Additives to prevent superheater corrosion- Comparison of FB and grate combustion

Transcription

1 Additives to prevent superheater corrosion- Comparison of FB and grate combustion VII Flame Days, Tampere, Finland By: Martti Aho Hao Wu*, Peter Glarborg*, Juha Roppo** *DTU, Denmark ** Valmet

2 24/1/214 2 Goals of study To compare sulphation power power of additives (against alkali chlorides) at grate combustion and bubbling bed combustion conditions To compare combustion technologies in view of additive dosage To explain reasons to the selected findings by modeling tools

3 24/1/214 3 Properties of the studied fuels BFB fuels: REF = refuse derived fuel, bark = fir bark, burnt as mixture 4% REF 6% bark in energy Grate combustion fuels: CS = corn stover, wood = heartwood chips burnt as mixture 4% CS 6% wood in energy fuel Cl S K Na Ca Ca/S Al Si REF4/ bark CS4/chips

4 24/1/ kw grate combustor and 2 kw bubbling bed To stack Observation port Gas probe Gas cooling Deposit probe Gas sample Temperature control Sampling port Bag filter Sampling port Heating zone 4 Obervation port/ Deposit probe Heating zone 3 Sampling port Cyclone Tertiary air optional Obervation port Tertiary air optional Tertiary air (preheated) Heating zone 2/ Cooling zone 2 Fuel container 2 Fuel container 1 Additive container Obervation port Heating zone 1/ Cooling zone 1 Obervation port BED made of quarz PC control and data logging system Secondary air (preheated) Primary gas heating Nitrogen Air REF: EU project Enercorn REF: TEKES project Corraway

5 24/1/214 5 Temperature vs. time in grate and BFB furnaces chemical spraying BFB grate T, C chemical spraying (Y3) furnace top furnace top (Y5) t, S

6 24/1/214 6 Impactor sampling to determine Cl in alkali chlorides and deposit sampling to SEM EDXA analysis to determine Cl deposition

7 24/1/214 7 Comparison of flue gas composition Remarks: Huge differences in SO2 formation between GC and BFB 7 1 kw grate combustor 2 kw BFB 7 BFB, 4% REF / 6% bark mg/nm3 6% O HCl SO2 Al/Cl = 4. post flame S/Cl =.8 to fuel S/Cl =.8 as spray post flame S/Cl =.8 to fuel mg/nm3 6% O HCl SO2 Al/Cl = 4.35 Al/Cl = 4.35 S/Cl =.75 S/Cl = No Kaolin Y3 Fe2(SO4)3 Fe2(SO4)3 Y3 S No Kaolin fb Kaolin bed Fe2(SO4)3 (NH4)2SO3

8 24/1/214 8 Comparison of mass flows of chlorine present in alkali chlorides Remarks 1: Kaolin has no decomposition power at grate combustion conditions Remarks 2: Huge increase was found in power for ferric sulphate when sprayed at post flame compared to its mixing with fuel Cl mg/nm3 6% O kw grate combustor 2 kw BFB Grate combustor, 4% corn stover, 6% wood chips Al/Cl = 4. post flame S/Cl =.8 to fuel S/Cl =.8 to fuel Cl mg/nm3 6% O2 in: µm µm µm µm.3-.1 µm Cl mg/nm3 6% O BFB, 4% REF / 6% bark Al/Cl = 4.35 Cl mg/nm3 6% O2 in: µm µm µm µm.3-.1 µm S/Cl =.75 as spray post flame 2 S/Cl =.8 as spray post flame 2 S/Cl =.75 as spray post flame No Kaolin Y3 Fe2(SO4)3 Fe2(SO4)3 Y3 S No Kaolin fb Kaolin bed Fe2(SO4)3 (NH4)2SO3

9 24/1/214 9 Cl deposition results 1 kw grate combustor 2 kw BFB Grate combustor 4% corn stover 6% wood chips 4 BFB, 4% REF / 6% bark 35 Al/Cl = wind 3 wind 25 Cl wt% 2 side lee 25 Cl wt% 2 Al/Cl = 4.35 side lee S/Cl = Al/Cl = 4.35 S/Cl =.75 S/Cl =.75 No Kaolin Y3 Fe2(SO4)3 No Kaolin fb Kaolin bed Fe2(SO4)3 (NH4)2SO3

10 S conversion to SO 2 (%) 24/1/214 1 DTU tube reactor measurement on thermal decomposition of ferric sulphate: - (1%-6% SO2) = 4% of sulphur forms SO3 at 7-1 C: - Real situation is far from equilibrium Experimental Equilibrium calculation Temperature ( o C) Ref. Wu et al. AICHEJ 214

11 Cl in aerosols (mg/nm 3 ) 24/1/ Modeling result (DTU): comparison of sulphation powers of ammonium and ferric sulphates, SO2 and SO3 to KCl when injected at post flame zone, Sadded/KClmeasured =.5 (theoretic minimum to destroy all KCl) (Sadd/Clfuel =.184) T vs t used was taken from VTT s BFB Ferric sulfate Ammonium sulfate SO 3 SO Injection temperature ( o C) Used in combustion tests Ref: Wu et al, Energy& Fuels 214

12 Concentrations (ppmv) Temperature ( o C) Concentrations (ppmv) Temperature ( o C) 24/1/ BFB combustion modeling result (DTU): progress of sulphation during spraying of ammonium and ferric sulphates (S/Cl =.75) at post flame zone (NH4)2SO4: too fast decomposition=> weaker sulphation power against KCl Fe2(SO4)3: decomposition is slower enabling stronger sulphation of KCl * Means calculated SO3 concentration in undecomposed sulphate SO 2 HCl 9 15 HCl 9 SO SO 3 * K 2 SO 4 KCl SO KHSO 3 4 T K 2 SO 4 (s) SO 3 * K 2 SO 4 KCl KHSO 4 T K 2 SO 4 (s) Residence time (s) SO Residence time (s) Ref: Wu, H. et. al. Energy&Fuels 214

13 Concentrations (ppmv) Temperature ( o C) Concentrations (ppmv) Temperature ( o C) 24/1/ Grate combustion: Modeling result (DTU) S/Cl = 1 Ferric sulphate Mixing with the fuel Spraying at post flame zone (port Y3) Poor sulphation of KCl Efficient sulphation of KCl due to too high T at SO3 rich zone Y3 Y5 SO 2 SO 3 * HCl KCl (a) SO 3 KHSO 4 K 2 SO 4 K 2 SO 4 (s) Distance from port Y1 (mm) Y3 Y5 HCl SO 2 (b) SO 4 3 * K 2 SO 4 K 2 SO 4 (s) 2 KCl KHSO 4 SO Distance from port Y1 (mm) * means calculated SO3 concentration in undecomposed ferric sulphate Ref: Wu, H. et. al, Finnish-Swedish Flame days 213

14 24/1/ Conclusions ( BFB = bubbling bed, GC = grate combustion) The two fuel blends studied contained equal amounts of Cl but otherwise their composition differed Sulphur capture did not work at GC conditios: high SO2 slip (due to too high T) Kaolin had significant decomposition power to KCl power only at BFB conditions The following experimental results were explained by modeling: - GC: Why the sulphation power of ferric sulphate drops strongly when mixed to the fuel compared to its power when sprayed at post flame zone - BFB: Why ferric sulphate is more effective than ammonium sulphate at the spraying temperatures used

15 24/1/ Acknowledgements EU (project Enercorn ) TEKES, Metso Power, Kemira, PVO (project Corraway 24-26)

16 24/1/ VTT creates business from technology