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Final Report: EFP 95 Project No. 1323/95-0008
HIGH TElMPERATURE CORROSION IN BIOMASS INCINERATION PLANTS
DTU FGKE INST ITU TE Authors: 7 Klaus Gotthjaelp Povl Brendsted Materials Department, RIS0 National Laboratory
Piet Jansen Jacob Markussen The Force Institute, Division for Materials and Maintenance.
Melanie Montgomery Ernst Maahn Corrosion and Surface Technology, Institute of Manufacturing Engineering, Technical University of Denmark.
Edited by Melanie Montgomery @TU), March 1997. ISBN 87-550-2305-3 Portions of this document may be illegible in electronic image produttc fmncs are produced fmm the best available original dO~~t -,
Final Report: EFP 95 Project No. 1323/95-0008
HIGH TEMPERATURE CORROSION IN BIOMASS INCINERATION PLANTS
DTU
K>RcE INSTITUTE Authors: 7 Klaus Gotthjaelp Povl Brendsted Materials Department, IUS0 National Laboratory
Piet Jansen Jacob Markussen The Force Institute, Divisionfor Materials and Maintenance.
Melanie Montgomery Ernst Maahn Corrosion and Surface Technology, Institute of Manufacturing Engineering, Technical University of Denmark.
Edited by Melanie Montgomery @TU), March 1997. ISBN 87-550-2305-3 .
CONTENTS
ABSTRACT ...... 3 1. INTRODUCTION ...... 5 2. CELARACTERISATION OF ASH DEPOSITS FROM VARIOUS BIOMASS FIRED POWER PLANTS IN DENMARK ...... 7 2.1 INTRODUCTION...... 7 2.2 LABORATORYANALYSIS ...... 9 2.3 RESULTS ...... 9 2.4 DISCUSSIONAND Co~~s...... 20 3. EXPOSURE OF SPECIMENS IN GAS ENVIRONMENTS WITH ASH DEPOSITS ...... 24 3.1 EXPERTMENTALPROCEDURE ...... 24 3.2 RESULTS ...... 28 3.2.1 Sandvik 8LR30 ...... 28 3.2.I .I Exposure at a gas and metal temperature of 800 ‘17 and 600 ‘17 ...... 28 3.2.I .2 Exposure at a gas and metal temperature of 600 ‘17 ...... 33 3.2.2 Sanicro 28...... 38 3.2.2.I Exposure at a gas and metal temperature of 800 ‘17 and 600 ‘17 ...... 38 3.2.2.2 Exposure at a gas and metal temperature of 600 T ...... 44 3.3. DISCUSSIONAND MODELLINGOFRESULTS ...... 48 3.4. CONCLUSIONS...... 57 4 . CONCLUDING REMARKS...... 58 APPENDIX 1 ...... 60
2 Abstract
The aim of the present project is to study the role of ash deposits in high temperature corrosion of superheater materials in biomass and refuse fired combined heat and power plants. The project has included the two main activities:
An chemical characterisation of ash deposits collected from a major number of biomass and refuse fired combined heat and power plant boilers.
Laboratory exposures and metallurgical examinations of material specimens with ash deposits in well-defined gas environments with HC1 and SO2 in a furnace.
In the first part of the project ash deposits were collected from the radiation chamber, superheater and economiser sections in both waste incineration and straw-firedwood chip fired power plants. Thirteen plants participated in the investigations giving a total of 52 ash deposit samples. These were analysed using SEM-EDX. For refuse incineration, predominant elements are sodium, potassium, sulphur, chlorine, magnesium and calcium and in some cases zinc and lead. For straw or wood chip fired power plants, predominant elements are chlorine, sulphur, potassium and magnesium. Various combinations of these elements can lead to eutectic melts which have been found in some of the superheater deposits.
The laboratory experiments were conducted using the electrically heated furnace rig for gaseous exposures. Two types of high temperature resistant steels, Sandvik 8LR30 (18Cr lONi Ti) and Sanicro 28 (27Cr 31Ni 4Mo) were investigated at 600°C and 800°C flue gas temperature and at a 600°C metal temperature for up to 300 hours. Specimens which were embedded in ash deposits from a straw fired power plant were exposed to HCl(200 ppm) and SO2 (300 ppm).
The results show that the presence of both aggressive gases and ash deposits increase the corrosion rate synergistically. The reaction of potassium chloride with sulphur dioxide and oxygen is proposed to be the initiation reaction for the active oxidation mechanism at a flue gas and metal temperature of 600°C. When the flue gas is at 800"C, this reaction is fbrther encouraged and a more porous unprotective oxide forms. A melt is also formed at the deposit-flue gas interface presumably of KCl and KzS04 which can lead to hot corrosion where dissolution and precipitation of oxide occurs. For both the corrosion at a flue gas temperature of 600°C and at 800"C, Sanicro 28 had greater corrosion resistance than Sandvik 8LR30. By correlating the thermodynamical calculations and the experimental analysis results, the important corrosion reactions are identified.
This work has been conducted under the Danish Ministry of Energy Research Programme EFP 95 project No. 1323/95-0008.
3 ResumC:
FomGlet med projektet har varet at skabe et udbygget kendskab til den rolle askebelagninger influerer pH korrosionsforholdene i materialer anvendt i biobrandselsfLrede forbrandingsanlag. Dette kendskab er opbygget ved at karakterisere askebelagninger fia et anta1 biomassemede krafivarker og ved i velddefinerede korrosive gasblandinger at eksponere udvalgte materialer indpakket i aske.
Askeprodukter blev indsamlet fia b&deaflalds- og haldtraflisfjrede forbraendingsanlag. Aske fia 13 anlag blev inddraget i undersnrgelserne som gav totalt 52 forskellige prmer, der blev analyseret i SEM-EDX.
Det blev fbndet, at fia aflaldsanlag er de dominerende elementer Nay K, S, C1, Mg, og Ca og i visse tilfalde Zn og Pb. Fra halm eller traflis anlag fandtes de dominerende elementer at vare C1, S, K og Mg. Adskillige kombinationer af disse stoffer kan give eutektiske smelter som forhager hsjtemperaturkorrosion. og vekselvirkninger mellem gasser og salte kan fnrre til fiigmelse afklor hvilket resulterer i aktiv korrosion.
Laboratorie eksperimenter blev gennemfnrrt i en oxidationsudstyr opbygget i forbindelse med et tidligere projekt (EFP94 1324/94-006). To typer af hsjtemperatur stdl (Sandvik 8LR30 (18Cr 1ONi Ti) and Sanicro 28 (27Cr 31Ni 4Mo)) blev eksponeret ved 600°C i HCl og SO2 holdige gasser (gastemperaturer 600°C og 8OOOC) i op til 300 timer indpakket i akse fia et halmet kraftvark.
Resultaterne fia laboratorieeksponeringeerne viser at tilstedevzrelse af bide aggressiv gas og askebelagninger fornrger korrosionshastigheden. SANICRO 28 udviser en storre korrosionsmodstand end Sandvik 8LR30. Ved at sammenholde termodynamiske beregninger og de eksperimentelle analyseresultater kan de vigtigste korrosionsreaktioner identificeres.
Projektet viser, at en betydelig viden omkring korosionsprocesserne kan opnds ved en velkontrolleret identifikation og kontrol af de individuelle processer, der forekommer under de meget komplekse forhold der forefindes i forbraendingsanlaeg under virkelige forhold. Den viden kan i hsj grad medvirke til et bedre materialevalg og hewed give betydelige okonomiske gevinster for brugerne.
Projektet er gennemfarrt med financiel state fra Energiministeriets Forskningsprogram, EFP 95 1323/95-0008.
4 1
1. INTRODUCTION
Due to progressive environmental attention with respect to energy production in Denmark, straw, wood chip and domestic refuse are considered to be significant energy resources, which should not be just destroyed but utilised as valuable sources of energy. The research into high temperature superheater materials for power production through biomass and refuse started in 1991. The emphasis on this research has been developed through the Danish Ministry of Energy Research Programmes in 1991-95 (EFP projects) with participation from the Danish power industry and research institutes.
In 1991 Vnrlund A/S (producer of power boilers) RIS0 National Laboratory and the FORCE Institute (Materials and Research Laboratories) commenced the project EFP 91: “High Temperature Corrosion of Superheater Materials”’. The project aimed to establish an overview of the materials developed and commercially used for superheaters operated at 400-600°C in municipal solid waste fired boilers. The project also aimed to provide an up-to-date knowledge on the corrosion processes active in these environments. Based on the results of the EFP 91 project, it was planned to test a range of superheater materials.
The EFP 93 project “Testing of Superheater Materials for Waste and Biomass Incinerat~rs~~~was conducted in collaboration between Verlund, ELSAM and ELKRAFT (utility operators), the FORCE Institute and RIS0 National Laboratory. In this project, candidate materials were tested in-situ in 4 different boilers: 3 waste-fired boilers and 1 straw-fired boiler. Using water and air-cooled probes, 13 different materials were tested at operating conditions of 475-575OC for 500-700 hours. Following these tests, a small superheater coil was constructed to be inserted in one boiler, thus 8 materials were further tested for prolonged duration. These tests have produced data on material loss rate in the different boiler types and fuels. The observed corrosion of materials has also been described. From the preliminary results of EFP 93, it was discovered that it was difficult to relate specific parameters to corrosion rates for the alloys tested presumably because so many parameters are continually fluctuating such as gas composition, temperature, and ash composition.
The EFP 943 project “High Temperature Corrosion of Superheater Materials for Power Production through Biomass” concentrated on laboratory research to analyse the relationship of specific parameters and thus give a deeper understanding of corrosion
5 processes. Two main contributory factors to corrosion were studied based on straw-fired combustion: corrosion from a gaseous environment and corrosion fiom ash deposits found in a straw-fired power plant. The gaseous environment consisted of a synthetic flue gas to which 200 ppm HCl and/or 300 ppm SO;?were added at a gas temperature of 800°C and a metal temperature of 600OC. The fly-ash used contained potassium chloride and potassium sulphate and was fiom a superheater at Haslev power plant. Two alloy types are investigated, an AISI 321 type steel - Sandvik 8LR30 - and a high alloy steel - Sanicro 28. Sanicro 28 has a greater amount of chromium and nickel than an AISI 321 steel, and it is these alloying elements that are considered to give improved corrosion resistance. Based on these experiments it was found that the presence of an ash deposit greatly increased the corrosion attack compared to gaseous aggressive gas corrosion.
The present work, EFP 95, - is a direct continuation of the work in EFP 94 focusing on the importance of ash deposits. The major part of the project involves a systematic collection of ash samples of different types from superheaters in biomass and rehse fired heat and power plants. This part of the project has been undertaken by the FORCE Institute Division for Materials and Maintenance. The involved heat and power plants have beside taking the samples provided information on physical characteristics of the deposits, corrosion failures and tube replacements in the boilers. SEM-EDX elemental analysis has been carried out on the ash samples to reveal the distribution and concentration of chemical elements present in the ash deposits fiom refuse incineration and strawlwood chip fired plants.
The laboratory experiments have been conducted in a high temperature exposure rig at the Materials Department, RIS0 National Laboratory as a direct continuation of the work in EFP 94. The exposure of specimens in either mixed gas environments or under ash deposits has been conducted in EFP 94. Using the same material, exposure is undertaken where the specimens are embedded in ash surrounded by a mixed gas environment. In this way the interaction between both gaseous and deposit corrosion is investigated. The analytical and metallurgical examinations of the specimens after the exposure has been undertaken by The Corrosion and Surface Technology Department - The Institute of Manufacturing Engineering at Technical University of Denmark in co- operation with the Division for Materials and Maintenance at The FORCE Institute.
6 2. CHARACTERISATION OF ASH DEPOSITS FROM VARIOUS BIOMASS FIRED POWER PLANTS IN DENMARK
2.1 INTRODUCTION A systematic collection and examination was conducted of the composition of ash deposits from biomass &maces with superheaters for heat production. The investigation has strived to be as complete as possible in that all biomass fired plants with power production in Denmark were invited to participate.
In total, twenty plants have been contacted and they all were prepared to contribute by taking the necessary samples and giving the required information for the investigation. Table 2.1 gives a summary over the 15 biomass fired plants with production of superheated steam and electricity which have contributed to the investigation. Since ash deposits can only be taken out in connection with the shutdown of the plant, we have not received samples fiom plants that had not experienced shutdown during the investigation period.
The aim of the investigation has been to develop a better understanding of the ash deposits which constitute the non-volatile part of the corrosion environment in a biomass fired superheater. The ash deposits enter into chemical interactions with the hot and corrosive constituents of the flue gas. Together these make up the corrosive environment to which construction materials are actually exposed. The investigation thus contributes to the foundation for the building of models of corrosion environment and corrosion mechanisms for metallic materials in superheaters and boilers in biomass fired plants.
The practical work of taking ash and deposit samples from the individual plants was conducted by the operation personnel at the specific plants. The investigation includes samples taken from the following boiler areas:
1. The radiation chamber above the walls 2. The final superheater 3. The economiser section
7 EFP 95 Ash Analyses
Plant IBoiler IType IYear Hours
Amagerforbrzendingen Fuel Domestic and industrial refuse 1' Radiation chamber with Sic protection tiles Boiler no. 2 Velund Eckrohr 1991 21000 Boiler no. 3 Velund Eckrohr 1992 26000
KAVO Slagelse
rstrial refuse~~ Boiler No.1 Aalborg Boilers AT 23 W 1990 38000 18 Vh 430 Superheater partly renewed in 1994 heavy corrosion in the lowest 3-4 tube rows in SH3.
Slagelse Kraftvarmevierk Fuel: straw Boiler No.1 Aalborg Boiler AT23 1990 26000 40,3tih 450
Junckers industrier AIS Fuel Wood chips, saw cuttings and dust Boiler no. 3 B&W Boiler no. 6 B&W Energi water 1986 66000 60th 525 tube
{aslev kraftvarmevzerk Fuel: Straw i large bales, mainly wheat(lO,d%water) Soot blowing of the SH: Steam to remove dust and stee balls 10mirVh.ECO: Ball cleaning 30 min/h Boiler Vslund Eckrohr 1988 33000 26,4 Vh 455 Final SH has a general loss of metal of 0.3 mm
Zudkebing Kraftarmevzerk Fuel: . Straw -wheat, barley and triticale Boiler B8W Energi 1989 3oooO 13,9Vh 450
(olding Kraftvarmevmk Fuel: Domestic and industrial refuse Boiler Aalborg Boilers AT 23 W 1993 31,3Vh 420 laderslev kraftvarmevzerk Fuel: Domestic and industrial refuse Gas fired final SH Boiler Aalborg Boilers AT 23 W 1993 13.5 Vh 4301520
'hided Kraftvannevark Fuel: Domestic and industrial refuse Boiler Velund Eckrohr 1991 35000 6,4t/h 400-430 ahus Nord Kraftvarmeanlieg Fuel: Domestic and industrial refuse Boiler Aalborg Ciserv AT 23 1994 8000 24,7 Vh 430 irena Kraftvarmevzerk Fuel: Straw and coal SH No. 3 renewed in 1993 Boiler Ahlstrom Fluid bed 1991 3OOOO 100th 505 lovopan Traeindustri AIS Fuel Wood chips, saw cuttings and dust SH renewed in 1985 and 1990 Boiler Vslund Eckrohr 1980 120000 35Vh 450
.en0 Syd - Skanderborg Fuel: Domestic and industrial refuse (50/50%) Boiler Aalborg Boilers 1992 20000 18th 430 orsens Kraftvarmevark Fuel Domestic and industrial refuse Boiler no. 1 Velund Eckrohr 1991 22OOO 15.1 Vh 425 Boiler no. 2 Vslund Eckrohr 1991 21OOO 15.4 Vh 525 Final SH: 2 lower tube rows renewed eno Nord Aalborg Fuel: Domestic and industrial refuse Boiler No.1 Aalborg Boilers 19901 34000 35Vh 425
Table 2.1: Description of biomass plants involved in investigation.
8 As well as collecting samples, the operating staff have assessed the character of the deposits in the plant in a questionnaire. The samples were then packed in bags and sent to the FORCE Institutes laboratory.
2.2 LABORATORYANALYSIS At the FORCE Institutes laboratory, elemental analysis is undertaken using SEM/EDX to reveal the composition of the ash deposits and the content of corrosive elements. The analyses are undertaken on ground homogenised samples with a particle size of <100pm.
Elemental analysis is undertaken on FORCE Institutes scanning electron microscope Philips SEM 535 equipped with BSE and SE detector as well as a new advanced DX-4 EDX analyser with a high resolution detector. The detector is equipped with a polymer window that make it possible to detect X-ray signals from the lighter elements as well, i.e. from Z= 5 boron upwards. In this way it is possible to conduct a quantitative analysis of the samples content of oxygen, nitrogen and carbon. These elements cannot be analysed quantitatively by the majority of other types of SEM-EDX equipment.
For analysis, a homogenised sample is utilised which is placed in a sample cup, 5mm in diameter and 1 mm deep: the sample is compressed and its surface is levelled. The analysis is undertaken with an acceleration voltage of 25 kV, an analysis spot size of 64 mm2 and an analysis time of minimum 500 live seconds.
For the purpose of phase identification, one set of samples, samples from Reno-S, X-ray diffraction analysis was undertaken on ground powder with a Rigaku X-ray diffractometer using Cu-Ka radiation.
Further information on analysis technique and reliability has been investigated and is described in Chapter 5 of the EFP 94 final report3.
2.3 RESULTS The results of the SEM-EDX analyses are given in a complete summary for all the detected elements from waste incineration plants in Table 2.2 (including an analysis from a plant in Gibralter GBK). The results for the most important elements in the ash samples are illustrated graphically in Figures 2.1-2.8.
The results of the SEM-EDX analyses are given in a complete summary for all the detected elements from straw and wood chip fired plants in Table 2.3. The results for the most important elements in the ash samples are illustrated graphically in Figures 2.9- 2.14.
Furthermore a visual description of the structure of the individual ash deposits is given in Table 2.4.
9 EFP 95 Materials and Corrosion by biofuels - Analyses of ashes --__~_____~___ - Ref. Plant I No. Fuel Pos. -_-18b ___Amager K2 STR __-18c -~______(refuse) OH 18a ECO --12b Amager K3 ____STR 12a --- ______(refuse) ~-OH 12c ECO 15b Slagelse KV STR ------. - -- - __- .- 15c--- (refuse) ___OH3 15a ECO 19b Kolding KV STR
-__19c ---______.___(refuse) OH 19a ECO 14c HaderslevKV STR ~ - ~. 14b (refuse) OH 14a ECO 17b Thisted KV STR ____~17c - (refuse) OH 17a ECO
Reno-S (refuse)
__ 21d Irefuse) OH - L___ 21a I I ECO -__2oc I- HorsensK2 - I STR -__20d (refuse) OH . 20a ECO 24a __ Reno-N STR -__24b - (refuse) __~OH 24c ECO 7c - GBK STR 7b Irefuse) OH 1
Table 2.2: SEM-EDX analyses for deposits from waste incineration plants.
I Element: Chlorine (CI) Fuel: Refuse
25.0
H Radiation chamber 20.0 1 BSuperheater I 0 Economiser
15.0 s 3 3 10.0
5.0
0,o Amager Amager Slagelse Kolding Haderslev Thisted ArhusN Reno-S Horsens Horsens Reno-N GBK K2 . K3 Kv Kv Kv Kv Kv K1 K2 Plant
Figure 2.1: Presence of chlorine in ash deposits from waste incineration plants.
Element: Sulphur (S) Fuel: Refuse
Superheater 25'0 T' 0 Economiser 20'o I
Amager Amager Slagelse Kolding Haderslev Thisted ArhusN Reno-S Horsens Horsens Reno-N GBK K2 K3 KV KV 1 KV KV KV K1 K2 Plant
Figure 2.2: Presence of sulphur in ash deposits from waste incineration plants.
11 Element: Sodium (Na) Fuel: Refuse
25.(
Ei Radiation chamber H Superheater 2o.c 0 Economiser
15.0 s 3 3 10,o
5.0
0,o Arnager Arnager Slagelse Kolding Hadenlev Thisted husN Reno-S Honens Honens Reno-N GBK K2 K3 Kv Kv Kv KV Kv K1 K2 Plant
Figure 2.3:Presence of sodium in ash deposits from waste incineration plants.
Element: Potassium (K) Fuel: Refuse
25.0
Superheater 20,o Economiser
15.0 s 3 3 10.0
a Amager Arnager Slagelse Kolding Haderslev Thisted Amus N Reno Honens Horsens Reno GBK K2 K3 Kv Kv Kv Kv Kv K1 K2 Plant
Figure 2.4:Presence of potassium in ash deposits from waste incineration plants.
12 25.0 -r
20.0 Superheater 0 Economiser
15.0 s 3 3 10.0
5.0
Arnager Arnager Slagelse Kolding Hadenlev Thisted Arhus N Reno-S Honens Honens Reno-N GBK K2 K3 KV KV Kv KV Kv K1 K2 Plant
Figure 2.5: Presence of magnesium in ash deposits from waste incineration plants.
Element: Calcium (Ca) Fuel: Refuse
25.0 H Radiation chamber W Superheater 0 Economiser 20.0
15.0 s 3 3 10.0
5.0
0.0 Arnager Amager Slagel Kolding Haderslev Thisted Arhus Reno-S Hone Horse1 Reno-N GBK K2 K3 KV KV KV KV KV K1 K2 Plant
Figure 2.6: Presence of calcium in ash deposits from waste incineration plants.
13 Element: Zinc (Zn) Fuel: Refuse
25.0
Radiation chamber Superheater 20.0 0 Economiser
15.0 s 3 3 10,o
5.0 r
0,o Amager Amager Slagelse Kolding Hadenlev Thisted Arhus N Reno-S Horsens Honens Reno-N GBK K2 K3 Kv Kv Kv Kv Kv K1 K2 Plant
Figure 2.7: Presence of Zinc in ash deposits from waste incineration plants.
Element: Lead (Pb) Fuel: Refuse
25.0
FA Radiation chamber Superheater 20.0 ‘I1 0 Economiser
15.0 s 3 3 10.0
5.0
0.0 Amager Amager Slagelse Kolding Hadenlev Thisted Arhus N Reno-S Horsens Horsens Reno-N GBK u2 K3 Kv KV Kv KV KV K1 K2 Plant
Figure 2.8: Presence of lead in ash deposits from waste incineration plants.
14 Table 2.3: SEM-EDX analyses for deposits from straw/wood chip fired plants. Element: Chlorine (CI) Fuel: Strawlwood chips
50.0
45.0 Superheater 40.0 Economiser
35.0
30,O s 3 25,O 3
10.0
5.0
0.0 n Haslev KV Rudkabing KV Slagelse KV Juncken K6 Juncken K3 Gren! KV Plant
Figure 2.9: Presence of chlorine in ash deposits from straw/wood chip fired plants.
Element: Sulphur (S) Fuel: StrawMlood chips
25.0
E4 Radiation chamber Superheater 20.0 - ! 0 Economiser
n n n
0.0 Haslev KV Rudkabing KV Slagelse KV Juncken K6 Juncken K3 Gren6 KV Plant
Figure 2.10: Presence of sulphur in ash deposits from straw/wood chip fired plants.
16 Element: Sodium (Na) Fuel: Strawlwood chip
25.0
FA Radiation chamber Superheater 20,o I 0 Economiser I
15.0 s 3 3 10.0
5,O
Haslev KV Rudkabing KV Slagelse KV Junckers K6 Juncken K3 Greni KV Boiler plant
Figure 2.1 1: Presence of sodium in ash deposits from straw/wood chip fired plants.
Element: Potassium (K) Fuel: StrawMlood chips
50.0
EA Radiation chamber
40.0 . H Superheater Economiser 35.0 - n
30.0 - s 3 25.0 - 3 20.0 -
15.0 -
10.0 -
5.0 - 0.0 - - 1 Haslev KV Rudkebing Kv Slagelse KV Junckers K6 Junckers K3 GrenS KV Plant
Figure 2.12: Presence of potassium in ash deposits from straw/wood chip fired plants.
17 Element: Magnesium (Mg) Fuel: StrawWood chips
25.0 7
j Ed Radiation chamber i H Superheater 20,o 0 Economiser
15.0 s 3 3 10.0
Haslev KV Rudkrabing KV Slagelse KV Junckers K6 Junckers K3 GrenA KV Boiler plant
Figure 2.13 : Presence of magnesium in ash deposits from straw/wood chip fired plants.
Element: Calcium (Ca) Fuel: StrawWood chips
1"
45 ~ EA Radiation chamber Superheater 40 - Economiser
35 -
30 . s: 3 25 n 3 20
15
10
5
0 Haslev KV Rudkrabing KV Slagelse KV Juncken K6 Juncken K3 GrenA KV Boiler plant
Figure 2.14: Presence of calcium in ash deposits from straw/wood chip fired plants.
18 Table 2.4:Visual description of the structure of ash deposits collected
19 2.4 DISCUSSIONAND COMMENTS
The results of the analyses of the deposits taken from the investigated plants show that there is significant variation in composition of ash and deposit between the individual plants and between the samples from different zones in the boiler (radiation chamber, superheater and economiser). This is the case for both waste incineration and straw/wood chip fired plants.
The waste incineration plants are characterised by a relatively high content of the elements sodium, potassium, sulphur, chlorine, magnesium and calcium. Essential elements with respect to high temperature corrosion are additionally zinc and lead, which are present in high concentrations in several plants. The variations in composition must to a great extent reflect differences in he1 composition for the individual plants in the period when the sample was taken. Sources of lead may be assumed to be in the non domestic rehse in the form of old electricity and telephone cables, solders as well as lead accumulators etc. The high concentration of lead in the ash is remarkable today where much focus is set on environmental protection and recycling of chemicals and hazardous waste.
There is no significant difference in the concentration of the elements sodium, potassium, magnesium, calcium and chlorine in the different zones in the boiler. However sulphur only occurs in small concentrations in the radiation chamber whereas there is no marked difference between deposits taken from the superheater and the economiser. A similar but less significant distribution is found for the elements zinc and lead. An explanation for the observed differences in elemental distribution is presumed to be that the elements concerned are parts of salts and other chemical compounds which at the high temperature in the radiation chamber have a relatively high vapour pressure i.e. they are volatile.
The survey of the structure and character of the tube deposits has shown the majority of deposits from the refhe boilers to be either powdery or sintered from a light to a hard cement like deposit on the tubes. The colour of the deposits varies from dark grey to ochre-yellow. The deposits from the straw fired boilers is generally white or light grey and powdery in the economiser sections. Molten deposits with a dense crystalline structure in the deposits are found in the deposits ftom the final superheater in plants in Slagelse and in Rudbbing. In Haslev the corresponding deposits were swollen and coke like.
X-ray analysis of samples from Reno-S identified the presence of potassium sulphate in the superheater and economiser sections and potassium lead sulphate in one of the superheater sections. Small amounts of calcium sulphate, silica and potassium sulphate was also found in all samples. The presence of sodium zinc sulphate, potassium zinc sulphate or potassium lead sulphate is indicated.
20 Elements and compounds in ash deposits interact with the gaseous combustion products and hereby directly influence the corrosion mechanism. A more detailed description of some of the corrosion processes are given in the reports from earlier EFP projects EFP91', EFP93, and EFP943.
The following section summarises the essential elements which occur in the ash deposits with a short description of their influence on corrosion and the ash deposits properties:
Carbon (C) Carbon in the ash samples exists both as partially unburned soot or coke particles and as carbonate compounds of particularly calcium and magnesium. This is formed when calcium or magnesium oxide after cooling react with absorbed carbon dioxide and water vapour from flue gas or from the atmosphere. Unburned carbon and carbon monoxide in the flue gas do not participate directly in the corrosion mechanisms but have a significant influence on the corrosion environment by reacting with parts of the ash and by binding up the oxygen excess in the flue gas. Carbon monoxide is at high temperature strongly reducing and thereby can both work to destroy the metals protective oxide layer and also have a reducing effect on chlorine and sulphur compounds. Carbon monoxide is a gas type that can be detected in flue gas but not in collected samples of deposit.
Ovgen (0) Oxygen is for metallic materials the primary corroding medium by which the metallic materials are converted to corrosion products in the form of oxides. The formed oxide layer on the metals surface constitutes a protective barrier layer which greatly reduces the hrther reaction between oxygen and the underlying metal. The resistance of the protective layer is of extreme importance for the materials corrosion resistance. The negative effect of many of the elements which are found in ash or in flue gas results in the degradation of this oxide layer and thereby initiates a severe increase in the corrosion process.
Sodium (Na) and potassium (K) Sodium and potassium both constitute a great deal of inorganic chemical compounds which are found as salts in fuels and which later can be found in the ash. Both sodium and potassium are primarily found as chlorides, sulphates and silicates. Sodium and potassium alone do not contribute to the corrosion process but as salts of chlorine or sulphur they contribute actively in the corrosion process. In ash from straw, the potassium content is much greater than the sodium content due to the use of artificial fertilisers which contain a high content of potassium. Sodium and potassium salts are part of the majority of salts with a low melting point which can occur in the hottest areas of the boiler.
Calcium (Ca) and magnesium (Mg) Calcium and magnesium are found in ash during operation as salts of sulphur and chlorine as well as oxides formed at the high temperature in the boilers by dissociation of carbon dioxide (CO,) from carbonate compounds found in the
21 hels. Furthermore calcium and magnesium contribute to a range of silicate compounds. Since the majority of calcium and magnesium compounds have high melting points and their oxides are alkaline, the influence of calcium and magnesium compounds must be more accurately defined as corrosion inhibiting.
Sulphur (S) Sulphur is found in fuels as inorganic chemical compounds in the form of sulphate with sodium, potassium, magnesium and calcium. These compounds can also be found again in the ash. Sulphur is also present in various organic compounds which are converted to sulphur dioxide (SOz) and sulphur trioxide (SO,) during combustion. In reducing conditions with low oxygen content in the flue gas and also the presence of carbon monoxide, sulphate (SOS-), sulphur dioxide (SOZ) and sulphur trioxide (SO3) are reduced to sulphur (S) and sulphide( S-). Sulphur reacts in the same way as oxygen with metals and forms sulphides as corrosion products. Sulphides form however a poor and chemically unstable protective layer on the materials surface. Moreover sulphur and nickel can form a nickel-nickel sulphide mixture which is molten at temperatures above 640°C. Therefore for nickel containing alloys in reducing conditions, there exists the possibility for a severe increased corrosion which often is seen occurring along the grain boundaries of the metal.
Chlorine (CI) Chlorine is found in fuels primarily as inorganic chemical compounds of salts in the form of chlorides predominantly with sodium and potassium. Chlorine can in addition occur in waste with PVC plastic which when combusted releases hydrochloric acid (HCI). Chlorine salts in the form of chlorides can from reduction with carbon monoxide and with reaction of sulphur dioxide (SOz) be converted to chlorine gas (Clz) or hydrogen chloride (HCl). Both are especially corrosive in that low melting metal chlorides form which are thus volatile. Iron (HI) chloride has a sublimation temperature of 32OOC where it is directly converted to a vapour phase. The presence of chlorine and hydrogen chloride destroys the protective layer on the surface of materials which leads to a severely accelerated corrosion.
Zinc (Zn) Zinc onIy occurs only in ash from waste in large concentrations. The sources are mostly galvanised parts that are found in waste. The majority of this comes from industry. Zinc forms oxides and salts with chlorine and sulphur which can act as catalysts for corrosion attack on materials. Zinc chlorides are moreover severely hygroscopic which could have great importance for the corrosion environment at lower temperatures.
Lead (Pb) Lead, just like zinc, only occurs only in ash from waste in large concentrations. The origination of the significant amount of lead which occurs in certain plants is
22 not obvious. Sources for this must be assumed primarily to be fiom industrial and other non domestic rehse sources like old electricity cables, solder and lead accumulators. Lead easily forms oxides and salts of both chlorine and sulphur. Since lead can easily change valency, it also acts as a catalyst for both corrosion processes and conversion of the various aggressive chlorine and sulphur compounds in the flue gases.
The remaining elements in the ash samples have a lesser importance with respect to corrosion. They interact of course with the others and form chemical compounds or mixtures which can both effect the melting point of the ash mostly towards the higher temperatures and dilute the more aggressive components.
23 3. EXPOSURE OF SPECIMENS IN GAS ENVIRONMENTS WITH ASH DEPOSITS
3.1 EXPERIMENTALPROCEDURE The two metal types under investigation were Sandvik 8LR30 (an AIS1 321 type steel; SS 14 23 37; Wkstf Nr. 1.4541) and Sanicro 28 (SS 14 25 84; Wkstf Nr. 1.4563) whose specifications are given below.
C% Si% Mn% P% S% Cr% Ni% Mo% Cu% Ti% Sandvik Min. - - - 17.0 9.0 SX(C+N) 8LR30 Max. 0.08 1.0 2.0 0.045 0.030 19.0 32.0 0.80 SS 14 23 37 Sanicro 28 Min. - - - - 26.0 30.0 3.0 0.6 SS 1425 84 Max. 0.025 1.0 2.0 0.030 0.020 28.0 34.0 4.0 1.4
Table 3.1: Specifications of steel types employed in these experiments.
The specimens fabricated for exposure were cylindrical specimens 15 mm in length and 16 mm external diameter, 10 mm internal diameter. The external surface of the specimens was ground to 600 grit and electropolished.
The fly-ash from Haslev power station designated as Haslev OH II was used; a deposit was analysed using SEM with EDX and X-ray diffiaction. The SEM-EDX results are given below in Table 2. This together with XRD analyses show that the fly-ash from Haslev contains potassium sulphate (JCPDS 5-6 13) and potassium chloride (JCPDS 41-1476) with a small amount of calcium magnesium silicate (JCPDS 35-592). From X-ray and SEM-EDX analysis it is estimated that the Haslev OH I1 ash contained - 25 wt.% KC1 + 65 wt.% KzS04 + 10 wt.% silicates.
24 Element Mg Si P S c1 K Ca Fe Wt.% 0.07 3.51 1.59 15.59 19.35 53.01 5.1 1.78 0.39 2.83 0.83 16.83 17.75 55.04 5.42 0.90
Table 3.2: SEM-EDX analysis of elements in wt.% constituting fly-ash from straw- fired boilers utilised as deposits in this study.
Two sets of exposure have been undertaken in the experimental rig at RIS0 (Figure 3.1) according to the Table 3.3.
Furnace #l Furnace #2
Specimen holder lwh#===
-TT'- Heat coil: ca. 80°C
Pump sensor ' Venti- < lation -> -> I
.vw w y: rl Gas washer pxY /' 0 V + / s X v3 Humidifier (4O-6O0C,
Figure 3.1: Laboratory rig at Ris0 used for exposure of specimens in corrosive gases.
The flue gas mixture contains 7.3% H20, 19% C02, 8% 02plus 300 ppm SO2 and 200 ppm HC1; the balance was nitrogen. The samples were buried in a bed of gently packed ash. The exposure was undertaken in the experimental exposure rig at RIS0 as seen in the diagram above. From the three specimens that were exposed for each run, two were exposed with a flue gas temperature of 800°C and metal temperature of 600°C and one specimen was exposed where both the flue gas and metal temperature were 600°C. More detailed information on laboratory exposure procedures and equipment is given in the report from EFP 943.
25 4-9-2 200 ppm HCl I 1-9-1 Sanicro 25 300 ppm SOz 60OOC 6OOOC 3 IO h Ash deposits 200 ppm HCI
Table 3.3: Exposures undertaken in the laboratory testing rig at RIS0. The specimens were photographed before and after exposure (Figure 3.2a and 3.2b respectively). The specimen which was not air cooled, that is to say metal and flue gas temperature were both 600°C is placed in a ceramic cup surrounded by ash. Specimens with a metal temperature of 600°C and gas temperature of 800°C were mounted on a tube so that air of 600°C is delivered to give to the inner surface a metal temperature of 600°C. Specimens are supported in a ceramic tube, in which ash is lightly packed. Figure 3.2~shows the same specimens afier they had been taken out revealing that the ash deposit had sintered. The surface of the specimen was then analysed using SEM-EDX. In some cases spallation of oxide with ash deposit had occurred so these “spalls” were also mounted on a SEM-stub and analysed. The cylindrical specimens were cross-sectioned and mounted diametrically and axially and examined with light optical and scanning electron microscopy. Analysis of the true cross-section has been undertaken in this case in preference to the tapered cross-section, as the corrosion product is not so thin as was the case with specimens oxidised in a gas environment and analysis of the true section gives a more accurate picture of corrosion, especially with regard to corrosion attack, re€er to EFP 94 Final report’.
Figure 3 .?a: Specimens prepared for exposure.
26 Figure 3.2b: Specimens removed from exposure rig. Note that there is a sintered deposit layer on the specimen.
Figure 3.2~:Specimen removed from holder - note sintered deposit.
27 Specimen Material GasMetal Exposure Oxide Spalled oxide Depth of Total Temperature duration thickness thickness internal corrosion (Po (w) corrosion affected (elm) depth (pm) 3-9-0 Sandvik 800/6OO0C 263 h 5 150-200 120 325 3-9-2 8LR30 3-9-1 Sandvik 600/6OO0C 263 h 15 50 15 80 8LR30 4-9-0 Sanicro 80O/60O0C 310 h None 75-200 50 250 4-9-2 28 4-9-1 Sanicro 600/600°C 310 h 15 Couldnotbe 10 25 28 mounted.
Table 3.4: Oxide thickness measured for exposed specimens. From these results a trend could be seen that Sandvik 8LR30 experienced more corrosion than Sanicro 28 and that the specimens with a higher flue gas temperature of 800°C corroded faster than those at 600°C. It would be expected that a higher alloyed steel would perform better than a lower alloyed steel, and a higher flue gas temperature would give more corrosion than a lower flue gas temperature despite the fact that the metal temperature was the same in both cases.
3.2.1 Sandvik 8LR30
3.2.1.1 Exposure at a gas and metal temperature of 800 "c and 600 "C XRD analysis of the surface showed the presence of a magnetite type inverse spinel. The surface morphology viewed with SEM was uneven and prone to spallation (Figure 3.3). SEM-EDX analysis (Table 3.4) of the surface of the spalled layer was high in potassium, chlorine, calcium and oxygen. The underside of a spalled scale was rich in chromium and oxygen. The surface of the specimen from which the scale had spalled was rich in iron, nickel and oxygen, but not chromium. A cross-section of the spalled deposit (Figure 3.4) showed a white phase at the gas-scale interface which was rich in iron, calcium potassium and sulphur and a darker phase of inner oxide which was rich in iron and chromium. A cross-section of the specimen revealed an almost black corrosion product which had grown into the metal surface so that the outer surface was a layer consisting of metal particles and corrosion product whereas deeper into the metal, intercrystalline corrosion was observed (Figure 3.5). A pure oxide layer on the surface of the specimen was very thin. SEM-EDX analysis of the corrosion products showed that the overlying corrosion product was nickel based oxide with some iron and a little sulphur. Chromium was totally absent from the corrosion product. The dark phase was a nickel iron oxide and the light phase was nickel and iron. A small amount of chlorine was also detected in the corrosion product.
28 Figure 3.3: Surface of Sandvik 8LR30 exposed for 263 h. in the asWgas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively.
Table 3.5: Elemental analysis for Sandvik 8LR30 exposed for 263 h. in the ashlgas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively.
29 * Analysis for oqgen could not be undertaken.
Figure 3.4: LOM micrographs of cross-section of (a) spalled scale and (b) corrosion attack from Sandvik 8LWO (Specimen 3-9-2) exposed for 263 hours in the ash/gas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively. Areas of point analyses giving elemental composition in wt.%.
30 Figure 3.5a. Area of x-ray mapping. Figure 3.5b. Iron x-ray map.
Figure 3 Sc. Chromium x-ray map. Figure 3.5d. Nickel x-ray map.
Figure 3.53. Sulphur x-ray map Figure 3.X Chlorine x-ray map
Figure 3.5: SEM micrograph of cross-section of Sandvik 8LR30 (Specimen 3-9-0) exposed for 263 hours in the ash/gas corrosive environment at a metal and gas temperature of 600 and 800°C respectively.
31 A = Metal close to corrosion product B = Dark phase in corrosion product C = Blackiwhite corrosion product D = Light phase at interface E = Corrosion product close to sas-oxide interface F = Light phase in the middle of corrosion product.
Figure 3.6: SEM micrograph of cross-section of Sandvik 8Lft'O (Specimen 3-9-0) exposed for 263 hours in the ashlgas corrosive environment at a metal and gas temperature of 600 and 800°C respectivefy. Areas of point analyses giving elemental composition in w.%.
32 3.2.1.2 Exposure at a gas and metal temperature of 600 “c For specimen 3-9-1, the surface morphology viewed with SEM was uneven and grainy (Figure 3.7). SEM-EDX analysis (Table 3.5.) detected that the surface of the spalled layer was rich in sulphur, potassium, oxygen and phosphorus. The underside of the spall was rich in chromium, iron and oxygen. The surface of the specimen was rich in iron and nickel and oxygen with some chromium. A cross-section of the spalled deposit (Figure 3.8) showed that the inner oxide is an Fe-Cr oxide and that the outer layer contains iron oxide and calcium silicates. Potassium and sulphur are not present.
The cross section of the specimen revealed an outer corrosion product - mostly blacWdark grey - which also showed the presence of white particles within this oxide. Some intercrystalline attack could be seen but not to the great extent as that observed for the previous specimen with flue gas temperature of 800°C. The thickness of the affected corrosion region was also more even. From the elemental mapping and point analyses (Figures 3.9, 3.10), it could be seen that that the outer layer was a chromium sulphur compound. with some iron and nickel, probably a Fe-Ni oxide. The inner corrosion product contained nickel and iron, however more nickel than iron. A little chlorine was also detected within the corrosion product. The metal adjacent to the corrosion front was slightly depleted in chromium and was slightly richer in nickel.
33 Figure 3.7: Surface of Sandvik 8LR30 specimen exposed for 263 hrs in the ashigas corrosive environment at a metal and gas temperature of 600°C.
Table 3.6: Elemental analysis for Sandvik 8LR30 exposed for 263 hrs in the ashlgas corrosive environment at a metal and gas temperature of 600°C.
34 * Analysis for oxygen could not be undertaken.
Figure 3.8: LOM micrographs for (a) spalled scale and (b) corrosion attack from Sandvik 8LR30 (Specimen 3-9-1) exposed for 263 hours in the ashlgas corrosive environment at a metal and gas temperature of 600°C. Areas of point analyses giving elemental composition in wt.%.
35 Figure 3.9a. L4reaof x-ray mapping Figure 3.9b. Iron x-ray map.
Figure 3.9~Chromium x-ray map. Figure 3.9d. Nickel x-ray map.
Figure 3.9e Sulphur x-ray map. Figure 3.9f. Chlorine x-ray map.
Figure 3.9: SEM micrograph of cross-section of Sandvik 8LR30 (Specimen 3-9-1) exposed for 263 hours in the asWgas corrosive environment at a metal and gas temperature of 600°C.
36 G - 1.05 - - - 17.80 1.78 70.11 9.25 * Analysis for o'qgen could not be undertaken.
A = Corrosion product at gas-oxide interface B = Middle of blacldwhite corrosion product
C = Dark corrosion product at oxide-metal interface
D = Light phase in corrosion product E = Metal close to metal-oxide interface
F = Dark phase in the middle of corrosion product G = Metal Figure 3.10: SEM micrograph of cross-section of Sandvik 8LR30 (Specimen 3-9-1) exposed for 263 hours in the askigas corrosive environment at a metal and gas temperature of 600°C. Areas of point analyses giving elemental composition in wt.%.
37 3.2.2 Sanicro 28
3.2.2.1 Exposure at a gas and metal temperature of 800 “c and 600 “c The surface of the spalled layer analysed using SEM-EDX (Table 3.6) was high in oxygen, potassium and sulphur however no chlorine was present. The underside of the spdled scale was high in oxygen and chromium indicating chromium oxide. The surface morphology of the specimen viewed from SEM revealed two distinctive surface morphologies. The majority of the specimen surface was as in Figure 3.11 where the oxide had cracked and almost spalled; these areas were high in chlorine. Other areas showed a sponge like formation (Figure 3.12) and these areas were high in molybdenum, nickel with a little oxygen. The corrosion product on the specimen (two specimens were available for investigation) was scraped off and analysed with XRD. The result showed a magnetite inverse spinel crystallographic structure however it was also seen that there was a large amount of unidentifiable amorphous compound.
A cross-section of the spalled deposit (Figure 3.15) showed a white phase at the scale- gas interface which was rich in potassium, calcium, silicon and iron with a little nickel, and a light grey phase of inner oxide which was iron and chromium rich but also contain some silicon.
A cross-section of the specimen revealed no actual corrosion product alone but corrosion attack which consisted of metal lamella interspersed with corrosion products containing nickel or chromium and oxygen. However nickel has the greatest concentration in the corrosion product and in the metallic lamella within the corrosion product. In addition nickel and chloride rich particles were found in the area where chlorine was seen on the distribution maps (Figures 3.14, 3.15). Thus it was small nickel chloride particles that showed the presence of chlorine on the maps. Sulphur and molybdenum were also detected within the oxide.
38 0.16 0.47 0 Ca 1.39 57.75 17.91 5.31 Cr 0.44 Mn 0 0.09 0 0.28 Fe 0.29 4.29 6.64 4.94 Ni 0 0.57 17.07 42.57
Table 3.7: Elemental analysis for Sanicro 28 exposed for 3 10 hrs in the asWgas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively.
39 . , , . . "."
Figure 3.11: Surface of Sanicro 28 exposed for 3 10 hrs in the ashlgas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively -
-11 " I
-- - -I Fi.gure- 3.12: Surface of Sanicro 28 exposed for 310 hrs in the ash/gas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively - spongy oxide.
40 100 vm I
Figure 3.13 LOM micrographs for a) spalled scale and b) corrosion attack Sanicro 28 (Specimen 4-9-2) exposed for 310 hours in the ash/gas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively. Areas of point analyses give elemental composition in UT.%.
41 Figure 3.14a. Area of x-ray mapping. Figure 3.14b. Iron x-ray map.
Figure 3.14~Chromium x-ray map. Figure 3.14d . Nickel x-ray map.
Figure 3.14e Sulphurlmolybdenum Figure 3.14f. Chlorine x-ray map. x-ray map.
Figure 3.16: SEM micrograph of cross-section of Sanicro 28 (Specimen 4-9-0) exposed for 310 hours in the ash/gas corrosive environment at a metal and gas temperature of 600 and 800°C respectively.
42 A = Metal close to corrosion front B = Phase in corrosion product C = Metal island within corrosion affected area D = Corrosion area E = Dark corrosion product
Figure 3.15: SEM micrograph of cross-section of Sanicro 28 (Specimen 4-9-0) exposed €or 310 hours in the asldgas corrosive environment at a metal and gas temperature of 600°C and 800°C respectively. Areas of point analyses giving elemental composition in wt.%. 3.2.2.2 Exposure at a gas and metal temperature of 600 “c For Specimen 4-9-1, only the surface of the specimen was analysed as the spalled scale was so thin and fragile and could not be collected and mounted on a SEM stub.
The surface morphology as viewed with SEM is seen in Figure 3.16 with analysis adjacent to it. The surface is uneven and is prone to spallation. Little chlorine, potassium, calcium or silicon was found on the specimen surface, however the concentration of molybdenum and sulphur is relatively high.
A cross-section of the specimen (Figure 3.17, 3.18, 3.19) revealed an irregularly shaped oxide. Below the oxide is an area where the metal is porous. The oxide layer was rich in chromium and iron. There was no marked difference between inner and outer oxide. This is substantiated by the LOM micrograph, where the oxide is the same shade of grey all the way through. There was also a little sulphur with chromium, iron and nickel in the area where the oxide was weakly attached to the substrate. The porous metal was rich in nickel and there was also some molybdenum in this area.
44 Element a1 composition of surface in wt.%. 0 16.44, Si 1.45, Mo 8.65, S 5.08, C10.61, K 1.12, Ca 0.17, Cr 22.54, Mn 0.59, Fe 18.49, Ni 24.86.
Figure 3.16: Surface of Sanicro 28 exposed for 3 10 hrs in the asldgas corrosive environment at a metal and gas temperature of 600°C.
Figure 3.17: LOM micrograph showing corrosion attack for Sanicro 28 (Specimen 4- 9- 1) exposed for 3 10 hours in the ashlgas corrosive environment at a metd and gas temperature of 600°C.
45 Figure 3.1Sa. Area of x-ray mapping. Figure 3.18b. Iron x-ray map
Figure 3.18~Chromium x-ray map. Figure 3.18d . Nickel x-ray map.
Figure 3.18e Sulphurfmolybdenum Figure 3.18f Chlorine x-ray map. x-ray map.
Figure 3.18: SEM micrograph of cross-section of Sanicro 28 (Specimen 4-9- 1) exposed for 310 hours in the asldgas corrosive environment at a metal and gas temperature of 600°C.
46 * Analysis for ohygen could not be undertaken. A = Outer oxide B = Inner oxide C = Oxide at oxide-rnetal interface D = Hole E = Metal close to oxide-metal interface F = Dark hole within metal G = Metal close to porous metal section €3 = Metal analysis Figure 3.19: SEM micrograph of cross-section of Sanicro 28 (Specimen 4-9-1) exposed for 310 hours in the askdgas corrosive environment at a metal and gas temperature of 600°C. Areas of point analyses giving elemental composition in wt.%.
47 3.3. DISCUSSIONAND MODELLINGOF RESULTS Based on the results obtained, schematic diagrams (Figure 3.20-3.23) were built up showing the presence and concentrations of Fe, Cr, Ni, Si, C1 and S (and also 0) within the corrosion product. For the specimens exposed with a gas temperature of 800"C, oxygen has been included in the diagram as the majority of analyses undertaken also included analysis for oxygen. For the spalled oxide, the oxygen content was estimated to be 20 wt. %. For specimens where the gas temperature was 6OO0C, oxygen has been taken out of the diagram as only a few of the results included the analysis of oxygen. Based on the experimental observations and thermodynamic calculations, models describing the course of corrosion are given in Figures 3.24 and 3.25. The models proposed are working models which are open to revision and discussion.
In all cases, iron is more predominant in the outer oxide. Chromium is then present in the inner oxide and the inner corrosion products.
For the Sandvik 8LR30 specimen with flue gas at 800°C (Specimen 3-9-0), nickel and iron are the main constituents of the oxides attached to the specimens (Figures 3.5 and 3.6); chromium was absent from the corrosion product attached to the specimen but was present in the inner oxide (Figure 3.5). Small amounts (1-2%) of sulphur and chlorine were present in the corrosion products.
For the Sandvik 8LR30 specimen with flue gas at 600°C (Specimen 3-9-l), a similar picture is seen however there is a Cr and S rich layer in the outer corrosion product attached to the specimen (Figure 3.9). For both specimens, approx. 5% of silicon was also detected within the scale.
For Sanicro 28 at a flue gas temperature of 800°C (Specimen 4-9-0) a similar picture of Fe rich spalled outer oxide and Fe/Cr rich spalled inner oxide was observed (Figure 3.13). However within the attached corrosion product, there was not much iron or chromium. Chromium was predominantly present close to the corrosion front and nickel was present throughout the corroded region (Figure 3.14). In addition the metal adjacent to the corrosion front was depleted in iron and chromium and enriched in nickel. This indicates that iron and chromium are selectively form corrosion products. When they are lacking, this allows a higher oxygen potential to be present, and then reaction with nickel occurs. The presence of nickel and chlorine containing components was also detected within the attached corrosion product (Figure 3.15). A small amount of sulphur was also detected on the specimen surface.
For the Sanicro 28 with flue gas temperature of 600°C (Specimen 4-9-0) where minor spallation had occurred, the outer corrosion products were iron and chromium oxides with chromium being richer at the corrosion front. Thus the metal adjacent to the corrosion front was again rich in nickel and depleted in iron and chromium (Fig. 3.18). Some sulphur was detected within the corrosion product. The metal was porous at the corrosion front (Fig. 3.17, 3.19). The highest chlorine concentration (0.5%) was detected at the corrosion front.
48 -r , ,. ,.,,." ..,.", ,.....~ ,..., " .. 1. ,.~.-...I 60 50 40 30 20 10 0
Figure 3.20: Elemental composition across corrosion product on specimen 3-9-0.
70 T" $ 6o $ 50 40 c, 30 E -a 20 10 0 Spall Outer Inner Spall Surfaceof Outer Inner Corrosion Metal Bulkmetal surface oxide oxide underside specimen corrosion corrosion front adjacent product product to corrosion
I I
Figure 3.21: Elemental composition across corrosion product on specimen 3-9-1
49 Figure 3.22: Elemental composition across corrosion product on specimen 4-9-0.
70 60
10 0 Surface Outer inner Corrosion Metal Bulk mtal of corrosion corrosion front adjacent specimen product product to corrosion
Figure 3.23: Elemental composition across corrosion product on specimen 4-9-1.
50 An important result which was not seen in the gas only exposures is that there is a significant difference if the gas is 800°C or 600°C for a metal temperature of 600°C. For both Sanicro 28 and Sandvik 8LR.30, increased corrosion was seen at higher flue gas temperatures which indicates that the corrosion mechanisms are different when a deposit is involved. This implies that the heat transfer from the flue gas through the deposit results in temperatures above 600°C at the metal surface. In any case the deposit surface-flue gas interface will be at 8OO"C, thus giving temperatures between 600°C and 800°C within the deposit/oxide. The temperature of the specimen surface will depend on the extent the deposit and oxide promote or insulate against heat transfer.
There is no doubt that the presence of an ash surface deposit greatly accelerates the amount of corrosion. The question is does ash together with corrosive gases result in synergistic or additive accelerated corrosion. In the EFP 94 project, specimens exposed to flyash in an air environment at 600°C were conducted over 1 week resulting in a depth of corrosion of 24pm for Sandvik 8LR30 and 3 pm for Sanicro 28 (see EFP 94 Ch 63). For the specimens exposed to aggressive flue gases at a gas and metal temperature of 600°C conducted over 2 weeks, the depth of corrosion was up to 12 pm for Sandvik 8LR30 and <1pm for Sanicro 28 (see EFP 94 Ch 43). If the the corrosion rate with both mixed gas and ash deposits gave an additive accelerated corrosion and assuming a linear corrosion rate with respect to exposure time, this would give a calculated depth of corrosion of 60 pm for Sandvik 8LR30 and 7 pm for Sanicro 28. From the experiments in this project, for flue gas and metal temperatures of 600"C, the depth of corrosion was 80pm for Sandvik 8LR30 and 25 pm for Sanicro 28. These results give a corrosion rate which is above the calculated additive corrosion rate thus it can be concluded that the presence of a gas mixture has accelerated the ash reaction synergistically.
A interesting result seen for Sandvik 8LR30 is that in the experiments in this project with ash deposits and mixed gases, nickel forms corrosion products whereas in tests either in mixed flue-gas only atmospheres or salt deposits in air, the nickel did not form corrosion products except for a few percent incorporated in the oxide layer. Instead there was found a nickel enriched area at the metal adjacent to the corrosion fi-ont which is probably nickel as free metal. The reason for the encouraged reaction of nickel is that the surface metal is depleted of chromium and iron, which then results in corrosion of nickel as the alternative. The reaction is presumably accelerated by the synergistic interaction of aggressive gases and salt deposits.
For specimens with gas and metal temperature of 800°C and 600°C respectively, a temperature gradient will be seen from flue gas side to inner metal temperature4 so the temperature at the surface of the corrosion product plus deposit is above 691°C (temperature where KCl and &SO4 form a eutectic melt). This gives molten salt at the top of the corrosion product which can lead to the dissolution of oxide that comes in contact with the melt. The temperature at the oxide surface will probably be lower and closer to the 6OO0C, so although the ash in contact with flue gas is a molten phase, no molten phase will be in contact with the metal or oxide. However for Sanicro 28, the morphology of the specimen surface (Figure 3.11) indicates the presence of a melted phase containing nickel, iron, chromium and chlorine which has then cracked when the
51 specimen was cooled. CrC12 has been reported to form a melt with FeCl2 at temperatures slightly above 670°C'. No sign of melted phases or chlorine rich phases were found for Sandvik 8LR30.The reaction at the oxide front is therefore assumed to be gaseous reaction resulting in a porous unprotective oxide layer.
Thermodynamic Modelling Based on these experimental observations and the work in EFP943, the models are suggested for Sandvik 8LR30 and Sanicro 28 in Figures 3.24 and 3.25. The thermodynamic calculations for these models are given as. follows: AG" values are given at equilibrium, i.e. when AG" = -RTlnK at the temperature of 600°C and are calculated fiom the Thermocalc computer program'.
At the gas-oxide interface, the following initiation reaction occurs:
A. 2KC1+ SO2 + 02= &so4 + Cl2 AG9=-77kJ p(Cl2) = 0.965 atm.
Analysis of specimen surfaces detects potassium sulphate and a lesser amount or an absence of potassium chloride. The high presence of potassium sulphate without chloride supports the model where reaction A is the initiating reaction.
The chlorine migrates through the imperfections in the oxide scale, either via grain boundaries, pores or by volume difksion and reacts with the alloy below the oxide. In addition chlorine migrates down the grain boundaries within the alloy. Chlorine reacts predominantly with iron and chromium to form iron and chromium chlorides.
B. Cr + Cl2 = CrCl2 AG" = -286 W p(C12) = 7.7 x 10-"atm
C. Fe+ Cl2 =Fee12 AG" = -232 kJ p(C12) = 1.3 x 10-l4atm.
At higher chlorine partial pressures hrther reaction with nickel, chromium and iron occurs:
D. Ni + C12 = NiCl2 AG"= -174 kJ p(C4) = 3.8 x lo-" atm.
E. CrC12 + W12 = CrCb AGe = -72 kJ p(Cl2) = 2.1 x 10"atm.
F. Feel2 + %C12 = FeCl3 AG" = -9 W p(Cl2) = 0.084 atm.
At 6OO0C, FeCl3, FeC12, CrCl3 and NiC12 have vapour pressures above lxlOd atm. which is regarded as the critical value above which evaporation is importantg. Thus at
52 this high temperature, these chlorides are volatile and therefore evaporate from the metal surface. This can give rise to voids in the metal located at grain boundaries.
As the chlorides migrate out, they meet a gradually increasing partial pressure of oxygen. This will result in the formation of oxides and release of chlorine which again migrates into the metal and results in metal chloride formation.
G. 2CrC12 +1%02 = Cr203 + 2C12 AG" = -332 W
H. 3FeC12 +202= Fe304 + 3Clz AG" = -134 kJ
The iron chloride, which has a higher vapour pressure migrates hrther out from the metal and therefore forms oxide closer to the gas-oxide interface. This type of oxide is has a tendency to porosity. However in the case of nickel chloride:
I. NiCl2 + 1/02 = NiO + C12 AGe = +13kJ
The oxidation of nickel chloride has a positive Gibbs fiee energy implying that it will not occur spontaneously. Thus if nickel is converted to nickel chloride, it is more likely to be detected within the corrosion products where the partial pressure of chloride is relatively high. At lower partial pressures of chlorine close to the specimen surface, the nickel chloride will be converted to oxide or nickel metal. Chloride migrating out could also react with sulphur dioxide and oxygen close to the surface of the deposit. The presence of chromium in association with sulphur has been seen which indicates that the reaction J occurs. A similar reaction involving iron could also occur.
J. 2CrC12 + 302 + 3so2 = Cr2(S04)3 + 2C12 AGe = -474 kJ
K. 2FeC12 + 302 + 3so2 = Fe2(S04)3 + 2C12 AGe = -275 kJ
A subsequent reaction of sulphate formation could be as below or a similar reaction with chromium sulphate:
L. Fe2(S04)3= Fez03 + 3s03 AGe= +95kJ
With a higher flue gas temperature of 800°C, similar reactions will occur resulting in chloride and oxide formation to a greater degree. The increased formation of chloride will result in a more porous unprotective oxide. This will give greater corrosion since the reaction of metal and gas will not be slowed down by gaseous diffusion through a protective oxide layer.
For specimens exposed with a flue gas temperature of 8OO0C, it is presumed that the
53 surface of the oxide does not reach a temperature of at least 700OC. The presence of a melt at the corrosion fiont is not observed however there may be an indication of a melt fiom the cracking of the surface for the Sanicro 28 specimen (Figure 3.11). Potassium sulphate and potassium chloride form a eutectic mixture at 691°C with the composition 44 wt.% K2S04and 56 wt.% KCl. If this temperature was reached, a eutectic melt would allow ions to migrate freely and not be inhibited by solid state diffision barriers therefore resulting in greater corrosion.
To summarise: For corrosion at 600°C, the presence of chlorine was not detected in the previous work (EFP 943) with either gaseous corrosion or corrosion under deposits in air so this work reveals an interaction between the two corrosion processes: thus the amount of chlorine is continually replenished fiom the aggressive environment. The high presence of Ni on the surface of the Sandvik 8LR30 specimens indicates a nickel rich oxide. Such a nickel rich oxide did not form in either the gaseous or ash deposit corrosion. There is therefore a clear indication that the reactions that occur with the presence of both aggressive gases and ash deposits give greater corrosion attack than the aggressive gases alone or ash deposits alone.
It is therefore suggested that the initial reaction is:
2KC1+ SO2 + 02= K2S04 + C12 which can only occur when both deposits and aggressive gases are present. This gives the increased corrosion attack observed. The chlorine released accelerates the formation of oxides by the "active oxidation mechanism", however the occurrence of metal chlorides hinders the formation of a protective oxide layer. Chlorine diffises to the corrosion fiont to form metal chlorides which are converted to oxide as they migrates out to the surface of the corrosion product. This mechanism causes the formation of a porous oxide which is not protective.
At both flue gas temperatures, Iess corrosion is seen for Sanicro 28. Thus considering the vapour pressures of the relevant metal chlorides in descending order are FeC13, FeC12, CrC13, NiCl2 and CrC12, the specimens containing more iron are susceptible to greater corrosion attack than those with a high nickel or chromium content. For both steel types where the flue gas temperature was higher than the metal temperature, corrosion was greater which indicates the importance of flue gas temperature resulting in an increased temperature on the oxide surface.
54 SS so, Ha co2 40 o*
K2S0.4 KCI Silicates
Fe,Cr,Ni oxides
bo3
Cr/Fe oxide Cr sulphate/sulphide
Fe(Ni)oxide
Porous metal containig nickel
Sanicro 28
Figure 3.25a: Sanicro 28 at a metal and gas temperature of 600’C.
HCI SO, H,O CO, 0,
K2S04 KCI Silicates %atso, t o2= KJO, t q
Silicates K,SO,
Fe,O, Silicates K2S04 Cr/Fe oxide silicates Cr(Fe) oxide silicates
Ni (Fe,Cr) oxides, Ni,Cr,Fe chloride, Mo oxide Ni-Cr oxides NICI,
Sanicro 28 Fecl,t’hcI,=FecI,
Figure 3.25b: Sanicro 28 at a metal and gas temperature of 600°C and 800°C respectively.
56 3.4. CONCLUSIONS
The following conclusions can be drawn from this chapter:
The presence of both aggressive gases and potassium sulphate/chloride containing ash has a synergistic effect on the corrosion rate in that potassium chloride reacts with oxygen and sulphur dioxide to liberate chlorine. This chlorine gives rise to the active oxidation mechanism.
The presence of a flue gas temperature (800°C) which is higher than the metal temperature (6OOOC) leads to increased corrosion attack compared to specimens where the flue gas temperature and metal temperature are both 600°C. Thus the flue gas temperature and subsequently the temperature this imparts to the deposit in contact with the oxide is important.
The greater amount of nickel and chromium in Sanicro 28 gives improved resistant to corrosion attack.
57 4. CONCLUDING REMARKS
From this report the importance of ash deposits in the high temperature corrosion scenario is highlighted. It is seen that corrosion is accelerated when both ash deposits and corrosive gases are present, and that corrosion is less in the high alloy steel. A marked increase in corrosion rate is when the flue gas is raised from 600°C to 800°C indicating the importance of flue gas temperature.
The characterisation of ash deposits has indicated that deposits vary from different plants and different areas in the plants, thus a type of corrosion attack due to certain deposits which may be relevant in the radiation chamber may not be so relevant to the economiser section even though the same deposits are present. This is also substantiated from the laboratory work where an increase in flue gas temperature results in a different mode of corrosion. It is also of interest that the indication of molten deposits was observed in straw-fired plants and not in waste incineration plants.
Based on these results and the past EFP projects, it is seen that ash deposit corrosion is sigdicant in biomass fired power stations. This type of corrosion is very temperature dependent and dependent on deposit components. Certain deposits can give rise to molten phases either from the deposit components themselves or from a mixture of deposit components and corrosion products and this area deserves hrther investigation.
58 References 1. EFP 91 Nye materialer i affaldsenergianlaeg 2. EFP 93 Projekt J.nr. 1323/93/ 0012 AfpreMling af‘Overhedermaterialer for Affalds- og Biobrandsel-Energianlag” 3. EF’P 94 Projekt J.nr 1323/94-0006 High Temperature of Corrosion of Superheater Materials for Power Production through Biomass” ISBN 87-5 50-2205-7 4. European Federation of Corrosion Publication No 14 “ Guidelines for Methods of Testing and Research in High Temperature Corrosion” Edited by H.J. Grabke and D.B. Meadowcroft 1995. 5. N. Otsuka and T. Kudo Proc. Internat. Cod. Stainless Steels ‘91 Chiba ISIJ pp. 402-407 1991. 6. N. Otsuka and T. Kudo, High Temperature Corrosion of Advanced Materials and Protective Coatings Edited Y. Saito, B. Onay and T. Maruyama pp. 205-211 1992. 7. Phase Diagrams for Ceramists 1975 Supplement p. 340 Edt. American Ceramic Society 1969. 8. Thermocalc database KTH Stockholm (1995) 9. V.A.C.Haanapel et al, High Temp. Mat. Proc. 10 (2) pp 67-89 1992.
59 Appendix 1 X-rav diffraction (XRD)of 4 samdes from Reno South
Description of samples
The following 4 deposit samples have been selected for XRD analysis: Sample 10: Reno South superheater Sample 9 : Reno South economiser Sample 8A:Reno South superheater 3 Sample 8C:Reno South radiation chamber
Sample procedure
The deposit material from the four samples was homogenised in an agate ball mill to a particle size of <0.125 mm. The material was then partitioned into four parts. Powder diffraction was conducted in a Rigaku x-ray difiactometer using Cu Ka radiation. The above mentioned analysis method can determine crystalline phases in the sample material. Phases represented by a content of less that 5 vol.% cannot be detected with certainty.
Results The samples all contained a great number of different phases which made identification complicated. The dieactogram did not reveal a great amount of amorphous phases.
The phases mentioned below were identified as main constituents of the samples: Sample 10: KNaS04 Sample 9: KNaS04, K3Na(S04)2 Sample 8A: K2Pb(S0& Sample 8C: No main components could be identified with certainty due to lack of time.
The four samples contain almost certainly small amounts of the phases: CaS04 Si02 &So4
The difiactograms for the 4 samples all indicate that there is also the presence of “combined” sulphates in the samples. Here are some examples: Na&( SO& N&n( SO& K2Zn2( Sod3 K2Pb(S04)2 as well as K,,N&( S04)(,+,,,p
60 The "combined" sulphates can occur by heating of the "original" sulphates. For example, an equal amount of Na2S04 and K2S04 when heated for an appropriate period forms KNaS04. Zinc and lead sulphates will in a similar way be mixed with sodium and potassium sulphates. Hence a numerous amount of different sulphates can occur with varying content of lead, zinc, potassium, sodium and perhaps calcium. Many of these phases have an almost identical crystal structure which of course makes identification uncertain.
EDX analysis shows that the sulphur and oxygen ratio in samples amounts more or less to that seen in sulphate. XRD analysis also indicates that there are primarily sulphates and not oxide that are present.
Jacob B. Markussen The Force Institute
61