Fusibility Characteristics of Fine Chars from Pilot-Scale Fluidized-Bed

Oct 8, 2014 - The particle size of FSM had a multipeak distribution, whereas that of FJC had a two-peak distribution. During sintering, molten, low-me...
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Fusibility Characteristics of Fine Chars from Pilot-Scale Fluidized-Bed Gasification Fenghai Li,†,‡,§ Huixia Xiao,‡ Jiejie Huang,§ Yitian Fang,§ and Zhaomin Xue*,† †

Department of Chemistry and Chemical Engineering, Heze University, Heze 274000, People’s Republic of China School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, People’s Republic of China § State Key Laboratory of Coal Conversion, Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS), Taiyuan 030001, People’s Republic of China ‡

ABSTRACT: To explore the fusibility characteristics of fine chars from gasification using an ash agglomeration fluidized bed, fine char samples of Shenmu bituminite (FSM) and Jincheng anthracite (FJC) were analyzed by X-ray fluorescence spectrometry, pressure-drop sintering technique, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses. Differences in elemental distributions and surface morphologies of the samples due to variations in petrographic composition and reactivity of their respective raw coals were observed. The ash fusion temperatures and sintering temperatures increased in the order FSM < raw Shenmu bituminite < FJC < raw Jincheng anthracite, because of the increase in total base content of their ash. The particle size of FSM had a multipeak distribution, whereas that of FJC had a two-peak distribution. During sintering, molten, low-melting minerals flowed between the surfaces of adjacent ash particles and led to particle aggregation and aggregate densification. investigated.7 Fusion and composition characteristics of residual ashes (slag, agglomerate, and gangue ashes) from lignite AFB gasification and their formation mechanism were explored.8−10 The slagging tendency of fly ashes from AFB gasification of Jincheng anthracite was found to increase via enhancement of calcium and iron flux into the fly ashes rather than into raw coal.11 On the basis of reaction rate calculations, the gasification reactivities of fine chars from AFB gasification of Xiangyuan bituminous coal and Jincheng anthracite increase as the temperature increases, and the controlling step of the reaction mechanism from reaction shifts to gas diffusion at high temperature.12,13 Some of the fine spherical particles with high calcium content attach to the surfaces of fly ash from Shell gasifiers. However, these particles easily aggregate, promoting adhesion and deposition of fly ash.14 The adhesion of sodium silicate is the culprit behind slag formation during the gasification of high-sodium lignite. Reduction of the operating temperature and addition of meta-kaolin can prevent this problem.15,16 However, little work about the fusibility characteristics of fine chars from AFB gasification tests in a pilot plant has been published, despite the fact that such characteristics are important to adjustments of the secondary oxygen concentrator and to the prevention of slag formation during coal gasification in a MFB. Thus, experiments on coal gasification in a pilot-scale fluidized bed were conducted at the Coal Gasification Engineering Center (ICC, CAS) to understand the characteristics of fine chars from AFB gasification. We believe that the study can provide some references for developing MFB gasification technology.

1. INTRODUCTION Coal gasification, which is a key technology for coal process and power generation, is increasingly attracting considerable interest worldwide.1,2 Among the three gasification technologies, namely, fluidized-bed, entrained-flow bed, and fixed-bed gasification, the first one is considered to be one of the most promising technologies for coal conversion,3,4 because it allows for wide fuel flexibility, provides uniform bed temperature, and is environmentally friendly. However, its operating temperature is lower than that of entrained-flow bed, its particle mixture is uniform, and syngas entrains its fine chars; thus, its carbon conversion ratio is usually lower, compared with that of entrained-flow bed gasification. On the basis of research and development of an ashagglomeration fluidized bed (AFB), the Institute of Coal Chemistry (ICC) at the Chinese Academy of Sciences (CAS) proposed the concept of a multistage conversion integrated fluidized bed (MFB; see Figure 1) to improve the conversion ratio and energy efficiency of the AFB. A technology for introducing oxygen and coal samples in batches was adopted in the MFB. This technology reduced the combustion intensity in the local region, as well as increased the utilization ratio of the gasifier volume, the residence time and concentration of fine chars, and energy efficiency. The characteristics of fine chars from an AFB are fundamental to the design of a MFB and to the selection of its operating parameters. Thus, investigating the characteristics of fine chars in the research and development of the MFB is necessary. In recent years, the characteristics of coal gasification residuals were examined by researchers from different disciplines. Fusibility, flow, and rheology of the slag from Shell gasification and from Texaco gasification were explored.5,6 The chemical composition of glassy and crystalline phases in coarsecoal gasification ashes from a Sasol−Lurgi fixed bed were © XXXX American Chemical Society

Received: July 8, 2014 Revised: October 8, 2014

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

RJC

Proximate Analysis (wt, ad %) moisture volatile matter ash fixed carbon carbon hydrogen nitrogen total sulfur oxygena a

6.54 27.25 12.06 54.15 Ultimate Analysis (wt, ad %) 63.36 4.15 0.95 0.37 11.49

4.32 5.97 25.30 64.41 63.08 1.36 0.74 1.83 3.11

By differences.

Table 2. Ash Compositions and Ash Fusion Temperatures (AFTs) of RSM and RJC composition SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2 P2O5 ash fusion temperatures deformation temperature, DT soft temperature, ST hemispheric temperature, HT flow temperature, FT

Figure 1. Schematic of the multistage conversion integrated fluidized bed (MFB). [Legend: 1, ash falling pipe; 2, circular separation tube; 3, gasification agent inlet pipe on the distribution plate; 4, cone distribution plate inlet chamber; 5, crushed coal feed tube; 6, fine pulverized coal feeding tube; 7, oven feed tube; 8, central section of fast pyrolysis gasification reaction; 9, heat insulation materials; 10, thermal insulation material; 11, furnace; 12, upper section that chars hightemperature pyrolysis; 13, gasifier gas exports; 14, first cyclone separator; 15, fine chars circulation riser; 16, fine chars circulation control valve; 17, steam blow pipe; 18, fine chars feed tube; 19, bottom section that dense phase fluidized bed gasification; 20, cone distribution board; 21, center jet pipe; 22, center pipe gasification agent inlet pipe; 23, circular tube gasification agent inlet pipe; and 24, discharge port.]

RJC

RSM

34.06 wt % 14.32 wt % 16.95 wt % 27.62 wt % 1.88 wt % 0.97 wt % 3.08 wt % 0.22 wt % 0.62 wt % 0.28 wt %

47.00 wt % 33.55 wt % 7.99 wt % 5.16 wt % 1.60 wt % 2.92 wt % 0.38 wt % 0.46 wt % 0.85 wt % 0.01 wt %

1170 °C 1185 °C 1204 °C 1210 °C

1485 °C >1500 °C >1500 °C >1500 °C

performed at 950−1030 °C and at 1050−1150 °C, respectively, at a pressure of 1.5 MPa. The fine char samples, referenced here as FSM and FJC, respectively, were collected in receptacles for fine char after the second cyclone separator. A flow diagram of the PFAB pilot-plant gasifier (described in a previous paper9) including the receptacle for fine chars is depicted in Figure 2. 2.2.2. Preparation of the Fine-Char Samples of Various Particle Size Distributions. The particle size distributions of FSM and FJC were analyzed through a method using an American standard sieve (ASTME-11-61). Chars from AFB gasification were divided into seven groups by passing them through 40M, 60M, 80M, 100M, 120M, and 140 M standard sieves. 2.2.3. Preparation of Ash Samples Processed at Different Temperatures. Samples of FJC were prepared according to the Chinese Standard procedure (GB/T1574-2001). Low-temperature ash samples of FJC were prepared at 180 °C by using a low-temperature plasma asher (230 V, 50 Hz; ProSciTech, Model EMS1050X, U.K.). Ash samples of the FJC samples were heated in a fixed-tube furnace (see Figure 3) at 920, 950, 980, 1000, and 1050 °C under a reducing atmosphere (1:1 H2/CO2, volume ratio). The structure and preparation process of the ash samples heated at different temperatures are expounded in a previous paper on low-temperature ash samples of FJC.8 The ash samples were subsequently removed from the fixed-tube furnace and immediately immersed in ice water to prevent phase transformation and crystal segregation.17,18 The quenched samples were then dried in a vacuum chamber at 105 °C, ground to a size of 1500 °C

Figure 7. B values of four samples.

FJC increase with the particle size. The variation in AFTs mainly resulted from the difference in composition, since there were great differences in elemental distributions, along with the differences in particle sizes. Smaller particles of the fine chars formed during coal gasification usually contained more iron and calcium, because of the interactions of pyrite with other minerals and the transformation of exchanged calcium from carboxylate and organometallic complexes.7 The ash compositions of FSM and FJC (Table 7) show that the flux of CaO and Fe2O3 decreases as the particle size of FJC increases. This trend explains the AFT increase of FJC with its particle size. The AFT of FSM decreased first with the increase in particle size, because of the decrease in the amounts of the refractory Al2O3 and CaO. It subsequently increased because of the increase in Al2O3 content and the decrease in CaO content. The AFT decreased as the CaO content increased until the latter reached 35%, because of the formation of low-melting anorthite and gehlenite; at higher CaO contents, the AFT increased, because of its high melting temperature.32 3.4. Sintering Characteristics of Ashes from FJC. 3.4.1. Change in Surface Morphology. Sintering refers to a process of particle softening and surface flow, which leads to particle cohesion. 7 Sintering is one of the dominant mechanisms behind operational problems during fluidizedbed gasification or combustion of coal (e.g., agglomeration, defluidization, deposition on the gasifier wall, and clinker

Table 6. Sintering Temperatures of Four Samples under Reducing Atmosphere type of ash

sintering temperature (°C)

RSM RJC FSM FJC

835 980 820 975

tendency to terminate polymer formation and lower the AFT and sintering temperature. Oxides of coal ashes lower the AFT and sintering temperature in the following order: SO3 > CaO > MgO > Fe2O3 > Na2O.29−31 Thus, SO3 is classified as a basic composition. B values for the four samples, which represent the total amounts of SO3, CaO, MgO, Fe2O3, Na2O, and K2O, are shown in Figure 7. The B values decrease in the order FSM > RSM > FJC > RJC, which may be explained by the differences in the AFTs and sintering temperatures of the four samples. 3.3.2. AFT Differences in FSM and FJC Samples of Various Particle Sizes. AFTs of FSM and FJC with different particle sizes are presented in Figure 8. The AFTs of FSM decrease and then increase as the particle size increases, whereas those of

Figure 8. Ash-forming temperatures (AFTs) of two fine chars with different particle sizes. G

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Table 7. Ash Compositions of FSM and FJC with Different Sizes Ash Composition of Lignite (wt %) size

SiO2

Al2O3

Fe2O3

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

25.19 28.29 32.82 36.09 42.99 44.04 46.71

13.85 12.11 11.47 10.51 10.99 13.60 15.38

21.85 20.81 20.20 19.62 19.73 18.93 18.96

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

44.50 44.28 45.64 47.95 48.72 46.29 47.75

28.35 30.17 32.16 31.09 32.10 35.57 36.34

11.23 10.87 9.79 9.17 8.23 7.63 7.16

CaO

MgO

Sample FSM 37.41 1.35 35.21 1.15 30.62 2.07 29.77 1.38 21.16 2.78 18.89 2.22 14.97 1.58 Sample FJC 10.51 1.75 9.38 1.72 7.01 1.79 6.62 1.65 5.74 1.66 5.23 1.75 3.36 1.85

SO3

K2O

Na2O

TiO2

P2O5

0.57 0.53 0.48 0.53 0.42 0.50 0.45

0.51 0.50 0.47 0.46 0.38 0.38 0.39

0.13 0.10 0.12 0.12 0.11 0.08 0.08

1.21 1.07 1.20 1.15 0.96 1.07 1.23

0.28 0.23 0.37 0.33 0.38 0.29 0.25

1.98 2.01 1.99 1.89 1.95 1.98 1.94

0.31 0.25 0.30 0.28 0.26 0.27 0.33

0.38 0.36 0.38 0.34 0.32 0.30 0.28

0.96 0.95 0.93 0.99 0.97 0.95 0.97

0.03 0.01 0.01 0.02 0.02 0.03 0.02

Figure 9. Surface morphologies of FJC ash samples at different temperatures: (a) 920, (b) 950, (c) 980, (d) 1000, and (e) 1050 °C.

formation).33−35 Since ash sintering temperatures are lower than its deformation temperature (DT, sometimes corresponding to 80%−90% of the DT),36 the sintering temperature may be a simpler tool for evaluating the agglomeration potential.37 To explore the sintering process of fine chars under a gasification atmosphere, FJC ash samples were heated at 920, 950, 980, 1000, and 1050 °C under a reducing atmosphere. These temperatures were used because they were near the sintering temperature of FJC (975 °C). The surface morphologies of ash samples at different temperatures are displayed in Figure 9. At 920 °C, the ash was mainly composed of many fine, irregular particles and larger particles. Some fine ash particles adhered to the surface of

larger spherical particles (Figure 9a). As the temperature increased, some fine particles agglomerated with the spherical particles and led to an increase in spherical volume (Figure 9b). The larger spherical particles then began to agglomerate, while the closed aperture in the ash samples widened, and open channels formed; ash sintering could occur at the aforementioned temperatures (Figure 9c). At 1000 °C, particles began to aggregate and some channels became smaller (Figure 9d). Finally, the adjacent, larger spherical particles began to merge and formed even larger spherical particles, because of the reduction in free surface energy, and finally led to densification (Figure 9e).38−40 H

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(4) During sintering, molten, low-melting-point minerals flowed between the surface of adjacent ash particles, leading to an accumulation of particles and densification of aggregates.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Contract No. XDA07050103), and the Foundation of State Key Laboratory of Coal Conversion (Contract No. J12-13-102). We are thankful to all workers in the coal gasification pilot scale center, ICC, CAS.

Figure 10. XRD patterns of FJC ashes at different temperatures. [Legend: 1, quartz (SiO2); 2, sodium iron sulfide (NaFeS2); 3, mullite (Al6Si2O13); 4, hercynite (FeAl2O4); 5, microcline, ordered (KAlSi3O8); and 6, anorthite (CaAl2Si2O8).]

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