Extracts from text by Andrei Veksha and Grzegorz Lisak: Two scientists in Singapore who may have found a solution to our plastic waste problem.
Mountains of waste found all around the globe are a pressing issue for humanity.
The evolution of plastic revolutionised the modern world. However, there is a dark side to these advancements: Namely disposable plastics, also known as single-use plastics, which are an ever-growing global concern. Single-use plastics, such as plastic bags, food wrappers, and plastic bottles, contribute significantly to the global plastic consumption crisis and may continue to do so for a hundred more years.
Rapid plastic development follows an increasing carbon footprint. The sources of carbon footprint in the case of plastics are the fossil fuels used for production, energy-intensive processes during manufacturing, and carbon emissions during incineration of plastic waste. About over a century ago, fossil fuels were the only source to manufacture plastics. Currently, there are concurrent options to derive plastic from natural gas, oils, or even plants. A concerning rise in the emission of greenhouse gases due to the production of plastic has caused a huge ripple effect.
While the alarming levels of plastic pollution are a worldwide plight, the world has responded to the crisis with groundbreaking technology and global community campaigns. In the same vein, the scientists at Nanyang Technological University, Singapore
(NTU) decided to explore the thermal decomposition of plastic as an environmentally-friendly solution to find the global plastic crisis.
A Pressing Situation
From a waste management perspective, conventional practices fail to sustainably address plastic waste treatment. Tackling plastic waste relies heavily on landfills and incineration. Only small quantities of plastic are recycled using mechanical methods. Due to landfill mismanagement and illegal waste dumping, considerable amounts of plastics leak back into the environment, creating devastating and persistent land and marine pollution. Moreover, incineration does not allow for the recycling of plastics into value-added products while contributing to unwanted carbon dioxide (CO2) emissions.
Plastics can only be recycled for a limited number of cycles before they begin to degrade and eventually, lose their quality. Thus, final treatment of mechanical unrecyclable plastics is always needed. However, mechanical recycling of plastics into polymer resins that can be used again to manufacture goods is not a scalable solution.
Recent data by the National Environment Agency of Singapore found that only 4 percent of disposed-of plastic waste was recycled in the country in 2020. This is because mechanical recycling is not well suited for the processing of mixed and contaminated plastic waste streams, which represent the main waste fraction, due to high labour requirements, excessive consumption of fresh water, and increased wastewater disposal costs.
The chemical recycling of plastic waste via thermal routes, such as pyrolysis, could be a way forward.
During low-temperature pyrolysis (about 250– 500°C), plastics are thermo-chemically broken down into smaller fragments (hydrocarbons) that can serve as fuel or as feedstock for a variety of chemical processes that nowadays rely mostly on fossil-based hydrocarbons. Low-temperature pyrolysis is executed in the absence of oxygen and produces liquid oil as the main product. Currently, the avant-garde technique of plastic pyrolysis has been implemented in many industrial practices and has many benefits.
The use of pyrolysis oil could potentially make petrochemical manufacturing more sustainable due to the avoidance of crude oil, which has the potential to decrease the carbon footprint of the process. By using pyrolysis, plastic waste could be re-processed back into monomers (building blocks of polymers) for manufacturing plastic articles.
However, using pyrolysis oil to produce new plastics as a sustainable practice requires a separate assessment. A recent study published in the Science journal suggests that 11 percent of all plastics generated globally in 2016 ended up in aquatic environments. This means that by creating more plastics, we may generate more marine and land pollution, unless plastics already existing in the environment are extracted and subjected to chemical recycling.
A major breakthrough
Plastic waste mainly comprises two chemical elements: Carbon and hydrogen. Researchers from NTU found a way to treat plastic waste by using a catalytic thermal process to convert it into carbon nanotubes – a form of solid carbon – and hydrogen-rich gas. The process consists of two stages. First, plastic waste is decomposed via heating into a gaseous stream of hydrocarbons. Then, hydrocarbons are directed into another reactor filled with a catalyst. Upon contact with the catalyst, hydrocarbons are transformed into carbon particles and hydrogen gas. Hydrogen is considered a “green” fuel since its use does not produce carbon emissions.
Depending on its origin, the mixed plastic waste contains about 7–15 wt.percent hydrogen, meaning that from one tonne of plastic waste, up to 70–150 kilograms of hydrogen fuel can be recovered. If used for fuelling hydrogen cars, that is enough for 7,000–15,000 kilometres of travel.
In addition to hydrogen, approximately 600–800 kilograms of solid carbon can be obtained from one tonne of mixed plastic waste, which is equivalent to 2,500–3,400 kilograms of captured CO2. Solid carbon is easier to store and handle compared to gaseous CO2 emissions from petrochemical plants. The energy requirements for the tedious plastics-to-hydrogen process amounts only to a small fraction of energy contained in waste, and it can also be replaced by energy derived from renewable sources, such as solar and wind, to further decrease carbon footprint.
Recent life cycle assessment studies have shown that converting plastic waste to hydrogen-rich gas and solid carbon can provide substantial environmental benefits compared to the pyrolysis-to-oil process.
The researchers established a start-up company backed by NTU Singapore: Nanomatics. The company aims to commercialise the already-developed plastics-to-hydrogen technology and explore this advanced concept on a deeper level. The novel plastic-to-hydrogen process has already shown promising results in solving the global plastic pollution conundrum. With the help of a local social enterprise, the Ocean Purpose Project, the team at NTU Singapore obtained plastic waste from seashore sites in Singapore and Lombok, Indonesia, and successfully converted these plastics into hydrogen.
It’s anticipated that owing to high hydrogen value and high carbon capture potential, combined with the suitability of the process to treat problematic waste streams, the technology can be beneficial not only for multinational corporations, but for small communities struggling with waste pollution as well.
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