Gasification Reactivities and Pore Structure Characteristics of Feed

Moreover, gasification of the mixture of raw coal and gasification residues under a temperature higher than ash fusion temperature can improve the uti...
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Gasification Reactivities and Pore Structure Characteristics of Feed Coal and Residues in an Industrial Gasification Plant Wei Huo, Zhijie Zhou,* Qinghua Guo, and Guangsuo Yu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, PR China ABSTRACT: Gasification reactivities and pore structure characteristics of raw coal, coarse slag, filter cake, and demister slag in industrial gasification plant were investigated and compared with each other in this study. Results show that the reactivity of raw coal gasification is the highest because of the biggest specific surface area, the most developed pore structure, the most micropores and combustible matter contents of raw coal. The reactivities of filter cake and demister slag are lower than those of raw coal because these two samples have experienced a similar high temperature gasification process. After that process, the amount of micropores decreased and many pore structures were blocked by molten materials. Coarse slag has experienced the longest gasification process, the combustible matter contents are fewest, the specific surface area is the smallest, and the micropore size distribution is the narrowest. Therefore, the gasification reactivity of coarse slag is the lowest. Moreover, gasification of the mixture of raw coal and gasification residues under a temperature higher than ash fusion temperature can improve the utilization of residues because neither the main pore structures of raw coal nor the process of gasification can be affected by adding the gasification residues to raw coal.

1. INTRODUCTION Entrained flow gasification is a key technology for clean and efficient usage of coal.1 During the entrained flow gasification process, the coal with higher gasification reactivity has a higher gasification rate and carbon conversion. Additionally it ensures that the gasification process goes on continuously and steadily. However, many residues are generated during the gasification process, which consist of inorganic minerals and partially converted combustibles. It has been proven that these gasification residues contain “unburnt carbon”, which are referred to as ungasified or residual carbon.2 The unburnt carbon content is one of the indicators of gasification efficiency. In recent years, “zero emission” has become important in the development of the large-scale coal gasification process.3 This implies that the gasification residues must be further converted or recycled to improve the overall economics of coal gasification and effective utilization of the resources. Therefore, in order to utilize the gasification residues more efficiently, it is necessary to thoroughly investigate the physical structure, chemical composition, and gasification characteristics of the gasification residues.4,5 In general, two kinds of residues are generated during the entrained flow gasification process. They are coarse slag and fine ash.5,6 There are extensive studies on the characterization of fine slag and coarse slag.7−12 Wu et al.3 studied the composition and distribution of the residues in the gasifier. Wagner et al.13 studied the mineral distribution, physical, and chemical properties of the gasification ashes in gasifiers systematically and classified the unburned carbons in the coarse slag into three major categories. These studies mainly focused on the physical and chemical properties of the gasification residues. Only a few studies related to the reactivity of the gasification residue were reported. Zhao et al.14 discussed the difference of reactivities between coarse slag and fine slag. Xu et al.15 also compared the reactivities between coarse slag and fine slag. They found that the difference of reactivities between coarse slag and fine slag © 2015 American Chemical Society

might be attributed to the catalytic component content of the slag. However, little attention has been paid to the gasification characteristics, as well as the conversion and the recycling of the gasification residues in the entrained-flow gasifiers. In this study, in order to investigate the gasification characteristics of the coal and its gasification residues in more detail, CO2 and steam gasification characteristics of feed coal and residues in the industrial gasification plants are discussed. The specific surface areas, pore structures, and pore size distributions of raw coal and gasification residues were thoroughly studied. Moreover, in order to provide the theoretical foundation for the efficient use of gasification residues, the gasification reaction characteristics and physical structure characteristics of the mixtures of raw coal and gasification residues with different loading amount were also investigated in this study.

2. MATERIALS AND METHODS 2.1. Sample Preparation. The process of coal gasification15,16 in the entrained-flow gasifier is shown in Figure 1. After being fed into the entrained-flow gasifier, the raw coal forms coal char after the removal of volatile matter and graphitization of particles. Then, after diffusing to the surface of the coal char, the gasification agent is reacted with coal char. When the gasification goes into a certain state, the particles start breaking and generate coarse slags and fine slags with homogeneous and heterogeneous reactions. Therefore, in order to investigate the gasification characteristics of the raw coal and various types of gasification residues clearly, gasification experiments of raw coal (RC), coarse slag (CS), demister slag (DS), and filter cake (FC) from the industrial gasification plant (as shown in Figure 2) were carried out in this study. From Figure 2, it can be seen that the gasification residues in this study come from the following three sources: (1) The low-density fine ashes that are generated after coal gasification are carried by the gas, go through the synthesis gas processing Received: January 29, 2015 Revised: May 15, 2015 Published: May 15, 2015 3525

DOI: 10.1021/acs.energyfuels.5b00221 Energy Fuels 2015, 29, 3525−3531

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Energy & Fuels

Figure 1. Schematic diagram of coal gasification process in entrained-flow gasifier.

Table 2. Ash Fusion Temperature of RC ash fusion temperature/°C samples

DT

ST

HT

FT

RC

1113

1137

1143

1156

During char preparation, all samples were heated at 25 °C/min to 850 °C and held 30 min under a high purity nitrogen atmosphere in a fixed bed reactor. After the temperature dropped to room temperature, the prepared char samples were taken out for further analysis. 2.3. Char Gasification Tests. Char gasification tests were carried out with a NETZSCH STA449F3 thermogravimetric analyzer (TGA) using the isothermal method. In all cases, about 5 mg of char samples was added into the crucible and heated at 25 °C/min to the prescribed temperature under a continuous high purity nitrogen flow of 80 mL/min.. Then, nitrogen was replaced by the gasifying agent (CO2 or steam) to initiate the gasification test. For CO2 gasification, the gasification temperatures were 900, 1000, 1100, 1200, and 1300 °C, respectively. For steam gasification, the gasification temperatures were 850, 900, 950, and 1000 °C, respectively. The effect of external diffusion had been eliminated by previous tests.17 2.4. Pore Structure Characteristics Tests. To study the characteristics of the pore structure, most researchers conducted adsorption tests using CO2 or N2 as the adsorbate.18,19 For carbonaceous materials such as coal, during the gasification process, the gasifying agent directly diffuses to the pore surface of the material and then reacts. Although the sizes of CO2 and N2 molecules are almost the same (0.33 nm for CO2 molecule and 0.364 nm for N2 molecule), N2 adsorption is carried out at −196 °C; thus the molecular diffusion is severely limited by the temperature and N2 molecular cannot enter the pore whose size is smaller than 2 nm. On the other hand, CO2 adsorption is carried out at 0 °C, so molecular diffusion is not seriously limited by the temperature and CO2 molecules can effectively enter the pore structure.18 Owing to this fact, CO2 was chosen as the adsorbate in this study. The pore structure characteristics of the char samples were carried out with a Micromeritics ASAP2020 physical adsorption analyzer. The test steps are as follows: the samples were degassed for 12 h at 0.33 Pa and 105 °C to remove the moisture and impurities within the pores and at the surfaces. Then, CO2 adsorption test of the degassed samples was carried out at 0 °C. The specific surface areas and pore size distributions were determined by density functional theory (DFT).

Figure 2. Schematic diagram of the industrial gasification plant. system, and are then collected in the demister. This part of the slags is named the demister slag (DS). (2) The high-density fine ashes that are generated after coal gasification are carried by the chilled water, go through the black water processing system, and are then filter-pressed by the pressure filter, forming the filter cake (FC). (3) The large-particle high-density slags generated after coal gasification are quenched in the quench chamber and then directly discharged via the slag removal system into the slag pool. These collections in the pool are the coarse slags (CS). The proximate analysis and ultimate analysis of these samples are listed in Table 1. Moreover, the ash fusion temperatures are shown in Table 2. 2.2. Char Preparation. To determine the gasification reaction characteristics of raw coal and its gasification residue, samples with particle sizes of 80−120 μm were chosen for the gasification reactivity test. In addition, the raw coal was mixed with gasification residues in certain proportions to study the gasification reaction characteristics of the mixtures. The load of the gasification residue was either 5% or 20% (The mixtures are denoted as 5% FC, 20% FC, 5% DS and 20% DS, respectively.) The mixtures were prepared with the following method: the raw coal and its gasification residues were weighed accurately according to their proportions. A certain amount of deionized water was added to mix them totally, and then the mixtures were stirred for 30 min and dried at 105 °C for 2 h.

Table 1. Proximate Analysis and Ultimate Analysis of Samples proximate analysis (wt %, ad)

ultimate analysis (wt %, ad)

sample

M

A

V

FC

C

H

N

S

RC FC DS CS

1.65 0.80 2.30 0.11

13.85 76.23 62.62 96.22

30.83 4.24 11.05 1.98

53.67 18.73 24.03 1.69

68.69 23.09 33.44 3.92

3.62 2.19 2.45 3.32

0.79 0.21 0.39 0.18

1.11 0.34 1.79 0.74

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Figure 3. (a, b) Variation of reactivity index with gasification temperature. In order to obtain the scanning electron microscope (SEM) images, a HITACHI SU1510 scanning electron microscope was used to observe the surfaces of the samples.

3. RESULTS AND DISCUSSION 3.1. Gasification Reactivities of Raw Coal and Gasification Residues. Because the particle size and pore structure of char sample keep changing during the gasification process and the gasification rate is not a constant, the reactivity index R0.5 (R0.5 = 0.5/t50) proposed by Takayuki20 is used to express the reactivity of char gasification. t50 is the time of the carbon conversion of 50%. Because of the high content (96.22%) of ash in coarse slags (shown in Table 1), the contents of combustibles and unburned carbon are very low. The carbon conversion of coarse slag hardly changes with gasification time at any temperature. This means the reactivity index of the coarse slag is near zero. Hence, the gasification reactivity of coarse slag is the lowest, and it is unnecessary to recycle coarse slag. Figure 3 shows the variation of reactivity index with the gasification temperature of the other three samples. It can be observed that the reactivity indexes of the three samples increase gradually with the gasification temperature. For steam gasification, at the same gasification temperature, the reactivity index of raw coal is about 3 times higher than that of filter cake and demister slag. For CO2 gasification, the reactivity index of raw coal is about twice as high as that of filter cake and demister slag. The reactivity index of filter cake is found to be very close to but slightly less than that of the demister slag, regardless of whether the chars are reacted with CO2 or steam. Coupled with the proximate and ultimate analysis results and gasification process, it can be seen that the gasification experiences of filter cake and demister slag are almost the same, but coarse slag has experienced more gasification time than filter cake and demister slag. Consequently, the combustible contents (e.g., C and H) of filter cake and demister slag are high due to incomplete conversion. In addition, raw coal has not undergone a gasification process, so it contains the highest proportion of combustibles (e.g., C and H), which is 2−3 times that of the demister slag and the filter cake. 3.2. Effect of Pore Structure on the Gasification Reaction. The specific surface areas of four chars are shown in Figure 4. The specific surface area of raw coal is 147.84 m2/g, which is larger than that of demister slag or filter cake. In

Figure 4. Specific surface areas of various samples.

addition, the specific surface area of coarse slag is 9.84 m2/g, which is the smallest among these four chars. This result shows that after the raw coal underwent high-temperature gasification, more and more pores were broken down with gradual gasification of the combustibles. So the specific surface area of the materials became smaller and smaller. In addition, because the gasification process operated at a temperature higher than 1400 °C, which exceeded the ash melting point of the raw coal as shown in Table 2, the melted ashes accumulated during the reaction, thereby further blocking the pore structure of the materials and resulting in reduction of the specific surface area of the materials. Figure 5 shows the proportions of specific surface area contributed by different pores. It can be seen from this figure that the meso-macropores contribute less than 30% of the specific surface area to raw coal, whereas the rest of the 70% specific surface area is contributed by the micropores. The contribution made by the meso-macropores and the micropores in demister slag is almost the same (50% each). For filter cake, the contribution of the meso-macropores to the specific surface area is up to about 67%, and the remaining 33% is contributed by the micropores. For coarse slag, a higher proportion of the specific surface area is contributed by meso-macropores; the proportion reaches about 69%. The results show that many micropores are either broken or blocked during the gasification process, resulting in reduction of their contribution to the specific surface areas. The SEM images of the four chars are shown in Figure 6. It can be observed that the surface of raw coal is compact, and no 3527

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Figure 5. Proportions of specific surface area contributed by different pores.

Figure 7. Micropore size distribution of samples.

The micropore size distribution curves of the four chars are shown in Figure 7. For raw coal, many micropores are continuously distributed within the ranges of 0.5−0.65 nm and 0.8−0.9 nm, and a smaller number of micropores are distributed within the range of 0.65−0.8 nm. For demister slag and filter cake, only a small number of micropores are distributed within the ranges of 0.5−0.65 nm and 0.8−0.9 nm, and almost no micropore is distributed within other pore size ranges. For coarse slag, micropores can hardly be observed within the whole pore size ranges investigated in this study except within the range of 0.8−0.9 nm. Moreover, the quantity of micropores distributed within the range of 0.8−0.9 nm of coarse slag is significantly less than that of the other three samples. This phenomenon confirms the result that many micropores were either broken or blocked during the gasification reaction, which is obtained in Figure 5. This not only reduces the collision probability between pores and gasifying agents, but also leads the Knudsen diffusion to be more significant and then further reduces the reaction rate. To sum up, raw coal possesses a larger specific surface area and more advanced micropores, so it is favorable for gasification. For demister slag and filter cake, the specific surface areas are smaller, the micropore distribution ranges are narrower, and the pores are less advanced, so the reactivities of demister slag or filter cake are lower than those of raw coal. In addition, coarse slag has undergone the longest gasification time among these samples, so the number of micropores broken or blocked is the most, the specific surface area is the smallest, and micropore size distribution is the narrowest. In this regard, the coarse slag provides the smallest area contacted with the gasification agent, so it is unfavorable for the gasification. 3.3. Gasification Reactivity of the Mixture of the Raw Coal and Its Gasification Residues. As mentioned above, a large amount of residual carbon in the demister slag and filter cake were not fully gasified and thus could be recycled. But the gasification reactivities of demister slag and filter cake are very low, especially when they are compared to the gasification reactivity of raw coal. From the perspective of gasification kinetics, gasifying the gasification residues separately again cannot ensure that these residues could be fully and efficiently utilized. Hence, in order to improve the utility of the residues, the residues and the raw coal were mixed in certain proportions in this study, and the gasification reactivities of the mixtures were investigated. Figure 8 shows the gasification reactivity indexes of various mixtures. As for the CO2 gasification, the reactivity index curves

Figure 6. (a−d) SEM images of four samples.

obvious macropores appears. In contrast to this, the surfaces of demister slag and filter cake are coarse, and obviously many macropores are present. This observation is consistent with the analysis of Figure 5. Unlike raw coal, many white bright spherical molten materials appear at the surface of demister slag and filter cake, some of which directly block the pores on the surfaces of the chars. Moreover, pores can hardly be observed at the surface of coarse slag because the surface is full of molten materials. From the specific surface area analysis and the SEM images, it can be inferred that even though no obvious pore structure can be seen at the surface of raw coal, the internal micropore structures of raw coal are more advanced than those of demister slag, coarse slag, and filter cake. According to the literature,21 the reason for this phenomenon is that almost all the internal specific surface areas of the coal lay in the micropores. Moreover, since raw coal has not experienced the gasification reaction, the pore structures were neither broken nor blocked. So the specific area is the highest, and no molten materials can be found at the surface. On the other hand, demister slag, coarse slag, and the filter cake all have undergone a hightemperature gasification process. But the gasification time for the coarse slag was relatively long; hence, as compared to the filter cake and demister slag, a larger quantity of molten materials was generated and more pores were blocked. 3528

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Figure 8. (a, b) Reactivity indexes for various mixtures.

This means that at that temperature, the added gasification residue impedes CO2 gasification. However, when the gasification temperature is higher than 1100 °C, the curve for the carbon conversion of mixture-CO2 gasification almost overlaps the curve for the weighted average. This means that at this temperature, the added gasification residue neither impedes nor stimulates CO2 gasification. On the other hand, steam gasification was carried out at 850−1000 °C. At this temperature, the curve for the experimental carbon conversion of mixture-steam gasification is always below the curve for the weighted average, which means that the added gasification residue also impedes steam gasification. These phenomena can also be observed in the gasification process of the mixture that contained 5% loading amount of gasification residues. 3.4. Pore Structure Characteristics of Mixtures. To investigate why the added gasification residue impedes raw coal gasification at temperatures ≤1100 °C, the pore structures of the chars obtained by pyrolyzing the mixture at different temperatures were analyzed. To study whether the added gasification residue would further block the pores in raw coal or not, the specific surface area of the mixture was analyzed and compared with the weighted average of the specific surface. The weighted average of the specific surface area can be calculated as follows.

of raw coal and four different mixtures are very similar to each other. The reactivity index of raw coal is slightly higher than that of the mixture that contained 5% loading amount of gasification residues. The reactivity index of the latter is slightly higher than that of the mixture that contained 20% loading amount of gasification residues. This implies that with an increase in the loading amount of gasification residues, the reactivity index of the mixture decreases slightly. For steam gasification, the reactivity index curve of 5% FC is close to that of the 5% DS, and the reactivity index of the 20% FC is almost equal to that of the 20% DS at the same temperature. However, the reactivity index of the mixture that contained 5% loading amount of gasification residues is substantially higher than the mixture that contained 20% loading amount of gasification residues. Furthermore, at a given temperature, the steam gasification of the raw coal shows the highest reactivity index. This indicates that with a gradual increase in the loading amount of gasification residues, the mixture-steam gasification is considerably affected and its reactivity decreases significantly. Comparison of the gasification reactivity of the mixture with that of the raw coal shows that for the CO2 gasification, adding some gasification residues to the raw coal can substantially improve the utilization of the gasification residues. However, for steam gasification, adding more residues results in more obstacles to the steam gasification process, due to the high sensitivity of the gasification reaction to the gasification residue loading amount. Therefore, adding too many gasification residues to the raw coal is unfavorable for steam gasification. To study whether the added gasification residues impede the gasification reaction, the carbon conversion of the mixture obtained in the experiment can be compared to the weighted average of the carbon conversion. The weighted average of the carbon conversion can be calculated as follows: xm,t = Crxr,t + (1 − Cr)xc,t

Sm,T = CrSr,T + (1 − Cr)Sc,T

(2)

Here, Sm,T, Sr,T, and Sc,T denote the specific surface areas of the chars obtained from the mixture, gasification residue, and raw coal at a specific pyrolysis temperature (T), respectively. In Table 3, the mixture that contained 20% loading amount of gasification residues is also used as an example to compare the experimental value of the specific surface area with the weighted average value of the chars. It is observed from this table that when the pyrolysis temperature is raised from 850 to 1300 °C, the specific surface area of the char decreases. Referring to the ash fusion temperatures in Table 2, the ash fusion temperature of raw coal is 1156 °C. So it can be inferred that when the pyrolysis temperature is 1300 °C, which is higher than the ash fusion temperature, the fused ashes block the pores of the char, so the specific surface area of the char is smaller than that of the char obtained at 850 °C. In addition, at the pyrolysis temperature of 850 °C, the experimental values for the specific surface areas of the 20% FC and 20% DS are 106.85 m2/g and 140.56 m2/g, respectively, which are substantially smaller than the weighted average of the mixture’s

(1)

Here, Cr denotes the mass fractions of the gasification residues. xm,t, xr,t, and xc,t denote the carbon conversion of the mixture, gasification residue, and raw coal, respectively. In Figure 9, the mixture that contained 20% loading amount of gasification residues is used as an example to compare the experimental carbon conversion curve of the mixture with the weighted average curve of the carbon conversion. It is observed that when the gasification temperature is ≤1100 °C, the curve for the experimental carbon conversion of mixture-CO2 gasification is always below the curve for the weighted average. 3529

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Figure 9. (a−d) Curves of conversion vs time for 20% FC and 20% DS gasification.

Table 3. Specific Surface Areas of Various Charsa Sm,T (m2/g) Ex

a

20% FC

20% DS

pyrolysis temperature/°C

RC

FC

DS

Ex

Cal

Ex

Cal

850 1300

147.84 86.31

62.03 50.28

129.96 76.56

106.85 79.94

130.68 79.10

140.56 86.01

144.26 84.36

Ex = Experimental data; Cal = calculated data (weighted average data).

specific surface area (130.68 m2/g and 144.26 m2/g). This means that at this temperature, with the additions of the gasification residues, the pores in the raw coal are further blocked. So the experimental value of the specific surface area is smaller than that of the weighted average. However, at the pyrolysis temperature of 1300 °C, the experimental values of the specific surface area of the 20% FC and 20% DS are 79.94 m2/g and 86.01 m2/g, respectively, which are almost equal to the weighted average of the mixture (79.10 m2/g and 84.36 m2/g). Results show that when the temperature is higher than the ash fusion temperature, the fused ashes could have block some pores in the chars, so that even if the gasification residue is added, the pores are not further blocked. Therefore, the experimental value of the specific surface area of the mixture is almost equal to the weighted average value. In Figure 10, the micropore size distribution curves of various chars obtained at 850 °C is used as an example to illustrate the effect of the added gasification residues on the micropore size distribution of raw coal. This figure shows that the micropore size distribution curve of 20% FC or 20% DS is similar to that of raw coal, because most pores are continuously

distributed between 0.5 and 0.65 nm and 0.8−0.9 nm, and a small number of pores are distributed between 0.65 and 0.8 nm. However, the incremental pore volume per unit weight of char in the 20% FC or 20% DS is smaller than that in the raw coal. This means that compared to raw coal, there are fewer pores within each pore size range in the mixture. Adding gasification residues to raw coal can neither eliminate the pores nor change the main characteristics of the micropore size distribution, but it can reduce the number of micropores within each pore size range. This phenomenon is also observed in the mixtures that contained 5% loading amount of gasification residues. According to the analysis above in addition to the analysis in section 3.3, after the raw coal and the gasification residues are mixed, the effects of the gasification residues on the gasification characteristics vary with the temperature due to the following reasons. When the mixture is gasified at a temperature which is below the ash fusion temperature, the pore structures of raw coal are advanced because the raw coal is not fused. After it is mixed with the gasification residues, the fused slags in the residues block the pores of the raw coal, so the experimental value of the specific surface area is smaller than the weighted 3530

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ACKNOWLEDGMENTS This work has been partially supported by the National High Technology Research and Development of China (863 Program, 2012AA053101) and the National Natural Science Foundation of China (21376081).

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Figure 10. Pore size distribution of chars (pyrolysis temperature: 850 °C).

average, impeding the gasification. When the gasification occurs at a temperature above the ash fusion temperature, the ashes in the coal are fused, so the fused ashes block the pores in the raw coal, resulting in the reduction of the specific surface area. After it is mixed with the gasification residues, the fused ashes in the residues cannot further block the pores in the raw coal; thus the experimental value of the specific surface area is almost equal to the weighted average and the gasification characteristics of the mixture are not further impeded. Therefore, for the hightemperature entrained flow gasifier, adding some gasification residues to the raw coal can efficiently utilize the carbons left in the gasification residues.

4. CONCLUSIONS The gasification reactivities of the raw coal, coarse slag, demister slag, and filter cake are quite different from each other. Raw coal possesses the highest gasification reactivity, and it has the most combustibles, highest specific surface area, greatest number of micropores, and most advanced pore structures. Filter cake and demister slag have experienced similar hightemperature gasification processes, both of them have similar pore structures, and their pore structures are less advanced than those of raw coal. In addition the reactivities of these two samples are quite lower than those of the raw coal. Coarse slag has experienced the longest gasification process, so the specific surface area is the smallest, micropore size distribution is the narrowest, and the gasification reactivity of coarse slag is the lowest. For CO2 gasification, adding gasification residues to the raw coal does not greatly affect the gasification reactivity. But for steam gasification, adding gasification residues greatly reduces the gasification reactivity. Moreover, if the gasification temperature is below the ash fusion temperature, adding gasification residues to the raw coal blocks the advanced pore structures in the raw coal, thus impeding gasification. If the gasification temperature is above the ash fusion temperature, adding gasification residues neither affects the pore structures in the raw coke nor impedes the gasification.

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Corresponding Authors

*(Z.J.Z.) Tel.: +86-21-64252974. Fax: +86-21-64251312. E-mail: [email protected]. *(G.S.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3531

DOI: 10.1021/acs.energyfuels.5b00221 Energy Fuels 2015, 29, 3525−3531