3D numerical simulation of gas-solid hydrodynamics and coal combustion in an industrial scale circulating fluidized bed

Massoud Massoudi Farid Hyo Jae Jeong

Jong-Min Lee (KEPCO) Dong-Won Kim (KEPCO)

Jungho Hwang

Department of Mechanical Engineering Yonsei University

Introduction

Circulating Fluidized Bed

After 10 seconds no combustion occurred ( it took around 10 days)

Cyclone

Return leg

Furnace (Riser)

Air inlet

Wing Wall tubes

Steam inlets

Coal Feeder

1) Air enters the furnace through nozzles located at the bottom of the furnace and fluidizes sand particles which are packed initially in the furnace.

2) Air reacts with coal particles which are injected to the furnace by coal feeders located around the furnace.

3) Sand particles carried by mixture of air and combustion products to the top area of the furnace, enter the cyclones and are separated from the combustion products.

4) Combustion products then leave the cyclones through exit ports and sand particles return to the furnace through return legs.

5) Water steam flow into wing wall tubes, receive heat from gas-solid mixture inside the furnace.

Eulerian-Eulerian

In Eulerian frame

In Eulerian frame

Assumptions 1) It is a multiphase flow method and N.S equations are solved for both phases. 2) Solid and fluid phases are treated in the same way as interpenetrating continua. 3) granular properties are added to solid phase by solving an additional equation for solids

fluctuating energy or granular temperature. 4) There is no limitation for volume fraction. Hence effects of volume fraction should be considered in N.S equations

advantages: 1) It predicts well main features

of the flow. 2) lower computational time

compared to other methods

Multiphase Eulerian-Granular Method(MEGM) (For interaction of Gas and Sand particles)

Discrete Phase Method(DPM) (For interaction of Gas and Coal)

Assumptions 1) Solid particles have low Volume fraction

and they can not affect fluid motion. Hence Volume fraction is not considered in N.S equations.

2) There is no particles interaction. 3) Trajectory of a discrete phase particle is

predicted by integrating the force balance equation of the particle.

Eulerian-Lagrangian

In Lagrangian frame

In Eulerian frame

Particle force Balance for Solid Phase

xp

ppD

p Fg

uuFt

u +

−+−=

∂∂

ρρρ )(

)( 24Re18

2D

ppD

Cd

Fρ

µ=

Momentum of Fluid Phase

)()()( otherDPM FFgpvvvt

++ρ+τ⋅∇+−∇=ρ⋅∇+ρ

∂∂

Boiler Geometry

After 10 seconds no combustion occurred ( it took around 10 days)

5 Super heater wing walls

1 Evaporator wing wall Due to

computational barriers

Real CFB

340 MWe CFB boiler located in Yeosu, Korea.

Boundary Conditions and Coal properties

After 10 seconds no combustion occurred ( it took around 10 days)

Size Avg(mm) [%] Acc.[%]

12 0 0

6 10.7 10.7

3 24.7 35.4

1.7 16.8 52.2

1.2 14.1 66.3

0.75 20.1 86.4

0.428 4.6 91

0.303 3 94

0.215 2 96

0.075 2 98

0.1 1 99

0.057 1 100

0.019 0 100

Proximate analysis Wt. % Ultimate

analysis Wt. %

Moisture 15.3 Moisture 0 Fixed Carbon 41.7 Carbon 72 Ash 2.5 Hydrogen 5.1 Volatiles 40.5 Nitrogen 1.0 Sulfur 0.3 Oxygen 21.6 Total 100 Total 100

Coal feed rate per nozzle 4.49 kg/s

Air feed rate

Total Furnace inlet 50.5 kg/s

Return leg inlet 0.62 kg/s

Secondary air inlet per

nozzle

Lower 3.6 kg/s

Upper 2.3 kg/s

Outlet pressure -1304 Pa

Sand size 283.12 ㎛

Bed height 800 mm

Temperature

Inlet temperature 482 K

Wall temperature 632 K

Wing wall tube Super heater 692 K

Evaporator 632 K

Sand Eulerian Granular method Coal DPM

Coal information

Boundary Conditions

Numerical Methods

After 10 seconds no combustion occurred ( it took around 10 days)

Software ANSYS-FLUENT

Time Dependency Unsteady

Gas-Sand Particles interaction Multiphase Granular Method

Gas-Coal particles interaction Discrete Phase Method

Turbulence standard k–ε model

Turbulent dispersion effect of coal particles Stochastic tracking method

Species transport Finite-Rate/Eddy-Dissipation Model

Radiation Discrete Ordinate (DO) radiation model

Pressure-velocity coupling Phased Coupled SIMPLE scheme

Gradient discretization Least Squares Cell Based scheme

Momentum/ Energy, Volume fraction, and species transport discretization

Second order upwind /

first order upwind

Hydrodynamic Results

t=0s t=0.3s t=10s t=15s t=20s t=25s t=30s t=35s t=40s t=60s

Once air was introduced to the bottom of the furnace, the bed began to expand (around t=0.3s) , with some particles hitting the top wall and falling down and some particles close to the cyclone were dragged by gas into the cyclone. At t=35s bed height reached to the highest level and from then onward the bed height remained almost constant which shows the quasi-steady state situation where the input and output rates of particles for each of the flowing units involved are equal.

Bed expansion by time steps

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2

Sand Volume Fraction

H

Sand-Volume fraction

In the riser, solid volume fraction is high at the bottom and low at the top (the so called S-Shaped Profile)

1) There is a dense bottom coexisting with dilute top in both the furnace chamber and the return leg.

2) A clear boundary between solid phase and gas phase in the return leg can be discerned which is a bubbling fluidized bed, and packed at the beginning of the simulation to serve as solid seal to prevent riser gas leakage via the return leg.

3) The solids volume fraction was normally high near the walls and low in the center of the furnace, except from some clusters,

1

2

Grace Et. al

Sand-Velocity-Vectors (colored by sand Z velocity value )

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1) Wherever there is a velocity vector it means that there is a particle at that point.

2) Solids vertical velocity is mainly positive in those areas where volume fraction is

low and negative those areas where volume fraction is high or on the other words negative velocity owes to clustering of particles (points 1 and 2)

1

2

Core-annulus

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Z=15m Z=20m Z=30m

1) The volume fraction curves are typical to the so-called core–annulus structure, which causes solid back-mixing.

2) The core–annulus structure can be confirmed by Z velocity curves showing a downward movement of solids near-wall region, while rising particles in the center.

3) At Z=15 m the cross section is under the wing wall tubes and core-annulus structure is more obvious.

4) At Z=20 m the cross section is at the bottom part of wing wall tubes. At this point the core-annulus structure is affected little by wing wall tubes.

5) There are some local core annulus between wing wall tubes.

6) At Z=30 solid concentration is low and solid concentration is higher at the entrance of the cyclone. Hence core-annulus structure is weaker compared to previous pictures but still near the walls and wing wall tubes solid concentration is high and solid Z velocity is negative.

Gas-Velocity

ATmV total

cal ×ρ=

)(

Z=20m Varea-av-num=7.24m/s

Vcal =5.38m/s based on T=1356.6 K ρ=0.255936

Z=30m Varea-av-num=6m/s

Vcal =5.23m/s based on T=1314 K ρ=0.262421

Z=10m Varea-av-num=7.2m/s

Vcal =4.91m/s based on T=1291 K ρ=0.280238

1) In some regions due to existence of solid particles gas velocity is higher than other regions and in general gas velocity is higher at the middle of the furnace and lower near the walls due to no slip boundary condition. At the entrances and the exit of the cyclone gas velocity is high.

2) Theoretically velocity can be calculate by knowing inlet mass flow rate, furnace cross section and averaged density.

3) Differences between theoretical values and numerical ones could be due to existence of solid particles, coal combustion, and gas turbulence flow.

Sand-Velocity-Cyclone (colored by sand Z velocity value )

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Y=-3.1 Y=-2.5 Y=0 Z=35

Most particles congregate at the wall immediately after the inlet and then descend in strands in a cyclone. also there is particle accumulation in the apex region where particle–particle interaction is intensive. These phenomena have been reported before by other authors.

Gas velocity vectors in cyclone (colored by gas Z velocity)

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%96.99)/(157)/(06.01100)1( =−=×−=

skgskg

mm

efficiencyCycloneinletletSand

outletSand

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Pressure Loop and solid circulation rate

a

b

c d

f

g

e

inlet Cyclone inlet Cyclone outlet

numerical real numerical real numerical real

P(pa) 9778.273 8002 -1791.72 0 -1304 -1304

Gs(kg/s-m2) 3.31

h

Pcyc-en=0 pa

Pnuz-out=8002 pa

a

b

c

e

f

g

d

0

1

2

3

4

51 56 61G

s t

0

1

2

3

4

5

0 20 40 60

Gs

t

Combustion Results

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Gas Temperature.

1) Gas Temperature is high at the bottom of the furnace due to combustion reactions.

2) Due to heat transfer with wing walls and furnace wall, temperature falls and reaches at 1095K at the cyclone outlet.

3) The outlet Temperature in real CFB is around 1127K; therefore the numerical error is only around 2.84%.

Z=10m

Z=20m

Z=30m Coal Feeder No.2 Y=3.024

Coal feeder No.1 Y=0.56

CO Mole fraction

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1) Due to complete combustion and conversion of CO to CO2 the amount of CO in the domain is generally low.

2) The majority of CO exists at the bottom of the furnace and near the coal feeders due to large number of carbon particles and de-volatilization.

Z=10m

Z=20m

Z=30m

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CO2 Mole fraction.

1) The amount of CO2 in the domain is high especially at the upper side of the furnace due to conversion of CO to CO2 by water gas shift reaction and CO and O2 reaction, and also volatile combustion.

2) The reason that at X=0 cross-section CO2 mole fraction is higher at the right side is because of the position of the coal feeders.

Coal Feeder No.2 Y=3.024

Coal feeder No.1 Y=0.56

Z=10m

Z=20m

Z=30m

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Volatiles Mole fraction

Similar to CO mole fraction the amount of coal volatiles mole fraction is low in the domain due to burning and converting to combustion products.

Z=10m

Z=20m

Z=30m

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Char Concentration

the concentration of char is high at the bottom of the furnace and near the coal feeders.

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Outlet dry based spices

N2 O2 CO2 H2 CO CH4 volatiles Mol % 79.673 8.261 12.066 0 0 0 0

0

20

40

60

80

N2O2

CO2

Conclusion

• Eulerian-Eulerian method was used (multiphase granular model) for modeling sand and gas hydrodynamics

• Hydrodynamic simulation results such as sand circulating rate, pressure loop, and sand distribution in furnace were reasonable.

• 3D numerical simulation of air and sand particle hydrodynamics along with coal combustion in an industrial scale circulating fluidized bed was done.

• Eulerian-Lagrangian method was used (discreet phase model (DPM)) for modeling coal particle trajectories

• Combustion simulation results such as combustion products distribution in the furnace were reasonable.

• Exit gas temperature was in good agreement with real data obtained from a 340 MWe CFB boiler located in Yeosu, Korea.

Thank you for your attention

Q & A

Appendix

)())((

)()(

,,1

qvmqliftq

n

pqpqppqpqqppq

qqqqqqqqqqq

FFFvmvmvvK

gpvvvt

+++−+−

++⋅∇+∇−=⋅∇+∂∂

∑=

ραταραρα Momentum of Fluid Phase

)())((

)()(

,,1

svmslifts

N

lslsllslsslls

ssssssssssss

FFFvmvmvvK

gppvvvt

+++−+−+

+⋅∇+∇−∇−=⋅∇+∂∂

∑=

ραταραρα Momentum of Solid Phase

lss

ssssssssss

ssk

vIpvt

φγ

τραρα

+−Θ∇⋅∇

+−∇+−=Θ⋅∇+Θ∂∂

ΘΘ )(

:)()()( Granular

Temperature

Equations:

Multiphase Eulerian-Granular Method(MEGM)

Reactions

After 10 seconds no combustion occurred ( it took around 10 days)

Gas Phase Reactions Ar Er (J/kmol) m a b c

CH2.84O0.761 → 0.239CH4 + 0.761CO + 0.942H2 4.26×106 [1/s] 1.08×108 0 1 1 0

CH2.84O0.761 + 1.33O2 → CO2 + 1.42H2O 2.12×1012 [1/K/s] 2.03×107 0 1 1 0

CO +0.5O2 → CO2 2.239×1012 [(m3/kmol)0.75/s] 1.674×108 0 1 0.25 0.5 [H2O]

H2 + 0.5O2 → H2O 6.8×1015 [(m3/kmol)0.75/K-1/s] 1.67×108 -1 0.25 1.5 0

CO + H2O → CO2 + H2 275 [(m3/kmol)0.5/s] 8.374×107 0 1 1 0

H2 + CO2 → CO + H2O 0.0265 [(m3/kmol)0.5/s] 3960 0 1 1 0

CH4 + 0.5O2 → CO + 2H2 4.4×1011[(m3/kmol)0.75/s] 1.25×108 0 0.5 1.25 0

CH4 + H2O → CO+ 3H2 8.7×107[(m3/kmol)0.5/s] 2.51×108 0 0.5 1 0

Solid Phase reactions Ar [kg/m2/sec/Pa0.5] Er

(J/kmol)

C(s) + 0.5O2 →CO 0.052 6.1 x 107

C(s) + CO2 →2CO 0.0732 1.125 x 108

C(s) + H2O→CO + H2 0.0782 1.15 x 108

Since we have combustion Co concentration is low in the domain.

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CO-Coal nozzle effect

Coal Feeder No.2 Y=3.024

Coal feeder No.1 Y=0.56

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CO2-Coal nozzle effect

High Co2 concentration can be seen in the domain.

Coal Feeder No.2 Y=3.024

Coal feeder No.1 Y=0.56

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Volatiles-Coal nozzle effect

Same as Co, amount of Volatiles also is low because combustion occurs.

Coal Feeder No.2 Y=3.024

Coal feeder No.1 Y=0.56

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Char Concentration-Coal nozzle effect

Coal concentration is high at the bottom of the furnace and low at top because of the combustion.

t=33s

Coal Feeder No.2 Y=3.024

Coal feeder No.1 Y=0.56

0

1

2

3

4

5

0 10 20 30 40 50 60 70

Gs

t

∑=

−=⋅∇+∂∂ n

pqppqqqqqq mmv

t 1)()()(

ραρα

∑=

=n

qq

11α

)())((

)()(

,,1

qvmqliftq

n

pqpqppqpqqppq

qqqqqqqqqqq

FFFvmvmvvK

gpvvvt

+++−+−

++⋅∇+∇−=⋅∇+∂∂

∑=

ραταραρα

Volume Fraction Equation

Continuity

Momentum of Fluid Phase

lss

ssssssssss

ssk

vIpvt

φγ

τραρα

+−Θ∇⋅∇

+−∇+−=Θ⋅∇+Θ∂∂

ΘΘ )(

:)()()( Granular

Temperature

)())((

)()(

,,1

svmslifts

N

lslsllslsslls

ssssssssssss

FFFvmvmvvK

gppvvvt

+++−+−+

+⋅∇+∇−∇−=⋅∇+∂∂

∑=

ραταραραMomentum

of Solid Phase

α Volume Fraction ρ Density

v Velocity m Mass transfer between Phases

p Pressure

τ Phase Stress-Strain Tensor g Gravitational accelerate

K Interphase momentum exchange coefficient

F

External Body Force

liftF

Lift Force

vmF

Virtual Mass Force

sΘ Granular Temperature

)()(5.0 qpqpplift vvvF ×∇×−−= αρ

φφφ)()()(

∇⋅+∂

= qq v

dtdtd

−=

dtvd

dtvd

F ppqqqpvm

ρα5.0

65.2

43 −−

= ls

lsllsdsl d

vvCK α

ραα

[ ]687.0)Re(15.01Re

24sl

sldC α

α+=

l

lssls

vvdµ

ρ −

=Re

s

lssl

sl

llssl d

vvd

K

−+

−=

αρα

µαα 75.1)1(150 2

For

For

8.0>lα

8.0≤lα

sssssssssss gep Θ++Θ= ,02)1(2 αρρα

sse Coefficient of restitution for particle collision (around 0.9)

ssg ,0 Radial distribution function

Kinetic Term Collision Term

1

31

max,0 1

−

−=

s

sgαα

max,sα Max Packing Volume fraction

Ivvv ssssTsssss

⋅∇−+∇+∇= )

32()( µλαµατ

Stress-strain tensor for solid phase:

Shear viscosity =sµ kinscols ,, µµ +=

ss

ssssssscols egd απ

ραµ2/1

,0, )1(54

Θ+=

sssssssssss

ssskins eg

egd

αααρ

µ2

,0,0

, )1(541

)1(9610

++

+Θ

=

2/1

,0 )1(34

Θ+=π

ρα ssssssss egdBulk viscosity =sλ

Accounts for the resistance of the granular particles to compression and expansion

The collisional dissipation of energy

The generation of energy by the solid stress tensor

lss

ssssssssss

ssk

vIpvt

φγ

τραρα

+−Θ∇⋅∇

+−∇+−=Θ⋅∇+Θ∂∂

ΘΘ )(

:)()()(

sss vIp −∇+− :)( τ

ssk Θ∇Θ

The diffusion of energy

sΘγ

lsφ The energy exchange between phases

=Θsk Diffusion Coefficient

παρ

απρ

ssssssss

sssssssss

sss

egd

egeg

d

Θ+

+

++

+Θ

=

)1(2

)1(561

)1(348)(150

,02

2

,0,0

2/32,02 )1(12

ssss

ssss

dge

sΘ

−=Θ αρ

πλ

slsls K Θ−= 3φ

||,0max,

36 sss

s

ss Ug

Θ−= ρ

ααφπτ

Granular Boundary condition:

Liquid Phase:

Wall: No slip

Inlet: Velocity inlet

Outlet: Atmosphere Pressure

Solid Phase:

Wall: Partial slip:

Outlet: Atmosphere Pressure

23

02

max,||,||,0

max,

)1(34

36 sssw

s

sssss

s

ss geUUgq Θ−−⋅Θ= ρ

ααπρ

ααφπ