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 October, 2012 (For Graduation December 2012)

i

TABLE OF CONTENTS -- Cover Sheet-------------------------------------------------------------------------------------------------------------- i -- Table of Contents ----------------------------------------------------------------------------------------------------- ii -- List of Figures -------------------------------------------------------------------------------------------------------- iii -- List of Tables---------------------------------------------------------------------------------------------------------- iv -- Acknowledgment------------------------------------------------------------------------------------------------------ v -- Abstract ---------------------------------------------------------------------------------------------------------------- vi 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 --------------------------------------------------------------------------------------- 7 2.6.3.1. Non-Transferred Torch --------------------------------------------------------------- 8 2.6.4. The Plasma Reactor ------------------------------------------------------------------------------- 9 3. Results and Discussion ----------------------------------------------------------------------------------------------12 3.1. Analysis of Chemical Reactions ------------------------------------------------------------------------12 3.2. Determination of Heat of Gasification -----------------------------------------------------------------16 3.3. Material and Energy Balances ---------------------------------------------------------------------------19 4. Conclusion -------------------------------------------------------------------------------------------------------------23 5. References -------------------------------------------------------------------------------------------------------------24

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LIST OF FIGURES Figure 1: Plastic Waste at a Public Place and Landfill in a Less Developed Country ................................. vi Figure 2: Materials Discarded in Municipal Solid Waste, 2008 ................................................................... 1 Figure 3: The Plastic Waste Categories ........................................................................................................ 2 Figure 4: Terepthalic Acid (left) and Ethylene Glycol (right) ...................................................................... 3 Figure 5: PET Monomer - (C10H8O4) ............................................................................................................ 3 Figure 6: Cascade Process of Ionization. Electrons are “e – “, neutral atoms “o”, and cations “+” ............. 5 Figure 7: Plasma Gasification Process Layout ............................................................................................. 6 Figure 8: Typical Model of a Transferred (b) and Non-transferred (a) Plasma Arc Torch .......................... 8 Figure 9: Non-transferred Plasma Arc Torch ............................................................................................... 8 Figure 10: Typical Model of a Non-transferred Plasma Arc Torch .............................................................. 9 Figure 11: Typical Concept of a Plasma Reactor ....................................................................................... 10 Figure 12: Chemical Structure of Polyethylene Terephthalate ................................................................... 13 Figure 13: Schematic Diagram of Plasma Gasification Process ................................................................. 18 Figure 14: Schematic of Reactions 3 and 4................................................................................................. 21

iii

LIST OF TABLES Table 1: Typical Chemical Composition of Polyethylene Terephthalate ..................................................... 4 Table 2: Values of Heats of Formation of Reaction Products (gas) ........................................................... 13 Table 3: Values of Heats of Formation of Reaction Reactants ................................................................... 14 Table 4: Parameters describing Dependence of Measured Heat Capacity on Temperature ....................... 17 Table 5: Temperature and Heat of Fusion .................................................................................................. 17 Table 6: Temperature and Heat of Decomposition ..................................................................................... 18

iv

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.

v

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.'' 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. In this paper, recent progress in the recovery of plastic solid waste using plasma assisted gasification conversion technology will be reviewed. Also, a plasma assisted gasification process and its technical viability will be investigated for the PET plastic waste. Note that the fully plasma based gasification processing of PET waste, in the absence of partial combustion, will not be considered in this paper, as it will not be economically feasible for PET plastic waste treatment.

Figure 1: Plastic Waste at a Public Place and Landfill in a Less Developed Country

vi

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

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. 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. However, the incineration of plastics has various disadvantages because plastic has a base of oil or natural gas and does not biodegrade in a landfill. 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.

Figure 3: The Plastic Waste Categories

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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 in brief detail.

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)

Figure 5: PET Monomer - (C10H8O4)

The 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.

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Table 1: Typical Chemical Composition of Polyethylene Terephthalate

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.

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. The Figure 4 describes the Cascade process of ionization.

The 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. It 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 “+”

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.

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 however, 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

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.

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 3. 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.

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.

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Figure 8: Typical Model of a Transferred (b) and Non-transferred (a) Plasma Arc Torch

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 gives a good mixing of the both of them.

Figure 9: Non-transferred Plasma Arc Torch

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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 abatement and have to be watercooled to handle the high excursion of temperatures. They have low efficiencies 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.

Figure 10: Typical Model of a Non-transferred Plasma Arc Torch

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 an ordinary air. Furnace is equipped with one of its primary component, 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 metals and ash form a liquid pool at the bottom of the reactor. This metal and slag is usually drained out from the outlet provided at the bottom of the reactor. 9

As discussed in the plasma torch section, either transferred or 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 organic constituents are gasified, transformed into a synthesis gas containing mostly hydrogen, carbon monoxide and nitrogen, and exits at the top of the reactor. The 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 is therefore required cleaning through various process equipment. However, it is not included in this paper as it is outside the scope of this paper.

Figure 11: Typical Concept of a Plasma Reactor

10

Plasma reactor is well suited for the gasification process as there is better heat transfer to the waste and it can be brought up to a higher temperature, resulting in more complete waste conversion. Furthermore, plasma reactor consistently exhibits much lower environmental levels for both air emissions and slag leachate toxicity than competing technologies like incineration. The plasma reactors overall functionality usually does not discriminate between 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 reactor is to gasify the organic compounds 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 dissipates PET waste in an oxygen-starved environment and decomposes PET waste into the basic molecules of CO and 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 hydrogen (H2) and carbon monoxide (CO), instead of oxidizing them to water (H20) and carbon dioxide (CO2). The gases used in addition 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. The main reaction occurring in the reactor with polyethylene terephthalate and CO2 as reactants is: Reaction 1


   + 6
  16
 + 4  ∆ 

The initial temperature of the PET waste is assumed at room temperature, 25 °C, and the temperature in the reactor is approximated to be at 1100 °C. Now, if oxygen is added to support the energy supply, second reaction is Reaction 2


   + 3   10
 + 4  ∆ 

As stated earlier, gasification is the breakdown of the organic part of the waste into a syngas that is a mixture of CO and H2, by controlling the amount of oxygen present.

12

Now it is deemed to calculate the enthalpy of formation of polyethylene terephthalate. The free enthalpy of formation is calculated based on the knowledge of energy contributions of polymer structure. For polymeric reactants that 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 12: Chemical Structure of Polyethylene Terephthalate

The heats of formation of the polyethylene terephthalate constituent groups at T = 25 °C = 298 K are: $% $&' $%     2 ∗ 120,000 + 70 ∗  !"#!  198,280 $&' $% 
   2 ∗ 22,000 + 102 ∗  !"#!  104,792 $&' -----------------------------------------------------------------------------------------

2 
   2 ∗ 132,000 + 40 ∗  !"#!  240,160

+&"'  333,648

$% $&'

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.

∆°-  333,648

$% , " !&&  !"#! .&/011&/2, +  25 °
$&'

Values of heats of formation of reaction constituents are given in the table below for the evaluation of heat balance in plasma reactor: Table 2: Values of Heats of Formation of Reaction Products (gas)

Products of Reactions

Enthalpy

H2 (1100 °C)

32,282 kJ/kmol

CO (1100 °C)

-76,090 kJ/kmol 13

Table 3: Values of Heats of Formation of Reaction Reactants

Reactants of Reactions

Enthalpy

Polyethylene Terephthalate (298 K)

-333,648 kJ/kmol

CO2 (25 °C)

-393,448 kJ/kmol

O2 (25 °C)

0 kJ/kmol

Thus, the overall enthalpy of a reaction is the simply the sum of the enthalpies of the component reactions. In practice, the heat of combustion of the reaction can be calculated by subtracting the heat of formation of the products from the heat of formation of the reactants. Equation 1

∆!  4 56 ∆ℎ°-,6 ± 4 5 ∆ℎ°-, Where p and r denote products and reactants, respectively, in the standard state at temperature, T = 25 °C. Thus, to gasify polyethylene terephthalate with CO2: Equation 2

∆!  9:5ℎ°- ;



Equation 3

$% $%  + 4 $&'32,282 D $&' $&' $% $%  C1 $&' E333,648 F + 6 $&'393,448 D $&' $&'

∆!1  C16 $&'76,090

According to the enthalpies of components and the stoichiometric reaction:

∆!1  1606024

$% $%  1606.024 $&' &'

Addition of oxygen will introduce the second reaction and the heat balance is:

14

Equation 4

$% $% F + 4 $&' E32,282 FD $&' $&' $% $%  C1 $&' E333,648 F + 3 $&' E0 FD $&' $&'

∆!2  C10 $&' E76,090

According to the enthalpies of components and the stoichiometric reaction:

∆!2  298124

$% $%  298.124 $&' &'

The gross heat of combustion per unit mass is then Equation 5

$% 1606.024 ∆! &' HI JK+   L6 192.16 M  &' &N JK+ Where Mp is the molecular weight of the polymer repeat unit, PET: Equation 6

L6 
    10 ∗ 12.0107 + 8 ∗ 1.0079 + 4 ∗ 15.9994  192.16 M Therefore, the gross heat of combustion is, HI JK+  8.35

$% M

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.

The second material property is 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, calculated as the difference of heat of combustion and 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.

15

3.2 Determination of Heat of Gasification of Polyethylene Terephthalate The amount of heat that is required to gasify unit mass of material is one of the key properties required in order to determine its response to the plasma arc in the plasma reactor. The heat of gasification will assist in determining the plasma torches as it will be required to provide the calculated heat of gasification while at the same time melts the ash into vitrified products. Here, a methodology is applied to a polyethylene terephthalate plastic 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.

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 7

O 

B[\]^_

P
QRS 0+ + -TUVWX + YZI

B\]\`\^_

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 8

P

B[\]^_

B\]\`\^_


QRS 0+ 
a +SRXU  +VXVSVRb  + +


c   +-VXRb  +SRXU  2


a   +SRXU  +VXVSVRb  +
c :+-VXRb  +SRXU ; 2

16

The parameters supporting equation 8 above are given in Table 4. These values are obtained from DSC experiment documented in Reference 18.

Table 4: Parameters (of equation 8) describing Dependence of Measured Heat Capacity on Temperature

Polymer PET

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

By using the parameters defined in Table 4, the heat capacity of PET can be calculated as follow.

P

B[\]^_

B\]\`\^_


QRS 0+  0.97 253  25 + +

0.00453 253  25  + 1.72 433  253 2

0.00086 433  253  2 P

B[\]^_

B\]\`\^_


QRS 0+  727 %/M

The following Table 5 contains temperature and heat of fusion of PET obtained from polymer handbook, Reference 17. Table 5: Temperature and Heat of Fusion

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 is summarized in Table 5. These values are also determined by experiments and documented in Reference 17.

17

Table 6: Temperature and Heat of Decomposition

Polymer PET

Tdec

Hdec

(°C)

(J/g)

433

1800

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 below. Based on the information given above, the heat of gasification can be calculated as follow:

O 

B[\]^_

P
QRS 0+ + -TUVWX + YZI  727

B\]\`\^_

O  2564

% % % + 37 + 1800 M M M

% M

The heat of gasification can also lead to the temperature required to gasify the material. However it is not considered in this paper due to its necessity. Also, Heat of combustion of Syngas can also be calculated once we have the heat of gasification and heat of combustion. The gasification process and the significance of the above defined parameters are given below in the schematic diagram.

Figure 13: Schematic Diagram of Plasma Gasification Process

18

3.3 Material and Energy Balances for Plasma Gasification Process 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 focusing on PET. The one of the things this paper looks at is the material and energy balances of the plasma assisted gasification process and its attributes 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.

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.

Polyethylene Terephthalate plastic waste has a relatively high calorific value, which can be approximated,

JK+
"'&!1N1. e"'#  30

L% " $M

!&f1"0

The above defined calorific value corresponds to a chemical heat input of about 8.33 kWh/kg of PET waste. Therefore a ton of PET waste contains 7560 kWh of chemical energy. This was derived by the following conversion. Equation 9

30

L% $% $hℎ  30000 ∗ 0.252 .&/g!21&/ N".&!  7560 $M $M &/

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 process consists of a partial combustion and gasification of the waste followed by use of the syngas to power a gas engine or turbine of assumed 50% thermal efficiency. 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:

19

Reaction 3


   + 3  → 10
 + 4  + ∆!2 $hℎ, ! &/ &N JK+ j"2 Here, ∆!2  298.124

kl

QWb

Equation 10

298.124

$% 1&'JK+ 1000M $% ∗E F∗  1550 ∗ .252 .&/g!21&/ N".&!  390 $hℎ &' 192.16 M 1$M $M
   + 3  → 10
 + 4  + 390 $hℎ, ! &/ &N JK+ j"2

This assumes the gasification by means of partial combustion with oxygen and, for the time being, assuming zero reactor heat loss Gasification takes the same pathway as combustion but stops at an intermediate level, hence yielding hydrogen and carbon monoxide, instead of oxidizing them to water and carbon dioxide. Gasification turbine combustion, for the time being, assuming zero turbine heat loss: Reaction 4

10
 + 4  + 7  → 10
 + 4   + ∆!3 $hℎ, ! &/ &N JK+ j"2 Here, Equation 11

$% $% F + 4 $&' E285,830 FD $&' $&' $% $% $%  C10 $&' E76,090 F + 4 $&' E32,282 F + 7 $&' E0 FD $&' $&' $&'

∆!3  C10 $&' E393,448

∆!3  4446

$% &'

Now, by converting chemical energy to kWh, Equation 12

4446

$% 1&'JK+ 1000M $% ∗E F∗  23144 ∗ .252 .&/g!21&/ N".&!  5832 $hℎ &' 192.16 M 1$M $M 20

Therefore,

10
 + 4  + 7  → 10
 + 4   + 5832 $hℎ, ! &/ &N JK+ j"2

Figure 14: Schematic of Reactions 3 and 4

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, an assumed 50% thermal efficiency for the gas turbine, the electricity generated is: Equation 13

5832 $hℎ ∗ 50%  2916 $hℎ Furthermore, heat may be recovered from the high-temperature syngas as well as from the exhaust gas of the turbine. This heat can be used to produce steam, and the steam can be used to generate more electricity or to heat water for district heating.

In case of the steam turbine generator, with assumed thermal efficiency of 32%, and also now assuming 10% heat loss in the gasifier and 10% heat loss in the steam boiler, the maximum amount of additional electricity that may be generated from the syngas flow is: Equation 14

390 $jℎ ∗ 80% ∗ 32%  99.84 $hℎ, ! &/ &N JK+ j"2 It will also be necessary to produce industrial oxygen for such a process and use some of the power generated to power the plasma torches.

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The production of one ton of industrial oxygen (95% O2) requires about 250 KWh of electricity. The reaction of gasification (Reaction 3) shows that one mole of combustible corresponds to 3moles of oxygen. We know that the molecular weight of PET compound is determined as,


    192.16 M On the basis of their respective molecular weights, we calculate that to gasify 192 kg of C10H8O4 it will take: Equation 15

  2 ∗ 15.9994  32,

And

3 ∗ 32  96 $M &N &fnM/

Thus, the amount of oxygen required to gasify one ton of PET waste is: Equation 16

1000 $M ∗

96 $M &N &fnM/  499.3 $M &N &fnM/ 192.16 $M &N JK+

Therefore, the electricity needed to produce enough oxygen to gasify one ton of PET waste can be calculated as follow: Equation 17

499.3 $M &N &fnM/ ∗ 250 $hℎ 1000 $M

 124.89 $hℎ, &N '.!1.1n ! &/ &N JK+ j"2 M"21N10

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. However, the above calculation

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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 not to experiment with PET plastic waste and plasma assisted gasification but to provide basic description and analysis of reaction taking place in gasification reactor.

There is no doubt that plasma assisted gasification is increasingly viewed as a possibility in the waste-toenergy domain. Plasma assisted gasification is an interesting process with potential for future application. 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. The hydrogen to carbon monoxide ratio can be modified easily according to the needs of the user.

In this paper, 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.

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5 References 1) Mountouris, E. Voutsas, D. Tassios, Solid waste plasma gasification: Equilibrium model development and exergy analysis, Energy Conversion and Management, Volume 47, Issues 13–14, August 2006, Pages 1723-1737, ISSN 0196-8904, 10.1016/j.enconman.2005.10.015. . 2) Mountouris, E. Voutsas, D. Tassios, Plasma gasification of sewage sludge: Process development and energy optimization, Energy Conversion and Management, Volume 49, Issue 8, August 2008, Pages 22642271, ISSN 0196-8904, 10.1016/j.enconman.2008.01.025. . 3) Dummersdorf Hans-Ulrich; Waldmann Helmut, Process for Converting Plastic Waste into Power, U. S. Patent Number 5,369,947, December 1994. 4) Orr, Doug, and David Maxwell. A Comparison of Gasification and Incineration of Hazardous Wastes. Rep. no. DCN 99.803931.02. Austin, Texas: Radian International LLC, 2000. Web. 9 May 2012. . 5) R. W. Beck Inc., City of Honolulu Review of Plasma Arc Gasification and Vitrification Technology for Waste Disposal. Rep. 2003. Web. 10 May 2012. . 6) "Safe Waste and Power - The Plasma Gasification Process." Safe Waste and Power. 2003. Web. 15 May 2012. . 7) "Plastics, Common Wastes & Materials." EPA. Environmental Protection Agency, 16 Apr. 2012. Web. 16 May 2012. . 8) "Plastic Waste Types." Plastic Waste. Nothwest Polymers. Web. 17 May 2012. . 9) Brandrup J., Immergut E.H., Grulke E.A., Abe A., Bloch D.R., eds., Polymer Handbook, Fourth Edition, John Wiley & Sons, New York, 1999. 10) 2. Frederick W.J. and Mentzer C.C., “Determination of Heats of Volatilization for Polymers by Differential Scanning Calorimetry,” Journal of Applied Polymer Science, Vol. 19, 1975, pp. 1799-1804. 11) W. Thornton, “The Role of Oxygen to the Heat of Combustion of Organic Compounds,” Philosphical Magazine and J. of Science, 33(196) (1917). 12) D.W. Van Krevlen, “Thermochemical Properties: Calculation of the Free Enthalpy of Reaction from Group Contributions,” in Properties of Polymers, 3rd Ed., Chapter 20, pp. 629-639, Elsevier, Amsterdam (1990). 13) Pourali, M., (2010), Application of Plasma Gasification Technology in Waste to Energy—Challenges and Opportunities, The IEEE Xplore digital library (Institute of Electrical and Electronics Engineers), 1(3), pp 125-130.

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14) Lisa Zyga, (2012), Plasma Gasification Transforms Garbage into Clean Energy, Science Blogger, InventorSpot.com, via: Popular Science. 15) S.F. Paul, Review of the thermal plasma research and development for hazardous waste remediation in the United States, in: R. Benocci, G. Bonizzoni, E. Sindoni (Eds.), Proceedings of the International School of Plasma Physics workshop on the Thermal Plasmas for Hazardous Waste Treatment, Varenna, Italy, 1995. 16) G. Bonizzoni, Design of a plasma torch for toxic waste treatments, in: R. Benocci, G. Bonizzoni, E. Sindoni (Eds.), Proceedings of the International School of Plasma Physics workshop on the Thermal Plasmas for Hazardous Waste Treatment, Varenna, Italy, 1995. 17) Brandrup J., Immergut E.H., Grulke E.A., Abe A., Bloch D.R., eds., Polymer Handbook, Fourth Edition, John Wiley & Sons, New York, 1999.

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