FeO Low-Temperature Eutectics on

Article

The effects of SiO2-A12O3-CaO/FeO low temperature eutectics on slagging characteristics of coal ash Xianglong Cheng, Yonggang Wang, Xiongchao Lin, Jicheng Bi, Rong Zhang, and Lei Bai Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The Effects of SiO2-A12O3-CaO/FeO Low Temperature Eutectics on Slagging Characteristics of Coal Ash †

†,*

†

‡

‡

†

Xianglong Cheng , Yonggang Wang , Xiongchao Lin , Jicheng Bi , Rong Zhang , and Lei Bai †

School of Chemical and Environmental Engineering , China University of Mining and Technology (Beijing), Beijing, 100083; ‡

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001

ABSTRACT: The effects of low temperature eutectics (LTEs) in SiO2-A12O3-CaO/FeO on the slagging characteristics were comprehensively studied in the light of ternary phase diagram of 264 typical coal samples; and two novel criteria for coal ash slagging characteristics were proposed. The results show that LTEs are essential reason inducing the coal ash slagging, and their amounts have strong relationship with the slagging characteristics. Moreover, the sum amount of SiO2 and A12O3 (A, wt%) as well as that of CaO and FeO (B, wt%) can be considered as important factors regarding the formation of LTEs. In addition, both A and B have significant linear correlation with ash slagging index. Furthermore, the values of A and B can be employed to define the slagging level (slight, moderate, serious) with higher accuracy, compared with common criteria (e.g., ash softening temperature, base/acid ratio, SiO2-Al2O3 mass ratio and FeO-CaO mass ratio, etc.).

Keywords: coal ash; low temperature eutectic; slagging characteristics; deformation temperature; criterion

1 Environment ACS Paragon Plus

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. INTRODUCTION Coal ash slagging characteristics is a very important parameter for the design and operation of large-scale gasifier/boiler. Many studies concerning the effects of coal ash chemical composition, mineral composition and reaction atmosphere on coal ash slagging mechanism have been performed 1-7. Some practical methods to judge the coal ash slagging characteristics have been proposed, such as base/acid ratio, ash fusion temperature, the SiO2-Al2O3 mass ratio, the FeOCaO mass ratio.1-4,8 However, these criteria are not able to offer satisfying prediction, with the accuracy of lower than 70%, or even only 20-30%.4,8 Recently, some new methods have been developed,4-11 such as catastrophe progression method, gravity screening method, fuzzy pattern recognition method, fuzzy neural network, thermal equilibrium phase diagram, silicon carbide tube method, thermal microscopy method, and magnetic analysis. These new methods have higher accuracy than the old ones, but companied with disadvantages as overlaborate tests, tedious calculations or arbitrary input variables. These drawbacks limit the utilization of such methods, especially in the industrialized gasification or combustion process. Coal ash slagging is mainly caused by the formation of low temperature eutectics(LTEs). Gupta et al.1 and Yan et al.10 studied coal ash fusion characteristics, and found that when the coal ash was heated, the pore frameworks swelled up and micropores produced gradually. With increasing of heating time and temperature, larger pores were slowly developed as pores expanding. When several minerals generating LTEs, initial liquid phase emerged and flowed into nearby micropores and larger pores. Therefore, heat exchange and chemical reaction happened between liquid phase and ash particles, leading to ash particles gradually melting and shrinking to dense and hard solids. Nowok et al.2 indicated that LTEs could generate a viscous fluid. This viscous fluid made adjacent particles adhere to each other, touching, bonding and then shpaing a


Page 2 of 25

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

closed small porosity. Finally, an architecture dense solid was formed in favor of liquid surface tension. Li et al.4-7 found that during lignite fluid-bed gasification, the adhesive glassy material melted slowly due to LTEs, and then the mergence of individual melted particles happened. From aforementioned researches, it can be seen that the LTEs play an obvious induction role in the initial stage of coal ash slagging. However, due to variations of types and crystal structures of minerals in the coal ash, evolved from those minerals at high temperature, LTEs also have different types, which are represented by various phase diagrams, such as SiO2-A12O3 binary phase diagram, SiO2-A12O3-CaO ternary phase diagram, and SiO2-A12O3-MgO-ZnO quaternary phase diagram, etc. Among these phase diagrams, SiO2-A12O3-CaO and SiO2-A12O3-FeO ternary phase diagrams attract more attentions than multicomponent (≥4) phase diagrams.9-15 On the one hand, SiO2, A12O3, CaO, FeO are the main components of the ash, and always have a sum concentration more than 70 wt%. Statistically, for 264 coal samples in this study, the four oxides show sum content even more than 90 wt% of 70% coal samples. Naturally, the generating amount of SiO2-A12O3-CaO/FeO LTEs are larger than that of other LTEs composed of any other three oxides. Al-Otoom et al.16 studied factors that affecting coal ash slagging in a pressurized fluidized gasifier, and found that agglomerated slag sample contained a large amount of sintered calcium silicoaluminate which could generate LTEs. Li et al.17 studied coal ash slagging mechanism in fluidized bed gasifier for Jincheng anthracite, and found the formation of slag during gasification was mainly caused by hercynite and anorthite (low temperature molten matrix), which were formed at about 1100℃. Mao et al.18 studied the effects of alkali catalyst addition and different chemical compositions of nine coal ashes at ash sintering temperature. They found added potassium minerals reacted easily with iron and calcium minerals to produce LTEs which accelerated the sintering and


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

agglomeration. The presence of iron and calcium minerals could promote the slagging by forming SiO2-A12O3-CaO/FeO LTEs. Wu et al.19 studied coal ash mineral melting evolution in coal gasification process, and found the presence of calcium and its LTEs significantly accelerated the ash slagging. Many conclusions based on SiO2-A12O3-CaO/FeO ternary phase diagram have been applied and verified in production and engineering design, especially for the prediction and adjustment of ash fusion temperature or ash slagging of single/blended coal.6,8,17,20-25 On the other hand, the establishment of multicomponen (≥4) phase diagram needs a great deal of experiment data, as data created by the thermodynamic simulation software is incredulous with no experimental validation. In addition, the generating amount of minerals is decreasing with the increased number of oxides types. Naturally, the generating amount of LTEs decreases with the increase of oxides types. Usually, small amount of LTEs have less effect on coal ash fusibility and slagging. This is the possible reason that researchers proposed composite ternary phase diagram, e,g., alkaline oxide - acidic fluxing oxides-acid non-fluxing oxide ternary phase diagram.29,30 In this study, the ash composition, ash fusion temperature and clinker ratio of 32 coal samples from Midwest of China were tested, and the same data of 232 coal samples from literature was collected. With 264 coal samples, the effect of the generating amount of SiO2-A12O3-CaO/FeO LTEs on coal ash slagging characteristics was studied. Moreover, the relationship between the sum amount of SiO2 and A12O3 (A, wt%) in SiO2-A12O3-CaO/FeO LTEs and slagging index was discussed, so was the relationship between the sum amount of CaO and FeO (B, wt%) and slagging index. Finally, two new criteria for predicating coal ash slagging characteristics were proposed and compared with the common criteria. 2. MATERIALS AND METHODS


Page 4 of 25

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

32 coal samples from Central and Western China, including Henan Yima (YM), Qinghai Muli (ML) and Xinjiang Beitun/Dahuangshan (BD) were studied. Ash composition, ash fusion temperature and clinker ratio of such samples were analyzed according to GB/T1574-2007, GB/T219-2008 and GB/T1572-2001 respectively, and were shown in Table 1 and Table 2. Table 1. Chemical composition of 32 coal samples Chemical composition % Coal NO. Coal rank* source SiO2 A12O3 Fe2O3 CaO 1 ML Non-caking coal 27.66 14.27 15.74 16.43 2 ML Non-caking coal 38.06 13.74 13.46 14.39 3 BD Gas coal 50.16 18.37 11.44 7.81 4 BD Gas coal 28.54 15.07 15.23 17.28 5 BD Non-caking coal 16.74 6.71 28.75 16.02 6 BD Long-flame coal 47.42 30.44 3.01 5.55 7 BD Long-flame coal 28.84 15.89 10.3 23.95 8 BD Non-caking coal 24.88 10.11 13.23 31.84 9 BD Non-caking coal 25.48 10.96 12.27 30.2 10 YM Weakly caking coal 52.2 19.15 15.53 2.73 11 YM Medium caking coal 44.12 27.54 13.46 6.35 12 YM Coking coal 40.88 33.14 14.46 4.84 13 YM Long-flame coal 44.88 33.95 6.16 4.31 14 YM Long-flame coal 58.98 21.36 6.4 4.12 15 YM Long-flame coal 44.42 35.79 6.7 3.37 16 YM Long-flame coal 58.38 23.45 5.83 3.69 17 YM Long-flame coal 52.26 25.44 7.95 3.46 18 YM Long-flame coal 44.7 39.11 5.71 1.98 19 YM Lean coal 36.44 31.96 8.8 10.92 20 YM Lean coal 44.22 30.65 11.27 5.26 21 YM Lean coal 15.68 14.42 42.07 14.32 22 BD Long-flame coal 48.8 30.49 4.66 3.75 23 YM Non-caking coal 50.62 40.13 2.22 1.49 24 YM Coking coal 43.28 35.23 10.92 4.09 25 YM Coking coal 42.44 34.44 13.12 3.94 26 YM Long-flame coal 60.04 20.07 3.27 3.45 27 YM Long-flame coal 45.3 42.15 3.06 1.87 28 YM Lean coal 46.66 31.56 8.2 5.36 29 YM Anthracite 42.7 35.55 7.4 5.87 30 YM Anthracite 46.36 31.89 6.07 5.87 31 YM Anthracite 47.6 34.29 6.07 4.74 32 YM Weakly caking coal 47.42 35.58 9.67 2.06 *according to GB5751-86(China); ** the sum content(wt%) of K2O and Na2O.

NO. 1 2 3 4

Coal source ML ML BD BD

KNaO** 1.48 1.29 1.22 3.64 5.61 3.43 5.82 3.14 3.19 2.18 1.7 0.46 2.57 2.4 3.15 3.27 3.07 3.23 0.64 1.74 0.11 3.1 0.77 0.4 0.38 2.38 2.42 1.12 1.23 1.83 1.38 0.84

Table 2. Ash fusion temperature and clinker ratio of 32 coal samples Ash fusion temperature* /℃ Clinker ratio* /% Coal rank* DT ST FT 0.1m/s 0.2 m/s 0.3 m/s Non-caking coal 1120 1130 1140 52.4 82.5 90.0 Non-caking coal 1130 1140 1180 36.7 88 88.4 Gas caol 1190 1220 1290 7.6 25.9 35 Gas caol 1110 1150 1160 80.8 66.5 80.8


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5 BD Non-caking coal 1180 1190 1200 44.9 6 BD Long-flame coal 1330 1420 1440 3.9 7 BD Long-flame coal 1200 1230 1250 43.4 8 BD Non-caking coal 1280 1310 1340 42.9 9 BD Non-caking coal 1290 1310 1330 41.9 10 YM Weakly caking coal 1140 1250 1360 39.7 11 YM Medium caking coal 1240 1280 1350 36.9 12 YM Coking coal 1400 1480 >1500 4.2 13 YM Long-flame coal 1250 1310 1370 7.1 14 YM Long-flame coal 1250 1330 1380 15 15 YM Long-flame coal 1290 1360 1420 36.2 16 YM Long-flame coal 1230 1290 1360 4.6 17 YM Long-flame coal 1210 1310 1350 16.5 18 YM Long-flame coal 1370 1430 1460 33.0 19 YM Lean coal 1360 1370 1390 21.1 20 YM Lean coal 1340 1430 1470 41.4 21 YM Lean coal 1150 1170 1190 57.5 22 BD Long-flame coal 1350 >1500 >1500 10.0 23 YM Non-caking coal >1500 >1500 >1500 27.1 24 YM Coking coal 1460 >1500 >1500 41.5 25 YM Coking coal 1400 >1500 >1500 6.2 26 YM Long-flame coal 1460 >1500 >1500 12.3 27 YM Long-flame coal 1460 >1500 >1500 31.6 28 YM Lean coal 1420 >1500 >1500 26.7 29 YM Anthracite 1410 >1500 >1500 37.9 30 YM Anthracite 1410 >1500 >1500 33.9 31 YM Anthracite 1430 >1500 >1500 14.4 32 YM Weakly caking coal >1500 >1500 >1500 21.5 *according to GB5751-86, GB/T219-2008 and GB/T1572-2001 (China) respectively

Page 6 of 25

51.0 48.6 56.5 54.1 52 63.2 56.1 11.3 14.3 33.9 59.4 26.9 56.2 32.8 36.8 40.9 50.1 29.9 38.8 51.9 10.8 25.1 61 42.3 29.5 44 43.9 42

69.2 69.9 80.9 81.9 74.7 73.9 72.6 30 43.2 69 65.4 53.8 81.4 57.1 78.1 62.6 51.8 72.9 47.5 37.6 37.5 51.9 70.8 42.1 39.2 54.3 56.8 56.1

Table 3. Ash fusion temperature and clinker ratio of coal samples Clinker ratio* /% Ash fusion temperature* /℃ DT ST FT 0.1m/s 0.2 m/s 0.3 m/s YL non-caking coal 1146 1240 1296 31.6 61.0 70.8 QX weakly-caking coal 1100 1180 1286 47.1 54.3 83.2 YM Long-flame coal 1370 1430 1460 33.0 32.8 57.1 MC Long-flame coal 1460 >1500 >1500 12.3 25.1 51.9 *according to GB5751-86, GB/T219-2008 and GB/T1572-2001 (China) respectively Samples

Coal rank*

More experiments were done to further illustrate the phase variation of LTEs in ash slagging process. Shortly, coal samples showing serious slagging (Yulin coal & Qixian coal) and slightly slagging (Yima coal & Mianchi coal) were chosen to prepare ash samples at 800℃ according to GB/T212-2001 (slowly ashing method). Then, ash samples were heated to the deformation temperature according to GB/T219-2001(in atmosphere). After that, samples were rapidly quenched in water. Finally, ash samples were dried and grounded to a suitable size for XRD


Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

analysis (X'Pert PRO powder diffractometer). Ash fusion temperatures and clinker ratios of coal samples were shown in Table 3. 3. RESULTS AND DISCUSSION 3.1 Relationship between SiO2-A12O3-CaO/FeO Low Temperature Eutectics and Slagging Characteristics During coal ash fusion, LTEs form the initial liquid which made adjacent particles close to each other, and play an obvious induction role in the early stage of coal ash slagging1,2,4-7,10. Moreover, a large amount of liquid phase has arisen at deformation temperature,1,10,11 i.e., deformation temperature can be taken as the characteristic temperature to study the phase and crystal type of coal ash mineral at high temperature. X-ray diffraction patterns of coal samples with serious slagging(YL and QX coal) and slightly slagging (YM and MC coal) were shown in Figure 1. YL ash sample mainly contained gehlenite, rankinite, hematite, wollastonite and calcium sulfate at the deformation temperature, while QX ash sample contained gehlenite, rankinite, wollastonite, anorthite, sanidine and potassium iron oxide. According to the SiO2A12O3-CaO ternary phase diagram, gehlenite, rankinite, wollastonite, anorthite can form SiO2A12O3-CaO LTEs which can cause coal ash slagging. The YL coal, as feedstock in industrialized U-GAS fluidized-bed gasification, had a narrow appropriate gasification temperature range of 950-970℃. Once the temperature rose to 980℃, fluidization deteriorated resulting from obvious increase of differential pressure amplitude of bed layer. If this temperature was hold, slag discharge would be difficult, and even emergency shutdown of gasifier might be caused. However, when YL coal was used in entrained-flow gasifier, e.g., SHELL gasifier, a stable operation and comparative large appropriate gasification temperature range could be obtained. It was due to the production of low


Energy & Fuels

ash fusion temperature in favor of liquid slag. Bhattacharya et al.31 studied the gasification of lignites from Southern Australia in fluidized bed at 800℃, and found that gasifier could only run less than 30 hours because of serious agglomeration and slagging caused by the formation of LTEs and the interaction between granules and bed layer. Other researchers9,17,22,23 also found that the formation of SiO2-A12O3-CaO/FeO LTEs decreased the ash fusion temperature, and caused coal ash slagging. 2700

3000

(a)

(b)

2500

4,8

6,7

2000

2400

2100

intensity(counts)

1800

3

1500

5 1200

5

4 1

900

3,5

7

4,6 intensity(counts)

1,10

2700

2400

5 4 5,7 1 3

5 1

26

3 7

7 6

4

4

6 5

4

2

1500

2100

9 10 9 10 4

1000

1800

20

1500

4 10

9

24

28

3

1200

4 7

10

5 10 7 4

3 10 5

4

7,8

5

32

36

7 7

10

900 600 20

25

30

35

40

45

50

55

20

25

30

35

40

2θ/(°)

45

50

55

1

11

60

65

2θ/(°)

6000

(c) 5000

(d)

4000

1

1

3500 4000

intensity(counts)

intensity(counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

3000

2000

1000

1 1,12

12

12 12 1 1

2500

11

2000

1500

1

1

3000

1 11

12

11 12 11

1

11,12 1

11

1 11

1000 15

20

25

30

35

40

45

50

55

60

65

15

20

25

2θ/(°)

30

35

40

45

50

55

60

65

2θ/(°)

Figure 1. XRD patterns of ash samples at deformation temperature 1- silicon oxide, 99-0088# 2-lime, 99-0070# 3-Iron(III) oxide, 89-0599# 4-gehlenite, 74-1067# 5-rankinite, 22-0539 6-Calcium sulphate, 99-0010# 7-wollastonite 43-1460# 8-potassium iron oxide,480692# 9-sanidine84-1504# 10-anorthite 99-0012# 11-mullite89-2645# 12-sillimanite99-0093# (a):Yulin coal sample; (b):Qixian coal sample; (c):Yima coal sample; (d):Mianchi coal sample


Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

YM ash sample mainly contained refractory minerals such as silica and sillimanite (Figure 2), while MC ash sample contained silica and mullite. In large fluidized bed gasification, Yima/Mianchi coal were with gasification temperature of 1100-1140℃, higher than that of YL coal, 950-970℃. LTEs could induce the formation of coal ash slagging to a certain extent. 3.2 Effect of SiO2-A12O3-CaO/FeO Low Temperature Eutectics Amount on Slagging Characteristics At eutectic point, several minerals are involved in the formation of low temperature eutectics; however, it is difficult to detect the amount of LTEs, let alone the amount of every single mineral involved in the formation of LTEs. The mass ratio of every oxide involved in the formation of LTEs can be obtained by phase diagram. Based on the SiO2-A12O3-CaO ternary phase diagram, LTEs arise at 1170℃ and 1265℃ respectively in accordance with the weight percentage ratio of 4.3:1:1.6and 2.09:1:1.91(SiO2:Al2O3:CaO). In this study, two LTEs are named LTECa1 and LTECa2 respectively for short. Similarly, from the SiO2-A12O3-FeO ternary phase diagram, LTEs is formed in accordance with the weight percentage ratio of 2.66:1:3.24(SiO2:Al2O3: FeO) at 1083℃, with LTEFe for short. Furthermore, the generating amount of LTEs of coal ash can be calculated according to the content of the oxide with minimum mass fraction in coal ash. For example, the content of SiO2, Al2O3, FeO in coal ash (26#, YM coal, Table 1) was 60.04%, 20.07%, 3.27%, respectively. With the ratio of SiO2:Al2O3: FeO being 2.66:1:3.24 in phase diagram, the generating amount of LTEFe was calculated by the following formula employing FeO as minimum content in ash: FeO(wt%) + FeO(wt%)/3.24×1+ FeO(wt%)/3.24×2.66 According to GB/T1572-2001, the coal sample in 3-6mm was filled in a given device for gasification/combustion under different blast strength (0.1, 0.2, 0.3m/s). The slag was weighed


Energy & Fuels

and sieved after burning. Clinker ratio was the mass ratio of slag greater than 6mm and all slag. Here, we used the arithmetical average value of coal clinker ratios under there different blast strength to indicate the coal clinker property. The relationship between the average clinker ratio and the generating amount of SiO2-A12O3-CaO/FeO LTEs was shown in Figure 2-4. It could be seen that the average clinker ratio increased with the increasing of the generating amount of LTECa1, LTECa2 or LTEFe. Namely, the larger the generating amount was, the easier the coal ash slagged. A few extremely scattered points were ignored which might be caused by analysis deviation or other reasons. When x and y were employed to express average clinker ratio and generating amount of LTEs, the formulas of fitting lines were shown in Table 4. Table 4. Fitting result between average clinker ratio and generating amount of low temperature eutectics Square error Eutectics type Formula LTECa1

y=0.325x+37.73

R2=0.08

LTECa2

y=0.439x+37.58

R2=0.17

LTEFe

y=0.618x+33.16

R2=0.11

In Figure 1, YL coal and QX coal were easy to agglomerate mainly because gehlenite, rankinite, wollastonite, anorthite formed SiO2-A12O3-CaO LTEs which accelerated the slagging. On the contrary, YM and MC coal ashes were difficult to slag mainly because refractory minerals, such as silica sillimanite and mullite, greatly reduced the possibility of the ash melting. 100

Average clinker ratio /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

80 60 40 20 0 0

10

20

30

40

50

60

70

Content of LTEFe %

Figure 2. Effect of low temperature eutectic LTEFe on coal ash average clinker ratio


Page 11 of 25


80

60

40

20

0 0

10

20

30

40

50

60

70

Content of LTECa1 %

Figure 3. Effect of low temperature eutectic LTECa1 on coal ash average clinker ratio 80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60

40

20

0 0

10

20

30

40

50

60

70

Content of LTECa2 %

Figure 4. Effect of low temperature eutectic LTECa2 on coal ash average clinker ratio

In order to further study the relationship between the amount of SiO2-A12O3-CaO/FeO LTEs and slagging characteristics, the ash composition and fusion temperature of 232 coal samples from literature13-15,17,25-29,32-43 were collected, and 32 coal samples(Table 1) from Midwest China was tested. Accordingly, coal ash slagging index was calculated by a formula in JB/T104402004 which has been applied and verified widely in production and engineering design in China.38 The relationship between the amount of LTEs and slagging index was shown in Figure 5-7. It could be seen that with the increase of the generating amount of LTECa1, LTECa2 or LTEFe, coal ash slagging index increased obviously, i.e., the LTEs accelerated the ash slagging.


Energy & Fuels

6

Slagging index /%

5 4 3 2 1 0 -10

0

10

20

30

40

50

60

70

80

Content of LTEFe %

Figure 5. Effect of low temperature eutectic LTECa1 on coal ash slagging index

6

Slagging index /%

5 4 3 2 1 0 -10

0

10

20

30

40

50

60

70

80

Content of LTECa1 %


6 5

Slagging index /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

4 3 2 1 0 -10

0

10

20

30

40

50

60

70

80

Content of LTECa2 %



Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3.3 Definition of Slagging Characteristics Using the Sum Content of SiO2 and A12O3/(CaO and FeO) in SiO2-A12O3-CaO/FeO Low Temperature Eutectics Minerals composed of SiO2, Al2O3 and CaO can form LTEs, so can the minerals composed of SiO2, Al2O3 and FeO. As a whole, those LTEs can be considered as reaction products of component SiO2+A12O3 and component CaO+FeO. Both CaO and FeO presented fluxing action in coal ash slagging, as shown in Figure 8. The phase transformation was predicted by “FactSage” for the SiO2-Al2O3-CaO-Fe2O3 system in equilibrium with 0.7/0.3 of SiO2/Al2O3 mass ratio(the average value of ML coal, 1-2#, Table 1). The phase transformations primarily occurred at 1200-1700℃. The region below the liquidus temperatures represented where only homogeneous metastable liquids exist. Apparently, anorthite and wollastonite dominated the phase transformation. The composition with low CaO and Fe2O3 content were located in the mullite primary phase field, thus, the liquidus temperatures were predicted to be above 1600oC. In contrast, Fe2O3 was able to accelerate the formation of low melting composition in the lower liquidus temperature region. CaO was demonstrated to have similar effect on the system. Nevertheless, the high amount of CaO (which is hardly retained in coal ash) tended to generate monoxide composition (i.e., CaO residue) with high melting temperature. The increasing trends in slagging with the content of CaO/Fe2O3 was a good confirmation to Figure 5-7. Moreover, From Figure 8, it was also found that, by and large, ash liquidus temperatures decreased with the increasing of the sum conetent of Fe2O3 and CaO, which was a good confirmation to Figure 9. Here, the effects of oxidizing and reducing atmosphere on Fe existence forms was ignored, because the difference of deformation temperature under oxidizing and reducing atmosphere was only 20-40℃, less than analytical error 40-50℃.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

Figure 8. Phase diagram of the Al2O3-SiO2-CaO-Fe2O3 system with SiO2(wt%)/Al2O3(wt%)=0.7/0.3(the average value of ML coal, 1-2#, in Table 1)

However, it is very difficult to detect the amount of component SiO2+A12O3 and that of CaO+FeO involved in the formation of LTEs due to the various types of minerals and their crystal structures in coal ash. In order to develop a convenient method to predict slagging characteristics in production and engineering design, the sum amount of SiO2 and A12O3 (A, wt%) in coal ash was employed to represent the amount of component SiO2+A12O3 involved in the formation of LTEs. Similarly, the sum amount of CaO and FeO (B, wt%) represents the component CaO+FeO. The generating amount of LTECa1, LTECa2 or LTEFe was calculated based on the method introduced in section 3.2. It indicated that there were no residue of SiO2 and A12O3, and little residue of CaO and FeO for 94% of the 264 coal samples. The relationship between A and slagging index calculated by the formula used widely in China38 was shown in Figure 9(a). It could be seen that they presented a good linear relation, and coal ash slagging index linearly decreased with the increase of A. Likewise, a good linear relation was also revealed between B and coal ash slagging index, and coal ash slagging index linearly increased with the increase of B, shown in Figure 9(b).


Page 15 of 25

6

6

(a)

(b)

5

Slagging index /%

5

Slagging index /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4 3 2 1

4 3 2 1

0

0 0

10

20

30

40

50

60

70

Content of (SiO2+Al2O3 ) %

80

90

100

0

5

10

15

20

25

30

35

40

45

50

55

60

Content of (Fe2O3+CaO ) %

Figure 9. Relationship between the sum amount of SiO2 and A12O3/(CaO and FeO) and coal ash slagging index

In light of that A and B had a perfect linear relationship with coal ash slagging index, they could be served as criteria for coal ash slagging characteristics. When component SiO2+A12O3 and component CaO+FeO form LTEs, A and B are used as independent variables of the generating amount of LTEs, i.e., only when both A and B are known, the generating amount of LTEs can be calculated for a certain coal ash sample. At the same time, SiO2, A12O3, CaO and FeO as the main oxides of coal ash, it is reasonable to assume that the sum content(wt%) of them is equal to 1, ignoring the existence of other oxides with little amount. Consequently, if A is known, theoretically B can be calculated and used as a criterion for slagging characteristics. However, the errors of B as a criterion is unneglectable. There are several reasons. First, B is deduced by A under some assumptions, thus there is inevitable error accumulation and transmission. Second, as criterion, B is from theoretical calculation, and it is lack of direct support and verification by large amounts of experimental data. Finally, B is not a strict one-toone linear function relationship with slagging index(Figure 8 and 9), and the optimal partitioning method is employed when identifying the value of A, B. Therefore, in this study, A and B were respectively used as a criterion for coal ash slagging characteristics and were respectively made sure directly by the data of 264 coal samples.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

According to Chinese standards JB/T10440-2004, coal was classified into three types in terms of slagging levels, i.e, slightly, moderate and serious slagging, corresponding three interval of slagging index [0, 1.5], (1.5, 2.5), [2.5,∞], demonstrated in Figure 8 and 9. The values of A and B for 264 coal samples were respectively processed by optimal partitioning method. Two sufficient-necessary criteria for coal ash slagging characteristics were obtained. Criterion(I)( wt%) A≤64.1

serious slagging

64.1< A 0.4 0.206-0.4 2.65 1.87-2.65

FeO Low-Temperature Eutectics on

Recommend Documents