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Hydrogen production by partial oxidative gasification ofbiomass and its model compounds in supercritical water

Hui Jin, Youjun Lu, Liejin Guo*, Changqing Cao, Ximin Zhang

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

a r t i c l e i n f o

Article history:

Received 10 June 2009

Accepted 19 June 2009

Available online 15 July 2009

Keywords:

Partial oxidation

Supercritical water

Hydrogen production

Biomass

Glucose

* Corresponding author. Tel.: þ86 29 8266389E-mail address: lj-guo@mail.xjtu.edu.cn (

0360-3199/$ – see front matter Crown Copyright ª 2

doi:10.1016/j.ijhydene.2009.06.059

a b s t r a c t

Partial oxidative gasification in supercritical water is a new technology for hydrogen

production from biomass. Firstly in this paper, supercritical water partial oxidative gasi-

fication process was analyzed from the perspective of theory and chemical equilibrium

gaseous product was calculated using the thermodynamic model. Secondly, the influence

of oxidant equivalent ratio on partial oxidative gasification of model compounds (glucose,

lignin) and real biomass (corn cob) in supercritical water was investigated in a fluidized bed

system. Experimental results show that oxidant can improve the gasification efficiency,

and an appropriate addition of oxidant can improve the yield of hydrogen in certain

reaction condition. When ER equaled 0.4, the gasification efficiency of lignin was 3.1 times

of that without oxidant. When ER equaled 0.1, the yield of hydrogen from lignin increased

by 25.8% compared with that without oxidant. Thirdly, the effects of operation parameters

including temperature, pressure, concentration, and flow rate of feedstock on the gasifi-

cation were investigated. The optimal operation parameters for supercritical water partial

oxidative gasification were obtained.

Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.

All rights reserved.

1. Introduction directly deal with high-moisture biomass to produce

Biomass is a renewable energy resource, and the net emission

of CO2 during its utilization process is zero [1]. In spite of the

large worldwide biomass resource potential and the opti-

mistic long-term contribution, quite a lot of biomass is not

efficiently used at present. Due to gradual depletion of fossil

energy and the deterioration of the ecological environment in

the process of energy utilization, the rational use of biomass

has attracted great interest [2].

Hydrogen production by biomass supercritical water gasi-

fication (SCWG) is an advanced technology developed

recently. In the conventional gasification technology, high

energy-consumption drying process is necessary. SCWG can

5; fax: þ86 29 82669033.L. Guo).009 Published by Elsevier Ltd

hydrogen-rich gas without drying process. High gasification

efficiency is achieved and the emission of criteria pollution is

negligible. Many researchers studied the supercritical water

gasification of various biomass and its model compounds

[3–10].

However, in certain reaction conditions, the problems of

slow heating and coking still exist. Supercritical water partial

oxidative (SCWPO) gasification can solve the problems

mentioned above by adding certain amount of oxidant less

than the stoichiometric requirements for complete oxidation

to the system, and the reasons are as follows. (1) Biomass

supercritical water gasification is an endothermic reaction,

moreover, high heating rate is needed to achieve complete

on behalf of Professor T. Nejat Veziroglu. All rights reserved.

Nomenclature

ER oxidant equivalent ratio, amount of oxidant

added divided by the required amount for

complete oxidation by stoichiometry

calculation

GE gasification efficiency, mass of gaseous product

divided by mass of dry matter in feedstock

CE carbon gasification efficiency, mass of carbon

elementary in gaseous product divided by mass

of carbon elementary in dry matter in feedstock

HGE hydrogen gasification efficiency, mass of

hydrogen elementary in gaseous product

divided by mass of hydrogen elementary in dry

matter in feedstock

gas yield the mass of certain gas product divided by the

mass of dry matter in feedstock

4 volume fraction of certain gas product

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 03002

gasification. If the temperature of the reaction increases

slowly, more tar and oil are generated in the heating process.

And the tar or oil once generated is difficult to be gasified. The

heat resistance exists inevitably in the process of external

heating method. However, in the process of partial oxidative

gasification, in situ heat generated from the oxidation reaction

rapidly heat up the reaction fluid which reduces the char or tar

formation and favors the process of hydrogen production [11–

14]. (2) In the process of hydrogen production by biomass

supercritical water gasification, some reactants or interme-

diate products with complex structures, for instance, phenols

are difficult to be gasified and are referred to the ‘‘last hurdle’’

to get over for complete gasification [8,15]. However, hydrogen

peroxide is usually used as a source of free radicals to help

decompose the phenolic compound [16]. So the activation

energy of the reaction process is reduced, and the gasification

efficiency is improved [17–20]. (3) In some reaction conditions,

intermediate phenols are prone to polymerize with formal-

dehyde to produce tar and coke, and oxidant can fast react

with formaldehyde to prevent the polymerization reaction so

as to reduce coking problems [19,20].

Watanabe [21] reported the gasification of lignin and found

that the yield of hydrogen can be improved by supercritical

water partial oxidation. Some researchers studied the partial

oxidation of hydrocarbon [12,13,22]. GA in the USA has

established the demonstration plant of partial oxidative

gasification for hydrogen production, and planed to build an

industrial-scale waste treatment plant [14]. But till now, the

continuous operation of supercritical water partial oxidative

gasification from biomass for hydrogen production has

seldom been reported. In this paper, the process of super-

critical water partial oxidative gasification was analyzed from

the point of view of thermodynamics, and the supercritical

water partial oxidative gasification characteristics of glucose,

lignin and corn cob were experimentally studied in super-

critical water fluidized bed system and the effects of opera-

tion parameters upon gasification results were investigated.

The research work in this paper provides the theoretical

foundation and basic experimental results for the

development of supercritical water partial oxidative gasifi-

cation technology.

2. Thermodynamics analysis

Biomass gasification in supercritical water is a complex

process, and Antal [17] proposed the reaction process mainly

includes three reactions: steam reforming (1), water gas shift

reaction (2) and methanation reaction (3). Ethanol is taken as

an example.

C2H5OH(g)þH2O(g) / 2CO(g)þ 4H2(g) DH¼ 256 kJ/mol (1)

CO(g)þH2O(g) / CO2(g)þH2(g) DH¼�41 kJ/mol (2)

CO(g)þ 3H2(g) / CH4(g)þH2O(g) DH¼�206 kJ/mol (3)

If ethanol is fully converted into H2 and CO2, the reaction

equation is:

C2H5OH(g)þ 3H2O(g) / 2CO2(g)þ 6H2(g) DH¼ 174 kJ/mol (4)

If oxidant is added, the following reactions also occur:

C2H5OH(g)þ 3/2O2(g) / 2CO2(g)þ 3H2(g) DH¼�552 kJ/mol (5)

CH4(g)þ 1/2O2(g) / CO(g)þ 2H2(g) DH¼�36 kJ/mol (6)

CO(g)þ 1/2O2(g) / CO2(g) DH¼�283 kJ/mol (7)

The complete gasification reaction (4) is endothermal, and

the reactions (5)–(7) are exothermal, so partial oxidation

technology can integrate the endothermic gasification reac-

tion and exothermic oxidation reaction in one reactor to

realize the internal coupling of heat transfer and reaction

process. This kind of internal heating method eliminates the

heat resistance existing in the indirect external heating

process, so the heat transfer in the system is enhanced and

the gasification efficiency is improved. Simultaneously,

oxidant added can accelerate the reaction to increase the

biomass gasification efficiency.

Yan reported [23] a chemical equilibrium model for

hydrogen production by glucose and real biomass supercrit-

ical water gasification. Process of the reaction was analyzed,

and the thermodynamic rules were obtained. In present

paper, glucose was used as the model compound of biomass

and oxygen was used as oxidant, and the process of biomass

supercritical water partial oxidative gasification was analyzed

using the chemical equilibrium model. Fig. 1 shows, at 500 �C,

25 MPa, when the concentration of glucose is 10wt%, the

effects of ER on the amount of equilibrium gas production. As

can be seen from Fig. 1, when the chemical reaction reaches

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

6

0.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

H2 CH4 CO2

Gas yield

(m

ol/m

ol g

lu

co

se)

ER

CO

CO

yield

(m

ol/m

ol g

lu

co

se)

Fig. 1 – Effects of ER to the amount of equilibrium gas

production. (Temperature, 5008C; Pressure, 25MPa;

Feedstock, 10wt%glucose).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 0 3003

equilibrium, glucose is completely gasified, and the main

product is hydrogen, carbon dioxide, carbon monoxide and

methane. When the amount of oxidant is small, the amount

order of produced gases is H2>CO2>CH4>CO, within which

the amount of carbon monoxide is very small, almost can be

neglected compared with other gases. As ER increases, the

production of hydrogen, methane, carbon monoxide gradu-

ally decreases, while the amount of carbon dioxide increases.

When ER is comparatively large, the amount order of gas

produced is: CO2>H2>CH4>CO, It can be seen that from the

point of view of thermodynamics, the addition of oxidant

decreases hydrogen yield. Fig. 2 shows the effects of ER on the

equilibrium gas fraction, from which we can see, the regu-

larity of gas fraction in equilibrium with ER is in close agree-

ment with the regularity of the amount of gas produced with

ER. When ER equals 1.0, that is to say the amount of oxidant

equals the required amount for complete oxidation by stoi-

chiometry calculation. As can be seen from Fig. 2, in this

reaction condition by chemical equilibrium calculation, the

amount of hydrogen, methane and carbon monoxide

decreases to zero. Although oxidant can accelerate the rate of

the gasification reaction, too much oxidant leads to reaction

between combustible product such as hydrogen, methane and

carbon monoxide. As a result, finding the optimal oxidant

amount for hydrogen production is very important.

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100

ER

H2CH4CO2

/%

Fig. 2 – Effects of ER to the equilibrium gas composition

(Temperature, 5008C; Pressure, 25MPa; Feedstock,

10wt%glucose).

3. Apparatus and experimental procedures

The experimental system in this paper is shown in Fig. 3. A

fluidized bed reactor made of 316 stainless steel is equipped in

the experimental system. The inside diameters of the fluid-

ization part and suspension part are 30 mm and 40 mm,

respectively. The whole length of the fluidized bed reactor is

915 mm. The designed temperature and pressure are 650 �C

and 30 MPa, respectively. Five K-armored thermocouples are

equipped along the axial direction of the reactor to measure the

fluid temperature in the reactor. Quartz sand with the diameter

of 0.1–0.15 mm is used as the bed material. Detailed experi-

mental device and method can be seen in the literature [22].

The corn cob used in the experiment was collected in the

city of Xi’an, Shaanxi Province, China. The ultimate and

proximate analysis of the corn cob is shown in Table 1. The

corn cob was pulverized into particles smaller than 60 mesh

and mixed with sodium carboxymethyl cellulose (CMC) to

realize continuous delivery under high pressure. Alkali lignin,

substitute of lignin, was purchased from Wuhan Donghua

chemical Co., Ltd. CMC was purchased from Shanghai Shanpu

chemical Co., Ltd. Glucose and 30wt% analytically pure H2O2

was purchased from Tianjin Jinbei fine chemical Co., Ltd. H2O2

was used as oxidant in the experiment.

The composition of the gaseous phase was measured by

HP6890 gas chromatograph, which is equipped with thermal

conductivity detector and capillary column C-2000 that was

purchased from LanZhou Institute of Chemical Physics in

China. High purity He was used as carrier gas with a flow rate

of 10 ml/min.

4. Partial oxidative gasification of biomass

The three main components of biomass are cellulose, semi-

cellulose and lignin. Glucose, as the monomer of cellulose, is

a representative biomass model compound. Lignin consists of

phenolic structure not only is difficult to be gasified but

also adversely affects the hydrogen production [21,24,25],

and it is also usually used as the model compound of

biomass to study the rules of hydrogen production from

biomass in supercritical water. Corn cob is a typical agricul-

tural residual frequently seen. So glucose, lignin and corn cob

were selected to study the gasification characteristics of

biomass by supercritical water partial oxidation. Gasification

efficiency (GE), carbon gasification efficiency (CE) and

hydrogen gasification efficiency (HGE) are used to evaluate the

effect of gasification.

4.1. Glucose

Fig. 4 shows the influence of ER upon the gasification results in

the reaction condition: temperature (without special decla-

ration in this paper, temperature means the average temper-

ature of the external wall of the reactor) 600 �C, pressure

25 MPa and flow rate of glucose 24.7 g/min, concentration

10wt%. It can be seen from Fig. 4(c) that as the amount of

oxidant added increased, the GE and CE increased. Carbon

monoxide is a typical partial oxidant product [26] through the

Fig. 3 – Scheme diagram of experimental system.1:feedstock tank; 2,3: feeder; 4: fluidization bed reactor; 5: heat exchanger;

6: pre-heater; 7: cooler; 8,9,10: back pressure regulator; 11: high pressure separator; 12: low pressure separator; 13,14: wet

test meter; 15,16,17,18: high pressure metering pump; 19,20,21,22: mass flow meter; 23: water tank.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 03004

partial oxidative reaction (8), so as can be seen in Fig. 4(b), the

amount of carbon monoxide increased as ER increased.

CmHn / COþ other hydrocarbons (8)

It can be seen from Fig. 4(a) that the fraction of hydrogen

and carbon dioxide in gas production decreased and the

fraction of carbon monoxide increased as ER increased. It can

be seen from Fig. 4(b) that the yield of hydrogen increased first

and then decreased. GE of glucose was improved by the

oxidant added comparatively faster when ER was small, even

though hydrogen fraction in the gas production decreased,

when ER equaled 0.2, the yield of hydrogen obtained its

maximum value at the investigated experimental conditions.

The trend of HGE with oxidant amount was in close agree-

ment with the yield of hydrogen. Thus, oxidant could improve

the gasification efficiency of glucose and appropriate amount

of oxidant can improve hydrogen production. The experiment

by Williams [25] was in agreement with our experimental

results: as oxidant concentration increased, the yield of

hydrogen increased first and then decreased, but the yield of

hydrogen peaked when ER¼ 0.53. The reason for the deviation

of the optimal ER value maybe the experiment done by

Williams was in relatively low temperature 350 �C, and then

the reaction rate was lower, so more oxidant was needed to

achieve the maximum yield of hydrogen.

Table 1 – The ultimate and proximate analysis of corn coba.

w/% (Ultimate analysis)

C H N S Ob M

40.22 4.11 0.39 0.04 42.56 9.71

a Air dry basis.

b Difference.

The fraction of carbon monoxide was relatively high,

which was not consistent with thermodynamic result

mentioned above. That is mainly because the gas fraction

calculated by chemical equilibrium model only shows the

thermodynamic limit of the reaction. However, the real gasi-

fication reaction is controlled by kinetics of the reactions.

4.2. Lignin

It can be seen from Fig. 5(c) that as the ER increased from 0 to

0.4, GE increased from 0.216 to 0.679, CE increased more than

threefold, from 0.165 to 0.518. Formaldehyde, benzene and

polymers with phenolic structure are generated from lignin in

supercritical water, as reaction (9), and phenolic resins are

easily produced through the polymerization reaction (10)

between phenolic compounds and formaldehyde. Thus,

formaldehyde must decompose fast before the polymerization

reaction with phenolic compounds [21]. When oxidant is

present, on one hand, as mentioned above, oxidant can help

decompose compounds with phenolic structure to improve the

gasification efficiency. On the other hand, hydrogen peroxide

in supercritical water can form free radicals OH� through

reaction (11), and the presence of free radicals OH� can greatly

facilitate the reaction (12), and decrease the concentration of

formaldehyde, so the reaction (10) is inhibited greatly. Thus the

production of char and tar decreased and the gasification

w/% (Proximate analysis) Heat value/kJ kg�1

A V FC

2.97 71.21 16.11 16650.0

0.0 0.2 0.4 0.6 0.80

10

20

30

40

50

H2 CO

CH4 CO2

ER

a

0.0 0.2 0.4 0.6 0.80

4

8

12

G

as Y

ield

/m

ol.kg

-1

ER

H2 CO

CH4 CO2

b

0.0 0.2 0.4 0.6 0.80.2

0.4

0.6

0.8

CE

, G

E o

r H

GE

ER

GECEHGE

c

/%

Fig. 4 – Effects of ER on gasification characteristics of

glucose: (a) molar fraction; (b) gas yield; (c) CE, GE and HGE.

(Temperature, 6008C; Pressure, 25MPa; Feedstock, 10wt%

glucose; Feedstock flow rate, 24.7g/min).

0.0 0.1 0.2 0.3 0.40

20

40

60

ER

H2

COCH4

CO2

0.0 0.1 0.2 0.3 0.40

4

8

12

16

H2

COCH4

CO2

Gas Y

ield

/m

ol.kg

-1

ER

b

0.0 0.1 0.2 0.3 0.40.1

0.2

0.3

0.4

0.5

0.6

0.7

GE

, H

GE

, o

r C

E

ER

GEHGECE

c

a

/%

/%

Fig. 5 – Effects of ER on gasification characteristics of lignin:

(a) molar fraction; (b) gas yield; (c) CE, GE and HGE.

(Temperature, 6008C; Pressure, 25MPa; Feedstock, 2wt%

lignin D 2wt%CMC; Feedstock flow rate, 25g/min).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 0 3005

20

40

60

H2 CO

CH4 CO2

a

%

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 03006

efficiency of lignin increases .So the continuous operation of

the system is guaranteed, as it was observed that as ER

increases, the colour of the liquid products become lighter.

Lignin / HCHOþ phenolic compoundsþ others (9)

HCHOþ phenolic compounds / resins (10)

0.0 0.1 0.2 0.3 0.40

ER

0.0 0.1 0.2 0.3 0.4

0

5

10

15

20

Gas Y

ield

/m

ol.kg

-1

ER

H2 CO

CH4 CO2

b

0.8

0.9c

H2O2 / 2OH� (11)

HCHO / COþH2 (12)

It can be seen from Fig. 5(b) and (c) that as ER increases

from 0 to 0.4, the yield of hydrogen and hydrogen gasification

efficiency increased first and then decreased. This trend is

mainly caused by two aspects. On one hand, the presence of

oxidant inhibits the reaction (10), so more hydrogen and

carbon elements in the liquid intermediate product are

released. Reaction (12) is also promoted, and then the carbon

monoxide produces carbon dioxide and hydrogen through the

water gas shift reaction (2). On the other hand, as ER increases,

as can be seen in Fig. 5(a), the fraction of hydrogen and carbon

monoxide decreased, because the reaction between oxidant

and hydrogen became intense and resulted in the decrease of

hydrogen yield. Summing up the above factors, experiment

results show that the yield of hydrogen peaked when ER

equaled 0.1, and the yield of hydrogen increased by 25.8%

compared with that without oxidant.

It can be seen from Fig. 5(a) and (b), the yield of carbon

monoxide was lower than that in Fig. 4(a) and (b), because the

feedstock was alkali lignin as the substitute of lignin, and

reaction (2) was catalyzed by the presence of alkali so the yield

of carbon monoxide decreased greatly.

0.0 0.20.1 0.3 0.40.3

0.4

0.5

0.6

0.7

GE

, C

E o

r H

GE

ER

GE

CE

HGE

Fig. 6 – Effects of ER on gasification characteristics of corn

cob: (a) molar fraction; (b) gas yield; (c) CE, GE and HGE

(Temperature, 6508C; Pressure, 25MPa; Feedstock,

5wt%corn cobD2wt% CMC; Feedstock flow rate, 25.1g/min).

4.3. Corn cob

Fig. 6 shows the relation of corn cob gasification results and the

amount of oxidant added in the condition temperature 650 �C,

pressure 25 MPa and flow rate of feedstock 25.1 g/min, and the

concentrations of corn cob and CMC are 5wt% and 2wt%,

respectively. As can be seen from the Fig. 6(c) and (b), GE and CE

increasedrapidlyandHGEdecreased.WhentheERwasrelatively

small, the hydrogen yield’s deviation was small, and when ER

was more than 0.1, the yield of hydrogen decreased rapidly. Itcan

be seen from Fig. 6(a) that the fraction of hydrogen in gas

production gradually decreased as the concentration of oxidant

increased. When the ER was 0.4, the hydrogen fraction was only

24.20%. The fraction of carbon dioxide increased as ER increased,

because increasing ER enhanced direct oxidation to carbon

dioxide. Compared with the glucose partial oxidative gasification

result, the fraction of carbon monoxide was much lower, that is

mainlybecauseof thesmallamountofalkalinecompoundK2CO3

in real biomass, which can be a catalyst to accelerate the water

gas shift reaction (2) to convert carbon monoxide to hydrogen.

Compared with the gasification characteristics of model

compounds mentioned above, there was no maximum yield of

hydrogen as ER increased. Average external wall temperature

as high as 650 �C was adopted to realize the continuous

23 25 270

10

20

30

40

50

Pressure/MPa

H2

COCH4

CO2

a

23 25 270

4

8

12

16

Gas Y

ield

/m

ol.kg

-1

Pressure/MPa

H2

CO CH4

CO2

b

23 25 270.0

0.2

0.4

0.6

0.8

1.0

GE

, H

GE

o

r C

E

Pressure/MPa

GE HGE CE

c

/%

Fig. 8 – Effects of pressure on the gasification

characteristics: (a) molar fraction; (b) gas yield; (c) CE, GE

and HGE(Temperature, 6508C; Feedstock, 10wt% glucose;

Feedstock flow rate, 25g/min; ER[0.4).

550 600 6500

10

20

30

40

50

Temperature/°C

H2

COCH4

CO2

a

b

550 600 6500

4

8

12

Gas Y

ield

/m

ol.kg

-1

Temperature/°C

H2 CO

CH4 CO2

550 600 6500.0

0.2

0.4

0.6

0.8

1.0

GE

, H

GE

o

r C

E

Temperature/°C

GEHGECE

c

/%

Fig. 7 – Effects of temperature on the gasification

characteristics: (a) molar fraction; (b) gas yield; (c) CE, GE

and HGE (Pressure, 25MPa; Feedstock,10wt% glucose;

Feedstock flow rate, 25g/min, ER[0.4).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 0 3007

operation of the system, and the temperature was high enough

for the fast gasification reaction for corn cob, and the addition

of oxidant can also combust the hydrogen and carbon

monoxide fast in high temperature. Portela [16] also reported

that at 500 �C the effect of hydrogen peroxide appears to be

negligible. In our experiment system, when the average

external wall temperature is 650 �C, the average temperature of

the fluid is about 500 �C [22], so the effect of oxidant for

hydrogen yield is more obvious in lower temperature.

10 17.5 240

20

40

60

Concentration/%

H2

COCH4

CO2

a

10 17.5 240

4

8

12

16

Gas Y

ield

/m

ol.kg

-1

Concentration/%

H2

CO

CH4

CO2

b

10 17.5 240.0

0.2

0.4

0.6

0.8

1.0

GE

, H

GE

o

r C

E

Concentration/%

GEHGECE

c

/%

Fig. 9 – Effects of concentration on the gasification

characteristics: (a) molar fraction; (b) gas yield; (c) CE, GE

and HGE(Temperature, 6508C; Pressure, 25MPa; Feedstock

flow rate, 25g/min; ER[0.4).

14 25 370

10

20

30

40

50

Flow rate/g.min-1

H2

CO

CH4

CO2

14 25 370

4

8

12

16

Gas Y

ield

/m

ol.kg

-1

Flow rate/g.min-1

H2

COCH4

CO2

14 25 370.0

0.2

0.4

0.6

0.8

1.0

GE

, H

GE

o

r C

E

Flow rate/g.min-1

GEHGECE

/%

a

b

c

Fig. 10 – Effects of flow rate of feedstock on the gasification

characteristics:(a) molar fraction; (b) gas yield; (c) CE, GE

and HGE (Temperature, 6008C; Pressure, 25MPa; Feedstock,

10wt% glucose; ER[0.4).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 03008

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 0 3009

5. Effects of operation parameters

5.1. Temperature

From Fig. 7(a) and (b), we can see that when the temperature

increased from 550 �C to 650 �C the fraction of hydrogen

increased from 14.68% to 29.30%, and the yield of hydrogen

increased from 4.425 mol/kg to 8.939 mol/kg. Temperature

showed a great effect on supercritical water partial oxidative

gasification. The occurrence of two competing reaction ways

was reported [27]. One way consists of ionic reaction step,

which is preferred at higher pressure and/or lower tempera-

ture. The second reaction pathway is a free radical degradation

and dominates at lower pressures and/or higher temperatures.

High temperature favors the free radical reaction in which gas

is produced. What’s more, high temperature accelerates the

approaching to the chemical equilibrium state. Based on the

thermodynamics analysis above, gasification is complete in

the chemical equilibrium state, so high temperature favors

complete gasification. As is shown in Fig. 7(c), CE, GE, HGE

increased with temperature. It also can be seen that the yield of

methane decreased with temperature, which was determined

by the exothermic nature of the methanation reaction.

5.2. Pressure

The mechanisms of pressure on supercritical water partial

oxidative gasification are very complicated. (1) If the other

operation parameters such as temperature, concentration,

flowrateare kept constant, thedensity and ionproduct of water

increase as pressure increases. Increased density and ion

product of water accelerate the hydrolysis reaction, water gas

shift reaction and oxidant reaction, which favors gas produc-

tion. (2) Gases are typical products of the free radical reactions,

therefore, high pressure inhibits gas formation reaction [27]. (3)

In general, a reaction that increases the number of molecules is

inhibited in thehigh-pressureregion according to Le Chatelier’s

principle [28]. Therefore, lower pressure favors gasification

process. In conclusion, the influence of pressure on gasification

results from the integrated actions of the factors mentioned

above [10]. The experimental results as shown in Fig. 8 indi-

cated that the pressures had no significant effect on the partial

oxidative gasification of biomass. However, when the pressure

is below the supercritical pressure, the special physical and

chemical properties disappear, which goes against hydrogen

production. At the same time, higher pressure causes trouble

for the design and maintenance of the system and also

increases the operating costs. Experiment experience shows

that the ideal operation pressure is about 25 MPa.

5.3. Concentration

From Fig. 9(a) and (b), we can see that as the concentration of

glucose increased from 10wt% to 24wt%, the fraction of

hydrogen decreased from 29.3% to 10.14% and the yield

of hydrogen decreased from 8.939 mol/kg to 1.933 mol/kg.

From Fig. 9(c) we can see that GE, HGE and CE decreased in

varying degrees. High concentration of feedstock means that

the concentration of water around reactants decrease, so the

hydrolysis reaction of reactants and the water gas shift reac-

tion are inhibited. Therefore, high concentration biomass is

difficult to be gasified. But high concentration gasification is the

basis for large-scaled commercial application, so how to gasify

high concentration feedstock is the focus of future research.

5.4. Flow rate of feedstock

Different flow rate affects the partial oxidative gasification

from the following two aspects. (1) Volume of the reactor kept

constant, when the flow rate is higher, the resident time is

limited, so high flow rate of feedstock probably leads to

incomplete reaction. So it can be seen from Fig. 10(c) and (b) the

GE and the yield of hydrogen were lower. (2) According to the

report by Matsumura [29], the range of flow rate investigated in

this paper made sure that the velocity of bed material was

between the minimum fluidization velocity and terminal

velocity. Higher flow rate leads to a fiercer fluidization condi-

tion and enhances heat and mass transfer so as to favor the

gasification reaction. It can be seen from Fig. 10(a) that when

the flow rate of feedstock increased from 14 g/min to 37 g/min,

the fraction of hydrogen decreased from 29.30% to 10.14%.

From Fig. 10(b) we can see that the yield of hydrogen decreased

from 8.75 mol/kg to 4.68 mol/kg, and the fraction of carbon

monoxide increased. It can be seen from Fig. 10(c), GE, HGE and

CE decreased in different degrees. From the experiment result

we can deduce that the former factor was the dominant one in

the experiment range investigated, in another words, low flow

rate of feedstock favored the hydrogen production.

6. Conclusions

Supercritical water partial oxidative gasification is an effective

method to produce hydrogen from biomass. Oxidant can

improve the gasification efficiency and decreases the produc-

tion of char and tar to guarantee the system’s continuous

operation. Thermodynamic analysis shows that when the

chemical reaction reaches equilibrium, the gasification is

complete. The amount order of produced gases is

H2>CO2>CH4>CO. when the oxidant amount is small. As ER

increases, yields of hydrogen, methane and carbon monoxide

and the molar fraction of hydrogen decrease, and the yield of

carbon dioxide increases. Experimental results show that yield

of hydrogen from lignin increased from 6.45 mol/kg to 8.11 mol/

kg when ER increased from 0 to 0.1. GE of lignin increased from

0.216 to 0.679 when ER increased from 0 to 0.4. Supercritical

water partial oxidative gasification is influenced by operating

parameters. High temperature favors hydrogen production.

Pressure has no significant effect on the gasification result. High

flow rate of feedstock leads to incomplete gasification and low

concentration is favorable for hydrogen production and GE.

Acknowledgements

This work is currently supported by the National Key Project

for Basic Research of China through Contract No.

2009CB220000 and the National High Technology Research

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 3 0 0 1 – 3 0 1 03010

and Development Program of China (863 Program) through

contract No. 2007AA05Z147.

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