Energy & Fuels 1998, 12, 963-968
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Synergetic Effects in the Copyrolysis of Coal/Petroleum Residue Mixtures by Pyrolysis/Gas Chromatography: Influence of Temperature, Pressure, and Coal Nature R. Moliner,* I. Suelves, and M. J. La´zaro Instituto de Carboquimica, CSIC, Maria de Luna, 12, 50015 Zaragoza, Spain Received February 19, 1998
The treatment of wastes is one of the most important concerns of modern society, and pyrolysis is claimed as an alternative method to improve the quality of the products obtained from materials of a hydrocarbon nature such as coal and residues from petroleum distillation. The main objectives of this paper are (i) to evaluate the interactions between coal and petroleum residues (PR) which lead to a synergetic effect on the production of light olefins C1-C4 and BTX and (ii) to study the influence of variables such as temperature, pressure, and coal nature on the intensity of these effects. The study has been carried out by pyrolysis/gas chromatography in a pyroprobe using two coals (Samca and Figaredo), a petroleum residue, and mixtures of them in a mass ratio (coal/ residue) of 70/30. Two temperatures, 800-900 °C, and two pressures, 0.1-1 MPa, have been studied. Different results are obtained for the individual components and the mixtures depending on the nature of the coal. In general, an increase in temperature promotes the production of both light olefins and BTX and the synergetic effects desired from the copyrolysis of the mixtures. On the other hand, increasing the pressure does not favor the production of light olefins.
Introduction The treatment of wastes is one of the most important concerns of modern society. Different methods have been developed, and coprocessing of coal and residual products has been widely studied in the past few years. The most important works are related to the catalytic processing of coal and residues in the presence of H2.1,2 Hayashi et al.3,4 consider the copyrolysis of coal and plastic wastes, polymers, and other materials of similar nature as an alternative method of improving the liquid yield obtained from the pyrolysis of coal. The influence of different variables such as temperature, pressure, and the nature of the coal in the final products obtained are studied. The pyrolysis of coal is a good method for producing chemicals such as BTX and light olefins, but the yields of these products are limited because of the low hydrogento-carbon ratio in coal.5,6 For this reason, it is necessary to supply H2 from other sources. Petroleum residues, which are hydrogen-rich compounds, can act as H2 donors in coprocessing reactions.7 However, there are * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Orr, E. C.; Shi, Y.; Liang, J.; Ding, W.; Anderson, L.; Eyring, E. M. Prepr.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (3), 633. (2) Palmer, S. R.; Hippo, E. J.; Tandon, D.; Blankenship, M. Prepr.s Am. Chem. Soc., Div. Fuel Chem. 1995, 40 (1), 29. (3) Hayashi, J.; Kawakami, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993, 7, 1118-1122. (4) Hayashi, J.; Mizuta, H.; Kusakabe, K.; Morooka, S. Energy Fuels 1994, 8, 1353-1359. (5) Miura, K.; Mae, K.; Asaoka, S.; Yoshimura, T.; Hashimoto, K. Energy Fuels 1991, 5, 340-346. (6) Miura, K.; Mae, K.; Yoshimura, T.; Masuda, K.; Hashimoto, K. Energy Fuels 1991, 5, 803-808. (7) Ceylan, K.; Stock, L. M. Fuel 1990, 69, 1386-1393.
few references in the literature to the copyrolysis of coal and materials such as hydrocarbons and petroleum residues. Moliner et al.8 have shown the possibility of improving the quality of the products obtained from the copyrolysis of this kind of material. In a previous paper9 we have shown that there are interactions between coal and residues when they are copyrolized as a mixture. The desirable effects sought from these interactions are an increment on the production of some valuable products related to those obtained when coal and residue are pyrolyzed separately, which are usually referred as synergetic effects. Light olefins and light aromatic compounds are the products normally desired when a synergetic effect occurs. These products are more valuable for the chemical industry than aliphatics and PAHs. Pyrolysis reactions are affected by different parameters such as temperature, residence time of the volatiles in the reaction zone, pressure, and nature of the components.10 It has been shown in the literature11 that aromatic compounds are produced from pyrolysis of aliphatic compounds by means of light olefins as intermediate products. For this reason, it is expected that petroleum residues which contain an important aliphatic fraction can act as a source of light aromatic compounds. Temperature seems to be the most important variable that controls the pyrolysis reactions. In previous works (8) Moliner, R. Final Report. ECC Project No. 7220-EC/763; 1996. (9) Suelves, I.; Moliner, R.; La´zaro, M. J.; Ibarra, J. V. Coal Science and Technology 24; Pajares, J. A., Tasco´n, J. M. D., Eds.; Elsevier: Amsterdam, 1995; pp 873-876. (10) Cypres, R. Demonstration Project LG, Final Report 103/84; 1990. (11) Cypres, R. Fuel Process. Technol. 1987, 15, 1.
S0887-0624(98)00033-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/07/1998
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Table 1. Ultimate Analysis of the Samples Samca Figaredo PR
ash
volatiles
C
H
N
O
S
22.8 3.9 0.2
36.9 17.1 80
75.9 91.2 83.41
5.3 4.2 9.75
0.7 1.8 0.55
12.27 0.6 0.98
5.8 2.3 5.31
on coal pyrolysis12,13 it has been proved that an increase in the production of gaseous compounds and light aromatics occurs as temperature increases. The increasing of pressure reduces liquid and gas yields, but it has a lesser effect on products distribution than temperature. In this paper, our attention will be focused on two types of compounds: light olefins (especially ethylene, propylene, and butadiene) and the aromatic compounds (benzene, toluene, and xylene, namely BTX). The main objectives of the study are (i) to evaluate the interactions between coal and petroleum residues (PR) and the synergetic effect for the production of light olefins C1C4 and BTX and (ii) to study the influence of variables such as temperature, pressure, and coal nature on the intensity of these effects. Experimental Section Pyrolysis runs were carried out on an analytical scale, by pyrolysis-gas chromatography in a pyroprobe 1000 CDS. This device consists of a microoven (a platinum coil, length 2 cm and i.d. 0.3 cm) where a small quartz tube (i.d. 0.2 cm) is introduced. The sample (between 2 and 5 mg) is heated with a very high nominal heating rate up to the temperature set. The microoven is pressurized with nitrogen. This kind of experimentation has several advantages: Short periods of time are required for each experiment since the total heating process lasts 20 s. In addition, the probe can easily be linked online to the gas chromatograph, which prevents the loss of the less heavy compounds in the tubes. The nominal temperatures used were 800 and 900 °C and pressure was 0.1 and 1 MPa. One selected run was repeated five times to calculate the experimental error. The confidence interval calculated this way for the chromatographic peak areas was in all cases below 1%. The other runs were repeated twice, and the mean value was assumed as the true value. Coal/Petroleum Residue Samples. Two coals, Samca and Figaredo, and a petroleum vacuum residue (PR) have been used in this work. Samca is a subbituminous coal with a high volatile content, and Figaredo is a bituminous one, of higher rank, with a low volatile content. The petroleum residue proceeds from the distillation of different crudes and has been submitted by REPSOL, Puertollano. The main characteristics of these materials are shown in Table 1. The coals and the residue were pyrolyzed alone and then as a mixture in a mass ratio (coal/residue) of 70/30. Preparation of the samples was as follows: petroleum residue was first dissolved in tetrahydrofuran (THF) and then coal was added. The mixtures were subsequently sonicated. After 15 min, THF was evaporated by heating under vacuum. The dried samples are ground to C6 compounds) were analyzed by GC/MS. All compounds present in liquids were identified by using a computerized library of mass spectra. Tri- and tetramethylbenzene were accounted for as alkylbenzenes. All compounds cromatographed between naphthalene and phenanthrene were accounted for as 2,3 rings. The quantitative composition of liquids was determined by a solution of external standards. For the study of the C1-C6 fraction, the pyroprobe was not directly linked to the chromatograph. The experimental device consists of a small chamber, with a septum and a gas inlet, that can be easily pressurized. The pyroprobe is introduced into the chamber, the chamber is purged by a small N2 flow, and then the pyrolysis is carried out. A sample of the pyrolysis gas produced is taken through the septum by a chromatography syringe. The volume of the chamber was previously calculated by injecting a known volume of a standard and measuring the dilution. A different experimental device was used for the >C6 compounds. In this case, the pyroprobe was linked to the chromatograph in order to avoid condensation of the heavier compounds on the tubes. The pyroprobe is introduced in a small programmable oven that is connected to the chromatograph through a system of heated valves. After purging, pyrolysis is carried out at ambient temperature to prevent the sample from being modified before pyrolysis. Subsequently, the oven is heated to 250 °C and kept at this temperature for 5 min. Then, the compounds are directly introduced into the chromatograph by a helium flow. The chromatographic column is cooled to 0 °C by cryogenic CO2 in order to retain the lighter compounds at the head of the column and improve the chromatographic separation. This device does not allow working at high pressure because of the limits imposed by the chromatograph injector.
Results and Discussion Evolution of Yields with Temperature. The influence of temperature in the copyrolysis yields of the mixtures of Samca/PR and Figaredo/PR has been evaluated. First, each component of the mixture was pyrolyzed alone. Yields of the gaseous compounds obtained at 800 and 900 °C are shown in Table 2. For the Samca coal, the global yields increase as temperature does. This fact is particularly important for CO, CO2, and CH4. Temperature does not seem to affect the rest of the compounds. The effect of increasing temperature is also important for Figaredo coal, enhancing the formation of CO, CO2, methane, ethane, ethylene, and propylene. The behavior of the petroleum residue is rather different from that of both coals. The global gas yield is unaffected by increasing temperature. However, an increase is observed for the production of methane, ethylene, and butadiene. The yields of the rest of the compounds decrease as temperature increases. Some authors13,14 have previously pointed out that the increase of pyrolysis temperature produces an increase in the yields of some gaseous compounds, especially (14) Fairburn, J. A.; Behie, L. A.; Svreek, W. Y. Fuel 1990, 69, 15371545.
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Energy & Fuels, Vol. 12, No. 5, 1998 965
Table 2. Yields (% wt) of the Main C1-C6 Compounds Obtained in the Copyrolysis of Coals and PR, P ) 0.1 MPa Samca
Figaredo
residue
800 °C
900 °C
800 °C
900 °C
800 °C
900 °C
2.93 7.76 1.51 1.01 0.14 0.13 0.05 0.12 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.03 0.08
6.68 10.28 1.88 1.92 0.12 0.19 0.03 0.14
0.60 1.38 0.03 3.13 0.04 0.01 0.06 0.06 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01
2.06 2.92 0.12 6.76 0.29 0.31 0.06 0.11 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02
0.36 0.30 1.84 6.86 1.47 2.53 0.66 2.53 0.03 0.29 0.09 0.46 0.27 0.26 0.69 0.98 1.35
0.27 0.18 1.89 9.37 1.20 4.48 0.34 2.32 0.02 0.12 0.05 0.29 0.20 0.13 0.96 0.60 1.02
CO CO2 SH2 CH4 C2H6 C2H4 C3H8 C3H6 I-butane N-butane 1-butene I-butene C-butene T-butene butadiene C5 C6
0.01 0.01 0.01 0.02 0.01 0.03 0.02 0.03
Table 3. Yields (% wt) of the >C6 Compounds Obtained in the Copyrolysis of Coals and PR, P ) 0.1 MPa Samca
Figaredo
Table 4. Yields (% wt) of the Main C1-C6 Compounds Obtained in the Copyrolysis of Mixtures 70/30, P ) 0.1 MPa Samca/residue 900 °C
800 °C
900 °C
3.24 6.11 1.70 2.35 0.28 0.61 0.13 0.35 0.01 0.05 0.01 0.05 0.04 0.02 0.06 0.13 0.19
5.41 7.73 2.10 3.79 0.39 1.63 0.11 0.73 0.01 0.03 0.01 0.06 0.06 0.04 0.28 0.21 0.29
0.57 1.36 0.43 4.48 0.47 0.37 0.19 0.35 0.01 0.07 0.01 0.06 0.04 0.03 0.08 0.17 0.21
1.40 1.57 0.57 7.68 0.65 2.16 0.14 0.95 0.01 0.04 0.02 0.08 0.07 0.04 0.36 0.27 0.32
CO CO2 SH2 CH4 C2H6 C2H4 C3H8 C3H6 I-butane N-butane 1-butene I-butene C-butene T-butene butadiene C5 C6
Table 5. Yields (% wt) of the >C6 Compounds in the Copyrolysis of Mixtures 70/30, P ) 0.1 MPa
residue
Samca/residue
800 °C 900 °C 800 °C 900 °C 800 °C 900 °C benzene toluene xylene alkylbenzene naphthalene 2,3 rings
0.17 0.18 0.12 0.35 0.01 0.08
0.11 0.09 0.04 0.32 0.02 0.08
0.10 0.16 0.14 0.09 0.01 0.08
0.14 0.19 0.15 0.55 0.04 0.16
1.87 2.26 1.91 3.04 0.22 0.79
1.15 0.57 0.31 0.56 0.53 1.15
methane and ethylene, either as subproducts of aromatization reactions or because of the increase of decomposition processes of larger chains. Table 3 shows the yields of some selected compounds of the >C6 fraction: benzene, toluene, xylene, alkylbenzene, naphthalene, and the heaviest aromatic compounds (2,3 rings). The yields of the >C6 compounds obtained from the pyrolysis of both coals are lower than those obtained from PR. The evolution with temperature is different for both coals. For Samca coal, the increase of temperature produces an important decrease in the production of aromatic compounds, especially for the light ones, benzene, toluene, and xylene. For Figaredo coal there is a slight increase for all compounds, particularly for the heavy ones. The influence of increasing temperature is negative for the pyrolysis yields from petroleum residue: BTX production is higher at 800 °C. Otherwise, the yield of the heaviest compounds is favored by the increase of temperature, probably because of the condensation reactions of the lightest ones.10,15 Table 4 shows the yields of the main gaseous compounds obtained from the pyrolysis of the Samca/PR and Figaredo/PR mixtures. In both cases, temperature has a benefit effect on the whole gas yield. If we analyze the individual components of the gas phase, similar trends for both mixtures are also observed: a notable increase in the production of methane and light olefins such as ethylene, propylene, and butadiene at the detriment of the C4-C6 aliphatic compounds. (15) Bredael, P. Ann. Mines de Belgique, 1975, 11, 1046.
Figaredo/residue
800 °C
benzene toluene xylene alkylbenzene naphthalene 2,3 rings
Figaredo/residue
800 °C
900 °C
800 °C
900 °C
0.84 0.67 0.43 1.45 0.09 0.39
0.69 0.39 0.17 0.44 0.28 0.32
0.47 0.33 0.19 0.47 0.07 0.32
0.44 0.30 0.12 0.44 0.24 0.32
Table 6. Yields (% wt) of the Main C1-C6 Compounds Obtained in the Copyrolysis of Coals and PR, P ) 1 MPa Samca CH4 C2H6 C2H4 C3H8 C3H6 I-butane N-butane 1-butene I-butene C-butene T-butene butadiene C5 C6
Figaredo
residue
800 °C
900 °C
800 °C
900 °C
800 °C
900 °C
1.28 0.30 0.19 0.10 0.20
1.86 0.30 0.27 0.07 0.30
0.5 0.10 0.02 0.03 0.02 0.01 0.01 0.01
1.05 0.13 0.05 0.03 0.03 0.01 0.01
9.86 3.31 5.64 1.56 3.70
0.01
0.02
5.53 2.61 2.01 0.28 1.19 0.30 0.72 0.28 0.65 0.30 0.26 0.30 1.25 0.92
0.09
0.08 0.10
0.31 0.28 0.64 0.25 0.26 0.64 1.53 0.97
Table 5 shows the variation with temperature of the yields of the >C6 compounds for both mixtures. Temperature has a negative effect on the yields of all compounds, except for the naphthalene yield for the Samca/PR mixture. However, for the Figaredo/PR mixture, temperature does not have a great influence on BTX, but the trend is similar to that of Samca/PR for naphthalene yield. For these preliminary results we can conclude that a higher temperature enhances retrogressive reactions, probably by cleavage of larger saturated chains, which increases the yields of small gas compounds such as light olefins. Simultaneously, the increase of temperature promotes condensation of BTX to produce heavier aromatic compounds. Evolution of Yields with Pressure. Table 6 shows the results obtained from the co-pyrolysis of Samca coal, Figaredo coal, and PR at 800 and 900 °C at a pressure
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Table 7. Yields (% wt) of the Main C1-C6 Compounds Obtained in the Copyrolysis of Mixtures 70/30, P ) 1 MPa Samca/residue CH4 C2H6 C2H4 C3H8 C3H6 I-butane N-butane 1-butene I-butene C-butene T-butene butadiene C5 C6
Figaredo/residue
800 °C
900 °C
800 °C
900 °C
1.46 0.48 0.60 0.19 0.47
2.43 0.71 1.28 0.18 0.85
0.06 0.08 0.10 0.09 0.07 0.05 0.28 0.19
0.08 0.06 0.07 0.10 0.09
0.72 0.34 0.18 0.21 0.27 0.02 0.09 0.03 0.08 0.04 0.04 0.05 0.31 0.43
0.92 0.28 0.32 0.11 0.32 0.01 0.03 0.02 0.04 0.03 0.03 0.09 0.11 0.25
0.46 0.10
of 1 MPa. The analysis of CO, CO2, and SH2 was carried out with TCD detection as was previously mentioned in the Experimental Section. This detector is not as sensitive as the FID one, thus yields obtained for the permanent gases under pressure could not be evaluated. For this reason, the study of the evolution of yields under pressure has been limited to the C1-C6 compounds. The comparison of results in Tables 2 and 6 shows that there is a higher production of C2-C3 compounds at high pressure and a more important decrease for the C4-C6 ones than that for Samca coal. This effect is observed at both temperatures tested. Pressure has a higher negative effect on the yields of all compounds produced from the pyrolysis of Figaredo coal. The important decrease in the methane production at 1 MPa should also be noted. A decrease in the global yield of the gas phase as pressure increases from 0.1 MPa to 1 MPa is observed for the PR. However, yields of some gas compounds increase as the pressure does. This effect depends on temperature and the compound considered: At 800 °C, the increase of pressure produces an increase in the yields of ethane and C4 saturated compounds and a decrease in propylene and butadiene. In contrast, at 900 °C there is an increase in the yields of methane, propylene, and butadiene and an increase for C4 olefinic compounds. Table 7 shows the results obtained from the copyrolysis of the mixtures at both temperatures and 1 MPa. In all cases there is a decrease in the production of methane, ethane, ethylene, and propylene. However, the production of saturated compounds is enhanced. Ettinger et al.16 reported that the effect of pressure on this type of process is not clear at all. From our experiments, it can be concluded that pressure has a negative influence on the yields of the most interesting products of copyrolysis. If the influence of both temperature and pressure as a whole are considered, then, the conclusion is that temperature is therefore the leading variable of the process. Synergetic Effect. If there were no interaction between the components of a mixture, the yields obtained in its copyrolysis (actual values) should be equal to the sum of the yields obtained in the pyrolysis of the (16) Ettinger, M. D.; Stock, L.; Gatsis, J. G. Division of Petroleum Chemistry, Inc. 205th National Meeting, American Chemical Society Denver, CO, 1993.
Figure 1. Theoretical and actual yields for the C1-C6 compounds from the copyrolysis of Samca/PR at P ) 0.1 MPa.
individual components (theoretical values). On this basis, the existence of a synergetic effect has been evaluated by comparing actual and theoretical values, and the influence of the experimental conditions has been evaluated. Influence of Temperature. Figure 1 shows the comparison between the experimental and the theoretical values obtained from copyrolysis of Samca coal/PR mixture (70/30) for the yields of the C1-C6 compounds at 800 and 900 °C. Figure 1 shows that there is a general decrease in the actual yields of all compounds compared with the theoretical values at 800 °C. The actual decrease is important specially for light olefins (ethylene, propylene, and butadiene), and there is also a slight decrease in the actual production for methane. The actual C4-C6 yields are also lower than the theoretical ones. On the other hand, there is an increase in the actual production for CO and CO2. The behavior of the mixture is different at 900 °C. CO and CO2 actual yields remain higher than the theoretical ones, but the increase is lower. For the other compounds the theoretical and actual values are very similar. In addition, there is a slight synergetic effect on the experimental yield of ethylene. Figure 2 shows the comparison between the asynergetic and experimental values for the >C6 compounds obtained in the copyrolysis of the Samca coal/PR mixture (70/30) at 800 and 900 °C. There is an increase in the actual production of benzene and alkylbenzene versus the theoretical one and a slighter increase for the production of heavier aromatic compounds at 800 °C. On the contrary, there is a decrease in the production of toluene and xylene versus the theoretical value. At 900 °C, a higher production of all aromatic compounds in relation to the asynergetic values is observed. The increase is more important for the light ones, especially for benzene, toluene, and xylene. Different types of reactions have been reported for the pyrolysis of coal and petroleum residues.11,15 Most of
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Energy & Fuels, Vol. 12, No. 5, 1998 967
Figure 2. Theoretical and actual yields for the >C6 compounds from the copyrolysis of Samca/PR at P ) 0.1 MPa.
them are degradative processes as well as recombination and Diels-Alder reactions. If the predominant pathways of the copyrolysis were recombination and DielsAlder reactions, there would be an increase in the production of BTX, this causing a decrease in the production of light olefins that act as precursors for the aromatic compounds. Samca/PR mixtures, particularly at 900 °C, show a synergetic production of BTX versus a countersynergic effect for the light olefins, and so, it can be assumed that the pathways of the reaction are mainly recombination and Diels-Alder reactions. The slight experimental increase of ethylene in this case might be as a subproduct of these reactions. Therefore, the accurate description of the mechanism of this type of process is not an easy task, as pyrolysis of these types of materials involves a multitude of single reactions proceeding simultaneously that cannot be easily controlled.17-19 Figure 3 shows the comparison between the theoretical and actual values for the C1-C6 compounds obtained in the pyrolysis of the mixture of PR with Figaredo coal at 800 and 900 °C. It can be observed that there is a slight increase in the actual production of CO2 and CH4 while there is a decrease in the actual yields for the other compounds at 800 °C. This effect is similar to that observed for the mixture with Samca coal. The main difference between both mixtures concerns the synergic effect observed in the PR/Figaredo mixture for the production of methane. As stated above, Figaredo coal produces a higher yield of methane than Samca. Calkins et al. and Conesa et al.20,21 have suggested that the precursors of methane in pyrolysis of hydrocarbon materials are different from the precursors of light olefinic compounds. Moreover, Ofosu-Asante et al.22 (17) Barketova, E.; Bajus, M. Pet. Coal 1995, 37 (3). (18) van Heek, K. H.; Hodek, W. Fuel 1994, 73 (6), 886-896. (19) Hodel, W.; Kirschstein, J.; van Heek, K. Fuel 1991, 70, 425428. (20) Conesa, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. Energy Fuels 1994, 8, 1238-1246. (21) Calkins, W. H.; Hagaman, E.; Zeldes, H. Fuel 1984, 63, 1131118.
Figure 3. Theoretical and actual yields for the C1-C6 compounds from the copyrolysis of Figaredo/PR at P ) 0.1 MPa.
studied copyrolysis reactions of coal and paraffinic compounds mixtures and suggested that CH4 precursors proceed mainly from coal. These facts could explain the different behavior of both mixtures. The Figaredo/PR mixture produces a higher experimental methane yield versus the theoretical one probably as a consequence of the high methane production of Figaredo coal. Samca coal produces less methane than Figaredo, and it is likely that because of that Samca/PR mixtures show no synergetic behavior regarding methane production. There is a decrease in the actual production of CO, CO2, and H2S and an increase in the actual yields for ethane, ethylene, propylene, and butadiene at 900 °C. The comparison between the actual and theoretical values for the >C6 compounds obtained in the copyrolysis of the mixture with Figaredo coal are shown in Figure 4. There is a decrease for the yields of BTX at 800 °C, although the actual and theoretical values are similar for heavier aromatic compounds, and there is even a slight increase in the actual production for the 2,3 rings compounds. For pyrolysis at 900 °C there is a slight increase in the actual yields for benzene and naphthalene and a decrease for the other ones. However, as a whole, the aromatic yield decreases slightly. In accordance with the results shown above, there is a countersynergetic effect for all compounds at 800 °C, and at 900 °C production of olefins is favored toward the BTX compounds. This suggests that the pathways of the reaction are different in this case from those of Samca coal, and so the most important reactions that occur are the degradative ones. Additional works8 at bench scale have shown that copyrolysis of Figaredo coal and petroleum residues produces a higher experimental char yield, toward a decrease in the experimental production of liquids. This fact could explain the (22) Ofosu-Asante, K.; Stock, M.; Zabransky, R. F. Fuel 1989, 68, 567-572.
968 Energy & Fuels, Vol. 12, No. 5, 1998
Figure 4. Theoretical and actual yields for the >C6 compounds from the copyrolysis of Figaredo/PR at P ) 0.1 MPa.
Figure 5. Theoretical and actual yields for the C1-C6 compounds from the copyrolysis of Samca/PR at P ) 1 MPa.
absence of synergetic behavior observed for the mixtures of Figaredo/PR for the production of BTX on an analytical scale, where no mass balance can be carried out.
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Figure 6. Theoretical and actual yields for the C1-C6 compounds from the copyrolysis of Figaredo/PR at P ) 1 MPa.
Influence of Pressure. The comparison between the theoretical and actual values obtained for the copyrolysis of Figaredo/PR and Samca/PR at 1 MPa are shown in Figures 5 and 6. It can be observed that for Samca coal there is always a countersynergetic behavior for C1-C6 compounds at both temperatures, and so the increase of pressure is not appropriate to enhance the production of light olefins. As stated above, the measure of the >C6 compounds production at 1 MPa could not be carried out due to experimental limitations, and for that reason, it is not known how increasing pressure would affect BTX yields. Diels-Alder reactions seem to govern the mechanisms of the pyrolysis of Samca/PR mixtures at 0.1 MPa and might also govern the process at higher pressure. Additional bench scale works are being carried out to elucidate this matter. For Figaredo coal/PR mixtures, pressure has a negative effect on the copyrolysis products at both temperatures but especially for 900 °C. Previous results obtained at bench scale8 have shown that pressure enhances the production of char in detriment of liquid and gaseous products. If one supposes that the reactions that occur during copyrolysis on an analytical scale are similar to those observed on a bench scale, it could be inferred that the countersynergetic effect that is observed for Figaredo/PR mixtures is a consequence of a higher char production. EF980033O
