EDX study of bed agglomerates formed during fluidized bed combustion of three biomass fuels

EDX study of bed agglomerates formed during fluidized bed combustion of three biomass fuels

ARTICLE IN PRESS BIOMASS AND BIOENERGY 32 (2008) 252 – 266 Available at www.sciencedirect.com http://www.elsevier.com/locate/biombioe An SEM/EDX s...

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ARTICLE IN PRESS BIOMASS AND BIOENERGY

32 (2008) 252 – 266

Available at www.sciencedirect.com

http://www.elsevier.com/locate/biombioe

An SEM/EDX study of bed agglomerates formed during fluidized bed combustion of three biomass fuels Fabrizio Scala, Riccardo Chirone Istituto di Ricerche sulla Combustione—Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio, 80, 80125 Napoli, Italy

ar t ic l e i n f o

abs tra ct

Article history:

The agglomeration behaviour of three biomass fuels (exhausted and virgin olive husk and

Received 21 July 2006

pine seed shells) during fluidized bed combustion in a lab-scale reactor was studied by

Received in revised form

means of SEM/EDX analysis of bed agglomerate samples. The effect of the fuel ash

6 September 2007

composition, bed temperature and sand particle size on agglomeration was investigated.

Accepted 16 September 2007

The study was focused on the main fuel ash components and on their interaction with the

Available online 24 October 2007

bed sand particles.

Keywords: Fluidized bed combustion Bed agglomeration Biomass

Agglomeration was favoured by high temperature, small sand size, a high fraction of K and Na and a low fraction of Ca and Mg in the fuel ash. An initial fuel ash composition close to the low-melting point eutectic composition appears to enhance agglomeration. The agglomerates examined by SEM showed a hollow structure, with an internal region enriched in K and Na where extensive melting is evident and an external one where sand

Alkali

particles are only attached by a limited number of fused necks. Non-molten or partially molten ash structures deposited on the sand surface and enriched in Ca and Mg were also observed. These results support an ash deposition–melting mechanism: the ash released by burning char particles inside the agglomerates is quantitatively deposited on the sand surface and then gradually embedded in the melt. The low-melting point compounds in the ash migrate towards the sand surface enriching the outermost layer, while the ash structure is progressively depleted of these compounds. & 2007 Elsevier Ltd. All rights reserved.

1.

Introduction

Bed agglomeration/defluidization problems have often been reported when burning biomass fuels in a fluidized bed of silica sand [1–3]. Operational experience showed that bed defluidization is accompanied by a sharp drop in the differential pressure across the bed (as a result of air channelling through the bed), and a significant increase of the temperature in the lower freeboard section (due to stratified biomass combustion on the top of the agglomerated bed) [1,2,4]. This makes the operation uncontrollable, and leads to unscheduled shutdowns and costly maintenance stops for the boiler. Corresponding author. Tel.: +39 081 7682969; fax: +39 081 5936936.

E-mail address: [email protected] (F. Scala). 0961-9534/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2007.09.009

The onset of bed agglomeration is attributed to the presence of alkali species (potassium and/or sodium) in the ash of biomass fuels, which interact with silica sand under hightemperature conditions leading to the formation of low-melting point eutectics [2,4–6]. Under prolonged operation, this sticky superficial layer leads to the formation of permanent bonds between sand particles. Bed agglomeration is favoured by local temperature peaks associated with combustion of char particles [4,7,8]. Recently, a thorough review of the mechanisms for bed agglomeration during biomass fluidized bed combustion has been reported [4]. Based on experimental data available in the literature and on their own results, the authors concluded that alkali species most likely transfer by random collisions of sand

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with burning char particles. The authors further suggested that bed agglomerates start to form near burning char, where the higher temperatures enhance the formation of melt and, in turn, the particles’ stickiness. The variables that exert the largest influence on the agglomeration process are the operating temperature and the composition of the biomass fuel ash [4,9–11]. The effect of the first variable is straightforward, as the formation of agglomerates is enhanced at higher temperatures, where it is easier to reach low-melting point eutectics at the inert particle surface. On the other hand, the effect of biomass ash composition is more uncertain. In general, the higher the fraction of alkali metals and the lower the fraction of highmelting point compounds (Al, Ca, Mg), the easier the agglomerates are formed. However, differences in the behaviour of a number of biomass fuels still remain unclear. In this work, an SEM/EDX study of the agglomerates formed during the fluidized bed combustion of three biomass fuels in a lab-scale reactor has been carried out to investigate the effect of fuel ash composition, bed temperature and particle size on their agglomeration behaviour. The study is focused on the main fuel ash components and on their interaction with the bed sand particles. The overall fluidized bed combustion behaviour of the three biomass fuels at hand and the corresponding fluid-dynamic characterization of bed defluidization conditions have been recently reported and discussed in detail elsewhere [4,12].

2.

Experimental

2.1.

Apparatus

The experimental apparatus, sketched in Fig. 1, consisted in a stainless-steel cylindrical fluidized bed with an inner

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diameter of 0.102 m and a height of 1.625 m. The vessel was fitted with a stainless-steel perforated distributor plate with 518 holes, 0.5 mm in diameter, disposed in triangular pitch. The windbox was packed with ceramic rings to act as a gas preheater. The vessel was equipped with five tubes flush to the column wall located at a vertical distance z from the distributor plate equal to 0.045, 0.095, 0.145, 0.245 and 0.775 m. The tube located at z ¼ 0.095 m was used for fuel feeding, those located at z ¼ 0.045, 0.145 and 0.245 m were fitted with a pressure tap and a thermocouple (K type), while the tube located at z ¼ 0.775 m was fitted only with a thermocouple. The combustor was electrically heated by means of ceramic heaters. The reactor temperature was controlled by two PID controllers driven by the signal from the thermocouples inserted near the column wall by means of the tubes located at z ¼ 0.045 and 0.775 m. The fluidizing gas was preheated up to 500 1C in an electrical heater. The flue gas exiting the combustor was directed to a high-efficiency cyclone for fine particles collection. After the cyclone, the flue gas was sampled for gas analysis (O2, CO, CO2, SO2 and CH4). The fluidization column was equipped with an air-assisted solids metering/feeding system for continuous injection of the fuel at the bottom of the bed. The feeding system consists of a fuel hopper mounted over a screw feeder that further delivers the powder in a mixing chamber where a swirled air flow pneumatically conveys the fuel within the bed. Fluidizing and fuel-feeding air flows were metered with two mass flow controllers.

2.2.

Materials

Technical air was used both as primary fluidizing gas and to assist fuel feeding. Fresh quartz sand, sieved in the two size ranges 212–400 and 600–850 mm, was used as bed inert

Fig. 1 – Experimental apparatus. (1) Thermocouple; (2) temperature PID controller; (3) preheater; (4) thermocouple and pressure transducer; (5) acquisition data unit; (6) personal computer; (7) gas analysers; (8) condenser; (9) filter; (10) cyclone; (11) gas distributor; (12) mass flow controller; (13) ceramic heaters; (14) windbox; (15) fuel–air mixer; (16) screw feeder; (17) fuel hopper.

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material. The bed inventory was kept constant at 3.3 kg, corresponding to a bed height at minimum fluidization conditions of 0.3 m. The three biomass fuels used in this work were olive husk, either exhausted or virgin, and shells of pine seeds (Pinus pinea), whose properties are reported in Table 1. Olive husk is a solid residue of the olive oil production industry. Virgin husk is obtained after mechanical extraction of oil by olive pressing, while exhausted husk is obtained after solvent extraction of residual oil from virgin husk. The particle size of the husk used in the experiments was in the range of 20–4000 mm. Pine seed shells have in origin an irregular and drop-like shape (approximately 20  10  5 mm in size) and good mechanical resistance. In order to ensure reliable fuel feeding, pine seed shells were crushed and sieved in the size range 1.0–3.3 mm.

2.3.

Procedures

Steady combustion tests were carried out at a fluidization velocity varying between 0.38 and 0.92 m s1. Bed temperature was fixed at either 850 or 900 1C. The combustor start-up was accomplished by electrically heating the bed of fresh inert sand. When the bed temperature reached a value of 750 1C, fuel feeding was started. The fuel feed was adjusted in the range 0.33–0.82 kg h1 to reach the desired excess air value (in the range of 17–110%, i.e. e ¼ 1.17–2.1). During the run,

Table 1 – Properties of biomass fuels tested Exhausted husk

Virgin husk

Pine seed shells

1250

1310

1200

17.5

19.8

15.2

13.04 56.25 26.23 4.43

8.54 73.21 16.77 1.48

11.87 67.51 19.27 1.35

Ultimate analysis (dry basis) (%w) Carbon 51.76 Hydrogen 5.46 Nitrogen 1.25 Sulphur 0.09 Ash 5.09 Oxygen (diff) 36.35

47.06 6.82 0.82 0.11 1.62 43.57

49.80 6.17 0.34 0.04 1.52 42.13

Ash composition (%w) CaO MgO K2O Na2O Fe2O3 Al2O3 SiO2 P2O5 SO3

12.34 4.70 41.65 0.50 2.34 3.23 18.99 8.81 2.03

2.57 2.74 15.02 1.84 0.41 0.34 72.86 0.60 2.21

Particle density (kg m3) LHV (MJ kg1) Proximate analysis (%w) Moisture Volatiles Fixed carbon Ash

22.34 1.80 25.56 1.74 2.75 6.22 34.78 1.77 0.92

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temperature, pressure and gas concentration data were continuously logged in a PC. Every 15–20 min elutriated material collected at the cyclone was measured and analysed for carbon concentration, for calculation of the unburned carbon flow rate at the exhaust. Mass balance closures on carbon and oxygen were always within 3% error. The run ended when defluidization of the bed occurred, as indicated by a jump in the temperature and pressure profiles within the bed. The total time interval from the beginning of the fuel feeding until the defluidization onset was recorded. After the end of the run, the bed was cooled down and discharged from the combustor, weighed and sieved for agglomerates separation. Selected agglomerate samples were observed under a scanning electron microscope (SEM; Philips XL30 with LaB6 filament) and subjected to energy dispersive X-ray (EDX; Edax DX-4) elemental analysis. Few samples were embedded in epoxy resin and then cut and polished for SEM observation and EDX analysis of agglomerate cross-section.

3.

Results and discussion

Table 2 reports a summary of the operating conditions and of the main results of the defluidization experiments carried out with the three different biomass fuels. The steady combustion tests were performed at different bed temperatures (Tbed), fluidization velocities (U), excess air factors (e) and inert bed particle sizes (dS). The overall results of the experiments are reported in terms of combustion efficiency (Z), defluidization time (tD) and percent of ash accumulated in the bed at defluidization (Wash). Further details on the combustion behaviour, gas emissions, pressure and temperature profiles and fluid-dynamic conditions during the runs and upon bed defluidization have been reported thoroughly elsewhere [4,12]. All the experiments eventually ended with bed defluidization, whose occurrence was clearly detectable from temperature and pressure measurements in the combustor [2,4]. Upon defluidization, the pressure within the bed abruptly peaked down because of the onset of bed channelling, while the temperature in the lower section of the bed peaked down, and that in the upper section peaked up. This trend can be related to the decrease of the bed heat transfer coefficient upon defluidization and to the segregated fuel combustion in the upper bed region. Figs. 2 and 3 report the measured defluidization time (tD) and the maximum weight fraction of ash accumulated in the bed (Wash) at the end of each combustion run for the three fuels, as a function of the excess air factor (adapted from Refs. [4,12]). Defluidization time is defined as the time interval between the start of the biomass fuel feeding and the bed defluidization occurrence, while Wash is calculated as the total ash inlet with the fuel feed during the run divided by the inert bed weight and multiplied by 100. It must be underlined that this is a theoretical quantity as the elutriation loss of both sand and ash during the run is not taken into account in the calculation. Analysis of data reported in Fig. 2 clearly shows that for each fuel faster agglomeration occurred with a higher

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Table 2 – Summary of operating conditions and main results of the combustion experiments Run #

Fuel

T (1C)

U (m s1)

e ()

dS (mm)

Z (%)

tD (min)

Wash (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Exhausted husk Exhausted husk Exhausted husk Exhausted husk Exhausted husk Exhausted husk Exhausted husk Exhausted husk Exhausted husk Virgin husk Virgin husk Virgin husk Virgin husk Virgin husk Pine seed shells Pine seed shells Pine seed shells Pine seed shells Pine seed shells Pine seed shells Pine seed shells

850 850 850 850 850 850 900 900 900 850 850 850 850 850 850 850 850 850 900 900 850

0.48 0.48 0.48 0.47 0.44 0.47 0.49 0.46 0.45 0.59 0.61 0.92 0.38 0.61 0.52 0.71 0.55 0.50 0.53 0.52 0.54

1.17 1.17 1.42 1.44 1.67 2.10 1.22 1.38 1.65 1.47 1.77 1.76 1.78 1.76 1.40 1.89 1.35 1.75 1.78 1.23 1.58

212–400 212–400 212–400 212–400 212–400 212–400 212–400 212–400 212–400 212–400 212–400 212–400 212–400 600–850 212–400 212–400 212–400 212–400 212–400 212–400 600–850

96.1 96.1 97.3 97.1 99.2 99.7 96.0 97.1 98.8 98.5 99.6 98.7 98.3 99.2 96.9 98.6 96.9 97.7 97.8 96.8 97.8

196 200 207 287 285 310 156 168 225 147 197 119 230 348 320 318 320 388 31 35 702

3.0 3.2 2.7 3.4 2.9 2.7 2.4 2.1 2.3 0.48 0.57 0.51 0.41 1.0 0.95 1.2 1.3 1.1 0.21 0.14 2.3

operating temperature, with a lower excess air and with a higher fluidization velocity. The first effect is consistent with literature data [4,11] reporting that agglomerate formation is enhanced at higher temperatures. The other two effects are simply a consequence of the larger fuel feed rate adopted when a lower excess air value or when a higher fluidization velocity was chosen. This point is more evident upon inspection of Fig. 3, which shows that irrespective of the excess air value and fluidization velocity, a well-defined ash content was necessary to defluidize the bed, depending on the bed temperature. In the experiments carried out with a larger sand particle size (runs # 14 and 21) both the defluidization time (Fig. 2) and the weight fraction of ash accumulated in the bed upon defluidization (Fig. 3) approximately doubled with respect to the corresponding experiments where the smaller sand was used (runs # 11 and 15). A possible explanation of this result relies on the consideration that larger bed particles have more inertia and consequently are associated with more energetic collisions, so that adhesion of the particles one to the other to form agglomerates should be more difficult [4]. Comparison of data for the three different fuels in Figs. 2 and 3 allows us to draw the following conclusions: (a) at 850 1C combustion of virgin olive husk was associated with the shortest defluidization time and combustion of pine seed shells with the longest one; (b) at the same operating temperature the lowest ash content in the bed at defluidization was found with virgin olive husk and the largest one with exhausted olive husk; (c) the increase of operating temperature to 900 1C brought a moderate decrease of tD and Wash for exhausted olive husk, a dramatic decrease of both quantities for pine seed shells (the effect of temperature was not tested for virgin olive husk); (d) the increase of sand particle size had a similar influence on both virgin olive husk and pine seed

shells (this effect was not tested for exhausted olive husk). Explanation of some of these results is not straightforward; therefore a thorough campaign of SEM/EDX analyses of the agglomerates collected after defluidization runs was carried out to investigate the effect of the fuel composition on their agglomeration behaviour at different operating conditions.

3.1.

SEM/EDX analysis of agglomerate samples

At the end of each combustion test the bed was discharged for further characterization. The agglomerates discharged from the bed appeared to be quite weak and were easily broken into smaller ones. Approximately 2% of the bed material was found to have a particle size larger than the initial size range of the fresh sand. However, the bed discharge procedure is likely to have influenced the particle size distribution of the bed by partly breaking the agglomerates. About 10% of this larger bed material was made up by big agglomerates containing a considerable number of sand particles stuck together. In the following, agglomerates collected in selected runs will be examined separately. However, a general feature of the agglomerates collected in all the runs was a hollow structure, indicating that a burning fuel particle was located inside and initiated the agglomeration process [4].

3.1.1.

Exhausted olive husk: T ¼ 850 1C, dS ¼ 212–400 mm

Figs. 4A and B show SEM micrographs of two typical agglomerate samples discharged from the bed after defluidization at 850 1C. In particular, Fig. 4B clearly shows the hollow structure of the agglomerate. Inspection of the figures indicates the presence of two different zones: in the first one the sand particles appear to be completely embedded in a fused layer of material, while in the second one the sand

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800 600

T = 850 °C T = 900 °C

Exhausted olive husk

T = 850 °C

Virgin olive husk

400

Defluidization time, min

200 0 800 600 ds = 600-850 µm

400

U = 0.38 m/s 200 U = 0. 92 m/s 0 800 Pine seed shells

T = 850 °C T = 900 °C

600

ds = 600-850 µm

400 20 0 U = 0.71 m/s 0 1.0

1.2

1.4

1.6

1.8

2.0

2.2

e, Fig. 2 – Measured defluidization time as a function of the excess air factor for all combustion experiments with the three biomass fuels (adapted from Refs. [4,12]).

particles are only attached one to the other by a limited number of contact points, in the form of fused material necks. The first kind of zone was typically found in the internal part of the agglomerates, indicating that higher temperatures were experienced inside. Figs. 4C and D show SEM micrographs at higher magnification of selected zones of the external surface of the agglomerate samples. Many zones appear to be fused and re-solidified. Within the fused material, however, non-molten or partially molten ash inclusions are clearly visible. These ash inclusions are likely to have been first deposited on the sticky fused surface and then progressively embedded in the melt. In particular, Fig. 4D clearly shows an ash structure deposited on the bed material, but that did not undergo any significant melting yet. Results of EDX spot analyses of selected zones of the agglomerates (points a–d in Fig. 4) are reported in Table 3. An enrichment of potassium (and sodium) can be clearly observed in the fused layer.

3.1.2.

and internal surface of the agglomerate samples. As for the agglomerates formed at 850 1C, two different zones can be clearly distinguished. The external surface appears to be composed of sand particles attached one to the other by a limited number of contact points (Fig. 5C). Fused zones or necks are less evident than for the sample obtained at 850 1C. On the other hand, the internal surface appears to be very rough (Fig. 5D). At a closer examination of Fig. 5D a smooth fused zone is visible at a deeper level, while the outermost layer is much rougher. This result supports the ash deposition–melting mechanism recalled before: the ash released by burning char particles inside the agglomerate is quantitatively deposited on the sticky sand surface and then gradually embedded in the melt. In this case, this process appears to be much more intense than for the samples obtained at 850 1C with the same fuel. Results of EDX spot analyses of selected zones of the agglomerates (points e–g in Fig. 5) are reported in Table 3. Again, an enrichment of potassium (and sodium) is found in the fused layer (point g).

Exhausted olive husk: T ¼ 900 1C, dS ¼ 212–400 mm

Fig. 5A shows the SEM micrograph of a typical agglomerate sample discharged from the bed after defluidization at 900 1C. Fig. 5B shows the internal surface of another (hollow) agglomerate that was broken into pieces before observation. Figs. 5C and D show at a higher magnification the external

3.1.3.

Virgin olive husk: T ¼ 850 1C, dS ¼ 212–400 mm

After tests with virgin husk at 850 1C, the agglomerates appear similar to those obtained after tests with exhausted husk at the same temperature. Figs. 6A and B show SEM micrographs of a typical agglomerate sample and a piece of another

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4 Exhausted olive husk 3 2

% Accumulated ash at defluidization

1

T = 850°C T = 900°C

0 4 Virgin olive husk

T = 850°C 3 ds = 600-850 µm 2 1 0 4

Pine seed shells

T = 850°C T = 900°C

3

ds = 600-850 µm

2 1 0 1.0

1.2

1.4

1.6

1.8

2.0

2.2

e, Fig. 3 – Weight fraction of ash accumulated in the bed at defluidization as a function of the excess air factor for all combustion experiments with the three biomass fuels (adapted from Refs. [4,12]).

Fig. 4 – SEM micrographs at different magnifications of an agglomerate sample (A), details of the hollow structure of another agglomerate (B), details of a fused zone (C), and an ash structure attached on the sand surface (D). Exhausted olive husk: T ¼ 850 1C, dS ¼ 212–400 lm.

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Table 3 – EDX spot analysis of selected zones of the agglomerates shown in Figs. 4–11 Spot

Na

Mg

Al

Si

P

K

Ca

Fe

Exhausted husk a b c d e f g

1.32 6.11 5.57 2.11 1.39 1.23 6.28

1.30 1.69 2.33 4.81 0.98 4.49 2.44

2.70 2.95 1.98 5.66 1.71 4.51 1.29

51.02 6.12 6.11 32.21 77.38 12.97 52.99

5.63 3.23 2.12 6.19 2.96 3.21 2.94

16.94 66.81 35.94 16.19 7.09 4.63 17.17

14.14 8.60 2.21 25.84 6.22 57.37 12.59

6.95 4.49 43.74 6.99 2.27 11.59 4.30

Virgin husk h i j k l m

0.85 1.00 11.55 5.98 3.53 3.91

1.06 3.83 3.30 2.44 3.37 2.67

9.05 2.38 2.39 1.98 2.25 2.26

65.99 22.63 57.74 63.89 45.84 32.47

2.02 22.00 – – 14.30 5.26

18.77 7.44 6.81 13.04 6.77 3.01

1.16 39.89 16.71 11.18 20.90 48.47

1.10 0.83 1.50 1.49 3.04 1.95

Pine seed shells n o p q r s t u

4.62 15.22 2.23 18.36 13.11 5.94 3.67 5.21

8.58 1.15 0.78 14.35 4.38 2.50 7.73 2.37

5.42 1.90 1.61 1.24 1.79 0.92 1.85 2.76

47.84 69.66 88.94 2.74 67.34 66.52 63.13 75.48

5.45 – 4.41 44.20 4.74 5.06 10.93 3.45

5.69 9.31 0.88 0.73 6.66 11.19 1.75 4.41

8.80 1.42 0.50 17.93 1.51 6.63 9.20 2.56

13.60 1.34 0.65 0.45 0.47 1.24 1.74 3.76

Data are in %w of the element (carbon and oxygen free basis).

Fig. 5 – SEM micrographs at different magnifications of an agglomerate sample (A), the interior of a broken agglomerate (B), details of the external surface (C), and details of the internal surface (D). Exhausted olive husk: T ¼ 900 1C, dS ¼ 212–400 lm.

agglomerate that was broken before observation. Figs. 6C and D show at a higher magnification selected zones of the external surface of the agglomerate samples. In particular,

Fig. 6D shows a partially molten ash structure deposited on the bed material. Results of EDX spot analyses in points h–i in Fig. 6 are reported in Table 3.

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Fig. 6 – SEM micrographs at different magnifications of a typical agglomerate sample (A), a piece of broken agglomerate (B), details of the external surface (C), and an ash structure attached on the sand surface (D). Virgin olive husk: T ¼ 850 1C, dS ¼ 212–400 lm.

3.1.4.

Virgin olive husk: T ¼ 850 1C, dS ¼ 600–850 mm

For the run with the larger sand size the agglomerates discharged from the bed appear slightly different with respect to those obtained with the smaller sand. Fig. 7A shows the SEM micrograph of a typical agglomerate sample, while Fig. 7B shows the internal surface of another agglomerate that was broken into pieces before observation. Figs. 7C and D show SEM micrographs at higher magnification of selected zones of the external surface of the agglomerate samples. Inspection of the figures indicates that in this case both on the external surface and on the internal one melting was much more extensive than for the samples obtained with the smaller sand. This is likely the result of the larger accumulation of ash in the bed in this run (see Fig. 3). Results of EDX spot analyses in points j–m in Fig. 7 are reported in Table 3. Noteworthy, a significant enrichment of Na was found in the fused material, especially on the internal surface of the agglomerate (point j).

3.1.5.

Pine seed shells: T ¼ 850 1C, dS ¼ 212-400 mm

After tests with pine seed shells at 850 1C, the agglomerates appear similar but more porous than those obtained after tests with olive husk. Figs. 8A and B show SEM micrographs of a typical agglomerate and of a detail of the external surface at a higher magnification. Extensive melting on the external surface is evident and large voids are found throughout the agglomerate. Figs. 8C and D show SEM micrographs of the cross-section of an agglomerate embedded in epoxy resin and then cut and polished, and details at higher magnification. The sand particles (grey) in the agglomerate appear to be

loosely connected one to the other by fused necks (light grey), while large voids are filled with epoxy resin (dark grey). Results of EDX spot analyses in points n–o in Fig. 8 are reported in Table 3.

3.1.6.

Pine seed shells: T ¼ 900 1C, dS ¼ 212–400 mm

After tests with pine seed shells at 900 1C, the agglomerates appear more compact than those obtained at 850 1C. Figs. 9A and B show SEM micrographs of a typical agglomerate and detail of the external surface at a higher magnification. As for the agglomerate obtained with olive husk at 900 1C, fused zones or necks are much less evident than for the sample at 850 1C on the external surface. This might be explained by the extremely short agglomeration time for these runs and, in turn, by the scarce accumulation of ash on the sand particles. Figs. 9C and D show SEM micrographs of the crosssection of an agglomerate embedded in epoxy resin and then cut and polished, and details at higher magnification. Figs. 9E and F show selected zones of the internal surface of an agglomerate that was broken before observation. It is evident that in this case the internal surface appears to be very rough, even if several fused smooth zones can be observed. Again, Fig. 9F shows a non-molten ash structure deposited on the internal surface of the agglomerate. Results of EDX spot analyses in points p–t in Fig. 9 are reported in Table 3.

3.1.7.

Pine seed shells: T ¼ 850 1C, dS ¼ 600–850 mm

Fig. 10A shows the SEM micrograph of a typical agglomerate sample, while Fig. 10B shows details of the external surface

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Fig. 7 – SEM micrographs at different magnifications of a typical agglomerate sample (A), the interior of a broken agglomerate (B), details of the external surface (C), and ash debris attached on the external surface (D). Virgin olive husk: T ¼ 850 1C, dS ¼ 600–850 lm.

Fig. 8 – SEM micrographs at different magnifications of a typical agglomerate sample (A), details of the external surface (B), cross-section of an agglomerate (C), and details of the agglomerate cross-section (D). Pine seed shells: T ¼ 850 1C, dS ¼ 212–400 lm.

of the agglomerate. As was found for the olive husk sample (Fig. 7), with the larger bed sand extensive melting is observed on the external surface. In this case, however, the

agglomerate appears more porous than those obtained with olive husk. Results of EDX spot analysis in point u in Fig. 10 are reported in Table 3.

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Fig. 9 – SEM micrographs at different magnifications of a typical agglomerate sample (A), details of the external surface (B), cross-section of an agglomerate (C), details of the agglomerate cross-section (D), details of the interior of a broken agglomerate (E), and details of an ash structure attached on the internal surface (F). Pine seed shells: T ¼ 900 1C, dS ¼ 212–400 lm.

3.2.

Analysis of EDX composition data

To better understand the agglomeration behaviour of the three fuels, the EDX analyses carried out on the samples have been organized and compared with each other, on the basis of the appearance of the spot in the SEM micrograph where the analysis was performed. In particular, three different zones were distinguished: sand particles with visible molten material; ash structures deposited on the sand surface; and sand particles without visible molten material. This comparison was made on a silica-free basis, to get rid of the influence of the sand particles. Figs. 11–13 report the average composition of the different zones on the surface of the agglomerated samples, for the three fuels. In the same figures the fuel ash compositions (on a silica-free basis) are also reported for comparison. It must be recalled here that EDX is a semiquantitative analysis, so that comparison with ash compositions should be carried out with care.

The following conclusions can be drawn from Figs. 11–13. For all the fuels K and Na are enriched in the molten material and depleted in the deposited ash, while on the contrary Ca and Mg are depleted in the molten material and enriched in the deposited ash. Differences among samples from the three fuels mostly reflect the differences in the composition of the initial fuel ashes. In all the samples K appears to be depleted with respect to the initial fuel ash, indicating that some loss by vaporization is probable. Al, P and Fe, instead, are enriched with respect to the initial fuel ash. Al and Fe most likely derive from sand impurities and from reactor walls, respectively. As regards P, it must be noted that EDX analysis is somewhat more uncertain for this species, because the P peak is located very near the Si peak (that is always the broadest one in the measured spectrum). The composition of the non-molten sand surface is always very close to that of the molten material (Figs. 12 and 13). As it will be shown later, the difference between these two zones mostly relies on

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Fig. 10 – SEM micrographs at different magnifications of a typical agglomerate sample (A), and details of the external surface (B). Pine seed shells: T ¼ 850 1C, dS ¼ 600–850 lm.

100 Molten material - T = 850°C Molten material - T = 900°C Deposited ash - T = 850°C Deposited ash - T = 900°C Fuel ash

Exhausted olive husk

Composition, wt %

80

60

40

20

0 Ca

K

Na

Mg

Al

P

Fe

Fig. 11 – Average composition (on a silica-free basis) of different zones on the surface of agglomerated samples, obtained by semiquantitative SEM/EDX spot analyses. Exhausted olive husk.

the proportion of silica with respect to the other species. This result indicates that the non-molten sand surface represents an early step of ash accumulation on the sand, where the lowmelting point eutectic composition has not yet been reached. As regards the samples obtained at different temperatures (Figs. 11 and 13), no significant difference is evident in the molten material composition, while in the deposited ash an enrichment of Ca and a depletion of K at 900 1C is noticed. For the samples obtained using a larger sand size (Figs. 12 and 13) Ca is enriched and K depleted both in the molten material and in the deposited ash with respect to the samples with a smaller sand size. In analysing the composition of the different samples it appears that Si, Ca and K are often the three most abundant species. For this reason the agglomeration behaviour of the three fuels was also examined in the light of the ternary system K2O–CaO–SiO2. To this end, all the EDX analyses carried out on the samples have been rearranged and reported in the ternary diagram shown in Fig. 14, together with the initial fuel ash compositions, for comparison. Fig. 15

reports for reference the high SiO2 corner of the phase diagram of the system K2O–CaO–SiO2 [13]. Comparison between data in Fig. 14 and the phase diagram in Fig. 15 should be carried out with care, because of the semiquantitative nature of the EDX analyses and also because the influence on the phase diagram of other species present in the samples (Na, Mg, Al, P and Fe) is neglected. Results reported in Fig. 14 are qualitatively similar to those ¨ hman et al. [14] for a number of other biomass reported by O fuels. Broadly speaking, the sample composition data in Fig. 14 can be grouped in three well-defined zones in the ternary diagram. Samples with visible molten material are all confined in the low CaO region of the diagram with a K2O content in the range 5–25%. This zone is clearly located in the phase diagram region where low-melting point eutectics appear (Fig. 15). Samples showing sand surface without visible molten material are confined in the nearby region on the right, with a K2O content in the range 0–10%. On the other hand, the composition of the ash structures deposited on the sand surface is always included in a region with a

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100 Molten material - 212-400µm Molten material - 600-850µm Deposited ash - 212-400µm Deposited ash - 600-850µm Non-molten sand - 212-400µm Fuel ash

Virgin olive husk

Composition, wt %

80

60

40

20

0 Ca

K

Na

Mg

Al

P

Fe

Fig. 12 – Average composition (on a silica-free basis) of different zones on the surface of agglomerated samples, obtained by semiquantitative SEM/EDX spot analyses. Virgin olive husk.

100 Molten material - T = 850°C Molten material - T = 900°C Molten material - 600-850µm Deposited ash - T = 900°C Non-molten sand - T = 900°C Non-molten sand - 600-850µm Fuel ash

Pine seed shells

Composition, wt %

80

60

40

20

0 Ca

K

Na

Mg

Al

P

Fe

Fig. 13 – Average composition (on a silica-free basis) of different zones on the surface of agglomerated samples, obtained by semiquantitative SEM/EDX spot analyses. Pine seed shells.

moderate-to-high CaO content (20–80%). It is interesting to note that a well-defined correlation between the melting appearance of the ash structures and the CaO content was found: the lower the CaO fraction, the higher the melting degree of the ash sample. For completeness, two analyses performed on one particular sample (shown in Fig. 4C) gave a very large K content (and also Fe), and are the only ones reported on the K2O-rich side of the diagram. On the whole, it appears that both the fuel type and the operating conditions have a limited influence on the location of the composition data points in the K2O–CaO–SiO2 ternary diagram. On the other hand, the previous observation that K

is enriched in the molten material and depleted in the deposited ash is confirmed, while Ca is depleted in the molten material and enriched in the deposited ash. This result is in line with the ash deposition–melting mechanism: once the fuel ash is deposited on the sand, the low-melting point compounds (K and Na) migrate towards the sand surface enriching the outermost layer, while the ash structure is depleted of these compounds (and at the same time enriched with Ca and Mg). From the above reasoning, however, the different agglomeration behaviour of the three fuels (as evidenced in Figs. 2 and 3) is not explained. At this point it is interesting to locate

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CaO Exhausted olive husk Virgin olive husk Pine seed shells Fuel ash

0

100

10

90

20

80

30

Ash deposited on sand

70

40

60

Exhausted olive husk 50 De

50

ee gr of

60

40

me

Virgin olive husk

g ltin

70

30

80

Sand particles without visible molten material

20

90

10

100

K2O

0

0 10

20

30

40

Partially molten ash deposited on sand

50

60

70

Pine seed shells

80

90

100

SiO2

Sand particles with visible molten material

Fig. 14 – Ternary diagram of the system K2O–CaO–SiO2 showing the measured composition of different zones on the surface of agglomerated samples, obtained by semiquantitative SEM/EDX spot analyses.

in Fig. 14 the initial fuel ash compositions. It is noted that the virgin olive husk ash has a much higher K content with respect to the exhausted husk. This means that a much lower quantity of ash should be deposited on the sand surface to reach the low-melting point eutectic composition. This may well explain the trends in Fig. 3 for these fuels. On the other hand, the K content of the pine seed shells ash is lower (but not on a silica-free basis, see Fig. 13) with respect to the two husks. Now, two points must be noted: first, the (high-melting point) Ca content of the pine seed shells ash is very low; second, because of the high Si content, the initial ash composition belongs to the region of the ternary phase diagram where the low-melting point eutectics appear (see Figs. 14 and 15). This indicates that ash melting may already begin before the ash structure comes in contact with the sand particle. Of course, the higher the operating temperature, the higher would be the melting degree and the stickiness of the ash. These considerations might explain the agglomeration behaviour of the pine seed shells with respect to the olive husks, and the strong enhancement of the agglomeration tendency of the pine seed shells with the increase of operating temperature to 900 1C (Figs. 2 and 3). Salour et al. [1] reported quite a similar agglomeration behaviour difference between rice straw and wood waste: while the two fuels ash potassium content was similar, rice straw had a much higher agglomeration tendency. In the light of the above considerations, the high silica content of the rice straw ash and the high calcium content of the wood waste ash might explain this behaviour.

4.

Conclusions

An SEM/EDX study of the agglomerates formed during the fluidized bed combustion of three biomass fuels (exhausted and virgin olive husk and pine seed shells) in a lab-scale reactor has been carried out to investigate the effect of the fuel ash composition, bed temperature and particle size on their agglomeration behaviour. The study is focused on the main fuel ash components and on their interaction with the bed sand particles. The following conclusions resulted from the analysis:

 for a particular fuel, agglomeration is favoured by higher temperatures and smaller sand sizes;

 a high fraction of low-melting point compounds (K, Na)





and a low fraction of high-melting point compounds (Ca, Mg) in the fuel ash favours agglomeration. In addition, an initial fuel ash composition close to the lowmelting point eutectic composition appears to enhance agglomeration; a general feature shown by the agglomerates is a hollow structure, indicating that a burning fuel particle was located inside and initiated the agglomeration process; two different zones were noticed on the agglomerates’ surface: in the internal region the sand particles appear to be completely embedded in a fused layer of material, while in the external one the sand particles are only attached by a limited number of fused necks;

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Fig. 15 – The high SiO2 corner of the phase diagram of the system K2O–CaO–SiO2. Adapted from Levin et al. [13]. Reprinted with the permission of The American Ceramic Society, www.ceramics.org (1956). All rights reserved.

 non-molten or partially molten ash structures deposited 





on the sand surface were also observed; in all the samples K and Na are enriched in the molten material and depleted in the deposited ash, while on the contrary Ca and Mg are depleted in the molten material and enriched in the deposited ash; a well-defined correlation between the melting appearance of the ash structures and the CaO content was found: the lower the CaO fraction, the higher the melting degree of the ash sample; the results support the ash deposition–melting mechanism: the ash released by burning char particles inside the agglomerate is quantitatively deposited on the sand surface and then gradually embedded in the melt. The lowmelting point compounds (K and Na) migrate towards the sand surface enriching the outermost layer, while the ash structure is progressively depleted of these compounds (and enriched with Ca and Mg).

Acknowledgements The authors are indebted to Mr. A. Cammarota, Mr. M. Fascelli, Ms. A. Narducci and Mr. F. Tartaglione for their assistance in performing experimental tests. The support of Mrs. C. Zucchini and Mr. S. Russo in performing the SEM/EDX analyses is gratefully acknowledged. The authors are grateful

to ENEL Produzione-PSI-Ricerca for providing olive husk samples. R E F E R E N C E S

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