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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 Available online at www.sciencedirect.com Fuel 87 (2008) 583–591 www.fuelfirst.com 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 tem- peratures has considerable influences on the compositions of coal ash and little effect on the amounts of unburned carbon. The amor- phous 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 prep- aration 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.
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