Monitoring of corrosion due to RDF co-combustion in a coal fired power plant

Transcription

1 Monitoring of corrosion due to RDF co-combustion in a coal fired power plant V. Fantini Generation Technologies and Materials Department RSE- Milan, Italy vincenzo.fantini@rse-web.it

2 Outline RSE corrosion monitoring system boiler long-term corrosion monitoring in a large RDF-coal co-firing plant corrosion mechanism found corrosion monitoring in a MWI conclusions Work performed in the frame of: Biomass Project, financed by the Research Fund for the Italian Electrical System; UE DEBCO Project - Demonstration of Large Scale Biomass Co-firing and Supply Chain Integration

3 T 1 T 3 T 2 Workshop Which future for the SFR market? Milan, Nov. 20,2013 RSE corrosion monitoring system central unit remotely controlled suitable for operation in harsh environment up to 4 air cooled probes T-type probe: exposure in the membrane wall of the combustion chamber off-line measurement of the material consumption by postexposure metallographic section average material consumption vs. the inserted reference Pt pin T controlled by air cooling (at 2 mm from the sample surface) T&R-type probe: exposure into the boiler convective pass on-line measurement of residual thickness of a corroded material tubular shape simulates a section of a bundle tube T controlled in 2 points of the sample by air (at 1 mm from the sample surface) MM-type multi-material probe: exposure into the boiler at very high T (flue gas T up 1100 C) off-line meas. up to 6 samples of different materials bare or with cladding T 1 and T 2 independently controlled, T 3 adjusted close to T 1 and T 2 comparison among samples of same material at different T characterization of welded joints T controlled by air cooling

4 On-line measurement of the residual thickness of a corroded sample high accuracy measurement: ±5 μm resistive measurement T 1 and T 2 independently controlled Industrial PC PC industriale Data Acquisition Acquisitore and Control Unit Measure Zona di misura zone Current alimentatore Stabilized Power Supply Sistema di inversione Polarity Switch della polarità Unit Thermocouples termocoppie Differenza Potential di potenzi difference tubular shape sample: replicated fluid dynamics effects not uniform corrosion on real tubes flue gas flow 1 or 2 controlled probe T flue gas T user friendly interface

5 Long-term boiler corrosion monitoring in 320 MW Unit 4 of ENEL Andrea Palladio co-firing power plant Fusina-Venice (I) tangential fired dry-bottom boiler with LNCFS co-firing: bituminous coal + 5% of RDFsince 2009; t/y of RDF (RDF mass flow/unit: ave. 6.7 t/h, max. 9.2 t/h; ave. h RDF /h fire = 72% ) 4 long-term (8-15 months) monitoring campaigns performed, two into the combustion chamber and two in the convective pass measurement of long-term consumption rates and characterization of the corrosion attacks of various materials, both in the current operating conditions of the plant and simulating contemporary operation of co-firing + Ultra Super Critical steam cycle

6 Fireside corrosion monitoring in the combustion chamber RDF pellets are treated in dedicated blade-type high speed mills and injected mixed with the coal in 2 burner levels in the center zone of the boiler (above and below only coal fed nozzles) probe positions were selected on the basis of chlorine content at boiler walls Cl distribution at the membrane wall was calculated by ENEL CFD model 4 corrosion probes were installed two materials were characterized (16Mo3, A105) two campaigns were carried out (f. s. 240 ppmv) Workshop Which future for the SFR market? Milan, Nov. 20,2013 Expected chlorine map at the boiler membrane wall (by ENEL CFD model) probe probe position details expected Cl content 1 side close to RDF-coal nozzles high ( ppmv) 2 centre of front wall medium-high 3 side- below RDF-coal nozzles medium-low 4 side close to coal nozzles (baseline) very low based on CFD model, it was assumed the consumption of probe in pos. 4 as the baseline consumption for operation without co-firing (only coal as fuel)

7 Corrosion monitoring in combustion chamber (1 st campaign) Aim: characterization of material resistance to the corrosion, simulating operation in co-firing + USC steam cycle h of fire ( h of RDF) 3 extractions at different exposure times and consumption measurements first step of exposure (800 h RDF ) was repeated, in order to evaluate the variability of the consumption values 4 probes exposed tested material: 16Mo3 at T=450 C (material and T used in a large Italian USC p.p. in operation) Main chemical composition of the tested material material Cr Ni Mo Mn Fe others 16Mo bal. C ;Si 0.35; Cu 0.3; S 0.01; P 0.025

8 Corrosion monitoring in combustion chamber: results of 1 st campaign Sonda 15 Posizione h tot (CDR 2600 h) consumption ( m) Consumption of the sensitive element of the probes in combustion chamber of Fusina p.p.- Unit pos. 1 pos.3 pos. 4 material 16Mo3, T = 450 C y = 0,4231x 0,817 R 2 = 0,9979 y = 0,381x 0,784 R 2 = 0,9944 y = 0,3606x 0,7588 R 2 = 0, exposure time (h RDF) Profondità da altezza pin centrale (micron) ,6 3,2 2,8 2,4 lato bruciatore 2,0 1,6 1,2 0,8 Distanza dal pin centrale (mm) depending on the probe position 0,4 max. consumption in position 1 (side close to RDF-coal injection nozzles), min. in pos. 4 (considered reference for the operation with only coal as fuel) consumption vs. exposure time higher than a parabolic law: unstable and porous scale is formed detaching re-onset of the corrosion 0,0 0,4 0,8 1,2 1,6 2,0 2,4 2,8 3,2 3,6 4,0

9 Estimation of long-term material consumption Inputs for long-term consumption estimation: material/temperature: 16Mo3/450 C estimation at 8000 h fire /y (considered the ave.yearly up-time of this kind of plants) RDF max. 5% of the boiler thermal load RDF mass flow: ave.6.7 t/h, max. 9.2 t/h ave.h RDF /h fire =72% Estimation of yearly consumptions (8000 h fire /y) material / T Co-firing (probes 1 and 3) No co-firing (probe 4) 16Mo3 / 450 C 0.43±0.1 mm 0.26±0.1 mm

10 Post-exposure analysisof the samples (1 st campaign) EDS maps of exposed samples 16Mo3 Pt pin trace elements S products, silicates and aluminates in the deposit; compact Fe oxide in inner zone close to the interface to base metal; Na, K e Ca presence only in trace into the deposit; Pb and Zn not found

11 Corrosion monitoring in the combustion chamber 2 nd campaign Aim: measurement of the corrosion rate of the used material (A105) in the current operating temperature of 440 C (3135 h of RDF) material Cr Ni Mo Mn Fe others A bal. C 0.35 ;Si ; Cu 0.4; S 0.04; P 0.035; V 0.08 Main chemical composition of the tested material Comparison of results of 1 st and 2 nd campaign in the c.c. AISI S304H metallographic sections Probe 1 Probe 2 Probe 3 Probe 4 Workshop Which future for the SFR market? Milan, Nov. 20,2013 A105 consumption is more homogeneous in the different positions compared to 16Mo3 in 1 st campaign A105 sample in no.2 position was destroyed because of overheating due to half an hour lack of cooling air (T over 800 C) 16Mo3 consumption is higher in pos. 1 (close to RDFcoal nozzles) and 3; similar consumption of both materials in pos. 4

12 Corrosion monitoring in the convective pass Operating conditions: flue gas T: C estimated Cl content in flue gas: ppmv estimated alkali compounds concentration: ppmv 1 st campaign Aim: measurement of corrosion rates in current operation 5000 h of fire, 3500 h of RDF 2 probes exposed material AISI 347H (currently in use at the p.p.) T=570 C (operating T of bundle tubes RH HT in unit 4 of the p.p.) 2 nd campaign Aim: measurement of corrosion rates simulating USC + co-firing operation T=630 C (used in a large Italian USC p.p. in operation) first probe of AISI S304H (used in the above USC p.p.) second probe of AISI 347H at T=630 C, in order to compare its behavior in USC vs. SC conditions S304H sample: 5125 h fire, 4050 h RDF 347H sample: 5655 h fire, 4430 h RDF SH HT RH HT probes position Monitoring: on-line measurement of the residual thickness of the corroded samples post-exposure metallographic analysis

13 Results of 1 st campaign in the convective pass On-line measurement: negligible material consumption Post-exposure metallographic analysis: negligible corrosion penetration into the base material also in upstream zone (worst) two-layers Fe/Cr oxide formation Cr and Fe depletion zones into the base material close to the interface with oxide

14 Results of 2 nd campaign in the convective pass/1 Main composition of the tested steels: Material Cr Ni Mo Mn Fe Others AISI Super 304H bal. C 0.1 Si 0.2 Cu 3 Nb 0.4 AISI 347H bal. C 0.05; Si 0.29 plant summer outage for maintenance 5 μm On-line measurement along 13 months exposure of AISI S304H sample at 630 C: only a small thickness decreasing since summer 2011

15 Results of 2 nd campaign in convective pass/2 AISI S304H 5125 h of fire AISI 347H 5655 h of fire Position as angle with reference to top Corrosion penetration at 630 C (µm) 347H S304H Top Flue gas upstream side Down Flue gas downstream side negligible material consumption by metallographic analysis corrosion penetration generally low max. penetration in upstream zone 347H better than S304H (results of lab corrosion tests confirmed DEBCO Project - WP1) corrosion rate (µm/1000 h) Materials corrosion rates in 1000 h laboratory test (T spec 630 C; salt 50%KCl+50%K 2 SO 4 wt.; gas co-combustion Fusina p.p.) IN625 IN740 SAN28 AISI 347H AISI S304H R156 ave max

16 Post-exposure metallographic analysis (2 nd campaign) EDS maps similar behavior of AISI S304H and 347H samples unstable scale formation: Cr (inner) and Fe (outer) oxides Al Si S Cr and Fe depletion zones into base material Fe oxide not continuous, but limited to a few points SEM O Cr Al and Ca rich deposits outside Ca Cu Fe AISI S304H metallographic section and EDS maps at centre of upstream zone

17 Cl-corrosion mechanism: Active Oxidation (1) Cl, Cl - penetrate the scale to the interface FeCl 2, CrCl 2 are formed depleting Cr and Fe from zones close to metal-scale interface volatile FeCl 2, CrCl 2 back flow through the scale to the gas phase, leaving a porous and unstable scale in oxidative conditions Fe 2 O 3, Cr 2 O 3 are formed delivering Cl to the gas phase Cl re-onset the corrosion cycle Cr Fe Mechanism final results: SEM porous not compact scale, voids, frequent detaching Cr, Fe depletion zones into the base material two-layers scale formation: inner Cr oxide, outer Fe oxide (1)-H.P. Nielsen et al., The Implications of Chlorine-associated Corrosion on the Operation of Biomass-Fired Boilers, Progress in Energy and Combustion Science 26, pp , 2000 O AISI S304H EDS maps

18 Corrosion monitoring in WtE plants RSE system was installed in line 3 of an Italian MWI (capacity t/y), fed by 6 t/h of waste; steam parameters: 400 C/40 bar long-term corrosion monitoring of the bundle tubes in the convective is in progress (flue gas T= C; expected Cl content 500 ppmv) samples of pipe steel P11 with and without alloy 625 cladding are under testing at 400 C (operating T) and 450 C on-line and multi-material probes were installed On-line measurement of the consumption of the sample of steel P11 at 450 C along the first month: fast material consumption in the first 15 days followed by the long-term rate

19 Conclusions 4 long-term corrosion monitoring campaigns were carried out in RDF-coal co-firing Fusina p. p. by RSE corrosion monitoring system, exposing samples in the current operating conditions or simulating the operation in co-firing + USC steam cycle tested steelss304hand347h(currently inusein theplant) in theconvectivepass showed a negligible corrosion both in the current and USC conditions onthecontrarytestedsteels16mo3anda105(currentlyinuseintheplant)atthe membrane wall indicated that fireside corrosion was depending on the probe position, generally higher close to RDF-coal nozzles; estimated long-term 16Mo3 consumption in co-combustion operation was higher compared to that one for operation with coal as fuel the long-term campaigns in the plants also showed that on-line measurements of material consumption performed by RSE system can supply the plant managers with relevant and timely information on material degradation in different operating conditions useful for the optimization, which are in good agreement with postexposure metallographic examinations

20 Thank you for your attention!

21 Composition and quality of RDF at the power plant UNI Parameter UM normal high Values range for Fusina RDF Moisture % ,8-23,1 Volatiles % d.b. 66,0-77,8 Ash % d.b ,6-20,0 C % d.b. 42,1-54,6 H % d.b. 3,6-7,8 N % d.b. 0,5-2,6 S % a.r. 0,6 0,3 0,15-0,58 Cl % a.r. 0,9 0,7 0,61-0,9 F ppm d.b PCI kj/kg a.r Main elements in the ash (dry samples) Al % d.b. 0,5-1,7 Ca % d.b. 3,0-5,0 Fe % d.b. 0,3-0,6 K % d.b. 0,16-0,61 Mg % d.b. 0,3-0,8 Mn mg/kg ss Na % d.b. 0,4-1,1 P % d.b. 0,02-0,47 Si % d.b. 1,7-4,2 Ti % d.b. 0,1-0,4 Trace elements in the ash (dry samples) Hg ppm d.b. 0,19-1,59 As ppm d.b ,9-9,6 Cd ppm d.b. 1,0-6,6 Cd+Hg ppm d.b ,4-6,9 Cr ppm d.b Ni ppm d.b Pb ppm d.b Zn ppm d.b d. b. = dry basis; a. r. = as received Jan Dec Ash from Coal Fly Ash from Coal-RDF Average Value (mg/kg a. r.) Feb Mar Feb Mar Al Cr Cu Fe Pb Zn Mg 1680 n. a SiO2 n..a Ca K Na Chloride n. a. 8 n. a. 232 Sulphate n. a n. a. 2776