Modeling of fuel flow in fluidized beds Markku Nikku, Payman Jalali, Omid Joneydi Content of the presentation − − − − Introduction to fluidized beds Modeling of fluidized beds Fuel flow and modeling at LUT Conclusions Metso Power Introduction to fluidized beds − − − − Fluidization is a multiphase phenomena with it’s own characteristics ”A bed” of solid material set to ”a fluid-like state” with a fluid flow Properties of the bed affected by the flow rate of the fluid Fluidized beds widely used in energy production with gas fluidization (air, oxygen+fluegas) − Packed fluidized beds − grate firing/gasifiers/reactors − packed bed reactors − Bubbling fluidized beds − BFB boilers/gasifiers/reactors − external heat exchanger − Circulating fluidized beds − CFB boilers/gasifiers/reactors − looping processes − Pneumatic transport − pneumatic conveyers Modeling of fluidized beds − Interconnected “puzzle” of hydrodynamics, chemical reactions and heat transfer in various scales − Computational fluid dynamics (CFD) simulations of fluidized beds have to consider − Fluctuating multiphase flow with (millions of) particles − Interaction between phases and between particles − Reactions and heat transfer − Other issues − Compromises have to be made in order to model commercial scale units − Level of details vs. computational costs Micro-scale 1 year 1 h...1 d Meso-scale Steady state Quasi steady Averaged CFD 2D/3D Empirical and semi-empirical models 1D/1.5D/3D Transient Time scale Macro-scale Lumped scale Correlation models 0D 1s Eulerian-Eulerian continuum models CFD / TFM 2D/3D 1 ms 1 µs Particle scale DNS,LBM,DEM/DPM Lagrangian-Eulerian DEM/DPM-CFD,DSMC 2D/3D 2D/3D 1 µm 1 mm 0.1 m 1m Global 10...50 m Space scale Myöhänen, K. and Hyppänen, T. 2011. A Three-Dimensional Model Frame for Modelling Combustion and Gasification in Circulating Fluidized Bed Furnaces, International Journal of Chemical Reactor Engineering 9(1) Approaches to modeling and scale-up Bench scale Pilot scale Boiler scale EXPERIMENTAL SCALES 1D-MODEL flue gas to stack v MODELS AND DESIGN TOOLS Model analyses Vo lati le, mo istu r re ele bu stio n+1 n-1 ase n-2 dYCO dt CO combustion Mixing Ch ar com n b exp( A / T )(d / d ref ) n kef YCO k ef 1 /(1 / kCO rc dmc dt m) n 3 2 n kmc X O2 Models for phenomena Secondary air 1 Primary air 1-D process models 3-D process models Fuel flow and modeling − Fuel flow and mixing determine where the reactions take place, directly resulting to − fuel concentration − changes in gas species CFD − temperature field… − Three dimensional solution required − Process characteristics and behavior determined by fuel flow − Flue flow can be modeled in large 3D Semi-empirical scale in CFD with and time− Single fuel particles – Lagrangian averaged CFD − As continuous phase – Eulerian − Still many large scale fluidized bed furnace models are semi-empirical as ”pure” CFD would be too computationally expensive − Many use dispersion/diffusion type of mixing for fuel due to steady state nature of the simulations − Following slides presents some research done in LUT Energy/Laboratory of modeling of energy system Fuel flow: Particle scale with DEM approach − Single particle force balance solved for each dicrete particle − Every collision between particles recorded and affecting to the particle movement − Very detailed results but simulations computationally heavy − Study of drag force between the fuel and bed particles Large fuel particle (red) in a bed of small particles. Force on the fuel particle recorded as function of time. Fuel flow: Particle scale with DEM approach Jalali, P., Nikku, M., Hyppänen, T. 2013. Particle-Cloud Drag Force in Dilute Particle Systems: Discrete Element Method versus Eulerian Simulations, Industrial & Engineering Chemistry Research 52 Fuel flow: Meso scale CFD − CFD (3 phases, Eulerian approach) used to model transient mixing of fuel from the feeding without reactions − 3 mesh sizes used and compared − Time and space averaging − Finding corrections to the drag force from fine mesh to be applied with coarser meshes 0s 5s 10s 15s 20s 25s 30s 35s 40s Omid Joneydi. 2013. CFD Modeling of Circulating Fluidized Bed: Effect of Volume Averaging on Solid-Solid Drag Force. Master’s Thesis. LUT. Fuel flow – 3D large scale modeling − Existing 3D semi-empirical, steady state, holistic furnace model − Convective solution for fuel momentum equation added − Target profile + dispersion type solution in the old model − Dispersion still used in the modeling of mixing − Improves modeling of biofuels which are significantly lighter than coal and bed material − Model developed and validated in ERA-NET Biomodelling project Nikku, M., Myöhänen, K., Ritvanen, J. Hyppänen, T. 2014. Modeling of fuel flow in circulating fluidized bed furnaces. 11 th International conference on fluidized bed technology, 14th-17th May 2014 Beijing. In review. Nikku, M., Myöhänen, K., Hyppänen, T., 2012. ERA-NET Biomodelling - final presentation of LUT, 24th September 2012, Vienna. Conclusions − Fluidized beds and fuel flow modeling − Several scales − Varying level of details − Practical applicability to different cases − Micro scale models help to understand the phenomena involved − Meso scale models bring insight when several different phenomena are combined, might be applicable to laboratory and pilot fluidized beds − Macro scale models are combining all the related phenomena and simplifying the physics and chemistry in order to achieve reasonable compational times − As the computational power increases, more detailed models can be utilized in modeling of commercial scale fluidized bed applications Thank you for your attention! Questions, comments… ?