Metal wastage in fluidized-bed combustors

Materials Science and Engineering, 88 (1987) 295-301

295

Metal Wastage in Fluidized-bed Combustors* D. M. LLOYD, E. A. ROGERS, J. E. OAKEY and A. J. PITTAWAY

British Coal, Coal Research Establishment, Stoke Orchard, Glos. GL52 4RZ (U.K.) (Received May 26, 1986)

ABSTRACT

1. INTRODUCTION

The fluidized-bed combustion o f coal is finding increasing applications in industry. By employing heat exchange surfaces located in and above the fluidized bed, it is possible to generate h o t water, steam or h o t gas which can be used for a range o f heating and process applications. For some types o f c o m b u s t o r design, it has been observed that unacceptably high rates o f metal wastage can occur on heat exchanger surfaces which come into contact with the fluidized bed. The reason w h y only some units exhibit wastage is unclear and is probably masked by the wide range o f operating conditions and boiler designs employed. Additionally, the precise wastage mechanism is u n k n o w n ; however, it is widely believed that the wearing action o f the fluidized bed on metallic surfaces is largely responsible. Whether wear occurs as a result o f erosion and/or abrasion, and what role is played by corrosion, is open to question. In an a t t e m p t to resolve these issues, the metallographic nature o f heat exchanger surfaces removed from a n u m b e r o f fluidized-bed combustors, some o f which have suffered wear, has been studied using microscopic analysis. Also, laboratory-based experiments have been devised in order to simulate the type o f wastage experienced in industrial units. A laboratory rig has been constructed, which allows erosion tests to be performed at low particle velocities and at elevated temperatures. Erosion tests at a low velocity and at ambient temperature are reported. Erosion rates and morphologies produced in the laboratory are compared with those observed in commercial fluidized-bed combustors. Areas o f future work are also identified.

To keep coal competitive with ot her fuels and to m eet the exacting requirements of industry, m uch research and devel opm ent into the attractive t echnol ogy o f fluidized-bed com bust i on of coal are taking place. The process involves burning coal in an inert bed of sand, or coal ash, which is fluidized at velocities o f around 3 m s-1 . For the industrial market, fluidized-bed com bust i on is carried o u t at atmospheric pressure. The heat generated is t hen used to raise h o t water, low grade steam or h o t air for space heating, process heat and drying purposes. Additionally, pressurized fluidized-bed combustion is being considered for power generation. Electricity is generated using a combined cycle t hat incorporates b o t h a steam turbine and a gas turbine. The gas turbine is run o f f the cleaned h o t exhaust gas from the combustor while the steam turbine is pow ered by steam generated in tubes t hat are totally, or partially, immersed in the fluidized bed. Over the last 5 years or so, incidences of enhanced wastage of tubes t hat com e into c o n t a c t with the fluidized bed have been report ed for some industrial [ 1] and pressurized [2] units. Wear rates as great as 1 m m per 1000 h have been measured. Although the precise mechanisms of this wastage are unclear, it is believed t hat wear on in-bed heat exchanger surfaces is caused by the aggressive action of the bed material. Cold m odel experiments [ 3] suggest t hat wastage of boiler walls and tubes immersed in deep beds is largely attributable to abrasion while wear of tubes in the expanded bed region is t h o u g h t to be due to erosion. These studies [3] have also shown t hat the t ube bank g e o m e t r y and location, the fluidizing velocity and the fluidization state are also i m p o r t a n t variables influencing wear. Additionally, when wear is occurring on alloy tubes operating at elevated temperatures,

*Paper presented at the International Symposium on High Temperature Corrosion, Universit~ de Provence, Marseille 13331, France, July 7-11, 1986. 0025-5416/87/$3.50

© Elsevier Sequoia/Printed in The Netherlands

296 the rate of metal wastage will be controlled by physicochemical interactions. For some alloys, continuous oxide scales that are capable of resisting wear may form if the operating temperatures are conducive. Alternatively, corrosion and wear mechanisms can interact in a synergistic manner, resulting in appreciable metal loss. Finally, regardless of the wear mechanism the characteristics of the bed material will also influence the wastage process. These characteristics include the particle type, size and shape and the ease with which they are subject to degradation, or attrition, during their exposure in fluidized beds. To combat the problem of wear in fluidized-bed combustors, a large programme of work funded by the European Coal and Steel Community is being carried out at the British Coal's Coal Research Establishment. A significant portion of this programme is dedicated to the study of materials issues influencing wear in fluidized-bed combustors. In this paper, some of the initial results obtained from these materials studies (the objectives of which are outlined in Section 2) are presented.

2. EXPERIMENTALPROGRAMME The scope of the work programme studying materials issues relating to wear in fluidizedbed combustors entails (a) the study of wear mechanisms using laboratory erosion and abrasion rigs, (b) the examination of surface morphologies of worn and unworn components removed from fluidized-bed combustors, comparing them with those produced under control laboratory conditions, and (c) ranking wear-resistance steels, protective coatings and surface-hardened alloys exposed in laboratory rigs and commercial boilers. This paper concentrates on erosion phenomena. The subject of abrasive wear in fluidized-bed combustors and the influence of bed material properties will be the topics of a future pubhcation.

3. EXPERIMENTALPROCEDURE

3.1. Apparatus development: recirculating material erosion rig To study the erosive behaviour of materials under conditions similar to those found in

r.....~- TARGET(AT 45° TODRAUGHTTUBE) DRAUGHTTUBE FLUIDIZING NOZZLES

FLUIDIZING AIR

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Fig. 1. Recirculating material erosion rig.

fluidized-bed combustors, it was necessary to construct a rig capable of running for long periods of time at low particle velocities (less than 10 m s-1 ). Additionally, it is important to establish the influence of temperature on wear. The recirculating material erosion rig, shown in Fig. 1, satisfies these requirements. The apparatus consists of a fluidized bed containing a draught tube. Located immediately above this draught tube is a polished target (the mass of which is determined previously) inclined at an angle of 45 ° to the normal. Target wear is produced by firing bed material up the draught tube. After hitting the target, the erodent falls back into the fluidized bed. The fluidizing velocity and the spout velocity are controlled by two separate compressed air supplies. When required, these supplies can be heated to 600 °C using two resistance-wound heaters. In this way the target can be tested at temperatures recorded by a thermocouple that is buried in the specimen's bulk. Two methods were employed to determine the velocities of the particles at the point where they would hit the target. An Eastman Kodak high speed cine camera running at 2500 frames s-1 was used to record the motion and hence the velocity of the particles. The second method (a correlation technique) involved the impacting particles passing two illuminated photo-transistors, spaced 5 m m apart, one above the other, at the exit from the transport tube. The two signals produced by the shadowing effect of the particles were fed to a Hewlett-Packard 3721A correlator. The most probable velocity could then be de-

297 termined b y cross-correlation of these signals. Using these two techniques, it was possible to correlate particle velocities in terms of spout air flow rates. Th e degree of erosion experienced during each test was quantified in terms of mass loss measurements carried o u t using a Sartorius analytical balance. 3.2. Wear morphologies The surfaces of alloys exposed in the labora t o r y a n d commercial fluidized-bed boilers were examined using a Nikon SMZ-10 stereooptical microscope and a Philips 501-B scanning electron microscope. The electron microscope was fitted with an electron probe and an EDAX 711 energy-dispersive X-ray microanalyser, allowing the acquisition o f chemical data. 4. RESULTS AND DISCUSSION 4.1. Erosion behaviour o f steel in the laboratory 4.1.1. Erosion rates at low particle velocities A survey o f the literature revealed t hat little in f o r matio n exists relating to the erosion o f steel b y sand, or coal ash, at low particle velocities. To eradicate this situation, the erosion o f mild steel at low particle velocities was studied using the recirculating material erosion rig (Fig. 1). The tests were carried o u t using silica sand, having a size range of 6001180 pm, which was accelerated up the spout to hit the polished mild steel targets at velocities o f either 2.1 m s-1 or 4.5 m s-~. Several tests o f varying duration were performed, the results of which are presented in Table 1. It is apparent f r o m these results t ha t an initial period o f rapid mass loss occurs. This is followed b y a transient stage o f reduced mass loss which then leads into a further period of more rapid wastage. The length of the transient period appears to be inversely portional to the increase in particle velocity. Mechanistically, the initial period of high wastage probably represents the removal of surface undulations on the surfaces o f the targets. It is possible t h a t the subsequent transient period corresponds to the competing process of particle e m b e d m e n t in the targets. However, it is unclear at this stage in the work w h y a f u r t h e r increase in the rate o f wastage should occur.

TABLE 1 Mild steel target mass loss due to erosion by silica sand in the size range 600-1180 pm using the recirculating material erosion rig Total m a s s of impacting particles (kg)

Exposure time (h)

Particle velocity (ms -1 )

Target mass loss (rag)

44 88 132 264

1 2 3 6

4.5 4.5 4.5 4.5

3.8 3.4 3.6 7.4

199 1912 2868

5 48 72

2.1 2.1 2.1

1.2 4.6 13.2

At this poi nt it is i m p o r t a n t to compare the mass loss data given in Table 1 with the wastage rates measured in commercial fluidizedbed boilers. As q u o t e d in Section 1, a metal recession rate of 1 mm per 1000 h is representative for a fluidized-bed c o m b u s t o r suffering wear. Such boilers use a fluidizing velocity of 2-3 m s-1 while the particle loading on metal surfaces com pl et el y immersed in the bed will be somewhat less than 1000 kg cm -2 h -1. In order to achieve a direct comparison, it is necessary to convert the surface recession rate of 1 m m per 1000 h into a mass loss value. Since the wear scars observed on the metal targets exposed in the recirculating material erosion rig are roughly circular with a diameter of approxi m at el y 10 mm, it is convenient to calculate the surface recession rate in terms of the mass loss from a cylinder having a crosssectional diameter of 10 m m and a d e p t h of 1 m m (Fig. 2). Therefore, in 1000 h the mass of mild steel removed from such a cylinder is 0.6 g or 0.6 mg h -1 . It is observed f r o m Table 1 t hat the rates o f metal wastage produced under laboratory conditions compare very favourably with those measured in fluidizedbed boilers even though the particle loadings in the laboratory rig are a factor of 30 less than the estimated loadings in a fluidized bed. Therefore, it appears to be unnecessary to have to explain c o m p o n e n t wastage in boilers in terms of erosion which arises because a small fraction of the bed material particles travel at high velocities (of the order of 10

298 par'ic[e ve[ocify,,~2 m s-I

Fig. 2. Schematic diagram of the cylindrical volume used to compare the metal loss rate in the fluidizedbed combustor which is about 1 m m per 1000 h (equivalent to 0.6 mg h -1 ) with the metal loss rate in the laboratory which is about 0.2 mg h -1 (particle loading (fluidized-bed combustor), less than 1000 kg cm -2 h -1 ; particle loading (laboratory), about 40 kg cm -2 h-l).

m s-1 ) since equivalent wastage rates can be reproduced in the laboratory using low velocity particles, albeit in the absence of elevated temperatures.

4.1.2. Erosion morphologies The surface of a mild steel target removed f r o m the recirculating erosion rig after being eroded by 44 kg o f 6 0 0 - 1 1 8 0 pm silica sand impinging at an angle of 45 ° and a velocity of 4.5 m s-1 is shown in Fig. 3. Figure 3(a) is a scanning image of the most severely eroded region. It is clear that considerable disturbance of the target's surface has occurred. Additionally, as indicated by the X-ray spectrum {Fig. 3(b)) obtained f r om this region, appreciable SiO2 contamination of the surface has resulted. Such contamination may arise because small pieces of er odent are left behind in the impact craters (Fig. 4(a)) o r , as is more frequently the case, larger particles becom e embedded in the target's surface (Fig. 4(b)). The mechanisms of alloy wastage under erosive conditions can best be deduced by examining the erosion damage present on the periphery o f the exposed targets. In this region, particle impacts are few and it is possible to observe the damage pr oduc e d by singleparticle impacts. The scars and impact craters shown in Fig. 4(c) are those observed on the periphery of the target. It is observed that metal is displaced, or extruded, to the edges, or tips, o f these indentations. Subsequent im-

I I Fe

AL

Au Fig. 3. (a) A scanning electron micrograph of a severely eroded region of a mild steel target (each white dash equals 10 pm); (b) an X-ray spectrum obtained from the surface shown in (a).

pacts would then result in the removal of the e x t r u d e d material. These observations are not unique and are consistent with those associated with ductile erosion [ 4]. It is also interesting to note that the dimensions of the erosion craters and scars are of the order o f 10 pm while the diameters of the sand are of the order of 1000 pm. It appears t herefore t hat the observed damage is produced by the cutting action of the sharp edges of the sand. However, the sand does n o t remain completely unaffected by its impact with the steel targets and some particle breakdow n does occur, generating smaller angular particles. On a macroscopic scale the eroded surfaces are rippled. Again, such an observation is con-

299

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o o

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o o

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o o

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°

o r °

J

\ $ f Q f i c bed he,ghl

Fig. 5. (a) A schematic diagram of the in-bed heat exchanger layout associated with furnace A (o, SiC tubes; o, type 310 stainless steel tubes); (b) a scanning electron micrograph showing the surface morphology of the fireside heat exchanger surface (each white dash equals 100/~m); (c) a higher magnification image of the area shown in (b) (each white dash equals 10 pm).

Fig. 4. Scanning electron micrographs of an eroded mild steel target (each white dash equals 10 pm): (a) erodent contamination in a wear scar; (b) particle embedding in the surface; (c) erosion damage.

sistent w i t h ductile erosion and has b e e n rep o r t e d b y o t h e r w o r k e r s [5].

4.2. Field experience By e x a m i n i n g the surfaces o f w o r n and unw o r n c o m p o n e n t s r e m o v e d f r o m fluidized-bed c o m b u s t o r s , it is possible to establish w h y wear has, or has n o t , o c c u r r e d . A d d i t i o n a l l y , b y c o m p a r i n g t h e wear m o r p h o l o g i e s w i t h

t h o s e p r o d u c e d in the l a b o r a t o r y u n d e r controlled c o n d i t i o n s , it m a y be possible to ident i f y w h i c h wear m e c h a n i s m is o p e r a t i n g w i t h i n any particular boiler. It is the overall i n t e n t i o n o f this research p r o g r a m m e to categorize the surface t o p o g r a p h i e s o f alloys e x p o s e d in m a n y industrial fluidized-bed boilers; in the following a c c o u n t s , some o f the e x a m i n a t i o n s carried o u t so far are described.

4.2.1. Furnace A Figure 5(a) is a schematic diagram showing t h e in-bed h e a t e x c h a n g e r a r r a n g e m e n t in a f u r n a c e used to heat air for d r y i n g purposes. T h e m a j o r i t y o f the t u b e s in this unit are manu f a c t u r e d f r o m SiC. H o w e v e r , for trial purposes, f o u r t y p e 310 stainless steel t u b e s w e r e also installed in the locations indicated in Fig. 5(a). During o p e r a t i o n , these t u b e s are pred i c t e d t o possess m e t a l t e m p e r a t u r e s in the range 6 0 0 - 8 5 0 °C. A f t e r e x p o s u r e f o r a p p r o x i m a t e l y 4 0 0 0 h a steel t u b e was r e m o v e d {Fig. 5(a)) f o r e x a m i n a t i o n .

300

Visual inspection revealed that the tube had not wasted significantly. The b o t t o m and the sides of the tube were smooth, the b o t t o m being matt black and the sides dark brown. The upper surface of the tube was covered with a red deposit. Figure 5(b) is an electron image showing the typical surface morphology found on the lower surface of the stainless steel tube. The surface is covered by an oxide scale although in some places it had been chipped away, presumably by the impacting bed material. Figure 5(c) is a higher magnification image, revealing that the individual damage craters possess a layered structure. Electron probe microanalysis demonstrated that each layer has a different chemical composition. The free surface consists of an iron-rich scale containing some chromium as well as embedded bed material. Below this outer layer lies a Cr2Oa-rich scale. Finally beneath the Cr20~ rich layer exists a chromium-depleted region of the alloy. It appears therefore that the different layers of oxide formed on this alloy protect the tube from wear. Even though some scale removal does occur, it is probably quickly replaced by freshly grown oxide. Hence, it seems reasonable to postulate that the use of a highly alloyed stainless steel exposed at high temperatures in the range 600-850 °C affords some protection against wear. Similar experiences have been reported for tubes removed in the atmospheric fluidized bed at the Netherlands Organisation for Applied Research (TNO) [6] and in the pressurized unit at Grimethorpe [2]. 4.2.2. Boiler B Wear of tubes extending into the expanded bed region of this boiler has been particularly severe (Fig. 6(a)). These mild steel tubes generate low grade steam and operate at a temperature of about 250 °C. A fluidizing velocity of 2.5 m s-1 is used. Visual examination of a badly worn section of tube removed from the boiler revealed that the wear was concentrated on the lower surface. This region was dark brown and extremely smooth. The electron images of the worn surface (Figs. 6(b) and 6(c)) demonstrate that this smoothness exists even at higher magnifications. In some regions, isolated colonies of iron oxide scale are present on the worn surface.

Specimen examined

Static bed height " - -

(o)

~m.

@

.

C]

Fig. 6. (a) A schematic diagram of the in-bed heat exchanger associated with boiler B; (b) a scanning electron micrograph of the worn surface (each white dash equals 10 pro); (c) an area possessing oxide scale (each white dash equals 10 pro).

This examination shows that it is not easy to establish the wastage mechanisms operating. It is difficult to decide from the surface topography whether erosion or abrasion is occurring. The presence of some oxide scale on the wearing surface implies that wear-corrosion interactions may be occurring. However, because of the low metal temperature, corrosion is likely to play only a minor role.

5. CONCLUSIONS

(i) The erosion rate of mild steel by sand at low particle velocities at ambient temperature is not linear with time. (ii) Erosion rates similar to those measured in commercial fluidized-bed boilers can be reproduced at ambient temperature in laboratory erosion rigs for particle velocities of about 2 m s-z . (iii) Erosion morphologies observed on mild steel eroded in the laboratory are typical of

301 ductile erosion. A p p r e c i a b l e e m b e d m e n t o f b e d m a t e r i a l in t h e surface o f steel was also detected. (iv) Stainless steel ( t y p e 310) e x p o s e d at elevated t e m p e r a t u r e s o v e r t h e r a n g e 6 0 0 8 5 0 °C a p p e a r s to resist w e a r b e c a u s e o f t h e f o r m a t i o n o f p r o t e c t i v e o x i d e scales. (v) T h e surface m o r p h o l o g y o f a w o r n boiler t u b e o p e r a t i n g at a m e t a l t e m p e r a t u r e o f a b o u t 2 5 0 °C w a s largely featureless, m a k i n g it dificult t o d e d u c e w e a r m e c h a n i s m s .

o f t h e a u t h o r s and n o t necessarily t h o s e o f British Coal. This w o r k w a s f u n d e d b y a E u r o p e a n Coal and Steel C o m m u n i t y grant.

REFERENCES 1 W. Stockdale, D. Toth, C. G. Wells and F. Ellis, Foster Wheeler operating experience on commercial fluid bed boilers, Fluidised Combustion: Is It Achieving its Promise?, Proc. 3rd Int. Fluidised Conf., October 1984, Institute of Energy, London

6. FUTURE WORK F u t u r e r e s e a r c h will c o n c e n t r a t e o n establishing (a) t h e a b r a s i o n m e c h a n i s m s , (b) t h e i n f l u e n c e o f t e m p e r a t u r e on w e a r , (c) a r a n k ing o f w e a r - r e s i s t a n t alloys, coatings a n d surf a c e - h a r d e n e d alloys a n d (d) t h e w e a r m e c h anisms o p e r a t i n g in c o m m e r c i a l f l u i d i z e d - b e d combustors.

ACKNOWLEDGMENTS T h e a u t h o r s are g r a t e f u l f o r t h e assistance p r o v i d e d b y colleagues d u r i n g t h e p r o d u c t i o n o f this p a p e r . A n y views e x p r e s s e d are t h o s e

1984, p. KN/1AI-1. 2 Materials related activities during test series 2.2 and 2.3, Vol. 1, Grimethorpe Pressurised Fluidised Bed Combustion Project Rep. GEF/U/84/15, 1984. 3 M. J. Parkinson, Cold model studies of PFBC erosion, Proc. Workshop on Materials Issues in Fluidised Bed Combustion, Electric Power Research Institute, Palo Alto, CA, 1985, to be published. 4 A.W. Ruff and S. M. Widerhorn, Erosion by solid particle impact, Treatise Mater. Sci. Technol., 16 (1979), p. 69. 5 I. Finnie and Y. J. Kabil, On the formation of surface ripples during erosion, Wear, 8 (1965) 60. 6 P. L. F. Radermaker, L. Bos, J. C. VanWortel and B. H. Kilster, Corrosion/erosion tests in a 4 MWth atmospheric fluidised bed boiler, Fluidized Combustion: Is It Achieving its Promise?, Proc. 3rd Int. Fluidised Conf., October 1984, Institute o f Energy,

London, 1984, p. DISC/41/366.