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

This paper evaluates the catalytic effects of olivine and Fe-impregnated olivine 10%wtFe/ olivine Catalyst with reference to silica sand in the MIUN dualfluidised bed DFB gasifier.. This p

An experimental study on catalytic bed materials in a biomass dual

fluidised bed gasifier

K G€oransson*, U S€oderlind, P Engstrand, W Zhang

Department of Chemical Engineering, Mid Sweden University, Sundsvall SE-85170, Sweden

a r t i c l e i n f o

Article history:

Received 9 April 2014

Accepted 10 March 2015

Available online 1 April 2015

Keywords:

Biomass gasification

Tar reforming

Catalytic bed material

Dual fluidised bed

a b s t r a c t

A study on in-bed material catalytic reforming of tar/CH4has 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/CH4catalytic 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 dualfluidised 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 CH4content together with higher H2þ 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/CH4catalytic reforming

© 2015 Elsevier Ltd All rights reserved

1 Introduction

Bio-automotive fuels and chemicals can be produced from

high-quality syngas (mainly hydrogen and carbon monoxide)[1] Ef

fi-cient 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 biomassfluidised-bed gasification to

pro-duce high-quality syngas, is the reforming of tars and CH4(except

for methanation application) to a minimum allowable limit

Reduction of tars and CH4to an acceptable low level is usually

achieved by high temperature thermal cracking, low temperature

catalytic cracking, or physical tar treatment like water scrubbing

and sedimentation or oil scrubbing and combustion[2e10] Cata-lytic cracking efficiency can reach 90e95 % at reaction tempera-tures about 800e900C[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 thefluidised-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 dualfluidised 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 endo-thermic 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 H2content, and perform catalytic activity, CO2capture, oxygen transportation, etc The use of catalytically active bed materials promote char gasi fi-cation, water-gas-shift (WGS) and steam reforming reactions, which enhance tar/CH4reforming and increase the H2content in

* Corresponding author.

E-mail address: kristina.goransson@miun.se (K G€oransson).

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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)2SiO4), 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

700e900C 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

hydro-carbon 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

Fe1 xO (wustite),a-Fe2O3(haematite),g-Fe2O3(maghemite), and

Fe3O4(magnetite) The ideal and stoichiometric FeO consists of Fe2þ

-ions, thea-Fe2O3andg-Fe2O3of Fe3þ-ions, and the Fe3O4of 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)2SiO4þ 0.05O2/ 0.1Fe2O3þ 0.1SiO2þ 0.9Mg2SiO4(1)

The binary iron oxide diffuses to the surface of the bed material

Fe2O3enters the steam gasifier, where it reacts with hydrocarbons

and is reduced to FeO, reaction(2):

5Fe2O3þ 2CxHy/ 10FeO þ 2xCO2þ 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):

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 veryflexible 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 CH4steam 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 regenera-tion of the catalytic bed-material, where the carbon deposit is burned away

CH4is the most recalcitrant hydrocarbon to reform The steam reforming of CH4consists 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]

CH4þ H2O4CO þ 3H2 DH298¼ þ206 kJ=mol (4)

COþ H2O4CO2þ H2 DH298¼ 41 kJ=mol (5)

CH4þ 2H2O4CO2þ 4H2 DH298¼ þ165 kJ=mol (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 CH4conversion

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 dualfluidised beds (DFB) Furthermore,

a comparative experimental test is carried out with the same oper-ation condition and bed-materials when the gasifier is operated in the mode of single bubblingfluidised 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, thefluidisation agent is air, which results in an oxidation of the char that produces heat at the temperature of 950e1050C The hot

bed-material separates from theflue 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 sepa-rate operation of the steam gasifier as a BFB gasifier At BFB oper-ation, 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]

K G€oransson et al / Renewable Energy 81 (2015) 251e261 252

The tar content in raw untreated syngas from the MIUN gasifier

with silica sand is approx 20 g/Nm3or 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 CH4in the syngas from the MIUN

gasifier needs to be reduced to an acceptable low level

Hence, internal tar/CH4reforming 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/CH4reforming test runs in

three cases: 1) basic condition with silica sand (no catalytic

activ-ity), 2) calcined olivine, 3) Fe-impregnated olivine (10%wtFe/olivine

Catalyst), at the temperatures of 750, 800, 850 and 900C, 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 ¼m_steamþ nH 2 O _mbiomass

wherem_steamrepresents the massflow of steam (kg/s) _

mbiomassrepresents theflow of biomass (kg/s)

nCrepresents the carbon mass fraction in the biomass

nH 2 Orepresents 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 forfluidisation of the upper pressure lock, and for aeration of the

MIUN biomass DFBG gasifier.

lower pressure lock The bed material and operation conditions for

the tests are described inTables 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

accumu-late over 6 tests for each bed material The gas and tar sampling

were carried out when the gasifier has reached steady state

condition

2.2 The catalytic bed materials

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 900C 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

900C for 10 h, with air at slightly elevated pressure The 10

wt%Fe-olivine 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 propaneflare 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

temper-ature for the Fe/olivine is 250e300 C Finally the Fe/olivine is

calcined at 900C for 10 h as the natural olivine described above

2.3 Analysis of gas composition and tars

The main syngas stream from the gasifier is led to an incinerator

for complete combustion A slip stream of the syngas passes

Table 1

Fuel analysis of wood pellets (6 mm) produced by SCA BioNorr AB,

Sweden.

Net calorific value as rec 18.849 MJ/kg

Table 2

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

Minimum fluidisation velocity a U mf (m/s) 0.02 0.03

Gasifier superficial velocity (U/U mf ) 11e16 7e10

a Steam at bed temperature 750e900 

 C)

K G€oransson et al / Renewable Energy 81 (2015) 251e261 254

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 400C 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

off-line in a parallel FID and TCD GC-detection system The TCD

de-tector is used for H2, CO, CH4, CO2, O2, ethene, and ethane; the FID

detector is used for C3and C4 The gas was sampled four times in

each experimental test under the same gasifier operation

condi-tion, and analysed within 24 h

Heavy tars, which are mostly oily liquid or solid at low

tem-perature under 100C, are a considerable problem for

commerci-alisation of thefluid 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 clogfilters and valves with potentially efficiency loss and plant

stop

Most heavy tars are GC-undetectable and here are

gravimetri-cally 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

(0.3mm)) held at 400C during sampling, and cooled down to 30C

while passing through two tar capture glassfibre filters in series

(seeFig 2) After each sampling, the glassfibre-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

possiblefilter 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 roundflask the

evaporation isfinished The weight difference, i.e the tar content,

canfinally be calculated The analytical balance applied is a Precisa

XR 205SM-DR, readability 0.01 mg/0.1 mg, with a built-in self

cali-bration 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

However, a hydrocarbon reformer is usually employed before downstream synthesis

3 Results and discussion The measurement results of the main gas components, H2, CO,

CO2and CH4are presented inFigs 3e8which 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.%), C3and C4(tot.< 0.5 Vol %)

3.1 Effect of temperature on gas composition, CH4and gravimetric tar

The effect of gasification temperature on the syngas composi-tion including CH4is shown throughFig 3for 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 H2and CO, while the CO2

content slightly decreases since the exothermic shift reaction is favoured by low temperature

Tar/CH4reforming is a strongly endothermic reaction, and fav-oured by high temperature to a great degree CH4 is the most recalcitrant hydrocarbon to reform, which very much depends on the temperature (seeFig 4) The methane content is not sensitive to the temperature change up to the temperature 800C.From tem-perature 800 Ce900C, 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 900C, especially for reforming with catalytic bed materials

Fig 5shows 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 850C, 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 temper-ature 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 770e860C.

3.2 Effect of steam-to-carbon ratio on gas composition, CH4and gravimetric tar

Fig 6shows 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 850C For all the cases of different bed materials, both the H2content 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

fibre filters in series at ambient temperature whereas the tars condense).

reaction (COþ H2O4 CO2 þ H2) to the right hand side of the

equilibrium equation, and produces more H2at 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

CH4for all the bed materials, which can be seen more clearly in

Fig 7where the methane content decreases from around 11% to

around 9%.Fig 8shows 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

In the present experimental test, the measurement date is scattering much when the tar content is plotted against S/C ratio as seen inFig 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 4 Methane content vs gasifier temperature at S/C 1.2.

Fig 5 The temperature dependency of the gravimetric tar content in the syngas when 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.

K G€oransson et al / Renewable Energy 81 (2015) 251e261 256

3.3 Effects of catalytic bed materials on gas composition, CH4and

gravimetric tar

From overview of the measurement results shown inFigs 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 H2and CO in the syngas for different bed

mate-rials, 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 H2þ 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 H2þ CO in the cases of catalytic bed ma-terials 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 H2þ CO

as indicated inFig 3 Concerning methane catalytic reforming in the test as seen in

Figs 4 and 7, the methane content in the syngas is reduced 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.

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

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 900C, 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 inFigs 5 and 8 The

gravi-metric 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 850C 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 H2þ CO, the lowest concentration of CH4and 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, CH4and gravimetric tar

content in the two different gasification modes, BFB and DFB

As shown inFigs 3e8, for both the BFB and DFB modes, the H2/

CO ratio is insensitive to the temperature within the temperature

range of 750Ce900C, 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

concen-tration of H2þ 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

H2þ 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/C¼ 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 H2þ 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

H2þ 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

counter-current 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

sug-gest 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/CH4reforming

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 9shows 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 850C and S/C 1.2 In

the DFB mode, the temperature profile has a peak point of 865C 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 thefine

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 dif-fers 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 CH4content compared to DFB mode This might be attributed to a higher tem-perature in the freeboard and higher concentrations of char and ash

in the dense bed of the BFB while thefine 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 11shows the Fe/olivine bed material particles directly after the drying procedure A coating on the surface of the bed material particles is notable.Fig 12shows the Fe/olivine after use in the

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 850C and S/C 1.2.

K G€oransson et al / Renewable Energy 81 (2015) 251e261 258

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

cat-alytic bed material before use (inFig 11) and after use (inFig 12) in

the gasifier, it can be seen that the particle size distribution become

narrow and the particles becomes uniform when thefine 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 concentra-tion of H þ CO, the ratio of H /CO, the CH concentration and the

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).

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

concentra-tion of H2þ CO, the lowest CH4and 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/CH4catalytic reforming based on the results of this test

A much lower concentration of H2þ 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

trans-port from the riser to the gasifier The lower tar and CH4contents in

the BFB mode might be attributed to the higher concentrations of

char and ash (while thefine 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/CH4reforming performance by the

cata-lytic 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

Acknowledgements

The authors would like to acknowledge the project support of

EU Regional Development Fund, Ångpannef€oreningen's Foundation

for Research and Development (ÅForsk), LKAB, L€anstyrelsen

V€asternorrland, Swedish Gasification Centre (SFC) and SCA BioNorr

AB, H€arn€osand The authors are grateful to Dr Christina Dahlstr€om

for the SEM-images in this paper

Abbreviations

BFB bubblingfluidised bed

CFB circulatingfluidised bed

CLC chemical loop combustion

DFB dualfluidised bed

DFBG dualfluidised bed gasifier

FID flame ionization detector

GC gas chromatography

GCMS gas chromatography-mass spectrometry

MIUN Mid Sweden University

Nm3 normal cubic meter

SEM scanning electron microscope

S/C steam-to-carbon ratio [kg/kg]

TCD thermal conductivity detector

WGS water-gas-shift

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