## 2 Experimental
### Furnace  
Figure 2 shows a schematic of the multi-fuel combustion system with a vertically down-fired furnace and a maximum thermal loading of 300-kW\({}_{\text{th}}\)[17]. The overall structure of the combustion system can be divided into three parts: radiative, convective, and flue gas sections. The vertical refractory-lined radiant chamber in the furnace has an inner diameter of 0.56 m and a length of 3.05 m and can be supplied with various types of fuel, such as diesel, HFO, emulsion fuel, pulverized coal, solid/liquid biofuel, etc. The radiative section (the vertical combustion chamber) is composed of 5 sub-sections, each of which is equipped with an R-type thermocouple to measure the wall temperature of the chamber, and an observation window to facilitate observation of the flame appearance and combustion stability. The set of two burners for burning solid and liquid fuels installed on top of the vertical furnace were designed and manufactured by C&C Engineering, Inc. (Taipei, Taiwan), with each one having a maximum thermal loading of 300-kW\({}_{\text{th}}\). Both burners are equipped with a pilot flame, flame detector and pressure gauge for igniting the flame and monitoring its behavior.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Properties** & **Diesel** & **FPBO** & **5\% FPBO + 95\% Diesel** \\ \hline Density (@20 \({}^{\circ}\)C) & & 1.15 g/cm\({}^{3}\) & 0.849 g/cm\({}^{3}\) \\ Density (@15 \({}^{\circ}\)C) & 0.8334 g/cm\({}^{3}\) & & \\ Flash point & 74 \({}^{\circ}\)C & 79 \({}^{\circ}\)C & 75 \({}^{\circ}\)C \\ Pour point & \(-\)9 \({}^{\circ}\)C & \(-\)8 \({}^{\circ}\)C & \(-\)9 \({}^{\circ}\)C \\ Boiling point & & 85–90 \({}^{\circ}\)C & \\ Pyroysis solids content & & 1.15 wt \% & \\ Kinematic viscosity (@40 \({}^{\circ}\)C) & 2.763 cSt & 48 cSt & 4.98 cSt \\ Water content & 0.0 wt \% & 54.5 wt \% & 3.69 wt \% \\ Water and sediment & 0.005 vol \% & & \\ Ash content & 0.0 wt \% & 1.21 wt \% & 0.083 wt \% \\ Carbon & 85.41 wt \% & 33.05 wt \% & 81.86 wt \% \\ Hydrogen & 13.58 wt \% & 4.87 wt \% & 12.99 wt \% \\ Oxygen & 0 wt \% & 56.31 wt \% & 3.81 wt \% \\ Nitrogen & 0.45 wt \% & 0.695 wt \% & 0.47 wt \% \\ Sulfur & 5.5 ppmw & 0.11 wt \% & 0.008 wt \% \\ Carbon residue & 0.08 wt \% & & \\ pH & & 3.6 & \\ Lower heating value (LHV) & 43.13 MJ/kg & 14.41 MJ/kg & 41.19 MJ/kg \\ Higher heating value (HHV) & 46.01 MJ/kg & 16.40 MJ/kg & 44.00 MJ/kg \\ \hline \hline \end{tabular}
\end{table}
Table 3: Properties of diesel, FPBO (50 vol % oily-phase oil + 50 vol % aqueous-phase oil), and the emulsion of 5% FPBO + 95% diesel.  
In the present study, the vertical combustion chamber was utilized to conduct the experiments. Diesel alone and FPBO/diesel blends with 5 vol % FPBO were used as the fuels. A low-pressure air-assisted atomizer was employed for oil atomization [17]. The fuel stream was injected from the center of the nozzle with primary air (atomizing air), and the annular secondary air stream (combustion air) was injected into the combustion chamber. The primary air used to atomize the oil in the spray nozzle was kept at a constant value throughout the experiments. The secondary air was introduced by the tangential injection in a circumferential direction, thereby establishing a swirling-flow field in the centerline. The tubular oil spray flame was initially ignited by a pilot flame of liquid petroleum gas, which in turn led to a stabilized spray flame in the combustion chamber.  
In the horizontal convective section, both water-cooled and air-cooled heat exchangers were installed to remove heat from the high-temperature flue gas. The flue gas zone was located next to the convective section. Prior to the stack, there was a 11.2 kW induced-draft fan, which was employed to remove the flue gas from the furnace and force the flue gas out through the stack. The induced draft fan could also be used to adjust the operating pressure.