ISSN 1068-364X, Coke and Chemistry, 2007, Vol. 50, No. 8, pp. 226–231. © Allerton Press, Inc., 2007.
Original Russian Text © O.I. Platonov, 2007, published in Koks i Khimiya, 2007, No. 8, pp. 27–33.
CHEMISTRY
Desulfurization of Coke-Oven Gas
O. I. Platonov
OOO Institut Gipronikel, St Petersburg, Russia
DOI: 10.3103/S1068364X07080054
As we know, coke-oven gas contains ammonia and
hydrogen sulfide, which must be removed (to residual
contents of ≤0.03 and ≤0.5 g/m3 according to European
standards). Therefore, the removal of hydrogen sulfide
from coke-oven gas by the ammonia method is in
increasing use [1]. This method includes the capture of
hydrogen sulfide by an aqueous solution of ammonia
and subsequent treatment of the acidic gas by the Claus
process, to obtain elementary sulfur, which is a commercial product.
After this method was introduced at enterprises in
North America [2], two lines for sulfur production from
coke-oven gas went into operation in Poland, in 1997
and 1999 [3, 4]. There is also information on the design,
construction, and startup of similar units for removing
sulfur from coke-oven gas in the Czech Republic (Nova
Huta, Ostrava), in France (Dunkirk), China (five units),
South Korea, and elsewhere [5].
Factors encouraging the wide use of the ammonia
method of hydrogen-sulfide removal from coke-oven
gas include the absence of toxic emissions and the generation of a product (elementary sulfur) capable of
withstanding transportation and prolonged storage.
This is especially important in view of the global overproduction of sulfur and sulfuric acid and the geographic separation of regions that produce and consume sulfur.
Accordingly, there are good prospects for wider use
of the ammonia method. In this context, experience in
introducing this method in the coke plant at OAO Magnitogorskii Metallurgicheskii Kombinat (MMK) may
be of interest for Russian coke specialists.
In the present work, we analyze experience at MMK
in introducing the least-developed part of the technology: the processing of gas that contains hydrogen sulfide so as to obtain commercial sulfur.
DESULFURIZATION OF COKE-OVEN GAS
The extraction of hydrogen sulfide from coke-oven
gas by ammonia liquor is well established in the coke
industry and may be regarded as sufficiently welldeveloped [1, 6]. However, the processing of acidic gas
containing hydrogen sulfide and ammonia in the coke
industry differs from the classical Claus processing of
hydrogen sulfide [7].
Profound sulfur extraction in classical Claus equipment involves four or more catalytic-conversion stages,
with corresponding capital expenditures [2, 7]. Both the
capital expenditures and the losses of sulfur are reduced
in a design employed by Krupp–Koppers (now Thyssen–Krupp EnCoke, Germany) [8].
In this design, after sulfur extraction in a condenser
and trapping in a separator, the tail gas from the sulfurremoval unit (SRU) is cooled to ~60°C in a contact
cooling unit by excess water from the gas collector,
according to patent [8]. This gas is then returned to the
incoming coke-oven gas, thereby boosting the reserves
of hydrogen sulfide. To reduce the sulfur losses with
SO2 in the coolant and simplify the regulation of the
process, treatment of the acidic gas by the Claus
method is characterized by high Claus ratio (HCR technology) [9]. The air supply to the Claus furnace is regulated so that the Claus ratio at the SRU output is CR ≡
[H2S]/[SO2] = 10.
The first SRU of Krupp–Koppers type in the former
Soviet countries went into operation in September 2000
in purification shop 2 for coke-oven gas at OAO MMK.
Operational experience in this shop indicates that the
desulfurization technology is highly efficient [10]. The
existing system with a closed hydrogen-sulfide cycle
differs somewhat from a classical SRU. This must be
taken into account in the design of future equipment
(shops) for the purification of coke-oven gas, with sulfur production.
In the existing system for sulfur removal from acidic
gas in purification shop 2 at MMK, the hydrogen-sulfide concentration is low, while the ammonia concentration in the acidic gas is high. According to measurements throughout shop operation, the H2S concentration is 5–8 vol % and the NH3 concentration is 12–
17 vol %. This may be attributed to inadequate initial
data on the design (the H2S reserves in the coke-oven
gas are overestimated by a factor of 2–3) and to the
design of the section for trapping hydrogen sulfide [11].
As a result, Superclaus processing of concentrated
(24 vol %) hydrogen sulfide cannot be fully implemented in MMK purification shop 2.
In the Combiclaus processing system for acidic gas
at Ostrava, the gas supplied to the SRU in 2000 contained 3.5–9.3 vol % H2S and 16.3–34.6 vol % NH3.
226
DESULFURIZATION OF COKE-OVEN GAS
Similar H2S and NH3 concentrations in the acidic gas
are typical for equipment purifying coke-oven gas at
Moscow coke plant. Thus, existing (admittedly limited)
experience does not provide examples of the successful
implementation of ammonia technology for selective
trapping of hydrogen sulfide in the coke industry. In all
cases, the ratio of components in the acidic vapor
[NH3]/[H2S] ≈ 3 is determined by their proportions in
saturated water, which means that existing reducing
agents are inefficient and the technology differs significantly from classical Claus processing of acidic gas.
CLAUS PROCESSING OF ACIDIC GAS
In processing acidic vapor at coke plants, as in the
classical treatment technology for acidic gas, one third
of the hydrogen sulfide present is first consumed [7]
2H2S + SO2 + H2O,
(1)
3H2S + 3/2O2
and then the partial-oxidation products of H2S undergo
Claus reaction
2H2S + SO2
2H2O + 3/nSn,
(2)
which occurs most effectively at 230–250°C with aluminum-oxide catalyst.
As in any Claus unit, controlling sulfur production
from the acidic gas reduces to regulating the supply of
the oxidizing agent (air) to the Claus furnace for combustion of the hydrogen sulfide by Eq. (1). The presence of considerable quantities of ammonia in the
acidic gas may create a secondary channel of oxygen
consumption, with the formation of water, nitrogen,
and nitrogen oxides, which hinders the regulation of
hydrogen-sulfide processing to obtain sulfur by Eqs. (1)
and (2).
Nevertheless, as shown by prolonged operational
experience with the MMK SRU, the use of a catalyst of
ammonia decomposition in the Claus furnace permits
stable sulfur extraction without excessive of hydrogen
sulfide and the formation of NOx and ammonium sulfate, even at low initial H2S concentrations. To prevent
spoiling of the catalyst by the hydrogen sulfide and to
increase ammonia decomposition, high temperatures
are required (>1100°C) [12]. Consequently, all the
existing technologies for low-temperature catalytic oxidation of hydrogen sulfide (and the corresponding catalysts of H2S oxidation) for processing acidic gas that
contains ammonia are inapplicable here.
Accordingly, in the case of joint trapping of hydrogen sulfide and ammonia from coke-oven gas by Amasulf technology, the most promising approach is the
partial oxidation of H2S with simultaneous NH3
decomposition on a nickel catalyst at 1100–1200°C;
this technology has undergone sufficient industrial
development. If the technological oxidation conditions
are observed, the guaranteed working life of BASF G1-11
catalyst for ammonia decomposition (2 yr) may be
extended severalfold. The actual lifetime of this catalyst in ammonia-decomposition furnaces is ~10 yr [13];
COKE AND CHEMISTRY
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2007
227
this is acceptable for industrial conditions. However, in
view of the fairly high cost of this catalyst (at least
20 euro/kg), it would be expedient to develop a cheaper
Russian analog, perhaps on the basis of known lowtemperature catalysts of ammonia dissociation [13].
A high-temperature reactor of new design permits significant reduction in the size of the catalyst granules,
which makes the main contribution to the gas-dynamic
drag of the Claus line [14]. This provides additional
reserves of activity and stability for the new Russian
catalyst.
The low initial concentration of hydrogen sulfide in
the acid coke-oven gas calls for additional fuel consumption in order to reach a temperature of 1150–
1200°C in the Claus furnace. When using the natural
fuel for coke production—coke-oven gas or natural
gas—carbon is introduced in the gas, so as to considerably increase the yield of organic sulfur byproducts
(COS and CS2) at the thermal stage. These byproducts
may contain up to a quarter of the sulfur. Consequently,
their conversion is a priority in the Claus processing of
the acidic gas. HCR technology may lead to the formation of secondary CS2 in the catalytic stage of the SRU,
as shown by the analysis of data from an industrial
experiment [15]; this is unacceptable.
Another problem observed at an early stage in the
operation of MMK purification shop 2 is the presence
of elementary sulfur (~1 g/m3) in the tail gas of the
Claus line [10]. Adhesion of this sulfur causes problems of repeated startup (rotor freezing) of the gas
blower. Although elementary sulfur is present in the tail
gases of all existing Claus units, it is especially hazardous for those that operate on a closed cycle. According
to available information, operation of the SRU in
Dunkirk had to be interrupted after a few months on
account of sulfur deposition on the walls and clogging
of the inverse-gas lines. Purification of the tail gases
from Claus units by Shell Claus Off-Gas Treating
(SCOT technology) and other methods widely used in
the gas industry is too complicated and capital-intensive in the present context [16, 17]. Other methods must
be sought.
A considerably simplified SCOT process suitable
for the coke industry has been implemented at the SRU
in MMK purification shop 2 [18]. It involves the conversion of all the sulfur-containing components of the
SRU tail gas to hydrogen sulfide [19]. This hydrogen
sulfide is then returned to the incoming coke-oven gas
by means of the existing system [8, 10]. To this end, the
tail gas in the Claus line is heated to 330–350°C and
passes through a catalytic reactor, where the reactions
at the catalyst include hydrolysis
COS + H 2 O
H 2 S + CO 2 ,
(3)
CS 2 + 2H 2 O
2H 2 S + CO 2
(4)
2H 2 S,
(5)
H 2 S + 2H 2 O.
(6)
and hydrogenation
2H 2 + S 2
3H 2 + SO 2
228
PLATONOV
Acidic gas
ηH2S, % (rel.)
6
3
Air
7
90
2
60
1
4
5
Initial coke-oven gas
Sulfur
Fig. 1. Improved unit for sulfur extraction: (1) Claus furnace; (2) waste-heat boiler; (3) heat exchanger; (4) Claus
reactor; (5) sulfur condenser; (6) controllable bypass;
(7) catalytic converter (pyrolytic reactor).
The reagents required here (H2O and H2) are present
in excess in the tail gas and the fuel (coke-oven gas or
natural gas) sent to the gas heater. Hydrolysis of the elementary sulfur by the inverse Claus reaction in Eq. (2)
at an aluminum-oxide catalyst at >330°C is practically
complete, as shown by direct measurements. The sulfur
leaves the reactor predominantly in the form of hydrogen sulfide (>87%). along with carbonyl sulfide
(<10%) and sulfur dioxide (≤3%) [18]. Thanks to the
high degree of SO2 reduction in Eq. (6) and in the
inverse reaction in Eq. (2), there is no need for HCR
technology; Claus conversion is possible with CR ≈ 2,
which is optimal for sulfur production in Eq. (2). Practically complete sulfur removal from coke-oven gas is
ensured, with minimal capital and operating expenditures,
by the improved desulfurization technology in [19].
If hydrogen-sulfide recycling is considered at the
stage of SRU design, the system for tail-gas hydrolysis
may be significantly simpler than that in [18], in which
the hydrolysis reactor is inserted in the existing SRU at
MMK purification shop 2. In particular, it is possible to
eliminate the additional cooling unit for the emitted gas
that is intended to reduce the pressure losses associated
with the gas-dynamic drag of the line in the system
described in [18].
A simplified and economical unit for processing
concentrated (≥23 vol % H2S) acidic gas is shown in
Fig. 1. The stages of processing include H2S oxidation
in Claus furnace 1, conversion in Claus catalytic reactor 4,
sulfur extraction in condenser 5, and inverse conversion
of the sulfur-bearing components (SO2 and S0) to H2S
in Eqs. (2)–(6) at 330–350°C, in catalytic converter 7.
The process gas is heated from 145–150°C to ~320°C
in heat exchanger 3 and then, where necessary, by
another 10–30°C, as a result of mixing with hot gas
(~430°C) supplied through controllable bypass 6.
30
0
2
4
6
8
10
[H2S]0, vol %
Fig. 2. Dependence of hydrogen-sulfide conversion at
AO-MK-2 catalyst on its initial concentration, at a mean
temperature of 252°C.
MONITORING THE TECHNOLOGY
Stable operation of the Claus SRU depends on monitoring the activity of the catalysts and predicting their
remaining life. In practice, the composition of the process gas is monitored every day (where necessary,
every shift) [20]. The results are analyzed by appropriate methods. Increased carbon content in the incoming
acidic gas calls for especially careful monitoring of the
catalyst.
The catalyst activity in the Claus unit cannot be adequately evaluated by the conventional method of monitoring the actual conversion, determined from the final
SO2 and H2S concentrations in the SRU tail gas, in conditions where the initial composition of the acidic gas
varies, as is typical in the coke industry [21].
This is illustrated in Fig. 2, which shows the measured degree of hydrogen-sulfide conversion η H2 S as a
function of its initial concentration [H2S]0. The unfilled
symbols in Fig. 2 correspond to the experimental conversion values (different shapes indicate different CR
values), and the filled symbols to monitoring data for
the Claus reactor in the SRU at MMK purification
shop 2 [22]. The continuous curve is obtained from a
semiempirical model with the corresponding bulk flow
rate w ~ 2000 h–1 [22]. We see that, when the initial H2S
concentration in the acidic vapor varies (which is typical for MMK purification shop 2) and catalyst activity
is constant, the observed conversion coefficient varies
in the range 35–47% (rel.).
Sulfur deposition in the micropores of the catalyst
produces even greater (real, not apparent) reduction in
catalyst activity in Eq. (2). This (reversible) deactivation of the Claus catalyst, occurring after about a
COKE AND CHEMISTRY
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DESULFURIZATION OF COKE-OVEN GAS
229
Table
Process parameters
Averaging period
Conversion, % (rel.)
Ki, s–1 atm–1/2
T, K
τ, s
H2S
COS
K2
K8
518
522
558
535
522
525
527
527
526
529
530
529
527
526
527
527
500
532
1.75
1.73
1.72
1.68
1.73
1.70
1.71
1.71
1.71
1.70
1.70
1.70
1.71
1.70
1.71
1.71
1.80
1.68
50
45
37
44
38
46.6
47.7
39.8
47.6
51
47.4
44
46
38
42
44
46.6
35
68
42
50
50
35
12
33.5
42.5
40
40
47
33
36
19
44
25
40
40
2.6
2.0
1.4
1.8
1.7
2.3
2.3
1.5
2.3
2.1
1.9
1.9
1.9
1.8
1.8
1.9
2.2
1.6
6.8
2.9
3.8
3.5
2.1
0.50
1.7
1.9
2.0
1.7
2.8
1.4
1.9
1.3
2.6
1.4
2.2
2.2
April 11–20, 2003
September 15–October 15, 2003
November 15–December 14, 2003
March 19–30, 2004
July 23–August 2, 2004
October 27–November 18, 2004*
February 27–March 14, 2005
May 22–June 8, 2005
August 12–September 1, 2005
September 12–28, 2005
November 14–30, 2005
February 4–15, 2006
June 11–26, 2006
July 10–19, 2006*
August 24–September 3, 2006
November 10–22, 2006*
December 10, 2006
March 22–25, 2007
* After shutdown, with no thermal regeneration of the catalyst.
month, is completely eliminated by thermal regeneration of the catalyst, which is undertaken every quarter
at MMK purification shop 2, without change in operating conditions of the Claus furnace [22]. This maintains
the mean catalyst activity at an acceptable level (>90%
of the maximum).
Irreversible reduction in catalyst activity, which
reflects its aging, requires the development of a special
monitoring method that is free of the influence of the
process variables: the initial gas composition and the
temperature. It is very important here to extend the catalyst’s working life, whose guaranteed (and often
actual) value is usually no more than 2–3 yr [7]. This is
true of the CR-31 aluminum-oxide catalyst (La Roche
Chemicals, USA) used in the two-stage Claus catalytic
reactor at MMK purification shop 2, whose guaranteed
life is no more than 3 yr.
Monitoring of the catalyst activity and prediction of
its remaining life may be based on a phenomenological
formal–kinetic model of the Claus process, in which
the catalyst activity is estimated by the rate constant K
of H2S consumption at the aluminum-oxide catalyst [15]
K = ( 1 – c /c 0 )/ ( τc )
1/2
= ( c 0 /c
1/2
1/2
1/2
1/2
(7)
– 1 )/ ( τc 0 ).
1/2
The initial (c0 ≡ [H2S]0) and final (c ≡ [H2S]) hydrogen-sulfide concentrations in the Claus reactor are sepCOKE AND CHEMISTRY
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No. 8
2007
arated by a time τ, with the initial reagent ratio [H2S]0 =
2[SO2]0. As shown by analysis, this model may also be
used to describe the conversion of carbonyl sulfide in
the Claus reactor.
In fact, in the working-temperature range of Claus
reactors (<260°C), the rate of hydrolysis of the organic
sulfur components—in particular, carbonyl sulfide—at
aluminum-oxide catalysts is small [7]; their contribution to the measured COS conversion is slight. Therefore, for industrial Claus reactors, the contribution of
Eq. (3) may be neglected. We may assume that the conversion of carbonyl sulfide is mainly determined by the
reduction of sulfur dioxide
2COS + SO 2
2CO 2 + 3/nS n ,
(8)
which is stoichiometrically similar to the Claus reaction in Eq. (2). On the basis of this stoichiometric similarity, the reactions in Eqs. (2) and (8) may be
described by the same formal kinetic equation. The
COS consumption is then characterized by Eq. (7), in
which the H2S concentration is replaced by the COS
concentration: c0 ≡ [COS]0; c ≡ [COS].
The table presents the rate constants Ki of hydrogensulfide and carbonyl-sulfide conversion at AO-MK-2
catalyst calculated from Eq. (7) for different periods in
the MMK Claus reactor between April 2, 2003 and
March 25, 2007. Note that the values of the contact time τ
in the table are hypothetical (i.e., referred to the total
230
PLATONOV
Yi, %
100
80
H2S
60
40
COS
20
0
200
300
350
tc, °C
Fig. 3. Temperature dependence of the relative yield of carbonyl sulfide and hydrogen sulfide in a catalytic converter.
catalyst volume). The corresponding values of the rate
constants K2 and K8 for Eqs. (2) and (8) are determined
from measurements after thermal regeneration of the
catalyst or shutdown of the line, when the influence of
sulfur deposits in the micropores on the catalyst’s activity is minimal.
As follows from the table, the rate of irreversible
reduction in the rate constant of Eq. (2), which corresponds to the aging of AO-NKZ-2 catalyst, is no more
than 3–5%/yr after four years of operation in the MMK
Claus reactor. This corresponds to a subsequent life of
at least 7–10 yr. However, the catalyst deactivation in
carbonyl processing is somewhat unpredictable, on
account of the double mechanism of COS conversion.
After loss of activity in Eq. (3) within the first weeks of
operation, the conversion of carbonyl sulfide is based
on Eq. (8), for which the rate at which catalyst activity
is lost is determined by its hydrothermal aging [23].
The deactivation rate of AO-NKZ-2 catalyst in this
period, determined against the background of the statistical error in calculating K8, is no more than 12%/yr, on
average. Note, however, the brief declines in COS-conversion activity observed after SRU shutdown (restart),
on account of sulfating of the catalyst; such behavior is
hardly observed in the conversion of hydrogen sulfide
(and in the value of K2).
Thus, the deactivation of the catalyst in the Claus
reactor and its remaining life may be reliably predicted
on the basis of a phenomenological model of low-temperature catalytic conversion [22]. This is not true,
however, of the hydrolysis of organic sulfur components, for which there is no adequate mathematical
description suitable for engineering use. In this case,
deactivation of the catalyst is more rapid than for the
Claus catalyst [7].
The aging of the catalyst for COS hydrolysis is illustrated in Fig. 3, where the relative yield of the basic sul[i]
fur-bearing products Yi(t) ≡ ----------- (i ≡ H2S or COS) is
Si
plotted as a function of the time. These data are given
∑
separately for different operating periods of the hydrolytic reactor in the SRU at MMK purification shop 2;
they were given in generalized form in [24]. The dashed
line in Fig. 3 denotes the equilibrium yield of hydrogen
sulfide, which exceeds 98% (rel.) in the given temperature range.
As is evident, the maximum H2S yield is ~10%
lower in the fourth year of reactor operation (filled symbols in Fig. 3) than in the first year (open symbols). The
fact that the maximum value of Y H2 S declines over time
and is attained at lower temperatures may be attributed
to shrinkage of the active surface in hydrothermal aging
of the catalyst. Consequently, we need to formulate a
model of the process suitable for quantitative estimation of
the catalyst activity; no such model currently exists.
Overall, operational experience at the MMK SRU
indicates that the deactivation of aluminum-oxide catalysts of the Claus process and hydrolysis in the desulfurization of coke-oven gas may be regarded as essentially the same as their aging in traditional sulfurremoval systems [7]. The characteristics of the acidic
gas obtained in the coke industry necessitate particular
attention to monitoring and processing of organic sulfur
compounds (notably carbonyl sulfide). The different
aging rates of aluminum-oxide catalyst in the conversion of carbonyl sulfide and hydrogen sulfide entail
individual selection of different catalysts for the
hydrolysis of organic sulfur compounds and the Claus
conversion of hydrogen sulfide.
CONCLUSIONS
(1) Operational experience with the sulfur-removal
line in purification shop 2 at the MMK coke plant for
six years shows that the industrial removal of hydrogen-sulfide from coke-oven gas by means of ammonia
in the Claus technology, with the production of sulfur,
is sufficiently reliable and effective. The processing of
acidic gas (containing not only a low concentration of
hydrogen sulfide but also considerable quantities of
ammonia) is not fundamentally different from the traditional Claus technology.
(2) The characteristic features of acidic-gas processing in the coke industry may (and should) be taken into
account in SRU control. It is important to develop a
method of monitoring the activity of the Claus catalysts
with variable H2S concentration in the acidic gas.
Improved desulfurization of coke-oven gas with a
closed hydrogen-sulfide cycle is most promising for
coke plants.
(3) The operational experience at MMK in producing elementary sulfur from acidic gas, as well as the
corresponding improvements in the technology
employed, may be taken into account in the design and
operation of similar SRU—in particular, when the initial coke-oven gas has a high H2S content. This experience may prove useful for Ukrainian coke plants, for
example.
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DESULFURIZATION OF COKE-OVEN GAS
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