Cogasification of Polyethylene and Lignite in a Dual Fluidized Bed

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Cogasification of Polyethylene and Lignite in a Dual Fluidized Bed Gasifier Stefan J. Kern,* Christoph Pfeifer, and Hermann Hofbauer Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, 1060 Vienna, Austria S Supporting Information *

ABSTRACT: This work focuses on the cogasification of lignite and fresh polyethylene (PE) granulate at lignite ratios ranging from 0 to 100% in the dual fluidized bed pilot plant at Vienna University of Technology. The operating parameters for all gasification and cogasification tests were kept constant. The input fuel power was 90 kWth, while the gasification temperature was 850 °C. Olivine was used as the bed material. In addition to standard online measurements of the permanent components of the product gas, the tar content was also measured. Using lignite in the feedstock improved quality in terms of reducing tar load and increasing the possibility of adjusting the main components of the product gas. The beneficial effects of lignite could even be attained at low concentrations of lignite in the fuel mix with PE. flexibility in the type of feedstock that can be used and the quality of the product gas.7 In most common large-scale gasification plants, air or a mixture of oxygen and steam is used as the gasification agent, which drives the process autothermally. The utilization of pure oxygen turned out to be cost intensive, whereas using air causes the product gas quality, especially the heating value, to suffer due to the high nitrogen content that dilutes the gas. With H2O or CO2 as the gasification agent, these drawbacks of oxygen and air can be avoided, but then the process becomes allothermal, meaning that the heat for endothermic gasification reactions has to be provided externally. A way of externally introducing the heat for gasification at an industrial scale is provided by dual fluidized bed gasification (DFB). This technology separates the combustion reactor, which provides the energy for gasification, from the gasification reactor where pure steam is used as the gasification agent. Circulating bed material between these two reactors carries the heat from the combustion reactor to the gasification reactor.8 This technology is commercially available, and four plants using this methodology are currently in operation in Europe, generating fuel power ranging from 8 to 15 MW.8−11 Plastics display a huge variety in their types. There are currently more than 20 types of plastics available, and each exhibits different potentials for recycling and thermal treatment. The main types of plastics used in the European Union (EU) are shown in Figure 1, where the most frequently used is polyethylene (PE), a thermoplastic resin that is mostly used for packaging. Figure 1 also gives information about the use of plastics in the EU and confirms the fact that most of the plastics are PE and are used for packaging purposes. This is the reason why pure PE was used as the representative fuel for plastics in this study.

1. INTRODUCTION Plastics occur in large amounts mostly in consumer goods, packaging materials, and many other single-use applications. Therefore, huge quantities of plastic wastes are produced. In the countries of the European Union, more than 250 × 106 tons of municipal solid wastes are produced annually.1 In 2007 in the United States, 12.1% of the waste produced contained plastics, giving an indication of the large proportion of plastics occurring as wastes.2 The European Directive 94/62/EC promotes recycling, reuse, and other forms of plastic waste recovery. One of the targets is that between 55 and 80% by weight of packaging waste has to be recycled. Recycling involves either conversion back into the monomer or into another product. Plastic waste recycling and treatment processes can be divided into four major categories: reextrusion (primary), mechanical (secondary), chemical (tertiary), and energy recovery (quaternary).3 Since plastics have a very high lower heating value (LHV) of up to 43 MJ/kg, they can be burned immediately, for example, in municipal waste incinerators to recover heat and produce electricity, or they can be used as an additional fuel for blast furnaces or cement kilns. Thus, energy recycling by incinerating plastics that result in energy recovery is popular and state-of-the-art. However, it has a drawback in that the chemical structure of the plastics is lost. Gasification of plastic waste is an interesting and promising option for turning plastics into valuable products, and can be used for liquid and gaseous fuel production as well as for the synthesis of chemicals. Moreover, it can convert plastic waste into electric power with high efficiency or allow the cofiring of the product gas in combustion processes that cannot deal with the physical properties (like the low melting point) of plastics, such as pulverized coal firing.4 Another advantage of gasification over the incineration of plastic waste is that the process gas volume is lower, which reduces the costs for gas cleaning.5 Furthermore, the production of undesired components such as dioxins is decreased by the operating conditions in a gasifier.6 For the gasification of wastes, especially plastics, fluidized bed gasification turned out to be the most appropriate in terms of © 2013 American Chemical Society

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Figure 1. Shares of plastic types used and applications for plastics in the European Union12.

(which corresponded to a range of PE in the feedstock from 0 to 77.4% in terms of energy). The H2 and CO contents of the resulting product gas were affected only up to PE ratios of 20 wt % in the fuel mix: H2 content increased by about 73% in relative numbers, and CO content decreased by about 27%. Levels of CO2 and CH4 and higher hydrocarbons were constant in this range, but for PE ratios higher than 20 wt %, concentrations of CO2 decreased, and those of CH4 increased. In the same gasification facility, also a mixture of coal, pine wood, and PE was gasified with a mass fraction of 60% of coal, 20% of pine wood, and 20% of PE in the feedstock.16 These tests showed that the presence of plastics favored hydrocarbon and tar release. Mixtures of coal, wood, and plastics were used as a solid feedstock for gasification in a bubbling fluidized bed by Mastellone et al.17 In addition to mixtures with wood, a fuel mix of 50 wt % of coal with mixed plastics was gasified, with the plastics ratio in terms of energy being 64%. The net effect of the plastic in the plastic/coal mixture was an increase in the specific gas yield and a higher LHV of the gas due to increased levels of light hydrocarbons (CH4 + CnHm). A mixture of 23 wt % polyethylene terephthalate (PET) and 77 wt % brown coal was gasified using a bubbling fluidized bed reactor, with a mixture of 10 vol % O2 and 90 vol % N2 as the gasification agent and silica sand as the bed material.18 Compared to the tar yield from gasification of pure coal, the researchers found that the mixture with PET (21.6% in terms of energy) led to a tar yield 3× as high (7.9 g/Nm3). A downdraft gasifier was used for investigations by GarciaBacaicoa and co-workers regarding cogasification of wood and high-density polyethylene (HDPE), with HDPE ratios of up to 17.4 wt % and a total fuel feed rate between 30 and 40 kg/h. An increase of the HDPE ratio caused an increase of the gasification temperature, leading to enhanced fuel conversion and an increase in the heating value of the product gas from 3.2 MJ/Nm3 using pure wood to 6.9 MJ/Nm3 using the HDPE/ wood mixture.19 Plastics and mixtures of plastics with wood pellets have already been gasified in the DFB gasifier at Vienna University of Technology.20 There were two types of plastics used, a shredder light fraction (SLF) from end-of-life vehicles and

Generally, cofiring involves the use of different fuels at the same time for combustion. Cofiring can be accomplished via three different ways: direct, indirect, and parallel cofiring. Direct cofiring uses blends of fuels, while indirect cofiring is based on the thermochemical conversion of biomass or waste into gaseous or liquid fuels and the cofiring of these converted fuels with the main fuel. For parallel cofiring, the fuels are fed into separated boilers to produce steam which drives one joint turbine. In this study, direct cogasification was used as a blended fuel of two types of feedstock, which were fed simultaneously into the gasification reactor. Direct cogasification offers various advantages. The most important is cost, since this method allows flexibility in choosing the type of fuel, which is a key issue for gasification plants. Furthermore, it offers the possibility of building larger plants with higher efficiencies. A very important aspect for cogasification with coal is that the composition and properties of the product gas can be adjusted by adding a different type of feedstock. A previous study investigated cogasification of wood pellets and lignite in a DFB, demonstrating a beneficial effect on product gas quality and tar content even at low lignite ratios.13 For gasification of plastics, high tar contents are usually present in the product gas. Cogasification with fuels like coal offers the possibility to improve product gas quality, especially in terms of condensable products (tars). Aznar et al.14 studied the gasification behavior of feedstock mixtures of coal, biomass, and plastics (mostly PE waste) with plastic ratios of up to 20 wt %, as well as the gasification of 100% plastic waste. Gasification with air as the gasification agent was performed in a bubbling fluidized bed reactor with a silica sand/dolomite blend as the bed material. The authors demonstrated that higher plastic ratios elicited a higher LHV of the product gas, while higher contents of coal in the feedstock reduced tar yields. The product gas generated by gasification of pure plastics resulted in an LHV of more than 8 MJ/Nm3db, which is considerably higher than that usually gained by airblown gasifiers. Cogasification of pine wood and PE in a bubbling fluidized bed reactor fluidized with pure steam was investigated by Pinto et al.15 In that study, PE ratios ranged from 0 to 60 wt % 4361

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Figure 2. Scheme of the DFB gasification pilot plant at Vienna University of Technology.

pilot plant at Vienna University of Technology, and the results are presented herein.

pellets made of selected plastics from municipal solid waste. Tar yields increased up to 70 g/Nm3 in dry product gas generated by the gasification of pure SLF plastics. An increasing percentage of plastics in the fuel mixture also induced an increase in the levels of CH4 and other light hydrocarbons, while the CO content dropped from values higher than 25 vol %db using SLF ratios of 0% down to values below 5 vol %db using 100% SLF plastics. CO2 stayed more or less constant for the SLF ratios. Cogasification of plastics with coal is promising as the completely different fuel properties of coal (i.e., volatile components and fixed carbon content) can be beneficial for the operation of a gasifier using plastics. A large-scale prime example of the cogasification of these two fuels is the hightemperature Winkler (HTW) process with plastic wastes as the fuel. The addition of 50 wt % lignite stabilizes the operation and counterbalances the variations in the properties of plastic wastes.21 To date, research on the cogasification of PE and lignite is lacking for lignite ratios ranging from 0 to 100% using pure steam as the gasification agent. To gain insight into the influence of the cogasification of PE and lignite on the performance of the DFB system as well as on the product gas quality, gasification tests were performed at the 100 kW DFB

2. MATERIALS AND METHODS 2.1. The Dual Fluidized Bed Pilot Plant. For experiments carried out at pilot scale, Vienna University of Technology operates a 100 kW dual fluidized bed (DFB) gasification reactor. A schematic drawing of the pilot rig is shown in Figure 2. A detailed description of the pilot plant including its basic geometric data and operating conditions can be found in previous articles.13,22 The pilot plant is equipped with three different hoppers to feed fuel into the gasification reactor at different locations as well as to give the possibility of cogasification of several feedstocks at any mixing ratio. The three hoppers are used for the following feeding locations or fuel requirements: • Hopper 1: to feed solid fuels into the bubbling bed. The screw conveyor introduces the fuel about 0.3 m below the splash zone of the bubbling bed. This hopper is used in most cases. • Hopper 2: to feed solid fuels from the side into the freeboard of the gasifier. The screw conveyor introduces 4362

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Koppatz et al.,25 and the mechanical properties of the olivine bed material are mentioned in Koppatz et al.29 Due to its high level of hardness and heat capacity, olivine is considered ideal for fluidized bed applications. 2.4. Feedstock. Lignite and polyethylene granulate were used for the investigations. The lignite used for the tests originates from the Rhenish lignite mining region (Germany). The feedstock was provided with a particle size of 2−6 mm. This type of lignite is characterized by a relatively low content of sulfur, nitrogen, and ash, compared to other types of lignite. The polyethylene used was not waste material; fresh PE granulate was used to avoid any variability of the composition. The proximate and ultimate analyses of these two fuels are listed in Table 1.

the fuel about 0.3 m above the splash zone of the bubbling bed. • Hopper 3: to feed solid fuels from the top of the gasifier (top-down feeding). This feeding position is designed for fuels with a low melting point, like plastics, and ensures that the fuels do not contact hot surfaces (free-falling into the reactor) before coming into contact with the hot bed material. For the experiments discussed in this article, hopper 1 was used to feed the lignite and hopper 3 to introduce PE into the reactor 2.2. Analytics. 2.2.1. Measurement of Main Product Gas Composition. The composition of product gas was measured after it left the gasification reactor. The permanent gas components CH4, H2, CO, CO2, and O2 were measured by a Rosemount NGA 2000. The components N2, C2H4, and C2H6 were measured using an online gas chromatograph (PerkinElmer Clarus 500). 2.2.2. Tar Measurement. Tar was sampled isokinetically with impinger bottles; afterward, gravimetric as well as GC/MS tars were analyzed. Tar sampling was performed discontinuously by condensing and dissolving the tar components out of the product gas. The measurement method was based on the tar protocol of CEN/TS 15439.23 The method applied here differed in the solvent used, as CEN/TS 15439 proposes isopropanol (IPA), but here toluene was used. This allows a simultaneous detection of the water content in the product gas because water can be measured as a separate phase in the impinger bottles. Unlike IPA, toluene as a solvent does not enable the detection of the tar components benzene, toluene, and xylene (BTX). However, with toluene, the separation of tar components larger than BTX is more efficient than with IPA. As isokinetic sampling conditions were applied with this measurement method, entrained dust and char in the product gas were collected in combination with the tar measurement by an upstream filter cartridge. A scheme of the arrangement of the tar sampling line can be found in ref 24. 2.3. Used Bed Material. The bed material used was olivine, which is a naturally occurring mineral composed of silicate tetrahedra that also contains iron and magnesium in the form (Mg1−X, FeX)SiO2, although the contents of these latter two elements vary with mining location. The catalytic tar reduction effect caused by using olivine as the bed material was reported by Koppatz et al.25 for biomass gasification. Moreover, for the gasification of PE, olivine from the same mining location was used by Arena et al.5 who reported a massive abatement of tar in the product gas compared to gasification when silica sand was employed as bed material. The olivine was precalcined. This process considerably improves catalytic activity.26,27 In the case for gasification of pure plastics, Toledo and co-workers28 already documented the effect of different olivine contents in the bed material on the product gas quality by gasification of PP in a bubbling fluidized bed pilot rig fluidized with air. These researchers found that by increasing the olivine content up to 100% led to an abatement of the tar content with the lowest values found to be around 2 g/Nm3. Another positive effect was an increase of the heating value of the product gas. In the present study, the olivine used in the tests was provided by the Austrian manufacturer Magnolithe GmbH and calcined at temperatures of up to 1600 °C. As a result of this procedure, the material was sintered. The mean diameter was approximately 375 μm. The results of XRF analysis are listed at

Table 1. Proximate and Ultimate Analyses of the Feedstock water content ash content C H N O S Cl volatile matter fixed carbon LHV

unit

lignite

PE

wt % wt %db wt %db wt %db wt %db wt %db wt %db wt %db wt % wt % MJ/kg

18.6 4.23 65.53 3.75 0.84 25.22 0.38 0.05 42.2 39.2 19.33

0.0 0.0 85.9 14.1 0.0 0.0 0.0 0.0 >99 0, which was accompanied by high H2 and CO2 contents, the products of the water−gas shift reaction, and a low amount of H2O (one of the reactants) in the gas. The main reason for pδeq,CO‑shift being below 0 with lower lignite ratios involves the CO2 content of the product gas, which was nearly absent during pure PE gasification. Several further interesting specific data were determined from the results obtained from the test runs and are also shown in Table 4. These data describe the operation performance in more detail, but no additional explanations are necessary.

4. CONCLUSION Dual fluidized bed gasification is a fuel-flexible technology capable of converting carbonaceous feedstock into high quality product gas. Originally designed for wood chips, it can now use other types of fuel, increasing its economic value. The combination of coal with polyethylene offers various advan-

9.0 − 10.0

m/s

(7)

To quantify the distance to equilibrium, the model parameter can be expressed as the logarithm of the ratio of the actual partial pressure product to the equilibrium constant (eq 8). If pδeq,CO‑shift < 0, the actual state is still on the side of the reactants, and so, another reaction is thermodynamically possible. If pδeq,CO‑shift = 0, the water−gas shift equilibrium is achieved by the product gas composition, whereas if pδeq,CO‑shift > 0, there is a shift toward the products.

Table 4. Specific Data and Characteristic Values of the Tests OP2, 33% lignite

(6)

The lowest cold gas efficiency was calculated for gasification of pure PE (OP1), obviously as nearly 49 kW of additional fuel for the combustion reactor had to be used to fulfill the heat demand in absence of char in the combustion reactor. This was rapidly improved by the addition of lignite to the fuel mix. A lignite ratio of 33% reduced the requirement of additional fuel for combustion already down to 27 kW. As for OP3 the chemical power in the product gas was in the same range as for OP2, but the demand of additional fuel was again reduced, and the highest cold gas efficiency was found here. In general, for steam gasification, the water−gas shift reaction is favored taking place in the reactor as it increases H2 yield and promotes water conversion (eq 7).

Figure 13. Carbon conversion vs lignite ratio.

OP1, 0% lignite

(Pfuel , G

̇ ·LHVPG VPG + Pfuel , C − Q̇ PP + Q̇ IP) ·3600

4.0

0.8 − 0.9

4368

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Notes

tages. This study showed that the system could handle lignite and PE, in pure forms or as blends, without any drawbacks. Higher ratios of PE increased tar contents significantly, while the addition of lignite limited the tar content. A mix of the two fuels had a synergetic effect, increasing the cold gas efficiency as less fuel had to be added for combustion if lignite was present during PE gasification. If PE is added to lignite gasification, the chemical energy delivered by the product gas rises due to the high calorific gas components CH4, C2H4 and C2H6. In summary, the effect of PE on the system is: • an increase in the tar content; • changes in the composition of GC/MS tars, with increases in components such as acetylene, pyrene, and styrene and decreases in species such as indene or fluorene; • a lower amount of entrained particles (dust and char) in the product gas; • an increase of the heating value of the product gas; • an almost total conversion of PE into product gas under certain conditions; • A reduced amount of residual char, and consequently, a higher demand for fuel for combustion which could be satisfied by recycling or a part of the product gas. However, linear trends for the measured values or process parameters versus the lignite ratio could only be observed for the composition of the main gas components in the product gas. Especially the conversion performance of steam in the gasification reactor did not show clear trends. This might be caused by the changed solids circulation rate of bed material between the two reactors and different temperature of the recycled hot bed material from the combustion reactor. The solids circulation rate changed as the fluidization (steam) of the gasification reactor varied due to holding the steam-to-carbon ratio (φSC) for all tests constant between 9 kg/h for OP1 and 13 kg/h for OP4. However, there has to be kept in mind that not only the above-mentioned process parameters are influencing the results of cogasification. Also the fuel blend itself can cause a nonlinear behavior as the species can interact with each other. Such a nonlinear behavior was already documented for cogasification of plastics and wood pellets.20 The only major drawback of gasifying PE, a representative of the gasification of plastic wastes, is the increased tar content. This can be solved most probably by a new design of the gasification reactor, as proposed by Hofbauer and co-workers from Vienna University of Technology,36,37 who suggested the conversion of high-molecular weight condensable components by a novel reactor design that promotes gas−solid interaction between hot bed material and product gas.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support granted by the European Commission as this study was carried out within the framework of the Fecundus project, funded by the Research Fund for Coal and Steel of the European Union (Contract No. RFCR-CT-2010-00009).

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NOMENCLATURESYMBOLS ΔHR,850 Heat of reaction at 850 °C, kJ/mol Kp,CO‑shift Equilibrium constant of CO-shift, LHVPG Lower heating value of the product gas (dry), MJ/ Nm3db ṁ CFG Carbon flux in the flue gas stream, kg/h ṁ CPG Carbon flux in the product gas stream, kg/h ṁ fuel Mass flux of solid fuel into the gasification reactor, kg/h ṁ H2O,con. Amount of water that is converted to product gas, kg/h ṁ steam Mass flux of steam in the gasification reactor, kg/h pδeq,CO‑shift Logarithmic deviation from CO-shift equilibrium, Pfuel,C Input fuel power of fuel for the combustion reactor (light heating oil), kW Pfuel,G Input fuel power of solid fuel into the gasification reactor, kW pi Actual measured gas phase partial pressure of the species i, Pa Q̇ IP Heat loss from an industrial-sized plant with the same fuel power as the pilot plant, kW Q̇ PP Heat loss from the pilot plant, kW T Temperature, °C Ug Superficial velocity gasification reactor, m/s Uc Superficial velocity combustion reactor, m/s V̇ PG Volumetric flow rate of product gas (dry), Nm3db/h XC Carbon conversion in the gasification reactor, % XC,DFB Carbon conversion in the complete DFB system, % XH2O Water conversion in the gasifier, related to total amount of introduced water, % XH2O,rel Water conversion in the gasifier, related to the dry and ash free fuel input, kgH2O/kgfuel,daf x Molarity of carbon in the fuel (dry, ash, N, Cl, and S free basis), mol/kgC,H,O y Molarity of hydrogen in the fuel (dry, ash, N, Cl, and S free basis), mol/kgC,H,O Greek letters

ηC φSF,wt φSC,wt τF

ASSOCIATED CONTENT

S Supporting Information *

Table S1: Classification of GC/MS tar components according to Milne et al. and ECN; Table S2: Amount of GC/MS tar components detected; Table S3: Weight fraction of individual components of GC/MS tar. This material is available free of charge via the Internet at http://pubs.acs.org.

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τC νash νC νi νH2O

AUTHOR INFORMATION

Cold gas efficiency calculated based on heat loss in an industrial-sized plant, Steam-to-fuel ratio, kgH2O/kgfuel,daf Steam-to-carbon ratio, kgH2O/kgC Product gas residence time in the freeboard of the gasification reactor, s Flue gas residence time in the combustion reactor, s Ash mass fraction in the fue, Carbon mass fraction in the fuel, Fraction of component i, Water mass fraction in the fuel, -

Corresponding Author

Abbreviations and subscripts

*Tel.: +43 1 58801 166382. Fax: +43 1 58801 16699. E-mail: [email protected].

BTX c 4369

Benzene, toluene, xylene Carbon, cold gas (efficiency), combustion reactor dx.doi.org/10.1021/ie303453e | Ind. Eng. Chem. Res. 2013, 52, 4360−4371

Industrial & Engineering Chemistry Research CHP daf db DFB E&E EPS HDPE HTW g GC/MS IPA LHV PE PET PG PP PS PUR PVC SLF VUT XRF

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Combined heat and power plant Dry and ash free basis Dry basis Dual fluidized bed Electrics and electronics Expanded polystyrene High-density polyethylene High-temperature Winkler Gasification reactor Gas chromatography/mass spectrometry Isopropanol Lower heating value Polyethylene Polyethylene terephthalate Product gas Polypropylene Polystyrene Polyurethane Polyvinyl chloride Shredder-light fraction Vienna University of Technology X-ray fluorescence

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