INTERNATIONAL FLAME RESEARCH FOUNDATION 1 18 th IFRF Members’ Conference – Flexible and clean fuel conversion to industry Freising, Germany, 1, 2, 3 June 2015 – Paper n. 26 EXTERNALLY BIOMASS-FIRED GAS TURBINE: DEVELOPMENT AND VALIDATION OF THE BOILER NUMERICAL MODEL Chiara Galletti 1 , chiara.galletti@unipi.it, Valentina Giomo 1 , valentinagiomo@gmail.com, Simone Giorgetti 1 , simone.giorgetti90@gmail.com, Paolo Leoni 2 , paolo.leoni@enel.it, Marco Paci 2 , marco.paci@enel.it, Nicola Rossi 2 , nicola.rossi@enel.it, Leonardo Tognotti 1 , leonardo.tognotti@unipi.it 1 DIPARTIMENTO DI INGEGNERIA CIVILE E INDUSTRIALE – UNIVERSITÀ DI PISA, Largo L. Lazzarino 2, 56122 Pisa, Italy 2 ENEL INGEGNERIA E RICERCA S.P.A, Via Andrea Pisano 120, 56122 Pisa, Italy ABSTRACT Grate-firing is one of the main technologies currently used for biomass combustion for heat and power production. However, grate-firing systems are often reported to have relatively high un-burnout, low efficiency and high emissions and they need to be optimized. The purpose of this report is to establish a reliable baseline CFD model for a biomass grate-fired boiler for an EFMGT (Externally Fired Micro Gas Turbine) plant. The numerical model can be used for diagnosis and to signal critical regions, such as high temperature zones, and the spatial distribution of chemical species within the boiler. Different modelling of the fuel bed and their impacts on CFD analysis are discussed and compared. The prediction clearly show that a detailed fluid dynamic above the fuel bed (e.g. modelling the air injection nozzles) is required to obtain reliable results such as exact temperature field and a correct distribution of chemical species, whereas the model is rather insensitive to the conditions of the gas phase above the fuel bed. 1 INTRODUCTION The worldwide concern about global warming and the limited availability of fossil fuels has motivated the use of biomass for energy production. Among the technologies for electricity generation from biomass, small Externally Fired Gas Turbine (EFGT) plants are receiving some attention as they offer the possibility of burning “dirty” fuels such a biomass “in situ”, without the complexity of gasification. EFGT cycle attempts to combine the advantages of gas turbines (low operational costs, high lifetime and reliability, relatively high energy efficiency even at small size) and the capability of using low quality biofuel. As for the boiler, grate-fired systems are usually employed because of their larger fuel flexibility, as they can operate with up 100% raw biomass. The main limitations and problems of the EFGT cycles are related to thermal stresses and fouling on the high- temperature heat exchanger [1]. Computational Fluid Dynamics (CFD) could potentially help investigating the boiler behavior, providing a manner of calculation of the thermo-chemical field and thus giving information useful to detect regions of the boiler with high thermal stresses. However, CFD modelling of grate-fired systems is far more complex than of other solid fuel combustion technologies, based on suspended injection systems. The solid biomass bed undergoes a series of non-stationary processes, including drying, pyrolysis and char oxidation. Such processes interact with the freeboard, characterised by a reactive turbulent flows. Things are further complicated by convective and radiative heat transfer. It appears clear that a comprehensive CFD model is unfeasible, especially because its accuracy would be affected by many assumptions and hypothesis that should be made to model the system.