sign in sign up
<<
Home , Fuel oil

INFLUENCE OF ASPHALTENE CONCENTRATION ON THE COMBUSTION OF A HEAVY DROPLET

Item Type Article

Authors Alkhateeb, Abdulrahman; Elbaz, Ayman M.; Guida, Paolo; Roberts, William L.

Citation Khateeb A, Elbaz AM, Guida P, Roberts WL (2018) INFLUENCE OF ASPHALTENE CONCENTRATION ON THE COMBUSTION OF A HEAVY FUEL OIL DROPLET. Energy & . Available: http:// dx.doi.org/10.1021/acs.energyfuels.8b03260.

Eprint version Post-print

DOI 10.1021/acs.energyfuels.8b03260

Publisher American Chemical Society (ACS)

Journal Energy & Fuels

Rights This document is the Accepted Manuscript version of a Published Work that appeared in final form in Energy & Fuels, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/ acs.energyfuels.8b03260.

Download date 27/09/2021 18:19:19

Link to Item http://hdl.handle.net/10754/630014 Subscriber access provided by King Abdullah University of Science and Technology Library Combustion INFLUENCE OF ASPHALTENE CONCENTRATION ON THE COMBUSTION OF A HEAVY FUEL OIL DROPLET Abdulrahman Khateeb, Ayman M. Elbaz, Paolo Guida, and William L. Roberts Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03260 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

Just Accepted

“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 48 Energy & Fuels

1 2 3 4 INFLUENCE OF ASPHALTENE CONCENTRATION ON 5 6 7 THE COMBUSTION OF A HEAVY FUEL OIL DROPLET 8 9 Abdulrahman A. Khateeba*, Ayman M. Elbaza, Paolo Guidaa, William L. 10 11 12 Robertsa 13 14 15 a 16 Clean Combustion Research Center, King Abdullah University of Science and 17 18 Technology, 19 Thuwal, 23955-6900, Saudi Arabia. 20 21 22 *Corresponding author e-mail: [email protected] 23 24 25 Highlights 26 27  Thermogravimetric analysis of heavy fuel oil samples at different asphaltene 28 concentration is discussed. 29 30  The influence of high asphaltene concentration on the droplet burning stages is 31 illustrated. 32 33  The relation between the droplet ignition delay time and droplet size is discussed 34 35 at different asphaltene concentration. 36  The effect of asphaltene concentration on the droplet evolution with time is 37 38 measured. 39 40 Keywords 41 42 Asphaltene, Burning stages, Heavy fuel oil, Thermogravimetric analysis. 43 44 45 Nomenclature 46 퐴 47 frequency factor 48 퐸 activation energy 49 푘 50 rate constant 51 푛 reaction order 52 53 푅 universal gas constant 54 푡 time 55 56 푇 absolute temperature 57 58 59 60 ACS Paragon Plus Environment Energy & Fuels Page 2 of 48

1 2 3 4 wt% weight percentage 5 훼 fraction decomposition 6 7 API American institute 8 ASTM American society for testing and materials 9 10 DTG differential thermogravimetric 11 FD fuel decomposition 12 13 FT-ICR Fourier transform ion cyclotron resonance 14 GDT glowing delay time 15 16 HFO heavy fuel oil 17 18 HTO high-temperature oxidation 19 IDT ignition delay time 20 21 Le Lewis number 22 LTO low-temperature oxidation 23 24 NMR nuclear magnetic resonance 25 Slpm standard liter per minute 26 27 TGA thermogravimetric analysis 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 1 59 60 ACS Paragon Plus Environment Page 3 of 48 Energy & Fuels

1 2 3 4 Abstract 5 6 7 Heavy fuel consist of a blend of middle distillates, mainly , and 8 9 10 heavy oil residuals. Varying the fraction of the mixture changes the weight percentage 11 12 13 of the asphaltene in the heavy fuel oil (HFO) sample. Asphaltene is a very high molecular 14 15 16 weight complex component in the fuel which increases the fuel , surface tension, 17 18 19 and chemical reaction rate. Here, we investigate the influence of high asphaltene 20 21 22 concentration on the combustion of a single HFO droplet. In this experimental work, we 23 24 25 used the thermogravimetric analysis (TGA) and the suspended droplet techniques. We 26 27 28 tested HFO samples containing asphaltene at 8, 16, 24 wt% (HFO8, HFO16, and 29 30 31 HFO24). The TGA result shows a residual amount of approximately 2.4 wt% of the 32 33 34 HFO24 compared to no residuals for the HFO8 at the end of the process. The suspended 35 36 37 38 droplet technique results reveal the following seven consecutive burning stages for the 39 40 41 entire burning process of the liquid and solid phases: 1) pre-heating, 2) flame startup, 42 43 44 3) inner evaporation, 4) thermal decomposition, 5) solidification, 6) coke pre-ignition, and 45 46 47 7) smoldering. The temperature range of the various burning stages is seen to be 48 49 50 independent of both the concentration of the asphaltene and the initial size of the droplet. 51 52 53 On the other hand, both the total burning time and ignition delay time become longer 54 55 56 by 40% and 26% respectively as the content of the asphaltene increases from 8 to 24 57 58 2 59 60 ACS Paragon Plus Environment Energy & Fuels Page 4 of 48

1 2 3 4 wt% in the HFO sample. The evolution of the droplet’s size in time shows that the 5 6 7 maximum size of the droplet becomes larger by a factor of 2 for the HFO24 compared 8 9 10 to the HFO8 sample. 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 3 59 60 ACS Paragon Plus Environment Page 5 of 48 Energy & Fuels

1 2 3 4 1. Introduction 5 6 7 As the demand for fuels continues to increase rapidly, oil production 8 9 10 industries and consumers are paying considerable attention to the dynamic performance 11 12 13 and reliability of the combustion process of these fuels. Heavy fuel oil (HFO) has 14 15 16 generated increasing interest due to its lower costs, higher durability and heat efficiency 17 18 19 compared to similar conventional light oils. Since 2015, HFO market prices have remained 20 21 22 approximately 30% lower than crude oil prices 1. Additionally, the HFO products from a 23 24 25 gasification process, mainly CO and H2, can be primarily used in power generation 26 27 28 systems with lower emissions 2-4. The use of different HFO methodologies, such as the 29 30 31 gasification process, besides the projected fuel prices, indicates a promising consumption 32 33 34 of the heavy fuel oil resources for small-scale power generation systems 5. However, 35 36 37 38 the use of HFO is facing many challenges due to its negative impacts on the environment, 39 40 41 resulting mainly from the emissions of cenospheres and oxides of . Additionally, 42 43 44 HFO has harmful corrosive effects on the equipment used in power generation systems, 45 46 47 due to its high content of sulfur. For these reasons, the use of HFO as a source of 48 49 50 energy has drastically decreased after reaching the peak in the late of 1970s 5. 51 52 53 The complete combustion of the HFO requires the complete burning of the 54 55 56 volatile constituents, in addition to the coke residues. Otherwise, incomplete combustion 57 58 4 59 60 ACS Paragon Plus Environment Energy & Fuels Page 6 of 48

1 2 3 4 of the fuel occurs and thus is more likely to generate and coke particles. The poor 5 6 7 carbon efficiency of HFO leads to a significantly higher amount of emissions in 8 9 10 the surrounding environment, compared to light fuel oils 6 7. The composition of heavy 11 12 13 fuel oils can vary greatly; as any HFO consists mostly of a blend of middle distillates, 14 15 16 mainly diesel, and a heavy oil residual. Varying the fraction of the middle distillate plays 17 18 19 a critical role on the key properties such as viscosity, surface tension, and content of 20 21 22 asphaltene, all of those being determinant factors for the HFO combustion characteristics. 23 24 25 This concept forms the primary objective of this study, that is, to generalize the effects 26 27 28 of the concentration of asphaltene on the burning stages of the heavy fuel oil. 29 30 31 In , the HFO is atomized via assisted or air blast 32 33 34 nozzles. Most of the literature is therefore based on sprays of fuel oils. The goal of 35 36 37 38 such studies is to collect qualitative and quantitative information about the combustion 39 40 2 5 8-10 41 process and burning stages of the fuel droplets . Even though generating a spray 42 43 44 is implemented in almost every industrial application, the single suspended droplet 45 46 47 technique is a powerful tool that permits the simplification of the problem and studies 48 49 50 the effect of different technologies, such as the modification of the asphaltene content 51 52 53 or the addition of water, on the burning process. Therefore, studying a single suspended 54 55 56 droplet is used to explore the fundamental combustion characteristics of heavy fuel oils. 57 58 5 59 60 ACS Paragon Plus Environment Page 7 of 48 Energy & Fuels

1 2 3 4 Previous research studies on a single suspended droplet revealed various HFO 5 6 7 combustion stages that can be divided into physical and chemical phenomena, both 8 9 10 taking place in liquid and gas phase 10-14. 11 12 13 Moreover, the single droplet technique has been used to study the ignition and 14 15 16 combustion characteristics of a wide range of different fuel samples. For example, heavy 17 18 19 oil residues 15, diesel and fuels 16-18, and slurry fuels 19 20. Another group of 20 21 22 researchers used the suspended droplet technique to investigate the use of non-organic 23 24 25 dopants on waste-derived fuel and coal-water slurry to prepare compositions which are 26 27 28 environmentally friendly and relatively high in power efficiency 21-24. 29 30 31 Braide et al.6 successfully divided the liquid phase into two steps: preheating and 32 33 34 flaming, and Baert et al.25 showed that the flaming step was a complicated process 35 36 37 38 involving pyrolysis and a polymerization chemical reaction. The pyrolysis produces soot 39 40 41 around the droplet, and the polymerization forms a solid particle at the end of the liquid 42 43 9 44 phase. Additional studies by Ikegami et al. suggested that the liquid phase could be 45 46 47 subdivided into four consecutive stages: pre-heating, evaporation, , and 48 49 50 polymerization towards coke residuals. For more than 40 years, the solid phase was 51 52 53 considered as a one-step oxidization process. More recently, it has been shown to be 54 55 56 composed of two steps, glowing and smoldering 5, and that these stages are mainly 57 58 6 59 60 ACS Paragon Plus Environment Energy & Fuels Page 8 of 48

1 2 3 4 affected by the fuel composition, droplet size, and combustion environment around the 5 6 7 droplet. Asphaltene is a very complex component in the heavy fuel oil consisting of 8 9 10 aromatic and naphthenic rings linked by chemical bonds, resulting in a high fuel viscosity 11 12 13 and surface tension 26. 14 15 16 One of the main causes of the formation of a cenosphere is the presence of 17 18 19 asphaltene. The cenosphere is the name given to the hollow carbonaceous spheres 20 21 22 formed during the combustion of heavy fuel oil droplets 27. Several studies using the 23 24 25 HFO suspended droplet technique 2 5 9 10, showed variable weight percentage of 26 27 28 asphaltene ranging between 3 and 17 wt% (Table 1). In our current study, we tested 29 30 31 HFO samples with a higher percentage of asphaltene reaching up to 24 wt%. 32 33 34 Table 1: Summary of the weight percentage of asphaltene in heavy fuel oils from the 35 36 related studies using the suspended droplet technique. 37 38 Authors (Year) Asphaltene wt% 39 Bartle et al. (2013) 2 3 6 9 12 40 41 Ikegami et al. (2003) 9 11 17 - - 42 Xu et al. (2002) 5 11 17 - - 43 44 Villasenor et al. (1999) 10 13 17 - - 45 46 Current study 8 16 24 - 47 48 49 50 The content of asphaltene can be adjusted by varying the fraction of the light oil 51 52 53 blended with the HFO. As the demand for light oil cuts increases, HFOs will be blended 54 55 56 57 58 7 59 60 ACS Paragon Plus Environment Page 9 of 48 Energy & Fuels

1 2 3 4 with a lower fraction of light oils, which, in turn, would increase the percentage of the 5 6 7 concentration of asphaltene. Therefore, in compliance with the future economic demand 8 9 10 in both light and heavy fuel oils, HFO with higher content of asphaltene is expected to 11 12 13 be used in power generation systems 5. With this in mind, we investigate the influence 14 15 16 of high asphaltene concentration through the thermogravimetric analysis and the burning 17 18 19 stages of suspended droplets using HFO samples with 8, 16, and 24 weight percent of 20 21 22 asphaltene. The main goal of this study is to help modelers, and burner designers better 23 24 25 understand the influence of a high concentration of asphaltene on the burning process, 26 27 28 and ultimately increase the HFO combustion efficiency while reducing maintenance 29 30 31 expenses. 32 33 34 2. Experimental procedure 35 36 37 38 A heavy fuel oil (HFO) sample collected from the Shuaiba power plant in Saudi 39 40 41 Arabia was tested experimentally, using the suspended droplet technique. HFO8, the 42 43 44 base oil sample containing approximately 8 wt% asphaltene, was used as a reference. 45 46 47 Its physical properties and elemental compositions are presented in Table 2. Two other 48 49 50 HFO samples were prepared, one containing 16 wt% in asphaltene (HFO16) and the 51 52 53 other 24 wt% (HFO24). These two samples of higher concentration of asphaltene were 54 55 56 prepared in our laboratory, where the asphaltene component was extracted from a portion 57 58 8 59 60 ACS Paragon Plus Environment Energy & Fuels Page 10 of 48

1 2 3 4 of the base oil, HFO8, following the ASTM D3279 standard separation process for 5 6 7 asphaltene. The HFO8 sample was treated with n-heptane to separate the asphaltene 8 9 10 from the HFO sample. HFO8 was added to n-heptane at a 1: 70 weight ratios and the 11 12 13 mixture was subsequently stirred for one hour at 60 oC, followed by three hours at 14 15 16 ambient temperature. The solution was then left untouched overnight, before filtering. A 17 18 19 filtering pad was used under vacuum to separate and extract the asphaltene component 20 21 22 from the mixture. The extracted asphaltene was subsequently left in an oven at 90 oC 23 24 25 to dry out the remaining n-heptane moisture. It was then added, by weight percentage, 26 27 28 to the base oil HFO8 to prepare the HFO16 and HFO24 samples. 29 30 31 32 33 Table 2: HFO8 physical properties and compositional data. 34 35 Physical properties Units Results 36 Density at 288 K kg/m3 970.5 37 o 38 Specific gravity (60/60 F) - 0.9711 39 Kinematic viscosity at 40 oC cSt 618 40 41 Compositional data 42 43 Sulfur mass% 3.3% 44 Asphaltene content wt% 8.2% 45 46 Vanadium mg/kg 18.0 47 Nickel mg/kg 11.0 48 49 Sodium mg/kg 3.4 50 Zinc mg/kg <1.0 51 52 Lead mg/kg 0.4 53 Potassium mg/kg 0.1 54 55 Carbon mass% 85.0% 56 57 58 9 59 60 ACS Paragon Plus Environment Page 11 of 48 Energy & Fuels

1 2 3 4 Hydrogen mass% 10.9% 5 Oxygen mass% 0.03% 6 7 Nitrogen mass% 0.24% 8 Heating values 9 10 Higher heating value BTU/IB 18258 11 12 Lower heating value BTU/IB 17255 13 14 15 16 Before conducting suspended droplet experiments, the thermal oxidation behavior 17 18 19 of the HFO8 and HFO24 samples was investigated under non-isothermal 20 21 22 thermogravimetric (TG) conditions. For the various content of asphaltene, 23 24 25 thermogravimetric data were used to perform kinetic calculations. For a given content of 26 27 28 asphaltene, the HFO sample was studied using different techniques including 29 30 31 32 thermogravimetric and differential thermogravimetric (TG/DTG), Fourier transforms Ion 33 34 35 cyclotron resonance (FT-ICR) mass spectroscopy, and nuclear magnetic resonance 36 37 28-30 38 (NMR) spectroscopy . Thermal analysis techniques have been used to investigate 39 40 41 the combustion characteristics of different fuel samples 30-35. The first application of the 42 43 44 thermo-analytical method to study the combustion of crude oil was applied by Tadema 45 46 47 36. Subsequently, different research groups have this used the same methodology of 48 49 50 TGA to evaluate the combustion, pyrolysis kinetic parameters of different fuel samples, 51 52 53 for example, HFO 31, individual oil fractions 32 33 37, medium and heavy crude oil 34. 54 55 56 57 58 10 59 60 ACS Paragon Plus Environment Energy & Fuels Page 12 of 48

1 2 3 4 Recently, with the TGA data, the kinetic parameters of Chinese HFO sample were 5 6 35 7 evaluated using the distribution activation energy method . In our study, the 8 9 10 thermogravimetric analysis (TG/DTG) was conducted using a Mettler Toledo TGA/DSC 11 12 13 (thermogravimetric analysis/differential scanning calorimetry) instrument. This instrument 14 15 16 was used to measure the sample weight loss for a wide range of temperature (20 – 17 18 19 1100 oC), at a high resolution of 0.1 µg. The instrument was equipped with a GC 200 20 21 22 gas controller and an auto-sampler. For each experiment, a sample of approximately 10 23 24 25 mg was weighted and placed inside an Al2O3 crucible. Pure air at a 50 mL/min flow 26 27 28 rate was used for the experimental combustion tests, whereas nitrogen gas was used 29 30 31 at a 25 mL/min flow rate for purging. Experiments were performed, at a heating rate of 32 33 34 5 oC/min over a 25 – 1000 oC temperature range, under non-isothermal conditions. Before 35 36 37 38 each experiment, the TG analyzer was tested and calibrated using a calcium oxalate 39 40 41 monohydrate standard. A blank test was conducted before each experiment to correct 42 43 44 for the buoyancy effects, with the same crucible as that used for the TG analysis. 45 46 47 Additionally, to ensure the reproducibility of the results, tests were repeated three 48 49 50 times. The main goals of these experiments were to investigate the effects of the 51 52 53 asphaltene concentration on the thermal oxidation behavior. Consequently, an Arrhenius 54 55 56 57 58 11 59 60 ACS Paragon Plus Environment Page 13 of 48 Energy & Fuels

1 2 3 4 model was applied to the TG/DTG non-isothermal data to predict the apparent activation 5 6 7 energy for the two samples HFO8 and HFO24. 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Figure 1: Schematic of the experimental setup presenting the main equipment. 27 28 Figure 1 shows a schematic of the suspended droplet experimental setup. To 29 30 31 support the droplet and to also measure its internal temperature, we used a type S 32 33 34 35 thermocouple made of platinum (Pt) and Platinum-rhodium (Pt-Rh, 13 wt% Rh) wires 36 37 38 with a diameter of 75 µm. A vertical quartz tube (inner diameter = 134 mm, and length 39 40 41 = 750 mm) was placed inside a vertically oriented cylindrical (Lindberg/blue, 42 43 44 HTF55000 series) with control consoles from the standard Lindberg 58000 series. An 45 46 47 inline heater (Sylvania heater, F074719, 8000 Watt) was used to preheat the convective 48 49 50 air stream flowing inside the quartz tube. The preheated air was supplied from the 51 52 53 bottom of the furnace, passing through a ceramic honeycomb, to ensure a uniform air 54 55 56 distribution around the droplet. At the droplet location, the air flow rate setting point is 57 58 12 59 60 ACS Paragon Plus Environment Energy & Fuels Page 14 of 48

1 2 3 4 kept at 20 slpm, yielding a velocity of 8.2 cm/s, and the diameter is kept constant, at a 5 6 7 set value between 720 and 750 µm. Then, the suspended droplet was placed into the 8 9 10 heated air stream that is flowing through the quartz tube. A micro syringe containing the 11 12 13 heavy fuel oil sample was used to hang the droplet on the thermocouple bead. The 14 15 16 syringe was filled with the quantity corresponding to one droplet only, to control the 17 18 19 droplet size. An image analysis MATLAB code was developed to measure the size of 20 21 22 the droplet at the beginning of the experiment, and to track its evolution in time. This 23 24 25 code can identify the equivalent droplet diameter, as it transformed high-speed images 26 27 28 into a binary matrix based on the color gradient measured from the RGB matrix. The 29 30 31 gradient was then compared with a threshold that allowed us to discriminate among the 32 33 34 phases. The binary matrix of the droplet obtained from this procedure was then 35 36 37 38 normalized using the area of the first image. This procedure generated a value 39 40 41 corresponding to the normalized squared diameter. This procedure is important in 42 43 44 measuring the droplet size since the droplet did not remain spherical during evaporation, 45 46 47 but its shape was altered both by the influence of the thermocouple and the micro- 48 49 50 behavior. The uncertainty associated with this procedure was directly related 51 52 53 to not taking into account the third dimension. 54 55 56 57 58 13 59 60 ACS Paragon Plus Environment Page 15 of 48 Energy & Fuels

1 2 3 4 Moreover, the noise of the captured images created an experimental uncertainty, 5 6 7 in the order of one pixel. A 2-D translation system controlled the thermocouple stem 8 9 10 movement inside the furnace through a side channel in the quartz tube. This allowed 11 12 13 the droplet burning location inside the furnace to stay in a fixed stable position, and for 14 15 16 the camera imaging to not show variations in the position of the droplet. An air-cooled 17 18 19 shield surrounding the thermocouple carried the droplet until it reached the reaction 20 21 22 position with the hot air stream to reduce the heat transfer from the heated air to the 23 24 25 heavy fuel oil droplet during the insertion of the droplet inside the furnace. Prior to 26 27 28 experiments, the radial temperature readings of the heated co-flow air were recorded in 29 30 31 a range of 25 mm radius around the central position, showing a uniform temperature 32 33 34 distribution of 950 K. The temperature was recorded using an NI114 data acquisition 35 36 37 38 card, at a 14 Hz sampling rate, while droplet images were acquired through a high- 39 40 41 speed camera (Imager HS), at a 1000 frame per second, capturing all the combustion 42 43 44 stages. 45 46 47 48 49 50 51 52 53 54 55 56 57 58 14 59 60 ACS Paragon Plus Environment Energy & Fuels Page 16 of 48

1 2 3 4 3. Results and discussion 5 6 7 3.1 Thermogravimetric analysis 8 9 10 The heavy fuel oil combustion process involved several physical and chemical 11 12 13 reactions. The description of the combustion behavior is complex because it involves a 14 15 16 wide variety of having boiling temperatures within a wide range from 160 17 18 19 to 700 oC. The presence of heavy hydrocarbons leads to liquid phase reactions such as 20 21 22 pyrolysis and polymerization. The lighter fraction (e.g., diesel fuel) commonly used to 23 24 25 reduce the viscosity of the fuel and to reduce the ignition delay time (IDT) corresponded 26 27 28 to approximately 40 wt% of the HFO. Figure 2 shows TG data obtained during the 29 30 31 combustion of the two samples HFO8 and HFO24, with their TG/DTG curves and 32 33 34 combustion stages shown in Fig. 2b and Fig. 2c respectively. 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 15 59 60 ACS Paragon Plus Environment Page 17 of 48 Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Figure 2. (a) Non-isothermal thermogravimetric data for HFO8 and HFO24 samples, (b) 20 TG/DTG combustion curve of the HFO8 sample, and (c) TGA/DTG combustion curve of 21 22 the HFO24 sample, all at a heating rate of 5 oC/min 23 24 As indicated in Fig. 2, the sample with the highest content of asphaltene, HFO24, 25 26 27 led to a time delay of the sample mass reduction; it showed that there was no further 28 29 30 31 apparent thermal degradation in the HFO24 sample beyond a temperature of 32

33 o 34 approximately 580 C. A residual amount of approximately 2.4 wt% was found at 700 35 36 o 37 C (Fig. 2), whereas, for the raw sample, no residue was obtained at the end of the 38 39 40 process. A close inspection of the TG/DTG plots presented in Fig. 2b and Fig. 2c 41 42 43 highlighted three distinct regions from the DTG, in the oxidation of both samples. These 44 45 46 three regions correspond to 1) low-temperature oxidation (LTO), 2) fuel decomposition 47 48 49 (FD), and 3) high-temperature oxidation (HTO). In the LTO region, the fuel was heated 50 51 52 and started to volatilize. Different processes belonged to this stage, with a major 53 54 55 contribution coming from the evaporation of the light fraction, similarly to the commercial 56 57 58 16 59 60 ACS Paragon Plus Environment Energy & Fuels Page 18 of 48

1 2 3 4 diesel cut. In both samples, the evaporation occurred in two consecutive steps, revealed 5 6 7 by the two peaks in the DTG profile. After a mass reduction of approximately 1 wt%, a 8 9 10 smooth continuous decrease in the TG curve was observed, indicating a rapid mass 11 12 13 loss. The DTG profile reached a peak value, defined as P1, at a temperature of 147 oC 14 15 16 for the HFO8 sample and 172 oC for the HFO24. Up to this point, the low boiling gases 17 18 19 left out the fuel, but as the temperature further increased, the rate of volatilization 20 21 22 decreased, reaching a minimum value after P1, first for the HFO8, and then for the 23 24 25 HFO24 sample, at a higher temperature. The second phase of the devolatilization started 26 27 28 from the minimum point on the DTG curve (after P1) when the vaporization started to 29 30 31 become more rapid again. This showed a faster decrease in the sample mass 32 33 34 degradation than the first volatilization stage, reaching a peak point P2 on the DTG 35 36

37 o o 38 curve, at a temperature of 352 C for the HFO8, and 347 C for the HFO24 sample. 39 40 41 The occurrence of two different volatilization stages was attributed to the polymerization 42 43 44 reactions forming a layer on the surface of the sample, resulting in a limitation of the 45 46 47 mass transfer retaining the hydrocarbons with a higher boiling point. This phenomenon 48 49 50 (called the skin-like effect) was revealed in previous studies 39. The side chains of the 51 52 53 heavy compounds were subsequently split and subjected to the low-temperature oxidation 54 55 56 27 30 40. 57 58 17 59 60 ACS Paragon Plus Environment Page 19 of 48 Energy & Fuels

1 2 3 4 During the FD phase, oxidation reaction between the gases generated from the 5 6 7 previous step (LTO region) continued with the liquid hydrocarbon phase. The higher 8 9 10 temperature within this zone and the existence of heavy metals such as Vanadium and 11 12 13 Nickel (see Table 2 for oil compositional and elemental analysis) catalyzed the liquid 14 15 16 phase oxidation 41. The catalytic effect of the metals explained the steep decrease in 17 18 19 the mass of the sample observed in the FD region. In this region, the DTG plots revealed 20 21 22 sharp fluctuations (up and down), corresponding to a breakdown of the skin-like shape 23 24 25 by the volatile fractions formed in the LTO region. The FD region ranged from 371 to 26 27 28 466 oC for the HFO8, and from 376 to 472 oC for the HFO24 sample, with more 29 30 31 fluctuations for the HFO24 sample, indicating a high resistance for mass transfer. As the 32 33 34 FD region came to an end, the rate of mass degradation decreased again, due to a 35 36 37 38 mass transfer resistance by the formation of carbonaceous particles. This process was 39 40 41 found to be similar to the creation of shell type structures (cenosphere) during the 42 43 27 42 43 44 combustion of the HFO droplets . We expected to observe a high ratio of carbon 45 46 47 to hydrogen residues at the end of the FD region with holes and cracks within the 48 49 50 sample due to the emission of gases through the shell-like surface, similarly was observed 51 52 53 in previous experimental studies 2. The high-temperature oxidation, HTO, was the final 54 55 56 region of the combustion of the HFO sample and it extended from 466 oC to 577 oC 57 58 18 59 60 ACS Paragon Plus Environment Energy & Fuels Page 20 of 48

1 2 3 o o 4 for the HFO8 sample, and from 472 C to 566 C for the HFO24 sample. Within this 5 6 7 region, the reactions with the oxygen occurred heterogeneously on the surface of the 8 9 10 formed shell. In this stage, the oxygen diffused into the shell inside the surface holes 11 12 13 and cracks showing the highest rate of mass degradation. As a result of the higher 14 15 16 content of asphaltene, for the HFO24 sample and at the end of the HTO region, nearly 17 18 19 97.5 wt% of the initial mass was lost, leaving a residual mass of 2.5 wt% at 700 oC. A 20 21 22 value of nearly 0.1 wt% was found at the same temperature, for the HFO8 sample. The 23 24 25 cenosphere size was found to be directly proportional to the initial asphaltene 26 27 28 concentration 27. Thus, we found the residual ash more abundant in the HFO24 sample 29 30 31 than in the HFO8. The regions for which reactions occur and the temperature of the 32 33 34 peak points, for the two samples, are summarized in Table 3. 35 36 37 38 Table 3: Reaction regions, and the temperature of the peak points of the combustion of 39 the HFO8 and the HFO24 samples at a heating rate of 5 oC/min. 40 41 Sample LTO FD HTO P1 P2 Peak at HTO 42 HFO8 41-371 371-466 466-577 147 352 542 43 44 HFO24 72-376 376-472 472-566 172 347 532 45 46 47 A kinetic study of the HFO combustion is a challenging problem due to the 48 49 50 complexity of the fuel composition and the variety of physical reactions associated with 51 52 53 the combustion process, and which makes this topic different from a purely chemical 54 55 56 57 58 19 59 60 ACS Paragon Plus Environment Page 21 of 48 Energy & Fuels

1 2 3 The wide range of molecular weights and boiling points of the constituents of HFO makes the 4 problem. 5 6 combustion process complicated, and therefore, the fuel non-isothermal weight loss kinetics is a complex 7 8 phenomenon. This combustion complexity is due to the multiple simultaneous reactions; hence, the 9 10 evaluated kinetics parameters should be regarded as apparent kinetic values representing complex parallel 11 12 13 and serial reactions. In this work, we assumed that the mass transfer constraints have a negligible effect on 14 15 the reaction progress because of the small size of the sample, and there is an excess of purge gas supply 16 17 around the sample crucible. We also assumed that the rate of mass loss of the initial sample is only 18 19 dependent on the rate constant where the mass of the sample remains with a reaction order of unity and 20 21 temperature. Therefore, it is reasonable to assume that the first-order rate equation can describe the non- 22 23 isothermal combustion 44. 24 In the following analysis, we assumed that the limitations on the 25 26 44 27 mass transfer had no clear effect on the reaction behaviors . Here, we aimed to 28 29 30 investigate the effects of asphaltene concentration on the kinetic parameters. The 31 32 33 Arrhenius model 31-38 45 was used to estimate the apparent activation energy. The kinetic 34 35 36 data calculation was based on the following kinetic equation, where 훼 is the sample 37 38 39 fraction decomposition at any time 푡 , 푘 is the rate constant, and 푛 is the reaction order. 40 41 푑훼 푛 42 푑푡 = 푘(1 ― 훼) (1) 43 44 45 The temperature dependence of 푘 is expressed by the following Arrhenius equation, 46 47 48 where 퐴 is the frequency factor (1/s), 퐸 is the activation energy (kJ/mol), 푅 is the 49 50 51 universal gas constant (J/mol. K), and 푇 is the absolute temperature (K). 52 53 퐸 54 푘 = 퐴푒푥푝 ― 55 ( 푅푇) (2) 56 57 58 20 59 60 ACS Paragon Plus Environment Energy & Fuels Page 22 of 48

1 2 3 4 The Arrhenius model assumes that the rate of mass loss of the total sample depends 5 6 7 only on the rate constant, the remaining mass sample, and the temperature. Accordingly, 8 9 10 the Arrhenius type kinetic model takes the following form assuming the first order of 11 12 푑푤 13 reaction, where 푑푡 is the rate of mass change (g/s). 14 15 푑푤 16 푑푡 퐸 17 log 푤 = log 퐴 ― 2.303푅푇 (3) 18 19 20 We applied the Arrhenius model to the TG/DTG presented in Fig. 2. by plotting log [( 21 22 푑푤 23 )/ 푤 ] on the vertical axis versus 1/T on the horizontal axis; the activation energies 24 푑푡 25 26 27 of the HFO samples within the LTO and HTO regions were obtained via equating the 28 29 30 slope of the straight line to E/2.303 RT (Fig. 3), both for the HFO8 and the HFO24 31 32 33 samples. The estimated activation energy is listed in Table 4, and the Arrhenius model 34 35 36 is shown in Fig. 3. 37 38 39 Table 4: Activation energy obtained for different reaction regions of HFO8 and HFO24 40 samples. 41 42 Arrhenius model, 푬 (kJ/mole) 43 44 LTO region Sample 45 LTO up to HTO 46 LTO up to P1 47 P2 48 49 HFO8 26 39 177 50 HFO24 38 50 238 51 52 53 54 55 56 57 58 21 59 60 ACS Paragon Plus Environment Page 23 of 48 Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Figure 3. Arrhenius model to evaluate the apparent activation energy in the LTO and 20 HTO regions for (a) HFO8 sample, and (b) HFO24 sample. 21 22 23 As indicated by the TG/DTG curves in Fig. 2, the LTO regions have two stages 24 25 26 of volatilization for the low and high boiling volatile gases, leaving the sample up to the 27 28 29 peak points P1 and P2. Accordingly, as illustrated in Fig. 3, two straight lines could fit 30 31 32 the Arrhenius data model for the low and high boiling gases, in the LTO region. We 33 34 35 found that, for both samples, the second stage of volatilization, during which the high 36 37 38 boiling temperature, volatiles (aromatics) were released up to the peak point P2, had 39 40 41 apparent activation energy higher than in the first stage (which ends at the peak point 42 43 44 45 P1). At higher temperatures (Table 4), the estimated value of the activation energy value 46 47 48 was higher for the HTO region than for the LTO evaporation phases. This indicates that 49 50 51 the three distinct regions, LTO, FD, and HTO have different activation energies due to 52 53 54 the change in the combustion mode. In the LTO region, the reaction occurred in a 55 56 57 58 22 59 60 ACS Paragon Plus Environment Energy & Fuels Page 24 of 48

1 2 3 4 heterogeneous liquid phase, with two consecutive stages of low and high boiling volatiles. 5 6 7 The phase subsequently became a homogeneous gas phase in the FD region during 8 9 10 which the fuel compounds for the HTO region were formed. In the high-temperature 11 12 13 oxidation region, the hydrocarbon/char reactions played a dominant role. As shown in 14 15 16 Fig. 3, the increase in asphaltene contents led to an increase of the activation energy. 17 18 19 Slightly higher activation energy in the LTO region of the HFO24 compared to the HFO8 20 21 22 as the oxidation reaction of the asphaltene is slow in this region 46. However, as the 23 24 25 concentration of asphaltene increased, a significant increase in the activation energy was 26 27 28 observed in the HTO region during which the phase of the reaction changed into solid. 29 30 31 Similar observations were made for the activation energy across the different combustion 32 33 34 stages 34 46. It was also observed that the apparent activation energy increased as the 35 36 37 38 API gravity of the oil was decreased, due to the presence of higher asphaltene content 39 40 34 41 . 42 43 44 3.2 Burning stages 45 46 47 The burning stages of a heavy fuel oil droplet were illustrated using the suspended 48 49 50 droplet technique, where the initial diameter is approximately 720 – 750 µm. The air 51 52 53 stream temperature inside the furnace is set to 950 K, and the co-flow air rate is 54 55 56 constant at 20 slpm, giving a velocity of 8.2 cm/s. In previous work 27, the droplet size, 57 58 23 59 60 ACS Paragon Plus Environment Page 25 of 48 Energy & Fuels

1 2 3 4 air stream temperature, and air flow rate were extensively investigated to identify the 5 6 7 autoignition temperature, using a similar fuel sample and experimental setup. For this 8 9 10 study, the experimental parameters were selected, based on our previous work 27, in 11 12 13 order to provide the necessary conditions for the autoignition of the droplet. Figure 4 14 15 16 shows the droplet temperature profile, starting from its insertion into the preheated air 17 18 19 stream to the end of the solid combustion process and the extinction of the coke. As 20 21 22 shown in Fig. 4, the entire burning process of the HFO droplet comprises two phases: 23 24 25 a liquid phase followed by a solid phase. The liquid phase combustion was found to be 26 27 28 a complex event consisting of several consecutive stages, each of them characterized 29 30 31 by distinctive physical and chemical reactions. The solid coke phase represented a period 32 33 34 of heterogeneous surface oxidation of the carbonaceous residue 9. Referring to the 35 36 37 38 thermogravimetric analysis, the liquid phase combustion occurs during the LTO and FD 39 40 41 regions, while the solid phase combustion occurs during the HTO region. The entire 42 43 44 burning process was composed of seven consecutive stages: 1) pre-heating, 2) flame 45 46 47 startup, 3) inner evaporation, 4) thermal decomposition, 5) solidification, 6) coke pre- 48 49 50 ignition and 7) smoldering. 51 52 53 54 55 56 57 58 24 59 60 ACS Paragon Plus Environment Energy & Fuels Page 26 of 48

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 Figure 4: Temperature profile of a suspended HFO8 droplet, with an initial diameter of 27 750 µm, showing high-speed images of the combustion stages. The heated co-flow air 28 29 temperature is set to 950 K, and the velocity to 8.2 cm/s. 30 31 The burning process started with the heating of the particle through convective 32 33 34 heat transfer, at this stage, several fluctuations affect the shape of the droplet due to 35 36 37 the micro-explosions behavior. Part of the volatile fraction evaporated superficially, but a 38 39 40 41 significant amount was trapped inside the droplet, leading to periodic expansions ending 42 43 44 with a ‘jet-like’ release of both the fuel and the gaseous compounds. Figure 2 shows 45 46 47 this behavior by a change in the TGA/DTG slope as a release of low boiling gases 48 49 50 around the peak point, P1, in the LTO region at 420 K. After this point, the rate of 51 52 53 volatilization decreases while the temperature keeps increasing. This behavior was 54 55 56 different from that described by Baert et al. 25 who describes a distillation like process, 57 58 25 59 60 ACS Paragon Plus Environment Page 27 of 48 Energy & Fuels

1 2 3 4 while, for the HFO, inner evaporation of the light compounds was observed. As discussed 5 6 2 7 in a previous study , the weakest bonds of the heavy fuel oil start to break at 8 9 10 approximately 625 K, with the heavy molecules of the residual starting to decompose. 11 12 13 At this temperature, Fig. 2 shows a peak, P2, on the TGA/DTG curve as a sign of 14 15 16 releasing the high boiling gasses. Then, at the FD region, more mass transfer of the 17 18 19 gaseous compound escapes from the droplet due to the oil bonds breaking process. 20 21 22 The release of the gases is more intense than the at low boiling gases, P1. A flame 23 24 25 event started after ignition of the vapor-air mixture and terminated shortly before the 26 27 28 solid coke particle starts forming at the end of the solidification stage. When the vapor/air 29 30 31 mixture reached critical ignition temperature, the mixture autoignition created a visible 32 33 34 flame around the droplet with no splashing or micro-explosion in the first step, Fig. 4 35 36 37 38 image 2. The ignition delay time, IDT, is defined as the amount of time between the 39 40 41 entrance of the fuel droplet in the hot environment and the appearance of the flame. 42 43 44 The first sharp rise in the temperature profile revealed the ignition, mainly through the 45 46 47 radiative effect of the flame. The calculation of the ignition delay time was performed 48 49 50 either by identifying the peak in the first derivative corresponding to the ignition, or by 51 52 53 observing the formation of the flame from the high-speed images. In any case, we found 54 55 56 that the increase in the temperature profile was a bit delayed, concerning the formation 57 58 26 59 60 ACS Paragon Plus Environment Energy & Fuels Page 28 of 48

1 2 3 4 of the flame, due to the limitation of the time response of the thermocouple. The absolute 5 6 7 error associated with measuring the IDT using the images was found to be inversely 8 9 10 proportional to the frame rate, 1000 frame/s in the present study. The droplet burning 11 12 13 process displayed four distinctive stages, starting from the ignition of the droplet to the 14 15 16 flame extinction: the flame startup, the inner evaporation, the thermal decomposition, and 17 18 19 the solidification (Fig. 4). The four distinctive stages occur during the FD region causing 20 21 22 disruptions and fluctuations to the DTG curve Fig. 2 as a result of the chemical reactions 23 24 25 of different species in the heavy fuel oil. During the flame startup stage, the flame 26 27 28 gradually developed, featuring a slight swelling of the droplet, indicating that the 29 30 31 evaporation process continued with the remaining volatiles and the products of cracking. 32 33 34 The diameter grew exponentially, up to more than four times the initial diameter. Ikegami 35 36

37 9 38 et al. related this phenomenon to the Lewis number, Le, as per equation 4. The Lewis 39 40 훼 41 number is defined as the ratio between the thermal conductivity , and the mass 42 43 44 diffusivity Γ: 45 46 47 훼 48 퐿푒 = Γ (4) 49 50 51 52 For this type of fuel, the heat transfer rate is greater than the rate of mass 53 54 55 transfer, so Le>>1 and the volatile species are trapped inside the droplet at high 56 57 58 27 59 60 ACS Paragon Plus Environment Page 29 of 48 Energy & Fuels

1 2 3 4 temperature. Hence these species evaporate and react while the heavier ones, such as 5 6 7 asphaltene, form a solid shell. As the evaporation of the light fraction proceeded, the 8 9 10 heavier species accumulated on the surface of the droplet. This accumulation leads to 11 12 13 a higher resistance to the mass transfer. The inner evaporation started after the flame 14 15 16 startup stage and ended with a thermal decomposition occurring during the initial phase 17 18 19 of the fuel evaporation, as described by Chen & EI-Wakil 47. During this stage, the 20 21 22 temperature increased at a low rate, but the droplet displayed a large swollen, as 23 24 25 illustrated in Fig. 4 image 3, due to the release of the high boiling gases. Ikegami et 26 27 28 al. 9 explained the decrease in temperature rate by thermal cracking, an endothermic 29 30 31 process requiring a relatively high amount of energy, as indicated by the activation 32 33 34 energy in Table 4. Splashing occurred during the early phase of the inner evaporation 35 36 37 38 stage when the droplet surface viscosity was not high enough to prevent it. At this stage, 39 40 41 the flame was very sensitive to the droplet as it swelled and returned to its original size 42 43 44 continuously. At the end of this stage, the droplet surface viscosity progressively 45 46 47 increased, leading to a higher tendency to have a micro-explosive behavior, as illustrated 48 49 50 in Fig. 4 image 4. The next stage, i.e., the thermal decomposition stage, started with a 51 52 53 sharp increase in temperature, as a result of the decomposition of the fuel constituents 54 55 56 and the production of the volatile gases. 57 58 28 59 60 ACS Paragon Plus Environment Energy & Fuels Page 30 of 48

1 2 3 4 The non-aromatic part produces volatile gases as a result of the thermal cracking, 5 6 48 7 decreasing the molecular weight of the heavy fuel . Consequently, the micro-explosions 8 9 10 became more intense and frequent, due to the increase in surface tension caused by 11 12 13 the evaporation of superficial volatiles trapping the gaseous inside the droplet. The 14 15 16 trapped gases could only escape from the droplet because of the micro-explosive 17 18 19 behavior. The droplet subsequently entered the solidification stage, also called the 20 21 22 polymerization stage, which represented a transient period between the liquid and solid 23 24 25 phases. The DTG curve in Fig. 2 shows this transition occurring at low mass degradation 26 27 28 point between the FD and HTO regions. During this transition, the rate of mass 29 30 31 degradation was decreased, due to resistance in mass transfer by the formation of 32 33 34 carbonaceous particles, as previously described (in the thermogravimetric analysis 35 36 37 38 section). The solidification involved mostly a combination of the aromatic parts. The 39 40 41 thermal cracking reduced the heavy fuel molecules to their aromatic nuclei, which are 42 43 48 44 very stable and only began to decompose at 1000 K . 45 46 47 The aromatic nuclei either evaporated or recombined to form polymers from the 48 49 50 residual components 48. During this period, the droplet temperature was no longer 51 52 53 controlled by the fuel evaporation process, and the droplet reached its maximum 54 55 56 temperature, forming a solid particle of coke residue. The flame intensity was then 57 58 29 59 60 ACS Paragon Plus Environment Page 31 of 48 Energy & Fuels

1 2 3 4 reduced steadily Fig. 4 image 5. Accordingly, the droplet temperature decreases gradually 5 6 7 until the flame became extinct (Fig. 4 (6)), while the solid phase was being initiated. 8 9 10 Due to the extinction of the flame, the residue collapsed around the thermocouple bead, 11 12 13 to form a cenosphere. The co-flow of air was then able to diffuse to the surface of the 14 15 16 coke particle to react. Thus, a significant amount of energy was released during the 17 18 19 high-temperature oxidation region, which ended by a glowing of the coke, a final stage 20 21 22 called ‘smoldering stage’ Fig. 4 image 7. This energy is much higher than that in the 23 24 25 LTO region as discussed in the thermogravimetric analysis section. 26 27 28 The amount of time from flame extinction to the coke glowing is defined as the 29 30 31 glowing delay time, GDT. The smoldering stage takes place to burn the remaining coke 32 33 34 particles until quenching occurs, as heat losses exceed the heat generation, leaving 35 36 37 38 unburnt coke. During this stage, the temperature displays a gradual increase to a peak 39 40 41 temperature. The droplet lifetime can be calculated as the amount of time between 42 43 44 insertion of the droplet into the furnace and the coke extinction Fig. 4 image 8. 45 46 47 3.3 Influence of high asphaltene concentration 48 49 50 Figure 5a and the zoomed window in Fig. 5b, compare the droplet temperature 51 52 53 histories for the three different samples HFO8, HFO16, and HFO24, featuring the main 54 55 56 burning stages. For the three HFO samples, the initial droplet diameter for the tests is 57 58 30 59 60 ACS Paragon Plus Environment Energy & Fuels Page 32 of 48

1 2 3 4 approximately 720 - 750 µm, the air flow temperature is set to 950 K, and the velocity 5 6 7 is set to 8.2 cm/s. 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Figure 5: (a) Suspended droplet temperature profiles for HFO8, HFO16, and HFO24; (b) 25 zoomed window of the temperature profile showing closely the burning stages. The initial 26 27 droplet diameter is 750 µm, the airflow temperature is set to 950 K, and the velocity is 28 set to 8.2 cm/s. 29 30 31 Increasing the asphaltene concentration does not affect the range of temperature 32 33 34 associated with each combustion stage; it only delays or shifts the start of the droplet 35 36 37 ignition and burning stages. For instance, the ignition temperature is almost the same 38 39 40 for the three samples, approximately 775 K. However, compared to the HFO8, the 41 42 43 ignition of the HFO16, and HFO24 droplets occurred at a later time corresponding to 44 45 46 the larger content of asphaltene concentration. This means that a longer duration time 47 48 49 of the preheating stage was required for the flame to start up for the HFO24 than the 50 51 52 HFO8 (Fig. 6), and this matches the TGA findings. Accordingly, the total burning time 53 54 55 of the droplet increased by 40% for the HFO24 compared to the HFO8. A higher 56 57 58 31 59 60 ACS Paragon Plus Environment Page 33 of 48 Energy & Fuels

1 2 3 4 percentage of asphaltene leads to a reduction of the weight percentage of the light 5 6 7 components, in particular, since the blending procedure is usually carried out with 8 9 10 approximately 40 wt% of lighter cut in the HFO8, the reduction of the diesel-like fraction 11 12 13 is about 36.5 wt% for the HFO16 and 33 wt% for the HFO24. Figure 6 summarizes the 14 15 16 duration time in seconds, for each burning stage for the three HFO samples. 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Figure 6. Duration time of the seven burning stages for the HFO8, HFO16, and HFO24 34 droplets related to the temperature profile shown in Fig. 5. 35 36 37 The duration time for each stage is sensitive to the asphaltene concentration as 38 39 40 illustrated in Fig. 6. The pre-heating stage contributes most of the combustion process 41 42 43 of the droplet with approximately 50% of the total burning time. Increasing the asphaltene 44 45 46 concentration increases the pre-heating stage length as the asphaltene increases the 47 48 49 droplet viscosity. The second stage, flame startup, also increases with increasing the 50 51 52 asphaltene concentration. This is because of creating ignitable environment depends 53 54 55 mainly on forming the right mixture between the fuel gasses and hot air during the pre- 56 57 58 32 59 60 ACS Paragon Plus Environment Energy & Fuels Page 34 of 48

1 2 3 4 heating stage. We found that the length of the flame startup stage is almost double for 5 6 7 HFO24 than it is for HFO8 (Fig. 6). On the other hand, the length of the inner evaporation 8 9 10 stage has an inverse relationship with the asphaltene concentration. Figure 6 shows that 11 12 13 the inner evaporation time dropped from 0.2 seconds, in the HFO8, to 0.025 seconds, 14 15 16 in the HFO24. This can be explained by the larger skin-like effect which limits the mass 17 18 19 transfer of the inner gases with higher asphaltene concentration. Both thermal 20 21 22 decomposition and solidification stages increase proportionally with increasing the 23 24 25 asphaltene percentage. Also, the entire solid phase including the coke pre-ignition and 26 27 28 smoldering stages increases with increasing the asphaltene percentage. The HFO24 29 30 31 contained heavier components, and therefore induced longer polymerization reactions 32 33 34 than the HFO8. Additionally, the residual of the HFO24 was much larger, as shown in 35 36 37 38 Fig. 2, thus, required a longer time for the heterogeneous reaction to be ignited. 39 40 41 Accordingly, the coke pre-ignition stage, also called glowing delay time, increased from 42 43 44 0.47 seconds to 0.60 seconds, as we increased the concentration of asphaltene from 8 45 46 47 to 24 wt.% respectively. Furthermore, we observed an increase in the duration of the 48 49 50 smoldering stage of the solid particle, from 0.85 seconds for HFO8, to 2 seconds for 51 52 53 HFO24 Figs. 6. The coke burnout time, from coke ignition to coke extinction, increased 54 55 56 from 15% to 25% of the total droplet’s lifetime. 57 58 33 59 60 ACS Paragon Plus Environment Page 35 of 48 Energy & Fuels

1 2 3 4 Figure 7 presents high-speed images of the seven different burning stages, for 5 6 7 the three samples HFO8, HFO16, and HFO24. The results show qualitative combustion 8 9 10 characteristics, similar to those illustrated in Fig. 4. The droplet images at each stage 11 12 13 show similar behavior despite the variations in the asphaltene concentration. 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 Figure 7: Transient images of the HFO8, HFO16, and HFO24 droplets and flames related 49 50 to the plots in Fig. 5. 51 52 During the pre-heating stage Fig. 7 (1), the droplet was preheated in a spherical 53 54 55 56 shape with less surface distortion and more stability for the HFO24 sample. However, 57 58 34 59 60 ACS Paragon Plus Environment Energy & Fuels Page 36 of 48

1 2 3 4 the size of the droplet becomes larger than the original size due to the low boiling 5 6 7 gases. In the second stage, flame startup, similar behavior of visible flame was observed 8 9 10 in Fig. 7 (2) for the three, although HFO8 experienced the highest increase in temperature 11 12 13 during the flame startup stage Fig. 5 (a-b). Very little change was observed between 14 15 16 HFO16 and HFO24. Additionally, a steady decrease in the time required for the inner 17 18 19 evaporation stage was obtained by increasing the asphaltene concentration Fig. 5 (b-c). 20 21 22 The slower rate of the inner evaporation in HFO24 than HFO8 shows a bigger size of 23 24 25 the droplet Fig. 7 (3) proportionally with the amount the trapped inner gasses. The 26 27 28 thermal decomposition stage Fig. 5 (c-d) occurred at an earlier time for a higher content 29 30 31 of asphaltene. This can be explained by the fact that the thermal decomposition and 32 33 34 solidification stages are mostly a function of the heavier components, and therefore, 35 36 37 38 excessive micro-explosions occurred with a higher content of asphaltene (Fig. 7 (4)). 39 40 41 The effect of increasing the asphaltene on the thermal decomposition stage appeared 42 43 44 clearly in the TGA/DTG results (Fig. 2) through more fluctuations during the FD region 45 46 47 for the HFO24 than the HFO8. Increasing the concentration of asphaltene led to the 48 49 50 formation of bigger coke particles during the solidification stage (Fig. 7 (5-7)), requiring 51 52 53 a longer coke pre-ignition and smoldering time Fig. 6. Accordingly, a larger coke particle 54 55 56 was left unburnt, Fig. 7 (8), when the content of asphaltene increased from 8 to 24 57 58 35 59 60 ACS Paragon Plus Environment Page 37 of 48 Energy & Fuels

1 2 3 4 wt.%. In compliance with the residual mass of 2.4 wt% was obtained for the HFO24 5 6 7 sample, whereas no residue was found for the HFO8 sample. Those experimental results 8 9 10 are consistent with the TGA results. 11 12 13 Figure 8 shows the droplet normalized square diameter for the three HFO samples 14 15 16 (HFO8, HFO16, HFO24), and a zoomed view of the micro-explosive zone during the 17 18 19 pre-heating stage, for the three samples. Tracking the droplet evolution with time is 20 21 22 23 fundamentally important to explore the effect of the asphaltenes on the micro- 24 25 26 explosions. This principle helps to characterize fuels with a wide boiling range 27 28 29 and to understand the relevant effect on the combustion efficiency 49 50. 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 36 59 60 ACS Paragon Plus Environment Energy & Fuels Page 38 of 48

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 Figure 8: Normalized squared diameter for the three samples, a) HFO8, b) HFO16, c) 32 33 HFO24; with a zoomed view of the micro-explosions during the pre-heating stage. 34 35 Increasing the asphaltene content increases the droplet’s viscosity and reduces 36 37 38 39 the reactivity of the mixture. The evolution of the normalized squared droplet diameter 40 41 42 profile highlighted these features. We observed that the amplitude of the oscillations of 43 44 45 the droplet size in the evaporative regions was much more consistent for the sample 46 47 48 with the highest content of asphaltene, in comparison with the raw fuel. Oscillations are 49 50 51 strictly related to the micro-explosive behavior; as this is because the formation of the 52 53 54 bubbles inside the fuel matrix is the step preceding this phenomenon. Samples HFO16 55 56 57 58 37 59 60 ACS Paragon Plus Environment Page 39 of 48 Energy & Fuels

1 2 3 4 and HFO24 were more capable of restraining the super-heated vapors inside the particles, 5 6 9 51 7 resulting in a larger peak in the droplet size than the HFO8 as indicated in Fig 8. 8 9 10 As shown in Fig. 8, the size of the HFO8 droplet is smaller at the end of the micro- 11 12 13 explosive step than it was initially, due to the loss in the fuel in the form of micro- 14 15 16 satellites. This process is of relevant importance in the industrial sector, particularly for 17 18 19 spray combustors, because it generates secondary atomization that improves the 20 21 22 evaporation rate and increases the surface area of the fuel in contact with the surrounding 23 24 25 atmosphere 51. A fast consumption of fuel is observed after the ignition Fig. 8 a, b, and 26 27 28 c, it corresponds to a drastic reduction of the droplet size followed by a rapid expansion 29 30 31 which generates a maximum size that is again proportional to the asphaltene content. 32 33 34 The maximum size of the droplet becomes larger by a factor of 2 with increasing the 35 36 37 38 asphaltene concentration from 8 to 24 wt% in the HFO sample, again a symptom of a 39 40 41 remarkable capacity of the asphaltene to create a shell and trap the vapors. All the 42 43 44 three samples present a disruptive behavior at the end of the combustion stage when 45 46 47 the shell collapses releasing the vapors and forming the cenosphere. 48 49 50 3.4 Ignition delay time 51 52 53 In this experimental work, we investigate the influence of high asphaltene 54 55 56 concentration on the ignition delay time for different droplet initial size. The ignition delay 57 58 38 59 60 ACS Paragon Plus Environment Energy & Fuels Page 40 of 48

1 2 3 4 time was examined as a function of the initial droplet diameter for the HFO8, HFO16, 5 6 7 and HFO24 samples. Figure 9 presents the ignition delay time for approximately fifty 8 9 10 different droplets for each sample, and for a diameter size ranging from 400 µm and 11 12 13 900 µm. 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Figure 9: Ignition delay time for different HFO8, HFO16, and HFO24 droplet size. 32 33 34 The ignition delay time, IDT, is the time required for the combustible mixture to 35 36 37 form around the droplet plus the time necessary for the early stage chemical reactions 38 39 40 to ignite the droplet. In our previous work 27, we discussed the required conditions for 41 42 43 HFO droplet autoignition to occur. The IDT could be divided into a heat-up time and an 44 45 46 evaporation/mixing time. For the present study, the droplet’s asphaltene concentration 47 48 49 control the ignition delay by a positive linear relationship between the heating time and 50 51 52 the droplet size. As shown in Fig. 9, the ignition delay time increases as the diameter 53 54 55 56 of the droplet increases for the three HFO samples. The time delay for the ignition of 57 58 39 59 60 ACS Paragon Plus Environment Page 41 of 48 Energy & Fuels

1 2 3 4 the larger HFO droplet is due to the difficulty in forming the air/fuel mixture, which, in 5 6 7 turn, leads to the droplet’s autoignition. Moreover, the evaporation/mixing time increases 8 9 10 with the delay time for samples with a high concentration of asphaltene. We found the 11 12 13 droplet ignition delay time to be a function of both the droplet size and the content of 14 15 16 asphaltene. For example, the IDT increases by 26% when the asphaltene concentration 17 18 19 increases from 8 to 24 wt% in the HFO sample. On the other hand, the temperature of 20 21 22 the droplet during the ignition step does not strongly depend on these two parameters. 23 24 25 3.5 Experimental error 26 27 28 One of the largest sources of error in the temperature measurements arises from 29 30 31 uncertainty in the location of the thermocouple in the droplet. The droplet position on 32 33 34 the thermocouple junction changes due to the oscillations of the droplet interface during 35 36 37 38 the combustion process. The temperature measurements obtained is an average of the 39 40 41 inner liquid phase temperature except for some instants where the micro-explosions move 42 43 44 the droplet leaving the junction uncovered. The characteristic time of these events is 45 46 47 much smaller compared to the data acquisition rate by the thermocouple and are 48 49 50 effectively averaged out. 51 52 53 The experimental error associated with the interface measurements and tracking 54 55 56 is of the same order of magnitude of a single pixel. The major issue is represented by 57 58 40 59 60 ACS Paragon Plus Environment Energy & Fuels Page 42 of 48

1 2 3 4 the fact that the third dimension is completely neglected since the images can observe 5 6 7 no depth in the actual experimental setup. The measurements of the normalized square 8 9 10 diameter are based on the assumption that the ratio between the surface of the droplet 11 12 13 at a certain instant and the original surface has the same ratio between the measured 14 15 16 2-D area and the original area of the droplet. 17 18 19 4. Conclusions 20 21 22 The primary purpose of this work was to experimentally investigate the influence 23 24 25 of the asphaltene concentration on the combustion of a heavy fuel oil droplet. We tested 26 27 28 HFO samples containing asphaltene concentration of 8, 16, and 24 wt%. This work 29 30 31 provides quantitative information about the combustion of HFO from the thermogravimetric 32 33 34 analysis and the combustion of a suspended droplet. 35 36 37 38 TGA results have shown that increasing the content of asphaltene from 8 to 24 39 40 41 wt% in the HFO sample delays the mass degradation starting time and leaves a residual 42 43 44 of 2.4 wt% at the end of the process. The results have revealed seven burning stages 45 46 47 during the complete liquid and solid phases: 1) pre-heating, 2) flame startup, 3) inner 48 49 50 evaporation, 4) thermal decomposition, 5) solidification, 6) coke pre-ignition, and 7) 51 52 53 smoldering. Increasing the asphaltene concentration increases the duration time of six 54 55 56 out of the seven burning stages. The inner evaporation stage, by contrast, decreases 57 58 41 59 60 ACS Paragon Plus Environment Page 43 of 48 Energy & Fuels

1 2 3 4 with increasing the asphaltene concentration. For the same droplet size, the total burning 5 6 7 time increases by 40% when increasing the asphaltene concentration from 8 to 24 wt%. 8 9 10 Likewise, the ignition delay time increases by 26%. On the other hand, increasing the 11 12 13 asphaltene concentration does not affect the temperature range of the various burning 14 15 16 stages. 17 18 19 The micro-explosive behavior is still present even when increasing the amount of 20 21 22 asphaltene inside the mixture up to 24 wt%. The higher asphaltene content leads to a 23 24 25 larger maximum size of the droplet over the burning time by approximately a factor of 26 27 28 2 for the HFO24 sample compared to the HFO8. In our future work, we will consider 29 30 31 the effects of these phenomena on the spray combustion behavior. 32 33 34 Acknowledgments 35 36 37 38 The research reported in this publication was supported by Saudi 39 40 41 Company (SEC) under grant number RGC/3/3466-01-01 and by King Abdullah University 42 43 44 of Science and Technology (KAUST). 45 46 47 48 49 50 51 52 53 54 55 56 57 58 42 59 60 ACS Paragon Plus Environment Energy & Fuels Page 44 of 48

1 2 3 4 References 5 6 7 (1) Roy, B.; Comer, B. Alternatives to heavy fuel oil use in the Arctic: Economic and 8 9 10 environmental tradeoffs; 2017. 11 12 13 (2) Bartle, K.; Jones, J.; Lea-Langton, A.; Pourkashanian, M.; Ross, A.; Thillaimuthu, J.; 14 15 16 Waller, P.; Williams, A. Fuel 2013, 103, 835-842. 17 18 19 (3) Reyhani, H. A.; Meratizaman, M.; Ebrahimi, A.; Pourali, O.; Amidpour, M. Energy 2016, 20 21 22 107, 141-164. 23 24 25 26 (4) Gomez, J.; Mmbaga, J.; Hayes, R.; Toledo, M.; Gracia, F. International Journal of 27 28 29 Hydrogen Energy 2016, 41, 17933-17943. 30 31 32 (5) Xu, G.; Ikegami, M.; Honma, S.; Ikeda, K.; Nagaishi, H.; Takeshita, Y. Combustion 33 34 35 science and technology 2002, 174, 115-145. 36 37 38 (6) Braide, K.; Isles, G.; Jordan, J.; Williams, A. Journal of the Institute of Energy 1979, 52, 39 40 41 115-124. 42 43 44 (7) Costa, M.; Costen, P.; Lockwood, F. Journal of the Institute of Energy 1991, 64, 64-76. 45 46 47 (8) Ocampo-Barrera, R.; Villasenor, R.; Diego-Marin, A. Combustion and flame 2001, 126, 48 49 50 1845-1855. 51 52 53 54 55 56 57 58 43 59 60 ACS Paragon Plus Environment Page 45 of 48 Energy & Fuels

1 2 3 4 (9) Ikegami, M.; Xu, G.; Ikeda, K.; Honma, S.; Nagaishi, H.; Dietrich, D.; Takeshita, Y. Fuel 5 6 7 2003, 82, 293-304. 8 9 10 (10)Villasenor, R.; Garcia, F. Fuel 1999, 78, 933-944. 11 12 13 (11)Allen, G.; Hocking, W.; Watson, D.; Wild, R.; Street, P. Journal of the Institute of Energy 14 15 16 1984, 57, 260-265. 17 18 19 (12)Kobayasi, K. An experimental study on the combustion of a fuel droplet. Symposium 20 21 22 (International) on Combustion. 1955, 141-148. 23 24 25 (13)Lightman, P.; Street, P. Journal of the Institute of Energy 1983, 56, 3-11. 26 27 28 (14)Marrone, N. J.; Kennedy, I. M.; Dryer, F. L. Combustion science and technology 1984, 29 30 31 36, 149-170. 32 33 34 (15)Zhu, M.; Zhang, Z.; Zhang, Y.; Setyawan, H.; Liu, P.; Zhang, D. Proceedings of the 35 36 37 38 Combustion Institute 2017, 36, 2475 - 2482. 39 40 41 (16)Zhu, M., Ma, Y., Zhang, D. Proceedings of the Combustion Institute 2013, 34, 1537 - 42 43 44 1544. 45 46 47 (17)Zhu, M.; Ma, Y.; Zhang, Z.; Chan, Y. L.; Zhang, D. Fuel 2015, 150, 88 - 95. 48 49 50 (18)Setyawan, H. Y.; Zhu, M.; Zhang, Z.; Zhang, D. Energy 2016,113, 153 - 159. 51 52 53 (19)Zhu, M.; Zhang, Z.; Zhang, Y.; Liu, P.; Zhang, D. Applied Energy 2017, 185, 2160 - 54 55 56 2167. 57 58 44 59 60 ACS Paragon Plus Environment Energy & Fuels Page 46 of 48

1 2 3 4 (20)Zhu, M.; Zhang, Z.; Zhang, Y.; Setyawan, H.; Liu, P.; Zhang, D. Proceedings of the 5 6 7 Combustion Institute 2017, 36, 2475 - 2482. 8 9 10 (21)Kuznetsov, G. V.; Vershinina, K. Y.; Valiullin, T. R.; Strizhak, P. A. Fuel Processing 11 12 13 Technology 2018, 179, 407 - 421. 14 15 16 (22)Glushkov, D. O.; Kuznetsov, G. V.; Strizhak, P. A.; Syrodoy, S. V. Energy & Fuels 17 18 19 2018, 32, 8789-8802. 20 21 22 (23)Nyashina, G. S.; Strizhak, P. A. Environmental 2018, 242, 31 – 41. 23 24 25 (24)Egorov, R. I.; Valiullin, T. R.; Strizhak, P. A. Combustion and Flame 2018, 193, 335 – 26 27 28 343. 29 30 31 (25)Baert, R. Combustion Science and Technology 1993, 90, 125-147. 32 33 34 (26)Alshareef, A. H.; Azyat, K.; Tykwinski, R. R.; Gray, M. R. Energy & Fuels 2010, 24, 35 36 37 38 3998-4004. 39 40 41 (27)Elbaz, A. M.; Khateeb, A. A.; Roberts, W. L. Energy 2018, 152, 455-465. 42 43 44 (28)Elbaz AM, Gani A, Hourani N, Emwas AH, Sarathy SM, Roberts WL. Energy & Fuels. 45 46 47 2015, 12, 7825-35. 48 49 50 (29)Abdul Jameel, A. G.; Elbaz, A. M.; Emwas, A.-H.; Roberts, W. L.; Sarathy, S. M. Energy 51 52 53 & Fuels 2016, 30, 3894-3905. 54 55 56 57 58 45 59 60 ACS Paragon Plus Environment Page 47 of 48 Energy & Fuels

1 2 3 4 (30)Jameel, A. G. A.; Han, Y.; Brignoli, O.; Telalovic, S.; Elbaz, A. M.; Im, H. G.; Roberts, 5 6 7 W. L.; Sarathy, S. M. Journal of Analytical and Applied Pyrolysis 2017, 127, 183-195. 8 9 10 (31)Ciajolo, A.; Barbella, R. Fuel 1984, 63, 657−661. 11 12 13 (32)Freitag, P.; Verkoczy, B. Journal of Canadian Petroleum Technology 2005, 44, 54−61. 14 15 16 (33)Kok, M. V.; Karacan, O. Energy & Fuels 1998, 12, 580−588. 17 18 19 (34)Kok, M. V. Fuel Processing Technology 2011, 92, 1026-1031. 20 21 22 (35)Cheng, F.; Cheng, Z.; Qiang, Z.; Desheng, M.; Yue, C.; Hang, J.; Lin, S.; Fei, W. Fuel 23 24 25 Processing Technology 2014, 119, 149−153. 26 27 28 (36)Tadema, H. J. 5th World Petroleum Congress 1959, 279–287. 29 30 31 (37)Ranjbar, M. Journal of Canadian Petroleum Technology 1995, 34, 25 – 30. 32 33 34 (38)Kok, M.; Acar, C. Journal of Thermal Analysis and Calorimetry 2005, 83, 445-449. 35 36 37 38 (39)Pereira, A. N.; Trevisan, O. V. Journal of the Brazilian Society of Mechanical Sciences 39 40 41 and 2014, 36, 393-401. 42 43 44 (40)Millington, A.; Price, D.; Hughes, R. Journal of Thermal Analysis and Calorimetry 1993, 45 46 47 40, 225-238. 48 49 50 (41)Gureyev, A.; Sablina, Z. The Oxidation of Hydrocarbons in the Liquid Phase; Elsevier, 51 52 53 1965; pp 362-374. 54 55 56 57 58 46 59 60 ACS Paragon Plus Environment Energy & Fuels Page 48 of 48

1 2 3 4 (42)Reddy, V. M.; Rahman, M. M.; Gandi, A. N.; Elbaz, A. M.; Schrecengost, R. A.; Roberts, 5 6 7 W. L. Combustion Theory and Modelling 2016, 20, 154-172. 8 9 10 (43)Urban, D. L.; Dryer, F. L. New results on coke formation in the combustion of heavy- 11 12 13 fuel droplets. Symposium (International) on Combustion. 1991; pp 1437-1443. 14 15 16 (44)Murugan, P.; Mahinpey, N.; Mani, T.; Freitag, N. Fuel 2009, 88, 1708-1713. 17 18 19 (45)Gundogar, A. S.; Kok, M. V. Fuel 2014, 123, 59-65. 20 21 22 (46)Ambalae, A.; Mahinpey, N.; Freitag, N. Energy & fuels 2006, 20, 560-565. 23 24 25 (47)Chen, C.; El-Wakil, M. Experimental and theoretical studies of burning drops of hydro- 26 27 28 carbon mixtures. Proceedings of the Institution of Mechanical Engineers, Conference 29 30 31 Proceedings. 1969; pp 95-108. 32 33 34 (48)Garaniya, V.; Goldsworthy L.; McWilliam D. Chemeca 2011: Engineering a Better World: 35 36 37 38 Sydney (Australia) 2011, 251. 39 40 41 (49)Rao, D. C. K.; Karmakar, S.; Basu, S. Scientific Reports 2017, 7, 8925. 42 43 44 (50)Mura, E.; Calabria, R.; Califano, V.; Massoli, P.; Bellettre, Journal Experimental Thermal 45 46 47 and Fluid Science, 2014, 56, 69-74. 48 49 50 (51)Watanabe, H.; Suzuki, Y.; Harada, T.; Matsushita, Y.; Aoki, H.; Miura, T. Energy 2010, 51 52 53 35, 806-813. 54 55 56 57 58 47 59 60 ACS Paragon Plus Environment