Numerical simulation of nitrogen oxides and carbon monoxide emissions of biodiesel diffusion flame

Biodiesel is one of the most promising fossil fuel replacements for automotive engines, furnaces, and turbines due to its sustainability, energy savings, and reduced carbon emissions. While commonly reported in engine studies, nitrogen oxides (NO_x) and carbon monoxide (CO) released from combustion of biodiesel have not been studied in laminar diffusion flames. This numerical study examines the concentrations of NO_x and CO emissions of the laminar biodiesel diffusion flames at different carbon flow rates and then compares its emissions with those of two liquid hydrocarbon fuel surrogates, n-heptane and iso-octane. A consistent carbon flow rate of 17.2 g/h is applied at the fuel inlet to compare the NO_x and CO emissions of the three liquid fuels. The results show that biodiesel diffusion flame produces greater NO_x and CO emissions with increasing carbon flow rate. At the same flow rate, n-heptane produces the greatest NO with 2.1% greater than biodiesel and 4.2% greater than iso-octane. The primary pathway for generating NO in biodiesel flame is the prompt pathway, with significant contributions from the thermal and NO₂ decomposition pathways. While the NO productions in n-heptane and iso-octane flames are predominantly through the thermal pathway. It is also observed that biodiesel produces the greatest CO emission with 3.2% more than those of n-heptane and iso-octane. The oxidisation reaction of CO, CO + OH = CO₂ + H primarily controls the CO mass fraction in the product for all fuels.