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Research on the slagging characteristics of Shenhua coal.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2007; 2: 171–176
Published online 22 May 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.037
Research Article
Research on the slagging characteristics of Shenhua coal†
Weijuan Yang,* Junhu Zhaou, Xinyu Cao, Xiaohu Zhao, Zequan Lan, Jianzhong Liu and Kefa Cen
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, P. R. China
Received 20 July 2006; Revised 12 October 2006; Accepted 18 December 2006
ABSTRACT: Shenhua coal is one of high-quality power coals of high calorific value that has large reserves in China.
But the low fusion point of its ash poses some problems for its application in thermal power plants. The experiments
were carried out in a 0.25 MW combustion facility to make a comparative study of the ash deposition and slagging
process of Shenhua coal and XinWen coal. The results show that the ash deposition process of Shenhua coal lasts
longer than that of Xinwen, and self-cleaning of the slag does not take place with the Shenhua coal. Also, the ash
deposition of Shenhua coal decreases the heat transfer of the furnace wall severely. The three slag or ash specimens,
sampled in the furnace, near the furnace wall and in the flue duct, were tested and analyzed. It was indicated that the
furnace wall slag is rich in iron while in the fly ash calcium content is higher. The slag near furnace wall had a larger
ratio of silicon to aluminum, more glassy matter and smaller crystal characteristic peaks in the X-ray diffraction (XRD)
pattern. The crystalline matter is mostly quartz in the furnace wall slag. The crystal characteristic peaks in XRD in the
fly ash are larger than those of the furnace wall slag. The crystalline matter of the fly ash are quartz, andalusite and
hematite.  2007 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: slagging; ash deposition; coal; boiler operation
INTRODUCTION
The Shenhua Dongsheng coalfield is one of eight
largest coalfields in the world, which lies in Shanxi
and Nemenggu. It is estimated that there is 31 200 km2
of coal and the coal reserve is up to 223 billion tons.
A firm coal bed and low gas content make it suitable to build modern large mines. Shenhua coal is
a high-quality power coal with high calorific value,
and has low ash, low sulfur and low phosphorus.
Therefore, the Shenhua Dongsheng coalfield is considered as one of the strategic energy source bases.
Shenhua coal is young bituminous coal and has high
moisture content, low ash fusion point and high calcium in the ash. The quality of merchandise coal is
as follows: 12–15% moisture, 4–9% ash, 24–34%
volatile matter as arrived, sulfur lower than 0.5%, net
calorific value 23.42–25.09 MJ/kg, ash fusion temperature 1150–1250 ◦ C (Jianguo, 2003).
ASH CHARACTERISTICS
Though Shenhua coal has large reserves and high
calorific value, its low ash fusion point poses some
*Correspondence to: Weijuan Yang, State Key Laboratory of
Clean Energy Utilization, Zhejiang University, Hangzhou, 310027,
P.R. China. E-mail: yangwj@zju.edu.cn
†
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.
problems in its application to thermal power plants.
Table 1 lists the ash contents of several types of typical Shenhua coal (Jiwu, 2005; Yanjun et al ., 2005). It
is shown that the iron and calcium contents of most
Shenhua coal are high, which makes it easy to slag
and deposit. The boilers in our power plants are mostly
designed for tapping the solid slag and require that the
coal has high ash fusion point. Serious accidents due
to slagging have occurred in the boilers of the Xiaxing power plant in Zhejiang and the Shajiao power
plant in Guangdong (Xiongya and Lin, 2003), which
were designed to use Shenhua coal. The key problem is to prevent and control slagging when Shenhua coal is used, thereby extending its use in more
boilers.
Iron in ash plays an important role in slagging during
coal combustion: especially iron in the pyrite form as
we know. Table 2 shows several types of iron contents
in coal minerals and in ash, in which FeS2 in the mineral
form was calculated using exponential equation by
Raask (1985). The maximum ratio of Fe2 O3 converted
from FeS2 to the total Fe2 O3 is 22.3% and FeS2 is not
the most abundant mineral containing iron in Shenhua
coal. Therefore it can be concluded that other types of
iron minerals besides FeS2 cause Shenhua coal to slag
easily. Those iron minerals could possibly be silicate
and siderite.
172
Y. WEIJUAN ET AL.
Asia-Pacific Journal of Chemical Engineering
Table 1. Ash contents of several typical Shenhua coals (%).
Diggings
SiO2
Al2 O3
Fe2 O3
CaO
MgO
TiO2
SO3
K2 O
Na2 O
Huojitu
Wulanmulun
Wujiata
Pulianta
Majiata
Daliuta
Guojiawan
Shigetai
38.1
24.02
30.23
18.18
31.74
33.26
68.95
58.97
11.98
12.17
11.1
8.33
10.44
11.7
8.36
17.54
7.99
14.97
15.24
26.95
14.16
9.3
4.92
4.29
25.47
26.78
26.67
27.44
26.72
26.34
9.14
7.30
3.29
2.26
2.88
2.12
2.68
3.83
1.25
0.94
0.88
0.86
0.89
0.74
0.88
0.72
0.44
1.5
8.74
14.1
8.8
11.55
9.82
8.78
–
–
1.03
0.48
0.3
0.12
0.31
0.96
0.09
2.00
1.08
2.18
2.4
2.87
1.71
1.54
1.77
0.75
Table 2. Several types of iron contents in coal mineral and in ash.
Index
A
B
C
D
Items (%)
Huojitu
Wulanmulun
Wujiata
Pulianta
Majiata
Daliuta
FeS2 in mineral
Fe2 O3 converted from FeS2
Total Fe2 O3
B/C
1.66
1.09
7.99
13.66
3.72
2.45
14.97
16.36
5.17
3.40
15.24
22.30
4.67
3.07
26.95
11.39
3.36
2.21
14.16
15.62
1.23
0.81
9.30
8.71
SLAGGING PROCESS EXPERIMENTS
heat flux and Qini is the maximum of heat flux at the
beginning of the test when the HFM is clean.
Facility and coal samples
Rq= Qt /Qini
Slagging experiments were conducted in a 0.25 MW
coal combustion facility (Xinyu et al ., 2004; Zequan
et al ., 2004), in which the furnace was 3.5 m high and
the inner diameter 0.3 m. A heat flux meter (HFM)
was inserted into the furnace during the slagging test
and connected to a computer, so that the online heat
flux during the dynamic slag process could be obtained.
The HFM was designed specially, and its principle and
structure have been described in detail by Zequan et al .
(2004). The principle of the HFM was the same as that
in Paist et al . (2002) and Su (2001). The experiments
were carried out with both Shenhua coal and Xinwen
coal. The Shenhua coal we used was a blended coal of
several diggings and was a kind of commercial product,
and the Xinwen coal had a high ash fusion point and low
slagging tendency. The ash content and fusion points of
Shenhua and Xinwen coals are shown in Table 3.
Results on heat transfer
The head of the HFM was covered with an ash layer
after the HFM was put into furnace, and it was the ash
layer that increased the resistance to the heat transfer.
The ash layer of the Xinwen coal was very thin and tight
and composed of very fine ash particles, while that of
the Shenhua coal was thicker and composed of bigger
particles. The heat flux (Q) and heat flux ratio (Rq) were
adopted to estimate the degree of slagging severity. Rq
is defined as Eqn. (1), in which Qt is instantaneous
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
(1)
The change in dynamic Q and Rq during the slagging
process is shown in Fig. 1. The heat load of the facility
was kept at full load during the tests and the gas
temperature near the HFM was kept at 1300–1380 ◦ C.
The clean HFM was inserted into the same test hole
of the furnace and the inserted depth was also kept
the same. Thus the heat transfer condition of the HFM
was almost same in the tests of Xinwen and Shenhua
coal slagging. The heat flux is weakened as time goes
on. Shenhua coal slagged more severely than Xinwen
coal, and the value of Rq can reduce to 0.70, while that
of Xinwen coal is over 0.85 at any time. The average
heat flux (Qa ) is defined as the average of the heat flux
during the whole slagging test, and the average heat
flux ratio calculated by Rq a = Qa /Qini · Qa of Xinwen
coal was 224 kW/m2 and that of Shenhua coal was
254 kW/m2 . Also, the difference was due to the fact
that the flue gas temperature in the furnace was slightly
different for the different calorific value of the coal and
feed coal flow. So Rq a can reflect more clearly the
serious effect of different coal slags on heat transfer.
Rq a of Xinwen was 0.91 and that of Shenhua was 0.83,
which meant that the absorbed heat decrease of the dirty
surface was 12% more when burning Shenhua coal than
Xinwen coal.
Heat flux of the Shenhua coal decreased straightly,
while that of the Xinwen coal appeared to be fluctuating
periodically. There are three obvious valleys in the Q
and Rq curves of Xinwen coal. These are due to the
fact that some ash particles that adhere to the slag
Asia-Pac. J. Chem. Eng. 2007; 2: 171–176
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
THE SLAGGING CHARACTERISTICS OF SHENHUA COAL
Table 3. Ash content and ash fusion points in a reducing ambient.
Coal
Xinwen
Shenhua
Coal
Xinwen
Shenhua
SiO2 (%)
Al2 O3 (%)
Fe2 O3 (%)
CaO (%)
MgO (%)
Na2 O (%)
K2 O (%)
TiO2 (%)
46.94
53.51
38.03
19.62
7.84
8.38
3.76
9.84
1.07
1.73
0.59
1.81
0.78
1.13
1.01
3.55
Distortion temperature (◦ C) Softening temperature (◦ C) Hemispherical temperature (◦ C) Flow temperature (◦ C)
1346
1208
1395
1260
1410
1276
1422
1299
(a) Shenhua coal
(b) Xinwen coal
Figure 1. Q and Rq change during the slagging process.
surface break off and fall down by gravity when the
particles’ viscidity was smaller than the gravity or the
whole slagging piece had grown too big and heavy
to adhere to the surface. This slag-break makes the
HFM cleaner and the Q bigger, which can be called
the slag self-clean function. As shown in Fig. 1(b),
the slag self-clean happens periodically when burning
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
the Xinwen coal and the period was about 1 h. But
the self-clean function did not happen during the
Shenhua coal slagging process in the experiment. The
ash particle of the Shenhua coal had better viscidity
and the adhering force was enough to form bigger slag
and prevent it from breaking. Compared to that of the
Xinwen coal, the slagging process of Shenhua coal
Asia-Pac. J. Chem. Eng. 2007; 2: 171–176
DOI: 10.1002/apj
173
Y. WEIJUAN ET AL.
Asia-Pacific Journal of Chemical Engineering
lasted for a longer time and impacted heat transfer more
severely.
Iron exists in primary coal as organic compounds,
carbonate (siderite), sulfide (mostly FeS2 ) and silicate.
The organic compound is decomposed and oxygenated
in combustion and does not produce any melting particles. Siderite is not decomposed to melting particles
directly, but it can produce low-melting-point matter
with other minerals. Some FeS2 is oxygenated insufficiently and converted into melting sulfides and oxides,
such as Fe1 – x S and FeO, which have high viscidity.
Some silicate is converted by vitrification into melting ferruginous silicate particles whose fusion point is
lower than 1100 ◦ C such as CaO–FeO–SiO2 –MgO and
SiO2 –Al2 O3 –Fe2 O3 . It is shown that there is more SiO2
in SFW than in FA and APFS (Fig. 2), which is the
same as iron. This is the evidence that there are more
ferruginous silicates in SFW. FeS2 is not the mineral
containing most iron in the Shenhua coal as shown in
Table 2, and the other ferruginous inorganic matter such
as silicate and siderite also plays an important role along
with FeS2 in Shenhua coal slagging.
Figure 3 gives three SEM photographs of Shenhua
coal slag and ash. There are more round particles and
less transformed particles that adhere to each other in
the fly ash than in APFS. In the SEM photographs,
particles in the fly ash are a little bigger than those
in APFS. It is the slice structure and not the particle
structure that is observed in SFW. SFW has fewer pores
than FA and APFS and has been sintered in a way.
Figure 4 gives three XRD graphs of the Shenhua coal
slag and ash. Slag and ash are composed of crystalline
and vitreous matter. Crystal characteristic peaks of
SFW XRD graph are smaller than those of APFS
and FA, which indicates that SFW has more vitreous
matter while APFS and FA have more crystalline
particles. Most crystals of SFW are quartz, hematite
and anorthite. And most crystals of APFS are quartz,
mullite (3Al2 O3 ·2SiO2 ), calcite, anhydrite, hematite and
albite (Na2 O·Al2 O3 ·6SiO2 ). Albite does not exist in fly
ash, but the oxide (CaAll 8 Fe4 O19 ) does.
ASH CONTENTS AND STRUCTURE
The three ash specimens of Shenhua coal, sampled in
the furnace space, near the furnace wall and in the
tail flue duct, were collected and analyzed. The ash
particles in the furnace were drawn out by a watercooled sampling gun, which was designed specially for
sampling ash particles. The gun was inserted into the
furnace from a hole that was 1330 mm away from the
burner and was equipped on the top of the furnace. Also,
the gun head was inserted into the furnace’s center to
draw out ash particles in furnace space (APFS) and
just into the furnace and near the wall to draw out
slag particles near furnace wall (SFW). Fly ash (FA)
was drawn out from the tail flue duct after all the flue
gas coolers by an isokinetic sampling instrument. The
results of ash content analysis are listed in Table 4. As
shown in Table 4, there is much more Fe2 O3 in SFW
and APFS than in FA and coal ash. The mineral contents
in coal ash can redistribute in SFW, APFS and FA
during the combustion process, and the redistribution
is shown in Fig. 2, in which the relative contents were
calculated on the basis of the contents of coal ash. It
is indicated in Fig. 2 that SFW and APFS are enriched
in iron, while the calcium and aluminium contents are
higher in FA than in the coal ash.
Coal ash
SFW
APFS
FA
160
Relatively content %
174
120
80
40
CONCLUSIONS
0
SiO2 Al2O3 Fe2O3 CaO
MgO Na2O
K2O
TiO2
1. Shenhua coal is easy to slag because of its low
fusion point and high Fe and Ca contents. Slag
process of the Shenhua coal can reduce the heat flux
by about 30% after having slagged for 270 min in
Redistribution of mineral elements in the
combustion process. This figure is available in colour online
at www.apjChemEng.com.
Figure 2.
Table 4. Ash contents of three samples (%).
Sample
SiO2
Al2 O3
Fe2 O3
CaO
MgO
Na2 O
K2 O
TiO2
SFW
APFS
FA
60.01
55.02
55.58
17.64
19.42
23.88
13.29
10.91
6.49
5.01
7.26
6.91
1.32
1.96
1.63
0.76
0.85
1.01
1.02
1.36
1.56
0.87
2.68
2.99
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 171–176
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
THE SLAGGING CHARACTERISTICS OF SHENHUA COAL
(a) APFS
(b) FA
(c) SFW
Figure 3. SEM photographs of the Shenhua coal slag and ash.
Figure 4. XRD graphs of the Shenhua coal slag and ash.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 171–176
DOI: 10.1002/apj
175
176
Y. WEIJUAN ET AL.
the experiment. And slag self-clean function seldom
happens, while it happens each about 1 h during
Xinwen coal slagging test. So the soot should be
blown more frequently in the boilers that burn the
Shenhua coal.
2. The three ash specimens of the Shenhua coal sampled, in furnace space, near furnace wall and in the
tail flue duct, were analyzed. The results show that
SFW and APFS are enriched in iron, while the calcium and aluminium contents are higher in FA than
in coal ash. SFW is enriched by both iron and SiO2 .
Also, FeS2 is not the most iron-containing mineral
compound in the Shenhua coal. It can be concluded
that other ferruginous inorganic matter such as silicates and siderites also play an important role along
with FeS2 in Shenhua coal slagging.
3. SEM photographs show that there are more round
particles and less transformed particles that adhere
with each other in fly ash than in APFS, and SFW
appears to be of a slice structure. XRD graphs show
that SFW has more vitreous matter, while APFS and
FA have more crystalline particles, and most crystal
is quartz.
4. Furnace cross-section heat release rate, burner zones
heat release rate, single burner heat load and furnace
exit temperature should be chosen near the lower
limit when designing the boiler structure that aims
at using Shenhua coal. Blending coal is also a good
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
way to control slagging in boiler operation, such as
blending Shenhua coal with high-fusion-point coal
and blending several diggings of Shenhua coal to
adjust its ash content.
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Asia-Pac. J. Chem. Eng. 2007; 2: 171–176
DOI: 10.1002/apj
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