This is the solid residue remaining after combustion made up of a material with a high inert content (incombustible fraction). Examples include the residue from coal combustion, also known as heavy ash, or from incineration of solid urban waste. Specifically, the residue from an urban-waste incineration process, made up of the incombustible fraction of the waste, represents approximately 30% of the weight and 10% of the original waste volume.
Source: Courtesy of Ambiente, an Eni Group Company

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