Influence of slag properties and operating conditionson slag flow in a coal gasifier

Insoo Ye1, Changkook Ryu1, Bongkeun Kim2

1School of Mechanical Engineering, Sungkyunkwan University2Energy Conversion System Research Team,

Doosan Heavy Industries & Construction Co., LTD.

8th International Freiberg Conference on IGCC & XtL TechnologiesInnovative Coal Value Chains

12-16 June 2016, Cologne, Germany

Coal gasification technology and process¨ Coal gasification technology

l Conversion of coal into syngas (CO, H2)l Used for power, fuel and chemical production

¨ Entrained bed coal gasifiersl Typical operating conditions

¨ Slag layer formation on the gasifier walll Deposition of molten ash onto the walll Protection of the wall from chemical, physical

and thermal damagesl Ash discharge by slag flow on the surface

2

Coal size (μm) ~50

Particle residence time (sec) 5– 10

Carbon conversion (%) ~ 99

Wall condition Slagging

Entrained-bed gasifiers (up) and slag layer on the wall (down)

Slag layer formed on the gasifier wall¨ Structure of the slag layer

l Solid slag layer facing the cold walll Liquid slag flowing downward on the surfacel Tcv at viscosity of 25 Pa.s considered as

the interface temperature of the two layers

¨ Key parameters influencing the slag layerl Ash content of coall Ash composition (SiO2, Al2O3, CaO, etc.)l Gas temperaturel Reactor shape and flow pattern, etc.

¨ Understanding the slag layer behaviorl Difficult to directly measure/monitor

during gasifier operationl Numerical models required to understand its behavior

Wat

er/s

team

tube

Ref

ract

ory

Liqu

id s

lag

Solid

sla

g

Coal

Ash

Molten ash

Deposition

Pre

ssur

e ve

ssel

Gasifier(T > 1500°C)

Slag formation on the wall in a gasifier

3

Existing models for slag flow¨ Seggiani (1998) model

l Assumption: linear temperature profile in the slag layerl Slag viscosity:l Adopted in

ü Li et al. (2009): Vertical wall ü Kittel et al. (2009): Siemens gasifierü Ni et al. (2010) : Lower part of pilot scale PC gasifier

¨ Yong et al. (2012) modell Assumption: cubic temperature profile

in the liquid slag layerl Slag viscosity: constant across the liquid slag layerl Adopted in

ü Chen et al. (2012) : Vertically-oriented oxy-coal combustor

( ) ( ) ( ) ( )LS δarTμrμTμ -=º exp

Wat

er/s

team

tube

Ref

ract

ory

Pres

sure

ves

sel

Solid

sla

g

Liqu

id s

lag

TSTCVTRTSteam

Seggiani

Yong et al.

r

Temperature profile assumed within the slag layer

4

Objective and methods¨ Research objective

l To propose and validate a new slag flow modell To evaluate the influence of key design/operating parameters

¨ Methodsl Target gasifier: PRENFLO gasifier (Entrained bed, Spain)l Part I: model development and evaluation

ü In comparison to existing models (Seggiani, Yong et al.)

l Part II: parametric analysisü Gas temperature, ash deposition, slag propertiesü Reactor geometry (bottom cone angle)ü Tcv

l Part III: model expansion to a transient system

PRENFLO Gasifier

5

Approach of the new numerical model¨ Model for the liquid slag layer: direct solution of the governing equations

l Solution along the direction (i) perpendicular to the wall, and marching downward (j)ü No assumptions required for temperature profile or slag viscosity.

l Control volumes inherit the same amount of mass from above (mi,j = mi,j+1)

l New ash deposition: a new control volume added on the surfacel Programing with Excel Visual Basic for Application (VBA)

¨ Governing equations (steady states)Mass

Momentum

Energy

depositinout mmm +=

gravityviscousdepositinout MMMMM +++=

GLdepositconductionphaseinout QHQHHH ++++= Δ

Solidslag

TCV

r0 r1 ri-1∆r1

T1,v1 Ti,vi

∆ri

Tsurf

TJ,vJ

Refrac-tory

Coolanttube

mdep,Hdep

rIrI-1

TRTtubeTC

Qgas

Qsteam = QR = Qsolid Qcond

ri

Δyj

Tcv

Solid

slag

laye

r

Deposit

Deposit

Liquid slag layer

Deposit

i

j- Critical viscosity temperature - Interface temperature between solid and liquid slag layer (μC= 25 Pa·s)

6

Slag thickness and properties¨ Slag thickness

l Liquid slag thickness : summation of each cell width l Solid slag thickness : from ,

¨ Slag propertiesl Heat capacity, Cp [kJ/kg.K]

l Density, ρ [kg/m3]

l Thermal conductivity, k [W/mK]

ü Thermal diffusivity of slag, α [m2/s] = 4.5 ´10-7 →

l Emissivity, ɛ = 0.83 (constant)

l Viscosity, μ [Pa.s]

å=

D=I

ijijS r

1,,d

å= PliqP XCC , )1000( 2, å -+= TcbTaXC glassP

)MnO%%OFeFeO%(182460 wt.wt.32wt. +++=r

Pslag Ck ××= ra

)/exp( TbTaμ ×1000××=

jCjSjcond QQQ ,,,0, ==

)( ,,,,, jCjRjCjRCjC TTAUQ -=)/ln( ,,,

,,,,

jjRjR

jRcvjSjRjS rrr

TTkAQ

0

-=

jjRjS rr ,0,, -=d

and

6steam

tube

refractory

Solid slag

rR,i

r0,j

ri,J

rtube,j

Liquid slag7

Geometry and Input conditions: Reference case¨ Gasifier: PRENFLO gasifier¨ Input and boundary conditions

l Main slag composition (%wt.)

l Tcv : 1548 K (at 25 Pa·s)l Uniform ash deposition (mdep) along the heightl Boundary condition

¨ Control volumel 60 sections (20 cells in each part)

Al2O3 CaO Fe2O3 K2O SiO2 SiO3 Misc.

22.05 24.27 3.39 0.52 44.24 2.29 3.24

Refractory k = 8 W/m∙K

Tube wall k = 43 W/m∙K

Water/steam T = 523 K

Wat

er/s

team

Tube

wal

l

Ref

ract

ory

Tube

wal

l

166.3

Tgas (1800 K)Tdep (1750 K)

mdep (4.5 kg/s)

8

Results: Reference case¨ Liquid slag layer

l Slag thickness (δ): increase in the in the downward directionü At the bottom cone: rapid increase due to inclined wall (angle: 78°)

l Temperature: linear increase to the surface from Tcv on the wall (x=0)l Velocity: fast flow near the surface due to low slag viscosity

Liquid Slag Thickness (mm)

0 5 10 15 20 25

Hei

ght (

m)

0

2

4

6

8

Top cone

Bottom cone

Main body

1500

1550

1600

1650

1700

1750

1800[Temperature, K]

Solid slag thickness (mm)

0 20 40 60 80

Hei

ght (

m)

0

2

4

6

8

Heat flux (kW/m2)

0 50 100 150

Heat flux (qout)

Solid slag thickness (δS)

x

9

Liquid Slag Thickness (mm)

0 5 10 15 20 25

Hei

ght (

m)

0

2

4

6

8

0

5

10

15

20

25[viscosity, Pa·s]

Liquid Slag Thickness (mm)

0 5 10 15 20 25

Hei

ght (

m)

0

2

4

6

8

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08[velocity, m/s]

Part I: Model Comparison and Validation

¨ Comparison with existing modelsl Seggiani: Good agreementl Yong et al.: Slag thickness under-estimated

by 10% (due to constant slag viscosity)

¨ Constant slag viscosity conditionl Good agreement with Yong et al’s model

confirms that the under-estimation of slag thickness is by the assumption of constant viscosity

Liquid slag thickness andsurface temperature

Liquid slag thickness (mm)

0 5 10 15 20 25H

eigh

t (m

)0

2

4

6

8

Surface temperature (K)

1500 1550 1600 1650 1700 1750 1800

This studyYong et al's modelSeggiani's model

dL

Tsurf

Liquid slag thickness (mm)

0 5 10 15 20 25

Hei

ght (

m)

0

2

4

6

8

Surface temperature (K)

1500 1550 1600 1650 1700 1750 1800

This studyYong et al's model

dL

Tsurf

μ=7.12 Pa·s

μ=7.12 Pa·s

10

Solid slag thickness (mm)

0 100 200 300 400 500 6500 7000

Hei

ght (

m)

0

2

4

6

8

This studyYong et al's modelSeggiani's model

0 100 200 3000.0

0.1

0.2

0.3

Position in the liquid slag, r (mm)

0 5 10 15 20 25

Tem

pera

ture

, K

1500

1520

1540

1560

1580

1600

1620

1640This studyYong et al's modelSeggiani's model

Part I: Model Comparison and Validation (cont’d)¨ Case of low Tgas at the bottom cone (Tgas=1518 K or Tcv-30K)

l Liquid slag thickness (δL): three models give similar valuesl Solid slag thickness (δS): Seggiani’s model over-estimates due to the linear temperature profile

in the liquid slag (zero heat flux to the solid slag = infinite value of δS)

Solid slag layer thickness

11

Temperature profile in the liquid layer at slag tap

Solid slag thickness (mm)

0 100 200 300 400 500 6500 7000

Hei

ght (

m)

0

2

4

6

8

This studyYong et al's modelSeggiani's model

0 100 200 3000.0

0.1

0.2

0.3

Position in the liquid slag, r (mm)

0 5 10 15 20 25

Tem

pera

ture

, K

1500

1520

1540

1560

1580

1600

1620

1640This studyYong et al's modelSeggiani's model

12Part II : Parametric analysis¨ ±10% changes from reference values¨ Gas temperature

l Largest influence on the slag flow and heat transferü Radiation (~T4) dominant at high temperature

¨ Ash deposition ratel Influence on the slag behavior is not large

(~ 3%)

¨ Slag properties: viscosity, thermal conductivity, emissivityl Very small influences on δ (< 6%)

δL Liquid slag thickness at the slag tap

δS Solid slag thickness at the slag tap

QLS Heat transfer rate to the solid layer from liquid slag

Varied Parameters δL (%) δS (%) QLS (%)

Gas temperature(ref. 1800 K)

+10% −17.4 −54.8 107.1

−10% 24.0 405.5 −78.2

Ash deposition rate(ref. 4.5 kg/s)

+10% 3.2 3.1 −2.0

−10% −3.4 −3.4 2.3

Slag viscosity(ref. f(T) Pa.s)

+10% 1.2 6.0 −4.3

−10% −1.3 −6.1 4.7

Slag conductivity(ref. 1.58 W/mK)

+10% 0.3 1.3 6.9

−10% −0.3 −1.3 −7.3

Slag emissivity(ref. 0.83)

+10% −0.3 −1.7 2.4

−10% 0.4 2.1 −2.8

Part II : Parametric analysis – Bottom cone angle¨ Influence of bo om cone angle: 78 → 60°

l Surface velocity: 0.075 m/s → 0.102 m/sü By increase in the gravity force

l Liquid slag thickness: 17 mm → 13 mml Solid slag thickness: 69 mm → 53 mml Thermal conduction rate to the solid layer

strengthened and affected its thickness

13

Position in the liquid slag, r (mm)

0 2 4 6 8 10 12 14 16 18 20V

eloc

ity (m

/s)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Reference (12o)18o

24o

30o

72°66°60°

Reference (78°)

Steeper angle

Part II : Parametric analysis - Tcv¨ Relative changes in δ assessed for Tcv under

various conditionsl Exponential increase in δ at high values of Tcvl High Tcv means Small temp. difference between

the liquid slag surface and Tcv(Low heat flux to the solid slag → Thick solid slag)

l Little contribution of liquid slag thickness

14

Tcv (K)

1200 1300 1400 1500 1600 1700

Rel

ativ

e m

agni

tude

of s

lag

laye

r thi

ckne

ss (%

)

0

100

200

300

400

500At the slag tapAt the end of main bodyBottom cone angle = 60o

Tgas= 1750 KTgas = 1850 K

Hollow symbols: Shifts in TcvSolid symbols: Different ash:flux ratios

Influence of Tcv on the slag layer thickness

Part III : Model expansion to a transient system

sec

Initial condition(Tgas: 1800 K)

[Temperature, K] [velocity, m/s]

15

Time (sec)

0 600 1200 1800 2400 3000 3600

Sla

g la

yer t

hick

ness

(mm

)

0

20

40

60

80

100

Liquid slag layer

Solid slag layer

¨ Example results: Instant change in gas temperature from 1800 K → 1980 Kl It requires more than 1200 sec (20 min) to reach the new steady-statel For the first 100 seconds, slag thickness at the slag tap increases by faster flow of liquid slagl Then solid slag turns into liquid slag and flows down

At the slag tapTime constant for 63.2% change sec

δ at slag tap 449

heat transfer to the wall 387

Conclusions¨ A new numerical model developed for slag flow and heat transfer

l Evaluated by comparison with existing modelsl No assumptions required for gas temperature profile or slag viscosity

¨ Influence of key parameters on slag thickness and heat transfer to the walll Gas temperature has a dominant influence on the slag flow and heat transferl Steep bottom cone decreases the slag thicknessl Ash deposition and slag properties are not very influentiall Exponential increase of solid slag thickness with increase in Tcv

¨ Model expansion to a transient system l Slag thickness and heat transfer rate on the wall adjust very slowly (~ several minutes) to the

changes in the operating conditions

16