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Preparation, characterization and engine performance of water in diesel nanoemulsions
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Preparation, characterization and engine performance of water in diesel nanoemulsions Q6
B.S. Bidita a, A.R. Suraya a, b, *, M.A. Shazed a, M.A.Mohd Salleh a, b, A. Idris a a
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 b
Q2
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2014 Received in revised form 8 March 2015 Accepted 9 March 2015 Available online xxx
Water in diesel (W/D) nanoemulsions were prepared by the aid of high energy emulsification method. The formulation was accomplished in the presence of Triton X-100 surfactant. A wide range of surfactant concentration (0.25%e0.40% v/v) with varying amount of water percentage (0.50%e0.90% v/v) was used in the preparation of W/D nanoemulsion fuels. The droplet size of the nanoemulsions at different water:surfactant:diesel ratio increased as surfactant concentration decreased. High kinetic stability was observed in the nanoemulsions. The stability of nanoemulsions with 0.40% surfactant concentration was persisted more than two weeks without phase separation. The droplet size of the nanoemulsions increased with time proving the influence of breakdown processes such as Ostwald ripening. Combustion characteristics of W/D nanoemulsions were studied in terms of different formulating compositions. An engine test bed of diesel engine was used to combust the nanoemulsions to study the exhaust emission concentrations such as CO, CO2, NH3 and NO, and performance parameters include brake power, thermal efficiency. The highest reduction in the exhaust gas emissions concentrations was notified by using surfactant concentration of 0.40% with 0.90% water content. The lowest calorific value of prepared W/D nanoemulsions was achieved 38.48 MJ/kg by using surfactant concentration of 0.40% with 0.90% water. The highest brake power and thermal efficiency was also obtained with 0.40% surfactant concentration and 0.90% water content. In addition, the characteristic evaluation of W/D nanoemulsions was made on the basis of emission characteristics of neat diesel. It has been observed that the use of W/D nanoemulsions in diesel engine has evidently led to the reduction in exhaust emissions, anticipating its application as an alternative eco-friendly fuel in the internal combustion engine. © 2015 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Water in diesel nanoemulsion Surfactant Stability Exhaust gas emission Calorific value Environmental fuel
1. Introduction Nanoemulsion is a type of emulsion systems which consists of nanoscale droplets [1]. The droplet size of nanoemulsion typically ranges from 20 to 500 nm [2,3]. Nanoemulsion can be transparent or translucent depending on the compositional ratio [4]. Both microemulsion and nanoemulsion comprise of three components such as water, surfactant and oil, however, the major difference between them is thermodynamic stability. Compared to microemulsions, nanoemulsions are thermodynamically unstable [5e7]. Nonetheless, due to the small droplet size in the nanoemulsions, they have high kinetic stability which makes them much steady against sedimentation and creaming. Kinetic stability of nanoemulsions is the manifestation of the interplay between surface forces and Brownian movement [8e10]. Presently, a growing interest is observed in the field of nanoemulsion fuel since it can be considered as an alternative eco-friendly fuel in the internal combustion engine. Apart from the fuel technology, nanoemulsified dispersions are mostly used in various applications such as in pharmaceutical field, in cosmetics, in agrochemicals etc [11].
* Corresponding author. Materials Processing and Technology Laboratory, Institute of Advanced Technology, University Putra Malaysia, Selangor 43400, Malaysia. Tel.: þ60 3 89466285; fax: þ60 3 86567120. Q1 E-mail address: [email protected] (A.R. Suraya). http://dx.doi.org/10.1016/j.joei.2015.03.004 1743-9671/© 2015 Published by Elsevier Ltd on behalf of Energy Institute.
Please cite this article in press as: B.S. Bidita, et al., Preparation, characterization and engine performance of water in diesel nanoemulsions, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.03.004
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In general, there are two distinct types of emulsion, oil-in-water type (O/W) and water-in-oil type (W/O). Oil-in-water possesses dispersed oil droplets in water whereas water-in-oil type contains dispersed water droplets in the oil phase, based upon the dispersion mechanism [12]. However, the basic method of dispersion is homogenization. Moreover, the emulsion can be used as an alternative fuel for diesel engine [13]. A potential advantage of W/D emulsion fuel is the simultaneous occurrence of micro-explosion and secondary atomization phenomenon, resulting in vigorous vaporization and better mixing of air, in turn increasing the combustion reaction in the engine chamber [14]. The consideration of nanoemulsions has been taken by researchers due to its fuel efficiency and the environmentally friendly impacts. Although the stability of the nanoemulsion system is minimal, it can be accentuated by surface-active agents [15]. In view of the fact that attractive interactions between the molecules of the two liquid phases in nanoemulsion are different. For this reason, an interfacial tension exists between two liquid phases. Surface-active agents or surfactants have the potential to reduce this interfacial tension significantly because of their molecular structures. They have non-polar hydrocarbon tails that favor to be in nonpolar liquids, for instance, oils, and polar head groups that prefer to exist in polar liquids, for example, water. Eventually, they adsorb at interfaces [16]. Nevertheless, the dispersion of the largest quantity of water with the smallest quantity of surfactant can generate emulsions with long-term stability [17]. Triton X-100 (iso-octylphenoxypolyethoxy ethanol) is used in the preparation of water in diesel (W/D) emulsions due to its high availability and inexpensiveness [18]. It is important to consider coalescence and Ostwald ripening which are two breakdown processes responsible to destabilize the nanoemulsions. Coalescence is a process in which the emulsion breaks up into bulk oil and water phases. This is generally governed by four different droplet loss mechanisms, i.e., Brownian flocculation, creaming, sedimentation flocculation and disproportion [19]. It acts as a driving force to facilitate the instability of nanoemulsion. Besides, smaller droplets form larger ones by molecular diffusion through the continuous phase in Ostwald ripening [20]. Therefore, it is necessary to apply extreme shear to rupture the droplets of nanoemulsions [16]. Ultrasonic vibration is a feasible option during the emulsification process for the preparation of W/D nanoemulsion as ultrasound has been found effective in terms of improving droplet size and energy efficiency [21]. At present, the diesel engine is the most powerful combustion engine and being vastly used all over the world. However, air pollution caused by incomplete fuel combustion in the diesel engine chamber has always been a major concern. The pollutants, such as unburnt hydrocarbon, carbon monoxide (CO), oxides of nitrogen (NOx), and smoke emissions, may result in severe health hazards to living beings and damage to the environment [22]. Because of the harmful global effects caused by the increase of carbon dioxide (CO2) in the atmosphere which is a decisive factor behind the Kyoto protocol of 1997, it is desirable to limit the increase of CO2 which could be possible by using environmentally friendly fuels in the transport and industrial sectors. The emissions of nitrogen oxides (NOx) and particulate matter are health hazards that are strictly regulated by legislation [23]. Some researchers have shown that the nitric oxide (NO) can be significantly reduced by nanoemulsion fuel. In addition, nanoemulsions can produce lower exhaust gas temperature which is favorable for combustion [24]. Water emulsified fuel are now being used in power plant engines for NOx control without affecting the maintenance costs as a standard engine design permits the addition of some 20% of water at full load [25,26]. Higher brake thermal efficiencies are noticed in emulsion fuels compared to that of neat diesel [27]. Ultrasonic emulsification is generally occurred through two mechanisms. Firstly, the use of an acoustic field generates interfacial waves which turn out to be unstable, thus causing the eruption of the water droplets into the oil [28]. Secondly, the use of low frequency ultrasound produces acoustic cavitations. Acoustic cavitation is the formation and subsequent collapse of micro bubbles through the pressure fluctuations of a simple sound wave. An extreme level of highly localized turbulence is caused by each bubble collapse (an implosion on a microscopic scale) event. The turbulent micro-implosions act as a useful method to break up primary droplets of dispersed oil into droplets of sub-micron size [29]. Engine Test Bed (SOLTEQ TH03) and the diesel engine (YANMAR L48N) were used to combust the nanoemulsion fuel in order to analyze the exhaust temperature and gas emissions. Gas monitors were used to measure the exhaust gases such as CO, CO2, NH3 and NO [30]. The main aim of this study is to prepare W/D nanoemulsion fuel with different surfactant concentrations using a high energy emulsification method. Triton X-100 was used as a surfactant to reduce the water/oil interfacial tension. Ultrasonic process was used in the preparation of W/D nanoemulsion fuel. The stability of the prepared nanoemulsions was evaluated. Exhaust temperature, emission of CO2, CO, NH3, NO, calorific value, brake power and thermal efficiency of combusted W/D nanoemulsion were also studied. All of the characteristic properties were compared to neat diesel.
2. Methods and materials 2.1. Materials Commercial diesel fuel (Petroleum Nasional Bhd., Malaysia) was utilized as the continuous phase of the nanoemulsion system. The physical properties of this diesel oil are summarized in Table 1. The surfactant used throughout the investigation namely Triton X-100 was obtained from Scharlab S.L, Spain. The water in all experiments was demineralized and doubly distilled.
Table 1 Physical properties of diesel fuel. Properties
Guaranteed Level
Test Method (ASTM)
Color Cetane index Pour point, C Flash point, C Kinematic viscosity @40 C, cSt Water by distillation, vol%
2.5 47 15 60 16-58 0.05
D1500/D6045 D976/D4737 D97/D5950 D93 D445 D95
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2.2. Preparation of W/D nanoemulsions W/D nanoemulsion fuel was prepared by pouring water (dispersed phase) into diesel (continuous phase) at 25 C by using an ultrasonic processor (ColeeParmer 500-W Ultrasonic Homogenizer, 115 VAC). Surfactant was added in different percentages (0.25%e0.40% v/v with 0.05% v/v increment) with varying amounts of water (0.5%e0.9% v/v by an increment of 0.1% v/v) into diesel fuel. Table 2 shows water, diesel and surfactant compositions in the preparation of W/D nanoemulsion. Each surfactant concentration was used with different percentages of water at 30% ultrasonic amplitude for 10 min processing time. All experiments were carried out at 25 C. Fig. 1 shows the schematic process flow diagram of the preparation method of W/D nanoemulsion. 2.3. Droplet size and stability analysis Dynamic light scattering (DLS) equipment (Zetasizer Nano-ZS 90, Malvern, UK) was used to determine the mean droplet size and to evaluate the stability of the prepared W/D nanoemulsion. DLS monitors the speed at which the particles are diffusing due to Brownian motion by recording the rate at which the intensity of the scattered light fluctuates [31]. All of the experimental evaluation process was performed at 25 C. Stability of the nanoemulsions was also determined by observing physical appearance within a limited time duration. 2.4. Exhaust emission analysis of prepared W/D nanoemulsions In order to analyze the exhaust gas emission, an engine Test Bed (Model: SOLTEQ TH03) and the diesel engine (Model: YANMAR L48N) were used to combust the nanoemulsion fuel. Engine test bed consists of self contained, compact and easily installed bench mounted unit, four stroke diesel engine, eddy current dynamometer and controller and fully instrumented for air and fuel flow, temperatures, speed and power. In engine test bed, the nanoemulsions and a conventional diesel fuel were tested upon pouring into the fuel tank. At 2500 rpm, the parameters used to combust the fuels were recorded during the time of test with constant load (100%). During the changing of fuels, the existing tank fuels were drained prior to filling them with the next fuel. The engine was then warmed with the new fuel for at least 30 min in order to purge any of the remaining previously tested fuel from the engine fuel system. Afterward, once the engine was ready with the new fuel, it ran nearly 15 min for the measurement of emission. The effect of density variation such as air temperature and fuel temperature of emulsions was displayed from engine test bed. Two gas monitors were used to measure the exhaust gas component concentrations; MultiRae IR (PGM-54, RAE Systems, USA) for CO, CO2 and Vrae (PGM-7800 and 7840, RAE Systems, USA) for NH3. NO emissions was determined by using a gas analyzer (Horiba, USA). The combustion characteristics of neat diesel were utilized to compare with the results found from W/D emulsified fuels. Specifications of diesel engine, gas monitors and gas analyzer are detailed in S1eS4. 2.5. 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 (Parr 1341). The measurement was followed by ASTM-D240:64, ASTM-D271:70, National Bureau of Standards Monograph No. 7 and Experimental Thermochemistry Measurement of Heats of Reaction. The heat obtained from the W/D
Table 2 Water, diesel and surfactant compositions in the preparation of W/D nanoemulsion. Triton X-100
Water
Diesel
(v/v %)
mL
%
mL
%
mL
0.25
0.375
0.30
0.450
0.35
0.525
0.40
0.600
0.5 0.6 0.7 0.8 0.9 0.5 0.6 0.7 0.8 0.9 0.5 0.6 0.7 0.8 0.9 0.5 0.6 0.7 0.8 0.9
0.75 0.9 1.05 1.20 1.35 0.75 0.9 1.05 1.20 1.35 0.75 0.9 1.05 1.20 1.35 0.75 0.9 1.05 1.20 1.35
99.25 99.13 99.05 98.95 98.85 99.20 99.10 99.00 98.90 99.80 99.15 99.05 98.90 98.86 99.73 99.10 99.00 98.90 98.88 98.70
148.9 148.7 148.6 148.4 148.2 148.8 148.6 148.5 148.4 148.2 148.7 148.6 148.4 148.3 148.1 148.6 148.3 148.4 148.2 148.1
Please cite this article in press as: B.S. Bidita, et al., Preparation, characterization and engine performance of water in diesel nanoemulsions, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.03.004
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Fig. 1. Schematic flow diagram representing the preparation method of W/D nanoemulsion.
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:
Hg ¼
ðTemperature riseÞ ðEnergy equivalent of standard benzoic acidÞ W ðWeight of W=D nanoemulsion fuelÞ
(1)
By multiplying Hg by 1.8, the heat of combustion is converted to calorific value (Btu per pound) which can be further converted to MJ/kg unit,
Calorific value ¼ Hg 1:8
(2)
2.6. Measurement of brake power and thermal efficiency In order to calculate brake power, it is important to evaluate barke on time. Brakeontime; t ¼ v=a:g
(3) 2
where, t is brake on time (s), v is test speed (m/sec), a is deceleration (g units) and g is acceleration due to gravity (m/sec ). Thermal efficiency is measured by following equation: h ¼ 3600PoweroutputðkWÞ=M:K
(4)
where, h is thermal efficiency, M is mass of fuel used per hour in kilograms and K is the calorific value of the fuel in kilojoules per kilograms. 3. Results and discussion 3.1. Droplet size and stability of nanoemulsion Fig. 2 shows the droplet size of W/D nanoemulsions for different surfactant concentration together with several water contents obtained using 30% amplitude in 10 min emulsification time. It was found that the smallest droplet size of W/D nanoemulsions was found to be 17 nm using surfactant concentration of 0.40% for 0.9% water content whilst the largest droplet size in W/D nanoemulsion was found to be 325 nm from 0.5% water content incorporated with 0.25% surfactant concentration. As the amount of water increased, the droplet size in W/D nanoemulsion decreased. The droplet size of nanoemulsion depends on the equilibrium between the droplet break up and coalescences process. The decrease in droplet size is possibly produced from the coalescences of the internal water drops with the diesel phase while some movement of water through the nanoemulsion apparently occurred as well [32]. The phenomenon demonstrates that water has an influence on the formation of specific droplet size in W/D nanoemulsions. To determine the stability of the nanoemulsion, surfactant concentration of 0.40% with varying amounts of water (0.5%, 0.6%, 0.7%, 0.8%, 0.9 v/v) were chosen. In this effort, W/D nanoemulsions were observed to be stable for more than two weeks (16 days) without any phase separation which indicates good stability behavior. However, Ostwald ripening is considered as the major Please cite this article in press as: B.S. Bidita, et al., Preparation, characterization and engine performance of water in diesel nanoemulsions, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.03.004
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Fig. 2. Droplet size (nm) of W/D nanoemulsion with different water content and surfactant concentrations (error bars denote the standard deviation of the measurement).
destabilization process for the nanoemulsions. To express Ostwald ripening rate (u), The Lifshitz-Slezovand Wagner (LSW) theory [33,34] is used: u ¼ dr3 =dt
(5)
Here, r is the average radius of the droplet and t is the storage time. As shown in Fig. 3(a), a graph of r3 as a function of time was plotted whereby a linear relationship between the cube of the droplet radius and time was found by using Eq. (5). The slope of each straight line in Fig. 3(a) indicates Ostwald ripening rate. Fig. 3(b) illustrates the size distribution by intensity which is proportional to the number of water nuclei producing in the nanoemulsion for 0.40% surfactant with varying amounts of water. The results indicate that as water percentage increases, droplet size decreases. Ostwald ripening rate for 0.40% surfactant concentration with different percentages of water are shown in Table 3 in which the highest Ostwald
Fig. 3. Representative plots of (a) the linear relationship of the droplet size (r3) as a function of time; and (b) size distribution by intensity for 0.40% surfactant with different percentages of water in W/D nanoemulsion at day 1.
Please cite this article in press as: B.S. Bidita, et al., Preparation, characterization and engine performance of water in diesel nanoemulsions, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.03.004
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B.S. Bidita et al. / Journal of the Energy Institute xxx (2015) 1e12 Table 3 Ostwald ripening rate (u) shown for different percentage of water using 0.40% surfactant concentration.
0.40% Surfactant
Water content, %
u, m3/s1
0.5 0.6 0.7 0.8 0.9
5.10 3.60 5.07 5.90 1.24
1027 1037 1036 1037 1036
ripening rate was found for 0.5% water whereas the lowest was achieved for 0.6% water. Ostwald ripening arises due to the decrease in the kinetics of nanoemulsions which was caused by the increases of droplet sizes in the nanoemulsions [24]. Table 3 shows the nanoemulsion fuel using 0.40% surfactant incorporated with 0.5% water content exhibited more stable nanoemulsion compared to other nanoemulsions, since a small mean droplet size and a large Ostwald ripening rate is required in order to formulate a stable emulsion [24].
3.2. . Evaluation of engine performance 3.2.1. Combustion properties of neat diesel of neat diesel An engine test bed was employed to characterize neat diesel in terms of its exhaust temperature, and emission concentrations as well as calorific value. Fig. 4 shows the outcomes of neat diesel which will be further utilized to compare with the nanoemulsion fuels. Diesel fuel produces power due to its atomization and mixing with air in the combustion chamber of the diesel engine. This produces high pressure in the chamber which contributes to the rapid increase of temperature and emission concentrations of exhaust gases [35]. 3.2.2. Exhaust temperature of W/D nanoemulsion Fig. 5 shows the variation of exhaust temperature for different ratios of water:surfactant:diesel. A slight anomaly was observed for surfactant concentration of 0.30% with 0.6% water, but a decreasing trend was found for the other amounts of surfactant containing nanoemulsions. The anomalies may have appeared due to the distribution of ununiformed water droplets in the nanoemulsion fuel. The lowest exhaust gas temperature was found to be 187 C while using a surfactant concentration of 0.4% with 0.9% water content. It shows that as water content increases, the exhaust temperature decreases. This might be possible due to the formation of finely dispersed water droplets by the rapid evaporation of water [36]. Thus, a relatively homogeneous nanoemulsion with more efficient combustion can be produced with optimum water:surfactant ratio. Although slight fluctuations were observed, in general there was an overall decrease in exhaust temperature obtained for all W/D nanoemulsions compared to the neat diesel. This was due to a heat sink effect which is caused by the presence of water in the nanoemulsions. A larger heat sink, a lower flame temperature, and a shorter burning time can occur for the nanoemulsions having greater water content due to the micro-explosion phenomenon from atomized droplets. The presence of water forms latent heat throughout the vaporization process. Accordingly, it reduces the rate of the increase in temperature of the droplet [37]. For the duration of this process, the explosion produces sufficient energy to emit torn droplets to a distance of several millimeters away from the spray boundary, breaking them into fine fuel droplets, leading to improved air/fuel mixing, atomization, and evaporation process, hence speeding up the flame propagation [38]. The change of water droplets in W/D nanoemulsion is shown in Fig. 6. Generally, the rapid vaporization of water droplets in the emulsion results in the micro-explosion phenomenon as water is more volatile than diesel. Explosion of the continuous hydrocarbon phase will be caused by this vaporization at a temperature much higher than the boiling point of water, which is referred to as the superheat limit temperature and that will help the atomization process [39e42]. 3.2.3. Exhaust gas concentration of W/D nanoemulsion Fig. 7 shows the emission concentrations of CO produced from the combustion of different water:surfactant:diesel ratios.
Fig. 4. Combustion and emission characteristics of neat diesel.
Please cite this article in press as: B.S. Bidita, et al., Preparation, characterization and engine performance of water in diesel nanoemulsions, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.03.004
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Fig. 5. Exhaust temperature from engine test bed for the combustion of nanoemulsions with different water:surfactant:diesel ratios.
Fig. 6. Water droplets in W/D nanoemulsion fuel (a) before, (b) during, and (c) after micro-explosion effect.
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Fig. 7. Emission concentrations of CO produced from the combustion of nanoemulsions with different water:surfactant:diesel ratios.
It is observed that all CO concentrations followed a decreasing trend with increase in water content and surfactant concentrations. Although 0.30% surfactant incorporated with 0.5% and 0.6% showed some anomalies, but overall a reduction in CO emissions was found using W/D nanoemulsions compared to neat diesel. A surfactant concentration of 0.4% for all water content exhibited the largest and relatively uniform decrease; approximately 85% decrease compared to the neat diesel. This is because a large extent of micro-explosions can be generated by the combustion of W/D nanoemulsion, leading to a larger degree of mixing of reactant mixture hence more efficient combustion. Water in nanoemulsion fuel supplies oxygen for the reaction, hydrogen is liberated that may make explosive mixtures with air. Therefore a larger amount of fuel can be burnt in a given amount of air [43]. The CO emissions are The CO emissions are generally reduced at full load due to this increased air-fuel ratio; hence a more complete combustion is achieved. Thus, the CO emission decreases as the air-fuel mixture ratio increases. The evaporation of water occurs due to its latent heat which has been absorbed from the system. The cylinder average temperature following injection and before ignition becomes lower as the water percentage increases. These results in a lower peak combustion temperature leading to an overall decrease in CO emissions obtained for all W/D nanoemulsions compared to the neat diesel. The formula for calculating CO oxygenating rate KCO is: CO þ OH$CO2 þ H;
(6)
KCO ¼ 6:76 1010 exp½T=1102cm3 =gmol According to this formula, there is a proportional relation between temperature and CO emissions. As the temperature is lower, the CO oxygenation rate also becomes lower; hence less CO emissions is produced. Furthermore, adding water in the fuel has an effect to increase the oxygen availability in the fuel, leading to better combustion efficiency, thereby causing lower CO emissions [44,45]. Fig. 8 shows the emission concentrations of CO2 produced from the combustion of different water:surfactant:diesel ratios. Despite having some variations in the data, the general observation was an overall reduction in CO2 emissions using W/D nanoemulsions compared to the neat diesel. The most significant decrease in CO2 emissions was achieved using surfactant concentration of 0.4% for a water content of 0.9%; a decrease of approximately 80% compared to the neat diesel. A possible reason of this decrease was the micro-explosions phenomenon
Fig. 8. Emission concentrations of CO2 produced from the combustion of nanoemulsions with different water:surfactant:diesel ratios.
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Fig. 9. Emission concentrations of NH3 produced from the combustion of nanoemulsions with different water: surfactant: diesel ratios.
during the combustion, which led to a larger degree of mixing of reactant mixture; therefore combustion efficiency was enhanced. It has been observed that the most favorable emissions results were found for surfactant concentration of 0.40%. Therefore, 0.40% surfactant can be considered as the optimum surfactant concentration. At normal combustion, N2 absolutely inert (no reaction) C12 H23 þ 18:75ðO2 þ 3:76N2 Þ ¼ 12CO2 þ 11:5H2 O þ O2 þ 70:5N2
(7)
At high combustion temperature, N þ O2 ¼ NO þ O
(8)
N2 þ 3H2 ¼ 2NH3
(9)
Particulate matter (PM), such as ammonium nitrate (NH4NO3) or ammonium sulfates (NH4SO4, NH4(SO4)2) can be produced by NH3. It can also produce NOx by further oxidation in the atmosphere. Thus, it is necessary to reduce NH3 from the exhaust emissions. Fig. 9 shows the emission concentrations of NH3 for different ratio of water:surfactant:diesel. There was an overall reduction in NH3 emissions using W/D nanoemulsions compared to the neat diesel. Approximately 40% decrease was found by using surfactant concentration of 0.4% with 0.9% water which was the largest decrease compared to other nanoemulsions. This is again due to the effect of micro-explosion during the combustion. Fig. 10 shows the emission concentrations of NO produced from the combustion of different water:surfactant:diesel ratios. Overall, there is a decrease in the NO emission using W/D nanoemulsions compared to neat diesel. As the water percentages increased, the NO emission decreased. The highest reduction was found for 0.40% surfactant with 0.9% water. This is possibly due to the evaporation of water that requires sensible and latent heat from the cylinder, thereby lowering the cylinder temperature leading to reduction in NO emission. 3.3. Calorific value Calorific value is typically referred to as the amount of heat produced from the complete combustion of a specific amount of fuel. Fig. 11 shows that as the amount of water was increased, the calorific value of the nanoemulsion fuel decreased.
Fig. 10. Emission concentrations of NO produced from the combustion of nanoemulsions with different water:surfactant:diesel ratios.
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Fig. 11. Calorific values of W/D nanoemulsions with different amount of surfactant concentrations.
Fig. 12. Brake power of W/D nanoemulsions with different amount of surfactant concentrations.
The surfactant concentration of 0.4% for a water content of 0.9% exhibited the largest decrease in calorific value. This decrease is probably due to the lesser amount of diesel fuel in the nanoemulsion formulations. Generally, nanoemulsion fuels contain water in their composition which absorbs heat for their vaporization process. Consequently, the vaporization of the water content of the nanoemulsion is caused by using some portion of calorific value [24,45,46]. Eventually, higher water content is responsible to generate lower calorific value.
Fig. 13. The thermal efficiency of W/D nanoemulsions with different surfactant concentrations.
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3.4. Brake power and thermal efficiency Fig. 12 shows the brake power produced from the combustion of different water:surfactant:diesel ratios. The brake power of neat diesel was found to be 13 KW. Although there are some fluctuations, but in general, there is a rise in the brake power using W/D nanoemulsions compared to neat diesel. This might be because of some factors such as finer dispersion of fuel droplets, injection timing, reaction time etc. in the combustion chamber. Some other reasons, for example, brake energy, brake on time are responsible for the increase in brake power. While the brake is applied (but rotating), energy is being dissipated in the brake system. The energy to each brake depends on the number of brakes and the proportion of braking on each axle. In addition, brake on time is important to get brake power which mainly depends on test speed, deceleration, and acceleration due to gravity. If the brake energy increase, brake power also increase [47,48]. Fig. 13 shows thermal efficiency of W/D nanoemulsions with different water:surfactant:diesel ratios. The thermal efficiency of neat diesel was found to be 21%. In overall, there is an increase in the thermal efficiency using W/D nanoemulsions compared to neat diesel. Thermal efficiency increased with increasing water content. This might be due to the smaller the amount of water, the finer the dispersion of fuel that causes higher contact with the air during the burning process, thereby increasing thermal efficiency [23]. 4. Conclusions W/D nanoemulsions have been prepared by high energy emulsification method. The droplet sizes in the nanoemulsions were formed in the range of 17e400 nm provide with different surfactant:water ratios. The nanoemulsions were stable more than two weeks (16 days) which indicated good fuel stability. The change in droplet size with time was attributed to Ostwald ripening. The most significant W/D nanoemulsion with optimum emission properties was found by using 0.4% surfactant concentration. Lower exhaust gas temperature and lower CO, CO2, NH3 and NO emissions was found by using W/D nanoemulsion fuel containing increasing amounts of water. Highest decrease was found in all emissions by using W/D nanoemulsions with surfactant concentration of 0.4% and 0.9% water content. Calorific value was also slightly reduced with the increase of water. Engine performance studies were conducted and the brake power and thermal efficiency was found to be higher in nanoemulsions compared to neat diesel. This work demonstrates that W/D nanoemulsions can be used as an alternative fuel in internal combustion engines without any prior modification. Acknowledgments The authors gratefully acknowledge the financial support of the Exploratory Research Grant Scheme (ERGS), Ministry of Higher EduQ3Q4 cation, Malaysia.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.joei.2015.03.004. References
Q5
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Preparation, characterization and engine performance of water in diesel nanoemulsions Q6
B.S. Bidita a, A.R. Suraya a, b, *, M.A. Shazed a, M.A.Mohd Salleh a, b, A. Idris a a
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 b
Q2
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2014 Received in revised form 8 March 2015 Accepted 9 March 2015 Available online xxx
Water in diesel (W/D) nanoemulsions were prepared by the aid of high energy emulsification method. The formulation was accomplished in the presence of Triton X-100 surfactant. A wide range of surfactant concentration (0.25%e0.40% v/v) with varying amount of water percentage (0.50%e0.90% v/v) was used in the preparation of W/D nanoemulsion fuels. The droplet size of the nanoemulsions at different water:surfactant:diesel ratio increased as surfactant concentration decreased. High kinetic stability was observed in the nanoemulsions. The stability of nanoemulsions with 0.40% surfactant concentration was persisted more than two weeks without phase separation. The droplet size of the nanoemulsions increased with time proving the influence of breakdown processes such as Ostwald ripening. Combustion characteristics of W/D nanoemulsions were studied in terms of different formulating compositions. An engine test bed of diesel engine was used to combust the nanoemulsions to study the exhaust emission concentrations such as CO, CO2, NH3 and NO, and performance parameters include brake power, thermal efficiency. The highest reduction in the exhaust gas emissions concentrations was notified by using surfactant concentration of 0.40% with 0.90% water content. The lowest calorific value of prepared W/D nanoemulsions was achieved 38.48 MJ/kg by using surfactant concentration of 0.40% with 0.90% water. The highest brake power and thermal efficiency was also obtained with 0.40% surfactant concentration and 0.90% water content. In addition, the characteristic evaluation of W/D nanoemulsions was made on the basis of emission characteristics of neat diesel. It has been observed that the use of W/D nanoemulsions in diesel engine has evidently led to the reduction in exhaust emissions, anticipating its application as an alternative eco-friendly fuel in the internal combustion engine. © 2015 Published by Elsevier Ltd on behalf of Energy Institute.
Keywords: Water in diesel nanoemulsion Surfactant Stability Exhaust gas emission Calorific value Environmental fuel
1. Introduction Nanoemulsion is a type of emulsion systems which consists of nanoscale droplets [1]. The droplet size of nanoemulsion typically ranges from 20 to 500 nm [2,3]. Nanoemulsion can be transparent or translucent depending on the compositional ratio [4]. Both microemulsion and nanoemulsion comprise of three components such as water, surfactant and oil, however, the major difference between them is thermodynamic stability. Compared to microemulsions, nanoemulsions are thermodynamically unstable [5e7]. Nonetheless, due to the small droplet size in the nanoemulsions, they have high kinetic stability which makes them much steady against sedimentation and creaming. Kinetic stability of nanoemulsions is the manifestation of the interplay between surface forces and Brownian movement [8e10]. Presently, a growing interest is observed in the field of nanoemulsion fuel since it can be considered as an alternative eco-friendly fuel in the internal combustion engine. Apart from the fuel technology, nanoemulsified dispersions are mostly used in various applications such as in pharmaceutical field, in cosmetics, in agrochemicals etc [11].
* Corresponding author. Materials Processing and Technology Laboratory, Institute of Advanced Technology, University Putra Malaysia, Selangor 43400, Malaysia. Tel.: þ60 3 89466285; fax: þ60 3 86567120. Q1 E-mail address: [email protected] (A.R. Suraya). http://dx.doi.org/10.1016/j.joei.2015.03.004 1743-9671/© 2015 Published by Elsevier Ltd on behalf of Energy Institute.
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In general, there are two distinct types of emulsion, oil-in-water type (O/W) and water-in-oil type (W/O). Oil-in-water possesses dispersed oil droplets in water whereas water-in-oil type contains dispersed water droplets in the oil phase, based upon the dispersion mechanism [12]. However, the basic method of dispersion is homogenization. Moreover, the emulsion can be used as an alternative fuel for diesel engine [13]. A potential advantage of W/D emulsion fuel is the simultaneous occurrence of micro-explosion and secondary atomization phenomenon, resulting in vigorous vaporization and better mixing of air, in turn increasing the combustion reaction in the engine chamber [14]. The consideration of nanoemulsions has been taken by researchers due to its fuel efficiency and the environmentally friendly impacts. Although the stability of the nanoemulsion system is minimal, it can be accentuated by surface-active agents [15]. In view of the fact that attractive interactions between the molecules of the two liquid phases in nanoemulsion are different. For this reason, an interfacial tension exists between two liquid phases. Surface-active agents or surfactants have the potential to reduce this interfacial tension significantly because of their molecular structures. They have non-polar hydrocarbon tails that favor to be in nonpolar liquids, for instance, oils, and polar head groups that prefer to exist in polar liquids, for example, water. Eventually, they adsorb at interfaces [16]. Nevertheless, the dispersion of the largest quantity of water with the smallest quantity of surfactant can generate emulsions with long-term stability [17]. Triton X-100 (iso-octylphenoxypolyethoxy ethanol) is used in the preparation of water in diesel (W/D) emulsions due to its high availability and inexpensiveness [18]. It is important to consider coalescence and Ostwald ripening which are two breakdown processes responsible to destabilize the nanoemulsions. Coalescence is a process in which the emulsion breaks up into bulk oil and water phases. This is generally governed by four different droplet loss mechanisms, i.e., Brownian flocculation, creaming, sedimentation flocculation and disproportion [19]. It acts as a driving force to facilitate the instability of nanoemulsion. Besides, smaller droplets form larger ones by molecular diffusion through the continuous phase in Ostwald ripening [20]. Therefore, it is necessary to apply extreme shear to rupture the droplets of nanoemulsions [16]. Ultrasonic vibration is a feasible option during the emulsification process for the preparation of W/D nanoemulsion as ultrasound has been found effective in terms of improving droplet size and energy efficiency [21]. At present, the diesel engine is the most powerful combustion engine and being vastly used all over the world. However, air pollution caused by incomplete fuel combustion in the diesel engine chamber has always been a major concern. The pollutants, such as unburnt hydrocarbon, carbon monoxide (CO), oxides of nitrogen (NOx), and smoke emissions, may result in severe health hazards to living beings and damage to the environment [22]. Because of the harmful global effects caused by the increase of carbon dioxide (CO2) in the atmosphere which is a decisive factor behind the Kyoto protocol of 1997, it is desirable to limit the increase of CO2 which could be possible by using environmentally friendly fuels in the transport and industrial sectors. The emissions of nitrogen oxides (NOx) and particulate matter are health hazards that are strictly regulated by legislation [23]. Some researchers have shown that the nitric oxide (NO) can be significantly reduced by nanoemulsion fuel. In addition, nanoemulsions can produce lower exhaust gas temperature which is favorable for combustion [24]. Water emulsified fuel are now being used in power plant engines for NOx control without affecting the maintenance costs as a standard engine design permits the addition of some 20% of water at full load [25,26]. Higher brake thermal efficiencies are noticed in emulsion fuels compared to that of neat diesel [27]. Ultrasonic emulsification is generally occurred through two mechanisms. Firstly, the use of an acoustic field generates interfacial waves which turn out to be unstable, thus causing the eruption of the water droplets into the oil [28]. Secondly, the use of low frequency ultrasound produces acoustic cavitations. Acoustic cavitation is the formation and subsequent collapse of micro bubbles through the pressure fluctuations of a simple sound wave. An extreme level of highly localized turbulence is caused by each bubble collapse (an implosion on a microscopic scale) event. The turbulent micro-implosions act as a useful method to break up primary droplets of dispersed oil into droplets of sub-micron size [29]. Engine Test Bed (SOLTEQ TH03) and the diesel engine (YANMAR L48N) were used to combust the nanoemulsion fuel in order to analyze the exhaust temperature and gas emissions. Gas monitors were used to measure the exhaust gases such as CO, CO2, NH3 and NO [30]. The main aim of this study is to prepare W/D nanoemulsion fuel with different surfactant concentrations using a high energy emulsification method. Triton X-100 was used as a surfactant to reduce the water/oil interfacial tension. Ultrasonic process was used in the preparation of W/D nanoemulsion fuel. The stability of the prepared nanoemulsions was evaluated. Exhaust temperature, emission of CO2, CO, NH3, NO, calorific value, brake power and thermal efficiency of combusted W/D nanoemulsion were also studied. All of the characteristic properties were compared to neat diesel.
2. Methods and materials 2.1. Materials Commercial diesel fuel (Petroleum Nasional Bhd., Malaysia) was utilized as the continuous phase of the nanoemulsion system. The physical properties of this diesel oil are summarized in Table 1. The surfactant used throughout the investigation namely Triton X-100 was obtained from Scharlab S.L, Spain. The water in all experiments was demineralized and doubly distilled.
Table 1 Physical properties of diesel fuel. Properties
Guaranteed Level
Test Method (ASTM)
Color Cetane index Pour point, C Flash point, C Kinematic viscosity @40 C, cSt Water by distillation, vol%
2.5 47 15 60 16-58 0.05
D1500/D6045 D976/D4737 D97/D5950 D93 D445 D95
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2.2. Preparation of W/D nanoemulsions W/D nanoemulsion fuel was prepared by pouring water (dispersed phase) into diesel (continuous phase) at 25 C by using an ultrasonic processor (ColeeParmer 500-W Ultrasonic Homogenizer, 115 VAC). Surfactant was added in different percentages (0.25%e0.40% v/v with 0.05% v/v increment) with varying amounts of water (0.5%e0.9% v/v by an increment of 0.1% v/v) into diesel fuel. Table 2 shows water, diesel and surfactant compositions in the preparation of W/D nanoemulsion. Each surfactant concentration was used with different percentages of water at 30% ultrasonic amplitude for 10 min processing time. All experiments were carried out at 25 C. Fig. 1 shows the schematic process flow diagram of the preparation method of W/D nanoemulsion. 2.3. Droplet size and stability analysis Dynamic light scattering (DLS) equipment (Zetasizer Nano-ZS 90, Malvern, UK) was used to determine the mean droplet size and to evaluate the stability of the prepared W/D nanoemulsion. DLS monitors the speed at which the particles are diffusing due to Brownian motion by recording the rate at which the intensity of the scattered light fluctuates [31]. All of the experimental evaluation process was performed at 25 C. Stability of the nanoemulsions was also determined by observing physical appearance within a limited time duration. 2.4. Exhaust emission analysis of prepared W/D nanoemulsions In order to analyze the exhaust gas emission, an engine Test Bed (Model: SOLTEQ TH03) and the diesel engine (Model: YANMAR L48N) were used to combust the nanoemulsion fuel. Engine test bed consists of self contained, compact and easily installed bench mounted unit, four stroke diesel engine, eddy current dynamometer and controller and fully instrumented for air and fuel flow, temperatures, speed and power. In engine test bed, the nanoemulsions and a conventional diesel fuel were tested upon pouring into the fuel tank. At 2500 rpm, the parameters used to combust the fuels were recorded during the time of test with constant load (100%). During the changing of fuels, the existing tank fuels were drained prior to filling them with the next fuel. The engine was then warmed with the new fuel for at least 30 min in order to purge any of the remaining previously tested fuel from the engine fuel system. Afterward, once the engine was ready with the new fuel, it ran nearly 15 min for the measurement of emission. The effect of density variation such as air temperature and fuel temperature of emulsions was displayed from engine test bed. Two gas monitors were used to measure the exhaust gas component concentrations; MultiRae IR (PGM-54, RAE Systems, USA) for CO, CO2 and Vrae (PGM-7800 and 7840, RAE Systems, USA) for NH3. NO emissions was determined by using a gas analyzer (Horiba, USA). The combustion characteristics of neat diesel were utilized to compare with the results found from W/D emulsified fuels. Specifications of diesel engine, gas monitors and gas analyzer are detailed in S1eS4. 2.5. 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 (Parr 1341). The measurement was followed by ASTM-D240:64, ASTM-D271:70, National Bureau of Standards Monograph No. 7 and Experimental Thermochemistry Measurement of Heats of Reaction. The heat obtained from the W/D
Table 2 Water, diesel and surfactant compositions in the preparation of W/D nanoemulsion. Triton X-100
Water
Diesel
(v/v %)
mL
%
mL
%
mL
0.25
0.375
0.30
0.450
0.35
0.525
0.40
0.600
0.5 0.6 0.7 0.8 0.9 0.5 0.6 0.7 0.8 0.9 0.5 0.6 0.7 0.8 0.9 0.5 0.6 0.7 0.8 0.9
0.75 0.9 1.05 1.20 1.35 0.75 0.9 1.05 1.20 1.35 0.75 0.9 1.05 1.20 1.35 0.75 0.9 1.05 1.20 1.35
99.25 99.13 99.05 98.95 98.85 99.20 99.10 99.00 98.90 99.80 99.15 99.05 98.90 98.86 99.73 99.10 99.00 98.90 98.88 98.70
148.9 148.7 148.6 148.4 148.2 148.8 148.6 148.5 148.4 148.2 148.7 148.6 148.4 148.3 148.1 148.6 148.3 148.4 148.2 148.1
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Fig. 1. Schematic flow diagram representing the preparation method of W/D nanoemulsion.
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:
Hg ¼
ðTemperature riseÞ ðEnergy equivalent of standard benzoic acidÞ W ðWeight of W=D nanoemulsion fuelÞ
(1)
By multiplying Hg by 1.8, the heat of combustion is converted to calorific value (Btu per pound) which can be further converted to MJ/kg unit,
Calorific value ¼ Hg 1:8
(2)
2.6. Measurement of brake power and thermal efficiency In order to calculate brake power, it is important to evaluate barke on time. Brakeontime; t ¼ v=a:g
(3) 2
where, t is brake on time (s), v is test speed (m/sec), a is deceleration (g units) and g is acceleration due to gravity (m/sec ). Thermal efficiency is measured by following equation: h ¼ 3600PoweroutputðkWÞ=M:K
(4)
where, h is thermal efficiency, M is mass of fuel used per hour in kilograms and K is the calorific value of the fuel in kilojoules per kilograms. 3. Results and discussion 3.1. Droplet size and stability of nanoemulsion Fig. 2 shows the droplet size of W/D nanoemulsions for different surfactant concentration together with several water contents obtained using 30% amplitude in 10 min emulsification time. It was found that the smallest droplet size of W/D nanoemulsions was found to be 17 nm using surfactant concentration of 0.40% for 0.9% water content whilst the largest droplet size in W/D nanoemulsion was found to be 325 nm from 0.5% water content incorporated with 0.25% surfactant concentration. As the amount of water increased, the droplet size in W/D nanoemulsion decreased. The droplet size of nanoemulsion depends on the equilibrium between the droplet break up and coalescences process. The decrease in droplet size is possibly produced from the coalescences of the internal water drops with the diesel phase while some movement of water through the nanoemulsion apparently occurred as well [32]. The phenomenon demonstrates that water has an influence on the formation of specific droplet size in W/D nanoemulsions. To determine the stability of the nanoemulsion, surfactant concentration of 0.40% with varying amounts of water (0.5%, 0.6%, 0.7%, 0.8%, 0.9 v/v) were chosen. In this effort, W/D nanoemulsions were observed to be stable for more than two weeks (16 days) without any phase separation which indicates good stability behavior. However, Ostwald ripening is considered as the major Please cite this article in press as: B.S. Bidita, et al., Preparation, characterization and engine performance of water in diesel nanoemulsions, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.03.004
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Fig. 2. Droplet size (nm) of W/D nanoemulsion with different water content and surfactant concentrations (error bars denote the standard deviation of the measurement).
destabilization process for the nanoemulsions. To express Ostwald ripening rate (u), The Lifshitz-Slezovand Wagner (LSW) theory [33,34] is used: u ¼ dr3 =dt
(5)
Here, r is the average radius of the droplet and t is the storage time. As shown in Fig. 3(a), a graph of r3 as a function of time was plotted whereby a linear relationship between the cube of the droplet radius and time was found by using Eq. (5). The slope of each straight line in Fig. 3(a) indicates Ostwald ripening rate. Fig. 3(b) illustrates the size distribution by intensity which is proportional to the number of water nuclei producing in the nanoemulsion for 0.40% surfactant with varying amounts of water. The results indicate that as water percentage increases, droplet size decreases. Ostwald ripening rate for 0.40% surfactant concentration with different percentages of water are shown in Table 3 in which the highest Ostwald
Fig. 3. Representative plots of (a) the linear relationship of the droplet size (r3) as a function of time; and (b) size distribution by intensity for 0.40% surfactant with different percentages of water in W/D nanoemulsion at day 1.
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B.S. Bidita et al. / Journal of the Energy Institute xxx (2015) 1e12 Table 3 Ostwald ripening rate (u) shown for different percentage of water using 0.40% surfactant concentration.
0.40% Surfactant
Water content, %
u, m3/s1
0.5 0.6 0.7 0.8 0.9
5.10 3.60 5.07 5.90 1.24
1027 1037 1036 1037 1036
ripening rate was found for 0.5% water whereas the lowest was achieved for 0.6% water. Ostwald ripening arises due to the decrease in the kinetics of nanoemulsions which was caused by the increases of droplet sizes in the nanoemulsions [24]. Table 3 shows the nanoemulsion fuel using 0.40% surfactant incorporated with 0.5% water content exhibited more stable nanoemulsion compared to other nanoemulsions, since a small mean droplet size and a large Ostwald ripening rate is required in order to formulate a stable emulsion [24].
3.2. . Evaluation of engine performance 3.2.1. Combustion properties of neat diesel of neat diesel An engine test bed was employed to characterize neat diesel in terms of its exhaust temperature, and emission concentrations as well as calorific value. Fig. 4 shows the outcomes of neat diesel which will be further utilized to compare with the nanoemulsion fuels. Diesel fuel produces power due to its atomization and mixing with air in the combustion chamber of the diesel engine. This produces high pressure in the chamber which contributes to the rapid increase of temperature and emission concentrations of exhaust gases [35]. 3.2.2. Exhaust temperature of W/D nanoemulsion Fig. 5 shows the variation of exhaust temperature for different ratios of water:surfactant:diesel. A slight anomaly was observed for surfactant concentration of 0.30% with 0.6% water, but a decreasing trend was found for the other amounts of surfactant containing nanoemulsions. The anomalies may have appeared due to the distribution of ununiformed water droplets in the nanoemulsion fuel. The lowest exhaust gas temperature was found to be 187 C while using a surfactant concentration of 0.4% with 0.9% water content. It shows that as water content increases, the exhaust temperature decreases. This might be possible due to the formation of finely dispersed water droplets by the rapid evaporation of water [36]. Thus, a relatively homogeneous nanoemulsion with more efficient combustion can be produced with optimum water:surfactant ratio. Although slight fluctuations were observed, in general there was an overall decrease in exhaust temperature obtained for all W/D nanoemulsions compared to the neat diesel. This was due to a heat sink effect which is caused by the presence of water in the nanoemulsions. A larger heat sink, a lower flame temperature, and a shorter burning time can occur for the nanoemulsions having greater water content due to the micro-explosion phenomenon from atomized droplets. The presence of water forms latent heat throughout the vaporization process. Accordingly, it reduces the rate of the increase in temperature of the droplet [37]. For the duration of this process, the explosion produces sufficient energy to emit torn droplets to a distance of several millimeters away from the spray boundary, breaking them into fine fuel droplets, leading to improved air/fuel mixing, atomization, and evaporation process, hence speeding up the flame propagation [38]. The change of water droplets in W/D nanoemulsion is shown in Fig. 6. Generally, the rapid vaporization of water droplets in the emulsion results in the micro-explosion phenomenon as water is more volatile than diesel. Explosion of the continuous hydrocarbon phase will be caused by this vaporization at a temperature much higher than the boiling point of water, which is referred to as the superheat limit temperature and that will help the atomization process [39e42]. 3.2.3. Exhaust gas concentration of W/D nanoemulsion Fig. 7 shows the emission concentrations of CO produced from the combustion of different water:surfactant:diesel ratios.
Fig. 4. Combustion and emission characteristics of neat diesel.
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Fig. 5. Exhaust temperature from engine test bed for the combustion of nanoemulsions with different water:surfactant:diesel ratios.
Fig. 6. Water droplets in W/D nanoemulsion fuel (a) before, (b) during, and (c) after micro-explosion effect.
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Fig. 7. Emission concentrations of CO produced from the combustion of nanoemulsions with different water:surfactant:diesel ratios.
It is observed that all CO concentrations followed a decreasing trend with increase in water content and surfactant concentrations. Although 0.30% surfactant incorporated with 0.5% and 0.6% showed some anomalies, but overall a reduction in CO emissions was found using W/D nanoemulsions compared to neat diesel. A surfactant concentration of 0.4% for all water content exhibited the largest and relatively uniform decrease; approximately 85% decrease compared to the neat diesel. This is because a large extent of micro-explosions can be generated by the combustion of W/D nanoemulsion, leading to a larger degree of mixing of reactant mixture hence more efficient combustion. Water in nanoemulsion fuel supplies oxygen for the reaction, hydrogen is liberated that may make explosive mixtures with air. Therefore a larger amount of fuel can be burnt in a given amount of air [43]. The CO emissions are The CO emissions are generally reduced at full load due to this increased air-fuel ratio; hence a more complete combustion is achieved. Thus, the CO emission decreases as the air-fuel mixture ratio increases. The evaporation of water occurs due to its latent heat which has been absorbed from the system. The cylinder average temperature following injection and before ignition becomes lower as the water percentage increases. These results in a lower peak combustion temperature leading to an overall decrease in CO emissions obtained for all W/D nanoemulsions compared to the neat diesel. The formula for calculating CO oxygenating rate KCO is: CO þ OH$CO2 þ H;
(6)
KCO ¼ 6:76 1010 exp½T=1102cm3 =gmol According to this formula, there is a proportional relation between temperature and CO emissions. As the temperature is lower, the CO oxygenation rate also becomes lower; hence less CO emissions is produced. Furthermore, adding water in the fuel has an effect to increase the oxygen availability in the fuel, leading to better combustion efficiency, thereby causing lower CO emissions [44,45]. Fig. 8 shows the emission concentrations of CO2 produced from the combustion of different water:surfactant:diesel ratios. Despite having some variations in the data, the general observation was an overall reduction in CO2 emissions using W/D nanoemulsions compared to the neat diesel. The most significant decrease in CO2 emissions was achieved using surfactant concentration of 0.4% for a water content of 0.9%; a decrease of approximately 80% compared to the neat diesel. A possible reason of this decrease was the micro-explosions phenomenon
Fig. 8. Emission concentrations of CO2 produced from the combustion of nanoemulsions with different water:surfactant:diesel ratios.
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Fig. 9. Emission concentrations of NH3 produced from the combustion of nanoemulsions with different water: surfactant: diesel ratios.
during the combustion, which led to a larger degree of mixing of reactant mixture; therefore combustion efficiency was enhanced. It has been observed that the most favorable emissions results were found for surfactant concentration of 0.40%. Therefore, 0.40% surfactant can be considered as the optimum surfactant concentration. At normal combustion, N2 absolutely inert (no reaction) C12 H23 þ 18:75ðO2 þ 3:76N2 Þ ¼ 12CO2 þ 11:5H2 O þ O2 þ 70:5N2
(7)
At high combustion temperature, N þ O2 ¼ NO þ O
(8)
N2 þ 3H2 ¼ 2NH3
(9)
Particulate matter (PM), such as ammonium nitrate (NH4NO3) or ammonium sulfates (NH4SO4, NH4(SO4)2) can be produced by NH3. It can also produce NOx by further oxidation in the atmosphere. Thus, it is necessary to reduce NH3 from the exhaust emissions. Fig. 9 shows the emission concentrations of NH3 for different ratio of water:surfactant:diesel. There was an overall reduction in NH3 emissions using W/D nanoemulsions compared to the neat diesel. Approximately 40% decrease was found by using surfactant concentration of 0.4% with 0.9% water which was the largest decrease compared to other nanoemulsions. This is again due to the effect of micro-explosion during the combustion. Fig. 10 shows the emission concentrations of NO produced from the combustion of different water:surfactant:diesel ratios. Overall, there is a decrease in the NO emission using W/D nanoemulsions compared to neat diesel. As the water percentages increased, the NO emission decreased. The highest reduction was found for 0.40% surfactant with 0.9% water. This is possibly due to the evaporation of water that requires sensible and latent heat from the cylinder, thereby lowering the cylinder temperature leading to reduction in NO emission. 3.3. Calorific value Calorific value is typically referred to as the amount of heat produced from the complete combustion of a specific amount of fuel. Fig. 11 shows that as the amount of water was increased, the calorific value of the nanoemulsion fuel decreased.
Fig. 10. Emission concentrations of NO produced from the combustion of nanoemulsions with different water:surfactant:diesel ratios.
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Fig. 11. Calorific values of W/D nanoemulsions with different amount of surfactant concentrations.
Fig. 12. Brake power of W/D nanoemulsions with different amount of surfactant concentrations.
The surfactant concentration of 0.4% for a water content of 0.9% exhibited the largest decrease in calorific value. This decrease is probably due to the lesser amount of diesel fuel in the nanoemulsion formulations. Generally, nanoemulsion fuels contain water in their composition which absorbs heat for their vaporization process. Consequently, the vaporization of the water content of the nanoemulsion is caused by using some portion of calorific value [24,45,46]. Eventually, higher water content is responsible to generate lower calorific value.
Fig. 13. The thermal efficiency of W/D nanoemulsions with different surfactant concentrations.
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3.4. Brake power and thermal efficiency Fig. 12 shows the brake power produced from the combustion of different water:surfactant:diesel ratios. The brake power of neat diesel was found to be 13 KW. Although there are some fluctuations, but in general, there is a rise in the brake power using W/D nanoemulsions compared to neat diesel. This might be because of some factors such as finer dispersion of fuel droplets, injection timing, reaction time etc. in the combustion chamber. Some other reasons, for example, brake energy, brake on time are responsible for the increase in brake power. While the brake is applied (but rotating), energy is being dissipated in the brake system. The energy to each brake depends on the number of brakes and the proportion of braking on each axle. In addition, brake on time is important to get brake power which mainly depends on test speed, deceleration, and acceleration due to gravity. If the brake energy increase, brake power also increase [47,48]. Fig. 13 shows thermal efficiency of W/D nanoemulsions with different water:surfactant:diesel ratios. The thermal efficiency of neat diesel was found to be 21%. In overall, there is an increase in the thermal efficiency using W/D nanoemulsions compared to neat diesel. Thermal efficiency increased with increasing water content. This might be due to the smaller the amount of water, the finer the dispersion of fuel that causes higher contact with the air during the burning process, thereby increasing thermal efficiency [23]. 4. Conclusions W/D nanoemulsions have been prepared by high energy emulsification method. The droplet sizes in the nanoemulsions were formed in the range of 17e400 nm provide with different surfactant:water ratios. The nanoemulsions were stable more than two weeks (16 days) which indicated good fuel stability. The change in droplet size with time was attributed to Ostwald ripening. The most significant W/D nanoemulsion with optimum emission properties was found by using 0.4% surfactant concentration. Lower exhaust gas temperature and lower CO, CO2, NH3 and NO emissions was found by using W/D nanoemulsion fuel containing increasing amounts of water. Highest decrease was found in all emissions by using W/D nanoemulsions with surfactant concentration of 0.4% and 0.9% water content. Calorific value was also slightly reduced with the increase of water. Engine performance studies were conducted and the brake power and thermal efficiency was found to be higher in nanoemulsions compared to neat diesel. This work demonstrates that W/D nanoemulsions can be used as an alternative fuel in internal combustion engines without any prior modification. Acknowledgments The authors gratefully acknowledge the financial support of the Exploratory Research Grant Scheme (ERGS), Ministry of Higher EduQ3Q4 cation, Malaysia.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.joei.2015.03.004. References
Q5
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