Environmental Engineering Reference
In-Depth Information
takes place during the cooling cycle (engine side, smoke side and oil engine side) of
the machine. This brings to total efficiency even higher than 80/90%. The
temperature of the thermal energy recovery is about 80°C in case of water use, or 6-8
bar of pressure in case of steam production.
The solid wastes of gasification are transformed into glass, thanks to high temperatures,
and are completely inert.
2.4. Comparison among Systems for Waste Disposal with Energy Recovery
It is necessary to point out which is the most efficient technology from an environmental
and an energetic point of view. This comparison considers the following aspects:
•
structure-process;
•
performance;
•
environmental impact.
From the structural-processing point of view,
system a
(
waste to energy plant
described
on paragraph 2.1 and here called
system a
) reduces waste in ash, by using again combustion
smokes to produce electric and/or thermal energy, whereas
system b
(
dissociator,
described in
paragraph 2.2 and here called
system b
) and
system c (gasifier
, described in paragraph 2.3 and
here called
system c
) transform (in different ways) the raw material into a synthetic gas used
then as a fuel.
The main body of
system a
is the furnace of combustion, in
system b
is the cell and in
system c
is mono-tube reactor with vertical development, the last two produce syngas.
Another important aspect is the very high chimney for
system a
compared to the chimney a
few meters high for
system b
.
Despite the fact that the final product is the same, there are a lot of differences between
b
and c systems:
the main body of the system, the working temperature, the procedures and the
scheme for syngas production.
System b
works at about 400°C, in presence of oxygen at 4-5%, so the raw material
transformation is exothermic; so it only triggers the reaction with a moderate electric
resistance (200-300kW per 100 t/g of waste).
System c
has a different mechanism. Its reactor has a variable thermic profile (optimum
temperature at 1200°C). The waste falls down in the reactor from top to bottom, and it is
burnt by flames generated by thermic lances which are fed with methane and oxygen. This
lets the process be controlled, independently from changes of waste calorific value, but with
unavoidable economic consequences (262 m
3
/h of methane and 560 m
3
/hof oxygen per 3 t/h
of input waste are necessary).
System b
has a lower self-consumption (less than 2%) and it can treat any kind of waste
without a pre-treatment. Furthermore, both the gasification technologies dimensions fill a
minimum space, and have a modular process;
system a
does not present these advantages. The
energetic results depend on the choice of the system for the production of electric and/or
thermic energy. Analysis results, listed in Figure 9, show how syngas brings higher
