Powder biomass fast pyrolysis as in combustion conditions: Numerical prediction and validation

Abstract

Current pyrolysis models given in the literature are presented for slow combustion conditions, which are not applicable for real behavior of powder biomass combustion in boilers. The current study aims at presenting advanced combustion kinetics models to predict char, tar and gases produced during powder biomass pyrolysis under combustion conditions. Five different pyrolysis models, similar to those found in commercial coal-fired boilers, are presented and examined. The models have been coupled to a CFD code that offers a more detailed handling for the fluid dynamics involved in the boiler along with intra-particle heat and mass transfer by diffusion and advection during particle heating and fast pyrolysis process. The models are compared against experimental data from literature and results showed a satisfactory modelling prediction for biomass yield (char, tar and gases) at fast pyrolysis. The paper also presents a new phenomenon, which is related to the effects of volatile releases (during fast pyrolysis) on motion of powder biomass particles. Experimental measurement and numerical simulations have been carried out to study this phenomenon. Results indicated that, for most biomass particles, the release of gases is not isotropic along the particle surface and, in turn, biomass particles are subjected to a so-called jet force. The influence of the jet force on different particle sizes is examined; comparison between the jet force and other forces acting on the particles motion is studied. The effects of jet force on the particle velocity, particle distribution and the combustion process are discussed as well.