The porosity selective (500–600°C). This indicates that

The influence of pyrolysis temperature, heating rate, and
residence

time on the yields and composition of pyrolysis products
have been

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discussed in the literature. Williams and Williams 139
conducted the

pyrolysis of plastic waste (high- and low-density polyethylene,

polypropylene, and PVC) using a fixed-bed reactor at
temperatures

between 500°C and 700°C. The study showed that increased

temperature led to a dramatic increase in the yield of gas.
It was also

found that higher temperatures enhanced the yield of
aromatic

compounds in the liquid product. Chen et al. 140 described
the

influence of pyrolysis temperature on the adsorption
capacity of

organic molecules of the char by characterizing chars as
being polarity

selective (produced at 200–400°C) and porosity selective
(500–600°C).

This indicates that the polarity of the adsorbed compound is
the most

important factor for adsorption on low-temperature chars,
whereas

the size of the adsorbed compound is more crucial for
adsorption

on high-temperature chars 140. Char produced at 700°C and

conventional activated carbon were nonselective because of
less

amount of functionalities and larger pore size, and these
chars also

demonstrated the highest adsorption capacities 140. It has
also been

reported that pyrolysis at high temperatures will remove
volatiles in

the thermoplastic phase, which increase the pore size,
resulting in a

nonselective type of char 141. This type of char has
properties similar

to commercial activated carbon and has good adsorption capacity

because of the large surface area.

The residence time for feedingMSWremaining in pyrolysis
reactors

can vary from a few seconds to 2 h 142. It has been
reported that

increased residence time could lead to enhanced tar cracking
and

result in higher gas yields 142. However, long residence
times could

reduce the processing efficiency in terms of the capacity of
MSW

treatment. Heating rates for MSW pyrolysis reported in
literature

varied from approximately 10°C/min in slow pyrolysis 143
to 600°C/

s in flash pyrolysis 144. It was commonly recognized that
high

heating rate could lead to a higher yield of volatile
products and lower

char yield 132. In the case that volatiles are extracted
immediately in

the cooling trap, a higher yield of liquid products can be
obtained.

The effect of temperature on the stability, nutrient
availability, and

hydrophobicity of chars derived from MSW have been studied

recently 116. It was found that increasing the pyrolysis
temperature

increased the aromatization and stability of the char,
whereas

lowering the temperature (300–400°C) could improve the
cation

exchange capacity and nutrient availability of the produced
chars.

Di Blasi 145 has reported on the influence of particle
size on MSW

pyrolysis and concluded that reduced particle size in
general resulted in

increasing heating rate and consequently higher yield of the
liquid

product. Another study, however, showed that the influence
of

particle size on the yield of the liquid product was minor
when the

temperature was sufficiently high 146. Additionally,
operating

pressure is also an influencing factor. High-pressure
pyrolysis

increases the yield of char, although practically it may be
difficult to

operate 147.

The environmental impact of the formation of inorganic
(i.e., HCl and

heavy metals) and organic (i.e., polychlorinated
dibenzo-p-dioxins and

dibenzofurans) contaminants in the pyrolysis of solid waste
containing

PVC, discarded tires, and demolition waste wood has been
investigated

148,149,150,151. Organic pollutants and heavy metals
tended to

largely remain in the liquid and char fractions,
respectively. Studies also

showed that the formation of some undesired inorganic
by-products

containing Cl, Br, and S could be reduced with the use of
catalysts 152.

A study compared the behavior of heavy metals in
MSWpyrolysis with

that in incineration 153 and showed that the release of
heavy metals

in rapid-heating combustion generally exceeded that observed
in

pyrolysis. The volatilities of some heavy metals, e.g., Zn
and Pb, were

reduced in pyrolysis compared to those in combustion. It was
concluded

from that study that pyrolysis is a better choice for MSW
treatment in

terms of controlling heavy metal contamination. To control
the emission

of organic and inorganic pollutants associated with MSW
pyrolysis, flue

gas
scrubbing is to date the most efficient method 132.

x

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