(167e) Inhibition of Catalyst Poisoning in Supercritical Water Gasification of Sewage Sludge: Technical Study On Reactor Design

Supercritical water gasification is a thermal method which enables us to transform biomass to H₂ rich gas with reduced tar and char formation. The technology not only exhibits essentially high conversion but also takes advantages on process concerns. Easy gas separation and fast reaction rate (compared to anaerobic digestion) leading to compact equipment as well as elimination of post-drying step unlike conventional thermo-chemical treatment processes are likely to be featured as significant advantages of this technology.

Although the process seems to be promising, supercritical water gasification for real biomass is still under development. The main challenges are problems concerning with salts originating from biomass. Alkaline salts tend to increase hydrogen yield, but at the same time it is associated with the risk of plugging and catalyst poisoning. The issue still remains as a challenge being the major obstacle to technical application.

This work focuses on the geometry of the gasification reactor which will lead to the inhibition of catalyst poisoning in supercritical water. In this work, a lab-scale batch reactor with two compartments, so called two-bed reactor, was introduced to the supercritical water gasification of sewage sludge. The reactor consists of two compartments, physically separating biomass/water feedstock and catalyst. This reactor configuration aims to reduce poisoning caused by the solid residue.  Therefore, pore size of the frit filter used to separate the two compartments is set to be small enough to avoid unnecessary intercommunication between sewage sludge particles and heterogeneous catalysts. Char and tar which is likely to pass the filter causing catalyst poisoning, can be resolved by using an additive inhibiting their formation. In our experiments, K₂CO₃ which is reported to function as a char and tar formation inhibitor is added to either of the compartment.

Under the condition of 400^oC, 27 MPa and reaction time of 30 minutes with Ni catalyst presence, the reactor was capable to suppress the deposition of byproducts and residuals on catalyst surface attaining higher gasification efficiencies. Furthermore, acceleration of hydrogen gas production was achieved when K₂CO₃ was loaded with the presence of Ni catalyst. Studies have revealed that loading location of K₂CO₃ influenced the gasification behavior dramatically. Besides Ni catalyst, various combinations of catalyst/additive to gas yield and efficiency were quantitatively identified. Catalyst condition before and after reaction was also compared by microscopic observation. In prospect of future application, heavy metal concentration in the effluent will also discussed in our presentation.