Download - Catalytic decomposition of ammonia gas with metal cations present naturally in low rank coals
Catalytic decomposition of ammonia gas with metal cations
present naturally in low rank coals
Chunbao Xu1, Naoto Tsubouchi, Hiroyuki Hashimoto, Yasuo Ohtsuka*
Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
Received 3 December 2004; received in revised form 17 March 2005; accepted 17 March 2005
Available online 19 April 2005
Abstract
A novel hot gas cleanup method to decompose a low concentration of NH3 to N2 with metal cations present inherently in low rank coals
has been studied with a quartz reactor under the conditions of 750–850 8C, 0.1 MPa and high space velocity of 45,000 hK1. Each coal is
pyrolyzed at 900 8C to prepare the char, which is subjected to the decomposition of 2000 ppm NH3 after pretreatment with H2. All of five
chars examined promote NH3 decomposition in inert gas, but the promotion effect depends strongly on the kind of char and can correlate
more closely with the Fe content than with the Ca content. This result may indicate that the Fe plays a crucial role in the reaction. A
commercial activated carbon with a very low Fe content of !0.05 wt% exhibits lower conversion of NH3 to N2 than five chars. The TEM
pictures reveal the formation of nanoscale particles of Fe and Ca components on a brown coal char that provides the largest catalytic
performance. The char maintains the high conversion level of 80% during 25 h reaction at 750 8C and achieves the complete decomposition
of NH3 at 850 8C. The co-feeding of a mixture of H2, CO, and CO2 does not change significantly the catalytic activity of the char at a steady
state, whereas the coexistence of 2000 ppm H2S lowers it in the whole range of time on stream. It is proposed by combining the XRD and
TPD observations with our previous results that the catalytic decomposition of NH3 in inert gas with the chars derived from low rank coals
proceeds through two cycle mechanisms involving iron metal, iron nitrides, CaO and CaCN2.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Ammonia decomposition; Hot gas cleanup; Low rank coals; Nanoparticles; Metallic iron; CaO
1. Introduction
Catalytic decomposition of a low concentration (usually
1000–5000 ppm in volume) of NH3 gas has attracted
increasing attention from a view point of hot gas cleanup
of raw fuel gas produced in coal gasification for an
integrated gasification combined cycle (IGCC) or fuel cell
(IGFC) technology [1–6], because such a cleaning method
can increase the power generation efficiency of IGCC and
IGFC, which may lead to more efficient reduction of CO2
emissions, compared with pulverized coal-fired power
plants. A hot gas cleanup system for IGCC is based on
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2005.03.019
* Corresponding author. Tel.: C81 22 217 5653; fax: C81 22 217 5655.
E-mail address: [email protected] (Y. Ohtsuka).1 Present address: Syncrude Edmonton Research Center, 9421-17
Avenue, Edmonton, Alta., Canada.
a sequence of unit operations for the removal of particulates,
halides, H2S and NH3 at high temperatures [2].
Although several catalysts for the decomposition of NH3
to N2 and H2, such as Ni-, Mo-, or Ru-based catalysts, have
been proposed [1–4,7–9], the present authors have been
studying on the utilization of inexpensive Fe catalysts for
this purpose [10–13]. It is readily expectable that metallic
Fe can promote the NH3 decomposition, because this
reaction is the reverse one of NH3 synthesis in which the Fe
is catalytically active [14]. According to earlier work [8,15],
cheap raw materials, such as ferrous dolomite and sintered
iron ore, were effective for decomposing a low concen-
tration of NH3 at 900 8C, but the temperature was too high,
and the catalytic effects were insufficient. We have recently
shown that nanoscale metallic Fe, which is formed by
heating simply Fe ions incorporated into oxygen functional
groups in brown coals [16–18], can achieve the complete
decomposition of 2000 ppm NH3 diluted with inert gas at
750 8C [10,13], and that fine particles of CaO prepared from
Fuel 84 (2005) 1957–1967
www.fuelfirst.com
C. Xu et al. / Fuel 84 (2005) 1957–19671958
ion-exchanged Ca added to an Australian brown coal can
also catalyze this reaction [13].
It is well known that Fe and Ca cations are naturally
present as ion-exchangeable forms in low rank coals
[19–21] and transformed into fine particles of Fe and CaO
upon pyrolysis, respectively [22,23]. The present work
therefore aims at examining the catalytic effect of such
inherent cations on the NH3 decomposition, and in
particular it focuses on the catalytic performance of Fe
nanoparticles formed from Fe ions. The high performance,
if it can be achieved, should be interesting from a practical
point of view, because low rank coals can be used as the
catalysts for this purpose without any catalyst components
added.
2. Experimental
2.1. Coal samples and char preparation
Five kinds of low rank coals from different countries
were used as raw materials of the catalysts for NH3
decomposition. Every coal was dried in laboratory air,
ground with a mortar and sieved to the particles with size
fraction of 150–250 mm. The ultimate and proximate
analyses are shown in Table 1, where each coal is
represented as a corresponding code name throughout the
present paper. Carbon and ash contents in the five coals
were in the range of 66–72 wt% (daf) and 1.5–5.3 wt%
(dry), respectively. Among them, RB coal from Germany
was used, unless otherwise described.
The coal sample was first dried in a stream of pure N2 at
about 110 8C, and the pyrolysis was then carried out with a
quartz-made fluidized-bed reactor to prepare the char
sample as the catalyst, the bed zone being 2.5 cm i.d. and
35 cm long. In a typical run, about 0.50 g of the coal was
heated at a relatively large rate of 400–500 8C/min up to
900 8C under the flow of high purity He (O99.99999%).
After 10 min holding, the reactor was quenched to room
temperature to recover the char. The apparatus and
procedure have been described in more detail elsewhere
[18]. As shown in Table 2, all char samples prepared had
Table 1
Elemental and proximate analyses of low rank coals as catalyst precursors
Coal Code Country Elemental analysis, wt% (daf)
C H N
Rhein
Braun
RB Germany 66.3 5.1 0.9
Aban AB Russia 66.9 4.6 1.0
Zalainuour ZN China 69.9 4.8 1.7
Adaro AD Indonesia 70.8 5.0 1.2
South
Banko
SB Indonesia 71.8 5.2 1.4
a VM, volatile matter; FC, fixed carbon.b Determined by difference.
carbon contents of 88–92 wt% (daf). Every char was treated
with pure H2 before use, as shown below.
Iron and calcium catalysts, which were prepared by
pyrolysis of Fe- and Ca-loaded brown coals with the
fluidized bed reactor [13], were used for comparison with
the above-mentioned char samples, metal loading being
1.6 wt% Fe for 2% Fe catalyst, 5.9 wt% Fe for 6% Fe
catalyst, and 5.8 wt% Ca for 6% Ca catalyst. A Victorian
brown coal from Australia with a low ash content of
0.6 wt% (dry) was employed as the catalyst support in this
preparation. The details of the method and procedure have
been provided in our previous papers [13,17,24].
Since NH3 gas reacts with activated carbons to form
N-containing functional groups in the carbons [25–27], the
reaction of NH3 with the carbon in the present char may
occur. To examine this point, a commercially available
activated carbon, denoted as AC, was used in place of the
char. The Fe and Ca contents of the AC were both as low as
!0.05 wt%, and the BET surface area was 870 m2/g.
2.2. Ammonia decomposition and gas analysis
Catalytic decomposition of NH3 was performed iso-
thermally at ambient pressure with a vertical, cylindrical
quartz reactor (8 mm i.d.) placed in a glass-made electric
furnace [13]. The height of catalyst bed was kept at 8 mm,
and the temperature was measured with a thermocouple on
the outer surface of the reactor. The experimental procedure
has been provided in detail previously [13] and is thus
simply described below. Special precautions against any
leakages were taken to determine precisely N2 as a product
of NH3 decomposition. After a sufficient amount of high
purity He was passed over the char held in the reactor, the
concentration of N2 in the outlet gas was ensured to be less
than 20 ppm with a high-speed micro gas chromatograph
(GC). Then, the char was heated up to 500 8C and pretreated
with pure H2 at this temperature for 2 h. Finally, the reactor
was held at a predetermined reaction temperature, and high
purity He was replaced with reactant gas to start the
reaction. The standard conditions are: NH3 concentration,
2000 ppm; balance gas, high purity He; temperature,
750 8C; space velocity, 45,000 hK1; apparent contact time
Proximate analysisa, wt% (dry)
S Ob Ash VM FCb
0.6 27.1 4.1 56.9 39.0
0.6 26.9 5.3 44.6 50.1
0.4 23.2 4.1 42.5 53.4
0.1 22.9 1.5 45.7 52.8
0.5 21.1 4.2 48.9 46.9
Table 2
Elemental analysis of five chars used
Chara C, wt% (daf) H, wt% (daf)
RB char 88.9 0.7
AB char 87.7 0.8
ZN char 87.5 1.4
AD char 89.0 1.1
SB char 91.5 0.8
a Prepared at 900 8C with a fluidized bed pyrolyzer.
C. Xu et al. / Fuel 84 (2005) 1957–1967 1959
between gas and catalyst, 0.080 s; time on stream, 4 h. A
simulated gas including 2000 ppm NH3, H2, N2, syngas
(a mixture of H2 and CO), CO2, CH4, and H2S was used in
several runs.
The initial concentration of NH3 in reactant gas was
determined with a photo-acoustic multi-gas monitor, and the
amount of N2 evolved was analyzed at 2.5 min intervals
using the high-speed micro GC [13]. Conversion of NH3 to
N2 can be estimated by using the amounts of NH3 fed and N2
formed on the assumption that NH3 decomposition proceeds
according to the following equation.
2NH3/2N2 C3H2 (1)
The conversion to N2 is expressed in % on a nitrogen
basis. When 2000 ppm NH3/He was passed over the AC
under the above-mentioned standard conditions, the
elemental analysis of the AC recovered after 4 h reaction
revealed that about 4% of total amount of NH3 fed was
introduced into the AC as N-containing functional groups.
However, this NH3 consumption was not taken into account
on the calculation of conversion of NH3 to N2, because no
N2 was formed in this reaction.
0
20
40
60
80
100
0 50 100 150 200 250Time on stream, min
Con
vers
ion
of N
H3
to N
2, %
RB char
AB char
SB char
ZN char
AD char
Quartz woolActivated carbon
Fig. 1. Catalytic effects of different chars derived from low rank coals on
NH3 decomposition in inert gas at 750 8C.
2.3. Characterization of coal and char
To analyze the content of Fe or Ca element present
naturally in the coal, it was burned out at 815 8C to obtain
the ash, which was subjected to acid leaching, and these
metal cations leached were determined by the inductively
coupled plasma method [22]. The surface area of the char
prepared at 900 8C was determined by the BET method after
N2 adsorption at 77 K. The crystalline forms of Fe and Ca
species in the char were analyzed with a powder X-ray
diffraction (XRD) method with Ni-filtered Cu Karadiation (40 kV!30 mA). The dispersion states of these
components were measured by the high resolution
transmission electron microscope (TEM) with an energy-
dispersive X-ray analyzer (EDX), the acceleration energy
being 200 kV. To carry out the temperature programmed
desorption (TPD) run of the used char after NH3
decomposition, the sample held in the reactor was first
quenched to room temperature in a stream of high purity He
and then heated again at 10 8C/min up to 850 8C, and N2
desorbed from the char in the TPD process was monitored
by the micro GC.
3. Results and discussion
3.1. Decomposition of ammonia in inert helium
The profiles for conversion of NH3 to N2 against time on
stream over different chars at 750 8C are shown in Fig. 1,
where the result of a blank run with quartz wool or AC alone
is also plotted. The conversion was !1% and 11–13% with
the wool and AC throughout the 4 h reaction, respectively.
The carbon in the AC may promote the NH3 decomposition,
because the contents of the catalytically active species, such
Fe and Ca components, were very low. As mentioned above,
the nitrogen in NH3 reacted with the AC to be introduced
into it as N-functional groups, and about 4% of total amount
of NH3 supplied was thus consumed. However, this
consumption was not included in conversion of NH3 to N2
plotted in Fig. 1. As seen in Fig. 1, all of five chars examined
showed the larger promotion effects on the NH3 decompo-
sition than the AC. With most chars, the NH3 conversion
decreased with increasing time on steam to be almost
steady, and the value after 3–4 h reaction increased in the
sequence of AD!ZNzSB!AB!RB. The RB char thus
provided the largest conversion of 80% at a steady state.
Since Fe and Ca catalysts supported on Victorian brown
coal chars can promote NH3 decomposition [13], it is
probable that the enhanced conversions by use of these
chars originate from the catalysis by Fe and Ca components
present naturally in them. The performances of the present
chars, Fe and Ca catalysts are compared in Table 3, where
the latter data are cited from our preceding paper [13]. At
750 8C, NH3 conversions with RB and AB chars were lower
than that with 6% Fe catalyst but larger than those with 2%
Fe and 6% Ca catalysts. Table 3 also showed that
surface areas of RB, AB and ZN chars were in the range
of 100–230 m2/g, which were smaller than those of 2–6% Fe
Table 3
NH3 conversion over chars and catalysts with different surface areas
Catalyst Surface areaa, m2/g Conversion of
NH3 to N2b, %
RB char 230 80
AB char 230 68
ZN char 100 51
Activated carbon 870 13
2 wt % Fec 360 57
6 wt % Fec 340 96
6 wt % Cac n.a.d 33
a Measured by the BET method.b After 3.5 h reaction at 750 8C.c Supported on brown coal char [13].d Not analyzed.
vers
ion
of N
H3
to N
2, %
60
80
100
Almost constantconversion
C. Xu et al. / Fuel 84 (2005) 1957–19671960
catalysts. Although activated carbon had the largest surface
area of 870 m2/g, it provided the lowest NH3 conversion of
13%. This observation suggests that the carbon in the
present char may promote the decomposition reaction, but
the promotion effect of the char may be small, because the
char possesses the much lower surface area than the AC.
The high performances of RB and AB chars should be
noteworthy from a practical point of view, because no unit
operations for catalyst addition are needed.
In order to clarify the catalytic roles of the Fe and Ca
species present originally in the chars used in this work, it is
of interest to examine the relationship between the inherent
content and the extent of NH3 decomposition. The results
are shown in Fig. 2, where conversion of NH3 to N2 after
3–3.5 h reaction at 750 8C is provided, and the data with AC
are plotted at zero of a horizontal scale. The Fe or Ca
content in the dried char was determined to be 0.2–2.0 wt%
or 0.8–5.0 wt%, respectively. As expectable, the NH3
conversion increased with increasing inherent Fe or Ca
content, but there was a stronger correlation between
the Fe and the conversion. This may be reasonable,
0
20
40
60
80
100
Con
vers
ion
of N
H3
to N
2, %
0 2 4 6
Fe or Ca content in char, wt%
Fe content
Ca content
Fig. 2. Correlation between inherent Fe or Ca in char and NH3 conversion
at 750 8C.
since the Fe catalyst was more active than the Ca catalyst
as seen in Table 3. The comparison of Fig. 2 and Table 3
indicates that the RB char exhibits much higher activity than
the 2% Fe catalyst, in spite that the Fe content is almost the
same between the two. This difference may be explained by
the catalytic effect of 3% Ca in the RB char.
Fig. 1 demonstrated the largest catalyst effectiveness of
the RB char for NH3 decomposition. Some influential
factors were thus investigated using this char below. Fig. 3
shows the catalytic performance of the RB char under
several conditions. As seen in Fig. 3A, the initial NH3
conversion of about 90% at 750 8C decreased to less than
80% after 2–3 h reaction, but the conversion level was
almost unchanged when time on stream was further
prolonged to 1500 min. This observation may prove the
long catalyst life of the RB char under the present
conditions.
The catalyst deactivation at the initial stage of reaction in
Fig. 3A was also observed with the 2 and 6% Fe catalysts, and
the extent was larger at the smaller Fe loading [13]. To make
clear the deactivation mechanism, the RB char after 4 h
reaction at 750 8C was in situ treated with H2. The result is
shown in Fig. 3B. After conversion of NH3 to N2 decreased to
about 80%, 2000 ppm NH3 was replaced with pure H2, and
the H2 treatment was carried out for 1 h. When the reactant
gas in place of pure H2 was passed again over the treated char,
Con
400 300 600 900 1200 1500
A
Con
vers
ion
of N
H3
to N
2, %
40
60
80
100
0 100 200 300 400
Time on stream, min
750°C
H2 treatment at 750°Cwithout feeding NH3
850°C750°C
B
Fig. 3. Catalytic performance of RB char in NH3 decomposition under
several conditions: (A) change in NH3 conversion for a prolonged time of
1500 min at 750 8C; (B) NH3 conversion at 850 8C against time on stream
and effect of in situ H2 treatment on NH3 conversion at 750 8C.
Fig. 5. Changes in CH4 concentration and temperature in the process of H2
pretreatment.
C. Xu et al. / Fuel 84 (2005) 1957–1967 1961
the NH3 conversion was restored to the almost initial level. A
significant amount of CH4 could be detected at the beginning
of the H2 treatment, and the CH4 decreased steeply with
increasing time. It is thus likely that the surface of metallic Fe
present in the RB char is partly carburized by the reaction
with carbon atoms of the char in the process of NH3
decomposition, and part of the catalytic activity is conse-
quently lost, whereas the H2 treatment can transform the
carbide species to the metallic form and thus restore the
activity of the char to the original state.
When the decomposition temperature was increased to
850 8C, as seen in Fig. 3B, the NH3 conversion over the RB
char reached 100%, and no significant deactivation took
place during 4 h reaction. Almost the same phenomenon
was observed with the 2% Fe catalyst [13]. The H2 evolved
by NH3 decomposition might prevent the surface of metallic
Fe from being carburized due possibly to the larger reaction
rate of the carbide species and H2 at 850 8C.
As indicated in Fig. 3B, the in situ H2 treatment of the RB
char at 750 8C was effective for recovery of the decreased
catalytic activity. Although every char was first pretreated
with H2 at 500 8C and then subjected to NH3 decomposition,
the result in Fig. 3B suggests that the H2 pretreatment of the
char at a higher temperature than the usual 500 8C may
improve the catalytic performance. Fig. 4 shows the effect
of the pretreatment temperature on NH3 decomposition with
the AB char. Conversion of NH3 to N2 over the char
pretreated at 500 8C decreased gradually with increasing
time on stream, but the degree of the decrease almost
leveled off after 6 h. When the pretreatment was carried out
at a higher temperature of 700 8C, on the other hand, the
char maintained the high conversion of 93–96% for 6 h, and
the difference in the NH3 conversion with the char
pretreated at 500 and 700 8C reached 30% after 6 h. It is
evident that the high temperature H2 pretreatment can
increase the catalyst life of the char.
0
20
40
60
80
100
Con
vers
ion
of N
H3
to N
2, %
0 100 200 300 400
Time on stream, min
Pretreated with H2 at 700°C
Pretreated with H2 at 500°C
Fig. 4. Effect of H2 pretreatment temperature on NH3 decomposition with
AB char at 750 8C.
The change in the concentration of CH4 evolved in the
process of H2 pretreatment of the AB char at 700 8C is
illustrated in Fig. 5, where the temperature profile is also
given on a right-hand vertical axis. A small amount of CH4
was observed during heating up in inert He and may be
ascribed to CH4 adsorbed in the pores of the char after
pyrolysis. When the He was replaced with pure H2 at
700 8C, CH4 concentration initially increased quickly, then
had the maximal value, and finally decreased steeply with
increasing time on stream. After the 2 h pretreatment, the
concentration was almost zero by switching from H2 to inert
He. The sum of CH4 evolved roughly corresponds to a few
per cent of the carbon present in the char. The CH4 may
originate from the two sources of reactive carbon atoms and
surface iron carbides in the char. Metallic Fe present
inherently in the char may catalyze the gasification of the
highly reactive carbon with H2 to provide CH4, because
highly dispersed Fe catalyst supported on brown coal char
can promote the hydrogasification at a temperature as low as
600 8C under ambient pressure according to the following
equation [28].
C C2H2/CH4 (2)
The removal of the highly reactive carbon from the char
by this equation may suppress the formation of iron carbides
by reaction of the carbon and metallic Fe, and it may thus
lead to the stable catalytic activity of the AB char observed
in Fig. 4. Iron carbides in the char are another possible
source of the CH4, which may be formed by the reaction of
the carbide species with H2 in the pretreatment process. If
the carbide can be regarded as cementite (Fe3C), the CH4
formation can be expressed as follows:
Fe3C C2H2/3Fe CCH4 (3)
It has been reported that Fe3C is readily formed when Fe
ions incorporated into brown coals are pyrolyzed to prepare
C. Xu et al. / Fuel 84 (2005) 1957–19671962
Fe catalysts supported on the chars [13,18,28]. The
transformation of the carbide species to metallic Fe can
increase the number of catalytically active sites for NH3
decomposition and consequently maintain the high catalytic
performance of the AB char.
3.2. Effect of the coexistence of fuel gas components
on ammonia decomposition
As well known, the composition of the raw fuel gas
produced in an actual coal gasification process is dependent
on the kind of coal, the type of gasifier and the reaction
conditions. For example, in an air blown type gasification
process, N2 from air comprises about half proportion of the
raw gas, and the sum of H2 and CO is approximately
40 vol% with the mole ratio of roughly 1, the rest being
CO2, CH4, H2O, H2S and NH3 [2]. This section describes
the effect of these gas components on NH3 decomposition
with not only the RB char but also 2% Fe and 6% Ca
catalysts as references.
The results are provided in Fig. 6, where NH3
decomposition at 750 8C is carried out in a stream of
13 vol% H2/13 vol% CO/7 vol% CO2/1 vol% CH4 balanced
by He, unless otherwise denoted, and the RB char alone is
used in the run in the presence of 20 vol% H2/40 vol% N2.
The catalytic activity of the char was stable in the
coexistence of H2 and N2, which maintained the high
level (about 95%) of NH3 conversion for 4 h, in contrast
with the gradual decrease in the activity without H2 and N2
(Figs. 1 and 3B). Almost the same phenomenon was
observed with 2% Fe catalyst [13]. The H2 coexisted is
likely to work for protecting metallic Fe against the carbide
formation and thus keeping the metallic surface active and/
or increasing the number of the metallic sites according to
Eq. (3). Although one may doubt that the presence of
20 vol% H2/40 vol% N2 is unfavorable thermodynamically
0
20
40
60
80
100
Con
vers
ion
of N
H3
to N
2, %
Time on stream, min
0 50 100 150 200 250
2 % Fe
RB char
SB char
6 % Ca
RB char(H2/N2)
Fig. 6. Performances of chars and catalysts in NH3 decomposition at 750 8C
in the coexistence of H2/N2 (for RB char alone) and H2/CO/CO2/CH4
(unless otherwise denoted).
for the decomposition of NH3 to H2 and N2, it can be
estimated that this atmosphere does not affect equilibrium
conversion of NH3 to N2 significantly under the present
reaction conditions.
Fig. 6 also shows the effect of the simulated gas
containing H2, CO, CO2 and CH4 on NH3 decomposition
with several chars and catalysts. Initial NH3 conversions
over RB and SB chars were lower than the corresponding
values in inert He (Fig. 1). In contrast with the decrease of
their catalytic activity at the latter stage of reaction observed
in Fig. 1, conversions of NH3 to N2 over these chars were
larger at a longer time, and the values after 4 h reached
approximately 70–80%, which were nearly equal to and
rather higher than those in inert He. As seen in Fig. 6, the 2%
Fe catalyst exhibited the similar trend in the time change in
the NH3 conversion and provided the higher conversion at a
longer time, but contrarily the 6% Ca catalyst was almost
inactive in the whole range of reaction. The CO in the gas
mixture of H2/CO/CO2/CH4 should be responsible
dominantly for the catalyst deactivation, because the Ca
was also deactivated almost completely in the coexistence
of syngas alone [13]. The considerable activity difference
between the Fe and the Ca in Fig. 6 indicates that only the Fe
species present inherently in the RB and SB chars can
account for their catalytic effects in the simulated gas.
It is unclear at present why the RB (or SB) char exhibited
the quite different profiles in Figs. 1 and 6. One may expect
that the catalytic activity of the char should be lower in the
H2/CO/CO2/CH4 than in inert He, because it is probable that
the CO in the former deactivates the Ca component present
in the char, from the previous result that the coexistence of
syngas caused the almost complete deactivation of the 6%
Ca catalyst [13]. This might be acceptable at the beginning
stage of reaction where the NH3 conversion was lower in the
simulated gas than in inert He. At the latter stage where the
Ca was still inactive, however, the conversion was almost
the same regardless of the gas atmosphere. Part of the Fe
species in the RB char may initially be in the higher
oxidation state in the H2/CO/CO2/CH4 than in inert gas.
Since the reaction of H2 and CO2 present in the former
gas proceeded to produce H2O and CO during NH3
decomposition, the oxidation state might change in this
process, which may result in the different profiles in both
gas atmospheres.
Although several thousands ppm of H2S (plus COS) in the
raw fuel gas can be removed prior to NH3 decomposition in a
hot gas cleanup system for IGCC [2], it is of interest to
examine the tolerance of the RB char to H2S. Fig. 7 shows the
effect of 2000 ppm H2S balanced by He alone on NH3
decomposition at 750 8C. The comparison of Figs. 1 and 7
reveals that, although the H2S decreases the catalytic activity
of the RB char, the char can still catalyze the reaction and
provide the almost constant NH3 conversion of 40%, which is
about half of that (Fig. 1) without H2S added. As shown in
Fig. 7, the sulfur had the distinct effect on the catalytic
activity of the 2% Fe and the 6% Ca. In other words, the Fe
0
20
40
60
80
100C
onve
rsio
n of
NH
3 to
N2,
%
Time on stream, %0 50 100 150 200 250
2 % Fe catalyst
RB char
6 % Ca catalyst
Fig. 7. Effect of 2000 ppm H2S on NH3 decomposition with RB char, Fe
and Ca catalysts at 750 8C.
Fig. 8. XRD profiles for fresh chars before NH3 decomposition.
C. Xu et al. / Fuel 84 (2005) 1957–1967 1963
was active, and the NH3 conversion over the Fe after 4 h was
50%, which corresponded to be about 85% of that (Table 3) in
inert gas, whereas the Ca was almost inactive. This
observation indicates that the catalysis by the RB char arises
from the Fe component present in the char.
The almost complete deactivation of the Ca catalyst
observed in Fig. 7 strongly suggests the capture of H2S by
the Ca. Actually, the XRD measurements of the used
catalyst after NH3 decomposition revealed the formation of
CaS, as shown later. Based on these observations, one may
propose a sequential operation process using low rank coal
chars for the removal of H2S and NH3, where a Ca-rich char
(for example, the AB char) first captures H2S, and a Fe-rich
char (for example, the RB char) then decomposes NH3.
3.3. Characterization results and possible mechanisms
Fig. 8 shows the XRD profiles for some chars before NH3
decomposition. The fresh RB and AB chars provided the
strong C(002) and weak C(10) diffraction lines at 2q
(Cu Ka) of 25.5–25.6 and 43.3–43.78, respectively, whereas
the XRD spectra in these regions were much broader with
the AD char. These XRD peaks observed with the RB and
AB chars may be assigned to be a carbon that is composed
of turbostratic structures [29–31]. The turbostratic carbon
can be regarded as crystallized (partly graphitized) carbon
[29], though it differs from a graphite carbon, which
possesses well-organized and three dimensional structures
and shows very sharp C(002) line at 2q (Cu Ka) of 26.78. It
has been accepted that Fe catalysts added to polymers and
brown coals promote crystallization reactions of the
corresponding carbons and chars formed during pyrolysis
at temperatures of %1000 8C [29,32–34]. It is thus likely
that the turbostratic carbon is detectable for the RB and AB
chars with larger Fe contents of 1.5–2.0%, whereas it is not
formed significantly for the AD char poor in the Fe
component.
As seen in Fig. 8, no distinct XRD peaks attributable to
Fe species were detectable with the fresh RB, AB, and AD
chars, whereas only very weak diffraction lines of CaO
appeared with the RB and AB chars. These observations
suggest that the Fe and Ca species present in these chars are
highly dispersed, because the Fe and Ca contents are
1.5–2% and 3–5%, respectively, and may be sufficient to be
detected by XRD.
Fig. 9 shows the XRD profiles for the RB and AB chars
recovered after 4 h reaction at 750 8C. With the RB char, the
diffraction lines of a-Fe appeared at 2q (Cu Ka) of 44.6 and
82.38 (not provided in Fig. 9), and the XRD peak of iron
carbide (Fe3C) might overlap with the C(10) line observed
at 40–458. On the other hand, the presence of a-Fe in the AB
char was not clear due to overlapping with the C(10) line.
With the Ca species, the XRD intensities of CaO observed
in the fresh RB and AB chars seemed to decrease, and
instead CaCN2 appeared in both chars. It has been reported
that the transformation of CaO into CaCN2 takes place in the
NH3 decomposition with the 6% Ca catalyst under the same
conditions as in this work according to the following
equation [13]
CaO C2NH3 C2C/CaCN2 C3H2 CCO (4)
This reaction is favorable thermodynamically under the
present conditions, because the standard Gibbs free energy
changes can be estimated to beK27 kJ/mol at 727 8C.
Fig. 9. XRD profiles for RB and AB chars after NH3 decomposition.
Fig. 11. Magnified picture of part of Fig. 10.
C. Xu et al. / Fuel 84 (2005) 1957–19671964
Fig. 10 shows a TEM picture of the fresh RB char before
reaction. Fine particles with the size of 10–20 nm existed on
the char. The EDX analysis of this field revealed that Fe and
Ca were the major elements, whereas Si and Al were minor.
Part of Fig. 10 is magnified in Fig. 11. The particle with the
size of 20 nm was present, and the EDX analysis indicated
that the nanoscale particle almost comprised Fe element
alone. Lamella structures were also observed clearly in
Fig. 11, though not so developed. This observation is in
good agreement of the XRD results that can prove the
formation of the crystallized carbon (Fig. 8). A dissolution/
precipitation mechanism has been suggested for the
Fe-catalyzed crystallization of amorphous carbon in
brown coals at 750–900 8C [34].
Crystalline forms identified by the XRD measurements
of the RB char, AB char, Fe and Ca catalysts are
summarized in Table 4, where part of the results for
Fig. 10. TEM picture of fresh RB char before reaction.
the latter two is cited from our previous work [13] for
comparison. Although any crystalline Fe species were not
observed significantly with the fresh RB and AB chars, Fe3C
was detectable with the 2% Fe catalyst, and metallic Fe
(a-Fe) could also be detected at higher loading of 6% Fe
[13]. After NH3 decomposition in inert He, a-Fe existed
with the RB char, and Fe3C might also be formed, whereas
the presence of these species in the AB char were not clear.
It has been reported that the diffraction lines of Fe4N appear
after the reaction with 8% Fe catalyst supported on activated
carbon [13]. However, any nitride compounds could not be
identified with the RB char, the AB char, and the 2% Fe
catalyst, because, if formed, the identification by XRD was
difficult due to the slight amount and/or the high dispersion.
It appeared that the XRD forms of the Fe species in the RB
char were unchanged apparently after NH3 decomposition
in the different atmospheres. As seen in Table 4, on the other
hand, very weak diffraction lines of FeS were detectable
with the 2% Fe catalyst used in the decomposition reaction
in the presence of 2000 ppm H2S, indicating the transform-
ation of part of a-Fe into the sulfide. The sulfur poisoning
may be responsible for the decreased activity of both the Fe
catalyst and the RB char in this atmosphere (Fig. 7).
Table 4 also shows the changes in the crystalline forms of
Ca species during NH3 decomposition. As mentioned in
Figs. 8 and 9, the CaO observed in the fresh RB and AB
chars as well as the 6% Ca catalyst was transformed into
CaCN2 after the reaction in inert He. On the other hand, this
transformation did not occur with the RB char and the Ca
catalyst in the coexistence of the H2/CO/CO2/CH4 gas. The
CO coexisted may inhibit the formation of CaCN2
according to Eq. (4) and result in the almost no activity of
the Ca catalyst (Fig. 6), because the CaCN2 is suggested to
be the intermediate species of the Ca-catalyzed NH3
decomposition [13]. In the presence of 2000 ppm H2S, the
CaO in both the RB char and the Ca catalyst disappeared
almost completely, and CaS appeared newly. The Ca
component in the char as well as the Ca catalyst was
deactivated by the sulfur capture. No crystallized carbons
Table 4
Changes in crystalline forms of chars and catalysts before and after NH3 decomposition
Catalyst Species identified by XRDa
Before run After NH3 decomposition
In pure He In H2/CO/CO2/CH4/He In 2000 ppm H2S/He
RB char a-Fe? (vw) a-Fe (w), Fe3C? (vw) a-Fe? (vw), Fe3C? (vw) a-Fe? (vw), Fe3C? (vw)
CaO (vw) CaCN2 (w), CaO (vw) CaO (vw) CaS (m)
Cb (s) Cb (s) Cb (s) Cb (s)
AB char a-Fe? (vw) n.a.c n.a.c
CaO (w) CaCN2 (vw), CaO (vw) n.a.c n.a.c
Cb (s) Cb (s) n.a.c n.a.c
2% Fed Fe3C (w) a-Fe (w), Fe3C (w) a-Fe (w), Fe3C (w) a-Fe (vw), Fe3C (w), FeS (vw)
Cb (s) Cb (s) Cb (s) Cb (s)
6% Cad CaO (s) CaCN2 (s), CaO (w) CaO (s) CaS (s)
a XRD intensities designated by vw (very weak), w (weak), m (medium), and s (strong).b Crystallized carbon observed at 2q (Cu Ka) of 25.5–25.98 (see Figs. 8 and 9 for RB and AB chars).c Not analyzed.d Cited partly from Ref. [13].
Rat
e of
N2
deso
rbed
, µm
ol/m
in/g
0
1
2
3
4
5
90 min holding
850500 850
Temperature, °C
Fig. 12. Formation of N2 in the TPD run of RB char after NH3
decomposition.
C. Xu et al. / Fuel 84 (2005) 1957–1967 1965
were present with the fresh Ca catalyst, in contrast with the
strong XRD peaks observed with the RB char, the AB char,
and the Fe catalyst. When Ca cations were added to low
rank coals by the ion exchange method, the Ca was effective
for carbon crystallization at high temperatures of R1000 8C
[31]. It has been reported that about 2at.% of carbon can
dissolve into Ca metal at 1000 8C [35].
When Fe catalysts after NH3 decomposition, supported
on brown coal chars and activated carbon, were subjected to
the TPD measurements, the desorption of N2 proceeded at
temperatures of R500 8C and arose from the decomposition
of Fe4N [13]. Fig. 12 shows the profile for N2 evolved from
the used RB char in the TPD run, where the char after
reaction is first quenched in high purity He to room
temperature, then heated up to 850 8C and held at this
temperature for 90 min. A measurable amount of N2 was
detectable at R700 8C, and the evolution rate decreased
during soaking at 850 8C. The N2 formation in the wide
temperature range indicates the occurrence of the different
sources of N2 in the used char. It is likely that the N2 evolved
originates from the decomposition of not only Fe nitrides
but also CaCN2, though the former species could not be
identified by the XRD. The nitrides may be the main source
of the low temperature N2 desorption, because Fe nitrides,
for example Fe4N, undergo decomposition reactions at a
larger rate than CaCN2 does [13]. The XRD measurement
after the TPD run of the used RB char revealed that,
although the intensities of CaCN2 lowered, this species still
remained in the char, which means that the decomposition
of CaCN2 to N2 is slow. The CaCN2 may thus be the major
source of the high temperature N2 evolution in Fig. 12.
It has been proposed that the decomposition of 2000 ppm
NH3 in inert gas with Fe catalysts supported on brown coal
chars takes place according to the following equations [13]
8Fe C2NH3/2Fe4N C3H2 (5)
2Fe4N/8Fe CN2 (6)
The overall reaction can thus be expressed as Eq. (1).
Both Eqs. (5) and (6) are thermodynamically favorable
under the present conditions [13]. It is reasonable to
understand that nanoscale particles of metallic Fe present
in the RB and AB chars, as observed in Figs. 10 and 11, can
catalyze the decomposition reaction through the same
mechanism as above, though the nitride species may work
as the forms other than Fe4N, for example non-stoichio-
metric nitrides (FexNy). When iron carbides, for example
Fe3C, are initially present in the RB and AB chars or formed
in the process of NH3 decomposition, their catalytic activity
should be small or decreased, because it is unlikely that the
carbides are transformed into the nitride intermediates by
the reaction with NH3 [14]. The H2 pretreatment of the AB
char at a higher temperature (Fig. 4) and the in situ H2
treatment of the RB char (Fig. 3) enhanced the catalytic
performance of each char, because such treatments can
convert iron carbides to metallic Fe (Eq. (3)) and
thus increase the number of catalytically active sites.
C. Xu et al. / Fuel 84 (2005) 1957–19671966
The coexistence of H2 also increased conversion of NH3 to
N2 over the RB char probably by protecting the surface of
metallic Fe against the carbide formation (Fig. 6).
The RB and AB chars included 3–5 wt % Ca, and the 6%
Ca catalyst had the catalytic effect on NH3 decomposition in
inert gas (Table 3) [13]. It is therefore probable that the Ca
species in these chars are also catalytically active. In the
decomposition reaction with the 6% Ca catalyst, CaCN2
may initially be formed according to Eq. (4) and then be
decomposed to N2 and a Ca-containing interstitial com-
pound, which might react with NH3 to form CaCN2 again
[13]. The TPD measurement of the Ca catalyst after NH3
decomposition and the XRD analysis after the TPD run
indicated the formation of N2 from the CaCN2 [13].
However, it is not clear what the interstitial species is and
how CaCN2 is formed again. Since the XRD signals of CaO
and CaCN2 were detectable in the RB and AB chars
similarly as the case of the Ca catalyst, the CaO in these
chars may also promote NH3 decomposition through the
mechanism involving CaCN2. As seen in Table 4, the co-
feeding of CO prevented the CaO in the catalyst from
forming CaCN2 according to Eq. (4), and the coexistence of
H2S caused the formation of chemically stable CaS, which
could not take part in Eq. (4). The Ca catalyst was thus
nearly inactive in these gas atmospheres (Figs. 6 and 7).
Since fine particles of CaO present inherently in the chars
are deactivated almost completely by CO and H2S in the
same manner as above, metallic Fe alone in the RB char can
account for the catalytic performance observed in Figs. 6
and 7.
4. Conclusions
Several chars, which can simply be prepared by pyrolysis
of low rank coals without any metal cations added, work as
the catalysts for decomposing 2000 ppm NH3 diluted with
inert He under the conditions of 750–850 8C, 0.1 MPa and
large space velocity of 45,000 hK1. Significant amounts of
Fe and Ca ions present inherently in the raw coals are
transformed into nanoscale particles of metallic Fe and
CaO, which both show catalytic activity for this reaction.
Conversion of NH3 to N2 at 750 8C can correlate more
closely with Fe content of each char, suggesting a more
important role of metallic Fe in NH3 decomposition. A
brown coal char with the largest Fe content of 2 wt% shows
the stable catalytic performance of maintaining the NH3
conversion of 80% during 25 h reaction at 750 8C, and it
achieves the almost complete decomposition of NH3 at
850 8C. When the reaction is carried out under co-feeding
simulated fuel gas components, the coexistence of H2, CO,
and CO2 changes only the initial catalytic activity of the
brown coal char, whereas the presence of 2000 ppm H2S
diluted with inert He decreases the activity throughout the
reaction. It is suggested that the catalytic decomposition
of NH3 in inert gas with the char proceeds through
the mechanisms involving iron nitrides and CaCN2 as
intermediate species. The results described here indicate
that Fe-rich low rank coals may be promising as catalyst
materials for a hot gas cleanup method of removing a low
concentration of NH3 from fuel gas, because these coals are
readily available, and there are no unit operations for
catalyst preparation needed.
Acknowledgements
The present work was supported by a Grant-in-Aid for
Scientific Research on Priority Areas (B) from the Ministry
of Education, Science, Sports, and Culture, Japan (No.
11218202). One of the authors (C.X.) gratefully acknowl-
edges Japan Society for the Promotion of Science for the
Postdoctoral Fellowship. The authors are indebted to Ms
Keiko Matsukura and Mr Dapeng Kong for their assistance
in carrying out experiments.
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