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Investigation of ash fusion characteristics and migration of sodium during co-combustion of Zhundong coal and oil shale
Accepted Manuscript Investigation of ash fusion characteristics and migration of sodium during cocombustion of Zhundong coal and oil shale Yang Lu, Ying Wang, Ying Xu, Yuan Li, Wuxing Hao, Yongfa Zhang PII: DOI: Reference:
S1359-4311(16)34167-9 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.062 ATE 10211
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
16 December 2016 22 February 2017 12 April 2017
Please cite this article as: Y. Lu, Y. Wang, Y. Xu, Y. Li, W. Hao, Y. Zhang, Investigation of ash fusion characteristics and migration of sodium during co-combustion of Zhundong coal and oil shale, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng.2017.04.062
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Investigation of ash fusion characteristics and migration of sodium during co-combustion of Zhundong coal and oil shale
Yang Lu, Ying Wang, Ying Xu, Yuan Li, Wuxing Hao, Yongfa Zhang *
Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province,
Taiyuan University of Technology, Taiyuan 030024, China
ABSTRACT: Zhundong coal, which has high sodium content, induces severe fouling and slagging during its combustion in a boiler. Co-combustion is an effective method for mitigating these processes. In this study, oil shale is selected as an additive during combustion in order to study the properties of mixed fuel. The study results reveal that water-soluble sodium is the predominant component of Zhundong coal, which originates from the salt in seawater. With an increase in the oil shale blending ratio, the ash fusion temperatures of the mixed coal and the difference between softening temperature and deformation temperature first decrease and then increase. The maximum sodium retention of 86.01% occurs at a blending ratio of 10% and a final temperature of 815°C. With an increase in temperature, aluminosilicates, such as albite and nepheline, form in the ash. Therefore sodium gradually migrates from the water-soluble phase to the aluminosilicate phase. With an increase in the SiO2/Al2O3 ratio, the ash fusion temperature of mixed coal decreases gradually, which is attributed to the effects of the SiO 2 and Al2O3 contents on the melting point. Additionally, a mathematical model for sodium volatilization in the coal is constructed by MATLAB R2014a for fitting and optimization.
KEYWORDS: High-sodium coal; Oil shale; Ash fusion characteristic; Sodium retention; Sodium migration
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* Corresponding Author Tel./Fax: +86-0351-6018676. E-mail: [email protected]
1. Introduction Zhundong region, located in Xinjiang in China, has huge coal reserves. The predicted coal reserve of this region is about 390 billion tons, and its proven coal resource reserve is about 213.6 billion tons. Zhundong coalfield has a coal formation area of 14,000 km2, and it is therefore the largest integrated coalfield in China [1]. The coal found in Zhundong region is mainly high-sodium coal, with sodium content (by ash) higher than 2%. The coal in this region is characterized by its low rank, inflammability, ease of burnout, medium/high water content, low ash content, high volatile content, moderate calorific value, high slagging tendency, high alkali-metal content, and high fouling tendency [2-4]. These distinct properties of Zhundong coal result in problems of fouling, slagging, and corrosion of the heat exchange surface and boiler wall during its combustion in boilers in power plants. These processes not only reduce the heat transfer efficiency of the heat exchange surface but also affect the operation safety and life of boilers in power plants. Thus, the examination of the ash fusion characteristics of high-sodium coal, investigation of the migration mechanism and control technique of sodium volatilization are economically significant and practically meaningful. The problems of slagging and fouling of the sodium in Zhundong coal are mainly caused by two reasons [5]. One is that sodium is highly prone to volatilization at high temperatures, and the other is that eutectics are formed during the combustion of Zhundong coal in the boiler. Wang et al. [6] studied the ash deposition mechanism in boiler burning Zhundong coal and concluded that 80% of sodium was released in the gas phase when the temperature reached 400–800°C. Furthermore, when the temperature reached 800–1000°C, all the remaining sodium volatilized, and sulfur began to be released. Sodium sulfate with a fusion point of 884°C forms by a reaction of sodium with sulfur. The low-melting-point sodium sulfate 2
fouls easily, slags the heat exchange surface and boiler wall, and forms a dense and viscous layer. It captures the fly ash particles in the flue gas, which results in sticky ash deposition at high temperature. Further, with an increase in temperature, sodium might transform into sodium silicates, which mix with iron, calcium, and magnesium oxides to form eutectics [7]. These low-melting-point eutectics also tightly adhere to the heat exchange surface and boiler wall. At present, most Zhundong coals are burnt by being co-fired with low-fouling coal. This technique mitigates fouling to some extent but does not thoroughly solve the practical problems associated with it [8]. In addition, it is rather difficult to select an appropriate type of low-fouling coal. Some researchers proposed the use of pretreated Zhundong coal for this purpose [9]. Despite its effectiveness in the removal of sodium, this approach has a complicated process, rigorous requirements for equipment, and high production cost and equipment cost; furthermore, the waste liquid may result in environmental pollution because of difficulties in its treatment [10]. Shen et al. [11] studied the effects of co-firing of Zhundong coal and kaolin on sodium retention and ash fusion characteristics. Although kaolin co-firing provided a sodium retention effect, kaolin inhibited fuel combustion and reduced the utilization ratio of fuel because of its incombustibility. A soot blower or high-pressure steam purge device was added to the boilers in some power plants; however, this approach has some disadvantages such as a high construction cost, complicated boiler reconstruction technology, unknown risks of the rehabilitated equipment, and incomplete elimination of fouling. Zhang et al. [12] used distilled water, ammonium acetate, dilute hydrochloric acid, and hydrofluoric acid to study the changes in potassium and sodium in bituminous coal and anthracite with changes in the coal rank and particle size, and they analyzed the alkali-metal behaviors at the beginning of combustion. Some researchers believed that the sodium contained in high-sodium coal was mainly water-soluble sodium.
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With an increase in combustion temperature, the water-soluble sodium gradually volatilized to the gas phase [13,14], and this volatilization of water-soluble sodium was the main cause of fouling during ash deposition. The ash fusion temperature was one of the commonly used indices for prediction of fouling caused by high-sodium coal, which was significantly affected by the mineral components in the coal. Zhang et al. [15] used mixed coal or a mineral additive to adjust the coal ash fusion temperature in order to inhibit sodium volatilization and fouling. Kyi et al. [16] studied the adsorbability of 12 minerals in compounds such as NaCl, etc., and found that minerals containing high amounts of elements, such as Si and Al had strong sodium adsorbability. Wang et al. [17] mixed Zhundong coal with high-Si coal extracted from Heilongjiang in different ratios for ashing at different final temperatures and found that when the SiO2/NaO2 ratio was more than 20 or the (SiO2 + Al2O3)/Na2O ratio was more than 25, a small amount of sodium-containing substance volatilized and the substance containing Si and Al captured the sodium element in aluminosilicate, resulting in effective mitigation of fouling. A new fuel, oil shale, was discovered in Changji region in Xinjiang. Its predicted reserve is 54.8 billion tons, and its proven reserve is 459 million tons [18]. Oil shale is flammable sedimentary rock with high Si and Al contents [19]. These two elements, being the beneficial components of coal ash, inhibit sodium volatilization and fouling during high-sodium-coal combustion and enhance the ash fusion temperature of Zhundong coal as a refractory. Thus, in the present study, the co-combustion of Manasi high-sodium coal and oil shale extracted from Changji region was used to extensively study the properties of mixed fuel.
2. Experimental Section 2.1 Coal Quality Analysis The experimental study employed Manasi (a county in Xinjiang Province) high-sodium coal (MNS) and Changji region oil shale (CJR), which were crushed, ground, and sieved to less than 74 μm. Results of
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proximate analysis and ultimate analysis of Manasi coal and oil shale are listed in Table 1. Manasi coal was mixed with 5%, 10%, 15%, 20%, and 25% oil shale to obtain mixed coal samples, termed MC1, MC2, MC3, MC4, and MC5, respectively. These samples were dried in a thermostatic drying oven at 105°C for 2 h and stored in sealed bottles for later use.
2.2 Experiment and Analysis Method The Manasi coal sample was crushed in the GJ-1 series laboratory sealed sample crusher, sieved using a GZS standard automatic screening machine, immersed in a water bath with a DF-101S heat-gathering constant magnetic stirrer, and incinerated in an SRJK-2.5-13 high-temperature tubular furnace; the furnace has a maximum temperature of 1350°C, hearth dimensions of φ22 × 180 mm, voltage range of 0–220 V, and power of 2.5 kW. The minerals in the ash sample were analyzed using the MiniFlex 600 X-ray diffractometer (Rigaku Corporation) with Cu Kα as the radiation source at a tube voltage of 40 kV, tube current of 15 mA, scanning speed of 8°·min-1, and scanning range of 20°–60° and the calculations were performed using Jade 6.5 software. Four characteristic fusion temperatures of the ash sample were determined using the computer-controlled SJHR-3 smart ash fusion point analyzer consisting of a high-temperature furnace (maximum temperature of up to 1600°C), temperature control system, image acquisition system, and computer control system. The sodium content of the ash sample was determined using the ICAP 6000 Series inductively coupled plasma (ICP) atomic emission spectrometer (Thermo Fisher Scientific) at a radio-frequency power of 1.0 kW, plasma gas flow rate of 15.0 L·min-1, auxiliary gas flow rate of 0.5 L·min-1, atomization gas flow rate of 0.75 L·min-1, peristaltic pump flow rate of 1.5 mL·min-1, and integration time of 5 s (average of three tests). Radial observation.
2.3 Analysis of Occurrence Mode of Sodium The occurrence mode of sodium was analyzed by the sequential extraction method. The Manasi
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high-sodium coal sample with a particle size of less than 74 μm was used in the experiment. Air-dried coal (1 g) was immersed in 50 mL of highly purified water and heated in a water bath at 60°C for over 24 h and filtered with a microfilter before the filtrate was diluted to 100 mL. Residual solid was treated with 1 mol/L ammonium acetate and 1 mol/L dilute hydrochloric acid by the above procedure. The final residue was dried and digested with a mixture of hydrofluoric acid and perchloric acid before the digestion solution and filtrate were analyzed by ICP-optical emission spectrometry (ICP-OES).
2.4 Low-Temperature Ashing Some minerals escaped in the form of gases at a certain temperature during coal combustion. The higher the slagging tendency of the coal, the larger were the amounts of corresponding minerals that escaped. Fan et al. [20] found that the sodium escape ratio of an ash sample determined by the national standard method of China was 27.36%. Zhang et al. [21] studied the difference in composition between Zhundong coal ashes subjected to incineration at 575°C and 815°C and concluded that the coal sample was completely incinerated and the sodium escape ratio was minimized at the final temperature of 575°C. Because of a large error in the sodium content (by ash) of Zhundong coal after incineration at high temperature, low-temperature ashing was employed in this study. Manasi coal was experimentally incinerated in a tubular furnace at a heating rate of 10°C·min-1, with a final temperature of 575°C, and kept for 2 h under air atmosphere with an airflow rate of 0.5 L·min-1 in order to achieve complete incineration. Oil shale was incinerated at 815°C in accordance with the national standard method of China. The resultant ash sample was analyzed according to the GB/T1574-2007 Chinese standard. Results of the ash component analysis are listed in Table 2.
2.5 High-temperature Ashing Manasi coal and oil shale were mixed in five blending ratios which were mentioned in 2.1. Then, the
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mixed coal sample was incinerated in a high-temperature tubular furnace at a heating rate of 10°C·min-1 below 900°C and at a heating rate of 5°C·min-1 above 900°C under air atmosphere with an air flow rate of 0.5 L·min-1; subsequently, the sample was kept at final temperatures of 815°C, 900°C, 1000°C, 1100°C, and 1200°C for 2 h in order to achieve complete incineration. Then, ash samples with different blending ratios were obtained by ashing at different temperatures.
2.6 Chilling and Drying of Ash at High Temperature The ash samples obtained by high-temperature ashing were rapidly immersed in liquid nitrogen in order to chill them to less than 300°C within 2 s to prevent a crystal phase change during the temperature drop. After the samples cooled down to room temperature, they were dried in a thermostatic drying oven at 105°C for 2 h. The mineral contents and sodium contents of the cooled samples were determined by X-ray diffraction (XRD) and ICP–OES, respectively.
2.7 Ash Fusion Characteristics of Coal Pyramids of the ash sample and dextrin were produced according to the GB/T219-2008 standard. They were then tested in the high-temperature furnace by using an ash fusion point testing apparatus to determine the following four characteristic ash fusion temperatures of the Manasi coal, oil shale, and mixed coal samples under a weakly reducing gas atmosphere: deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT).
2.8 Sodium Content Determination In accordance with the MT/T1014-2006 standard, the mixed coal samples were incinerated at various target final temperatures and then digested with a hydrofluoric acid-perchloric acid mixture to obtain the sample solution before the following tasks: preparation of blank solutions and standard solutions, standard curve plotting, determination of the absorbency of the blank solutions and sample solutions by ICP-atomic
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emission spectroscopy (ICP-AES), and calculation of the sodium contents of ash samples.
2.9 Sodium Retention and Sodium Volatility To better describe the volatilization characteristics of sodium during mixed coal combustion, the following parameters were introduced for characterization. Sodium content, m1 (mg·g−1), of raw-coal ash
m1 m1' A
(1)
where m1 = sodium content of coal ash after combustion, mg·g−1, and '
A = percentage of ash produced from coal combustion at temperature T. Percentage of sodium residue in raw-coal ash, i (%)
i
m1 m' A 100% 1 100% (2) m0 m0
where m0 = sodium content of coal before combustion, mg·g−1. Sodium volatility, j (%), at combustion temperature T
j=1 i
(3)
The sodium retention effect of oil shale was described as the sodium retention η, which was calculated as follows:
η
M Na ,MC (a , T) M Na，MNS M Na，CJR
100%
(4)
where MNa,MC(a,T) = mass of sodium in the mixed coal with an oil shale blending ratio of a and final combustion temperature of T; MNa,MNS = mass of sodium in Manasi coal; and MNa,CJR = mass of sodium in Changji region oil shale.
2.10 Development of Prediction Model of Sodium Volatility by MATLAB The binary relationship among the sodium volatility V T, oil shale ratio X, and temperature T, i.e., VT =
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f(X, T), was obtained with MATLAB R2014a by using the particle swarm optimization algorithm, a polynomial fitting method, and the MATLAB Curve Fitting Toolbox. This equation was determined in order to obtain the curve fitting equation for sodium volatility at different ratios and ashing temperatures. The curve fitting images were obtained, and then, the sum of squared errors and goodness of fit were calculated by the cftool function in MATLAB.
3. Results and Discussion 3.1 Occurrence Mode of Sodium in Manasi Coal The occurrence mode of sodium in Manasi coal was determined by the sequential extraction method [22]. Highly purified water, ammonium acetate, and dilute hydrochloric acid were used as the extractants. The sodium occurrence modes in the coals as determined using different extractants are listed in Table 3. The sodium present in Zhundong coal included water-soluble sodium, ammonium-acetate-soluble sodium, hydrochloric-acid-soluble sodium, and insoluble sodium [23]. Highly purified water could act as an extractant to extract NaCl crystals and inorganic sodium containing hydrates. The ammonium acetate solution could extract ion-exchangeable ions in the form of carboxylates. Organic sodium in the coordinate form on nitrogen-containing or oxygen-containing functional groups in the coal structure could be extracted only by dilute hydrochloric acid, but the sodium present in the residue obtained after sequential extraction was believed to be insoluble sodium, which mainly included some aluminosilicates and some salts in the coal matrix. It can be seen from Table 3 that the sodium present in Manasi coal was mainly water-soluble sodium, and the content of this water-soluble sodium accounted for 74.36% of the total sodium. Also, the contents of ammonium-acetate-soluble sodium, hydrochloric-acid-soluble sodium, and insoluble sodium were 14.53%, 7.85%, and 3.26%, respectively. These contents were related to the formation and evolution of
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Manasi coal. Salt produced by seawater evaporation during geological changes and inorganic salt formed when the coal-forming plants absorbed water were the main sources of the water-soluble sodium. The organic sodium originated from the organic components of the coal-forming plants, whereas the insoluble sodium originated from the environment during evolution [24]. Volatilization of water-soluble sodium to the gas phase was one of the main reasons for boiler fouling and slagging during the combustion of Manasi coal [12].
3.2 Ash Fusion Characteristics of Manasi Coal, Oil Shale, and Mixed Coal Sodium escaped from Zhundong high-sodium coal during high-temperature ashing, and the reduction in the sodium content led to a high ash fusion temperature and large experimental error. Therefore, the ash fusion temperature of Manasi coal was determined by low-temperature ashing and combustion at the final temperature of 575°C to preserve the raw minerals in Manasi coal. The ash fusion temperature of oil shale was determined at a final temperature of 815°C. The ash fusion temperature of the mixed coal was determined by ashing at 1000°C because of the formation of a high-fusion-point substance at about 1000°C. The four characteristic ash fusion temperatures of Manasi coal, oil shale, and mixed coal are listed in Table 4. It can be seen from Table 4 that the four characteristic ash fusion temperatures of Manasi coal, oil shale, and mixed coal were not linearly correlated. The ash fusion temperature of coal was related to several factors such as the mineral components of ash, the reaction between mineral components during ashing, and the formation of a low-temperature eutectic. Acidic oxides such as SiO2, Al2O3, and TiO2 are refractory minerals; the higher their content, the higher is the ash fusion temperature of coal. On the other hand, acidic oxides such as Fe2O3, CaO, MgO, Na2O, and K2O are fluxing minerals; the higher their content, the lower is the ash fusion temperature of coal [25]. Manasi coal had high contents of alkali metals and sulfur.
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It could be seen from Table 2 that high-sodium coal ash had CaO, Na2O, and SO3 contents of 27.28%, 4.21%, and 22.56%, respectively. Substances with a low fusion point and high viscosity, such as NaSO4 and CaSO4, which were formed during high-temperature ashing, lowered the four characteristic ash fusion temperatures of high-sodium coal. Thus, this was one of the crucial reasons for fouling during the combustion of high-sodium coal. It can be seen from Fig. 1 that with an increase in the oil shale blending ratio, the four characteristic ash fusion temperatures of the mixed coal sample first decreased and then increased. First, when the oil shale blending ratio was in the range of 0%–5%, the four characteristic ash fusion temperatures decreased gradually; when the ratio was in the range of 5%–10%, the four characteristic ash fusion temperatures increased rapidly; finally, when the ratio was in the range of 10%–25%, the four temperatures increased stably. The mixed coal sample consisting of Manasi coal and oil shale had higher contents of refractory substances such as SiO2 and Al2O3 but lower contents of fluxing substances, such as Na 2O and CaO. Hence, more products with high fusion points, such as SiO2, mullite, and nepheline were formed during high-temperature ashing. The fusion point of crystals was affected by their binding energy. Atom crystals and ion crystals had high binding energies. The higher the binding energy of ion crystals, the higher is the fusion point of the product [26]. SiO2, mullite, and nepheline have high crystal binding energies and stable molecular structures. An increase in the content of minerals with high crystal binding energies during combustion of the oil-shale-containing mixed coal increased the ash fusion temperature of coal. Al2O3 in coal ash functioned as a visible skeleton during fusion. The higher the Al2O3 content of coal ash, the higher is its ash fusion temperature [27]. With an increase in the oil shale blending ratio between 0% and 25%, the DT increased by 87°C and the FT increased by 141°C, indicating that oil shale improved the coal ash fusion temperature to a great extent. The mineral component ratio of the ash sample resulting from the
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combustion of Manasi coal with an oil shale blending ratio of 5% was in the low-temperature eutectic region, finally resulting in a lowered fusion point of the ash sample. This is in agreement with the explanation of a low-temperature eutectic region in the ternary phase diagram provided by Li et al. [28,29]. Wei et al. [30] believed that the difference between the ST and the DT led to the formation of highly viscous ash at lower temperature and severer fouling during combustion of Zhundong coal. It can be seen from Fig. 2 that with an increase in the oil shale blending ratio, the difference between the ST and the DT first decreased rapidly and then increased rapidly. The difference between these two temperatures was minimum when the blending ratio was 10%; and the fouling and slagging tendencies were also lowest at this ratio.
3.3 Effect of Oil Shale Blending Ratio on Sodium Retention It can be seen from Table 5 that the sodium content of the Manasi raw coal ash accounted for 50%–60% of the total sodium. Therefore, 40%–50% of sodium-containing substance volatilized from raw coal to the gas phase during combustion and reacted with sulfides to form viscous substances, resulting in fouling and slagging of the coal-fired boiler [31]. After oil shale was added to Manasi raw coal, a higher amount of sodium was retained in coal ash, with the sodium retention being as high as 70%–80%. This indicates that oil shale had a strong sodium retention effect, and the Si and Al in oil shale captured the sodium in Manasi coal to form high-fusion-point aluminosilicates, which were discharged from the boiler by the deslagging system. It can be seen from the curve of the sodium retention effect in Fig. 3 that with an increase in the blending ratio, the sodium capture efficiency first increased rapidly and then decreased gradually. When the oil shale blending ratio was between 5% and 10%, the oil shale reactivity was high and it increased rapidly; when the blending ratio increased beyond 10%, the sodium retention effect weakened stably
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because most of the sodium had participated in the reaction. The maximum sodium retention of 86.01% occurred at the oil shale blending ratio of 10% and combustion temperature of 815°C. With an increase in temperature from 815°C to 1200°C, the capture efficiency of sodium in the mixed coal ash decreased slightly, suggesting that oil shale provided a better sodium retention effect at low temperature than at high temperature. With increasing combustion temperature, both the sodium release rate and the sodium capture rate of oil shale increased. The release rate was slightly higher than the capture rate, so the sodium capture efficiency decreased slightly. With increasing temperature, the sintering property of coal improved gradually, and the surface areas of particles of Manasi coal and oil shale decreased and hindered reaction occurrence. Furthermore, some of the SiO 2 and Al2O3 in oil shale reacted to form mullite at high temperature, resulting in inhibition of the reaction with sodium and a further decrease in the sodium capture efficiency [32].
3.4 Mineral Evolution in Manasi Coal and Mixed Coal at High Temperatures The XRD patterns of Manasi coal ash at the final temperature of 575°C and those of mixed coal ash at final temperatures of 815°C, 900°C, 1000°C, 1100°C, and 1200°C are shown in Fig. 4. Only the 20°–60° XRD patterns are shown here, because the coal sample had weak absorption peaks at 10°–20° and 60°–90°. Fig. 4(a) shows the XRD pattern of the ash obtained from combustion of Manasi coal at the final temperature of 575°C. It can be seen from Fig. 4(a) that the primary minerals in Manasi coal were mainly calcite (CaCO3), anhydrite (CaSO4), and quartz (SiO2); This coal also contained thenardite (NaSO4) and rankinite (CaSiO3) in minor quantities. Manasi coal was also high calcium coal, and contained many kinds of calcareous minerals and had high calcareous mineral content, which was consistent with the ash composition analysis results in Table 2.
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Figs. 4(b)–(d) show XRD patterns of the ash obtained from combustion of the mixed coal sample consisting of 90% Manasi coal and 10% oil shale at final temperatures of 815°C, 900°C, 1000°C, 1100°C, and 1200°C. It can be seen from Figs. 4(b)–(d) that the ash obtained from the combustion of Manasi coal and mixed coal at each final temperature mainly contained quartz with a fusion point of 1710°C. All the absorption peaks of NaSO4 disappeared at the combustion temperature of 815°C, and the entire amount of NaSO4 volatilized to the gas phase. CaO in Manasi coal reacted with SiO2 and Al2O3 in oil shale to form anorthite (CaAl2Si2O8) with a fusion point of 1553°C. This reaction is given as CaO + 2 Al2O3 + SiO2 → CaAl2Si2O8. Calcite disappeared when the combustion temperature reached 900°C, suggesting that calcite decomposed into CaO and CO2 as CaCO3 → CaO + CO2. Moreover, NaCl in Manasi coal reacted with SiO2 and Al2O3 in oil shale to form albite (NaAlSi3O8) with a fusion point of 1180°C. This reaction is given as 2 NaCl + 2 SiO2 + Al2O3 + H2O → 2 NaAlSiO4 + 2 HCl. Albite readily reacted with CaO and other aluminosilicates to form a eutectic with a low fusion point, resulting in a decrease in the fusion point of the mixed coal ash. When the combustion temperature reached 1000°C, albite further decomposed into nepheline (NaAlSiO4) with a fusion point of 1550°C and SiO2 as NaAlSi3O8 → NaAlSiO4 + 2 SiO2. Nepheline was also produced by the reaction of NaCl with SiO2 and Al2O3 as 2 NaCl + 6 SiO2 + Al2O3 + H2O → 2 NaAlSi3O8 + 2 HCl. In addition, SiO2 and Al2O3 reacted to form mullite (3Al2O3·2SiO2) with a fusion point as high as 1850°C. This reaction is given as 3 Al2O3 + 2 SiO2 → 3Al2O3·2SiO2. Rankinite decomposed into CaO and SiO 2 as CaSiO3 → CaO + SiO2. When the combustion temperature reached 1100°C, MgO and SiO2 reacted to form sepiolite (Mg4Si6O15(OH)2) with a fusion point of 1500–1700°C. This reaction is given as 4 MgO + 6 SiO2 + H2O → Mg4Si6O15(OH)2. Further, NaCl and CaO in Manasi coal reacted with the SiO2 and Al2O3 in oil shale to form feldspar minerals ((Ca, Na)(Si, Al)4O8) as NaCl + CaO + SiO2 + Al2O3 → (Ca, Na)(Si, Al)4O8. When the temperature reached 1100°C, all anhydrite
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disappeared, indicating its decomposition into CaO and SO 2 as CaSO4 → CaO + SO2. When the combustion temperature reached 1200°C, the reaction product was in accord with that at 1100°C, suggesting that no new substance was formed, because of the complete reaction of all the mineral elements in the mixed coal. The XRD patterns obtained at the various final combustion temperatures showed that the sodium gradually migrated from the water-soluble phase to the aluminosilicate phase, whereas it was deposited in coal ash in the solid form by the Si and Al in oil shale and discharged along with slag. Li et al. [26] concluded from a comparison of Fukui functions that nucleophiles preferably attacked Si(6) and Si(8) to break and reconstruct Si-O bonds whereas electrophiles attacked O(26) and O(22) to break Al-O bonds. Ma et al. [33] pointed out that when silicate reacted with R-O bonds (where R = alkali-metal ions or alkaline-earth-metal ions) in a coal ash fusant, the oxygen ions from the R-O bonds could become attracted to their surroundings to break the bridging oxygen bonds, i.e. Si-O-Si. Thus, the oxides of the alkali metal Na and the alkaline-earth metal Ca in coal nucleophilically reacted with Si(6) and Si(8) in oil shale in the presence of O2- as a nucleophile at different temperatures to produce anorthite, albite, nepheline, etc. These reactions are given as CaO + 2 Al2O3 + SiO2 → CaAl2Si2O8, Na2O + 6 SiO2 + Al2O3 → 2 NaAlSi3O8, and Na2O + 2 SiO2 + Al2O3 → 2 NaAlSiO4.
3.5 Comparison of Sodium Retention and Ash Fusion Characteristics Fig. 5 shows a comparison of the sodium retention and coal ash fusion characteristic (i.e., the ST) at different blending ratios at the combustion temperature of 1200°C. It can be seen from Fig. 5 that a blending ratio of about 5% (1 in Fig. 5), ash fusion point of about 1200°C, and sodium retention of about 70% were favorable for liquid slag removal, whereas a blending ratio in the range of 15%–25% (2 in Fig. 5), ash fusion point higher than 1300°C, and sodium retention of 75%–80% were favorable for solid slag
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removal. To meet the requirements of different boilers in power plants for slag removal method and sodium retention, an appropriate blending ratio could be selected according to the actual combustion temperature in the boilers in power plants.
3.6 Effects of SiO2/Al2O3 Ratio on Ash Fusion Characteristics of Manasi Coal The effects of the SiO 2/Al2O3 ratio on the ash fusion characteristics of Manasi coal were studied by selecting an oil shale–bauxite (Al2O3) mixture with different ratios as the composite mineral additive and adjusting the SiO2/Al2O3 ratios in the mixed ash sample. The results are listed in Table 6. The mineral composition of coal ash is one of the main factors governing its fusion characteristics. The SiO2/Al2O3 ratio has been reported to significantly affect the coal ash fusion temperature [34]. It can be seen from Table 6 that the Manasi coal ash fusion temperature decreased gradually with increasing SiO2/Al2O3 ratio, which is attributed to the effects of the SiO 2 and Al2O3 contents. Al2O3 in coal always enhances the ash fusion temperature, but the SiO2 content is not significantly related to the ash fusion temperature [35]. When the SiO2/Al2O3 ratio was between 2:1 and 3:2, the ash fusion temperature did not change significantly, because a high SiO2/Al2O3 ratio meant more SiO2, which existed in the noncrystalline form in coal ash and was a smaller contributor to the ash fusion temperature [36]. When the SiO 2/Al2O3 ratio was 1:1, a considerable amount of mullite with a high ash fusion point was produced during combustion, resulting in a significant increase in the ash fusion temperature. When the SiO 2/Al2O3 ratio was above 1:1, the SiO2 content was high and SiO2, silicate, and calcareous minerals in the mixed ash sample formed a low-temperature eutectic, which resulted in a decrease in the ash fusion temperature. When the SiO 2/Al2O3 ratio was between 2:3 and 1:2, the ash fusion temperature increased greatly because a low SiO2/Al2O3 ratio meant more Al2O3, which existed in the form of ion crystal and had a high binding energy and fusion point; this resulted in an increase in the ash fusion temperature. When the SiO2/Al2O3
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ratio was 1:2, a considerable amount of Al2O3 existed in the ash sample, so the ST exceeded 1500°C. However, the SiO2/Al2O3 ratio in raw coal was about 2:1, and addition of composite additives enhanced the ash fusion temperature of the mixed coal samples slightly because the composite additives reacted with the mineral components of raw coal to form substances with a high crystal binding energy and high fusion point during combustion.
3.7 Prediction Model of Sodium Volatility in Mixed Coal 3.7.1 Sodium Volatility in Mixed Coal The effects of blending ratio and temperature on the sodium volatility in mixed coal are shown in Fig. 6. (1) Effects of blending ratio - It can be seen from Fig. 6(a) that the mixtures of Manasi coal and oil shale at different ratios reduced the sodium volatility to a much greater extent than did raw coal. With an increase in the blending ratio, the sodium volatility first decreased and then increased because the sodium in Manasi coal reacted with the Si and Al in oil shale to form albite and nepheline. Sodium was attached to aluminosilicate, which inhibited its volatilization. The maximum sodium volatility of 49.60% occurred at the raw coal combustion temperature of 1200°C, whereas the minimum sodium volatility of 13.99% occurred at the oil shale blending ratio of 10% and final combustion temperature of 815°C. (2) Effects of temperature - It can be seen from Fig. 6(b) that with an increase in temperature, the sodium volatility in the Manasi coal and mixed coal samples increased and the residual ratio gradually decreased. This is because with an increase in temperature, the coefficient of sodium diffusion from within the coal particles to their surfaces and from their surfaces to their surroundings increased, the sodium diffusion rate in coal increased, and the escape rates of moisture and volatile compounds from coal also increased, resulting in the formation of new micropores in the inner structure of coal and release of more sodium.
3.7.2 Sodium Volatility Prediction Model
17
It was found from the above analysis that the sodium volatility in coal was closely related to the combustion temperature and blending ratio, and oil shale strongly inhibited the sodium volatility. Fig. 7 shows the three-dimensional fitting curve for sodium volatility in the mixed coal sample at different ratios and different final combustion temperatures, as obtained by MATLAB. It can be seen from Fig. 7 that the sodium volatility in the mixed coal sample first decreased rapidly and then increased stably at different temperatures, and the minimum sodium volatility occurred at the oil shale blending ratio of 10%. At the same oil shale blending ratio, the higher the temperature, the higher was the sodium volatility. To describe the sodium volatility in mixed coal, its prediction model, i.e., the binary function relationship among sodium volatility VT, oil shale blending ratio X, and combustion temperature T, was proposed as VT = f (X, T). MATLAB R2014a was used for fitting and optimization. VT = (a0 + a1X + a2X2 + a3X3 + a4X4) × (b0 + b1T + b2T2 + b3T3) (5) where VT = Total sodium volatility in the mixed coal sample at temperature T; X = Mass fraction of oil shale; and a0 = 1.1891 × 10-6, a1 = -2.5116 × 10-5, a2 = 2.4482 × 10-4, a3 = -9.9666 × 10-4, a4 = 1.4685 × 10-3, b0 = 0.9444, b1 = 1.2946, b2 = 1.2815, and b3 = -6.7697 × 10-4. The goodness of fit, i.e., the R-squared value, was 98.11%; the sum of squared errors, SSE, was 0.07%; the maximum error was 1.29%; and the minimum error was 0.09%. The error range was smaller and the degree of fitting was higher. The sodium volatility in the mixed coal sample was calculated at 900°C and 1100°C at different blending ratios according to the model given in (5), and the calculation results were compared with experimental values and weighted values as shown in Fig. 8. It can be seen from Fig. 8 that the calculation results obtained using the sodium volatility prediction
18
model in (5) for the mixed coal sample at different temperatures and blending ratios agreed well with the experimental values but differed considerably from simply weighted values. This model was more suitable for prediction of sodium volatilization characteristics in mixed coal. The sodium volatilization characteristics in coal are related to several factors such as the contents of ash and volatile compounds, the content and occurrence mode of sodium, mineral components, and combustion conditions [37]. Sodium volatilization in Manasi coal was observed to be a highly complicated physicochemical change. A prediction model of sodium volatilization characteristics in coal has rarely been reported in the literature. In the present work, on the basis of existing research techniques and calculation methods, a prediction model of sodium volatility in mixed coal was preliminarily studied, and the sodium volatilization characteristics in a mixed coal sample were predicted at different temperatures and oil shale blending ratios in order to provide technical guidelines for ensuring combustion safety of Zhundong coal in the boilers in power plants.
4. Conclusions (1)
The sodium contained in Manasi coal was in the form of
ammonium-acetate-soluble
sodium,
hydrochloric-acid-soluble
sodium,
water-soluble sodium,
and
insoluble
sodium.
Water-soluble sodium was found to be the predominant chemical form of sodium in Manasi coal, which accounted for 74.36% of the total sodium. This result was related to the formation and evolution of Manasi coal. (2) With an increase in the oil shale blending ratio, the ash fusion temperatures and the difference between the ST and the DT first decreased and then increased. When the blending ratio reached 5%, the ash fusion temperatures decreased owing to the formation of a low-temperature eutectic. The formation of SiO2, mullite, and nepheline led to an increase in the ash fusion temperature. When the blending ratio was 10%,
19
the fouling and slagging tendencies were observed to be lowest. (3) During combustion, 40%–50% of sodium volatilized to the gas phase and reacted with sulfides to form viscous substances, resulting in fouling and slagging of a coal-fired boiler. With an increase in the oil shale blending ratio, the sodium retention first increased and then decreased. Oil shale provided a better sodium retention effect at low temperature than at high temperature. The maximum sodium retention of 86.01% occurred at the blending ratio of 10% and final temperature of 815°C. (4) The primary minerals in Manasi coal were mainly calcite, anhydrite, and quartz; this coal also contained thenardite and rankinite in minor amounts. With an increase in temperature, aluminosilicates such as albite and nepheline formed in the ash after blending with oil shale. The sodium gradually migrated from the water-soluble phase to the aluminosilicate phase, which was eventually discharged from the boiler by the deslagging system. (5) The sodium retention was about 70% at a blending ratio of about 5% and ash fusion temperature of about 1200°C; these conditions were favorable for liquid slag removal. Further, the sodium retention was 75%–80% at a blending ratio between 15% and 25% and an ash fusion temperature above 1300°C; these conditions were favorable for solid slag removal. (6) With an increase in the SiO2/Al2O3 ratio, the ash fusion temperature of Manasi coal decreased gradually. The fusion point did not change significantly when the SiO 2/Al2O3 ratio was between 2:1 and 3:2, but it increased greatly when the SiO 2/Al2O3 ratio was between 1:1 and 1:2. This behavior was related to the effects of the SiO2 and Al2O3 contents. (7) A mathematical model for prediction of sodium volatility in mixed fuel was constructed using MATLAB R2014a by the particle swarm optimization algorithm and polynomial fitting methods. This model is given as VT = (a0 + a1X + a2X2 + a3X3 + a4X4) × (b0 + b1T + b2T2 + b3T3). The calculation results
20
of this model agreed well with experimental results.
Acknowledgments
This work was supported by the National Key Technology Support Program of China (Grant No. 2012BAA04B03) and the National Natural Science Foundation of China (Grant No. 21576182).
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24
The figures are listed there. Fig 1 Variation trend of AFTs of mixed coals at different blending ratio
Fig 2 Variation trend of ST-DT of mixed coals at different blending ratio Fig 3 Variation trend of sodium retention rate at different blending ratio and temperature Fig 4 X-ray diffraction patterns of ash samples of single coal and mixed coal Fig 5 Comparison of sodium retention（1200℃）and ash fusion temperature（ST） Fig 6
Sodium volatilization of mixed coal at different blending and different temperature
Fig7
Three-dimensional pattern of sodium volatilization of mixed coal at different blending and different temperature Fig 8 Modeling sodium volatility vs experimental and weighted result of mixed coal
25
1500 DT ST HT LT
1450
AFTs /
℃
1400 1350 1300 1250 1200 1150
0
5
10
15
20
25
mass ratio of oil shale / %
Fig 1 Variation trend of AFTs of mixed coals at different blending ratio
26
60
T/
℃
55
50
45
40 0
5
10 15 20 mass ratio of oil shale / %
25
Fig 2 Variation trend of ST-DT of mixed coals at different blending ratio
27
90
sodium retention rate / %
85 80 75 70
815℃ 900℃ 1000℃ 1100℃ 1200℃
65 60 55 50 0
5
10 15 20 mass ratio of oil shale / %
25
Fig 3 Variation trend of sodium retention rate at different blending ratio and temperature
28
2
2 4 1
3 13
20
2
30
40 2θ / (° )
50
60
℃
20
1 1 41 3 13 1 2 3 4
30
40 2θ / (° )
62 4
50
60
(a)
1
℃
（ d） 1000
℃
intensity/(a.u.)
intensity/(a,u)
（ c） 900
4
1
6
3 6 8 1 31 8
6
8
20
30
4
40 2θ /( ° )
3
1
50
11 9
30
20
40
40 2θ /(° )
1
60
3
9
9
50
60
℃
5
9 6
11
2θ / (° )
1 9
（ f ） 1200
9 10
6 50
17 1
2
111 9
1
1
intensity/(a.u.) 1
6 71
30
6 1
7
℃
10 11 6
9
9
60
11 10 9 9
3
3 5 3 4 6
（ e ） 1100
1
20
3 6 2
6
3
2
1
1
1 4
3
2 5
（ b） 815
3
1
2
intensity/(a.u.)
1 ℃
intensity/(a.u.)
intensity/(a.u.)
（ a） 575
20
30
9 1
1
6
1
11
40 2θ / (° )
50
Fig 4 X-ray diffraction patterns of ash samples of single coal and mixed coal 1：quartz；2：calcite；3：anhydrite；4：rankinite；5：thenardite；6：anorthite； 7：mullite；8：albite；9：nepheline；10：sepiolite；11：anorthite Na-rich
29
60
1350
85 η(1200
℃
)
80 75
ST
ST /
65
2
1
η/%
70
℃
1300
60 1250
55 50 0
5
10
15
20
25
mass ratio of oil shale / %
Fig 5 Comparison of sodium retention（1200℃）and ash fusion temperature（ST）
30
100
100 815 900 1000 1100 1200
0% 5% 10% 15% 20% 25%
℃
80
volatilization of sodium / %
volatilization of sodium / %
℃
℃
℃
℃
60
40
20
0
0
5
10 15 20 Mass ratio of oil shale / % (a)
80
60
40
20
0
25
815
℃
900
℃
1000 1100 temperature / % （ b） ℃
℃
1200
Fig 6 Sodium volatilization of mixed coal at different blending and different temperature
31
℃
Fig7
Three-dimensional pattern of sodium volatilization of mixed coal at different blending and different temperature
32
55
45
50
volatilization of sodium / %
volatilization of sodium / %
50
40 experimental modeling weighted
35 30 25 20 15 10
45
experimental modeling weighted
40 35 30 25 20 15
5
10
15 20 mass ratio of oil shale / % (a)900
25
0.05
0.10 0.15 0.20 mass ratio of oil shale / x (b)1100
℃
0.25
℃
Fig 8 Modeling sodium volatility vs experimental and weighted result of mixed coal
33
The tables are listed there.
Table 1 The proximate analysis and ultimate analysis Table 2 Ash component analysis Table 3 Distribution of Na in Manasi coal Table4 AFTs of Manasi coal, oil shale and mixed coals Table 5 Sodium retention rate at different blending ratio and temperature Table 6 AFTs of mixed coals at different SiO2 / Al2 O3 ratios
34
Table 1 The proximate analysis and ultimate analysis proximate analysis/wad %
ultimate analysis/wad %
Sample M
A
V
FC
C
H
O*
N
St
MNS
13.10
5.36
23.80
57.74
64.38
3.46
14.15
0.60
0.15
CJR
2.99
81.18
10.76
5.07
9.92
1.67
5.24
0.47
0.18
ad =air dried.
*by difference
35
Table 2 Ash component analysis component
SiO2
Fe2O3
TiO 2
P2O5
CaO
MgO
Al2O3
SO3
K2O
Na2O
MnO2
MNS
17.42
7.58
0.34
0.36
27.28
9.06
9.84
22.56
0.50
4.21
0.36
CJR
65.52
4.59
0.9
0.09
0.33
1.48
23.38
0.06
2.44
0.27
0.12
36
Table 3 Distribution of Na in Manasi coal Content ω / % Element
Na
Water solute
NH4AC solute
HCl solute
insolute
74.36
14.53
7.85
3.26
37
Table4 AFTs of Manasi coal, oil shale and mixed coals Sample
DT（o C）
ST（o C）
HT（o C）
FT（o C）
ST-DT（o C）
MNS
1202
1261
1287
1310
59
CJR
1399
1457
>1500
>1500
58
MC1
1190
1242
1273
1302
52
MC2
1253
1294
1364
1392
41
MC3
1259
1302
1367
1398
43
MC4
1266
1315
1391
1425
48
MC5
1289
1340
1409
1451
51
38
Table 5 Sodium retention rate at different blending ratio and temperature η/% T/℃ MNS
MC1
MC2
MC3
MC4
MC5
815℃
63.89
79.12
86.01
84.21
81.98
81.56
900℃
56.75
76.00
84.27
83.00
81.38
78.34
1000℃
53.57
73.20
83.45
81.78
80.43
77.83
1100℃
51.59
71.60
82.93
80.73
78.80
75.36
1200℃
50.40
70.90
80.69
79.52
78.29
74.88
39
Table 6 AFTs of mixed coals at different SiO2 / Al2 O3 ratios AFTs t / ℃ Sample
Addictive
SiO2 / Al2O3 DT
ST
HT
FT
Sample1
—
2:1
1202
1261
1287
1310
Sample2
oil shale + alumina
2:1
1247
1322
1349
1377
Sample3
oil shale + alumina
3:2
1263
1348
1356
1383
Sample4
oil shale + alumina
1:1
1316
1400
1433
1456
Sample5
oil shale + alumina
2:3
1360
1455
1482
1490
Sample6
oil shale + alumina
1:2
1413
1499
>1500
>1500
40
Highlights: 1.The co-combustion of Zhungdong coal and oil shale was used to study the properties. 2.The AFTs of mixed coal was measured by ashing at final temperature of 1000℃. 3.The minimum fouling and slagging tendency was occurred at blending ratio of 10%. 4.The prediction model of Na volatilization ratio was proposed.
41
S1359-4311(16)34167-9 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.062 ATE 10211
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
16 December 2016 22 February 2017 12 April 2017
Please cite this article as: Y. Lu, Y. Wang, Y. Xu, Y. Li, W. Hao, Y. Zhang, Investigation of ash fusion characteristics and migration of sodium during co-combustion of Zhundong coal and oil shale, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng.2017.04.062
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Investigation of ash fusion characteristics and migration of sodium during co-combustion of Zhundong coal and oil shale
Yang Lu, Ying Wang, Ying Xu, Yuan Li, Wuxing Hao, Yongfa Zhang *
Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province,
Taiyuan University of Technology, Taiyuan 030024, China
ABSTRACT: Zhundong coal, which has high sodium content, induces severe fouling and slagging during its combustion in a boiler. Co-combustion is an effective method for mitigating these processes. In this study, oil shale is selected as an additive during combustion in order to study the properties of mixed fuel. The study results reveal that water-soluble sodium is the predominant component of Zhundong coal, which originates from the salt in seawater. With an increase in the oil shale blending ratio, the ash fusion temperatures of the mixed coal and the difference between softening temperature and deformation temperature first decrease and then increase. The maximum sodium retention of 86.01% occurs at a blending ratio of 10% and a final temperature of 815°C. With an increase in temperature, aluminosilicates, such as albite and nepheline, form in the ash. Therefore sodium gradually migrates from the water-soluble phase to the aluminosilicate phase. With an increase in the SiO2/Al2O3 ratio, the ash fusion temperature of mixed coal decreases gradually, which is attributed to the effects of the SiO 2 and Al2O3 contents on the melting point. Additionally, a mathematical model for sodium volatilization in the coal is constructed by MATLAB R2014a for fitting and optimization.
KEYWORDS: High-sodium coal; Oil shale; Ash fusion characteristic; Sodium retention; Sodium migration
1
* Corresponding Author Tel./Fax: +86-0351-6018676. E-mail: [email protected]
1. Introduction Zhundong region, located in Xinjiang in China, has huge coal reserves. The predicted coal reserve of this region is about 390 billion tons, and its proven coal resource reserve is about 213.6 billion tons. Zhundong coalfield has a coal formation area of 14,000 km2, and it is therefore the largest integrated coalfield in China [1]. The coal found in Zhundong region is mainly high-sodium coal, with sodium content (by ash) higher than 2%. The coal in this region is characterized by its low rank, inflammability, ease of burnout, medium/high water content, low ash content, high volatile content, moderate calorific value, high slagging tendency, high alkali-metal content, and high fouling tendency [2-4]. These distinct properties of Zhundong coal result in problems of fouling, slagging, and corrosion of the heat exchange surface and boiler wall during its combustion in boilers in power plants. These processes not only reduce the heat transfer efficiency of the heat exchange surface but also affect the operation safety and life of boilers in power plants. Thus, the examination of the ash fusion characteristics of high-sodium coal, investigation of the migration mechanism and control technique of sodium volatilization are economically significant and practically meaningful. The problems of slagging and fouling of the sodium in Zhundong coal are mainly caused by two reasons [5]. One is that sodium is highly prone to volatilization at high temperatures, and the other is that eutectics are formed during the combustion of Zhundong coal in the boiler. Wang et al. [6] studied the ash deposition mechanism in boiler burning Zhundong coal and concluded that 80% of sodium was released in the gas phase when the temperature reached 400–800°C. Furthermore, when the temperature reached 800–1000°C, all the remaining sodium volatilized, and sulfur began to be released. Sodium sulfate with a fusion point of 884°C forms by a reaction of sodium with sulfur. The low-melting-point sodium sulfate 2
fouls easily, slags the heat exchange surface and boiler wall, and forms a dense and viscous layer. It captures the fly ash particles in the flue gas, which results in sticky ash deposition at high temperature. Further, with an increase in temperature, sodium might transform into sodium silicates, which mix with iron, calcium, and magnesium oxides to form eutectics [7]. These low-melting-point eutectics also tightly adhere to the heat exchange surface and boiler wall. At present, most Zhundong coals are burnt by being co-fired with low-fouling coal. This technique mitigates fouling to some extent but does not thoroughly solve the practical problems associated with it [8]. In addition, it is rather difficult to select an appropriate type of low-fouling coal. Some researchers proposed the use of pretreated Zhundong coal for this purpose [9]. Despite its effectiveness in the removal of sodium, this approach has a complicated process, rigorous requirements for equipment, and high production cost and equipment cost; furthermore, the waste liquid may result in environmental pollution because of difficulties in its treatment [10]. Shen et al. [11] studied the effects of co-firing of Zhundong coal and kaolin on sodium retention and ash fusion characteristics. Although kaolin co-firing provided a sodium retention effect, kaolin inhibited fuel combustion and reduced the utilization ratio of fuel because of its incombustibility. A soot blower or high-pressure steam purge device was added to the boilers in some power plants; however, this approach has some disadvantages such as a high construction cost, complicated boiler reconstruction technology, unknown risks of the rehabilitated equipment, and incomplete elimination of fouling. Zhang et al. [12] used distilled water, ammonium acetate, dilute hydrochloric acid, and hydrofluoric acid to study the changes in potassium and sodium in bituminous coal and anthracite with changes in the coal rank and particle size, and they analyzed the alkali-metal behaviors at the beginning of combustion. Some researchers believed that the sodium contained in high-sodium coal was mainly water-soluble sodium.
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With an increase in combustion temperature, the water-soluble sodium gradually volatilized to the gas phase [13,14], and this volatilization of water-soluble sodium was the main cause of fouling during ash deposition. The ash fusion temperature was one of the commonly used indices for prediction of fouling caused by high-sodium coal, which was significantly affected by the mineral components in the coal. Zhang et al. [15] used mixed coal or a mineral additive to adjust the coal ash fusion temperature in order to inhibit sodium volatilization and fouling. Kyi et al. [16] studied the adsorbability of 12 minerals in compounds such as NaCl, etc., and found that minerals containing high amounts of elements, such as Si and Al had strong sodium adsorbability. Wang et al. [17] mixed Zhundong coal with high-Si coal extracted from Heilongjiang in different ratios for ashing at different final temperatures and found that when the SiO2/NaO2 ratio was more than 20 or the (SiO2 + Al2O3)/Na2O ratio was more than 25, a small amount of sodium-containing substance volatilized and the substance containing Si and Al captured the sodium element in aluminosilicate, resulting in effective mitigation of fouling. A new fuel, oil shale, was discovered in Changji region in Xinjiang. Its predicted reserve is 54.8 billion tons, and its proven reserve is 459 million tons [18]. Oil shale is flammable sedimentary rock with high Si and Al contents [19]. These two elements, being the beneficial components of coal ash, inhibit sodium volatilization and fouling during high-sodium-coal combustion and enhance the ash fusion temperature of Zhundong coal as a refractory. Thus, in the present study, the co-combustion of Manasi high-sodium coal and oil shale extracted from Changji region was used to extensively study the properties of mixed fuel.
2. Experimental Section 2.1 Coal Quality Analysis The experimental study employed Manasi (a county in Xinjiang Province) high-sodium coal (MNS) and Changji region oil shale (CJR), which were crushed, ground, and sieved to less than 74 μm. Results of
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proximate analysis and ultimate analysis of Manasi coal and oil shale are listed in Table 1. Manasi coal was mixed with 5%, 10%, 15%, 20%, and 25% oil shale to obtain mixed coal samples, termed MC1, MC2, MC3, MC4, and MC5, respectively. These samples were dried in a thermostatic drying oven at 105°C for 2 h and stored in sealed bottles for later use.
2.2 Experiment and Analysis Method The Manasi coal sample was crushed in the GJ-1 series laboratory sealed sample crusher, sieved using a GZS standard automatic screening machine, immersed in a water bath with a DF-101S heat-gathering constant magnetic stirrer, and incinerated in an SRJK-2.5-13 high-temperature tubular furnace; the furnace has a maximum temperature of 1350°C, hearth dimensions of φ22 × 180 mm, voltage range of 0–220 V, and power of 2.5 kW. The minerals in the ash sample were analyzed using the MiniFlex 600 X-ray diffractometer (Rigaku Corporation) with Cu Kα as the radiation source at a tube voltage of 40 kV, tube current of 15 mA, scanning speed of 8°·min-1, and scanning range of 20°–60° and the calculations were performed using Jade 6.5 software. Four characteristic fusion temperatures of the ash sample were determined using the computer-controlled SJHR-3 smart ash fusion point analyzer consisting of a high-temperature furnace (maximum temperature of up to 1600°C), temperature control system, image acquisition system, and computer control system. The sodium content of the ash sample was determined using the ICAP 6000 Series inductively coupled plasma (ICP) atomic emission spectrometer (Thermo Fisher Scientific) at a radio-frequency power of 1.0 kW, plasma gas flow rate of 15.0 L·min-1, auxiliary gas flow rate of 0.5 L·min-1, atomization gas flow rate of 0.75 L·min-1, peristaltic pump flow rate of 1.5 mL·min-1, and integration time of 5 s (average of three tests). Radial observation.
2.3 Analysis of Occurrence Mode of Sodium The occurrence mode of sodium was analyzed by the sequential extraction method. The Manasi
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high-sodium coal sample with a particle size of less than 74 μm was used in the experiment. Air-dried coal (1 g) was immersed in 50 mL of highly purified water and heated in a water bath at 60°C for over 24 h and filtered with a microfilter before the filtrate was diluted to 100 mL. Residual solid was treated with 1 mol/L ammonium acetate and 1 mol/L dilute hydrochloric acid by the above procedure. The final residue was dried and digested with a mixture of hydrofluoric acid and perchloric acid before the digestion solution and filtrate were analyzed by ICP-optical emission spectrometry (ICP-OES).
2.4 Low-Temperature Ashing Some minerals escaped in the form of gases at a certain temperature during coal combustion. The higher the slagging tendency of the coal, the larger were the amounts of corresponding minerals that escaped. Fan et al. [20] found that the sodium escape ratio of an ash sample determined by the national standard method of China was 27.36%. Zhang et al. [21] studied the difference in composition between Zhundong coal ashes subjected to incineration at 575°C and 815°C and concluded that the coal sample was completely incinerated and the sodium escape ratio was minimized at the final temperature of 575°C. Because of a large error in the sodium content (by ash) of Zhundong coal after incineration at high temperature, low-temperature ashing was employed in this study. Manasi coal was experimentally incinerated in a tubular furnace at a heating rate of 10°C·min-1, with a final temperature of 575°C, and kept for 2 h under air atmosphere with an airflow rate of 0.5 L·min-1 in order to achieve complete incineration. Oil shale was incinerated at 815°C in accordance with the national standard method of China. The resultant ash sample was analyzed according to the GB/T1574-2007 Chinese standard. Results of the ash component analysis are listed in Table 2.
2.5 High-temperature Ashing Manasi coal and oil shale were mixed in five blending ratios which were mentioned in 2.1. Then, the
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mixed coal sample was incinerated in a high-temperature tubular furnace at a heating rate of 10°C·min-1 below 900°C and at a heating rate of 5°C·min-1 above 900°C under air atmosphere with an air flow rate of 0.5 L·min-1; subsequently, the sample was kept at final temperatures of 815°C, 900°C, 1000°C, 1100°C, and 1200°C for 2 h in order to achieve complete incineration. Then, ash samples with different blending ratios were obtained by ashing at different temperatures.
2.6 Chilling and Drying of Ash at High Temperature The ash samples obtained by high-temperature ashing were rapidly immersed in liquid nitrogen in order to chill them to less than 300°C within 2 s to prevent a crystal phase change during the temperature drop. After the samples cooled down to room temperature, they were dried in a thermostatic drying oven at 105°C for 2 h. The mineral contents and sodium contents of the cooled samples were determined by X-ray diffraction (XRD) and ICP–OES, respectively.
2.7 Ash Fusion Characteristics of Coal Pyramids of the ash sample and dextrin were produced according to the GB/T219-2008 standard. They were then tested in the high-temperature furnace by using an ash fusion point testing apparatus to determine the following four characteristic ash fusion temperatures of the Manasi coal, oil shale, and mixed coal samples under a weakly reducing gas atmosphere: deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT).
2.8 Sodium Content Determination In accordance with the MT/T1014-2006 standard, the mixed coal samples were incinerated at various target final temperatures and then digested with a hydrofluoric acid-perchloric acid mixture to obtain the sample solution before the following tasks: preparation of blank solutions and standard solutions, standard curve plotting, determination of the absorbency of the blank solutions and sample solutions by ICP-atomic
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emission spectroscopy (ICP-AES), and calculation of the sodium contents of ash samples.
2.9 Sodium Retention and Sodium Volatility To better describe the volatilization characteristics of sodium during mixed coal combustion, the following parameters were introduced for characterization. Sodium content, m1 (mg·g−1), of raw-coal ash
m1 m1' A
(1)
where m1 = sodium content of coal ash after combustion, mg·g−1, and '
A = percentage of ash produced from coal combustion at temperature T. Percentage of sodium residue in raw-coal ash, i (%)
i
m1 m' A 100% 1 100% (2) m0 m0
where m0 = sodium content of coal before combustion, mg·g−1. Sodium volatility, j (%), at combustion temperature T
j=1 i
(3)
The sodium retention effect of oil shale was described as the sodium retention η, which was calculated as follows:
η
M Na ,MC (a , T) M Na，MNS M Na，CJR
100%
(4)
where MNa,MC(a,T) = mass of sodium in the mixed coal with an oil shale blending ratio of a and final combustion temperature of T; MNa,MNS = mass of sodium in Manasi coal; and MNa,CJR = mass of sodium in Changji region oil shale.
2.10 Development of Prediction Model of Sodium Volatility by MATLAB The binary relationship among the sodium volatility V T, oil shale ratio X, and temperature T, i.e., VT =
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f(X, T), was obtained with MATLAB R2014a by using the particle swarm optimization algorithm, a polynomial fitting method, and the MATLAB Curve Fitting Toolbox. This equation was determined in order to obtain the curve fitting equation for sodium volatility at different ratios and ashing temperatures. The curve fitting images were obtained, and then, the sum of squared errors and goodness of fit were calculated by the cftool function in MATLAB.
3. Results and Discussion 3.1 Occurrence Mode of Sodium in Manasi Coal The occurrence mode of sodium in Manasi coal was determined by the sequential extraction method [22]. Highly purified water, ammonium acetate, and dilute hydrochloric acid were used as the extractants. The sodium occurrence modes in the coals as determined using different extractants are listed in Table 3. The sodium present in Zhundong coal included water-soluble sodium, ammonium-acetate-soluble sodium, hydrochloric-acid-soluble sodium, and insoluble sodium [23]. Highly purified water could act as an extractant to extract NaCl crystals and inorganic sodium containing hydrates. The ammonium acetate solution could extract ion-exchangeable ions in the form of carboxylates. Organic sodium in the coordinate form on nitrogen-containing or oxygen-containing functional groups in the coal structure could be extracted only by dilute hydrochloric acid, but the sodium present in the residue obtained after sequential extraction was believed to be insoluble sodium, which mainly included some aluminosilicates and some salts in the coal matrix. It can be seen from Table 3 that the sodium present in Manasi coal was mainly water-soluble sodium, and the content of this water-soluble sodium accounted for 74.36% of the total sodium. Also, the contents of ammonium-acetate-soluble sodium, hydrochloric-acid-soluble sodium, and insoluble sodium were 14.53%, 7.85%, and 3.26%, respectively. These contents were related to the formation and evolution of
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Manasi coal. Salt produced by seawater evaporation during geological changes and inorganic salt formed when the coal-forming plants absorbed water were the main sources of the water-soluble sodium. The organic sodium originated from the organic components of the coal-forming plants, whereas the insoluble sodium originated from the environment during evolution [24]. Volatilization of water-soluble sodium to the gas phase was one of the main reasons for boiler fouling and slagging during the combustion of Manasi coal [12].
3.2 Ash Fusion Characteristics of Manasi Coal, Oil Shale, and Mixed Coal Sodium escaped from Zhundong high-sodium coal during high-temperature ashing, and the reduction in the sodium content led to a high ash fusion temperature and large experimental error. Therefore, the ash fusion temperature of Manasi coal was determined by low-temperature ashing and combustion at the final temperature of 575°C to preserve the raw minerals in Manasi coal. The ash fusion temperature of oil shale was determined at a final temperature of 815°C. The ash fusion temperature of the mixed coal was determined by ashing at 1000°C because of the formation of a high-fusion-point substance at about 1000°C. The four characteristic ash fusion temperatures of Manasi coal, oil shale, and mixed coal are listed in Table 4. It can be seen from Table 4 that the four characteristic ash fusion temperatures of Manasi coal, oil shale, and mixed coal were not linearly correlated. The ash fusion temperature of coal was related to several factors such as the mineral components of ash, the reaction between mineral components during ashing, and the formation of a low-temperature eutectic. Acidic oxides such as SiO2, Al2O3, and TiO2 are refractory minerals; the higher their content, the higher is the ash fusion temperature of coal. On the other hand, acidic oxides such as Fe2O3, CaO, MgO, Na2O, and K2O are fluxing minerals; the higher their content, the lower is the ash fusion temperature of coal [25]. Manasi coal had high contents of alkali metals and sulfur.
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It could be seen from Table 2 that high-sodium coal ash had CaO, Na2O, and SO3 contents of 27.28%, 4.21%, and 22.56%, respectively. Substances with a low fusion point and high viscosity, such as NaSO4 and CaSO4, which were formed during high-temperature ashing, lowered the four characteristic ash fusion temperatures of high-sodium coal. Thus, this was one of the crucial reasons for fouling during the combustion of high-sodium coal. It can be seen from Fig. 1 that with an increase in the oil shale blending ratio, the four characteristic ash fusion temperatures of the mixed coal sample first decreased and then increased. First, when the oil shale blending ratio was in the range of 0%–5%, the four characteristic ash fusion temperatures decreased gradually; when the ratio was in the range of 5%–10%, the four characteristic ash fusion temperatures increased rapidly; finally, when the ratio was in the range of 10%–25%, the four temperatures increased stably. The mixed coal sample consisting of Manasi coal and oil shale had higher contents of refractory substances such as SiO2 and Al2O3 but lower contents of fluxing substances, such as Na 2O and CaO. Hence, more products with high fusion points, such as SiO2, mullite, and nepheline were formed during high-temperature ashing. The fusion point of crystals was affected by their binding energy. Atom crystals and ion crystals had high binding energies. The higher the binding energy of ion crystals, the higher is the fusion point of the product [26]. SiO2, mullite, and nepheline have high crystal binding energies and stable molecular structures. An increase in the content of minerals with high crystal binding energies during combustion of the oil-shale-containing mixed coal increased the ash fusion temperature of coal. Al2O3 in coal ash functioned as a visible skeleton during fusion. The higher the Al2O3 content of coal ash, the higher is its ash fusion temperature [27]. With an increase in the oil shale blending ratio between 0% and 25%, the DT increased by 87°C and the FT increased by 141°C, indicating that oil shale improved the coal ash fusion temperature to a great extent. The mineral component ratio of the ash sample resulting from the
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combustion of Manasi coal with an oil shale blending ratio of 5% was in the low-temperature eutectic region, finally resulting in a lowered fusion point of the ash sample. This is in agreement with the explanation of a low-temperature eutectic region in the ternary phase diagram provided by Li et al. [28,29]. Wei et al. [30] believed that the difference between the ST and the DT led to the formation of highly viscous ash at lower temperature and severer fouling during combustion of Zhundong coal. It can be seen from Fig. 2 that with an increase in the oil shale blending ratio, the difference between the ST and the DT first decreased rapidly and then increased rapidly. The difference between these two temperatures was minimum when the blending ratio was 10%; and the fouling and slagging tendencies were also lowest at this ratio.
3.3 Effect of Oil Shale Blending Ratio on Sodium Retention It can be seen from Table 5 that the sodium content of the Manasi raw coal ash accounted for 50%–60% of the total sodium. Therefore, 40%–50% of sodium-containing substance volatilized from raw coal to the gas phase during combustion and reacted with sulfides to form viscous substances, resulting in fouling and slagging of the coal-fired boiler [31]. After oil shale was added to Manasi raw coal, a higher amount of sodium was retained in coal ash, with the sodium retention being as high as 70%–80%. This indicates that oil shale had a strong sodium retention effect, and the Si and Al in oil shale captured the sodium in Manasi coal to form high-fusion-point aluminosilicates, which were discharged from the boiler by the deslagging system. It can be seen from the curve of the sodium retention effect in Fig. 3 that with an increase in the blending ratio, the sodium capture efficiency first increased rapidly and then decreased gradually. When the oil shale blending ratio was between 5% and 10%, the oil shale reactivity was high and it increased rapidly; when the blending ratio increased beyond 10%, the sodium retention effect weakened stably
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because most of the sodium had participated in the reaction. The maximum sodium retention of 86.01% occurred at the oil shale blending ratio of 10% and combustion temperature of 815°C. With an increase in temperature from 815°C to 1200°C, the capture efficiency of sodium in the mixed coal ash decreased slightly, suggesting that oil shale provided a better sodium retention effect at low temperature than at high temperature. With increasing combustion temperature, both the sodium release rate and the sodium capture rate of oil shale increased. The release rate was slightly higher than the capture rate, so the sodium capture efficiency decreased slightly. With increasing temperature, the sintering property of coal improved gradually, and the surface areas of particles of Manasi coal and oil shale decreased and hindered reaction occurrence. Furthermore, some of the SiO 2 and Al2O3 in oil shale reacted to form mullite at high temperature, resulting in inhibition of the reaction with sodium and a further decrease in the sodium capture efficiency [32].
3.4 Mineral Evolution in Manasi Coal and Mixed Coal at High Temperatures The XRD patterns of Manasi coal ash at the final temperature of 575°C and those of mixed coal ash at final temperatures of 815°C, 900°C, 1000°C, 1100°C, and 1200°C are shown in Fig. 4. Only the 20°–60° XRD patterns are shown here, because the coal sample had weak absorption peaks at 10°–20° and 60°–90°. Fig. 4(a) shows the XRD pattern of the ash obtained from combustion of Manasi coal at the final temperature of 575°C. It can be seen from Fig. 4(a) that the primary minerals in Manasi coal were mainly calcite (CaCO3), anhydrite (CaSO4), and quartz (SiO2); This coal also contained thenardite (NaSO4) and rankinite (CaSiO3) in minor quantities. Manasi coal was also high calcium coal, and contained many kinds of calcareous minerals and had high calcareous mineral content, which was consistent with the ash composition analysis results in Table 2.
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Figs. 4(b)–(d) show XRD patterns of the ash obtained from combustion of the mixed coal sample consisting of 90% Manasi coal and 10% oil shale at final temperatures of 815°C, 900°C, 1000°C, 1100°C, and 1200°C. It can be seen from Figs. 4(b)–(d) that the ash obtained from the combustion of Manasi coal and mixed coal at each final temperature mainly contained quartz with a fusion point of 1710°C. All the absorption peaks of NaSO4 disappeared at the combustion temperature of 815°C, and the entire amount of NaSO4 volatilized to the gas phase. CaO in Manasi coal reacted with SiO2 and Al2O3 in oil shale to form anorthite (CaAl2Si2O8) with a fusion point of 1553°C. This reaction is given as CaO + 2 Al2O3 + SiO2 → CaAl2Si2O8. Calcite disappeared when the combustion temperature reached 900°C, suggesting that calcite decomposed into CaO and CO2 as CaCO3 → CaO + CO2. Moreover, NaCl in Manasi coal reacted with SiO2 and Al2O3 in oil shale to form albite (NaAlSi3O8) with a fusion point of 1180°C. This reaction is given as 2 NaCl + 2 SiO2 + Al2O3 + H2O → 2 NaAlSiO4 + 2 HCl. Albite readily reacted with CaO and other aluminosilicates to form a eutectic with a low fusion point, resulting in a decrease in the fusion point of the mixed coal ash. When the combustion temperature reached 1000°C, albite further decomposed into nepheline (NaAlSiO4) with a fusion point of 1550°C and SiO2 as NaAlSi3O8 → NaAlSiO4 + 2 SiO2. Nepheline was also produced by the reaction of NaCl with SiO2 and Al2O3 as 2 NaCl + 6 SiO2 + Al2O3 + H2O → 2 NaAlSi3O8 + 2 HCl. In addition, SiO2 and Al2O3 reacted to form mullite (3Al2O3·2SiO2) with a fusion point as high as 1850°C. This reaction is given as 3 Al2O3 + 2 SiO2 → 3Al2O3·2SiO2. Rankinite decomposed into CaO and SiO 2 as CaSiO3 → CaO + SiO2. When the combustion temperature reached 1100°C, MgO and SiO2 reacted to form sepiolite (Mg4Si6O15(OH)2) with a fusion point of 1500–1700°C. This reaction is given as 4 MgO + 6 SiO2 + H2O → Mg4Si6O15(OH)2. Further, NaCl and CaO in Manasi coal reacted with the SiO2 and Al2O3 in oil shale to form feldspar minerals ((Ca, Na)(Si, Al)4O8) as NaCl + CaO + SiO2 + Al2O3 → (Ca, Na)(Si, Al)4O8. When the temperature reached 1100°C, all anhydrite
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disappeared, indicating its decomposition into CaO and SO 2 as CaSO4 → CaO + SO2. When the combustion temperature reached 1200°C, the reaction product was in accord with that at 1100°C, suggesting that no new substance was formed, because of the complete reaction of all the mineral elements in the mixed coal. The XRD patterns obtained at the various final combustion temperatures showed that the sodium gradually migrated from the water-soluble phase to the aluminosilicate phase, whereas it was deposited in coal ash in the solid form by the Si and Al in oil shale and discharged along with slag. Li et al. [26] concluded from a comparison of Fukui functions that nucleophiles preferably attacked Si(6) and Si(8) to break and reconstruct Si-O bonds whereas electrophiles attacked O(26) and O(22) to break Al-O bonds. Ma et al. [33] pointed out that when silicate reacted with R-O bonds (where R = alkali-metal ions or alkaline-earth-metal ions) in a coal ash fusant, the oxygen ions from the R-O bonds could become attracted to their surroundings to break the bridging oxygen bonds, i.e. Si-O-Si. Thus, the oxides of the alkali metal Na and the alkaline-earth metal Ca in coal nucleophilically reacted with Si(6) and Si(8) in oil shale in the presence of O2- as a nucleophile at different temperatures to produce anorthite, albite, nepheline, etc. These reactions are given as CaO + 2 Al2O3 + SiO2 → CaAl2Si2O8, Na2O + 6 SiO2 + Al2O3 → 2 NaAlSi3O8, and Na2O + 2 SiO2 + Al2O3 → 2 NaAlSiO4.
3.5 Comparison of Sodium Retention and Ash Fusion Characteristics Fig. 5 shows a comparison of the sodium retention and coal ash fusion characteristic (i.e., the ST) at different blending ratios at the combustion temperature of 1200°C. It can be seen from Fig. 5 that a blending ratio of about 5% (1 in Fig. 5), ash fusion point of about 1200°C, and sodium retention of about 70% were favorable for liquid slag removal, whereas a blending ratio in the range of 15%–25% (2 in Fig. 5), ash fusion point higher than 1300°C, and sodium retention of 75%–80% were favorable for solid slag
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removal. To meet the requirements of different boilers in power plants for slag removal method and sodium retention, an appropriate blending ratio could be selected according to the actual combustion temperature in the boilers in power plants.
3.6 Effects of SiO2/Al2O3 Ratio on Ash Fusion Characteristics of Manasi Coal The effects of the SiO 2/Al2O3 ratio on the ash fusion characteristics of Manasi coal were studied by selecting an oil shale–bauxite (Al2O3) mixture with different ratios as the composite mineral additive and adjusting the SiO2/Al2O3 ratios in the mixed ash sample. The results are listed in Table 6. The mineral composition of coal ash is one of the main factors governing its fusion characteristics. The SiO2/Al2O3 ratio has been reported to significantly affect the coal ash fusion temperature [34]. It can be seen from Table 6 that the Manasi coal ash fusion temperature decreased gradually with increasing SiO2/Al2O3 ratio, which is attributed to the effects of the SiO 2 and Al2O3 contents. Al2O3 in coal always enhances the ash fusion temperature, but the SiO2 content is not significantly related to the ash fusion temperature [35]. When the SiO2/Al2O3 ratio was between 2:1 and 3:2, the ash fusion temperature did not change significantly, because a high SiO2/Al2O3 ratio meant more SiO2, which existed in the noncrystalline form in coal ash and was a smaller contributor to the ash fusion temperature [36]. When the SiO 2/Al2O3 ratio was 1:1, a considerable amount of mullite with a high ash fusion point was produced during combustion, resulting in a significant increase in the ash fusion temperature. When the SiO 2/Al2O3 ratio was above 1:1, the SiO2 content was high and SiO2, silicate, and calcareous minerals in the mixed ash sample formed a low-temperature eutectic, which resulted in a decrease in the ash fusion temperature. When the SiO 2/Al2O3 ratio was between 2:3 and 1:2, the ash fusion temperature increased greatly because a low SiO2/Al2O3 ratio meant more Al2O3, which existed in the form of ion crystal and had a high binding energy and fusion point; this resulted in an increase in the ash fusion temperature. When the SiO2/Al2O3
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ratio was 1:2, a considerable amount of Al2O3 existed in the ash sample, so the ST exceeded 1500°C. However, the SiO2/Al2O3 ratio in raw coal was about 2:1, and addition of composite additives enhanced the ash fusion temperature of the mixed coal samples slightly because the composite additives reacted with the mineral components of raw coal to form substances with a high crystal binding energy and high fusion point during combustion.
3.7 Prediction Model of Sodium Volatility in Mixed Coal 3.7.1 Sodium Volatility in Mixed Coal The effects of blending ratio and temperature on the sodium volatility in mixed coal are shown in Fig. 6. (1) Effects of blending ratio - It can be seen from Fig. 6(a) that the mixtures of Manasi coal and oil shale at different ratios reduced the sodium volatility to a much greater extent than did raw coal. With an increase in the blending ratio, the sodium volatility first decreased and then increased because the sodium in Manasi coal reacted with the Si and Al in oil shale to form albite and nepheline. Sodium was attached to aluminosilicate, which inhibited its volatilization. The maximum sodium volatility of 49.60% occurred at the raw coal combustion temperature of 1200°C, whereas the minimum sodium volatility of 13.99% occurred at the oil shale blending ratio of 10% and final combustion temperature of 815°C. (2) Effects of temperature - It can be seen from Fig. 6(b) that with an increase in temperature, the sodium volatility in the Manasi coal and mixed coal samples increased and the residual ratio gradually decreased. This is because with an increase in temperature, the coefficient of sodium diffusion from within the coal particles to their surfaces and from their surfaces to their surroundings increased, the sodium diffusion rate in coal increased, and the escape rates of moisture and volatile compounds from coal also increased, resulting in the formation of new micropores in the inner structure of coal and release of more sodium.
3.7.2 Sodium Volatility Prediction Model
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It was found from the above analysis that the sodium volatility in coal was closely related to the combustion temperature and blending ratio, and oil shale strongly inhibited the sodium volatility. Fig. 7 shows the three-dimensional fitting curve for sodium volatility in the mixed coal sample at different ratios and different final combustion temperatures, as obtained by MATLAB. It can be seen from Fig. 7 that the sodium volatility in the mixed coal sample first decreased rapidly and then increased stably at different temperatures, and the minimum sodium volatility occurred at the oil shale blending ratio of 10%. At the same oil shale blending ratio, the higher the temperature, the higher was the sodium volatility. To describe the sodium volatility in mixed coal, its prediction model, i.e., the binary function relationship among sodium volatility VT, oil shale blending ratio X, and combustion temperature T, was proposed as VT = f (X, T). MATLAB R2014a was used for fitting and optimization. VT = (a0 + a1X + a2X2 + a3X3 + a4X4) × (b0 + b1T + b2T2 + b3T3) (5) where VT = Total sodium volatility in the mixed coal sample at temperature T; X = Mass fraction of oil shale; and a0 = 1.1891 × 10-6, a1 = -2.5116 × 10-5, a2 = 2.4482 × 10-4, a3 = -9.9666 × 10-4, a4 = 1.4685 × 10-3, b0 = 0.9444, b1 = 1.2946, b2 = 1.2815, and b3 = -6.7697 × 10-4. The goodness of fit, i.e., the R-squared value, was 98.11%; the sum of squared errors, SSE, was 0.07%; the maximum error was 1.29%; and the minimum error was 0.09%. The error range was smaller and the degree of fitting was higher. The sodium volatility in the mixed coal sample was calculated at 900°C and 1100°C at different blending ratios according to the model given in (5), and the calculation results were compared with experimental values and weighted values as shown in Fig. 8. It can be seen from Fig. 8 that the calculation results obtained using the sodium volatility prediction
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model in (5) for the mixed coal sample at different temperatures and blending ratios agreed well with the experimental values but differed considerably from simply weighted values. This model was more suitable for prediction of sodium volatilization characteristics in mixed coal. The sodium volatilization characteristics in coal are related to several factors such as the contents of ash and volatile compounds, the content and occurrence mode of sodium, mineral components, and combustion conditions [37]. Sodium volatilization in Manasi coal was observed to be a highly complicated physicochemical change. A prediction model of sodium volatilization characteristics in coal has rarely been reported in the literature. In the present work, on the basis of existing research techniques and calculation methods, a prediction model of sodium volatility in mixed coal was preliminarily studied, and the sodium volatilization characteristics in a mixed coal sample were predicted at different temperatures and oil shale blending ratios in order to provide technical guidelines for ensuring combustion safety of Zhundong coal in the boilers in power plants.
4. Conclusions (1)
The sodium contained in Manasi coal was in the form of
ammonium-acetate-soluble
sodium,
hydrochloric-acid-soluble
sodium,
water-soluble sodium,
and
insoluble
sodium.
Water-soluble sodium was found to be the predominant chemical form of sodium in Manasi coal, which accounted for 74.36% of the total sodium. This result was related to the formation and evolution of Manasi coal. (2) With an increase in the oil shale blending ratio, the ash fusion temperatures and the difference between the ST and the DT first decreased and then increased. When the blending ratio reached 5%, the ash fusion temperatures decreased owing to the formation of a low-temperature eutectic. The formation of SiO2, mullite, and nepheline led to an increase in the ash fusion temperature. When the blending ratio was 10%,
19
the fouling and slagging tendencies were observed to be lowest. (3) During combustion, 40%–50% of sodium volatilized to the gas phase and reacted with sulfides to form viscous substances, resulting in fouling and slagging of a coal-fired boiler. With an increase in the oil shale blending ratio, the sodium retention first increased and then decreased. Oil shale provided a better sodium retention effect at low temperature than at high temperature. The maximum sodium retention of 86.01% occurred at the blending ratio of 10% and final temperature of 815°C. (4) The primary minerals in Manasi coal were mainly calcite, anhydrite, and quartz; this coal also contained thenardite and rankinite in minor amounts. With an increase in temperature, aluminosilicates such as albite and nepheline formed in the ash after blending with oil shale. The sodium gradually migrated from the water-soluble phase to the aluminosilicate phase, which was eventually discharged from the boiler by the deslagging system. (5) The sodium retention was about 70% at a blending ratio of about 5% and ash fusion temperature of about 1200°C; these conditions were favorable for liquid slag removal. Further, the sodium retention was 75%–80% at a blending ratio between 15% and 25% and an ash fusion temperature above 1300°C; these conditions were favorable for solid slag removal. (6) With an increase in the SiO2/Al2O3 ratio, the ash fusion temperature of Manasi coal decreased gradually. The fusion point did not change significantly when the SiO 2/Al2O3 ratio was between 2:1 and 3:2, but it increased greatly when the SiO 2/Al2O3 ratio was between 1:1 and 1:2. This behavior was related to the effects of the SiO2 and Al2O3 contents. (7) A mathematical model for prediction of sodium volatility in mixed fuel was constructed using MATLAB R2014a by the particle swarm optimization algorithm and polynomial fitting methods. This model is given as VT = (a0 + a1X + a2X2 + a3X3 + a4X4) × (b0 + b1T + b2T2 + b3T3). The calculation results
20
of this model agreed well with experimental results.
Acknowledgments
This work was supported by the National Key Technology Support Program of China (Grant No. 2012BAA04B03) and the National Natural Science Foundation of China (Grant No. 21576182).
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The figures are listed there. Fig 1 Variation trend of AFTs of mixed coals at different blending ratio
Fig 2 Variation trend of ST-DT of mixed coals at different blending ratio Fig 3 Variation trend of sodium retention rate at different blending ratio and temperature Fig 4 X-ray diffraction patterns of ash samples of single coal and mixed coal Fig 5 Comparison of sodium retention（1200℃）and ash fusion temperature（ST） Fig 6
Sodium volatilization of mixed coal at different blending and different temperature
Fig7
Three-dimensional pattern of sodium volatilization of mixed coal at different blending and different temperature Fig 8 Modeling sodium volatility vs experimental and weighted result of mixed coal
25
1500 DT ST HT LT
1450
AFTs /
℃
1400 1350 1300 1250 1200 1150
0
5
10
15
20
25
mass ratio of oil shale / %
Fig 1 Variation trend of AFTs of mixed coals at different blending ratio
26
60
T/
℃
55
50
45
40 0
5
10 15 20 mass ratio of oil shale / %
25
Fig 2 Variation trend of ST-DT of mixed coals at different blending ratio
27
90
sodium retention rate / %
85 80 75 70
815℃ 900℃ 1000℃ 1100℃ 1200℃
65 60 55 50 0
5
10 15 20 mass ratio of oil shale / %
25
Fig 3 Variation trend of sodium retention rate at different blending ratio and temperature
28
2
2 4 1
3 13
20
2
30
40 2θ / (° )
50
60
℃
20
1 1 41 3 13 1 2 3 4
30
40 2θ / (° )
62 4
50
60
(a)
1
℃
（ d） 1000
℃
intensity/(a.u.)
intensity/(a,u)
（ c） 900
4
1
6
3 6 8 1 31 8
6
8
20
30
4
40 2θ /( ° )
3
1
50
11 9
30
20
40
40 2θ /(° )
1
60
3
9
9
50
60
℃
5
9 6
11
2θ / (° )
1 9
（ f ） 1200
9 10
6 50
17 1
2
111 9
1
1
intensity/(a.u.) 1
6 71
30
6 1
7
℃
10 11 6
9
9
60
11 10 9 9
3
3 5 3 4 6
（ e ） 1100
1
20
3 6 2
6
3
2
1
1
1 4
3
2 5
（ b） 815
3
1
2
intensity/(a.u.)
1 ℃
intensity/(a.u.)
intensity/(a.u.)
（ a） 575
20
30
9 1
1
6
1
11
40 2θ / (° )
50
Fig 4 X-ray diffraction patterns of ash samples of single coal and mixed coal 1：quartz；2：calcite；3：anhydrite；4：rankinite；5：thenardite；6：anorthite； 7：mullite；8：albite；9：nepheline；10：sepiolite；11：anorthite Na-rich
29
60
1350
85 η(1200
℃
)
80 75
ST
ST /
65
2
1
η/%
70
℃
1300
60 1250
55 50 0
5
10
15
20
25
mass ratio of oil shale / %
Fig 5 Comparison of sodium retention（1200℃）and ash fusion temperature（ST）
30
100
100 815 900 1000 1100 1200
0% 5% 10% 15% 20% 25%
℃
80
volatilization of sodium / %
volatilization of sodium / %
℃
℃
℃
℃
60
40
20
0
0
5
10 15 20 Mass ratio of oil shale / % (a)
80
60
40
20
0
25
815
℃
900
℃
1000 1100 temperature / % （ b） ℃
℃
1200
Fig 6 Sodium volatilization of mixed coal at different blending and different temperature
31
℃
Fig7
Three-dimensional pattern of sodium volatilization of mixed coal at different blending and different temperature
32
55
45
50
volatilization of sodium / %
volatilization of sodium / %
50
40 experimental modeling weighted
35 30 25 20 15 10
45
experimental modeling weighted
40 35 30 25 20 15
5
10
15 20 mass ratio of oil shale / % (a)900
25
0.05
0.10 0.15 0.20 mass ratio of oil shale / x (b)1100
℃
0.25
℃
Fig 8 Modeling sodium volatility vs experimental and weighted result of mixed coal
33
The tables are listed there.
Table 1 The proximate analysis and ultimate analysis Table 2 Ash component analysis Table 3 Distribution of Na in Manasi coal Table4 AFTs of Manasi coal, oil shale and mixed coals Table 5 Sodium retention rate at different blending ratio and temperature Table 6 AFTs of mixed coals at different SiO2 / Al2 O3 ratios
34
Table 1 The proximate analysis and ultimate analysis proximate analysis/wad %
ultimate analysis/wad %
Sample M
A
V
FC
C
H
O*
N
St
MNS
13.10
5.36
23.80
57.74
64.38
3.46
14.15
0.60
0.15
CJR
2.99
81.18
10.76
5.07
9.92
1.67
5.24
0.47
0.18
ad =air dried.
*by difference
35
Table 2 Ash component analysis component
SiO2
Fe2O3
TiO 2
P2O5
CaO
MgO
Al2O3
SO3
K2O
Na2O
MnO2
MNS
17.42
7.58
0.34
0.36
27.28
9.06
9.84
22.56
0.50
4.21
0.36
CJR
65.52
4.59
0.9
0.09
0.33
1.48
23.38
0.06
2.44
0.27
0.12
36
Table 3 Distribution of Na in Manasi coal Content ω / % Element
Na
Water solute
NH4AC solute
HCl solute
insolute
74.36
14.53
7.85
3.26
37
Table4 AFTs of Manasi coal, oil shale and mixed coals Sample
DT（o C）
ST（o C）
HT（o C）
FT（o C）
ST-DT（o C）
MNS
1202
1261
1287
1310
59
CJR
1399
1457
>1500
>1500
58
MC1
1190
1242
1273
1302
52
MC2
1253
1294
1364
1392
41
MC3
1259
1302
1367
1398
43
MC4
1266
1315
1391
1425
48
MC5
1289
1340
1409
1451
51
38
Table 5 Sodium retention rate at different blending ratio and temperature η/% T/℃ MNS
MC1
MC2
MC3
MC4
MC5
815℃
63.89
79.12
86.01
84.21
81.98
81.56
900℃
56.75
76.00
84.27
83.00
81.38
78.34
1000℃
53.57
73.20
83.45
81.78
80.43
77.83
1100℃
51.59
71.60
82.93
80.73
78.80
75.36
1200℃
50.40
70.90
80.69
79.52
78.29
74.88
39
Table 6 AFTs of mixed coals at different SiO2 / Al2 O3 ratios AFTs t / ℃ Sample
Addictive
SiO2 / Al2O3 DT
ST
HT
FT
Sample1
—
2:1
1202
1261
1287
1310
Sample2
oil shale + alumina
2:1
1247
1322
1349
1377
Sample3
oil shale + alumina
3:2
1263
1348
1356
1383
Sample4
oil shale + alumina
1:1
1316
1400
1433
1456
Sample5
oil shale + alumina
2:3
1360
1455
1482
1490
Sample6
oil shale + alumina
1:2
1413
1499
>1500
>1500
40
Highlights: 1.The co-combustion of Zhungdong coal and oil shale was used to study the properties. 2.The AFTs of mixed coal was measured by ashing at final temperature of 1000℃. 3.The minimum fouling and slagging tendency was occurred at blending ratio of 10%. 4.The prediction model of Na volatilization ratio was proposed.
41