Bioresource Technology 110 (2012) 603–609

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Bioresource Technology

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Syngas production by two-stage method of biomass catalytic pyrolysisand gasification

Qinglong Xie a, Sifang Kong a, Yangsheng Liu b,⇑, Hui Zeng a,c,⇑a School of Urban Planning and Design, Shenzhen Graduate School, Peking University, Shenzhen Key Laboratory of Circular Economy, Shenzhen 518055, Chinab College of Environmental Sciences and Engineering, Peking University, Beijing 100871, Chinac College of Urban of Environmental Sciences, Peking University, Beijing 100871, China

a r t i c l e i n f o

Article history:Received 11 November 2011Received in revised form 5 January 2012Accepted 7 January 2012Available online 14 January 2012

Keywords:Two-stage methodBiomass pyrolysisGasificationKineticsNickel based catalyst

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.biortech.2012.01.028

⇑ Corresponding authors. Tel./fax: +86 10 62751756E-mail addresses: yshliu@pku.edu.cn (Y. Liu), zeng

a b s t r a c t

A two-stage technology integrated with biomass catalytic pyrolysis and gasification processes wasutilized to produce syngas (H2 + CO). In the presence of different nickel based catalysts, effects ofpyrolysis temperature and gasification temperature on gas production were investigated. Experimentalresults showed that more syngas and char of high quality could be obtained at a temperature of750 �C in the stage of pyrolysis, and in the stage of gasification, pyrolysis char (produced at 750 �C)reacted with steam and the maximum yield of syngas was obtained at 850 �C. Syngas yield in this studywas greatly increased compared with previous studies, up to 3.29 N m3/kg biomass. The pyrolysis processcould be well explained by Arrhenius kinetic first-order rate equation. XRD analyses suggested that for-mation of Mg0.4Ni0.6O and increase of Ni0 crystallite size were two main reasons for the deactivation ofnickel based catalysts at higher temperature.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Energy crises and environmental problems have led to anincreasing focus on renewable energy sources, alternative to tradi-tional fossil fuels. Biomass is the only carbon-containing renewablesource, making it suitable for fuel production and chemical feed-stock (Kantarelis and Zabaniotou, 2009). Among biomass utiliza-tion technologies, thermochemical methods including pyrolysisand gasification are currently most appropriate and widely com-mercially used (Encinar et al., 2000). Syngas produced from bio-mass pyrolysis and gasification is an important intermediate forsynthesis of large numbers of industrial products (e.g., methanoland ammonia) (Chmielniak and Sciazko, 2003; Hamelinck andFaaij, 2002; Tijmensen et al., 2002). Thus, maximizing the syngasyield from biomass will largely promote the biomass utilizationwith high efficiency.

Up to date, research about syngas production from biomassmainly focuses on gasification technologies. According to differenttar removal technologies involved, biomass gasification processcan broadly be divided into two approaches: primary methodsand secondary methods (Devi et al., 2003). In primary methods,gasification process and tar elimination are carried out simulta-neously in gasifier; while in secondary methods, gas cleanup isconducted in a separate reformer in downstream of the gasifier.

ll rights reserved.

.h@szpku.edu.cn (H. Zeng).

The primary methods have gained much attention (Ahmed andGupta, 2009; Göransson et al., 2011; Karmakar and Datta, 2011;Moghtaderi, 2007; Ueki et al., 2011). Ahmed and Gupta (2009)examined steam gasification of cardboard using a batch reactor,obtaining syngas of about 1.2 N m3/kg biomass at 600 �C. Karmakarand Datta (2011) studied steam gasification of rice husk in a fluid-ized bed reactor and generated syngas with a maximum yieldof 1.21 N m3/kg biomass and lower heating value (LHV) of 11.18MJ/N m3 at 750 �C. Moghtaderi (2007) investigated steam gasifica-tion of pine sawdust catalyzed by Ni/Al2O3 and observed a maxi-mum H2 yield of 1.6 N m3/kg biomass at 600 �C. Although primarymethods eliminate the need for downstream cleanup, they cannoteffectively solve the purpose of tar reduction without affecting theuseful gas composition and heating value (Devi et al., 2003). As aresult, the syngas yields in primary methods will be relatively lowcompared with those in secondary methods.

Extensive studies on secondary methods of biomass gasificationhave also been conducted (Gao et al., 2009; Lv et al., 2007; Wanget al., 2006; Xiao et al., 2011; Yang et al., 2010). By steam gasifica-tion of pine sawdust using an updraft gasifier combined with a por-ous ceramic reformer, Gao et al. (2009) obtained syngas with amaximum yield of 1.72 N m3/kg biomass and lower heating value(LHV) of 11.73 MJ/N m3 at 950 �C. Employing a fluidized bed gasifierand a downstream fixed bed as the reactors, Lv et al. (2007) studiedcatalytic gasification of pine sawdust and the maximum gas yieldreached 2.41 N m3/kg biomass at 850 �C. Similarly, Xiao et al.(2011) utilized primary fluidized bed and secondary reforming

Table 1Characteristics of pine sawdust (dry basis).

Biomasssample

Proximate analysis/% Elemental analysis/% Component analysis HHVa (MJ/kg) LHVb (MJ/kg)

Moisture Volatile Ash Fix carbon C H O N S Cellulose Hemicellulose Lignin

Pine sawdust 13.43 73.45 2.01 11.11 42.74 6.11 34.88 0.14 0.70 56.85 15.16 12.70 16.71 15.00

a Higher heating value.b Lower heating value.

Table 2Comparison of nickel based catalysts with mineral catalysts in biomass pyrolysis at 800 �C.

Catalyst Gas yield (N m3/kg biomass) Syngas (% v/v) H2/CO BET surface area of char (m2/g) Micropore volume of char (�10�2 N cm3/g)

Dolomite 0.49 25.14 0.60 17.00 1.88Olivine 0.51 37.27 0.67 7.24 0.66Z402 + Z405 0.63 87.63 0.90 14.28 1.38Z405 + Z409 0.64 83.09 0.92 20.33 1.59Z412 + Z413 0.56 83.26 0.94 46.42 5.49

10152025303540455055606570a GAS LIQUID SOLID

45

50

55

60 GAS LIQUID SOLID

biom

ass)

b

604 Q. Xie et al. / Bioresource Technology 110 (2012) 603–609

fixed bed to investigate steam gasification of waste biomass withNi/BCC as catalyst and obtained syngas with a yield of 2 N m3/kgbiomass and LHV of 14 MJ/N m3 at about 600 �C. Secondary meth-ods are effective in reducing tar content and improving syngas yield,but additional equipments required will increase the investment.

Despite the large number of studies on syngas production, inmost cases, the processes of pyrolysis and gasification are con-ducted at the same temperature. However, effects of temperatureon syngas production may differ in these two completely distinctprocesses. Therefore, if pyrolysis and gasification processes canbe separated and investigated, respectively, effect of temperatureon syngas yield in each process will be clearer, and the optimumparameters for syngas yield would be found accordingly.

In this study, pyrolysis and gasification processes were sepa-rately conducted using the same apparatus with the presence ofcatalysts, and effect of temperature on syngas yield was considered,respectively in each process. The purpose of this study is to explorethe optimum temperature for both pyrolysis and gasification pro-cesses, thereby maximizing syngas yield. The mechanism of catalystdeactivation in higher temperature was also investigated.

600 650 700 750 800 850 9005

10152025303540455055606570

Yiel

d(w

t-% o

f dry

bio

mas

s)

c GAS LIQUID SOLID

Temperature ( )

15

20

25

30

35

40

Yiel

d(w

t-% o

f dry

Fig. 1. Effects of temperature on the major products distribution in pine sawdustcatalytic pyrolysis process. (a) Z402 + Z405, (b) Z405 + Z409, and (c) Z412 + Z413.

2. Methods

2.1. Materials and catalysts

The pine sawdust was obtained from Shaoguan City inGuangdong Province, China. Its physico-chemical properties includ-ing proximate and element analyses are shown in Table 1. The highcontent of cellulose and low content of ash favor the pyrolysisprocess and syngas production. According to the elemental analysis,the chemical formula of the raw material that derives is CH1.72O0.61.Prior to its use, the sawdust samples were ground using a rotarycutting mill and then screened to limit the particle size smaller than0.3 mm. Afterwards, these ground samples were dried for 24 h at105 ± 0.5 �C.

The catalysts used in the experiments were primarily nickelbased catalysts. Five types of nickel based catalysts were chosen,i.e., Z402,�Z405,�Z409,�Z412 and Z413, which were commerciallyavailable from Qilu PetroChemical Company in Shandong Province,China. The catalyst supports for Z402 and Z405 are MgO�nAl2O3

(n = 1–2) and CaO�nAl2O3 (n = 1–2), respectively, and a-Al2O3 actas the support for the catalysts including Z409,�Z412 and Z413.The mass contents of the active component of Ni for these five cat-alysts are (wt.%): 12.6, 10.1, 17.3, 11.0 and 11.0, respectively. Inorder to further improve the catalytic activities, five catalysts were

600 650 700 750 800 850 9000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8c

Temperature ( )

Yiel

d(N

m3 /k

g bi

omas

s)

H2 CO CH4 CO2 Total

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7b H2 CO CH4 CO2 Total

Yiel

d(N

m3 /k

g bi

omas

s)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8aYi

eld

(Nm

3 /kg

biom

ass)

H2 CO CH4 CO2 Total

Fig. 2. Effect of temperature on major gases yield in pine sawdust catalyticpyrolysis process. (a) Z402 + Z405, (b) Z405 + Z409, and (c) Z412 + Z413.

Q. Xie et al. / Bioresource Technology 110 (2012) 603–609 605

combined into three groups for experimental use. These combina-tions were Z402 + Z405, Z405 + Z409, Z412 + Z413, and the massratio of two catalysts in each group equaled 1:1. According to prac-tical experience, these combinations were proved to be more appli-cable to syngas production than each individual catalyst. Prior toapplication, the catalysts were first ground using a ball mill toachieve a particle size smaller than 3 mm, and then reduced inthe atmosphere of H2 at 800 �C for 3 h. In addition, two mineralcatalysts were also selected for comparison: dolomite and olivine.The two catalysts were first crushed to obtain a fraction with a par-ticle sized 0.15–0.2 mm and then calcined in muffle at 900 �C for4 h.

2.2. Apparatus

The pyrolysis and gasification processes were examined in thesame system. The major components include carrier gas and steam

generator, tube furnace reactor, gas condenser and purifier, flowmeters and gas collecting bag.

The reactor is made of quartz tube which is externally heated byan electric furnace (Model OTF-1200X, Hefei Kejing MaterialsTechnology Co. Ltd., Anhui, China). Its length is 100 mm with an in-ner diameter of 42 mm. In the stage of pyrolysis, the biomass sam-ple (2.00 g) was first loaded in a small quartz boat, which was puton the left side of the tube. The carrier gas (pure N2) was pumpedinto the system to expel all the air in it. When the temperature offurnace reached the designated value, the quartz boat was quicklypushed into the middle of the tube, and the reaction occurred.Flowing through the condenser and filter, the product gas wouldbe purified, and was then collected into sampling bags made of alu-minum (volume 15 L, Shanghai Eler Co. Ltd., Shanghai, China) foroffline analysis. The flow and volume of product gas were mea-sured by the rotameter (Model LZB-3, Yuyao Industrial AutomationInstrument and Meters Factory, Zhejiang, China) and the wet meter(Model LMP-1, Changchun Automobile Filter Co. Ltd., Jilin, China),respectively. When it came to the gasification process, these proce-dures were repeatedly conducted, and the only difference was thatthe valve of carrier gas was closed and steam produced in thesteam generator (Model DZFZ-0.4, Shanghai Diye Ironing Equip-ment Manufacturing Co. Ltd., Shanghai, China) was pumped intothe tube to react with pyrolysis product in the quartz boat.

The measurement accuracies for temperature sensor, rotameterand wet meter are ±1 �C, ±0.025 N L/min, ±0.02 L, respectively.These accuracy numbers were obtained from the manufacturers.

2.3. Gas and char analysis

Offline gas analysis was performed using an Agilent Technologies6890N gas chromatograph (GC) equipped with thermal-conductivitydetector (TCD). Gas samples were introduced to GC through a six-way valve with the sample loop of 1 mL. The two columns used werePlot Q and Molesieve with helium as carrier gas. Among them, Plot Qwas used to analyze CO2 and light hydrocarbons, while Molesievewas used for CO,�H2 and CH4 analysis. The column temperature ofthe gas chromatograph was set at 40 �C, and the analysis retentiontime was 38 min. The standard gas mixture used for the calibrationof the method was CO,�CO2,�H2,�CH4,�C2H4 and C2H6 1% v/v balancedin helium and the volume concentration was calculated by an exter-nal standard method. The produced gas was mainly composed ofCO,�CO2,�H2,�CH4, and a small amount of light hydrocarbons. Lighthydrocarbons joined as CmHn with the exception of CH4 due to itshigher concentration.

BET surface area and pore size distribution of the pyrolysis charwere determined by N2 adsorption using a JW-BK static nitrogenadsorption instrument. The sample was degassed at 105 �C for5 h in high vacuum before measurement.

2.4. Catalyst characterization

The X-ray powder diffraction (XRD) patterns, obtained on a Sie-mens D500 X-ray diffractometer instrument with a Cu�Ka radiationat 40 kV and 30 mA, were used to identify the major phases pres-ent in the nickel based catalysts.

3. Results and discussion

3.1. Comparison of nickel based catalysts with mineral catalysts

In order to investigate the performance of catalysts, three kindsof nickel based catalysts and two mineral catalysts (dolomiteand olivine) were tested in pyrolysis experiments. As shown inTable 2, at the temperature of 800 �C, all the nickel based catalysts

Table 3Pine sawdust catalytic pyrolysis-produced gas characteristics.

Temperature(�C)

Z402 + Z405 Z405 + Z409 Z412 + Z413

Syngas(% v/v)

H2/CO

LHV(MJ/Nm3)

HHV(MJ/Nm3)

Syngas(% v/v)

H2/CO

LHV(MJ/Nm3)

HHV(MJ/Nm3)

Syngas(% v/v)

H2/CO

LHV(MJ/Nm3)

HHV(MJ/Nm3)

600 56.15 0.70 10.80 11.72 58.56 0.53 11.39 12.28 54.49 0.48 11.06 11.91650 61.53 0.90 11.09 12.09 64.25 0.71 11.85 12.85 60.93 0.70 11.59 12.58700 82.26 0.85 14.01 15.24 75.53 0.81 13.46 14.64 74.63 0.78 13.46 14.62750 81.39 0.89 14.00 15.25 75.80 0.85 13.33 14.51 83.35 0.77 14.75 16.01800 87.63 0.90 14.48 15.77 83.09 0.92 13.87 15.11 83.26 0.94 13.46 14.67850 79.26 1.00 12.32 13.44 65.91 0.80 10.64 11.54 91.75 0.97 14.31 15.60900 83.54 1.24 11.01 12.07 67.36 1.07 9.68 10.57 84.49 1.09 12.52 13.69

600 650 700 750 800 850 9000.00.20.40.60.81.01.21.41.61.82.02.22.4

Yiel

d(N

m3 /k

g bi

omas

s)

Temperature ( )

H2-Gasification CO-Gasification H2+CO-Gasification H2+CO-Pyrolysis+Gasification

Fig. 3. Effects of char obtained from different pyrolysis temperature on major gasesyield in char gasification process. Gasification temperature: 750 �C, catalyst:Z402 + Z405.

1/T (1/K)0.00090 0.00095 0.00100 0.00105 0.00110 0.00115 0.00120

ln (k

)

-6.8

-6.6

-6.4

-6.2

-6.0

-5.8

-5.6Z402+Z405 Z405+Z409 Z412+Z413

Fig. 4. Arrhenius plots for syngas production in pine sawdust catalytic pyrolysisprocess. Temperature range: 600–800 �C.

Table 5Kinetic parameters for syngas production in catalytic pyrolysis.

Catalyst Kinetic equation R2 E (kJ/mol) k0 (10�3 s�1)

Z402 + Z405 ln(k) = �4000.8(1/T) � 2.0472

0.9267 33.26 129.10

Z405 + Z409 ln(k) = �2945.1 0.9679 24.49 41.33

Table 4BET surface area and miropore volume of pyrolysis char.

Temperature(�C)

Z402 + Z405 Z405 + Z409 Z412 + Z413

BET surface area(m2/g)

Micropore volume(�10�2 N cm3/g)

BET surface area(m2/g)

Micropore volume(�10�2 N cm3/g)

BET surface area(m2/g)

Micropore volume(�10�2 N cm3/g)

600 55.83 6.46 52.59 5.88 50.22 6.80650 48.14 5.16 47.14 5.74 45.69 5.30700 48.69 5.60 38.65 4.06 47.03 6.81750 55.60 7.31 53.98 5.93 50.76 6.85800 14.28 1.38 20.33 1.59 46.42 5.49850 23.23 1.85 27.13 2.20 47.59 6.45900 38.51 4.24 16.21 1.31 16.83 1.51

606 Q. Xie et al. / Bioresource Technology 110 (2012) 603–609

presented better performance than the mineral catalysts in bothgas yield and char quality. The result is consistent with previousstudies (Asadullah et al., 2003; Li et al., 2009; Magrini-Bair et al.,2007; Tomishige et al., 2004; Yung et al., 2010). Therefore, nickelbased catalysts were chosen as the catalysts for the followingexperiments.

(1/T) � 3.1861Z412 + Z413 ln(k) = �2692.2

(1/T) � 3.45670.8247 22.38 31.53

3.2. Effect of temperature on pyrolysis products

Catalyzed by nickel based catalysts, pyrolysis experiments werecarried at a temperature range of 600–900 �C to examine effect oftemperature on products distribution between gas, liquid and solidphases. As Fig. 1 shows, for three catalysts, the variation trends ofgas, liquid or solid fraction yield during the sawdust pyrolysis weremuch similar with an increase of temperature. The yield of solidfraction was almost kept stable as temperature increased. Whenthe temperature was higher than 500 �C, solid fraction had a verylow content of volatile matter (Kantarelis and Zabaniotou, 2009),

so no further (or negligible) devolatilization took place. The varia-tion curves of yield with temperature for liquid and gas fractionswere nearly opposite. When temperature was below 700 �C, gasproduction was increased, while liquid production decreased. Whentemperature was at 700–850 �C, high gas yield was obtained andthere was an optimum temperature at which maximum gas yieldwas observed (for Z402 + Z405, Z405 + Z409 and Z412 + Z413 were

700 750 800 850 9000.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

H2 CO H2+COYi

eld

(Nm

3 /kg

biom

ass)

Temperature ( )

Fig. 5. Effect of temperature on major gases yield in pyrolysis-char gasificationprocess. (char resulting from sawdust pyrolysis at 750 �C), catalyst: Z402 + Z405,gasifying agent: steam.

Table 6Influence of gasification temperature on char elemental composition.

Temperature (�C) C (wt.%) H (wt.%) Oa (wt.%) Ash (wt.%)

700 48.11 3.12 30.54 15.53750 31.73 2.89 25.22 37.51800 13.28 2.33 8.85 72.91850 4.35 1.20 6.80 85.16900 1.80 0.93 3.75 91.04

a By difference.

Q. Xie et al. / Bioresource Technology 110 (2012) 603–609 607

at 700, 800 and 700 �C, respectively). Further temperature increaseresulted in decline of gas production.

The main reaction occurred during the catalytic pyrolysis pro-cess was thermal cracking of the biomass. Thermal cracking is anendothermic reaction which is favored by the rose of temperature(Encinar et al., 2000), indicating the increase of gas production anddecrease of liquid production with an increase of temperature.However, when temperature reached above 800 �C, yield of gasfraction was found to decline due to the decrease of catalyst activ-ity resulting from sintering at high temperature (Sehested et al.,2006; Yung et al., 2010; Hashemnejad and Parvari, 2011). There-fore, the optimum temperature range for catalytic pyrolysis of pinesawdust to produce more gas is 700–850 �C.

Major gases (H2,�CO,�CO2,�CH4) yield was also affected by temper-ature, as shown in Fig. 2. There was also an optimum temperaturefor each catalyst at which the maximum overall gas productionwas obtained. For Z402 + Z405, Z405 + Z409 and Z412 + Z413, theoptimun temperatures are 800, 800 and 850 �C, and their corre-sponding gas yields are 0.720, 0.685, 0.726 N m3/kg biomass (drybasis), respectively. It is obvious that effects of temperature onmajor gas yields were also related with the catalyst activities.

The lower heating value of the produced gas was calculated bythe following equation (Lv et al., 2004; Yang et al., 2006).

LHV ¼ ð30½CO� þ 25:7½H2� þ 85:4½CH4� þ 151:3½CmHn�Þ� 4:2=1000 MJ=N m3 ð1Þ

while the higher heating value was calculated using the followingcorrelation (Li et al., 2004).

HHV ¼ ð12:63½CO� þ 12:75½H2� þ 39:82½CH4�þ 63:43½CmHn�Þ=100 MJ=N m3 ð2Þ

where [H2],�[CO],�[CH4] and [CmHn] are the molar fractions ofH2,�CO,�CH4 and CmHn in the produced gas.

As shown in Table 3, the LHV and HHV of produced gases at thistemperature range (600–900 �C) varied from 9.7 to 14.7 MJ/N m3

and 10.6–16.0 MJ/N m3, respectively. From Fig. 2 and Table 3, theconclusion could be made that the optimum temperature rangefor catalytic pyrolysis of pine sawdust is 700–850 �C.

Char was the major component of solid fraction produced in thepyrolysis process, which then reacted with steam in the gasifica-tion process to produce more syngas. Therefore, the property ofpyrolysis char is another important factor affecting the ultimatesyngas production. Table 4 shows the BET surface area and micro-pore volume of pyrolysis char. It can be found that for each cata-lyst, the pyrolysis char achieved the maximum BET surface areaand micropore volume at 750 �C. Large surface area and mircoporevolume provide more sites for char to react with steam, thereforefavoring the syngas production during gasification process.

To further examine effects of pyrolysis char on gasificationproducts, chars produced at different pyrolysis temperatures weregasified at 750 �C, and Z402 + Z405 was employed as the catalyst.As shown in Fig. 3, the maximum yields of H2 and H2 + CO wereboth obtained at the pyrolysis temperature of 750 �C, at whichthe optimum char quality was observed. In addition, the total yieldof H2 + CO produced in pyrolysis and gasification processes alsoachieved the maximum value at 750 �C.Considering both the gasesyield and char quality, the optimum temperature of pine sawdustcatalytic pyrolysis is 750 �C.

3.3. Kinetic model for syngas production in pyrolysis process

The objective of sawdust thermochemical conversion was to ob-tain more syngas with higher heating value. The kinetics of reac-tions emerging in the conversion inevitably produced effect onthe yield and quality of syngas, so it is necessary to study the reac-tion kinetics. The following classical equation is often used for thekinetic interpretation of the thermal decomposition of wood andits components (Capart, 1991):

dq=dt ¼ �kq ð3Þ

q refers to the weight percentage of raw material in time t. Eq. (3)can be used in the form of an equation of Avrami–Erofeev type,applicable to materials whose porosity changes during the reaction,as

K ¼ ½� lnð1� XÞ�=s ð4Þ

X refers to the weight percentage of syngas in time s.Also, for k, it abides by the Arrhenius equation as

k ¼ k0 expð�E=RTÞ ð5Þ

Pre-exponential factor (k0) is in s�1, whereas activation energy (E) inkJ mol�1.

Eq. (5) can be further transformed as

lnðkÞ ¼ lnðk0Þ � E=ðRTÞ ð6Þ

Eq. (6) is linear. By applying k (determined by Eq. (4)) and T to it, thevalues of E and k0 can be determined. The curves lnðkÞ ¼ lnðk0Þ�E=ðRTÞ for three nickel based catalysts are presented in Fig. 4,which shows good linearity. The E and k0 values determined byFig. 4 are listed in Table 5. These values ranged between those re-ported by Barooah and Long (1976) (18.0 kJ/mol) and Fong and Ross(1980) (45.1 kJ/mol).

3.4. Effect of temperature on gasification products

After pyrolysis, steam was pumped into the quartz tube to reactwith pyrolysis char which was obtained at 750 �C, thereby syngaswas further produced. The reaction temperature is also an impor-tant factor with regard to the composition of final syngas.

700 750 800 850 90014

15

16

17

18

19

20d

Ni0 c

ryst

allit

e si

ze (n

m)

Temperature ( )700 750 800 850 900

3000

3500

4000

4500

5000

5500

6000c

Mg 0.

4Ni 0.

6O a

rea

at 3

7.2

a.u.

)

Temperature ( )

R2=0.9399

36 37 38 39 40 41 42

Blank

900

850

800

750

700

Mg0.4

Ni0.6

O

Inte

nsity

(a.u

.)

2θ2θ

b

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Blank

900

850

800

750

700

NiO ANiOAA NiNiIn

tens

ity (a

.u.)

Nia

Fig. 6. (a) XRD patterns for Z402 + Z405 before and after gasification at different temperatures, with nickel phases labeled as: Ni (cubic Ni0), NiO (NiO), and A (Mg0.4Ni0.6O).(b) XRD patterns for the Mg0.4Ni0.6O phase on post-reaction catalysts, with fresh catalyst as comparison. (c) Areas of the Mg0.4Ni0.6O phase on post-reaction catalysts, underthe peak at 37.2�; and (d) Ni0 crystallite sizes on post-reaction catalysts.

608 Q. Xie et al. / Bioresource Technology 110 (2012) 603–609

Gasification experiments were carried out at temperatures be-tween 700 and 900 �C, and the results were shown in Fig. 5. H2

and CO yields drastically increased from 1.31 to 2.25 N m3/kg bio-mass and from 0.22 to 0.54 N m3/kg biomass, respectively, whentemperature rose from 700 to 850 �C. H2 and CO yields both beganto decrease with temperature over 850 �C.A simplified model thatexplains gases evolution during gasification is described as follow-ing (Fryda, 2006):

CþCO2!2CO DH¼�173:8 kJ=mol ðBoudouard reactionÞ ð7ÞCþH2O!COþH2 DH¼�132 kJ=mol ðWater-gas shift reactionÞ ð8ÞCOþH2O!CO2þH2 DH¼�41:2 kJ=mol ðWater-gas shift reactionÞ ð9ÞCH4þH2O!COþ3H2 DH¼�206 kJ=mol ðMethane steam reformingÞ ð10Þ

Since reactions (7)–(10) are all endothermic reactions, highertemperature favored gasification of char when the temperaturewas below 850 �C. The maximum syngas yield was obtained at850 �C, i.e., 2.78 N m3/kg biomass. The decrease of gas yield withtemperature over 850 �C may be due to catalyst deactivationcaused by sintering. Therefore, the optimum temperature of pyro-lysis char gasification is 850 �C. Furthermore, under optimum con-ditions, the total yield of syngas produced in pyrolysis andgasification processes could achieve 3.29 N m3/kg biomass, withLHV and HHV calculated by Eqs. (1) and (2) up to 9.6 and10.9 MJ/N m3, respectively. Compared with previous studies, muchhigher syngas yield was obtained in this study, favoring synthesisof liquid fuels in subsequent processes.

The elemental analyses of char resulting from steam gasifica-tion at different temperatures were given in Table 6. As tempera-ture increased from 700 to 900 �C, the carbon content decreasedfrom 48.11% to 1.80%, while ash content rose from 15.53% to91.04%, indicating that higher temperature significantly reducedcarbon content and increased ash content in the char.

3.5. Catalyst characterization

To investigate the relation between the decrease of gas yieldand catalyst deactivation at high temperature, catalyst character-ization was analyzed. Catalysts (Z402 + Z405) before and afterthe gasification process at different temperatures were analyzedwith X-ray diffraction to determine effect of temperature on cata-lyst structure (Fig. 6a). The main nickel phase emerging in the freshcatalyst was metallic nickel, Ni0 (2h = 44.5�, 51.9� and 76.3�), andlittle nickel oxide (NiO) (2h = 62.8� and 75.3�). Nickel–magnesiumcompound Mg0.4Ni0.6O (2h = 37.2�, 43.2� and 79.1�) can be ob-served in all five catalysts after gasification.

For fresh catalyst, nickel phase existed mainly in the form of Ni0

with little or no NiO, which indicated that the H2-reduction treat-ment was sufficient to reduce NiO. The post-reaction samplesshowed the presence of Ni0 as well as NiO. It should be noted thatMg0.4Ni0.6O was also seen in all post-reaction samples and itsintensity was generally increased as gasification temperature rose.

Fig. 6b and c showed a part of XRD patterns of post-reaction cat-alysts and corresponding areas under the peak at 37.2�, which was

Q. Xie et al. / Bioresource Technology 110 (2012) 603–609 609

attributed to Mg0.4Ni0.6O. The intensity and area of the Mg0.4Ni0.6Ophase were observed to increase as temperature rose from 700 to900 �C, which demonstrated an increasing amount of nickel inter-acted with the support to form the spinel Mg0.4Ni0.6O. Moreover,there was a good linear relationship between area of theMg0.4Ni0.6O phase and temperature, as shown in Fig. 6c. The nickelincorporation into the spinel phase reduced the amount of Ni0

which was the active component of the catalyst, thus the catalystactivity was decreased. As shown in Fig. 6a, the formation of NiOphase may also be responsible for the loss of catalyst activity dur-ing the gasification process.

The Ni0 crystallite sizes in post-reaction catalysts were shown inFig. 6d. The Ni0 size was increased from 149 Å at 700 �C to 193 Å at900 �C, which corresponded to a 22.8% decrease in the nickel sur-face area from a geometric argument. The result indicated thatsome sintering occurred during reaction at higher temperature,which also resulted in the catalyst deactivation, and the higherreaction temperature would cause the more serious sintering.

4. Conclusion

In this study, two-stage catalytic pyrolysis and gasification ofpine sawdust has been applied to investigate effects of pyrolysistemperature and gasification temperature on syngas production.Results illustrated that higher temperature was needed in the gas-ification process (850 �C) than in the pyrolysis process (750 �C) tomaximize syngas yield, and the maximum syngas yield couldachieve up to 3.29 N m3/kg biomass (dry wt.), much higher thanprevious studies. Catalyst characterization analyses indicated thatformation of Mg0.4Ni0.6O and increase of Ni0 crystallite size weretwo main reasons for catalyst deactivation. Further study shouldfocus on the industrialization of the technique.

Acknowledgements

The authors would like to express their great appreciation tothe National Science Foundation of China (grants 21077002 and20877002) and the ‘‘Double-Hundred Talents’’ Program of Shenz-hen Municipal Government for the financial support to this study.

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