## 4 Conclusions  
The properties and combustion performance of heavy fuel oil, HFO, and the products of the fatty acids distillation residues, FADR, were analyzed in this study. It was observed that FADR can be considered as a substitute for HFO due to the similarity in calorific as well as physical properties (density and viscosity). It is worth underscoring that FADR includes significantly lower sulphur, which is directly related to the lower SO2 emission. On the other hand, FADR calorific value (LHV 36,619 kJ/kg) is lower compared with the HFO (LHV 42,251 kJ/kg); nevertheless, due to the higher density of FADR (i.e., 882 kg/m\({}^{3}\) at 60 \({}^{\circ}\)C) compared with HFO (i.e., 864 kg/m\({}^{3}\) at 60 \({}^{\circ}\)C), the energy density of FADR (i.e., 32.298 GJ/m\({}^{3}\) (from LHV)) is 88.5% of the value for HFO (i.e., 36.505 GJ/m\({}^{3}\) (from LHV)). FADR is characterized by the lower energy demand on heating up fuel and pressurization (i.e., 710 kJ/kg) compared with the value for HFO (i.e., 806 kJ/kg). It was caused by the required lower pressure after the fuel pump, as well as lower temperature to obtain the required viscosity. The combustion performance of HFO and FADR was stable. The  
\begin{table}
\begin{tabular}{c c c c} \hline
**Compound** & \multicolumn{3}{c}{**Emission**} \\ \hline  & **HFO** & **FADR** & **Micro Wax Data from [22]** \\ \hline naphthalene & 8.5 & 10.9 & 2.1 \\ acenaphthylene & 10.2 & 57.4 & 2.0 \\ acenaphthene & 12.6 & 49.8 & 1.0 \\ fluorene & 14 & 48.2 & 5.0 \\ phenanthrene & 72.2 & 53.5 & 12.2 \\ anthracene & 34.2 & 50.6 & 1.7 \\ fluoranthene & 75.4 & 29.2 & 9.7 \\ pyrene & 81.4 & 26.2 & 4.8 \\ benzo(a)anthracene & 15.1 & 3.5 & 1.0 \\ chrysene & 9.5 & 1.4 & 13.3 \\ benzo (b + k) fluoranthene & 30.5 & 20.7 & 110.6 \\ benzo(e)pyrene & 290.3 & 0.55 & 1.0 \\ benzo(a)pyrene & 30.6 & 0.73 & 22.7 \\ perylene & 11.5 & 2.8 & 8.2 \\ bibenzo(a, h) anthracene + indeno (1,2,3) pyrene & 98.4 & 0.02 & 1.8 \\ benzo(g, h, i)perylene & 16 & 0.04 & 1.4 \\ Total PAHs & 810.4 & 355.5 & 198.5 \\ \hline \end{tabular}
\end{table}
Table 6: Concentration (\(\upmu\)g/m\({}^{3}\)n) of polycyclic aromatic hydrocarbons (PAHs) in dry flue gas (ref to 3% O2).  
emission of NO during the combustion of FADR (i.e., 114 ppm at 3% O2) was slightly higher compared with the emission during the combustion of HFO (i.e., 90 ppm at 3% O2). Finally, lower emission of PAHs was observed during the combustion of FADR (i.e., 355.5 \(\mathrm{\SIUnitSymbolMicro m}\)/\(\mathrm{m}^{3}\mathrm{n}\)) compared with the emission of HFO (i.e., 810.4 emission \(\mathrm{\SIUnitSymbolMicro m}\)/\(\mathrm{m}^{3}\mathrm{n}\)). Thus, due to the many benefits related to the FADR application, future work should contain the analysis of FADR utilization as a fuel in energy systems.  
Conceptualization, J.L., K.G., A.C., Y.-H.L. and C.-W.H.; methodology, J.L., K.G., A.C., Y.-H.L. and C.-W.H.; investigation, J.L. and K.G.; data curation, J.L.; writing--original draft preparation, J.L.; writing--review and editing, J.L.; supervision, J.L. and Y.-H.L., project administration, J.L., K.G. and A.C.; funding acquisition, J.L., K.G., A.C., Y.-H.L. and C.-W.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by a grant from the Ministry of Education and Science, the Republic of Poland. Grant title and number: \"Utrzymanie potencjalu badawczego ZTE\"; No. ITPE 11.23.017.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
[PERSON] is acknowledged for support in the uncertainty analysis.  
The authors declare no conflict of interest.