Poto?nik P. (Ed.) Natural Gas
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Natural gas hydrates 161
Diffusion processes appeared to be the dominant driving mechanism in supplying CO
2
to
the methane hydrate reaction sites and the concomitant increase of methane in the fracture.
The exchange process continued over several weeks. When methane production ceased, the
spacer was again flushed with CO
2
to accelerate the reaction by supplying fresh and pure
liquid CO
2
to the system. The methane production curve found from the average MRI
intensity in the fracture is shown for three separate experiments in Figure 11. Two of them
are duplicate experiments with initial water saturation of 50 % and 5 wt% NaCl (published
in Graue et al., 2008). The agreement between the two is very good. As shown in Figure 11,
the methane molar volumes by far exceeded any free methane that might have remained in
the pores after hydrate formation (diffusion experiment). Mass balance calculations and the
molar production curve from MRI intensities in the fracture suggest that between 50-85 per
cent of methane originally in hydrates was recovered by CO
2
replacement. Another
observation is the apparent absence of large-scale melting of hydrates during the CO
2
-CH
4
-
exchange. All the experiments run in this system did not detect any significant increase in
MRI signal in the hydrate saturated cores that would indicate the presence of free water
during CO
2
exchange. This was verified by the evaluation of the MRI signal intensity in the
core halves once CO
2
exchange began. MRI intensity remained constant or was even less
than the baseline value after the completion of hydrate formation. The exchange process did
not cause significant dissociation of the hydrate, at least on the scale of the MRI’s spatial
resolution of ~0.8 mm
3
. These experiments were run at CO
2
partial pressures significantly
greater than CO
2
saturation levels, in contrast to earlier studies where the CO
2
levels were
only slightly in excess to saturation or were undersaturated. This portion of the work shows
that methane can be produced by CO
2
replacement in within sandstone pores.
0
0.1
0.2
0.3
0.4
0.5
0.6
0 100 200 300 400 500 600 700
Time (Hours)
Molar methane consentration in fracture
[fractions]
Free gas diffusion level at
S
wi
=50%
after 1st CO
2
flush - duplicate experiments, S
wi
=50%
1st flush, S
wi
=45 %
after 2nd CO
2
flush - duplicate experiments, S
wi
=50%
2nd flush, S
wi
=45 %
diffusion
experiment
Fig. 11. Methane produced from methane hydrate by CO
2
replacement. Duplicate
experiments with S
wi
=50 % (5wt % NaCl) and one with S
wi
=50 % (0.1 wt % NaCl).
10. Conclusion
The experimental set-up with the MRI monitoring apparatus was capable of forming large
quantities of methane hydrates in sandstone pores and monitor hydrate growth patterns for
various initial conditions. Spontaneous conversion of methane hydrate to carbon dioxide
hydrate occurred when methane hydrate, in porous media, was exposed to liquid carbon
dioxide. The MRI images did not detect any significant increase in signal in the hydrate
saturated cores that would indicate the presence of free water during the carbon dioxide
replacement.
11. Acknowledgements
The authors are indebted to the Norwegian Research Council and ConocoPhillips for
financial support and thank Jim Stevens, James Howard and Bernie Baldwin for their
contribution in acquiring the MRI data.
12. References
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Jadhawar, P.; Yang, J.; Jadhawar, J.; Tohidi, B. (2005). Preliminary experimental investigation
on replacing methane in hydrate structure with carbon dioxide in porous media.
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th
International Conference on Gas Hydates, Trondheim, Norway.
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Replacement of CH4 in the hydrate by use of liquid CO2". Energy Conversion and
Management, 46 (11-12): 1680-1691.
Hester, K. & Brewer, P. G. (2009). Clathrate Hydrates in Nature. Annual Reviews of Marine
Science, 1 303-327
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Accumulations Under Various Geologic Conditions, LBNL-52568, presented at the
TOUGH Symposium, Berkeley, CA, May 12-14.
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McIver, R. D. (1977) Hydrates of natural gas – an important agent in geologic processes, In
Abstracts with Programs, pages 1089––1090. Geological Society of America.
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Fall Newsletter, 5-7.
Graue A.; Kvamme B.; Baldwin B.A.; Stevens J.; Howard J.; E. Aspenes, Ersland G.; Husebø
J. & Zornes D.. (2008). MRI Visualization of Spontaneous Methane Production From
Hydrates in Sandstone Core Plugs When Exposed to CO
2
. SPE Journal (SPE 118851),
13 (2). p. 146-152.
Phale, H. A.; Zhu, T.; White, M. D. & McGrail, B. P. (2006).Simulation study on injection of
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Technology Symposium, Calgary, Alberta, Canada
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Kvamme B., Graue A., Buanes T., Ersland G.. (2007). Storage of CO
2
in natural gas hydrate
reservoirs and the effect of hydrate as an extra sealing in cold aquifers. International
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Husebø, J. Monitoring depressurization and CO2-CH4 exchange production scenarios for natural
gas hydrates. (2008). PhD thesis, University of Bergen, Norway.
Makogon, YF., Trebin, F.A., Trofimuk, A.A., Tsarev, V.P., and Cherskiy, N.V. (1971).
Detection of a pool of natural gas in a solid (hydrated gas) state," Doklady Akademii
Nauk SSSR, 196, pp 203-206 (Translation in Doklady-Earth Science Section, 196, pp
197-200, 1972).
Soloviev, V.A. (2002). Global estimation of gas content in submarine gas hydrate
accumulations”, Russian Geology and Geophysics 43, pp. 609–624
Trofimuk, A.A.; Cherskiy, N.V. & Tsarev, V.P.( 1973) Accumulation of natural gases in
zones of hydrate—formation in the hydrosphere. Doklady Akademii Nauk SSSR, 212,
pp. 931–934.
Kvenvolden, K.A. (1988). Methane hydrate—a major reservoir of carbon in the shallow
geosphere. Chemical Geology 71, pp. 41–51.
Kennedy, M., Mrofka, D., Borch, C. (2008). Snowball Earth termination by destabilization of
equatorial permafrost methane clathrate”, Nature, 453: 642-645.
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Pooladi-Darvish, M, Riedel, M, Ripmeester, JA, Shipp, C and Willoughby, E. (2008)
Energy From Gas Hydrates: Assessing the Opportunities & Challenges for Canada, Report
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diapirism and gas hydrates at the head of the Cape Fear slide. Geo-Mar. Lett.,
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Abstracts with Programs, pages 1089––1090. Geological Society of America.
Ohgaki, K.; Takano, K.; Moritoki, M. (1994). Exploitation of CH
4
Hydrates under the Nankai
Trough in Combination with CO
2
storage”, Kagaku Kogaku Ronbunshu, 20 121-123
Ohgaki K; Takano K, Sangawa H; Matsubara T; Nakano S. (1996) Methane exploitation by
carbon dioxide from gas hydrates – phase equilibria for CO
2
-CH
4
mixed hydrate
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Hirohama S.; Shimoyama Y.; Tatsuta S.; Nishida N. (1996). Conversion of CH4 hydrate to
CO
2
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2
, J. Chem. Eng. Jpn. 29(6): 1014-1020
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The effect of H
2
S on hydrogen and carbon black production from sour natural gas 163
The effect of H
2
S on hydrogen and carbon black production from sour
natural gas
M. Javadi and M. Moghiman
X
The effect of H
2
S on hydrogen and carbon
black production from sour natural gas
1
M. Javadi,
1
M. Moghiman and
2
Seyyed Iman Pishbin
1
Ferdowsi University of Mashhad,
2
Khorasan Gas company
Iran
1. Introduction
Hydrogen is well known as an ideal and clean source of energy which is believed to reduce
the emission of carbon dioxide and therefore play a major role in decreasing the global
warming problem [Ryu et al, 2007]. Eventual realization of a hydrogen economy requires
cheap and readily available hydrogen sources and a technology to convert them into pure
hydrogen in an efficient and sustainable manner [Abdel et al, 1998]. In addition to water that
is an ideal hydrogen source, CH
4
and H
2
S are considered as alternative sources of hydrogen
[Jang et al, 2007; T-Raissi, 2003]. On the other hand, there is ample scope for CH
4
and H
2
S as
the raw source of H
2
, because the energy required for CH
4
and H
2
S splitting (ΔH
CH4
=74.9
kJ/mol and ΔH
H2S
=79.9 kJ/mol) is much less than water splitting (H
water
= 284.7 kJ/mol)
[Jang et al, 2007]. There are several convenient technologies for production of H
2
from CH
4
,
including steam methane reforming (SMR), partial oxidation, pyrolysis, autothermal
pyrolysis, and autothermal SMR [Huang & T-Raissi, 2007a]. Methane decomposition is a
moderately endothermic reaction. It requires much less thermal energy (only 37.8 kJ per mol
of hydrogen produced) than SMR (69 kJ/mol H
2
). Besides, the decrease in the required
energy, the CO
2
emission is also decreased in this method. Methane which is the main
component of the high quality natural gas can be decomposed to hydrogen and carbon
black in pyrolysis reactors [Abanades & Flamant , 2007; Moghima & Bashirnezhad, 2007].
Carbon black is an industrial form of soot produced by subjecting hydrocarbon feedstock to
extremely high temperatures in a carefully controlled combustion process. Carbon black is
widely used as filler in elastomers, tires, plastics and paints to modify the mechanical,
electrical and optical properties of materials in which it is used [Ghosh, 2007; Petrasch et al,
2007].
As the prices of fossil fuel increase, abundant sour natural gas, so called sub-quality natural
gas resources become important alternatives to replace increasingly exhausted reserves of
high quality natural gases for the production of hydrogen and carbon black [Huang & T-
Raissi, 2007b; Abdel et al, 1998]. At oil flow stations it is common practice to flare or vent
SQNG, which is produced along with crude oil. This accounts for more than 100 million
cubic meters (m
3
) world-wide per day, and approximately equals to France’s annual gas
consumption [Gruenberger et al, 2002]. Clearly this is of considerable concern in terms of
8
Natural Gas164
global resource utilization and climate change implications. Gas flaring has also been
blamed for environmental and human health problems such as acid rain, asthma, skin and
breathing diseases [Lambert et al, 2006]. The removal of H
2
S from sub-quality natural gas is
expensive and not commercially viable for large-scale plants. When H
2
S concentration in
natural gas is higher than about 1.0%, the high separation cost makes the sour natural gas
uneconomical to use [Huang & T-Raissi, 2007b]. As mentioned above, production of
hydrogen and carbon black from sour natural gas is one viable option utilizing this
untapped energy resource while at the same time reducing carbon oxides and hydrogen
sulfide emissions.
There is a massive back ground literature on thermal decomposition of high quality natural
gas using different types of reactors. Petrasch & Steinfeld (2007) have studied hydrogen
production process using solar reactors with SMR method. Abanades & Flamant (2007) also
have investigated the effect of different parameters and system geometry on methane
conversion and hydrogen yield using thermal decomposition method in solar reactors. Their
results show that the solar reactor producing pure H
2
has high efficiency in CH
4
conversion.
Cho et al (2009) have studied on the development of a microwave plasma-catalytic reaction
process to produce hydrogen and carbon black from pure natural gas. The direct conversion
of methane, using various plasma technologies has widely been studied in order to obtain
more valuable chemical products. Gruenberger et al (2002) and Moghiman & Bashirnezhad
(2007) have investigated the effect of feedstock parameters on methane decomposition in
carbon black furnace.
Although many studies have been carried out on high quality natural gas pyroysis, sour
natural gas pyrolysis have received much less attention. Towler & Lynn (1996) introduced
thermal decomposition of hydrogen sulfide at high temperature as an alternative of Clause
process. The main advantage of the thermal decomposition is reduction of produced tail gas
rather than Clauses process. They have investigated the effect of CO
2
presence in feed gas
and temperature on decomposition and sulfur compounds production. Also, Huang and T-
Raissi et al (2007b, 2007c and 2008) have performed the thermodynamic analyses of
hydrogen production from sub-quality natural gas using a Gibbs reactor operation in the
AspenPlusTM chemical process simulator. Javadi and Moghiman (2010) have investigated
carbon disulfide, hydrogen and solid carbon production from sub-quality natural gas. Their
results show that the maximum yield of C(s) is in 1000 °K and then decreases due to
increasing of CS2 production.
Based on the importance of sub-quality natural gas pyrolysis, the effects of feedstock
parameters, reactor temperature and H
2
S/CH
4
molar ratio of feedstock on decomposition
process have been studied using the proposed carbon black furnace by Gruenberger et al
(2002).
2. Gas furnace carbon black
Hydrogen and carbon black production via thermal decomposition of natural gas have been
achieved using a carbon black furnace [Gruenberger et al, 2000 & 2002], plasma
[Gaudernack & Lynum 1998], solar radiation [Abanades et al, 2007 & 2008], a molten metal
bath and thermal reactors with and without catalyst [Steinberg, 1998; Ishihara et al, 2002;
Muradov et al, 1998 & Kim et al, 2004].
Depending on the way that heat is supplied to sour natural gas, carbon black furnaces can
be classified as follows:
Type 1: Part of the natural gas or any other fuel burns inside the reactor to provide heat
needed to decompose the sour natural gas.
Type 2: Direct heat transfer from inert hot gases introduced into the reactor. This
method is known as “Hydrogen Sulfide- Methane Reformation”.
The carbon black furnace used in this investigation is a small-scale axial flow reactor
identical to that reported previously by Gruenberger et al (2002). The furnace has been
designed on the basis of using gaseous fuels as feedstock hydrocarbon, with a maximum
output of 10 kg carbon black per hour. The basic geometry of the carbon black furnace is
shown in Fig. 1, consisting of a pre-combustor, a mixing zone and a reactor. In the pre-
combustor, the axially injected natural gas burns with inlet air introduced through two
tangential inlets. Then, the highly swirling hot combustion gases mix with the sub-quality
natural gas injected radially into the pre-combustor in the proximity of the mixing zone. A
sudden increase in the tube diameter at the exit of the choke promotes vigorous mixing of
the SQNG fuel with the hot gases leading to thermal decomposition of CH
4
+H
2
S and
formation of hydrogen, carbon black, sulfur compounds and other precursor species for the
formation of carbon black [Lockwood et al, 1995].
Fig. 1. Carbon black gas furnace[Gruenberger et al, 2002]
3. Chemical reaction modelling
Production of carbon black through thermolysis of SQNG involves a complex series of
chemical reactions which control conversion of both
4
CH
and H
2
S as follows [Huange & T-
Raissi, 2007b; Towler & Lynn, 1996]:
The effect of H
2
S on hydrogen and carbon black production from sour natural gas 165
global resource utilization and climate change implications. Gas flaring has also been
blamed for environmental and human health problems such as acid rain, asthma, skin and
breathing diseases [Lambert et al, 2006]. The removal of H
2
S from sub-quality natural gas is
expensive and not commercially viable for large-scale plants. When H
2
S concentration in
natural gas is higher than about 1.0%, the high separation cost makes the sour natural gas
uneconomical to use [Huang & T-Raissi, 2007b]. As mentioned above, production of
hydrogen and carbon black from sour natural gas is one viable option utilizing this
untapped energy resource while at the same time reducing carbon oxides and hydrogen
sulfide emissions.
There is a massive back ground literature on thermal decomposition of high quality natural
gas using different types of reactors. Petrasch & Steinfeld (2007) have studied hydrogen
production process using solar reactors with SMR method. Abanades & Flamant (2007) also
have investigated the effect of different parameters and system geometry on methane
conversion and hydrogen yield using thermal decomposition method in solar reactors. Their
results show that the solar reactor producing pure H
2
has high efficiency in CH
4
conversion.
Cho et al (2009) have studied on the development of a microwave plasma-catalytic reaction
process to produce hydrogen and carbon black from pure natural gas. The direct conversion
of methane, using various plasma technologies has widely been studied in order to obtain
more valuable chemical products. Gruenberger et al (2002) and Moghiman & Bashirnezhad
(2007) have investigated the effect of feedstock parameters on methane decomposition in
carbon black furnace.
Although many studies have been carried out on high quality natural gas pyroysis, sour
natural gas pyrolysis have received much less attention. Towler & Lynn (1996) introduced
thermal decomposition of hydrogen sulfide at high temperature as an alternative of Clause
process. The main advantage of the thermal decomposition is reduction of produced tail gas
rather than Clauses process. They have investigated the effect of CO
2
presence in feed gas
and temperature on decomposition and sulfur compounds production. Also, Huang and T-
Raissi et al (2007b, 2007c and 2008) have performed the thermodynamic analyses of
hydrogen production from sub-quality natural gas using a Gibbs reactor operation in the
AspenPlusTM chemical process simulator. Javadi and Moghiman (2010) have investigated
carbon disulfide, hydrogen and solid carbon production from sub-quality natural gas. Their
results show that the maximum yield of C(s) is in 1000 °K and then decreases due to
increasing of CS2 production.
Based on the importance of sub-quality natural gas pyrolysis, the effects of feedstock
parameters, reactor temperature and H
2
S/CH
4
molar ratio of feedstock on decomposition
process have been studied using the proposed carbon black furnace by Gruenberger et al
(2002).
2. Gas furnace carbon black
Hydrogen and carbon black production via thermal decomposition of natural gas have been
achieved using a carbon black furnace [Gruenberger et al, 2000 & 2002], plasma
[Gaudernack & Lynum 1998], solar radiation [Abanades et al, 2007 & 2008], a molten metal
bath and thermal reactors with and without catalyst [Steinberg, 1998; Ishihara et al, 2002;
Muradov et al, 1998 & Kim et al, 2004].
Depending on the way that heat is supplied to sour natural gas, carbon black furnaces can
be classified as follows:
Type 1: Part of the natural gas or any other fuel burns inside the reactor to provide heat
needed to decompose the sour natural gas.
Type 2: Direct heat transfer from inert hot gases introduced into the reactor. This
method is known as “Hydrogen Sulfide- Methane Reformation”.
The carbon black furnace used in this investigation is a small-scale axial flow reactor
identical to that reported previously by Gruenberger et al (2002). The furnace has been
designed on the basis of using gaseous fuels as feedstock hydrocarbon, with a maximum
output of 10 kg carbon black per hour. The basic geometry of the carbon black furnace is
shown in Fig. 1, consisting of a pre-combustor, a mixing zone and a reactor. In the pre-
combustor, the axially injected natural gas burns with inlet air introduced through two
tangential inlets. Then, the highly swirling hot combustion gases mix with the sub-quality
natural gas injected radially into the pre-combustor in the proximity of the mixing zone. A
sudden increase in the tube diameter at the exit of the choke promotes vigorous mixing of
the SQNG fuel with the hot gases leading to thermal decomposition of CH
4
+H
2
S and
formation of hydrogen, carbon black, sulfur compounds and other precursor species for the
formation of carbon black [Lockwood et al, 1995].
Fig. 1. Carbon black gas furnace[Gruenberger et al, 2002]
3. Chemical reaction modelling
Production of carbon black through thermolysis of SQNG involves a complex series of
chemical reactions which control conversion of both
4
CH
and H
2
S as follows [Huange & T-
Raissi, 2007b; Towler & Lynn, 1996]:
Natural Gas166
mol/kJ9.74HH2)S(CCH
29824
Reaction (1)
mol/kJ9.79HHS
2
1
SH
298222
Reaction (2)
Since reaction 1 is mildly endothermic, it requires temperatures higher than
K850
to
proceed at reasonable rates [Dunker et al, 2006], and, as reaction 2 is highly endothermic,
temperatures in excess of
K1500
is required for achieving reasonable rates [Huang & T-
Raissi, 2008].
Under special circumstances including using catalyst H
2
S can react with methane producing
carbon disulfide (CS
2
) and H
2
[Huange & T-Raissi, 2008].
2 4 2 2 298K
2H S CH CS 4H H 232kJ / mol
Reaction (3)
A portion of
4
CH
and
SH
2
can oxidize to produce
CO
,
2
CO
and
2
SO
[ Abdel
et al, 1998]:
4 2 2 2 2 2
1 1
CH O CO H H O and CO O CO
2 2
Reaction (4)
2 2 2 2
3
H S O SO H O
2
Reaction (5)
H
2
S can also react with
2
CO
producing
COS
[Sakanishi et al, 2005]:
OHCOSCOSH
222
Reaction (6)
4. Turbulence–chemistry interaction
The mixture fractionPDF method is used to model the turbulent chemical reactions
occurring in the diffusion, combustion and thermal decomposition of natural gas in the
carbon black furnace. This method, which assumes the chemistry is fast enough for a
chemical equilibrium to always exist at molecular level, enables handling of large numbers
of reacting species, including intermediate species. Transport equations are solved for the
mean mixture fraction
f
, its variance
2
f
and for enthalpy h . Calculations and PDF
integrations are performed using a preprocessing code, assuming chemical equilibrium
between 30 different species. The results of the chemical equilibrium calculations are stored
in look-up tables which relate the mean thermochemical variables (species mass fractions,
temperature and density) to the values of
f
,
2
f
and h [Saario & Rebola, 2005].
In non-adiabatic systems, where change in enthalpy, due to heat transfer, affects the mixture
state, the instantaneous thermo chemical state of the mixture, resulting from the chemical
equilibrium model, is related to a strictly conserved scalar quantity known as the mixture
fraction, f, and the instantaneous enthalpy,
)H,f(,H
*
ii
*
. The effects of turbulence on
the thermo chemical state are accounted for with the help of a probability density function
(PDF):
1
0
*
ii
.df)f(p)H,f(
(1)
In this work, the
-probability density function is used to relate the time-averaged values of
individual species mass fraction, temperature and fluid density of the mixture to
instantaneous mixture fraction fluctuations. The
-PDF in terms of the mean mixture
fraction
f
and its variance
2
f
, can be written as:
1f0,
df)f1(f
)f1(f
)f(P
1
0
11
11
(2)
where:
,1
f
)f1(f
f
2
.1
f
)f1(f
)f1(
2
(3)
Using the unweighted averaging [Jones & Whitelaw, 1982], the values of the two parameters
f
and
2
f
at each point in the flow domain are computed through the solution of the
following conservation equations [Warnatz, 2006]:
),
x
f
(
x
)fu(
x
it
t
i
i
(4)
,f
k
C
x
f
C)
x
f
(
x
)fu(
x
2
d
2
i
tg
i
2
t
t
i
2
i
(5)
where the constants
t
, )/2(C
tg
and
d
C
take the values 0.7, 2.86 and 2.0, respectively.
The distribution of the instantaneous enthalpy is calculated from a transport equation as
follows:
h
k
i
ik
i
*
p
t
i
*
i
S
x
u
)
x
H
c
k
(
x
)Hu(
x
(6)
where
t
k
is turbulent thermal conductivity and
h
S
includes the heat generated by the
chemical reaction and radiation. The instantaneous enthalpy is defined as:
j j
T
T
)j,ref
jj,pjjj
*
j,ref
T(hdTcmHmH
(7)
where
j
m is the mass fraction of species j and
)T(h
j,ref
j
is the formation enthalpy of species j
at the reference temperature
j,ref
T
.
The effect of H
2
S on hydrogen and carbon black production from sour natural gas 167
mol/kJ9.74HH2)S(CCH
29824
Reaction (1)
mol/kJ9.79HHS
2
1
SH
298222
Reaction (2)
Since reaction 1 is mildly endothermic, it requires temperatures higher than
K850
to
proceed at reasonable rates [Dunker et al, 2006], and, as reaction 2 is highly endothermic,
temperatures in excess of
K1500
is required for achieving reasonable rates [Huang & T-
Raissi, 2008].
Under special circumstances including using catalyst H
2
S can react with methane producing
carbon disulfide (CS
2
) and H
2
[Huange & T-Raissi, 2008].
2 4 2 2 298K
2H S CH CS 4H H 232kJ / mol
Reaction (3)
A portion of
4
CH
and
SH
2
can oxidize to produce
CO
,
2
CO
and
2
SO
[ Abdel
et al, 1998]:
4 2 2 2 2 2
1 1
CH O CO H H O and CO O CO
2 2
Reaction (4)
2 2 2 2
3
H S O SO H O
2
Reaction (5)
H
2
S can also react with
2
CO
producing
COS
[Sakanishi et al, 2005]:
OHCOSCOSH
222
Reaction (6)
4. Turbulence–chemistry interaction
The mixture fractionPDF method is used to model the turbulent chemical reactions
occurring in the diffusion, combustion and thermal decomposition of natural gas in the
carbon black furnace. This method, which assumes the chemistry is fast enough for a
chemical equilibrium to always exist at molecular level, enables handling of large numbers
of reacting species, including intermediate species. Transport equations are solved for the
mean mixture fraction
f
, its variance
2
f
and for enthalpy h . Calculations and PDF
integrations are performed using a preprocessing code, assuming chemical equilibrium
between 30 different species. The results of the chemical equilibrium calculations are stored
in look-up tables which relate the mean thermochemical variables (species mass fractions,
temperature and density) to the values of
f
,
2
f
and h [Saario & Rebola, 2005].
In non-adiabatic systems, where change in enthalpy, due to heat transfer, affects the mixture
state, the instantaneous thermo chemical state of the mixture, resulting from the chemical
equilibrium model, is related to a strictly conserved scalar quantity known as the mixture
fraction, f, and the instantaneous enthalpy,
)H,f(,H
*
ii
*
. The effects of turbulence on
the thermo chemical state are accounted for with the help of a probability density function
(PDF):
1
0
*
ii
.df)f(p)H,f(
(1)
In this work, the
-probability density function is used to relate the time-averaged values of
individual species mass fraction, temperature and fluid density of the mixture to
instantaneous mixture fraction fluctuations. The
-PDF in terms of the mean mixture
fraction
f
and its variance
2
f
, can be written as:
1f0,
df)f1(f
)f1(f
)f(P
1
0
11
11
(2)
where:
,1
f
)f1(f
f
2
.1
f
)f1(f
)f1(
2
(3)
Using the unweighted averaging [Jones & Whitelaw, 1982], the values of the two parameters
f
and
2
f
at each point in the flow domain are computed through the solution of the
following conservation equations [Warnatz, 2006]:
),
x
f
(
x
)fu(
x
it
t
i
i
(4)
,f
k
C
x
f
C)
x
f
(
x
)fu(
x
2
d
2
i
tg
i
2
t
t
i
2
i
(5)
where the constants
t
, )/2(C
tg
and
d
C
take the values 0.7, 2.86 and 2.0, respectively.
The distribution of the instantaneous enthalpy is calculated from a transport equation as
follows:
h
k
i
ik
i
*
p
t
i
*
i
S
x
u
)
x
H
c
k
(
x
)Hu(
x
(6)
where
t
k
is turbulent thermal conductivity and
h
S
includes the heat generated by the
chemical reaction and radiation. The instantaneous enthalpy is defined as:
j j
T
T
)j,ref
jj,pjjj
*
j,ref
T(hdTcmHmH
(7)
where
j
m is the mass fraction of species j and
)T(h
j,ref
j
is the formation enthalpy of species j
at the reference temperature
j,ref
T
.
Natural Gas168
5.
Fl
u
re
f
th
r
gr
i
te
t
Fi
g
T
h
ra
t
an
th
e
T
h
th
e
st
r
in
s
th
e
eq
u
tu
r
re
g
d
u
co
n
fu
r
A
o
p
fo
r
1
%
e
m
ab
Numerical sol
u
ent CFD softwa
r
f
inement techni
q
r
ee dimensional
i
d is represented
t
rahedral cells.
g
. 2. Three-dime
n
h
e conservation
e
t
e, mixture fracti
o
al
y
sis, usin
g
a s
e
e
transport equat
i
h
e radiative heat
t
e
Discrete Ordin
a
r
ess model) is us
s
ide the combus
t
e
Re
y
nolds-ave
r
u
ations for Re
yn
r
bulence kinetic
e
g
ion. At the inl
e
u
rin
g
the cours
e
n
ditions are app
r
nace.
g
rid dependenc
e
p
timal accurac
y
a
n
r
typical set ope
r
%
after the num
b
m
issivity coeffici
e
sorption). Wall e
m
ution procedu
r
e has been use
d
q
ue to solve the
3
g
eometr
y
creati
o
in Fig 2. The do
m
n
sional tetrahedr
a
e
quations for m
a
o
n and its varian
c
e
cond-order up
w
i
ons.
t
ransfer in the ab
s
a
tes (DO) radiati
o
s
ed for predictio
n
t
or. Abandonin
g
r
a
g
ed Navier-St
o
n
olds stresses,
t
e
ner
g
y
. The con
v
e
t boundar
y
, co
n
e
of the solutio
n
lied. We assum
e
e
stud
y
was con
n
d efficienc
y
. T
h
r
ating conditions
b
er of
g
rid poi
n
e
nt at the flow
m
issivit
y
was se
t
re
d
to model the fu
r
3
D problem. Ga
m
o
ns and unstruc
t
m
ain is discretiz
e
a
l
g
rid
a
ss, momentum,
c
e, and concentr
a
w
ind scheme for
d
s
orbin
g
, emittin
g
o
n model [Murt
h
n
of anisotropic,
the isotropic ed
d
o
kes equations
t
o
g
ether with a
n
v
entional wall-fu
n
n
ditions are spec
n
procedure. A
t
e
d an isothermal
ducted to arrive
h
e number of
g
ri
d
. We observed t
h
n
ts increased be
inlets and outl
e
t
at 0.6, a t
y
pical
v
r
nace emplo
y
in
g
m
bit preprocess
o
t
ured
g
rid
g
ene
r
e
d into a grid of
ener
gy
, Re
y
nol
d
a
tion of soot are
s
d
iscretisation of
t
g
and scatterin
g
m
hy
& Mathur, 199
highly swirling
dy
-viscosit
y
h
y
p
o
b
y
solvin
g
six
n
equation for
t
n
ction approach
i
ified once and
d
t
the outlet bo
u
boundar
y
cond
i
at the appropri
a
d
points was vari
h
at the field qua
y
ond 20493. Fo
r
e
ts were taken
t
v
alue for combu
s
g
solution-adapti
v
o
r is used for th
e
r
ation. The 3D
v
20493 nodes an
d
d
's stresses, diss
i
s
olved b
y
finite-
v
t
he convective te
m
edium is calcul
a
9
8]. The RSM (Re
y
and recirculatin
g
o
thesis, the RSM
differential tra
t
he dissipation
r
i
s used in the ne
a
d
id not need up
u
ndar
y
, zero
gr
i
tion at the wall
a
te size of the
gr
ed from 17231 t
o
ntities varied le
s
r
the radiation
m
t
o be 1.0 (blac
k
s
tio
n
gases.
v
e
g
rid
e
full
y
v
olume
d
82745
i
pation
v
olume
rms in
a
ted b
y
y
nolds
g
flow
closes
nsport
r
ate of
a
r-wall
datin
g
r
adient
of the
r
id for
o
36387
s
s than
m
odel,
k
bod
y
6. Results and discussion
As mentioned above, the processes of methane pyrolysis differ mainly by the way heat is
supplied to the furnace. In this study, sour natural gas decomposition in a carbon black
furnace has been investigated for two types of supplying heat. In the first type, the natural
gas burns inside the pre-combustor (Fig.1) to provide required heat for decomposing feed
sour gas. In this case, the problem is the effect of combustion product (process gases) and
excess air which extremely affect on sour natural gas decomposition and furnace product. In
the second type the heat transfers from inert hot gases to feed sour gas. In this case only
reactions 1 to 3 are involved and there is not the problem of excess air and combustion
products. In this study the sour natural gas thermal decomposition inside the axial flow gas
furnace designed by Gruenberger et al (2002) has investigated. The results of two types of
supplying heat for pyrolysis are as follows:
6.1 Type 1: Pyrolysis by hot combustion gases
The total pre-combustor inlet airflow rate is
s/m1019
33
, at the temperature of 690 K and
pressure of 1 bar. The equivalence ratio used for the pre-combustor is 0.92. The accuracy of
the quantitative or even the qualitative trends for the combustion and decomposition
parameters depend on the accuracy with which the temperature and species concentration
fields are determined from the numerical calculation of the present model. To establish the
accuracy of our model, we have been calculated and compared the model predictions to the
experimental measurements of Gruenberger [Gruenberger et al, 2002] with no H
2
S. For
comparison purposes, we first conducted computations without H
2
S in feed gas.
A comparison of reactor outlet average temperature and carbon black yield (kg carbon
black/kg feedstock) predicted by this model and by experimental results is given in Figs. 3
and 4. Results of Fig. 3 depict that the model predicts lower temperatures than the
experimental data, especially at high feed flow rates. The discrepancy between the two
results might be due to the fundamental assumption made in the combustion model (PDF
fast chemistry combustion model), which assumes that chemistry is fast enough for a
chemical equilibrium. Results of Fig. 4 show that the predicted and measured carbon black
yields are in very good agreement and maximum carbon black yield is reached at the
equivalence ratio of 3. The discrepancy between the two results can be attributed to the
temperature levels obtained by the two methods (see Fig. 3). The lower temperature levels
computed by the model might be due to higher decomposition of CH
4
. Fig. 5 presents the
calculated distributions for CH
4
, H
2
S, temperature and mass fraction of soot, carbon black,
COS and gaseous sulfur predicted by the model at feed rate of
s/kg103
3
. H
2
S mass
fraction in natural gas is assumed to be 10%. Of particular interest are Figs. 5d-f that show
soot formation due to incomplete combustion of inlet methane and production of solid
carbon and gaseous sulfur by pyrolysis of methane- hydrogen sulfide jet interaction with
hot surroundings. Results from the model calculations seem to indicate that the use of more
inlet injection ports for SQNG feed would increase the yield of carbon black and sulfur
compounds.
Fig. 6 shows the reactor outlet temperature as a function of inlet mass flow rate for two cases
a) with H
2
S, b) without H
2
S. It can be seen that the results obtained for these two cases are
similar. The small discrepancy between the results may be due to CH
4
decomposition
The effect of H
2
S on hydrogen and carbon black production from sour natural gas 169
5.
Fl
u
re
f
th
r
g
r
i
te
t
Fi
g
T
h
ra
t
an
th
e
T
h
th
e
st
r
in
s
th
e
eq
u
tu
r
re
g
d
u
co
n
fu
r
A
o
p
fo
r
1
%
e
m
ab
Numerical sol
u
ent CFD softwa
r
f
inement techni
q
r
ee dimensional
i
d is represented
t
rahedral cells.
g
. 2. Three-dime
n
h
e conservation
e
t
e, mixture fracti
o
al
y
sis, usin
g
a s
e
e
transport equat
i
h
e radiative heat
t
e
Discrete Ordin
a
r
ess model) is u
s
s
ide the combus
t
e
Re
y
nolds-ave
r
u
ations for Re
yn
r
bulence kinetic
e
g
ion. At the inl
e
u
rin
g
the cours
e
n
ditions are app
r
nace.
g
rid dependenc
e
p
timal accurac
y
a
n
r
t
y
pical set ope
r
%
after the num
b
m
issivity coeffici
e
sorption). Wall e
m
ution procedu
r
e has been use
d
q
ue to solve the
3
g
eometr
y
creati
o
in Fi
g
2. The do
m
n
sional tetrahedr
a
e
quations for m
a
o
n and its varian
c
e
cond-order up
w
i
ons.
t
ransfer in the ab
s
a
tes (DO) radiati
o
s
ed for predictio
n
t
or. Abandonin
g
r
a
g
ed Navier-St
o
n
olds stresses,
t
e
ner
g
y
. The con
v
e
t boundar
y
, co
n
e
of the solutio
n
lied. We assum
e
e
stud
y
was con
n
d efficienc
y
. T
h
r
atin
g
conditions
b
er of
g
rid poi
n
e
nt at the flow
m
issivit
y
was se
t
re
d
to model the fu
r
3
D problem. Ga
m
o
ns and unstruc
t
m
ain is discretiz
e
a
l
g
rid
a
ss, momentum,
c
e, and concentr
a
w
ind scheme for
d
s
orbin
g
, emittin
g
o
n model [Murt
h
n
of anisotropic,
the isotropic ed
d
o
kes equations
t
o
g
ether with a
n
v
entional wall-fu
n
n
ditions are spec
n
procedure. A
t
e
d an isothermal
ducted to arrive
h
e number of
g
ri
d
. We observed t
h
n
ts increased be
inlets and outl
e
t
at 0.6, a t
y
pical
v
r
nace emplo
y
in
g
m
bit preprocess
o
t
ured
g
rid
g
ene
r
e
d into a
g
rid of
ener
gy
, Re
y
nol
d
a
tion of soot are
s
d
iscretisation of
t
g
and scatterin
g
m
hy
& Mathur, 199
hi
g
hl
y
swirlin
g
dy
-viscosit
y
h
y
p
o
b
y
solvin
g
six
n
equation for
t
n
ction approach
i
ified once and
d
t
the outlet bo
u
boundar
y
cond
i
at the appropri
a
d
points was vari
h
at the field qua
y
ond 20493. Fo
r
e
ts were taken
t
v
alue for combu
s
g
solution-adapti
v
o
r is used for th
e
r
ation. The 3D
v
20493 nodes an
d
d
's stresses, diss
i
s
olved b
y
finite-
v
t
he convective te
m
edium is calcul
a
9
8]. The RSM (Re
y
and recirculatin
g
o
thesis, the RSM
differential tra
t
he dissipation
r
i
s used in the ne
a
d
id not need up
u
ndar
y
, zero
gr
i
tion at the wall
a
te size of the
gr
ed from 17231 t
o
ntities varied le
s
r
the radiation
m
t
o be 1.0 (blac
k
s
tio
n
gases.
v
e
g
rid
e
full
y
v
olume
d
82745
i
pation
v
olume
rms in
a
ted b
y
y
nolds
g
flow
closes
nsport
r
ate of
a
r-wall
datin
g
r
adient
of the
r
id for
o
36387
s
s than
m
odel,
k
bod
y
6. Results and discussion
As mentioned above, the processes of methane pyrolysis differ mainly by the way heat is
supplied to the furnace. In this study, sour natural gas decomposition in a carbon black
furnace has been investigated for two types of supplying heat. In the first type, the natural
gas burns inside the pre-combustor (Fig.1) to provide required heat for decomposing feed
sour gas. In this case, the problem is the effect of combustion product (process gases) and
excess air which extremely affect on sour natural gas decomposition and furnace product. In
the second type the heat transfers from inert hot gases to feed sour gas. In this case only
reactions 1 to 3 are involved and there is not the problem of excess air and combustion
products. In this study the sour natural gas thermal decomposition inside the axial flow gas
furnace designed by Gruenberger et al (2002) has investigated. The results of two types of
supplying heat for pyrolysis are as follows:
6.1 Type 1: Pyrolysis by hot combustion gases
The total pre-combustor inlet airflow rate is
s/m1019
33
, at the temperature of 690 K and
pressure of 1 bar. The equivalence ratio used for the pre-combustor is 0.92. The accuracy of
the quantitative or even the qualitative trends for the combustion and decomposition
parameters depend on the accuracy with which the temperature and species concentration
fields are determined from the numerical calculation of the present model. To establish the
accuracy of our model, we have been calculated and compared the model predictions to the
experimental measurements of Gruenberger [Gruenberger et al, 2002] with no H
2
S. For
comparison purposes, we first conducted computations without H
2
S in feed gas.
A comparison of reactor outlet average temperature and carbon black yield (kg carbon
black/kg feedstock) predicted by this model and by experimental results is given in Figs. 3
and 4. Results of Fig. 3 depict that the model predicts lower temperatures than the
experimental data, especially at high feed flow rates. The discrepancy between the two
results might be due to the fundamental assumption made in the combustion model (PDF
fast chemistry combustion model), which assumes that chemistry is fast enough for a
chemical equilibrium. Results of Fig. 4 show that the predicted and measured carbon black
yields are in very good agreement and maximum carbon black yield is reached at the
equivalence ratio of 3. The discrepancy between the two results can be attributed to the
temperature levels obtained by the two methods (see Fig. 3). The lower temperature levels
computed by the model might be due to higher decomposition of CH
4
. Fig. 5 presents the
calculated distributions for CH
4
, H
2
S, temperature and mass fraction of soot, carbon black,
COS and gaseous sulfur predicted by the model at feed rate of
s/kg103
3
. H
2
S mass
fraction in natural gas is assumed to be 10%. Of particular interest are Figs. 5d-f that show
soot formation due to incomplete combustion of inlet methane and production of solid
carbon and gaseous sulfur by pyrolysis of methane- hydrogen sulfide jet interaction with
hot surroundings. Results from the model calculations seem to indicate that the use of more
inlet injection ports for SQNG feed would increase the yield of carbon black and sulfur
compounds.
Fig. 6 shows the reactor outlet temperature as a function of inlet mass flow rate for two cases
a) with H
2
S, b) without H
2
S. It can be seen that the results obtained for these two cases are
similar. The small discrepancy between the results may be due to CH
4
decomposition
Natural Gas170
reaction that begins at lower temperatures than that of H
2
S. Also, Fig. 6 depicts that
temperature drops precipitously with increasing flow rate of feed gas due to the
endothermic nature of both CH
4
and H
2
S decompositions.
Fig. 3. Comparison of the predicted reactor outlet temperature with the experimental data
Fig. 4. Comparison of the predicted carbon black yield with the experimental data
1100
1300
1500
1700
1900
2100
0 0,5 1 1,5 2 2,5 3 3,5 4
Temprature(k)
Feedstock Mass Flow Rate (1000*kg/s)
Exp. [4 ]
Pre. [this study]
0
10
20
30
40
0,0 1,0 2,0 3,0 4,0 5,0 6,0
Carbon Black yield [%]
Equivalence Ratio
Exp. [4 ]
Pre. [This study]
Fig. 5. Contour of species mass fractions and temperature (K)