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Theinvestigation of SCR reaction on sulfated CaO.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2012; 7: 55–62
Published online 4 August 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.491
Research article
The investigation of SCR reaction on sulfated CaO
Xinfang Yang, Bo Zhao, Yuqun Zhuo,* Changhe Chen and Xuchang Xu
Key Laboratory for Thermal Science and Power Engineering of Minister of Education, Department of Thermal Engineering, Tsinghua University,
Beijing 100084, China
Received 7 April 2010; Revised 29 May 2010; Accepted 11 June 2010
ABSTRACT: The catalytic activity of sulfated CaO in the selective catalytic reduction (SCR) reaction of NO with NH3
in the presence of O2 was investigated. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS),
Brunauer–Emmett–Teller (BET), SEM, XRD, NH3 -TPD (temperature programmed desorption), and NO-TPD were
used to characterize the catalyst of sulfated CaO and analytical grade CaSO4 . The BET results indicated that the surface
area decreased as the sulfation extent increased due to pore plug. SEM analysis revealed the surface morphology of
the sulfated CaO with different sulfation extent. XRD results suggested that the sulfated CaO was hexagonal CaSO4 .
NH3 -TPD and NO-TPD experiments indicated that NH3 was more likely to be absorbed on the sulfated CaO than
NO, and as the sulfation extent increased, the acid sites reduced because of the decreasing surface area, but the acid
strength of surface acidity increased. Based on the characterization and SCR reaction results, NH3 adsorption on CaO
and CaSO4 , and then reaction with O2 or NO would be the possible reaction route on sulfated CaO.  2010 Curtin
University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: DRIFTS; NO reduction; calcium sulfate; ammonia; mechanism
INTRODUCTION
The emissions of SO2 and NOx from coal combustion
damage environment and human health, and it is
imperative to control the air pollutants from coal
combustion.[1 – 3] For SO2 emission control, though
wet scrubbing technique has high desulphurization
efficiency, its drawback of large water consumption
limits its wide application. For such cases, the lowcost calcium-based dry flue gas desulfurization (FGD)
technique has become an attractive alternative. Previous
research on a pilot-scale circulating fluidized bed - flue
gas dusulfurization (CFB-FGD) system indicated that
SO2 removal efficiency could be as high as 85–95%
with Ca/S molar ratio at 2 under 700–800 ◦ C, as the
competition effect of CO2 on the active sites of the
calcium-based sorbent could be eliminated under these
temperature.[4,5] Moreover, Li et al .[6,7] have indicated
that trace elements such as selenium and arsenic can
be captured by CaO in this temperature window, which
implies a possibility of simultaneous removal of multipollutants in the high-temperature window. For NOx
emission control, the so-called SCR process involving
the reduction of NOx by ammonia in the presence of
*Correspondence to: Yuqun Zhuo, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084,
China. E-mail: zhuoyq@tsinghua.edu.cn
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
oxygen according to reaction in Eqn (1):
4NO + 4NH3 + O2 → 4N2 + 6H2 O
(1)
is presently used largely to reduce the emission of
NOx . Recently, simultaneous removal of SO2 and NOx
has been extensively investigated at the same hightemperature window to reduce the emission control cost.
The research by Lee et al .[8] and Li et al .[9] have shown
that the sulfated CaO had a catalytic effect on NO
reduction by NH3 in the presence of O2 at 700–850 ◦ C.
Hence, the simultaneous removal of SO2 and NOx
might be achieved by low-cost calcium-based absorbent
at 700–850 ◦ C. However, few studies have been done
on the detailed catalytic mechanism of the sulfated CaO.
It is known that the NH3 oxidation by O2 and the NO
reduction by NH3 are the two competitive reactions
during SCR reaction. Inhibition of NH3 oxidation to
NO and the enhancement of NO reduction by NH3 to
N2 will both be beneficial to the overall NO reduction.
Laboratory experiments have indicated that CaO is an
effective catalyst for NH3 oxidation to NO. However,
CaSO4 is inert to oxidize NH3 to NO, and promote
NO reduction by NH3 .[8 – 13] Moreover, Li et al .[9] have
shown that during CaO sulfation process, the catalytic
activity of sulfated CaO for SCR reaction first increased
and then decreased when the sulfation extent was large
enough, but the reason was not definite. Therefore,
the catalytic mechanism of the sulfation products and
56
X. YANG et al.
Asia-Pacific Journal of Chemical Engineering
the relation between the catalytic activity and the CaO
sulfation extent need further investigation.
Many studies have been done on the sulfated metal
oxides as SCR catalysts,[14 – 19] other than CaO, indicating that the surface-sulfated species produce acidic sites
and favor ammonia absorption, which may lead to high
reactivity of the SCR reaction. However, for sulfated
CaO, the correlation between SCR activity and surface
acidity has not been clarified. An in-depth study is necessary to understand the catalytic mechanism of sulfated
CaO in SCR reaction to improve the overall reactivity
of sulfated CaO, and achieve simultaneous removal of
SO2 and NOx .
The purpose of this research is to identify the CaO
sulphation products, find out the relation between the
catalytic activity and the sulfation extent, and propose
the SCR reaction mechanism of sulfated CaO.
EXPERIMENTS
Catalyst preparation
The analytical grade CaO lump was first crushed and
sieved to a size range of 0.35–0.50 and 0.3–0.35 mm.
The sulfated CaO was prepared by passing 3.2 L/min
gas mixture containing 2000 ppm SO2 and 5% O2 (N2
as the balance gas) at 850 ◦ C in a bubbling bed. This is
an improved catalyst preparation method developed by
Li et al .[9] . Catalyst prepared in a bubbling bed enabled
more homogeneous sulfation extent for all the materials
in the bed. In each run, the initial CaO was 5.50 g, and
the different CaO sulphation extent was obtained by
changing the sulfation time. The extent of CaO sulfation
(XCa ) was calculated from the mass increase of the solid
sample defined as:
XCa (%) =
nS
× 100
nCa
(2)
where nS was the molar of sulfur element in the sulfated
CaO, and nCa was the molar of calcium element in the
sulfated CaO.
For comparison purposes, analytical grade CaSO4
particles with diameter of 0.35–0.50 mm was prepared,
and then calcined at 800 ◦ C for 8 h in N2 atmosphere.
The CaSO4 content is more than 99%, MgO is about
0.2%, carbonate calcium is about 0.05%, and the carbonate calcium would be converted to CaO at 800 ◦ C.
Characterization techniques
The characterization techniques used are shown in
Table 1.
The DRIFTS was used to identify the surface species
of the samples. Spectra were collected in an infrared
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Table 1. Characterization techniques.
Technique
Objective
Apparatus
BET
SEM
XRD
Surface area
Surface morphology
Crystal type
DRIFTS
Surface species
NH3 -TPD
NO-TPD
Surface acidity
Absorption tests
Model ASAP2010
JSM – 6301F
Bruker D8 advance
X-ray diffractometer
Thermo Nicolet
Corporation, NEXUS
Mass spectrometer
Mass spectrometer
spectrometer equipped with an MCT detector, collecting
100 scans at a resolution of 4 cm−1 . The IR spectra
of KBr were collected as the background spectra. Pure
samples were placed in an IR quartz cell allowing
heating in N2 atmosphere. Then the spectra of the
samples were obtained by subtracting the background
spectra. The sample used for DRIFTS characterization
needed to be in powder form. The sulfated CaO powder
was directly prepared from the sulfation of powdered
CaO. The particle size used for BET, SEM, XRD, NH3 TPD, NO-TPD, and the catalytic activity tests were
0.30–0.50 mm.
The NH3 -TPD or NO-TPD (temperature programmed
desorption) spectrum was obtained by monitoring the
absorbed NH3 or NO when increasing the sample
temperature at 5 ◦ C/min in the 230 mL/min Ar flow,
after the NH3 or NO adsorption at room temperature
for 1 h. The mass of the pure and sulphated CaO was
0.5 g. The outlet gas was detected during the heating
process by mass spectrometer.
Activity measurements
The catalytic activity was tested in a fixed-bed apparatus
including the gas feeding, the reactor, and the gas
analyzing sections. All the configurations were the
same as those used by Li et al .[9] . The inlet gas
was carefully controlled by the mass flow controllers
(MFC) to obtain the desired gas flow rate and gas
concentration. The total flow rate of the inlet gas
mixture was set at 1000 mL/min (STP). SCR process
(NH3 + NO + O2 ) was conducted by feeding the premixed gas (500 ppm NH3 , 500 ppm NO, 5% O2 ,
balance N2 ) into the system. The outlet gas was
continuously monitored by a Fourier transform infrared
(FT-IR) spectrometer (Thermo Nicolet Corporation,
NEXUS670) equipped by an MCT detector and a 2 m
gas cell, collecting 32 scans at a resolution of 0.5 cm−1 .
The gas-cell temperature was maintained at 150 ◦ C. The
FT-IR spectrometer had been calibrated beforehand.
The measurement accuracy was estimated to be ±2%
for a single gas, and ±2% (NO), ±3% (NH3 ), ±3%
(SO2 ) for simultaneous monitoring of multiple gases,
Asia-Pac. J. Chem. Eng. 2012; 7: 55–62
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
INVESTIGATION OF SCR REACTION ON SULFATED CaO
which had been tested and verified in the previous work
of Li et al .[9,20,21] . The pipeline between the reactor and
the gas analyzer was heated to 90 ◦ C to prevent the
potential reaction between the gas mixtures.
Li et al .[9] and Iisa et al .[22] indicated that the formation of N2 O and NO2 were negligible during the NO
reduction experiments. Hence, during the catalyst activity test, only the outlet concentration of NO and NH3
was measured to calculate the NO conversion (XNO ),
NH3 conversion (XNH3 ), and the apparent selectivity of
NH3 for NO reduction to N2 (SNO ).These parameters
were defined as in Eqns (3–5):
out CNO
XNO (%) = 1 − in × 100
(3)
CNO
out
CNH
(4)
XNH3 (%) = 1 − in 3 × 100
CNH3
SNO =
CNO,in − CNO,out
× 100
CNH3 ,in − CNH3 ,out
(5)
in
where CNO
was the NO concentration at the inlet of
out
the quartz reactor, CNO
was the NO concentration at
in
was the NH3
the outlet of the quartz reactor; CNH
3
out
concentration at the inlet of the quartz reactor, and CNH
3
was the NH3 concentration at the outlet of the quartz
reactor.
RESULTS AND DISCUSSIONS
BET and SEM characterization
The BET surface area of the raw and sulfated CaO with
various sulfation extents is shown in Table 2. The BET
surface area decreased with the increase of the sulfation
extent, for the pore blocking during the CaO sulfation
process. The BET surface area of the calcined analytical
grade CaSO4 was 3.07 (m2 /g).
Figure 1 shows the apparent morphology of the
sulfated CaO with different sulfation extent, which
indicates the change of the surface properties during
the process that the surface CaO gradually changes into
Table 2. BET surface area of the raw and sulfated CaO
with various sulfation extent.
XCa (%)
0
5.38
10.90
16.15
23.97
30
34.1
BET area (m2 /g)
10.2
6.19
3.59
3.55
3.08
2.23
1.20
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
CaSO4 . During the sulfation of particles, a CaSO4 layer
builds up from the outer surface and develops inwards.
As the sulfation extent increases, the surface CaO is
gradually changed into CaSO4 , and the produced CaSO4
accumulates, which results in the CaSO4 crystallization
and further reduces the BET surface area.
XRD characterization
Figure 2 shows the typical XRD patterns for analytical
grade CaSO4 and sulfated CaO with 38.5% conversion.
The XRD spectra analysis indicates that the analytical
grade CaSO4 and sulfated CaO are both hexagonal
CaSO4 . The peak position in the sulfated CaO is almost
the same with that of the analytical grade CaSO4
besides the three peaks of CaO. The difference is
that the baseline of the analytical grade CaSO4 is
slightly offset from zero line, indicating a high degree
of crystallization of CaSO4 . In general, XRD results
indicate that the crystal type of the analytical grade
CaSO4 is the same as that of the sulfated CaO with
a little difference in crystallization degree.
DRIFTS study of the surface sulfates
After heated at 500 ◦ C in N2 atmosphere for 1 h,
and cooled to room temperature, spectra of analytical
grade CaO powder, analytical grade CaSO4 powder,
and sulfated CaO powder with 35.6% CaO conversion (XCa (%) = 35.6) were collected. The spectra consisted of several bands in the 650–1400 cm−1 region,
as shown in Fig. 3. Comparing with fresh CaO, sulfated
CaO formed the peaks with wave numbers at 672, 983,
1014, 1103, 1251, 1282 cm−1 . Previous research[23]
suggests that the polyhedra sulphate anion belonging
to the Td symmetry in its free state have four fundamental internal modes of vibration, i.e. the symmetric
stretching mode v1 (A1 ), the doubly degenerate symmetric bending mode v3 (E ), the triply degenerate asymmetric stretching v3 (F2 ), and bending modes v4 (F2 ). These
modes respectively correspond to the wave numbers 981
(v1 ), 451 (v2 ), 1108 (v3 ), and 613 cm−1 (v4 ). Therefore, the absorption band at 983 cm−1 corresponds to
the v1 mode vibration of the SO4 2− , while the adsorption band at 1103 cm−1 corresponds to the v3 vibration
of the SO4 2− .[23 – 25] For SO4 2− , its coordinate modes are
described in Fig.4, including the free state SO4 2− , the
monodentate state SO4 2− , the bidentate (chelate ring)
state SO4 2− , and the bidentate (bridged ring) SO4 2− .[26]
The free state SO4 2− has the highest symmetry, whilst
the bidentate state has the lowest symmetry. With the
decrease of the symmetry, the number of the FT-IR
spectra bands increase. Combined with the spectra in
Fig.3, the peak of 983 cm−1 does not exist in analytical CaSO4 , which means that the symmetric stretching
Asia-Pac. J. Chem. Eng. 2012; 7: 55–62
DOI: 10.1002/apj
57
X. YANG et al.
Asia-Pacific Journal of Chemical Engineering
Figure 1. SEM photograph of sulfated CaO with different sulfation extent.
6000
5000
Analytical grade CaSO4
4000
3000
2000
Intensity (Counts)
58
1000
0
6000 30
32
34
36
38
40
42
44
46
48
50
52
54
CaO
5000
4000
Sulfated CaO with 38.5% conversion
CaO
3000
CaO
2000
1000
0
30
32
34
36
38
40
42
44
46
48
50
52
54
2q(°)
Figure 2. XRD pattern of analytical grade CaSO4 and
the sulfated CaO with 38.5% conversion.
mode v1 (A1 ) vibration is not obvious in analytical grade
CaSO4 . The peaks of 672, 1014 cm−1 are typical of the
vS – O band and the peak 1251, 1282, and 1291 cm−1 are
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
suggested to be the vS O band in covalent organiclike sulfates,[18,27 – 31] and the bidentate SO4 2− exists
in both sulfated CaO and analytical grade CaSO4 . The
peaks of 694, 856, 879 cm−1 in sulfated CaO and 714,
856, 879 cm−1 in fresh CaO arise from the CO3 2−
vibration,[29] as the sample could easily absorb CO2
when it is exposed to the air. Comparing the spectra of
the sulfated CaO with 35.6% CaO conversion and the
analytical grade CaSO4 , the peak of 983 cm−1 exists
obviously in the sulfated CaO, but not in analytical
CaSO4 ; and the peak position of suggested vS O band
wave numbers have some differences between 1251,
1282 cm−1 in sulfated CaO and 1291 cm−1 in analytical grade CaSO4 . The FT-IR spectra of sulfated CaO
with different sulfation extent was investigated to verify whether the difference between 1251, 1282 cm−1 in
sulfated CaO and 1291 cm−1 in analytical grade CaSO4
was caused by the different sulfation degree.
Figure 5 shows FT-IR spectra of sulfated CaO
with different sulfation extent. Between 1248 and
1282 cm−1 , as the sulfation extent increases, the peak
position shifts from 1248 to 1257 cm−1 , then to
1264 cm−1 . Also, the relative peak value at 1248 and
1282 m−1 , 1257 and 1282 cm−1 , 1264 and 1282 cm−1
changed with the sulfation extent. For sulfated CaO
Asia-Pac. J. Chem. Eng. 2012; 7: 55–62
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
INVESTIGATION OF SCR REACTION ON SULFATED CaO
When sulfation extent further increases to 50.5%, the
peak value near 1264 cm−1 is no longer higher than that
at 1282 cm−1 . The wave number shift toward higher
wave number with increasing SO4 -content in sample
has been previously observed on sulfated ZrO2 and copper containing sulfated ZrO2 , which is ascribed to a parallel increase in the ratio of polynuclear to mononuclear
sulfates.[24,29] With the increasing CaO sulfation extent,
the produced sulfates changed gradually from dispersed
mononuclear form to polynuclear sulfates and drove the
corresponding wave numbers to the higher end. Hence,
1251, 1282 cm−1 in sulfated CaO and 1291 cm−1 in
analytical grade CaSO4 was identical to vS O in sulfates.
1291
Analytical grade CaSO4
1103
694
672
1014
694
1251
Sulfated CaO (35.6% conversion)
672
Absorbance
848
1282
1103
983 1014
1176
879
714
879
1.2
Fresh CaO
0.8
848
1176
1072
0.4
0.0
800
900
1000
1100
1200
1300
1400
Wavenumbers / cm-1
NH3 -TPD and NO-TPD
Figure 3. FT-IR spectra of fresh CaO, sulfated CaO and
analytical grade CaSO4 .
M
O
O
S
O
O
S
O
O
free
O
O
M O
O
O
S
M
O
S
O M
O
O
O
bidentate
monodentate (chelate ring)
bidentate
(bridged ring)
Absorbance
Figure 4. Coordinate modes of the SO4 2− .
1.6
694
1.4 672
1.2
1.0
0.8
0.6
694
0.4
1.6
1.4 672
1.2
1.0
0.8
0.6
694
0.4
1.6
672
1.4
1.2
1.0
0.8
0.6
0.4
700
Sulfated CaO (50.5% conversion)
1103
983 1014
1264 1 282
1176
848 879
Sulfated CaO (38.5% conversion)
983
848
1014
1257 1282
Figure 6 shows the NH3 -TPD curves of analytical grade
CaO and the sulfated CaO with 20, 30, 36.5% conversion with the particle diameter of 0.30–0.35 mm.
The NH3 -TPD curve of analytical grade CaO indicates that there are two kinds of NH3 desorption peaks,
one near 350 ◦ C, and the other near 550 ◦ C. For sulfated CaO (XCa (%) = 20), the adsorbed NH3 peaks
near 350 and 550 ◦ C both decreases, for the decrease
of the BET surface area during the sulfation process.
The peak value labeled for the NH3 -TPD curves indicates that the absorbed NH3 decreases largely as the
sulfation extent increases, because of the decrease of
the BET surface area. Besides, the relative peak positions of higher sulfation extent CaO (XCa (%) = 30 and
XCa (%) = 36.5) change to 550 and 650 ◦ C, whereas the
peaks of analytical grade CaO and lower sulfation extent
sulfated CaO (XCa (%) = 20) mainly at 350 and 550 ◦ C.
This observation indicates that the CaO active site is
gradually changed into CaSO4 active site, and the
1103 1 176
879
Sulfated CaO (36.5% conversion)
0.40E-012
Sulfated CaO (31.4% conversion)
1014
1103 1176
1248
1282
983
Sulfated CaO (30% conversion)
848 879
800
900
0.50E-012
1000
1100
1200
1300
1400
Intensity
700
Sulfated CaO (20% conversion)
3.10E-012
Wavenumbers / cm-1
Figure 5. FT-IR spectra of sulfated CaO with different
sulfation extent.
with 31.4% conversion, the peak value at 1248 cm−1
is apparently higher than that at 1282 cm−1 ; as the sulfation extent increased to 38.5%, the peak value at
1257 cm−1 is a little higher than that at 1282 cm−1
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Analytical grade CaO
1.15E-011
100
200
300
400
500
600
700
800
Temperature / °C
Figure 6. NH3 -TPD on the fresh CaO and sulfated CaO.
Asia-Pac. J. Chem. Eng. 2012; 7: 55–62
DOI: 10.1002/apj
59
X. YANG et al.
Asia-Pacific Journal of Chemical Engineering
adsorption strength (temperature for complete NH3 desorption) increases.
The NO-TPD experiment was also done on the
catalyst, and the outlet NO concentration was below
the detection limit of the instrument during the heating
process.
The NH3 -TPD and NO-TPD experiments indicated
that NH3 was more easily absorbed on the catalysts
during the SCR reaction. NH3 adsorption ability is associated with the surface acidity of the catalysts. The acid
strength (temperature for complete NH3 desorption) of
the catalyst depends on the surface sulfates, and the
concentration of the acid sites (total amount of NH3
desorbed) depends on the surface area.
Lietti et al .[32 – 34] , Xie et al .[28] , and Liu et al .[35]
have pointed out that the surface acidity of a catalyst
determines its SCR activity at high temperature. The
sulfation provides specific acid properties, which may
be the reason that the sulfated CaO has catalytic effect
on the SCR reaction.
Catalysis
Before the SCR experiments, the blank experiment in
empty reactor tube was conducted to investigate the
selective non-catalytic reduction (SNCR) effects. In
blank tests, the NO and NH3 conversions were all below
5% when the temperature was below 800 ◦ C, and then
the catalyst activity tests of sulfated CaO and analytical
grade CaSO4 were conducted at 650–800 ◦ C with the
SNCR effect neglected.
Figure 7 indicates the conversions of NO and NH3
in NH3 + NO + O2 reaction at 800 ◦ C over sulfated
CaO with different sulfation extent. The diameter of the
CaO particle was 0.30 ∼ 0.35 mm and 0.35–0.50 mm
respectively. The sulfated CaO catalyst with different
sulfation extent was prepared by 5.5 g CaO sulfating for
different time, and then all the sulfated products were
used for SCR catalytic activity test. At the initial stage
of the test, the NO conversion over CaO was negative
because of the NH3 oxidation to NO as catalyzed by
CaO, which made the outlet NO concentration higher
than the inlet NO concentration. As the CaO conversion increased, the NO conversion first increased and
then decrease after the CaO conversion was higher more
than a certain value, whilst the NH3 conversion always
decreased with the increase of the CaO conversion. At
first, the NO conversion increased because of the CaO
active sites that were active for NH3 oxidation gradually
changed into CaSO4 active sites that were active for NO
reduction by NH3 ; and then, the NO decreased because
of the decrease of BET surface area. Figure 8 shows
the apparent selectivity of NH3 for NO reduction to N2
(SNO ) over sulfated CaO with different sulfation extent
in NH3 + NO + O2 reaction at 800 ◦ C. As the CaO sulfation extent increased, the SNO increased, which agreed
with the results that CaSO4 had high selectivity for NH3
on NO reduction as reported by Li et al .[9] . Because
the sulfated CaO was prepared in the bubbling bed,
and the particles collided among each other, the pore
plugging was not serious so that CaO could still oxidize NH3 , resulting in decreased SNO . Figure 9 shows
the conversions of NO and NH3 in NH3 + NO + O2
reaction system over 4.0 g calcined analytical grade
CaSO4 of 0.35–0.50 mm at 650–800 ◦ C. The NO and
NH3 conversion both increased with the increasing
reaction temperature. Figure 10 shows the SNO in the
NH3 + NO + O2 reaction over calcined analytical grade
CaSO4 , and the results indicates that the SNO is kept at
about 40% over the CaSO4 catalyst, which might be
due to the small amount of impurities of CaO and MgO
in the sample, for their high catalytic activity on NH3
oxidation.[13,36] The catalytic activity of analytical grade
100
0.6
80
0.4
60
40
SNO / (100%)
NO (NH3) Conversion (%)
60
20
0
-20
Dp0.30~0.35mm NO Conversion (%)
Dp0.30~0.35mm NH3 Conversion (%)
Dp0.35~0.50mm NO Conversion (%)
-40
-60
0.2
0.0
-0.2
-0.4
Dp0.30~0.35 mmSNO
-0.6
Dp0.35~0.50mm NH3 Conversion (%)
Dp0.35~0.50 mm SNO
-0.8
0
5
10
15
20
25
30
35
CaO Conversion (%)
NO and NH3 conversion in NH3 + NO + O2
reaction at 800 ◦ C. This figure is available in colour online at
www.apjChemEng.com.
Figure 7.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
0
5
10
15
20
25
30
35
40
CaO Conversion (%)
Figure 8. SNO in NH3 + NO + O2 reaction at 800 ◦ C. This
figure is available in colour online at www.apjChemEng.
com.
Asia-Pac. J. Chem. Eng. 2012; 7: 55–62
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
INVESTIGATION OF SCR REACTION ON SULFATED CaO
CaSO4 implies that the active substance of the sulfated
CaO would not disappear under high sulfation extent,
and the decrease of the NO conversion after certain CaO
conversion might be due to the decrease of the surface
area.
In conclusion, the results in Figs 7–10 have indicated
that sulfated CaO and analytical grade CaSO4 both have
catalytic activity on SCR reaction. The catalytic activity
of sulfated CaO in the close-to-real simulated flue gas
has been investigated. The results indicated that the
sulfated CaO could catalyze the SCR reaction when
CO2 , SO2 , and H2 O coexisted, suggesting the industrial
potentials of the catalysts.
Analysis and discussion
The SCR activity tests indicated that sulfated CaO and
analytical grade CaSO4 both had a catalytic effect on
NO / NH3 Conversion / (%)
70
NO Conversion
NH3 Conversion
60
50
40
30
20
10
650
700
750
800
Temperature / °C
Figure 9. NO and NH3 conversion in NH3 + NO + O2
reaction over calcined analytical grade CaSO4 . This figure
is available in colour online at www.apjChemEng.com.
100
SNO over calcined analytical grade CaSO4
SNO / %
80
60
40
20
0
650
700
750
Temperature / °C
800
Figure 10. SNO in NH3 + NO + O2 reaction over
calcined analytical grade CaSO4 .
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
the SCR reaction of NH3 + NO + O2 . Moreover, the
XRD characterization indicated that the sulfated CaO
and the analytical grade CaSO4 had the same crystal
type, and the DRIFTS study showed that there were
no significant differences between the SO4 content in
the sulfated CaO and the SO4 content in the analytical
grade CaSO4 . Hence, the formed sulfate was inferred
to be the active matter for the SCR reaction, and it
would not disappear as the sulfation extent increased.
As the S O has a strong inductive effect of adsorbing
electrons, thereby increasing the surface acidity of the
catalyst,[37 – 39] which would favor the NH3 adsorption.
Hence, it could also be concluded that the S–O and
S O bands played an important role in the SCR
reaction mechanism, and the H-abstraction of NH3 or
NH4 + might occur on the weakly bonded oxygen in
SO4 2− to form NH2 , and then NH2 reacts with NO
to form N2 .[40] Therefore, combined with the BET
and SEM results, the NO conversion decreased when
CaO conversion was higher than certain XCa (as shown
in Fig. 7), which was mainly caused by the surface
loss and CaSO4 accumulating during the sulfation
process.
The NH3 -TPD results indicated that there were two
NH3 desorption peaks in the CaO desorption curves.
Combining the two peaks with the conversion of the
NO and NH3 when CaO conversion is zero (as shown
in Fig. 7), it could be deduced that the NH3 desorbing
near 350 ◦ C would be converted to NO or N2 , the NH3
desorbing near 550 ◦ C would be converted to N2 . For
CaO, the desorption peak intensity at 350 ◦ C was higher
than the peak intensity at 550 ◦ C, hence CaO made the
NO outlet concentration be higher than the NO inlet
concentration 500 ppm, and do not exhibit denitrification effect. As the CaO converted to CaSO4 , desorption
peak intensities at 350 and 550 ◦ C both decreased when
the BET surface area decreased, but the relative proportion of the desorption peak intensity at 550 ◦ C became
larger, then the sulfated products became effective for
NO reduction. The relative proportion of desorption
peak increasing at 550 ◦ C indicated that the absorbed
NH3 on the active sites of CaSO4 would be converted
to N2 , which agreed well with the sulfated CaO catalytic
activity. Nonetheless, the TPD experiment analysis supports the SCR activity experiments on CaO and the
sulfated CaO. The quantitative analysis of adsorbing
and desorbing reactions as a function of temperature
and other conditions is important to further find out the
SCR reaction mechanism. Due to the research focus of
this article is to evaluate the feasibility of NOx reduction
over sulfated CaO, only the NH3 -TPD and NO-TPD at
N2 atmosphere has been conducted. The in situ FT-IR
experiments will be carried out to further study the NH3
absorption intermediates and the reactions on CaSO4
surface under different temperatures and atmospheres,
to clarify the SCR reaction mechanism in our later work.
Asia-Pac. J. Chem. Eng. 2012; 7: 55–62
DOI: 10.1002/apj
61
62
X. YANG et al.
Asia-Pacific Journal of Chemical Engineering
CONCLUSIONS
The experiments and related analysis show that:
1. The sulfated CaO and the calcined analytical grade
CaSO4 both had catalytic activity on SCR reaction.
2. XRD characterization and the DRIFTS study showed
that there were no significant differences between the
SO4 content in the sulfated CaO and the SO4 content
in the analytical grade CaSO4 .
3. When the sulfation extent was larger than a certain
value, the decrease of the catalytic activity was
caused by the decrease of the BET surface area, not
by the active matter’s disappearance.
4. NH3 -TPD and NO-TPD experiments indicated that
NH3 was easier than NO to be absorbed on the
sulfated CaO, and as the sulfation extent increased,
the strength of surface acidity increased.
5. NH3 absorption on CaO and CaSO4 , and then
reacting with O2 or NO would be the possible
reaction route on sulfated CaO. It was also inferred
that the S–O and S O played an important role in
the SCR reaction mechanism over CaSO4 .
Acknowledgements
Financial support from the National Key Basic Research
and Development Program of China (2006CB200301)
is gratefully acknowledged.
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DOI: 10.1002/apj
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