Influence of metal contents on the characteristics of biodiesel and petrol blends as transportation fuel

Abstract

The development of renewable energy seems to be the key player in addressing global climate crisis, caused by the global warming and climate change. To produce less emissions, new engine systems have been developed i.e., petrol compression ignition engines, which promise to produce less emissions, while generating high efficiency, and facilitating economic, social, and environmental sustainability. With the novelty of blending biodiesel with petrol, the challenges caused by biodiesel, and petrol individually, are lessened. Nevertheless, biodiesel’s proneness to corrosion and degradation overtime, due to its chemical nature and storage conditions require continuous evaluation. In the initial part of this study, the bi-functional catalyst (75%CaO/25%Al2O3) was synthesised via adjusted wet impregnation method. Results showed that, the catalyst presented enhanced activity, good porosity, and type H1 desorption hysteresis loop, with the high surface area and the pore diameter of 13.006 m2/g, 24.0371 nm. The catalyst characterisations were conducted through BET, XRD, FTIR and SEM. The obtained bifunctional catalyst favoured the transesterification reaction of high free fatty acids feedstocks, with high yields of above 98% of methyl esters in biodiesel produced from waste sunflower oil. With the use of GC, fatty acids compositions of waste sunflower and waste palm oils were determined. The results also showed that the chemical composition of these different feedstocks i.e., degree of saturation, chain length, produced biodiesels with varying fuel properties. While sunflower biodiesel indicated better viscosity, palm biodiesel had excellent oxidation stability. Additionally, sunflower biodiesel met the international biodiesel specifications, with the exception of increased Ca concentration within the biodiesel, as a result of CaO/Al2O3 catalyst use in the biodiesel synthesis. This soft metal, along with Mg, K were introduced in the biodiesel through the synthesis process. While soil, seed, fertiliser, and contamination in the vegetable oils, may have contributed to the high content of P, and trivial Fe, Al, and Zn. The use of ICP - OES allowed for the determination of these metals. To commercialise biodiesel, optimisation can be performed in reducing the cost and time necessary to produce biodiesel. After optimising sunflower biodiesel using response surface methodology and central composite design, the optimal reaction conditions observed were 5 h for reaction time, 60 C for temperature and 2.5wt% for catalyst weight. With the use of a linear regression model that had 95 % confidence, the predicted and experimental yields were confirmed to be comparable.

In accessing fuel quality of biodiesel and the biodiesel-petrol blends, analysis of the viscosity, acidity, oxidation, density, volatility, moisture content, cetane number, metal contamination and particulate matter were conducted. Palm biodiesel had an increased thermal stability which rendered the palm biodiesel-petrol blended fuels superiority over the sunflower biodiesel-petrol blends. The blended fuels were observed to have enhanced fuel characteristics, better than pure petrol, increasing with increase in biodiesel content with 75% petrol 25% biodiesel (PB25) showing quality like petrodiesel. The addition of petrol into the biodiesel diminished the Ca concentrations, and obstructed moisture absorption, while improving low temperature fluidity loads, air-fuel mixing, and characteristics of good performance with high efficiency. Sunflower biodiesel-petrol blends were observed to be less acidic, have more energy content and subsequently more power. While palm biodiesel-petrol blends had more thermal stability and better cold start. Moreover, addition of petrol reduced particulate matter of sulphates. In the final part of this study, the effect of Cu, Fe and Zn on the characteristics of fuel quality were evaluated for the purpose of the storage and transportation of biodiesel and the biodiesel-petrol blends. From the results obtained for pure biodiesel, the highest degradation was caused by exposure to Fe concentrations, while degradation in the biodiesel-petrol blends was caused by exposure to Cu. Sunflower biodiesel-petrol blends degraded in order of Fe > Cu > Zn, while palm biodiesel-petrol blends were degraded by Cu > Fe > Zn, and with Cu affecting pure palm biodiesel the most. Increase in oxidation instability for biodiesel-petrol blends was due to rise in Cu concentrations. The fuel quality was observed to decrease the most in palm biodiesel and palm biodiesel-petrol blends.