Although in laboratory experiments excellent results have been achieved and the technology possesses many potential benefit s over the conventional methods of processing biomass to biofuels or chemicals, there are certain issues, which need to be addressed.

— Biomass feeding at high pressure: As a “rule of thumb,” the solid loading in excess of 15-20 wt.% is considered economical on commercial point of view.

Feeding slurries at high pressure is always challenging especially for the lab scale studies since low capacity slurry pumps are rarely available. Pumping slurry at large scale is less of a problem, where progressive cavity or similar pumps are commercially available.

— Salt precipitation: Plugging of reactors caused by the precipitation of inorganic salts above supercritical temperature and low density conditions. At room tem­perature, water is an excellent solvent for most salts. On the other hand, solubility of most salts is very low (typically 1-100 ppm) in supercritical water (low density) and precipitating salts may plug the reactors even at high flow velocities [59]. However, the problem may be used as an opportunity to produce a valuable fertilizer by-product of the process, if managed properly.

— Corrosion: The halogens, such as sulfur or phosphorous, present in the organic matter are converted to the respective acids, which may cause severe corrosion on the reactor wall under harsh reaction conditions. The corrosion problem can be reduced or avoided by selecting the right material of construction and or a slightly modified reactor concept.

— Coking and deactivation of heterogeneous catalyst: Some catalyst supports degrade or oxidize in hydrothermal conditions. Decline in catalyst activity is also observed with long period of exposure of catalyst during continuous process [94].