**4. Production and feedstock value chain**

The technology for petroleum-based fuel production has been in existence for many years. The same catalyst, reactor type, and distillation facilities used for the production of fossil-based fuel are also applicable for hydro-processing of vegetable oil-derived feedstocks to obtain high-quality hydrocarbon. Thus, massive savings are achievable since the same production plant facilities can be used for the purpose. Conversion of used vegetable oil via hydro processing using stand-alone units can be achieved by optimizing and controlling the facilities to obtain high yield green diesel. The design and construction of this facility as an attachment to existing plants and hydrogen in the refinery can be streamlined as recycled gas. The only shortcoming of a stand-alone unit as a production facility is the high cost of construction. In recent times the feedstock used for the production of HDRD has been plant-derived oils such as rapeseed, soybeans, and palm [55–58] which are edible oils, with the non-edible oils like Jatropha, algal oils and waste cooking oil products being the most popular feedstock ion recent times [57, 59]. Many researchers have confirmed UCO as being the most viable, cost-effective, and available feedstock. Researchers are still investigating the most suitable technology for the production of green diesel that is costeffective and CI engine compatible. No research has been done yet on the viability of biodiesel oil as a potential feedstock for HDRD production. When this is explored it will boost the supply of renewable fuel in energy sector. Green diesel obtained from biomass can be processed through four technologies: (i) Pyrolysis and upgrading of bio-oil, (ii) Hydro-processing, (iii) Catalytic upgrading of sugars, starches, and alcohols. (iv) Biomass to liquid (BTL) thermochemical processes. FT green diesel is produced by the Fischer-Tropsch [60, 61]. Hydrocracking and hydrotreating are the two major step in hydroprocessing technique [62]. The following yield was obtained via hydro processed used vegetable oil; biofuel (85%), non-condensable gases (10%), and water(5%) [63]. Green diesel is a biofuel product that comprises of branched saturated hydrocarbons and straight chain which contain carbon atoms of C15–C18. The properties of green diesel is similar to fossil fuel making it compatible with CI engines [64, 65]. Therefore, green diesel has superior fuel properties like high cetane number, oxidation stability, and cold flow and cloud point compared to FAME and petroleum based diesel [66]. Othman et al. investigated the value of cetane number, the outcome shows that green diesel has value of cetane number between 80 to 90 which is higher than the petrol based diesel standard [67]. The density and the net heating value of green diesel are in the range 0.77 g/ml to 0.83 g/ml and 42 MJ/kg and 44 MJ/kg respectfully, which also meets the biodiesel and petrol diesel standard [68].
