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6 Waste Plastics Management and Conversion into Liquid …
Stabilization and anoxic pyrolysis carbonization
Plastics such as polyacrylonitrile (PAN), polyolefins and PVC do not have oxygen
atoms in their structures, so they cannot be directly carbonized by heat treatment and
they require stabilization processes such as pre-oxidation and pre-chemical treatment
before carbonization in order to stabilize carbon chains [26]. The most common
stabilization method is thermal oxidation in the presence of oxygen, while chemicals treatments such as sulfonation by sulfuric acids are usually used to stabilize
the PE and PS chains by the introduction of crosslinks. Oxidative stabilization is
an important stage in the preparation of carbon fibers from PAN fibers. The final
carbon yield of carbon fiber from PAN is usually in the range of 50–55%, depending
on the constituent of PAN precursor, degree of oxidation and final carbonization
temperature. In the case of PVC, direct pyrolysis under an inert atmosphere led to a
low carbon yield (as low as 10 wt%), but with a pre-oxidation process, much higher
carbon yields of around 20 wt% could be obtained [26].
Catalytic carbonization
In this method, the plastics are directly pyrolyzed in the presence of catalysts. The
process usually takes place in two steps: (1) decomposition of the plastics into liquid
or gaseous products, (2) growth of carbon nanomaterials on the catalysts using the
decomposed products as carbon source. It can be performed in a one-pot or a stepwise approach [26]. In the one-pot approach, the plastics are mixed with the catalysts
and are placed in the reactor, where the decomposition and product formation occur.
In a step-wise approach, the decomposition of the plastics takes place in a reactor
and then the carbonization products form in a subsequent reactor where the catalysts
are loaded. Transitional metals such as nickel, iron, or compounds such as nickel
oxides, ferrocene, and ferrous chloride are mostly used catalysts for the growth of
carbon products, while solid acids such as HZSM-5 and zeolite are usually used
for the decomposition of plastics. A combination of chlorinated compounds and/or
activated carbon is also used as catalysts for the decomposition of plastics. Chlorine radicals released from the chlorinated compounds promote the dehydrogenation and aromatization reactions during the plastic decomposition. Activated carbon
catalysts promote the formation of light hydrocarbons and aromatic compounds as
well as facilitating the dehydrogenation and aromatization reactions of the aromatic
intermediates or polycyclic aromatic hydrocarbons to form carbon materials [26].
One of the drawbacks of the catalytic carbonization method is the high costs due to
the use of metal catalysts. In addition, the metal elements will remain in the products
after carbonization and affect the subsequent applications. The use of these carbon
products might be subjected to some restrictions due to the environmental risk of the
residual metals and the highly crystallized carbon [26].
Pressure carbonization
Pressure carbonization occurs at high pressure and can prepare carbon materials
with a relatively higher yield compared to atmospheric carbonization. The process
can be classified as the direct pressure carbonization in which the high pressure of
6 Waste Plastics Management and Conversion into Liquid …
Stabilization and anoxic pyrolysis carbonization
Plastics such as polyacrylonitrile (PAN), polyolefins and PVC do not have oxygen
atoms in their structures, so they cannot be directly carbonized by heat treatment and
they require stabilization processes such as pre-oxidation and pre-chemical treatment
before carbonization in order to stabilize carbon chains [26]. The most common
stabilization method is thermal oxidation in the presence of oxygen, while chemicals treatments such as sulfonation by sulfuric acids are usually used to stabilize
the PE and PS chains by the introduction of crosslinks. Oxidative stabilization is
an important stage in the preparation of carbon fibers from PAN fibers. The final
carbon yield of carbon fiber from PAN is usually in the range of 50–55%, depending
on the constituent of PAN precursor, degree of oxidation and final carbonization
temperature. In the case of PVC, direct pyrolysis under an inert atmosphere led to a
low carbon yield (as low as 10 wt%), but with a pre-oxidation process, much higher
carbon yields of around 20 wt% could be obtained [26].
Catalytic carbonization
In this method, the plastics are directly pyrolyzed in the presence of catalysts. The
process usually takes place in two steps: (1) decomposition of the plastics into liquid
or gaseous products, (2) growth of carbon nanomaterials on the catalysts using the
decomposed products as carbon source. It can be performed in a one-pot or a stepwise approach [26]. In the one-pot approach, the plastics are mixed with the catalysts
and are placed in the reactor, where the decomposition and product formation occur.
In a step-wise approach, the decomposition of the plastics takes place in a reactor
and then the carbonization products form in a subsequent reactor where the catalysts
are loaded. Transitional metals such as nickel, iron, or compounds such as nickel
oxides, ferrocene, and ferrous chloride are mostly used catalysts for the growth of
carbon products, while solid acids such as HZSM-5 and zeolite are usually used
for the decomposition of plastics. A combination of chlorinated compounds and/or
activated carbon is also used as catalysts for the decomposition of plastics. Chlorine radicals released from the chlorinated compounds promote the dehydrogenation and aromatization reactions during the plastic decomposition. Activated carbon
catalysts promote the formation of light hydrocarbons and aromatic compounds as
well as facilitating the dehydrogenation and aromatization reactions of the aromatic
intermediates or polycyclic aromatic hydrocarbons to form carbon materials [26].
One of the drawbacks of the catalytic carbonization method is the high costs due to
the use of metal catalysts. In addition, the metal elements will remain in the products
after carbonization and affect the subsequent applications. The use of these carbon
products might be subjected to some restrictions due to the environmental risk of the
residual metals and the highly crystallized carbon [26].
Pressure carbonization
Pressure carbonization occurs at high pressure and can prepare carbon materials
with a relatively higher yield compared to atmospheric carbonization. The process
can be classified as the direct pressure carbonization in which the high pressure of