Biodiesel NOx penalty and control measures - a review

Biodiesel fuels comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats serve as a potential alternative to fossil diesel. The biodiesel fueled engine does not require any major hardware modifications in the existing configuration. Also, the uses of biodiesel in compression ignition engines are found to significantly lower soot emissions. Thus, the conventional nitric oxide-soot trade-off problem narrows down to a unidirectional control of nitric oxide emissions which are generally known to increase with several candidate biodiesel fuels. The formation of oxides of nitrogen (NOx) from biodiesel fuel is dependent on both fuel and engine characteristics. The present work reviews the effect of biodiesel fuel composition and properties on NOx emissions emanating from biodiesel fueled engines and describes fuel modifications and engine tuning measures for their mitigation. The other outcomes of biodiesel NOx control measures in terms of engine performance and emission changes are also highlighted and the best possible ways for achieving biodiesel-NOx neutrality are outlined.