Effect of a homogeneous combustion catalyst on the nanostructure and oxidative properties of soot from biodiesel combustion in a compression ignition engine

Abstract

The effect of a ferrous picrate based homogeneous combustion catalyst on the properties of soot from the combustion of a biodiesel in a compression ignition engine was systematically studied. Soot samples from the combustion of a reference petroleum diesel (RD) and the biodiesel (BD), with and without the catalyst treatment, were collected when the engine was maintained under a constant condition of 2800rpm speed and 5.5Nm load, and subsequently analysed using transmission electron microscopy (TEM) for their nanostructures and thermogravimetric analyser (TGA) for the oxidative reactivity. The results showed that compared to the soot from BD, the sizes of both primary soot and aggregates from the catalyst treated biodiesel were consistently smaller; while the nanostructure and fractal dimension remained similar, indicating that there were no dramatic changes in the mechanisms of the soot nucleation and agglomeration. Furthermore, soot from the catalyst treated biodiesel possessed higher oxidative reactivity as indicated by the lower ignition temperature and faster oxidation rate than those of soot from BD. In addition, the primary soot and aggregates from BD were smaller and the soot had a higher order in its nanostructure with greater oxidative reactivity, in comparison to those from RD. A mechanism of the working of the catalyst was proposed in that the catalyst promotes the biodiesel combustion process leaving fewer soot precursors and also accelerates the oxidation of soot particles, resulting in smaller sizes of the primary soot and soot aggregates and the reduced overall soot emissions.