Biodiesel produced from renewable energy sources has been widely researched by different countries as a potential and ecologically acceptable substitute for the conventional fuel. Considering the increasing material cost and the human consumption of edible vegetable oils, low-grade raw materials involving non-edible oils, waste cooking oils, soapstocks and animal fats have drawn much interest for biodiesel production. This paper reviews the transesterification of low-grade feedstocks to convert into biodiesel with supercritical fluid technology that is more efficient and eco-friendly. This technonogy leads to simpler separation and purification steps compared with the conventional catalytic methods. The supercritical process is insensitive to free fatty acids or water in feedstocks and requires relatively short reaction time with high ester conversion yield. Besides, potential intensified technology has also been provided for reducing the biodiesel production cost to expect an early industrial application.
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