Characterization of Low-Temperature Coal Ash Behaviors at High Temperatures Under Reducing Atmosphere

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Characterization of low-temperature coal ash behaviors at high temperatures under reducing atmosphere

Jin Bai a,b, Wen Li a,*, Baoqing Li a

a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China b Graduate University of Chinese Academy of Sciences, Beijing 100039, China

Received 23 November 2006; received in revised form 12 January 2007; accepted 7 February 2007 Available online 9 March 2007

Abstract

The coal ash obtained at 815 °C under oxidizing atmosphere was further treated at 1300 °C and 1400 °C under reducing atmosphere. The resultant ashes were examined by XRD, SEM/EDX and FTIR. The results show that the residence time of coal ash at high temperatures has considerable influences on the compositions of coal ash and little effect on the amounts of unburned carbon. The amorphous phase of mineral matters increases with the increasing temperature. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD, and supply additional information of amorphous phase which cannot be detected in XRD. The ash samples generated from a fixed bed reactor during char gasification were also studied with FTIR. The temperatures of char preparation are responsible for the different transformation of minerals during high temperature gasification. Ó 2007 Elsevier Ltd. All rights reserved.

Keywords: Coal ash; High temperature; Residence time

1. Introduction transformation under oxidizing and reducing atmospheres. They found that the ash melting behavior was controlled Coal consists of organic components and a range of by iron rich corner in a reducing atmosphere. The coal minerals. Coal ash is the main product of mineral matters ashes prepared at 350 °C and 850 °C have been character- in coal during its utilization. The coal ash causes many ized carefully, and Mukherjee et al. found that the quartz is problems during combustion and gasification. The deposits the dominant phase in coal and its ash [9]. Vassilev et al. on heating transfer surfaces affect the heat transfer inten- [10] studied the composition of fly ashes generated from sity and lead to unexpected shutdowns as well as decreasing power stations. The fly ashes consist basically of alumino- the lifetime of equipments [1–3]. And also the ash particles, silicate glass, to a lesser extent of mineral matter and mod- which emitted around have a serious adverse health effect erate char occurrence. The ashes generated from three for human [4–7]. Hence, the understanding of the behav- coals were mixed by different ratios to predict the ash iors of coal ash is the basis for effective and clean coal behaviors of blend coal mines, and the mineral transforma- utilization. tions and phase changes under DT (initial deforming tem- The operating temperatures in an entrained flow gasifier perature), ST (soften temperature), and FT (fusing are typically in excess of 1400 °C. At high temperatures, the temperature), which were consistent with the CaO–SiO2– mineral matter within coal may oxidize, decompose, fuse, Al2O3 phase diagram [11]. disintegrate or agglomerate [6]. Huffman et al. [8] studied The physical and chemical characteristics of coal ash are relations between ash melting behavior and mineral matter controlled by the coal, reactor and its operation conditions, such as residence time. During entrained flow gasification * Corresponding author. Tel.: +86 351 4044335; fax: +86 351 4050320. the coal ash generated during combustion and gasification E-mail address: [email protected] (W. Li). processes would go through the reducing reaction zone,

0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.02.010 584 J. Bai et al. / Fuel 87 (2008) 583–591 where the coal ash could influence the gasification such as procedure is designed. The sample (CA) was firstly pushed the content of unreacted carbon in ash and the agglomera- to the low-temperature zone (around 800 °C) of the elec- tion of ash. However, few investigations were focused on tricity tube furnace and then held for 15 s. After that it the mineral transformation in coal ash under the certain was moved to the constant temperature zone (above temperature above 1300 °C and the influence of adequate 1300 °C) for a given time. The residence time was set as residence time. In this study, the ash behaviors from 3, 5, 10, 15, 20, 25 min. As which finished, the sample 1300 °C to 1400 °C were discussed and the residence time was taken out and immerged into the ice water immedi- is an important parameter to be considered about. In addi- ately. The phase transformation and the segregation of tion, the comparisons between ashes from different sources the crystal would be prevented with the sharp cooling of were carried out. X-ray diffraction, scanning electron the sample. The coal ash samples quenched from high tem- microscopy coupled with EDX and FTIR were employed peratures were denoted as CAR. to study the ash behaviors with different temperature and residence time, respectively, under reducing atmosphere. 2.3. Preparation of chars for gasification

2. Experimental The original coal was ground to 80 meshes. The chars were made in a drop tube furnace and the final temperature 2.1. Ash preparation was 1200 °C, 1300 °C, 1400 °C and 1500 °C, respectively. The residence time was about 1 s and N2 was used during One representative Chinese coal, Shenhua coal from pyrolysis. Inner Mongolia, was used in the study. The ash samples, denoted as CA, were prepared in a muffle furnace at 2.4. Gasification of chars 815 °C according to the Chinese Standard GB/T1574- 1995. Briefly, the temperature rises to 500 °C within The chars prepared with the coal mentioned above 30 min, and then stays for another 30 min. After that the under four different conditions were ground to 80 meshes temperature rises to 815 °C and then keeps for about and 4 g chars were put in the corundum dish. The chars 60 min. were gasified with CO2/Ar (6:4, mole ratio) in a fixed bed reactor and the flow rate is 500 ml/min. The temperature 2.2. High temperature treatments of coal ash rises to 1100 °CinN2 atmosphere within 40 min from room temperature, and then the atmosphere was switched In order to investigate the behaviors of coal ash at differ- to CO2/Ar for 3 min gasification. Then, the temperature ent temperatures and residence times under reducing atmo- furthers rises to 1200 °C from 1100 °C within 4 min under sphere (CO/CO2 = 6:4, mole ratio), the following N2 atmosphere, and the gasification lasts for 3 min in the

Table 1 Ash composition and melting temperature of the coal (by wt%)

SiO2 Al2O3 Fe2O3 CaO MgO TiO2 K2ONa2O 25.29 11.26 12.89 34.62 3.91 0.90 0.71 1.60 Ash fusion temperature (°C) Ash particle size distribution (by wt%) DTa STb HTc FTd <74 lm 74–154 lm 154–180 lm 180–280 lm >280 lm 1125 1210 1220 1240 48.38 31.94 8.01 9.97 1.70 a DT: deformation temperature. b ST: sphere temperature. c HT: hemisphere temperature. d FT: ﬂow temperature.

Table 2 Chemical compositions of the CAR under diﬀerent temperatures and residence times Constituent Composition (by wt%) 1300 °C 1400 °C Residence time: 3 min 5 min 10 min 15 min 20 min 25 min 3 min 5 min 10 min 15 min 20 min 25 min

SiO2 29.42 28.75 29.16 28.98 28.72 28.53 29.98 32.24 31.32 29.94 31.47 29.26 Al2O3 13.07 12.91 13.3 13.33 12.48 12.12 12.69 13.86 13.59 12.82 13.2 12.88 Fe3O4 14.77 14.71 14.32 11.71 14.77 14.75 12.13 13.92 11.6 8.89 9.97 12.76 CaO 35.83 36.23 36.1 33.16 35.3 34.76 36.9 36.37 36.1 34.49 35.3 35.8 MgO 0.96 1.06 0.77 2.11 0.77 1.73 1.73 1.59 2.59 2.5 2.88 1.88

TiO2 1.69 1.32 1.33 5.6 3.23 2.57 2.29 1.69 2.11 4.55 4.57 2.35 LOI 0.5 0.2 0.1 –a –a –a 0.3 0.1 0.1 –a –a –a a The weight loss is less than 0.01 mg. J. Bai et al. / Fuel 87 (2008) 583–591 585 switched CO2/Ar. The same procedure was repeated until 2.5. Ash analysis methods the temperature was up to 1500 °C. After char gasiﬁcation, the corundum dish was taken out immediately and the ash A high resolution scanning electron microscope (JEOL particles (denoted as GA) picked and collected for analysis. JSM-6360LV) was employed for observing the morphology

Fig. 1. Micrograph and EDX spectrum of CAR particle under (a) typical coal ash particle above 1300 °C and (b) internal structure of the particle above 1300 °C and (c) crystal of anorthite (presumable) in CAR at 1300 °C and (d) eutectic of FeS–FeO in CAR at 1300 °C. 586 J. Bai et al. / Fuel 87 (2008) 583–591 of the samples, and an energy dispersive spectrometer Table 3 (Oxford ZNCAX-sight-7582) was used for elemental anal- The components of CAR at 1300 °C ysis. The elemental analysis was performed in ‘‘spot mode’’ Residence time Components (by wt%) in which the beam is localized on a single area manually (min) chosen. The spot is represented on the SEM images by a 3 Gehlenite (41.91) akermanite (40.27) anorthite (9) circle or cross. The XRD data were collected using an pyrrhotite (2.92) glass(5.94) ARL diffractometer. And the reference intensity ratio 5 Gehlenite (41.52) akermanite (41.52) anorthite (6) forsterite ferroan (5) forsterite (1.25 ) glass( 5.65) (RIR) method was applied for quantitative analysis to 10 Gehlenite (41.95) akermanite (38.72) ringwoodite (2.89) relate peak intensity ratio to the mass fraction of each anorthite (10) glass (6.44) phase in the mixture. RIR values determined from PDF- 15 Gehlenite (43.09) akermanite (32.51) cordierite (11.24) 2 (powder diffraction file) date base [12] were used for this sekaninaite (6.97) glass (8.13) study. Calibration constants (RIRs) can vary significantly 20 Gehlenite (42.70) akermanite (27.30) rankinite (12) sekaninaite (5) cordierite (3) glass(10) depending on actual phase compositions. The precision 25 Gehlenite (42.14) akermanite (33.11) anorthite (14.5) of the analysis is estimated as ±10% for strongly diffracting ringwoodite (4.89) glass (8.25) phases such as gehlenite, and ±25% for weakly diffracting phases such as iron oxide. The NICOLET NEXUS 470 FTIR could supply the structural information of composi- formed by partial oxidization of pyrite or marcasite and tions. The NETZSCH STA TG was used to check LOI would be stable under reducing atmosphere [2]. The FeO (loss on ignition). The typical applications of SEM, XRD formed on the iron particles covers the surface of the iron and FTIR in the study of coal ash can be found in Refs. particles which could be stable at high temperature. When [9,13–17]. the temperature is above 1000 °C, kaolinite, the most com- mon mineral in coal ash, is recrystallized to generate mull- 3. Results and discussion ite and liberates extra SiO2. After consideration of the high calcium content of the ash, the content of mullite could be 3.1. Characterization of raw ash CA below the detectable limit of XRD. And the porous structure over 25 lm may be attributed to FeS–FeO eutectic The properties of raw ash are given in Table 1. The CA which could not be interfused with the aluminosilicate sys- was also analyzed with XRD, and the main components tem. Outgassing and swelling during the conversion from are calcium sulfate (30%), hematite (6%), silicon oxide crystal to noncrystal result in the pores around 10 lm. (29%), portland (8%), iron sulfide (5%), metakaolinite (10%) and amorphous phase (12%). 3.2.3. XRD and FTIR analysis The content of all the components at 1300 °C determined 3.2. Characterization of CAR by RIR are listed in Table 3. The major components at 1300 °C are akermanite ((Ca Na )(Mg Al Fe ) 3.2.1. Chemical composition 1.53 0.51 0.39 0.41 0.16 Si O , PDF#72-2127) and gehlenite ((Ca Na ) The chemical compositions of the CAR at different tem- 2.0 7 1.96 0.5 (Mg Al Fe ) (Si Al O ), PDF#72-2128), as peratures and under different residence times are present in 0.24 0.64 0.12 1.39 0.61 7 shown in Fig. 2. According to the elemental analysis, the Table 2. The CAR particles are rich in calcium and have ratio of Ca:Si:Al is 46:38:17. In the CaO–SiO –Al O low silicon and aluminum contents. Compared with CA, 2 2 3 phase diagram in Fig. 3, this region is gehlenite. The follow- CAR has a slight increase in calcium, aluminum and silicon ing reaction is believed to occur at high temperature [18]: content. A decrease in LOI with increasing residence time was observed. As the temperature is above 1300 °C, the Metakaolinite þ calcite þ MgO ! gehlenite þ akermanite LOI is quite low. Metakaolinite, magnesia and calcium from decomposition 3.2.2. Morphology characteristics of anhydrite or portlandite react to generate gehlenite and The ash was melted and was driven to form spherical akermanite. According to XRD results, the alteration of particle above 1300 °C, as seen in Fig. 1a. And according residence time has no obvious effect on the composition. to the EDX spectrum, the particles are mainly formed by However, the contents of gehlenite and akermanite are af- Ca aluminosilicate, which is consistent with the elemental fected. Gehlenite and akermanite in solid solution have compositions determined by chemical analysis in Table 2. very similar patterns in XRD. The variations of distance EDX result is an average value from three spots marked between d(310), d(211), d(400) and d(300) demonstrate in the image. The internal structure of the ash particle is that the content of akermanite decreases from 3 to porous in Fig. 1b. The sizes of the particles observed in this 20 min and finally increases at 25 min, while the content study are almost 1700 lm. The crystals of anorthite (pre- of gehlenite increases. The increase is attributed that gehl- sumable) and FeS–FeO are found in the internal part of enite is more stable than akermanite at 1300 °C. The simi- CAR according to EDX in Fig. 1c and d, and the existence lar trend is observed in FTIR spectra. The IR transmission of FeS is confirmed in XRD. The eutectic of FeS–FeO is spectra of the ash sample processed for different residence J. Bai et al. / Fuel 87 (2008) 583–591 587

7500

25 min 5000 20 min 15 min 10 min 2500 5 min Intensity (Counts) 3 min

0 SQR(I) Gehlenite- (Ca1.96Na.05)(Mg.24Al.64Fe.12)(Si1.39Al.61O7)(Major)

SQR(I) Akermanite- (Ca1.53Na0.51)(Mg0.39Al0.41Fe0.16)Si2.0O7(Major)

10 20 30 40 50 60 70 2-Theta (°)

Fig. 2. XRD patterns of ash samples at 1300 °C.

SiO One strong band during 950–1100 cm1 is due to anti-sym- 2 metric Si–O–Si or Si–O–Al stretching vibrations. The 0.00 1.00 polarization effect of cations in the center of tetrahedron structure would cause band split. The effect is obvious in the samples with longer residence time. The other strong 1 0.25 band during 400–500 cm is caused by Si–O or Al–O 0.75 bending vibrations. And the third band between 550 and 850 cm1 affected by polymerizations of aluminosilicate structure is medium. While the extent of polymerization in- 0.50 0.50 creases, the band goes stronger. And the extent can be influenced by Al in the tetrahedron structure. When Al represented Si in the tetrahedral coordination, the intensity of 0.75 the bridge bond between the monomer is weaken. And the Gehlenite 0.25 2 structure of gehlenite is Al tetrahedron ([Al(AlSi)O7] ), so the content of gehlenite affects this band. In Fig. 5, while the content of gehlenite determined by RIR increases, the 1.00 0.00 intensity of the band decreases. In the samples of 3 and 0.00 0.25 0.50 0.75 1.00 5 min residence time, there is a very weak peak which is CaO Al O 1 2 3 attributed to Fe–O at around 560 cm . As discussed above, Fe2+ or Fe3+ has strong polarization, and it is not Fig. 3. The distribution of Ca, Si and Al in the phase diagram. easy to form Fe-aluminosilicate. After a longer residence time, iron oxide would react with aluminosilicate [20]. time at 1300 °C are compared in Fig. 4. The spectra are At 1400 °C the XRD patterns, shown in Fig. 6, show normalized to the intensity of the 3800–400 cm1 band. that most of the phases turn into amorphous. Sillimanite, The bands in the region 3500 cm1 and 1620 cm1 are com- fayalite, ferrosilite and microcline are the main components monly assigned to O–H in the planar water. The band at of crystalline which is lower than 10% of amorphous by 1040 cm1 is due to anti-symmetric Si–O–Si or Si–O–Al weight. Sillimanite is the major aluminosilicate below stretching vibrations. And the broad band with two shoul- 1400 °C. As the residence time increasing, the content of der peaks at 915 cm1 and 845 cm1 is attributed to Al–O sillimanite decreases. The transition between ferrosilite stretching, which become stronger as the residence time in- and fayalite is observed from 10 min residence time. The creases. The vibrations of Si–O cause the band around hedenbergite would be more stable in the Ca rich mixtures, 667 cm1 and the Si–O–Al vibrations are around but it could not be detected in XRD [21]. At 1400 °C, the 800 cm1 and 720 cm1. In general, IR bands caused by bands in IR spectrum caused by aluminosilicates are 950– aluminosilicates could be divided into three parts [19]. 1100 cm1, 400–500 cm1 and 550–850 cm1, which is 588 J. Bai et al. / Fuel 87 (2008) 583–591

44 25 min 350 Peak intensity Gehlenite content 300 Weight percent

250

20 min Area 200 42

150

100 15 min 0 5 10 15 20 25 Residence time/ min

Fig. 5. Relation between the content of gehlenite and the peak intensity of IR band at 1400 °C. T/%

10 min CaO–SiO2–Al2O3 phase diagram. The major changes caused by increasing temperature should be more obvious in amorphous phase. The content of amorphous phase increased greatly from 1300 to 1400 °C according to XRD pattern of CAR (residence time, 3 min) in Figs. 2 and 6. The crystallinity of gehlenite and akermanite is 5 min about 83.17% and 89.22% at 3 min. The crystallinity and Fe-O crystalline fraction at 1400 °C is quite low, which caused the increase of viscosity of CAR melted. Above the melting temperature, the viscosity of ash hardly depends on the

Al-O temperature, but the higher temperature causes more alkali metals and earth metals to release [23]. High viscosity Si-O-Al Fe-O would make the melt over-cooling and turn to amorphous 3 min O-H deformation Si-O-Al phase [24]. The residence time should also be responsible for the change of amorphous phase due to the similar rea- son above. However, the compositions in amorphous could Al-O O-H stretching Si-O not be analyzed in this study because the precision of XRD 3600 3200 2800 2400 2000 1600 1200 800 400 analysis is not good enough [17]. And the accurate analysis of amorphous phase should be studied further. σ/cm-1

Fig. 4. FTIR for different residence time at 1300 °C. 3.3. Characterization of GA consistent with those at 1300 °C. However, the major com- 3.3.1. Morphology characteristics ponent is glass at 1400 °C, and there are barely any differ- Mineral matters in coal also form a spherical particle ences in shape of IR bands of ash with different residence which is similar to the situation during CAR formations. times. The IR spectrum of 3 min is shown in Fig. 7. The fine And the particle diameter is approximately 760 lm. Besides structure bands below 1100 cm1 disappear due to the dis- that, a semispherical luminous particle (SLP) is sticking on ordered structures, and the band from 900 cm1 to the spherical particle (SP) and also some small irregular 1100 cm1 has no peak split, which could be used to predict particles are embodied into the surface of the spherical par- the content of Al in the samples. While the content of Al ticle, as shown in Fig. 9a. EDX spectrums of different posi- increases, the barycenter of the band would shift to the tions (semispherical, spherical and embodied particles) are low frequency field over 15 cm1 [22], but the results from given in Fig. 9a1, a2 and a3. The GA spherical particle con- IR spectrum were not fitted well with the element analysis tains more sulfur than the CAR particle in which most sul- in Table 2. The trend of the position (900–1100 cm1) shift fur has emitted during the ashing procedure. The spherical and the Al2O3 content were compared in Fig. 8. The disre- particle is rich in calcium. The element analysis from EDX lation between the content and position shift at 20 min in Fig. 9a1 indicates the existence of CaS. The fusible min- should be attributed to experimental errors. The transfor- erals form the spherical particles firstly, and then the semi- mations from 1300 to 1400 °C cannot be explained with spherical part coagulates on it. The iron rich particles J. Bai et al. / Fuel 87 (2008) 583–591 589

44 25 min

1 1 20 min 2 1 2

1 1 2 1 2 15 min

1 1 3 3 10 min 2 1 2 Intensity (counts) 1 3 3 1 5 min

1 1 33 1 3 min 1 1 0 1020304050607080 2-Theta (º)

1.Sillimanite (Al2(SiO4)O) 2.Ferrosilite (FeSiO3) 3.Fayalite (FeMgSiO4) 4.Microcline (K0.92Na0.08Al0.99Si3.01O8)

Fig. 6. XRD patterns of ash samples at 1400 °C.

(Fig. 9a2) seem to be generated from pyrite [25]. Inside the 85 SP, some smaller spherical particles (SSP) and irregular shaped particles (ISP) are found. The composition of SSP 80 3 min is similar with that of SLP. The crystals in Fig. 9b are com- 75 posed almost entirely of iron oxide. The iron-bearing phase Si-O-Al is identiﬁed as magnetite or hematite with element analysis 70 (O:Fe 1.4) supplied by EDX and crystal structure.

T/% 65 According to the EDX spectrum, carbon was only found

60 on the surface of the SLP and in the iron-rich particle, sug- gesting that the unburned carbon trended to combine with 55 coal ash particles selectively. At high temperature, the Si-O unburned carbon would react with SiO to form SiC, which 50 Si-O-Al 2 could react on the surface of Fe rich particles during 3600 3200 2800 2400 2000 1600 1200 800 400 quenching [26].InFig. 9c, EDX shows that there are σ /cm-1 approximately 10% Si, 40% C and 50% Fe on the surface. Fig. 7. FTIR patterns of ash sample at 1400 °C. Considering the experimental procedure, the following reactions are expected to occur:

3C þ SiO2 ! SiC þ 2CO

1030 14.0 SiC þ 3Fe2O3 þ 9CO ! Fe3C þ Fe3Si þ 9CO2

13.8 1020 Peak positions Al O weight percent Al 2 3 13.6 2 O 3.3.2. FTIR analysis 1010 3 weight percent The FTIR spectrums of GA generated from four diﬀer- 13.4 1000 ent chars are shown in Fig. 10. The temperatures of char 13.2 preparation are 1200 °C, 1300 °C, 1400 °C and 1500 °C, 990 13.0 respectively. The arrangements of the bands in IR are sim-

Positions of peaks ilar with those of CAR. As the temperature increases, the 980 12.8 ﬁne structures of aluminosilicate during 1000–400 cm1 970 12.6 weaken, which indicates that the amount of non-crystal phase and the eﬀect of polymerization increase. This 05 10 15 20 25 increase should be attributed to the loss of Na, K, Mg Residence time/ minute and Fe during high temperature pyrolysis of coal. These

Fig. 8. The relation between peak positions and Al2O3 content. elements are grid alteration agents, which could break the 590 J. Bai et al. / Fuel 87 (2008) 583–591

Fig. 9. Micrograph and EDX spectrum of GA particle and (a) hollow spherical particles and (b) octahedral crystals and (c) formation of FeC3 and Fe3Si. bridge bond of aluminosilicates to slow down the forma- bending variations at 480 cm1 also indicate the polymeri- tion and polymerization of amorphous phase [23]. The zation of Si–O is not strong in A, B, C, but it is strong in D. shoulder of the band at 1100 cm1 is due to Si–O–Si, which So the sharp peak around band 1000 cm1 should belong 1 1 has a wide bond angle. The band at 1000 cm is caused by to SiO2. The variations of the bands at 1000 cm and 1 Si–O bond. And the very weak peaks caused by O–Si–O 450 cm indicate that the content of SiO2 decreases and J. Bai et al. / Fuel 87 (2008) 583–591 591

D 1500oC (2) The residence time varied from 3 to 25 min inﬂuences the composition of gehlenite–akermanite solid solution. The content of gehlenite would increase with the residence time. The transformations from crystalline to amorphous phase caused by residence time variations should be studied further. (3) The temperatures of char preparation aﬀect the com-

C 1400oC positions of coal ash generated from gasiﬁcation. FTIR indicates that the preparation temperature should be responsible for accelerative transformation of SiO2 and iron silicates during gasiﬁcation.

o Acknowledgments T/% B 1300 C

This work was supported by National Basic Research Program of China (No. 2004CB217704-3) and Interna- Si-O Al-O-Si tional Joint Project of MOST (2005DFA60220).

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