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Ash fusion properties and mineral transformation behavior of gasified semi-char at high temperature under oxidizing atmosphere Yukui Zhang, Qiangqiang Ren, Hongxiang Deng, and Qinggang Lyu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02756 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017
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Energy & Fuels
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Ash fusion properties and mineral transformation behavior of gasified
3
semi-char at high temperature under oxidizing atmosphere
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Yukui Zhang1,2, Qiangqiang Ren*,1,2, Hongxiang Deng1,2, Qinggang Lyu1,2
5
6
1
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
7
8
2
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
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1
ABSTRACT
2
The carbon conversion of fluidized bed gasification has been restricted by the entrainment of semi-chars
3
with high carbon content. Recently, the disposal of the semi-char has become an intractable problem. In this
4
study, the ash fusion properties and mineral transformation behavior at high temperature of the semi-char
5
from an industrial circulating fluidized bed (CFB) gasifier were investigated under oxidizing condition. In
6
comparison with the raw coal, the total base content increases from 27.92 to 29.45 after partial gasification,
7
which results in a decrease in the ash fusion temperatures (AFTs) of the semi-char. Kaolinite decomposed
8
into quartz, alumina and water via dihydroxylation reaction and anhydrite was transformed into oldhamite in
9
the CFB gasifier. As the particle size increases, the AFTs of the semi-char decrease first and then remain
10
almost unchanged, which coincides with the variations in acid/base ratio; however, the disparity is relatively
11
small, which favors its melting utilization. Quartz, anorthite, pyroxene and hematite are the main crystalline
12
phases of the semi-char ash at high temperature and the major molten phase turns out as anorthite. The
13
liquidus temperature (1325 °C) predicted by FactSage is slightly higher than the flow temperature (1310 °C)
14
of the semi-char ash due to kinetic limits and mass transfer. In-situ observation of the semi-char ash melting
15
process indicates that low-melting-point minerals first fuse and flow between the adjacent particles; as the
16
temperature increases, high-melting-point minerals then start to melt, the molten phase content increases and
17
the flow behavior accelerates.
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Energy & Fuels
1. Introduction Gasification, which converts carbon-rich solid fuels (coal, biomass and semi-coke, etc.) into gaseous [1, 2]
3
products, e.g., syngas (H2 and CO), CH4 and CO2 with the presence of air, steam and oxygen
, has been
4
considered one of the most significant technologies for power generation, chemical synthesis and IGCC
5
systems. Despite the availability of diverse techniques, fluidized bed gasification has attracted substantial
6
interests because of its feedstock flexibility, low-cost operation and high environmental performance
7
However, large amounts of carbon-containing semi-chars are entrained out by crude gas, which reduces
8
carbon conversion of the gasification system (< 90%). As byproducts from gasification, the semi-chars are
9
generally considered as solid wastes; and serious environmental problems would occur if improperly
10
disposed [5]. Besides, the semi-chars always contain large quantities of unburned carbon (30-70%) compared
11
with combustion fly ash. Thus, it would be considered feasible if the semi-chars are reclaimed and reused for
12
further utilization.
[3, 4]
.
13
Currently, considerable studies have been carried out to explore effective methods for the semi-char
14
utilization. Semi-char re-injection could increase the carbon concentration of dense phase and improve the
15
carbon conversion
16
residence time
17
entrained flow gasification. However, complete conversion of the semi-char is impossible to achieve in
18
consideration of the fact that the fly ash from entrained flow gasifiers also contain certain amounts of
19
unburned carbon (30-50%) [10]. Fluidized bed combustion has proven to be an effective way to fully recover
20
residual carbons in the semi-char
21
aggravate atmospheric pollution problems, such as acid rain and photochemical smog. Furthermore, the
22
combusted fly ashes are of low density as with the semi-char, and thus volume reduction could not be
23
attained. Therefore, it is necessary to develop new techniques to obtain clean and efficient conversion of the
[8]
[6, 7]
, whereas the benefit is restricted by kinetic limits at given temperature and solids
. Wu et al.
[9]
proposed the integrated gasification concept, i.e., fluidized bed coupled with
[11]
. Nevertheless, the NOx and SO2 emitted from combustion would
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semi-char.
2
Considering the above situations, we proposed the concept of fluidized bed gasification combined with
3
combustion melting system to make effective use of the residual carbon in the semi-char. Coal was fed into
4
the fluidized bed gasifier and the generated semi-chars were reclaimed and reused as fuel of the melting
5
furnace. The organic material in the semi-char was completely combusted and the high temperature flue gas
6
was guided directly into the gasifier to elevate the temperature of dilute zone and accelerate the gasification
7
reactions. As a result, the residual carbon was extracted from the semi-char to increase total carbon
8
conversion, and the heat released during combustion was recovered to enhance the cold gas efficiency. The
9
NOx and SO2 were converted into N2, NH3 and H2S under reducing atmosphere and removed downstream to
10
avoid air pollution. Simultaneously, the mineral matters in the semi-char were transformed into liquid slags
11
at high temperature and volume reduction was achieved. However, preliminary tests indicated that severe
12
slagging problem occurred at high temperature and long-term operation of the melting furnace was impeded.
13
Therefore, investigations on the fusion properties of the semi-char are essential for the research and
14
development of the integrated system.
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In recent years, the fusion characteristics of coal ashes and slags from entrained flow gasifiers have been
16
extensively investigated. van Dyk [12] concluded that the addition of acid components, e.g., SiO2, Al2O3 and
17
TiO2, could raise the ash fusion temperatures (AFTs) of South African coal. The AFTs of coal ashes were
18
found to decrease first, reach a minimum and increase again when the contents of CaO, Fe2O3 and MgO
19
increased
20
reported that iron presented as iron oxides in Ar atmosphere, which was reduced to metallic iron in H2
21
atmosphere, resulting in a decrease in AFTs. The fusibility and flow properties of coal ash and slag from
22
Shell gasifier were compared and the results indicated that the AFTs and viscosity of coal ash were higher
23
than those of slag [16]. Kong et al. [17] evaluated the effect of CaCO3 on the flow properties of slag and found
[13]
; while it decreased continuously with an increase in SiO2/Al2O3 ratio
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[14]
. Song et al.
[15]
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that the slag viscosity changed notably with the addition of CaCO3 owing to the difference in formation
2
route of solid phase. Song et al. [18] explored the rheology of slag from Texaco gasifiers and obtained that the
3
slag behaved as non-Newtonian fluid below its liquidus temperature because of the increase in solid content.
4
Furthermore, the fusion behavior of coal ashes are closely related to the transformation mechanism of
5
minerals at high temperature
6
Indian coals and correlated them to the AFTs. Bai et al.
7
low-temperature ash under reducing atmosphere and found that the residence time had major influence on
8
the mineral compositions at high temperature. The amounts of crystalline phases and glassy materials in
9
coarse gasification ash were determined to predict its melting behavior
[19]
. Chakravarty et al.
[20]
analyzed the mineralogical compositions of three [21]
characterized the fusion behavior of
[22]
. van Dyk et al.
[23]
investigated
10
the mineral transformation mechanism and slagging behavior of coal resources at high temperature during
11
Sasol-Lurgi fixed bed gasification. With reference to the characteristics of the semi-char from fluidized bed
12
gasification, lots of literatures
13
However, little work has been reported concerning the fusion properties and mineral transformation
14
behavior of the semi-char at high temperature, despite the fact that these characteristics are crucial for the
15
design and operation of the melting furnace.
[24-27]
regarding its physical and chemical properties have been published.
16
In this study, the basic properties and fusion characteristics of the semi-char from an industrial circulating
17
fluidized bed (CFB) gasifier were analyzed. Meanwhile, the transformation behavior of minerals at high
18
temperature
19
thermogravimetric-differential thermal analysis (TG-DTG-DTA), X-ray diffraction (XRD) and FactSage
20
simulation. Moreover, in-situ melting and crystallization of the semi-char ash were observed by using a
21
heating stage microscopy. This work aims to get a thorough understanding of the ash fusion behavior and
22
mineral transformation mechanism of the semi-char at high temperature and provide valuable information
23
for its melting utilization.
under
oxidizing
atmosphere
was
investigated
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by
thermogravimetric-differential
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2. Experimental section
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2.1. Raw materials. The semi-char was obtained from an industrial CFB gasifier, the flowsheet of which
3
has been elaborated elsewhere [28]. The gasifier, located in Liaocheng city of Shandong province, China, was
4
established to provide clean fuel gas for alumina calcination. The feed coal is a bituminous coal, and the
5
gasifying agents are the mixture of air and steam. Generally, the gasifier operates at 950 °C under
6
atmospheric pressure and the gas-producing capacity is approximately 40,000 Nm3/h. The high-temperature
7
semi-char is separated by the cyclone separator and transported directly back into the riser via a loop seal.
8
The semi-char sample was derived from the bag dust collector, and the feed coal was collected from the coal
9
bunker. The conventional properties of the raw coal and semi-char, including proximate and ultimate
10
analyses, ash compositions and AFTs under an oxidizing atmosphere (21:79 O2/N2, volume ratio) are
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presented in Table 1.
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Table 1. Conventional properties of the raw coal and gasified semi-char.
Item
Raw coal
Semi-char
Proximate analysis (wt. %, ar) Moisture
12.48
0.31
Volatile matter
29.06
3.86
Fixed carbon
47.23
75.67
Ash
11.23
20.16
Ultimate analysis (wt. %, ar) Carbon
61.73
76.66
Hydrogen
3.27
0.58
Oxygen
9.88
0.00
Nitrogen
0.44
0.57
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Sulfur
0.98
1.72
Ash fusion temperatures (°C) DT
1255
1180
ST
1290
1250
HT
1320
1290
FT
1345
1310
SiO2
51.10
50.59
Al2O3
17.95
18.13
Fe2O3
9.26
6.80
CaO
8.93
9.36
MgO
2.13
2.30
SO3
5.35
8.70
TiO2
0.64
0.81
P2O5
1.89
0.42
K2O
1.10
0.98
Na2O
1.15
1.31
Ash compositions (wt. %)
a
Abbreviations: ar, as received basis; DT, deformation
temperature; ST, sphere temperature; HT, hemisphere temperature; FT, flow temperature. 1
2.2. Samples preparation.
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2.2.1. Preparation of the semi-char sample of various particle sizes. The particle size distribution of the
3
semi-char was analyzed by a laser diffractometer (Mastersizer 2000, Malvern, Britain). The semi-char was 7 ACS Paragon Plus Environment
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divided into five groups with different particle sizes, namely, 45 µm, via sieving method.
3
2.2.2. Ash preparation. The raw coal (less than 100 µm), gasified semi-char and the semi-char of various
4
particle sizes were ashed at 815 °C in a muffle furnace according to Chinese standard GB/T 1574-2007.
5
Briefly, the furnace was first heated from ambient temperature to 500 °C within 30 min, and then held for
6
another 30 min. Subsequently, the furnace was raised to 815 °C and remained at this temperature for
7
approximately 120 min. The resultant ash samples were preserved in a desiccator for further treatment and
8
analysis.
9
2.2.3. High temperature treatment of the semi-char ash. The semi-char ash prepared previously was retreated
10
at higher temperatures (1100 °C, 1150 °C, 1200 °C, 1250 °C, 1300 °C and 1350 °C) under an oxidizing
11
atmosphere (21:79 O2/N2, volume ratio). The schematic diagram of the horizontal tube furnace is illustrated
12
in Figure 1. The treatment procedure was designed as follows. First, the alumina crucible loaded with 1 g ash
13
samples was pushed into the reaction zone by a pushing rod. Then, the tube furnace was heated to the
14
prescribed temperature at 15 °C/min and stayed for 5 min. Consequently, the samples after treatment were
15
withdrawn from the tube furnace and quenched in ice water to prevent phase transformation and crystal
16
segregation. The quenched samples were dried at 105 °C, ground to a size of Ca>Mg>Fe>Na [34, 35]. Thus, the augmented sulfur content should be responsible for the decrease in AFTs
10
[32]
. The B value, which represents the total amounts
[33]
. The B value of the raw coal is 27.92, which increases to 29.45 for the semi-char, resulting in a
of the semi-char.
11
3.1.3. Mineral transformation during gasification. The mineralogical properties of the raw coal and
12
gasified semi-char were determined by powder XRD and the crystalline phases were identified and labeled
13
in Figure 3. Clearly, A broad hump is observed between 20° and 35° (2θ) for both samples, which is mainly
14
due to the presence of organic matter
15
(Al4(OH)8(Si4O10)), anhydrite (CaSO4) and hematite (Fe2O3). After gasification, the peak representing quartz
16
ascended, while kaolinite was not identified. This was caused by dehydroxylation of kaolinite at high
17
temperature: Al4Si4O10(OH)8(s) → 2Al2O3(s) + 4SiO2(s) + 4H2O(g)
18
amounts of oldhamite (CaS) present in the semi-char, while anhydrite vanished. This indicated that anhydrite
19
was transformed into oldhamite under reducing atmosphere in the CFB gasifier according to: CaSO4(s) +
20
4CO(g) → CaS(s) + 4CO2(g) [38].
[36]
. The main minerals in coal are quartz (SiO2), kaolinite
[37]
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. Moreover, there are considerable
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1 Raw coal Semi-char Relative intensity (CPS)
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1 5 1
2
23
10
1 1 1 51 1
4
1
1
4
1
1
1 2
20
3
3 11 1 1 1
30
40
1 50
60
70
80
2θ(°)
1 2
1-quartz (SiO2); 2-Kaolinite (Al4(OH)8(Si4O10)); 3-anhydrite (CaSO4);
3
4-hematite (Fe2O3); 5-oldhamite (CaS).
4
Figure 3. XRD patterns of the raw coal and gasified semi-char.
5
The SEM micrographs of the raw coal and gasified semi-char are displayed in Figure 4. The surface of the
6
raw coal appears clean and smooth, with few fragments stacked. Scarcely any pore structure is found. In
7
contrast, the semi-char comprises many fine, irregular particles. The surface seems rough and loose, with
8
clear pore texture being visible. The morphology differences are closely related to the drastic coal
9
devolatilization process occurred in the gasifier
10
[39]
. Particularly, no agglomerates are observed on the
surface of the semi-char.
11
Raw coal 12
Gasified semi-char
Figure 4. SEM micrographs (×500) of the raw coal and gasified semi-char. 13 ACS Paragon Plus Environment
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3.2. AFTs variation with particle size. Investigators have reported that elemental compositions of the
2
gasified semi-char were greatly affected by its particle size due to different processes the inorganic matters
3
experienced during coal gasification [27, 40]. Thus, the ash fusion properties of the semi-char might vary with
4
its particle size, causing difficulties in operation of the melting furnace. Therefore, the semi-char were
5
sieved into five groups according to its particle size distribution and the AFTs of the semi-char with various
6
particle sizes are illustrated in Figure 5. As the particle size increases, the AFTs first decrease and then
7
remain almost unchanged. The chemical compositions of the semi-char with different particle sizes were
8
determined by XRF analysis and shown in Figure 6. Clearly, the major mineral elements in the semi-char are
9
Si, Al, Ca, Fe and S. The fractions of SiO2 and SO3 increase when the particle size is less than 45 µm, and
10
then decrease; the inverse trend is observed for Al2O3 and Fe2O3. However, none of these compounds
11
exhibits the same variation tendency as the AFTs. This implies that the variation in AFTs is not controlled
12
by a certain compound. The acid/base (A/B) ratio [41], which is developed to predict the ash fusion behavior
13
of coal ashes, is defined as below:
A/B ratio=
14
SiO2 +Al2O3 +TiO2 CaO+MgO+Fe2O3 +Na 2O+K 2O+SO3
15
where the chemical formulas represent the mass fraction of the compounds in the ash. The A/B ratios of the
16
semi-char of various particle sizes are calculated and also depicted in Figure 7. Notably, the variation trend
17
of A/B ratio is similar to that of the AFTs. It has been acknowledged that the AFTs of coal ashes increase as
18
the A/B ratio increases
19
particle sizes.
[41]
, which explains the variation trend of the AFTs of the semi-char of various
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1320
DT HT
ST FT
Temperature (°C)
1280
1240
1200
45
Particle size (µm)
1 2
Figure 5. The variations of AFTs with particle size. 60 45µm
50
Ash composition (%)
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Energy & Fuels
40
30
20
10
0 Na2O MgO Al2O3 SiO2 P2O5
SO3
K2O
CaO
TiO2 Fe2O3
3 4
Figure 6. The variations of ash compositions with particle size.
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2.74 2.72 2.70
A/B ratio (%/%)
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2.68 2.66 2.64 2.62 2.60 45
Particle size (µm)
1
Figure 7. The variations of A/B ratio with particle size.
2 3
Despite the fact that the AFTs of the semi-char vary with its particle size, the disparity is not remarkable,
4
e.g., the FTs falls between 1285 °C and 1320 °C. Li et al. [27] evaluated the influence of particle sizes on the
5
AFTs of Shenmu and Jincheng fine chars separated from a pilot-scale ash-agglomerated fluidized bed (AFB)
6
gasifier. The FTs vary within the range of 1220-1310 °C and 1380-1500 °C for Shenmu and Jincheng
7
semi-char, respectively. The temperature intervals (90 °C and 120 °C) are noticeably larger than our results
8
(35 °C). This is directly relevant to the variations of chemical compositions of the semi-char with different
9
particle sizes and further attributable to the type and structure of reactors and operational parameters of the
10
gasifier [42]. In our study, the gasifier is a CFB reactor with a superficial velocity of about 4 m/s. The larger
11
particles are mostly collected by the cyclone separator and delivered into the riser for multi-circulated
12
gasification. As a result, the semi-chars entrained out by crude gas are of ultrafine particle size characterized
13
by a narrow distribution (Figure 2); thus, the elemental contents of the semi-char differ slightly with its
14
particle size. Accordingly, the discrepancy of the AFTs is relatively small. In contrast, the AFB gasifier
15
emerges as optimization and upgrading of bubbling fluidized bed reactor and the superficial velocity is low
16
(
