Mineral Transformation in Char and Its Effect on Coal Char

Feb 24, 2014 - aDT, deformation temperature; ST, soften temperature; HT, hemisphere temperature; and FT, ..... Shenhua Group Corporation, Ltd. (U12612...
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Mineral Transformation in Char and Its Effect on Coal Char Gasification Reactivity at High Temperatures, Part 2: Char Gasification Zhibin Ma,†,‡ Jin Bai,*,† Zongqing Bai,† Lingxue Kong,† Zhenxing Guo,† Jingchong Yan,†,‡ and Wen Li† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: The effect of mineral transformation on the gasification reaction of char with CO2 was investigated from 950 to 1500 °C by comparing the gasification rate and gasification reactivity of raw char, demineralized char, and demineralized char blended with different minerals. The gasification experiments were performed by thermogravimetry (TG) using an isothermal method. Differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were used to investigate the melting process of minerals during char gasification at around the ash melting temperature and to evaluate the effect of melting of minerals on the char gasification at high temperatures. The results show that the minerals in the char could obviously catalyze the gasification reaction below the deformation temperature of coal ash. The catalytic minerals are anhydrite (CaSO4), oldhamite (CaS), hematite (Fe2O3), and magnetite (Fe3O4). No remarkable catalytic effect on the gasification by the vitreous minerals themselves was found, but they could transform to crystalline minerals, which exhibit a catalytic effect during char gasification. The melting process of minerals during coal gasification could be divided into three stages according to the DSC curve. It is found that the hindering effect on the char gasification by mineral melting is also influenced by the carbon content in char. This process was also revealed by SEM analysis of some partially gasified chars.

1. INTRODUCTION Gasification has been widely applied for coal clean utilization.1 In the past 10 years, the high-temperature gasification technology by companies, such as Shell and Texaco, has become increasingly important and widely used in China because of its high carbon conversion and low pollutant emission. Mineral transformation has an important effect on the gasification reactivity of char at high temperatures.2,3 Many studies about catalytic action of inorganic matters on the char gasification reactivity were reported in the literature,4−10 but a large number of previous studies mostly focused on the effect of a certain mineral species or pure minerals, such as Ca(Ac)2, Na2CO3, K2CO3, and Fe(NO3)3, on the gasification below 1000 °C. The alkali metal and alkaline earth metal in coal could catalyze the char gasification at low temperatures.11 However, these minerals may lose the catalytic effect after reacting with other minerals in coal or even the coal/char matrix to form silicates, carbide, etc. at high temperatures. It is well-accepted1,2,12,13 that the formation of aluminosilicates in coal at high temperatures is responsible for the weakening catalytic performance of minerals in char, but no direct evidence has been provided for explaining this phenomenon. Wu et al.14 found that the carbon thermal reactions between metal oxides and carbon black at high temperatures weakened the catalytic effect of metal oxides. However, the carbon thermal reactions of metal oxides were not possible in many coals during coal gasification. Bai et al.2 found that iron oxides were the only catalytic mineral matter existing at high temperatures by comparing the gasification reactivity of raw char, demineralized char, and demineralized char blended with coal ash, but the compositions of the added coal ash were largely different from the inherent minerals in coal. It is not clear © 2014 American Chemical Society

whether the inherent minerals, such as oldhamite (CaS), magnetite (Fe3O4), etc., in char catalyze the char gasification reaction at high temperatures. In addition, the minerals melt to form vitreous matter (denoted as VM) easily during hightemperature coal gasification. To date, the effect of mineral transformation, especially that of the VM, on the hightemperature gasification reaction is not clearly elucidated. Besides, the morphology of minerals is largely different during coal gasification at high and low temperatures. During the gasification process, mineral matter in char not only reacts with each other but also undergos morphological changes and fusion at high temperatures. Makino et al.15 found that the diffusional resistance of gasification agents through the ash layer increased at around the ash melting temperature. There is a process about the melt of minerals in char from solid to liquid. Lin et al.16 described the ash melting process in char, including the following steps: (1) dry solid ash, (2) high-viscosity melting ash adhered to carbon, and (3) low-viscosity spherical slag. Wu et al.17 found that the initial melting behavior of ash usually occurs at the middle or later stage of the char CO2/H2O reaction because of the endothermic reaction and higher reactivity of the char gasification reaction, as compared to that of mineral melting reactions in ash. The morphological change of minerals during char gasification varies with diverse coals. There is no final conclusion about how the melt of minerals affects the gasification reaction thus far. Received: December 4, 2013 Revised: January 27, 2014 Published: February 24, 2014 1846

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2.3. Preparation of VM. The raw ash was heated to 1500 °C at 6 °C/min in a horizontal tube furnace under a reducing atmosphere (VCO/VCO2 = 6:4) and kept for 30 min to prepare the VM. The X-ray diffraction (XRD) pattern (Figure 1) shows that there are no crystalline

The mineral transformation in char during gasification and the morphological variation of char at high temperatures were investigated in our previous work.18 In this part, the effect of mineral transformation on the gasification reaction of char with CO2 was investigated from 950 to 1500 °C by comparing the gasification rate and reactivity of raw char, demineralized char, and demineralized char blended with various minerals. The major objectives are (1) to investigate that the variations of the catalytic performance of minerals on the char gasification with the elevated temperature and the reasons for that, (2) to check whether VM catalyzes char gasification at high temperatures, and (3) to explain the adverse effect of melt of minerals in char on the gasification.

2. EXPERIMENTAL SECTION 2.1. Coal Sample and Raw Ash. A typical Chinese coal from Yunnan, Xiaolongtan (denoted as XLT) lignite, was used in this work. It was demineralized by HCl and HF solution, and the demineralized coal was denoted as XLTD. The properties of XLT coal were listed in Table 1.18

Figure 1. XRD pattern of VM from the raw ash. minerals in VM. The mineral transformation in VM during char gasification was investigated according to the following method. The VM was heated in a horizontal tube furnace at 950, 1100, 1300, 1400, and 1500 °C and kept for 30 min. The treated samples were taken out and immersed in an ice water bath immediately, and then the minerals in them were determined by XRD and quantified by Siroquant software. 2.4. Preparation of Char Used for the Gasification Experiment. The char samples used for the gasification experiment were prepared from XLT raw coal and XLTD in a horizontal tube furnace at 950 °C. The procedures were given in section 2.2. The raw char and demineralized char were denoted as 950S and 950SD, respectively. The LTAs prepared in section 2.2 were separately added to 950SD according to the ash content (21.91%) of raw char to investigate the effect of minerals in XLT char on its gasification reactivity. The mixtures were ground homogeneously in an agate mortar and were denoted as 950SD−LTA. Mullite was added to demineralized char to guarantee that the content of fixed carbon in all of the char samples used for the gasification experiment is constant. Mullite was obtained from decomposition of kaolinite above 1500 °C, and it has minimal influence on the char gasification reactivity below its melting point (1810 °C). The mulliteadded char sample was denoted as 950SD−M. To explore the effect of VM on the char gasification, the VM was added to 950SD and the mixtures were denoted as 950SD−VM. The partially gasified chars were prepared at 1300 °C to investigate the melting process of minerals during char gasification by scanning electron microscopy (SEM). The carbon conversion of partially gasified char was adjusted by the reaction time. 2.5. Analysis Methods. 2.5.1. XRD Analysis. The XRD patterns of the samples were recorded using a Rigaku Miniflex II Desktop X-ray diffractometer with Cu Kα radiation. A step size of 0.02° at the speed of 4° (2θ)/min over 10−80° was applied. The minerals in VM treated between 950 and 1300 °C were quantified by Siroquant softwere.23 2.5.2. Physical Adsorption. The Brunauer−Emmett−Teller (BET) surface area of the mesopore in the raw char samples prepared from 950 to 1500 °C under an argon atmosphere was determined by isothermal N2 adsorption at 77 K using an ASAP 2020 physisorption analyzer (Miromeritics).

Table 1. Proximate and Ultimate Analyses of XLT Coal proximate analysis (wt %, dry basis)

a

ultimate analysis (wt %, dry and ash-free basis)

ash

volatile matter

fixed carbon

C

H

N

S

Oa

11.30

42.79

45.91

66.41

3.07

1.39

1.81

27.32

By difference.

The ash prepared with XLT coal at 815 °C in a muffle furnace following the procedures of the Chinese Standard (GB/T 212-2008) was defined as raw ash in this work according to the traditional expression in the previous studies.18−21 The procedures were described concisely here. The XLT coal was oxidized by air in a muffle furnace from ambient temperature to 815 °C. The temperature of the muffle furnace rises to 500 °C within 30 min and is maintained for another 30 min. After that, the temperature rises to 815 °C and then is kept for 60 min.19 The basic properties of the raw ash were presented in Table 2.18 2.2. Preparation of Minerals in High-Temperature Char. The minerals in the high-temperature char were prepared by oxygen plasma oxidation technology to investigate the gasification characteristic of demineralized char blended with minerals. The high-temperature char samples were prepared with XLT coal at 950, 1100, 1300, 1400, and 1500 °C under an inert atmosphere in a horizontal tube furnace. The coal placed in the corundum crucible was heated to a final temperature at 6 °C/min and maintained for 30 min. To ensure that the minerals in char did not transform during cooling, the corundum crucible containing char was taken out and immersed into an enclosed small steel container immediately when the treatment finished. The upper oxidized layer in the crucible was discarded.18 To obtain the intrinsic minerals in char, the organic matters in these char samples were removed by oxygen plasma oxidation in a K1050X plasma furnace (Quorum Technologies, Ltd.) and the product was defined as the lowtemperature ash (denoted as LTA). The oxygen plasma oxidation has a minimal effect on the mineral species; therefore, the minerals in the LTA represent the inherent minerals in high-temperature char.22

Table 2. Main Compositions and Melting Temperatures of XLT Raw Ash melting temperature (°C)a

main composition (wt %)

a

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

DT

ST

HT

FT

33.97

10.88

11.67

27.9

2.8

9.72

1243

1271

1287

1299

DT, deformation temperature; ST, soften temperature; HT, hemisphere temperature; and FT, flow temperature. 1847

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Figure 2. Variation of (a) initial gasification rate, (b) maximum gasification rate, and (c) RS for demineralized char, mullite-added char, LTA-added char, and raw char, with gasification temperatures. The surface area of the micropore in the raw char samples was determined from CO2 adsorption isotherms at 273 K using a NOVA e4200 surface area analyzer (Quantachrome). 2.5.3. Thermogravimetry (TG) Measurement for Char Gasification. The gasification reactivity of raw char, demineralized char, and demineralized char blended with diverse minerals under a CO2 atmosphere was performed in TG (Setaram Setsys Evo) using an isothermal method. The gasification temperature is from 950 to 1500 °C. About 28 mg of char sample with a particle size less than 77 μm was gasified in a 140 mL/min CO2 atmosphere. The char samples were heated under an argon atmosphere at 40 °C/min. CO2 replaced Ar when the desired temperature was reached. The carbon conversion (XC) was calculated with the following equation:1 m − mt XC = 0 × 100% m0 − ma (1)

To evaluate quantitatively the effect of various minerals on the char gasification, the mineral factor index (denoted as MFI) is defined as MFI(X , T ) =

dX C dt

(2)

The gasification reactivity is usually quantified by a reactivity index, RS (h−1), which is defined as24 RS =

0.5 τ0.5

R S(950SD−M,T)

(4)

where X is the mineral in char, T is the gasification temperature, RS(950SD−X, T) is the gasification reactivity index at T, defined in eq 3, of 950SD blended with mineral X, and RS(950SD−M, T) is the gasification reactivity index at T of demineralized chars blended with mullite. MFI is a dimensionless factor. While MFI > 1, the minerals obviously catalyze the char gasification, and the bigger the value, the stronger the catalytic performance of the mineral on the gasification reaction. While MFI = 1, the catalytic performance of the mineral disappears or is counteracted by other factors. While MFI < 1, the aggregation or melt of minerals hinders the char gasification, indicating that minerals have an inhibition effect on the gasification reaction. 2.5.4. Thermal Analysis. Differential scanning calorimetry (DSC, Setaram Setsys Evo) was used to measure the exothermic and endothermic transitions during the char gasification. The effect of mineral transformation on the char gasification was also studied by comparing the DSC curves of 950S and 950SD. 2.5.5. SEM Analysis. A high-resolution scanning electron microscope (JSM-6700F) was employed to investigate the melting process of minerals in XLT char during gasification.

where m0 is the initial mass of sample at the constant temperature stage, mt is the mass of sample at the gasification time t, and ma is mass of gasified residue. The gasification rate is defined as the differential of conversion to gasification time.

r=

R S(950SD−X ,T)

3. RESULTS AND DISCUSSION 3.1. Effect of Minerals in Char on the Char Gasification at High Temperatures. The gasification characteristics of 950SD, 950SD−M, 950SD−LTA, and 950S from 950 to 1500 °C were shown in Figure 2. The gasification rate and reactivity of

(3)

where τ0.5 (h) is the time needed for the carbon conversion of 50%. The initial gasification rate, maximum gasification rate, and RS were used to evaluate the gasification characteristics of various char samples. 1848

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Figure 3. Variation of the BET surface area of meso- and micropores in the raw char samples prepared from 950 to 1500 °C.

Figure 5. XRD patterns of major minerals in VM during heating.

Figure 4. Variation of the MFI of minerals in LTA and VM with the gasification temperature.

950SD are similar to those of 950SD−M below 1200 °C, and they are obviously lower than those of 950SD−M above 1200 °C, which are aroused by the diffusional effect on the char gasification at high temperatures. Tang et al.25 and Ochoa et al.26 found that two regions during gasification may be identified: (1) kinetic control prevails for chars below 1060 °C, and (2) diffusional effects appear to become significant and affect the overall reaction rates above 1060 °C. In addition, the content of the fixed carbon in the char samples is different. The addition of mullite in 950SD−M could decrease the diffusional effects dramatically on the char gasification at high temperatures and guarantee that the content of the fixed carbon in 950SD−M is consistent with that of the other three char samples. Therefore, the gasification rate and reactivity of 950SD−M at high temperatures are more accurate than those of 950SD. The minerals in 950S and 950SD−LTA could catalyze the gasification reaction strongly between 950 and 1200 °C because both their initial and maximum gasification rates are larger than those of 950SD−M. According to our previous work,18 the major minerals in XLT char between 950 and 1200 °C during a reducing atmosphere are oldhamite (CaS), anhydrite (CaSO4), hematite (Fe2O3), and magnetite (Fe3O4). During char gasification, oldhamite and anhydrite in char readily transform into CaO, which further turns into gehlenite when it contacts SiO2

Figure 6. Content of crystalline minerals generated in VM during heating at different temperatures.

and Al2O3 closely, but the rate of this transformation is relatively slower than that of the former transformation. Therefore, the LTA catalyzes the char gasification reaction strongly between 950 and 1200 °C because of these catalytic minerals, including oldhamite, anhydrite, hematite, and magnetite. In Figure 2a, the initial gasification rate of the char samples intersect at about 1250 °C, which is the deformation temperature (generally denoted as DT) of XLT coal ash, and the initial gasification rate of 950SD− M exceeds that of 950S and 950SD−LTA above this temperature, which indicates that the minerals in 950S and 950SD− 1849

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inhibition effect of minerals in char on the gasification reaction coexist within 1300−1400 °C, but they occur at different stages of gasification. On the whole, the catalytic performance of minerals is weaker than the inhibition effect at 1300 and 1400 °C from the observation of lower values of RS for 950S and 950SD− LTA (Figure 2c). Both the initial and maximum gasification rates of 950S and 950SD−LTA at 1500 °C are lower than those of 950SD−M, and their gasification reaction times are longer than that of 950SD−M, indicating that the minerals in 950S and 950SD−LTA only show an inhibition effect to the gasification reaction of char at 1500 °C. The initial gasification rate of 950S and 950SD−LTA levels off or even decreases when the gasification temperature exceeds the DT of coal ash, but an increasing gasification temperature could raise the maximum gasification rate of char samples. Therefore, a balance point between the two effects exists. 3.2. Effect of VM on the Char Gasification. In Figure 4, the MFI of VM first increases with the elevated temperature, reaches the maximum at about 1050 °C, and then declines with an increasing temperature. The variance of MFI indicates that the catalytic performance of VM on char gasification is negligible at 950 °C but increases rapidly with rising temperatures until 1100 °C. Then, its catalytic effect gradually weakens and totally disappears above 1300 °C. The VM is unstable at high temperatures, and it transforms to crystalline minerals easily during char gasification. The mineral transformation in VM during coal gasification was investigated according to the method described in section 2.3. The results were summarized in Figure 5. A part of oxides in VM transforms to the crystalline minerals, including gehlenite, anorthite, hercynite (FeAl2O4), and calcium silicate, during char gasification between 950 and 1300 °C. Figure 6 demonstrates that the variation tendency of MFI of VM is consistent with that of the content of crystalline minerals generated from VM, which implies that the different catalytic performances of VM are due to the mineral transformation in VM. The major crystalline minerals generated from VM are gehlenite and hercynite between 1000 and 1200 °C. It is wellaccepted that aluminosilicates cannot catalyze char gasification, but no direct evidence has been found to verify it. Floess et al.27 thought that the calcium in char may change the energy distribution of active sites, which results in a lower energy barrier of the rate-controlling step. The calcium in the aluminosilicates may also have a similar effect. Hence, it is probable that the minerals generated from VM catalyze the char gasification between 1000 and 1200 °C. These clearly indicate that there is no remarkable catalytic performance on the gasification for the VM itself at high temperatures, but they transform to catalytic crystalline minerals when they are reheated to a certain temperature. The catalytic performance of VM disappears above 1300 °C because of the melt of minerals. Both LTA and VM come from the inorganic matter in XLT coal, and their chemical composition is the same; however, the species of minerals in them are different. Figure 4 shows that the catalytic performance of LTA is obviously stronger than that of VM at the same gasification temperature, which indicates that a part of catalytic minerals in coal loses the catalytic action during the process of heat treatment and the change is irreversible. The formation of VM weakens the catalytic performance of crystalline minerals on the char gasification. Figure 4 demonstrates that the catalytic performance of minerals in char on the char gasification weakens with the elevated temperature. From the discussions mentioned above, there are two primary reasons for that: (1) the

Figure 7. DSC of gasification of raw char and demineralized char at 1100 and 1300 °C (XC, carbon conversion; the letters from a to g indicate the different conversions at certain reaction times).

LTA hinder the gasification reaction at the initial stage of reaction above the DT of coal ash. The minerals in char aggregate seriously and develop to a smooth sphere adhered to the surface of char above 1200 °C.18 The aggregation of minerals reduces the effective contact area of catalytic minerals with carbon, hinders the penetration of the gasification agent into carbon, and blocks a part of pores in char, which were confirmed by the variation of the BET surface area of XLT char samples (Figure 3). In Figure 3, the surface area of meso- and micropores in char decreases dramatically above 1250 °C. In addition, the initial gasification rate of 950S is lower than that of 950SD−LTA above 1250 °C, which implies that the mineral aggregation blocks the internal pores easily because the minerals in 950S disperse more homogeneously than that in 950SD−LTA. Hence, the mineral aggregation in char above the DT of coal ash decreases the initial gasification rate of char. Although the initial gasification rate of 950S and 950SD−LTA is lower than that of 950SD−M at 1300 and 1400 °C, the maximum gasification rate of 950S and 950SD−LTA is higher than that of 950SD−M. This indicates that the minerals in 950S and 950SD−LTA catalyze the gasification reaction at the middle or later stage of reaction in this temperature range. There are two main reasons for that: (1) the spheres adhered to the surface of char disappear during char gasification, which will be discussed in section 3.3 in detail, and (2) anhydrite and oldhamite transform to CaO during char gasification, which has a catalytic effect on the char gasification reaction. Therefore, the catalytic behavior and 1850

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Figure 8. SEM images of XLT partially gasified chars at 1300 °C.

aggregation of minerals above 1300 °C leads to the decline of an

minerals transform to VM during char gasification at high temperatures. 3.3. Effect of the Melt of Minerals on the Char Gasification. DSC and SEM were used to investigate the effect

effective contract area between CO2 and carbon and the blocking of the pores within the char, and (2) the catalytic crystalline 1851

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Hence, according to DSC and SEM, the melting process of minerals in XLT char during gasification above the deformation temperature of coal ash could be divided into three stages (illustrated in Figure 7): (1) spherical mineral particles entrapped in char or adhered to its surface of char (stage I in Figure 7), (2) molten minerals spread out on the surface of char (stage II), and (3) low-viscosity flow of minerals (stage III). In stage I, the minerals aggregate to form spheres adhered to the char surface and the spheres hinder the contact of reactant gas with carbon, so that the initial gasification rate of 950S and 950SD−LTA is lower than that of 950SD−M above 1250 °C. The minerals melt gradually with the consumption of carbon in char during stage II and then spread out on the char surface. This may have a hindering effect on the gasification reaction of chars because of the diffusion resistance. In addition, the slope of the DSC curve of 950S at 1300 °C is lower than that of 950SD in stage II, which also indicates that the gasification rate of 950S is lower than that of 950SD. The maximum gasification rate of 950S is higher than that of 950SD at 1300 °C according to the peak valley in the DSC curve. This observation is also consistent with the results in section 3.1. The plot between points f and g is steeper than that in stage II, which implies that the gasification rate in stage III is higher than that in stage II. A low carbon content accelerates the melt of minerals. Moreover, the viscosity of the mixture between melted mineral and carbon decreases with carbon consumption. This is the reason that the maximum gasification rate occurs at a higher carbon conversion for 950S at high temperatures (Figure 9), because the inhibition effect is weakened with the decline of viscosity of the melting minerals. Hence, the maximum gasification rates of 950S and 950SD−LTA exceed that of 950SD−M.

Figure 9. Variation of the gasification rate of 950S with carbon conversion from 1000 to 1500 °C.

of mineral melt on the char gasification at high temperatures. The DSC curves of 950S and 950SD gasified at 1100 and 1300 °C were shown in Figure 7. The gasification reaction of coal char with CO2 is endothermic.28 The endothermic rate could be reflected by the slope of the DSC curve, and it has a positive correlation with the gasification rate of coal char. In other words, the high slope of the DSC curve indicates the high gasification rate of coal char, and the maximum gasification rate appears at the peak valley. As shown in Figure 7, at 1100 °C, the gasification rate of 950S is obviously higher than that of 950SD because of the catalytic minerals in 950S and the maximum gasification rate of 950S is higher from its lower value of the peak valley of the DSC curve. This is consistent with the results discussed in section 3.1. Both the DSC curves of 950S and 950SD are smooth at 1100 °C. However, at 1300 °C, the DSC curve of 950S represents the other case. It is not as smooth as that at 1100 °C and becomes tortuous before the point f, but that of 950SD is still smooth at 1300 °C. From section 3.1, the mineral transformation hinders the initial gasification rate of 950S and 950SD−LTA above 1250 °C. This implies that the variation of the DSC curve of 950S at 1300 °C is related to the mineral transformation, such as aggregation, fusion, etc., during char gasification and, thus, affects the change of heat. To seek the reason for that, the partially gasified chars with different carbon conversions (shown in Figure 7) were prepared in the thermal analyzer, and their SEM images were illustrated in Figure 8. Figure 8a shows the morphology of ungasified char at 1300 °C. The minerals aggregate seriously and develop a sphere adhered to the smooth surface of unreacted char. At the initial stage of the gasification reaction, the number of spheres adhered to the surface of char declines and the surface of char becomes very rough (Figure 8b). In Figure 8c, the amount of spheres on the surface decreases obviously with the proceeding of the gasification reaction. In Figure 8d, the melting minerals spread out on the surface of char after the inflection point, shown in Figure 7. The inorganic matter enriches gradually as the gasification reaction continues, which accelerates the melt of minerals (panels e−g of Figure 8), because the existence of carbon also delays the aggregation and then the melting of inorganic matter. Above the ash flow temperature, the melting process of minerals during char gasification is controlled by carbon consumption.

4. CONCLUSION The effect of mineral transformation on the gasification reaction of char with CO2 was investigated from 950 to 1500 °C through comparing the gasification characteristics of raw char, demineralized char, and demineralized char blended with various minerals. The gasification experiments were performed in TG using an isothermal method. SEM was employed to investigate the melt process of minerals during char gasification above the deformation temperature of coal ash. TG−DSC was used to evaluate the effect of mineral melt on the char gasification at high temperatures. The following main conclusions could be drawn from this work: (1) The minerals in XLT char could catalyze the gasification reaction obviously below the DT of coal ash. The major catalytic minerals in XLT char are anhydrite, oldhamite, hematite, and magnetite. The aggregation of minerals blocks the meso- and micropores in char, which reduces the initial gasification rate of 950S and 950SD−LTA above the DT of coal ash. (2) No remarkable catalytic action on the gasification of the VM itself was observed, and it transforms to catalytic crystalline minerals during char gasification. The catalytic performance of minerals in VM has a positive correlation with the content of crystalline minerals generated from VM during the char gasification. (3) The melting process of minerals in XLT char during gasification could be divided into three stages: (i) spherical mineral particles entrapped in char or adhered to the surface of char, (ii) molten minerals spread out on the surface of char, and (iii) low-viscosity flow of minerals. The aggregation and melt of minerals have an adverse effect on the char gasification in the former two stages of gasification, but the adverse effect decreases with the consumption of carbon in char during char gasification. 1852

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(24) Gu, J.; Wu, Sh. Y.; Zhang, X.; Wu, Y. Q.; Gao, J. Sh. Energy Sources, Part A 2009, 31, 232−243. (25) Tang, L. H.; Wu, Y. Q.; Zhu, X. D.; Zhu, Zh. B. J. Fuel Chem. Technol. 2002, 30, 16−20. (26) Ochoa, J.; Cassanello, M. C.; Bonelli, P. R.; Cukierman, A. L. Fuel Proc. Technol. 2001, 74, 161−176. (27) Floess, J. K.; Longwell, J. P.; Sarofim, A. F. Energy Fuels 1988, 2, 756−764. (28) Zong, N. F.; Liu, Y. Thermochim. Acta 2012, 22−26.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-0351-4040289. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2010CB227003), the National Natural Science Foundation of China (21006121), and the Joint Foundation of the Natural Science Foundation of China and Shenhua Group Corporation, Ltd. (U1261209).

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NOMENCLATURE

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REFERENCES

XLT = Xiaolongtan XLTD = Xiaolongtan demineralized coal LTA = low-temperature ash VM = vitreous matter 950S = slow pyrolysis char prepared with XLT raw coal at 950 °C 950SD = slow pyrolysis char prepared with XLT demineralized coal at 950 °C 950SD−LTA = 950SD blended with low-temperature ash 950SD−M = 950SD blended with mullite 950SD−VM = 950SD blended with vitreous matter MFI = mineral factor index

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dx.doi.org/10.1021/ef402382m | Energy Fuels 2014, 28, 1846−1853