol g/mol 1063 g/mol

In water−gas phase reaction, C (12) g/mol

+

H2O (1 × 2 + 16) g/mol 30 g/mol

CO (12 + 16) g/mol

=

CO (12 + 16) g/mol

+

efficiency which is desirable. Two gas monitors and gas analyzer were used to measure the concentrations of exhaust gas components, and reduces emissions of CO2, NH3, and NO were notified from the combustion of fuel additive rich nanoemulsions; although emissions of CO were increased in the fuel additive incorporated nanoemulsion. Although there may be some variation regarding physical and chemical characteristics caused by both schemes, the overall properties of nanoemulsions have been superior compared to the neat diesel. Based on the overall analysis, it can be concluded that the exhaust emission concentrations were less in nanoemulsion compared to neat diesel, and thus, the W/D nanoemulsion has the potential be considered as an environment friendly fuel.

H2 (1 × 2) g/mol

30 g/mol

1

+

+

/2O2 ( /2 × 16 × 2) g/mol 44 g/mol

=

1

=

CO2 (12 + 16 × 2) g/mol 44 g/mol

By volume, air ∼ 21% O2 + 79% N2. Hence, 1 kmol of O2 is remains with 3.76 (= 79/21) kmol of N2. Considering the inertness of N2 in standard assumption of normal combustion: C12H23 + 18.75 (O2 + 3.76 N2) [12 × 12 + 1 × 23] [18.75(16 × 2 + 3.76 × g/mol 14 × 2)] g/mol 2741 g/mol

=

12CO2 + 11.5 H2O + [12(12 + 16 × 2)] (11.5(1 × 2 + 16)] g/mol g/mol = 2741 g/mol

O2 (16 × 2) g/mol

+

70.5N2 (70.5 × 14 × 2) g/mol

At high combustion temperature, N (14) g/mol

+

O2 (16 × 2) g/mol

46 g/mol

N2 (14 × 2) g/mol

+

=

=

3H2 (3 × 1 × 2) g/mol 34 g/mol

NO + (14 + 16) g/mol 46 g/mol

= =

■

O (16) g/mol

AUTHOR INFORMATION

Corresponding Author

*Tel.: +60 3 89466285. Fax: +60 3 86567120. Email: suraya_ar@ upm.edu.my.

2NH3 [2(14 + 1 × 3)] g/mol 34 g/mol

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Exploratory Research Grant Scheme (ERGS/1-2012/5527133), Ministry of Higher Education (MOHE), Malaysia.

From the above discussion, it can be seen that the mass conservation throughout the combustion process of nanoemulsion fuel is fully maintained. The mass balance equations K


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