Exploration of Slagging Behaviors during Multistage Conversion

Nov 16, 2015 - Fine mineral particles contain much more iron and calcium than other particles. The possible mechanism of slag formation may involve mi...
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Exploration of Slagging Behaviors during Multistage Conversion Fluidized-Bed (MFB) Gasification of Low-Rank Coals Fenghai Li,†,‡,§ Hongli Fan,† and Yitian Fang*,§ †

Department of Chemistry and Chemical Engineering, Heze University, Heze, Shandong 274015, People’s Republic of China School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan 454003, People’s Republic of China § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS), Taiyuan, Shanxi 030001, People’s Republic of China ‡

ABSTRACT: Four slag samples of Shenmu bituminite and Huolinhe lignite (SSM(A), SSM(B), SHLH(A), and SHLH(B)) selected from two upper parts of a multistage conversion integrated fluidized bed (MFB) were analyzed by X-ray fluorescence spectrometry, scanning electron microscopy, and X-ray diffraction analyses. The results show that the ash sintering temperature and ash fusion temperature of slag samples are below those of the raw coal ashes and increase in the order SSM(A) < SSM(B) < SHLH(A) < SHLH(B) because of differences in acid/base composition. The slag samples are composed of amorphous matter and some crystals, and the crystal content of the two SHLH samples are higher than that of the two SSM samples. The section of the MFB above the site of oxygen injection may be divided into three zones based on particle movement. Fine mineral particles contain much more iron and calcium than other particles. The possible mechanism of slag formation may involve mineral exposure by char gasification, interaction of minerals, formation of particles of different sizes, formation of an initial adhesive layer, and emergence of slag.

1. INTRODUCTION Low-rank coal has increasingly played an important role in energy and chemical resources,1 especially in the United States, Germany, Australia, and China. In comparison to other rank coals, low-rank coal has high concentrations of oxygencontaining functional groups, transitional and macropores, and catalytic inorganic constituents.2 These distinguishing characteristics make it suitable for gasification. Gasification is a widely used technology in clean conversion technologies for coal, for example, in the production of synthetic natural gas, coal-to-liquid fuel, and chemicals, as well as integrated gasification combined cycles.3−6 Because of its minimal environmental impact, uniform bed temperature, and fuel flexibility, fluidizedbed gasification is considered as a promising technology for coal conversion.7,8 However, the carbon conversion ratio of a fluidized-bed is usually low compared to that of the entrainedflow bed because of its lower operating temperature, uniform particle mixture, and fine chars entrained by syngas.9 To increase the coal conversion ratio, a multistage conversion integrated fluidized bed (MFB; see Figure 1)9 was used at the Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS). The MFB was based on many years of operation characteristics of the ash agglomerate fluidized bed (AFB). In MFB gasification, the temperature in the upper part of the MFB increases because of introduced oxygen, which improves the coal conversion ratio. However, slag sometimes forms on the upper part of the MFB during pilot-plant testing of MFB gasification of low-rank coal because of its low ash fusion temperature (AFT) and high ash content. This slag may destabilize the MFB, reduce gasification efficiency, and even cause shutdown of the gasification system. Therefore, it is necessary to explore the slag formation process during MFB gasification of low-rank coals. © 2015 American Chemical Society

The formation of slag during coal conversion is strongly related to coal properties (ash constituents and their fusibility characteristics, mineral content, distribution, etc.), operating conditions (atmosphere, pressure, temperature and its distribution, oxygen/carbon ratio, fluidized state, etc.), particle flow dynamics, etc.10,11 Models for slag initiation and growth during coal conversion have been proposed and elaborated.12,13 Ash particle agglomeration can occur when an amorphous phase forms a bond or when ashes adhere to molten phases in local high-temperature zones.14,15 Adhesion of amorphous ferric silicate or sodium silicate particles during coal gasification results in the slag formation.16,17 Some indices have been proposed recently to predict the slagging tendency during coal conversion based on its ash composition and mineral properties.18,19 Slag characteristics are fundamental to the development of a method for preventing slag formation. The mineral composition of slag may be effectively analyzed by computer-controlled scanning electron microscopy.20 The formation of large amounts of anorthite may lead to slag formation during Shell gasification. Large amounts of calcium aluminosilicate have been found in slag samples from fluidized-bed gasification.21,22 Alkali (sodium and potassium), alkaline earth (calcium and magnesium), or ferrous aluminosilicates are enriched in slag deposits from coal gasification.13,19 The fusion, fluid, and rheology characteristics of the slags from Shell gasification or from Texaco gasification have been investigated.23,24 The chemical compositions of amorphous and crystalline phases in coarse ashes from a Lurgi gasifier have Received: July 27, 2015 Revised: November 16, 2015 Published: November 16, 2015 7816

DOI: 10.1021/acs.energyfuels.5b01711 Energy Fuels 2015, 29, 7816−7824

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Figure 1. Flow sheet of the MFB pilot-plant gasifier.

Table 1. Proximate and Ultimate Analyses of RSM and RHLH RSM

RHLH

Proximate Analysis on an Air-Dried Basis (wt %) moisture 6.54 9.39 volatile matter 27.25 28.69 ash 12.06 18.16 fixed carbon 54.15 43.76 Ultimate Analysis on an Air-Dried Basis (wt %) carbon 63.36 75.54 hydrogen 4.15 3.63 nitrogen 0.95 1.32 sulfura 0.37 1.53 oxygenb 11.49 17.99 a

Figure 2. Location where the slag samples were collected.

b

Total sulfur. From difference.

Table 2. Ash Compositions and Ash Fusion Characterstics of RHLH and RSM constituent SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2 P2O5

DT ST HT FT

RSM Composition (wt %) 34.06 14.32 16.95 27.62 1.88 0.97 3.08 0.22 0.62 0.28 Ts (°C) 635 AFTa (°C) 1170 1185 1204 1210

RHLH 49.19 21.85 11.73 8.04 1.79 2.20 1.23 1.04 1.45 0.26

Figure 3. Schematic diagram of the fixed-bed tube furnace.

been explored.25 Fusion characteristics and composition of slag from Jingcheng anthracite ash AFB gasification have also been analyzed.26 However, there is little published work on the characterization of slag fusion from MFB gasification of Chinese low-rank coal. In comparison to other rank coal, the content of inorganic minerals combined with organic minerals in the low-rank coal includes high levels of magnesium, iron, calcium, potassium, and sodium, which may reach 60% of the total mineral content.27,28 The objectives of the present study, therefore, were to examine the characteristics of slag from MFB gasification of low-rank coal and to explore the formation mechanism in terms of flow dynamics. The results may provide a reference for the development of MFB gasification of lowrank coals.

738 1233 1296 1323 1380

a

AFT, ash fusion temperature; DT, deformation temperature; ST, soft temperature; HT, hemispheric temperature; and FT, flow temperature. 7817

DOI: 10.1021/acs.energyfuels.5b01711 Energy Fuels 2015, 29, 7816−7824

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Energy & Fuels Table 3. Ts and AFT of Slag Samples

and AFT under a reducing condition (1:1 H2/CO2 volume ratio) and ash compositions are listed in Table 2. Levels of volatiles (28.26 and 27.25%, respectively) and oxygen (17.99 and11.49%, respectively) in the two raw coals were very high. RSM and RHLH are typical low-rank coals. Ts and AFT of RSM and RHLH were relatively low, but those of RSM were lower than those of RHLH, probably because RHLH has levels of acid oxides higher than those of RSM. 2.2. Preparation of Slag Samples. Gasification tests on a pilotscale MFB were performed at the Coal Gasification Engineering Center, ICC, CAS. Gasification of RSM and of RHLH was conducted at 950−1030 °C at a pressure of 1.8 MPa. However, the gasification system was occasionally shut down in the two-coal gasification processes because of slag formation on the upper part of the MFB refractory wall. The four slag samples were collected by hand from two different points of the refractory wall (Figure 2) and were referred to as SSM(A), SSM(B), SHLH(A), and SHLH(B), respectively. Slag samples for the analyses for scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements were ground to particle sizes of SSM(B) > SHLH(A) > SHLH(B). This is consistent with the chemical composition of the slag samples in Table 4. 3.2. Surface Morphologies of the Four Slag Samples. The surface morphologies and elemental compositions of the four slag samples obtained by SEM−EDS are presented in Figure 5. Elemental analyses were conducted in “spot mode”, in which the beam is localized on a manually selected circular area that represents the SEM images. Most areas of the SSM(A) surface were molten, and some small particles adhered to them. For SSM(B) particles, only small parts of the surface were molten and numerous particles attached to large areas of their surface. Apertures appeared on most surfaces of the larger molten particles probably because of sintering and agglomeration of fine SHLH(A) particles. Apertures of SHLH(A) samples on the aggregate surface were smoother and larger than those of the SHLH(B) sample. Most of the large particles of SHLH(B) were hardly visible. The initial and mean elemental compositions of the five different spots based on EDS are shown in Table 6. The amounts of silicon and aluminum decreased while those of iron and calcium increased in the order SSM(A) < SSM(B) < SHLH(A) < SHLH(B). These trends indicate that two SM slag samples contained more low-melting-point (MP) calcium and iron eutectic compounds than the two HLH slag samples. 3.3. Mineral Compositions of the Four Slag Samples. The peak height ratio in the XRD pattern is proportional to the mineral concentration.23,31 The diffraction intensity approximately reflects a change in the content of a given mineral. The mineral composition of the four slag samples is shown in Figure 6, and the amounts of the components as determined by the RIR method are listed in Table 7. Distorted baselines in the 2θ range of 15−30° (Figure 6) clearly indicate an abundance of amorphous matter in the four slag samples. This result is consistent with the morphological observations (Figure 5). The mineral content of the two SHLH samples is higher than that of the two SSM samples, as deduced by their diffraction intensities. The two SHLH samples contained mullite, which has a high MP (1850 °C). These results may be explained by the higher AFT of the SHLH samples compared to that of the SSM samples.

Table 6. Element Composition of Slag Samples Determined by EDS (Line Weight Percent) element

1

2

3

4

5

mean value

14.47 9.94 17.75 33.52 15.02 5.04 2.03 0.48 1.75

14.51 9.92 17.70 33.50 15.05 5.00 2.08 0.45 1.79

14.48 9.95 17.73 33.49 15.02 5.03 2.06 0.47 1.77 0.00

O Al Si Ca Fe S K Mg C Na

14.45 9.97 17.75 33.52 14.98 5.06 2.08 0.46 1.73

14.50 9.94 17.71 33.57 15.00 5.01 2.02 0.45 1.80

(a) SSM(A) 14.45 9.98 17.74 33.36 15.07 5.05 2.09 0.50 1.76

O Al Si Ca Fe S K Mg C Na

28.41 14.40 24.46 16.42 6.67 3.08 3.46 0.65 2.45

28.46 14.42 24.63 16.37 6.62 3.07 3.42 0.59 2.42

(b) SSM(B) 28.45 14.45 24.57 16.38 6.64 3.02 3.44 0.60 2.45

28.43 14.38 24.68 16.35 6.61 3.09 3.42 0.61 2.43

28.47 14.36 24.53 16.41 6.60 3.05 3.45 0.64 2.49

28.44 14.40 24.57 16.39 6.63 3.06 3.44 0.62 2.45 0.00

O Al Si Ca Fe S K Mg C Na

27.50 19.02 30.07 12.53 6.70

27.49 18.97 30.10 12.51 6.71

(c) SHLH(A) 27.48 19.05 29.99 12.55 6.74

27.52 18.98 30.10 12.50 6.72

27.45 19.04 30.11 12.49 6.76

1.86 0.09 1.49 0.74

1.89 0.12 1.52 0.69

1.85 0.12 1.47 0.71

O Al Si Ca Fe S K Mg C Na

22.20 23.52 32.06 2.68 13.44

22.17 23.50 32.19 2.66 13.43

1.83 1.88 0.11 0.08 1.51 1.52 0.74 0.70 (d) SHLH(B) 22.15 22.16 23.54 23.50 32.01 32.06 2.71 2.72 13.49 13.40

27.49 19.01 30.07 12.52 6.73 0.00 1.86 0.10 1.50 0.72

2.34 0.48 2.95 0.33

2.32 0.42 2.92 0.39

2.32 0.43 2.98 0.37

2.34 0.47 2.98 0.37

22.21 23.49 32.11 2.67 13.48 2.36 0.44 2.90 0.34

22.18 23.51 32.09 2.69 13.45 0.00 2.34 0.44 2.94 0.36

Anorthite, quartz, hedenbergite, and gehlenite were present in SSM(A), and hercynite was present in SSM(B). The high-MP quartz content of the SSM(B) was higher than that of SSM(A), illustrating the difference in ash fusion characteristics (Ts and AFT) of the two SSM samples. Six minerals (anorthite, quartz, hedenbergite, gehlenite, hercynite, and mullite) were found in both SHLH samples, with mullite found to be more abundant in SHLH(B) than in SHLH(A). These minerals led to the higher Ts value and AFT of SHLH(B) compared to those of SHLH(A). 3.4. Process of Slag Formation. 3.4.1. Mineral Transformation during Ash Heating. The main minerals in the two ash samples at different temperatures under a reducing atmosphere (H2/CO2 volume ratio of 1:1) are shown in Figure 7. Mineral matters in the HLH ashes at 815 °C consisted of 7820

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Figure 6. XRD patterns of four slag samples. 1-anorthite (CaO·Al2O3·2SiO2); 2-quartz (SiO2); 3-hedenbergite (CaO·FeO·2SiO2); 4-gehlenite (2CaO·Al2O3·SiO2); 5-hercynite (FeO·Al2O3); 6-mullite (3Al2O3·2SiO2).

anorthite + calcium oxide → gehlenite (2CaO·Al2O3·SiO2).11 Figures 6 and 7 suggest that more low-MP minerals were present in the slag samples than in the corresponding ash samples at 900−1100 °C. 3.4.2. Characteristics of Fine Char. The characteristics of fine char are fundamental to the investigation of the slagging behavior in the upper region of the MFB. Thus, the particle distribution and fusibility characteristics of fine chars from the second cyclone in the pressurized ash agglomerate fluidized bed (detailed in a previous paper9) used in SM and HLH gasification were investigated. The particle size distributions of the two fine chars were determined by passing them through 40, 60, 80, 100, 120, and 140 mesh standard sieves (ASTME11-61) (results are presented in Figure 8). The particle size distributions of two fine chars were different: fine chars of SM had a multi-peak distribution, whereas HLH had a two-peak distribution that was similar to the particle size distributions of fly ashes from the combustion of pulverized coal.34 Table 8 shows the ash compositions of fine chars with different sizes. Smaller particles of the two fine char ashes contain more iron and calcium because pyrite interacts with other minerals and calcium exchanged from the complexes of carboxylate and organometal transformed during gasification.25 3.4.3. Mechanism of Slag Formation. On the basis of the above discussion, the process of slag formation in the upper region of the refractory wall during coal MFB gasification may be deduced as follows.

Table 7. Mineralogical Composition of Four Slag Samples by RIR type of slag sample mineral (wt %)

SSM(A)

SSM(B)

SHLH(A)

SHLH(B)

anorthite quartz hedenbergite gehlenite hercynite mullite amorphous mattera

21.72 13.34 8.13 9.28

19.02 18.71 7.89 8.74 3.28

47.53

42.36

17.46 21.09 8.01 7.64 3.07 3.45 40.28

16.06 25.17 7.29 6.92 2.85 5.37 37.24

a

Includes both the amorphous phase and any carbon (char) components.

quartz, anhydrite, enstatite, and hematite. Olivine appeared at 900 °C because of the reactions of shematite (Fe2O3) + H2 (CO) → wustite (FeO) and magnesium oxide (MgO) + quartz (SiO2) + wustite → olivine [(MgFe)2SiO4]. Mullite and hercynite emerged at 1000 °C because of the reactions of metakaolin (3Al2O3·2SiO2) → mullite (Al2O3·2SiO2) and mullite + wustite → fayalite (2FeO·SiO2) + hercynite (FeO· Al2O3).32 Anorthite formed at 1100 °C because of the reaction mullite + calcium oxide (CaO) → anorthite (CaO·Al2O3· 2SiO2).33 Calcite was present in the ash samples of SM at 815 °C. Because of the calcium oxide content of SM (27.25%), which is higher than that of HLH (8.04%), gehlenite formed at 1000 °C through the four slag samples, reaction 7821

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Figure 8. Particle size distribution of two fine chars from PAFB. Figure 7. XRD patterns of ash samples at different temperatures. 1 quartz (SiO2); 2 anhydrite (CaSO4); 3 enstatite (MgSiO3); 4 hematite (Fe2O3); 5 olivine ((Mg·Fe)2SiO4); 6 hercynite (FeO·Al2O3); 7 mullite (3Al2O3·2SiO2); 8 anorthite (CaO·Al2O3·2SiO2); 9 calcite (CaCO3); 10 gehlenite (2CaO·Al2O3·SiO2).

disturbed, these fine mineral particles can move to the zone near the inner faces of the upper parts of the gasifier. Interactions of fine particles with high reactivity (large surface specific area) lead to the formation of low-MP spherical grains, which can agglomerate into adhesive amorphous matter. Such amorphous matter can adhere to the inner face of the gasifier (section I in Figure 2) and form an initial adhesion layer. Because of the core−annulus flow through the fluidized bed (upward in the core and then downward in the annulus37), the region above the site of oxygen injection may be divided into three different zones on the basis of the movement of mineral particles, as presented in Figure 9. Results for stress analyses of the particles are also shown in Figure 9. Particles in the transition zone tend to move to the boundary zone during their transformation. Interaction of these particles and fine particles in the boundary zone under a high temperature may lead to the formation of adhesive amorphous matter. Under thermophoretic force, the amorphous matter moves to the inner face of the gasifier and forms an initial adhesion layer in section II (see Figure 2). Finally, fine particles with low MP can settle near the inner walls of the gasifier. The reaction of sodium vapors and sulfur

First, fine chars from ash from the lower part of the AFB of the MFB rise to the oxygen injection zone (high-temperature zone), where the fine chars are gasified quickly. During its gasification, non-liberated or included minerals in the fine char become exposed, interact, and form mineral particles of various sizes. Some of these fine particles are enriched in iron and calcium. During gasification, pyrite transforms into pyrrhotite, which then decomposes and undergoes oxidation from the surface inward, producing molten FeO−FeS.35 Sulfur dioxide generated in the FeO−FeS molten droplets (MP of 940 °C) leads to the rupture of ash particles.36 Thus, the ferrous salt ferroaluminate or ferroaluminosilicate forms in the fine ash particles. The ferrous salt in liquid aluminate or aluminosilicate is then partially substituted by calcium because the polarity of calcium is lower than that of ferrous ion.11 Thus, the amounts of low-MP eutectics of hercynite, anorthite, and gehlenite increase in the fine ash particles.16 When oxygen injection is 7822

DOI: 10.1021/acs.energyfuels.5b01711 Energy Fuels 2015, 29, 7816−7824

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Energy & Fuels Table 8. Ash Compositions of Two Fine Chars with Different Sizes ash compositions of fine chars (wt %) sample SM

HLH

size (mesh)

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

K2O

Na2O

TiO2

P2O5

−140 140−120 120−100 100−80 80−60 60−40 +40 −140 140−120 120−100 100−80 80−60 60−40 +40

26.16 28.19 31.82 35.13 42.89 44.06 46.70 44.50 44.28 45.64 47.95 48.72 46.29 47.75

13.38 12.21 12.47 11.51 11.09 13.58 15.39 18.35 20.17 22.16 21.09 22.12 25.57 25.34

21.35 20.80 20.18 19.82 19.63 18.90 18.76 16.23 16.87 1 4.79 1 4.17 1 3.23 1 2.63 1 2.16

35.06 34.22 30.72 29.57 21.26 18.92 15.17 15.51 1 3.38 1 2.01 11.62 1 0.74 1 0.22 9.36

1.34 1.65 2.17 1.28 2.68 2.20 1.56 1.77 1.72 1.79 1.65 1.67 1.76 1.85

0.58 0.73 0.48 0.53 0.52 0.52 0.47 1.96 2.01 1.97 1.89 1.95 1.98 1.95

0.50 0.50 0.47 0.56 0.39 0.36 0.38 0.32 0.25 0.32 0.28 0.26 0.25 0.32

0.14 0.10 0.12 0.12 0.11 0.10 0.09 0.37 0.36 0.39 0.34 0.31 0.30 0.29

1.21 1.37 1.30 1.18 0.98 1.09 1.22 0.97 0.95 0.92 0.99 0.98 0.97 0.96

0.28 0.23 0.27 0.30 0.36 0.27 0.26 0.02 0.01 0.01 0.02 0.02 0.03 0.02

refractory material and reacts with ferrous or calcium salt, leading to the formation of low-MP matter. Fe2+ in the ashes forms low-MP eutectic compounds, which increase the degree of bonding between the molten ash and refractory liner.39 This mechanism of slag formation at different sections may explain the differences in elemental composition and ash fusion characteristics of the slag samples. 3.5. Possible Methods of Operation That Prevent Slag Formation. As deduced above, slag formation is related to the fusion of low-MP minerals and their eutectics and to the appearance of the initial layer. Thus, the decrease in the temperature of the MFB section above the site of oxygen injection may provide an efficient way to prevent the slag formation. This may be performed by specifically decreasing the amount of oxygen injected or the amount of water vapor purged from the walls of the gasifier at regular intervals, which washes off the initial deposit through purging pipes.

4. CONCLUSION The Ts values and AFT of the four slags increase in the order SSM(A) < SSM(B) < SHLH(A) < SHLH(B), because of differences in the A/B ratio and mineral type. The slags are composed of amorphous matter and crystals, with the crystal content of the SHLH samples being higher than that of the SSM samples. The section of the MFB above the site of oxygen injection may be divided into three zones based on particle movement. Iron and calcium are more abundant in fine particles than in other particles. The formation of slag (the upper part of the MFB) might occur in the following way: via mineral exposure by char gasification, mineral interaction, formation of particles of different sizes, formation of an initial adhesive layer, and slag generation. The reduction of the amount of injected oxygen content may provide an efficient way to prevent the slag formation.

Figure 9. Zone distribution and stress analyses of particles.

dioxide generates liquid silicate, which then evolves into lowmelting eutectics and Ca−S−Na sulfate by reaction with calcium minerals.38 These processes lower the MP of the particles. Under certain conditions, these particles can adhere to the initial adhesion layer of the inner faces of the gasifier. Some of these particles subsequently adhere to section I, forming slag A, through effects of collision with other particles, and others adhere to section II, forming slag B. During gasification, refractory material in the gasifier wall can react with the minerals that reach the inner wall, thereby increasing the tendency for slagging on the inner face of the gasifier. Silicon dioxide in the refractory material can undergo reduction and convert into gaseous silicon(II) oxide [SiO2(s) + H2(g) → SiO(g)↑ + H2O]. SiO2 then escapes from the

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Corresponding Author

*Telephone: +86-3512021137. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7823

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ACKNOWLEDGMENTS This work is financially supported by the Strategic Priority Research Program of the CAS (XDA07050100 and XDA07050103) and the Natural Science Foundation of Shandong Province, China (ZR2014BM014). The authors are thankful to all workers in the Coal Gasification Pilot-Scale Center, ICC, CAS.

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DOI: 10.1021/acs.energyfuels.5b01711 Energy Fuels 2015, 29, 7816−7824