Effect of leaching on the ash behavior of olive residue during fluidized bed gasification

Effect of leaching on the ash behavior of olive residue during fluidized bed gasification

Biomass and Bioenergy 22 (2002) 55–69 Eect of leaching on the ash behavior of olive residue during uidized bed gasi!cation S. Arvelakisa; ∗ , H. Ge...

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Biomass and Bioenergy 22 (2002) 55–69

Eect of leaching on the ash behavior of olive residue during uidized bed gasi!cation S. Arvelakisa; ∗ , H. Gehrmannb , M. Beckmannb , E.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+23, D-38678, Clausthal-Zellerfeld, Germany Received 17 January 2001; accepted 26 August 2001

Abstract Fluidized bed gasi!cation is considered to be the most advanced method for thermochemical conversion of various biomass fuels, e.g., wood, energy crops, agroresidues, etc., to energy oering economical and environmental bene!ts. Ash-related problems including sintering, agglomeration, deposition, erosion and corrosion resulting due to the low melting point ash of agroresidues are the main obstacles for economical and viable application of this conversion method for energy exploitation of the speci!c residues. The eect of leaching (washing) the olive residue in order to improve its ash thermal behavior under the gasi!cation conditions was studied. Gasi!cation tests were performed in a lab scale uidized bed gasi!er with both leached and non-leached samples and the results concerning the ash thermal behavior of the used samples were associated with their ash elemental analysis, while the deposits produced during the gasi!cation tests were analyzed using SEM-EDX analysis technique. The produced results clearly show that leaching could help signi!cantly in avoiding to a great extent, the operational problems associated with the problematic ash thermal behavior in the case of olive residue. In particular, all gasi!cation tests performed using the non-leached olive residue material as feedstock resulted in a rapid agglomeration of the reactor’s bed material and consequently to the end of the gasi!cation tests. On the contrary, gasi!cation tests performed using the leached olive residue material ended without any agglomeration-deposition problems, while the operation time was c 2002 Published by Elsevier Science Ltd. in all cases longer compared with the tests with the non-leached olive material.  Keywords: Olive residue; Ash; Gasi!cation; Sintering; Deposition; Agglomeration; Leaching

1. Introduction Biomass fuels for generating heat and power are of interest because biomass is a renewable form of energy with low ash and sulfur content, which can contribute signi!cantly to the reduction of the greenhouse eect ∗ Corresponding author. Tel.: +30-1-772-3163; fax: +30-1-7723163. E-mail address: [email protected] (S. Arvelakis).

and to the increase of the energy independence of the user countries. Biomass can be divided into two big categories, woody biomass which includes the biomass resulting from conventional forests and=or from tree energy plantations, and the agroresidues e.g., straws, hulls, pits, cobs, etc., resulting as byproducts from various agricultural and=or agro-industrial processes. Agroresidues have the advantage to be truly renewable since they are produced every year and are free,

c 2002 Published by Elsevier Science Ltd. 0961-9534/02/$ - see front matter  PII: S 0 9 6 1 - 9 5 3 4 ( 0 1 ) 0 0 0 5 9 - 9

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with the exception of the straw residues, from transport expenses resulting as byproducts of agro-industrial processes which makes the thought of their use for decentralized energy production even more attractive. Nevertheless, the use of agroresidues for energy production today is limited due to problems related with their ash thermal behavior during combustion and gasi!cation processes. With the term ash we refer to all inorganic constituents included in biomass materials in a variety of forms such as organically bound cations, inorganic salts, minerals included in biomass and minerals excluded in biomass. These inorganic constituents undergo complex chemical and physical transformations during thermochemical reactions to produce ash in the form of vapors, liquids and solids sometimes characterized as intermediates. Participation of inorganic components in these complex reactions to form ash intermediates depends, to a great extent, on their nature and chemical characteristics, and the reaction conditions. Agroresidues contain mainly alkali metals such as potassium present in the form of inorganic salts e.g., KCl dissolved in inherent moisture, or connected to carboxylic or other functional groups, or as complex ions and chemisorbed material, and silicon in the form of hydrated silica or as deposits on the cell walls as their principal ash forming inorganic constituents. They are also rich in elements such as chlorine and sulfur compared to wood fuels and also contain small amounts of alkali earth materials like calcium that is present either in the cell walls or as crystalline calcium oxalate in cytoplasm, and magnesium and also phosphorous, with the latter two present mainly in biological forms [1,2]. Alkali metals especially potassium which has high mobility tends to react with silica, even at tempera◦ tures far below 900 C by breaking the Si–O–Si bond, and forming low melting point silicates or with sulfur to produce alkali sulfates. Chlorine acts as a facilitator of these reactions by 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 vaporize during combustion or gasi!cation. 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 signi!cant problem [3–5]. The produced alkali silicates and=or sulfates have ◦ very low melting points that may reach 700 C and tend to deposit on the reactor walls or in heat exchange surfaces in the case of the conventional grate !red systems, while in the case of the uidized bed reactors contribute signi!cantly in bed sintering and deuidization of the bed inert material through the development of a sticky deposit on the surface of the bed particles. This particular ash behavior result in a number of problems such as the lowering of the heat transfer coeJcients, restriction of the gas ow through the reactor due to increased deposits and eventual decline on plant eJciency. Large scale fouling, deposition and=or agglomeration can lead to unscheduled shut-down of the plant and subsequent deterioration of the plant economics [6 –11]. 2. Materials and methods Two dierent Greek olive residue samples, untreated olive residue and leached olive residue, were used as feedstock material during the gasi!cation tests. Olive residue consists of kernels, pulp, leaves and limbs and it is produced as a byproduct after the extraction of olive oil. The leached olive residue samples used in the gasi!cation tests were pretreated with tap water in order to extract a part of its inorganic constituents, mainly alkali metals and chlorine, which combine in large quantities into its ash and are thought to cause ash-related problems during biomass gasi!cation. Speci!cally, leaching comprises the submergence of the treated material into tap water using speci!c 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. Additional information concerning leaching pre-treatment technique and its eect on the ash behavior of the pretreated samples can be found elsewhere [12–16]. A pilot-scale gasi!er depicted in Fig. 1 situated at CUTEC-GmbH institute in Germany was used in conducting the gasi!cation tests. The uidized bed reactor consists of the following parts, uidized bed, free board, hot gas cyclone, post-combustion chamber system and ue gas puri!cation. A conveyor worm was used for feeding the raw material to the reactor

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Fig. 1. Pilot-scale Fluidized bed gasi!er.

about 300 mm above the sieve of the uidized bed. A maximum mass ow of about 20 kg=h can be attained, while the maximum uidized bed height at steady height mode is about 300 mm. The heating of the uidized 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 the tests was silica sand with particle diameter varying from 0.4 –1:4 mm and a mean particle diameter of 0:82 mm. Sand particles had a theoretical speci!c 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 . Approximately, 15 kg of silica sand was used as bed inert material during each gasi!cation test. The gasi!cation agent used in all the tests was a mixture of air and nitrogen in order to derive the appropriate equivalence ratio for the gasi!cation of the speci!c biofuel, while all the tests were conducted under atmospheric pressure. Samples of the silica sand bed material before and after the gasi!cation 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 gasi!cation process. The examination was focused in analyzing the surface of the sand particles using the EDS 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 gasi!cation process, while photos of the analyzed surface areas helped in a better understanding and evaluation of the results. Several analyses (15 –20) were performed for each “class” of particles covering different magni!cations and surface areas in order to be sure about the credibility of the produced results. 3. Results and discussion Table 1 presents the analysis and characterization of the olive residue samples, while Table 2 gives the ash elemental analysis of the samples. Two gasi!cation tests were performed using the material untreated olive residue. In both tests, the operating tempera◦ ture was set at 850 C while the feeding rate used was

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12 kg=h. In total, an amount of 35 kg of the speci!c olive residue sample was fed into the reactor in each ◦ test. The gasi!er was !rst heated up to 850 C using the gas burner until good uidization conditions were established. Then the gas burner was put o and the feeding of the olive material in the gasi!er was started. The temperature at the reaction zone of the gasi!er was !rst observed to decline for the !rst few minutes of the test until the gasi!cation reaction was established. The reactor was observed to operate at the temper◦ ature range of 840 –850 C for about 90 min without problems at an equivalence ratio of 0.5. At that particular moment, the reaction temperature was observed to gradually decline and this led us to modify the air to nitrogen mixture used as gasi!cation agent in order ◦ to keep the operating temperature above 850 C. The total ow of the uidizing medium remained stable during the test. Nevertheless, after 10 min of operation of the gasi!er at this temperature range the temperature in the reaction zone was observed to decline again gradually Table 1 Analysis and characterization of olive residue samples Proximate analysis (% dry basis)

Olive residue

Leached olive residue

Moisture Ash Volatiles Fixed carbon

5.5 4.6 76.0 19.4

7.83 2.43 78.5 19.07

1.36 50.7 5.89 0.3 0.18 36.97 21.2

1.90 54.05 5.80 0.297 0.03 35.49 21.24

Ultimate analysis (% dry basis) Nitrogen Carbon Hydrogen Sulfur Chlorine Oxygen Gross calori!c value (MJ=kg)



reaching 840 C despite the increase in the air ratio we performed. This kind of operation of the reactor was continued for about 40 min with the temperature in the reaction zone changing rapidly, varying between 820 ◦ and 870 C despite our eorts to stabilize the operation of the reactor by changing the air ratio that was increased, reaching 0.58 at the end of the test. After 40 min of unstable operation the temperature in the reaction zone of the uidized bed was observed ◦ to suddenly rise to 870 C while the temperature at ◦ the freeboard reached 1000 C. At this point, the feeding of the gasi!er was stopped while the olive material having been fed into the reactor was observed to burn out giving bright red ames at the upper part of the bed. After 2 min, the feeding of the gasi!er with olive material started again but it was immediately cut o due to the observation that the uidization of the bed was stopped and the material was burnt in the upper part of the bed increasing the local temperature while temperature in the other parts of the bed was decreased rapidly giving a clear evidence of bed agglomeration and deuidization. Fig. 2 displays the temperature pro!le in the reaction zone of the gasi!er while Fig. 3 shows the concentrations of the main gasi!cation gases produced during the gasi!cation of untreated olive residue sample. The bed temperature pro!le shows great variations during the test that reach a maximum at the agglomerating point with the tem◦ perature at the upper part of the bed reaching 1000 C, while the temperature at the lower part declines to ◦ 400 C. Moreover, the product gas appears to contain large amounts of CO2 and low amounts of CO and H2 compared with the product gas from the gasi!cation of the leached olive residue. ◦ The gasi!er was left to cool down to 300 C and then was opened and inspected for the determination of agglomeration and=or deposition phenomena. As is seen in Fig. 4, the bed inert material appeared to be fully agglomerated forming clusters that varied in size from few millimeters to several centimeters.

Table 2 Ash analysis of olive residue samples Ash basis %

K2 O

Na2 O

CaO

MgO

SiO2

Al2 O3

Fe2 O3

TiO2

SO3

Cl

Untreated olive residue Leached olive residue

27.23 4.9

4.18 0.05

10.21 23.38

3.79 1.12

32.60 43.57

2.95 0.62

1.9 2.4

0.1 0.14

4.97 3.29

1.43 0.03

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T-WSF-01 [˚C]

T-WSF-02 [˚C]

T-WSF-03 [˚C]

59

T-WSF-04 [˚C]

1000

900

Reaction end point

Reaction start point 800

700

Temperature (C)

600

500

400

300

200

100

0 0.0

1.0

2.0

3.0

4.0

4.5

Time (h)

Fig. 2. Bed temperature pro!le during the gasi!cation of untreated olive residue.

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 port. Samples of both the bed inert material before and after the gasi!cation tests were examined with the use of SEM-EDX analysis in order to determine the eects of agglomeration on the morphology and composition of the sand particles surface. Figs. 5 and 6 depict the surface of silica sand particles in its initial condition before the gasi!cation test. The surface appears to be irregular with holes

and voids, while its elemental analysis depicted in Table 3 shows that the main forming element is silicon (¿ 99% as silica), while traces of Ca and K were also detected. On the other hand, SEM-EDX analyses of the silica sand particles after the gasi!cation test with the untreated olive residue sample revealed a dramatic change in the composition of the sand particles. Speci!cally three dierent types of silica sand particles were observed to form the bed inert material after the gasi!cation test. Figs. 7–9 depict the dierent types of sand particles detected after the gasi!cation

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C-G1-01/02[Vol.-%]

C-G1-02/CO2[Vol.-%]

C-G1-08/H2 [Vol.-%]

C-G1-03/CO [Vol.-%]

C-G1-07/CH2 [Vol.-%]

18

16

Gas concentration (%vol)

14

12

10 Reaction start point

8

Reaction end point

6

4

2

0 0.0

1.0

2.0

3.0

4.0

4.5

Time (h)

Fig. 3. Gas concentrations during the gasi!cation of untreated olive residue.

test, while Table 3 depicts the SEM-EDX elemental analysis of each dierent type. Particularly, Figs. 7–9 depict the gradual impact of alkali and alkali-earth metals on the silica sand particles during the gasi!cation tests with the speci!c olive residue sample. As is seen, the !rst “class” of particles depicted in Fig. 7 appears to have almost the same surface morphology as the non-reacted silica sand particles. SEM-EDX elemental analysis of the particle’s surface depicted in Table 3 veri!es this observation, showing that the main forming element of the surface is silicon, while traces of potassium and calcium were also detected. On the other hand, the second “class” silica sand particles depicted in Fig. 8 appears to be totally dif-

ferent compared with the former one. The surface of both sand particles forming the agglomerate depicted in Fig. 8 appears to be covered with a thin layer, which is responsible for the “polished” surface of the particles, while they are connected through a “bridge”-like bond. SEM-EDX elemental analysis of the particles surfaces depicted in Table 3 shows that the thin surface layer is mainly composed of potassium and silicon while small quantities of calcium, sulfur and chlorine were also detected. Moreover, the bridge-like bond connecting the two particles forming the agglomerate mainly consists of a complex silicate mixture containing silicon, potassium, and calcium as its main forming elements. In both the surface of the two silica particles and of the connecting bond, small y ash particles were detected to have been stuck on

S. Arvelakis et al. / Biomass and Bioenergy 22 (2002) 55–69

Fig. 4. Bed inert material agglomeration during the gasi!cation of untreated olive residue sample.

Fig. 5. SEM morphological surface analysis of non-reacted silica sand particles. Analysis of whole particle.

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Fig. 6. SEM morphological surface analysis of non-reacted silica sand particles. Spot analysis. Table 3 SEM-EDX elemental analysis of various silica sand particles Elements % dry basis

Na2 O K2 O CaO MgO Al2 O3 SiO2 Cl SO3 Fe2 O3 P2 O 5

Non-reacted particles

First “class” sand particles

Second “class” sand particles

Third “class” sand particles

Silica sand particles the leached test

Whole area Spot Whole area Spot Whole area Spot analysis analysis analysis analysis analysis analysis

Whole area Spot Whole area Fly ash analysis analysis analysis particle analysis

0.00 0.01 0.01 0.00 0.5 99.3 0.00 0.00 0.00 0.00

0.00 10.1 39.5 0.00 0.00 46.2 0.00 0.00 2.8 1.2

0.00 0.01 0.01 0.00 0.5 99.3 0.00 0.00 0.00 0.00

0.00 2.1 1.5 0.00 1.5 95 0.00 0.00 0.00 0.00

0.00 2.1 1.5 0.00 1.5 95 0.00 0.00 0.00 0.00

2.00 55.1 10.5 1.25 1.5 23.7 2.25 3.5 0.5 0.00

forming a second class deposition layer on the surface of the already alkali-infected silica sand particles. Silicon, potassium and calcium were observed to be the basic forming elements of the deposited y ash particles. Nevertheless, the extent of this layer observed

0.00 28.1 18.5 1.25 0.00 45.7 2.25 2.5 3.5 0.00

0.00 32.6 9.5 0.00 0.00 56.2 0.00 0.00 1. 2 0.3

0.5 2.3 15 0.00 1.2 79 0.01 0.5 0.00 1.5

0.00 6.5 53.5 0.00 0.00 35.8 0.01 0.01 4.2 0.00

in the sand particles belongs to the second “class” of sand particles that appeared to be limited. Finally, the third “class’ sand particles depicted in Fig. 9 appear to have a totally dierent morphology compared with the former two. The surface of the

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Fig. 7. SEM morphological analysis of the dierent types of silica sand resulted after the gasi!cation of untreated olive residue for !rst “class” particles.

particles now appears to be covered mainly with a secondary y ash layer while some spots of “polished” surface are also observed, as in the case of the previous agglomerate. SEM-EDX elemental analysis of this secondary ash layer revealed that it mainly consists of calcium silicates while the “polished” spots include mainly potassium and silicon as the main forming elements. The results observed during the gasi!cation of the untreated olive residue sample and from SEM-EDX analysis of the resulted silica sand bed inert material were found to be in accordance with the ash elemental analysis of the speci!c ash sample depicted in Table 2. Agglomeration was seen to proceed as a result of the reactions among the alkali and alkali-earth metals, especially potassium, which are present in large quantities in the material’s ash, and of the silica sand particles used as bed inert material during the gasi!cation of the speci!c olive residue material, as it has been observed by many other authors [1,2,9,11].

These reactions lead to the formation of a sticky potassium surface layer that enhances the deposition of ash particles, produced during the reactions occurring in the olive residue particles during the gasi!cation process, while it increases the cohesion forces among the particles leading to gradual clustering and !nally agglomeration and de-uidization of the bed. The majority of the sand particles after the gasi!cation tests with the untreated olive residue sample was seen to belong to the second “class” of particles described above, forming white agglomerates having potassium as the main element of their sticky surface. Furthermore, the amount of particles remained intact after the gasi!cation process was observed to be minimum. Two gasi!cation tests were also conducted using the material leached olive residue as gasi!cation feedstock. The operating parameters applied to these tests were the same applied during the tests with the untreated olive residue sample. Both tests were observed to start and !nish without any problem.

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Fig. 8. SEM morphological analysis of the dierent types of silica sand resulted after the gasi!cation of untreated olive residue for second “class” particles.

Inspection of both the bed inert material and the interior surfaces of the reactor after the end of each test with the leached olive residue sample revealed no agglomeration=deposition problems. Fig. 10 depicts the temperature pro!le in the reaction zone of the gasi!er while Fig. 11 shows the concentrations of the main gasi!cation gases produced during the gasi!cation of untreated olive residue sample. As is seen from these !gures, the temperature pro!le in the reaction zone appears to be more constant avoiding uctuations while the product gas appears to have a better composition containing less amounts of CO2 and increased amounts of CO and H2 compared with the product gas from the gasi!cation of untreated olive residue. Samples of the bed material after the gasi!cation tests were analyzed using SEM-EDX analysis. Figs. 12 and 13 depict the results obtained from the morphological analysis of the sand particles surface after the speci!c gasi!cation test, while the SEM-EDX elemental analysis of the particles surfaces is depicted in Table 3.

As can be seen in Fig. 12, the surface of the examined sand particles shows the same morphology with the surface of the non-reacted silica sand particles depicted in Figs. 5 and 7, while a few signs of ash deposition were also observed. According to SEM-EDX elemental analysis depicted in Table 3, silicon is the main forming element of the particle surface, while a small amount of calcium and traces of potassium were also detected. SEM-EDX elemental analysis of the ash particle, depicted in Fig. 13 stuck on the silica sand’s surface revealed that silicon, calcium and potassium were the main forming elements. SEM-EDX analysis results of the sand particles after the gasi!cation tests of the leached olive residue sample were found to be in order with the ash elemental analysis of the speci!c sample depicted in Table 2. The low amounts of potassium and chlorine included in the ash of the speci!c olive residue sample are thought to be responsible for the improved thermal behavior during the gasi!cation reaction.

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Fig. 9. SEM morphological analysis of the dierent types of silica sand resulted after the gasi!cation of untreated olive residue for third “class” particles.

Furthermore, this leads to a dierent reaction– deposition mechanism of ash particles, generated during the reactions that take place in the olive residue particles during the gasi!cation process, on the silica sand particles where calcium-silica reaction was found to be the leading mechanism showing lower rates compared with the potassium–silica mechanism. 4. Conclusions The results obtained from the gasi!cation tests of olive residue samples veri!ed that ash behavior remains one of the main obstacles for viable and economical exploitation of these materials for energy production. SEM-EDX analysis of the bed inert material after uidized bed gasi!cation of untreated olive residue showed that agglomeration takes place in two steps. First, alkali metals, mainly potassium reacts with the silica sand particles forming an eutectic glue-like sil-

icate coating on their surface that gradually !lls all the voids and holes of their surface. This process is followed in a second step by the deposition of y ash particles on the newly formed surface alkali layer and the reaction among them forming new complex silicate mixtures having calcium, silicon and potassium as their main forming elements. These reactions increase the tendency of the sand particles to stick together forming clusters and lead to agglomeration and to the !nal deuidization of the bed. On the contrary, leaching pretreatment technique showed a positive eect as far as it concerned the ash thermal behavior of olive residue samples. In particular, leaching proved to signi!cantly improve the ash thermal behavior of olive residue samples by securing the unproblematic operation of the gasi!er during all tests with the leached treated samples. Leaching resulted, according to Table 2, in an almost total expulsion of alkali metals, chlorine and sulfur from the materials ash ensuring its neutral behavior at elevated temperatures.

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Reaction start point

T -W S F -02 [˚ C ]

T -W S F -03 [˚ C ]

T -W S F -04 [˚ C ]

T -W S F -01 [˚ C ]

1000

900

Reaction end point

800

Temperature (C )

700

600

500

400

300

200

100

0 0 .0

1 .0

2 .0

3 .0

4 .0

5 .0

6 .0

Time (h ) Fig. 10. Bed temperature pro!le during the gasi!cation of leached olive residue.

SEM-EDX analysis of silica sand particles from the gasi!cation tests with the leached olive residue sample showed that these particles were only slightly infected by alkalies and alkali-earth metals with calcium being the main deposited element. Thus, it is obvious that leaching causes a change as far as it concerns the deposition mechanism of alkalies onto the surface of the silica sand particles, setting calcium as the main troublesome element, while the

reaction rates and also the harmful results seem to delay to a great extent. Nevertheless, since the gasi!cation tests with the leached olive residue sample were performed for less than 4 h, much more work is needed in order to investigate the eect of calcium deposition– reaction mechanism with the silica sand particles on the behavior of the infected sand particles during the gasi!cation process and also the extent and

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C-G1-01/O2 [ Vol.-%] C-G1-03/CO[ Vol.-%]

67

C- G1-02/CO2 [ Vol .- %]

C-G1-08/H2 [ Vol.-%]

C-G1-07/CH4 [ Vol.-%]

Gas concentration (%vol)

25

20

15

10 Reaction start point

Reaction end point

5

0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

Fig. 11. Gas concentrations during the gasi!cation of leached olive residue.

Fig. 12. SEM morphological analysis of silica sand and deposited y ash particles after the gasi!cation of leached olive residue for silica sand particle.

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Fig. 13. SEM morphological analysis of silica sand and deposited y ash particles after the gasi!cation of leached olive residue for y ash particle.

progress of the phenomenon versus the gasi!cation time. [3]

Acknowledgements The research work was !nanced by European Commission through the COPES research program and thus its contribution is gratefully acknowledged by the authors.

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