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DOI: 10.1039/D3EE00969F (Paper) Energy Environ. Sci., 2023, 16, 3074-3087

Nonviable carbon neutrality with plastic waste-to-energy†

Serang Kwon‡ a, Jieun Kang‡ a, Beomhui Lee‡ a, Soonwook Hong b, Yongseok Jeon c, Moonsoo Bak d and Seong-kyun Im *ae
aMechanical Engineering, Korea University, Seoul 02841, Republic of Korea. E-mail: sim3@korea.ac.kr
bMechanical Engineering, Chonnam National University, Gwangju 61186, Republic of Korea
cMechanical Engineering, Korea Maritime and Ocean University, Busan 49112, Republic of Korea
dMechanical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
eOJEong Resilience Institute, Korea University, Seoul 02841, Republic of Korea

Received 27th March 2023 , Accepted 7th June 2023

First published on 8th June 2023

Abstract

Incineration, pyrolysis, and gasification during plastic waste treatment are inevitable to reduce the volume of landfilled plastic waste and recover energy; however, they cause severe carbon emissions. We show that the current practices of plastic waste-to-energy will significantly impact carbon neutrality. Various energy recovery systems, such as combined power cycles and fuel cells, were modeled to evaluate the power generated and CO2 emitted from treating the current and projected plastic waste by 2050. The CO2 emissions from plastic waste-to-energy systems are higher than those from current fossil fuel-based power systems per unit of power generated, even after considering the contribution of carbon capture and storage. Power generation using plastic waste will significantly increase by 2050, and therefore, we suggest technologies required for achieving carbon neutrality.


Broader context

The environmental issues caused by plastic waste have overwhelmed human societies in recent years. Thus, there have been global trends to reduce the volume of plastic waste while recovering energy by using plastic waste-to-energy via incineration, pyrolysis, and gasification. Plastic waste-to-energy is a plastic waste treatment method that can solve outstanding environmental and energy issues. However, the impact of plastic waste-to-energy on carbon neutrality has been overlooked. This study modeled various energy recovery systems with incineration, pyrolysis, and gasification to evaluate carbon emissions during energy recovery processes. The results showed that plastic waste-to-energy systems produce significant CO2, and the amount of CO2 emissions will rapidly increase with increasing plastic waste by 2050. Plastic waste-to-energy will severely influence carbon neutrality even with carbon capture and storage. This study implies that carbon must be separated during energy recovery processes. Otherwise, the fight against global warming would fail due to plastic waste. Plasma gasification technology was suggested as a carbon separation technology.

Introduction

Plastics: waste or energy source

The convenience of plastic has made it an indispensable element in our daily lives. In addition, global plastic use has inevitably increased globally because of the prolonged COVID-19 pandemic,1 which caused various environmental issues throughout production, distribution, consumption, and disposal. Examples include increased soil and marine water pollution caused by microplastics.2 Several efforts have been made to alleviate the environmental impacts of plastic waste by recycling, reducing the volume of waste, and recovering valuable resources from plastic waste.3 In recent years, plastic waste has been regarded as an attractive alternative for energy generation sources, which can solve outstanding waste and energy crises. Thus, various government agencies classify municipal solid waste (MSW) containing plastic as a renewable energy source for power generation. The US Energy Information Administration (EIA) classifies MSW as biomass, which is a renewable energy source,4 and the OECD indicates “the renewable fraction of municipal waste” to be renewable energy.5 Furthermore, other countries, such as China and South Korea are progressing towards treating plastic waste as a clean and renewable energy source. Given this global trend, it is necessary to evaluate the environmental impacts of using plastic waste as an energy source and whether it is environmentally benign. Although there have been efforts to evaluate the carbon neutrality of plastic waste-to-energy,6 a study that comprehensively addresses the carbon neutrality of plastic waste-to-energy under the current plastic waste generation and treatment trends is necessary.

Incineration: “final destination” of plastic waste

Plastic waste is currently treated in three ways: landfilling, recycling, and incineration (Fig. 1). The total amount of plastic waste treatment was 353.29 Mt in 2019, of which recycling was 32.83 Mt, landfill was 173.84 Mt, and incineration was 67.30 Mt.7
image file: d3ee00969f-f1.tif
Fig. 1 Global material, electricity, and GHG gas emission flow due to plastic waste and fossil fuel in 2019. The material, energy, and GHG emission flow occurring due to fossil fuel8 and plastic wastes.9,10

Landfill

The landfill method buries plastic waste at landfill sites. Landfill has the advantage of less carbon dioxide generation per weight due to the low decomposability of plastics as compared to other waste that produce gas emissions during decomposition. The degradation rate of the plastic fraction of landfilled municipal waste has been 1% over 100 years, while that of the paper fraction is 27%.11 However, plastic waste requires a large landfill space owing to its high volume-to-mass ratio. Thus, landfill cannot accommodate the increasing amounts of plastic waste. In addition, plastic waste from landfills can contaminate groundwater, leading to unpredicted long-term pollution issues. Therefore, landfills cannot be a solution for handling plastic waste.

Recycling

Recycling is a physical process of converting waste into new products12 and is an environmentally benign method of plastic waste treatment. However, plastics cannot be recycled indefinitely as they will degrade, and thus, the multi-recycled plastics eventually end up contributing to landfilling or get incinerated. Furthermore, the plastic waste that can be recycled is very limited.13

Incineration

Incineration is the combustion method of waste disposal, and drastically reduces plastic waste volume. During incineration, heat is released and recovered mostly via steam generation. Thus, plastic waste can be treated as fuel by utilizing the energy conversion. This management has a pivotal energy generation potential, though the amount of plastic waste treated by incineration is less than that of mismanaged. For example, incineration of plastic waste generates approximately 140.2 TW h of electricity globally in 2019 (Fig. 1).14

Greenhouse gas (GHG) emissions are inevitably produced during plastic waste treatment. In 2019, carbon dioxide generated from plastic waste was 369.39 Mt CO2-eq, of which landfill, recycling, and incineration emitted 17.39 Mt CO2-eq, 29.55 Mt CO2-eq, and 322.46 Mt CO2-eq, respectively.14 Landfills are the least affecting way to treat plastic waste, followed by recycling, with respect to their impact on global warming. However, due to the aforementioned limitations, incineration is the most practical way to treat plastic waste because it can reduce the volume of waste and also produce energy simultaneously, which can reduce the consumption of hydrocarbon fuels such as coal, resulting in potential carbon emission reduction. However, the incineration of plastic waste is inevitably accompanied by GHG emissions. Therefore, many countries are looking for ways to minimize GHG emissions while also recovering energy from plastic waste.

Materials and methods

Materials

Mixed plastic11 shown in Table 1 was fed to the designed power generation cycles. Additionally, four different types of plastics were selected to demonstrate the effect of material composition on waste-to-energy cycles. The selected plastics were polyethylene15 (PE), polypropylene16 (PP), polyethylene terephthalate17 (PET), and polycarbonate17 (PC). PE and PP have higher hydrogen-to-carbon ratios than PET and PC. The mixed plastics contain intermediate hydrogen-to-carbon fractions. In contrast, PET and PC have high oxygen fractions, whereas that of PE and PP were negligible. The oxygen fraction in the plastic waste acts as an additional oxidant during gasification.
Table 1 Ultimate analysis and heating values of various plastics
Mixed plastic11 PE15 PP16 PET17 PC17
PP: polypropylene; PE: polyethylene; PET: polyethylene terephthalate; PC: polycarbonate
C (wt%) 63.36 85.81 86.42 62.8 69.56
H (wt%) 10.59 13.86 12.28 4.3 5.33
O (wt%) 7.37 0.0 0.0 32.79 24.86
N (wt%) 0.61 0.12 0.72 0.07 0.08
Moisture (wt%) 15.3 0.02 — — —
Higher heating value (MJ kg−1) 34.05 — 44.7 22.77 26.71
Lower heating value (MJ kg−1) 30.79 38.04 42.09 21.60 24.94

Methods

Cycle modeling scenarios. The incineration method has been widely adopted to manage plastic waste. This method can reduce the volume of the waste to less than 10% while recovering energy by utilizing the heat released from incineration.18 However, large amounts of carbon dioxide (CO2) are generated during incineration. Thus, alternatives such as gasification and pyrolysis, have been suggested for plastic waste energy recovery.

Gasification and pyrolysis occur in oxygen-insufficient/depleted environments and generate syngas or synthetic oils. The syngas produced from the gasification was fed into an energy-recovery system. Under certain circumstances, hydrogen in syngas can be selectively collected for fuel-cell power generation or other purposes. Similarly, the synthesis of oil from pyrolysis has been employed as a fuel for power generation. Incineration, gasification, and pyrolysis retrieve energy from plastic waste under various thermodynamic conditions. Therefore, the efficiencies of power generation from these three processes differ considerably. Furthermore, the carbon emissions during energy recovery vary depending on the energy recovery method. During gasification, a fraction of the carbon in plastic waste is captured as char. Thus, the syngas produced during this method contains fewer carbon atoms than the raw material. In contrast, the synthesis of oil by pyrolysis contains as much carbon as the raw material.

Therefore, this study models the energy efficiency and GHG emissions of plastic waste using incineration and gasification cycles, with and without carbon capture and storage (CCS). The pyrolysis method was not considered in this analysis, as the major product of this process is synthetic oil, and the yield of gaseous products is nearly 30 wt% during pyrolysis, which is much smaller than the number produced in gasification (>85%).19 In addition, pyrolysis is generally adopted for homogeneous materials, such as biomass, and not plastic waste.18 Below is a list of the modeled cycles. Detailed descriptions of these cycles are provided in the following sections.

p-I: incineration and steam cycle (Fig. 2(a)).


image file: d3ee00969f-f2.tif
Fig. 2 Schematic diagrams of the modeled cycles. Gray dashed carbon capture and storage (CCS) unit of each subfigure is employed if the modeled cycle includes CCS. Subfigures indicate (a) incineration and steam cycle (p-I) and incineration and steam cycle with CCS (p-I/CCS), (b) gasification and steam cycle (p-G) and gasification and steam cycle with CCS (p-G/CCS), (c) gasification and fuel cell (p-FC) and gasification and fuel cell with CCS (p-FC/CCS), and (d) gasification and combined cycle (p-CC) and gasification and combined cycle with CCS (p-CC/CCS).

p-I/CCS: incineration and steam cycle with CCS (Fig. 2(a)).

p-G: gasification and steam cycle (Fig. 2(b)).

p-G/CCS: gasification and steam cycle with CCS (Fig. 2(b)).

p-FC: gasification and fuel cell (Fig. 2(c)).

p-FC/CCS: gasification and fuel cell with CCS (Fig. 2(c)).

p-CC: gasification and combined cycle (Fig. 2(d)).

p-CC/CCS: gasification and combined cycle with CCS (Fig. 2(d)).

Gibbs energy minimization method. The Gibbs energy minimization (GEM) method was used to predict the yield and composition of syngas for the gasification process.20 The GEM method calculates the fraction of products at the minimum total Gibbs energy with the conservation of elements at a given temperature and equivalence ratio (ER). During the calculations, the molecular distributions of the products varied among the species considered. Therefore, the inclusion or exclusion of certain species can lead to different product compositions. This study considered 14 species in the products, namely, methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), water vapor (H2O), nitrogen (N2), nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), propane (C3H8), and solid carbon (C(s)). These are the major species for the plastic gasified products. The composition of these products varied at different gasification temperatures. This study investigated the temperature range of 500–1000 °C and ER of 0.0–0.5. For instance, Fig. 3 shows the molar yields of 5 species and cold gas efficiency (CGE) of the mixed plastic at a temperature range of 500–1000 °C for ER of 0.3. The molar yield, that is the number of moles produced per mole of gasified plastic of the species, increased for H2 and CO but decreased for CO2. As the increasing molar yields of H2 and CO were greater than that of CH4, the CGE, that is the ratio of the total lower heating value of syngas to gasified material, increased with temperature. Moreover, the molar yield of C(s) was higher at low temperatures. The amount of carbon captured as C(s) can be greater at lower temperatures, producing less GHG when syngas is utilized.
image file: d3ee00969f-f3.tif
Fig. 3 Gasification products and cold gas efficiency (CGE) of various plastics calculated using Gibbs energy minimization method at an equivalence ratio of 0.3. PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate; PC: polycarbonate.
Incineration. The first two cycles, p-I and p-I/CCS, included combustion chambers and boilers. During incineration, 98% of the carbon content of plastic was oxidized to CO2, and this was used to estimate the CO2 emissions from incineration power plants.21 As incineration occurs in an O2-rich environment, the flue gas downstream of the incinerator primarily comprises of CO2, H2O, O2, and N2. It was assumed that the molar amount of CO2 was 98% of the carbon molar amount of the raw material. The heat generated during incineration was transferred to the steam cycle through the boiler. The efficiency of a boiler is defined as the ratio of heat transferred to the steam cycle to that of the lower heating value of the raw material, and this was assumed to be 90% in this study. The assumed value refers to the boiler efficiency of coal-fired power plant with subcritical or supercritical steam cycles.22 Therefore, the amount of heat transferred to the steam cycle can be calculated directly from the lower heating value (LHV) of the raw material using eqn (1).
 
Qsteam cycle = ηboiler × LHVmaterial(1)
Gasification. Gasification is the process of converting feed material into syngas in an O2-deficient environment (ER from 0.2–0.5). This reaction is endothermic and the thermal energy is supplied during the process. Gasification types are distinguished by their heat supply methods. The heat supply from conventional thermal gasification consumes extra fuel, such as natural gas or waste materials. Plasma gasification uses an electrically-driven plasma source for heat supply.23 In both methods, energy was supplied for gasification. Therefore, the energy consumption of a gasification unit is assumed to be in the range of 500–1500 kW h per ton of fed plastic. From the literature, it can be found that the general power consumption of a plasma torch in plasma gasification is 800–1000 kW h per ton of feed material.24 Therefore, in this study, a wider range of power consumption was examined. Gasification products were predicted using the GEM method.25
Steam cycle. The modeled cycles, except for p-FC and p-FC/CCS, included the steam cycle for power generation. The identical steam cycle was used for these models. A schematic of the steam cycle is presented in Fig. 4, it has a high-pressure (HP) of 78.14 bar (77.12 atm) and 456.7 °C and low-pressure (LP) of 10.65 bar (10.51 atm) and 232.2 °C. With the state properties summarized in Table 2, the steam cycle attains an energy efficiency of 25.22%. The GT Pro software26 (Thermoflow, Inc.) was used to calculate the net efficiency of the steam cycle. The steam cycle refers to the steam temperature and pressure in incineration power plants. The steam cycle of incineration power plants fed with MSW has a typical superheated steam pressure of 30–70 bar, and temperature range of 400–450 °C.18 These properties are similar to those of the bottoming cycle for a combined cycle fueled by natural gas, provided by Thermoflow's GT Pro software. Therefore, an identical steam cycle can be employed throughout the modeled power cycles.
image file: d3ee00969f-f4.tif
Fig. 4 Schematic diagram of the steam cycle in incineration and steam cycle (p-I), incineration and steam cycle with carbon capture and storage (CCS) (p-I/CCS), gasification and steam cycle (p-G), gasification and steam cycle with CCS (p-G/CCS), gasification and combined cycle (p-CC), and gasification and combined cycle with CCS (p-CC/CCS). HRSG: heat recovery steam generator; HP: high-pressure; IP: intermediate-pressure; Eco: Economizer.
Table 2 Temperature and pressure of steam cycle at each state
State Temperature (°C) Pressure (atm)
s1 456.7 77.12
s2 232.2 10.51
s3 38.7 0.07
s4 38.7 0.07
s5 38.6 1.20
s6 93.3 1.17
s7 104.4 1.17
s8 104.4 1.17
s9 104.4 1.17
s10 104.8 11.15
s11 178.4 10.83
s12 184.0 10.83
s13 180.6 81.81
s14 252.7 80.30
s15 289.8 79.43
s16 295.4 79.43

Fuel cell. Solid oxide fuel cells (SOFC) operating with direct internal reforming of syngas have been used in p-FC and p-FC/CCS.27 A schematic of an SOFC is shown in Fig. 5(a). The modeled fuel cell system allowed for the recirculation of the cathode and anode sides. In addition, the afterburner burns the unreacted fuel and air that exit the cathode and anode. The syngas and air at the inlets of the cathode and anode were preheated by the flue gas. The flue gas temperature was obtained from the energy balance between the input and output of the burner.
image file: d3ee00969f-f5.tif
Fig. 5 (a) Schematic diagram of fuel cell cycle of the solid oxide fuel cell in the gasification and fuel cell (p-FC) and gasification and fuel cell with carbon capture and storage (p-FC/CCS). (b) Energy efficiency of the solid oxide fuel cell as function of syngas properties depending on equivalence ratio (ER) and gasification temperature (Temp). (c) Operating current density of solid oxide fuel cell upon ER of gasification. The current density is kept constant on fixed ER values. PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate; PC: polycarbonate.

For the SOFC, the electrical efficiency was obtained using the assumed operating parameters, including the fuel utilization ratio, that is the ratio of reacted H2 at the cathode side to injected H2, as shown in eqn (2), respectively.28 Subsequently, the composition of syngas at the outlet of the cathode and air at the outlet of the anode were determined from a system of equations, and the amount of power generated was estimated using the Nernst voltage and voltage losses. The other parameters for the voltage loss calculations are summarized in Table 3.29 The operation condition of SOFC was 850 °C, and the recirculation factor at the anode and cathode outlet was 0.3.27 The fuel utilization factor was fixed at 0.85, whereas the current density was designated based on the diverse compositions of the syngas. Fig. 5(b) and (c) illustrate the energy efficiencies and the varied current densities for different materials and gasification conditions. The electrical efficiency of a SOFC depends on the syngas composition at different gasification temperatures.

 
Uf = H2,electrochemical reaction/H2,syngas(2)

Table 3 Operation parameters of solid oxide fuel cell29
Operation parameters Quantity
Pressure 1 bar
Exit temperature 850 °C
Inlet temperature 750 °C
Active surface area 100 cm2
Exchange current density of anode 0.65 A cm−2
Exchange current density of cathode 0.25 A cm−2
Effective gaseous diffusivity through anode 0.2 cm2 s−1
Effective gaseous diffusivity through cathode 0.05 cm2 s−1
Thickness of anode 0.5 mm
Thickness of electrolyte 0.01 mm
Thickness of cathode 0.05 mm

Gas turbine cycle. A gas turbine cycle was used for the modeled p-CC and p-CC/CCS systems for power generation. The gas turbine cycle consists of a fuel compressor, air compressor, combustor, and turbine, as shown in Fig. 6(a). The performance of a gas turbine cycle varies with different operating parameters. The representative operating parameters are the pressure ratio (PR) and turbine inlet temperature (TIT). The operational parameters used in this study refer to a gas turbine cycle fueled by fossil fuels.22,30–32 The performance of the gas turbine cycle with a range of PR (15–19) and TIT (1200–1600 °C) was evaluated to select representative values for the current modeling. Fig. 6(b), (c) and (d) show the turbine outlet temperature (TOT), energy efficiency of the gas turbine cycle (simple cycle), and energy efficiency of the combined cycle for different ER, TIT, and syngas properties at the gasification temperature on the x-axis. Because PR and TIT of 17 and 1400 °C showed near average values among the considered range, it was then selected as the operating parameters of the gas turbine cycles in this study.
image file: d3ee00969f-f6.tif
Fig. 6 (a) Schematic diagram of gas turbine cycle of the combined cycle in the gasification and combined cycle (p-CC) and gasification and combined cycle with carbon capture and storage (p-CC/CCS). (b) Turbine outlet temperature (TOT), (c) energy efficiency of gas turbine cycle, and (d) energy efficiency of combined cycle upon different turbine inlet temperature (TIT) and pressure ratio (PR). Purple (PR 15), blue (PR 17), and green (PR 19) circles, squares, and triangles with solid, dotted and dashed lines represent a TIT at 1200 °C, 1300 °C, and 1400 °C, respectively. comp: compressor; turb: turbine.
Carbon capture and storage. CCS can be applied in either pre- or post-combustion processes.33,34 The pre-combustion capture process collects CO2 from the syngas, whereas the post-combustion process removes CO2 from the flue gas. In this study, the cycles modeled with CCS were post-combustion processes. The syngas of the modeled cycles was incinerated in the energy generation units, that were the combustion chambers and fuel cell. Therefore, post-combustion CCS can reduce the total amount of GHG emissions compared with the pre-combustion process. Representative CCS methods include absorption, adsorption, membrane, and chemical looping.34 In this study, an absorption method employing methyl diethanolamine (MDEA) as the absorbent was chosen, which is adequate for the post-combustion process.35 The carbon capture efficiency and power consumption of the process were assumed to be 90% and 0.0950 kW h kg−1 CO2.33 To transport the captured CO2 to a storage site, the captured gas was compressed to 110 bar with a power consumption of 0.146 kW h kg−1 CO2.33,36 Conventionally, pipeline transportation was performed by using several compressors along the path to the storage site. Typical storage sites for captured CO2 include the deep ocean, saline aquifers, and unmineable coal beds.33 The compressed gas in this study was assumed to be transported to saline aquifers. The distance between the saline aquifer and power plant was 800 km.37 Recompression with a power consumption of 7 kW h per ton CO2 and the leakage of 3.28% during transport were also considered.37 The construction of pipelines at the site and storage facility can generate CO2, as discussed in the following section.

CCS has been extensively studied, and there are many ongoing CCS projects globally because CCS can permanently remove carbon unless there is accidental leakage.38 In addition, CCS could be an economical way to reduce carbon emissions, particularly for point emission sources like plastic waste-to-energy power plants. However, the distance between the emission sources and storage sites could be a hindrance to the development of CCS.39 Furthermore, CO2 storage at geological sites still has a risk of leakage and induced seismicity.34 Despite the difficulties mentioned above, CCS is regarded as one of the most viable technologies for carbon neutrality because CO2 pipeline technology is matured, and CCS can permanently remove carbon. Thus, in this study, CCS was used as a carbon mitigation method for plastic waste-to-energy power plants. More discussions for other carbon mitigation methods are provided in the ESI.†

Construction of infrastructure. The GHG emissions from constructing a power plant should be included in the analysis. Therefore, GHG emissions from construction were obtained by considering the GHG intensity40 of the materials used for construction.41 The GHG emissions from building the power plant and CCS infrastructure were also determined by taking into consideration the type of material used for construction, as shown in Tables 4 and 5.41
Table 4 Required amounts of materials and the greenhouse gas (GHG) intensity of each material for construction of the power plant40–42
Fuel type Co-firing41 (coal and wood)
Electricity output (MW) 400
Lifetime (year) 30
Lifetime output (MW h) 89[thin space (1/6-em)]352[thin space (1/6-em)]000
Materials Consumption (ton) GHG intensity40 (kg CO2-eq per kg)
Concrete 310[thin space (1/6-em)]000 0.22
Reinforcing steel 13[thin space (1/6-em)]600 2.03
Low-alloyed steel 8540 1.39
Chromium steel 22[thin space (1/6-em)]300 2.03
Aluminum 1[thin space (1/6-em)]420 22
Copper 419 4.7
Zinc 62.8 3.41
Lead 41.9 1.64
PVC 1[thin space (1/6-em)]100 2.14
PE and PP 141 2.4
Glass fiber 329 2.63
Epoxy 125 3.07
Brass 147 —
Bitumen 200 —
Lubricant 522 2.93
Rubber 71.5 3.18
Copolymer 15.8 —
Cast iron 592 1.25
Ceramic tile 237 —
Glass 345 —
GHG emission (g CO2-eq per kW h) 2.16

Table 5 Required amounts of materials and the greenhouse gas (GHG) intensity of each material for construction of carbon capture and storage infrastructure
Material Quantity41 (kg) GHG intensity40 (kg CO2-eq per kg)
Steel 317[thin space (1/6-em)]000 1.39
Concrete 2400 0.22
Total amount of CO2 captured over lifetime (Mt) 94
GHG emission per captured CO2 (kg CO2-eq per CO2 captured kg) 0.0000047

Transportation of plastic waste. The transportation of plastic waste from collection sites to power plants were also included in the total GHG emissions estimates. An emission amount of 0.0331 g CO2-eq per g of plastic waste was used, based on the literature for GHG emissions of plastic waste transported to combustion sites.42
Pyrolysis. The GHG emissions from pyrolysis were calculated based on Miandad et al.,43 who provided experimental results for various feedstock types. The experimental conditions were as follows: quantity of the feedstock was 1 kg, retention time was 75 minutes, temperature was 450 °C, and the heating rate was 10 °C min−1. Polystyrene (PS), PP, and PE were used as feedstock types. In addition, mixtures of PS/PP, PS/PE, and PP/PE in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio, PS/PE/PP in 2[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio, and PS/PE/PP/PET in 2[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio were used. Under these seven conditions, the GHG emissions from the utilization of pyrolysis oil were calculated for electricity generation (Table 6). To calculate the amount of electricity, the efficiency was assumed to be 0.2, and the heating value was calculated based on the equation by Demirbas.44
Table 6 Greenhouse gas (GHG) emission from utilization of the pyrolysis oil from PS, PE, and mixtures of PS/PP, PS/PE, PP/PE, PS/PP/PE, and PS/PP/PE/PET. PS: polystyrene; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate
Material Product Quantity (kg) Heating value (MJ kg−1) Electricity (kW h) GHG emission (kg CO2-eq per kg) GHG emission (kg CO2-eq per kW h)
PS Styrene 0.390 27.2 0.590 3.31 831.5
Ethylbenzene 0.171 119 1.13 3.25
Toluene 0.207 114 1.32 3.28
PE Alpha-methylstyrene 0.0206 137 0.157 3.28 456.9
Ethylbenzene 0.0307 134 0.229 3.25
Benzene 0.0328 134 0.244 3.31
Propylbenzene 0.0983 117 0.638 3.23
Xyplene 0.0328 134 0.244 3.25
Naphthalene 0.0223 137 0.169 3.36
Methylnaphthalene 0.0353 133 0.261 3.34
Biphenyl 0.0172 138 0.132 3.31
Phenanthrene 0.0319 134 0.238 3.39
2-Phenylnaphthalene 0.0139 139 0.107 3.38
PS/PP Styrene 0.0613 116 0.394 3.31 568.0
Ethylbenzene 0.0165 135 0.124 3.25
Benzene 0.0265 131 0.193 3.31
Propylbenzene 0.0158 135 0.119 3.23
Methylstryrene 0.130 85.7 0.619 3.28
PS/PE Ethylbenzene 0.0734 128 0.520 3.25 563.1
Toluene 0.0778 127 0.547 3.28
Annulene 0.267 88.6 1.31 3.31
Benzene 0.0427 134 0.317 3.31
PP/PE Azulene 0.0247 131 0.180 3.36 1068
Phenanthrene 0.00768 139 0.0592 3.39
Phenol 0.0166 88.4 0.00813 2.75
Naphthalene, 2-ethenyl 0.0101 138 0.0771 3.36
1-Docosanol, acetate 0.0319 37.1 0.0658 2.91
Benzenedicarboxylic acid 0.123 16.4 0.112 2.08
PS/PP/PE Styrene 0.0960 121 0.645 3.31 507.3
Ethylbenzene 0.0470 132 0.345 3.25
Benzene 0.0314 135 0.236 3.31
Benzene 1-propenyl 0.0225 137 0.172 3.28
Propylbenzene 0.0250 137 0.190 3.23
Alpha-methylstyrene 0.18228 102 1.03 3.28
PS/PP/PE/PET Azulene 0.0404 131 0.295 3.36 932.3
Biphenyl 0.0168 138 0.129 3.31
Phenanthrene 0.0212 137 0.161 3.39
Cyclononasiloxane, octadecamethyl 0.0536 128 0.0380 1.16
Phenol 0.02 2020 0.00911 2.75
1,2-Benzenedicarboxylic acid 0.0732 5.38 0.0219 2.08
.beta.-sitosterol 0.0232 18.6 0.0240 3.02
Stigmasta-5, 24(28)-dien-3-ol, (3.beta.) 0.028 18.9 0.0294 3.01
Terphenyl, 5′-phenyl- 0.0124 139 0.0957 3.38
Cyclodecasiloxane, eicosamethyl 0.0316 10.9 0.0192 1.16

GHG emission of the modeled cycles. The GHG emissions from the modeled cycles were calculated based on the composition and amount of exhaust gas. The GHG emissions from the p-I and p-I/CCS were based on the carbon content of the plastics. For other energy recovery cycles that utilize gasification, the GHG emissions depend on the carbon content of the syngas, excluding C(s). Because all the modeled cycles included the combustion stage of syngas or plastic, the global warming potential (GWP) of other chemicals was not required. The gases emitted include CO2, N2, O2, and H2O.
Life cycle of GHG emission analysis of fuels. The life cycle of GHG emissions analysis of renewable sources is in Table 7 for the average and 95% confidence intervals in Fig. 3. The life cycle of GHG emissions analysis of fossil fuels is in Table 7, too.
Table 7 Greenhouse gas emission values of life cycle analysis of renewable sources and fossil fuels
Energy source Quantity (g CO2-eq per kW h)
Mean Standard deviation
PV: photovoltaic; CSP: concentrated solar power; CI: confidence interval.
Solar PV45 49.9 43.3
Wind power 41 34.1 67.23
Hydropower46 (reservoir type) 21.05 6.25
Hydropower46 (run-of-river type) 27.18 10.38
CSP47 (parabolic trough) 79.8 67.82
CSP47 (central receiver) 85.67 78.48
Nuclear48 (boiling water reactor) 14.52 9.37
Nuclear48 (pressurized water reactor) 11.87 10.24
Nuclear48 (light water reactor) 20.5 16.71
Energy source Mean CI 90%− CI 90%+
Coal49 823.2 729.4 917
Natural gas49 420.8 391.5 450.2

Results and discussion

Energy efficiency

The energy efficiencies of the modeled cycles were analyzed for each plastic material, as shown in Fig. 7(a). An ER of 0.3 was assumed for gasification. For incineration, 98% of the total carbon was oxidized, and 90% of the total LHV was involved in energy generation. Fig. 7(a) showed that the energy efficiency increased with increasing gasification temperature, as the CGE increased with increasing temperature shown in Fig. 3. The energy efficiency decreased with increasing power consumption of the gasification unit. The mixed plastic, PE, and PP exhibited higher overall energy efficiencies than PET and PC. This indicates that plastics with a higher hydrogen-to-carbon content are more suitable for gasification.
image file: d3ee00969f-f7.tif
Fig. 7 (a) Energy efficiency and (b) greenhouse gas (GHG) emission of the modeled cycles when applied to various plastics. Wgasif: power consumption of the gasification unit; Temp: gasification temperature; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate; PC: polycarbonate; p-I: Incineration and steam cycle; p-I/CCS: Incineration and steam cycle with carbon capture and storage (CCS); p-G: gasification and steam cycle; p-G/CCS: gasification and steam cycle with CCS; p-FC: gasification and fuel cell; p-FC/CCS: gasification and fuel cell with CCS; p-CC: gasification and combined cycle; p-CC/CCS: gasification and combined cycle with CCS.

The energy efficiency of p-I was approximately 20%. The energy efficiency of p-G was lower than that of p-I, or negative for some of the gasification conditions. However, when p-FC or p-CC was employed instead of the steam cycle, the gasification cycles resulted in higher efficiencies than the incineration cycles at gasification temperatures over 600–700 °C. Furthermore, when CCS was included, the energy efficiency decreased by 2–3% for the modeled cycles.

GHG emission per power generation

The GHG emissions per unit of power generation are shown in Fig. 7(b) for each type of plastic material. The GHG emissions per power generation were not estimated if the energy efficiency was under 10% because the low energy efficiency was regarded as inappropriate for energy recovery.

The mixed plastic, PE, and PP exhibited different tendencies for PET and PC, as shown in Fig. 7(b). The emissions per power generation were the highest for p-G, followed by p-I, when mixed plastic, PE, and PP were used. The emissions from the other gasification cycles (p-FC and p-CC) were generally low. As the energy efficiencies of p-CC for all the plastics were higher than those of the other gasification cycles, p-CC resulted in lower GHG emissions than p-G, p-FC, and p-CC. The cycles with CCS exhibited lower emissions than those without CCS.

Effect of ER on energy efficiency and GHG emission

Fig. 8(a) shows the effect of ER on energy efficiency. The plotted values are the average energy efficiencies of the cycles fed with the mixed plastic. Energy efficiency decreased when CCS was included. The energy efficiency of the gasification cycles was the highest for p-CC, followed by p-FC and p-G. The energy efficiency of the gasification cycles peaked at an ER between 0.25–0.35, point p-CC achieved a higher energy efficiency than p-I, and the energy efficiency of p-FC was similar to that of p-I. In contrast, p-G resulted in a lower energy efficiency than p-I or p-I/CCS over the entire ER range.
image file: d3ee00969f-f8.tif
Fig. 8 (a) Energy efficiency and (b) greenhouse gas (GHG) emission of the modeled cycles on equivalence ratio (ER) for each plastic. Dotted line and dashed line indicate average values for gasification at 500–700 °C and 800–1000 °C each. Solid lines are the average values for gasification at 500–1000 °C. PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate; PC: polycarbonate; p-I: Incineration and steam cycle; p-I/CCS: Incineration and steam cycle with carbon capture and storage (CCS); p-G: gasification and steam cycle; p-G/CCS: gasification and steam cycle with CCS; p-FC: gasification and fuel cell; p-FC/CCS: gasification and fuel cell with CCS; p-CC: gasification and combined cycle; p-CC/CCS: gasification and combined cycle with CCS.

Fig. 8(b) shows the effect of ER on GHG emissions per unit of power generation. Likewise, in Fig. 7(b), the value of GHG emissions was not considered if the energy efficiency was below 10%. Therefore, the p-G results were discontinuous at low ER for PE and PP. In addition, those of PET and PC were discontinued along most ER values. The emissions per power generation increased for the gasification cycles with an increase in the ER. At a low ER, the emissions per power generation of the gasification cycles without CCS were lower than those of p-I in most cases. Despite the decreased energy efficiency of the cycles with CCS, the decreased emissions per power generation compensated for the emissions so that the emissions per power generation can be reduced with CCS.

Overrated carbon neutrality of plastic waste-to-energy

Pyrolysis and gasification are advanced and more environmentally benign technologies than incineration with respect to their environmental impact and energy recovery from plastic waste. Therefore, there is a global trend of transitioning plastic waste-to-energy recovery methods from incineration to pyrolysis and gasification. Pyrolysis is a thermochemical process that occurs at high temperatures in the absence or presence of a small amount of oxygen, producing pyrolysis oil.50 Gasification is a tertiary recycling method used for syngas production.51 Pyrolysis and gasification can be performed in a more controlled manner than incineration. Consequently, using pyrolysis and gasification to treat plastic waste can reduce harmful emissions during the energy recovery processes. Therefore, they are recognized as promising technologies that can solve plastic waste and sustainable energy crises. However, GHG production during the pyrolysis and gasification processes has been overlooked. Therefore, the impact of plastic waste-to-energy recovery via pyrolysis and gasification on global warming needs to be evaluated.

According to a thermodynamics modeling study of waste-to-power generation cycles, the emissions from incineration are 980–1128.5 g CO2 per kW h and 228–774 g CO2 per kW h during steam cycle without and with carbon capturing, respectively. The total CO2 emission from pyrolysis oil for power generation was calculated as 456.9–1068 g CO2 per kW h (Table 6). For fossil fuels, the emissions from unabated oil, coal, and natural gas are 600–839 g CO2 per kW h,8,52 688–1404 g CO2 per kW h,8,30,52 and 306–987 g CO2 per kW h,53 respectively. The modeling results indicate that the pyrolysis and gasification methods for treating plastic waste have little superiority to fossil fuel-based power generation in terms of CO2 emissions (Fig. 1), because the former are accompanied by oxidation processes, resulting in a large amount of CO2 production. On the contrary, pyrolysis is not limited in its oxygen supply; hence, the emission during the production of useful oils is lesser than that during other processes.54 However, pyrolyzed oil contains a high percentage of carbon like fossil fuels, and eventually carbon is emitted during its utilization. Therefore, it is difficult to determine pyrolysis and gasification as optimal carbon-neutral countermeasures against overflowing plastic waste because of the non-negotiable amount of CO2 emissions (Fig. 9).


image file: d3ee00969f-f9.tif
Fig. 9 Current projected plastic waste management methods and associated emission. Plastic waste was directly used as feedstock for incineration and gasification. Conversion of waste to pyrolysis oil preceded during pyrolysis resulting in lesser emission and usage of the oil as feedstock for electricity generation.43,54

Achieving the emission level close to those from renewable sources is necessary for achieving carbon neutrality, such as 34.1 g CO2 per kW h from wind, 49.9 g CO2 per kW h from solar-photovoltaic (PV) techniques, and 79.8–85.67 g CO2 per kW h from concentrated solar power (CSP) resources.45,47 Therefore, significant reductions in CO2 emissions and management of plastic waste are still required to generate a satisfactory amount of electricity and achieve carbon neutrality.

Required technologies for carbon neutrality

An ideal energy recovery process from plastic utilizes hydrogen in the plastic while retaining the carbon content to avoid large-scale emission of CO2. However, current incineration with energy recovery is almost incapable of capturing carbon, except for the CCS process. Total 98% of the carbon in plastics is converted to CO2 during incineration.21 Gasification is capable of capturing the carbon content as solid carbons more efficiently and has been adopted to reform fossil fuels and MSW to syngas.55 Accordingly, various energy recovery systems with plastic gasification are designed in this study, and their GHG emissions and potential power generation have been compared with those of renewable energy and fossil fuels. The modeled cycles fed with plastics included incineration cycles (p-I or p-I/CCS), gasification and steam cycles (p-G or p-G/CCS), gasification and fuel cells (p-FC or p-FC/CCS), and gasification and combined cycles (p-CC or p-CC/CCS) with or without CCS.

The common products of plastic gasification, including solid carbon, were predicted based on the GEM method.20 Gasification at temperatures below 800 °C yielded high amounts of solid carbon, whereas that above 800 °C produced syngas with a higher total heating value (Fig. 3). High-temperature operation conditions are attainable with an external power source, such as a plasma torch used for integrated plasma gasification combined cycles (IPGCCs).20 The high content of solid carbon can be captured by the filter system if the plastic waste was gasified at low temperatures. Syngas with a high hydrogen content can be sent to the energy recovery components if the gasification temperature is high.

Power generation and GHG emissions under various available energy recovery components were estimated using the predicted gasification products. Typically, lower GHG emissions can be achieved using CCS, which is expected to be widely used in fossil fuel power plants. In 2040, electricity generation from fossil fuels with CCS will possibly exceed that of unbated fossil fuels.8Fig. 10 describes the GHG emissions per electricity generation of renewable energy, fossil fuels, and plastic waste. The gasification of plastic waste with a combined cycle or fuel cell showed low emission levels among the energy recovery methods from plastics. However, highly efficient CCS must be applied in the energy recovery system of plastic waste to achieve low CO2 emissions. Plastic wastes will possibly become the next equivalent to coal and produce high CO2 during energy conversion processes.


image file: d3ee00969f-f10.tif
Fig. 10 Greenhouse gas (GHG) emission from various energy sources with a confidence interval of 95%. Renewable energy sources include solar photovoltaic45 (Solar PV), wind energy45 (Wind), hydropower46 of reservoir type (Hydro-R) and run-of-river type (Hydro-ROR), concentrated solar power47 (CSP) of parabolic trough type (CSP-PT) and central receiver type (CSP-CR), nuclear power48 of boiling water reactor type (Nuclear-BWR), pressurized water reactor type (Nuclear-PWR), and light water reactor type (Nuclear-LWR). The 95% confidence interval is obtained from the mean value ± standard deviation for the energy sources except for coal49 (*Coal) and natural gas49 (*NG). Error bars of the two sources with asterisk indicate 90% confidence intervals. Energy recovery cycles of plastic wastes include incineration (p-I), incineration with CCS (p-I/CCS), and gasification and steam cycle (p-G), steam cycle and CCS (p-G/CCS), fuel cell (p-FC), fuel cell with CCS (p-FC/CCS), combined cycle (p-CC), and combined cycle with CCS (p-CC/CCS).

Conclusion

If the current increasing trend of plastic consumption persists, which is highly likely to with the growth of the global economy, the contribution of CO2 emissions during the energy recovery process of plastic waste to global warming will become more significant in the future. Simultaneously, the potential for plastic waste to be used as an energy source will increase. Fig. 11 depicts the current electricity generation from various sources and projected generation in 2050, indicating that the electricity generation by plastic waste is comparable to that by fossil fuels and nuclear power. Therefore, sustainable development without furthering the global warming would be impossible without technologies for environmentally benign energy recovery from plastic waste. Therefore, technologies for efficient and effective CCS, and carbon separation from plastic waste during conversion processes are required, which do not exist at the present time.
image file: d3ee00969f-f11.tif
Fig. 11 Global annual electricity generation from (a) various energy sources in 2020 and (b) that is projected by 2050. Global annual electricity generation in 2020 includes electricity generation8 from renewable sources, such as solar, wind, hydropower, nuclear, and concentrated solar power (CSP), and fossil fuels, such as coal with carbon capture utilization and storage (Coal/CCUS), natural gas with CCUS (NG/CCUS), coal, natural gas (NG), and oil. The electricity generation from waste plastics is obtained from the amount of plastic waste generated in 2020 with incineration fraction of 29%. In 2050, the projected value of plastic waste generation is 1100 Mt per year56,57 to 1600 Mt per year,58 and the predicted value of incineration rate is 50%.56,58,59 Various energy recovery methods have been compared based on the ratio of amount of plastic waste produced to that undergoing incineration. p-I: Incineration and steam cycle; p-I/CCS: Incineration and steam cycle with carbon capture and storage (CCS); p-G: gasification and steam cycle; p-G/CCS: gasification and steam cycle with CCS; p-FC: gasification and fuel cell; p-FC/CCS: gasification and fuel cell with CCS; p-CC: gasification and combined cycle; p-CC/CCS: gasification and combined cycle with CCS.

Author contributions

M. B. and S. I. contributed to conceptualization of the project. S. K., J. K., B. L., S. H., Y. J., M. B., and S. I. contributed to methodology of the project. S. K., J. K., B. L., and Y. J. contributed to investigation of the project. S. K., J. K., and B. L. contributed to visualization of the project. S. H., Y. J., M. B., and S. I. acquired funding of the project. S. I. contributed to administration and supervision of the project. S. K., J. K., B. L., and S. I. contributed to writing of the original draft. S. H., Y. J., M. B., and S. I. contributed to review and editing of the draft.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. K., J. K., B. L., S. H., Y. J., M. B., and S. I. were supported by the Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2022R1A4A3023960). S. K., J. K., B. L., and S. I. were also supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (2020R1C1C1006837). S. K. and S. I. were supported by the OJEong Eco-Resilience Institute (OJERI) of Korea University (K2208841).

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Footnotes

† Electronic supplementary information (ESI) available: Methodological details, supporting tables and figures, PDF. See DOI: https://doi.org/10.1039/d3ee00969f
‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2023
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