CFD-DEM modeling of biomass combustion on a reciprocating grate

The Cornia 2 hybrid geothermal/biomass power plant of Enel Green Power represents an innovative technology integrating renewable and geothermal energy. The plant is located in the Larderello geothermal area in Tuscany and uses biomass to superheat the geothermal steam entering the power station from a temperature of 150-160 to 370-380 Celsius degrees, thus increasing the efficiency of the geothermal cycle and the net electric power output from 12 to 16 MW.  The super-heater boiler is based on reciprocating grate technology and is fed with two kinds of biomass: local virgin biomass wood-chips and low-quality agricultural residues. A numerical model of the biomass combustion on the moving grate is developed through the coupling of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In particular the eXtended Discrete Element Method (XDEM) [1] is employed to describe the granular behaviour of the two kinds of biomass particles. The peculiar feature of XDEM is that it encompasses chemical and thermodynamic particle properties (temperature profile, species distribution, porosity, diffusivity, etc.) in addition to kinematic (position, velocity and orientation) ones, thus allowing taking into account evaporation, pyrolysis and char oxidation. The CFD model is handled with OpenFOAM®-Extend and solves mass, momentum, species transport end energy equations in the gaseous phase. Hence, the CFD solution provides boundary conditions to the particles within the XDEM treatment whereas particles constitute sources/sinks of mass, momentum, chemical species and energy in the gas phase CFD equations. The results highlight the unsteady behaviour of the biomass combustion on the grate, showing the presence of gas streaks coming out from the bed. The approach is very useful as it provides a full characterisation of both gaseous and particle phases without the need of any a priori hypothesis on the biomass conversion. Hence it may be used to estimate the amount of unburnt biomass and thus to plan strategies to optimize biomass combustion, thus further increasing the net efficiency of the plant.