Waste tyres are a major problem, with thousands of tonnes per year being disposed of across Europe. Recent European legislation on the restriction of disposing of tyres at landfill sites could lead to a major environmental issue, as illegal dumping leads to large stockyards brimming with a potential air and water polluting fire hazard.
But of course, the other way to look at the situation is that waste tyres could be another serious alternative to the use of fossil fuels. In a vehicle-driven society this source of readily available cheap fuel is exciting for forward-thinking companies. Tyres include a make up of rubber hydrocarbon, carbon black and oil, producing a heat value of 6000 Kcal/kg, which is 20% greater than coal and seven times more efficient than natural gas. Heat and energy producers have understood this for many years, but it is only relatively recently that, together with climate change and fossil fuel pricing, waste tyre energy has become more attractive.
Use of biomass as a fuel is also encouraged by the EU Renewables Directive because it is carbon-neutral and therefore gives rise to less ‘greenhouse gas’. Boilers can be adapted for firing with up to 20% biomass in the fuel mixture, equivalent to a reduction in CO2 emissions of around 40%. Some energy industry experts predict that ‘clean’ coal will be the most suitable fuel for generating energy for at least the next 10 years.
With the clean burn benefits and low cost fuels, its no surprise that more biomass power stations are planned throughout Europe. Power generation is one of the latest industries that has turned to waste-derived environmentally safe fuels. Meaning the market for ‘waste-to-energy’ equipment is growing rapidly throughout Europe.
Which industries use waste tyre fuel?
A major use for waste tyres is the firing of cement kilns, and many cement producers have embraced the tyre as an alternative source of fuel. The process of tyre chipping has been used to reduce the volume taken up by the whole tyre, making it far easier to handle, but this has posed other problems:
- the extra cost considerations involved with tyre chipping
- there is a tight tolerance of chip size, which is critical for effective handling and use
- the extra process and equipment needed
- also, the tyre chips are non flowing, so are prone to clogging within storage bins.
Equipment such as the push floor conveyor were developed, to effectively slice the chips out of the storage bin and on to a belt conveyor for transport to the kiln. Belt conveyors and screw conveyors were designed specifically to ensure the smooth flow of tyre chips.
An example of a tyre chip system was recently installed at a lime quarrying and process plant in Derbyshire, UK, which feeds the furnace with five tonnes per hour from a tyre chip handling plant, with a 500 tonne capacity. This is how the process works:
- tyre chips are delivered by lorry to storage
- chips are then transferred from storage to the push-floor hopper by a loader truck
- the push floor then is operated automatically by the demands set by the weigh feeder hopper to give a controlled feed of chips into the screw conveyor, which feeds onto the long gantry conveyor
- the gantry conveyor discharges into a weigh feeder buffer hopper
- the weigh feeder controls the flow through the screw feed, and a twin valve system into the furnace.
What about whole tyres?
Cost reductions, minimizing double handling, meant a rethink on tyre chipping. Could a whole process be eliminated? The engineers went back to the drawing board.
 Each screw conveyor is designed and constructed to suit each application. |
Using such bulky heavy objects in great numbers poses its own problems. Experience of other industries, such as airport baggage handling were drawn upon, where there were many similarities such as systems for sizing, orientating and loading that could be redesigned for waste tyres.
Waste tyre feed systems, for transporting tyres directly from the delivery point to the kiln, were developed. Starting at the reception hopper, the tyres were unloaded via delivery truck directly into the hopper. The base of the hopper has a slat conveyor feeding the tyres towards a singulator, which separates, aligns and evenly spaces the tyres, onto a belt conveyor which transports the tyres to the furnace. Results were that, once the tyres had left the delivery truck, the system did all that was needed, no other manual or mechanical handling had to be undertaken.
Biomass fuel: a good alternative
Although tyres are readily available and are energy efficient when used as fuel, some would argue a cleaner sustainable solution should be used. Alternatives include biomass derived from waste wood, grain husks, nut shells, and forestry waste, and among other natural materials, poultry litter. These options fall into two distinct categories: woody biomass – including forest products, untreated wood products, energy crops and short rotation coppice (SRC), which are quick-growing trees like willow; and non-woody biomass – including animal waste, industrial and biodegradable municipal products from food processing and high-energy crops. Although it should be remembered that biomass fuels do bring their own handling challenges.
Developments for the handling of environmental-friendly fuels were needed giving rise to a new generation of conveyors specifically designed to handle alternative and non-standard fuels. With a minimum of moving parts within the conveyor casing, Airglide conveyors are ideally suited for fuels derived from vegetation. With dust-producing products the Airglide conveyor designed to conform to the EU ATEX (Explosive Atmospheres) regulations ensures safety and minimizes any risk of dust explosion.
A recent installation to handle biomass fuel was at a power station in Warrington, UK. Valued in the region of £5 million (US$8.8 million), the system will initially handle palm kernel chips, which will be burnt alongside coal at the power station. The conveyor’s innovative design can also handle a wide variety of supplementary fuels derived from vegetation such as olive kernels, wood pellets, citrus pulp, sunflower pellets, maize pellets and olive cake. It is currently feeding two of power stations’ four boilers, with provision for extension to the others in due course.
The system transfers the biomass from the initial delivery point and after passing through a metal detector and screening for oversized matter, the biomass is transferred into storage by inclined gantry conveyors and directed through shuttle chutes into either of the silos supplying the boilers. It is withdrawn on demand through variable-speed screw conveyors to be milled down to the appropriate particle size for combustion, and then delivered to the boiler burners independently of the pulverised coal.
The design of a fully enclosed Airglide conveyor for the gantry conveyors eliminates the need to enclose the complete gantry structure. The Airglide conveyor differs from traditional designs by allowing the loaded belt to be supported on a cushion of air along its longitudinal length. This eliminates the need for carrying idlers whilst drastically reducing motor power and energy consumption.
The use of conveyors in waste-to-energy systems
The incinerator has been a viable alternative to landfill for many years. The constant feeding of the incinerator has been shown to be problematic with early systems – especially with feed rates and sizing of waste. Current and new technology had to be combined to create a feed system to give a desired feed rate, whilst shredding the waste materials to allow optimum incineration.
 ATEX rated Airglide conveyors |
Designed primarily for controlled feed of raw domestic and industrial refuse into fluidized bed incinerators, the waste feeder is a robust twin screw unit providing infinitely variable feed rate to match incinerator demand. A hydraulically movable shaft is fitted to relieve potential blockages detected by torque monitoring devices, plus the automatic control of stripper and hydraulically operated pusher quadrant fitted to the mouth of the feed hopper to again prevent blockages. The machine is capable of passing solid objects up to 300 mm³.
What goes in must come out …
The resulting ash deposits have to be constantly removed to ensure smooth running of the incinerator. Heavy-duty screws are designed to handle the ash, sand and incombustibles discharged at the bottom of fluidized bed incinerators and can withstand temperatures of up to 900ºC. Often sited in extreme environments, the screw conveyors are specifically designed to perform under the most demanding conditions. The robust design of the screw conveyor incorporates sealed bearings, an external gearbox and drive motor, and minimizes the service cycles and maintenance downtime. The screw conveyor is the ideal design to transport the ash to storage bunker or awaiting vehicle.
The flexibility of the screw conveyor means that the design can be adapted to handle wet or dry materials and is often used inclined to convey and dewater sludge cake or biomass, materials used for power generation fuel, or waste processing. The standard screw conveyor has only one moving part within the casing, with the bearings and drive mounted externally. This ensures a safe enclosure ideal for dust or gas emitting materials or ATEX (Explosive Atmospheres) zoned areas.
In this time of climate change and environmental concern, meeting the challenges of ever-changing materials and processes, offering designs and systems to accommodate the alterative fuel processing, material recycling, waste handling and waste processing worldwide, is challenging for all materials handling companies. Innovation, new technologies and research and development have never been in so much demand.
Jon Skidmore of Geo Robson & Co (conveyors) Ltd
e-mail: SkidmoreJ@robson.co.uk
Conveyors for materials that need dewatering
Side walled conveyors
The side walled conveyor is a cross between a belt conveyor and an elevator, able to convey and elevate wet or dry material. Used inclined, the side walled conveyor is often used for dewatering, as the wet material is elevated and the water content drains leaving the solids within the belt compartments. The side walled belt conveyor is ideally suited to handle coarse stringy screenings discharged from rakes etc.
Solids removal conveyors
For use in sewage treatment works, inclined screw units are used for grit removal and sidewalled belt conveyors to handle coarse, stringy screenings and retain liquid that is then drained away safely via a gradual fall. Stainless steel construction and external bearings provides increased resistance to corrosion, coupled with minimal maintenance.
Sludge cake handling conveyors
Many industries produce waste sludge or, as part of the production process, sludge is a component part. The handling of sludge and sludge cake has been refined over many years and systems are readily available. Handling companies with extensive experience of handling sludge and other cake from filter presses, belt presses and centrifuges can offer systems well suited to the individual customers requirements. From the processes of filter pressing, belt pressing and centrifuging the cake produced can be handled on a variety of conveyors including:
- pushplate units (twin strand chain and scraper)
- conventional troughed belt
- screw conveyors.
Receiving hoppers under filter presses are full width and incorporate breaker bars to pre-break the individual cakes, which are pre-treated with polymer binder. Wide pushplate units convey the pieces of cake to secondary cutters, which reduce the bulk still further, allowing transport to storage or trailer in narrower conveyors.
