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Low temperature gasification of olive kernels in a 5-kWfluidized bed reactor for H2-rich producer gas

V. Skouloua, G. Koufodimosb, Z. Samarasb, A. Zabaniotoua,*aDepartment of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124, GreecebDepartment of Mechanical Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece

a r t i c l e i n f o

Article history:

Received 19 May 2008

Received in revised form

15 July 2008

Accepted 16 July 2008

Available online 12 October 2008

Keywords:

Air fluidized bed gasification

Olive kernel

Biomass

Hydrogen

Olivine

* Corresponding author. Tel.: þ30 2310 99627E-mail address: sonia@cheng.auth.gr (A.

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.07.074

a b s t r a c t

Air gasification of olive kernels in a 5 kW bench scale, bubbling fluidized bed gasifier, aimed

at H2 enrichment of the producer gas, was the target of this study. The effects of reactor

temperature (T¼ 750–850 �C) and equivalence ratio (ER¼ 0.2–0.4), representing the under

stoichiometric amount of air inserted into the reactor to that necessary for complete

combustion, on producer gas quality were determined. The experimental results revealed

that producer gas H2 content increased at the temperature of T¼ 750 �C and ER¼ 0.2,

resembling the high-temperature pyrolysis conditions that favour H2 and CO production.

Further increase in ER deteriorated producer gas quality, decreased H2 content and fav-

oured CO2, thus lowering producer gas heating value. The data obtained from several

experiments indicate that olive kernels produced a medium heating value gas

(LHV¼ 6.54 MJ/Nm3) at 750 �C and ER¼ 0.2, while H2 and CO production were maximized at

the same conditions (H2: 24%vv, CO: 14.3%vv).

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction The overall reaction (Reaction (1)) that takes place in the

Gasification is a thermochemical process that converts a solid

fuel (biomass) into a combustible gaseous product (producer

gas). Gasification provides a means of converting a difficult to

process material, such as solid biomass, into a versatile

energy and hydrogen gaseous carrier, rendering lignocellu-

losic material an important renewable source. Main process

products are the producer gas, which is a mixture of mainly

H2, CO, CO2, CH4, C2H4, C2H2, C2H6 and N2 if air is the oxidizing

medium, a solid phase consisting of char and inorganic ash

(mainly received as fly-ash) and a condensable gaseous/

aquatic phase consisting of aromatic hydrocarbon

compounds known as tar. Apart from the energy content of

producer gas, char and tar could be further treated for high

added value material recovery.

4; fax: þ30 2310 996209.Zabaniotou).ational Association for H

gasifier could be written as follows [1], assuming the complete

transformation of the char residue to gas:

BiomassþAir �!HEATH2þCOþCO2þCH4þCnHmþN2þTarsþAsh

(1)

Producer gas utilization either for high added value products

(e.g., 2nd generation biofuels through Fischer Tropsch

synthesis) or for power generation requires compliance with

high-quality standards in terms of composition and purity.

Composition and quality of the producer gas depend on

several factors including biomass physicochemical properties

(e.g., moisture, ash content, etc.), bed material (e.g., silica

sand, olivine, etc.), oxidation medium (e.g., air, pure oxygen,

steam, or special mixtures) and several process operating

conditions (e.g., temperature, equivalence ratio, etc.). When

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 46516

optimum quality of producer gas is obtained then it can be

exploited in boilers, gas turbines, internal combustion engines

(ICE) or even in fuel cells (FC).

Concerning electricity production in ICE, electrical effi-

ciency (ne) could reach w28% [2], but a thorough gas cleaning

is a prerequisite of primary importance in gasification systems

[3,4]. Electricity production integrated systems of fluidized bed

gasifiers in conjunction with FC (usually SOFCs) can reach

ne w 60% [5,6].

Gasification usually takes place at higher temperatures than

pyrolysis, in the range of 750–850 �C. Fluidized bed gasifiers

(FBG) constitute a promising type of reactor for large-scale

applications of biomass thermochemical conversion, due to

their ability of achieving high heat and mass transfer rates [2].

Such operations lead to a high rate of production and flexibility

towards the raw material feeding. However, problems like bed

sintering, agglomeration and deposition that appear due to the

low melting point of agro-residues’ ash, as well as tar produc-

tion, are obstacles for their viable application. Both problems

can be of major or minor importance according to the physi-

cochemical characteristics of the agro fuel.

Data concerning olive kernel gasification are scarce in the

scientific literature and almost all studies for olive kernel

thermochemical conversion are reported by researchers from

the Mediterranean. In Mediterranean countries olive oil

production is an agricultural activity of great importance,

producing annually large amounts of olive kernel as a valuable

byproduct. Spain, Greece, Italy and Tunisia [7] represent 65% of

the olive tree cultivation area, 76% of the olive trees in

production and 74% of the olive production worldwide. On

a global scale, the olive oil production reaches 1,600,000 tons

annually. Main solid by-products from olive oil production are

olive kernel, as well as, olive tree pruning and harvest residues.

This special agro-residue, olive kernel, presents some

particularities compared to other lignocellulosic residues, due

to its aliphatic and aromatic content, since it still contains an

oil amount. However, this agro-residue is mainly exploited

with conventional combustion for heat and sometimes elec-

tricity production, because it posses a considerable heating

value per mass unit, comparable to the LHV of the low-quality

Greek lignite.

Gasification technology of olive industry wastes emerges

nowadays as a promising alternative method of biogenic

energy production, fuel by the need for greenhouse gas miti-

gation. In Spain, Garcıa-Ibanez et al. [8] studied the effect of

experimental conditions on olive kernels gasification in a pilot

plant. The aim was to enhance gas production and improve its

composition for energetic uses. They concluded that a carbon

conversion efficiency of 81–86% was obtained at 800 �C using

air as the gasification agent. Ganan et al. [9], from Portugal,

reported that olive waste has a pattern typical of lignocellulosic

materials, and therefore can be used in gasification process,

producing combustible gases that can be further exploited to

generate electricity due to their attractive heating value.

In Portugal, again, Rui Neto et al. [10] studied the co-gasi-

fication of olive oil industry wastes and coal, in a pilot unit

utilizing a mixture of air and steam as gasification medium.

Aim was to enhance and optimize the producer gas compo-

sition in CO and H2. They concluded that at 890 �C steam

reforming leads to 45% increase in hydrogen concentration.

In Italy, Seneca [11] demonstrated that under thermo-

chemical conversion treatment olive husks were more reac-

tive than pine wood chips and seed shells. They concluded

that this behavior was linked mainly to the high content of

mineral compounds imposing a pronounced catalytic activity.

In Greece, Fryda et al. [12] tested – among others – olive

bagasse and its agglomeration tendency in an atmospheric,

lab scale, air fluidized bed gasifier. They found that fluidized

bed gasification of olive bagasse resisted to defluidisation

tendencies to temperatures over 850 �C, due to its lower

potassium (K) and higher calcium (Ca) content, compared to

energy crops such as giant reed and sweet sorghum. Zaba-

niotou et al. [13] have also studied fast pyrolysis of olive

kernels and olive tree cuttings. As pyrolysis is the main stage –

and of main importance in the fluidized bed gasification

process – results of the above study proved to be useful in

order to analyse in depth the fluidized bed gasification process

of olive kernels.

In the present study, a bench scale fluidized bed gasifier

was developed and air was used as the gasification medium.

The aim was to investigate the effects of some critical

parameters of olive kernel on gasification performance and

primarily H2 maximization while ultimate purpose is the

reactor scale up in a commercial application.

2. The bench scale FBG unit

A 5 kW, bench scale, bubbling fluidized bed gasifier (FBG) was

designed and manufactured for biogenic material gasification.

It is an atmospheric air blown FBG unit that consists of four

sections: (a) the biomass feeding section, (b) the air supply,

control and preheating section, (c) the gasification facility and

(d) the gas sampling and off-line analysis section.

Producer gas was sampled using airtight gas sampling bags

and analyzed with a GC system for quantitative determination

of CO, CO2, H2, CH4, C2H4 and C2H6.

2.1. The olive kernel feeding section

The FBG unit is equipped with a feeding hopper suitable of

feeding low-density biomass. Feeding takes place through

a system of a constant speed screw feeder and a variable

speed screw feeder (motorized via inverter), separated by

a rotary valve in order to avoid the hot gas backflow that could

pyrolyse olive kernels and cause serious tar fouling problems.

Additionally, a small proportion of fluidization air (w1 l/min)

is supplied for backflow prevention. The motorized screw

feeder is able to control the biomass feeding, based on a series

of calibration curves and the maximum mass flow rate of olive

kernel could reach 120 g/min. Olive kernel pass through the

metal to metal rotary valve and is introduced into the reactor

by the second screw feeder, into the hot zone of the reactor,

120 mm above the air distributor.

2.2. The air supply, control and preheating section

Laboratory’s air supply unit provides the FBG unit with the

primary and secondary air, necessary for the fluidization and

gasification process, always in hypo-stoichiometric amounts

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 4 6517

obtaining the desired ER ratio. It consists of a central

compressor, a main valve to control the air flow rate and an air

flow meter with maximum flow capacity of 200 l/min. The

primary air for gasification process is introduced from the

bottom of the FBG and a uniformly perforated plate (151 holes

of 1 mm diameter) is used in order to retain bed fluidization.

Primary air is preheated up to 350 �C, by passing through

a stainless steel tube that is kept in touch with the hot zone of

the reactor inside the furnace. In the beginning of each oper-

ation, while the reactor is still cold, air preheating is obtained

via an electric resistance placed at the bottom of the FBG.

Secondary air contributes in producer gas temperature

increase, which results in further tar cracking improving

carbon conversion efficiency.

2.3. The 5 kW FBG test unit

Fig. 1 shows the bench scale FBG reactor with nominal capacity

of 5 kW. Bed and freeboard sections are made of stainless steel

with diameters of 60 and 90 mm, respectively; the total reactor

height is 1400 mm. A more-detailed analysis about the reactor

design and construction can be found in a previous published

work [14]. FBG design enables the easy dismantling and thor-

ough cleaning, in order to avoid not only memory effects, but

also possible tar fouling under the hot flow experimental

conditions. The gas cleaning system consists of a cyclone of

10 mm cut point. Limited experimental work has done using

high-temperature ceramic filters and various types of metallic

foam for particulate abatement and tar elimination.

2.4. Producer gas sampling and analysis

The sampling line is kept at a constant temperature of

w300 �C in order to avoid tar condensation and sampling line

Fig. 1 – The 5 kW, bench scale FBG unit, 1-biomass hopper; 2-s

distributor; 6-fluidized bed reactor; 7-cyclone; 8-electric furnace;

12-isolation valve; 13-electrical heater; 14-differential pressure

temperature measurement.

fouling. Tar condensation and particle removal takes place in

the gas cleaning section (Fig. 2) consisting of a water washing,

moisture trap, impinger bottles with isopropanol, a fiber filter

and a silica gel filter.

When the appropriate gasification conditions are achieved

producer gas is sampled, using a membrane pump and airtight

gas sampling bags and analyzed in laboratory’s GC system

(Model 6890 N, Agilent Technologies equipped with FID and

TCD detectors connected in series). The gas chromatograph is

fitted with two columns, HP-Plot Q (30 m� 0.530 mm� 40 mm)

and HP-Molsiv (30 m� 0.530 mm� 50 mm), with helium as

carrier gas. GC’s temperature profile was an isothermal at

50 �C and the retention time of the analysis process was

38 min. The standard gas mixture used GC calibration

composed from CO, CO2, H2, CH4, C2H4 and C2H6 1%vv of each

one of the above components balanced in helium.

3. Experimental

3.1. Raw material

Olive kernel is the final solid residue emerging from olive oil

production industries. Olives are processed in an oil extracting

plant to recover the oil content. After a first residue drying,

residual oil is hexane extracted generating a residual solid

product with a moisture content of around 10–12%wt called

olive kernel. Traditionally, such kind of residue is sold as fuel

for combustion in small boilers and specially designed

furnaces due to its significant calorific value (HHV w 21 MJ/kg).

Nowadays, there are several boilers fuelled with olive kernels

destined for residential and greenhouse heating or in some

cases to generate steam for electricity production in conven-

tional steam turbines. However, a more efficient and

crew feeder (inverter); 3-rotary valve; 4-screw feeder; 5-air

9-air compressor; 10-pressure regulator; 11-air flow meter;

transmitter; P1 to P4 pressure measurement; T1 to T8

Fig. 2 – Producer gas sampling section.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 46518

environmental friendly alternative with respect to CO2 and

CH4 mitigation to the atmosphere is olive kernel gasification,

which aims to produce a high calorific value gas to fuel a gas

engine or even a gas turbine in a combined cycle of heat/

power production [15].

Untreated olive kernel used as the feeding material to the

FBG unit and its physicochemical characteristics are pre-

sented in Table 1.

The molecular formula of the olive kernel estimated from its

ultimate analysis is CH1,41O0,68. Olive kernels also present low

sulfur (S) and nitrogen (N) contents, eliminating the probability

of SOx and NOx formation, and hence the possibility of envi-

ronmental problems and corrosion of metallic components.

3.2. Bed material

The bed material is of great importance in a fluidized bed

reactor as it plays a dual role acting as heat transfer medium

and, if it is not an inert material, as an internal catalyst for

extended tar cracking. A major problem of gasification appli-

cability is tar formation and its minimisation is seen as one of

the greatest technical challenges to overcome for the

successful development of commercially attractive gasifica-

tion technologies [16]. According to the literature, tar produc-

tion in an air-steam gasification process could be as low as

w20 g/Nm3 [17]. Tar treatment technologies are categorised

Table 1 – Physicochemical characteristics of olive kernel

Proximate analysis Ultimate analysis (%wt)

HHVa (MJ/kg) 20.96 Carbon, C 48.59

Moistureb (%wt) 12.3 Hydrogen, H 5.73

Ashb (%wt) 1.9 Oxygen, O 44.06

Combustiblesb (%wt) 85.8 Nitrogen, N 1.57

Bulk density (kg/m3) 573 Sulfur, S 0.05

a Dry base.

b Wet base.

either as upstream, when the tar problem is tackled inside the

gasifier (named also primary or internal methods) or down-

stream, when tar is cleaned in separate hot gas devices

(named also secondary or external methods). Although the

secondary methods are proven to be effective and fully

controlled, primary treatment technologies are gaining more

interest due to their economic benefits [18]. In primary

methods, operating parameters such as temperature, gasi-

fying agent, bed material, equivalence ratio and residence

time can play an important role in the formation and decom-

position of tars. Cao et al. [16] concluded that the utilization of

some catalysts in the gasifier and the concepts of two-stage

gasification are of prime importance. They worked on a labo-

ratory scale, two-region, combined fluidized bed reactor for air

gasification of sawdust producing a H2-rich gas, with an LHV of

5 MJ/Nm3 and tar concentration below 10 mg/Nm3.

Corella et al. [17] stated that several in-gasifier materials

have catalytic action and eliminate in-bed tar by activating

reforming reactions. Such materials modify the kinetic

constants of most catalytic reactions involved in gasification.

The in-bed additives are classified into two categories: (i)

laboratory scale catalysts, which are very active but expen-

sive; those catalysts are deactivated by coking when used at

high (above 1000 kg/h m2 cross-sectional area) biomass

throughputs (realistic for commercial applications) and (ii)

naturally occurring solids, such as dolomite, limestone,

olivine, high iron-content solids, which have been proved to

have only a relatively small tar-elimination activity [17].

Nordgreen et al. [18] used metallic iron and iron oxides to

catalytically crack tars in a secondary reactor, downstream

the atmospheric fluidized bed gasifier fuelled with Swedish

birch. They worked at a temperature range of 700–900 �C and

low equivalence ratio almost near to pyrolysis conditions

(0< ER< 0.2), concluding that catalytic breakdown of the tar

reached 100%.

In the present study, the first set of experiments was

carried out with quartz sand (Table 2) as bed material, in the

size range 500–425 mm. Although quartz sand was an easy and

Table 2 – Comparison of physicochemical characteristicsof commercial quartz sand and olivine

Physical characteristics

Bed material Silica sand Olivine

Bed material

solid density (kg/m3)

2650 3300

Bed material

bulk density (kg/m3)

1700 1900

Chemical characteristics

SiO2 (%wt) 99.5 43.5%

Fe2O3 (%wt) 0.10 7.5–8.2%

Al2O3 (%wt) 0.10 0.3%

CaO (%wt) 0.03 0.4%

MgO (%wt) n.a. 46–48%

Fig. 4 – Effect of temperature and ER on H2 production.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 4 6519

cheap to find material, it caused severe defluidization due to

inevitable agglomeration and tar formation tendency at rela-

tively low gasification temperature (T< 800 �C). Thus, taking

into account that an iron containing bed material might act as

a catalyst for tar destruction, quartz sand was replaced with

olivine (Table 2). A 500–425 mm size fraction of olivine was also

used and it was found to be durable for a prolonged operating

period and in the temperature range 800–950 �C.

4. Experimental results

4.1. Effect of reactor temperature and air equivalenceratio in the producer gas composition

Temperature and air equivalence ratio (ER) are two of the most

important parameters affecting the overall biomass gasifica-

tion process. In the present work, reactor temperature varied

from 750 to 850 �C and ER between 0.2 and 0.4. Producer gas

composition (CO, H2, CO and CH4) as a function of temperature

and ER is depicted in Figs. 3–6. Fig. 3 shows the impact of ER

and temperature on CO production. It can be seen that as

Fig. 3 – Effect of temperature and ER on CO production.

temperature increases, there is a slight increase in CO, which

can be attributed to the endothermic (Reactions (2) and (3)).

Temperature increase favours CO production at low ER (0.2),

while at higher ER (0.4) CO production is hindered by the

complete oxidation of carbon to CO2. At the lowest ER and the

highest temperature (T¼ 850 �C) steam reforming of CH4

(Reaction (3)) or even tar thermal cracking (Reaction (4)) might

contribute to the slight further increase of CO [20].

CþH2O/COþH2 þ 131 MJ=kmol (2)

CH4 þH2O/COþ 3H2 þ 206 MJ=kmol (3)

Tar �!HeatCO2 þ COþH2 þ CH4 þ lighter H=C (4)

H2 production (Fig. 4) on the other hand, peaked at the lower

gasification temperature (T¼ 750 �C) and the lowest ER¼ 0.2.

This finding was validated with pyrolysis experiments per-

formed in a laboratory flash pyrolysis reactor: olive kernel in

an inert helium atmosphere (ER¼ 0) and temperature of

T¼ 765 �C produced a gas with 41%vv H2, 42% CO, 13.5%vv CH4

and some traces of light hydrocarbons.

Fig. 5 – Effect of temperature and ER on CO2 production.

0

2

4

6

8

10

750 800 850

Temperature [oC]

ER=0,4 ER=0,3 ER=0,2

Pro

duce

r G

as L

HV

[M

J/N

m3 ]

Fig. 7 – Effect of temperature and ER on LHV of the producer

gas.

Fig. 6 – Effect of temperature and ER on CH4 production.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 46520

On the other hand, carbon dioxide (Fig. 5) increased with

temperature for ER¼ 0.3 and ER¼ 0.4. The opposite trend was

noticed for the lowest achieved ER¼ 0.2 and this could be

possibly attributed to the fact that the endothermic Bou-

douard reaction (Reaction (5)) possibly took place; thus the

unburned fixed carbon of char reacted further in favour of

carbon monoxide. These results are also in accordance with

the trend that Fig. 3 depicts for CO production.

Cþ CO2/2CO þ 172 MJ=kmol (5)

Finally, the methane was generally produced (Fig. 6) at low

concentrations (CH4< 5%vv) under all test conditions.

Temperature increase at low ER¼ 0.2 favoured methane

reforming due to Reactions (3) and (4). This could be well-

explained probably due to (a) the catalytic effect of olivine

(iron presence) and (b) the steam reforming reactions activa-

tion that take place at high temperature (T¼ 850 �C) [19,20].

4.2. Producer gas LHV

Temperature and ER alsosignificantly affect the heating value of

producer gas. Main research target is the production of

aproducergasenriched inCO,H2 and CH4. Thepresenceofthese

combustible species contributes to the production of a medium

to high heating value gas, suitable for further exploitation in

internal combustion engines (ICE) and turbines for power

production. LHV calculation was made using Eq. (1) [21]:

LHVgas ¼ð30COþ 25:7H2 þ 85:4CH4 þ 151:3CXHYÞ4:2

1000MJ=Nm3

(1)

where CO, H2, CH4 and CXHY are the molar ratios of the species

in the producer gas as measured in our gasification

experiments.

In Fig. 7 the effect of temperature and ER on producer gas

LHV is depicted. Higher ER lead to lower gas heating value, not

only due to nitrogen dilution, but also, because of the

promotion of oxidation reactions. At low ERs temperature

increase reduced the heating value of the producer gas;

however, this effect is not clear at medium and high ERs.

Experimental results indicated the optimum achieved

LHVgas¼ 6.54 MJ/Nm3 at conditions of 750 �C and ER¼ 0.2.

Finally, a comparison of the experimental results from the

present study with literature data is presented in Table 3

[9,10,13,22–26].

4.3. H2/CO ratio in producer gas

High yields of syngas (H2 and CO) were obtained from olive

kernel gasification with air and this behavior could be attrib-

uted to the increased content of cellulose and hemicellulose in

the raw material. Such information was known from previous

experimentation in a laboratory scale, fixed bed gasification

unit [15].

Uses of syngas could be distinguished according to the H2/

CO molar ratio. Producer gas with H2/CO ratio between 1 and 2

can be used for the production of 2nd generation biofuels via

Fischer Tropsch synthesis (H2/CO w 2) and high added value

chemical products like methanol [27–29]. The present study

indicates that producer gas of olive kernels gasification at

750 �C and ER¼ 0.2 with a molar ratio of H2/CO¼ 1.7 could be

useful for Fisher Tropsch synthesis. The effect of temperature

on H2/CO molar ratio is presented in Fig. 8.

Further syngas quality improvements could possibly make

the producer gas suitable for energy production not only in

conjunction with ICEs, but also, with FCs. Especially con-

cerning SOFCs, research has shown that a fraction of CO of

w24%vv in syngas could improve the SOFC performance by

23% compared to pure H2 usage [30].

5. Discussion

The experimental data gained in this study reveal that in olive

kernel air gasification the optimum conditions for H2

production maximization were temperature of 750 �C and

ER¼ 0.2. The relatively low gasification temperature might be

attributed to various reasons such as pyrolytic behavior of

fluidized bed gasification, high reactivity of olive stones char,

Ta

ble

3–

Co

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[10]

[22]

[13]

[24]

[9]

[25]

[26]

[10]

[23]

[13]

Bio

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Oli

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kern

els

Oli

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kern

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Oli

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kern

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Oli

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kern

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kern

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tin

gte

mp

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(�C

)750

750

700

600

824

700

800

780

800

750

600

600

ER

b0.2

00.4

20.0

00.0

00.6

4n

.a.

0.0

00.4

10.5

90.4

20.0

00.0

0

Gas

com

pos

itio

n%

vv

%v

v%

vv

%w

t%

vv

%w

t%

wt

%v

v%

vv

%v

v%

wt

CO

14.2

622.7

915.0

095.5

37.5

00.2

446.2

08.6

08.4

015.2

59.0

099.5

0

CO

219.4

242.4

848.0

00.4

919.7

00.1

529.0

021.7

019.0

055.1

25.0

00.4

2

H2

23.9

826.6

830.0

0n

.a.

7.6

00.1

51.3

05.4

09.3

019.5

512.5

0n

.a.

CH

43.7

56.7

38.5

03.9

81.8

00.1

116.2

03.0

01.9

07.9

717.0

00.0

8

C2H

41.5

20.3

53.5

0n

.a.

0.9

0n

.d.

4.4

01.6

00.8

00.6

30.2

0n

.a.

C2H

60.2

90.9

62.5

0n

.a.

0.2

0n

.d.

0.3

00.2

00.4

70.4

0n

.a.

LH

Vga

s(M

J/N

m3)

6.5

49.0

03.0

010.4

03.8

03.3

08.2

3

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Fig. 8 – Effect of temperature and ER on H2/CO molar ratio.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 4 6521

combustion rate of cellulosic char, high volatile content of

biomass and probably catalytic effects associated with the

high alkali content of olive kernel. The experimental findings

of the current study are fully in line with the results of other

researchers [9,10,17,18,22,23].

5.1. Pyrolytic behavior of fluidized bed gasification

It is widely known that pyrolysis consists a step of primary

importance in biomass fluidized bed gasification process. For

this reason, pyrolysis results could be used to obtain useful

information in order to be implemented in the development of

bench and pilot scale fluidized bed gasification units. In

a previous work, Zabaniotou et al. [13] studied olive kernel

rapid pyrolysis in order to define optimum conditions for H2

production and to acquire useful information about fluidized

bed gasification units design. Results revealed that high

pyrolysis (ER¼ 0) temperatures (T> 600 �C) favour the

cracking of heaviest hydrocarbons (tars) in the gaseous

product, giving rise to H2 yield. Maximum H2 yield was

obtained at 700 �C and that served as a guideline for the

present study. When the FBG was operated close to pyrolytic

conditions, i.e., at the lowest ER, the highest H2 yield was

achieved.

5.2. High reactivity of the char produced by olive kernels

It is known that lignocellulosic materials gasification involves

a complex set of reactions apart from pyrolysis reactions.

Biomass gasification steps embody the drying process as the

first step and char gasification as the final step (third step).

Pyrolysis is the second step, which is fast in fluidization

conditions, resulting in high volatiles and low char yield. This

kind of char has a considerable reactivity and it is valuable for

the production of high added value materials. Among those

two individual processes, pyrolysis and char gasification, the

latter is the rate-limiting step determining the required resi-

dence time in the reactor. Char reactivity is important for

combustion and gasification systems optimization and

development. It is desirable for the gasification process to

reach the lowest char yield in combination with its highest

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 46522

reactivity, during the pyrolysis step. Thus the residual carbon,

that is embodied in the char, is further gasified and combusted

in the bottom region of the fluidized bed reactor.

According to Di Blasi et al. [27] lignocellulosic char reac-

tivity, after a rapid rate of oxidative weight loss (at temper-

atures< 400 �C), presents two distinct exothermic stages

corresponding to combustion of aliphatic and aromatic

components, respectively. The relative importance of the two

combustion stages depends on char formation temperature,

with the aliphatic groups to be the predominant at the less

severe pyrolysis conditions. Char reactivity decreases during

conversion and gives the depletion of the more reactive

components and the changes in the inorganics, which become

less catalytically active [27].

Additionally, the combustion rate of cellulosic char was

assumed to consist of the following main adsorption (Reaction

(6)) and desorption reactions (Reaction (7) and (8)). Therefore,

the results presented here can be directly applicable to the

above reactions:

2CþO2/2CðOÞ (6)

2CðOÞ/CO2 (7)

CðOÞ þ 2CðOÞ/CO2 þ C (8)

It can be expected that, at low temperatures, the desorption

process (physical process) is the controlling step and at high

temperatures the controlling step is the chemical sorption of

oxygen (chemical process).

Higher H2 production at low gasification temperatures,

a main experimental result in the present study, is justified

also by the findings of Cao et al. [16]. They considered that the

following reactions are involved in biomass gasification:

CnHmOz �!Heat

aCO2 þ bH2Oþ cCH4 þ dCOþ eH2 þ fðC2 � C5Þ (9)

CþO2/CO2 (10)

CþO2/CO (11)

COþH2O/CO2 þH2 (12)

Reaction (9) indicates that lignocellulosic biomass under

thermal treatment releases high amount of volatiles; thus at the

beginning biomass is rapidly devolatized without the necessity

of large amounts of energy. On the other hand, the energy needs

for the pyrolysis step can be provided from the complete and

partial oxidation reactions of the fixed carbon that is left in the

char residue (exothermic Reactions (10) and (11)). Additionally,

the water–gas shift (Reaction (12)) also occurs in the gasification

process due to the presence of process steam or raw material

moisture and enhances H2 production. Carbon also reacts with

moisture according to Reaction (2) and carbon dioxide (Reaction

(5)) in order to produce carbon monoxide. However, the steam

and carbon dioxide gasification processes are both intensive

endothermic reactions, which will result in temperature drop in

the gasifier. According to Le Chatelier’s principle, higher

temperatures favour reactants in exothermic reactions and

products in endothermic reactions. At lower temperature,

according to the endothermic Reactions (2), (5) and (12) the

occurrence of H2 and CO seemed that were not favoured, as

steam reforming reactions take place at higher than 750 �C

temperature [18]. However, low gasification temperature

(750 �C) in combination with the impose of as low as possible ER

(0.2) and olivine presence, another phenomenon seemed to be

prevailed over steam reforming reactions. Possibly the compo-

nents of tar, and mainly toluene, have been thermally broken

down, under the catalytic effect of iron-based olivine, releasing

at the same time as intermediate compounds H2 and CO [31].

5.3. The volatile content of olive kernel

Olive kernel releases a considerably high proportion of vola-

tiles during its thermal treatment [10]. Volatiles then react

further with the char residue in order to produce more fuel

gas. Additionally, Ollero et al. [19] concluded that external

diffusion resistance may limit the char reactivity, especially at

low CO2 partial pressures ðpCO2 � 0:2 barÞ and high tempera-

tures (T� 900 �C). Lower temperatures may lead to an increase

in the char reactivity and that supports the results of the

present study. They have also reported that CO2 adsorption on

char surface, followed by CO desorption and are the

predominant steps that determine the gasification rate

according to Reactions (13)–(15):

Cf þ CO2/CðOÞ þ CO (13)

CðOÞ þ CO/Cf þ CO2 (14)

CðOÞ/Cf þ CO (15)

where Cf represents the active carbon site and C(O) the

carbon–oxygen complex.

According to their results, CO reduces the steady-state

concentration of C(O) due to the inverse reaction and has an

adverse effect on it. They have also noted that the char reac-

tion with CO has a significant inhibition effect and the acti-

vation energy of char gasification of olive kernel is

significantly lower than other biomass chars. This happens

probably due to the catalytic effect associated to olive kernels

high alkali content [21].

5.4. Alkali content of olive kernels

It is known that the gasification products distribution depend,

apart from temperature and ER, on the type of biomass,

something that also affects both biomass devolatilization and

char conversion. Di Blasi et al. [27] investigated the char reac-

tivities of wheat straw, olive husks and grape residues under

pyrolysis and established that the higher reactivity of chars

emerging from olive husk pyrolysis – in comparison to other

agricultural residues – may be attributed to the lower pyrolysis

temperatures; such conclusion was made as a consequence of

the higher bed density. They have also connected this obser-

vation to another fact; ash from the olive kernel is Na, Ca and K

rich – compared to wheat straw and grape residues – and

therefore the char catalysis during combustion is stronger.

In addition, pulverized olive kernel allows a better heat flow

rate in comparison with other residues that are not easy to

pulverize. Since smaller particles have larger surface area and

therefore faster heating rate, it is expected that the size of the

biomass particles will influence the product gas composition

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 5 1 5 – 6 5 2 4 6523

and yield. A possible explanation is that for smaller particle

sizes pyrolysis process is mainly controlled by reaction

kinetics; but as the particle size increases, the product gas (that

emerges from the interior of the particle) is more difficult to

diffuse out and as a result the process is mainly controlled by

gas desorption. It is generally accepted that gas yield and

composition are related to the heating rate of biomass particles:

high heating rates produce mainly light gases and less char and

condensates. Due to a simple power law, global mechanisms

are observed to give a good prediction of combustion kinetics,

with reactivities that increase significantly as heating rate

increases and/or temperature is decreased [8,21].

6. Conclusions

Olive kernels have a significant position amongst other agri-

cultural residues in Greece. The present study concerned the

air gasification process of olive kernel in an FBG at the

temperature range of 750–850 �C and air equivalence ratio in

the range 0.2–0.4. Olivine was chosen as bed material playing

a dual role: as fluidization medium and as an in-bed catalyst.

Experimentalresultsindicatedtheeffectoftemperatureand

air equivalence ratio on producer gas composition and LHV.

Suchexperimentalresultsgaveconsiderableinformationabout

gasifier’s performance and reactor’s scaling up. An important

remark was that the highest H2 proportion in the producer gas

was reached at 750 �C and ER¼ 0.2 where the maximization of

LHV was also detected. Further increase in air led to a consid-

erable increase of CO2 (due to promotion of oxidation reactions)

and deterioration of the producer gas LHV.

Acknowledgments

The authors are grateful to the European Social Fund (ESF),

Greek Ministry of Development - General Secretariat of

Research and Technology (GSRT), for the financial support,

under the frames of the research program PENED03.

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