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: ohtsukay@tagen.tohoku.ac.jp (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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