energy recovery from waste; comparision

ENERGY RECOVERY FROM WASTE: COMPARISON
OF DIFFERENT TECHNOLOGY COMBINATIONS
The thermal treatment of solid residues was introduced as the most suitable way to reduce the mass (up to 70–80% reduction) and the volume (up to 90% reduction) of waste and to clear up their potential for putrefaction, with the connected sanitary risks Further, according with the growing awareness of conventional fuels availability reduction and along with the increasing costs of traditional energy sources, the solid waste thermal treatment collected attention also in reference to the possibility of associating, to the simple disposal, the energy recovery process. Nowadays, the waste thermal treatment represents an inescapable part of the integrated waste management and treatment system and the challenge for the future lays in the improvement of energy recovery, keeping a high level
of plant reliability. The term “thermal treatment” means a process which takes place at relatively high
Temperatures involving several different chemical reactions evolving, in the case of waste, from the compounds present in the solid mass. Whether the oxygen is present or not it is possible to obtain different thermo-chemical processes.
The most common thermo-chemical process applicable to solid materials—and hence
waste—is combustion . It takes place in a large excess of oxygen with respect to the stoichiometric ratio, since the aim is the complete oxidation of organic material made of mainly carbon and hydrogen. The combustion is an exothermic process, and since its outputs are combustion gases and bottom ashes—both of which are completely oxidised materials—there is no energy content left.
When oxygen is added in sub stoichiometric ratio to solid materials, partial oxidation reactions of the organic material take place, releasing thermal energy and heating up the system. At increasing temperature, the bonds of long chain molecules of solid materials are broken to obtain smaller molecules in gaseous form. Hence, the process outputs are a gaseous stream (called the syngas), which still has an energy content, carried by partially oxidized compounds and small hydrocarbons, and solid residues which may contain unreacted compounds. Such a process is called gasification.
When the solid material undergoes a process that takes place at a relatively high temperature and in complete absence of oxygen, this is called the pyrolysis . Such a process requires a heat supply from the external environment and its outputs are in general—with different yields depending mainly on the temperature—a gaseous stream (called the syngas), a liquid stream (called the tar) and a solid stream (called the char), with all three having combustible characteristics.
In this lecture the application of the above-mentioned thermo-chemical processes to Municipal Solid Waste (MSW), coupled with appropriate energy recovery systems, is considered from a process analysis point of view, with the aim of comparing the energy recovery potential in the different cases.
Of course, while combustion can be applied to MSW directly, gasification and pyrolysis