5-7 OCTOBERISFR2011

103

TOLEDO (SPAIN)HYDROGEN PRODUCTION FROM PYROLYSIS-GASIFICATION OF
POLYPROPYLENE USING A SCREW KILN REACTOR SYSTEM
C. Wu and P.T. Williams
Energy & Resources Research Institute, The University of Leeds, Leeds LS2 9JT, UKe-mail: c.wu@leeds.ac.uk (C. Wu) and p.t.williams@leeds.ac.uk (P.T. Williams)

Abstract
A screw kiln continuous reaction system was used and investigated for hydrogen production from the catalytic steam
pyrolysis-gasification of polypropylene. A Ni-Mg-Al catalyst was investigated in relation to gasification temperature. Theresults showed that the introduction of the catalyst in the gasification bed dramatically improved the hydrogen production. Forexample, the potential H2 production increased from only 2.53 wt.% (without catalyst) to 21.01 wt.% in the presence of theNi-Mg-Al catalyst, when the pyrolysis temperature, gasification temperature and water injection rate was 500, 800 °C and9.49 g h-1, respectively. The gas and hydrogen production and amount of reacted water per hour were increased with theincrease of the gasification temperature from 600 to 900 °C for the Ni-Mg-Al catalyst.

Keywords: Polypropylene; Nickel; catalyst; Screw kiln

1. Introduction
There is increasing interest in hydrogen energy due to its
clean combustion and potential use in advanced
technologies such as in fuel cells etc. [1,2]. The
production of hydrogen from the thermal decompositionof plastics has been suggested to be a good alternative
for waste plastics treatment and a possible way toproduce extra hydrogen for a future energy system.
Among various reactors, the continuous screw kiln has
been proposed to be a preferred reaction system forindustrial commercialization [3]. The screw kiln reactor
was suggested to have the potential to be used for theprocessing of waste plastics, which possess complex
chemical and physical properties. Furthermore, the screw
kiln reaction system might reduce the over-cracking of
raw materials, and generate a comparatively narrow
range of products that would be more easily reformed ina second stage to produce hydrogen gas.
In this paper, polypropylene was used as a raw material
to be pyrolysed and gasified to generate hydrogen by
combining screw kiln pyrolysis reactor with a fixed bed
gasification reaction system. A laboratory prepared NiMg-Al catalyst and a commercial Ni catalyst wereinvestigated for the pyrolysis-steam catalytic gasificationof polypropylene. The experimental parameters such as
gasification temperature and water injection rate werealso investigated.

2. Materials and Methods
2.1. Materials and catalyst.
Polypropylene (PP) was obtained as 2 mm virgin polymer
pellets provided by BP Chemicals UK.
A laboratory-prepared Ni-Mg-Al catalyst was used in thiswork. Ni-Mg-Al catalysts were prepared using the risinginteriores libro.indd 103

pH technique according to the method reported by
Garcia et al. [4] and our previous report [5].
2.2. Reaction system.
A schematic diagram of the screw kiln reactor is shown in
Figure 1. The screw kiln reaction system mainly includes
a materials feeding system, screw kiln pyrolysis systemand the fixed-bed catalytic gasification system. The
plastic materials were continuously fed into the pyrolysis
reactor (62 mm diameter and 540 mm length), where ascrew was placed through the reactor and was rotated byan electric motor. The plastics were screwed forward andpyrolysed in the screw kiln reactor. The screw kiln washeated by a small pre-heat furnace and a large main
furnace. The derived pyrolysis gases passed through afixed-bed vertical gasification reactor (25 mm diameterand 260 mm length) which was heated and controlled
separately and also contained the catalyst bed. Steamwas introduced into the gasification reaction system forthe catalytic steam gasification of the product pyrolysis
gases. The connection of the pyrolysis reactor with thegasification reactor was also heated by a small heater toavoid gas condensation before entering into thegasification bed. The generated gases were firstly
condensed to collect any liquid condensable products
and gases were then collected with a Tedlar TM gas
sample bag, and analysed for gas concentration off-line
using packed column gas chromatography.
The screw kiln rotation rate and polypropylene feeding
rate was kept at 13.0 rpm and 11 g h-1, respectively. The
carrier gas (N2) was introduced with a total flow rate
about 200 ml min-1. For comparative experiments in theabsence of the catalyst, silica sand was used torepresent base-line reaction conditions, to compare withthe Ni-Mg-Al catalyst data.

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6th International Symposium on Feedstock Recycling of Polymeric Materials

The gaseous products collected in the Tedlar™ sample
bag were analysed by packed column gas
chromatography (Varian 3380) to determine theconcentration of hydrocarbons and permanent gases.
OutletRaw Materials

Gasification Furnace

Catalyst

Nitrogen

Agitator
Band Heated Furnace
Agitator

Steam

Pre-heated Furnacepresented in Table 2. From Table 2, the reacted water
was increased with the increase of gasification
temperature for the Ni-Mg-Al catalyst. The hydrogenproduction rate and potential hydrogen production werealso increased with increasing gasification temperaturefor both catalysts. In our previous work using a fixed bed
reaction system, the potential H2 production was found to
be increased from 13.4 to 52.0 wt.% withaNi/CeO2/Al2O3 catalyst when the gasification temperature
was increased from 600 to 900 °C [6]. In addition, a 57.1
wt.% potential hydrogen production was obtained in thepresence of a Ni-Mg-Al catalyst at a gasification
temperature of 800 °C using the two-stage fixed bed
reaction system [6].

Main Furnace

Table 2 Influence of gasification temperature

CharFig.1. Schematic diagram of screw kiln reactor system

3. Results and Discussion
3.1. Influence of steam and catalyst on the gas andhydrogen production.
Experiments were carried out to investigate the influenceof steam and catalyst on gas and hydrogen production
from the pyrolysis-gasification of polypropylene. The
pyrolysis temperature and gasification temperature werecontrolled at 500 and 800 °C, respectively. The gas
production and hydrogen production results arepresented in Table 1.
Table 1 Gas and hydrogen production
SandSand

NiWater injection rate (g h-1)
Reacted water (g h-1)
Gas production (g h-1)000.75

9.49
0.58
2.35

9.49
5.14
9.08

H2 production (g h-1)
Potential H2 production (wt.%)

0.03
0.61

0.12
2.53

0.99
21.01

The results shown in Table 1 demonstrate that with theintroduction of steam in the gasification process, gas
production increased from 0.8 to 2.4 g h-1 and the H2
production increased from 0.03 to 0.12 g h-1. The
presence of catalyst largely increases the gas andhydrogen production. In this paper, the potential H2
production is presented corresponding to the maximum
H2 production. The maximum theoretical amount of H2
produced from polypropyelene steam gasification isestimated to be 42.9 g per 100 g polypropylene. [5]3.2. Influence of gasification temperature.
The gasification temperature has been investigatedextensively during the pyrolysis/gasification process due
to its importance. In this section, the gasification
temperatures of 600, 700, 800 and 900 °C were studied
with the continuous screw kiln reactor system. The water
injection rate was controlled at 14.23 g h -1. Pyrolysis
temperature was kept at 500 °C.
The gas production and hydrogen production at different
gasification temperatures with the Ni-Mg-Al catalyst areinteriores libro.indd 104

Reacted water (gh-1)
Gas production (gh-1)
H2 production (gh-1)
Potential H2production (wt.%)

600°C

700°C

800°C

900°C

3.69

8.78

9.73

12.59

5.49

14.35

13.79

21.59

0.53

1.56

1.68

1.97

11.27

33.01

35.58

41.65

4. Conclusions
In this paper, a screw kiln continuous reaction system
was tested for hydrogen production from the pyrolysisgasification of polypropylene. The process conditions
such as gasification temperature and water injection rate
were investigated with a Ni-Mg-Al catalyst. The results
show that the amount of reacted water, gas productionand H2 production were increased with the increase ofgasification temperature from 600 to 900 °C

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[2] J.A. Onwudili, P.T. Williams. Role of sodium hydroxidein the production of hydrogen gas from the hydrothermal
gasification of biomass. Int. J. Hydrogen Energ. 34(2009): 5654-5655.
[3] D.P. Serrano, J. Aguado, J.M. Escola, E. Garagorri.
Performance of a continuous screw kiln reactor for thethermal and catalytic conversion of polyethylenelubricating oil base mixtures. Appl. Catal. B: Environ.
44(2003): 95-105.
[4] L. Garcia, A. Benedicto, E. Romeo, M.L. Salvador, J.
Arauzo, R. Bilbao. Hydrogen production by steam
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[5] C. Wu, P.T. Williams. Hydrogen production by steam
gasification of polypropylene with various nickel catalysts
P.T. Appl. Catal. B: Environ. 2009, 87, 152-161.
[6] C. Wu, P.T. Williams. Effects of gasification
temperature and catalyst ratio on hydrogen production
from catalytic steam pyrolysis-gasification ofpolypropylene. Energy Fuel 22 (2008): 4125-4132

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