OCR Text |
Show The phenomena described include turbulent flow field of gas phase and trajectories and the behaviour of fuel particles. Homogeneous gas phase as well as heterogeneous gas-particle surface reactions including NO-chemistry are modelled with various approaches. Special attention has been put to turbulence-chemistry interaction and radiative heat transfer. There are several submodels of different levels of complexity for the same physical phenomenon. The user can choose the combination of submodels so that the process at interest can be simulated with reasonable accuracy in acceptable computing time. In the sense of modelling, the B F B furnace can be separated into the regions of the dense bed and the dilute freeboard above the bed. The models developed for the pulverised combustion are valid with reasonable accuracy also in the freeboard. This is because the particle - particle interactions are negligible in the dilute suspension of sand and fuel particles above the more or less definite surface of fluidised bed. Moreover, due to the absence of the complicated burners, the combustion aerodynamics is in many cases essentially simpler than in pulverised combustion. However, the stoichiometric ratio SR and temperatures between the bed and the over fire air (OFA) region are significantly lower (SR ~ 0.4, T < 1000 °C) than the values in pulverised combustion (SR = 0.8.0,9, T > 1200 °C). The models and data for chemical kinetics of combustion and emission formation are not necessarily valid in these conditions. Fortunately, the reactions can be assumed to be slow in this region with low temperature and short residence time. In BFB boiler modelling, additional difficulties arise from the complex phenomena in bed region, which have to be described in the extent to place a reasonable inlet boundary condition for freeboard computation. From the viewpoint of freeboard, the bed behaves like a gasifier, from where the evaporated water and the pyrolysed and flue gases are released to the freeboard. The existing C F D models cannot be used to solve complex hydrodynamics of the dense bed region within a reasonable computation time. Here the boundary condition information is obtained from a simple one-dimensional global system model (i.e. a model which is not based on 3D fluid dynamics) that is modified from the model presented by Van den Bleek et al., 1990. The model is based on the mass and energy balances of the bed. The bed is assumed to be heated by the char burning in the bed and by heat radiation from the freeboard. The amount of char bumed is determined by the global heat balance. All pyrolysed gases are assumed to be released from the bed surface in the region determined by fuel feeding. In the BFB boilers burning peat or biofuel, the fuel is typically fed into the furnace through a few droptubes above the bed. The fuel travels its way from the droptube to the bed as quite a coherent flow and meets the bed surface in a relatively small area. According to observations through the furnace cameras, pyrolysis gas is mainly released near this contact area, which is due to a relatively slow horizontal mixing in the bed and quite a rapid pyrolysis process. For the freeboard modelling, this release area can be estimated either by computing the fuel particle trajectories or, when an existing boiler is analysed, from the furnace camera information. Above the bed surface the solid phase concentration decreases exponentially until a certain level called transport disengaging height (TDH) is reached, after which the particle concentration remains constant. This also means that all the particles above the T D H will be driven out of the furnace. Here only the char and ash particles are modelled by Lagrangian description like in the case of pulverised combustion. The effect of sand particles is considered only in respect to radiative heat transfer. This is done by estimating their number density distribution as a function of height from which their contribution to the local absorption and scattering coefficients can be predicted. EMISSIONS NQ, V NEAR BURNER ZONE Advanced NQ, Locally denser gird COMBUSTION TURBULENCE {Eddy breakup Eddy dissipation concept Extended gas chemistry Local extinctior and reignition \J RADIATION Flux-method I DT-method Local radiation properties D-SOLVER f ^_ k-e-model ^\ r > Multiple-time-scale- model 1 PDrAyRinTgI CLES ( Pyrolysis f Char ignitior [ Combust f Dispersion Lagrangian description J ior J • I J J < J Figure I. The structure of A R D E M U S program 2 |