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Experimental study on the gasification characteristics and sodium transformation behavior of high-sodium Zhundong coal Yukui Zhang, and Haixia Zhang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017
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Experimental study on the gasification characteristics and sodium transformation behavior of high-sodium Zhundong coal
Yukui Zhanga,b, Haixia Zhang*,a a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China b
University of Chinese Academy of Sciences, Beijing 100049, China
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ABSTRACT
Zhundong coalfield is a newly discovered super-huge coal deposit in China. Zhundong coal is considered as a potential resource for gasification, due to its high reactivity. However, the high sodium content in Zhundong coal could cause serious ash-related problems during its thermal utilization. This study was conducted to investigate the gasification characteristics and sodium transformation behavior of a typical high-sodium Zhundong coal. The gasification performance of Zhundong coal is greatly improved at elevated temperature, which is verified both by thermogravimetric analysis and bench-scale study in a bubbling fluidized bed reactor. Compared to Shigouyi coal, Zhundong coal shows better gasification reactivity at the same temperature, indicating better adaptability of Zhundong coal to fluidized bed gasification. Sodium in Zhundong coal chiefly exists as water-soluble form. The temperature is a crucial parameter affecting the release and transformation of sodium. With increasing temperature, more sodium is released to gas phase, primarily in the form of NaCl(g). Sodium in the gasification fly ashes mainly exists as NaCl and a small amount of NaAlSiO4 is also identified, whereas the major crystalline phase of sodium in the bottom ash is NaAlSiO4. The formation of high melting point sodium containing compound NaAlSiO4 is beneficial for the mitigation of bed agglomeration problems during high sodium coal gasification.
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1. Introduction Coal has dominated the primary energy mix of China for the past few decades, and will still be the fundamental resources in the foreseeable future.
[1]
Zhundong coalfield, located in Eastern Junggar Basin,
Xinjiang Province, Northwest China, is newly explored with estimated reserves amounting to 164Gt.[2] Due to its super-huge reserves, low-cost mining, high reactivity and low contents of ash and sulfur, the utilization of Zhundong coal currently attracts growing concern.[3, 4] Unfortunately, a series of ash-related problems occur during thermal conversion of Zhundong coal, e.g., fouling, ash depositing and bed agglomeration. It is reported that these problems are attributable to the high content of sodium in Zhundong coal. [5-7] Therefore, it appears imperative to get a clear understanding of the release behavior and transformation mechanism of alkalis for the clean and efficient utilization of Zhundong coal. Recently, research on Zhundong coal has become a hot spot in China and some valuable investigations have been published. Zhang et al.[8] explored the release characteristic of sodium during Zhundong coal ashing. Their results showed that the increase in ashing temperature largely promoted the volatile ability of sodium and chlorine. Yang et al.[9] obtained that the volatilization of sodium during the ashing process mainly attributed to water-soluble sodium, and the contents of acid-soluble and insoluble sodium in ash showed little change with treating temperature. Li et al.[10] investigated the release and transformation characteristic of sodium during Zhundong coal combustion in a lab-scale reactor. The results showed that sodium mainly volatilized as NaCl(g) between 600 and 800°C, and the ash deposition tendency was closely related to occurrence modes of alkali metals. Wang et al.[11] reported that the water-soluble sodium mostly volatilized as gaseous form during Zhundong coal pyrolysis and the retained sodium was partly converted into insoluble form. Wang et al.[6] investigated the transformation characteristic of sodium in a 350Mw industrial boiler and a fixed bed reactor, and brought out the ash depositing mechanism during Zhundong coal combustion. The ash experienced three different evaporating stages with elevated temperature. The 3 ACS Paragon Plus Environment
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released sodium generally took the form of atom, oxide, and chloride. Meanwhile, when the temperature was raised up to 1000°C, part of the alkali and alkaline earth metals (AAEMs) were found to release as gaseous sulfates. Such research clearly indicates that the release of sodium and its transformation among different occurrence modes are enormously affected by the operating temperature during coal thermal conversion. The reacting atmosphere is another significant factor influencing the transformation behavior of AAEMs. Wang et al.[11] found that the release of sodium and its transformation to insoluble form was prevented by CO2, in comparison to that in N2. Song et al.[12] compared the evolution characteristic of sodium during circulating fluidized bed (CFB) combustion/gasification of Zhundong coal. Their results showed that a larger amount of sodium was released into gas phase during combustion, and the transformation process of sodium differed significantly from gasification. Song et al.[13] also found that oxygen-enriched air gasification with water steam could be beneficial for mitigating the slagging and bed agglomeration problems in a CFB gasifier. Zhou et al.[14] evaluated the sintering characteristic of Zhundong coal ash at different combustion temperatures under O2/CO2 atmosphere. They revealed that oxy-fuel atmosphere did not alter the existence forms of mineral matters, but did change the relative content of mineral phases in the ash dramatically. The addition of additives, e.g. kaolin and silica, is considered an effective way to relieve the ash-related problems during thermal conversion of coals containing high amounts of alkali metals. Dai et al.[15] investigated the transformation characteristic of sodium during Zhundong coal combustion using silica as additives in a 30 MWth pulverized coal-fired boiler. Their results showed that the silica additive was beneficial for capturing AAEMs, and the fouling and slagging problems were effectively controlled. Yang et al.[9] also confirmed that silica additives exhibited an effective function in sodium capturing at high temperature. Kosiminski et al.[16] found that sodium disilicate (Na2Si2O5) was the major reaction product of 4 ACS Paragon Plus Environment
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sodium and silica under N2, H2O and CO2 atmosphere. Xu et al.[17] reported that kaolin could react with NaCl to form high melting aluminosilicate, such as nepheline and albite, and the nepheline-forming reaction dominated the reaction mechanism. Kosiminski et al.[18] also found that sodium aluminosilicate, principally nepheline (Na2O·Al2O3·2SiO2), was formed during reactions of sodium and kaolin. Ge et al.[19] applied chemical looping combustion (CLC) technique to the utilization of Zhundong coal, and investigated the sodium transformation characteristic when hematite was used as oxygen carrier. Their results also indicated that high melting aluminosilicate, e.g., Na2O·Al2O3·6SiO2, rather than low melting temperature sodium silicate was found in the CLC fly ash. The formation of solid aluminosilicate should help mitigate the problems associated with slagging and bed agglomeration in a fluidized bed. Fluidized bed gasification is a promising gas-producing technology owing to its wide fuel adaptability, lower operational cost and high environmental performance.[20-22] Either air, steam, pure oxygen or a mixture of them could be employed as the gasifying agents. Air-blown gasification, providing low-calorific fuel gases with heating value of 4~7 MJ/Nm3, is most widely adopted in application, because of its low-cost availability and simpler operation.[23] Zhundong coal, as a low-rank bituminous coal with high reactivity, has been considered a potential resource for fluidized bed gasification. Moreover, the catalytic effects of AAEMs on coal gasification have been extensively authenticated.[24] However, most previous works have concentrated on the combustion utilization of Zhundong coal,[10, 15, 25] with practical operation experience of gasification being relatively poor. Despite the high reactivity of Zhundong coal has been acknowledged, studies on its gasification characteristics have rarely been carried out. Furthermore, the release and transformation behavior, when Zhundong coal gasification was implemented in a fluidized bed, has scarcely been reported. In this paper, the gasification performance of a typical Zhundong coal was evaluated by thermogravimetric analysis and bench-scale study in a bubbling fluidized bed (BFB) reactor. Meanwhile, the occurrence modes 5 ACS Paragon Plus Environment
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of sodium in the coal were clarified and the release behavior of sodium at different temperatures was investigated. Moreover, the transformation mechanism of sodium in the gasification fly ashes and bottom ash was discussed. 2. Experiments 2.1. Fuel characteristics. In this study, a high-sodium Zhundong coal, obtained from Junggar Basin in Xinjiang, was chosen as feed material. The coal was crushed and sieved to less than 2.5 mm for BFB gasification. The mean diameter of the coal is 0.77 mm, and the particle size distribution is shown in Figure 1. The proximate and ultimate analyses, ash compositions and ash fusion temperatures are presented in Table 1. Shigouyi coal has been adopted as feedstocks for industrial fluidized bed gasifiers in China. For comparison, the properties of Shigouyi coal are also listed in Table 1. Zhundong coal is a low-rank bituminous coal, while Shigouyi coal is low-grade lignite. The volatile content in Zhundong coal is 34.08%, which is lower than that of Shigouyi coal (40.16%). Thus, the coalification degree of Zhundong coal is higher than that of Shigouyi coal. The sodium content of Zhundong coal is fairly high, with the mass fraction of Na2O up to 8.53%. Table 1. Properties of Zhundong and Shigouyi coal. Zhundong coal
Shigouyi coal
Moisture
15.64
5.9
Volatile matter
34.08
40.16
Fixed carbon
45.27
42.06
5.03
11.88
54.41
49.96
1.70
3.37
Oxygen
22.13
27.08
Nitrogen
0.69
0.75
Item Proximate analysis (wt. %, ad)
Ash Ultimate analysis (wt. %, ad) Carbon Hydrogen
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Sulfur
0.40
1.06
16.63
15.50
DT
1310
1340
ST
1320
1360
HT
1340
1390
FT
1350
1410
SiO2
17.24
52.95
Al2O3
11.90
21.4
Fe2O3
5.76
9.36
CaO
28.74
4.28
MgO
5.34
2.06
SO3
19.58
3.55
TiO2
0.60
0.94
P2O5
0.05
0.26
K2O
0.38
2.63
Na2O
8.53
1.42
Lower heating value (MJ/kg, ad) Ash fusion temperatures (°C)
Ash compositions (wt. %)
Note: ad, as air dried basis; DT, deformation temperature; ST, softening temperature; HT, Hemispherical temperature; FT, flowing temperature. 100
Zhundong coal Cumularative mass fraction (%)
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80
60
40
20
0 0.1
1
Particle size (mm)
Figure 1. Particle size distribution of Zhundong coal.
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2.2. Thermogravimetric analysis. The rapid pyrolyzed char was prepared in an electrical tube reactor. The configuration of the pyrolysis system and the experimental procedure have been depicted elaborately in our previous work.[26] The gasification reactivity measurement of the derived char was conducted by isothermal analyses in a NETZSCH STA-449 F3 Jupiter® analyzer. For each run, about 10 mg of samples, sieved to less than 100 µm, was loaded in an alumina crucible. The samples were first heated at 20°C/min to reach the preset temperature, and were retained for five minutes with an Ar flow of 150 mL/min. Next, the atmosphere was substituted with a binary flow of Ar at 20 mL/min and CO2 at 130 mL/min. The sample was then subjected to CO2 gasification at constant temperatures for an hour. The gasification temperatures were prescribed at 850, 900, 950, and 1000°C. The carbon conversion (x) and reaction rate (r) were obtained by the following equations: m0 -m m0 ⋅ (1 − Ad − Vd )
(1)
1 dm dx ⋅ = m0 ⋅ (1 − Ad − Vd ) dt dt
(2)
x=
r=
where m0 and m is the initial and instantaneous mass of the sample, Ad and Vd are the ash and volatile content in the sample, t is the reaction time. 2.3. Bench-scale BFB apparatus: description and operating parameters. Figure 2 illustrates the schematic diagram of the bench-scale BFB gasifier, where the gasification tests were conducted. The internal diameter and height of the riser are 100mm and 1750 mm, respectively. A screw feeder is equipped to introduce the coal into the gasifier. The feed port is situated at approximately 230 mm above the air distributor. Air is supplied by an air compressor and employed as the gasifying and fluidizing agent. The bed temperature and pressure are monitored by K-type thermocouples and pressure transducers, respectively. A sampling port is located close to the outlet of the BFB furnace, allowing for the measurement of fuel gas compositions. 8 ACS Paragon Plus Environment
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10
3
1 2
5
8 6
9
7
Figure 2. Schematic diagram of the bench-scale BFB apparatus 1, gasifier; 2, electric heater; 3, primary cyclone separator; 4, secondary cyclone separator; 5, screw feeder; 6, air chamber; 7, air compressor; 8, N2 cylinder; 9, mass flowmeter; 10, gas sampling port. Before the experiments, 4 kg of quartz sand (0.5~2.5 mm) was added into the furnace. The dense phase region was preheated to over 500°C by the auxiliary heater. Then, coal was fed in to raise the reactor to the prescribed level. Finally, the air/coal ratio was regulated to switch the running state from combustion to gasification. The operating temperatures were set at 850, 900, 950 and 1000°C. For all the cases studied, the coal feeding rate is 1.8 kg/h, and the air/coal ratio is set at 1.8 m3/kg. The corresponding O/C molar ratio is proximately 1.05. 2.4. Sampling and analysis methods. The fuel gases were analyzed by an on-line INFICON 3000 micro GC (gas chromatograph), and the fractions of H2, CO, CO2 and CH4 were determined. The gas heating value Qgas,net was calculated by
Q gas, net = X H 2 × 10.78 + X CO × 12.63 + X CH 4 × 35.86 (MJ/m 3 )
(3)
where XH2, XCO and XCH4 are the volume fraction of H2, CO and CH4 in the fuel gas, respectively. The 9 ACS Paragon Plus Environment
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gasification fly ashes were collected using a tank sampler seated at the bottom of the primary cyclone separator. The bottom ash was discharged from bottom of the gasifier, when the BFB system cooled down to ambient temperature. Inductively coupled plasma-optical emission spectrometer (ICP-OES) (710-ES, Varian, USA) was used to determine the amount of various chemical forms of sodium in the raw coal and the total sodium in the gasification fly ashes. X-ray fluorescence (XRF) (XRF-1800, Shimadu, Japan) was selected to analyze the elemental compositions of the samples. The qualitative analyses of crystallized phases were performed by powder X-ray diffraction (XRD) (D8 Advanced, Bruker, Germany). Before the XRF and XRD tests, all the samples were subject to ashing at 575°C, to burn out the residual carbon. Scanning electron microscopy (SEM) (S-4300, Hitachi, Japan) was applied to examine the morphology of the samples, assembled with an energy dispersive X-ray spectroscopy (EDX) to determine the surface elements. The particle size distributions of gasification fly ashes were analyzed by a laser diffraction meter (Mastersizer 2000, Malvern, Britain). 3. Results and Discussion 3.1. Thermogravimetric analysis. 3.1.1 Isothermal gasification behavior. The curves of carbon conversion (x) to reaction time (t) and gasification reaction rate (dx/dt) to carbon conversion (x) are presented in Figure 3(a) and (b), respectively. Figure 3(a) clearly indicates that the chemical reactivity of the Zhundong coal char is significantly enhanced at higher temperature. As the temperature rises from 850°C to 1000°C, the reaction time required to reach a carbon conversion of 0.9 decreases from 19.9 min to 3.3 min. In Figure 3(b), the reaction rate initially increases with the carbon conversion, reaching a maximum at a conversion rate less than 15%, and then decreases gradually. This behavior might attribute to the variations in microstructures and activation sites of the char during gasification. As the reaction was initiated, the originally blocked pores opened, followed by the widening of 10 ACS Paragon Plus Environment
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the micro pores,[27] resulting in a sharp increase of the activated sites. With the proceeding of the reaction, the neighboring pores started to coalescence, and the macro pores collapsed, causing the progressive decrease in the surface area;[28] subsequently, the reaction rate started to drop. 1.0
0.8
850°C 900°C 950°C 1000°C
x
0.6
0.4
0.2
0.0 0
4
8
12
16
20
24
t (min)
0.4
(b)
850°C 900°C 950°C 1000°C
0.3
-1
dx/dt (min )
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0.2
0.1
0.0 0.0
0.2
0.4
0.6
0.8
1.0
x
Figure 3. Isothermal gasification reactivity of Zhundong coal char: (a) carbon conversion versus time; (b) gasification reaction rate versus carbon conversion. 3.1.2 Reactivity index and activation energy. To further evaluate the gasification reactivity of Zhundong coal at moderate temperatures, the reactivity index (Rs)[29] and activation energy (E) are discussed. The definition of reactivity index Rs was R s =
0.5 , t 0.5
where t0.5 represents the time elapsed to reach 50% carbon conversion. The higher the Rs value is, the better the gasification reactivity is. The gasification behavior of Shigouyi coal has been detailedly investigated by 11 ACS Paragon Plus Environment
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Fan et al.[30] in the temperature range of 850~1000°C. The obtained reactivity indexes of Zhundong coal char and Shigouyi coal char are compared in Figure 4. Obviously, the Rs value of both Zhundong and Shigouyi coal char increases significantly as the temperature increases. Within the temperature range studied, the reactivity indexes of Zhundong coal char are clearly higher than those of Shigouyi coal char, which reveals higher gasification reactivity. The activation energy E is deducted by using the simplified DAEM (distributed activation energy model) method[31, 32], which is defined as follows 1 − (1 − x )1− n E 1 ln t = ln − lnk 0 + R T 1 − n
(4)
where t is the reaction time, x is the carbon conversion, n is the reaction order, k0 is the frequency factor, E is the activation energy, T is the temperature, and R represents the universal gas constant. The value of E and k0 at different carbon conversions could be derived by linear fitting of lnt with 1/T. The variation of activation energy with carbon conversion of Zhundong and Shigouyi coal char[30] is illustrated in Figure 5. Clearly, the E value increases with the increasing carbon conversion for both the two chars. As has been discussed in the previous section, the surface area decreases with the reaction progressing, which leads to the decrease in activated sites; therefore, much more resistance is encountered, and the gasification reactions become more difficult to proceed. Thus, the derived activation energy increases with the increase in carbon conversion. The average activation energy of Shigouyi coal char (152.49 kJ/mol) is 1.64 times that of Zhundong coal char (92.97 kJ/mol). The relatively lower activation energy generally means less reaction resistance, which suggests that the gasification reactions of Zhundong coal might be even easier to proceed than Shigouyi coal.
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0.30 Zhundong Shigouyi
0.25
Rs(min-1)
0.20
0.15
0.10
0.05
0.00 850
900
950
1000
Temperature (°C)
Figure 4. Gasification reactivity index (Rs) of Zhundong coal char and Shigouyi coal char.[30] 200 Zhundong Shigouyi
180 160
E (kJ/mol)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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140 120 100 80 60 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
x
Figure 5. Comparison of activation energy between Zhundong and Shigouyi coal char.[30] The gasification reactivity is significantly influenced by coal rank and ash behavior. As mentioned above, the coalification degree of Zhundong coal is higher than that of Shigouyi coal. Therefore, the exhibited higher gasification reactivity of Zhundong coal probably benefits from its ash constituents, especially alkali metals. The alkali index[33], which is generally used to assess the catalytic effects of mineral matters in coal, is defined as below: Alkali index = Wash ×
Fe 2 O3 + CaO + MgO + Na 2 O + K 2 O SiO 2 + Al2 O3
(5)
where Wash is the ash content in coal and the content of the compounds in the ash was indicated by their
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chemical formulas. The alkali indexes of Zhundong and Shigouyi coal are 8.41 and 6.77, respectively. The catalyst abilities of Zhundong coal ash are much higher than Shigouyi coal. Researchers believe that the catalysts did not change the kinetic routes radically, but did promote the development of the active sites on the surfaces of the coal char particles. [34]
3.2. BFB gasification experiments. 3.2.1 Temperature profiles. The temperature profiles of the BFB gasification system along the flue gas flow direction are shown in Figure 6. The variations of the temperature profiles are caused by the reactions occurred and heat losses. As air and fuel are fed into the system from bottom of the gasifier, the coal particles undergo successive reaction processes, such as drying, pyrolysis and gasification.[35] The highest temperature occurs at the bottom of the gasifier, and the temperature drops slightly along the height of the riser in the dense phase, causing a cold spot at 580 mm above the air distribution tube. The cold spot is mainly caused by the endothermic effect of gasification. It is also noteworthy that the cold spot appears at the same height for all of the temperatures studied. This attributes to the similar gas-solid flow and reaction course under different temperatures. As the temperature increases from 850°C to 1000°C, the fluidizing velocity increases from 0.43 m/s to 0.49 m/s. The increment is so small that the state of gas-solid flow remains almost unchanged and the endothermic gasification reactions occurred at the same location in the gasifier. As a result, the cold spot emerges at the same height under different temperatures. Besides, the temperatures of the dilute phase maintain stable due to external heaters, and the temperature declines significantly as the fuel gas flows along the tail flue, caused by heat loss.
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1000
850°C 900°C 950°C 1000°C
900
Temperature (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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800 700 Sampling port 600 500 Primary cyclone separator
400 0
400
800
1200
1600
2000
Distance from the bottom of the BFB along the flue gas flow direction (mm)
Figure 6. Temperature profiles of the BFB system along the flue gas flow direction.
3.2.2 Fuel gas compositions and heat value. Figure 7 depicted the fuel gas compositions and heat values under different operating temperature. It is evident that the temperature has a pronounced effect on the gasification process. As the gasification temperature rises from 850 to 1000°C, the concentration of carbon monoxide increases from 14.68% to 19.75%, while carbon dioxide decreases from 15.56% to 12.61%. These results are presumably caused by the enhanced Boudouard reaction (C+CO2 2CO) at elevated temperatures.[36] The hydrogen concentration increases from 5.73% to 7.60%, resulting from the decomposition of higher hydrocarbon and the condensation of aromatic ring. The fraction of methane is relatively low, with a proportion of 0.50~0.59%, and remains almost unchanged within the temperature range studied. Methane is produced mainly from pyrolysis process of volatiles in the coal, which tends to complete at low temperatures, generally below 850°C. Meanwhile, the decomposition of methane, CH4 C+2H2, is an endothermic reaction, which is relatively weak below 1000°C. In addition, the heat value of the resultant fuel gas increases from 2.65 MJ/m3 to 3.52 MJ/m3 due to the increased proportion of CO and H2.
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3.8 CO CO2 CH4 H2 Heat value
16
3.6
3.4
12
3.2
8
3.0
Heat value (MJ/m3)
20
Gas compositions (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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2.8 4 2.6 0 850
900
950
1000
Temperature (°C)
Figure 7. Fuel gas compositions and heat value.
3.2.3 Carbon conversion and cold gas efficiency. The definitions of carbon conversion XC and cold gas efficiency η of the BFB system are given by the equations below: XC =
0.79Fair (X CO +X CO2 +X CH 4 ) × 12 ⋅ × 100% X N2 M C 22.4 × Car
(6)
0.79 Fair Qgas ,net ⋅ × 100% X N 2 M C Qar ,net
(7)
η=
where Fair and MC are the air flow rate (m3/h) and coal feeding rate (kg/h), Car is the mass fraction of carbon in the raw coal (wt. %, as received basis), Qgas,net is the gas heating value (MJ/m3), Qar,net is the lower heating value of the feed coal (MJ/kg), and XN2, XCO, XCO2 and XCH4 represent the volumetric fraction of N2, CO, CO2 and CH4 in the fuel gas, respectively. Jiang et al.[37] investigated the air-blown gasification characteristic of Shigouyi coal in the same BFB gasifier. The coal feeding rate was 1.8 kg/h and the air/coal ratio was 1.6 m3/kg. For comparison, the obtained results of Zhundong coal, together with the data of Shigouyi coal,[37] are both depicted in Figure 8. As is shown in Figure 8, both the carbon conversion and cold gas efficiency increase at elevated gasification temperature.[35] Meanwhile, compared to the results of Shigouyi coal, the carbon conversion and cold gas efficiency of Zhundong coal obviously occupy higher values. This might be caused by the catalytic effects of alkali metals in Zhundong coal, as has been 16 ACS Paragon Plus Environment
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discussed in the previous section. Alkali metals can decrease the gasification temperature, increase reaction rates, and increase the production of target gases by modifying the selectivity of the reactions.[38] Hence, it can be concluded that Zhundong coal would exhibit higher chemical reactivity and better adaptability than Shigouyi coal when applied to fluidized bed gasification. 80
70
45
65 40 60
Cold gas efficiency (%)
50
Zhundong Shigouyi
75
Carbon conversion (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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35
55 850
900
950
1000
Temperature (°C)
Figure 8. Carbon conversion and cold gas efficiency of Zhundong and Shigouyi coal.[37]
3.3 Sodium release behavior during gasification. 3.3.1 Sodium in Zhundong coal. The chemical forms of alkali metals have a remarkable influence on their release and transformation behavior during thermal conversion. Generally, alkali metals in coal can present in the following occurrence modes: soluble salts (e.g., NaCl), organic state (e.g., carboxylate, -COONa), acid-soluble inorganic species, and insoluble silicate or aluminosilicate (e.g., Na-Al-Si). Not all occurrence modes of alkalis would give rise to ash-related problems. As a matter of fact, the water-soluble and organic alkali species are generally recognized as most harmful.[39] Herewith, it appears crucial to firstly clarify the occurrence modes of sodium in Zhundong coal. In this study, a three-step sequential chemical extraction method of leaching with H2O, CH3COONH4 and HCl solutions was employed to determine the chemical forms of sodium, quantifying in terms of “H2O-soluble”, “NH4Ac-soluble”, “HCl-soluble” and “insoluble”, respectively.[40, 41] Sodium in Zhundong coal mainly exists as water-soluble form, taking up 71.27% of the 17 ACS Paragon Plus Environment
Energy & Fuels
total sodium. The fractions of NH4Ac-soluble and HCl-soluble sodium account for 11.58% and 12.69%, respectively. The sodium present as silicate or aluminosilicate is only 4.45%. The crystalline phases in the coal ash are illustrated in Figure 9. Clearly, large quantities of CaSO4, CaCO3, CaO and SiO2 are observed. The results are consistent with Zhundong coal ash compositions listed in Table 1, which is rich in calcium, sulfur and silica. Sodium mainly exists as NaCl in the ash (ashed at 575°C). 1 1000
Intensity (Counts)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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800
1
4
600
4
400
7
5 36 3 5
2
1 1 2
5 1
3
8
1
200
0 10
20
30
40
50
60
70
2θ(°)
Figure 9. XRD patterns of Zhundong coal ash: 1-CaSO4; 2-CaCO3; 3-CaO; 4-SiO2; 5-NaCl; 6-Al2Ca3(SiO4)3; 7-Mg2SiO4; 8-Fe2O3.
3.3.2. Sodium release with gasification temperature. In order to evaluate the release behavior of sodium, the retention ratio (β, wt. %) is used, which is defined by the following equation
β (%)=
m Nat × 100 mNa 0
(8)
where mNa0 and mNat represent the content of sodium in the raw coal and gasification fly ashes, respectively. The variations of sodium retention ratio in the gasification fly ashes are shown in Figure 10. It can be seen that sodium in the ash decreases with an increase in temperature, indicating that more sodium volatilizes into the gas phase. The temperature is a crucial parameter influencing the release behavior of sodium during Zhundong coal gasification. When the temperature is 1000°C, only 17.86% of the total sodium in coal is retained in the gasification fly ash. This implies that most of the water-soluble and NH4Ac-soluble sodium 18 ACS Paragon Plus Environment
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volatilizes as gaseous form. Similar conclusions have been obtained in previous studies. Song et al.[42] found that about 40~65% of the sodium in Zhundong coal released within the temperature from 850°C to 1000°C during circulating fluidized bed gasification. Wei et al.[43] and Sugawara et al.[44] also reported the increasing trend of sodium volatile ability at higher gasification temperatures. The gaseous sodium released has a significant influence on ash deposition and is considered as the major reason for the severe ash-related problems.
60
Retention ratio of sodium (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
50
40
30
20
10 850
900
950
1000
Temperature (°C)
Figure 10. Retention ratio of sodium in the gasification fly ashes versus gasification temperature.
3.4 Sodium transformation in the gasification fly ashes. 3.4.1. Crystalline phase analyses. The mineralogical analyses of the samples were performed by powder XRD, and the results of crystalline phase analyses are given in Figure 11. Unlike Zhundong coal ash, calcium in the gasification fly ashes mainly exists as calcium sulfate (CaSO4) and calcium oxide (CaO), while calcite (CaCO3) is not observed. This is due to the decomposition of CaCO3 at high temperature and calcium is transformed into ((CaO)12(Al2O3)7). Sodium mainly takes the form of NaCl, and minor amount of NaAlSiO4 is also detected. With the increase in temperature, the peak intensity of NaCl decreases, indicating that more sodium volatilizes into gas phase. Under reducing atmosphere, the chemcial equilibrium of NaCl(s) and NaCl(g) is 19 ACS Paragon Plus Environment
Energy & Fuels
considered the major transformation mechanism of sodium.[45,
46]
When the temperature increases, the
equilibrium shifts toward the formation of NaCl(g), and more sodium is released, coincide with the results in Figure 10. A similar result was obtained by Marc et al.[47] in a lab-scale tube furnace under gasification condition. The identification of the crystalline phases NaAlSiO4 suggests that small amounts of sodium could interact with Al and Si and form sodium aluminosilicate according to Na 2 O + Al 2 O 3 ⋅ 2SiO 2 ⋅ 2H 2 O → 2NaAlSiO 4 + 2H 2 O
(9)
This means that part of the sodium would be retained in the solid phase and the amount of gaseous sodium would be decreased. This is beneficial for the mitigation of the ash depositing in the flue duct, but the catalytic effect of sodium on coal gasification would inevitably be reduced. 3000
1
2000
4
1000
Intensity (CPS)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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850°C 3
6
1
2000
4
1000
1
75
0 3000
12 1 1 3
4
1
2 1
8
900°C 1
75
6
0 3000
6
3
6
121 1
3 4
1
2 1
8
1
950°C
2000 1000
6
4 3
75
0 3000
1
6
1
21 1 3 4
1
2 1
8
1
1000°C
2000 1000 0 10
6
75
20
2
1
4 3
6
1
30
1 1
40
3 4
2 1
1
50
8
60
70
2θ (°)
Figure 11. XRD patterns of the gasification fly ashes: 1-CaSO4; 2-CaO; 3-SiO2; 4-NaCl; 5-Mg2SiO4; 6-(CaO)12(Al2O3)7; 7-NaAlSiO4; 8-Fe2O3.
3.4.2. Elemental compositions. The ash compositions of fly ashes analyzed by XRF are illustrated in Figure 12. The sodium content in the fly ashes reduces considerably with increasing temperature, which is consistent with the results obtained by ICP-OES and XRD analysis. The calcium content increases obviously with increasing temperature, which is due to its weak volatility. The contents of Mg, Fe and Al remain almost unchanged. The sulfur content 20 ACS Paragon Plus Environment
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shows an decreasing trend as the temperature is elevated from 850°C to 950°C, and then declines slightly when further increases to 1000°C, which might be caused by the decomposition of CaSO4 at relatively higher temperature. The release and transformation of alkali metals is strongly affected by the evolution behavior of chlorine. Sodium present as NaCl in coal could vaporize into gas phase at substantially lower temperature. The volatized sodium still takes the form of NaCl(g), which tends to deposit and react with other compounds, e.g., Al2O3, Fe2O3 and SO2, to form adhesive film on the surface of heat-exchangers downstream.[48] In addition to NaCl(g), chlorine could also vaporize in the form of HCl(g). Molar ratios of Na/Cl released are considered an effective parameter to get more information about the release and transformation behavior of sodium.[49] Therefore, the augmented amount of the released sodium and chlorine was calculated, and the results are depicted in Figure 13. The increment of sodium content is slightly lower than that of Cl, when the temperature rises from 850°C to 900°C. This indicated that not all chlorine was released in the form of NaCl(g), minor fractions of Cl was probably vaporized as HCl(g). As the temperature is further raised to 950 and 1000°C, the increment of sodium content is notably higher than that of Cl, which implies that NaCl(g) is not the only form released. Sodium could vaporize into gas phase as other forms during gasification, especially in the temperature range of 950~1000°C. Some researchers[50] believed that atom-sodium(g) was another form discharged from coal structure, which might result from the reducing free-radical species in the reaction atmosphere. Unlike water-soluble sodium, organic sodium, which is attached to the coal structure, is more stable and less volatile. Therefore, it is likely that only minor amount of organic sodium has been released to the gas phase below 900°C. When the temperature was raised to 950°C and 1000°C, the organic sodium could be released via the following mechanism.[50] (-COONa) + C → (-CNa) + CO2
(10)
(-CNa) → Na(g)
(11)
21 ACS Paragon Plus Environment
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40
850°C 900°C 950°C 1000°C
35
Elemental content (%)
30 25 20 15 10 5 0 Na
K
Ca
Mg
Fe
Al
Si
S
Cl
Figure 12. Elemental compositions of the gasification fly ashes. 3.0 Sodium Chlorine
2.5
2.0 Elemental content (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1.5
1.0
0.5
0.0 850
900
950
1000
Temperarure (°C)
Figure 13. Augmented amount of released sodium and chlorine calculated on the basis of 850°C.
3.4.3. Morphology and microstructures. The SEM micrographs of the gasification fly ashes are shown in Figure 14. Clearly, more fine particles emerge with the increasing temperature. The surface of fly ash at 850°C appears clean and smooth. At 900°C, the surface becomes rough and loose, with few particles attached. Some amounts of small pieces are seen to adhere to the surface at 950°C. At 1000°C, the surface of the fly ash is covered with fine particles. It is noteworthy that no obvious melting compounds are observed in any of the samples. Table 2 presents the 22 ACS Paragon Plus Environment
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particle sizes of the gasification fly ashes. The d50 value of the fly ashes decreases from 63.1µm to 45.0µm, when the temperature is raised from 850°C to 1000°C. As the gasification temperature increases, the fluidizing velocity increases. More fine ash particles are collected owing to the enhanced separation efficiency of the cyclone.
(a) at 850°C
(b) at 900°C
(c) at 950°C
(d) at 1000°C
Figure 14. SEM micrographs (×5000) of the gasification fly ashes. Table 2. Particle sizes of the gasification fly ashes. Temperature
d10
d50
d90
°C
µm
µm
µm
850
12.7
63.1
211.1
900
12.2
56.3
199.4
950
11.8
51.6
200.1
1000
11.4
45.0
163.0
3.5 Analysis of bottom ash. 23 ACS Paragon Plus Environment
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The high sodium content in coal is widely considered as the main reason for agglomeration in fluidized bed during thermal conversion. However, the temperature and pressure were stable in these experiments studied, and no agglomeration or defluidization phenomena were observed. As a representative, the bottom ash after gasification at 1000°C was selected for detail analysis. The microstructures and element contents determined by SEM-EDX analysis are shown in Figure 15. As a contrast, the SEM micrograph of Zhundong coal is also listed below. While the surface of the raw coal seems clean and smooth, with scarce pore textures being observed, the surface of bottom ash is noticeably more rough and incompact, stacked with fine fragments. This difference was inevitably related to the drastic devolatilization process occurred in the gasifier.[35] Similar to the gasification fly ashes, no agglomerates were observed on the surface of the bottom ash. As is shown by EDX analysis, a large amount of Na, Ca, Mg, Fe, Al, Si and S was found on the surface of the bottom ash. However, in contrast to the high Na content, no Cl is detected, which indicates that little Na in the bottom ash exists as NaCl.
(a) Zhundong coal
(b) Bottom ash
Elements
Na
K
Ca
Mg
Fe
Al
Si
S
Wt. %
5.2
1.1
14.5
4.3
24.9
9.3
17.5
3.9
Figure 15. SEM micrographs (×5000) and elemental compositions of bottom ash. To acquire the existence forms of elements in the bottom ash, XRD analysis was applied after ashed at 575°C. The results of crystalline phase analyses are presented in Figure 16. It can be seen that calcium sulfate (CaSO4) and quartz (SiO2) take up the main constituents of bottom ash. Sodium mainly present as 24 ACS Paragon Plus Environment
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NaAlSiO4 in the bottom ash, and NaCl is not identified, which is consistent with the EDX results. Compared to the gasification fly ashes, the diffraction peaks of NaAlSiO4 are much higher, suggesting that more NaAlSiO4 is formed in bottom ash. This indicates that the content of sodium aluminosilicate in bottom ash is enriched, which might be caused by the transformation of sodium from sodium chloride to sodium aluminosilicate. Zhang et al.[8] also observed the conversion of sodium chloride to sodium aluminosilicate during Zhundong coal ashing. The melting point of NaAlSiO4 is high, taking a value of about 1550°C, which is beneficial for relieving the problems of bed agglomeration and defluidization. Zhang et al.[51] investigated the agglomeration tendency of the same Zhundong coal using the ternary phase diagram of Na2O-SiO2-Al2O3. Their results showed that the Zhundong coal ash was located in a high melting temperature zone (carnegiete region), and agglomeration did not easily happen. This agrees well with our experimental results that no agglomeration occurred in any case studied. 1
2000
Intensity (Counts)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1500
3 1000
500
0 10
1 34 3
20
5 5
30
1 1 3
1
1 2
40
6
50
1
5
5
60
70
2θ(°)
Figure 16. XRD patterns of the bottom ash: 1-CaSO4; 2-CaO; 3-SiO2; 4-NaAlSiO4; 5-MgFeAlO4; 6-Fe2O3. 4. Conclusion In this paper, the gasification characteristics of Zhundong coal were studied by thermogravimetric analysis and bench-scale study in a bubbling fluidized bed reactor. Meanwhile, the release and transformation behavior of sodium during fluidized bed gasification was also investigated. The major conclusions are as 25 ACS Paragon Plus Environment
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follows: (1). The increased temperature greatly favors the gasification performance of Zhundong coal. Compared to Shigouyi coal char, the reactivity index Rs of Zhundong coal char is higher, and the activation energy E is lower, which indicates much higher chemical reactivity of Zhundong coal. (2). The concentrations of CO and H2, and the heat value of fuel gas are significantly influenced by operating temperature. Zhundong coal exhibits higher carbon conversion and cold gas efficiency than Shigouyi coal at the same temperature, which indicates its better adaptability to fluidized bed gasification. (3). Sodium in Zhundong coal mainly takes the form of water-soluble form. NaCl is the major crystalline phase of sodium in the coal ash. The operating temperature plays a dominant role in the volatilization process of sodium during fluidized bed gasification. While the release behavior of sodium is intimately associated with the content of chlorine, NaCl(g) is not the only form sodium released. (4). No agglomeration was observed for gasification fly ashes and bottom ash. While most of sodium in the gasification fly ashes exists in the form of NaCl, and small amount of NaAlSiO4 is also identified; sodium in the bottom ash mainly presents as NaAlSiO4. The formation of high melting point sodium-containing compound NaAlSiO4 is essential to inhibit the bed agglomeration and defluidization problems.
AUTHOR INFORMATION Corresponding Author *Telephone: +86-010-82543191, Email:
[email protected].
ACKNOWLEDGMENT 26 ACS Paragon Plus Environment
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The authors gratefully acknowledge the supports of the National Natural Science Foundation of China (No. 21306193).
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