PROCESSING WASTE PLASTIC INTO FUEL
By : Joanne Pui Chen Hui
ABSTRACT
The disposal of plastic have been a large issue in society. The purpose of this research is to
investigate if pyrolysis is suitable to obtain fuel. It is found out that both plastics and petroleum are
hydrocarbons containing the elements of carbon and hydrogen. Plastic are thermally degraded to
produce functional liquid hydrocarbons that can be used as fuel or returned to a refinery where they
can be added into raw materials to be processed. So, the process of pyrolysis of plastic are discussed
in detail and the factors affecting are described briefly. Test shows that the output products are
pyrolysis oil, hydrocarbon gas and bio-char. Besides, the oil produced has very high quality and close
to the commercial petroleum derived liquid fuels. In this research, pyrolysis oil may not be able to
substitute petrol but it can be use as an alternative fuel. Therefore, reduce the usage of petrol. It is
also a big step forward to save the environment.
1. INTRODUCTION
In 2011, the production of plastic has been increasing rapidly to 280 million tonnes, according to first
rough estimates published by Plastics Europe. Plastic have been one of the material used in
development in different industry because of their wide range of applications due to versatility and
cheap. In consequence of the plastic production, an increase of the usage of petroleum is seen.
However in recent years, petroleum has become less and may be used up at anytime. In Malaysia,
the rapid economic growth has generated 0.5-0.8 kg waste material/person/day and in rural areas
the figure increased to 1.7 kg/person/day (Kathirvale et al., 2003). Waste plastic disposal and
excessive use of fossil fuels have caused environment concerns in the world.
There are a few ways to dispose plastic which include recycling, composting, incineration and land
filling. Recycling waste plastic is now often practiced but the disadvantage of recycling waste plastic
is that the product no longer has the special characteristics of the plastics used to make it.
Composting isn’t a new thing but it requires space like land fillings and takes up a long time to
compose. Incineration is a costly process and is less practice. Open air burning of plastic normally
releases toxic fumes which is harmful. All those methods have major drawbacks.
Therefore, pyrolysis of waste plastic is a viable way to dispose and recycle plastic. Pyrolysis is
process of thermal decomposition of a material in the absence of oxygen. Both plastics and
petroleum are hydrocarbons containing the elements of carbon and hydrogen. (Pyrolysis of Waste
Plastics into Fuels, 2010) Based on research, the difference among them is that plastic molecules has
more longer carbon chain than petroleum. Thus, it is possible to convert plastic back into fuel.
Production of plastics from petroleum requires 62 to 108 MJ of energy per kilogram which is taken
into account the average efficiency of US utility stations of 35%.And by adjusting operating
conditions, the rate and extent of decomposition can be controlled. In this way, it is possible to
obtain a predominantly liquid hydrocarbon product with potential for use as a fuel or a refinery
feedstock (The Feedstock Option, 1995). Pyrolysis processes are frequently classified according to
their operating temperature (Basic Principles of Waste Pyrolysis and Review of European
Processes,1980 ). When high yields of liquid hydrocarbons are desired, at temperature which is
below 550oC, processes are employed; these yield mainly oils or tars with smaller amounts of gaseous
and solid products. As the operating temperature is increased, the gas yield increases and the liquid
yield decreases. Pyrolysis processes can also be classified according to the type of reactor used.
Fluidised bed reactors are widely accepted and utilised because they have excellent heat and mass
transfer characteristics and maintain a highly uniform temperature across the fluid bed (Pyrolysis with
respect to recycling of Polymer, 1995).
2. METHODS OF CONVERTING PLASTIC TO FUEL
Plastic is continuously treated in a cylindrical chamber and the gases condensed in a specialized
sysem to obtain hydrocarbon distillate and branched chain aliphatics, cyclic aliphatics and aromatic
hydrocarbons. The resulting distillate is basically the same compared to petroleum distillate. The
plastic is pyrolised at 370ºC-420ºC and the pyrolysis gases are condensed and liquid separated using
fractional distillation to produce the liquid fuel products. (Cynar Technology).
Figure 1. Pyrolysis Process of generating fuel oil from plastics
Depending on the pyrolysis condition and type of plastic used, carbonous matter gradually develop
on the inner surface of the reactor. (UNEP,2009) For example, char is accumulated at the surface
where the reactions occurred. Therefore if this occurs in the reactor, the char cannot be removed
easily. The present of char affects the heat exchange and limits the maximum process running time.
(Gao, Feng ,2010) After pyrolysis, the carbonouse matter, acting as a heat insulator, should be
remove from the reactor to achieve the heat conduction efficiency of the reactor and it is removed
after the resulting oil is distilled. . (UNEP,2009)
In normal operation in the pyrolysis reactor, the energy transferred to the plastics is same the
energy requirement for the pyrolysis and the vaporization. In the same time, the mass of the plastic
fed in is equal to the plastic vaporized. Hence, the liquid level should remain constant in the reactor.
The temperature of the melted plastics also remains constant near to the cracking temperature. The
resulting oil which is the mixture of liquid hydrocarbons is constantly distilled once the waste plastic
is decomposed in the reactor until it can evaporate upon reaching reaction temperature. The
evaporated oil is further cracked with a catalyst.
When the liquid comes out from the pyrolysis reactor, the liquid is passed through a condenser to
extract fuel gas. Then, the evaporated oil from the condenser is a mixture of hydrocarbons with
various molecular weights. This resulting oil needs to be separated through distillation into different
fractions in order to obtain liquid products which match the current retail liquid fuels. The function
of the separation section on the apparatus is to separate hydrocarbons into different ranges which
correspond to retails products such as diesel, petrol, wax and LPG. According to the boiling points of
the products obtained , the products can be separated into four groups: hydrocarbons heavier than
diesel (wax), hydrocarbons in the range of diesel, hydrocarbons lighter than the diesel; and noncondensable gases (LPG). (Gao Feng, 2010)
2.1 Factors affecting plastic pyrolysis
2.1.1
Cracking temperature and heating rate
One of the main operating variable is temperature since temperature dominates the
cracking reaction of polymer substances. The difference in temperature influenced the
amount of yield. Not all polymer can be cracked by increasing the temperature. The
other operating variable is the heating rate. In this field, heating rate is define as
increase of temperature per unit time. The effect of heating rate on the pyrolysis
process is different due to the difference in the pyrolysis reactor, temperature and
pressure.
2.1.2
Pressure
Pressure has a very big influence on both the pyrolysis process and the products. The
boiling points of the pyrolysis products are increased under higher
pressure, therefore, under pressurised environment heavy hydrocarbons are further
pyrolyzed instead of vaporized at given operation temperature
2.1.3
Type of reactor
The type of reactor effects the rate of heat transfer. Based on the feeding and product
removal process, the reactors can be classify into three which is batch, semi-batch and
continuous reactors. In batch reactor, the substance for pyrolysis are added into the
reactor in batches either at the start or after all the added substance are processed. In
semi-batch, the reactor eliminates the product continuously once they are generated.
However, the feed is added before the pyrolysis start. In the continuous reactor, the
feed is added at one part and the products are led out from the other part in the
reactor.
2.1.4
Time
The definition of residence time differs in various studies. In fast pyrolysis or
continuous pyrolysis process, it refers to the contact time of the plastic on the hot
surface throughout the reactor. However in slow pyrolysis and batch process, the
residence time means the duration from the time when feedstock plastic start to be
heated to the time when the products are removed. Longer residence time favours a
further conversion of the primary products thus yielding more thermal stable products
such as light molecular weight hydrocarbons, non-condensable petroleum gases. In a
slow pyrolysis, long residence time encourages the carbonization process and produces
more tar and char in the products. (Introduction to Feedstock Recycling of Plastics,2006)
2.1.5
Catalyst
One of the main purposes of using catalysts is to shorten the carbon chain length of the
pyrolysis products and thus to decrease the boiling point of the products. Catalysts are
found to be mainly applied to PE pyrolysis because the primary product from other
plastics, such as PP and PS, are mainly light hydrocarbons, with similar carbon chain
length to the range of commercial fuels.
3. Product and by-products
Bio-oil is used in engines or to power up machines as a substitute for petroleum. There are
three main products from plastic pyrolysis which are bio-oil, bio-char/charcoal and
hydrocarbon gas. Table 3.1 shows the properties of pyrolysis oil and petroleum fuels.
Sample A and sample B are a whole distillate and middle distillate of pyrolysis oil
respectively. (UNEP,2009)
Category
Specific gravity (15 o C),
g/cm3
Flashing point (o C)
Kinetic viscosity (30 o C/
50 o C, mm2/s)
Carbon residue on 10%
bottoms; wt%
Ash weight (%)
Gross heating value
(cal/g)
Total chlorine (wt ppm)
Nitrogen (wt%)
Sulfur (wt ppm)
Cetane Index
Distillation Temperature
Initial
10%
50%
90%
End
Sample
A (Whole
distillate)
0.8306
Sample B
(Middle
distillate)
0.8430
Diesel fuel
Heavy oil
0.8284
0.8511
-18 (PM)
1.041-/
-/1.73
-/1.73
69.0 (Tag)
3.822/-
64 (PM)
-/2.29
-
0.85
0.01
0.46
0.00
11294
<0.001
10746
-
0.006
10708
47
0.14
100
27.0
10
0.033
910
42.9
<1
310
58.4
1.6
0.015
0.41%
46.3
47.0
69.0
148.0
294.5
374.0
180.0
199.0
233.0
323.5
351.5
344.0
Table 3.1 : The properties of pyrolysis oil and petroleum fuels
164
195
276
347
370<
During pyrolysis , some plastic produce residue substances such as carbonous matter and
other inorganic matter. Carbonous matter can be used as raw material. Other inorganic
matter may be contained depeding on the level of waste composition. Figure 3.1 shows the
output of plastic pyrolysis
Figure 3.1 : Output of Plastic Pyrolysis, Waste tyre & plastic tyre pyrolysis(2013)
4. CONCLUSION
In conclusion, Pyrolysis of waste plastic is a viable way to dispose and recycle plastic. This research
have show that the process of thermal degradation of plastic within an enclosed, oxygen free
environment is a more efficient way to dispose of waste plastics. The oil produced has very high
quality and close to the commercial petroleum derived liquid fuels. This method can produce fuel
substitutes. It has shown that this method of recycling plastic can reduce the usage of petroleum
and it could yield economical and environmental benefits.
5. ACKNOWLEDGEMENTS
First and foremost, I would like to thank the Principal of Sekolah Menengah Kebangsaan Lutong, Mr
Marcus Hugo Matu Lejau for giving me the chance to participate in this programme. Secondly, I
would like to show appreciation and thanks to my mentor, Prof. Dr. Yun Hin Taufiq-Yap for his help
and patience when dealing with all my questions.
6. REFERENCES
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Feedstock Recycling and Pyrolysis of Waste Plastics, J. Scheirs and W. Kaminsky, Editors.
2006, John Wiley & Sons, Ltd: Mostoles, Spain. p. 73-110.
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High-Density Polyethylene (HDPE) in decalin solvent: Effect of the reaction time and
temperature. Industrial and Engineering Chemistry Research, 2007. 46(11): p. 34973504.
3. Buekens, A., Introduction to Feedstock Recycling of Plastics, in Feedstock Recycling and
Pyrolysis of Waste Plastics, J. Scheirs and W. Kaminsky, Editors. 2006, John Wiley & Sons,
Ltd: Brussels, Belgium. p. 3-41
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Websites :
1. http://www.coveyconsulting.com.au/Documents/paper_gc_plastic_waste_to_liquid_fue
l.pdf
2. http://www.hindawi.com/isrn/renewable.energy/2013/902053/
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4. http://www.plastic-pyrolysis.com/plastic-tyre-pyrolysis/
5. http://cynarplc.com/cynar_technology.asp
6. http://hdl.handle.net/10092/4303