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Rapidly developing economy and increasingly strict environmental ... The analysis data of the WJT bituminous coal from Inner Mongolia are listed in Ta...
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Sintering Characteristic in Catalytic Gasification of China Inner Mongolia Bituminous Coal Ash Yandong Mao,*,†,‡ Yadan Jin,† Kezhong Li,† Jicheng Bi,†,§ Jinlai Li,† and Feng Xin‡ †

State Key Lab of Coal-Based Low Carbon Energy, Enn Technology & Development Co. Ltd., Huaxiang Road, Langfang 065001, China ‡ School of Chemical Engineering and Technology, Tianjin University, Weijin Road,Tianjin 300072, China § State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan Road, Taiyuan 030001, China ABSTRACT: To determine the ash characteristics, catalysts transformation and the sintering mechanism during catalytic coal gasification, the investigation of the effect of ashing temperature, K2CO3 and CaO catalysts addition on the ash sintering behavior of bituminous coal was conducted under catalytic gasification conditions. The ash sintering temperature was determined at 3.5 MPa using a pressure-drop sintering device with H2O, H2, CO inlets. The ash morphology was analyzed using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDS). An X-ray diffractometer (XRD) in combination with FactSage were used to predict the reactions occurring between minerals as well as the mineral transformation and slag formation. The results showed that different ashing temperature affected both ash transformation reactions and volatilization amount of added catalyst, which influenced the ash fusion temperatures and the sintering temperature. The ash-550 °C was chosen to investigate the sintering behavior. The ash fusion temperatures and the sintering temperature both dropped markedly to a limit value as the concentration of K2CO3 increased and increased again. Besides, the morphology changes from SEM-EDS validated the trend of the sintering temperature, and the molten degree of the tested C-K-5 sample was more serious. The sintering happened at several hundred Celsius below the initial deformation temperature; it is more accurate using FactSage to predict slagging compared with the fusion temperatures in catalytic coal gasification process. Furthermore, the FactSage and XRD results revealed that the existence of massive kaliophilite was the main cause of the greatly decreased sintering temperature, the Kbearing aluminosilicate could react with Fe, Ca-containing aluminosilicates to form eutectic mixtures, which led to a dramatic drop of the sintering temperature. In addition, the effect of the CaO addition on the slagging property depends on the amount of CaO, reaction condition (pressure, reaction atmosphere) and raw coal property, especially to high iron coal. The existing Fe(II) components easily reacted with the Ca-bearing feldspar minerals to produce low-temperature eutectics and formed the liquid phases, which accelerated the sintering and agglomeration. At last, from the view of K, Ca-containing catalysts transformation using XRD and FactSage calculation, the additional catalyst partly transformed into silica, aluminosilicates and entered into cofusions during sintering, which led to the decreasing catalytic activity and increasing difficulty of catalyst recovery in catalytic coal gasification process.

1. INTRODUCTION Rapidly developing economy and increasingly strict environmental protection regulation demand clean energy to solve the issue of plentiful coal and short natural gas. Methane can be synthesized by reversed steam reforming of methane through the syngas from coal. A developing technology is to directly produce methane by catalytic coal gasification, which is a more efficient and less costly approach of all the reactions taking place in one reactor.1,2 The catalyst can not only reduce the reaction temperature but also increase the yield of methane.3,4 Catalytic gasification of coal has attracted much attention recently.5−7 As well-known, both alkali and alkaline earth metals (AAEM) and transition metal (Fe, Ni etc.) salts have been used to catalyze coal gasification; the catalytic activities of AAEM salts are particularly significant.8−11 However, some undesirable ashrelated problems, such as agglomeration, deposition, and erosion due to the presence of AAEM catalysts have been affecting the operation stability and increasing the maintenance costs.12−14 Because AAEM catalysts could react with minerals © 2016 American Chemical Society

in coal to produce low temperature eutectics, the agglomeration of ash was accelerated.15−17 For instance, potassium can be incorporated into the structures of silicates to form eutectics with low melting temperatures of about 540−600 °C in the K2O−SiO2 binary system.18 The melting behavior appears to be accelerated by the presence of calcium to form eutectic mixtures in the FeO−SiO2−Al2O3 and CaO−SiO2−Al2O3 system, and many low-temperature eutectic points (700− 1000 °C) occur in the K2O−FeO−SiO2 and K2O−SiO2−Al2O3 phase diagrams, which enable coal to begin melting.19 Wangjiata (WJT) coal as one of typical bituminous coals in China Inner Mongolia has abundant oxygen-containing functional groups on the coal surface and low ash content, which is beneficial to uniform load and recovery of catalyst. Our research result showed that a commercially acceptable gasification rate could be obtained at 700 °C by adding K2CO3 Received: March 3, 2016 Revised: April 29, 2016 Published: May 3, 2016 3975

DOI: 10.1021/acs.energyfuels.6b00514 Energy Fuels 2016, 30, 3975−3985

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Energy & Fuels Table 1. Relevant Analysis Data of Coal Proximate Analysis (wt %) Mad

Ad

Vd

10.21

9.54 Ultimate Analysis (wt %)

30.29

Cd

Hd

Nd

Sd

Oda

70.35

4.01

0.81 Ash Analysisb (%)

0.15

15.14

Al2O3

CaO

Fe2O3

11.56

17.48

14.16

K2O

Na2O

SO3

TiO2

SiO2

1.03 2.27 0.59 4.47 Ash Fusion Temperaturesc (°C) (Reducing Atmosphered)

MgO

MnO2

7.14

0.5

40.8

DTe

STe

HTe

FTe

1158

1287

1303

1321

a

By difference. bNormalized ash composition. cThe testing method was in accordance with the Chinese Standard GB/T 219-2008. dMolar ratio of H2 and CO2 is 1:1. eDT, deformation temperature; ST, softening temperature; HT, hemispherical temperature; FT, flow temperature. These are the same as the abbreviation in Table 2, Figure 3, and Figure 7.

catalyst, compared to a required temperature of about 930 °C without catalyst. However, agglomeration in a fluidized bed gasifier happened during catalytic WJT coal gasification. The agglomerates were observed at far below the initial deformation temperature of coal ash tested by the ASTM procedure.20,21 The agglomeration caused by sintering of solids has greatly influenced the design and operation of fluidized bed gasifier.22 The ash sintering behavior was mainly determined by the ash chemistry, reaction atmosphere, temperature, and pressure. AlOtoom et al.22,23 indicated that the pressure-drop technique could provide a more accurate indication of the sintering temperature of coal ash than other sintering temperature measuring techniques and studied the sintering temperature of five coal ashes from a pressurized fluidized bed combustor (PFBC), the ash with a sintering temperature lower than the operating temperature of the PFBC showed agglomeration. Wang et al.24,25 researched the effects of reaction atmosphere and ash composition on coal ash sintering behavior, concluded that the sintering temperature increased with the ratio of the acidic oxide to the basic oxide in coal ash, and it was lower under reducing reaction temperature atmosphere. Jing et al.26−28 investigated the effect of pressure, reaction atmospheres, and ash composition on the sintering behavior and indicated that the addition of Fe2O3, CaO, and Na2O could obviously reduce the sintering temperatures under gasification atmospheres. The sintering temperatures decreased with an increase of pressure from 0.1 to 1.0 MPa and pressure influenced sintering temperatures by affecting reaction rate and mineral transformation in coal ash, but they did not study the sintering temperatures at higher pressure. Although numerous studies have been carried out on sintering behavior, no record is found in our special operating condition of 3.5 MPa and high concentration of steam reaction atmosphere. The presence of H2O enhanced the reaction of alkali metal and alkaline earth metal with silica or aluminosilicate in coal ash29,30 and furthermore to form low melting eutectics, which accelerated the sintering and agglomeration in fluidized bed. This paper focuses on the effect of ashing temperature, K2CO3 and CaO catalysts addition on the ash sintering behavior of bituminous coal, the interactions among coal minerals, K2CO3 and CaO under catalytic gasification conditions, and further studies of K, Ca

catalysts transformation mechanisms and the sintering mechanism during catalytic coal gasification.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ashes. The analysis data of the WJT bituminous coal from Inner Mongolia are listed in Table 1. The coal was pulverized and sieved to 100−180 μm in size, chemical reagents of K2CO3 and CaO were impregnated on coal samples by aqueous solutions. After thoroughly mixing by vigorous stirring, the samples were dried in an oven at 105 °C for 24 h and named as C-K-X and CCa-X, where X represented the mass percent of added K2CO3 or CaO (relative to the mass of coal sample), respectively. Three ashing methods were chosen to prepare C-K-X and C-Ca-X ash samples. According to the Chinese Standard (NY/T 1881.5-2010), the first was prepared by heating to 250 °C in a muffle furnace with a heating rate of 5 °C/min and kept for 1 h in the air, then heated to 550 °C and kept for 6 h to complete ashing, namely, ash-550 °C. The second was prepared according to the Chinese Standard (GB/T2122008) by heating to 500 °C with a heating rate of 15 °C/min and kept for 0.5 h in air, then the residue was heated to 815 °C in air and kept for 2 h to complete ashing, namely, ash-815 °C. The third was heated in an oxygen plasma ashing set at about 160 °C for 7 days to complete ashing, namely, ash-160 °C. Finally, the resultant ash samples were cooled down to room temperature and then ground to a particle size less than 180 μm in a mortar for preparing the ash pellet. 2.2. Pressure-Drop Measurements. A sintering device of the pressure-drop method was used to determine the ash sintering temperature in the catalytic coal gasification condition. A schematic diagram of the experimental setup is shown in Figure 1. The device consists of a horizontal tube furnace, a measurement system, three gas inlets of H2, CO, and N2, a water vaporizer, and a gas−liquid separating and drying unit. The measurement system comprises of a pressure difference transmitter, mass flow meters, pump, and thermocouples. Ash sintering refers to the process in which some partial melting particles become sticky and bond together with adjacent solid particles. These bonded particles will potentially grow up and finally form a bulk of ash deposits.31 A cylindrical ash pellet (12 mm diameter and height) prepared using an ash pellet compaction device with a compaction pressure of 25 MPa was filled into an Incoloy 800H stainless steel tube of 12 mm i.d. and 94 cm length. The experimental pressure is 3.5 MPa, H2 and CO gas from the gas cylinder passed through the mixer separately at the flow rates of 0.26 L/min and 0.087 L/min, respectively. Also water pumped at a flow rate of 0.65 g/min through the vaporizer to mix with H2 and CO in the mixer. The mixed gas was preheated to 400 °C in the preheater and passed through the tubular reactor at a flow rate of 3976

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Table 2. Effect of the Ashing Temperature on the K Content in Ash, The Ash Fusion Temperatures and the Sintering Temperature of the C-K-10 Sample ashes K2O DT ST HT FT

Figure 1. Schematic diagram of the experiment setup: (1) mass flow meter; (2) metering pump; (3) water vaporizer; (4) mixer; (5) preheater; (6) safety valve; (7) pressurized pressure-drop measurement system; (8) pressure difference transmitter; (9) gas−liquid separator; (10) dryer; (11) back pressure valve.

C-K-10 (160 °C)

C-K-10 (550 °C)

C-K-10 (815 °C)

C-K10

Ash Analysis (%) 40.9 41.6 9.1 42.2a Ash Fusion Temperatures (°C) (Reducing Atmosphere) 1150 1142 1146 1164 1167 1190 1201 1193 1223 1223 1221 1246 Sintering Temperatures (°C) 650 650 730

C-K-0 1.03b

a

Calculated value based on the content of ash, K2O in WJT raw coal, and K2CO3 catalyst additive (convert to K2O). bThe testing method was in accordance with the Chinese Standard GB/T1574-2007.

in air at intervals of 500 and 815 °C. It is noteworthy that the initial sintering or melting can occur below the standard ashing temperature,32 and the volatilization of inorganic elements at high temperature, i.e., potassium, can change the ash composition. High-temperature ashing has faster reaction rate, it is easy to prepare, and is time-saving. However, reactions of minerals in coal occur during the high-temperature ashing procedure, which will destroy the existing form, composition, and structure of original minerals in coal, then change the property of ash, and the accuracy of experiment result may be affected if the high temperature ash was used as the raw material. The minerals of the ash prepared by low temperature oxygen plasma method are the most close to original minerals in coal. Organic matters in coal are rejected, and the crystal structure, chemical composition of inorganic matters are conserved, then the composition, structure and property of ash including added catalyst never change, which can help us to know what would bring about slagging. However, the lowtemperature oxygen plasma ashing method has the disadvantages such as lower operating temperature, slower reaction rate, time-consuming and incomplete organic removal. Combining the features of different ash preparation processes, three ashing procedures as shown in section 2.1 were chosen to prepare C-K-10 ash samples: C-K-10 ash-160 °C, C-K-10 ash-550 °C, and C-K-10 ash-815 °C to study the effect of ashing temperature on the temperatures of ash sintering and ash fusion. The obtained data is listed in Table 2. The lowest K content found in the ash-815 °C. The K amounts are almost the same for the ash-550 °C and the ash-160 °C, which are basically identical with the calculated value of K2O in C−K-10. The result indicates that there was basically no K loss for the ash-550 °C and the ash-160 °C, but K loss happened when ashing temperature is above 550 °C, and the ash-815 °C has higher fusion temperature than the others because of the difference in ash compositions, especially the K content. The ash sintering temperatures of the ash-160 °C, the ash-550 °C, and the ash-815 °C are 650 °C, 650 and 730 °C, respectively, which shows that the two low-temperature prepared ash samples have the same ash sintering temperature. The results show that the ashing temperature can influence the ash fusion temperatures and the sintering temperature. Figure 2 shows the XRD patterns of the C-K-10 coal and the ash samples of the C-K-10 prepared by three ashing procedures. As seen in Figure 2, the major phases of the C−K-10 coal are

1.16 L/min to replace the inert N2. The coal ash pellet was heated from ambient temperature to reaction temperature at a rate of 10 °C/ min and reacted with the mixed gas. A pressure-drop between two ends of the ash pellet was produced when the gas passed through the ash pellet. Pressure-drop increased with the increasing temperature, and the pressure-drop and temperature of the bed were recorded simultaneously during the experiment. There will be a significant pressure-drop reduction when the sintering process of the ash particles takes place. The sintering temperature is defined as the point at which the pressure-drop reaches a maximum during the heating process.22,23 The pressure-drop technique is a sensitive measurement with a quick response to any changes in the dimensions of the pellet, and the reproducibility of the sintering temperature was satisfied with a deviation ±10 °C. 2.3. XRD and SEM-EDS Measurements. The prepared ash samples and the sintered ash samples were ground in a mortar to a particle size less than 75 μm and then scanned using a Bruker D8 Focus X-ray diffraction (XRD) at accelerating voltage 40 kV and current 40 mA with copper Kα radiation from 5° to 80° of 2θ at 8°/ min. The patterns will be used to study the mineralogical transformation of the ash samples. The morphological characteristics and spot composition analysis of the samples after the sintering test were performed using a HITACHI S-4800 scanning electronic microscopy (SEM) analyzer aided with an Oxford X-ray energy dispersive spectroscopy (EDS). The spot location is represented on the provided SEM images by a star symbol. 2.4. Factsage Calculation. In order to understand the ash behavior of WJT coal with different added amounts of AAEM catalysts, the FactSage Thermodynamics Model (FactSage 7.0) was used to calculate the initial formation temperature of slag, the liquidus temperature of ash sample, and ash transformation with the temperature increase under the catalytic coal gasification process conditions. Calculation conditions for FactSage Thermodynamic Model: (1) Chemical composition: The ash composition of Al2O3, CaO, Fe2O3, Na2O, K2O, MgO, SiO2, and SO3 was input in the table of the software. (2) Gas atmosphere: The catalytic gasification condition of 70 mol % H2O, 7.5 mol % CO, and 22.5 mol % H2 was employed in the FactSage calculation. (3) Pressure: The pressure for the FactSage calculation was 3.5 MPa.

3. RESULTS AND DISCUSSION 3.1. Effect of Ashing Temperature. The existing standards of ash preparation usually choose a higher ashing temperature. The standard ashing procedure is to heat the coal 3977

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Figure 2. XRD patterns of the C-K-10 coal sample and the ash samples of C-K-10 prepared at three ashing temperatures (160 °C, 550 °C, 815 °C). (F, fairchildite K2Ca(CO3)2; P, potassium carbonates K2CO3; C, calcite CaCO3; Q, quartz SiO2; Ka, kaolinite Al2O3·2SiO2·2H2O; Cs, calcium sulfate hydrate CaSO4·0.15H2O; SI, sodium iron sulfide NaFeS2; Wa, wairakite CaO·Al2O3·6SiO2; Li, lime CaO; Wo, wollastonite CaSiO3; A, anhydrite CaSO4; An, annite K2(Fe2+5Al)Si5Al3O20(OH)4).

Figure 3. Effect of K2CO3 addition on the ash fusion temperatures and the sintering temperature.

Figure 5. Ash transformation of the C-K-5 sample as the function of temperature under catalytic gasification conditions.

calcite (CaCO3), quartz (SiO2), kaolinite (Al2O3·2SiO2·2H2O), calcium sulfate hydrate (CaSO4·0.15H2O), wairakite (CaO· Al2O3·6SiO2), and sodium iron sulfide (NaFeS2). The added potassium carbonate is not detected by XRD because it

interacted with the organic macromolecular groups in coal.33,34 The main minerals of the ash-160 °C are fairchildite (K 2 Ca(CO 3 ) 2 ), potassium carbonates (K 2 CO 3 ), calcite (CaCO3), quartz (SiO2), kaolinite (Al2O3·2SiO2·2H2O),

Figure 4. SEM-EDS analyses of C-K-X samples after the sintering test (800 °C). 3978

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Figure 6. XRD patterns of the ash-550 °C samples (a) and sintered ash samples (b) with different K2CO3 addition. (F, fairchildite K2Ca(CO3)2; P, potassium carbonates K2CO3; C, calcite CaCO3; Q, quartz SiO2; H, hematite Fe2O3; A, anhydrite CaSO4; M, magnetite Fe3O4; W, wuestite FeO; G, gehlenite 2CaO·Al2O3·SiO2; S, spurrite 4CaO·2SiO2·CaCO3; K, kaliophilite KAlSiO4).

°C, which are different from those in the C-K-10 coal and the ash-550 °C, the ash-160 °C. This shows that the minerals in coal decomposed and transferred to other phases and the added potassium reacted with Fe minerals to produce potassium iron aluminosilicate. Comparing the XRD patterns and K amounts of three ash samples, we find that new minerals formed and K loss happened in the ashing procedure of the ash-815 °C, meanwhile the main minerals in coal and the added K-catalyst phase are still not decomposed and transferred for the ash-160 °C and the ash-550 °C, no K catalyst lost and more K-bearings are detected which are prone to produce low-melting-point eutectics with the minerals of coal, it would accelerate sintering and agglomeration in catalytic coal gasification.21,27,35 The formation of high temperature minerals of lime, wollastonite,

which come from the coal and the added K-catalyst. The added K-catalyst phase is identified in the ash-160 °C; it is because the organic macromolecular groups interacted with catalyst were decomposed and organic matters in coal were rejected during ashing process of the ash-160 °C. Compared to the major phases of the C−K-10 coal, no new mineral phases are identified except for the added K-catalyst phases. The major minerals in coal and added K-containing catalyst have not decomposed and transferred yet. For the ash-550 °C, the major species are the same as that of the ash-160 °C, and more Kcontaining catalyst phases appear. However, the main minerals in coal and the added catalyst are still not decomposed and transferred. However, the phases of lime (CaO), wollastonite (CaSiO3), anhydrite (CaSO4), and potassium iron aluminosilicate (K2(Fe52+Al)Si5Al3O20(OH)4) are detected in the ash-815 3979

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Energy & Fuels Table 3. Main Reactions Involving WJT Coal Minerals, K2CO3 and CaO Catalysts in Catalytic Coal Gasification Conditions CaCO3(calcite) → CaO + CO2 ↑

(1)

CaSO4 (anhydrite) → CaO + SO3

(2)

CaO + Al 2O3 + SiO2 → 2CaO·Al 2O3 · SiO2 (gehlenite)

(3)

CaO + CaCO3 + SiO2 → 4CaO·2SiO2 ·CaCO3(spurrite)

(4)

Fe2O3(hematite) → FeO(wuestite) + Fe3O4 (magnetite) (H 2 /CO atmosphere)

Fe3O4 (magnetite) → FeO(wuestite)(H 2 /CO atmosphere)

(5) (6)

2CaO· Al 2O3 · SiO2 (gehlenite) + FeO(wuerstite) → 2FeO· SiO2 (fayalite) + FeO·Al 2O3(hercynite)

(7)

+ 3FeO· Al 2O3 · 3SiO2 (almandine) + other aluminosilicates K 2CO3 + Al 2O3 + SiO2 → KAlSiO4 (kaliophilite)

(8)

CaO + Al 2O3 + SiO2 → CaO·Al 2O3 ·2SiO2 (anorthite)

(9)

Al 2O3 ·2SiO2 ·2H 2O(kaolinite) → 3Al 2O3 ·2SiO2 (mullite)

(10)

Figure 9. Ash transformation of the C-Ca-5 sample as the function of temperature under catalytic gasification conditions.

coal and K content and whether the experimental conditions allow the interaction to occur before further transformation of the minerals in coal and K evaporation. It means the ashing temperature affects both ash transformation reactions and K catalyst content in the coal. In view of the long preparation period, incomplete decomposition of organics of the ash-160 °C and formed high temperature minerals different from minerals in raw coal, K loss of the ash-815 °C, the ash-550 °C is chosen to investigate the sintering behavior of WJT coal. The following ash is specialized to the ash-550 °C. 3.2. Effect of the K2CO3 Addition. The effect of the K2CO3 catalyst addition on the fusion temperatures and sintering temperature is given in Figure 3, and this figure also shows two curves adapted from FactSage calculation representing the change of initial formation temperature of slag and liquidus temperature with K2CO3 concentration. The fusion temperatures drop as addition of K2CO3 catalyst increases until the addition is 5%; while the addition of K2CO3 increases to 10%, the fusion temperatures increase again, but they are still lower than the fusion temperatures of raw coal without K2CO3 catalyst addition. Mao et al.36 and Huggins et al.37 also presented that the ash fusion temperatures dropped to a limit value as the concentration of K2CO3 increased and increased again when alkali additive concentration was higher. They thought that the addition of K2CO3 as a alkali fluxing mineral greatly reduced the ash fusion temperatures, and the ash fusion temperatures at higher addition of K2CO3 increased, which could be attributed to the formation of new higher melting point minerals.

Figure 7. Effect of CaO addition on the ash fusion temperatures and the sintering temperature.

and aluminosilicate salts and the reduction or absence of the fluxing K-bearing minerals result in higher fusion temperatures and sintering temperature of the ash-815 °C than those of the low temperature prepared ash samples. The result indicates that the effect of added potassium carbonate catalyst on sintering and slagging was dependent on whether the K-bearings can interact with the minerals in the

Figure 8. SEM-EDS analyses of C-Ca-X samples after sintering test (800 °C). 3980

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Figure 10. XRD patterns of the ash-550 °C samples (a) and sintered ash samples (b) with different CaO addition (C, calcite CaCO3; Q, quartz SiO2; H, hematite, Fe2O3; A, anhydrite CaSO4; M, magnetite Fe3O4; W, wuestite FeO; G, gehlenite 2CaO·Al2O3·SiO2; S, spurrite 4CaO·2SiO2·CaCO3; An, anorthite CaO·Al2O3·2SiO2; Mu, mullite 3Al2O3·2SiO2).

the condition of atmospheric pressure and H2, CO2 reaction atmosphere. However, the sintering temperature was tested in our special operating condition of 3.5 MPa and high concentration of steam reaction atmosphere. Our previous study35 revealed that sintering temperature decreased with increasing pressure in the range of 0.1−3.5 MPa, and the existence of a water vapor atmosphere lowered the sintering temperature significantly. The presence of water vapor led to formation of low melting point materials, such as KOH, and Kbearing materials were easier to react with minerals of coal ash to produce low temperature eutectics in water vapor atmosphere which accelerated the sintering and agglomeration in fluidized bed. Therefore, the fusion temperatures of coal cannot predict agglomeration in the fluidized bed of the catalytic coal gasification process.

As shown in Figure 3, the ash sintering temperature decreases to the lowest point of 610 °C when the concentration of K2CO3 is increased to 5%, which decreases by 215 °C compared with that of sample without K2CO3 catalyst addition. Also then the sintering temperature begins to increase as the concentration of K2CO3 increases. The sintering temperature of C-K-10 ash increases to 650 °C, but it is far lower than the sintering temperature 825 °C of C-K-0 ash sample. From Figure 3, it is shown that the sintering temperatures decline markedly after adding potassium carbonate catalyst, and the amount of added K2CO3 affects the ash sintering temperatures. The sintering happens at about 300−500 ° C below the initial deformation temperature of coal ash, especially for the ash from the coal with K2 CO3 catalyst addition. The standard determination method of the fusion temperatures stipulates 3981

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Energy & Fuels

and then dramatically decrease, and Ca/Na-silicates disappear at about 900 °C. The increase of Ca/Fe-olivine is due to the reaction of FeO and SiO2, Ca-silicates, and subsequently the dramatic decrease of Ca/Fe-olivine, Ca/Na-silicates, and kaliophilite indicates they reacted to produce low-meltingpoint eutectics, which results in a rapid increase in the proportion of liquid phase. Ca/Fe-olivine phase and kaliophilite phase disappear at about 1000 °C, and the proportion of slag phase increases sharply and reaches more than 90%. The FactSage calculation for ash transformation and liquid phases formation tallies with the EDS analysis and deduction above. In order to gain a better understanding of the transformation mechanism of K2CO3 addition in the ash sintering process, the mineralogical transformations of some typical ash samples are investigated using XRD. Figure 6a shows the XRD patterns of the ash samples prepared at 550 °C from WJT coal with different K2CO3 addition amounts. It can be seen that the main minerals in the coal ash without K2CO3 addition are calcite (CaCO3), quartz (SiO2), hematite (Fe2O3), and anhydrite (CaSO4). Among them, calcite, hematite, and anhydrite are fluxing minerals and are prone to produce low-temperature eutectics with other minerals,19 which reduces the fusion temperatures and the sintering temperature and makes the coal sintering and slagging. When the K2CO3 addition increases to 5%, new phases fairchildite (K2Ca(CO3)2), potassium carbonates (K2CO3) appear. The existence of the fluxing mineralsfairchildite and potassium carbonates which react easily with Al, Si minerals in coal to produce low-melting-point eutectics38,39 leads to a lower ash sintering temperature. Meanwhile, the addition of alkali catalyst also dilutes the refractory acid oxides in coal ash, seen in Figure 6a, and the peak of quartz decreases, which results in a further decrease of the sintering temperature. The major species of the C-K-10 ash are the same as that of the C-K-5 ash sample besides the higher amounts of potassium carbonates. After the ash preparation process, the ash pellets are placed in a pressure-drop sintering device to determine the ash sintering temperature in catalytic coal gasification condition and obtain the sintered ash samples. Also the XRD patterns of sintered ash samples are shown in Figure 6b. Gehlenite (2CaO· Al 2 O 3 ·SiO 2 ), spurrite (4CaO·2SiO 2 ·CaCO 3 ), magnetite (Fe3O4), wuestite (FeO), and calcite (CaCO3) are observed in the C-K-0 sintered ash sample. Comparing with the XRD pattern of the ash-550 °C sample (before the sintering temperature testing) shown in Figure 6a, it notes that gehlenite and spurrite appear while anhydrite and quartz disappear and the content of calcite decreases. The formed gehlenite and spurrite are from the reaction of decomposing products of calcite, anhydrite and Al, Si minerals in coal ash such as quartz et al. (see reactions 1−4 in Table 3). Also the hematite transforms into magnetite and wuestite (see reactions 5 and 6 in Table 3). Magnetite and wuestite as fluxing minerals easily react with K, Ca-containing aluminosilicates to form low temperature cofusions. However, gehlenite, spurrite, and wuestite disappeare in the C-K-5 sintered ash sample and kaliophilite (KAlSiO4) forms. The kaliophilite is a kind of Kbearing aluminosilicate formed as a reaction product from K2CO3, SiO2, and Al2O3 (see reaction 8 in Table 3), which is capable to react with Fe, Ca-containing aluminosilicates (see reaction 7 in Table 3) to form eutectic mixtures and decrease the sintering temperature,27 and the XRD technique cannot detect these amorphous materials such as eutectic mixtures. It can explain the disappearance of gehlenite, spurrite, and

In Figure 3, the trends of liquidus temperature from Factsage calculation and the ash fusion temperatures show similarities and parallelism, and the variation trend of the sintering temperature is consistent with that of initial formation temperature of slag adapted from FactSage calculation except no close parallelism; it is because ash compositions taking the form of oxides are input in FactSage thermodynamic model software but not the real existing form of minerals in coal. The difference between the sintering temperature and the calculated initial formation temperature of slag is in the range of several tens to more than 100 °C, which is far less than the gap between the sintering temperature and the initial deformation temperature. The result shows that it is more accurate using FactSage to predict slagging compared with the fusion temperatures in catalytic coal gasification process. The gap between the real sintering temperature and the calculated initial formation temperature of slag from FactSage is attributed to the input form of ash compositions and the set ideal state of thermodynamic equilibrium calculation in FactSage thermodynamic model software. In order to understand the ash surface morphology change of C-K-X samples produced by sintering temperature testing which terminated at 800 °C and cooled to ambient temperature, some typical tested ash samples are examined by SEMEDS and the results are shown in Figure 4. It is shown that the tested C-K-0 sample presents a significant amount of loose and dispersed particles with irregular shape, in which no obvious melting or agglomeration phenomena occurred. The tested CK-5 sample and the tested C-K-10 sample are obvious agglomerated and stuck together with molten surfaces. The occurrence of molten surface indicates that liquid phases have formed, and those liquid phases can act as glues bonding particles together. In Figure 4, the molten degree of the tested C-K-5 sample is more serious than the tested C-K-10 sample, the surface is almost completely molten and no clear particles are observed. On the whole, the morphology changes validate the trend of the sintering temperature results in Figure 3. Besides, from the analysis of EDS, the chosen spot in the tested C-K-0 sample is mainly composed of O, Ca, Si, Al, implying that the particle may contain Ca-bearing silicates and aluminosilicates. For the tested C-K-5 and C-K-10 samples, higher content of K present in the elemental analysis and the EDS results show that the melting surfaces of them are mainly composed of O, K, Si, Ca, Al, Fe, indicating that the melting surfaces are mainly composed of K, Ca, Fe-bearing silicates and aluminosilicates, and these minerals can react with each other to form the low temperature eutectics, which facilitate the formation of liquid phases. To validate the deduction from EDS analysis, FactSage has been used to predict the ash behavior and calculate ash transformation under catalytic coal gasification process conditions. Figure 5 shows the main calculation results for ash transformation of the C-K-5 ash sample (with the lowest sintering temperature) with the temperature increase. The data shows that the main minerals are kaliophilite, K2CO3, Ca/Nasilicates, Ca/Fe-olivine, K-silicates, and FeO at 700 °C. As the temperature increases to about 800 °C, K-silicates disappear, Ca/Na-silicates decrease smoothly and liquid(slag) starts to form, which indicates K-silicates reacted with Ca/Na-silicates to produce low-melting-point eutectics to form slag. From 800 to 900 °C, FeO phase markedly decreases and disappears, Ca/Feolivine first increases and then decreases as the temperature increases, Ca/Na-silicates, kaliophilite first decrease smoothly 3982

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Energy & Fuels

between the sintering temperature and the calculated initial formation temperature of slag is in the range of several tens to more than 100 °C, which is far less than the gap between the sintering temperature and the initial deformation temperature. Research on the influence of CaO catalyst addition on the slagging property using FactSage is more accurate compared to the fusion temperatures in the catalytic coal gasification process. The gap between the real sintering temperature and the calculated initial formation temperature of slag from FactSage is because ash compositions taking the form of oxides are input in the FactSage thermodynamic model software but not real existing forms of minerals in coal. In Figure 8, there are three images presented: three tested CCa-X (X is 0, 5, 10) samples from sintering temperature testing (terminated at 800 °C and cooled). As shown in Figure 8, both of the tested C-Ca-0 sample and C-Ca-10 sample present a significant amount of irregularly shaped and dispersed particles, in which no obvious melting or agglomeration phenomena occurred. However, for the tested C-Ca-5 sample, the ash particles hold together and become agglomerated, the surface of which is obviously molten, which indicates that some liquid phases have already formed and the sintering has occurred. Conclusively, the morphology changes validate the trend of sintering temperature results in Figure 7. From Figure 8, the EDS results show the chosen spot in the tested C-Ca-10 sample is mainly composed of O, Ca, Al, Si, implying that the particle may contain Ca-bearing aluminosilicates including feldspar minerals. Compared with the EDS results of the tested C-Ca-0 sample (shown in section 3.2), higher contents of Ca, Al present in the elemental analysis of the tested C-Ca-10 sample, which indicates that more Ca-bearing feldspar minerals and aluminates formed. For the tested C-Ca-5 sample, the EDS results show that the melting surfaces are mainly composed of O, Si, Ca, Al, Fe, indicating that the melting surfaces are mainly composed of Ca, Fe-bearing silicates and aluminosilicates. It is reported that those minerals could react with each other to form the low-temperature eutectics, which accelerated the agglomeration and slagging.40 As shown in Figure 9, ash transformation of the C-Ca-5 sample with the temperature increase is calculated using FactSage under catalytic coal gasification process condition. It is shown that the main minerals are Ca-olivine, Ca/Fe-olivine, Ca-feldspar, and FeO at 800 °C. As the temperature increases to about 900 °C, FeO phase markedly decreases and disappears, Ca/Fe-olivine decreases smoothly and 1iquid(slag) forms, which indicates FeO reacted with SiO2 and silicates in ash to produce low-melting-point cofusions to form slag. From 900 to 1100 °C, Ca/Fe-olivine phase markedly decreases and disappears, Ca-feldspar decreases smoothly as the temperature increases, they reacted to produce low-melting-point eutectics, and the proportion of liquid phase rapidly increases. This validates the deduction from EDS analysis above. Meanwhile, Ca−olivine dramatically decreases and disappears, Ca2SiO4 appears and increases rapidly, which shows some Ca−olivine transforms into Ca2SiO4 with the temperature increases. When the temperature is above 1100 °C, Ca2SiO4 and Ca-feldspar phases dramatically decrease and Ca-feldspar phase disappears, which indicates they reacted to produce low-melting-point eutectics to lead to a sharp increase in the proportion of the liquid phase. The XRD technique is also used to further investigate the mineralogical transformations and mechanism of CaO addition in the ash sintering process. Figure 10 shows the XRD patterns

wuestite in the XRD pattern of the C-K-5 sintered ash sample. The FactSage calculation result for ash transformation of the CK-5 ash sample in the Figure 5 is consistent with the XRD result, which can verify the inferred mineralogical transformation mechanism from XRD analysis. On the whole, the existence of massive kaliophilite is the main cause of the greatly decreased sintering temperature of the C-K-5 sample. Meanwhile, the presence of these fluxing fairchildite and potassium carbonates also leads to a dramatic drop of the sintering temperature. From Figure 5b, the main phases of the C-K-10 sintered ash sample are fairchildite, potassium carbonates, magnetite, kaliophilite, and a spot of calcite, but the amount of kaliophilite is lower than that in the C−K-5 sintered ash sample; the added K catalyst is mainly in the forms of fairchildite and potassium carbonates. The lower content of kaliophilite, and the presence of calcite result in the higher sintering temperature of the C-K-10 ash sample than that of the C-K-5 ash sample. The existence of massive kaliophilite greatly decreases the sintering temperature. Also the additional K catalyst partly transforms into kaliophilite and enters into cofusions during sintering, and it will lead to the decreasing catalytic activity and increasing difficulty of catalyst recovery in the catalytic coal gasification process. 3.3. Effect of the CaO Addition. Figure 7 shows the effect of CaO catalyst addition on the fusion temperatures, sintering temperature, and initial formation temperature of slag, liquidus temperature from the FactSage calculation. As shown in Figure 7, the fusion temperatures increase as the increase of CaO catalyst addition. However, the ash sintering temperature decreases with increasing CaO addition, it decreases to the lowest point of 770 °C when the concentration of CaO is increased to 5%, and then the sintering temperature begins to increase as the concentration of CaO increases. The sintering temperature of C-Ca-10 ash sample is higher than the sintering temperature 825 °C of the C-Ca-0 ash sample. There is large deviation between the sintering temperature and the fusion temperatures, and their trends are different. The sintering happens at about 300−500 °C below the initial deformation temperature of C-Ca-X coal ash. As mentioned above (see in section 3.2), the different determination conditions (pressure, reaction atmosphere) mainly resulted in the large deviation of the two temperatures. The reaction rate increased under elevated pressure, and the decomposition of some minerals was inhibited; those minerals were easier to react with minerals in coal ash to produce low temperature cofusions to accelerate the sintering and agglomeration.34 The presence of H2O enhanced the reaction of Ca with silica or aluminosilicates in coal ash and furthermore to form low melting eutectics, which could be due to the formation of transient Ca(OH)2 and accelerate the sintering.30 Especially for coal with high content iron (as our chosen WJT coal), the existing form of iron in reducing atmosphere was mainly Fe(II), and the Fe(II) components easily reacted with the Ca-bearing feldspar minerals to produce low-temperature eutectics and formed the liquid phases,40 which accelerated the sintering and agglomeration. Therefore, the effect of the CaO addition on the slagging property depends on the amount of CaO, reaction conditions, and the nature of the chosen coal. From Figure 3, it is shown that the trends of liquidus temperature from the Factsage calculation and the ash fusion temperatures show similarities, and the variation trend of the sintering temperature is consistent with that of the initial formation temperature of slag adapted from the FactSage calculation and parallelism. The difference 3983

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Energy & Fuels of the ash-550 °C and the sintered ash samples with different CaO addition amounts. In Figure 10a, it is shown that the main phases of three ash-550 °C samples are concordant, and the content of calcite increases in the ash with CaO addition increases because the added CaO catalyst reacted with CO2 in air to form calcite during sample preparation. Comparing with the phases of the ash-550 °C, we find the disappearance of anhydrite, quartz, the decrease of calcite, and the appearance of gehlenite, spurrite produced by the reaction of decomposing products of calcite, anhydrite and Al, Si minerals in coal ash such as quartz et al. (see reactions 1−4 in Table 3) in Figure 10b. Meanwhile, the hematite transforms into the fluxing magnetite and wuestite (see reactions 5 and 6 in Table 3). As shown in Figure 10b, it can be seen that the main minerals for the C-Ca-5 sintered ash sample are spurrite and calcite. Comparing with the XRD pattern of the C-Ca-0 sintered ash sample, it shows that the peaks of magnetite, wuestite, and gehlenite disappear, which indicates that gehlenite reacted with FeO (see reaction 7 in Table 3), and those generated Fe, Cabearing minerals interacted with each other to form lowtemperature eutectics and melted into glass phase which cannot be detected by XRD40 and finally led to a dramatic drop of the sintering temperature. It can explain the disappearance of gehlenite, magnetite, and wuestite in the XRD pattern of the CCa-5 sintered ash sample. Meanwhile, this verifies the analysis of SEM-EDS in Figure 7, and the FactSage calculation results for ash transformation of the C-Ca-5 ash sample in the Figure 8 is consistent with the XRD result. For the C-Ca-10 sintered ash sample, the main phases are gehlenite, magnetite, anorthite (CaO·Al2O3·2SiO2) and mullite (3Al2O3·2SiO2). The formed anorthite is from the reaction of decomposing products of calcite, anhydrite, and Al, Si minerals in coal ash (see reactions 1, 2, and 9 in Table 3). Also, the mullite is transformed from kaolinite in coal ash during heating (see reaction 10 in Table 3). The existence of the refractory mullite leads to a higher sintering temperature. Therefore, the CaO addition on the ash sintering temperature in the catalytic coal gasification process is strongly influenced by the additional amount and raw coal property, especially in high iron coal.41

(3) It is more accurate using FactSage to predict slagging compared with the fusion temperatures in the catalytic coal gasification process. FactSage in combination with XRD could be used to predict the reactions occurring between minerals as well as the mineral transformation and slag formation. (4) The fusion temperatures increased as the increase of CaO catalyst addition, but the sintering temperature had a minimum. The different determination conditions mainly resulted in the large deviation of the two temperatures. The existing Fe(II) components easily reacted with the Ca-bearing feldspar minerals to produce low-temperature eutectics and formed the liquid phases, which accelerated the sintering and agglomeration. The effect of the CaO addition on the slagging property depends on the amount of CaO, reaction conditions, and raw coal property, especially high iron coal. (5) The additional K, Ca-containing catalysts partly transformed into silica and aluminosilicates and subsequently entered into cofusions during sintering, which led to the decreasing catalytic activity and increasing difficulty of catalyst recovery in the catalytic coal gasification process. (6) These results could provide useful support for the practical application of the catalytic coal gasification process.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-03162595824. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support from the National Key T e c h n o lo g y R & D P r o g r am C h i n a ( Pr o je ct No . 2009BAA25B00) and the National Basic Program Research of China (Project No. 2011CB201305).

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4. CONCLUSIONS This paper focused on the effect of ashing temperature and K2CO3 and CaO catalysts addition on the ash sintering behavior of bituminous coal, the interactions among coal minerals, K2CO3 and CaO under catalytic gasification conditions, and further studies of the K, Ca catalysts transformation mechanism, and the sintering mechanism during catalytic coal gasification. The following conclusions can be drawn: (1) Different ash preparation temperature affected both ash transformation reactions and volatilization amount of the added catalyst, which influenced the ash fusion temperatures and the sintering temperature. The ash-550 °C was chosen to investigate the sintering behavior. (2) The ash fusion temperatures and the sintering temperature declined markedly after adding K2CO3 catalyst, and they both dropped to a limit value as the concentration of K2CO3 increased and increased again. The existence of massive kaliophilite was the main cause of the greatly decreased sintering temperature, and the K-bearing aluminosilicate could react with Fe, Ca-containing aluminosilicates to form eutectic mixtures, which led to a dramatic drop of the sintering temperature. 3984

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