Gasification and pyrolysis

Combustion by means of incineration can be one of the solutions for recovering the energy content of waste, however it involves numerous difficulties, among which the emission of gas effluents that require a costly purification treatment and that have induced researchers and engineers to search for more solutions for the plants. Among these are gasification and pyrolysis, which are being experimented as a potential alternative to the Waste to Energy systems. Even if in the waste sector, innovative technologies are being considered, gasification and pyrolysis have a more ancient history that dates back to the 18th century. The first applicative examples made use of coal, while waste started being used from the 90s.
How do these Waste to Energy systems differ one from the other? During combustion, the combustible elements that are present in the waste are oxidized in the presence of excess oxygen, which produces a release of heat and waste products, such as combustion smokes and inert solid residue. Diversely, during gasification the conversion of a solid or liquid material into a combustible gas (syngas) takes place through partial oxidation in which air is used in minor amounts than what would be necessary in order to complete the reaction, and a gas, enriched with carbon oxide (CO) and hydrogen, is obtained. Finally, unlike in the case of combustion, pyrolysis is carried out in absence of oxygen and consequently it is possible to obtain three products in different phases, all are fuels: syngas, tar (a condensable substance that is present in syngas, in the form of a liquid product) and char (carbon residue).
But what are its uses? Syngas can be used as a fuel or raw material in the chemical industries, tar can be used in various ways, among which for co-combustion with coal for the production of electric energy, as fertilizers, as fuel for thermo-electric power plants, etc., finally, char can be treated with hydrochloric acid for the production of coal, or with carbon dioxide for the production of activated carbon, a material that is used for water purification. From 1 kg of MSW, by means of a pyrolysis process, 0.15 to 0.3 kg of syngas, 0.5 to 0.6 kg of tar and 0.2 to 0.3 kg of char, are obtained. Gasification involves a greater production of gas than the other two components.

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