Plastics are synthetic polymers derived from petro-fossil feedstock and made-up of long chain hydrocarbons with additives
Two types of plastics
Thermoplastics(Recyclable)-become soft when heated, can be moulded or shaped with pressure when in plastic state
Thermosets (Non Recyclable)-Once set cannot be remoulded /softened by applying heat.
Environmental Issues
Plastic waste disposal on land makes it infertile
Burning generates toxic emissions -CO, HCl, Dioxin, Furans
Leaching out of toxic chemicals added as additives
Littering
Affects waste processing facilities like composting
In efficient usage of crude oil resources
Recycling of plastics
Primary Recycling: involves processing of a waste into a product with characteristics similar to those of original product.
Secondary (Mechanical)Recycling: processing of waste plastics into products that have characteristics dissimilar from those of original plastic products
Tertiary(Chemical) Recycling: extraction of chemicals from plastic wastes
Quaternary Recycling: recovery of energy from wastes
Sorting Of Plastic Wastes Density sorting
Hydrocyclones- uses centrifugal force, enhance material wettability.
Heavy medium separation -using tetrabromoethane
Triboelectric separation- sorts materials on the basis of a surface charge transfer phenomenon
Speed accelerators
Paint Removal
Grinding & Solvent stripping
Mechanical RecyclingCan only be performed on single-polymer plastic
Steps involved
Crushing/Shredding
Contaminant separation:
Floatation
Milling
Washing and Drying
Agglutination
Extrusion
Existing recycling processes & Issues
Selection of waste:
Toxic wastes are separated
Segregation of plastics waste: Segregation shall be done in accordance with IS 14535
Processing (extrusion process)
Washing generates waste water having high pollution load & needs treatment before proper disposal
Exposure of workers in reprocessing plant to toxic dyes and additives
CONVERTING WASTE INTO A RESOURCEquality concerns when converting waste plastics into fuel
resources are
Smooth feeding to conversion equipment
Effective conversion into fuel products
Well-controlled combustion and clean flue gas in fuel user facilities
SOLID FUEL PRODUCTIONTwo types of solid fuel
refuse derived fuel (RDF)
refuse-derived paper and plastic densified fuel (RPF)
o Thermoplastics act as a binder for the other components
o Contamination of plastics with other plastics containing halogens (Cl, Br, F), N, S and other hazardous substances may cause air and soil pollution
Production method
Involves two steps- pretreatment and pellet production
Two types of commercial production systems
large-scale model(3 ton/hour)with pretreatment for the separation of undesirable contamination
small-scale model (150 kg/hour) without pretreatment equipment
Heating values
RDF: 4000 – 5000 kcal/kg
RPF: 6000 – 8000 kcal/kg
Liquid Fuel Production
Based on the pyrolysis of the plastics and the condensation of the resulting hydrocarbons
Thermoplastics (PE, PP, and PS)are used for conversion.
Decomposition occur at 450 to 550 °C.
The products generated from include gasoline, diesel, benzene, naphtha, and fuel oil
The by-products from the process - sludge and gas are reused.
PVC plastics waste is not used as the chlorine can be converted into HCl as a by-product
Schematic diagram of a liquid fuel production plant (UNEP, 2009)
Gaseous Fuel Production Waste plastics undergo thermal decomposition
in a tank reactor for an extended time at a reaction temperature
Occurs at higher temperatures 800 -1000 °C
Two types of gaseous fuel are produced:
Gaseous hydrocarbon & Syngas
Calorific value of syngas ranges between the calorific value of biogas and LNG/LPG.
Observations Impact, Los Angels Abrasion & Crushing Value
increased with the increase in the percentage of plastics.
Better skid resistance
Unevenness index values were low (3000 mm/km)
indicating good surface evenness.
Better resistance towards water stagnation i
Co-processing of Plastic waste in Cement Kiln Plastic waste is used as an alternative fuel in cement Kilns Primary fuel and raw material are substituted by waste
Pre-processing of plastic waste
Plastic waste is sun dried and subjected to shredding before feeding into cement kilns
PVC containing plastic waste is not used as it impair the cement quality
Plasma Pyrolysis Disintegration of organic compounds into gases and
non leachable solid residues in an oxygen-starved environment
Uses ions and excited molecules with high energy radiation to decompose chemicals
High temperature decomposes waste material into simple molecules
Efficient way of treating chlorinated plastics
Dioxins and furans emissions are below the prescribed limits ie, the range of 0.005-0.009 ng/m3
Ideal for decentralized disposal of plastic waste
Reduction in volume of organic matter > 99%
Segregation of the waste is not necessary
DEGRADABLE PLASTICSTypes
Poly-Lactic Acid (PLA)
produced in a two-step fermentation and chemical polymerization process
Polyhydroxyalkanoate (PHA)
synthesized by bacteria as intracellular carbon and energy storage compounds
Poly olefins modified by adding transition metals/compounds
Oxo-biodegradable plastics
Moisture and heat attack PLA polymer chains, splitting them apart through hydrolysis, to lactic acid monomers which are consumed by microbes
PHA plastics are attacked by microorganisms that secrete PHA depolymerizer, and most products take three to nine months to degrade
Polyolefins are decomposed by UV or heat-initiated oxidation
Plastic degradation processes
Photo degradation: Degradation caused through the action of sunlight on the polymer
Biodegradation :Degradation that occurs through the action of microorganisms such as bacteria, yeast, fungi, and algae etc.
Biodeterioration: Degradation that occurs through the action of microorganisms such as beetles, slugs, etc.
Autooxidation :Degradation caused by chemical reactions with oxygen.
Hydrolysis: Degradation that occurs when water cleaves the backbone of a polymer, resulting in a decrease in molecular weight and a loss of physical properties
Solubilization: Dissolution of polymers that occurs when a water-soluble link is included in the polymer
Compatibility of Degradable Plastics with Current SWM Practices Composting
Require specific levels of moisture and oxygen & inadequate temperatures may not initiate the key hydrolysis reaction for PLAs
Recycling
The increase in input heterogeneity will reduce the quality of the recovered plastic
Waste-to-Energy Incineration
Less CO2 emissions
Landfilling
Decay and release of more methane , production of higher strength leachate & sequestration of carbon,
Drawbacks Higher costs
Difficulty in recycling
concerns regarding potential later degradation of the end products
Requirement of specific levels of moisture and oxygen for initial reactions to occur
References
Recycling and recovery routes of plastic solid waste (PSW): A review, S.M. Al- Salem , P.Lettieri, J. Baeyens
Environmental evaluation of plastic waste management scenarios: L. Rigamontia, M.Grossoa, J. Møllerb, V. Martinez Sanchezb,S. Magnania, T.H. Christensen
A Review of Plastic Waste Management Strategies:JaveriyaSiddiqui and Govind Pandey, International Research Journal of Environment Sciences
Development of process for disposal of plastic waste using plasma pyrolysis technology and option for energy recovery: M. Puncochara, B. Rujb, P. K. Chatterjee
Degradable Plastics and Solid Waste Management Systems: David J. Tonjes ,Krista L. Greene
IS 14534 1998 -GUIDELINES FOR RECYCLING OF PLASTICS
Preliminary Study On The Conversion Of Different Waste Plastics Into Fuel Oil : Yasabie Abatneh, Omprakash Sahu
Biodegradability of Plastics : Yutaka Tokiwa, Buenaventurada P. Calabia ,Charles U. Ugwu and Seiichi Aiba
Recycling of plastic: accounting of greenhouse gases and global warming contributions : Thomas Astrup, Thilde Fruergaard, Thomas H. Christensen