Abstract:
The main emphasis of this work was to investigate experimentally the particulate matter (PM) emissions from a dual fuel engine operated on alternative gaseous fuels such as natural gas and simulated biogas in comparison to the diesel fueling. A literature review suggested research needs in respect of PM emissions from dual fuel engines especially for biogas fueling. A number of techniques were employed to determine and characterize the PM emissions from the dual fuel engine in this study. PM mass emissions were measured by the conventional gravimetric method and using an online technique, a light scattering photometer. In respect of chemical characterizations of the emitted PM, thermogravimetric analysis (TGA), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy (RS) were used. For the PM size distribution and morphological studies, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were used. In addition to the PM measurements, the regulated gaseous emissions (CO, CO2, NOx and HC) were also measured for different fueling. About 70 to 75 percent reduction in PM mass emissions was achieved for dual fueling compared to diesel fueling. The volatile material/total PM fraction of the PM decreased sharply from about 94% to 30% when the engine load increased from light (-.3 Nm) to high load (-28 Nm). Higher volatile fractions were also obtained in the dual fuel (about 82% on an average) PM. Chainlike elongated structure was observed for diesel particulates while smaller and rounder particulates were observed for the dual fuel PM from SEM analysis. Size distribution obtained by SEM, was bimodal for all the different PM samples investigated in this work. TEM showed complementary results with SEM. Primary particle diameters ranged from 25 to 30 nm for all the cases investigated here. PM fractal dimensions were determined in the range from 1.69 to 1.88 for different PM samples. Dual fuel PM showed higher fractals indicating less chainlike agglomeration compared to that obtained for diesel high load condition. Carbon and oxygen were the main elements in the PM surface as indicated by the XPS analysis. The other trace elements were zinc and calcium. The maximum contribution of the graphitic carbon or aliphatic carbon such as hydrocarbons and paraffins (C=C or C-C) were found in the topmost atomic layers of all PM samples. The other chemical states were found to be the carbon-oxygen functional groups such as alcohol, carbonyl or ether, carboxyl or ester groups in the PM surface. Sulfur was only detected for the diesel-biogas3A PM sample which was found to be mainly in sulfate (SO4) form. According to the Raman studies the disorder of the graphite structures decreased with the increase in engine load. The measurements indicated that the carbon in the diesel PM sample was more disordered than the dual fuel PM. The carbon in dual fuel PM, on the other hand, were found to be more amorphous compared to diesel PM. The crystalline size of the carbon for dual fuel PM (— 7 nm) was found to be higher than that for diesel PM (— 4 nm). It might be possible that the main structural change was passing from graphite to nanocrystalline graphite and from nanocrystalline graphite to a-C (amorphous carbon) for the carbon structure of diesel and dual fuel PM respectively.