Pyrolysis is the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam.
Waste, which is too rich in calorific value for classic incineration, is called refuse-derived fuel (RDF) or solid recovered fuel (SRF) and should be pyrolised. This is made in hermetically sealed, indirectly heated rotary kilns, which chemically convert and - in this way - homogenize inhomogeneous RDFs/SRFs to produce gas and coke. In a next step, the process gas is incinerated in a combustion chamber to obtain hot flue gas. Downstream a Heat Recovery Steam Generator is connected with an electricity producing Turbine Generator Set. Compared to classic incineration, the above mentioned process takes place at much less temperature and therefore produces considerably less pollutants such as nitrogen oxides (NOx). In opposition to classic incineration the production of significant amounts of dioxins and furans have not to be feared. Therefore the effort in flue gas cleaning of power plants which base on pyrolysis is minimized.
Soil, which is contaminated with oil, hydrocarbons, dioxins, furans or mercury can be purified by pyrolysis. Direct and indirect heated kilns can be used for this application. The volatiles are converted to vapour, which is incinerated in a downstream combustion chamber. Compared to a direct heated kiln an indirect heated kiln produces much less process gas so that the flue gas cleaning process is minimized, whereas the costs for the heating devices are more expensive. Mercury has to be separated in a specially designed flue gas cleaning process. Dioxins and furans are decomposed in absence of oxygen above approximately 500 °C - for this application only an indirectly heated kiln is of use.
Pyrolysis of Biomass
Biomass with a high calorific value can be pyrolised to generate oil. The most common application for rape-seed, wood, straw or other so-called energy plants is the flash pyrolysis which means, that the heat transfer is maximized, wheras the retention time in the rotary kiln is minimized.
End-of-life tyres contain a lot of chemical bound energy. As a result of pyrolysis of waste tyres one obtains: high energy gas, hydrocarbon oils, char and steel. The ratios of the first three products depend on process conditions, mainly on temperature. The high energy gas may be utilized as a source of energy for the pyrolysis process. The pyrolytic oil may be used as a fuel independently or mixed with diesel fuel in different ratios. The char after additional processing may be used as a carbon black substitute or precursor of carbon adsorbents.
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