Environmental Engineering Reference
In-Depth Information
2.1. Waste to Energy Plant - System
a
Although the
traditional waste to energy plant uses a more obsolete technology, studying
it helps to understand evolution and innovations in energy efficiency occurred in the last few
years [8].The primary purpose of waste to energy plants, also called incinerators (waste
burning plants) with energy recovery, is to burn waste (at 900°-1000°C) and make it
biologically and chemically inert, by reducing its volume, with a great advantage that is the
production of electricity and thermal energy. The heat produced by the combustion, generates
steam which is used for energy production and for the heating [9].
Municipal waste incineration is a controlled combustion process which oxidizes the
organic substances contained in waste, producing simple molecules in a gaseous state at
standard temperature (gas): organic carbon is oxidized to carbon dioxide (CO
2
), hydrogen to
water (H
2
O) and sulphur to sulphur dioxide (SO
2
); generally the inorganic part of the waste is
not subject to reactions and becomes a solid waste to be disposed of and/or to be recovered
(ash or slag). Also unpleasant chemical transformations take place, such as the creation of
nitrogen oxide (NO), hydrochloric acid and hydrofluoric acid, which toxic characteristics.
The cycle of wastes combustion is divided into the following phases:
•
acceptance and storage;
•
feeding;
•
combustion;
•
cooling of combustion smokes and warmth recovery;
•
treatment of combustion rests: ashes, smokes, slags, waste waters.
Waste combustion phase is performed in furnaces and it involves three phases: drying,
starting and combustion. In order to meet various needs, several kinds of furnaces have been
introduced: moving grid, fluid bed and rotary drum. During combustion the smokes come out
at a temperature of 1000°C, so it is necessary an intermediate phase of cooling in a
combustion section and the extraction of heat from the smokes. But these smokes embed
pollution elements. It is possible to treat these smokes. Polluting gas control
equipment
can
be performed by mixing smokes with chemical elements (solids, liquids or gas) able to
remove one or more polluting components. Often the absorption process on solids are used,
like active carbon.
Another problem of wastes incineration is due to solid residual products and to their
impact on the environment. The incinerators produce two kinds of solid wastes: slags
discharged by furnaces, and fine particles (flying ash mixed to smoke) seized by smoke
treatment. It is difficult to manage them because they embed large quantities of toxic
compounds, like heavy metals and organ chlorinated compounds easily released in the
environment through percolation.
2.2. Molecule Dissociation - System
b
The molecular dissociation process includes the thermo chemical conversion of a carbon-
composite into a burning gas [10].
