Wednesday, July 21, 2010

Nuts and Bolts - Pyrolysis of Tyres

Burning 1 ton of waste tyres produces about 420 litres of pyrolysis oil, 150 kg of steel wire and 270 kg of carbon black. The damage to the environment is obvious. Performing pyrolysis of scrap tyres saves the environment and gives yield to 3 valuable materials.

Pyrolysis gas can be condensed recovering pyrolysis oil. The recovered oil usually has specific gravity about 0.93, a sulphur content (1.1%) as well as the residual carbon content. This oil can be further filtered prior to be used mainly as heating oil. It is a type of Light Fuel Oil or commonly named LFO. Its kinematic viscosity is 2.6 centi-Stokes (cSt) which makes it a non-viscous liquid.

If the oil is filtered through a filter press, we can expect the residual carbon in the oil to be smaller than 10 micron and therefore suitable even for packaged burner system which typically will have 0.3 mm or 300 micron nozzle diameter. Any depositing of residual carbon will be avoided.

The calorific value of the tyre oil is 43.8 MJ or 10,500 kcal per kg which is similar to that of diesel and gasoline.

Another by-product related to the scrap tyres pyrolysis is the carbon black. After separation from the steel wire, solid residual from the pyrolysis process is the carbon black usually in the powdered form. From laboratory analysis, tyre carbon black has 76% carbon, 10% ash, 3% sulphur and other metals such as magnesium, copper, iron and zinc.

The calorific value of the tyre carbon black is 26 MJ or 6,200 kcal per kg which makes it good as a solid fuel. It will require further processing from powder into briquette form to make it suitable for combustion.

The steel wire recovered from the process is usually sold as scrap steel for recycling.
  • Heat required for Pyrolysis of Tyres
For every 1 ton of tyres, the fuel needed to complete the conversion of the tyres into oil and carbon black, is about 52 litres of diesel or 2,100 MJ of heat.

The heat from the by-products will generate 16,800 MJ from combusting the oil and another 7,000 MJ from combusting the carbon ie a total heat content of 23,800 MJ. There is therefore a 11 times gain in heat by recovering heat energy from waste tyres which makes it worthwhile.