• Biomass Combustion
• CFD
• Chemical Processes
• Moving Grate Boiler
• Multi-phase Flows
• Numerical Modelling
• XDEM

In the present thesis a numerical model of the combustion chamber of a 16 MW geothermal steam super-heater, which is part of the Enel Green Power "Cornia 2" power plant, has been developed. The super-heater is fed with biomass and, by increasing the temperature of the geothermal steam, allows the augmentation of the net plant generation capacity; hence it represents an innovative example of integration of geothermal and biomass energy.
The super-heater is equipped with a moving grate. The first numerical approach exploits a coupling between a three-dimensional model of the gas flow inside the chamber, realised with Computational Fluid Dynamics (CFD) techniques, and an off-line sub-model of the biomass bed. The last one is based on a zone schematization of the bed, which is treated as a series of perfectly stirred reactors in which the biomass undergoes evaporation, pyrolysis and char oxidation. This bed sub-model provides flow rates, compositions and temperatures of the inlet flow to the CFD domain. The results of this approach have pointed out a strong sensitivity to the turbulent characteristics of the flow. It is indeed the turbulent mixing of the reactants that controls the reaction rates and thus the resulting thermochemical field inside the chamber. For this reason a second approach able to solve the detailed dynamics of the bed particles has been tested. Given the numerical complexity of the problem, a two-dimensional representation of the super-heater has been considered in this case. The XDEM code, developed at the University of Luxembourg, has been employed to analyse the granular flow of the bed. This model simulates the grate motion and the dynamical and thermodynamic behaviour of the biomass particles above it. Preliminary results, with a non-reactive biomass, have shown high levels of turbulence over the bed. These data have been used to provide more accurate boundary conditions to the three-dimensional CFD model. Potentialities and limits of both the approaches, other than aspects relative to the implementation of the chemical kinetics, are discussed in the thesis.