sulphur behaviour

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copyright reserved 2005, Sasol Technology, 31 January 2005

SULPHUR BEHAVIOUR IN THESASOL-LURGI FIXED BED DRYBOTTOM GASIFICATION

PROCESS15-17 May 2007

M.P Skhonde, RH Matjie, TJ van der Walt, JR Bunt

2007 Topsoe Catalysis Forum: Sulphur Management


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Background

Sasol-Lurgi Fixed Bed Dry Bottom gasification process asdeployed at the Sasol in Secunda, South Africa consumes morethan 30million tons of coal per annum

Production of more than 150000 barrels/day of fuels andchemicals

Coal is an abundant resource with a promising future in energy

and petrochemicals productionCoal is also a potential source of pollution, with increasedutilisation leading to even more environmental footprints

Coals used in the Secunda operation gasification plant have low

sulphur content (1-2%) compared to other coals.


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Sulphur formation in coal

Sulphur in coal originates from the original plant materials and fromthe ambient fluids in the coal-forming environment

Sea water interaction with the peat during coal formation will resultin elevated levels of sulphur in coal

Low sulphur coal seams were deposited in an alluvial environmentand the peat was not influenced by seawater and is derived from

the parent plant materialHigh sulphur coals are associated with marine strata e.g. HerrinCoal in the Illinois Basin that is overlain by the marine Anna Shale

and Brereton Limestone

Organic sulphur is formed by reaction of reduced sulphur specieswith the premaceral humic substances formed by bacterialdecomposition of the peat


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SO4-2 in sea water

Bacterial reduction

Dissolved H2S

Precipitation of pyrite

Reaction with Fe

Sulphur formation in coalSulphur formation in coal

PyriteFramboids

CellInfillings

Reaction withPre-maceral humic substance

Organically bound

sulphur


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Sulphur in coal

Forms of sulphurin coal

Organicallybound

sulphurSulphideminerals

Sulphate

minerals


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Gasification process


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The fate of sulphur in the Secunda plant

Sulphur incoals from

mines

Coalpreparation

plant

Steam plant

gasification

SOx

S in ash

Gas cooling

S in gas liquortar/oil/naphtha

H2S inraw gas

S recovery unit

Elementalsulphurproduct


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H2S emissions reduction options


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Coal destoning

Coals from themines

Coalscreening

Physicalseparation

gasification

Raw gas to gas

purification

Fine coal tosteam plant Discard coal

Cleancoal


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Sulpholin process

Upstream gasproduction units Sulpholin units

Off-gasGranulation unit

Bagged andunbagged sulphur

granulesSodium sulphate

unit

Sodium sulphatecrystals

Off-gas stacks toatmosphere

Untreatedoff-gas Treated

off-gas

Moltensulphur

Sulphur

granulesSulpholinsolution

Sodiumsulphatecrystals


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Sulphur production by chemical absorption

Upstream gasproduction

unitsabsorber regenerator

Clausereactor

Sulphurproduct

Aminesolvent

Off-gas stack toatmosphere

Treated off-gases

Selectedoff-gasstreams


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Sulphuric acid production

Upstream gas

productionunits Incineration

SO2 converter and

acid plant

Sulphuric acidproduct

air

Offgas stacks toatmosphere

Selectedoff-gasstreams


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Sulphur fate in a gasification process


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Methodology

SampleNumber

E32 Top of Gasifier

Bosman skirt

D24

Pyrolysis zone

C16

Gasification zoneB8

Ash bedA1

Bottom of Gasifier

Gas

Ash

Steam,oxygenor air

coal

zone

drying

pyrolysis

gasification

combustion

Ash bed


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Analysis done

Sulphur content

Total sulphur (ASTM D4239)Mineral sulphur(ISO157)Sulphate sulphur(ISO157)Organic sulphur(ISO157)

XRDQuantification on Siroquant

Computer Controlled Scanning Electron Microscopy(CCSEM)


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Gasifier temperature profile (Bunt, 2005)

0

5

10

15

20

25

30

35

0200 400 600 800 1000 1200 1400 1600

Temperature (oC)

Sample number

Average temperature Surface temperature Peak temperature

Drying zone

Reduction

zone

Pyrolysis

zone

Slow pyrolysis

with gasification

Fast pyrolysis

Gasification

Oxidation

Start of combustion zone

and ash bed

E32

D24

C16

B8

A1

Gasifier height

top

bottom


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Sulphur content across the gasifier

FeS2 + H2 Fe(1-x)S + H2S

Fe(1-x)S + H2 + 2O2

FeO/Fe2O3 + H2S

Sulfur behaviour

0.00

0.50

1.00

1.50

2.00

2.50

S0 01 S0 08 S0 16 S0 24 S0 32

sample

sulfur

content%

total sulfur

mineral sulfur

sulfate sulfur

organic sulfu

Bottom gasifier height top


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Total sulphur to mineral sulphur correlation

R2 = 0.9679

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0.00 0.50 1.00 1.50 2.00 2.50

Total sulphur

mineralsulphur


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Sulphur bearing molecular composition-XRD

FeS2 + H2 Fe(1-x)S + H2S

Fe(1-x)S + H2 + 2O2 FeO/Fe2O3 + H2S

Bottom gasifier height top

Sulphur species in the crysta lline phase in ash (bas ed o n XR D )

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

1 8 16 24 32

pyrite

pyr r hoti teanhydri te

Hemati te

Magnetite


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Sulphur association in ash

Sulfur bearing mineral association in a gasifier

0

1

2

3

4

5

6

1 8 16 24 32

Gasification samples

%com

position

pyrite cleats

Kaolinite (pyrite)

Kaolinite(carbonate,

pyrite)

Quartz(carbonate,pyrite)

Pyrrhotite/Fe-S-O/Fe-Oxide

Fe-Ca-Al2Si2O5 interactionforming from Pyrite,carbonates & kaolinite

Some of the FeOparticipate inslagging

Fe-Al2Si2O5 interactionforming from pyrite &kaolinite

Pyrite cleatstransformation

Bottom gasifier height topFe-Ca-SiO2interaction formingfrom pyritecarbonates & quartz


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Concluding remarks

Most of the sulphur is released as H2S with small amounts ending up in the ashMost of the H2S comes from the reduction of pyrite to pyrhottite which takesplace in the pyrolysis zoneSome of the pyrhottite is further converted to various oxides of iron leading to

formation of H2S in the gasification zoneVery small amount end up in the ash as anhydrite as well as part of interstitialand matrix glass compounds such as Fe-Ca-Al2Si2O5, Fe-Al2Si2O5, Fe-Ca-SiO2, and pyrrhotite/Fe-S-O/Fe-OSulphur in ash is retained mainly in:

Sulphur oxide that reacted with high temperature transformation product (CaO) ofcalcite or dolomite to form anhydrite (CaSO4)As sulphur that reacted with the glassOrganically bound sulphur in unburnt carbon

Understanding of the mode of occurrence and behaviour of sulphur important

for development of sulphur capture methodsUnderstanding the behaviour of sulphur in a Sasol-Lurgi Fixed Bed Dry BottomGasification process and applying proven gas conditioning techniques,solutions to an emissions-free gas island are in reach


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Thank you for your attention!!!


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Minerals transformation at elevated temperatures

Understanding of minerals transformation necessary for understanding ofpollutants emissions during coal processing

During Combustion pyrite transforms to iron oxides, and

In presence of C, Fe is produced and can react with other inorganic elements to

form spinnel, srebrodolskite (Ca2Fe2O5), magnesioferrite (MgFe2O4), andbrownmillerite (Ca4Al2Fe2O10)

Kaolinite (Al4Si4O10(OH)8) forms mullite(3Al2O3.2SiO2), alumina (Al2O3) andcristoballite (SiO2).

Illite (K1.5Al4(Si6.5Al1.5)O(OH)4) forms spinnel and mulliteQuartz (SiO2) is non reactive but transforms to other forms -quartz, -quartz(573-870C), 2-tridymite (870-1470C), and 2 cristobalite (1470- hightemperatures) to form a liquid.Excluded carbonate minerals calcite (CaCO3), dolomite (CaMg(CO3)2), siderite(FeCO

3), ankerite ((Fe, Ca, Mg, (CO

3)3) are found in most bituminous coals.

Calcite fragments at high temps(1000C) to form qui cklime (CaO).CaO react with water and sulphur oxides to form portlandite (Ca(OH)2) andanhydrite (CaSO4)At high temperatures (>1000C) CaO interacts with m ore reactive aluminium

silicate such metakaolinite to form gehlenite (Ca2Al2SiO7) andanorthite (CaAl2Si2O8)

sulphur behaviour

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