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Catalysts for waterЦgas shift processing of coal-derived syngases.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2010; 5: 585–592
Published online 7 May 2010 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.439
Special Theme Research Article
Catalysts for water–gas shift processing of coal-derived
syngases‡
San Shwe Hla,1 * G. J. Duffy,2 L. D. Morpeth,1 A. Cousins,1 D. G. Roberts,1 J. H. Edwards 2 and D. Park1†
1
2
Commonwealth Scientific and Industrial Research Organisation (CSIRO), Energy Technology, Pullenvale, QLD 4069, Australia
Commonwealth Scientific and Industrial Research Organisation (CSIRO), Energy Technology, Newcastle, NSW 2300, Australia
Received 27 October 2009; Revised 16 February 2010; Accepted 26 February 2010
ABSTRACT: Although the gasification of coal is an efficient means of producing syngas, the carbon content of coal
is such that gasification produces significantly higher ratios of carbon oxides to hydrogen than those obtained by the
steam reforming of natural gas. The CO : H2 ratio can be adjusted, and more hydrogen produced, by the subsequent
application of the water–gas shift (WGS) reaction. This article presents a review of technologies associated with the
catalytic WGS reaction in a fixed-bed reactor that might be incorporated into a coal gasification-based system for H2
production with CO2 capture. The main output from this review is the identification of key project areas requiring further
research. The performance of existing, commercially available catalysts – designed for use in natural gas reforming
processes – with coal-derived syngases is an important aspect of developing technologies for coal-based H2 production.
This article presents an experimental assessment of the performance of selected commercially available WGS catalysts,
two high-temperature catalysts (HT01 and HT02) and a sour shift catalyst (SS01), with such syngases. For the three
commercial catalysts investigated in this study, CO reaction order is found to be in a range of 0.75–1. The effect of
changes in H2 O concentration over HT01 is insignificant, whereas H2 O reaction orders determined using HT02 and
SS01 are found to be significantly positive even at high H2 O : C ratios. The CO conversion rate is significantly reduced
by increasing CO2 concentration, whereas increasing H2 concentration also causes a slight reduction in CO conversion
rate for the three commercial catalysts investigated.  2010 Curtin University of Technology and John Wiley & Sons,
Ltd.
KEYWORDS: water–gas shift reaction; WGS catalysts; coal-derived syngas; reaction orders
INTRODUCTION
To reduce the environmental impacts created by continued application of traditional pulverized coal fired
power systems, integrated gasification combined cycle
(IGCC) systems are widely accepted as potential
future low emission technologies for power generation. Although efficiency of gasification-based power
systems is relatively high in overall terms, the carbon
content of coal is such that the gasification produces
significantly higher ratios of carbon oxides to hydrogen
than might be obtained by the steam reforming of natural gas. Syngases from the gasification of coal typically
contain 30% hydrogen, 60% CO and 10% CO2 .[1]
If an IGCC system is to have low or zero CO2 emissions, then it would be preferable to remove the CO2
prior to syngas combustion or utilisation of hydrogen
in fuel cells. Hydrogen-rich gas production by gasification is a primary low greenhouse gas (GHG) emissions
route for power generation from coal provided that the
CO2 can be captured and sequestered.[2] The CO : H2
ratio can be adjusted, and more hydrogen and CO2 produced prior to the separation process, by the subsequent
application of the water–gas shift (WGS) reaction:
CO + H2 O ↔ CO2 + H2
*Correspondence to: San Shwe Hla, Commonwealth Scientific and
Industrial Research Organisation (CSIRO), Energy Technology, PO
Box 883, Pullenvale, QLD 4069, Australia. E-mail: san.hla@csiro.au
†
Current address: Origin Energy, PO Box 148, Brisbane, QLD 4001
Australia.
‡
This article was published online on 7 May 2010. Errors were
subsequently identified. This notice is included in the online and
print versions to indicate that both have been corrected [25 May
2010].
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
H298K = −41.1 kJ mol−1
(1)
This is an equilibrium reaction that favours the
formation of CO2 and H2 at low temperatures.[3] In
practice, catalysts are required to increase the rate of
the forward reaction to ensure that the equilibrium
conversion of CO is approached under the selected
reaction conditions. The concept of an IGCC system
together with a WGS reactor and CO2 capture is
586
S. S. HLA et al.
Asia-Pacific Journal of Chemical Engineering
Gas turbine &
Steam turbine
High T
Shift
Gasifier
Low T
Shift
Electricity
Separation
H2
Coal
Gas
Cleaner
H2
CO2
Figure 1. IGCC with CO2 removal. This figure is available in colour online at www.apjChemEng.com.
presented in Fig. 1. Hydrogen production from syngases
can be promoted by conventional high temperature
(HT) and low temperature (LT) WGS catalysts operated
sequentially, followed by separation of the CO2 from
the hydrogen. This separation would at present involve
the use of suitable solvent-based systems but current
research is being directed at the use of membranes
to reduce the cost and increase the overall efficiency
of the process.[4] With current technology one would
use a combination of a HT followed by a LT reactor
(as shown in Fig. 1) to take advantage of the faster
kinetics achieved in the former and the better approach
to equilibrium conversion achieved in the latter. These
would then be followed by a solvent-based system
to recover the CO2 . However, if a membrane reactor
can be used, combining the catalytic reactor with
simultaneous removal of one of the products, then it
may be possible to use only the HT catalyst as the
removal of a reaction product will drive the equilibrium
in Eqn 1 to the right.
Syngas can be produced from coal gasification with a
range of compositions dependant on the application, e.g.
power generation, metallurgical reductants, chemicals
or hydrogen fuel for vehicles.[5] The WGS reaction will
have a major role to play in any scenario involving
the partial or complete recovery of a CO2 co-product.
The WGS reaction is a mature technology that has been
applied for many decades to the processing of syngases,
largely produced by the reforming or partial oxidation
of oil and natural gas. However, coal-derived syngases
contain much higher CO and lower hydrogen levels
than those produced from natural gas, putting greater
demands on the performance of WGS catalysts. Unlike
syngases from natural gas, which are essentially free
of impurities such as sulfur and nitrogen compounds,
particulates etc., coal-derived syngases contain a wide
range of major and trace impurities depending on
the composition of the coal and the type of gasifier
used.[6] These major and trace impurities, many of
which are not expected to be removed by conventional
or emerging gas cleaning technologies, could have
deleterious impacts on the downstream processing of
syngases.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
The purposes of this article were twofold: the first
one was to review the available technologies associated
with the WGS reaction which might be applied to
the processing of coal-derived syngases as part of an
overall scheme to achieve power generation with lowGHG emissions and to identify the key areas requiring
further research. The second one was to present recent
research work that has been conducted according to the
requirements identified in this review.
STATE OF THE ART
Catalysts for WGS reaction
The WGS reaction is the most important step in terms
of the processing of coal-derived syngases, both as a
precursor to fuel gas decarbonisation and for adjusting
the CO/H2 ratio for downstream synfuel production.
Even though the equilibrium reaction to form CO2 and
H2 favours lower temperatures, whether equilibrium
conversions are achieved in a reactor will depend on
the kinetics of the reaction. Catalysts are normally used
to speed up reaction rates to ensure that equilibrium
is achieved in practice, and high reaction rates are
favoured by higher temperatures.[7]
There are essentially three types of commercial catalysts that have been developed for the WGS reaction:
• iron-based HT catalysts;
• copper-based LT catalysts;
• sulfur-tolerant catalysts (sour shift).
Current technology is based on fixed-bed catalytic
reactors using catalysts provided by companies such
as Haldor Topsoe, Süd-Chemie, Johnson Matthey, and
Englehard, all of whom closely guard information on
the performance of their catalysts.
The iron-based catalysts used today for the HT
shift reaction have hardly changed since the days
when Badische Anilin & Soda-Fabrik (BASF) first
developed them in the late 19th century.[3,8] Typical iron-based catalysts would have a composition
in the range of Fe2 O3 (80–95%), Cr2 O3 (5–15%)
Asia-Pac. J. Chem. Eng. 2010; 5: 585–592
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
CATALYSTS FOR WGS PROCESSING OF COAL-DERIVED SYNGASES
◦
and CuO (1–5%). The Cr2 O3 additive acts as a stabiliser rather than a promoter, preventing high temperature sintering and subsequent loss of surface
area. The Fe2 O3 must be reduced to Fe3 O4 , which
is thought to be the active component of the ferrochrome catalyst at temperatures of 250–400 ◦ C, with
higher temperatures resulting in decreased activity
due to sintering of the catalyst.[9] These HT catalysts can promote the formation of carbon by the
dis-proportionation of CO via the Boudouard reaction
and the formation of hydrocarbons by Fischer–Tropsch
reactions.[10 – 12] A large excess of steam (steam to carbon ratios of 3–5) is required to avoid such problems although the use of a Cu promoter reduces the
steam requirement by about 20%. Small amounts of
sulfur (<50 ppm) have a negligible effect on ironbased catalysts. Chlorine acts to poison the catalyst irreversibly, but can be tolerated up to about 1 ppm. Phosphorus and silicon compounds can also have adverse
effects.[13]
Cu-based catalysts are used for performing the WGS
reaction at low temperatures where a high degree of CO
conversion can be achieved. A typical composition for
a copper-based catalyst (Johnson Matthey’s Katalco 833) is Cu (51%), ZnO (31%) with the balance alumina
(Al2 O3 ). The oxides of Zn, Cr and Cu by themselves
are not particularly good catalysts for the WGS reaction, whereas the mixtures of these metals have been
found to have excellent activity.[14] Activation requires
the reduction of CuO to Cu which is done by heating the catalyst while a carrier gas (nitrogen or natural
gas), to which a small amount of hydrogen has been
added, is passed over the catalyst. LT WGS catalysts are
still very susceptible to sintering if the catalyst temperature exceeds certain limits (around 300 ◦ C). A typical
operating range for a Cu/Zn/alumina catalyst would be
190–275 C, although they can be operated at temperatures as low as 170 ◦ C. They are also very susceptible to
poisoning, being intolerant to sulfur (<0.1 ppm), chlorine and silica. Sulfur reduces activity by covering the
active Cu-surface, and chlorine promotes sintering of
both the copper and the support, whereas silica blocks
the catalyst surface and pores.[15]
Metals or mixed metals of Groups VI (Cr, Mo,
W) and VIII (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt)
other than Fe and Cr, supported on alumina, provide
a family of sulfur-tolerant WGS catalysts. The activity
of these catalysts (referred to as sour shift catalysts)
is actually enhanced by the presence of sulfur, and
they require at least 20 ppm sulfur in the feed gas to
maintain sulphidation.[9] They would normally operate
in the range of 230–450 ◦ C although temperatures
as high as 500 ◦ C are acceptable (Johnson Matthey
Katalco series). A typical composition of a sour shift
catalyst in its pre-sulfided state is CoO (4%) and
MoO3 (10%), with the balance being the support.
The need to pre-sulfide the catalyst before it is used
for the first time presents further complications for
the initial start-up of plants that incorporate such
catalysts.[16]
Syngas compositions
Carbon monoxide and hydrogen are the major constituents of the raw syngas from a coal gasifier, with
lesser amounts of carbon dioxide, argon, nitrogen,
water, methane, and reduced gas species such as ammonia, hydrogen sulphide and carbonyl sulphide. The relative proportions of the major components (H2 and CO)
in coal-derived syngases will depend not only on the
composition of the coal but also on the type of gasifier used (Table 1). Such variations in H2 : CO ratio
Table 1. Raw syngas composition for various slagging gasifier technologies and feedstocks.[17]
Bed type
Moving
Moving
Entrained
Entrained
Entrained
Entrained
Entrained
Feed form
Dry
Dry
Unknown
Dry
Slurry
Slurry
Slurry
Illinois #6
Illinois #6
Illinois #5
SUFCO Low S
Illinois #6
Pittsburgh #8
26.4
45.8
2.9
NR
3.3
16.3
38 000
10 000
1000
2000
2000
52.2
29.5
5.6
NR
1.5
5.1
44 000
9000
400
5000
3000
26.7
63.1
1.5
NR
5.2
2
300
13 000
1000
200
NR
37.6
41.8
19.8
0.08
0.69
NR
2400
1260
23.2
2.3
NR
37.3
44.0
16.9
0.08
1.1
NR
1570
9570
153
0.58a
NR
37.9
42.7
17.3
0.07
1.41
NR
1930
7590
176
0.62a
NR
Coal
H2 , vol %
CO, vol%
CO2 , vol%
Ar, vol%
N2 , vol%
H2 0, vol%
CH4 , ppmv
H2 S, ppmv
COS, ppmv
NH3 , ppmv
THC, ppmv
30.33
47.72
17.88
0.83
1.27
0.12
100
10 760
20
0.00a
NR
THC, total hydrocarbons (excluding methane) expressed as methane; NR, not reported.
a
Measured after particulate scrubbing and gas cooling (i.e., after ammonia removal).
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 585–592
DOI: 10.1002/apj
587
588
S. S. HLA et al.
Asia-Pacific Journal of Chemical Engineering
will have a direct impact on the level of conversion
required and the suitability of ‘traditional’ catalysts in
the WGS reactor. As shown in Table 1, the quantities
of reduced sulfur and nitrogen species will also be very
much dependent on the composition of the coal.
Processing coal-derived syngases in IGCC
systems
As discussed earlier, catalysts for the WGS reaction
are commercially available from several suppliers. As
a result of development over the past 50–60 years, the
technology could well be ranked as mature for application to natural gas-derived syngases.[18] Whilst this may
be true for the processing of syngases derived from the
steam reforming of natural gas where the CO concentrations are in the range of 5–10%, this is not necessarily
the case for coal-derived syngases where CO concentrations are in the range of 30–60% (Table 1). Therefore, a
considerably higher degree of shifting is required. Coalderived syngases are also likely to have much higher
H2 S concentrations which may severely degrade catalyst activity (for non-sour shift catalysts). Coal-derived
syngas may also contain a number of other impurities
(such as mercury, nitrogen compounds such as ammonia, chlorine, alkali and trace heavy metals) which, even
after syngas cleaning, may be present at levels that
could have an impact on catalyst performance. In addition, if such catalysts are used as a packed-bed in a
membrane reactor then they are likely to come into contact with high concentrations of CO2 or H2 depending
on which gas permeates through the membrane. Commercially available catalysts might have limitations with
respect to the high levels of CO2 or H2 concentrations
over which catalytic activity can be maintained. The
operating envelope for these catalysts with respect to
the WGS of coal-derived syngas is not known.
Research areas requiring further investigation
The above brief review of WGS technology for application to coal derived syngases has highlighted the following major research areas requiring further investigation:
• WGS catalyst technology is a mature technology
for the processing of natural gas-derived syngases,
but the operating envelope is less well defined for
coal-derived syngases where large amounts of CO
must be shifted, and particularly where the catalyst
is integrated with a membrane for in situ CO2 /H2
separation where high concentrations of H2 and/or
CO2 may be encountered.
• The impact of impurities (such as sulfur, nitrogen compounds, mercury and particulates) typically
encountered in coal-derived syngases on the performance of commercially available catalysts, especially
HT WGS catalysts, is not known. This information
will give a clear indication of the extent of syngas
clean-up required in upstream processing.
• The reaction engineering aspect of integrating the
WGS process into IGCC technology for pre-combustion decarbonisation needs to be investigated from
both a technical and an economic viewpoint.
This work, therefore, attempts to provide data and
analysis in response to these areas. The next section presents information on how commercially available WGS catalysts perform in environments relevant
to coal-derived syngases. Detailed kinetics studies of
both HT01 and HT02 and the effect of H2 S concentration on performance of HT02 have been presented
elsewhere[19,20] and are therefore not repeated here.
Figure 2. Schematic diagram of the experimental system and fixed-bed
reactor used for WGS catalytic reactions. This figure is available in colour
online at www.apjChemEng.com.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 585–592
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
CATALYSTS FOR WGS PROCESSING OF COAL-DERIVED SYNGASES
Table 2. Specifications of HT01, HT02 and SS01.
HT catalyst
Sour shift catalyst
Composition
HT01
HT02
Composition
SS01
Fe2 O3 (wt%)
Cr2 O3 (wt%)
CuO (wt%)
80–90
8–13
1–2
80–95
5–10
1–5
CoO (wt%)
MoO3 (wt%)
MgO (wt%)
Al2 O3 (wt%)
Promoter (wt%)
Pellet
6 × 6 mm
Pellet
6 × 6 mm
≥2
≥8
≥24
≥50
Balance
Pellet
2.5 (D) × 18 (L) mm
Shape
Size
EXPERIMENTAL
WGS reactor system
A schematic diagram of the experimental apparatus and
the configuration of the fixed-bed reactor are shown in
Fig. 2. On the inlet side to the reactor, the individual
mass flow rates of CO, H2 , CO2 , N2 and H2 S were
controlled using mass flow controllers. The gas stream
was introduced into the electrically heated 13 mm
internal diameter (ID) quartz tube reactor together with
a steady flow of de-ionised water metered by a highperformance liquid chromatography pump. A weighed
sample of catalyst was packed in the reactor between
layers of inert material (quartz wool). Concentrations
of the dried product gases (CO, CO2 , H2 and N2 ) were
determined by gas chromatography (Varian CP–4900).
Catalyst sample
HT catalyst 1 (HT01) and HT catalyst 2 (HT02)
are copper promoted iron–chromium oxide-based HT
shift catalysts purchased from two different commercial
suppliers. SS01 is a commercial Co–Mo catalyst, which
is an active sour shift catalyst that can be used for
CO conversion in syngases without prior removal of
the sulfur compounds. Vendor-supplied details for these
catalysts are given in Table 2. The catalyst pellets were
ground and sieved to a particle size of +53–150 µm to
reduce the possibility of internal diffusion limitations
on the rate measurements.
Test procedure
Approximately, 0.2 g of HT catalyst was mixed with
1 g of α-alumina (prepared to the same particle size
as the catalyst sample) and the mixture was loaded
into the reactor which was located in the centre of the
electric furnace. Initial reduction was carried out using
a gas mixture containing H2 , CO, N2 and H2 O with
a gas velocity of 5.1 cm s−1 at 250 ◦ C for 2 h. The
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
sample was then heated to 450 ◦ C under nitrogen and
steam, and a gas mixture of the desired composition
was then introduced to the reactor. For the case of
the sour shift catalyst (SS01), 1.2 g of catalyst was
loaded to the reactor without mixing with α-alumina.
To sulfide SS01, the catalyst sample was first dried at
110 ◦ C for 1 h in a steady flow of N2 . After drying, the
N2 stream was switched to the sulfiding gas mixture
consisting of H2 S (4%) and H2 (96%) with a gas flow
at atmospheric pressure of 100 ml min−1 . The reactor
temperature was then increased from 110 to 450 ◦ C.
The catalyst was sulfided at 450 ◦ C for 4 h before it
was tested under the desired gas stream with a constant
level of H2 S (1000 ppm).
To investigate the effect of CO, H2 O and H2 , a syngas
composition relevant to a dry-feed coal gasifier (65%
CO, 30% H2 , 2% CO2 and 3% N2 with steam : carbon
ratio of 3) was selected. To investigate the effect of
CO2 on the performance of WGS catalysts, a syngas
composition that might be encountered at the back end
of a catalytic membrane reactor (retentate side) was
selected. A typical gas composition at the backend of a
catalytic membrane reactor (post-hydrogen separation)
was calculated to be 7% CO, 12% H2 , 78% CO2 and 3%
N2 , assuming 90% CO conversion and 90% H2 removal.
Steam was added to both gas mixtures to give a constant
inlet steam : carbon molar ratio of 3 : 1.
RESULTS AND DISCUSSION
In this section, the effects of the individual gas species
on WGS reaction kinetics over HT01, HT02 and SS01
are discussed.
Experimental runs to examine the effects of CO,
H2 O, CO2 and H2 were carried out by varying the
composition of one component by replacing it with
N2 while keeping the other gas compositions constant.
Figures 3–6 show the plots of ln [WGS reaction
rate/(1 − β)] vs ln [component partial pressure] over
HT01, HT02 and SS01. β is the term reflecting the
reverse reaction or approach to equilibrium, and is
Asia-Pac. J. Chem. Eng. 2010; 5: 585–592
DOI: 10.1002/apj
589
S. S. HLA et al.
Asia-Pacific Journal of Chemical Engineering
Ln P (kPa)
1 PCO2 PH2
β=
K PCO PH2 O
(2)
where K is the equilibrium constant for the WGS
reaction. The equilibrium constant for the WGS reaction
for a wide range of reaction temperatures is given by
Twigg[3] and the calculated equilibrium constant of the
WGS reaction is 7.3369 at 450 ◦ C.
Ln P (kPa)
Ln [rate/(1−β)], mol/g/s
1.6
−8.5
−9.5
2.1
1.2
−10.5
2.6
3.1
−11.0
Ln [rate/(1−β)], mol/g/s
defined as
−11.5
1.7
2.2
2.7
3.2
HT01CO2 = −0.36
HT02CO2 = −0.156
−12.0
−12.5
HT01
HT02
SS01
−13.0
SS01CO2 = −0.073
−13.5
HT01
HT02
SS01
HT01CO = 1.0
−14.0
HT02CO = 0.9
Figure 5. Variation in CO conversion rate over
HT01, HT02 and SS01 catalysts under backend
of a catalytic membrane reactor condition with
CO2 partial pressure (450 ◦ C, wet gas velocity
= 79.7 cm s−1 ). This figure is available in colour
online at www.apjChemEng.com.
−10.5
−11.5
SS01CO = 0.75
Ln P (kPa)
0.5
−8.5
−12.5
1.0
Ln P (kPa)
3.5
−8.5
3.7
3.9
4.1
Ln [rate/(1−β)], mol/g/s
HT01, HT02 and SS01 catalysts under dry-feed
coal-derived syngas condition with CO partial
pressure (450 ◦ C, wet gas velocity = 79.7 cm s−1 ,
H2 O : C ratio of 3). This figure is available in colour
online at www.apjChemEng.com.
HT02H2O = 0.31
2.0
HT02H2 = −0.05
−9.5
−10.5
−11.5
HT01H2O = 0
−9.5
1.5
HT01H2 = −0.09
Figure 3. Variation in CO conversion rate over
Ln [rate/(1−β)], mol/g/s
590
HT01
HT02
SS01
SS01H2 = −0.094
−12.5
Figure 6. Variation in CO conversion rate over
−10.5
HT01, HT02 and SS01 catalysts under dry-feed
coal-derived syngas condition with H2 partial
pressure (450 ◦ C, wet gas velocity = 79.7 cm s−1 ,
H2 O : C ratio of 3). This figure is available in colour
online at www.apjChemEng.com.
HT01
HT02
SS01
−11.5
SS01H2O = 0.31
−12.5
Figure 4. Variation in CO conversion rate over
HT01, HT02 and SS01 catalysts under dry-feed
coal-derived syngas condition with H2 O partial
pressure (450 ◦ C, wet gas velocity = 79.7 cm s−1 ).
[Correction made to figure caption after initial
online publication]. This figure is available in colour
online at www.apjChemEng.com.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
From Figs 3–6, it can be seen that all the plots
are linear. The reaction orders with respect to CO,
H2 O, CO2 and H2 over HT01, HT02 and SS01 were
calculated from the slopes of the graphs and these values
are also shown in Figs 3–6.
The effect of CO concentration on CO conversion
rate over HT01, HT02 and SS01 is shown in Fig. 3,
where it is clear that CO conversion rate increases with
CO concentration over the three catalysts investigated.
Asia-Pac. J. Chem. Eng. 2010; 5: 585–592
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
CATALYSTS FOR WGS PROCESSING OF COAL-DERIVED SYNGASES
◦
The reaction order with respect to CO at 450 C was
found to be 1.0, 0.9 and 0.75 for HT01, HT02 and
SS01, respectively. This finding indicates that CO
concentration has the greatest effect on reaction rate
over HT01 and least effect on reaction rate over SS01.
The effect of H2 O concentration on CO conversion
rate over HT01, HT02 and SS01 is shown in Fig. 4.
Podolski and Kim[21] found the WGS reaction rate to be
independent of steam concentration when it is in excess
of the stoichiometric amount. This is true for the case of
HT01 in the current work, where we have found that the
effect of changes in H2 O concentration is insignificant
for the range of 33–69% H2 O contents investigated and
its reaction order can be taken as zero for H2 O : carbon
ratios greater than 1.5 : 1. However, it is interesting
that the H2 O reaction order determined using HT02
was found to be significantly positive even at high
steam:carbon ratios. This difference could be associated
with the microstructure and with the compositional
difference between the two HT catalysts. Reaction order
for H2 O at 450 ◦ C over SS01 was found to be 0.31
(Fig. 4) which is the same as the reaction order over
HT02 at 450 ◦ C.
The effect of CO2 concentration on CO conversion
rate over HT01, HT02 and SS01 is shown in Fig. 5.
Here, it is shown that CO2 concentration has a negative
effect on CO conversion rate over three catalysts
investigated. The reaction order for CO2 at 450 ◦ C was
found to be −0.36, −0.156 and −0.073 over HT01,
HT02 and SS01 catalysts, respectively (Fig. 5). As the
reaction order for CO2 over HT01 was −0.36, HT01
is the most sensitive to CO2 partial pressure among the
three catalysts investigated. The reaction order for CO2
over SS01 was found to be −0.073 (Fig. 5) which is
the least negative for the catalysts tested. This finding
indicates that the inhibition effect of CO2 on the forward
WGS reaction rate is less with SS01 than for HT01
and HT02; that is, the SS01 catalyst is more suitable
for use with a gas stream that contains a high CO2
concentration.
While increasing CO2 concentration will have a
negative effect on the CO conversion rate, the fact that
the order of the reaction with respect to CO2 is constant
over the entire range of conditions for the three catalysts
investigated (Fig. 5) indicates that there is no additional
inhibition effect from high CO2 concentrations other
than that resulting from the reverse WGS reaction.
This implies that the use of a membrane reactor with
in situ H2 separation, avoiding the need for a separate
downstream H2 recovery step, is a potentially viable
alternative for conducting the WGS reaction.
The effect of H2 content on CO conversion rate over
HT01, HT02 and SS01 is shown in Fig. 6 where it can
be seen that H2 has a slightly negative effect on the
WGS reaction rate over the three catalysts investigated.
The reaction order for H2 at 450 ◦ C was found to be
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
−0.09, −0.05 and −0.094 over HT01, HT02 and SS01
catalyst, respectively (Fig. 6).
The reaction orders discussed above imply that the
presence of CO2 , and to a lesser extent H2 , in the syngas
can have a significant effect on the CO conversion rate,
particularly for high concentrations of CO2 and H2 .
This is due to the rate of the reverse WGS reaction
increasing significantly as the concentrations of CO2
and H2 increase. This effect is found to be most
significant over HT01 and least significant over SS01.
CONCLUSIONS
This article includes a review of technology developments in the area of WGS catalysts for use in a fixed-bed
reactor that might be incorporated into a coal-based
IGCC power generation system with CO2 capture. This
article also presents the effect of individual gas components (CO, H2 O, CO2 , H2 ) on the performance of
selected commercially available WGS catalysts in environments relevant to coal-derived syngases.
The review of the literature led to the conclusions
that:
(1) WGS catalyst technology is a mature technology
for the processing of natural gas-derived syngases,
but the operating envelope is less well defined for
coal-derived syngases where large amounts of CO
must be shifted. This is particularly the case where
the catalyst is to be integrated with a membrane
separator when the in situ CO2 /H2 separation will
result in high concentrations of H2 and/or CO2 .
(2) The impacts of impurities, such as sulfur, nitrogen compounds, mercury and particulates (those
usually contained in coal-derived syngases) on the
performance of commercially available catalysts,
especially HT WGS catalysts, are not known. Information on such issues is required to give a clear
indication of the extent of syngas clean-up required
in upstream processing.
(3) The reaction engineering aspect of integrating
the WGS process into IGCC technology for precombustion decarbonisation needs to be investigated from both a technical and an economic viewpoint.
Subsequent experimental investigations showed that:
(1) The reaction orders obtained in this study generally
agree well with those given in the literature in which
inlet CO concentrations are lower than the dryfeed, coal-derived syngas and the slurry-feed, coalderived syngas compositions used in this study.
(2) For both HT catalysts, the rate of the WGS reaction
is approximately proportional to the CO concentration (i.e. n ∼ 1); the rate is retarded by increasing CO2 concentration (n is negative), and to
Asia-Pac. J. Chem. Eng. 2010; 5: 585–592
DOI: 10.1002/apj
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592
S. S. HLA et al.
Asia-Pacific Journal of Chemical Engineering
a lesser extent with increasing H2 concentration
which causes a slight reduction in reaction rate (n
is slightly negative).
(3) For the sour shift catalyst (SS01) tested, the reaction
order with respect to CO was found to be less
than those for HT01 and HT02. CO2 reaction order
over SS01 was found be less negative than those
for HT01 and HT02. This indicates that the SS01
catalyst is suitable for use with gas streams that
contain less CO and high CO2 concentrations, such
as the gas composition encountered at the backend
of a catalytic membrane reactor.
Acknowledgement
The authors wish to acknowledge the support provided
through the Centre for Low Emission Technology
(cLET) for this work.
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DOI: 10.1002/apj
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