**1. Introduction**

As to the wide application of heavy fuel oil (HFO) in utility boilers, industrial furnaces, and marine engines, the burning of HFO has been broadly tested in laboratory research and engineering development. With the gradual reduction of conventional light and medium oil resources, efficiently and economically recovering vast reserves of unconventional heavy oil and asphalt has attracted increasing attention. HFO has an energy density content similar to distilled fuel, but it has a very high viscosity and requires additional heating before spray combustion in the boiler [1]. The specific characteristics of HFO are the high contents of asphaltenes, carbon residues, trace metals, such as vanadium and nickel, and fuel-bound nitrogen and sulfur. Asphaltenes are heavy polycyclic aromatic compounds with embedded heteroatoms, making the fuel difficult to burn, and the inefficient combustion process leads to the formation of large cenospheres (lightweight, inert, hollow spheres).

Asphaltenes are compounds in crude oil, heavy fuels (high-boiling and nonboiling petroleum fractions), and oil sand bitumen. They are insoluble in n-alkanes (e.g., n-heptane) and only soluble in aromatic solvents (e.g., toluene). Their chemical structures are still not fully understood, so asphaltenes are characterized by their solubility class instead of their chemical properties. Asphaltenes have a significant impact on the physicochemical properties of heavy fuels and residues. It has been shown that asphaltenes are the most

**Citation:** Pei, X.; Tian, H.; Roberts, W.L. Swirling Flame Combustion of Heavy Fuel Oil Blended with Diesel: Effect of Asphaltene Concentration. *Energies* **2022**, *15*, 6156. https:// doi.org/10.3390/en15176156

Academic Editors: Monika Kosowska-Golachowska and Tomasz Czakiert

Received: 27 July 2022 Accepted: 19 August 2022 Published: 24 August 2022

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aromatic part of heavy fuels and, therefore, increase the viscosity of the fuel [2]. Thus, HFO with high asphaltene content requires the adding of solvent or the preheating of the fuel to improve fluidity. Asphaltenes in crude oil precipitate and deposit along the walls of the oil reservoir, thereby clogging the oil well and causing flowline fouling problems. For all these reasons, asphaltenes have been called 'bad guys' in petroleum fuels [3].

The well-referred results established by HFO combustion were to distinguish two combustion stages. The first is the liquid phase of evaporating volatile substances, and, then, the solid phase of oxidized coke, carbonaceous cenosphere particles formed during the liquid phase. This liquid combustion stage is a complex process that involves heat and mass transfer and chemical reactions, such as pyrolysis (thermal cracking) and polymerization. The solid combustion phase occurs immediately after the liquid combustion phase and forms a hollow shell cenosphere [4]. Most of the research on HFO combustion reported in the literature was carried out using suspended [5–7] and falling droplet [8–10] techniques, and many researchers used numerical simulations [11–13]. Elbaz et al. [7] studied the formation and oxidation of heavy oil, HFO, and particles produced by the combustion of droplets. They also used a scanning electron microscope (SEM) and an energy dispersive X-ray (EDX) to analyze the microstructure of the particles. The study concluded that the droplet ignition temperature is independent of the droplet size, but the liquid phase ignition delay time and droplet life are proportional to the initial droplet diameter. Kwack et al. [14] burned No. 6 fuel in a closed burner, observed the combustion residue through a scanning electron microscope, and studied the qualitative relationship between the shape of these particles and the temperatures to which they were subjected. Xu et al. [4] divided the process of heavy oil droplet combustion into four steps: ignition delay, flame lifetime, coke glowing delay, and coke ember time. Using the mixed oil of mixed heavy oil residue (HOR) and diesel light oil (LO), they further analyzed their composition characteristics in terms of oil composition and combustion conditions. They found that increasing the temperature of the combustion chamber significantly reduced the ignition delay and the coke luminescence delay, but hardly changed the flame lifetime. Ambalae et al. [15] used a thermogravimetric analyzer (TGA) to obtain information on the pyrolysis and combustion behavior of crude oil (Neilburg) and its asphaltenes. The study found that asphaltenes contributed the most to coke formation among all saturated, aromatic, resin, and asphaltene fractions. They analyzed the temperature rise of whole oil and asphaltenes and conducted isothermal pyrolysis experiments to determine the temperature at which coke formation was maximized. In addition, they obtained isothermal combustion curves of coke derived from whole oil and asphaltenes. Atiku et al. [16] explored the mechanism of forming fine particulate soot and cenospheres and studied the chemical structure of petroleum asphaltenes through pyrolysis technology. Jameel et al. [17] used non-isothermal thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectrometers to study the pyrolysis and combustion of heavy oil in nitrogen and air, respectively, and deeply understood the three stages of heavy oil combustion.

Some work has been conducted on the influences of oil compositions on HFO burning. The effect of oil composition on HFO burning was studied by changing asphaltene content [5,18,19], sulfur [6,20], metal (vanadium) [21], viscosity [22,23], and fuel nitrogen [24]. The viscosity of HFO largely depends on the volume fraction, chemical structure, and physicochemical properties of its asphaltenes. Asphaltenes are the most polar and heaviest components in HFO [25]. Fakher et al. [26] explained the main components of crude oil and its relationship with asphaltenes and the methods for quantifying asphaltenes in crude oils, showing more discussed models for asphaltene modeling, and mentioned the chemistry used to characterize and study asphaltene analysis methods. In addition, they also introduced the methods by which asphaltenes destroy oil recovery. The structure of asphaltenes was studied through atomic force microscopy with atomic resolution imaging and scanning tunnelling microscopy with molecular orbital imaging to study more than 100 asphaltene molecules [27]. Peng et al. [28] conducted experimental and theoretical studies on the specific effects of asphaltene content on the viscosity of heavy oil at different

temperatures. They determined four important parameters to characterize the reconstituted heavy oil samples: solvation constant, shape factor, intrinsic viscosity, and maximum filling volume fraction. The study showed, through the results of nonlinear regression, that the state of asphaltene particles in heavy oil changed with the change of asphaltene content and temperature, and this change greatly influenced the viscosity of heavy oil. Bartle [29] showed that asphaltene reduced the ignition delay time of heavy oil but did not affect the burning time of fuel droplets. The reduction in ignition delay was attributed to the volatiles produced by the pyrolysis of asphaltenes.

The current work is dedicated to showing how blending asphaltene influences the combustion of heavy fuel oil. The emission characteristics (gaseous emissions and particulate matter) of different asphaltene constituent oils were tested in the swirling flame burning system, while the inspected parameters were mainly about coking characteristics. This research used measurements to analyze blending fuel characteristics, combustion performance, and pollution, considered necessary for future energy supplies using diversified fuels. In this research, fuel samples with five different HFO blending fuels with asphaltene mass fractions of 4%, 6%, 8%, 16%, and 24% were compared in swirling flame experiments. Pure HFO containing 8% (by mass) asphaltenes was used as a basis for comparison. Low asphaltene content fuel oils were prepared by blending HFO with diesel light oil, and extra asphaltene was added to the HFO to produce high asphaltene oils. This work also intends to illustrate how the mixing of asphaltenes affects the emissions of HFO combustion.
