Modification of a Sub-bituminous Coal by Hydrothermal Treatment

(19) found that the aqueous environment allowed for ionic pathways to proceed for the cleavage reactions of activated aryl ethers and ester types of C...
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Modification of a Sub-bituminous Coal by Hydrothermal Treatment with the Addition of CaO: Extraction and Caking Properties Hengfu Shui,* Xiangyu Zhang, Zhicai Wang, Changhui Lin, Zhiping Lei, Shibiao Ren, and Shigang Kang School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Coal Clean Conversion and Utilization, Anhui University of Technology, Ma’anshan 243002, Anhui Province, People’s Republic of China ABSTRACT: A Chinese sub-bituminous coal, i.e., Shengfu (SF) coal, was hydrothermally treated with and without CaO addition at different temperatures, and the extraction yields and caking indexes (GRI) of the treated coals were measured. The action of CaO during hydrothermal treatment was probed in this study. The results show that hydrothermal treatment can obviously increase the extraction yield and GRI of SF coal, and CaO can further promote this effect of hydrothermal treatment. The removal of oxygen functional groups, especially the hydroxyl group, dissociating the aggregated structure of coal, is responsible for the modifying results. CaO can supply a basic environment for the hydrothermal treatment, which is beneficial for the removal of acidic oxygen functional groups by acid−base interactions and promotes the effect of hydrothermal treatment for SF coal, resulting in additional increases in the extraction yield and GRI. bonds,1,8 and the decrease in oxygen functional groups, which are thought to be responsible for cross-linking reactions.10 Oxygen groups in coal are a disadvantage for coke making because they can deplete the supply of donor hydrogen and diminish the amount of solubles and the extent of fluidity development in the plastic state.11 Hydrothermal treatment can promote to remove the oxygen groups in coal; therefore, it should be beneficial for coking coal pretreatment, especially for SF coal, which has a high content of oxygen. It can be speculated that hydrothermal treatment in a basic environment should further promote the effects for removal of oxygen groups, therefore modifying the caking properties of coal. However, some strong bases, such as KOH or NaOH, cannot be used in the hydrothermal treatment of coal, because induced alkali metals will strongly enhance the gasification reaction rate of coke, thus increasing the reactivity of coke when the treated coal is used in coking coal blends.12 In this study, the hydrothermal treatment of a sub-bituminous SF coal was carried out with the addition of CaO to supply a basic environment and mechanisms of the hydrothermal treatment were also discussed.

1. INTRODUCTION Coal cleaning and efficient use have been becoming part of the hot concerning issues for energy supply in the world, especially for China, which has abundant reserves of coal. However, the decreased reserves of coking coal in China and increased production of coke promote us to look for new sources of coking coals. Shenfu (SF) coal is a Chinese sub-bituminous coal with low S and ash contents, and its reserve is abundant. However, its noncoking property makes its limited use in coal blends of cokemaking. Accordingly, effective pretreatment for SF coal to modify its caking property can increase its amount of use in coal blends of cokemaking, therefore opening the coking coal resources as soon as possible. Hydrothermal treatment is one of the effective pretreatment methods for modifying the solvent extraction property.1−5 It has been established that the material responsible for the caking behavior of coal is extractable from the coal.6 Therefore, it can be speculated that hydrothermal treatment should be an effective method for modifying the caking property of coal. Bienkowski et al.7 found that the conversion of toluene solubles of Wyodak coal in liquefaction at 400 °C increased from 27.3 to 38.4% by steam treatment at 200 °C. Iino et al.1,8 also found that water treatments of three Argonne Premium coals at 600 K increased their extraction yields greatly. For Beulah-Zap (BZ) lignite, the extraction yield enhancements by water treatment were attributed to the removal of oxygen functional groups and the breaking of hydrogen bonds to a greater extent than those for bituminous coals. We2 also found that hydrothermal treatment to some bituminous coals enhanced the extraction yields greatly. The decrease of total oxygen and hydroxyl oxygen in hydrothermal treatment because of catalysis by water, where water acted more effectively as an acid or basic catalyst, may be responsible for the enhancement of solubility for the treated coals. Therefore, there have been proposed various mechanisms for the enhancement of extraction after hydrothermal treatment of coal, for example the rupture of weak covalent bonds, such as ether bonds,9 the breaking of hydrogen © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Coal Extraction. SF coal was used in this study. The properties of SF coal are shown in Table 1. The coal sample was ground and sieved to particles of size less than 200 μm, stored under a nitrogen atmosphere, and dried for 12 h under vacuum at 80 °C before use. Coal extraction was carried out at room temperature. A mixed solvent of carbon disulfide/N-methyl-2-pyrrolidinone (CS2/NMP, 1:1 by volume) was used as the solvent, as described in details elsewhere.2,13 Briefly, 1.5 g of coal and about 40 mL of CS2/NMP mixed solvent were charged into a centrifuge tube and extracted for 30 min under ultrasonic (38 Hz) irradiation at room temperature. The mixture was filtered after centrifugation for 50 min at 14 000 rpm. The Received: March 5, 2012 Revised: April 16, 2012 Published: April 26, 2012 2928

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Table 1. Ultimate and Proximate Analyses of SF Raw Coal ultimate analysis (wt %, daf)

a

proximate analysis (wt %)

sample

C

H

N

S

Oa

ashd

volatile matterdaf

moisturead

GRI

SF coal

79.4

4.7

1.0

0.5

14.4

5.2

39.9

10.1

0

By difference. min under 50 mL/min nitrogen gas flow. The elemental analysis was carried out with an Elementar Vario EL III. All elemental analyses were duplicated, and the experimental error was within 5%.

residue was thoroughly extracted with the fresh mixed solvent repeatedly for about 6 times in the same way until the filtrate became colorless. The residue was washed with acetone 3 times and dried for 12 h under vacuum at 80 °C. The extraction yield was then determined from the weight of the residue

extraction yield=

3. RESULTS AND DISCUSSION 3.1. Effect of the Hydrothermal Treatment on the Solvent Extraction. SF coal is a sub-bituminous coal, and its extraction yield in the CS2/NMP mixed solvent (1:1 by volume) is low (5.3%). Hydrothermal treatment can obviously increase the extraction yield of SF coal, and the extraction yield increased with the increase of the hydrothermal treatment temperature, as shown in Figure 1. The extraction yield

1 − M r /Mcoal × 100% (100 − Ad )/100

where Mr is the weight of the dried residue (g), Mcoal is the weight of the dried coal (g), and Ad is the ash content of coal (%, db). The ash content of each sample for the extraction yield measurement was carried out, including the hydrothermally treated one, with or without CaO addition to ensure the reproducibility of the extraction measurement. All of the extraction yields presented in this paper were the average values from two experimental runs, and the error in the extraction yields between the two runs was within 3%. 2.2. Hydrothermal Treatment. The hydrothermal treatment of coal was performed using a 0.25 L autoclave. In each run, 20 g of dried coal (200 meshes) mixed with or without the required amount of CaO and 40 g of water were loaded into the autoclave and purged with nitrogen gas 3 times to displace the residual air in the reactor. Then, the autoclave was pressurized with nitrogen to 0.1 MPa at room temperature. The autoclave was then heated to the desired temperature with stirring, maintained for 1 h under autogenous pressure, and then cooled to room temperature in 2−3 h. The treated coal was filtered to remove excess water and then dried under vacuum at 80 °C for 12 h. 2.3. Caking Index Measurement. The caking index (GRI) was used to characterize the caking property of coal. The measurement was carried out according to the National Standard of China (GB5447-85), which is based on the measurement of the Roga index. Briefly, 1 g of coal (1 mm) after the first and second drum test, respectively. If GRI is less than 18, then the measurement should be retested and the amount of sample will be changed to 3 g of coal mixing with 3 g of standard anthracite. Therefore, the caking index GRI was calculated as

G RI =

30m1 + 70m2 5m

Two trials were performed for each sample, and the experimental error against the GRI was within 3%. 2.4. Characterization of SF Coal and Its Treated Coals. The SF raw coal and the hydrothermally treated coals with or without CaO addition were characterized by infrared (IR) spectra using a PESpectrum One IR spectrometer at a resolution of 4 cm−1. Samples for the Fourier transform infrared (FTIR) measurement were prepared by mixing 5 mg of coal sample with 200 mg of KBr, and the mixture was pressed to form a pellet. Thermogravimetric (TG) analysis was carried out on a Shimadzu TG60 analyzer. About 10 mg of sample was placed in an alumina pan and heated from 25 to 800 °C at a rate of 10 °C/ 2929

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It is believed that the enhancement of the extraction yield after hydrothermal treatment of coal is attributed to the rupture of weak covalent bonds, such as ether bonds,9 and the breaking of hydrogen bonds,1,7 leading to the treated coal with a less aggregated state. Therefore, the results suggest that CaO can promote these effects of hydrothermal treatment. Table 2 Table 2. Volatile Yield of Hydrothermally Treated Coals with and without CaO Addition (wt %, daf) treatment temperature (°C)

without CaO

with 1% CaO

with 2% CaO

150 200 250 300

39.6 36.3 33.6 30.6

37.9 35.8 33.3 30.0

37.2 35.0 32.6 29.4

Figure 2. Caking indexes of SF raw coal and its hydrothermally treated coals at different temperatures (GRI is measured by blending with an equivalent amount of standard rich coal with a GRI of 98).

shows the volatile yields of hydrothermally treated coals with and without CaO addition. It can be observed that, with the addition of CaO, the volatile yields of hydrothermally treated coals are lower than those corresponding coals without CaO addition. This suggests that CaO can promote the hydrolysis reactions in hydrothermal treatment, resulting in the less aggregated structure formation and the extraction yield enhancement compared to single hydrothermal treatment. However, hydrothermal treatment at 300 °C with CaO addition resulted in a dramatic decrease in the extraction yield. This is because of the formation of strong cross-linking because of the enhanced thermolytic reactions at this temperature by CaO and the lack of supply of active hydrogen in time. We also found that that, with the increase of the hydrothermal treatment temperature to 350 °C without CaO addition, the extraction yield of the treated SF coal decreased to 3.8%, which was less than that of raw coal. 3.2. Effect of the Hydrothermal Treatment on the Caking Property. The caking index GRI is one of the most important indexes for characterizing the caking property of coking coals.14,15 To probe the effect of the hydrothermal treatment on the caking property of SF coal, GRI was used to reflect the change of the caking property after hydrothermal treatment. Because SF coal is a non-caking coal, its GRI is 0, as shown in Table 1. Therefore, it is difficult to observe the changing of GRI and to evaluate the effects of hydrothermal treatment on the caking property. To differentiate the variety of GRI after hydrothermal treatment, SF coal or its hydrothermally treated coal was mixed with an equal weight of rich coal and the mixed coal was used to determine GRI, instead of SF raw coal or its hydrothermally treated coal in this study. The GRI indices shown in Figures 2 and 3 correspond to 1:1 by the mass ratio blending of SF raw or treated coal with a standard rich coal with GRI of 98. Figure 2 shows the change of GRI after hydrothermal treatment with or without CaO addition. With the same changes of the extraction yield mentioned above, it can be observed from Figure 2 that hydrothermal treatment can modify GRI for SF coal. With the increase of the hydrothermal treatment temperature, GRI of hydrothermally treated coal increased and the maximal GRI of 52.2 was obtained at 300 °C hydrothermal treatment, which was much higher than that of raw coal (46.7). With the addition of CaO, the effect of hydrothermal treatment on the caking property was enhanced and additional increasing of GRI can be observed. Figure 2 shows that, with the addition of CaO to SF raw coal, the change of GRI is negligible. This means that CaO can not directly modify the caking property of SF raw coal. The additional

Figure 3. Relationship between the extraction yield and GRI (measured by blending with an equivalent amount of standard rich coal with a GRI of 98).

increase of GRI after hydrothermal treatment is because of the enhanced extraction yield by CaO addition. Hydrothermal treatment at 300 °C with the addition of CaO also resulted in a dramatic decrease in GRI, as shown in Figure 2. In comparison of Figures 1 and 2, it can be observed that there is a good relation between the extraction yield and GRI. Figure 3 shows that GRI is only dependent upon the extraction yield, regardless of the addition of CaO or not. It is very interesting that there is a point of inflection in Figure 3 at the extraction yield about 11%. When the extraction yield was more than the point of inflection (11%), the GRI increased obviously. In our previous work,14 we have found that not only the amount of soluble (extraction yield) but also the soluble component has a great effect on the caking property of coal. The content of the constituents with suited molecular masses and low melting donor hydrogen-rich species presented within the coal structure dominates the development of the plastic layer. The lighter constituents in coal, in which much of the content is hydrogen, are easy to become volatile materials released from coal macromolecules before the plastic temperature of coal, resulting in the decrease of the caking index because of the lack of hydrogen donation of the coal structure.14 It is easy to understand that hydrothermal treatment at higher temperatures or with the addition of CaO can break some stronger associative interactions from the heavy constituents of coal; 2930

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hydrothermally treated coals without CaO addition, and hydrothermally treated coals with CaO addition. The ratios of the height of peak 3400 cm−1 to peak 1600 cm−1 (assigned to aromatic ring vibration17) are 2.8, 2.2, and 2.0 for SF raw coal, hydrothermally treated coals without CaO addition, and hydrothermally treated coals with CaO addition, respectively. The decrease in the intensity of the band near 3400 cm−1 suggested the decrease in the self-associated OH hydrogen bonds of hydrothermally treated coals.1 Mukherjee et al.18 found that the relative decreases of total oxygen and hydroxyl oxygen were greater in hydrothermal treatment than in thermal treatment without water for coal. Siskin et al.19 found that the aqueous environment allowed for ionic pathways to proceed for the cleavage reactions of activated aryl ethers and ester types of C−O bonds present in coals that were not available via thermal/free-radical routes. Ross et al.20 described a picture of coal pyrolysis, in which the generated fragments were high in arenol content, the catechol unit is the source of much of the reactivity, and the reactivity is largely dehydroxylation yielding water and polymerization retaining oxygen, as shown in Figure 5. The acid sites could be supported by the mineral phase of coal. Water at hydrothermal conditions likely swells the coal and dissolves the mobile, thermolytic fragments. In this case, the fragments can be readily carried from the matrix, avoiding the reaction with reactive centers there, while being stabilized by incipient hydrogen obtainable either through the interplay of carbon and water or from water alone.15 In addition, in a basic aqueous environment, the acidity of the acid sites is substantially reduced, reducing the tendency for polymerization and condensation.20 In our previous work,21 we found that the FTIR spectra of thermally treated SF coal with or without CaO addition were similar, suggesting that CaO has little action during this thermal treatment without water existing. Therefore, it suggested that, during hydrothermal treatment with CaO addition, CaO can be converted to Ca(OH)2, which will supply a basic environment for the hydrolysis, thus being beneficial for the removal of acidic oxygen functional groups including dehydroxylation and promoting the hydrothermal effects, as shown in Figure 5. It can also be observed from Figure 4 that the intensity of the band near 1370 cm−1, which is assigned to −CH3 deformation for the hydrothermally treated coal with CaO addition, decreased obviously compared to that of the hydrothermally treated coal without CaO addition or SF raw coal. Considering the decrease in the O content for the hydrothermally treated coal with CaO addition, the result suggests that hydrothermal treatment with CaO addition can promote to break some weak aliphatic ester bands, such as −OCH3, by hydrolysis. In addition, the decreases in the intensity of bands near 1100 cm−1 in the spectra of hydrothermally treated coals with or without CaO addition compared to that of raw coal imply the decrease of minerals in the treated coals,2 as shown in Figure 4. Figure 6 shows the TG curves of SF raw coal and its hydrothermally treated coals at 250 °C with and without CaO addition. The weight losses of SF raw coal were larger than those of hydrothermally treated coals before 250 °C because of the lowering of the volatile yield of hydrothermally treated coals, as shown in Table 2. The weight losses of SF raw coal at 150−350 °C were little, and then an obvious weight loss could be observed with an increasing temperature, suggesting the beginning of thermolytic reactions for the SF raw coal. However, in the range of 150−350 °C, the weight losses of hydrothermally treated coals were steady, reflecting a steady

therefore, the solubles formed (dissociated) should be more suited molecular masses with a low content of volatile constituents, resulting in an obvious increase in GRI 3.3. Characterization of Hydrothermally Treated Coal. To probe the mechanism of the effect of CaO addition on the hydrothermal treatment of SF coal, the hydrothermally treated SF coals with and without CaO addition were characterized by elemental analysis, FTIR, and TG measurements. Table 3 Table 3. Elemental Analyses of SF Coal and Its Hydrothermally Treated Coals at 250 °C (wt %, daf) temperature (°C)

C

H

N

S

Oa

C/H

raw coal without CaO with 1% CaO with 2% CaO

79.38 80.76 81.75 82.12

4.74 5.03 5.20 5.21

1.02 1.06 1.69 1.57

0.52 0.50 0.59 0.50

14.34 12.65 10.77 10.60

1.39 1.34 1.31 1.31

a

By difference.

shows the elemental analyses of SF raw coal and its hydrothermally treated coals at 250 °C with and without CaO addition. It can be observed from Table 3 that, after hydrothermal treatment at 250 °C without CaO addition, the O content decreased obviously from 14.34% of raw coal to 12.65% of hydrothermally treated coal. This suggests that hydrothermal treatment can remove the oxygen groups in coal molecules. With the addition of CaO, this removal of oxygen groups was further promoted, and the O content in the hydrothermally treated coal with 1% CaO addition decreased to 10.77%. Table 3 also shows that there is a little increased tendency of the H content for the hydrothermally treated coals with or without CaO addition compared to that of raw coal, suggesting that hydrothermal treatment with or without CaO addition has promotion to a certain extent for the hydrogenation of SF coal. Figure 4 shows the FTIR spectra of SF raw coal and its hydrothermally treated coals with and without CaO addition at 250 °C. It can be observed that the intensity of the band near 3400 cm−1, which is assigned to self-associated OH hydrogen bonds 16 in coal, decreases in order of SF raw coal,

Figure 4. FTIR spectra of SF raw coal and its hydrothermally treated coals at 250 °C with and without CaO addition. 2931

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Figure 5. Coal thermolytic and hydrolysis reaction route.

acidic oxygen functional groups, including dehydroxylation, and promotes the effect of hydrothermal treatment for SF coal.

4. CONCLUSION Hydrothermal treatment can obviously increase the extraction yield and GRI of SF coal. With the increase of hydrothermal treatment temperatures from 150 to 250 °C, the extraction yield and GRI increased. The addition of CaO during hydrothermal treatment can further promote the effect of hydrothermal treatment. However, hydrothermal treatment at 300 °C with CaO addition resulted in a dramatic decrease in the extraction yield and GRI. This is because of the formation of strong cross-linking as a result of the enhanced thermolytic reactions at this temperature by CaO and the lack of supply of active hydrogen in time. In comparison to the elemental compositions, FTIR spectra, and TG curves of SF raw coal and its hydrothermally treated coals with and without CaO addition, it suggests that hydrothermal treatment can partly break self-associated OH hydrogen bonds and dissociate the treated coal to a less aggregated structure. The addition of CaO can supply a basic environment for the hydrothermal treatment, which is beneficial for the hydrolysis and removal of acidic oxygen functional groups, including dehydroxylation, and promotes the effect of hydrothermal treatment for SF coal, resulting in additional increases in the extraction yield and GRI of hydrothermally treated coals.

Figure 6. TG curves of SF raw coal and its hydrothermally treated coals at 250 °C with and without CaO addition.

evaporation of pre-existing fragments rather than thermolytic generation fragments. This may suggest that hydrothermally treated coals are a less aggregated structure in comparison to that of SF raw coal.22 This is consistent with their different extraction behaviors mentioned above. It can also be observed that, in the range of 250−400 °C, the weight losses of hydrothermally treated coal with CaO addition were obviously lower than those of hydrothermally treated coal without CaO addition, suggesting that the solubles formed (dissociated) in hydrothermally treated coal with CaO addition were more suited molecular masses with a lower content of volatile constituents in comparison to those in the hydrothermally treated coal without CaO addition. This may be one of the reasons for the obvious increase in the GRI index for the hydrothermally treated coal with CaO addition. The weight losses of hydrothermally treated coal with CaO addition were a little larger than those of hydrothermally treated coal without CaO addition after 600 °C because of the dehydration of Ca(OH)2 taking place. On the basis of the elemental, FTIR, and TG analyses above, it may be suggested that one of the mechanisms of hydrothermal treatment of coal is to remove the OH group, thus decreasing self-associated OH hydrogen bonds, and to make the treated coal be a less aggregated structure, resulting in the solubility and caking index enhancements for the treated coal. CaO can supply a basic environment for the hydrothermal treatment, which is beneficial for the hydrolysis and removal of

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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 supported by the National Basic Research Program of China (973 Program, 2011CB201302), the National Natural Scientific Foundation of China (21076001, 20936007, 21176001, and 51174254), and the State Key Laboratory of Coal Conversion (Grant 11-12-904). The authors are also appreciative for the financial support from the Anhui Provincial Innovative Group for Processing and Clean Utilization of Coal Resource. 2932

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