Supercritical Water Gasification of Biomass

Finding new ways to produce renewable energy is among the most important and strategic challenges of technology nowadays. Biomass is one of the ideal candidates for reliable and abundant renewable energy production, since it is largely available, universally distributed and potentially CO2 neutral, if utilized in a sustainable way. There are several processes for energy exploitation of biomass, including combustion, pyrolysis and gasification. However, traditional thermochemical technologies can be only effective with dry biomass, owing to energy considerations. As a consequence, wet biomass (e.g. municipal or agro-industrial wastes), which represents the greatest part of the overall biomass, cannot be converted into energy. This strong limitation can be overcome by a novel technology: supercritical water gasification (SCWG). SCWG is based on reacting biomass with water above its critical point (T > 374.1°C; P > 22.1 MPa). Thanks to the unique properties of supercritical water, high gaseous yields can be achieved, as well as reduced (or even null) tar and char production. Moreover, a H2-rich gas can be obtained. Therefore, high moisture content is not a drawback anymore, being water part of the process itself. In this thesis, SCWG is analyzed under different aspects. First of all, a comprehensive state of the art is traced. The work is then divided into two main sections: mathematical modeling and experimental activities. The first section reports three different modeling approaches for SCWG. In thermodynamic equilibrium modeling, a two-phase thermodynamic equilibrium model was built, enabling to predict products composition as a function of process parameters, as well as solids formation at equilibrium. Energy balances were also performed by means of such tool. The kinetics modeling approach was applied to methanol SCWG, developing an elementary reactions model able to highlight the main reaction pathways. Process modeling was then used to calculate the energy needs of a possible industrial SCWG process scheme, enabling to prove its energetic feasibility. The second part of the thesis deals with experimental tests, which were executed with both real biomass and model compounds. A first campaign was performed with glucose and glucose/phenol mixtures in small metallic batch autoclaves. The catalytic effect of the reactor material (stainless steel and Inconel 625) on the gasification products composition was discussed, as well as the influence of subcritical (350°C) and supercritical (400°C) reaction conditions. Moreover, the effect of phenol addiction, inhibiting glucose gasification, was observed. In a subsequent campaign, real biomass was gasified, including beech sawdust, municipal wastes, malt spent grains and hydrothermal char. The effect of the reactor material was studied, as well as the system behavior after long time runs (16 h) and the addiction of K2CO3 as a catalyst. Finally, glucose/phenol mixtures, with increasing phenol contents, were gasified in a continuous tubular reactor at 400°C and 25 MPa, between 10 and 240 s of residence time. Results showed that phenol is hardly gasified and that methanol is a key intermediate product.