6.3 Waste Plastics Generation and Disposal
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Recycling of the plastics can be divided into four different stages [6, 9]:
(1) Primary Recycling or the mechanical re-processing: In this process, the waste
plastic is fed into the original production process of the basic material. Thus,
the recycled material has the same specification as that of the original one and
can be used for substituting the virgin material. This requires clean and not
contaminated waste as the feedstock. Since the process is very expensive and
complex requiring intensive sorting, it is not a popular choice by the recycling
plants.
(2) Secondary Recycling: in which the recycled materials are used for different
applications than the original one and require less demanding material specification. The secondary recycling can use contaminated or less separated waste,
thus most of the plastics are recycled using this method.
(3) Tertiary Recycling: This process involves the conversion or recovery of chemicals or raw materials, such as monomers, from the plastics through thermal
degradation (such as pyrolysis) or catalytic de-polymerization. Pyrolysis is the
most common method for producing a variety of hydrocarbons from the plastic
waste to be used as a chemical feedstock or for energy generation.
(4) Quaternary Recycling: that includes the recovery of energy content. Since the
waste plastic has a high heating value, it can be incinerated to produce heat. It can
also be converted into different types of fuels based on the conversion technologies, e.g., controlled combustion, gasification, the manufacture of refuse-derived
fuel (RDF), pyrolysis or direct liquefaction. In Europe, most of the plastic wastes
are used for energy recovery (39.5%) compared to recycling (29.7%).
6.4 Conversion of Waste Plastics and Resins into Liquid
Fuels and Carbon Materials
Considering the challenges for recycling such as the labor-intensive need for sorting,
the conversion of waste plastics into liquid fuels and other high-value products
such as carbon materials becomes a promising approach to achieve economic and
environmental benefits of waste plastics management. This is also beneficial as a
portion of the plastic waste stream cannot be recycled due to contamination. Conversion of the waste plastics into liquid fuels and carbon materials can be achieved
by different methods mainly including pyrolysis, gasification, hydrogenation and
devolatilization/carbonization, etc. [9, 10].
6.4.1 Liquid Fuels or Oils from Waste Plastics
Plastics can be converted into liquid products such as crude oils, fuel oils and lubricants mainly through thermal cracking or pyrolysis, other thermochemical technologies (gasification, hydrogenation), hydrocracking and catalytic cracking. Thermal
163
Recycling of the plastics can be divided into four different stages [6, 9]:
(1) Primary Recycling or the mechanical re-processing: In this process, the waste
plastic is fed into the original production process of the basic material. Thus,
the recycled material has the same specification as that of the original one and
can be used for substituting the virgin material. This requires clean and not
contaminated waste as the feedstock. Since the process is very expensive and
complex requiring intensive sorting, it is not a popular choice by the recycling
plants.
(2) Secondary Recycling: in which the recycled materials are used for different
applications than the original one and require less demanding material specification. The secondary recycling can use contaminated or less separated waste,
thus most of the plastics are recycled using this method.
(3) Tertiary Recycling: This process involves the conversion or recovery of chemicals or raw materials, such as monomers, from the plastics through thermal
degradation (such as pyrolysis) or catalytic de-polymerization. Pyrolysis is the
most common method for producing a variety of hydrocarbons from the plastic
waste to be used as a chemical feedstock or for energy generation.
(4) Quaternary Recycling: that includes the recovery of energy content. Since the
waste plastic has a high heating value, it can be incinerated to produce heat. It can
also be converted into different types of fuels based on the conversion technologies, e.g., controlled combustion, gasification, the manufacture of refuse-derived
fuel (RDF), pyrolysis or direct liquefaction. In Europe, most of the plastic wastes
are used for energy recovery (39.5%) compared to recycling (29.7%).
6.4 Conversion of Waste Plastics and Resins into Liquid
Fuels and Carbon Materials
Considering the challenges for recycling such as the labor-intensive need for sorting,
the conversion of waste plastics into liquid fuels and other high-value products
such as carbon materials becomes a promising approach to achieve economic and
environmental benefits of waste plastics management. This is also beneficial as a
portion of the plastic waste stream cannot be recycled due to contamination. Conversion of the waste plastics into liquid fuels and carbon materials can be achieved
by different methods mainly including pyrolysis, gasification, hydrogenation and
devolatilization/carbonization, etc. [9, 10].
6.4.1 Liquid Fuels or Oils from Waste Plastics
Plastics can be converted into liquid products such as crude oils, fuel oils and lubricants mainly through thermal cracking or pyrolysis, other thermochemical technologies (gasification, hydrogenation), hydrocracking and catalytic cracking. Thermal