*3.7. Bio-Jet Fuel Properties*

To evaluate the suitability of the bio-jet fuel produced in this work as alternative jet fuel, some of the major properties of bio-jet fuel were analyzed and compared with American Society for Testing and Materials (ASTM) D1655, the standard specification for aviation turbine fuels. The test results and the standard limit are summarized in Table 8.

**Table 8.** Comparison of properties between bio-jet fuel of the current work with ASTM D1655 standard.


It can be seen that the kinematic viscosities and densities of the bio-jet fuel is lower than the maximum value provided by the ASTM D1655 standard (<8 cSt and 775–840 kg/m3, respectively). The measurement of kinematic viscosity and density of fuel are important and should be within the acceptable limits established in the standard to avoid clogging in the fuel injectors and to achieve good fuel atomization. From the table, it is noticed that the density and kinematic viscosity of the bio-jet fuel in this study is higher compared to soybean, but lower than palm. The lower heating value of the bio-jet fuel in this study is 43.5 MJ/kg, which is well below the limit given in the ASTM D1655 standard, slightly lower than that for diesel (>42.8 MJ/kg). The flashpoint of the bio-jet fuel from FAEE coconut oil is 55 ◦C, which is significantly higher than the limit given in the ASTM D1655 (>38 ◦C). The flashpoint is higher than soybean, and much lower than the palm. As shown, bio-jet fuel obtained in the current study does not meet the criteria for the freezing point. A similar result has been reported by [42]. This showed that freezing point is the biggest challenge for bio-jet fuel technologies. This might be because bio-jet fuel consists of nearly no aromatics or cycloalkanes that are required for jet fuel [54]. Freezing point is one of the properties that affects the operability of bio-jet fuel at low temperature. Hence, further upgrading or blending is needed to bring down the freezing point and improve its jet fuel characteristics. Cheng and Brewer [55] suggested that blending of bio-jet fuel with alkyl-benzens might be the best, as it has a low molecular weight and emit less soot combustion as compared to other aromatics compounds. However, the effect of different aromatic compounds on the freezing point of bio-jet fuel has not been explored. As this objective is beyond the scope of this paper, it is suggested as a future work.

#### **4. Conclusions**

Other than the depletion issue of non-renewable fossil fuel, the increasing greenhouse gas emission has also driven the aviation industry toward sustainable development, such as exploration and commercialization of alternative renewable aviation fuels. This paper has investigated the effect of three parameters, including coconut oil to ethanol molar ratio, reaction time, and microwave power, on bio-jet fuel production. Bio-jet fuel production data was modeled using RSM and ANN. Statistical analysis proved that the ANN modeling was better than RSM. The optimal parameters predicted using ANN- ACO were, ethanol to oil molar ratio: 1:9.25, reaction time: 12.66 min and microwave power 74.8 W. Optimal yield of 74.45% were obtained experimentally in this optimal parameter conditions. Lastly, it was found that the bio-jet fuel collected in this work had comparable properties with the ASTM D1655, except freezing point. Hence, the aromatic additive is suggested to be added for properties enhancement. Besides, future work on evaluating the effect of catalyst and deoxygenation method on the improvement of bio-jet fuel properties will be conducted.

**Author Contributions:** Conceptualization, S.N. and M.Y.O.; methodology, M.Y.O., F.K. and R.M.H.R.S.; software, M.Y.O. and F.K.; validation, R.M.H.R.S. and F.K.; resources, F.K.; data curation, R.M.H.R.S.; writing—original draft preparation, M.Y.O.; writing—review and editing, M.Y.O., F.K. and S.N.; supervision, S.N.; funding acquisition, A.H.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by AAIBE Chair of Renewable Energy (Project code: 202006KET-THA) and BOLD 2020 research grant (grant number: RJO010517844/116) under iRMC, UNITEN.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in Ong, M.Y.; Nomanbhay, S.; Kusumo, F.; Shahruzzaman, R.M.H.R.; Shamsuddin, A.H. Modeling and Optimization of Microwave-Based Bio-Jet Fuel from Coconut Oil: Investigation of Response Surface Methodology (RSM) and Artificial Neural Network Methodology (ANN). *Energies* **2021**, *14*, x. https: //doi.org/10.3390/xxxxx.

**Acknowledgments:** The authors would like to acknowledge UNITEN for the research facilities. M.Y.O would also like to thank UNITEN for the UNITEN Postgraduate Excellence Scholarship 2019 (YCU-COGS).

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