Environmental Engineering Reference
In-Depth Information
I
NTRODUCTION
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 (Tchobanoglous et
al.,1993).
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 (Tchobanoglous et al., 1993). 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 oxidised
compounds and small hydrocarbons, and solid residues which may contain unreacted
compounds. Such a process is called gasification (Reed, 1981).
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 (Bridgewater et
al., 1999). 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 chapter 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
require a more homogeneous entering material, in terms of size, calorific value and
composition. In order to obtain an adequate stream from MSW to be fed to gasification and
pyrolysis, a pre-treatment, based on mechanical sorting of MSW, is applied with the aim of
