Assessment of Plasma Assisted Gasification for Effective
Polyethylene Terephthalate (PET) Plastic Waste Treatment
by
Nandan A. Patel
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANIAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
November, 2012
(For Graduation December 2012)
TABLE OF CONTENTS
-- Cover Sheet-------------------------------------------------------------------------------------------------------------- i
-- Table of Contents ----------------------------------------------------------------------------------------------------- ii
-- List of Figures -------------------------------------------------------------------------------------------------------- iii
-- List of Tables---------------------------------------------------------------------------------------------------------- iv
-- List of Symbols -------------------------------------------------------------------------------------------------------- v
-- Acknowledgment----------------------------------------------------------------------------------------------------- vi
-- Abstract --------------------------------------------------------------------------------------------------------------- vii
1. Introduction and Background --------------------------------------------------------------------------------------- 1
1.1. The Problem of Plastic Waste Management ----------------------------------------------------------- 1
2. Theory and Methodology -------------------------------------------------------------------------------------------- 2
2.1. Problem Description ---------------------------------------------------------------------------------------- 2
2.2. Plastic Waste Properties ----------------------------------------------------------------------------------- 2
2.3. Polyethylene Terephthalate ------------------------------------------------------------------------------- 3
2.4. Plasma Technology Background ------------------------------------------------------------------------- 4
2.5. Generation of Artificial Plasma -------------------------------------------------------------------------- 4
2.6. The Plasma Assisted Gasification Technology for the Treatment of PET Waste -------------- 5
2.6.1. The Plasma Gasification Process Overview -------------------------------------------------- 5
2.6.2. Waste Feed Handling Unit – Cleaner/Shredder/Crusher ---------------------------------- 7
2.6.3. Plasma Torch --------------------------------------------------------------------------------------- 8
2.6.3.1. Non-Transferred Torch --------------------------------------------------------------- 8
2.6.4. The Plasma Reactor ------------------------------------------------------------------------------10
3. Results and Discussion ----------------------------------------------------------------------------------------------12
3.1. Analysis of Chemical Reactions ------------------------------------------------------------------------12
3.2. Determination of Heat of Gasification -----------------------------------------------------------------17
3.3. Material and Energy Balances ---------------------------------------------------------------------------20
4. Conclusion -------------------------------------------------------------------------------------------------------------24
5. References -------------------------------------------------------------------------------------------------------------25
ii
LIST OF FIGURES
Figure 1: Plastic Waste in Ocean and Landfill in a Less Developed Country [2, 3] .................................. vii
Figure 2: Materials Discarded in Municipal Solid Waste, 2008 [4] ............................................................. 1
Figure 3: The Plastic Waste Categories [4, 5]............................................................................................... 2
Figure 4: Terepthalic Acid (left) and Ethylene Glycol (right) [4]................................................................. 3
Figure 5: PET Monomer - (C10H8O4) [4] ...................................................................................................... 3
Figure 6: Cascade Process of Ionization. Electrons are “e – “, neutral atoms “o”, and cations “+” [10] ..... 5
Figure 7: Plasma Gasification Process Layout [9] ........................................................................................ 6
Figure 8: Possible PET Waste Feed Handling Unit [11] .............................................................................. 7
Figure 9: Typical Model of a Non-transferred (a) and Transferred (b)Plasma Arc Torch [12] .................... 8
Figure 10: Non-transferred Plasma Arc Torch [13] ...................................................................................... 9
Figure 11: Schematic of a Non-transferred Plasma Arc Torch [11] ........................................................... 10
Figure 12: Typical Concept of a Plasma Reactor [11] ................................................................................ 11
Figure 13: Typical Non-Transferred Plasma Arc Torch Configuration [14] .............................................. 13
Figure 14: Chemical Structure of Polyethylene Terephthalate [15] ........................................................... 14
Figure 15: Schematic Diagram of Plasma Gasification Process ................................................................. 19
Figure 16: Schematic of Reactions 3 and 4 [20] ......................................................................................... 21
iii
LIST OF TABLES
Table 1: Typical Chemical Composition of Polyethylene Terephthalate [6]................................................ 4
Table 2: Values of Heats of Formation of Reaction Products (gas) [17] .................................................... 15
Table 3: Values of Heats of Formation of Reaction Reactants [17] ........................................................... 15
Table 4: Parameters describing Dependence of Measured Heat Capacity on Temperature [18] ................ 18
Table 5: Temperature and Heat of Fusion [19] ........................................................................................... 18
Table 6: Temperature and Heat of Decomposition [18] ............................................................................. 19
iv
LIST OF SYMBOLS
∆𝐻𝑟
Enthalpy of Reaction (kJ)
𝑁
Number of Moles (mol)
𝑁𝑝
Number of Moles (p denote Product) (mol)
∆ℎ°𝑓,𝑝
Enthalpy of Formation (p denote product) (J/mol)
𝑁𝑟
Number of Moles (r denote reactants) (mol)
∆ℎ°𝑓,𝑟
Enthalpy of Formation (r denote reactant) (J/mol)
∆𝐻𝑟1
Heat of Reaction w/carbon dioxide (kJ)
∆𝐻𝑟2
Heat of Reaction w/oxygen (kJ)
𝑀𝑝
Molecular Weight of the Polymer Repeat Unit (g)
𝑄𝑐
Gross Heat of Combustion (kJ/g)
𝐻𝑔
Heat of Gasification (kJ/kg)
𝐶𝑚𝑎𝑡
Heat Capacity of Material (J/g)
𝐶𝐿0
Temperature Dependent Heat Capacity Constraint (J/g-°C)
CL1
Temperature Dependent Heat Capacity Constraint (J/g-°C²)
CR0
Temperature Dependent Heat Capacity Constraint (J/g-°C)
CR1
Temperature Dependent Heat Capacity Constraint (J/g-°C²)
𝐻𝑓𝑢𝑠𝑖𝑜𝑛
Heat of Fusion (J/g)
𝐻𝑑𝑒𝑐
Heat of Decomposition (J/g)
𝑇𝑓𝑢𝑠𝑖𝑜𝑛
Fusion Temperature of PET Waste (°C)
𝑇𝑑𝑒𝑐
Decomposition Temperature of PET Waste (°C)
𝑇𝑡𝑟𝑎𝑛𝑠
Final Temperature of PET Waste (°C)
𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙
Initial Temperature of PET Waste (°C)
∆𝐻𝑟3
Energy Output from Turbine (kWh/ton (short))
v
ACKNOWLEDGMENT
I would like to thank Professor Ernesto Gutierrez-Miravete for his guidance and patience throughout the
completion of my master’s project. I would also like to thank all the other professors from UCONN and
RPI for sharing their knowledge and passion. Lastly, I would like to thank my family and friends for their
support throughout my entire academic career.
vi
ABSTRACT
In today’s day and age, the consumption habits of our modern lifestyles are causing a huge worldwide
waste management and energy concerns. With each passing day, engineers and scientists are constantly
challenged to develop new ideas and unique solutions to decrease our dependence on fossil fuels. With
the growing demand of finding sustainable sources of energy, the need to dispose of growing amounts of
solid waste along with the pressure to reduce the amount of waste going to landfill, minimize
environmental liabilities and to reduce greenhouse gas emission, are resulting in an increasing interest in
utilizing the waste-to-energy (WTE) conversion technologies.
Plastic solid waste, especially Polyethylene Terephthalate (PET), is one of the major players in waste
management as it presents challenges and opportunities for the less developed societies. As stated in
various news media, "Plastic pollution is a major global phenomenon that has crept up on us over the
decades, and it really requires a global and comprehensive solution that includes systemic rethinks about
usage and production'' [1]. Increasing interest in focusing on plasma assisted gasification applied to the
treatment of plastic waste worldwide. The plasma assisted gasification process has been demonstrated in
many of the most recent studies as one of the most effective and environmentally friendly methods for
waste treatment and energy utilization. This method will be assessed here for the treatment of
plastics/polymers. A special emphasis will be paid on waste generated from polyester sources, which
makes up a great percentage of our daily life cycle, called PET. This paper will utilize recent progress in
the recovery of plastic solid waste using gasification conversion technology and will apply it to the
production of syngas. The syngas produced from PET plastic waste will then be evaluated for the
generation of energy. Also, a plasma assisted gasification process and its technical viability will be
investigated for the PET plastic waste.
Figure 1: Plastic Waste in Ocean and Landfill in a Less Developed Country [2, 3]
vii
1 Introduction and Background
1.1 The Problem of Plastic Waste Management
Plastics play an important role in almost every aspect of our lives. Plastics have opened the way for new
inventions and have replaced other materials in existing products. They are light, durable and versatile, as
well as resistant to moisture, chemicals and decay. Plastics are used to manufacture everyday products
such as beverage containers, toys, and even furniture. Yet these are the same properties that present
environmental challenges and demands proper end of life management. Today, the world produces plastic
waste at a rate that outpaces its capacity to collect and dispose it of in a safe and environmentally sound
manner. Plastic constitute a significant and increasing segment of the municipal solid waste (MSW)
stream, as can be seen in Figure 2 below.
Figure 2: Materials Discarded in Municipal Solid Waste, 2008 [4]
In 2010, the world produced 300 million tons of plastic out of which 31 million tons of plastic waste was
generated. Only 8 percent of the total plastic waste generated in 2010 was recovered for recycling.
Plastics make up more than 13 percent of the municipal solid waste stream, a dramatic increase from
1960, when plastics were less than one percent of the waste stream [6]. The largest categories of plastic
wastes are found in containers and packaging but they also are found in durable and nondurable goods.
Plastic waste is spreading at a high pace and can found at various public places such as railways, airports,
parks, and floating in the ocean.
1
2 Theory and Methodology
2.1 Problem Description
The utilization of plastic waste is of considerable social significance. At the present time, disposal at
waste disposal sites is still the most common form of final disposal although the incineration of plastics is
likely to be preferred. Disposal of plastic waste at waste disposal sites is very disadvantages because
plastic has a base of oil or natural gas and does not biodegrade in a landfill. Similarly, the incineration is
disadvantageous in two factors: first the incinerator ash; and second, the emission from the incineration
process. The problem addressed by various disposal facilities is to eliminate the need for disposal at waste
disposal sites and another problem addressed by various environmentalists is to reduce emission produced
by the incineration process, which is only possible to a limited extent.
2.2 Plastic/Polymer Waste Properties
The Society of Plastics Industry (SPI) defines a plastic material as “any one of a large group of materials
consisting wholly or partly of combinations of carbon with oxygen, hydrogen, nitrogen and other organic
or inorganic elements, which, while solid in the finished state, at some stages in its manufacture is made
liquid, and thus capable of being formed into various shapes, most usually through the application, either
singly or together, of heat and pressure.”
In 1988, the SPI developed the seven resin identification codes to differentiate the six major resins
suitable for recycling as shown in Figure 3. The resins codes are (1) PET, (2) HDPE, (3) PVC, (4) LDPE,
(5) PP, (6) PS, and (7) other (O). The “Other” category is not to be confused with non-recyclable
thermosets [4].
Figure 3: The Plastic Waste Categories [4, 5]
2
This paper examines one of the most commonly used plastic and major players in the plastic waste,
Polyethylene Terephthalate. The following section describes the composition of PET.
2.3 Polyethylene Terephthalate (PET)
PET is the most common thermoplastic polyester and was first known as fiber. As a member of the
polyester family, PET is used extensively in the formation of synthetic fibers. PET has good clarity and
toughness and is a good barrier to gases such as oxygen and carbon dioxide. Polyethylene terephthalate
has the molecular formula (C10H8O4), Figures 4 & 5, and is unique among the major polymers for its high
oxygen content. The oxygen content makes the plastic impervious to gas diffusion, which is crucial in
keeping carbonated soft drinks fresh.
Figure 4: Terepthalic Acid (left) and Ethylene Glycol (right) [4]
Figure 5: PET Monomer - (C10H8O4) [4]
Most plastics are limited to a handful of resins that differ only slightly in makeup. PET is notable for
containing a larger amount oxygen. Its chemical composition is shown in Table 1 below.
3
Table 1: Typical Chemical Composition of Polyethylene Terephthalate [6]
Symbol
Element
Atomic Weight
Number of
Atoms
Mass Percent
Chemical Formula – C10H8O4
C
Carbon
12.01078
10
62.5011%
H
Hydrogen
1.007947
8
4.1961%
O
Oxygen
15.99943
4
33.3028%
2.4 Plasma Technology Background
In a simplistic view, a plasma torch is a way to generate heat, via the passage of an electric current
through a gas flow. Plasma as a method to generate heat is a proven, well-demonstrated commercial
technology at work around the world. In the 19th century, plasma technology was developed and used in
Europe for the metals industry. At the beginning of the 20th century, the chemical industry used plasma
heaters to extract acetylene gas from natural gas. In the early 1960s, the United States National
Aeronautics and Space Administration used plasma technology to simulate the high temperatures that
orbiting space vehicles would encounter when reentering earth’s dense atmosphere. In the 1980s, largescale plasma heater processes were built and commissioned for a variety of industrial applications,
particularly for metals and chemicals [8].
Although plasma technology has a long track record, its application to waste disposal is more limited.
During the past twenty years, the use of plasma technology for waste disposal has undergone extensive
research and small-scale development. Plasma technology has been used for a long time for surface
coating and for destruction of hazardous wastes but its application in plastic waste, especially
polyethylene terephthalate, has not been explored fully because of the high cost of using electricity as a
source of energy.
2.5 Generation of Artificial Plasma
There are several means for the generation of Plasma, however, one principle is common to all of them,
that there must be energy input to produce and sustain it. Plasma refers to every gas of which at least a
percentage of its atoms or molecules are partially or totally ionized. In a plasma state of matter, the free
4
electrons occur at reasonably high concentrations and the charges of electrons are balanced by positive
ions. Plasma is generated when an electrical current is applied across a dielectric gas or fluid. The
potential difference and subsequent electric field causes ionization of material and electrons are pulled
toward the anode while the nucleus pulled towards cathode. Figure 6 describes the Cascade process of
ionization [9].
Plasma torches and plasma arc technology have been used in a variety of industrial, military, space and
other applications due to its sizable temperature and density ranges. The technology is being used in
industrial and extractive metallurgy surface treatments such as thermal spraying (coating), etching in
microelectronics, metal cutting and welding etc.
Figure 6: Cascade Process of Ionization. Electrons are “e – “, neutral atoms “o”, and cations “+” [10]
Thermal plasmas have the potential to play an important role in a variety of chemical processes.
Compared to most gases even at elevated temperatures and pressures, the chemical reactivity and
quenching rates that are characteristic of these plasmas is far greater. Plasma technology is very drastic
due to the presence of highly reactive atomic and ionic species and the achievement of higher
temperatures in comparison with other thermal methods. In fact, the extremely high temperatures (several
thousand degrees in Celsius scale) occur only in the core of the plasma, while the temperature decreases
substantially in the marginal zones [10].
2.6 The Plasma Gasification Technology for the Treatment of PET Waste
2.6.1
The Plasma Gasification Process Overview
Plasma gasification is a technologically advanced and environmentally friendly process of disposing solid
wastes and converting them to commercially usable gas called syngas. It is a non-incineration thermal
process that uses extremely high temperature in an oxygen starved environment to decompose input waste
5
material completely into very simple molecules. Plasma as a method to generate heat is a proven, welldemonstrated commercial technology at work around the world. The unique characteristics of plasmas
should result in much broader applications in environmental mitigation and are increasingly becoming
valuable environmental tools in future research of environmental areas. Although plasma technology has
a long track record, its application to waste disposal is more limited. This paper focuses on plasma
assisted technology as an innovative thermal waste treatment technique, which is very effective and
presents great prospects especially in PET waste management. The block diagram given below presents
the main sections of the established plasma gasification process. Furthermore, this paper doesn’t describe
the plasma gasification process in detail but briefly looks at the waste feed unit and plasma furnace along
with analysis of chemical reaction that takes place inside the furnace.
Figure 7: Plasma Gasification Process Layout [9]
The plasma gasification technology has been applied by the National Technical University of Athens,
which set the relevant specifications for the development of the pertinent pilot unit, trying to explore its
potential in waste management. Plasma gasification is an efficient and environmentally responsible form
of thermal treatment of wastes which occurs in oxygen starved environment so the waste is gasified, not
incinerated. As a result, most of the carbon is converted to fuel gas. Waste treatment applications exploit
the plasma’s ability to rapidly initiate a variety of chemical reactions including decomposition,
evaporation, pyrolysis and oxidation [9].
A typical plasma gasification process consists of many sections including feeding system, a gasification
furnace, one or more plasma torches with their associated power supplies and controls, pollution
abatement and monitoring hardware, plus associated gas and slag handling equipment and energy
6
recovery units. The entire system is illustrated in Figure 7. However, this paper will only demonstrate the
primary unit of the plasma gasification process, plasma furnace, which leads to production of the primary
synthesis gas. The following subsections will provide a brief schematic description of the gasification
sections however; it will not be covered in depth due to the scope of this paper.
2.6.2
Waste Feed Handling Unit – Cleaner/Shredder/Crusher
The waste feed unit is used for pre-treatment of the waste in order to meet the inlet requirements of the
plasma furnace. For PET waste with high volume, a shredder and crusher will be required to reduce the
volume of the waste with air tight screw feeders to drive the plastic waste into the furnace. For some PET
processes, rinsing/washing can be done during and after shredding/crushing. Also, there can be two types
of grinding: the wet and dry method. In the dry grinding, the materials are ground to the desired size and
bagged in a plastic sacks to meet the desired weight capacity. In wet grinding, water is continuously fed
on the material during and after grinding is performed. In some cases, detergent is added to efficiently
remove the stubborn dirt, adhesives or glues in the plastic waste. The feed rate is adjustable by varying
the speed of the screw conveyor. The waste can be manually loaded into the hopper connected to the
screw conveyor. Multiple inlet ports in the furnace are desired in design to ensure that the waste is evenly
distributed within the furnace, and air tight feeding operation ensures that the reducing atmosphere of the
furnace can be fully controlled and no synthesis gas can escape from the furnace to the local
surroundings.
Figure 8: Possible PET Waste Feed Handling Unit [11]
7
2.6.3
The Plasma Torch
There are two types of plasma torches, the transferred torch and the non-transferred torch that are
typically used in the plasma reactor. In each case the electrical source for the torch is direct current. When
using a transferred torch, one electrode extends into the plasma reactor (i.e., the metal slag in the reactor
bottom or a conducting wall), allowing the electric arc to generate between the tip of the torch and the
conducting receiver. The low pressure gas is heated in the external arc and therefore preventing heat loss
– as in the case with non-transferred torch as discussed below.
Figure 9: Typical Model of a Non-transferred (a) and Transferred (b) Plasma Arc Torch [12]
Alternatively, a non-transferred torch can be used in which the ionized gas is created within the torch and
is projected onto the waste. Both types of torches have been in commercial operation for a decade.
Eventually, despite its lower thermal efficiencies, the most commonly used torch is the non-transferred
because it allows the good mixing of the plasma and the waste; and the treatment of the waste does not
require the high heat fluxes achieved by the transferred arc. Since the non-transferred arc torch is more
commonly used device for the waste treatment, it is merits more discussion.
2.6.3.1 Non-transferred Arc Torch
For the non-transferred arc troch, electricity is transformed into thermal energy by means of electric
discharges from cathode to anode within a water cooled torch and heats the plasma jet issued from the
torch. It provides a plasma flow for treating the waste and provides effective plasma flow coverage.
8
Figure 10: Non-transferred Plasma Arc Torch [13]
The arc is established between an axial cathode and an annular anode. The gas crosses the boundary layer
between the gas column and the anode inner surface and is pushed downstream by the pressure of the gas
flow. The electrodes are large components able to tolerate the gradual reduction and have to be watercooled to handle the high excursion of temperatures. Inert argon in a cathode chamber prevents any
chemical reactions on the cathode surface. The water mainly determines power balance in torch and store
most of the energy taken from arc. Water is considered good cooling median because of high enthalpies
of hydrogen and oxygen. Water injected in the system plays two roles. The first objective of the water is
to cool the chamber. Second, the water is the arc-stabilizer.
Non-transferred torch has low efficiency due to heat losses in the cold boundary layer region; therefore its
power output can be as low as 50% of the power input. However, it gives a very uniform temperature
distribution due to the mixing of the waste within the plasma jet and is easily scaled down to small
installations. This device can be used in two configurations: with hot electrodes (temperature of the
plasma is between 6000 K to 15000K) and cold electrodes (temperature below 7,000K). The main
producers worldwide are Europlasma and Westinghouse.
9
Figure 11: Schematic of a Non-transferred Plasma Arc Torch [11]
The quality of plasma produced is a function of pressure, temperature and torch power (the greater the
better). Every manufacture build torches in accordance with their proprietary technologies and therefore it
can vary in regards to efficiency of the torch itself.
2.6.4
The Plasma Reactor
The plasma furnace is the central component of the system where the gasification process takes place.
The plasma gasification takes place in a closed plasma chamber called the plasma rector or furnace which
is a sealed, usually stainless steel vessel filled with ordinary air. The furnace is equipped with one of its
primary components, the plasma torch. Typically, the waste enters the reactor through a point at the top or
the side of the reactor and, after contact with the ionized gas, the waste starts to gasify and the ash is
formed in a liquid pool at the bottom of the reactor. This slag is usually drained out from the outlet
provided at the bottom of the reactor.
As discussed in the plasma torch section, either a transferred or a non-transferred torch can be used in the
reactor and can be positioned to suit design and/or to increase efficiency. The gas introduced between the
electrode and the anode that becomes plasma can be oxygen, helium or other, but use of air is very
common due to its low cost. The gases in the reactor can be heated by one or more plasma torches or
electrodes.
At the temperatures maintained within the plasma reactor, the organic molecules contained in the plastic
waste begin to break down and react with the gases/air to form carbon monoxide, hydrogen and carbon
dioxide. As a result of these reactions, the plastic waste is gasified, transformed into a synthesis gas
containing mostly hydrogen, carbon monoxide and nitrogen, and exit at the top of the reactor. The
10
synthesis gas from the reactor has a low to medium calorific value, and is therefore suitable as fuel for a
gas fired power generation unit. However, after leaving the reactor, the gas is still contaminated with a
number of undesirable compounds, such as hydrogen chloride which can cause damage to machinery and
the environment. The gas therefore requires cleaning through various process equipments. However, the
cleaning system is not included here as it is outside the scope of this paper.
Figure 12: Typical Concept of a Plasma Reactor [11]
A plasma reactor specific to the waste configuration is usually preferred for the gasification process as
better heat transfer to the waste can be achieved along with higher temperature, resulting in more
complete waste conversion. Furthermore, a plasma reactor consistently exhibits much lower
environmental levels for both air emissions and slag leachate toxicity than competing technologies like
incineration. The plasma reactor’s overall functionality usually does not discriminate among any types of
plastic wastes. The only variable for the reactor is the amount of energy that it takes to destroy the waste.
11
3 Results and Discussion
3.1 Analysis of Chemical Reactions
The primary goal of the plasma furnace/reactor is to gasify the organic compounds of the PET waste to a
high quality syngas. The thermal chemical conversion process that takes place inside the plasma furnace
can be described well by the term “gasification”. Plasma assisted gasification decomposes PET waste in
an oxygen-starved environment into the basic molecules of carbon monoxide (CO) and hydrogen (H2).
Gasification is conducted in sub-stoichiometric amounts of oxygen in order to obtain only partially
oxidized products. Gasification takes the same pathway as combustion but stops at an intermediate level,
hence yielding H2 and CO, instead of oxidizing them to water (H20) and carbon dioxide (CO2).
The gases used as additive to perform gasification are basic CO2 and O2, effect of which is analyzed in
this section. The reason for the addition of oxygen is because PET contains more carbon atoms than
oxygen atoms and therefore more oxygen has to be added to gasify all carbon. During the plasma
gasification process, various chemical reactions take place. However, the main overall reaction occurring
in the reactor with polyethylene terephthalate and CO2 as reactants is:
Reaction 1
𝐶10 𝐻8 𝑂4 + 6 𝐶𝑂2 →
∆𝐻𝑟1
16 𝐶𝑂 + 4 𝐻2
The initial temperature of the PET waste is assumed room temperature, 25 °C, and the temperature in the
reactor is approximated 1100 °C. The plasma reactor temperature can be selected based on the properties
of the waste. In this case, the temperature required for gasification of the PET waste is determined to be
around 500 °C. This is further explained in Section 3.2.
If oxygen is added to support the energy supply, the second reaction is:
Reaction 2
𝐶10 𝐻8 𝑂4 + 3 𝑂2 →
∆𝐻𝑟2
10 𝐶𝑂 + 4 𝐻2
As stated earlier, gasification is the breakdown of the organic part of the waste into a syngas that produces
a given mixture of CO and H2, by controlling the amount of oxygen present.
12
Reactions 1 and 2 are achievable because while the PET plastic waste descends down through the top in
the plasma reactor, as shown in Figure 12, it passes through the plasma jet and it dissociates and ionizes.
This is indeed possible as the plasma jet has a centerline temperature of up to 25,000 K and the
surrounding temperature in the plasma furnace is assumed to be controlled around 1100 °C. This results
in a generation of synthesis gas which is composed of simple molecules, CO and H2. To create a plasma
jet and to maintain the plasma furnace temperature, a specially designed plasma torch can be utilized. A
normal setting for the plasma torch needed to achieve the temperature required for gasification consist of,
torch current = 400 A and voltage = 380 V. In Figure 13, the schematic of a typical non-transferred
plasma torch is shown. As discussed in Section 2.6.3.1, non-transferred torches usually have low output
efficiency and it can be seen from Figure 13, all the given models have an output efficiency of 60%. This
paper will consider an efficiency of the plasma torch to be, ƞtorch = 0.60.
Figure 13: Typical Non-Transferred Plasma Arc Torch Configuration [14]
Now, by given all the parameters of the torch, consider the model PPT-150AC in Figure 13. For this
torch, a power input of 150 kW is required by the electric source. Knowing the efficiency of the torch, it
can be concluded that only 90 kW remains in the plasma jet due to an efficiency loss. This paper do not
consider any heat losses through the reactor walls, but if we assume the heat losses through the reactor
walls to be equal to 15 kW, only 75 kW of the initial energy remains available for the reaction in terms of
thermal energy. Furthermore, addition or losses of power can come from the reaction taking place.
However, to simplify the process, the reaction constituents are assumed balanced to produce a
stoichiometric mixture of CO and H2. The gases used in addition are CO2 and O2, effects of which are
analyzed above.
13
The enthalpy of formation of PET is calculated based on the knowledge of energy contributions of
polymer structure. For polymeric reactants the molar heat of formation can be estimated from the
tabulated molar contributions of the chemical groups which constitute the monomer or repeat unit. The
chemical structure of the PET polymer is shown below.
Figure 14: Chemical Structure of Polyethylene Terephthalate [15]
Here, one of the first developed estimation method of additivity of group properties of Van KrevelenChermin is used. According to Van Krevelen-Chermin, the heat of formation can be found by,
Equation 1
∆𝐻°𝑓 = ∑
𝑐𝑜𝑛𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛𝑠 𝑜𝑓
𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑎𝑙
+ ∑
= 𝐴 + 𝐵 ∗ 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑔𝑟𝑜𝑢𝑝
𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑖𝑜𝑛𝑠
Next to the group contributions, also corrections must be taken into consideration, which allow for
structural corrections and for the degree of symmetry of the molecule. The group contributions are
considered as linear functions of the temperature. The group of contributions and structural corrections
are based on experimental data. The heats of formation of the polyethylene terephthalate constituent
groups at T = 25 °C = 298 K are [21 and 22]:
𝐽
𝑚𝑜𝑙
𝐽
2 ∗ (− 𝑂 −) = 2 ∗ (−120,000 + 70 ∗ 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒) = −198,280
𝑚𝑜𝑙
𝐽
2 ∗ (−𝐶𝐻2 −) = 2 ∗ (−22,000 + 102 ∗ 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒) = 16,792
𝑚𝑜𝑙
2 ∗ (𝐶 = 𝑂) = 2 ∗ (−132,000 + 40 ∗ 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒) = −240,160
= (96,000 + 185 ∗ 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒) = 151,130
𝐽
𝑚𝑜𝑙
----------------------------------------------------------------------------------------𝑇𝑜𝑡𝑎𝑙 = −270,518
𝐽
𝑚𝑜𝑙
Summing these group contributions gives the molar heat of formation of the PET monomer. It is the
amount of heat needed to make 1 mole of compound from its elemental components.
∆𝐻°𝑓 = −270,518
𝐽
, 𝑎𝑡 𝑟𝑜𝑜𝑚 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑠, 𝑇 = 25 °𝐶
𝑚𝑜𝑙
14
Values of heats of formation of all the other reaction constituents are given in the table below for the
calculation of the overall heat of reaction balance in the plasma reactor.
Table 2: Values of Heats of Formation of Reaction Products (gas) [17]
Products of Reactions
Enthalpy
H2 (1100 °C)
32,282 J/mol
CO (1100 °C)
-76,090 J/mol
Table 3: Values of Heats of Formation of Reaction Reactants [17]
Reactants of Reactions
Enthalpy
Polyethylene Terephthalate (298 K)
-270,518 J/mol
CO2 (25 °C)
-393,448 J/mol
O2 (25 °C)
0 J/mol
The overall enthalpy of the reaction is obtained from the formation enthalpies of the reactants and
products. By subtracting the heat of formation of the products from the heat of formation of the reactants:
Equation 2
∆𝐻𝑟 = ∑ 𝑁𝑝 ∆ℎ°𝑓,𝑝 ± ∑ 𝑁𝑟 ∆ℎ°𝑓,𝑟
where p and r denote products and reactants, respectively, in the standard state at temperature, T = 25 °C.
Thus, to gasify one mole of polyethylene terephthalate with CO2:
Equation 3
∆𝐻𝑟 = {(𝑁ℎ°𝑓 )
𝐶𝑂
+ (𝑁ℎ°𝑓 ) } − {(𝑁ℎ°𝑓 )
+ (𝑁ℎ°𝑓 ) }
𝐻
𝑃𝐸𝑇
𝐶𝑂
2
2
Equation 4
𝐽
𝐽
) + (4 𝑚𝑜𝑙)(32,282
)}
𝑚𝑜𝑙
𝑚𝑜𝑙
𝐽
𝐽
− {(1 𝑚𝑜𝑙) (−270,518
) + (6 𝑚𝑜𝑙)(−393,448
)}
𝑚𝑜𝑙
𝑚𝑜𝑙
∆𝐻𝑟1 = {(16 𝑚𝑜𝑙)(−76,090
∆𝐻𝑟1 = 1,542 𝑘𝐽, 𝑓𝑜𝑟 𝑒𝑎𝑐ℎ 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑃𝐸𝑇
Addition of oxygen will introduce the second reaction and the heat of reaction is:
15
Equation 5
𝐽
𝐽
) + (4 𝑚𝑜𝑙) (32,282
)}
𝑚𝑜𝑙
𝑚𝑜𝑙
𝐽
𝐽
− {(1 𝑚𝑜𝑙) (−270,518
) + (3 𝑚𝑜𝑙) (0
)}
𝑚𝑜𝑙
𝑚𝑜𝑙
∆𝐻𝑟2 = {(10 𝑚𝑜𝑙) (−76,090
∆𝐻𝑟2 = −361 𝑘𝐽, 𝑓𝑜𝑟 𝑒𝑎𝑐ℎ 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑃𝐸𝑇
The molecular weight of the polymer repeat unit, PET is given by:
Equation 6
𝑀𝑝 = 𝐶10 𝐻8 𝑂4 = 10 ∗ (12.0107) + 8 ∗ (1.0079) + 4 ∗ (15.9994) = 192.16 𝑔
The gross heat of combustion with CO2 per unit mass is then:
Equation 7
𝑄𝑐 (𝑃𝐸𝑇) =
∆𝐻𝑟1
1,542 𝑘𝐽, 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑃𝐸𝑇
=
= 8 𝑘𝐽/𝑔
𝑀𝑝
192.16 𝑔, 𝑚𝑜𝑙 𝑜𝑓 𝑃𝐸𝑇
where Mp is the molecular weight of PET and ∆𝐻𝑟1 is the energy from an endothermic Reaction (1)
Similarly, the gross heat of combustion with O2 per unit mass is then:
Equation 8
𝑄𝑐 (𝑃𝐸𝑇) =
∆𝐻𝑟2
−361 𝑘𝐽, 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑃𝐸𝑇
=
= −1.88 𝑘𝐽/𝑔
𝑀𝑝
192.16 𝑔, 𝑚𝑜𝑙 𝑜𝑓 𝑃𝐸𝑇
where Mp is the molecular weight of PET and ∆𝐻𝑟2 is the energy from an exothermic Reaction (2)
There are two properties that are related to heat release rate and those are heat of combustion and heat of
gasification. The heat of combustion is the ratio of heat release rate to mass loss rate, as calculated in
Equations (7 and 8). The second material property is the heat of gasification, which is defined as the net
heat flow into the material required to convert one unit mass of solid material to volatiles. The heat of
gasification, i.e. production of syngas with composition given in Reaction (2), can be calculated as the
difference between the heat of combustion of PET and the heat of combustion of syngas. However, this
paper discusses the heat of gasification as a sum of contribution of heat capacity and heats of processes
that occur when material is gasified in the following section.
16
3.2 Determination of Heat of Gasification of Polyethylene Terephthalate
The amount of heat required to gasify a unit mass of material is one of the key properties required in order
to determine its response to the plasma jet in the plasma reactor. The heat of gasification will assist in
determining the plasma torches required to provide the calculated heat of gasification while at the same
time melting the ash into vitrified products. Here, a methodology is applied to a polyethylene
terephthalate plastic waste for determining the heat of gasification. This methodology is in agreement
with U.S. Department of Transportation Federal Aviation Administration methodology for determining
heat of gasification using differential scanning calorimetry as documented in Report No. DOT/FAA/ARTN07/62, Reference 18.
The heat of gasification (Hg) is a thermodynamic quantity that is equal to the amount of energy required
to gasify unit mass of material at a constant atmospheric pressure. Hg depends on the initial and final
temperatures of the material along with its composition. The heat of gasification is presented as a sum of
contributions of heat capacity and heats of processes that occur when material is gasified.
Equation 9
𝑇𝑓𝑖𝑛𝑎𝑙
𝐻𝑔 =
∫ 𝐶𝑚𝑎𝑡 𝑑𝑇 + 𝐻𝑓𝑢𝑠𝑖𝑜𝑛 + 𝐻𝑑𝑒𝑐
𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙
Here, Cmat is temperature-dependent heat capacity of the PET. Hfusion is the heat of fusion. Hdec is the heat
of decomposition, which also includes the heat of vaporization of volatiles formed during the
decomposition. Tinitial is the initial temperature of PET and Tfinal is the final temperature (decomposition
temperature) of its gasification product.
The above defined parameters of PET can be obtained from differential scanning calorimetry (DSC). This
paper will utilize the established literatures and experiments to calculate the heat of gasification.
Equation 10
𝑇𝑓𝑖𝑛𝑎𝑙
∫
𝐶𝑚𝑎𝑡 𝑑𝑇 = 𝐶𝐿0 (𝑇𝑡𝑟𝑎𝑛𝑠 − 𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙 ) +
𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙
+
𝐶𝐿1 2
2
(𝑇𝑡𝑟𝑎𝑛𝑠 − 𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙
) + 𝐶𝑅0 (𝑇𝑓𝑖𝑛𝑎𝑙 − 𝑇𝑡𝑟𝑎𝑛𝑠 )
2
𝐶𝑅1 2
2
(𝑇𝑓𝑖𝑛𝑎𝑙 − 𝑇𝑡𝑟𝑎𝑛𝑠
)
2
17
The parameters supporting Equation (10) above are given in Table 4. These values are obtained from
DSC experiment documented in Reference 18.
Table 4: Parameters (of Equation 10) describing Dependence of Measured Heat Capacity on Temperature [18]
Polymer
CL0
CL1
Ttrans
CR0
CR1
(J/g-°C)
(J/g-°C²)
(°C)
(J/g-°C)
(J/g-°C²)
0.97
0.00453
253
1.72
0.00086
PET
By using the parameters defined in Table 4, the enthalpy associated with heat capacity of PET can be
calculated as follow.
𝑇𝑓𝑖𝑛𝑎𝑙
∫
𝐶𝑚𝑎𝑡 𝑑𝑇 = 0.97 (253 − 25) +
𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙
+
0.00453
(2532 − 252 ) + 1.72 (433 − 253)
2
0.00086
(4332 − 2532 )
2
𝑇𝑓𝑖𝑛𝑎𝑙
∫
𝐶𝑚𝑎𝑡 𝑑𝑇 = 727 𝐽/𝑔
𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙
The following Table 5 contains temperature and heat of fusion of PET obtained from the polymer
handbook, Reference 19.
Table 5: Temperature and Heat of Fusion [19]
Polymer
PET
Tfusion
Hfusion
(°C)
(J/g)
253
37
Each material decomposes over a range of temperatures, a single characteristic temperature (Tdec)
corresponding to the maximum of the decomposition of PET as well as the heat of decomposition are
given in Table 5. These values are also determined by experiments and documented in Reference 18.
18
Table 6: Temperature and Heat of Decomposition [18]
Polymer
PET
Tdec
Hdec
(°C)
(J/g)
433
1,800
The parametric description of Cmat, together with Hfusion and Hdec can be used within the structure of a
gasification to describe the thermal behavior of a PET. These parameters can also be substituted into
Equation (7) to obtain an integral value for the heat of gasification for specific initial and final
temperatures. The value of Hg for Tinitial = 25 °C and Tfinal = Tdec is calculated as:
𝑇𝑓𝑖𝑛𝑎𝑙
𝐻𝑔 =
∫ 𝐶𝑚𝑎𝑡 𝑑𝑇 + 𝐻𝑓𝑢𝑠𝑖𝑜𝑛 + 𝐻𝑑𝑒𝑐 = 727
𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙
𝐻𝑔 = 2,564
𝐽
𝐽
𝐽
+ 37 + 1,800
𝑔
𝑔
𝑔
𝐽
𝑘𝐽
𝑘𝐽
= 2.564 = 2,564
𝑔
𝑔
𝑘𝑔
As stated earlier in Section 3.1, the heat of gasification can be calculated from the difference of the Heat
of Combustion and the Heat of Combustion of Syngas. Similarly, the Heat of combustion of Syngas can
also be calculated once we have the heat of gasification and heat of combustion of the reactant. However,
it is not considered in this paper. Also, the gasification process and the significance of the above defined
parameters is summarized in Figure 15.
Figure 15: Schematic Diagram of Plasma Gasification Process
19
3.3 Material and Energy Balances
The most advanced companies are developing plasma gasification processes for processing solid
municipal wastes however, there are not many companies focusing on the 100% plastic waste, especially
on PET. One of the things this paper looks at is the material and energy balances with regard to
polyethylene terephthalate plastic waste treatment. As discussed earlier in the paper, the overall idea of
the plasma assisted gasification process is to use partial oxidation of the waste followed by thermal
treatment done by small plasma torches, like PPT-150AC (Figure 13).
This section examines the chemical reaction and corresponding material and energy balances involved in
partial combustion of PET waste. This will allow us to understand the energy content of the syngas that
can be recovered and the electricity requirements of the plasma torches.
One parameter that plays an important role in the energy balance is the calorific value. The calorific value
is the quantity of heat produced by its combustion at constant pressure and under “standard” conditions.
The calorific value is usually represented the units of Mega Joules per kg. Polyethylene Terephthalate
plastic waste has a relatively high calorific value [16].
Equation 11
𝑃𝐸𝑇 𝐶𝑎𝑙𝑜𝑟𝑖𝑓𝑖𝑐 𝑉𝑎𝑙𝑢𝑒 = 30
𝑘𝐽
𝑘𝑊ℎ
𝑘𝑊ℎ
(𝑎𝑝𝑝𝑟𝑜𝑥𝑖𝑚𝑎𝑡𝑒𝑑) = 8.33
= 7,560
𝑘𝑔
𝑘𝑔
𝑡𝑜𝑛 (𝑠ℎ𝑜𝑟𝑡)
This calorific value is the basis of the chemical heat content of PET waste, expressed in kWh, and
therefore will have to be taken into consideration while examining the energy generation.
The meaning of above defined calorific value can be further understood by comparing it with that of
different fuel types. It is broadly known that all fuels are different. For example, burning PET plastic
waste compared to coal or natural gas will result in different amount of heat. Average calorific values of
natural gas and coal are 52.2 MJ/kg and 27.3 MJ/kg, respectively. As stated previously, the higher the
calorific value, the more energy someone can derive from the fuel. Compared to the values given above,
PET plastic waste could be a good source of energy if it can be properly utilized.
The process described in this paper consists of the partial combustion and gasification of the waste
followed by use of the syngas to power a gas turbine. Gas turbines can vary significantly in regards their
thermal efficiency. Without going into further discussion on gas turbines, this paper assumes 50% thermal
20
efficiency for the gas turbine. If we assume that bulk oxygen is used in the gasifier instead of air and that
the solids are brought to the gasification temperature by some means, the process involves the following
stages:
Reaction 3
𝐶10 𝐻8 𝑂4 + 3 𝑂2 → 10 𝐶𝑂 + 4 𝐻2 + 473
𝑘𝑊ℎ
, 𝑜𝑓 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒
𝑡𝑜𝑛
Here, 473 kWh was derived from ΔHr2, Equation (5) as follow,
Equation 12
∆𝐻𝑟2 = −361
𝑘𝐽
1 𝑚𝑜𝑙 𝑃𝐸𝑇 1,000𝑔
𝑘𝐽
∗(
)∗
= −1878
∗ .252 (𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑓𝑎𝑐𝑜𝑡𝑟)
𝑚𝑜𝑙
192.16 𝑔
1𝑘𝑔
𝑘𝑔
∆𝐻𝑟2 = −473
𝑘𝑊ℎ
, 𝑜𝑓 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒
𝑡𝑜𝑛 (𝑠ℎ𝑜𝑟𝑡)
Reaction (3) is the exothermic reaction which refers to a transformation of PET plastic waste into energy
(heat). This assumes the gasification by means of partial combustion with oxygen and assumes zero
reactor heat loss. So far what we have is the energy produced by the gasification process. This is
illustrated in Figure 16.
Figure 16: Schematic of Reactions 3 and 4 [20]
Now, this paper will consider the second part of the process which is the combustion of syngas in the gas
turbine. Note that the turbine combustion assumes zero turbine heat loss:
Reaction 4
10 𝐶𝑂 + 4 𝐻2 + 7 𝑂2 → 10 𝐶𝑂2 + 4 𝐻2 𝑂 + ∆𝐻𝑟3 𝑘𝑊ℎ, 𝑝𝑒𝑟 𝑡𝑜𝑛 𝑜𝑓 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒
Here, ∆𝐻𝑟3 is the energy output from the gas turbine and is calculated as follow:
21
Equation 13
𝐽
𝐽
) + (4 𝑚𝑜𝑙) (−285,830
)}
𝑚𝑜𝑙
𝑚𝑜𝑙
𝐽
𝐽
𝐽
− {(10 𝑚𝑜𝑙) (−76,090
) + (4 𝑚𝑜𝑙) (32,282
) + (7 𝑚𝑜𝑙) (0
)}
𝑚𝑜𝑙
𝑚𝑜𝑙
𝑚𝑜𝑙
∆𝐻𝑟3 = {(10 𝑚𝑜𝑙) (−393,448
∆𝐻𝑟3 = −4,446
𝑘𝐽
𝑘𝐽
𝑘𝑊ℎ
= −23,144
= −5,832
, 𝑜𝑓 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒
𝑚𝑜𝑙
𝑘𝑔
𝑡𝑜𝑛 (𝑠ℎ𝑜𝑟𝑡)
Therefore,
10 𝐶𝑂 + 4 𝐻2 + 7 𝑂2 → 10 𝐶𝑂2 + 4 𝐻2 𝑂 + 5,832 𝑘𝑊ℎ, 𝑝𝑒𝑟 𝑡𝑜𝑛 𝑜𝑓 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒
When using in a gas engine or turbine, natural gas has to be added to the syngas to raise the calorific
value so that it can be processed by the gas turbine. As discussed earlier, assuming 50% thermal
efficiency for the gas turbine, the electricity generated is:
Equation 14
5,832
𝑘𝑊ℎ
𝑘𝑊ℎ
∗ 50% = 2,916
𝑡𝑜𝑛 (𝑠ℎ𝑜𝑟𝑡), 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒
𝑡𝑜𝑛 (𝑠ℎ𝑜𝑟𝑡), 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒
Furthermore, heat may be recovered from the exhaust gas of the turbine as well as from the hightemperature syngas.
As we discussed earlier in Section 3.1, the plasma torch plays a key role in the gasification process
however, it requires electric power to operate. Likewise, it will also be necessary to produce industrial
oxygen for this process, to achieve Reaction (2).
The production of one ton (short) of industrial oxygen (95% O2) requires about 250 kWh of electricity
[11]. The reaction of gasification (Reaction 3) shows that one mole of combustible corresponds to 3moles
of oxygen. Since the molecular weight of PET compound is 192.16 g, as given in Equation (6), we
calculate that to gasify 192 kg of C10H8O4 it will require:
Equation 15
𝑂2 = 2 ∗ (15.9994
𝑘𝑔
𝑘𝑔
) = 32
𝑘𝑚𝑜𝑙
𝑘𝑚𝑜𝑙
and
22
3 𝑘𝑚𝑜𝑙 ∗ 32
𝑘𝑔
= 96 𝑘𝑔 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛
𝑘𝑚𝑜𝑙
Thus, the amount of oxygen required to gasify one ton (short) of PET waste is:
Equation 16
907 𝑘𝑔
96 𝑘𝑔 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛
∗
= 453 𝑘𝑔 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛
1 𝑡𝑜𝑛(𝑠ℎ𝑜𝑟𝑡) 192.16 𝑘𝑔 𝑜𝑓 𝑃𝐸𝑇
Therefore, the electricity needed to produce enough oxygen to gasify one ton (short) of PET waste can be
calculated as follow:
Equation 17
453 𝑘𝑔 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛
∗ 250 𝑘𝑊ℎ
907 𝑘𝑔
= 125 𝑘𝑊ℎ, 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 𝑝𝑒𝑟 𝑡𝑜𝑛 (𝑠ℎ𝑜𝑟𝑡) 𝑜𝑓 𝑃𝐸𝑇 𝑤𝑎𝑠𝑡𝑒 𝑔𝑎𝑠𝑖𝑓𝑖𝑒𝑑
This amount must be provided by the electricity output of the syngas turbine. The electricity needed for
the plasma torches that will crack the syngas and vitrify the ash depends on the capacity and the number
of plasma torches used in the plasma gasification process, also discussed in Section 3.1. All the
information given in Methodology sections can be combined to generate the “Net” energy produced by
the gasification process. However, due to the scope of this paper it is not considered herein.
23
4 Conclusion
Due to rapid pace of urbanization there is an increasing challenge to waste management. The research of
plasma assisted gasification has been started as a response for a need of more efficient utilization of
plastic waste for energy production. The intent of this paper was to provide technical assessment of
feasibility to effectively treat a plastic waste, specifically polyethylene terephthalate in a plasma
gasification unit. There is no doubt that plasma assisted gasification is increasingly viewed as a possibility
in the waste-to-energy domain. Plasma assisted gasification is an interesting process with potential for
future applications. First, it is a convenient way to provide thermal energy in a gasification process.
Second, controlling the amount of heat input to the process by means of the plasma torches allows
controlling the composition of the syngas. Third, the hydrogen to carbon monoxide ratio can be modified
easily to suit the needs of the end use.
In this paper, the gasification process was reviewed to understand the characteristics of various
components that are comprised in the system along with generation of plasma. Similarly, the gasification
process cannot be successfully executed till it is designed for a specific use and therefore, PET plastic
waste properties and characteristics were also evaluated. The primary focus of this paper was on the
chemical reaction taking place inside the plasma reactor due to a gasification of a PET plastic waste. In
order to activate the chemical reaction, the plasma rector has to produce the heat required for the reaction.
Consequently, the heat of gasification was defined as a function of the initial and final temperatures of the
gasification process. The determining parameters of this function were captured from literature data.
These parameters were used to obtain integral values of the heat of gasification for heating PET plastic
waste from room temperature through its decomposition. Additionally, analysis of the reactions and
power balance were done for the reactions of gasification of PET plastic waste. Enthalpies were derived
based on the standard enthalpies of formation for reaction components for energy analysis and using
group contribution method. As a result, energy produced during gasification and later combustion was
derived.
The sustainability of any plastic waste management system depends on numerous factors; however, the
most important factor is the will of the people to change the existing system and develop
something better. Adoption of latest technologies have to be taken into consideration while selection of a
waste management system and plasma assisted gasification is a leading waste management option.
24
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