3.4. Char properties
The final char results obtained from pyrolysis of the RDF and
straw at 500 C are shown in Fig. 9. A porous structure can be detected
on the surface of RDF char whereas the straw pellet char
shows a more compact structure. The formation of pores results
from the escape of volatiles from the non-char materials when
the plastic groups at the outer layer are melted during pyrolysis.
The diameters of the final RDF and straw char are shown in
Fig. 10, and the diameter of the RDF char is slightly larger than that
of the original RDF.
Scanning electron microscopy was performed on the char obtained
from pyrolysis to examine the difference in the swelling/
shrinkage behaviors of the recovered solid waste and the straw
pellets. The SEM images of the char are shown in Fig. 11A–F. As
shown in the figure, the straw char pellets display a compact structure
with a regular shape. In Fig. 11C and D, the fibers are stacked
together layer-by-layer due to the compression during the pelletized
process and remain as a multilayered structure in the longitudinal
direction. The surface is quite clear. Abundant pores also can
be detected from the cross area of the fibers. In Fig. 11A–C, the RDF
char consists of filamentous structures and irregular particles. The
remaining entangled filamentous structures originate from the cellulosic
fibers contained in the recovered solid waste, and the irregular
particles dispersed over the surface of the filamentous fiber
are attributed to the char and mineral matter obtained from the
plastic and wood components [42]. In Fig. 11A, the surface of the
char is covered by one thin layer of smooth material, which is generated
and left behind during the plastic melting and pyrolysis processes
and could be compared with that of the char from higher
temperature decomposition. Circles that resulted after the rupture
of bubbles are dotted randomly over the surface in Fig. 11A. This
appearance suggests that the recovered particles exhibited a plasticity
process during the pyrolysis. With the increase of the temperature,
the fluffy structure of the char becomes more visible.
The fluffy filamentous fibers, which could increase the integrity
of the recovered solid waste particles, play a crucial important role
in the pyrolysis process. Compared with a typical coal particle or
biomass particle, the recovered solid waste particle has a more heterogeneous
structure. Because the softening temperature of most
of the plastics falls in the temperature zone of 110–260 C, the
plastic groups contained in the RDF particles first exhibit plasticity
during the heating phase. Meanwhile, the volatiles from the pyrolysis
of the cellulosic groups with lower decomposition temperatures
begin to burst and escape from the particle. Thus, an
abundance of bubbles forms throughout the RDF particle mass
and causes the swelling phenomena.
To understand the differences in the surface area of the chars
obtained at 550 C, the BET surface area, the total pore volume
and the average pore diameter are reported and compared in Table
2. The pellets particles were ground to a particle size of less
than 2 mm. The RDF char displays a lower BET surface area and total
pore volume compared with those of the straw char. This result
suggests that the thin layer of smooth material generated from the