Article pubs.acs.org/EF
Gasification of Mixed Plastic Wastes in a Moving-Grate Gasifier and Application of the Producer Gas to a Power Generation Engine Jeung Woo Lee,†,‡ Tae U Yu,†,‡ Jae Wook Lee,‡ Ji Hong Moon,‡,§ Hyo Jae Jeong,§ Sang Shin Park,§ Won Yang,†,‡ and Uen Do Lee*,†,‡ †
Department of Green Process and System Engineering, Korea University of Science and Technology, 217 Gajung-ro, Yuseung-gu, Daejeon, Republic of Korea 305-350 ‡ Energy System R&D Group, Korea Institute of Industrial Technology, 35-3, Hongcheon, Ipjang, Cheonan, Chungnam, Republic of Korea 331-825 § Department of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodamun-gu, Seoul, Republic of Korea 120-749 ABSTRACT: Production and utilization of producer gas from mixed plastic wastes were investigated in a pilot-scale movinggrate gasification process. A 500 kWth moving-grate gasifier was developed, including a flame-assisted tar reformer and sequential gas cleaning processes comprising a gas cooler, bag filter, and wet scrubber. The cleaned producer gas, composed mainly of hydrogen, carbon monoxide, carbon dioxide, and methane, was used in a 30 kWe gas engine to generate power. Optimal operating conditions of the integrated system were investigated for various parameters. As a result, producer gas with a higher heating value of greater than 10 MJ/(N m3) and a cold gas efficiency of more than 55% was obtained under oxygen-blown conditions. Due to the flame-assisted tar reforming with oxy-combustion of natural gas, the hydrogen content was significantly increased, resulting in an increase in the syngas caloric value and a decrease in the gas cleaning load downstream. In addition, the performance of the syngas power generation engine was tested with the slipstream of the producer gas. A power output of greater than 20 kWe and a power generation efficiency of about 22% were obtained. (e.g., coal and biomass).7−14 Gasification is a thermochemical route for conversion of solids to fuel gas, which mainly consists of hydrogen, carbon monoxide, carbon dioxide, and methane. Most combustible solid wastes can be used as feedstock for the gasification process, and the producer gas can be used in various conventional energy production systems, such as electric power generation, poly generation, and synthetic fuel production systems.15 However, compared to combustion, gasification technologies have low long-term reliability and some technical barriers to commercialization, including the low caloric value of producer gas relative to energy production and operational challenges such as tar and dust impurities in the producer gas.16 The tar content should be less than 100 mg/Nm3, and the dust size should be less than 10 μm.17 These are important criteria for producer gas for use in a syngas engine. Recent technical and economic studies have identified fluidized bed systems as the most promising technology for industrial application of plastics-to-energy cogeneration systems.5,18 However, moving-grate systems have long been applied in conventional incineration processes in the field of thermal waste treatment and have some advantages over fluidized bed systems. For example, the residence time of the solid fuel in the reactor is relatively long and can be easily controlled based on reaction characteristics, which is advantageous for waste fuel with varying chemical properties. Moreover, lumped feedstock can be directly fed into the system, which is also convenient for waste fuel with various physical properties; in other types of gasifiers such
1. INTRODUCTION Due to global climate change and the unstable price of fossil fuels, current high-priority global energy issues include CO2 emissions and flexible fuel sources for energy systems. Among the many options, utilization of solid waste is a promising approach to reducing greenhouse gas emissions and developing a recyclingoriented society. Moreover, according to the renewable energy portfolio standard (RPS) recently announced in South Korea, electricity suppliers in South Korea are required to include 2% renewable energy by 2012 and 10% by 2022. Each renewable resource has its own multiplier that equalizes the economic differences between these resource technologies. The multiplier for waste utilization, including plastic wastes, has been set at 1, the same as that for wind and tidal power.1 In general, plastics recycling can be divided into three methods: mechanical recycling, chemical recycling, and energy recovery.2−4 Arena et al.5 categorized the recycling streams of plastic wastes as follows: (1) materials primarily made of polyethylene (PE) and polyethylene terephthalate (PET), which can be used in environmentally and economically sustainable recycling processes; (2) mixed plastic wastes, predominantly polyolefins, which have a very high caloric value and can be preferentially used as wastederived fuel; and (3) “out of target” materials, which cannot be used in any recycling or energy recovery process. The energy of plastic wastes can be recovered through three main thermochemical processes: combustion, pyrolysis, and gasification. Combustion of plastic wastes presents severe constraints related to a very low softening temperature with consequent high risk of sintering in the combustion chamber.6 As an alternative to combustion, gasification technologies have been extensively studied for energy recovery from plastic wastes or in a mixture with other solid fuels © 2013 American Chemical Society
Received: October 30, 2012 Revised: February 23, 2013 Published: March 15, 2013 2092
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Figure 1. Moving-grate gasification and utilization system. (a) Integrated process diagram of the system and (b) view of the overall process: (1) moving-grate gasifier, (2) gas cleaning system, (3) tar reformer, and (4) gas engine power generator. South Korea. An integrated process diagram of the system and an overview of the entire process are shown in Figure 1. The process mainly consists of a moving-grate gasifier, gas cleaning systems, and a gas engine power generation system. The waste treatment capacity of the gasifier is greater than 80 kg/h, the gas cleaning system has a treatment capacity of ∼300 N m3/h, and a syngas engine of 30 kWe is equipped for power generation using the slipstream of the producer gas. The remaining gas is fed into a cofired oil boiler. The unreacted char and ash are collected in a cooled residue container installed at the end of the moving-grate gasifier. Figure 2 shows a schematic diagram of the moving-grate gasifier. The main component designs (e.g., grate and lining) were adapted from a conventional grate-type incinerator. The gasification process was designed for a multistage reaction divided into three stages: a drying stage, a pyrolysis stage, and a char gasification stage. However, in actual operation, the staging function is highly dependent on the fuel characteristics (e.g., fuel type, size, and moisture content) and the operational scheme. In contrast to the fluidized bed reactors normally used for waste gasification processes, the conversion steps for solid waste in grate systems do not take place simultaneously in the bed but along the length of the fuel flow path. Thermal treatment of plastic wastes presents severe constraints related to the very low softening temperature of the plastics, with the risk of plugging the feeding system.5
as entrained bed systems, the fuel size must be carefully controlled. Accumulated knowledge and experience in the industry of construction methods and operation of the moving-grate system are another advantage. Retrofitting or conversion from the combustion mode to the gasification mode can be accomplished without major modification of the hardware.19 Thus, the aim of this study was to develop an integrated gasification and syngas power generation system based on a movinggrate system and gas engine. Pure oxygen was used as the gasifying agent to produce high heating value gas, and a flame-assisted tar reformer was adapted as a dry syngas cleaning system. Optimal operating conditions of the integrated system were determined for the main parameters (e.g., waste feeding rate and excess oxygen ratio) taking into account performance of the syngas power generation engine.
2. EXPERIMENTAL METHODS 2.1. Gasification and Power Generation System. A 500 kWth scale pilot moving-grate gasification system was recently constructed and is operated by the Korea Institute of Industrial Technology (KITECH), 2093
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Table 1. Specifications for the Gas Engine Power Generator engine type rotational speed intake system power generator power generation capacity
four-cycle gas engine 1800 rpm turbo charger induction motor maximum 30 kWe
temperature of the gas mixture to increase the volumetric efficiency of the mixer, and the turbocharger increases the output power of the engine with producer gas that has a low caloric value compared to natural gas or diesel, that is, the design fuel for this engine. 2.2. Feedstock Materials. Mixed plastic wastes are typically made up of various kinds of plastics. For these experimental tests, refused plastic fuel (RPF) from a commercial waste sorting facility was selected as the main feedstock material for evaluating the performance of the gasification system and for characterizing the producer gas generated during gasification. The RPF was composed of plastic wastes (polypropylene, polystyrene, polyvinylchloride, etc.) mixed with other materials such as textiles and ceramics and was shredded into lumps of about 50 mm. RPF has many economic benefits and advantages compared to other fuels because it is manufactured from the remaining residues after recycling waste plastics and in particular because of its high caloric value. In the early stages of the experiments, wood chips were used for stabilization of the gasifier. The wood chips were a blend of 50% pine and 50% fir and were shredded to about 50 mm. The results of the used feedstock analysis are presented in Table 2.
Figure 2. Schematic diagram of the moving-grate gasifier. To avoid this problem, the waste feeding system consists of double gate with a pusher sequentially operated by a hydraulic system. The grates are composed of moving parts and stationary parts with several tuyeres on the front of each grate. To control the residence time of solid wastes in the gasifier, the movement speed and sequencing of the moving grates can be automatically controlled by a programmed logic control system. The refractory lining inside the gasifier was designed to withstand high temperatures of less than or equal to 1100 °C. The lining was constructed of high alumina fire bricks that are resistant to reducing atmospheres as well as high temperatures. Pure oxygen is used as the gasifying agent and is introduced to the gasifier below the grates. An auxiliary burner was installed in front of each stage for preheating the inside of the gasifier. The producer gas flow exit was installed in the middle of the gasifier at the top of the pyrolysis stage, followed by a tar reformer. As noted above, the presence of tar in the producer gas is one of the main technical barriers in gasification systems. Tar can cause several problems, such as formation of coke, plugging filters, and condensing in cold spots, resulting in serious operational interruptions.20 Moreover, certain tar concentrations can damage or lead to unacceptable levels of maintenance for engines and turbines. In the present study, the producer gas leaving the gasifier contained a significant amount of tar, because the gas exit was located in the pyrolysis stage. However, by using a flame-assisted tar reformer with oxy-combustion of natural gas, most of the tar in the producer gas was removed. The tar reformer is a cylindrical reactor (0.6 m i.d. × 3 m long) constructed of refractory and insulated materials, designed to withstand up to 1300 °C. An oxygen burner of natural gas was installed to thermally crack the tar. The temperature distribution inside the tar reformer was measured using an R-type thermocouple. The reformed gas was cooled in a heat exchanger and was then cleaned with several sequential gas cleaning processes, including a bag filter and a wet scrubber. Because the gasifier and gas cleaning system were operated at ambient pressure, a buffer tank and a ring blower were used to increase the supplied pressure of the cleaned gas before being fed to the gas engine system; supplementary gas was sent to the cofired boiler. After passing through the gas cleaning system, the producer gas was supplied to a gas engine to generate electric power. The specifications of the syngas engine are presented in Table 1. The gas engine used in this study was equipped with a turbocharger and an intercooler to increase the efficiency of the engine. The intercooler lowers the
Table 2. Results of the Feedstock Analysis refused plastic fuel Proximate Analysis (wt %) moisture volatiles fixed carbon ash Ultimate Analysis (daf wt %)b C H O N S Cl Lower Heating Value (MJ/kg)
wood chipsa
0.5 71.9 6.6 21.1
6.4 75.9 17.4 0.3
65.2 8.7 19.2 1.0 0.5 5.1
50.8 5.4 43.6 − 0.2 −
21.2
16.1
a
Used for stabilization of the gasifier in the early stage of experiments. b daf = dry ash free. 2.3. Test Procedures and Measurements. Before the tests, the gasifier was heated up to ∼500 °C with the auxiliary natural gas burner. However, it was necessary to wait longer before beginning solid waste feeding to avoid tar condensation in the bag filter. Once the temperature upstream of the bag filter was ∼180 °C, the gasifier was ready for waste feeding, and wastes were fed slowly into the first stage, the drying zone. Under excess oxygen conditions, the solid waste oxidized, and the gasifier rapidly reached the desired temperature. The transition from combustion to gasification was effected by increasing the solid waste feeding rate to decrease the excess oxygen ratio. Once the process conditions were stable, continuous solid feeding was conducted. To prevent explosion of entrained ambient air into the gasifier during negative pressure operation, the static pressure of the gasifier was maintained at a slight positive pressure of 0−0.5 kPa by controlling the fan speed. The temperature of the wet scrubber outlet was maintained at approximately ambient temperature by supplying sufficient scrubbing water. After the induction fan, the producer gas was collected in a buffer tank placed upstream of the gas engine power generator. The volume of gas supplied to the gas engine was lower 2094
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traps, which were connected in series and maintained at −4 °C by a cooling system.
than the total volume of the producer gas to prevent inflow of ambient air. Thus, the pressure in the buffer tank was held above atmospheric pressure. To evaluate the performance of the gasifier and the gas engine power generation system, the following variables were defined and determined. The variables analyzed included gas composition, caloric value, cold gas efficiency (CGE), carbon conversion efficiency (CCE), and power generation efficiency (PGE):
HHVproduct gas =
Table 4. Effect of the Excess Oxygen Ratio on Producer Gas Composition
(12.75 H 2% + 12.36 CO% + 39.82 CH4%) 100 (1)
ER =
oxygen flow rate stoichiometric oxygen flow rate
CCE =
CGE =
PGE =
(2) a
C product gas Cwaste
run
1
2
3
4
excess oxygen ratio gas composition (dry vol %) H2 CO CO2 CH4 gas flow rate ((N m3)/h, dry)a
0.15
0.30
0.45
0.60
30.1 21.9 8.2 10.0 95
41.1 33.4 15.2 6.8 109
34.5 28.4 20.0 4.5 119
29.0 28.0 22.0 4.3 122
Measured after the demister system and can be regarded as dry base.
(3)
HHVproduct gas × Vproduct gas HHVwaste × ṁ waste + HHVNG × ṁ NG
(4)
Pe LHVwaste × Vproduct gas
(5)
Data for the operational parameters of the gasification system, including temperature, pressure, gasifying agent flow rate, and producer gas flow rate, were automatically stored in a data acquisition system at 1 min intervals. The main producer gas components (H2, CO, CO2, O2, and CH4) were analyzed every 2 s with an online gas analyzer (model: AO2020; ABB Co. Ltd., Germany). Tar was sampled in the
Table 3. Main Process Parameters for Testing the Effect of the Excess Oxygen Ratio parameter
characteristics, value
feedstock feeding rate solid residence time in bed gasifying agenta excess oxygen ratio auxiliary heat sourceb auxiliary heat load operating pressure
refused plastic fuel 80 kg/h ∼1 h pure oxygen 0.15−0.65 natural gas 58.3 kWth 0−50 Pa
a
Injection position: char gasification zone. bBurner position: pyrolysis zone, stoichiometric conditions.
Figure 4. Effects of the ER on the HHV of the producer gas, process efficiency, and temperature of the gasifier.
Figure 3. Variations in the gas composition during the tests in terms of ER. (a) ER = 0.3 and (b) ER = 0.6. 2095
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3. RESULTS AND DISCUSSION 3.1. Performance of the Gasifier. Prior to integrated operation of the overall process, the performance of the gasifier was tested independently. The experimental conditions for integrated operation were determined based on single-run test results for the gasifier, performed by varying the excess oxygen ratio (ER). The main process parameters are presented in Table 3. Figure 3 shows the variations in gas composition during the tests in terms of ER. Stable operation was achieved during the tests. However, the producer gas composition fluctuated because the RPF was not fed into the bed continuously but intermittently using a pusher type feeder. Table 4 and Figure 4 show the gasification results in terms of ER. An increase in the ER from 0.15 to 0.30 led to obvious changes in the producer gas composition. Most producer gas species (H2, CO, and CO2) increased other than CH4. Considering that the temperature was increased, H2 and CO in the producer gas likely increased because volatile gases escaped more rapidly and hydrocarbons including CH4 were reformed into H2 and CO,21−23 and char oxidation promoted CO2 generation. An increase in the ER from 0.30 to 0.60 led to a slight increase in carbon conversion but decreased the HHV and CGE. The gasifier
temperature increased due to the promotion of char oxidation as the ER increased. There is a trade-off between the CCE and the caloric value of the producer gas (Figure 4). To optimize both the caloric value of the producer gas and the CCE, we assessed carbon conversion and the amount of unreacted char in the residues. The experimental conditions used in these experiments were the same as those presented in Table 3, except for the ER. As the ER was varied, the weight of residues collected in the cooled residue container was measured. The unreacted char in the residues was analyzed using the standard test method for loss on ignition of solid combustion residues (ASTM D7348) and was determined by eq 6: unreacted char in the residues (%) w − wash = residues × 100 wresidues − wmoist
(6)
A decrease in the ER from 0.72 to 0.15 led to a clear increase in the HHV of the producer gas but also an increase in the amount of unreacted char in the residues and a decrease in carbon conversion (Figure 5). On the basis of the amount of unreacted char in the residues, the CCEs should have been greater than 75−95%. However, the CCEs based on the product gas without the tar reforming process were 57−88%, according to eq 3. These results imply that the unconverted tar, which should be transferred to the producer gas, decreased the CCE and that tar reforming is necessary to improve the overall system efficiency. Overall, the performance tests indicated that the average HHV of the producer gas was greater than 10 MJ/(N m3), the CGE was greater than 55%, and the CCE was ∼70% with an ER of 0.3−0.4. 3.2. Tar Reforming. The tar reforming test was performed under the same operating conditions shown in Table 3, except for the ER. The ER was set at 0.1 to generate a large amount of tar during the gasification process. The temperature of the tar reformer was held at ∼1000 °C using the auxiliary oxygen burner by controlling the amount of auxiliary fuel (natural gas) and oxygen to stoichiometric conditions. Figure 6 shows changes in the amounts of the generated gas species before and after the tar
Figure 5. Unreacted char percentage in the residues and carbon conversion efficiency with variations in the ER.
Figure 6. Variations in the gas composition and HHV before and after the tar reformer (RPF = 80 kg/h, ER = 0.1, tar reformer was held at ∼1000 °C). 2096
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waste plastics that decomposed into its monomer (styrene).10 Two- and three-ring aromatic compounds were primarily detected after the tar reformer. This implies that secondary tar species such as naphthalene and biphenyl were formed during the hightemperature tar reforming process.25 Downstream, secondary tar was removed from the producer gas in the gas cleaning system, composed of a bag filter and a scrubber. However, fouling of the bag filter and contamination of the scrubbing water could be an issue after long operation; thus, additional gas treatment systems are needed to prevent downstream problems related to the secondary tars. 3.3. Gas Engine Power Generation. Under optimal operating conditions (ER = 0.3−0.4) for the gasification system, the cleaned producer gas was fed into the gas engine, and power generation tests were carried out. The main process parameters for the tests are summarized in Table 6. Figure 7 shows the fuel gas flow rate and power generation efficiency according to each case (different LHV of fuel gas) for producing certain electric power. For these case studies, the power output of the engine was set at 5, 10, 15, and 20 kWe. The consumed fuel gas flow rate increased as the power generation load increased; with higher LHV fuel gas, the fuel gas flow rate required to maintain the same power output was reduced. A power output of greater than 20 kWe and a power generation efficiency of ∼22% were obtained.
reformer. After thermal treatment, the percentages of most gas species (H2, CO, and CH4) increased. Compared to the other gas species, H2 and CH4 increased more in the producer gas because higher molecular weight hydrocarbons decomposed into H2 and CH4 at high temperatures.24 Figure 6 also shows the change in HHV of the producer gas after thermal treatment. Higher HHV was obtained due to the increases in H2 and CH4. The tar samples were qualitatively analyzed using a gas chromatography/ mass selective detector. For simplicity, the tar compounds have been grouped into three classes of compounds by the number of aromatic rings. Table 5 shows the primary tar compounds detected before and after the tar reformer. Styrene was detected during the test, likely originating from polystyrene mixed with Table 5. List of Tar Compounds before and after the Tar Reformer compd before
1-ring
after
2-ring 3-ring 1-ring 2-ring 3-ring
a
toluene, ethylbenzene, benzene, o,p-xylene, styrene, methylstyrene nda nda toluene, ethylbenzene, benzene, o,p-xylene, styrene, methylstyrene indene, naphthalene, methylnaphthalene, biphenyl phenanthrene
4. CONCLUSIONS Production and use of fuel gas from mixed plastic wastes were investigated in an integrated pilot-scale moving-grate gasification and power generation process, and the following results were obtained: • For oxygen-blown gasification of RPF, the optimal ER was 0.3−0.4. Under these operating conditions, the higher heating value of the producer gas was greater than 10 MJ/(N m3), the cold gas efficiency was greater than 55%, and a carbon conversion efficiency of ∼70% was obtained. However, unconverted tar transferred to the producer gas decreased the system efficiency. • The producer gas from gasification of waste plastic contained substantial amounts of tar. With the flame-assisted tar reformer, a high percentage of H2 and CH4 in the reformed gas was obtained, resulting in a higher caloric value producer gas. After tar reforming, secondary tar species such as naphthalene and biphenyl, which were not detected before reforming, were present. It was observed
nd = not detected.
Table 6. Main Process Parameters for the Power Generation Tests
a
parameter
1
feedstock feeding rate (kg/h) gasifying agent excess oxygen ratio auxiliary heat load gas composition (dry vol %)a H2 CO CO2 CH4 gas temperature (°C) lower heating value (MJ/(N m3))
refused plastic fuel 80 oxygen 0.4 58.3 kWth 28 25 19 5.0 52 8.0
2
3
0.35
0.3
26 24 19 6.6 29 8.2
24 20 18 10 25 8.7
Mean value in each case.
Figure 7. Electric power output set value vs consumed fuel gas flow rate and power generation efficiency. 2097
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that tar was effectively removed from the producer gas through the gas cleaning system, which consisted of a bag filter and a scrubber. To prevent problems related to secondary tar in the future, secondary tar treatment systems downstream are needed for long-term application of this system. • The performance of the gas engine power generation system was tested. Under the optimal operating conditions of the gasification system, a power output of greater than 20 kWe and a power generation efficiency of ∼22% were obtained. In summary, the technical feasibility of a moving-grate gasification system was demonstrated through integrated operation at a pilot scale. The operating characteristics developed in this study will be reflected in the design of a commercial-scale system in the future.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +82-41-589-8574; fax: +82-41-589-8353; e-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the project Development of Renewable Resource Based Materials and Energy Production System with Low Carbon Emission funded by the Korea government Ministry of Strategy and Finance.
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NOMENCLATURE
Abbreviations
CCE=Carbon conversion efficiency CGE=Cold gas efficiency ER=Excess oxygen ratio HHV=Higher heating value (MJ/(N m3)) LHV=Lower heating value (MJ/(N m3)) MPW=Mixed plastic wastes NG=Natural gas PGE=Power generation efficiency (−) RPF=Refused plastic fuel RPS=Renewable energy portfolio standard Symbols
C ṁ P V w
Mass of carbon (kg) Feeding rate of waste (kg/h) Electric power (kW) Flow rate of gas ((N m3)/h) Weight of sample (g)
Subscripts
e Electricity output N Normal state th Thermal input/output
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REFERENCES
(1) Moon, J. H.; Lee, J. W.; Lee, U. D. Bioresour. Technol. 2011, 102, 9550−9557. 2098
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