878
Energy & Fuels 2002, 16, 878-886
Characterization of Chars Obtained from Co-pyrolysis of Coal and Petroleum Residues I. Suelves, M. J. La´zaro, M. A. Diez,† and R. Moliner* Instituto de Carboquı´mica, CSIC, Marı´a de Luna 12, 50015 Zaragoza, Spain, and Instituto Nacional del Carbo´ n (INCAR), CSIC, Apartado 73, 33080 Oviedo, Spain Received November 6, 2001. Revised Manuscript Received April 6, 2002
Co-pyrolysis of coal and petroleum residue has been carried out in a bench scale unit in order to study the influence of the coal nature and the experimental conditions on the characteristics of the char obtained. Two coals of different rank, Samca (sub-bituminous) and Figaredo (bituminous), and a petroleum residue from the Maya crude have been used. Temperatures of 600, 650, and 700 °C, pressures of 0.1, 0.5, and 1 MPa, and mass ratios (Coal/PR) of 70/30 and 50/50 have been studied. A synergistic effect on char yields, which increases as temperature, pressure, and PR/coal ratio increase, is observed for both coals studied. Some relevant char characteristics of sulfur content, reactivity, and coking properties have been analyzed in order to determine a final use for the chars obtained. It is concluded that Samca/PR chars, especially for the 70/30 mixture, could be gasified to produce syn-gas. Coal /PR ratio higher than 50/50, temperature lower than 700 °C, and atmospheric pressure should be used in co-pyrolysis with Samca coal, to keep chars reactive enough to be gasified. Figaredo/PR chars, having a low sulfur content, good optical properties, and a lower reactivity could be used as a coke feedstock.
Introduction Pyrolysis of hydrocarbon materials such as coal and petroleum residue has the advantage versus other conversion technologies of promoting transfer of hydrogen from the parent material to the gas and liquid products, concentrating carbon in the residual char. This is an important advantage in the present energetic scenario in which carbon-free or at least low-carbon fuels need to be promoted to reduce CO2 emissions. Gases produced from pyrolysis of coal contain high concentration of hydrogen and light olefins, whereas liquids contain BTX, PCX, and other aromatic compounds that can be upgraded into desired hydroaromatic fuel components or used as chemical feedstocks. However, the yields of these products are limited because of the low hydrogen-to-carbon ratio in coal.1-3 For this reason, it is necessary to supply hydrogen from external sources to increase the liquid yields obtained from coal pyrolysis. One of the most promising ways to provide hydrogen at a reasonable cost is to take it from hydrocarbon wastes that can be co-pyrolyzed with coal. There are many works in the literature4-8 related to the co-pyrolysis of coal and materials of a hydrocarbon nature such as tires, plastic, and other wastes, whose aim is to enrich two materials of low value. Moliner et al.9 conducted a research project to study the valorization of coal by co-pyrolysis with lubricant oil wastes and petroleum residue. * Author to whom correspondence should be addressed: E-mail:
[email protected]. † Instituto Nacional del Carbo ´ n (INCAR), CSIC. (1) Jime´nez, J. Ingenierı´a Quı´mica 1999, 5, 149-155. (2) Ofosu-Asante, K.; Stock, L. M.; Zabransky, R. F. Fuel 1989, 68, 567-572. (3) Hayashi, J.; Kawakami, T.; Taniguchi, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993, 7, 1118-1122.
On the other hand refineries need new technologies to convert the heavy streams into lighter or more valuable products in order to improve its strategic position.10 Low-value petroleum feedstocks are usually converted in oil refineries into useful products using the delayed coke process which is a kind of mild pyrolysis.11,12 Coal and petroleum residue can be co-pyrolyzed and two benefits could be expected: (i) petroleum residue containing hydrogen-rich compounds will act as hydrogen-donors promoting the conversion of coal and improving the quality of the products obtained; and (ii) the presence of coal could improve some properties of the products obtained from petroleum residue. There are few references in the literature on this subject because most of the works on co-utilization of coal and petroleum residues are related to catalytic coprocessing reactions using high hydrogen pressures. Tomic and Schobert,13 Martin et al.,14 and Fickinger et al.15-17 studied coal/petroleum resid interaction during (4) Miura, K.; Mae, K.; Asaoka, S.; Hashimoto, K. Energy Fuels 1991, 5, 340-346. (5) Fontana, A.; Braekman-Danheux, C.; Laurent, P. Coal Sci. Technol. 1995, 24, 1089-1092. (6) Laurent, P.; Braekman-Danheux, C.; Fontana, A.; Lecharlier, M. In Ziegler et al., Eds., ICCS’9, Vol. II, Essen, 1997, 837-840. (7) Palmer, S. R.; Hippo, E. J.; Tandon, D.; Blankenship, M. Coal Sci. Technol. 1995, 24, 29-33. (8) Mastral, A. M.; Alvarez, R.; Callen, M. S.; Clemente, C.; Murillo, R. Ind. Eng. Chem. Res. 1999, 38, 2856-2860. (9) Moliner, R. Final Report Project CECA 7220-EC/763, 1995. (10) Klose, W.; Stuke, V. Fuel Process. Technol. 1993, 36, 283-289. (11) Rodrı´guez-Reinoso, F.; Santana, P.; Romero-Palazo´n, E.; Diez, M-A.; Marsh, H. Carbon 1998, 36, 105-116. (12) Martı´nez-Escandell, M.; Torregrosa, P.; Marsh, H.; Rodrı´guezReinoso, F.; Santamarı´a-Ramı´rez, R.; Go´mez de Salazar, C.; RomeroPalazo´n, E. Carbon 1999, 37, 1567-1582. (13) Tomic, J.; Schobert, H. H. Energy Fuels 1997, 11, 116-125. (14) Martin, S. C.; Tomic, J.; Schobert, H. H. Abstracts of papers of the Am. Chem. Soc., Div. Fuel Chem. 1997, 42, 121-124.
10.1021/ef010264m CCC: $22.00 © 2002 American Chemical Society Published on Web 06/11/2002
Chars from Co-pyrolysis of Coal and Petroleum Residues
co-processing under nitrogen and noncatalytic conditions using a lab-scale simulated delayed coker and at temperatures lower than 450 °C. In previous works,18,19 carried out at analytical scale, it has been shown that there is a synergistic effect on the production of some interesting compounds such as light olefins and BTX when coal and petroleum residue are co-pyrolyzed. However, in the experimental device used, a pyroprobe 1000, it was not possible to determine the char yields and as a consequence to determine the influence of co-pyrolysis on char formation and char characteristics. In this paper, the co-pyrolysis of coal/ petroleum residue mixtures has been carried out in a bench scale unit at different temperatures, pressures, and coal/resid ratios. This experimental device allows us to recover all the pyrolysis products in order to calculate the mass balances and to characterize all the products obtained from co-pyrolysis. The evaluation of char characteristics is of a great importance because char is the main product and so, the global feasibility of the process strongly depends on them. Two ways of utilization of co-pyrolysis chars may be considered as the most promising ones: (i) gasification for syn-gas production or power generation; and (ii) blending with high quality coke-making coals in order to reduce the final coke price. Regarding the first alternative, steam gasification, chars should be kept reactive enough. It has been shown20 that char reactivity in gasification and combustion processes depends on several facts like coal precursor rank and pyrolysis conditions. In the second alternative, it has been shown21,22 that the residues from petroleum distillation are one of the most typically used materials for coke production. Coke yields of around 20-25% are obtained from these residues. In this context, the incorporation of petroleum coke as a minor component to coal blends can be considered as a good way to modify coal plastic properties and improve metallurgical coke properties.23-25 The main objective of this work is to study the properties that can be considered as the most relevant to decide the final use of the chars. In addition, the use of the optical microscopy in char characterization allows the evaluation of the interactions between coal and petroleum residue and how they affect the development of optical anisotropy and coke forming structures. (15) Fickinger, A. E.; Badger, M. W.; Mitchell, G. D.; Schober, H. H. Abstracts of papers of the Am. Chem. Soc., Div. Fuel Chem. 1999, 44, 106-109. (16) Fickinger, A. E.; Badger, M. W.; Mitchell, G. D.; Schobert, H. H. Abstracts of papers of the Am. Chem. Soc., Div. Fuel Chem. 1999, 44, 684-687. (17) Fickinger, A. E.; Badger, M. W.; Mitchell, G. D.; Schobert, H. H. Abstracts of papers of the Am. Chem. Soc., Div. Fuel Chem. 2000, 45, 299-303. (18) Moline, R.; Suelves, I.; La´zaro, M. J. Energy Fuels 1998, 12, 963-968. (19) Suelves, I.; Moliner, R.; La´zaro, M. J. J. Anal. Appl. Pyrolysis 2000, 55, 29-41. (20) Miura, K.; Makino, M.; Silveston, P. Fuel 1990, 69, 580-589. (21) Rodrı´guez-Reinoso, F.; Santana, P.; Romero-Palazo´n, E.; Dı´ez, M. A.; Marsh, H. Carbon 1998, 36, 105-116. (22) Adams, H. A. In Introduction to Carbon Technologies; Marsh, H., Heintz, E. A., Rodriguez-Reinoso, F., Eds.; Universidad de Alicante: Spain, 1997; Chapter 10. (23) Mene´ndez, J. A.; Pis, J. J.; A Ä lva´rez, R.; Barriocanal, C.; Canga, C. S.; Diez, M. A. Energy Fuels 1997, 11, 379-384. (24) Ruiz, O.; Romero-Palazo´n, E.; Diez, M. A.; Marsh, H. Fuel 1990, 69, 456-459. (25) Sakurovs, R.; Lynch, L. J. Fuel 1993, 72, 743-749.
Energy & Fuels, Vol. 16, No. 4, 2002 879 Table 1. Proximate and Ultimate Analysis of the Coal Sample Samca Figaredo
C
H
N
O
S
ash
volatiles
H/C
75.9 91.2
5.3 4.2
0.7 1.8
12.27 0.6
5.8 2.3
22.8 3.9
36.9 17.1
0.84 0.55
Experimental Section Samples. Two coals of different rank, the Samca coal (Teruel, Spain) and the Figaredo coal (Asturias, Spain), and a petroleum residue have been used. The Samca coal is a subbituminous coal with high volatile matter and high sulfur and oxygen contents, while the Figaredo coal is a bituminous coal with low volatile sulfur and oxygen contents. Table 1 shows the main characteristics of both coals. It can be observed that the Figaredo coal has a more condensed structure (lower H/C ratio) than the Samca coal. The petroleum residue proceeds from vacuum distillation of a Maya crude and has been provided by REPSOL (Puertollano, Spain). The aliphatic/ aromatic nature of PR has been evaluated by thin-layer chromatography,26, 27 showing that the PR used in this work has 44.2% of aliphatic fraction, 26.6% of aromatic fraction, and 29.2% of nonmoving material. Bench Scale Pyrolysis Unit. Figure 1 shows the bench scale pyrolysis unit used. Briefly, it consists of a drop tube reactor heated by an electrical oven with two different parts; the bottom of the reactor, the reaction zone, is filled with ceramic rings. The sample is fed from a hopper situated on the upper part and falls into the reactor by gravity. The hopper is slightly over-pressurized and the sample is introduced once the reactor has reached the selected pressure and temperature. The pyrolysis products leave the reactor from the bottom and they are cooled in a trap cooled by the Peltier effect. The liquids are collected in a liquid-cyclone, placed below the cool trap, and the gases are collected in a gas sampling bag whose volume was measured after each run. The coal/PR mixtures were prepared as follows: PR was dissolved in tetrahydrofuran (THF) and, then, coal added to the solution. To promote mixing of PR and coal, the mixture was sonicated for 15 min. THF was removed by heating under vacuum. The dried samples were ground to
