The demand for energy around the world is increasing, specifically the demand for petroleum-based energy. Appeasing this growing energy demand without irreparably damaging the environment is of primary concern. With rising fuel prices and environmental concerns, alternative fuels could satisfy the need for renewable energy with low environmental impact. Some of the more popular alternative fuels for new vehicles are ethanol, hydrogen, and biodiesel. Although gasoline engines are expected to be replaced by hydrogen-powered fuel cells, compression-ignition engines, the diesel engines, are expected to remain in use for high-power applications because of limitations of hydrogen-storage densities. The viable environmental friendly alternative fuel for compression-ignition engines is methyl esters (commonly known as biodiesel), which is derived from vegetable oils or animal fats. Using biodiesel instead of conventional diesel fuel reduces emissions such as the overall life cycle of carbon dioxide (CO2) particulate matter, carbon monoxide, sulfur oxides (SOx), volatile organic Compounds (VOCs), and unburned hydrocarbons significantly. However, biodiesel increases nitroaen oxides (NOx) emissions, mostly NO and NO2, which are considered as zone A hazardous compounds. This paper reviews the kinetics of NOx formation in relation to thermal, prompt, and fuel NOx formation processes and critically reviews the techniques that have been attempted to reduce NOx emissions from mechanisms to effectively reduce the NOx formation with biodiesel fuel.
