Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Effect of Nanosilver/Water-in-Kerosene Emulsion on NOx Reduction and Enhancement of Thermal Characteristics of a Liquid Fuel Burner S. H. Pourhoseini*,† and R. Esmaeeli† †
Department of Mechanical Engineering, Faculty of Engineering, University of Gonabad, Gonabad, Iran ABSTRACT: This study investigated the effect of nanosilver/water-in-kerosene emulsion on thermal and radiative characteristics and pollutant emissions of a liquid burner. The results were compared with the corresponding values resulted from neat kerosene. Light microscopic imaging was used to study microexplosion of the fuel droplets. Also, combining chemiluminescence technique and measurement of pollutant concentrations, a qualitative study was carried out to find out the effect of CO, CO2, water vapor, and intermediate soot particles on the flame emissivity coefficient. The results showed that the Brownian motion of silver nanoparticles and localized convection between nanoparticles and the adjacent fluids increased the evaporation rate of nanoemulsion droplets and the rate of their mixing with oxidizing air. Besides the nanoparticles effect, the secondary injection of nanoemulsion due to microexplosion of water droplets enhanced the mixing rate of nanoemulsion fuel and oxidizing air. Although absorption of heat by water content of nanoemulsion fuel decreased the flame temperature, the nanoemulsion fuel, compared with neat kerosene, enhanced the average radiation of flame by 23%. Further, the results indicated that although CO and CO2 did not change significantly in amount, NOx emission reduced as much as 23.7% when the nanosilver/water-in-kerosene emulsion, instead of neat kerosene, was used.
1. INTRODUCTION Global decline in oil production and climate change due to pollutant emission are among the most important problems of liquid fossil fuels.1−4 Using renewable energy resources5 such as solar energy,6−8 wind energy,9−12 and hydroelectric power13,14 is a strategy used to decrease the consumption rate of fossil fuels. Hydrogen has been referred to as another potential energy source.15,16 However, there are significant economical and technological challenges involved in hydrogen storage, transportation, and utilization, which restrict the use of the fuel.17,18 The essential disadvantages of the other sources of energy, some of which are mentioned above, have caused fossil fuels, especially liquid fossil fuels, to be still among the dominant sources of energy in the modern world.19,20 Therefore, in the recent years, many companies and research projects, with the purpose of reaching higher efficiency and lower pollutant emissions, have been seeking methods to enhance the performance of devices which make use of liquid fossil fuels.21−24 Nanoparticles are small-scale particles of metals, oxides, carbides, nitrides, or carbon nanotubes with their typical size being smaller than 100 nm which, due to high surface area and catalytic activity, have recently attracted great interest.25,26 Moreover, the physical properties of nanofluids can be extremely different from those of base fluids. For instance, nanofluids significantly increase the random Brownian motion of nanoparticles in a liquid, which results in the enhancement of thermal conductivity.27,28 Consequently, the addition of low-concentration nanoparticles to conventional liquid hydrocarbon fuels is a novel favorable method to improve physical, ignition, and combustion properties of the fluids. Fangsuwannarak and Triratanasirichai29 investigated physical properties of palm oil biodiesel and diesel in the presence of TiO2 nanoparticles as an additive. They observed that the calorific value was increased as a result of increasing the concentration of nanoadditives. However, raising the concen© XXXX American Chemical Society
tration of nano-TiO2 particles decreased the fuel kinematic viscosity. In a similar study, following the addition of zinc nanoparticles into diesel−pomoplion stearin wax biodiesel blends, improvement in calorific value and increase in cetan number was observed.30 Imdadul et al.31 carried out a comprehensive review on fuel additive effects on combustion behavior in CI engine fuelled with diesel biodiesel blends and reported increase in heat release rate, cylinder gas pressure, and peak pressure and reduction in ignition delay. Sajeevan and Sajith32 studied the effect of cerium oxide nanoparticles as an additive to diesel fuel on brake thermal efficiency and pollutant emissions of CI engine and observed an increase in brake thermal efficiency and decrease in NOx and HC as a result of using nanofuel. Sadhik and Anand33 did an experimental study on performance and pollutant emissions of a CI engine using Al2O3 nanoadditives. They found that the use of nanoadditives reduced the emissions of NOx, HC, and smoke and intensified the CO pollutant emission. Furthermore, the brake thermal efficiency increased in the presence of nanoadditives. Control of emissions of NOx, HC, and CO from diesel engine using nanoadditives is the primary aim set in the literature. Some of the papers report that nanoadditives decrease NOx emission,34−36 and some others mention an increase in NOx emission.37−40 They believe that the increase is probably due to the catalyst effect of nanoparticles in the combustion process. Also, the studies concerning the effect of nanoparticle additives on CO emission show that nanoparticles may increase or decrease the CO emission.41−43 Besides being used as additives in diesel engines, nanoparticles can ignite and burn themselves alone. Although there is still no agreement on the mechanisms Received: October 6, 2017 Revised: November 23, 2017
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DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. AFM image of synthesized silver nanoparticles. citrate (35 mM, 10 mL) was immediately added to the flask. It took a few minutes for the solution to gradually turn yellow, which was a sign of the formation of Ag nanoparticles. The solution was kept boiling for an additional 6 min. Then, the heating mantle was removed, and the solution was allowed to cool to produce the silver−water nanofluid of the concentration of 100 ppm (100 mg/L). Figure 1 shows the AFM picture of the synthesized solution of nanoparticles. The average size of the synthesized silver nanoparticles was 16.2 nm. 2.2. Production of Nanosilver/Water-in-Kerosene Emulsion Fuel. Nanoemulsion fuel is composed of kerosene as the base fuel, silver−water nanofluid, and surfactant. The chemical composition of kerosene as determined by a CHNS elemental analyzer VARIO ELCUBE is given in Table 1.49
of combustion of nanoparticles, a rough estimation based on the conventional d2 law demonstrated that a 100 nm nanoparticle could be burned within tens of nanoseconds in the absence of passivation oxide layers.44 Therefore, energetic nanoparticles such as boron, aluminum, and iron nanoparticles have been used as solid fuel additives for enhanced propulsion, airbag igniters/boosters, and aerial decoy flares.45,46 As mentioned above, most of the reviewed studies have focused on using nanoparticles as additives to diesel fuel to enhance engine performance and reduce pollutant emissions. However, besides engines, liquid fossil fuels have widespread applications in industrial liquid burners while, from the experimental point of view, the effects of nanoparticles on liquid fuel flame characteristics, including flame structure, thermal and radiation characteristics, and pollutant emissions, have been rarely studied. Liquid burners use spray injectors to atomize liquid fossil fuels into tiny droplets. In such spray combustion systems, the mean diameter of droplets is generally on the micrometer scale. Therefore, when nanoparticles are added to liquid fuels used in the systems, the droplets of fuel have a number of nanoparticles inside them. Solid nanoparticles have better thermal characteristics than pure liquid fossil fuel droplets do. Therefore, they can change the evaporation rate and mixing process of droplets, combustion characteristics, and pollutant emissions. Furthermore, when nanoparticles are dispersed in water and the resulted nanofluid is added to liquid fossil fuel, it is expected that, due to the volatility difference of water and fuel droplets, occur microexplosion and secondary atomization phenomena occur. These two phenomena, besides nanoparticles characteristics, can improve the quality of the mixing of liquid fuel and oxidizing air in a liquid burner and change the flame thermal characteristics and pollutant emission rate. The present study investigates how using nanosilver/water-in-kerosene emulsion in a liquid burner affects flame structure, thermal and radiation characteristics, and NOx emission.
Table 1. Chemical Composition of Kerosene Determined by CHNS Elemental Analyzer C (%) H (%) N (%) S (%) density (kg/m3) viscosity (mPa·s)
84.475 15.435 0.090 0.000 786 1.51
All glassware was washed in a mixture of distilled water and nonionic detergent, which was followed by rinsing with distilled water and ethanol many times to make sure of removing any remains of the nonionic detergent. The glassware was afterward dried to be ready for use. To prepare the nanosilver/water-in-kerosene emulsion, 100 mL of silver−water nanofluid of the concentration of 100 ppm (see the previous section) was mixed with 6.35 mL of Tween 80 (Merck/ Germany) through a process of magnetic stirring at 500 rpm for 30 min. In the next step, 33.65 mL of Span 80 (Fluka) and 1860 mL of kerosene were, in turn, added to the mixture, which was stirred for 10 min. Tween 80 and Span 80 are well-known surfactants, which promote the chemical stability of the suspension.50,51 Finally, for complete homogenization of the suspension, a 180 W supersonic homogenizer was used. The solution turned cream in color after the process (see Figure 2). It is worth mentioning that if the water content of emulsified fuel increases, the flame will be unstable. Furthermore, in such a case, it is difficalt to create a homogeneous and stable emulsified fuel composed of kerosene and nanosilver water. 2.3. Experimental Setup. Experiments were done on a laboratory cylindrical furnace which was 1200 mm long and 500 mm in diameter (Figure 3). The body of the furnace was made of high-temperature resistant steel AISI316. Five measuring holes, each with an i.d. of 2.5 cm, were located all along the furnace wall to measure temperature and
2. EXPERIMENTAL DETAILS 2.1. Synthesis of Silver−Water Nanofluid. Silver nanoparticles were produced by a chemical reduction method.47,48 In this method, AgNO3 (17.0 mg) was dissolved in 100 mL of water in a 250 mL trineck flask. The solution was heated to boiling point with a hemisphere heating mantle under vigorous magnetic stirring. When the solution was boiled for 2 min, an aqueous solution of sodium B
DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
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flame radiation heat flux. It can sense thermal radiations of wavelengths shorter than 50 μm and intensities of up to 100 KW/ m2. Also, to avoid the interfering radiation emitted from furance walls, the radiation heat flux was measured outside the furnace. Thermal radiation consists of visible and infrared (IR) radiation. In combustion research, the visible wavelengths emitted from a flame are known as luminosity. To obtain the luminosity of the flame, we used a TES1332A digital luminance meter. The luminance meter has spectral sensitivity close to the CIE photopic curve. Further, the flame photography technique was used to visualize in colors the dominant visible spectra in the flame picture. A KIGAS 310 gas analyzer (KIMO Instrument Company) was used to obtain the concentrations of CO, CO2, and NOx emission at the exhaust of the furnace. The accuracies of CO and NO measurement were ±10 and ±5 ppm, respectively. To make sure of the accuracy of the results, all tests were done twice, and, for all the measurements, the expanded uncertainties based on the accuracy of the equipment and repeatability of experiment were calculated by 95% confidence level and reported in the related figures in the “Results and Discussion” section.
3. RESULTS AND DISCUSSION Figure 4 shows the light microscopy image of the nanosilver/ water-in-kerosene emulsion fuel. It can be seen that all the droplets are smaller than 100 μm in diameter and the nanoemulsion fuel is suitably homogeneous. The nanoemulsion fuel dominantly consists of the three components of kerosene droplets, water droplets, and silver nanoparticles dispersed in the water. From microscopic observation (Figure 4), it can be seen that the water droplets are smaller than kerosene droplets and every nanoemulsion droplet consists of a kerosene droplet inside which a number of submicrometer water droplets are trapped. Since the boiling temperature of water is lower than that of kerosene, this pattern of the distribution of nanoemulsion droplets has been shown to be effective in the occurrence of microexplosion and secondary injection. Furthermore, if the sizes of the droplets are compared with the average size of the synthesized silver nanoparticles, resulting from what is shown in the AFM picture in Figure 1, it can be seen that the silver nanoparticles are significantly smaller than both water and kerosene droplets. Therefore, a large number of silver nanoparticles are inside the nanoemulsion droplets. Silver nanoparticles have higher absorption coefficients than water and kerosene do. Therefore, they absorb heat from the flame reaction zone faster than water and kerosene droplets do, and the rate of temperature rise of silver nanoparticles will be greater than that of the adjacent fluids. Consequently, since there are temperature differences between nanoparticles and adjacent fluids and silver nanoparticles have Brownian motion, probable localized convection can occur between nanoparticles and adjacent fluids. This will disperse the absorbed heat energy to the base fluid and thus result in elevated droplet temperature, which enhances the evaporation rate of nanoemulsion fuel droplets in comparison with neat kerosene and also strengthens the microexplosion of water droplets inside the kerosene droplets. In Figure 5, the flame image for neat kerosene and nanoemulsion fuel is depicted. It is seen that nanoemulsion fuel, in comparison with neat kerosene, shrinks the visible region of the flame. In diffusion flame, the mixing rate of fuel and oxidizing air determines the rate of combustion reaction and the visible flame region, which, in turn, specifies the boundary of the flame reaction zone. Therefore, the enhancement of the mixing process shrinks the flame reaction zone and visible flame region. As mentioned above in the case of Figure 4, the existence of silver nanoparticles, whose absorption
Figure 2. Nanosilver/water-in-kerosene emulsion fuel. radiation heat flux. They were 5, 10, 15, 20, and 80 cm distant from the furnace inlet. A liquid fuel burner with the maximum heat capacity of 120 000 kcal/h was selected and installed in front of the furnace to produce a diffusion flame by maintaining separate supplies of the fuel and the oxidant. Liquid fuel was sprayed by a hollow-cone nozzle embedded in the center of the exit section of the burner when air stream flowed around the droplets. The pressure of injection was 10 bar, and the spray cone angle was 60°. The volume flow rate of fuel injection was 2.4 L/h. A standard DT-619 anemometer was used to measure the volume flow rate of combustion air. The volume flow rate of combustion air was 2.01 m3/min. The air velocity ranged between 0.4 and 30 m/s, and the accuracy of the anemometer was ±(3% of the read velocity + 0.2 m/s). An S-type thermocouple was used to gauge the axial flame temperature. The maximum operating temperature and accuracy of the thermocouple were 1600 and ±2.5 °C, respectively. Furthermore, an SBG01 water-cooled heat flux sensor measured the C
DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. Schematic diagram of the furnace and experimental setup.
increases the mixing rate of nanoemulsion fuel droplets and oxidizing air and decreases the flame reaction zone and visible flame region. It is worthy of mention that nanoemulsion fuel will disperse the absorbed heat energy more quickly and easily than neat kerosene does. The reason has to be sought in the temperature difference between the silver nanoparticles and adjacent liquid phase, which results in Brownian motion of nanoparticles and localized convection between nanoparticles and the adjacent fluids. This, in comparison with what happens in neat kerosene, enhances the evaporation rate and mixing process of nanoemulsion fuel droplets and reduces the flame reaction zone and visible region of flame. Also, besides the effect of silver nanoparticles on evaporation rate and mixing process, the volatility difference between water and kerosene droplets results in microexplosion of water droplets inside the kerosene droplets. Consequently, there occurs a secondary atomization that results in a spray composed of droplets much finer than the droplets produced in the initial spray by the injection nozzle. Since small droplets take advantage of easier, better, and more complete mixing process than large droplets do, the microexplosion and secondary atomization also improve the mixing process and make the flame reaction zone small. The axial temperature profile for neat kerosene and nanoemulsion fuel is shown in Figure 6. It is seen that for both fuel flames, there exists a peak in temperature profile, which is a remarkable characteristic of diffusion flames. However, the maximum measured temperature of nanoemulsion fuel flame is 111 K smaller than that of neat kerosene flame. The reason behind this is that the water content of nanoemulsion fuel increases the specific heat capacity of the fuel. Consequently, a higher amount of heat energy required for water evaporation process is absorbed from the flame, which decreases the flame temperature. Further, for nanoemulsion fuel, the maximum temperature is shifted to the flame upstream region, which is a result of the enhancement of the mixing rate of fuel and oxidizing air (see the explanation of Figure 5). Moreover, it is worth mentioning that, unlike energetic nanoparticles such as Al nanoparticles, nonenergetic and noncombustible silver nanoparticles do not ignite and burn in the flame and only participate in heat transfer by absorbing and scattering heat energy.
Figure 4. Light microscopy image of nanosilver/water-in-kerosene emulsion fuel.
Figure 5. Flame image of neat kerosene (a) and nanosilver/water-inkerosene emulsion fuel (b).
coefficient is higher than those of liquid phases, inside the nanoemulsion fuel droplets enhances the heat absorption by and evaporation rate of droplets. Therefore, nanoemulsion fuel droplets evaporate faster than neat kerosene droplets, which D
DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 6. Axial flame temperature for neat kerosene and nanosilver/ water-in-kerosene emulsion fuel.
Figure 8. CO emissions for neat kerosene and nanosilver/water-inkerosene emulsion fuel.
In Figure 7, the radiation heat flux on the furnace wall is compared for both fuels of neat kerosene and nanosilver/water-
Figure 9. Concentration of CO2 at the exhaust of the furnace for neat kerosene and nanosilver/water-in-kerosene emulsion fuel. Figure 7. Radiation flux on the furnace wall for neat kerosene and nanosilver/water-in-kerosene emulsion fuel.
trations of CO and CO2. Also, from the environmental point of view, in both cases the CO emissions are smaller than the standard value (150 ppm). Furthermore, since the concentration of emitted unburned hydrocarbon (UHC) was negligible, it is found from carbon balance that the rate of soot emission is almost the same for both cases. Consequently, water vapor is the only species which can effectively contribute to the enhancement of radiation heat transfer. It is well-known that water has significant effect on radiative heat transfer process through absorption, scattering, and emission of thermal radiation.54,55 Therefore, any changes in the water vapor concentration within a furnace can change the radiation heat flux value. As mentioned above, a great amount of flame heat energy was absorbed by the silver nanoparticles while the liquid phases absorb very little amount. The strong absorption of heat energy by silver nanoparticles results in elevated droplet temperature and enhances evaporation rate of water content of nanoemulsion fuel. This increases the concentration of H2O in the flame reaction zone and, as a result, the radiation heat transfer from flame. As mentioned in the previous paragraph, from carbon balance, it is found that the rate of soot emission is almost the same for both neat kerosene and nanoemulsion fuels. For
in-kerosene emulsion. It can be seen that the radiation heat flux for nanoemulsion fuel is significantly greater than that of neat kerosene. In other words, the average flame radiation of nanoemulsion fuel is 23% greater than that of neat kerosene. The temperature, emissivity coefficient, and volume (of reaction zone) of flame are three parameters that affect the radiation of flame. Based on the results from Figures 5 and 6, flame reaction zone and temperature in the case of nanoemulsion fuel are smaller and lower than those of neat kerosene. Therefore, these two factors tend to decrease the radiation rate of nanoemulsion fuel flame. This leads us to the conclusion that the enhancement of radiation of flame in the case of nanoemulsion fuel is a result of significant increase in flame emissivity coefficient. CO2, water vapor (H2O), and intermediate soot particles along with polar CO molecules are the most important radiative species in the flame.52,53 Figures 8 and 9 illustrate the concentrations of CO and CO2 emissions at the exhaust of the furnace, compared with the corresponding emissions from neat kerosene. As seen, in comparison with neat kerosene, nanoemulsion fuel does not significantly change the concenE
DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels further explanation, luminosity measurement can be used in combination with chemiluminescence approach to determine and compare the radiation of intermediate soot particles in the flame. Figure 10 illustrates the luminosity of flame for both neat
Figure 11. NOx emissions for neat kerosene and nanosilver/water-inkerosene emulsion fuel.
the rate of radiation heat transfer of flame. Since radiation heat transfer is the fastest and most important method of heat transfer from flame, it rapidly discharges the heat energy of the flame leaving no hot-spot region in the flame. This decreases the rate of NOx formation.
Figure 10. Luminosity of flame for neat kerosene and nanosilver/ water-in-kerosene emulsion fuel.
kerosene and nanoemulsion fuels. The sum of visible spectral intensities emitted from all the flame species is called luminous radiation or luminosity, which can be measured by means of a luminance meter. The blue color of a flame is attributed to CO2 and H2O gaseous species while intermediate soot particles, when burning in the flame, create a yellow color. Therefore, if we use a luminance meter which is sensitive to yellow spectrum, the instrument indication will serve as a measure of intermediate soot concentration in the flame. The luminance meter used in the measurements has spectral sensitivity close to the CIE photopic curve. The CIE photopic luminosity function has low sensitivity to blue spectrum. In addition, the yellow spectrum has the greatest impact on CIE photopic luminosity function. Consequently, the measured luminosity is only related to intermediate soot particles. It can be seen that neat kerosene and nanoemulsion fuels result in values of the luminosity of flame that nearly fit. Therefore, the concentration of intermediate soot particles and their contribution to flame radiation are the same for both fuels. In Figure 11, the concentration of NOx emission at the exhaust of the furnace, which was corrected to 8% O2 in the flue gas, is shown for both fuels of neat kerosene and nanosilver/ water-in-kerosene emulsion. It can be seen that the use of nanoemulsion fuel, compared with neat kerosene, reduces the NOx emissions as much as 23.7%. Thermal NO, prompt NO, and fuel NO are three well-known mechanisms of NOx generation. Among the above mechanisms, prompt NO and fuel NO are important in the rich combustion regime. However, the present study is in the lean combustion regime (equivalence ratio of ϕ = 0.50). Therefore, thermal NO is expected to be the most important mechanism of NOx generation. Thermal NOx formation is strongly dependent on flame temperature and accelerates exponentially at high temperatures. Therefore, the first reason for lower concentration of NOx emission in the case of nanoemulsion fuel is the reduction of flame temperature due to heat absorption demanded by water evaporation (see Figure 6). Furthermore, as shown in Figure 7, the use of nanoemulsion fuel enhances
4. CONCLUSION The present study investigated the structure, thermal, and radiative characteristics and pollutant emissions of a liquid fuel burner when nanosilver/water-in-kerosene emulsion fuel is in use. The main findings are as follows: • Higher absorption coefficient of silver nanoparticles in comparison with water and kerosene increases the rate of temperature rise of particles. It also creates the Brownian motion of silver nanoparticles and localized convection between nanoparticles and the adjacent fluids. • The Brownian motion and localized convection enhance the evaporation rate of nanoemulsion fuel droplets and enhance the mixing rate of droplets and oxidizing air. • Besides the nanoparticles effect, the secondary injection of nanoemulsion fuel, through microexplosion of water droplets, enhances the mixing rate of fuel and oxidizing air. • Although nanoemulsion fuel decreases the flame temperature, it enhances the average radiation of flame as much as 23% in comparison with when neat kerosene is used. • The NOx emission at the exhaust of the furnace reduces as much as 23.7% when, instead of neat kerosene, the nanoemulsion fuel is used.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
S. H. Pourhoseini: 0000-0003-0682-1718 Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.7b02981 Energy Fuels XXXX, XXX, XXX−XXX
