**7. Conclusions**

The continuous search for sustainable renewable energy has stimulated researchers to develop solutions to the global environmental crises and demand for renewable biofuel. HDRD has received significant attention across the global economy due to its higher stability, higher heating value, and emission of harmless pollutants. The use of biodiesel as a feedstock is more viable as it reduces the production cost and has the potential to settle the concerns related to the edible oil market. This critical review has identified biodiesel fuel as a potential feedstock for hydrogenation into HDRD. The main conclusions are:

