Sulfur Recovery from Acid Gas Using the Claus Process and High Temperature Air Combustion (HiTAC) Technology

Abstract

Sulfur-bearing compounds are very detrimental to the environment and to industrial process equipment. They are often obtained or formed as a by-product of separation and thermal processing of fuels containing sulfur, such as coal, crude oil and natural gas. The two sulfur compounds, which need special attention, are: hydrogen sulfide (H2S) and sulfur dioxide (SO2). H2S is a highly corrosive gas with a foul smell. SO2 is a toxic gas responsible for acid rain formation and equipment corrosion. Various methods of reducing pollutants containing sulfur are described in this paper, with a focus on the modified Claus process, enhanced by the use of High Temperature Air Combustion (HiTAC) technology in the Claus furnace. The Claus process has been known and used in the industry for over 100 years. It involves thermal oxidation of hydrogen sulfide and its reaction with sulfur dioxide to form sulfur and water vapor. This process is equilibrium-limited and usually achieves efficiencies in the range of 94-97%, which have been regarded as acceptable in the past years. Nowadays strict air pollution regulations regarding hydrogen sulfide and sulfur dioxide emissions call for nearly 100% efficiency, which can only be achieved with process modifications. High temperature air combustion technology or otherwise called flameless (or colorless) combustion is proposed here for application in Claus furnaces, especially those employing lean acid gas streams, which cannot be burned without the use of auxiliary fuel or oxygen enrichment under standard conditions. With the use of HiTAC it has been shown, however, that fuel-lean, Low Calorific Value (LCV) fuels can be burned with very uniform thermal fields without the need for fuel enrichment or oxygen addition. The uniform temperature distribution favors clean and efficient burning with an additional advantage of significant reduction of NOx, CO and hydrocarbon emission.

Full text

American Journal of Environmental Sciences 4 (5): 502-511, 2008
ISSN 1553-345X
© 2008 Science Publications
Corresponding Author: Mohamed Sassi, Department of Chemical and Mechanical Engineering, The Petroleum Institute,
P.O. Box 2533, Abu Dhabi, UAE 502
Sulfur Recovery from Acid Gas Using the Claus Process and
High Temperature Air Combustion (HiTAC) Technology
1Mohamed Sassi and 2Ashwani K. Gupta
1Department of Chemical and Mechanical Engineering,
The Petroleum Institute, P.O. Box 2533, Abu Dhabi, UAE
2Department of Mechanical Engineering, University of Maryland,
College Park, MD 20742, USA
Abstract: Sulfur-bearing compounds are very detrimental to the environment and to industrial process
equipment. They are often obtained or formed as a by-product of separation and thermal processing of
fuels containing sulfur, such as coal, crude oil and natural gas. The two sulfur compounds, which need
special attention, are: hydrogen sulfide (H2S) and sulfur dioxide (SO2). H2S is a highly corrosive gas
with a foul smell. SO2 is a toxic gas responsible for acid rain formation and equipment corrosion.
Various methods of reducing pollutants containing sulfur are described in this paper, with a focus on
the modified Claus process, enhanced by the use of High Temperature Air Combustion (HiTAC)
technology in the Claus furnace. The Claus process has been known and used in the industry for over
100 years. It involves thermal oxidation of hydrogen sulfide and its reaction with sulfur dioxide to
form sulfur and water vapor. This process is equilibrium-limited and usually achieves efficiencies in
the range of 94-97%, which have been regarded as acceptable in the past years. Nowadays strict air
pollution regulations regarding hydrogen sulfide and sulfur dioxide emissions call for nearly 100%
efficiency, which can only be achieved with process modifications. High temperature air combustion
technology or otherwise called flameless (or colorless) combustion is proposed here for application in
Claus furnaces, especially those employing lean acid gas streams, which cannot be burned without the
use of auxiliary fuel or oxygen enrichment under standard conditions. With the use of HiTAC it has
been shown, however, that fuel-lean, Low Calorific Value (LCV) fuels can be burned with very
uniform thermal fields without the need for fuel enrichment or oxygen addition. The uniform
temperature distribution favors clean and efficient burning with an additional advantage of significant
reduction of NOx, CO and hydrocarbon emission.
Key words: Sulfur recovery, acid gas, Claus process, flameless or colorless combustion
INTRODUCTION
Hydrogen sulfide is present in numerous gaseous
waste streams from natural gas plants, oil refineries,
wastewater treatment, among other processes. These
streams usually also contain carbon dioxide, water-
vapor, trace quantities of hydrocarbons, sulfur and
ammonia. Waste gases with ammonia are called sour
gases, while those without ammonia are called acid
gases. Sulfur must be recovered from these waste
streams before flaring them.
Sulfur recovery from sour or acid gas typically
involves application of the famous Claus process[1]
using the reaction between hydrogen sulfide and sulfur
dioxide (produced in the Claus process furnace from the
combustion of H2S with air and/or oxygen) yielding
elemental sulfur and water vapor:
2H2S(g) + SO2(g)  (3/n) Sn(g) + 2H2O(g)
with
Hr = −108 kJ moL−1
Therefore, higher conversions for this exothermic,
equilibrium-limited reaction call for low temperatures
which lead to low reaction rates, imposing the use of a
catalyst. The catalytic conversion is usually carried out
in a multi-stage fixed-bed adsorptive reactors process,
to counteract the severe equilibrium limitations at high
conversions. This technology process can possibly
provide about 96-97% conversion of the influent sulfur
in H2S to S. However, higher removal requires critical
examination of the process and use of near isothermal
reactor since the conversion is critically dependent

Am. J. Environ. Sci., 4 (5): 502-511, 2008
503
upon exothermic and endothermic conditions of the
reactions. Flameless combustion has been shown to
provide uniform thermal field in the reactor so that the
reactor temperature is near uniform[2,3,4]. In addition it
has been shown to result in compact size of the reactor,
reduce combustion generated pollutants emission up to
50% and increase energy efficiency up to 30%[5]. The
application of this technology appears to offer great
advantages for the process under consideration.
The UAE, which pumps about 2.4 million bpd of
crude oil, is also home of the world’s fifth biggest gas
reserves at about 200 trillion cubic feet. Abu Dhabi Gas
Industries (GASCO), an operating company of the Abu
Dhabi National Oil Company (ADNOC), is leading a
drive to boost gas production in the UAE from five to
seven billion cubic feet per day. This calls for sulfur
recovery capacity of over 3000 metric tons per day with
the associated SOx and NOx emissions. Therefore, the
adoption and further development of flameless
combustion technology for sulfur recovery among other
commercial and industrial heating processes is expected
to be very crucial and beneficial, both economically and
environmentally.
The conventional Sulfur recovery process is based
upon the withdrawal of sulfur by in-situ condensation
within the reactor. The selective removal of water
should, however, be a far more effective technique as
its effect on the equilibrium composition in the mass
action equation is much greater. The in-situ
combination of the heterogeneously catalyzed Claus
reaction and an adsorptive water separation seems
especially promising, as both reaction and adsorption
exhibit similar kinetics and pressure can be adapted to
the needs of the adsorptive separation. Such an
adsorptive reactor will lead to almost complete
conversion as long as the adsorption capacity is not
exhausted. There are numerous possibilities for
implementing these two functionalities, ranging from
fixed-beds with homogeneous catalyst/adsorbent
mixtures to spatially structured distributions or even
fluidized beds.
For the sulfur recovery process most of the
previous studies have concentrated on the Claus
catalytic conversion reactors and the Tail Gas
Treatment Unit TGTU[6]. However, some previous
studies have identified the Claus furnace as one of the
most important yet least understood parts of the
modified Claus process[7]. The furnace is where the
combustion reaction and the initial sulfur conversion
(through an endothermic gaseous reaction) take place
and also where the SO2 required by the downstream
catalytic stages is produced and the contaminants (such
as ammonia and BTX (benzene, toluene, xylene) are
supposedly destroyed. The main two reactions in the
Claus furnace are:
H2S + 3/2O2  SO2 + H2O (1)
with
Hr = −518 kJ moL−1
2H2S + SO2  3/2S2 + 2H2O (2)
with
Hr = +47 kJ moL−1
This last endothermic reaction is responsible for up
to 67% conversion of the sulfur at about 12000C.
Moreover, many side reactions take place in the
furnace, which reduce sulfur recovery and/or produce
unwanted components that end up as ambient pollutant
emissions. Therefore, it would be useful to combine the
endothermic and exothermic process using an
isothermal reactor offered by the flameless (or
colorless) oxidation combustion.
A vast majority (about 92%) of the 8 million metric
tons of sulfur produced in the United States in 2005 was
recovered from industrial by-products using the Claus
process[8]. However, the traditional Claus process does
face limitations and various process improvements have
been investigated in order to satisfy the increasingly
stringent emission regulations and the need to process
gas streams and fuels with higher sulfur content. New
technologies have to be developed in order to achieve
100% removal of sulfur compounds from industrial flue
gases. The Claus process and its various derivatives and
improvements are described here for treatment of H2S
containing streams. The use of HiTAC technology as a
reliable and cost-effective alternative for improvement
of lean acid gas treatment in the Claus process is
proposed and described. Finally, the flameless or
colorless combustion is proposed and described for
processing acid-rich gas.
The Traditional Sulfur Recovery Process: The three
main steps of sulfur recovery from sour gas are
described below.
Amine Extraction: Gas containing H2S is passed
through an absorber containing an amine solution
(Monoethanolamine (MEA), Diethanolamine (DEA),
Methyldiethanolamine (MDEA), Diisopropylamine
(DIPA), or Diglycolamine (DGA)), where the hydrogen
sulfide is absorbed along with carbon dioxide. A typical
amine gas treating process, shown in Fig. 1, includes an
absorber unit and a regenerator unit as well as

Am. J. Environ. Sci., 4 (5): 502-511, 2008
504
Fig. 1: Flow diagram of a typical amine treating process
used in industrial plants[9]
accessory equipment[9]. In the absorber, the down-
flowing amine solution absorbs H2S and CO2 (referred
to as acid gases) from the up-flowing sour gas to
produce a sweetened gas stream (i.e., an H2S-free gas)
as a product and an amine solution rich in the absorbed
acid gases. The resultant rich amine is then routed into
the regenerator (a stripper with a re-boiler) to produce
regenerated or lean amine that is recycled for reuse in
the absorber. The stripped overhead gas from the
regenerator is concentrated H2S and CO2. The extracted
mixture of H2S and CO2, referred to as an acid gas, is
passed into the Claus unit for sulfur recovery. The
process is also known as Gas sweetening and Acid gas
removal. Amines are also used in many oil refineries to
remove acid gases from liquid hydrocarbons such as
Liquefied Petroleum Gas (LPG).
Claus Thermal Stage: H2S is partially oxidized with
air (one-third of H2S is converted into SO2) in the Claus
furnace. The acid gas/air mixture is passed into a
furnace operating at temperatures from 1300-1700 K,
where the reactions are allowed sufficient time to reach
equilibrium. The products from this step are: sulfur
dioxide, water and unreacted hydrogen sulfide.
Additionally some of the sulfur dioxide produced here
reacts with hydrogen sulfide inside the furnace to
produce sulfur according to reactions 1 and 2 shown
earlier. The furnace products flow then into a waste
heat boiler to condense the sulfur and produce high
pressure steam for the Claus catalytic stages.
Depending on the calorific value of the acid gas,
various methods of stable burning are achieved. If very
lean acid gases are involved (low calorific value) then
auxiliary fuel, oxygen enrichment or a by-pass stream
has to be used. The H2S-content and the concentration
of other combustible components (hydrocarbons or
ammonia) determine the location where the feed gas is
burned. Claus gases (acid gas) with no further
combustible contents apart from H2S are burned in
lances surrounding a central muffle. Gases containing
ammonia, such as the gas from the refinery's Sour
Water Stripper (SWS) or hydrocarbons are converted in
the burner muffle.
Claus Catalytic Stage: The remaining H2S, from the
Claus furnace, is reacted with the SO2 at lower
temperatures (about 470-620 K) over an alumina- or
titanium dioxide-based catalyst to make more sulfur:
2H2S + SO2  3/8S8 + 2H2O (3)
Hr = −108 kJ moL−1
On average, about 70% of H2S and SO2 will react
via reaction (3). Note that in the catalytic stage mostly
S8 is produced, which is an exothermic reaction
whereas in the thermal stage S2 is the major product and
the reaction is endothermic. Other allotropes of sulfur
may also be present in smaller quantities.
The overall reaction for the entire process is:
3H2S + 1.5O2  3/nSn + 3H2O (4)
Hr = −626 kJ moL−1
Reactions 1 and 3 are exothermic and a cooling
stage is needed following these steps in order to
condense the sulfur produced. The condensed phase is
then separated from the gas stream by draining it into a
container. An interesting property of liquid sulfur is its
increase in viscosity with temperature[1]. This is due to
polymerization of sulfur at around 430 K. Therefore,
the temperature of condensed sulfur should be closely
monitored to prevent polymerization and clogging of
pipes used in the process. Care must also be taken in
order not to pass condensed sulfur through the catalyst,
which would become fouled and inefficient. Liquid
sulfur adsorbs to the pores and deactivates the catalytic
surface. Therefore reheat stages using the previously
generated steam are needed in order to keep the sulfur
in gas phase while in the catalytic stage. Several
methods of reheating used in industry are:

Am. J. Environ. Sci., 4 (5): 502-511, 2008
505
Fig. 2: Flow diagram of a typical Claus process
Hot-Gas Bypass: involves mixing the two process gas
streams from the process gas cooler (cold gas) and the
bypass (hot gas) from the first pass of the waste heat
boiler.
Indirect Steam Reheaters: the gas can also be heated
with high pressure steam in a heat exchanger.
Gas/Gas Exchangers: whereby the cooled gas from
the process gas cooler is indirectly heated from the hot
gas coming out of an upstream catalytic reactor in a
gas-to-gas exchanger.
Direct-fired Heaters: fired reheaters utilizing acid gas
or fuel gas, which is burned substoichiometrically to
avoid oxygen breakthrough and damage to Claus
catalyst.
A typical Claus process involves one thermal stage
followed by multiple catalytic stages in series to
maximize efficiency. The need for multiple catalytic
stages increases complexity and cost. Therefore,
various methods of minimizing these steps in the
process have been proposed.
A schematic of the process flow diagram along
with approximate gas temperatures is shown in Fig. 2.
High-pressure steam (40 atm) is generated in the boiler
stage and low-pressure steam (3-4 atm) is produced in
the condensers. A total of two to four catalytic stages
are typically used in order to maximize efficiency. The
tail gas is either routed to a clean-up unit or to a thermal
oxidizer to incinerate the remaining sulfur compounds
into SO2. Where an incineration or tail-gas treatment
unit (TGTU) is added downstream of the Claus plant,
only two catalytic stages are usually installed. Before
storage and downstream processing, liquid sulfur
streams from the process gas cooler, the sulfur
condensers and from the final sulfur separator are
routed to the degassing unit, where the gases (primarily
H2S) dissolved in the sulfur are removed. Over 2.6 tons
of steam will be generated for each ton of sulfur yield.
The Claus process is equilibrium-limited. In the
furnace stage the SO2 produced from the combustion
process (reaction 1) recombines with H2S in an
endothermic reaction to form S2 (reaction 2). Adequate
residence time has to be provided in order to allow this
reaction, responsible for 60-70% of sulfur conversion,
to reach equilibrium[10]. Since the main Claus reaction 3
is exothermic, this stage calls for the use of low
temperatures in order to shift the equilibrium constant
towards higher product yields. The low temperatures,
however, lead to decreased reaction rates, hence the
need for a catalyst. The law of mass action for the Claus
reaction is as follows:
2 8
2 2
2 3/ 8
H O S
p2H S SO
p p
K (T) p p

(5)
Where, Kp(T) is the chemical equilibrium constant
and pH2O , pS8 are partial pressures of the products and
pH2S , pSO2 and partial pressures of the reactants.
This equation illustrates the nature of equilibrium
limitations involved in the Claus process; decreasing
the process temperature can increase the equilibrium
constant and thus increase conversion, but the lower
limit of this temperature and hence the upper limit of
equilibrium conversion is set by the condensation
temperature of sulfur. A typical arrangement for the
Claus sulfur recovery process is shown in Fig. 3.
Acid gas
Air
Burner Furnace Boiler Condenser
Liquid
sulfur
Re-heater Catalyst Condenser Tail
gas
T [K]
Liquid
sulfur
300 K
1500 K
650 K 450 K 500 K 600 K 450 K
2000 K

Am. J. Environ. Sci., 4 (5): 502-511, 2008
506
Fig. 3: Typical arrangement of a Claus unit[11]
Fig. 4: Hydrogen sulfide conversion as a function of
time[6]
Improvements on Claus Process: The traditional
Claus process has been a reliable and relatively efficient
way of removing hydrogen sulfide from the flue gas
and converting it into elemental sulfur. It has, however,
faced some shortcomings and limitations. Increasingly
stringent air pollution regulations from oil, gas and
chemical processing facilities combined with the fact
that lower-grade, higher sulfur-content fuels will have
to be used in the near future, call for improved
efficiency of the process.
Elsner, et al.[6] proposed an adsorptive water
separation process applied in the catalytic reactor stage.
Taking advantage of Le Chatelier’s principle, this
process removes H2O (one of the products) from the
reaction, shifting equilibrium towards higher
conversion (Eq. 5). An adsorptive reactor of this type
could produce complete conversion in a single catalytic
stage.
The Zeolite adsorbent beads saturate with water
after a certain time and therefore need to be
regenerated. This calls for a cyclic process where the
flow of gas is reversed and hot gas is used to vaporize
the adsorbed water off of the surface of Zeolite spheres
and remove them from the reactor. The process can
then be reversed again to regenerate the second
adsorptive reactor (Fig. 4).
Fig. 5: Cold bed adsorption process diagram[12]
Figure 4 shows that 100% conversion can be
achieved in the reactor for a longer time than in a
conventional Claus reactor with no water adsorption.
The decline in conversion efficiency after a period of
about 1.3 hrs is due to the fact that the Zeolite spheres
are saturated with steam and they need to be
regenerated. It was also found that as a side effect of the
water adsorption, the chemisorption of SO2 on the
surface of the alumina catalyst occurs.
A Cold Bed Adsorption (CBA) process, also
known as the sub-dew point process developed by the
Amoco Corporation has been shown to produce
efficiencies in the range of 97.5-99.5%[12]. In the CBA
process the heterogeneous catalytic reaction is allowed
to take place at low temperatures (below sulfur dew
point), thus increasing equilibrium conversion.
Additionally since the Claus reaction occurs in the gas
phase, this liquid sulfur does not inhibit the reaction
like sulfur vapor does, effectively removing one of the
reaction products to result in a favorable shift in the
reaction equilibrium and higher sulfur conversion. The
condensed phase is then periodically desorbed from the
catalytic surface by flowing hot gas through the unit to
vaporize the condensate, thus regenerating the reactor.
Therefore, this process is inherently a cyclic one.
There are normally two or more CBA reactors in
series so that at least one can be operating sub-dew
point while the other is being regenerated, Fig. 5. Due
to the cyclic nature of the CBA process, the CBA
switching valves are subjected to very demanding
sulfur vapor service that has caused significant
operation and maintenance problems in many of the
CBA plants designed by others. Sulfur recoveries in
excess of 99.5% have been achieved with the Modified
Claus process with tail gas cleanup developed by

Am. J. Environ. Sci., 4 (5): 502-511, 2008
507
Fig. 6: Calculated hydrogen sulfide conversion as a
function of reactor temperature for different
oxygen concentrations[16]
Fig. 7: Calculated hydrogen sulfide conversion as a
function of reactor temperature for different
water concentrations[16]
Ortloff[13]. In this process the sulfur-bearing compounds
(COS, CS2, SO2, Sn) in the tail gas are converted to H2S
using hydrolysis and hydrogenation and recycled back
into the Claus unit. Amine-based tail-gas cleanup is
also used to recover the remaining hydrogen sulfide in
the tail gas.
The Modified Claus Process with Tail Gas Cleanup
Unit (TGCU) is used when very high sulfur recovery is
necessary, such as for sulfur plants in petroleum
refineries in the U.S. The U.S. EPA regulations
normally require that the incinerated effluent from
refinery sulfur plants contain no more than 250 ppmv
SO2 on a dry, oxygen-free basis. This usually
corresponds to an overall sulfur recovery of 99.8-
99.9%. The problem with any TGCU is that it usually
costs as much as the whole Claus plant while it adds
only about 2% in the total sulfur recovery.
Lagas, et al.[14] describe a selective oxidation process,
in which the tail gas is selectively oxidized in the
presence of active metal oxides to produce sulfur
Fig. 8: Calculated concentrations of sulfur species as a
function of temperature[16]
and small quantities of SO2. Total sulfur recovery of
99% has been achieved this way (99.4% with an
additional hydrogenation step).
Oxygen enrichment technologies have been
proposed to increase sulfur recovery, throughput of the
system and decrease the size of the unit by reducing the
amount of inert nitrogen from the process[15,16]. The
resultant high flame temperatures have to be dealt with
using techniques such as staged combustion and water
spraying because of material limitations. The increased
complexity of the system is offset by the fact that better
mixing, higher reaction rates, conversion and
throughput for a given size of the unit are achieved.
Figure 6 suggests that it is desirable to remove
water from the reaction furnace during the process. As
water is one of the products of the reaction, its removal
will lead to the shift in equilibrium towards the product
side and hence more conversion is achieved.
The removal of nitrogen and introduction of
oxygen into the process has many effects. First,
removal of the diluent nitrogen results in the increased
partial pressure of each of the reacting species; second,
the reduced volume of reacting gases is easier to mix;
and, third, higher temperatures can be obtained. All
three increases in the process rate (Fig. 8).
The use of a gas recycling process has been
proposed by the CNG group[17]. The effluent gas from
the first condenser was recycled back into the burner to
attain overall sulfur recovery of 100%. However,
intermediate stages had to be used to remove water
vapor and nitrogen from the recycled gas to achieve
efficient conversion and stable flame regime. A
separator membrane can typically be used to separate
nitrogen out of the stream. However, if pure oxygen is
used in the combustion process, the membrane is not
necessary and only water condensation is needed before
the tail gas can be recycled back into the unit.

Am. J. Environ. Sci., 4 (5): 502-511, 2008
508
Fig. 9: Claus process with recycling[18]
Fig. 10: Adiabatic flame temperature calculation with
and without tail gas recycling as a function of
N2/O2 ratio[18]
The heat recovery for this process is increased,
since the water condensation heat can also be extracted
out of the stream. In a recent work, El-Bishtawi, et
al.[18] described a Claus recycle with double combustion
process (Fig. 9). The acid gas was partially combusted
in the first furnace and the hot exhaust was passed into
the second furnace where the remainder of oxygen was
added to complete the reaction. The second furnace
operated at a high temperature air combustion regime,
since the inlet gas was above its auto-ignition
temperature.
One sulfur condenser was used following the two
furnaces. Part of the effluent gas was recycled back into
the first furnace. It was reported that 100% conversion
could be achieved without the use of catalytic reactors
and with only one condenser. Such an arrangement
should reduce the cost and complexity of the system by
removing the catalytic stages. It was also found that the
oxygen content should not exceed 78% in order not to
exceed the maximum temperature limitations of the
equipment materials.
Fig. 11: Schematic of the reciprocal flow burner[20]
Claus Process with HiTAC: In the case of lean acid
gas feeds (<15% H2S) special considerations have to be
taken in order to maintain a stable flame in the burner
and achieve good combustion efficiency. Common
approaches include: oxygen enrichment, a split-flow
process and use of auxiliary fuel[15]. In the case of
oxygen enrichment the flame temperature is increased
by removing part or all of inert nitrogen from air, thus
decreasing the thermal loading of the system. In the
split-flow process part of the acid gas is allowed to
bypass the burner, which leaves adequate fuel/air
proportions in the burner and higher flame temperature.
The by-pass flow is then reintroduced into the furnace
at a later stage in order to keep the H2S:SO2 ratio 2:1
(Eq. 2 and 3). With the use of auxiliary fuel the
calorific value of the gas is increased. Stable flame of a
higher temperature is therefore possible.
Paskall[19] collected a substantial amount of field
data and reviewed the literature data on sulfur
conversion in Claus furnaces and recommended that
sulfur conversions are greater in furnaces that are
designed for greater gas mixing and turbulence and
equipped with burners that provide for good mixing of
the feed gas and oxidizer and in furnaces of smaller
volume. HiTAC or Flameless or colorless combustion
furnaces can achieve all of these recommendations and
beyond, providing the highest sulfur recovery.
Furthermore, Khudenko et al.[10] through several
thermodynamic and process simulation scenarios
showed that a dual thermal stage system with cold
products recycle (very similar to flameless concept)
provides the greatest capacity reserve. They claimed
that, with the dual stage system, no changes in the
existing process train are required, even when the
throughput capacity of the existing conventional system
is more than doubled.

Am. J. Environ. Sci., 4 (5): 502-511, 2008
509
Fig. 12: Proposed Claus System With High Temperature Air Combustion
Economically this is very wise and attractive for
increasing sour gas production in the oil and gas
industry due to the exploitation of aging reservoirs. A
reciprocal flow filtration combustor with embedded
heat exchangers for super-adiabatic combustion has
been proposed and studied by the Gas Technology
Institute (GTI) and the University of Illinois at
Chicago[20] (Fig. 11). The motion of the flame zone to
the downstream of the reactant gas mixture results in
positive enthalpy flux to the cold gas and thus
increasing the reactant temperature prior to combustion.
This is similar to the principles of HiTAC. A prototype
was build and tested for sulfur recovery at GTI. The
results showed that the super-adiabatic combustion
(which is very similar to flameless or colorless
combustion in principle, but taking place in a non-
catalytic porous medium) significantly extends
conventional flammability limits to the region of the
ultra-low heat content mixtures (such as lean acid gas)
and features ultra low emissions for NOx and CO.
Therefore High Temperature Air Combustion
(HiTAC) technology is proposed here as an alternative
treatment of lean to very lean (<15% H2S) Low
Calorific Value (LCV) acid gases. While a stable
conventional flame is usually not achievable in this
regime, HiTAC provides very lean homogeneous
thermal field uniformity flames[5,21-24]. Moreover,
uniform thermal characteristics with high and uniform
heat flux distribution in the combustion chamber are
achievable. This produces good overall conversion, low
emissions and uniform heat loading of the equipment,
which reduces mechanical stresses.
In fact, it has been reported that HiTAC technology
has shown significant reduction in pollutants emissions
(about 50%), reduction in the size of the combustion
chamber (about 25%), reduced thermal losses to the
environment and significant energy savings (about
30%)[5,23,24]. High temperature air combustion is
especially useful for reducing NOx emissions due to its
uniform thermal field and overall lower operating
temperature and no adiabatic flame with hot spots that
are responsible for thermal NOx formation. With the
use of HiTAC the need for by-pass feed stream, oxygen
enrichment and multiple furnaces could be eliminated
as the lean acid gas could be oxidized in a single
furnace operating above the auto-ignition temperature,
with good conversion.
As far as practical considerations are concerned,
the Claus process is well suited for the use of HiTAC
technology, as steam generated in the waste heat boiler
as well as the condensers is readily available to preheat
the incoming air stream in a heat exchanger, (Fig. 12).
In High Temperature Air Combustion, the air is brought
to above the auto-ignition temperature of the fuel to
obtain uniform ignition and combustion characteristics
across the reactor. The reported auto-ignition
temperature of hydrogen sulfide (563 K or 290ºC) is
lower than a typical auto-ignition temperature for
hydrocarbon fuels (400-600 ºC) and therefore requires
less energy extraction from the high-pressure steam to
achieve ignition and sustained combustion[17]. During
the transient start-up period, preheating with an
electrical heater or auxiliary fuel can be used after
which self-sustained operation at steady-state
Acid gas
Hot Air
Furnace
Cooling
water
Waste
Heat
Boiler
HP steam Reheater
Condenser
LP
steam
Reheater
Condenser
Catalyst Catalyst
Tail Gas
S8
Heat
Exchanger
Air

Am. J. Environ. Sci., 4 (5): 502-511, 2008
510
conditions can be maintained. Issues of air/fuel mixing,
flame characteristics, such as temperature, size and
flammability limits, that are relevant for the Claus
process, must first be investigated. The resultant
uniform thermal field in the flameless combustor plus
gas recycling is expected to produce close to 100%
conversion.
For rich acid gas oxidation, flammability limits and
flame stability are not an issue due to the high calorific
value of the gas. However, thermal field uniformity
offered by flameless or colorless combustion would
always promote better conversion and lower pollutant
emissions, among other benefits as mentioned above.
Furthermore the super-adiabatic flame studies,
discussed earlier, featured that fuel rich (much more
than stoichiometric H2S to oxygen ratio) conditions
promote H2S conversion to H2 and S2 rather than H2O
and SO2. Their numerical results showed that at a
super-adiabatic temperature of about 1650K and an
equivalence ratio of about 10, an overall H2S
conversion of 50% resulted with an H2/H2O selectivity
of 57/43 and an S2/SO2 selectivity of 99/1. These
conditions, with even higher temperature, would be
easily attained under flameless combustion with H2S
recycling and pre-heating. This flameless combustion
assisted-thermal decomposition of H2S would then
eliminate any catalytic stage use and produce hydrogen
which is highly needed in fuel processing and power
production.
Of course thermal decomposition of H2S is a well
researched route for the production of hydrogen and
Cox et al.[25] presented a study on the economics of
thermal dissociation of H2S to produce hydrogen and
some studies are even at the pilot plant stage. However,
none of the early studies address the problem of heat
transfer. Due to the endothermic heat of reaction, heat
transfer limits the overall rate of reaction resulting in
low conversions. However, with flameless or colorless
combustion the H2S rich mixture reacts in a very hot
homogeneous medium with no heat transfer limitations
and therefore will present much higher conversions.
CONCLUSION
A review of the sulfur recovery process from acid
gases has been presented. The conventional modified
Claus process and its derivatives have been presented
and discussed. It is shown that all improvements
towards very high sulfur recovery induce very high cost
additions to an already economically deficient process.
HiTAC has been shown to feature great potential for an
almost complete sulfur recovery from lean acid gases.
The flameless or colorless combustion has been
proposed to be a very promising process for sulfur
recovery and hydrogen production from rich acid gases.
Therefore, these last two technologies feature the
potential for reducing the complexity and the cost of the
sulfur recovery process.
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