Chalmers University of Technology

Mixing phenomena in fluidized beds

– diagnostics and observations

Filip Johnsson, David Pallarès, Erik Sette

Department of Energy and Environment

Chalmers University of Technology, 412 96, Göteborg

The 68th IEA-FBC meeting

Beijing, China, 12-13 May, 2014

Chalmers University of Technology

Bubbling fluidized bed boiler (BFBC)

Circulating fluidized bed boiler (CFBC)

Chalmers University of Technology

Dual bed systems – indirect gasifier

2-4 MW integrated in Chalmers 12 MW CFB

Gasifier

fuel

Combustor

fuel

Chalmers University of Technology

Biomass gasification

Research and development at

Chalmers with associated

industries

Chalmers lab-reactor

Chalmers 2-4 MW

pilot plant

GoBiGas Phase 1

20 MW SNG demonstration plant

Göteborg Energi

GoBiGas Phase 2

80 MW SNG

Commercial plant

Göteborg Energi

2008 2012 2016

Chalmers University of Technology

Oxyfuel in fluidized-bed combustion

CFB Technology Metso Power (Now Valmet)

4 MW CFB Oxyfuel

Chalmers University of Technology

CFB & BFB characteristics • Group B solids

– CFB: Primary gas velocity > ut for major part of bed solids

– BFB: Primary gas velocity < ut for major part of bed solids

• Furnace height-to-width ratio < 10

– Large cross section, Lcharact 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: Gs < 10 kg/m2 s (oxyfuel: higher Gs may be required)

– BFB: No external solids flux

Chalmers University of Technology

Back-mixing:

Bottom-region

clustering/bubble flow

Back-mixing:

Splash-zone solids

cluster flow

Furnace wall-layer backmixing

(dispersed core region flow)

Bottom region/bed Splash zone Transport zone

Chalmers University of Technology

CFB characteristics result in:

• 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)

Chalmers University of Technology

Fuel mixing

Chalmers University of Technology

Da

Fuel maldistribution: consequences

Fuel concentr

atio

n

- Higher air-to-fuel ratio needed

- Lateral gas concentration gradients

- More fuel feeding ports needed

Fuel conversion (drying, devolatilization, combustion)

Fuel

mixing transport τ kinetics τ Da =

Chalmers University of Technology

Experimental observations - Chalmers

2D cold tests

3D cold down-scaled tests

3D hot tests

3D cold tests

Qualitative

Quantitative (?)

Quantitative

Qualitative

Chalmers University of Technology

3D hot conditions (12 MWth CFB Chalmers)

Chalmers University of Technology

2D cold tests

Chalmers University of Technology

Hb~ 0.33 m

u0=1.5 m/s

X [mm] X [cm]

y [cm] y [mm] Dh=0.93∙10-2 m2/s Dh=1.23∙10-2 m2/s

2D cold tests – wide vs narrow unit

Chalmers University of Technology

SCgradDdivt

C

Fuel “dispersion” is the sum of convection (dominating) and diffusion.

Dispersion coefficient can be determined using a diffusion analogue

Expressing fuel mixing

Chalmers University of Technology

CDt

C 2

Diffusion analogue only on macroscopic level

Diffusion analogue only

on macroscopic

(bubble path) level

in large cross sections

with homogeneous

nozzle distribution

Chalmers University of Technology

2D cold tests

– velocity and bed-height dependency

Red symbols - narrow unit

Chalmers University of Technology

u: 0.6 m/s, 1 m/s

H0: 0.4 m

Tracer particles: Wood chips, Bark pellets

Y

X Fuel

Camera

3D cold conditions (Chalmers gasifier bed)

Chalmers University of Technology

3D cold conditions (Chalmers gasifier bed)

Batch of fuel

particles

Fuel inlet

Chalmers University of Technology

3D cold conditions (Chalmers gasifier bed)

Fuel inlet

Batch of fuel

particles

Chalmers University of Technology

3D cold conditions (Chalmers gasifier bed)

Single fuel

particle

Fuel inlet

Chalmers University of Technology

u/umf = 5

u/umf = 7.5

3D cold conditions (Chalmers gasifier bed)

Single fuel

particle

Chalmers University of Technology

Bed geometry scaled by a factor 1/6

gL

u 2

0

f

s

f

ps du

0

f

f Lu

0

0u

G

s

s

rdistributo

bed

P

P

Parameter value

Length 1.8 m

Width 0.8 m

u0 0.32 m/s

ρs 2600 kg/m3

ρf 0.18 kg/m3

Parameter value

Length 0.3 m

Width 0.13 m

u0 0.14 m/s

ρs 15700 (8900) kg/m3

ρf 1.21 kg/m3

3D cold tests – downscaled unit -Chalmers gasifier bed

Chalmers University of Technology

Sy

CD

yx

CD

xt

C

3D cold tests – downscaled Chalmers gasifier bed

Dispersion of inert solids -fitting dispersion equation to outlet sampling of tracer

Chalmers University of Technology

3D cold tests – downscaled Chalmers gasifier bed

Dispersion of inert solids

Chalmers University of Technology

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

Chalmers University of Technology

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

Chalmers University of Technology

3D cold downscaled test

– ability to provide quantitative results

850 ºC

Scaling

rules

Camera mounted 45 degrees downwards

Approximation of the region which is visible with camera probe

3D hot conditions - Chalmers gasifier bed

Chalmers University of Technology

U=0.11 m/s U=0.19 m/s

3D hot conditions - Chalmers gasifier bed

Work in progress – development of camera probe

Chalmers University of Technology

Improvement under way

– to minimize/eliminate

- Deposits on lens

- Condensation on lens

- Camera resolution

- Camera adjustments

3D hot conditions - Chalmers gasifier bed

Work in progress – development of camera probe

Gasification: The possibility to increase residence time of fuel particles

From laboratory to pilot scale

Fuel

Steam

Fuel

Steam

Bed-material

Bed-material

Bed-material

Bed-material

Chalmers University of Technology

With baffle

Without baffle

In gasifier bed – fuel dispersion should be limited

- Insertion of baffles

In-bed tube bundles reduce bubble size

Air distributor

Air distributor

Andersson, Johnsson, Leckner, Proc Int. Conf. Fluidized Bed Combustion, 1989, BookNO-I0290A

Without tubes Sparse tube packing Dense tube packing

Gasification: Influence of tube bundles on fuel residence time (cross flow of solids)

Velocity field, u, induced by cross-flow of solids

Solids flow in and out of gasifier

Tube Bundle

Application of

scaling laws Fuel feed inlet

Cross section of gasifier

Chalmers University of Technology

Oxyfuel in fluidized-bed combustion

CFB Technology Metso Power (Now Valmet)

4 MW CFB Oxyfuel

Chalmers University of Technology

4 MW Oxyfuel runs

Chalmers University of Technology

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)

Extras

Chalmers University of Technology

Chalmers CFB model

Example of ongoing work

Chalmers University of Technology

Heat extraction

Chalmers University of Technology

Optimization: heat transfer

Heat extraction panels

Test varying

- locations

- shapes

- functions (EV, SH)

and optimize by evaluating

the pdf of W/m2

Chalmers University of Technology

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

Chalmers University of Technology

Bottom-region clustering/bubble flow

• u0 > ut 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

Primary gas distributor

u0 = 2.7 m/s

ut = 2.1 m/s

Chalmers University of Technology

0 5 10

Time [s]

-25000

-20000

-15000

-10000

-5000

0

5000

Imp

act

pre

ssu

re [

Pa

]

0 5 10

Time [s]

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0

5000

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act

pre

ssu

re [

Pa

]

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Time [s]

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-15000

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0

5000

Impa

ct

pre

ssu

re [

Pa

]

0 5 10

Time [s]

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-15000

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0

5000

Impa

ct

pre

ssu

re [

Pa

]

ER, 50 mm from wall ER, 2550 mm from wall

L2f3, 50 mm from wallL2f3, 2000 mm from wall

36.7 m above air distributor

3.8 m aboveair distributor

0 5 10

Time [s]

-25000

-20000

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0

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ct

pre

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re [

Pa]

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ct

pre

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re [

Pa]

17.7 m above air distributor

L5f, 50 mm from wall L5f, 2500 mm from wall

Front wall

Wall layer Core

Solids flux – Momentum measurements (235 MWe boiler)

Furnace wall-layer backmixing

Johnsson, et al.

Chalmers University of Technology

Bottom region

0 5 10 15 20 25 30 35 40HEIGHT ABOVE AIR DISTRIBUTOR, z [m]

0

2

4

6

8

10

PR

ES

SU

RE

DR

OP

, p

- p

exit [k

Pa

]

Chalmers 12 MWthTurow 235 MWe0 1 2 3

0

2

4

6

8

Cold unit (exploding bubble regime)Cold unit (transport condition)

Large boiler > 200 MWe

dp/dh b < 0.5

dp/dh b < 0.5

b = (- mf)/(1-mf)