State of the art of NOx mitigation technologies and their effect on the performance and emission characteristics of biodiesel-fueled Compression Ignition engines

Biodiesel fuels have the potential to become a reliable substitute for diesel which is used moderately to meet the current energy demands. This fuel can be produced from new or used vegetable oils, non-edible sources and animal fats, which are non-toxic, biodegradable and renewable. In spite of the many advantages of using biodiesel, most of the researchers have reported that they produce higher NOx emissions compared to diesel, which is a deterrent to the market expansion of these fuels. In this study, the different paths to reduce NOx emissions from diesel engines by applying several technologies, such as using additives into fuel, exhaust gas recirculation (EGR), water injection (WI), emulsion technology (ET), injection timing retardation (ITR), simultaneous technology (ST) and low temperature combustion (LTC) mode are reviewed briefly. The impacts of different NOx mitigation technologies on biodiesel-fueled diesel engine performance and emissions are also analyzed critically and different methods of their implementation are shown. This paper also provides a comparison of different NOx mitigation technologies based on previous articles related to this topic. From this comparative study, it was found that the average reduction of NOx emissions by using additives, EGR, WI & ET, ITR, ST and LTC are in the ranges 4–45%, 26–84%, 10–38%, 9.77–37%, 22–95% and 66–93% respectively, compared to biodiesel combustion without applying technologies. However, the average reduction of NOx emissions by using those technologies for biodiesel are reasonable, 36–46%, 3–34%, 21–37%, 33–92% and 8.68–70% respectively, when compared to diesel. The results of this paper show that using new NOx mitigation technology, such as LTC, is more efficient and promising than the others because of its having the capability to reduce both NOx and particulate matter (PM) emissions simultaneously without significant penalties with regard to engine performance. However, applying this technology has increased CO and HC emissions in several cases due to the reduction in their oxidation rates in the combustion chamber.