Microwave-Induced Plasma Torch for Thermal Decomposition of H2S
into Hydrogen and Sulfur
Mohamed Sassi and Naji Amira
Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, UAE
Abstract:
The conventional treatment method for H2S is the Claus process, which produces
sulfur and water. This results in a loss of the valuable potential product hydrogen.
H2S treatment would be more economically valuable if both hydrogen and sulfur
products could be recovered. Based on standard heats of formation analysis, the
theoretical energy required to produce hydrogen from H2S dissociation is only
20.6 kJ/mol of H2 as compared to 63.2 kJ/mol of H2 from steam-methanereforming and 285.8 kJ/mol of H2 from water electrolysis. Among the many
thermal decomposition methods that have been explored in the literature, Microwave plasma dissociation of H2S prevails as the method of choice to attain the
best conversion and energy efficiency. Equilibrium and chemical kinetics
simulations have been carried out on the Chemkin-Pro software package and they
support these last findings. In addition a MW plasma torch experimental
apparatus has been designed and is being built to investigate this developing
technology. Optical diagnostics experimental methods in addition to gas analysis
and CFD simulations using the Fluent software package coupled with chemical
kinetics studies of the MW plasma torch decomposition of H2S for hydrogen and
sulfur production are therefore investigated in this study.
Keywords: Microwave plasma, H2S dissociation, Chemical Kinetics, Optical
Diagnostics
Introduction
Plasma treatment of gases has been broadly studied
in literature as an alternative scheme for dissociation
instead of the conventional thermal method or
combustion. Harkness, Doctor and Daniels [1]
explained in their report how dissociation of
hydrogen sulfide has been already practiced in soviet
literature; and their experiments were duplicated in
the Argonne national laboratory. The nonequilibrium nature of the plasma resulted in high
conversions with low energies of dissociation;
resulting in a capital cost which is less than half the
costs of a Claus sulfur recovery unit with the tail-gas
cleanup unit. Some of the plasma systems used in
the dissociation of H2S using plasma include the use
of a pulsed corona discharge reactor [2, 3], a gliding
arc discharge [4, 5], and an ozonizer discharge [6].
Hadidi and Woskov [7] have developed an
atmospheric pressure microwave plasma torch
powered by combining two 2.45 GHz microwave
plasma sources arranged in series (figure 1). The
microwave power coupling efficiency reached
approximately 95%, using reflected power
measurements. Plasma gas temperatures of 5500K
were measured using spontaneous emission
spectroscopy. This shows that coupling more than
one magnetron together is scalable and can be used
as a cheap and efficient source for thermal gas
treatment. MW plasma is electrode-less and hence
there is no problem of limited electrode lifetime (no
wearing out). RF induction-coupled plasma
production is the only other way for producing
electrode-less plasma, however it is not efficient.
In this work, thermal dissociation of H2S is
simulated using two different chemical kinetics
mechanisms in two ideal flow reactor models; and
the results are discussed. In addition, an
experimental setup, reproducing the one in figure 1,
is being built to validate and support the simulations.
The Two models
Chemical kinetics simulations were carried out on
two different thermal mechanisms for the
dissociation of H2S; one which was proposed by
Alexander Friedman [8] (table 1), and the other was
extracted from the sulfur species mechanism which
was develop in the University of Leeds (table 2).
The Fridman mechanism includes 6 species and 5
overall molecular reactions; where the Arrhenius
reaction coefficient of the first reaction was
extracted from [9] while the second mechanism
includes 8 species and 16 reactions, two of which
were taken from the work on Ar/H2 plasma by
Beuthe and Chang [10]. The CHEMKIN PRO
computational package was used in this analysis.
Two reactor models were used for each of the
mechanisms, the perfectly stirred reactor (PSR) and
the plug flow reactor (PFR).
S2+H+M=HS2+M
HS2+H=S2+H2
HS2+S=S2+SH
HS2+H+M=H2S2+M
H2S2+H=HS2+H2
H2S2+S=HS2+SH
H+H+M=H2+M
H+H+H2=H2+H2
1.00E+16
1.20E+07
8.30E+13
1.00E+16
1.20E+07
8.30E+13
1.87E+18
9.79E+16
0
2.1
0
0
2.1
0
-1
-0.6
0
352.42
3700
0
360
3700
0
0
Figure 1.The future experimental rig
Results and discussion
The PSR model
Table 1. Mechanism 1
Reaction
A(cm^3/mole-sec) ᵝ
Ea(K)
H2S= SH+H
64341.47 1 23550.57
H2S+H=H2+SH
7.83E+12 0
860
SH+SH=H2+S2
1.28E+14 0
0
SH+SH=H2S+S
7.23E+12 0
0
H2S+S=H2+S2
6.02E+12 0
2500
Table 2. Mechanism 2
Reaction
H2S+M=S+H2+M
H2S+H=SH+H2
H2S+S=2SH
H2S+S=HS2+H
S+H2=SH+H
2SH=S2+H2
SH+S=S2+H
S2+M=2S+M
A(cm^3/
mole-sec)
1.60E+24
1.20E+07
8.30E+13
2.00E+13
1.40E+14
1.00E+12
1.00E+13
4.80E+13
ᵝ
-2.61
2.1
0
0
0
0
0
0
Ea(K)
44800
350
3700
3723.8
9700
0
0
38800
Figures 2 and 3 show the species profiles as a
function of the power input to the PSR; for
mechanisms 1 and 2, respectively. The inlet stream
consists of pure H2S at a flow rate of 0.5 l/sec, a
temperature of 300K, and a pressure of 1 atm. The
power input was varied from 0-2000W; which is the
range of power that can be provided by the
aforementioned plasma torch.
It can be seen that both mechanisms predict very
similar trends in the gas mixture temperature and
species composition. However, mechanism 1
predicts a lower breakdown power (600W) as
compared to the second mechanism (1000W); which
is due to the difference in the kinetic parameters that
determine the initial H2S dissociation reaction rate.
The sulfur (S2) and hydrogen gas yields were
identical.
both mechanisms. It can be seen that both kinetic
models predict almost identical profiles for the
common species. However, an obvious depression in
the temperature profile associated with mechanism 2
is noticed at a distance of 7 cm from the reactor
inlet. This is due to some thermal energy input being
used for H2S thermal dissociation instead of
temperature increase of the gas mixture.
Figure 2. Mechanism 1- species and temperature profiles (PSR)
Figure 5. Mechanism 1-species and temperature profiles (PFR)
Figure 3. Mechanism 2- species and temperature profiles (PSR)
The PFR model
The PFR provides a better representation of the
decomposition that occurs in the MW plasma tube.
Microwave power is released into the plasma tube
only across the wave guide cross-sectional area. The
simulated reactor has an inner diameter of 25.4 cm, a
length of 30 cm, and operates at a pressure of 1 atm.
The reactant inlet consists of pure H2s at a flow rate
of 0.5 l/sec and a temperature of 300K. To represent
conditions of figure 1, the power is input at a linear
rate of 200 W/cm from the 5 to 15 cm length along
the plasma tube.
Figures 5 and 6 represent the spatial distribution of
the major species profiles and temperature as a
function of the axial distance along the reactor for
Figure 6. Mechanism 2-species and temperature profiles (PFR)
The plasma thermal quenching effect on the reverse
reactions that might destroy the hydrogen and sulfur
products were simulated by adding a heat removal
section on the rest of the plasma tube from 15 to 30
cm at a heat removal rate of 100 W/cm.
distance along the reactor. This is attributed to the
energy consumed by the initial H2S dissociation
reaction.
References
[1] J.B. Harkness, R.D. Doctor & E.J. Daniels,
"Plasma-chemical waste treatment of acid gases,"
Argonne National Laboratory Report, 1993.
Figure 8. Mechanism 1-species and temperature profiles (PFR)
[2] G. Zhao, S. John & J.Zhang, "Production of
hydrogen and sulfur from the hydrogen sulfide in a
nonthermal-plasma pulsed corona discharge
reactor," Chemical Eng. Science, vol. 62, pp. 2216–
2227, 2007.
[3] S. John, J.C. Hamann & S.S. Muknahallipatna,
"Energy
efficiency
of
hydrogen
sulfide
decomposition in a pulsed corona discharge reactor,"
Chemical Eng. Science, vol. 64, pp. 4826–4834,
2009.
[4] K.R. Gutsol, A. Rabinovich & A.Fridman,
"Dissociation of H2S in non-equilibrium gliding arc
tornado discharge," Int. J. Hydrogen Energy, vol. 34,
pp. 7618–7625, 2009.
Figure 8. Mechanism 2-species and temperature profiles (PFR)
Figures 8 and 9 show that the initial effect of cooling
brought the H2 and S2 products concentrations up
until they reach a maximum at an optimum
temperature of about 2300K similar to that obtained
with chemical equilibrium calculations. Further
cooling starts to reform the original reactant gas
H2S. This plasma thermal quenching analysis is
very important to help design the optimal heat
exchanger that would save the hydrogen and sulfur
products.
Conclusion
This paper compares two chemical kinetics
mechanisms for the thermal dissociation of pure
H2S gas into hydrogen and sulfur using a
microwave plasma torch. The simulation results
show that both mechanisms predict similar product
yields. Some discrepancy is only observed in the
temperature profile of the PFR as a function of
[5] V. Dalaine, J.M. Cormier, S.Pellerin &
P.Lefaucheux, "H2S destruction in 50 hz and 25 khz
gliding arc reactors," J. Applied Physics, vol. 84, no.
3, pp. 1215–1221, 1998.
[6] I. Traus, H. Suhr & J.E. Harry, "Application of a
rotating high-pressure glow discharge for the
dissociation of hydrogen sulfide," Plasma Chemistry
and Plasma Processing, vol. 13, 1993
[7] K. Hadidi and P. Woskov, "Efficient, modular
microwave plasma torch for thermal treatment,"
Plasma Science and Fusion Center Report, MIT.
[8] Alexander Friedman, Plasma chemistry.
Cambridge University Press, 2009, pp. 744–745.
[9] V. Kaloidas & Papyannakos, "Kinetics of
thermal, non-catalytic decomposition of hydrogen
sulfide," Chemical Eng. Science, vol. 44, no. 11, pp.
2493–2500, 1989.
[10] T.G. Beuthe & J.S. Chang, "Chemical kinetics
modelling of non-equilibrium Ar-H2 thermal
plasmas," Japanese J. Applied Physics, vol. 38, pp.
4576–4580, Apr.1999.