Znd. E n g . C h e m . Res. 1987, 26, 1329-1335
Sherwood, T. K.; Ryan, J. M. Chem. Eng. Sci. 1959,Il,81. Storch, D. M.; Dymek, C. J.; Davis, L. P. J. Am. Chem. SOC.1983, 105, 1765.
Takahashi, T.;Hatanaka, M.; Konaka, R. Can. J . Chem. Eng. 1967, 45,145. Waeeermann, H. H.;Murray, R. W. Singlet Oxygen; Academic: New York, 1978.
1329
Weast, R. C.; Astle, M. J. CRC Handbook of Chemistry and Physics, 63rd ed.; CRC: Boca Raton, FL, 1982. Wilke, C. R.; Chang, Pin AZChE J . 1955,I , 264. Received for review December 24, 1985 Revised manuscript received January 27, 1987 Accepted February 25, 1987
Solvent Extract Liquefaction of Coal with Fractionated Anthracene Oil and Recycle Solvent Koji Chiba, Hideyuki Tagaya,* Tsunayoshi Kobayashi, and Yuji Shibuya Faculty of Engineering, Yamagata University, Yonezawa, Yamagata 992, J a p a n
T o estimate a solvent ability for coal liquefaction, 26 kinds of anthracene oils with widely ranging characteristics were used for Yallourn and Taiheiyo coal liquefactions. Hydrogen contents of solvents were correlated well with Yallourn coal conversion having a large coefficient of correlation (0.90), whereas less value was obtained for Taiheiyo coal (0.81). The hydrogen donating ability (HDA) defined by the amount of dihydroanthracene produced in the reaction of the solvent with anthracene was correlated well with conversion to TS for both coals (coefficients of correlation were 0.90 and 0.94). Their relationship was applied to recycle solvents; however, smaller conversions than predicted by the regression curve were obtained. Removing n-paraffin from the recycle solvents increased coal conversion. It was considered that the saturates in the recycle solvents were detrimental t o coal liquefaction. It has long been known that, in coal liquefaction, the selection of the solvent has a pronounced effect upon the yield of liquid products from coal. In this paper, to quantitatively define the characteristics of a good solvent for coal dissolution, coal liquefaction using 26 kinds of anthracene oils was carried out. Anthracene oil was hydrogenated under three different conditions. Raw anthracene oil and three types of hydrogenated oils were fractionated into 5 or 6 fractions by vacuum distillation, obtaining 26 kinds of anthracene oils. We already reported the results of the liquefaction of Miike coal using other 20 kinds of anthracene oils (Chiba et al., 1984). In this report, two coals (Taiheiyo and Yallourn coals) whose carbon contents were less than that of Miike coal were liquefied using the anthracene oils. The hydrogen distributions of these oils were measured by ‘H NMR. The structural parameters were calculated by using the Brown-Ladner (1960) method. The main constituents of the oils were determined by GC, and the concentrations of acidic and basic compounds were determined by alkali and acid washing. Furthermore, anthracene was hydrogenated in these oils, and the amount of dihydroanthracene produced was defined as the hydrogen-donating ability of the oils. These solvent characteristics were correlated with conversions of Taiheiyo and Yallourn coals. Moreover, the correlation between the hydrogen donating ability and the Taiheiyo coal conversion was applied to recycle solvents provided by the Mitsui Engineering and Shipbuilding Co., Ltd. Literature Survey
Because solvent quality for coal liquefaction is an important factor (Whitehurst et al., 1980; Shah, 1981), many attempts have been made to correlate solvent characteristics with coal conversion efficiency. Curtis et al. (1981) characterized liquefaction solvents by spectroscopic measurements of their hydrogen distribution and determined the optimum solvent parameter ranges for effective coal dissolution. Miller and Silver 0888-5885/87/2626-1329$01,50/0
(1980),and Miller et al. (1982)reported that a Watson-type characterization factor, K,, was shown to be a good indicator of a solvent’s ability to physically dissolve coal fragments. K , values were calculated using the solvent‘s average volumetric boiling point and specific gravity at 298 K. Weight percent hydrogen in a solvent at its maximum liquefaction effectiveness has been shown to be a linear function of K,. They suggested that basic differences exist in the composition of recycle solvents derived from different coals and different processes. Furlong et al. (1976) defined a parameter called solvent quality index that varied with the degree of solvent hydrogenation in an EDS process. Coal conversion decreased significantly when the quality index of the solvent was less than 10. Details of the solvent quality index are proprietary. Curran et al. (1967) first described the results of hydrogen transfer in terms of a free-radicalmechanism where the free radicals produced by the decomposition of coal were stabilized by abstraction of a hydrogen from the solvents. According to this mechanism, a solvent’s hydrogen donor ability is very important. Hydrogen shuttling ability is closely related to the hydrogen donor ability because the shuttling reaction contributes to an increase in the amount of transferred hydrogen. Measurements of a solvent’s hydrogen donor ability have been attempted by many investigators. Hydroaromatic compounds such as tetralin are wellknown hydrogen donor solvents (Obara et al., 1983). Although hydroaromaticity of coal-derived liquids was measured by lH and 13Cmagnetic resonance and correlated with liquefaction conversions (Aiura et al., 1984; Curtis et al., 1985), assignments of shift range are still questioned. Uchino et al. (1984) proposed a proton donatable quality index (PDQI) which was calculated from the donatable naphthenic hydrogen content in hydroaromatic species as analyzed by HPLC and GC/MS methods. Methods to measure hydrogen donor ability using coal model compounds were proposed by many investigators because of its ease of measurement. 0 1987 American Chemical Society
1330 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 Table I. Analyses of Coals (wt 70) ultimate analysis (daf) coal C H N Y allourn 67.5 5.7 0.6 Taiheiyo 77.0 6.2 0.8 Wandoan 77.6 6.2 0.8 Miike 84.4 6.1 1.3 Fixed carbon.
0 diff 26.1 16.0 15.4 8.2
proximate analysis (coal) moisture FC" 13.2 41.0 5.2 36.4 9.2 38.6 1.2 46.4
ash 2.2 4.2 7.3 8.5
VMb 43.6 44.3 44.9 43.9
Volatile matter.
Dibenzyldiazene and dibenzylmercury were used as precursors for the benzyl radical (Bockrath and Noceti, 1981). Both easily decompose a t moderate temperature (130-170 "C). Indan has the highest donor index of the hydrocarbons. Indan seems to be a good rather than poor donor; however, indan is not a effective liquefaction solvent (Chiba et al., 1985a). It suggests the difference in its reaction mechanism a t low and high temperatures. Raaen and Roark (1978) indicated that benzophenone was a very useful reagent for ranking a series of hydrogen donors. However, Curtis et al. (1984) indicated no correlation between coal conversion and benzophenone conversion. The selection of a suitable coal model compound other than benzophenone should be considered. Other coal model compounds such as 1,l'-binaphthyl (Kline et al., 1982), anthracene (Yokono et al., 1981), and stilbene (Kamiya et al., 1983) were reacted with hydrogen donors. Their hydrogenated forms such as perylene, dihydroanthracene, dibenzyl, etc., were determined to calculate the hydrogen donated. A correlation between the hydrogen donor ability determined in these manners and coal conversion needs to be further evaluated.
Experimental Section Three coals were used in this study. These were a brown coal (Yallourn coal, Australia) and two subbituminous coals (Taiheiyo coal, Japan; Wandoan coal, Australia). They were ground to pass through 100-mesh (149-pm) screen and dried at 107 "C in a nitrogen atmosphere. Their analytical data are shown in Table I. Table I also shows the analytical data of Miike coal which was used in the previous report (Chiba et al., 1984). Anthracene oil (AO-10) was hydrogenated at the following three conditions: (1)hydrogenation a t 350 "C for 2 h with 1wt % MOO,; the hydrogen reaction pressure was 15 MPa; (2) hydrogenation at 420 "C with 3 wt % MOO,; after 1h the reaction pressure dropped to 10-12 MPa; the pressure was raised to 15 MPa by hydrogen, and hydrogenation was continued for 2 h more; (3) hydrogenation a t 420 "C with 3 wt % MOO,, 3 wt % Fe(OH),, and 3 wt % sulfur; the pressure was raised to 15 MPa every hour for 5 h. The extent of hydrogenation increased in the above order. Hydrogenated anthracene oils obtained under the conditions described above were designated as AO-20, -30, and -40, respectively. AO-10, -20, -30, and -40 were fractionated into five or six portions by vacuum distillation, and the obtained fractions were designated as AO-11-15, -21-25, -31-36, and -41-46. Their structural parameters were calculated by using the Brown-Ladner method (Brown and Ladner, 1960; Takeya et al., 1964; Makabe and Ouchi, 1980). Peak assignment (lH NMR shift, 6) included the peaks due to hydrogen on aromatic carbon atoms (Ha) a t 6-9 ppm, hydrogen on a-carbon atoms (H,) a t 2-4.5 ppm, and hydrogen on other nonaromatic carbon atoms (H,) a t 0.3-2 ppm. The quantitative separation of anthracene oil into three fractions of acidic, basic, and neutral cuts was carried out by successive extractions using 1 M NaOH and 1 M HC1
aqueous solutions (Morita et al., 1983). The oil (3 g) and anthracene (AN, 3 g) were heated a t 400 "C for 1 h in a nitrogen atmosphere. It was found by GC that most of the reaction products were dihydroanthracene (DHA). The amount of DHA was determined by 'H NMR. The determination was carried out by comparing the peak height at 3.87 ppm due to 9,lO-dihydroanthracene with the peak height at 3.33 ppm due to acenaphthene which was added as an internal standard. The hydrogen donating ability of the oil was defined as DHA/(DHA AN). Removal of n-paraffin from the recycle solvents was done by the urea adduct method. Recycle solvent (2.5 g), urea (3.3 g), methanol (0.4 g), and benzene (4.5 g) were stirred at 0 "C for 10 h and extracted with cold benzene. About 10 wt % of the recycle solvent was removed as n-paraffin. Coal liquefaction was carried out by the previously described method (Tagaya et al., 1983). A nominal reaction time was 60 min at 400 "C. We already confirmed that, in the liquefaction of Taiheiyo and Yallourn coals a t 400 "C without a catalyst, the effect of hydrogen gas on the coal conversion was fairly small (Tagaya et al., 1983). Therefore, liquefaction was carried out in a nitrogen atmosphere (5.1 MPa a t room temperature). Conversions to benzene-soluble material (BS) and to THF-soluble material (TS) were calculated according to (Chiba et al., 1985a) conversion to BS or T S (wt % daf coal basis) = lOO(coa1 charged benzene- or THF-insoluble material) /daf coal
+
Results The boiling point range and the carbon and the hydrogen contents of the anthracene oil fractions are shown in Table 11. Hydrogen contents of the unfractionated anthracene oils (AO-10, -20, -30, and -40) increased from 5.7% to 6.6% with an increase in the severity of the hydrogenation conditions. This indicated the smooth addition of hydrogens to the oils. Furthermore, the hydrogen content decreased as the boiling point range increased. Hydrogen on aromatic carbons (HJH) of unfractionated anthracene oils increased from 0.64 to 0.67 with an increase in the severity of hydrogenation conditions. Hydrogen contents from 5.4 to 8.1 and H,/H ratios from 0.47 to 0.80 indicate that these oils are solvents having a wide range of characteristics. Yallourn coal was liquefied using the anthracene oils as shown in Figure 1. The abscissa is the boiling point range. In the case of Miike coal liquefaction, conversion to TS was in the 50-75% range and the effect of solvents on conversion to TS was small (Chiba et al., 1984). On the other hand, conversion to TS of Yallourn coal increased from about 10% to 75% with an increase in the severity of hydrogenation conditions, in every boiling point range. Furthermore, conversion to TS of Yallourn coal decreased with the rise of boiling point cuts of the oils. Taiheiyo coal, whose carbon content of 77.0% is between those of Miike coal (84.4%) and Yallourn coal (67.5%),was
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1331
-
0 All
252- 290- 325- 342- 352-252 290 325 342 352
O
Hydrogen content ( ~ 1 % )
Boiling point ('Y)
Figure 1. Conversion to TS of Yallourn coal as a function of the boiling point range for anthracene oils. Liquefaction was carried out at 400 OC for 60 min in a nitrogen atmosphere (5.1 MPa at room temperature) using AO-10-15 (A), A0-20-25 ( O ) ,A0-30-36 (01,and A0-40-46 ( 0 ) .
'7 80
-
s P
8 8 " Q
0
60-
3
0
Q
c
A
0
0
A
A
2o
I
01
* a o Q
A
g 40-
V
1.
0 0
.5 2.k
Figure 3. Conversion to TS of Yallourn coal as a function of the hydrogen content for fractionated anthracene oils. Key as in Figure
I
-
2
4 5 6 I 8 9
'
A
0
20
A
252- 290- 325- 341- 352-252 290 325 342 352
I
Boiling p i n t ('C)
Figure 2. Conversion to TS of Taiheiyo coal as a function of the boiling point range for fractionated anthracene oils. Key as in Figure 1.
liquefied using the anthracene oils. Conversions to TS are shown in Figure 2. Conversion to TS increased as did the severity of hydrogenation conditions of anthracene oils. The difference between minimum and maximum conversion to TS was about 50%, which was smaller than that of Yallourn coal. The effect of the compounds contained in the high boiling point c u b on conversion to TS was smaller than that in Yallourn coal. Hydrogen content of a solvent is a fundamental characteristic of the solvent and may be a qualitative indication of the solvent ability. Therefore, conversions to TS were correlated with the hydrogen contents of anthracene oils. Conversion to TS of Yallourn coal increased as did the hydrogen content as shown in Figure 3. A coefficient of correlation was 0.90. Similarly, the conversions to TS of Taiheiyo coal were correlated with the hydrogen contents as shown in Figure 4, but a coefficient of correlation was 0.81 which was 0.09 smaller than that of the Yalloum coal. For the liquefaction using the anthracene oil whose hydrogen content is 6%, 55% conversion to TS is expected from the regression curve. However, conversions to TS
-
Hydrogen content (wt*/.)
Figure 4. Conversion to TS of Taiheiyo coal as a function of the hydrogen content for fractionated anthracene oils. Key as in Figure 1.
which were obtained using about 6% hydrogen content oils were in the range 3 5 4 0 % . Other characteristics such as hydrogen distribution, aromaticity ( f a ) , and acidic and basic fractions were also correlated with conversion to TS. The hydrogen on acarbon atoms or the hydrogen on other nonaromatic carbons was expected to correlate with the amount of transferable hydrogen of the oil. However, no good relationship between them or a f a and conversion to TS was observed. Basic fractions varied from 2.1% to 6.5% as shown in Table 11, but no good correlation between the amount of basic fraction and conversion to TS was obtained. The hydrogen donating ability of a solvent (DHA/(DHA AN)) as shown in Table I1 increased with an increase in the severity of hydrogenation conditions. The proportion of tetralin (tetralin/ (tetralin + naphthalene)) was also shown in Table 11. The proportion of tetralin increased with an increase in the severity of hydrogenation conditions. I t suggests that the contribution of hydroaromatic compounds such as tetralin to DHA/ (DHA + AN) is large. DHA/(DHA + AN) was correlated with conversion to T S as shown in Figures 5 and 6. In the case of Yallourn
+
1332 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 Table 11. Characteristics of Raw Anthracene Oils (AO-10-15) and Hydrogenated Anthracene Oils (Weak, Medium, Strong; AO-20-46) A0 bP C, % H, 70 HJH H,/H fa HDA" BF, %* TE' 0.098 6.2 91.9 5.7 0.64 0.22 0.87 all 10 raw 0.093 -0 0.14 0.92 89.9 6.0 0.77 11 252-290 raw 0.094 -0 0.72 0.23 0.90 91.2 6.0 290-325 raw 12 0.094 91.9 5.9 0.69 0.19 0.89 325-342 13 raw 0.095 0.93 91.2 5.7 0.80 342-352 0.17 14 raw 0.093 0.17 0.93 92.2 5.4 0.78 352 raw 15 -0 0.118 5.9 0.65 0.22 0.87 91.7 5.9 all 20 weak 0.89 0.130 4.3 -0 89.6 6.5 0.72 0.20 21 252-290 weak 0.22 0.82 0.134 6.8 91.0 6.4 0.56 290-325 22 weak 5.5 0.23 0.89 0.163 91.4 6.1 0.70 325-342 weak 23 5.0 0.123 0.72 0.18 0.89 91.9 6.0 342-352 weak 24 0.16 0.92 5.4 0.136 91.5 5.5 0.77 352 weak 25 4.8 0.22 0.85 0.136 0.65 91.7 6.4 medium all 30 0.33 0.77 4.0 0.215 88.0 7.5 0.27 0.57 252 medium 31 5.4 0.20 0.81 0.273 0.25 90.1 7.3 0.59 medium 252-290 32 0.14 6.5 0.337 91.1 6.8 0.83 0.27 0.60 290-325 medium 33 5.2 0.185 0.85 0.25 91.4 7.0 0.68 325-342 medium 34 6.5 0.171 0.88 0.22 89.7 6.2 0.69 342-352 medium 35 6.0 0.160 0.91 0.17 90.1 5.7 0.75 medium 352 36 3.2 0.158 0.86 0.67 0.21 92.9 6.6 strong all 40 2.1 0.179 0.72 0.58 0.27 88.5 8.1 0.47 252 strong 41 4.7 0.42 0.199 0.75 0.30 89.9 7.7 0.49 252-290 strong 42 4.2 0.33 0.54 0.243 0.79 0.28 91.5 290-325 7.2 strong 43 4.3 0.63 0.185 0.84 0.25 325-342 90.6 6.9 strong 44 0.190 0.86 3.4 0.23 0.66 342-352 92.8 6.6 strong 45 0.179 0.73 0.90 0.20 93.0 6.0 strong 352 46
" The hydrogen donating ability
of solvent. bBasic fractions (wt % solvent base). 'Tetralin/(tetralin
Table 111. Liquefaction of Taiheiyo Coal Using Neutral (AO-30-N and AO-40-N), Acidic (AO-30-A and AO-40-A), and Basic (AO-30-B and AO-40-B) Fractions of Anthracene Oils
injection wt, g 18 18 2 + 16 2 + 16 2 + 16 18 18 2 + 16 2 16 2 + 16
solvent AO-30 AO-30-N AO-30-N MN" AO-30-A MN" AO-30-B MN" AO-40 AO-40-N AO-40-N + MN" AO-40-A MN" AO-40-B + MN"
+ + +
+
+
a
conversion to BS TS 47.1 68.1 50.1 76.6 29.1 45.9 25.3 39.6 36.4 54.0 76.5 52.5 53.2 78.1 31.3 46.9 21.1 41.8 24.2 44.1
-s
+ naphthalene).
80 0
'i
+
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R = 0.94
.-
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40-
C
8 20
-
1-Methylnaphthalene, O'
o.;o
a
60-
0
e
f
40-
C
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20-
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O'
related well, with hydrogen content having a coefficient of correlation of 0.90; however, a correlation between the hydrogen content and DHA/(DHA AN) was poor. In Figure 5, even though solvents having a DHA/(DHA + AN) of over 0.20 were used, almost the same conversion to TS was obtained. This might result from the presence of "inherent inert" in Yallourn coal. When Yallourn coal was liquefied a t 380 O C in tetralin, the conversion for 180 min was smaller than the conversion for 120 min (Chiba et al., 1986). Under these liquefaction conditions, inherent inert (Nagaishi et al., 1984) in Yallourn coal is present, and the longer reaction time appears not to increase the coal conversion. Also in the case of Taiheiyo coal, a large coefficient of correlation, 0.94, was observed between the hydrogen donating ability and conversion to TS as shown in Figure 6. The tendency that conversion to TS does not increase in the solvents having the hydrogen donating
+
._
E
I-)
Figure 6. Conversion to TS of Taiheiyo coal as a function of the hydrogen donating ability for fractionated anthracene oils and recycle solvents. Recycle solvents were RS-1 and RS-2 (0)and RS1DP and RS-PDP (a). DHA and AN are dihydroantbracene and anthracene. Key as in Figure 1.
-s
-e5
0.30
0.20 DHAI(0HA. AN)
0:10
0.20
0.30
I
DHA/(DHA * AN) (-)
Figure 5. Conversion to T S of Yallourn coal as a function of the hydrogen donating ability for fractionated anthracene oils. DHA and AN are dihydroanthracene and anthracene. Key as in Figure 1.
coal, a large coefficient of correlation (0.90) was obtained. As shown in Figure 3, Yallourn coal conversion was cor-
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1333 Table IV. Analyses of Recycle Solvents solvent c, % H, % 9.2 87.7 RS-1 8.4 86.6 RS-1DP 9.7 87.5 RS-2 8.9 87.9 RS-2DP
N, %
Ha/H
H,/H
fa
6
1.1 1.5 0.9
0.22 0.34 0.17 0.32
0.24 0.21 0.32 0.29
0.47 0.61 0.46 0.62
0.30 0.50
1.4
0.36 0.35
n 3.22 2.37 2.55 2.29
HDA" 0.243 0.240 0.263 0.260
The hydrogen donating ability of the solvent.
ability over 0.20 was the same as in Yallourn coal. From the above results, the conversions to TS of both coals were found to be successfully correlated with the hydrogen donating ability of anthracene oil. We tried to apply the relationship between Taiheiyo coal conversion and the hydrogen donating ability to the recycle solvents (RS-1 and RS-2). RS-1 and RS-2 were recycle solvents which were obtained from Wandoan coal by using a bench scale unit (0.1 ton/day) of the Mitsui Engineering and Shipbuilding Co., Ltd. Their analytical data are shown in Table IV. The hydrogen donating ability for RS-1 was 24.3 and 26.3 for RS-2. From these values, an 80% conversion to TS was expected. But conversion to TS was 65.3% for RS-1 and 68.8% for RS-2 as shown in Figure 6. These values were 12-15% lower than expected. The hydrogen contents of RS-1 and RS-2 were 9.2% and 9.7%, respectively. They were higher than those of anthracene oils (5.4-8.1%). The amounts of hydrogen, especially nonaromatic hydrogen, and the value of the structural parameters such as chain length are larger than those of anthracene oils. This suggests that the recycle solvents contain more saturates than the anthracene oils. Murata and Fukuji (1985) reported the presence of nparaffin accumulation in the recycle solvents. We tried to remove the n-paraffin by using the urea adduct method described above. About 10 wt % RS-1 and RS-2 was removed as n-paraffin. Oils obtained in this manner were designated as RS-1DP and RS-BDP, respectively. It had already been confirmed that n-paraffins over C18 were completely removed by this urea adduct method, but nparaffins below C17 were not completely removed. Analytical data of RS-1DP and RS-2DP are shown in Table IV. Decreased hydrogen content and chain length and increased aromaticity were observed after the removal of the paraffins. But the hydrogen content and chain length of RS-1DP and RS-2DP are still higher than those of the anthracene oils? and aromaticity is lower. On the other hand, the hydrogen donating abilities were not affected by the removal of n-paraffin. Conversions to TS increased from 65.3% to 73.6% for RS-1 and from 68.8% to 71.8% for RS-2. The 73.6% conversion to TS in RS-1DP was still about 6% lower than expected. This may be because the unperfect removal of n-paraffin and/or the presence of long alkyl chain in the recycle solvents which act similar to saturates. However, these facts indicate that the removal of n-paraffin is beneficial.
Discussion Conversion of Yallourn coal increased from 10% to 75% with an increase in the severity of hydrogenation conditions of anthracene oils. Similarly the conversion of Taiheiyo coal increased from 30% to 80%. In this manner the effect of solvents on Taiheiyo coal conversion was larger than on Miike coal conversion; however, it was smaller than on Yallourn coal conversion. When Yallourn coal was liquefied at 400 "C in tetralin, more hydrogen (2.6 wt % daf) was consumed in order to obtain the same conversion (60%) as that of Taiheiyo coal (1.5 wt % daf) and Miike coal (1.2 wt % daf). From the facts the difference among coals is interpreted from the difference in
the amounts of hydrogen required during liquefaction. It is not hard to appreciate why Yallourn coal is more sensitive to the characteristics of the solvent than other coals. Davies et al. (1977) reported that coal (carbon content, 84%) conversion became higher in the higher boiling point cut of anthracene oil. They suggested a hydrogen shuttling mechanism by some fraction of polynuclear compounds. In this study Yallourn coal conversion decreased with the rise of boiling point cut of solvent although the tendency was not observed in the liquefaction of Taiheiyo coal. The hydrogen contents decrease with the rise of boiling point range, and a more condensed structure than for the lower boiling point range was presumed for the higher boiling point range as shown in Table 11. It is considered that the higher boiling point cuts are not sufficient to liquefy lower carbon content coal like Yallourn. On the other hand, higher carbon content coals like Miike coal are easily liquefied even using a poor donor because their requirement of hydrogen during liquefaction is small. A quantitative discussion could not be done considering the physical solvent ability of higher boiling point cut. However, the possibility that the effective dissolution is obtained by a good compatibility between higher rank coal and higher boiling point cuts because of their similarity to the structures is not excluded. Conversions of Yallourn and Taiheiyo coals increased with hydrogen contents as shown in Figures 3 and 4. Saturated compounds such as fully saturated cyclic compound and paraffin are not donors in spite of their higher hydrogen content. Therefore, the utilization of hydrogen contents as the solvent characteristics is restricted. Nevertheless, if the solvents contain small amounts of saturates like anthracene oil, hydrogen contents can be sufficiently employed as a solvent ability indicator. No good correlation between the amounts of basic fractions, and Yallourn, Taiheiyo coal conversions was observed. It is well-known that tetrahydroquinoline is a hydrogen donor and good physical solvent for coal liquefaction (Guin et al., 1983; Panvelker et al., 1984; Hellgeth and Taylor, 1984). Such basic compounds were reported to be more effective liquefaction solvents than acidic and neutral compounds (Spencer, 1982). To obtain information about the contribution of basic fraction on the liquefaction, Taiheiyo coal was liquefied using neutral, acidic, and basic fractions. AO-30 and AO-40 contained 93.3 and 95.0 wt 70neutral, 1.9 and 1.8wt % acidic, and 4.8 and 3.2 wt % basic fractions. Under these liquefaction conditions, conversions to BS and TS in neutral fraction (AO-30-N and AO-40-N) were higher than those in unfractionated oils (AO-30 and AO-40) as shown in Table 111. Especially, conversion to TS using AO-30-N was 9% higher than that using AO-30. To compare the ability as a liquefaction solvent, every fraction was used as the liquefaction solvent of Taiheiyo coal. Here, as the amounts of acidic and basic fractions were small, every fraction was mixed with 1methylnaphthalene. Conversion to TS using the basic fraction of AO-30 was 54% and larger than 46% of the neutral fraction as shown in Table 111. Although in the liquefaction using AO-40 a clear effect of the basic fraction was not observed, the acidic fraction was less effective than
1334 Ind. Eng. Chem. Res., Vol. 26, No. 7 , 1987
the neutral and the basic fractions similar to AO-30. The condensation reaction of acidic fraction such as dehydration of two hydroxyl groups was considered. We must pay attention to the role of the basic and the acidic fractions in the liquefaction; however, under these liquefaction conditions, their effects on conversion were less than the effect of other characteristics like the hydrogen content. The hydrogen donor ability (HDA) of a solvent is a critical for coal liquefaction. As shown in Figures 5 and 6 , the hydrogen donating ability defined in this study is well correlated with coal conversions. Anthracene was chosen as a hydrogen acceptor because of its high capability as a hydrogen acceptor. Free radicals produced by the coal decomposition are expected to be much stronger hydrogen acceptors than anthracene. We already reported the effects of various solvents on the decomposition of bridged compounds, dibenzyl and dibenzyl ether (Chiba et al., 1985b). The strong hydrogen acceptors (benzyl and benzyloxy radicals) were produced by the decomposition of them. However, the hydrogen abstraction reaction seemed to be fast, and ranking of the solvent donor ability was unsuccessful. Because the decomposition was accelerated by the molecular induced homolysis instead of hydrogen donation, poor donors such as diphenylmethane and diphenylamine were more effective than the donor tetralin. It suggests that the selection of a radical source is difficult. In this study using anthracene as the hydrogen acceptor, we could not distinguish added anthracene and originally present anthracene in the anthracene oil. Further the NMR peaks of dihydroanthracene and fluorene overlapped. However, the largest amount of anthracene originally present in oil was only 12% in AN-14, and the largest amount of fluorene present was less than 10%. We could not find out a particular relation between HDA and the amount of anthracene or fluorene originally present in oil. Therefore, the interference of original compounds in oils was ignored. The HDA calculated in this manner is a good indicator for coal liquefaction solvent as shown in Figures 5 and 6. McMillen et al. (1985) reported a solvent radical mediated hydrogenolysis (SRMH) mechanism in which the strongest aryl-phenyl bonds are cleaved upon transfer of hydrogen to the ipso position of arylphenyl. It suggests that the hydrogenation of aromatic rings is one of the first steps of coal degradation. The good acceptor of anthracene on the solvent ability indicator is explicable by the above facts. Smaller conversions than predicted by the hydrogen donor abilities were obtained when the recycle solvents were used as the liquefaction solvents. The fact that removing n-paraffin increased coal conversion indicates a presence of a detrimental effect of n-paraffin. To confirm this, the mixture of 15% n-octadecane and AO-43 was used as the liquefaction solvent of Wandoan coal. Although 77.9% conversion to TS was obtained when Wandoan coal was liquefied using AO-43, 4% lower conversion to TS (73.9%) was obtained in the mixture. We already reported that the detrimental effect of n-paraffin mixing was also observed even when the cosolvent was donor (tetralin) (Chiba et al., 1985~).Conversion of Wandoan coal to TS using the mixture of 25% tetralin and 1-methylnaphthalene was 67.8%, whereas that using the mixture of 25% tetralin and n-octadecane was 45.8%. 1-Methylnaphthalene is not a donor and regarded as an inert diluent. These results indicate that the detrimental effect of n-paraffin addition does not result from only a role as an inert diluent of n-paraffin. One other aspect is an effect of n-paraffin on the filtration of the product slurry. Kimber (1985) reported about the effect of saturates on
the filtration rate of a slurry. He showed that the cake specific resistivity reduced considerably as the saturate level in the solvent rose. Furthermore he explained that the saturates caused some agglomeration. To obtain the information about a physical effect of n-paraffin, the solubility of SRC in benzene was investigated in the presence of n-octadecane. SRC from Morwell brown coal which was provided by Mitsui SRC Development Co., Ltd., was used in this investigation. An amount equal to 49.5% SRC was soluble in benzene, and 50.8% SRC was soluble in benzene in the presence of the same weight of n-octadecane with SRC. This result indicates that the solubility of slurry is little affected by the presence of saturates such as n-octadecane. Lower molecular weight produds were confirmed by GC after the thermal treatment of n-paraffin. During the decomposition reaction, intermediate radicals abstract hydrogen from coal or the solvent. The hydrogen transfer reaction is known to proceed by a chain reaction. The hydrogen abstraction of n-paraffin from coal or the solvent decreases the useful hydrogen of them and causes the termination of the chain of the hydrogen transfer reaction.
Summary a n d Conclusions Important conclusions derived from this work include the following. 1. Twenty-six kinds of solvents with widely ranging characteristics were obtained by the hydrogenation under three different reaction conditions and the fractionation by vacuum distillation. They were used as coal liquefaction solvents. Yallourn coal and Taiheiyo coal conversions increased from 10% to 75% and from 30% to 80% with an increase in the severity of hydrogenation conditions of oils. 2. Y allourn and Taiheiyo coals conversions increased with an increase in the hydrogen contents of anthracene oils, having coefficients of correlation of 0.90 and 0.81. However, other solvent characteristics such as a hydrogen distribution, aromaticity, and the amounts of acidic and basic fractions were not a sufficient indicator of the solvent ability. 3. The hydrogen donating ability (HDA) of solvent was defined by the amount of dihydroanthracene produced in the reaction of the solvent with anthracene. Such HDAs of the oils were good indicators for the solvent ability, and Yallourn and Taiheiyo coal conversion increased with an increase in the hydrogen donating ability, having coefficients of correlation of 0.90 and 0.94. 4. Relationship between Taiheiyo coal conversion and HDA was applied to the recycle solvents. Smaller conversions than predicted by the regression curve were obtained. The detrimental effect of n-paraffin in the recycle solvents was confirmed by the improvement of conversion by the removing of n-paraffin. To estimate the solvent ability for coal liquefaction, not only positive effects such as the concentration of transferable hydrogen, but also detrimental effects such as hydrogen consumption of solvents must be considered. If the contribution of such detrimental effects on coal liquefaction was estimated exactly, HDA calculated from the amount of dihydroanthracene was further useful to evaluate coal liquefaction solvents. Acknowledgment We thank Dr. K. Murata of Mitsui Engineering and Shipbuilding Co., Ltd., and Dr. H. Sugimura of Mitsui SRC Development Co., Ltd., for providing the recycle
Ind. E n g . Chem. Res. 1987,26, 1335-1339
solvents and SRC used in this study. We are also grateful to T. Chiba for his technical assistance.
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1335
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Receiued for review January 14, 1986 Revised manuscript received February 18, 1987 Accepted April 11, 1987
Steam Gasification of Biomass with Nickel Secondary Catalysts Eddie G. Baker,* Lyle K. Mudge, and Michael D. Brown Pacific Northwest Laboratory,' Richland, Washington 99352
Nickel secondary catalysts are effective for increasing the gas yield from steam gasification of biomass by converting tars and other hydrocarbons t o gas. Coke buildup on the catalyst can cause loss of catalyst effectiveness in a very short time. Bench-scale gasification tests showed that coke buildup can be prevented if the catalyst is used in a separate fluid bed reactor. When the catalyst was used in the primary gasification reactor or in a separate fixed bed reactor, rapid catalyst deactivation occurred. Gasification or pyrolysis of biomass produces a wide spectrum of products including gases, char, light oils, and tar. Pacific Northwest Laboratory (PNL) has been exploring the use of catalysts to produce gases from biomass. In particular, supported nickel catalysts were found to be effective in producing high yields of synthesis gas from the hot, raw gas produced by steam gasification of biomass (Mitchell et al., 1980). Several other investigators have obtained similar results (Tanaka et al., 1984; Yokoyama et al., 1983; Ekstrom et al., 1985; Corte et al., 1985). We refer to these catalysts as secondary to differentiate them from catalysts such as alkali carbonates which catalyze pyrolysis and gasification reactions. In tests in a small fixed bed laboratory reactor (10-12 g/h wood feed) several catalysts were found that retained their activity over a long period of time without regeneration. One catalyst was tested for over lo00 h and was still active when the test was terminated (Baker et al., 1983; +Operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76 RLO 1830. 0888-5885187 f 2626-I335$0I.50/0
Mudge, et al., 1985). These tests also determined the optimum conditions for synthesis gas production to be 750 "C and a stream/wood mass ratio of about 0.8. When these same catalysts were tested in a 1 tpd (ton/day) fluidized bed process research unit (PRU), they quickly lost their activity. Fouling due to coke buildup was the cause of catalyst deactivation (Baker et al., 1984; Mudge et al., 1985). In the PRU, the catalyst was used in a fluidizable form in the gasifier vessel. A bench-scale gasification unit (BSG) was built to resolve the apparent discrepancy in results between the laboratory and PRU tests. The BSG has provision for placing the catalyst in the fluidized bed gasification vessel similar to the PRU or downstream in a separate catalytic reactor to more closely simulate the original laboratory studies.
Background Table I shows the most important secondary reactions in steam gasification of biomass. With a nickel secondary catalyst, the yields of methane and light hydrocarbon gases 0 1987 American Chemical Society
