Mismanaged plastic waste is a growing problem that pollutes and contaminates vast parts of the natural environment and threatens the health of the entire global ecosystem[1] [2]. After nearly 50 years of different campaigns, regulations, incentives and compliance regimes, the rate of recycling of plastic waste is still less than 10% globally.
We propose adapting electrified gasification to use pelletized plastic waste, and use the syngas in an oxyfuel CHP system to achieve ultra-low emissions plastic waste elimination with energy recovery. But first, we provide more background, context and motivation.
Look to Scandinavia
Some regions and nations are making progress by building specialized facilities to automatically sort plastic waste into the main types of polymers in the plastic items. In Sweden, Denmark and Norway, such facilities can sort up to 400000 tons of plastic waste yearly into 7-11 different types by polymer. This allows effective mechanical recycling which aims to create single polymer pellets as close to new quality as possible, which can then be the raw material for new plastic products and packaging.
However, the technical limits of mechanical recycling plastic result in direct losses of material and material degradation. The process of mechanical recycling begins with shredding the items into millimetre-sized bits, which unavoidably creates dust-sized particles which are lost in the process. The evidence from rigorous study of this is limited but indicates that 6-13% of the plastic waste input to a mechanical recycling facility cannot be recycled due to its being blown out as dust in the facility’s ventilation system or otherwise washed out with wastewater from the facility’s cleaning of the shredded plastic.
Mechanically shredding and re-melting plastic also degrades its physical properties, such that it can no longer function as specified after only 2-3 uses/recycling. After this it can be ‘downcycled into low-quality items, which have less strength and can be easily cracked. The item then becomes unrecyclable waste.
The theoretical limit to mechanical recycling rates can therefore never exceed about 70%.
Only one example has been found of achieving this highest possible rate. In Norway, PET bottles used for water and soft drinks are returned into specialised bottle-return automats at a rate of over 95% due to the deposit value in the bottles. These are then sent to a closed system (no other plastic packaging types are included) to a separate mechanical recycling facility for PET bottles only. The operator of this system reports about 69% overall recycling rate.
The conclusion is that a large fraction of plastic will not be recyclable even after all possible mechanical recycling is implemented. The main enabler of maximizing mechanical recycling is centralized collection of sorted plastic waste, such that it can be further sorted into polymer types suitable for high-quality mechanical recycling.
It is at the centralized plastic waste sorting facility that the first fraction of non-recyclable plastic is sorted out and becomes an identified waste stream entailing costs for disposal.
The seriously insufficient methods we use today
Landfilling is currently the main method used to dispose of non-recyclable plastic waste, but it is broadly recognized that this is not a long-term solution. The drawbacks of landfilling are destruction of land, transport costs and pollution from the transport to the landfill and leaching of toxic liquids from the landfilled waste into the ground and surface waters. In the case of plastic waste, landfilling constitutes a kind of ‘reverse mining’ in which the imbedded energy in the largely petroleum-based plastic is re-buried, forfeiting the energy value of the plastic produced by the intense process of producing the petroleum to make the plastic in the first place.
Incineration with energy recovery is practiced by the cement industry, municipal waste management industry, district heating and more. It is the second most common means of disposal of plastic waste and is also widely used. But incinerating plastic waste produces carbon dioxide, which contributes to global warming. Increasing incineration of plastic waste will ameliorate the pollution from mismanaged waste but exacerbate global warming
Therefore, there is scope for cost and performance improvements in incinerating plastic waste to eliminate carbon dioxide emissions while maximizing energy conversion. The current standard solution is to retrofit carbon dioxide capture to existing moving-grate mixed municipal waste incinerators. This is a costly solution which reduces energy output of the process significantly. It also requires a steady stream of chemicals to facilitate the carbon dioxide capture process.
Furthermore, there are upper limits to using plastic waste as a feedstock in moving-grate incinerators. The limiting factor is maximum operating temperature and corrosion rates in the moving grate incinerator. Plastic has about the same energy content as crude oil, and using too much plastic creates operating temperatures high enough to cause rapid corrosion degradation of the incinerator. This shortens the technical life of the incinerator in an unacceptable way.
Now we present our improved best-practice technology for plastic waste elimination
Our invention solves these problems in a novel, new configuration that allows 100% plastic waste as the fuel and produces energy without carbon dioxide emissions. It achieves this through the process integration three subsystems that together form an energy and material flow symbiosis that is greater than the case of these being separate subsystems.
The solid fuel is the near-pure plastic waste from centralized plastic waste sorting facilities.
Fuel gas is produced by gasification of shredded plastic waste injected into a device heated by a direct current plasma generator. The gasifying agent used is carbon dioxide, which is injected into the gasification device. This allows the system to operate without air, which if used would introduce unwanted nitrogen into the system. By avoiding nitrogen into the system, our invention enables lowest-cost carbon dioxide removal downstream of the gasification unit. It also avoids the need for a separate source of pure oxygen for the gasification process.
The ratio of injected carbon dioxide and plastic waste injected into the gasification unit is carefully controlled to achieve optimal conversion of the injected plastic waste into an energy-rich, combustible gaseous mixture called syngas, free of nitrogen, which is then treated to be compatible for injection into an oxyfuel combustion device.
Furthermore, there are no hydrocarbon fuels directly involved in heating the gasification unit because the plasma generators are powered by electricity. Thus, the system can be fully powered by electricity made from solar power, wind power, hydroelectric power, geothermal power, nuclear power or other low-emissions energy technologies.
The syngas from the gassified plastic waste feedstock is then ready for conversion into energy. This is achieved with near-zero carbon dioxide emissions by using the syngas from the gasification unit in a process called oxyfuel combustion. The key input in addition to a combustible gas for oxyfuel combustion is pure oxygen from water electrolysis. There is a global growth of water electrolysis buildout to produce green hydrogen, which produces eight times more pure hydrogen by mass. The vast majority of the known electrolysis projects do not report using any significant quantity of the biproduct pure oxygen for any commercial purposes. By using this stranded byproduct oxygen from water electrolysis, the need for a dedicated unit to separate oxygen from air is avoided, saving significant capital expenditures.
The flue gas from oxyfuel combustion consists of water vapour and carbon dioxide. Capturing of the carbon dioxide is achieved by cooling the flue gas in a condensing unit, converting the water vapor to flowing water, leaving carbon dioxide gas to be transferred from the condenser for compression and transport. This carbon dioxide capture process uses no chemicals and is the lowest-cost, simplest CO2 separation process possible. The water condensed from the flue gas can then be used by the electrolyser, reducing the net usage of imported water by up to 50%.
Synergies from integrating the gasifier, electrolyser and oxyfuel combustor are achieved by maximizing heat recovery by engineered heat exchangers, by using a fraction of the captured carbon dioxide for re-injection into the direct current plasma gasification unit for use as the gasifying agent, re-use of the condensed water from the oxyfuel combustion unit flue gas condenser as feed for the water electrolyser and use of the stranded biproduct pure oxygen from the water electrolyser.
We have analyzed our concept in more detail. A copy of our report, which includes a fit-for-purpose Computional Fluid Dynamics simulation of the plasma torch, can be sent upon request (use the 'Contact Us' form).
[1] https://www.newyorker.com/magazine/2023/07/03/book-reviews-plastic-waste
[2] For a quick summary of the latest peer-reviewed research on plastic pollution, see this link https://threadreaderapp.com/thread/1767619382547456207.html?utm_campaign=topunroll
