6.4 Conversion of Waste Plastics and Resins into Liquid Fuels and Carbon Materials
167
from carbonaceous materials (biomass, coal, coke or waste plastics) by chemical and
thermal treatment at elevated temperatures. The main processes for activated carbon
production include carbonization (devolatilization) and activation of the produced
charcoal.
6.4.2.1 Carbonization
Carbonization or devolatilization is the process of reducing the volatile matter content
of carbon-rich materials and converting them into pure carbon by heating. The process
takes place at medium to high-temperature ranges of 500–1200 °C and can be divided
into four stages: (1) the initial drying of raw material at temperatures less than
200 °C, (2) production of small amounts of pyroligneous liquids and non-condensable
gases at a temperature range of 170–300 °C, (3) elimination of a large portion of
pyroligneous liquids and tars from the previous stage and production of charcoal
at a temperature range of 250–300 °C, (4) increasing the carbon content of the
charcoal by removal of the remaining volatile matter content [25]. Carbonization
process can be performed in oxidative or inert atmosphere, in the absence or presence
of catalyst, and under atmospheric or high-pressure conditions [26]. The catalytic
processes result in the production of graphite carbon nanomaterials such as carbon
nanotubes, nanofibers, graphene, etc., while other processes result in the production of amorphous carbons such as activated carbon, carbon fibers, nanostructured
carbon, carbon spheres, etc. [26]. The processes for the production of amorphous
carbons include anoxic pyrolysis, stabilization and anoxic pyrolysis, and pressure
carbonization. Table 6.3 compares the different pathways for plastics carbonization
and their products.
Anoxic pyrolysis carbonization
In this process, the plastics are directly heated under atmospheric pressure and with
the presence of inert gases such as N 2 or Ar. During the process, most of the carbon
atoms of plastics are converted to carbon materials by aromatization mechanism and
the non-carbon atoms such as H, O, N, Cl are removed from the products in the
form of gas. Phenolic formaldehyde (PF) resin or phenolic resin, is one of the most
popular thermoset resins for amorphous carbon production. Pyrolysis of this resin
could result in a high carbon yield of ≈60% at 1000 °C [27]. Carbonization of PET
separately or in a mixture with coal/pitch can also result in high-quality activated
carbon, however the carbon yield is usually low and in the range of 20–25%. The
activated carbons produced from PET have a high surface area of up to 2500 m
2 /g
after activation [26].
The advantages of this method are simple operation, low equipment cost, and
suitability for most types of polymers. In addition, functional carbon materials (e.g.,
activated carbon, mesoporous carbon, carbon fibers) can be produced by a combination of modification and activation steps with no specific restrictions for the products
applications. Thus, this method is a feasible process for the utilization of waste
plastics [26].
167
from carbonaceous materials (biomass, coal, coke or waste plastics) by chemical and
thermal treatment at elevated temperatures. The main processes for activated carbon
production include carbonization (devolatilization) and activation of the produced
charcoal.
6.4.2.1 Carbonization
Carbonization or devolatilization is the process of reducing the volatile matter content
of carbon-rich materials and converting them into pure carbon by heating. The process
takes place at medium to high-temperature ranges of 500–1200 °C and can be divided
into four stages: (1) the initial drying of raw material at temperatures less than
200 °C, (2) production of small amounts of pyroligneous liquids and non-condensable
gases at a temperature range of 170–300 °C, (3) elimination of a large portion of
pyroligneous liquids and tars from the previous stage and production of charcoal
at a temperature range of 250–300 °C, (4) increasing the carbon content of the
charcoal by removal of the remaining volatile matter content [25]. Carbonization
process can be performed in oxidative or inert atmosphere, in the absence or presence
of catalyst, and under atmospheric or high-pressure conditions [26]. The catalytic
processes result in the production of graphite carbon nanomaterials such as carbon
nanotubes, nanofibers, graphene, etc., while other processes result in the production of amorphous carbons such as activated carbon, carbon fibers, nanostructured
carbon, carbon spheres, etc. [26]. The processes for the production of amorphous
carbons include anoxic pyrolysis, stabilization and anoxic pyrolysis, and pressure
carbonization. Table 6.3 compares the different pathways for plastics carbonization
and their products.
Anoxic pyrolysis carbonization
In this process, the plastics are directly heated under atmospheric pressure and with
the presence of inert gases such as N 2 or Ar. During the process, most of the carbon
atoms of plastics are converted to carbon materials by aromatization mechanism and
the non-carbon atoms such as H, O, N, Cl are removed from the products in the
form of gas. Phenolic formaldehyde (PF) resin or phenolic resin, is one of the most
popular thermoset resins for amorphous carbon production. Pyrolysis of this resin
could result in a high carbon yield of ≈60% at 1000 °C [27]. Carbonization of PET
separately or in a mixture with coal/pitch can also result in high-quality activated
carbon, however the carbon yield is usually low and in the range of 20–25%. The
activated carbons produced from PET have a high surface area of up to 2500 m
2 /g
after activation [26].
The advantages of this method are simple operation, low equipment cost, and
suitability for most types of polymers. In addition, functional carbon materials (e.g.,
activated carbon, mesoporous carbon, carbon fibers) can be produced by a combination of modification and activation steps with no specific restrictions for the products
applications. Thus, this method is a feasible process for the utilization of waste
plastics [26].