Fuel ash behavior importance of melting

Transcription

1 Fuel ash behavior importance of melting

2 Why is ash melting important? Bed agglomeration in fluidized bed boilers Bed behavior in BL recovery boilers Deposit formation and build up Corrosion of superheaters

3 Temperature [ C Pure substance melting points Na 2 S (1180) K 2 SO 4 (1069) K 2 S (948) K 2 CO 3 (901) Na 2 SO 4 (884) Na 2 CO 3 (858) NaCl (801) KCl (771) K 2 S 2 O 7 (415) KOH (404) Na 2 S 2 O 7 (402) NaOH (320) KHSO 4 (197) NaHSO 4 (187) 0 Chemistry in Combustion Processes II - Hupa

4 Ash Melting at Increasing Temperatures Initial cone First melting Sticky Radical Complete deformation melting Temperature, o C Chemistry in Combustion Processes II - Hupa

5 Amount of melt, w-% Melting behavior of different alkali salts T 70 T Temperature, o C

6 Melt fraction [Wt%] Percentage Melt vs Temperature for an Alkali Salt Mixture T 0 T 15 T 70 T T [ C] Chemistry in Combustion Processes II - Hupa T 0 First melting temperature T 15 Sticky temperature T 70 Flow temperature T 100 Complete melting temp.

7 Temperature [ C] Percentage of molten phase lever rule T T LIQUID 800 T T % Na 2 SO 4 (s) + NaCl(s) L + NaCl(s) Composition [wt-% NaCl] 628

8 Sticky ash and T15 Chemistry in Combustion Processes II - Hupa

9 500 C 700 C 950 C

10 Melt fraction [Wt%] Percentage Melt vs Temperature for an Alkali Salt Mixture T 0 T 15 T 70 T T [ C] Chemistry in Combustion Processes II - Hupa T 0 First melting temperature T 15 Sticky temperature T 70 Flow temperature T 100 Complete melting temp.

11 Entrained Flow Particle Reactor University of Toronto Prov matning Gasbrännare Partiklar Ugn 9 m Mullitrör Våg Sond VCR 25.4 mm Chemistry in Combustion Processes II

12 Deposition Rate (mg/g/cm 2 /min) Stickiness of Salt Particles vs. Temperature and Composition 0.10 Na 2 SO 4 - NaCl blandningar % % 5% mol% Na 2 Cl Temperature ( o C) Chemistry in Combustion Processes II - Hupa

13 Deposition (mg/g-cm 2 -min) Stickiness of Partially Molten Particles Entrained Flow Reactor Tests in Toronto Fraction molten phase (wt-%) Chemistry in Combustion Processes II

14 Temperature around a tube wall Chemistry in Combustion Processes II - Hupa

15 Flue-gas-to-steam heat transfer - Clean tube Tube wall Steam 500 C Heat flux Temperature profile? Flue gas 1000 C Chemistry in Combustion Processes II - Hupa

16 Flue-gas-to-steam heat transfer Tube wall - Clean tube T gas = 1000 C Heat flux 110 kw/m 2 T steam = 500 C 570 C 600 C Chemistry in Combustion Processes II - Hupa

17 Flue-gas-to-steam heat transfer - Clean tube Convection Conduction Radiation & convection Chemistry in Combustion Processes II - Hupa

18 Flue-gas-to-steam heat transfer - Clean tube s Convective/conductive heat transfer Heat resistances in series Heat flux = k (T gas - T steam ) 1/k = 1/α gas + s/λ steel + 1/α steam Heat transfer coefficients: α gas = 40 λ steel /s = 4000 α steam = 1590 k = 39 Radiative heat transfer Flue gas to steel Heat flux = ε σ (T gas 4 T steel4 ), ε = steel emissivity σ = Stefan-Boltzmann constant

19 Flue-gas-to-steam heat transfer - Tube with deposit Tube wall Ash deposit Steam 500 C Heat flux Temperature profile? Flue gas 1000 C Chemistry in Combustion Processes II - Hupa

20 Chemistry in Combustion Processes II - Hupa Flue-gas-to-steam heat transfer - Tube with deposit Tube wall (5 mm) Ash deposit (1mm) T gas = 1000 C T steam = 500 C 560 C 680 C 580 C Case Heat flux Clean 110 kw/m 2 Deposit 95 kw/m 2 Thermal conductivities λ (W/m-K) Steel: 20 Deposit: 1 Conduction heat flux = λ ΔT/Δx

21 110 kw/m 2 95 kw/m 2 Chemistry in Combustion Processes II - Hupa

22 Deposit thickness and surface temperature Tube wall (5 mm) Ash deposit (2mm) T gas = 1000 C 740 C T steam = 500 C 550 C 570 C Chemistry in Combustion Processes II - Hupa

23 Deposit thickness and surface temperature 1 mm deposit 2 mm deposit 95 kw/m 2 82 kw/m 2 Chemistry in Combustion Processes II - Hupa

24 Flowing ash deposit and T70 Chemistry in Combustion Processes II - Hupa

25 Melt fraction [Wt%] Percentage Melt vs Temperature for an Alkali Salt Mixture T 0 T 15 T 70 T T [ C] T 0 Chemistry in Combustion Processes II - Hupa First melting temperature T 15 Sticky temperature T 70 Flow temperature T 100 Complete melting temp.

26 Steady-state deposit thickness T 70 Tube wall (5 mm) Ash deposit (2.3mm) T gas = 1000 C T steam = 500 C 550 C 750 C 570 C Case Heat flux Clean 110 kw/m 2 1 mm dep. 95 kw/m 2 2 mm dep. 82 kw/m 2 Steady-state deposit 79 kw/m 2 Chemistry in Combustion Processes II - Hupa

27 Air Cooled Probes after Exposure in Flue Gases 60 min 20 min Probe Surface Temp 500 C Flue Gas Temp 950 C 1 min Chemistry in Combustion Processes II

28 Chemistry in Combustion Processes II - Hupa

29 High temperature corrosion and T0 Chemistry in Combustion Processes II - Hupa

30 STEADY-STATE DEPOSIT THICKNESS T 0 T 70 Tube wall T gas Q. Steam Flue gas T steam Deposit Chemistry in Combustion Processes II

31 High temperature corrosion and T 0 T 0 = 540 C T 0 T 0 T steam T 0 79 kw/m 2 T kw/m 2 79 kw/m kw/m 2 T flue gas

32 Corrosion test with alkali salt deposits on steel at 550 C 0 % molten phase in deposit 5 % molten phase in deposit Chemistry in Combustion Processes II

33 Thermodynamic modeling of ash chemistry and melting

34 Melt fraction [Wt%] Thermodynamic modeling of ash chemistry and melting T 0 T 15 T 70 T T [ C] T 0 First melting temperature T 15 Sticky temperature T 70 Flow temperature T 100 Complete melting temp.

35 Modeling Experimental Thermodynamic modeling of ash chemistry and melting Laboratory Pilot / Full scale Melting behavior Corrosion experiments Well defined conditions, synthetic ash Melting behavior (Stable species ~ ash corrosivity) Synthetic ash Understanding boiler ash chemistry and melting corrosion Corrosion probes Actual boiler ash Melting behavior (Stable species ~ ash corrosivity) Accurate prediction of ash composition and conditions at steel surface???

36 At chemical equilibrium No thermal or mechanical (pressure) gradients No chemical potential gradients No apparent change in the amounts and the chemical composition of the phases in the system Forward and reverse reactions occur at the same rate Gibbs energy minimum of a system at constant P,T 3

37 Global chemical equilibrium calculation Input parameters Chemical composition Physical conditions Pressure, Temperature, Volume, Heat balance Thermodynamic database Gibbs energy functions of all compounds and phases considered in calculation Gibbs energy minimizer = Software to calculate chemical equilibrium Results Chemical equilibrium (e.g. Melting curves) Energy balances (e.g. Adiabatic flame temperatures)

38 Thermodynamic modeling successfully utilized in understanding BL smelt chemistry and melting

39 Pb & Zn in waste combustion Spectrum At-% Na 1.8 K 10.7 Ca 2.4 Pb 6.5 Al 0.7 Cl 4.5 S 13.0 O 60.4 Condensation behavior of zinc and lead, S.Enestam

40 Approach Thermodynamic equilibrium calculations Input: fuel composition Data: 16 elements (C, H, N, O, S, Cl, Ca, Mg, Na, K, Al, Si, Fe, P, Pb, and Zn), 1 gas phase 2 liquid solutions 2 solid solutions 12 pure liquid compounds 269 pure solid compounds Zone 2: T = C λ = 1.2 Zone 1: T = C λ = 0.5 Condensation behavior of zinc and lead, S.Enestam

41 Zone 1 λ = 0.5 Reference case Zone 2 λ = 1.2 Condensation behavior of zinc and lead, S.Enestam

42 Furnace Zone 1 λ = 0.5 bottom Condensation behavior of zinc and lead, S.Enestam

43 Furnace wall deposit (Pb,K,Cl 2 ) Zone 2 λ = 1.2 (NaCl) (PbCl 2 ) (Pb) (PbS) (KCl) At-% O Na S Cl K Condensation behavior of zinc and lead, S.Enestam Pb

44 Global chemical equilibrium calculation Input parameters Chemical composition Physical conditions Pressure, Temperature, Volume, Heat balance Thermodynamic database Gibbs energy functions of all compounds and phases considered in calculation Gibbs energy minimizer = Software to calculate chemical equilibrium Results Chemical equilibrium (e.g. Melting curves) Energy balances (e.g. Adiabatic flame temperatures)

45 Predicting Ash Chemistry Fuel sample 1 Chemical fractionation & SEM & Lab tests Reactive Thermodynamic equilibrium Chemical composition Inert

46 Comparison of experiments & predictions Combustion temperature: 700 C (wheat) Fuel Major fly ash phases (XRD) Major fly ash phases (model) HCl (exp.) HCl (model) SO 2 (exp.) SO 2 (model) Wheat straw K 2 SO 4, KCl, Ca-phosphate K-silicate, K 2 SO 4, Ca-phopshate, Ca-Mg silicates <1 ppm 110 ppm 4 ppm <1 ppm Wheat straw Reactive fractions K 2 SO 4, KCl, Ca-phosphate K 2 SO 4 -K 2 CO 3 -KCl liquid K 2 (SO 4,CO 3 ) (ss) CaCO 3, Ca-phosphate <1 ppm <1 ppm 4 ppm <1 ppm Fractionation data: All Si in inert fraction, Alkali in reactive fractions Predictions for Wheat Straw similar to observations

47 Mueller, Skrifvars, Backman, Hupa (2003) in Progress in CFD 47

48 48

49 Thermodynamic modeling - Summary Succesfully used for predicting smelt and dust chemistry in black liquor combustion For biomass combustion, additional process or fuels specific parameters often needed Element speciation, release -> reactivity Parametric studies to give general understanding of chemistry