Graphite Oxide Nanoparticle as a Diesel Fuel Additive for Cleaner

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Graphite Oxide Nanoparticles as Diesel Fuel Additive for Cleaner Emissions and Lower Fuel Consumption Jong Boon Ooi, Harun Mohamed Ismail, Varghese Swamy, Xin Wang, Akshaya Kumar Swain, and Jeevan Raj Rajanren Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02162 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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Graphite Oxide Nanoparticles as Diesel Fuel Additive for Cleaner Emissions and Lower Fuel Consumption

Jong Boon Ooi,1 Harun Mohamed Ismail,*1 Varghese Swamy,1 Xin Wang,1 Akshaya Kumar Swain,2 and Jeevan Raj Rajanren1

1

School of Engineering, Monash University Malaysia, Jalan Lagoon Selatan, 47500 Bandar

Sunway Selangor Darul Ehsan, Malaysia 2

IITB-Monash Research Academy, Department of Metallurgical Engineering and Materials

Science, Indian Institute of Technology-Bombay, Mumbai, Maharashtra, India, 400076

KEYWORDS: energetic nanoparticles, graphite oxide, diesel fuel additives, droplet combustion.

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ABSTRACT: Graphene and its derivatives have drawn interest across many disciplines due to their remarkable properties. We investigated the influence of graphite oxide (GO), aluminum oxide (Al2O3) and cerium oxide (CeO2) nanoparticles at 0.1% and 0.01% dosing concentration on the combustion characteristics of diesel fuel by using the single droplet combustion experiment. Shortened ignition delay (ID) by up to 46.5%, increased burn-rate constant (up to 29.4%), reduced peak temperature (up to 13.8%), and shortened burnout time (up to 19.2%) are observed when GO nanoparticle is dosed in diesel fuel. These remarkable features may substantially improve the combustion efficiency and reduce harmful emissions in diesel engine applications.

1. INTRODUCTION Diesel fuel is widely used in road transportation, locomotives, agriculture, military, construction, mining, maritime, propulsion, and stationary electricity production.1 Oil based fossil fuels (e.g., diesel fuel) have contributed about 33% in the global energy consumption in 2013, as shown in Figure 1. In the next 50 years, consumption of diesel fuel is expected to rise with more diesel-operated passenger cars worldwide.2 The increased uptake of diesel fuel for diesel engines will contribute to a surge in urban pollutions, greenhouse gases, and fossil fuel depletion.3 Diesel engine operated with diesel fuel typically releases carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), and polycyclic aromatic hydrocarbons (PAHs) that are harmful to the environment and human health.4-6 The NOx and PM emissions are principally responsible for smog pollution,7-11 while PAHs (e.g., benzene and formaldehyde) are mutagenic and potentially carcinogenic to human.12-14


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Figure 1. Global energy consumption in 2013 segmented by source.15

In view of these, stricter emission standards such as Euro 6 were recently enforced on automobile manufacturers to further reduce exhaust-out NOx levels by approximately 50% and CO by approximately 90% as compared to the current Euro 5 requirements.16, 17 For the lightduty diesel car category, NOx emissions have to be reduced from 0.18 g/km to 0.08 g/km whereas CO emissions are to be reduced from 0.50 g/km to 0.05 g/km under the new Euro 6 ruling.18, 19 Current approaches to reduce undesirable diesel engine emissions in response to the stringent emission legislations include pre-combustion treatments, post-combustion treatments, engine configuration strategies, use of biofuels, and fuel additives. For instance, post-combustion treatments such as diesel particulate filter exhaust gas recirculation, and diesel catalytic converter are equipped in light-duty diesel cars to reduce exhaust-out emissions.20 Similarly, effective strategies of fuel injection (e.g., high pressure common rail , multiple injections and multi-point fuel injections) and air intake (e.g., turbocharging and supercharging) have been integrated into diesel engines for emission reduction.21,

22

Further, fueling strategies for reducing emissions

including the use of biodiesels and diesel-biodiesel blends are being explored.23,

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Although 3

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existing approaches can reduce diesel engine emissions, they are relatively expensive with limited operating efficiencies.25,

26

Zero-emission technologies such as hydrogen fuel cell and

battery operated vehicle have also been developed to replace petroleum liquid fuel (i.e., gasoline and diesel fuels). However, in the near future, diesel fuel will still be one of the dominant energy sources to power transportation globally due to its relatively lower cost over zero-emission technologies. One of the cost-effective ways to mitigate the ever-increasing diesel pollutant emissions is the use of fuel additives. Significant efforts are underway exploring the use of nanoscale materials as environmentally beneficial fuel additives for diesel engines. Nanoscale particles can form uniform dispersions within liquid fuel. They have higher surface area-to-volume ratio compared to micron-sized particles which allows for better heat and mass transfer within liquid fuel.27,

28

Recently, much of the research in fuel additives is focused on the use of energetic

metal-based nanoparticles as diesel fuel additives for improving fuel consumption and emission reduction. Metal oxide nanoparticles such as Al2O3 and CeO2 have shown to improve the engine performance and reduce exhaust-out emissions when added in diesel fuel or biodiesel. For instance, diesel fuel dosed with Al2O3 nanoparticles yields increased linear burn-rate,29 shortened ignition delay,30 increased hot-plate ignition probability,31 and lower CO emission.32 Similarly, diesel fuel dosed with CeO2 nanoparticles produces improved fuel consumption, reduced NOx emissions,33 and reduced soot accumulation.34 Reduced PM, CO and NOx emissions, and increased brake thermal efficiency have also been reported for biodiesel dosed with CeO2.35 Notwithstanding the above-mentioned benefits, the metal oxide additives were found to release airborne particulates as a combustion by-product that cause breathing and lung related problems as well as skin allergies.36-39 Graphite oxide, on the other hand, is potentially a more 4 ACS Paragon Plus Environment

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environmentally benign diesel-fuel additive with a host of desirable chemical, physical, and combustion characteristics. Chemically, GO is predominantly carbon with traces of oxygen and hydrogen, but devoid of any metal components. The GO nanostructure is made up of 2dimensional graphene (carbon) sheet stacks separated by oxygen functional groups.40-42 Thus, GO can provide large surface area-to-volume ratios and oxygen reactive sites in the combustion process. A recent study showed that the addition of functionalized graphene sheets (i.e., reduced GO) significantly improved the combustion characteristics (e.g., increased linear burn-rate and lowered ignition temperature) of liquid monopropellant nitromethane.29 Encouraged by these results, we investigate the combustion characteristics of diesel fuel dosed with GO by using single droplet experiment to assess the viability of GO as a potentially environmentally benign diesel fuel additive. We evaluate the comparative performances of the baseline diesel fuel and the same diesel dosed with GO, Al2O3, and CeO2 at 0.01% and 0.1% dosing ratios. The desired combustion characteristics of a fuel from single droplet experiment and the expected outcomes in diesel engines are summarized in Table 1. For example, with higher burn-rate, micro-explosion and lower burn-out time, may lead to lower fuel consumption as different engine calibration can be utilized to reduce the amount of fuel injected to produce while maintaining similar power output. The fuel combustion characteristics and the pollutant emissions from a diesel engine are discussed in detail in Section 3. Table 1. The desired combustion characteristics of a fuel and the expected outcomes in diesel engines No.

Desired combustion characteristics

Expected outcomes in diesel engines

1

↓ Ignition delay (ID)

↓ NOx

2

↓ Temperature at Point of Ignition (TPI)

↓ NOx


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3

↑ Burn-rate constant

↓ CO, ↓PAHs, ↓ PM, ↓ Fuel consumption

4

↑ Micro-explosion frequency


5

↓ Peak temperature

↓ NOx

6

↓ Burnout time


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2. EXPERIMENTAL METHOD 2.1. Nanoparticle Additives. Commercially available nanocrystalline Al2O3 and CeO2 with nominal average particle size of 50 nm were used as received from Sigma-Aldrich. GO was synthesized via chemical oxidation based on the improved Hummers’ method.43 The presence of GO was confirmed by dispersing the product in deionized water: a brown colored solution suggests the presence of exfoliated and highly dispersed graphite oxide nanoparticles.44 Fourier transform infrared (FTIR) spectrum of the GO indicated the presence of O-H (3350 cm-1), C=O (1720 cm-1), C=C (1640– 1670 cm-1), and C-O (1100 cm-1) functional groups, as shown in Figure 2. The FTIR spectrum is similar to that reported for GO by Marcano et al.43 The presence of C=C indicates partially unoxidized sp2 carbon-carbon bonds in the GO. Images of nanoparticles with a resolution of 3296 × 2563 pixels were taken by Transmission electron micrographs (TEM) operating at an accelerating voltage of 120 kV. The nanoparticles were dried on carbon-coated copper grids overnight at room temperature before the imaging. We have taken 3 samples for each nanoparticle to ensure consistency of the size and structure of nanoparticles captured. TEM of the GO, Al2O3, and CeO2 nanomaterials used (Figure 3) suggest average particles sizes of 100 nm (Al2O3), 200 nm (CeO2), and 500 nm (GO). It may be noted that the GO consists of single- or multi-layered graphene sheets (Figures 3c and 3d). The dominant 2-dimensional morphology of 6 ACS Paragon Plus Environment

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GO nanoparticles leads to high surface area-to-volume ratios that promote relatively more abundant chemically active sites and high surface area contact between the nanoparticles and the diesel fuel.45 On the other hand, the more equant shaped CeO2 and Al2O3 nanoparticles with much reduced surface-to-volume rations provide much lower fuel-particle surface area contact.

0.6 Measured

Marcano et al. [43]

O-H 0.5

C-O

Absorbance (arb. u.)

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C=O 0.4

C=C

0.3

0.2

0.1

0 1000

1500

2000

2500

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4000

Wavenumber (cm-1)

Figure 2. FTIR spectrum of the synthesized GO.


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Figure 3. TEM of (a) Al2O3, (b) CeO2, (c) single layer GO, and (d) multilayer GO. Scale bar: 200 nm for (c) and (d).

2.2. Fuel Blends Preparation. A commercial diesel fuel was used as the baseline fuel without any treatment or modification. GO dosed in diesel fuel (GDD), Al2O3 dosed in diesel fuel (ADD) and CeO2 dosed in diesel fuel (CDD) at 0.01% and 0.1% dosing concentrations (i.e., mass over volume percent), respectively were also prepared. The dosed diesel mixtures were stirred vigorously and subsequently ultrasonicated for 30 minutes to fully disperse the nanoparticles. However, the dispersion stability of all the fuel blends decreased after 10 minutes and sedimentation was obvious within 60 minutes. The agglomeration of nanoparticle in diesel fuel is probably due to the weak van der Waals forces between the nanoparticles that attract each other. This issue can be solved by applying a surfactant.27 However, the addition of a surfactant is not considered in this study because the surfactant can affect the combustion characteristics of the dosed diesel fuel and the 8 ACS Paragon Plus Environment

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effects of various nanoparticle addition discussed cannot be quantified separately.46 Therefore, fuel blends were prepared without any surfactant for the single droplet experiments in order to identify the important contributions of individual nanoparticle type to the combustion characteristics of diesel fuel. The fuel properties of the baseline fuel and the dosed diesel fuel are summarized in Table 2. Slight increment in density is observed for the fuel blends at 0.1% dosing concentration compared to diesel fuel. On the other hand, the density of fuel blends at 0.01% dosing concentration is seen to be reduced compared to diesel fuel. Furthermore, reduction in boiling point, flash point, cetane number, and carbon residue are found for all the dosed diesel fuel. Table 2. Comparison of fuel properties Properties Diesel

Density a (°C) 848.8

Boiling point b (°C) 258.0

Viscosity c (mm2/s) 4.74

Flash point d (°C ) 83.0

Cetane number e 46

Carbon residue f (%) 0.25

0.01% GDD

822.4

250.0

3.27

71.0

45

0.01

0.01% ADD

822.4

280.0

3.20

69.0

43

0.01

0.01% CDD

822.3

240.0

3.17

68.0

42

0.01

0.1% GDD

849.1

243.0

4.33

79.0

40

0.17

0.1% ADD

849.6

250.0

5.05

82.0

40

0.18

0.1% CDD

849.7

239.0

4.71

73.0

41

0.20

Test Method: a ASTM D1298, b ASTM D7169, c ASTM D445, d ASTM D93, e ASTM D613, f ASTM D4530.

2.3. Single Droplet Experiment. In order to study the combustion characteristics of the fuel samples, a well-established single droplet experiment was employed in this work.27,

47

The single droplet experimental setup is

schematically shown in Figure 4. Droplet combustion experiments were conducted in an 9 ACS Paragon Plus Environment

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enclosed and insulated chamber to minimize heat convection. The 20 cm x 20 cm x 28 cm chamber is equipped with viewing windows on two opposite sides. The viewing windows allow access for a high-speed camera and high intensity backlight (Figure 4). The other opposed viewing windows were utilized for igniter access to the chamber and for optical access, respectively. A droplet with 1.2 ± 0.1 mm diameter was carefully suspended on a K-type thermocouple (0.3 mm thickness) using a micro-syringe.48, 49 The thermocouple was connected to a Graphtec GL800ASTM midi data logger for recording the droplet thermal history at 200 ms interval. Ignition of a suspended droplet was achieved by a hot surface igniter (10 mm in diameter and 70 mm in length) fixed at 5 mm below the droplet. Once the droplet is ignited, the igniter was immediately removed from the chamber by a horizontal mechanized roller. Droplet shadow images were captured at 1280 x 800 pixels resolutions and 500 frames per seconds using a high speed camera (Phantom Miro M310 monochrome) coupled with extended 12x optical zoom lens. Image-processing code was developed using MatlabTM to obtain the cross-section area and diameter of the droplets from the shadow images. The image-processing approach used in this study is similar to the one carried out by Javed et al.48 Firstly, in this method, the grayscale images captured were converted into binary images to distinguish between the droplet and holder (white zone) and the background (black zone), as shown in Figure 5. Then, a threshold value was set to count the pixels in the white zone to obtain the cross-section area of droplet and holder. Consequently, the droplet area was obtained by subtracting the cross-section area of droplet and holder with the cross-section area of holder (this was obtained before the droplet is suspended on the holder). Finally, the diameter of the droplet was estimated from the droplet area using the area of circle.48 In some cases, the droplet is being distorted during the combustion process. However, by using this code, the diameter of irregular-shaped droplet was 10 ACS Paragon Plus Environment

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calculated within a reasonable accuracy. This method was executed iteratively for each image in order to obtain the temporal variation of droplet area and droplet diameter during combustion.

Thermocouple

High-speed Camera

Midi Data Logger

High intensity back light Igniter

Horizontal mechanized roller

Computer

Figure 4. Schematic diagram of the single droplet combustion experiment.

Figure 5. Conversion from (a) a grayscale image to (b) a binary image with fixed reference lines for determining the droplet cross-section area.

3. RESULTS AND DISCUSSIONS


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This section summarizes a generic trend for normalized droplet area (A/Ao) and droplet temperature histories observed for all the fuels as illustrated in Figure 6. The observed droplet combustion behaviors with time can be categorized into three distinctive combustion stages. First, preheating and ignition (Stage 1) was identified between the start of droplet expansion (preheating) and the appearance of a small peak (ignition) found on the A/Ao curve. In this stage, ignition delay marked as point ii, and temperature at point of ignition (TPI) marked as point iii, were directly determined as depicted in Figure 6. Secondly, the steady burning droplet (Stage 2) was defined as a quasi-linear decrement of the A/Ao curve (point iv). Finally, the unsteady burning droplet (Stage 3) was identified by rapid fluctuations of the A/Ao curve (point v). At the end of droplet combustion, peak temperature (point vi) and burnout time (point vii) were determined as shown in Figure 6. The aforementioned definitions and naming conventions are used for the analysis in the following sections.

800

1.4 Stage 1

Stage 2

Stage 3 vi

1.2

700

1 600

iv

i

ii

500

v

iii

400

300

0.8

Stage 1: Preheating and ignition Stage 2: Steady burning Stage 3: Unsteady burning i: Expansion ii. ID iii: TPI Iv: Linear decrement v: Bubbling and explosion vi: Peak temperature vii: Burnout time

A/Ao

Droplet temperature / (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

0.2

vii

200

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Time / (s)


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Figure 6. Generic trend of droplet combustion behavior observed in this study.

3.1. Preheating and Ignition (Stage 1) Initially, all the test fuel droplets undergo preheating for a short period before ignition. During this period, the baseline diesel fuel and the dosed diesel at 0.01% and 0.1% dosing starts to vaporize and expand gradually as illustrated in Figures 7a and 7b, respectively. The observed trend can be attributed to thermal expansion and cavitation build up in the fuel droplet during preheating.27 After a short period of heating (i.e., ignition delay), the droplets of all the fuels eventually ignite at different time intervals, as depicted in Figure 8.


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800

1.4

(a)

Diesel

i

iv

0.01% GDD

700

0.01% CDD 1

600 0.8

A/Ao

Temperature / (K)

1.2

0.01% ADD ii

500

i: ii: iii: iv: v:

iii 400

Ignition Linear decrement Bubbling and Explosion Peak temperature Burnout time

0.6

0.4

300

0.2 v

200

0 0.0

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1.0

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5.0

Time / (s) 800

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(b)

i

Diesel 0.1% GDD

iv

700

1.2

0.1% ADD

ii

0.1% CDD 1 600 0.8

iii

A/Ao

Temperature / (K)

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500

i: ii: iii: iv: v:

400

Ignition Linear decrement Bubbling and Explosion Peak temperature Burnout time

0.6

0.4

300

0.2 v

200

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Time / (s)

Figure 7. Normalized droplet area and temperature histories for the baseline fuel and the dosed diesel fuels at (a) 0.01% dosing and (b) 0.1% dosing.


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Figure 8. Droplet burning stages of the (a) baseline fuel, (b) 0.01% GDD, (c) 0.01% ADD, (d) 0.01% CDD, (e) 0.1% GDD, (f) 0.1% ADD, and (g) 0.1% CDD (1= Stage 1, 2 = Stage 2, 3 = Stage 3).


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3.1.1. Ignition Delay (ID). ID indicates how fast a fuel droplet is ignited upon heating. Generally, shorter ID corresponds to higher cetane number in the diesel fuel and lower NOx due to lower flame temperature.50-52 In this study, ID of a droplet is defined as the time interval between the start of heating and the start of ignition. In order to precisely determine the start of droplet ignition, a classical thermal definition of ignition conditions was adopted.53 Here, the start of ignition was determined when the lowest value of droplet gradient temperature (i.e., dT/dt ≥ 0) and the first successive point of inflection for second derivative of droplet gradient temperature (i.e., d2T/dt2 = 0) are reached. Our results show that ID of the diesel fuel is shortened by up to 48.4% when the fuel is dosed with nanoparticles at 0.01% and 0.1%, as seen in Figure 9. First, this is attributed to higher evaporation rate as a result of additional heat being absorbed through radiation by the nanoparticles that are distributed within the liquid droplet as indicated in Figures 10a and 10b. The additional heat is then scattered to the surrounding liquid within the liquid droplet causing more fuel to be vaporized. Secondly, more oxygen is readily available to form higher concentrations of combustible mixtures due to fuel-borne oxygen content provided by mixing of the nanoparticle oxides. In depth investigations also revealed significant change in ID for different nanoparticle type and different dosing ratios as depicted in Figure 9. The greatest reduction in ID is found to be 48.4% for 0.1% CDD, while the percentage reduction in ID is lower for 0.1% GDD (reduction in ID by 38.2%) and 0.1% ADD (reduction in ID by 35.1%). The aforementioned trend can be attributed to higher evaporation of CDD due to ceria’s superior radiative heat absorbance in the visible light range as illustrated in Figure 10a. On the other hand, a counterintuitive trend in ID is observed for GDD compared to ADD and CDD when 16 ACS Paragon Plus Environment

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dosing ratio is increased from 0.01% to 0.1% as summarized in Figure 9. This is most likely due to the relatively larger particle size of GO caused by agglomeration at the 0.1% dosing level over 0.01% dosing as shown in Figure 11. Consequently, less surface area of larger GO particles exposed to the surrounding fuel (i.e., lower surface area to volume ratio) lowers the vaporization rate of the fuel droplet and hence longer period is required to form an ignitable fuel-air mixture. A significant reduction in the ID observed for all the dosed fuel droplets compared to undosed diesel fuel droplets is desirable for diesel engine applications. Based on the findings (shortened ID for the dosed fuels) from single droplet experiment, we can deduce that less amount of fuel is burned in the premixed combustion, thus resulting in lower flame temperature. Consequently, the NOx formation is reduced in diesel engines.50-52


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Figure 9. ID comparison between the baseline diesel fuel and the dosed diesel fuels at 0.01% and 0.1% dosing ratio.

2.5

(a) UV-Visible Range

Diesel 0.1% GDD


2

0.1% ADD 0.1% CDD

1.5

Overall radiative absorbance in visible light range: CeO2 > GO >Al2O3 > Diesel

1

0.5

0 200

300

400

500

600

700

800

Wavelength / (nm) 2.5

(b) Infrared Range

Diesel 0.1% GDD

GO>CeO2>Al2O3>Diesel

2

0.1% ADD 0.1% CDD


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1.5 High intensity flame radiation (1500-3000K) at wavelength 1100-2000 nm

1

0.5

0 500

1000

1500

2000

2500

3000

3500

4000

Wavelength / (nm)

Figure 10. Radiative absorbance spectra of the baseline diesel fuel and the dosed diesel fuels at 0.1% dosing ratio in (a) the visible light and (b) infrared regions.


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Figure 11. Average particle size of GDD, ADD, and CDD at 0.01% and 0.1% dosing ratio.

3.1.2. Temperature at Point of Ignition (TPI). The droplet temperature at the point of ignition is interpolated from the droplet temperature histories and indicated as TPI in Figure 6. TPI is determined to elucidate the auto-ignition temperature of the fuel droplet. In this study, lower TPI is observed for all the dosed diesel fuels except for 0.1% GDD with reduction between 2.8% and 8.5% compared to the baseline diesel fuel as illustrated in Figure 12. This is attributed to the higher diffusion rate (enhanced mass transfer) and higher radiative heat absorption (enhanced heat transfer) by the nanoparticles as shown in Figure 10. Additional heat absorbed by the nanoparticles is transported away from the droplet to the fuel vapor-air mixing region by diffusion as illustrated in Figure 13. As a result, the droplet is cooled, while more heat is supplied by the nanoparticles at the fuel vapor and air 19 ACS Paragon Plus Environment

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mixing region to be ignited earlier (shorter ID). In contrast, the increased TPI (about 1.0%) for 0.1% GDD is due to the larger GO particles (i.e., agglomerated particle) inside the droplet, as illustrated in Figure 11. Here, diffusion rate is reduced with larger and heavier GO particles from the droplet surface. This causes most GO particles to be trapped inside the droplet and absorb heat, thus resulting in higher TPI.54 The highest reduction in TPI is achieved for 0.01% GDD with 8.5% reduction as seen in Figure 12. This is likely caused by the combined effects of smaller average particle size (see Figure 11) and higher radiative absorbance (see Figure 10) for the 0.01% GDD.

Figure 12. TPI comparison between the baseline diesel fuel and the dosed diesel fuels at 0.01% and 0.1% dosing ratio.


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Heat

Fuel vapor phase

Ambient air

Heat

Fuel vapor and nanoparticles diffuse away to flame front to combust Diffusion-Controlled Flame (Flame front)

Fuel droplet

Air (Oxidizer)

Figure 13. Diffusion-controlled flame of a fuel droplet with nanoparticles.

There was no significant difference in the TPI between 0.01% and 0.1% dosing for ADD and CDD as summarized in Figure 12. This may be attributed to the relatively smaller percentage difference in average particle size between 0.01% and 0.1% dosing for Al2O3 (difference by 30%) and CeO2 (difference by 14%) compared to GO (difference by 61%) as depicted in Figure 11. Lower TPI observed for all the dosed diesel fuels except for 0.1% GDD can yield significant advantages in diesel engine operation. For instance, lower TPI can potentially improve engine cold-start and enhance smoothness of engine operation at low temperature.50

3.2. Steady Burning Droplet (Stage 2). 3.2.1. Burn-rate constant. 21 ACS Paragon Plus Environment

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After ignition, steady burning is observed for all fuel droplets where A/Ao decreased linearly with time as shown in Figure 7. A linear decrement in D2-t plot was obtained for all fuels droplet as illustrated in Figures 14a and 14b. The D2 law equation55 was utilized in this study based on the D2 law assumptions that: 1) the droplet is spherical, 2) the square of the droplet diameter decreases linearly with time, 3) heat convection from the droplet to air is minimized, and 4) the droplet experiment is conducted in a constant pressure environment. Similar works conducted by other researchers have also adopted the D2 law based on these assumptions.48, 49, 56 The burn-rate constant, k was calculated using D2 = Do2 – kt where, D is the droplet diameter, Do is the initial droplet diameter, and t is the duration taken between Do and D. The higher magnitude of burnrate constant is attributed to higher combustion rate between air and fuel vapor within a diffusion-controlled combustion51 as illustrated in Figure 13.


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1.8

(a) D2-t plots for 0.01% dosing ratio

Linear (Diesel) Linear (0.01% GDD)

1.6

Linear (0.01% ADD) Linear (0.01% CDD)

D2 / (mm2)

1.4

1.2

1

0.8

0.6 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Time / (s) 1.8

(b) D2-t plots for 0.1% dosing ratio

Linear (Diesel) Linear (0.1% GDD)

1.6

Linear (0.1% ADD) Linear (0.1% CDD)

1.4

D2 / (mm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2

1

0.8

0.6 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Time / (s)

Figure 14. D2-t plots for the baseline diesel fuel and the dosed diesel fuels at (a) 0.01% dosing ratio and (b) 0.1% dosing ratio.

In the present analysis, the burn-rate constant of the dosed diesel fuels increased up to 29% compared to the baseline diesel fuel as summarized in Figure 15. First, this can be attributed to the presence of oxygen content in the nanoparticles, resulting in more rapid combustion within 23 ACS Paragon Plus Environment

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the flame front. Secondly, more heat is scattered by the nanoparticles owing to their greater radiative heat absorbance (see Figure 13) thereby causing higher vaporization rate of the fuel droplet for more rapid burning. The highest increase in burn-rate constant is found for 0.1% GDD by 29.4% followed by 0.01% GDD with 27.9% as presented in Figure 15. The exceptionally higher burn-rate constant of GDD can be explained by the superior radiative heat absorbance of GO in the range between 1100 nm and 2000 nm as depicted in Figure 10b. In this range, high intensity radiation emitted by the flame at 1500 K to 3000 K57, 58 is greatly absorbed by GO resulting in higher vaporization rate of the fuel droplet and hence burning occurs more rapidly. On the other hand, the burn-rate constant of ADD and CDD increased significantly when the dosing ratio is increased from 0.01% to 0.1% as outlined in Figure 15. The observed trend may be attributed to the increased reactivity of Al2O3 and CeO2 nanoparticles with air-fuel vapor at higher dosing ratio, thus resulting in increased burn-rate. Overall, the higher burn-rate constant observed for all the dosed fuel compared to diesel fuel denotes more complete combustion due to the additional oxygen and high heating values supplied during the decomposition of the nanoparticles.59 The high heating value increases the kinetic energy of both oxygen and fuel molecules, thereby leading to more effective collisions between the fuel and oxygen. Since more molecules are reacting in the same period of time, this increases the rate of combustion. Fuel droplet with high burn-rate is beneficial in diesel engine applications for achieving low fuel consumption and low emissions (i.e., CO, soot, and unburned hydrocarbons).60


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Figure 15. Burn-rate constant comparison between the baseline diesel fuel and the dosed diesel fuels at 0.01% and 0.1% dosing ratio.

3.2.2. Micro-explosions. Micro-explosion is an important factor that can promote droplet breakup into smaller droplets during the combustion process allowing smaller droplets to burn more rapidly.47, 60 It is caused by heterogeneous nucleation, where nucleation occurs at the droplet surface.61 In this work, micro-explosion frequency is determined for all the fuels by the total occurrences of microexplosion observed62-64 as shown in Figure 8. The 0.1% GDD produced the greatest microexplosion frequency followed by 0.01% GDD with 200% and 100% increase over diesel fuel, respectively as summarized in Figure 16. On the other hand, the micro-explosion frequency of 0.01% ADD and 0.01% CDD are lower by 30% and 40%, respectively compared to diesel fuel. 25 ACS Paragon Plus Environment

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The reasons for the higher micro-explosion frequency of GDD droplet can be explained by the energetic decomposition of GO particles40, 65 at elevated temperature as presented in Figure 17. It may be mentioned that GO is bonded by the weaker van der Waals forces, whereas Al2O3 and CeO2 are held together by the stronger ionic bonds, giving the latter two higher resistance to thermal decomposition. The increased micro-explosion frequency of GDD can speed up the combustion process in diesel engine.

Figure 16. Micro-explosion frequency comparison between the baseline diesel fuel and the dosed diesel fuels at 0.01% and 0.1% dosing ratio.


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100

80

GO GO Al2O3 Al2O3

Weight / (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

CeO2 CeO2

60

GO decomposes at 180˚C

40

Ionic bonds in Al2O3 and CeO2 are stronger than GO van der Waals forces.

20

0 50

200

350

500

650

800

Temperature / (˚C)

Figure 17. Thermal decomposition of GO, Al2O3, and CeO2 nanoparticles using thermogravimetric analysis.

During the fuel injection, micro-explosions from GDD droplets further extend the air-fuel vapor region as shown in Figures 18a and 18b. As the spray droplets travel further downstream, coalescence decreases due to expansion of the spray and breakup ceases due to the reduced relative velocity between the droplets and entrained air.66 At this region, micro-explosions from GDD droplets discharge smaller droplets and extend the air-fuel vapor region to achieve more rapid burning as illustrated in Figure 18b. As a result, higher fuel conversion efficiency and increased work output can be achieved in diesel engine.


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Figure 18. Micro-explosions extend further the distribution of droplets as illustrated in (a) the crosssection view and (b) the top view of a typical diesel engine combustion chamber.

3.3. Unsteady Burning Droplet (Stage 3). The unsteady burning droplet is defined as the disruption of the droplet. Droplet disruption behavior is observed for all the fuels as continuous bubble generation, and bubble explosion occurs during the droplet burning as illustrated in Figures 7 and 8. Bubble generation is caused by the homogeneous nucleation, where bubbling sites occur inside the droplet.67 As the droplet shrinks, cavities within the droplet start to merge together into a larger bubble. This is followed by the expansion of the bubble and it eventually explodes. At the end of droplet burning, flame extinction is identified by the visible smoke for all the test fuels as illustrated in Figure 8.

3.3.1. Peak temperature. Peak temperature (point iv) was obtained for all the test fuels near the end of the droplet burning, as illustrated in Figure 7. Here, as the droplet shrinks, the flame gets closer to the thermocouple holder, hence causing the temperature to be higher with time. Therefore, the peak 28 ACS Paragon Plus Environment

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temperature can closely estimate the flame temperature of the droplet that are responsible the formation of NOx.50 In this investigation, the peak temperature of all the dosed diesel fuel is found to be reduced and can reach up to 13.8% reduction compared to the baseline diesel fuel as summarized in Figure 19. This can be associated with the high heat absorptivity in the infrared and visible light range (see Figure 10) for the unburned nanoparticles that are deposited on the thermocouple wire. Similar observation for the combustion residue of aluminum nanoparticle that is formed on the tip of the fiber was also reported by Gan and Qiao.27 Consequently, this causes more heat to be absorbed by the nanoparticles, thus resulting in lower peak temperature measured at the thermocouple wire. During the droplet burning, additional heat absorbed by the nanoparticles is transported away from the fuel droplet by diffusion. The peak temperature is thus reduced. The highest and lowest reduction in the peak temperature is found for 0.01% GDD (reduction by 13.8%) and 0.1% GDD (reduction by 7.8%), respectively. The contrasting trend observed between 0.01% GDD and 0.1% GDD is likely related to their relatively large difference in the particle size (difference by 61%) as depicted in Figure 11: the smaller GO particles that are relatively lighter tend to diffuse away heat absorbed more rapidly over larger GO particles, thereby resulting in lower peak temperature for 0.1% GDD.


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Figure 19. Peak temperature comparison between the baseline diesel fuel and the dosed diesel fuels at 0.01% and 0.1% dosing ratio.

By contrast, higher reduction in the peak temperature is observed when the dosing ratio of ADD and CDD is increased from 0.01% to 0.1% as shown in Figure 19. The aforementioned trend is likely due to the accumulation of more particles in the droplet as the droplet shrinks. As a result, more incomplete combustion occurs thus resulting in low peak temperature. Overall, the significantly lower peak temperature observed for 0.1% GDD over the baseline diesel fuel is interesting as it indicates lower flame temperature that may hinder the formation of NOx.

3.3.2. Burnout time. The burnout time (point v) for the baseline diesel fuel and the dosed diesel fuels is obtained as illustrated in Figure 7. The burnout time of the dosed diesel fuel is reduced by up to 19.2% 30 ACS Paragon Plus Environment

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Energy & Fuels

compared to the baseline diesel fuel as shown in Figure 20. This can be attributed to the shorter ID and higher burn-rate constant in all the dosed diesel fuel as illustrated in Figure 9 and 15, respectively. Here, the shortened ID and increased burn-rate constant can collectively result in a more rapid consumption of the fuel droplet and hence, shorter burnout time is expected.

Figure 20. Burnout time comparison between the baseline diesel fuel and the dosed diesel fuels at 0.01% and 0.1% dosing ratio.

Significant difference in the burnout time is also observed for different nanoparticle type and dosing ratio as summarized Figure 20. The highest reduction in burnout time is found for 0.1% GDD (reduction by 19.2%), followed by 0.1% CDD (reduction by 16.7%), and 0.1% ADD (reduction by 13.8%). The shortest burnout time for 0.1% GDD is mainly caused by the highest burn-rate and highest micro-explosion frequency as illustrated in Figure 16. Therefore, the 31 ACS Paragon Plus Environment

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aforementioned factors can contribute to higher rate of consumption of the fuel droplet. On the other hand, the burnout time for ADD and CDD is shortened when dosing ratio is increased from 0.01% to 0.1% as presented in Figure 20. The shorter ID and higher burn-rate constant found for ADD and CDD at 0.1% dosing over ADD and CDD at 0.01% dosing as depicted in Figures 9 and 15 contributed to shorter burnout time. In diesel engine operation, shorter burnout time in diesel fuel minimizes late combustion during the expansion stroke. As a result, this can reduce the incomplete combustion that causes fuel-rich combustion products (e.g., soot and CO).50 Furthermore, shorter burnout time allows the droplet combustion to occur near the top dead center, thus resulting in higher in-cylinder pressure.51,

52

Consequently, higher work output and higher brake thermal efficiency can be

achieved.

4. CONCLUSIONS A comprehensive study of the combustion characteristics of GDD, ADD, and CDD at 0.01% and 0.1% dosing ratio was carried out using single droplet experiment. Preheating and ignition, steady burning, and unsteady burning were identified as the three distinctive droplet burning stages for all the tested fuels. All dosed diesel fuels showed improvements in the combustion characteristics over the baseline diesel fuels. 0.01% GDD and 0.1% GDD showed exceptionally higher burn-rate constant, higher micro-explosion frequency and shorter burnout time over ADD, CDD and the baseline diesel fuel. This suggests that GDD undergoes more complete combustion thereby releasing lesser CO, PM, and PAHs compared to ADD, CDD and the baseline diesel fuel. Additionally, more complete combustion in GDD will result in lower fuel consumption when used in a diesel engine. 0.01% GDD achieved lowest TPI and lowest peak temperature 32 ACS Paragon Plus Environment

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Energy & Fuels

which indicates GDD combust at lower flame temperature and hence resulting in more reduced NOx. Overall, in this study, GO was found to be very effective in improving the combustion characteristics of diesel fuel and could reduce pollutant emissions (e.g., PM, NOx, PAHs, and CO). Therefore, GO could be a potential diesel fuel additive that can dampen the continuous increase of pollutant emissions and the depletion of fossil fuel in near future.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Ooi Jong Boon acknowledges the PhD stipend provided by the School of Engineering, Monash University Malaysia. Varghese Swamy acknowledges Advanced Engineering Platform grant (Monash University Malaysia). Special thanks are due to Chai Siang Pao and Tan Ming Kuang for providing laboratory facilities.

ABBREVIATIONS A/Ao

=

Normalized Droplet Area

ADD

=

Aluminum Oxide Dosed in Diesel Fuel

Al2O3

=

Aluminum Oxide

ASTM

=

American Society for Testing and Materials 33 ACS Paragon Plus Environment

Energy & Fuels

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CDD

=

Cerium (IV) Oxide Dosed in Diesel Fuel

CeO2

=

Cerium (IV) Oxide

CO

=

Carbon Monoxide

D

=

Droplet Diameter

Do

=

Initial Droplet Diameter

dT/dt

=

Droplet Gradient Temperature

d2T/dt2

=

Second Derivative of Droplet Gradient Temperature

FTIR

=

Fourier Transform Infrared Spectroscopy

GDD

=

Graphite Oxide Dosed in Diesel Fuel

GO

=

Graphite Oxide

ID

=

Ignition Delay

k

=

Burn-rate Constant

NOx

=

Nitrogen Oxides

PAHs

=

Polycyclic Aromatic Hydrocarbons

PM

=

Particulate Matter

t

=

Time taken between Do and D

TEM

=

Transmission Electron Microscopy

TPI

=

Temperature at Point of Ignition

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