Fusibility Characteristics of Residual Ash from Lignite Fluidized-Bed

Jun 28, 2012 - Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People's Republic of China. Energy Fuels , 2012, 26 (8), pp 5...
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Fusibility Characteristics of Residual Ash from Lignite Fluidized-Bed Gasification To Understand Its Formation Feng-hai Li,† Jie-jie Huang,‡ Yi-tian Fang,*,‡ and Quan-run Liu† †

School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, People’s Republic of China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China

‡

ABSTRACT: To investigate fusibility characteristics of residual ash from Xiaolongtan lignite (XLT) pressurized fluidized-bed ash agglomerate (PFBA) gasification and its formation mechanism, the ash samples were examined by press-drop sintering technique, scanning electron microscope with energy-dispersive X-ray detector (SEM−EDX), and X-ray diffraction (XRD) analyses. The results show that both sintering temperatures and ash fusion temperatures (AFTs) of ash samples decrease from the gangue ashes to agglomerates to slag, because of the increase of the total base content and the differences in their mineral compositions accordingly. The surfaces of the slag were obviously molten; the agglomerates were covered with some small spheres and large particle aggregates, and the gangue ashes were composed of some irregular prismatic particles. During XLT PFBA gasification, the agglomerates originate from the collision and reunification of low-melting minerals under the dragging force of rising gases and the gangue ashes result from some thermally stable mineral particles with a high-melting point. When the PFBA gasification deviates from the suitable operation parameters (e.g., the gas velocity of the distribution plate is low, the gas velocity of the tubular loop is high, or the steam/oxygen ratio is low), the slag might be formed in the gasifier.

1. INTRODUCTION Lignite is an abundant resource, which accounts for approximately 40% of global coal reserves. It is forecast to play an important role in energy markets and chemical engineering products in the near future.1 However, lignite usually has a high water and oxygen content and a low calorific value and is prone to break down, weathering, and spontaneous combustion. In addition, it is not easily transportable. All of these characteristics limit its high-efficient, clean use. Fortunately, lignite fluidized-bed gasification is considered as a promising gas-producing technology because it is more adaptable to the poorer quality coal and is more environmentally friendly.2 However, owing to the low ash fusion temperature of lignite and its high ash content3 (especially for sulfur in the form of pyrite, which reduces the ash fusion temperature to levels as low as 900 °C.4), this makes lignite highly susceptible to slagging. The slag could lead to defluidization, reducing the efficiency of the gasification process, and may even cause the fluidized-bed gasification system to shut down. The slag formation is strongly related to the transformation and fusion characteristics of coal mineral matter under high temperature and pressure during coal conversion.5−8 The ash fusion test provides an enhanced understanding of the processes occurring in the ash fusion furnace,9,10 and the ash fusion temperature (AFT) is still the most acceptable method for assessing the ash-slag propensity, although it has some shortcomings.11 The effects of CaO, Fe2O3, K2CO3, MgO, the ratio of silica/alumina (SiO2/Al2O3), as well as acid and basic fluxes on the AFTs of coal ashes have been studied by both means of the thermodynamic computer package FactSage and experimentally.7,12−14 Some indexes to predict the slagging tendency have been developed recently15−18 based on the composition and properties of coal ashes. When the operating © 2012 American Chemical Society

temperature is higher than the ash sintering temperature, the ash particles sticking to molten or sintered in the hightemperature regions in the fluidized bed leads to the slag formation.19−21 The sintering characteristics are mostly associated with alkali-metal content and the adhesion of ash deposit constituents to the boiler tubes, starting with smallparticle retention as a result of the van der Waals, electrostatic, and liquid film surface tension forces.22,23 In recent years, the gasification center of the Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS), has devoted much effort into the research and development (R&D) of lignite pressurized fluidized-bed ash agglomerate (PFBA) gasification (Figure 1). In an ideal fluidized-bed gasification process, the inorganic matter goes into bottom ashes and the organic matter produces syngas. When pilot-plant tests of the Xiaolongtan lignite (XLT) PFBA gasification process ran smoothly, two types of ash particles were found in the bottom ashes: one was spherical (agglomerates), and the other was prismatic (gangue ashes). When the operational parameter deviates from the suitable condition, the lump block (slag) might be formed on the inner surface or near the distribution plate of the gasifier. The fusibility characteristics of residual ashes play important roles in the deduction of mineral behaviors and the slag formation during gasification. Thus, it is necessary to study the characteristics of residual ashes. Texaco gasification ashes contain a relatively large proportion of the glassy phase, which is mostly composed of silicon dioxide, alumina, calcium oxide, and residual carbon.12,24 The microparticles of Shell gasification ashes are mostly molten or semi-molten spheres, and its fusion temperature and the Received: March 30, 2012 Revised: June 28, 2012 Published: June 28, 2012 5020

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Table 1. Ash Compositions and AFT of XLT constituent

composition (wt %)

SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2 P2O5 temperaturea

33.14 17.56 8.95 21.64 1.79 13.16 0.99 0.94 1.44 0.28 AFT (°C)

DT ST HT FT

1096 1158 1169 1189

a

DT, deformation temperature; ST, soften temperature; HT, hemispherical temperature; and FT, flow temperature.

2.2. Preparation of Three Residual Ash Samples. XLT gasification tests were conducted in the pilot-scale PFBA gasification engineering center, ICC, CAS. The gasification was operated at 900− 1000 °C with a pressure of 0.8 MPa. During the process of gasification, the gasification system was sometimes shut down because of the formation of slag. The agglomerates and gangue ashes were handpicked from the bottom ashes, and the slag samples were selected from the lump block that formed near the distribution plate of the gasifier. The shapes of three residual ashes are presented in Figure 2. The three residual ashes were ground to less than 0.074 mm and stored in a cabinet dryer before measurement.

Figure 1. Schematic presentation of the PFBA.

temperature of critical viscosity are lower than the corresponding laboratory ashes.7,25 Large amounts of anorthite crystals lead to the formation of slag during Shell gasification, and much sintered calcium aluminosilicate is found in the slagging samples during fluidized-bed gasification.26 However, little work has been published about the characteristics of residual ashes from Chinese lignite fluidizedbed gasification. This study reports some preliminary results on fusibility characteristics of three residual ashes (slag, agglomerates, and gangue ashes) from XLT PFBA gasification and the investigation on their formation processes. It is expected that the study can provide some guidance in the R&D of lignite fluidized-bed gasification technology. Figure 2. Shapes of three residual ash samples.

2. EXPERIMENTAL SECTION 2.1. Laboratory Ash Preparation. In this study, the air-dried XLT samples (originating from the Yunnan province, southwestern China) were selected. The samples were ground to a particle size of less than 0.200 mm and dried at 105 °C for 24 h in a nitrogen (N2) atmosphere.27 The XLT samples were preheated to 540 °C in a muffle furnace under air for 0.5 h. Then, the samples were heated to 820 °C and kept for 2 h in the muffle furnace, according to the Chinese standard GB/T212-2001. Finally, the samples were cooled to room temperature and stored in a cabinet dryer. The compositions of laboratory ashes were performed following analytical methods set out in the Chinese standard GB/1574-1995 for the ash compositions of coal, coke, and gangue and endorsed by the Chinese State Bureau of Technical Supervision (CBTS) on January 12, 1995. The AFT of XLT ashes under a reducing atmosphere (H2/CO2 = 1:1 volume ratio) were tested and are shown in Table 1. It can be observed that calcium oxide and sulfur trioxide in XLT ash composition are very high, and as expected, its AFT is comparatively low.

2.3. Experimental Apparatus and Method of the Sintering Temperatures. 2.3.1. Experimental Apparatus. The sintering temperatures of the ash samples were tested by the press-drop technique, which has been described in detail in previous work.28−32 The XLT laboratory ash and three residual ash samples were tested in the self-made pressurized sintering analyzer, which is schematically shown in Figure 3. The pressurized sintering analyzer mainly consists of five parts: quartz reactor, silicon−graphite tube furnace, pressure container, differential pressure transmitter, and collecting system. A thermocouple is inserted in the middle of the quartz reactor to measure and control the temperature. The pressure difference and temperature signal data are transmitted into the curve of pressure differences with the temperature. 2.3.2. Experimental Method. The sintering temperatures of ash samples were measured according to the following procedure. The ash sample (1.00 g) was formed into a cylinder with a diameter of 8 mm 5021

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3. RESULTS AND DISCUSSION 3.1. Fusion Characteristics of Three Residual Ashes. 3.1.1. Sintering Characteristics of Ash Samples. During the

Figure 5. Agglomerate curve of pressure differences with temperature. Figure 3. Schematic diagram of the pressurized sintering temperature analyzer: 1, thermoelectric couple; 2, gas inlet; 3, pressure vessel; 4, silicon−graphite tube furnace; 5, quartz tube; 6, connecting electric wire point; 7, ring flange connect; 8, temperature controller; 9, differential pressure transmitter; 10, computer processor; 11, CO2; 12, O2; 13, H2; 14, CO; 15, N2; 16, pressure gauge; 17, mass flow meter; and 18, disconnecting valve. on a tablet press with a pressure of 0.8 MPa. The cylinder was put into the quartz reactor with a sheath (Figure 4) and then inserted into the

Figure 4. Quartz reactor with sheath with and without sample.

Figure 6. Sintering temperatures of laboratory and three residual ash samples.

constant temperature zone of the silicon−graphite tube furnace. Then, a mixture of 50% hydrogen and 50% carbon dioxide (volume ratio) was introduced into the reactor to displace the air until the reducing atmosphere was reached. When the system steady state was achieved, the temperature was increased at a constant heating rate. At the same time, the data collecting system acquired the curve of pressure differences with the temperature. The temperature of the pressure difference turning point is the sintering temperature of ash samples. 2.4. Analytical Measurements. The surface morphology and composition of three as-received residual ash samples were investigated using FEI, NavaNano 430 scanning electron microscopy (SEM) equipped with a KEVEX, Sigma energy-dispersive spectroscopy X-ray (EDX) analyzer. For SEM observations, the fine ash powder was placed carefully on conducting glue and then coated with gold vapor to make them conductive. The accuracy of such morphological, geometrical, and elemental composition analyses is highly sampledependent.33 The concentrations of elements are only semiquantitative, although the data value has a precision of 0.01 wt % because of the retestability of the EDX analyzer. The mineral compositions of the residual ash samples were analyzed by X-ray diffraction (XRD). The XRD patterns of the complexes were recorded on a RIGAKU D/max-rB X-ray powder diffractometer with Cu Ka radiation (40 kV, 100 mA, Ka1 = 0.154 08 nm), and a step size of 0.01° at 5° 2θ/min scanning speed between 2θ = 15° and 80° was applied.

Figure 7. AFTs of three residual ash samples.

processes of sintering, the atoms in the powdered particles diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. The pressuredrop techniques are believed to provide a more accurate 5022

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Figure 8. SEM photos and elemental compositions of three residual ash samples: (a) slag, (b) agglomerates, and (c) gangue ashes.

agglomerates to slag, which is the same as the changing trend of their sintering temperatures. 3.2. Surface Characteristics of Three Residual Ash Samples. SEM (FEI, NavaNano 430) was used to examine the surface morphologies of three residual ash samples. Qualitative element identification and semi-quantitative analyses of the samples were obtained by EDX (KEVEX, Sigma). The elemental analyses were performed in “spot mode”, in which the beam is localized on a single manually chosen area. The spot is represented on the SEM images by a circle. The results of the SEM photos and elemental compositions of the three residual ash samples are shown in Figure 8. It can be seen that the slag is composed of larger particle aggregates with obvious apertures, the surfaces of which were obviously molten and smoothed (Figure 8a), which indicated that the slag contained a certain amount of glassy material. The agglomerate samples were covered with some small spheres and large-particle aggregates (Figure 8b), and the gangue ashes were composed of

indication of the sintering temperature of coal ashes because of the constant monitoring of ash structural changes and the inherent sensitivity to structural changes.28 The variation in pressure difference with the temperature increase of the agglomerate under a reducing atmosphere (H2/CO2 = 1:1) is presented in Figure 5. It can be seen that the pressure difference reaches a maximum at 670 °C. According to Darcy’s law,30 670 °C is the sintering temperature of the agglomerate. The sintering temperatures of laboratory and three residual ashes are presented in Figure 6. It can be seen that the sintering temperature of the laboratory and three ash samples decreases in the order of gangue ashes, laboratory ashes, agglomerates, and slag. 3.1.2. AFTs of Three Residual Ashes. The AFTs of three residual ash samples under a reducing atmosphere (H2/CO2 = 1:1) were tested on the HR-A5 ash-melting-point analyzer and are shown in Figure 7. It can be seen that the AFTs of three residual ash samples are decreased from gangue ashes to 5023

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Table 2. Element Composition of Three Residual Ashes Determined by EDX (Line Weight Percent) element

1

2

O Al Si S K Ca Fe

44.47 8.95 14.20 2.05 1.06 18.48 10.79

44.60 8.82 14.18 1.92 1.12 18.53 10.83

O Al Si S K Ca Fe

51.70 8.97 16.13 2.06 1.36 12.05 7.73

51.71 9.02 16.09 2.10 1.30 12.03 7.75

O Al Si S K Ca Fe

54.17 10.06 18.63 2.76 1.59 7.77 4.91

54.30 9.98 18.72 2.69 1.65 7.83 4.83

3

5

6

mean value

44.60 8.80 14.10 2.03 0.96 18.67 10.83

44.52 8.98 14.20 1.90 1.10 18.62 10.68

44.58 8.89 14.16 2.00 1.05 18.54 10.78

51.75 9.04 16.05 2.04 1.30 12.00 7.82

51.58 9.03 16.09 2.08 1.38 11.97 7.87

51.69 9.01 16.10 2.06 1.34 12.00 7.80

54.31 9.96 18.64 2.77 1.64 7.86 4.81

54.22 10.07 18.73 2.72 1.59 7.82 4.85

54.21 10.03 18.69 2.74 1.62 7.83 4.88

4

(a) Slag 44.77 44.51 8.93 8.86 14.15 14.13 2.03 2.07 0.98 1.08 18.43 18.51 10.71 10.84 (b) Agglomerates 51.66 51.76 8.95 9.05 16.09 16.15 2.08 2.00 1.32 1.38 12.03 11.92 7.87 7.74 (c) Gangue Ashes 54.16 53.95 10.05 10.08 18.70 18.72 2.70 2.80 1.61 1.64 7.83 7.87 4.95 4.94

Table 3. Compositions of Three Residual Ash Samples composition (wt %) constituent

slag

agglomerates

gangue ashes

residual carbon SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2 P2O5

4.85 39.33 17.44 13.90 19.01 0.66 2.14 1.04 0.82 0.48 0.15

8.47 39.26 16.76 13.65 18.38 0.54 0.93 0.89 1.06 0.37 0.28

5.36 63.46 20.24 3.64 4.18 0.35 0.78 0.17 0.52 0.30

Figure 10. XRD patterns of three residual ash samples: (a) slag, (b) agglomerates, and (c) gangue ashes, with peaks indicated as 1, quartz (SiO2); 2, anorthite (CaO·Al2O3·2SiO2); 3, hedenbergite (CaO·FeO·2SiO2); 4, gehlenite (2CaO·Al2O3·SiO2); 5, chloritoid (FeO·Al2O3·SiO2); and 6, mullite (3Al2O3·2SiO2).

some irregular prismatic particles (Figure 8c). Such morphological differences may be attributed to the different formation processes of the three residual ash samples. On the basis of EDX analyses, the original and mean elemental compositions of six different spots of the three residual ash samples are shown in Table 2. It can be seen that the contents of silicon and aluminum decrease, and the contents of iron and calcium increase from the gangue ashes to agglomerates to slag. This indicates that the slag and agglomerates contain more low-melting-point calcium and iron eutectic compounds than gangue ashes.

Figure 9. Total base contents of laboratory and three residual ashes. 5024

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the laboratory and three residual ashes have been obtained and are shown in Figure 9. It can be seen that the total base content increases in the order of gangue ashes, laboratory ashes, agglomerates, and slag, which can explain the sintering temperature or AFT variation in the three residual ash samples. Base oxides present in the coal are known to be fluxing materials, which tend to lower the AFT or sintering temperature.38 Furthermore, the different structures and contents of residual carbon in three residual ashes might have effects on their fusion characteristics.39 3.3.2. Crystal Differences in the Three Residual Ash Samples. The ratio of the peak heights in the XRD patterns is proportional to the mineral concentrations.40,41 For the same mineral, the change of diffraction intensity can approximately reflect the variation in its content.42 The mineral compositions of the three residual ash samples are presented in Figure 10, and the contents of all of the components determined by the reference intensity ratio (RIR) are listed in Table 4. It can be seen that the slag is mostly composed of anorthite, quartz, hedenberite, and small quantities of gehlenite (Figure 10a). Clearly distorted baselines in the 2θ range of 5−30° indicate the existence of much glassy material in the slag.1 This is consistent with the morphological results observed by SEM. The formation of low-melting-point eutectics of gehlenite, anorthite, and other calcium compounds makes the sintering temperature and AFT of slag lower than the other two ashes. The contents of high-melting-point quartz and mullite in the gangue ashes are much higher than those in agglomerates, along with the low-melting-point chloritoid. Consequently, the sintering temperature and AFT of agglomerates are lower than those of gangue ashes.43 3.4. Formation Mechanism of Three Residual Ashes. On the basis of the fusion characteristics, surface morphologies, and XRD patterns of three residual ashes, the formation of three residual ashes during XLT PFBA gasification might occur in the following way. When lignite is fed into the PFBA gasifier, the volatiles are quickly changed into gases by devolatilization and form a porous coke. Then, the porous cokes gasify through the disintegration of the char, mineral dehydration, decomposition, and interactions, and the coalescence of ashes on the surfaces of char particles or ashes.44 Some alkali metal elements (e.g., sodium and potassium) might evaporate at high temperature, and other mineral matters might undergo a series of interactions and gradually form the ash particles with different sizes, densities, and compositions. The gas (steam/oxygen) distribution is presented in Figure 11, and the formation processes of three residual ashes are shown in Figure 12. When the XLT PFBA gasification runs smoothly, a large proportion of non-mineral inorganic elements (organically associated bound inorganic elements, exchangeable ions attached to carboxylates, e.g., Ca2+, Na+, organometallic complexes, and other functional groups) tends to partially melt and form liquid phases,45 together with adhesive glassy materials, with the transformations of some included minerals (e.g., 50 wt % of included iron mineral incorporated into iron alumino-silicate glass materials15),46 and forms the center of the adherend. Under certain temperatures and pressures, some melting entities are formed on the surface of the adherend because of the low-melting-point eutectic formation of fayalite, hercynite, gehlenite, and anorthite. At the central tube hightemperature zone, these melting entities, fly ash particles, or ash particles with a certain amount of residual carbon collide,

Table 4. Mineralogical Composition of Three Residual Ashes by the RIR types of residual ashes mineral (wt %)

slag

agglomerates

gangue ashes

quartz anorthite hedenbergite gehlenite chloritoid mullite glassa

17.46 20.32 10.62 5.37

47.60 9.87 8.34 9.43 4.38

60.27

46.23

20.38

10.35 8.58 6.53 14.27

a

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

Figure 11. Schematic diagram of gas distribution.

3.3. Composition and Mineralogical Differences in the Three Residual Ash Samples. 3.3.1. Differences in Ash Composition. The content of residual carbon in the three residual ash samples was analyzed by SC-444 equipment (Leco Corp., St. Joseph, MI). The remaining constituents of three residual ash samples were performed following the Chinese standard GB/1574-1995. The results of the analyses are presented in Table 3. It can be seen that the contents of iron and silicon in the slag or agglomerate ashes are higher than those in the gangue ashes, because the formation of the lowmelting-point adhesive ferrosilicate (the ternary phase diagram of the FeO−Al2O3−SiO2 system indicates the formations of two kinds of low eutectics of ferrosilicate, and the melting points are 1083 and 1148 °C34,35) during gasification leads to the enrichment of iron and silicon in block slag.36 The content of the residual carbon in the agglomerates is higher than in the slag, which might be due to the residence time of the slag, which is longer than that of the agglomerate. It was found that there is a good correlation between the sintering temperature or AFT and its total base composition content. The main base components (CaO and Fe2O3) usually comprise more than 85% of its total base content. Therefore, it is reasonable to assume that the properties of the ashes may be predicted from these main components.37 Here, B is defined as the total base content of CaO and Fe2O3. The values of B for 5025

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Figure 12. Formation process of three residual ashes.

the agglomerates result from the low-melting minerals with low-carbon collision and reunification under the dragging force of rising gases and the gangue ashes were derived from some high-melting-point and thermally stable mineral particles. When the operational conditions were deviated from the suitable operation parameters, the slag might be formed in the gasifier.

combine, and reunify with each other under the dragging force of rising gases and form larger agglomerate particles. The agglomerates drop into the ash bucket when they exceed the dragging forces of the gases. Meanwhile, some high-meltingpoint and thermally stable mineral particles with high densities (e.g., quartz particles pass through the gasifier without significant alteration45,47) do not melt when they go through the high-temperature zone of the central tube and come out of the gasifier in the form of prismatic gangue ashes. When the operational conditions of PFBA gasification deviate from the normal situation, it might lead to the formation of block slag. For example, when the gas velocity of the distribution plate is low, some melting or partial melting entities might agglomerate in the inactive zone, which leads to a lower gas velocity and eventually results in the formation of a larger slag. When the gas velocity of the tubular loop is high, the increase of dragging forces causes the residence time of the agglomerates to increase and the residual carbon content decreases and leads to the chances of a slagging increase, because the residual carbon prevents the mineral aggregation during gasification.27 When the steam/oxygen ratio is low, the local or bed temperature increases and causes more mineral matter to melt at certain places in the gasifier; therefore, the possibility of slagging increases accordingly.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National High-Tech R&D Program of China (863 Program, 2008AA050302), the Knowledge Innovation Programs of the Chinese Academy of Sciences (KGCX2-YW-397), and the Opening Foundation of State Key Laboratory of Coal Conversion (J12-13-102). We are grateful to the anonymous reviewers of the manuscript for their valuable suggestions and the editors working at the International Science Editing (ISE) for their help with the English editing of this paper.

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4. CONCLUSION The main conclusions from this work can be summarized as follows: (1) Under a reducing atmosphere (CO2/H2 = 1:1), both the sintering temperatures and AFTs of three residual ash samples decrease from gangue ashes to agglomerates to slag, because of the increase of the total base content and the differences in their mineral compositions accordingly. (2) The surfaces of slag were obviously molten; the agglomerate samples were covered with some small spheres and largeparticle aggregates, and the gangue ashes were composed of small prismatic particles. (3) During XLT PFBA gasification,

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dx.doi.org/10.1021/ef300543x | Energy Fuels 2012, 26, 5020−5027