Enhancement of radiation characteristics and reduction of NOx emission in natural gas flame through silver-water nanofluid injection

Journal Pre-proof Enhancement of radiation characteristics and reduction of NOx emission in natural gas flame through silver-water nanofluid injection

S.H. Pourhoseini PII:

S0360-5442(20)30007-4

DOI:

https://doi.org/10.1016/j.energy.2020.116900

Reference:

EGY 116900

To appear in:

Energy

Received Date:

24 October 2019

Accepted Date:

02 January 2020

Please cite this article as: S.H. Pourhoseini, Enhancement of radiation characteristics and reduction of NOx emission in natural gas flame through silver-water nanofluid injection, Energy (2020), https://doi.org/10.1016/j.energy.2020.116900

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Journal Pre-proof

Enhancement of radiation characteristics and reduction of NOx emission in natural gas flame through silver-water nanofluid injection S.H. Pourhoseini1 Department of Mechanical Engineering, Faculty of Engineering, University of Gonabad, Gonabad, Iran.

Abstract The aim of the present work is to investigate the effect of low-flow-rate injection of silver-water nanofluid on the enhancement of thermal and radiative characteristics and reduction of NOx emission in natural gas flame. In our method, 0.009 Lit/min of 100ppm silver-water nanofluid is directly injected into the natural gas flame at the equivalence ratio of 0.65. An SBG01 water cooled heat flux sensor is used to measure the total radiation of flame and IR photography method and luminosity measurement are used to determine the spectral emissivity of flame in infrared and visible wavelengths. Furthermore, a KIGAS 310 gas analyzer measures the concentrations of pollutant emissions. The results indicate that nanofluid injection enhances the IR radiation of natural gas flame and creates a high-value and uniform radiative heat flux distribution for flame. Solid nanoparticles, as foreign surfaces, enhance the nucleation rate and surface growth of soot particles, which are the best radiative species in flame. The other parameters which affect the flame radiation are increase in H2O concentration, as another radiative species, and increased number of tiny high temperature dark nanoparticles, as highly emissive bodies. Furthermore, although the nanofluid injection does not significantly change the concentration of carbonaceous pollutant emission, it decreases the NOx emission as much as 22.6%. The reduction of NOx emission is caused by decrease in the maximum flame temperature, due to absorption of heat by nanofluid droplets, and increase in the concentration of OH radicals, due to dissociation of H2O at the high temperatures of flame reaction zone.

Key Words: Silver-water nanofluid injection, IR radiation, Natural gas flame, NOX emission. 1

Corresponding author

[email protected]

1

Journal Pre-proof 1. Introduction Liquid and solid fossil fuels are among the most important sources of thermal energy generation in industrial and domestic applications [1-2]. They are categorized as heavy fossil fuels and have high carbon numbers in their chemical structures [3]. Even if they have a complete combustion, they generate and emit a great amount of NOx emission, which participates as a pollutant and greenhouse gas in environmental problems such as global warming, acid rain and climate change [4-6]. To reduce NOx emission in the solid and liquid combustors, it is crucial to well evaluate the mechanism of NOx formation. However, the mechanism of NOx formation in solid and liquid combustion systems is complex. NOx formations in solid and liquid combustion processes result from a combination of a thermal generation process and the process of fuel nitrogen oxidation [7]. At very high temperatures, thermal generation of NOx from the air nitrogen becomes very important, while at low temperatures found in circulating fluidized bed (CFB) and dual-fluidized bed (DFB) systems, the dominant factors participating in the competing formation and destruction mechanisms of NOx are fuel properties (nitrogen

and

volatile

content,

particle

size

distribution),

primary/secondary gas ratio, local temperature and oxygen partial pressure in the furnace, the presence of calcined limestone in the combustion chamber, gas velocity in the furnace, gas-recycle ratio and the geometry of the system [8]. Krzywanski et al. [9] investigated the effects of O2 concentration of flue gas, N/C ratio of fuel and NO feed to the fluidizing gas, on NOx emissions from a calcium looping (CaL) DFB system. They observed that when O2 and NO content in gas as well as the N/C molar ratio increase, the NOx emissions also increase whilst the increase in fixed carbon content lead to the decrease of the NOx 2

Journal Pre-proof concentrations in flue gas. In another work, Krzywanski et al. [10] suggested the Artificial Neural Network (ANN) approach to predict the NOx emissions from CFB facilities in different scales and under a wide range of operating conditions, both for air-fired and oxy-fuel conditions. The results of this work showed that the ANN model gives quick and accurate results for both large and pilot – scale CFB combustors as an answer to the input pattern, i.e. under different conditions and geometry parameters of the combustion chambers. Gungor et al. [11] investigated the effect of biomass share on SO2 and NOx emissions in a CFB combustor. Simulation results show that emission of NOx decreases dramatically when the mass ratio of biomass (rice husk) to coal increases. In the past decade, the lack of petroleum resources, increase of world petroleum price and the growing awareness of environmental problems associated with the use of solid and liquid fossil fuels have led to the widespread use of natural gas as a clean alternative fuel in industrial and domestic applications [12]. Natural gas includes mainly CH4 and is the lightest hydrocarbon with the lowest carbon number among prevalent hydrocarbon fossil fuels. Therefore, during a combustion process, it generates low levels of CO and unburned natural gas (UHC) pollutants [13]. Furthermore, compared with solid and liquid fossil fuels, there is typically no nitrogen component in natural gas, which rules out the formation of fuel-NOx emission. However, from the point of view of heat transfer, natural gas has lower heat transfer efficiency than liquid and solid fossil fuels do. Consequently, a great deal of research has gone on the methods of enhancing thermal characteristics of natural gas flame in industrial applications such as natural gas burners [14]. As a result of high temperatures of flames, radiation is the most important of heat transfer mechanisms and a large part of heat must be transferred via 3

Journal Pre-proof radiation [15-17]. Therefore, to enhance heat transfer characteristics of natural gas flame, the radiative characteristics of natural gas flame must be enhanced. Based on Stephan Boltzmann law, temperature and total emissivity of flame both influence flame radiation heat transfer [18-19]. According to combustion knowledge, flame temperature directly depends on fuel-oxidant mixing rate [20]. Since natural gas and air are in the same phase (gas phase), natural gas typically produces a flame with more appropriate mixing rate than solid and liquid fossil fuels do. This is testified by low emission of CO and UHC pollutants in gas burners. Also, in connection with emissivity of flame, it was found that, in comparison with the highly efficient radiation of a black body at the same temperature range of flame, emissivity of natural gas flame directly depends on the IR wavelengths emitted from natural gas flame [21]. H2O and CO2 are dominant radiative species of natural gas flame and have non-continuous and weak radiation bands in near IR wavelengths [22-24]. Therefore, to enhance radiation characteristics of natural gas flame, the focus should be on emissivity and the factors affecting it. The physical state (phase) of a matter is a factor influencing its emissivity. In general, solid particles have better radiation characteristics than gaseous species. Solid particles have continuous radiation while gaseous species have radiation bands [25]. Soot particles are among the most important solid particles with effective continuous radiation in near IR wavelengths and act as highly emissive gray bodies in flame structure [26]. Unlike natural gas, solid and liquid fossil fuels have flames that are rich in soot particles. This enhances their flame’s emissivity in comparison with natural gas flame [27-28]. This is why some researchers have advocated boosting the concentration of intermediate soot particles in natural gas flame for the purpose of enhancing the emissivity and radiation characteristics of 4

Journal Pre-proof natural gas flame [29-30]. As the most well-known of the methods suggested for realizing the desired boost, we should mention synchronous combustion of natural gas along with fuels with high sooting tendency such as coal particles and kerosene [31-33], thermal decomposition of natural gas to soot particles through high temperature preheating of natural gas or through preheating of combustion air [34-36]. Nanoparticles are small-scale solid particles which, due to high area/volume ratio and catalytic activity, have recently attracted great interest [37-40]. The addition of low-concentration of nanoparticles to conventional liquid fuels is a novel favorable method to improve physical, ignition, and combustion properties of the liquid fuels [41-46]. However, there has been no comprehensive study on the effect of nanoparticle additives on thermal and heat transfer characteristics of flame and pollutant emissions of natural gas flame. Therefore, to reach the aim of enhancing radiation characteristics of natural gas flame and reducing NOx emission in the present study, we study the effect of lowflow-rate injection of an optimum concentration of silver-water nanofluid on thermal and radiation characteristics and pollutant emission of a natural gas burner. It is worth mentioning that the reason behind the choice of silver nanoparticles is that they have high thermal conductivity (429 W/m.K at 300 K). Therefore, they are able of absorbing and distributing heat in the flame reaction zone and enhancing the nucleation rate and surface growth of intermediate soot particles. This way, they bring about changes in the thermal and radiative characteristics of flame and NOx pollutant emission. Furthermore, in connection with the environmental consequences of the silver nanoparticles, it should be mentioned that they are harmless to the environment. They are used for catalyzing chemical reactions, Raman imaging, and antimicrobial 5

Journal Pre-proof sterilization. In addition to their antimicrobial properties, their low mammalian cell toxicity makes the particles a common addition to consumer products [47-57]. 2. Details of the experiments The experiments and measurements were conducted by using a laboratory cylindrical furnace (Fig. 1). In order to measure features of combustion such as temperature, luminosity and radiation heat flux, five measuring holes, each with an ID of 2.5 cm, were embedded on the furnace wall. They were 0.07, 0.13, 0.19, 0.24 and 0.30 m distant from the inlet of the burner. Furthermore, for the sake of durability at high temperatures of flame, the furnace body was made of high temperature resistant steel AISI316 and a chimney, which was 3000 mm long and 150 mm in diameter, was placed at the end of the furnace to suck and exhaust the combustion products. The burner used in the experiments was a natural gas burner with the maximum heat capacity of 139 kW, which was installed in front of the furnace. A calibrated counter measured the volume flow rate of the natural gas fed to the burner. Also, a standard DT-619 anemometer measured the volume flow rate of combustion air. Its accuracy was ±(3% of the velocity read + 0.2 m/sec). The air velocity ranged between 0.4 and 30 m/sec. The volume flow rates of natural gas and combustion air were respectively 0.124 m3/min and 1.956 m3/min, which correspond to the equivalence ratio of ϕ= 0.65. Also, a micro nozzle, 100 𝞵m in diameter, was installed in the outlet of the burner to inject the silver-water nanofluid droplets into the flame. The silver-water nanofluid concentration and volume flow rate of nanofluid injection were 100 ppm (mg/Lit) and 0.009 Lit/min, respectively. The nanofluid concentration of 100 ppm is among the most attractive nanofluid concentrations recommended in the literature, which noticeably improves 6

Journal Pre-proof thermal conductivity in comparison with the water-based fluid [58-59]. Also, the laboratory observations showed that the 100 ppm silver-water nanofluid had not changed in color and appearance in comparison with the initial state. Besides, the silver particles were fully dispersed in the solution, which indicates great stability of the nanofluid at this concentration. Also, the volume flow rate of 0.009 Lit/min was the maximum rate after which, due to aggregation of nanoparticles in the nozzle injector, there was a disturbance in the operation of nozzle injector. Based on the volume flow rates of nanofluid injection and natural gas and, considering the density of nanofluid and natural gas (1000 kg/m3 and 0.65 kg/ m3), the mass ratio of silver-water nanofluid to natural gas was 0.11 kg/kg. Flame photography and image processing method were used to investigate the physical flame properties such as flame dimension and color. The flame temperature along the furnace was measured by an S-type thermocouple. Technical specifications of this type of thermocouple make it as an appropriate choice in flame temperature measurements. The maximum operating temperature and accuracy of the thermocouple were 1600˚C and ±2.5˚C, respectively. Furthermore, the radiative characteristics of flame including the flame radiation heat flux and luminosity were determined by an SBG01 water cooled heat flux sensor and a TES-1332A digital luminance meter, respectively. The accuracy and repeatability of the luminance meter were ±3% rdg (% of reading value) and ±2%, respectively. It has a detector, which was placed in front of the measuring holes and determined the luminous radiation. Also, the operating range of the heat flux sensor was 0-100 kW/m2 and it was sensitive to thermal radiation with wavelengths under 50 𝞵m. A KIGAS 310 gas analyzer (KIMO Instrument Company) was located at the exhaust of the furnace to measure the concentrations of 7

Journal Pre-proof CO, UHC, CO2 and NOx emission. The accuracies of CO and NOx measurement

were

±10

ppm

and

±5ppm,

respectively.

From

environmental point of view, it is desirable to have minimum concentration of CO emission at the exhaust of the furnace. Furthermore, at low concentration of CO emission, it can be sure that we have complete combustion regime. Therefore, in the first step of the experiments, the burner was lit up without nanofluid injection and the concentrations of CO emissions at the exhaust of the furnace in steady state condition were measured in different equivalence ratios. The results show that for natural gas volume flow rate of 0.124 m3/min, the concentration of CO emission of the burner was in minimum state. Consequently, the experiments related to the effect of silver-water nanofluid injection on thermal and radiation characteristics and NOx pollutant emission of natural gas burner was investigated in the equivalence ratio of ϕ= 0.65. To make sure of the accuracy of the results, all tests were done twice and for all the measurements, the expanded uncertainty based on the accuracy of the equipment and repeatability of experiment were calculated by 95% confidence level and the uncertainty range was presented with the symbol (I) on the related figures in the “Results and Discussion” section. 3. Results and discussion Fig. 2 illustrates the effect of silver-water nanofluid injection on the radiation heat flux from the flame of the natural gas burner. It shows that the injection of silver-water nanofluid into the natural gas flame enhances the radiation heat flux and creates a uniform higher-value distribution of radiative heat flux in the flame. Based on Stephan Boltzmann law, the intensity of flame radiation is related to temperature and emissivity. 8

Journal Pre-proof Therefore, to study the radiation heat transfer characteristics of flame, it is necessary to focus on flame temperature and emissivity. Fig. 3 shows the axial flame temperature of burner for natural gas as well as natural gas enriched with silver nanofluid injection. A decline in temperature is observed in the flame upstream region and the decline is especially observed for the value of maximum temperature. It can be explained by two reasons. The first one is that the evaporation of the droplets absorbs heat and reduces temperature. The second reason for decline in temperature in this region relates to melting of the silver nanoparticles. The melting point of silver is 961 °C and silver oxide particles decompose at temperatures above 280 °C. Therefore, in the flame upstream region, since the temperature is higher than the melting point of silver, the silver nanoparticles produced by thermal decomposition of the silver oxide particles can absorb heat from the surrounding flame and melt and then coat a surface. There was no instrument to measure the concentration of silver nanoparticles in the flame. Therefore, to investigate the above mentioned phenomenon, we placed a number of iron bars in the flame and the iron bars was observed to change color from dark brown to white, which proves that the silver nanoparticles coated some surfaces (see Fig. 4). Therefore, we expect that if the size of furnace is very small after long-term operation of the furnace, some parts of its wall will be silver-coated. However, the concentration of silver nanoparticles was so low and the surface area of the furnace was so large that we didn’t observe significant change in color of the furnace wall. Unlike the flame upstream region, it can be seen that, in the other regions, injection of silver-water nanofluid makes only a slight increase in flame temperature. The little temperature rise can be explained as follows: after the absorption of heat by silver nanoparticles and 9

Journal Pre-proof evaporation of the droplets in the flame upstream region, a large number of high temperature silver nanoparticles remain in the flame. They act as a moving heat source in the flame and, through accelerating the heat transfer to different areas of flame, increase the flame temperature. In the downstream region, due to the low concentration of the particles, there occurs a slight increase in the flame temperature. As mentioned above, flame temperature is among the factors that affect the rate of radiation heat transfer (see the explanation given for Fig. 2). However, based on the results of Fig. 3, the average temperature difference between the flames of natural gas and natural gas enriched with silver-water nanofluid injection is small (less than 1%). Consequently, it is concluded that the temperature difference between the flames of natural gas and natural gas enriched with silver-water nanofluid injection has a negligible effect on radiation heat flux. Therefore, enhancement of flame emissivity must be the main way of obtaining higher radiation from natural gas flame when there is silver-water nanofluid injection. Thermal radiation, as electromagnetic radiation, is emitted in wavelengths from the visible spectrum to infrared (IR) wavelengths. The former is called luminous radiation and the latter IR radiation. Fig. 5 illustrates the luminous radiation of flame. Although the flame luminosity is strengthened by silver nanoparticles, luminous radiation has a negligible contribution to the total radiation of flame and the principal part of radiation occurs at IR wavelengths (compare Fig. 2 with Fig. 5). The intensity and distribution of radiant energy varies across wavelengths. The difference between visible and infrared wavelengths is reflected in visible emissivity and IR emissivity. To depict the IR 10

Journal Pre-proof emissivity of flame, an IR photograph of flame was taken (see Fig. 6). The red tone color represents the IR emissivity of flame and, as shown, silver nanoparticles increase the IR emissivity of flame. However, regarding natural gas flame, it is observed that there is no red tone color in the IR picture. This proves that, when silver nanoparticles are not injected into natural gas flame, it has lower IR radiation and emissivity. CO2 and H2O, as high concentration gaseous species of flame, and soot particles are powerful determinants of the emissivity of flame. Therefore, to find out why natural gas flame enriched with silver-water nanofluid injection has an enhanced IR emissivity, we first discuss the concentration of the above mentioned radiative species. In Fig. 7, the CO2 emissions at the exhaust of the furnace are shown for natural gas flame and natural gas flame enriched with silver-water nanofluid injection. Silver nanofluid injection can be seen to have a negligible effect on CO2 concentration. H2O, as another radiative species of flame, can enhance the IR emissivity of flame in case its concentration is increased. When injected in the flame, silver-water nanofluid creates many submicrometer droplets inside which a number of silver nanoparticles are trapped. The silver nanoparticles have high thermal conductivity and high area/volume ratio. Therefore, they quickly absorb heat from the flame reaction zone, heat up the droplets and evaporate them. This phenomenon increases the concentration of H2O species in the flame and enhances the emissivity and radiation of flame. Besides, after the evaporation of the droplets, a large number of tiny black or dark brown silver oxide nanoparticles remain in the flame (see Fig. 8). These dark brown or black silver oxide nanoparticles do two things in the flame. First, these tiny high temperature dark nanoparticles act as highly emissive bodies in the flame and take part in the enhancement of flame radiation characteristics at IR 11

Journal Pre-proof wavelengths. The second and more important role of the solid nanoparticles is that, as foreign surfaces, they enhance the nucleation rate and surface growth of soot particles. Soot particles are the main radiative species of flame in IR radiation. Therefore, increase in soot particles concentration enhances the emissivity and total radiation of flame. It is worth mentioning that, from the environmental point of view, one may think that an elevated rate of soot nucleation increases the concentration of soot emission as a pollutant. However, increase in H2O concentration in the presence of the high temperature of flame reaction zone, which is caused by evaporation of the water content of silver-water nanofluid, intensifies the dissociation rate of H2O. This increases the concentration of OH radicals in the flame reaction zone. The OH radicals are very effective in the oxidation of soot particles to CO and CO2. Therefore, the rates of nucleation and oxidation of soot particles increase simultaneously and, consequently, there will be no soot emission at the exhaust of the furnace. It is the concentration of intermediate soot particles that is expected to increase by the injection of silver-water nanofluid. Such particles enhance the radiation characteristics of flame while they generate no soot pollutant emission. To invalidate this argument, we measure the concentration of CO and UHC at the exhaust of the furnace. The concentrations of CO and UHC emissions, respectively, are 3 ppm and 1 ppm for natural gas flame and 10 ppm and 2 ppm for natural gas flame enriched with silver-water nanofluid injection. In either case, the pollutant emissions are lower than the standard levels (the standard values for CO and UHC emissions are 150 ppm for CO and 56 ppm for UHC [9]). Further, silver-water nanofluid injection leads to no significant change of CO and UHC pollutant emissions. If we combine the above mentioned pollutant concentrations with the results of CO2 (Fig. 7), it can 12

Journal Pre-proof be seen that the concentration of carbonaceous compounds at the exhaust of the furnace is almost the same. Since all carbon atoms of natural gas are finally converted to CO, UHC, CO2 and soot emission, carbon balance shows that, natural gas flame, whether enriched with silver-water nanofluid injection or not, results in the same concentration of soot particles at the exhaust of the furnace. Consequently, as we predicted, the silver-water

nanofluid

injection

increases

the

concentration

of

intermediate soot particles. However, a significant part of them are finally oxidized to CO and CO2. Furthermore, to obtain the rate of soot emission at the exhaust of the furnace, the gravity method was used. In the method, a sample of combustion products was sucked from the chimney by a suction tube which was coupled into a vacuum pump. Combustion products passed through a soot filter and soot particles were trapped in the filter. By weighing the filter at the end of the test, the average rate of soot emission was calculated. They were 0.0010 gr/min for natural gas flame and 0.0011 gr/min for natural gas enriched with silver-water nanofluid, which confirmed that the rates of soot emission were very small in both flames. Fig. 9 illustrates the concentration of NOx emission (the sum of NO and NO2) for natural gas flame and natural gas flame enriched with silverwater nanofluid injection. It can be seen that silver-water nanofluid injection decreases the NOx emission of natural gas flame as much as 22.6%. NOx is the most important of pollutant emissions from natural gas burners. There is no fuel nitrogen in natural gas and therefore the only source of NOx is air nitrogen. Besides, thermal NOx formation mechanism, described by Zeldowich, is the dominant mechanism of NOx generation in industrial natural gas burners. Based on Zeldowich mechanism, at high temperatures, the nitrogen molecules present in the 13

Journal Pre-proof combustion air can break and react with oxygen atoms to generate NOx pollutant. Based on Fig. 3, injection of silver-water nanofluid makes only a slight increase in flame temperature at downstream region. Nevertheless, in the flame upstream region, there is no increase, whatsoever, in temperature and a decline in temperature is observed with silver-water nanofluid injection. The decline is especially observed for the value of maximum temperature. Consequently, silver-water nanofluid injection decreases the maximum temperature of flame compared with natural gas lacking silver-water nanofluid injection. Since thermal NOx formation mechanism is strongly dependent on temperature and thermal NOx formation accelerates exponentially at high temperatures, decrease in maximum flame temperature with silver-water nanofluid injection is the main reason for decrease in NOx emission. Furthermore, as mentioned above, the dissociation of the water content of silver-water nanofluid can increase the concentration of OH radicals, which are effective species in the oxidation of soot precursors to CO and CO2. Since NOx is formed after the completion of combustion, OH radicals consume O atoms to oxidize the intermediate soot particles to CO and CO2. Consequently, the concentration of O atoms in the flame reaction zone reduces and there will be fewer oxygen atoms to combine with nitrogen and generate NOx pollutant. This decreases NOx emission. 4. Conclusion 0.009 Lit/min of 100 ppm silver-water nanofluid was directly injected into natural gas flame. Then, thermal and radiation characteristics of flame and NOx emission were studied. The main findings are as follows:

14

Journal Pre-proof •

The injection of silver-water nanofluid into natural gas flame

enhances radiation heat flux and creates an elevated and uniform radiative heat flux distribution for flame. •

Effect of silver-water nanofluid injection on IR flame emissivity is

the main reason for flame radiation enhancement. •

Silver nanoparticles, as foreign surfaces, enhance the nucleation

rate and surface growth of intermediate soot particles which are the best radiative species of flame in IR wavelengths. •

Increase in H2O concentration, as a gaseous radiative species of

natural gas flame, and an increased number of tiny high temperature dark nanoparticles, as highly emissive bodies, are the other parameters which affect the flame radiation. •

When the silver-water nanofluid is injected, the flame temperature

in the upstream region and, especially, the maximum flame temperature decrease and the concentration of OH radicals increases, which is caused by dissociation of H2O at high temperatures of flame reaction zone. Thease lead to decrease in the NOx emission as much as 22.6%.

15

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Fig. 1. Schematic diagram of the experimental setup

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Fig. 2. Effect of silver-water nanofluid injection on flame radiation heat flux

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Fig. 3. Axial temperature of flame for natural gas and natural gas enriched with silver-water nanofluid injection

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Fig. 4. Iron bar after being coated by melted silver nanoparticles

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Fig. 5. Flame luminosity for natural gas and natural gas enriched with silver-water nanofluid injection

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Fig. 6. IR picture of natural gas flame (A) and natural gas enriched with silver-water nanofluid injection (B)

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Fig. 7. CO2 emission for natural gas and natural gas enriched with silver-water nanofluid injection

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Fig. 8. Picture of dark brown or black silver oxide nanoparticles

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Fig. 9. NOx emission for natural gas and natural gas enriched with silver-water nanofluid injection

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Journal Pre-proof 1 Author declaration 1. Conflict of Interest Potential conflict of interest exists: We wish to draw the attention of the Editor to the following facts, which may be considered as potential conflicts of interest, and to significant financial contributions to this work: The nature of potential conflict of interest is described below: No conflict of interest exists. We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. 2. Funding Funding was received for this work. All of the sources of funding for the work described in this publication are acknowledged below: [List funding sources and their role in study design, data analysis, and result interpretation] No funding was received for this work. 3. Intellectual Property We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. 4. Research Ethics We further confirm that any aspect of the work covered in this manuscript that has involved human patients has been conducted with the ethical approval of all relevant bodies and that such approvals are acknowledged within the manuscript. IRB approval was obtained (required for studies and series of 3 or more cases) Written consent to publish potentially identifying information, such as details or the case and photographs, was obtained from the patient(s) or their legal guardian(s).

Journal Pre-proof 2 5. Authorship The International Committee of Medical Journal Editors (ICMJE) recommends that authorship be based on the following four criteria: 1. Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND 2. Drafting the work or revising it critically for important intellectual content; AND 3. Final approval of the version to be published; AND 4. Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All those designated as authors should meet all four criteria for authorship, and all who meet the four criteria should be identified as authors. For more information on authorship, please see http://www.icmje.org/recommendations/browse/roles-andresponsibilities/defining-the-role-of-authors-and-contributors.html#two. All listed authors meet the ICMJE criteria. 
We attest that all authors contributed significantly to the creation of this manuscript, each having fulfilled criteria as established by the ICMJE. One or more listed authors do(es) not meet the ICMJE criteria. We believe these individuals should be listed as authors because: [Please elaborate below] 
 We confirm that the manuscript has been read and approved by all named authors. We confirm that the order of authors listed in the manuscript has been approved by all named authors. 6. Contact with the Editorial Office The Corresponding Author declared on the title page of the manuscript is: [S.H. Pourhoseini] This author submitted this manuscript using his/her account in EVISE. We understand that this Corresponding Author is the sole contact for the Editorial process (including EVISE and direct communications with the office). He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs.

Journal Pre-proof 3 We confirm that the email address shown below is accessible by the Corresponding Author, is the address to which Corresponding Author’s EVISE account is linked, and has been configured to accept email from the editorial office of American Journal of Ophthalmology Case Reports: [[email protected]] Someone other than the Corresponding Author declared above submitted this manuscript from his/her account in EVISE: [Insert name below] We understand that this author is the sole contact for the Editorial process (including EVISE and direct communications with the office). He/she is responsible for communicating with the other authors, including the Corresponding Author, about progress, submissions of revisions and final approval of proofs.

We the undersigned agree with all of the above.

Author’s name (Fist, Last)

1. S.H. Pourhoseini

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Date

Friday, October 24, 2019.

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Highlights •

Silver-Water nanofluid injection enhances the IR radiation of natural gas flame

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Nanofluid injection creates an elevated uniform radiative flux distribution for flame

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Nanofluid injection decreases the NOx emission as much as 22.6%

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Nanofluid injection doesn't change carbonaceous pollutant emission significantly