The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel

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Fuel Processing Technology 84 (2003) 229–241

The influence of fine char particles burnout on bed

agglomeration during the fluidized bed

combustion of a biomass fuel

Fabrizio Scala a,*, Riccardo Chirone a, Piero Salatino b

aIstituto di Ricerche sulla Combustione, CNR, P.le V. Tecchio, 80-80125 Naples, ItalybDipartimento di Ingegneria Chimica, Universita degli Studi di Napoli Federico II, P.le V. Tecchio,

80-80125 Naples, Italy

Received 5 August 2002; received in revised form 21 April 2003; accepted 24 April 2003

Abstract

The combustion of biomass char in a bubbling fluidized bed is hereby addressed, with

specific reference to the influence that the combustion of fine char particles may exert on ash

deposition and bed agglomeration phenomena. Experiments of steady fluidized bed combustion

(FBC) of powdered biomass were carried out with the aim of mimicking the postcombustion of

attrited char fines generated in the fluidized bed combustion of coarse char. Experimental results

showed that the char elutriation rate is much smaller than expected on the basis of the average

size of the biomass powder and of the carbon loading in the combustor. Samples of bed

material collected after prolonged operation of the combustor were characterized by scanning

electron microscopy (SEM)–EDX analysis and revealed the formation of relatively coarse

sand–ash–carbon aggregates. The phenomenology is consistent with the establishment of a char

phase attached to the bed material as a consequence of adhesion of char fines onto the sand

particles.

Combustion under sound-assisted fluidization conditions was also tested. As expected,

enhancement of fines adhesion on bed material and further reduction of the elutriation rate were

observed.

Experimental results are interpreted in the light of a simple model which accounts for

elutriation of free fines, adhesion of free fines onto bed material and detachment of attached fines

by attrition of char–sand aggregates. Combustion of both free and attached char fines is

considered. The parameters of the model are assessed on the basis of the measured carbon

loadings and elutriation rates. Model computations are directed to estimate the effective size and

0378-3820/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0378-3820(03)00108-5

* Corresponding author. Tel.: +39-081-7682969; fax: +39-081-5936936.

E-mail address: [email protected] (F. Scala).

F. Scala et al. / Fuel Processing Technology 84 (2003) 229–241230

the peak temperature of char–sand aggregates. The theoretical estimates of the effective aggregate

size match fairly well those observed in the experiments.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Fluidized bed combustion; Biomass; Bed agglomeration

1. Introduction

The attractiveness of biomass fuels as a renewable and CO2-neutral energy source has

boosted research on technologies suitable for burning this class of fuels. Among the others,

fluidized bed combustion (FBC) has been indicated as one of the most promising ones [1–

3], because of its fuel flexibility, high combustion efficiency and low environmental

impact. However, a number of operational problems, mostly related to the fate of volatile

matter (mixing/segregation, spatial burning profiles) [4,5] and of the ash components

(fouling, slagging, bed agglomeration/defluidization) [6–9], call for deeper investigation

on the combustion behavior of these fuels. In particular, the occurrence of bed agglom-

eration and defluidization has been often reported during operation of combustors fuelled

with biomass, eventually leading to unscheduled boiler shutdown. Alkalis, which are

abundant in the ash of most biomass fuels, are responsible for the formation of melts in

combination with the inert bed material. Similar problems are encountered in the fluidized

bed combustion of lignites [10,11]. Despite a considerable research effort has been

devoted to this subject [12–15], the mechanisms of ash-bed material interaction and of

bed agglomeration are not as yet well understood.

The present study moves from the consideration that the fluidized bed combustion of

most biogenous fuels takes place with extensive generation of carbon fines. This is a

consequence of the tendency of these fuels to yield highly porous or even incoherent chars

after pyrolysis that are very susceptible to attrition and/or percolative fragmentation along

with burnout. Arena et al. [16], Salatino et al. [17] and Scala et al. [18] highlighted that,

even when coarsely grained fuel feedings are considered, a large fraction of the resulting

char is actually converted as fines along a pathway consisting of the following phenomena:

(1) generation of char fines by attrition/percolative fragmentation; (2) afterburning of the

attrited char over its residence time in the reactor, typically favoured by large intrinsic

combustion reactivities.

Attrited char may experience, upon further burn-off, peak temperatures largely exceed-

ing the bed temperature. Modifications of the ash constituents, like softening, melting or

even vaporization, might occur even at nominal bed temperatures at which no such change

in mineral matter would take place. Chirone et al. [19] have discussed at depth this feature,

concluding that the combination of attrition-induced generation of fines particles and

combustion-induced overheating of the same char particles be responsible for the

formation of ash-layered bed material eventually leading to bed agglomeration and

defluidization.

Experimental results reported by Chirone et al. [20] coherently fit in this picture. They

observed that powdered biomass char was elutriated at a much lower rate from a fluidized

bed combustor than coal- or tyre-derived chars of comparable size. The phenomenology

F. Scala et al. / Fuel Processing Technology 84 (2003) 229–241 231

was consistent with occurrence of adhesion of char fines onto bed solids. In addition, some

tests were carried out with the use of acoustic fields, with the purpose of enhancing

adhesion of fine char onto coarse bed particles [21,22]. The application of sound of

suitable intensity and frequency consistently resulted in further decrease of the char

elutriation rate.

The present study moves one step further towards the characterization of the

mechanisms of ash-bed interaction and of bed agglomeration in the fluidized bed

combustion of biogenous fuels. The study is based on the concept that, regardless of

whether fuel feeding to the fluidized bed combustor consists of coarse or fine biomass

particles, extensive attrition phenomena taking place during combustion ultimately lead

to the generation of large amounts of elutriable char fines. Indeed, the combustion of

these fines represents a major pathway to overall char burnout for both powdered or

coarsely grained biomass feedings, reflected by the char time–temperature history and

by the fate of ash components. The burnout of attrited carbon fines generated during

fluidized bed combustion of coarse biomass particles was mimicked in experiments in

which powdered biomass, of size comparable with that of attrited fines, was steadily fed

to the combustor. The biomass fuel was Robinia Pseudoacacia. Sound-assisted com-

bustion was also tested, to better assess the influence of the interaction between char

fines and bed solids on char adhesion and bed agglomeration. The fate of char fines is

analyzed in the light of a simple model based on the following processes: elutriation,

adhesion of fines onto bed material, detachment of elutriable fines from the coarse bed

solids by abrasion, combustion.

2. Experimental

2.1. Experimental apparatus

The experiments were carried out in a quartz atmospheric fluidized bed combustor 40

mm ID and 1 m high (Fig. 1). The gas distributor is a stainless steel sintered porous plate.

The fluidization column and the stainless steel preheating section are heated by two

electric furnaces. The temperature of the bed, measured by a type K thermocouple

immersed in the bed 40 mm above the gas distributor, is kept constant by a PID controller.

The freeboard is kept unlagged in order to minimize fines postcombustion in this section.

Gases are fed to the column via two high-precision digital mass flowmeters. The

fluidization column is equipped with a solids metering/feeding system for continuous

injection of fine powders at the bottom of the bed, a set of high-efficiency cellulose filters

(10 Am nominal particle retention) for the collection of elutriated material at the column

exit, a double-elbow at the top of the freeboard acting as a sound wave guide and a sound

generation system. The solids feeding system consists of a mechanically vibrated fuel

hopper mounted over a screw feeder that further delivers the powder in a pneumatic

conveyor for injection above the bed distributor. The sound generation system consists of a

digital signal generator of sine waves of specified frequency whose signal is amplified by

means of a power audio amplifier rated at 40 W. The signal is sent to an 8 V woofer

loudspeaker located downstream the double-elbow. A paramagnetic analyzer and two

Fig. 1. Sound-assisted fluidized bed apparatus. (1) Air pre-heater; (2) electric furnace; (3) quartz tube; (4) sound

wave guide; (5) microphonic probe; (6) loudspeaker; (7) flue gas filter; (8) gas analysis system; (9)

micromanometer.


NDIR analyzers are used for on-line measurement of O2, CO and CO2 concentrations,

respectively, in the exhaust gases. Further details of the apparatus are given elsewhere

[22,23].

2.2. Materials

The bed material consisted of 280 g of silica sand, corresponding to an unexpanded

bed height of 0.15 m. Sand was double sieved in the nominal size range 0.3–0.4 mm

with Sauter mean diameter of 0.36 mm. Incipient fluidization velocity was 0.05 m/s at

850 jC.Experiments have been carried out using powdered Robinia Pseudoacacia, a ligneous

biomass whose properties are reported in Table 1, as a fuel. Robinia branches were

milled into fine particles and sieved in the nominal size range 0.075–0.18 mm.

Table 1

Properties of Robinia Pseudoacacia fines

Particle density, g/cm3 0.38

Char density, g/cm3 0.24

LHV, kJ/kg 15,600

Proximate analysis (dry basis), %w

Volatiles 79.2

Fixed carbon 19.3

Ash 1.5

Ultimate analysis (dry basis), %w

Carbon 43.9

Hydrogen 7.8

Nitrogen 0.02

Sulfur –

Ash 1.5

Oxygen (diff.) 46.78

Ash composition, %w

CaO 76.2

MgO 11.8

K2O 9.2

Na2O 0.78

Fe2O3 0.25

Al2O3 0.69

SiO2 0.07

SO4 < 0.1


Fluidization gas consisted of technical grade air and nitrogen or mixtures of the two.

Inlet oxygen concentration in the combustor was varied between 7.6% and 21% on

volume basis.

2.3. Procedures

Steady combustion tests were performed by continuously feeding the powdered fuel

in the fluidized bed kept at a nominal temperature of 850 jC with a gas superficial

velocity of 0.5 m/s. Several experimental runs were carried out under sound-assisted

conditions: following [20] an optimal sound field of 150 dB (measured at the top of the

bed) and 120 Hz was used. The attainment of a steady state was revealed by the

steadiness of flue gas concentrations. During steady combustion, elutriated material was

collected by filters, weighted and analyzed to determine carbon content. Closure of the

carbon mass balance between fed, elutriated and burned carbon (worked out from steady

CO and CO2 concentrations in the exhaust gases) was always within F 5%. At the end

of each run, the fuel feeding was suddenly switched off and the residual carbon in the

bed was burnt batchwise at a gas velocity lower than that used during combustion

experiments, in order to prevent fine particles elutriation. The carbon loading in the

combustor at steady state was obtained by time-integration of CO and CO2 concentration

profiles at the exhaust during this stage. At the end of the run, bed material was

discharged from the reactor for further characterization. The morphology of sand

samples was characterized by the use of a scanning electron microscope (Philips


XL30 with LaB6 filament). SEM observations were complemented by elemental analysis

of particle surface by means of an EDX probe (EDAX DX-4).

3. Theory

The fate of carbon fines burning in a fluidized bed of coarse particles is analyzed with

reference to the network in Fig. 2. Carbon fines may be present in the bed either as fines

freely moving in the interstices of the bed (free fines, whose carbon loading is Wf) or as

fines stuck onto the surface of coarse bed solids (attached fines, whose carbon loading is

Wa). These ‘‘phases’’ are represented as square-shaped blocks in Fig. 2. Carbon fluxes

from and to each phase are dependent on four competitive phenomena: elutriation of free

fines from the bed, adhesion of free fines onto bed material, detachment of attached fines

by attrition of fine–coarse aggregates and combustion of both free and attached fines in

the bed. It is assumed that the carbon mass flow rates departing from each phase along the

different paths depend linearly on the mass of carbon present in that phase (either Wa or

Wf). Rate constants corresponding to each path are indicated in Fig. 2.

Further assumptions are:

(i) Adhesion of free fines may be promoted by the oscillatory motion induced by sound.

On the contrary, the other processes are negligibly influenced by the establishment of

the acoustic field [24]. Accordingly, only the value of kad is influenced by the sound.

(ii) The combustion rate of both attached and free fines is controlled by diffusion of

oxygen across the particle boundary layer. This assumption is the result of an order of

magnitude evaluation of the burning particles Damkohler number, representing the

relative importance of the apparent particle combustion kinetics versus the external

Fig. 2. Fate of fixed carbon in ordinary or sound-assisted fluidized bed combustion of carbon powders.


mass transfer rate. Using the intrinsic kinetics data for Robinia biomass char given by

Masi et al. [25], it can be shown that at 850 jC the char particles burning rate is

controlled by boundary layer diffusion for particles with size down to 0.075 mm. It

must be underlined that in principle the combustion rates of free and attached fines are

different one from the other as a consequence of the different relevant particle sizes.

(iii) The gas flow pattern in the bed corresponds to perfect mixing: a large bubble-dense

phase mass transfer index (>10) was evaluated under the experimental conditions used

and, in addition, U/UmfH1.

(iv) Postcombustion of elutriated fines in the freeboard is negligible. This is partly justified

by the consideration that the freeboard has been purposely kept cold in the

experiments.

(v) Evolution and combustion of volatiles is assumed to occur rapidly and uniformly

throughout the bed, consistently with the small size of the fuel particles.

The balance on fixed carbon in the reactor corresponding to steady combustion of fixed

carbon fed to the reactor at a mass rate Fc reads:

Fc þ katWa ¼ kelWf þ kfcWf þ kadWf ð1Þ

for the free fines and:

kadWf ¼ kacWa þ katWa ð2Þ

for the attached fines. kel, kad and kat are elutriation, adhesion and attrition rate constants,

respectively. kca and kc

f are the combustion kinetic constants of attached and free fines,

respectively, embodying the dependence of combustion rate on temperature and oxygen

concentration. The global balance on fixed carbon may be written as:

Fc � kelWf ¼ kfcWf þ kacWa ð3Þ

The working hypothesis will be hereinafter made that the loading of free fines is much

smaller than the loading of attached fines (WfbWa) and, accordingly, that the contribution

of free fines to carbon burnout is negligible. Thus, Eqs. (1) and (3) can be simplified into:

Fc þ katWa ¼ kelWf þ kadWf ð1VÞ

Fc � kelWf ¼ kacWa ð3VÞ

An ‘‘adhesion factor’’ fa is defined as:

fa ¼kat þ kac þ kad

kat þ kacð4Þ

whose value increases with the importance of adhesion: the lower bound fa = 1 applies

when adhesion is absent. According to hypothesis (i), the establishment of an acoustic

field, all other operating variables being the same, affects only the value of kad and, in turn,

of fa.


The overall carbon loading, i.e. the sum of the mass of carbon in both free and attached

fines, the carbon elutriation rate and the fixed carbon combustion efficiency are given,

respectively, by:

Wa þWf ¼Fc

kelVþ kacð5Þ

Ec ¼ kelWf ¼ kelVðWa þWf Þ ð6Þ

gcf ¼ 1� Ec

Fc

¼ 1

1þ kelV

kac

ð7Þ

where kelV= kel/fa. Eq. (6) is formally equal to expressions of the elutriation rate from

fluidized beds based on the elutriation constant concept. The factor fa accounts for the

adhesion–detachment mechanism.

Eqs. (5) and (6) can be used to estimate the kinetic constant of the combustion rate of

attached fines kca and the apparent elutriation rate constant kelV. According to hypothesis (ii),

the steady material and energy balances around a burning attached fine particle read:

kac ¼6Kg12kCO2

daqfc

ð8Þ

kacqfcda

6

njDHCOj þ jDHCO2j

nþ 1¼ hðTa � TbedÞ þ reeff ðT4

a � T4bedÞ ð9Þ

where da is the relevant particle diameter, qfc is the char density, CO2is the bulk oxygen

concentration in the bed, Kg =DO2Sh/da is the boundary layer mass transfer coefficient, Sh

is the particle Sherwood number, DO2is the oxygen diffusivity in the particle boundary

layer, Ta is the fine particles temperature, Tbed is the bed temperature, h= kgNu/da is the

particle heat transfer coefficient, Nu is the particle Nusselt number, kg is the gas thermal

conductivity, eeff is the effective particle emissivity, k=(1 + n)/(1 + n/2), n is the primary

CO/CO2 ratio for combustion of char, DHCO and DHCO2are the heats of formation of CO

and CO2, respectively.

Provided that the values of the apparent kinetic constant kca, of bed temperature and of

oxygen concentration are known, Eqs. (8) and (9) can be used to compute particle

temperature and size. The following values of the parameters have been used: n = 0.3 [26];

eeff = 0.65 [27]. Particle Sherwood and Nusselt numbers have been evaluated following

Palchonok [27].

4. Results and discussion

Table 2 reports the operating conditions and the experimental results of steady

combustion experiments carried out at 850 jC, with variable inlet oxygen concentration

under both ordinary and sound-assisted conditions.

Table 2

Results from experiments and model calculations

Run # Experimental data Calculated data

O2IN

(%)

O2OUT

(%)

Fc

(g/min)

Ec

(g/min)

Wa +Wf

(g)

gfc kelV(min� 1)

Wf

(g)

fa Ta(jC)

da(mm)

Without sound

1 7.6 0.94 0.117 0.0088 0.135 0.925 0.065 0.0029 46 852 0.79

2 7.6 2.07 0.098 0.0075 0.130 0.923 0.058 0.0025 52 852 1.26

3 10.6 2.52 0.143 0.0065 0.075 0.955 0.087 0.0022 35 854 0.86

4 10.6 4.27 0.112 0.0068 0.087 0.939 0.078 0.0023 38 853 1.38

5 21 4.00 0.257 0.0068 n.m. 0.973 – – – – –

6 21 6.00 0.224 0.0052 n.m. 0.977 – – – – –

With sound

7 7.6 0.55 0.123 0.0081 0.163 0.934 0.050 0.0027 60 852 0.65

8 7.6 1.37 0.109 0.0046 0.115 0.958 0.040 0.0015 75 852 0.90

9 7.6 1.69 0.103 0.0055 0.130 0.947 0.042 0.0018 71 852 1.10

10 10.6 3.49 0.125 0.0056 0.110 0.955 0.051 0.0019 59 853 1.31

11 10.6 4.49 0.107 0.0048 0.092 0.955 0.052 0.0016 58 853 1.47

n.m.: not measured.


Analysis of data in Table 2 suggests that oxygen concentration at the exhaust

significantly departs from the value at the inlet, i.e. the combustor behaves as an integral

reactor with respect to oxygen feedings. Carbon elutriation rates are always smaller than

8% of the fixed carbon feed rate. Correspondingly, combustion efficiencies are always

above 92%. Overall carbon loadings (here Wa +Wf) establishing at steady state in the bed

are in the order of 0.1 g.

The apparent elutriation constant kelV, obtained as the ratio of the elutriation rate and the

total carbon loading, is in the order of 0.05 min� 1. This result is consistent with residence

times of fines in the reactor reported by Chirone et al. [20]. It further justifies assumption

(iv), being the residence time of the fines in the bed several orders of magnitude larger than

in the freeboard. It is interesting to compare the value of kelVwith values obtained from

available literature correlations. The Zenz and Weil [29] correlation (following Geldart

[28]) yields kel = 0.05 s� 1 under the operating conditions of the present study. Similar

values are obtained with other correlations. It can be confidently stated that the elutriation

constant kel estimated if char fines were all freely moving in the bed is far larger than the

actual constant kelV, the ratio between the two being fai50 (Table 2). The value of kel can

be used to assess the carbon loading of free fines in the bed (Wf) after Eq. (6). Results are

reported in Table 2 and show that the free fines carbon loadings that would be consistent

with the observed elutriation rates would be nearly two orders of magnitude smaller than

the total carbon loadings actually measured in the experimental runs. Despite approx-

imations and uncertainties associated with this estimate, it can be confidently stated that

the working hypothesis made in the Theory, namely that WfbWa, is correct. This

hypothesis enables using Eq. (5) to determine the combustion kinetic constant of the

attached fines kca.

The application of sound results in further decrease of the apparent ‘‘elutriation’’

constant kelV compared to kel. Values of the char residence time in the reactor are


correspondingly larger by about 50% than values obtained in the absence of sound. This

result is consistent with the likely influence that the application of sound exerts on the

occurrence of collisions and adhesion of fine particles on bed material [21,22].

Samples of bed material were discharged from the reactor after the tests and

characterized by scanning electron microscopy (SEM). Fig. 3 shows a typical SEM

micrograph of a bed material sample after test. Coarse agglomerates of three to six sand

particles firmly stuck together are observed. EDX analysis carried out for all the samples

reveals a strong enrichment of potassium and, to a lesser extent, of sodium on the surface

of the sand particles, especially on fused bridges keeping sand particles together. These

findings recall similar results obtained by Chirone et al. [19] in the combustion of coarse

particles of the same biomass in a bed of pure quartz particles at 850 jC.Table 2 reports the particle temperature and the size of attached fines computed by

means of Eqs. (8) and (9). The active particle size da, relevant to char burnout, turns out

to be of the order of 1.0 mm, under both ordinary and sound-assisted combustion

conditions. The finding that this size is much larger than particle sizes of both the parent

fuel and the bed material implies that attached fines burn as agglomerates. It can be

considered that aggregates of about five sand particles would be consistent with a value

of da = 1 mm. This finding is in agreement with the above reported occurrence of

agglomerates of three to six particles in the bed material retrieved at the end of the

combustion experiments.

Table 2 further indicates that the temperature of the attached char fines is very close to

the bed temperature for all the experiments. On the one hand, this result is the consequence

of carbon burning rates much lower than if the char were burning as freely moving

particles. On the other hand, the limited overheating is a consequence of effective heat

transfer between char–sand agglomerates and the bed material. It must be noted that there

Fig. 3. SEM micrograph of a bed sample after biomass combustion at 850 jC.


are no experimental data available to date in the literature on the temperature of burning

fines in a dense fluidized bed, that is actually beyond the present measurement capabilities.

Fig. 4 reports a comparison between experimental and computed values of the fixed

carbon combustion efficiency. Theoretical curves correspond to computations carried out

assuming values of da = 1.0 mm and da = 0.1 mm, under both ordinary and sound-assisted

conditions. It is noted that the combustion efficiency increases by decreasing the particle

size. This is a consequence of the high reactivity of the biomass tested: in fact, the biomass

fines have typically sufficient residence time for complete burn-out, as opposed to fines

from low-reactivity fuels (coal) that are elutriated with significant unburned carbon.

Curves relative to da = 1.0 mm fit experimental data points better than those obtained

assuming da = 0.1 mm (that is, the average particle size of powder biomass in the feeding).

In the latter case, loss of combustion efficiency would be negligible throughout the range

of oxygen concentrations investigated.

Altogether, results of the present study confirm and support previous findings of

Chirone et al. [19]. The scenario according to which burnout of biomass char fines in a bed

of sand is associated with formation of millimeter-sized char–sand aggregates represent-

ing the major carbon-bearing phase in the bed receives here additional confirmation.

Phenomenologies observed during burn-off of coarse biomass particles [19] and of

powdered biomass are very similar, as far as the formation of ash layers and the occurrence

of bed agglomeration is concerned. This suggests that burnout of attrited char fines should

be relevant to the fate of ash even when fuel feeding consists of coarse biomass. Adhesion

of attrited char fines onto inert bed particles is favoured by higher peak burning

temperatures, and is associated with the formation of alkali-rich surface layers. At bed

Fig. 4. Efficiency of fixed carbon combustion gfc versus oxygen concentration at the combustor outlet. Symbols:

experimental results; lines: model computations.


temperatures larger than the alkali-silicate eutectic, bed particle stickiness is enhanced by

the formation of melts and bed agglomeration onsets.

5. Conclusions

The mechanism of bed agglomeration during the fluidized bed combustion of biomass

is studied by means of steady combustion experiments of a biomass powder under both

ordinary and sound-assisted conditions. The application of sound is used as a tool to

influence the extent of adhesion/agglomeration of fine char without influencing the other

operating variables.

Results indicate that carbon elutriation rate is rather small, despite the small size of

particles in the feeding. It is about two orders of magnitude smaller than carbon elutriation

rates predicted on the basis of the measured carbon loading in the bed and of elutriation

rate constants for fine particles freely moving in the bed. Consistently with this result,

inspection by SEM of bed samples collected after several hours of operation of the reactor

indicated that extensive formation of char–sand aggregates takes place. It is therefore

concluded that fixed carbon conversion takes place to a large extent via the adhesion of

biomass powder onto bed particles followed by carbon burn-off as a captive, non-

elutriable, phase. The application of sound further enhances particle adhesion and

ultimately results in a decrease of the apparent fine particle elutriation rate. Combustion

efficiency increases accordingly.

A phenomenological model of carbon conversion has been developed in order to assess

the relative importance of free fines versus aggregate carbon burn-off. Input variables to

the model are measured carbon loadings and elutriation rates from the experiments under

the hypothesis that fixed carbon is mostly associated with char–sand aggregates. The

model output is represented by the size of aggregates established in the bed at steady state

and by their temperature. In particular, the former turns out to be significantly larger than

the size of feed material, and of the same order of magnitude as the aggregate size found in

the bed.

In conclusion, experimental and theoretical results highlight the importance of char/ash

fines adhesion onto bed particles as the process responsible for the formation of large bed

aggregates that may ultimately lead to agglomeration/defluidization problems.

Acknowledgements

The support of Mrs. C. Zucchini and Mr. S. Russo in SEM/EDX analysis and of Mr. A.

Cammarota and Mr. M. Serpi in fluidized bed experiments is gratefully acknowledged.

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The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel