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Energy & Fuels 1994,8, 978-983
Heat Treatment of Coals in Hydrogen-DonatingSolvents at Temperatures as Low as 175-300 "C Jianli Shen and Masashi Iino* Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1 -1, Aoba-ku, Sendai 980, Japan Received February 22, 1994. Revised Manuscript Received May 10, 1994'
Heat treatments of Zao Zhuang and Upper Freeport coals were carried out in several hydrogendonating solvents at 175-300 "C under N2 atmosphere. Retrogressive reaction of the coal, i.e., the mixed solvent soluble fraction, compared decrease of carbon disulfide-N-methyl-2-pyrrolidinone with that originally existed in the raw coal, was observed in tetralin even at temperature as low as which are much stronger 175"C, while in 9,lO-dihydroanthracene or 1,4,5,8,9,10-hexahydroanthracene, hydrogen donors than tetralin, dissolution reactions, Le., the increase of the mixed solvent soluble fraction, was observed at 175-300 "C. The quantity of hydrogen transferred from the solvent to the coal was found to be well correlated with the extent of the dissolution reactions, and spin concentration in coal components decreased after the dissolution reactions, suggesting that the hydrogen donation to coal radicals from the solvent occurs during the heat treatment. The origin of the participating radicals, i.e., whether they are formed by the scission of weak covalent bonds or are indigenous radicals activated by heat, and the mechanisms of the retrogressive and dissolution reactions are discussed.
Introduction The mechanisms of retrogressive reactions of coal liquefaction and pyrolysis and how to prevent them efficiently have received increased Although the details of retrogressive reactions are still obscure, hydrogen-donating reactions from liquefaction solvents and/or hydrogen to coal fragments are known to be a key reaction for the suppression of the retrogressive reactions in coal liquefaction. Especially,the radicals formed during the initial stages of coal liquefaction must be stabilized efficiently to prevent retrogressive reactions which could lead to the formation of refractory, high-molecular-weight species. Carbon disulfide-N-methyl-2-pyrrolidinone(CSrNMP) mixed solvent (1:l by volume) has been reported to give high extraction yields (40-65%,daf) at room temperature for many bituminous coals.- No significant occurrence of reactions of the solvents with the coals, which would result in an increase of the extraction yields, is indicated for this extraction, suggesting that the extracts obtained originally existed in the coals.7 We also found that the extracts obtained with the CS2-NMP mixed solvent include a considerable amount of the very heavy extract component which is not soluble in THF or pyridine, but soluble in the mixed solvent.8 Abstract published in Adoance ACS Abstracts, June 1, 1994. (1)Suuberg, E.M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987,I, 30.5-m. - .- - - -. (2)Berkowitz, N.;Calderon, J.;Liron, A. Fuel 1988,67,626-631,10171019,1139-1142. (3)Solomon, P. R.;Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990,4,42-54. (4)McMillen, D. F.;Malhotra, R. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1992,37(I),385-392. (5)Saini, A. K.; Coleman, M. M.; Song, c.; Schobert, H.€3. Energy Fuels 1993,7,328-330. (6)Iino, M.; Kumagai, J.; Ito, 0. J . Fuel SOC.Jpn. 1986,64,210-212. (7)Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel, 1988,67, 1639-1647. (8)Iino, M.; Takanohashi, T., Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989,68,1588-1593. 9
We have used the mixed solvent as an extraction solvent for the heat treatment products of the bituminous coals at 300-450 "C in tetralin (TET) or naphthalene (NAP) without a ~ a t a l y s t .The ~ results obtained showed that retrogressive reactions occur more readily for the heaviest fraction, i.e., THF-insoluble, the CSpNMP mixed solvent soluble fraction (TIMS), than for other lighter fractions such as benzene-insoluble, THF-soluble (preasphaltene) and n-hexane-insoluble, benzene-soluble (asphaltene) fractions. The heat treatment of TIMS itself, which was obtained from the extraction of Zao Zhuang raw coal with the CS2-NMP mixed solvent at room temperature, in several solvents at 1W350 "C, showed that the retrogressive reactions, Le., the conversion of TIMS to the mixed solvent insoluble fraction (MI),was suppressed by adding a strong hydrogen-donating solvent such as 9,lO-dihydroanthracene (DHA)or 1,4,5,8,9,10-hexahydroanthracene (HHA). It was also found that, as more hydrogen was transferred from the solvent to TIMS, the extent of the retrogressive reactions decreased.1° In this study, heat treatments of raw Zao Zhuang and Upper Freeport coals were carried out at 175-300 "C in various solvents such as TET, NAP, 1-methylnaphthalene (MNA),DHA, and HHA. The coal was found to undergo either retrogressive reactions or dissolution reactions even at temDeratures as low as 175 "C. deDending on the hydrogen donatability of the solvent used. The mechanisms for these reactions are discussed. Experimental Section Materials. The samples used in this study were Zao Zhuang (China) and Upper Freeport (the Premium Coal Sample Bank at Argonne National Laboratory, USA) coals. Zao Zhumg cod (9)Wei, X.; Shen, 3.; Takanohashi, T.; Iino, M. Energy Fuels 1989,3, 575-579. (10)Shen, J.; Takanohashi, T.; Iino, M. Energy Fuels 1992,6, 854858.
0887-0624/94/2508-0978$04.50/00 1994 American Chemical Society
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Heat Treatment of Coal Table 1. Ultimate, Proximate and Maceral Analyses of the Coals and the Solvent proximate maceral ultimate analysis analysis' analysis (wt % , db) (~01% , dmm0 (wt %, dan C H N S Ob VM ash FC VC Id Le Zao 86.9 5.1 1.5 1.6 4.9 28.6 7.4 64.0 85.8 13.7 0.5 Zhuang Upper 86.2 5.1 1.9 2.2 4.6 28.2 13.1 58.7 89.7 9.5 0.8 Freeport LRS 88.5 9.7 0.5 0.1 1.2 0.0 - - a Data of Zao Zhuang coal from ref 15. By difference. Vitrinites, including pseudovitrinite. d Inertinites. e Liptinites. f Liquefaction recycle solvent.
-
Y
500 .
2
5b
300
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200
e 2 5 0 " C -E+175"C
.-j
w
3
2
CSZ-NMP mixed solvent CSz-NMP soluble
THF soluble
THF insoluble (TIMS)
CrS)
Figure 2. Extraction and fractionation procedures for the heat treatment mixture.
-0- 300 "C 400
v
z
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100 0 0
10
20
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Reaction Time ( min )
Figure 1. Temperature-time profile for the heat treatment mixture in the autoclave. was ground to -60 mesh (-250 wm). Both of the coals are dried in vacuum at 107 "C to constant weight (2-3 h). The ultimate, proximate, and maceral analyses are given in Table 1. Reagent grade solvents were used without further purification. Heat treatment solvents used were TET, NAP, MNA, DHA, HHA, and liquefaction recycle solvent (LRS). LRS was obtained from the 1 t/day liquefaction plant operated by NED0 (New Energy and Development Organization, Japan) with Wyoming coal; 5-70% of LRS was distilled at 240-338 "C. The ultimate analysis of LRS (fa = 0.50) is also given in Table 1. Heat Treatment. Heat treatment of the coal was performed in a 50-mL magnetically stirred autoclave at 175,250,and 300 "C, respectively; 1 g of the coal and 5 g of the solvent were charged to the autoclave. The autoclave was purged with nitrogen for 3-5 times to remove remaining air in the autoclave and then was pressurized with nitrogen to 5.0 MPa a t room temperature. The autoclave was placed in a preheated molten tin bath for the treatment at 250 and 300 "C or a silicon oil bath for that at 175 "C. These baths brought the heat treatment mixture quickly to the desired reaction temperature in 1 or 2 min. After the heat treatment, the autoclave was removed from the bath and cooled in ice water bath to room temperature for 1-2 min. A typical profile of heat treatment temperature versus time is shown in Figure 1. Pressure increases in the autoclave after the treatment at 300 "C were all less than 0.5 atm, indicating little formation of gaseous products. GC analysis showed that main product is carbon dioxide. The heat treatment mixture was then fractionated into the CSz-NMP mixed solvent insoluble fraction (MI) and soluble fraction (MS), and then the MS was further fractionated into TIMS and TS (and the solvent), at room temperature with the mixed solvent and THF under ultrasonic irradiation, as shown in Figure 2. The quantity of MI and TIMS was determined after drying overnight under vacuum at 80 "C and that of TS was calculated by difference, Le., 100 - MI - TIMS. The detailed extraction procedures are described elsewhere.9JO The dissolution yield was defined here as the sum of TIMS and TS. Reproducibility of the yield of each fractions were within 10% for duplicate runs of selected reactions, like that reported the previous paper.10
Analyses of the Products and Solvents. The spin concentration of the raw coal and ita heat treatment products was determined by ESR spectra (Varian E-4 spectrometer) by using l,l-diphenyl-2-picryl-hydrazyl(DPPH) as a standard. The reproducibility of the data was within 10% for duplicate runs and measurements. The quantity of hydrogen transferred from DHA and HHA to the coal was determined from GC analysis. After the solvent fractionation of the heat treatment mixture with the CS2-NMP mixed solvent and THF, respectively, a part of the THF-soluble fraction (TS),which includes the solvent and its derivatives, was extracted with benzene to separate them from benzene-insoluble coal products. GC of benzene solution obtained showed that the main solvent-related products are DHA and anthracene (ANT), and HHA, tetrahydroanthracene (THA), and octahydroanthracene (OHA), for the heat treatment in DHA and HHA, respectively. The quantity (mol) of hydrogen transferred in DHA (0.028mol, 5 g) was determined from the area ratio (S) of ANT/ DHA, using the equation of 0.028S/(1+ S). In this calculation, the area ratio of ANT/DHA is assumed to be equal to their mole ratio. For the heat treatment in HHA (0.027mol, 5 g), hydrogen transferred was similarly determined using the equation 0.027(SI- Sz)/(l + SI + SZ). Here, SI and SZare the area ratios of THA/HHA and OHA/HHA, respectively. Although we did not check the reproducibility in this study, the previous study10 showed that reproducibility within 5% was obtained for similar heat treatments of TIMS.
Results Figure 3 shows the fraction distribution after the heat treatment of Zao Zhuang coal in 5 solvents at 175,250, and 300 "C, together with that for the raw coal, which was obtained from the fractionation of the extract of the raw coal with the CS2-NMP mixed solvent at room temperature. A t 175 and 250 "C in NAP and TET, MI increased and TIMS decreased, compared with those for the raw coal
(MI,37.O%;TIMS,49.2%;TS,13.8%;wt%,daf),while, in a strong hydrogen-donating solvent such as DHA and HHA, MI decreased and TS increased by the treatment. Contrary to the results at 175 and 250 "C, MI decreased and TS increased in TET at 300 "C. The same tendency was observed, to a lesser extent, in NAP than TET. In DHA, HHA and LRS the decrease of MI and the increase of TS was further enhanced at 300 "C. Especially,in HHA, TS increased to 42.15% from 13.8% ofthe raw coal and the dissolution yield; i.e., TIMS + TS increased to 83.4% from 63.0% of the raw coal. The dissolution yield and TS are the order of HHA > DHA > LRS. Figure 4 shows that Upper Freeport coal gave similar results as those for Zao Zhuang coal, except that MI did not decreaes at 300 "C with MNA and TET, and the dissolution yields are 86.8
980 Energy & Fuels, Vol. 8, No. 4, 1994
Shen and Iino 100
MI
(a)
0 TS
TIMS
80 60 40
20 0 100
80 60 40
20 0
Raw NAP "ET
DHA HHA LRS
Raw MNA "ET
coal
Figure 3. Fraction distribution after the heat treatmentof Zao Zhuang coal in NAP, TET, DHA, HHA, and LRS at 175 "C (a), 250 "C (b),and 300 "C (c) for 1 h, together with that for the raw coal.
and 90.6 % at 300 "C in DHA and HHA, respectively,which are higher than that for Zao Zhuang coal. Figures 5 and 6 show the dependence of the fraction distribution on the heat treatment time for Zao Zhuang coal in HHA and for Upper Freeport coal in DHA at 250 "C,respectively. From the figures it can be seen that for the reaction times up to 1h MI decreases and TS increases with increasingreaction time, but little change occurs over 1h. Figures 7 and 8 show the dependence of hydrogen transferred from the solvent to the coal and the dissolution yield on the heat treatment (1h) temperature. The figures indicate that the increase of the dissolutionyield is closely related to the hydrogen transferred. The figures also indicate that hydrogen transferred is always higher for HHA than that for DHA, indicating that HHA is a stronger hydrogen-donatingsolventthan DHA. Figure 9 alsoshows that the quantity of hydrogen transferred is well correlated with the dissolution yield when the heat treatment time was varied. Table 2 shows that the spin concentration of the acetoneinsoluble fraction (AI) of the products, which includes MI and ash, after the heat treatment of Zao Zhuang coal in HHA is smaller than that of the raw coal, and it decreased with increasing heat treatment temperature. The spin concentration of the acetone-soluble fraction (AS) could not be measured due to the existence of HHA; the yields of AI are higher than 90% (based on the raw coal), and spin concentration of AS is known to be much lower than AI: suggesting that spin concentrations of the whole
DHA HHA LRS
coal
Figure4. Fractiondistributionafter the heattreatment of Upper Freeport coal in MNA, TET, DHA, HHA, and LRS at 175 "C (a), 250 "C (b), and 300 "C (c) for 1 h, together with that for the raw coal.
3 &
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o Raw Coal
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Figure 5. Fraction distribution after the heat treatment of Zao Zhuang coal in HHA at 250 "Cfor 1-360 min, together with that for the raw coal.
products after the heat treatment are lower than that of the raw coal.
Discussion The result that MI increased and TIMS decreased at 175 and 250 "C in TET indicates the occurrence of retrogressive reactions in this solvent. Although TET is often used as a hydrogen-donatingsolvent in the study of
Energy & Fuels, Vol. 8, No. 4, 1994 981
Heat Treatment of Coal 100
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#
e
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Figure 6. Fractiondistributionafter the heat treatmentof Upper Freeport coal in DHA at 250 "C for 1-360 min, together with that for the raw coal.
8o 60
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i
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Heat Treatment Temperature ( "C) Figure 8. Plots of dissolution yield (0)and H2 transferred (A) with heattreatment temperature for Upper Freeport coal in DHA (a) and HHA (b) for 1 h.
0.020
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Figure 7. Plots of dissolution yield (0)and H2 transferred (A) with heat treatment temperature for Zao Zhuang coal in DHA (a) and HHA (b) for 1 h.
coal liquefaction above 350 OC,11J2 this result indicates that TET is a poor hydrogen donor like NAP or MNA at the low temperatures,while, in a much stronger hydrogendonating solvent than TET, i.e., DHA or HHA,l0 the hydrogen donation from the solvent to the coal occurs even at 175 and 250 "C, resulting in the dissolution reactions, i.e., the decrease of MI and the increase of TS. These dissolution reactions occur more easily when temperature rises to 300OC. Bedell and Curtis13has shown that HHA gave higher coal conversion than DHA, when Western Kentucky No. 9 coal was heated in those solvents in nitrogen atmosphere,being in agreementwith the result obtained here. When temperature rises to 300 "C, the hydrogen donatability of TET and the reactivity of the coal radicals may increase. So, the retrogressive reaction in TET was suppressed. NAP, which has no donatable hydrogens, gave no suppression of the retrogressive reaction even at 300 O C . MNA also gave a similar result as NAP. It should be (11) Neavel, R. C. Fuel, 1976,55,237-242. (12) McMillen, D. F.; Malhotra, R.; Chang, S.; Ogier, W. C.; Nigenda, S. E.; Fleming, R. H. Fuel 1987,66,1611-1620. (13) Bedell, M. W.; Curtis, C. W. Energy Fuels 1991,5,469-476.
40L, I
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E
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0.000 60 120 180 240 300 360 420 Heat Treatment Time ( min )
Figure 9. Plots of dissolutionyield (0)and H2 transferred (A) with heat treatment time for Zao Zhuang coal in HHA (a) and Upper Freeport coal in DHA (b) at 250 "C. Table 2. Spin Concentrations of Zao Zhuang Coal and Acetone-Insoluble Fraction (AI) of Coal after the Heat Treatment in HHA yielda spin concentration heat treatment (wt % ,db) (1019 spin/g, daf) (HHA, 1h) 2.34 raw cal AI 175 "C 96.3 2.04 94.4 1.68 AI 250 "C AI 300 "C 89.8 0.91 a
Based on the raw coal.
noticed that LRS is a better solvent than TET in this treatment. It is probably due to the fact that LRS contains a lot of hydrogenated condensed aromatic compounds which can easily donate hydrogens. The close relationship between the dissolutionyield and the quantity of hydrogen transferred from the solvent to
Shen and Iino
982 Energy &Fuels, Vol. 8, No. 4, 1994
Scheme 1. Reaction Scheme for the Heat Treatment of the Coal 6o
ki
I
Scheme 2. Possible Reaction Paths for the Heat Treatment of the Coal in DHA and HHA MI
k,
TIMS
ki
TS
the coal and the decrease of spin concentration after the heat treatment indicates that coal radicals are responsible for the results observed here. Thus, when the radicals are stabilized by the hydrogen donation from the solvent, the dissolution occurs, while in a poor hydrogen-donating solvent the radicals hardly donate hydrogen and retrogressive reactions such as the addition to aromatic rings of coal network and coupling reactions between them may occur. All the possible reactions in the heat treatment of coals are shown in Scheme 1. kl, kp, and k3 represent the rate constant for dissolution, while k-1, k-2, and k-8 represent those for retrogressive reactions. Our preliminary experiment showed that the heat treatment of TS from Zao Zhuang raw coal in DHA at 175 and 250 "C for 1 h gave negligible amounts of TIMS and MI, suggesting that k-2 and k-3 can be negligible. When MI or TIMS was heat treated in DHA and HHA, TS was formed for both the cases, but TS from MI was much smaller than that from TIMS, suggesting that kp is larger than k3. When the heat treatment is carried out in DHA or HHA, in which little retrogressive reactions occur, k-1 can also be negligible. If we neglect the rate constants estimated above, we can get Scheme 2. From Scheme 2, we can consider the sum of the increase of TS and the decrease of MI, Le., ATS + (-AMI) as a net conversion of the dissolution reaction, where ATS = TSp - TSR, AMI = MIp - MIR, and the subscripts R and P denote the raw coal and after the heat treatment, respectively. The plot of the net dissolution reaction of Zao Zhuang and Upper Freeport coals versus the amount of hydrogen transferred from DHA or HHA to the coal is shown in Figure 10. Figure 10 shows that the net dissolution reaction is well correlated with the amount of hydrogen transferred during the reaction, suggesting again that hydrogen donation to coal radicals from the solvent is a key step for the dissolution reaction. It is not clear that the radicals, which are responsible for the dissolution and retrogressive reactions, are formed by the scission of weak covalent bonds and/or indigenous radicals activated by heat at 175-300 "C. The fact that the extent of the dissolution reactions increased with the heat treatment time up to 1h suggests that the dissolution reactions are rather slow. So, the radicals may be formed by slow bond scissions, not indigenous ones, since the thermal activation of the latter radicals seems to be a rapid process. There are several kinds of weak covalent bonds which break at these low temperatures. For example, it is well-known that as the number of phenyl groups replacing hydrogen atoms in ethane increases, its central C-C bond strength markedly decreases, and pentaphe-
0
) I ,
0,000
0.005
0.010
0.015
0.020
Hz Transferred ( g / g coal, daf ) Figure 10. Plot of net dissolution yield versus Hz transferred and Upper Freeport duringthe heat treatments of Zao Zhuang (0) (+) coals in DHA and HHA at 175-300 "C.
nylethane readily dissociatesinto the triphenylmethyl and diphenylmethyl radicals below 100 It may be reasonable to consider that MI and TIMS are more susceptible to the dissolution and retrogressive reactions than TS, since they have much more easily broken bonds due to higher content of phenyl groups, Le., higher fa! and higher concentration of the radicals than the lighter TS.8Jo Stronger bonds than considered above are possibly broken at these low temperatures by solvent-mediated hydrogenolysis proposed by Malhotra and M~Mi1len.l~ The maceral distribution of coals (Table 1)shows that the quantity of inertinite group are 13.7 and 9.5% for Zao Zhuang and Upper Freeport coals, respectively. Among the macerals of inertinite group, fusinite, macrinite, and micrinite were found to be hardly extracted with the CS2NMP mixed solvent (1:l by volume).16 Their total quantities are 4.4'6 and 8.0% and the highest dissolution yields obtained in this study are 83.4 and 90.6% in HHA at 300 "C for 1 h for Zao Zhuang and Upper Freeport coals, respectively. The heat treatment of Upper Freeport coal for a longer time, 3 h, also gave a similar yield of 90.8 % . This suggests that the yield obtained for Upper Freeport coal may be the maximum dissolution yield which can be obtained at the low temperature heat treatments. Finally, it should be noted that the results above described were obtained by the use of the CS2-NMP mixed solvent as an extraction solvent for the reaction mixture. If THF is used as an extraction solvent instead of the mixed solvent, we can only see small change of TS by the heat treatments carried out here. Conclusions
Heat treatments of Zao Zhuang and Upper Freeport coals were carried out in several hydrogen-donating (14) Bachmann, W. E.; Osborn, C. J. Org. Chem. 1940,5, 29-39. (15)Malhotra, R.; McMillen, D. F. Energy Fuels 1993, 7 , 227-233. (16) Takanohashi, T.; Ohkawa, T.; Yanagida, T.; Iino, M. Fuel 1993, 72, 51-55.
Heat Treatment of Coal
solvents at 175-300 O C under N2 atmosphere. Retrogressive reaction of the coal was observed in TET at temperatures as low as 175-250 OC, while, in DHA or HHA, which are much stronger hydrogen donors than TET, the coal underwent dissolution reactions at 175-300 "C. The quantity of hydrogen transferred from the solvents to coal was found to be well correlated with the degree of the dissolution reactions, and spin concentration in coal components decreased after the dissolution reactions, suggesting the hydrogen donation to coal radicals, which
Energy & Fuels, Vol. 8, No. 4, 1994 983
are formed by weak covalent bond scission and/or are indigenous radicals activated by heat, in this heat treatment.
Acknowledgment. The authors thank NED0 (New EnergyandDevelopmentOrganization,Japan) andNippon Coal Oil Co. for providing the LRS sample used in this research. This work was supported by a Grant-in-Aid for Energy Research from Ministry of Education, Scienceand Culture, Japan.
