Flash Pyrolysis of Coal Modified through Liquid Phase Oxidation and

donor solvent,1,2 flash pyrolysis of coal swollen by a tetralin vapor,3 and flash ... spheric pressure of helium, total volatiles reached 67% and liqu...
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Energy & Fuels 1996, 10, 364-370

Flash Pyrolysis of Coal Modified through Liquid Phase Oxidation and Solvent Swelling Kazuhiro Mae,† Shinji Inoue, and Kouichi Miura* Department of Chemical Engineering and Research Laboratory of Carbonaceous Resources Conversion Technology, Kyoto University, Kyoto, 606 Japan Received June 13, 1995. Revised Manuscript Received December 4, 1995X

A new flash pyrolysis method of coal is presented for increasing the total volatiles and the tar yield through suppression of the cross-linking reactions during the pyrolysis. Coal was oxidized in liquid phase at 25-60 °C for several hours using H2O2, NaOH, or Na2Cr2O7; then the coal was swollen by tetralin at 100-220 °C under 1 MPa of nitrogen. The oxidation pretreatment significantly enhanced the solvent swellability of the coal. Pyrolysis of the coal samples was performed using a Curie-point pyrolyzer at 590-920 °C in atmospheric pressure of helium. When the oxidized coal alone was pyrolyzed, the tar yield decreased as compared with the raw coal. When the coal oxidized and swollen by tetralin was pyrolyzed, on the other hand, the tar yield successfully increased as compared with that of the raw coal swollen by tetralin. The net increase in the tar yield by the tetralin swelling reached up to 19.9 kg/100 kg of coal for a Morwell brown coal oxidized with hydrogen peroxide at 25 °C, and 16.9 kg/100 kg of coal for a Taiheiyo subbituminous coal oxidized with hydrogen peroxide at 25 °C. These values were respectively 1.7 times and 2.3 times larger than the tar yields of the raw coals. The tar yield increased further when a solvent mixture of tetralin and tetrahydrofuran was used as a swelling solvent. Evaluating the amounts of hydrogen transferred during the pyrolysis, it was found that these increases in the tar yield were brought about through the suppression of cross-linking reaction forming H2O as well as the effective hydrogen transfer from tetralin to coal.

Introduction We have previously presented several flash pyrolysis methods of coal for increasing the total volatiles and the tar yield: flash pyrolysis of coal preswollen by hydrogen donor solvent,1,2 flash pyrolysis of coal swollen by a tetralin vapor,3 and flash pyrolysis of coal swollen sequentially by a pyridine vapor and a liquid tetralin.4 The idea of these methods lies in the realization of effective hydrogen transfer from the hydrogen donor solvent to the coal fragment during the flash pyrolysis. The validity of the proposed methods was tested using 10 different coals swollen by tetralin at 100-220 °C. Pyrolyzing an Australian brown coal Morwell (MW) using a Curie-point pyrolyzer at 920 °C in an atmospheric pressure of helium, total volatiles reached 67% and liquid yield surprisingly reached more than 42%, which was 2.3 times larger than the yield from the pyrolysis of the raw coal. This significant increase was found to be brought about by the suppression of the H2O forming cross-linking reaction as well as the effective hydrogen transfer from tetralin to coal fragments. Thus the proposed method realized significant increases in both total volatiles and the liquid yield for low-rank coals, but the method was not so effective for higher † Research Laboratory of Carbonaceous Resources Conversion Technology. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Miura, K.; Mae, K.; Asaoka, S.; Yoshimura, T.; Hashimoto, K. Energy Fuels 1991, 5, 340-346. (2) Miura, K.; Mae, K.; Yoshimura, T.; Masuda, K.; Hashimoto, K. Energy Fuels 1991, 5, 803-808. (3) Miura, K.; Mae, K.; Sakurada, K.; Hashimoto, K. Energy Fuels 1993, 7, 434-435. (4) Mae, K.; Hoshika, N.; Hashimoto, K.; Miura, K. Energy Fuels 1994, 8, 868-873.

0887-0624/96/2510-0364$12.00/0

rank coals. The effectiveness of the proposed method is related to the amount of hydrogen donated to coal fragments during pyrolysis. The amount of donated hydrogen was found to be closely related to the degree of swelling which would be correlated with the covalent cross-link density and the amount of noncovalent bondings in the coal. Then pretreatments breaking a part of the covalent cross-links and simultaneously introducing oxygen functional groups into the coal were expected to be effective to increase the swellability of the coal. Oxygen functional groups are known to act as hydrogen bonding sites in the coal. It is well-known that oxygen functional groups can be introduced by the oxidation.5,6 Many studies have been performed on air oxidation of coal at low temperatures to examine the effect of the weathering on the coal properties,7 and to diminish the caking properties of coal.8,9 When the coal was oxidized by air, its reactivity to the pyrolysis and the gasification generally decreased because the air oxidation produced ester and/ or ether cross-links as Van Krevelen,10 Painter et al.,9 and Liotta et al.11 demonstrated. On the other hand, oxidation in liquid phase by NaOH aqueous solution, (5) Wender, I.; Heredy, L. A.; Neuworth, M. B.; Dryden, I. G. C. In Chemistry of Coal Utilization, Second Supplementary volume; Elliott, M. A., Ed.; John Wiley & Sons: New York, 1981; pp 455-468. (6) Berkowitz, N. The Chemistry of Coal; Elsevier: New York, 1985; Chapter 5, pp 143-170. (7) Kahn, M. R.; Jenkins, R. G. Fuel 1985, 64, 189-195. (8) Huffman, G. H.; Huggins, F. E.; Dunmyre, G. R.; Pignocco, A. J.; Lin, M. C. Fuel 1985, 64, 849-854. (9) Painter, P. C.; Snyder, R. W.; Pearson, D. E.; Kwong, J. Fuel 1980, 59, 282-288. (10) van Krevelen, D. W. Coal, 2nd ed.; Elsevier Scientific: New York, 1981. (11) Liotta, R.; Bron, G.; Isaac, J. Fuel 1983, 62, 781-786. (12) Francis, W. Fuel 1938, 17, 363-372.

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hydrogen peroxide, etc. has been performed to obtain humic acids and aromatic carboxyl acids.5,6 This means that a part of the covalent cross-linkings is broken and many carboxyl groups are formed through the liquid phase oxidation. Thus the oxidation in liquid phase was expected to be suitable to partly degrade coal and to introduce oxygen functional groups which will act as hydrogen-bonding sites. The oxidized coal would be swollen in solvent to larger extent than the untreated coal. On the basis of this concept, we tried to oxidize the coal in liquid phase using hydrogen peroxide, NaOH, and Na2Cr2O7 as oxidizing agents. Then, the oxidized coals were swollen by tetralin and used in the pyrolysis experiments to examine the validity of the oxidation pretreatment as a means to increase the tar yield during the pyrolysis. Experimental Section Oxidation of Coal. An Australian brown coal (Morwell; abbreviated to MW) and a Japanese subbituminous coal (Taiheiyo; TC) were used as raw coals. Each coal was ground into fine particles of less than 74 µm, and dried in vacuo at 110 °C for 24 h before use. The oxidation of the coals was performed by four different methods described below. 1. Oxidation by hydrogen peroxide: 2 g of coal was swollen by 3 mL of methanol in a 200 mL flask and then 20 mL of 30% aqueous hydrogen peroxide was added to the swollen coal. The mixture was kept for 4 h in a water bath kept at a constant temperature of either 25 or 60 °C. The coals oxidized at 25 and 60 °C are abbreviated as HPO25 and HPO60, respectively. 2. Oxidation by NaOH: 20 mL of 2 N aqueous NaOH was added to 1 g of coal in a 200 mL flask and then the mixture was stirred for 8 h at 60 °C. This oxidized coal is abbreviated as NA60. 3. Oxidation by Na2Cr2O7: 2.5 g of Na2Cr2O7 was dissolved in 50 g of 97% sulfuric acid and 4 g of distilled water. This solution was added to 1 g of coal and then the mixture was stirred for 3 h at 25 °C. This oxidized coal is abbreviated as CR25. 4. Oxidation by air: The TC coal was oxidized by air for 2 h at 250 °C in a thermogravimetric analyzer (Shimadzu Co., TG-50) to trace the weight change during the oxidation. The weight reached an equilibrium state after 1.5 h of oxidation. This oxidized coal is abbreviated as AO250. All the coals oxidized in the liquid phase were filtered and dried in vacuo at 40 °C for 24 h. Swelling of Coal by Tetralin. Coal samples were treated with tetralin to incorporate tetralin within the particles as follows: the coal particles were mixed with tetralin in a stainless steel tube reactor and then they were heated to a temperature between 100 and 220 °C under 1 MPa of nitrogen by immersing the reactor into a temperature-regulated sand bath. The tetralin to coal ratio was maintained within 0.6-1.0 by weight. Through this treatment coal particles were swollen, and all the tetralin was incorporated in the coal matrix. The content of tetralin in the swollen coal was estimated from the weight change before and after the treatment. To swell the coal to a large extent, the coals oxidized by H2O2 at 25 °C (HPO25) were also swollen by a mixture of 20 wt % tetrahydrofuran (THF) and 80 wt % tetralin. The swelling ratio of the solvent-treated coal was measured by the volumetric technique13 by the procedure described in the previous paper.1 Characterization of Oxidized Coals. The changes in coal properties through the oxidation were examined from the ultimate analysis, the FTIR analysis, and the values of swelling ratio at 25 °C in tetrahydrofuran (THF), dioxane, and (13) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935-938.

Figure 1. Ultimate analyses and swelling ratios of MW and TC coals oxidized by several methods. benzene. The swelling ratio was measured every 24 h until an equilibrium was reached. One week was long enough to confirm the equilibrium swelling. Flash Pyrolysis. The samples prepared above were pyrolyzed in an inert atmosphere using a Curie-point pyrolyzer (Japan Analytical Ind., JHP-2S). About 2 mg of sample wrapped up tightly in a ferromagnetic foil was placed in a small quartz reactor (4.0 mm i.d.) and heated to a temperature between 590 and 920 °C at the rate of 3000 K/s by an induction heating coil to be pyrolyzed rapidly. The tar produced was completely trapped by the quartz wool placed just below the foil. Gaseous products were all led to a gas chromatograph equipped with a Porapak Q column to analyze inorganic gases (IOG; H2, CO, CO2, and H2O) and hydrocarbon gases (HCG; C1 to C6 gaseous compounds, benzene, toluene, and xylene). To analyze H2O, lines connecting the pyrolyzer and the GC were all heated to 150 °C to prevent the condensation of H2O. The calibration curve of H2O was constructed by pyrolyzing carefully weighed CuSO4‚5H2O. The yields of char and tar were measured from the weight changes of the foil and the reactor. The experimental apparatus and the procedure were described in detail in the previous paper.1 The product yield of each component during the pyrolysis of the solvent treated coal was represented on the basis of 100 kg of dry and ashfree (daf) coal. It was compared with the sum of the yields obtained by pyrolyzing the raw coal and the solvent independently to estimate the effect of the solvent treatment.

Results and Discussion Changes in Coal Structure through Oxidation. First, the changes in the properties of coal through the oxidation were examined. Figure 1 compares the ultimate analyses and the swelling ratios measured in tetrahydrofuran, where they are represented on both raw coal basis and oxidized coal basis. The ultimate analyses on oxidized coal basis are also given for the

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Figure 2. Comparison of the FTIR spectra of the coals oxidized in the liquid phase with those of the raw coals. Table 1. Properties of Oxidized Coals ultimate analysis (wt %, daf) MW Raw MW(HPO25) MW(HPO60) MW(CR) TC Raw TC(HPO25) TC(HPO60) TC(CR) TC(NA)

C

H

N

O(diff)

H/C

O/C

64.0 61.6 57.6 57.8 72.4 70.2 64.4 64.0 67.2

4.7 4.6 4.9 4.6 5.6 5.8 6.1 6.7 6.8

0.7 0.6 0.6 0.4 1.4 1.6 1.0 1.0 1.1

30.6 33.2 36.9 37.2 20.6 22.4 28.5 28.3 24.9

0.881 0.896 1.021 0.955 0.928 0.991 1.137 1.256 1.214

0.359 0.404 0.480 0.483 0.213 0.239 0.332 0.332 0.278

coals oxidized in liquid phase in Table 1. The total height of the bar in the middle figure represents the yield through the oxidation. For MW, the yields of CR25 and HPO60 are less than unity, but the yield of HPO25 is slightly greater than unity. The atomic ratios of hydrogen to carbon (H/C) and oxygen to carbon (O/C) of the oxidized coals are all larger than those of the raw coal. The decreases in the yields for CR25 and HPO60 are mainly due to the consumption of carbon. The increase in the yield for HPO25 is realized by addition of oxygen and hydrogen by consuming little carbon. On the other hand, the effect of oxidation on the yield was small for TC. The yields through five different oxidation methods were all within 95-105 wt %. The oxygen contents of all the oxidized coals slightly increased as compared with the oxygen content of the raw coal. Both the H/C and O/C ratios increased by the oxidation except for AO250. For both MW and TC, the swelling ratios of the oxidized coals in THF were all larger than those of the raw coals except for AO250 prepared from TC. The swelling ratios of HPO60 prepared from TC surprisingly reached as large as 2. Figure 2 compares the FTIR spectra of the coals oxidized in liquid phase with those of the raw coals. For both MW and TC, the peak intensity of 1710 cm-1 which is assigned to carboxyl groups15,16 increased by the (14) Lucht, L. M.; Peppas, N. A. Fuel 1987, 66, 815-826.

Figure 3. Product yields during the pyrolysis of the coals oxidized by several methods for MW and TC.

oxidation. The oxidation by Na2Cr2O7 (CR25) brought about the increase in the peak intensity between 1000 and 1100 cm-1 that is assigned to alcoholic OH groups. From the above discussion, it is judged that the liquid phase oxidation forms oxygen functional groups such as -COOH and -OH in the coal and simultaneously breaks some covalent bondings. Part of the functional groups introduced will act as hydrogen-bonding sites in the coal. When the oxidation is performed under severe conditions, carbon is consumed to decrease the yield of the oxidized coal. It is desirable to minimize the loss of carbon during the oxidation from a practical viewpoint. The oxidation by H2O2 at 25 °C is judged to be mild enough for both coals. Flash Pyrolysis of the Oxidized Coal. The flash pyrolysis of the oxidized coal was performed at 764 °C to examine the effect of oxidation on the pyrolysis yields as shown in Figure 3. The yields shown in Figure 3 are the average values of three to five experiments. The standard deviations of the total volatiles and tar were within (1% and those of the yields of H2O, CO, CO2, and the hydrocarbon gases were less than (0.1%. Each yield of the oxidized coal is represented on the basis of 100 kg of the raw coal (daf); namely, each pyrolysis yield from the oxidized coal was multiplied by the yield of oxidized coal to prepare Figure 3. Noticeable changes through the oxidation are the increases in the yields of CO2 and CO and the decrease in the tar yield for MW. The yields of CO2 and CO of HPO60, whose oxidation yield was only 75%, are significantly larger than those of the raw coal even on the raw coal basis. For TC, the yields of CO2 and CO increased, and the tar yield decreased for AO250 and HPO25. On the other hand, both the total volatiles and the tar yield increased for HPO60, although the yields of CO2 and CO increased simultaneously. (15) Landais, P.; Rochdi, A. Fuel 1993, 72, 1393-1401. (16) Calemma, V.; Iwanski, P.; Rausa, R.; Girardi, E. Fuel 1994, 73, 700-707.

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Figure 4. Effect of the oxidation method on the product yields during the pyrolysis of the coals oxidized and swollen by tetralin for MW and TC.

Formation of CO2 and/or CO is related to the decomposition of functional groups such as -COOH, -OH, and CdO, which is closely related to the extent of the cross-linking reactions during the pyrolysis at 400-600 °C as Solomon et al.17 and Suuberg et al.18,19 reported. The large increases in the yields of CO2 and CO through the oxidation, therfore, indicate that the oxygen functional groups introduced through the oxidation will enhance the cross-linking reaction during the pyrolysis. Flash Pyrolysis of the Oxidized Coal Swollen by Tetralin. Next, all the oxidized coals were swollen by tetralin and then pyrolyzed at 764 °C. Figure 4 compares the effect of the oxidation on pyrolysis yields for MW and TC. Each yield is represented on the basis of 100 kg of dry and ash-free (daf) raw coal. The black bars represent the yields obtained by pyrolyzing the solvent-swollen coals. The white bars are the yields for the raw coal or the oxidized coals, and the hatched bars are the yields for the pyrolysis of tetralin. Comparison between the black bar and the sum of the white and hatched bars gives the effect of tetralin swelling. No difference means no interaction between the coal and tetralin during the pyrolysis. The figures in the graphs of tar, H2O, and H2 represent the difference in the values between the black bar and the sum of the white and hatched bars. The positive value means that the yield was increased through the tetralin swelling. For all the oxidized coals as well as the raw coals, the tetralin swelling brought about the increases in the total volatiles and the tar yield and the decreases in the yields of H2O and H2. The yields of the hydrocarbon gases (HCG) changed little. The yields of CO and CO2 changed little except for HPO25 and HPO60. Thus, the main effects of the tetralin swelling are the suppression (17) Solomon, P. R.; Serio, M. A.; Despade, G. V.; Kroo, E. Energy Fuels 1990, 4, 42-54. (18) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 16681671. (19) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987, 1, 305-308.

of the H2O forming cross-linking reaction and the acceleration of hydrogen utilization during the pyrolysis,1-3 as is indicated by the decreases in the yields of H2O and H2, respectively. For MW, the amounts of the increase in the tar yield brought about by the tetralin swelling were 13.0 kg/100 kg of coal for CR25, 19.9 kg/100 kg of coal for HPO25, and 14.1 kg/100 kg of coal for HPO60, whereas the amount was only 11.5 kg/100 kg of coal for the raw coal. This indicates that the oxidation by H2O2 is very effective to increase the tar yield, but excess oxidation reduces the effect as shown in the tar yield for HPO60. The yields of H2O and H2 decreased corresponding to the increase in the tar yields. For TC, the amounts of the increase in the tar yield brought about by the tetralin swelling reached up to 13.0 kg/100 kg of coal for NA60, 11.9 kg/100 kg of coal for CR25, 16.9 kg/100 kg of coal for HPO25, and 13.0 kg/100 kg of coal for HPO60. They are much larger than the increase for the raw coal, 7.3 kg/100 kg of coal. The yields of H2O and H2 again significantly decreased when the tar yield increased. Judging from the amounts of the increase in the tar yield, the oxidation pretreatment is more effective to TC than to MW. The degree of the effect decreased with the increase in the severity of oxidation as the results for HPO25 and HPO60 indicate. The above discussion summarizes that the increase in the tar yield is brought about by the suppression of the H2O-forming cross-linking reaction in addition to the effective hydrogen transfer from tetralin to coal. Swelling Characteristics of the Oxidized Coal. From Figures 3 and 4 it was found that the oxidation pretreatment of coal accelerated the cross-linking reaction during the pyrolysis, but the cross-linking was suppressed and a large amount of tar was produced by combining the oxidation pretreatment and the tetralin swelling. This was presumed to be because the swellability of the coal with tetralin increased through the oxidation pretreatment. Then we examined the effect

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Figure 6. Comparison of the swelling ratio measured in tetralin and that measured in a tetralin-THF mixture for the raw coal and HPO25.

Figure 5. Comparison of the swelling ratios between the raw coal and the coal oxidized by H2O2 at 25 °C (HPO25) for MW and TC.

of the oxidation in more detail using HPO25, because the effect of the oxidation swelling was most advantageous for HPO25. Figure 5 shows the swelling ratios of both HPO25 sample and the raw coal for MW and TC measured in three solvents: benzene, dioxane, and tetrahydrofuran at 25 °C. The abscissa represents the difference between the donor number (DN) and the acceptor number (AN) of the solvent, DN-AN, which is one of the representative indices for the solvent polarity.20 First, we compared the swelling characteristics of the raw coals. The raw MW coal was not swollen by benzene, but swollen largely by the polar solvent, THF. On the other hand, the swelling ratio of the raw TC coal gradually increased with the increase in the solvent polarity. Next, we compared the change in the swelling ratios of the both coals through the oxidation. The swelling ratios of the HPO25 prepared from MW were only 1.1-1.2 with the weak polar solvents, but reached up to 1.9 with THF. In general, highly oxygenated coals do not swell to such an extent as higher rank coals. Solvent swelling of coal is affected by several factors. Of the factors the covalent cross-link density and the hydrogen-bonding density are the main factors controlling the swelling of the lower rank coal. Song et al.21 reported that the mild oxidation changed the structure of coal, indicating that the covalent bondings were affected by the oxidation. We have also shown that some covalent bondings were surely broken in the oxidized coal.22 Then the cross-linking density of the oxidized coal would be smaller than the untreated coal, and a part of the oxygen functional groups introduced would form hydrogen bondings. These hydrogen bondings would be broken by THF.14 Resulting from these effects caused by the oxidation, the swelling ratio of the HPO25 prepared from MW was believed larger than that of the untreated coal. Judging from Figure 5, the HPO25 prepared from MW would not be swollen by tetralin to a large extent. (20) Marzec, A.; Juzwa, M.; Betley, K.; Sobkowiak, J. Fuel Process. Technol. 1979, 35-49. (21) Song, C.; Saini, A. K.; Schobert, H. H. Energy Fuels 1994, 8, 301-312. (22) Miura, K.; Mae, K.; Maki, T.; Araki, J. Chem. Lett. 1995, 909910.

This suggests that the tar yield will become larger than that shown in Figure 4 if the swelling by tetralin can be promoted by some means. To realize it, the coals were swollen by the mixture of tetralin and THF (4/1 weight ratio). This method utilizes THF as a promoter of the swelling. Figure 6 compares the swelling ratios measured by this method and those measured with only tetralin for the raw coals and the HPO25 samples. In this figure the raw coal is denoted as Raw, and the swelling solvents, tetralin and the mixture of tetralin and THF, are denoted as Tet and Tet-THF, respectively. For MW, the swelling ratio of the HPO25-Tet was almost same as that of the Raw-Tet. On the other hand, the swelling ratio of the HPO25-Tet was significantly larger than that of the Raw-Tet for TC. This coincided with the result that a significant increase was brought about in the tar yield for the HPO25 prepared from TC in Figure 4. The addition of THF significantly increased the swelling ratio of the HPO25 for MW. THF will play the role of breaking hydrogen bondings of coal, which will facilitate tetralin to penetrate into the interior of the coal matrix as reported by Joseph23 and Song et al.24 On the other hand, the swelling ratio of the HPO25Tet-THF was almost similar to that of HPO25-Tet for TC as anticipated from Figure 5. Thus, the swelling utilizing THF as a promoter was found to be more effective to enhance the tetralin swellability of the lower rank coal. Flash Pyrolysis of Oxidized Coal Swollen by Tetralin-THF Mixture. Figure 7 compares the effect of the THF addition during the tetralin swelling on the pyrolysis yields of the HPO25 and the raw coal. Most of the THF was vaporized during the pretreatment of the swollen coal at 150 °C in an inert atmosphere, but most of the tetralin was retained in the swollen coal. The amounts of THF remaining in the swollen coals were estimated from the amounts of THF detected by GC analysis during the pyrolysis of the swollen coal at 386 °C. Since the amounts were less than 2 kg/100 kg of coal, the contribution of THF to the calculation of pyrolysis yields of the swollen coal was neglected. All the yields are represented in the same manner as in Figure 4. The figures in the graphs showing the yields of tar, H2O, and H2 also have the same meaning as in Figure 4. Fow MW, the amounts of the increases in the tar yields by solvent swelling were 11.5 kg/100 kg of coal (23) Joseph, J. T. Fuel 1991, 70, 139-144. (24) Song, C.; Hou, L.; Saini, A. K.; Hatcher, P. G.; Schobert, H. H. Fuel Process. Technol. 1993, 34, 249-276.

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Figure 8. Relationship between the amounts of hydrogen transferred by the two mechanisms.

Figure 7. Effect of tetrahydrofuran addition during the tetralin swelling on the pyrolysis yields of the raw coal and HPO25.

for the Raw-Tet, 14.2 kg/100 kg of coal for the Raw-TetTHF, 19.9 kg/100 kg of coal for the HPO25-Tet, and 22.9 kg/100 kg of coal for the HPO25-Tet-THF. These results clearly indicate that the addition of THF during the tetralin swelling brought about the increase in the tar yield as compared with the case of no THF addition. The effect was larger for the HPO25 than for the Raw. The amount of the increase in the tar yield for the HPO25Tet-THF was larger than the tar yield of the raw coal (white bar). The yields of H2O and H2 decreased in parallel with the increase in the tar yields for all the samples. The H2O yield for the HPO25-Tet-THF was only 4.1 kg/100 kg of coal which was less than a half of the H2O yield for the raw coal, indicating that the crosslinking was significantly suppressed by the swelling with the tetralin-THF mixture. The addition of THF during the tetralin swelling also brought about the increase in the tar yield for both the Raw and the HPO25 for TC, but the extent was not so large as for MW. The difference in the effect of the THF addition between MW and TC reflects the difference in the swelling ratios of the Raw-Tet-THF, the HPO25Tet-THF, the Raw-THF, and the Raw-Tet-THF between the coals (as shown in Figure 6). Thus, the addition of THF during the tetralin swelling was found to be more effective if it increases the swelling ratio further. Estimation of the Amount of Hydrogen Radicals Utilized. The oxidation of coal in liquid phase broke some covalent cross-links and simultaneously increased the atomic ratios of oxygen to carbon and of hydrogen to carbon through the introduction of functional groups such as phenolic OH and COOH. When the oxidized coals were pyrolyzed, the tar yield decreased in general, and the yields of CO and CO2 increased. This indicates that the oxygen functional groups introduced through the liquid phase oxidation acted as cross-linking sites during the pyrolysis. However, the oxidized coal was swollen to a large extent with solvents such as tetralin, THF, etc. The more intimate contact is effected between solvent and the functional groups of the coal. As a result of this, the tar yield significantly increased and

the yields of H2O and H2 decreased when the coal oxidized and swollen by tetralin was pyrolyzed. The suppression of the H2O forming cross-linking reaction (mechanism 2) and the effective utilization of hydrogen radicals originating from tetralin and coal itself (mechanism 1) are presumed to be the main mechanisms by which the increase in the tar yield was brought about. To confirm this, we examined quantitatively the hydrogen transfer during the pyrolysis of the tetralinswollen coal. In the previous paper,2 we showed that the amounts of radicals transferred by the two mechanisms can be estimated from the pyrolysis yields. The difference between the sum of the H2 yields of the coal and the solvent and the H2 yield of the solvent swollen coal is equal to twice the amount of H radical transferred by mechanism 1, RH(H2). It is difficult to identify the radical species transferred by mechanism 2. However, the decrease in the H2O yield surely corresponds to the amount of the radicals transferred. Then we estimated the amount of radicals transferred by mechanism 2, RH(H2O), by assuming that the phenolic condensation is the main mechanism to produce H2O. The difference between the sum of the H2O yields of the raw coal and the solvent and the H2O yield of the solvent swollen coal will correspond to the amount twice as large as RH(H2O).2 Figure 8 plots the RH(H2) value against the RH(H2O) value for examining the relative importance of the two mechanisms. The values for the raw coals and all the oxidized coals, all of which were swollen by tetralin, are included in the figure. The values of RH(H2O) and RH(H2) values are comparable for the both coals, indicating that both the mechanisms are significant. However, the relative importance of the two mechanisms seems to be different between MW and TC. Comparing the data among the Raw-Tet, the HPO25-Tet, and the HPO25Tet-THF for MW, we can say that the RH(H2O) value mainly increased with the increases in the degree of the oxidation and in the swelling ratio by solvent. This shows that the mechanism 2 is more significant for the oxidized and swollen coals prepared from MW. On the other hand, comparing the data of the HPO25-Tet, HPO60-Tet, and HPO25-Tet-THF for TC, we can say that the RH(H2) value solely increased with the increases in the degree of the oxidation and in the swelling ratio by solvent. This clearly shows that the mechanism 1 as well as the mechanism 2 is accelerated by promoting the contact between solvent and coal.

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It is interesting to note that the solvent swelling of coal also enhances the liquefaction conversion, although we are not sure if the mechanism increasing the conversion is the same in pyrolysis and liquefaction. Presoaking of coal in tetralin in temperature-programmed liquefaction of coal was reported to increase the liquefaction conversion.25 It was also reported that coals preswollen in THF gave higher liquefaction conversion in tetralin solvent and other H-donor solvents.26

Figure 9. Relationship between the increase in the tar yields and the sum of the amounts of hydrogen transferred by the two mechanisms.

If the increase in the tar yield was brought about by the two mechanisms, there should be some relation between the amount of tar increased and the sum of RH(H2) and RH(H2O) values. Figure 9 shows the increase in the tar yield brought about by the tetralin swelling, ∆YTar, against the sum of RH(H2) and RH(H2O) values. For the both coals, a fairly good correlation was obtained irrespective of coal type and the degree of the coal oxidation, indicating that the increase in the tar yield was certainly brought about by the two mechanisms. The coal oxidized in liquid phase is largely swollen by solvent. Pyrolysis of the oxidized and swollen coal increases the tar yield by stabilizing large coal fragments which would become char in the absence of solvent as tar components. This is brought about by the suppression of the H2O-forming cross-linking reaction as well as the effective hydrogen utilization. (25) Song, C.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1992, 6, 326-328. (26) Rincon, J. M.; Cruz, S. Fuel 1988, 67, 1162-1163.

Conclusion A new flash pyrolysis method which increases both total volatiles and tar yield was developed. The method is based on a new coal pretreatment method consisting of two steps. The first step is the liquid phase oxidation to break some covalent cross-links and to introduce oxygen functional groups such as -COOH and -OH, and the second step is the swelling of the oxidized coal with a hydrogen donor solvent such as tetralin. Of the several oxidation methods examined, liquid phase oxidation using H2O2 at 25 °C was found to be the best from the practical viewpoint. The addition of tetrahydrofuran as a swelling promoter was found to be effective to increase the swelling ratio with tetralin for the lower rank coal. The validity of the proposed method was clarified using an Australian brown coal, Morwell, and a Japanese subbituminous coal, Taiheiyo. The tar yield increased by this method was 1.7-2.3 times larger than the tar yield increased pyrolyzing the raw coal swollen with tetralin. Such significant increase in the tar yield was brought about by the suppression of the H2O-forming cross-linking reaction and the enhancement of hydrogen utilization during the pyrolysis, both of which were realized by the new coal pretreatment method. EF950108D