Critical high temperature corrosion issues in biomass-fired powerplants

Transcription

1 Critical high temperature corrosion issues in biomass-fired powerplants L-G. Johansson J Liske, A Olivas, C Geers, D Zhao, E Larsson, JE Svensson, J Enqvist, K Hellström, N Israelsson, S Karlsson, T Jonsson High Temperature Corrosion Centre (HTC) Chalmers University of Technology, Göteborg, Sweden

2 GWh High Temperature Corrosion Centre Biomass- and waste-to-energy are increasing in Sweden Fuel mix used at the Händelö plant in Norrköping, source E.on

3 Biomass Waste Forest residue Recycled wood Straw Energy crops

4 What is the problem? Fireside corrosion reduces the lifetime of the superheaters. To mitigate corrosion, biomass- and wastefired boilers operate at lower steam temperatures, resulting in decreased electrical efficiency compared to coal-fired boilers.

5 Corrosion problems limit Green Electricty Production Material Environment Most important parameters to investigate? Boiler design Fuel

6 Characterisation of fuels, source Foster Wheeler 600ºC/185b 500ºC/110b 400ºC/65b

7 Elements important for deposit formation and corrosion Content [weight %] K+Na Coal Peat Wood Forestry waste Willow Demolition wood Wheat straw Chlorine Sulphur

8 Typical materials and temperatures in a biomass-fired boiler Superheater materials: Low alloyed steels Ferritic chromium steels Stainless austenitic steels High alloyed austenitic steels Material temperature Material cost

9 The fireside environment in a biomass/waste fired boiler The chemical environment at the superheater is very complex! CO 2, H 2 O, O 2, HCl, SO 2, NO x and particles High Temperature Corrosion Centre NaCl, KCl, CaSO 4, K 2 SO 4, K 2 CO 3, SiO 2 K 2 CrO 4, CaCrO 4 (Fe,Cr) 2 O 3, Fe 2 O 3, (Fe,Cr,Ni) 3 O 4 FeCl 2, CrCl 3 Fe, Cr, Ni, Mn, Mo Steam Deposit Oxide/Corrosion products Steel tube

10 Knowledge Validation Knowledge Knowledge Validation Knowledge High Temperature Corrosion Centre Research on model systems 2SO 2 (g) + O 2 (g) + 4KCl(s) + 2H 2 O(g) 2K 2 SO 4 (s) + 4HCl(g) 2,5 2 1,5 1 0,5 O 2 O 2 with SO Understanding of corrosion mechanisms Intelligent exposures Multiple analytical approach Field exposures Sanicro 28, 304L, T22 Wall Air outlet Air inlet

11 The protective scale Steam superheaters are often made from ferritic FeCr and austenitic FeCrNi alloys that can withstand high temperature corrosion because they form a protective (Cr,Fe) 2 O 3 scale. The Cr/Fe ratio of the solid solution determines its protective properties: Cr 2 O 3 (Cr,Fe) 2 O 3 Fe 2 O 3 Slow growing protective grows rapidly poorly protective

12 Reactions that deplete the oxide in chromia tend to destroy the protective properties of the oxide! Protective Non - Protective (Cr,Fe) 2 O 3 - Cr 2 O 3 Fe 2 O 3 Two important reactions that act as sinks for Cr: (Cr,Fe) 2 O 3 O 2 +H 2 O(g) Fe 2 O 3 + CrO 2 (OH) 2 (g) (Cr,Fe) 2 O 3 O 2 + (K +, Na +, Ca 2+ ) Fe 2 O 3 + ((K, Na) 2 or Ca)CrO 4 (s)

13 The role of alkali High Temperature Corrosion Centre

14 KCl High Temperature Corrosion Centre Some alkali salts cause stainless steel corrosion because they deplete the protective oxide in chromium! 304L, 5% O %H 2 O, 600 C Before After Chromate formed Mass gain (mg/cm 2 ) K 2 CO 3 Chromate formed O 2 (no salt) Exposure time (h) K 2 SO 4 No chromate formed (Cr,Fe) 2 O 3 O 2 + H 2 O + K KCl 2 SO CO 43 Fe 2 O 3 + K 2 CrO 4 + HCl CO SO 2 Pettersson, J., Folkeson, N., Johansson, LG and Svensson, JE OXIDATION OF METALS 76(1-2)

15 O 2 + H 2 O High Temperature Corrosion Centre Suggested mechanism for KCl-induced corrosion of chromia formers Cr depleted KCl (s) (Cr,Fe) 2 OK 3 2 oxide CrO 4 KCl (ads) O 2-, Cl - KCl (g) Fe 2 O 3 HCl(g) KCl (ads) K 2 CrOFe 4 2 O 3 (Fe,Cr,Ni) 3 O 4 MeCl x Metal Voids Propagation Initiation

16 The role of chlorine High Temperature Corrosion Centre HCl and alkali chlorides are very corrosive towards high temperature steels in oxidizing environment The resulting scales tend to be porous and have complex convoluted morphologies. Cl penetrates the oxide scale forming transition metal chlorides (e.g. FeCl 2 ) at the scale/metal interface. When exposed to the O 2 - containing gas, iron chloride tends to decompose, leaving a porous metal oxide behind. The process is termed active oxidation.

17 ESEM in-situ investigation of KCl induced corrosion of a low alloy steel Before exposure T22 at 400ºC with KCl(s). Exposure time: 1 hour. Ramp rate: 50 C/min. Atmosphere: ambient air, 2.5 Torr. 100 µm

18 Evolution of corrosion overview 330 C 355 C 360 C KCl KCl 10 µm 370 C 375 C 400 C (0 min) 400 C (2 min) 400 C (7 min) 400 C (20 min)

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21 O The presence of metal chloride causes: Poor scale adhesion Increased ion transport Porous oxide Material loss by vaporisation Fe Cl

22 High Temperature Corrosion Centre Electrochemical alloy chlorination in the presence of KCl 2KCl + 1/2O 2 + H 2 O + 2e - 2KOH + 2Cl - Fe Fe e - (at oxide/gas interface) (at oxide/alloy interface) Cl - (ads) Iron oxide e - Fe Cl - FeCl 2 Metal

23 Can we use the knowledge on the mechanism of corrosion to mitigate corrosion in the field? - If the same mechanisms at play we should be able to mitigate fireside corrosion in by converting alkali chlorides to sulfates, e.g. by using fuel additives: 2KCl + SO 2 +½O 2 + H 2 O K 2 SO 4 + 2HCl Alkali Chlorides Alkali Sulphates Adding sulphur

24 Experience from biomass and waste fired boilers Sulphur-rich additives ~50% decrease in corrosion rate with elemental sulphur 50-90% decrease in corrosion rate with sulphur recirculation Up to 80% decrease in corrosion rate with sewage sludge

25 Bringing laboratory results to a full scale commercial boiler Corrosion tests at the waste fired boiler at Renova, Gothenburg Sulphur Recirculation Turning a waste fraction from the flue gas cleaning into a corrosion mitigation technique!

26 Exposed probe rings with and without sulfur recycling Reference exposure (525 C, 1000h, 16Mo3) Deposit Sulphur recirculation exposure (525 C, 1000h, 16Mo3) Corrosion product layer Corrosion product layer Deposit Steel ring Steel ring

27 What about the materials? Alloys forming iron oxide scales are no good in alkali chloride environments The protection afforded by Cr 2 O 3 scales (stainless steels and other Cr alloys) tends to be lost by alkali chromate formation What about alloys that form Al 2 O 3 scales?

28 What about the materials? Alumina is electronically insulating and is much less reactive with alkali compared to Cr 2 O 3 Hence, alloys that form protective scales based on Al 2 O 3 have the potential to solve the fireside corrosion issue in biomass and waste fired boilers!

29 What about the materials? Work is in progress within HTC to develop new materials and coatings that form protective scales based on alumina However, there are many challenges Some recent results on the FeCrAl type alloys:

30 What about pre-oxidation? High Temperature Corrosion Centre

31 The work was performed by the Swedish High Temperature Corrosion Centre HTC is sponsored by: