Simulation of Circulating Fluidized Bed Reactors Using ASPEN PLUS

ELSEVIER PII: SOO16-2361(97)00211-l

Fuel Vol. 77, No. 4, 327-337, 1998 pp. 0 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0016-2361/98 $19.00+0.00

Simulation of circulating fluidized bed reactors using ASPEN PLUS

R. Sotudeh-Gharebaagh, R. Legros*, J. Chaouki and J. Paris Department 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 (CFBC). The proposed model integrates hydrodynamic 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 ASPEN PLUS reactor models and several subroutines. The developed subroutines were 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 CANMET 0.8 MWth CFBC pilot plant. 0 1998 Elsevier Science Ltd. All rights reserved.

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

INTRODUCTION

Circulating fluidized bed combustors (CFBCs) are considered as an improvement over the traditional methods associated with coal combustion. The CFBC exhibits several advantages over conventional coal combustion methods, especially when high sulfur coal is used’. Operation of CFBCs 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 characteristics assure an ever-increasing number of successful commer- cializations of CFBC in power generation applications. Although CFBC technology is becoming more common from these commercial applications, there are some significant uncertainties in predicting their performance in large-scale systems.

Technical knowledge about design and operation of CFBC is widely available for pilot plant and large scale units. However, little has been done in the field of mathematical modeling and simulation of combustion in CFBCs. This might be attributed to the fact that the combustion process occurring in a CFBC involves complex phenomena including chemical reactions, heat and mass transfer, particle size reduction due to combustion, attrition, fragmentation and other mechanisms, gas and solid flow structure, etc. Weiss et al.* introduced a CFBC model by dividing it into 11 blocks, each corresponding to a CSTR reactor for both gas and solid phase. Five of these blocks related to the CFBC riser. Basu et aL3 developed a CFBC model in which a plug flow regime for both the gas and solids is assumed. Lee and Hypanen4 presented a CFBC

*Corresponding author

model which considers the riser as a plug flow reactor for the gas phase and a CSTR reactor for the solid phase. The model also 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 CFBC riser into four blocks, each corresponding to a separate reactor. Three of these blocks related to the CFBC riser. The hydrodynamic parameters were considered uniform within each section and were used in various kinetic models to predict char conversion. Wong6 proposed a model for the hydrodynamics of CFBC risers to characterize the effect of the internal flow structure within the riser, the particle size distribution and the operating conditions on CFBC 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 circulating 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 CFBC riser and to develop mathematical 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 the Massachusetts Institute of Technology (MIT) under a United States Department of Energy project to simulate coal conversion processes. It has now become a powerful tool for engineers to model chemical, power generation and other processes. The work of Young’* entails the modeling and simulation of AFBC using ASPEN. Herein, the ‘black

Fuel 1998 Volume 77 Number 4 327

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

box’ approach with one ASPEN PLUS stoichiometric reactor was used to calculate the mass balances based on given combustion and sulfur capture efficiencies. CFBC simulation work was also initiated at CERCHAR13 to provide the technical information required for the evaluation and optimization of CFBCs under steady-state condition in power generation applications. The study of pollutant emissions such as SO*, NO, and N20, as well as the ash composition leaving the CFBC was not included in this work; instead several ASPEN PLUS user subroutines were used in order to study the hydrodynamic, combustion and heat transfer phenomena in the bed. The approach used at CERCHAR is similar to that of Young’s, but was extended to cover CFBCs. Combustion Engineering Inc.14 also used ASPEN in modeling a Lurgi circulating fluid bed. The approach, similar to that of Young’s’* used here, has a low 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 as ASPEN PLUS) has been limited to simple mass and energy balances, without predictive capabilities.

ASPEN PLUS is widely accepted in the chemical industry as a design tool because of its ability to simulate a variety of steady-state processes ranging from single unit operation to complex processes involving many units. Consequently, ASPEN PLUS was chosen as a framework for the development of a CFBC process simulation. Since there is no CFBC model provided by ASPEN PLUS, we must develop our own using the tools offered by ASPEN PLus15,16. In addition to its conventional reactor models, ASPEN PLUS has the flexibility to allow the insertion of Fortran blocks and user kinetic subroutines into the simulation.

In this work, several ASPEN PLUS reactor models interact with their corresponding user-written kinetic subroutines to perform calculation during the simulation. This flexible structure of ASPEN PLUS permits handling of complex processes, such as those occurring in a CFBC. Hence, an attempt is made to develop a model which includes several features that were neglected or simplified in the previous studies as outlined above, in order to produce a predictive tool. This paper presents the details of the modeling approaches taken to obtain a process simulation programme for coal combustion in a CFBC.

MODELING APPROACHES

In a typical CFBC used for coal combustion, crushed coal together with limestone or dolomite and ash particles are fluidized by the combustion air entering at the bottom of the bed and at one or several secondary air injection points. A large portion of the bed particles exits the riser of the CFBC with the flue gas due to the high superficial gas velocities utilized. The particles are then separated from the exhaust gas in a gas/solid separator (often a cyclone) and recycled into the riser to promote complete combustion of the coal. Because coal combustion in a CFBC is directly affected by its hydrodynamic parameters, both hydrodynamic and combustion models must be treated simultaneously to yield a predictive model for the CFBC. The description of the method followed in developing the hydrodynamic and reaction models is given below.

Hydrodynamic model The hydrodynamic model enables the variation of the

void fraction with height in the riser to be determined. The

328 Fuel 1998 Volume 77 Number 4

general hypotheses of the hydrodynamic model along with the modeling procedure are presented below.

General hypotheses of the hydrodynamic model. For steady state conditions, the assumptions regarding the hydrodynamic model are the following:

(1) The CFBC is naturally divided into two hydrodynamic 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 the height of the secondary air injection point. (2) There is perfect mixing of solids (individual ash, char particles and sorbents) in the lower region and in each zone of the upper region17. This assumption is justified by the high internal and external recirculation of solids in the bed. (3) Plug flow regime for gas is assumed in the bed. This is consistent with the results of gas backmixing ex eriments in the CFBC risers as reported in the literature p7 . (4) The gas velocity throughout the bed is uniform and constant for each region of the bed. (5) For a given superficial gas velocity, the mean voidage in the lower region of the CFBC is constant. This assumption is justified by the results of experiments of Chehbouni et al. l8 for group B particles considering the lower region to be operated under the turbulent fluidization condition. (6) In the upper region of the CFBC, the voidage decreases with the vertical position along the riser.

Modeling procedure. The model considers that the CFBC is divided into two regions: a dense lower region with a constant suspension density (turbulent fluidized bed) and a more dilute upper region with a decaying suspension density with height. Detail related to the gas-solid structures chosen to represent two regions of the riser are given below:

Lower region of the CFBC. The lower region is fluidized by the primary air supply. Kunii and Levenspiel’, Saraiva et al. ‘O, and Kwaulk et al. ’ ’ treated the lower region of CFBC using the models developed originally for bubbling fluidized beds. This is inconsistent with the fact that the gas superficial velocity in this region is usually higher than a certain critical value, U,, where the region becomes turbu- lent18. At this condition, solid velocity, bubble diameters and velocities are quite different from the bubbling regime7,18. However, for simulation purposes, perfect mixing between the solids and the gas phases is assumed in this region. Under these conditions, the mean voidage of the dense region is considered constant and may be obtained using the correlation proposed by Kunii and Levenspiel’.

Upper region of the CFBC. The upper region is suspended both by the combustion gases from the lower region and the secondary air supply which determines the boundary between the two regions. Hydrodynamic models, as proposed in most CFBC literature regarding the upper region, are classified into three broad groups’: (1) those predicting the axial profile of the solids suspension density but failing to predict the radial variation; (2) those assuming two or more 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 CFBC reactors. On the other hand, type 3 models, which are differential models, become rapidly tedious when coupled to reaction models because 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 of the CFBC, assuming that this region consists of two zones: an acceleration zone and a fully developed zone.

In the acceleration zone, the axial voidage decreases with the vertical position along the riser’:

e * - E(Z) =e

-az E* -El

(1)

In the lumped modeling approach used in this work, the riser will be divided into a discrete number of intervals. Based on the void fraction variation in the acceleration zone given by eqn (I), the mean value of voidage in a certain riser interval between height Zi_l and Zi can be calculated using the expression proposed by Kunii and Levenspiel’:

1 Ei=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:

1 E4 = (3)

1+ (PG,

U2Ps

where 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 CFBC is quite complex, only the major steps of coal combustion are considered in the model 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. For 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 circulating bed.

(4) Since char combustion is slower, it is assumed to occur after 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 temperatures are considered 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 gas media. (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 assumed to bum with a constant diameter. This diameter is the mean char particle diameter based on the experimental particle size distribution. (8) The attrition rate constant for char particles in the CFBC is smal15. Therefore, the attrition-assisted combustion rate is deemed negligible. (9) The effects of the primary fragmentation of coal and the secondary fragmentation of char in the overall coal combustion process are neglected5. (10) Any char particle size reductions caused by ash particles or the walls of the CFBC are neglected.

Modeling procedure. For simulation purposes, the combustion of coal particles can be modeled using the following reactions:

(1) (2) (3) (4)

devolatilization and volatile combustion; char combustion; NO, formation; SO2 absorption.These reaction steps occur in the different 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 were selected and combined in a program flowsheet representing the CFBC. The CFBC riser is divided into two regions: a lower region and an upper region. The lower region is represented by a single CSTR (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 CSTR in series 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 simulates the fully developed zone. Since the height of the acceleration zone predicted by model is relatively high with a considerable solid fraction variation, 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 ASPEN PLUS is justified. Figure I shows the mean solids fraction (1 - Ei) corresponding to the four reactors (lower region and three sections of the upper region). The reactors are numbered as 1 for the lower region reactor and 2, 3 and 4 for the three upper region reactors. Description of reaction steps involved in each reactor, with the corresponding ASPEN PLUS unit operation blocks, are presented below.


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

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

I

I

I (l-e3 I

- - -MepnsolidfktionUinthekm#tion @

I

Upper&on @ I

_--

1 (l-e,) ; I

I Lowetregion 0

I I I + b

Solid fraction, (1 e)

Figure 1 Variation of void fraction with height in the riser

Devolatilization and volatile combustion. When coal is introduced into a CFBC, it decomposes into two parts: hydrogen-rich volatile and char. The char remains in the bed and burns slowly. Based on the plume model, coal devolatilization and complete combustion of the volatile occur at the feed entry point23. Two steps will then be considered in the simulation: decomposition and volatile combustion.

Decomposition. In this step, coal is converted into its constituting components such as carbon, hydrogen, sulfur, nitrogen and ash. This step occurs in the lower region reactor only and RYTELD (ASPEN PLUS yield reactor) is used to model this process by speciying the yield distribution vector according to the coal ultimate analysis.

Volatile combustion. To simulate the volatile combustion 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 where the coal is introduced and RSTOIC (ASPEN PLUS Stoi- chiometric reactor) is used to model the volatile combustion process. The combustion of the volatile matter is based on the following hypotheses:

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

hydrogen content of the coal is found in the volatile matter. The volatile carbon fraction (X,) reacts to form CO only during the volatile combustion process because of the oxygen depletion in the lower region of the riser. (ii) The coal hydrogen content is entirely consumed during the volatile combustion process. (iii) The coal sulfur content is assumed to be converted completely to SO2 during the volatile combustion process.

Char combustion kinetic model. The char particles resulting from the devolatilization process consist of the remaining carbon fraction (1 - X,) and ash only. These particles are then burned to produce a mixture of CO and CO*. Three main reactions for char combustion are considered here 24:

c+@**co

c+co*=+2co

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

The first and third reactions are heterogeneous and the second is homogeneous. Since the temperature of the burning particles in the CFBC is not sufficiently high, the effect of the third reaction on the combustion rate is 10w*~, and this reaction is neglected in the model. For the remaining two reactions, the reaction rate expressions must be developed. The first reaction is a gas-solid reaction and the chemical changes take place on both the external and the internal surface of the char particles’. The following expression for the char combustion rate, to form CO, per unit volume of the ith interval is obtained25.

rl,i = 3v02kcrFchar, i( 1 - fi) PcharrC( 1 - EC)Fsolid, i

CO,

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

k,, = kolexp

(5)

The following equation is used to calculate the mean particle radius based on the experimental particle size distribution:

1 rc= m (7)

t r,(k) Carbon monoxide produced during the heterogeneous combustion of char reacts with 02 in the homogeneous gas phase reaction to form CO*. Factors contributing to CO emission levels are the bed temperature, 02, CO and Hz0 concentration. The following ex ression is used for the CO combustion 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 combustion in a CFBC, the air used for the combustion is divided into two or more streams: the first one is supplied through the bottom air distributor and secondary air streams are injected in the upper region of the riser. NO, formations in combustion processes result from a combination of a thermal generation process and fuel nitrogen oxidation. At very high temperatures, thermal generation of NO, from the air nitrogen becomes very important, while at low temperatures found in a CFBC, the dominant source of NO, is fuel nitrogen oxidation. It is also important to emphasize the complexity involved in NO, chemistry within the CFBC because of many catalytic reactions involving, for example, 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 CFBC can be calculated by the following formula which considers the thermal generation and fuel nitrogen oxidation as detailed below:

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

Since CaCOs is unstable under CFBC conditions, the calcination process is assumed to occur instantaneously and completely in the lower region of the bed. The second reaction, representing the SOi capture in the riser, is considered 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. RSTOIC 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 CaS40 is strongly affected by the physical and chemical properties of limestone, hydrodynamic parameters, mass transfer resistance, temperature, reactive 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)

Vcao x - 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 expressed by:

P C=---.-

RlTb (10)

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

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

REQUIL (ASPEN PLUS equilibrium reactor) is used to predict the amount of thermal NO, formed during coal combustion based on equilibrium conditions considering the nitrogen present in the riser.

CY = 35D”.3 P

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

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

via fuel nitrogen oxidation is modeled using the following overall reaction:

VCaOFl rso~*i = 1 _ E,Au * 100

RSTOIC block is used to calculate the fuel nitrogen oxidation with a given value of o, which is taken from the literature. The NO, formation calculations are then applied in each of the four reactors using the combined REQUIL and RSTOIC blocks.

X

I

3aCYS02, i - j&

SO2 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 expressed by

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

6

CaO + SO2 + & 3 CaS04 and

The formation of SOI to SO3 is assumed to be instanta- neous. In CFBC reactors, the maximum concentration of SO3 is governed by the thermodynamic and kinetic considerations. Equilibrium conditions predict a certain conversion of SO2 to S03. However, 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,i 1 + (e”” - 1)

R,Kv RS 1

1 + 3oCYsoz,,

R&v (e”” - 1)

(11)

(12)

lume

(13)

(15)

(lo)



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 the unknown 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 simultaneous 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 reaction stoichiometry when only a few reactions approach equilibrium

Table 2 The reactor model parameters utilized in the simulation

Once Go,, i is calculated for the ith reactor, it is used as the fractional conversion of the key component necessary to run the RSTOIC 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 VALIDATION

The process simulation program for a CFBC was developed using four ASPEN PLUS reactor blocks-KYIELD, RSTOIC, RCSTR and REQUIL-to represent the phenomena identified in the coal combustion process. A detailed description of the ASPEN PLUS reactor blocks along with their re uirement is given in the ASPEN PLUS user manualsq5,t6. A brief description of the reactor blocks is given in Table 1. Table 2 presents the reactor model parameters and input variables required for the simulation.

Considering the reaction and hydrodynamic models, the entire CFBC system is divided into three sub-flowsheets: lower, upper and sep-flowsheets. 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 mixed in each interval of the bed, an overall SO* balance gives

Y soz, I = -5 @so,, I - rso,, I)

GUI (17)

and

Y SO*,i =

WRso,,i - rso2,J i z 1

cu2 (18)

where

(19)

and

R (1 - Xso,, i - I Yc ws

so*,i = 32AAL i#l (20)

The value of Yso2, i 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 (Xso,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 assumed 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, CYCLONE and FSPLIT. CYCLONE 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 ASPEN PLUS, is used. The resulting solid stream from CYCLONE is fed to FSPLIT where it is divided 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 with the unit operation blocks provided by ASPEN PLUS, several complete Fortran programs and an external subroutine for the kinetic models were used in the simulation. The Fortran codes contain the following four blocks which are required to complete the model. The first block, ‘KINETIC’, is the external kinetic subroutine developed for both heterogeneous gas/solid and homogeneous 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 the CFBC. This calculation is required in order to execute the ‘KINETIC’ subroutine. The third block, ‘HYDRO’, is inserted into the ASPEN PLUS 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 &B4 s4

- BS Rl

1 :

I- HYDRODYNAMIC AND KINETIC MODELS mdro,IMtimcdKilldics~)

a Bl7 s2n B16 #s&y $1 ‘I B9 z BS

- FSPLIT

S1Q CYCLOM R4 RSm1c - REQUIL

??All square blocks arc given by 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. A comprehensive simulation diagram for the CFBC is illustrated in Figure 2. Table 3 gives the value of the parameters used in the simulation. The input and output variables of the process simulation program are summarized 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 1.70 832.0 0.43 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 CFBC 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 flowsheet, which contains the recycle loop, must be solved iteratively and the tear streams, convergence methods and calculation sequence must be specified. ASPEN PLUS 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 Wegstein method, which normally converges rapidly15. It should be mentioned that for convergence to occur, the value of the tear stream variables needs to be correctly initialized. Such an initialization will enable a rapid 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 Efficiency (W)

Figure 3 Between the predicted and experimental combustion efficiency

350

300

250

g 200

8

t 150

k 100

50

0 L

0 Equation 24 (proposed correlation)

0 Equation 8 (original correlation)

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 feed rate. This procedure ensured a rapid and stable convergence process.

Model validation In order to validate the proposed model, 14 different sets

of operating data from various CANMET runs*’ were used to validate the simulation (see Table 5). A detailed description of the CANMET 0.8 MWth CFBC pilot plant is presented by Desai et ~1.~~. The predicted simulation results in terms of combustion efficiency, emission levels of CO, SO2 and NO, and 2O and CO concentration profiles are compared with the experimental data. The results are detailed below.

!z 1000

B 2 800

F .fi 800

400

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 so2

Combustion eficiency. In order to estimate combustion efficiency, the two outlet streams, S20 and S22 (see Figure 2), are used. These streams contain small amount of unburnt char particles that controls the combustion efficiency (vc), which 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 combustion efficiency were used to compare with the model predictions. 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 calculated for the cyclone efficiency. ASPEN calculated 99.99% efficiency for the cyclone used in the pilot plant, which is generally higher than that reported experimentally. Conse- quently, the carbon content of the fly ash predicted by the model becomes substantially smaller than the experimental value. This smaller amount of carbon in flyash causes the model to overestimate the combustion efficiency.

CO emission levels. Although CO combustion rates have been widely studied1*6*24,2 , the extension of these expressions to CFBC conditions is limited. The validity of the proposed CO combustion rates from the literature was examined by inserting them into the simulation program. Following the simulation results, a new correlation, similar to the Robinson’s expression26, with two adjustable parameters wa;proposed to predict the CO range reported by Desai 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 various sources are required in order to confirm this correlation. The CO emission levels predicted by the model ranged from 119 to 271 ppm, while the experimental data varied between 112 and 3 16 ppm. Therefore, the model based on the new correlation estimates CO emission levels relatively well. The comparison between predicted and experimental CO levels is presented in Figure 4.

SO* emission levels. The Ca/S molar ratio ranged between 1.6 and 2.3 for the various runs considered. Such values of Ca/S are usually considered sufficient to achieve reasonably 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=tm~tNO,(ppm)

Figure 6 Comparison between the predicted and experimental NO,

observed between predicted and experimental SO* concentration in the flue gas.

NO, emission levels. As mentioned earlier, two formation mechanisms were considered in the modeling of NO, formation in the CFBC: thermal generation and fuel nitrogen oxidations. Thermal generation was calculated considering equilibrium conditions, while NO, formation from fuel nitrogen oxidation was calculated using an overall conversion factor (cY~) from the literature. Since the aim of this work was not to study the NO, formation and reduction processes in detail, this overall approach was taken to simulate the fuel nitrogen oxidation. Fuel nitrogen conversion entails relatively complex reactions schemes involving several heterogeneous reaction steps and therefore attains a lower overall conversion26. Typical values of fuel N2 to NO, conversion factors, as reported by Legros et CZZ.*~ and Becker 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 agreement between predicted and experimental NO, was chosen as the overall conversion factor. As reported in most CFBC literature, the results also confirmed that thermal NO, formation, which leads to between 18 and 65 ppm of NO,, is unimportant compared to that of fuel nitrogen oxidation which approximately lies between 84 and 104 ppm of NO,. The predicted emission levels ranged from 130 to 267 ppm, while NO, emissions for the experimental CFBC ranged from 107 to 309 ppm.27. Figure 6 appears to indicate a reasonable agreement between predicted and experimental NO,. The difference between experimental data and those of the simulation prediction is attributable to the fact that a constant value of CY I = 0.05 is used throughout the entire simulation.

In recent years, several comprehensive studies have been reported6.3 ’ -34 regarding NO, formation and reduction processes. These were conducted to develop an improved understanding of the fundamental nature of NO, chemistry and underlying physical processes in CEBCs, and to support the needs for experimental work in this field. Emphasis is

9

6- 0 8 Redi*edCOConcartration(ppm) _ 2500 p

.g 8 5- -

E 2000 i

!'

E - 1500 $ E 3-

0" 0 - 1000 "

2- . .

l- *. - '..Q 500 _. . . . .

0 .-...._._.. I I I I I I 00

0 1 2 3 4 5 6

BedHeight

Figure 7 O2 and CO concentration profiles within the CFBC 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 be controlled by adding chemical components such as carbon monoxide, char hydrogen, ammonia, unburnt hydrocarbons and limestone due to the catalytic reactions found in CFBC32V33.

02 and CO concentration profiles. The emission data from the CANMET CFBC pilot plant used to validate the model consisted only of flue gas concentration and did not include the various gas concentration profiles along the riser. However, since the model can predict these concentration profiles within the riser, oxygen and carbon monoxide profiles were chosen to be compared qualitatively with data from the literature. The overall trends observed in experimental concentration profiles along the riser height are in close agreement with those predicted by the model. In the lower region a significant change in the oxygen concentration is found, while in the upper region there is a gradual decrease in the oxygen concentration. The CO concentration is constantly high in the lower region, while it sharply decreases in the upper region due to the injection of secondary air. Typical O2 and CO concentration profiles provided by Hansen et a1.35 and experimental data reported by Brereton et al.33 and Grace et al.34 are similar with those predicted by the present model. In Figure 7, the predicted profiles are presented.

Due to the relatively high dense bed found at the bottom of 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 the upper region. Since the operating and bed design data reported in those references differ with that of CANMET, the predicted concentration profiles have been compared qualitatively with the trend reported in these references.

CONCLUSION 10

A model was developed for the combustion of coal in a circulating fluidized bed using the ASPEN PLUS simulator. To provide such a CFBC model, several ASPEN PLUS unit operation blocks were combined and, where necessary, kinetic expressions and hydrodynamic model were developed using data and models from the literature. The developed models were then inserted into the flowsheet to provide a complete representation of the CFBC. The resulting model was used to predict the performance of the CANMET CFBC pilot plant in terms of combustion efficiency, emission levels of CO, SO2 and NO,, and 02 and CO 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 but more experimental data are still required to confirm the proposed CFBC model in order to make it more comprehensive and reliable. The model can now be used to represent a CFBC unit in various process simulation flowsheets plants.

such as power generation

ACKNOWLEDGEMENTS

This project was supported by CANMET, part of Energy, Mines and Resources, Canada. This financial assistance is gratefully 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 also appreciated. We greatly acknowledge Aspen Technology for having granted special permission for the use of the ASPEN PLUS under the condition of the academic licensing agreement.

REFERENCES

1

2

3

4

5

6

7

8

9

11

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13

14

1.5

16

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18

19

20

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Rajan. R., Krishnan, R. and Wen, C. Y., Simulation of fiuidized bed combustors-part II. In Fluidizatioti application to coal conversion process. AIChE Sym., 1978, 74, 112. Levenspiel, O., Chemical Reaction Engineering. 2nd ed. Wiley, New York, 1972. LaNauze R. D., in Fundamentals of Coal Combustion, in Fluidization, 2nd ed., ed. J. F. Davidson, R. Clift and D. Harrison. Acadamic, New York, 1985, pp. 63 l-674. Gordon, A. L. and Amundson, N. R., Modelling ofjuidized bed reactor--IV. Chem. Engng Sci., 1978,31, 1163. Sotudeh-Gharebaagh, R., Simulation of a circulating fluidized bed combustor using ASPEN PLUS. M.Sc.A Thesis, Ecole Polytechnique de Montreal, Canada, 1994. Robinson, W. D., The Solid Waste Handbook: a Practical Guide. Wiley, New York, 1986. Desai, D. L., Lau, I. and Anthony, E. J., Study of NzO formation in CRL’s circulating fluidized bed combustor. Final report, ERL’s, CANMET, Ottawa, 1991. Desai, D. L., Friedrich, F. D. and Lee, C. K., Pilot-scale circulating fluidized bed combustion research facility at CCRL’s. CANMET, Ottawa, 1990. Legros, R., Brereton, C. M. H., Lim, C. J., Li, H., Grace, J. R. and Anthony, E. J., in Combustion characteristics of different fuels in a pilot scale circulating fluidized bed combustor. 12th International Conference on FBC, Proceedings, Vo1.2, ed. Lynn N. Rubow. ASME, New York, 1993, pp. 66 l-666. Becker, H. A., Code, R. K., McCleave, R. and Stephenson, J. R., Pilot plant studies of fluidized coal combustion. Technical Report QFBC.TR.85. I, Kingston, Ontario, Canada, 1985. Zhao, J., Nitrogen oxide emissions from circulating fluidized bed combustion. Ph.D. Dissertation, University of British Columbia, Vancouver, Canada, 1992. Furusawa, T., Koyama, M. and Tsujimura, M., Nitric oxide reduction by carbon monoxide over calcined limestone enhanced by simultaneous sulfur retention. Fuel, 1985, 64, 413. Brereton, C., Grace, J. R., Lim, C. J. Zhu, J., Legros, R., Muir, J. R., Zhao, J., Senior, R. C., Luukos, A., Numaru, N., Zhang. J. and Hwang, I., Environmental aspects, control and scale-up of circulating fluidized bed combustion for application in western Canada. University of British Columbia, Final report, prepared for Energy, Mines and Resources Canada, 199 1. Grace, J. R., Brereton, C. M. H., Lim, C. J., Legros, R., Zhao, J., Senior, R. C., Wu, R. L., Muir, J. R. and Engman, R., Circulating fluidized bed combustion of western Canadian fuels. UBC, Final report, prepared for Energy, Mines and Resources Canada, 1989. Hanson, P. F. B, Dam-Johansen, K., Bank, L. H. and Ostergaard, K.. Sulphur retention on limestone under fluidized bed combustion conditions-an experimental study. Proceedings of the 11th International Conference on FBC, ed. E. J. Anthony. ASME, New York, 1991.

NOMENCLATURE

c Cd? CO? DP D, 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, Figure 2, (I = 1, 2, 3,...,17) combustion gas concentration (kmole m-‘) calcium to sulfur ratio concentration 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, I Fwr-2

Fc

FdlK,

FL-,,,

Fl

F lhnr F \“ld., fco fH!O

fo2 F, F* G,

IK” kol k LL’ LI P P(k) R RI RI 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,,

X so,. , Y SO?., 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-‘)-‘I four 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 [kmole (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 th 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 conditions combustion 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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Simulation of Circulating Fluidized Bed Reactors Using ASPEN PLUS