## 2 Experimental  
### Preparation of Test Fuel  
The fast pyrolysis bio-oil (FPBO) was produced from the fast pyrolysis of a bio-waste source (coffee bean residue) in a fluidized bed reactor system using CO\({}_{2}\) as the fluidized gas [29]. Two phases of FPBO were produced, i.e., an oily-phase (Figure 1a) and an aqueous-phase (Figure 1b), depending on the condensation collection temperature. More specifically, the oily phase was collected mainly at higher condensation temperatures (380-400 K), whereas the aqueous phase required lower condensation temperatures (300-330 K). The FPBO was prepared by blending the oily-phase oil and aqueous-phase oil at a ratio of 50:50 by vol %. Emulsification was necessary because bio-oil produced from a cellulosic biomass cannot be directly mixed with diesel. Surfactants were added to support the mixing stability. A surfactant molecule has two parts: one has affinity for water and the other has affinity for oil. The HLB (hydrophilic-lipophilic balance) of a surfactant is a measure of the degree to which it is hydrophilic or lipophilic, and it is determined by calculating values for the different regions of the molecule. Surfactants are classified according to their HLB value, which affects their usage. An optimal value of HLB is necessary for the stabilization of emulsion [30]. Span 80 (sorbitan monooleate; C\({}_{24}\)H\({}_{44}\)O\({}_{6}\); HLB value of 4.3) is a lipophilic emulsifier, whereas Tween 80 (polyoxyethylene sorbitan monooleate; C\({}_{32}\)H\({}_{60}\)O\({}_{10}\); HLB value of 15) is a hydrophilic emulsifier. According to well established emulsification principles, a combination of a high and low emulsifier is more effective than the use of a single emulsifier. In this study, we found that the emulsion of FPBO/diesel with HLB 9.9 results in the highest stability when using Span 80 and Tween 80 together. Therefore, Span 80 and Tween 80 were added to the diesel and bio-oil, respectively. Firstly, a 1.5 vol % of Span 80 was blended with the diesel, and a 1.5 vol % Tween 80 was blended with the FPBO (prepared by blending the oily-phase oil and aqueous-phase oil at a ratio of 50:50 by vol %). Then, emulsification (Figure 1c) was subsequently conducted to produce superior emulsified fuels based on the mixing ratio, which in this case was set at 5 vol % FPBO in the blend. Table 1 shows the properties of the oily phase oil and aqueous phase oil. As illustrated in Table 1, the oily phase differed greatly from the aqueous phase in terms of carbon content, oxygen content, water content, lower heating value (LHV), and viscosity. The oily and aqueous phases contained 42.56 wt % and 23.54 wt % carbon, respectively. The oily-phase oil contained 46.19 wt % oxygen,  
Figure 1: (a) Oily phase; (b) aqueous phase; and (c) emulsion of 5% FPBO + 95% diesel (FPBO = 50 vol % oily-phase oil + 50 vol % aqueous-phase oil). FPBO: fast pyrolysis bio-oil. and the aqueous phase comprised 66.31 wt % oxygen. The oily and aqueous phases contained 22.0 wt % and 87.0 wt % water content, respectively. Significant differences in heating value and viscosity were caused by differences in oil composition. The nitrogen content of both phases was low, while the hydrogen content of both phases was similar. Moreover, the oily phase was darker than the aqueous phase (Figure 1). The components of oily-phase oil and aqueous-phase oil of FPBO are also shown in Table 2. The oily phase was composed of compounds such as 2-cyclopent-1-one and 2,6-dimethoxy-phenol, and the aqueous phase was composed mainly of acetone, acetic acid, propionic acid, and a substantial amount of water. \begin{table}
\begin{tabular}{c c c} \hline
**Element Analysis** & **Oily Phase** & **Aqueous Phase** \\ \hline Carbon (wt \%) & 42.56 & 23.54 \\ Hydrogen (wt \%) & 4.32 & 5.41 \\ Nitrogen (wt \%) & 0.62 & 0.77 \\ Oxygen (wt \%) & 46.19 & 66.31 \\ Water content (wt \%) & 22.0 & 87.0 \\ pH & 3.6 & 3.6 \\ Viscosity@40 \({}^{\circ}\)C (cP) & 221.0 & 0.75 \\ Lower heating value (LHV) (MJ/Kg) & 21.59 & 8.32 \\ \hline \end{tabular}
\end{table}