The influence of aerosol particles on the melting behavior of ash deposits in biomass fired boilers

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1 The influence of aerosol particles on the melting behavior of ash deposits in biomass fired boilers Rainer Backman (*, Bengt-Johan Skrifvars, Patrik Yrjas Åbo Akademi University Process Chemistry Centre FI Åbo Finland *) Umeå University Energy Techology and Thermal Process Chemistry SE Umeå Sweden

2 CONCLUSIONS Alkali chloride in ash and deposits gives first melting temperature around 550 o C. Lead and zinc together with alkali chlorides decrease the first melting temperature further to around 200 o C. The amount of melt as a function of composition and temperature can be estimated using theoretical methods.

3 OUTLINE Background Melting properties of salt mixtures Estimation of ash fraction composition Melting of ash fractions Influence of lead and zinc on melting Conclusions

4 BACKGROUND Novel way of predicting ash behavior Advanced fuel analysis Basic chemistry knowledge Chemical thermodynamic modeling Submodels for other models

5 Ash reactions in boiler flue gases Deposition Particulate emissions Transformations Volatilisation Condensation Melting Release Fine and Coarse

6 MELTING OF SALT MIXTURES Pure melting points Stepwise melting Characteristic temperatures Complex melting models

7 Melting temperatures Pure compounds PbSO o C K 2 SO Na 2 SO NaCl 801 KCl 771 ZnSO PbCl ZnCl Eutectics PbSO 4 -K 2 SO o C PbSO 4 -Na 2 SO ZnSO 4 -K 2 SO PbSO 4 -ZnSO ZnSO 4 -Na 2 SO PbCl 2 -NaCl 444 PbCl 2 -KCl 435 ZnCl 2 -NaCl 294 ZnCl 2 -KCl 288 PbCl 2 -ZnCl 2 243

8 Stepwise melting Melting curve Calculated Na 2 SO 4 -NaCl Portion molten phase vs. temperature, Na 2 SO 4 -NaCl Temperature [ C] A B C Liquid 628 Na 2 SO 4 (s) + NaCl(s) 801 Portion molten phase [wt-%] C B A T 100 T 70 T Na 2 SO 4 Composition [wt-% NaCl] NaCl Temperature [ C] T 0

9 Characteristic temperatures T 0 T 15 T 70 T 100 First melting temperature Sticky temperature Flow temperature Final melting temperature

10 SO 4 Na Na Na Cl K Na SO 4 CO 3 S SO 4 SO 4 Cl Na K Na Na Meltest K Calculation of melting properties in multicomponent systems S CO 3 CO 3 Cl S Cl 1200 Na min G(T, p, {N k}) = K K C S Cl Σ P p Σ i N i (p) µi (p) S SO 4 SO 4 S K K K CO Cl Na ÅA/CCRG/95 K S CO 3 CO 3 Cl SO 4 CO 3 K meltest.cdr Na

11 Melting curves for alkali ash K/Na = 90/10, SO 4 /CO 3 = 80/ T 100 Portion melt [wt-%] % Cl 0 % Cl T 70 T Temperature [ o C] T 0

12 ESTIMATION OF COMPOSITION Prediction based on fuel composition Aerosol formation models Condensation chemistry Influence of total alkali Influence of Na, Cl, S

13 Condensation sequences at cooling O 2 = 6 % in flue gases 1400 Wood chips Bark Waste wood Zn 2 SiO ZnO 1185 ZnO 1100 K 2 SO K 2 SO K 2 SO Temperature [ C] 800 K 2 CO Na 2 SO KCl 620 PbO 590 KCl KCl 485 Zn 2 SiO CdO 360 (PbO) 3 PbSO PbSiO CdO 445 Zn 2 SiO Pb 3 O Pb 2 SiO PbSiO (PbO) 3 PbSO CdO 420 CdSiO PbSiO 3 300

14 5 Alkali condensation pure compounds Na/(K+Na) = 0.2, Cl/(K+Na) = 0.45, S/(K 2 +Na 2 ) = Amount [g/100 kg dry fuel] K 2 SO 4 Na 2 SO 4 K 2 CO 3 KCl Formation rate, dm/dt, [mg/ C] Temperature [ C]

15 Alkali condensation melt formation Na/(K+Na) = 0.2, Cl/(K+Na) = 0.45, S/(K 2 +Na 2 ) = 0.45 Amount [g] Melt (sulfate-rich) Solid sulfate/carbonate Melt (chloride-rich) Solid chloride Formation rate, dm/dt, [mg/ C] Temperature [ C]

16 Alkali condensation melt composition Na/(K+Na) = 0.2, Cl/(K+Na) = 0.45, S/(K 2 +Na 2 ) = Sulfate Mol fraction Sulfate Chloride Carbonate Temperature [ C]

17 Melt composition Na/(K+Na) = 0.2, Cl/(K+Na) = 0.8, S/(K 2 +Na 2 ) = SO 4 Cl Temperature [ o C] Composition [mol fraction] solids only gas only CO 3

18 Influence of potassium Na/(K+Na) = 0.1, Cl/(K+Na) = 0.05, S/(K 2 +Na 2 ) = 0.95 Temperature [ o C] Gas Solid + Melt Solid K in dry fuel [wt-%]

19 Influence of sodium K in dry fuel = 0.2 %, Cl/(K+Na) = 0.05, S/(K 2 +Na 2 ) = 0.95 Temperature [ o C] Gas Solid + Melt Solid Na in fuel [% of K + Na]

20 MELTING OF ASH FRACTIONS Impactor measurements Biomass fired BFBC (105 MW) Chemical composition by SEM/EDX Al, Ca, Fe, K, Mg, Mn, Na, Si, Ti, Cl, S, P Estimation of speciation (compounds) Calculated melting curves Coarse ash fraction Biomass grate firing

21 Alkali part (K, Na) of fine particles Biomass combustion, BFBC Portion [wt-%] nm 82 nm 140 nm 240 nm 380 nm 640 nm 1.0 µm 1.7 µm 2.6 µm 4.2 µm 7.1 µm 10.8 µm >10 µm Impactor stage

22 Calcium salt part (SO 4, CO 3 ) of fine particles Biomass combustion, BFBC Portion [wt-%] nm 82 nm 140 nm 240 nm 380 nm 640 nm 1.0 µm 1.7 µm 2.6 µm 4.2 µm 7.1 µm 10.8 µm >10 µm Impactor stage

23 Inert part (oxides silicates) of fine particles Biomass combustion, BFBC Portion [wt-%] nm 82 nm 140 nm 240 nm 380 nm 640 nm 1.0 µm 1.7 µm 2.6 µm 4.2 µm 7.1 µm 10.8 µm >10 µm Impactor stage

24 Chloride and sulfate of fine particles Biomass combustion, BFBC Cl 2 /(Cl 2 +SO 4 ) [mol-%] nm 82 nm 140 nm 240 nm 380 nm 640 nm 1.0 µm 1.7 µm 2.6 µm 4.2 µm 7.1 µm 10.8 µm >10 µm Impactor stage

25 Melting curves of impactor samples Biomass combustion, BFBC nm 45 nm 82 nm nm 140 nm 240 nm Portion melt [wt%] nm 240 nm 380 nm 380 nm 640 nm 1.0 µm 1.7 µm 2.6 µm 4.2 µm 7.1 µm 10.8 µm >10 µm T Temperature [ C]

26 100 Melting curves of coarse ash Grate biomass firing Portion melt [wt-%] Waste wood Beech Temperature [ o C]

27 INFLUENCE OF LEAD AND ZINC Alkali chlorides + Pb and Zn Biomass grate firing Calculated cases

28 Calculated phase diagram KCl-PbCl 2 ÅAU-model 1200 Temperature [ o C] Liquid KCl KCl [mol fraction] PbCl 2

29 Calculated phase diagram KCl-ZnCl 2 ÅAU-model 1200 Temperature [ o C] Liquid ZnCl 2 KCl KCl [mol fraction]

30 Calculated melting curves for condensing fume with Pb and Zn. a) low Cl, b) high Cl 100 a. 100 b. Portion melt [wt-%] Temperature [ o C] Temperature [ o C]

31 Calculated cases for condensing fume with Pb and Zn Case Cl 2 /A (* Na 2 /A Pb/A Zn/A # m/m m/m m/m m/m ') A = K 2 + Na 2 + Pb + Zn = Cl 2 + SO 4 mol

32 Sticky temperature dependency of Pb and Zn LC: low chloride, HC: high chloride 1000 Sticky temperature [ o C] LC HC LC HC LC HC LC HC K + Na + Pb + Zn + Pb + Zn

33 CONCLUSIONS Melting properties of ash fractions affect deposit formation and growth Melting properties can be estimated if chemical composition is known Composition can be estimated based on thermodynamic considerations Stickiness strongly dependent of composition Ratio Cl/S critical for alkali fume melting T o C Presence of lead and zinc in alkali fume can decrease T 0 to 200 o C

34 ACKNOWLEDGEMENTS European Commission Tekes, the National Technology Agency of Finland Nordic Energy Research Foster-Wheeler Energia Oy Kvaerner Pulping Oy