**4. Conclusions**

This study introduces an experimental study on the specific effects of asphaltene content on the fuel properties and swirling flame combustion performance of heavy oil fuels under five different asphaltene contents of 4%, 6%, 8%, 16%, and 24%. The analysis of the properties of fuel samples with various asphaltene contents, including composition, density, viscosity, heating value, and TG analysis, showed the importance of all fuel properties that must be considered prior to implementation and combustion. The asphaltene content determines the high viscosity of heavy oil. The viscosity of heavy oil can be significantly reduced at higher temperatures, especially at high asphaltene contents, as the interparti-

cle interactions among the dispersed asphaltene particles become weaker. The nonlinear viscosity reduction result revealed that the state of the asphaltene particles in the heavy oil varies with asphaltene concentration and temperature, and this change in state has a significant impact on the viscosity of the heavy oil. Since blended light oil has a high heating value, the heating values of the HFO samples drop as the asphaltene percentage increases. The TGA and morphological studies showed that asphaltene is vital for cenosphere formation during HFO combustion. The reduction in asphaltene percentage in HFO enhances spray characteristics and combustion performance, as well as lowers particle emissions, including those of sulfur and metal components, such as vanadium, avoiding hot and cold corrosion on the burner's surface. The mechanism of NOx formation varies with the fuel asphaltene content, due to the different combustion performances. The 4% asphaltene sample had an accumulation distribution that was more powdery and more tiny particles but less cenospheres than the 8% asphaltene sample, attributed to the small spray droplet size, fine spray, and efficient combustion. The high amount of metal components in solid particle emissions corresponds to pure asphaltene compositions. As a result, asphaltene contributes to the formation of cenospheres. In general, the combustion emissions of various asphaltene fuels are summarized by investigating the impact of asphaltenes on the fuel properties and burning behavior of HFO blends. The research may indicate a potential method of burning HFO by combining it with low-viscosity and high-volatility fuels. In this instance, the combustion properties of the blending fuels proposed in this study are critical for technological advancement, and the further achieving of low carbon emission targets.

**Author Contributions:** Conceptualization, X.P. and W.L.R.; methodology, X.P. and H.T.; formal analysis, X.P.; investigation, X.P.; writing—review and editing, X.P. and H.T.; funding acquisition, X.P. and W.L.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China (grant number 52006122) and National Science and Technology Major Project (grant number 2019-III-0014-0058).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The research was in collaboration with the Clean Combustion Research Center (CCRC) at King Abdullah University of Science and Technology (KAUST).

**Conflicts of Interest:** The authors declare no conflict of interest.
