Simulation of Circulating Fluized Bed Reactors Using Aspen Plus

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ELSEVIERPII: SOO16-2361(97)00211-l

Fuel Vol . 77, No. 4, 327-337, 1998p.

0 1998 E lsev ie r Sc ience L td . A l l r i gh ts reserved

Pr in ted i n Grea t B r i ta in

0016-2361/98 $19.00+0.00

Simulation of circulating fluidizedbed reactors using ASPEN PLUS

R. Sotudeh-Gh arebaagh, R. Legros*, J. Chaou ki an d J. ParisDepartment of Chemical Engineering, &o/e Polytechnique, PO Box 6079, Station ‘Centre-Vi//e’,

Montrkal, PO, Canada H3C 3A7

(Received 4 April 1995; revised 18 August 1997)

A comprehensive model is developed for the combustion of coal in a circulating fluidized bed combustor (CFB C).

The proposed model integrates hydrodyn amic parameters, reaction model and kinetic subroutines necessary to

simulate coal combustion in a CFBC . Kinetic expressions were developed for the char combustion rates and the

SO2 absorption in the bed using data from the literature. The reaction model, which considers only the important

steps of coal combustion, was simulated using four ASP EN PLU S reactor models and several subroutines. T he

developed subroutines w ere then nested in the ASPEN PLUS input file, so that the CF’EK may be represented. The

validity of the model was demonstrated using 14 different sets of operating conditions for the CAN ME T

0.8 MW th CFBC pilot plant. 0 1998 Elsevier Science Ltd. All rights reserved.

(Keywords: ASPEN PLUS; CFBC ; circulating fluidized bed reactors; fluidized bed com bustion)

INTRODUCTION

Circulating fluidized bed combus tors (CFB Cs) are consid-ered as an improvement over the traditional metho ds

associated with coal combustion. The CFB C exhibits

several advantages over conventional coal combustion

metho ds, especially when high sulfur coal is used’.

Operation of CFB Cs at industrial levels has confirmed

many advantages that include fuel flexibility, broad turn-

down ratio, high combustion efficiency, low NO , emissions

and high sulfur capture efficiency. These chara cteristics

assure an ever-increasing number of successful comm er-

cializations of CFB C in powe r generation applications.

Although CFBC technology is becoming more common

from these comm ercial applications, there are some

significant uncertainties in predicting their performa nce inlarge-scale systems.

Technical knowledge about design and operation of

CFB C is widely available for pilot plant and large scale

units. Ho weve r, little has been done in the field of

mathem atical modeling and simulation of combustion in

CFB Cs. This might be attributed to the fact that the

combustion process occurring in a CFB C involves complex

phenomena including chemical reactions, h eat and mass

transfer, particle size reduction due to combustion, attrition,

fragmentation and other mechanism s, gas and solid flow

structure, etc. Weiss et al.* introduced a CFBC model by

dividing it into 11 blocks, each corresponding to a CST R

reactor for both gas and solid phase. Five of these blocksrelated to the CFBC riser. Basu et aL3 developed a CFBC

model in which a plug flow regime for both the gas and

solids is assum ed. Lee and Hypanen4 presented a CFB C

*Corresponding author

model which considers the riser as a plug flow reactor for

the gas phase and a CSTR reactor for the solid phase. Themodel a lso considers the feed particle size distribution and

the attrition phenomena. Using a lumped-modeling

approach, Arena et aL5 introduced the means for predictive

calculation by dividing the CFB C riser into four blocks,

each corresponding to a separate reactor. Three of these

blocks related to the CFB C riser. The hydrodynamic

param eters were considered uniform within each section

and were used in various kinetic models to predict char

conversion. Wong6 prop osed a model for the hydrody-

namics of CFBC risers to characterize the effect of the

internal flow structure within the riser, the particle size

distribution and the operating conditions on CFB C behav-

ior. To estimate the axial voidage profile, a core-annulus

model was developed. The predictive hydrodynamic model

was then applied to a CFBC design. A comprehensive

review of relevant work on the hydrodynamics of circulat-

ing fluidized bed risers is presented by Berruti et aL7

Moreover, Senior’ conducted some theoretical and

experimental investigations to improve the understanding

of the fluid and particle mechanics in the CFB C riser and to

develop m athematical models to represent riser suspension

flows. On the other hand, a few CFBC modeling efforts have

been based on extension of bubbling AFBC hydrodynamic

concepts’-’ ‘.

Beyond those mentioned above, some modeling work

have been developed using ASPEN (advanced system for

process engineering). ASPEN was developed at theMas sachuse tts Institute of Technology (MIT) under a

United S tates Department of Energy project to simulate

coal conversion process es. It has now become a powerful

tool for engineers to model chem ical, powe r generation and

other processes. The work of Young’* entails the modeling

and simulation of AFB C using ASPEN . Herein, the ‘black

Fuel 1998 Volume 77 Number 4 32 7



Circulating fiuidized bed reactors: R . Soutdeh-Gharebaagh et al.

box’ approach with one ASPEN PLUS stoichiometricreactor was used to calculate the mass balances based ongiven combustion and sulfur capture efficiencies. CFBCsimulation work was also initiated at CERCHAR13 toprovide the technical information required for the evalua-

tion and optimization of CFBCs under steady-state con-dition in power generation applications. The study of

pollutant emissions such as SO*, NO, and N20, as well asthe ash composition leaving the CF BC w as not included inthis work; instead several ASPEN PLUS user subroutines

were used in order to study the hydrodynam ic, comb ustionand heat transfer phenom ena in the bed. The approach usedat CERCHAR is similar to that of Young’s, but wasextended to cover CFBCs. Combustion Engineering Inc.14also used ASPEN in modeling a Lurgi circulating fluid bed.The approach, similar to that of Young’s’* used here, has alow level of complexity since the goal was the calculation of

the mass and energy balances for the CFBC. Up to now,modeling of CFBCs using a process simulator (such asASPEN PLUS) has been limited to simple mass and energy

balances, without predictive capabilities.ASPEN PLUS is widely accepted in the chemicalindustry as a design tool because of its ability to simulate

a variety of steady-state processes ranging from single unitoperation to complex processes involving many units.Consequently, ASPEN PLUS w as chosen as a frameworkfor the developmen t of a CFB C process simu lation. Sincethere is no CFBC model provided by ASPEN PLUS, wemust develop our own using the tools offered by ASPENPLus15,16. In addition to its conventional reactor m odels,ASPEN PLUS has the flexibility to allow the insertion ofFortran blocks and user kinetic subroutines into thesimulation.

In this work, several ASPEN PLUS reactor modelsinteract with their corresponding user-written kineticsubroutines to perform calculation during the simulation.This flexible structure of ASPEN PLUS permits handling ofcomplex processes, such as those occurring in a CFBC.Hence, an attempt is made to develop a model whichincludes several features that were neglected or simplified in

the previous studies as outlined above, in order to produce apredictive tool. This paper presents the details of themodeling approaches taken to obtain a process simulationprogramme for coal combustion in a CFBC .

MODELING APPROACHES

In a typical CFBC used for coal combustion, crushed coaltogether with limestone or dolomite and ash particles arefluidized by the combustion air entering at the bottom of thebed and at one or several secondary air injection points. Alarge portion of the bed particles exits the riser of the CFBC

with the flue gas due to the high superficial gas velocitiesutilized. The particles are then separated from the exhaustgas in a gas/solid separator (often a cyclone) and recycledinto the riser to promote complete combustion of the coal.Because coal combustion in a CFBC is directly affected byits hydrodynam ic param eters, both hydrodynam ic andcombustion models must be treated simultaneously toyield a predictive model for the CFB C. The description of

the method followed in developing the hydrodynam ic andreaction models is given below.

Hydrodynamic model

The hydrodynamic model enables the variation of thevoid fraction with height in the riser to be determined. The

32 8 Fuel 1998 Volume 77 Number 4

general hypotheses of the hydrodynam ic model along withthe modeling procedure are presented below.

General hypotheses of the hydrodynamic model. Forsteady state conditions, the assumptions regarding the

hydrodynam ic model are the following:

(1) The CFBC is naturally divided into two hydrody-

namic regions:

(i) a lower region-turbulent fluidized bed (dense bed);(ii) an upper region (dilute bed).

The boundary between the two regions is defined by theheight of the secondary air injection point.(2) There is perfect mixing of solids (individual ash, charparticles and sorbents) in the lower region and in eachzone of the upper region17. This assu mp tion is justified bythe high internal and external recirculation of solids in thebed.(3) Plug flow regime for gas is assumed in the bed. This isconsistent with the results of gas backmixing ex erimentsin the CFB C risers as reported in the literature p7.

(4) The gas velocity througho ut the bed is uniform andconstant for each region of the bed.(5) For a given superficial gas velocity, the mean voidagein the lower region of the CFBC is constant. This assum p-tion is justified by the results of experiments ofChehbouni et al . l8 for group B particles considering thelower region to be operated under the turbulent fluidiza-tion condition.(6) In the upper region of the CFBC, the voidage

decreases with the vertical position along the riser.

M odeling procedure. The model considers that theCFBC is divided into two regions: a dense lower regionwith a constant suspension density (turbulent fluidizedbed) and a more dilute upper region with a decaying suspen-sion density with height. Detail related to the gas-solidstructures chosen to represent two regions of the riser aregiven below:

Lower region of the CFBC. The lower region is fluidizedby the primary air supply. Kunii and Levenspiel’, Saraiva et

al. ‘O ,and Kwaulk et al. ’ ’ treated the lower region of CFBCusing the models developed originally for bubbling flui-dized beds. This is inconsistent with the fact that the gas

superficial velocity in this region is usually higher than acertain critical value, U,, where the region becomes turbu-lent18. At this condition, solid velocity, bubble diametersand velocities are quite different from the bubblingregime7,18. However, for simulation purposes, perfectmixing between the solids and the gas phases is assumedin this region. Under these conditions, the mean voidage ofthe dense region is considered constant and may be obtainedusing the correlation proposed by Kunii and Levenspiel’.

U pper region of the CFBC. The upper region is suspendedboth by the combustion gases from the lower region and thesecondary air supply which determines the boundary

between the two regions. Hydrody namic mod els, as pro-posed in most CFBC literature regarding the upper region,are classified into three broad groups’: (1) those predictingthe axial profile of the solids suspension density but failingto predict the radial variation; (2) those assuming two ormore regions considering either the core annulus or the



Circulating fluidized bed reactors: R. Soutdeh-Gharebaagh et al.

cluster models to predict the radial variation; (3) those

applying the fundamental equations of fluid mechanics to

model gas-solid flow structure. Type 1 and 2 models, which

are lumped models, can be easily coupled with reaction

models to simulate a CFB C reactors. On the other hand,

type 3 models, which are differential models, become rapidly

tedious when coupled to reaction models bec ause of the

numerical complexity.For simulation purposes, we chose to apply the type 1

model to predict the mean axial voidage profile in the upper

region o f the CFBC , assum ing that this region consists of

two zones: an acceleration zone and a fully developed zone.

In the acceleration zone, the axial voidage decrea ses with

the vertical position along the riser’:

e * - E(Z)=e

-az

E* -El(1)

In the lumped modeling approac h used in this work, the riser

will be divided into a discrete number of intervals. Base d on

the void fraction variation in the acceleration zone given by

eqn (I), the mean value of voidage in a certain riser intervalbetween height Zi_l and Zi can be calculated using the

expression proposed by Kunii and Levenspiel’:

1Ei=E* - -&El -e*)(exp-“z -exp-“&-I) i=2,3

(2)

In the fully developed zone, the mean axial voidage is esti-

mated by the following equation:

1E4 = (3 )

1+(PG,

U2Ps

wher e the slip factor Q is19:

+ = 1 + $ + o.47fi4’ (4)r

The variation of void fraction with length in the riser is

illustrated in Figure 1.

Reaction model

The reaction model allows for the determination of the

chemical changes and the heat released during combustion.

Since coal combustion in the CFB C is quite complex, only

the major steps of coal combustion are considered in themodel with some simplifying hypotheses. The general

hypotheses of the reaction model along with the modeling

procedure is presented below.

General hypotheses of the reaction model. Fo r

steady-state conditions, the assumptions regarding the

reaction model are the following:

(I) The coal and limestone are fed into the bottom of the

bed at a uniform temperature” . This is largely encoun-

tered in industrial units operating at high feed rate,

because in these conditions the temperature gradient

within the feed is negligible.(2) Since the time required for volatile combustion is very

short, the devolatilization process is considered instanta-

neous and to take place at the bottom of the bed’.

(3) Char is uniformly distributed throughout the circulat-

ing bed.

(4) Since cha r combustion is slower, it is assume d to

occur a fter all the volatile products have been burned2’.

This is an acceptable hypothesis considering the very

short time required for volatile combustion,

(5) Burning coal particle and gas tempera tures are con-

sidered constant and equal to the bed temperature. This is

a simpliying hypothesis considering the fact that the coal

particle temperature is higher than the temperature in gasmedia.

(6) The contribution of the cyclone and the circulation

loop on the overall combustion process is neglected.

Arena et al5 have considered the cyclone as a reaction

block in their simulation, but due to the small particle

residence time in the cyclone and the lack of excess

oxygen in the recirculation loop, the hypothesis appears

reasonable.

(7) Char particles are assume d to bum with a constant

diameter. This diameter is the mean ch ar particle diam-

eter based on the experimental particle size distribution.

(8) The attrition rate constant for char particles in the

CFB C is smal15. Therefore , the attrition-assisted combus-tion rate is deem ed negligible.

(9) The effects of the primary fragmentation of coal and

the secondary fragmentation of char in the overall c oal

combustion process are neglected5.

(10) Any char particle size reductions caused by ash par-

ticles or the walls of the CFBC are neglected.

Modeling procedure. For simulation purposes, the com-

bustion of coal particles can be modeled using the following

reactions:

(1 )(2 )(3 )

(4 )

devolatiliza tion and volatile comb ustion;

char combustion;

NO, formation;

SO2 absorption.These reaction steps occur in the differ-

ent regions of the riser which can be divided into a

number of individual reactors. To carry out the required

calculations for each of those reactors, ASPEN PLUS

reactor blocks w ere selected and combined in a program

flowshee t representing the CFB C. The CFB C riser is

divided into two regions: a lower region and an upper

region. The lower region is represented by a single

CST R (continuous stirred tank reactor), while in the

upper region a plug flow regime for both gas and

solid phases is assumed. A series of CSTR reactors

are used to simulate the corresponding plug flow

regime in the upper region2’. The number of CST R inseries is determined based on the hydrodynamic

description of the upper region. As mentioned pre-

viously, this region is divided into two zones, a fully

developed zone and an acceleration zone. One CSTR

reactor sim ulates the fully developed zone. Since the

height of the acceleration zone predicted by model is

relatively high with a considerable solid fraction varia-

tion, this zone is then modeled using two CSTR reactors

with a different mean solid fraction for each reactor.

With these arguments, the use of four CSTR reactors

in ASPE N PLU S is justified. Figure I shows the mean

solids fraction (1 - Ei) corresponding to the four reac-

tors (lower region and three sections of the upperregion). The reactors are numbered as 1 for the lower

region reactor and 2, 3 and 4 for the three u pper region

reactors. Description of reaction steps involved in each

reactor, with the corresponding ASPE N PLU S unit

operation blocks, are presented below.

Fuel 1998 Volume 77 Number 4 32 9



Circulating fiuidized bed reactors: R. Soutdeh-Gharebaagh et al.

Reactor number.j................................

I

I

I l-e3I

- - -MepnsolidfktionUinthekm#tion @

I

Upper&on @

I

_--

1 l-e,) ;I

ILowetregion 0

III + b

Solid fraction,(1 )

Figure 1 Variation of void fraction with height in the riser

Devolatilization and volatile combustion. When coal isintroduced into a CFBC, it decomposes into two parts:hydrogen-rich volatile and char. The char rema ins in the

bed and burns slowly. Based on the plume model, coaldevolatilization and complete comb ustion of the volatileoccur at the feed entry poin t23. Two steps will then be con-sidered in the simulation : decompo sition and volatilecombustion.

Decom position. In this step, coal is converted into itsconstituting com ponents such as carbon, hydrogen, sulfur,nitrogen and ash. This step occurs in the lower region reac-tor only and RYTELD (ASPEN PLUS yield reactor) is usedto model this process by speciying the yield distributionvector according to the coal ultimate analysis.

Volatile comb ustion. To simulate the volatile comb us-tion step, three reactions are considered in the model:

c+$**co

s+o**so*

H2 + $02 * H20

These reactions occur in the lower region reactor only wherethe coal is introduced and RSTOIC (ASPEN PLUS Stoi-chiometric reactor) is used to model the volatile com bustionprocess. The combustion of the volatile matter is based onthe following hypotheses:

(i) Considering that the volatile matter (VM) in the coal,(obtained from a proximate analysis) cons ists exclusivelyof carbon, hydrogen and sulfur, the fraction of total coalcarbon associated to volatile combustion is given by X, =VM - H - S, where H and S are the fraction of hydrogenand sulfur in the coal. This supposes that the entire

hydrogen content of the coal is found in the volatilematter. The volatile carbon fraction (X,) reacts to formCO only during the volatile combustion process becauseof the oxygen depletion in the lower region of the riser.(ii) The coal hydrogen content is entirely consumedduring the volatile combustion process.

(iii) The coal sulfur content is assumed to be converted

completely to SO2 during the volatile com bustion proce ss.

Char combustion kinetic model. The char particles result-

ing from the devolatilization process co nsist of the remain-ing carbon fraction (1 - X,) and ash only. These particlesare then burned to produce a mixture of CO and CO *. Threemain reactions for char combustion are considered here 24:

c+@**co

c+co*=+2co

These reactions occur in the entire riser, hence in the fourCSTR reactors, and RCSTR (ASPEN PLUS CSTR reactor)is used to model this process. This block requires the knowl-edge of the reaction kinetic mod el which is presented below.

The first and third reactions are heterogeneous andthe second is hom ogeneous. Since the temperature of theburning particles in the CFBC is not sufficiently high, theeffect of the third reaction on the combustion rate is 10w*~,and this reaction is neglected in the model. For theremaining two reactions, the reaction rate expressions

mu st be developed. Th e first reaction is a gas-solid reactionand the chemical changes take place on both the externaland the internal surface of the char particles’. The following

expression for the char combustion rate, to form CO, perunit volume of the ith interval is obtained25.

rl,i =

3v02kcrFchar, i( 1 - fi)

PcharrC( 1 - EC)Fsolid, iCO,

k, , can be expressed by an Arrhenius form as follows:

k,, = kolexp

(5 )

The following equation is used to calculate the mean p ar-ticle radius based on the experimen tal particle size

distribution:1

rc= m (7)

t r,(k)

Carbon monoxide produced during the heterogeneous com-bustion of char reacts with 02 in the homogeneous gas phasereaction to form CO* . Factors con tributing to CO emissionlevels are the bed temperature, 02, C O and Hz0 concentra-tion. The following ex ression is used for the CO comb us-tion rate in the model 2g

:

rco,i= 1.18 10’3f&6~~o()exp

x (-F)CEi (8 )

NO, formation. Staged combustion remains an attractive





method for reducing NO , emissions in various combustion

systems. For staged com bustion in a CFB C, the air used for

the combustion is divided into two or more strea ms: the first

one is supplied through the bottom air distributor and sec-

ondary air streams are injected in the upper region of the

riser. NO , formations in combustion process es result from a

combination of a thermal generation process and fuel nitro-

gen oxidation. At very high tempera tures, thermal genera-tion of NO , from the air nitrogen becom es very important,

while at low temperatures found in a CFB C, the dominant

source of NO , is fuel nitrogen oxidation. It is also important

to empha size the complexity involved in NO , chemistry

within the CFB C because of many catalytic reactions invol-

ving, for exam ple, char, ash and sorbent particles.

V205 , is often present in fly ash from heavy oil combustion,

for example. However, in our case such an element was not

present and it is therefore correct to assume negligible SO2

to SO3 conversion.

Therefore , the total NO , formation in the CFB C can be

calculated by the following formula w hich considers the

thermal generation and fuel nitrogen oxidation as detailed

below:

]N0,1,,,,1 = ]NOx]thermar+ ~1WQlfuel

where

Since CaC Os is unstable under CFBC conditions, the

calcination process is assum ed to occur instantaneously and

completely in the lower region of the bed. The second

reaction, representing the SOi capture in the riser, isconsidered to occur in both the lower and the upper regions.

The corresponding fractional conversion for SO2 (Xso,, i) is

calculated using a Fortran program according to the CaO

conversion model presented below. RS TOIC is then used to

model the capture of sulfur in the riser from the calculated

value Of Xso*,i. The fractional conversion of CaO to CaS 40

is strongly affected by the physical and chemical properties

of limestone, hydrodynam ic param eters, mass transfer

resistance, temperature, rea ctive concentration, particle

size distribution (PSD ) and operatin%

conditions, and can

be calculated according to Couturier from the following

expression:

cy, = Overall fuel nitrogen to NO , conversion factor

X(O<cr, < 1)

Vcaox -CaO,i’ I __,

‘I--

3aCYso,,i KV

(9 )

Thermal generation ([NO ,] thermal). Three main reactions

are used to represent this process in the model:

;N~+&*NO

Considering the ideal gas law for the combustion products

in the riser, the total gas concentration is express ed by:

PC=---.-

RlTb(10)

The values of parameters a I and a! may be written as*‘:

N2 + &02 * N20 a, = 3.33 * 10e4eyR\

REQU IL (ASPEN PLUS equilibrium reactor) is used to

predict th e amount of thermal NO , formed during coal com-

bustion based on equilibrium conditions considering the

nitrogen present in the riser.

CY 35D”.3P

Using eqn (9), the moles of SO2 remove d per unit v Ol

becomeFuel nitrogen oxidation ([NO ,] fuel). The NO, formation

via fuel nitrogen oxidation is modeled using the following

overall reaction:

VCaOFlrso~*i1 _ E,Au * 100

RSTO IC block is used to calculate the fuel nitrogen oxida-

tion with a given value of o, which is taken from the litera-ture. The NO , formation calculations are then applied in

each of the four reactors using the combined REQ UIL and

RSTOIC blocks.

X

I

3aCYS02, - j&

SO 2 absorption. The SO2 capture by limestone can be

represented by the following ureactions:

If the sorbent particles in the bed are well mixed, their

residence time is independent of particle size*‘. Therefore ,

the mean residence time is express ed by

CaCOs =+ CaO + CO2 ALI?I =W%p-

6

CaO + SO2 + & 3 CaS04and

The forma tion of SOI to SO3 is assume d to be instanta-

neous. In CFB C reactors, the maximum concentration ofSO3 is governed by the thermodynamic and kinetic consid-

erations. Equilibrium conditions predict a certain conver-

sion of SO 2 to S03. How ever, kinetic considerations

predict very a small reaction rate without the presence of

a catalyst for the SO2 to SO3 conversion. Such a catalyst,

where

3acyS02,i1 + (e”” - 1)

R,Kv

RS 1

1 +3oCYsoz,,

R& v(e”” - 1)

(11)

(12)

lume

(13)

(15)

(lo)

Fuel 1998 Volume 77 Number 4 331




RCSTR

REQUIL

Table 1 The reactor models description utilized in the

simulation’5”6

Reactor block

RYIELD

RSTOIC

Description

To simulate a reactor by speciying

yield distribution data or correlation

when reaction stochiometry and

kinetics are unknown

To simulate a reactor with theunknown or unimportant reaction

kinetic and Imown stoichiometry by

speciying the extent of reaction or the

fractional component of the key

component

To handle any number of simulta-

neous or series reactions

To model CSTR reactors with known

reaction kinetic

To require user supplied kinetics

subroutine when solids, such as char,

are participating in the reactions

To calculate simultaneous phase

chemical equilibrium by solving

stoichiometric chemical and phase

equilibrium equations

To be convenient to the known reac-

tion stoichiometry when only a few

reactions approach equilibrium

Table 2 The reactor model parameters utilized in the simulation

Once o,, i is calculated for the ith reactor, it is used as the

fractional conversion of the key component necessary to run

the RSTO IC block corresponding to this reactor. Calcula-

tions start at the first reactor (lower region) and continue

upward until the top of the riser.

SIMULATION RESULTS AND MODEL VALIDATIONThe process simulation program for a CFBC was developed

using four ASPEN PLUS reactor blocks-KYIELD,

RSTOIC, RCSTR and REQUIL-to represent the phenom-

ena identified in the coal combustion process. A detailed

description of the ASPE N P LUS reactor blocks along with

their re uirement is given in the ASPE N PLU S user

manualsq5,t6. A brief description of the reactor blocks is

given in Table 1. Table 2 presents the reactor m odel

param eters and input variables required for the simulation.

Considering the reaction and hydrodynamic models, the

entire CFB C system is divided into three sub-flowsheets:

lower, upper and sep-flowshee ts. The first sub-flowsheet

represents the dense region, the first reactor of the riser,where the phenomena associated with coal devolatilization

and volatile combustion, char combustion, NO , formation,

limestone calcination and SO2 capture take place. The

Phenomena

(I) Devolatilization and volatile combustion

(2) Char combustion

(3) NQ formation

(4) SO* capture

Reactor block Input variables

(1) RYIELD Tbr p. Fti)

(2) RSTOIC Tb, P, Xc, Xn, Xs chemical reactions

(3) RCSTR Ts, P, ri,,, rco,, chemical reactions

(4) REQUIL Tb, P chemical reactions

(5) RSTOIC Tbr p, &CO, 3 X,o,. i chemical reactions

Since SO2 is well m ixed in each interval of the bed, an

overall S O* balance gives

Ysoz, =

-5 @so,,- rso,, )

GUI(17)

and

YSO*,iWR so,,i - rso2,J i z 1

cu2

(18)

where

(19)

and

R(1 - Xso,, i - IYc ws

so*,i =32AAL

i#l (20)

The value of Yso2, is calculated by simultaneous solving of

eqns (13) and (17) for i = 1, or eqns (13) and (18) for i f 1.

The fractional sulfur capture for each reactor (Xs o,J can

then be calculated from

(21)

&02.i=1- [ Fc,j$fzi_,J i# 1 (22)

second sub-flowsheet represents the dilute region of the riser

which is divided into three intervals; each one represented

by an individual reactor. Since the devolatilization and

volatile combustion process is assume d to take place

exclusively in the lower region, only char combustion,

NO , formation and SO2 capture are considered to occur in

each reactor of the upper region. The last sub-flowsheet

contains two unit operation blocks, CYC LO NE and

FSPLIT. CYC LON E is used to represent the gas/solid

separation at the riser outlet. To maintain the required level

of solid inventory in the bed, a solid drain valve, represented

as FSPLIT in ASP EN PLU S, is used. The resulting solid

stream from CYC LON E is fed to FSPLIT where it isdivided into two streams; the first one is recycled into the

lower region of the riser and the second one exits the system

to satisy the material balance.

Along w ith the unit operation blocks provided by ASP EN

PLU S, several complete Fortran program s and an external

subroutine for the kinetic models were used in the

simulation. The Fortran code s contain the following four

blocks which are required to complete the model. The first

block, ‘KINET IC’, is the external kinetic subroutine

developed for both heterogeneous gas/solid and homoge -

neous gas phase reactions. The second Fortran block, called

‘KESTIME’, when integrated with the ASPEN PLUS input

file, calculates the residence time of char particles in theCFB C. T his calculation is required in order to execute the

‘KINET IC’ subroutine. The third block, ‘HYD RO ’, is

inserted into the ASP EN PLU S input file to calculate the

mean void fraction in each section of the upper region and in

the dense bed of the riser. The fourth block of Fortran codes,





B2 ~2, B3 &B4s4

- BSRl

1 :

I-HYDRODYNAMIC AND KINETIC M OD ELS

mdro,IMtimcdKilldics~)

aBl7

s2n B16 #s&y$1 ‘I B9 z BS

-

FSPLITS1Q

CYCLOM R4 RSm1c - REQUIL

??All squareblocksarc givenby ASPEN PLUS

Figure 2 A comprehensive simulation diagram for the CFBC

Table 3 Value of the fixed parameters used in the simulation

Parameter Source

El = 1.247*10* (J kmole-‘)

kO, = 1.55*10’ (m s-‘)

Kv = 8*10e4 [kmole (m’ s-‘)-‘I

Vcao = 1.69*10-’ (m’ kmole-‘)y = - 0.0226

Ec = 0.30, E, = 0.52

pchar = 1500 (kg m-‘)

pL = 2710 (kg m-‘)

ps = 800 (kg m-‘)

CFBC geometery A = 0.13 m2

L = 6.7 m

Dr = 0.405 m

Gordon and Amundsonz4

Gordon and Amundsot?

Couturie?

Wang’Couturier”

Wong6

Wang’

Wang’

Gordon and Amundson24

Desai et a1.27

Table 4 List of the input and output variables of the model

Input variables Output variables

Cross-sectional area (A)

Height of the bed (L)

Height of the dense bed (L,)Superficial gas velocity

Solid circulation flux (Gs)

Temperature (Tb)

Pressure (P)

Coal (feed rate, PSD, analysis)

Limestone (feed rate, Ca/S, PSD)

Air (flowrate, SIP, composition)

Kinetic constants, physical

properties and fixed parameters

value

Combustion efficiency

Sulphur capture efficiency

NO, and CO emission levels

Outlet O2 concentration and flow rate

Hydrodynamic parameters (interval

positions and voidages)

02 and CO concentrations profiles

Outlet gas stream composition and

flowrate

Outlet solid stream composition

and flowrate

‘SO2’, calculates the sulfur capture efficiencv in each

section of the upper region aid in the dense bed. Acomprehensive simulation diagram for the CFB C is

illustrated in Figure 2. Table 3 gives th e value of the

param eters used in the simulation. The input and output

variables of the proces s simulation program are summ arized

in Table 4.

Table 5 The CFBC operating conditions”

Run Data

Th 6) Fcoai

(kg h-‘) (k;‘?)

cab F,,, SIP LI (m)

(kg h-‘)

1 1140 67.30 19.20 2.28 799.0 0.45 1.37

2 1106 70.20 15.90 1.70 832.00.43 2.59

3 1146 64.60 13.20 1.61 778.0 0.43 2.59

4A 1155 74.90 23.50 2.38 807.0 0.43 1.37

4B 1155 62.10 16.10 1.97 796.0 0.43 1.37

5 1187 61.20 17.60 2.13 757.0 0.42 2.59

6 1180 60.40 16.60 2.07 749.0 0.41 2.59

7 1192 65.10 17.90 2.10 768.0 0.85 1.37

8 I183 63.30 17.80 2.10 773.0 0.85 1.37

9 1155 66.60 17.90 2.10 792.0 0.86 1.37

10 1152 66.30 18.10 2.06 791.0 0.86 2.59

11 1109 70.00 18.20 1.91 836.0 0.84 2.59

12A 1105 69.80 18.00 2.08 734.0 0.85 1.37

12B 1104 70.00 18.70 2.15 831.0 0.85 I 37

Simulation convergence

One important feature of a CFB C is the recirculation of

solids, captured by the cyclone at the top of the riser and

recycled back to the base of the riser. Therefore , the

simulation flowshee t, wh ich contains the recycle loop, must

be solved iteratively and the tear streams , convergence

metho ds and calculation sequence must be specified.

ASP EN PLU S can perform all these functions automatically

or the user can supply them . To achieve convergence of the

recirculation stream in the simulation, we used the classical

bounded We gstein m ethod, which normally converges

rapidly15. It should be mentioned that for convergence to

occur, the value of the tear stream variables needs to be

corre ctly initialized. Such an initialization will enable arapid convergence of the tear streams. Thus, to initialize the

tear stream variables (see Figure 2), a greater G, value was

considered for the initial tear stream flux. Since the char

conversion during one pass in the riser is less than lo% , the

initial amount of char in the tear stream was considered





Q8

90 92 94 Q8 Q8 100

Expeflmental Combustion Eff i c i ency W

Figure 3 Betw een the predicted and experimental combustion

efficiency

35 0

30 0

25 0

g 200

8

t

15 0

k10 0

50

0 L

0 Equation24 (proposed orrelation)

0 Equation8 (original orrelation)

0 50 100 150 200 250 300 350

mntal co @pm)

Figure 4 Comparison between the predicted and experimental

co

approximately 10 times the carbon fraction in the coal feedrate. This procedure ensured a rapid and stable convergenceprocess.

Model validation

In order to validate the proposed model, 14 different setsof operating data from various CANMET runs*’ were usedto validate the simulation (see Table 5). A detailed

description of the CANME T 0.8 MW th CFBC pilot plantis presented by Desai et ~1.~~.The predicted simulationresults in terms of comb ustion efficiency, emission levels ofCO, SO 2 and NO, and 2O and CO concentration profiles arecompared with the experimental data. The results aredetailed below.

!z1000

B2 800

F.fi 800

40 0

0 I I 1 I I I I

0 200 400 800 800 1000 1200 1400

Waa so2 @pm)

Figure 5 Comparison between the predicted and experimental

so 2

Combustion eficiency. In order to estimate combustionefficiency, the two outlet streams, S 20 and S22 (seeFigure 2), are used. These streams contain small amountof unburnt char particles that controls the combustionefficiency (vc), wh ich is defined as

vc=l-(

Total rate of carbon in the outlet stream

Total rate of carbon in the feed stream 1

(23)Thirteen sets of expenmental data reported values for the

comb ustion efficiency were used to compare with the modelpredictions. In Figure 3, it is found that the model consis-tently overpredicts the combustion efficiency. The differ-

ences between the predicted values and experimental data,which are less than 3%, are related to the value calculatedfor the cyclone efficiency. ASPEN calculated 99.99% effi-ciency for the cyclone used in the pilot plant, which isgenerally higher than that reported experim entally. Conse-quently, the carbon content of the fly ash predicted by themodel becomes substantially smaller than the experimentalvalue. This smaller amount of carbon in flyash causes the

model to overestimate the combustion efficiency.

CO emission levels. Although CO combustion rateshave been widely studied1*6 *24,2 the extension of theseexpressions to CFB C conditions is limited. The validity ofthe proposed CO combustion rates from the literature wasexamined by inserting them into the simulation program.Following the simulation res ults, a new correlation, similarto the Robinson’s expression26, with two adjustable par-ameters wa;prop osed to predict the CO range reported byDesai et al. :

with(24)

/3, = 1.8 * lOI

p* = 0.21





These parameters were obtained by fitting the CANMET

experimental data. However, more data from varioussources are required in order to confirm this correlation.

The CO emission levels predicted by the model rangedfrom 119 to 271 ppm, while the experimental data variedbetween 112 and 3 16 ppm. Therefore, the model based onthe new correlation estimates CO emission levels relatively

well. The comparison between predicted and experimentalCO levels is presented in Figure 4.

SO* emission levels. The Ca/S molar ratio rangedbetween 1.6 and 2.3 for the various runs considered. Suchvalues of Ca/S are usually considered sufficient to achievereasonably high sulfur capture efficiencies. Without using

any fitting parameters, Figure 5 shows the close agreement

ii‘: 200

8

%150

%100

0

0 50 100 150 200 250 300 350

Ew=t m~t NO, ( ppm

Figure 6 Comparison between the predicted a nd experimental

NO ,

observed between p redicted an d experimen tal SO* concen-tration in the flue gas.

NO, emission levels. As mentioned earlier, two forma-tion mechanisms were considered in the modeling of NO,formation in the CFBC: thermal generation and fuel nitro-gen oxidations. Th ermal generation was calculated con-

sidering equilibrium conditions, while NO, formationfrom fuel nitrogen oxidation was calculated using an overallconversion factor (cY~)rom the literature. Since the aim ofthis work was not to study the NO, formation and reductionprocesses in detail, this overall approach was taken to simu-late the fuel nitrogen oxidation. Fuel nitrogen conversionentails relatively complex reactions schemes involving sev-eral heterogeneous reaction step s and therefore attains alower overall conversion26. Typical values of fuel N2 toNO , conversion factors, as reported by Legros et CZZ.*~ndBecker et d3’, vary approximately between 0.05 to 0.25,

depending on coal properties, feed particle size distribution,excess air level and operating conditions. In our simulation,

a value from that interval, which gave the best agreementbetween predicted and experimental NO, was chosen as theoverall conversion factor. As reported in mos t CFB Cliterature, the results also confirmed that thermal NO, for-

mation, which leads to between 18 and 65 ppm of NO,, isunimportant compared to that of fuel nitrogen oxidationwhich approximately lies between 84 and 104 ppm ofNO ,. The predicted em ission levels ranged from 130 to267 ppm, while NO, emissions for the experimentalCFBC ranged from 107 to 309 ppm.27. Figure 6 appearsto indicate a reasonable agreemen t between predicted andexperimental NO,. The difference between experimentaldata and those of the simulation prediction is attributable

to the fact that a constant value of CY= 0.05 is used through-out the entire simulation.In recent years, several compreh ensive studies have been

reported6.3 -34 regarding NO, formation and reduction

processes. These were conducted to develop an improvedunderstanding of the fundamental nature of NO, chemistryand underlying physical processes in CEB Cs, and to supportthe needs for experimental work in this field. Em phasis is

9

6- 08 Redi *edCOConcart rati on( ppm 2500 p

. g 8- -

E 2000 i

! '

E

- 1500 E-

0"0 - 1000 "

2-

.

l -* . -. . Q 500

_ ..0 . - . . . . _ . _ . .

I I I I I I 00

0 1 2 3 4 5 6

BedHei ght

Figure 7 O2 and CO concentration profiles within the CFB C predicted by the model





also given to develop reliable techniques to control NO,emissions from fluidized bed combustion. For example,many published studies suggest that NO, emissions could becontrolled by adding chemical comp onents such as carbonmonoxide, char hydrogen, ammonia, unburnt hydrocarbonsand limestone due to the catalytic reactions found inCFBC32V33.

02 and CO concentration profiles. The emission data

from the CANMET CFBC pilot plant used to validate themodel co nsisted only of flue gas concentration and did notinclude the various gas concentration profiles along theriser. However, since the model can predict these concen-

tration profiles within the riser, oxygen and carbon monox-ide profiles were chosen to be compared qualitatively withdata from the literature. The overall trends observed inexperimental concentration profiles along the riser heightare in close agreement with those predicted by the model.In the lower region a significant change in the oxygen con-centration is found, while in the upper region there is a

gradu al decrease in the oxygen con centration. The COconcentration is constantly high in the lower region, wh ileit sharply decreases in the upper region due to the injection

of secondary air. Typical O2 and CO concentration profilesprovided by Hansen et a1.35and experimental data reportedby Brereton et al .33 and Grace et al .34 are similar with those

predicted by the present model. In Figure 7, the predictedprofiles are presented.

Due to the relatively high dense b ed found at the bottomof the CFBC reactor, Brereton et al.33 and Grace et aZ.34

have not measured the O2 and CO concentrations. There-fore, the experimental data have only been reported for theupper region. Since the operating and bed design datareported in those references differ with that of CA NM ET,the predicted concen tration profiles have been comparedqualitatively with the trend reported in these references.

CONCLUSION 10

A mod el was developed for the comb ustion of coal in acirculating fluidized bed using the ASPEN PLUS simulator.To provide such a CFBC model, several ASPEN PL US unitoperation blocks were combined and, where necessary,kinetic expressions and hydrodynam ic model were devel-oped using data and models from the literature. Thedeveloped models were then inserted into the flowsheet toprovide a complete representation of the CFB C. The

resulting m odel was used to predict the performance ofthe CANMET CFBC pilot plant in terms of combustionefficiency, emission levels of CO , SO2 and NO,, an d 02 andCO concentration profiles. The predictions of CO and NO,were achieved using two and one fitting parameters,respectively. The agreement between the model prediction

and experimental data is satisfactory b ut more experim entaldata are still required to confirm the proposed CFBC modelin order to mak e it more com prehensive and reliable. Themodel can now be used to represent a CFBC unit in variousprocess simulation flowsheetsplants.

such as power generation

ACKNOWLEDGEMENTS

This project was supported by CANMET, part of Energy,Mines and Resources, Canada. This financial assistance isgratefully acknowledged. Special thanks are due to the

Ministry of Culture and Higher Education of Iran for

providing a scholarship to Mr R. Sotudeh-Gharebaagh.Helpful discussions from F. Preto and E. J. Anthony are alsoappreciated. We greatly acknowledge Aspen Technology

for having granted special permission for the use of theASPEN PLUS under the condition of the academiclicensing agreement.

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1

2

3

4

5

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7

8

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NOMENCLATURE

cCd?

CO?DPD,El

W)F,,‘

cross-sectional area of bed (m’)

decay constant (m-l)

parameter defined in eqn (9) (s-l)

unit operation blocks given by ASPEN PLUS, Figure2, (I = 1, 2,

3,...,17)

combustion gas concentration (kmole m-‘)

calcium to sulfur ratioconcentration of oxygen (kmole m -‘)

average sorbent surface particle diameter (cm)

riser diameter (m)

apparent activation energy (J kmole-‘)

yield distribution vector

mass Rowrate of air (kg h-‘)

Fat,

Fwr-2

Fc

FdlK,

FL-,,,

Fl

F lhnr

F \ “ld.,

fco

fH!O

fo2

F,

F*

G,

IK”

kolk

LL’

LI

P

P(k)

R

RIRI

4

RS

Rso,.,

rc

rc(k)

YC0.I

r1.r

rso,

SJ

SIP

Tb

T,

t,

Ul

u2

UC

vc.0

W,

xc

XH

xs

&co,

&Tao,,

Xso,. ,YSO?.,Z

Z,

Z,-I

primary air mass flowrate (kg h-‘)

secondary air mass flowrate (kg h-‘)

mass flowrate of coal (kg s-‘)

flux of the char particles entering the ith interval (kg s-‘)

mass flowrate of coal (kg h-‘)

mass flowrate of limestone in the feed (kg s-‘)

mass flowrate of limestone in the feed (kg h_-‘)

flux of solids entering the ith interval (m- s ‘)

mole fraction of CO

mole fraction of Hz0

mole fraction of O2

Froude number

particle Froude number

net solids circulation flux (kg mm2 riser s-‘)

reactor number i

volumetric rate constant [kmole (m’ s-‘)-‘I

pre-exponential factor (m s-‘)

chemical reaction rate constant (m s-‘)

height of the bed (m)

height of dense bed (m)bed pressure (atm)

weight fraction vector of char particles in the recirculation stream

universal gas constant [kcal (kmole K-‘)-‘I

universal gas constant [atm cm-3 (gmole K-‘)-‘Ifour reactors representing the riser (I = 1,2,3,4)

universal gas constant [J (kmole K-l)]mean sorbent particle radius (cm)

rate of SO2 per unit volume of the ith interval (kmole me3 s-‘)

mean coal particle radius (m)

coal particle radius vector (m)

CO combustion rate per unit volume of the ith interval

[kmole (m’s_‘)-‘]

char combustion rate per unit volume of the ith interval

[kmole (m’ s-‘)-‘I

mole of SO2 removed per unit volume of the ith interval [kmo-

le (m’ s-‘)-‘I

stream number, Figure 2 (J = 1,2,3,. .,22)

secondary to primary air ratio

bed temperature (K)temperature of the char particles (K)mean residence time of sorbent particles in ith interval of the bed (s)

superficial gas velocity (m s-‘)superficial gas velocity in the dilute bed (m 8)

onset of the turbulent regime (m s-‘)

molar volume of CaO (m’ kmole-‘)

sulfur weight fraction in the dry-based coal

fractional conversion of carbon in the volatile combustion

fractional conversion of hydrogen in the volatile combustion

fractional conversion of sulfur in the volatile combustion

fractional conversion of CaS04 in the dense bed

fractional conversion of CaO in the ith interval

fractional sulfur capture in the ith interval of the riser

mole fraction of SO2 in the ith interval

riser height (m)

corresponding distance for the ith interval above the lower region

(m)

corresponding distance for the i - I h interval above the lower

region (m)

Greek letters

external mass transfer coefficient (cm s-‘)

overall N? to NO I conversion factor

parameter used in eqn (I 1) (cm-‘)

hight of the ith interval (m)

volume fraction occupied by sorbent particles

char porosity

porosity of particle after the calcination

mean voidage of the lower region

mean voidage of the fully developed zone

mean axial voidage in the ith interval of the riser

axial voidage in the acceleration zone

axial voidage at saturated conditionscombustion efficiency

effectiveness factor

density of char particles (kg mm3)

density of limestone particles (kg m-‘)

density of bed solids particles (kg m-j)

slip factor


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