A two-step approach to the hydrothermal gasification of carbohydrate-rich wastes: Process design and economic evaluation

A two-step approach to hydrothermal gasification of carbohydrate-rich wastes and wastewaters is a promising route for H₂ production and simultaneously as a water clean-up technology. Experimental data and kinetic models are used to further develop the process for industrial scale. In this work, a preliminary process design is conducted in order to assess the market potential of the two-step process. For stabilisation, two cases are considered: the use of excess vs. stoichiometric H₂, and for gasification, the utilisation of sequential reactors for gasification housing Pt and Ru catalysts is compared to a single reactor with Pt alone. A total of four options are conceptually designed and economically evaluated. Using state-of-the-art insights and process techniques and the current market scenario, a minimum H₂ selling price of 3.4 $ kg⁻¹ was obtained. A sensitivity study showed that the feedstock price, concentration and quantity, played a crucial role in the selling price of H₂. These variables are all correlated and are dependent on the industry from where the feedstock is obtained. Industrial wastewater streams rich in carbohydrate residues and associated with gate fees were found to be promising feedstock for the process. Further advancement in the areas of catalyst development (hydrothermal stability, affordability) as well as increased H₂ yields are necessary in order to improve the economics of this process on industrial scale.