## 3 Results and Discussion  
### Combustion Tuning  
During combustion, the carbon content of the fuel burns in oxygen to form carbon dioxide. When oxygen appears in the flue exhaust, this indicates that more combustion air (excess air or excess oxygen) was supplied than was needed for complete combustion. However, when a supply of combustion air is inadequate, the fuel cannot get enough oxygen to burn completely, and incomplete combustion occurs, leading to CO formation. When the excess oxygen is reduced below a critical excess oxygen amount (critical point, e.g., point E in Figure 4a), CO formation increases rapidly. Therefore, to achieve a complete combustion situation, the excess oxygen amount must be adjusted above this critical point, below which incomplete combustion occurs, and CO increases significantly, leading to possible smoke generation because the amount of excess oxygen is too low. Figure 4a,b show the variations in the emissions with residual oxygen measured in the flue exhaust during the combustion tuning process for burning pure diesel and the blend (5% FPBO + 95% diesel), respectively. The fuel flow rate (\(\dot{Q}_{F}\)) was fixed at 20 L/h in both the combustion tuning and stable combustion processes. Therefore, to determine the minimum oxygen required for complete combustion, the airflow rate was gradually adjusted from high to low levels, but the \(\dot{Q}_{F}\) was maintained a constant value of 20 L/h. As shown in Figure 4a, points A, B, C, D, E, F, and G correspond to the airflow rates (\(\dot{Q}_{A}\)) of 265, 261, 256, 251, 246, 241, and 236 Nm\({}^{3}\)/h, respectively. The residual oxygen concentration decreased with decreases in the airflow rate from point A (265 Nm\({}^{3}\)/h) to point G (236 Nm\({}^{3}\)/h). For the airflow rate greater than the critical point (point E, 246 Nm\({}^{3}\)/h) for complete combustion, the CO emission was nearly 0 ppm. However, when the air flow rate was smaller than the critical point, the CO emission increased gradually, and consequently, a large amount of CO was generated, e.g., at point F and point G. Increases in the amount of combustion air corresponding to increases in excess oxygen thus in turn led to increases in the residual oxygen concentration. NO formation was found to increase with increases in the amount of excess oxygen from point G to point E, and then NO emission was nearly maintained at a slightly lower level from point D to point A. Formation of SO\({}_{2}\) is directly related to the sulfur content of the fuel. Thus, when burning the same fuel, the SO\({}_{2}\) emissions remain nearly at a constant value. A better adjustment of combustion air was found to at point E. With an airflow rate of 246 Nm\({}^{3}\)/h, both CO and NO emissions were low. Therefore, point E was taken as the optimum air-supply rate for the minimum excess-oxygen requirement with low CO emission and flue-gas heat loss. This operating condition was adopted for the subsequent stable complete-combustion diesel experiment. Similarly, for the blend of 5% FPBO + 95% diesel, point C (\(\dot{Q}_{A}\) = 256 Nm\({}^{3}\)/h) was the optimum operating condition (which met the minimum residual oxygen in the flue gas required for complete combustion) obtained from the combustion-tuning procedures, as shown in Figure 4b. This optimum operating condition was also adopted for the subsequent stable complete-combustion experiment. From the results of Figure 4a,b, the air-feed rates for the minimum excess-oxygen requirements were 246 and 256 Nm\({}^{3}\)/h for diesel alone and the blend of 5% FPBO + 95% diesel, respectively. That is, the airflow rate of the blend of 5% FPBO + 95% diesel at the critical point was slightly greater than that of diesel alone. More specifically, the theoretical air-to-fuel ratios (A/F ratios) for diesel and FPBO are 14.44 and 3.06, respectively, whereas the theoretical A/F ratio for the bio-oil/diesel blend with 5 vol % bio-oil is 13.67.