Biodiesel and oxides of nitrogen: investigations into their relationship

Abstract

Biodiesel is an alternative fuel that can be produced from a variety of lipid feedstocks. It has a number of perceived advantages over conventional petroleum diesel and as a result world production of biodiesel has increased dramatically since the turn of the century. Amongst its reported disadvantages is a widely observed increase in emissions of oxides of nitrogen, or NOx. Several explanations have been proposed for this phenomenon; in reality it is likely to be due to a combination of factors. The interplay of multiple factors affecting NOx emissions means that the increase in NOx when fuelling on biodiesel is not consistent or ubiquitous, but is instead dependent upon operating conditions and the specifics of the fuels being compared. The work documented in this thesis explores the nature and causes of the change in NOx emissions associated with biodiesel. The intention was that, by adjusting operating conditions, and using a wide range of fuels, doped with additives to achieve an even broader range of combustion characteristics, the impact of important variables would be made clearer, making it possible to reduce the problem to its lowest common denominators. In early experiments it was found that NOx emissions from biodiesel tended to be lower than those of petrodiesel under conditions where combustion was relatively highly premixed, but higher under more conventional diesel conditions where diffusion combustion constituted a larger proportion of heat release. The main experimental set revealed a definite increase in NOx emissions when fuelling on biodiesel, for a fixed start of combustion and equivalent degree of premixing. The addition of an oxygenate to petrodiesel elicited comparable NOx emissions to biodiesel, as a function of fuel-bound oxygen content; the data implies that the like-for-like biodiesel NOx increase may be a direct result of fuelbound oxygen. However, the like-for-like biodiesel NOx increase varies dependent upon operating conditions. In part, this may be related to higher apparent heat release rate (AHRR) through the diffusion burn phase when fuelling on biodiesel. This may result from the extended biodiesel injection duration. Across operating conditions, the extent to which smoke emissions when fuelling on petrodiesel exceeded those when fuelling on biodiesel was generally correlated with the magnitude of the biodiesel NOx increase; where the difference in smoke emissions was small, the biodiesel NOx increase was small, and where the difference in smoke emissions was more substantial, so was the difference in NOx emissions. This suggests a possible connection to changes in mixture stoichiometry. When differentiating between fuels, increased cetane number reduces NOx, and increased oxygen content increases NOx. Biodiesel does not necessarily have higher NOx emissions than petrodiesel: the biodiesel NOx increase exists where the difference in cetane number is insuffi cient to counteract the effects of fuel-bound oxygen content.