Storage degradation of palm-derived biodiesels: Its effects on chemical properties and engine performance

Palm olein and palm stearin are co-products of palm oil refining processes having different melting point ranges. This study compares the storage degradation characteristics of biodiesels derived from these two palm products, which are palm olein and palm stearin, in terms of chemical properties, engine performance and exhaust emission. The degradation study was carried out by keeping biodiesels in dark closed-lid containers at room temperature for up to 6 months. It was found that the oxygen present in the container led to slow degradation of biodiesels through oxidative reaction with the double bonds in biodiesel. Within 6 months, the majority of oxidative products were composed of shorter hydroperoxide compounds and other short secondary products. These changes resulted in lower heating value and higher density of biodiesels, which in turn caused reductions in fuel combustion efficiency and fuel economy. In terms of emission, the degraded biodiesel produced more complete combustion as indicated by lower emissions of black smoke and carbon monoxide but with higher emission of NOx. In terms of palm oil type, even though palm olein biodiesel possessed higher degree of unsaturation and produced higher peroxide value and acid values from the degradation, its combustion efficiency and fuel economy were still superior to the biodiesel produced from palm stearin possibly due to its higher chain lengths.

► Palm olein has higher degree of unsaturation and chain length than palm stearin.
► PO biodiesel gave better combustion efficiency and fuel economy than PS biodiesel.
► PO biodiesel degraded faster than PS biodiesel.
► Degradation of biodiesel led to lower heating value and higher density.
► The degraded biodiesels produced lower black smoke and CO2 but higher NOx.