H2S / SO2 removal andvalorization

S. Vigneron, PhDConsultant

F-13600 La Ciotatwww.passair.orginfo@passair.org

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

HistoryA 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.

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.

SULFATREAT (cont’d)

Simple alkali scrubbingCa(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

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

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

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

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 VanadiumStretford, Unisulf and Sulfolin, are vanadium based processesLOCAT and Sulferox are iron based processes. Hiperion process uses iron and quinine as catalyst

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°

Hiperion process (cont’d)

SULFEROX processH2S 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.

Sulferox (cont’d)

SULFEROX range

LOCATLOCAT 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.

LOCAT (Cont’d)

SULFOLIN Process

SULFOLIN2 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

SULFONIN

Regeneration of the reduced vanadate

The strip of CO 2

NaHCO 3 ------> CO 2 + NaOH

La réaction globale:

Stretford processIn 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 → V4O92- + 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

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.

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

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

Regenerative thermal oxidiser

10 TPD H2S can give you 18,5 TPD SO2

Conversion to H2SO42 SO2 + O2 → 2 SO3 H = -99 kJ/mol

SO2 concentration is the basis of the choice of a wet sulfuric acid converter

raw gas

fue l

air

filter

concen trationcolum n

reactor

com bustion cham ber

sulphuricacid

electrostaticprecip ita to r

ta il gas

heat transfersystem

fue la ir

concen trationcolum n

reactor

inc ine ra to r

e lectrostaticp rec ip ita to r

ta il gasw astegas

liquid salt

evapora to r

boile r feed w a te r

sulphuricacid

am bienta ir

SULFOX/WSA unitsBritish Gas, Sfax, Tunisie, HC, 95000 Nm³/h

10 TPD H2S can give you 27,5 TPD H2SO4

Regarding FGD

DRY

Filtre à manches

Quench

Réacteur

Injection réactif/Charbon actif

Mist eliminationFinal step of H2SO4 recovery must include mist elimination

CANDLES (Atephos) or WESP

FGD: semi-dry

Filtre à manches

Atomiseur

Préparationsolutionréactive

InjectionCharbon 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

Semi-dry absorption: Niro

Niro process – Semi-dry abasorption - SDA

Source: http://www.gea-pe.fr/NFR/cmsdoc.nsf/webdoc/ndkw74emvy

SDA: Research-Cottrell

Source: http://www.hamonusa.com/hamonresearchcottrell

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

FGD : comparison wet/dryExample of a coal fired electricity power station

FGD wet: PFD

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

Basic reagent

Limestone calcium sulfite : CaCO3 + SO2 → CaSO3 + CO2 Lime calcium sulfite : Ca(OH)2 + SO2 → CaSO3 + H2OValorization to gypsum (forced oxidation):

CaSO3 + H2O + ½ O2 → CaSO4 + H2OCaustic soda soda sulfite or bisulfite (NaHSO3)

2 NaOH + SO2 → Na2SO3 + H2OSodium bicarbonate sodium sulfate2 NaHCO3 + SO2 + ½ O2 Na2SO4 + H2O +

2 CO2

Magnesium Hydroxyde magnesium sulfiteMg(OH)2 + SO2 → MgSO3 + H2O

Sea water: SO2 + H2O + ½O2 → SO42- + 2H+

Economical assetsOperating 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 €,-