Simultaneous production of carbon nanotubes and hydrogen gas from the thermal treatment of waste plastics

In this work a two-stage reaction system was used for the
simultaneous production of carbon nanotubes (CNTs) and
hydrogen gas from plastic feedstocks. Initial pyrolysis was
undertaken in the first stage heated at 600 C, before the evolved
gases were passed to a second stage where a catalyst was held.
Carbon deposition builds up on the surface of the catalyst, whilst
hydrogen and other gases are then collected downstream. A series
of analytical techniques were used to characterise the carbon
deposition on the catalysts, as well as any oils or gases produced.
Initially, a two stage pyrolysis-gasification process was undertaken
with plastics from waste electrical and electronic equipment for
investigation into hydrogen production. The introduction of a nickel
catalyst led to increased hydrogen production, with small amounts
of CNTs observed in the carbon deposits on the catalyst surface. In
order to increase the yield of CNTs, different plastics including
polyethylene, polypropylene and polystyrene were investigated.
The rate of steam injection into the two stage pyrolysis-gasification
process was also investigated, and proved significant to obtaining
high yields of CNTs and hydrogen. All of the plastics produced
CNTs, with the largest yield obtained from the pyrolysis gasification
of polystyrene at a steam injection rate of 0.25 g h
-1
. Additionally,
the use of different catalysts was investigated, with iron, nickel,
cobalt and copper catalysts all tested. CNTs were produced on the
iron, nickel and cobalt catalysts, with the iron catalyst producing the
largest yield. The calcination temperature and metal loading on the
nickel catalyst was also studied, with catalyst support interactions
of intermediate strength and a higher metal loading producing
larger CNT yields. Finally, investigations into the temperature of the
second stage where the catalyst was held were undertaken, along
with using different ratios of catalyst: plastic sample. These too
proved important in achieving large yields of both CNTs and
hydrogen.