An overview on glycerol-free processes for the production of renewable liquid biofuels, applicable in diesel engines

Biodiesel is a biofuel obtained from vegetable oils or animal fats by transesterification with methanol, so that it offers a very promising alternative respect to diesel fuel, since it is able to provide a suitable substitute for the fossil diesel in unmodified internal combustion engines, pure or in blends. However, a major barrier exists to consolidate this conventional biodiesel, as the more suitable biofuel for the replacement of fossil fuel. This drawback is related to the significant amount of glycerol obtained as a byproduct in the transesterification process, which exceeds at least 10% by weight of oil used as raw material. Thus, future widespread use of biofuels depends on developing new process technologies to produce high quality transportation fuels from biologically derived feedstocks, which avoid this key handicap. These new biofuels, like the biodiesel, need to be also compatible with the fossil fuel as well as with existing transportation infrastructures to be economically feasible. In this respect, various alternative methods are currently under development to convert vegetable oils into a high quality diesel fuel, fully compatible with petroleum derived diesel fuel but avoiding the existing glycerol glut. The present review aims to explore the current state of available technologies and recent information in research, production practices and engineering developed to produce alternative high-quality diesel fuel from vegetable oils, by hydrotreating of triglycerides in conventional oil refineries (green diesel) as well as those novel biofuels that integrate glycerol into their composition ( Gliperol®, DMC-Biod® and Ecodiesel®) and the respective technologies for their productions. These very recent biofuels obtained from oils and fats, seek to achieve greater atom efficiency (ideally 100%) because nor glycerol neither other byproduct is generated, avoiding any purification treatment, so that the overall production process of the biofuel is in large extension simplified.