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Recycling plastics

Interview with Professor Jan Baeyens, Visiting Professor at the University of Warwick

Published January 2011

Plastic is notoriously difficult to recycle - currently only 12% of household plastic waste is reprocessed. The rest is either burned or goes to landfill, a situation that is made worse by the high cost of collection of these materials and the lack of recycling plants that deal with them. However, Professor Jan Baeyens and his research team have discovered a way to extract valuable chemicals from waste plastics, a process that would ease the strain on landfill sites, and would produce an output that could be sold at profit.

Plastic wasteOne of the latest breakthroughs, however, has done just this. Professor Jan Baeyens, a Visiting Professor at Warwick, along with his team, have found a way to extract valuable chemicals from the common mix of waste plastics found in our rubbish bins. Although this process takes time and money, the input rubbish is better than free – since you avoid paying landfill taxes - and the various outputs, gases liquids and solids can be sold at a high price. Such products include styrene and char (or carbonaceous residue) that can be sold at £400-£700 per tonne. In addition to the financial incentives this could ease the strain on Britain’s landfill sites. European directives for increasingly reducing landfill are not being met and it is getting increasingly expensive to bury our waste.When it comes to rubbish we tend to bury most of it, recycle some, and burn the rest. In 2007, 260 million tonnes of plastic waste was produced globally, of which the EU produced 25%, yet only 20.4% was recycled. Finding a process to break it into its constituent parts in order to recoup the valuable bits is something that sounds good in theory but is most probably impossible in practice.

Laboratory experimentation has found that a process called pyrolysis can be used to crack waste polymers down into monomers that can then be refined, purified and re-used. Pyrolysis is basically decomposition brought about by high temperatures in the absence of oxygen. In this case the processor heats waste plastics to 400-600°C within a bubbling and circulating fluidised-bed reactor, previously developed for biomass decomposition. During the breakdown of the plastics are vapourised - the resultant gases can be burnt to power the process, the valuable chemical liquids can be condensed and collected and the solids separated out by the fluidised sand in the bed.

The pilot project’s aim, funded by Advantage West Midlands, was to prove that this was possible in a pilot-scale plant. The next step is to scale it up and trial it out on an actual rubbish dump where a larger plant could pump-out products on a commercial scale. The products are much denser than the rubbish they come from and therefore are easily transportable.

Although hardly glamourous, waste processing is fast becoming a global-scale business. In 2008 the top 4 UK waste processors generated some £2.8 bn turnover with gross profits averaging 8%. The installation of a plant on a refuse site would cost in the region of £750,000-£1,000,000 but initial estimates suggest sale of the recyclants could recoup this outgoing within a year.

How does it work?

It is possible to convert and upgrade Municipal Plastic Solid Waste (MPSW) by applying fast pyrolysis where the operating parameters are designed to yield different valuable products of unusually high molecular weight.

The bubbling and circulatory fluidised bed offers the best potential of high heat and mass transfer, excellent control of the waste material residence time and temperatures and easy scale up. As part of the AWM sponsored Hydrogen project, the University of Warwick has acquired a bubbling and circulating fluidised-bed reactor, capable of operating at the required pyrolysis temperatures. It is currently being used for the thermal conversion of biomass. By modifying this reactor and adding some additional equipment, they have created a pilot-plant for waste plastics capable of handling between 10kg/hr and 20 kg/hr of waste material.

The focus of this work is on the pivotal and product yield controlling pyrolysis reactor, and proving the practicality at scale, and with mixed plastics of a.) the technical viability, b.) the market attractiveness and c.) the adaptations required in order to handle raw plastic waste and capture the chemical products.

A typical large-scale plant has an average capacity of 10000 tonnes of MPSW pa. Thus over 1000 of these plants would be needed in Europe and some 50 in the UK. Each plant would generate output chemicals of a value around £5m, and save another £0.5m in landfill taxes at 2010 rates. With energy costs in the region of £50,000 per annum, the system should be commercially attractive and give a rapid payback of capital costs.

So why has this not been done already?

Due to the difficulty of reprocessing mixed and dirty waste, currently only 12% of household plastic waste is reprocessed. This is despite household waste being by-far the largest source of plastic waste (approx 2/3rds). The rest is either burnt or goes to landfill. There are no fully commercial recycling plants in operation for this totality of mixed waste, only for selected fractions. This situation is made worse by the high cost of collection of these materials due to the high volume to mass ratio of household plastic goods.

Currently the largest amount of recycling of waste is by re-use in the plastic moulding facility itself. Here the plastic remains pure, is easily segregated and is clean. The waste can simply be reground and fed back to the moulding machine. This way there is only a small amount of material property degradation. The second largest recycling technique involves the physical separation of mixed plastic types followed by regrinding, again taking place pre-consumer. Here the disadvantage of having mixed colours, retardants and coatings in the collected material causes significant degradation of properties and thus output is of significantly lesser value and demand is limited.

Are there alternatives?

There are other processes in development which have been suggested to be an alternative to the suggested technique; namely hydro-cracking and gasification.

Gasification produces hydrogen, methane and carbon monoxide which can then be burnt or used as chemical feedstock. However the higher temperatures required by the process causes the valuable complex chemicals to be cracked to less valuable chemical species and any heavy metals in pigments to be volatised. In addition the energy required is greater than 1000kJ/kg as opposed to the 100’s kJ/kg required for pyrolysis. Practically a third of the feedstock is burnt to achieve gasification temperatures.

Warwick's engineers are now working with our technology transfer arm, Warwick Ventures, who expect that their work will be of great interest to local authorities and waste disposal companies who could use the technology to create large scale reactor units at municipal tips which would produce tanker loads of reusable material.

Professor Jan Baeyens studied Nuclear Engineering (Brussels) and Chemical Engineering (Leuven). He obtained his Ph.D. further to research in the team of Prof. Derek Geldart at the Postgraduate School of Powder Technology (Bradford-U.K.). After 13 years of employment in divisions of various Belgian environmental engineering companies, he became a part-time professor at the University of Leuven (B) and worked as a process and project consultant in Europe and overseas. Between 2003 and 2005, he joined the University of Antwerpen (B) as a full-time professor, responsible for teaching and research in the fields of environmental, powder en process technology. Since 2005, he became part-time Visiting Professor at the University of Birmingham (U.K.). In 2009 he joined the University College London and the University of Warwick where he lectures on process design, sustainable development and renewable energy at the School of Engineering. He has contributed to over 250 publications in international and national journals, is co-author/editor of 14 books and a regular speaker at international congresses.