Air gasification of mixed plastic wastes in a two-stage gasifier using olivine andactivated carbon for the production of a low-tar and hydrogen-rich producer gas
Min-Hwan Cho, Young-Gon Choi and Joo-Sik Kim*
Department of Energy and Environmental System Engineering, University of Seoul,
90 Jeonnong-Dong, Dongdaemun-Gu, Seoul 130-743, Republic of Korea
e-mail: joosik@uos.ac.kr

Abstract
In this work, air gasification was conducted with a fraction of mixed plastic wastes using a two-stage gasifier. For theproduction of a low- tar and hydrogen-rich producer gas, olivine as the fluidized-bed material and activated carbon as atar-cracking additive were simultaneously used. When the amount of activated carbon increased from 0 to 1,500 g, theconcentrations of H2 and CO in the producer gas increased sharply from 9.3 to 31.1 vol% and from 3.2 to 8.6 vol%,respectively. When 1,500 g of activated carbon was applied, the total tar amount was about 97.5 % less than thatobtained with only olivine as the bed material. The maximum Lower Heating Value (LHV) of the producer gas was about
8.2 MJ/Nm3.

Keywords: Mixed plastic wastes, Two-stage gasifier, Tar removal, Olivine, Activated carbon

1. Introduction
With the rapid population growth and increase ofindustrial activity, the worldwide production andconsumption of plastics has increased. Recently, theresearch for the recycling of plastic wastes has been
carried out to obtain environmental and economical
benefits [1]. Among a variety of the different alternatives
for plastic wastes treatment, gasification is one ofthermo-chemical conversion processes, which converts
waste plastics into a combustible producer gas, mainly
made up of hydrogen, carbon monoxide and methane. In
the gasification process, however, a huge amount of tar
is released, which can cause operational troubles, forinstance blocking joint pipe lines or fouling the processequipments [2]. In this study, a fraction of plastic wastes
was gasified in a two-stage gasifier consisting of abubbling fluidized bed (lower reactor) and a tar-cracking
zone (upper reactor) in series using olivine as thefluidized bed material and activated carbon as the tarcracking agent in the upper reactor.

2. Materials and Methods
Feed Material and additives. Pellet-type mixed plastic
wastes used for this study were supplied by a recycling
company, which consisted of various types of plastics
such as polyethylene, polypropylene, polystyrene,
polyvinyl chloride, polymethylmetacrylate (PMMA) andpolyethylene terephtalate (PET). The proximate analysisof the mixed plastic fraction showed that its moisture,
volatile matter, fixed carbon and ash content was 0.6wt%, 94.2 wt%, 0.4 wt% and 4.6 wt%, respectively. In theelement analysis, it was revealed that the feed material
was comprised of 80.8 wt% of carbon, 12.8 wt% ofhydrogen, 5.1 wt% of oxygen, 0.2 wt% of nitrogen and1.1 wt% of chlorine. The bed material, applied in theseexperiments, was natural olivine. Olivine (2.5 kg) with adiameter of 150-300 μm was used as the fluidized bedmaterial. The olivine was mainly composed of 44.2 wt%
SiO2, 8.0 wt% Fe2O3, 43.5 wt% MgO and other minor
components, such as Al2O3 and CaO. Moreover,commercially available coal-based activated carbonpellets with a size of about 3.35-10 mm were applied inthe upper reactor of the gasifier.
Gasifier and experimental procedure. The bench-scale
gasification process consists of four main sections:feeding system, two-stage gasifier, char removal systemand quenching system. The two-stage gasifier comprises
a bubbling fluidized bed (lower reactor) and a fixed bed
reactor (upper reactor) in series, made of a 310 SS tube,
which was indirectly heated with electricity. The lower
reactor has an inner diameter of 110 mm and a height of390 mm, with four thermocouples. The upper reactor has
an inner diameter of 160 mm and a height of 340 mm,
with two thermocouples. The producer gas obtained in
two-stage gasifier passed the char removal systemcomposed of a high-efficiency cyclone and hot filter
designed to capture particles larger than 100 and 10 μm,respectively. After char particles removal, condensable
components in the producer gas were removed in thequenching system consisting of three steel condensers
which operated with cool water at 10 °C. The aerosols
and very fine particles after the quenching system werecaptured with an electrostatic precipitator. Some of noncondensable producer gas was sampled by means ofteflon gas bags (1L) at intervals of 10 min to analyze its
composition and the remaining gas stream was burned ina flare stack after its gas volume and temperature weremeasured. A diagram of the two-stage gasifier is shownin Fig. 1.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

Fig. 1. A diagram of the two-stage gasifier

Reaction conditions. Gasification experiments (Runs 1to 3) were conducted to investigate the influence of theamount of activated carbon on the composition ofproducer gas and the amount of tar produced. Run4 wasconducted without EP, the results of which can becompared with those of Run3. Run5 was performed
using an activated carbon filter instead of EP, the resultsof which can be compared with those of Runs 3 and 4.
Product analysis. After gas samples were taken fromthe exit of the gasifier, the gas samples were measured
by means of gas chromatography (GCs) using a thermal
conductivity detector (TCD) for N2, H2, CO, CH4, CO2 anda flame ionization detector (FID) for light hydrocarbons.
The tar contents in the producer gas were defined asorganic molecules with a molecular weight larger thanthat of benzene, and it was quantitatively analyzed byusing a GC-FID.

vol.%) in the producer gas. This is because tar cracking
reactions were enhanced by activated carbon which
could adsorb tars on its surface and therefore, prolong
the residence of tars till they were cracked. When 1500 gof activated carbon was applied, the tar content inproducer gas was sharply decreased to 5 mg/Nm3.
Regarding the composition and tar content in theproducer gas, there was no observable difference
between Run 4 and Run 3, which showed the possibilityof successful gasification operation without an EP. The
comparison between Run 4 and Run 5 did not indicate a
discrete difference on the composition of the producer
gas and the amount of tar generated. In this study, thetotal amount of tar was defined as the sum of the tarcollected in the condensers and tubes after quenching,
the tar deposited on the apparatus and the tar capturedby the electrostatic precipitator. Fig. 2 shows the total tar
production and tar removal efficiency under various
reaction conditions. As the amount of activated carbon
increased (Runs 1 to 3), the amount of total tar
decreased. When 1,500 g of activated carbon wasapplied (Runs 3 to 5), the tar removal efficieny reached
up to 97.5 %.
Total tar yield (g/kg mixed plastic wastes)
Tar removal efficiency (%)

With AF/Without EP (Run 5)

Without EP (Run 4)

A.C. 1,500 g (Run 3)

A.C. 900 g (Run 2)

3. Results and Discussion
No additive (Run 1)

Table 1 shows compositions, tar contents and LHVs ofproducer gases.
Table 1. Reaction conditions and producer gas compositions.
Parameter

Run1

Run2

Run3

Run4Run5

Upper-reactor temperature (°C)

812

806

806

801804Lower-reactor temperature (°C)

831

833

823

814

826

Feed rate (g/min)

4.67

4.68

4.56

4.65

4.83

Operation time (min)

64

64

65

64

62

Equivalence Ratio

0.31

0.31

0.31

0.31

0.3

Activated carbon (g)

None

900

1500

1500

1500

Electrostatic precipitator

OOOX

X(AF)

N2

60.23

51.14

48.47

46.31

52.51

CO213.04

8.56

7.76

8.46

4.61

H2

9.34

27.09

30.25

31.05

30.63

CO

3.19

6.7

7.62

8.6

7.66

CH4

9.1

6.39

5.88

5.57

4.59

C2H6

0.19

0.01

0.001

< 0.001< 0.001

C2H4

3.37

0.05

0.002

0.001

< 0.001

C2H2

0.27

0.01

0.01

N.D.

N.D.

C3+C4+C5

0.11

0.001

< 0.001

N.D.

N.D.

Benzene

1.09

0.05

0.002

0.004

N.D.

Toluene

0.04

0.001

< 0.001< 0.001

N.D.

Tar in producer gas (mg/Nm3)

2170

53

59

N.D.

Lower heating value (MJ/Nm3)

8.22

5.81

6.03

6.05

5.56

Composition (vol%)O: applied, X: not applied, AF: Activated carbon filter

The increase in the amount of activated carbon led to alarge increase in the concentrations of hydrogen (from9.3 to 30.3 vol.%) and carbon monoxide (from 3.2 to 7.6020406080100120140160180

Fig. 2. Amount of total tar and tar removal efficiency

4. Conclusions
In this study, air gasification of a fraction of mixed plasticwastes was carried out to investigate the influence ofoperating conditions and additives on the composition ofproducer gas and the amount of tar. The maximum LHVof the producer gas was about 8.2 MJ/Nm3. When 1,500
g of activated carbon was applied, the concentration ofhydrogen was 31.1 vol% and the total tar amount was
about 97.5 % less than that obtained with only olivine as
the bed material. To summarize, the producer gas
generated in the experiments was considered suitable forthe use as a fuel in internal combustion engines or fuelcells.

References[1] M.L. Mastellone, L. Zaccariello and U. Arena. Fuel 89(2010) 2991-3000.
[2] A. Paethanom, S. Nakahara, M. Kobayashi, P.
Prawisudha and K. Yoshikawa. Fuel ProcessingTechnology 104 (2012) 144–54.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013