The purpose of this study is to use biogas produced in industrial wastewater treatment plants to generate energy by co-combustion with diesel fuel or biodiesel to avoid the unstable flow rate of biogas and the variable methane ratio, which determines the biogas energy content. An experimental analysis was conducted in this study to determine the combustion and emissions performance of a 350 KW industrial burner fuelled with three different percentages of biogas: Biogas1 (CH4 75%, CO2 25%), Biogas2 (CH4 70%, CO2 30%), and Biogas3 (CH4 60%, CO2 40%) co-combusted with diesel or waste cooking oil biodiesel. Practical tests have demonstrated that in comparison to biogas and diesel fuel, the CO emission level of co-combustion biogases and biodiesel for B1000Biogas1, B100Biogas2, and B100Biogas3 was reduced by 60%, 50%, and 42%, while NOx emission increased by 52%, 47%, and 43%, along with the maximum flame temperature, by 9%, 10%, and 12%, respectively. The flame structures of the fuels in the swirl burner were investigated using flame pictures and contour temperature. The flame color for biodiesel and biogas was more brilliant and intense than for diesel and biogas. All the fuel test results demonstrate that inert CO2 in biogas composition has a substantial influence on the chemical reactions occurring in the flame and pollutant emissions due to its dilution effect and slowing oxidation reaction. The higher inert CO2 gas ratio in biogas caused a reduction in reaction intensity, which resulted in a weaker, unstable flame and also decreased flame temperature and NOx emissions.