Environ. Sci. Technol. 2004, 38, 3189-3194
Complete Study of the Pyrolysis and Gasification of Scrap Tires in a Pilot Plant Reactor
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J U A N A . C O N E S A , * I . M A R T IÄ N - G U L L O Ä N, R. FONT, AND J. JAUHIAINEN Department Chemical Engineering, University of Alicante, Ap. 99, E-03080 Alicante, Spain
The pyrolysis and gasification of tires was studied in a pilot plant reactor provided with a system for condensation of semivolatile matter. The study comprises experiments at 450, 750, and 1000 °C both in nitrogen and 10% oxygen atmospheres. Analysis of all the products obtained (gases, liquids, char, and soot) are presented. In the gas phase only methane and benzene yields increase with temperature until 1000 °C. In the liquids the main components are styrene, limonene, and isoprene. The solid fraction (including soot) increases with temperature. Zinc content of the char decreases with increasing temperature.
Introduction One of the ways of recycling organic wastes such as used tires is the energetic revalorization by combustion, although another way is its pyrolysis in order to obtain chemicals (1). Tires are mostly composed of different rubbers, such as natural rubber (NR), butile rubber (BR), or styrene-butadiene rubber (SBR) as well as mineral oils and carbon black. Despite the fact that scrap tires represent slightly more than 12% of all solid waste, scrap tires present a special disposal and reuse challenge because of their size, shape, and physicochemical nature. Tire wastes have a high amount of energy, so energy recovery could be a good recycling strategy. Considering the low calorific value of several solid residues (for example, wood 10 178 kJ/kg, municipal solid wastes 12 374 kJ/kg, lignite 16 983 kJ/kg, subbituminous coal 24 428 kJ/kg, tires 33 035 kJ/kg), it can be seen that, for example, tire is more “energetic” than bituminous coal, a classical energy source. The thermal degradation of tires produces a wide variety of products in the oil and gas phase, in addition to the residual char. The thermal degradation of the individual rubber components of tires has been analyzed using a variety of techniques to determine the pyrolysis products and their degradation mechanism. Groves et al. (2) analyzed the oil derived from the pyrolysis of natural rubber in a pyrolysisgas chromatograph at 500 °C. They showed that the major products were monomer (isoprene) and dimer (dipentene), with other oligomers up to hexamer in significant concentrations. They suggested that the isoprene monomer was formed via a depropagating mechanism in the polymer chain and that the dipentene dimer was formed by either intramolecular cyclization followed by scission or by monomer recombination via a Diels-Alder reaction. Tamura et al. (3) have also shown that isoprene and dipentene are formed in high concentration in natural rubber pyrolysis and have * Corresponding author phone: 34+6-590-3400 ext 2324; fax: 34+6-590-3826; e-mail:
[email protected]. 10.1021/es034608u CCC: $27.50 Published on Web 04/23/2004
2004 American Chemical Society
suggested that both are produced by depolymerization from polymer radicals occurring by beta-scission at double bonds. The polymer radicals are liable to form six-membered rings, especially under mild pyrolysis conditions, so the dipentene is formed predominantly at lower temperature. Bhowmick et al. (4) also examined the pyrolysis of natural rubber using TGA. They showed that degradation started at 330 °C in nitrogen, with a peak weight loss at 400 °C. They suggested that decomposition followed from radical generation via polymer chain scission and the formation of isoprene, dipentene, and other smaller compounds. Chien and Kiang (5) pyrolyzed natural rubber in helium at 384 °C and identified isoprene and dipentene as the main products. They also identified a wide range of other products, including alkane and alkene gases, toluene, xylene, octene, and hydrocarbons up to C16H26. They also suggested a mechanism similar to that of Bhowmick et al. (4), in which isoprene and dipentene were formed by polymer chain scission and minor compounds were formed via chain propagation with or without intramolecular hydrogen transfer. The pyrolysis of polybutadiene rubber to 550 °C in nitrogen was examined by Brazier and Schwartz (6) using TGA. They stated that decomposition took place in two stages, with maximum decomposition rates at 370 and 470 °C, which depended on the heating rate and sample size. The material which did not decompose to yield gaseous products forms a solid residue by cyclation and cross-linking, which is degraded in the second stage yielding more gases. The products of pyrolysis from the second stage were a complex mixture of hydrocarbons. Madorsky et al. (7) have also examined the pyrolysis of polybutadiene rubber and similarly showed that butadiene, vinylcyclohexene, and dipentene were formed in high concentrations. Other compounds formed were ethylene, ethane, propylene, propane, butylene, butane, cyclopentadienes, etc. The pyrolysis of styrene-butadiene rubber (SBR) between 240 and 450 °C was investigated by Erdogan et al. (8) using a pyrolysis-mass spectrometer. They identified butadiene and butadiene fragments at lower pyrolysis temperatures and styrene and/or benzene at higher temperatures. They suggested that the thermal degradation of SBR started with the butadiene elements of the copolymer and that higher temperatures were required to degrade the styrene elements. Other compounds identified in tire pyrolysis oils in significant concentrations have included benzene, toluene, xylene, styrene, limonene, indane, indene, polycyclic aromatic hydrocarbons such as naphthalene, fluorene, and phenanthrene, vinylalkenes, alkanes. The main gases produced during the pyrolysis of tires are CO2, CO, H2, CH4, C2H6, C3H6, C3H8, and C4H6, with lower concentrations of other hydrocarbon gases. Bhowmick et al. (4) and Chien and Kiang (9) have suggested that the other products of pyrolysis can be accounted for by the thermal decomposition of isoprene and dipentene. Tamura et al. (3) have also suggested that benzene may be formed as a direct result of the thermal degradation of the rubber polymer via the formation of conjugated double bonds in the polymer chain. Where extensive secondary reactions of the pyrolysis vapors occur, the formation of aromatic and polycyclic aromatic compounds has been attributed to a Diels-Alder cyclization reaction to alkenes, particularly at either high temperatures or long residence times (10, 11). The pyrolysis of tires leads to the production of ethene, propene, and 1,3-butadiene, which react to form cyclic alkenes. Dehydrogenation of the cyclic alkene compounds with six carbon atoms occurs, producing single-ring aromatic compounds such as benzene and, as a VOL. 38, NO. 11, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Proximate and Ultimate Analysis of the Tire proximate analysis
% weight
moisture volatile matter fixed carbon ash
0.9 65.5 29.4 3.7
ultimate analysis
% weight
C H N S
89.4 7.0 0.2 2.0
result of subsequent associative reactions, may lead to the formation of polycyclic aromatic hydrocarbons such as naphthalene and phenanthrene and its substituted derivates. In previous papers of our research group, the kinetics of the decomposition of tire wastes when heating at different rates during thermogravimetric measurements was studied, both in nitrogen and oxygen atmospheres (12, 13), and a complete kinetic model was also developed. The products formed during pyrolysis (1) and partial oxygen supply (14) were also studied. In this last paper, special interest was placed in the pollutant formation and control. A paper presenting the gas evolution in different equipments was published (15). It was shown that the decomposition of tire wastes at three different heating rates (1, 5, and 25 °C/min) is explained by means of a kinetic model including 3-organic fractions that do not form char and an inorganic fraction that is inert to pyrolysis. The mass spectrometer showed that the evolution of gases takes place in three different stages, confirming that three different groups of compounds are pyrolyzed. The decomposition in an oxidizing atmosphere was explained by means of a kinetic model including the first step of pyrolysis and a step of combustion of the residue formed. On the other hand, the yield of total gas obtained in the pyrolysis in a fluidized bed reactor increased in the range 600-800 °C from 6.3 to 37.1%. At higher temperatures, the yield of total gas decreased slightly. The formation of methane, hydrogen, benzene, and toluene is favored by high residence times, but ethane, ethylene, propane, propylene, butane, butylene, acetylene, 1,3-butadiene, and pentane undergo cracking to different extents at increasing residence times and/or temperature. In other work (14), over 100 volatile and semivolatile compounds evolved in the combustion of scrap tires in a horizontal laboratory furnace were identified and quantified by gas chromatography mass spectrometry, plotting their yields vs the bulk air ratio and temperature. Five different behaviors considering the bulk air ratio and the temperature were identified. The objective of the present study is to investigate the pyrolysis and gasification of scrap tires in a pilot plant equipment at different temperatures, with semicontinuous feeding of the material. An exhaustive analysis of the products evolved is done, including the quantification and analysis of the gas, liquid, and solid fractions.
Materials and Equipment The tires used for the study were supplied by a local recycling plant. Tires were crushed by physical procedures (no cryogenic immersion was needed) until a particle size of approximately 4 mm. Both proximate and ultimate analysis of the material are presented in Table 1. An scaled scheme of the reactor and condensation system can be seen in Figure 1; the length of the reactor that is inside the furnace is 654 mm. The whole system has four different zones: 3190
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FIGURE 1. Schematic of the reactor system. I. Solid feeder. The scrap tires are fed to the reactor by a two-valve manual system. This systems allows the introduction of the tire particles without gas venting through the feeder. The appropriate amount of tire is placed in the hopper and passes first the upper valve, and, after closing it, the lower valve is opened. The feeding rate has been 5 g each 10 min during at least 2 h. II. Gas line. The carrier gas is first preheated by circulating vertically between two cylindrical tubes: the outer one is the calandria and the inner one is the reactor itself, where the solids are fed. The gas flow rate was 1.5 L/min. In the pyrolysis runs industrial nitrogen (purity = 99.5%) was used. In the gasification runs, a nitrogen-synthetic air mixture, with 10% of oxygen, was fed. III. Reactor. This is the part that is inside the vertical electric furnace. The nominal temperatures used in the runs have been 450, 750, and 1000 °C. The temperature is controlled by a type-K thermocouple sited inside the furnace near to the reactor. Measure of the temperature inside the reactor has been performed by introducing a thermocouple by its upper side. The measurements at different positions shows a maximum difference of 10 °C between the furnace and the inner part of the reactor. IV. Cooling and condensation system. This was done by means of a jacket filled with solid CO2 (-78 °C), approximately 300 g in each run. The temperature inside the system in this zone is lower than 13 °C in all the runs, so a very good condensation of semivolatile species is expected. Description of an Experiment. All the experiments and analysis of the different fractions (except that of the solid) were repeated twice in order to check for the reproducibility. The steps followed to run an experiment are as follows: 1. The temperature is selected in the furnace and begins the heating. A high flow of air is used during the heating, to eliminate rests (coke) of the previous run. 2. Dry ice is placed in the condensation jacket.
3. Once the reactor is at the selected temperature, the flow rate of the gases was fixed at 1.5 L/min. After 15 min scrap tires were fed 5 g each 10 min. Note that in this way, the residue solid (char) accumulates inside the reactor during the experiment. 4. After 1 h, a gas sample is taken directly from the upper part of the reactor. A trap for condensable species is inserted after the probe (water jacket, XAD-2 resin and silica gel), and a pump is used to control the flow rate of the sampling system (maintained at = 300 mL/min). The gas sample is collected throughout 15 min (approximately 5 L of gas sample) in a Tedlar bag sited after the pump. After the second hour of experiment another gas sample is taken. Later, the feeding of the tires is interrupted. In the first runs a gas sample was also taken from the exit of the system, near to the burner; it was checked that the gas analysis is very similar to that performed over the sample taken directly from the furnace. 5. The furnace is maintained at the reaction temperature for 2 more hours, to get a solid completely reacted. The carrier gas flow rate is also maintained during this time. In a gasification run performed at 750 °C the gas was kept only during 30 min, to see the effect of this heating. 6. The furnace is switched off and after cooling, and the solid char was recovered from the reactor. The tars condensed in the cold zone are removed by opening valve 8 in Figure 1, and the condensation system is cleaned with acetone; the acetone with the rests of tars are also collected, and later the acetone is evaporated at vacuum and low temperature (
