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Energy & Fuels 2006, 20, 754-758
Production of Hydrogen from Plastics by Pyrolysis and Catalytic Steam Reform Stefan Czernik* and Richard J. French National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado 80401 ReceiVed October 28, 2005. ReVised Manuscript ReceiVed December 29, 2005
Thermal decomposition of the most common plastics such as polyethylene and polypropylene produces a mixture of many different hydrocarbons that can be used as a feedstock for producing hydrogen. The proposed process includes two steps: pyrolysis of plastics and catalytic steam reforming of pyrolysis gases and vapors. This research provides a proof of concept supported by experiments on selected polymers performed using a microscale reactor system interfaced with a molecular beam mass spectrometer and demonstrates process performance using a two-reactor bench-scale system. From 60 g/h polypropylene fed to the system 20.5 g/h hydrogen was produced, which corresponds to 80% of the theoretical potential.
Introduction At present, hydrogen is commercially produced mostly by catalytic steam reforming of hydrocarbons: natural gas and naphtha. However, processes using other raw materials to produce hydrogen, especially wastes and byproducts, are also attractive because of promising economic and environmental benefits. In this research we explored possibilities for producing hydrogen from post-consumer waste plastics. Plastics, especially polyolefins, have significant potential but are not yet used as a resource for hydrogen production. Plastics account for 8-9% of today’s waste stream, or about 15 million tons annually,1 that are mostly disposed of in landfills. Potentially, these waste plastics could be used to generate six million tons of hydrogen per year. Though recycling of plastics has a positive environmental impact, in most cases it is not yet economically attractive. So far, industry has focused efforts in plastics recycling on the recovery and reuse of polymers by mechanical processing. However, mechanical technologies require relatively clean feedstocks that are expensive to collect and separate. For this reason, commercial recycling has not had a significant impact with the collection rate of less than 5% of total annual resin sales. Producing hydrogen from waste plastics could complement conventional recycling techniques because it could use more complex materials that are not handled by existing processes, e.g. mixed plastics, polyester-cotton blends, rigid polyurethane foams. The challenge is to efficiently convert these polymers to hydrogen at a cost similar to that for the existing natural-gas-based technologies. There has been considerable work conducted on pyrolysis of polymers, some of which aimed at pyrolytic recycling of plastics to monomers and fuels. The chemistry of plastic pyrolysis was reviewed by Wall et al.2 who concluded that thermal treatment at sufficiently high temperature results in breaking polymer chains and subsequent formation of smaller * To whom correspondence should be addressed. E-mail:
[email protected]. (1) A Research Needs Assessment for Waste Plastics Recycling; DOE/ ER-30168, Vol. 2; December 1994. (2) Wall, L. A. The Mechanism of Pyrolysis, Oxidation, and Burning of Organic Materials; NBS Spec. Pub. 357.
molecules. Some polymers, e.g., acrylics, decompose thermally and generate high yields of monomers. However, the most common plastics such as polyethylene and polypropylene do not appear to give high yields of monomers but rather a mixture of many different hydrocarbons with proportions depending on the process conditions (mostly temperature and catalyst). In most cases, it is not practical to isolate a single chemical or a fraction from those mixed hydrocarbons or oxygenated compounds derived from other polymers. The most straightforward application for the whole product stream is the use as fuel. Thermal conversion of plastics, both pyrolysis and gasification, has been extensively studied, and commercial processes have been developed3,4 to convert waste plastics to fuels. However, very few works focused on hydrogen as the main product from waste plastics. Pinto et al.5 studied a fluidized bed co-gasification of coal, biomass, and plastics to generate hydrogen-rich gas. Gebauer et al.6 patented a process for manufacturing synthesis gas from petroleum residues and waste plastics. Recently, Tsuji et al.7 reported on catalytic steam reforming of oils produced by pyrolysis of plastics at 350-400 °C. We propose an integrated process, in which the whole volatile product of plastics pyrolysis would be directly used as a feedstock for catalytic steam reforming to optimize the production of hydrogen according to the equations:
CnHmOk + (n - k)H2O f nCO + (m/2 + n - k)H2 nCO + nH2O T nCO2 + nH2 CnHmOk + (2n - k)H2O f nCO2 + (m/2 + 2n - k)H2 This research comprises two parts: first the proof of the pyrolysis/reforming concept for producing hydrogen from plastics using a microscale reactor system interfaced with a (3) Plastics Recycling. In Uhlmann’s Encyclopedia of Industrial Chemistry; VCH: Weinheim, 1992; Vol. A21, pp 64-68. (4) Menges, G.; Fischer, R.; Lackner, V. Int. Polym. Process. 1992, 7, 291-302. (5) Pinto, F.; Franco, C.; Andre, R.; Tavares, C.; Dias, M.; Gulyurtlu, I.; Cabrita, I. Fuel 2003, 82, 1967-1976.
10.1021/ef050354h CCC: $33.50 © 2006 American Chemical Society Published on Web 01/26/2006
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Figure 1. Schematic of the MBMS-pyrolysis/reforming reactor system.
molecular beam mass spectrometer (MBMS), and second the bench-scale demonstration of process performance including yields of hydrogen and mass balances. Experimental Section Microreactor/Molecular Beam Mass Spectrometer System. The experimental setup used for the tests included a vertical, dualbed quartz reactor interfaced with a molecular beam mass spectrometer, as shown in Figure 1. The microreactor was housed in a tubular furnace with four independently controlled temperature zones. An outer flow provided dilution of samples with helium gas to obtain an adequate molecular beam and high signal-to-noise ratios. Steam was generated in situ in the reactor by vaporizing water injected with a HPLC pump through the bottom inlet. Samples of several types of plastics such as polyethylene, polypropylene, polystyrene, poly(ethylene terephthalate), nylon, polyurethane, and poly(vinyl chloride), were cryo-ground to obtain particles of 250350 µm diameter. Known amounts of those plastic particles (1-4 mg) were placed in quartz boats and inserted into the pyrolysis zone of the vertical reactor operating at 550-750 °C using a special sample holder. The released vapors and gases were then steam reformed in a catalyst bed in the upper part of the reactor. C11-NK, a commercial naphtha reforming catalyst obtained from Su¨d-Chemie, was ground and sieved. Particles in the size range of 350-500 µm mixed with quartz chips of the same particle size were placed in a quartz tube between two layers of quartz wool. A K-type thermocouple measured temperature inside the catalyst bed. The reactor was heated to the desired operating temperature, and the catalyst was reduced with 50% hydrogen in helium for 2 h before the tests. The product gas obtained from plastics by thermal decomposition/reforming expanded through an orifice, which led (6) Gebauer, M.; Pfitzner, R.; Dommaschk, V.; Schneider, J.; Reuss, J.; Wolf, B.; Meissner., R.; Wrackmeyer, T. Synthesis gas manufacture from petroleum residues and waste plastics or mixed waste plastics after removal of inorganics, metallic, mineral, and organic impurities. Patent DE 4329436, 1995. (7) Tsuji, T.; Okajima, S.; Sasaki, A.; Masuda, T. J. Chem. Eng. Jpn. 2005, 38, 859-864.
to the formation of a molecular beam of the reaction products. These product molecules were then ionized and analyzed by a quadrupole mass spectrometer. The time for completion of the process (release of the volatile product) was in the range of 15-20 s. The quantities of hydrogen, CO, CO2, and methane in the gas were calculated by integrating areas under the respective evolution curves and comparing with the calibration standards for those compounds. The accuracy of the quantification was estimated at (10%. Fluidized Bed Pyrolysis/Reforming System. The bench-scale system for producing hydrogen from plastics is shown in Figure 2. It is comprised of two inconel, tubular, fluidized bed reactors. Each one is supplied with a perforated gas distribution plate and externally heated by a three-zone electric furnace. In the first, a 1.5 in. diameter reactor, silica sand of the particle size 180-350 µm was used as a medium to transfer heat necessary for thermal decomposition of the polymers to lower-molecular-weight volatile compounds. In the second, a 2 in. diameter reformer, those volatile compounds catalytically reacted with steam in the fluid bed of a commercial catalyst. In both reactors, superheated steam was used as the fluidizing gas. A KCL24T20 K-Tron screw feeder was used to meter polymer beads to a small lock hopper comprised of two pneumatically controlled valves. This lock hopper was connected to a centrifugal mixing chamber and a pneumatic transfer line, which had a cooling jacket in its end section to maintain the reactor inlet temperature at below the plastics softening point. The plastic particles were transported to the pyrolyzer pneumatically in a stream of nitrogen. The gases and vapors formed by thermal decomposition of plastics entered the reformer through a 0.25 in. diameter tube located axially, whose outlet was located in the catalyst bed about 5 mm above the distribution plate. Downstream of the reformer, a cyclone and a hot-gas filter captured fine catalyst particles and, possibly, char generated in the reactor. The product collection line included two heat exchangers to condense excess steam and any remaining organic liquids. The condensate was collected in a vessel whose weight was continuously monitored. The dry gas flow rate was measured by a mass flow meter (instantaneous flow rate) and by a dry test meter (flow rate in 1 min intervals). The concentrations of CO2, CO, and CH4 in the reforming gas were continuously
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Figure 2. Schematic of the pyrolysis/reforming reactor system.
monitored by a nondispersive infrared analyzer (NDIR Model 300 from California Analytical Instruments) and that of hydrogen by a thermal conductivity monitor (TCM4 from Gerhard Wagner, Germany). In addition, the gas was analyzed every 5 min by an on-line MTI gas chromatograph, which provided concentrations of hydrogen, carbon monoxide, carbon dioxide, methane, ethylene, and nitrogen as a function of time. The temperatures in the system as well as the flows were recorded and controlled by an OPTO data acquisition and control system. In this series of tests the pyrolyzer operated with 120 g of sand in the fluidized bed at 600-700 °C. Superheated steam at a flow rate of 180-240 g/h was used as a fluidizing gas. In certain experiments, air at a flow rate of 3 L/min was added to the steam to partially oxidize polypropylene (this amount was equal to 25% of that needed for the complete oxidation) and thus provide energy necessary for its thermal decomposition. Isotactic polypropylene beads, less than 500 µm particle size, were fed to the pyrolyzer at rates up to 60 g/h. Nitrogen at a flow rate of 3 L/min was used as the feed transport gas. The reformer operated at 850 °C with 250 g of commercial nickel-based catalyst (C11-NK) developed for reforming moderately heavy petroleum fractions. The catalyst, originally obtained as pellets, was ground to a particle size of 300500 µm, which allowed for uniform fluidization while avoiding entrainment from the reactor. Before reforming, the catalyst in the reactor was activated in H2/N2 stream at the process temperature for approximately 2 h. This reduced NiO to Ni and also resulted in the catalyst weight loss of about 7%. Superheated steam (120 g/h), in addition to that flowing in from the first reactor, was used as both the fluidizing gas and also a reactant in the reforming process. In most experiments, the process conditions were very similar to those used in commercial natural gas reformers; molar steam-tocarbon ratio (S/C) equaled 4.6 and methane-equivalent space velocity, GC1VHSV, was ≈1600 h-1.
Figure 3. Mass spectrum of polyethylene pyrolysis product.
Results and Discussion
spectrum of the product obtained from polyethylene pyrolyzed at 650 °C is shown in Figure 3. At these conditions polyethylene totally volatilized forming a wide range of hydrocarbons. The most abundant are ethylene (m/z 28), propylene (m/z 41, 42), butylenes (m/z 54), benzene (m/z 78), and toluene (m/z 91, 92). Also, small amounts of naphthalene (m/z 128) were detected in the pyrolysis products. Qualitative tests of pyrolysis/reforming of these seven polymers were then performed to identify suitable reforming process conditions and the product composition for the integrated process. At S/C ) 5 and GC1VHSV ) 5000 h-1 (defined as the volumetric gas flow that would be observed if all carbon in the feed was in the form of CH4 per unit volume of the catalyst), the gas obtained from all those plastics contained only hydrogen and carbon oxides as shown in Figure 4 for PET. At lower S/C and higher GC1VHSV small amounts of methane and aromatic hydrocarbons were also produced. More aromatic compounds were obtained from polystyrene and PET than from polyolefins. Similar to the series of pyrolysis experiments, polyethylene volatilized completely within 15-20 s but PET left 5-10%
Microscale Pyrolysis/Reforming of Plastics. The initial tests were carried out to study only thermal decomposition of seven selected plastics: polyethylene, polypropylene, polystyrene, poly(ethylene terephthalate) (PET), nylon, polyurethane, and poly(vinyl chloride) (PVC) in the microreactor (without a catalyst bed). The results were similar to those reported in the literature8-13 for these polymers. For example, the mass
(8) Kaminsky, W. J. Anal. Appl. Pyrolysis 1985, 8, 439. (9) Scott, D. S.; Czernik, S.; Piskorz, J.; Radlein, D. Energy Fuels 1990, 4, 407-411. (10) Conesa, J. A.; Font, R.; Marcilla, A. Energy Fuels 1997, 11, 126136. (11) Guaita, M. Br. Polym. J. 1986, 18, 226-230. (12) Wlodarczyk, D. J. Appl. Polym. Sci. 1988, 36, 377-386. (13) Alajbeg, A. J. Anal. Appl. Pyrolysis 1987, 12, 275-291.
Production of Hydrogen from Plastics
Figure 4. Mass spectrum of the product gas from pyrolysis/reforming of PET; tpyr ) 700 °C, tref ) 800 °C, S/C ) 5.2, GC1VHSV ) 5300 h-1.
Figure 5. Evolution of the gas produced by pyrolysis/reforming of polyethylene; S/C ) 5, GC1VHSV ) 20 000 h-1.
carbonaceous residues that slowly gasified (>5 min) releasing CO2. The nitrogen-containing polymers (polyurethane, nylon, polyacrylonitrile) were totally converted to hydrogen, carbon oxides, and nitrogen with only traces of benzene detected at the reactor outlet. HCl released from PVC was first adsorbed in the catalyst bed then slowly desorbed. After the contact with HCl, the catalyst almost completely lost its reforming activity. Therefore, processing plastic mixtures containing PVC will require a trap to remove HCl from pyrolysis gas before reforming. Quantitative microreactor tests using polyethylene and PET were carried out at 650 °C in pyrolysis zone followed by steam reforming of the polymer-derived hydrocarbons at 800 °C. A reforming bed of C11-NK catalyst operated at a molar steam-to-carbon ratio ranging from 5 to 20 and a space velocity GC1VHSV from 6000 to 24000 h-1 depending on the size of pyrolyzed plastic samples. At such process conditions polyethylene was totally converted to hydrogen, CO, and CO2, and only traces of methane were detected in the product gas. Bimodality of the curves representing the concentration of these components as a function of time (Figure 5) probably resulted from heat and mass transfer limitations within the pyrolyzed sample (for 1-2 mg samples bimodality was not observed). On the basis of the amounts of CO, CO2, and CH4 found in the product gas, the carbon mass balance closure (conversion from polyethylene to gas) for all the tests was in the range 101109%. Yields of hydrogen calculated from the areas under the peaks compared to the calibration values were 77-83% of the stoichiometric potential and did not seem to change within the studied range of S/C and space velocity. If CO were totally converted to CO2 and H2 by water-gas shift, the yield of hydrogen would approach 100% of the theoretical yield. During processing PET carbonaceous solids were formed that remained in the quartz dish. Also, for larger samples (higher space velocities), small amounts of benzene were detected in the reforming product. Consequently, carbon to gas conversion was
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Figure 6. Gas composition obtained by pyrolysis/reforming of polypropylene.
86-98% and hydrogen yields were 66-78% of the stoichiometric potential. Bench-Scale Fluidized Bed Pyrolysis/Reforming Tests. The microscale experiments proved the concept of producing hydrogen from plastics by the pyrolysis/steam reforming approach. Further experiments were carried out to demonstrate the technical feasibility of the method using a reactor system that allowed us to collect tangible amounts of the product, to calculate reliable mass balances, and to test the longer-duration performance of the catalyst. A series of tests was performed using polypropylene as the feedstock. Figure 6 shows the gas compositions produced by pyrolysis of polypropylene performed at 650 °C (then 600 °C) followed by the catalytic reforming at 850 °C, with S/C ) 4.6 and GC1VHSV ) 1600 h-1. During a 10 h long test, the product gas composition stayed unchanged, with hydrogen exceeding 70% of the volume of that gas (on a nitrogen-free basis). A total of 92% of carbon from polypropylene was converted to carbon oxides, and only 8% remained as hydrocarbons, methane and ethylene, whose concentration is shown on the right-hand axis. No coke deposits were observed, and the global and elemental mass balance closures for the whole process were almost 100%. The change in pyrolysis temperature had no visible influence on the product gas composition within the studied range. This means that the thermal decomposition of polypropylene even at 600 °C produced hydrocarbons that were easily steam reformed. The yield of hydrogen produced in this experiment was 34 g per 100 g polypropylene. The maximum amount of hydrogen that could be produced if the polymer were completely converted to CO2 and H2 is 42.9 g per 100 g polypropylene, which means that the achieved yield was 80% of the stoichiometric potential. If a water-gas shift reactor followed the reformer similar to commercial plants producing hydrogen from natural gas, the yield of hydrogen from polypropylene would increase by 1012% as shown in Figure 7. Both pyrolysis and steam reforming are endothermic processes requiring energy to proceed. In the laboratory tests, electric-powered heaters provided energy needed to thermally decompose polypropylene and to reform the pyrolysis gases and vapors to produce hydrogen. In larger-scale facilities, however, the energy will most likely come from the feedstock itself. This can be done either in an indirect mode (heat exchangers using gases from plastics combustion as a source of energy) or directly by partial oxidation (POX) of plastics in the first reactor and of pyrolysis gases in the reformer. The latter option, which is an auto-thermal process, seems to have a significant advantage with respect to energy efficiency and reactor design. We tested POX/ reforming in the bench-scale system adding air at a flow rate equal to 25% of that necessary for total oxidation of polypropylene to the fluidizing gas (steam) in the first reactor. Because of the significant heat losses in the small-scale reactor, partial oxidation of polypropylene did not provide enough energy to
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from the latter was almost 80% of the stoichiometric potential, POX/reforming produced only 56% of the theoretical maximum. In both cases the production of hydrogen would increase by 10-12% if CO in the gas was further converted by a subsequent water-gas shift process. The reduction in the hydrogen yield resulted from the oxidation of a part of the plastic feedstock in the first reactor, which subsequently decreased the potential for producing hydrogen in the reformer. However, the partial oxidation directly provided energy to the process that otherwise would have to be supplied in a different, most likely less efficient, way. It is also possible that in a larger scale process less than 25% of the polymers would be used for energy, which would increase the amount of hydrogen generated from plastics. Figure 7. Yield of hydrogen produced by pyrolysis/reforming of polypropylene.
Figure 8. Gas composition obtained by pyrolysis/reforming and by POX/reforming of polypropylene.
Figure 9. Yield of hydrogen produced by pyrolysis/reforming and by POX/reforming of polypropylene.
maintain the required process temperature but resulted in a 4050% reduction of the external power input to the unit. Figures 8 and 9 present the results obtained in a test that was carried out for 2.25 h in pyrolysis mode, and then air was gradually added to the pyrolysis reactor so that stable partial oxidation conditions (air coefficient 0.25) were achieved in the time span from 5 to 7.5 h. As expected, during the POX/reforming operation mode the concentration of hydrogen decreased and that of CO2 increased compared to the gas produced by the pyrolysis/reforming process. Also, the yield of hydrogen was 24 g per 100 g polypropylene compared to 34 g per 100 g polypropylene obtained by pyrolysis/reforming. While the yield of hydrogen
Summary and Conclusions 1. Using a microscale pyrolysis/reforming reactor interfaced with MBMS we proved that many common plastics could be efficiently converted to hydrogen and carbon oxides by this thermocatalytic process. 2. With the pyrolysis zone operating at 650 °C and the reforming at 800 °C (commercial nickel-based catalyst, steamto-carbon ratio and space velocity similar to those used for reforming natural gas), polyethylene was completely converted to gas yielding hydrogen at 80% of the stoichiometric potential. At these conditions, PET did not totally volatilize, formed about 5-10% of carbonaceous residue, and yielded hydrogen at 70% of the theoretical potential. These hydrogen yields would be 15-20% higher if CO in the product gas were converted further by water-gas shift. 3. At the bench-scale, we demonstrated a two-step process for producing hydrogen from plastics using polypropylene as a representative synthetic polymer with the pyrolysis reactor operated at 600-700 °C and the reforming conditions similar to those used for reforming natural gas. The two-reactor system performed very efficiently in both pyrolysis and POX mode with the global and elemental mass balance closures ranging from 99 to 102%. From 60 g/h polypropylene fed to the system 20.5 g/h hydrogen was produced during the pyrolysis/reforming operation and 14.3 g/h hydrogen during POX/reforming, which corresponds respectively to 80% and 56% of the stoichiometric potential. These yields could be 12% greater if a secondary water-gas shift reactor followed the reformer. 4. The commercial catalyst accumulated 60 h on stream with no observed deactivation. If the proposed two-step approach is applied to processing mixed plastics containing PVC or rubber that release HCl or H2S during pyrolysis, a gas clean up unit must be installed between the pyrolysis and reforming stage to protect the reforming catalyst from poisoning. 5. The proposed process is more complex and will not deliver as high yields of hydrogen per weight unit of feedstock as the commercial natural-gas-based technology (SMR). However, it produces hydrogen from a low-cost waste feedstock contributing to both increasing hydrogen production and reducing environmental impact of plastic wastes. Acknowledgment. This work was supported by the DOE Hydrogen, Fuel Cells & Infrastructure Technologies Program. EF050354H
