Page 48 - Impiantistica industriale

NEW TECHNOLOGIES

fractions which alternatively would have been disposed zone on top of reactor, where a further introduction of
best through – at least – landfi lling or – incineration with auxiliary fuel and oxygen lead to an increase of temper-
energy recovery. The waste-to-chemical process allows ature ensuring tar degradation, full decomposition of the
The concept of circular carbon and hydrogen recovery, i.e. long chain organic molecules and inhibition of dioxins
contextually material and energy
formation.
economy is currently recovery.
“re-designing many industrial Refuse Derived Fuel (RDF), dry

fields with the aim of waste fraction of unsorted fraction of
Municipal Solid Waste (MSW), and
streams valorization unrecycled fraction of plastic sort-
ed waste, are kind of waste eligible
for the waste-to-chemical process. An innovative route
for circular Hydrogen production is here presented and
described from technical, economic, and environmental
point view.

1. High temperature gasification
for waste valorization
As already mentioned, waste feedstock like Municipal
Solid Waste, Refuse Derived Fuel (RDF) and plastics res-
idues, due to the high content of carbon and hydrogen,
may be considered a sort of alternative and sustainable Figure 1 - High temperature gasifi cation reactor
feedstock to be used in substitution of conventional fos-
sil. Typical compositions for the above-mentioned waste Multiple injection of oxygen and auxiliary fuel along the
are reported in the Table 1. As shown by the elementary reactor, take temperature in order of 1600-2000°C in the
composition, carbon content may vary in the range 30- melting zone, 600-800°C in the gasifi cation zone up to
60%w while hydrogen in the range 4-7%w. 1100-1200°C on the top. Such temperature profi le as-
If properly converted into syngas, these kinds of wastes sures a full conversion of waste into two products: a high
may be used for the synthesis of a wide range of chem- valuable syngas rich in H , CO and free of char, tar, di-
2
icals [4]. oxin and furans discharged from the top of reactor and
Under this scenario technology plays a major role in the an inert vitrifi ed material discharged on the bottom [5].
fully implementation of circular economy around the con- The high temperature held on the melting zone allows to
cept of waste as feedstock for industrial processes. This discharge the inert components of waste (mineral and
paradigm implies a robust and reliable technology able metals), in a granulated and vitrifi ed state ideally carbon
to manage the heterogeneous nature of waste as well as free. Depending on local legislation, such material can be
their pollutants content. valorized into cement or construction industry otherwise
The proposed technology allowing to convert waste into disposed as standard waste.
chemicals, is based on a high temperature gasifi cation As reported by Salladini et al [6], the syngas yield and
process carried out under pure oxygen environment. A relevant composition, are mainly affected by the LHV val-
schematic view of gasifi er reactor allowing to perform ue and C/O ratio. On the overall higher LHV results into
such conversion is shown in Figure 1. higher syngas yield as well as higher content in terms
The gasifi er reactor consists of three sections: the melt- of CO an H and lower concentration of CO . Produced
2
2
ing zone on the bottom of reactor, where exothermic re- syngas contains as major components CO, H , CO and
2
2
actions and melting of inert compounds take place; the under minor content volatile metals and any particles up
gasifi cation zone in the middle, where low oxygen-con- drafted with the syngas. Figure 2 reports a block dia-
tent brings to partial oxidation reactions; the stabilization gram of the gasifi cation section, together with preliminary
cleaning and syngas purifi cation section.
Component Value RDF PW As fi rst step the hot gas exiting the reactor is routed to
Wet basis an evaporative quench where temperature is abruptly
C % weight 33-38 47-61 reduced down to 85-90°C by direct injection of water.
Although there is a loss of high temperature heat, this
H % weight 4-5 5-7 rapid cooling freezes chemical composition achieved at
O % weight 16-18 14-20 high temperature avoiding any undesired reaction. The
N % weight 0.2-1.0 0.2-0.5 two-phase mixture at the bottom exit of quench is rout-
ed to a sedimentation tank. This unit allows to collect
S % weight 0.02-0.15 0.02-0.3
on the bottom the sludge, continuously removed from
Cl % weight 0.8-1.5 0.8-1.5 the system, and clarifi ed water reused as cooling wa-
Moisture % weight 17-21 5-9 ter in the quench. The Sedimentation works under low
Inert % weight 17-25 7-20 pH condition (1.5-3) in order to promote the migration
of volatile metals in liquid phase. The syngas exiting the
LHVwet MJ/kg 14-16 21-24 sedimentation tank is routed to an acidic column that
Table 1 - Typical elementary composition of PW and RDF and relevant LHV values. further promotes the metal removal.

44 44 Impiantistica Italiana - Gennaio-Febbraio 2022