Energy & Fuels 1997, 11, 813-818
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Iron-Catalyzed Coal-Tire Coprocessing. Influence on Conversion Products Distribution Ana M. Mastral,* Ramo´n Murillo, Jose´ M. Palacios,† M. Carmen Mayoral, and Marisol Calle´n Instituto de Carboquı´mica, CSIC, P.O. Box 589, 50080 Zaragoza, Spain, and Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus UAM, 28049 Madrid, Spain Received October 7, 1996X
This paper reports the hydrocoprocessing of a low-rank coal and rubber from discarded tires in the presence and absence of Red Mud as an iron catalytic precursor. A subbituminous coal from northeast Spain is processed with ground rubber from a mixture of old tires, free of steel thread and textile netting. This is the first time that the influence of the hydrogen pressure on conversion products is deeply analyzed. In addition, the influence of the coal-tire ratio on conversion and product distribution is also studied. Experiments have been conducted in small tubing bomb reactors, held by an oscillation device, and immersed in a preheated fluidized sand bath. Temperature (400 °C) and the reaction time (30 min) were kept constant. The THF-soluble and n-hexane-soluble products have been characterized by thin layer chromatography. Gas and asphaltene formations are commented, and THF insolubles are thoroughly studied following the iron evolution, both from coal mineral matter and those added as catalyst precursors by SEMEDX. It is concluded that iron activity in coal processing is dependent on the hydrogen pressure: at high hydrogen pressure (10 MPa), it has no effect because of the high conversions; at medium pressures (7.5 and 5 MPa), it has a positive effect mostly increasing the asphaltene formation. Iron addition to tire processing has no effect due to the high conversions reached at the working conditions. Iron addition to coal-tire coprocessing produces higher polar conversion products, and its catalytic activity is mainly reflected in higher asphaltene formation. Higher hydrogen pressures produce higher oil percentages.
Introduction Today, one of the main problems to be solved in coal liquefaction is to obtain high-quality liquids at a competitive cost. The high price of hydrogen and the pressure needed to achieve high conversions make these processes not competitive compared to petroleumderived products. A way to reduce the costs of coal liquefaction might be the coprocessing of coal with other hydrogen source materials which could allow lower hydrogen pressure in the liquefaction reactions and, in this way, to improve coal conversion. Coprocessing coal with some waste materials is being paid growing interest.1-3 In this way, a double positive goal is reached: on one hand coal liquefaction4 is improved and on the other hand waste materials that cause environmental disturbances are reused.5,6 Typical examples of the above-mentioned materials are plastics present in urban wastes which mainly are †
Instituto de Cata´lisis y Petroleoquı´mica. Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Symposium on Co-Utilization of Coal and Wastes. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 and bibliography there cited. (2) Symposium on Co-Utilization of Coal and Wastes Materials. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 and bibliography there cited. (3) International Congress on Solid Residues. Proceedings of the ANQUE, Tenerife, Diciembre 1994 and bibliography there cited. (4) Williams, P. T.; Besler, S.; Taylor, D. T. Proc. Inst. Mech. Eng. 1993, 207, 55. (5) Farcasiu, M. CHEMTECH 1993 (January), 22. (6) Mastral, A. M.; Murillo, R.; Perez-Surio, M. J.; Callen, M.; Clemente, M. C. 3rd International Congress on Solids Residues, ANQUE, Tenerife, December 1994. X
S0887-0624(96)00171-5 CCC: $14.00
used for packing purposes. The total amount of waste plastics generated in the U.S. is about 30 million tons per year,7 and most of it is taken to landfills. These plastic materials have been coprocessed with coal,8,9 and results show that the addition of these materials has a positive effect on coal liquefaction (total conversion and oil yields are increased in the presence of plastics). The problem with these kinds of residues is that a previous selection is necessary because they usually appear together with other residues which cannot be coprocessed with coal like glass, tinplate, etc. Moreover, every kind of plastic has a different behavior in liquefaction10 conditions and this means that posterior selection is needed in order to reach optimum results. Old tires are another residue which is generated by modern society and which could be coprocessed with coal. In the 1980s, 220 million tires were generated per year in the U.S.11 These tires are usually dumped in landfills or left in the open air, generating significant environmental disturbances as nonbiodegradable residues and dangerous situations as the risks of fire. There are several choices to eliminate this residue like combustion,12 pyrolysis,13,14 and hydrocoprocessing15 with coal. Despite combustion being an easy application (7) Harrison, G.; Ross, A. B. Fuel 1996, 75 (8), 1009. (8) Liu, Z.; Zondlo, J. W.; Dadyburjor, D. B. Energy Fuels 1994, 8, 607. (9) Liu, Z.; Zondlo, J. W.; Dadyburjor, D. B. Energy Fuels 1995, 9, 673. (10) Taghiei, M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1228. (11) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69 (12), 1474. (12) 8th Pittsburgh Coal Conference Proceedings, 1991; p 859. (13) Hodek, W. ICCS Proceedings, Newcastle, U.K., 1991; p 782.
© 1997 American Chemical Society
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Table 1. Tire and Coal Proximate and Ultimate Analyses tire
coal
C (daf) H (daf) N (daf) S (mf)
88.64 8.26 0.43 1.43
80.17 6.69 1.01 5.68
moisture (af) ash (mf) volatiles fixed carbon
0.94 3.83 67.30 31.14
22.05 26.93 48.62 28.45
calorific value (kcal/kg)
9.159
3.714
to obtain energy, the emissions produced (dioxins, PAC, particulate matter, etc.) make this possibility disfavorable from an environmental point of view. The method described in this paper is hydrocoprocessing with coal because results in coal-tire coprocessing have been encouraging and various authors8,9,16 have found synergism between coal and old tires. The selection problems with this residue are fewer than those existing with plastics, but every tire is different depending on its purpose (for example, tires used in cars are different than tires used in lorries) and on the brand. A study with different kinds of tires or with a mixture of them would be necessary due to the fact that different papers report different results.9,16 Another way to improve the competitiveness of coal liquefaction is to add a catalyst17 active enough to catalyze hydrogenation reactions without promoting hydrocraking reactions which mainly lead to gas formation. Iron catalysts have proven to be good hydrogenating and deoxygenating catalysts.18 In this paper, the coal-old tire coprocessing is performed in the absence of any solvent and iron oxide, as Red Mud (RM), has been added as the catalyst precursor. The influence of the feed coal-tire ratios and the pressure of the coprocessing are studied when a fixed catalyst load, 5 wt % Fe (daf) coal, is added, keeping constant the process temperature and reaction time. Experimental Section Discarded tires, supplied by AMSA (A. Mesalles, S.A., rubber recycling enterprise), ground and sifted to a particle size of 0.9 mm, were used. The steel threads and the textile nettings had been previously removed. Low-rank SAMCA coal from Utrillas, Spain, was used. The coal was ground and sifted to a particle size between 0.25 and 0.50 mm and stored in 250 g containers under a nitrogen atmosphere. The low-rank coal used in this work was selected because of the good conversions shown in previous experiments and already reported.19 The rubber used originated from nonselected discarded old tires. Coal and tire characteristics are shown in Table 1, where daf means dry and ash free, mf means moisture free, and af means ash free basis. The iron is added as iron oxide in Red Mud (dry basis, Fe2O3, 33.6%; Al2O3, 28.9%; TiO2, 11.1%; SiO2, 10.9%; and CaO, 4.6%; moisture 2.0% as received). The experimental installation used for hydrocoprocessing has been described in detail in a previous paper.19 The workup (14) Serio, A.; Wojtowicz, H. A.; Tang, H.; Pines, D. S.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 906. (15) Anderson, L. L.; Tuntawiroon, W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 816. (16) Farcasiu, M.; Smith, C. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 472. (17) Debyrshire, F.; Hager, T. Fuel 1994, 73, 7. (18) Mastral, A. M.; Mayoral, M. C.; Palacios, J. M. Energy Fuels 1994, 8, 94. (19) Mastral, A. M.; Murillo, R.; Pe´rez-Surio, M. J.; Calle´n, M. Energy Fuels 1996, 10 (4), 941.
of the conversion products was performed according to the procedure described in ref 20, and the experimental percentages were calculated according to the same reference. The gases were analyzed by GC with dual-column molecular sieve and Porapak N. C1-C4, H2S, and COx percentages in the gas mixture were calculated, and the total gas formation was calculated by difference. Oils were characterized by their elemental analysis and their hydrocarbon-type by thin layer chromatography (TLC-FID) in an Iatroscan MK-5 in its three main groups, saturated, aromatics, and polars, by eluting with n-hexane, toluene, and dichloromethane/methanol. The residues were characterized by SEM-EDX in a (SEM) ISI DS-130 with a Si/Li Kevex detector. The diffractograms were obtained in a Seifert 3000 diffractometer with Cu KR filtered with Ni radiation.
Results and Discussion Some results obtained with these two materials have been previously published.19 Now, the data obtained as a function of the composition of the mixture feed and starting hydrogen pressure when both are coprocessed after adding 5wt % Fe (daf) coal are shown in Figures 1-5. The other variables of the coprocessing have been fixed at 30 min and 400 °C according to the previous experience. As a reference point, the data obtained in the absence of catalyst are also shown in Figures 1-5. Afterward, the influence of each variable studied is thoroughly discussed. Influence of Feed Composition on Conversion Products. 1. Coal Feed. This low-rank coal exhibits a high level of reactivity under the operation conditions studied. At 10 MPa of initial pressure, the conversion obtained is higher than 90%. The product distribution shown in Figure 1 shows that the main product is the asphaltene fraction. The distribution of light products in favor of oils with a decrease in gas is the only effect that the addition of iron catalyst precursor involves. By decreasing the initial hydrogen pressure, the reaction atmosphere is less favorable to the hydrogenation mechanism, leading to a progressive decrease in conversion values. At 7.5 and 5 MPa, this decrease is clearly due to a minor extent of asphaltenic products, with the oil and gas fraction in similar percentages. This could imply that the thermal breakdown of the structure produces fragments of polar high molecular weight with a certain number of heteroatoms in their structures. The presence of catalyst at these pressures leads to an increase in coal structure breaking, but it has a negligible effect in terms of oil production. Initial pressures of 2.5 and 1 MPa of H2 are insufficient to promote the hydrocracking and hydrogenation reactions as can be deduced from the low conversions achieved. At these conditions the process is not positively affected by the catalyst presence. In fact, the precursor could not reach the active form (sulfide), as explained below, due to the absence of reductive atmosphere. According to these results, high hydrogen pressures are more influencing due to the promotion of hydrogenation and hydrocracking reactions, while Fe catalyst seems to be effective at promoting hydrogenation but not hydrocracking reactions. The gas formation, which was calculated by difference, does not seem to be affected by the catalyst (see Figure 1). These results (20) Mastral, A. M.; Rubio, B.; Izquierdo, M. T.; Mayoral, M. C.; Pe´rez-Surio, M. J. Fuel 1994, 73, 897.
Fe-Catalyzed Coal-Tire Coprocessing
Energy & Fuels, Vol. 11, No. 4, 1997 815
Figure 1. Percentage and conversion product distribution reached in processing of a 100% coal feed at 400 °C for 30 min depending on H2 pressure.
Figure 2. Percentage and conversion product distribution reached in processing of a 100% tire feed at 400 °C for 30 min depending on H2 pressure.
corroborate those obtained previously,18-21 according to which Fe catalysts have a medium activity in hydrogenation reactions which mainly lead to asphaltene formation from the coal structure breaking, but they do not show activity in hydrocracking reactions. 2. Tire Feed. The polymeric material in the rubber tire is completely solubilized during the thermal treatment under hydrogen atmosphere at all of the pressures studied, even under inert (low nitrogen pressure) atmosphere. Only the carbon black fraction is unconverted. The product distribution is very similar in all (21) Mastral, A. M.; Mayoral, M. C.; Izquierdo, M. T.; Rubio, B. Energy Fuels 1995, 9, 753.
of the conditions studied, and it is not affected by the presence of catalyst (see Figure 2). These results indicate that the addition of rubber tire to coal for coprocessing implies the addition of a material that can act as a liquefaction solvent in the conditions studied with two different roles: as a liquid medium that favors the mass transport and as a hydrogenating agent. This effect is studied thoughout different coaltire coprocessing ratios within the same pressure range. 3. Coal-Tire Mixture Feeds. With rich coal feed (80% coal-20% tire) (Figure 3), the trend followed is almost the same as when only coal is processed, but the differences in total conversions between catalyzed and
816 Energy & Fuels, Vol. 11, No. 4, 1997
Mastral et al.
Figure 3. Percentage and conversion product distribution reached in coprocessing of 80% coal and 20% tire feed at 400 °C for 30 min depending on H2 pressure.
Figure 4. Percentage and conversion product distribution reached in coprocessing of 20% coal and 80% tire feed at 400 °C for 30 min depending on H2 pressure.
noncatalyzed processes are higher at 10 and 7.5 MPa hydrogen pressures. The higher conversions do not lead to higher oil formation but to higher asphaltene percentages. However, by comparison with the theoretical conversions that could be expected, due to the data knowledge with pure materials, the positive effect of coprocessing coal with tire in the presence of Fe catalyst is reflected in the oils obtained at 7.5 and 5 MPa, which reach values 2.7% and 2.6%, respectively, higher than the theoretical ones. At 2.5 MPa hydrogen pressure, a synergism is observed with a dramatic increase (8.4% points) in conversion compared to the theoretical datum, due to the dramatic increase in asphaltene formation. Results are shown in Figure 3.
With rich tire feed (20% coal-80% tire) (see Figure 4), the conversion is so high that it screens the effect of the Fe, so the catalyst addition does not seem to be effective. Furthermore, there is even a slow decrease in total conversion in the presence of RM which is reflected in lower asphaltene formation which is, on the other side, accompanied by a slight increase in oil formation, about 1% higher. With a 50%-50% mixture feed (see Figure 5), higher conversions are observed in the presence of RM. The increase in conversion products is distributed in the same proportion between oils and asphaltenes. By comparison between the experimental and the theoretical calculated values, a synergism can be observed22 at
Fe-Catalyzed Coal-Tire Coprocessing
Energy & Fuels, Vol. 11, No. 4, 1997 817
Figure 5. Percentage and conversion product distribution reached in coprocessing of 50% coal and 50% tire feed at 400 °C for 30 min depending on H2 pressure. Table 2. Composition of Gases Obtained in 80% Coal-20% Tire Catalyzed and Noncatalyzed Coprocessing at 400 °C for 30 min and 10, 7.5, and 5 MPa of Cold H2
the lowest pressures, mainly at 2.5 MPa at which an increase of 8%, mainly corresponding to asphaltene formation. It is worth commenting that, even in an inert atmosphere, coal and tire promote each other; conversion is 3 percentage points higher than the theoretical one, showing a tire slight hydrogenating effect which is reflected in a higher asphaltene formation. Catalyst Evolution. The morphological studies have been performed by SEM on powder samples metalized with a thin layer of Au/Pd to avoid static electricity. The morphological appearance dramatically changes versus hydrogenation pressures. At low pressures, the solid appearance looks heterogeneous with different porous sizes. As the hydrogen pressure increases, an increase in homogeneity is observed, decreasing both the particle size and the porous size (see Figures 6-9). The XRD and the EDX analyses show a S, Ca, and Fe relative abundance in the raw coal mineral matter. The sulfur distribution is parallel to the Fe distribution, due to pyrite. Ca is associated to S as gypsum and also appears as calcite. Pyrite and gypsum are the main components of the mineral matter. Other detected species by SEM-EDX are Al and K silicates, and when the process is going on, gypsum is transformed into anhydrite and Fe from pyrite into pyrrhotite. TiO2 undergoes no variation. With rich tire feeds and at 1 MPa hydrogen pressure, the iron oxide from the catalyst remains unaltered, as it happens with TiO2 without being converted in sulfur retention centers. With increasing hydrogen pressures, Fe is converted into pyrrhotite, Fe1-xS and FeS. While the later sulfide has no vacancies, the stoichiometry of the former shows the existence of Fe vacancies. The hydrogen pressure influences very clearly both Fe chemical species. The higher the hydrogen pressure, the higher the S/Fe ratio. Consequently, S retention
by the catalyst increases with the hydrogen pressure which is corroborated by the lower H2S percentage in gas formation according to the GC analysis, as it is shown in Table 2. Previous results18 show higher conversions for the same coal and catalyst when only coal is processed, in comparison to the coprocessing results despite using the same reactors. It must be due to the fact that a rubber layer continuously coats the surface of coal, clearly observed at 1 MPa of hydrogen pressure interfering with the coal-hydrogen contact (see Figure 6). This effect decreases when the hydrogen pressure increases but, anyway, could be the responsible for the lower conversions obtained in coprocessing. Conversion Product Characterization. According to the results shown in this paper, the Fe addition is relevant to asphaltenes formation but its influence concerning oil and gas formation is very low. The production of volatile products is the consequence of both the hydrogenation reactions and the hydrocracking reactions.23 However, asphaltenes have their origin mainly in coal structure disruption and, to a lesser extent, from retrogressive and repolymerization reactions because of the high severity of this process.20 The lightest conversion products, gases and oils, have been characterized by gas chromatography and thin layer chromatography, respectively. As Table 2 shows C1-C3, H2S, and COx (CO + CO2) have been analayzed
(22) Mastral, A. M.; Murillo, R.; Mayoral, M. C.; Calle´n, M. S. Energy Fuels, in press.
(23) Mastral, A. M.; Mayoral, M. C.; Izquierdo, M. T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (1), 121.
pressure (MPa) 10 7.5 5
no catalyst %C1-C3 %COx %H2S 14.1 21.2 20.6
66.4 52.3 54.2
19.6 26.5 25.4
Fe catalyst (Red Mud) %C1-C3 %COx %H2S 23.2 22.0 18.7
66.5 69.9 74.5
10.3 8.1 6.7
818 Energy & Fuels, Vol. 11, No. 4, 1997
Mastral et al. Table 5. Oil Hydrocarbon-Type Composition from Tire Catalyzed and Noncatalyzed Processing at 400 °C for 30 min and 10, 7.5, and 5 MPa of Cold H2
Figure 6. Morphological aspect of the THF insoluble from 20% coal-80% tire hydrocoprocessing at 400 °C for 30 min and 1 MPa of starting hydrogen pressure. Table 3. Oil Hydrocarbon-Type Composition from 80% Coal-20% Tire Catalyzed and Noncatalyzed Coprocessing at 400 °C for 30 min and 10, 7.5, and 5 MPa of Cold H2 pressure (MPa)
sat.
10 7.5 5
11 10 8
no catalyst arom. polar 45 40 41
44 50 51
Fe catalyst (Red Mud) sat. arom. polar 11 10 11
42 44 45
47 46 44
Table 4. Oil Hydrocarbon-Type Composition from Coal Catalyzed and Noncatalyzed Processing at 400 °C for 30 min and 10, 7.5, and 5 MPa of Cold H2 pressure (MPa)
sat.
10 7.5 5
2 2 4
no catalyst arom. polar 36 23 21
62 75 75
Fe catalyst (Red Mud) sat. arom. polar 1 1 1
21 15 18
78 84 81
for 80% coal-20% tire catalyzed and noncatalyzed experiments. If catalyzed and noncatalyzed results in Table 2 are compared, it is observed that there is a clear H2S retention because, as it has been commented in the above section, it is captured by iron to produce iron sulfide. Also, a higher oxygen elimination in catalyzed processes can be observed due to the increase in COx formation. These results corroborate the desoxygenation effect of the iron catalyst.18 So, the Fe catalyst performs a double effect: there is a sulfur retention (the iron oxide is converted into pyrrhotite, the catalytic active species), and a heteroatom release is performed by the iron catalyst (there is an increase in COx formation). Tables 3-5 show, respectively, the hydrocarbon distribution in oils from coal-tire, coal, and tire processing. The thin layer chromatography shows that the three main components are saturated, aromatic, and polar hydrocarbons. Polar hydrocarbons are the main components when coal is processed alone (see Table 4) and when coal and tire are coprocessed using a feed mixture of 80% coal-20% tire by weight (see Table 3). Aromatic and polar hydrocarbons are the main components of oils from tire (see Table 5). It is worth commenting that
pressure (MPa)
sat.
10 7.5 5
11 8 7
no catalyst arom. polar 49 42 42
40 50 51
Fe catalyst (Red Mud) sat. arom. polar 19 14 13
58 53 55
23 33 32
the aromatic hydrocarbons peak in tire-derived oils shows two maxima, related to two different kinds of compounds, monocyclic and polycyclic aromatics. Oils obtained from coal in the presence of catalyst have been shown to be of higher polar nature than in the absence of Fe. Quantitatively, the amount of saturates and aromatics is the same; the presence of iron only involved an increase in the polar fraction, due to the hydrocracking effect which can decompose the coal network structure. In tire processing, the effect of iron catalyst is not reflected on oil formation but in oil composition: the polar fraction undergoes an important decrease. This effect can be due to two different catalyst mechanisms: the prevention of a retrogressive recombination of radicals formed during depolymerization or the hydrocraking of high molecular weight species formed by retrogressive reactions. In this way, the percentage of polar hydrocarbons decreases in favor of saturates and aromatics in similar proportions at all pressures studied, although to different extents. In oils from both material coprocessings, both effects, higher oils selectivity and higher saturated and aromatic percentages, are reached in catalyzed processes versus noncatalyzed ones. Conclusions In conclusion, it can be summarized that Fe is not a very active catalyst in coal-tire coprocessing. Added as RM, the iron oxide is converted into iron sulfide in such a way that the higher the hydrogen pressure, the higher the conversion into the monosulfide species. The main fact of the catalytic role of the Fe is reflected in a higher amount of asphaltene formation. The weak Fe catalytic activity shown in coal-tire coprocessing seems to be due to (a) the excellent conversions reached in tire hydrogenation, making unnecessary the Fe addition, and (b) the tire polymeric nature: its plasticity makes a thin layer, the thickness depends on the hydrogen pressure, that partially covers the reacting coal surface, hindering the interactions with the hydrogenating radicals involved in the process. The higher the coprocessing pressure, the higher the aromatic formation. The TLC-FID shows two types of aromatic hydrocarbons when tire-derived oils were analyzed: the first one probably composed of mainly alkylbenzenes and the second one composed of polycyclic compounds. The RM addition to the coprocessing makes both gas and oil formation show higher polarities. When RM is added to tire processing, the polar hydrocarbons in oils are converted into aromatic and saturate hydrocarbons. EF960171I
