Mixing phenomena in fluidized beds diagnostics and observations

Transcription

1 Mixing phenomena in fluidized beds diagnostics and observations Filip Johnsson, David Pallarès, Erik Sette Department of Energy and Environment Chalmers University of Technology, , Göteborg The 68th IEA-FBC meeting Beijing, China, May, 2014

2 Circulating fluidized bed boiler (CFBC) Bubbling fluidized bed boiler (BFBC)

3 Dual bed systems indirect gasifier 2-4 MW integrated in Chalmers 12 MW CFB Gasifier fuel Combustor fuel

4 Biomass gasification Research and development at Chalmers with associated industries Chalmers 2-4 MW pilot plant GoBiGas Phase 1 20 MW SNG demonstration plant Göteborg Energi Chalmers University of Technology GoBiGas Phase 2 80 MW SNG Commercial plant Göteborg Energi Chalmers lab-reactor

5 Oxyfuel in fluidized-bed combustion CFB Technology Metso Power (Now Valmet) 4 MW CFB Oxyfuel

6 CFB & BFB characteristics Group B solids CFB: Primary gas velocity > u t for major part of bed solids BFB: Primary gas velocity < u t for major part of bed solids Furnace height-to-width ratio < 10 Large cross section, L charact up to 10 meters Dense bottom-region height << furnace width Main solids backmixing processes Bottom-region clustering/bubble flow (highly dynamic) Splash-zone solids cluster flow Furnace wall-layer backmixing (dispersed core region flow) Low solids recirculation flux CFB: G s < 10 kg/m 2 s (oxyfuel: higher G s may be required) BFB: No external solids flux

7 Back-mixing: Bottom-region clustering/bubble flow Back-mixing: Splash-zone solids cluster flow Bottom region/bed Splash zone Furnace wall-layer backmixing (dispersed core region flow) Transport zone

8 CFB characteristics result in: Chalmers University of Technology Good vertical solids mixing Limited lateral solids mixing Fuel mixing is crucial Important to establish basis for modeling of fuel mixing from known parameters (gas velocity, gas and solid properties, bed and gas distributor geometry)

9 Fuel mixing Chalmers University of Technology

10 Fuel concentration Chalmers University of Technology Fuel maldistribution: consequences Da - Higher air-to-fuel ratio needed - Lateral gas concentration gradients - More fuel feeding ports needed Fuel mixing Fuel conversion (drying, devolatilization, combustion) Da = τ τ transport kinetics

11 Experimental observations - Chalmers 2D cold tests 3D cold tests 3D cold down-scaled tests 3D hot tests Qualitative Qualitative Quantitative (?) Quantitative

12 3D hot conditions (12 MW th CFB Chalmers)

13 2D cold tests Chalmers University of Technology

14 H b ~ 0.33 m u 0 =1.5 m/s 2D cold tests wide vs narrow unit y [mm] D h = m 2 /s D h = m 2 /s y [cm] X [mm] X [cm]

15 Expressing fuel mixing Chalmers University of Technology Fuel dispersion is the sum of convection (dominating) and diffusion. Dispersion coefficient can be determined using a diffusion analogue C t div D grad C S

16 Diffusion analogue only on macroscopic level t C 2 D C Diffusion analogue only on macroscopic (bubble path) level in large cross sections with homogeneous nozzle distribution

17 2D cold tests velocity and bed-height dependency Red symbols - narrow unit

18 3D cold conditions (Chalmers gasifier bed) Y Camera X Fuel u: 0.6 m/s, 1 m/s H 0 : 0.4 m Tracer particles: Wood chips, Bark pellets

19 3D cold conditions (Chalmers gasifier bed) Batch of fuel particles Fuel inlet

20 3D cold conditions (Chalmers gasifier bed) Batch of fuel particles Fuel inlet

21 3D cold conditions (Chalmers gasifier bed) Single fuel particle Fuel inlet

22 3D cold conditions (Chalmers gasifier bed) u/u mf = 5 Single fuel particle u/u mf = 7.5

23 3D cold tests downscaled unit -Chalmers gasifier bed Chalmers University of Technology u 2 0 gl s f su 0 d p f f u 0 f L G s su 0 P P bed distributor Parameter Length Width u0 ρs ρf value Parameter 1.8 m Length 0.8 m Width 0.32 m/s u kg/m3 ρs 0.18 kg/m3 ρf Bed geometry scaled by a factor 1/6 value 0.3 m 0.13 m 0.14 m/s (8900) kg/m kg/m3

24 S y C D y x C D x t C 3D cold tests downscaled Chalmers gasifier bed Dispersion of inert solids -fitting dispersion equation to outlet sampling of tracer

25 3D cold tests downscaled Chalmers gasifier bed Dispersion of inert solids

26 3D cold tests downscaled Chalmers gasifier bed -UV light tracing

27 3D cold tests downscaled Chalmers gasifier bed -UV light tracing

28 3D cold downscaled test ability to provide quantitative results Chalmers University of Technology 850 ºC Scaling rules

29 3D hot conditions - Chalmers gasifier bed Camera mounted 45 degrees downwards Approximation of the region which is visible with camera probe

30 3D hot conditions - Chalmers gasifier bed Work in progress development of camera probe U=0.11 m/s U=0.19 m/s

31 3D hot conditions - Chalmers gasifier bed Work in progress development of camera probe Improvement under way to minimize/eliminate - Deposits on lens - Condensation on lens - Camera resolution - Camera adjustments

32 Gasification: The possibility to increase residence time of fuel particles From laboratory to pilot scale Fuel Fuel Bedmaterial Bedmaterial Bedmaterial Bedmaterial Steam Steam

33 In gasifier bed fuel dispersion should be limited - Insertion of baffles With baffle Without baffle

34 In-bed tube bundles reduce bubble size Without tubes Sparse tube packing Dense tube packing Air distributor Air distributor Andersson, Johnsson, Leckner, Proc Int. Conf. Fluidized Bed Combustion, 1989, BookNO-I0290A

35 Gasification: Influence of tube bundles on fuel residence time (cross flow of solids) Cross section of gasifier Tube Bundle Solids flow in and out of gasifier Velocity field, u, induced by crossflow of solids Application of scaling laws Fuel feed inlet

36 Oxyfuel in fluidized-bed combustion CFB Technology Metso Power (Now Valmet) 4 MW CFB Oxyfuel

37 4 MW Oxyfuel runs Chalmers University of Technology

38 Summary Fuel mixing crucial for modeling CFB (BFB) performance Need for experimental data and measurement methods Measurements carried out so far indicate: Highly convective mixing process Fuel vortex structures related to bubble flow Possible to relate fuel mixing to bubble flow, i.e. to known parameters (which determine bubble flow) Dynamic modeling required Bed internals can be used to control mixing application to indirect gasification in a dual-bed arrangement to enhance gas yield Active bed material can enhance mixing Need for continued development of fuel mixing measurement methods/technologies (2D and 3D)

39 Extras

40 Chalmers CFB model Example of ongoing work

41 Heat extraction Chalmers University of Technology

42 Optimization: heat transfer Chalmers University of Technology Heat extraction panels Test varying - locations - shapes - functions (EV, SH) and optimize by evaluating the pdf of W/m 2

43 Supporting experiments new cold CFB To determine how solids flow/circulation is influenced by: - Tapered walls - Internals - 2y air or bottom FGR - Furnace exit geometry

44 Bottom-region clustering/bubble flow u 0 > u t of major part of solids. Yet, a dense region can be maintained Limited air-distributor pressure drop Velocity at distributor varies in time and over cross section u 0 = 2.7 m/s u t = 2.1 m/s Primary gas distributor

45 Impact pressure [Pa] Impact pressure [Pa] Impact pressure [Pa] Impact pressure [Pa] Impact pressure [Pa] Impact pressure [Pa] Furnace wall-layer backmixing Solids flux Momentum measurements (235 MW e boiler) Chalmers University of Technology Time [s] Wall layer ER, 50 mm from wall L5f, 50 mm from wall Time [s] L2f3, 50 mm from wall Time [s] Time [s] Core ER, 2550 mm from wall L5f, 2500 mm from wall Time [s] L2f3, 2000 mm from wall Time [s] 36.7 m above air distributor 17.7 m above air distributor 3.8 m above air distributor Front wall Johnsson, et al.

46 PRESSURE DROP, p - p exit [kpa] Chalmers University of Technology Bottom region 10 8 dp/dh b < 0.5 b = (- mf )/(1- mf ) 8 6 dp/dh b < Chalmers 12 MW th Turow Large boiler 235 MW > 200 e MW e Cold unit (exploding bubble regime) Cold unit (transport condition) HEIGHT ABOVE AIR DISTRIBUTOR, z [m]