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Effect of CaO on NH3 + NO + O2 reaction system in the absence and presence of high concentration CO2.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2010; 5: 287–293
Published online 15 June 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.262
Special Theme Research Article
Effect of CaO on NH3+ NO + O2 reaction system in the
absence and presence of high concentration CO2
Tianjin Li, Yuqun Zhuo,* Changhe Chen and Xuchang Xu
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University,
Beijing 100084, China
Received 18 August 2008; Revised 11 December 2008; Accepted 12 December 2008
ABSTRACT: The effect of CaO on the NH3 + NO + O2 reaction system at 650–850 ◦ C was investigated. High CO2
concentration was added to investigate the effect of CaCO3 on this reaction system also. Experimental results showed
that CaO had a strong catalytic effect on NH3 decomposition, NH3 oxidation by O2 to NO, and NO reduction by NH3
in the absence of O2 . The overall effect of CaO on the NO + NH3 + O2 reaction was to enhance NH3 oxidation by
O2 to produce more NO. A small amount of NO2 and no N2 O was detected in the outlet gas stream over CaO in the
NH3 + NO + O2 reaction. NO2 formation decreased with temperature increase. NO2 formation in the NH3 + NO + O2
reaction over CaO was attributed to the oxidization of NH3 and NO by O2 . The performance of CaCO3 was different
from CaO. NH3 decomposition was promoted, but NH3 oxidation to NO was inhibited after CaO was converted to
CaCO3 . No catalytic activity for NO reduction was detected in NO + NH3 reaction over CaCO3 , but strong activity
for NH3 decomposition was observed. NO and NH3 outlet concentration over CaCO3 in the NH3 + NO + O2 reaction
was lower and higher, respectively, than that of CaO, which was mainly due to the difference of CaO and CaCO3
for NH3 oxidation. NO2 formation was inhibited, but N2 O was observed over CaCO3 . N2 O formation increased with
temperature increase at 650–750 ◦ C, and then decreased at 750–850 ◦ C.  2009 Curtin University of Technology and
John Wiley & Sons, Ltd.
KEYWORDS: NO reduction; calcium oxide; carbon dioxide; NH3 oxidation; multi-pollutant control
INTRODUCTION
Because of the very large coal reserves, most of the
energy supplied in China will still be from coal in
the near future. Removal of SO2 and NOx from coalfired power plants has received much attention due
to many environmental problems caused by SO2 and
NOx emissions, such as acid rain and threat to human
health.[1]
Wet and semi-dry scrubbing flue gas desulfurization (FGD) techniques have been found to be very
effective for SO2 removal. However, wet scrubbing
method requires a large amount of water and high initial investment and operating expense. Semi-dry FGD
needs less water than wet FGD, but it still needs considerable amounts of water to achieve high desulphurization efficiency.[2] In many regions, water shortage is a
significant problem. These drawbacks of wet and semidry methods limit their application in some developing
*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@mail.tsinghua.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
countries and arid areas. Therefore, a dry FGD technique with the characteristics of low initial investment,
low operating expense, and low water consumption, is
necessary to be developed.
In our previous work, the medium-temperature dry
FGD technology with the aforementioned characteristics has been developed for the FGD process based
on the highly active sorbents prepared from lime and
fly ash.[3 – 5] TGA experimental results showed that
high calcium utility efficiency (above 90%) could be
achieved at 700–800 ◦ C.[6] The pilot scale CFB-FGD
experimental results showed that SO2 removal efficiency reached 80–95% at Ca/S molar ratio of 2 at
700–800 ◦ C.[7]
It is obvious that, simultaneous removal of SO2 and
NOx in the same temperature window in a single reactor
will be more attractive from both the economic and
equipment simplicity points of view than alternative
techniques which remove SO2 and NOx separately.
In the temperature range 700–800 ◦ C, CaO is the
main effective composition for SO2 removal, since the
decomposition temperature of Ca(OH)2 is usually below
450 ◦ C. It is reported that calcined limestone has a
catalytic effect on NH3 oxidation to NO.[8 – 11] Lee et al .
288
T. LI ET AL.
reported that calcined Reed limestone had a significant
catalytic effect on the NH3 + NO + O2 reaction in
increasing NO outlet concentration at 727 and 827 ◦ C.[8]
However, Iisa et al . reported that calcined Sweden
limestone decreased NO outlet concentration in the
NH3 + NO + O2 reaction at 750–950 ◦ C.[12] Catalytic
reduction of NO by NH3 in the presence of excess O2
over a calcined limestone bed was also observed by Hou
et al .[13] The reason for the discrepancy of the effect of
calcined limestone on the NH3 + NO + O2 reaction was
unclear.
Furthermore, although the reaction system of NH3 +
NO + O2 with calcined limestone as catalysts had
been previously studied by other researchers,[8,12,13] the
effect of CO2 was not considered. Dam-Johansen et al .
reported that the presence of CO2 was found to inhibit
NO catalytic reduction by CO in the absence of O2
over calcined limestone.[14] A similar inhibiting effect
was found by Xu et al . in the effect of CO2 on NO
reduction by CH4 over CaO catalysts.[15] When using
NH3 as the reducing agent for NO reduction over CaO,
the effect of CO2 on NO reduction has not been reported
in literature.
The aim of this study is to make a further understanding of the effect of CaO on the NH3 + NO + O2
reaction system at 650–850 ◦ C in the absence and in the
presence of CO2 . The CO2 concentration used in this
study is 75.4%, which is above the equilibrium concentration of CO2 and CaO system at 850 ◦ C. Therefore,
the effect of CaO with 75.4% CO2 presence is indeed
the effect of CaCO3 on the NH3 + NO + O2 reaction
system, since CaO is in the form of CaCO3 from the
equilibrium point of view.
EXPERIMENTAL
Apparatus
Experiments were carried out in a quartz fixed-bed
reactor. Figure 1 shows the schematic diagram of the
experimental apparatus. It consists of three parts, i.e.
gas mixing system, heating and reaction system, and
gas analyzing system.
The reactant gases, e.g. NO and NH3 , were supplied
in gas cylinders. Nitrogen was used as balance gas. The
flow rate of each cylinder gas was controlled by Mass
Flow Controller (MFC) to obtain the desired inlet gas
concentration.
The reactor is similar to that used by Dam-Johansen
et al . for heterogeneous reaction research.[16] NH3 and
NO were separated from O2 and directly injected onto
the bed to reduce early selective non-catalytic reduction
(SNCR), since SNCR effect might play an important
role when reaction temperature was above 750 ◦ C.[17]
Both ends of the outer quartz tube were sealed by
silica gel plugs which could be easily removed. The
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Schematic diagram of experimental apparatus. This figure is available in colour online at
www.apjChemEng.com.
Figure 1.
bed material was placed on a sintered porous quartz
disc (height 5 mm) located in the middle section of the
inner reactor. The diameter of the inner reactor was
20 mm. Detailed information about the quartz reactor
was shown in Figure 2. The reactor was placed in
an electrically heated furnace which was capable of
maintaining the reaction zone at constant temperature.
The concentration of NO, NO2 , N2 O and NH3 were
continuously monitored by a precalibrated FTIR spectrometer (Nicolet Corporation, NEXUS670) aided by a
liquid N2 -cooled MCT detector. NO conversion (XNO ),
NH3 conversion (XNH3 ) and NO formation in NH3 oxidation experiment (fNO ) were defined as follows:
C out
XNO (%) = 1 − NO
× 100
in
CNO
out
CNH
3
× 100
XNH3 (%) = 1 − in
CNH3
out
CNO
× 100
fNO (%) =
in
out
CNH
− CNH
3
3
(1)
(2)
(3)
in
out
in
out
, CNO
, CNH
and CNH
were the NO and NH3
where CNO
3
3
concentrations (in units of ppm) at the inlet and outlet
of the quartz reactor, respectively.
Experimental procedures
The raw material used in the study was of analytic
grade CaO (purity >98.0 wt%) lump in order to avoid
the side effects of impurities. CaO lumps were crushed
and sieved to 300–500 µm. The typical reactant gas
concentration used in this study was as follows: NO
500 ppm, NH3 500 ppm, O2 3%, CO2 75.4%, and
Asia-Pac. J. Chem. Eng. 2010; 5: 287–293
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF CAO ON NH3 + NO + O2 WITH AND WITHOUT CO2
Figure 2. The quartz fixed-bed reactor (measurements in mm).
balanced by N2 . The total flow rate of inlet gas mixture
was 1000 ml/min (under standard conditions). 6.00 g
CaO was used as bed materials in each experiment,
corresponding to approximately 10 000 h−1 (GHSV).
CaO may react with CO2 to form CaCO3 . The
production of CaCO3 depends on many factors, such as
temperature, CO2 concentration, and reaction time. The
CO2 concentration used in this study is 75.4%, which
is above the equilibrium concentration of the CO2 and
CaO systems at 850 ◦ C.
Before the CaO bed experiments, CaO was loaded
into the reactor and calcined at 950 ◦ C for 3 h in order
to reduce further sintering during reaction. After this
pretreatment, the temperature was decreased to and
maintained at the desired temperature to conduct the
CaO bed experiment. The thermal decomposition of
NO and NH3 over CaO were initially studied. Then,
NO + NH3 reaction was conducted to investigate NO
reduction by NH3 in the absence of O2 . The NH3 + O2
and NO + O2 reactions were studied to determine the
oxidization of NH3 to NO and NO to NO2 , respectively.
Finally, the NO + NH3 + O2 reaction over CaO were
studied. A set of experiments had been performed
with the decrease of temperature from 850 to 650 ◦ C.
Experiments with high CO2 presence were conducted in
a similar way. Data were collected after the reactions
reached steady-state.
RESULTS AND DISCUSSIONS
NH3 and NO decomposition
Figure 3 shows the effect of CaO and CaCO3 on
NH3 decomposition (Reaction 4) in the temperature
range 650–850 ◦ C. The results showed that CaO had
a catalytic effect on NH3 decomposition, and XNH3
increased with temperature increasing. This result was
in accordance with the study of Cooper et al . who
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 3. Effect of CaO and CaCO3 on NH3 decomposition.
investigated the catalytic decomposition of NH3 over
calcined limestone at 725–950 ◦ C.[18]
For a certain temperature, XNH3 was much higher in
the CaCO3 bed than in that of the CaO bed. The surface
area of CaO decreased after adding 75.4% CO2 to
form CaCO3 . These results suggested that the catalytic
activity of CaCO3 for NH3 decomposition was much
higher than that of CaO.
No obvious NO decomposition (Reaction 5) was
detected in a blank experiment, CaO bed and CaCO3
bed experiments in the temperature range 650–850 ◦ C.
This result was in agreement with the study of Lee et al .
who investigated the effect of NO decomposition over
calcined limestone at 727 and 827 ◦ C.[8]
2 NH3 −−→ 2 N2 + 3 H2
(4)
2 NO −−→ N2 + O2
(5)
NO + NH3 reaction
Figure 4 shows the effect of CaO on the NO + NH3
reaction at 650–850 ◦ C. In blank tests, NO and NH3
Asia-Pac. J. Chem. Eng. 2010; 5: 287–293
DOI: 10.1002/apj
289
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T. LI ET AL.
Asia-Pacific Journal of Chemical Engineering
detected. N2 O formation increased from 2.4 ppm to
30.6 ppm with temperature increase. The mechanism
of N2 O formation in this process was unclear. The
incomplete reduction of NO by NH3 to N2 O over
CaCO3 surface might be a possible explanation. After
revising NO conversions by considering N2 O byproduct
formation, NO conversion was almost the same with
the blank tests. These results suggested that CaCO3
had no catalytic effect on NO + NH3 reaction for NO
reduction, but had strong catalytic activity for NH3
decomposition.
(a)
NH3 + O2 reaction
(b)
Effect of CaO and CaCO3 on NO + NH3
reaction. This figure is available in colour online at
www.apjChemEng.com.
Figure 4(a–b).
conversion were both below 5% at 650–850 ◦ C. In the
presence of CaO, NO and NH3 conversion increased
from 56 to 100% and from 56 to 94%, respectively,
with the increase in temperature. In the absence of O2 ,
the stoichiometric ratio of NH3 to NO is 2/3 (Reaction
6). NO conversion was higher than NH3 conversion
when reaction temperature was above 700 ◦ C, since the
inlet NH3 /NO molar ratio was larger than stoichiometric
ratio. No NO2 or N2 O was detected in either blank tests
or CaO bed experiments. These results showed that CaO
had a strong catalytic effect on NO + NH3 reaction for
NO reduction. This was similar with the study of Lee
et al .[8] using calcined limestone as bed material at 727
and 827 ◦ C.
6 NO + 4 NH3 −−→ 5 N2 + 6 H2 O
(6)
The effect of CaCO3 on NO + NH3 reaction at
650–850 ◦ C are also shown in Figure 4. The results
were quite different from that of the CaO bed. NO
conversion was only slightly higher than that of blank
tests. NH3 conversion was almost the same with that
of the CaO bed. N2 O was detected and no NO2 was
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 5 shows the effect of CaO and CaCO3 on
NH3 oxidation at 650–850 ◦ C. In blank tests, NH3
conversion was below 5% when the temperature was
below 850 ◦ C, and NO, NO2 , N2 O outlet concentration
were negligible. In CaO bed tests, NH3 was consumed
completely and NO formation was above 80%. NO2
was detected as the minor product and no N2 O was
detected in the outlet gas mixtures. NO2 outlet concentration decreased from 12.8 ppm to 4.4 ppm with
the increase in temperature. NO formation increased
with the increase in temperature at 650–800 ◦ C, and
decreased slightly when reaction temperature was above
800 ◦ C because of the SNCR effect. These results indicated that the main product of NH3 oxidation over CaO
were NO (Reaction 7) and N2 (Reaction 8), with a small
amount of NO2 (Reaction 9).
4 NH3 + 5 O2 −−→ 4 NO + 6 H2 O
(7)
4 NH3 + 3 O2 −−→ 2 N2 + 6 H2 O
(8)
4 NH3 + 7 O2 −−→ 4 NO2 + 6 H2 O
(9)
With 75.4% CO2 presence in CaO bed, NH3 conversion and NO formation were strongly inhibited in
CaCO3 bed tests at relatively low temperatures compared with that of CaO bed. At 700 ◦ C, NH3 conversion
and NO formation decreased from 100 to 60% and
from 85 to 12%, respectively. NO2 byproduct formation also was inhibited. Considerable amount of N2 O
was detected in the outlet gas mixtures, which is different from the CaO bed. N2 O formation increased with
the increase of temperature at 650–750 ◦ C, and then
decreased at 750–850 ◦ C. The decrease of N2 O formation at >750 ◦ C might be attributed to N2 O decomposition over the surface of the bed materials.
The decrease of surface area might lead to the
decrease of NH3 conversion. Experiment with 1/12
of the initial CaO weight (0.50 g) was conducted to
investigate the role of surface area decreasing. NH3 was
also converted almost completely, and NO formation
was above ∼80%. Therefore, the main reason for the
inhibition of NH3 oxidation after adding 75.4% CO2
Asia-Pac. J. Chem. Eng. 2010; 5: 287–293
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF CAO ON NH3 + NO + O2 WITH AND WITHOUT CO2
study during NH3 oxidation over CaO with 75.4% CO2
presence at 850 ◦ C. The reason for this discrepancy
was unclear. A possible reason was that the property
of the bed materials used was different. In the present
study, CaCO3 was produced by adding 75.4% CO2 to
the analytical grade CaO bed. However, there were
some MgCO3 , Al2 O3 and Fe2 O3 impurities in the
limestone used in the study of Shimizu et al . The
N2 O observed in the study of Shimizu et al . might
be formed by NH3 oxidation (Reaction 10) over these
impurities. There was still another possibility that the
discrepancy was due to the impurities in the gases
(e.g. hydrocarbons) in CO2 gas bottle. N2 O formation
will be promoted by the presence of hydrocarbons
when NO, NH3 and O2 was coexisted in the gas
mixtures.[21]
(a)
(b)
2 NH3 + 2 O2 −−→ N2 O + 3 H2 O
(10)
NO + O2 reaction
Figure 5(a–b).
oxidation.
Effect of CaO and CaCO3 on NH3
over the CaO bed should not be the decrease of surface
area of CaO, but the higher reduction in catalytic
activity of CaCO3 than that of CaO.
Zijlma et al . investigated the effect of CO2 concentration variation from 0 to 16% on reactivity of NH3 oxidation over calcined limestone at 850 ◦ C.[19] The result
showed that adding CO2 generally did not decrease
NO formation except for a small decrease at low CO2
concentration. It was reported that this result was in
contradiction with the result of Shimizu et al .[20] who
investigated the effect of calcined limestone on NH3
oxidation with 75% CO2 presence at 850 ◦ C. In the
experiment of Zijlma et al .,[19] with 16% CO2 presence,
CaO remained in the form of CaO after the reaction
reached steady-state. However, in the experiment of
Shimizu et al .,[20] CaO was in the form of CaCO3 after
the reaction reached steady-state from the point of view
of equilibrium. The discrepancy of the two studies could
be explained by the difference in CaO and CaCO3 for
NH3 oxidation, since experimental data in this study
showed that CaCO3 was less active than CaO for NH3
oxidation to NO.
In the study of Shimizu et al ., a considerable amount
of N2 O was detected in NH3 oxidation over limestone
with 75% CO2 presence at 850 ◦ C.[20] However, N2 O
was below the detection limit (2 ppm) in the present
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 6 shows the effect of CaO and CaCO3 on NO +
O2 reaction at 650–850 ◦ C. In blank tests, NO oxidation
was neglectable. With CaO and CaCO3 presence, the
reaction rate of NO oxidation was still very slow. Only
a small amount of NO2 (Reaction 11) was detected in
the outlet gas mixtures. NO2 formation decreased from
15.8 ppm to 4 ppm with the increase of temperature
over the CaO bed. NO2 formation was inhibited by
adding 75.4% CO2 to CaO bed at temperatures below
800 ◦ C.
(11)
2 NO + O2 −−→ 2 NO2
NO + NH3 + O2 reaction
Figure 7 shows the effect of CaO and CaCO3 on
the NO + NH3 + O2 reaction at 650–850 ◦ C. In blank
Figure 6. Effect of CaO and CaCO3 on NO + O2 reaction.
Asia-Pac. J. Chem. Eng. 2010; 5: 287–293
DOI: 10.1002/apj
291
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T. LI ET AL.
(a)
(b)
Asia-Pacific Journal of Chemical Engineering
(Reactions 9 and 11) over CaO, since NO2 formation
in the NH3 + NO + O2 reaction at a certain temperature
was approximately the sum of that in the NH3 + O2 and
NO + O2 reaction.
The NH3 + NO + O2 reaction system over the CaO
bed was somehow complicated. Reactions 4–12 might
describe what happened during experiments. The overall effect of CaO on this system was depended on
the competition between NH3 oxidization by O2 to
form NO (Reaction 7) and the reduction of NO
by NH3 (Reaction 6) over CaO. The increase of
NO outlet concentration over CaO bed indicated that
NH3 oxidization to NO was more favored compared with NO reduction. The selective catalytic
reduction (SCR) of NO by NH3 in the presence
of excess O2 (Reaction 12) was not dominated at
650–850 ◦ C.
4 NO + 4 NH3 + O2 −−→ 4 N2 + 6 H2 O
(c)
Figure 7(a–c). Effect of CaO and CaCO3 on NO +
NH3 + O2 reaction.
tests, NO and NH3 conversion was both below 5% at
650–800 ◦ C. Above 800 ◦ C, the SNCR effect became
obvious. No NO2 or N2 O was detected at 650–850 ◦ C.
In the presence of the CaO bed, NH3 was completely
consumed and the NO outlet concentration was higher
than the inlet level. NO2 was detected but no N2 O
was detected in the outlet gas mixtures. NO2 outlet
concentration decreased from 26 ppm to 8.8 ppm with
the increase of temperature. The formation of NO2 over
CaO bed in the NH3 + NO + O2 reaction system was
attributed to the oxidization of NH3 and NO by O2
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
(12)
The NH3 + NO + O2 reaction system with calcined
limestone as catalysts was previously studied by some
researchers.[8,12,13] Since the composition and physical characteristics of the limestone used in the experiments were different, and the reaction conditions
were not completely identical, different results were
obtained. In this study, experimental data with analytic grade CaO as catalysts showed that the overall
effect of CaO on the NO + NH3 + O2 reaction system at 650–850 ◦ C was to enhance the oxidization of
NH3 and form more NO. This conclusion was similar
to Lee et al .,[8] but different from Iisa et al .[12] and Hou
et al .[13]
After CaO was converted to CaCO3 by adding 75.4%
CO2 , NO outlet concentration was lower and NH3 outlet
concentration was higher than that of CaO bed, respectively. NO2 formation was inhibited and considerable
amount of N2 O was detected. These differences were
mainly due to the difference of CaO and CaCO3 for
NH3 oxidation, which have been shown in Figure 5.
The formation behavior of N2 O over CaCO3 in the
NH3 + NO + O2 reaction system (Fig. 7c) was similar to that in the NH3 + O2 reaction (Fig. 5b). The
detailed mechanism of the effect of CaCO3 on the
NH3 + NO + O2 reaction system was unclear. It was
suggested that NO formation over the active site of CaO
surface was partly replaced by N2 O formation over the
active site of CaCO3 surface after converting CaO to
CaCO3 .
From the point of view of practical application, the
effect of CaO on the NO + NH3 + O2 reaction system was not favored. However, it has been reported
that NH3 oxidation over CaO could also be strongly
inhibited by the presence of H2 O[9,19] and SO2 .[22 – 25]
Asia-Pac. J. Chem. Eng. 2010; 5: 287–293
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF CAO ON NH3 + NO + O2 WITH AND WITHOUT CO2
Calcined limestone sulfation product even had catalytic effect for NO reduction by NH3 in the presence of excess oxygen.[22] This system is quite complicated. Further work is needed to evaluate the applicability of simultaneous NO and SO2 removal based
on the dry FGD process developed in our previous
work by injecting NH3 as reductant for NO reduction.
CONCLUSIONS
The effect of CaO on the NH3 + NO + O2 reaction
system at 650–850 ◦ C was investigated. The effect of
CaCO3 on this reaction system was also investigated
by adding 75.4% CO2 to convert CaO to CaCO3 .
Experimental results showed that CaO had a strong
catalytic effect on NH3 decomposition, NH3 oxidization
by O2 to form NO, and NO + NH3 reaction for NO
reduction in the absence of O2 . The overall effect
of CaO on the NO + NH3 + O2 reaction system at
650–850 ◦ C was to enhance NH3 oxidation by O2 to
produce more NO. A small amount of NO2 , and no
N2 O was detected in the outlet gas stream over the CaO
bed in the NH3 + NO + O2 reaction. NO2 formation
decreased with temperature increase at 650–850 ◦ C.
The formation of NO2 over the CaO bed in the NH3 +
NO + O2 reaction was attributed to the oxidization of
NH3 and NO by O2 .
CaCO3 had a strong catalytic effect on NH3 decomposition, a moderate catalytic effect on NH3 oxidization by O2 to form NO, and no catalytic effect on
the NH3 + NO reaction for NO reduction, but NH3
decomposition. The overall effect of CaCO3 on the
NO + NH3 + O2 reaction system at 650–850 ◦ C was
to enhance NH3 decomposition and NH3 oxidation by
O2 , and form more NO than blank tests, but less NO
than in the CaO bed. NO2 formation was inhibited, but
considerable amount of N2 O was observed over CaCO3
bed. NO2 formation increased slightly with temperature
increase. N2 O formation increased with the increase
of temperature at 650–750 ◦ C, and then decreased at
750–850 ◦ C.
Acknowledgements
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The financial support by the National Key Basic
Research and Development Program of China (2006CB200301) for this study is gratefully acknowledged.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 287–293
DOI: 10.1002/apj
293
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