Prof. Dr. techn. G. Scheffknecht

Institute of Combustion and Power Plant Technology

Experiences and Results of SO3

Measurements Performed under

Oxy‐Coal Fired Conditions

IEA-GHG – Special Workshop

“Addressing the SO2/SO3/Hg and Corrosion Issues in

Oxyfuel Combustion Boiler and Flue Gas Processing Units”

Rembrandt Hotel, London - 25/26 January 2011

Reinhold SpörlJörg Maier

Günter Scheffknecht

reinhold.spoerl@ifk.uni-stuttgart.dejoerg.maier@ifk.uni-stuttgart.de

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Outline

1. Background: SO3 generation; H2SO4 formation; Acid dew point;

Controlled condensation method

2. Experiences measuring SO3:

• Choice of the filter material

• Temperature of the SO3 condenser

• SOx concentrations in air and oxyfuel firing mode

• Influence of SCR catalyst on acid due point

3. Conclusions

4. Outlook

SO3 sources (1/2)

• Variable amounts of sulfur (e.g. pyrite FeS2) in coal, e.g.:

Sulfur content in central European brown coals: 0.2 up to 5 % [1]

• Oxidation of sulfur to SO2 during coal combustion

• Further oxidation of part of the SO2 to SO3

Predominant oxidation reactions in the boiler [2]:

M indicates a caytalyst (e.g. Fe2O3); Most efficient at 700 to 800°C.

Relatively slow; Equilibrium on left side at high temperatures

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MSO M OSO 32 +→++

32 SO OSO →+

SO3 sources (2/2)

• SO3 generation mainly depends on:

− O2 concentration in boiler: possible increase in oxyfuel firings

− SO2 concentration in boiler: increase by factor 3-4 in oxyfuel firings

− Presence of catalytically active material (e.g. Fe2O3 in fly ash)

− Residence time of the flue gas between 700 and 800°C

• SO2 oxidation takes place:

−In the boiler and economizer: depending on coal and boiler (in air firings: up to

1.6% of toal SO2 converted) [3]

−In the SCR-reactor (in air firings up to 1.25% of total SO2 converted) [3]

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H2SO4 formation and dew point (1/3)

• SO3 and H2O form vapour sulfuric acid H2SO4 [4]:

− Formation starts at about 400°C.

− At about 200°C almost all SO3 is

consumed.

5

)g(42

(g)2

(g)3 SOH OHSO →+

H2SO4 formation and dew point (2/3)

• Condensation of H2SO4 when temperature falls below acid dew

point temperature: TDew = f(pSO3, pH2O)

• TDew calculation at IFK according

to Zarenezhad [5]

• TDew formerly calculated according

to Verhoff/Banchero [6] or Okkes [7]:

−Verhoff/Banchero equation overpredicts TDew

−Okkes equation underpredicts TDew

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H2SO4 formation and dew point (3/3)

• Acid dew points calculated according to Zarenezhad equation:

• Typical dew points in air fired power plants: 95-150°C

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Controlled condensation method (1/1)

• No other flue gas component condensing at 100-200°C

• Separation of H2SO4 through selective condensation

• Condenser: Glas coil tempered between water and acid dew point

• Sample train:

• Sample analysis by titration or chromatography8

in out

H2SO4 condenser

Experiences: Choice of filter material (1/5)

• At IFK in-stack dust removal is considered to be the most reliable

configuration, to avoid H2SO4 condensation on the filter

• Possible filter material for in-stack dust removal:

−Glass wool: glass fibers consisting of SiO2 (approx. 70 %), Al2O3, CaO, K2O,

MgO and NaO

−Quarz wool: quarz fibers consisting of >99% SiO2

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Experiences: Choice of filter material (2/5)

• Handling of glass and quarz wool:

−Glass wool:

cheap

easy to handle

−Quarz wool:

very brittle

breaks into small fibrous dust when force is applied

difficult to apply on the filter cartrige of the probe

possible contamination of the sample by small fibre

pieces

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Experiences: Choice of filter material (3/5)

• Experiments on IFK‘s 20 kW entrain flow reactor (air fired)

• Firing of 1.5 kg/h El Cerrejon hard coal

• SO2 content in the flue gas: ~ 1300 mg/m3 (STP)

• SO3 measurements at a SCR catalyst (Tin • 395°C)11

• Sampling with glass wool for dust removal (all samples taken behind catalyst)

−Variation of the sampling volume:

acid mist not or late (after 30 min of sampling) visible in H2SO4 collector

−Addition of SO2 with secondary air (sampling volume 190l (STP)):

acid mist visible after 10-15 minutes of sampling

Saturation behaviour

Experiences: Choice of filter material (4/5)

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No. Sampling volume [l] (STP) SO3 in fluegas [mg/m3] (STP)

A 96 0.3

B 218 0.2

C 247 0.3

D 349 3.6

Shorter saturation time

No. SO2 in fluegas [mg/m3] (STP) SO2 in fluegas [mg/m3] (STP)

E 5580 9.4

F 5380 29.9

G 5200 1.9

Significant variations

Experiences: Choice of filter material (5/5)

• Sampling with quarz wool for dust removal

−Sampling volume ~ 200l (STP)

−Sampling before and behind SCR catalyst:

acid mist visible after 1-2 minutes of sampling, minor

variation of concentrations

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No. Sampling position SO3 in fluegas [mg/m3] (STP)

H Before catalyst 5.8

I Before catalyst 4.4

J Behind catalyst 52.0

K Behind catalyst 53.3

For comparison (sampling with glass wool)

B Before catalyst 0.2

Dust removal by quarz wool

Dust removal by glass woolNo saturation behaviour with quarz wool

Experiences: SO3 condenser temperature (1/2)

• Temperature of condenser coil between acid and water dew point

−H2SO4 condensation follows the phase

equilibrium according to the acid dew point:

If the condenser coil is tempered to 121°C,

1.7 ppm H2SO4 remain in the gas phase

−Temperature far below acid dew point, to

asure a high level of H2SO4 condensation

−Temperature high enough above water

dewpoint to avoid water condensation in coil:

20-30K above water dew point14

Water dewpoints for water contents of 12,

28 and 35 Vol-%

Experiences: SO3 condenser temperature (2/2)

• Water and acid dew points in air and oxyfuel mode measured at

IFK‘s 500kW PC combustion rig:

(calculated from H2SO4 and water concentrations)

− Maximum water dewpoint (with soot blower: water content >35 Vol-%): 75°C

• Recommended coil temperature:

−Air firing: 75-80°C

−Oxyfuel firing: 85-95°C (depending on water concentration)

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Air firing Oxyfuel firing

Water concentration [Vol-%] 11.8 28.3

Water dewpoint [°C] 49.1 67.8

Acid dewpoint [°C] 130-140 ~160

Measurements at IFK‘s 500kW combustion rig (1/3)

• Measurements at IFK‘s 500kW PC combustion rig

• Firing of ~50kg/h high sulfur brown coal

• Operation in air and oxyfuel mode16

Bottomash

ID fanESPSCR

Storagetanks

FD/ RG fan

O2

CO2

Stack

Air O2 CO2

Pre-Heater

Coal feeding

Air

By-passes

Measurements at IFK‘s 500kW combustion rig (2/3)

• SO2 concentrations:

−Concentrations increased by a factor of 2.6 to 3.1 (air : oxyfuel)

−Maximum SO2 concentrations in oxyfuel firing mode: approx. 11700 mg/m3

(STP) [reference O2: 6 Vol-%]

• SO3 concentrations (measured at different positions along the

flue gas path):

SO3 concentrations in air firing mode:

up to 40 mg/m3 (STP) [reference O2: 6 Vol-%]

High SO3 concentrations in oxyfuel firing mode:

up to 200 mg/m3 (STP) [reference O2: 6 Vol-%]

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Measurements at IFK‘s 500kW combustion rig (3/3)

− Increase of acid dew point over SCR catalyst (qualitative):

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Increase of approx.

30mg/Nm3

Increase of approx.

30mg/Nm3

~2 K

~15 K

Air firing

Oxyfuel firing

Conclusions (1/1)

• Controlled condensation method at IFK:

−Filter material for in-stack dust removal: Quarzwool

−H2SO4 condenser coil temperature:

Air firing: 75-80°C

Oxyfuel firing: 85-95°C (depending on H2O content)

• Significantly higher SO3 concentrations in oxyfuel mode

• Influence of SCR catalyst on acid due point is less pronounced in

oxyfuel firings

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Outlook

• Future research topics:

−Test and evaluation of continous SO3 analysers (e.g.: Severn Science

analyser, FTIR)

−Evaluation of the influence of fly ash in the gas sampling probe on SO3

measurements

−Evaluation of the SO3 evolution over the flue gas path in air and oxyfuel

combustion, e.g.:

SO3 capture in the ESP

SO3 formation in the boiler

Possibility of SO3 enrichment due to flue gas recirculation

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Literature

[1] P. ADOLPHI ; M. STÖRR ; P.G. MAHLBERG ; H.H. MURRAY ; E.M. RIPLEY: Sulfur sources and sulfur bonding of some central European attrital brown coals. In: International Journal of Coal Geology 16 (1990), S. 185–188

[2] T. L. JORGENSEN, H. LIVBJERG, P. GLARBORG: Shorter Communication - Homogeneous and heterogeneously catalyzed oxidation of SO2; In: Chemical Engineering Science 62 (2007), S. 4496 – 4499

[3] R.K. SRIVASTAVA ; C.A. MILLER ; C. ERICKSON ; R. JAMBHEKAR: Emissions of sulfur trioxide from coal fired power plants. In: Journal of the Air & Waste Management Association 54 (2004), S. 750–762

[4] R. HARDMAN ; R. STACY: Estimating Sulfuric Acid Aerosol Emissions from Coal-Fired Power Plants. Conference on Formation, Distribution, Impact, and Fate of Sulfur Trioxide in Utility Flue Gas Streams, 1998

[5] B. ZARENEZHAD: New correlation predicts flue gas sulfuric acid dewpoints. In: Oil & Gas Journal 107 (2009), S. 60–63

[6] F.H. VERHOFF ; J.T. BANCHERO: Predicting dewpoints of flue gases. In: Chemical Engineering Progress 70 (1974), S. 71–72

[7] A. G. OKKES: Get acid dewpoint of flue gas. In: Hydrocarbon Processing 7 (1987), S. 53–55

Literature recommendation (background information on SO3 measurement):

• Workshop Proceedings on Primary Sulfate Emissions from Combustion Sources. Volume I. Measurement Technology., 1978

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Thank you for your attention!

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IEA-GHG – Special Workshop�“Addressing the SO2/SO3/Hg and Corrosion Issues in �Oxyfuel Combustion Boiler and Flue Gas Processing Units”�Rembrandt Hotel, London - 25/26 January 2011OutlineSO3 sources (1/2)SO3 sources (2/2)H2SO4 formation and dew point (1/3)H2SO4 formation and dew point (2/3)H2SO4 formation and dew point (3/3)Controlled condensation method (1/1)Experiences: Choice of filter material (1/5)Experiences: Choice of filter material (2/5)Experiences: Choice of filter material (3/5)Experiences: Choice of filter material (4/5)Experiences: Choice of filter material (5/5)Experiences: SO3 condenser temperature (1/2)Experiences: SO3 condenser temperature (2/2)Measurements at IFK‘s 500kW combustion rig (1/3)Measurements at IFK‘s 500kW combustion rig (2/3)Measurements at IFK‘s 500kW combustion rig (3/3)Conclusions (1/1)OutlookLiteratureSlide Number 22