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INTRODUCTION
Plastics are polymeric materials, a material built up from long repeating chains of
molecules. Polymers such as rubber occur naturally, but it wasn't until the development of
synthetic polymers around 1910 that the polymers tailored to the needs of the engineer
first started to appear. One of the first commercial plastics developed was Bakelite and
was used for the casing of early radios. Because the early plastics were not completely
chemically stable, they gained a reputation for being cheap and unreliable. However,
advances in plastic technology since then, mean that plastics are a very important and
reliable class of materials for product design.
Plastic is a marvel of polymer chemistry, plastics have become an indispensable
part of our daily life. But repeated reprocessing of plastic waste, and its disposal cause
environmental problems, pose health hazards, in addition to being a public nuisance. The
biggest current threat to the conventional plastics industry is likely to be environmental
concerns, including the release of toxic pollutants, greenhouse gas and non-biodegrable
landfill impact as a result of the production and disposal of petroleum and petroleum
based plastics.
Various methodologies have been tried and tested to process waste plastics for
many years, with recycling becoming the most common method reflecting today’s
environmental requirements. Liquefaction of plastic is a superior method of reusing this
resource. The distillate product is an excellent fuel and makes Thermo-Fuel one of the
best, economically feasible and environmentally sensitive recycling systems in the world
today. Thermo-Fuel diesels can be used in any standard diesel engine, trucks, buses,
trains, boats, heavy equipment and generators.
India has been used as a dumping ground for plastic waste, mostly from
industrialized countries like Canada, Denmark, Germany, UK, Netherlands, Japan, France
and the United States.
Each year more than 100 million tonnes of plastic are produced worldwide.
Though plastics have opened the way for a plethora of new inventions and devices it has
also ended up clogging the drains and becoming a health hazard. The plastic waste
accounts to about 10 thousand tons per day in India [1]. At these alarming levels of waste
generation, India needs to set up facilities for recycling and disposing the waste.
Plastics
The term "plastic" covers a wide range of synthetic polymer materials. What they
have in common is that they are all made by joining together or "polymerizing" a bunch
of molecules (monomers). There are two main families of plastics, thermosetting and
thermoplastics.
Thermoplastics can repeatedly soften and melt if enough heat is applied and
hardened on cooling, so that they can be made into new plastics products.
Examples are Polyethylene, Polystyrene and Polyvinyl Chloride.
Thermo-sets or thermo-settings can melt and take shape only once. They are not
suitable for repeated heat treatments; therefore after they have solidified, they stay
solid.
Examples are phenol formaldehyde and urea formaldehyde.
CONVERTING WASTE PLASTIC INTO LIQUID FUEL
Many may not realize throwing away plastic is throwing away a ready fuel source.
Plastic is primarily petroleum and burns with high efficiency. Plastics are commonly
made from fossil fuels which is usually an irreversible process, process have been
developed which recycles plastic waste back into oil.
Thermo-Fuel technology is used to convert Waste Plastic into Liquid Fuel.
Thermo-fuel is a ten year old commercially proven technology with nine operational
plants in Japan. Thermo-fuel is a process where scrap and waste plastics are converted
into synthetic fuel. The system uses liquefaction, pyrolysis and the catalytic breakdown of
plastics. The system can handle almost all the plastic that is currently being sent to
landfills. A major advantage of the process is its ability to handle unsorted, unwashed
plastic and its extremely high efficiency. A Thermo-Fuel plant can produce up to 9,500
liter of high-grade synthetic fuel from 10 tonnes of waste plastics, with systems ranging
from 10 to 20 tonnes per day. This means that heavily contaminated plastics can be
processed without difficulty.
2.1 Structure of the system
The system consists of stock in feed system, pyrolysis gasification chamber,
catalytic converter, condensers, centrifuge, oil recovery line, off-gas cleaning, and
adulterant removal. Fig 2.1 shows below the structure of the system.
Waste plastics are loaded via a hot-melt in feed system directly into main
Pyrolysis chamber. When the chamber temperature is raised, agitation commences to
even the temperature and homogenize the feed stocks. Pyrolysis then commences to the
point of product gasification. Non-plastic materials fall to the bottom of the chamber. The
gas goes through the (patented) catalytic converter and is converted into the distillate
fractions by the catalytic cracking process [2]. The distillate then passes into the recovery
tank after cooling in the condensers. From the recovery tank, the product is sent to a
centrifuge to remove contaminants such as water or carbon. The cleaned distillate is then
pumped to the reserve tank, then to the storage tanks.
Pre-Treatment
Pre-treatment, depending on the form of delivery of plastics to the plant, may
include shredding and granulating. The system accepts granulated to a flake size of 2.5
cm or less in order for it to be conveyed and metered uniformly via a melt in feed system
into the chamber. However, Thermo-fuel can process most sizes and types of plastics
with suitable pre-treatment equipment. Fig.3.1 presents a schematic diagram of a liquid
fuel production plant.
3.2 Pyrolysis
The heart of the pyrolysis system is the prime chamber, which performs the
essential functions of homogenization, controlled decomposition and out gassing in a
single process. The process requires minimal maintenance apart from carbon residue
removal, and produces consistent quality distillate from mixed and low-grade plastic
waste. The key to an efficient pyrolysis process is to ensure the plastic is heated
uniformly and rapidly. If temperature gradients develop in the molten plastic mass then
different degrees of cracking will occur and products with a wide distribution of chain
lengths will be formed.
Pyrolysis is a process of thermal degradation in the absence of oxygen. Plastic
waste is continuously treated in a cylindrical chamber and the pyrolytic gases condensed
in a specially-designed condenser system to yield a hydrocarbon distillate comprising
straight and branched chain aliphatic, cyclic aliphatic and aromatic hydrocarbons [3]. The
resulting mixture is essentially equivalent to petroleum distillate. The plastic is pyrolised
at 370ºC-420ºC and the pyrolysis gases are condensed in a 2-stage condenser to give a
low sulphur content distillate.
3.2.1 The essential steps in the Pyrolysis of plastics involve
Evenly heating the plastic to a narrow temperature range without excessive
temperature variations.
Purging oxygen from Pyrolysis chamber.
Managing the carbonaceous char by-product before it acts as a thermal insulator
and lowers the heat transfer to the plastic.
Careful condensation and fractionation of the pyrolysis vapours to produce
distillate of good quality and consistency.
Pyrolysis is used as a form of thermal treatment to reduce waste volumes and produce
liquid or gaseous fuels as a byproduct. There is also the possibility of using pyrolysis
systems integrated with other processes such as mechanical biological treatment and
anaerobic digestion. The agricultural waste is pyrolised at a temperature of 450 to 550c
3.2.2 There are 3 pyrolysis products
A combustible gas that is burned to generate the heat required for the endothermic
pyrolysis reaction. No extra heat or fuel source is required.
A liquid bio-oil that can be used as a fuel. The bio-oil cannot be used directly in
car engines. It is converted to a synthetic gas from which clean fuels and
petrochemicals can be synthesized, using well-established technologies.
A solid char that can either be burned for energy or recycled as a fertilizer
The catalytic breakdown
The core technology of the Thermo-fuel process is the catalytic reaction tower (or
catalytic converter) on the exit side of the main pyrolysis chamber. The catalyst is
important because it lowers the amount of energy that is required to break down the
structure of the waste plastics. As well as promoting the initial cracking of the polymers,
the catalysts are used to promote the production of a heavier fuel suitable for the
manufacture of diesel and gasoline. Only a small amount of catalyst material is lost
during the conversion process.
The catalytic reaction tower contains a system of plates made from a special
catalytic metal alloy. Thermo-fuel requires no additives or consumable catalytic
consumables. The metal plates do get fouled with a tar-like residue and terephthalic acid
and therefore the reaction tower needs to be stripped down periodically and the plates
polished, generally every 6 months to once a year. The maintenance service can be
quickly performed (approx. 1 hour) with minimal plant down time, using spare catalytic
plates. The catalyst chamber is heated using the exhaust gases from the furnace of the
pyrolysis chamber.
The gas from the pyrolysis chamber is feed to the catalytic converter and is
converted into the distillate fractions by the catalytic cracking process. The metal catalyst
’cracks’ paraffinic chains longer than C25 and ’reforms’ chains shorter thanC6. The
catalyst ensures that the final fuel has a carbon chain distribution in the range C8-C25 and
peaking at C16 (Cetane), which is very similar to standard fuels [4]. The liquid distillate
then passes into the operating tank after cooling in the condensers. From the operating
tank, the product is sent to a centrifuge to remove contaminants such as water or carbon.
3.4 Collection of liquid fuel
As the plastics are reduced, the gases are collected and cooled, yielding liquid
fuel. This liquid fuel or crude oil is a complex mixture that has to be separated in a
fraction chamber to form gasoline and diesel. The remaining incondensable gases pass
through the top of the fraction chamber and are either burnt off in a flare stack or fed back
to the initial stage of the process where they are used as an additional fuel to heat the
incoming plastic materials.
CHARACTERISTICS OF THERMO-FUEL
4.1 Plastics suitable for treatment
Thermo-Fuel process can be done on waste plastics such as
1. Plastic packaging scrap from material recovery/sorting facilities
2. Oil and detergent bottles
3. Mixed post-consumer plastics,
4. Caps/Labels/Rejected bottles from bottle recycling operations,
5. Commercial stretch and shrink wrap.
The table below shows the different types of plastic and there suitability for
thermo-fuel system
Pre-treatment
Input feedstock plastics do not require washing or sorting. The plastics can be
shredded or granulated prior to being fed through a melt-in feed system into the chamber
so almost any shape or size of waste plastics can be handled. The system is designed to
cope with these foreign materials up to approximately 10% by weight or volume. So no
pre-treatment is needed
4.3 Maintenance
Coking occurs in the chamber when the pyrolysis of the waste plastics is almost
complete. However, Thermo Fuel is designed to minimize coking by stabilizing at
conductivity within the pyrolysis chamber. The pyrolysis chamber requires cleaning
every second process, and takes just 30 minutes. The catalytic reaction tower needs to be
stripped down periodically and the plates polished, generally every 6months to once a
year. The maintenance service can be quickly performed (approx. 1hour) with minimal
plant down time [5].
4.4 Pollution
Thermo-Fuel produces extremely low level of emissions, due to the capture of
almost all of the output, both liquids and gases, inside the system. Pyrolysis of plastics
tends to occur on irregular basis, hence the carbon chain lengths of the pyrolytic gases
vary between 1-25. Most of the gas is liquefied in the condensers but some remains as
gas. This high calorific gas contains methane, ethane, propane, butane, etc. This gas is
reused to heat the Pyrolysis chamber.
BYPRODUCTS
5.1 The char stream
A carbonaceous char is formed in the chamber during pyrolysis. The char residue
produced is generally proportional to the level of contaminants which are adhering to the
feed stocks. Since the char passes acid leaching tests it can simply be land filled.
Inorganic additives such as cadmium pigments from the plastics end up in the char
stream.
5.2 Off gas
Pyrolysis of plastics tends to occur on an irregular basis. Hence, the carbon chain
lengths of the pyrolytic gases vary from 1 to 25. Most of the gas is liquefied in the
condensers but some gas remains uncondensed. Hydrocarbons with carbon count of 4 and
lower remain as a gas under room temperature. This off-gas contains methane, ethane,
propane, butane, etc. Although volume of the gas differentiates, depending upon the types
of the plastics, it is generally no more than 2 to 5% [6].The incondensable gases pass
through the top of the fraction chamber and are either burnt off in a flare stack or fed back
to the initial stage of the process where they are used as an additional fuel to heat the
incoming plastic materials.
5.3 Output fuel
The typical mass balance for one tonne of mixed polyolefin plastic entering the
process is approximately 90% hydrocarbon distillate, 5% char, as well as 5%gaseous
material known as non-condensable gases. The non-condensable gas from the ThermoFuel
plant is passed through a water scrubber and then fed into the natural gas flow for
the burner, which heats the unit so there are no net hydrocarbon emissions.
The hydrocarbon fraction in turn comprises approximately 75% distillate cut and
25% paraffin material. The paraffin fraction is continuously cracked after the first
condenser until it reaches the desired chain-length range and then added to the primary
fuel stream.
The table.6.1 shows the different fuel types and there percentage conversion of
fuel per unit volume.
5.4 Comparison
A comparison of the distillate produced from a commingled plastic mix compared
with regular synthetic fuel has been conducted by gas chromatography, and shows good
similarity between fuels. A key indicator of diesel is the Cetane Number which is
analogous to the octane rating for petrol. Cetane is a measure of the ignition delay, that is,
the time between injection into the cylinder and the moment of auto ignition. This is most
significant in relation to low-temperature start ability, warm-up, and smooth, balanced
combustion.
Distillates with a higher Cetane rating show increased power and superior
performance characteristics. Ideal diesel will have a high proportion of hydrocarbon
chains that are 16 carbon atoms. Thermo Fuel-produced diesel has a Cetane number in the
range of 57, similar to or higher conventional diesel, which averages 51-54. Most engine
manufacturers recommend diesel fuels with a Cetane number of at least 50 [7].
5.5 Applications
The distillate is designed to operate in a diesel engine where it is injected into the
compressed, high-temperature air in the combustion chamber and ignites spontaneously.
Thermo Fuel is perfectly suited to any standard application.
5.6 Lubricity
Thermo Fuel is extremely high in lubricity. In diesel engines some components
like fuel pumps and injectors are lubricated by the fuel, so good lubricity is a key element
in reducing wear on these parts.
CONVERTING WASTE PLASTIC INTO FUEL IN INDIA
A plant, Unique Plastic Waste Management & Research Co Pvt Ltd, was setup at
the industrial estate in Nagpur .Industrial units in the area are running their captive power
plants on this fuel and are happy with its pricing and performance. The fuel is priced at
Rs30 per liters.
The fuel scores over petrol/diesel because it ignites faster. Besides, several test.
Reports by government and non-government institutions say it has smaller sulphur
content and low reaction temperature. Above all, the Maharashtra Pollution Control
Board has found that the conversion of plastic waste into fuel is non-polluting.
Engine output is nearly as much as produced by other fuels. A test drive on a
Kinetic Honda gave a mileage of 44 km/liter on plastic fuel as compared to 44.4km/liter
petrol. It accelerated from 0 to 60 km in 18 seconds against 22.5 seconds on petrol.
Today a 25 MT plant supplies fuel to neighboring industrial units. Their ultimate
aim is to take the capacity up to 450 MT [8]. Most of their raw material comes from the
plastic waste dumped in their premises by the factories in the area.
CONCLUSION
Thermo-Fuel is a truly sustainable waste solution, diverting plastic waste from
landfills, utilizing the embodied energy content of plastics and producing a highly usable
commodity that is more environmentally friendly than any conventional distillate. The
Thermo fuel system converts these waste plastics into high-grade “green" distillate fuel.
The result of this process is claimed to be a virtually nonpolluting,(100%) synthetic fuel
that does not require engine modification for maximum efficiency. Post-consumer, postindustrial
unwashed and unsorted waste plastics are the feedstock for the Thermo-fuel
process, and with an expected production efficiency of over 93%, the resultant diesel
output would almost equal the waste material input.