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
[email protected]@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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