Environmental Engineering Reference
In-Depth Information
9.2 PYROLYSIS OF PLASTIC WASTE FOR
FEEDSTOCK RECOVERY
9.2.1 Direct Thermolysis
Pyrolysis, also referred to as “cracking,” is a relatively low-cost treatment
option where waste is converted into gas and liquid fuel. Presorting of MSW
to select the plastic fraction and shredding it down to a suitable size (both
steps requiring energy) is necessary to ensure good yield of fuel. Plastic
waste is heated with or without a catalyst (Al-Salem et al., 2010; de Marco
et al., 2009; Kaminsky et al., 1995) in the absence of air and is thermally
degraded into small molecules. Using a catalyst (such as silica-alumina,
zeolite, zirconia, or clay) allows for pyrolysis at lower temperatures and
increases the conversion rates into useful products (Ding et al., 1997; Panda
et al., 2010). The composition of fuel products as well as the ratio of gas to
liquid yield in pyrolysis is determined by the type of catalyst used.
Non-catalytic pyrolysis is carried out at 650-900°C and is designed to
maximize the yield of fuel oils (Tukker et al., 1999). The process yields
low-grade gasoline or oil and is relatively inefficient. One of the most
important of these, the BP process for instance, based on a fluidized bed
reactor, is operated at 500°C in the absence of air. About 80% of the plastic
is converted into a liquid mix of hydrocarbons (oil) and the remaining
(8-10%) into a gas rich in monomers, under these pyrolysis conditions.
A broad range of hydrocarbons ranging from C
5
to C
28
results from
high-temperature thermolysis.
Pyrolysis at moderate temperatures (300-500°C) is achieved in catalyzed
reactions (Ding et al., 1997; Park et al., 1999) and yields a wax suitable for
further catalytic cracking (Lee, 2012) in a second step, to produce useful
petrochemicals. These can be converted into a high-grade gasoline
substitute with a high content of isoparaffins (Songip et al., 1993).
Pyrolysis-based recovery can yield fuels such as the naphtha, furnace oil,
and heavy fuel that might also be used without further refining. Oils from
low-temperature pyrolysis generally contain less of olefins and more
branched hydrocarbon and aromatics.
The most studied and already demonstrated pyrolytic technique is
fluidized-bed pyrolysis of waste (Conesa et al., 1997; Williams and Williams,
1999). The process is also carried out in two-steps where the plastics are
