08-07-2014, 02:06 PM
CONVERTING WASTE
PLASTICS INTO
A RESOURCE
CONVERTING WASTE.pdf (Size: 1.36 MB / Downloads: 14)
Introduction
1.1 Overview
This compendium of technologies aims to present an overview of the technologies available
for converting waste plastics into a resource. It emphasizes the typical methods for converting
waste plastics into solid, liquid and gaseous fuels as well as the direct combustion of waste plastics
for specific applications.
The compendium is divided into two parts: Part 1 focuses on the technical aspects of the
technologies and Part 2 presents technologies examples.
Within Part 1, Chapter 2 displays the plastics which are suitable for each type of fuel and the
potential problems posed by contamination by undesirable materials. Chapters 3 through 5 depict
flowcharts of typical production systems for solid, liquid and gaseous fuels and Chapter 6 refers to
other technologies used in the steel, cement and lime manufacturers.
Part 2 presents examples of waste plastic applications in several industries.
1.2 Plastics
As a brief introduction to plastics, it can be said that plastics are synthetic organic materials
produced by polymerization. They are typically of high molecular mass, and may contain other
substances besides polymers to improve performance and/or reduce costs. These polymers can be
molded or extruded into desired shapes.
There are two main types of plastics: thermoplastics and thermosetting polymers.
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, among others.
Thermosets or thermosettings 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.
Target Waste Plastics
Waste plastics are one of the most promising resources for fuel production because of its high
heat of combustion and due to the increasing availability in local communities. Unlike paper and
wood, plastics do not absorb much moisture and the water content of plastics is far lower than the
water content of biomass such as crops and kitchen wastes.
The conversion methods of waste plastics into fuel depend on the types of plastics to be
targeted and the properties of other wastes that might be used in the process. Additionally the
effective conversion requires appropriate technologies to be selected according to local economic,
environmental, social and technical characteristics.
In general, the conversion of waste plastic into fuel requires feedstocks which are
non-hazardous and combustible. In particular each type of waste plastic conversion method has its
own suitable feedstock. The composition of the plastics used as feedstock may be very different and
some plastic articles might contain undesirable substances (e.g. additives such as flame-retardants
containing bromine and antimony compounds or plastics containing nitrogen, halogens, sulfur or
any other hazardous substances) which pose potential risks to humans and to the environment.
The types of plastics and their composition will condition the conversion process and will
determine the pretreatment requirements, the combustion temperature for the conversion and
therefore the energy consumption required, the fuel quality output, the flue gas composition (e.g.
formation of hazardous flue gases such as NOx and HCl), the fly ash and bottom ash composition,
and the potential of chemical corrosion of the equipment,
Therefore the major quality concerns when converting waste plastics into fuel resources are as
follows:
1) Smooth feeding to conversion equipment: Prior to their conversion into fuel resources, waste
plastics are subject to various methods of pretreatment to facilitate the smooth and efficient
treatment during the subsequent conversion process. Depending on their structures (e.g. rigid,
films, sheets or expanded (foamed) material) the pretreatment equipment used for each type of
plastic (crushing or shredding) is often different.
2) Effective conversion into fuel products: In solid fuel production, thermoplastics act as
binders which form pellets or briquettes by melting and adhering to other non-melting
substances such as paper, wood and thermosetting plastics. Although wooden materials are
formed into pellets using a pelletizer, mixing plastics with wood or paper complicates the pellet
preparation process. Suitable heating is required to produce pellets from thermoplastics and
8
other combustible waste.
In liquid fuel production, thermoplastics containing liquid hydrocarbon can be used as
feedstock. The type of plastic being used determines the processing rate as well as the product
yield. Contamination by undesirable substances and the presence of moisture increases energy
consumption and promotes the formation of byproducts in the fuel production process.
3) Well-controlled combustion and clean flue gas in fuel user facilities: It is important to
match the fuel type and its quality to the burner in order to improve heat recovery efficiency.
Contamination by nitrogen, chlorine, and inorganic species, for instance, can affect the flue
gas composition and the amount of ash produced. When using fuel prepared from waste
plastics, it must be assured that the flue gas composition complies with local air pollution
regulations. In the same way, ash quality must also be in compliance with local regulations
when disposed at the landfill. If there aren’t any relevant regulations, both the producers and
consumers of the recycled fuel should control the fuel quality and the emissions at combustion
in order to minimize their environmental impact.
Table 2.1 classifies various plastics according to the types of fuel they can produce. It can be
observed that thermoplastics consisting of carbon and hydrogen are the most important feedstock
for fuel production either in solid or liquid form.
As shown in Table 2.2, PE, PP and PS thermosplastics are preferable as feedstock in the
production of liquid hydrocarbons. The addition of thermosetting plastics, wood, and paper to the
feedstock leads to the formation of carbonous substances and lowers the rate and yield of liquid
products.
Scope of liquid fuel in this compendium
Liquid fuel within this compendium is defined as plastic-derived liquid hydrocarbons at a
normal temperature and pressure. Only several types of thermoplastics undergo thermal
decomposition to yield liquid hydrocarbons used as liquid fuel. PE, PP, and PS, are preferred for the
feedstock of the production of liquid hydrocarbons. The addition of thermosetting plastics, wood,
and paper to feedstock leads to the formation of carbonous substance. It lowers the rate and yields
of liquid products.
Depending on the components of the waste plastic being used as feedstock for fuel production,
the resulting liquid fuel may contain other contaminants such as amines, alcohols, waxy
hydrocarbons and some inorganic substances. Contamination of nitrogen, sulfur and halogens gives
flu gas pollution. Unexpected contamination and high water contents may lower the product yields
and shorten the lifetime of a reactor for pyrolysis
Liquid fuel users require petroleum substitutes such as gasoline, diesel fuel and heavy oil. In
these fuels, various additives are often mixed with the liquid hydrocarbons to improve the burner or
the engine performance. The fuel properties such as viscosity and ash content should conform to the
specifications of the fuel user’s burners or engines. No additives would be needed for fuel used in a
boiler. A JIS technical specification was proposed for pyrolytic oil generated from waste plastic for
use as boiler and diesel generator fuel (TS Z 0025:2004).
Skillful operators and a well-equipped facility are required due to the formation of highly
flammable liquids and gases.
4.2 Production method
The production method for the conversion of plastics to liquid fuel is based on the pyrolysis of
the plastics and the condensation of the resulting hydrocarbons. Pyrolysis refers to the thermal
decomposition of the matter under an inert gas like nitrogen.
For the production process of liquid fuel, the plastics that are suitable for the conversion are
introduced into a reactor where they will decompose at 450 to 550 C. Depending on the pyrolysis
conditions and the type of plastic used, carbonous matter gradually develops as a deposit on the
inner surface of the reactor. After pyrolysis, this deposit should be removed from the reactor in
order to maintain the heat conduction efficiency of the reactor.
The resulting oil (mixture of liquid hydrocarbons) is continuously distilled once the waste
16
plastics inside the reactor are decomposed enough to evaporate upon reaching the reaction
temperature. The evaporated oil is further cracked with a catalyst. The boiling point of the produced
oil is controlled by the operation conditions of the reactor, the cracker and the condenser. In some
cases, distillation equipment is installed to perform fractional distillation to meet the user’s
requirements.
After the resulting hydrocarbons are distilled from the reactor, some hydrocarbons with high
boiling points such as diesel, kerosene and gasoline are condensed in a water-cooled condenser. The
liquid hydrocarbons are then collected in a storage tank through a receiver tank. Gaseous
hydrocarbons such as methane, ethane, propylene and butanes cannot be condensed and are
therefore incinerated in a flare stack. This flare stack is required when the volume of the exhaust
gas emitted from the reactor is expected to be large.
Technical description
Figure 8.4 shows a flow diagram of the fuel production plant at Mogami-Kiko Co., Ltd. in
Yamagata Prefecture. Two tank reactors are installed. Upon primary thermal treatment of waste
plastics containing PVC at 400 C, hydrogen chloride is released into a combustor followed by an
absorber. After the primary treatment, the reactor temperature is raised to 500 C and thermal
decomposition is performed for 12 to 15 hours. After cooling, the decomposition residue is removed
with a vacuum cleaner.
Annexure 1: List of Technology Providers
A list of plant manufacturers and consultants is provided below. Note that some companies
may have limited ability to communicate with overseas customers without the assistance of
experienced trading companies and may have changed their products from those currently
advertised on the internet or ceased to sell waste plastic conversion technologies for business
reasons.
Some companies supply smaller equipment for thermal decomposition of waste plastics with a
capacity of less than 100kg/h. However, waste plastic conversion using these micro-reactors
requires special business and economic feasibility considerations.