Visible Plume Abatement, GHG

Emissions and Energy

Conservation for Sulphur Plant
Stacks – A Balancing Act!
JOHN SAMES
Sulphur Experts Inc.
St. Peter, Barbados

AL WAKELIN
Energy Experts
Canada

Operation of Sulphur Recovery processes and their associated Tail Gas Incinerators is a
field in which technology advances and regulatory requirements have significantly increased
the complexity of operations and require a much more sophisticated approach than before.
Sulphur Recovery technology is capable of achieving extremely high efficiencies, however,
this efficiency comes at a price and the overall environmental performance must look at a
trade-off between sulphur emissions and other considerations such as energy efficiency and
greenhouse gas emissions. Most operating facilities have to adjust to new pressures to
maintain acceptable environmental performance and stakeholder expectations as these
facilities age.
Sulphur Plant Incinerators, when operated inefficiently, often produce a visible plume that
is both environmentally and aesthetically unacceptable. Vigilant attention to the control
parameters in the SRU and TGTU will minimise the SO2 sent to the stack from the thermal or
catalytic incinerator, thereby minimising the contribution of sulphur species to the plume. But
what else is in the plume? We face the dilemma of increasing CO2 emissions (GHG -
greenhouse gases) associated with any attempt to reduce SO2 emissions. This needs to be
considered seriously.
On the energy conservation side, reductions in fuel usage go hand-in-hand with
decreasing GHG emissions and there are many parameters that play a role from incinerator

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design, to residual oxygen, breech temperature, and consumption of the H2 and CO,
abundant in the tail gas.
The correctly operated, energy efficient sulphur plant incinerator stack will minimise the
use of fuel to provide the necessary thermal lift and effluent dispersion, minimise GHG
emissions and eliminate any sign of a visible plume.

INTRODUCTION
Sulphur Plants are required to achieve excellent performance in a number of areas, all of which can be
addressed through proper design and operation. Sulphur recovery technology selection plays a major role
in meeting the increasing demand for high sulphur recovery efficiency and reduced emissions of sulphur
dioxide. However, there is a significant trade-off between sulphur recovery efficiency and energy efficiency
associated primarily with the operation of amine-based
tail-gas units. This has a significant impact on greenhouse gas emissions associated with these facilities.
In addition, existing facilities are coming under increasing pressure to minimize overall emissions. When
operated inefficiently, tail gas incinerators can fail to meet prescribed emissions limits and may generate a
visible plume from the stack that is both environmentally and aesthetically unacceptable.
Incinerator design should address energy efficiency with the objective of minimizing emission of
greenhouse gases (GHGs). Energy conservation is primarily tied to reductions in fuel usage and this goes
hand-in-hand with decreasing GHG emissions. The routes to energy efficiency involve manipulation of
incinerator design, residual oxygen, breech temperature, and use of all combustible compounds in the tail
gas stream including hydrogen (H2) and carbon monoxide (CO).
Visible plumes from sulphur plant stacks manifest themselves in many colours and each is influenced by a
different set of incinerator conditions. Sulphur trioxide (SO3), even at very low concentrations, produces a
characteristic bluish-white plume, while oxides of nitrogen may result in an orangey-brown plume. Both of
these species are formed at high temperatures with high excess air, conditions often needed to promote
the oxidation of sulphur species as well as the combustion of H2 and CO.
Incinerator optimisation must be undertaken with due regard for the competing requirements and
demands. On the one hand, we face the dilemma of increasing carbon dioxide (CO2) emissions (GHG -
greenhouse gases) associated with any attempt to reduce emissions of reduced sulphur compounds. On
the other hand, it is possible to reduce energy consumption and GHG emissions by selecting appropriate
operating parameters such as incinerator temperature and excess air and by taking advantage of the
heating value found in H2 and CO in the tail gas.
A properly designed and operated tail-gas incinerator will meet multiple objectives, including:
 oxidation of TRS to SO2;
 provision of thermal lift to promote plume dispersion;
 minimisation of fuel consumption and GHG generation; and
 elimination of any sign of a visible plume.
IMPACT OF SULPHUR RECOVERY TECHNOLOGY ON EMISSIONS
Sulphur recovery technologies are capable of achieving very high sulphur recovery efficiencies. In
particular, facilities employing amine-based tail-gas units (e.g. SCOT plants) can meet the most stringent
emission requirements and are capable of reliably achieving sulphur recovery efficiencies in excess of
99.9%. In some jurisdictions, this technology is required to meet regulatory requirements or to ensure
acceptable ground-level concentrations for sulphur compounds in highly industrialised areas. However,
this sulphur recovery efficiency comes at a significant cost in terms of energy efficiency and the associated
greenhouse gas emissions.
For new facilities, it will be important to weigh any competing emission requirements in the initial selection
of the sulphur recovery technology. In one optimisation evaluation for a 1500 tonne per day sulphur plant
undertaken by Sulphur Experts, it was found that an improvement of 2% in recovery efficiency, reducing
SO2 emissions by almost 60 tonnes per day, by installing an amine-based tail gas clean-up unit on the

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 Visible Plume Abatement, GHG Emissions and Energy Conservation for Sulphur Plant Stacks

SRU increased GHG emissions of CO2 by 170 tonnes per day. The impact on CO2 emissions associated
with increasing sulphur recovery efficiency using current technology is illustrated in Figure 1. This is a good
example of the need for an educated dialogue to establish the relative environmental impacts
commensurate with reduced SO2 emissions and increased GHG emissions.
In most cases however, operators are dealing with installed facilities and are forced to address
environmental issues within the constraints of their existing infrastructure. The remainder of this paper
focuses on some of the more common challenges faced by existing facilities and identifies some of the
opportunities to address operations, regulator or stakeholder challenges, through optimisation of existing
equipment.
TAIL GAS INCINERATOR/STACK OPTIMISATION
Optimisation of tail-gas incinerators has historically focused on only a single objective - the proper
oxidation of reduced sulphur compounds such as H2S, COS and CS2 and their near complete conversion
to SO2. Under this limited approach, it was relatively easy to achieve this task by ensuring that a
combination of high operating temperatures, plentiful excess air and long residence time was provided.
In recent times it has become necessary to take fuel consumption into consideration during optimisation
and certainly more recently it has been necessary to address the impacts that incineration has on the
environment in the larger sense.
The demands of regulatory agencies and the public require that optimization addresses not only pollutants
of concern and the dispersion of these pollutants, but also plume visibility, GHG emissions, and energy
efficiency. These sometimes competing requirements have made the job of incinerator optimisation quite
complex and have also raised the issues of what should take priority.
Optimisation needs to be a shared responsibility among designers, plant operators, regulatory agencies
and the public.
In order to better understand the task at hand, the more significant requirements are briefly set forth below:
Regulators
 Lowest achievable levels of pollutants discharged from the stack. H2S and SO2 are often the
primary concerns but there are others;
 Ever lower levels of GHG emissions. This may be a defined mass emission or an intensity
measure.
Maintenance of acceptable ground level concentrations of specified emissions, particularly H2S and SO2

Sulphur 2012 International Conference (Berlin, Germany 28-31 October 2012) 145
 J. Sames, A. Wakelin

350.0
CO2 and SO2 Emissions for a 1500 Tonne/Day Sulphur
300.0 Inlet Loading
Emissions (Tonnes/day
260.5
250.0

CO2
200.0 Emissions

139.9
150.0

89.3 93.3 90.3

100.0
60.0
44.5
50.0 30.0
1.8 0.3
0.0
3 Stage Claus 2 Stage + Sub- 2 Stage + 2 Stage + 2 Stage +
(99.8.0%) dewpoint (98.5%) Selective Absorption Absorption
Oxidation (99.0%) (99.94%) (99.99%)

Fig. 1: Comparison of Emission Trade-offs associated with Existing SRU Technology

Plant operators
 Reasonable capital and operating costs.
 Availability of technology to measure and meet regulated emission limits.
 Clarity of regulations with reliance on objective measures. (Plume visibility is highly subjective -
influenced by the observer, meteorological conditions and distance from the source).
 Harmony within regulations. (GHG emissions tend to increase as efforts are made to reduce
ground-level H2S and SO2 emissions)
Public
 Invisible plumes.
 Absence of odours.
 Comfort with overall emission levels.
Achieving the balance
While it is impossible to achieve zero emissions from a tail-gas incinerator, there is much that can be done
to improve current performance.
In order to undertake the optimisation of a tail gas incinerator, it is necessary to understand the underlying
fundamentals associated with the principal emissions encountered and what can be done to mitigate
these. Table 1 provides an introduction to the issues and the species of concern.

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 Visible Plume Abatement, GHG Emissions and Energy Conservation for Sulphur Plant Stacks

Table 1
Emission-Related Issues and Contributing Factors
Issue Compounds of Contributing Factors Potential Solutions
Interest
Visible plume H2O Product of combustion Stack gas heating
NOx High flame temperature and high Lo-NOx burners, excess air
excess air control, fuel/air staging
SO3 High bulk temperature and high Minimise temperature and
excess air excess air
GHG emissions CO2 High incineration temperature, high Temperature and excess air
excess air optimization
MGLC1 SO2 Terrain and meteorological conditions Stack height
Stack exit temperature and velocity
Energy HC Higher than necessary temperature Temperature and excess air
Consumption and excess air optimization
Inadequate mixing Incinerator design, burner
design
H2 and CO slip Delayed quenching, thorough
mixing
Non-sulphur CO Majority is introduced via tail gas Sophisticated design,
pollutants additional technology
1
MGLC – Maximum ground-level concentration

Depending on the goals, there are complimentary actions that can be implemented in an optimisation
program.
Reductions in the incinerator bulk temperature and excess air will have a positive outcome in terms of
GHG reduction and are likely to reduce the concentrations of SO3 as well as NOx.
On the other hand these actions may result in significantly higher ground level concentrations of SO2 due
to poor thermal lift (plume buoyancy) and increased concentrations of reduced sulphur species in the
effluent.
Improvements in incinerator design and the use of forced draft high intensity burners can go a long way to
improving mixing, thereby allowing lower incinerator temperatures and lower excess air levels. This, in turn
will reduce fuel consumption and GHG emissions. Improvements in incinerator design are also essential in
taking advantage of the heating value of H2 and CO present in the tail gas (allowing a reduction in the
equivalent quantity of fuel gas used).
Incinerator optimisation must necessarily include a review of the upstream process and here, the
regulatory demand for higher sulphur recovery and lower SO2 emissions, imposes a significant penalty of
fuel requirements and associated GHG emissions.
Similarly, in Figure 2, we see that requirements to achieve very low levels of reduced sulphur compounds
such as H2S, COS and CS2 require higher incinerator operating temperatures and/or an increase in
excess air thereby necessitating a significant increase in fuel gas consumption and the corresponding
increase in GHG emissions.
Regulators in several jurisdictions have recently introduced requirements for CO emissions from tail gas
incinerators as displayed below.
CO limit (ppm)
UAE 500
Germany 100
Netherlands 50
California 10
Given that CO can be more than 1 mole % (10 000 ppm) in the tail gas (it is created in the reaction
furnace) and that currently, about 50% of the inlet CO to the incinerator can be burned in a modern tail gas
incinerator, there are perhaps only two options that can be pursued:

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 Application of incinerator technology changes to effectively force much higher operating

temperatures to consume CO as a fuel
 Application of upstream technology changes, such as hydrogenation to convert the CO before it
reaches the incinerator

Attainment of TRS Levels

90.0
TRS = H2S + COS + 2 x CS2 (ppmv) Residual
Oxygen
80.0
5.0%
Fuel Gas Consumption (103m3/d)

70.0 4.0%

3.0%
60.0
2.0%
TRS = 150
50.0 1.0%
TRS = 300
TRS = 600
40.0

30.0

20.0

10.0
300 350 400 450 500 550
Stack Exit Temperature (°C)
Fig. 2: Stack TRS Emission Levels Under Different Operating Scenarios

Maintaining high operating temperatures requires higher fuel consumption and generates higher GHG
emissions. In addition, it could require the installation of capital intensive heat recovery equipment. Clearly
less energy intensive methods of reducing CO emissions will be required but this will inevitably impact the
selection of SRU technology, which in turn may or may not impact SO2 emissions.
INCINERATION ESSENTIALS
Changes to the system require that those involved have adequate understanding of, and experience in, the
fields of combustion, chemistry, measurement and analysis, process control systems, modelling and
meteorology. The role of each is detailed below:
Combustion
The heart of any tail gas incinerator is a burner or burners that must provide the necessary mixing and
temperature to promote the combustion of reduced sulphur compounds and provide sufficient thermal lift to
achieve acceptable ground level concentrations of pollutants of concern under the worst-case
meteorological conditions that can be expected. Advances in burner technology have been dramatic in the
past 30 years but more is also required of a burner than was the case even a few years ago. Properly
designed and operated burners must now:
Provide complete combustion of the fuel,
Promote mixing of the burner effluent and the tail gas,

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Oxidise reduced sulphur compounds and combust tail gas “fuel” (hydrogen and CO),
Minimize the formation of NOX and SO3.
Older style (and still commonly employed) induced draft burners will not adequately address all of these
requirements, and if used will require that additional means be provided to overcome the deficiencies. As
an example, an induced draft burner may require the installation of a checker wall to promote mixing and
may require more excess air to achieve high combustion efficiencies.
Chemistry and reactions
Within an incinerator there exists a continuously variable mixture of chemical species and it is necessary to
understand the interactions among, tail gas source reagents, fuel, air and the products of combustion. Not
only are these products influenced by variations in the inlet tail gas composition but also by reactions that
are occurring as gas passes through the incinerator. Some, but not all, of the numerous reactions are well
understood in terms of thermodynamics and kinetics while others are barely understood at all. Over time
we have developed an improved understanding of these processes and can identify, within reasonable
bands, what the reaction products at the discharge of the incinerator are likely to be. It is also possible to
estimate which variables impact combustion products and the manner in which they do so. This
combination of chemistry and empirical observation has enabled a satisfactory projection of incinerator
performance over the range of operating conditions encountered in industry.
Measurement and analysis
Predicting and optimising incinerator performance necessarily starts with the complete characterisation of
the input stream such as tail gas, fuel and oxidising agents. It must also be remembered that streams
other than tail gas such as sour water stripper off-gas, pit vents and flash gases may be introduced into the
incinerator and these must be taken into consideration.
Process control systems
Process control schemes have become much more sophisticated and can provide consistent performance
over a wide range of input variables. Unfortunately this same level of sophistication has resulted in few
people being able to properly establish and troubleshoot control logic and even fewer understand how to
tune systems in light of changing operating conditions or regulatory requirements.
Modelling
Increasingly, process models are employed to predict outcomes and troubleshoot on-line systems. In
many cases available models only accept a limited number of input parameters and thus are not capable
of identifying the full mix of effluent products. This is especially evident when attempting to predict the fate
of compounds such as SO3 and NOx, species not historically thought to be an issue in tail gas incineration.
Modelling is also heavily relied upon to predict the ground-level concentration of the pollutants present in
the incinerator effluent. It is therefore necessary that the operator be cognisant of the impact changed
incinerator conditions might have on plume dispersion and plume visibility.
Meteorology
Since one important function of an incinerator is to disperse pollutants, site specific meteorological data
must be applied at the design, operating and optimising stages. Variation in meteorological conditions may
require different operating parameters and these requirements must be properly understood and
incorporated in the control of plume visibility.
Incinerator design
Older incinerator designs have deficiencies that were not apparent when these were put into operation but
these weaknesses are exposed when efforts are made to balance the competing requirements presented
above. Earlier designs that employed natural draft burners are, in many cases unable to achieve emission
parameters unless they are operated at very high temperatures and with significant excess air. This
imposes a significant fuel penalty compared to modern installations.
The availability and performance of high intensity burners has been instrumental in allowing improvement
in the performance of tail gas incinerators with particular impact on fuel consumption. High intensity

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burners not only achieve very high combustion efficiencies at low excess air levels but also result in a short
flame and a very turbulent effluent stream. As a result it is possible to induce rapid mixing with the tail gas
and virtually eliminate premature quenching of the burner flame. A further advance is staging the tail gas
introduction into the incinerator chamber which has proved to be an effective way of increasing the
combustion of CO and H2 in the tail gas.
In a major revamp of a tail gas incinerator, Sulphur Experts, in conjunction with an EPC company and a
specialty burner company, incorporated several unique features to enable varying degrees of tail gas
staging in combination with a new high intensity burner. The success of this was demonstrated in a
prolonged set of test runs that proved more than 50% utilisation of the CO and H2 present in the tail gas
and a corresponding 45% reduction in the overall fuel gas consumption. This was achieved without any
increase in MGLC of SO2 and shows the benefits of an integrated design team.
PLUME VISIBILITY - CAUSES AND ELIMINATION
Formation of so3 and subsequent H2SO4 aerosol (the bluish-white plume)
The visible plume resulting from SO3 emissions is due to the presence of condensed sulphuric acid mist.
Sulphuric acid mist is produced by the condensation of water vapour onto gaseous SO3 in the air. The
aerosol droplets resulting from this condensation reaction are very small, typically around 1 μm, so they
scatter light very effectively. If sulphuric acid is present in the gases at the stack exit, the condensing
aerosol forms a bluish-white plume. Once formed, such plumes are very persistent and can negatively
impact visibility a long distance from the source.
It is possible to influence plume opacity at quite low levels of SO3. Stack gas SO3 concentrations in the
range of 5 – 10 ppm may result in a visible, whitish plume. The plume takes on a distinct blue colour as the
SO3 concentration increases. There is very little chance of producing a visible plume when the SO3
concentration is less than 2 ppm.
SO3 formation can be expected in tail gas incinerator effluent given the high concentrations of SO2, excess
oxygen levels and temperatures that typically occur. Visible plumes may result when excess air levels are
above 8 to 10% and may be promoted by the presence of vanadium and iron, both of which have been
found in tail gas incinerator effluent. The formation of SO3 is favoured in the temperature range of 650°C to
1000°C but in most tail gas incinerators such bulk temperatures are avoided.
Formation of NOx (the orangey-brown plume)
The visible plume produced by nitrogen oxides (NOx) can be light yellow, to orange/brown in colour
depending on concentration. This coloured plume is primarily a result of scattering and absorbance of
sunlight and hence may appear differently when viewed from different angles.
The most significant factors resulting in the formation of NOx are temperature, oxygen concentration and
residence time.
In tail gas incinerators, flame temperatures can be well above 1200°C when excess air is tightly controlled
and this is sufficient to oxidize nitrogen in the combustion air to NO but as the gas cools, NO2 is formed.
Increasing residence time and oxygen will also result in the formation of more NOx. Tail gas incinerators
usually operate with about 3 - 6% residual O2. Higher levels result in higher NO. With even higher residual
O2 levels, the flame temperature may also fall below 1200°C at which the formation of thermal NO
essentially ceases due to kinetic limitations.
High intensity burners operate with relatively short flames thereby limiting the residence time that
combustion products and air have to react. When these burners are employed in combination with air
staging, NOx formation can be virtually eliminated.
Limiting excess O2 is not only advantageous in reducing NOx formation but can also reduce energy input to
the tail gas incinerator. Although excess air is required to oxidise reduced sulphur compounds in the tail
gas, this can usually be accomplished with no more than 3% residual oxygen in the stack.
If the tail gas incinerator design allows for independent introduction of burner air and combustion air, this
allows fuel rich conditions to be used in the burner, thereby limiting the flame temperature, and subsequent
addition of air to promote oxidation of tail gas compounds.

150 Sulphur 2012 International Conference (Berlin, Germany 28-31 October 2012)
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In some jurisdictions, catalytic incinerators can be employed to allow combustion temperatures to be

reduced below the lower NOx formation temperature of about 740°C
GOING, GOING, GONE – ELIMINATING THE VISIBLE PLUME
In early 2012, Sulphur Experts was called in to assess the causes for a highly visible plume from a Tail
Gas Incinerator. As a result of full spectrum analysis of gas streams into and out of the incinerator it was
determined that adjustments to both combustion air and temperature of the incinerator were needed to
reduce the formation of SO3. As these changes were implemented, the plume was eliminated as shown in
Figure 3. The high temperature and O2 content were, in this case, resulting in formation of a visible mix of
SO2 and SO3 and water vapour. In this example the SRU plant operation remained unchanged and
therefore, the sulphur load to the incinerator was unchanged. A reduction of the breech temperature and
the O2 residual in the stack eliminated the offending plume. But is this the entire solution?
STRIVING FOR A BALANCED APPROACH
In spite of major advances in incinerator design and performance over the last 30 years, the need to
achieve ever better performance will remain a significant challenge.
Regulators will continue to press for reduced SO2 emissions and reduced CO2 emissions. Carbon taxes
are likely to become more prevalent and carry a higher cost even though it is rather contradictory that a
regulator can impose new conditions that directly result in an increase in another taxable emission.
Public perception on issues surrounding the environment is seldom based on facts but is driven by media
clips that may, or may not, be relevant. Too often, what they see is not the most damaging to them or to
the environment.
In summary, we need to understand that the selection of technology and optimisation of a process should
make the process as effective and functional as possible. It should have a broad, rather than a narrow
focus and it must provide a cost effective outcome as opposed to maximizing a particular environmental
outcome.

Figure 3

Going, Going Gone

As Found
900°C - 8% O
2

Step 1
790°C - 7%O
2

Step 2
650°C - 6% O
2

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