High Value Products from Scrap Tyres
The European Union has recognised scrap tyres as a ‘priority waste stream’ requiring special treatment and disposal and has recommended that a target of 65% recovery of scrap tyres should be set by the member states. This recommendation is supported by the UK Government in their report on sustainable waste management - ‘Making Waste Work’. This report recommends that there should be increased waste minimisation, re-use and recycling of waste, including energy recovery and that landfill should be the last resort for waste disposal.
The treatment and disposal option for tyres most commonly used throughout the European Union is landfilling, however, proposed EC legislation in the form of the European Waste Landfill Directive has the specific proposal to prohibit the landfilling of whole or shredded tyres. In addition, as the costs of disposal inevitably increase illegal dumping is likely to increase. Open dumping represents a problem for local communities in that they are an eyesore and may result in accidental fires with high pollution emissions. In addition, tyres can be a breeding ground for insects and a home for vermin.
The current disposal methods of landfilling of tyres is clearly a waste of a valuable resource and with increasing emphasis on recycling there are attempts to move the treatment of this waste stream higher up the hierarchy of waste management. Consequently alternative treatment and disposal routes for tyres are urgently being sought.
The application of a novel alternative process - pyrolysis - as a means of reusing scrap tyres has recently been the subject of renewed interest and is the basis for Dr Williams’ research programme. Pyrolysis is the degradation of the rubber of the tyre using heat in the absence of oxygen. The tyres rather than burn, breakdown to give an oil and gas leaving a residual solid carbon and the steel casing of the tyre. Pyrolysis of tyres therefore produces an oil, carbon and gas product, in addition to the steel cord, all of which have the potential to be recycled. The yield of oil can be up to 58 wt% of tyre and the oil has broad fuel properties similar to commercial grade light fuel oil/diesel fuel. For example, the energy value of the oil is 42 MJ kg-1 and sulphur content between 0.5 and 1.5 wt% depending on process conditions, and therefore the pyrolysis oils may be combusted directly or added to petroleum-derived fuels. The pyrolysis gases are composed of mainly hydrogen, methane and other hydrocarbons and have sufficient energy value that they can be used to provide the energy requirements of the pyrolysis process. The solid carbon residue left after pyrolysis has potential as a solid fuel or as a low grade carbon black.
Pyrolysis of tyres has been around for decades but has not taken off as an alternative treatment technology due in part to the lack of commercial interest in the derived products. However, the process has the potential to produce higher value products which could off-set the costs of treatment and make the process economically attractive.
Paul Williams has been researching the process of pyrolysis of tyres and other waste materials for over 10 years. The EPSRC grant awarded to Dr Williams has the aim of improving the efficiency of the tyre pyrolysis process to optimise the production of high value products. The targeted products are the valuable aromatic chemicals found in the oil which can be used in the petrochemicals industry and the residual carbon which is being upgraded to produce a high grade activated carbon.
The EPSRC research grant has involved the design, commissioning and operation of experimental equipment in the department of Fuel and Energy at the University which uses fluidised bed and fixed bed technology to process the scrap tyres at temperatures between 450 °C and 700 ° C. The scrap tyres thermally breakdown at such temperatures to produce the oils and gases and leave behind the residual carbon. The oils are condensed using a novel selective temperature condensation system, developed as part of the research grant, which concentrates the higher value aromatic chemicals as a separate fraction.
The tyre pyrolysis oil is very aromatic and contains certain chemicals in high concentration. One example is a chemical called ‘limonene’ which has an extremely fast growing wide industrial application. It is used in the formulation of industrial solvents, resins and adhesives and as a feedstock for the production of fragrances and flavourings. Most importantly it is biodegradable and environmentally safe and can be used as a replacement for CFC’s - the ozone destroying group of chemicals. The yearly growth demand for limonene has reached 50%/year in recent years and current prices are of the order of £7.80/kg. Limonene concentrations in selectively condensed fractions have reached 14% by weight in the experiments at Leeds, a concentration which can be separated using conventional processing to produce a commercially viable product.
Also found in the tyre oil is indene which is also regarded as being of particular technical importance since it is used to produce indene/coumarone resins which find extensive application, especially in the production of adhesives, as reinforcers and tackifiers in the production of commercial rubber products, and in paint manufacture. Production of indene-derived resins in Western Europe is around 110,000 tonnes/year and current prices are of the order of £30.0/kg.
Other high value chemicals also found in the oil are, styrene, xylene and naphthalene from the derived light oil fraction. Styrene is used in the production of synthetic rubbers and plastics, xylene is used for polyester production, and naphthalene used in the dyestuffs industry and for the production of plasticizers and in paint manufacture. These products also have a significant commercial value.
The research at Leeds has also shown that the residual carbon obtained from the process may also be further upgraded to produce a high value product. The carbon after pyrolysis is high in ash and has surface areas of about 80 m2/g and is generally only useful for low grade use such as a solid fuel. The surface area is low when compared with commercially available activated carbons which are highly porous carbonaceous materials with high surface areas, typically in the range 400 to 1500 m2 g-1. These properties mean that activated carbon is an excellent adsorbent and is commonly used to remove pollutants from gas or liquid streams. The most commonly used feedstocks in the production of activated carbon are wood, coal, lignite, coconut husks and peat respectively.
The research work at Leeds has produced an upgrading process for the tyre derived carbon which produces activated carbons of similar quality to those obtainable commercially. For example, surface areas of over 600 m2/g have been obtained in the experiments at the University. The process involves removal of the ash from the carbon using a simple acid wash procedure followed by activation at high temperatures of about 900 ° C in the presence of steam or carbon dioxide. A wide variety of process parameters has been investigated to optimise the process, including activation temperature, particle grain size, the ratio of steam/carbon dioxide activation gas, activation time etc.
Of further significance, the work at Leeds has shown that the activated carbons have a porosity and sulphur content which is particularly suitable for the removal of cadmium and mercury from industrial aqueous waste-waters and flue gases. The results from the work has shown that the tyre derived activated carbons have removal capacities for cadmium and mercury which are several times greater than those obtained for commercial activated carbons. The removal of heavy metals from waste streams is particularly important to industry since the metals are known to be toxic, but also the emission levels are heavily regulated and require expensive clean-up. The use of a low cost activated carbon derived from a waste such as scrap tyres would off-set the costs of clean-up. Price levels for activated carbons used currently in the clean-up of mercury in incinerator flue gases are between £500 - 750 per tonne.
The research work being undertaken by Paul Williams is helping to solve the disposal problem for scrap tyres. With greater emphasis on recycling of waste, the pyrolysis process has the potential to recycle tyres to produce high value products which make recycling not only environmentally attractive, but also commercially attractive. The project has industrial links with three SME’s (Small, Medium Enterprises), which consist of two tyre pyrolysis companies and an activated carbon company. These companies are closely watching the developments at Leeds to take the research into the commercial world and establish this novel process as an alternative to wasteful landfill and open dumping of scrap tyres.
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Paul T. Williams
Department of Fuel and Energy
The University of Leeds, Leeds, LS2 9JT, UK