Supercritical Water Gasification of Biomass and Organic Wastes

With the exhaustion of fossil fuels and the severe pollution of the environment, it’s urgent to find substitute renewable energy resources. Biomass is a kind of widespread and abundant renewable resource with lower sulfur content than fossil fuels, so the pollution to environment from biomass usage is much slighter than that from fossil fuels usage. Besides, zero emission of CO2 can be realized in the cycle of growing and energy conversion of biomass. Supercritical water gasification (SCWG) is an innovation biomass conversion process which takes advantage of the special properties of supercritical water (temperature above 374 ̊C and pressure above 22.1MPa) to transform biomass into hydrogen-rich gaseous products. Comparing with conventional gasification and pyrolysis in normal pressure, SCWG has many advantages: • Most organic materials of biomass can be dissolved in supercritical water for the relatively high dielectric constant of supercritical water (SCW), thus SCWG of biomass is homogeneous reaction, which can decrease the mass transfer resistance between phases. • High pressure of the gaseous product make it easy for transportation, usage, carbon capture and further purification of the product gas through steam reforming or PSA (pressure swing adsorption). • Higher energy efficiency can be achieved in SCWG of biomass especially for high moisture content biomass as the drying process is not required in SCWG. • The reaction temperature is much lower than that in conventional gasification and pyrolysis. For example, the temperature of conventional steam gasification is always above 1000°C, while the complete gasification of glucose can be achieved at 650°C, 35.4MPa in supercritical water gasification. • The gaseous product can be very clean. As almost no NOx and SOx were generated in supercritical water gasification, and the CO concentration is very low, especially with the catalyst to enhance the water-gas shift reaction. Supercritical water gasification was firstly described by Modell in reforming of glucose and wood residues (Modell 1977; Modell 1980). In recent decades, much important progress has been made in SCWG technology by the researchers around the world. Elliott et al in PNNL (Pacific Northwest National Laboratory) have done a series of research on the reactions of kinds of biomass and organic wastes in high-pressure aqueous environments since 1980s.