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Pyrolysis Analysis and Solid Residue Stabilization of Polymers,
Waste Tyres, Spruce Sawdust and Sewage Sludge
M. Grigiante •M. Ischia •M. Baratieri •
R. Dal Maschio •M. Ragazzi
Received: 16 February 2010 / Accepted: 5 August 2010 / Published online: 4 September 2010
ÓSpringer Science+Business Media B.V. 2010
Abstract
Purpose The pyrolysis thermal treatment of several waste
such as polymers (PE, PVC, PS), sewage sludge, tyres,
waste wood as spruce sawdust and the successive stabil-
ization of the pyrolysis residue has been investigated on
analytical and energetic point of view. This thermal pro-
cess has been considered as it allows the reduction of the
waste mass with the recovery of its energy content, through
the exploitation of the produced gas phase as fuel.
Methods Analyzed plastics are pure polymers: Polyeth-
ylene ‘‘Riblene FF22’’ and polystyrene ‘‘Edistir 1910’’
furnished by Enichem, while polyvinylchloride has been a
K57 PVC furnished by EVC. The sewage sludge sample
derives from the urban wastewater treatment plant of
Trento, while the waste tyre is a SMR 10 Marangoni tyre.
Spruce sawdust has been furnished by a neighbouring
sawmill. The pyrolysis of the above indicated solid waste
was studied by thermogravimetry coupled to mass spec-
trometry, TG-MS and TG-GC–MS. This analytical
approach was followed by pyrplysis tests, carried out on a
selection of the waste materials, by using a pyrolysis bench
scale reactor.
Result The pyrolysis of all the wastes takes place in the
range of 400–600°C and leads the reduction of the 90% of
the mass for plastics, 50% for sludge, and ca. 60% for
tyres, with production of a fuel gas phase particularly rich
in hydrocarbons, with a estimated LHV from 15 to
32.8 MJ/kg for sewage sludge and plastics, respectively. A
schematic energetic analysis is proposed implementing the
pyrolysis stage with a vitrification process in order to
obtain, in particular for sewage sludge residue, a product
environmental friendly to use as raw material in industry.
Conclusions The promising perspective of a two steps
pyrolysis–vitrification process has been investigated to
exploit the heating power of the resulting gas phase and to
solve the environmental impact of heavy metals. The
proposed analytical and energetic analysis looks promising
for future improvements of this type of processes.
Keywords Pyrolysis study Thermogravimetric-mass
spectrometric-gas chromatographic analyses Waste
disposal Vitrification
Introduction
In the European Union many regions apply municipal solid
waste (MSW) management strategies, based on selective
collection and thermal valorization, in order to recuperate
energy and to minimize landfilling volumes. In Trentino,
an alpine region with little industry, mountain agriculture
and high tourism vocation, the main produced wastes are:
residual MSW (about 73,000 ton/year), sewage sludge
(50,000 ton/year), polymers and tyres (3,000 and
10,000 ton/year, respectively) wood waste, other special
waste and a few streams of MSW collected at the source.
To this concern selective collection of MSW has reached
M. Grigiante (&)M. Ragazzi
Department of Civil and Environmental Engineering,
University of Trento, Trent, Italy
e-mail: maurizio.grigiante@ing.unitn.it
M. Ischia R. Dal Maschio
Department of Industrial Technologies,
University of Trento, Trent, Italy
M. Baratieri
Faculty of Science and Technologies,
Free University of Bozen, Bozen, Italy
123
Waste Biomass Valor (2010) 1:381–393
DOI 10.1007/s12649-010-9038-2
65% of efficiency. Moreover, an incineration plant of
103,000 ton/year is planned, for the non selective collected
waste and a small part of special waste, as a consequence
that many sites used to landfill have exhausted their
capabilities.
Concerning the selective collected waste, pyrolysis was
taken into account as strategy to obtain the reduction of the
waste mass, with the recovery of its energy content,
through the exploitation of the produced gas phase as fuel.
Respect to incineration, pyrolysis appears to the public
opinion as an environmentally friendly strategy, with minor
health and landscape impact of incineration plants. This
scenario encourages the exploitation of alternative pro-
cesses such as pyrolysis and gasification, able to dispose
biomasses and MSW and to recuperate their energy content
[1–4].
The solid residue that remains as by product of the
pyrolysis process can present high concentrations of heavy
metals and requires then a stabilization strategy. For a
disposal solution close to the zero-landfilling scenario, the
vitrification process has been investigated as it satisfies the
following two specifications: it furnishes an inert vitreous
matrix that can be used as raw material in the glass industry
or as additive for cements, mortars and plasters; moreover,
it allows the contemporaneous stabilization of other wastes
with disposal problems, such as the incineration fly ashes,
whose landfilling is not an optimized solution [5,6].
The aim of this paper is an experimental study on the
complete disposal of the considered waste through: (i) a
pyrolysis study, carried out by thermogravimetric-mass
spectrometric analysis and lab-scale reactor tests, (ii) a
stabilization study of the main pyrolysis residue, by vitri-
fication in a lab-scale furnace, (iii) an energy analysis of
the pyrolysis–vitrification process.
Materials and Methods
Samples
The analyzed plastics were pure polymers: Polyethylene
(PE) ‘‘Riblene FF22’’ and polystyrene (PS) ‘‘Edistir 1910’’
were furnished by Enichem, while polyvinylchloride
(PVC) was a K57 PVC furnished by EVC.
The analyzed sewage sludge sample was furnished by
the urban wastewater treatment plant of Trento (population
about 100,000 inhabitants). The sludge was previously
dehydrated in the plant by belt-press filtration and dried.
The pyrolyzed waste tyre is a SMR 10 Marangoni tyre,
shredded and crumbed to produce an 8–10 mm size,
without steel in it.
The spruce sawdust was furnished by a neighbouring
sawmill, while the fly ashes were furnished by the
incinerator plant of Bolzano, also a neighbouring plant with
a capacity of 300 ton/day.
Instrumentation
In the characterization of the samples, elemental analyses
were carried out through an E.A. 1108 Fisons analyzer by
the Analytical Chemistry Laboratory, Department of
Chemistry, University of Padua, Italy.
The analyses of metals, contained in particular in the
dried sludge, were carried out on a Ciros
CCD
ICP-OES
Spectro spectrometer. The differential scanning calorime-
try on plastics was made by using a DSC 92 SETARAM
instrument. For experimental procedure, about 20 mg of
sample were used. This instrument operates in a range that
goes from environmental temperature (25°C) until 600°C.
The chosen speed of heating was 10°C/min, in a nitrogen
atmosphere.
The Mahler calorimeter used for the determination of
the calorific value of substances is a Calorimat CBM
Cecchinato. The calorimetry bomb, after the sample
charge, is saturated with 21–25 bar of pure oxygen.
Thermogravimetric (TG) and differential thermal anal-
yses (DTA) were performed on a LabSys Setaram ther-
mobalance operating in the 20–1,000°C range, with a
heating rate of 10°C min
-1
, under 100 cm
3
min
-1
He flow.
Samples were analysed by using a 0.1 cm
3
alumina cru-
cible and a-Al
2
O
3
as reference.
Gas chromatographic (GC) analyses were carried out on
an HRGC Carlo Erba Instruments chromatograph, equip-
ped with a GR8 Bimatic thermostated micro-valve for gas
sampling, and a VG-QMD-1000 Carlo Erba Instruments
quadrupole mass spectrometer as detector. Chromato-
graphic elutions were performed by using OV1 Mega
(15 m, 0.32 mm) and poraPLOT Q Chrompack (25 m,
0.32 mm) capillary columns with a temperature program of
30°C for 5 min, followed by 10°C min
-1
heating rate up to
200°C, held for 15 min. Helium was used as carrier gas
with 15 kPa inlet pressure. Electron impact mass spectra
(70 eV) were continuously registered and stored with fre-
quency of 1 scans
-1
ranging from 2 to 500 amu.
The pyrolysis, carried out on a small amount of the
above indicated samples, was studied by thermogravimetry
coupled to mass spectrometry TG-MS and gas chroma-
tography TG-GC–MS [7], by using a home-assembled
instrumental plant.
The themogravimetric balance was connected by the
mass spectrometer or by the gas chromatograph equipped
with the mass spectrometer as detector, by two types of
interface, realized by modifying the gas inlet system of the
thermobalance.
In TG-MS configuration, gas species, released from the
solid sample during thermal analysis, were drawn into an
382 Waste Biomass Valor (2010) 1:381–393
123
alumina tube fixed inside the furnace of the thermobalance
close to the sample crucible and then connected to a cap-
illary silica column heated at 300°C. The gases were then
directly sucked and introduced into the ionization chamber
of the mass spectrometer.
This instrumental interface allows the graph of the total
ion current (TIC), obtained from the contribution of all ions
present in each recorded mass spectra, plotted versus time
(or pyrolysis temperature). Moreover the plot of a selected
m/zion current (IC) can be showed. This fact allows to
detect any chemical species released from the sample,
following the trend of a typical ion of its fragmentation
pattern.
When the contemporaneous evolution of more species
takes place, the revealed mass spectrum in TG-MS con-
figuration is the sum of the contribution of all evolved
species. In this case, the second instrumental configuration
TG-GC–MS allows an accurate analysis, through the
chromatographic separation of the contemporaneously
evolved compounds and their identification by the suc-
cessive mass spectrometric analysis.
This second instrumental interface is realized by con-
necting the capillary silica column, heated at 300°C, to the
inlet port of a microsample valve. In this case, an external
vacuum pump continuously sucked a fraction of the gas
flow from the thermobalance through the valve, whereas,
an appropriate gas chromatographic capillary column was
connected to the outport of the valve. Gas sample injec-
tions were made for temperatures corresponding to the
most important mass losses during TG analyses. This
second instrumental configuration allows to separate and to
identify different compounds that are released during the
same mass loss.
The pyrolysis study of significant amount of samples
was carried out on a pyrolysis bench scale reactor, that
consists of a cylindrical-shaped vessel – manufactured
using Incoloy 800 (30%Ni, 21%Cr, 46%Fe) – and having
internal diameter of 142 mm, height of 400 mm and a
thickness of 4 mm. The reactor is inserted from above in a
cylindrical furnace, open on the top side, where all the
control and acquisition devices are connected. The heating
is provided by several electric resistances (maximum
power 10.5 kW at 230 V, 50 Hz), up to a maximum work
temperature of 1,000°C.
The power supplied to the resistances is regulated by a
PID temperature controller controlling both the maximum
temperature (i.e. the desired temperature) and the heating
rate (temperature ramp). The temperature is measured
using two K-type thermocouples insulated by a refractory
sheath and inserted into the furnace through the insulation
layer at 26 cm from the top side of the furnace. The data
acquisition unit consists of an Agilent34970 multimeter
(DMM 6accuracy) with a 20-channel plug-in module
connected to a PC by means of a RS-232 port. For a more
detailed description of this apparatus reference is made to
[8].
Finally, for the lab-scale pyrolysis tests on worn-out
tyres, a quartz tube immersed into a horizontal furnace was
used. The quartz tube reactor was heated up to 400°C. The
tyre sample, in scales, was previously wrapped by a
metallic mesh, that keep the pyrolysis solid char. After the
process, the pyrolysis oil was collected around the quartz
tube and recovered for its characterization.
Results
Ultimate analysis carried out on the investigated samples
allows characterising their compositions with respect to
carbon (C), hydrogen (H) and other elements. The analysis
results are reported on the following Tables 1and 2; the
polymers analyzed were crude PE, PS and PVC.
Characterization of the Pyrolysis Processes by TG-MS/
TG-GC–MS and Reactor Tests
The pyrolysis was studied by a double instrumental
approach: firstly an analytical study by thermogravimetry
(TG) coupled with mass spectrometry (MS) and gas
chromatography (GC) analyses TG-MS/TG-GC–MS.
These analyses were followed by several experimental tests
carried out on a pyrolysis lab-scale reactor and a gas
chromatograph (GC) as analyzer, in order to identify the
evolved gas species.
Table 1 Proximate analysis of the sewage sludge and the spruce
sawdust samples
Waste Moisture Volatile solids Ash
Sewage sludge 1.5 67.0 31.5
Spruce sawdust 20 79.21 0.79
Table 2 Ultimate analysis of the selected waste samples. Spruce
sawdust refers to a moisture content of 20%
Element or specie
wt%
Sewage
sludge
Automobile
tyres
Spruce
sawdust
C 36.0 85.9 40.49
H 4.5 7.8 4.5
N 5.6 0.4 0.09
S 0.5 0.5 0.0
Ashes – 5.0 0.79
O (by difference) – 0.4 34.13
Waste Biomass Valor (2010) 1:381–393 383
123
Although TG-MS/TG-GC–MS analyses allow to pro-
cess a very small amount of waste (10–30 mg of sample),
this instrumental analysis furnishes relevant information
about the sample that are indispensable to study and scaling
up a possible process treatment based on pyrolysis [9,10].
TG analysis furnishes both the thermal stability range of
the solid waste, its mass loss % and its thermal decompo-
sition temperature interval. Moreover, by changing the
atmosphere (inert or oxidizing conditions), or the heating
rate, it allows to study the behaviour of the sample at
different thermal treatments [11,12]. Finally, the interface
with a mass spectrometer or a gas chromatograph allows an
accurate characterization of the gas species evolved from
the waste during its thermal treatment.
To obtain a more realistic behaviour of the investigated
process and to study the effects of the most important
thermodynamic variables, a significant amount of the
selected materials has been treated on a pyrolysis bench
scale reactor having a volume of 6.3 l, operating at a
maximum temperature of 800°C and a pressure of at
3 barg. A micro gas-chromatograph has been utilized to
detect the most important gaseous compounds coming
from the process: H
2
,O
2
,N
2
,CH
4
, CO, CO
2
,C
2
H
4
,C
2
H
6
and C
2
H
2
.
Pyrolysis of Plastics
The pyrolysis of PE takes place in a single step in the
400–510°C temperature range. The observed mass loss was
95.2%, which corresponds to the evolution of more
hydrocarbons. These were identified by a gas sampling at
490°C (TG-GC–MS analysis). 1-Alkenes were the major
observed species, beside other alkenes, alkanes and dialk-
enes [13,14]. Figure 1shows the TG plots of all the ana-
lyzed samples.
Polystyrene pyrolysis takes place in a single step in the
300–450°C interval. Thermogravimetric analysis shows for
this polymer a mass loss of 96.3%. The thermal depoly-
merization is the main reaction that occurs into the mass of
the plastics subjected to pyrolysis process. In fact, mass
spectrometry reveals the evolution of a gas phase that is
mainly constituted by styrene (98.6%). The very small
amounts of a-methylstyrene and toluene that were
observed, indicate a non-regular breaking of the polymeric
chains, with hydrogen transfer reactions [15,16].
Two separated events take place during the pyrolysis of
polyvinylchloride. Firstly, between 225 and 370°C the
polymer undergoes the dehydrochlorination of the poly-
meric chains that successive rearrange by aromatization
reactions with evolution of HCl and benzene [17,18]. The
mass loss observed in this temperature interval is 56.9%.
The successive thermogravimetric event, in the 400–550°C
interval, with a mass loss of 24.3%, leads to the evolution
of a mixture of hydrocarbons, similar to that observed in
the pyrolysis of the polyethylene sample, as product of the
thermal decomposition reaction of the solid residue.
The pyrolysis of these polymers was studied also on a
blend (40% PS and 30% PVC and PE, respectively). A
continuous mass loss, with intensity of 90.1%, was
observed in the 250–550°C temperature interval. Events,
centred at 295, 420 and 480°C, respectively, describe the
pyrolysis of the blend, that is characterized, at 295°C, by
the reactions that occur on the PVC sample, i.e. dehydro-
alogenation and aromatization. The following Fig. 2makes
evidence, in the TG-MS analysis, the correspondence
between the mass loss and the evolution of the detected gas
species.
Pyrolysis of Sewage Sludge
TG-MS analysis of sewage sludge presents a mass loss of
61.4% in the 20–1,000°C interval. Two events are showed
during the thermal treatment in inert atmosphere: the first,
more intense (51.8%, 100–600°C range), is produced by the
evolution of CO
2
and H
2
O, and hydrocarbons, which max-
imum release takes place at 325°C and 460°C, respectively.
Regarding the hydrocarbons released at 460°C, several
compounds are observed in TG-GC–MS analyses: CO
2
,
CO, H
2
O, alkanes and alkenes up to C
5
, toluene, styrene,
C
8
and C
9
hydrocarbons are the main chemical species
produced.
The second event observed in TG-MS analysis (mass
loss 9.6%, 600–1,000°C range), is produced by elimination
of CO
2
from carbonates and small amount of CO from
organic residues.
At lower temperatures, CO and CO
2
arise from pyrolytic
decomposition of partially oxygenated organic compounds
(lipids, carbohydrates, cellulose, lignin into the sewage
sludge), also acetaldehyde and acetone derive from oxy-
genated organic compounds, while cyano compounds, can
be formed by dehydrogenation of amino groups that are
present in proteins, nucleic acids and dead micro-organ-
isms present in the solid sludge, as observed in other
studies [2].
Pyrolysis of Automobile Waste Tyre
The pyrolysis of an automobile waste tyre was studied also
by lab-scale reactor tests, because the themogravimetric
coupled mass spectrometric analysis is not sufficient for the
characterization of the pyrolysis process. Although TG
analysis shows an intense mass loss in the 300–500°C
interval, with intensity of –59.2%, the real time monitoring
of the evolved gas phase does not show all the products of
the thermal decomposition of tyres. In fact, the tyres
pyrolysis oil (TPO) condenses in the transfer line of the
384 Waste Biomass Valor (2010) 1:381–393
123
TG-MS plant, and it is not identified by the mass spectro-
metric analysis [19].
For this reason, scrap tyres pyrolysis reactions were
carried out on a quartz tube batch reactor immersed in a
lab-scale furnace, heated up to 400°C. The pyrolysis
reaction lead to the production of an oil (40%wt of the
scrap tyre mass), a 20%wt gas phase previously charac-
terized by TG-GC–MS and a 40% of solid residue. The
amount of tar and char is very similar to that observed by
Berrueco et al. and by Boxiong et al. [20,21].
The liquid and solid phases were characterized by ele-
mental analysis and heating value tests; TPO shows C and
H percentages of 84.6% and 14.2%, respectively (C/H
molar ratio close to ), with very small amount of N
(0.2%), while S was not detected. The absence of sulphur
in the TPO gives to this pyrolysis oil high interest. The
measure of calorific values, carried out with an adiabatic
oxygen bomb calorimeter, shown high heating value
(HHV) of 49.5 MJ/kg and low heating value (LHV) of
46.7 MJ/kg, respectively.
Fig. 1 Thermogravimetric
analyses of polyethylene,
polyvinylchloride, polystyrene,
a blend of the three polymers
(30% PE, 30% PVC and 40%
PS), wastewater sewage sludge,
spruce sawdust, and worn out
tyres samples
Waste Biomass Valor (2010) 1:381–393 385
123
The characterization of the solid char (40%wt of the
scrap tyre mass), reveals this chemical composition: C
82.2%, H 4.9%, N 0.4%, S 1.0%, O 0.4%, and inert ashes.
The combustion of the solid residue in an adiabatic oxygen
bomb calorimeter gives HHV and LHV of 34.7 and
33.7 MJ/kg, respectively. The amount of tar and char is
very similar to that observed in other studies [20,21].
Pyrolysis of Spruce Sawdust
Thermogravimetric analysis, in inert atmosphere, of the
spruce sawdust shows two events, during its thermal
treatment. The evolution of water is the first event, in the
80–200°C interval (mass loss 8.3%). The second event
takes place in the 200–500°C interval and is characterized
by the thermal decomposition of the sawdust to volatile
compounds (mass loss 69.8%). The char content has been
measured analyzing the residual matter after a pyrolysis
experimental run, and results 21.9% of the biomass sample.
Other pyrolysis runs were carried out on a lab-scale
reactor. On more spruce sawdust samples, moisture deter-
mination analysis were carried out, by performing several
runs at 105°C; values ranges from 7.65 to 8.73%. The dried
sample was then pyrolyzed, and the produced gas phase
was analyzed by gas chromatograpy. The gaseous effluent
mainly consists of, CO, CO
2
,H
2
, and CH
4
, also with low
concentrations of C
2
H
4
(ethene), C
2
H
6
(ethane) and traces
of C
2
H
2
(ethyne). The small hydrocarbons molecules
(alkanes, alkenes and alkynes compounds) found in the
analyzed mixture are the results of the cracking action of
Fig. 2 TG-MS analyses of
polymer samples and plastics
blend. Trends of derivative
thermogravimetric curve (DTG)
and evolved gas analysis,
detected as total ion current
(TIC) by the mass spectrometer.
For all analyses is evidenced the
correspondence between the
mass loss and the evolution of
the detected gas species
386 Waste Biomass Valor (2010) 1:381–393
123
the pyrolysis process that tends to split large molecules
(tar) into smaller ones [22,23].
The char remaining on the bottom of the reactor after an
experimental run (a similar amount observed in thermo-
gravimetric analysis), has been also characterized by the
elemental analysis. The ultimate analysis on the residual
solid matter showed that it consists mainly of solid carbon
(95.1% mass fraction), ash and other elements (H and N) in
traces.
Summary of the Pyrolysis Experiments
The analytical characterization of the pyrolysis process
carried out on the considered wastes allows to investigate
their thermal decomposition, in terms of temperature range,
and to estimate both the waste mass loss intensity and the
composition of the gas phase produced during this thermal
treatment.
Table 3reports the produced gas species and the cor-
respondent investigated temperature range. A more
detailed composition of the gas phase evolved during the
pyrolysis process of PE, PVC, sewage sludge, worn out
tyres, and spruce sawdust are reported on Table 4, in which
the percent values are calculated from peak areas of gas
chromatographic elution (TG-GC–MS analyses).
Stabilization of Pyrolysis Residue by Vitrification
The pyrolysis study carried out on the selected wastes
makes evidence a significant reduction of the mass waste
and the resulting gas phase contains species having interest
both as chemicals and energy source. However, pyrolysis
does not represent a definitive solution for the waste
treatment. As matter of fact, in some cases, the remaining
solid residue of the pyrolysis process must be stabilized.
This is mainly the case of the wastewater sewage sludge
whose pyrolysis residue is particularly rich in metals. For
the other analysed wastes, the treatment of the solid residue
is less critical as it mainly consists in graphitized carbon.
The following Table 5reports the content of metals pre-
sents on the dried sewage sludge sample utilised during this
study.
After the pyrolysis of wastewater sewage sludge, the
26% of SiO
2
inside the pre-treated waste reaches a value of
about 60%, while the Al
2
O
3
of about 5%. This sufficient
amount of SiO
2
suggests the stabilization of the sludge
pyrolysis residue by means of the vitrification process.
Vitrification is a reliable method that considerably reduces
the environmental impact of the heavy metals. These are
thus immobilized in a stable vitreous-matrix. Moreover, the
high temperatures (1,300–1,450°C) of the process lead to
the thermal destruction of organic-chlorinated compounds
that can be present in the solid waste.
The so obtained vitreous matrix presents high resistance
to leaching, and high durability; it does not need landfill
disposal and it can be used as sintering additive in the
ceramics industry or as roadbed with considerable savings
on raw materials. Since 30 years, vitrification is considered
the best available technology for the immobilization of
Table 3 Summary of the main quantities and proximate analysis of
the pyrolysis gas of the selected wastes
Sample Temperature
range (°C)
Mass loss
intensity (%)
Produced gas
species
PE 400–500 95.2 Hydrocarbons
PVC 225–370 56.9 HCl, benzene
400–550 24.3 Hydrocarbons
PS 300–450 96.3 Styrene
Sewage
sludge
100–600 51.8 CO
2
,H
2
O,
Hydrocarbons
600–1000 9.6 CO, CO
2
,
Tyres 300–500 59.2 Oil, Hydrocarbons
Spruce
sawdust
200–500 69.8 CO
2
,
Hydrocarbons
Table 4 TG-GC–MS analyses
characterization of the gas phase
that originates from the
pyrolysis process
Sample Polyethylene Polyvinylchloride Sewage sludge Tyres Spruce sawdust
Tof the sampling
species
T=490°CT=455°CT=460°CT=390°CT=500°C
Molar % Molar % Molar % Molar % Molar %
CO
2
– – 12.9 – 21.8
CO – – – – 32.2
H
2
– – – – 30.5
C
1
–C
3
15.5 34.3 20.8 2.1 8.0
C
4
14.0 15.6 15.7 2.5 –
C
5
11.6 16.1 9.1 82.6 –
[C
5
58.9 34.0 20.4 12.8 –
H
2
O – – 14.7 – 15.5
N-compounds – – 6.4 – (N
2
free)
Waste Biomass Valor (2010) 1:381–393 387
123
radioactive wastes. The vitrification process can stabilize
other wastes with disposal problems, such as ashes from
thermoelectric power plants, or from incinerators, absestos,
muds of different origin, and metallurgical slag, and gives
an excellent leaching resistant material [5,6].
The costs of the treatment, given by the high con-
sumption of energy during melting, can be reduced by the
transformation of the obtained glass to a marketable
product. For a good vitrification of the solid residue, the
charge mass must present 40–50% of silica; this compo-
sition can be obtained by adjusting the amount of wastes
devoted to vitrification. Moreover, the presence of a suf-
ficient amount of CaO, makes the resultant vitreous matrix
resistant against leaching tests [24,25].
Vitrification tests, carried out on pyrolysis solid residue
samples by using a lab-scale oxy-fuel fired kiln, have
produced the glass showed in Fig. 3. This glass presents a
good behaviour on leaching tests of heavy metals. In fact,
the percolate solution shows heavy metals concentration
values which are lower than the imposed limits determined
by IRSA and US-EPA rules. In most cases, these values are
lower even of an order of magnitude, as shown in Table 6.
Energy Analysis of the Pyrolysis–Vitrification Process
Case 1 – Sewage Sludge
A plant layout for the complete thermal treatment of
sewage sludge is proposed on the following Fig. 4, where
the terms M, R, S refer both to mass flux, recovered energy
and energy demand, respectively. Table 7describes the
mass and energy fluxes arising from the indicated plant
units. As the proposed plant refers to a general scheme, the
mass fluxes are only indicated while, for all the energy
fluxes, the quantities have been calculated on the basis of
1kgh
-1
of dried sludge (kg
DS
) and indicated in Table 7in
terms of kWh kg
DS
-1
. Due to its high moisture content,
sewage sludge flux (M1) is pretreated on a dryer unit to
reduce its moisture content to 10–20% making then the
sludge available as a quality biosolids fuel. The dried
biosolids sludge (M2) is fed into the pyrolysis conversion
stage while the evaporated water is removed (M4). The
char and ashes residual fraction discharged from the
pyrolysis unit, flux (M3), undergoes the vitrification in
the oxy-fuel unit, to be stabilized. The synthesis gas
(or pyrolysis–gas) (M5) is burned on the burner unit.
Table 5 Chemical species and content of metals in the dried
(105°C) sewage sludge
Chemical species wt% Metals mg kg
-1
SiO
2
26.0 As –
Al
2
O
3
1.0 B 62
CaCO
3
13.0 Ba 460
MgCO
3
1.0 Bi 6
Fe
2
O
3
6.7 Cd \4
Na
2
O 0.2 Co 4
K
2
O 0.1 Cr 130
Cu 240
Ga 4
Hg –
In 2.2
Li 1.34 910
3
Mn 128
Ni 24
Pb 16
V–
Zn 540
Fig. 3 The pyrolysis residue of the wastewater sewage sludge,
vitrified in a lab-scale oxy-fuel fired kiln
Table 6 Detected values of heavy metals concentration in leaching
tests, compared with their limits
Heavy metals Detected value/mg L
-1
Italian limit value/mg L
–1
As \0.001 0.5
Cd \0.001 0.02
Cr 0.007 0.2
Cu 0.09 0.1
Mn 0.006 2.0
Ni 0.011 2.0
Pb 0.007 0.2
Zn 0.044 0.5
Hg 0.0006 0.005
Se \0.003 0.03
Ba 0.03 20.0
Fe 0.03 2.0
Sn \0.001 10.0
Al 0.005 1.0
388 Waste Biomass Valor (2010) 1:381–393
123
The energy flux obtained from the syngas (R1) is
recovered to support the energy demand for the pyrolysis
conversion stage (D2) and dryer unit (D1). For the vitrifi-
cation stage both the energy demand (D3) and the energy
recovery (R2) carried out on the off-gas stream have been
considered. All these contribution allow to determine the
energy balance of the entire process expressed by the fol-
lowing Eq. 1:
Energy balance ¼R1þR2D1þD2þD3ðÞð1Þ
Two scenarios have been examined selecting two temper-
ature values of 600°C and 800°C for the pyrolysis process
and, for each temperature, two moisture content percentage
of 10% and 20% have been considered. Table 7reports
also the temperature values assumed to calculate the energy
fluxes for each of the stages of the described process. For
the evaluation of the syngas produced by the pyrolysis
stage, a thermochemical equilibrium approach has been
adopted. This methodology allows to obtain only a rough
estimation of the syngas composition because complete
thermodynamic equilibrium conditions cannot be reached
inside a pyrolysis reactor. This approach was investigated
and tested by the Authors on biomass gasification processes
[8,26] and the obtained results are quite satisfactory if
compared with published experimental results [27,28].
The approach adopted in this work is based on the Gibbs
energy minimization method and consists in evaluating the
concentrations of the species present that minimize the
PYROLYSERDRIER
TDRY
VITRIFICATION
SYNGAS
BURNER
2M1M
M4
M3
M3
D1
D2 D3
R2
M5
R1
HEAT RECOVERY
ENERGY DEMAND
TPYR,MIN ÷ TPYR,MAX TVIT
TOFF-GAS
Fig. 4 Proposed plant layout
for the sewage sludge treatment
Table 7 Energy balance of the process, mass fluxes indication and process parameters
Sewage sludge
Pyrolysis temperature 600°C 800°C
Dried Sample moisture content (%mass) 10 20 10 20
Energy fluxes (kWh kg
DS
-1
)
D1 Drier energy demand (water evaporation) 2.565 2.200 2.565 2.200
D2 Pyrolysis stage energy demand 0.404 0.492 0.565 0.690
D3 Vitrification stage energy demand 0.826 0.734 0.826 0.734
R1 Energy recovered form the syngas
combustion
1.438 1.491 1.506 1.658
R2 Energy recovered form the vitrification stage 0.099 0.088 0.099 0.088
Global energy
balance
–2.258 –1.847 –2.351 –1.878
M1 Wet sludge into the dryer
M2 Dried sludge discharged from the dryer
M3 Ash discharged from the pyrolysis process
M4 Water evaporated from the drier
M5 Synthesis gas from the pyrolysis process
Process temperature (°C)
T
DRY
Dryer temperature 105
T
VIT
Vitrification temperature 1,500
T
F
Off-gas temperature 1,170
Waste Biomass Valor (2010) 1:381–393 389
123
total Gibbs energy of the products, in accordance with the
constraints imposed by the principle of conservation of
mass and of the stoichiometry (elements conservation). The
chief advantage of this method is that it does not require to
select a number of ‘‘representative’’ chemical reactions
allowing the formation of (equilibrium) products; it is nev-
ertheless necessary to establish a list of chemical species
inclusive of the ones expected in the product mixture.
A Matlab code has been developed using the Cantera soft-
ware library (a collection of object-oriented software tools
for problems involving chemical kinetics, thermodynamics,
and transport processes [29]. The solver implemented in
Cantera [29] is a version of the Villars-Cruise-Smith (VCS)
algorithm (a well suited method to handle multiphase
problems), that finds the composition minimizing the total
Gibbs free energy of an ideal mixture [30]. The NASA
(McBride et al.) [31] and the GRI-MECH (Smith et al.) [32]
databases have been used to evaluate the thermodynamic
properties of the chemical species considered in the
model.
Case 2 – Spruce Sawdust
The proposed plant layout for the complete spruce sawdust
thermal treatment is proposed on the following Fig. 5. The
terms M, R, S, have the same meaning previously adopted.
The pretreatment on a dryer unit is not necessary in this
case as, usually, this type of biomass presents a moisture
content on the range from 10% to 20%. The M1 spruce
sawdust flux feeds directly the pyrolysis conversion stage
while producing the synthesis gas (or pyrolysis–gas) (M4)
which burns on the burner unit. The char flux (M2) intro-
duced on the char burner (M2) allows to recover the energy
flux (R2)while the ashes flux (M3), due to its very limited
quantity, is discharged from the pyrolysis unit. For the
same reason, the vitrification stage is not taken into account
in this case. The global energy balance of the process is
expressed by the following Eq. 2:
Energy balance ¼R1þR2D1ð2Þ
For this case too, two scenarios were examined for tem-
perature values of 600°C and 800°C and two moisture
content percentages of 10% and 20%. The following
Table 8reports the obtained results with the use, as done
for sewage sludge, of a thermodynamic equilibrium
approach to determine the composition of the gas phase
produced by the pyrolysis unit. We have considered, as
ultimate analysis for the spruce sawdust, the values
reported on the next Table 9, evaluated on the basis of 1 kg
of wet biomass.
Discussion
The main effect of the experimental pyrolysis tests so
carried out consists in the relevant reduction of mass of the
processed wastes. In fact, pyrolysis leads to the reduction
of about the 90% for plastics, the 50% for sludge, roughly
the 60% for tyres and the 80% for spruce sawdust. Making
reference to the indicated process parameter values, the
estimated LHV for the gas phase obtained from the pyro-
lysis stage is 15.22 MJ kg
-1
for sewage sludge, 19.78
MJ kg
-1
for plastics and a mean value of 15.3 MJ kg
-1
for
the spruce sawdust. These values, on a dry basis, were
computed by means of the Dulong formula.
On the basis of the energy balance, for the case of
sewage sludge and the reference to the mean of the four
values reported on Table 7, the total energy demand
(D1?D2?D3) amounts to 3.7 kWh kg
DS
-1
while the total
recovered energy (R1?R2) amounts to 1.616 kWh kg
DS
-1
providing then the 43.6% of the total energy demand. This
value corresponds to 67.8% of the energy required by the
drying unit. As general results, the energy balance of the
plant points out that the process involving the sewage
sludge does not allow to obtain a self-sustainable process
as indicated by the negative values of the global energy
balance reported on Table 7. This is mainly due to the
energy consumed to dry this waste which usually presents a
very high moisture content. On the other hand, the com-
bined vitrification process allows stabilizing all the
obtained residue. The final advantage is that a vitreous
matrix of the total residue, not requiring landfill disposal,
can be obtained. However, due to the consistent energy
amount required (about 50%), this solution must be deeper
investigated in particular in terms of an optimizing energy
procedure.
For the case of spruce sawdust, the proposed process
allows obtaining a self-sustainable solution as indicated by
the positive values of the energy balance reported on
PYROLYSER
CHAR
BURNER
SYNGAS
BURNER
M1 M3M2
M3
D1
R2
M4
R1
HEAT RECOVERY
ENERGY DEMAND
T
PYR
Fig. 5 Proposed plant layout for the spruce sawdust treatment
390 Waste Biomass Valor (2010) 1:381–393
123
Table 8. As the spruce sawdust is characterized by a
reduced value of moisture content with respect to the
considered sewage sludge, the required energy for the
drying process is completely saved. Moreover a greater
energy productivity (in terms of pyrolysis products) is
realized with respect to the sewage one and the ash content
is lower at all. In this case, on the basis of the composition
reported on Table 9, the estimated LHV for the gas phase
obtained from the pyrolysis presents a value of
14.30 MJ kg
-1
for the biomass moisture content of 20%
and 16.40 MJ kg
-1
for the case having a moisture content
of 10%.
Considering that for developed and emerging countries
the amount of wastes is particularly high and increasing in
time, the proposed process stands out as a strategic
opportunity. A real example can be deduced considering
the update amount of waste produced in authors country
(Italy): 3,000 ton/year of plastics, 50,000 ton/year of dried
sewage sludge, 10,000 ton/year of worn out tyres. Taking
into account the mass reduction percentage that were
observed in this experimental work, a considerable amount
of gas phase with a satisfactory LHV can be obtained. The
proposed disposal process can represent an effective real
strategy, since a similar plant exists in Italy and operates in
Terni, in the Middle Italy. The plant is a pyrolysis pilot
plant for the thermal valorisation of a mix of wastes
managed by ASM-Terni [33]. The pyrolysis reactor is a
rotative kiln, indirectly heated. The kiln is enclosed in a
refractory fire-box. The pyrolysis oil, that evolves from the
solid during the process as aerosol with the syngas, is
condensed and recycled for the optimization of syngas
production.
Advanced vitrification technology can however improve
the global efficiency of the entire process as the applica-
tion, in this field, of the oxy-combustion process that has
been revealed, in recent years, as a valuable option. The
oxy-combustion process, if compared with traditional
processes in air or with electric furnaces, allows a flame
temperature higher than the one of a traditional combustion
process and a higher efficiency, with the drastic decrease of
the off-gas volume (by over 70% [5]) and of the particulate
content. Although energetic consumption sensitively
affects the cost of production, the final product presents
features, for remunerative applications that can compensate
the costs of this technology.
Conclusions
A two steps pyrolysis–vitrification oxy-fuel furnace pro-
cess was investigated and applied to sewage sludge, tyres,
polymers and spruce sawdust pyrolysis residues. The pro-
posed work was mainly dedicated to the experimental
characterization of the pyrolysis reaction products of the
treated materials by using several techniques, such as
proximate and ultimate analyses, TG-DTA, DSC, TG-MS
and TG-GC–MS analyses, ICP-AES. The promising per-
spective of vitrification process was analysed to solve the
environmental impact of heavy metals. This process allows
Table 8 Energy balance, mass fluxes indication and process parameters of spruce sawdust
Spruce sawdust
Pyrolysis temperature 600°C 800°C
Dried sample moisture content (%mass) 10 20 10 20
Energy fluxes (kWh kg
DS
-1
)
D1 Pyrolysis stage energy demand 0.069 0.293 0.338 0.587
R1 Energy recovered form the syngas combustion 1.301 1.415 1.713 1.873
R2 Energy recovered form the char combustion 1.309 1.046 1.137 0.853
Global energy balance 2.541 2.168 2.512 2.139
M1 Wet spruce sawdust feeding the pyrolysis stage
M2 Residual char discharged from the pyrolysis process
M3 Ash discharged from the pyrolysis process
M4 Synthesis gas from the pyrolysis process
Process temperature (°C)
T
F
Off-gas temperature 1,170
Table 9 Ultimate analysis of the spruce sawdust and ashes content
Element or specie
wt%
Moisture content
10%
Moisture content
20%
C 44.93 40.49
H 5.37 4.5
O 38.77 38.84
N 0.10 0.09
S 0.0 0.0
Ashes 0.89 0.79
Waste Biomass Valor (2010) 1:381–393 391
123
their immobilization in a leach-resistant material without
requiring a landfill collocation as it can be utilized as raw
material in several fields of ceramic industry or as road
bed.
In the case of sewage sludge and spruce sawdust thermal
treatments, the present study estimates in particular the
energy balance of the entire pyrolysis–vitrification process.
It clearly emerges that process that involves sewage sludge
requires a supply of energy as the recovered amount
reaches only the 43.6% of the total energy demand. This is
due to the relevant amount required to reduce the moisture
content from 80% to the assumed 20–10% content. Con-
siderable advantages appear in the case of spruce sawdust:
the lower moisture content avoid the use of a dryer unit and
the global energy balance of the plant, considering the
combustion of the gas and char, allows to obtain a positive
value. Due to the very low ashes content and the absence of
the heavy metals, the stabilization of residual ashes through
a vitrification unit is not required.
Considering the pyrolysis process, the proposed analysis
looks promising to improve the knowledge of this thermal
treatment as it can be applied to existing cases as the cited
Terni plant. Regarding the vitrification process, this per-
spective, evaluated in terms of the stabilization of the solid
residue, emerges as particularly attractive. Even though
this last process requires a deep analysis in particular on the
energy point of view, the proposed work looks promising
and stands as a basic contribution to enhance the effective
perspectives for the exploitation of this specific type of
wastes.
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