An experimental study on catalytic bed materials in a biomass dual fluidised bed gasifier

Renewable Energy 81 (2015) 251e261

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Renewable Energy
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An experimental study on catalytic bed materials in a biomass dual
fluidised bed gasifier
€ ransson*, U. So
€derlind, P. Engstrand, W. Zhang
K. Go
Department of Chemical Engineering, Mid Sweden University, Sundsvall SE-85170, Sweden

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 9 April 2014
Accepted 10 March 2015
Available online 1 April 2015

A study on in-bed material catalytic reforming of tar/CH4 has been performed in the 150 kW allothermal
gasifier at Mid Sweden University (MIUN). The major challenge in biomass fluidised-bed gasification to
produce high-quality syngas, is the reforming of tars and CH4. The MIUN gasifier has a unique design
suitable for in-bed tar/CH4 catalytic reforming and continuously internal regeneration of the reactive bed
material. This paper evaluates the catalytic effects of olivine and Fe-impregnated olivine (10%wtFe/
olivine Catalyst) with reference to silica sand in the MIUN dual fluidised bed (DFB) gasifier. Furthermore,
a comparative experimental test is carried out with the same operation condition and bed-materials
when the gasifier is operated in the mode of single bubbling fluidised bed (BFB), in order to detect

the internal regeneration of the catalytic bed materials in the DFB operation. The behaviour of catalytic
and non-catalytic bed materials differs when they are used in the DFB and the BFB. Fe/olivine and olivine
in the BFB mode give lower tar and CH4 content together with higher H2 þ CO concentration, and higher
H2/CO ratio, compared to DFB mode. It is hard to show a clear advantage of Fe/olivine over olivine
regarding tar/CH4 catalytic reforming.
© 2015 Elsevier Ltd. All rights reserved.

Keywords:
Biomass gasification
Tar reforming
Catalytic bed material
Dual fluidised bed

1. Introduction
Bio-automotive fuels and chemicals can be produced from highquality syngas (mainly hydrogen and carbon monoxide) [1]. Efficient cleaning of raw syngas from biomass gasification is important
for commercialization of the technology for applications such as
electricity generation and synthetic fuel production. The syngas
from a typical indirect gasifier contains H2, CO, CO2, CH4, H2O, trace
amounts of higher hydrocarbons, possible inert gases from
biomass, gasification agent and various contaminants. There has
been much experience gained from gas cleaning related to engine
and turbine applications. Product gas for synthesis normally has a
much stricter specification of impurities than these applications [2].
The major challenge in biomass fluidised-bed gasification to produce high-quality syngas, is the reforming of tars and CH4 (except
for methanation application) to a minimum allowable limit.
Reduction of tars and CH4 to an acceptable low level is usually
achieved by high temperature thermal cracking, low temperature
catalytic cracking, or physical tar treatment like water scrubbing

* Corresponding author.

€ ransson).
E-mail address: (K. Go
/>0960-1481/© 2015 Elsevier Ltd. All rights reserved.

and sedimentation or oil scrubbing and combustion [2e10]. Catalytic cracking efficiency can reach 90e95 % at reaction temperatures about 800e900  C [11], whereas thermal cracking requires
temperatures above 1200  C to reach the same efficiency at
expense of energy losses and big investments on high temperature
materials.
The catalysts can be used in downstream catalytic reactors
[12,13], such as catalytic beds, monoliths and filters, or added
directly in the fluidised-bed gasifier as the bed material. Use of bed
materials as catalyst for tar reduction is simple, reliable and can be
reactivated by the combination of combustion and gasification in
the dual fluidised bed gasifier (DFBG).
The main function of the bed material in a DFBG is to transfer
heat from the combustor reactor to supply energy for the endothermic gasification of biomass in the gasification reactor. In
addition, the bed material makes in-situ gas conditioning possible.
Reactive bed materials can be applied to improve agglomeration
behaviour, to enhance tar cracking and to increase H2 content, and
perform catalytic activity, CO2 capture, oxygen transportation, etc.
The use of catalytically active bed materials promote char gasification, water-gas-shift (WGS) and steam reforming reactions,
which enhance tar/CH4 reforming and increase the H2 content in


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the syngas. In the same time, the bed material behaviour can be

improved with respect to a reduced risk of bed agglomeration [2].
One potential catalytic bed-material is olivine ((Mg, Fe)2 SiO4), a
natural mineral containing magnesium, iron oxide and silica. The
oxygen transport capacity of olivine can be 0.5wt% [14]. Hence, the
produced gas in the gasifier will be partially oxidized by olivine as
an oxygen carrier in DFB operation. Reduction of bed material in the
steam gasifier with the following oxidation in the air combustor
achieves a catalyst recovery cycle, similar to the chemical looping
combustion (CLC) [15].
Catalytic activity of olivine in cracking and reforming of tars and
enhanced steam and dry reforming of hydrocarbons are reported in
a number of articles [15e20]. The catalytic activity of iron species is
considered to be related to their oxidation state. Some researchers
suggest that the efficiency of olivine in tar cracking relies on free
iron (III) oxides present at the surface, while others have the
opposite opinion. Nordgreen et al. [21] studied the decomposition
of tars on metallic iron and iron oxides in the temperature range of
700e900  C. In this study, only iron in the metallic state showed
considerable activity for tar decomposition. At 900  C the tar
decomposition activity was similar to calcined dolomite [21].
Metallic iron is known to be an active species for aromatic hydrocarbon decomposition and iron oxides are known to be a good
catalyst for the WGS reaction [22].
Compounds of iron and oxygen occurring in nature, include
Fe1ÀxO (wustite), a-Fe2O3 (haematite), g-Fe2O3 (maghemite), and
Fe3O4 (magnetite). The ideal and stoichiometric FeO consists of Fe2þ
-ions, the a-Fe2O3 and g-Fe2O3 of Fe3þ -ions, and the Fe3O4 of Fe2þ and Fe3þ -ions [23]. In the air combustor, the olivine decomposes to
binary iron oxide, silica oxide and magnesium silicate, reaction (1):
(Fe0.1, Mg0.9)2 SiO4 þ 0.05O2 / 0.1Fe2O3 þ 0.1SiO2 þ 0.9Mg2SiO4(1)
The binary iron oxide diffuses to the surface of the bed material.
Fe2O3 enters the steam gasifier, where it reacts with hydrocarbons

and is reduced to FeO, reaction (2):
5Fe2O3 þ 2CxHy / 10FeO þ 2xCO2 þ yH2O

(2)

Reduced iron oxide (FeO) is transported back to the air
combustor where it reacts with air and is oxidized to Fe2O3 [15],
reaction (3):
2FeO þ 0.5O2 / Fe2O3

(3)

The catalytic bed material can be pre-treated by calcination [17]
to increase the free iron (III) concentration on the olivine surface for
better catalytic activity. Besides, olivine is a very flexible structure
that can be a host for transition metal [24]. A better conversion can
be achieved by the use of modified olivine, such as Ni-supported
olivine or Fe-supported olivine. Ni-supported olivine is highly
effective in reduction of tars and CH4 [25], but an important
drawback is the toxicity of nickel and the volatiles particles that
occurs in FB gasifiers. Fe/olivine, however, is a relatively harmless
and cheap catalyst [26]. Many investigations, e.g. the research
project UNIQUE [27], have shown that Fe/olivine is efficient in tar
reforming and also active in CH4 steam reforming [22,24,28e30].
Biomass ash can be treated as a catalyst which may significantly
improve the performance of biomass gasification. In the ash, the
calcium-rich compounds interact with the bed material, and build
calcium-rich layers around the particles. The catalytic effect could
be dominated by the calcium-rich layer [18,31].
The catalyst can be deactivated due to carbon deposition,

chloride, sulphur poisoning, oxidation, and sintering. However, the
lifetime of the catalyst can be prolonged by the oxygen balance in a

DFBG [13,25]. This can be seen as continuously internal regeneration of the catalytic bed-material, where the carbon deposit is
burned away.
CH4 is the most recalcitrant hydrocarbon to reform. The steam
reforming of CH4 consists of three reversible reactions: the strongly
endothermic reforming reactions (4) and (6) and the moderately
exothermic WGS reaction (5). It is found that the WGS reaction is
very fast at reforming conditions, and hence, the WGS equilibrium
is always established during steam reforming [32].


CH4 þ H2 O4CO þ 3H2
CO þ H2 O4CO2 þ H2

DH298 ¼ þ206 kJ=mol


DH298 ¼ À41 kJ=mol

CH4 þ 2H2 O4CO2 þ 4H2



DH298 ¼ þ165 kJ=mol

(4)
(5)
(6)

Steam reforming is favoured by high temperature and low
pressure; in contrast the exothermic shift reaction is favoured by
low temperature, while unaffected by changes in pressure. The
amount of steam will enhance the CH4 conversion.
A 150 kW DFBG was built at Mid Sweden University (MIUN) in
2007 [33] which has a unique design suitable for in-bed tar/CH4
catalytic reforming and continuously internal regeneration of the
reactive bed material. This paper evaluates the catalytic effects of
calcined olivine and Fe-doped olivine (10%wt Fe/olivine Catalyst)
with reference to non-catalytic silica sand in the MIUN gasifier when
it is operated in the mode of dual fluidised beds (DFB). Furthermore,
a comparative experimental test is carried out with the same operation condition and bed-materials when the gasifier is operated in
the mode of single bubbling fluidised bed (BFB), in order to detect
the internal regeneration of the bed materials in the DFB operation.
The measurement results have been evaluated by comparing the
syngas composition and tar/CH4 content in the syngas from the
gasifier operated in the two modes under different operation
conditions.

2. Experimental
2.1. Gasification test in the MIUN gasifier
The MIUN gasifier is a DFBG (see Fig. 1) and consists of an
endothermic steam BFB gasifier and an exothermal circulating
fluidised bed (CFB) riser combustor, and has the biomass treatment
capacity of 150 kWth, i.e. approx. 25 kg biomass feed per hour. The
heat carrier between the reactors is the bed-material. The biomass
is fed into the dense bed in the gasifier. The fluidisation agent in the
gasifier is steam and the syngas exits from the top of the gasifier.
The residual biomass char is then transferred by bed-material into

the combustor through the lower pressure lock. In the combustor,
the fluidisation agent is air, which results in an oxidation of the char
that produces heat at the temperature of 950e1050  C. The hot
bed-material separates from the flue gas in the particle separator to
be recycled into the gasifier through the upper pressure lock, which
prevents gas leakage between the separate environments in the
gasifier and the combustor. The gasifier is supported by electrical
heaters and is heavily insulated. The electrical heaters allow separate operation of the steam gasifier as a BFB gasifier. At BFB operation, the interconnections between the gasifier and the riser are
blocked, and hence are the vessels divided. The only heat source for
BFB operation is the electrical heaters. A large part of heat for DFB
gasification comes from electricity energy as well. The gasifier and
the combustor have a height of 2.5 and 3.1 m, and inner diameters
(i.d.) of 300 and 90 mm, respectively. The MIUN gasifier has been
described in detail in a previous article [33].


€ransson et al. / Renewable Energy 81 (2015) 251e261
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The tar content in raw untreated syngas from the MIUN gasifier
with silica sand is approx. 20 g/Nm3 or more. The concentration of
CH4 is about 10% corresponding to one third of syngas energy,
which cannot join the downstream synthesis reaction for liquid
fuels. The content of tars and CH4 in the syngas from the MIUN
gasifier needs to be reduced to an acceptable low level.
Hence, internal tar/CH4 reforming with catalytic bed materials
needs to be investigated in detail. These tests are carried out in both
BFB mode and DFB mode to compare the bed materials under
different gasification conditions. The tar/CH4 reforming test runs in
three cases: 1) basic condition with silica sand (no catalytic activity), 2) calcined olivine, 3) Fe-impregnated olivine (10%wtFe/olivine

Catalyst), at the temperatures of 750, 800, 850 and 900  C, and at
the steam-to-carbon (S/C) ratios of 0.6, 1.2 and 1.8 in weight (kg/kg).
The S/C ratio is calculated according to equation (7).

S=C ¼

m_ steam þ nH2 O Â m_ biomass
nC Â m_ biomass

253

(7)

where m_ steam represents the mass flow of steam (kg/s)
m_ biomass represents the flow of biomass (kg/s)
nC represents the carbon mass fraction in the biomass
nH2 O represents the water mass fraction in the biomass
Silica sand is the reference case for comparing the activity of the
catalytic bed materials. The biomass feedstock is wood pellets (see
Table 1).
In the experiment, the gasifier is fluidised with steam and the
riser with air at atmospheric pressure. Default, air or argon is used
for fluidisation of the upper pressure lock, and for aeration of the

Fig. 1. The 150 kWth MIUN biomass DFBG gasifier.


850
1.8
10

8
0.30
1.7e2.2
The space time of total gas in gasifier bed (bed height 0.50e0.65 m).
a

Fe/Ol

850
0.6
14
3
0.29
1.7e2.3
900
1.2
10
5
0.26
1.9e2.5

Fe/Ol
Fe/Ol

850
1.2
10
5
0.25
2.0e2.6

800
1.2
10
5
0.24
2.1e2.7

Fe/Ol
Fe/Ol

750
1.2
10
5
0.23
2.2e2.9
850
1.8
10
8
0.30
1.7e2.2

Olivine
Olivine

850
0.6
14
3

0.29
1.7e2.3
900
1.2
10
5
0.26
1.9e2.5

Olivine
Olivine

850
1.2
10
5
0.25
2.0e2.6
800
1.2
10
5
0.24
2.1e2.7

Olivine
Olivine

750
1.2

10
5
0.23
2.2e2.9
850
1.8
10
8
0.30
1.7e2.2

Sand
Sand

850
0.6
14
3
0.29
1.7e2.3
900
1.2
10
5
0.26
1.9e2.5

8
26
7

25
6
24
5
23
4
22
3
21
2
20

750
1.2
10
5
0.23
2.2e2.9

Steam at bed temperature 750e900  C.

Olivine
223
3300
0.03
2.8
7e10

Sand

a

Silica sand
200
2650
0.02
2.2
11e16

Temperature ( C)
S/C (kg/kg-)
Biomass feed rate (kg/h)
Steam flow rate (kg/h)
Gasifier Superficial Velocity (m/s)
The space time in bed (s)a

Material
Mean particle size (mm)
Density (kg/m3)
Minimum fluidisation velocitya Umf (m/s)
Terminal velocity Ut (m/s)
Gasifier superficial velocity (U/Umf)

Bed Material

Table 2
Bed material (Fe/olivine is here considered to have equal properties as olivine).

1
19

The main syngas stream from the gasifier is led to an incinerator
for complete combustion. A slip stream of the syngas passes

Test BFB
Test DFB

2.3. Analysis of gas composition and tars

Table 3
Gasifier operation conditions for BFB and DFB mode, atmospheric pressure.

Two catalytic bed materials are used in this test, olivine and 10%
wt Fe/olivine. A sufficient high content of free iron oxide in olivine
requires a high calcination temperature and a long calcination time.
Calcination at temperature below 900  C causes reduction of the
surface iron oxide at low temperature, which is not convenient for
steam reforming of hydrocarbons due to sintering and carbon
deposition [22]. A very high temperature to improve iron reduction
is unnecessary and not gainful.
The olivine in these tests is calcined inside the DFB reactor at
900  C for 10 h, with air at slightly elevated pressure. The 10 wt%Feolivine catalyst is synthesized by impregnation of an aqueous iron
nitrate solution. The iron nitrate is received as an aqueous solution
(9.3e9.7 wt% Fe; 40.3e42.0 wt% Fe(NO3)39H2O). The natural olivine
((Mg,Fe)2SiO4) is added (1.0 kg olivine to 1.1 kg aqueous iron nitrate
solution) and stirred vigorously in the aqueous solution of iron
nitrate (for approx. 15 min). The next step is solvent evaporation
and drying in a vertically revolving kiln with a propane flare as heat
source (for approx. 1 h), until all liquid is evaporated and the olivine
particles are rust-coloured. In the rotating vessel, the bed material

has a steep temperature gradient; the estimated average temperature for the Fe/olivine is 250e300  C. Finally the Fe/olivine is
calcined at 900  C for 10 h as the natural olivine described above.

9
27

2.2. The catalytic bed materials

Sand

16
34
15
33
14
32
10
28

11
29

12
30

13
31

lower pressure lock. The bed material and operation conditions for
the tests are described in Tables 2 and 3. Each test started after

stabilization of the gasification temperature, which ran for
approximately 6 h. The same batch of each bed material (silica sand,
olivine or Fe/olivine) was used during the whole test series with
each bed material. This means that the biomass ash may accumulate over 6 tests for each bed material. The gas and tar sampling
were carried out when the gasifier has reached steady state
condition.

850
1.2
10
5
0.25
2.0e2.6

42.7
6.5%
98.1%
667 kg/m3
18.849 MJ/kg
20.159 MJ/kg

Sand

Oxygen O (calc.)
Moisture
Durability of pellets
Bulk density
Net calorific value as rec.
Net calorific value db

800
1.2
10
5
0.24
2.1e2.7

0.4
<0.01
<0.01
50.9
5.9
<0.1

Sand

% db

Ash cont.
Sulphur S
Chlorine Cl
Carbon C
Hydrogen H
Nitrogen N

17
35

Elementary analysis

18
36

Table 1
Fuel analysis of wood pellets (6 mm) produced by SCA BioNorr AB,
Sweden.

Fe/Ol

€ransson et al. / Renewable Energy 81 (2015) 251e261
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254


€ransson et al. / Renewable Energy 81 (2015) 251e261
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through the gas sampling and analysis system. To avoid tars
condensation in the pipes, electrical heater is used to keep the
syngas temperature at about 400  C. For the measurements of the
gas composition, the syngas is drawn by a vacuum pump and
sampled manually in Tedlar gas sampling bags and analysed offline in a parallel FID and TCD GC-detection system. The TCD detector is used for H2, CO, CH4, CO2, O2, ethene, and ethane; the FID
detector is used for C3 and C4. The gas was sampled four times in
each experimental test under the same gasifier operation condition, and analysed within 24 h.
Heavy tars, which are mostly oily liquid or solid at low temperature under 100  C, are a considerable problem for commercialisation of the fluid bed gasification technology and should be paid
a special attention. Heavy tars dominate the tar dew point, even at
low concentrations. The condensation temperature increases
dramatically with increasing molecular sizes of the tars. Condensed
tars clog filters and valves with potentially efficiency loss and plant

stop.
Most heavy tars are GC-undetectable and here are gravimetrically analysed. The results include GC-undetectable heavy tar
compounds together with some GC-detectable tars (from 2 to 3
rings) [34] in addition to all compounds larger than 3 rings [35]. It
took about 45 s to collect 4 L product gas from the gasifier. First, the
gas passes a high temperature filter (penetration <0.002% DOP
(0.3 mm)) held at 400  C during sampling, and cooled down to 30  C
while passing through two tar capture glass fibre filters in series
(see Fig. 2). After each sampling, the glass fibre-filter adaptors are
washed with isopropanol. Finally, the tars are collected in a round
flask containing isopropanol. The detailed sampling procedure can
be found elsewhere [33].
The solution containing the tar sample together with some
possible filter fibres is filtered through a glass fibre funnel (pore size
10e16 mm) and collected to a new round flask that has been
weighted. The glass fibre funnel is used instead of the time
consuming Soxhlet extraction method, which can shorten the
analysis time at a risk of e.g. entrained bed material particles.
Finally, the round flask is inserted into an evaporation
condenser. When the tar sticks on the inside of the round flask the
evaporation is finished. The weight difference, i.e. the tar content,
can finally be calculated. The analytical balance applied is a Precisa
XR 205SM-DR, readability 0.01 mg/0.1 mg, with a built-in self calibration system.
The remaining tars are referred to “gravimetric tar”. A part of the
light tar produced at this experimental conditions consist of
nonpolar aromatic compounds that might be removed during the
evaporation [36]. Nevertheless, this method is simple and can give
sufficient reliable gravimetric tar measurement that is employed to
evaluate catalytic effects of different bed materials at various
gasifier operation conditions. The light tars without condensing on

heat exchanger can be harmful to downstream synthesis catalyst.

255

However, a hydrocarbon reformer is usually employed before
downstream synthesis.
3. Results and discussion
The measurement results of the main gas components, H2, CO,
CO2 and CH4 are presented in Figs. 3e8 which account for 78e93
Vol. % of the total product gas. The remaining components are, in
general, O2 (<0.5 Vol.%), N2 (<10 Vol.% for DFB mode), C2H4 (<5
Vol.%), C2H6 (<1 Vol.%), C3 and C4 (tot. < 0.5 Vol. %).
3.1. Effect of temperature on gas composition, CH4 and gravimetric
tar
The effect of gasification temperature on the syngas composition including CH4 is shown through Fig. 3 for three different bed
materials when the S/C ratio is held at 1.2. It can be seen from the
figures that higher temperature enhances the tar/CH4 reforming
reactions and results in higher content of H2 and CO, while the CO2
content slightly decreases since the exothermic shift reaction is
favoured by low temperature.
Tar/CH4 reforming is a strongly endothermic reaction, and favoured by high temperature to a great degree. CH4 is the most
recalcitrant hydrocarbon to reform, which very much depends on
the temperature (see Fig. 4). The methane content is not sensitive to
the temperature change up to the temperature 800  C.From temperature 800  Ce900  C, the methane content decreases clearly
from about 11% to 9%, and even down to 7% when catalytic bed
materials are used. These results indicate that hydrocarbon
reforming is strongly sensitive to the temperature around 900  C,
especially for reforming with catalytic bed materials.
Fig. 5 shows the temperature dependency of the gravimetric tar
content in the syngas when the S/C ratio is held constant at 1.2. The

tar content decreases with increasing of temperature as a general
trend. At 850  C, Fe/olivine gives a slightly higher tar content which
is a vague result compared to olivine. But the tar content follows the
downward tendency for Fe/olivine when the gasification temperature increases to 900  C. Similar behaviour with Fe/olivine for
GCMS tars can be found in another article [37], where no significant
change can be recognized in the temperature range of 770e860  C.
3.2. Effect of steam-to-carbon ratio on gas composition, CH4 and
gravimetric tar
Fig. 6 shows the syngas composition at different S/C ratio of 0.6,
1.2 and 1.8 for three different bed materials when the gasification
temperature is held constant at 850  C. For all the cases of different
bed materials, both the H2 content and the ratio of H2/CO increase
with S/C ratio due to the enhanced WGS reaction. A higher S/C ratio
means a higher steam partial pressure that pushes the WGS

Fig. 2. Gravimetric tars sampling system (the syngas passes through two tar capture glass fibre filters in series at ambient temperature whereas the tars condense).


256

€ransson et al. / Renewable Energy 81 (2015) 251e261
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Fig. 3. The effect of the gasification temperature on the syngas composition for three different bed materials when the S/C ratio is held at 1.2.

reaction (CO þ H2O 4 CO2 þ H2) to the right hand side of the
equilibrium equation, and produces more H2 at the expense of CO,
i.e. the CO yield decreases with S/C.
Increased S/C ratio from 0.6 to 1.8 results in a slightly decrease of
CH4 for all the bed materials, which can be seen more clearly in

Fig. 7 where the methane content decreases from around 11% to
around 9%. Fig. 8 shows the gravimetric tar content at S/C ratios of
0.6e1.8. Both olivine and silica sand show a decreasing tendency at
higher S/C ratios, which can be explained by an enhanced steam
reforming of tar. This trend is not clear for Fe/olivine from the
present test.

Fig. 4. Methane content vs. gasifier temperature at S/C 1.2.

In the present experimental test, the measurement date is
scattering much when the tar content is plotted against S/C ratio as
seen in Fig. 8. A decreasing trend of tar content with S/C cannot be
clearly seen here. This might be explained by several reasons: 1) a
much weaker effect of S/C on the gravimetric tar comparing to the
catalytic bed materials and the difference between the BFB and DFB
modes; 2) a narrow range of S/C used, which is beyond the effective
range for tar reforming [38]; 3) a bad mixing of steam with catalytic
bed material and volatile; 4) a low tar reforming rate by steam [16];
5) sampling procedure and deviations.

Fig. 5. The temperature dependency of the gravimetric tar content in the syngas when
the S/C ratio is held constant at 1.2.


€ransson et al. / Renewable Energy 81 (2015) 251e261
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257

Fig. 6. The syngas composition at different S/C ratio for three different bed materials when the gasification temperature is held constant at 850  C.

From overview of the measurement results shown in Figs. 3 and
6, generally speaking, the gas compositions in the use of Olivine and
Fe/olivine not differs much to the results with sand. Comparing the
concentration of H2 and CO in the syngas for different bed materials, the improvements by catalytic bed materials are unclear. This
was also shown by Freda et al. with similar experiment for Fe/
olivine [36].
On the other hand, the concentration of H2 þ CO is higher for
silica sand than the catalytic bed materials in the mode of DFB,

while this situation is reversed in the mode of BFB. This indicates
catalytic effect of hydrocarbon reforming in the BFB mode. The
lower concentration of H2 þ CO in the cases of catalytic bed materials used in the DFB mode can be explained by oxygen transfer by
the catalytic bed materials from the riser into the gasifier, which
results in partially oxidation of H2 and CO to H2O and CO2. The
oxygen transport capacity of olivine can be 0.5wt% [14] due to the
Fe contained in olivine. Fe/olivine will have a higher oxygen
transport capacity, which gives the lowest concentration of H2 þ CO
as indicated in Fig. 3.
Concerning methane catalytic reforming in the test as seen in
Figs. 4 and 7, the methane content in the syngas is reduced

Fig. 7. Methane content vs. S/C ratio at 850  C.

Fig. 8. Gravimetric tar content at S/C 0.6e1.8, gasification temperature 850  C.

3.3. Effects of catalytic bed materials on gas composition, CH4 and
gravimetric tar

258

€ransson et al. / Renewable Energy 81 (2015) 251e261
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somewhat when the catalytic bed materials are used. This catalytic
effect is seen clearly at higher temperatures for the BFB mode, and
is independent of S/C ratio. Olivine is more efficient than Fe/olivine,
and shows much lower methane content than silica sand at the
high temperature of 900  C, whereas Fe/olivine catalytic effect is
not clear from the present test.
Comparing to methane, a similar tendency of tar reforming by
the catalytic bed materials can be seen in Figs. 5 and 8. The gravimetric tar content is slightly lower for the catalytic bed materials
than the silica sand with an overview of the present test results
except for the case at the temperature of 850  C. Higher gasification
temperature leads to a lower gravimetric tar content and a better
catalytic reforming of tar, which is significant for olivine used in the
BFB mode.
In summary, the olivine used in the BFB mode gives the highest
concentration of H2 þ CO, the lowest concentration of CH4 and the
lowest content of tar in the syngas. This tendency is clearer at
higher temperature but insensitive to the S/C ratio. Fe/olivine didn't
show an advantage over olivine regarding tar/CH4 catalytic
reforming based on the results of this test.
3.4. Comparison of gas composition, CH4 and gravimetric tar
content in the two different gasification modes, BFB and DFB
As shown in Figs. 3e8, for both the BFB and DFB modes, the H2/
CO ratio is insensitive to the temperature within the temperature
range of 750  Ce900  C, but increases clearly with S/C ratio. The
ratio of H2/CO is above 1 in the BFB mode but below 1 in the DFB

mode. When silica sand is used as the bed material, the concentration of H2 þ CO in the DFB mode is slightly higher compared to
the BFB mode for all the cases of different temperatures and
different S/C ratios. Correspondingly to the concentration of
H2 þ CO, the gravimetric tar content in the DFB mode is lower for all
cases of different temperatures compared to the BFB mode. The
effect of S/C ratio on the gravimetric tar content in DFB mode is
slightly observed for S/C ¼ 0.6 and more clear for S/C ¼ 1.2. This
might be explained by a better reforming of hydrocarbon through
the contact between the hotter circulated bed material from the
riser and the product gas in the freeboard of the fluidized bed
gasifier.
For olivine and Fe/olivine catalytic bed materials on the other
hand, the concentration of H2 þ CO in the BFB mode is much higher
with slightly lower CH4 concentration and the gravimetric tar
content compared to the DFB mode for all the cases of different
temperatures and different S/C ratios. The lower concentration of
H2 þ CO in the DFB mode can be well-explained by the Fe-based
catalyst oxygen transport from the riser to the gasifier. When
olivine particles circulate from the riser and fall down in a countercurrent mode from the upper loop-seal into the gasifier, a part of
the product gas can be immediately oxidized with a rapid oxidation
reaction in the particle falling section. This phenomenon cannot
explain the higher CH4 and tar contents for the DFB mode
compared to the BFB mode. The above measurement results suggest that the catalytic steam reforming of hydrocarbon in the BFB
mode performs better than the DFB mode.
Catalytic bed materials usually promote char gasification, WGS
and steam reforming reactions, which enhance tar/CH4 reforming
and increase the H2 content in the syngas. Some investigations
[22,39,40] have shown promoted tar reforming activity with Fe/
olivine in externally heated bench-scale tests. However, the
improvement is not clear in DFB pilot scale tests [37], as is shown

from this test in the pilot scale 150 kW MIUN gasifier.
Fig. 9 shows a typical case of the temperature profile along the
height of the MIUN gasifier in the BFB and DFB modes respectively
for same operation conditions of temperature 850  C and S/C 1.2. In

Fig. 9. The temperature profile along the height of the MIUN gasifier in the BFB and
DFB modes respectively for the same operation conditions of temperature 850  C and
S/C 1.2.

the DFB mode, the temperature profile has a peak point of 865  C at
the dense bed surface where the hot solid particles entering the
riser.
The temperature holds constant over the dense bed but drops
down in the freeboard due to the wall cooling effect (no electrical
heaters around the gasifier freeboard), steam reforming and
cracking reactions as well as pyrolysis and gasification of entrained
biomass and char particles in the freeboard. A bad mixing of the hot
falling particles with the volatile gas in the freeboard may lead to an
insufficient heat transfer between the gas and solid phases. In the
BFB mode, the vertical temperature profile is almost uniform with
the height of the gasifier up to the top of the freeboard as seen in
Fig. 9.
In the experimental test, the total bed material was adjusted to
hold a similar dense bed height of the gasifier in both the DFB and
BFB modes. In the DFB mode, the char and ash produced in the
gasifier will be transferred to the riser by the bed material return. A
part of char and ash will be burned away or carried over for the fine
fly ash. Thus, the residence time of char and ash in the gasifier are
short and the concentrations are low. In the BFB mode, on the other
hand, the char and ash in the gasifier accumulate, the residence

time is long and the concentrations are high. Both char and ash
have been recognized to be a good catalyst for hydrocarbon
reforming. In the case that calcium enriches in the ash layer
covering bed material particles, the catalytic reforming of tar can be
enhanced to a great degree.
The behaviour of catalytic and non-catalytic bed materials differs when they are used in the DFB mode and in the BFB mode. Fe/
olivine and olivine in the BFB mode give lower tar and CH4 content
compared to DFB mode. This might be attributed to a higher temperature in the freeboard and higher concentrations of char and ash
in the dense bed of the BFB while the fine catalyst particles are
carried over from the DFB riser. The internal regeneration of the
catalytic bed materials in the DFB mode leading to better reforming
of hydrocarbon has not been observed from this experimental test.
3.5. Scanning electron microscopy of the Fe/olivine particles
The olivine and Fe/olivine bed material was investigated by
using scanning electron microscopy (SEM). Fig. 10 shows the
calcined olivine after use in the gasifier.
Fig. 11 shows the Fe/olivine bed material particles directly after
the drying procedure. A coating on the surface of the bed material
particles is notable. Fig. 12 shows the Fe/olivine after use in the


€ransson et al. / Renewable Energy 81 (2015) 251e261
K. Go

259

Fig. 10. Calcined olivine after use in gasifier.

Fig. 11. Fe/olivine after impregnation (a coating on the surface of the bed material particles is notable).

Fig. 12. Calcined Fe/olivine after use in gasifier (the particle surface is abraded and there is no coating visible).

gasifier. It can be seen that the particle surface is abraded and there
is no coating visible.
One reason could be the too low temperature employed in the
iron impregnation. This may have resulted in a porous surface on
the Fe/olivine particle that was removed during the calcination and
gasification process. Attrition phenomena with loss of a part of the
iron have also been reported by other researchers [40e42].
Comparing the particle size distributions of the Fe/olivine catalytic bed material before use (in Fig. 11) and after use (in Fig. 12) in
the gasifier, it can be seen that the particle size distribution become
narrow and the particles becomes uniform when the fine is carried
over. Thus, the overall surface of catalyst will be reduced and the
catalytic effect becomes low.

It was found in this experimental test that Fe/olivine causes an
increased particulate load of rusty red colour particles in the gas
stream, especially in the DFB mode, which had also been observed
by others [36]. This could be one reason that the trend of tar
reduction by the catalyst such as Fe/olivine is not clear.
4. Conclusions
An experimental test on the in-bed catalytic materials, olivine
and 10%Fe/olivine (with reference to silica sand), were carried out
in the BFB and DFB modes of the pilot-scale MIUN gasifier at
different gasification temperatures and S/C ratios. The concentration of H2 þ CO, the ratio of H2/CO, the CH4 concentration and the


€ransson et al. / Renewable Energy 81 (2015) 251e261
K. Go

260

tar content in the syngas measured in the test follow a reasonable
trend with respect to steam biomass gasification, WGS reaction as
well as hydrocarbon reforming.
The olivine used in the BFB mode gives the highest concentration of H2 þ CO, the lowest CH4 and tar contents in the syngas. This
tendency is clearer at higher temperature but insensitive to the S/C
ratio. Fe/olivine did not show an advantage over olivine regarding
tar/CH4 catalytic reforming based on the results of this test.
A much lower concentration of H2 þ CO in the DFB mode with
Fe/olivine and olivine catalytic bed materials suggests the syngas
partial oxidation by the Fe-based catalysts through oxygen transport from the riser to the gasifier. The lower tar and CH4 contents in
the BFB mode might be attributed to the higher concentrations of
char and ash (while the fine catalyst particles are carried over from
the DFB riser) in the BFB gasifier, and a higher temperature in the
freeboard.
From this test, the tar/CH4 reforming performance by the catalytic bed materials has not been as clear as in small bench-scale
tests published in literature, especially for Fe/olivine catalyst. This
can be explained by insufficient catalyst coating on the particle
surface, the fine catalyst particle carry-over and the evaluation
method based on gravimetric tar.

[8]

[9]

[10]
[11]
[12]

[13]

[14]

[15]

[16]
[17]

[18]

Acknowledgements
[19]

The authors would like to acknowledge the project support of
€reningen's Foundation
EU Regional Development Fund, Ångpannefo
€nstyrelsen
for Research and Development (ÅForsk), LKAB, La
V€
asternorrland, Swedish Gasification Centre (SFC) and SCA BioNorr
€rno
€sand. The authors are grateful to Dr. Christina Dahlstro
€m
AB, Ha
for the SEM-images in this paper.

[22]

Abbreviations

[23]

BFB
CFB
CLC
DFB
DFBG
FID
GC
GCMS
MIUN
Nm3
SEM
S/C
TCD
WGS

bubbling fluidised bed
circulating fluidised bed
chemical loop combustion
dual fluidised bed
dual fluidised bed gasifier
flame ionization detector
gas chromatography
gas chromatography-mass spectrometry
Mid Sweden University
normal cubic meter
scanning electron microscope
steam-to-carbon ratio [kg/kg]

thermal conductivity detector
water-gas-shift

[20]

[21]

[24]

[25]

[26]

[27]
[28]

[29]

[30]

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