Modeling of a strongly coupled Thermal, Hydraulic and

Modeling of a strongly coupled Modeling of a strongly coupled Thermal, Hydraulic and Chemical Thermal, Hydraulic and Chemical

problem: problem: problem: problem: drying and lowdrying and low--temperature pyrolysis of temperature pyrolysis of

chromated copper arsenate (CCA)chromated copper arsenate (CCA)--wood waste wood waste

particles in a moving bed reactorparticles in a moving bed reactor

Ir. Joan Govaerts Ir. Joan Govaerts Prof. Dr. Ir. Lieve HelsenProf. Dr. Ir. Lieve Helsen

Presented at the COMSOL Conference 2010 Paris

http://www.comsol.com/conf_cd_2011_eu

OutlineOutline

�� Introduction Introduction

�� Description of the mathematical modelDescription of the mathematical model

�� Simulation resultsSimulation results

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�� Simulation resultsSimulation results

�� ConclusionsConclusions

CCACCA--impregnated woodimpregnated wood

�� CCA stands for CCA stands for Chromated Copper Chromated Copper ArsenateArsenate

Preserves wood from Preserves wood from

IntroductionIntroduction Mathematical modelMathematical model Results Results

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�� Preserves wood from Preserves wood from insects, fungi and insects, fungi and water damagewater damage

�� Decks, fences, utility Decks, fences, utility poles, playground poles, playground equipment… equipment…

CCACCA--impregnated woodimpregnated wood�� Since 1970, phaseSince 1970, phase--out in 2005 out in 2005 �� Nowadays restricted to a limited number of industrial applicationsNowadays restricted to a limited number of industrial applications�� Classified as hazardous wasteClassified as hazardous waste�� Service life of 10Service life of 10––40 years: disposal will continue long into the future40 years: disposal will continue long into the future�� Worldwide problem (U.S.: peak disposal rate of 9.7 million m³ Worldwide problem (U.S.: peak disposal rate of 9.7 million m³

wastewood in 2008)wastewood in 2008)


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wastewood in 2008)wastewood in 2008)�� Need for a sustainable disposal methodNeed for a sustainable disposal method

Low T Low T -- carbonisationcarbonisation�� Promising technologyPromising technology

�� < 370< 370°°CC�� Wood chips are converted to Wood chips are converted to

carbon and volatile organic carbon and volatile organic compounds: energy recuperationcompounds: energy recuperation

�� Material recuperation: Material recuperation: Heavy metals remain in solid Heavy metals remain in solid


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�� Heavy metals remain in solid Heavy metals remain in solid phasephase

�� Carbon productCarbon product

�� Low tar production/emission Low tar production/emission

�� Simulation modelSimulation model�� Influence of operational Influence of operational

parametersparameters�� Optimal working conditionsOptimal working conditions�� Controlling metal and tar Controlling metal and tar

emissionsemissions

Description of the mathematical Description of the mathematical modelmodel

�� Model assumptionsModel assumptions

�� Governing equationsGoverning equations

�� SubmodelsSubmodels


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�� SubmodelsSubmodels

�� Initial and boundary conditionsInitial and boundary conditions

�� Numerical SolutionNumerical Solution

Model assumptionsModel assumptions

�� Volume Averaging Volume Averaging –– continuum approachcontinuum approach�� 1D anisotropic porous medium1D anisotropic porous medium�� reaction products are lumped into three main reaction products are lumped into three main

groups: char, tar and volatilesgroups: char, tar and volatilesOnly AsOnly As--Oxide consideredOxide considered


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�� Only AsOnly As--Oxide consideredOxide considered�� Cu, Cr very stableCu, Cr very stable�� bound, condensed, gaseousbound, condensed, gaseous

�� Water: bound, vapor Water: bound, vapor �� Solid and gas phase at different TSolid and gas phase at different T�� No secondary reactions (low T)No secondary reactions (low T)

�� Cracking of tarsCracking of tars�� Secondary char formationSecondary char formation

Governing equationsGoverning equations

�� Continuity gas phase Continuity gas phase

�� Darcy LawDarcy Law


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�� Species conservationSpecies conservation

Governing equationsGoverning equations

�� Energy conservationEnergy conservation


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SubmodelsSubmodels�� Drag force; heat and mass dispersionDrag force; heat and mass dispersion

�� Darcy coefficientsDarcy coefficients

�� Dispersion tensorDispersion tensor

�� Experimentally determined (Govaerts & Mayerhofer, 2010)Experimentally determined (Govaerts & Mayerhofer, 2010)

�� Convective heat transferConvective heat transfer(Wakao & Kugei, 1982)(Wakao & Kugei, 1982)

1/3 0.6(2 1.1Pr Re ) /h k dξ= +


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(Wakao & Kugei, 1982)(Wakao & Kugei, 1982)

�� Solid conductivitySolid conductivity

(Yagi & Kunii, 1957)(Yagi & Kunii, 1957)

1/3 0.6(2 1.1Pr Re ) /sg g p

h k dξ= +

,(1 )

g

s eff g p rs

kk d hε

= − +

Ψ

�3

εwith 0.227

2 ε 100

s

s

rs

Th

= −

SubmodelsSubmodels

�� Thermal degradation of woodThermal degradation of wood1

2

( )

( )

wood volatiles k

wood tar k

→

→

exp( / )i i ik A E RT=


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�� No secondary reactions No secondary reactions (cracking/secondary char formation)(cracking/secondary char formation)

3( )wood char k →

SubmodelsSubmodels�� AsAs--release release

�� first order single reaction scheme with a first order single reaction scheme with a Arrhenius temperature dependency. Arrhenius temperature dependency.

�� A A = 6.5 = 6.5 ×× 1010--3 s3 s--11 and and EEaa == 20.4 kJ/mol. 20.4 kJ/mol. ((Helsen and Van den Bulck, 2000)Helsen and Van den Bulck, 2000)


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((Helsen and Van den Bulck, 2000)Helsen and Van den Bulck, 2000)

�� AsAs--release is restricted to range of 280release is restricted to range of 280°°CC--450450°°CC

�� AsAs--condensation/recondensation/re--evaporation: diffusionevaporation: diffusion--limitedlimited

,

1/3 0.6

( )

with (2 1.1Sc Re ) /

gs

As m As sat As

g

m g p

Am k p p

V

k D d

= −

= +

SubmodelsSubmodels�� Drying/condensation: diffusionDrying/condensation: diffusion--limitedlimited


2 2,

1/3 0.6

( )


gs

As m H O sat H O

g

Am k p p

V

k D d

= −

= +

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�� To avoid overTo avoid over--and undershoots Heaviside and undershoots Heaviside functions are usedfunctions are used

1/3 0.6with (2 1.1Sc Re ) /m g pk D d= +

Latest addittions: Latest addittions: AsAs--release/condensationrelease/condensation

�� AsAs--release is restricted to range of 280release is restricted to range of 280°°CC--450450°°CC

�� AsAs--condensation/recondensation/re--evaporation: diffusionevaporation: diffusion--limitedlimited

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limitedlimited,

1/3 0.6

( )


gs

As m As sat As

g

m g p

Am k p p

V

k D d

= −

= +

Boundary and initial conditionsBoundary and initial conditions

�� Initial conditionsInitial conditions

�� Ambient temperature, Ambient temperature, pressurepressure Vsolid


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pressurepressure

�� VVgg = 0= 0

�� Boundary conditionsBoundary conditions

�� Inlet: TInlet: Tgg=370=370°°C, VC, Vgg=V=Vinin

�� Outlet: P=POutlet: P=Patmatm; gradients=0; ; gradients=0; VVsolidsolid

CouplingsCouplingsIntroductionIntroduction Mathematical modelMathematical model Results Results

Solid Species eq.Solid Species eq.

Gas Species eq.Gas Species eq.

TTs, s, CCp,sp,s

SSggSSkk

MMgg

CCp,gp,g

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Gas Energy eq.Gas Energy eq.

Solid Energy eq.Solid Energy eq.

Darcy eq.Darcy eq.

Gas densityGas density

Gas velocityGas velocity

SSThTh

TTgg

hhsgsg

Numerical solutionNumerical solution

Solve gas & solid species conservation equation

t = t + ∆t

START

Ou

ter

ite

rati

on

Tim

e ite

rati

on


Update physico-chemical properties

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Update physico-chemical properties

Convergence?

t = tmax ?

yes

yes

STOP

Ou

ter

ite

rati

on

Tim

e ite

rati

on

no

no

Solve Darcy & Energy equations

Simulation ResultsSimulation Results

�� ObjectivesObjectives

�� Maximise wood conversion and char Maximise wood conversion and char productionproduction


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productionproduction

�� Minimise tarMinimise tar-- and Asand As--emissionemission

�� Short hot zoneShort hot zone

�� Long cold zoneLong cold zone

Simulation ResultsSimulation Results�� drying efficiency of about 100% drying efficiency of about 100%

�� wood conversion of 99.4 %, wood conversion of 99.4 %,

�� 29.9% charcoal29.9% charcoal

�� 22.1% volatiles 22.1% volatiles


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�� 47.4% tars 47.4% tars

�� No condensation of tar and secondary char formationNo condensation of tar and secondary char formation

�� tar emissions are probably overestimatedtar emissions are probably overestimated

�� the overall product efficiency of the process underestimated. the overall product efficiency of the process underestimated.

�� Relative mass and energy balance errorsRelative mass and energy balance errors

�� 0.029 % 0.029 %

�� --0.077 %0.077 %

Simulation ResultsSimulation ResultsAxial temperature profile at nominal flow ratesAxial temperature profile at nominal flow rates


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Simulation ResultsSimulation ResultsAxial AsAxial As--oxide concentrations at nominal flow ratesoxide concentrations at nominal flow rates


�� 13.2 % of the initial As is 13.2 % of the initial As is released due to thermal released due to thermal decomposition decomposition

�� 12.9 % of the initial As12.9 % of the initial As--

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�� 12.9 % of the initial As12.9 % of the initial As--content will leave the reactor content will leave the reactor as a volatile compound. as a volatile compound.

�� 0.3% gets condensed in the 0.3% gets condensed in the middle part of the reactor. middle part of the reactor.

�� adsorption/desorption, adsorption/desorption, formation of stable metalformation of stable metal--mineral compounds not mineral compounds not consideredconsidered

ConclusionsConclusions

�� model for the simulation of the thermochemical decomposition of model for the simulation of the thermochemical decomposition of CCACCA--wood in a packed bed reactor.wood in a packed bed reactor.�� unsteady, oneunsteady, one--dimensional conservation equations of heat and mass for dimensional conservation equations of heat and mass for

the solid and the gas phase, the solid and the gas phase, �� Darcy’s lawDarcy’s law�� a competitive reaction mechanism for wood decompositiona competitive reaction mechanism for wood decomposition


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�� a competitive reaction mechanism for wood decompositiona competitive reaction mechanism for wood decomposition�� drying drying �� arsenic oxide release/condensation. arsenic oxide release/condensation.

�� This model allows to investigate the influence of design parameters This model allows to investigate the influence of design parameters (e.g. the volumetric flow rate of the hot gas supplied at the bottom (e.g. the volumetric flow rate of the hot gas supplied at the bottom and wood residence time) and wood residence time) �� product distributionproduct distribution�� AsAs--release release �� temperature profiles temperature profiles

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Modeling of a strongly coupled Thermal, Hydraulic and