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Research on alkali-catalyzed gasification of coal black liquor slurry cokes made up by five different coals.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2007; 2: 152?157
Published online 31 July 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.034
Research Article
Research on alkali-catalyzed gasification of coal black liquor
slurry cokes made up by five different coals?
Kuang Jian-ping,* Zhou Jun-hu, Zhou Zhi-jun, Liu Jian-zhong and Cen Ke-fa
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China
Received 20 July 2006; Revised 10 October 2006; Accepted 13 December 2006
ABSTRACT: The black liquor from paper mills contains large quantities of sodium compounds and other organic
matter, such as lignin and cellulose. The sodium compounds will provide the catalytic action in coal black liquor slurry
(CBLS) gasification, while lignin and cellulose can enhance the heat value in the process of gasification. Five black
liquor slurries were made from coals from different regions: Xin Wen, Huang Ling, Zao Zhuang, Shen Mu and Shen
Hua. Alkali-catalyzed gasification experiments on the different samples of CBLS and coal water slurry (CWS) were
made on a thermobalance and a fixed-bed reactor. The residues of gasification were analyzed by X-ray diffraction
(XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The results reveal
that many mesopores and micropores exist on the surface of the CBLS coke, which play a key role in the catalytic
gasification process, and sodium as a catalyst can quicken the gasification reaction rate. XRD shows that NaCl and
sodium silicate are the main crystal components in dry samples of CBLS and CWS. The C-O stretching vibration peak
shifting to a lower wavenumber means that the energy for the C-O stretching vibration in the CBLS carbon matrix
decreases after partial gasification. Not only the coal rank but also the oxygen-containing groups and minerals influence
coal coke?s gasification activity. Of the five different CBLS, the gasification reactivity of CBLS made by the Huang
Ling coal was found to be higher than that of the others. The higher the degree of coalification, the lower the activity
of the coke. ? 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: coal black liquor slurry coke; mineral; XRD; catalyzed gasification; FTIR
INTRODUCTION
The coal water slurry (CWS) and coal black liquor
slurry (CBLS) are new clean coal utilization technologies that can be used as fuel for gasification. CWS is
made up of 60?70% coal and 30?40% water, while
CBLS contain 60?70% coal and 30?40% black liquor.
The black liquor comes from paper mill wastewater,
which contains large quantities of cellulose, lignin and
sodium compounds. Sodium compounds can act as catalyst in the CBLS gasification process (Dewu et al .,
1994; Chenzhi and Hanxu, 2002). The characteristics
of gasification of CBLS made by different coals seem
to vary according to the coal rank and the sodium compounds present.
Some researchers pay more attention to the effective utilization of wastewater and coal. Several authors
(Baosheng and Zhaoping, 2004; Guang and Jie, 2005;
Hongcang and Baosheng, 2005) have studied the characteristics of the synthesis gas in coal gasification.
Jongwon Kim (Kim, 1999) has researched on the
liquefaction of coal and black liquor, and indicated that
the lignin in black liquor can break the methylene key,
and act as a live carrier in coal pyrolysis. Yonghao
et al . (Yonghao et al ., 1991) hold that the surface complex compound made by the alkali metal ion and the
oxygen-containing group in coal are the activity centers
in the process of gasification. Puertolas and Gea (Puertolas and Gea, 2001) have studied the characteristics
of black liquor in pyrolysis. Jiyu and Shijie (Jiyu and
Shijie, 2002) have investigated the catalytic effect on
anthracite by adding waster water and confirmed that the
organic and inorganic matter in the black liquor plays
a key role in catalytic combustion and gasification.
The coal rank and the black liquor of CBLS will influence the characteristics of gasification of CBLS. Five
different coals (show in Table 1) have been selected to
analyze the gasification activity of five CBLS made by
different coals.
EXPERIMENTAL
*Correspondence to: Kuang Jian-ping, State Key Laboratory of
Clean Energy Utilization, Zhejiang University, Hangzhou, 310027,
China. E-mail: jpkuang@163.com
?
Presented at the 2006 Sino-Australia Symposium on Advanced Coal
Utilization Technology, July 12?14, 2006, Wuhan, China.
? 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Samples
The experimental samples included five different coal
varieties (Xin Wen (XW), Huang Ling (HL), Zao
Asia-Pacific Journal of Chemical Engineering ALKALI-CATALYZED GASIFICATION OF COAL BLACK LIQUOR SLURRY COKES
Table 1. Proximate analysis and ultimate analysis of coal, coal water slurry (CWS) and coal black liquor slurry
(CBLS).
Proximate analysis w(%)
HL
ZZ
XW
SM
SH
CWS
CBLS
Ultimate analysis war(%)
Mt
Aar
Var
FCar
Qnet,ar kJ/kg
Car
Har
Nar
St, ar
Oar
3.63
2.49
2.20
3.61
3.14
42.99
37.63
4.22
8.70
2.44
8.91
3.73
6.79
7.32
30.80
29.90
35.66
29.95
27.77
19.99
22.16
61.35
58.91
59.70
57.53
65.36
30.23
32.89
28 085
29 759
32 864
27 744
29 430
15 707
17 261.4
68.90
73.00
80.10
67.99
72.29
42.94
46.47
3.65
3.74
4.14
3.52
3.65
2.10
3.18
0.92
1.34
1.41
0.85
0.97
0.79
0.92
0.29
0.68
1.57
0.75
0.39
0.73
0.77
18.39
11.05
8.14
14.37
15.83
3.66
3.71
Zhuang (ZZ), Shen Mu (SM), Shen Hua (SH)) and two
coal water slurries (CWS and CBLS). The proximate
and ultimate analyses of the coal samples and the coal
water slurries are shown in Table 1.
CWS is made up of 57.01% XW coal and 42.99%
water, while CBLS comprise 62.37% XW coal and
37.63% black liquor. The components of the black
liquor are shown in Table 2. It contains complex compounds, such as lignin, cellulose, sodium compounds,
etc. The weight ratio of Na in the black liquor is nearly
14.4%.
Before the experiment, all the cokes were made
at 800 ? C for 30 min in an inert environment (in
presence of N2 gas). A TGA-SDTA 851e thermobalance
experiment was conducted to predict the gasification
activity of different coals, CWS and CBLS. The CO2
gasification experiments were performed in a fixedbed reactor. After externally heating the reactor to
the desired temperature of 1200 ? C, weighed samples
(100 mg) were introduced at the top of the reactor. The
gaseous products from the gasification of CWS and
CBLS, CO and CO2 were analyzed in a Rousemount
analyzer.
and thereby obtain the sample?s surface area and pore
diameter distribution. The BET model assumes that the
surface of an adsorbent is covered by an adsorbate
monolayer, and that the monolayer coverage can be calculated if the molecular dimensions of the adsorbate and
the quantity adsorbed are known (Khalili et al ., 2000):
1
c?1 p
p/p0
=
+
(1)
V (1 ? p/p0 )
Vm C
Vm C p0
where p is the equilibrium partial pressure, p0 is the
saturation vapor pressure, V is the volume adsorbed,
Vm is the monolayer capacity and C is a dimensionless
constant. In Table 3 the results of five different coals,
CWS and CBLS are reported.
The specific surface area of the different coals varies
from 2 to 6 m2 g?1 , and the SM coal?s surface area is
Table 3. Experiment results of different coals by
nitrogen adsorption.
Granularity/mm
RESULTS AND DISCUSSION
XW
SM
SH
ZZ
HL
CWS
CBLS
Analysis of the surface physical properties
Isothermal adsorption method is based on multilayer
adsorption theory, which can be used to solve the
Brunauer, Emmet and Teller (BET) model (Eqn (1)),
Total pore
volume
(mm3 g?1 )
�?2
Specific
surface area
(m2 g?1 )
�?2
Average pore
diameter (nm)
1.219
2.568
1.803
1.626
1.496
1.759
9.539
2.338
6.181
4.426
3.122
3.156
2.752
97.15
208.5
166.2
163.0
208.3
189.6
255.7
39.28
Table 2. Component analysis of the black liquor.
C (%)
H (%)
O (%)
38.56
4.03
Na2 O
(g l?1 )
23.40
40.82
1.05
Availability
alkali (g l?1 )
0.33
Total
alkali (%)
19.44
N (%)
S (%)
Si (%)
Na (%)
Inert
oxides (%)
0.47
0.45
Qnet , ar
(MJ kg?1 )
14.17
14.4
0.2
SiO2 /%
0.96
? 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Organic
compounds (%)
56.32
Solid matter
(g l?1 )
pH
120.4
10.22
Inorganic
compounds (g l?1 )
40.55
Asia-Pac. J. Chem. Eng. 2007; 2: 152?157
DOI: 10.1002/apj
153
K. JIAN-PING ET AL.
Asia-Pacific Journal of Chemical Engineering
larger than that of the others; the average pore diameter
of different coals are ?200 nm except for CBLS; the
total pore volume of SM coal is greater than that of the
others. There are no marked differences in the specific
surface area for XW coal and CWS, but for CBLS there
are some differences: the specific surface area and total
pore volume of CBLS are larger than those of others,
but the average pore diameter is much less than that of
the others.
These results show that more micropores have been
generated in the dry sample?s surface of CBLS in the
drying process. Owing to the fact that many lignin
and cellulose fractions exist in the black liquor, porous
materials are generated on the surface of particles during
the water evaporation process from CBLS.
Thermogravimetric analysis
100
90
80
70
60
50
40
30
20
10
0
(1) CBLS Coke
(2) CWS Coke
(3) XW Coke
(4) HL Coke
(5) SM Coke
(6) SH Coke
(7) ZZ Coke
(8) Temperature
0
500
(4)
(5)
(7)
(1)
(3)
(2)
(8)
(6)
1000
1500 2000
Time /s
2500
1200
1100
1000
900
800
700
600
500
400
300
200
100
Temperature /癈
Figure 1 shows the carbon conversion of different coke
samples. The cokes were made from XW, HL, SM,
SH, and ZZ coals, and CWS and CBLS, which were
all carbonized under the same conditions: carbonization
temperature 800 ? C; duration 30 min. The TGA study
was conducted using pure (99.99%) CO2 gas at a flow
rate of 50 ml min?1 and different heating rates: 30 ? C
min?1 till 900 ? C; 12 ? C min?1 from 900 to 1200 ? C.
The average particle size of the coke was about 100 祄.
Tx =0.5 is an active index that is used to indicate the
coke?s gasification activity when the carbon conversion
is equal to 50%. The lower the temperature at which the
Carbon conversion / %
154
carbon conversion reaches 50% in C?CO2 gasification,
the more the activity of the coke. As shown in Table 4,
the order of gasification activity of the different cokes
is as follows: HL>SM>SH>ZZ>CBLS>XW>CWS.
Cdaf shows the carbon content based on moisture and
ash content in coal, and in Table 4 the Cdaf of HL,
SHand SM cokes are less than 80%, but the carbon
conversion all reach 98%. Once Cdaf is greater than
80%, the carbon conversion is lower than before.
These results indicate that the higher the degree of
coalification, the lower the coke?s activity. Takarada
et al . (Takarada et al ., 1985) also support this, and
consider that the reactivity of coal is related not
only to the degree of coalification but also to the
oxygen-containing functional group and the inorganic
compounds in coal.
The carbon conversion of CBLS is greater than
that of XW and CWS cokes, though the Cdaf of the
three cokes are almost equal. There are some sodium
compounds in the CBLS that can act as catalyst in the
gasification process, weaken the C?O bond and make
the gasification reaction easier than before (Xie, 2002).
C?CO2 gasification
The C?CO2 gasification of different CBLS made by
the five coal samples were performed at 1200 ? C in the
fixed-bed reactor. The flow rate of CO2 was 1000 ml
min?1 , and the sample weight in different reaction conditions was all 100 mg. The CO concentration of the
different CBLS are shown in Fig. 2. In Fig. 2, the gasification reaction rates of the different CBLS are remarkably different. The reaction times of the five samples
are 170, 180, 175, 235 and 325 s, respectively, while
the total volume of CO generated in the gasification
are 365.69, 398.65, 394.54, 353.166 and 330.66 ml. So
the gasification reactivity of CBLS made by HL coal
is superior to that of the others because of its high CO
volume and less reaction time in the process of gasification, whereas the CBLS made by ZZ coal is the worst.
This result is consistent with the conclusion arrived at
from the thermogravimetric analysis.
3000
X-ray diffraction
Carbon conversion of the different samples. This figure is available in colour online at
www.apjChemEng.com.
Figure 1.
X-ray diffraction (XRD) was used to determine the
type of crystal structure present in the matrix of the
Table 4. Active index of different cokes, CWS coke and CBLS coke.
Sample
Cdaf (%)
Tx =0.5 (? C)
Conversion
CBLS coke
CWS coke
XW coke
HL coke
SH coke
SM coke
ZZ coke
84.41
922.5
84.96
85.5
870
76.28
84.0
868.7
82.53
74.76
829.4
98.72
77.62
839.4
97.73
77.72
898.6
98.63
82.20
847
90.07
? 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 152?157
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering ALKALI-CATALYZED GASIFICATION OF COAL BLACK LIQUOR SLURRY COKES
35
XW coal for CBLS
HL coal for CBLS
SH coal for CBLS
SM coal for CBLS
ZZ coal for CBLS
15
10
5
0
350
400
#
1
30
3
2
600
40
1 3
50
60
d=2.0016
2
d=2.8294
d=2.6574
d=2.4545
2
900
20
d=3.3612
10
d=3.5930
1200
d=7.2956
0
1500
70
1
#
Scanning electron microscopic analysis and
FTIR
1
300
0
10
20
30
40
2?/ 癇
50
60
70
Figure 3. XRD pattern of a dry sample of CBLS and CWS
1# : CBLS; 2# : CWS; 1?NaCl; 2 ? Iron silicon carbide
(Fe?Si?C); 3 ? sodium silicate (Na6 Si8 O19 ).
sample. By using X-rays of a known wavelength ?
and measuring ? , the plane spacing d in the crystal
is determined using Eqn (2).
d=
?
2 sin ?
(2)
The d values identify the types of crystals present in
the samples.
Figure 3 shows that the crystal structure in CBLS
and CWS are not clear. For the first sample (1# ) the
peaks at d = 3.3612, 3.5910, 2.8291 are attributable to
NaCl and sodium silicate. In the second sample, the
peaks at d = 2.8340, 3.3658, 3.6068, 7.3072 are also
attributable to NaCl and sodium silicate. But the peak
value of sodium silicate in the first sample is 50% more
than that of the second sample, which is mainly due to
the high content of Na in the first sample.
? 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
SEM images of the sample were measured using a
SIRION field-emission scanning electron microscope.
Figure 5 shows the SEM image of a dry sample of
CBLS, which indicates the surface image of a CBLS
particle.
1200
900
600
300
0
3 2
3
3
1
d=2.5932
3
300
2
1
10
20
1200
900
600
300
0
30
4
2
4
40
d=2.6348
1
600
Intensity (cps)
2
d=2.0028
900
2 3
d=3.3658
2
d=2.8340
d=2.6588
1200
d=7.3072
1500
d=3.6068
Figure 2. CO concentration in the C?CO2 reaction for
different CBLS.
2
1
50
60
d=2.5989
300
d=3.2977
250
Time / s
d=3.0295
d=2.6362
200
d=3.0335
150
d=3.7410
100
d=3.7362
50
d=4.2280
0
d=3.8901
20
d=4.2471
CO /%
25
Intensity (cps)
30
The residues of gasification in the fixed-bed reactor were detected by XRD as shown in Fig. 4. In
the first sample (1# ) the peak at d = 2.5932, 3.7410
are attributable to nepheline and nosean. The peaks in
another sample at d = 3.7352, 4.2471, 2.5989, 6.5046
are attributable to the calcium iron oxide and nosean.
There are remarkable differences in the crystal structures of the two residues: the peak value of nosean in
the CBLS residue is greater than that of CWS, and at the
same time, a new mineral (calcium iron oxide) is generated in the residue of CBLS. These changes indicate that
thermal metamorphism has occurred in the gasification
process of CBLS because of the lignin, cellulose and
sodium compounds in the black liquor. Sodium is an
active metal, which can reduce residue?s fusion temperature, and generate other low-melting compounds with
Si, Al, Fe, Ca, K. Lothar and Plogmann (Lothar and
Plogmann, 1983) also consider that the alkali metal can
react with a mineral, producing inert substances such as
nepheline, and weaken the catalytic reaction.
Mckee and Spiro (Mckee and Spiro, 1983) discovered
that the catalyst partially lose its activity in gasification
when adding Na2 CO3 (wt 5%) to coal; but this deactivation phenomenon does not occur when coal is changed
into graphite. Once the alkali metal reacts with the minerals in coal and generates silicate and aluminosilicate,
its activity, to some extent, will be lost.
#
70
2
#
4
10
20
30
40
50
60
70
2? / 癇
Figure 4.
XRD pattern of the gasification products
of CBLS coke and CWS coke 1# : CWS coke; 2# :
CBLS coke;1 ? Nepheline (Na6 K1.2 Al7.1 Si8.9 O32 ); 2 ? Nosean
(Na8 Al16 Si6 O24 SO4 );3 ? Nepheline (Na2.8 K0.6 Ca0.2 Al3.8 Si4.2
O16 ); 4 ? Calcium iron oxide.
Asia-Pac. J. Chem. Eng. 2007; 2: 152?157
DOI: 10.1002/apj
155
156
K. JIAN-PING ET AL.
Asia-Pacific Journal of Chemical Engineering
Figure 5. SEM image of a dry sample of CBLS.
Figure 7. FTIR spectra of CBLS, Na2 CO3 and CBLS coke
after partial gasification.
Figure 6 shows the SEM image of the partial gasification residue of CBLS. This picture displays that there
are many of micro- and mesopores on the surface of the
residue. After partial gasification, these micropores and
mesopores have been generated on the surface of the
carbon, which will increase the specific surface area of
coke in the gasification. The zigzag faces that exist on
the surface of CBLS coke are likely the active sites in
the process of gasification.
Figure 7 shows FTIR spectra of CBLS, Na2 CO3
and CBLS coke after partial gasification. The 1450
and 2361 cm?1 bands appear in CBLS and Na2 CO3 .
After partial gasification, the 1450 cm?1 band has
disappeared in CBLS coke. This indicates that Na2 CO3
has been decomposed. At the same time, the 1060 cm?1
band in CBLS has shifted to 1026 cm?1 in the CBLS
coke after partial gasification. The C?O stretching
vibration peak shifting to lower wavenumber means
that the energy of stretching vibration in the CBLS
carbon matrix has decreased, so the structure of the
carbon matrix is liable to react with an oxygen ion or a
hydroxide ion on gasification.
CONCLUSIONS
The characteristics of gasification of five different coal
samples (XW, HL, SH, SM and ZZ), CWS and CBLS
have been studied by thermogravimetry and a fixedbed reactor. From the data we obtained from the
experiments, some conclusion can be made:
1. The gasification activity of the different cokes is the
following order: HL>SM>SH>ZZ>CBLS>XW
>CWS. The higher the degree of coalification, the
lower the activity of the coke. The reactivity of the
five different CBLS in the C?CO2 gasification are
also consistent with this result.
2. Owing to the many lignin, cellulose matter and
sodium compounds that exist in the black liquor,
porous materials are distributed on the surface of the
particles of CBLS after drying. The specific surface
area of CBLS is larger than that of CWS.
3. The C?O stretching vibration peak is the 1060 cm?1
band in CBLS, and it shifts to 1026 cm?1 in the
CBLS coke after partial gasification. This means
that the energy for C?O stretching vibration in the
CBLS carbon matrix decreases, so the structure of
the carbon matrix is liable to react with an oxygen
ion or a hydroxide ion in gasification
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Figure 6. SEM image of the partial gasification
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? 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
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DOI: 10.1002/apj
157
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