Latin American Applied Research 32:123-129 (2002)

PLATINUM MONOLITHIC CATALYST FOR SO2 ABATEMENT IN DUST-FREE

FLUE-GAS FROM COMBUSTION UNITS

E. ALVAREZ, J. BLANCO*, J. OTERO DE BECERRA1, J.OLIVARES DEL VALLE2 and L.

SALVADOR2.
Instituto de Catálisis y Petroleoquímica, CSIC., Cantoblanco, 28049 Madrid, SPAIN.
1
Departamento de Combustión y Gasificación, CIEMAT, 28040, Madrid, SPAIN.
2
Departamento de Ingeniería Química y Ambiental, ESII, 41092 Sevilla, SPAIN.

Abstract A catalytic oxidation process been developed through improvements in pre,

of traces of SO2 to SO3 for the treatment of post and in-combustion treatments. In post-
dust-free flue-gas from power stations and combustion methods, the most widespread
some special waste incinerators has been procedure uses wet lime/limestone scrubbers
developed. The catalyst was prepared by and spray dryers for flue-gas desulfuration;
impregnation of platinum precursors on latterly, the catalytic oxidation of sulfur
alumina/silicates-based material. The dioxide to sulfur trioxide and further sulfuric
behavior of this industrial sized honeycomb- acid production has emerged as an alternative,
type catalyst was studied at lab and pilot- which could avoid the usual problematic
plant scale. The evolution of SO2 conversion disposal of the calcium sulfates produced from
and some physico-chemical properties of the the conventional methods. In these catalytic
catalyst with time in operation were systems, a useful SO3 byproduct is obtained
examined. The 7-8 wt.% of sulfate deposition from which various commercial products may
on the fresh catalyst had no influence on the be marketed, such as sulfuric acid and
catalytic activity at steady state conditions. ammonium sulfate (Borio and Kingston.,
The catalyst showed SO2 to SO3 conversion (1993); Blumrich and Engler, (1993)).
value of 80-90 vol.% operating at a Obviously, catalysts produced for commercial
temperature of 450ºC and space velocities of application should require dust-free flue-gas
3000-6150 h-1 (NTP). conditions.
The process might also be suitable for SO2
Keywords SO2 oxidation, industrial removal from flue-gas produced by combined-
size, honeycomb-type, dust-free gas combustion of coal and waste, such as tanned
leather residue incineration. In Spain, tanned
I. INTRODUCTION leather residues have been estimated as 104 T
Nowadays, one of the tasks in environmental per year produced from shoe and leather
protection is directed towards the development accessories factories. The average content of
of technologies for flue-gas SOx removal, chromium is about 4 wt.% as Cr2O3 while the
where further reduction in its emission levels sulfur content is around 2 wt.%, although this
will be required in accordance with the future value can widely vary depending on the tanned
application of more stringent regulations. leather. The chromium oxidation state in
Emission control techniques for the leather is Cr (III) with very low mobility in the
combustion of coal and other fossil fuels have environment and low toxicity for living

*
Author to whom correspondence should be addressed
Instituto de Catálisis, Cantoblanco, 28049 Madrid, Spain
jblanco@icp.csic.es

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Latin American Applied Research 32:123-129 (2002)

organisms. Nevertheless, the large volume of 15 cm length were manufactured and plugging
the leather residues is giving rise to several problems between layers by dust arose.
problems as the storage capacities of the In the development of Pt-impregnated
deposits are reached and the weathered honeycomb catalyst for SO2 oxidation, the
chromium salts restrict dumping alternatives. objective of seeking a sulfate-proof support
The action of water and air produces the slow has given rise to a certain number of
conversion of Cr (III) to Cr (VI) which could publications and patents. For alumina, Ziolek
be lixiviated and transported away from the et al.(1996) reported that SO2 alone is
stored residues. The hexavalent chromium is a chemisorbed at 350°C. Moreover, the SO2
common toxic pollutant in soils and waters. Its adsorption on alumina mainly takes place via
action on human beings covers allergic contact the hydroxyl groups, giving rise to hydrogen
dermatitis and cancers. Taking into account sulfite species (Mohammed-Saad et al., 1995).
these considerations, incineration could be the When O2 is present, significant quantities of
best solution to get rid of this waste material. SO2 are oxidized to SO3 and subsequently
The combined-combustion of leather residues stored on the alumina. When Pt is added, the
and coal is also an interesting alternative. SO2 oxidation process is catalyzed by the
In the incineration of leather residues, the noble metal and SO2/SO3 are adsorbed on
generated ashes, enriched in chromium (the different alumina sites (Summers, 1979). Li-
content varies between 30-60 wt.%, depending Dun et al., (1991) pointed out two types of
on raw material and incineration conditions), sulfate species on the alumina having different
can be used in ceramics, including decorative environments: free sulfates and sulfates
ones, to manufacture pavements and bricks, associated with the carrier.
etc. During the incineration process, additional No information about SO2/SO3 adsorption
drawbacks arise as the leather residues contain on sepiolite or aluminium silicates behavior
significant sulfur content. The addition of has been reported.
limestone to retain the SO2 emissions produces The role of the platinum loading and
the chromium (VI) release. The total oxidation dispersion on the carrier for the SO2 oxidation
of Cr (III) to Cr (VI) takes place if limestone is (Xue et al., 1996) has also been pointed out.
fed into the reactor, in the range of temperature When a Pt/alumina catalyst is used, the catalytic
between 450-600°C. In this case, the catalytic activity is not strongly affected by the Pt particle
oxidation of SO2 containing flue-gas and size and, there is a maximum loading
subsequently sulfuric acid recovery might be a percentage at which the excess of noble metal
suitable solution. does not enhance the performance of the
In dust-free flue-gas treatment, the abrasion catalyst (Matsuda et al., 1982).
effect is minimized and noble-metal In this study, a Pt-impregnated on
impregnated catalyst is currently used in the alumina/silicates-based support catalyst has
abatement systems (Truex et al., 1992). In the been developed and tested both at laboratory
SO2 reduction field, some reports described and pilot-plant scale. The pilot plant has been
noble-metal catalysts for SO2 oxidation, where designed to test the industrial sized catalyst at
the SO3 obtained is further converted to real operational conditions in order to evaluate
sulfuric acid. As an example in the DeSONOx the performance of the material. The evolution
process (Blumrich and Engel (1993); Ohlms, of SO2 conversion and some physico-chemical
1993), the previously dust cleaned flue-gas, properties of both fresh and used catalyst were
was passed through a precious metal examined.
monolithic catalyst; afterwards, the SO3-rich
flue-gas was cooled and sulfuric acid (70
wt.%) condensed. In this work, short pieces of

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 E. ÁLVAREZ, J. BLANCO, J. OTERO DE BECERRA, J. OLIVARES DEL VALLE, L. SALVADOR

II. EXPERIMENTAL SECTION was monitored by Atomic Absorption

A. Catalyst Preparation technique. After impregnation, the monoliths
The preparation method followed four steps: the were treated in ammonia flow at 400°C for 4
carrier conformation, drying and calcination, Pt- hours to produce the total Pt species reduction.
species impregnation and subsequent reducing. XPS analysis of the prepared catalyst gave an
The material of the support was prepared by average percentage of 0.12 wt.% of metallic
mixing the corresponding quantities of three platinum.
different components: a) a boehmite with an
average particle size of 5 µm, b) a natural α- B. Characterization Techniques
sepiolite containing ca. 36% of impurities, N2 adsorption/desorption isotherms at 77 K
mainly other clays and calcium carbonate with were determined using a Micromeritics ASAP
an average aggregate-size of about 11 µm, and 2300-10D. Samples of the catalyst were
c) a natural high-purity aluminum silicate with outgassed overnight at 140°C to a vacuum of <
an average particle size of 9 µm. The support 10-4 torr to ensure a dry clean surface. Surface
composition (by weight) was 2/2/1 for alumina, area determinations were made by application
α-sepiolite and aluminum silicate, respectively. of the BET equation. Total pore volume was
The three components of the support were determined using a CE Instrument Pascal
thoroughly dry mixed before adding water. 140/240 porosimeter. Samples were also dried
Kneading of this slurry was performed until at 140°C overnight. The mechanical strength
homogeneous dough was obtained. The material of the monolithic catalyst was determined
was conformed using an industrial-scale using a Chatillon LTCM Universal Tensile
extruder. Parallelepiped monoliths of Compression and Spring Tester with a cylinder
10x10x100 cm external dimensions were test head of 1 mm diameter. The test head was
obtained with square cells of 4x4 mm open-cell positioned over a channel wall and the
and 1 mm wall thickness. In the case of pressure slowly increased until rupture of the
operation in low-dust conditions as a result of a wall was caused; the average of ten
malfunction of the dust collector, the 1 m length consecutive measurements was taken to ensure
would avoid the interlayer plugging problems, the accuracy of the result.
which arise when shorter monoliths are used. Thermal Gravimetric Analysis (TGA)
The conformed structures were dried at determination was made using a
110°C and calcined at 500°C for 4 hours thermobalance Perkin Elmer TGA 7 connected
causing 12% shrinkage. The bulk density of to a Fisons MD 800 mass spectrometer using a
the prepared carrier was 1.3 g cm-3 and the heating ramp of 10 °C min-1 within the
mechanical strength was 119 kg cm-2. temperature range of 30-950°C in 20 cm3 min-1
The impregnation of this support was carried helium flow rate.
out using a 600 ppm aqueous H2Cl6Pt solution. Platinum concentration analysis was carried
The impregnation process was performed using a Perkin Elmer 3030 Atomic Absorption
immersing one monolith each time. A movable (AA) Spectrophotometer. Standard AA
cage holding the material and allowing a conditions for Pt were 265.9 nm wavelength,
constant vertical motion was built. This way, 0.7 nm slit and air/acetylene flame.
the Pt concentration inside the monolith X-Ray photoelectron spectroscopy (XPS)
channels was kept constant. The average time analysis of Pt was carried out using a Perkin
for each impregnation was 15 minutes. After Elmer spectrometer model Phi 5400, with a
each impregnation process, the released liquid monochromatic Mg source (1254 eV),
volume was replaced trying to maintain the operating at 12 kV and 20 mA. The samples
initial Pt concentration of 600 ppm. The were outgassed under a vacuum of 10-8 bar
evolution of Pt concentration in the solutions before placing in the analysis chamber.

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A Varian 3400 model Gas Chromatograph LV: Linear Velocity; AV: Area Velocity
coupled to a flame photometric detector was
used to monitor the inlet/outlet SO2 B. Pilot plant catalytic activity test
concentration in the lab-scale experiment. A scheme of the pilot plant installation is shown
The inlet/outlet SO2 and O2 measures in the in Fig. 1. The main body is a catalytic reactor
pilot plant were performed using an ESSA containing a basket with the package of
ML9850 model Ultraviolet Fluorescence honeycomb-type monoliths arranged vertically.
analyzer. The prismatic basket was built to provide two
The EPA Method (Method 8, 1990) was separate layers of 5x4 monoliths of 9.5x9.5x100
selected for SO3 sampling procedure; the cm with a maximum catalyst load of ca. 330
collected sulfuric acid mist was measured by liters. The dimensions of the reactor and
the barium-thorine titration method. coupling accessories to the main gas duct
assured an isokinetic gas flow at the entrance of
III. RESULTS AND DISCUSSION the catalytic bed. The head unit was a gas
A. Lab-Scale Catalytic Activity Test generator constituted by a gas-oil combustion
A sample from the industrial sized monolith chamber with a capacity of 200-500 STD m3 h-1
was selected to test the catalytic performance of simulated flue-gas in the range of 350-500ºC.
at lab-scale conditions. The geometric
dimension of the monolith piece corresponded AIR FLY-ASH DOSAGE
SO2/O2 SAMPLING
SOLID/GAS
to a 6-open-channel structure of 25 cm length. MIXER
FAN

The catalytic activity measurement was carried BURNER BOILER

out in a reactor working close to the isothermal GAS

axial profile, where carborundum was used to AIR

VALVES

fix and isolate the catalyst. In order to avoid

the entrance of SO3 to the GC, a 98 wt.%
sulfuric acid trap was placed at the reactor REACTOR

outlet. The operation conditions are SULFUR-DOPED

summarized in Table 1. GAS-OIL

AIR-COOLER

The SO2 conversion to SO3 increased with

time in operation and reached a constant value AIR
of 90 vol.% after the first 6 hours. No FAN

deactivation of the catalyst was observed SO2/SO3 SAMPLING

during the following 25 hours of the test, and

the conversion was kept stable at the indicated Figure 1. Scheme of the pilot plant installation.
value. Varying the amount of carbon sulfide added
Table 1. Operation conditions (NTP) for the to the gas-oil as the doping agent, the desired
lab-scale test. SO2 concentration in the flue gas was
450 °C Temperature obtained. The air supplier valve placed in the
120 kPa Pressure burner controlled the variation of O2
6140 h-1 GHSV concentration.
0.6 m s-1 LV The catalyst placed in the prismatic holder
8.1 m h-1 AV occupied 151 liters total volume with a cross-
Gas Composition (by vol.) section of 0.17 m2. Gas generation was about
1000 ppm SO2 430-470 Nm3 h-1 in order to achieve a GHSV
7% O2 of around 3000 h-1. The pressure drop through
N2 balance the catalytic bed was in the range of 21-24 mm
GHSV: Gas Hourly Space Velocity;

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 E. ÁLVAREZ, J. BLANCO, J. OTERO DE BECERRA, J. OLIVARES DEL VALLE, L. SALVADOR

SO2 inlet
500

[SOx] (ppm); Flow (Nm 3.h-1); Temperature (°C)

Total Flow
of H2O m-1. The main operational parameters Temperature

are summarized in Table 2. 400 SO3 outlet

In addition to the above-indicated (SO2 + SO3)outlet
compounds, the flue gas contained CO2, CO, 300

NOx (ca. 300 ppm) and unburned

hydrocarbons (ca. 600 ppm). 200

Table 2. Operation conditions (NTP) for the 100

SO2 outlet
pilot plant-scale test.
475 °C Temperature 0
0 5 10 15 20 25
120 kPa Pressure Time in operation (hours)
2980 h-1 GHSV Figure 2. SO2 inlet and (SO2+SO3) outlet
1.1 m s-1 LV concentration, flow-rate and temperature
3.8 m h-1 AV versus time for the first 20 hours in pilot-plant
Gas Composition (by vol.) operation.
500 ppm SO2
5% O2 Textural properties comparison between the
7% H2O fresh and used catalysts has been performed in
order to evaluate the effect of the SOx uptake
Two glass heated lined probes were during the pilot plant run. The fresh catalyst
connected upstream and downstream of the presented a BET surface area (SBET) of 159 m2
reactor to withdraw an aliquot for the SOx g-1 and a total pore volume (Vp) of 0.53 cm3 g-
analysis. The temperature was recorded at the 1
. After reaction, the SBET and Vp decreased to
top and bottom of the reactor. A further 123 m2 g-1 and 0.48 cm3 g-1, respectively. Pore
thermocouple was located at 15 cm depth size distributions of both fresh and used
along a selected central monolith channel from catalysts are plotted in Fig. 3.
the top of the reactor.
In Fig. 2, the temperature of the 1.2
- - - - Fresh Catalyst

thermocouple inside the monolithic channel, _____ Used Catalyst

the total flow and the SO2 inlet and SO2/SO3

dV/dlogD (cm3 g-1)

outlet are plotted against the time in operation. 0.8

Only data from the first 20 hours were plotted.

The reaction system reached the stationary
state after 13 hours, with SO2 to SO3 0.4

conversion values of about 80% until the run

stopped after 40 hours in operation. Stable
flow, temperature and O2 concentration values
0
1 10 100 1000 10000 100000

were observed over the test. Pore Diameter (nm)

From Fig. 2 it can be clearly seen that the Figure 3. Pore size distribution of fresh and
catalyst underwent a sulfating process during used Pt-catalysts.
the first 13 hours in operation as also observed
in the lab-scale test. After this step, the catalyst The fresh catalyst presented a mesopores
presented stable behavior with an 80 vol.% with an average pore diameter centered at ca.
SO2 to SO3 conversion, lower than that 10 nm and a broad range between 15-100 nm
observed in the lab-scale experiment. in the mesoporosity zone; in the macropore
range a broad peak within 0.6 and 10 µm pore
diameter would correspond to interparticulate

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Latin American Applied Research 32:123-129 (2002)

space. For the used catalyst, it can be seen that catalyst became more hygroscopic as a
pores with a diameter greater than 30 nm were consequence of the sulfite/sulfate groups
affected by the sulfate deposition. Otherwise, deposition. No water MS signal at about 800°C
sulfating process did not markedly alter the was recorded, therefore, the hydroxyl groups
mesopores with diameter below 30 nm surface were also altered.
existing in the fresh catalyst. Moreover, the With respect to CO2, in the fresh catalyst, the
slight decrease of the BET surface area values carbonates from sepiolite were removed
indicated that the sulfatization was mainly between 500 and 750°C. No CO2 in the used
taking place in the range of meso and sample in the range of 500-700°C was detected
macropores. and the carbonate decomposition took place at
Thermal gravimetric (TG) and mass higher temperature (700-900°C) than that in the
spectrometry (MS) combined techniques were fresh sample. The CO2 peak in the used catalyst
used to monitor weight loss and gas released started to show up when the SO2 peak reached
with temperature. The results corresponding to its maximum value. This CO2 might come from
the fresh and used catalysts are shown in Fig. 4. the unreacted carbonate, which would have
been covered by the sulfate species.
Fresh Catalyst
CO2 With respect to the sulfate deposition, at
H2O
least two types of sulfates were removed from
the used catalyst. The SO2 starting to be
MS Signal (Arbitrary Units)

Used Catalyst
released at ca. 500°C might correspond to the
sulfates produced from the interaction between
H2O
silicates and SO3. A further SO2 peak showing
SO2
up when the temperature reached ca. 700°C
might be assigned to the aluminum sulfate
previously formed.
CO2
As the TG analysis indicated, the total
0 200 400 600

Temperature (°C)
800 1000
estimated amount of sulfite/sulfate uptake by
the catalyst was 7.5%. This value may be
Figure 4. MS signal evolution with temperature compared with that obtained from the pilot
for the fresh and used catalysts. plant results. A simple calculation from the
SO2 inlet, SO2+SO3 outlet concentrations and
For the fresh catalyst the 6.3% total weight total flow data found that 6% (as SO3) would
loss between 20 and 950°C mainly be the amount of deposited on the catalysts.
corresponded to H2O and CO2. In the used The mechanical strength of the monolithic
sample, TG analysis showed between 20 and catalyst was not altered by the 6-7 wt.% of
400°C an 8% weight loss due to H2O release. deposited sulfate, maintaining the fresh sample
Between 500-950°C second range presented a value of 119 kg cm-2.
7.5% weight loss, which was mainly related to
the sulfite/sulfate decomposition. IV. CONCLUSIONS
With respect to the water signal, in the fresh The fresh catalyst adsorbed up to 7-8% (as
catalyst, different H2O or hydroxyl groups were SO3) of SOx. Afterwards, a stable 90-80 vol.%
removed as the temperature was increased. Two SO2 to SO3 conversion was achieved at
types of water molecules can be distinguished. laboratory and pilot-plant scales respectively.
The absorbed water leaving the catalyst between The textural characteristics were not
80-200°C and hydroxyl groups probably strongly affected by the sulfate deposition.
coming from the alumina at 800°C. BET surface area and total pore volume were
In the used catalyst, the MS results showed not dramatically altered. Mesopores of less
the water signal between 20-400°C. The used

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 E. ÁLVAREZ, J. BLANCO, J. OTERO DE BECERRA, J. OLIVARES DEL VALLE, L. SALVADOR

than 10 nm were unchanged, but the decrease Matsuda, S., A. Kato, T. Mori, T. Kumagai, Y.
of wider mesopores and macropores was the Hishinuma, H. Akimoto and F. Nakajima,
more significant effect. “Process for treating flue-gas”, U.S. Patent
The difference of 10 points in the 4,350,670, (1982).
conversion value between the laboratory and Method 8- “Determination of sulfuric acid
pilot-plant runs might be mainly assigned to mist and sulfur dioxide emissions from
the presence of water vapor and unburned stationary sources”, Environmental
hydrocarbons presenting the pilot-plant gas Protection Agency. Pt. 60, App. A, Method.
stream. 8. 40 CFR Ch. I (7-1-90 Edition)(1990).
The catalyst presents an acceptable catalytic Mohammed Saad, A.B., O. Saur, Y. Wang, C.P.
activity with no observed poisoning action by Tripp, B.A. Morrow and J.C. Lavalley,
SOx along with adequate mechanical “Effect of sodium on the adsorption of SO2
properties. Therefore, this feasible Pt- on Al2O3 and on its reaction with H2S”, J.
impregnated monolithic catalyst based on Phys. Chem. 99, 4620-4625 (1995).
silicates/alumina might be applied in the SO2 Ohlms N., “The DESONOX process for flue
emission reduction from dust-free flue-gas. gas cleaning”, Catalysis Today 16, 247-261
(1993).
Acknowledgments Summers J.C., “Reaction of sulfur oxides with
The authors are thankful to CICYT, ECSC and alumina and platinum/alumina”,
CSE-ENDESA-OCICARBON for their Environmental Science & Technology 13(3),
financial support. The authors gratefully 321-325 (1979).
acknowledge the support from the ECSC Truex, T.J., R.A. Searles, D.C. Sun, “The
project (Nº 7220-ED/080). opportunity for new technology to
We are also grateful to Dr. Yates for his complement platinum group metal
help in obtaining and discussing the textural autocatalysts”, Platinum Metals Review
data and to Dr J. Pérez-Pariente and Dr.E. 36(1), 2-11 (1992).
Sastre for the TPD-MS analysis. Also, we wish Xue, E., K. Seshean and J.R.H. Ross, “Catalytic
to thank Dr. C. Knapp for his valuable scientific control of diesel engine particulate emission:
support. studies on model reactions over a EuroPt-1
(Pt/SiO2) catalyst”, Applied Catalysis B:
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