Advances in Chemical Engi neering and Science , 2011, 1, 183-190
doi:10.4236/aces.2011.14027 Published Online October 2011 (
Copyright © 2011 SciRes. ACES
Formation and Elimination of Pollutant during Sludge
Decomposition in the Presence of Cement Raw Material
and Other Catalysts
Juan A. Conesa, Araceli Gálvez, Ignacio Martín-Gullón, Rafael Font
Department of Chemical Engineering, University of Alicante, Alicante, Spain
Recieved January 28, 2011; revised March 25, 2011; accepted May 12, 2011
The use of a waste-based secondary fuel in clinker kilns is a widely used practice. Nevertheless, specific
studies to understand the destruction mechanism of exhaust pollutants in cement raw material (CRM) are
limited. This work focuses on the possible catalytic effect of the interaction of exhaust gases from the com-
bustion of sewage sludge with various solids beds, including CRM. Catalyst based on vanadium pentoxide
and deNOx commercial catalyst, based on Ti/Zr/Pt, were used. The behaviors of volatile compounds,
polycyclic aromatic compounds and dioxins and furans are analyzed in the presence or absence of the
different materials. Some compounds are produced when interacting the pollutants with the beds, and some
others are destroyed. Results show that the presence of CRM at the outlet of the combustion gases is
beneficial for the decrease in pollutant emission, confirmed by a catalytic effect of CRM at medium
Keywords: Dioxin, Polycyclic Aromatic Hydrocarbons, Sewage Sludge, Cement, Emissions
1. Introduction
Currently, a common way of eliminating sewage sludge
is its combustion in either specifically designed incin-
erators or in cement kilns. Incineration is the destruction
by thermal oxidation at high temperature to convert
waste material into a much smaller inert volume (not
dangerous), i.e. the combustion of waste at high tem-
perature in the presence of large amounts of oxygen in
order to eliminate it. It can be useful to take advantage of
these thermal resources while si multaneously eliminatin g
waste. This is the case in using waste as an alternative
fuel or energy recovery in industrial boilers and furnaces
of clinker in the cement industry [1].
The use of alternative fuels has been an established
practice in most developed countries for over the last
thirty years. This is an activity being developed in
Europe with total environmental qua lity since 1975. This
activity has both societal and industrial benefits, such as
decreasing non-renewable fossil fuel consumption, low-
ering the emission of greenhouse gases (75% of green-
house gases emissions produced from waste management
are due to methane generated in land filling), reducing
the amount of waste deposited in landfills, recovering
latent energy contained in waste and the reduction of
indirect energy costs arising from the use of fossil fuels
(payment of allowances for CO2).
In 2007, the use of alternative fuels from treated bio-
mass in Spain reduced atmospheric CO2 emissions by
300,000 tonnes, equivalent to the emissions of 100,000
cars in one year. The average unit emissions in Spain in
2005-2006 were 860 kg CO2/t clinker against the Euro-
pean average of 872 kg CO2/t clinker and the world av-
erage of 873 kg CO2/t clinker [2].
In Spain, approximately one third of installed cement
furnaces are using alternative fuels, equivalent to a con-
sumption of 3 million tonnes of coal. In 2001, the Span-
ish cement sector managed almost 52,000 tonnes of
waste as alternative fuels, accounting for just over 1% of
the theoretical co nsumption of clink er kilns. However, in
recent years a significant increase has been realized. In
2007, more than 290,000 t of waste was managed [2].
Nevertheless, absolute amounts are still very small:
72000 t of liquid wastes (oils, solvents, ...); 223,000 t of
solid fuels as meat meal (88,000 t), tires (42,000 t), saw-
dust (35,000 t), wood (27,00 0 t), WWTP sludge (9000 t),
lightweight plastic (5000 t) and lesser amounts of other
waste types. This represents 8% of total fuel used in
2006, but indicating a significant increase in 5 years.
In recent literature [3,4] it is shown that while the use
of sewage sludge as secondary fuel is beneficial for the
reduction in greenhouse gas emissions, no additional
health risks for the populatio n derived fr om PCDD/F and
metal emissions are estimated. Concerning the properties
of the clinker produced by using sludge as fuel, several
research groups are working on the properties of the ce-
ment (clinker) when mixed with different wastes’ ash. In
literature [5] it is shown that the inclusion of ash improve
mechanical properties and that mortars fabricated with
10 wt% sewage sludge ash replacement meet the me-
chanical requirements of the European standard in terms
of early age compressive strength and nominal compres-
sive strength.
It is well known that cement kiln exhaust gases are nor-
mally clean, and no gas washing processes are necessary.
In regular cement plant schemes, the cement raw mate-
rial (CRM) is fed to the kiln at the opposite end to the
main burner. Combustion gases, together with clinkeri-
zation exhaust gases, flow through the whole process
counter-currently to solids. This disposition allows a
direct contact between the fresh CRM with exhaust gases
in a cyclone system, preheating CRM prior to the kiln
entrance, thus saving energy. During the cooling of the
exhaust gases some pollutants could be produced, such
as dioxins by de-novo synthesis [6-8] or other organic
compounds from the desorption on CRM [9,10]. Addi-
tionally, during the contact of flue gas-CRM in the cy-
clones, the elimination of the pollutants could also be
possible by the action of CRM through adsorption or
catalytic oxidation. There are many papers about the
study of the adsorption of organic pollutants in different
substances like fly ash, coke or active carbon [11,12]
where PCDD/Fs are adsorbed up to 90%, but unfortu-
nately, there are not enough scientific references to ana-
lyze this fact over other materials.
Several works [13-17] report that a catalyst based on
V2O5/WO3 supported on TiO2 produced a sharp decrease
of organochlorinated compounds, including dioxins and
furans (PCDD/Fs) at low temperatures (200˚C - 400˚C).
This catalyst is commercially available due to its being
commonly used in the SCR process for the elimination of
NOx of flue gas stream (DeNOx). Some researchers [17,
18] carried out experiments with a V2O5/TiO2 cata- lyst
in relation to PAHs and found that the efficacy for PAHs
destruction increases with temperature and the efficacy
for PAHs adsorption on the catalyst pellets decreases.
With respect to dioxin elimination, some papers have
been published on this subject [12,17-21]. The re- sults
of these studies revealed that the use of V2O5/TiO2 cata-
lysts results in a decrease of PCDD/Fs emissions of 90%
- 99% at 200˚C - 300˚C. According to [17], the elimina-
tion of dioxins is 98% at 230˚C with 1% adsorption on
the catalyst. On the other hand, it has been shown that
the adsorption decreases with temperature [18].
In the work presented in this paper, the direct contact
of a stream of gases produced during combustion of
sewage sludge with a fixed bed of different materials was
considered. The analysis of the volatile compounds, se-
mivolatile (PAHs) and dioxins and furans resulting in
emissions was recorded, in order to analyze the effect of
the presence of the different particle beds. This study
was conducted in two stages. First, a few preliminary
experiments were conducted in the laboratory furnace
described elsewhere [22]. In a second stage, a new ex-
perimental system was designed, with the aim to cor-
roborate some results and to extend the study with the
use of new materials and different operating conditions.
2. Material and Methods
Dried sewage sludge from a waste water facility in Tar-
ragona, Spain was used. In addition, milled cement raw
material (CRM), supplied by a cement industry close to
the University of Alicante was also used. An exhaustive
analysis of the two materials used made, and the results
are shown elsewhere [22]. A previous crushing was car-
ried out to improve homogenization.
Other materials used to directly compare the catalytic
over the flue gases are:
V2O5—1 wt% of vanadium pentoxide was incorpora-
ted to SiO2. It is known the catalytic activity of V2O5 in
the removal of NOx emissions in chimneys. In recent
years, it has also been proven that reduces the emission
of PCDD/Fs [19].
PD—Alumina/Palladium catalyst beads with 1% of
active phase provided by Enghelard.
CA—Automotive catalyst honeycomb extracted for a
car muffler. Table 1 shows the metallic composition of
Table 1. Analysis of the automotive catalyst.
Oxide Weight%
Al 26.64
Si 13.41
Mg 3.61
La 0.72
Fe 0.47
Ce 0.34
Ti 0.26
Zr 0.25
P 0.14
Pt 0.10
Copyright © 2011 SciRes. ACES
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this catalyst obtained by X-Ray Fluorescence, where the
active metals or promoters seem to be La, Ce and Pt.
The equipment used to carry out the present work, this
catalyst obtained by X-Ray Fluorescence, where the
shown in Figure 1, has been exclusively developed to
accurately control the ratio of oxygen in combustion
processes. It consists of a moving tubular reactor with
the sewage sludge carefully placed along the tube, which
is introduced at a precisely controlled speed in a furnace
while passing through a constant flow of air. In this way,
a constant flow of air and a constant mass rate of sludge
were used. This permits to simulate during several min-
utes the continuous combustion.
The equipment comprises a sample introduction area,
the combustion zone, the gas transfer zone and the zone
of interaction of gases with the different catalyst, which
is separately heated by a blanket heater, simulating the
gas temperature in the cyclonic system in a cement plant.
Before carrying out the experiments shown in this paper,
it was done a comprehensive study on the capacity of the
equipment with respect to leaks, reproducibility of expe-
riments, and homogeneity of the obtained gas flow.
The ratio between carbon monoxide and carbon diox-
ide is maintained in all experiments to approximately
0.38, indicating that the combustion process has been
carried out very similarly in all runs.
In the case of powdery substances, as is the case of
V2O5/SiO2 catalyst, quartz wool was intercalated in order
to reduce pressure drop. The mass ratio between the
amount of sludge used in each experiment and the cata-
lyst was 4: 1.
During the study, several operating conditions were
modified, such as temperature and catalyst nature. Oxy-
gen ratio, as defined by [23], was kept constant at λ =
0.14, a sub-stoichiometric value in order to simulate poor
areas and poor combustion furnace mixing and therefore
mass production of organic contaminants. To achieve
these combustion conditions, a sample size of approxi-
mately 1200 mg spread over 280 mm was used. The
sample introduction rate was 1.8 mm/s and the air flow
was 300 ml/min. The temperatures were 850˚C in the
combustion oven and 300˚C in the transfer and interact-
tion-catalyst line. The particles used in the interaction
zone were introduced into tubes of 8 mm internal diame-
ter and along the length 10 - 15 mm.
Comparing these experimental conditions with that
present in the cement plant, it is interesting to note that
the temperature in the lab reactor is much lower than that
of the plant. Furthermore, the preheater and precalciner
stage, from the flue gas, and the quenching of the hot
kiln gases before air pollution treatment are not sepa-
rately considered. Although the ultimate application of
the study is the use of sewage sludge in cement plants,
we must take into account that the conditions are not
reproducing the industrial plant. The experimental condi-
tions, with a very low air supply, are selected in order to
maximize the production of pollutants such as PAHs or
dioxins, as previously presented [23,24].
On the other hand, temperature of the actual interact-
tion of CRM with the hot gases is not known. The inter-
action takes place somewhere between the gas exit of the
cement kiln (approx. 1500˚C) and the input of the meal
solids (approx. 300˚C in the preheater). In the present
study an intermediate temperature of 850˚C has been
Sampling was carried out at the end of the interaction
zone. Three types of pollutants have been analyzed:
- Volatile organic compounds (C1-C6) gases were
collected using Tedlar bags and then analyzed by gas
chromatograph with flame ignition detector (GC-FID)
and with thermal conductivity de tector (GC-TCD).
- Semivolatile organic compounds (PAHs): a XAD2
resin w as used as ad sorbe nt and af ter a pro cess of ex trac -
tion and concentration was analyzed by high resolution
gas chromatography and spectrometry (HRGC/MS). The
analysis of semivolatile compounds has focused exclu-
sively on the group of 16 polyaromatic hydrocarbons
established by the American Environmental Protection
Agency (USEPA) as priority pollutants and potential
- Dioxins and furans: collected similarly to PAHs with
subsequent analysis by HRGC/MS. Method EPA 1613
was used to analyze the samples.
850 ºC
Furnace (850 ºC)
Adsorbent resin
Heating blanket
line zone
Feed ing zone
(sewage sludge)
Air input
Horizontal actuator
(mechanic movement at
constant velocity)
Figure 1. Scheme of the laboratory e quipme nt used.
3. Results and Discussion
3.1. Volatile Organic Compounds (VOCs)
VOCs analyzed in each of the conditions did not show
changes in yields when using any catalyst. Figure 2
shows the results achieved with the different runs con-
sidering the sewage sludge combustion without any ma-
terial in the bed and with different materials inside the
bed. It can be observed that the profile of compounds in
all cases is quite similar, and no specific trend can be
seen in the elimination of compounds in this range of
This result coincides with that found in the prelimi-
nary runs [22] in which smaller-scale experiments al-
ready indicated that none of the substances tested caused
the removal of hydrocarbons, even heavier substances
such as benzene, toluene and xylenes. The main com-
pounds are methane, ethylene and benzene, as already
shown in the work of [24] and [22].
3.2. Polycyclic Aromatic Hydrocarbons (PAHs)
Table 2 shows the results for the experiments done with
the different materials in the bed at the post-combustion
zone. The profile of compounds is very similar in all
experiments, with naphthalene, acenaphthylene, fluorene
and phenanthrene as major compound in all cases, re-
gardless of catalyst used. Considering the data, we can
observe that the lightest PAHs present relative high
yields. The exhaust catalysts from the bed used in the
runs were also analyzed for PAHs, and virtually no ad-
sorbed hydrocarbons were found. In this way, the power
of removing this type of compounds is mainly due to
thermal destruction or decay, not adsorption in the bed.
In order to analyze the experimental data, it is useful
Figure 2. Emission of VOCs. SS: sew age sludge c ombustion; SS + CC: use of automotive honeycomb catalyst; SS + Pd: use o f
bead paladium catalyst; SS + CRM: use of cement r aw mate r ial; SS + V2O5: use of Si + V2O5.
Table 2. Emission of PAHs.
MW (g/mol) SS SS + CCSS + CRM SS + Pd SS + V2O5
Naphthalene 128 2472 3393 1696 1393 1862
Acenaphtylene 152 505 211 385 293 503
Acenaphtene 154 39.8 39.7 24.0 148 25.1
Fluorene 166 178 144 135 149 163
Phenanthrene 188 330 337 215 275 256
Anthracene 188 99.6 123.3 75.4 83.5 107
Fluoranthene 202 58.8 61.5 45.9 45.4 60.9
Pyrene 202 61.2 76.7 51.0 54.4 69.4
Benzo(a)anthracene 228 12.3 32.7 20.8 23.8 32.2
Chrysene 228 24.3 26.4 28.4 24.3 22.4
Benzo(b)fluoranthene 252 7.5 11.2 4.2 5.3 6.2
Benzo(k)fluoranthene 252 1.8 3.3 3.7 6.3 6.2
Benzo(a)pyrene 252 6.5 12.0 7.2 9.1 12.7
Indeno(1,2,3-cd)pyrene 276 3.4 2.5 2.4 2.2 3.1
Dibenz(a,h)anthracene 278 2.4 1.6 1.5 0.7 0.9
Benzo(g,h,i)perylene 276 2.7 1.2 1.8 1.7 1.8
SS = sewa g e sludge; CC = automotive c atalyst; CRM = cement raw material; Pd = palladium b ased catalyst.
Copyright © 2011 SciRes. ACES
Copyright © 2011 SciRes. ACES
to calculate the production percentage of each of the
compounds compared with the experiment in which there
is no active solid phase. The production percentage will
be calculated for each PAH compound analyzed based
on the equation:
during sludgedecomposition
emited mgmg
where mgduring sludge decomposition represents the emission of
the particular compounds with no bed in the post-com-
bustion zone. Thus, a positive output percentage indi-
cates a greater emission, probably due to a greater for-
mation of that particular compound in the experiment
only with sludge. The opposite will happen if we obtain a
negative value, indicating in this case a greater destruct-
tion. Figure 3 shows the results for the PAHs analysed.
One can see that the cement raw material (SS + CRM)
is the material that produces fewer compounds and
therefore has more activity, regardless of the PAH con-
sidered. The results using vanadium pentoxide are simi-
lar to SS + CRM, probably due to the fact that bed con-
taining V2O5 is composed mostly of silica, one of the
major components of cement raw material, so it must be
that silica has an effect of elimination similar to crude.
On the other hand, the automotive catalyst and the alu-
mina balls lead to an increase production of some pol-
yaromatic hydrocarbons, reaching values greater than
400% for the production of acenaphthene. The higher
molecular weight compounds undergo high removal
rates in all experiments.
There are many processes occurring in the catalytic
bed reactor system, such as are the reactions of cracking
of high molecular weight compounds giving lower mo-
lecular weight compounds as final products hydrogen
and carbon [25] and pyrosynthesis reactions, which are
responsible for the formation of high molecular weight
compounds from smaller compounds [26,27]. The final
product of these reactions is the soot.
Based on these patterns, Figure 3 shows the general
behavior of the production of compounds of intermediate
molecular weight, whereas both compounds with low or
high MW, or are not produced, or are destroyed properly.
This behavior is logical bearing in mind that, due to
cracking and pyrosynthesis reactions, the lower and the
higher MW compounds are producing intermediate MW
In the results obtained during the decomposition of
sewage sludge by [24], this behavior is also observed, so
that compounds such as acenaphthylene, phenanthrene,
anthracene, and benzo (a) anthracene have maximum
yields at intermediate temperatures, i.e. at intermediate
cracking conditions, what indicates that these compounds
are producing lower and higher molecular weight com-
pounds. It should be kept in mind that the experimental
conditions (λ = 0.14) are very close to pyrolytic condi-
tions, which permits the catalyst to show their potential
as radical generators causing the decomposition of pol-
yaromatic hydrocarbons [28].
3.3. Polychlorinated Dioxins and Furans
The results of the analysis of dioxins and furans for each
of the types of experiments are found in Table 3, which
shows both the absolute concentration and that obtained
by applying toxic equivalency factors and the total toxic-
Figure 3. Percentage of production of each individual PAH.
Table 3. Emission of PCDD/Fs.
SS = sewa g e sludge; CC = automotive c atalyst; CRM = cement raw material; Pd = palladium b ased catalyst.
ity. For these types of compounds there is a decrease in
the toxicity that varies between 30% and 60%, with the
maximum removal in the presence of cement raw mate-
rial. From the data, we observe an increase of over 30%
with the use of V2O5. These behaviors are due to the de-
cline of some selected congeners and also to a shift in the
congener from more to less toxic (higher chlorinated
Regarding the distribution of dioxins and furans, in all
experiments there is a profile change for dioxins, going
from 6.5% to almost 40%. The cement raw material and
vanadium pentoxide are the substances most responsible
for this effect.
To more clearly assess the emission of each congener,
the production percentage has been calculated in a man-
ner analogous to the case of PAHs. Figure 4 shows the
result. It is clear from this Figure that there is a higher
production of dioxins, instead of furans, and even lead-
ing to a production increase of over 500%. The removal
rates of furans can reach up to 100% in some congeners.
Note the high levels of production found in the runs done
in the presence of vanadium pentoxide in virtually all
congeners; and it is remarkable that is the only bed that
produces higher levels of furans. On the other hand, it is
worthy to note that cement crude achieves the highest
removal rates of virtually all congeners, including diox-
4. Conclusions
The use of raw cement and other substances such as beds
of catalysts interacting with the flue gas of sewage
sludge has had different effects depending on the nature
of the materials used and the type of compounds to re-
The concentrations of volatile compounds are unaf-
fected by the use of any of the materials used in the beds.
There is no significant change in concentration in any
The emission of polyaromatic h ydrocarbons is charac-
terized by the presence of naphthalene, which reaches
more than 50% of the total PAHs emitted. Crude cement
is the substance that has the greatest power of elimina-
tion on this compound, followed by V2O5, which has a
very similar behavior probably because the silica is
among major components of the crude cement. The
CRM has a chemical composition represented by amount
of oxides (mainly Al2O3, CaO, Fe2O3, P2O5, SiO2 and
minor amounts of MgO, TiO2, Cr2O3, ZnO, CuO) that
actually form different minerals. Indeed, these mineral
compounds can cause reactions on the CRM surface due
to their acid-basic characteristics.
The use of two commercial catalysts has not caused
the desired effect in the conditions used in the present
As far as the emission of dioxins and furans, it is ob-
served that the use of raw cement causes a decrease of
over 60% in the toxicity of the sample due to both a
change in the congener profile and a pronounced de-
crease in emissions. On the other hand, the use of vana-
dium pentoxide has caused a completely opposite be-
havior of that expected, the increased toxicity of the
sample due to the increase of congeners of low degree of
SS SS + CC SS + CRM SS + Pd SS + V2O5
pg/g pg I-TEQ/g pg/g pg I-TEQ/gpg/g pg I-TEQ/gpg/g pg I-TEQ/g pg/g pg I-TEQ/g
2378-TCDF 133.6 13.36 20.87 2.09 12.661.27 40.984.10 10.56 1.06
12378-PeCDF 75.12 3.76 46.56 2.33 30.711.54 68.703.44 103.74 5.19
23478-PeCDF 171.01 85.50 110.14 55.07 53.8626.93 141.6470.82 151.37 75.69
123478-HxCDF 296.38 29.64 102.11 10.21 41.424.14 134.9713.50 154.20 15.42
123678-HxCDF 177.04 17.70 178.13 17.81 43.194.32 130.7413.07 187.03 18.70
234678-HxCDF 262.75 26.28 155.64 15.56 68.616.86 130.5013.05 295.18 29.52
123789-HxCDF 64.51 6.45 39.58 3.96 19.801.98 49.604.96 95.43 9.54
1234678-HpCDF 852.22 8.52 584.78 5.85 199.612.00 345.963.46 833.83 8.34
1234789-HpCDF 78.37 0.78 41.60 0.42 6.44 0.06 46.590.47 100.83 1.01
OCDF 209.79 0.21 192.57 0.19 22.970.02 102.850.10 149.38 0.15
2378-TCDD 7.96 7.96 12.98 12.98 7.21 7.21 2.44 2.44 83.29 83.29
12378-PeCDD 7.07 3.54 10.60 5.30 42.97 21.49 12.936.46 38.90 19.45
123478-HxCDD 3.95 0.43 21.53 2.15 7.21 0.72 21.872.19 24.02 2.40
123678-HxCDD 5.18 0.52 20.97 2.10 6.61 0.66 17.161.72 25.19 2.52
123789-HxCDD 4.03 0.40 23.77 2.38 8.70 0.87 27.402.74 27.68 2.77
1234678-HpCDD 36.91 0.37 39.57 0.40 13.640.14 29.350.29 56.44 0.56
OCDD 32.78 0.03 41.53 0.04 31.430.03 21.300.02 106.48 0.11
% elimination 31.84 60.61 29.88 -35.36
TOTAL 205.45 138.83 80.23 142.82 275.71
%PCDD 4.05 6.45 10.40 18.26 19.0938.78 10.0011.11 85.20 40.30
%PCDF 95.95 93.55 89.60 81.74 80.9161.22 90.0088.89 14.81 59.70
Copyright © 2011 SciRes. ACES
Copyright © 2011 SciRes. ACES
Figure 4. Percentage of production of each individual congener of PCDD/Fs.
Probably, the use of sewage sludge as alternative fuel
in cement kilns does not cause an increase in organic
pollutant emissions because the compounds are reduced
both by the gas cleaning system and by the reactions
occurring in the preheat zone of raw materials. In the
cyclones, the interaction between crude cement and
gases is produced, causing the elimination of a signify-
ncant portion of the organic contaminants that may have
been produced during the combustion process of waste.
For this reason, the use of sewage sludge in cement kilns
is a sure alternative to manage the excess waste.
5. Aknowledgements
Support for this work was provided by Generalitat Va-
lenciana (Spain) with projects Prometeo/2009/043 and
ACOM2009/135, and by the Spanish MCT CTQ2008-
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