Energy & Fuels 1994,8, 607-612
607
Tire Liquefaction and Its Effect on Coal Liquefaction Z. Liu, J. W. Zondlo, and D. B. Dadyburjor' Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506-6101 Received May 24, 1993. Revised Manuscript Received January 31, 1994@
Liquefaction experiments were run using ground tire rubber and ground coal, individually and in combination. The runs, in a batch tubing-bomb reactor, were carried out in the presence and absence of tetralin as asolvent, and at high hydrogen pressures. The tire conversion is temperature dependent. At 400 "C in 30 min, almost a11 of the organic matter in the tire is converted to hexane-soluble material, in the absence of solvent and Hz. The conversion of the tire-coal mixture is greater than the combination of the individual conversions. The incremental conversion,i.e., the difference between the conversion of the mixture and the combination of the individual conversions, increases with the tire/coal ratio. The incremental yield of hexane-solubles, defined analogously, shows trends that are similar but smaller in magnitude. The effect of the tire appears similar to that of the solvent tetralin, even though the tetralin is more effective in converting coal. The differences between the two may be due to the different radical-stabilizing capabilities between tire products and tetralin.
Introduction The chemical composition of the solvent used in direct coal liquefaction is an important factor in determining the quantity and quality of the products formed. Solvents function by furnishing caps to the radicals formed during the depolymerization of coal upon liquefaction, thus preventing the radicals from recombining to form unwanted heavy products. These caps may be either hydrogen or other radicals. A solvent may also function by dispersing coal radicals, thereby preventing retrogressive reactions. A solvent should therefore have radicaldonor, hydrogen-donor, or hydrogen-transfer abilities and should physically dissolve coal pr0ducts.l Recently, McMillen et ala2proposed that, besides being stabilizing agents, radicals from the solvent are also active reagents that alter the cleavage of strong bonds by hydrogenolysis. Many organic solvents have been studied for coal liquefaction. Tetralin is known to be a good hydrogen donor and is easily dehydrogenated to naphthalene via:
Although tetralin can significantly increase coal liquefac~ tion yields, such a solvent is relatively e ~ p e n s i v e .The use of inexpensive solvents, such as fluid catalytic cracking bottoms and heavy oils, for coal liquefaction has been r e p ~ r t e d .In~ practice, recycle loops are proposed, whereby a fraction of the liquefied product may be hydrogenated and returned to the reactor as the solvent.
* T o whom correspondence should be addressed. *Abstract published in Advance ACS Abstracts, March 15,1994. (1)Whitehurat,D. D.; Mitchell, T. D.; Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980. (2)McMillen, D. F.; Malhotra, R.; Tse, D. S. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36 (2),498. (3)Schindler, H. D. 'Coal Liquefaction-A Research and Development Vol. Needs Assessment"; Final Report, Contract DE-AC01-87ER30110, 2,pp 4-27 e t seq. (4)Yan,T. Y.;Espenscheid, W. Fuel Process. Technol. 1983,7,121. Moschopedis, S. E.;Hawkins, R. W.; Fryer, J. F. Fuel 1980,59,647.
Discarded automobile tires are waste products in the United States. In the 1980s,the US generated 220 million used tires per year, and 75 % of them were sent to landfiis.5 Today, however, landfilling is not an environmentally acceptable method of tire disposal. A clue to alternative methods may be found in the composition of tire rubber, a typical value for which is shown in Table 1. Note that the tires contain more than 60% of organic materials and are rich in hydrogen. Hence, waste tires can be used as a fuel source or pyrolyzed to produce oils and carbon black, or reclaimed,6-10but none of these options are economical in the USa5Alternative methods for the reuse of discarded tires include supercritical extraction for oi1"J2 and pyrolysis to produce activated Results from tire pyrolysis in a nitrogen atmosphere indicate6that the primary products are styrene, butadiene, and other alkanes. Further reactions involving styrene lead to benzene, toluene, naphthalene, and phenanthrene. Increasing the temperature increases the conversion and the yield to oils and gas, at least upto 600 "C. The oilrange products show a decrease in aliphatic fraction and a concomitant increase in aromatics with increasing temperature. At higher temperatures, no appreciable change is observed. The maximum conversion is 65%, ( 5 )Paul, J. Kirk-Othmer Encyclopedia of Chemical Technology,3rd ed.; Wiley: New York, 1982;Vol. 19,p 1002. (6) Williams, P. T.; Beater, S.; Taylor, D. T. Fuel 1990,69, 1474. (7)Larsen, J. W.; Chang, B. Rubber Sci. Technol. 1976,49,1120. (8)Schulman, B. L.,White, P. A. Prepr., ACS 175th Symp. Solid Wastes Residues Conuersion, Anaheim, March 13-1 7,1987 1987. (9)Chambers, C.; Larsen, J. W.; Li, W.; Wlesen, B. Znd. Eng. Chem. Process Des. Deu. 1984,23, 648. (10)Sitting, M. Organic and Polymer Waste RecloimingEncyclopedia; Noyes Data Corp.: Park Ridge, NJ, 1981. (11)Funazukuri, T.; Takanaahi, T.; Wakao, N. J . Chem. Eng. Jpn. 1987,20, 23. (12)Funazukuri, T.; Ogasawara, S.; Wakao, N.; Smith, J. M. J . Chem. Eng. Jpn. 1986, 18,455. (13)Ogasawara, S.;Kuroda, M.; Wakao, N. Znd. Eng. Chem.Res. 1987, 26, 2552. (14)Merchant, A. A.; Petrich, M. A. Chem. Eng. Commun. 1992,118, 251. (15)Merchant, A. A.; Petrich, M. A. AZChE J . 1993,39,1370.
0887-0624/94/2508-0607$04.50/0 0 1994 American Chemical Society
608 Energy & Fuels, Vol. 8,No. 3, 1994
Table 1. Composition of Rubber Compounding composition wt% 62.1 styrene-butadiene rubber 31.0 carbon black 1.9 extender oil 1.9 zinc oxide 1.2 stearic acid 1.1 sulfur 0.7 accelerator total 99.9
with 55% oil and 10% gas. Gas-phase products include mainly CO, C02, H2, and C1-Cd hydrocarbons. The assertion of Williams et al.6 that only the styrenebutadiene rubber (SBR) reacts is reasonable; see Table 1. SBR is a polymer with the repeating unit
Hence, this is also consistent with the formation of the light hydrocarbons during pyrolysis. Earlier, Larson and Chang7 investigated the products from tire pyrolysis with and without molten salts as catalysts. At temperatures greater than 450 "C, the uncatalyzed pyrolysis results in similar conversions and yields to those of Williams et al.6 The presence of molten metal chloride catalysts at 500 "C results in higher oil yields and lower gas yields. The gas product after catalyzed pyrolysis shows significant amounts of C1 to Cq hydrocarbons. CO and C02 were not reported. The liquid products of catalyzed pyrolysis contain mainly paraffins, while aromatics are reported to form the majority of liquid products in uncatalyzed pyrolysis. It has been suggested16J7that the high hydrogen content and aromaticity of the rubber (see Table 1)can be used in the coliquefaction of tires and coal to improve coal conversion. Farcasiu and Smith17report that, at 425 "C, lo00 psig of Ha (at room temperature) and 60 min of batch reaction time, 100% of the tire rubber can be converted to heptane-soluble materials. The conversion of Illinois No. 6 coal (to methylene chloride-soluble materials) increases by 10% when the tire rubber-to-coal ratio is increased from 0.74 to 1.4. The heptane-soluble materials produced from the coprocessingcan be used as an aromatic oil for tire manufacture or as transportation fuel. The presence of carbon black (of surface area 138 m2/g) in the reaction mixture appears to improve the quality and quantity of the products. The total number of tires discarded in the US corresponds to only two million tons per year, comparable to the amount of coal expected to be used annually for a single commercial-scalecoal liquefaction plant, five million tons of coal per year. Even so, indications are that tire material can be useful at least as a recycle solvent for coal liquefaction, while simultaneously eliminating a serious environmental problem. Therefore, the coliquefaction process deserves further study for detailed information. In this paper, we report on the effect of temperature and Hz pressure on tire liquefaction. The effects of reaction parameters on the coliquefaction of tire and coal are presented. The effect of tire-derived liquids on coal conversions are discussed. (16)Farcasiu, M.Chemtech 1993, 23, 22. (17)Farcasiu, M.;Smith, C. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1992, 37 (l), 472.
Liu et al. Table 2. Proximate Analysis of Coal and Tire sample tire coal moisture (% ,as received) 0.31 1.8 ash ( % ,dry basis) 5.4 6.3 volatile matter (% ,d e ) 67 49 fixed carbon ( 5%, d e ) 33 51 daf: dry and ash-free basis.
Experimental Method A discarded Goodyear "Invicta" tire was used in the study. Samples from the front and side walls of the tire were combined and ground into particles with a diameter of less than 1mm. The particles were water-washed and dried, and the metal parts were removed by float/sink and magnetic separation. Proximate analysis of the tire sample thus prepared is shown in Table 2. The results are consistent with those in Table 1;comparing the percentages of carbon black (Table 1)and fixed carbon (Table 2), it is reasonable to assume that most of the fixed carbon in the tire consists of carbon black. The coal used is a high-volatile-A bituminous sample (DECS-6) from the Blind Canyon Seam. Its proximate analysis is also shown in Table 2. The coal was ground to -60 mesh under nitrogen. A tubing bomb with a volume of 57 cm3was used as the reactor. The feedstock (tire alone, coal alone, or tire and coal, 6 gin total) was loaded into the reactor, with or without tetralin. The reactor was then purged, pressurized with hydrogen to the required pressure (typically lo00 psig at room temperature, i.e., cold), and sealed. The reaction started when the reactor was immersed vertically in a preheated fluidized sand bath. Reaction temperatures were from 350 to 400 OC. The reactor was agitated vertically by a crank mechanism at 300 rpm for all the experiments. Previous experiments indicated that the reaction temperature can be reached in 2 min at these conditions. The batch time for the reaction was typically 30 min, although a few runs lasted 1 h. After the reaction, the reactor was quenched in water to room temperature. The gas phase of the reactor was vented, and the solid and liquid products were washed from the reactor with tetrahydrofuran (THF). The THF-solubles were extracted in a Soxhlet unit for 3 days, after which the THF-insoluble portion (TI) was dried and weighed. Liquefaction conversion ( X ) was calculated on a dry, ash-free basis (daf) using coal
+ tire - H,O - TI
(%) = coal + tire - H,O
- ash x 100
(1)
(It should be noted that there is a water contribution and an ash contribution from both coal and tire). THF was then removed from the THF-soluble portion by a rotary evaporator. The THFsoluble portion was refluxed in hexane for 2 h and then filtered. The solid portion on the filter was dried and weighed. This is the hexane-insolubleportion (HI),whichcontains the asphaltenes and preasphaltenes. The percentage of asphaltenes and preasphaltenes ( A ) was calculated from: HI
A (%) = coal
+ tire - H,O - ash x 100
The yield to oil + gas (OG) was calculated by difference:
OG(%)=X-A (3) In a few cases, the gases present after the reactor was quenched were expanded toa fixed volume, and an aliquot was then injected into a gas chromatograph equipped with a thermal conductivity detector. The amount of product gas (less hydrogen) and its composition could thus be determined. In most cases, duplicate liquefaction reactions were carried out. The experimental errors were typically less than two percentage points.
Results and Discussion Tire Liquefaction. In this subsection and the next, we present results for tire alone and for coal alone under
Tire Liquefaction Table 3. Conversion and Yield for Liquefaction of Tire Alone (lo00 psig of Hr cold, 300 rpm) 30 min 60 min tetralin no temp(OC) R P 350 375 400 400 conversion ( % ) 7.9 43 60 66 64 7.5 42 60 65 63 oil+gas ( % ) yesb temp ("C) R P 350 375 400 conversion ('7%) 8.4 63 64 67 oil+ gas ('7%) 8.4 63 64 65 a R T room temperature. Tetralin/tire= 9.7/6.0(g/g,raw basis).
liquefaction conditions. The effect of adding tetralin is also noted in each case. These results allow us to compare base-case results with the results of coal-tire coliquefaction. Liquefaction results when the feedstock is tire alone are listed in Table 3. At lo00 psig of H2 (cold) and 30 min reaction time, the tire conversion is strongly dependent on reaction temperature and the presence of tetralin. When the tire material at room temperature is simply extracted with THF and hexane, the "conversion" and the "OG yield" (as defined above) are both approximately 8 % ;i.e., about 8% of the unreacted tire material is soluble in THF and hexane. The addition of tetralin has no effect in this case. On heating to higher temperatures, tire particles appear to keep their physical shape but are seen to consist of carbon-black particles surrounded by oily liquid. With increasing temperature for a reaction time of 30 min, X and OG increase. The presence of tetralin increases these values still further, but this effect is more significant at 350 "C than at higher temperatures. At 400 "C, neither the addition of tetralin nor an increase of reaction time (to 60 min) serves to increase liquefaction conversion or OG yield over 66%. This indicates that a maximum reachable conversion is about 66% for this tire. The same value was obtained for pyrolysis in a nitrogen atmosphere by Williams et a1.6 Note that this maximum conversion is approximately equal to the volatile matter (VM) content of the tire. Further, virtually all of the conversion is to oil + gas. (The small difference between X and OG in Table 3 probably results from the transfer of fine carbon black particles through the Soxhlet extraction thimbles to the extractives; the fine particles are then collected on a filter as HI material.) This suggests that all of the organic portion of the tire, other than carbon black, is converted into oil + gas at 400 "C in 30 min and that a solvent is not needed under these conditions. This 100% conversion of the rubber itself is consistent with the results reported by Farcasiu and Smith,17obtained at a higher temperature and longer reaction time. The result is also consistent with the assertion of Williams et that only the SBR material is converted during pyrolysis. One explanation of the temperature dependence of the solvent effect may be as follows. It is reasonable to expect that heating-up is required to break the long-chainpolymer present in the tire. It may further be that tire liquefaction follows a thermal cracking mechanism and/or a hydrogenation mechanism depending upon the reaction temperature. A t a relatively low temperature (350 "C, for example), hydrogenation is a necessary step before bond breakage, and therefore the hydrogen-donor solvent tetralin helps the conversion. At a higher temperature (400"C, for example), however, thermal cracking may be the predominant step, and the presence of hydrogen-donor
Energy & Fuels, Vol. 8, No. 3, 1994 609
t
400
1000
1600
2200
2800
3400
4000
Wavenumber, c m - l
Figure 1. Fourier-transforminfrared spectrum for THF-insoluble residue after liquefaction of tire alone.
solvent and/or hydrogen would have little effect. Free radicals would be expected to be generated at these higher temperatures, and these would then combine to form aliphatic products. The existence of aliphatic free radicals containing a few carbon atoms is consistent with the results of Williams et a1.6 In addition, the aromatization of the styrene component into multiple-ring aromatics may generate hydrogen free radicals. We return later to the role of tire-generated free radicals. Analysis of the gas-phase products of tire liquefaction at the highest temperature in hydrogen results in the following,on a hydrogen-free basis: CO, 2.5% ;CHI, 7.2% ; C02,8.2 5% ;C2 hydrocarbons, 34% ;C3 hydrocarbons, 14% ; C4 hydrocarbons, 22%; and C5 hydrocarbons, 12%. Compared to the pyrolysis results,6p7there is a significant contribution from the C5 hydrocarbons as well. Furthermore, the total gas yield is approximately 6.2%, significantly smaller than the value of 10% during pyrolysis. Hence the presence of hydrogen in the gas phase influences the gas products formed during the reaction of the tire material at the higher temperatures and may influence the fate of the free radicals formed under these conditions. A typical FTIR spectrum for the unreacted (THFinsoluble) material from tire liquefaction a t 400"Cis shown in Figure 1. Peaks for aromatic hydrogen would be expected at wave numbers of 3000-3100 cm-l and around 800 cm-l, while peaks of aliphatic hydrogen would be expected at 2800-3000 cm-l and around 1450 cm-1. No detectable amount of aromatic or aliphatic hydrogen is present in Figure 1. Thus, Figure 1 confirms that all organic matter, except carbon black, in the tire can be liquefied when the maximum conversion is reached. Since tire rubber can be fully liquefied at 400 "Cwithout a solvent, it is of interest to determine the effect of H2 pressure at this condition. Figure 2shows the results when the hydrogen partial pressure is increased from 0 psi to 1000 psig (cold). The results of 0 psi hydrogen were obtained under 15 psi (cold) nitrogen. X and OG for tire liquefaction are practically unchanged under these conditions. The nitrogen results are consistent with those of Williams et al.6 and by Larson and Chang.7 All these results imply that, with the possible (and minor) exception of the gas yield, the so-called tire liquefaction is actually tire pyrolysis, at least at temperatures of 400 "C or higher. We reiterate that virtually all the tire conversion is to oil + gas. The virtual absence of asphaltenic products and the absence of effects from either a hydrogen-donor solvent or vapor-phase Hz at 400 "C (again, with the possible exception of the gas yield) indicate that radicals
Energy & Fuels, Vol. 8,No. 3, 1994
-1
0
conversion
A
oil-tgas yield
0
a
A
u
conversion oil+gas
I
I
A
t
2
4
A
40 4 0
1200
Hydrogen Pressure [psig. cold]
u
0
Tire/Coal [g/g]
Figure 2. Effect of hydrogen pressure on tire-aloneliquefaction at 400 'C, 300 rpm, 30 min. Table 4. Conversion and Yield for Liquefaction of Coal Alone (400 'C, 1000 psig of Ha cold, 300 rpm, 30 min) coal coal + tetralina conversion ( % ) 37 81 asphaltenes ( % ) 15 55
oil+gas(%) 22 a Tetralin/coal = 9.7/6.0 [g/g, raw basis]
t
c
600
26
formed during tire liquefaction, when they recombine, do not form asphaltenes. Hence these radicals must be small in size. Such small-sized radicals may be useful in coal liquefaction in order to stabilize the coal radicals. This suggests that tire material can be added to coal to serve as a solvent for coal liquefaction. Of course, tire material is more diverse than pure tetralin; hence the tire as asolvent can be expected to have a more complex role than does tetralin. In order to determine without ambiguity the effect of tire liquefaction on coal liquefaction yield, standard conditions for further study were set at 400 "C, lo00 psig of H2 (cold), and 30 min reaction time. These are the mildest conditions at which all of the tire (except the carbon black) is completelyconverted to oil + gas products. Hence the fate of the tire feed can be unambiguously known under these standard conditions. Coal Liquefaction Coal liquefaction results at the standard conditions mentioned above are listed in Table 4. In contrast to the results from tire liquefaction, a significant portion of the coal liquefaction products is insoluble in hexane, i.e., is asphaltenic. The gas yield is 5.7 % , approximately equal to that from the tire. Analysis, again on a hydrogen-free basis,yields: CO, 2.5%; CH4,9%;CO2,15%;C2,4O%;C3, 14%; C4,6%; and CS, 12%. Compared to the tire-alone analysis, the gas from coal contains more CO2 and C2 at the expense of Cq. Further, the solvent tetralin significantly improves the liquefaction conversion, X. However, the increase is mainly in asphaltenes, although OG also increases. As noted earlier, one of the main functions of the solvent is to furnish hydrogen to the radicals formed from coal depolymerization,thus preventing them from recombining. Upon the addition of solvent tetralin, the increased yield of asphaltene (relativeto OG) implies that radicals formed during coal liquefaction are mainly in the asphaltenic size range. Hence these radicals are larger than tire-generated radicals, which are in the oil range.
Figure 3. Effect of tire/coal ratio (raw weight basis) on coliquefaction at 400 'C, lo00 psig of HZ(cold),300 rpm, 30 min.
Coliquefaction of Tire and Coal Effect of Tire. Based on the arguments above, we examined the products obtained when coal and tire are coliquefied in the absence of tetralin. From these runs, values of X and OG were calculated using eqs 1-3, as before. However, it is useful to describe the results on a slightly different basis. Since the liquefaction results of the tire under the standard conditions are not affected by tetralin, H2 pressure, or reaction time of greater than 0.5 h, it is reasonable to assume that the conversion and oil gas yield of the tire under the standard conditions in the presence of coal are the same as those values without coal. Hence, the values of conversion and oil + gas yield for the coal in the presence of the tire, X,*and OG,* respectively, can be obtained by subtracting the proportional conversion and oil + gas yield of tire from the overall conversion and oil + gas yield. Thus,
+
where XCtand Xt are daf-basis conversions for the tirecoal mixture and for the tire alone, and wt is the (daf) weight fraction of tire in the tire-coal mixture. An analogous relation holds for OG,*. Values of X,*and OG,* mirror values of the corresponding incremental values, viz., the difference between the mixture values and combinations of the individual values for coal and tire. Addition of an equal mass of tire to coal results in an increase in coal conversion at the standard conditions from X, = 37% (in the absence of the tire, Table 4)to X,* = 50%, and relatively little change in the oil + gas yield from the coal. (Note that these numbers are based on the coal alone, the tire contribution having already been as described above.) The gas yield subtracted off for Xc*, from the coal, again subtracting off the tire contribution, is approximately 1.6% ,much less than that from the coal alone. The individual analyses are CO, 2%; CHr, 33%; C02, 52%; C2, 6 % ; C3, 9%; C4, 1%, and Cg, -3%. Compared to the gas from the coal-alone liquefaction, the gas here contains approximately the same amount of CO, CH4, and C02, but much smaller amounts of C2+ hydrocarbons. Clearly, the higher hydrocarbons formed by liquefaction of the tire alone are being converted into liquid-phase products during the liquefaction of coal. When the relative amount of tire in the mixture is increased, the effect on the values of X,*and OG,* can be seen in Figure 3. As noted above, the conversion of coal in the tire-coal mixture increases with an increase of
Energy &Fuels, Vol. 8, No. 3, 1994 611
id
0 A
0
conversion oiltgas
Conversion,
A
O i I + G a s Yield
-
U
c .-
eW
;" -
5 0
-
0
A
1000
2000
Liquefaction Pressure [psig]
Figure 5. Effect of hydrogen pressure on coal liquefaction at 400 O C , 300 rpm, 30 min, and a tetralin/coal ratio of 9.7/6.0.
(18) Ibrahim,M. M.;Seehra,M.S.Prepr.Pap.-Am. Chem. Soc.,Diu. Fuel Chem. 1993, 38 (3), 841.
(cold). At higher Hz pressures, the conversion and oil + gas yield are relatively insensitive to the pressure. As shown in Figure 5, the same behavior can be found qualitatively for coal liquefaction in the absence of the tire but in the presence of tetralin, at the same reaction conditions. Clearly,coal liquefaction needs some hydrogen from the gas phase, whether the solvent is tetralin or liquefied tire. In either case, a H2 pressure of greater than 500 psig is not necessary with this particular coal under these reaction conditions. Further, the asymptotic values of conversion at Hz pressures greater than 500 psig are different in Figure 4 (coal plus tire) and Figure 5 (coal plus tetralin). This suggests a difference between the actions of tetralin and liquefied tire. Of course, we are using 3.0 g of tire, as compared to 9.7 g of tetralin. However, from Figure 3, we can estimate the conversion using 14.7 g of tire, corresponding to 9.7 g of tire-based liquid. The value of this conversion is not much greater than 68%, compared with a conversion of as high as 80% using 9.7 g of tetralin from Figure 5. Hence the difference is probably not related just to the different amounts of tire and tetralin used. Concentrating, as above, on the radical-donating role of the solvent, the tire-based radical is larger than the tetralinbased hydrogen radical. Hence, it is reasonable to believe that the capping process involving the former is less efficient than that involving the latter. This could account for the higher total conversion using tetralin relative to that using tire. Effect of Added Tetralin to Coal/Tire Coliquefaction. The effects of tetralin on coal liquefaction and coaltire coliquefaction are shown in Figure 6. For the points representing coliquefaction, the tire/coal ratio is maintained at unity. The results listed in Table 4 for coal liquefaction (without tire) are also shown in Figure 6 for comparison. Addition of tetralin increases the conversion of coal, with or without tire. Hence the relationship between tetralin and coal conversion is not affected by the presence of tire. Consistent with the arguments made above, this may be because tetralin is a better radical donor than the (liquefied) tire for coalliquefaction, and the effect of tetralin overwhelms the effect of the tire. This result also seems to indicate that the solids in the tire (carbon black, zinc oxide, sulfur, etc.) apparently do not have significant catalytic activity. Otherwise, the presence of the tire would have increased the conversion and oil + gas yield of the coal. Use of Liquefied Tire as a Solvent. To investigate further the effect of the tire solids on the coliquefaction,
Liu et al.
0 0 A
0
A
L
9
Open symbols Coal only Filled symbols Coal+Tire
conversion oll+gas
100.-
L
V
Tetralin/Coal [g/g]
Figure 6. Effect of tetralin on coal liquefaction (open symbols) and coliquefaction (filled symbols) at 400 O C , lo00 psig of Hz (cold),300 rpm, 30 min. oils obtained from the liquefaction of tire (alone) were used as the solvent in coal liquefaction, instead of raw tire material. The tire-oillcoal ratio was 213, equivalent to a tire/coal ratio of 313. Under the standard conditions, the values of conversion and oil gas yield were found to be no different from the corresponding values during the coliquefaction of (equal parts of) tire and coal as indicated in Figure 3. Hence, it would appear that the tire solids play no catalytic role during tire-coal coliquefaction. This would seem to contradict the results of Farcasiu and Smith,17 who compared conversions and yields of coal plus the individual tire components without carbon black with coal plus components including 138 m2/g carbon black. However, there is a large variety of carbon blacks with different particle sizes, shapes, and surface areas. Perhaps the characteristics of the carbon black used by Farcasiu and Smith are different from those of the carbon black which was present in our tire sample. Other Possible Roles for Tire in Liquefaction. The effects of tire materials on coal liquefaction under various reaction conditions have been ascribed above to the radicaldonating capability of the SBR component of the tire. However, as mentioned earlier, a solvent can have several roles in coal liquefaction. Here we briefly discuss the importance of the remaining roles in the light of our experimental results. One possible role is that of a donor of hydrogen radicals. However, preliminary FTIR spectroscopy on the tire products indicates that the aromatic content is very low. The literature6 further indicates that hydrogenation of the tire at 400 "C yields mainly single-ringaromatics, which can be expected to have little if any hydrogen-donor capability. This is confirmed by experiments that we ran using the tire and phenanthrene at liquefaction conditions. With or without vapor-phase hydrogen, there was negligible incremental conversionof 9,lO-dihydrophenanthrene (DHP) by the tire. Hence, the hydrogen-donor capability of the tire is negligible, or at least less than that of DHP. Another possible role is to disperse coal radicals to prevent retrogressive reactions. This is of course possible,
+
but it is not clear how the dispersal of coal radicals alone could explain the conversion and yield effects noted above. Further, because of the physical attributes of the particles after liquefaction, described earlier, flow is difficult, so dispersal will not be significant. A nonsolvent role for the tire is the catalytic effect of carbon black or other solids present in the tire. However, the addition of tire to coal and tetralin does not improve the conversion or yield, relative to coal plus tetralin. Further, the effect of adding tire to coal is the same as the effect of adding the products of tire liquefaction to the coal. Finally, the presence of carbon black does not increase the conversion of phenanthrene to DHP under the standard reaction conditions. Hence carbon black probably has no catalytic effect, a t least in the present case. The present work cannot unambiguously assign the role of the tire to only the generation and donation of radicals. However, all the available data, including FTIR spectra, conversions, yields, and vapor composition described above, are all consistent with this hypothesis.
Conclusions The products from tire liquefaction are entirely hexanesoluble materials. The tire conversion is dependent on temperature and is improved by addition of a solvent for reaction at temperatures below 400 "C for 30 min. At 400 "C, the presence of tetralin or hydrogen has no effect on the conversion of the tire. The tire may be used in coal liquefaction. The effect of the tire is qualitatively similar to that of tetralin, in that the total conversion and the asphaltenic fraction of the products is increased. However, the effect of the tire is less than that of tetralin. Under the present conditions, the coal liquefaction conversion increases with an increase of tire-to-coal ratio upto about 2. The pressure of hydrogen affects tirelcoal coliquefaction up to a H2 pressure of approximately 500 psig. At higher pressures, the liquefaction results are insensitive to H2 pressure. Solids from the tire during the coliquefaction do not appear to increase coal liquefaction conversion and oil + gas yield. These data are consistent with the hypothesis that the SBR component of the tire generates small aliphatic free radicals; these radicals cap coal-based radicals to decrease retrogressive reactions, to increase the overall coal conversion (THF-soluble material), and to increase the yield of the asphaltenic fraction (hexane-insoluble material).
Acknowledgment. The work was conducted under US. Department of Energy Contract No. DE-FC2290PC90029 under the Cooperative Agreement to the Consortium for Fossil Fuel Liquefaction Science. The authors gratefully acknowledgethis support. The authors also thank Mr. D. Tian, J. Zheng and Dr. J. P. Wann for their help in the early stages of this work, J. Yang for FTIR analysis, and Prof. P. Stansberry, Prof. B. Zhong, and Dr. R. Sharma for valuable discussions.
