Supercritical water has some of the desired properties of its liquid phase as well as the gaseous phase. Supercritical water was first applied in hydrothermal conversion of organic compounds to gases in the 1970s. Supercritical water is being researched for its application in biomass oxidation as well as gasification. Supercritical water oxidation is generally used for neutralizing toxic organic matter. At supercritical conditions, organic matter and oxygen become fully miscible in water. Heterogenous catalysts have a few benefits over homogenous catalysts such as selectivity, being environment friendly, and recyclability. Residence time is the time for which the biomass stays in the reactor at the desired reaction temperature. Reactor design and process control is one of the most challenging tasks while dealing with hydrothermal reactions. Lignocellulosic biomass, which makes up almost 90% of total land biomass available, mostly comes from agricultural by-products, forests, or dedicated energy crops. Microalgae can be grown in marine and freshwater in the presence of CO2 and sunlight through photosynthesis.