Agglomeration problems during fluidized bed gasification of olive-oil residue: evaluation of fractionation and leaching as pre-treatments☆

Agglomeration problems during fluidized bed gasification of olive-oil residue: evaluation of fractionation and leaching as pre-treatments☆

Fuel 82 (2003) 1261–1270 www.fuelfirst.com Agglomeration problems during fluidized bed gasification of olive-oil residue: evaluation of fractionation...

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Fuel 82 (2003) 1261–1270 www.fuelfirst.com

Agglomeration problems during fluidized bed gasification of olive-oil residue: evaluation of fractionation and leaching as pre-treatmentsq Stelios Arvelakisa,*, Hans Gehrmannb, Michael Beckmannc, Emmanuel G. Koukiosa a

Bioresource Technology Unit, Laboratory of Organic and Environmental Technologies, Department of Chemical Engineering, National Technical University of Athens, Zografou Campus, Athens GR-15700, Greece b Clausthaler Umwelttechnik Institut GmbH, Leibnizstrasse 21 þ 23, Clausthal-Zellerfeld D-38678, Germany c Department of Process Engineering and Environment, Bauhaus Universitat, Weimar Coudray Straı´e 13C, Weimar D-99423, Germany Received 28 April 2002; accepted 3 January 2003; available online 5 April 2003

Abstract The effect of two pre-treatment techniques leaching and fractionation on the agglomeration problems observed during the gasification of the olive-oil residue material in a fluidized bed reactor was studied. The obtained results proved to be very positive in the case of leaching showing a substantial decrease of the agglomeration phenomena in the bed during the gasification tests compared with the results from the tests with the un-treated material. On the other hand, fractionation appeared to increase the ash reactivity leading to a dramatic increase of the agglomeration problems during the gasification process. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Leaching; Fractionation; Ash behavior

1. Introduction Agricultural residues and wastes are largely CO2-neutral because of their short regeneration time and due to the fact that they contribute to CO2-increase only through the fossil fuels consumed during their cultivation and transport to the energy production plants. However, the use of this kind of biomass fuels for energy production is so far restricted by the fact that they have a high content of potentially deposit/agglomerating forming and corrosive elements (K, Na, Cl, S, Ca, Si, P). The ash-forming elements occur in these biofuels as internal or external mineral grains, simple salts such as chlorides (KCl), and sulfates (K2SO4), or associated with the organic matrix of the fuel. Depending on the gas/particle temperature and the redox conditions during fuel particle heat-up, devolatilisation and char burn out, the simple salts may vaporize, while the mineral grains will undergo phase transformations and approach each other to form fly ash particles. Vapors and fly ash particles may be deposited on * Corresponding author. Tel.: þ 30-1-772-3191; fax: þ30-1-772-3163. E-mail address: [email protected] (S. Arvelakis). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

heat transfer surfaces in boilers and/or react with the particles of the bed inert material in fluidized bed reactors initiating the formation of deposits and agglomerates. The reactions can take place either in the solid phase during the burn-out of the biomass char particles or in the gas phase where the fly ash particles have been formed [2,3, 12,13,15,16,22,23]. Among the different ash constituents chlorine plays a key role on these reactions increasing the mobility of potassium since most of it is present as KCl. Potassium chloride is among the most stable high-temperature gas-phase alkali containing species while the amount of chlorine in the fuel often dictates the amount of the alkali possible to be vaporized during combustion or gasification. Calcium also appears to react with sulfur to form sulfates but the lower mobility of calcium in combination with its limited quantity in these biofuels does not make it a significant problem. The produced alkali silicates and/or sulfates have very low melting points that may reach 700 8C and tend to deposit on the reactor walls or in heat exchange surfaces in the case of the conventional grate fired systems, accompanied by the deposition of alkali vapors, forming fouling and slagging problems. Furthermore, in the case of the fluidized bed reactors contribute significantly in bed sintering

0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00013-9

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and defluidization of the bed inert material through the development of a sticky deposit on the surface of the bed particles through solid – solid and gas – solid reactions taking place during the burn-out of the char particles in contact with the bed material and during deposition and condensation of silicate fly ash particles and alkali vapors on the bed material surface [1,4,6,7,9,11 – 14]. The aim of this work is to study the effect of leaching (washing), and fractionation pre-treatment techniques on the ash-related problems caused during the gasification of olive-oil residue in a lab-scale gasifier. Olive-oil residue material in both un-treated and pre-treated forms was gasified in a fluidized bed reactor in order to study the effect of the pre-treatment techniques on the ash behavior of the material. The results showed to be very promising for the leached samples expanding the operation time of the reactor from 3 to 6 times compared with the tests with the untreated olive-oil residue. Agglomeration of the inert bed material during the tests with the leached olive-oil residue materials was observed to proceed after long operation time and the produced agglomerates varied significantly in size, shape and composition compared with the agglomerates from the tests with the untreated olive-oil residue. On the contrary, the tests with the fractionated material resulted in a significantly higher agglomeration tendency of the bed material, decreasing the operation time of the reactor to almost the half compared with the tests with the un-treated olive-oil residue.

consists of kernels, pulp, leaves and limbs and it is produced as byproduct after the extraction of olive oil. The leached olive residue samples used in the gasification tests were pretreated with tap water in order to extract a part of its inorganic constituents, mainly alkali metals and chlorine, which meet in large quantities into its ash and are thought to cause ash-related problems during biomass gasification. In specific, leaching comprises the submerge of the treated material into tap water using specific water/mass ratios for a certain period of time in order to achieve the extraction of water soluble inorganic elements (K, Na, Cl) of its ash. Three different water/mass ratios were used during the leaching treatment and the produced samples were characterised as ORL1, ORL2 and ORL3 depending on the water/mass ratio used that decreases with the increase of the number. On the other hand, fractionation included the split of the material in two fractions the ORF1 with particle size Dp . 1 mm and the ORF2 with particle size Dp , 1 mm. The ORF1 fraction that accounted for approximately 70% of the initial sample was selected as sample material for the investigation. Finally, the code name OR was given to the un-treated olive-oil residue material. An analytical description regarding the pre-treatments of the OR material used in the gasification tests is given by Arvelakis et al. [10]. Additional information concerning leaching pre-treatment technique and its effect on the ash behavior of the treated samples can be found elsewhere [5,8]. 2.2. Experimental set-up

2. Materials and methods 2.1. Materials Greek olive-oil residue samples were used as feedstock material during the gasification tests. Olive-oil residue

A lab-scale gasifier depicted in Fig. 1 situated at CUTEC-GmbH institute in Germany was used in conducting the gasification tests. The fluidized bed reactor consists of the fluidized bed, the free board, the hot gas cyclone, the post-combustion chamber system and the flue gas purification system. A conveyor worm was used for feeding

Fig. 1. Lab-scale fluidized bed gasifier CUTEC Institute GmbH Germany.

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the gasification process. The examination was focused in analysing the surface of the sand particles using the EDX microprobe of the microscope in order to calculate the concentrations of the main inorganic elements forming the surface of the particle before and after the gasification process, while photos of the analysed surface areas helped in better understanding and evaluation of the results. Several analyses (15 – 20) were performed for each ‘class’ of particles covering different magnifications and surface areas in order to be sure for the credibility of the produced results.

Table 1 Analysis and characterization of OR samples OR

ORF1

Proximate analysis (%d. basis) Moisture 9.5 8.76 Ash 4.6 1.9 Volatiles 76.0 77.9 Fixed carbon 19.4 20.2 Ultimate analysis (%d. basis) Nitrogen 1.36 1.7 Carbon 50.7 50.5 Hydrogen 5.89 6.19 Sulfur 0.3 0.28 Chlorine 0.18 0.23 Oxygen 36.97 33.37 GCV (MJ/kg) 21.2 19.84

ORL1

ORL2

ORL3

16.2 2.48 78.1 19.42

11.13 2.73 78.7 18.57

7.82 2.78 76.25 20.97

1.84 51.77 5.6 0.357 0.085 37.87 21.4

1.64 51.1 5.58 0.302 0.1 38.6 21.3

1.85 52.15 5.74 0.305 0.13 37.05 21.1

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3. Experimental 3.1. Samples characterization

the raw material to the reactor about 300 mm above the sieve of the bed. A maximum mass flow of about 20 kg/h can be attained, while the maximum fluidized bed height at steady height mode is about 300 mm. The heating of the fluidized bed is achieved with a controllable natural gas burner situated at about the same height as the input supply pipe connection. The bed inert material used in all tests was silica sand with particle diameter varying from 0.4 to 1.4 mm and a mean particle diameter of 0.82 mm. Sand particles had a theoretical specific surface of 29 cm2/g, in combination with an apparent density of 2.65 g/cm3 and a true density of 1.5 t/m3. The SEM-EDX analysis shows that the particle’s surface appears to be irregular with holes and empties, while its main forming element is silicon (. 99% as silica), while traces of Ca, K were also detected. Approximately 15 kg of silica sand was used as bed inert material during each gasification test. The gasification agent used in all tests was a mixture of air and nitrogen in order to have the appropriate equivalence ratio for the gasification of the specific biofuel, while all tests were conducted under atmospheric pressure. Samples of the silica sand bed material before and after the gasification tests were collected and examined using a JEOL 6300 scanning electron microscope in order to have a clear view regarding the reactions among the silica sand particles and the inorganic elements of the olive residue ash during

The treatment of the OR material using both the fractionation and the leaching processes appears to have a positive effect on the main properties of the OR samples such as ash content, volatiles and gross calorific value according to the results depicted in Table 1. The positive effect of leaching is seen to decline when the water/mass ratios used for the treatment of the raw material are reduced, while in the case of fractionation the only negative result is the chlorine content that is kept in similar levels as in the case of the un-treated material. The property mainly affected by the leaching process was the ash content that appeared to be reduced from 40 to 60% (w/w) approximately in all cases. The ash elemental analysis of the OR samples depicted in Table 2 shows that the amounts of alkali metals and chlorine contained in the ash of the leached samples were reduced from 21 to 65% (w/ w) and from 72 to 93% (w/w), respectively, depending on the water/mass ratios applied to the raw material during the leaching process. The observed ash chemistry improvement after the leaching process is believed to lead to an improved ash behavior during the gasification of these samples. On the contrary, the ash chemistry of the fractionated sample appears to deteriorate with the amounts of alkali metals showing significant increase, while the concentration of chlorine appears to be at the same levels as in the case of the un-treated material and the amounts of aluminum and

Table 2 Ash elemental analysis of OR samples Ash basis (%)

K2 O

Na2O

CaO

MgO

SiO2

Al2O3

OR ORF1 Difference% ORL3 Difference% ORL2 Difference % ORL1 Difference %

27.23 34.82 27.89 21.32 221.70 10.033 263.15 9.42 265.38

4.18 3.94 25.68 0.06 298.38 0.05 298.87 0.07 298.38

10.21 9.79 24.19 19.37 89.56 23.94 134.28 25.76 152.09

3.79 3.32 212.54 1.07 271.85 0.92 275.58 1.23 267.42

32.6 31.86 22.27 35.11 7.69 39.32 20.61 41.28 26.64

2.95 2.52 214.83 0.59 279.9 0.93 268.46 0.68 276.95

Fe2O3 1.9 2100 2.57 35.6 2.82 48.57 2.34 23.53

TiO2 0.1 2100 0.19 91.6 0.16 60 0.15 55

SO3

Cl

4.97 4.68 25.84 2.32 253.25 5.85 17.84 7.54 51.64

1.43 1.42 20.7 0.39 272.7 0.03 297.9 0.08 294.3

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Fig. 2. SEM-EDX micrograph of silica sand particle before the gasification tests with the OR sample.

Fig. 4. SEM-EDX micrograph of silica sand particle after the 2nd gasification test with the OR sample.

Two gasification tests were performed using the material OR (Figs. 2– 5). The main operating parameters and also the main findings of both tests are presented in Table 3. As it is seen from Table 3 the first test ended after 70 min of operation without visible agglomeration problems. After the end of the test inspection of the reactor’s bed showed the presence of small silica sand agglomerates forming clusters of 2 –5 sand particles all over the bed and especially nearby the feeding point. The SEM-EDX analysis of these agglomerates presented in Table 4 shows signs of reaction

and deposition processes among the sand particles and the ash constituents generated during the various steps of the gasification process. The sand particles appeared to be covered with a thin sintered layer of alkali and alkali earth metals, mainly potassium followed by calcium, silicate that gives to their surface a ‘polished’ like shape, while a secondary layer of silicate particles generated during the gasification process appear to have stuck on the sintered surface layer of the silica particles forming a new irregular surface with calcium and secondary potassium as the main substances [9,15,18,20]. The second test using the OR sample lasted for approximately 3 h and ended with the complete agglomeration and defluidization of the bed. Signs of agglomeration observed to appear after approximately 90 min from the test’s start point and were detected from the large temperature fluctuations observed in the bed. The inspection of the gasifier after the end of the test revealed that the bed inert material was fully agglomerated forming clusters that varied in size from few milimeters to several centimeters. The presence of char particles acting as

Fig. 3. SEM-EDX micrograph of silica sand particle after the 1st gasification test with the OR sample.

Fig. 5. SEM-EDX micrograph of silica sand and char agglomerate after the 2nd gastification test with the OR sample.

silicon are decreased. These results demonstrate that fractionation removes mainly ash fractions such as extraneous minerals and mineral inclusions with basically silicate and aluminosilicate structure that have medium to low reactivity and tendency to cause ash-related problems. As a result, the ash melting tendency is expected to increase significantly for the fractionated sample. 3.2. Gasification tests using the untreated olive-oil residue (OR) material

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Table 3 Operating parameters and produced results from the various biomass gasification tests Samples

Temperature (8C)

Pressure (atm)

Air ratio

Bed material particle size (mm)

Feeding rate (kg/h)

Operation time (min)

Alkali1 input in the bed (g)

Chlorine1 input in the bed (g)

Number of tests

OR ORF1 ORL1 ORL2 ORL3

840 –860 840 –860 840 –860 840 –860 840 –860

1 1 1 1 1

0.5– 0.6 0.5– 0.6 0.5– 0.57 0.5– 0.54 0.5– 0.54

0.4– 1.4 0.4– 1.4 0.4– 1.4 0.4– 1.4 0.4– 1.4

12–15 12–13 12–14 14–16 14–16

70,165 90 390 900 495

16.38–33.7 10.31 13.43 30.97 35.66

0.75–1.54 0.38 0.11 0.09 0.65

2 2 1 1 1

1

Alkali and chlorine inputs are given in g/kg of bed material.

bonds among the sand particles forming the agglomerates, was observed for the majority of the small-sized (2 –5 particles), agglomerates. Moreover, inspection of the reactor’s inner surface revealed the presence of silica sand agglomerates deposited on the reactor walls. The deposition problem appeared to be more intense near the feeding point. As in the case of the first test with the OR sample, the sand particles appear to have been heavily attacked by alkali metals, mainly potassium, and in a second hand by calcium in the form of potassium –calcium silicates. The sand particles are divided in two categories. The first category includes the sand particles having a white sintered-like surface color, already observed in the first test with the OR material. These particles form small and large agglomerates and incorporate char particles in their structure as it is seen in Fig. 5. The surface of these particles is covered with a potassium – calcium silicate layer, while signs of fly ash particles deposition having mainly a calcium –potassium silicate structure are also observed as it is shown in Table 4. The second category includes the yellow-colored sand particles that in principal form small to medium size agglomerates, while the presence of char particles in their structure appears to be limited. The surface of these particles is covered mainly with calcium –potassium silicates, while there are also signs of the initially formed potassium – calcium silicate layer on the surface of the agglomerated particles as it is seen from Table 4 and Figs. 3 and 4. This eutectic layer increases the stickiness of the sand particles and also the capture and deposition rate of fly ash particles, generated during the gasification process, on the surface of the silica sand particles. As a result the shape and tendency of the sand particles to coalesce and form clusters increase significantly, causing a significant deterioration of the fluidization-mixing conditions prevail in the bed and the formation of ‘hot’ spots that further increase the rate of the agglomeration process leading to the final agglomeration and de-fluidization of the bed. The concentrations of potassium and calcium on the surface of the sand particles after the second OR test in both the white and the yellowcolored particles appear to be higher now compared with the concentrations in the case of the analysis of correspondent areas of silica sand particles during the first gasification test.

Moreover, the amount of the yellow-color sand agglomerates appears to be substantially higher than the amount of the white ones. These observations are attributed to the significant higher operation time of the gasifier during the second test compared with the former one. The results from the SEM-EDX analyses verify that potassium appears to be the most reactive ash constituent and plays an important role as far as it concerns the initiation of the agglomeration phenomena observed during the gasification process. Furthermore, the reactivity of potassium is seen to be strongly affected, by its mineral form and especially from the presence of chlorine in the ash fraction as it is demonstrated from the results depicted in Figs. 6 and 7 and in Table 2 [15,18 –21,24 – 32]. These results verify that potassium reactivity increases significantly when potassium is present in the form of KCl, or when enough chlorine is available for reaction with potassium and formation of KCl during the various decomposition steps of biomass through the gasification process. On the other hand, calcium shows to have lower reactivity and it appears to affect the sand particles mainly in a second stage in the form of silicate or/and sulphate fly ash particles that stick on the initially formed potassium silicate layer. The presence of calcium also in moderate levels in the initial sticky potassium silicate layers covering the surface Table 4 SEM-EDX analysis of various silica sand particles during the gasification tests with the OR sample Samples

K2 O Na2O CaO MgO SiO2 Al2O3 Fe2O3 P2O5 SO3 Cl

First test

Second test

White particle

White particle

Yellow particle

Sintered surface

Deposit layer

Sintered surface

Deposit layer

Sintered surface

Deposit layer

21.78 3.09 1.77 0.91 71.41 0.00 0.20 0.86 0.00 0.00

8.71 0.50 22.69 9.38 46.35 1.66 1.48 8.58 0.56 0.00

28.07 0.14 16.52 0.23 52.61 0.00 2.36 0.00 0.00 0.00

6.49 0.05 69.76 0.34 12.05 0.00 9.90 1.27 0.00 0.11

45.78 0.02 13.12 0.00 40.11 0.00 1.05 0.00 0.00 0.00

16.14 0.00 53.51 0.24 23.23 0.00 6.50 0.00 0.00 0.07

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Fig. 6. Alkali input in the bed during the gasification tests with various OR samples.

of the sand particles is attributed to reactions of the calcium containing species with the initial potassium silicate layer in a second stage. 3.3. Gasification tests using the fractionated (ORF1) olive-oil residue material The evaluation of the fractionation pre-treatment technique regarding its effect on the ash behavior of the OR during the gasification process was studied by performing two tests in the fluidized bed reactor. Both tests were performed using the same parameters as in the case of the tests with the OR sample. The tests resulted in a complete agglomeration and defluidization of the bed material in a substantially shorter time compared with the case of the OR gasification tests. As it is seen from Table 3 and Figs. 6 and 7 fractionation appears to decrease the operation time of the reactor up to almost 50% compared with the tests with the OR sample though the total alkali and chlorine input in the fluidized bed during the tests with the ORF1 sample is also reduced by almost 70% (w/w) compared with the tests with the OR sample. These results correlate well with the results depicted in Tables 1 and 2

concerning the ash content and chemistry of the ORF1 sample and demonstrate a large increase on the reactivity of the remained ash fraction in the olive material after the fractionation pre-treatment. The alkali metals remained in the ash fraction of the ORF1 sample show to have a much higher reactivity leading to a significant speeding of the agglomeration phenomena observed in the fluidized bed. The formed agglomerates appear to have variable sizes, while their majority appear to be yellow-colored as in the case of the tests with the OR sample. The SEM-EDX analysis depicted in Table 5 shows that the amounts of potassium appear now to be at the same level in both the sintered areas and the deposited layer areas of the silica sand particles, as in the case of the tests with the OR sample. 3.4. Gasification tests using the leached (ORL1, ORL2, ORL3) olive-oil residue material The results and the operating parameters from the gasification tests with the leached OR samples are summarized in Table 3. As it is seen from Table 3 the operating time during the gasification tests with all the three different leached samples was substantially extended

Fig. 7. Chlorine input in the bed during the gasification tests with various OR samples.

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Table 5 SEM-EDX analysis of various silica sand particles during the gasification tests with the ORF1 and ORL1 samples Samples

K2O Na2O CaO MgO SiO2 Al2O3 Fe2O3 P2O5 SO3 Cl

Sample ORF1

Sample ORL1

White particle

Yellow particle

Yellow particle

Sintered surface

Deposit layer

Sintered surface

Deposit layer

Sintered surface

Deposit layer

24.73 0.02 8.57 0.20 65.17 0.00 1.31 0.00 0.00 0.00

11.34 0.00 26.93 0.38 58.12 0.00 1.47 1.76 0.00 0.00

40.05 0.04 11.71 0.00 44.37 0.00 3.95 0.00 0.00 0.00

10.42 0.04 56.90 0.11 25.02 0.00 7.34 0.00 0.00 0.00

25.17 0.00 29.54 0.00 28.65 1.62 13.32 1.71 0.00 0.00

2.53 0.00 48.85 7.33 29.29 2.12 3.56 5.94 0.38 0.00

compared to the cases of the OR and ORF samples tests showing a significant improvement of their ash behavior. As it is seen from Table 3 the gasification test with the sample ORL1 ends without agglomeration problems. The specific test is conducted for a limited amount of time due to the lack of available feeding material, while the tests performed using the samples ORL2 and ORL3 conducted for longer periods of time and ended with agglomeration and de-fluidization of the bed. However, since the ORL1 sample is, according to Tables 1 and 2, the most wellleached OR sample the total operation time in its case is anticipated to be higher compared to the cases of the ORL2 and ORL3 samples under the same operating conditions as it is also indicated in Figs. 6 and 7. The inspection of the silica sand material and of the reactor inner surfaces after the end of the test with the ORL1 sample verified the absence of agglomeration/deposition problems. The SEM-EDX elemental analysis performed in several sand particles revealed signs of reactions among the sand particles and the alkali and alkali earth metals. As it is seen in Table 5 and in Fig. 8 the surface of the sand particles was divided, as in the case of the tests with the OR sample, in two areas again. The first area that appeared now to cover only a small part of the surface includes the sintered parts where mainly calcium and potassium reacted with the silica particles forming a sticky ‘polished’ like silicate surface layer. On the other hand, the second area of the sand particles surface includes the silicate deposit layers that have been formed during the deposition and reaction of fly ash particles with the sticky particle surface. The amounts of potassium in both the initial sintered silicate layer and the secondary formed deposit area of the sand particles appear to be lower now compared with the amounts in the case of the OR and ORF1 samples. In addition, calcium appears to be the dominant element in both types of the silicate layers formed on the surface of the silica sand particles.

Fig. 8. SEM-EDX micrograph of silica sand particle from the gasification test with the ORL1 sample.

The inspection of the reactor’s bed and inner surfaces after the end of the test with the ORL2 sample showed that the bed was fully agglomerated with the sand particles forming small clusters of 3 –5 particles having mainly yellow, and in a few cases white color. As it is seen from Fig. 9 and Table 6 the sand particles appear to have the same morphology and in an extent the same composition as in the case of the previous test with the ORL1 sample. Calcium appears again to be the dominant element in the areas of the particles surface where deposition of fly ash particles has been seen to take place on a second stage. On the other hand, the sintered areas of the particles appear to have, as in the case of the tests with the OR and ORF1 samples, potassium as the dominant element though its amounts appear to be lower now. The final test using the leached OR samples as feeding material performed with the ORL3 sample. The test was started using the same conditions as in the case with the ORL2 sample. A bed material sampling was performed after having fed 17 kg of feeding material into the reactor, in order to compare the produced results with the results from the first test with the OR sample. The test ran without problems for approximately 7.5 h when the bed temperature

Fig. 9. SEM-EDX micrograph of silica sand particle from the gasification test with the ORL2 sample.

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Table 6 SEM-EDX analysis of various silica sand particles during the gasification tests with the ORL2 sample Samples

Sample ORL2 White particle

K2 O Na2O CaO MgO SiO2 Al2O3 Fe2O3 P2O5 SO3 Cl

Yellow particle

Sintered surface

Deposit layer

Sintered surface

Deposit layer

18.69 0.12 11.10 1.64 57.78 0.00 2.05 1.50 0.00 0.00

9.97 1.30 23.95 7.16 50.32 0.31 2.86 4.13 0.00 0.00

30.07 2.32 8.57 0.80 55.23 0.00 2.11 0.91 0.00 0.00

6.34 0.00 52.16 7.15 20.27 2.28 5.05 6.08 0.43 0.17

started to fluctuate again as in the case of the test with the ORL2 sample. The bed appears to fully agglomerate within 20 min from the moment the temperature fluctuation began, while approximately 120 kg of the ORL3 sample had been fed into the reactor until this point. The produced agglomerates appear to have bigger dimensions, while the presence of white colored agglomerates is more distinct now compared to the gasification test with the ORL2 sample. The SEM-EDX analysis of the sand particles from the sampling performed at the beginning of the gasification test showed very small signs of alkali –silica interactions, with potassium and calcium as the main constituents but in concentrations below 15% (w/w) as it is seen in Table 7, while many particles appeared to be clear without signs of alkali reaction-deposition processes now. These results demonstrate a clear improvement compared to the case of the test with the OR sample where all the examined sand Table 7 SEM-EDX analysis of various silica sand particles during the gasification tests with the ORL3 sample Samples

K2 O Na2O CaO MgO SiO2 Al2O3 Fe2O3 P2O5 SO3 Cl

Fig. 10. SEM-EDX micrograph of silica sand agglomerates from the gasification test with the ORL3 sample.

particles appeared to be heavily affected mainly by potassium and calcium. In addition, the SEM-EDX analysis of the agglomerates at the end of the test depicted in Table 7 and Figs. 10 and 11 showed that the surface of the sand particles is heavily affected by potassium and calcium. The silica particles appear to be mainly covered by the same ash deposit layer that has been observed to develop progressively on the surface of the sand particles during all the gasification tests. The analysis of the deposited silicate layer shows that calcium appears again to be the dominant element followed by low to medium amounts of potassium and other constituents such as magnesium, iron and phosphorus. The sintered surface areas of the bed particles appear to be scarce as in the case of the ORL2 test due to the high operation time. The analysis of the sintered areas shows that the amounts of potassium increase significantly compared with the results from the gasification tests with the ORL1 mainly and in a less extent with the ORL2 samples and are closer to those from the tests with the OR and ORF1 samples.

Sample ORL3 Yellow particle (65 min)

White particle

Yellow particle

Sintered surface

Deposit layer

Sintered surface

Deposit layer

Sintered urface

Deposit layer

20.71 1.56 4.96 1.15 69.78 0.00 0.75 1.07 0.10 0.00

7.69 0.49 19.62 3.57 62.02 1.08 0.93 4.04 0.42 0.09

21.68 5.71 6.84 1.12 62.77 0.00 0.42 1.44 0.00 0.00

12.27 0.84 30.19 4.02 45.36 0.82 1.77 4.34 0.31 0.00

36.07 1.32 9.50 0.80 52.23 0.00 2.01 0.31 0.00 0.00

2.35 0.00 62.27 8.71 10.96 1.94 4.87 8.53 0.39 0.00

Fig. 11. SEM-EDX magnification X1000 on the surface of a sintered sand particle from the gasification test with the ORL3 sample.

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As it is seen from Table 2 and Figs. 6 and 7 the amount of alkali metals and chlorine in the ash fraction of the feeding material plays a critical role regarding the agglomeration phenomena observed during the gasification process. Chlorine appears to have the most important role regarding the reactivity level of the ash fraction contained in the feeding material. The almost total elimination of chlorine input in the bed during the gasification tests with the samples ORL1 and ORL2 leads to a significant expansion of the operation time of the gasifier compared to the case of the gasification tests with the OR sample. The increase of chlorine input in the bed observed in the case of the test with the ORL3 sample, which reaches now the 42% (w/w) of the chlorine input in the case of the OR sample, leads to the initiation of the agglomeration phenomena in the bed in a shorter time compared with the cases of the ORL1 and ORL2 samples, which is still substantially higher compared to the case of the test with the OR sample. The agglomeration problems observed in the case of the tests with the leached OR samples are also attributed to the increased amounts of ash material in the bed due to the large operating time that leads to bad fluidization conditions in the bed and to the gradual formation of stationary ‘hot’ spots in parts of the bed where the temperature increases significantly and as a result also the melting fraction of the ash material. On the other hand, the high amount of chlorine in the ash of the ORF1 sample leads to an increased reactivity of its ash fraction also due to the elimination of different forms of alkali metals (carbonates, sulfates) and of less reactive ash constituents such as silicates, as a result of the fractionation process [10], resulting in the initiation of the agglomeration process in the bed in almost half the time needed in the case of the test with the OR sample. At the same time the total amounts of alkali and chlorine input in the bed appear to reach only the 24% and the 30% (w/w) of the corresponding amounts in the case of the test with the OR sample and are also lower even from the case of the ORL3 sample.

metals and chlorine input in the bed appear to be substantially lower now. On the contrary, leaching pre-treatment technique showed a highly positive effect as far as it concerns the ash thermal behavior of the OR samples. Leaching proved to extend significantly the operation of the gasifier compared to the tests with the un-treated and the fractionated OR material. According to Table 2 leaching led to a significant expulsion of alkali metals and chlorine from the material’s ash for the samples ORL1 and ORL2 and in a moderate to low expulsion of alkali metals followed by a significant expulsion of chlorine in the case of the ORL3 sample. As a result the ash of the leached samples appears to have a very low tendency to cause agglomeration/deposition problems. Chlorine, followed by potassium, appears to play the most important role regarding the reactivity of the ash fraction in biomass and its behavior during the gasification process, which is consistent with the results from other researchers [11,17,24,27]. In specific, the reactivity of potassium in the ash was severely affected by the correspondent amount of available chlorine in the ash according to the results from the SEM-EDX analysis of the produced agglomerates during the different gasification tests. As it is seen from the SEM-EDX results depicted in Tables 4 – 7 the participation of potassium to the formation of both the sintered areas on the surface of the silica sand particles and also to the composition of the ash particles deposited in a second stage on the surface of the particles is decreased proportionally with the decrease of the chlorine content of the feeding material, while the participation of calcium increases. Furthermore, The amount of chlorine present on the surface of the agglomerated sand particles was found to be minimal (, 0.5%) in all cases verifying the intermediate role it has during the formation of the agglomerates [11,17]. According to the produced results, the elimination of chlorine in the ash fraction of biomass leads to the production of low reactivity ash that could be managed easier during the biomass thermochemical treatment.

4. Conclusions

Acknowledgements

The obtained results from the gasification tests of OR samples verified that ash behavior remains one of the main obstacles for viable and economical exploitation of these materials for energy production. Fractionation pre-treatment shows to lead to a substantial increase of the agglomeration problems during the gasification process. The removal of the fine particles from the olive material during the fractionation procedure leads, according to Table 2 and Figs. 6 and 7, to a substantial increase of the ash reactivity. As a result agglomeration in the case of the ORF1 sample tests appears in a significantly shorter time compared to the case of the OR sample tests, though alkali

This research work was financed by the European Commission through the COPES research program and thus its contribution is gratefully acknowledged by the authors.

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