High temperature water-gas shift step in the production of clean hydrogen rich synthesis gas from gasified biomass

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) S 1 2 3 eS 1 3 1

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High temperature water-gas shift step in the production of clean hydrogen rich synthesis gas from gasified biomass Jessica Einvall a, Charlotte Parsland a, Patricia Benito b, Francesco Basile b, Jan Brandin a,* a b

School of Engineering, Department of Bioenergy Technology, 351 95 Va¨xjo¨, Sweden Dip. Chimica Industriale e dei Materiali, Alma Mater Studiorum-Universita` di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy

article info

abstract

Article history:

The possibility of using the water-gas shift (WGS) step for tuning the H2/CO-ratio in

Received 27 August 2010

synthesis gas produced from gasified biomass has been investigated in the CHRISGAS

Received in revised form

(Clean Hydrogen Rich Synthesis Gas) project. The synthesis gas produced will contain

25 April 2011

contaminants such as H2S, NH3 and chloride components. As the most promising

Accepted 30 April 2011

candidate FeCr catalyst, prepared in the laboratory, was tested. One part of the work was

Available online 31 May 2011

conducted in a laboratory set up with simulated gases and another part in real gases in the 100 kW Circulating Fluidized Bed (CFB) gasifier at Delft University of Technology.

Keywords:

Used catalysts from both tests have been characterized by XRD and N2 adsoption/

Biomass gasification

desorption at
196  C.

Synthesis gas

In the first part of the laboratory investigation a laboratory set up was built. The main

Water-gas shift

gas mixture consisted of CO, CO2, H2, H2O and N2 with the possibility to add gas or water-

FeCr catalyst

soluble contaminants, like H2S, NH3 and HCl, in low concentration (0e3 dm3 m
3). The set

Catalyst poisons

up can be operated up to 2 MPa pressure at 200e600  C and run un-attendant for 100 h or

Slipstreams

more. For the second part of the work a catalytic probe was developed that allowed exposure of the catalyst by inserting the probe into the flowing gas from gasified biomass. The catalyst deactivates by two different causes. The initial deactivation is caused by the growth of the crystals in the active phase (magnetite) and the higher crystallinity the lower specific surface area. The second deactivation is caused by the presence of catalytic poisons in the gas, such as H2S, NH3 and chloride that block the active surface. The catalyst subjected to sulphur poisoning shows decreased but stable activity. The activity shows strong decrease for the ammonia and HCl poisoned catalysts. It seems important to investigate the levels of these compounds before putting a FeCr based shift step in industrial operation. The activity also decreased after the catalysts had been exposed to gas from gasified biomass. The exposed catalysts are not re-activated by time on stream in the laboratory set up, which indicates that the decrease in CO2-ratio must be attributed to irreversible poisoning from compounds present in the gas from the gasifier. It is most likely that the FeCr catalyst is suitable to be used in a high temperature shift step, for industrial production of synthesis gas from gasified biomass if sulphur is the only poison needed to be taken into account. The ammonia should be decomposed in the previous catalytic reformer step but the levels of volatile chloride in the gas at the shift step position are not known. ª 2011 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ46 733969465; fax: þ46 470708756. E-mail address: [email protected] (J. Brandin). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.04.052

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1.

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Table 1 e Estimated concentrations of the synthesis gas (Thermodynamic calculations) from Kirm et al. [3].

Introduction

The production of a clean hydrogen rich synthesis gas (CO þ H2) by gasification was the goal of the CHRISGAS project [1]. The gasification of a carbon containing fuel generates a gas containing a mixture of hydrogen, carbon monoxide, carbon dioxide and water to be used as a feedstock for various synthesis processes. The actual composition of the gas depends on many different factors such as type of fuel, type of gasifier, mode of operation of the gasifier, etc. The product gas, i.e. the gas after the gasification step, usually needs upgrading since it contains lower hydrocarbons and tars that need to be converted. This upgrading, from produced gas into synthesis gas, is done in the reformer step. The resulting synthesis gas is not necessarily suited for the subsequent synthesis step; it may need to be processed further. For instance the carbon dioxide level may need to be decreased and/or the hydrogencarbon dioxide ratio be adjusted. The water-gas shift (WGS) process is the process where the ratio between hydrogen and carbon monoxide in the synthesis gas can be tuned. To investigate the effect of different poisons present in the gas a FeCr catalyst was exposed to poisons both in a laboratory set up and in the fuel gas from an atmospheric 100 kW CFB gasifier. Typical concentrations of H2S and NH3 are around 100 cm3 m
3 and 150 cm3 m
3 respectively when clean wood is used as fuel in this gasifier. The WGS [2] (Eq. (1)) is a moderately exothermic reaction (to the right) that is limited by the equilibrium: CO þ H2 O4CO2 þ H2

(1)

The estimated gas composition from Kirm et al. [3] after the gasifier and after upgrading of the gas by Auto-Thermal Reforming (ATR) or Partial Oxidation (POX) is presented in Table 1.

2.

Method

2.1.

Laboratory set up and analysis

The laboratory set up consists of a stainless steel reactor with an inside diameter of 10 mm. The catalyst is loaded into the reactor, and the gas and steam are fed into the heated reactor. The gas composition can be altered and the catalyst can be exposed to poisons in gas phase and water-soluble poisons can be fed into the system with the water injection. Both the catalyst temperature and the gas temperature can be measured by a thermocouple type K. The water in the gas will be condensed before the gas is analysed. The set up is presented in Fig. 1. Early on in the project, a gas composition (H2, CO, CO2, H2O) identical to the one given in Table 1 (After ATR) was used for the experiments (e.g. experiments per Section 3.1.1 ”Activation of the catalyst”). However, later on in the project, due to the thermodynamic calculations in [3] concerns arose that the water content was too low to maintain catalyst stability. Therefore to ensure catalyst stability the water content of the gas was doubled in line with the calculations in [3]. This new gas composition, as shown in Table 2, applies to all

Component

After gasifier After ATR After POX 1300  C volume 1000  C volume volume fraction (%) fraction (%) fraction (%)

Inlet O2 Inlet Temperature,  C C2-hydrocarbons CH4 CO CO2 H2 H2O NH3 H2S Tars Massflow ratio kg kg
1 a Supplied heat MJ kg
1 b LHV MJ m
3 c LHV MJ kg
1 LHV  MFR d MJ kg
1 Efficiency e

7 800 1.6 8.2 11.9 27.9 11.8 37.7 0.3 0.01 0.3 1

e 23.8 19.8 23.0 33.4 0.2 0.01 e 1.13

10 800 e e 24.3 19.2 16.1 39.7 0.2 0.01 e 1.19

e

0.25

0.37

7.3 6.6 6.6

5.4 5.6 6.3

4.8 4.8 5.7

e

0.92

0.80

a Gasifier outlet. b By the injection stream (H2O(l) þ O2 (g) (25  C) / H2O(g) þ O2 (g) (800  C) MJ (kg gas after gasifier)
1). c At 101 kPa and 0  C. d MFR ¼ Massflow ratio. e (LHVout  MFRt esupplied heat) (LHVin MFR)
1.

experiments presented in this paper with the exception of Section 3.1.1 as stated above. The pressure in all experiments was 100 kPa. In the H2S experiment, the gas mixing had to be adjusted to achieve the same total volume since the addition of H2S was in the presence of N2 (see Table 2). The concentration of H2, CO and CO2 was measured with a Varian CP-4900 Micro Gas Chromatograph equipped with thermo conductivity detectors. A molecular sieve column and nitrogen as carrier were used to analyse H2. A molecular sieve column and helium as carrier were used to analyse CO. A Pora Pac column and helium as carrier were used to analyse CO2. In the long time reference measurement at SV (the space velocity calculated as the volumetric flow versus the volume of the catalyst bed) 10,000 h
1 an IR instrument (Fuji ZKJ) with gas conditioning equipment Ankersmid Sampling APS 311 was used to measure the CO and CO2 concentrations. Since the inlet gas contains a mixture of CO, CO2, H2 and water, the conversion of CO or production of CO2 and H2 is not a satisfactory description of the state of the gas after the treatment in the integral reactor, without recalculating the new concentrations or partial pressures. Instead the CO2-ratio (RCO2 ) of the total ingoing carbon (CO þ CO2) during the experiments was used: RCO2 ¼ ðCO2 =ðCO þ CO2 ÞÞ

(2)

These measured ratios can also be directly compared to the ratios of CO2 calculated at the thermodynamic equilibrium of

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pressure guards

H2

CO

CO2

N2

H2S

flow meter

flow meter

flow meter

flow meter

flow meter

evapo-

To analysis: micro-GC IR

Condensor

rator flow meter

N2 vacuum N2 vacuum

cooling water

reactor and oven

catalyst temperature storage vessels

gas temperature

Exit Fig. 1 e The laboratory set up.

the WGS reaction, at the given conditions. The equilibrium concentrations were calculated with HSC Chemistry version 5 (by Outokumpu Research Oy, Finland). This means that the inlet mixture of CO, CO2, H2 and H2O has an initial CO2-ratio of 45%.

[4]. The active phase, in activated state, is magnetite (Fe3O4) and the structural promoter is Cr in the form of Cr2O3. Catalyst granules in the size 0.71e1 mm were used. The particle size was chosen considering the reactor diameter to avoid wall effects.

2.2.

2.4.

Experimental set up for product gas experiment

The instrument set up during catalyst exposure is presented in Fig. 2. The H2 and CO concentrations were measured on-line before passing through the catalyst bed using a Varian CP-4900 Micro Gas Chromatograph.

2.3.

Catalyst preparation

The catalyst was a commercial-like catalyst with FeCr as the active component prepared according to Catalysts Manufacture

Table 2 e The gas composition. Gas CO CO2 H2 H 2O N2 þ H2S

Volume fraction (%)

Volume fraction (%) when 150 cm3 m
3 H2S is present

17.8 14.8 17.3 50.1 e

15.2 12.6 14.8 42.8 14.6

Characterization techniques

X-Ray Diffraction (XRD) powder analyses were carried out using a Philips PW1050/81 diffractometer equipped with a graphite monochromator in the diffracted beam and controlled by a PW1710 unit (CuKa-Ni filtered, l ¼ 0.15418 nm). A 2q range from 10 to 80 was investigated at a scanning speed of 70 h
1. Average sizes of crystallites of the Fe3O4 phase were calculated by using the Scherrer equation: D ¼ Kl/ (b cos q), where D is the average crystallite size, K is the shape factor for the average crystallite (the expected shape factor is ˚ for Cu Ka1), q is the 0.9), l is the X-ray wavelength (1.54056 A Bragg angle, and b is the full width at half-maximum (FWHM) in radians [b ¼ (B2 - b2)1/2, where B is the measured FWHM and b is the instrumental broadening, which was determined by collecting the diffraction pattern of a silicon line-width standard]. Specific surface area BET (Brunauer, Emmet and Teller) measurements were carried out in a Micromeritics ASAP 2020 instrument by N2 adsorption/desorption at
196  C. Samples were previously degassed under vacuum and heated up to 250  C and maintained for 30 min.

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b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) S 1 2 3 eS 1 3 1

Pump

Excess flow

condensor

Cooling water

Condensate

Fig. 2 e Instrument set up during catalyst exposure.

3.

Experimental and results

3.1.

Laboratory experiments: fresh catalyst

3.1.1.

Activation of the catalyst

To investigate the effect of the temperature on the catalyst activity, the CO2-ratio of the catalyst was measured at a temperature from 350  C to 450  C at ATR conditions according to “After ATR 1000  C” in Table 1. According to Fig. 3, at 350  C a slow activation occurs, and at a temperature above 350  C the activation is fast. The laboratory tests show that the FeCr catalysts can be activated at a temperature of at least 350  C in the synthesis gas stream.

3.1.2.

Reference measurements-long time experiments

To investigate the long time stability of the catalyst the reactor was loaded with 1 ml of fresh catalyst and the reaction run for

approximately 73 h at 400  C at SV 30000 h
1. In the second part of this experiment, the reactor was loaded with 3 ml of fresh catalyst and the reaction run for approximately 69 h at 400  C and SV 10000 h
1. According to Fig. 4, the activity decreases during the first 10 h from approximately 75%e70%, and is relatively stable near 70% during the following measurement at SV 30000 h
1. The CO2-ratio is close to 88% in the beginning, and decreases through the experiment to 75% at SV 10000 h
1.

3.1.3.

Start and stop experiment

To investigate if shut downs and restarts of the industrial plant has any effect on the catalyst, the following start stop experiment was conducted: The reactor was loaded with 1 ml catalyst. The reaction was run for approximately 4 h at 400  C and SV 30000 h
1 and the reaction gases and the heating of the oven were then stopped. A low flow of N2 was fed through the reactor until the next start. This experiment was conducted

Fig. 3 e Activation of the FeCr in the synthesis gas stream with the composition corresponding to Table 2 “After ATR”.

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100 90 80

CO2 ratio(%)

70 60

SV 30000 h-1 SV 10000 h-1 Equilibrium

50 40 30 20 10 0 0

20

40

60

80

Time (hours) Fig. 4 e The CO2-ratio at SV 30000 hL1 and 10000 hL1.

The reaction ran for approximately 66 h at 400  C and SV 11700 h
1. The results are presented in Fig. 6. The CO2-ratio decreases from 89% when no H2S is added down to 76% after 6 h H2S exposure. The CO2-ratio is then relatively stable for the next 60 h.

three times during one week. The CO2-ratio is presented in Fig. 5. There is an initial deactivation in both cases but the difference is small and is probably within the experimental error range. The CO2-ratio does not seem to be affected because of system shut down. The initial deactivation was later connected to the growth of magnetite crystals; a natural process that is unavoidable.

3.1.4.2. Catalyst exposed to 150 cm3 m
3 HCl. In this experi3.1.4.

H2S, HCl and NH3 exposure

ment, the reactor was loaded with 3 ml fresh catalyst. The reaction ran for approximately 44 h at 400  C and SV 10000 h
1. The CO2-ratio decreases from 89% when no HCl is added down to 50% after 9 h HCl exposure (see Fig. 7). The CO2-ratio is then reasonably stable for the next 35 h.

The synthesis gas produced will contain contaminants such as 50e200 cm3 m
3 of H2S, NH3 (see Table 1) and chloride components originating from the biomass. To investigate if any of these compounds have any effect on the catalyst activity the catalyst was exposed to 150 cm3 m
3 H2S, 150 cm3 m
3 HCl and 150 cm3 m
3 NH3 in three separate experiments.

3.1.4.3. Catalyst exposed to 150 cm3 m
3 NH3. In this experiment, the reactor was loaded with 3 ml fresh catalyst. The reaction ran for approximately 47 h at 400  C and SV 10000 h
1. The CO2-ratio decreases slowly from 85% when no NH3 is added down to 60% after 18 h NH3 exposure (see Fig. 8). The decrease is fairly slow over the following 29 h.

3.1.4.1. Catalyst exposed to 150 cm3 m
3 H2S. In this experiment the reactor was loaded with 3 ml fresh catalyst. The gas composition with H2S in N2 added was according to Table 2.

100 90 80

CO2 ratio(%)

70 60

Reference SV 30000 h-1 Start stop Equilibrium

50 40 30 20 10 0 0

5

10

15

20

Time (hours) Fig. 5 e Start Stop experiment with the FeCr catalyst for three days compared to the reference measurement at SV 30000 hL1.

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100 90 80

CO2 ratio (%)

70 60

No poison

50

H2S exposure

40

Equilibrium

30 20 10 0 0

20

40

60

80

Time (hours) Fig. 6 e The effect of H2S on the CO2-ratio.

3.2. Experiments with catalyst exposed to gas from gasifier

3.2.2. Forced flow of product gas through catalyst bed, experiment B

To evaluate the effect of the gas from a biomass gasifier on the catalyst, the catalyst was exposed to real gas in two experiments using the atmospheric 100kWmax CFBG at the Process and Energy department at Delft University of Technology. The gasifier is described in detail by Siedlecki et al. [5].

3.2.1.

Exposure to product gas from gasification, experiment A

In this experiment the catalyst was exposed to the gas generated by gasification of a clean wood fuel. The catalyst was placed inside a probe in the syngas duct after the high temperature filter. The temperature at this position varied during the day from 100  C at start up to above 350  C after some hours. The catalyst was exposed for 24 h. The reactor was loaded with 1 ml exposed catalyst. The reaction ran for approximately 40 h at SV 30 000 h
1. The gas composition was as according to Table 2. The CO2-ratio is presented in Fig. 9. The CO2-ratio is near 50% during the whole measurement.

In this experiment a flow was forced through the catalyst bed during two gasification days and an SV of 3000 h
1 was desired during the exposure. However, the actual SV s at the start of the first day was 2822 h
1, and 1374 h
1 at the end of the first day. On the second day the SV varied only from 2932 h
1 at start, to 3096 h
1 at the end. The fuel gas was forced through the catalyst bed for 15 h during the two days. The temperature of the gas increased during the gasification time from 150  C at the start up to approximately 280  C at the end of the experiment. The catalyst was exposed to the gas generated by gasification of miscanthus. The average CO concentration was 13%, and the average H2 concentration was 32% during the exposure. The ratio of CO/H2 ratio during exposure is around 1:3 in this experiment, and not 1:1 which is expected after ATR according to Table 1. In the laboratory measurement, the reactor was loaded with 1 ml catalyst. The reaction ran for approximately 5 h at SV 30 000 h
1. The CO2-ratio is presented in Fig. 10. The CO2-ratio

HCl injection started

100 90

CO2 ratio (%)

80 70 60

No poison HCl exposure Equilibrium

50 40 30 20 10 0 0

20

40

60

Time (hours) Fig. 7 e The effect of HCl on the CO2-ratio.

80

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NH3 injection started

100 90 80

CO2 ratio (%)

70 60

No poison NH3 exposure Equilibrium

50 40 30 20 10 0 0

20

40

60

80

Time (hours) Fig. 8 e The effect of NH3 on the CO2-ratio.

starts at almost 55% and decreases to 45% during the measurement; in principle the CO2-ratio reaches the inlet ratio.

3.3.

Characterization

XRD characterization of the reference catalyst indicated that at the working temperature the CO and H2 present in the gas mixture reduced the haematite phase (Fe2O3) to active magnetite (Fe3O4) [6]; the formation of FeO and Fe was avoided [7] in agreement with the catalytic data and with thermodynamic calculation results obtained with the same gas mixture composition [3]. The large water content of the gas stream reduced the carbide formation and suppressed the metallic Fe formation [3]. The exposure of the catalyst to the poisons (H2S, NH3 and HCl) decreased the average crystallite size of the Fe3O4 phase (Table 3). No FeS was detected in the catalyst after the test with H2S, probably related to the reversibility of the sulfidation reaction [8], favoured by the large amounts of water in the stream, or to the formation of very low amounts of this phase not detectable by this technique. On the other hand, both Fe2O3 and Fe3O4 phases co-exist in the catalysts exposed to the gas coming from the gasifier, and the catalyst exposed to the forced flow shows a lower amount of reduced

magnetite. The smaller amount of Fe3O4 in the samples exposed to gas from the gasifier could be related to a low degree of reduction of the Fe2O3 phase due to low temperatures and/or to short exposure at this temperature; or during discharge operation since the catalyst is phyrophoric and reacts with oxygen in the air. It should also be reiterated that these catalysts were characterized before they were used in the laboratory WGS rig. Specific surface area and pore volume values of the catalysts are summarized in Table 3. After catalytic tests both the surface areas and the pore volumes decrease, which can be related to the thermal sintering of the particles; a well-known and unavoidable deactivation process of FeCr catalysts during the start up and reduction of the catalyst [8]. In the catalysts exposed to the gasification stream the large values in surface area are related to the presence of Fe2O3.

4.

Discussion

The results from the experiments have shown that the CO2ratio is stable over long time, and that the activity of the catalyst is not affected by plant shut down. The CO2-ratio is

100 90 80

CO2 ratio(%)

70 60

Fresh catalyst Experiment A Equilibrium

50 40 30 20 10 0 0

20

40

60

80

Time (hours) Fig. 9 e The CO2-ratio after 24 h exposure to product gas from a biomass gasifier (experiment A), compared to fresh catalyst at SV 30000 hL1.

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100 90

CO2 ratio (%)

80 70 60

Fresh catalyst Experiment B Equilibrium

50 40 30 20 10 0 0

2

4

6

8

10

Time (hours) Fig. 10 e The CO2-ratio after 15 h forced flow of product gas through the catalyst bed (experiment B), compared to fresh catalyst at SV 30000 hL1.

Table 3 e Specific surface area (SBET) and pore volume (Vp) values of the FeeCr catalysts. Sample Fresh Reference sample Exposed to H2S Exposed to NH3 Exposed to HCl Exposed to product gas from gasification, experiment A Exposed to forced flow of product gas through catalyst bed, experiment B

Crystalline phase

D/nm

SBET/m2g
1

Vp/cm3g
1

Fe2O3 Fe3O4 Fe3O4 Fe3O4 Fe3O4 Fe2O3 þ Fe3O4

e 18.6 16.4 10.7 12.4 7.2

84.9 37.7 39.2 53.0 42.9 60.8

0.27 0.21 0.20 0.23 0.23 0.26

Fe2O3 þ Fe3O4

5.5

68.1

0.26

D: average crystallite sized of Fe3O4 calculated from the (220) plane.

affected initially when H2S is injected but then appears to stabilize. The CO2-ratio decreases remarkably in the presence of HCl in particular, but also in the presence of NH3 too. The catalyst exposed to chloride has only low activity which is close to the inlet CO2-ratio. The effect of these poisons must be considered when a FeCr WGS catalyst is to be used. The ammonia should be converted to nitrogen and hydrogen in the previous step i.e. the catalytic reformer. However, a reduction from 3 dm3 m
3 (See “After gasifier” in Table 1) to 150 cm3 m
3 (the poisoning experiment) represents a CO2ratio of 95% so one could expect some ammonia left in the gas at the shift step. The amount of volatile chloride in the gas at the shift step position is not known, but it seems to be an important factor to investigate. The activity decreased after the catalysts had been exposed to gas from gasified biomass. The exposed catalysts are not activated by time on stream in the laboratory set up, which indicates that the decrease in CO2-ratio must be attributed to poisons present in the gas from the gasifier.

5.

Conclusions

Several measurements to investigate how different compounds affect the water-gas shift catalyst during long time

experiments have been made. The catalyst has also been exposed to gas from gasified biomass. The FeCr catalyst can be activated in the produced synthesis gas at 350  C or above. The inlet reactor temperature should be at least 350  C. The FeCr catalyst can be used to shift the synthesis gas at the expected H2S-levels (0e150 cm3 m
3). The presence of other poisons in the gas phase, like HCl and NH3, must be considered since the experiments show a strong impact on the catalyst activity when exposed to HCl, NH3 and gas from a biomass gasifier. The impact of different poisons on catalyst activity must be further investigated before a FeCr catalyst can be used in industrial scale hydrogen rich synthesis gas production.

Acknowledgements The financial support provided via the European Commission (EC) 6th Framework Programme (CHRISGAS Project contract number SES6-CT-2004-502587) and by the Swedish Energy Agency is gratefully acknowledged. We would also like to thank and acknowledge the work performed by Mehri Sanati, Simone Albertazzi, Ilham Kirm

b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) S 1 2 3 eS 1 3 1

and Hussam Abdulhamid early on in the CHRISGAS project; and also Marcin Siedlecki, Eleonora Simeone and Wiebren de Jong at Delft University of Technology with respect to their work performed relating to the gasifier.

references

[1] Albertazzi S, Basile F, Brandin J, Einvall J, Hulteberg C, Fornasari G, et al. The technical feasibility of biomass gasification for hydrogen production. Catal Today 2005;106: 297e300. [2] Twigg MV. Catalyst handbook. 2nd ed. London: Manson Publishing Ltd; 1996.

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[3] Kirm I, Brandin J, Sanati M. Shift catalysts in biomass generated synthesis gas. Top Catal 2007;45:1e4. [4] Stiles AB, Koch TA. Catalysts manufacture. 2nd ed. New York: Marcel Dekker Inc; 1995. [5] Siedlecki M, de Jong W. Biomass gasification as the first hot step in clean syngas production process - gas quality optimization and primary tar reduction measures in a 100 kW thermal input steam-oxygen blown CFB gasifier. Biomass & Bioenergy 2011;35(S1):S40e62. [6] Lloyd L, Ridler DE. The water gas shift reaction. In: Twigg MV, editor. Catalyst handbook. 2nd ed. London: Wolfe Publishing; 1989. p. 283e339. [7] Rhodes C, Hutchings GJ, Ward AM. Water-gas shift reaction: finding the mechanistic boundary. Catal Today 1995;23:43e58. [8] Ratnasamy C, Wagner JP. Water gas shift catalysis. Catal Rev 2009;51:325e440.