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Chalmers University of Technology

Fuel mixing in fluidized beds

- experimental observations

Filip Johnsson, David Pallarès, Erik Sette

Department of Energy and Environment

Chalmers University of Technology, 412 96, Göteborg

64th IEA-FBC Meeting, Naples, June 3, 2012


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


Bubbling fluidized bed boiler (BFBC)

Circulating fluidized bed boiler (CFBC)


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


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


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)


Splash-zone solids cluster flow

Splash zone

Dense bed


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

-10000

-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 [P

a]

0 5 10

Time [s]

-25000

-20000

-15000

-10000

-5000

0

5000

Impa

ct pre

ssu

re [P

a]

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

Impact

pre

ssu

re [

Pa]

0 5 10

Time [s]

-25000

-20000

-15000

-10000

-5000

0

5000

Impact

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.


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 concentration

Fuel conversion (drying, devolatilization, combustion)

Fuel

mixing transport τ kinetics τ

Da =

Da Da

Fuel distribution – determined by Da number

Maldistribution

- Low lateral mixing

- Large cross section

- High volatile fuel

- High char reactivity


Continuous fuel feeding as a sum of time-delayed fuel batches

Fuel conversion fuel particle terminal velocity is reduced with time

2m

kg

Modeling fuel mixing and conversion


Fuel mixing – experimental observations

• 2 Dimensional cold measurements

• 3 Dimensional clod measurements - large unit

• 3 Dimensional cold scaled measurements

• 3 Dimensional hot measurements


General pattern – highly convective 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

Pallarès, D., Johnsson, F., 2010

3 D cold fuel mixing measurements (The Chalmers gasifier)

u: 0.6 m/s, 1 m/s

H0: 0.4 m

Tracer particles: Wood chips, Bark pellets

Y

X Fuel

Camera

3 D cold

Fuel (tracer)

Feed position

3 D cold


Single wood

particle

24 cases

u/umf = 7.5

u/umf = 5

u/umf = 7.5


Bed geometry scaled by a factor 1/6

gL

u 20

f

s

f

ps du

0

f

f Lu

0

0u

G

s

s

rdistributo

bed

P

P

Parameter Value

Length L (m)

Width W (m)

Superficial velocity (m/s)

ρs 2600 (kg/m3)

ρf 0.18 (kg/m3)

Parameter Value

Length L/6 (m)

Width W/6 (m)

Superficial Velocity (m/s)

ρs 15700 (8900) (kg/m3)

ρf 1.21 (kg/m3)

6

2

0U

0U

3 D cold fuel mixing measurements

Downscaled unit

Tracer measurement – UV light

U = 0.33 m/s. Fuel particle density 200 – 1200 k/m3


Downscaled unit

0 200 400 600 800 1000 12000

0.1

0.2

0.3

0.4

0.5

0.6

0.7Specific Dispersion Coefficient, x direction

Time [s]

Dis

pers

ion c

oeff

icie

nt

of

one p

art

icle

[m

2/s

]


Downscaled unit

Camera mounted 45 degrees downwards

Approximation of the region which is visible with camera probe

3 D hot fuel mixing measurements (The Chalmers gasifier)

u = 0.15 m/s, wood pellets, 800 C

u = 0.27 m/s, wood pellets, 800 C

Macroscopic pattern of fuel flow

Homogeneously-distributed ”mixing cells” can be seen as isotropic mixing

CDt

C 2

More detailed modeling required


Summary

• Fuel mixing crucial for modeling CFB (BFB) perfromance

• 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

• Need for continued development of fuel mixing measurement methods/technologies (2D and 3D)


Extras


Fuel mixing in fluidized beds

- simulations

Filip Johnsson, David Pallarès

Department of Energy and Environment

Meisam Farzaneh, Srdjan Sasic

Department of Applied Mechanics

Chalmers University of Technology, 412 96, Göteborg

64th IEA-FBC Meeting, Naples, June 3, 2012


Fuel mixing simulation

Eulerian-Eulerian-Lagrangian(E-E-L)

• Gas and inert particle phases are resolved using

Eulerian-Eulerian scheme

• Properties of mixture (gas and inert) such as density,

viscosity, … are obtained

• Fuel particle is tracked using Eulerian-Lagrangian


Results of E-E-L

Fluidization velocity = 0.4 m/s

Bed dimension = 0.4 m * 1.0 m

Static bed height = 0.56 m

Plenum included

0.33 mm inert particles

12 mm fuel particle


Fuel Particle concentration

Simulations and experiments

Simulation Measurements


Fuel Particle concentration - simulations


Importance of fuel mixing - test in a the Chalmers gasifier

Intro 2D Cold 3D Cold 3D Cold down-

scaled 3D Hot

Fuel mixing controls residence

time of fuel particles

Gasification reactions slow

(compared to combustion)

Long residence time required

22 HCOOHC

Fuel inlet

Solids

outlet

Bed material

inlet

Gas outlet


dTP :10 mm u0: 0.4 m/s

ρTP : 985 kg/m3

RUN 1

mb: 1.5 kg

RUN 16

mb: 5.0 kg

Fuel mixing

0.059 m/s 0.431 m/s

Pallarès, D., Johnsson, F., 2010 2 D cold


Dispersion of fuel particles

Intro 2D Cold 3D Cold 3D Cold down-

scaled 3D Hot

U = 0.36 m/s. Particle density corresponds to chipped wood.


Model for fuel conversion Main assumptions

- Fuel particle approximated to an ideal geometry (∞-plane, ∞-cylinder, sphere)

- Quasi-steady state

- Convective term shown to be neglectable


0

500

1000

1500

2000

2500

3000

3500

4000

2005 2010 2015 2020 2025 2030 2035 2040 2045 2050

Ele

ctr

icity g

enera

tion [

TW

h]

Year

Hydro

Nuclear

Lignite

Hard coal

Gas

biomass & waste

New Wind

New Biomass & waste

New Hard coal

New Gas

Hydro replacements

Hard coal CCS

Lignite CCS

Nuclear reinvestments

Wind

New Lignite

Role of thermal conversion → gas-solids flow: - Same level of importance

- From fossil fuel dominated to renewables and CCS

Johnsson, F., Odenberger, M., Energy Procedia 4 (2011) 5869–5876


Bubbling fluidized bed boiler (BFBC)

Circulating fluidized bed boiler (CFBC)


CFBC with and without EHE

+ 55,4 m

+ 2,3 m

External Heat Exchanger (EHE)


Bottom bed definition

Time averaged pressure drop

Johnsson, et al. 1991


Gas flow distribution in bottom bed

Dynamic in time and space

Pallarès, D., Johnsson, F., 2010


FBC characteristics → Key features/problems • Ratio of mixing and fuel conversion

• Solids segregation

• Dynamics of mixing

200 m2 bed surface (235 MW CFB Turow)

Air distributor


Back-mixing:

Splash-zone solids

cluster flow

Splash zone

fuel mixing in fluidized beds - experimental observations › iea-fbc.org › upload ›...

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