Conveyor News

Waste tyres to fuel cement production

Cement Producers may be coming to the aid of the automotive industry, struggling to cope with the demands of a growing mountain of used tyres and the requirements of the forthcoming Landfill Directive. With approval from the Environment Agency Blue Circle Cement has started to burn tyre chips in two of its cement kilns, with a combined capacity of 35,000 tonnes per year. Castle Cement has also recently started to use whole tyres as an alternative fuel source in its Rutland plant and estimates indicate that up to 200,000 tonnes of tyres could be used for this purpose within five years. As the UK produces around 400,000 tonnes of waste tyres each year, the initiative taken by the cement industry has the potential to go a long way to solving the problem of what to do with used tyres.

Energy efficient

As well as providing a positive solution to used tyre disposal, burning tyre chips is an efficient way to generate energy, providing a heating value of 6,000 kcal/kg or 20% greater than coal. Tyres consist mainly of rubber hydrocarbon (51% by weight), carbon black (26%) and oil (13%).

At a recent symposium on recycling arranged by the Concrete Society, speakers from Kansas State University showed that scrap tyres are seven times more energy efficient compared with natural gas in cement kilns. During the seminar Graham Worthington, alternative fuels manager of Blue Circle Cement, confirmed that in 1999 his company used over 750,000 tonnes of coal and 120,000 tonnes of petrol coke, so the potential savings are considerable.

The environment

The commercial benefits are matched by the environmental advantages of conserving fossil fuels. Based on Blue Circle Cement’s estimate the potential savings throughout the industry could easily exceed a million tonnes of fossil fuel, a worthwhile reduction in use of scare resources and an example to other industries to follow their example, reducing Carbon Dioxide and Nitrous Oxide emissions.

Handling tyre chips in bulk

However the existing plants needed to be modified with specially designed handling equipment to take in tyre chips and feed them in bulk at a controlled rate into the kilns. Specialist bulk conveyor manufacturer Geo Robson worked closely with both Blue Circle and Castle Cement to design and install vertical and horizontal conveyors and hoppers, in order to provide a constant supply of tyre chips to the kilns.

The first installation for Blue Circle’s Hope Works in Derbyshire was designed to handle up to 70 tonnes of shredded tyres each day, taking deliveries from the ground floor up to the kiln reception hopper 42m above ground. Robson also had to cope with limited space inside the plant and the tendency of tyre chips to stick together. Using a combination of screw conveyors and vertical bucket elevators, tyre chips are taken up five floors to the kiln. The hopper base was designed with a special push floor which uses rams to separate the chips and prevent blockages.

To maintain the required feed rate of two tonnes per hour, Robson fitted a stripper drum that controls the depth of material on the belt, and an integral weighing facility.

Automation

The recent installation for Castle Cement has increased the level of automation and takes raw materials through a more complex process and path, due to the design of the building. Unlike Blue Circle, Castle Cement burns whole tyres, replacing up to 25% of the coal previously used, and the handling system was designed to eliminate manual offloading of tyres weighing some 10 kg each, into the kiln. Robson designed a complex of overhead conveyors, chutes and carriers which feeds tyres into the kiln at up to eight each minute, then returns to the ground to collect incoming tyres.

Landfill directive

Tyres are a controlled waste under the Environmental Protection Act 1990, which places a duty of care on waste producers to ensure safe disposal through licensed carriers to licensed sites. Under the EU Landfill Directive whole tyres are prohibited from being used for landfill after 2003 and after 2006 even shredded tyres will be banned. When used for landfill, tyres in large volumes can cause instability by rising to the surface, which affects future use and possible land reclamation. In addition tyres contain organic chemical with possibly harmful long-term effects from leaching.

The Department of the Environment recommends that only shredded tyres should be used for landfill to a maximum of 5% by volume. As tyres are turned away from landfill sites and gate fees rise, fly tipping has become a problem – and with tyre fires lasting months of even years, they represent a major potential health and environmental hazard.

Retreads

Western Europe generates more than 200 million scrap tyres each year and around 40 million are taken from UK vehicles each year. Volumes are expected to rise as car ownership inexorably increases and as the number of miles driven per vehicle year is growing, tyre usage will increase even more rapidly. Until recently re-treading was an effective way of recycling rather than scrapping used tyres. However some 60 % of UK tyres are too worn to retread and low-cost imports from the Far East and Eastern Europe have made the process less economic. As a result the demand for retreads has steadily fallen to an estimated one million tyres per annum. The Retread Manufacturing Association is making efforts to promote the benefits of professionally produced retreads and is aiming to meet an EU target of retreading 25% of all tyres.

Crumbing and other methods

There are a number of other processes to re-use old tyres in a more constructive and environmentally responsible manner. Crumbing is a process which reduces rubber compounds to a fine granular or powdered form, for use in sports and play surfaces, manufactured items such as shoe soles and carpet underlay, or even in road asphalt. Rubberised asphalt can increase road surface elasticity, temperature range and resistance to oxidisation, with fewer ruts and potholes. The US government has sought ways to encourage greater use of crumbed materials in road surfaces but the development of steel braced tyres has made the process more costly. Compared with the potential for using shredded tyres as an alternative fuel source, the openings for crumbing are minimal.

Cryogenic fragmentation involves shredding tyres at a very low temperature - below minus 80°C - and pounding the chips into granules. The resulting material can be used in a similar way to rubber crumb but the high energy costs involved make it uneconomic for large scale production.

Even the public at large has discovered a number of practical and innovative uses for old tyres. They can be seen in use in boatyards and marinas, protecting young trees, in playgrounds, as crash barriers on go-kart circuits and on motorway embankments.
One entrepreneur has even patented the idea of sinking tyre sections into the ground to act as water reservoirs beneath lawns.

Only a large-scale initiative can tackle a large scale problem and using tyres as an alternative fuel source may be the most promising solution. The fact that tyres are highly efficient energy producers and reduce the use of fossil fuels makes the case even more powerful. Let us hope that where the cement industry leads, other major energy consumer will follow.

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