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Pyrolysis can be defined as the thermal decomposition of organic material through the application of heat without the addition of extra air or oxygen. Some authors define it as the thermal decomposition "in the absence of oxygen" (Resource Recycling, 1995), however this is a wrong concept as air can be trapped in the waste or the chemical composition of waste may also include oxygen. Although pyrolysis can be considered as an alternative to reduce waste volume and a method for obtaining energy from wastes, it "appears to be best suited for processing organic feedstocks with high heat value” (Rhyner et al., 1995).
At a temperature of around 450°C and under no addition of air, the hydrocarbon content of the waste reacts and generates pyrolysis products, such as pyrolysis gas, pyrolysis coke and tar. The pyrolysis process can be represented by Equation 1 (Tillman, 1991). The produced fuel gas, consisting mainly of carbon monoxide and hydrogen (FOE, 2009), is "suitable for either electricity generation or to provide heat in boiler applications – with no need for flue-gas treatment” (Energos, 2004). The gas has a calorific value between 22 and 30 MJ/m3 depending on the waste material being processed.
Equation 1: CaHbOc + heat → H2O + CO2 + H2 + CO + CH4 + C2H6 + CH2O + tar + char
The solid products of the pyrolysis process consist of metals, sand, glass and pyrolysis coke, which contains residual carbon that is not converted to gas in the process. Although the pyrolysis coke can be further processed to release the energy content of the carbon or utilized in other thermal processes (Energos, 2004), there is no solid market for the pyrolysis coke (Bilitewski, 1980). In addition, Pyrolysis is an endothermic process, meaning that it does not generate heat but instead requires heat for the reaction to be sustained.
As mentioned before, the EU intends to increase the reuse and recycling rates of paper, plastics and biowaste waste streams. Such increase will mean that WtE plants base on pyrolysis and gasification technologies may not be able to operate profitably (FOE, 2009), as their feedstock is rich on these types of waste.
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