br Acknowledgement br Introduction The

Acknowledgement
Introduction The world is currently challenged with global warming, depletion of non-renewable fossil fuel and environmental pollution. The major sources of greenhouse gas emissions are fossil fuels (Endalew et al., 2011). To overcome these challenges, there is a need to find alternative Sirolimus sources that are renewable, economically feasible and environmentally friendly. Biodiesel has a great potential as an alternative fuel. Biodiesel is a biodegradable and non-toxic fuel, characterised by carbon dioxide emission reduction attributable to recycling by photosynthesis. This minimises the impact of biodiesel combustion on the greenhouse effect (Körbitz, 1999; Brito et al., 2007; Chung et al., 2008). The main advantage of the application of biodiesel fuel is its better quality exhaust gas emissions (Glisic and Orlović, 2014). Further, biodiesel has an advantage of good fuel properties such as high flash point which makes it easy to handle and good lubricity (Kouzu and Hidaka, 2012; Glisic and Orlović, 2014). Other properties such as cetane number and cloud point depend significantly on the feedstock type. The challenges associated with the development of alternative fuels continue to attract intensive studies (Yagiz et al., 2007; Kotwal et al., 2009). Most investigations on biodiesel production have been focused on the use of vegetable oil or animal fat as feedstock in the presence of a catalyst or enzyme via transesterification process (Demirbas, 2005; Yagiz et al., 2007; Asakuma et al., 2009; Lee et al., 2011; Olutoye et al., 2011; Atabani et al., 2012). Biodiesel fuel is expensive as compared to petroleum-based fuel, as 60–80% of the cost is associated with the feedstock oil, due to the use of expensive, high quality virgin oils such as soybean, sunflower, olive, palm, canola, cottonseed, peanut and linseed (Parawira, 2009; Christopher et al., 2014). This has created a bottleneck for biodiesel commercialisation (Raman, 2013; Talebian-Kiakalaieh et al., 2013). Due to price increase in food commodities and discharge of waste in the environment, there is a current shift towards the use of waste vegetable oil (WVO) and non-edible oils as low grade feedstock in biodiesel production (Zhang et al., 2003; Shu et al., 2007; Wang et al., 2011). The use of these oils as feedstock in the production of biodiesel are viable with respect to cost reduction and they have attracted much attention since they are renewable and readily available (Yagiz et al., 2007). While biodiesel production using WVO has been well reported in the literature, there are major drawbacks such as presence of impurities and high concentrations of free fatty acid, characterising this feedstock, which hinder conventional homogeneously-catalysed transesterification process (Lam et al., 2010). Further, the use of homogeneous catalyst and waste vegetable oil in biodiesel production have shown several drawbacks such as equipment corrosion, formation of soap and total consumption of catalyst (Shu et al., 2007). Presently, there are several heterogeneous base catalysts available for biodiesel production from feedstock characterised with high FFA content. Heterogeneous catalysts are characterised by a number of advantages in comparison to the homogeneous catalysts. Several washing steps can be eliminated, the separation process is easy to handle, low cost, environmentally friendly, catalyst can be reused, less toxicity, high catalytic activity, and consequent reduction of production cost (Lee et al., 2009). Recent studies have been conducted on the use of ash from waste shell as heterogeneous catalyst for biodiesel production (Chakraborty et al., 2010). Heterogeneous base catalysts of alkaline earth metal oxides have been investigated for biodiesel production (Khemthong et al., 2012). It has been shown that the derived CaO contained in ash from waste materials is a potential heterogeneous catalyst. This new orientation for biodiesel production is eco-friendly and economical (Khemthong et al., 2012).