Energy &Fuels 1994, 8, 1353-1359
1353
Flash Copyrolysis of Coal and Polyolefin Jun-ichiro Hayashi, Hirotaka Mizuta, Katsuki Kusakabe, and Shigeharu Morooka* Department of Chemical Science and Technology, Kyushu University, Fukuoka 812, Japan Received May 23, 1994. Revised Manuscript Received August 15, 1994@
Pulverized Yallourn and Taiheiyo coals were mixed with a fine powder of homo-polypropylene, low-density polyethylene or high-density polyethylene. Then the mixture was heat-treated above the melting point of the polyolefin, and composite particles prepared were subjected to flash copyrolysis in a Curie-point pyrolyzer. The copyrolysis decreased total char yield and increased total tar yield for all combinations of coals and polyolefins in comparison with the sum of corresponding yields that were obtained by separate pyrolysis of each component. The promotion of tar evolution was well explained by the transfer of hydrogen and hydrocarbon radicals from polyolefin to coal and by the suppression of cross-linking formation in the coal. These reactions were confirmed from the decrease in yields of hydrogen, paraffinic and olefinic hydrocarbon gases, and inorganic gases. The increment of tar yield was dependent on the temperature of copyrolysis and the coal-polyolefin combination and was well correlated by the decrement of gas yield on a molar basis. Flash pyrolysis was also performed in an entrained-flow pyrolyzer, where secondary pyrolysis in the gas phase occurred. Increase in yield of light aromatics was characteristic of the secondary pyrolysis of initial tar, and unsubstituted aromatics such as benzene and naphthalene were produced more abundantly than substituted ones.
Introduction Coal is composed of natural macromolecules, in which aromatic units are linked by various covalent and noncovalent bonds into a three-dimensional network.' During pyrolysis, bond-breaking and cross-linking reactions proceed simultaneously. To increase liquid yield, radical stabilization should be promoted and crosslinking should be suppressed. However, the hydrogen content in coal is not sufficient to stabilize fragment radicals by themselves. Thus radical stabilizers such as hydrogen or methyl radicals should be supplied to coal at a comparable rate to that of the generation of fragment radicals. Since the primary reaction of flash pyrolysis ends within 10-100 ms,2 the diffusion of radical stabilizers into the matrix is frequently ratecontrolling. Miura et al.3 found that flash pyrolysis of coal preswollen with hydrogen donor solvents increased the yield of total volatile matter and tar by enhancing the hydrogen transfer from solvent to coal. Their result demonstrates the importance of contact between coal and radical source. It is well-known that 0-alkylation of coal is an effective pretreatment to promote the coal degradation in flash p y r o l ~ s i s . ~ The - ~ primary role of the 0alkylation is the removal of hydrogen bonds that induce
* Author to whom all correspondence should be addressed. @Abstractpublished in Advance ACSAbstracts, September 15,1994. (1)van Krevelen, D. W. Coal: Typology-Chemistry-Physics-Constitution; Elsevier: Amsterdam, The Netherlands, 1993;Chapter 25. (2) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987,1 , 138. (3)Miura, K.;Mae, K.;Yoshimura, T.; Masuda, K.; Hashimoto, K. Energy Fuels 1991,5 , 803. (4)Ofosu-Asante, K.;Stock, L. M.; Zabranski, R. F. Fuel 1989,68, 567. ( 5 ) Chatterjee, K.; Stock, L. M.; Zabranski, R. F. Fuel 1989,68,1349. (6)Chu, C. J.;Cannon, S. A.; Hauge, R. H.; Margrave, J. L. Fuel
1986.65. 1740. (7j Rose, G. R.; Zabranski, R. F.; Stock, L. M.; Huang, C.-b.; Srinivas, V. R.; Tse, K.4. Fuel 1984,63,1339.
low-temperature cross-linking reactions among acidic OH groups. Alkyl groups introduced also behave as donors of hydrogen and hydrocarbon radicals. Hayashi et al.9 investigated the flash pyrolysis of coal impregnated with long-chain paraffinic compounds and found that these compounds effectively increased the yield of total volatile matter and tar. Their results indicate that long alkyl chains physically retained in the coal matrix play dual roles of supplying hydrogen and hydrocarbon radicals and of inhibiting the cross-linking reaction, as seen with chemically incorporated 0-alkyl groups. In the present study, we propose a novel flash pyrolysis of coal, utilizing polyolefins as hydrogen and hydrocarbon radical donors. This process is related to the final disposition of plastic wastes. In spite of many proposals including landfill, combustion, and pyrolysis, the problem becomes more and more serious on a worldwide scale.1° Copyrolysis with coal is one of the most attractive and feasible means of disposing polyolefins. The reasons are as follows: (1)generation of hydrogen radicals or hydrocarbon radicals from polyolefins requires no such high pressure as does molecular hydrogen in hydropyrolysis, and (2) the yield of volatiles from coal is possibly increased. In copyrolysis of different materials, however, we should consider the balance between the decomposition and generation rates of radicals. Wall et al.I1 studied the pyrolysis of polyethylene and polymethylene a t ca. 400 "C. The kinetics of their thermolysis was well explained by random decomposition of linear chains, (8) Ofosu-Asante, K.; Stock, L. M.; Zabranski, R. F. Energy Fuels 1988,2,511. (9)Hayashi, J.-i.;Kawakami, K.; Kusakabe, K.;Morooka, S. Energy Fuels 1993,7 , 1118. (10)Plastic Waste Management: Disposal, Recycling, and Reuse; Mustafa, N., Ed.; Marcel Dekker: New York, 1993. (11)Wall, L.A,; Madorsky, S. L.; Brown, D. W.; S t r a w , S.; Simha, R. J.A m . Chem. SOC.1954,76,3430.
0887-0624/94/2508-1353$04.50/00 1994 American Chemical Society
Hayashi et al.
1354 Energy & Fuels, Vol. 8,No. 6, 1994 Table 1. Elemental Composition (wt %, daf) of Coals and Polyolefins
Yallourn Taiheiyo HDPEb LDPEb HPPb BPpb RPpb
C
H
65.9 74.4 85.5 85.0 85.4 85.6 85.5
4.9 6.2 14.3 14.2 14.2 14.1 14.3
N 0.6 1.3
-
-
-
O+Sa
ash*%
28.7 18.1 0.3 0.7 0.4 0.3 0.2
-
-
and the following rate constant was obtained.
= 1.93 x 10l6exp(-285 kJ mol-'/RT)
x
I I k l n J e c t b nnozzle
Reactor tube
ElecMc furnace
(1)
Recently, more realistic pyrolysis models were proposed, considering the macromolecular structure of coal. The FG-DVC model of Solomon et a1.12 is applicable to the pyrolysis of various coals with a wide spectrum of heating rates. The rate constant of bridge breaking was expressed by
kcoa,/s-' = 0.86 x 1015exp(-230 kJ mol-'/RT)
I Temperaturecontrollc
-K
2.0 7.8
By difference. Samples heat-treated at 350 "C for 0.5 h.
kp&-'
Nitrogengas I
coal pal!
(2)
At 800 "C, kcod is at most 10 times larger than k p ~ , suggesting rapid radical transfer between coal and polyolefins. Hodek13 studied the copyrolysis of a bituminous coal and polyethylene and polystyrene at 400 "C for 3 h. Yield and selectivity of paraffins and ethylbenzene were increased by the copyrolysis, while those of olefins and styrene were decreased. This indicates that hydrogen radicals are transferred from coal to polyethylene and polystyrene. On the contrary, we found that the hydrogen and hydrocarbon radicals were transferred from polyolefins to coal in flash copyrolysis above 700 "C. In the present study we examined the interaction between polyolefins and coals in flash copyrolysis. The primary and secondary pyrolysis reactions were evaluated separately by using a Curie-point pyrolyzer and an entrained-flow pyrolyzer.
Experimental Section Preparation of CoaVPolyolefin Composites. Yallourn brown coal (YL) and Taiheiyo subbituminous coal (TH)were pulverized and sized to 0.074-0.125 mm and dried at 100 "C under vacuum. Polyolefins-highdensity polyethylene(HDPE), homo-polypropylene (HPP), block-polypropylene (BPP), and random polypropylene (RPP)-were supplied as pellets of 1-4 mm in size. Low-density polyethylene (LDPE)was supplied as a powder. They were heat treated at 350 "C for 0.5 h in nitrogen atmosphere under an initial pressure of 1.0 MPa and gradually cooled to room temperature. The solidified samples were pulverized with an alumina mortar mill and sieved to 0.074-0.210 mm. The weight loss during the heat treatment was at most 1 wt % of the initial sample mass. Table 1 summarizes the elemental compositions of the coals and polyolefins used. Pulverized coal and polyolefin were mixed well, the mass ratio of coaVpolyolefin being U0.25. The coallpolyolefin mixture was charged in a stainless-steel tube bomb of ca. 15 mL and was heat-treated at 200 "C for 1h in nitrogen of 1.0 MPa. After cooling to room temperature, the agglomerate was placed on a stainless-steel mesh of 0.210-mm openings and was (12)Solomon, P. R.; Serio, M. A.; Hamblen, D. G.;Yu, Z. Z.; Charpenay, S. Energy Fuels 1988,2,405. (13) Hodek, W.Proc. Znt. Conf. Coal Sci., Newcastle 1991,782.
Kept at 2M'C
\
U
Cold trap
Figure 1. Schematic diagram of entrained-flow pyrolyzer. disintegrated by vibration to pass the mesh. The surface of the compositeparticles was observed with an scanning electron microscope (SEM, Hitachi, S-2300). The coal particles were evenly coated with polyolefin, and the surface morphology was similar for any combination of coal and polyolefin. Char particles prepared by pyrolyzing YL coal at 1000 "C for 1 h were also used as the inert support of polyolefins. It was confirmed that only a small amount of CO whose yield was at most 0.3 wt % was produced from the char at 800 "C. Flash Pyrolysis with Curie-Point Pyrolyzer. Raw coals, polyolefins supported on the inert char, and coalpolyolefin composites were pyrolyzed with a Curie-point pyrolyzer (CPP, JHP-2, Japan Analytical Ind.). As reported by Xu and Tomita,14the secondary pyrolysis of volatiles in the gas phase is negligible in the pyrolyzer. Typically, 1-3 mg of composite particles was tightly wrapped in a ferromagnetic foil (pyrofoil),the Curie point of which was 764 or 920 "C. Then the pyrofoil was set in a quartz tube and heated inductively for 5.0 s at a heating rate of about 3000 "C/s. Gaseous products were introduced directly into an FID-GC and a TCD-GC, while tar was completely condensed in a quartz tube packed densely with quartz wool placed just downstream of the pyrofoil. The pyrolysis was conducted at least five times under the same conditionsto ensure the reliability of data. The reproducibility of the total volatile matter was within 1%based on the initial coal mass, and the mass balance of 98-102% was attained. Flash Pyrolysis with Entrained-Flow Reactor. Figure 1 is a schematic diagram of the entrained-flowreador (EFR). This experiment was carried out to elucidate the effectiveness of copyrolysis under circumstances where the secondary pyrolysis of volatiles in the gas phase proceeded concurrently with the primary pyrolysis in the coal. Particles were charged (14)Xu, W.-C.;Tomita, A. Fuel 1987,66,627.
Energy & Fuels, Vol. 8, No. 6, 1994 1355
Flash Copyrolysis of Coal and Polyolefin in a gas-tight hopper. Then they were entrained by nitrogen flowing at a rate of 25 mLJs into the reactor through a nozzle of 4.0 mm i.d. The feed rate of particles was fixed at 5.0 mg/ s. The reactor tube was made of a SUS 316 tube of 15.0 mm id., and the heated and isothermal regions were respectively 1.0 and 0.85 m in length. Char particles formed were collected with a cyclone. Heavy tar was completelytrapped with a glass filter thimble at 200 "C, and light tar was trapped in a fixed bed of glass beads kept in a dry ice/methanol bath of -70 "C. The light tar was analyzed with an FID-GC afier it was dissolved in n-hexane or methanol. Gaseous products were collected in an impermeable gas bag and were analyzed with gas chromatographs as in the Curie-point pyrolysis. Pyrolytic water was collected in the cold trap along with the light tar and was dissolved in methanol. The concentration of water was determined by Karl Fischer titrimetry. The pyrolysis was carried out at 800 "C in nitrogen at atmospheric pressure and was repeated at least twice under the same conditions. The reproducibility of the total volatile yield was within 0.5 wt % on the basis of initial coal mass, and mass balances were within 98-101%.
Table 2. Product Yield by Flash Pyrolysis of Polyolefins at 784 "C in CPP
product Hz
HPP
yield (g/lOO g of polyolefin) BPP RPP LDPE HDPE
0.77 1.44 0.11
co
c02 HzO
n.d.
CH4 c2H4
CzHfi CiHi C3Hs C4He C4H10 c5
HCGCI-C~) tar(> CS) char
0.99 1.62 1.19 11.04 0.51 3.36 0.13 3.36 22.2 69.8 5.76
0.73 0.25 0.03
0.76 0.28 0.04 n.d. 1.04 1.91 1.21 11.09 0.55 3.46 0.21 3.53 23.0 71.1 4.85
n.d. 0.68 1.56 1.32 10.01 0.51 3.00 0.19 3.53 20.8 73.8 4.48
0.60 0.64 0.11 n.d. 0.65 3.07 0.64 1.94 0.60 1.66 0.35 1.39 10.3 85.0 3.32
0.52 0.62 0.10
n.d. 0.72 3.53 0.75 1.76 0.76 1.70 0.37 1.61 11.2 84.2 3.24
Results and Discussion Interaction between Coal and Polyolefin. The Hildebrand solubility parameters (SP) of polyethylene and polypropylene are respectively 8.0 and 7.4 ~alO.~/ These values are much smaller than the SP of Illinois No. 6 coal, 11.4 ~ a l ~ . ~ / c estimated m l . ~ , by Painter et al.15 Larsen et a1.16 investigated the swelling behavior of Illinois No. 6 coal in nonpolar solvents. The swelling ratio at equilibrium state was highest when the solubility parameter was 9-10 ~ a l ~ . ~ / c mThe ' . ~ coal . was hardly swollen with solvents whose SP value was smaller than 8.0 ca1°,5/cm1,5.This result suggests that molten polyolefins behave as poor solvents, like paraffinic solvents. Previously, we impregnated a paraffinic compound, 1-octadecanol, into Monvell brown c o d g When the impregnation ratio, (mass of compoundMmass of coal), was smaller than 0.2-0.3, the compound was mostly absorbed in intrapores of the coal, and the color of the coal particles remained brown. However, the YL coal turned black after heat treatment with polyolefins. This color change means that molten polyolefins were not completely absorbed into pores. Those left on the outer surface formed a thick layer in the present experiment. Flash Pyrolysis of Polyolefins. Table 2 summarizes the product distributions of polyolefins in the Curie-point pyrolysis a t 764 "C. The most abundant gaseous products from polyethylene and polypropylene were their monomers, ethylene and propylene, respectively. Their yields were, however, 4-10 w t % at most, while the yield of tar, defined as a portion heavier than c6, reached 70-85 wt %. This indicates that pyrolysis fragments escaped from the folded pyrofoil without further decomposition to monomer or dimer gases. The total yield of hydrocarbon gases from polypropylene was a little higher than that from polyethylene, but the difference in their chain configuration did not much affect the product distributions. The hydrogen yield was 0.5-0.8 w t %, the highest among gaseous products evaluated on the molar basis, and 3.5-5% of hydrogen in the initial polyolefins was converted to hydrogen (15) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990,4, 379. (16)Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985,50,
4729.
Char
100
HCG
Tar
H2
50
1
CH4
C2H4
C2H6
C3H6
C3Ha
C4Ha
B T X S I N MN Figure 2. Product yield by flash pyrolysis of polyolefins in entrained-flow pyrolyzer at 800 "C. IOG; inorganic gases (Hz,
CO and COz); HCG, hydrocarbon gases; tar, liquid products heavier than C6; B, benzene; T, toluene; X, xylene; S, styrene; I, indene; N, naphthalene; MN, methylnaphthalene. molecules. As indicated in Table 2, a small amount of CO and COn and virtually no water were produced. Figure 2 shows the pyrolysis products of HPP, HDPE, and LDPE at 800 "C in the entrained-flow reactor. The yields of hydrocarbon gases in the EFR where the secondary pyrolysis proceeded were much higher than those in the CPP. The total yield of monoaromatics produced in the EFR was 4-5 wt %, while the yield of aromatics with larger ring size was much lower.
Hayashi et al.
1366 Energy & Fuels, Vol. 8, No. 6, 1994
an
'20>
r
3 764'C E.
8
2
Ei
eo a
B
1
E
F 0
CH4
C2H4
CPHB
C3He
CaHe
C4Hs
C4Hio
CH4
C2H4
CzHs
C3Hs
C3Ha
C4He
C4Hio
c 8
2
E
. -i f f ul
rr
.-* 0
Figure 3. Comparison of product yield by copyrolysis of YL, and HPP with sum yield by their separate pyrolysis.
radicals generated from HPP were effectively transferred to fragment radicals from YL coal during the primary pyrolysis that was completed within 10- 100 ms. Radicals generated from n coal also abstracted hydrogen atoms of HPP and suppressed the evolution of hydrogen. Ofosu-Asante et al.5 studied the flash pyrolysis of Illinois No. 6 coal chemically modified by 0-alkylation with pentadecyl or octadecyl groups and found that these long alkyl chains behaved as hydrogen donors. In this study, as mentioned in the Experimental Section, polyolefins were less molecularly retained in the coal matrix than alkyl chains by 0-alkylation. Nevertheless, hydrogen was transferred from HPP, the hydrogen donor, to YL coal, the hydrogen acceptor. In the copyrolysis, the evolution of hydrocarbons was also suppressed. The extent of suppression of C2H4, C3H6, and C4H8 hardly changed with temperature but that of CH4 was more significant a t 920 "C than a t 764 "C. At both temperatures the methane yield in the copyrolysis was nearly equal to that of YL coal alone, as seen with the hydrogen gas evolution. Thus, the decrease in the total methane yield at 920 "C implies that methyl radicals formed from HPP in the pyrolysis of HPP alone were stabilized to CH4, while they were consumed in the copyrolysis by the stabilization with radicals formed from YL coal. It may be possible to ascribe the decrease of methane and hydrogen in the copyrolysis to their transfer to polyolefin. Hodek13 studied copyrolysis of bituminous coal and polyethylene at 400 "C and found that hydrogen radicals were transferred from coal to polyethylene. They formed this opinion from the result that olefinic products derived from polyethylene disappeared in the copyrolysis. In the present study, however, the evolution of both paraffinic and olefinic gases was both suppressed by the copyrolysis of YL coal and HPP. Thus it is reasonable to conclude that the radical transfer was directed from HPP to YL coal. In the case of pyrolysis of polypropylene, olefinic gases such as propylene are produced by @-bond scission followed by radical formation via C-C bond scission or abstraction of methyl or methenyl hydrogen. -CH(CH3)-CH2-CH(CH3) -CH(CH3) -+
Copyrolysis in Curie-PointPyrolyzer. Copyrolysis of YL Coal and HPP. Figure 3 shows product distributions obtained by copyrolysis of the YL-HPP composite particles a t 764 and 920 "C in the CPP. The yield of each product is expressed in the unit of mass per 100-hold mass of raw coal on a dry basis. If there is no interaction between coal and polyolefin in the primary pyrolysis, the yield of each product from the copyrolysis should be equal to the sum of the yields obtained by pyrolyses of coal and polyolefin, conducted separately. However, a synergistic effect was recognized for most products. As shown in Figure 3, the copyrolysis of n coal and HPP decreased the char yield by 5-7 g per 100 g of raw coal. The increase in the tar yield was 10-12 g, larger than the decrease of the char, and was comparable to the decrease in the yield of hydrocarbon and inorganic gases (H2, CO, C02, and H20). These results suggest that HPP promoted the pyrolysis of YL coal. The hydrogen yield in the copyrolysis was equivalent to that in the pyrolysis of YL coal alone. Hydrogen
-CH,-CH(CH3)-CH2
-. -CH2
+ CH,=CH(CH,)
(3)
+ CH(CH,)=CH,
(4)
Recombination of chain-end radical with coal is possible as is that between chain-end radicals. -CH2-CH(CH3)-CH2
+ R'
-+
-CH,-CH(CH3)-CH2-R
(5)
where R' indicates a coal radical. Reaction 5 explains the suppression of olefinic hydrocarbons by the copyrolysis. It is, however, difficult to clarify how reaction 5 contributes to the net degradation of the coal. The yield of inorganic gases, CO, COz, and H20, was also decreased by the copyrolysis. Suuberg and Unger,17 (17)Suuberg, E.M.; Unger, P. E. Energy Fuels 1987, 1, 305.
Flash Copyrolysis of Coal and Polyolefin Solomon et al.,18 and Ibarra et al.19 reported that the amount of C02 and H2O was well correlated to the number of cross-links formed in the pyrolysis of subbituminous coals and lignites. Carbon monoxide is generally produced by decomposition of phenolic OH groups.2o Thus, the decrease in yield of inorganic gases strongly suggests that cross-linking among -COOH and -OH groups in YL coal was suppressed by the copyrolysis. All other combinations of coals and polyolefins tested in the present study showed a trend similar to that shown in Figure 3. Miura et al.,3 who conducted the flash pyrolysis of coals preswollen with tetralin, found that the yield of pyrolytic water was much decreased, from 14 to 9 wt %, for Monvell brown coal. Then they concluded that this result was due to the relaxation of hydrogen bonds by the swelling. The polyolefins employed in the present study, however, are typical "poor solvents" for coals and are not expected to swell coals extensively. The suppression of cross-linking reactions in the copyrolysis is, therefore, a result of promoting the degradation of coals. Copyrolysis of T H Coal and HPP. Figure 4 shows the result of the copyrolysis of TH coal and HPP. The pyrolysis conditions are the same as in Figure 3. This combination was not as effective as that of YL coal and HPP. The decrease in char yield and the increase in tar yield were respectively 2-3 and 3-6 g per 100 g of dried raw coal, smaller than those in the copyrolysis of YL coal and HPP. The suppression of inorganic gases was similar t o that in YL-HPP copyrolysis. At 764 "C, the evolution of paraffinic gases, CH4, C2H6, and C3H8, was not suppressed and that of olefinic gases, C2H4, C3H6, and C4H8, was slightly promoted, contrary to the results with the YL-HPP combination. Thus, hydrocarbon radicals such as methyl radical were not transferred from HPP to TH coal at 764 "C, although the yield of these hydrocarbons was decreased at 920 "C. It is not clear how coal species and temperature affected the transfer of hydrogen and hydrocarbon radicals, but the following points are evident: (1)pyrolysis rates of TH coal and HPP are affected by temperature; (2) reactivities of fragment radicals vary with coal species; and (3) contact between coal and polyolefin depends on coalpolyolefin combinations. The sum of hydrogen, inorganic gases (CO, C 0 2 , and H20), and paraffinic and olefinic hydrocarbon gases produced in the copyrolysis was smaller than the sum of gases produced in the separate pyrolyses of coal and polyolefin. Figure 5 shows the difference between them on a molar basis, as functions of gas composition, pyrolysis temperature, and coal-polyolefin combination. The fraction of hydrocarbon gases in the total gas production difference was much smaller than that of hydrogen and inorganic gases for the copyrolysis of YLHDPE and YL-LDPE composites, while it was appreciable for the copyrolysis of YL-HPP composite. Solomon et a1.18 investigated low-temperature crosslinkmg in pyrolysis of low-rank coals. The formation of cross-linking, which was evaluated from the pyridine swelling ratio of char, was well predicted by their model12 in which the evolution of one C 0 2 or H2O molecule was supposed t o form one cross-link. In a (18)Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990,4 , 42. (19)Ibarra, J. V.; Moliner, R.: Gavilan, M. P. Fuel 1991,70. 408. (20) Juntgen, H. Fuel 1984,63,731
Energy & Fuels, Vol. 8, No. 6,1994 1367 60
1 764'C
U
764'C
Tar
Char
H2
60 _.
a
CO
C02
H2O
9
50
40
30
0
2
3
20
E
*
2 10 n Tar
Char
s a
8
3
E s" s
-
2
s E
H2
CO
COz
HzO
i 0
'1
CH4
CnH4
C2Hs
C3H6
C3Ha
C4He
C4Hia
J
920'C
.
CH4
C2H4
CzH6
C3H6
C3H8
C4Ha
C4Hio
Figure 4. Comparison of product yield by copyrolysis of TH and HPP with sum yield by their separate pyrolysis. ul
3.6
3.0
....-
........ 920'C
764% 920'C
7M'C 920.C
YL-HDPE
YL-LDPE
YL-HPP
7M'C
Bh
'
7°C
92O'C
.
I
-11
61 Olefinic HCG
H ParatfinicHCG
TH-HPP
0 H2
IOG
Figure 5. Distribution of decreased amount of gaseous
products by copyrolysis in Curie-point pyrolyzer. similar manner, a 1-mol decrease in the formation of hydrogen gas corresponds to the stabilization of two moles of coal radicals generated by breaking 1 mol of cross-links. Thus, the decrease in yield of hydrogen and hydrocarbon gases is the result of stabilization of coal radicals and of suppression of cross-linking formation.
1358 Energy & Fuels, Vol. 8, No. 6,1994 15
1
.
.
1
I
' , ' I
Hayashi et al.
-
= a 8 P e
A YL-HDPE YL-LDPE O YL-HPP
8
,/'
,**'
0
s"
0
BE
,/'
40
30
20 10
9)
F 5 .
0 Char
= a 8 0.1
0.2
0.3
0.4
0.5
Total decrease of gas (mol/lOOg raw coal] Figure 6. Increased amount of tar by copyrolysis as a function of total decreased amount of gaseous products on molar basis (Curie-point pyrolyzer, 764 "C). Figure 6 shows the relationship between the decrease in total gas yield on a molar basis and the increase in tar yield by the copyrolysis. The scattering of data points suggests that various gases contribute a t different efficiencies to the promotion of bond scission or the suppression of cross-linking in the copyrolysis. The slope of the broken lines means that 20-40 g of tar was formed by consuming 1 mol of gas. Miura et al.4 conducted flash pyrolysis of tetralin-swollen coals at 764 "C with a Curie-point pyrolyzer. The tar was increased roughly by 30 g when the sum of hydrogen gas and pyrolytic water was decreased by 1mol. This suggests that the efficiency of radical transfer and cross-linking suppression is not largely changed in cases where tetralin or polyolefins is employed as pyrolysis promoter. Continuous Copyrolysis in Entrained-Flow Pyrolyzer. Composite particles of YL coal combined with any polyolefin were easily fed into the reactor. However, the TH coal particles were difficult to feed at a constant rate without tapping the feed line when the weight ratio of polyolefin to coal was larger than 0.25. Clogging also occurred in the injection nozzle. This difference was apparently caused by the degree of macroporosity developed in the coal particles. Figure 7 shows the result of the copyrolysis of YLHPP and YL-HDPE composite particles. The char yield was decreased by ca. 4 g per 100 g of dry raw coal for both composites. The increase in tar yield by the copyrolysis was smaller than that in the CPP shown in Figure 3. Figure 7 also demonstrates that the copyrolysis increased light aromatics in the entrained-flow reactor. Secondary cracking of initial tar proceeded, and light aromatics were produced in the vapor phase. The increase of aromatics without substituted groups, benzene and naphthalene, was significant. The amount of benzene produced by the pyrolysis of the polyolefins alone was ca. 4 wt % of the initial polyolefin mass. This value was 4-5 times larger than that from the pyrolysis of YL coal alone a t 800 "C. I t may be thought that the increment of the benzene yield comes from the polyolefins. However, the yield of naphthalene derivatives, which were not produced from the pyrolysis of the polyolefins, was increased by the copyrolysis. Furthermore, the yield of hydrocarbon gas from the copyrolysis was nearly equal to the total yield of hydrocarbon gas
HCG
IOG
H2 I
-
YL HDPE HDPE
0 YL
P
0
Tar
3,
e
E
BE 9)
F
= m 8 3 e
s"
BE 0
B
T
P
Char
Tar
HCG
B
T
P
N
MN
100
H2
N
MN
40
30 20 10
F 0
-a 0
u
s
e
2
s" 5
Y
1
E 0
F 0
Figure 7. Comparison of product yield by copyrolysis of YL
and HDPE or HPP with sum yield by their separate pyrolyses (entrained-flowpyrolyzer, 800 "C). B, benzene; T, toluene; P, phenol; N, naphthalene; MN, methylnaphthalene. from the separate pyrolyses of YL coal and polyolefins. If aromatization of polyolefins is accelerated by contact with YL coal, the evolution of hydrocarbon gases should be suppressed. On the contrary, their yield was little changed by the copyrolysis. Then a reasonable explanation for the increase in the benzene yield is the promotion of benzene evolution from YL coal in the presence of polyolefins. The yield of hydrogen gas was decreased by the copyrolysis in the EFP, but the difference was smaller than that obtained in the CPP. Hydrogen transfer from polyolefin volatiles to coal volatiles was not significant in the vapor phase. In the previous study,21we investigated the secondary reaction of flash pyrolysis tar in the freeboard region of a fluidized-bed reactor. Flash (21)Hayashi, J.-i.; Kawakami, T.; Taniguchi, T.; Kusakabe, K.; Morooka, S.; Yumura, M. Energy Fuels 1993,7,57.
Flash Copyrolysis of Coal and Polyolefin pyrolysis tar, generated from Wandoan coal (C= 76 wt %) at 600 "C,was continuously pyrolyzed a t 800 "C,and n-octane was fed into the freeboard at a feed rate of 0.2 relative to the mass feed rate of the coal. The copyrolysis of tar and n-octane in the vapor phase did not change the production of benzene and naphthalene. Also the addition of octane vapor did not affect the hydrogen gas yield. These results mean that hydrogen transfer in the copyrolysis hardly occurs in the vapor phase.
Conclusions 1. By the copyrolysis of coal-polyolefin composites, the total yield of char was decreased and that of tar was increased compared with those obtained by pyrolyzing each component separately under the same conditions. 2. The enhancement of tar evolution by the copyrolysis was explained by transfer of hydrogen and hydrocarbon radicals from olefin to coal as well as by suppression of cross-linking due to decomposition of oxygen-containing
Energy & Fuels, Vol. 8, No. 6,1994 1359 groups. This explanation was confirmed by the suppression of hydrogen and hydrocarbon and inorganic gases. 3. The extent of decrease in the yield of hydrocarbon gas by copyrolysis was smaller with YL-HDPE and YL-LDPE combinations than with the YL-HPP combination. The yields of hydrogen and inorganic gases were decreased similarly by copyrolysis of any combination. 4. The decrease in total gas yield on a molar basis was strongly related to the increase in the tar yield. The former contributed to the latter at an efficiency of ca. 30 g/mol. 5 . The copyrolysis performed in the EFP was especially effective in increasing light aromatics such as benzene and naphthalene.
Acknowledgment. This work was financially supported by the Nissan Science Foundation and the Plastic Waste Management Institute.
