Article pubs.acs.org/EF
Transformation Characteristics of Sodium of Zhundong Coal Combustion/Gasification in Circulating Fluidized Bed Guoliang Song,*,† Weijian Song,†,‡ Xiaobin Qi,†,‡ and Qinggang Lu† †
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
‡
ABSTRACT: In this study, the transformation characteristics of sodium in Zhundong coal were investigated, following its combustion (oxidizing atmosphere) and gasification (reducing atmosphere) in a circulating fluidized bed (CFB) experimental system of 0.25 t/d high alkali coal thermochemical conversion. The equilibrium distribution of Na is predicted under the oxidizing and reducing atmospheres by the thermodynamic equilibrium calculation. Na in bottom ash and fly ash evidently decreases as the temperature increases in both the combustion and gasification processes. More Na is retained in the ash during the gasification process. Na exists as sodium aluminosilicate in both combustion bottom ash and gasification bottom ash, and mainly as sodium sulfate (Na2SO4) in combustion fly ash and sodium chloride (NaCl) in gasification fly ash. Na undergoes different transformation processes through combustion and gasification. Gaseous metallic sodium, Na2O and NaCl are released from the coal and condense directly on the cold tube surface during gasification. When SO2 is present, Na reacts with it, forming a large amount of gaseous sodium sulfates during combustion. results show that the heat flux declines with ash deposit growth. Li et al.3 investigated the fine particulate formation and ash deposition during Zhundong lignite combustion in a downfired furnace. Zhundong lignite presents the higher deposition propensity and produces more fine particles, compared to other fuels. Wang et al.8 investigated the ash deposition mechanism by burning Zhundong coal in a full-scale boiler (350MW) and by condensing ash evaporating in a fixed bed reactor. The condensation and deposition of Na2SO4 and Ca2SO4 play an important role on ash depositing on convection heating surfaces. Dai et al.9−11 researched the properties of ash deposits during the combustion of Zhundong coal mixed with silica additive in a 30 MW pulverized-fuel boiler. The results indicate that adding silica (>4 wt %) is essential in promoting the agglomeration of ash. Such research mainly focuses on the Zhundong coal combustion process, while it lacks the occurrence of the gasification process. The results carried out under pulverized coal fired systems may not be applicable to CFBs, as CFBs are operating at lower temperatures. Similar to Zhundong coal, the Victorian brown coal in Australia is also characterized by its high Na content. Many efforts have been made to investigate the transformation of Na and the ways to control the slagging and fouling problems. Li et al.12−17 investigated the volatilization and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Kosminski et al.18−20 studied the transformation of sodium during the gasification of South Australian lignite, and confirmed the reaction of sodium chloride with hydrogen from coal carboxylic acid groups during gasification. They also investigated the reaction between sodium and silica or kaolin, and they found that liquid sodium
1. INTRODUCTION The consumption of coal in China has increased rapidly in the past decade and it continues to increase during this decade.1 A significantly large coalfield, named Zhundong, has been found east of Junggar Basin in Xinjiang, China. It is estimated that the Zhundong coalfield contains more than 390 billion tons of lignite.2 Zhundong coal is a promising fuel for power plants due to its high volatility and low of ash and sulfur (S) content. However, there is a series of issues associated with coal utilization in Zhundong, such as severe slagging, fouling, and ash deposition inside both the furnaces and the convective passes. The high contents of alkali and alkaline earth metal elements, especially Na in the Zhundong coal, are the main sources for these issues.3,4 Circulating fluidized bed (CFB) combustion/gasification is a promising choice for highly efficient and clean utilization of coal. The CFBs are operated at lower temperatures (about 850−950 °C) and different atmospheres (oxidizing or reducing) compared to pulverized coal fired systems. The transformation of inorganic matter in coal is influenced by the temperatures and reacting atmosphere. Hence, the research work carried out under pulverized coal fired systems may not be applicable to CFBs and it is critically important to study Zhundong coal in CFBs. Some recent investigations have been carried out on Zhundong coal. Li et al.5 used a laboratory-scale tube-quartz reactor to investigate the release and transformation of sodium during combustion, the results show that chlorine has a significant influence on the transformation of sodium, with NaCl (g) being the main form of sodium volatized from coal between 600 and 800 °C. Wang et al.6 compared the conversion of sodium in CO2 and in N2, and concluded that the release of sodium and its conversion from water-soluble form to insoluble form in CO2 would be inhibited compared to N2. Zhou et al.7 measured the effective heat conductivity of ash deposit in a pilot plant, which is burning Zhundong coal. The © 2016 American Chemical Society
Received: January 5, 2016 Revised: March 9, 2016 Published: March 30, 2016 3473
DOI: 10.1021/acs.energyfuels.6b00028 Energy Fuels 2016, 30, 3473−3478
Article
Energy & Fuels silicates are major reaction products of sodium and silica, which should be considered as a potential source for the agglomeration and defluidization during the fluidized bed gasification of coal. Aluminosilicates are the main reaction product of sodium with kaolin, the formation of solid aluminosilicates should avoid the formation of liquid silicates, as well as the issues of agglomeration and defluidization. Some additives, such as kaolin and dolomite, were introduced to control defluidization during fluidized bed combustion for the high- sodium low-rank coal. Wang et al.21 investigated the fly ash deposition of a bituminous coal under both oxy-fuel and air combustion conditions in a bench-scale fluidized bed. The results show that the most significant deposition propensity appears under a 21% O2/79% CO2 atmosphere, and only a small difference is observed between 30% O2/70% CO2 and air. To date, there are a serial of studies on alkali metal behaviors during the thermal conversion of biomass, focusing mainly on the transformation characteristics of potassium.22−26 Alkali mental in biomass shows catalytic behavior26−30 and can increase the gasification reaction rate, meanwhile induces slagging and agglomeration during the utilization of biomass. All these investigations provide numerous references for further investigation on Zhundong coal. In this study, a CFB experimental system of 0.25 t/d was applied to investigate the transformation of sodium in Zhundong coal during combustion (oxidizing atmosphere) and gasification (reducing atmosphere), respectively, at different temperatures. The transformation of sodium was investigated. Meanwhile, the equilibrium distribution of sodium under both the oxidizing and the reducing atmospheres was simulated by FactsSage 6.1. Finally, the transformation mechanism of sodium during Zhundong coal combustion and gasification in circulating fluidized bed is discussed.
Figure 1. CFB system of 0.25t/d for high alkali coal thermochemical conversion experiments. The chemical fractionation analysis was applied in order to measure the occurrence of sodium in the samples (bottom ash, fly ash and coal). The samples were sequentially leached by ultrapure water and hydrochloric acid (HCl, 1 mol/L). The residual ash was digested by nitric acid (HNO3) and hydrofluoric acid (HF).The leached and digested solutions were analyzed by inductive coupled plasma equipped with an atomic emission spectrometer (ICP-AES, Varian, America), in order to determine the amount of sodium in different occurrences. The test was applied in the air condition. Sodium was separated into different groups, including water-soluble Na, HClsoluble Na and insoluble Na. The crystalline phases in the samples were determined by X-ray diffraction (XRD, PANalytical, Netherlands). The detector uses Cu Kα radiation (λ = 1.5406 Å). Samples were crashed and screened with 200 μm sieve. Bottom ash and fly ash collected during the gasification experiments were burnt at 575 °C in a muffle furnace in the air condition before the XRD test to burnt out the left carbon in the ash. Bottom ash and fly ash collected during the combustion experiments, with little carbon left, were tested directly without any treatment. The microstructure and elements distribution on the surface of the samples were analyzed by scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM-EDX, S-4800, Hitachi, Japan). 2.3. Fuel. A specific, typical Zhundong coal, from Wucaiwan, was chosen as the experiment fuel. The coal was crushed and sieved to the range of 0.1−1 mm. The proximate analysis, ultimate analysis, ash composition and the ash fusion temperatures (DT, ST, HT, and FT) are presented in Table 1. The coal was ashed at 575 °C for the sake of decrease the release of sodium during ashing, more details can be found in our previous study.25 The ash content is ultralow (5.03%, wt %, air-dry basis). There is 3.92% Na2O, 28.74% CaO and 19.58% SO3 by weight basis in the ash composition. The swelling index of the coal is 0. According to chemical fractionation analysis, the sodium in Zhundong coal exists mainly as water-soluble Na, about 81.53% by weight, and 14.22% as HCl-soluble Na, 4.52% as insoluble Na, respectively. The crystalline phases in the coal are presented in Figure 2; the coal was ashed at 575 °C before XRD test. It can be observed that the Zhundong coal contains a large quantity of CaCO3 and CaSO4, which is in accordance with the high content of Ca in the coal ash. Na in the coal ash exists mainly as NaCl.
2. EXPERIMENTAL SECTION 2.1. Test System. Combustion and gasification experiments were undertaken in a CFB system with a capacity of coal feeding rate of 0.25t/d, the schematic diagram is shown in Figure 1. The reactor used was a stainless steel cylindrical tube with an inner diameter of 100 mm and a height of 4.0 m. The system comprised of a reactor, a cyclone separator and a loop seal. Compressed air was introduced into the reactor through the air distributer at the bottom of the reactor. The air distributor is an internal counter flow wind cap with inner tube. Fuel was fed into the bed through a screw feeder. The reactor was equipped with K type thermocouples and differential pressure probes along its height. The bottom of the reactor was equipped with electric wire for ignition. The reactor was coated with insulated cotton for heat preservation. The temperatures, pressures and air flows were monitored by Agilent data acquisition system. To start up the system, 6 kg of silica sand, with a size range from 0.18 mm to 0.71 mm, was added to the reactor, and 1 kg silica sand was added into the loop seal. Air flow was set at 50 L/min. The electrical wire was switched on to heat up the bed material and the reactor. Coal was fed into the reactor, as the temperature of the bed was over 500 °C. During the experiments, bed temperature was maintained at 850, 900, and 950 °C for 3−4 h, respectively. The excess air ratio was maintained at 1.2 (the O2 content is 3.5% in the flue gas) in the combustion experiments, while the air stoichiometry was maintained at 0.4 (the O2 content is 0% in the flue gas) in the gasification experiments. 2.2. Sampling and Analysis Methods. Bottom ash and fly ash were sampled during the experiments. Bottom ash was collected in a bottom ash can, which was located 100 mm above the air distributor, at the bottom of the reactor (A in Figure 1). Fly ash was collected in a natural sedimentation ash can, which was located at the outlet of the CFB cyclone (B in Figure 1). 3474
DOI: 10.1021/acs.energyfuels.6b00028 Energy Fuels 2016, 30, 3473−3478
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Energy & Fuels Table 1. Properties of Zhundong Coala
In particular, considering the transformation of sodium, the solution databases of FToxid SLAGB and FTsaltA, including Na2O and NaCl, were used for the thermodynamic equilibrium calculation.
proximate analysis (wt %) Water content (ad) Ash (ad) Volatile matter (ad) Fixed carbon Lower heating value (MJ/kg, ad) ultimate analysis (ad, wt %) C H N O St Cl ash fusion temperature (°C) DT ST HT FT chemical components in ash (wt %) SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O
15.64 5.03 34.06 45.27 17.63
3. RESULTS AND DISCUSSION 3.1. Distribution of Sodium in Ash. Figure 3 shows the distribution of sodium in ash from combustion and gasification,
54.41 1.7 0.69 22.03 0.4 0.104 1320 1320 1330 1340 12.98 14.87 6.84 31.42 8.69 0.69 16.55 0.06 0.25 5.06
Figure 3. Distribution of sodium in the ash during combustion and gasification.
respectively. The data were taken from the chemical fraction analysis. The content of sodium in ash was calculated on a silica-free basis, in order to eliminate the influence of the bed material (mainly as silica). It was shown that sodium in the ash evidently decreases with an increase in bed temperatures in both combustion and gasification. Accordingly, more gaseous Na is released with increasing temperature. The temperature has an important effect on the release of sodium during combustion and gasification of Zhundong coal. Similar conclusion was drawn in other published works.5,12 Meanwhile, the influence of atmosphere on the release of Na cannot be ignored. Figure 3 gives the different behavior of Na release during combustion and gasification. The content of sodium in fly ash and bottom ash during gasification is higher than that during combustion, which means that more sodium was retained in the ash during gasification. During the thermal utilization of Zhundong coal, the content of gaseous Na has a significant effect on fouling and ash deposition problems in the stationary plant.4 The air stoichiometry was maintained at 0.4 during the gasification experiments, whereas a large proportion of the carbon in the coal remained unreacted, and a proportion of sodium was retained in the ash, as it was bound to the coal matrix.32 Wei et al.33 reached to a similar conclusion when they investigated the thermal utilization of biomass. 3.2. Transformation of Sodium in the Ash. Mineralogical analyses were conducted by XRD. Figure 4 gives the crystalline phase analysis of bottom ash and fly ash of combustion or gasification. It can be seen that quartz and calcium sulfate make up the main phases of bottom ash, which is in accordance with the high content of silica in the bed material and the high content of calcium in the coal. Calcium sulfate exists as one stable solid phase, which has little influence on ash deposition8 in the CFBs operating temperature range (about 850−950 °C). The presence of sodium aluminosilicate in bottom ash illustrated that sodium transformed from sodium chloride to sodium aluminosilicate or that enrichment of the original sodium aluminosilicate in raw coal occurred, both in
a
Note: ad-as air-dried basis; DT-deformation temperature; STsoftening temperature; HT-hemispherical temperature; FT -flowing temperature.
Figure 2. XRD patterns of Zhundong coal ash. a-CaCO3;b-CaSO4;cSiO2;d-NaCl. 2.4. Factsage Thermodynamic Equilibrium Calculations. Thermodynamic equilibrium calculation was carried out in an attempt to predict the equilibrium distribution of sodium under oxidizing atmosphere and reducing atmosphere, using the software program of FactSage 6.1.31 The program uses the method of minimization of the total Gibbs free energy of the system. The calculation includes the following elements: C, H, O, N, S, Cl, Si, Al, Fe, Ca, Mg, K, and Na. The calculation input date is shown in Table 1. The calculation temperature range was from 800 to 1000 °C, in incremental steps of 50 °C. The calculation was carried out at atmospheric pressure. The excess air ratio was set at 1.2 for the combustion, whereas at 0.4 for the gasification, which is consistent with the experimental data. The air was assumed to be composed of 21 mol % oxygen and 79 mol % nitrogen. 3475
DOI: 10.1021/acs.energyfuels.6b00028 Energy Fuels 2016, 30, 3473−3478
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Figure 4. XRD patterns of Zhundong coal ash from combustion and gasificationa (a, CaSO4; b, SiO2; c, Ca2Al2SiO7; d, CaO; e, Ca2SiO5; f, Ca12Al14O33; g, CaS; h, NaCl; i, Na2SO4; j, KNaFeSi4O10; k, NaAlSiO4; l, NaAlSi3O8; m, MgO).
sulfated in the presence of SO2 and SO3 during combustion. According to the study of Vutnaluru et al.,35 Na2SO4 forms when the temperature exceeds 800 °C. Considering the melting point of NaCl is 801 °C and the melting point of Na2SO4 is 884 °C, when the combustion temperature exceeds 884 °C, the reaction of gaseous phase plays an important role during the sulfation of NaCl. The main reaction is as follows: NaCl (g) + H2O (g) + SO2 (g) + 1/2 O2 (g) = Na2SO4(g) + 2 HCl (g).36 It is responsible for the absence of NaCl in fly ash combusted at 900 and 950 °C.NaCl was detected in the fly ash of gasification, whereas Na2SO4 was not detected. In the absence of SO2 and SO3 during gasification, the equilibrium of NaCl (g) and NaCl (l) plays a key role for sodium transformation. As the temperature increases, the equilibrium shifts toward NaCl (g) and more gaseous Na is released,28,37−39 which agrees with the results in Figure 3.Basic investigation has been performed in an atmospheric lab-scale tube furnace under both gasification and
Figure 5. Micro structures of fly ashes.
combustion and gasification. Zhang et al.34 got a similar conclusion during Zhundong coal ashing. Sodium exists as NaCl and Na2SO4 in fly ash gasified at 850 °C. Na2SO4 was detected in the combustion fly ash at 900 °Cand 950 °C, while NaCl was not found. It can be concluded that NaCl was 3476
DOI: 10.1021/acs.energyfuels.6b00028 Energy Fuels 2016, 30, 3473−3478
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Energy & Fuels Table 2. Element Contents in Fly Ashes (wt%) position
C
O
Na
Mg
Al
Si
S
Cl
K
Ca
Fe
A B
9.50 73.94
45.46 7.96
11.40 0.35
0.48 0.86
2.00 0.9
0.53 0.45
12.27 3.86
0.43 1.11
0.55 0
14.35 5.84
3.02 1.02
Figure 6. Equilibrium distribution of sodium in different atmospheric conditions.
Figure 7. Mechanism of sodium transformation.
combustion conditions by Marc et al.40 and a similar result was obtained. NaAlSiO4 (melting point 1550 °C) was detected in the bottom ash of combustion when the combustion temperature reached 950 °C and NaAlSi3O8 (melting point 1100 °C) was detected in the bottom ash of gasification when the gasification temperature reached 950 °C. During the thermal conversion, sodium in the coal can be trapped and interact with Al and Si to form sodium aluminum silicate18,25,26,28,29 according to
calculation using FactSage 6.1 are shown in Figure 6. Sodium is strongly dependent on the surrounding atmosphere. As shown in Figure 6 (a), Na2SO4(s) is predicted to be the most stable species under an oxidizing atmosphere in the bed temperature range of 800 to 1000 °C. Gaseous NaCl (g) increases from 3.76% to 9.71% when the bed temperature increases from 800 to 1000 °C. The liquid phase of Na2O (slag) begins to form when the bed temperature reaches a maximum of 950 °C, which is considered to be a potential source for bed agglomeration and defluidization during the circulating fluidized bed combustion. At the same time, NaAlSiO4 disappears at 950 °C. As to the reducing atmosphere, the results seem to be more complex (Figure 6 (b)). Up to 48.3% of the total sodium is predicted to be Na2Ca2Si3O9 (s) and 34.76% to be Na2O (slag) at 800 °C in a reducing atmosphere. Na2O (slag) is the dominant phase of sodium above 850 °C in the reducing atmosphere, indicating that agglomeration and defluidization in the reducing atmosphere occur more readily when compared to the oxidizing atmosphere. As in the oxidizing atmosphere, gaseous NaCl (g) increases from 9.7% to 13.26% when the temperature in the reducing atmosphere increases from 800 to 1000 °C.In the absence of SO2, Na2SO4 is not predicted in the reducing atmosphere, which is in accordance with the experimental results of XRD. Metallic sodium vapor is predicted and the content of metallic sodium increases with increasing temperature. 3.5. Mechanism of Sodium Transformation. Combining the experiment results and thermodynamic equilibrium calculation, sodium in Zhundong coal undergoes different transformation processes during combustion and gasification. At the beginning of combustion and gasification, sodium is released from the coal as gaseous metallic sodium, Na2O and NaCl. In the oxidizing atmosphere, in which SO2 is present,
Na 2O + Al 2O3 ·SiO2 ·2H 2O = 2NaAlSiO4 + 2H 2O
The formation of Na−Al−Si compound would inhibit the sodium catalytic activity and retained sodium in the solid phase. It can be concluded that bed agglomeration and defluidization readily occur during gasification. 3.3. Morphology of Ash. The micro structures and element contents in fly ash from combustion or gasification are shown in Figure 5. It can be seen that more fine particles were produced during the combustion process. Some fine particles adhere onto the char surface of gasification fly ash. The EDX analyses results are presented in Table 2. The results indicate that the gasification fly ash (position B) contains a significant proportion of C, O, S, Cl, Na, Ca, and Al. However, the ash (position A) formed in combustion, contains a large amount of O, Na, Al, S, Ca, and Fe. Also, there is little Cl compared to the relatively high content of Na. This indicates that NaCl is not present in the combustion fly ash in significant proportion. The proportion of Si is rather small when compared to other inorganic elements in the combustion fly ash, providing evidence that the combustion fly ash consists mostly of sulfates and oxides,41 which is in accordance with the XRD results. 3.4. Results of Thermodynamic Equilibrium Calculation. The results of the thermodynamic equilibrium 3477
DOI: 10.1021/acs.energyfuels.6b00028 Energy Fuels 2016, 30, 3473−3478
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sodium reacts with SO2, forming gaseous sodium sulfates. In the reducing atmosphere, is maintained as gaseous metallic sodium, Na2O and NaCl. When fly ash and gaseous sodium as NaCl or Na2SO4 leaves the fluidized bed reactor, the gas temperature decreases, and the gaseous sodium condenses onto the fly ash and the heat exchanger tubes, causing severe deposition. Since the surface of the heat exchanger tubes is quite sticky, once the gaseous sodium condenses on the surfaces, the fly ash particles adhere onto the surface. The mechanism of sodium transformation is shown in Figure 7.
4. CONCLUSIONS In the present study, the mechanism of sodium transformation during the combustion and gasification of Zhundong coal in a circulating fluidized bed reactor was investigated and the FactSage 6.1 equilibrium software was used to predict the equilibrium distribution of sodium. The main conclusions are as follows: (1) Sodium in ash evidently decreases with increasing temperature, both during combustion and gasification. Accordingly, more sodium is released in the gas phase as the temperature increases. (2) The content of sodium in fly ash and bottom ash in gasification is higher than that in combustion, which means that more sodium is retained in the ash during the gasification process. (3) Sodium exists mainly as sodium aluminosilicate in bottom ash in both combustion and gasification, and mainly as sodium sulfate in combustion fly ash and sodium chloride in gasification fly ash. (4) Sodium undergoes different transformation processes in combustion and gasification. Gaseous metallic sodium, Na2O and NaCl are released from coal and condense directly onto the cold surfaces through the process of gasification. In the presence of SO2, sodium reacts with SO2, forming gaseous sodium sulfates through combustion.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-010-82543129; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (No.XDA07030100) and the International Science & Technology Cooperation Program of China (No. 2014DFG61680).
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b00028 Energy Fuels 2016, 30, 3473−3478
