## 3 Results and Discussion
### Emissions  
Figure 7a,b show the concentration of flue gas composition versus time for burning the pure diesel and the blend of 5% FPBO + 95% diesel, respectively, at a constant fuel supply rate of 20 L/h under the minimum residual oxygen in the flue exhaust required for complete combustion of the fuel. The values of CO, CO\({}_{2}\), SO\({}_{2}\) and NO reported in this study were all corrected to 6% O\({}_{2}\)[17]. It was found that the variations in the O\({}_{2}\) and CO\({}_{2}\) concentrations with time for these two liquid fuels were very small. Figure 7 also indicates that the CO emissions for firing 5% FPBO have larger fluctuations than those for firing pure diesel. This may be due to the higher viscosity of FPBO than diesel. The higher viscosity of FPBO led to poorer atomization of the fuel spray, which linked to incomplete combustion (corresponding to higher CO emissions [16]) and larger fluctuations observed in the CO emissions.  
Figure 8 and Table 4 show that the average O\({}_{2}\) concentration appearing in the flue exhaust for firing the blend of 5% FPBO + 95% diesel was 5.2%, while that for firing diesel alone was 4.1%. Meanwhile, the average CO\({}_{2}\) emissions for firing the blend of 5% FPBO + 95% diesel and diesel alone were 11.2% and 12.0%, respectively, which were comparable.  
Figure 6: Comparison of average temperatures for burning diesel alone and the blend of 5% FPBO + 95% diesel: (**a**) the wall temperatures (\(T_{\text{w1}}\)-\(T_{\text{w5}}\)) in the radiative section; and (**b**) gas temperatures (\(T_{\text{g1}}\)-\(T_{\text{g5}}\)) in the flue gas section.  
Both firing cases were adjusted to meet the minimum excess oxygen conditions required for complete combustion. Consequently, the combustion was almost complete, resulting in low CO emissions (less than 35 ppm). The average CO emissions for firing the blend of 5% FPBO + 95% diesel and diesel alone were 20 and 34 ppm, respectively. Additionally, NO emissions were very low (smaller than 57 ppm) and were comparable for these two firing cases. It should be noted that when the wall temperature of a furnace reaches a near-steady state, the factor affecting NO\({}_{X}\) emission is only the gas temperature [31], not the wall temperature. Thus, the formation of nitrogen oxides has a clear dependence on excess oxygen, temperature in combustion and fuel-nitrogen (fuel-N). During firing, the blend of 5% FPBO + 95% diesel exhibited a lower temperature but higher excess oxygen (or higher residual O\({}_{2}\) in the flue gas) and slightly higher fuel-N than was the case when firing diesel alone. The combined effects of temperature, excess oxygen and fuel-N resulted in slightly higher NO\({}_{X}\) emissions (56.1 ppm) for burning the blend as compared with burning diesel alone (45.9 ppm).  
Figure 8: Average emissions when burning diesel alone and the blend of 5% FPBO + 95% diesel.  
Figure 7: Emissions versus time: (**a**) when burning diesel alone; and (**b**) when burning the blend of 5% FPBO + 95% diesel.  
The amount of SO\({}_{2}\) emissions significantly depends on the sulfur content in the fuel. Both FPBO and diesel have a very low sulfur content. As expected, the SO\({}_{2}\) emissions for burning the blend of 5% FPBO + 95% diesel and the diesel alone were nearly zero.  
In view of the emissions results, it can be summarized that the use of 5% FPBO in the blends produces comparable emission levels of CO, NO and SO\({}_{2}\) when compared to pristine diesel.