Experiences and Results of SO 3 Measurements Performed under Oxy-Coal Fired Conditions

Transcription

1 Institute of Combustion and Power Plant Technology Prof. Dr. techn. G. Scheffknecht 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 Experiences and Results of SO 3 Measurements Performed under Oxy-Coal Fired Conditions Reinhold Spörl Jörg Maier Günter Scheffknecht reinhold.spoerl@ifk.uni-stuttgart.de joerg.maier@ifk.uni-stuttgart.de

2 Outline 1. Background: SO 3 generation; H 2 SO 4 formation; Acid dew point; Controlled condensation method 2. Experiences measuring SO 3 : Choice of the filter material Temperature of the SO 3 condenser SO x concentrations in air and oxyfuel firing mode Influence of SCR catalyst on acid due point 3. Conclusions 4. Outlook 2

3 SO 3 sources (1/2) Variable amounts of sulfur (e.g. pyrite FeS 2 ) in coal, e.g.: Sulfur content in central European brown coals: 0.2 up to 5 % [1] Oxidation of sulfur to SO 2 during coal combustion Further oxidation of part of the SO 2 to SO 3 Predominant oxidation reactions in the boiler [2]: SO + O + M SO3 2 + M M indicates a caytalyst (e.g. Fe 2 O 3 ); Most efficient at 700 to 800 C. SO + O 2 SO 3 Relatively slow; Equilibrium on left side at high temperatures 3

4 SO 3 sources (2/2) SO 3 generation mainly depends on: O 2 concentration in boiler: SO 2 concentration in boiler: possible increase in oxyfuel firings 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 SO 2 oxidation takes place: In the boiler and economizer: depending on coal and boiler (in air firings: up to 1.6% of toal SO 2 converted) [3] In the SCR-reactor (in air firings up to 1.25% of total SO 2 converted) [3] 4

5 H 2 SO 4 formation and dew point (1/3) SO 3 and H 2 O form vapour sulfuric acid H 2 SO 4 [4]: SO (g) (g) (g) 3 + H2O H2SO4 Formation starts at about 400 C. At about 200 C almost all SO 3 is consumed. 5

6 H 2 SO 4 formation and dew point (2/3) Condensation of H 2 SO 4 when temperature falls below acid dew point temperature: T Dew = f(p SO3, p H2O ) T Dew calculation at IFK according to Zarenezhad [5] T Dew formerly calculated according to Verhoff/Banchero [6] or Okkes [7]: Verhoff/Banchero equation overpredicts T Dew Okkes equation underpredicts T Dew 6

7 H 2 SO 4 formation and dew point (3/3) Acid dew points calculated according to Zarenezhad equation: Typical dew points in air fired power plants: C 7

8 Controlled condensation method (1/1) No other flue gas component condensing at C Separation of H 2 SO 4 through selective condensation Condenser: Glas coil tempered between water and acid dew point Sample train: in out H 2 SO 4 condenser Sample analysis by titration or chromatography 8

9 Experiences: Choice of filter material (1/5) At IFK in-stack dust removal is considered to be the most reliable configuration, to avoid H 2 SO 4 condensation on the filter Possible filter material for in-stack dust removal: Glass wool: glass fibers consisting of SiO 2 (approx. 70 %), Al 2 O 3, CaO, K 2 O, MgO and NaO Quarz wool: quarz fibers consisting of >99% SiO 2 9

10 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 10

11 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 SO 2 content in the flue gas: ~ 1300 mg/m 3 (STP) SO 3 measurements at a SCR catalyst (T in 395 C) 11

12 Experiences: Choice of filter material (4/5) Sampling with glass wool for dust removal (all samples taken behind catalyst) Variation of the sampling volume: No. Sampling volume [l] (STP) SO 3 in fluegas [mg/m3] (STP) A B C D Saturation behaviour acid mist not or late (after 30 min of sampling) visible in H 2 SO 4 collector Addition of SO 2 with secondary air (sampling volume 190l (STP)): No. SO 2 in fluegas [mg/m3] (STP) SO 2 in fluegas [mg/m3] (STP) E F G acid mist visible after minutes of sampling Significant variations Shorter saturation time 12

13 Experiences: Choice of filter material (5/5) Sampling with quarz wool for dust removal Sampling volume ~ 200l (STP) Sampling before and behind SCR catalyst: No. Sampling position SO 3 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 acid mist visible after 1-2 minutes of sampling, minor variation of concentrations No saturation behaviour with quarz wool Dust removal by glass wool 13

14 Experiences: SO 3 condenser temperature (1/2) Temperature of condenser coil between acid and water dew point H 2 SO 4 condensation follows the phase equilibrium according to the acid dew point: If the condenser coil is tempered to 121 C, 1.7 ppm H 2 SO 4 remain in the gas phase Temperature far below acid dew point, to asure a high level of H 2 SO 4 condensation Temperature high enough above water dewpoint to avoid water condensation in coil: Water dewpoints for water contents of 12, 28 and 35 Vol-% 20-30K above water dew point 14

15 Experiences: SO 3 condenser temperature (2/2) Water and acid dew points in air and oxyfuel mode measured at IFK s 500kW PC combustion rig: Air firing Oxyfuel firing Water concentration [Vol-%] Water dewpoint [ C] Acid dewpoint [ C] ~160 (calculated from H 2 SO 4 and water concentrations) Maximum water dewpoint (with soot blower: water content >35 Vol-%): 75 C Recommended coil temperature: Air firing: Oxyfuel firing: C C (depending on water concentration) 15

16 Measurements at IFK s 500kW combustion rig (1/3) Measurements at IFK s 500kW PC combustion rig Coal feeding O 2 Air CO 2 FD/ RG fan Storage tanks Air O 2 CO 2 Bottom ash Pre- Heater SCR Firing of ~50kg/h high sulfur brown coal Operation in air and oxyfuel mode By-passes ESP ID fan Stack 16

17 Measurements at IFK s 500kW combustion rig (2/3) SO 2 concentrations: Concentrations increased by a factor of 2.6 to 3.1 (air : oxyfuel) Maximum SO 2 concentrations in oxyfuel firing mode: approx mg/m 3 (STP) [reference O 2 : 6 Vol-%] SO 3 concentrations (measured at different positions along the flue gas path): SO 3 concentrations in air firing mode: up to 40 mg/m 3 (STP) [reference O 2 : 6 Vol-%] High SO 3 concentrations in oxyfuel firing mode: up to 200 mg/m 3 (STP) [reference O 2 : 6 Vol-%] 17

18 Measurements at IFK s 500kW combustion rig (3/3) Increase of acid dew point over SCR catalyst (qualitative): ~2 K Increase of approx. 30mg/Nm 3 ~15 K Increase of approx. 30mg/Nm 3 Oxyfuel firing Air firing 18

19 Conclusions (1/1) Controlled condensation method at IFK: Filter material for in-stack dust removal: Quarzwool H 2 SO 4 condenser coil temperature: Air firing: Oxyfuel firing: C C (depending on H 2 O content) Significantly higher SO 3 concentrations in oxyfuel mode Influence of SCR catalyst on acid due point is less pronounced in oxyfuel firings 19

20 Outlook Future research topics: Test and evaluation of continous SO 3 analysers (e.g.: Severn Science analyser, FTIR) Evaluation of the influence of fly ash in the gas sampling probe on SO 3 measurements Evaluation of the SO 3 evolution over the flue gas path in air and oxyfuel combustion, e.g.: SO 3 capture in the ESP SO 3 formation in the boiler Possibility of SO 3 enrichment due to flue gas recirculation 20

21 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 [2] T. L. JORGENSEN, H. LIVBJERG, P. GLARBORG: Shorter Communication - Homogeneous and heterogeneously catalyzed oxidation of SO2; In: Chemical Engineering Science 62 (2007), S [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 [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 [6] F.H. VERHOFF ; J.T. BANCHERO: Predicting dewpoints of flue gases. In: Chemical Engineering Progress 70 (1974), S [7] A. G. OKKES: Get acid dewpoint of flue gas. In: Hydrocarbon Processing 7 (1987), S Literature recommendation (background information on SO 3 measurement): Workshop Proceedings on Primary Sulfate Emissions from Combustion Sources. Volume I. Measurement Technology.,

22 Thank you for your attention! 22