model development and validation_co-combustion of residual char, gases and volatile fuels in the...

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JFUE 3518 No. of Pages 9, Model 5+

17 April 2007 Disk UsedARTICLE IN PRESS

www.fuelfirst.com

Fuel xxx (2007) xxx–xxx

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Model development and validation: Co-combustion of residualchar, gases and volatile fuels in the fast fluidized combustion

chamber of a dual fluidized bed biomass gasifier

Priyanka Kaushal *, Tobias Proll, Hermann Hofbauer

Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, A-1060, Vienna, Austria

Received 20 November 2006; received in revised form 14 March 2007; accepted 20 March 2007

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PAbstract

A one-dimensional steady state model has been developed for the combustion reactor of a dual fluidized bed biomass steam gasifi-cation system. The combustion reactor is operated as fast fluidized bed (riser) with staged air introduction (bottom, primary and second-ary air). The main fuel i.e., residual biomass char (from the gasifier), is introduced together with the circulating bed material at thebottom of the riser. The riser is divided into two zones: bottom zone (modelled according to modified two phase theory) and upper zone(modelled with core-annulus approach). The model consists of sub-model for bed hydrodynamic, conversion and conservation. Biomasschar is assumed to be a homogeneous matrix of C, H and O and is modelled as partially volatile fuel. The exit gas composition and thetemperature profile predicted by the model are in good agreement with the measured value.� 2007 Published by Elsevier Ltd.

Keywords: Model; Fluidized bed; Char combustion

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RR1. Introduction

Currently, biomass fuel provides 15% of world’s pri-mary energy and this fulfilment establishes biomass asworld’s fourth largest energy source after oil, coal andgas. Pyrolysis, gasification and combustion are the primetechnologies for thermal conversion of biomass. Gasifica-tion is the process where, through thermal decompositionin a low oxygen environment at temperatures of 800–900 �C, organic matter is converted to combustible gaseswith tars and chars as possible side-products. Air gasifica-tion results in the inferior quality gas (4–7 MJ/Nm3 LHV).Oxygen as a gasification agent produces superior qualitygas (10–16 MJ/Nm3 LHV) but requires additional costsfor oxygen production. A similar high quality gas can beproduced by using a dual fluidized bed (DFB) system with

51525354

0016-2361/$ - see front matter � 2007 Published by Elsevier Ltd.

doi:10.1016/j.fuel.2007.03.032

* Corresponding author. Tel.: +43 1 58801x15965; fax: +43 1 5880115999.

E-mail address: [email protected] (P. Kaushal).

Please cite this article in press as: Kaushal P et al., Model developmdoi:10.1016/j.fuel.2007.03.032

steam as a gasification agent [1]. The fundamental idea ofthe DFB concept is to divide the gasifier into two fluidizedbed reactors, a gasification reactor and a combustion reac-tor. The energy needed for the endothermic gasificationreactions in the gasification reactor is provided by combus-tion of residual biomass called char in a combustion reac-tor. The energy released during combustion is transportedto the gasification reactor along with the circulating bedmaterial, thus making DFB gasifier a self sustainedprocess.

The DFB gasifier can be regarded as a circulating fluid-ized bed (CFB) system with the bubbling bed gasifier in thereturn loop of the solids. A number of different CFB mod-els have been developed and reviewed [2,3] but most ofthem are developed for coal combustion, i.e., for combus-tion of low volatile fuels. So far, biomass processing inCFBs has received little attention [4] and only a fewattempts have been made to model biomass char as partialvolatile fuel in CFB covering hydrodynamics and reactionkinetics. Further, confidentiality, numerous assumptions

ent and validation: Co-combustion of residual ..., Fuel (2007),

mailto:[email protected]

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63646566676869707172

7374757677787980818283848586878889909192

Nomenclature

A area (cross-sectional area) (m2)a decay constant (m�1)Cp heat capacity (J/mol K)D diameter of column (m)D molecular diffusivity (m2/s)Da diameter of annulus (m)Dc diameter of core (m)dB bubble diameter (equivalent diameter of a

sphere) (m)dp particle diameter (m)dchar char diameter (m)g gravity (m/s2)_H enthalpy flow (J/s)

H(T) enthalpy at temperature T (J/mol)DH 0

f ;298 enthalpy of formation at 298.15 K (J/mol)hm mass-transfer coefficient (kg/m2 s bar)KB,E mass-transfer coefficient between bubble and

emulsion (s�1)Ki1 elutriation rate constant (kg/m2 s)kB,E mass-transfer coefficient between bubble and

emulsion (m/s)k reaction rate constant (dependent)Mc molecular weight of carbon (kg/mol)_m mass flow rate (kg/s)mbed mass hold-up (bed material) (kg)mchar mass hold-up (char) (kg)N no of cells (–)Nor number of orifices (–)_n molar flow rate (mol/s)n order of reaction (–)P partial pressure (bar)QB volumetric flow rate in bubble (m3/s)

QEX gas exchange (m3/s)q specific combustion rate of carbon on external

surface (kgcarbon/m2s)R universal gas constant (J/mol K)SB,E surface area of bubble in contact with emulsion

(m2)Schar surface area of char (m2)Sh Sherwood number (–)T temperature (K)U0 superficial velocity (m/s)UB bubble velocity (m/s)UB1 velocity of a single isolated bubble (m/s)UE velocity of gas in emulsion (m/s)Umf minimum fluidization velocity (m/s)Ut terminal velocity (m/s)Y correction factor for modified two phase theory

(–)y mole fraction (–)z height (m)z0 height of dense zone (m)

Greek symbols

dB bubble fraction (–)emf porosity at minimum fluidization condition (–)ez average porosity at height z (–)e0 average porosity in dense zone (–)e1 Porosity above TDH (–)/ mechanism factor for primary surface product

(1 for CO2, 2 for CO) (–)k air ratio (–)l viscosity (Pa s)qp density of particle (kg/m3)

2 P. Kaushal et al. / Fuel xxx (2007) xxx–xxx



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Rvaried in a wide range makes the evaluation and compari-son of these models very difficult. Hence, a mathematicalmodel is developed for the riser to gain the better under-standing of the process. The model is based on knownapproaches and with respect to the data available (for val-idation) from the DFB gasification plant at Guessing/Austria.

2. Model development

The present work focuses on the mathematical model-ling of the combustion reactor indicated by the broken linein Fig. 1. The boundaries of the investigated system are theconnecting chute at the bottom of the riser where bed mate-rial enters together with the residual biomass char from thegasification reactor and the cyclone at the top of the riserwhere the hot solids are separated from the flue gas stream.The objective of this study is to develop a model to predictthe gas phase concentration and temperature profile alongthe height of the reactor.


The combustion reactor is divided into two zonesnamely dense and transport zone, having different hydro-dynamic characteristics. Transport zone is further subdi-vided into middle and upper zone as shown in Fig. 1.Preheated air is introduced into the riser at three differentpoints which are termed as bottom air, primary air and sec-ondary air. Preheated gaseous fuel (i.e., recycled producergas from the gasifier) is also introduced into the reactorto control the temperature of the entire system.

Each zone is further divided into cells and within eachcell the local hydrodynamic, kinetic and thermodynamicvariables are evaluated. Cells are solved sequentially withoutput of the Nth cell considered as input for the(N + 1)th cell. The energy conservation equation is solvedacross the entire zone, and it is assumed that within a zonethe temperature is uniform. This assumption is based on,efficient solids mixing and the dominance of the solid’s heatcapacity within a fluidized bed. The inputs for the modelare the riser geometry, particle properties, gas compositionand flow rate (Table 1)


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111112113114115

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118119120121122123

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137138139

141141

Flue gas

Solid

Solid

(Riser)

Secondaryair

Primaryair

Bottom air

Producergas

Producer gas

Biomass

Gasifier

Steam

Combustor

Modelboundary

Transportzone

Densezone

Fig. 1. Model structure and boundaries (1: dense zone, 2: middle zone, 3:upper zone).

P. Kaushal et al. / Fuel xxx (2007) xxx–xxx 3



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REModel assumptions:

The system is one-dimensional and in steady state.Gases are ideal and in plug flow.Solids are uniform in size and well mixed.Each of the three main zones has a uniformtemperature.The mass transfer of a single gas species between theemulsion phase and bubble phase is modelled by amass-transfer coefficient (kBE).

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Table 1Input parameters to the model (typical operating condition from Guessingplant)

Diameter of column 0.61 (m)

Height of column 12 (m)Diameter of particle 500E�6 (m)Diameter of coke 0.001 (m)Density of bed material 2960 (kg/m3)Density of coke 600 (kg/m3)Bed circulation 37 (kg/s)Temperature of gasifier 883 (C)Bottom air feed rate 700 (Nm3/hr)Primary air feed rate 2222 (Nm3/hr)Secondary air feed rate 1043 (Nm3/hr)Char composition (C, H, O) (0.82, 0.04, 0.14) (wt/wt)


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The hydrodynamic parameters are evaluated based onthe properties of bed which is classified as Geldart groupB.

The flow regime map of Grace [5] between dimensionlessvelocity (U*) and dimensionless particle diameter (d�p) con-firmed that the fluidization state of the dense bed is in bub-bling mode. Gas balance based on superficial velocity is

U 0 ¼ UBdB þ ð1� dBÞemfU E: ð1ÞTwo phase theory (TPT) assumes that visible bubble flowcorresponds to the excess gas velocity i.e., (U0 � Umf).However, this theory tends to overestimate the bubbleflow. In order to limit the deviation from observations,the modified two phase theory [6] is used, where

QB ¼ Y ðU 0 � UmfÞ: ð2ÞY is always below unity and usually in the range from 0.7to 0.8. Y can also be 0.3 for coarse particles. The reportedvalues of Y in literature scatter in a wide range [7]. Y is afunction of the height above the gas distributor and hencethe correlation proposed by Johnsson et al. [8] is used toestimate the value of Y.

Y ¼ 0:26þ 0:7 expð�0:0033dpÞ0:15þ ðU 0 � U mfÞ½ �0:33

zþ 4

ffiffiffiffiffiffiffiA

Nor

r� �0:4

: ð3Þ

Bubbles are formed as soon as the superficial gas veloc-ity exceeds minimum fluidization velocity. The bubble sizeis one of the most critical parameters when modelling fluid-ized beds, it affects the bubble rising velocity, the bubblefraction and the mass transfer between bubble and emul-sion phase. As the bed height increases, bubbles coalesceand the bubble size increases. The bubble size is calculatedas a function of bed height and it is assumed that all bub-bles at any cross section are of uniform size [9].

dB ¼ 0:54ðU 0 � U mfÞ0:4 zþ 4

ffiffiffiffiffiffiffiA

Nor

r� �0:8

g�0:2: ð4Þ

Bubble’s interaction influences the bubble rising velocityand the common adaptation for bubble rising velocitygiven by Davidson and Harrison [10] is used in this work

UB ¼ UB;1 þ ðU 0 � UmfÞ: ð5Þ

For bubbles, the rising velocity of a single isolated bubbleis given below.

UB;1 ¼ 0:71ffiffiffiffiffiffiffiffigdB

pð6Þ

There is no theoretical basis to support this correlation,however, numerical simulations [11,12] show that thisapproximation is reasonable.

It is evident that a major part of the gas flows in the bub-ble phase. Thus, the interphase mass transfer between bub-ble and emulsion phase is essential for the gas–solidreactions. The interphase mass transfer is modelled by asemi-empirical approach using a mass-transfer coefficient,the exchange area and the concentration gradient accord-ing to Sit and Grace [13]


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KBE ¼Umf

4þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4emfDU B

pdB

s: ð7Þ

Fig. 2 shows the gas flow streams in the dense zone and thegas exchange between the bubble and emulsion phase.After the injection of primary air into the combustion reac-tor, the superficial gas velocity is increased and the reactoroperates in fast fluidization regime. Fast fluidized beds ex-hibit strong radial gradients with respect to solid hold-upand solid movement. Near the column walls the particlemoves downwards and the concentration of particles ishigher than in the core of the reactor, where particles aretransported upwards. Different approaches are proposedto describe fast fluidized bed reactors [14–16]. In the pres-ent work, the transport zone is modelled by a simplifiedcore-annulus approach. The axial mean voidage is calcu-lated using an exponential decay function as proposed byZenz and Weil [17]

ez � e1e0 � e1

¼ exp½�aðz� z0Þ�: ð8Þ

Unknown parameters in Eq. (8) are the decay factor ‘a’ andthe porosity above transport disengaging height ‘e1’. Thevalue of ‘a’ varies widely in the literature and is approxi-mately in the range of 2–6.4 m�1 [18]. Cold flow studiesof the process showed that the decay constant dependson the fluidizing velocity to a higher power than one. Thus,the correlation proposed by Adanez et al. [19] was adopted.

aðU 0 � U tÞ2D0:6 ¼ K: ð9Þ

Based on the pressure drop measurement at standard con-ditions the constant K was determined to be K = 8.6. Thevalue of e1 is determined from the particle elutriation rateconstant Ki1 for a mono-sized bed material:

1� e1 ¼K i1

qpðU 0 � U tÞð10Þ

K i1 ¼ 0:011qp 1� U t

U 0

� �2

: ð11Þ

The voidage in the core at a given height is calculated fromthe mean porosity ez at that height [20]

eC ¼ 1� 0:6ð1� ezÞ: ð12Þ

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LSIO

N

BUBB

LE

out (N−1)EQ BQ

EXQ

out (N) out (N)EQ BQ

out (N−1)

Fig. 2. Gas balances in dense zone.


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It is assumed that the annulus is maintained at minimumfluidization conditions. The diameter of the core increaseswith the height of the riser and is estimated from Eq. (13).

eaD2a þ ecD2

c ¼ ezD2: ð13Þ

Unlike dense zone in the transport zone it is assumed that,there is a single gas stream across both the core and annu-lus. The local variables (e.g., reaction rate, hold-ups, etc.)are calculated separately for core and annulus but the massbalances are applied across the whole cell as shown by thebroken line in Fig. 3.

Char combustion is in general a complex process due tothe combination of different mechanisms as mass transfer,chemical reaction and heat transfer. In general, combustionof char begins after the evolution of volatiles. Sometimes,especially for large particles, there is a significant overlap-ping of these processes. The char composition is an impor-tant model parameter and is assumed to be a homogeneousmatrix of carbon, hydrogen and oxygen. Carbon present inchar is modelled as non-volatile fuel subject to heteroge-neous oxidation with O2 and if no O2 is present then withH2O. The primary products in the case of char combustionare CO and CO2 represented by reaction A.

ð1þ bÞCþ ð1þ b2ÞO2 ! bCOþ CO2; ðAÞ

where b is a stoichiometric coefficient for char combustiondescribing the ratio of CO to CO2 and is a function of sur-face temperature [21]. Char gasification reaction (B) is alsoconsidered in the model.

CþH2O !k COþH2: ðBÞ

The carbon combustion model is based on the combinedeffects of gas film diffusion and reaction kinetics on ashrinking particle and it is assumed that the diameter ofthe char particle decreases while the density of the char re-mains constant. Oxidation occurs in a thin layer close tothe particle surface and no ash layer is formed. The com-bustion rate in term of mass transfer through the gas filmcan be described as

q ¼ hmðP O2;g � P O2;sÞ ð14Þ

hm ¼M c/ShDdcharRT

: ð15Þ

N-1

N+1

EXQ

Fig. 3. Gas balances in transport zone.


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The chemical reaction rate of carbon with oxygen per unittime and per unit external surface area of the particle is

q ¼ kP nO2;S

Schar: ð16Þ

Eqs. (14) and (16) are solved under steady state assump-tion,. The data obtained from experiments allow minimiza-tion of the root of mean square for the Arrheniusexpression [22].

The hydrogen and oxygen present in char are modelledas volatile fuel. Hydrogen and oxygen are released in a waythat the char composition remains constant as conversionproceeds. The hydrogen and oxygen released react to formwater (C). The ratio of CO, CO2, H2, O2 and H2O release iscalculated from the combustion kinetics of carbon and bal-ances of char.

H2 þ1

2O2 () H2O: ðCÞ

Carbon monoxide released from char is modelled to under-go combustion reaction and shift reaction

COþ 1

2O2 () CO2 ðDÞ

COþH2O() CO2 þH2: ðEÞ

The effect of the CO shift reaction becomes pronouncedwhen oxygen within a zone is consumed. As shown inFig. 1, a small amount of producer gas is also recycled intothe middle zone of the combustion reactor. The combus-tion kinetics and composition of the producer gas are givenin Tables 2 and 3, respectively.

The elementary mass balances inside every single cell iscovered by the stoichiometry of the homogeneous and het-erogeneous reaction formulations. The mass balance forsolid carbon and the energy balance are applied globallyacross each of the three zones as indicated in Fig. 1.

Char is evenly distributed in each cell of dense zone as

mcK ¼1

NðmcÞ; ð17Þ

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Table 2Reaction rates

Reactions Reaction rate

A (1+b)C +(1+b/2)O2! CO + CO2 q = K[PO2]0.5 SB C + H2O! CO + H2 q = K[PH2O]0.5

C H2 +1/2 O2! H2O rH2 = K[H2]1.5[D CO + 1/2 O2! CO2 rCO = K[CO][OE CO +H2O! H2 + CO2 rCO = K[CO][HF CH4 + 3/2 O2! CO + 2H2O rCH4 = K[CH4]G C2H4 + 2O2! 2CO + 2H2O rC2H4 = K[C2HH C2H6 + 5/2 O2! 2CO + 3H2O rC2H6 = K[C2HI C3H8! C2H4+ CH4 rC3H8 = K[C3H

Table 3Producer gas composition

LHV (dry) Composition (vol%)

(MJ/Nm3) CO CO2 CH4 C2H4

13.1 21 20 10 2


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where as in transport zone char is distributed in cells insuch a way that the ratio of bed hold-up in adjacent cellis the same as the ratio of char hold-up.

mbedkþ1

mbedk

¼ mcharkþ1

mchark

: ð18Þ

Globally across the entire zone, the char balance is formu-lated as

_mChar;out þ _mChar;reacted ¼ _mChar;in: ð19Þ

The general carbon balance of each zone is written as

_mcin¼ _mcout þ

XN

k¼1

dmc

dt: ð20Þ

All feed rates are input parameters to the model. Becauseof the assumption of uniform char concentration in the sol-ids, the char exit rate and the char hold-up in the zone aredirectly linked to each other. The char conversion ratefrom solid to gas is dependent on several parameters withineach cell and depends directly on the available char surfacearea within the zone, the zone temperature and partialpressures of the reactants, i.e., O2 or H2O.

The energy balance is formulated globally across theentire zone using total enthalpies including the standardenthalpy of formation with the assumption that the totalamount of energy entering the system is actually leavingthe system.X

_Hout ¼X

_H in ð21Þ

HðT Þ ¼ DH 0f;298 þ

Z T

T¼298:15

CpðT ÞdT : ð22Þ

The enthalpy of ideal gas mixtures is calculated using theNASA polynomials with coefficients reported by Burcatand McBride [29]. The enthalpy of the inert bed materialis calculated by interpolation of data reported by Barin[30]. The sensible heat of char is calculated using the corre-lations reported by Merrick [31]. The enthalpy of forma-

K Ref.

char 8.56 · 10�2 exp(�2237/T) [22]7 2.62 · 108 exp(�237000/T) [23]O2] 1.63 · 109 T3/2 exp(�3420/T) [24]

2]0.5[H2O]0.5 3.25 · 107 exp(�15098/T) [25]

2O] 0.03 e exp(�7249/T) [26]0.7[O2]0.8 4.68 · 1018 T0.5 exp(�20087/T) [27]

4][O2] 1.0 · 1012 exp(�20844/T) [28]

6][O2] 2.34 · 1018 T0.5 exp(�20087/T) [27]

8] 1.0 · 1012 exp(�21145/T) [28]

C3H8 H2 H2O O2 N2

1 35 9 0 2


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0

0.5

1

1.5

0.00 0.05 0.10 0.15 0.20

Hei

ght o

f ris

er (m

)

0 0.01 0.02Mole fraction (-)

BCO2

ECO2

BO2

EO2

EH2O

ECO

*EH2

BH2O

* Sec. axis

BCO




tion for char is calculated from the heating value, estimatedfrom ultimate analysis using the Boie formula [32]. _Hout is afunction of the exit temperature of the respective streams.Since a uniform temperature is assumed inside the zone,the problem is reduced to the determination of the twovariables char hold-up and temperature in order to fulfilEqs. (19) and (21).

A two-dimensional Newton–Raphson algorithm isapplied where the components of the Jacobian matrix arecalculated analytically. The iteration is performed for eachof the three zones where the order is again from bottomtowards top of the combustion reactor. After convergence,the model yields the gas phase concentration profiles, thetemperature in each zone, the solid char flow rate leavingthe reactor and the apparent global air ratio k which isdefined by neglecting the bypassing fraction of the char(i.e., only the actually converted char is considered).

PR

Mole fraction (-)

Fig. 4a. Gas composition profile in dense bed. B: bubble; E: emulsion.

k ¼½ _nyO2

�in½ _nyO2

�in � _nfyO2� 0:5yCO �

PCxHyðxþ 0:5yÞyCxHy

gh i

out

:

ð23Þ

TED

2

4

6

8

10

Hei

ght o

f ris

er (m

)

COCO2H2OO2
C
3. Results and discussion

The solution of the mathematical model representssimultaneous convergence of the conservation equation(for which different subroutines have been defined). Table1 lists the input parameters for a typical operation case.The average residence time of the gas in the combustionreactor is in the range of 1.0–1.5 s while the air ratio inthe standard condition is approximately 1.02.
E
357358359360361362363364365366367368369370371372373

00.00 0.10 0.20

Mole fraction (-)

Fig. 4b. Average gas composition profile along height of riser.

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Fig. 4a shows the concentration profile inside the bot-tom bed (dense zone) separately for the bubble and emul-sion phases. As can be seen, the concentration of oxygenin emulsion phase goes down swiftly and when O2 is com-pletely consumed, the char gasification and shift reactiondominates; this can be observed by the increased concen-tration of H2 and CO2 in emulsion phase at the relevantheights. In the bubble phase, the concentrations of COand H2 are very low while that of oxygen is relatively high.This is one of the major limitations of one-dimensionalmodel that it fails to explain the three-dimensional mixingeffect. Nevertheless such behaviour of dense bed is oftenreported [4,33]. The overall gas composition profiles alongthe riser height are shown in Fig. 4b. The concentrationprofiles of CO, CO2, H2 and H2O locally go down at theheight of 2 and 4 m due to dilution effect caused by primaryand secondary air addition. Results show that the producergas added in the middle zone (rich in combustible gas)quickly combusts to CO2 and H2O. The predicted gas-pro-file inside the riser could not be verified at the demonstra-


tion plant at Guessing because of the lack of measurents.Therefore, to validate the model, the operating parametersof the present model were adjusted in order to match withthe operating parameters of the fluidized bed boiler atChalmers University (Sweden), reported by Adanez et al.[4]. Newton–Raphson method was used to iterate the woodfeed rate to fit the excess air ratio reported by Adanez et al.Fig. 5 shows a comparison between the measured and pre-dicted oxygen concentrations along the height of riser. Theoxygen profile predicted by the present model is in fairagreement with the measured values [4,34]. As seen inFig. 5, an increase in oxygen concentration with height isnot realistic and as indicated by Amand et al. [33], it canbe attributed to an insufficient penetration depth and mix-ing of the secondary air flow. Nevertheless Adanez et al.investigated various possibilities to deal the mixing prob-lem in one-dimensional model.


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EC

T

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375376377378379380

381

382383384385386387388389390391392393394395396397398399400401402403404405

0

2

4

6

8

10

0 0.1 0.2 0.3Mole fraction O2 (-)

Hei

ght o

f ris

er (m

)

Adanez et al. [4]Present model

Measured value from the fluidized bed boiler at ChalmersUniversity Sweden [4,34].

Center lineAverage

Fig. 5. Comparison of model prediction for O2 concentration.

0

2

4

6

8

10

880 900 920 940 960Temperature (°C)

Hei

ght o

f ris

er (m

)

Avg. temperature of zone( model prediction) Measured temperature

Den

sezo

ne

Mid

dle

zone

Upp

erzo

ne

Fig. 6. Measured and predicted temperature.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8Char feed

Ap

par

ent

air

rati

o (

-)

dc = 4 mm

dc = 8 mm

dc = 12 mm

dc = 16 mm

dc = 20 mm

dc = 24 mm

Fig. 7. Effect of char feed ra





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3.2. Predicted temperature profile

At the demonstration plant at Guessing, the tempera-ture inside the riser is measured at three different heights.The predicted temperature profile is in good agreementwith the measured value (Fig. 6). A little over-predictionin temperature is observed but the deviation in temperatureprediction is less than 5 K.

3.3. Effect of char diameter

Fig. 7 shows the effect of char feed rate on air ratio atdifferent char sizes. It should be noted that all the otheroperating parameter were fixed except char feed rate andsize. Each curve in Fig. 7 were generated by changing thechar feed rate at a given constant char size. Result showsthat with increase in char feed rate, the air ratio decreases.The slope of the curve changes at an air ratio of 1, suggest-ing that for lower char feed rates, char size have a veryprominent effect on char conversion. Simulation showsthat for constant char feed of 0.8 kg/s air ratio changesfrom 0.85 to 0.93 when char size is changed from 4 to8 mm. Therefore, it can be concluded that smaller sizefavours char conversion. The simulation results also showthat the residual char from the gasifier is only partly con-verted in the riser and the remaining un-combusted charis circulated back into the gasification reactor and poten-tially leads to an increase of char hold up in the dual fluid-ized bed system. Fig. 8 shows the exit CO concentration forthe various points shown in Fig. 7. It can be seen in the fig-ure, that for air ratio below 1, the CO concentrationincreases abruptly. It also shows that irrespective of thechar size the CO concentration follows the same trend linesuggesting that exit CO concentration is a strong functionof air ratio and is independent of char size.

1 1.2 1.4 1.6 1.8 rate (kg/s)

Increasing char diameter

te on apparent air ratio.


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Fig. 8. CO concentration in flue gas.




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4. Conclusions

A one-dimensional steady state model of a riser has beendeveloped to analyze the combustion of biomass char incirculating fluidized bed. The model is based on mass andenergy balances. The model includes different sub-modelsthat are linked together to describe the overall combustionprocess. The hydrodynamic sub-model highlights the phys-ical characteristics of the bed material while the reactionsub-model deals with the chemical reactions in the differentzones of the combustion chamber (riser). The model is fuelflexible and offers the opportunity to evaluate different fueltypes over wide range of composition both for solid andgases. Global reaction rates are used for the descriptionof chemical kinetics. Model validations have been per-formed with measured results obtained at both 8 MWthdual fluidized bed gasification plant at Guessing, Austriaand with literature data on the 12 MWth circulating fluid-ized bed boiler at Chalmers University (Sweden). The pre-dicted trends are qualitatively compared with the publisheddata [4,33] and it turns out that they are in fair agreement.The model predictions show that the gas composition var-ies significantly in the upper section above the secondaryair injection. Simulation results also show that smaller sizeof wood or char favours the overall performance of thegasifier.
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Acknowledgement

The authors gratefully acknowledge the financial sup-port from RENET Austria (Knet/Kind public fund pro-gram, Austria).


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http://www.entek.chalmers.se/~leam/Input_model/Fuel_loading1/Charload_rev1.pdf

http://www.entek.chalmers.se/~leam/Input_model/Fuel_loading1/Charload_rev1.pdf

model development and validation_co-combustion of residual char, gases and volatile fuels in the...

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