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Introduction to Incinerating Tires for Energy
Worn vehicle tires are notoriously difficult to dispose of due to their durability and sheer volume.
Most states have now banned them from landfill sites as they were creating a hazard from inherent chemicals and heavy metals leaching into the ground.
Recycling tires has become big business, especially in the Far East, where we export millions of tires every year.
Tires can also be used to produce energy for a variety of purposes and oil can also be produced from specialist processing of scrap tires.
This is an article on the use of scrapped tires to produce energy; we begin by examining the components of a typical vehicle tire, and move on to the different processing techniques.
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Composition of a Typical Vehicle Tire
Tire manufacturers regard the exact make-up of their tires as confidential; however most of them use the same components, albeit in various quantities.
- Natural and synthetic rubber – 41%
- Carbon black – 28%
- Steel wire– 15%
- Chemicals, nylon and fillers – 16%
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Processing Scraped Tires to Rubber Crumb
The tires are hand-fed into a splitter from where the two halves are conveyed to a shredding machine that chops them up into smaller pieces.
These are fed into a grinder or hammer mill, where depending on the particular application, various sizes of rubber crumb are produced.
From here the rubber crumb is conveyed under an electromagnet where the pieces steel wire are removed, the remaining crumb passing into a set of cyclone filters/screens.
Compressed air is injected into the cyclone as the crumb passes through the screens, removing dust and pieces of nylon fluff, before dropping into canvas 2000lb bags.
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Processing Tires to Produce Oil.
There are several methods being used to produce oil from tires;
In this process the tires are cut up and shredded into quite small 1" pieces, before being fed into a liquefaction reactor vessel containing hot oil @ 700F.
Here the rubber pieces are melted in the oil (old engine oil is sometimes be used) and are liquefied into two types of oil; light condensate oil and a heavier more viscous tar-like oil.
The light oil is extracted from the top of the reactor, with the heavy tire oil being drawn of from the reactor bottom section.
The light oil can be further processed to diesel or heating oil; the heavier oil being further processed to marine, heating or lube-oil.
Both types of oil require further processing including filtration and electromagnetic separation to remove the other components such as nylon fluff and reinforcing wire.
The heavy oil can also be processed into a syngas; this process will be covered later.
The tires are shredded into 2" pieces, and put through a vibrating screen that removes oversize pieces, returning these to the shredder.
The remaining pieces are conveyed to a reactor where hot hydrocarbon vapors containing combustible gas and vaporized oil are produced. This leaves solids of carbon black and steel which are removed from the reactor for further processing and recycling.
The solids are removed by water-washing/ spraying techniques, the gases being fed into a condenser where most of the oil vapors are condensed and the resultant liquid oil drawn of from the bottom of the condenser, and stored.
The remaining gases in the condenser are pressurized and used as fuel in the pyrolysis process, making this system self-sufficient in energy, with any surplus gas being used to fuel steam boilers in the plant.
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Syngas from Scrapped Tires
The heavy oil that was produced from the liquefaction process is fed into a gasifier, which is maintained at a temperature of 2500F.
Oxygen and steam are injected and the oil flashes off into hydrogen (H2), hydrogen sulphide (H2S), carbon monoxide (CO), and carbon dioxide (CO2).
The CO2 and hydrogen sulphide are then removed from the gas. The CO2 can be removed for sequestration using a membrane, and the sulphur extracted from the hydrogen sulphide by processing the gas through a sorbent desulfurization tower. The resultant sulfur is then collected for selling to the chemical industry.
The reinforcing steel wire from the tire processing is also collected and sent to a steel recycling facility.
Syngas has a high calorific value, and because of the high temperatures and post gas processing treatment, the gas contains no organic materials or metals.
It is used to drive gas turbines in power stations, usually in a combined cycle mode.
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Incineration of Scrapped Tires to Produce Thermal Energy
Scrapped tires can be used to produce energy through incineration; current methods are listed below;
Tire Derived Fuel (TDF)
- Rubber Crumb (Co-firing with Coal or Wood)
This process consists of shredding the tires into tiny components known as rubber crumb, which after removal of the reinforcing steel wire can be fed into a steam boiler furnace along with solid fuel. Rubber crumb is used to supplement coal and wood in Power Stations, pulp and paper mills, hospitals, and other institutions.
- Used Whole
Tires are used as fuel by the cement industry where they are loaded into the kiln. Here they not only provide heat but the clinker produced is used in the cement making process being mixed with gypsum to form Portland cement.