Energy & Fuels 1991,5,803-808
803
Mechanism of Radical Transfer during the Flash Pyrolysis of Solvent-Swollen Coal Kouichi Miura,* Kazuhiro Mae, Tomonori Yoshimura, Kazuhiro Masuda, and Kenji Hashimotof Research Laboratory of Carbonaceous Resources Conversion Technology and Department of Chemical Engineering, Kyoto University, Kyoto, 606 Japan Received May 29,1991. Revised Manuscript Received August 23, 1991
We have previously presented a new coal flash pyrolysis method for drastically increasing the total volatile matter and the tar yield, in which the coal swollen by tetralin at 100-250 "C was pyrolyzed in an atmospheric pressure of He. To examine the validity of the proposed method further and to examine the pyrolysis mechanism of the solvent-swollen coal, the flash pyrolysis of coal swollen by solvent was carried out for several coal-solvent combinations. Seven kinds of solvent were used to test the effect of the kind and the amount of radicals transferred. Ten coals of different rank were used to examine the radical acceptability of coal. Of the seven solventa tested, tetralin, a liquid derived from coal liquefaction, and phenol were very effective as far as the formation rate of the radicals from the solvent matches that of the coal fragment. The proposed method was more effective for the lower rank coals which have more oxygen functional groups. It was clarified that at least two mechanisms were involved in increasing the tar yield. The amount of radicals transferred by the two mechanisms, RH,'and RH2,were estimated from the yields of H2and HzO,respectively. The value of represented the degree of hydrogen transfer from the solvent to coal, which was controlled by the hydrogen donability of solvent and the hydrogen acceptability of coal. RH2 was related to the coal intramolecular radical transfer which is caused by the suppression of the cross-linking reaction of OH-associated hydrogen bonding.
Introduction We have previously presented a new coal flash pyrolysis method for considerably increasing the total volatile matter and the tar yield, in which the coal preswollen by tetralin at 100-250 "C was pyrolyzed in an atmospheric pressure of He.' The original idea of this method lies in the realization of effective hydrogen transfer from the hydrogen donor solvent to the coal fragment during the flash pyrolysis. When two coals were swollen by tetralin, tetralin was found to be retained in the micropores of its molecular dimension. Pyrolyzing the swollen coal in a Curie-point pyrolyzer at 670,764,and 920 "C in a helium stream at atmospheric pressure resulted in the production of 56% of volatile matter for Taiheiyo (TC; Japanese subbituminous coal) and surprisingly 67% for Morwell (MW, Australian brown coal). The maximum liquid yield from MW reached more than 42%, which was 2.3 times larger than that from the raw coal. This significant increase was brought about by 8, physical effect as well as the effective hydrogen transfer from tetralin. Although the validity of the proposed method was clarified in the previous paper as briefly stated above, the method was applied to only two coals swollen by tetralin. It is expected that radicals other than H' are also effective and that the coal type, namely the radical acceptability of the coal, will affect the validity of the method. This paper examines the validity of the proposed method in more detail. Ten kinds of coal of different rank were used to examine the radical acceptability of the coal. Seven kinds of solvent which produce different radicals such as OH' and CH; at different temperatures were used to examine the effect of the kind of radicals donated. Based on these results, the mechanism of the flash pyrolysis of solvent-treated coal is discussed. The amount of radical transferred from the solvent to the coal fragment 'Department of Chemical Engineering.
was estimated to quantitatively examine the radical donability of solvent and the radical acceptability of the coal fragment.
Experimental Section Sample Preparation. Ten kinds of coal of different rank were used as raw coal. Their properties are given in Table I. Each coal was ground into fine particles of less than 74 pm, and dried in vacuo at 110 "C for 24 h before use. Seven kinds of solvent given in Table I1 were used to swell the coal. CL is a coal liquid derived from a liquefaction process of MW brown coal, and OIL was an oil derived from the hydropyrolysis of Great Greta subbituminous coal. Their preparation conditions are given at the bottom of Table I. Coal samples retaining the solvent within the particles were prepared as follows: the coal particles were mixed with solvent in a stainless steel tube reactor; then they were heated to a temperature between 100 and 250 "C under 1 MPa of nitrogen by immersing the reactor into a temperature regulated sand bath. The solvent to coal ratio was maintained within 0.4-1.0 kg/kg of coal. This small ratio was employed to avoid the extraction of coal by the solvent. By this treatment coal particles were swollen, and all the solvent was retained in the coal matrix. The solvent swollen coal was abbreviated by 'coal name-solvent" such as TC-Tet, MW-CL, and so on. To estimate the contribution of pyrolysis of the solvent to the product yields from the solvent-treated coal, a char produced under high-temperature pyrolysis of Taiheiyo coal, which was confirmed not to be pyrolyzed further, was also treated by each solvent in the same manner as was employed to prepare the solvent-treated coal. The amount of each solvent in the char was adjusted to be nearly equal to that of the solvent-treated coal. This sample was pyrolyzed to obtain the yield of each component from the solvent. The swelling ratio of the solvent-treated coal was measured by the volumetric technique* described in a previous paper.' (1)Miura, K.; Mae, K.; Asaoka, S.;Yoshimura, T.; Hashimoto, K. Energy Fuels 1991,5, 340-346. (2) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 936-938.
0887-0624/91/2505-08~3~02.50/0 0 1991 American Chemical Society
Miura et al.
804 Energy & Fuels, Vol. 5, No. 6,1991
coal .__.
Morwell (MW) Jacobsranch (JR) Highvale (HV) Taiheiyo (TC) Decker No. 1 (DK) Illinois No. 6 (IL) Wandoan (WD) Datong (DC) Liddel (LD) Beatrice (BR)
Table I. Properties of Coals proximate analysis, wt % FC VM ash C 48.2 50.3 1.5 67.1 68.3 39.4 11.8 48.8 74.0 35.4 12.0 52.6 11.0 74.5 43.2 45.8 75.2 43.4 6.0 50.6 7.4 77.1 57.8 34.8 78.2 45.7 8.2 46.1 81.5 7.3 59.0 33.7 8.1 83.4 57.4 34.5 5.1 89.8 18.4 76.5
ultimate analysis, wt %, daf H N S 4.9 0.6 0.3 5.5 0.5 0.9 0.2 1.0 4.6 1.3 6.0 0.4 0.4 1.3 5.0 1.5 5.6 3.9 0.9 5.9 0.3 1.0 0.9 4.8 0.6 5.5 2.2 0.7 4.7 1.0
0 27.1 24.8 20.2 18.0 18.5 11.9 14.7 11.8 8.3 3.8
Table 11. Properties of Solvents ultimate anal,
struct parameter solvent C H 0 H/C f. 1.20 0.60 tetralin (Tet) 90.91 9.09 0 76.60 6.38 17.02 1.00 phenol (Ph) 0.91 0.91 1-methylnaphthalene (1-Me) 92.96 7.04 0 CL4 86.62 4.90 3.88 1.18 0.61 OILb 73.90 4.90 13.80 0.79 0.92 wt%
CL: Coal liquids from liquefaction of Morwell coal (liquefaction conditions 430 OC, 15 MPa; boiling point range, 180-350 "C). bOIL: Tar derived from flash hydropyrolysis of Great Greta coal (pyrolyzed in an entrained bed at 700 OC and 7 MPa).
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 O C at the rate of 3000 OC/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, COz, and HzO) and hydrocarbon gases (HCG: C1to CBgaseous compounds, benzene, toluene, and xylene). To analyze HzO, lines connecting the pyrolyzer and the GC were all heated to 150 OC to prevent the condensation of H20. The calibration curve of HzO was constructed by pyrolyzing carefully weighed CuS04.5Hz0. 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.' The product yield of each component during the pyrolysis of the solvent-treated coal was represented based on 100 kg of dry and ash-free (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 Pyrolysis of Solvent. Since we are expecting the transfer of radicals from the solvent decomposed to the coal fragment during the flash pyrolysis, it is essential to know the pyrolysis behavior of each solvent first. Figure 1 shows the product yields obtained at 764 "C from the flash pyrolysis of the solvents contained in the inert char. The tar yield in this figure is defined to include the amount of unreacted solvent, since we could not analyze the tar component. However, we can safely say that more than 90% of the tar is the unreacted solvent. The yields of hydrocarbon gases were small except for CL and 1-Me. It is reported that the C-H bonds of 1-Me are broken at around this temperaturea3p4The coal liquefaction liquid, CL, contains a large amount of alkylnaphthalenes. These (3) Run-Ling, R.; Itoh, H.; Makabe, M.; Ouchi, K. Fuel 1987, 66, 643-653. (4) Graber, W. D.; Huttinger, K. I. Fuel 1982, 61,505-509.
ze l f t
0.7
0
0.5
0
0
0
Tet.
CL
OIL
Ph
1-Me
Solvent F i g u r e 1. Product yields from the flash pyrolysis of solvents.
compounds are expected to produce H', CH3', and so on at this temperature through the decomposition of the alkyl chains. These are the reasons 1-Me and CL produced lots of hydrocarbon gases. The hydrogen yields of 1-Me and CL are also large for the same reasons. The fairly large hydrogen yield of tetralin mainly comes from the dehydrogenation of its hydrogenated ring, judging from the fact that naphthalene was detected in the pyrolysis products. A similar dehydrogenation mechanism may contribute to the large hydrogen yield of CL, since its f a value is almost same as that of tetralin as given in Table 11. Little hydrogen was produced from both phenol and OIL. Water was produced from only CL, Ph, and OIL, because tetralin and l-Me contain no oxygen. CO and COZ were produced from only CL and Ph. Their yields were smaller than H 2 0 yield. Effect of Solvent Type on Product Distribution. When two coals, MW and TC, swollen by tetralin were pyrolyzed in the previous paper,' a distinct difference was
Energy & Fuels, Vol. 5, No. 6, 1991 805
Pyrolysis of Solvent -Swollen Coal
pvomolbn coal
HCG
120
0 Rawcoal 2.3 2.4
80
=
40
=
0
. "1
Y
0 T-
I
co
m
c
5.1 5.1
0
.-
Q)
* c. 0
a 0 2 0
U
a
0
U
n
L.
10
n 0 0.4 0.2
0.0 Tet.
CL
OIL
Ph
1-Me
Solvent
Tet.
CL
OIL
Ph
1-Me
Solvent
Figure 2. Effect of solvent type on the swelling ratio and the product distribution during the flash pyrolysis of solvent-treated Morwell coal (MW): (a, left) total volatile matter, tar, water, and hydrogen; (b, right) hydrocarbon gases, CO, and COP
found in the tar yield at higher pyrolysis temperatures. For TC, the tar yield reached a maximum, which was ca. 10% larger than the yield of the raw TC coal, at around 764 "C, and then tended to decrease with further increase of pyrolysis temperature. On the other hand, the tar yield increased continuously with the increase of pyrolysis temperature for MW, and at 920 "C it reached 42 wt %, which is 2.3 times larger than the yield of the raw coal. This suggests that interaction between the coal and the solvent is largely dependent on the coal-solvent combination. Then we examined how the solvent-coal combination affects the pyrolysis behavior of the solvent-swollen coal. Figure 2, a and b, shows the swelling ratio and the product distributions during the flash pyrolysis at 764 "C of MW swollen by five different solvents. Each yield is represented on the basis of 100 kg of dry and ash free (daf) coal. The black bars represent the yields obtained by pyrolyzing the solvent-swollen coals. The white bars are the yields for the raw coal, and the dotted bars are those for the solvents. Comparison between the black bar and the sum of the white and dotted bars gives the effect of the solvent swelling. No difference means no interaction between the coal and the solvent during the pyrolysis. The solvent content was kept at 0.67 kg/kg of coal for all the solvent-swollencoals to compare the effect of the solvents on the same basis. The swelling ratio was around 1.3, and almost same in all cases except for MW-Ph. The large swelling ratio of MW-Ph is probably due to stronger interaction between phenol and the coal. For all solvent-swollencoals, the yields of CO and C02 were almost exactly equal to those of the raw coal. This means that the reactions forming CO and COBare not affected by the presence of solvents. For MW-Tet., the solvent swelling brought about the increase in the total volatile matter and tar, and the increases were compensated by the decrease in the yields of
char, Ha,and H20 as stated in the previous paper. Almost the same result was obtained for MW-CL. This was expected because CL has almost the same fa value as tetralin. Similar results were obtained for MW-1-Me, though the decrease in hydrogen yield was not so significant. The swelling ratio of MW-OIL was comparable to that of MW-Tet or MW-CL, but all the pyrolysis yields of MW-OIL were almost the same as the sums of the yields of raw coal and OIL. This is because little hydrogen was produced from OIL as shown in Figure 1. This indicates that the swelling of coal is essential but not sufficient for the proposed method to be valid. Solvents must also produce reactive radicals during the pyrolysis. The coal swollen by phenol, MW-Ph, showed the largest total volatile matter and tar yield of the five solventswollen coals. The tar yield increased by more than 26 kg/100 kg of coal. For MW-Ph, however, the hydrogen yield decreased little. The great increase in the tar yield was compensated by the decrease in the yields of char and H20. For this solvent-swollen coal, therefore, transfer of radicals related to H20 formation is judged to be the main contribution to the increases in total volatile matter and tar. Summarizing the effects of the solvents on the validity of the proposed pyrolysis method, we can say that tetralin, CL, and phenol are very effective solvents. The net effects of the solvents are the decreases in char, hydrogen, and/or H20 yields, resulting in the increase in the tar yield. The yields of CO and C02changed little. Neither the yield nor the composition of hydrocarbon gases changed significantly. Thus the proposed method is effective to increase the tar yield, but we must mention that the tar produced from the solvent-swollen coal consists of heavier components and contains more oxygen than the tar produced from the raw coal. This is because the increase of tar yield is compensated mainly by the decrease of the char and the
Miura et al.
806 Energy & Fuels, Vol. 5, No.6,1991
I
1
16
MW
JR
HV TC Coal
DK WD
IL
DC
LD
BR
Type
I
MW JR
HV TC Coal
DK WD
IL
DC
LD
BR
Type
Figure 3. Effect of coal type on the swelling ratio and the product distribution during the flash pyrolysis of solvent-treated c o d (a, left) total volatile matter, tar, water, and hydrogen; (b, right) hydrocarbon gases, CO, and COP
H20 yields on the weight basis. Effect of Coal Type on Product Distribution. To examine the effect of coal type, 10 kinds of coal swollen by tetralin were pyrolyzed. Figure 3, a and b, shows the swelling ratio and the pyrolysis yields at 764 "C of each solvent-treated coal. The yields are compared with the sums of the yields of the raw coal and tetralin as was done in Figure 2, a and b. There is a slight difference in the tetralin content among the coals, though the coal and tetralin all were mixed in the same ratio to prepare the tetralin-swollen coals. This is because some tetralin evaporates from the tetralin-swollen coals when they are stored at room temperature. Although the degree was different from coal to coal, the yields of all the tetralin-swollen coals showed similar trends as compared with the sum of the yields of the raw coal and tetralin. The total volatile matter and the tar yield increased, the H2 and the H20 yields decreased, and the CO, the COP and the hydrocarbon gas yields changed little. Presumed Pyrolysis Mechanism of Solvent-Swollen Coal. The net effect of solvent swelling on the product distribution is to increase the tar yield by decreasing the char, hydrogen, and/or H20 yields in all the cases. The CO and the C 0 2 yields changed little. Neither the yield nor the composition of the hydrocarbon gas changed. These results indicate that the increase of tar yield is brought about at least by the following two mechanisms: (1)Hydrogen radical produced from the solvent is transferred to coal fragments to increase the tar yield. Other radicals such as CH3' produced from the solvent were judged not to be utilized to increase the tar yield under the operating conditions employed in this work. (2) Radicals that form H20 during the pyrolysis of raw coal
are utilized to increase the tar yield when the solvent swollen coal is pyrolyzed. Thus, focusing on the yields of hydrogen and H20,we examined the pyrolysis mechanism of the solvent swollen coal. From the hydrogen yields shown in Figure 2a, we can say that almost all hydrogen radicals deriving from the solvent were utilized to increase the tar yield for MW-Tet and MW-CL. In the case of MW-1-Me, only a portion of the radicals were used. These results show that the timing as well as the amount of the hydrogen produced from the solvent are very important so that the hydrogen radicals may be utilized effectively. Tetralin and CL probably produce H radicals timingly to stabilize the coal fragments as tar. The timing of the hydrogen radical formation of 1-Me did not fit the formation of the coal fragments. It is clear from Figure 3a that hydrogen radicals produced from tetralin are effectively utilized to increase the tar yield for MW, JR,DK, and LD, but are not so effective for other coals. The difference in the degree of hydrogen utilization was not explained solely by the difference of coal rank or the swelling ratio. In addition to these properties, many other factors including reactivity of coal fragments and the strength of the coal-solvent interaction will affect the degree of hydrogen utilization. We cannot examine each effect separately. At the moment we will simply say that the hydrogen acceptability of coal, or the ability of hydrogen subtraction of coal, also largely affects the amount of hydrogen radicals utilized. The H20 yield of MW swollen by the five solvents decreased significantly except for MW-OIL as shown in Figure 2a. Since neither tetralin nor 1-Me produces H20 from itself, the decrease of H 2 0 yield for MW-Tet. and MW-1-Me comes from the radicals stabilized as H20 in
Pyrolysis of Solvent-Swollen Coal
Energy & Fuels, Vol. 5, No. 6,1991 807
the absence of the solvent. The H 2 0 yields of several tetralin swollen coals (JR, HV, TC, DK, WD,and IL) also decreased significantly as shown in Figure 3a. This again shows that the radicals stabilized as H 2 0 in the absence of tetralin were utilized to increase the tar yield. At 764 "C water is mainly produced through phenolic condensation reaction such a s 5 7 6 Ar-OH
+ Ar'-OH
-
-
Ar-OH + Af-OH
Ar-O-At
+
A -Af
(-1 U
+ '/*H20
H20
+
Swollen by tetralln at la0 "C
.a:, Y
:j
'12H2 (2)
8
+ H'
,
1
*L
:I
U
0
60
70
(4)
The H radicals produced will be utilized to stabilize the tar precursors through intramolecular hydrogen transfer. The above discussion partly clarifies the two mechanisms mentioned above. The solvent swelling of coal brings about effective hydrogen transfer from the solvent to the coal. This is realized by contact at the molecular level between the solvent and the functional groups of coal (mechanism 1). The solvent swelling of coal simultaneously suppresses the cross-linking reactions such as (1)and (2) through the breakage of some hydrogen bondings, resulting in other radical-transfer reactions which will contribute to stabilize the coal fragment (mechanism 2). The first mechanism is controlled by the hydrogen donability of the solvent and the hydrogen acceptability of the coal. The second mechanism will be closely related to the number of hydrogen bonds broken. Estimation of t h e Amount of Hydrogen Radicals Utilized. Then we tried to estimate the amount of radicals (5) Gavalae, G. R. Coal Pyrolysis; Elaevier: New York, 1982; pp 34-38. (6) Poutama, M. L.; Dyer, C. W. J. Org. Chem. 1981,47,3367-3377. (7) Painter, P. C.; Sobkowiak, M.; Youtcheff, J. Fuel 1987,66,973-978. (8)Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729-4735. (9) Larsen, J. W.; Shawver, S.Energy Fuels 1990, 4, 74-77. (10) Solomon, P. R.; Serio, M. A.; Despade, G. V.; Kroo, E. Energy
Fuels 1990.4.42-54. ~ .,.-, . -- - -(11) Klein, M. T.;Virk, P. S.Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1980,!?5, 18Ck187. (12) MacMillen, D. F.; Ogier, W. C.; Ross, D. S. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1981,26, 181-188.
80
90
100
c%
(3)
where - - - represents hydrogen bonding. This configuration is easily broken at around 500 OC to produce H 2 0 by reactions 1 and 2. On the basis of the above mechanism, the presence of the solvent is judged to have suppressed reactions such as (1)and (2). Solomon et al. also reported that the crosslinking reaction is suppressed by the presence of the solvent.1° In the swollen coal a part of the hydrogen bonds represented by eq 3 will be broken through the solvent swelling to produce free Ar-OH and Ar'-OH.*7s It is very difficult to know the fate of the free Ar-OH and A i-OH produced. They may form other hydrogen bondings with the solvent. Then the pyrolysis behavior of the Ar-OH will change significantly. Anyway other radical-transfer reactions will occur when the Ar-OH is pyrolyzed. This effect was detected by the decrease in the H20 yield and the increase in the tar yield. The next reaction is one of the possible reactions of the pyrolysis of the A P O H . ~ J ~ J ~
Ar=O
soo0 , 0 0 ,
4
I
-
0,
P =
H
Ar-OH
,
(1)
where Ar and Ar' represents segments of coal molecule. In lower rank coals Ar-OH and Ar'-OH form hydrogen bondings such as7+ Ar4-H--O-Ar'
0
Figure 4. Relationship between the amount of radical transferred and C% of coal. Hydrogen radical transferred from solvent to coal by mechanism 1 (upper);radicals transferred by mechanism 2 (lower). Swollen by tetralin at 100 "C Pyrolyzedat 764 "C
0
0
0
t 0
0
....(li"..+...... ......................... ........................
0.1
0.2
0.3
0.4
(-1 Figure 5. Correlation of the amount of radicals transferred by mechanism 2, RH,2,with the O/C ratio of coal. OIC
transferred during the pyrolysis by virtue of the presence of the solvent. Correlation of that amount with some coal property will give a clearer insight into the radical acceptability of the coal. The net effect of the solvent during the pyrolysis of the solvent-swollen coal is to increase the tar yield by the transfer of H radical from solvent to coal fragments (mechanism 1) and the suppression of the cross-linking reactions such as (1)and (2), which will contribute to the increase in tar yield by other radical-transfer reactions (mechanism 2) as discussed earlier. The difference between the sum of the H2 yields of the raw coal and the solvent and the H2yield of the solvent swollen coal is equal to twice the amount of H radical transferred by mechanism 1, RH.1. It is difficult to identify the radical species transferred by mechanism 2. However, the decrease of H 2 0yield surely corresponds to the amount of the radicals transferred. Then we estimated the amount of radicals transferred by mechanism 2, RH,2,by assuming that reaction 4 is the main reaction. The difference between the sum of the H20 yields of the raw coal and the solvent and H 2 0 yield of the solvent-swollen coal will correspond to the amount twice as large as RH,2as eqs 3 and 4 indicate.
Miura et al.
808 Energy & Fuels, Vol. 5, No. 6, 1991
is examined in Figure 6. Fairly good correlation was obtained irrespective of pyrolysis temperature and coalsolvent combinations, indicating the validity of the two mechanisms. This also indicates that the proposed pyrolysis method increases the tar yield by stabilizing fairly large coal fragments which would become char in the absence of solvent through the effective hydrogen transfer from solvent to coal and/or the intramolecular radical transfer.
0
4 Ryl+&,2
8
12
16
(mol /kg-cwl)
Figure 6. Correlation of the increase in tar yield realized by the proposed method with the amounts of radicals transferred by the two mechanisms, RH,l + RH,*.
Figure 4 shows the values of R H , 1 and R H , 2 calculated from Figure 3 against C% of the coals. Both values tend to decrease with the increase in C ?% , though they scatter largely. We can qualitatively say that the hydrogen transfer by mechanism 2 is more significant than that by mechanism 1 for lower rank coals, and that the contribution of mechanism 2 is negligible for higher rank coals. Correlation of RH,I and R H 3 with the swelling ratio was not successful, though some trends were found. Mechanism 2 is closely related to the amount of Ar-OH existing in the coals. RH,2 corresponds to the number of cross-linking reactions suppressed by mechanism 2. Then we correlated R H z with the atomic ratio of oxygen to carbon of the coals, O/C, as seen in Figure 5. R H , 2 increased almost proportionally with the increase of O/C when O/C is larger than ca. 0.1. In this region the value of R H 2 was estimated to be 1/3 to 1 / 2 of the amount of Ar-Ok in the coals. In the coals of small O/C ratio few AI-OH exist, which is well reflected by the zero RH,2 values for these coals. Thus, we could clarify that the amount of hydrogen radicals transferred by mechanism 2 is well correlated with the O/C ratio which is closely related to the amount of Ar-OH in the coal. This supports the validity of the discussion as regards mechanism 2. If the increase in the tar yield was brought about by the two mechanisms, there should be some relation between the amount of tar increase by solvent swelling and the s u m of RH,1 and RH,2 for all the cd-solvent combinations. This
Conclusion The validity and the mechanism of a new pyrolysis method which we proposed for increasing the tar yield were examined for several coal-solvent combinations. A coal liquefaction oil and phenol in addition to tetralin were very effective to increase the tar yield. Lower rank coals seemed to be more suitable than higher rank coals for the proposed method to be effective. In all cases when the tar yield increased, the hydrogen yield and/or the H20 yield decreased without affecting the yields of CO, C02and hydrocarbon gases. This suggested that at least two mechanisms exist to increase the tar yield one is the transfer of hydrogen radical from the solvent to coal (mechanism l),and the other is the coal intramolecular radical transfer which is realized by the suppression of the phenolic condensation reaction through the swelling of coal (mechanism 2). The amounts of radicals transferred by both mechanisms were estimated from the hydrogen and H 2 0 yields and are represented by RH,l and RH,2, respectively. The value of RH,1 was affected by both the hydrogen donability of the solvent and the hydrogen acceptability of coal. RH,2correlated well with the O/C ratio of coal, indicating that mechanism 2 is closely related to the breakage of -OH associated hydrogen bondings. The increase in tar yield realized by the proposed method was well correlated with the sum of RH,1 and RH,2for all coal-solvent combinations, which supports the validity of the presumed mechanisms. Acknowledgment. This work was financially supported by the Ministry of Education, Culture and Science of Japan through the Grant-in-Aid on Priority-Area Research (Grant No. 62603014, 63603014, and 01603012). The authors express their sincere thanks to Kobe Steel Co. Ltd., Osaka Gas Ltd., and NBCL Co. for supplying the samples. Registry No. Tet, 119-64-2; Ph, 108-95-2; 1-Me, 90-12-0.
