Fireside Corrosion: Implications and Solutions for Oxy-combustion Boilers. Jo hn E. Oakey

Transcription

1 Fireside Corrosion: Implications and Solutions for Oxy-combustion Boilers Jo hn E. Oakey Tanvir Hussain Adnan U. Syed Nigel J. Simms Nelia Jurado Hamid G Darabkhani

2 Outline Introduction What we think we know What conditions apply Cranfield Testing Activities Aim Pilot-scale oxy-combustor trials Laboratory-scale fireside corrosion trials Damage measurement Findings Corrosion Damage Modelling Influence of process and materials variables Implications for Oxy-combustion

3 W hat do we believe we know? 1 (comparing oxy-firing with air-firing) Boiler environments will be very different with higher levels of CO 2 and steam and lower levels of N 2 due to flue gas recycle (FGR) Contaminant levels, in particular SO x, will increase due to FGR and the ratio of SO 3 to SO 2 will be raised, increasing the flue gas acid dewpoint Higher heat flux will influence deposition and deposit evolution Some differences in ash deposition behaviour and deposit chemistry have been found but there do not seem to be any consistent trends

4 W hat do we believe we know? 2 (comparing oxy-firing with air-firing) Pilot plant SH/RH corrosion testing often show no definitive difference in damage rates, but some show higher levels of damage but testing durations are often short. Is there an incubation effect where tests are using commercial tubing? Laboratory simulation of SH/RH corrosion also shows varied behaviour, but usually higher levels of damage related to the use of aggressive conditions - i.e. where boiler SO x levels and alkali deposits combine NOTE: Wide variations are often found between labs carrying out tests to the same specification reflecting the difficulty in managing the test variables and in our understanding of the mechanisms.

5 W hat conditions will the SH/RH materials see? Temperatures between 580 and >700 o C depending on whether the plant is new build with increased efficiency (higher steam temperatures) or is a retrofit SO x levels will be significantly higher than in air-firing up to about 4x the level, but will depend the types and levels of flue gas recycling. Also true for HCl if present. Levels of other contaminants will also be enhanced, depending on the fuel properties Deposit chemistries are broadly similar

6 Aim of Cranfield Research To develop a broad understanding of the fireside corrosion behaviour of heat exchanger (SH/RH) materials across the likely temperature range in oxy-combustion environments (with coal firing and biomass co-firing) Cranfield Research Activities Pilot plant testing to investigate boiler environments and deposition behaviour/chemistry with different levels and types of flue gas recycle Long term laboratory fireside corrosion trials to provide reliable data on the impact of process variables for model development

7 Predicted Gas Compositions UK vs. S. American coal Air vs. Oxy-fired (worst case - with hot recycled flue gas) UK coal, air-fired S Amer, air-fired UK coal, oxy-fired S Amer, oxy-fired Volume % H2O CO2 O2 N2 Ar SO2 HCl Gas species

8 Sensitivity of SO 2 vs. HCl to changes in cereal co-product (CCP) or typical wheat straw co-firing with two coals compared to example biomass UK coal (Daw Mill) + straw (wheat, typical) UK coal (Daw Mill) + CCP South American coal (El Cerrajon) + straw (wheat, typical) South American coal (El Cerrajon) + CCP Biomass (examples) HCl (vpm) % wheat straw 100% CCP SOx (vpm) Increase in co-firing 100% UK. 100% 100% S.Am CCP

9 Sensitivity of SO 2 vs. HCl to changes in cereal co-product (CCP) or typical wheat straw co-firing with two coals compared to example biomass UK coal (Daw Mill) + straw (wheat, typical) UK coal (Daw Mill) + CCP South American coal (El Cerrajon) + straw (wheat, typical) South American coal (El Cerrajon) + CCP Biomass (examples) HCl (vpm) % wheat straw 100% CCP SOx (vpm) Increase in co-firing 100% UK. 100% 100% S.Am CCP

10 Cranfield Pilot Sca le O xypf Combustor Recycled Flue Gas CO 2 +O 2 O 2 Pulverised Fuel Secondary Flow Primary Flow Wet Recycle Oxy-Fuel Combustor Heat transfer measurement Burnout measurement Dry Recycle Water and SOx Removal Acid dew point measurement SO 3 measurement CO 2 Rich Flue Gas Flow Diagram of the Cranfield Oxy-fuel Pulverised Fuel-fired Combustor (Red: Recent Modifications)

11 Cranfield Pilot Sca le O xypf Combustor Pure Oxygen (O2) Recycled Flue Gases Pulverised coal / biomass/natural gas + CO2/O2 / Air Cooling water in Condenser Cooling water out Dry Gas Temperature monitoring Gas analysis Temperature monitoring Coolant out Coolant in Cooling water out Cooling water out Cooling water in Cyclone Exhaust Condensates Fan Ash removal system Gas analysis Temperature monitoring Cooling water in Diagram of 100kWth Oxy-Combustor with Condenser Fan

12 Nominal compositions of fuels used Proximate analysis (% wt, AR) Moisture Ash Volatile matter Calorific value (kj/kg) Gross Calorific value Net Calorific value Ultimate analysis (% wt, AR) Carbon Hydrogen Nitrogen Oxygen Sulphur Chlorine Ash composition (% wt, of total ash) SiO 2 Al 2 O 3 Fe 2 O 3 TiO 2 CaO MgO Na 2 O K 2 O Mn 3 O 4 P 2 O 5 SO 3 BaO CCP Daw Mill Coal CCP = Cereal Co-product

13 Deposits formed in oxy-firing trials a)daw Mill:CCP(50:50 %,wt) b)ccp(100%,w t)

14 Nominal composition of alloys used in laboratory fireside corrosion studies Alloys Cr Mo Ni Si Mn P S C Fe Others T Bal V; 2W; 0.09 Nb; 0.07 N 347 HFG Bal. 0.6< Nb + Ta< 1 HR3C Bal. 0.4 Nb; 0.2 N Alloy Bal Co; 0.4 Al (wt%)

15 Nominal gas compositions & deposits *Co-firing Daw Mill: CCP (80: 20) wt.% N 2 (vol.%) O 2 (vol.%) CO 2 (vol.%) H 2 O (vol.%) SO 2 (vpm) HCl (vpm) Air-firing Oxy-firing Na 2 SO 4 K 2 SO 4 Fe 2 O 3 D0 (bare) D * Deposit-recoat test methodology with 200 h cycle & total test duration 1000 h

16 Schematic of a controlled atmosphere furnace (air-firing)

17 Schematic of a controlled atmosphere furnace (oxy-firing) Mass flow Controller 1 Vent Gas A (HCl/N 2 /CO 2 ) Mass flow Controller 2 Gas B (SO 2 /CO 2 /O 2 ) Mass flow Controller 3 Gas C (CO 2 ) -5 C +5 C -5 C Alumina tube H o t z o n e Pump Condensate Scrubbers Thermostat DI water Pump Crucibles holding samples Alumina liner Stainless steel reaction vessel Hot water bath

18 Laboratory Fireside Corrosion Test Matrix Simulated Air-firing conditions Test No Temperature ( C) Time (hours) Materials Deposits T92, 347HFG, HR3C, 625 D0, D T92, 347HFG, HR3C, 625 D0, D T92, 347HFG, HR3C, 625 D0, D1 Simulated Oxy-firing conditions Test No Temperature ( C) Time (hours) Materials Deposits T92, 347HFG, HR3C, 625 D0, D T92, 347HFG, HR3C, 625 D0, D T92, 347HFG, HR3C, 625 D0, D T92, 347HFG, HR3C, 625 D0, D1

19 Simulated air-firing environment after 1000 h (bare alloys; 1300 vppm SO 2 /400 vppm HCl) T92 347HFG HR3C 600 C 650 C 700 C

20 Simulated oxy-firing environment after 1000h (bare alloys; 1300 vppm SO 2 /400 vppm HCl) T92 347HFG HR3C 600 C 118 μm 30.7 μm 19.5 μm 100 µm 100 µm 100 µm 650 C 128 μm 44.7 μm 103 μm 100 µm 100 µm 100 µm 700 C 623 μm 53.5 μm 16.3 μm 500 µm 100 µm 100 µm 750 C 377 μm 84.2 μm 10.2 μm 500 µm 100 µm 100 µm

21 Simulated air-firing environment after 1000 h (with D1 screening deposit; 6260 vppm SO 2 /1700 vppm HCl) T92 347HFG HR3C C 900 µm 900 µm 900 µm 900 µm 650 C 1 mm 1 mm 1 mm 900 µm 700 C 1 mm 1 mm 900 µm 900 µm

22 Simulated Oxy-firing environment after 1000 h (with D1 screening deposit; 6260 vppm SO 2 /1700 vppm HCl) T92 347HFG HR3C C 800 µm 800 µm 800 µm 800 µm 650 C 1 mm 800 µm 800 µm 800 µm 700 C 1 mm 1 mm 1 mm 800 µm 750 C 1 mm 1 mm 800 µm 800 µm

23 Sample Metrology & Data Analysis Post-exposure metrology of corroded samples x-y positions of features around samples Metal losses are calculated by comparing x-y datasets with measurements made prior to testing Y Direction (um) Data Point Distribution Y-distance X-distance X Direction (um) Origin Blade Polished cross section Rectangle sample Pins Motorised and calibrated X-Y stage

24 Sample Metrology & Data Analysis 0 Change in sound metal (um) Position around sample ( ) 1. Metal loss data ordered (most damage to least damage) 2. Metal loss data plotted against cumulative probability Change in metal (µm) T22 347HFG T Probability (%)

25 Change in metal vs. cumulative probability for 347HFG covered in D1 in simulated oxy-firing

26 Median metal loss on bare alloys after 1000 h Median metal loss (μm) T92 347HFG HR3C Median metal loss (μm) T92 347HFG HR3C Temperature ( C) Simulated air-firing environment Temperature ( C) Simulated oxy-firing environment

27 Median metal loss on alloys with screening deposit (D1) after 1000 h 1400 T92 347HFG 1400 T92 347HFG 1200 HR3C HR3C 625 Median metal loss (μm) Median metal loss (μm) Temperature ( C) Temperature ( C) Simulated air-firing environment Simulated oxy-firing environment In general, the median metal damage for alloys followed: T92> 347 HFG > HR3C > 625 (except at higher temp HR3C outperformed 625)

28 Trends in median metal loss (screening deposit D1)

29 Characteristics of fireside corrosion p e a k (1) Corrosion damage Oxidation of alloy - rate depends on: Alloy composition Gas composition Alloy surface preparation Metal surface temperature Oxidation damage Temperature

30 Characteristics of fireside corrosion peak (2) Corrosion damage Deposit-induced alloy corrosion increasing rate depends on: Alloy composition Deposit composition (e.g. liquid deposit formation) Gas composition (e.g. SO 3 above that needed to stabilise deposit) Deposition flux Alloy surface preparation Metal surface temperature (must be above that required for formation of stable deposit).. Stable deposit Oxidation damage Temperature

31 Characteristics of fireside corrosion peak (3) Corrosion damage Deposit-induced alloy corrosion decreasing rate depends on: Deposit instability (e.g. reduced gas SO 3 levels) Metal surface temperature (above that needed for stable deposits) Gas composition Alloy composition Deposition flux.. Stable deposit Deposit unstable Oxidation damage Temperature

32 Predicted SOx change with temperature Oxy-firing with pre-fgd recycle giving high SOx levels in the boiler Hussain et al, 2013

33 Characteristics of fireside corrosion peak (4) Corrosion damage Increasing deposition flux Increasing SO 3 Stable deposit Deposit unstable Oxidation damage Temperature

34 Overall characteristic bell-shaped fireside / hot corrosion peak Corrosion damage Stable deposit Deposit unstable Oxidation damage Temperature Simms, 2011

35 Corrosion mechanisms Corrosion damage Stable deposit Deposit unstable Oxidation damage Temperature Corrosive deposits Sulphate deposits Pyro-sulphates (eg K 2 S 2 O 7 ) Alkali-iron tri-sulphates (eg K 3 Fe(SO 4 ) 3 ) Mixed sulphates (eg K,Na,Fe) X SO 4 Chloride deposits - mixed Carbonates mixed Sulphate chloride carbonate soup Molten vs sticky vs solid Deposit instability Vapour condensation dewpoints SO 3 needed to stabilise some sulphate phases SO 3 /SO 2 balance favoured at lower temperatures Other phases more stable with change in deposit temperature

36 Implications for Understanding of Fireside Corrosion in Oxy-combustion Boilers Effect of simulated air-firing and oxy-firing combustion conditions on fireside corrosion of alloys :T92, 347 HFG, HR3C & 625 in C Tests targeted at environment anticipated around superheaters/ reheaters in future power plants Deposit (D1 used for alloy selection screening) induced significant corrosion damage compared to bare alloys Bell-shaped curves: characteristic of superheater/reheater fireside corrosion damage (with screening deposit ). Highest metal damage at 650 C in simulated air-firing 700 C in simulated oxy-firing Higher amount of SO x in the oxy-firing gases responsible for the shift in peak corrosion damage In general, the median metal damage for alloys followed: T92> 347 HFG > HR3C > 625 (except at higher temp HR3C outperformed 625)

37 Conclusions and what next? Increasing steam temperatures (to help offset energy penalties) will have implications for SH/RH fireside corrosion the peak damage temperature will change and may not be where the highest metal temperature are found. Corrosion mechanisms seem to follow closely the bell-shaped curve understanding developed over 50 years ago Need closer collaboration and alignment of materials testing approaches and data analysis - both in pilot/demonstration plant testing and laboratory testing if we are to develop a reliable understanding of fireside corrosion behaviour and a sound basis for materials selection (for alloys and coatings) (Cranfield is already collaborating with NETL and others)

38 Thank You Professor of Energy Technology Centre for Energy and Resource Technology (CERT) Cranfield University, UK