mixing phenomena in fluidized beds diagnostics and observations · · 2017-02-21mixing phenomena...

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


Bubbling fluidized bed boiler (BFBC)

Circulating fluidized bed boiler (CFBC)


Dual bed systems – indirect gasifier

2-4 MW integrated in Chalmers 12 MW CFB

Gasifier

fuel

Combustor

fuel


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


Oxyfuel in fluidized-bed combustion

CFB Technology Metso Power (Now Valmet)

4 MW CFB Oxyfuel


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


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


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)


Fuel mixing


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 =


Experimental observations - Chalmers

2D cold tests

3D cold down-scaled tests

3D hot tests

3D cold tests

Qualitative

Quantitative (?)

Quantitative

Qualitative


3D hot conditions (12 MWth CFB Chalmers)


2D cold tests


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


SCgradDdivt

C

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

Dispersion coefficient can be determined using a diffusion analogue

Expressing fuel mixing


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


2D cold tests

– velocity and bed-height dependency

Red symbols - narrow unit


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)



Batch of fuel

particles

Fuel inlet



Fuel inlet

Batch of fuel

particles



Single fuel

particle

Fuel inlet


u/umf = 5

u/umf = 7.5


Single fuel

particle


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


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


3D cold tests – downscaled Chalmers gasifier bed

Dispersion of inert solids


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


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


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


Work in progress – development of camera probe


Improvement under way

– to minimize/eliminate

- Deposits on lens

- Condensation on lens

- Camera resolution

- Camera adjustments


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


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


Oxyfuel in fluidized-bed combustion

CFB Technology Metso Power (Now Valmet)

4 MW CFB Oxyfuel


4 MW Oxyfuel runs


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 CFB model

Example of ongoing work


Heat extraction


Optimization: heat transfer

Heat extraction panels

Test varying

- locations

- shapes

- functions (EV, SH)

and optimize by evaluating

the pdf of W/m2


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


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


0 5 10

Time [s]

-25000

-20000

-15000

-10000

-5000

0

5000

Imp

act

pre

ssu

re [

Pa

]

0 5 10

Time [s]

-25000

-20000

-15000

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-5000

0

5000

Imp

act

pre

ssu

re [

Pa

]

0 5 10

Time [s]

-25000

-20000

-15000

-10000

-5000

0

5000

Impa

ct

pre

ssu

re [

Pa

]

0 5 10

Time [s]

-25000

-20000

-15000

-10000

-5000

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

-15000

-10000

-5000

0

5000

Impa

ct

pre

ssu

re [

Pa]

0 5 10

Time [s]

-25000

-20000

-15000

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-5000

0

5000

Impa

ct

pre

ssu

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.


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)

mixing phenomena in fluidized beds diagnostics and observations · · 2017-02-21mixing phenomena...

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