The plastic recycling industry handles a high proportion of plastic waste by mechanical recycling. Some plastics however, cannot be recycled by this method, and remain a problem in waste management. Systems have been developed that convert mixed non-recyclable plastics to fuel oil using scalable technology.
Plastics are created primarily from energy feedstocks, typically natural gas or oil, or coal in the case of South Africa. The hydrocarbons that make up plastics are embodied in the material itself, essentially making plastics a form of stored energy, which can be turned into a liquid fuel source.
South Africa produces about 1,5-million t of new plastic polymer every year. Much of the plastic is discarded after use and ends up in landfills or is discarded indiscriminately. Approximately 350 000 t is recycled. This represents the portion that is diverted from landfills .
The advantage of plastic as a material for containers has led to it being used extensively for many things. The problem however, is that unlike glass or paper products, plastic containers and plastic wrapping materials cannot be re-used and are generally discarded as garbage. Sorting-at-site systems have helped recycling programmes tremendously, but some plastics collected through recovery systems cannot be recycled for various reasons.
The main reasons include:
While the weighted average life (time to disposal) of all plastic is eight years, more than 40% of plastics have a life-span of less than one month, meaning that significant volumes of waste are generated annually. Post-consumer plastics make up a major portion of municipal solid waste and appear in waste streams from agriculture, distribution and packaging, construction and demolition, automotive, electrical and electronic applications. The fraction of plastic in municipal solid waste comprises 60% polyolefins (POs) such as high density polyethylene, low density polyethylene and polypropylene, which is desirable from a PTL standpoint since they are the most suitable candidates for quality plastic-to-liquid fuel production .
Plastic’s durability is also its worst enemy, as plastic in landfills can take several hundred years to degrade. One of the characteristics is that all plastics can theoretically be recycled, either into other materials, such as polyethylene terephthalate (PET) bottles which end up as pillow stuffing, or others which are returned to the resin state for re-use as other plastics.
Plastic which cannot be recycled, is generally incinerated in specially designed furnaces, together with other waste. Instead of burning it, plastic can be converted to fuel oils by a process known as pyrolysis, and a number of systems are available on the market for achieving this. Sizes range from 100 kg/day capacity to large units of 10 000 t/annum.
The benchmark for conversion is 1 l of fuel for every kg of plastic input. Most systems work way below this benchmark but some achieve close to 85% of this target.
A typical example would be the plastic tops of PET bottles. These are made of high density polyethylene (HDPE), and cannot be recycled with the PET material, and are usually discarded by the plastic collectors. The bottle cap contains 30% of the total material in the bottle.
Feedstock recycling and pyrolysis
Feedstock recycling is a process that attempts to recover the original feedstock used to manufacture the recycled product, rather than converting the product into another form. Feedstock recycling is a form of tertiary recycling and encompasses a number of thermal or chemical processes that recover fuels or raw chemicals from plastic waste.
|Polyvinyl chloride||18,0 MJ/kg|
|Liquefied petroleum gas||46,1 MJ/kg|
|Light fuel oil||41,9 MJ/kg|
Pyrolysis is a recycling technique which converts plastic waste into fuels, monomers, or other valuable materials by thermal and catalytic cracking processes. It allows the treatment of mixed, unwashed plastic waste. For many years research has been carried out on thermally converting waste plastic into useful hydrocarbons liquids such as crude oil and diesel fuel.
Recently, the technology has matured to the point where commercial plants are now available. Pyrolysis recycling of mixed waste plastics into generator and transportation fuels is seen as the answer for recovering value from unwashed, mixed plastics and achieving their desired diversion from landfill [2, 3].
Generally speaking, the intensity of pre-treatment required of the feedstock lies somewhere between that for mechanical recycling (intensive) and incineration (non-intensive). It should be noted that the liquid energy-carrying medium that it produces is easy to store and so can be used on demand, and has a higher economic value than, for example, electricity produced via energy recovery from waste. Furthermore, pyrolysis can be employed alongside mechanical recycling and incineration in a cascaded waste management infrastructure so should be viewed as a component of a waste management system rather than a competing technology .
Types of plastic
Plastics are classified into seven groups and products are identified by a number marked on product. Industry (SPI) defined a resin identification code system that divides plastics into the following seven groups based on the chemical structure and applications.
HDPE, LDPE, PP and PS are all hydrocarbons consisting entirely of carbon and hydrogen, which are similar to hydrocarbon fuels such as liquefied petroleum gas (LPG), petrol and diesel. Plastics are derived from petroleum and have calorific values in a similar range as those of LPG, petrol and diesel as given in Table 2 .
Not all plastics are suitable for pyrolytic conversion. The main branch of convertible plastics are known as polyolefins and comprise the following:
PET and PVC are not considered suitable for this process because of low yields and acid production. PET is normally mechanically recycled and is seen as a valuable feedstock for other processes. In 2015 some 170 000 t of polyolefin was handled by the recycling business in South Africa. This represents a potential 150 000 l of diesel fuel .
Fig. 2 shows the layout of a typical plastics-to-fuel (PTF) plant. The process varies with the size of the plant but all systems fundamentally incorporate the following stages:
Installations in South Africa
A number of systems are in operation in South Africa, and more are expected to be installed in future.
Kraaifontein waste management facility (KIWMF) in Cape Town
The City of Cape Town, in partnership with the Japan International Cooperation Agency (JICA), is operating a pilot plastics-to-oil conversion plant, a six-month pilot project that will provide invaluable insights into the potential for creating fuel from plastic waste diverted from landfill sites. The plant, taken into operation in February 2016, converts up to 500 kg of plastics into 500 l of cracked oil per day. The commercial plant has the capacity to convert a maximum of 8000 kg of plastics per day.
After harvesting the three types of plastic (polyethylene, polypropylene and polystyrene) from the stream processed at the KIWMF , these materials (which come in the form of all manner of plastic packaging) are brought to the processing plant where they are then washed, shredded, heated and converted to oil.
The yield of 500 kg of plastic materials per day works out to approximately 500 of fuel. These yields are being assessed to determine the quality and quantity of fuel being produced in different combinations and ratios of the three types of plastic. Ultimately, the aim is to test the best combinations to yield the highest quality.
Approximately 70% of fuel produced by the pilot plant is channeled back into the running of the plant, powering the 150 kW generator on site. The rest could be used to power any other machinery that runs on diesel if the oil is of a good enough quality.
Small PTF plant
A number of small PTF systems have been developed locally and there are several small units in operation in rural areas of South Africa. Units vary in size from a single batch feed capable of producing /100 l day to units that run continuously for eight days producing 2000 l.
 Plastics SA: “Plastics SA releases 2015 plastics recycling figures”, 30 May 2016, www.plasticsinfo.co.za/wp-content/uploads/2016/05/Plastics-recycling-figures-2015-1.pdf
 J Scheirs: “Feedstock recycling and pyrolysis of waste plastics: Converting Waste Plastics into Diesel and Other Fuels”, 2006, John Wiley & Sons.
 4R Sustainability: “Conversion technology: A complement to plastic recycling”, www.scribd.com/document/79709681/Conversion-Technology-A-Complement-to-Plastic-Recycling-Apr-11
 G Devlin: “Waste Polyolefins to Liquid Fuels via Pyrolysis: Review of Commercial State-of-the-Art and Recent Laboratory Research”, Waste and Biomass Valorization, August 2011.
 F Gao: “Pyrolysis of Waste Plastics into Fuels”, PHD thesis: University of Canterbury 2010.
 CFP group: “Waste Plastic-to-Oil Conversion Plant, Cape Town, South Africa”, www.cfp-eco.com/e/group/waste_div_capetown.php
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