Waste plastic to fuel oil: an under-exploited opportunity for energy generation



 

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.

Fig. 1: Waste plastic delivered to landfills or recycling centres consists of a mixture of different types of plastic.

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 [1].

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:

  • Contamination: This applies particularly to plastics used for food packaging and wrapping. It can also apply to discarded plastics recovered from unseparated garbage at landfills and other dumps. Plastic sealing and wrapping material, including plastic bags, is highly susceptible to contamination either during or prior to recovery.
  • Transport and distance: Transport costs for lightweight plastics which offer low recovery rates inhibit recycling endeavours.

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 [4].

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.

Table 1: Plastics are classified into seven groups.

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.

Conversion potential

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.

  • Depolymerisation is a process that yields resins with the same quality as new feedstock, and is used to convert “clean” plastic waste.
  • Pyrolysis is a thermochemical conversion technology that can be considered a “feedstock recycling” process and may play an increasing role in integrated waste management systems of the future. The advantage of pyrolysis is that it can accommodate relatively contaminated feedstocks and value added materials can, if desired, be recovered prior to conversion to fuels, such as metals or hydrochloric acid.
Material Calorific value
Polyethylene 46,3 MJ/kg
Polypropylene 46,4 MJ/kg
Polystyrene 41,4 MJ/kg
Polyvinyl chloride 18,0 MJ/kg
Coal 24,3 MJ/kg
Liquefied petroleum gas 46,1 MJ/kg
Petrol 44,0 MJ/kg
Kerosene 43,4 MJ/kg
Diesel 43,0 MJ/kg
Light fuel oil 41,9 MJ/kg

Pyrolysis

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 [4].

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.

Table 2: Comparison of calorific value of plastics and different types of fuels [5].

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 [5].

Not all plastics are suitable for pyrolytic conversion. The main branch of convertible plastics are known as polyolefins and comprise the following:

  • High density polyethylene (HDPE)
  • Low density polyethylene (LDPE)
  • Polystyrene
  • Polypropylene (PP)

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 [1].

Pyrolysis process

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:

  • Feedstock processing: Depending on the plant size, the feedstock will be cleaned and shredded or crushed. In smaller plant, this may be done by hand, but in larger systems, mechanical shredding and crushing are used. Feedstock can be used without cleaning, but this increases the amount of char generated, and could affect the feed process for continuous systems.
  • Feedstock melt or extrusion: This only applies to larger systems using continuous operations. The shredded or crushed plastic is melted and extruded into shapes that can be handled by the continuous feed system of the plant. The melt cycle also serves to exclude oxygen from the pyroliser.
  • Load into the pyroliser: In smaller systems running on a batch process loading is done on a manual basis.
  • Pyrolisis: plastic is subjected to heat in an oxygen free environment, which depolymerises the plastic into a gaseous form. Contaminants and other items such as fillers are converted to char, or carbon black. The temperature of pyrolysis and the catalysts used will determine the output product. The plastic is pyrolysed at 370 to 420°C, and the pyrolysis gases are condensed in a two-stage condenser to produce a low-sulphur distillate. During start up, the pyrolysis furnaces are supplemented with either natural gas or liquefied petroleum gas (LPG), depending on availability. After the process has stabilised, the syngas produced is used to heat the pyroliser.
  • The vapour is converted into various fractions, including raw diesel, in the distillation column and the distillates then pass into the recovery tanks.
    Many of the larger plants use the fuel produced to drive electrical generators which run the recycling plant [6]. Most plants are energy positive i.e. produce more energy or electricity than the required to run the plant. Surplus electricity is exported to the grid.

Fig 3: Fuel produced in the Kraaifontein facility [6].

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 [6], 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.

Fig. 4 Rural plastics to fuel plant (Recor).

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.

References

[1] 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
[2] J Scheirs: “Feedstock recycling and pyrolysis of waste plastics: Converting Waste Plastics into Diesel and Other Fuels”, 2006, John Wiley & Sons.
[3] 4R Sustainability: “Conversion technology: A complement to plastic recycling”, www.scribd.com/document/79709681/Conversion-Technology-A-Complement-to-Plastic-Recycling-Apr-11
[4] 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.
[5] F Gao: “Pyrolysis of Waste Plastics into Fuels”, PHD thesis: University of Canterbury 2010.
[6] 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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