The 5th ISFR (October 11-14, 2009, Chengdu, China)

CHARACTERISTICS OF THE PYROLYSIS ANDGASIFICATION OFLOW-DENSITYPOLYETHYLENE (LDPE)
Zheng Jiao*, Chi Yong
Institute for Thermal Power Engineering, State Key Lab of Clean Energy Utilization,
Zhejiang University, Hangzhou 310027, China
E-mail: msw_2@zju.edu.cn
1. Introduction
Plastic waste was considering as one of the most harmful component of municipal solid
waste, due to it’s non-biodegradability. However, on the other hand, plastics contain large
quantity of energy and shows great potential to be used as some raw material or energy
source[1]. Worldwidely, polyethylene is the most common used plastics, it takes about 25%of total plastic production of the world[2].
Nowadays, pyrolysis and gasification were developed to be one of the most effective
way of recycling and energy utilization of solid wastes. Previous studies uncovered somecharacteristics of PE pyrolysis and gasification, such as kinetic modeling[3,4] or products
distribution[5,6]. This work try to find the basic access of the decomposition of LDPE underboth pyrolysis and gasification condition by comparing the TGA/FTIR and fluidized bed
gasification experiments.
2. Experimental
2.1 Material
The raw material used in this study was LDPE particles produced by Sinopec Yangzi
petrochemical company with the particle size of 3mm, the density of 0.920g/cm3 and themelting point of 130~145°C. Tab.1 shows the proximate and ultimate analysis of the material.
Powdered LDPE was used for TGA/FTIR experiment and original particles were directly
used to carry out the fluidized bed gasification.
Tab. 1 Proximate and ultimate analysis of material/%
MadAadVadFCadQb,ad/kJ·kg-1

Cad

Had

Nad

SadOad

0.02

0.15

99.83

―①46362

85.83

14.38

0.16

0.07

―①①- 134 -Not detected.

PARTⅡ GASIFICATION
2.2 TGA/FTIRThe Nicolet Nexus 670 spectrometer and Mettler Toledo TGA/SDTA851e thermo
analyzer were coupled by a Thermo-Nicolet TGA interface model, of which the stainlesstransfer line and gas cell (20cm path length) were set to 180 C to minimize the change ofevolved gas. Heating rate was set to 15K/min, nitrogen was used as the purge gas for both
TGA and spectrometer. Resolution in FTIR was set as 4cm−1, number of scans per spectrum
was set as 20 times/min, and the spectral region was set as 4000~400cm−1.
2.3 Fluidized bed gasification
A fluidized bed gasifier with the capacity of 10kg/h was used in this study. Fig. 1 shows
the schematic diagram of the facility. The height of the gasifier is 5m, and the inner diameteris 0.1m.
The sampling point was located at horizontal section at the exit of the gasifier. The detail
description of sampling point is showed at Fig. 2. The filter was used to catch the entrained
ash/char and the condensable products were collected by the condenser, ice water was used as
the coolant, the composition of syngas was detected by the gas analyzer. The sampling
process was strictly in accordance with the requirements of isokinetic sampling. The
equivalent rate for gasification was set to 0.4 and the temperature was 550 ~750°C.

Fig. 1 Schematic diagram of the experimental facility

Fig. 2 Schematic diagram of the Sampling point- 135 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
3. Results and discussion
3.1 Pyrolysis behaviour and products

A, FTIR curve
B, TGA curve
Fig. 3 TGA/FTIR curves of pyrolysis of LDPE

Fig. 4 FTIR spectra of the product of LDPE pyrolysis

Fig. 5 FTIR spectra comparing

Fig. 3 shows the TGA/FTIR curves of pyrolysis of LDPE, the decomposition takes place
in a very narrow temperature range, from 450°C to 550°C and the maximum conversion
speed reached 0.9%/s at 500°C. As expected the FTIR curve presented the similar figure, anarrow absorption peak represents the gaseous product generated at the temperature range of450~550°C, as well. Fig. 4 shows the FTIR spectra of the product of LDPE pyrolysis, somemixture of alkanes (bands in the regions 2964-2885and 2853 cm-1) can be indentified.
Comparing with this FTIR spectra with methane, ethene, ethane , propane and polyethylene asshown in Fig. 5. It is found that the similarity increased with the molecular weight, that means
high molecular weight products takes a great part of the products of LDPE pyrolysis.
Therefore, the pyrolysis progress was deduced as following: LDPE [C2H4]n is a kind ofmixture caused by the switch of ‘n’ in a certain range. As the increase of ‘n’ some physicalcharacteristic such as density, soften temperature and boiling temperature can be raisedslightly.[7] Under the high temperature condition, some small ‘n’ ones vaporized first and as
the raising of temperature the decomposition decreased the number of ‘n’, that means most of
the small molecular products was generated by the decomposition of the steam of small ‘n’PE.
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PARTⅡ GASIFICATION
However, the experimental condition was not so good for the sufficient decomposition ofLDPE, because, the heating rate was only 20k/min and the residence time of gas in the reactor
was not long enough. In another words, when the steam leave the reactor, the temperature is
only a little higher than the boiling point. That’s why a lot of wax was found in the condensed
products of PE pyrolysis[8].
3.2 Distribution of products of fluidized bed gasification
Tab.2 Mass and energy distribution of LDPE gasification
Mass distribution /％Energy distributio /％Temperature/°C

SyngasWater

Tar

Ash/Char

SyngasTar

Ash/Char

550

73.75

3.42

0.86

1.12

79.93

1.31

3.2665088.70

4.35

1.09

0.53

81.03

1.67

1.50

750

93.30

5.20

1.04

0.32

80.70

1.61

0.86

Fig. 6 The composition of the syngas
Tab. 2 shows the mass and energy distribution of LDPE gasified in fluidized bed. Either
for mass or energy, syngas was the main product takes about 80%of energy and 90% of massof the products. Fig. 6 shows the composition of the syngas, ethene is the main combustible
component of syngas, takes about 10% vol. Obviously, the decomposition of LDPE in
fluidized bed was much more sufficient than in TGA/FTIR condition.
There may be three possible reasons for achieving sufficient decomposition, they arehigher heating rate, long residence time and the effect of oxygen. As shown by TGA/FTIR thedecomposition of LDPE which can generate small molecules such as ethene, should takeplace in the gaseous phase, therefore the temperature of gaseous phase is very important forachieving sufficient decomposition. In despite of the residence time of gaseous phase in
fluidized bed can not be very long, the dramatically high heating rate can be helpful to heat
the gaseous phase and make the decomposition sufficient. The exist of oxygen burn out some- 137 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
gas products and the high temperature tail gas directly increased the temperature of gaseous
phase. On the other hand, the incomplete oxidation of LDPE or the high molecular products
such as tar or wax also generated some small molecules. That’s why the decomposition undergasification condition can be much more sufficiently than pyrolysis condition at lowtemperature, although the reactors are both fluidized bed[9].
4. Conclusions
Comparing with the TGA/FTIR study and the fluidized bed gasification some conclusion
was drew as flowing:(1) The decomposition of LDPE take place at about 450~550°C and generate a lot ofhigh molecular products;(2) The temperature of gaseous phase is the key parameter of decomposition. Since theresidence time of gas can not be very long in both TGA/FTIR condition or fluidized bed
gasification, high temperature and heating rate can be very helpful for achieving sufficient
decomposition.
(3) The incomplete oxidation of both LDPE and it’s decomposition products is good forheating the gaseous phase to promote the reaction of decomposition and generate small
molecules as well.
(4) The gasification of LDPE can be very sufficient in fluidized bed reactor at low
temperature (about 550°C). Among the products, syngas takes more than 80% for both massand energy. Ethene is the main combustible component of the syngas, takes about 10% of thevolume.
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- 138 -PARTⅡ GASIFICATION[8] A. E. S. Green, S. M. Sadrameli, Analytical representations of experimental polyethylene
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