Fuel combustion behaviour analysis combining furnace measurements - PowerPoint PPT Presentation

Fuel combustion behaviour analysis combining furnace measurements with CFD modelling Jyväskylä, 23 rd September 2014 Sirpa Kallio VTT Technical Research Centre of Finland Contributors: Perttu Jukola, Marko Huttunen, Juho Peltola, Timo Niemi, Lars Kjäldman

Contents  Background on CFD approaches for FB’s  CFD modeling activities at VTT  CFD modeling of bubbling bed combustion at VTT  CFD modeling of CFB combustion at VTT  CFD modeling in combination with furnace measurements 23/09/2014 2 2

Background: CFD modeling of gas-solid flow 3

What is CFD and what is it used for?  Computational fluid dynamics (CFD) aims at describing the processes in 2D or 3D geometries, either as a function of time or by means of a time- averaged/steady-state description  Several methods available aimed at different applications  CFD is useful when there are large local variations in the process, mixing is important and/or the geometry plays a central role 23/09/2014 4 4

What do we mean by a CFD simulation Typically 3D Finite Volume Method (FVM) simulations: 1. Create a CAD model of the geometry 2. Split the 3D fluid volume into to small control volumes 3. Solve balance equations for desired properties in each of these control volumes Momentum, mass, energy ….  4. May include tracking of particles, droplets, bubbles etc. 5. Computational time: hours, days, weeks, months … 23/09/2014 5 5

What do we get from a CFD simulation Full 3D fields of: 1. Gas and solids velocities 2. Pressure 3. Volume fraction 4. Temperature 5. Turbulence properties ( mixing ) 6. Species concentrations 7. Reactions rates 8. Local size distribution 9. Fluctuations of all the above 10. Information on fouling, erosion etc. with specific submodels 23/09/2014 6 6

Alternative approaches to CFD modelling of Fluidization Small scale 1. DEM, Discrete Element Method 2. MP-PIC, Multi-Phase Particle-In-Cell 3. Transient Euler-Euler 4. Time-averaged Euler-Euler Large scale 23/09/2014 7 7

Alternative approaches: DEM  Individual particles are tracked and collision between the particles are calculated explicitly  Soft- or hard-sphere models for the collisions + Accurate, simple closure models - Computationally very, very demanding • Large, soft and light particles => less effort • Small, hard and heavy particles => more effort  Suitable for studies of small details (valves, injectors, guide vanes) and gas-solid-solid interaction models 23/09/2014 8 8

Alternative approaches: MP-PIC  Solid phase is modelled as parcels of particles that are tracked .  Collisions are not calculated explicitly , but modelled with a statistical model + Usable in reasonably large applications (pilot scale) + Easy description of solids diameter distribution and its evolution - Reasonably fine mesh resolution or sub-grid-closures are needed for accurate results - Solid-solid interaction: Currently available tools do not produce good results in dense conditions (BFB) 23/09/2014 9 9

Alternative approaches: Transient Euler-Euler  The most commonly used approach in fluidization simulations.  Both phases are modelled as continuous fluids  Can be coupled with Lagrangian fuel particles  Statistical interaction models + Most mature approach: interaction models, chemistry etc. + Usable in reasonably large applications (pilot scale) + Good results in dense conditions - Difficult to describe a solids diameter distribution - Reasonably fine mesh resolution or sub-grid-closures are needed for accurate results 23/09/2014 10 10

Alternative approaches: Time-averaged Euler- Euler  TASIF : A time-averaged version of the commonly used Euler- Euler model  The required closure models have been developed at VTT  Eulerian gas and bed mass phases + Lagrangian fuel particles + Drastically reduced the computational effort in large applications + Suitable for industrial scale applications + Allows for more complex process models - More reliant on the quality of the closure models - Fluctuations are modelled, not resolved => their effects on the process also have to be modelled. 23/09/2014 11 11

Avtivities of the CFD modeling team at VTT

Computational Fluid Dynamics (CFD) at VTT  Applied since 1982  CFD Modelling Team : 15 researchers  Research & application areas  Combustion & emissions, gasification, fast pyrolysis  Multiphase flows, rotating machinery, fluid- structure interactions, nuclear safety analysis, molecular modelling  Model development and testing  Applied research  Investigation of practical application cases  Utilize simulation to understand process behaviour and as a design tool  Co-operation within VTT and with several companies and universities 23/09/2014 13 13

Combustion & emissions  CFD application areas…  Pulverized fuel combustion  Bubbling and circulating fluidised bed combustion (BFB & CFB)  Grate fired combustion  inc. spreader stoker fired units  Recovery boilers & lime kilns  Gaseous & liquid fuel flames  Common issues in CFD studies  NOx emissions (e.g. IED) & emission reduction (low NOx techniques + thermal DeNOx / SNCR)  Heat transfer  Furnace availability  Slagging, fouling, corrosion (tendencies)  Co-firing of coal / biomass / peat / etc.  Fluidization issues (CFB) 23/09/2014 14 14

CFD modeling of bubbling fluidized bed combustion at VTT 15

Methods for Bubbling Fluidized Bed Boilers • Dense bottom bed described by simple empirical balance models • CFD used for the freeboard • Lagrangian particle tracking for fuel particles • CFD modeling for the bottom bed possible but nor currently linked to boiler simulations 23/09/2014 16 16

Example 1: An Air Distribution Study of a Bubbling Fluidized Bed Boiler  Furnace capacity: 175 Mw fuel  fuel mixture: peat 30 %, biomass 70 %  Main topic: NOx formation  effect of 2’ry air elevation  low furnace air distribution  Other topics  burnout (indicator: CO at furnace exit)  upper furnace fouling tendency (indicator: furnace exit gas temperature, FEGT) Jukola, Huttunen, Dernjatin & Heikkilä: New methods for NOx emission reduction in fluidized bed combustion: CFD modelling results from a stationary fluidised bed boiler, VGB Powertech Journal, 11 / 2013 23/09/2014 17 17

An Air Distribution Study of a Bubbling Fluidized Bed Boiler, cont. Simultaneous decrease in exit gas T, CO and NOx when “optimizing” Stoichiometric Ratio in Zone I (SR1) 23/09/2014 18 18

Example 2: Modelling of Combustion in BFB Furnaces: Slagging & Fouling Issues REDUCE FURNACE SLAGGING REDUCE SUPERHEATER /REHEATER SLAGGING Particle concentration near walls Combustion rate of char in freeboard (178 MWf; Bio/Peat = 45% / 55 % of Fuel Power) (294 MWf; Bio/Peat = 30% / 70 % of Fuel Power) Old New Old New 23/09/2014 19 19

CFD modeling of circulating fluidized bed combustion at VTT 20

Method developed at VTT: Steady-state CFD modelling Fast simulation method for large geometries. Time-averaged form of the continuity and momentum equations used in the transient simulations for phase q are:     U = 0 q q q                   τ U U = g p p q q q q q q q q q q                    ( 1 ) τ M ( 1 ) qs K u u p u " u " q q gs g s qs s q q q q instantaneous velocity u phase-weighted average velocity U    u / q q velocity fluctuation   23/09/2014 21 21 u " u U q q q

Description of the CFD model: hydrodynamics Gas and solids momentum Closures for drag , Reynolds stresses , volume fraction-pressure  correlation and solids pressure terms. Turbulence, Reynolds stresses Transport equations are solved for the solids normal  components of velocity correlations. Gas phase stresses are calculated with algebraic correlations  from the solid phase stresses. Fluctuation time scales are obtained from algebraic correlations.  Small scale and dilute suspension turbulence: dispersed k- ε model  with a modified turbulent viscosity 23/09/2014 22 22

Description of the CFD model: energy and material balance equations Energy: Specific enthalphy equations for both phases Unisotropic diffusion coefficients approximated from Reynolds • stresses and time scales Phase interaction (Gunn, 1978), wall heat transfer based on: Vijay& • Reddy (2005) Species equations for gas components Isotropic diffusion coefficient approximated from Reynolds stresses • and time scales . Gas reactions are assumed to be limited by mixing. • Species : O 2 , N 2 , CO 2 , CO, H 2 O, H 2 and CH x O y . • Reactions : • 1. CH x O y +(x/2+1-y)/2 O 2 → CO + x/2 H 2 O 2. CO +0.5 O 2 → CO 2 3. H 2 + 0.5 O 2 → H 2 O 23/09/2014 23 23