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IBP2941_10
SNOXTM – NEW INTEGRATED REFINERY SULPHUR
TREATMENT CONCEPT AND WHY PETROBRAS CHOSE THIS
TYPE OF SOLUTION FOR THEIR NEW RNEST REFINERY IN
PERNAMBUCO, BRAZIL
Laerte Michels1, Rodrigo Lavich2, Torben Christensen3
Copyright 2010, Brazilian Petroleum, Gas and Biofuels Institute - IBP
This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2010, held between September, 1316, 2010, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according
to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not
reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does
not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, nor that of its Members or Representatives.
Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2010 Proceedings.
Abstract
Today, large amounts of high-sulphur residuals, particularly heavy oil and petroleum coke, are produced in the
oil refining sector. Most of these have to be burned to produce power and steam. This paper describes a technology,
SNOXTM, for cleaning of flue gas from combustion of high-sulphur fuels in refineries. The SNOXTM process in a very
energy efficient way converts the NOx in the flue gas into nitrogen and the SOx into concentrated sulphuric acid of
commercial quality without using any absorbents and without producing waste products or waste water.
Simultaneously, all particulates are removed from the flue gas. Along with the flue gases, other sulphurous waste
streams from a refinery can be treated, such as H2S gas, SWS gas, Claus tail gas and elemental sulphur. The paper
further describes the SNOXTM solution chosen by Petrobras S.A. for the new RNEST refinery project in Pernambuco
and the considerations leading to the decision of choosing this type of technology. A reason of major importance for
adopting a flue gas cleaning technology based on SOx recovery as concentrated sulphuric acid, as SNOXTM, was the
fact that production of large amounts of gypsum and other kinds of residues had to be avoided and all efforts should be
made in order to minimize water demand.
1
Technical Consultant, Northeast Refinery Project, PETROBRAS S.A.
Process Engineer, Northeast Refinery Project, PETROBRAS S.A.
3
Sales Manager, Environmental Technology – HALDOR TOPSØE A/S
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1. Introduction
More and more heavy crude oil with high content of non-volatile carbon and high sulphur and metal content is
processed, and more and more process units are introduced in the oil refineries in order to squeeze out of the crude oil
as much valuable distillate product as possible.
Delayed cooking is the least expensive way of converting heavy crude oil to more valuable, lighter products in
refineries which are then left with petroleum coke (petcoke) as low-value by-product containing all the vanadium and
nickel and about half of the sulphur in the original crude oil. World production of petcoke is rapidly increasing and
expected to double every 10 years. Also its content of sulphur and heavy metals is expected to increase.
An increasing fraction of the petcoke and the heavy residual oil fractions must be used as fuel in the
production of heat and power. This can most advantageously take place in the steam and power generation sections of
the refineries. See figure 1 below.
All conventional flue gas desulphurization (FGD) technologies have the disadvantage of being increasingly
more expensive to operate the higher the SO2 content in the flue gas. Water and sorbents in large amounts are often
required, by-products are generated and, on top of that, the SO3 and the metal oxide dust created by burning of high
sulphur residual oils give severe environmental and operational problems in traditional FGD plants.
This paper describes:
1.
2.
3.
An efficient way to remove the dust, NOx and SOx in the flue gases from the boilers by use of SNOXTM
technology.
How this flue gas treatment can be combined with treatment of the amine strip gases (H2S), SWS gases and Claus
plant tail gases in the refinery.
Why Petrobras selected a type of technology as SNOXTM for its new RNEST refinery in Pernambuco, Brazil.
Why SNOXTM?
Other sulphurous
waste streams
More Sulphur
in crude oil
What to do with
the sulphurous
refinery
residues?
Less Sulphur
in refined
products
Clean
flue gas
Clean
the flue gas
in a SNOXTM
plant
High SO2
flue gas
Burn them
to produce
steam and
power
Sulphuric
acid
Energy
recovery
Steam
Power
Figure 1. Optimal Use of High Sulphur Residue in Refineries with SNOXTM
2. The SNOXTM Technology
2.1. General
The SNOX process is a regenerative, catalytic flue gas cleaning process which removes up to 99% of SO2 and
SO3 and up to 96% of the NOx in flue gases. The sulphur is recovered as 94-96% concentrated sulphuric acid of high
purity. NOx is catalytically reduced to N2 by NH3 added to the flue gas. Essentially all dust and particulates are removed
from the flue gas. The heat produced in the process and by cooling down the flue gas to 100°C is recovered as steam
and preheating of combustion air. Thus boiler thermal efficiency is increased, gross power production is increased and
the specific CO2 emission is reduced, and in turn increasing the gross power production gross power production and
reducing specific CO2 emission.
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The SNOXTM process generates no secondary sources of pollution such as waste water, slurries or solids. It consumes
no water or absorbent and chemicals, except for ammonia for the catalytic NOx reduction. The process is in particular
suited for purification of flue gas from combustion of high sulphur petroleum coke (petcoke) and other petroleum
residues such as heavy fuel oil, tars and sour gases. In reality there is no upper limit to the content of SO2 and SO3 in
the flue gas.
Figure 2. The SNOXTM process
2.2. The process steps of SNOXTM
The principal SNOXTM process steps are (see figure 2 above):






Dust removal in ESP (or bag filter) at about 200°C
Heating of the flue gas to about 400°C
Catalytic reduction of NOx by NH3 added to the gas upstream of the SCR reactor
Catalytic oxidation of SO2 to SO3 in the subsequent SO2 converter
Hydration starting in the SO2 converter and completed in the WSA Condenser
Further cooling of the gas to about 100°C in the WSA condenser, whereby the sulphuric acid vapour condenses
The dust removal is performed in an electrostatic precipitator. The dust removed contains heavy metals and
the electrostatic precipitator ensures that these are neither sent into the atmosphere nor disturbing the downstream
process steps. The dust is either deposited or the valuable heavy metals can be removed from the dust by a dedicated
recovery company.
The heating of the flue gas to the required 400oC for the DeNOx step takes place in a plate type 100% fully
welded feed effluent heat exchanger and by heating the flue gas in a heater by use of fuel gas. The heat exchanger
recovers the heat from the flue gas after the catalytic conversion of NOx and SOx. The last adjustment of the flue gas up
to 400oC takes place in a burner.
The catalytic reduction of NOx to N2 by use of NH3 is performed in a catalyst bed which is actually integrated
with the SO2 catalyst bed in one common reactor. The reaction:
4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O
and the similar reaction for NO2 are both exothermal and increase the gas temperature slightly. The catalyst is a
honeycomb type and the catalytic active materials are oxides of vanadium and tungsten.
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The catalytic oxidation of the SO2 to SO3 takes place in a catalyst installed subsequent to the NOx reduction.
The oxidation:
SO2 + ½O2 → SO3
is also exothermal and increases also the gas temperature slightly. The catalyst is a ring type and the main active
component is vanadium pentoxide.
The hydration of the SO3 gas with water to form sulphuric acid in gas phase:
SO3 + H2O → H2SO4 (gas)
takes place in the feed effluent heat exchanger whereby the SO3 reacts with the water vapour in the flue gas and forms
sulphuric acid in gas phase (also exothermal reaction).
Finally, the flue gas is cooled in the WSA condenser whereby condensation of the gas phase H2SO4 takes
place and the final product of the process is formed (liquid H2SO4) and the clean gas goes to the stack.
This final condensation of H2SO4 (gas) into H2SO4 (liquid) takes place in the heart of the SNOXTM plant, the
glass tube based WSA condenser (see below figure 3). The cooling air causing the condensation is then sent to the
boiler as preheated combustion air at 200oC. The condensation of the acid takes place inside the glass tubes and the acid
leaves the glass tubes by gravity into the brick lined acid collector. At the exit of the acid collector, the hot acid is
cooled by mixing with cold acid, cooled further in a plate heat exchanger and pumped to storage as commercial-grade
sulphuric acid product.
Figure 3. Principle sketch of WSA condenser in SNOXTM plant
2.3. The Use of The SNOXTM Technology
Integration with SNOXTM makes it possible to use conventional PC (powdered coal) boilers to burn petroleum
coke without blending it with bituminous coal or co-firing with fuel oil more than just necessary for efficient
combustion of the petcoke. High sulphur liquid fuels can also be burned in a conventional boiler. Essentially all SO3
and heavy metal oxides are removed from the flue gas in the SNOXTM plant without corrosion problems.
The process is well suited for treatment of flue gas from burning up to 100% petcoke in downshot fired PC
boilers. The high combustion temperatures and 3-4 vol.-% residual O2 required for high burn-out of petcoke and the
high content of vanadium in the fly ash lead to a high content of NOx and SO3 in the flue gas, both being efficiently
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removed without corrosion or other problems in a SNOXTM plant. The higher temperature from preheating the
combustion air with SNOXTM also improves the combustion efficiency of the petcoke.
The main attraction of the SNOXTM process is that it makes it possible to burn petcoke or other low cost high
sulphur residuals for generation of steam and power without producing any waste products. An additional advantage is
that even before credit is taken for sales of the produced sulphuric acid, the operating cost of SNOXTM units decreases
with increasing SOx content in the flue gas. This is due to the recovery of the heat of formation of H2SO4 from SO2
amounting to 8 MJ per kg S in the fuel. Finally, and as a result of the low content of SO3 and acid mist from the
SNOXTM unit, the flue gas can be further cooled down to approx. 100°C before entering the stack. The additional heat
recovered from such cooling is utilized to boost the temperature of the combustion air which in the end leads to a
higher overall efficiency of the boiler plant.
Today two refinery SNOXTM plants are in operation in Italy and Austria as follows:
Client
Plant Site
Upstream Unit
S-source
Process Gas
Flow Nm3/h
Start-up Year
Agip Petroli SpA
Gela
Italy
Heavy fuel oil and
petro coke fired steam
and power plant
1,200,000
1999
OMV Refining
Schwechat
Austria
Heavy oil fired steam
and power plant
820,000
2007
Steam generation system
Combustors for acid
gases
Figure 4. SNOXTM – Bird’s View; Petrobras RNEST Refinery
Another two SNOXTM plants are under design/construction for Petrobras RNEST Refinery in Pernambuco,
Brazil for 636,000 Nm3/h each.
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3. The Integrated SNOXTM Solution Chosen by Petrobras
The previous process description of the SNOXTM process is based on cleaning of the large volume relatively
dilute SO2 and NOx gas coming from the petcoke and heavy fuel oil burning boilers. A content of some few hundred
ppm NOx and a few thousand ppm SO2 is normal, depending on the crude treated in the refinery.
Not all nitrogen and sulphur is taken out with the petcoke. The remaining part is taken out by hydrotreating of
the various oil fractions. The oil product is here reacted catalytically with H2 and hereby N and S are converted to NH3
and H2S.
The NH3 and H2S are normally separated from the product in amine units and then treated in Claus plants
where the H2S is converted to elemental sulphur.
3.1. SNOXTM Plant, Consumption in General
As mentioned previously, an important advantage of the SNOXTM technology is that no waste products are
produced and no absorbents or chemicals are used. However, two streams have to be added to the SNOXTM process
when it is operating on flue gas, namely:
-
Fuel gas for adjusting the flue gas to reaction temperature
NH3 for the catalytic reduction of NOx
3.2. Fuel Gas Consumption
The fuel gas is needed to heat up the flue gas to the final reaction temperature at the inlet to the DeNOx
catalyst. Depending on the flue gas volume and its content of SO2 and NOx a typical figure of around 500 m3/h of fuel
gas (refinery fuel gas or natural gas) will be consumed.
In the Petrobras RNEST SNOXTM plants this fuel gas demand is minimized due the combustion heat released
by either amine H2S gas or SWS gas. Both gases contain sufficient energy to function as fuel gas and can burn directly
with a minimum of additional fuel. In this case when part (or all) of the amine gas and SWS gas is used in the SNOXTM
plant, smaller Claus plants (or no Claus plants) can be built.
Another big advantage by using the SWS gas as fuel is that the NH3 and HC's (hydrocarbons) present in
varying concentrations in the SWS gas is avoided in the much more sensitive Claus plants, which then operate more
easily and more efficiently without the NH3 and HCs.
In case amine gas and/or SWS gas is used in larger amounts than required for flue gas heating, the additional
energy developed by the combustion is exported from the SNOXTM plant as superheated high pressure steam.
3.3. NH3 Consumption
The NH3 required for the DeNOx step is in the Petrobras RNEST plant substituted by the use of SWS gas
which contains NH3.
The SWS gas is then mixed into the flue gas instead of NH3 and supplies the required NH3. The H2S in the
SWS gas is converted 100% into SO2 and SO3 on the DeNOx and SO2 catalysts. In this way NH3 import and storage in
the refinery is avoided. Also here the heat of combustion is extracted and exported from the SNOXTM plant as
superheated medium pressure steam and hot combustion air for boilers.
3.4. Claus Tail Gas Cleaning
In the Petrobras RNEST refinery the SNOXTM plants are also used to perform tail gas cleaning from the Claus
plants. The tail gas volume coming from the Claus plants is minor compared to the flue gas volume and does therefore
not influence the SNOXTM plant investment very much. However, the Claus plant does not need expensive tail gas
purification equipment as the efficiency is brought well above the required efficiency when using the SNOXTM plants
to do the tail gas cleaning.
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4. Why Did Petrobras Choose a Type of Technology as SNOXTM?
In the process of selecting the optimal flue gas treatment technology for its heavy oil and petcoke fired boilers
in the RNEST refinery, Petrobras studied various solutions. To do this the first important step was to identify the
challenges that the technology chosen had to meet. As the design of the refinery is based on the use of Marlin crude,
which has a high content of nitrogen and Carabobo crude, which has a high content of sulphur, it was obvious that the
flue gas treatment technology should be well suited to treat high content of NOx and SOx.
This was further underlined by the fact that the products of the refinery shall contain very small amounts of S
and N.
Having concluded the above, it was clear to Petrobras that the points which had to be addressed were:
-
Large amount of acid gases from sour water stripping and amine regeneration to be treated
The Claus plants would have to operate under high molar ratio between NH3 and H2S, or it would be necessary to
adopt a more complex sour water stripper with two towers and ammonia oxidation units
As the RNEST refinery is located in a resort area with hotels, tourist zone and beaches, very strict legislation was
imposed to the design of the refinery. These points plus the fact that the refinery wanted to establish a long term good relation
with the local community as well as to support the good image of Petrobras made it important to find the optimal solution.
Therefore, it was decided to investigate several potential solutions in details.
4.1. Alternative Technologies Investigated by Petrobras
SOx abatement with limestone:
- Large gypsum production, difficult to sell and requiring large storage facilities
- High cost for limestone and disposal of gypsum
-
SOx abatement with solvents:
Need a Claus plant to receive SO2
Bad mix ratio between SO2 and H2S
High water and energy consumption
SOx abatement with NH3
- No large scale industrial references
- Produces a 40% aqueous ammonium sulphate solution with impurities
- Necessary to store (seasonal use), transport and apply (needed to transform habits and systems from solid to liquid
fertiliser)
-
SOx abatement by bi-alkali process:
Consumes alkaline reagents and NaOH
High costs of NaOH
Consumes treated water
Large amount of residues to send to the ocean
All the above considerations lead to two important conclusions to be taken into account when selecting an
appropriate technology for flue gas purification and effluent treatment. The first point was to limit the use of water,
which is in limited supply and expensive in the area, and the second important point was that the production of waste
residues should be avoided completely due to the sensitive location of the refinery and long-term storage problems.
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4.2. Finally approved Solution/Configuration
Petrobras decided to build two Claus plants at a capacity of 250 t/d, each able to handle the amine gases.
Further they decided to build two SNOXTM plants in parallel able to handle flue gases from two heavy fuel oil
fired boilers, one petcoke fired boiler, four SWS single tower units, part or total of four amine units, and tail gases from
the two Claus plants (see figure 5). By locating the boilers, the SNOXTM plants and the Claus plants, SWS and amine
regeneration units close to each other, the amount of large dimension ducting and piping with acid gases was
minimized.
Combustion air
Petcoke
Heavy fuel oil
Flue gas
BOILERS
(3)
Superheated
HP Steam
SNOXTM
(2)
Steam Power
SWS gas (4 units)
H2S gas (4 units)
Sulphuric acid
Tail gas
CLAUS
(2)
Elemental sulphur
Figure 5. Petrobras RNEST, Conceptual SNOXTM Configuration
4.3. Main Reasons for Selecting a Type of Technology as SNOXTM
As mentioned earlier the technology principles for the flue gas treatment, as described by the SNOXTM
solution, was selected for several reasons. First of all it was important for Petrobras to avoid handling limestone and
gypsum in large amounts in the refinery. This would challenge infrastructure and cause other problem related to for
instance dust handling, storage capacity, etc. Further the limited use of water was an important factor for the decision,
and it was an attractive solution to combine flue gas treatment with effluent handling of amine gases, SWS gases and
Claus tail gases in one and the same unit.
Finally, the fact that the recycling of hot combustion air from the SNOXTM plant to the boilers in combination
with medium pressure steam production in the SNOXTM plants increases the thermal efficiency and output of the
boilers leading to a proportional reduction in CO2 emission, was another factor supporting the decision.
4.4. Flexibility/Reliability
To improve the reliability of the flue gas handling and effluent treatment in the refinery it was decided to
install two SNOXTM plants in parallel. This configuration ensures a certain redundancy as the combined capacity of the
two SNOXTM plants is larger than what is required in normal operation. Operation can then be split in the following
three scenarios:
Two SNOXTM plants in operation
Here everything is in normal operation as all loads are split equally between the two SNOXTM plants
U
One SNOXTM plant in operation
- One fuel oil fired boiler is switched from heavy fuel oil to low sulphur fuel oil or gas and the flue gas is sent
directly to the atmosphere and not to the SNOXTM plants.
- The flue gas from the remaining fuel oil fired boiler and the petcoke fired boiler is sent to the remaining SNOXTM
plant together with the tail gas from the two Claus plants.
Operation continues.
U
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No SNOXTM plants in operation
- Fuel oil fired boilers are switched to low sulphur fuel oil or gas and the flue gas is sent directly to the atmosphere
and not to the SNOXTM plants.
- If the petcoke fired boiler is running and, depending on the emission level, it can be reduced in load and then shut
down
- Shut down SWS units, store sour water in tanks, or if necessary and depending on the emission level, the SWS gas
can be flared
U
4.5. Market for Sulphuric Acid and Related Logistics
As a result of the flue gas cleaning process, the SNOXTM plants generate sulphuric acid and therefore
Petrobras studied the possible outlets for the acid.
One option was to produce ammonium sulphate for fertilizer use. This could be done by taking surplus of
ammonia from Sergipe and then use the acid from RNEST. However, this solution means cost for transporting
ammonia from Sergipe. On the market side the ammonium sulphate was considered for sugar cane fields which have to
apply imported fertilizer.
Another option for RNEST was simply to sell the acid in the local market, or try to develop some new
potential phosphate fertilizer customers.
The final option, which has now been introduced, is a pipeline from the refinery to the harbour so the
produced acid can be pumped to a ship and sent to the South/South East domestic markets where acid is currently being
imported.
5. Conclusion
If a refinery produces petcoke it is an obvious and advantageous solution to burn the petcoke in the refinery
power plant to produce steam and electric power.
The problematic flue gas with its content of heavy metal particulates, NOx and SOx needs to be treated, and
this can advantageously take place in a SNOXTM plant. Here the dust is removed, the NOx is reduced to harmless N2
and the SOx is converted to SO3 and recovered as commercial-grade, valuable sulphuric acid.
During the technology selection process for the flue gas treatment unit for RNEST refinery, Petrobras decided
to use a technology based on selective catalytic reduction of NOx combined with SOx removal as concentrated sulphuric
acid. When having analyzed the proposals, the SNOXTM was chosen for the grass root refinery in Pernambuco and it
was decided to add the SWS gases, some of the amine gases and the Claus plant tail gases to the SNOXTM plant. This
simplifies the Claus plant and SWS units design and at the same time the SNOXTM plants deliver hot combustion air to
the boilers and a considerable amount of medium pressure steam to the internal steam grid.
Finally, the solution fulfils all the stringent local environmental requirements and secures a good relation
between the refinery and the local community with its sensitive beaches and recreational areas long time into the future.
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