Particle-resolved numerical study of char conversion processes considering conditions in the British-Gas-Lurgi (BGL) gasifier

Transcription

1 Sebastian Schulze, Fred Compart, Andreas Richter, Petr A. Nikrityuk and Bernd Meyer IEC, TU Bergakademie Freiberg, Germany Department of Chemical and Materials Eng., UoA, Edmonton, Canada Particle-resolved numerical study of char conversion processes considering conditions in the British-Gas-Lurgi (BGL) gasifier June 13, 2016 International Freiberg Conference, June 2016, Cologne, Germany

2 Content Introduction & Motivation CFD model Validation: Air and 1 atm Influence of packed-bed structure: Air and 1 atm BGL conditions: Reducing atmosphere and 40 bar Conclusion TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

3 Introduction & Motivation Introduction The British Gas/Lurgi (BGL) gasifier with slag tapping poses advantages over the Lurgi Fixed-bed dry bottom gasification a. higher specific throughput high carbon conversion production of vitrified slag much lower steam consumption fines can be introduced via tuyeres a Schmalfeld (Ed.), Die Veredelung und Umwandlung von Kohle, DGMK, 2008 TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

4 Motivation The understanding of the phenomena occuring within the complex structure of the moving/fixed bed gasifiers is limited, especially in the part of the oxidation and gasification zones where conversion occurs rapidly. Additionally, due to the harsh conditions (extreme high temperatures in and above the slag bath, 40 bar) data is difficult to access via measurements a. Thus, detailed numerical simulations can give insights of the processes as they act as numerical experiment. a Schmalfeld (Ed.), Die Veredelung und Umwandlung von Kohle, DGMK, 2008 TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

5 Goals Apart from challenges in modeling of reacting granular media the following goals are set: Ensure that particle-resolved simulation can represent the reacting bed Investigation of the impact of the packed-bed structure on the fluid flow, heat and mass transfer as well as carbon conversion Applying boundary conditions which can be found in a BGL gasifier 1 for simulations 1 M. Olschar, O. Schulze, 5th Int. Freiberg Conf., Leipzig 2012 TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

6 CFD model Assumptions Applying the pseudo-steady-state approach (PSS) 2 following assumptions are introduced: The particle shape is spherical. The porosity of the particles is not taken into account, thus the intraparticle diffusion is neglected. The particles consists of carbon only. The devolatilization is not included due to the steady-state character. The gas radiation is included For 2-D simulation an axisymmetric domain is assumed. The buoyancy effect is neglected. 2 Amundson et al., Ind. Eng. Chemistry Fundamentals, TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

7 Reactions The chemistry is modeled using semi-global homogeneous and heterogeneous reactions 3 : heterogeneous (surface) reactions: C O 2 CO h 0 R1 = 9.2 MJ kg 1 C (R1) C + CO 2 2CO h 0 R2 = 14.4 MJ kg 1 C (R2) C + H 2 O CO + H 2 h 0 R3 = 10.9 MJ kg 1 C (R3) homogeneous (gas phase) reactions: CO O 2 + H 2 O CO 2 + H 2 O h 0 R4 = 10.1 MJ kg 1 CO (R4) CO + H 2 O CO 2 + H 2 h 0 R5 = 1.15 MJ kg 1 CO (R5) CO 2 + H 2 CO + H 2 O h 0 R6 = 1.15 MJ kg 1 CO (R6) 3 Turns, An Introduction to Combustion, TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

8 Validation: Air and 1 atm Validation case set-up: Combustion of coke particles with preheated air 4 product gas Tab. 1: Parameters for simulation of experiments Particle size 32 mm Void fraction 0.4 Inlet temperature 300, 478 and 700 K Inlet gas composition Y O Y H2 O Y N Inlet mass flux kg m 2 s m m sample holes steel grate preheated air 4 Nicholls, Bulletin 378, Bureau of Mines, Fig. 1: Scheme of experimental fixed bed combustion set-up TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

9 Results: Temperature and species distribution 1 Fig. 2: Temperature and O 2 mass fraction contour plots TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

10 Results: Temperature and species distribution 2 Fig. 3: CO 2 and CO mass fraction contour plots TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

11 Results: Comparison with experiments, 300 K Temperature [K] gas temperature CFD solid temperature CFD temperature experiment X i [-] Y CO2 experiment Y O2 experiment Y CO2 CFD Y O2 CFD X i [-] Y CO experiment Y CO CFD z [m] z [m] z [m] (a) (b) (c) Fig. 4: Validation: Temperature (a), CO 2 mole fraction (b) and CO mole fraction in dependence of height above grate. The different colors correspond to the inlet gas temperature. Results for 300 K are shown. TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

12 Results: Comparison with experiments, 700 K Temperature [K] gas temperature CFD solid temperature CFD temperature experiment X i [-] Y CO2 experiment Y CO2 CFD X i [-] Y CO experiment Y CO CFD z [m] z [m] z [m] (a) (b) (c) Fig. 5: Validation: Temperature (a), CO 2 mole fraction (b) and CO mole fraction in dependence of height above grate. The different colors correspond to the inlet gas temperature. Results for 700 K are shown. TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

13 Influence of packed-bed structure: Air and 1 atm Grid generation for random packed bed From DEM simulation of particle sedimentation to representive geometry (b) (a) (c) (d) Fig. 6: Column of monodisperse particles from DEM simulation (a), Cutting-out representative volume 2.5dP 2.5dP 10dP (b), bridge at contact points5,6 dbridge = 0.125dP (c) and grid comprising of ca tetrahedral cells (d) 5 6 Ookawara et al.,european Congress Chem.Eng Dixon et al., Computers Chem.Eng, TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

14 Set-up of simulation: Reacting flows in the combustion zone of fixed bed Tab. 2: Parameters for 3-D simulation Particle size 20 mm Void fraction 0.42 Inlet temperature 1000 K Inlet gas composition dry air Y O Y H2 O Y N Inlet velocity Re P = 100 ρ u dp Re P = µ ɛ (1) Fig. 7: Principal scheme of the computational domain TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

15 Results: Velocity isosurfaces Fig. 8: Isosurface of velocity magnitude of 3 m/s, the colour represents the gas temperature (K) for inlet gas composition of Y O2, = and Y H2 O, = TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

16 Results: Flame propagation into the fixed bed Comparison of random packed bed and simple cubic bed (a) (b) Fig. 9: Contour plot of gas temperature (K) at the symmetry planes for random fixed bed (a) and simple cubic packing (b) for inlet gas composition of Y O2, = and Y H2 O, = TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

17 Results: Combustion profiles within fixed bed 1 Carbon mass flux ṁ C [kg/(m2 s)] simple cubic packing random packed bed L/d P Fig. 10: Carbon mass flux ṁ C for random packed bed and simple cubic packing in dependence on depth in the bed. TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

18 BGL conditions: Reducing atmosphere and 40 bar Defining inflow conditions Boundary conditions within oxidation and gasification zone from basic flow sheet simulation of IEC s BGL gasifier 7 with AspenPlus. Tab. 3: Inlet gas properties Temperature CO 2 H 2 O CO H K 12.7 vol.-% 18.8 vol.-% 52.2 vol.-% 11.1 vol.-% Fig. 11: Flow diagram of BGL gasifier 7 M. Olschar, O. Schulze, 5th Int. Freiberg Conf., Leipzig 2012 TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

19 Applying suitable high-pressure kinetics for char gasification Tab. 4: Intrinsic reactivity data 8 Reaction A 0 [g/(m 2 s atm n )] E A [kj/mol] n C + O 2 3.0e C + CO 2 4.0e C + H 2 O 3.0e Discrepancy between intinsic reactivity data and solid particle with surface reactions Calculation of effectiveness factor: ( η = 1 1 Φ tanh(3 Φ) 1 3 Φ ) with Thiele modul Φ = d P 6 (n+1) k S c n 1 i 2 D eff 8 Hla et al., Research Rep. 80, CSIRO, TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

20 Results: Temperature and species distribution 1 Fig. 12: Temperature and CO 2 mass fraction contour plots TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

21 Results: Temperature and species distribution 2 Fig. 13: H 2 O and CO mass fraction contour plots TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

22 Conclusion Simulations of the validated particle-resolved CFD model were showing significantly different flame structure for the random packed bed compared to simple cubic packing Considering BGL-gasifer conditions, the carbon conversion in the fixed-bed is more mass transfer limited than the combustion with air at the same particle Reynolds number. TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

23 Acknowledgement This research has been funded by the Saxon State Ministry for Economic Affairs, Labour and Transport and the European Union in the frame of the project Schlackebadvergaser für die Nutzung schwieriger Brennstoffe (project number P ). TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23

24 Thank you for your attention. TU Bergakademie Freiberg IEC S. Schulze et al. Particle-resolved CFD-Study of Char Conversion IFC 2016, Cologne, Germany /23