H2S is a common pollutant in gas and air. This presentation is a review of different techniques to remove H2S ,and possible ways of valorization to sulfuric acid via SO2.

1. H2S / SO2 removal and
valorization
S. Vigneron, PhD
Consultant
F-13600 La Ciotat
www.passair.org
info@passair.org

2. Introduction
• Most of the time H2S or/and SO2 must be
removed because of its problematic, its
toxicity and its odour especially from fuel gas
• It could be capted following different ways:
– By using ‘simple’ alkali reagent in what some
valorizations are possible
– By using valorization process to elementar sulphur
or SO2
– By using valorization process to sulfuric acid

3. History
A simple way in order to remove H2S is to use iron oxides boxes :
• raw gas is passed at low pressure and at atmospheric temperature through a
vessel containing beds of wood shavings covered with crude iron oxide, beds of
pelleted crude iron oxide or iron oxide mixtures. The H2S reacts with the iron
oxide to form iron sulfide and water. Air is ordinarily added to the inlet gas in a
sufficient amount to supply about half of the theoretical oxygen to convert the
iron sulfide back to the oxide and to precipitate the elemental sulfur in the
bed. The mass will hold 40-55% of its dry weight in sulfur before excessive H2S
break through or excessive pressure drop occur. When the maximum sulfur
loading is attained the catalyst is removed, the sulfur burned for recovery of SO2
and the residue is discarded.
Some of continuous process are today available still with the
same principle using a chilled iron oxides solution for scrubbing
what could be continuously regenerated whereas sulfur is the
last product: Sulfatreat, Sulferox are using iron oxide like a
catalyst whereas process like Sulfalin or Lo-Cat are using vanadia
catalyst on a similar principle.

4. SULFATREAT
 Efficient SulfaTreat ® process is also iron based
using a porous form of iron oxide and therefore
has much larger capacity per unit volume of the
bed.
 Sulfatreat beds are much smaller for the same
gas capacity when compared to iron sponge beds.
Iron is a very strong oxidizing element and
oxidizes sulfur compounds and neutralize them
either to other forms of less harmful
sulfur compounds or to the elemental sulfur
itself.

5. SULFATREAT (cont’d)

6. Simple alkali scrubbing
Ca(OH)2, NaOH or NaOCl
• Ca(OH)2 + SO2 → CaSO3 + H2O
• Ca(OH)2 + H2S → CaS + 2 H2O
CaS is a salt what will precipitate; seeing the low solubility of Ca(OH)2,
CaS will be hydrolized and H2S will re-escape…
To be pointed out that in FGD solution, limestone can be used:
CaCO3 + SO2 → CaSO3 + CO2
Whereas gypsum can be obtained by forced oxidation:
CaSO3 + H2O + ½ O2 → CaSO4 + H2O

7. NaOH, NaOCl
• 2 NaOH + SO2 → Na2SO3 + H2O
• 2 NaOH + H2S → Na2S + 2 H2O
Regarding the Na2S discharge, a type of valorization is to reuse it
in paper industry but can also find another application.
For sodium hypochlorite:
• H2S + NaOCl → NaCl + H2O + S
• SO2 + NaOCl + H2O → NaCl + H2SO4
• H2S + 4NaOCl → 4NaCl + H2SO4
The hypochlorite scrubber also allows the oxidation of other compounds,
eg. organic sulfur compounds

8. Wellman–Lord process
Allow to regenerate SO2 trapped by a sulfide sodium
solution without any waste production.
Reversible reaction from what SO2 can be valorized
easier (to for ex H2SO4). Sulfide is reintroduced in
the process downstream.
Na2SO3 + SO2 + H2O → 2 NaHSO3
2 NaHSO3 + cooling → Na2S2O5↓ + H2O
Na2S2O5 + H2O → 2 NaHSO3
2 NaHSO3 + heating→ Na2SO3 + SO2 + H2O
8
Source: Sulfur Dioxide Removal, Kohl, Arthur L.; Nielsen, Richard B., pp. 554–555, Gas Purification, Gulf Professional Publishing, 1997

9. Liquid phase precipitation
With Ferrous sulphate
• 2NaOH + H2S → Na2S + 2H2O
• 3FeSO4.7H2O + 1.5Cl2 → Fe2(SO4)3 + FeCl3.H2O
• Fe2(SO4)3 + 3Na2S → Fe2S3 + 3Na2SO4↓
• Fe2(SO4)3 (hydrolysis) → 2FeS↓ + S↓
• FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl
= environmental disposal problems

10. Liquid phase catalytic oxidation
Iron is held in solution by an organic chelant which
participates in the absorption process as a catalyst. The basic
reaction :
Absorption:
• H2S + 2 [Fe3+] → S + 2[Fe2+] + 2H+
Regeneration:
• 2[Fe2+] + 1/2O2 + 2H+ → 2[Fe3+] + H2O
Provided catalysts are (Fe2+) or Vanadium
Stretford, Unisulf and Sulfolin, are vanadium based processes
LOCAT and Sulferox are iron based processes.
Hiperion process uses iron and quinine as catalyst

11. Hiperion process
In this process, the HS- ion is oxidized by the
naphthoquinone chelate to elemental sulfur and the
quinone is reduced to the hydroquinone form:
• NQ:Chelate + 2HS' --> HNQ:Chelate + 2S°
The hydroquinone chelate is subsequently reacted with
oxygen in atmospheric air to form the quinone chelate
and hydrogen peroxide:
• HNQ:Chelate + 0 2 --> NQ:Chelate + H202
Since hydrogen peroxide is an extremely active oxidation
agent, it quickly reacts with any residual HS' to form
sulfur and water:
• H202 + 2HS'--> 2H20 + 2S°

12. Hiperion process (cont’d)

13. SULFEROX process
H2S is oxidized to elemental sulfur by ferric ion chelated with
nitrilotriacetic acid (NTA) in aqueous solution of pH 3.5 to 4.5. The
ferrous ion formed in this reaction may be reoxidized with chemical
reaction with oxygen absorbed from a stream of air. The presence of
NTA catalyzes the reaction of O2 and Fe++ in the same pH range and
diffusion again controls the process. The overall chemical reactions :
Absorbtion:
• H2S + 2Fe3+.NTA → S + 2Fe2+ NTA + 2H+
Regeneration:
• 2H+ + 2Fe2+ NTA + ½ O2 → 2Fe3+. NTA + H2O
NTA serves two functions: to solubilize the ferric ion and prevent
formation of hydroxide at the pH of operation and catalyze the
reaction of Fe2+ with O2.

14. Sulferox (cont’d)

15. SULFEROX range

16. LOCAT
LOCAT is a chelated iron liquid redox process
Absorption:
• H2S + 2Fe+++ → 2H+ + S + 2Fe++
Regeneration:
• ½ O2 (gas) + H2O + 2Fe++ → 2(OH)- + 2Fe+++
LOCAT solution constitutes a ARI-310 catalytic reagent containing two
proprietary chemicals, a biocide and a surfactant, to ensure that sulfur
sill sink to the bottom of oxidizer from where it is removed as slurry.
ARI-310 is a third generation reagent that uses ethylene diamine
tetraacetic acid (EDTA) as a chelating agent to hold an iron solution of
500 to 1800 ppm. The solution serves as a catalyst in the overall
reaction of H2S with oxygen, which takes part in the reaction in the
oxidizer and absorber by transfer of electrons.

17. LOCAT (Cont’d)

18. SULFOLIN Process

19. SULFOLIN
2 reaction steps in the scrubber:
-1- absorption of H2S (and CO2).
Followed bu the reaction
High concentration of CO2 will decrease the efficiency of H2S removal.
-2- HS- oxidation to sulfur:
Regeneration of the carbonite:
The uptake of H 2 S has so if total reaction:
So here are hydroxide ions formed. These hydroxide ions may react with the bicarbonate ions which are formed in accordance with the reaction:
The overall reaction of CO2 uptake is then:
Global reaction:
2 H2S + 2 CO2 + 4 NaVO3 ---> Na2V4O9 + H2O + 2 HCO 3 - + 2 S

20. SULFONIN
Regeneration of the reduced vanadate
The strip of CO 2
NaHCO3 ------> CO2 + NaOH
La réaction globale:

21. Stretford process
In the first step, H2S is absorbed into a solution containing carbonate –
bicarbonate, where H2S is hydrolysed and dissociated to form bisulfide
ions (see data bank in Chapter 3).
Absorbtion:
• H2S(g) = H2S(l) = H+ + HS-
The HS is oxidized to elemental sulfur by vanadium (v), which
subsequently gets reduced to vanadium (IV):
• 4VO3
- + 2HS + H2O → V4O9
2- + 2S + 4OH-
Regeneration:
Stretford uses anthraquinone disulfonic acid (ADA) to catalyze the
oxygen transfer in regeneration of reduced vanadium:
• V4O9
2- + O2 + 2OH- → 4VO3- + H2O
The concentration of CO2 influences significant the extent of
desulfurization achieved in the process

22. STREDFORD/SULFOLIN
Since the regeneration of the Na2V4O9 occurs in two
steps, it is more likely to be byproducts here. This will
cause the decrease of the concentration of the carbonate
ions. The wash solution should be therefore more often
replaced.
Additionally three reaction steps are found in the wash
column of Stredford : the time required for this purpose
will therefore be more than in the Sulfolin process.
Considering these factors, but also the assumption that
the data on by-product formation and reaction rates are
correct, the preference is for the Sulfolin process.

23. H2S to SO2 conversion
This is possible for concentration of SO2 that will not exceed
around 22 g/Nm3 meaning around 12 g/Nm3 at the origin or
8000 ppmv.
H2S, CS2, and SO2 (and typically another C-S-…) are converted
catalytically or thermically following the oxidation reactions
what are exothermal:
H2S + 3⁄2O2 → H2O + SO2
H = -518 kJ/mol
CS2 + 3 O2 → 2 SO2 + CO2
2 SO2 + O2 → 2 SO3
H = -99 kJ/mol

24. Thermal oxidation
• Thermal oxidiser will allow for autothermicity
with thermal efficiency of heat exchanger at
80 % level for the max 8000 ppmv (12,162
g/Nm3) H2S concentration considering the
inlet temperature as ambient
• The maximum thermal efficiency to consider
is 96 % meaning a minimal concentration of
H2S around 1600-1700 ppmv.
• 1 g H2S will lead to 1,88 g SO2

25. Regenerative thermal oxidiser
10 TPD H2S can give you 18,5 TPD SO2

26. Conversion to H2SO4
2 SO2 + O2 → 2 SO3 H = -99 kJ/mol
SO2 concentration is the basis of the choice of a wet sulfuric acid
converter
raw gas
fuel
air
filter
concentration
column
reactor
combustion chamber
sulphuric
acid
electrostatic
precipitator
tail gas
heat transfer
system
fuel
air
concentration
column
reactor
incinerator
electrostatic
precipitator
tail gas
waste
gas
liquid salt
evaporator
boiler feed
water
sulphuric
acid
ambient
air

27. SULFOX/WSA units
British Gas, Sfax, Tunisie, HC, 95000 Nm³/h
10 TPD H2S can give you
27,5 TPD H2SO4

28. Regarding FGD
DRY
Filtre à manches
Quench
Réacteur
Injection réactif/
Charbon actif

29. Mist elimination
Final step of H2SO4 recovery must include mist
elimination
CANDLES (Atephos) or WESP

30. FGD: semi-dry
Filtre à manches
Atomiseur
Préparation
solution
réactive
Injection
Charbon actif
Source image: Mise en œuvre de procédés catalytiques :
Oxydation des COVs et SCR, P. Compain, S. Vigneron,
Ecole de catalyse et ses applications – Fès, mai 2011

31. Semi-dry absorption: Niro
Niro process – Semi-dry abasorption - SDA
Source: http://www.gea-pe.fr/NFR/cmsdoc.nsf/webdoc/ndkw74emvy

32. SDA: Research-Cottrell
Source: http://www.hamonusa.com/hamonresearchcottrell

33. FGD: semi-wet
Source picture: Mise en œuvre de procédés
catalytiques :
Oxydation des COVs et SCR, P. Compain, S.
Vigneron,
Ecole de catalyse et ses applications – Fès, mai
2011

34. FGD : comparison wet/dry
Example of a coal fired electricity power station

35. FGD wet: PFD

36. FGD wet versus dry
Gas velocity inside the tower: 5 m/s
If bag filter:
- 42328 bags 9 m height,
160 mm diameter
- 3 cells of 28 x 8,8 m
- Full height incl. penthouse:
25 m
- Preheating: 1275 kW
- Compressed Air 17220 m3/h
Source (exluding bag filter): New Flue Gas Treatment System for 1,050MWe Coal Fired Plant, T. Muramoto, T. Nakamoto, I. Morita, T. Katsube,
H. Kikkawa (Babcock-Hitachi K.K.,Japan), K. Chou & H. Murayama (Electric Power Development Co.,Japan), Avril 2000

37. Basic reagent
Limestone  calcium sulfite : CaCO3 + SO2 → CaSO3 + CO2
Lime  calcium sulfite : Ca(OH)2 + SO2 → CaSO3 + H2O
Valorization to gypsum (forced oxidation):
CaSO3 + H2O + ½ O2 → CaSO4 + H2O
Caustic soda  soda sulfite or bisulfite (NaHSO3)
2 NaOH + SO2 → Na2SO3 + H2O
Sodium bicarbonate  sodium sulfate
2 NaHCO3 + SO2 + ½ O2  Na2SO4 + H2O + 2 CO2
Magnesium Hydroxyde  magnesium sulfite
Mg(OH)2 + SO2 → MgSO3 + H2O
Sea water: SO2 + H2O + ½O2 → SO4
2- + 2H+

38. Economical assets
Operating costs:
Limestone is cheaper but more complicate to operate.
 Ca(OH)2 or NaOH.
NaOH  water soluble but complicate for reuse.
Power station < 300 MW : dry/semi-dry (< 106 Nm3/h)
Costs:
Investment, for 2 GW PST: 468 Moi€,-
Costs/ton SO2 by FGD
> 400 MW < wet > 165 €,- to 410 €,- / < 410 €,- to 5100 €,-
> 200 MW < semi-dry > 125 €,- to 250 €,- / < 410 €,- to 3300 €,-