5-7 OCTOBER

ISFR

2011

109

TOLEDO (SPAIN)

STEAM GASIFICATION OF PLASTICS IN A CONICAL SPOUTED BED REACTOR
A. Erkiaga, G. Lopez, M. Artetxe, M. Amutio and M. Olazar
University of the Basque Country, Department of Chemical Engineering
P.O. Box 644 - E48080 (Bilbao) Spaine-mail: aitziber.erkiaga@ehu.esAbstract
Steam gasification of HDPE has been carried out in a bench scale plant provided with a conical spouted bed reactor. The
effect of temperature over the gasification process has been studied in the 800-900 ºC range. The products obtained havebeen classified into three different fractions: gas, tar and char. The gas yield at 800 ºC is relatively low, but an increase of
100 ºC raises the gas amount and reduces the tar and char yields. Hydrogen content of syngas increased with temperature
due to the fact that the hydrogen formation reactions are endothermic. Finally, tar and char content are low so they can beused to provide heat to process.

Keywords: conical spouted bed reactor, HDPE, hydrogen, steam gasification, temperature.

1. Introduction
The amount of plastic wastes produced is growing year
by year and the fraction of plastics making up municipal
solid wastes and refuse derived fuels is also increasing
progressively in the world. Among the different chemical
recycling techniques, gasification process appears to be
an interesting option in the development of full scale
processes for the upgrading of solid wastes to more
usable and energy dense materials, such as gas fuel.
Hydrogen is an important raw material widely used in thechemical industry. Moreover, it is considered to be aclean alternative to fossil fuels provided that can beproduced from renewable. Steam has been used as
gasification agent because the main objective is tomaximize hydrogen yield. But the utilization of steam as
the only gasifying agent requires heat supply to theendothermic process [1].

2. Materials and Methods
2.1. Experimental equipment
The gasification unit consists of the following
components: (1) solid feeding device, (2) water feeding
device, (3) reaction system and (4) separation system.
The feeding system of the raw material to the gasification
reactor consists of a cylindrical piston which is actuatedelectrically. To generate the required steam for thegasification process, the plant has is a pump of type
HPLC outside the hot box which feeds water to thegasification reactor. The reaction system consists of thefollowing elements: gasification reactor, evaporators anda cyclone. All the elements are located inside a hot box
which is kept at a temperature of 400 ºC in order toprevent the condensation of heavy compounds. Thereactor is a spouted bed of conical geometry with acylindrical upper section. The bottom of the reactor is ainteriores libro.indd 109

gas preheater to raise the temperature of the steam tothe required by the gasification process. The gaseous
stream leaves the reactor and goes to a cyclone toremove particles entrained from the bed. The stream thatleaves the cyclone is conducted to a “T” which permits totake a sample for the analysis in gas chromatographconnected online and the rest of the stream in conducted
to a gas-liquid separation system.
2.2. Experimental procedure
The gasification runs have been carried out in continuousmode by feeding 1.5 g/min of HDPE. The bed was madeup of 70 g of silica sand (particle size 0.2-0.3 mm), andplastic to water ratio was maintained at 1:1 in all the runs.
In order to study the effect of gasification temperature on
product distribution, the runs have been conducted at800, 850 and 900 ºC. Products analysis has been carriedout on line, analyzing the reactor outlet stream by meansof a gas chromatograph (HP 6890). The line from thereactor outlet to the chromatograph is heated up to 250ºC in order to avoid the condensation of heavy
compounds. Furthermore, noncondensable products
have been monitored using a micro chromatograph(Varian 4900). The identification of the liquid products
has been performed in a GC/MS (Shimadzu QP2010S)
and the gaseous products have also been analyzed bymeans of a micro GC connected to a mass spectrometer(Agilent 5975B).

3. Results and Discussion
Gasification is a thermochemical process to produce highamounts of gas being the tar (containing benzene andcompounds of higher molecular weight) and char two
fractions formed at small extent.
The main effect of temperature is the increase of gas
fraction due to the more severe cracking of the moleculesthat lead to the formation of gaseous products. As a26/9/11 13:13:06

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6th International Symposium on Feedstock Recycling of Polymeric Materials

H2Composition (%vol.)CH 4CO40

CO 2Ethylene
Ethane30

Propylene

100750800

850900950

Temperature (ºC)

Fig. 1. Influence of temperature on the composition ofgaseous product.
The tar is the condensable fraction obtained in thegasification process and is mainly made up of aromatichydrocarbons, including benzene. Tars are formed during
gasification in a series of complex reactions.
Due to the high reaction temperature, secondary
reactions occur in the gas phase which converted heavyhydrocarbons compounds to light hydrocarbons andolefins, precisely these olefins are precursors of aromaticcompounds that lead to the formation of aromatic rings
by Diels-Alder reactions. Subsequently the condensationinteriores libro.indd 1108070

Light aromatics

60

Light polyaromatics
50

Heavy polyaromatics
Unidentified

4030
20
100750800

850

Temperature (ºC)

900950

Fig.2. Distribution of functional groups of tar (%wt) at
different temperatures.

4. Conclusions

References

Propane2090

The conical spouted bed reactor is a suitable technology
for the plastics steam gasification. The temperatureincrease leads to a higher gas yield (up to 96.5%) and areduction of tar and char yields. The main product ofgaseous fraction is hydrogen, the maximum yield has
been obtained at 900 ºC being around 60%. Together
with the increase of hydrogen the yield of CO increases
and decreases CO2 because the raise of temperature
disfavour exothermic reaction. The tar is composed
mainly of light aromatics.

6050

reactions take place and larger polyaromatichydrocarbons (PAH) are formed.
Based on the molecular weight of tar compounds, tar
was divided into four groups, as shown in Figure 2.
These four groups are considered like light aromatics,
light and heavy PAH and unidentified products byGC/MS.

Tar Composition (%wt)

result of the gas yield increase to 178.66 % for 100 gplastic fed at 900 ºC, the amount of tar is reduced to 4.8%, moreover the char gasification is also enhanced andconsequently the solid fraction yield is lower at hightemperatures. Thus, the gasification is more efficientobtaining a higher carbon conversion 92 % andconsequently most plastic is transformed into gaseousproducts.
Figure 1 shows the effect of temperature over thegaseous fraction composition, as observed the gaseous
fraction is mainly made up of hydrogen, carbon dioxide,carbon monoxide, methane and light olefins. The
temperature increase gives way to a reduction in theyield of light olefins, however an increasing trend of themethane yield is observed.
The use of steam as gasification agent leads to thetransformation of hydrocarbons into hydrogen andcarbon monoxide, in addition carbon monoxide formed isconverted into carbon dioxide and additional hydrogen.
As observed in Figure 1 hydrogen yield is influenced bytemperature, this is due to the fact that the hydrogenformation reactions are endothermic. The water gas shift
reaction is exothermic so when increase the temperature
this reaction loses importance [2]. Therefore the amountof carbon dioxide is less at higher temperatures. The gas
composition obtained at 900 ºC is especially interesting
because the molar ratio of H2/CO could be useful forchemical syntheses such as methanol, Fisher Tropsh,etc [3].

[1] U. Arena, L. Zaccariello and M.L. Mastellone. Tar
removal during the fluidized bed gasification of plastic
waste. Waste Management 29 (2009) 783-791.
[2] M. He, B. Xiao, Z. Hu, S. Liu, X. Guo and S. Luo.
Syngas production from catalytic gasification of waste
polyethylene: Influence of temperature on gas yield andcomposition. International Journal of Hydrogen Energy
34 (2009) 1342-1348.
[3] X. Song and Z. Guo. Technologies for direct
production of flexible H2/CO synthesis gas. Energy
Conversion and Management 47 (2006) 560-569.

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