CFD Modelling of Wet Flue Gas Desulphurization (WFGD) Unit: A New Era of Process System Control and Optimization

Transcription

1 CFD Modelling of Wet Flue Gas Desulphurization (WFGD) Unit: A New Era of Process System Control and Optimization A. Arif, R. C. Everson, H. W. J. P. Neomagus Emission Control North-West University, Potchefstroom Campus, Potchefstroom 50, South Africa

2 Outline Purpose Theoretical framework Results and conclusion Implications Acknowledgement

3 Purpose Overall objective of research Comprehensive CFD model for WFGD unit of high capacity coal fired power station to develop» Simulator for examining operation and trouble shooting» Simpler models for integrated system analysis for energy optimization» Recommendation for process modifications (sorbent, composition, control etc.) Objective of presentation CFD Model of an industrial WFGD to study» Hydrodynamics of flue gas and slurry droplets» Heat transfer between flue gas and slurry droplets» Slurry droplets characteristics (size, interaction, distortion etc)» Slurry droplets evaporation» Slurry distribution (nozzle location)

4 Introduction Power industry has been challenged by environmental initiatives Emission controls on modern power stations account for 10-0 % of the capital investment 1 Significant operation and maintenance cost Power generation represents the largest controllable source of SO emissions Dominating SO absorption technology in the world is wet flue gas Desulphurisation (WFGD) Where SO of flue gas is scrubbed by slurry of lime stone in counter current operation For an estimation of SO removal, It is important to know Exact flow characteristics Slurry droplets behavior Detailed modeling using computational fluid dynamics (CFD) platform 1 Marocco, L., 008. PhD Dissertation, Politecnico di Milano, Italy Bautsch, C., Fahlenkamp, H., 006, IClass-006, Kyoto, Japan

5 Introduction Open literature studies are limited to small scale / pilot plants, ignoring Droplet-wall interactions, distortion & size distribution Enhancement studies CO desorption & water condensation Natural oxidation of sulphite to sulphate Interphase chemistry with rate studies 5

6 South African power stations & WFGD

7 South African power stations & WFGD 7

8 WFGD's Absorber

9 Absorber geometry

10 Slurry spray bank Nozzle # 4

11 CFD modeling Modeling approach = Euler-Lagrange Phase interaction = Two way coupling Turbulence model = k-ε turbulence model Nozzles = 150 hollow cone point injectors Drag force = Liu dynamic drag coefficient model Droplet distortion = TAB distortion model Mist eliminator = Porous media with suitable pressure drop Droplet-wall interaction = Escape, Rebound and Bai-Gosman wall impingement model Droplet size distribution = Rosin Rammler particle size distribution model Domain discretization = Polyhedral and prism layer cells with surface remesher Evaporation = Quasi - steady state droplet evaporation model Absorption = Coupled specialized model

12 Rosin Rammler particle size distribution model D D ref 1 exp F D q Rosin-Rammler Exponent = 3.05 Rosin-Rammler Diameter = 650 μm Minimum Droplet Diameter = 68 μm Maximum Droplet Diameter = 5100 μm D R 100 exp D ref q 100 D log loge R D ref 100 log log q log D q log Dref log log e R 100 log log q log D C R q

13 Bai- Gosman wall impingement Incident Weber Number Laplace Number Boundary Temperature Wall State (Wet or Dry) Bai, C. and Gosman, A.D. 1996

14 Liu dynamic drag coefficient The Liu dynamic drag coefficient is intended to account for the dependence of the drag of a liquid droplet on its distortion under the action of aerodynamic forces. As the distortion of the droplet increases, its shape is assumed to become a disk whose axis is aligned with the relative velocity. This increases the drag on the droplet. The Liu drag coefficient models this effect by noting that the high Reynolds number limit of the drag coefficient of a disk is It then assumes that the disk drag is 1.54/0.44 higher than the sphere drag at all Reynolds numbers, and that the drag of intermediate shapes can be interpolated between those two extremes. So The interpolation factor y is 0 for a sphere and 1 for a disk. It is identified as the TAB distortion, and hence the Liu drag coefficient requires the TAB distortion model to calculate this quantity. Liu, 1993

15 TAB distortion model The TAB Distortion model is used to calculate the distortion of liquid droplets under the action of aerodynamic forces. It calculate the instantaneous displacement x of the droplet equator from its equilibrium position. Where y Distortion Rate Viscosity C t d We Damping Coefficient crit Critical Weber Number Surface Tension We Weber Number C k Stiffness Coefficient Baumgarten, 006, O'Rourke, 1987

16 Modeling equations Continuous Phase ρ = Density of flue gas τ = Shear stress u = Velocity of flue gas τ R = Reynolds stress tensor p = Pressure of flue gas ω A = Mass fraction of component A g = Acceleration due to gravity k c = Thermal conductivity of flue gas T = Temperature of flue gas D AB = Binary diffusion coefficient of A in B

17 Modeling equations Dispersed Phase After evaluation of flue gas velocity field, particles trajectories can be computed. The equation of single parcel takes the following forms The above equations can be solved by stepwise integration over discrete time steps, using the continuous phase flow properties at the current droplet positions. The evaluation of particles source terms allows the determination of gas source terms.

18 Mass transfer source terms Mass Source Term Mass transfer by evaporation of water from slurry to gas phase and absorption of SO from gas phase to slurry droplet, ignoring water vapor condensation and SO desorption. S S S m m k, mass k, H O k, SO H O SO N k P P H O H O, g H O, H O, i N k P H C SO SO, tot SO, SO SO,

19 SO absorption and chemical reaction M. Gerbec, A. Stergarsek, R. Kocjancic, Computer Chem. Engg.

20 NSO KG PSO P SOi P H C SOi SO SO SO SO SO SO, d N K P H C SO G SO, d o For C for Air-Water System, d, d...(1) A ln H B lnt G.Maurer-1980 SO d CTd D T Initially C N Then C SO absorption and chemical reaction 0 d K P SO G SO N SO Droplet Surface Area Droplet Residence Time Droplet Volume N K P SO G SO mole Mass transfer flux m s mole Global mass transfer coefficient m s pa pa Partial pressure of SO in gas bulk System absolute pressure Mole fraction SO 3 pa m H Henery's Constant SO mole mole C Concentration of SO in liquid bulk SO, d 3 m T K Droplet Temperature d

21 SO absorption and chemical reaction K k d D G 1 1 H k Dd, SO 10 d d, SO T d f, SO SO G SO d H O d kgrtdd Sh 0.55 Re D P Sc D D k Const f d d d f f, SO d vd, Re Sc d 1 1 M f M f, SO 1/3 1/3 P Vm, f Vm, SO T SO 1 k k G d D SO m s Gas side mass transfer coefficient mole m s pa Liquid side mass transfer coefficient 10 Enhancement Factor m d, SO s J R mole K d m P pa T d K m (Binary liquid diffusion coefficient of SO in water) Gas Constant Gas pressure in absolute values Gas Temperature Droplet diameter Volume m D f, SO Binary gas diffusion coefficient of SO in air s kg pa s Viscosity m s kg 3 Density m M MolecularWeight V Molecular

22 SO absorption and chemical reaction Soren Kill, Michael L. Michelsen and Kim Dam Johansen., 1998, Ind. Eng. Chem. Res.

23 SO absorption and chemical reaction Absorption (Spray Zone) Neutralization (Spray Zone) Eight nonlinear algebraic equations and eight unknowns dissolved species concentration i.e SO (aq), CO (aq), H +, OH -, HSO 3-, SO 3 -, HCO 3-, CO 3 - Marocco, L., 008. PhD Dissertation, Politecnico di Milano, Italy

24 L/G (1000*Vol Frac) Velocity (m/s) Pressure Drop (kpa) Results: Sensitivity analysis M Cell 3. M Cell.5 M Cell M Cell 3. M Cell.5 M Cell Iterations 7.4 M Cell 3. M Cell.5 M Cell Iterations Iterations

25 Results: Single nozzle dynamics 0 m 0 m 0 m 5

26 Results: Single nozzle dynamics

27 Results: Full absorber dynamics 7

28 Results: Velocity profile 10 cm upstream of ME 8

29 Results: Slurry droplets diameter 9

30 Results: Evaporation 30

31 Results: L/G (dm 3 of slurry / m 3 of gas) Axial Profile Level 1 & Level 3 & 4 Level & 3 31

32 Conclusion CFD analysis shows higher velocity close to the WFGD's wall which results in low L/G ratio near the wall and higher L/G ratio in the middle of the column. Concentrating more slurry nozzles near the wall, results in uniform L/G profile across the column. The estimated values of pressure drop across ME and nozzle dispersion of slurry are with close agreement with the manufacturer data. As the flue gas flows inside the WFGD cooling occur with an increase in moisture content which is due to counter current interaction with the slurry droplets, thereby exchange heat and mass transfer as a result of this interaction. Saturation is reached very soon after the gas enters the tower, which is very close to the real plant observations. 3

33 Future work Modeling of interphase mass transfer Natural oxidation of sulphite to sulphate due to the oxygen content in the flue gas. Studies with South African sorbents Modeling of interphase chemical reactions Complex chemistry with rate studies Enhancement studies Modeling of pilot plant for validation 33

34 Implications The modelling results can be used In the design phase of industrial full-scale WFGD and for the retrofitting of existing plants with WFGD unit Improve the efficiency of WFGD absorber at coal-fired power plants by revealing regions of poor gas/liquid contact and identifying a solution to eliminate them. Recommendations for major modifications to accommodate alternative sorbents, different gas compositions and new process control strategies. Understand the complex multiphase process inside the column and thereby will help to cope with trouble shooting and emergency conditions. 34

35 Acknowledgement

36 Thank You For Your Attention Any Questions??